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 ph otochemistry of Fe(dmpe)2H2 (dmpe = Me2PCH2CH2PMe2) are reported; this compound is uniquely effective among first-row transition metal compl exes in its ability to activate C-H bonds. Pulsed laser photolysis (30 8 nm) of alkane solutions of Fe(dmpe)2H2 at ambient temperature genera tes Fe(dmpe)2 (lambda(max) = 355 nm) within 30 ns. In the absence of a dded 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 cons tants 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 paramete rs for reaction with Et3SiH are dELTAH(double dagger) = 22.4 +/- 1.8 k J 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 exhib it saturation kinetics which are interpreted in terms of reversible fo rmation 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). Th e reaction may be partially reversed by selective photolysis. Irradiat ion in CO- or N2-doped matrices yields Fe(dMPe)2L (L = CO, N2); in a m ethane matrix the photoproduct is Fe(dmpe)2(CH3)H. Fe(dmpe)2 differs d rastically from Ru(dmpe)2 in its absorption spectrum and its reactivit y. Most notably, the rate constant for reaction with H-2 is a factor o f 7500 smaller for Fe(dmpe)2 than for Ru(dMpe)2. In contrast, reaction s of both complexes with CO are diffusion-controlled.