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Co X X L L N Me N Me N Me N Me Me N Me Me N Me Me N Me Me N Me Me 160 J. CHEM. RESEARCH (S) 1997 J. Chem. Research (S) 1997 160–161 J. Chem. Research (M) 1997 1075–1085 Proton and Carbon-13 NMR Spectra of Paramagnetic Cobalt(II) Complexes containing Mono- and Di-methylpyridines Maurizio Pulici,* Enrico Caneva and Sergio Crippa Dipartimento di Chimica Organica e Industriale Universit`a degli Studi di Milano via Venezian 21 I-20133 Milano Italy 1H and 13C NMR spectra were obtained for a series of sixteen paramagnetic complexes of general formula [CoX2LO2] where X is Cl or Br and L is a methylpyridine (picoline) or a dimethylpyridine (lutidine); use of inverse NMR and heteronuclear correlation 2D NMR spectroscopy allows full assignment of the carbon resonances for eight of them. Organocobalt derivatives especially CoII compounds have recently witnessed an increasing number of applications in organic synthesis.† Despite this 1H and 13C NMR spectra of these compounds which are very important tools in the hands of synthetic organic chemists have received little attention mainly as a consequence of the paramagnetic nature of CoII which can have either a low- or high-spin electronic configuration.With the intent of providing a framework for the rapid and reliable identification of nitrogen donors of the pyridine type when coordinated to a highspin CoII centre we report here 1H and 13C NMR spectra for a series of complexes of general formula [CoX2L2] where X is Cl or Br and L is a mono- or di-methyl-substituted pyridine (2- 3- and 4-picoline; 2,3- 2,4- 2,5- 3,4- and 3,5-lutidine see Fig.1 and Table 1). These are well characterized compounds whose synthesis magnetic moments and d–d electronic spectra have been reported.2 1H NMR spectra for some members of this series those having methyl substituents in the 3- 4- and 5-positions of the pyridine ring were reported long ago.8 The isotropic shifts of the proton resonances of the coordinated ligands relative to the free ligands were interpreted as arising primarily from a contact mechanism,8 with a dipolar mechanism playing only a negligible role and two different interpretations were given in order to explain the differences among the various ligands. Our investigation (Table 2) shows a trend significantly different from the one originally reported:8 (a) meta protons that are not equivalent show distinct signals; (b) although it is true that the shifts increase as the substituent in position 3 of the ring becomes larger this rule does not hold any more when considering complexes of 2-substituted pyridines.This behaviour is probably not related to any decrease in cobalt–pyridine bond covalence but rather to another factor that appears only near to the paramagnetic metal. It seems reasonable to assume that this factor is of a dipolar nature on enhancing the steric bulk of the ligand (either the anionic or neutral one) a lowering in the symmetry of the complex is expected and hence the appearance of a pseudo contact interaction becomes likely. None of the previous works dealing with this kind of complex included a study of 13C NMR spectra and indeed only a few high-spin cobalt(II) complexes have ever been characterized by means of this spectroscopic technique.In almost all the complexes investigated here the number of observed signals equals the number of chemically non-equivalent carbon atoms although there are a few readily explainable exceptions. The signals are spread out over a large range (Table 4); nevertheless it is possible also in this case to recognize a general trend there are always one or two signals at very low field (more than 600 ppm) one or two signals appearing around 300–400 ppm and all the other signals in the range from 15 to 300 ppm. In order to assign the carbon resonances HETCOR was extensively used. Heterocorrelation techniques however have several drawbacks.13 One major problem is for instance given by the fact that accumulation is performed on carbon a nucleus of low sensitivity.In an attempt to overcome this disadvantage the use of inverse nuclear magnetic resonance was attempted. However the only benefit we obtained was a shortening of the experimental time. Examination of the spectra obtained using the techniques mentioned above provided direct and indirect evidence for the full assignment of the carbon signals only of those complexes that do not contain a 2-methyl-substituted pyridine. 