New Perspective on the Formation and Reactivity of Metal−Metal-Bonded Dinuclear Rhodium and Iridium Complexes
journal contribution
posted on 1997-10-14, 00:00 authored by Cristina Tejel, Miguel A. Ciriano, José A. López, Fernando J. Lahoz, Luis A. OroReactions of the binuclear complexes
[{M(μ-Pz)(CNBut)2}2]
(M = Rh (1), Ir (2); Pz =
pyrazolate) with diiodine give the metal−metal-bonded complexes
[{M(μ-Pz)(I)(CNBut)2}2]
(M = Rh (3), Ir (4)). A further reaction
with diiodine afford the complexes
[{M(μ-Pz)(I)(CNBut)2}2(μ-I)]I, which react
cleanly with CF3SO3Me replacing the
ionic iodide by the triflate
group. The M(I) complexes 1 and 2 are easily
oxidized by mild oxidants such as
[Cp2Fe]+ to
give the metal−metal-bonded dications
[{M(μ-Pz)(CNBut)2(MeCN)}2]2+,
which add halide
ions yielding the neutral complexes
[{M(μ-Pz)(X)(CNBut)2}2]
(M = Rh, X = Cl (7); M = Rh,
Ir, X = I). These reactions indicate that the metal−metal bond
formation has an electron-transfer character. Evidence of the nucleophilic attack by the
metal−metal bond in complexes
3 and 4 of small molecules such as diiodine is
provided by the reaction of [I(Py)2]BF4
with
7, which adds the electrophilic iodine(I) to give the
bridging iodide complex [{Rh(μ-Pz)(Cl)(CNBut)2}2(μ-I)]+.
In addition, an electrophile such as
MeCF3SO3 abstracts one of the
iodide
ligands trans to the metal−metal bond in 3 to
give MeI and the cationic complex [Rh2(μ-Pz)2(I)(CNBut)4]CF3SO3.
This cation adds complex 3, which behaves as a
metalloligand
through an iodide atom, to give the tetranuclear complex
[{Rh2(μ-Pz)2(I)(CNBut)4}2(μ-I)]CF3SO3 (8). The X-ray structure of 8
contains two metal−metal-bonded binuclear
‘Rh2(μ-Pz)2(I)(CNBut)4' units linked through a
slightly asymmetric bridging iodine atom, showing an
unusual Rh−I−Rh bond angle of 129.99(4)°. Reaction of
3 with IC⋮C−Ph gives the acetylide
complex
[(CNBut)2(I)Rh(μ-Pz)2(μ-I)Rh(η1-C⋮C−Ph)(CNBut)2]I.
EHMO calculations on complexes
[Rh2(μ-Pz)2(CNBut)4]2+,
[Rh2(μ-Pz)2(I)(CNBut)4]+
[A]+, and
[{Rh(μ-Pz)(I)(CNBut)2}2]
successfully explain the reactivity at the metal−metal bond, that of
the iodide ligand in 3
with electrophiles, and those of [A]+
with nucleophiles. Complex 3 and the cation
[Rh2(μ-Pz)2(I)(CNBut)4]+
react with MeI in the presence of direct sunlight to give the
oxidative-addition product
[(CNBut)2(I)Rh(μ-Pz)2(μ-I)Rh(Me)(CNBut)2]+
in high yield.
