Synthesis and Molecular Structure of [<i>cis</i>-Rh(PMePh₂)₂(NCCH₃)₂][BC₂₈H₄₀O₄]

Abstract

Arylspiroborates are a remarkable family of compounds containing two catecholato groups bound to a central boron atom that are usually nontoxic, inexpensive and thermally, chemically and electrochemically stable. 1 An attractive advantage of using these compounds is that they display relatively low toxicities towards mammals. Of particular interest to us is the ability of these species to bind to transition metals using different coordination sites. Metal complexes containing these ligands are active and selective catalyst precursors for a number of reactions, including the hydroboration of alkenes. 1 Our group has been investigating the synthesis and reactivity of novel arylspiroborates and we have recently prepared Tl[B(1,2-O2C6H2-3,5-di-tert-butyl)2]. 2 To examine whether this anion could coordinate to metal centers we added one equivalent of the thallium salt to an acetonitrile solution containing the in situ mixture of [Rh(m-Cl)(coe)2]2 and 4PMePh2 (coe = cyclooctene).We found that this reaction leads to a mixture of products including the cationic species [cis-Rh(PMePh2)2(NCCH3)2] + and anionic [BC28H40O4] -(1), shown in Fig. 1, where the bulky arylspiroborate ligand does not appear to bind to the metal centre. Yellow, parallelepiped crystals of 1 precipitated and were collected by suction filtration in 23% yield. The 13 P{ 1 H} NMR data showed a doublet at d 29.7 ppm with a coupling constant of JPRh = 171 Hz and the 11 B{ 1 H} NMR spectrum remains unchanged with a sharp singlet at d 13.9 ppm, suggesting that coordination of the arylspiroborate ester to the metal center did not occur. A single crystal X-ray diffraction study was performed to ascertain the configuration of ligands bound to the metal center.Single-crystal X-ray diffraction measurements were performed and a hemisphere of data was collected on a Bruker AXS P4/SMART 1000 diffractometer using w and q scans with a scan width of 0.3˚ and 10 s exposure times. The detector distance was 5 cm. The data were reduced (SAINT) 3 and corrected for absorption (SADABS). 4 The structure was solved by direct methods and refined by full-matrix least squares on The rhodium compound, [cis-Rh (PMePh2)2(NCCH3)2][BC28H40O4], was isolated and the molecular structure was determined by a single crystal X-ray diffraction study at 173 K. The compound crystallizes in a triclinic system, space group P1 and Z = 2 with cell parameters of a = 13.870 (3)