Discrete iridium pyridonate chains with variable metal valence: nature and energetics of the Ir-Ir bonding from DFT calculations.

ABSTRACT

The structure of the Ir(I) complex [Ir2(mu-OPy)2(CO)4] (Opy = 2-pyridonate) has been fully characterized in its head-to-head (A) configuration as a "dimer of dimers" AA in which two binuclear complexes are connected by means of a weak, but unsupported, iridium-iridium interaction (Ir(2)...Ir(2A) 2.9808(6) A). The head-to-tail isomer, referred to as B, was found in equilibrium with A in solution. It has been shown that this complex can be oxidized by diiodine to give iridium chains with highly selective configurations and general formula I-[Ir2(mu-OPy)2(CO)4]n-I (n = 1-3). The synthesis of IAI (1), of the isomers IAAI (2AA) and IABI (2AB), and of IABAI (3) is reported. DFT calculations have been carried out on A and B and on the known isomers of 1-3, as well as on two isomers of the hypothetic chain of eight Ir(1.25) atoms corresponding to n = 4. The stability of the metal chain is assigned to a 2-electron/2n-center sigma bond delocalized along the metal backbone and supplemented with a weak attractive interaction of the metallophilic type. Calculations confirm that further oxidation of the Ir chains corresponding to n > 1 by iodine, yielding the cleavage of one or two unsupported bond(s), is a highly exothermic process. The formation of the I-[Ir2(mu-OPy)2(CO)4]n-I chains is also computed to be exothermic, either highly for n = 1 or still significantly for n = 2 and 3. At variance with these results, the formation of an octanuclear chain is predicted to be no more than marginally exothermic (DeltaG = 1.7 kcal.mol(-1)), mainly because of interligand strain induced by the steric bulk of the amidate rings.