ISSN:0306-0012    

DOI:10.1039/CS97201FX001

出版商:RSC    

年代:1972

数据来源: RSC

ISSN:0306-0012    

DOI:10.1039/CS97201BX003

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

Research in Chemical Education: A Reassessment1 By R. C.Whitfield DEPARTMENT AND FACULTY OF EDUCATION, CAMBRIDGE 1 Introduction For a chemist turned educationist, though the two labels are fortunately by no means mutually exclusive, the task of preparing a convincing article on research in chemical education for, predominantly, an audience of professional chemists is by no means easy. In the first place little real research has been done, reflecting the feelings of many that we can perhaps manage without it, for we seem to have done so in the past; progress in chemistry and its applications continues. Secondly, educational research has apparently been seen by many to have been tried in other areas, for example in intelligence and school organi- zation, and to have failed because of a lack of established and undisputable methodology. Finally, educational activities are often viewed by practitioners in the dlite disciplines, of which chemistry is one, as being inevitably bound up with intuition, passion, and a succession of ephemeral bandwagons rather than with disciplined reason based upon a developed system of concepts. It is the in-tention of this review to point to and admit the poverty of many ideas and ex- periments in chemical education research, to provide a map of the field and outline some current British work, and to chart possible routes by which exist- ing deficiencies and barriers to progress might be overcome. But first, at least for the purposes of this article, the area of concern must be further clarified by the specific exclusfonof two cognate aspects: the administrationof chemical education, by which is to be understood the institutional structure within which chemical education takes place (schools, universities, technical colleges, industry etc.), and the elementary hard and software of buildings, services, and apparatus required for the various courses of instruction; and development in chemical education by which one means the production of new courses and other less extensive innovations, together with their related aids and apparatus; developments may of course include a ‘research’ element but most, for example the NufField courses,8 have hither- to depended almost solely upon the relatively informal pooling of experience through which some consensus of ‘good‘ practice has emerged.‘This paper has arisen in a considerably extended and modified form from the author’s contributions to the symposium on ‘Research and Innovation in Chemical Education’ duringthe joint Annual Meeting of the Chemical Society and the Royal Institute of Chemistry,University of Sussex, April 1971. a Here there is no intended criticism of the Nuffield developments; few research findings exist as a base for constructing secondary school courses in chemistry; the distillation of experiencehas been the only viable means for obtaining revised courses. Research in Chemical Education: A Reassessment Research into chemical education implies the detailed, dispassionate enquiry into any aspect of the teaching and learning of chemistry at any stage in education. Some of this enquiry, perhaps more than we realize, can be done by disciplined ‘armchair’ methods, but much can only be performed by some kind of deliberate, imaginative, yet controlled empirical investigatioE ; implied is much more than mere fact-finding or opinion-gathering surveys.Although developments in, and the administration of chemical education have been with us in the formal education system for over a century, with a sig- nificant increase in, respectively, the pace and complexity of these undertakings particularly over the past decade, research into the field is a relative newcomer, and the need for this research has been intensified by the contempopary myriad of changes and developments in all our institutions of science education.Development leading to research in this field, rather than the reverse, is in contrast to much of scientific enquiry and its subsequent application in technology. But, as in many other spheres, we must see R and D as com- plementary partners. 2 What Educational Research Can and Cannot Provide The professional research chemist has little difficulty in understanding the potential contribution of his research to the theory and practice of chemistry. In the laboratory he discovers new ways of transforming materials, the physical and chemical properties of these materials, and an understanding of aspects of the processes of change. All this adds up to an ever-growing corpus of empirically gathered data which is systematized using the conceptual and theoretical tools which the discipline has evolved, chiefly over the past 200 years. The aim is an understanding leading to a control of natural phenomena using an ‘if-then’ causal attack.Theory and empirical findings march together in a spiral approach to the chemical heights, and a well-defined framework of concepts and methodologies exists both for classifying new discoveries and for publicly testing their validity. Would that the educational researcher were in such a fortunate position! In the educational ‘sciences’ we are perhaps 150 years behind the natural sciences in terms of both concepts, criteria and methodology. Only in this century have educational problems been considered worthy of disciplined investigation, and even today, many of those so engaged continue to do battle against con- servatism within the established disciplines and all the hoary problems of a ‘poor relation’.While the methodological problems of serious research into chemical education, or indeed into many other aspects of education, are immense (see further below) a good deal of popular misunderstanding let alone misguided research, arises from a lack of clarity about what research in this sphere can, and cannot, provide. And it should be noted that chemical education research is necessarily a branch of educational, as opposed to chemical research; it requires a practice of the relevant educational disciplines- particularly I would suggest philosophy and psychology-in an attack on the problems of chemistry in its educational setting, and it is characterized by Whitfield strengths and weaknesses similar to those which characterize the broad field of educational research.It must be understood that educational research cannot provide detailed solutions to many specific and important ‘in the field‘ problems-for example whether comprehensive schools are more effective than grammar schools, or what should be the timetable in school X, or whether ‘Nuffield‘ chemistry is more effective than ‘traditional’ chemistry, or whether it is better to buy an overhead rather than a film-loop projector, or whether undergraduates should have a research element in their degree courses.The diversity of valuations, objectives, and other variables within education is altogether too vast to permit valid direct transfer of many research or survey findings. Given an agreed framework of objectives, research may provide generalizable data about certain particular instructional problems, for example those of content sequencing and concept formation, but the area over which such transfer is applicable is probably far smaller than many might hope. What research can supply however, is a set of concepts, principles, and classifications which teachers and others can use in their own, often unique, situation in order to make wiser decisions; it can provide a basis for understanding the topology of the educational process and for designing educational programmes. We should expect to apply the emerging theory and principles, not the particular results: here there is some, if limited, similarity with the work of the research chemist.We need therefore to ask the right kinds of questions in research, and too rigid a view of research as the ‘systematic study in which major generalizations and the bases on which they are made are publicly reported in such a way as to permit independent ~erification’,~ pressed by many psychologists too anxious to adopt the physical science model, cannot but lead to disappointment, at least at this stage of our understanding. The generalization of all our findings to cases other than those which were the subject of study, is a worthy but premature objective, since in quests for this transfer we are driven to limit our experiments to the manipulation of a single variable at a time, and this often leads to a highly artificial situation and naive concluding statements (4.8.coloured children have a lower 1.Q.than white children) which teachers not surprisingly find difficult to translate into practical action. A single pupil or teacher is after all far more complex than asingle gram of even vitamin Bla;we cannot generalize about them as readily as we can about molecules. Educational research, in so far as it in- volves persons and their diverse valuations, should neither aspire to, nor be required to settle disputes in the neat and tidy manner which we now tend to expect in the physical sciences.3 Criteria for Research What criteria canwe therefore begin to tease out for sound research in chemical education out of which we can derive a much needed structure of criticism? The pertinence of the research must be a fundamental consideration, and to *R.W.Tyler, Journal of Research in Science Teaching, 1967,5,54. Research in Chemical Education: A Reassessment test any investigation in this respect we require a comprehensive yet cogent map of the terrain; an outline is suggested later in this review. Certainly research focused on the realities in which teachers are actually engaged stands the chance of influencing practice; research which does not help to explain some aspect of the educational process is worthless.Pertinence alone is however an insufficient criterion; to this we must add an adequate conceptualization of the phenomenon under investigation and appropriatemethodological techniques. Illustrations will here serve to clarify the first of these two further criteria. Research on teaching metho& is clearly of great importance but many of OUT concepts here have been unproductive. Typical classifications of methods as traditional/modern, didactic/discovery, or linear-syllabus/topic-projecttend to be gross oversimplifications of any teacher’s classroom technique. Direct comparison of methods so described can be futile, particularly since the under- pinning objectives which particular methods are intended to achieve may be different. A more fruitful conceptualization may be to view teaching methods as the teacher’s flexible way of manipulating and controlling the variables under his command*-for example the content sequence, aids in its presentation, form of pupil grouping, and rules of conduct-the flexibility being incorporated in order to emphasize variations in teachers’ actions which are dependent upon the particular pupils being taught.Unhelpful, or even naive, conceptualizations and stereotypes in learner research are the axes of able/less able, culturally deprived/ fortunate, middle class/working class, convergent/divergent, and scientist/non- scientist; these seem no more useful to educators than the notions of well nourished/malnourished are to the modern nutritionist. More useful descriptions might be concerned with the acquisition of particular skills and abilities and the extent to which these are fostered in particular children by particular manipu- lations of teaching method variables. What we require are more refined concepts to guide practical action, just as the nutritionist now has at his disposal the ideas of carbohydrate, fat, protein, vitamin, mineral etc.in diet prescription. Studies concerned with the prediction of student success as undergraduates usually compare A-level performance with degree class. A-level is usually ‘shown’ to be a poor predictor, but in this crude three-component conceptualization university experience is treated as a constant and the degree class as a wholly dependent variable. Manifestly, other more helpful and appropriate conceptions are possible, and potential variables such as the students’ atfiliations with his peers, the university teachers with whom he comes into contact, the mode of degree class assessment, the appropriateness of the students’ place of residence for study, and the kind of study support which the secondary school provided may be highly significant, affecting both the research conclusions and any possible action to be taken (e.g.ignore A-level scores and depend upon interview for student selection).In this example however, even if investigations are more appropriately A. J. Bishop and L.B. Levy, Educationfor Teaching, Summer 1968,6145, Whitfield conceived, accurate prediction depends upon a static situation-one which will rarely pertain.Methodological criteria in experimentally-based work tend to hinge around population and time-sampling techniques, the technical quality and range of the test instruments employed, and the statistical treatment of data. Each of these aspects involves at times complex administrative and technical considerations which it is inappropriate to describe in any detail here (but see bibliography). Investigations of a more philosophical kind are chiefly concerned with mapping and analysing values and concepts ;here there are well-charted methodological guidelines and forms in which arguments can be expressed. However it should be stressed that no research methodology, however elaborate and rigorous, can compensate for paucity on the pertinence and conceptual fronts.Many sophisti- cated research models continue to manipulate irrelevant variables and incon- sequential events. 4 A Perspective on Chemical Education Before we can evaluate existing findings and plan adequately for future research, it is essential for us to map what appears to be the anatomy of chemical education. This can be achieved by a model in which the dynamic inter-relations between key concepts and ideas are charted; such a model forms an outline theory which is required in order to understand and plan the process of chemical education. The lack of a map can lead only to isolated studies, short-term projects and opportunistic investigations which make little contribution towards solving the basic problems and crises before us.The chart shown here (Figure) is by no means prescriptive, but it is offered as an analytic aid for focusing our plans; its further testing and elaboration will be achieved through future researches which can potentially be carried out under almost all the headings shown. Central in this map are OUT aims and objectives in chemical education. The difficult task of identifying and clarifying these must be the logical starting point at any level, and in this respect the tertiary sector has much to learn from recent work in the schools. The description of the new undergraduate chemistry scheme at the University of Sussex6 for example devotes little space to a thorough-going consideration of objectives.A defined and argued philosophy which is the basis of teaching aims is a necessity, not a luxury, for every teacher and course designer. Whether we like it or not, all our teaching is based upon ‘theory’; responsible practice requires us to make this explicit, for ease of communication and for the provision of criteria for both course content selection and assessment schemes. 5 Current Research and Pertinent Growth Points Much of the published research in science teaching has its origin in the United States and papers of the post-sputnik era (1957 being a watershed in U.S. 6 C. Eaborn, Chemistry in Britain, 1970,6, 330. a institutional climate concepts syllabuses courses genera1 education b (education flirough bodies Chemical Research- Chemistry Figure The anatomy of chemical education.Whitfield science education) have been recently reviewed.O One respected critic of such work founds that not more than 10% of recent studies met reasonable technical criteria. The major areas of concern of U.S. investigations, which appeared mainly as postgraduate theses, divided approximately as : fact finding surveys ........................................... .25 % collections of opinions ........................................ .20% comparative studies of courses and teaching methods ..............30+ % Few of the reported researches were concerned with the objectives of science, and fewer still with learning theories compatible with different science teaching objectives.None of the investigations conducted empirical tests of different learning sequences in any of the natural sciences-an alarming fact, bearing in mind the highly structured nature of science itself. A single, yet meaningless, experiment concerned with the highly important ‘external relations’ of chemistry (see later) reported that when chemistry and mathematics were taught in the same course, students scored as well on tests in each subject as when the courses were taught separately. The majority of studies employed gross variables of students and teachers-such as I.Q., personality test scores, years of experience, and degree class-which as we have implied are of dubious value. The critic concludes3 ‘that the potential value of research for improving science teaching is not being realized‘.In Britain, where resources of finance and manpower for science education research have been comparatively meagre, there are a few pockets of activity which could hold out some promise if given adequate support. A cautious beginning can have its advantages since it gives those involved the opportunity to reflect before rushing into inadequately conceptualized empirical work-and an experiment in educational research cannot be halted or modified anything like so easily as an experiment in chemistry. Having drawn attention to the centrality of aims in the chemical education enterprise, which ultimately reduce to issues of value in relation to the various ‘pressures’ on chemical education from society, our idea of the person’ derived from ethics and psychology, and the discipline of chemistry itself (see chart), a brief account will now be given of significant recent British thinking and empirical study.This is described under some of the major headings of the anatomical map. G. A. Ramsey and R. W. Howe, Science Teacher, 1969,36,62. The possible aspect of student pressure on our aims has here been deliberately omitted. There are substantial philosophical arguments which suggest that it is educational nonsense for teachers to allow those uninitiated into a discipline to influence the aims, as opposed to method variables, in its teaching. Here, briefly, a telling quotation on the logical nature of education will have to suffice: Education is ...‘the transaction between generations in which newcomers to the scene are initiated into the world in which they are to inhabit.This is a world of understandings, imaginings, meanings, moral and religious beliefs, relationships, practices -states of mind in which the human condition is to be discerned as recognitions of and responses to the ordeal of consciousness. These states of mind can be entered into only by being themselves understood, and they can be understood only by learning to do so. To be initiated into this world is learning to become human; and to move within it freely is being human, which is an historic, not a natural condition’. (M. Oakeshott, Proceedings of the Philosophy of Education Society of Great Britain, 1971,5, 73.) Research in Chemical Education: A Reassessment A.The Assessment of Chemical Ability-This has recently been the most popular area for research in the U.K. and we are moving, at least in the school sector, to a more adequate and comprehensive characterization of chemical ability. There has been widespread recognition of the inadequacies of traditional tests and examinations, particularly in the high weightings given, often unconsciously, by examiners at all levels to the ability to remember. Curriculum developers have recognized that new courses must carry new complementary examinations if their underlying objectives are to be achieved. Work led, initially under the Nuffield development umbrella, by Mathews has made an outstanding contribution to chemical education and has set a fine example in techniques of assessment to many other curriculum subjects. Much of this unique work has been described elsewhere: and studies continue under Mathews at the University of Lancaster.Hitchman, Yeoman, and Brown of Nottingham University recently completed an interesting and thorough investigationa into the oral examining of chemistry at C.S.E. level in which five abilities were tested in the context of practical work. It proved possible to construct valid and reliable tests, and while the authors appreciate the complex logistics involved in widespread examining of this kind, they suggest that oral tests might form an effective moderating instrument for internal assessments at any level.The objective assessment of abilities fostered through practical work in science is also being carried out under the auspices of the International Association for the Evaluation of Educational Achievement (I.E.A.). lo The English national science committee for this project pleaded for the measurement of practical outcomes in addition to the cognitive and affective measures, and we became responsible with our Scottish friends for the development of test items in each major science to measure three abilities which were condensed from a larger and more detailed ‘Taxonomy of Practical Abi1ities’:ll (i) The ability to use simple apparatus and implement simple procedures. (ii) The ability to observe changes/differences in structures or systems under investigation and to record such changes/differences in ways which yield the maximum relevant information.(iii) The ability to select appropriate apparatus and/or procedures for a novel experimental problem. Although results of this vast survey of science achievement in some 20 different countries will not be published until 1973, it is disappointing that the optional prctical tests have been taken in very few countries; this is a reflection of the somewhat unique British approach to science teaching in which individual 8 See for example: J. C. Mathews, Educ. in Chem., 1967, 4, 2; 1969, 6, 205; also IUPAC Report, Pure Appl. Chem., 1965, 11, 587, and ‘Evaluation in Chemistry’, IUPAC/UNESCO, 1969.a Schools CounciI, Examinations Bulletin No. 21, Evans/Methuen, London, 1971. 10 I.E.A. Box 6701, S--113 85 Stockholm, Sweden (co-ordinating director: T. N. Postle- thwaite); document IEA/SCI/27, April 1971, by J. F. Eggleston describes the origin of the national option practical tests in science. 11 J. F. Eggleston and R. C. Whitfield, unpublished work; for a version with chemical examples see R. C. Whitfield, ‘The role of practical work in school chemistry’, UNESCO, Guidebook on the Teaching of Chemistry, to be published. Whitfield practical work and sensory experience is, be it noted in faith, valued highly as a means for attaining our objectives. B. Assessment of Courses.-The assessment of chemical abilities is closely related to the appraisal of courses designed to promote these abilities-this latter field being that of ‘curriculum evaluation’.l2 The I.E.A. studies referred to above are essentially comparisons of gross national school science curricula at four different age levels, but do not attempt a rigorous division of national populations into sub-gr0ups-e.g. in England, ‘NufXeld’ and ‘others’. The utilizable feedback from these studies as far as specific curriculum improvement is concerned may be minimal. What we require perhaps more urgently are more sensitive national studies on particular courses and sections of courses. But such studies are far from easy to design; the common question about the relative effectiveness of different courses has no simple answer.In course comparison we are not simply placing on trial two brands of something like washing powder, designed to do the same job. Revised curricula at any level often represent far more than an up-dating of content; they also frequently embody a marked shift in both the objectives being sought and the underpinning learning-theory upon which course materials are constructed. Comparing course A with course B at the student performance level, as opposed to the objectives level, can therefore often be a meaningless exercise since the underlying aims of the two courses may be different. Empirically it is usually far more helpful to look at individual courses or course units using both formative (on-going) and summative (terminal) evalu- ation techniques.The Nuf€ield chemistry courses have been compiled using informal formative feedback from trial school teachers, but uncertainties about practicable methodology and restricted resources have prevented any more elaborate assessments at the compilation and revision stages. Nevertheless, since the intended pupil outcomes from these courses have been framed with a commendable precision, which should be the envy of many other curriculum developers, interesting objective feedback of a summative kind from the exam- inations referred to earliex should be possible. However, important long-term aims which cannot be assessed in this way-for example that ‘pupils should gain an understanding that lasts throughout their lives of what it means to ap- proach a problem scientifically’ 14-should not be forgotten.Long-term researches on the persistence of independent learning and the external transfer of abilities acquired in the formal educational settings are urgently required. It is perhaps relevant to mention here that tests for use in curriculum evaluation differ from graded examinations in one important respect. Examiners must needs be concerned about the operational discrimination of questions included in For a readable introduction to this complex field see D. A. Pidgeon and S. Wiseman, ‘Curriculum Evaluation,’ 1970, National Foundation for Educational Research. la M. Scriven, ‘The Methodology of Evaluation’, in AERA monographs on curriculum evaluation, No.1, Rand McNally, Chicago, 1967. l4 H. F. Halliwell, RZC Reviews, 1968,1, 21 1. Research in Chemical Education: A Reassessment their papers; that is they plan, or ought to plan where grades are publicly re- quired, for the identification of individual differences in the population under test. Questions which are likely either to be very easy or very difficuIt for the population as a whole are consequently often rejected. Tn the appraisal of a curriculum on the other hand we are more concerned about the performance in each target ability of the whole group being taught, rather than the grades of individuals. Hence, ideally, tests for curriculum evaluation ought to be con- structed upon modified examining principles ;fortunately the diminished need for discrimination eases some of the test-construction problems.Nevertheless we are a long way from having at our disposal valid and reliable tests for the spectrum of chemical education objectives which our courses seek to attain, especially in the areas of practical work and attitudes. The interpretation of group test scores remains a professional value judgement. For some of our objectives, such as the acquisition of certain practical skills or perhaps knowledge of the structure of the Periodic Table, we aim for ‘mastery’ learning; in others, such as the ability to apply previous understanding of reaction rates and mechanisms to new problems, we may, depending upon the level of study, be satisfied with a good deal less before drastically revising our courses.The sole ultimate purpose of gathering data of a statistical nature on the effects of teaching is to provide information for wiser professional valuations and decisions. As has been indicated, incursions into curriculum evaluation l6 in providing information about the effectiveness of course materials shed light upon the feasibility of our objectives and diagnose learning problems and learning strengths of student groups. Johnstonel6 is presently engaged at the University of Glasgow on important follow-up work and problem diagnosis arising from the intro- duction of the Alternative Chemistry Syllabus in Scottish schools in the early 1960’s. Topics rated as ‘difficult’ by about lo00 ‘successful school chemists’ now studying at the Universities of Glasgow and Strathclyde fall into two basic groups: (a) work based on a facility in the use and interpretation of formulae and equations, and (b) formal organic chemistry.Although this finding may be in accord with the everyday experience of many chemistry teachers, proper diagnosis must precede causative speculation and rectification. One of our tasks in research is to gather hard data before hastily prescribing for intuitive diagnoses. Apart from the evaluation of programmes of national significance, it is possible to train the practising classroom teacher to be more sensitive in the day- to-day appraisal of his work. Evaluation is in any case an unavoidable element of teaching, and provided teachers are able to state their intentions at the right l6 See for example R.C. Whitfield, ‘Assessing Outcomes of a New Approach to Organic Chemistry’, in ‘Studies in Assessment’, ed. J. F. Kerr and J. F. Eggleston, English Univer- sities Press, London, 1969. Some problems which emerged during this study are discussed by J. F. Kerr and R. C. Whitfield in Teachers College Record, Columbia University, New York, 1970,72, (2), 268. l8A. H. Johnstone, Scottish Education Department Science Newsletter, 11’27. level of generality, some relatively simple assessment techniques are available to enable them to gather more objective data on their classroom performance as mirrored by student gains. C. Content Analysis.-Philosophical analysis of the nature of chemistry and chemical enquiry is an essential prerequisite for the construction of sound courses.This relatively neglected area would map the key concepts of our discipline, the ways in which they are and can be used, and the incorporated criteria for chemical truth; the criteria we apply to the concepts we use in dis-cussions of bonding for example are of a different kind to those used in establishing the rate of a particular chemical reaction. Content/conceptual analysis is likely to give us insight into teaching orders which might logically be most likely to succeed; any adequate ‘psychological’ theory of learning must build upon philosophical analysis of the logic of material to be communicated; such analyses could save much fruitless experimental investigation.Some possible hierarchical structures among concepts-which might be termed ‘concept runs’-are now given (see Table). Table Some possible structures among concepts Prerequisite Primary Secondary Tertiary experience concept concept concept 1. Vision Red, blue, etc. Colour Salmon-pink etc. 2. Handling things Weight, volume Density Expansion 3. Warmth, cold Temperature Heat Specific heat 4. Dissolving Solvent, solute Part icles Solution 5. Uniformity Pure substance, Chemical change Formulae, within stuffs mixture equations 6. Visible changes Conservation Combining Formulae, in things of weight weights, equations proportions 7. Walls, bricks, Composites, Atoms, Elements, sand, etc.simples, relative molecules compounds simples 8. Brownian Atom, molecule Element, gram Compound, motion, etc. atom, molecule mixture, chemical change 9. Electrolysis Ion Gram ion Structure, models 10. See-saw, mixing Steady state Dynamic Effect of different coloured Equilibrium constraints on balls, etc. steady state systems It should however be noted that little serious philosophical analysis has been done to chart these concepts accurately either vertically, horizontally, or diagonally; the ground in more complex areas such as quantum mechanics Research in Chemicat Education: A Reassessment and thermodynamics, which degree students often find difficult, is even more uncharted, and our present orders of presentation of chemical experience are little more than guesswork. In any future work we will need to distinguish between a pupil’s implicit (assumed) and explicit (articulated and verbalized) possession of a concept. The Nuffield 0-Level Sample Scheme for example assumes an implicit understanding of the ideas ‘pure chemical substance’, ‘compound’, ‘mixture’, and ‘chemical change’ by the time pupils reach topic A5 on ‘chemical elements’; but an explicit understanding of all these notions may be contingent upon an understanding of the idea of an atom which is not encountered until topic 11 (see runs 7 and 8 above).Certainly little sense can be attached to the idea of a ‘chemical element’ beyond that of a ‘pure chemical substance’ unless there is some concept of different kinds of atoms.D. Method Variables.-Some of the problems of research into teaching methods have already been mentioned, and it was implied that since activity in any class- room is highly complex, we might view ‘methods’ as a series of variables, per- haps not yet all definable, which are potentially under the teachers’ control. Here the analogy is perhaps more one of teaching as a craft rather than as an ‘art’ or a ‘science’. When an investigator enters a classroom he requires a con-ceptualization of what to look for other than the purely ‘physical’ aspects of the environment-teacher, children, apparatus, furniture etc. What gives meaning to the complex exchanges involved are the structures, relationships, and processes perceived by the skilled observer.Here a formal ‘model’ is essential in order to provide some means for viewing the diverse phenomena in orderly and meaning- ful patterns and to give a basis for ‘method‘ recommendations. The Schools Council project on the evaluation of science teaching methods1’ has since 1970 been engaged upon an important investigation to determine if pupils who are taught by contrasting teaching methods exhibit significant differences in both their attainment and attitudes towards science. Early work centres around the complex task of identifying and classifying the predominant ‘teaching styles’ of a representative sample of science teachers. In this connection an observation schedule has been developed as the major instrument for record- ing and analysing teacher behaviour.Carefully trained observers are now using this schedule to record accurately a selection of the intellectual transactions which take place between pupils and teachers in science lessons, the product being an estimate of the probability that a teacher may make a particular kind of statement, give a certain type of direction, ask a specific sort of question, and so on; estimation of the probability that pupils will engage in certain kinds of activity (e.g. seeking guidance when solving problems) is also incorporated in the schedule. It is therefore hoped that each teacher in the sample can be placed at some point upon an enquiry/didactic continuum by the time their pupils’ changing in thinking and attitudes over one year are measured.l7 Co-ordinating director J. F. Eggleston, School of Education, 21 University Road, Leicester, LE1 7RF. Whitfield We must however remember that many of our apparent problems in relation to teaching methods may be removed by a closer scrutiny of our objectives in which are incorporated important notions of desirability. If for example we do not expect our undergraduate chemistry students to be walking encyclopaedias of ‘in’ facts, albeit in abridged versions, then we will not trouble to lay on lecture series which consist of solo recitals of ‘in’ facts. If we believe that a sound chemical education consists of a broad coverage of key concepts in the context of a wide variety of elements and compounds, then we will view project or topic- centred work with some suspicion.If we believe that deep personal contact with practising chemists is very important for motivating students and endowing them with favourable attitudes towards the practice of chemistry, we may be disinclined to use programmed instruction for large sections of our courses. On the other hand, we must be cautious about action based upon un-substantiated beliefs about desirability. We may assert for example that there can be no substitute for individual practical work by students i.e. that it is desirable for all students of chemistry to be individually engaged at the bench. Curiously however it has yet to be shown that individual-pupil, or even group, practical work, as opposed to e.g. class assisted demonstrations, confers distinc- tive skills, abilities, and attitudes on students.This is a question of the greatest practical importance, for we invest massive resources of time and capital at all levels on this untested article of faith; there are after all many ways of providing a practical context. Or do we incorporate individual practical work so extensively in our courses for motivational reasons, while failing to appreciate that the motivations of 12 year-olds and undergraduates are likely to be significantly different? We may however be certain that much time and effort is wasted in teaching laboratories at all levels because we have failed to analyse practical work objectivesll with sufficient precision, both as a basis for experiment selection and for putting our hypotheses to the test; and so often we justify practices for all students in terms of the professional requirements of the small minority who reach or who want to reach the research level. E.Student’s Perception.-The anatomical map presented earlier placed the concept of perception central in certain considerations about students as learners, particularly in relation to their conceptual development and motivation. This factor is being increasingly drawn to our attention by social psychologists. How we react to any situation is largely governed by the way in which we ‘see’ the situation and the associations we can draw upon from our past experience. Classical experiments in psychology using visual stimuli show that several people looking at the same object ‘see’ different things.The same is true for 11-year-old seconda1y school pupils, freshman chemists, or new employees for example, who will perceive different objectives for themselves, tutor and employer intentions, and so on, and these perceptions, whether ‘correct’ or not, form the basis for much of their later action. Present evidence indicates, rather agonizingly, that our perceptions of persons, objects, and events form during our early contact with them and remain relatively stable despite later attempts to modify the Research in Chemical Education: A Reassessment inadequacies of the early perception. Practical work in chemistry, yet again, is an example in which student perceptions are likely to be vitally important for both motivation and effective learning; work has recently begun in Cambridge to investigate this area with lower-secondary- school children.One canon for practice which seems clear despite our ignorance of this field is that teachers are more likely to convey ‘desired perceptions’ particularly in the area of the purposes of learning tasks if they are able to dispassionately communicate and justify to students the objectives of the tasks in educational and professional terms. F. Chemistry and other Subjects :‘External’ Relations.-Halliwell has remarkedls that chemical education still has many of the characteristics of a cottage industry -a somewhat parochial attitude. While this is probably the case, there is no evidence to prove that chemists are any more inward-looking than specialists in other disciplines.Neverthless there are vital ‘external’ questions which all subject specialists must consider for the sake of both the health of the community and, perhaps paradoxically, the disciplines themselves. Considerations of chemistry’s logical and practical relationships with other subjects are particularly important for planning the whole curriculum of our where our main concern is education through chemistry. Schemes for the integration of the major sciences for teaching purposes at any level require a closer consideration of the relations between concepts in the three subjects than we have hitherto achieved for effective planning.Although in epistemo-logical and psychological terms, integration of the empirical sciences is logically permissible (there are similar concepts, methodologies, truth criteria and abilities developed), we have a long way to go before we have effectively ‘bridged the gaps’ or ‘removed the barriers’ between physics, chemistry, and modern biology. A move towards more ‘integrated science’ in schools seems imminent, and may enable us for the first time to provide all children with a balanced programme in science. Relations of chemistry with non-scientific subjects are however equally important and some current questions of interest are : (i) Which concepts and methods within mathematics constitute essential tools for modern chemistry at C.S.E., 0, A, and University level? How can we plan jointly with mathematicians and other scientists both for the developmentof mathematicsqua mathematics and mathematics as a tool ? (ii) To what extent is the development of scientific concepts and favourable attitudes towards learning contingent upon a sensitive use of language ? The need for detailed investigations into the structure and function of language in chemistry teaching, particularly during the pupil’s earliest l8 See ref.14, p. 205. 19 For a seminal workinrelation to questions about the whole curriculum for general education see R. C. Whitfield (ed.), ‘Disciplines of the Curriculum’, 1971, McGraw Hill, Maidenhead, 1971. Parts of Chapter 17, and an essay by E. H. Coulson on the contribution of chemistry to the curriculum are particularly pertinent.Whitfield chemistry lessons, has been highlighted by Barnes’ revealing preliminary explorat ions20 into language exchange in secondary schools. (iii) How might we make more effective use of chemical knowledge and understanding in ethical and moral issues? Can and should chemical educators avoid being moral educators? How may we help to promote moral awareness and sensitivity in our pupils and students through chemistry? (iv) What contribution does chemistry make to aesthetic experience? Has chemistry any relevance in the teaching of art, craft and creative design? G. The Training of Chemistry Teachers.-Rapid changes in curricula, our increasing understanding of the processes of education, when compounded with our British tradition of giving teachers a large measure of professional respon- sibility for their teaching programmes, now place teacher training at a focal point for the fulfilment of many of our aspirations.While the present structures within which the initial and in-service training of science teachers takes place have many shortcomings (felt not least by those of us endeavouring to work within them), there has been no shortage of new ideas and practices in training in the recent past in which chemists have played a full part. The Association of University Chemical Education Tutors (AUCET), the corporate body of chemistry method tutors in university departments and schools of education, has for several years acted as an informal forum for the exchange of ideas and the development of practice.More recently, after initial clarification of training objectives21 and the pooling of sample resource material, the association acted as a springboard for the Science Teacher Education Project (STEP) which has been sponsored since its inception in early 1970 by the Nuffield Foundation under the general direction of two chemistry method tutors.22 This project has accumulated some exciting materials in a dozen topic areas for its main trials with student teachers in the academic year 1971-72. These trial units demonstrate quite clearly that we are a long way past the naive and narrow conception of teacher training as instructing people how to teach.While it remains our duty to do all we can to provide new entrants to the teaching profession with a ‘survival kit’, our aim is more fundamentally to provide young teachers with the elements of a conceptual framework upon which they can modify their initial and build their future professional actions. STEP has therefore sought to incorporate aspects of educational philosophy, psychology and sociology, together with systems management, alongside material of a specifically scientific ’* D. Barnes, ‘Language, the Learner and the School’, Penguin, London, 1969. “See for example, R. C. Whitfield, Chem. in Britain, 1969, 5, 362. The training of chemistry teachers has its own sub-set of objectives, methods, content, student and assess- ment considerations, etc.which are not shown in the Figure. *a Drs. C. R. Sutton (Leicester) and J. T. Haysom (Reading); further information about the project, whose materials will be published in 1973 by McGraw Hill, can be obtained from S.T.E.P. School of Education, University of Reading, 24, London Road, Reading, RG1 5AQ; see also C. R. Sutton, Educationfor Teaching, 1970, Summer, 13, and C. R. Sutton and J. T. Haysom, School Science Review, 1970,52 (178), 7. Research in Chemical Education: A Reassessment nature. Some of the project’s units are likely to be put to good use in courses of in-service training. In Cambridge we are endeavouring to make a more adequate characterization of that elusive creature the ‘good‘ or ‘effective’ teacher by placing emphasis upon the teacher’s unavoidable role as decision-maker.We suggesta3 that the effective teacher is one who possesses: (a) an awareness of the variables under his control (these variables acting as a source for generating options from which teaching decisions are selected); (6) an awareness of the likely effects of manipulating these variables in different environments; and (c) an ability to manipulate variables in order to achieve his objectives, this last attribute being the realization of the potential mapped out by (a) and (6). In our work we make a distinction between ‘planned‘ and ‘on-the-spot’ decisions. The former have traditionally received significant attention in courses of teacher preparation by way of lesson planning exercises and so on.The more complex but crucial area of decisions which have to be made ‘on-the-spot’- whether in classroom, common room, corridor, or playground-has received relatively little attention. Our investigations are centred in this area through analyses of actual teaching situations, some of which are chemical in origin, though we must remember that the chemistry teacher has many decisions to make that are superficially not concerned with chemistry. Weare interested not solely in what effective teachers do, but more fundamentally how they decide what to do in any given situation. We suggest that the teacher’s decision frame- work is central in any consideration of ‘good’ or ‘effective’ teaching: -m Teacher’s Background, value system information -provides.-+ Teachkg situation* and experience * Objectiveswhich aid priorities 7r Decision implementation. This decision framework is undoubtedly complex, but we believe that among i@ key components are (i) a system for classifying teaching situations, and (ii) 83 A. J. Bishop and R. C. Whitfield, in introduction to ‘Situations in Teaching’, McGraw Hill, Maidenhead, 1972. Whitfield a system for appraising decisions taken. Analysis of over 200 situations in science and mathematics teaching together with some general school situations has led us to suggesta3 the following potential components of a classification system: A. Learning. A1 Cognition A2 Attitude B.Relationships. B1 Pupil-pupil B2 Pupil-teacher B3 Teacher-adult B4 Pupil-other adult C. Environment. C1 Physical-apparatus, aids etc. c2 Organization and administration. Every teaching decision involves a consideration of one or more of these elements, and in a practically-based subject like chemistry class C decisions seem fairly frequent. Take for example the following laboratory situation: a class of 15 year-old pupils is engaged on a volumetric analysis practical and the supply of standardized 0.1M-HCl becomes exhausted because some pupils have used too much of this solution for rinsing purposes. In this context a teacher could take a number of courses of action. The potential chief elements involved (with classification categories shown) seem to be: (i) a lack of understanding of volumetric analysis technique (Al) (ii) a careless and casual approach to laboratory work (A2) (iii) a defiance of the teacher’s specific instructions (B2) (iv) a problem of a deficiency of a laboratory material (Cl), and (v) a consideration of the time required to make up some more solution and its effects on the organization of the lesson (C2).The effective teacher will rapidZy assess these considerations and make an on-the- spot decision ;the ‘experienced’ teacher, i.e. one with a well-developed decision framework, may have a ‘standard way’ of dealing with this kind of situation and may not generate more than one line of action from which to choose. The ‘inexperienced’ teacher may only generate one frequently ineffective option- such as leaving the class while he himself goes to the preparation room to prepare some more solution! Possible more effective lines of action might be: (a) gather the class round the front bench, re-explain technique and give controlled questioning to test understanding; repeat experiment next lesson.(b) Admonish the class and do the experiment as a class-assisted teacher demonstration. (c) Send a pupil to ask the laboratory assistant if he can manage to prepare some more solution while leaving 30 minutes to the end of the lesson; demonstrate technique again in time gap and then carry on as planned if more solution is now available . . .and so on.The purpose of this simple illustration is to show not only the complexity of teaching-for any single day brings to the class teacher a myriad of such Research in Chemical Education: A Reassessment ‘critical incidents’ (how much easier it is to give a lecture!)-but also to indicate the inadequacy of the concept of the single ‘right’ decision in the context of teaching. Hence our belief in the importance of developing during training the teacher’s own professional decision framework. We postulate that practice in on-the-spot decision-making by student teachers using written and videotaped situations accelerates the development of this framework which we see as central for effective teaching. We are concerned not merely to enable student-teachers to respond in particular ways to particular situations but more basically to increase both their likelihood and their capability of responding in effective ways in their future careers.The analogy with the flight simulator for training airline pilots is appropriate in at least some of its aspects, though the ‘right’ decision concept tends to be one employed during their kind of training. In addition we believe that such simulated practice will act as a bridge between ‘advice about’ teaching and the ‘deep end‘ of teaching practice. We have however yet to test our postulates in any rigorous way, and before this can be done we need to investigate in much more detail the means by which teachers whose professional competence is widely respected actually make on-the-spot decisions.Methods of teacher training, like any other methods, can only be put to the test when we have an adequate theoretical basis for them. We therefore plan to gather introspective data on the on-the-spot decision- making process from a sample of effective teachers to refine our theories. We are encouraged by recent comment24 which suggests that introspection as opposed to the observation of overt behaviour has received in recent years all too little at tention in educational research. Introspective data have for example given insight into the processes of mathematical problem solving and chess playing. Many studies of teachers have investigated personality types and a knowledge of these does little directly to improve classroom performance.All too few studies seem to have troubled to examine in detail the work of effective teachers, so that when one dies it still remains too much akin to a light going out. The perceptions of learners have been mentioned as a pertinent area for research; the perceptions of teachers are no less important. Many of those con- cerned with the diffusion and uptake of new curriculum ideas are familiar with and often alarmed by the wide variation in what is taking place under apparently new banners. We therefore require studies of teachers’ perceptions of the objectives and methods of various courses and the roles which they are taking in relation to them, for there can sometimes be a significant difference between a teacher’s understanding and his overt teaching performance.6 Gaps in our Understanding and Barriers to Effective Research Having indicated some areas of current thinking and thereby the potential growth points for the immediate future, it will be all too apparent that there are many gaps in our understanding, and for some time yet we must in many areas proceed almost entirely by hunch. There is much to report about innovation but See for example L. S. Shulman, Review of Educational Research, 1970,40 (3), 371. Whitfield relatively little about research. A recent reviewz6 of research of a psychological nature into science education notes the dearth of work concerned with post- primary pupils-the stage at which serious science teaching begins !Apart from a trickle of M.Ed.theses, mostly done on a part-time basis by practising teachers (a U.K. doctorate in science education is a rarity), the output of research aimed at enlightening some of our practices is meagre. Yet we continue our vast invest- ments of manpower and capital at the practical level. There are of course reasons for this state of affairs, not all of which are commendable, and some of the major barriers impeding progress will now be outlined. Firstly there are the general difficulties associated with research into any human endeavour. There seem to be so many problems that it is difficult to focus upon priorities and ta get them adequately conceptualized; the numerous variables make empirical investigations far from easy and these are in addition beset by many practical difficulties such as co-operation between many persons in the field and pupil absences.The chemistry research laboratory becomes a relatively certain environment in which to work! These general difficulties are often exacerbated by the attitude of many social scientists anxious to make researches in this field ‘respectable’. Many would for example argue that attacks on ‘whole’ or ‘macro’ problems in natural settings-exemplified in this paper by, for example, the general area of curriculum evaluation and the introspective investi- gation of effective teachers-are by their nature ‘unscientific’. But have ‘micro’ investigations of individually inconsequential events in over-controlled environ- ments been all that productive for those in the front line of the educational process? Have not many of those engaged in educational research been labouring with a kind of reductionism in which it is very difficult, if not impossible, to put the pieces together again? How can we carry out controlled reality testing? Herein lies our dilemma, and heeding Shulman’s wisdom,zs which has no cant for national inadequacies, could carry us far : ‘On the one hand, in order to maximize the internal validity of OUT measure-ments, we must develop carefully controlled settings within which we can govern our research.This has long been recognized as a necessity, but it is likely that the experimental tradition in America overemphasized the importance of reliability and control at the expense of the characteristics affecting that other factor of equal importance in the development of experimental settings, external validity.. . . we] must . . . attempt to maximize the similarity between the conditions in which we study behaviour and those other conditions, whatever they may be, to which we may ultimately wish to make inferences.’ Secondly, there is a self-perpetuating dearth of suitably trained personnel for research into chemical education, for we require competent chemists with an understanding of either philosophy or one or more of the social sciences. Would it be heresy here to suggest that we might divert some potential chemistry I5 Schools Council, Curriculum Bulletin No. 3, EvansIMethuen, London, 1970.See ref. 24, p. 377. Research in Chemical Education: A Reassessment Ph.Ds. into the field of chemical education research after suitable preparative training? If we are concerned about the future careers of our Ph.D. students let alone the educational problems themselves, perhaps we should. In addition, more effective means must be found which will enable practising schoolteachers to engage in research of an educational kind. Secondments and day release from school are by no means the norm, and teachers can often assist in defining problems for investigation. Associated with this problem of personnel is, of course, finance for research student grants, secondments, research officer sal- aries, supporting soft- and hard-ware, and so on.Most of the British work reviewed in this paper has and is being carried out under conditions of man- power and finance which no research chemist would tolerate. Our require- ments are of course of a different nature, and even scale, but they are never- theless real, and expertise is already going untapped; some two thirds of a sample of AUCET members for example stated in a questionnaire that they would like to be more involved in research if circumstances permitted. On some projects which meet the criteria described earlier, progress is either so slow or in some cases non-existent because of inadequate institutional and ex- ternal support that unconstructive frustration results. One of the problems here is that chemical education is a ‘middle-ground’ area, and the buck of responsi- bility can be passed between several potential supportive institutions. Might not the chemical education division of the new Chemical Society help here? A third significant barrier to effective chemical education research is the highly diversified literature.The educational disciplines are a long way behind the sciences in the organization of journals, and there is almost no abstracting or retrieval service. Relevant papers tend to be published haphazardly, and even the conscientious student can easily miss a significant contribution-indeed the author may have missed several for this article! Allied to this problem of dis- semination is of course the uptake of research findings at the practical level-our papers, one is sure, often only preach to the converted.Finally there is still a certain complacency, subconsciously expressed in the feeling that research is a low priority in our educational concerns and that new horses for new courses, or at least new courses for old horses, is all that we need. This attitude is, however, irresponsible, yet it is to be found at times even in University Departments and Schools of Education where the limited view of education as ‘handing on advice from my years of experience’ often suffices. It is surely the professional duty of every chemist involved in education to seek to apply such similar high standards to educational decisions as he applies so meticulously to chemical decisions; in the University world in particular, the proceedings of many faculties are characterized by an ambivalent approach to ‘professional’ and ‘educational’ concerns, which today no longer goes unnoticed.There is an urgent need for those involved in shaping science and education policy to devote more resources to research if we are to gather intelligence for more informed action. Would that this plea came from someone eminent outside the field! But let support be unblinkered; any increasing commitment to Whitfield chemical education research carries a concomitant commitment for us all to act upon its findings. J am grateful to my colleague Dr. A. J. Bishop for many discussions which have helped to clarify some of the ideas here presented. Further Reading The following are useful introductions to the methodology of educational research : N. L. Gage (ed.), ‘Handbook of Research on Teaching,’ Rand McNally, Chicago, 1963. F. N. Kerlinger, ‘Foundations of Behavioural Research,’ Holt, Rinehart, and Winston, New York, 1964. D. G. Lewis, ‘Experimental Design in Education,’ University of London Press, 1968. K. Love11 and K. S. Lawson, ‘Understanding Research in Education,’ University of London Press, 1970. J. D. Nisbet and N. J. Entwistle, ‘Educational Research Methods,’ University of London Press, 1970.

