Influences of N-Heterocyclic Carbene and PMe₃Ligands on the Tautomerism between Methylene/Hydride and Bridging Methyl Complexes of Rh/Os and Unusual Examples of Ligand Deprotonation by the NHC Group

Abstract

The complexes [RhOsL(CO)3(dppm)2][CF3SO3] (L = IMe4 (1,3,4,5-tetramethylimidazol-2-ylidene) (6), PMe3 (8); dppm = μ-Ph2PCH2PPh2) were prepared by substitution of a carbonyl ligand in [RhOs(CO)4(dppm)2][CF3SO3]. Reaction of 6 with additional IMe4 resulted in deprotonation of a dppm ligand, yielding [RhOs(IMe4)(CO)3(μ-κ1:κ1-Ph2PCHPPh2)(dppm)] (7). Although reaction of 8 with diazomethane at −78 °C yielded the known methylene-bridged [RhOs(PMe3)(CO)3(μ-CH2)(dppm)2][CF3SO3] (3), compound 6 was unreactive toward diazomethane over a wide temperature range. The methylene-bridged species [RhOs(IMe4)(CO)2(μ-CH2)(dppm)2][CF3SO3] (9) was obtained by reaction of [RhOs(CO)3(μ-CH2)(dppm)2][CF3SO3] with IMe4, although [RhOs(CO)3(μ-CH2)(μ-κ1:η2-Ph2PCHPPh2)(dppm)] (10) was also obtained by competing dppm deprotonation by IMe4. Protonation of [RhOsL(CO)2(μ-CH2)(dppm)2][CF3SO3] (L = IMe4 (9), PMe3 (11)) with triflic acid at −78 °C yielded two isomers in each case. The more abundant isomer, [RhOsL(CO)2(μ-CH3)(dppm)2][CF3SO3]2, has a bridging agostic methyl group, while the minor isomer has a terminal, Os-bound methyl group. Upon warming, both isomers transformed to species having an Os-bound methyl group and a coordinated triflate ion, subsequently rearranging to the thermodynamic products [RhOsL(CO)2(μ-H)(μ-CH2)(dppm)2][CF3SO3]2 near ambient temperature. Attempts to prepare an IMe4-containing methyl species directly via triflate ion substitution in [RhOs(CH3)(OSO2CF3)(CO)3(dppm)2][CF3SO3] by IMe4 instead resulted in deprotonation of the methyl group to give the known product [RhOs(CO)3(μ-CH2)(dppm)2][CF3SO3]. Addition of methyl triflate to 6 gave no reaction, but protonation of 6 with triflic acid at −78 °C yielded the kinetic isomer of [RhOsH(IMe4)(CO)3(dppm)2][CF3SO3]2, in which the hydride is terminally bound to Os, and warming this product to ambient temperature resulted in rearrangement to the hydride-bridged, thermodynamic isomer.

Synthesis and Reactivity of Manganese(II) Complexes Containing N-Heterocyclic Carbene Ligands

“…The planar diamond-shaped Mn 2 C 2 core features nearly symmetric Mn–C bonds at 2.254(5) and 2.262(5) Å with internal angles of 72.79(14)° and 107.21(14)° for Mn(1)–C(29)–Mn­(1A) and C(29)–Mn(1)–C­(29A), respectively. Similar metric parameters were reported for the diamond Mn 2 C 2 motifs of other methyl-bridged Mn­(II) complexes. ,, To the best of our knowledge, 8 represents the only example of a bridging-methyl complex of any first-row transition metal containing NHC ligands …”

Methyl Complexes of the Transition Metals

“…Bridging μ‐CH 2 complexes can alternatively be formed through deprotonation of μ‐CH 3 ligands (Scheme b), as reported by the group of Cowie on a diphosphine Rh/Os complex . Other more classic elemental reactions have also been observed in the Cowie's system, such as migratory insertion of carbon monoxide (Scheme a) or olefins, reductive elimination with formation of new C−C bonds or hydrogenolysis accompanied by release of methane . The latter has also been described for a Ti/Ti system .…”

“…This offers a striking scenario still to be exploited in the search for new homogeneous catalysts for hydrocarbon functionalization. Cowie has reported this bimetallic reaction in Rh/Os 22 and Ir/Ir (Scheme 42a) 250 complexes built around diphosphines, while in a related Ir/Ru system the agostic μ-CH3 complex prevails and no α-hydrogen elimination is observed. 251 The reaction has also been described for an Os trinuclear species.…”

“…As further examples for the generation of M−CH 3 bonds, hydrogenation of Ir=CH 2 fragments and protonation of bridging CH 2 in dinuclear Os−Ru complexes (Scheme ) were recently reported to afford the corresponding methyl complexes.…”

Metal‐Metal Interactions in Bi‐, Tri‐ and Multinuclear Fe, Ru and Os N‐Heterocyclic Carbene Complexes and their Catalytic Applications

“…Cowie et. al., in the year 2011 [89] came up with heterobimetalic systems of Rh and Os involving NHC ligands with phosphine and CO bridging. The focus of this work was to study the effect of NHCs and phosphine ligands on the tautomerism in Rh/Os complexes and thus a variety of these complexes were synthesized.…”