The racemic and meso diastereomers of an electron-rich binucleating tetraphosphine ligand have been used to prepare homobimetallic rhodium norbornadiene complexes. The racemic bimetallic Rh complex is an excellent hydroformylation catalyst for 1-alkenes, giving both a high rate of reaction and high regioselectivity for linear aldehydes, whereas the meso complex is considerably slower and less selective. A mechanism involving bimetallic cooperativity between the two rhodium centers in the form of an intramolecular hydride transfer is proposed. Mono- and bimetallic model complexes in which the possibility for bimetallic cooperativity has been reduced or eliminated are very poor catalysts.
COMMUNICATIONS T = 25 "C, t = 3.5 h) also showed that complex 1 is able to perform more than 36 000 (yield 99.9 YO) without loss of activity.In order to rule out the participation in the catalysis of free, solvated [Rh(PPh,),]+ or similar cationic species formed by dissociation of complex 1, two independent experiments were made. First. the addition of four equivalents of Cs-2 did not alter the rate of reaction, which suggests that the common ion effect is absent. Second, hydrogenation experiments with added D,O provided further evidence to eliminate this hypothesis. If [Rh(PPh,),]+ or a related species were present, partially deuterated hexane would be found.["' By GC-MS analysis it was shown that the hexane formed in the reaction was not deuterium-enriched.The first e.w-monothiocarborane rhodium complex has been synthesized, and it catalyzed the hydrogenation of 1 -hexene eight times faster than Wilkinson's catalyst. The thioether group directly bonded to the cluster's open face enables discrimination between upper and lower belt B-H groups, leading to the formation of a B(l I)-H-Rh bond. This supports the initial hypothesis that the favoring of an exo-nido upper-belt form would readily enhance the hydrogenation activity of a rhodacarborane molecule.
Experimental Procedure
The X-ray structure of an unsymmetrical tetraphosphine bridged zwitterionic Rh-IRhI dinuclear tetracarbonyl complex is reported along with the facile CO addition equilibrium to form an R h o tetraphosphine pentacarbonyl bimetallic species; both of these are poor hydroformylation catalysts due to the relatively high electron density on the rhodium centres, in marked contrast to a dicationic bimetallic Rh" complex based on the same tetraphosphine ligand that is a highly active and regioselective hydroformylation catalyst.
The addition of even small amounts of PPh 3 to the dirhodium tetraphosphine hydroformylation catalyst generated from [rac-Rh 2 (nbd) 2 (et,ph-P4)](BF 4 ) 2 (nbd = norbornadiene; et,ph-P4 = Et 2 CH 2 CH 2 (Ph)PCH 2 P(Ph)CH 2 CH 2 PEt 2 ) causes a dramatic drop in the aldehyde linear-to-branched regioselectivity (25:1 to 3:1) in acetone solvent (90 °C, 6.1 bar, 1-hexene). Catalytic results are presented for differing amounts of added PPh 3 along with comparisons to the monometallic Rh catalyst family, HRh(CO) x (PPh 3 ) y (x = 1-3; y = 3 -x), generated from PPh 3 and Rh(acac)(CO) 2 (acac = acetylacetonate). The results point to the extremely effective inhibition of the regioselective bimetallic hydroformylation mechanism and the formation of an inefficient monometallic catalyst cycle, but not fragmentation to generate free HRh(CO)(PPh 3 ) 2 catalysts.
Der katalytisch aktive Komplex der homogen katalysierten, regioselektiven Hydroformylierung ist möglicherweise das zweikernige Komplex‐Dikation 1. Bemerkenswert ist die 1J(Rh,H)‐Kopplungskonstante von 164 Hz, die als typisch für zweikernige RhII‐Komplexe mit terminalen Hydridoliganden angesehen werden kann.magnified image
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