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-diaminobenzene
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(κ3P,CH,P-P­(CH)­PNMe-iPr)­(CO)3] and [Mo­(κ3P,CH,P-P­(CH)­PNMe-iPr)­(CO)3], featuring η2-Caryl–H bonds, were formed and no C–H
bond cleavage took place. With tungsten the expected seven-coordinate
hydrido carbonyl W­(II) complex [W­(κ3P,C,P-PCPNMe-iPr)­(CO)3H] 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.