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 Mo 2 Ir 2 (CO) 10 ( η 5 -C 5 H 4 Me) 2 ( 1 ) and 1-iodo-4-(oct-1‘-ynyl)benzene afforded the pseudooctahedral cluster Mo 2 Ir 2 { μ 4 - η 2 -Me(CH 2 ) 5 C 2 -4-C 6 H 4 I}(CO) 8 ( η 5 -C 5 H 4 Me) 2 ( 7 ). Similar reactions of 1 and W 2 Ir 2 (CO) 10 ( η 5 -C 5 H 4 Me) 2 ( 2 ) with di- and triynes afforded the related mono-, di-, and tricluster compounds [M 2 Ir 2 (CO) 8 ( η 5 -C 5 H 4 Me) 2 ] 3 { μ 12 -η 6 -Me(CH 2 ) 5 C 2 -4-C 6 H 4 C 2 C 6 H 4 -4-C 2 (CH 2 ) 5 Me} (M = Mo ( 12 ), W ( 14 )), [W 2 Ir 2 (CO) 8 ( η 5 -C 5 H 4 Me) 2 ] 2 { μ 8 - η 4 -Me(CH 2 ) 5 C 2 -4-C 6 H 4 C 2 C 6 H 4 -4-C⋮C(CH 2 ) 5 Me} ( 13 ), Mo 2 Ir 2 { μ 4 - η 2 -Me(CH 2 ) 5 C 2 C 6 H 3 -3,5-[C⋮C(CH 2 ) 5 Me] 2 }(CO) 8 ( η 5 -C 5 H 4 Me) 2 ( 15 ), and [Mo 2 Ir 2 (CO) 8 ( η 5 -C 5 H 4 Me) 2 ] 2 { μ 8 - η 4 -[Me(CH 2 ) 5 C 2 ] 2 -1,3-C 6 H 3 -5-C⋮C(CH 2 ) 5 Me} ( 16 ). Compound 13 corresponds to the 1,2-dicluster adduct of the linear triyne Me(CH 2 ) 5 C⋮C-4-C 6 H 4 C⋮C-4-C 6 H 4 C⋮C(CH 2 ) 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 SiC⋮CH and subsequent desilylation afforded Mo 2 Ir 2 { μ 4 - η 2 -Me(CH 2 ) 5 C 2 -4-C 6 H 4 C⋮CR}(CO) 8 ( η 5 -C 5 H 4 Me) 2 (R = SiMe 3 ( 8 ), H ( 9 )). Sonogashira coupling of 7 and 9 gave the 1,3-isomer [Mo 2 Ir 2 (CO) 8 ( η 5 -C 5 H 4 Me) 2 ] 2 { μ 8 - η 4 -Me(CH 2 ) 5 C 2 -4-C 6 H 4 C⋮CC 6 H 4 -4-C 2 (CH 2 ) 5 Me} ( 18 ), as well as the homocoupling product [Mo 2 Ir 2 (CO) 8 ( η 5 -C 5 H 4 Me) 2 ] 2 { μ 8 - η 4 -Me(CH 2 ) 5 C 2 -4-C 6 H 4 C⋮CC⋮CC 6 H 4 -4-C 2 (CH 2 ) 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 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.