Ring strain perturbation of the equilibria between hydroxycarbene complexes and metal acyl hydride complexes

The acyl anion complex [(CO)(2)ReC(=O)CH2CH2(eta(5)-C5H4)]Li--(+) (7) in which a two-carbon tether links the cyclopentadienyl ring to the acyl carbon was synthesized by attachment of a 2-lithioethyl side chain to the cyclopentadiene ring of CpRe(CO)(3) followed by intramolecular attack of the lithium reagent on a carbonyl group. Alkylation of 7 with (CH3)(3)O+BF4- occurred at oxygen to give the methoxycarbene complex (CO)(2)Re=C(OCH3)CH2CH2(eta(5)-C5H4) (9), which was shown by X-ray crystallography to have significant strain associated with the tethered ring. Protonation of 7 gave a mixture of hydroxycarbene complex (CO)(2)Re=C(OH)CH2CH2(eta(5)-C5H4) (2) and the metal acyl hydride complex trans-(CO)(2)HReC(=O)CH2CH2(eta(5)-C5H4) (3). The unusual observation of the metal acyl hydride is attributed to the two-carbon tether introducing strain into the three-legged piano stool geometry of 2 but leaving the four-legged piano stool geometry of 3 relatively unstrained. In agreement with this hypothesis, no strain was apparent in the X-ray structure of the three-carbon-tethered methoxycarbene complex (CO)(2)Re=C(OCH3)CH2CH2CH2(eta(5)-C5H4) (17) and only the hydroxycarbene tautomer (CO)(2)Re=C(OH)CH2CH2CH2(eta(5)-C5H4) (18) was observed at equilibrium. A two-carbon tether did not introduce sufficient strain into the aminocarbene complex (CO)(2)Re=C[NH(CH3)]CH2CH2(eta(5)-C5H4) (19) to allow observation of its iminoacyl hydride tautomer.