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C-H Activation / Functionalization Sequences in Diiridium Complexes

E. Sola, M. V. Jiménez, Y. Yuan, F. J. Lahoz, L. A. Oro

“What one metal can do, two metals can do better”

Considering our present understanding of the structure/reactivity relationships in di- and polynuclear metal compounds, this sentence, often found in articles, reviews and monographs, should be better regarded as the formulation of a reasonable expectation rather than as a fact.

Indeed, the chemical literature contains several particular examples describing unusual, sometimes extraordinary, chemical transforma-tions which can be eventually attributed to the presence of two or more metal atoms in close proximity. However, references can also be found describing polymetallic complexes that fail to give reactions easily achievable by related mononuclear species. This indicates that the chemistry of polynuclear compounds should be expected to differ from that of mononuclear relatives, although, not surprisingly, the differen-ces cannot always result into advantages. Due to this, the practical exploitation of this chemistry seems unlikely without a previous effort to further understand the peculiar chemistry of multimetallic sites and the mechanisms of inter-metallic cooperation.

Aimed at this goal, we initiated a systematic reactivity study of simple and NMR accessible dinuclear compounds of iridium such as those shown in Chart 1. Investigation of complex 1 led to the characterization of a mechanisms for intermetallic communication based on the transmission of trans effects, [1] which effectively operates in fast and selective catalytic hydrogenation cycles. [2] In contrast, the study of complex 2 and related compounds revealed the inadequacy of such open-book dinuclear Ir(I) compounds for simple concerted activations that readily take place at parent mononuclear derivatives.

Chart 1. Dinuclear iridium complexes typically used along this investigation.

Due to our interest in C-H activations of potential use for small molecule functionaliza-tion, we explored possible strategies to over-come the inertness of these Ir(I) dinuclear compounds toward concerted C-H oxidative additions. The rich chemistry of these complexes provided several successful ways by which their activation capability could be experimentally enhanced. In particular, we did the very counter-intuitive observation that oxidation of the com-pounds favors the oxidative addition of C-H bonds (Chart 2). [3]

Chart 2: Alkene C-H activations at dinuclear complexes can be promoted by oxidation.

The rationalization of this fact and other com-plementary results in the frame of the molecular orbital description of such dinuclear Ir(I) compounds concluded that the simplest require-ment to get C-H activating compounds is the breaking of their symmetry, since such operation is enough to liberate an empty orbital from the weak, but highly deactivating, metal-metal interactions existing in these compounds. [4]

A straightforward application of this conclu-sion has involved the use of bridging ligands that can asymmetrically coordinate the diiridium moiety. As can be seen in Chart 3, the asym-metric arrangement of such ligands led to complexes promoting C-H activation / insertion sequences of ethylene.

Chart 3: Asymmetric bridging systems promote C-H activation / insertion reaction sequences with ethylene.

The generalization and further development of this strategy to achieve other reaction sequences in route to catalytic functionalization processes in under current investigation.

Principal publication

Y. Yuan, M. V. Jiménez, E. Sola, F. J. Lahoz, L. A. Oro, J. Am. Chem. Soc. 2002, 124 , 752.

Acknowledgements

“Catálisis homogénea por complejos de metales de transición” BQU2000-1170 (IP: L. A. Oro)

References

[1] E. Sola, F. Torres, M. V. Jiménez, J. A. L-opez, S. E. Ruiz, F. J. Lahoz, A. Elduque, L. A. Oro, J. Am. Chem. Soc. 2001, 123 , 11925.

[2] F. Torres, E. Sola, A. Elduque, A. P. Martínez, F. J. Lahoz, L. A. Oro, Chem. Eur. J. 2000, 6 , 2120.

[3] M. V. Jiménez, E. Sola, J. Caballero, F. J. Lahoz, L. A. Oro, C. Angew. Chem. Int. Ed. 2002, 41 , 1208.

[4] L. A. Oro, E. Sola in Recent Advances in Hydride Chemistry , M. Peruzzini, R. Poli (Eds.), Elsevier, 2001, p. 299.

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