The known bis(ethene) dimer
[Ir(η2-C2H4)2Cl]2,
1, reacts with pyrazole in the presence of
NEt3 to form the deep purple pyrazolyl-bridged
diiridium(I) analogue
[Ir(η2-C2H4)2(μ-pz)]2
(2), which is however stable only in an atmosphere of
ethylene gas. Reaction of 2 with MeI
affords a pale orange product formulated as the
d7
2 diiridium(II) adduct
[Ir(Me)(η2-C2H4)2(μ-pz)2Ir(I)(η2-C2H4)2]
(3) on the basis of its 1H NMR spectrum, which
provides no evidence for
dissociation of 3 in solution (contrasting with the behavior
of the corresponding adduct formed
by the bis(cycloocta-1,5-diene) analogue of 2); like
2, 3 is unstable in the absence of
excess
ethylene. No congener of 2 could be obtained by similar
treatment of the perfluoroethylene
analogue
[Ir(η2-C2H4)(η2-C2F4)Cl]2
(4) of 1, although 4 was found to be
less unstable than
previously suggested and has been fully characterized in solution below
−70 °C by 19F as
well as 1H NMR measurements. Thus, in acetone (but not
in toluene or methylene chloride),
4 exists in equilibrium with two species, 4A and
4B, formulated as isomeric, mononuclear,
16e solvento complexes
[Ir(η2-C2H4)(η2-C2F4)Cl(S)],
S = acetone; 4:4A:4B (−90 °C)
ca. 67:13:20. The slow-limiting 19F NMR spectrum of
4, which has been assigned with the aid of
19F−19F COSY data, unambiguously identifies a
folded binuclear geometry that undergoes
facile ring inversion of the chloro-bridged core, with
ΔG
200
⧧ ca. 39 kJ
mol-1 estimated from
the temperature-dependent NMR behavior. Coalescence data for
signals attributed to
mononuclear 4A are consistent with “propeller rotation”
of the C2F4 ligand at d8
Ir(I), ΔG200
⧧
ca. 41 kJ mol-1. Mononuclear, 18e,
substituted cyclopentadienyl complexes
Ir(η5-C5H4R)(η2-C2H4)(η2-L)
(5, R = COCO2Et, L =
C2H4; 6, R = COOMe, L =
C2F4) can also be synthesized
by bridge-cleavage reactions of 1 and 4,
respectively.
