Synthesis, structure and reactivity of a novel bis(phosphite) tetrarhodium decacarbonyl hydroformylation catalyst precursor

The tetranuclear Rh₄(μ-bpnap)(CO)₁₀ (1) was synthesized in 68% yield by the reaction of Rh₂(CO)₄Cl₂ with bpnap {2,2′-bis[(1,1′-biphenyl-2,2′-diyl)phosphite]-1,1′- binaphthyl} (2) under CO. The crystal structure of 1, determined by X-ray diffraction, established that the bpnap ligand bridged two rhodium atoms of a tetrahedral metal framework. Recrystallization of 1 from CH₂Cl₂/THF led to its partial conversion to a structural isomer, 1a, which was characterized by solution spectroscopic methods. The reaction of Rh(acac)(CO)₂ and bpnap under 1:1 CO/H₂ led to a mixture of products consisting of 1, 1a and at least two hydridic compounds. The latter were also formed by the hydrogenation of Rh(acac)(CO)₂ and bpnap. Rh₄(μ-bpnap)(CO)₁₀ was shown to be an effective catalyst precursor for the isomerization and hydroformylation of 1-octene, giving, for the latter reaction, moderate linear/branched selectivity for the product aldehydes. The hydroformylation of a-methylstyrene with Rh(acac)(CO)₂ and R-(+)-bpnap gave essentially no enantiomeric excess of 3-phenylbutanal. The structural features of 1, along with comparison of its reactivity with other bis(phosphite) metal complexes are discussed. Crystal data for Rh₄(bpnap)(CO)₁₀ (1) (f.w. 1577.48): monoclinic, P2₁/c ; a = 14.624(4), b = 18.974(6), c = 21.343(5) Å; V = 5899(5) ų; β = 95.06(2)°, Z = 4; R = 6.4%.