Methylene-Bridged Complexes of Rhodium/Ruthenium as Models for Bimetallic Fischer−Tropsch Catalysts:  Comparisons with the Rh/Os and Ir/Ru Analogues

Abstract

The cationic heterobinuclear complex [RhRu(CO) 3 (µ-H) 2 (dppm) 2 ][X] (1) is prepared by protonation of the monohydride species [RhRu(CO) 3 (µ-H)(dppm) 2 ] (dppm ) µ-Ph 2 PCH 2 PPh 2 ; X -) BF 4 -, CF 3 SO 3 -). Treatment of 1 with carbon monoxide affords the tetracarbonyl complex ion [RhRu(CO) 4 (dppm) 2 ][X] (2), which yields the methylene-bridged product [RhRu(CO) 4 (µ-CH) 2 (dppm) 2 ][X] (3), upon treatment with excess diazomethane at -78 °C. Reaction of 3 with trimethylamine-N-oxide results in CO removal, affording the methylene-bridged tricarbonyl species [RhRu(CO) 3 (µ-CH) 2 (dppm) 2 ][X] (4). Although compound 3 is unreactive toward further treatment with CH 2 N 2 , compound 4 reacts readily with diazomethane, yielding a mixture of uncharacterized products. Compounds 3 and 4 react with PMe 3 to yield [RhRu(PMe 3 )(CO) 3 (µ-CH) 2 (dppm) 2 ][X] (5), in which coordination of PMe 3 at the unsaturated Rh center occurs. The structures of compounds 2, 3, and 5 have been determined by X-ray methods. Comparisons of the reactivity of compound 3 with that of the analogous Ir/Ru and Rh/Os complexes toward diazomethane are presented.

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

“…For isomer 13b , an additional 34 Hz coupling between the methyl carbon and the 31 P nucleus of PMe 3 is observed. Although three-bond 13 C− 31 P coupling is typically not this large, particularly when these groups occupy different metals, we have previously observed a number of cases in which such coupling through the metal−metal bond has been observed. , …”

20 Organometallic chemistry of bi- and poly-nuclear complexes

“…While these results, together with structural data and the comparative ease of CO displacement from the metal frameworks in these Rh/Os, Rh/Ru and related Ir/Ru systems do not clarify the precise and subtle effects which dictate the differing reaction pathways, they do clearly indicate that factors associated with the particular combination of metals employed are responsible for the reactivity. 132 Metal-stabilised carbo-cations are well-known, and are particularly useful intermediates in organic transformations. fragment which acts as an anionic 4e donor.…”

Bis(diethylphosphino)methane As a Bridging Ligand in Complexes of Ir2, Rh2, and IrRh: Geminal C–H Activation of α-Olefins

“…Interest in the reactivity of binuclear complexes containing either pairs of identical metals − or two metals that differ − has primarily been driven by the idea that these metals may act together in some way, either in a cooperative manner, ,− whereby the pair of adjacent metals give rise to reactivity that differs from that observed at single-metal sites, or in tandem, whereby one metal performs one transformation, followed by a second transformation at the other metal. − A binuclear compound studied by us, namely, [Ir 2 (CH 3 )­(CO) 2 (dppm) 2 ]­[X] (dppm = μ-Ph 2 PCH 2 PPh 2 , X = anion), has demonstrated a wealth of reactivity including C–H , and C–F ,, bond activation, in which the pair of adjacent metals play a pivotal role. In addition to the interesting reactivity displayed by this species, its simplicity and the fact that each ligand present has one or more convenient NMR-active nuclei ( 31 P, 13 C, 1 H) have often allowed the stepwise transformations to be conveniently followed by multinuclear and variable-temperature NMR studies, giving us an appreciation of the roles of the adjacent metals in the chemistry.…”