Conceptual Extension of the Degradation–Transformation of N-Heterocyclic Carbenes: Unusual Rearrangements on Osmium

Abstract

The range of processes of degradation−transformation of NHC ligands in the coordination sphere of a transition metal has been enlarged. The NHC-acyl ligand of the complex Os{κ 2-C,C-[C(O)CH 2 ImMe]}Cl(P i Pr 3) 2 (1) undergoes a complex rearrangement promoted by internal alkynes to give Os{κ 2-C,N-[CH 2 ImMe]}Cl(CO)(P i Pr 3) 2 (2). Mechanistic studies have revealed that the degradation involves a catalytic alkyne-mediated deinsertion of CO from the acyl moiety to afford Os{κ 2-C,C-[CH 2 ImMe]}Cl(CO)(P i Pr 3) 2 (3), followed by a thermally activated stoichiometric 1,2-methylene shift from N to C. The catalytic activity of the alkynes depends upon their substituents, decreasing in the sequence diphenylacetylene > 1-phenyl-1-propyne > 3-hexyne > 2-butyne. Phenylacetylene tautomerizes in the metal coordination sphere to afford the stable vinylidene Os{κ 2-C,C-[C(O)CH 2 ImMe]}Cl(CCHPh)(P i Pr 3) 2 (4), which experiences the coupling of the acyl moiety and the vinylidene ligand under a carbon monoxide atmosphere. The addition of HBF 4 •OEt 2 to the resulting complex Os{κ 2-C,C-[C(CHPh)C(O)CH 2 ImMe]}Cl-(CO) 2 (P i Pr 3) (5) leads to [Os{κ 2-O,C-[OC(CHCHPh)CH 2 ImMe]}Cl(CO) 2 (P i Pr 3)]BF 4 (6) containing an NHC-(α,βunsaturated ketone) ligand.

Reaction Mechanisms on Unusual 1,2‐Migrations of N‐Heterocyclic Carbene‐Ligated Transition Metal Complexes

“…As shown in Figure , there are three possible transition states for the 1,2‐carbon shift reaction. For the overall reaction process, metal carbene intermediates ( B ) have been proposed as previously reported . Consistent with these results, we successfully found the related transition states with ruthenium, osmium, and rhenium as the metal centers.…”

“…However, the reasons for the higher stability of the C‐binding imidazoles over the N ‐binding species are not fully understood. Very recently, Song group and Esteruelas group found that NHC‐ligated transition metal complexes can cause 1,2‐carbon shifts through an unusual four‐membered ring intermediate involved in intramolecular migration processes. With our continuing interest in aromaticity, aromaticity‐promoted migration reactions, the C−F and dinitrogen activation, here we carry out thorough density functional theory (DFT) calculations to investigate the origin for the 1,2‐group migration reactions of NHC‐ligated organometallics (Scheme b).…”

Silyl-Osmium(IV)-Trihydride Complexes Stabilized by a Pincer Ether-Diphosphine: Formation and Reactions with Alkynes

“…The metal-alkynyl bond lengths Os–C(16) and Os–C(31) of 2.068(3) Å are consistent with an Os–C(sp) single bond 33 and suggest a low degree of osmium-to-ligand back-bonding. 34 As in other osmium-vinylidene compounds, 35 the vinylidene ligand binds to the osmium atom in a nearly linear fashion establishing an angle Os–C(1)–C(2) of 175.2(3)°. Distances Os–C(1) and C(1)–C(2) of 1.820(3) and 1.316(4) Å, respectively, also compare well with those previously reported for complexes of this class and strongly support the presence of double bonds between the involved atoms.…”