CHAPTER 1
Nuclear Magnetic Resonance Spectroscopy
Introduction
Nuclear magnetic resonance spectroscopy, n.m.r., continues to play an increasingly important part in inorganic and organometallic chemistry. The number of papers this year has increased by 40% over the number reported in 1967, even after allowance has been made for a slightly increased journal coverage. Proton n.m.r. provides the bulk of the experimental information but the nuclei 2D, 7Li, 9Be, 11B, 13C, 14N, 17O, 19F, 23Na, 27Al, 31P, and 59Co have also been investigated. A1 in particular has had increased attention, though of course 11B, 19F, and 31P have been studied extensively and have separate sections devoted to them.
The subject matter is arranged very much as last year, the main differences being the omission of the section on less common resonances, since very few were reported, and the creation of a section on the Group IV elements. As before we begin with technique-oriented sections and follow with those dealing with particular elements. There is inevitably some overlap between sections and some arbitrary decisions have been made about where a particular paper should be discussed. In general, papers were placed where the most coherent n.m.r. picture would result and the order within sections and subsections follows the order of vertical groups of the Periodic Table.
Although this chapter has been written so as to emphasise the application of n.m.r. to each particular problem, it should be borne in mind that other techniques frequently play an important part and all of these should be considered in a balanced appraisal of the work.
Several books and reviews have appeared during the year. Volume 3 of 'Advances in Magnetic Resonance' contains articles on 'Correlation Functions of Molecular Motion' by R. G. Gordon; 'Time Correlation Functions in Nuclear Magnetic Relaxation' by J. M. Deutch and I. Oppenheim; 'Dynamic Nuclear Polarisation in Liquids' by K. H. Hausser and D. Stehlik; 'Magnetic Resonance in Hydrogen-bonded Ferroelectrics' by R. Blinc; and 'Thermodynamics of Spin Systems in Solids' by J. Jeener. Volume 2 of the same series (published 1967) contains articles on 'Sensitivity Enhancement in Magnetic Resonance' by R. R. Ernst; 'The Chemical Shift and Other Second-order Magnetic and Electric Properties of Small Molecules' by W. N. Lipscomb; 'Theory of the Chemical Shift' by J. I. Musher; and 'Nuclear Relaxation in Hydrogen Gas and Liquid' by J. M. Deutch and I. Oppenheim.
Volume 1 of a new series 'Annual Review of N.M.R. Spectroscopy' has appeared and contains articles on 'General Review of Proton Magnetic Resonance' by R. A. Y. Jones; 'Nuclear Magnetic Resonance Spectroscopy in Conformational Analysis' by W. A. Thomas; 'The Interpretation of High Resolution N.M.R. Spectra' by E. O. Bishop; 'Heteronuclear Magnetic Double Resonance' by W. McFarlane; 'The N.M.R. Spectra of Polymers' by P. R. Sewell; 'Signal to Noise Enhancement of N.M.R. Spectra' by G. E. Hall; and 'Fluorine-19 N.M.R. Spectroscopy' by E. F. Mooney and P. H. Winson. Volume 4 of 'Progress in N.M.R. Spectroscopy' contains articles on 'N.M.R. of Organic Charge-transfer Complexes' by R. Foster and C. A. Fyfe; 'The Application of N.M.R. to Organometallic Exchange Reactions' by N. S. Ham and T. Mole; 'The Study of Water in Hydrate Crystals by N.M.R.' by L. W. Reeves; 'N.M.R. Studies of Electrolyte Solutions' by C. Deverell; and 'N.M.R. in Magnetic Materials' by M. Bose.
'Intermolecular Forces' contains an article on 'Intermolecular Forces Determined by N.M.R.' by M. Bloom and L. Oppenheim. 'Physical Methods in Advanced Inorganic Chemistry' has a chapter on n.m.r. written by D. R. Eaton. 'Spectroscopy and Structure of Metal Chelate Compounds' has a chapter on n.m.r. spectroscopy written by P. J. McCarthy which includes a discussion of both paramagnetic and diamagnetic chelates, and 'Spectroscopic Tricks' edited by L. May contains notes on n.m.r. problems. A theoretical work 'Quantum Theory of Magnetic Resonance Parameters' has also been published dealing with the calculation of shielding parameters, coupling constants, and hyperfine coupling constants.
