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

Abstract

Several complexes of (tBu4PNP)Os (1) (tBu4PNP = C5NH3-2,6-(CH2PtBu2)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. © 2018 American Chemical Society.