Nuclear Magnetic Resonance Spectroscopy

BY B. E. MANN

1 Introduction

The growth of the use of n.m.r. spectroscopy by inorganic and organometallic chemists and the necessity to restrict the size of this chapter have forced a further reduction in the literature coverage afforded here. Only reviews and books directly relevant to this chapter are included, and the reader who requires a complete list of books and reviews is referred to the Specialist Periodical Reports on n.m.r. spectroscopy where a complete list of reviews and books is given. Reviews which are of direct relevance to a section of this Report are included in the beginning of that section rather than here. Papers where only 1H n.m.r. spectroscopy is used are only included when the 1H n.m.r. spectra make a non-routine contribution but complete coverage of relevant papers is still attempted where nuclei other than the proton are involved. Previously, a partial attempt has been made at cross-referencing between some of the sections, but this is now discontinued.

No books, and only a limited number of reviews, of direct relevance to this chapter have appeared, i.e. 'Fourier Transform Nuclear Magnetic Resonance Spectroscopy through the Periodic Table', 'Semi-empirical Calculations of the Chemical Shifts of Nuclei other than Protons', 'Nuclear Magnetic Resonance of Central Metal Ions in Octahedral Complexes', 'The Structure of Complexes of Mono- and Poly-nucleotides with Metal Ions of the First Transition Group. Part II. Nuclear Magnetic Resonance Studies', 'Structural Features of Hexafluorocomplexes of Noble Metals', 'Application of Halide Ion Magnetic Resonance to Bioinorganic Problems', and 'Conventions and Chemical Shifts in N.M.R.'. This last paper contains a justified attack on the inorganic chemists who quote chemical shifts without stating the sign convention or reference used. This behaviour is common for 11B, 19F, and 31P chemical shifts which are quoted using the 8 scale which implies, according to I.U.P.A.C., that high frequency (low field) is positive. Unfortunately in many cases it is clear that the authors intend the reverse convention and in many more cases it is not clear which sign convention is being used. It is therefore important, especially in papers quoting 11B, 19F, or 31P chemical shifts, for the sign convention to be clearly stated. It is to be hoped that editors and referees will draw the attention of authors to this problem. As far as possible, the sign convention that high frequency is positive has been used throughout this chapter.

A number of papers have been published which are too broadly based to fit into a later section and are included here. 1H N.m.r. spectra of edta and [R(O2CCH2)-NCH2CH2N(CH2CO2)2]3- (R = Me or CH2CH2OH) complexes of diamagnetic ions, including the alkali, alkaline-earth, and rare-earth metals, have been described. Splittings of the methylenic protons of the acetate groups, indicative of long-lived metal-nitrogen bonds, were found for each ligand in the complexes of the cations of higher charge density. The ligand proton chemical shifts were shown to correlate with the effective charge density of the metal ion. Computer analysis of the n.m.r. spectra of complexed pyrrole-A-carbodithiolate to CuII, CdII, PtII, PdII, CoII, and FeIII and of the free ligand indicates little positive charge build-up on the heterocyclic nitrogen atom. Finite-perturbation-theory INDO calculations have been reported for 1JPH, 1JSiH, 2JPCH, and 1JSiCH. These calculations are generally satisfactory. The calculated value of 1JPH in PH3 is too small and the effect of strongly electronegative substituents on the couplings was not completely accounted for. The role of diamagnetic and paramagnetic screening in determining chemical shifts in n.m.r. has been examined for AXn (A is the nucleus) and for AFn-1X (19F). K(A,19F) in [AFn]m- is negative for all compounds where A is a non-transition element (with the exception of compounds with markedly ionic A — F bonding) and most likely positive for compounds where A is a transition element. A correlation was established between the A ns orbital energy and the change in the completely reduced constants C(A,F) for the isoelectronic and isovalent [AFn]m- Pertubation theory was used to find the change in K(A,F) when going from [AF6]m- to [AF5L]m-. A relation was found between the nature of A and the signs and relative values of K(A,19Ftrans) and K(A,19Fcis). The theoretical predictions are in agreement with the experimental results. A comparative analysis was made of the relation between the change in K(A,19F) and the A — F bond strength in the compounds [AF5L]m- and [AF6]m-. 13C N.m.r. spectra can differentiate between S- and N-co-ordinated thiocyanate, and 31P n.m.r. spectroscopy can distinguish between ionic, uni-, and bi-dentate binding of [(RO)2PS2]- to a wide range of elements.

