1974 125Antiferromagnetism in Transition-metal Complexes. Part V1.l Low-lying Excited States of Dinuclear Copper(l1) Complexes with BridgingMultidentate Schiff's Base Groups and Some Related CompoundsBy (Mrs) J. P. Fishwick and R. W. Jotham, Department of Chemistry, The University, Sheffield S3 7HFSidney F. A. Kettle * and J. A. Marks, School of Chemical Sciences, The University of East Anglia,Variable-temperature magnetic susceptibility data for a number of multidentate Schiff's-base complexes of copper-(11) and related molecules are reinterpreted on a model which allows for the thermal population of excited statesother than those predicted by the vector-coupling model. The mechanism of superexchange through the oxygenbridges is discussed and its magnitude is found to increase sensibly through the series quadridentate Schiff's base< terdentate Schiff's base < bidentate Schiff's base -= aromatic N-oxide. This i s also the reverse order of theoverall, nephelauxetic effect of the ligands upon the metal a'-orbitals, so that the additional direct interaction is ofsimilar magnitude for the terdentate Schiff's base series as for copper carboxylates.Norwich NOR 88CTHE temperature-variation of the magnetic suscepti-bility of copper(I1) acetate and related compoundscontaining proximate copper( 11) ions has been commonlyinterpreted by the Heisenberg, Dirac, and Van Vleckapproach which leads to equation (1) for an exchangeinteraction of the form --JS,*S,.In this Series we have+ 2Ncc (1) 2Ng2p2 1 XU'(dimcr) = - - kT 1 + 3 exp (J/kT)tested the applicability of equation (2) as an alternative,this being distinguished from equation (1) by theinclusion of an additional low-lying singlet state.3, 2Np2ti2 1 XJI' = A ' kT 1 + 3 exp (J/KT) + exp ( - A / k T ) + 2Ncc (2)In the use of equation (2) it is desirable to use a fixedvalue for .g as determined from the e.s.r.study of thecompound concerned or one closely related to it.Similarly, Na should not be parameterized; we havepreferred to use a constant value of 75 x 10* c.g.s.u.Cu-l L0-92 x m3 Cu-l].lWe now complete our survey of dinuclear copper(I1)complexes by consideration of compounds containingbridging multidentate Schiff's bases and related species.4~~The copper-copper distances reported for these com-pounds are, roughly, intermediate between thosefound in the copper carboxylates and the pyridine1V-oxide complexe~.~-~~ Certain square-planar ' mono-1 Part V, R.W. Jotham, s. F. A. Kettle, and J. A. Marks,2 P. 0. Lowdin, Rev. Modern Phys., 1962, 34, 80.3 R. W. Jotham and S. F. A. Kettle, Inorg. Chem., 1970, 9,4 M. Kato, H. B. Jonassen, and J. C. Fanning, Chem. Rev.,E. Sinn, Co-ordination Chem. Rev., 1970, 5, 313.G. A. Barclav and B. F. Hoskins, J . Chem. Soc., 1965, 1979.J . A. Bertrand and J. A. Kelley, Inorg. Chim. Acta, 1970, 4,8 J . A. Bcrtrand, J. A. Iielley, and J. L. Breece, Inovg. Chiun.9 R. D. Willett, C. Dwiggins, jun., R. F. Kruh, and R. E.10 D. J . Hodgson, P. I<. Hale, J. A. Barnes, and W. E. Hatfield,11 /I.F. Cameron, I<. P. Forrest, D. W. Taylor, and R. H.l2 E. Sletten, Chern. Comm., 1967, 1119.13 M. Bonamico, G. Dessi, A. Mugnoli, A. Vaciago, andJ.C.S. Dalton, 1972, 1133.1390.64, 99.,003.Acta, 1970, 4, 247.liundle, J . Chew. Phys., 1963, 38, 2429.Chem. Comvn , 1970, 786.Nuttall, .I. Chetii. SOC. ( A ) , 1971, 2492.L. Zambonelli, Acla Cryst., 1965, 19, 856.meric ' bis(che1ate) complexes of Schiff's bases alsohave weakly linked dimeric structures (4 + 1 co-ordination) .20-z5Et Me(II) (m)As a background for our discussion we show in theFigure some representative dinuclear structures foundin the Schiff's base series.14 P. de Meester and A. C. Skapski, J . Chem. SOC. ( A ) , 1971,2167.l5 T. P. Mitchell, W. H.Bernard, and J. P. Wasson, ActaCryst., 1970, B26, 2096.l6 C. A. Bear, J. M. Waters, and T. N. Waters, Chew. Comm.,1971, 703.l 7 R. J. Majeste and E. A. Meyers, J . Phys. Chem., 1970, 74,3497.l8 A. T. Casey, B. F. Hoskins, and F. D. Whillans, Chem.Comm., 1970, 904.W. A. Franks and R. van der Helm, Acta Cryst., 1971, B27,1299.2o