Rhodium Chemistry, a Synthetical and Spectroscopic Study

Abstract

The reductant-catalysed synthesis of complexes of the type trans[RhL\(_4\)X\(_2\)]X.nH\(_2\)O has been used to prepare compounds with L=pyridine and substituted pyridines; X = Cl, Br, I and extended to include L = isoquinoline, pyrimidine, pyrazole and thiazole, as well as the bis­chelates with 1:10-phenanthroline and 2,2'-bipyridyl and a polymeric triamine complex with L = pyrazine. The exchange of chloride for bromide in the monoheterocyclic amine complexes is catalysed by ethanol; a five-hundred-fold rate increase is observed for the halide exchange when the L = thiazole complex is ethanol catalysed. The inhibiting effect of oxygen on the substitution reactions is related to the eventual formation of binuclear cationic complexes of the type [Rh\(_2\)L\(_8\)X\(_2\)O\(_2\)]Y\(_n\).mH\(_2\)O which have been isolated and characterised for L = pyridine, 4-methyl-pyridine, X = Cl, Y = ClO\(_4\)\(^-\), BF\(_4\)\(^-\) n = 3. They contain a bridging superoxide ligand whose relationship to the colours and magnetic properties of these and other complexes is discussed. The complexes with X = H\(_2\)O are weak acids and deprotonate to X = OH\(^-\). The µ-superoxodirhodium(III) complexes are strong one-electron oxidants in aqueous solution (E\(^o\)' ∼ +0.95 v.) and are converted to the corresponding µ-peroxodirhodium(III) complexes (n = 2) by base. These latter may be reoxidised quantitatively in acid solution, in which medium, however, they also disproportionate and hydrolyse slowly. The reactions of the trans[RhL\(_4\)X\(_2\)]X. salts (L = pyridine, X = Cl\(^-\), Br\(^-\)) with other ligands, Y, with and without added catalyst are studied. The species RhL\(_3\)XY\(_2\) (X = Cl, Y = N0\(_2\) ; X = Cl, Br, Y = NCO); [RhL\(_4\)Y\(_2\)]\(^+\) (Y = N\(_3\),I); RhL\(_3\)Y\(_3\) (Y = N0\(_2\), N\(_3\), I); [RhL\(_2\)Y\(_4\)]\(^-\) (Y = N\(_3\)) are prepared thus and characterised by analytical and spectroscopic methods. Hydrolysis of the trans[RhL\(_4\)X\(_2\)]\(^+\) ions is effected by fluoride, acetate and hydroxyl ions, and salts of the species trans[RhL\(_4\)XY]\(^n\)\(^+\) have been obtained (L = pyridine, X = Cl, Br ≠ Y = OH, OH\(_2\), Cl); the aqua-complexes are weak acids whose pK\(_a\) values have been determined. The compounds synthesised are surveyed polarographically, and comment made on the relationships between half-wave potentials, spectroscopic properties and (auto)catalytic behaviour. From the oxalato-complexes, RhL\(_3\)XOx, (L = pyridine, substituted pyridine; X = Cl, Br) the compounds RhL\(_3\)X\(_2\)Y (X = Cl, Br, Y = Cl, Br) are synthesised and characterised. Their electronic spectra and those of the other compounds are rationalised empirically. The low-frequency Raman and infrared spectra in the metal-ligand stretching region of these and several trans[ML\(_4\)X\(_2\)]\(^n\)\(^+\) species (M = Co, Rh, Pt, L = monoheterocyclic amine, X = Cl, Br) have been obtained and are discussed in terms of two simple valence force field models and rationalised on the basis of simplistic application of group theory. Aspects of the interaction of molecular oxygen with transition metal centres are presented in terms of a donor-acceptor interaction with oxygen acting as a Lewis acid. Solvates and adducts of both complex salts and uncharged complexes are prepared and examined with respect to their properties of stabilisation of unusual species and the probability of hydrogen bond formation with halo­carbons.