Reviews have appeared on 'Organotin Chemistry' including a section on n.m.r. spectroscopy; on 'Pyridine N-OxideComplexes of Platinum(II) including n.m.r. data; on 'The Study of Ion-solvent and Ion-ion Interactions by N.M.R. Techniques;' on 'The equivalence of Nuclei in High Resolution N.M.R.,' on 'Liquid Crystals as Solvents in N.M.R.,' and on general n.m.r. spectroscopy, this latter review containing 2227 references.
Coupling Constants and Chemical Shifts. — Papers are reviewed here in which the magnitudes and signs of coupling constants or the magnitude of chemical shifts have been determined in order to study the factors which affect these quantities in inorganic compounds. A few miscellaneous papers are also included which deal with 14N, 15N, and 59Co resonances, with analysis of butyl-lithium, and with the measurement of magnetic susceptibilities.
Homonuclear double-resonance studies have been used to determine the magnitudes and relative signs of the F — F coupling constants in perfluorophenyl mercury compounds, and also to determine the signs and magnitudes of Hg — F and Hg — H coupling constants and the mercury chemical shifts in aryl or pentafluoroaryl compounds. All the J(Hg — F) and 3J(Hg — Hortho) and 4J(Hg — Hmeta) are positive. In the series Ph2Hg, PhHgOAc, (C6F5)2Hg, and (C6F5)HgOAc it was found that the fluorine chemical shifts were not sensitive to the nature of the other substituent on the mercury while the 199Hg shift between the bi-aryl and the acetate were consistent with the electro-negative group of the latter making a large contribution to the paramagnetic shielding term. It is suggested that a study of solvent effects would be useful. The changes in inter-ring coupling constants, brought about by substituting OAc for C5X5, are small whereas changes in J(Hg — H) and J(Hg — F) are large, J(Hg — H) changing from 103 to 204 c./sec. between the biphenyl and acetate. This indicates that the changes occurring upon substitution must be confined to alterations in electron density and hybridisation at the mercury and the results further suggest that the Fermi contact mechanism dominates the interaction. The constant J(Hg — Me) has been measured in the compounds RCO2HgMe, where R = Me, Ph, p-NO2Ph, CH2Cl, or CF3, and was found to correlate with the pK of the acid RCO2H, varying from 212.4 c./sec. for R = Me, pK = 4.76, to 228.4 c./sec. for R = CF3, pK = -0.26. Thus electronegativity changes in RCO2 bring about changes in the s-character of the mercury–carbon bond and in the optical hyperfine structure constant of the mercury atom.
The values of J(Si — H) in the silanes SiHnPh4-n where n = 1, 2, or 3; SiHClmR3-m where R = Me or Ph and m = 1 or 2; SiH2XPh where X = Cl, Br, or Me; SiHPhMeY where Y = Ph, Me, or Cl; and SiHBr3 have been predicted from the values in other known silanes. A heteronuclear double resonance study has been made of Me3SnSnMe3, in which protons were observed while 119Sn or 13C was irradiated, and has given the value for the reduced coupling constant K(119Sn — 119Sn) as +2668 c./sec. and J(Sn — Sn) = 4460 c./sec. Well-resolved 119Sn INDOR spectra were obtained for the species Me3119SnSnMe3 and Me3119Sn117SnMe3. This is the first instance of the detection of direct coupling between non-first-row elements and its high value supports a Fermi contact mechanism. It is calculated that the Sn — Sn bond has 47% s-character.
Three groups of workers have reported data for deuteriated ammonium ions by use of either double-resonance methods or direct observation of 14N. The effects of isotopic substitution on the chemical shifts of the nitrogen and protons are given and the relative signs of coupling between 1H or 2H and 14N or 15N were determined. In the last paper the isotopic shifts were measured for the isoelectronic series BH4-, CH4, and NH4+. The 11B and 14N resonances shift upfield by 0.17 and 0.3 p.p.m. respectively if a hydrogen is replaced by deuterium while the protons of BH3D- and CH3D are both shifted to low field and those of NH3D to up field by 0.13 p.p.m. This is related to small changes in bond angles brought about by the substitution. Nitrogen has a higher electronegativity than either boron or carbon and this is thought to account for the change in direction of shift along the series.
