Fluxional molecular A-frames containing bridging acetylide ligands. Synthesis of [Rh2(.mu.-.eta.2-C2R)(CO)2(.mu.-Ph2PCH2PPh2)2]ClO4 (R = H, Ph, CMe3) and their reactions with nucleophiles

Reactivity of the Heterobinuclear Phenylacetylide-Bridged A-Frame [RhIr(CO)2(μ-CCPh)(Ph2PCH2PPh2)2][O3SCF3] with Small Molecules

“…In all subsequent chemistry, the chloride ion is replaced by triflate in order to reduce the possibility of anion coordination. Compound 1 is labile and readily loses CO under reflux in CH 2 Cl 2 /THF to form [RhIr(CO) 2 (μ-C 2 Ph)(dppm) 2 ][O 3 SCF 3 ] ( 2 ), a mixed-metal analogue of Grundy's alkynyl-bridged compounds, [Rh 2 (CO) 2 (μ-C 2 R)(dppm) 2 ][ClO 4 ] (R = H, Ph, t Bu). Compound 2 is another member of a series of A-frame complexes containing an anionic ligand at the bridgehead position and terminal carbonyl groups .…”

“…It has been shown that the dppm-bridged dirhodium alkynyl-bridged species, , along with the related species [Rh 2 (μ-O 2 CCH 3 )(μ-η 1 :η 2 -CCR)(CO) 2 (PCy 3 ) 2 ],8b [Rh 2 (μ-O 2 CCH 3 )(μ-η 1 :η 2 -CCPh)(COD) 2 ],8b and [NBu 4 ] 2 [(C 6 F 5 ) 2 Pt(μ-η 1 :η 2 -CCPh)(μ-η 2 :η 1 -CCPh)Pt(C 6 F 5 ) 2 ], is fluxional, having the alkynyl group moving from one metal to the other in a “windshield-wiper” fashion. In contrast, the heterobinuclear species 2 is static, as was also reported for [RhPtCl(μ-CCCH 3 )(CO)(dppm) 2 ][PF 6 ] …”

“…The reaction of 2 with phosphines is in contrast to the reaction involving the dirhodium analogue [Rh 2 (CO) 2 (μ-C 2 H)(dppm) 2 ][ClO 4 ], which yielded phosphonium vinylidenes, in which the PR 3 group coordinated to the alkynyl β-carbon. This is certainly not the case for compounds 3 and 4 ; for compound 4 the 13 C chemical shifts for C α and C β of the phenylacetylide group are inconsistent with those expected for a vinylidene (see compounds 7a , b for example), and neither signal displays significant coupling to the high-field phosphorus nucleus, as would be expected for a phosphonium vinylidene.…”

Diverse Reactivity of Alkynes with the Binuclear Methyl and Incipient Methyl Complexes [RhIr(CH3)(CO)3(Ph2PCH2PPh2)2][CF3SO3] and [Ir2H(CO)3(μ-CH2)(Ph2PCH2PPh2)2][CF3SO3]

“…We propose that the facility of this transformation, at least in related dppm-bridged complexes, results from the abilities of the acetylide and hydride ligands in the intermediates to function as either terminal or bridging groups and from the mobilities of both of these groups over the bimetallic core. The hydride ligand can readily migrate from one metal to the other and from one face of an M 2 P 4 unit to the other, , and the acetylide group can also move from metal to metal in a “windshield wiper” motion . It is proposed that a pivotal step in the conversion of the acetylide and hydride groups to a vinylidene group, in the presence of adjacent metals, is the alignment of the hydride ligand adjacent to the π-bound acetylide moiety, as shown for intermediate IV , since such an orientation can facilitate transfer of the hydride to the β-carbon atom in a concerted manner, as diagrammed.…”

“…The actual geometry at Ir in 13 is not known; however the transformations of 1 (+PhC⋮CH) to 13 and of 13 to 14 are reasonable when it is recalled that movement of a hydride ligand from one face of an MM‘P 4 unit to the other is a facile process 44,50 which, in combination with the known ability of bridging acetylides to undergo a windshield wiper movement from one metal to the other, could easily give rise to such rearrangements. We had considered that reductive elimination might be occurring from Rh since this process might be expected to be more facile from a second- rather than a third-row metal, due to the stronger M−H and M−CH 3 bonds with the latter and due to the greater kinetic lability of the second-row metal .…”