PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir⁺ Units

Several complexes of (^tBu₄PNP)Os (1) (^tBu₄PNP = C₅NH₃-2,6-(CH₂P^tBu₂)₂) are reported. 1 is isoelectronic with (^tBu₄PCP)Ir (2), which has played a leading role in homogeneous catalytic alkane dehydrogenation; the (^tBu₄PNP)Os complexes were investigated in this context. (^tBu₄PNP)OsH₄ (1-H₄) is analogous to (^tBu₄PCP)IrH₄ (2-H₄), but while 2-H₄ has some character of a dihydrogen dihydride, 1-H₄ is unambiguously a tetrahydride. Ethylene reacts with 1-H₄ to afford trans-(^tBu₄PNP)OsH₂(C₂H₄) (1-H₂(C₂H₄)). At 25 °C, 1-H₂(C₂H₄) readily undergoes reversible ethylene insertion into an Os-H bond to yield (^tBu₄PNP)OsH(C₂H₅) (1-EtH). DFT calculations indicate that alkane C-H addition to 1 is thermodynamically much more favorable than addition to 2. The favorable thermodynamics of 1-(alkyl)H, however, disfavor reductive elimination and formation of the free Os(0) fragment that is required for a catalytic cycle analogous to that reported for 2. The much greater favorability of C-H or H-H addition to 1 as compared with Ir(I) fragment 2 would typically be attributed to the lower oxidation state of 1. However, H₂ addition to the perfectly isoelectronic [(^tBu₄PNP)Ir(I)]⁺ cation is even more favorable than addition to 1; thus the thermodynamic differences result from the difference of the pincer ligand (PNP vs PCP) rather than the different metal centers (Os(0) vs Ir(I)). Although H₂ addition to Ir(I) is as favorable as addition to Os(0), addition of a second molecule of H₂ (to give tetrahydrides) is much more favorable for (^tBu₄PNP)Os. NBO analysis indicates that the MH₂/MH₄ additions are oxidative, whereas the M/MH₂ transformations are reductive.