|| Discrete Mixed-Valence Iridium Chains
C. Tejel, M. A. Ciriano, B. E. Villarroya, J. A. López, F. J. Lahoz,
L. A. Oro
Compounds containing discrete chains of metal-bonded atoms
are highly interesting not only from a theoretical point of view, but also because
of their potential applications, particularly as candidates for nanoscale electronic
devices. Reproducible and well-defined syntheses for these compounds include
ligand-assisted reactions of metal compounds with conjugated polyenes 
and with polydentate ligands, such as oligo- a -pyridylamino ligands. 
In these cases, the number of available coordination sites of the ligand determines
the length of the metallic chain.
Our approach to the synthesis of discrete metal-chains of iridium,
which are mostly unknown,  is different from the above described
systems. It involves the oxidation of dinuclear complexes of d 8 square-planar
metal centers to form metal-metal bonds between two dinuclear units, which leads
to a spontaneous linear condensation. This methodology has, in principle, no
other limitations on the growing of the metallic chain length than the thermodynamic
stability of the products, provided that the steric encumbrance between the
links is kept at minimal.
We have found that the binuclear complex [Ir 2 (
m -Opy) 2 (CO) 4 ] (Opy = 2-pyridonate) is an appropriate
precursor for discrete iridium chains. Thus, oxidation of [Ir 2 (
m -Opy) 2 (CO) 4 ] with diiodine gave the unsymmetrical
tetrametallic chain (HT,HH)-[Ir 4 ( m -Opy) 4 (I)
2 (CO) 8 ]  formed by a linear stack of HT-[Ir
2 ( m -Opy) 2 (I)(CO) 4 ] and HH-[Ir 2
( m -Opy) 2 (I)(CO) 4 ] units with an fractional averaged
oxidation state (+1.50) for the iridium atoms, as shown in Chart 1.
Chart 1. Schematic representation of (HT,HH)-[Ir
4 ( m -Opy) 4 (I) 2 (CO)
8 ]. The Opy ligands are represented as N-C-O for clarity.
The complex (HT,HH)-[Ir 4 ( m -Opy) 4
(CO) 8 ] was found to evolve to (HH,HH)-[Ir 4 ( m -Opy)
4 (CO) 8 ] on heating, which shows the HH,HH configuration
systematically found for the related tetranuclear platinum chains. What is remarkable
here is the demonstration that, contrary to previous speculations, binuclear
complexes with HT configurations are able to engage in tetrametallic chains.
Moreover, with this experimental evidence, one can imagine the growing of unidimensional
metallic chains through HT linkers, since they do not show steric hindrance
along the direction of the metal-metal bond, allowing the entrance of new dinuclear
units in this axis. In fact, we were able to add a new HH link to the tetrametallic
chain (HT,HH)-[Ir 4 ( m -Opy) 4 (CO) 8 ] to
form the hexametallic compound (HH,HT,HH)-[Ir 6 ( m -Opy) 6
(I) 2 (CO) 12 ] by careful oxidation of [Ir 2
( m -Opy) 2 (CO) 4 ] with diiodine. The crystal structure
of the new complex (Figure 1) revealed the almost linear array of the metals
in which two HH-[Ir 2 ( m -Opy) 2 (I)(CO) 4
] units sandwich an HT-[Ir 2 ( m -Opy) 2 (CO)Tsub] complex.
Figure 1. The almost linear array of metal
atoms (in yellow) in the first hexanuclear iridium-chain. This 1-D oligomer
is a molecular piece of a “metal-wire” related to the infinite mixed-valence
metal chains, such as the Krogmann salts. Crystalline samples of the Ir complex
have a copper-like appearence. (Taken from the Page Cover of Angew. Chem.
Int. Ed. )
Noteworthy is also the fractional oxidation state (+1.33) for the iridium atoms in the hexanuclear compound, which provides a new link in the electronic sequence connecting dinuclear Ir(I) and Ir(II) compounds:
C. Tejel, M. A. Ciriano, B. E. Villarroya, J. A. López, F. J. Lahoz, L. A. Oro, Angew. Chem. Int. Ed. 2003, 42 , 529.
“Síntesis, estructura y reactividad de complejos mono- y poli-metálicos
con posibles propiedades catalíticas o conducturas” BQU2002-00074 (IP:
M. A. Ciriano)
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