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Cooperative Bimetallic Effects on New Iridium(III) Pyrazolate Complexes: Hydrogen−Hydrogen, Carbon−Hydrogen, and Carbon−Chlorine Bond Activations
journal contribution
posted on 1998-01-30, 00:00 authored by Eduardo Sola, Vladimir I. Bakhmutov, Francisco Torres, Anabel Elduque, Jose A. López, Fernando J. Lahoz, Helmut Werner, Luis A. OroThe reaction of
fac-[IrH2(NCCH3)3(PiPr3)]BF4
(1) with potassium pyrazolate gave the
binuclear 34-electron complex
[Ir2(μ-H)(μ-Pz)2H3(NCCH3)(PiPr3)2]
(2). The structure of 2 was
determined by X-ray diffraction. An electrostatic potential
calculation located three terminal
hydride ligands and one hydride bridging both iridium centers. The
feasibility of this
arrangement was studied by EHMO calculations. The spectroscopic
data for 2 show that
the complex is rigid in solution on the NMR time scale. In
solution, the acetonitrile ligand
of 2 dissociates. The activation parameters for this
dissociation process in toluene-d8
are
ΔH⧧ = 20.9 ± 0.6 kcal
mol-1 and ΔS⧧ =
2.5 ± 1.3 e.u. Reaction of 2 with various
Lewis
bases (L) gives the substitution products
[Ir2(μ-H)(μ-Pz)2H3(L)(PiPr3)2]
(L = C2H4 (3), CO
(4), HPz (5)). The reaction of complex
5 with C2H4 yields the ethyl
derivative [Ir2(μ-H)(μ-Pz)2(C2H5)H2(HPz)(PiPr3)2]
(6); this reaction is reversible. Complexes
2 and 3 react with
CHCl3 to give CH2Cl2 and the
compounds
[Ir2(μ-H)(μ-Pz)2H2(Cl)(L)(PiPr3)2]
(L = NCCH3 (7),
C2H4 (8)). In the
1H NMR spectra of 2−6,
the signal of the bridging hydride ligand shows
two very different JHP couplings; in contrast,
for the chloride complexes 7 and 8, two
equal
JHP couplings are observed. NOE and
T1 measurements lead to the conclusion that
in
complexes 2−6 the hydride bridges
the iridium centers in a nonsymmetric fashion, whereas
for 7 and 8 the bridge is symmetrical. This
structural feature largely influences the
reactivity. Compounds 2 and 3 undergo H/D
exchange under a D2 atmosphere. Analysis
of
the isotopomeric mixtures of 2 reveals downfield isotopic
shifts in the 31P{1H} NMR
spectrum.
Downfield as well as high-field shifts are found for the hydride
signals in the 1H NMR
spectrum of partially deuterated 2. Further reaction of
3 with H2 gave ethane and the
dihydrogen complex
[Ir2(μ-H)(μ-Pz)2H3(η2-H2)(PiPr3)2]
(9). Under a deficiency of H2, in
toluene-d8 solution, 9 undergoes H/D
scrambling with the participation of the solvent. It has
also
been found that under H2 complex 3 catalyzes the
hydrogenation of cyclohexene.