The proton spectrum of freshly prepared HCN shows coupling to both 15N and 13C and the values J(H — 15N) = 8.7 and J(H — 13C) = 274 c./sec. are given. These values are compared with those in acetylene [1J(H — 13C) = 248.7 and 2J(H — C — 13C) = 49.3 c./sec.] and after allowance is made for different magnetogyric ratios it is concluded that J(H — 15N) is smaller than would be expected and that this is due to some property of the C [equivalent to] N bond.
A full analysis of the [AX]4 spin system (1) which has tetrahedral symmetry has been made. The fluorine spectrum is complex and contains 124 lines whereas the phosphorus spectrum is simple and values of J(P — P), J(P — F), and J(P — F) can be measured directly provided J(F — F) [much less than] J(P — P). The values J(P-F) = -1290, J(P – F') = +29.2, J(P — P) = + 17.7, and J(F — F) = 5.8 c./sec. were obtained. Double-resonance experiments are shown to establish the relative signs of J(A — X) and J(A – A') in AA'XX' spin systems even when J(X — X') is zero. The method is capable of extension to AA'XnXn' systems. The method was applied to the molecule HPO2·O·O2PH and it was found that 1J(P – H) = +667.5, J(P — O — P) = 10.7, 3J(P — H) = 1.5, and J(H — H) = [+ or - ] 0.5 c./sec. The variation of coupling constant 3J(P — H) along the series P(OCH2)3CMe (8 c./sec.), (CO)4FeP(OCH2)3CMe (0 c./sec.), (CO)3Fe[P(OCH2)3CMe]2 (0 c./sec.), and OP(COH2)3CMe (5 c./sec.) suggests that it changes in sign as the valency of the phosphorus changes from three to five. Results for the sulphur analogue of this phosphine are reported later (ref. 829).
It is considered also that the variation in the magnitude of 1J(P – P) in the series of directly bonded phosphorus compounds including F2PPF2, H2PPH2, R2PPR2 and R2P(X)P(Y)R2 is best explained by a change in sign of the coupling constant. A Pople-Santry plot is given in support, which shows J vs. the resonance integral between the two phosphorus s-orbitals (this term depends upon substituent electronegativity and bulk). All the coupling data are reported for the phosphinodifluorophosphine H2PPF2. Deceptively simple spectra are obtained for 1H and 19F but the phosphorus-phosphorus coupling was measured directly from the 31P spectrum. The results were (in c./sec.) 1J(P — H) = 191, 2J(P — H) = 17, 3J(H — F) = 22, 1J(P — F) = 1203, 2J(P — F) = 82, 1J(P — P) = 211. Values (c./sec.) are also reported for F2PPF2, which is an AA'XX'A" A" system (2), 1J (P — F) = [+ or -] 1198.5, 1J(P — P) = 227.4, 2J(P — F) = [+ or -]67.5, and 2J(F — F')gem = 300 [+ or -] 100 and 3J(F – F)vic = 34.4 and 0. A preliminary analysis has been made of P(PF2)3, which was prepared by thermal dissociation of P2F4, and whose 19F spectrum contains basically a triplet of doublets of triplets (3P) and a quartet of septets (1P) with much second-order structure. The approximate values of coupling constants (in c./sec.) are 2J(P — P — P) = 36, 2J(P — P — F) = 61, 1J(P — P) = 323, and 1J(P — F) = 1225. The fluorine spectra of the phosphines Et2(C3F7)P, Ph2 (C3F7)P, Bu(C3F7)P2, (CF3)2(C3F7)P, and (CF3)2(C3F7)PO consist of a series of doublets due to coupling to phosphorus which falls off in magnitude along the C3F7 chain. For fluorine-fluorine coupling 4J >3J both along the chain and across the phosphorus. When the oxide is formed 3J(P — F) increases as is usual for the change PIII -> PV3J(P — F) and 4J(P – F) both decrease.
