Influence of atomic size on mechanism: displacement of chelating ligands coordinating through sulfur from their chromium and molybdenum carbonyl complexes

Abstract

Variable-pressure kinetics studies (to 150 MPa) at various temperatures for chelate ring displacement from cfs-(S2)M(CO)4 complexes (S2 = dithiaalkane, dithiaalkene (2,5-dithiahexane, 3,6-dithiaoctane, and m-2,2,7,7-tetramethyl-3,6-dithiaoct-4-ene); M = Cr, Mo) by L (=Lewis liase (P(OR)3, R = Me, Et, -Pr, Ph)) in chlorobenzene (CB) and 1,2-dichloroethane (DCE) have been carried out. All reactions obey the rate law -d[(S2)M.(CO)4]/d< = fc[(S2)M(CO)4] [L]. However, calculated volumes ofactivation (AV*) and thermal activation parameters (AH*, AS*) indicate that ligand displacement in the Cr complexes takes place via initial, reversible Cr-S bond fission, while for Mo and S2 = 2,5-dithiahexane and 3,6-dithiaoctane an associative pathway involving significant initial L-Mo bond making is operative. For S2 = ti.s-2,2,7,7-tetramethyl-3,6-dithiaoct-4-ene and = Mo, activation parameters (the thermal data were taken over a 70 °C temperature range) are equivocal as to whether the mechanism is best described as an interchange process or in terms of competing dissociative and associative pathways. It is concluded that the mechanistic differences observed in the Cr and Mo systems result from the differing atomic sizes of these metals. The results are also discussed in terms of the properties of the chelating rings.

Thermal ring closure in Mo(CO)5L (L = bpy, dmbpy, dpbpy) transients generated by pulsed laser flash photolysis. Mechanistic information from high-pressure effects

“…A similar changeover in mechanism caused by steric hindrance was observed for ligand substitution reactions of (S2)Mo(CO)4 (S2 = 2,5dithioalkane) complexes. 25 In addition, this has also more generally been observed for ligand substitution of group VIB metal carbonyl complexes in going from the larger Wto the smaller Moand Cr-centered complexes. 21,22,26,35 Thus the sizes of the metal center and of the coordinated and entering ligands play a crucial role in determining the location of the transition state along the reaction coordinate which involves bond cleavage with the leaving and bond formation with the entering nucleophile.…”

A Kinetic Study of the Ring-Opening Process in Tungsten Carbonyl Complexes Containing Hemilabile Metallodithiolate Ligands

“…The quite negative entropy of activation determined for this reaction, which was carried out in the polar, aprotic solvent DMF, is most likely due to a solvent-assisted concerted ring-opening mechanism. Dobson and co-workers have demonstrated such a mechanism for ring-opening four-membered chelated dithioether derivatives of Mo and W in the presence of alkyl and aryl phosphites. −

7 Eyring plot for the conversion of complex 1 + CO → 2 .

…”

“…Dobson and co-workers have demonstrated such a mechanism for ring-opening fourmembered chelated dithioether derivatives of Mo and W in the presence of alkyl and aryl phosphites. [26][27][28] Upon carrying out the reaction outlined in Scheme 3 in the presence of 13 CO (slightly greater than 1 bar), an interesting observation was made. The rate of 13 CO incorporation into complex 1, as the result of the thus-formed 13 CO-labeled complex 2 undergoing CO loss, is stereoselective.…”