Chiral low valent clusters: Synthesis and molecular structures of nicotinyl clusters [M3(CO)10–x(PPh3)x(μ-H)(μ-η1,κ1-(S)-nic)] (M = Os, Ru; x = 0, 1)

• The novelties of this paper are • Maiden examples of crystallographically characterized ( S )-nicotinyl clusters of [M 3 (CO) 12 ] (M = Os, Ru) • Reactivity studies of the clusters [M 3 (CO) 10 (μ-H)(μ-η 1 ,κ 1 -( S )-2-nic)] towards PPh 3 and their unambiguous characterization. The Me 3 NO-initiated reaction of [Os 3 (CO) 12 ] and ( S )-nicotine under mild conditions results in selective C–H activation of the 2-position of the pyridine ring to yield the 2-nicotinyl (2-nic) cluster [Os 3 (CO) 10 (μ-H)(μ-η 1 ,κ 1 -( S )-2-nic)] ( 1 ), while a similar reaction of [Ru 3 (CO) 12 ] and ( S )-nicotine (in the absence of Me 3 NO) affords the analogous ruthenium cluster [Ru 3 (CO) 10 (μ-H)(μ-η 1 ,κ 1 -( S )-2-nic)] ( 3 ). Both are formed as a 1:1 mixture of diastereoisomers, which can be differentiated by ¹H NMR spectroscopy but cannot be separated by TLC. Molecular structures of both 1 and 3 have been carried out and confirm the selective 2-metalation. Carbonyl substitution occurs upon addition of PPh 3 at 40°C and in the presence of Me 3 NO, affording phosphine adducts. At the triruthenium center, 2-metalation is maintained and like its parent, [Ru 3 (CO) 9 (PPh 3 )(μ-H)(μ-η 1 ,κ 1 - ( S )-2-nic)] ( 6 ) also exists as a 1:1 mixture of diastereoisomers. Unexpectedly, at the triosmium center, along with the expected [Os 3 (CO) 9 (PPh 3 )(μ-H)(μ-η 1 ,κ 1 -( S )-2-nic)] ( 4 ), the isomeric 6-metalated cluster [Os 3 (CO) 9 (PPh 3 )(μ-H)(μ-η 1 ,κ 1 -( S )-6-nic)] ( 5 ) is also formed, and the latter is crystallographically characterized. Interestingly, the isomerization of the nicotinyl ligand from the 2- to the 6-site is due to a feasibly reversible oxidative addition of the C–H bond to the triosmium center, and it occurs after PPh 3 addition. Thus, we suggest that decarbonylation and initial phosphine-adduct formation may occur at the least electron-rich Os(CO) 4 center, to produce an unstable adduct and consequently the phosphine migrates to the nitrogen-bound osmium center, leading to reductive-elimination of the C–H bond to afford a σ-bound nicotine complex. This can then undergo subsequent oxidative addition of the 2- or 6-position of the pyridyl ring, leading to the formation of metalated products 4 and 5 , respectively. This speculation has also been supported by the molecular structures of 4 and 5 , which confirm that the phosphine substitution occurs at the N-bound osmium center, lying approximately cis to the bridged Os–Os vector, whether it is the 2- or 6-metalated coordination of the nicotinyl ligand. Chiral low valent nicotinyl clusters [M 3 (CO) 10–x (PPh 3 ) x (μ-H)(μ-η 1 ,κ 1 -(S)-nic)] (M = Os, Ru; x = 0, 1)