ISSN:0306-0012    

DOI:10.1039/CS9720100027

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

Dielectric Relaxation in Polymer Solutions By A. M. North DEPARTMENT OF PURE AND APPLIED CHEMISTRY, UNIVERSITY OF STRATHCLYDE, GLASGOW, C.l 1 Introduction A. Dielectric Relaxation.-When a molecular system is placed in an electric field, there is always the tendency for any electrically charged species to migrate along the field in the appropriate direction. If the charged species are completely mobile this results in the conductivity familiar in metals, electrolyte solutions, or semiconductors. However, if the charged entities can move only a certain distance, and then for some reason become localized, the net result is an electric polarization of the sample. For example, charge carriers may migrate across a sample, but be unable to cross the boundary between the sample and the electrodes.Under these condi- tions the trapped charges collect at the surface, causing interfacial polarization of the sample. At the other extreme on a distance scale, electrons may migrate across the atoms in a molecule but not between molecules. Thus, each molecule in the field suffers a slight distortion of electron distribution, called electronic polarization (forming in the molecules an induced dipole moment). The addition of each molecular dipole along the direction of the field again causes a resultant polarization of the whole sample. A third example of a polarization process, and the one of interest in this review, occurs when the two opposite charges in a molecular dipole attempt to migrate in the field, thus applying a turning couple to the molecule.This tends to align the dipole in the field. Again the resultant effect is an electric polarization of the sample, called in this case ‘orientation polarization’. The migration of charges lowers the energy of the system in the field, and so is called ‘relaxation’, and the adjective ‘dielectric’ is added when polarization, due to localized charges, is involved. It is evident that the formation of the polarization can take place only at a rate determined by the charge mobility. This is unimportant if the electric field changes slowly, but if the electric field is altered sufficiently rapidly (as with a high frequency alternating voltage or in the field of electromagnetic radiation) there is not time for an equilibrium polarization to be established.So a study of the frequency dependence of the macroscopic phenomena resulting from polarization can yield information on the charge carrier mobility. In the case of orientation polarization due to small fields, this mobility is the rate of Brownian rotational diffusion. The theory of dielectric relaxation is available in several introductory Dielectric Relaxation in Polymer Solutions reviews,1-8 and here we are concerned with the principal features of orientation relaxation as illustrated in Figure 1: f decades Figure 1 Relationship between complex permittivity and frequency for ideal orientation polariza- tion (a) As the frequency of the applied field is increased from below to above a possible frequency for dipole rotation, the macroscopic permittivity is complex and the real part, E' (measured as electrical capacitance) decreases from a value characteristic of orientation polarization to one in the absence of orientation polarization.(b) The imaginary part E" (measured as conductance, dielectric loss, or energy absorption) is zero when orientation can or cannot take place, but is a maximum when the frequencies of dipole rotation and electric field change are comparable. (c) Both these macroscopic phenomena occur over a rather wide frequency range. Thus, for an ideal system in which the angular position and rate of rotation of any dipole are independent of those of its neighbours, and all rota- tions are described by a single rate, the width at half-height of the loss curve is 1.1 decades.(d) The relationship between field frequency and dipole rotation rates in the ideal case is 2nfma~~~= OmllxT = 1 where&= is the field frequency in Hz at which there is a point of inflexion in Mansel Davies, Quart. Rev., 1954, 8, 250. * Mansel Davies, 'Some Electrical and Optical Aspects of Molecular Behaviour', Pergamon,Oxford and London, 1965. C. P. Smyth, Ann. Rev. Phys. Chem., 1966,17,433. North the €'-frequency relationship and a maximum in the &frequency curve, Omax is the same in radians per second, and T is the rotational relaxation time. For our purposes, T is defined simply as the time required for the extent of field-induced dipole alignment to drop to l/e of its value upon removal of the field.This equation of a macroscopic relaxation time with a molecular rotation time is a considerable oversimplification and a more detailed relationship has been discussed by several B. Molecular Motion in Polymers.-The technical significance of polymeric materials lies in their uses as plastics, rubbers, or fibres. Whether a particular polymer will behave as a glass, rubber, or leather, or whether it will be tough or brittle, is determined by the extent of molecular motion within the sample. Most polymers consist in whole, or in part, of linear backbone chains and the most significant mode of motion is the segmental 'worm-like' motion of these chains. Indeed, the major transition in solid polymers, the giass to rubber transition, is ascribed directly to the onset of large-scale segmental motion of this type.This segmental motion, in its turn, has its origin in the total or partial rotation which may take place around individual covalent bonds in the backbone. Con- sequently, it is affected by both intramolecular phenomena (such as the steric hindrance of groups substituted close to the bond about which rotation occurs) and intermolecular phenomena (such as chain entanglement, crystallization on to a relatively close-packed lattice, or simply immobilization by strong intermole- cular forces). Before a complete understanding of solid relaxation behaviour can be reached it is desirable to study separately the intra- and inter-molecular effects.Since gas-phase studies of such large molecules are impossible, the next best alternative is to study intramolecular effects in dilute solution, hoping to minimize inter-chain phenomena and to include sensibly the effect of polymer- solvent interactions. In this review emphasis will be placed on those dielectric studies of polymer solutions which give clear information on the relationship between chemical structure and intramolecular rotational motion. This means that certain subjects (such as the detailed theories of polymer chain conformations or polarization phenomena due to ion-atmosphere migration) will not receive full coverage. In these cases the interested reader is referred to more detailed of the topic.2 Modes of Motion of Dissolved Macromolecules A. The Basic Rotational Movements.-From the viewpoint of dielectric activity, 'N. E. Hill, Proc. Phys. SOC.,1954, B67, 149. R. Kubo, J. Phys. SOC.Japan, 1957, 12, 570. R. H. Cole, J. Chem. Phys., 1965, 42, 637.'L. De Brouckere and M. Mandel, Adv. Chem. Phys., 1958,1,77. W. H. Stockmayer, Pure Appl. Chem., 1967,15, 539. H. Block and A. M. North, Adv. Mol. Relaxation Processes, 1970,1, 309. Diekctric Relaxation in Polymer Solutions an important mode of motion of a dissolved macromolecule is gross rotational movement of the whole molecule. In its simplest form this requires all segments of the molecule to move in unison, and so implies a clearly defined interseg- mental geometry. This criterion exists in rod-like molecules (such as the helical form of certain polypeptides) where rotation may occur about the major or minor axes of the molecule.However, the molecular geometry need not be as perfectly defined as in an helix, and there is no difficulty in visualizing rotation of a random-coil molecule in which the segmental geometry remains fixed during the rotation. This is really introducing a time-dependent definition of molecular flexibility (or rigidity) in that we consider a molecule to be ‘rigid’ when the time required for changes in conformation by segmental rotation are greater than the time required for rotation of the whole molecule. This is an interesting definition because it introduces a molecular-weight dependence.Brownian whole-molecule rotation will be very rapid for small molecules (or short chains) and very slow for large molecules (or long chains). Consequently, for a given chemical structure there must be, in principle, a mole- cular weight at which the times required for segmental rearrangement and whole- molecule rotation are comparable. Below this value the molecule will be defined as ‘rigid’, and above this value the chain will be defined as ‘flexible’. This leads us to the second important mode of motion, which is a rearrange- ment of the backbone geometry in chains defined (on the time- and molecular weight-dependent definition) as ‘flexible’. A polar polymer molecule is considered to be made up of a large number of unit dipoles. In this form of motion re- orientation of each unit dipole occurs almost independently of the motion of the other dipoles in the same chain.The final intramolecular rotation of interest occurs when a dipole is contained in a side chain of the polymer molecule, and side-chain reorientation can take place independently of backbone motion. Thus, the three basic motions discussed above can be considered simply as a reduction in size of the independently orienting moiety from the whole molecule, through sections of the chain, to substituent side-groups. B. Processes other than Dipole Orientation.-Although this review is concerned primarily with the relaxation of orientation polarization, polymer solutions may exhibit a variety of different polarization phenomena which must be borne in mind.Of particular importance in this context are the charge-carrier phenomena exhibited by solutions of polyelectrolytes. It is assumed that effects due to polarization at electrodes can be eliminated, and that in a truly homogeneous solution there should be no Maxwell-Wagner-Sillarslo~llinterfacial polarization due to the large-scale migration of charge carriers across an occluded conducting phase. In this context we consider a separate ‘phase’ to contain many polymer molecules. However, ion migration can occur within the ion-atmosphere of a single dissolved polyelectrolyte molecule. This gives rise to a polarization which loK. W. Wagner, Arch. Electrotechnoi., 1914, 2, 371. R W.Sillars,J. inst. Elec. Engineers (London), 1937,80, 378.North is best treated by considering12 each molecule as a conducting occlusion in a less-conducting matrix. Polarization by means of a chemical reaction is always a distinct possibility. This is particularly the case if hydrogen-bond equilibria (involving differently polarized ‘bonded‘ and ‘non-bonded‘ states) are in existence. C. The Normal Co-ordinate Analysis for ‘Flexible’ Chains.-It is necessary now to be more specific about the ‘whole molecule’ mode of motion for flexible chains. The problem lies in describing the rotational behaviour of a deformable body. This is usually done using a normal-co-ordinate analysis.ls-16 The im- portant property of such an analysis is that motion in a many-particle elastic body can be described by a non-interacting set of differential equations which have, in principle, a definite solution.Basically, the random coil is resolved into three unidimensional arrays of beads, each experiencing frictional drag with its environment, and each being connected to its neighbour by a volumeless Hookean spring. Each bead corre- sponds to a subunit of the chain sufficiently large that end-to-end separations within the units follow Gaussian statistics, and the connecting ‘springiness’ has its origin in the entropic nature of rubber elasticity. That is, stretching a random- walk chain reduces the number of possible conformations, and so increases the free energy of the stretched state. Recently, TobolskylG has made an alternative suggestion in which the linear bead-spring arrays are replaced by damped torsional oscillators. However, the fundamental analysis remains unchanged.When this elastic array of subunits is placed in some field (shear or elastic) which exerts a turning movement, the molecule is both rotated and distorted (Figure 2). The whole process gives rise to a basic ‘breathing motion’, together with overtone distortions in which the normal mode number corresponds to the number of nodes in the whole assembly. For the dielectric case1’ we are interested in the first normal mode (which, incidentally, is responsible for about half the polymer contribution to solution viscosity). The significance of the normal-co-ordinate model is that it allows calculation of the relaxation times for the various normal modes from expressions containing only solute concentration c, polymer molecular weight M, and zero-shear-rate viscosity coefficients (for solvent qs,and solution q)as disposable parameters; see Table 1.In other words, as long as the subunits are sufficiently long to exhibit Gaussian behaviour there is no term in the expression for relaxation time, which is directly and obviously related to chemical structure. While this does give the analysis a very important generality, it also detracts from its usefulness in an attempt to correlate rate of molecular motion with chemical composition. lS C. T. O’Konski,J. Phys. Chem., 1960,64,605. lS P. E. Rouse, J. Chem. Phys., 1953,21, 1272. l4 F.Bueche, J.Chem. Phys., 1954,22, 603. l6 B. H. Zimm, J. Chem. Phys., 1956,24,269. la A. V. Tobolsky, J. Polymer Sci.,Part A-2, Polymer Phys., 1968,6, 1177. l7 W. H. Stodunayer and M. E. Bau, J. Amer. Chem. SOC.,1964,86, 3485. Dielectric Relaxation in Polymer Solutions No Gradicnt t Gradient Applied Time Zero Later Time 1st Mode 2nd Mode 3rd Mode Figure 2 Movement of a flexible coil in a turning field: zeroth and first two normal modes of motion Table 1 Relaxation times for first normal modes of motion Polymer molecular model 1st Mode relaxation time Flexible coil in which solvent movement is 0.61 (y -y~)M unhindered (free-draining coil) CRT Flexible coil within which solvent flow is impe ded 0.42 (q -7s) M (non-free-draining coil) cRT Rigid rod of large length :diameter ratio The normal mode, or ‘whole molecule’, analysis of molecular motion is relevant to a dielectric observation when a resultant displacement vector d~,of the molecule corresponds to the resultant dipole electric vector, PR.Thus, when the molecule is ‘rigid‘ any molecular movement corresponds exactly with North a movement of the total resultant dipole moment obtained by vector addition of all the individual unit dipoles. However, when the chain is flexible, and all the units in the chain are rotating with a considerable degree of independence, the displacement and electric dipole vectors correspond only when the di- poles are aligned parallel to, and unidirectional along, the chain contour; see Figure 3.This is a rather important restriction, because it implies that a dielec- 'tIcR (b) Figure 3 Correlation of displacement and electric dipole vectors for unidirectional parallel dipole units, (a); but no correlation for random or perpendicular dipole units, (b) tric relaxation rate observed for a flexible coil will be the rate of segmental (or side-group) rearrangement for all molecules except those with unidirectional parallel dipole components, when the observed rate will be that of the first normal mode of motion. D. The Geometry of the Active Dipole.-The unit dipoles in a polymer molecule may be classified into three major types according to the relative geometry of the dipole moment and the backbone contour.Thus, we may have: (a) the unit dipoles attached rigidly perpendicular to the chain backbone; (6) the unit dipoles attached rigidly parallel to the chain backbone; (c) the unit dipoles attached in a side group capable of movement indepen- dent of the chain backbone. Of course any dipole moment fixed at an angle to the backbone contour may be resolved into parallel and perpendicular components, and so may be a combination of the above types. Some examples are given in Table 2. 3 Dielectric Relaxation of Rod-like Molecules Considerable interest presently attaches to the study of rigid rod-like molecules in solution. A number of naturally occurring molecules such as proteins and Dielectric Relaxation in Polymer Solutions Table 2 Dipole geometries in some common polymers Polymer Dipole geometry Structure Poly(methy1 acrylate) One component in flexible side-chain, one component rigid per- pendicular Poly(viny1 chloride) Rigid perpendicular -CH2 -CH -I TCl QPoly(ethy1ene oxide) Rigid perpendicular -CH2 -0 -CH2 -4J 04Polyester Rigid perpendicular R -C and parallel components \0 -R’ 0 Poly(p-chlorophenyl-Rigid perpendicular DCXFC-acetylene) and parallel components Cl nucleic acids fall into this category.Besides, for all long-chain polymers with finite energy barriers opposing segmental rotation, it must be possible to reduce the chain length to a size where the molecule is essentially rod-like. Conversely, any real rod must have a degree of flexibility, so that if the length can be in- creased sufficiently, a point must be reached when even slight curvature can build up to cause an overall coil-like conformation (Figure 4).Consequently, the only difference between molecules conventionally described as ‘coils’ and those con- 56 North T I Figure 4 Schematic picture of a ‘bent rod’ or ‘truncated coil’ molecule. L is the projection length along the major axis, and B is the effective ‘rod radius’ or hydrodynamic minor radius of gyration; (a) shows the effect of increasing ‘flexibility’for a given rod length and (b) shows the effect of increasing molecular weight for a given flexibility Table 3 Form of certain polymers Polymer (moleculdr weight N lo5) ‘Flexibility’ Poly(met h yl met hacrylate) Flexible coil Poly(viny1 chloride) Flexible coil Cellulose esters Stiff coil Polysulphones Stiff coil Poly(n-butyl isocyanate) (molecular weight < 104) Stiff rod Poly(n-butyl isocyanate) (molecular weight > 105) Stiff coil Poly(y-benzyl L-glut amate) (a-helix) Stiff rod DNA (molecular weight < lo5) Stiff rod DNA (molecular weight > 10s) Stiff coil Dielectric Relaxation in Polymer Solutions ventionally described as ‘rods’ is the chain length, and hence length-to-diameter ratio, at some value of which the observed behaviour changes from ‘coil-like’ to ‘rod-like’.Some examples are given in Table 3. A. The Molecular Weight Dependence of the Dielectric Relaxation Time.-The rotational relaxation time of a rigid rod varies approximately as the cube of the rod length.Since the length of a rod-like polymer molecule varies linearly as the number of monomer units, the observed dielectric relaxation time associated with end-over-end rotation should vary approximately as the cube of the a I I t h 7LogMa Figure 5 Variation of relaxation time with weight-average molecular weight: 0poly(y-benzy-L-glutamate) (ref. 19) in benzene with ecaprolactam as deaggregant or in trans-dichlorol ethylene with NN-dimethylformamide as deaggregant; 0 poly(n-butyl isocyanate) in benzene North molecular weight. Such a dependence has been observed18 for dilute solutions of fractionated samples of poly(n-butyl isocyanate), of molecular weight below lo6; see Figure 5.However, if curvature gives rise to coil-like behaviour, the radius of gyration varies as the square root of the degree of polymerization, and the dielectric relaxation time for rotation of the resultant vector varies as the three- halves power of molecular weight. Such behaviour is observed for poly(n-butyl isocyanate) when the molecular weight is above lo6; see Figure 5. An intermediate state of affairs seems to exist1# in solutions of poly(y-benzyl L-glutamate);see Figure 5. In the low-molecular-weight polymers studied the relaxation time varies approximately as the square of the molecular weight. It is not yet clear whether this is due to imperfections in the a-helix conferring a degree of curvature on the molecule (the effect is exaggerated by incorporation of the D-enantiomorph) or is a ramification of the molecular-weight distribution in the polymers studied.B. Analysis of the End-over-end Relaxation Time.-Although dielectric observa- tion of a rigid polymer does not give a measure of the effect of chemical structure on the rate of conformational changes, it is possible to obtain information on the overall molecular shape. In the case of a rod this will be the rod length and diameter. The analysis is carried out assuming an equation for the end-over-end relaxation time which has the form Here 7is the solvent viscosity, n is the number of monomer units each of pro- jection length Loalong the rod, and B is the effective radius. y is a term which accounts for end corrections.According to BrOersmaBo y = 1.57 -7[1/h(nL0/B) -0.28Ia for perfect rods, or according to Perrin,2l y is 0.702 for prolate ellipsoids of revolution. The problem in using equation (1) lies in determining whether or not it is permissible to assume nLo/B% 1. This will be so for very long chains, but then these long chains will be beginning to exhibit curvature when different equations (or values of y) must be used. Many of the rod-like molecules which have been studied are biopolymers (such as deoxyribonucleic acid and ribonucleic acid) in which dipole orientation polarization is masked by ionic polarization phenomena. Despite these difficulties end-over-end relaxation times have been analysed for a number of polymers.In the case of poly(y-benzyl L-glutamate) dissolved in chloroform the dis- parity between an ideal Debye relaxation and the observed relaxation has been 18 A. J. Bur and D. E. Roberts,J. Chem. Phys., 1969,51,406. 19 H. Block, E. F. Hayes, and A. M. North, Trans. Furaday SOC.,1970,66,1095. 90 S. Broersma, J. Chem. Phys., 1960,32, 1626. I1 F. Perrin, J. Phys. Radium, 1934,5,497, Dielectric Relaxation in Polymer Solutions used [along with the Burgers form of equation (l)] to estimate the molecular weight polydispersity. In addition, estimates of Lo and B were more in accord with a 310helix than the standard a-helix. The problem in making such an assignment is that curvature of a helical molecule will result in the molecule functioning as a prolate ellipsoid of revolution and so yield large B values, Helix parameters estimated from these will always represent a lower pitch than is actually the case.The incorporation of y-benzyl D-glutamate into the chain decreases the stability of the helix and so affects the relaxation behaviour. It has been suggestedlQ that the helix dimensions are essentially unaltered up to a mole fraction of the D-enantiomorph of 0.1, but above this the helix is disrupted and larger B/L ratios result. Much the same is foundzz in the case of poly(n-butyl isocyanate). The monomer projection length along the rod is somewhat less than 1 and the rod diameter is between 5 and 10 A, suggesting that the molecule exists as a loose imperfect helix, and not as the tight helix described for the crystalline state.Increasing temperature decreases the apparent Lo/Bratio. As can be seen from Figure 4a, the effect is interpreted as an increase in possible curvature. The curvature causing these helices to depart from a perfect rod-like shape can be estimated from the molecular dipole moments. In this case the summation of the unit vectors depends on the chain ‘persistence length’, i.e. a measure of the distance in one direction over which the vector property of interest extends essentially unchanged. In rough qualitative terms this is the projection of the distance along a chain before curvature reverses the direction of our property of interest. For poly(y-benzyl L-glutamate) in chloroform this distance is about 200 A, and for poly(n-butyl isocyanate) it is about 220 A.These molecules have, therefore, rather similar curvature in solution. An interesting feature in the structure of these stiff chains is that the effective curvature seems to increase with molecular weight. The effect has been observed in both poly(n-butyl isocyanate)z2 and in DNA.23Presumably this is due to the greater torques exerted on the longer chains by thermal fluctuations of solvent. It is not only polymers with a clearly defined helical structure that exhibit rod-like properties. In low-molecular-weight poly(N-vinylcarbazole) the restric- tions on internal rotation are sufficiently large for the molecule to be rod-like in behaviour. An analysis22 of the dielectric relaxation times for polymers of molecular weight less than 5 x lo3suggests that the overall shape of the molecule is rather as portrayed in Figure 4b, with a persistence length of ca.250 A. 4 The Change: Stiff Coil-Flexible Coil In the preceding section we saw how increasing the molecular weight of a rod-like polymer brought it to a ‘stiff-coil’ geometry. As a result of this the molecular- weight dependence of the rotational relaxation time changed from a third to a three-halves power. Since for most polymers the end-to-end distances for a given molecular weight are rather similar (within the orders of magnitude under dis- aa S. B. Dev, R. Y. Lochhead, and A. M. North, Discuss. Faraday SOC.,1970, No. 49, 244. a* H. Eisenberg, Discuss, Faraday SOC.,1970, No.49,286; Biopoiymers., 1969,8,545. North cussion), the dielectric relaxation times in a solvent of given viscosity should also be similar. This is illustrated in Figure 6, where the broken lines enclose a log frequency-log molecular weight band in which should lie the relaxation times for most stiff coils in a normal non-viscous solvent. I I I I a=*-a10 'Flexible' coils 8 X l! 4 3cl 6 4 \' '\ \'.' I I I 2 3 4 5 6 I Log M Figure 6 Variation of relaxation frequency with molecular weight for dilute solutions in benzene or toluene at 25 "C:0poly(methy1 methacrylute); 0 poly(ethy1ene oxide) (ref. 8); @ poly-(N-vinylcarbuzole) (ref. 24); 0poly(hexene 1-sulphone) (ref. 25); ()poly(y-benzyl L-glutamate) (re6 19).Broken lines bound 'universal line' for stif coils Afurther increase in molecular weight must so increase the rotational relax- ation time that segmental motion becomes the operative process for dielectric re- laxation. When this occurs the dielectric relaxation time becomes independent of molecular weight. Such a situation has been observed24 for poly(N-vinyl- carbazole), and it can be seen from Figure 6 that the change occurs at a mole cular weight of ca. 104. By the same token, the change should occur at molecular weights below lo2for the very 'flexible' poly(ethy1ene oxide) chain,8 and above lo6for the very 'stiff' poly(hexene-l-sulphone)2sand poly(y-benzyl ~-glutamate)l* A. M.North and P. J. Phillips, Chem. Comm.,1968,1340. l6 T.W. Bates, K. J. Ivin, and G. Williams, Trans. Furuduy SOC.,1967,63, 1976. 61 Dielectric Relaxation in Polymer Solutions Table 4 Dielectric relaxation frequencies of some dissolved polymers with rigid perpendicular unit dipoles at 25 "C Po Iymer Solvent fmax (Hz) AH$ Ref. (kJ mol-1) -CH2 CH2 0- Benzene 1.5 x 1O1O 10.3 26 4H2-CH-Icl Tetrahydrofuran 2 x lo* 9.2 29 CHZ- CH- Cyclohexane 3 x 10' - 30 I Br Toluene 3~ 107 20.2 27 Toluene 8x10' -28 4HZ-a-Toluene 1 x 1Olo 6 27 (h.f. process) Cl 4H2 -CH-Toluene 9x105 42 24i M M. Davies, G. Williams, and G. D. Loveluck, 2. Elektrochem., 1960,64,575. '7 A. M. North and P. J. Phillips, Trans. Faraday SOC.,1968,64,3235. P. J. Phillips, Ph.D.Thesis, University of Liverpool, 1968. L. De Brouckere and R. van Nechel, Bull. SOC.chim. belges, 1952, 61,261,452. 30 M. Kryszewski and J. Marchal, J. Polymer Sci., 1958,29, 103. North chains. By comparison, the change for poly(methy1 methacrylate) should occur at molecular weights ca. lo3. It is interesting that these values bear a distinct resemblance to the sizes of the ‘equivalent freely rotating links’ estimated from time-averaged observations such as light scattering or dipole moments. 5 Dielectric Relaxation of ‘Flexible’ Polymers A. Polymers with a Rigid Perpendicular Unit Dipole.-One of the principal reasons for studying the dielectric relaxation of polymer solutions is to ascertain in some quantitative fashion how the chemical structure of a chain affects its flexibility.Intuitively, one might think that the larger, or more polar, a substituent on the chain, the greater would be the steric or electrostatic barriers to backbone rotation. That this is, indeed, part of the story can be seen from Table 4. In this Table are compared the relaxation frequencies and Arrhenius activation en- thalpies for a variety of polymers dissolved in non-polar solvents at 25 “C.In general, the relaxation frequencies (a measure of the rate of backbone segmental motion) decrease with increasing size of substituent groups. However, these results do present two rather strange features. In the first place the chlorophenyl-substituted chains do not fit neatly into the general scheme.Simple steric considerations would suggest that movement in m-chloro- phenyl polymers should be more hindered than in p-chlorophenyl analogues. However, the dielectric observations imply exactly the reverse of this. Indeed, the glass transition temperature^^^ of the solid polymers also suggest that the meta-polymer chain is more flexible than the para-polymer. The reason is unclear but an explanation might be that steric hindrance due to the m-chlorine raises the energy of the most stable conformations of the chain unit, but does not proportionately raise the energy of some eclipsed state presenting a barrier to conformational change. Under these circumstances the activation energy for conformational change would be lowered and so the rate increased.Even more surprising at first sight is the great speed of backbone rotation in the poly(pchlorophenylacety1ene)chain, which is ostensibly conjugated. In this case the steric hindrance of the substituents is less than in the saturated analogue, but is still sufficient that backbone and phenyl substituent cannot be coplanar. Under these circumstances rotation about a backbone single bond to destroy backbone planarity can be accompanied by rotation into conjugation of the phenyl side-group. Consequently, loss of resonance stabilization of the backbone is balanced by gain in the side group and the energy-angle profile is considerably flattened. That only a small energy is required to ‘break‘ backbone conjugation in these polymers has been confirmed by measurement of a temperature dependence in the visible a1 K.R. Dunham, J. W. H. Faber, J. Vandenberghe, and W. H. Fowler, J. Appl. Polymer Sci., 1963, I, 897. 3a A. G. Hankin and A. M. North, Trans. Faraday SOC.,1967, 63, 1525. 3 63 Dielectric Relaxation iri Polymer Solutions The second strange feature in Table 4 is the magnitude of the activation enthalpies. For poly(ethy1ene oxide) and poly(viny1 chloride) these are only of the order of magnitude of the activation energy for viscous flow of solvent, and are no larger than the barrier to internal rotation in ethane! Even the chains with bulky substituents [such as poly(p-chlorostyrene] and poly(N-vinylcar- bazole) have surprisingly low activation energies. The problem lies in ascertaining what exactly is the dipole orientation observed in the dielectric relaxation experi- ment.In general, the angular orientation imposed on each dipole by the field is about one radian, and so the backbone rotation observed may not involve crossing the highest energy barriers in the energy-angle profile. However, measurements of the energetics of conformational changes in poly- styrene solutions have also been made using acoustic technique^.^^ An activation enthalpy of 27.7 kJ mol-1 was found opposing the high-energy state to low- energy state transition, and this is not much higher than the dielectric activation energy observed for poly(pch1orostyrene). B. Polymers with a Unidirectional Parallel Unit Dipole.-In this class of polymers the resultant electric vector of the molecule coincides always with the end-to-end dimensional vector, and the dielectric observation is of the ‘whole molecule’ mode of motion.As mentioned in Section 2C, the dielectric relaxation time is that of the first normal mode of motion, and so is molecular-weight dependent. This means that in a chain of unknown flexibility it may be difficult to differentiate between whole-molecule rotation of a rigid entity (in which the unit dipoles are not necessarily unidirectional and parallel) and the normal mode of a perfectly flexible chain. However, an unambiguous assignment can be made when a flexible chain contains both perpendicular and unidirectional parallel components. Under these circumstances the ‘flexibility’ is observed in the higher-frequency molecular- weight-independent relaxation of the perpendicular components, and the parallel components relax by the lower-frequency molecular-weight-dependent normal mode.Two such relaxation processes have been observed in poly(propy1ene oxide)8 and in poly(p-substituted phenylacetylene~).~~ In the case of atactic poly(propy1ene oxide) the measurements were made on undiluted low-molecular-weight liquid polymers rather than on solutions. The unidirectional parallel dipole component arises in the different polarizations of the (CH ,)CH-0 and CH2-0 bonds. So long as the polymer is perfectly ‘head- to-tail’, this difference is unidirectional along the chain. The strengths of the two dipole components can be calculated from the magnitudes of the two relaxa- tion processer (either in E’ or in E”) and are 0.18 D and 1.0 D for the parallel and perpendicular components, respectively.In the case of the polyb-substituted phenylacetylenes) calculation of the dipole components was more difficult because the polymers were not of a sharp molecular-weight distribution and the two relaxation processes overlapped in s3 H.-J. Bauer: M. Immendorfer, and H. Hassler, Discuss. Faraday Soc., 1970, No. 49, 238. North the frequency plane. However, within an uncertainty of about 10% the parallel and perpendicular components are respectively 0.7 D and 1.4 D for poly(p-chloro- phenylacetylene). In this case, the parallel component arises because of the different polarizations of the CHSX and XC-CH bonds.The implication here is that conjugation in these systems is imperfect, the alternating double and single bonds retaining clear identities in the chain. C. Acrylic Polymers : Backboneand Side-Group Motion.-The acrylic polymers are interesting because the unit dipole, residing primarily on the ester carbonyl group, can be resolved into parallel and perpendicular components. These differ in that rotation of the perpendicular component requires backbone rotation, Table 5 Dielectric relaxation frequencies of some acrylic-type polymers dissolved in toluene at 25 "C Polymer AH1 (kJ mol-l) -CH,-CH-18.5 I OCOMe -CHz-CH-1.8 x loD 23 I C0,Me -CH z-C(Me)-3.9 x 107 27I C0,Me -CH,-C( Me)- 3.4 x 107 27 I COC02Bu -CH,-C(Me)-1 3.0 x 107 27 C02C9H 1D -CH-C(Me)-CH-C( Me)- 1.4 x 107 32 I 1 1 C0,MeCO CO v 0 (2 % citraconic anhydride) Dielectric Relaxation in Polymer Solutions whereas rotation of the parallel component can take place by bond rotation in the side-group alone.Since the two components can relax by two different modes of motion, in principle two relaxation processes could be observed. However, all observations on polymer solutions detect only a single process. This is significant and means that rotations around the backbone and side-group covalent bonds neighbouring the carbonyl are co-operative in nature. Consequently, the dielectric relaxation frequencies should be a measure of the steric constraints on backbone motion.An increase in the size, or number, of substituents on the backbone does, indeed, reduce the relaxation frequency, as can be seen in Table 5. Substitution of the a-hydrogen by methyl causes a fifty- fold decrease in the relaxation rate, and even quite small amounts of the hindered anhydride co-monomer bring about a further reduction. On the other hand, the dielectric process is relatively insensitive to the size of the alkyl group on the alcohol residue. This must be because the relatively small orientation of the carbonyl group in the electric field can be accommodated by partial rotation of the neighbouring bonds, and does not require movement of groups more than two or three atoms distant on the side-chain. This is in direct contrast to measure-ments of chain flexibility derived from the rate of diffusion-controlled reaction^,^^ where large-scale movement of the whole chain unit is affected by ester group size.D. The Distribution of Relaxation Times.-In many of the observations reported above, the relationship between complex dielectric constant and frequency does not follow the ‘ideal’ curves of Figure 1, but is broadened in the frequency plane. This is accommodated by postulating that the dipoles being observed exhibit a distribution of relaxation times. The obszrved data are then fitted to a semi-empirical relationship introducing a distribution parameter. This quantity is unity for the ‘ideal’ case, and decreases towards zero with increasing width of distribution.Some values of this parameter (calculated using the method of Davidson and are illustrated for poly(methy1 methacrylate) solutions in Table 6. Table 6 Distribution parameters for poly(methy1 mathacrylate) in toluene Concentration Temperature (K) (wt %> 10 190 0.51 230 0.72 270 0.78 310 0.90 5 0.74 0.84 0.86 0.93 2 0.84 0.97 1.oo 1.oo It can be seen that the distribution of relaxation times is broadened both by increasing the concentration and by reducing the temperature. However, at the A. M. North and G. A. Reed, J. Polymer Sci., Part A, Gen. Papers, 1963, 1, 1311. ai P. C. Scherer, D. W. Levi, and M. C. Hawkins, J. Polymer Sci., 1957, 24, 19. North highest dilutions and temperatures the relaxation becomes ‘ideal’.The exact molecular origin of the distribution is still uncertain, although current explana- tions pursue the fact that movement of a unit dipole may not be completely independent of the movement of neighbouring dipoles. A correlation in angular position between neighbouring dipoles can affect the observed dipole moment per molecule, and the time dependence of this correlation may introduce the non-ideal relaxation behaviour. It may be argued that the very presence of a limited number of covalent bonds between polymer unit dipoles must introduce some kind of dipole correlation and with it a broadening of the relaxation. However, the results of Table 6 show that this is not necessarily so for acrylic polymers, with four bonds between the two carbonyl groups.In poly(viny1 halides) there are formally only two bonds between the chain- halogen dipoles. In the case of poly(viny1 bromide),30 the distribution parameter is 0.94 at the highest temperatures and dilutions, so here too the dipole orienta- tion conforms to almost ‘ideal’ behaviour. On the other hand, a distribution parameter as low as 0.7 has been quotedz9 for poly(viny1 chloride), although here measurements were made only at relatively high polymer concentrations. 6 Dielectric Relaxation in Solutions of ‘Stiff-coil’ Polymers We have defined ‘stiff’ polymers as those chains for which segmental rearrange- ment is slower than some ‘whole-molecule’ modes of motion such as molecular rotation.Consequently, the dielectric relaxation frequency depends on molecular weight and gives little information on internal flexibility. A. Random-coil Polymers, including Cellulose Derivatives.-Cellulose esters and ethers form the most widely studied class of stiff random-coil polymers. The monomer units in these polymers contain separate dipolar groups which have components parallel and perpendicular to the chain contour, as well as possessing a component located in the ester or ether side-chains. The expectation that these different components might relax by different modes of motion is borne out, and two dielectric relaxation regions are The high-frequency process in cellulose acetate has been found3s to be inde- pendent of molecular weight, with a relaxation frequency of ca.10 MHz at room temperature. This is noticeably slower than in vinyl acetate, implying that in the cellulose derivatives the acetate side-groups are subjected to considerable steric hindrance or do not benefit from co-operative backbone movement. The dis- tribution parameter is unity, suggesting that it is permissible to consider each acetate group as an independently orienting moiety. A low-frequency molecular-weight-dependent relaxation has been observed in cellulose acetate,3s ethyl cellulose,35 and methyl cellulose.37 In each case the relaxation frequency varies inversely as the molecular weight raised to a power 30 A. Kheir, Ph.D. Thesis, University of Leiden, 1959. W. Kuhn and P. Moser,J.Polymer Sci., Part A, Gen. Papers, 1963, 1, 151. Dielectric Relaxation in Polymer Solutions between 1.2 and 2.4. The relaxation times for high molecular weights compare favourably with calculated times for non-free-draining random coils, whereas for lower molecular weights agreement is best with predictions for rod-like entities. Expressions relating relaxation frequency to molecular weight are given in Table 7. Another class of polymers exhibiting a low-frequency molecular-weight- dependent relaxation are the poly(o1efin sulphones). In this caseno second high- frequency process is observed, as is expected when the dipole components are rigidly attached to the backbone chain. R The dielectric observations can be explained either if the chains are 'stiff' or if they are flexible with unidirectional parallel components and a structure in which the perpendicular components cancel.The former explanation is preferred,39 although the fact that chain flexibility is so much lower than in vinyl polymers is somewhat surprising. Table 7 Molecular weight dependence of reldxation frequencies for stif coils at 25 "C Polymer Solvent Molecular weight, M, or degree of polymerization, P, related to relaxation fre-quency, fc. Log fc = Cellulose acetate Dioxan 14.9 -2.4 log M Ethyl cellulose Dioxan 5.5 -1.9 lOgP Carbon tetrachloride 4.2 -1.2 log P Benzene 4.8 -1.410g P Toluene 5.2 -1.6 log P Poly(hexene 1 -sulphone) Benzene or toluene 12.4 -1.5 log M Poly(2-methylpentene 1-sulphone) Benzene 14.9 -2.0 log M 38 P.C. Scherer, D. W. Levi, and M. C. Hawkins, J. Polymer Sci., 1958,31, 105. 3sT.W. Bates, K. J. Ivin, and G. Williams, Trans. Furuduy SOC.,1967, 63, 1964. North 7 Comparison with Other Measurements of Segmental Mobility While dielectric relaxation is the method which has been most extensively used to study segmental mobility in a variety of polymer structures, complementary information can be gained from a variety of other experimental techniques. A. The Rates of Diffusion-controlled Reactions.-A diffusion-controlled reaction is one in which the reaction rate is governed by the speed with which two reagents diffuse together, rather than by the probability of chemical reaction once they have come together.Obviously, in mobile liquid solutions only very reactive species will undergo reactions of this nature, but one such reaction is the mutual termination of two free radicals. In the case of dissolved macro radical^,^^ the rate of the reaction depends on the segmental mobility of the chain in the neigh- bourhood of the free radical unit. The movement observed by measurement of these reaction rates differs from that observed by dielectric relaxation in that the kinetic study involves movement of several monomer units over a distance equivalent to several solvent molecules (or polymer chain segments), whereas the dielectric movement involves partial rotation with a relatively small amplitude of the unit dipole. Despite this differ- ence, both rate processes show a similar variation with the backbone chain Table 8 Comparison of chain mobility measurements at 25 “C Polymer Dielectric Radical-radical Fluorescence relaxation rate constant depolarization frequency (Hz) (mol-l s-l) time (ns) Poly(ethy1ene oxide) 1.5 x 1O1O 1.0 x lo8 Poly(viny1 acetate) 2.0 x 109 2.0 x 108 - Pol y(me t hyl acryla te) 1.8 x los 2.0 x lo8 2.0 Poly(pch1orostyrene) Polystyrene 3.0 - x 107 7.7 2.5 x x 107 107 4.4 - Poly(methy1 met hacrylate) 3.9 x 107 1.6 x 107 4.0 Polyacry lamide - 1.2 x 107 4.0 Poly( bu t yl methacrylate) 3.5 x 107 2.5 x lo6 4.0 Poly(nony1 met hacrylat e) 3.0 x 107 1.3 x lo6 - Poly(met hyl methacrylate) 1.4 x 107 3.8 x - (citraconic anhydride) Pol y(N-vinyl carbazole) 9.0 x lo6 2.5 x 105 - 40 A.M. North, in ‘Structure and Mechanism in Vinyl Polymerisation’, ed. T. Tsuruta and K. F. O’Driscoll, Marcel Dekker, New York, 1969, ch. 4. Dielectric Relaxation in Polymer Solutiorrs structure (Table 8). The similarity disappears, however, when alterations are made to the structure of side-chains some atoms removed from the backbone. Thus, the orientation polarization of the carbonyl dipole in three alkyl metha- crylates is little affected by the nature of the alkyl group, whereas the diffusion- controlled reaction rate is reduced for large groups. B. Fluorescence Depo1arization.-Another method used in studying molecular rotation processes is the technique of fluorescence dep~larization.~~ A fluorescent molecule or group is excited with plane-polarized radiation, and the extent of depolarization in the resulting fluorescence is a measure of the random rotation which has occurred during the lifetime of the excited state.An independent measurement of excited-state lifetime then permits evaluation of the rotational rate, usually as a rotational relaxation time. In polymer studies a fluorescent dye molecule is either complexed to the chain or attached to one end by a radical transfer reaction during polymerization. The strength of the technique is the great sensitivity of fluorescence measure- ments, which allows study of a single group in a large chain. In principle, the fluorescent group could be situated in the middle or at the end of a chain, so allowing a comparison of mobility in different parts of the chain.In practice this has not yet been achieved. Indeed, in studies to date dye molecules such as fluorescein have been attached to chains either through several atoms (as in the reaction of fluorescein isothiocyanate with hydroxyl-ended chains) or through some indeterminate linkage. As a result, it is not certain how far rotation of the dye moiety truly reflects chain mobility, or whether the experiment merely detects completely independent rotation of the dye group. Of those few polymers which have been studied by both techniques (all with a dyestuff ostensibly on the chain end), only poly(methy1 acrylate) would be expected to have a rotational time markedly less than the others.As can be seen from Table 8, there is very little influence of chain structure on the rotational relaxation time. However, it still remains to be proved whether or not this is a true reflection of the mobility at the ends of polymer chains. C. Ultrasonic Relaxation.-A study of the absorption of ultrasound by a system of molecules capable of existing in two states can, in principle, yield the energy difference between the states and the interchange frequency. The tech- nique is particularly useful in examining the energetics of molecules which can exist as two rotational isomers. In the case of polymer chains it is assumed that rotation of each segment can be described as such a two-state isomerization. At the present time the technique has been applied successfully only to solutions of p~lystyrene.~~A relaxation is observed at about 10 MHz which is only slightly slower than would have been expected from dielectric measurements I1G.Weber, in ‘Fluorescence and Phosphorescence Analysis’, ed. D. M. Hercules, Inter-science, New York, 1966, ch. 8. IaH.-J. Bauer and H. Hassler, Kolloid-Z., 1969, 230, 194. North on polyCp-chlorostyrene). The activation energy (measured from the upper state) for the process is 28 kJ mol-l, again rather similar to the values encountered in dielectric studies. Indeed, the general agreement between the two techniques is most encouraging, and it is to be expected that further ultrasonic measure- ments on polymer solutions will be made in the near future.8 Comparison with Other Measurements of Molecular Rotation A number of techniques are available for the study of molecular rotational diffusion coefficients. Among these are pseudo-steady-state techniques such as the measurement of flow birefringence and electrical birefringence (Kerr effect), as well as dynamic electro-optic techniques in which birefringence or Rayleigh light scattering are measured in non-stationary electric fields. The steady-state techniques yield, generally, a rotational diffusion coefficient. This can be related to a relaxation time; for example, when the molecule is a rigid rod, is 0/6, where D is the rotational diffusion coefficient.When rotational relaxation times obtained from these techniques agree with those measured for a dielectric process, it is tempting to ascribe the dielectric phenomenon to a rotational relaxation. Indeed, the two techniques have been combined in an analysis of bovine serum albumin,43 yielding 0.23 and 0.11 ps as relaxation times for rotation about minor and major axes, respectively. However, agreement between dielectric and electro-optic studies is not a totally unambiguous proof that the dielectric process being observed is molecular rotation. For example, the evidence in the case of poly(sodium methacrylate) is that an ionic conductance mechanism is responsible for the observed dielectric behaviour, yet the relaxation time is similar to that obtained from birefringence measurements.These birefringence studies, being most easily made on anisometric molecules, have been applied mainly to biopolymers, and have recently been reviewed by O’K~nski.~~ In the case of poly(hexy1 isocyanate) it is possible to compare dielectricq5 and electro-optic light scatteringq6 studies. Both techniques suggest that the molecule exists as a somewhat imperfect (slightly bent) rod or highly prolate ellipsoid of revolution. The principle dipole moment of the molecule lies along the major axis, and agreement is obtained in the end-over-end rotational relaxation time for a given molecular weight. Although the optical techniques are most useful for rigid molecules, it is possible to measure the normal mode relaxation times for flexible molecules using the technique of viscoelastic rela~ation.~’ A comparison4* of dielectric and viscoelastic relaxation then aids the assignment of a particular mode of motion to the dielectric process.For example, dielectric relaxation in solutions 43 P. Moser, P. G. Squire, and C. T. O’Konski, .I. Phys. Cliem., 1966, 70, 744. O4 C. T. O’Konski. Encyclopaedia Polymer Sci. Technof.,1968,9, 551. 46 R. Y.Lockhead and A. M. North, unpublished work. 46 H. Plummer and B. R. Jennings, European Polymer J., 1970,6, 171. 47 J. Lamb and A. J. Matheson, Proc. Roy. SOC.(London),1964, A281,207 48 A. M. North and P. J. Phillips, Brit. Polymer J., 1969, 1, 76. Dielectric Relaxation in Polymer Solutions of acrylic polymers occurs at frequencies a factor of lo4higher than viscoelastic relaxation, confirming the segmental nature of the former process.On the other hand, the viscoelastic and low-frequency dielectric relaxations coincide for the poly(p-substituted phenylacetylenes), providing evidence for the coincidence of the electric and displacement vectors. 9 Present and Future Trends in Research Present trends in studies of dielectric relaxation in polymer solutions are moving towards an examination of the quantitative effects of detailed changes in chain structure. For example, the technique is being applied to regular head-head tail-tail polymers in order to ascertain whether the ease of backbone rotation (or the size of the rotating unit) is the same as in normal head-tail polymers.Similarly, the effect of increasing the separation between unit dipoles on dipole correlations and ease of movement is being studied in a quantitative way. It is to be expected that as chains with different backbone units (perhaps incor- porating hetero-atoms) are synthesized, the dielectric technique will continue to provide a relatively simple measurement of chain flexibility or stiffness. Of course, the hydrodynamic behaviour of stiff coils (or bent rods) will still be of interest. The next year or so should see an increasing integration of other experimental techniques with dielectric studies. Although certain of these are very elegant and yield a considerable amount of information, often the apparatus is complex or expensive.In this category fall two scattering techniques which are becoming available and which afford comparative evaluation of molecular and segmental rotational diffusion coefficients or relaxation times. One of these is an analysis of the frequency dependence of Rayleigh-scattered laser light. The major use of such studies lies in determining the translational diffusion coefficient of the dissolved macromolecule. However, under certain conditions (when the molecule is aniso- metric and observations are made at low scattering angles, preferably using polarized light) it is possible to measure the rotational diffusion coefficient and, hence, relaxation time. The only polymer molecule which has been studied extensively by this49 and a variety of techniques is tobacco mosaic virus.The general conclusion is that the molecule is rod-like with a rotational relaxation time of 1.7 x s, in agreement with results of flow and electric birefringence studies. As with dynamic electro-optic techniques, it is expected that in the future this method will be increasingly used to measure whole-molecule rotational processes, and so aid in the assignment of dielectric relaxation phenomena. Another technique of growing importance which will almost certainly be applied to polymer solutions is neutron scattering. In principle, these studies should yield information on segmental diffusive processes complementary to the information obtained by dielectric and ultrasonic measurements. The next few years, then, should see a rapid increase in quantitative knowledge relating chemical structure to chain rotational behaviour. ID A. Wada, N. Suda, T. Tsuda, and K. Soda, J. Chem. Phys., 1969, 50, 31.

