PHOTOSUBSTITUTION OF IRON CARBONYL PHOSPHINE COMPLEXES - QUANTUM YIELD, KINETIC, AND THERMOCHEMICAL STUDIES

The quantum yields and thermochemistry of photosubstitution of Fe(CO)4 PR3 and Fe(CO)3(PR3)2 (R = alkyl) in cyclohexane at 337 nm have been d etermined for the first time. In addition, time-resolved experiments p rovided the kinetics of the second step of the substitution, phosphine reaction with the intermediate formed upon CO dissociation. Several i nteresting results, with implications for understanding the interactio n of probe molecules with potentially reactive metals centers, were no ted. First, the quantum yields of photosubstitution of Fe(CO)4PR3 (R = Me, Et, and n-Bu, 1a-c, respectively) with PR3 are 0.56, 0.54, and 0. 62 (R is the same for coordinated and dispersed ligand). Second, analy sis of the products after photolysis of 1 with dispersed (CH3)3CNC or CO indicates that coordinated CO substitutes but not PR3. Third, the p hotolysis of Fe(CO)3(PMe3)2 (2) with PMe3 results in the substitution of CO (PHI = 0.50), while photolysis with CO results in the substituti on Of PMe3 (PHI = 0.03). On the other hand, the photolysis of Fe(CO)3( dmpe) (3, dmpe = 1,2-bis(dimethylphosphino)ethane) with PMe3 results i n the substitution of only CO (PHI = 0.47). Fourth, the enthalpies of CO dissociation in cyclohexane (DELTAH-1) for la, lb, 2, and 3 were 40 , 39, 30, and 36 kcal/mol, respectively, clearly indicating the change of the M-CO interaction caused by the phosphine ligand. Likewise, ent halpies of PR3 addition (structure of attacking PR3 is the same as the coordinated PR3) to the intermediate formed after CO dissociation (DE LTAH-2) were 41, 46, 20, and 33 kcal/mol, respectively. These results lead to the conclusion that the Fe(CO)3(PR3)-L bond is about the same strength for L = CO and PR3, while the Fe(CO)2(PR3)2-L bond is weaker for L = PR3 than for CO. The latter finding is counter to simple elect ronic arguments based on relative pi-acidity of ligands. The thermoche mical and kinetic trends point out the importance of steric effects in modulating ligand dissociation and addition at a metal center.