Transition metal NMR spectroscopy—a probe into organometallic structure and catalysis

Significant advances in NMR detection of insensitive, low-frequency spin-l/2 and quadrupolar nuclei have caused rapid progress in the application of transition metal NMR spectroscopy to organometallic chemistry. As a result, the structural dependence of metal NMR parameters, i.e. chemical shifts, heteronuclear and homonuclear spin coupling and relaxation times, has become a topical field of research. Following a survey of current detection methods, high-resolution and broad-line NMR spectra of 57Fe, 59Co and '°3Rh nuclei in monoand dinuclear olefin complexes are discussed. Particular attention is given to structural effects on nuclear shielding in low-valent metal complexes, to metal-carbon spin coupling, stereoelectronic effects in p,d-bonding, charge distribution in dinuclear complexes, and to intramolecular dynamic processes. For substituted CpCo(COD) and IndCo(COD) complexes quantitative correlations between 59Co shielding and the activity and regioselectivity in homogeneous catalysis of the pyridine synthesis permit a novel and effective screening of catalysts. The first report on 59Co-NMR spectra of cobalamins (vitaminB12 and methylcobalamin) suggests potential applications in biocatalysis. INTRODUCTION The thorough investigation of transition metal NMR spectroscopy of organometallic compounds was handicapped for a long time by a lack of sensitive detection techniques suitable to cover a wide range of resonance frequencies. During the past few years, the availability of high-field spectrometers, with provision for the use of large sample volumes in conjunction with broadbanded r.f. electronics and programmable pulse generators, has caused a real break-through in a field which may be considered of great potential for organometallic chemistry and homogeneous catalysis. METHODS Experimental problems for the detection and routine measurement of transition metal resonances may arise from either of the following facts. First, very small magnetic moments lead to low Larmor frequencies and sensitivities and, for spin-l/2 nuclei, to rather long spin-lattice relaxation times, e.g. 57Fe, '°3Rh, '°7Ag/'°9Ag. Second, nuclei with spin 1 may have large electric quadrupole moments (e.g. 59Co, 61Ni, '°5Pd) which cause extremely short nuclear relaxation times T1 and T2, the latter being responsible for large experimental line width (up to 30 KHz). In addition, low natural isotope abundance may lead to detection problems due to very small over-all receptivity. The magnetic properties of some typical transition metals of major chemical importance are summarized in Table 1. * Transition metal NMR spectroscopy Part VIII, Part VII: ref. 1.