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 determined for the first time. In addition, time-resolved experiments provided the kinetics of the second step of the substitution, phosphine reaction with the intermediate formed upon CO dissociation. Several interesting results, with implications for understanding the interaction of probe molecules with potentially reactive metals centers, were noted. 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, analysis of the products after photolysis of 1 with dispersed (CH3)3CNC or CO indicates that coordinated CO substitutes but not PR3. Third, the photolysis of Fe (CO)3(PMe3)2 (2) with PMe3 results in the substitution of CO (ʦ = 0.50), while photolysis with CO results in the substitution of PMe3 (ʦ = 0.03). On the other hand, the photolysis of Fe(CO)3(dmpe) (3, dmpe = l,2-bis (dimethylphosphino)ethane) with PMe3 results in the substitution of only CO (Φ = 0.47). Fourth, the enthalpies of CO dissociation in cyclohexane (ΔΗ1) for 1a, 1b, 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, enthalpies of PR3 addition (structure of attacking PR3 is the same as the coordinated PR3) to the intermediate formed after CO dissociation (ΔH2) 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 electronic arguments based on relative π-acidity of ligands. The thermochemical and kinetic trends point out the importance of steric effects in modulating ligand dissociation and addition at a metal center. © 1993, American Chemical Society. All rights reserved