Stepwise Construction of Polynuclear Complexes of Rhodium and Iridium Assisted by Benzimidazole-2-thiol. NMR and X-ray Diffraction Studies

Reactions of
[M2(μ-Cl)2(cod)2]
(cod = 1,5-cyclooctadiene, M = Rh, Ir) with benzimidazole-2-thiol
(H2Bzimt)
afford the mononuclear complexes
[MCl(H2Bzimt)(cod)] (M = Rh
(1), Ir (2)) for which a S-coordination of
the
ligand is proposed based on their spectroscopic data. The
dinuclear complexes
[M2(μ-HBzimt)2(cod)2]
(M = Rh
(3), Ir (4)) are isolated from the reaction of
[M(acac)(cod)] and benzimidazole-2-thiol. They contain
the
monodeprotonated ligand (HBzimt-) bridging the two metals
in a μ2-(1κN,2κS) coordination
mode and in a relative
cis,cis-HT arrangement. Complexes
3 and 4 react with the appropriate species
[M(cod)(Me2CO)2]+ to
afford the
trinuclear cationic aggregates
[M3(μ-HBzimt)2(cod)3]+
(M = Rh (5), Ir (6)) and with the
[M‘2(μ-OMe)2(cod)2]
compounds to give the homo- and heterotetranuclear complexes
[MM‘(μ-Bzimt)(cod)2]2 (M = M‘
= Rh (7), Ir
(8); M = Ir, M‘ = Rh (9)) containing the
dideprotonated ligand (Bzimt2-). The
trinuclear neutral complexes
[M3(μ-Bzimt)(μ-HBzimt)(cod)3]
are intermediates detected in the synthesis of the tetranuclear
complexes.
Protonation of 9 with HBF4 gives the
unsymmetrical complex
[Ir2Rh(μ-HBzimt)2(cod)3]BF4
(10). This reaction
involves the protonation of the bridging ligands followed by the
removal of one “Rh(cod)” moiety to give a
single isomer. The molecular structure of
[Rh2(μ-Bzimt)(cod)2]2
(7) has been determined by X-ray diffraction
methods. Crystals are monoclinic, space group
P21/n, a =
20.173(5) Å, b = 42.076(8) Å, c =
10.983(3) Å, β
= 93.32(2)°, Z = 8, 7145 reflections,
R = 0.0622, and Rw = 0.0779.
The complete assignment of the resonances
of the 1H NMR spectra of the complexes 3,
4, and 7−9 was carried out by
selective decoupling, NOE, and
H,H-COSY experiments. The differences in the chemical shifts of
the olefinic protons are discussed on the basis
of steric and magnetic anisotropy effects.