Pyrazolyl-Bridged Iridium Dimers. 8. Two-Center, Electrophilic Addition of Activated Acetylenes to Bis(cycloocta-1, 5-diene)bis(μ-pyrazolyl)diiridium(I) Leading to a Diiridacyclobutene Configuration: Regular, Parallel Coordination of Methyl Propiolate

The activated alkynes hexafluorobut-2-yne (HFB), dimethyl acetylenedicarboxylate (DMAD), and methyl propiolate (MPL) react under mild conditions with [Ir(COD)(μ-pz)]₂ (1) to afford 1:1 adducts 2–4, respectively, for which ¹H and ¹³C NMR measurements suggest “parallel” alkyne addition, generating a diiridacyclobutene configuration in which the two metal centers are formally d⁷ Ir(II). By contrast the related diiridium(I) complex [Ir(CO)(PPh₃)(μ-pz)]₂(5) forms a similar adduct (6) only with DMAD; with HFB alkyne polymerization occurs, attributed to PPh₃ dissociation from 5. Compounds 3 and 4 have been structurally characterized by using X-ray diffractometry, showing the geometry in each case to resemble that communicated earlier for 2 and substantiating the interpretation of the spectroscopic data. Crystal data for 3: space group P2₁/c, a = 11.318 (3) A, b = 11.543 (3) A, c = 42.091 (9) A, β = 98.07 (3)°. Crystal data for 4: space group P1, a = 10.983 (4) A, b = 12.585 (6) A, c = 9.862 (4) A, α = 92.96 (4)°, β= 110.29 (3)°, γ = 99.41 (4)°. The adjacent Ir(II) centers in each molecule are spin paired via Ir-Ir bond formation, with single bond distances of 2.633 (2), 2.642 (2) A (3, two independent molecules), and 2.637 (1) A (4), to accommodate which terminal COD ligands are rotated to occupy axial positions at each Ir with alkyne coordination in equatorial sites. Coordination of the unsymmetrical MPL bridging unit is regular with Ir-C_MPL distances equal at 2.094 (mean) A.