Early-late heterotetranuclear complexes (TiRh3) with bridging sulfide ligands:: Ligand replacement reactions and catalytic activity in hydroformylation of olefins

The tetranuclear early-late heterobimetallic (TiRh3) complexes [CpTi(mu(3)-S)(3){Rh(diolef)}(3)] (diolef = tfbb (1), cod (2)) in the presence of P-donor ligands are active precursors in the hydroformylation of hex-1-ene and styrene under mild conditions of pressure and temperature. A 96% conversion to aldehydes and 77% regioselectivity for the linear aldehyde is obtained in the hydroformylation of hex-1-ene rising PPh3 (P/Rh = 2 -4) as the P-donor ligand at 5 bar and 353 K. Hydroformylation of styrene under similar conditions (10 bar) gives 70% conversion and 88% regioselectivity in favor of 2-phenylpropanal. The reaction of [CpTi(mu(3)-S)(3)(Rh(CO)(2)}(3)] (3) with monodentate P-donor ligands results in the selective synthesis of the C3 isomer of the complexes [CPTi(mu(3)-S)(3)Rh-3(CO)(3)(PR3)(3)]. The compound [CpTi(mu(3)-S)(3)Rh-3(CO)(3)(PPh3)(3)] (5) reacts reversibly with carbon monoxide to give the 62-electron valence clusters [CPTi(mu(3)-S)(3)Rh-3(mu-CO)(CO)(4)(PPh3)(2)] (4b) and [CpTi(mu(3)-S)(3)Rh-3(mu-CO)(CO)(3)(PPh3)(3)] (4c), which are in equilibrium. High-pressure NMR spectroscopic studies have shown that both clusters are also formed under hydroformylation conditions. A multinuclear NMR spectroscopic investigation of the replacement reactions shows that [CpTi(mu(3)-S)(3)Rh-3(CO)(5)(PPh3)] (4a), 4b, and 4c are intermediary species in the replacement reactions leading to 5, the decarbonylation of 4c being the key step for the observed stereoselectivity. The reaction of 3 with diphosphines gives the complexes [CpTi(mu(3)-S)(3)Rh-3(mu-P-P)(CO)(4)] (P-P = 1,2-bis(diphenylphosphine)ethane (dppe); 1,3-bis(diphenylphosphine)propane, (dppp)), [CpTiC(mu(3)-S)(3)Rh-3(CO)(4)(eta(2)-(R)-BINAP)], and [CpTi(mu(3)-S)(3)Rh-3(mu-dppe)(CO)(2)(eta(2)-dppe)] in which the heterotetranuclear metal framework is maintained.