A Study of the Synthesis, Structure and Potential Uses of Some Complexes of Rhodium

Abstract

The complexes of rhodium(III) halides with uni-, bi- and terdentate N-heterocyclic ligands such as 4-phenyl pyridine, 4-methy] pyridine, 2.2'-bipyridy], 4.4'-dipheny] 2.2'-bipyridyl, 4.4'-dimethy] 2.2'-bipyridy]l, 1-10-phenanthroline and 2.2'.2"-terpyridy] are synthesized and characterized by various physical techniques. The rhodium(III) complexes with uni- and terdentate ligands are non-ionic while the complexes of bidentate ligands are ionic. The far infra-red spectra of these complexes are surveyed. it is concluded from the far infra-red stuay that when the ligand is of considerable molecular complexity e.g. terpyridyl, the assignments of V(RhX) (x = Cl, Br, I) are difficult. Also when the ligand js unidentate such as 4-methyl pyridine and 4-phenyl pyridine, differentiation of cis and trans isomers, using infra-red data alone is virtually impossible. However, far infra-red spectroscopy is more informative in the case of charged complexes. Thus cationic complexes e.g. [Rh(chelate)xp7" are readily differentiated from the anionic [Rh(chelate) x, ] (chelate = 2.2'-bipyridy], 4.4'-dipheny] 2.2'-bipyridy1, 4,4'-dimethy] 2.2'-bipyridy] and 1-10-phenanthroline). It is also noted that the range of frequencies for V(RhBr) is considerable and _ that the useful relationship V(RhBr) = V(RhC1) x 0.78 does not hold because of the coupling of V(RhBr) and V{RhN) which are expected in the same spectral region for complexes of nyridine derivatives. The polymorphus of [Rh{phen),.C1,].Ct. a and e are synthesized and it is found that. the spectra did not appear to be sensitive to polymorphism. The reduction of rhodium(TII)-2.2'-bipyridyl (1:2) with sodium borohydride in methanolic medium under an atmosphere of nitrogen is studied and it is considered that the reduction proceeds via labile hydride intermediate species which can be trapped by addition of triphenyl phosphine. Addition of organic halides to this medium affords new complexes of the type [Rh(bipy)2.RX](C104). When sodium borohydride is replaced by sodium amalgum the same compounds are formed. These complexes are considered to arise from the oxidative-addition of RX to a mono 2.2'-bipyridyl rhodium(III) species. The reaction of [Rh(chelate). Xo] x (chelate = 2.2'-bipyridyl, di-2-pyridy] amine and X = Cl, Br) in alkaline ethanol and in the ‘presence of excess triphenyl phosphine is studied. The known compound HRh (PPh3)4 is isolated from this reaction. The reactions of HRh (PPh3)4 with dilute mineral acids are studied. The reduction of nitrobenzene and nitrosobenzene by sodium borohydride in the presence of Co(bipy) 4" is studied. The reaction products aniline and azobenzene are isolated and characterized. Evidence is considered to favour a hydride transfer mechanism for the reduction rather than the electron transfer process originally proposed by Vlcek.