Mixed-metal cluster chemistry. 22. Synthesis and crystallographic, electrochemical, and theoretical studies of alkyne-coordinated group 6-iridium clusters linked by phenyleneethynylene groups

Reaction between the tetrahedral cluster compound Mo2Ir2(CO)(10)(eta(5)-C5H4Me)(2) (1) and 1-iodo-4-(oct-1'-ynyl)benzene afforded the pseudooctahedral cluster Mo2Ir2{mu(4)-eta(2)-Me(CH2)(5)C-2-4-C6H4I} (CO)(8)(eta(5)-C5H4Me)(2) (7). Similar reactions of 1 and W2Ir2(CO)(10)(eta(5)-C5H4Me)(2) (2) with di- and triynes afforded the related mono-, di-, and tricluster compounds [M2Ir2(CO)(8)(eta(5)-C5H4Me)(2)](3){mu(12)-eta(6)-Me(CH2)(5)C-2-4-C6H4C2C6H4-4-C-2(CH2)(5)Me} (M = Mo (12), W (14)), [W2Ir2-(CO)(8)(eta(5)-C5H4Me)(2)](2){mu(8)-eta(4)-Me(CH2)(5)C-2-4-C6H4C2C6H4-4-CdropC(CH2)(5)Me} (13), Mo2Ir2{mu(4)-eta(2)-Me(CH2)(5)C2H6H3Me)(2)](2){mu(8)-eta(4)-Me(CH2)(5)C2C6H3-3,5-[Cdrop C(CH2)(5)Me](2)}(CO)(8)(eta(5)-C5H4Me)(2) (15), and [Mo2Ir2(CO)(8)(eta(5)-C5H4Me)(2)](2)-{mu(8)-eta(4)-[Me(CH2)(5)C-2](2)-1,3-C6H3-5-CdropC(CH2)(5)Me} (16). Compound 13 corresponds to the 1,2-dicluster adduct of the linear triyne Me(CH2)(5)CdropC-4-C(6)H(4)CdropC-4-C(6)H(4)CdropC(CH2)(5)Me. 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 Me(3)SiCdropCH and subsequent desilylation afforded Mo2Ir2{mu(4)-eta(2)-Me(CH2)(5)C-2-4-C(6)H(4)CdropCR}(CO)(8)(eta(5)-C5H4Me)(2) (R = SiMe3 (8), H (9)). Sonogashira coupling of 7 and 9 gave the 1,3-isomer {Mo2Ir2(CO)(8)-(eta(5)-C5H4Me2](2) (eta(5)-C5H4Me)(2)](2) Me(CH2)(5)C-2-4-C(6)H(4)CdropCC(6)H(4)-4-C-2(CH2)(5)Me} (18), as well as the hompcoupling product [Mo2Ir2(CO)(8)(eta(5)-C5H4Me)(2)](2){mu(8)-eta(4)-Me(CH2)(5) -C2H-C(6)H(4)CdropCC(6)H(4)-4C(2)(CH2)(5)Me} (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 revers 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.