Tuning the Nature and Formation of Bis(dihydrogen)–Osmium Species

Abstract

The influence of chelate ligands in the formation and nature of bis(dihydrogen) units of OsH 4-complexes has been studied. The classical trihydride OsH 3 {κ 2-C,N-(C 6 H 4-py)}(P i Pr 3) 2 (1) reacts with HBF 4 •OEt 2 to give the Kubas-type dihydrogenelongated dihydrogen derivative [Os{κ 2-C,N-(C 6 H 4-py)}(η 2-H 2) 2 (P i Pr 3) 2 ]BF 4 (2), as a result of the protonation of one of the hydride ligands. Triflate (OTf) displaces the Kubas-type dihydrogen and elongates the elongated dihydrogen ligand, which is converted into a compressed dihydride. Thus, the addition of one equiv of HOTf to 1 leads to Os(H•••H){κ 2-C,N-(C 6 H 4-py)}(OTf)(P i Pr 3) 2 (3). Similarly to [OTf]-, acetone reacts with 2 to afford the related compressed dihydride [Os(H•••H){κ 2-C,N-(C 6 H 4-py)}(κ 1-OCMe 2)(P i Pr 3) 2 ]BF 4 (4), whereas acetonitrile leads to a 1:8 mixture of the monohydride [OsH(CH 3 CN) 3 (P i Pr 3) 2 ]BF 4 (5) and the trihydride [OsH 3 (CH 3 CN) 2 (P i Pr 3) 2 ]BF 4 (6). Reactions of 2 with toluene and p-xylene yield the half-sandwich derivatives [Os{κ 2-C,N-(C 6 H 4-py)}(η 6-toluene)(P i Pr 3)]BF 4 (7) and [Os{κ 2-C,N-(C 6 H 4-py)}(η 6-p-xylene)(P i Pr 3)]BF 4 (8), respectively. The acyl oxygen atom of the C,C-chelate ligand of the trihydride OsH 3 {κ 2-C,C-[C(O)CH 2 ImMe]}(P i Pr 3) 2 (10; Im = imidazolylidene) provides reliable and effective protection of the hydride ligands against the protonation. Thus, the addition of HBF 4 •OEt 2 or HOTf to 10 leads to the trihydride-hydroxycarbene cation [OsH 3 {κ 2-C,C-[C(OH)CH 2 ImMe]}(P i Pr 3) 2 ] + (11). The hydroxycarbene-NHC ligand of the latter is unstable, undergoing an intramolecular 1,3-hydrogen shift, which produces the rupture of the chelate ligand and the formation of the cis-hydride dihydrogen (12a) and trans-hydride-dihydrogen (12b) derivatives [OsH(η 2-H 2)(CO)(Me 2 Im)(P i Pr 3) 2 ] + (12). The protonation of 11 with a second equiv of HOTf affords the transient bis(Kubas-type dihydrogen) [Os(η 2-H 2) 2 {κ 2-C,C-[C(OH)CH 2 ImMe]}(P i Pr 3) 2 ](OTf) 2 , which undergoes the displacement of a coordinated hydrogen molecule by a [OTf]anion to give the Kubas-type dihydrogen [Os(OTf)(η 2-H 2){κ 2-C,C-[C(OH)CH 2 ImMe]}(P i Pr 3) 2 ]OTf (13). The addition of MeOTf to 10 leads to [OsH 3 {κ 2-C,C-[C(OMe)CH 2 ImMe]}(P i Pr 3) 2 ]OTf (14) which, in contrast to 11, is stable. Similarly to the latter, the protonation of 14 with HOTf yields the Kubas-type dihydrogen complex [Os(OTf)(η 2-H 2){κ 2-C,C-[C(OMe)CH 2 ImMe]}(P i Pr 3) 2 ]OTf (15).

Homogeneous catalysis with polyhydride complexes

“…[2][3][4][5] The chemistry of these species is quite poor, their main reaction being the dissociation of the hydrogen molecule. [6][7][8][9] However, the coordination of the hydrogen molecule can be also strong; in such case, the hydrogen-hydrogen bond length increases until a maximum of 1.3 Å. 10 The elongated dihydrogen ligand (1.0-1.3 Å) displays a noticeable acidity [11][12][13] that, in contrast to Kubas-type dihydrogen, provides to the complexes an interesting organometallic reactivity, in particularly with terminal alkynes.…”

Acceptorless Dehydrogenation of Aliphatics, Amines, and Alcohols with Homogeneous Catalytic Systems

“…Using o -xylene as the solvent and 2 mol% of Ru-3 as the catalyst, the CAD of benzofused N-heterocycles such as tetrahydroquinolines and indolines afforded the corresponding aromatic products in 58–93% yields (up to 87 TON) at 140 °C. 30c In 2017, a new osmium polyhydride Os-1 bearing an acetylacetonate ligand prepared by the Esteruelas group 30d was found to be a competent catalyst for the acceptorless dehydrogenation of cyclic amines, although the catalytic activity is relatively low: heating a solution of substrate and 10 mol% of Os-1 in p -xylene at 140 °C for 48 h afforded the corresponding N-heteroarenes in 29–100% yields (up to 20 TON).…”