FT-EPR study of alkyl radicals formed in the photochemical reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) complexes

Abstract

A Fourier transform EPR (FT-EPR) study was made of the photochemistry of [Re(R)(CO) 3 (a-diimine)] and [Ru(E)(R)(CO) 2 (a-diimine)] complexes, where R = alkyl or benzyl, E = I or SnPh 3 , and a-diimine = 4,4'-dimethyl-2,2'-bipyridine (DMB) or N,N'-diisopropyl-1,4-diazabutadiene (iPr-DAB). Photoexcitation of these complexes leads to homolysis of the metal-alkyl (benzyl) bonds as evident from the detection of the spectra of the alkyl (benzyl) radicals. FT-EPR spectra display strong spin polarization effects attributed to Triplet Mechanism (TM) and Radical Pair Mechanism (RPM) Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP patterns point to bond dissociation via a triplet state precursor. For a number of complexes, spin polarization was found to exhibit unusually large solvent effects, whereas for one complex the CIDEP pattern proved to be sensitive to the wavelength of laser light used to initiate bond dissociation. These effects reflect the strong dependence of CIDEP on the character of the excited states involved in the photochemical reactions and contribute to the understanding of the reaction mechanism.

A closer look at photochemical reactions of transition-metal compounds by time-resolved EPR

“…The FTEPR studies illustrated the remarkably strong effects that solvent dielectric constant and viscosity can have on CIDEP generated in photoinduced metal-alkyl bond cleavage reactions [16][17][18][19][20]. Also, one system was found to exhibit strong wavelength-dependent CIDEP [18]. This provides unequivocal evidence that bond cleavage occurs with a rate that competes with internal conversion processes following excitation in higher excited states of the complex.…”

“…With FTEPR the transient atkyl (or benzyl) radicals formed upon pulsed-laser excitation of the complexes in fluid solution at room temperature could be monitored directly with nanosecond time resolution. The FTEPR studies illustrated the remarkably strong effects that solvent dielectric constant and viscosity can have on CIDEP generated in photoinduced metal-alkyl bond cleavage reactions [16][17][18][19][20]. Also, one system was found to exhibit strong wavelength-dependent CIDEP [18].…”

“…Also, a series of FTEPR studies have been carried out on the bond homolysis reactions of the transition-metal complexes [Re(R)(CO)3(dmb)], [Ru(I)(R)(CO)2(iPr-DAB)], [Ru(CH3)(SnPh3)(CO)z(iPr-DAB)] and [Pt(CH3)4(iPr-DAB)] [16][17][18][19][20]. Here R is methyl, ethyl, isopropyl or benzyl, dmb is 4,4'-dimethyl-2,2'-bipyridine and iPr-DAB is N,N'-diisopropyl-l,4-diazabutadiene.…”

“…Kleverlaan et al [16,18,19] and van Slageren et al [17,20] used FTEPR to study the photoinduced metal-alkyl bond cleavage in a series of [Re(R)(CO)3(ctdiimine)] (III), [Ru(L)(R)(CO)z(ct-diimine)] (IV) and Pt(R)4(~-diimine ) (V) complexes (Fig. 2c-e).…”

“…In the following, salient results of the FTEPR studies concerned with these complexes will be reviewed. The focus primarily will be on metal-CH 3 bond homolysis reactions, complete accounts of the investigations can be found in the literature [16][17][18][19][20] Fig. 7 [19].…”