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

The structure of the IrI 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 Ir1.25 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.

双核铱配合物[Ir₂(μ-OPy)₂(CO)₄]（其中OPy为2-吡啶酮盐（2-pyridonate））的头对头（head-to-head）构型（记为A）已被完全表征，该结构属于“二聚体之二聚体”AA型，其中两个双核配合物通过弱但无支撑的铱-铱相互作用（Ir(2)···Ir(2A)键长为2.9808(6) Å）相连。被记为B的头-尾（head-to-tail）异构体在溶液中可与构型A形成平衡。研究表明，该配合物可被二碘单质氧化，得到具有高度选择性构型、通式为I−[Ir₂(μ-OPy)₂(CO)₄]ₙ−I（n=1~3）的铱链。本文报道了IAI（化合物1）、IAAI与IABI两种异构体（分别为2AA和2AB）以及IABAI（化合物3）的合成方法。密度泛函理论（DFT）计算已针对构型A、B，1~3的已知异构体，以及对应n=4的假想含8个Ir₁.₂₅原子的链的两种异构体展开。该金属链的稳定性可归因于沿金属主链离域的2电子/2n中心σ键，辅以弱的亲金属型吸引相互作用。计算结果证实，对于n>1的铱链，被碘进一步氧化并断裂一根或两根无支撑键的过程为强放热过程。计算同时表明，I−[Ir₂(μ-OPy)₂(CO)₄]ₙ−I型铱链的生成反应同样为放热反应：当n=1时放热量极高，n=2和3时放热量仍较为显著。与上述结果不同，八核铱链的生成反应被预测仅为微弱放热（吉布斯自由能变ΔG=1.7 kcal·mol⁻¹），这主要源于酰胺基环的空间位阻引发的配体间张力。