TRANSIENT AND MATRIX PHOTOCHEMISTRY OF FE(DMPE)2H2 (DMPE = ME2PCH2CH2PME2) - DYNAMICS OF C-H AND H-H ACTIVATION

The transient photochemistry in solution and low-temperature matrix photochemistry of Fe(dmpe)2H2 (dmpe = Me2PCH2CH2PMe2) are reported; this compound is uniquely effective among first-row transition metal complexes in its ability to activate C-H bonds. Pulsed laser photolysis (308 nm) of alkane solutions of Fe(dmpe)2H2 at ambient temperature generates Fe(dmpe)2 (lambda(max) = 355 nm) within 30 ns. In the absence of added quenchers, the latter decays by reaction with the alkane solvent (in pentane, k(obs) = 630 s-1 at 297 K, DELTAH(double dagger) = 25.0 +/- 5.9 kJ mol-1, DELTAS(double dagger) = -125 +/-22 J K-1 mol-1). The pseudo-first-order rate constants very with alkane by a factor of ca. 4. Fe(dmpe)2 is quenched by added reagents with second-order rate constants in the range 10(9)-10(4) dm3 mol-1 s-1, decreasing in the order CO >> H-2 > C2H4 > N2 > cyclopentene > Et3SiH. The activation parameters for reaction with Et3SiH are dELTAH(double dagger) = 22.4 +/- 1.8 kJ mol-1, DELTAS(double dagger) = -87 +/- 6 J K-1 mol-1. The product of reaction with CO is demonstrated by time-resolved IR spectroscopy to be Fe(dmpe)2(CO), which forms at the same rate as the transient decays. The reactions with arenes (benzene and toluene) in cyclohexane exhibit saturation kinetics which are interpreted in terms of reversible formation of Fe(dmpe)2(arene), followed by conversion to Fe(dmpe)2(aryl)H with rate constants of ca. 10(6) s-1. UV irradiation of Fe(dMPe)2H2 in Ar matrices at 12 K also yields Fe(dmpe)2 (UV and IR detection). The reaction may be partially reversed by selective photolysis. Irradiation in CO- or N2-doped matrices yields Fe(dMPe)2L (L = CO, N2); in a methane matrix the photoproduct is Fe(dmpe)2(CH3)H. Fe(dmpe)2 differs drastically from Ru(dmpe)2 in its absorption spectrum and its reactivity. Most notably, the rate constant for reaction with H-2 is a factor of 7500 smaller for Fe(dmpe)2 than for Ru(dMpe)2. In contrast, reactions of both complexes with CO are diffusion-controlled.