ISSN:0306-0012    

DOI:10.1039/CS9720100049

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

The Friedel-Crafts Acylation of Alkenes By J. K. Groves DEPARTMENT OF CHEMISTRY, MEMORIAL UNIVERSITY OF NEWFOUNDLAND, ST. JOHN’S, NEWFOUNDLAND, CANADA 1 Introduction Friedel-Crafts acylation refers to that substitution of a hydrogen atom by an acyl group which occurs under the influence of certain strongly acidic cata1ysts.l The reaction has been applied to alkenes since 1892 and has been used in the synthesis of acyclic, cyclic, terpenic, steroidal, and aromatic ketones. Several general reviewslsa on Friedel-Crafts react ions have included a discussion of alkene acylations but, despite this, textbooks still imply that only aromatic substrates acylate satisfactorily.s Although the reactive intermediates in alkene acylations do permit a considerably greater variety of reactions than their aromatic counterparts, the careful observance of optimum reaction conditions allows reactions that are of considerable synthetic ~tility.~ This review describes the interaction of non-functionally-substitutedalkenes and Friedel-Crafts acylating agents.Although the products obtained may be rationalized in terms of normal carbonium ion processes, it will become clear that little is known concerning the detailed mechanism of even the simplest reaction steps. A discussion of side reactions is included in view of their signi- ficance in synthetic work. Finally, since many reactions herein contravene the definition of acylation as a substitution process, for the purpose of this review the term acylation is used in a broader sense to include reactions involving addition to the alkene bond. 2 The Acylating Species The directive effects of substituents at the alkene bond leave little doubt that Friedel-Crafts acylation involves electrophilic attack by the acylating agent upon the alkene.The means by which catalyst-acylating agent interactions pro- duce an increased electrophilicity of the acylating agent are outlined below. Acyl halides and Lewis acids form oxonium complexes (l), acylium salts (2), or mixtures of these species.6 %Ray studies of isolated oxonium complexes (1) confirm co-ordination via oxygea6 Weakening of the C==Obond is evident from G. A. Olah, ‘Friedel-Crafts and Related Reactions’, Wiley, New York, 1963. * G. Baddeley, Quart.Rev., 1954,8,355; D. P. N. Satchel], ibid., 1963, 17, 160; P. F. G. Praill, ‘Acylation Reactions’, Pergamon Press, London, 1963. J. G. Sharefkin, J. Chem. Educ., 1962, 39, 206. H. 0. House, ‘Modern Synthetic Reactions’, Benjamin, New York, 1965. H. H. Perkampus and W. Weiss, Angew. Chem. Znternat. Edn., 1968,7, 70. S. E. Rasmussen and N. C. Broch, Chem. Comm., 1965, 289; W. Weiss and B. Chevrier, ibid., 1967, 145. The Friedel-Crafts Acylation of Alketies \ Y 'Y the increased bond length and from the decreased carbonyl stretching frequency in the i.r. spe~trum.~ In the n.m.r. spectra8 the a-protons exhibit a downfield shift of T ca. 05-1.0 as a result of such co-ordination, indicating the existence of a partial positive charge upon the carbonyl carbon. Although it seems highly likely that oxonium complexes are active acylating species it is difficult to pro- vide indubitable evidence of this, since the molecular predominance of (1) in an acylating system does not preclude acylation via a small equilibrium concentra- tion of acylium ions.Many acylium salts (2) have been isolated and all appear to be active in C-acylation~.~The shortened C=O bond lengthlo and the extent of increase in the carbonyl stretching frequency'' indicate that the positive charge is localized principally upon the carbonyl carbon. This is supported by the large downfield shift (7ca. 14-2.0) exhibited by the a-protons in the n.m.r. spectrum12 and more directly by the marked deshielding of the carbonyl carbon in the 13C n.m.1.~pectrurn.~~ Cryoscopic investigations1l9l2 indicate that little or no ion separation occurs even in highly polar media, hence it is not acylium ions but (sterically more demanding) ion-pairs which probably effect acylation. Aluminium chloride1* and other strong Lewis acids react with carboxylic anhydrides to afford acyl halides: (RC0)20 + 2AlC13-RCOC1,A1Cl3 + RCOzAlC12 'B. P. Susz and D. Cassimatis, Helv. Chim. Acta, 1961,44,943.* G. A. Olah, M. E. Moffat, S. J. Kuhn, and B. A. Hardie, J. Amer. Chem. Soc., 1964,86, 2198. @ E. Lindner, Angew. Chem. Internat. Edn., 1970, 9, 114. loJ. M. LeCarpentier and R. Weiss, Chem. Comm., 1968, 596; J. M. LeCarpentier, R. Weiss, and B. Chevrier, Bull. Soc. Franc. Mineral.Crist., 1968, 91, 544. l1 B. P. Susz and J. J. Wuhrmann, Helv. Chim. Acta, 1957,40, 971; G. A. Olah, S. J. Kuhn, W. S. Tolgyesi, and E. B. Baker, J. Amer. Chem. Soc., 1962,84,2733; see also D. Cassimatis, J. P. Bonnin, and T. Theophanides, Canad. J. Chem., 1970,48, 3860. l2 G. A. Olah, S. J. Kuhn, S. H. Flood, and B. A. Hardie, J. Amer. Chem. Soc., 1964, 86, 2203; G. A. Olah and M. B. Comisarow, ibid., 1966,88,4442; G. A. Olah and A. M. White, ibid., 1967, 89, 7072. l8 G. A. Olah, W. S. Tolgyesi, S. J. Kuhn, M. E. Moffat, I. J. Bastien, and E. B. Baker, J. Amer. Chem. SOC.,1963, 85, 1328. l4 R. E. VanDyke and H. E. Crawford, J. Amer. Chem. Soc., 1951, 73, 2018; G. Baddeleyand D. Voss, J. Chem. SOC.,1954,418. The Friedelkcrafts Acylation of Alkeiies These reactions are usually represented as proceeding via acylium ion formation, such as is known to occur under more highly acidic conditions.21 3 Reactivities The lack of kinetic studies in this field leads to the particularly unsatisfactory situation in which qualitative observations concerning reaction yields and velocities must be considered.Generally speaking, structural factors that enhance the nucleophilicity of the alkene or the electrophilicity of the acylating agent should favour reaction. Selective alkene acylation in the presence of an aromatic substituent is generally possible2, unless the alkene bears an electron-withdrawing group. In the acetylation of alkenes with zinc chloride-acetic anhydride, increasing alkene nucleophilicity is paralleled by an increase in yield :23 Alkene CH2 = CH, n-C5Hl1CH = CH, MeCH = CHMe Yield(%) 0 2 7 Alkene Me,C = CH, Me,C = CMe, Yield(%) 31 69 Reasonable yields (commonly 50 % and frequently much higher) may be obtained from the less-branched alkenes by the use of a more reactive acylating system. Particularly nucleophilic alkenes, such as diphenylethylene, will react with the more electrophilic acyl halides (e.g., RCOCl ;R = COCI, C02R1, Ph, CC13) in the absence of added A parallel generally exists between the reactivity of an acylating agent (RCOY) and the strength of the corresponding acid (HY).Acyl halides are more reactive than simple carboxylic anhydrides and the reactivity sequence RCOI > RCOBr > RCOCl > RCOF is frequently observed for C-acylations.Acetyl iodide is reported to acetylate cyclohexene in the absence of (added) The effect of the substituent (R) is less predictable since it may affect the reaction rate in two ways: (i) by altering theequilibria, producing acylating complexes and (ii) by altering the electrophilicity of the complexes so formed. Which of these opposing *l G. A. Olah, A. M. White, and D. H. O’Brien, Chem. Rev., 1970,70, 561. saE.Garbisch, J. Org. Chem., 1962, 27, 4243. 89 A. P. Mesheryakov and L. V. Petrova, Zzvest. Akad. Nauk S.S.S.R.,Otdel. Khim. Nauk, 1950, 98. F. Bergmann and J. Klein, J. Amer. Chem. SOC.,1952, 75, 4333. P. G. Stevens, J. Amer. Chem. SOC.,1934,56,450. Groves factors is dominant will depend upon the solvent, the catalyst, and the leaving group (Y).The activity of Lewis acid catalysts will depend largely upon the extent to which they produce an electron-deficient carbonyl carbon at the acylating agent. No general sequence of catalyst activity is possible since Lewis acid strengths are strongly dependent upon the reference base (e.g. Y-in acylium salts and C=O in oxonium complexes). Lewis acidity has recently been reviewed26 and here it will suffice to say that of the commonly used catalysts aluminium and antimony halides are usually very active, tin@) chloride and boron trifluoride are of intermediate activity, whilst zinc chloride is rather mild. A slight molar excess of catalyst is usually employed since the ketone complexes with the catalyst, removing it from the sphere of reaction.The reported use of sub- stoicheiometric quantities of zinc chloride2' and tin@) chloride2'? implies significant dissociation of the catalyst-ketone complexes. Solvents of high dielectric constant usually facilitate high reaction rates, although this need not be so if the solvent is also capable of co-ordinating with the catalyst (e.g. nitro-compo~nds).~~ 4 Interaction of the Alkene and the Acylating Species Electrophilic attack upon alkenes is frequently considered to involve formation of an intermediate n-complex in which the alkene r-electrons interact with the vacant orbitals of the electrophile. The extent to which such complexes actually participate in alkene reactions remains q~estionable~~ and pertinent data con- cerning acylations are unavailable.For the present purposes, reaction of the alkene and the acylating species may be considered to afford the cation (7), whatever its immediate progenitor, and it is the reactions of this ion with which the remainder of this review is primarily concerned. R' /R3 \ /R3+R-&=O -6-C-COR \R2 R4 R2 R4 5 Anion Addition to the Intermediate Ion One manner in which the intermediate ion (7) may react is by attack upon an anion. B6 D. P. N. Satchell and R. S. Satchell, Chem. Rev., 1969, 69, 251. M. Muhlstadt and P. Richter, Chem. Ber., 1967, 100, 1892. a8 J. Colonge and K. Mostafavi, Bull. SOC. chim. France, 1939,6, 342. 28 G. Hoonaert and P.J. Slootmaekers, Bull. SOC.chim. belges, 1969, 78, 257. 30D.V. Banthorpe, Chem. Rev., 1969, 69. The Friedel-Crafts Acylation of Alkenes R’ R1 R3\ \/6-c-/R3 COR + Y--Y-C-C-CCOR R2/ \R4 R2 (7) When the acylating agent is an acid chloride, P-chloroketone is found in the reaction Rather less anion addition is observed when acid anhydrides are used, particularly if the catalyst employed forms strong carboxylate ion complexes.23 The generally lower extent of addition of carboxylate relative to halide has been attributed to the more complex nature of the counter ion but the failure to isolate keto-esters has, at least in part, resulted from the ease with which they decompose to unsaturated ketone. Detectable amounts of P-chloroketone are a150 formed in the zinc-chloride- and tin(rv)- chloride-catalysed acylations of alkenes using acid anhydrides, a1 though acyl halide formation was not apparent.A cyclic transition state was invoked as an explanation (Scheme 1). COMe ___, a,!1 + ACOMCI,,_~ Scheme 1 The use of carboxylic acids may result in OH-addition although this is usually only apparent if structural factors restrain unsaturated ketone formation. Such a case is the acid-catalysed equilibration of 4-oxohomoadamantan-5-one and 4-hydroxyadamantan-2-one(Scheme 2).33 Alkenes which afford a tertiary ion (7) tend to give less addition product. This is typical of carbonium ion reactions although the reasons are not precisely unders to~d.~~ Determination of the stereochemistry of kinetically controlled addition products is fraught with difficulties.In the media employed the initial product may undergo re-ionization, enolization, elimination, or deacylation (see Section H. Wieland and L. Bettag, Chem. Ber., 1922, 55, 2246. D. P. N.Satchell and R. S. Satchell in ‘The Chemistry of the Carbonyl Group’, ed. S. Patai, J. Wiley, New York, 1966, p 233. 33 M. A. McKervey, D. Faulkner, and H. Hamill, Tetrahedron Letters, 1970, 1971. s4 D. Bethell and V. Gold, ‘Carbonium Ions’, Academic Press, London, 1967, p. 197. Groves CO,H CO OH t Scheme2 15). One investigation has avoided these difficulties by careful choice of the substrate and reaction conditions. The aluminium-chloride-catalysedcyclization of cyclo-oct-4-cis-ene-1-carboxylicacid chloride was shown to afford a mixture of stereoisomers in which the trans addition product predominated (Scheme 3).86 AlCIi'I u"8 (20%) Scheme 3 Unfortunately, the rigid geometry which makes this system so amenable to meaningful stereochemical investigation also introduces factors which may make 36 W.F. Erman and H. Kretschmar, J. Org. Chem., 1968, 33, 1545. The Friedel-Crafts Acylation of Alkenes the results atypical. Reaction of cyclohexene with acetyl chloride-aluminium chloride affords cis-and trans-1-acetyl-2-chlorocyclohexanein ca. 3 :1 ratio.s6 One may postulate that such cis addition arises from attack by the undissociated acylating complex followed by rapid collapse of the resulting ion pair.37 6 Tautomerism Before discussing the formation of unsaturated ketones it is necessary to draw attention to the acid- and basecatalysed prototropy of these compounds.Sub- stituent effects upon these tautomeric equilibria have been investigated and the following generalisations have emerged :38 (i) if the y-positions are unsubstituted the equilibria between ap-and &maturated ketones will favour the conjugated isomer; (ii) introduction of alkyl substituents into the y-position shifts the equilibrium towards the non-conjugated isomer; (iii) alkyl substitution at the a-position favours the conjugated isomer; (iv) alkyl substitution at the /%position has relatively little eft‘ect except in highly substituted systems when the non- conjugated isomer is frequently favoured.The effect of a-and y-substituents are consistent with the hyperconjugative stabilization which they afford to the two tautomers. The effect of /?-substitution in highly substituted systems prob- ably arises as a consequence of steric compressions enforcing non-planar con- formations of the conjugated enone These generalizations account for the proportions of tautomers obtained from many acylations in which the product undergoes equilibration during or before isolation. 7 Formation of @-Unsaturated Ketone ap-Unsaturated ketones are undoubtedly the most frequently reported product of alkene acylations. Examples of some intramolecular reactions are shown Isolation of conjugated ketone does not necessarily imply that it is a primary reaction product since it may also arise by an addition-elimination mechanism or via isomerization of non-conjugated ketone.If one considers proton ejection as involving attack by the cationic centre upon the electrons of a C-H a-bond then the relatively low electron density at the C-Ha bond is unlikely to be con-ducive to a-proton loss. On the other hand, if C-H bond fission is well developed in the transitior? state leading to unsaturated ketone formation, then both the intrinsic acidity of Ha in the ion (7) and the developing conjugation in the cup-unsaturated product should result in relatively ready transfer of Hato an adjacent nucleophile or solvent molecule. Whilst several workers1B2 admit tenta- m L.Otvos, H. Tudos, and L. Radics, Chem. and Ind., 1970, 597. 87 P. B. D. De La Mare, Sci. Progr. (Oxford), 1968,56, 243; G. Hoonaert and H. Martens, Tetrahedron Letters, 1970, 1821. D. Cram, ‘Fundamentals of Carbanion Chemistry’, Academic Press, New York, 1965. J. K. Groves and N. Jones, Tetrahedron, 1969,25,223. 40 P. Dostert and E. Kyburz, Helv. Chirn. Acta, 1970, 53, 897. 41 R. R. Sobli and S. Dev, Tetrahedron, 1970, 26, 649. G. Buchi and W. D. Macleod, J. Amer. Chem. SOC.,1962, 84, 3205. Groves CO2H 0v >('COCl SnClla 0 tive acceptance of the occurrence of direct a-proton loss, the importance of this mechanism as a general mode of conjugated ketone formation remains to be established conclusively.Acylation of unsymmetrical alkenes may afford stereoisomeric conjugated ketones; thus 2,4,4-trimethylpent-l-eneyields (8) and (9).43 Since kinetically- controlled acylation yields a /?y-unsaturated compound, the proportions of the stereoisomers obtained presumably reflect their relative thermodynamic stabilities. Me H Me Ac I \/ \ /Me CH,=C-CCH*CMe, --b AC+ C =c + C=C Ac /\ C€i,CMc, 1-1/\CH, CMe, 8 Formation of PyUnsaturated Ketones In certain instances Py-unsaturated ketones may, as a consequence of conforma-t ional factors, be the thermodynamically-con trolled acylation product .44 In many other cases Byunsaturated ketones are formed to the exclusion of thermo- dynamically more favourable conjugated isomers.45 Diacylation studies also 43 P.Amaud, Conzpt. rend., 1957, 244, 1785. 44 E. A. Braude and C. J. Timmons, J. Cliem. SOC.,1955, 3766. 45 J. K. Groves and N. Jones, J. Chenz SOC.(0,1968, 2215; 1969, 609. The Friedel-Crafts Acylation of Alkenes indicate that Py-unsaturated ketones are frequently the kinetically controlled monoacylation product .46 Several mechanisms have been proposed to account for the formation of non-conjugated products, the most convincing of which involves transfer of a y-proton to the carbonyl oxygen (lo), a process which occurs extensively in the R' R2 R' RZ /\ / KbO7-s -H CHR3 H-o=c 3jH //HO-C \ \R 'R chemistry of carbonyl compounds, This mechanism implies certain geometric limitations since it requires close proximity of the carbonyl oxygen and the y-hydrogen.The acetylation of 1-methylcycloalkenes (1 1) can afford two fly-unsaturated ketones and the observed effect of ring size upon the product distribution is perhaps related to the conformational preference of the acetyl group in the carbonium ions inv01ved.~' Studies upon conformationally more rigid systems could provide more conclusive evidence concerning the mechan- ism involved. n (12) (13)2 100% 0%. 3 64 36 4 17 67 46 A. T. Balaban, W. Schroth, and G. Fischer, Adv. Heterocyclic Chem., 1969,10,241. 47 J. K. Groves, Ph.D. Thesis, Lanchester Polytechnic, England, 1969; compare also J. A. Marshall, N. H. Anderson, and P. C. Johnson, J. Org. Chem., 1970, 35, 186. 82 Groves Acetylation of (+)-3-carene occurs trans to the cyclopropyl ring, probably as a consequence of the preferred conformation of this alkene.No evidence of cyclopropyi participation in the ion (14) was 9 Interaction of the Intermediate Ion with a Further Unsaturated Centre Acylation of 1,3-dienes should permit allylic rearrangements. 1,3-Cyclo-octadiene undergoes 1&addition of acetyl chloride but the resulting 3-acetyl- 8-chlorocyclo-octene (15) undergoes ready isomerization and eliminati~n.~’ ci ci Similarly, cycloheptatriene undergoes 1,6-addition of benzoyl chloride.60 Reaction at 0°C permits synthesis of 1-benzoylcycloheptatriene(16) whereas reaction at higher temperatures is accompanied by rearrangement to deoxy- benzoin (17). The extensive polymerization of cyclo-octatetraene which occurs upon attempted acylation of the free hydrocarbon can be circumvented by PhCOCl -HCI AlC13 c1 COPh I (16) (42%) COPh (17) acylating cyclo-octatetraene iron tricarbonyl (18). Subsequent displacement of the iron tricarbonyl unit affords the free ketones.s1 48 P.J. Kropp, D. C. Heckert, and T. J. Flautt, Tetrahedron, 1968,24, 1385. T. S. Cantrell and B. L. Strasser, submitted to J. Org. Chern., 1970. J. A. Blair and C. J. Tate, Chem. Comm., 1969, 1506. *l B. F. G. Johnson, J. Lewis, A. W. Parkins, and G. L. P. Randall, Chern. Comm., 1969,595. The Friedel-Crafrs Acylation of Alkenes ce4+ Free _L, ketones lY5-Dienes afford an intermediate ion in which the remaining alkene bond is suitably situated to permit nucleophilic attack upon the ionic centre with con- sequent formation of a new a-bond.1,5-Cyclo-octadiene affords 2-acetyl- 6-chIorobicyclo [3,3,0]octane (19) as a mixture of exo-and endo-isomer~.~~ Similar cyclizations of acyclic dienoic acid derivatives are also known, e.g. formation of (20).63vo&___I_, AIC13PPA Reaction of 1-methylcyclohexene with crotonic acid affords the normal acyla- tion-alkylation products (22) and (23), together with a small amount of a product (21) derived either by anti-Markovnikov addition of the acylating agent or by an alkylation-acylation rnechani~m.~~ According to the conditions employed, (31) (5%) (22) (1 5%) (23) (25%) 64 T. S. Cantrell, J. Org. Chem., 1967, 32, 1667.~3 M.F. Ansell and M. H. Palmer, J. Chem. Soc., 1960,5219. 15~S. B. Kulkarni and S. Dev, Tetrahedron, 1968, 24, 545. Groves trans-8-phenyl-5-octenoicacid (24) can be made to undergo alkylation-acyla- tion or acylation-alkylati~n.~~ An interesting new synthesis of cyclopentenones involves the treatment of ap-unsaturated esters [e.g. (25)] with polyphosphoric acid. Alkyl-oxygen heterolysis is followed either by (a) alkene formation and subsequent acylation or by (b) nucleophilic attack by the transient alkyl cation upon the carbonyl carbon (Scheme 4). Cyclization of the intermediate ion yields the five-membered ring ketone (26).66 (25) /PPA ,#'PPA 1 9 0PPA 0/PPA 0M Scheme 4 55 M. F. Ansell and S. S. Brown,J.Chem. SOC.,1958, 3956. 56 J. M. Conia and M. L. Levirend, Bull. SOC.chim. France, 1970,2981,2992. The Friedel-Crafts Acylation of Alkenes The failure of cyclododeca-l-cis-5-trans-9-trans-triene(27) to undergo intra- molecular cyclization is probably a consequence of rapid and essentially irrever- sible anion additi~n.~' 10 Intermolecular Hydride Transfer Nenitzescu and co-workers have shown that the use of a saturated hydrocarbon solvent for alkene acylations can provide a convenient synthesis of saturated ketones (Scheme 5)?8 Hydrogenation results from hydride abstraction from the solvent by the intermediate cation.* The ion formed by the solvent undergoes dehydrogenation and polymerization. Scheme 5 In the absence of large steric requirements the rate of such hydride transfer processes is a direct function of the relative stabilities of the incipient and dis- appearing cations5$ and hence should be most rapid with branched hydrocarbon solvents.In fact, cycloalkanes also appear capable of hydride donation at a practicable rate. Alkenes themselves are efficient hydride donors since they afford allylic cations but they are normally precluded from fulfilling this role by an adverse concentration factor. In certain instances the stereochemistry of products resulting from hydride abstraction have been established, for example, the l-acetyl-Z-methylcyclo- pentane formed by the dehydrogenation-acetylation of cyclohexane is pre- dominantly the more stable tr~ns-isomer.~~~~~ Delivery of hydride to the less hindered face of the intermediate ion (7) might be expected,61 but since the necessary excess of highly active catalyst provides conditions conducive to *Previous authors have suggested that reduction proceeds via chloroketone or unsaturated ketone.Recent '*C-labelling experiments show that reduction of the intermediate catien is considerably faster than that of either of these postulated intermediates. Personal com- munication, L. Otvos, H. Tudos, and A. Szabolcs, 1971. 67 J. Graefe, M. Muhlstadt, and D. M. Muller, Tetrahedron, 1970, 26, 2677. 68 C. D. Nenitzescu and E. Cioranescu, Chem. Ber., 1936,69, 1820. 6s N. C. Deno, G. Saines, and M. Spangler, J. Amer. Chem. SOC.,1962, 84, 3295. so H. Pines and N.E. Hoffman, J. Amer. Chem. SOC.,1954,76,4417. a R. M.Carlson and R. K. Hill, J. Org. Chem., 1969, 34, 4178. Groves enolate formation,62 reliable data concerning the stereochemistry of kinetically-controlled acylation products are difficult to obtain. 11 Intramolecular Hydride Transfer The intermediate ion (7) is destabilized by the electron-withdrawing effect of the adjacent carbonyl group. Accommodation of the positive charge at a position more remote from the carbonyl group by one or more hydride shifts is one manner in which the ion may increase its stability. The activation energy for 1,Zshifts is minimal when the vacant p-orbital at the cationic centre and the C-H bond of the migrating hydrogen are coplanar. It is also expected to decrease with increase in the positive charge at the migration terminus.63 Under certain conditions cyclopentene is reported to afford 86% y-chloro-ketone, presumably derived from a 1,Zhydride shift. In view of the inductive effect of the carbonyl group the hydride transfer process may continue beyond the y-carbon; thus 4% of 1-benzoyl-4-chlorocyclohexaneis obtained as a by- product from the benzoylation of cyclohe~ene.~~Similarly, cyclization of 3-(cyclohex-3-en-l-yl)propionylchloride (28) may afford the intramolecular hydride transfer product (29).66 Nenitzescu has shown that for acyclic systems the hydride shift process may continue until the positive charge reaches the most remote secondary, but not primary, carbon at0m.l Treatment of the resulting chloroketone with benzene results in alkylation of the aromatic compound in preparative yield: e.g.(30).66 i AlC13 C, H9 CH=CI I, + l%COCl 7 Ph(Me)CH(CH, ), COPh ii PhH (30) a H. 0. House, V. Paragamian, R. S. Ro, and D. J. Wluka, J. Amer. Chem. Soc., 1960, 82, 1457. J. L. Fry and G. J. Karabastos in ‘Carbonium Ions’,ed. G. A. OIah and P. von R. Schleyer,J. Wiley, New York, 1968, Vol. 2, p. 526. O4 C. L. Stevens and E. Farkas, J. Amer. Chem. SOC.,1953,75, 3306. 66 E. Marvell, R. S. Knutson, T. McEwen, D. Stunner, W. Ferderici, and K. Salisbury J. Org. Chem., 1970,35, 391. 6a A. D. Grebenyuk and N. F. Zaitseva, Zhur. org. Khim., 1968 4, 302 (Chm. Ah., 1968, 68, 95449). The Friedel-Crafts Acylation of Alkenes Nenitzescu has also shown that a tertiary carbon does not necessarily prevent hydride migrations which locate the charge in a position more remote from a carbonyl group.Such is the case in the formation of (31) from the acetylation of 1-met hylcyclohexene .67 In medium ring systems the favourable geometric arrangement of non-contiguous carbon atoms facilitates higher-order hydride shifts. cis-Cyclo-octene (32) has been shown to afford products in which 1,5-or 1,3-hydride shifts constitute the major reaction pathway.68 c1 12 More Complex Rearrangements The skeletal rearrangements that occur in alkene acylations may generally be represented as involving attack by the cationic centre upon the electrons of a C-C a-bond with subsequent or concurrent hydride migration.Unfortunately, the detailed mechanism of the ring-contraction reactions, which so commonly occur, remain a matter for conjecture. hcnAL.tl-, hl c' Me (33) 67 N. Dufort and J. Lafontaine Canad. J. Chem., 1968, 46, 1065; N. Dufort and J. Allard, ihid., 1969, 47, 2403. gs J. K. Groveb and N. Jones, J. Chem. SOC.(C),1969, 1718. 88 Groves The acetylcyclohexylium ion readily rearranges to the tertiary ion (33).g9 Acylation of cycloheptene can result in similar ring-contraction but in certain circumstances hydride migration may precede ring contraction .'l 0,A.GAc(i) hydride transfer' . (ii) +H-Complex rearrangements in the acetylcyclo-octylium ion have also been reported (Scheme 6).62972 QAc-HOAc d; 6Ti .Me c1 Et ( 14%) (53%) Q*-Scheme 6 In order to explain the formation of the aldehydes (34) as low-yield products from the acetylation and propionylation of cyclo-octatetrene, Cope suggested participation of the carbonyl oxygen.73 More convincing evidence of such par-d* L.&vos and H. Tudos, Chem. and Ind., 1969, 1140. 70 L. Rand and R. J. Dolinski, J. Org. Chem., 1966, 31, 3063. 71 S. L. Friess and R. Pinson, J. Amer. Chem. SOC.,1951,73,3512.'* L. Rand and R. J. Dolinski, J. Org. Chem., 1966, 31, 4061. A. C. Cope, T. A. Liss, and D. S. Smith, J. Amer. Chem. SOC.,1957,79,240. me Friedel-Crafls Acylation of Alkenes RCH, RCH, H (34) ticipation was obtained by Baddeley’s observation that acetylation of octa-hydronaphthalene (formed by in situ de-hydrogenation of decalin) results in the formation of 1,l’-epoxy-10-vinyl-trans-decalin(35) as the major This was interpreted as involving the formation of a four-membered cyclic bridged structure which rearranges to the less strained five-membered bridge.COMe Cl fCOMe H OH I-E (35) G. Saddeley, B. G. Heaton, and J. W. Rasburn, J. Chem. SOC.,1960,4713. Groves The products may alternatively be accounted for by bridging only after hydride migrations have located the charge at the geometrically more favourable y (and 6) positions. In the acetylation of camphene an exo-3,2-methyl shift appears to precede bridge formation to afford (36).75 Although (36) is probably a primary CH,AC product it should be noted that &unsaturated ketones can also form di- hydrofuran derivatives in acidic media.76 Benzoylation of camphene and sub- sequent hydrolysis was originally reported to afford 10-benzoylborneol but reinvestigation showed the product to be 10-benzoylisoborneol.77 Norbornene reportedly undergoes acetylation without rearrangement affording a chloro- ketone to which the authors tentatively assign a cis-ex0 configurati~n.~~ Finally, we mention the acylation of the quasi-unsaturated cyclopropane~.~~ Hart and Schlosberg suggest that the acetylation products of cyclopropane are best accounted for in terms of the ion (37) which undergoes equilibration with the more stable ions (38) and (39).Nucleophilic attack upon, or proton ejection from, these ions explains the products.Surprisingly, acetylcyclopropane forma- tion does not occur. The results obtained for the acetylation of methylcyclopro- pane do not yet permit a distinction between theinvolvement of conventional and bridged intermediates, whilst 1,l-dimethylcyclopropane undergoes isomerization to 2-methylbut-2-ene more rapidly than it acetylates. 75 J. A. Crosby and J. W. Rasburn, Chem. and Ind., 1967,1365. 76 D.D.Faulk, W. H. Corkern, I. Ookuni, and A. Fry,J. Org. Chem., 1970,35, 1518. 77 W.R.Vaughan, J. Wolinski, R. R. Dueltgen, S. Grey, and F. S. Seishter, J. Org. Chem,, 1970,35, 400.'* R. J. Poel, Diss Abs., 1965, B27, 766. 79 H.Hart and R. H. Schlosberg,J. Amer. Chem. SOC.,1968,90,5198.91 The Friedel-Crafts Acylation of Alkenes MeCO(CH2 C1 MeCOCH(Me)CH, C1 + +MCCOCH(C1)CHzMe 'M eCOC (Me)=CH2 13 Side reactions of Alkenes Alkenes readily undergo acid-catalysed isomerization and if the rate of acylation is not markedly greater than that of isomerization then the product composition will depend upon the rates of acylation and isomerization of the alkenes present. Bronsted acid catalysts are particularly prone to induce side reactions; thus polyphosphoric-acid-catalysedcyclizations of olefinic acids usually yield five- and six-membered ketones, irrespective of the initial positions of the acyl and alkenyl groups.so Obviously, the double bond migrates freely and the products reflect the acylation rates of the isomers present.Such bondmigration may be prevented 0 0 by choice of a more suitable catalyst. Trifluoroaeetic anhydride effects olefinic acid cyclizations without inducing bond migrations1 and acylation obviously forestalls isomerization in the aluminium-chloride-catalysedcyclization of 6-heptenoyl chloride to @-chlorocycloheptenone.8a Cyclohexanol, cyclohexyl chloride, and cyclohexane each react with aluminium chloride-acetyl chloride to afford a cyclohexylium cation. Reversible ring contraction, possibly by the mechanism shown in Scheme 7,permits formation of both cyclohexene and l-methylcyclopentene, but the acylation products are normally derived entirely from the more branched alkene.s3 Nitro-compounds tend to prevent ring contractions by assisting in rapid proton loss from the initial cycloalkyl cation.= no M.F. Ansell and M. H. Palmer, Quart. Rev., 1964, 18, 211; M. F. Ansell and T. M. Kafka, Tetrahedron, 1969,25,6025. M. F. Ansell, J. C. Emmet, and R. U. Coombs,J. Chem. Soc. (C), 1968,217. W. S. Trahanovsky, M. P. Doyle, P. W. Mullen, and Ching Ching Ong, J. Org. Chem., 1969,34,3679. as I, Tabushi, K. Fujita, and R. Oda, Tetrahedron Letters, 1968,4247. Groves 0 H 11-H+ 11 6I! 6'. Scheme 7 Apart from rearrangements, alkene protonation may also induce nucleophilic attack. Reactions employing carboxylic acids or anhydrides frequently result in carboxylate addition and the ester (or lactone)-to-ketone ratio of the product parallels the stability of the protonated alke~~e.'~ In certain instances the nucleo- phile involved is a second molecule of alkene, and thus the acylation of ethylene in the presence of an excess of aluminium chloride affords products which are, at least formally, derived from dimerization and rearrangement of the alker~e:~~ + 2 CH,=CH, AlClj Me\ RCO CORC=CH* -H+ Me' Me I-I 14 Side-reactions of the Acylating Agent The formation of acid chlorides from Lewis acids and acid anhydrides has already been noted.Acylium ions are reduced to aldehydes by hydrocarbon solventsE5 and for tertiary acyl halides such hydride abstraction may be accompanied by alkyl group migration.86 The decarbonylation reaction indicated in Scheme 8 occurs only if the resulting alkyl cation is tertiary and near planar.87 H.T.Taylor, J. Chem. SOC.,1958, 3922; T.Matsumoto, K. Hata, and T. Nishida, J. Org. Chem., 1958,23, 106. 86 G. Baddeley, E. Wrench, and R. Williams, J. Chem. SOC.,1956, 21 10. A. T.Balaban and C. D. Nenitzescu, Tetrahedron, 1960, 10, 55. D. G. Pratt and E. Rothstein, J. Chem. SOC.(C), 1968,2548. The FriedelLCrafts Acylation of Alkenes AICl3RCH,CR,COCl RCH,CR,CO+ RCH,zR, + co AlC1; AIClI;If 0I;: H-IIRCH2-C-6R, W RCH,-C-CHR, Scheme 8 In certain instances dehydrogenation of the alkyl portion of an acid chloride and subsequent intramolecular acylation have been reported, as in the formation of a-te t r alone from cyclo hex yl butyry1 chloride . 15 Side-reactions of the Ketones Diacylation studies have raised the question of the reversibility of monoacyla- tions.Baddeley postulated a deacetylation-reacetylation in the formation of the pyrylium salt (41)from 4-methyl-4-chloropentan-2-one(40) and acetyl chloride- aluminium Balaban and co-workers confirmed this by using labelled Me H+ -Hf Mey.Me7 Me\iS-CH,MeMe' Me = c,I-I AcCH, C(C1): Et Ac' 6 Me Me Me' 'Ac acetyl chloride and showing that the acetyl group of the ketone and acylating agent become equivalent during the reaction.90 Zinc chloride does not effect de- acylation and hence yields the different pyrylium salt (42).Other investigationsQ1 employing labelled ketones have confirmed deacylation of the chloroketone (40)but show that mesityl oxide undergoes deacylation less readily and that acetophenone does not at all.The reluctance of unsaturated ketones to undergo 88 N. Jones, E. Rudd, and H. T. Taylor, J. Chern. SOC.,1963,2354.** G. Baddeley and M. A. R. Khayat, Proc. Chem. SOC., 1961, 382. Bo M. Frangopol, A. Genunche, P. T. Frangopol, and A. T. Balaban, Tetrahedron, 1964, 20, 1881. 91 M. Frangopol, A. Genunche, N. Negoita, P. T. Frangopol, and A. T. Balaban, Tetrahedron, 1967,23, 841. Groves deacylation is perhaps related to conjugation energies. The ketone (43), which has a non-planar enone system, readily undergoes deacylation.92 0 CH, CH, if0CO &!1-CH2CH2 dMe a'\CH2CH2 H+ (b.(b.fifiOMeOMe Re-ionization of chloroketonesmay induce reactions other than deacylation ; thus 1-benzoyl-2-chlorocyclohexaneundergoesaluminium-chloride-ctalysediso-merization to the 4-chloro-isomer.64More surprisingly, treatment of l-acetyl-4-chlorocyclo-octanewith tin(Iv) chloride affords 4-acetylcyclo-octene(15 %) and 1-acetylcyclo-octene (25 %).93 Formation of the latter, which presumably involves ionization followed by hydride migration, appears contrary to Cope's generalization that transannular hydride shifts are important only when a more stable cation Interconversionsof tautomeric ketones are particularly common in Bronsted-acid-catalysed acylations,8o and base-catalysed isomerizationss6 of Py-un-saturated ketones to their conjugated tautomers may occur during isolation procedures.: The formation of an aromatic system may induce various further reactions of an initial product.Alka-2,4-dienoic acids afford dienones which enoliseto the correspondingphenol.e7Dehydrogenationreactions, e.g. formation of (44),may also occur.g8 Although /%chloroketones are normally readily dehydrohal~genated,~~in cer-tain instances the synthetic usefulness of alkene acylations is limited by an in-ability to effect dehydrochlorination without effecting concurrent isomerization. #See footnote to ref. 95, and ref. 96. sa J. A. Barltrop and N. A. J. Rogers, J. Chem. SOC.,1958, 2566. s3 T. S. Cantrell, personal communication. s4 A. C. Cope, M. M. Martin, and M. A. McKervey, Quart. Rev., 1966,20, 119. OE. J. K. Groves and N. Jones, J. Chem. SOC.(C), 1968,2898.$The reverse isomerisation maybe effected photochemically or by enol esterificationfollowed by mild hydrolysis (see ref. 961. D. Amar, V. Permutti, and Y. Mazur, Tetrahedron, 1969, 25, 1717. 97 G P. Chiusoli and G. Agnes, J. Chem. SOC.,1963, 310. s* R. L. Frank and R. C. Pierle, J. Amer. Chem. SOC.,1951, 73,724. gg R.Braidy, Compt. rend., 1966, 263C,810. 95 4 The Friedel-Crafts Acylation of Alkenes For example, dehydrochlorination of (45) results in isonierization at the alkene bond and at the ring junction.loO c1 @ 1+Po H Finally, we mention the dehydrochlorination of chloroketones in which chloride addition was preceded by hydride migration. These compounds are considerably more reluctant to undergo dehydrochlorination and, in certain instances, the major reaction is an intramolecular anionic displacement within the enolate anion.Useful syntheses of bicyclic systems [e.g. (46)] may result .Io1 c1-C1 16 Concluding Remarks An attempt has been made to provide a broad coverage of the various types of reaction possible. Despite the variety of reaction pathways available viable syntheses of unrearranged saturated or conjugated ketones are frequently possible. Syntheses of /3y-unsaturated ketones proceed satisfactorily for many alkenes which afford a tertiary intermediate ion, but anion addition reactions may preclude the application of this method to less branched alkenes. Additional unsaturated centres in the alkene or acylating agent behave in a relatively predictable manner but the same is only superficially true of rearrangement loo J.A. Marshall, N. H. Anderson, and J. W. Schlicher, J. Org. Chem., 1970, 35, 858. lol J. K. Groves and N. Jones, J Chem. SOC.(C), 1969,2350. Groves reactions. Friedel-Crafts acylations of alkenes clearly provide much interesting chemistry, the serious study of which has hardly begun. The author would like to thank Mr.N. Jones (Lanchester Polytechnic) and Dr. A. G. Fallis (Memorial University of Newfoundland) for helpful com- ments, and to acknowledge receipt of a National Research Council of Canada Research Fellowship.

