31P and 13C NMR of Transition Metal Phosphine Complexes by Paul S. Pregosin, Roland W. Kunz

By Paul S. Pregosin, Roland W. Kunz

For nearly 1 / 4 of a century the phrases "nuclear magnetic reso­ nance" have been synonymous with proton I,leasurements. in this interval the literature abounded with a doubtless limitless number of 1H NHR reviews involved basically with carbon chemistry. sometimes a "novel" nucleus used to be studied and, even in these early days, the poten- thirteen 14 31 19 tial provided through C, N, P and F used to be essentially well-known. regardless of the attract, the technical problems concerned with measuring a few of these nuclei have been faraway from trivial. Small magnetic moments and occasional normal abundance together with spin-spin coupling from different nuclei, typically protons, ended in a signal-to-noise challenge whose severity successfully excluded the research of steel complexes with unfa­ vorable solubility features. the 1st very important leap forward got here with the arrival of huge band 1H-decoupling. for instance, the featureless vast 31p resonance linked to the widely used ligand triphenyl phosphine is switched over to a pointy, extra comfortably ob­ served singlet whilst wide-band decoupling is hired (see Fig. 1). regardless of this development research of extra fascinating molecules, reminiscent of catalytically lively complexes was once compelled to watch for the devel­ opment of Fourier rework tools on account that simply with really swift sign averaging equipment might enough signal-to-noise ratios be achieved.

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Several values of 1J (AI,P) for trialkyl phosphine complexes of AICl 3 and AIBr 3 have been reported [168] as have some values for 1J (17 0 ,p), 90 - 205 Hz [169] and 1J (se,p), 358 - 955 Hz [170]. IV Two-Bond Phosphorus-Phosphorus Coupling Constants-Data Although the interest in 2J (p,p) is somewhat less than that for the one bond interaction, there has been an increase in the number of workers actively engaged in measuring this parameter. In most cases where values of 2J (p,p) are given, the motivation for the measurement stemmed from the expectation that 2J (p,p)t should be much greater 2 rans than J(P,P)cis In recent work the sign of this coupling has, generally, not been determined, although there is no reason to doubt the conclusion from earlier efforts [171] since these have been confirmed by other workers [172].

It is convenient to subdivide the rhodium studies into three sections: One-Bond Metal-Phosphorus Coupling Constants a) 4-coordinate planar Rh(I) complexes b) S-coordinate Rh(I) and Rh(III) complexes c) 6-coordinate octahedral Rh(III) complexes 39 Most of the work involving this metal concerns itself with the chemistry of these three classes of compounds and the transformations from one to another. Often the 31p NHR is incidental and is included primarily for the sake of completion. A noteworthy exception is a recent study in which the reduced one-bond rhodium-phosphorus couplings for complexes of Rh(I) and Rh(III) were shown to be positive [113].

Tri-t-butyl phosphine has the lowest field 31 p _ re _ sonance in the following list [64 ]. 3 182 PEt 3 PPh 3 103 87 Since the highest field shifts are associated with ligands with small C-P-C angles the suggestion has been made that the degree to which the C-P-C ang12 opens on complexation can be related to the 9. This approach may well have some validity; however, it is worth noting the calculations by Lehn and Munsch [67] as well as our comments in the previous chapter. 51 Chemical Shifts s-character in the phosphorus lone pair in the complex and therefore related to the 31p chemical shift.

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