SYNTHESIS AND CRYSTAL-STRUCTURE OF [PPH4]2[IR4(CO)10(CH2COOME)2] AND [PPH4][IR4(CO)11(CH2COOME)] - 1ST EXAMPLES OF IRIDIUM CLUSTERS BEARING AN ALKYL-LIKE LIGAND

The reaction of Na[Ir(CO)4] ([Na]1) with given amounts of BrCH2COOMe in THF, followed by treatment with PPh4Cl, gave high yields of the two novel clusters [PPh4]2[Ir4(CO)10(CH2COOMe)2] ([PPh4](2)2) and [PPh4] [Ir4(CO)11(CH2COOMe)] ([PPh4]3). The reaction takes place in two steps; the first compound formed is 2, which then reacts, in a slower reaction, with excess BrCH2COOMe to afford 3. To the best of our knowledge, 2 and 3 are among the first examples of clusters bearing an alkyl or alkyl-like substituent. X-ray diffraction analysis of the clusters was carried out on crystals obtained by using PPh4+ as a countercation. Compound [PPh4](2)2 is triclinic, space group P1BAR, with a = 11.163 (2) angstrom, b = 12.807 (2) angstrom, c = 22.849 (3) angstrom, alpha = 84.76 (1)-degrees, beta = 88.76 (1)-degrees, gamma = 73.53 (1)-degrees, Z = 2, R = 0.034, and R(w) = 0.042 for 6373 independent reflections with I > 3-sigma(I). Compound [PPh4]3 is monoclinic, space group P2(1)/c, with a = 13.155 (4) angstrom, b = 24.593 (5) angstrom, c = 38.401 (8) angstrom, beta = 91.00 (3)-degrees, Z = 12, R = 0.065, and R(w) = 0.069 for 5227 independent reflections with I > 3-sigma(I). Both anions contain a tetrahedron of iridium atoms with three bridging CO ligands around a triangular basal face of the metal tetrahedron, the alkyl-like substituents being axially bonded to the iridium atoms of this face. The reactivity of the sodium salt of [Ir(CO)4]- with BrCH2COOMe was quite different from that, already reported by us, of the same anion with PPN+ (PPN+ = (PPh3)2N+) as a countercation. This latter reaction forms only mononuclear species. The possible role of Na+ in the formation of the clusters is discussed.