ISSN:0306-0012    

DOI:10.1039/CS9720100073

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

Met al-Me t al Interact ions in Transition- me tal Complexes Containing Infinite Chains of Metal Atoms By T. W. Thomast and A. E. Underhill DEPARTMENT OF CHEMISTRY, UNIVERSITY COLLEGE OF NORTH WALES, BANGOR, CAERNS 1 Introduction Co-ordination compounds containing direct metal-metal interactions can be divided into two distinct types: Type A. Complexes containing discrete numbers of directly interacting metal atoms. Type B. Complexes containing an infinite number of directly interacting metal atoms arranged in linear chains throughout the crystal lattice. Type A complexes, which include dimeric complexes [e.g. copper(I1) acetate] and metal cluster compounds (e.g. K,Mo,Cl,) have been the subject of several extensive reviews.l However, no comprehensive review has been published of the TypeBcomplexes, and this present work attempts to classify the known examples of this type of complex and to present an assessment of the factors which affect the formation of such structures, together with a discussion of some of those properties which are unusual. This review includes only complexes in which direct metal-metal interactions T t Present address: Department of Chemistry, Temple University, Philadelphia 19 122, U.S.A.F. A. Cotton, Accounts Chem. Res., 1969,2,240; F. A. Cotton, Quart. Rev., 1966,20,389; B. R. Penfold in ‘Perspectives in Structural Chemistry’, Wiley, New York, 1968, Vol. 2, p. 71. Metal-Metal Interactions in Transition-metal Complexes occur, and excludes those structures in which the interaction occurs via a bridging atom or group.Thus many mixed-valence complexes, in which non- direct interaction is known to occur,2 and also many naturally occurring minerals in which the observed interaction3 may occur via bridging oxygen atoms, will not be discussed. Also excluded from this review are compounds such as Nb14 in which the metal atoms are grouped together in pairs so that the interaction is localized between pairs of adjacent metal atoms and does not extend along the whole metal-atom chain. A. Occurrence of Type B Complexes.-In general, these complexes have the columnar structure (1) in which planar or nearly planar monomer units are stacked above one another to form metal-atom chains. Most of the Type B complexes are formed from square co-planar monomers, and because such monomeric structures are mainly confined to ds metal complexes, this review deals predominantly with complexes of nickel(II), palladium(II), and platinum(n).It is however, more convenient to classify these complexes according to the ligand present rather than the central metal atom, and this classification is adopted in Sections 2-6. anionI.pz band cation pz band pz band anion dz2 band cation dz2 band dz2 band Figure 1 Diagrammatic representation of the band structure in d8 metal-atom chain compounds. Efect of (A) decreasing inter-metallic distance, (B) partial oxidation, and (C) alternatinganion-cation chain. Shaded portion indicates jilled band.M. B. Robin and P. Day, Adv. Inorg. Chem. Radiochem., 1967, 10, 247; G. C. Allen and N. S. Hush, Progr. Inorg. Chem., 1967, 8, 357. D. W. Robbins and R. G. J. Strens, Chem. Comm., 1968, 508. Thomas and Underhill B. Consideration of the Metal-Metal Interaction within the Chain.-Figure shows the relative energies of the orbitals of a metal atom situated at the centre of a square-coplanar array of ligands (D4h ~ymmetry).~In a columnar structure (l), N square-planar molecules may be assumed to be stacked along the z-axis. The interaction between the adjacent molecules in the column may be considered to be one of two types. (i) Purely electrostutic interaction. In this there is no overlap of orbitals of one metal atom withthose of its neighbours, and all the changes in spectroscopic properties are considered to be due to intramolecular transitions modified by the presence of the electrostatic crystal field of the neighbouring molecules in the chain.This approach has been discussed in detail recently by Day.6 (ii) Metal-metal orbital overlap. In this approach, developed by Rundle6 and Miller,’ the p, and dzz orbitals, or a combination of these orbitals, are con- sidered to overlap with the corresponding orbitals on adjacent metal atoms. If the p, and orbitals on adjacent metal atoms do overlap then N delocalized molecular orbitals are formed from the overlapping p, orbitals and N delocalized molecular orbitals from the dzz orbitals. Each of these groups of molecular orbitals constitutes a ‘band’ having energy limits ranging from a value above to a value below that of the unperturbed atomic orbital (see Figure 1).Ingrahama has calculated that the spread in energy of the band arising from the overlap of the 3dz2 orbitals in bisdimethylglyoximatonickel would vary from +0.509 to -0.492 eV compared with the energy of the unperturbed d,z orbital. Since the width of each band is proportional to the extent of overlap of the individual orbitals and therefore inversely proportional to the interatomic distance, a de- crease in interatomic distance produces a wider band, and hence a smaller ‘band gap’ between the top of the highest filled band and the bottom of the lowest empty band. The remaining d, s, and p orbitals on adjacent metal atoms are much less likely, for symmetry reasons, to overlap and form bands.These energy levels remain discrete, therefore, and the orbitals remain localized on each metal atom. The presence of another atom in close proximity along the z-axis will, however, perturb these orbitals to various degrees and, therefore, although they do not overlap with neighbouring metal orbitals, electronic transitions which involve these orbitals are sensitive to the presence of metal-metal interactions. For metal atoms with a dsconfiguration, the d,z band is full and thep, band empty. Although full and empty bands, when considered individually, are non- bonding, configuration interactions between them may result in a small net bonding in the ground ~tate.~ This effect is, however, very small in bisdimethyl- glyoximatonickel.s Oxidation of the central metal atom, however, will result in removal of electrons from the uppermost (antibonding) part of the dtz band and B.N. Figgis, ‘Introduction to Ligand Fields’, Interscience, New York, 1966, p. 313. P. Day, Inorg. Chim. Acta Rev., 1969,3, 81. R. E. Rundle, J. Phys. Chem., 1957, 61,45.’J. R. Miller, J. Chem. SOC.,1965, 713. L. L. Ingraham, Acta Chem. Scand., 1966, 20, 283. K. Krogmann, Angew. Chem. Znternat. Edn., 1969, 8, 35. Metal-Metal Interactions in Transition-metal Complexes hence result in a net increase in bonding. A series of compounds of this type, in which the inter-metallic distance is about 0.4 A shorter than in the corre- sponding compound with a full dzsband, has been reportedQ and is discussed in detail in Section 4.It can be seen from the preceding discussion that the metal orbital overlap approach does not necessarily imply the existence of metal-metal bonds in the ground state, and that both this approach and the purely electrostatic inter- action approach deal in a similar way with the modified ligand-field spectrum of crystals of these compounds. C. Investigation and Determination of Metal-Metal Interactions.--(i) X-&y crystal structure determination. The presence of chains of metal atoms running throughout the structure of single crystals of a compound can only be established by a full X-ray structure determination. In many of the compounds considered to contain interacting metal atoms, the inter-metallic distance along the chain, although short, is considerably longer than that found in the metal itself and does not indicate, apriori, the existence of any interaction.Thus, in all cases, further evidence from other experimental techniques is necessary to establish the presence of an interaction between the adjacent metal atoms. (ii) Spectroscopic measurements. The majority of the experimental evidence for the occurrence of interactions in metal-atom chain compounds is derived from a study of their U.V. and visible spectra. A reviews has recently appeared which deals in part with the spectroscopic properties of metal chain compounds and, therefore, only a brief account of this subject will be given here.The evidence is of two types. Diference between the solid-state (polymeric) and solution (monomeric) spectra. Compounds which possess a columnar stacked structure in which the metal atoms are separated by large inter-metallic distances (e.g.l0 K,PtCI,; Pt-Pt, 4-13A) exhibit very similar solution and solid-state spectra." In contrast to this, many of the compounds possessing a much shorter inter-metallic distance (-3-0-35 A) have solid-state spectra which are different from those of the same compounds in solution, often containing a band of considerably lower energy than any of the strong bands observed in solution. This 'solid-state' band has been assigned to the dzz-pz transition (often with a contribution from a metal --f ligand charge transfer transition) or to a transition from the dzp band to the pz band, the energy of which is very dependent on the proximity of the metal atoms in the z direction.In addition to the appearance of the new band there are also changes in the high-energy part of the spectrum. In general, the absorption bands observed in the solution spectra are still recognizable in the solid-state spectra but they undergo significant shifts in frequencies. Polarization of the low-energy band in the crystal spectra. The absorption bands of planar organic molecules are strongly polarized 11 to the plane of the lo R.C. Dickinson,J. Amer. Chem. SOC.,1922,44,2404. l1 D. S.Martin and C.A. Lenhardt, Inorg. Chem., 1964,3, 1368. Thomas and Underhill molecule.Yarnadal, found that the lowest energy band for certain planar transition-metal complexes was also 11 polarized but that for some columnar stacked compounds with fairly short inter-metallic distances the lowest energy band was more strongly polarized 1,than I[to the plane of the molecule.la This dichroism he termed 'unusual' and associated it with the presence of a metal- metal bond perpendicular to the plane of the molecule. Although the resolution of the spectra was poor in this early work, more recent studies1*J6 have confirmed that in many cases the lowest energy absorption band is predominantly 1. polarized. The lowest energy band of bis-N-methylsalicylaldiminatonickel,which also has a columnar structure is, however, 11 polarizedindicating that the presence of the columnar structure does not necessarily produce a 1_ polarized low- energy band.la It has been suggested more recently that the out-of-plane 1, polarization is not abnormal for planar complexes in which the transition is not a pure 7r-+n* transition of an organic molecule and particularly if it involves transition-metal orbitals." Thus the 1, polarization of the low-energy band cannot be taken as conclusive evidence of a metal-metal interaction.(iii) Mugnetism and e.s.r. measurements. As mentioned earlier, the majority of complexes containing chains of metal atoms involve metal ions with a da configuration located in a square-planar environment. The complexes are thus diamagnetic and little information has been gained from these techniques.(iv) Electrical conductivity measurements. Studies have been made of single crystals of some of these complexes and several have been found to be aniso- tropic semiconductors with greater electron delocalization along the line of the met al-metal chain. 2 Complexes of vic-Dioximes and Salicylaldiminates Some of the best known examples of infinite chains of interacting metal atoms occur in crystals of complexes containing anionic planar organic ligands. The neutral square co-planar monomeric units (ML,)are stacked above one another to form chains throughout the crystal lattice (see Figure 2). Discussion of the metal-metal interaction in these complexes has been based upon evidence from X-ray crystallographic, solubility, spectroscopic, and more recently from electrical conduction studies. It is useful to discuss the evidence from each technique separately as this illustrates the application of these techniques to the investigation of metal-metal chains.A. Structural Studies.-The general structure shown in Figure 2 has been found I2 S. Yamada, J. Amer. Chem. SOC.,1951,73, 1182. S. Yamada, J. Amer. Chem. SOC.,1951,73, 1579. lPB.G. Anex and F. K. Krist, J. Amer. Chem. SOC.,1967,89,6114. P. Day, A. F. Orchard, A. J. Thomson. and R.J. P. Williams, J. Chem. Phys., 1965, 43, 3763. J. Ferguson, J. Chem. Phys., 1961,34,611. G. Basu, G. M.Cook,and R. L. Belford, Inorg. Chem., 1964,3, 1361. 1' Metal-Metal Interactions in Transition-metalComplexes Figure 2 Structure of bisdimethylglyoximatonickelshowing how the methyl groups (represented by circles) on successive molecules interlock.for a number of the vic-dioximes of nickel@), palladium(II), and platinum(II).18-21 In bisdimethylglyoximatonickel [Ni(dmg),] the monomer units are stacked above one another in the line of the c-axis of the orthorhombic crystals with the planes of the individual molecules parallel to the (001) plane.18 The two ligands within each monomer are linked by short hydrogen bonds (2.40 A), conferring a rigidity to the planar molecule. Successive molecules axe staggered by 90" and because the inter-metallic distance is only 3.245 A the methyl groups, which form the thickest part of the molecule, interlock in the manner shown in Figure 2.This interlocking of adjacent molecules may add to the stability of the chain structure. The short metal-metal distances determined for Ni(dmg), (3.245 A)** and Pd(dmg), (3.253 A)l* led to the suggestion of metal-metal bonds in these complexes and that the presence of these bonds may further stabilize the columnar structure. Many complexes of Nil1, PdII, and PtI1 with vic-dioxime ligands have been L. E. Godycki and R. E. Rundle, Acta Cryst., 1953, 6,487; D. E. Williams, G. Wohlauer, and R. E. Rundle,J. Amer. Chem. SOC., 1959,81,755. lS C. Panattoni, E. Frasson, and R. Zannetti, Gazzetta, 1959,12, 2132. ao C. V. Banks and D. W. Barnum, J. Amer. Chem. SOC.,1958,80,4767. 21 E. Frasson, C. Panattoni, and R.Zannetti, Acta Cryst., 1959, 12, 1027. Thomas and Underhill studied but few possess the columnar structure analogous to that of Ni(dmg),. Table 1 shows the inter-metallic distances in those complexes which possess a Table 1 Complexes of vic-dioximes and salicylaldimines with short M-M distances Complex Nickel 4-isopropylnioxime M-M bond length (A) 3-19" Solid state colour band (cm-' x lo-*) 1 7.9Sa Nickel 4-t-amylnioxime 3.2" 1 8.28" Nickel nioxime 3*237a 1 8.1 2" (1 8.21 e) Nickel 4-methylnioxime 3~24~ 1 8.2ga Nickel dimethylglyoxime 3.24ssg 18*60*J(18.05a) /%Nickel ethylmethylglyoxime 3.4c 20.49 Nickel furil-a-dioxime 3~448~ 1 8-62a Nickel 3-met hylnioxime 3.47" 1 9*88a Nickel benzil-a-dioxime 3.547a 19-42a Nickel heptoxime 3,596" 21*9b (21 -51 ") Palladium nioxime 3.250" 20.75" (21*50"*e) Palladium dimethylglyoxime 3.26-f~~ 20.83" (22.17") Palladium heptoxime 3-329" 23-53" Palladium furil-a-dioxime 3.459a 21 -74a Palladium benzil-a-dioxime 3.517" 22-99a Platinum dimethylglyoxime 3-23) 16-28" Nickel N-methylsalicylaldiminate (orthorhombic form) Copper N-me thylsalicylaldiminate (orthorhombic form) 3.33f - $ Lowest energy band 16 500 cm-' reported polarized in plane of the molecu1e.m a C.V. Banks and D. W. Barnum, J. Amer. Chem. SOC.,1958, 80, 4767. * B. G. Anex and F. K. Krist, J. Amer. Chem. SOC.,1967, 89, 6114. A. G. Sharpe and D. B. Wakefield, J. Chem. Soc., 1957,281. d J. C. Zahner and H.G. Drickamer, J. Chem. Phys., 1960,33, 1625. e H. G. Drickamer and J. C. Zahner, Adv. Chem. Phys., 1962, 4, 161. f D. E. Williams, G. Wohlauer, and R. E. Rundle, J. Amer. Chem. SOC.,1959, 81, 755. E. Frasson and C. Panattoni, Acta Cryst., 1960, 13, 893. h M. R. Fox and E. C. Lingafelter, Acta Cryst., 1967, 22, 943. E. C. Lingafelter, G. L. Simmons,B. Morosin, C. Scheringer, and C. Freiberg,Acta Cryst., 1961, 14, 1222. J E. Frasson, C. Panattoni, and R. Zannetti, Acfu Crysf., 1959, 12, 1027. IC C. Panattoni, E. Frasson, and R. Zannetti, Gazzettu, 1959, 12, 2132. Y.Ohashi, I. Hanazaki, and S. Nagakura, Inorg. Chem., 1970, 9, 2551. J. Ferguson, J. Chem. Phys., 1961, 34, 611. columnar structure. It is of note that all the dioxime complexes which have short inter-metallic distances adopt an orthorhombic crystal habit.Surprisingly, the bis-complexes of glyoxime with NP, PdII, and PtII do not adopt the Metal-Metal Interactions in Transition-metal Complexes Ni(dn~g)~structure,22 although the molecules are planar and the ligand is similar in structure to dimethylglyoxime. The bismethylethylglyoximatonickel molecule is also planar, but the complex adopts a crystal structure which precludes the possibility of metal-metal interaction.,* It seems likely that the interlocking of successive planar molecules as described for Ni(dmg), is a requisite for a columnar structure with a short metal-metal distance in this type of compound, since this interlocking cannot occur either in the glyoximes, where the bulky methyl groups are replaced by hydrogen atoms, or in complexes with more bulky ligands such as methylethylglyoxime where the larger substituents prevent close packing of successive molecules.The complexes with nioxime ligands have short inter-metallic distances20 and may also interlock. Molecules of Cu(dmg), are not planar24 and this complex does not have a structure containing metal-atom chains. A study of some bis-salicylaldiminato and bis-N-methylsalicylaldiminato complexes show that whereas CulI 2s and NiII 26s27 complexes of bis-N-methyl- salicylaldiminato can exist in a form isomorphous to Ni(dmg),, with inter- metallic distances of 3-33 A and 3.29 A respectively, the bis-salicylaldiminato complexes ~~IIIIo~.~~~~~It would again appear that the ability of the ligands on successive molecules to interlock may be a prerequisite to the formation of this type of structure, and again the complexes of ligands with more bulky substituents such as bis-N-ethyl- and bis-N-butyl-salicylaldiminates adopt different structures.28 It is significant that complexes of NiII, PdII, and PtII which have the same ligand and possess the Ni(dmg),-type structure have similar inter-metallic distances.This indicates that the minimum inter-metallic distance in these com- plexes is probably determined by the packing of the ligand molecules. B. Solubility Studies.-The extremely low solubility of Ni(dmg), and Pd(dmg), in many solvents has been known for some time and has led to their use in the gravimetric determination of these metals.29 It has been suggested that this low solubility is due to the presence of metal-metal bonds in the solid ~tate~O-~~ and attempts have been made to relate the solubility of some vic-dioximes to the 2* M.Calleri, G. Ferraris, and D. Viterbo, Acta Cryst., 1967, 22,468; M. Calleri, G. Ferraris, and D. Viterbo, Inorg. Chim. Acta, 1967, 1, 297; G. Ferraris and D. Viterbo, Acta Cryst., 1969, B25,2066. *3 E. Frasson and C. Panattoni, Acta Cryst., 1960, 13, 893. 24 E. Frasson, R. Bardi, and S. Bezzi, Acta Cryst., 1959, 12, 201. rsE. C. Lingafelter, G. L. Simmons, B. Morosin, C. Scheringer, and C. Freiburg, Acta Cryst., 1961, 14, 1222. *8 M. R. Fox and E. C. Lingafelter, Acta Cryst., 1967,22,943.27 J. M. Stewart and E. C. Lingafelter, Acta Cryst., 1959,12, 842. E. Frasson, C. Panattoni, and L. Sacconi, Acta Cryst., 1964,17, 85,477. ** A. I. Vogel, ‘Textbook of Quantitative Inorganic Analysis’, Longmans, Green and Co., London, 1961. 3o A. G. Sharpe and D. B. Wakefield, J. Chem. Soc., 1957,281. 31 C. V. Banks and D. W. Barnum, J. Amer. Chern. Soc., 1958,80, 3579. 3p R. E. Rundle and C. V. Banks, J. Phys. Chem., 1963,67,508. C. V. Banks and S. Anderson, J. Amer. Chem. SOC.,1962.84, 1486. Thomas and Underhill length, and thus the strength, of the proposed metal-metal bond. The work by Banks and Barn~m~~ indicated, however, that the solubility of these complexes is also dependent on the nature of the ligands, but if a series of similar ligands is considered, a correlation does exist between solubility and inter-metallic distance.A comparison of the solubilities of Ni(dmg), and Ni(emg), led to anestimate of 9-1 1 kcal mol-1 for the strength of the Ni-Ni bond in the former This estimate is in good agreement with a value of 10 kcal mol-1 calculated by Rundle and BanksS2 from the crystal structures of the two complexes. C.U.V. and Visible Spectra Studies.-The absorption spectra of Ni(dmg), in suspension,17 polycrystalline films,17 and single ~rystal~,~~J~~~~~~~~~~ have received much attention during the past 15 years. Discussion of the spectral characteristics of single crystals of Ni(dmg)2 and structurally related vic-dioximes have centred on the presence of the lowest energy visible absorption band at about 18000-20000 cm-l which is polarized to the plane of the molecule and is not present in the solution spectrum (see Table 1).The band has been assigned to the 3dza -4pztransition with some 3dz2 -+ ~*blu~haracter,1~~~~-~~ and its presence interpreted as being due to the presence of metal-metal bondS.20sS4Day,5 however, has argued that the band is a result of electrostatic crys t al-field interact ion be tween neighbouring intramolecular transit ion dipoles, and that no metal-metal bond, or band, formation need be invoked to rationalize these phenomena. In a new study of this problem, Ohashi and co-workers86 conclude that the band is mainly due to the 3dza -4pz transition within a nickel atom, but also includes some interatomic 3dza (atom a) -+4pz(atom b) charge-transfer excitation.A close relationship exists between the energy of this absorption band and the inter-metallic distance for a series of vic-dioximes. This relationship has been successfully used to predict the inter-metallic distances in other related vic-dioximes. Drickamer and ZahneP also found this correlation when they studied the effect of pressure on the visible spectrum of Ni(dmg),, Pd(dmg),, and Pt(dmg),. They found that the energy of the dz2 3pz transition decreased markedly with increasing pressure due to the shortening of the inter- metallic distances. Molecular orbital calculations performed by Ingraham8 for Ni(dmg), indicate that an overlap of the dza orbitals is unlikely in the ground state.In the excited state, however, sufficient overlap of occupied 4pz orbitals may lead to inter- action between adjacent nickel atom~.~J~ D. Electrical Conduction Studies.-Recent studies of single crystals of Ni(dmg)28s have shown that the electrical conductivity along the axis of the metal-metal chain is 106 times greater than that of a compressed powder disc of the complex, 34 S. Yamada and R. Tsuchida, Bull. Chem. SOC.Japan, 1954, 27, 156. as Y. Ohashi, I. Hanazaki, and S. Nagakura, Inorg. Chem., 1970,9,2551. 86 J. R. Miller, J. Chem. SOC.,1961, 4452. 37 J. C. Zahner and H. G. Drickamer, J. Chem.Phys., 1960,33,1625. T. W. Thomas and A. E. Underhill, Chem. Comm., 1969, 725. 107 CI0 i$ 700 Table 2 Tetracyano-complexes Crystal system (z) polarized (xy) polarized fluorescence sComplex M-M distance Cotour (A) absorption absorption band F band band maximum R (cm-l x (cm-l x (cm-l x 2 violet monoclinic 3n’dark red ort horhombic 2 16-40c s*red tetragonal 18.00; 19*4Od 18.0Oe s-17.10C Y violet red monoclinic a” 17.90C s.g.18-9Oc yellow green monoclinic 22.70d 22-00e 19.50C $yellow green monoclinic 5yellow triclinic yellow orthorhombic 23*90d 22-80e 20.20c $21-70C k 24.W 22.10c $2colourless monoclinic 28~30~ 23.30C colourless colourless orthorhombic 30.30s colourless ort horhombic 3 1 *70gya colourless monoclinic 35.709 colourless0 Table 2 (continued) Ca[Ni(CN),],SH,O 3.221 orange orthorhombic 3-38 Li2[Ni(CN) J,3H20 3.631 Sr[Ni(CN),], 5H ,O 3.641 orange monoclinic 3.65 Na2[Ni(C"MH@ 3.671 K2"i(CN)41,3H20 3.69j a K.Krogmann and D. Stephan, 2.anorg. chem., 1968,362, 290. b K. KrogmaM, Angew. Chem. Internat. Edn., 1969, 8, 35. M. L. Moreau-Colin, Bull. SOC.roy. Sci. Liege, 1965,34, 778. d C. Moncuit and H. Poulet, J. Phys. Rad., 1962, 23, 353. e S. Yamada, Bull. Chem. SOC.Japan, 1951.24, 125. f H. Brasseur and A. de Rassenfosse, Bull. SOC. roy. Sci. Liege, 1939, 8, 24. 0 A. Macadre and C. Moncuit, Compt. rend., 1965,261, B, 2339. F. Fontaine, Bull. SOC.roy. Sci. Liege, 1964, 33, 178. R. LeBras and C. Moncuit, Compt. rend., 1968, 267, B, 1032. j M. L. Colin, Bull. SOC.roy. Sci. Liege, 1963, 34, 130. H. Brasseur and A. de Rassenfosse, Bull.SOC.roy. Sci. Liege, 1935, 4, 68. 1 R. M. Bozorth and L. Pauling, Phys. Rev., 1932, 39, 537. Metal-Metal Interactions in Transition-metal Complexes indicating some degree of electron delocalization along this chain. Crystals of Ni(dmg), behave as ohmic semiconductors and the energy of the band-gap closely corresponds to the lowest energy spectral transition present in the single crystal The observation of semiconducting behaviour rather than metallic conduction along the metal-metal chains indicates that it is necessary first to promote electrons to an excited state before conduction occurs. This parallels the arguments put forward by Anex and Kristl, and by Ingraham* that overlap of orbitals giving rise to metal-metal bonds is only probable in the excited state.3 Tetracyano-complexes %Ray show that in crystals of Mg[Pt(CN),],7H20 the square-planar [Pt(CN),I2- are stacked above one another with the Pt atoms forming chains which run through the crystal parallel to the c-axis. Successive [Pt(CN) J2-units are rotated by 45" and the Pt-Pt distance is 3.155 A. The metal-metal distances in a large number of PtII, PdII, and NiII cyano-complexes possessing a columnar structure of this type have been determined and are given in Table 2. The inter-metallic distance is strongly influenced by the cation but little affected by the presence of different central transition-metal atoms. This is seen in the two series Ca[M(CN),],SH,O (M = Ni, Pd, or Pt; Ni-Ni, 3.38; Pd-Pd, 3.42; Pt-Pt, 3.38 A) and Sr[M(CN),],SH,O (Ni-Ni, 3-65; Pd-Pd, 3.63; Pt-Pt, 3.60 A).A detailed study has been made of the platinum series of compounds and a variation in the Pt-Pt distance from 3.09 A in Sflt(CN)J,3H20 and 3.155 A in Mg[Pt(CN)J,7Ha0 to 3-60 A in Sr[Pt(CN),],SH,O is observed. This comparison illustrates not only the wide variation in inter-metallic distances found for a given central metal atom, but also the very marked effect of the degree of hydration on the metal-metal distance. Evidence for metal-metal interactions in these compounds, apart from the actual inter-metallic distances is again obtained from extensive studies of the U.V. and visible spectra of these compounds. A. Platinum(rr) Cyanides.