E. Fresson, R. Bardi, and S. Bezzi, Acta Cryst., 1959, 12, 201.21 D. Hall and T. Waters, J . Chem. SOG., 1960, 2644.22 J. A. Bevan, D. P. Graddon, and J. F. McConnell, Nature,23 V. F. Duckworth and N. C. Stephenson, A c f a C ~ y s t . , 1969,24 D. HdI, S. V. Sheat, and T. N. Waters, Chem. Coinm., 1966,Z5 G. J. Palenik, Acta Cryst., 1964, 1'7, 687.1963, 199, 573.25, 1795.436126 J.C.S. DaltonA complication arises because one cannot, in general,be certain that a given species is dinuclear.In par-ticular, chain structures may occur. Such species wouldbe expected to have a susceptibility given, at leastapproximately,26 by equation (3). In practice we(3)found that it was often difficult to distinguish betweenlinear and dinuclear models on the basis of the dataavailable without structural evidence, even when weconsidered a number of compounds which were originallythought to have linear-chain polymeric structures.Conversely, we have found several other species whichthan that adopted previously for the carboxylate andaromatic N-oxide series; l y 3 1 the differences betweenseries seem to be a reflection of the covalency of themetal-metal bonding.For some of the other relatedcompounds we found it difficult to estimate S reliablyand we have tended to adopt avalue of 2.16 in the absenceof e.s.r. data on a closely related compound. It isclear that this value could well be very high for someof the more covalent systems. Such cases will bereflected in a tendency to overestimate I J I and under-estimate A when finding best fits to equations (1) and(2). For all compounds the value of N u was fixedat 0.92 x c.g.s. for one inoleof dimer).m3 Cu-l (150 xSummary of resultsNo. of acceptable cases bType of copper(11) complexClassLinear polymersPre- Dimers ‘1.0 t a1No. of viously % with casescases described Average J infinite Average J con-rejected Total as such value/cm-l Total A value/cm-l sideretlBidentate Schiff’s base complexes 4 4 2 - 38 29 59 - 430 37Terdentate Schiff’s base complexcs 10 5 1 - 137 18 30 -281 33Quadridentate Schiff’s base 4 0 0 6 33 - 202 10Miscellaneous complexes 15 4 3 - 50 2s 9 - 203 41-complexesCarboxylate complexes a 16(25) 125(116) 42(43) -293(-296) 141A2romatic N-oxide complexes a 5P3) 37(32) 81(81) -S50(-893) 45a Parenthesised figures give the published data (where the criteria of acceptibility was 10% < 50 c.g.s.u.Cu-l). Criterion cfacceptibility 1060 < 75 c.8.s.u. Cu-l.may be linear-chain molecular rather than dinuclears ysterns.As in the previous Parts, we have found it necessaryto consider the possible presence of diamagnetic orparamagnetic impurities in the compounds reportedin the literature.It appears that the overall qualityof the data used on the present paper is rather lowerthan that in earlier papers. We therefore divided thecompounds into three sets, namely those for which thedeviation between the experimental and the calculatedsusceptibility data for at least one of the equations(1)-(3) fell in the ranges 0-50, 50-75, and >75 xlo* c.g.s.u. Cu-l. Compounds in the last set wererejected altogether but we found rather small differencesbetween the overall pattern of results in the first twosets. From data in the literature it is apparent that,in the absence of a direct measurement, a convenientvalue of g for substitution in equations (1) and (2)varies considerably with the type of compound con-~idered.~’ The g values of the multidentate Schiff’sbase complexes and related compounds cluster around2-13, and we have adopted this as the general valuefor complexes of bidentate, terdentate, and quadri-dentate Schiff’s ba~es.~*-~O This value is little lower* For details of Supplementary Publications see Notice to26 R.W. Jotham, Chem. Cowztm., 1973, 178.27 G. F. Kokoszka and G. Gordon, Tvansition Metal Chem..Authors No. 7 in J.C.S. Dalton, 1972, Index issue.1969, 5, 181.The results of our analysis of complexes of Scliiff’sbases and a number of related compounds are availabletogether with the relevant bibliography in Supplement-ary Publication No. SUP 20816 (19 pp., 1 microfiche) :Irand are summarised for the various classes in thefollowing sections and in the Table.Bidentate Sch@”s Bascs aizd Related Ligands.