CHAPTER 1
Nuclear Magnetic Resonance Spectroscopy
BY B. E. MANN
1 Introduction
Following the criteria established in earlier volumes, only books and reviews directly relevant to this chapter are included, and the reader who requires a complete list is referred to the Specialist Periodical Report 'Nuclear Magnetic Resonance', where a complete list of books and reviews 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.
Only one relevant book has appeared during 1980, namely 'E.P.R. and N.M.R. in the Chemistry of Coordination Compounds'. Numerous reviews have appeared, including 'Correlations in Nuclear Magnetic Shielding, Part II', 'N.M.R. Spectroscopy of Organometallic Compounds of the f-Elements; Practical Applications', 'N.M.R. for Rare Earth Complexes; Theory', 'N.M.R. of Other Nuclei', 'Application of N.M.R. in the Structure Elucidation of Coordination Compounds: Metal Nuclei', 'Nuclear Magnetic Resonance of the Less Common Quadrupolar Nuclei', 'Carbon-13 and Nitrogen-15 N.M.R. Studies in Organic and Organometallic Chemistry', σ,π-Conjugation in Organometallic Systems', 'Dynamic and Stereochemical Studies by Phosphorus-31 N.M.R. Spectroscopy on Polyphosphine Complexes of Heavy Metals', 'Phosphorus-31 N.M.R. Studies of Catalytic Systems Containing Rhodium Complexes of Chelating Diphosphines', 'Model Compounds as Aids in Interpreting N.M.R. Spectra of Haemoproteins', 'High Resolution N.M.R.', a review of n.m.r. properties of metal nuclei in metallo-proteins, 'Nuclear Magnetic Resonance Spectroscopy of Diamagnetic Porphyrins', 'Biophysical Applications of N.M.R. to Phosphoryl Transfer Enzymes and Metal Nuclei of Metalloproteins', 'Multinuclear N.M.R. Studies of Crown and Cryptand Complexes', 'Synthesis, Molecular Dynamics, and Reactivity of Mixed-Metal Clusters', and 'Nuclear Magnetic Resonance in Molten Salts and Molten Metal–Salt Mixtures. I. Chemical Shift'.
A number of papers have been published which are too broadly based to fit into a later section and are included here. J(15N, 13C) and 15N/14N isotope effects on 13C chemical shielding have been reported for [Cu(CN)4,]3-, [Ni(CN),]2-, [Pt(CN),]2-, [Cd(CN)4]2-, and [Hg(CN)4]2-. The isotope shifts, 0.02 — 0.04 p.p.m, are comparable to recently reported 16O/18O isotope effects on 13C chemical shielding in metal carbonyl derivatives. The H chemical shifts of 2-hydroxy-1,3-propanediamine-NNN'N'-tetra-acetic acid and 1,3-propanediamine- NNN'N'-tetra-acetic acid in 1:1 complexes with diamagnetic metal ions have been investigated as a function of the metal ion charge z and crystal ionic radius r. The chemical shifts correlate with z/r linearly. 1H and 31P n.m.r. spectra have been recorded for complexes of ATP with MgII, CaII, SrII, ZnII, CdII, SnII, PbII, HgII, AgI, and TlI. Each of these ions, except HgII, affected the 13P n.m.r. of ATP, usually shifting all three resonances to high frequency and decreasing J(31P,31P). PbII exerted the greatest shifts, whereas MgII caused the greatest change in coupling constant. In 13C n.m.r. spectroscopy the intensities of different signals become almost equal on adding Cr(acac)3 as a relaxation reagent.
15N n.m.r. spectra of nitrogen-containing crown ethers, cryptand ligands, and other ligands with pyridine-type nitrogens and their complexes with alkali, alkaline-earth, AgI, and TlI ions have been reported. An analytical LCAO MO theory of the Fermi contact term, 1K (A, 19F), has been developed for substituted planar-trigonal AF3 - n Ln and tetrahedral AF4 - n Ln fluorides of main-group elements. This treatment was extended to octahedral cases. For melts of transition, alkali, and alkaline-earth metal fluorides a theory about the effect of outer-sphere cations on the chemical shift of the 19F nucleus entering into the [MF4]n - 4 complex ion has been developed. The changes in the chemical shift of a central-atom nucleus during successive substitution of ligands in the AXk-nYn-type molecular systems have been explained.
2 Stereochemistry
This section is subdivided into ten parts which contain n.m.r. information about lithium, magnesium, and transition-metal complexes, presented by Groups according to the Periodic Table. Within each Group, classification by ligand type.
