SYNTHESIS AND X-RAY-ANALYSIS OF THE TETRANUCLEAR IRIDIUM COMPOUNDS HIR4(CO)9(MU-4-ETA-3-PH2PCCPH)(MU-PPH2)2 AND IR4(CO)7(MU-CO)(MU-3-ETA-2-HCCPH)(MU-PPH2)2 AND MULTINUCLEAR NMR-STUDIES OF THE LATTER COMPOUND

Reaction of HIr4(CO)10(mu-PPh2) (1) with Ph2PC=CPh yields the CO-substituted compounds HIr4(CO)10-n(Ph2PC=CPh)n(mu-PPh2), n = 1 (2) and 2 (3). The monosubstituted species (2) undergoes facile P-C bond cleavage, followed by acetylide insertion into the Ir-H bond to give Ir4(CO)7(mu3-eta2-HC2Ph)(mu-CO)(mu-PPh2)2 (4) in high yields, and under more forcing conditions, HIr4(CO)9(mu4-eta3-Ph2PCCPh)(mu-PPh2) (6) is also obtained. Compound 4 is also formed upon thermolysis of Ir4(CO)10(PPh2H)(Ph2PC=CPh) (7), which is obtained from carbonyl substitution in Ir4(CO)11(PPh2H) (8) with Ph2PC=CPh. Crystal data for 4: space group P2(1)/n, a = 8.94(2) angstrom, b = 39.05(2) angstrom, c = 11.954(6) angstrom, beta = 104.57(8)-degrees, V = 4037.1 angstrom3, Z = 4, and final R (R(w)) value 0.069 (0.073), for 2108 unique reflections [F(c)>4(F(o))]. Compound 4 exhibits a tetrahedral metal framework, with two elongated edges bridged by the mu-PPh2 ligands and a phenylacetylene bound to the cluster in the mu3-eta2-\\ fashion. H-1, C-13{H-1), and P-31{H-1} NMR spectroscopy studies have established that compound 4 is present in solution in the form of two isomers in a 1:2 ratio, which differ with respect to the orientation of the acetylene ligand and undergo two distinct dynamic processes: at low temperature, ca. 60-degrees rocking of the acetylene with DELTAG(double dagger)192 = 9.0 kcal mol-1, and estimated DELTAG(double dagger)169 = 5.4 kcal mol-1, and interconversion of the two isomers at 82-degrees-C, with DELTAG(double dagger) = 21.8 kcal mol-1 via rotation of the acetylene by 120-degrees steps. Crystal data for 6: space group P2(1)/n, a = 11.840(3) angstrom, b = 18.745(9) angstrom, c = 18.695(8) angstrom, beta = 100.63(3)-degrees, V = 4077.9 angstrom3, Z = 4; final R(R(w)) value 0.058 (0.060) for 2609 independent reflections observed [F(c) > 4(F(o))]. Compound 6 exhibits a flat butterfly arrangement of metal atoms, with an elongated edge bridged by a PPh2 ligand and all metal atoms interacting with Ph2PCCPh, which acts as a 6-electron ligand.