DIENE DERIVATIVES OF HEXARHODIUM HEXADECACARBONYL - MOLECULAR-STRUCTURES OF RH6(CO)14(ETA-4-CH2C(ME)C(ME)CH2) AND RH6(CO)12(ETA-4-CH2C(ME)C(ME)CH2)2, AND THEIR SUBSTITUTION-REACTIONS WITH BIS(DIPHENYLPHOSPHINO)METHANE

Reaction of Rh4(CO)12 with 2,3-dimethyl-1,3-butadiene (DMBD) in refluxing hexane gives Rh6(CO)14(η4-CH2C(Me)C(Me)CH2) (1) and Rh6(CO)12(Co)12(η4-CH2C(Me)C(Me)Ch2)2 (2), both of which have been characterized by X-ray crystallography. Complex 1 crystallizes in the orthorhombic space group P212121 with a 14.430(3), b 16.970(3), c 10.908(2) Å, and Z = 4. Refinement converged to R = 0.0425 and Rw = 0.0409 based on 2511 unique observations (NO) and 362 parameters varied (NV). The molecule consists of an octahedral cluster of rhodium atoms, bearing ten terminal and four face bridging carbonyl groups and a DMBD liband coordinated to one Rh atom in η4-s-cis form. The crystal data for 2 are as follows: monoclinic space group Cc, a 17.636(2), b 9.561(1), c 18.234(4) Å, β 97.24(1)°, Z = 4, R = 0.0286, Rw = 0.0317, NO = 2929, and NV = 380. Complex 2 has a similar structure as 1, and the two DMBD ligands are coordinated to two rhodium apices which are mutually trans to each other in the octahedral skeleton. Although the coordination of the DMBD ligands in 2 is rigid at room temperature, their rotation is observed at elevated temperatures, (activation energy Ea 9.7 kcal mol-1). Reactions of 1 and 2 with bis(diphenylphosphino)methane (DPPM) take place rapidly at room temperature to give Rh6(CO)14(DPPM) and Rh6(CO)12(DPPM)2 respectively. These reactions are much faster than the similar reaction of Rh6(CO)16. The reaction of 1 with DPPM has been found to be two to three times faster than that of 2, indicating that ligands attached to a metal apex can affect the rate of substitution occurring even at the remotest apex of the octahedron. © 1990.