2 Stereochemistry

This section is subdivided into ten parts which contain n.m.r. information about lithium, sodium, potassium, beryllium, magnesium, and transition-metal complexes, presented by Groups, according to the Periodic Table. Within each Group, classification is by ligand type.

Complexes of Group IA and IIA Elements. — 'N.M.R. in Alkali Molecules by Optical Pumping' and 'N.M.R. Spectral Change as a Probe of Chlorophyll Chemistry' have been reviewed.

A survey of the structure and bonding of alkyl-lithium compounds in hydro- carbon solvents has been made. These compounds exist in hydrocarbon solution as either hexamers or tetramers. Increasing chain length results in a lower degree of association. Li+ [CαMePhCH2CMe,]- and K+2[C2MePHCH2CH2C2MePh]2- have 1J(13Cα, 13C) consistent with an sp2-hybridized Cα with relatively little effect on the charge on the coupling constants. Li+[CH2CMe==CHBut]- gave much smaller coupling constants. The 13C n.m.r. spectra of indenyl- and cyclopentadienyl-lithium have been measured. The solvent dependence of the chemical shift was used to probe the π-electron density and hence ion pairing. There was, however, no solvent dependence of the average chemical shifts. 1H and 13C n.m.r. spectra were used to differentiate between (1; M = HgBr) and (2; M = Li, K, ZnBr, MgBr, SiMe3, Mg, or Hg). The nuclear magnetic resonance of Na2 and Cs2 has been measured by the atom-molecular exchange optical pumping method:

σ(Na) - σ(Na2) = (29 [+ or -] 16) x 10-6

σ(Cs) - σ(Cs2) = (221 [+ or -] 12) x 10-6

σ(Cs) - σ(Cs+) = (14 [+ or -] 12) x 10-6

1H N.m.r. spectroscopy has been used to investigate the solution conformation of ionophore A204A-Na+, antibiotic 3823A-Na+, and nonactin-K+.

The gas-phase 1H and 11B n.m.r. spectra of Be(BH4)2 show that it is fluxional. The inability to freeze out the fluxionality of Cp2Be in the n.m.r. spectrum has been discussed. The 9Be n.m.r. spectra of CpBeX (X = Me, Br, Cl, or Cp) have been measured and the half-height linewidths vary from 3 to 10 Hz. The 1H and 13C n.m.r. spectra of Mg(CH2But)2, [Mg(CH2SiMe3)2]3 and [Mg(CH2CMe2Ph2)]3 show bridge-terminal exchange. The 1H and 13C n.m.r. spectra of RCH=C(OM)But (M = MgBr or Na) and (MeCH=CButO)2Mg have been reported. The 13C n.m.r. spectrum of (MeCH=CButO)2Mg suggests that it exists as (3). N.m.r. data have also been reported for L2Mg, LGaMe2, and LTlMe2 [13C, 31P; L = (4)], chlorophyll a (13C), and diaqua{2,13-dimethyl-6,9-dioxa-3,12, 18-triazabicyclo[1 2,3,1]octadeca-1(18),2,12,14,16-pentaene}magnesium(II) dichloride hydrate (13C).

Complexes of Sc, Yb, Ti, and Zr. — From the 1H n.m.r. spectra of Sc(BH4)3,2THF, the BH4 protons appear to be equivalent, even at -80 °C. and the absence of coupling to 11B and 45Sc was attributed to quadrupole relaxation. The 19F n.m.r. spectrum of (C6F5)2Yb(THF)4 has been measured.

