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

Complex [OsTp(κ1-OCMe2)2(PiPr3)]BF4 (1; Tp = hydridotris(pyrazolyl)borate) reacts with 2-vinylpyridine to give the olefin derivative [OsTp(η2-CH2CH-C5H4N)(PiPr3)]BF4 (2). In methanol at 60 °C, complex 2 is in equilibrium with its alkylidene isomer [OsTp(CHCH2−C5H4N)(PiPr3)]BF4 (3). The rearrangement takes place via the intermediates OsTp{CH2CH(OMe)-C5H4N}(PiPr3) (4) and OsTp(CHCH-C5H4N)(PiPr3) (5), which have been isolated and characterized. The ruthenium complex [RuTp(κ1-OCMe2)2(PiPr3)]BF4 (6) reacts with 2-vinylpyridine, in a similar manner to 1, to give [RuTp(η2-CH2CH-C5H4N)(PiPr3)]BF4 (7). Like its osmium counterpart, complex 7 is transformed in an alkylidene species [RuTp(CHCH2-C5H4N)(PiPr3)]BF4 (10), via the ruthenium analogue to 4 and 5, RuTp{CH2CH(OMe)-C5H4N}(PiPr3) (8), and RuTp{CHCH-C5H4N}(PiPr3) (9). In contrast to 1 and 6, the cyclopentadienyl derivative [OsCp(CH3CN)2(PiPr3)]PF6 (11) reacts with 2-vinylpyridine to give the hydride-3-osmaindolizine derivative [OsHCp(CHCH-C5H4N)(PiPr3)]PF6 (12), by C−H bond activation of the CH2 group of the substituent of the heterocycle. Treatment of 12 with NaOMe in THF gives OsCp(CHCH-C5H4N)(PiPr3) (13), which can also be prepared by reaction of OsCpCl(PiPr3)2 (14) with 2-vinylpyridine. Protonation of 13 initially gives the alkylidene cation [OsCp(CHCH2-C5H4N)(PiPr3)]+ (15), which isomerizes into the olefin species [OsCp(η2-CH2CH-C5H4N)(PiPr3)]+ (16). The latter is transformed into 12 via the hydride-3-osmaindolizine intermediate 17 containing the hydride ligand transoid disposed to the nitrogen atom.