CHAPTER 1
Nuclear Magnetic Resonance Spectroscopy
By B. E. MANN
1 Introduction
This year no further reductions in literature coverage have been introduced, but this has the consequence that the treatment of each reference is even briefer than before. Once again, 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 Reports on 'N.M.R. Spectroscopy', 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.
Volume 6C of 'Annual Reports on N.M.R. Spectroscopy' has finally appeared, and is devoted to a referenced list of organic-metal carbonyl complexes that have had n.m.r. data reported on them during the period 1965 — 71. The book 'N.M.R. and the Periodic Table' is a comprehensive discussion of n.m.r. data on all nuclei other than 1H, 11B, 13C, 14N, 15N, 19F, and 31P up to the end of 1977. In addition to these books a number of reviews of direct relevance to this chapter have appeared, i.e. 'N.M.R. Spectra of Liquid Crystalline Solutions: A Route to Molecular Structures', 'Spectroscopy of Molten Salts', 'Multi-nuclear N.M.R. and the Preparative Coordination Chemist', 'C.I.D.N.P. from Bimolecular Reactions of Organometallic Compounds', 'Deuterium Magnetic Resonance, Applications in Chemistry, Physics, and Biology', 'Deuterium, Nitrogen-15, and Oxygen-17 N.M R and their applications', '17O N M.R. Spectroscopy as a Structural Probe', 'Fluorine-19 N.M.R. Spectroscopy of Fluoroalkyl and Fluoroaryl Derivatives of Transition Metals', 'Use of a Fluorine-19 N.M.R. Method for Studying Fluoro Complexes of d0-Transition Metals', 'Sodium-23 N.M.R. Spectroscopy', 'Alkali Metal, Magnesium-25, and Silver-109 N.M.R. Studies of Complex Compounds in Nonaqueous Solvents', and a review of tertiary phosphine ligands in co-ordination and organo-metallic chemistry includes the use of P n.m.r. data, especially 1J(M, 31P).
A number of papers have been published which are too broadly based to fit into a later section and are included here. The 1H n.m.r. shift for a given ring proton of cyclopentadienyl, benzene, and cycloheptatrienyl complexes relative to that for the appropriate free ligand shows a good linear correlation with the corresponding change in charge density at that proton with an intercept of 2.5 p.p.m. This intercept was attributed to an appreciable diminution in aromatic character of ligand rings on complex formation. 13C Chemical shifts for a variety of [M(CN)5X]n- complexes have been reported. δ(13C) Correlates extremely well with v (M — C) and v(C — N), and in the case of the iron complexes with Mössbauer isomer shifts and quadrupole splitting values. The 13C n.m.r. spectra of M(acac)n (M = H, Li, Na, K, Tl, Be, Mg, Ca, Cd, Al, Pt, Pd, Zn, In, Sc, Co, Zr, or Hf) have been recorded and the signals move to high frequency. J(195Pt, 13C) was recorded for Pt(acac)2. Be, Mg, Ca, Ba, and Zn β-diketonates have been investigated by 1H n.m.r. spectroscopy. Mg-, Ca-, and Ba-(acac)2 are polymeric while Mg- and Zn-(dpm)2 and the PhCHCOCHMe derivatives are monomeric. The 77Se n.m.r. spectra of twenty-nine selenium-containing compounds have been reported. The Se chemical shifts cover a range of ca. 100 p.p.m. In dialkyldiselenocarbamato metal complexes, the magnetic aniso-tropy associated with da8 nickel triad complexes contributes significantly to the 77Se chemical shielding, giving rise to upfield shifts with respect to the anionic ligands and zinc and cadmium complexes. Electronic effects arising from the ligand also significantly contribute to the shielding. Solvent temperature and concentration dependence studies were also carried out on a few of the diseleno-carbamate complexes. Both J(77Se, 31P) and J(195Pt, 77Se) and the n.m.r. trans influence argument were used in making peak assignments; the T1 range is 0.46 — 5 s. With Zn(Se2 CNEt2)2 and Pd(Se2CNBu12), the chemical shift anisotropy appears to be the dominant relaxation mechanism for the Se nuclei. An approximate formula for the second-order perturbation energy previously obtained has been used to calculate the geminal spin-spin coupling constants in ABn type molecular systems. The signs of cis- and trans-geminal coupling constants have been discussed for AB6-type molecular systems. The dependence of the nature of the A — Xi bond in octahedral and square planar complexes AX1X2 ... Xn - 11L on the properties of a variable ligand L has been analysed and used to interpret more than 600 experimental i.r., n.m.r., and n.q.r. spectral data. The use of 19F chemical shifts in the evaluation of the mutual effects of ligands in six-co-ordinate complexes has been examined.
