Discrete Iridium Pyridonate Chains with Variable Metal Valence:  Nature and Energetics of the Ir−Ir Bonding from DFT Calculations

The structure of the Ir^I complex [Ir₂(μ-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) Å). 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(μ-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 σ 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(μ-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 (ΔG = 1.7 kcal·mol^-1), mainly because of interligand strain induced by the steric bulk of the amidate rings.