STERIC AND ELECTRONIC EFFECTS OF MONODENTATE AND TRIDENTATE PHOSPHINE-LIGANDS ON THE BASICITIES OF THE METAL IN TUNGSTEN TRIS(PHOSPHINE) TRICARBONYL COMPLEXES

Titration calorimetry has been used to determine the heats of protonation (DELTAH(HM)) Of the faC-W(CO)3(PR3)3 (PR3 = PMePh2 (1), PEtPh2 (2), PMe2Ph (3), PEt2Ph (4), PMC3 (5), PEt3 (6)) and fac-W(CO)3(L3) (L3 - PhP(CH2CH2PPh2)2 (7), MeC(CH2PPh2)3 (8)) complexes with CF3SO3H in 1,2-dichloroethane solvent at 25.0-degrees-C. The W(CO)3(PR3)3 and W(CO)3(L3) complexes undergo protonation at the tungsten with 1 equiv of CF3SO3H to form [W(H)(CO)3(PR3)3]CF3SO3(1H+-6H+) and [W(H)(CO)3(L3)]CF3SO3(7H+,8H+), respectively. For the W(CO)3(PR3)3 (1-6) complexes, the metal basicity (-DELTAH(HM)) generally increases as phosphine basicity (-DELTAH(HP)) increases; the DELTAH(HM) values range from -1 5.1 kcal mol-1 (PR3 = PMePh2) to -25.0 kcal mol-1 (PR3 = PEt3). However, the trend in the DELTAH(HM) values is also influenced by the steric bulk of the phosphine ligand. Steric crowding in the fac-W(CO)3(PR3)3 complexes is relieved when the complexes are protonated and the phosphine ligands adopt a less crowded arrangement in which they are approximately coplanar with the metal; metal basicity increases as the cone angle (theta) of the phosphine increases. DELTAH(HM) of the tridentate phosphine complex 8 (-10.5 kcal mol-1) with the facially coordinating MeC(CH2PPh2)3 ligand is 6.2 kcal mol-1 less exothermic than that of 7 (-16.7 kcal mol-1) with the flexible PhP(CH2CH2PPh2)2 ligand. The lower basicity of 8 is attributed to a destabilization of the 8H+ product, which is forced by the MeC(CH2PPh2)3 ligand to adopt a structure less favorable than that of 7H+. The DELTAH(HM) values (-18.3 and -20.1 kcal mol-1, respectively) of the Cp*Re(CO)2(PR3) (PR3 - PMe2Ph (9), PMe3 (10)) complexes have also been determined.