-Yaniada first studiedM the polarized crystal spectra of Ca[Pt(CN),],SH,O, Mg[Pt(CN),],7H20, and B@t(CN),],4H20.He observed a very strong broad 1_ polarized band and a much sharper 11 polarized band both at lower frequencies than any of the absorption bands observed for these compounds in solution. The polarized spectra for several other [Pt(CN),12- complexes have now beem determined (see Table 3) and an approximately linear correlation between the Pt-Pt distance and the position of the 11 polarized band has been A similar relationship is found for the 1.polarized band using the data of Moncuit and Pouletq2 and for the band maximum observed in the as R. M. Bozorth and L. Pauling, Phys. Rev., 1932, 39, 537. 40 S. Yamada, Bull. Chem. SOC.Japan, 1951,24,125. 41 S.Yamada, 'Essays in Co-ordination Chemistry', ed. W. Schneider, G. Anderegg, and R. Gutt, Experentia, Supplementum IX, 1964. 4s C. Moncuit and H. Poulet,J. Phys. Rad., 1962,23,353. Thomas and Underhill Pt-Pt distance (A) 2*880a 2~887~ 2-985 2.96 2.80" 2-81" 2.81" 2-82d 2.82" 242d 2.83" 2.83" 2~84~ 2-84" 2.85" 2~85~ 2.W 2-85" 2-88" 2.8451 a K. Krogmann and H. D. Hausen, 2. anorg. Chem., 1968,358, 67. b K.KrogmaM and G. Ringwald, Z. Naturforsch., 1968,23b,11 12.C K.Krogmann and H. D. Hausen, Z. Narurforsch., 1968,23b,11 1 1. * K. Krogmann, Angew. Chem. Internat. Edn., 1969,8,35. 8 K.Krogmann, Z. anorg. Chem., 1968,358,97.f K.Krogmann, W. Binder, and H. D. Hausen, Angew. Chem. Internat. Edn., 1968,7, 812.fluorescence spectra as determined by M~reau-Colin~~ (see Figure 3). The _L polarized band has been assigned4a to the 5dz* -+ 6pz transition and it has been that the 11 polarized band occurring at approximately the same frequency also arises from this transition due to a vibronic coupling mechanism. There is some disagreement concerning the colourless compound Sr[Pt(CN),],5H20 in which the Pt-Pt distance is 3-60 A.46Yamada reportedq1 no additional bands present in the solid-state spectrum compared with the solution spectrum and therefore suggested that there could be no inter-metallic interaction. Moncuit and Po~let,~~ however, recorded a spectrum for the solid which is very similar in shape to those they obtained for the barium, magnesium, and calcium salts.The band system, however, occurs entirely in the U.V. and not partially in the visible region as observed for other salts and, in addition, the bands occur at much lower frequencies than those observed for [Pt(CN)Jz- in 43 M. L. Moreau-Colin, Bull. SOC. Roy. Sci. Lidge, 1965,34, 778. 44 C. Moncuit, J. Phys. Rad,, 1964,25, 833. 45 K.Krogmann and D. Stephan, Z. anorg. Chem., 1968,362,290. Metal-Metal Interactions in Transition-metal Complexes 30.0 -28.0 -26.0 -24.0 -cm-l x 10-~ 22.0 -20.0 -I X I I I I 3.1 3.2 3.3 3.4 3.5 3.6 Pt-Pt distance (A) Figure 3 Variation of band position with Pt-Pt distance for a series of platinocyanides. 0,z-polarized band; x ,fluorescence band maximum.solution. It can be seen from Figure 3 that both the1 polarized band maximum4a and the fluorescence band maximum43 fall on the same curves as those of the other complexes and this absorption therefore probably arises from a similar transition to that present in the other complexes. The observation of an intense colour associated with compounds having a Pt-Pt distance of less than 3.25 A is explained by the occurrence of the strong absorption bands in the visible region but, as discussed above, the absence of an intense colour does not necessarily indicate the absence of a metal-metal inter- action. These results also indicate that a weak interaction occurs even at a Pt-Pt distance of 3.60 A. B. Palladium(n) Cyanides.-The [Pd(CN), 12-ion exhibits no absorption bands in solution below 40 O00 cm-l but above this frequency there is an intense and complex band system in the 41-50000 cm-1 region.4s The polarized crystal spectra of the calcium, barium, and strontium salts have been and all three possess bands in the solid-state spectra below 40000 cm-l.The spectra have similar polarization properties to those observed for the platinocyanides but the band maxima occur at higher frequencies for the same inter-metallic l6A. Macadre and C. Moncuit, Compt. rend., 1965,261, B, 2339. Thomas and Underhill distance (see Table 2). The variation in the position of the 11 polarized band maximum with inter-metallic distance is similar to that found for the platinum complexes. C.Nickel(rr) Cyanides.-Nickel complexes of the type M[Ni(CN),] are coloured owing to the presence of ligand-field absorption bands in the visible region. A series of these complexes has been shown*' to exhibit intense absorption bands at about 20 OOO cm-l in the solid state, but as the position of these bands is not affected by the nature of the cation, the degree of hydration, or the inter-metallic distance, it appears that these bands are not due to inter-metallic interactions. 4 Partially Oxidized Chain Compounds It has been suggested in Section 1that for columnar stacked compounds in which the metal atoms have a dsconfiguration, the dza band of molecular orbitals is fully occupied. The upper part of this band has an antibonding effect on the complex because it is higher in energy than the unperturbed dz2 atomic orbital of the monomer unit which comprises the chain.Kr~gmann~~~~ suggested that the bonding within the metal-atom chain could be strengthened if electrons were removed from the upper part of this band by partial oxidation of the metal ions. This oxidation can be achieved in a columnar stacked PtII compound if the ratio of negatively to positively charged ions is increased either by the addition of anions (e.g. K,[Pt(CN),] --+ K2[Pt(CN)4]C10.3249)or by the removal of cations f e.g. Mg[Pt(C204)2],5.3H20 ). Several of the com- ---t Mg,.8,[Pt(C204)2],5*3H2060 pounds which have been examined in this manner are related to the tetracyano- complexes discussed in Section 3.In the partially oxidized compound K2[Pt(CN)JClo.32,2.6H20 the [Pt(CN)4]2- ions are stacked one above the otheF9 as in Mg[Pt(CN)4],7H20.Sg The C1- ion is in the centre of the unit cell surrounded by a tetrahedral arrangement of K+ ions. However, only 64% of the unit cells contain C1- ions, and this corresponds to 0.32 C1- per Pt atom, and results in an oxidation number of 2.32 for the platinum. The effect of the removal of electrons from the antibonding part of the dz2 band is to decrease the Pt-Pt inter-metallic distance from 3.155 A in Mg[Pt(CN)4],7H203gto 2.88 A in K2[Pt(CN)4]Cl,.32,2-6H20.49It is significant that the platinum atoms are all crystallographically identical, and that the structure does not contain PtII and PtIv complexes.It appears impossible to increase the oxidation number of the platinum above 2.32 and this may be due to the detailed structure of the dza band.Q A range of compounds containing partially oxidized chains due to vacancies in cation sites has been reported (see Table 3).Q The structure of Mg, 82[Pt(C204)2],5.3H20 has been shownso to contain the [Pt(C204)2]2- ions lirM. L. Colin, Bull. Classe Sci. Acad. Roy. Belg., 1963-64,49,973. laK. Krogmann, P. Dodel, and H. D. Hausen, Proc. VIII Internat. Conf. Coord. Chem., ed. V. Gutmann, p. 157. lSK. Krogmann and H. D. Hausen, 2.anorg. Chem., 1968,358,67, 6o K. Krogmann, Z. anorg. chem., 1968,358, 97. Metal-Metal Interactions in Transition-metal Complexes stacked one above the other to give a Pt-Pt chain with an inter-metallic distance of 2.85 A.Only 40% of the Mgz+ sites are occupied and this results in an oxidation number of 2.36 for the platinum atoms. Certain of the non-bond- ing distances between C and 0 atoms in adjacent [Pt(C,0,),J2- groups in the chain are shorter than is normally observed, and this too has been interpretedg as indicating strong Pt-Pt bonds. If oxidation of these chain compounds does result in a partially occupied dzz band then this should produce profound differences in the physical properties of these compounds compared with the unoxidized chain compounds. Partially occupied bands are responsible for metallic conduction in metals and some metal oxides, whereas systems possessing only completely filled or completely einp ty bands exhibit semiconducting properties.Metallic conduction is characterized by a high value for the specific conduction and a slight fall in conductivity with increasing temperature. The value of ohm-l cm-l in the chain direction observed*@for K,[Pt(CN),J Clo.32,2.6H20 is larger than the value reported for Magnus’s Green Salts1 and much larger than that reported for bisdimethylgly- oximatonickel?* but no work on the variation of u with temperature has been reported.* The observed conductivity is low for metallic conduction in general, but it has been suggestedg that lattice imperfections may hinder the movement of electrons in one-dimensional conductors to a greater extent than in two- or three-dimensional conductors.Preliminary datas2 on the strongly dichroic partially oxidized IrI compound, Ir(CO)2.93CIl.o,, indicate that its structure contains chains of metal atoms with an Ir-Ir distance of 2-85 A. The inter-metallic distance is similar to those found in the partially oxidized PtJI compounds and is much less than that present in unoxidized IrI metal chain compounds [e.g. Ir(CO),acac, 3.20 A,63 see later]. In the Ir(CO)2.9sC11.0, monomer units the positive charge arising from partial oxidation of the iridium atoms is compensated by a statistical replacement of some of the CO groups with C1-. The degree of partial oxidation of IrI (+0.07) is much smaller than that found in PtII (+0.3). 5 Anion-Cation Alternating Chains Magnus’s Green Salt (MGS) [Pt(NH3),] [PtCI,] crystallizes as small tetragonal needles containing planar [Pt(NH3)J2+ cations and [PtC1,I2- anions stacked * The electrical conduction properties of these compounds are now receiving a great deal of attention.The value of u in the chain direction in K,Pt(CN),Br,.,o,2~3H,0 has now been variously reported as lo-, ohm-’ cm-l (P. S.Gomm and A. E. Underhill, Chem. Cumm., 1971, 511), 4 ohm-’ cm-l (M. J. Minot and J. H. Perlstein, Phys. Rev. Letters, 1971, 26, 371), and lo2 ohm-l cm-1 (A. S. Berenblyum, L. 1. Buravov, M. D. Khidekel, 1. F. Shchegolev, and E. B. Yakimov, 2h.E.T.F. Pis. Red., 1971, 13, 619; D. Kuse and H. R. Zeller, Phys. Rev. Letters, 1971, 27, 1060). Berenblyum et al. also showed the conductivity to increase with increasing temperature.61 J. P. Collman, Chem. Eng. News, 1967,45, No.52,50. s2 K. Krogmann, W. Binder, and H. D. Hausen, Angew. Chem. Znternat. Edn., 1968, 7, 812. 53N.A. Bailey, E. Coates, G. B. Robertson, F. Bonati, and R. Ugo, Chem. Cumm., 1967, 1041. u Table 4 Magnus's Green Salt and Related Complexes Compound Absorption spectra (crn-' x IW3) COIOW M-M 'A10+lB1g 'AIg+'Eg 'Al#+'A to 'Alg-+SBlo 'A10-QEU 'A1e'A PP 'A,u-+'A tu distance (A) K aPtCI4 30*2(64) 25*5(59) -21.0(15) 17-7(2*6) Pink 28-5(57) 26*0(45) 20.q 17.5) 17*3(5)29.3(70) 20-2(20) -29.0 27.0 20.4 17.5 24*9( 170) 16*5(20) Green 24-9( 305) (23*0)( 190) 16*5( 150) 7.5(1*75)25.2 (23.0) 16.5 6.8 25.4 17-3 7.0 Green 27.0 19.0 Pink 25.1 18.9 Purple27.1 19.3 Pink (34) 25.5 18.3 Pink (32.5) 26.5 19.2 Pink (32.5) 26.3 24.5 18.8 16.5 Red 33.4 15.7 6-5 Green 23.2 16-3 6.8 23-7 16-6 24.4 17.5 22-6( 128) (20*0)(67) 18.0( 19)23*0(80) 17*0(7)(31.5) 21-5 18.0 (29) 20.2 (17.0) Pink (29.5) 20.0 (16.5) Pink Red Rb (25.0) 19.6 Red Rb (26.0) 20.0 b S, solution spectrum; R, reflectance spectrum; xyz, polarized light single crystal spectrum.$ does not possess a tetragonal StructureCJ' (I J. Chatt, G. A. Gamlen, and L. E. Orgel, J. Chem. SOC.,1958,486.b P. Day, A. F. Orchard, A. J. Thomson, and R. J. P. Williams, J. Chem. Phys.,1965,42, 1973; 1965, 43, 3763. C R. G.Dickinson, J. Amer. Chem. SOC.,1922,44, 2404. d M.Atoji, R. W. Richardson, and R. E. Rundle, f.Amer. Chem. Soc., 1957,79, 3017.8 J. R. Miller, J Chem. Soc., 1965, 713. f W. Theilacker, Z. anorg. Chem., 1937,234, 161.0 J. R. Miller, J. Chem. SOC., $ 1961,4452. 3- Metal-Metal Interactioiis in Transition-metnl Complexes alternately above one another in the direction of the c (needle)-a~is.~~In MGS alternate ions are staggered by 28" allowing a close approach of the platinum atoms (Pt-Pt, 3.25 A) along the chain. A range of Pt and Pd complexes exists with structures analogous to MGS7e56v5s and many of these have short inter- metallic distances in the direction of the metal-metal chain7 (see Table 4). A great deal of discussion has centred on the colours of the salts with short inter- metallic distances since the green colour of MGS cannot arise from the super- imposition of the colours of the anion (red) and the cation (colourless).Many early workers13 associated the green colour with metal-metal interactions. A pink modification of MGS also exists in which the closest metal-metal contact distance is about 5 The spectrum of this modification is similar to that of K2PtC1411 which possesses a columnar structure of [PtC1,I2-ions with an inter- metallic distance within the chains of 4.13 A.l0 Therefore there can be little or no metal-met a1 interact The spectrum of MGS contains three main bands (see Table 4) in the region 15-30 000 cm-1 which are related to the three bands observed in this region in the spectrum of K2PtC1,.I1 The cation, [Pt(NA,) J2+,possesses no absorption bands below 40OOO cm-I. In crystals of MGS, however, there is also a band at 6000 cm-l which is not present in compounds containing either the isolated cation or anion.This low-energy band occurs only in the spectra of complexes in which platinum is present in both the anion and the cation, and then only in those complexes which have the shorter metal-metal distances (see Table 4). The presence of this additional band is not, however, the cause of the green coloration, which is due to a red-shift in the positions of the higher energy bands and which will be discussed later. The occurrence of the band at -6OOO cm-' which is polarized in the direc- tion of the c-axis in MGS15 led to the postulation that this absorption band is due to an intermolecular transition from anion to cation.Miller suggested7 that, because the columnar structure in MGS consists of alternate cations and anions, the 5dz2(a,g) orbitals may interact to give two bands of molecular orbitals and, likewise, the empty 6pz(a2u) orbitals may give two further bands. The formation of these bands would be essentially non-bonding. These bands of molecular orbitals would extend throughout the crystals in the direction of the c-axis, with the lower two bands being occupied and the upper two bands unoccupied in the ground state. The transition 'Alg -'A2u (5h-6pz) from the top of the occupied band to the bottom of the unoccupied band (see Figure 1) is then dipole- allowed, and an absorption corresponding to such a transition would be polarized in the z direction, which is the c-axis of the crystal.Such a transition corresponds to partial electron transfer from anion 5dz2 to cation 6pz orbitals and would be expected to give rise to the greater intensity of the band which is observed for b' M.Atoji, J. W. Richardson, and R. E. Rundle, J. Amer. Chem. SOC.,1957, 79, 3017. s6 S. Yamada and R. Tsuchida, Bull. Chem. SOC.Japan, 1958,31,813. Day, A. F. Orchard, A. J. Thomson, and R. J. P. Williams, J. Chem. Phys., 1965, 42, 1973. 116 Thomas and Underhill shorter inter-metallic distances. Recently it has been suggested5vS7 that the 6OOO cm-l band is a N-H overtone of the cation. However, the close relationship between the energy of this transition and the band gap for conduction in the direction of the metal-atom chain,61 and the observation of a photoconduction threshold at 4650 cm-1 in MGS6l indicate an intermolecular origin for the transition.It may be that the absence of this low energy band in the salts con- taining palladium is due to less overlap of the relevant d andp orbitals. Since the spectra of the MGS analogues in the region 15-30000 cm-l are essentially similar to that of the [PtC1,I2- the shifts in the positions of the three main bands represent the perturbation of the anion molecular orbitals by the proximity of the cation neighbours. The assignment of the absorption bands in the [PtCl4I2- ion and the MGS type of comp~unds~~~~~~~ are given in Table 4. There is a markedred-shift in the 'Alg -'Egand 'Alg -lBlgtransitions com- pared with K,PtCl, and these correspond to transitions from the anion d,,, dyE, and dz2 to the anion dxz-yz orbitals respectively.Since the orbitals lie in the plane of the anions they are unlikely to be perturbed by the neighbouring cations, so that the band shifts are probably indicative of the perturbation of the anion dxz,dyr, and d,z orbitals by the cation dz2 orbital. This interaction can be produced either by electrostatic repulsion between pairs of electrons in neighbouring atoms or, possibly, by overlap of the d,2 orbitals. In as much as this shift is due to perturbation by neighbouring cations, the early ideas that the green coloration of MGS is due to metal-metal interactions were thus essentially correct. The absence of a green colour, however, cannot be taken as being indicative of the absence of metal-metal interaction, but any such interaction would be weak in these circumstances. In the bromo- and iodo-analogues of MGS only a small shift in the positions of the 'Alg -'Eg and 'A1g -'Big transitions is necessary to give the complex a green colour; thus many of these complexes appear green5* although the inter-metallic distance indicates that only weak interactions can occur.The polarizations of the U.V. bands in MGS have been determined5e using the specular reflection technique and have been discussed by Day.5 Promotion of electrons from the highest filled band to the lowest unoccupied band should facilitate electrical conduction in the direction of the c-axis of crystals of MGS, but not in the direction perpendicular to this axis.MGS behaves as an anisotropic semiconductor with a ratio of the conductivity along the c-axis to that perpendicular to this axis of about 100 : l.51960*61 Photocon-duction along the c-axis has also been observedS1 with a threshold of about 4500 cm-l, which corresponds to the onset of absorption of the low-energy band and suggests a correlation between the band gap for conduction and this band. Similar conduction studies62 on crystals of [Cu(NH,) J[PtCl,], which also has 67 Y.Kondo and C. K. Jargensen, personal communication. S. Yamada, Bull. Chem. SOC.Japan, 1962,35, 1427. 6B B. G. Anex, M. E. Ross,and M. W. Hedgcock, J. Chem. Phys., 1967,46,1090. P. S. Gomm, T.W. Thomas, and A. E. Underhill, J. Chem. SOC.(A), 1971,2154. C. N. R. Rao and S. N. Bhat, Inorg. Nuclear Chem. Letters, 1969, 5, 531. H. P. Fritzand H. J. Keller, 2.Narurforsch., 1965,20b, 1145. 117 Metal-Metal Interactions in Transition-metal Complexes the MGS structure, have shown them to be anisotropic conductors with the highest conduction again in the direction of the metal-metal chains. 6 Miscellaneous Compounds containing Metal-atom Chains A. Rh(CO),(acac), Ir(CO),(acac), andRelated Complexes.-Recently theprepara-tionssS and crystal of a series of bisdicarbonyl-p-diketonatesof RhI and IrI have been reported and these complexes shown to possess columnar structures with close metal-metal contact distances. Electrical conduction in single crystals of Rh(CO),(acac) and Ir(CO),(acac) has been studieds4 and the conductivity in the direction of the metal-metal chain is 500 times that per- pendicular to the chain, indicating electron delocalization along the chain.The conductivity in crystals of the Irl complex is lo6times greater than that observed in the RhI complex, probably owing to the shorter metal-metal distance (Ir-Ir, 3.20 A; Rh-Rh, 3.26 A) and the greater overlap of the larger orbitals in the IrI atoms. B. Dioxalato-complexes-Dioxalatoplatinates can often exist in two coloured forms both containing square-planar [Pt(C20,)2]2- ions. In the yellow forms there are no Pt-Pt interactions,6s but in the red phases, however, metal-metal interactions do occur and Krogmann has shown that the red modification of Ca[Pt(C,04)J,4H,0 possesses a Pt-Pt chain with an inter-metallic distance of only 3.18 A.66 C.AuI(dmg), Au*WI,.-The structure of this mixed valence compound contains square-planar [Au(dmg),]- and linear [AuCl,]+ ions stacked alternately along the c-axis of the crystal with an inter-metallic distance of 3.26 A.67The dichroisms8 and absorption spectrum of this complex indicate that any interaction must be weak, and the low electrical conduction (a -c lo-', ohm-' cm-l) along the c-axis of the crystalse supports this conclusion. D.Platinum Blue and Related Complexes.-Crystals of Platinum Blue [empirical composition Pt(MeCO*NH),H,O] are reported to exhibit red-blue dichroism similar to that found in bis(dimethylglyoximato)platinum, and it has been sug- gested that the compound contains polymeric Pt-Pt bonds.'O An X-ray diffrac- tion study of the dichroic blue crystals formed by the addition of sulphuric acid '8 F.Bonati and G. Wilkinson, J. Chem. Soc., 1964, 3156; F. Bonati and R. Ugo, Chimica e Industria, 1964,46, 1486. "C.G. Pitt, L. K. Monteith, L. F. Ballard, J. P. Collman, J. C. Morrow, W. R. Roper,and D. Ulku,J. Amer. Chem. Soc., 1966,88,4286. '6 R. Mattes and K. Krogmann, Z. anorg. Chem., 1964,332,247.''K. Krogmann, Z. Naturforsch., 1968, 23b, 1012. a7 R. E. Rundle, J. Amer. Chem. Soc., 1954,76, 3101. R. Tsuchida and S. Yamada, Ann. Report Fac. Sci. Osaka Univ. Japan, 1956,4, 79. 'a P. S. Gomm, T. W. Thomas, and A. E. Underhill, unpublished results.70 R. D. Gillard and G. Wilkinson, J. Chem. Soc., 1964,2835. 1i8 Thomas and Underhill to cis-dichlorodiammineplatinum(1r)indicates the presence of Pt atom chains in this compound with a Pt-Pt distance of only 3-06 7 Conclusions The formation of a columnar structure containing chains of interacting metal atoms is very dependent upon the monomeric units which constitute the columns. As this review shows, complexes containing chains of metal atoms almost invariably contain metal atoms with a d8configuration since this configuration favours the formation of square-planar monomers. Whereas PtIL forms planar complexes with nearly all ligands, Ni" only forms planar complexes with ligands situated at the strong-field end of the spectrochemical series.Under strong-field conditions, electrons in the d,n orbital are stabilized to a greater extent in a square-planar environment than in octahedral or tetragonal environments and this stabilization is further increased by the presence of positively charged ligands along the z-axis, and in these metal chain compounds the metal ions of the adjacent monomers act as 'positively charged Iigands'. The stabilization produced by the ligand field is opposed by the coulombic repulsion between the ligands of one monomer unit with those of the units above and below. In K,PtCl,, where lODq is small and the repulsion between the chloride ligands is large, a long inter-metallic distance (4.13 A) is found.1° In the platinocyanides lODq is much larger and the ligands are smaller, so that shorter inter-metallic distances (3.1-3.6 A) are ob~erved.~~*~~ Oxalate ligands are also small and their charge more dispersed, thus lessening the coulombic repulsion still further and allowing inter-metallic distances of 3.18 In the past, the short inter-metallic distances in Type B complexes and the observed changes in the energy levels of the monomers on formation of the columnar structure, have been interpreted as indicating the presence of metal- metal although more recently they have been discussed5 on the basis of purely electrostatic interactions.As discussed earlier (Section 1) the inter- action may lead to the formation of delocalized band systems in Type B com- plexes containing d8metal ions which are either non-bonding or only very weakly bonding.An examination of several series of d8metal chain compounds indicates that the inter-metallic distances are the same for the first, second, and third row transition metals. Thus in the bisdimethylglyoximes (Ni-Ni, 3.23 ;Pd-Pd, 3.25; Pt-Pt, 3.23 A) the inter-metallic distance appears to be primarily determined by repulsions between ligands, and in the series Ba[M(CN),],4H20 (M = Ni, Pd, or Pt; Ni-Ni, 3.31 ;Pd-Pd, 3.37; Pt-Pt, 3-32 A) by the size of the other ions present in the lattice. This is probably a reflection of the weakness of the bonding between the metal atoms in the chains, since partial oxidation of the cyano- and oxalato-PtII complexes, which is expected to result in stronger metal-metal bonds, produces a greatly reduced Pt-Pt di~tance.'~~~~ Although the inter-metallic distances are not significantly affected by the central transition metal in the vic-dioxime, tetracyano, or MGS series of Ni", PdII, and Pt" complexes (see Tables 1, 2, and 4) the extent of the interactions, as deduced from the spectra of these complexes, does decrease in the order 119 Metal-Metal Interactions in Transition-metal Complexes Pt > Pd > Ni.This decrease is probably due to the spatial distribution of the d-and p-orbitals which decreases in the order 5d > 4d > 3d and 5p > 4p > 3p. Further evidence of this effect is obtained from studies of the semiconduction properties of Rh(CO),(acac) and Ir(CO),(a~ac),~~ and of [Pt(NH,),][PtCI,], (Pd(NH3)4][PtC14], in which a much lower conductivity and [Pt(NH3)4][PdC14],60 and hence decreased interaction is observed in those complexes which contain the second-row transition-metal atom, Rh or Pd.It has been suggested1 that the strength of metal-metal bonding decreases across the transition series and increases from the first to the third transition series. However, for a dn configuration where n < 5 the formation of metal- metal bonds leads preferentially to the formation of dimers and metal cluster compounds. The work on partially oxidized chain systems indicates that metal ions with a d7configuration and low oxidation state should be capable of forming strongly-bonded metal-metal chains, but no examples have yet been observed. The complexes described in this review possess a unique one-dimensional array of metal atoms. If future work can lead to compounds in which the extent of electron delocalization along the metal atom is increased then the anisotropy of these compounds might be expected to find important application in the field of semiconductor technology.

