HYDROLYTIC OLIGOMERS OF RHODIUM(III) - A MULTINUCLEAR NMR-STUDY OF THE DOUBLY HYDROXO-BRIDGED DIMER AND TRIMER IN AQUEOUS-SOLUTION

Metastable oligomeric hydrolysis products of rhodium(III) have been chromatographically separated and studied by means of multinuclear NMR (Rh-103, O-17, and H-1). Two of these species, a dimer and a trimer, have been characterized in solution. For the dimer a centrosymmetric structure with a double hydroxo bridge was established, [(H2O)4Rh(mu-OH)2Rh(OH2)4]4+, giving a single Rh-103 NMR signal (9997.4 ppm at 298 K). O-17 NMR showed three signals resulting from oxygen in H2O coordinated cis and trans to the plane of the bridge (-122.2, -130.6 ppm) and from bridging OH (-320.1 ppm). From low-temperature studies of the hexaaquarhodium(III) ion in solutions with varying H:D ratios, the H-1 NMR chemical shift difference between H2O and HDO coordinated in the first hydration sphere of rhodium(III) was found to be 0.17 ppm. Similar studies on the dimer allowed us to differentiate H-1 NMR signals for protons at the three sites (cis H2O, trans H2O, and bridging OH) on the basis of the observation of split signals for H2O/HDO isotopic pairs. Our NMR data for the trimer are consistent with a linear bis(mu-hydroxo)-bridged structure, [(H2O)4Rh(mu-OH)2Rh(OH2)2(mu-OH)2Rh(OH2)4]5+, with two Rh-103 NMR signals at 9967.1 and 10004.3 ppm due to the terminal and central Rh sites, respectively. The two-bond spin-spin coupling constant of this species is 2J(Rh,Rh) = 1.5 Hz.