-Tlrecompounds in this series are copper halide complexeswith salicylaldimino-, 8-quinolinolato-, or @-amino-ethanolo-ligands. Few were found to have unacceptablesusceptibility data and the average value of the singlet-triplet separation was the highest of any of the seriesconsidered herein.These compounds seem to beintermediate between the carboxylate and aromaticN-oxide series in this respect. In most cases the valueof A was also found to be essentially consistent withthe intermediate Cu-Cu distances expected in this series.The lack of structural data is particularly unfortunatein this context since knowledge of the molecular geo-metry would provide scope for a more detailed under-standing of the antiferromagnetic coupling.Someindication of the planarity of the molecules is providedby the position of the d-d bands which are, as a whole,28 E. Sinn, Iytorg. Chent., 1970, 9, 2376.2s V. V. Zelentsov, Zhztr. strztkt. Klaim., 1966, 7, 543.30 G. 0. Carlisle, I<. I(. Ganguli, and L. J. Theriot, IPLOY~.31 R. W. Jotham, S. F. A. Ketttle, and J. A. Marks, J.C.S.Nuclear C h e w Lettevs, 1971, 7, 527.Dalton, 1972, 4281974 127at a much lower energy than those of correspondingtetrahedral bis (chelate) copper(I1) c ~ m p l e x e s . ~ ~ ~ ~ ~There is little evidence for linear polymeric structuresamong the salicylaldiminocopper halides, although thecdst- of this structure for 8-quinolinolatocopper fluorideseems to be strong.34 The complexes of the p-amino-ethanolo-1ig:tnds divide sharply into two groups.Intliree caws the data are of excellent quality and suggesta dinuclear- complex, electronically akin to the aromaticX-oxicle series, but in the other five compounds con-sidered tllere seemed to be moderate to poor evidencefor a more weakly interacting linear chain system.35Terdentate Sclzif ’s Bases and Related Ligands.-Mostcompounds in this series are complexes of salicylaldimino-ligand:; in which the nitrogen atom is substituted withan additional co-ordinating group. Almost one-thirdof the compounds were eventually rejected and severaloi the otliers are assigned to the class of linear polymers.The J and A values of the remaining, dinuclear, casesare similar to those found for the compounds withbridging carboxylate groups.A detailed study revealsa remarkably simple pattern, provided we distinguishnephelauxetic effects from specific effects on the super-exchange interaction within the bridge. A comparisonof aromatic X-oxide complexes with those of bidentateand terdeiitate Schiff’s bases shows that, essentially,the terminal halogen atoms are successively replacedby ligand groups connected to the bridging oxygen atomby a substantially conjugated framework of carbonand nitrogen atoms. In each case we may envisageelectron tranyfer from oxygen to copper of two types(which co-operate in superexchange despite promotinginetal-metal. overlap of different signs).l If G(n) andx(n) refer to non-bonding electrons labelled with respectto the bond joining the bridging oxygen atom to theligand framework, and the dx2-y2 orbital is referred tothe ZocaZ, near-tetragonal symmetry axes, then these twomechanisms are a(n) dG2-y2 and r(n) _+ dxz-y2.The extent of transfer from the c(n) orbital will reflect theinductive effect of the ligand framework, leading totrends in J values which have already been re-c ~ g n i s e d .~ ~ ~ 36-38 In the aromatic N-oxide series, thex(n) electroils appear to be conjugated to the electronsof the aromatic-ring * but in these Schiff’s base ligandssuch conjugation seems much reduced, as would beexpected on geometric grounds (Figure). If we acceptthis, the ~ ( n ) d,~-~z transfer will not be very sensi-tive to minor changes in ligand and will assume acharacteristic magnitude for each type of complex.For most of the Schiff’s base complexes studied the* It should bc noted that Watson and his co-workers currentlyprefer a x-bond superexchange mechanism in these systems, andnot the rn mechanisms (see Inorg.Chem., 1969, 1879 and referencesthercin).32 L. Sacconi, RI. Cismpolini, and U. Campigli, Inorg. Chenz.,1965, 4, 407.33 C. M. Harris, J. M. James, P. J. Milham, and E. Sinn,Imvg. C h i m Acta, 1969, 3, 81.34 M. J. Frazer, G. V. Robins, and F. B. Taylor, J . Chem. Scc.