ISSN:0306-0012    

DOI:10.1039/CS9720100099

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

Nitrogen Fixation (Incorporating the Liversidge Lecture, delivered in London, 18th November, 1971)* By J. Chatt, F.R.S. and G. J. Leigh A.R.C. UNIT OF NITROGEN FIXATION, UNIVERSITY OF SUSSEX, BRIGHTON, BNl 9QJ Nitrogen fixation is topical and the subject of many detailed reviews. Here the intention is to give a concise account of the present state of the subject in the light of our experience and that of our colleagues with reference to reviews and very recent papers only. We shall be concerned with developments over the past decade, which has seen a great surge of interest in the fixation of molecular nitrogen (i.e. dinitrogen) under mild conditions such as might simulate those found in biological nitrogen-fixing systems. Active extracts were obtained from bacteria only eleven years ago and the past six years have seen the development of the whole of the co-ordination chemistry of dinitrogen. This chemistry has added remarkably little to our knowledge of how the natural system works, but there is sufficient circumstantial evidence to suggest that it is relevant.There are two main approaches to the study of nitrogen fixation. One is to study the natural nitrogen-fixing systems found in bacteria and some other very simple forms of life. The other is to attempt to make chemical models of the natural system to see whether any fixation of nitrogen can be achieved under mild reaction conditions such as one would expect to find in bacteria. 1 Biological Nitrogen Fixation This, the more direct approach, has yielded very important biochemical informa- tion and much of interest to the chemist.The biochemical literature is vast (references 1-8 are recent and representative?) and here we can only summarize that of interest to chemists. Only very primitive bacteria and some blue-green algaee fix nitr~gen.l-~ * The lecture was also given in St. Andrews, 1 lth November, Aberystwyth, 25th November, 1971, Aberdeen, 8th March, and Glasgow, 9th March, 1972. t Phrases in square brackets in references are descriptions of the contents of review articles. ‘The Chemistry and Biochemistry of Nitrogen Fixation’, ed. J. R. Postgate, Plenum Press, London, 1971. R. W. F. Hardy and R. C. Burns, Ann. Rev. Biochem., 1968, 37, 331. [Enzymology and chemistry of biological nitrogen fixation.] R.W. F. Hardy and E. Knight, in ‘Progress in Phytochemistry’, ed. L. Rheinhold and Y.Liwshitz, Wiley, London, 1968, p. 407. [Biochemistry of nitrogen fixation.] J. R. Postgate, Symposium SOC.Gen. Microbiol,, 1971, 21, 287; ref. 1, p. 161. [Physiological chemistry of nitrogen fixation.] R. W. F. Hardy, R. C. Burns, and G. W. Parshall, Adv. Chem. Ser., 1971, No. 100, 219. [Biochemistry of nitrogen fixation.] 121 Nitrogen Fixatioti The bacteria have yielded most biochemical information. They are of two main types, symbiotic and free-living. The symbiotic, e.g. rhizobium, are the more important.10 They have normal bacterial lives in the soil but they do not fix nitrogen there. However, certain plants, especially the legumes, grown in soil deficient in fixed nitrogen, exude some substance which attracts a specific species of rhizobium to the root hairs. The organisms enter along the axis of the hair infecting certain cells and causing nodules to grow on the roots.The bacteria multiply briefly within a membrane in the infected cells, then change shape, increase in volume up to 40times, and lose the power of reproduction. In this altered state they are called 'bacteroids' and they can fix nitrogen, providing the plant with ammonia in exchange for carbohydrate. In contrast, the free-living bacteria live and fix nitrogen in the soil but are only important ecologically in barren soils.11 They are the more easily cultured in the laboratory and most of our information concerning the chemistry and biochemistry of the natural process of nitrogen fixation has been obtained from them.In 1930, Bortells showed that molybdenum is essential for nitrogen fixation by bacteria and is closely associated with the metabolism of dinitrogen. The bacteria grow well in the absence of added molybdenum if they are provided with a source of ammoniacal nitrogen but are unable to metabolize dinitrogen in the absence of molybdenum in excess of impurity levels. Indeed, in the presence of ammoniacal nitrogen the bacteria do not produce nitrogenase, the nitrogen-fixing enzyme. However, if the source of ammonia is removed, the bacteria can develop nitrogenase and metabolize dinitrogen within 1-10 h.The amount of molybdenum required by the bacteria is often very small, for example the bacteria Beoerinckia indica require only 0*004--0.034 p.p.m. for half optimal growth, and in culture 0.1 p.p.m. is ample. Vanadium, but no other metal, can be used by some cultures of bacteria in place of molybdenum. Iron, like molybdenum, is an essential trace element and magnesium ions are also necessary for nitrogen fixation. The low solubility of dinitrogen in water does not limit the rate of fixation in bacteria, which is independent of pressure down to 0.3 atm. It seems that the enzyme has an enormous affinity for molecular nitrogen. The temperature is also somewhat critical. Bacteria work best between 15 and 25 "C. They are very slow at 0 "C and most cut offsharply at about 35-40 "C.Studies using I6N2show that the first recognisable product from the natural system is ammonia. There is no evidence for the intermediate formation of 'R. W. F. Hardy and R. C. Burns, in 'Inorganic Biochemistry', ed. G. Eichorn, Elsevier, Amsterdam, 1972. [Bioinorgank chemistry of nitrogen fixation.] 'R. H. Burris, Proc. Roy. SOC.,1969, B172, 339; ref. 1, p. 105. [Fixation by free-living mic- robes: enzymology.] a R. M. Cox and P. Fay, Proc. Roy. SOC.,1969, B172, 357. [Fixation by blue-green algae.] a W. D. P. Stewart, 'Nitrogen Fixation in Plants', Athlone Press, London, 1966. lo F. Bergerson,Proc. Roy. SOC.,1969, B172,401; H.J. Evans and S. A. Russell, ref. I, p. 191. [Fixation in leguminous systems.] l1 W.D. P. Stewart, Proc. Roy. SOC.,1969, B172, 367. Chatt and Leigh hydroxylamine, hyponitrous acid, di-imine, hydrazhe, or any of the other substances which, on the basis of thermodynamic studies, might be intermediates. The dinitrogen in its intermediate stages of reduction is held by the enzyme, only ammonia being freed. Indeed, hydrazine is not reduced by the enzyme but poisons it. It is often said that the bacteria are very efficient nitrogen fixers because they operate under mild conditions of temperature and pressure, with carbohydrate or similar reducing agent. However, in normal laboratory culture, they consume enormous amounts of energy in the form of carbohydrate; by this criterion they are highly inefficient. The most efficient fix only about 15 mg of nitrogen per lg of glucose consumed, but laboratory systems devised to simulate natural condi- tions more precisely suggest that bacteria may actually be up to three times more efficient than this.It is claimed tha# the treatment of soil with waste molasses stimulates nitrogen fixation. The enzyme nitrogenase, when extracted and tested in vitro, requires consider- able amounts of adenosine triphosphate (ATP) to function. Approximately 15 moles ATP are hydrolysed to reduce 1 mole of dinitrogen to 2 of ammonia, a free energy dissipation in the region of 105 kcal mole-’ of dinitrogen. The seemingly high energy requirement of the reduction is puzzling. If dinitrogen and dihydrogen were to react to equilibrium at atmospheric pressure and temperature, heat would be evolved and the equilibrium concentration of ammonia would approach 100%.Thus only an efficient catalyst is necessary and no additional energy in any system which can generate dihydrogen. All nitrogen-fixing bacteria contain hydrogenase and can generate dihydrogen (although not all hydrogenase-containing bacteria fix nitrogen). The bacteria do not utilize dihydrogen as such, and indeed, dihydrogen is a competitive inhibitor of nitro- gen fixation. Presumably dihydrogen can compete with dinitrogen for the active site in the enzyme. The nitrogenase system is not as specific as most enzyme systems. It will reduce a number of dinitrogen-like molecules, such as acetylene, which are competitive inhibitors of nitrogen fixation.Carbon monoxide is not reduced, but is a strongly competitive inhibitor. Table 1 lists such ‘dinitrogen analogues’ and their reactions with the bacteria or enzyme extracts. Xn some of these the triple bond is reductively split as in dinitrogen, but acetylene is reduced to ethylene free from ethane or methane, and dideuterioacetylene yields cis-dideuterioethylene. All those substances in Table 1 which have a well-developed co-ordination chemistry also have a high affnity for class b transition-metal ionsla although other such substances, e.g. C2H4 or P(CH20H)s, do not inter- fere with the nitrogenase system, perhaps for steric reasons. Only small rod- like molecules interfere ;methylacetylene is reduced more slowly than acetylene and ethylacetylene slower still.Dimethylacetylene and diphenylacetylene are not reduced at all. S. Ahrland, J. Chatt, and N. R. Davies, Quarr. Rev., 1958, 12, 265. [Class a and class b metal ions.] Nitrogen Fixation Table 1 Substances which interact with the nitrogenase system Substance Interaction or reduction product Relative ratesa NZ 2NH3 1.0 CZH2 C2H4 4-0 N3-NH3, Nz 3.0 N2O Nz,HzO 3.0 MeNC CH4, MeNHz, traces of CzHz, CzHa, CzH6, etC. 0.8 MeCN C2H6 0.004 EtCN C3H8, NH3 0.003 CHZyCHCN C3H6, C3H8, NH3 0-2 CN-CH4, NH3, MeNHz 0.6 HZ Competitive inhibition of nitrogen fixation co Strong inhibition of nitrogen fixation NO Inhibition NZH* In hi bi tion Obtained from the rate of formation of reduction products from an enzyme saturated with substrate5 The reduction of acetylene to ethylene provides a very quick quantitative test for nitrogen fixation because the ratio of the two gases is readily deter- mined in minute concentration by gas-liquid chromatography.It is often useful to measure fixation in soil under natural conditions, or to screen bacteria for nitrogen-fixing activity, but positive acetylene tests on bacteria should always be confirmed by mass spectrometric tests of lsNz metabolism. No anomalous result has yet been reported from the acetylene test. Before 1960 all attempts to obtain active nitrogen-fixing cell-free extracts from bacteria failed, leading to the view that nitrogenase was very unstable. Then Carnahan, Mortenson, Mower, and Castle, working with Clostridium pasteurianum, discovered that it was essential to exclude dioxygen from the bacterial extracts.Their discovery sparked the present phase of interest in nitrogen fixation. They also found that nitrogenase alone did not cause the reduction of dinitrogen. The system also needed an electron carrier, such as ferredoxin, and a reducing agent (sodium pyruvate) which appeared to be somewhat specific because, as later discovered, it activated an ATP-generating system. The whole had to be buffered to about neutral pH. Later Bulen, Burns, and LeComte showed that sodium dithionite is an excellent artificial reducing agent which can replace both the electron carrier and the more natural reducing agent.They produced a very simple method for the assay of nitrogenase. Sodium dithionite and an ATP-generating system (adenosine diphosphate, creatine phosphate, and creatine kinase) were added to the buffered bacterial extract under argon. This caused the evolution of dihydrogen at a rate which was measured. In a parallel experiment under dinitrogen the rate of dihydrogen evolution was lower. The decrease in rate of dihydrogen evolution was exactly Chatt and Leigh equivalent to the rate of nitrogen fixation. An assay by this method can be done in 20 min, and its discovery facilitated the concentration and purification of nitrogenase fractions, although it has now been superceded by the acetylene test. The nitrogenase system produces electrons at a constant rate, regardless of the substrate, so that the total reduced product (e.g.dihydrogen plus ammonia or other reduction products, depending on the substrate) has the same chemical equivalence. Even under carbon monoxide, when the reduction of substrates is inhibited, dihydrogen is evolved as if no substrate were present. The relative rates of reduction in Table 1 indicate the efficiency of reduction. Thus acetylene, needing two electrons for reduction, should be reduced three times as fast as dinitrogen, which requires six. In fact it is reduced four times as fast, and proportionately less dihydrogen is produced. Thus it is reduced more efficiently. Nitrogenase from several sources has now been examined and has been obtained from three genera of bacteria in a state of what is believed to be high purity.It can be separated into two proteins, one of molecular weight 2-3 x lo5 and the other around 7 x 104; these may be broken down into sub-units. The larger protein contains molybdenum and iron, and the smaller only iron, as essential metallic constituents. There is still some doubt about the molecular weights and the exact metal content of the two proteins and of the character of the sub-units. Neither protein shows nitrogenase action on its own but a mix- ture is immediately active. There is obviously no winding together of protein chains or other slow process involved in their concerted action. The large pro- tein from one species of bacteria can be activated by the small protein from another if the bacteria are sufficiently closely related, but the activity falls as the difference between the two types of bacteria increases, This would indicate that the two proteins are essentially the same but have peripheral differences which prevent their fitting together when they come from widely different bac- teria. The nomenclature of the proteins is somewhat confusing as almost every worker in the field appears to have developed his own.The larger protein has been variously called enzyme 1, protein 1, fraction 1, component 1, molybdo-ferredoxin, azofermo, Fe-Mo protein, and Mo-Fe protein; the smaller has been referred to as enzyme 2, protein 2, fraction 2, component 2, azo ferredoxin, azofer, and Fe protein.We shall call them the molybdenum-iron (or Mo-Fe) and the iron (or Fe) proteins. The Mo-Fe protein is brown in aqueous solution; it appears to contain 1 or, less probably, 2 atoms of molybdenum and around 15 atoms of iron per molybdenum atom. One mole of dihydrogen sulphide per iron atom is liberated by acid. A solution of the Fe protein is yellow and contains 2 atoms of iron and 2 ions of ‘labile sulphide’. The proteins are unusual only in containing a somewhat higher than normal proportion of sulphur, leading to the suggestion that perhaps the iron and molybdenum are in a sulphur environment. There is no direct evidence as to how the metals are held, or even whether they are directly involved with dinitrogen during the fixation process. Mossbauer spectra of 67Fe-labelled proteins under various conditions and with various Nitrogen Fixation substrates have not given any definitive information, but they suggest that the iron atoms do not react directly with the dinitrogen and probably occur in pairs.It appears that, when all conditions for nitrogenase activity are satisfied (ATP, reductant etc.), the Fe protein catalyses a reduction of some of the iron in the Mo-Fe protein. E.s.r. spectra have shown signals typical of non-haem iron but no reproducible signals have been obtained from molybdenum. The electronic, o.r.d., etc. spectra have not yet given any useful information. The oxidation state of molybdenum is unknown and iron is probably present as iron(1r) and iron@) in the working enzyme.Both proteins are oxidized by air. If not over-exposed to air, the Mo-Fe protein can be reactivated by sodium dithionite, but the Fe protein is destroyed irreversibly. The aerobes, azotobacter, have a high respiratory rate and it has been suggested that this is necessary to consume dioxygen, so as to protect their nitrogenase. There is a limit to this mechanism of protection and as soon as the dioxygen tension in the environment becomes too high they switch off their nitrogen-fixing activity as though they had pulled down shutters to keep out the dioxygen (and dinitrogen). When the dioxygen tension drops the azotobacters start to fix nitrogen again at the critical threshold tension of dioxygen. The Mo-Fe protein has been obtained crystalline from Azotobacter vinelandii, but from no other source.The crystals are colourless and have curved faces, thus they are not suitable for X-ray structure determination. They mush very readily to a brown, highly active solution of the large protein. The two nitrogenase proteins have sometimes been attributed distinct func- tions, one as a dinitrogen-trapping agent and the other as the reducer, but apart from indications from Mossbauer spectroscopic methods, there is no definite evidence concerning their functions. The absence of reactions with bulky molecules related to those in Table 1 suggests that access to the active site is probably restricted. The site appears to have extreme class 6 metal character. The production of C2hydrocarbons from the reduction of methyl isocyanide has been held to support the hypothesis of a bimetal site, the two adjacent metal atoms being necessary to bring together the two terminal carbon atoms of the isocyanide, but it is equally consistent with a free-radical mechanism and only one site of attack.Indeed, most transition- metal ions in the presence of reducing agent produce some C2hydrocarbons from isocyanides. Studies of the relative rates of reduction of various substrates and of dinitrogen by nitrogenase fractions, and of the effects of inhibitors on the reductions and on hydrogen evolution, have given much valuable information about the biochemistry of nitrogen fixation. This is summarized well by Burris;' such studies give no hint as to the chemical mechanism of the reduction of dinitrogen on nitrogenase. 2 Chemical Nitrogen Fixation Although such evidence as described above points to the activation of dinitrogen by metal ions in the natural system, it is conceivable that highly reactive or strained organic species might use their energy to activate dinitrogen.Thus the Chatt and Leigh phenylsulphenium ion (PhS+) reacts with Nain trace amounts to form a material which may contain [PhSNJ+. Diazomethane is photolysed reversibly, carbene must thefore react with dinitrogen. Finally a salt, formulated NaJC {CHOP- (S)CS](I, is said to react with dinitrogen ‘greedily’ to give a product in which the nitrogen atoms are still linked. We have failed to confirm this last.None of these routes is likely to provide a ready source of fixed nitrogen. Studies of biological fixation suggest that it occurs at a metal site which is sensitive to dioxygen. The site may be in an anaerobic and/or anhydrous region of the enzyme, but there is no evidence concerning this. However, the rate of dihydrogen evolution from the enzyme increases in the absence of a substrate which indicates that protons can reach the reducing site. Most attempts to produce chemical model systems have been based on the assumption that moly- bdenum and iron are key elements. This has led, over the past quarter of a century, to the study of the interaction of dinitrogen with mixtures of transition- metal compounds and the development of three main types of chemical approach.These are now all being studied in various laboratories, and in chronological order of initiation they are as follows: (a) Reduction with aqueous reducing agents, Dinitrogen is bubbled through, or compressed on to, mixtures of reducing agent and transition-metal catalysts dissolved or suspended in water, and after reaction, if any, the solution is tested for ammonia or other reduced products such as hydrazine. (b) Nitriding. The aprotic reduction of dinitrogen is brought about by a transition-metal compound mixed with a very strong reducing agent such as an alkali metal. The product behaves as a nitride. (c) Study of dinitrogen complexes. This is a fundamental study of the inter- action of dinitrogen with transition-metal salts, which appears to be relevant to the functioning of the enzyme.Obviously these three approaches are related. Thus dinitrogen complexes are probably precursors to nitrogen fixation by reduced transition-metal systems, and may be intermediates in the reduction of dinitrogen to ammonia in the biological system. However, the chemical work at present falls neatly into the three sections noted above. Because it provides fundamental background information, the chemistry of dinitrogen complexes will be taken first. A. Dinitrogen Complexes.-Dinitrogen complexes are the only type of com- pound formed by elementary nitrogen under mild reaction conditions, i.e. in water or alcohols at ordinary temperature and pressure. There are two classes: terminal-dinitrogen complexes in which the dinitrogen molecule occupies one co-ordination place in a transition-metal complex, and bridging-dinitrogen complexes where dinitrogen bridges two metal atoms, one or both of which may be transition metals.Their general chemistry and their relation to bio- logical and abiological nitrogen fixation have been reviewed in detail.lS-O1 In J. Chatt, Proc. Roy. SOC.,1969, B172,327; Pure Appl. Chem., 1970,24,425.[Chemistry of nitrogen hation.] J. E. Fergusson and J. L. Love, Rev. Pure Appl. Chem., 1970,20,33.[Dinitrogencomplexes.] Nitrogen Fixation this outline we shall consider mainly those aspects relevant to nitrogen fixation. Terminal-dinitrogen Complexes. The electronic structure of dinitrogen is similar to that of carbon monoxide.The filled orbital of highest energy in both molecules has o-symmetry and low energy, but owing to the greater electronegativity of nitrogen relative to carbon, dinitrogen is a poorer electron donor. Its ionization potential of 15.6 eV is almost equal to that of argon and considerably higher than that of carbon monoxide (14.0 eV). Nevertheless, it forms a range of mononuclear complexes which are in many ways similar to those formed by carbon monoxide, and a similar bonding scheme has been postulated for them (Figure 1). The dinitrogen complexes are, however, far less numerous. Figure1 Bonding scheme for dinitrogen complexes. Electrons are donated from the ditritrogen 3ogorbital into a hybrid acceptor orbital on the metal ion, and a d (or dp hybrid) orbital of the metal donates electrons back into the anti-bonding dinitrogen lx,-orbital The first dinitrogen complex, [RU(NH,),(N,)]~+, was isolated by Allen and Senoff in 1965, during the attempted synthesis of [RU(NH,)~]~+ by the reaction of hydrazine with ruthenium trichloride trihydrate in water.Its solid salts were characterized by a very strong band in their i.r. spectra at about 2100 cm-I, the precise frequency being dependent upon the counter-anion. About 25 mononuclear complexes of dinitrogen have now been characterized. All have the dinitrogen bound in end-on configuration (i.e. they are monohapto-complexes) with the M-N-N system essentially linear in all cases investigated by X-ray analysis (see Table 2).In the complexes, the N-N distance is increased by less than 0.015 A from that in N2itself (1.098 A). The elements italicized in Table 3 are those which are at present known to form crystalline mononuclear dinitrogen complexes. They lie about a line drawn from nickel to tungsten in the Periodic Table, the most stable Complexes being formed by elements towards the tungsten end. All but a very few of the dinitrogen complexes obey the effective atomic number rule. A representative selection is shown in Table 4. It is evident that l6 Yu. G. Borodko and A. E. Shilov, Russ. Chem. Rev., 1969, 38, 355. [Dinitrogen complexes and nitrogen fixation.] R. Murray and D. C. Smith, Co-ord. Chem. Rev., 1968, 3, 429. [Activation of molecular nitrogen.]A.D. Allen and F. Bottomley, Accounts Chem. Res., 1968, 1, 360. [General review of dinitrogen complexes.] lEG.J. Leigh, Preparative Inorg. Reactions, 1971.7, 165. [Synthesis of dinitrogen complexes.] J. Chatt and R. L. Richards, ref. 1, p. 57. [Dinitrogen complexes and nitrogen fixation.] zo G. J. Leigh, ref. 1, p. 19. [Abiological nitrogen fixation.] *l G. Henrici-Olivk and S. Olive, Angew. Chem. Internat. Edn., 1969, 8, 650. [Non-enzymatic nitrogen fixation.] Chatt and Leigh Table 2 X-Ray strirctural data on some dinitrogen complexes A N-N (A) 1.16 M-N (A) 1 a80 M-N-N (") 175 1 * 101(1 2) 1.784(1 3) 178(2) N 1.03 1 * 12(08) 2-1O(04) 2-1qol) --180 1.1 24( 15) 1.928(6) 178*3(5) 1-106(11) 1.894(9) 179-3(9) 1 *055(30) 1 -966(21) 177( 1) Two crystallographically independent molecules in the unit cell; b en = ethylenediamine;distances and angles refer to the complexed dinitrogen dinitrogen differs from carbon monoxide mainly in its tendency to form mono- or occasionally bis-dinitrogen complexes, and only with metals in moderately low oxidation states (o-II), whereas carbon monoxide also tends to form poly- carbonyl complexes with metals in exceptionally low oxidation states (-r1-0).In its behaviour as a ligand, dinitrogen is thus closer to organic isonitriles than to carbon monoxide. The dinitrogen stretching frequencies, v(N2),in the i.r. spectra of dinitrogen complexes parallel those of v(C0) in the analogous carbonyl complexes, con- sistent with similar bonding systems for both carbon monoxide and dinitrogen in their complexes.Theoretical considerations, as well as i.r. and Mossbauer spectroscopy, suggest that dinitrogen is both a weaker a-donor and a weaker r-acceptor than carbon monoxide. In its terminal complexes, the dinitrogen molecule is strongly polarized, as indicated by the strong v(N2)bands in their i.r. spectra. The charge difference between the two nitrogen atoms in [ReCI(N,)-(Ph2PCH2CH2PPh,),] has been estimated from its X-ray-induced electron emission spectrum to be about 0.4 electrons. Dinitrogen might also form side-on, or dihapto-complexes, but none have been characterized. The main evidence concerning their stability comes from a study of the isomerization of [RU(NH,),(~~N~~N)]~+ to [RU(NH~)~(~~N~~N)]~+, which has an activation energy of about 21 kcal mol-l in aqueous solution.The aquation of this ion with loss of dinitrogen has an activation energy of 28 kcal mol-l. The isomerization occurs by tumbling of the dinitrogen molecule, passing through the dihupto-position. The energetics of the process indicate that the acti- vation energy of displacement of dinitrogen from the dihapto-position by water is of the order of 7 kcal mol-l, i.e. the dihapto-dinitrogen is almost free. It thus seems unlikely that dihapro-complexes could be stable relative to monohapto- complexes at any temperature, although they might exist at very low tem-perat ures. Terminal-dinitrogen complexes are usually prepared by the reduction of a transition-metal complex under dinitrogen, or from a two or three nitrogen atom chain in a ligand attached to the meta1.18~*g Dinitrogen is very specific as 129 Nitrogen Fixation to the type of transition-metal site which it will occupy and this appears always to possess strong class b character.The stabilities of dinitrogen complexes are very dependent upon the co-ligands with dinitrogen, and it is critical to have the right co-ligands in the reduced complex. The only guide to their choice is experience. Thus, the ruthenium(I1) and osmium(I1) dinitrogen complexes are particularly stable with five ammonia molecules as co-ligands {i.e. [(Ru,Os)-(NH3)S(N2)]2+}and not with many others, except ethylenediamine. A11 metals along the W-Ni line (Table 3) give isolable dinitrogen complexes with tertiary Table 3 The occurrence of dinitrogen complexes Ti V Cr Mn Fe co Ni Zr Nb Mo Tc Ru Rh Pd Hf Ta W Re 0s Ir Pt organic phosphines, usually also with hydride or halide ions as co-ligands.The lighter transition elements (Fe, Co, and Ni) tend to use hydride and the heavier (Re, Os, and Ir) use halide. Molybdenum(o), tungsten(o), and nickel(o) give dinitrogen complexes with phosphines only as co-ligands (Table 4). Only rarely have tertiary arsines been satisfactory co-ligands, and never yet have organic sulphides. The critical nature of the co-ligands is well illustrated in the series of complexes [Ir(N2)C1(PR3)2] (PR8 = tertiary phosphine). The complex is readily produced by reaction (1) when R = Ph, but analogous products containing similar phosphines such as methyldiphenylphosphine and tri-p-tolylphosphine are too unstable to be isolated in pure condition.Only two reasonably extensive series of stable dinitrogen complexes have been discovered. These are of the types [ReCI(N,)(PR,),] and [OsCI,(N,)(PR,),]. The critical dependence of stability on co-ligands with dinitrogen explains why the discovery of dinitrogen complexes was so long delayed in spite of many attempts to obtain them during the 1940's and ~O'S,and also why there are so few relative to the numbers of the analogous carbonyl complexes. For example, the carbonyl analogues of the above iridium(1) complex can be prepared from almost any tertiary phosphine, and are relatively stable substances.Typical reactions (2)-(7) of formation of dinitrogen complexes are as follows: (a) By direct reduction: THF [MoCI,(THF),] + Ph2PCH,CH2PPh, + Na-Hg + N2+ 15 "C trans-[Mo(N,),(Ph,PCH,CH,PPh,)J + NaCl (2) Et,O [Co(acac),] + AlEt, + PPh, + N2+[CoHCN,)(PPh,),] (3)0 "C Chatt and Leigh Table 4 Some typical terminal-dinitrogen complexes Metal electron Compound trans-[Mo(N2)2-(Ph ,PCH ,CH 2PPh 2) 2] [Mo(.rr-PhMe)(N,)(PPh,),l cis-IW(Na)d?'Me2Ph)J trans-[ReCl(N 2)(PMe2Ph)41 [ReCI(N,)(Ph,PCH,CH,PPh,),] + Colour Orange-yellow Orange Yellow Yellow Green or figuration con-ds ds de ds ds v(N2)a (cm-l) 2020, 1970(Nujol) 2005(toluene) 1931, 1998(benzene) 1925(chloroform) 2060(chloroform) ds 2105,2020, 1935 (chloroform) Yellow Orange Pale da ds d6 2057(Nujol) 2090(Nujol) 21 30(water) yellow White ds 2147(Nujol) Pale yellow White d6 ds 2220, 2190 (not stated) 2082(benzene) White Red Black Red- ds d@ dl0 d8 2057(benzene) 2093(THF) 1875(THF) 2096(Nujol) orange Yellow d8 21 O5(chloroform) Light d8 2152 (not stated) yellow Orange-yellow do 2076(hexane) d1 not i .r.-act ive ds 1910 (solid state, Raman) da 2100 (solid state, Raman) 2040 (not stated) not i .r.-active 2028( benzene) 0 Solvent or dispersion medium in brackets; b v(N3 and v(C0) are extensively coupled; cen = ethylenediamine; d Cy = cyclohexyl; i.r.-active 0,)probably due to dissociation in solution to [Ni(PCy,),(N,) J Nitrogen Fixation [FeHC1(Et2PCH2CH2PEt2),]+ Na [SPh,] + N, --+ [FeH(N,)(Et,PCH,CH,PEt,),] [BPh,] + NaCl (5) (c) By degradation of a nitrogen chain.Allen and Senoff's preparation and the formation of the iridium complex (see above) are of this type. A further example is : No\ MeOH[.