4-Methylpyridines resonate between 15 and 25.5 ppm while 3-methyl signals are generally found between 54 and 73 ppm. C-4 resonances are between 116 and 167 ppm while C-3 and C-5 resonances are the most downfield shifted. As a consequence of the above assignment we can deduce that C-2 and C-6 resonate between 316 and 384 ppm.The strong downfield shifts which we observed for the meta car- *To receive any correspondence. †See T. Kauffmann Angew. Chem. Int. Ed. Engl. 1996 35 386. Fig. 1 Structure of CoX2L2 complexes X and L are described in Table 1 Table 1 Entries for the compounds of general formula CoX2L2 Ligand L Halogen X Entry Cl Br Cl Br Cl Br Cl Br Cl Br Cl Br Cl Br Cl Br 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 J. CHEM. RESEARCH (S) 1997 161 bons which were larger than those experienced by the ortho ones are certainly related to the predominance of spinpolarization effects on the carbon 2s orbitals.16 This interpretation considers only as minor the intervention of a dipolar contribution to the shift (pseudocontact shift). Such an assumption is acceptable when considering 2-unsubstituted pyridine ligands but we cannot maintain it any more when dealing with complexes of 2-picoline or 2,n-lutidines.As we stated above magnetic anisotropy would in fact arise in these cases as a result of the easily predictable structural changes and a dipolar effect would contribute to the isotropic shift. However it is very likely also for the 2-picoline- and 2,n-lutidine- containing complexes that the meta carbon atoms remain the most downfield shifted while the ortho ones together with the ortho methyl occupy the central region of the spectra. In conclusion 1H NMR is a reliable tool for the identification of nitrogen donors of this type when coordinated to high-spin CoII. Resonances are always found in well differentiated regions of the spectra according to the nature of the proton.Dipolar effects arising from the introduction of distortion in the symmetry of the molecule cause only minor changes in the spectral features and assignment of all the resonances is always possible. 13C NMR spectra apart from requiring a longer accumulation time show analogous features only when concerning complexes that do not contain a 2-substituted pyridine. When a 2-picoline or a 2,n-lutidine is present owing to dipolar effects the spectra always display a less clear trend with the atoms close to the paramagnetic centre falling in the same albeit enlarged region. We are deeply indebted to Professor L. M. Vallarino of Virginia Commonwealth University (VA USA) for valuable assistance in obtaining some of the data reported in this work and for critical discussion of the results. Techniques used 1H and 13C NMR inverse NMT HETCOR References 17 Fig.2 1H NMR 13C NMR and HETCOR spectra of CoBr2(3,4- MeLut)2 as an example illustrating the procedure followed for the assignment Table 3 1H NMR isotropic shifts for the complexes of general formula CoX2L2 Received 17th December 1996; Accepted 17th February 1997 Paper E/6/08450B References 1 W. Darby and L. M. Vallarino Inorg. Chim. Acta 1981 48 215. 8 (a) B. B. Wayland and R. S. Drago J. Am. Chem. Soc. 1966 88 4957; (b) G. N. La Mar Inorg. Chem. 1967 6 1939. 13 R. R. Ernst G. Bodenhausen and A. Wokaun Principles of Nuclear Magnetic Resonance in One and Two Dimensions Oxford University Press London 1987. 16 M. Karplus and G. K. Fraenkel J. Chem. Phys. 1961 35 1312. Table 4 13C NMR chemical shifts assignments (dC) assignments in bold are tentativea Entry C(2),C(6) C(3),C(5) C(4) CH3(2) CH3(3) CH3(5) CH3(4) 12345678 b 9 10 11 12 13 14 15 16 419,377 446,389 326,316 354,345 347,347 369,369 509,417 510,426 456,396 453,375 424,424 433,415 358,332 384,359 321,321 351,351 509,466 523,487 593,575 599,579 591,591 588,588 529,509 529,516 549,504 529,494 550,495 546,499 601,598 608,608 598,598 610,610 142 158 120 127 132 137 145 156 158 167 146 151 130 141 116 128 258 313 ———— 254 217 298 332 284 330 ———— —— 64 65 —— 118 122 ———— 54 58 67 73 —————————— 61 63 ——67 73 ———— 24 22 —— 24 26 —— 15 15 —— aThe version of this table in the full text includes entries for the chemical shifts of the free ligands.bSpectrum recorded at 298 K. Table 2 1H NMR chemical shifts (dH) for the complexes of general formula CoX2L2 in parentheses are the data calculated from the previously reported isotropic shifts8b Entry H(2) H(6) H(3) H(5) H(4) CH3(2) CH3(3) CH3(5) CH3(4) 12345678 a 9 10 11 12 13 14 15 16 —— 165.1 (161.4) 168.5 (162.5) 161.2 (157.3) 163.8 (159.6) —————— 164 (161.8) 167.4 (168.3) 167 (165.8) 169.5 (168.1) 159.3 158.5 162.7 (161.4) 165.7 (162.5) 161.2 (157.3) 163.8 (159.6) 158.2 155.1 162.9 161.9 164.5 163.2 164 (161.8) 167.4 (168.3) 167 (165.8) 169.5 (168.1) 52.1 55.7 —— 44.0 (43.05) 45.8 (44.4) —— 52.1 54.1 55.8 57.8 ———— 42.5 44 46.9 (46.2) 48.6 (47.4) 44.0 (43.05) 45.8 (44.4) 45.6 45.7 43.4 44.7 —— 47.0 (46.5) 48.7 (46.1) —— 5.0 4.7 3.0 (3.0) 2.9 (2.9) —— 3.5 3.3 —— 4.4 4.0 —— 2.4 (2.3) 2.3 (2.1) 13.7 16.9 ———— 12.1 12.7 13.8 15.8 15.5 17.8 ———— —— 4.7 (4.6) 4.7 (4.5) —— 7.3 8.2 ———— 5.05 (5.0) 5.08 (4.9) 4.7 (4.5) 4.5 (4.5) —————————— 4.4 4.0 —— 4.7 (4.5) 4.5 (4.5) ———— 4.5 (4.3) 7.3 (6.2) —— 7.2 9.7 —— 5.05 (5.0) 7.3 (7.1) —— aSpectrum recorded at 298 K.

ISSN:0308-2342    

DOI:10.1039/a608450h

出版商:RSC    

年代:1997

数据来源: RSC