Pentamethylcyclopentadienylrhodaborane chemistry. Part 4. The isolation, molecular structure, and nuclear magnetic resonance study of an interesting phosphino-bridged arachno-type nine-vertex rhodaborane [2,5-(µ-PPh2)-1-(PHPh2)-2-(η5-C5Me5)-2-RhB8H9]

Abstract

Reaction of PHPh₂ with [6-(η⁵-C₅Me₅)-nido-6-RhB₉H₁₃] gives an air-stable product [(η⁵-C₅Me₅)Rh(µ-PPh₂)B₈H₉(PHPh₂)] which has been characterized by single-crystal X-ray diffraction analysis together with multielement, multiple resonance, and two-dimensional COSY n.m.r. spectroscopy. Crystals (of the 1 : 2 solvate with CH₂Cl₂) are yellow, monoclinic, space group P2₁/c, with a= 947.1(2), b= 2 062.2(6), c= 2 208.9(5) pm, β= 91.58(2)°, and Z= 4; final R factor 0.0498 for 4 727 observed reflections. The cluster structure is that of a nine-vertex RhB₈arachno system (iso-B₉H₁₅ skeletal type) in which the metal atom is on one of the projecting positions on the open face, and in which the PPh₂ group bridges between the metal and a (non-adjacent) projecting boron position on the open face. In spite of this arachno formulation, the cluster retains much nido nine-vertex character, as judged by geometrical and n.m.r. considerations.