Olefin−Alkylidene Equilibrium of 2-Vinylpyridine in Osmium- and Ruthenium-Hydrido-Tris(pyrazolyl)borate and Osmium-Cyclopentadienyl Complexes

Complex [OsTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 ( 1; Tp = hydridotris(pyrazolyl)borate) reacts with 2-vinylpyridine to give the olefin derivative [ OsTp(η 2 -CH 2 ═CH-C 5 H 4 N)(P i Pr 3 )]BF 4 ( 2 ). In methanol at 60 °C, complex 2 is in equilibrium with its alkylidene isomer [ OsTp(═CHCH 2 −C 5 H 4 N)(P i Pr 3 )]BF 4 ( 3 ). The rearrangement takes place via the intermediates OsTp{CH 2 CH(OMe)-C 5 H 4 N}(P i Pr 3 ) ( 4 ) and OsTp(CHCH-C 5 H 4 N)(P i Pr 3 ) ( 5 ), which have been isolated and characterized. The ruthenium complex [RuTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 ( 6 ) reacts with 2-vinylpyridine, in a similar manner to 1, to give [ RuTp(η 2 -CH 2 ═CH-C 5 H 4 N)(P i Pr 3 )]BF 4 ( 7 ). Like its osmium counterpart, complex 7 is transformed in an alkylidene species [ RuTp(═CHCH 2 -C 5 H 4 N)(P i Pr 3 )]BF 4 ( 10 ), via the ruthenium analogue to 4 and 5, RuTp{CH 2 CH(OMe)-C 5 H 4 N}(P i Pr 3 ) ( 8 ), and RuTp{CHCH-C 5 H 4 N}(P i Pr 3 ) ( 9 ). In contrast to 1 and 6, the cyclopentadienyl derivative [OsCp(CH 3 CN) 2 (P i Pr 3 )]PF 6 ( 11 ) reacts with 2-vinylpyridine to give the hydride-3-osmaindolizine derivative [ OsHCp(CHCH-C 5 H 4 N)(P i Pr 3 )]PF 6 ( 12 ), by C−H bond activation of the CH 2 group of the substituent of the heterocycle. Treatment of 12 with NaOMe in THF gives OsCp(CHCH-C 5 H 4 N)(P i Pr 3 ) ( 13 ), which can also be prepared by reaction of OsCpCl(P i Pr 3 ) 2 ( 14 ) with 2-vinylpyridine. Protonation of 13 initially gives the alkylidene cation [ OsCp(═CHCH 2 -C 5 H 4 N)(P i Pr 3 )] + ( 15 ), which isomerizes into the olefin species [ OsCp(η 2 -CH 2 ═CH-C 5 H 4 N)(P i Pr 3 )] + ( 16 ). The latter is transformed into 12 via the hydride-3-osmaindolizine intermediate 17 containing the hydride ligand transoid disposed to the nitrogen atom.