Sterically hindered chiral organometallic complexes: An X-ray crystallographic, NMR spectroscopic and EHMO study

Abstract

<p>Sterically crowded organometallic complexes present fascinating problems of structure and molecular dynamics. Tetrahedral clusters such as (RC≡CR')−(C₅H₅)₂M₂(CO)₄, where M = Mo or W, crystallize in conformations possessing three terminal carbonyls while the fourth is semi-bridging. However, these ligands undergo a rapid exchange process which can be followed by variable-temperature NMR spectroscopy. When the R substituent is derived from a chiral natural product, the low temperature NMR spectra reveal the presence of diastereomers which are interconvertible via rotations of the organometallic vertices. The fluxional behaviour of tetrahedral clusters containing such vertices as Co(CO)₃, Fe(CO)₃ or (C₅H₅)Mo(CO)₂ can be rationalized by means of molecular orbital calculations at the extended Huckel level of approximation. These studies show that the barriers to vertex rotation can usually be traced to one principal orbital interaction in each case. However, in (C₅H₅)₂Mo₂(CO)₄(R-C≡C-R') clusters, the barriers are primarily steric in character. The ability of transition metal clusters to delocalize electronic charge is well known and, in principle, could be used to stabilize intermediates of biochemical significance. Treatment of 2-methylcyclopentanone with an alkyne anion was carried out in order to generate 1-alkynyl-2-methylcyclopentanols in which the methyl and alkynyl groups are trans diaxial; the aim was to mimic the "D"-ring of the steroidal contraceptive mestranol. In fact, the major epimer was the one in which the methyl and alkynyl substituents were disposed in a cis manner. The conformation of 2-methyl-1-phenylethynylcyclopentanol 47 was elucidated by two-dimensional NMR techniques. Moreover, the structure of 47 and also of its Co₂(CO)₆ derivative have been determined crystallographically. Protonation of the dicobalt or dimolybdenum complexes of 47 lead to stable cations; treatment of these cations with nucleophiles results in elimination of water to yield 2-methylcyclopentene derivatives of which the dimolybdenum cluster has been characterized by x-ray crystallography. The sterically crowded complexes (C₆H₅)₃SiOH[Cr(CO)₃]n, where n = 1,2,3, have also been characterized X-ray crystallographically and all three have propellor-type geometries. The question of whether the phenyl ring rotations are correlated has been studied by variable-temperature NMR spectroscopy. These systems do not yield stable silicenium cations, but the analogous Cr(CO)₃ complex of triphenylcarbinol not only shows fluxional behaviour but also yields a metal-stabilized cation. The pathway for phenyl rotations in such systems can be followed via a Dunitz-type trajectory approach in which a number of x-ray structures were analyzed.</p>