(Ph3P)2ClzRe=N-N=C-P~)] + PR, reflux ?rans-[ReCI(N,)(PR3),] + PhC0,Me + PPh, + HCI (6) The azide ion has also been used as a source of co-ordinated N, and provides the best preparation of Allen and Senoff's compound. [Ru(NH~)~(H~O)]~++ N,-+[Ru(NH3),(N2)l2++ 4-N2 + H2O (7) Many terminal-dinitrogen complexes which are formed as impure products by such reactions as the above and recognised by their strong i.r.bands are too unstable to be isolated. Most reagents react with dinitrogen complexes with displacement of dinitrogen and formal oxidation of the metal, but certain ligand molecules such as carbon monoxide displace dinitrogen without the oxidation. The reaction (8) of dihydrogen with [CoH(N,)(PPh,),], and similarly with a number of related complexes, may be relevant to the natural pracess. [CoH(N&.PPh3)3] H2*[CoH3(PPh3)31 N2 (8) This reversible reaction occurs in ethanol and shows that dinitrogen and di- hydrogen can compete for the same metal site. Also carbon monoxide, which is a strong inhibitor of nitrogen fixation in the natural system, displaces dini- trogen irreversibly from this cobalt complex. Thus, if nitrogenase contained a metal site of the type in the cobalt complex, the competitive inhibition of nitrogen fixation by dihydrogen and the strong inhibition by carbon monoxide would be allowed for.When dideuterium is used in place of dihydrogen in reaction (8) it replaces all 18 o-hydrogen atoms from the phosphine ligands as well as the hydrogen on the metal, indicating that reversible formation of 0-C-Co bonds with loss of dihydrogen occurs. The less-stable dinitrogen complexes readily lose dinitrogen to leave highly reactive metal-containing species, e.g. [CoH(PPh,),], often excellent olefin polymerization catalysts. Dinitrogen complexes are especially sensitive to Chatt arid Leigh oxidation with loss of dinitrogen, but a few of the very stable complexes, e.g., trans-[ReCl(N,)(PMe,Ph),], are able to retain their dinitrogen and yield a pure product, e.g., trans-[ReCI(N,)(PMe,Ph),]+, when the metal is oxidized to the next highest oxidation state.The N-N stretching frequency is then raised by about 80-100 cm-l, owing to weaker back-bonding from the more positive metal of the oxidized species. All attempts to reduce well-defined terminal-dinitrogen complexes to ammonia have failed. Very strong reducing agents capable of producing nitride from elementary nitrogen in aprotic solvents are capable of producing nitride from the complexes, but this may occur through preliminary dissociation of the complex. A very few dinitrogen Complexes are sufficiently stable for ligand exchange reactions without loss of dinitrogen.One of the most remarkable, reaction (9), occurs in boiling toluene. trans-[ReCl(N,)(PMe,Ph),] + 2PhzPCH2CH2PPh--+ trans-[ReCl(N,)(Ph,PCH,CH,PPh,),] + 4PMe2Ph (9) Although the less stable dinitrogen complexes react with potential ligand molecules with irreversible displacement of dinitrogen, some react reversibly, e.g. reaction (10). This reaction is important because it shows that if dinitrogen were to be reduced on some metal sites, the ammonia produced could be displaced by dinitrogen so that the cycle of reduction would continue. The terminal nitrogen atom of co-ordinated dinitrogen in some of the more stable complexes is an electron donor, and reacts with various electron acceptor systems to form bridging-dinitrogen complexes (see below).Stable analogues of the dinitrogen complexes are usually obtained from carbon monoxide, organic cyanides, or organic isocyanides. In all such cases the stretching frequency of the triple bond is considerably lower than in the usual run of complexes of these ligands, indicating a very strong back-donation of d-electrons from the metal atom into the 7r-anti-bonding orbitals of the triply bonded system. This appears to be the main characteristic of a dinitrogen- bonding site. It has a high d-electron density on the metal, and this is probably responsible for the weak bonding of normally strong ligands such as water and ammonia, which have no mechanism for accommodating the d-electronic charge. Since the donor orbital on dinitrogen has very low energy, the a-acceptor on the metal will also have to be on a low energy for good mixing.Thus, for the binding of dinitrogen the metal must provide a very low energy a-orbital to match the o-orbital of the dinitrogen, and it must also have filled d-orbitals of sufficiently high energy to combine with the anti-bonding 7r-orbitals of the dinitrogen. These are conditions difficult to satisfy simultaneously. Their ful-fillment depends critically on the metal, the co-ligands with dinitrogen, and their positions relative to dinitrogen. Minor changes in any of these render the site much less capable of holding dinitrogen. Nitrogen Fixation Although the terminal nitrogen atom of some dinitrogen complexes serves as an electron donor to many acceptor systems, it does not appear to do so to the proton. If it did, the reduction of the dinitrogen ligand would be com- paratively easy. However, since the electron density on the dinitrogen ligand is enhanced largely by electron drift from the d-orbitals of the metal, it is evident that the metal must also have a high electron density.Thus protic acids appear to protonate the metal, leading to its oxidation and the release of the dinitrogen. It therefore appears unlikely that dinitrogen in terminal-dinitrogen complexes can be reduced as a consequence of attack by protons or by hydride ions. There is no evidence that dihydrogen is effective either. It may be that the electronic condition of bridging-dinitrogen is more favourable for reduction.Bridging-dinitrogen Complexes. Dinitrogen is also like carbon monoxide in being capable of bridging between metal atoms. Carbon monoxide normally bridges between two class b transition-metal atoms through the carbon atom, as in the usual form of (Co,(CO),]. However, in a very few cases it bridges through carbon and oxygen in structures of the type M-CEO-M1, where M is a class b transition metal and M1a strong class a acceptor, e.g. Me3AI. In contrast, only the M-N=N-M1 type of bridging has been found for dinitrogen and it occurs whether M and M1 are both strong class b transition-metal acceptors, or only M, with M1 a strong class a acceptor, e.g. Me,AI, CrCl,(THF),, or PF,. The first bridged complex was the ion [(NH3)5Ru(N2)Ru(NH3)5]4+,prepared in aqueous solution by a remarkable series of reactions: [RU@TH,),CI]~++ Zn + 2H+ -[Ru(NH3),(H,0)J2++ Zn2++ H, (11) [Ru(NH3)s(H20)la+-t-N2 -+[R~(NH~)S(N~)~~+ (12)+ H2O + +[Ru“H 3) 6(w2 + [Ru(NH3) 5(H20)l2 + -NNH 3) ,RU(N,)RU(NH 3) 6J4 + H20 (13) In this bridged ion the Ru(N,)Ru system is essentially linear, but the N-N separation is insignificantly different from that in [Ru(NH,),(N,)]~+, and only marginally greater than in dinitrogen (see Table 2).In solid [Ru(NH,),(N,)J- [BFJ,, v(N,) (Raman spectrum) is only 31 cm-l higher than in the correspond- ing salt of the dinuclear ion. Thus dinitrogen seems to have been altered very little on forming a bridge. A kinetic study of reaction (13) supports this by show-ing that the dinitrogen complex has about the same affinity for the aqua-complex as has dinitrogen itself. Other similar bridged species where dinitrogen connects closed-shell atoms are WH3) ,Ru(N 2)OS(NH3) 514 +, [(C,H,)(PPh 3) 2Mo(N2)Mo(PPh3) o(GH and [(P~,P),CO(N,)CO(PP~,)~].~~They have had little study.There is also a nickel complex with tricyclohexylphosphine which dissociates in solution : [(CY3P),Ni(N,)Ni(PCY,),I+ “i(N,)(PCY,)*l + “i(PCY3)J 22 M.L. H. Green and W. E. Silverthorn, Chem. Comm., 1971, 557. 2J M. Aresta, C. F. Nobile, M. ROSS~,and A. Sacco, Chem. Comm., 1971, 781. Chatt and Leigh In addition to the above bridged complexes, there is a series of dinitrogen- bridged complexes derived from [ReCl(N,)(PMe,Ph),], which has the lowest v(N,) (1925 cm-l) so far observed in any neutral complex.This may be an indication of a greater back-donation of d-electrons in this complex than in any other, with consequent raising of the electron density on the dinitrogen ligand. It appears to have donor properties very similar to those of acetonitrile, and interacts with a great number of acceptor molecules of all types, sometimes with evolution of dinitrogen (e.g. with BCI,), but often to form strongly coloured products which are essentially adducts. In these the N2molecule is believed to bridge between the rhenium atom of the original complex and the acceptor atom of the added reagent. Owing to the asymmetry of the product, v(N,) is i.r.-active and occurs at lower frequencies than in the parent rhenium complex. Table 5 Table 5 Adducts of trans-[ReCl(N,)(PMe,Ph),] with various Lewis acids Acid derivatives with Ratio configurationdo-d2 Colour v(N,) (cm-l) Re:acceptor atom Blue 1805 1 :1 Red not 1 :2 Brown assignable 1695 1 :I 1795 1 :1 Blue 1680 1 :2 [MoCI,(PMe,Ph) 2] Purple not 1 :2 assignable PFS Pale pink 1640 1 :1 Acid derivatives with configurationsd3-d10 or without available d-orbitals [CrCl3(THF)J Purple 1875 1 :1 CoCl,(THF),.s Blue 1855 a [Pt,Cb(PEt3)21 AlMe, Yellow Pale 1890 1894 a 1 :1 yellow a Dissociates upon work-up,pure adduct not obtained lists some of the acceptor molecules which are known to react, together with v(N,) of the product and the number of acceptor atoms in the rhenium dini-trogen-bridged product.Combination may occur with displacement of a ligand, e.g. with [CICla(THF),](reaction 14),or by direct addition, as with PFs (reaction 15). [ReCI(N,)(PMe,Ph),] + [CrCl,(THF),] -+[(PMe2Ph)4ClRe(Na)CrCI,(THF)2] [ReCl(N,)(PMe,Ph)J + PF6 -[(PMe2Ph),ClRe(N,)PFSJ + THF (14) (1 5) 135 Nitrogen Fixation These adducts and the symmetrical, bridged compounds first discussed appear to be of very similar type. The rhenium-containing adducts fall into two distinct classes, those where v(N2) is lower than in the mononuclear rhenium complex by <100 cm-l (formed by d3-d10 acceptors), and those where the lowering is considerably greater, up to 350 cm-l (formed by do, dl, and d2 acceptors).In some cases, e.g., 1 :2 adducts with titanium or molybdenum derivatives, it is not possible to assign a frequency to v(N2), presumably because it has been lowered into the region of the other ligand frequencies. The original rhenium complex can be recovered from all of these adducts by hydrolysis of the added molecule or otherwise, and so the N2molecule has remained intact. The reason for the formation of two classes of adduct is apparent from the simplified n-molecular-orbital scheme for the Re-N-N-M1 system given in Figure 2. The N2molecule provides the four electrons necessary to fill the le orbitals and the rhenium atom the six electrons necessary to fill the lb2, and the 2e orbitals which are anti-bonding on N2.Thus when the acceptor atom M1is in 3e (non-bonding) 9b2 I I I wI (non-bonding) -. I '"2 I 2eI I I 1 le Figure 2 Suggested molecular-orbital scheme for the x-system of an adduct of trans-[ReCI(N,)-(PMe,Ph),] with a transition-metal derivative Chatt and Leigh a door d1state, it can, by withdrawing electrons from the bonding system of the N, and feeding none back into the .rr-orbital system, cause a great lowering of v(N,). However, when it is a d3A10system, the additional electrons it contains must enter the 3e orbitals which are bonding on dinitrogen so compensating for any electron withdrawal from the N-N a-system or le r-system, and caus- ing very little lowering of v(N,) in the adduct. Thus, when both metals bridged by dinitrogen have closed electronic shells as in the compIex [(NH,),-Ru(N,)Ru(NH3)J4+, v(N,) is almost unchanged from that of the mononuclear analogue [Ru(NH,),(N,)]~ +.One of the most interesting of the [ReCI(N,)(PMe,Ph),] adducts is that with MoOC1,L (L = Et,O, PMe,Ph, etc.) attached to the terminal nitrogen atom (see Table 4). The low v(N,) may be a reflection of a weak N-N bond. It seems likely that, were it possible to attach two moieties of MoOC1,L type, one to each end of the dinitrogen molecule in a reducing medium, they might pull it apart. This is probably the basis of formation of nitride complexes from dinitrogen by transition-metal species which do not have closed electron shells. It suggests that transition metals with open electron shells should form dinitrogen complexes which might be unstable and reactive.Evidence is now appearing that dinitrogen can indeed form complexes with d1electronic systems. Thus there is electrochemical evidence that titanium(@ chloride in dimethyl sulphoxide or propylene carbonate forms complexes with dinitr~gen,,~and more recently a d1 complex, probably [(n-C6H5),PhTi(N2)- TiPh(r-C5H5),], has been It is formed by reaction of [TiPh(.rr-C,H,),] in toluene solution with dinitrogen. Reaction occurs at about -80 "C, and the solid complex is stable below 0 "C. None of the above dinitrogen complexes have been reduced to ammonia or to hydrazine. Indeed, since the 4e orbitals are on a higher energy than the corre- sponding anti-bonding orbitals in free dinitrogen (see Figure 2), the dinitrogen in the complexes is more difficult to reduce to nitride than is elementary nitrogen. Nevertheless, it is now becoming apparent that some very unstable bridging- dinitrogen complexes formed in aprotic media can be reduced to hydrazine and ammonia.These will be considered at the end of the next section. 3 Nitriding Lithium, magnesium, the alkaline-earth metals, and most transition metals of Groups IV-VII form nitrides from dinitrogen exothermically, generally at a dull red heat; indeed, titanium is reported to burn with incandescence when so heated. At room temperature, nitriding occurs rapidly on chemically clean metal surfaces but it does not occur in the bulk metal or is very slow, except on lithium and the alkaline-earth metals.These build up a visible layer, or even nitride completely through the bulk of the metal (e.g. Ca). If fresh lithium metal is continually exposed to dinitrogen, for example, by very slow addition of ethyl bromide to lithium in boiling dry pentane under dinitrogen, yields of UD to 40% T. C. Franklin and R. C. Byrd, Inorg. Chem., 1970,9,986. J. H. Teuben and H. J. de Liefde-Meijer, Rec. Truv. chim., 1971,90,360. I37 Nitrogen Fixation Li,N based on the metal can be obtained. Hydrolysis of the lithium nitride gives ammonia with traces of hydrazine. Workers with Zeigler catalysts (e.g. TiCl, + AIEt, in petrol) for polymeriza- tion of olefins became aware of systems which nitride at room temperature during the 1950’s.Dinitrogen can partially inhibit catalysis of olefin polymeriza- tion by some Ziegler-Natta systems. When the catalysts are formed under dinitrogen at atmospheric pressure and room temperature, the precipitated catalyst contains a small percentage of nitrogen which appears as ammonia when the catalyst is hydrolysed. This was discovered independently and deve- loped mainly by the work of Vol’pin in Russia26 and later, again independently, by Van Tamelen in the United States.a7 They showed that the reaction of a simple transition-metal compound, for example titanium tetrachloride or di-isopropoxydichloride,with a strong reduct- ant such as a reactive organometallic compound or an alkali metal, in ethereal soIution, yields products which react with dinitrogen.Details of the reaction are still uncertain, but generally when an organometallic reductant is used, there is an induction period after mixing the reagents before the absorption of dinitrogen begins, and during this period saturated and unsaturated hydrocarbons are evolved. Presumably organo-transition-metal compounds are formed and these decompose to form hydrides or lower-oxidation-state derivatives of the transi- tion metal. It is one or more of these species which react with dinitrogen to produce complexes, which on solvolysis with protic solvents give ammonia, sometimes with hydrazine. Generally, the nitrogen-containing products behave as solutions or suspensions of highly reactive nitride complexes.A highly active and much studied system which is homogenous is formed by the reaction of bis(.rr-cyclopentadieny1)dichlorotitaniumwith ethylmagnesium bromide. It shows an induction period, and the rate of reaction with dinitrogen is dependent on pressure. Russian workers have tended to use 100-150 atm, although others claim that pN, for half the maximum reaction rate is about 0.5 atm. Dinitrogen uptake is a maximum for rather over 4 mol of organometallic reagent or reductant per titanium atom in the above system and is partly in- hibited by strongly solvating solvents. Generally, carbon monoxide, acetylene, olefins, and dihydrogen inhibit fixation as they do in the natural system, indicat- ing that the fixation probably occurs on a class b metal site.However, some of the systems derived from titanium appear to show enhanced fixation in the presence of dihydrogen, which has been ascribed to the hydrogenation of dini-trogen. Dioxygen in small amounts is said not to affect the uptake of dinitrogen by some systems. The systems also reduce ‘dinitrogen analogues’ similarly to the reduction by nitrogenase.2s Any very strong reducing agent appears to be able to activate the titanium M. E. Vol’pin and V. B. Shut, Organometallic Reactions, 1970, 1, 55. [General review of nitriding reactions. 1 E. E. Van Tamelen, Accounts Chem. Res., 1970,3,361. [Systems for chemical modification of dinitrogen.J E. E. Van Tamelen, H. Rudler, and C. Bjorkiand, J. Amer. Chem. Soc., 1971, 93, 3526; G.J. Leigh, unpublished observations. Chatt and Leigh and other early transition-metal systems; Grignard reagents in ether, triethyl- aluminium, metallic sodium in diglyme, lithium-naphthalene in tetrahydrofuran, or reduction at a nichrome cathode have been used. The fixed nitrogen in the system behaves as nitride, and can be so reactive that even tetrahydrofuran solvolyses it slowly to ammonia. Undoubtedly dinitrogen complexes, possibly bridged-dinitrogen species, M-NEN-M (M = metal), are formed as inter- mediate~.~~-~~In titanium systems, titanium(r1) is believed to be the active species, and e.s.r. studies certainly suggest that the principal species in titanium systems is diamagneti~.~~ However, titanium complexes are so labile that any oxidation state which may be needed for nitrogen fixation could be produced by disproportionation of oxidation states, e.g.2TiII +Tio + TiIv. Despite much work and speculation, the mechanism of the reactions involved in the conversion of dinitrogen into nitride are still uncertain, but it is not now thought that hydridic species are important in the system. Most of the systems are not catalytic in the true sense, because solvolysis with destruction of the active species is needed to liberate ammonia. However, by controlled solvolysis followed by removal of the ammonia, a further cycle of reduction, dinitrogen absorption, and solvolysis can be made. Van Tamelen and co-workers have shown that the titanium retains activity through about nine cycles in the 2-propoxydichlorotitanium-diglyme-sodium system using propan- 2-01 for solvolysis.Vol'pin and co-workers, using a non-protic Lewis acid, aluminium tribromide, were able to demonstrate the truly catalytic effect of titanium by treating dini- trogen with a mixture of titanium tetrachloride, metallic aluminium, and alu- minium tribromide at 50 "C,either in the absence or in the presence of a solvent, e.g. benzene. As much as 200 mol of ammonia per mol of TiCl, was obtained after hydrolysis. It is clear that a system for the catalytic nitriding of aluminium had been evolved, since aluminium is not nitrided in the presence of its bromide alone. Circumstantial evidence suggests that titanium@) halides produced by the reaction with aluminium form an active adduct TiXz,nAlBr3, and that this takes up dinitrogen to form eventually a nitride which, with aluminium tri- bromide, generates titanium tribromide and forms aluminium nitride. The titanium(m) is then reduced again to titanium(@ and recycled.The above nitriding systems are also capable of producing nitrogen-containing organic compounds. Thus the bis(wcyclopentadieny1)dichlorotitanium system with an excess of phenyl-lithium in ether solution under dinitrogen at room temperature gives, after hydrolysis, 0.15 mol of aniline per atom of titanium, as well as ammonia (0.65 mol) and trace amounts of u-aminobiphenyl; both formation of a benzyne intermediate and insertion of N2into a titanium--carbon ns M.0. Broitman, N. T. Denisov, N. I. Shuvalova, and A. E. Shilov, Kinetika i Kataliz, 1971, 12, 504, and references therein. *O E. E. Van Tamelen, D. Seeley, S. Schneller, H. Rudler, and W. Cretney, J. Amer. Chem. SOC.,1970, 92, 5251, and references therein. s1 J. E. Bercaw and H. Brintzinger, J. Amer. Chem. SOC.,1971, 93, 2045; R. H. Marvich and H. Brintzinger, J. Amer. Chem. SOC.,1971,93,2046. sn E. Bayer and V. Schurig, Chem. Ber., 1969, 102, 3378. Nitrogen Fixation bond have been suggested as a mechanism, but dinitrogen has not yet been inserted into any stable transition-metalrarbon bonds. An alternative synthesis of organonitrogen compounds also supports the idea that the reduced titanium systems contain nitride. Thus when bis(7rcyclo- pentadieny1)titanium dichloride is reduced with magnesium metal in tetra- hydrofuran under dinitrogen, and the resultant mixture is allowed to react with ketones, secondary amines are formed upon subsequent hydrolysis :33 N3-f R'RCO ,If,O, RR'CHNH2 Apparently only titanium systems react in this way.Di-n-butyl ketone gives 5-nonyl- and di-(5-nonyl)-amines in about 25 % yield based on the total amount of nitrogen originally fixed. Benzaldehyde yields benzylamine and dibenzylamine. This reaction may be compared with that of lithium nitride with acid anhydrides to give nitriles or with benzoyl chloride to give triben~amide.~~ In these cases, nitride ions are almost certainly involved. Probably some combination of catalytic nitridation with this kind of nitride ion reaction could lead to the large- scale synthesis of organonitrogen compounds direct from molecular nitrogen.By careful control of the working-up, Van Tamelen and his co-workersYo have been able to catch the reduced complexed dinitrogen before the N-N bond has been completely severed. Under these conditions, hydrazine is also a solvo- lysis product. Lithium and barium nitrides are also known to produce some hydrazine upon solvolysis. Recently, from the Shilov have come some related developments concerning some very unstable, presumably bridging- dinitrogen, complexes of both early and late transition metals. The complexes may be related to [(~T-C~H~)~P~T~(N~)T~P~(~T-C~H~)~]discussed in the previous section.Apparently, however, that complex cannot be solvolysed to ammonia or hydrazine.26 The Shilov school has prepared a bright-blue solution by the reaction of [TiC12(7r-C6H6)2] with an excess of isopropylmagnesium chloride in ether at -60°C under dinitrogen. As with the phenyl complex26 this solution does not take up dinitrogen at room temperature but, on cooling, dinitrogen is absorbed and on warming is evolved again. However, if the cold solution con- taining dinitrogen is solvolysed with hydrogen chloride, hydrazine is obtained. Similarly [TiC12Et2] on treatment with ethylmagnesium bromide in ether at -80 "C produces a solution which contains a dinitrogen-trapping species. At -60 "C dinitrogen is released, but treatment with hydrogen chloride at -60"C before the dinitrogen is released produces hydrazine.Interestingly, the later transition metals also appear to undergo this type of reaction. When isopropylmagnesium chloride is added to ferric chloride in the presence of triphenylphosphine in ether, it also produces a solution which does not absorb dinitrogen at room temperature, but at -40°C and atmospheric pressure it does so to produce a red solution. This red solution, on solvolysis at ss E. E. Van Tamelen and H. Rudler, J. Amer. Chem. Soc., 1970, 92, 5253. s4 P.E. Koenig, J. M. Morris, E. J. Blanchard, and P. S. Mason, J. Org. Chem., 1961, 26, 4777. Chatt and Leigh -40 "C with hydrogen chloride, gives up to 10%of its complexed dinitrogen as hydrazine and the remainder as dinitrogen.On the other hand, if the red solution is allowed to warm to room temperature it becomes brown, the complex de- composes and dinitrogen is evolved, +N2per iron atom. Hydrazine is obtained only in the presence of triphenylphosphine. In its absence there is only very slight formation of a dinitrogen complex, but if the reaction is runat 90 "C and at over 20 atm pressure of dinitrogen in absence of triphenylphosphine, nitriding occurs, and yields of ammonia after hydrolysis reach 30% based on FeCl,, assuming one nitrogen atom per iron atom. The addition of triphenylphosphine reduces the yield of ammonia from the above reaction. The reaction of moly- bdenum(v) chloride with isopropylmagnesium chloride in ether at low tem- peratures similarly yields, on hydrolysis, ammonia and hydrazine.The above work has only just appeared and has not had appraisal in other laboratories. It is interesting in showing that easily dissociable, but nonetheless reactive, dinitrogen complexes can be formed at very low temperatures, although they are not formed at room temperature, and that they can be derived from transition metals whether at the beginning or near the end of the transition-metal series. It appears also that the formation of a dinitrogen complex precedes nitridation, but no well-characterized dinitrogen complex has been degraded to nitride, and the exact requirements for such a reaction of complexed dinitrogen are not yet clear.2o 4 Reduction by Aqueous Reducing Agents This has often been attempted, and many strong reducing agents in the presence of derivatives of transition metals such as molybdenum have been reported to produce minute traces of ammonia from dinitrogen. Typical aqueous systems are dihydrogen with platinum black or in the presence of reduced molybdenum compounds, fresh metallic iron rusting in dinitrogen with traces of dioxygen, and acidic solutions of sodium molybdate, sodium borohydride, and thio- organics, with or without ferrous salts.The apparent production of traces of ammonia from dinitrogen must always be checked by reduction of 15N2,and this relatively recent method of testing has been applied to very few systems. Spurious results are likely because the Nessler test for ammonia is not sufficiently specific and, where large volumes of dinitrogen are used, the system may scavenge from the gas phase traces of ammonia, or oxides of nitrogen which are subse- quently reduced.The apparent production of ammonia has even been traced to the rubber tubes used to convey the dinitrogen. Two types of aqueous system are now known to reduce dinitrogen, the one to hydrazine and the other to ammonia. The essential catalyst to produce hydrazine is a reduced molybdenum or vanadium salt in the presence of a substantial proportion of magnesium ions.35 Typically, dinitrogen is reduced by an aqueous or aqueous-alcoholic solution of sodium molybdate or molybdenum(v) tri- 36 A. Shilov, N. Denisov, 0. Efimov, N. Shuvalov, N. Shuvalova, and A. Shilova, Nurure, 1971,231,460.Nitrogen Fixation chloride oxide, mixed with titanium(iI1) chloride, at pH greater than or equal to 10.5. The mole ratio of magnesium to titanium for optimum reduction is 1:2. Hydrazine is produced at room temperature and atmospheric pressure of dinitrogen, but at elevated temperatures and pressures (50-100 "C,50-150 atm) yields of hydrazine as high as 100 mol per molybdenum atom have been obtained, and at the higher temperatures some systems produce ammonia. The reductant titanium(Ir1) can be replaced by vanadium(1x) or chromiwn(n). The system is poisoned by carbon monoxide, and indirect evidence suggests that carbon monoxide forms a 1:1 adduct with the molybdenum, which is in the oxidation state III. The mixture is heterogenous.It has been suggested that the function of the magnesium is to keep the Ti"' ions in the hydroxide gel apart so that their oxidation with dihydrogen evolution is retarded by the formation of Ti-0-Mg-O-Ti species. The following scheme has been proposed: This system parallels the natural system in many respects. However, its efficiency, as measured by the yield of reduced dinitrogen per molybdenum atom, is at best about 1% that of the natural system. Vanadium(n) can take the place of both the molybdenum and the titanium in the above system and at alkaline pH rapidly reduces the dinitrogen to hydrazine. At 10 atm pressure, essentially quantitative reduction based on vanadium according to the following equation is claimed: 4V2++ N2 + 4H2O --+ 4V3++ NaHI + 40H-The rate of reduction is faster than in the molybdenum system.Both systems also reduce acetylene to ethylene (and further to ethane). The molybdenum and vanadium systems apparently contain active metal compounds in the d3configuration. They are thus more deficient in electrons than those systems forming non-reducible stable dinitrogen complexes, in which at least one metal atom has a closed electron shell. They also differ from systems which nitride, in that the latter contain the metal atoms in exceptionally low oxidation states and even as finely divided metal. The second type of system known to reduce dinitrogen is the result of an attempt to produce chemical models on the basis of the scanty knowledge that nitrogenase contains iron, molybdenum, sulphide, and thiol groups.Schrauer Chatt and Leigh and his co-workers3* have reported that organic thiols, sodium molybdate, and ferrous sulphate in the presence of a reducing agent, e.g. Na2S20, or NaBH,, give good reduction of those dinitrogen analogues which are reduced by the natural system. There is a close similarity to the natural system except that reduction is not strongly inhibited by carbon monoxide. By optimizing condi- tions for acetylene reduction, they have produced a system which gives trace amounts of ammonia from dinitrogen at 2000 p.s.i. pressure, e.g., 3-5 pmol of NH, from about 5 mmol Na,MoO, + 2.5 mmol thioglycerol + 0-1 mmol FeSO,,SH,O and 0.25 g NaBH, in 50 ml water.The optimum ratio of moly- bdenum to iron is 2 :1, and no ammonia is obtained in the absence of moly- bdenum. Copper, nickel, or palladium could not be used in place of iron. Working with 15N2, Hill and Richards3' have been able to demonstrate the production of 2-3 pmol of 16NH3 under Schrauzer's conditions using cysteine, molybdate, and ferrous sulphate, but at 1 atm of 15N2. No 15NH3 was obtained when no ferrous sulphate was added. Further, using 2-aminoethanethiol and one hundredth the quantity of catalyst Na,MoO, (60 pmol), FeSO, (1 pmol), NaBH, (0.39 mmol) in water (1.5 ml) they have produced about 90 pmol of 16NH3 from 15N2 at atmospheric pressure. Shilov's, Schrauzer's, and the results from our laboratory are all very new. They demonstrate unequivocally that dinitrogen can be reduced catalytically in an aqueous environment to give substantial yields of hydrazine or ammonia and, with further development, may provide important and useful methods of nitrogen fixation.POSTSCRIPT. One of us has learnt more about the aprotic reduction of dini- trogen in the interesting system developed by the Shilov school, in discussion with Professor A. E. Shilov. The bright-blue solution mentioned above is obtained in a more concentrated state from ~iC1(.rr-C5H5),] than from the dich- loride, because the former is more soluble in ether. A blue solid complex [{Ti(Pri)(n-C,H5),),@I2)]has been isolated from it. Like its phenyl analogue, hydrogen chloride solvolyses it to give dinitrogen and not hydrazine in ether at -100 "C.However, if it is first treated with an ethereal solution of isopropyl- magnesium chloride, and then with hydrogen chloride at -100"C, hydrazine is obtained from it quantitatively (see Chem. Comm,, 1971, 1590). The active complex [Fe2(Pri),(PPh3),(N,)H(Et,0)n] has been isolated as a red solid from the ferric chloride system (see above). This decomposes slowly at 0 "C, and at -40 to -100"Cit is solvolysed by hydrogen chloride in ether to give a 10%yield of hydrazine, the remaining dinitrogen being evolved. Similar solvolysis at 0°C gives only dinitrogen as does solvolysis by cold methanol. Pretreatment with Grignard reagent is not necessary for the production of hydrazine from this complex (see Chem. Comm.,1971, 1185).By treating ferric chloride in very cold ether with phenyl-lithium (10-20 mol) G. N. Schrauzer, G. Schlesinger, and P. H. Doemeny, J. Amer. Chem. Soc., 1971,93,1803,and references therein. 37 R. E. E. Hill and R. L. Richards, Nature, 1971, 233, 114. Nitrogen Fixation under dinitrogen, a red complex is formed in solution, which by cold solvolysis with hydrogen chloride yields about 56 76 of hydrazine based on iron, assuming two iron atoms per molecule of bound dinitrogen. Ammonia and dinitrogen are produced together with the hydrazine. This work will be published in Kinetika i Katalk Fleischer and Krishnaniurthy (personal communication) report that sodium rneso-tetra(p-sulphonatophenyl)porphinatocobalt(III) in aqueous solution yields 06-0.8 mol NH3 by passage of air through its aqueous solution and with one addition of sodium hydroborate every 24 h; we have confirmed this.They claim that most of the ammonia is derived from the dinitrogen of the air, and some from the porphin. Hill and Richards (personal communication) report that their 2-aminoethane- thiol system depends for its action on some unknown factor, and that pure reagents give only marginal fixation, if any. When the fixation of 15N2 occurs on an appreciable scale, one third to one half as much 14NH3 is formed as 15NH3. It appears that some catalytically initiated condensation with elimination of ammonia from 2-aminoethanethiol is necessary to produce the ligand which forms the nitrogen-fixing catalyst complex, but the conditions for its formation are obscure. Sellmann (Angew. Chern. Internat. Edn., 1971, 10,919) has reported a dinitro- gen manganese complex, [(CO), (.rr-C,H,)(N,)Mn].