Complexes of Group IA and IIA. — The 3J (13C, 1H) and 1J(13C 13C) in EtM, Pr1M, and ButM (M = alkali metal or halogen) have been determined. The substituent electronegativity dependence of 3J(13C,1H) shows that the influence exerted by a β-Me substituent on 3J(13C,1H) and 2J (13C,1H) is not constant. The σ- vs. π-character of R2CR1=CR3CH 2M (M = Li, K, or MgBr) has been studied via13C n.m.r. spectroscopy.
INDO molecular-orbital calculations have suggested that 1J (13C, 7Li) in methyl-lithium monomer should be 16 Hz. The 13C n.m.r. shifts in straight-chain alkyl-lithium compounds in ether and cyclopentane are linear with those of the corresponding hydrocarbons, indicating that the whole series of compounds studied have similar electronic structures about the bridged carbon-lithium bonds. The C1, 13C n.m.r. resonance in PrLi broadens with decreasing temperature, indicating the operation of interaggregate bond exchange as well as lithium quadrupole relaxation. 13C n.m.r. spectra of allyl-lithium-1-d have been reported which are consistent with a symmetric rather than a rapidly equilibrating unsymmetric structure. 1H and 13C n.m.r. spectra of a series of para-substituted α-methyl-α-neopentylbenzyl-lithium indicate a Hückel charge distribution. For some 9-alkyl-10-lithio-9, 10-dihydroanthracenes the para n.m.r. shift has been investigated as a function of solvent.
The magnetic moment of 24mNa is 1.930(3) nuclear magnetons (uncorrected). The Na n.m.r. spectrum of Na{CH(PPh2CHPh)2} (thf) shows an asymmetric ligand arrangement with a 2200 Hz linewidth. 1H, 13C, and 31P n.m.r. spectra were also recorded. The 13C n.m.r. spectrum of C5H7 K has also been reported. The 31P skeleton of 6Li P7 has been proven by a complete analysis of the 31P n.m.r. spectrum of 6Li3P7. The 6Li isotopic substitution was carried out to avoid the line broadening caused by the large quadrupole moment of the 7Li isotope.
Mg(ATP) has been proposed as a thermometer for 31P n.m.r. studies in biological systems. The α — β chemical-shift difference is linearly proportional to temperature, changing by 0.012 p.p.m. °C-1. 13C chemical shifts, 1J (13C,13C), T1, and NOE have been determined for taurine complexes with CaII. N.m.r. data have also been reported for [Li{(Me2P)2BH2} 2]-, M{(Me2P)BH2}2 (M = Be, Mg, Zn, Cd, or Hg; 7Li, 11B, 13C, 31P), M{N(CHCH)2CH2}py2 (M = Mg or Zn; 13C), and some chlorophyll derivatives (13C).
Complexes of Y, La, U, Ti, Z, and Hf. — The spin-lattice relaxation mechanism for aqueous and DMSO solutions of Y(NO3) 3 are mainly spin-rotation and dipolar relaxation with solvent protons, unlike most heavy spin-1/2 metal ions which are relaxed mainly by spin-rotation and chemical-shift anisotropy. The theoretically maximum 89Y-{1H} NOE value of — 10.2 was observed when (i) τc for the ion was lengthened by lowering the temperature of the aqueous salt solution to 5 °C or (ii) the yttrium was complexed to an organic ligand. A polemic has appeared on a previous paper which stated that 89Y T2 values are not pH dependent. The ratios of the 138La and 138La Larmor frequencies, magnetic moments, and quadrupole moments have been determined. For 138La the chemical shifts and linewidths in aqueous lanthanum salt solutions, solvent isotope effects, linewidths as a function of temperature, and signals in the solid state were also investigated. Several LaIII-hydroxycarboxylate complexes in aqueous medium have been studied by means of 139La chemical-shift and linewidth measurements. The enrichment in 235U has been determined in UO2 (NO3)2 · nH2O by measuring T1 and T2 of zH nuclei. N.m.r. data have also been reported for complexes of (MeC5H4) Yb (13C), (CH2OCH2CONEt2)2 with Y3+ and La3+ (13C), Th(C8H8Bu)Cl2 (13C), UO2(hfac)2 (13C,1 9F), and UO2(2,6-diacetylpyridine) bis-(4-methoxybenzoylhydrazone) (13C).