The 19F n.m.r. spectra of aqueous solutions containing [48TiF6]2-/[49TiF6]2- and [74GeF6]2-/[73GeF6]2- and 45Sc n.m.r. spectra of an aqueous solution containing the [45ScF6]3- ion have been reported for a range of temperatures. Computer fitting of the observed lineshapes shows that both quadrupole-induced transitions between the spin states of a high-spin nucleus and chemical exchange of the fluorine atoms contribute to the lineshapes. The quadrupole coupling constants were estimated to be 4.5 MHz for [49TiF6]2-, 5.8 MHz for [73GeF6]2-, and 9.5 MHz for [45ScF6]3-. N.m.r. data have also been reported for Ti(BH4)3OR (11B), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (19F; M - TiCp2, ZrCp2, GeMe2, SnMe2, PbPh2, or PEt), CpTiMe(OPri)2, (13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C; M = Fe or Ru), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C, 31P; M = Ti, Nb, Al, or Sn), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

Complexes of V, Nb, and Ta. — In the 13C n.m.r. spectrum of [Ta(p-tolyl)6]- δ(13C — Ta) = 217.1 although the remaining signals are normal. On the basis of a comparative study of 51V chemical shifts and 1J(51V, 31P) in the complexes [CpV(CO)3L], cis-[CpV(CO)2L2], trans-[CpV(CO)2L], [V(CO)5L]-, and cis-[V(CO)4L2]-, the vanadium-phosphorus interaction via π-acceptor and σ-donor contributions has been investigated. Within a series of similar complexes, δ(51V) correlate with the overall ligand strength of the phosphine. The value of 1J(51V, 31P) was shown to be influenced mainly by the inductive effect of the substituent on phosphorus. A comparison of 51V n.m.r. data from [CpV(CO)3-PR3] with e.s.r. data from [Fe(NO)2BrPR3] reveals simple correlations between the respective basic resonance parameters depending on the phosphine ligand properties. In particular, 1J(51V and Aiso(31P) coupling constants exhibit a clear relationship. As was demonstrated on the basis of qualitative LCAO molecular orbital considerations, these coupling constants are related to each other by the phosphorus 5-orbital contribution to the molecular orbital relevant for the coupling. The simplified model leads to a relation of the form 1J(51V, 31P) varies Aiso(A - Aiso) which is in sufficient agreement with experimental findings. The correlation gives some insight into how the donor and the acceptor ability of the phosphorus atoms influence the observed parameters and gives some justification for the attempts to separate donor and acceptor strengths of the phosphines in a description of phosphine complexes of lower symmetry. The 51V and 55Mn n.m.r. shifts of metal carbonyls have been reported and discussed in relation to the bonding nature characteristic of carbonyl compounds of the [M(CO)6] and [Mn(CO)X5] types, where M = V or Mn. Theoretically evaluated chemical shift data based on Ramsey's expression agree well with those observed for the [M(CO)6]-type carbonyls. For [Mn(CO)5X], the variation of the shifts for halogens was elucidated theoretically and revealed to be dependent upon the π-bonding nature between the metal and the halogen atom. The 17O n.m.r. spectrum of [V11O28]6- has been used to investigate protonation sites. δ(51V) for a variety of [VO]3+ compounds range from -432 (liquid VOBr3) to +786 (VOF3 in MeCN) relative to liquid VOCl3. δ(51V) increase in the order VOBr3 [much less than] VOCl3< VO(NEt2)3< VO(OR)3< VOF3; VOCl3< VOCl2(OEt) < VOCl(acac)2< VOCl(OEt)2< VO(OEt)3; VOCl2(OMe) < VOCl2(OEt) < VOCl2(OPri) < VOCl2(OBut); V(OEt)3

Complexes of Cr, Mo, and W. — [(OC)5CrCPh2] has, for the carbene carbon, one of the lowest-frequency 13C resonances at δ399.4. N.m.r. data have also been reported for [MH5(PMePh2)4]+ (M = Mo or W; 31P), (OC)5WXH (13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (M = Mo or W; 31P), [M2Me8]4- (M = Cr or Mo; 13C), [W2Me8]4- (13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C), 31P), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (M = Cr or W; 13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C, 31P), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (M = Mo or W; 13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C, 31P), [Ph2CW(CO)5] [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] [(OC)5CrC(X)NEt2] (13C), [(OC)5MCR1OC(O)R2] (M - Cr or W; 13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (M = Cr, Mo, or W; 13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (M = Cr, Mo, or W; 13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C, 31P), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C, 31P), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (M = Cr or W; 13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (M1 = Mn or Re; M2 = Cr, Mo, or W; 13C), [Br(OC)4MCR] (M = Cr or W; 13C), [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (13C), and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].