Low-valent, heterobinuclear complexes of rhodium and osmium. Influence of the coordinatively unsaturated rhodium center on the reactivity

Article abstract of DOI:10.1021/om00047a065

The coordinatively unsaturated complex [RhOsH(CO)₃(dppm)₂] (1) is converted smoothly to the chloro analogue [RhOsCl(CO)₃(dppm)₂] (2) in carbon tetrachloride. Compound 1 also reacts with the electrophiles HBF₄·Et₂O and [AuPPh₃][BF₄], yielding [RhOs(CO)₃(μ-H)(μ-X)(dppm)₂][BF₄] (X = H (3), AuPPh₃ (4)). The bridging AuPPh₃ group is readily replaced by an iodo group in the reaction of 4 with I₂, yielding [RhOs(CO)₃(μ-H)(μ-I)(dppm)₂][BF₄] (5). Reductive elimination of H₂ from 3, in the presence of carbon monoxide, is reversible and yields [RhOs(CO)₄(dppm)₂][BF₄] (6), which also reacts with the above electrophiles, yielding [RhOs(CO)₄(μ-X)(dppm)₂][BF₄]₂ (X = H (7), AuPPh₃ (8)). Reaction of 6 with ^tBuNC leads to substitution of the carbonyl on Os that is trans to the Rh-Os bond to give [RhOs(CO)₃-(^tBuNC)(dppm)₂][BF₄] (9). Compound 6 also reacts with the alkynes dimethyl acetylenedicarboxylate (DMAD) and hexafluoro-2-butyne (HFB) to give the alkyne-bridged products [RhOs(CO)₃(μ-CO)(μ-DMAD)(dppm)₂][BF₄] (11) and [RhOs(CO)₃(μ-HFB)(dppm)₂][BF₄] (13), respectively. Carbonyl loss from 11 occurs under mild conditions to yield [RhOs(CO)₃(μ-DMAD)(dppm)₂][BF₄] (12), analogous to compound 13. This carbonyl loss is facilitated by donation of a pair of electrons from Rh to Os with formation of a Rh→Os dative bond. Addition of anionic or neutral ligands (L) to compound 12 yields the species [RhOs(CO)₂L(μ-CO)(μ-DMAD)(dppm)₂]ⁿ⁺ (n = 0, 1, respectively), in which L is coordinated on Rh and the original Rh-bound carbonyl has moved to the bridging site. Neutral ligands such as isocyanides react with 13 to yield products analogous to those obtained with 12; however, the I^- and S₂PMe₂^- anions (X^-) react with 12 with accompanying CO loss to yield [RhOsX(CO)₂(μ-HFB)(dppm)₂], in which X is bound to Rh with both carbonyls on Os. The other alkyne-bridged dicarbonyls [RhOs(CO)₂L(μ-RC≡CR)(dppm)₂]ⁿ⁺ can also be obtained by heating the tricarbonyl precursors. The structure of compound 12 has been determined by X-ray techniques. This compound crystallizes with two CH₂Cl₂ molecules in the triclinic space group P1 with a = 14.811 (2) A?, b = 19.250 (3) A?, c = 11.611 (1) A?, α = 103.27 (1)°, β = 94.26 (1)°, γ = 87.74 (1)°, V = 3212 (2) A?³, and Z = 2. Refinement has converged at R = 0.041 and R_w = 0.060 on the basis of 9594 unique observations and 512 parameters varied. The geometry about Rh is essentially square planar, ignoring the Rh-Os interaction, whereas that about Os is a distorted octahedron when the dative Rh→Os interaction is taken into account. The Rh-Os separation of 2.8744 (3) A? is consistent with a single bond.