( A ) , 1969, 2451.35 E. Uhlig and I<. Staiger, 2. anorg. Chem., 1968, 360, 39.groups which are substituted into the aromatic frame-work attached to the bridging oxygen atom are, first, a-CH=NR + Cu grouping and then R itself is replacedby an electron-withdrawing Cu-O-C,H,- group.Thesesuccessive substitutions with groups that withdrawelectrons both inductively and mesomerically presum-ably lead to a marked reduction in the extent of Cu-Opbonding as we move between these three differentseries (unfortunately, the available crystallographicdata are not sufficient to test this hypothesis). Ittherefore seems reasonable that electron-density whichpromotes superexchange in the aromatic N-oxide seriesis successively dispersed over the conjugated frameworkof the ligand in the Schiff’s base complexes. Thisprovides an explanation for the observation that theaverage value of I J I decreases sharply between theseseries of compounds.Conversely, the pattern of values obtained for Asuggest that any direct interaction between othersuitable orbitals, such as the local dZy orbitals, is negli-gible in the aromatic N-oxide series but increases wheneach terminal halogen atom is replaced by a Schiff’sbase ligand with an extra ‘ tooth ’.The overall nephelauxetic effect increases sharplyas halogen atoms are replaced by oxygen atoms in aconjugated ligand framework, an effect manifest in thediffering g-values obtained from e.s.r.measurementsas well as in the magnetic susceptibility data.Qziadridentate Schif ’ s Bases as Ligands.-The lowvalues of I J I for these so-called cis-dimers has alreadybeen the subject of c ~ m m e n t .~ ~ , ~ ~ It seems clear thatthe much weaker interaction in these complexes reflectsthe marked electronic asymmetry of the bridge systemand also delocalisation of electron density over thesingle quadridentate ligand. On the basis of thelimited number of acceptable cases, the A values alsoappear to be small, an observation which may becorrelated with the nephelauxicity of the ligand itself.This is so high that the Cu(ligand) complexes mayfunction as ligands towards numerous other systemsapart from cupric halide~.~O$~l Within this series, thetrend in IJ1 values also appears to reflect the stericrequirements of the unit which links the two salicyl-aldimino-sect ions together.Concluding Remarks.-This paper and the associatedsupplementary publication completes the survey of thesusceptibility data available for dinuclear copper (11)compounds. The results obtained indicate that thereis a strong case for the replacement of equation (1) by36 C . M. Harris and E. Sinn, J . Inovg. Nuclear Chew., 1968, 30,37 G. F. Kokoszka and R. W. Duerst, Co-ordination Chem. Rev.,38 W. E. Hatfield and F. L. Bunger, Inorg. Clzetvz., 1966, 5,39 S. J. Gruber, C. M. Harris, and E. Sinn, Inorg. Chem., 1968.40 IVI. Kato, U. Muto, H. B. Jonassen, K. Imai, and T. Tokii,41 S. J. Gruber, C. M. Harris, and E. Sinn, J . Inovg. Nuclear2723.1970, 5, 209.1161.7, 268.Bull. Chem. SOL Japan, 1970, 43, 1066.Chem., 1968, 30, 1805128 J.C.S. Daltonan equation derived from a more detailed model when- greater potential insight into the structure of a newever relatively short copper-copper separations or compound. However, we believe that it is essentialdinuclear complexes with highly nephelauxetic ligands to use sensible values for g and Na and to limit discussionare studied. In such cases, equation (2) represents a to those cases where the standard deviation betweenconsiderable improvement on equation (1) and has the calculated and experimental susceptibilities is notadvantage of reducing to equation (1) whenever these excessive.complicating factors are absent. It therefore offers [2/2208 Received, 21st Septembev, 1972