Ring Strain Perturbation of the Equilibria between Hydroxycarbene Complexes and Metal Acyl Hydride Complexes

The acyl anion complex [(CO) 2 ReC( O)CH 2 CH 2 (η 5 -C 5 H 4 )] - 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 (CH 3 ) 3 O + BF 4 - occurred at oxygen to give the methoxycarbene complex (CO) 2 Re C(OCH 3 )CH 2 CH 2 (η 5 -C 5 H 4 )( 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)CH 2 CH 2 (η 5 -C 5 H 4 ) ( 2 ) and the metal acyl hydride complex trans -(CO) 2 HReC( O)CH 2 CH 2 (η 5 -C 5 H 4 ) ( 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(OCH 3 )CH 2 CH 2 CH 2 (η 5 -C 5 H 4 ) ( 17 ) and only the hydroxycarbene tautomer (CO) 2 Re C(OH)CH 2 CH 2 CH 2 (η 5 -C 5 H 4 ) ( 18 ) was observed at equilibrium. A two-carbon tether did not introduce sufficient strain into the aminocarbene complex (CO) 2 Re C[NH(CH 3 )]CH 2 CH 2 (η 5 -C 5 H 4 ) ( 19 ) to allow observation of its iminoacyl hydride tautomer.