ISSN:0306-0012    

DOI:10.1039/CS9720100121

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

INDEXES Volume 1, 1972 Index INDEX OF AUTHORS Baker, A. D., 355 Grossert, J. S., 1 Ram, P. J., 259 Breslow, R., 553 Groves, J. K., 73 Rattee, I. D., 145 Brundle, C. R., 355 Gutteridge, N. J. A., 381 Sutherland, R. G., 241 Carabine, M. D., 411 Harmony, M. D., 211 Thomas, T. W., 99 Carless, H. A. J., 465 Leigh, G. J., 121 Thompson, M., 355 Chatt, J., 121 Linford, R. G., 445 Tolman, C. A., 337 Corfield, G. C., 523 Lipscomb, W. N., 319 Underhill, A. E., 99 Coulson, E. H., 495 Mason, R., 431 Waltz, W. L., 241 Coyle, J. D., 465 Menger, F. M., 229 Whitfield, R.C., 27 Fry, A,, 163 Mulheirn, L. J., 259 Griffiths, J., 481 North, A. M., 49 INDEX OF TITLES Acylation, Friedel-Crafts, of alkenes, Electron spectroscopy, 355 73 Elimination reactions, isotope effect Alkenes, the Friedel-Crafts acylation studies of, 163 of, 73 Enzymes, three-dimensional structures Atmosphere, interactions in, of drop- and chemical mechanisms of, 319 lets and gases, 41 1 Azobenzene and its derivatives, Fixation, of nitrogen, 121 photochemistry of, 481 Friedel-Crafts acylation of alkenes, 73 Biomimetic chemistry, 553 Gases, and droplets, interactions in the Biosynthesis of sterols, 259 atmosphere of, 41 1 Carbonyl compounds, photochemistry Homogeneous catalysis, and organo- of, 465 metallic chemistry, the 16 and 18 Catalysis, homogeneous, and organo- electron rule in, 337 metallic chemistry, the 16 and 18 electron rule in, 337 Interactions in the atmosphere of drop-CENTENARY LECTURE.Biomimetic lets and gases, 411 chemistry, 553 Interactions, metal-metal, in transi- CENTENARYLECTURE. Three-dimen- tion-metal complexes containing in- sional structures and chemical me- finite chains of metal atoms, 99 chanisms of enzymes, 319 Isotope effect studies of elimination Chemicals in rodent control, 381 react ions, 163 Chemistry of dyeing, 145 Conformational studies on small mole- Mechanisms, chemical, and three-cules, 293 dimensional structures of enzymes, Cyclopolymerization, 523 319 Metal-metal interactions in transition- Dielectric relaxation in polymer solu- metal complexes containing infinite tions, 49 chains of metal atoms, 99 Droplets and gases, interactions in the atmosphere of, 41 1 Natural products from echinoderms, 1 Dyeing, chemistry of, 145 Nitrogen fixation, 121 Echinoderms, 1 Organometallic chemistry and homo- Education, chemical, 8 reassessment of geneous catalysis, the 16 and 18 research in, 27 electron rule in, 337 582 Index Phase boundaries, reactivity of organicmolecules at, 229 Photochemistry, of azobenzene and its derivatives, 48 1 -, of carbonyl compounds, 465 -, of transition-metal co-ordination compounds -a survey, 241 Polymer solutions, dielectric relaxation in, 49 Quantum mechanical tunnelling in chemistry, 21 1 Reactivity of organic molecules at phase boundaries, 229 Research in chemical education: a reassessment, 27 Rodent control, chemicals in, 381 16 and 18 Electron rule in organo- metallic chemistry and homo-geneous catalysis, 337 Small molecules, studies on, conformational 293 Solids, surface energy of, 445 Some recent developments in chemistry teaching in schools, 495 Spectroscopy, electron, 355 Sterols, biosynthesis of, 259 Surface energy of solids, 445 Teaching, of chemistry in schools, some recent developments in, 495 Three-dimensional structures and chemical mechanisms of enzymes,319 TILDENLECTURE.Valence in transi- tion-metal complexes, 431 Transition-metal compIexes, con-taining infinite chains of metal atoms, metal-metal interactions in, 99 -, valence in, 431 Transition-metal co-ordination com-pounds, photochemistry of, 241 Valence in transition-metal complexes, 431 Specialist Periodical Reports The highly praised Chemical Society series which provides comprehensive and systematic review coverage of the major areas of research.Titles current/ y available include: Photochemistry Elect roc hem ist ry Mass Spectrometry Dielectric and Related Molecular Processes Nuclear Magnetic Resonance Spectroscopic Properties of lnorgan ic and Organome ,a1 Iic Compounds Electronic Structure and Mag net ism of InorganicCompounds Surface and Defect Properties of Solids Rad ioc hem is try Inorganic Reaction Mechanisms Inorganic Chemistry of the Transition Elements Organometallic Chemistry Organophosphorus Chemistry Organic Compounds of Sulphur, Selenium, and Tellurium Amino-acids, Peptides, and Proteins The Alkaloids Terpenoids and Steroids Carbohydrate Chemistry Fluorocarbon and Related Chemistry Foreign Compound Metabolism in Mammals Biosynthesis Further information on any of these publications can be obtained from :The Marketing Officer, The Chemical Society, Burlington House, London WIV OBN. THE CHEMICAL SOCIETY

ISSN:0306-0012    

DOI:10.1039/CS9720100581

出版商:RSC    

年代:1972

数据来源: RSC