Arene C-H Bond Coordination versus C-H Bond Cleavage in Low-Valent Group 6 Carbonyl Pincer Complexes

The reaction of hexacarbonyl group 6 complexes with the P(CH)P ligand N,N'-bis-(diisopropylphosphino)-N,N'-dimethyl-1,3-diaminobenzetie was investigated. The objective was to obtain PCP pincer complexes via directed C-H bond activation and cleavage. In the case of chromium and molybdenum, complexes [Cr-(kappa P-3,CH,P-P(CH)P-NME-iPr)(CO)(3)] and [Mo(kappa P-3,CH,P-P-(CH)P-Nme-iPr)(CO)(3)], featuring eta(2)-C-aryl-H bonds, were formed and no C-H bond cleavage took place. With tungsten the expected seven-coordinate hydrido carbonyl W(II) complex [W(kappa P-3,C,P-PCPNme-iPr)(CO)(3)H] was formed. DFT mechanistic studies corroborate the experimental observations and indicate that the M-H bond strength grows in the order Cr-H < Mo-H < W-H, explaining the preference for the hydride complex only in the case of W. X-ray structures of these complexes are presented.