The sign and magnitude of the coupling between 77Se and 195Pt have been obtained by a double-resonance experiment with cis- and trans-(Me2Se)2PtCl2. The magnitude of the coupling is smaller than expected, the values obtained being J(Se — Pt) = 480 (cis) and 365 c./sec. (trans), K(Se — Pt) = + 939 (cis) and + 745 c./sec. (trans). A large platinum chemical shift was observed between the cis- and trans-isomers. The magnitudes and signs of the indirect coupling constants are listed for the compounds Me2Te, Me2Se, MeSeH, and H2Se.
The protons ortho to Mg or Li in PhLi and PhMgBr are deshielded relative to benzene. This result is explained by the fact that Ph- is isoelectronic with pyridine in which the shifts are paramagnetic and related to the n -> π* transition, and it is pointed out that pyridine, PhLi, and PhMgBr all have similar u.v. spectra. The ortho-shift may be a qualitative measure of the ionicity of the Li — C or Mg — C bond. The chemical shifts of the ethyl protons in the germanium porphyrin (p), (3), pGe(OSiEt3)2, and the silicon and germanium phthalocyanins, (pc) pcSiEt(OSiEt3), pcGeMe(OSiEt3), and pcSi(OSiEt3)2, have been measured and it was found that protons further away from the central atom were less shielded by the ring current in the conjugated system. Thus in phthalocyanin (4) the CH part of the ethyl group attached to the central silicon appeared at τ ca. 16 while the CH2 groups of the OSiEt side-chain appeared at τ ca. 11.4. When the effect of the benzene ring currents was allowed for in the phthalocyanin ring it was found that the aromaticity of its porphyrin part is less than that of the porphyrin (3). A methyl group attached to the cobalt of cobalt phthalo-cyanine has also been observed at τ 16.1 indicating that in this compound also the total ring current is higher than in the prophyrin. The contributions to screening of the protons in (5) and its complexes, due to the several effects of ring currents, magnetic anisotropy of the nitrogen, and electric-field effect of the dipole at the nitrogen, have been calculated. These effects were subtracted from the measured shifts and the difference was found to correlate linearly with the calculated π-electron densities around the rings.
The 59Co shifts are reported for 35 cobalt complexes relative to [Co(CN)6]3-; [Coacac3] appears at -12,500 p.p.m. while [Co(CO3)3]3- resonates at - 13,900 p.p.m. Exceptions were found to the Dq correlation of shift with electron absorption frequencies. Better correlation was obtained with the total electronegativity ΣE of the ligand atoms, though the Dq correlation holds within a group of the same ΣE. The 59Co linewidths are determined by quadrupole relaxation and some point charge calculations are made which show that the electric-field gradient at Co depends on the stereochemistry as in (6), where the full and open circles represent different ligands. The linewidths of the complexes were in accord with these relative figures.
The structure of dinitrogen trioxide N0 has been investigated using 14N n.m.r. Two resonances were found, one at 165 p.p.m. to high field of NO2- in a similar position to the N2O4 resonance, and one 70 p.p.m. to low field of NO2- so that the structure is O2NxNO. The sample was made by liquefying N2O4 and NO gases and the spectrum was observed at low temperature. The nitroso-nitrogen is at low field because this possesses an easily excited electron (therefore low ΔE) which causes the blue colour of the complex.
Some hints have been given on the use of n.m.r. in determining volume diamagnetic susceptibilities and an instance of the use of the method has appeared. An n.m.r. method for the analysis of butyl-lithium is described which is claimed to be more accurate than hydrolysis followed by titration.
2 Stereochemistry of Complexes.
This important use of n.m.r. has expanded markedly during 1968. The number of references has doubled and the proportion of n.m.r. papers on this aspect has increased to some 18% of the references abstracted. The topics are dealt with in a series of sub-sections which have been arranged slightly differently from those in the 1967 review since the spread of work has changed. The subsections are: (a) π complexes, treated here in order of decreasing ring size or decreasing number of olefinic bonds in the ligand; (b) transition-metal hydride complexes; (c) complexes with phosphine ligands where the n.m.r. properties of the ligand are important; (d) complexes with organic ligands dealing first with fluorocarbon ligands, then with uni-, bi-, and ter-dentate ligands, and finally with amino-acid ligands; (e) organometallic compounds with bridging groups (except Group III compounds). Compounds of tin are dealt with in a section devoted to Group IV elements (p. 121).