2 Stereochemistry
This section is subdivided into ten parts which contain n.m.r. information about lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium, barium, 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. — The n.m.r. frequency of 113Cs+ ions along with the Zeeman transition frequency of 113Cs has been measured in a magnetic field of ca. 0.06 T. This gave the ratio of the nuclear g factor of the 133Cs+ ion to the electronic g factor in the ground electronic state of free 133Cs. Using this value and results of other researchers' calculations yield g1 (133Cs+)/g1(133Cs) = 0.9999917(21) as the ratio of the nuclear g factor of the Cs ions to that of the free Cs atoms; σ(Cs) - σ(Cs) = 7.7 (2.0) x 10-6, the difference in the dia-magnetic shielding constants of the free atoms and ions. 6Li, 7Li, and 13C T1 and n.O.e. data have been reported for MeLi, BunLi, and PhLi. 6Li T1 values are in the order of tens of seconds and their relaxation is two — three orders of magnitude less than that for 7Li. 6Li is substantially relaxed by the intramolecular 6Li — 1H mechanism whereas both quadrupolar and 6Li — 7Li dipolar relaxation are minor processes. The non-linearity of the Arrhenius curve for Me6Li is compatible with a small spin rotation contribution. Li in solution behaves as a spin-½ nucleus and is useful as an alternative to 7Li n.m.r. spectroscopy in spite of its lower magnetic moment and lower sensitivity. 13C N.m.r. spectroscopy has shown that (1) and its cis isomer contain a planar metallated carbon with Li+ in a stable chelated structure. The 13C n.m.r. spectra of 5,5-dimethylhexen-2-yl metals and 2,5,5-trimethylhexen-2-yl metals have been measured where the metal is Li, Na, K, Rb, Cs, Hg, or SiMe3. Substantial changes of the chemical shifts of the α-, β-, and γ-carbon atoms occur in the alkali-metal compounds as the counter-ions vary. The 13C chemical shifts of the α-carbon atoms in 1,3,3-trimethyl-1-phenylbutyl-lithium suggested an sp2 hybridization. The significant broadening of the 13C resonance of the α-carbon atom and the aromatic carbon atoms was related to an exchange reaction or to J (13C, 7Li) and was interpreted by an interaction between Li and the aromatic ring. The methyl protons of the i-propyl groups of Pr1 PhSiLi and Pr12PhGeLi are diastereotopic and anisochronous at room temperature in a wide range of solvents, while the benzyl protons of (PhCH2)2PhSiLi are accidentally isochronous even at 270 MHz. Because the 7Li resonances of Pr12PhMLi are very similar to those of Pr12PhCLi, the ionicities of the Si — Li and Ge — Li bonds cannot be greatly different from that of the C — Li bond. A lower limit to inversion about tervalent Si and Ge was set at 24 kcal mol-1. The 13C n.m.r. spectra of But COCH2Li, and o- and benzylic metallated Me2 NCH2Ph have been given. 31C N.m.r. spectra show that LiC[equivalent to]CLi, when irradiated in NH3, gives a species believed to be C4Li4, tetralithiotetrahedrane with σ(13C) at 57.2.
High-resolution 6Li, 7Li, and 23Na n.m.r. data in a liquid crystalline mesophase consisting of sodium decylsulphate, lithium sulphate, decanol, and water have been recorded. From the quadrupole splittings of the two isotopes, a ratio of the quadrupole moment of 7Li:6Li equal to 49 was obtained. By comparing the temperature dependence of the quadrupole splittings, it is apparent that lithium and sodium ions interact differently with the polar head group of the mesophase. Low-temperature 13C n.m.r. spectra of Li+ and Na+ enolates of [MeCOCRCOMe]- provide evidence concerning ionic bonding and the effect of steric hindrance on alkali-metal chelation. The n.m.r. signals of the carbons of (2) (R1, R2 = H or Me; R3 = p-BrC6H4 or EtO; M = Na or Cs) occur at lower fields than those of the corresponding ions. The chemical shifts and J(13C, 1H) of (2) (R1 = Me, R2 = H) decrease in the cation order Li > Na > K. > Cs. The 13C n.m.r. spectrum of 2,3-naphtho-20-crown-6 shows a relationship between the naphthalene shifts and the charge on the complexed metal, Na+, K+ Rb+, Cs+ , Ca+, and Ba2+.
Solutions of Mg(BH4)2.3THF in [2H 8] toluene have been characterized by 1H-, 11B-, and 1H-{11B} n.m.r. spectra at 30 °C to -80 °C. Neutralization of the 1H- 11B spin-spin interaction was observed and a singlet at -60 °C clearly indicates the equivalency of protons in Mg(BH4)2. 3THF under these conditions. The 1H chemical shift of the protons α to magnesium in EtMgBr and Et2Mg in various donor solvents provides a measure of the basicity of the solvents. The 1H and 13C n.m.r. spectra of (PhCH2) 2M, (M = Ba, Sr, or Ca) showed aromatic-proton resonances upfield from benzene, characteristic of benzyl carbanions. The C — M bonds of the benzyl carbanions in THF increase in ionic character in the order Mg < Ca < Sr < Li < Ba < K. 1H T1, n.O.e., and long-range couplings have been used to determine the assignments in chlorophylls and metalloporphyrins. The 1H n.m.r. spectra of (3) indicate that the two macrocycles are sandwiched directly on top of one another. 135B and 137Ba n.m.r. investigations have been reported in liquids. From these measurements ratios of g-factors, magnetic moments, atomic shielding constants, and the ratio of the quadrupole moments were evaluated using also data from the literature. N.m.r. spectra have also been reported for M(PO2Cl2)2 (p-dioxane)2 (31P; M = Mg, Ca, or Zn), {(PhO)2PO2}2Mg (31P), and [Mg4(OCH2CH2OMe)6 (dme)2]2+ [Hg(SiPhMe2)3] 2- (13C).
