||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,  which effectively operates in fast and selective
catalytic hydrogenation cycles.  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). 
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. 
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.
Y. Yuan, M. V. Jiménez, E. Sola, F. J. Lahoz, L. A. Oro, J. Am. Chem. Soc. 2002, 124 , 752.
“Catálisis homogénea por complejos de metales de transición”
BQU2000-1170 (IP: L. A. Oro)
 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
 F. Torres, E. Sola, A. Elduque, A. P. Martínez, F. J. Lahoz, L. A.
Oro, Chem. Eur. J. 2000, 6 , 2120.
 M. V. Jiménez, E. Sola, J. Caballero, F. J. Lahoz, L. A. Oro, C.
Angew. Chem. Int. Ed. 2002, 41 , 1208.
 L. A. Oro, E. Sola in Recent Advances in Hydride Chemistry , M.
Peruzzini, R. Poli (Eds.), Elsevier, 2001, p. 299.