Triphenylcyanoborate Complexes of Rhodium. A Nitrogen-15 NMR Study of [Rh(NCBPh₃)(PPh₃)₃] and [Rh(CNBPh₃)(PPh₃)₃] and Their Derivatives

Abstract

The isomeric compounds [Rh(NCBPh3)(PPh3)3] (1) (prepared from [RhCl(PPh3)3] and K[NCBPh3]) and [Rh(CNBPh3)(PPh3)3] (2) (prepared from [Rh(CN)(PPh3)3] and BPh3) undergo ligand exchange with pyridine (py) to give cis-[Rh(NCBPh3)(PPh3)2(py)] and trans-[Rh(CNBPh3)(PPh3)2(py)] and with L = ethylene (reversibly), carbon monoxide, and 2,6-xylyl isocyanide (XNC) to give trans-[Rh(X)(PPh3)2(L)] (X = NCBPh3, CNBPh3) and oxidative addition with hydrogen (reversibly) and triphenyltin hydride to give [Rh(X)(H)2(PPh3)3] and [Rh(X)(H)(SnPh3)(PPh3)2], respectively; triphenylsilane combines with 1 (reversibly) but not with 2. By heating 1 to 100 °C in an inert solvent, the rearrangement of 1 to 2 is observed. Complex 1 in DMSO combines with oxygen to yield a product formulated as a sulfonium salt, although in CH2Cl2 it is unreactive toward oxygen unlike 2 which forms an O2 adduct. In a solution of toluene containing pyridine or substituted pyridines in the presence of oxygen, 1 forms cis-[Rh(NCBPh3)(O2)(PPh3)2(py)] while 2 loses BPh3 to give [Rh(CN)(O2)(PPh3)2(py)]. Hydrogen is bound reversibly by trans-[Rh(CNBPh3)(PPh3)2(py)] and irreversibly by cis-[Rh(NCBPh3)(PPh3)2(py)]. A comparison of the properties of 1 and 2 and their derivatives shows the N-bonded isomers to be the more electrophilic and the N−B bond to be significantly more labile than the C−B bond. Nitrogen-15 NMR data are reported for complexes enriched to 99% in 15N.