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.

Date of Award	1972
Original language	English