As part of the characterization of some substituted bis-cyclopentadienyl TiIV and ZrIV compounds, 13C and 1H n.m.r. data were obtained which indicate that the effects of both metal shielding and ring substitution can be taken into account by a simple additive relationship so that it is possible to predict accurately chemical shifts for these systems. The 1H and 19F n.m.r. spectra of Cp2Ti(C6F5)(OMe) complexes show o-F inequivalence. 1H n.m.r. spectra suggest that MeC5H4Ti(S2CNMe2) 3 and HfCp(S2CNR2)3 are stereochemically rigid. The 13C n.m.r. spectrum of (1) gives only one signal for the C8H8 rings and is therefore fluxional. N.m.r. data have also been reported for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].
Complexes of V, Nb, and Ta. — The nuclear magnetic moment of Ta has been measured. The shielding of V in CpV(CO)3 PR3 complexes decreases with decreasing ligand strength (electronegativity of R) and increasing spatial requirement for the PR ligand as quantified by Tolman's cone angle. This correlation was explained by an increase in the paramagnetic contribution to the overall shielding resulting from diminished electronic interaction between vanadium and the phosphine. Chelate complexes of the types [V(CO)4L]-, CpV(CO)2L, HV(CO)4L, and C3H5V(CO)3L [L = Ph2P (CH2)PPh2 or cis-Ph2P CH=CHPPh2] exhibit a minimum V shielding in the 4-membered ring system, paralleled by a maximum 31P shielding; a maximum 51V shielding for a 5-membered chelate ring corresponds to a maximum 31P co-ordination shift. For [CpVH(CO)3] -, 51V n.m.r. data indicate that the VH(CO)3 moiety is probably C3v. The 51V n.m.r. spectra of [V(CO)6 - nLn]-, CpV(CO)4 - n Ln, and related compounds have been discussed on the basis of variations of the paramagnetic contribution to the overall shielding of the 51V nucleus. 1H, 13C, and 31P n.m.r. spectra of some alkylidene complexes of niobium and tantalum suggest that the metal attracts electron density from the CHα bond. N.m.r. data have also been reported for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].
For 13C and 15N spectra of XM(OCH2CH2) 3N (XM = B, MeSi, RGe, OV, O2Mo, or ButSn) the shift of the 15N signal occurs in different directions. The degree of charge transfer or M <- N bond strength increases in the series RGe ~ R'Sn < OV ≤ B ~ O2Mo. Multinuclear n.m.r. spectra of [V(PF3)6]+ and [Nb(PF3)6]+ show well resolved 93Nb coupling which is greater than 51V coupling. This observation was explained on the basis of greater valence s-electron density at the nucleus and enhanced cis-fluoride bridged chains. T1 and T2 of 27Al and 93Nb nuclei in [AlCln Br4 - n- and [NbClnBr6 - n anions in MeCN solution have been measured as a function of temperature. Calculated values of the electric-field gradient tensor are in good agreement with experimental data. N.m.r. data have also been reported for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].
Complexes of Cr, Mo, and W. — Protonation of 6-dimethylaminofulvenic complexes of chromium, molybdenum, and tungsten carbonyls has been shown to be at the metal atom. Deprotonation of Mo2Cp2 (CO)4(µ-H)(µ-PMe2) causes the 31P n.m.r. resonance to move from 146.4 to 61.6 p.p.m. At low temperature the 31P n.m.r. spectrum of WH6 (PPr12Ph) consists of two signals in the ratio 1:2.N.m.r. data have also been reported for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] exhibit relatively large values of 1J(13C,1H) for the α-carbon atom. 119Sn n.m.r. data of a number of base-stabilized stannylenes, LSnX2, have been compared with those of the corresponding tin ylid complexes M(CO)5L (M = Cr or W). 119Sn n.m.r. co-ordination shifts are strongly dependent on the transition metal. Increasing the number of base ligands attached to transition-metal- co-ordinated SnII leads to significant low-frequency shifts and to increasing 1J(119Sn, 183W). Low-temperature n.m.r. spectroscopic studies in conjunction with the McConnell relations allow the evaluation of the diamagnetic anisotropies of some metal-metal multiply bonded systems. N.m.r. data have also been reported for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].
13C chemical shifts have been measured for the acetylenic carbon atoms bound to a series of molybdenum(II) and tungsten(II) complexes. An empirical correlation between the number of electrons formally donated per alkyne ligand and the observed chemical shifts, which span ca. 90 p.p.m., was reported. The effective donation of 2, 3, 3 1/3, and 4 electrons per alkyne ligand was rationalized and the implication of the variable-electron donor properties exhibited by acetylenic linkages was discussed. Variable-temperature 19F n.m.r. spectra of CpMo-{C4(CF3)4}(S2CNMe2) have shown the existence of high barriers to rotation of the C ring about the metal-ligand axis. N.m.r. data have also been reported for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].
