Acceptor Pincer Chemistry of Osmium: Catalytic Alkane Dehydrogenation by (^CF₃PCP)Os(cod)(H)

Abstract

Syntheses of osmium analogues of acceptor pincer ( CF 3 PCP)Ru-(II) systems are reported. Treatment of [Et 4 N] 2 OsCl 6 with CF 3 PCPH at 130 °C in ethanol in the presence of Et 3 N gave the coordinatively saturated anionic carbonyl complex HNEt 3 + [( CF 3 PCP)Os(CO)Cl 2 ] − , which subsequently may be converted to cis-( CF 3 PCP)Os(CO) 2 Cl or cis-( CF 3 PCP)Os(CO) 2 H by reaction with Me 3 SiOTf or (Et 3 Si) 2 (μ-H) + B(C 6 F 5 ) 4 −, respectively. ( CF 3 PCP)Os(cod)H was obtained in modest yields by thermolysis of Os(cod)(η 3 -2-methylallyl) 2 with CF 3 PCPH in neat cod under 3 atm of H 2 at 130 °C. The alkane dehydrogenation activity of ( CF 3 PCP)Os(cod)H was examined: under identical conditions to previously studied ( CF 3 PCP)Ru(cod)H (1:1 cyclooctane/tert-butylethylene, 200 °C), the initial turnover rate for cyclooctene production was 1520 h −1 , 75% the rate observed for the ruthenium analogue, but with significantly enhanced catalyst lifetime. Acceptorless cyclodecane dehydrogenation under reflux conditions gave 125 turnovers of cyclodecenes in one hour. Spectroscopic evidence on the nature of the catalyst resting state and catalyst thermal and air stability is presented.

Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

“…At 200 C, the initial rate of the reaction (ca. 10 min) catalyzed by 69 (1.38 mM, 1,520 TO h À1 ) was comparable to that of ( tBu4 PCP)IrH 4 (11-H 2 ) [59] and about 75% that of Ru complex 67 [105][106][107]. The osmium pincer catalyst, however, has a significantly longer lifetime than its ruthenium analogue.…”

“…Following their success with pincer-ruthenium-catalyzed alkane dehydrogenation [105,107], Roddick and coworkers investigated transfer and acceptorless dehydrogenation of alkanes catalyzed by ( CF3 PCP)Os(COD)H (69) [106]. In contrast with ( CF3 PCP)Ru(COD)H (67), the Os analogue shows no activity at 150 C for the transfer dehydrogenation of COA with TBE (1:1; 3.9 M).…”

“…The osmium pincer catalyst, however, has a significantly longer lifetime than its ruthenium analogue. Thus use of 69 resulted in 610 turnovers after 8 h, whereas 67 was inactive after 20 min when a maximum of 350 TOs was obtained [106]. Control experiments to determine the stability of 69 at 200 C indicated that the decrease in activity over time resulted from catalyst decomposition to ( CF3 PCP)Os(COD)X (70) where X is likely to be CH 2 CH 2 t Bu [106].…”

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product

“…In addition to simple alkane dehydrogenation, these complexes have been employed for numerous other catalytic transformations of hydrocarbons, including alkane metathesis, 6,8,9,20,[24][25][26] alkyl group metathesis, 27 dehydroaromatization, 19,28,29 alkanealkene coupling reactions, [30][31][32] borylation of alkanes 23 and the dehydrogenation of several non-alkane substrates. 22,33,34 Several pincer motifs more recently explored, such as (CCC)Ir, [35][36][37][38] (PCP)Ru 39-41 , (PCP)Os 42 , and (NCN)Ir 43,44 have been found to show promise for alkane dehydrogenation but as of yet none have proven to be competitive with the well investigated PCP-type iridium-based systems. 26 In early alkane dehydrogenation studies 45 3,3-dimethyl-1-butene (TBE) was found by Crabtree to be a singularly effective hydrogen acceptor.…”

Catalytic alkane dehydrogenations

“…Following their investigation on the {(CF 3 ) 4 PCP}Ru(II) complexes in alkane dehydrogenation, Roddick and coworkers [36] reported the osmium analogue 8 containing the same ligand for catalytic transfer dehydrogenation of COA with TBE (Eq. (5)).…”