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...
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