Mixed-Metal Cluster Chemistry. 22. Synthesis and Crystallographic, Electrochemical, and Theoretical Studies of Alkyne-Coordinated Group 6−Iridium Clusters Linked by Phenyleneethynylene Groups

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Abstract

Reaction between the tetrahedral cluster compound Mo2Ir2(CO)10(η5-C5H4Me)2 (1) and 1-iodo-4-(oct-1‘-ynyl)benzene afforded the pseudooctahedral cluster Mo2Ir2{μ4-η2-Me(CH2)5C2-4-C6H4I}(CO)8(η5-C5H4Me)2 (7). Similar reactions of 1 and W2Ir2(CO)10(η5-C5H4Me)2 (2) with di- and triynes afforded the related mono-, di-, and tricluster compounds [M2Ir2(CO)8(η5-C5H4Me)2]3{μ12-η6-Me(CH2)5C2-4-C6H4C2C6H4-4-C2(CH2)5Me} (M = Mo (12), W (14)), [W2Ir2(CO)8(η5-C5H4Me)2]2{μ8-η4-Me(CH2)5C2-4-C6H4C2C6H4-4-C⋮C(CH2)5Me} (13), Mo2Ir2{μ4-η2-Me(CH2)5C2C6H3-3,5-[C⋮C(CH2)5Me]2}(CO)8(η5-C5H4Me)2 (15), and [Mo2Ir2(CO)8(η5-C5H4Me)2]2{μ8-η4-[Me(CH2)5C2]2-1,3-C6H3-5-C⋮C(CH2)5Me} (16). Compound 13 corresponds to the 1,2-dicluster adduct of the linear triyne Me(CH2)5C⋮C-4-C6H4C⋮C-4-C6H4C⋮C(CH2)5Me. No 1,3-dicluster isomer was isolated from direct reaction, but the related molybdenum-containing 1,3-dicluster isomer was prepared by exploiting organic reaction chemistry on precoordinated functionalized alkyne ligands. Thus, Sonogashira coupling of 7 with Me3SiC⋮CH and subsequent desilylation afforded Mo2Ir2{μ4-η2-Me(CH2)5C2-4-C6H4C⋮CR}(CO)8(η5-C5H4Me)2 (R = SiMe3 (8), H (9)). Sonogashira coupling of 7 and 9 gave the 1,3-isomer [Mo2Ir2(CO)8(η5-C5H4Me)2]2{μ8-η4-Me(CH2)5C2-4-C6H4C⋮CC6H4-4-C2(CH2)5Me} (18), as well as the homocoupling product [Mo2Ir2(CO)8(η5-C5H4Me)2]2{μ8-η4-Me(CH2)5C2-4-C6H4C⋮CC⋮CC6H4-4-C2(CH2)5Me} (19); the identity of the latter was confirmed by a single-crystal X-ray diffraction study. Cyclic voltammetric scans for 12−14, 18, and 19 all show a reversible/quasi-reversible oxidation followed by an irreversible oxidation process. Compounds 18 and 19 (in which clusters are linked by long unsaturated bridges) exhibit one irreversible reduction process, whereas 12−14 (in which n cluster cores are linked by a phenylene unit) show n irreversible reduction processes. Density functional calculations indicate that oxidation and reduction both proceed with retention of the pseudooctahedral core geometry but that loss of a carbonyl ligand concomitant with two-electron reduction is energetically accessible, suggesting that this accounts for the irreversibility of the reduction step.

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