Complexes of Y, U, Ti, and Zr. — For [Y(CH2 SiMe3)4]-, 1J(89Y, 13C) = 28 Hz, and the 13C n.m.r. spectrum of (4) has been reported.
1H N.m.r. studies of (5) reveal the 5-subunit maerocycle to be stereochemically dynamic. Analysis of ring current-induced 1H n.m.r. shifts indicates considerably reduced π-electron delocalization compared with phthalocyanin derivatives. Although the i.r. spectrum of U(SO3F)4 indicates the presence of both uni- and bi-dentate SO3F groups; the 19F n.m.r. spectrum consists of a singlet. A relation between the F chemical shifts and the heat of formation of M3UO2F5, (M = NH4, K, Rb, or Cs) has been presented. The n.m.r. method has been used to provide information concerning the isotopic content of in UF6 by measuring T2 of 19F nuclei in liquid UF6. The sources of errors in the T2 measurements were analysed and methods of reducing them were discussed.
The hydride in Zr(η5}-C5Me5) 2H2 is at abnormally high frequency, δ 7.46. In {(η5-C5H5)2ZrH}2 OCHCHO, 1J (13C, 13C) = 99 Hz. For Hf(BH4)2(η5-CH5Me) 2, exchange of bridge and terminal borohydride hydrogen atoms is rapid down to — 155 °C giving an upper limit for AG≠ of ca. 4.9 kcal mol-1. 13C N.m.r. spectra have been reported for (6) (including 31P), and (η 5- C5H5)2Ti(μ-N2) Ti(η5-C5H5)2. 1H N.m.r. spectroscopy has been used to investigate complexation between titanium (IV) complexes and pyrocatechol. Titanium Lewis acids are efficient n.m.r. shift reagents for lactams. 19F N.m.r. spectra of some pyridinium hexafluorotitanates have been reported.
Complexes of V, Nb, and Ta. — The activation energy for Ta — CH2 rotation in Ta(η5-C5 H5)(CH2)Me is in excess of 20 kcal mol-1 and the 13C n.m.r. spectrum was reported. In the 13C n.m.r. spectra of Ta(CH2CMe3)3(CHCMe 3), δ(13CH2) is 113.7 and δ (13CH) is 250.1. 1H and 13C n.m.r. spectra of some secondary carbene complexes of VI, Cr0, Mo0, W0, Mn1, Re1, Fe0, RuII, Co1, IrIII, PtIV, and HRhIII have been used to establish the stereochemistry of these complexes. 51V N.m.r. chemical shifts of V(CO)5EZ, (η5-C5H5)V(CO)3EZ3, and cis-(η5-C5H)V(CO)2 (BiEt3)2 show that the ligands can be arranged in a sequence of increasing overall ligand strength according to EPh3< Me2Ph ~ EEt3< E(OEt)3 (E = P or As) and BiEt3< AsZ3< PZ3< SbZ3. The results were discussed with respect to the ligand field parameter, the covalency of the metaHigand bond, nephelauscetic effects imparted by the donor atom, and the inductive influences of the substituents, Z. N.m.r. data have also been reported for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].
The 31P n.m.r. spectrum of Nb2Cl6.4 PhPMe2 shows three singlets due to isomers. Linewidths, Δv1/2, of 51V m.r. signals of VO(OPr13 dissolved in THF, range from 10 to 610 Hz, increasing with decreasing temperature according to a theoretically proposed T1/2 exp(E/RT) law, indicating that the molecular correlation time, τc, is the main factor influencing linewidths in this system. With lowering of temperature, 51V-shielding decreases, suggesting a loss of molecular symmetry due to solvation and association processes. In pentane, methylcyclohexane, and Et2O, the δv and Δv1/2/T correlations exhibit two inconsistencies at approximately 190 and 240 K which were attributed to the formation of clusters. Exchange interactions are discussed as a second factor affecting linewidths. For [V10O28]6-, the first protonation site differs both from the second protonation site and from the, site of preferential interaction with aqueous Mn2+. The 51V n.m.r. linewidths are almost independent of direct protonation but depend indirectly on more distant protonation. The equilibria of H3 + nPMo12 - n Vn, O40 in aqueous solution have been studied as a function of pH by 17O, 31P, and 51V n.m.r. spectroscopies. Chemical shifts were given for [PMo12O 40]-, [PMo11, VO40]4-, [PMo10V2O40]5-, and [PM09V3O40]6-. 17O N.m.r. spectra of [V2W4O19]4- have established the cis relationship between vanadium atoms in an octahedral metal array. Comparison of this spectrum with the spectrum of [HV2W4O19]3- demonstrates that the proton in [HV2W4O19]3- is bound to the unique oxygen atom which is bonded to both vanadium atoms. N.m.r. data have also been reported for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].
