ACETONITRILE COMPLEXES OF DIIRIDIUM .1. ISOLATION AND CHARACTERIZATION OF THE PARTIALLY SOLVATED CATIONS [IR-2(COD)(MU-FORM)(2)(MECN)(3)](2+) AND [IR-2(MU-FORM)(2)(MECN)(6)](2+) (COD=1,5-CYCLOOCTADIENE, FORM=N,N'-DI-P-TOLYFORMAMIDINATE)

Two partially solvated dinuclear complexes of the Ir-2(4+) core have been prepared from reactions of Ir-2(COD)(mu-form)(2)(O2CCF3),(H2O) (1) with an alkylating reagent in acetonitrile. The compound [Ir-2(COD)(mu-form)(2)(MeCN)(3)][BF4](2) (2) is produced in essentially quantitative yields from reactions of 1 with an excess of Et(3)OBF(4) in acetonitrile at room temperature whereas the thermal substitution product [Ir-2(mu-form)(2)(MeCN)(6)][BF4](2) (3) is isolated in similar to 60% yield when the identical reaction is refluxed. Both compounds were fully characterized by IR and NMR spectroscopies, electrochemistry and X-ray crystallography. Single crystal X-ray data for 2 .(CH3)(2)CO .(C2H5)(2)O: monoclinic, P2(1)/n, a=19.534(6), b=13.285(5), c=22.113(4) Angstrom, beta=105.07(2)degrees, V=5541(5) Angstrom(3), Z=4, R=0.074, R(w)=0.107; 3: orthorhombic, Pbca, a=21.670(6), b=31.407(5), c=14.543(6) Angstrom, V=9898(9) Angstrom(3), Z=8, R=0.047, R(w) = 0.053. The molecular structure of [Ir-2(COD)(mu-form)(2)(MeCN)(3)](2+) is very similar to the parent trifluoroacetate complex and consists of an Ir-Ir unit bridged by two cis-tolylformamidinate groups, with one Ir atom ligated by a cyclooctadiene ligand in the equatorial plane and the other Ir center ligated by three MeCN molecules. The absence of an axial ligand on the Ir center bonded to the COD ligand and the short Ir-NCCH3(axial) interaction of the other Ir atom (2.02(3) Angstrom) support the formulation of the compound as a mixed-valence Ir(I)-Ir(III) species. The more symmetrical cation [Ir-2(mu-form)(2)(MeCN)(6)](2+) is comprised of cis-tolylformamidinate ligands and six acetonitrile molecules that occupy four equatorial and two axial sites. In spite of the presence of two bridging ligands, the cations in 2 and 3 are considerably distorted from an eclipsed geometry with average torsional twist angles chi of 28 and 20 degrees, respectively. The Ir-Ir distances of 2.717(2) Angstrom for (2) and 2.601(1) Angstrom for (3) are shorter than the corresponding metal-metal interaction in the parent complex Ir-2(COD)(mu-form)(2)(O2CCF3)(2)(H2O) (2.774(1) Angstrom) but are appreciably longer than the Ir-Ir bond length in the previously reported tetra-bridged molecule Ir-2(mu-form)(4) (2.524(3) Angstrom). The H-1 NMR spectral properties of the new compounds are in accord with retention of the solid state structures in solution (C-s symmetry for 2 and C-2 nu symmetry for 3). Electrochemical measurements of 2 and 3 performed in acetone at a glassy carbon electrode revealed quite different electronic properties for the two diiridium species, with 2 exhibiting behavior similar to the starting compound 1 which is in agreement with its description as a mixed-valence species. The syntheses and structures of the two new compounds in this study are discussed in light of their potential use as precursors to other metal-metal bonded Ir-2(4+) complexes including bridging carboxylate compounds and a fully solvated dinuclear cation.