Novel group 6 complexes of cyclopentadienylidene ylides
Brownie, John Hugh
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Abstract Methyldiphenylphosphonium cyclopentadienylide, C5H4PMePh2 (II), has been synthesized and characterized spectroscopically and crystallographically, and has been found to exhibit properties consistent with the zwitterionic structure IIb. New group 6 complexes, (η5-C5H4PMePh2)M(CO)3, have been synthesized and fully characterized. Comparisons of ν(CO) of these complexes with those of the isoelectronic (η6-C6H6)M(CO)3 and [(η5-C5H5)M(CO)3]- suggest that the electron donating ability of the ylide is between that of the cyclopentadienyl anion (Cp-) and benzene, but closer to Cp-. The electronic structures of II and of (η5- C5H4PMePh2)Cr(CO)3 have been investigated using ab initio methodologies. Thermal and photochemical substitutions of the CO ligands of (η5-C5H4PMePh2)M(CO)3 (M = Cr, Mo) by equimolar amounts of PMe3 and PPh3 were not observed, but the ylide is displaced photochemically from (η5-C5H4PMePh2)Mo(CO)3 by excess PMe3 to form fac- Mo(CO)3(PMe3)3 while (η5-C5H4PMePh2)Mo(CO)3 reacts with I2 to form [(η5- C5H4PMePh2)Mo(CO)3I]I. One electron oxidations of (η5-C5H4PMePh2)M(CO)3 (M = Cr, Mo, W) have been performed to give the cationic radicals [(η5-C5H4PMePh2)M(CO)3]+, which undergo dimerization to give dicationic metal-metal bonded dimers ((η5-C5H4PMePh2)M(CO)3)2 2+ in the solid state. These complexes have been fully characterized spectroscopically and crystallographically. It has been determined that the chromium dimer ((η5- C5H4PMePh2)Cr(CO)3)2 2+ undergoes dissociation extensively in solution to the persistent radical cation monomer (η5-C5H4PMePh2)Cr(CO)3 +, but that the heavier metal analogues iii ((η5-C5H4PMePh2)M(CO)3)2 2+ (M = Mo, W) dissociate very little, if at all. The Cr-Cr bond distance of the chromium complex is 3.3509(7) Å, which is the longest Cr-Cr bond distance known for a compound not containing some type of ligand bridging the metalmetal bond. The hitherto unknown indenyl-derived ylide, C9H6PMePh2, has been synthesized and characterized spectroscopically and crystallographically. The chromium tricarbonyl complex of this ligand, (η5-C9H6PMePh2)Cr(CO)3, has been synthesized and characterized spectroscopically and crystallographically. This complex is a mixture of two isomers exhibiting planar chirality generated upon coordination of the ligand. This complex represents the first structurally characterized phosphorus noncyclopentadienylide coordinated in an η5 manner. The spectroscopic and crystallographic behaviour of C9H6PMePh2 demonstrates that this ylide behaves much like the related cyclopentadienylides.