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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 [1] and with polydentate ligands, such as oligo- a -pyridylamino ligands. [2] 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, [3] 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 ] [3] 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:

Principal publication

C. Tejel, M. A. Ciriano, B. E. Villarroya, J. A. López, F. J. Lahoz, L. A. Oro, Angew. Chem. Int. Ed. 2003, 42 , 529.

Acknowledgements

“Síntesis, estructura y reactividad de complejos mono- y poli-metálicos con posibles propiedades catalíticas o conducturas” BQU2002-00074 (IP: M. A. Ciriano)

References

[1] T. Murahashi, T Nagai, Y. Mino, E. Mochizuki, Y. Kai, H. Kurosawa, J. Am. Chem. Soc. 2001, 123, 6927.

[2] R. Clérac, F. A. Cotton, T. B. Dunbar, C. A. Murillo, X. P. Wang, J. Am. Chem. Soc. 2000, 122, 2272.

[3] C. Tejel, M. A. Ciriano, B. E. Villarroya, R. Gelpi, J. A. López, F. J. Lahoz, L. A. Oro, Angew. Chem. Int. Ed. 2001, 40, 4048

 

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