ORGANOMETALLIC SUPER REDUCING AGENTS - ELECTRON-TRANSFER CHEMISTRY OFPHOTOGENERATED 19E(-)W(CO)(5)(L)(-CENTER-DOT) RADICALS

The existence of ''19-valence electron radicals'' has been a controver sial issue in organometallic chemistry. The premise that photochemical excitation of [W-2(CO)(10)](2-) in the presence of donor solvents or phosphine ligands (L=THF, MeCN, 2-methyl THF, PEt3) leads to highly re ducing 19e(-) L-W(CO)(5)(-.) radicals was investigated by laser transi ent absorption (TA) spectroscopy in the UV-vis and IR regions and by e lectron transfer kinetics studies. Infrared transient absorption measu rements were found to be well suited to the [W-2(CO)(10)](2-) system. The IR transient absorption spectrum at <1 mu s following the laser pu lse in THF and MeCN is dominated by new IR absorptions at v(CO)>1960 c m(-1). The LR transient absorption spectrum at >10 mu s following the laser pulse is dominated by LR absorptions at v(CO)= 1860 and 1816 cm( -1). The significant lowering of v(CO) energy in the first 10 mu s is assigned to a 19e(-) solvent-W(CO)(5)(-.) (solvent=MeCN, THF) radical. Rate constants for second order recombination (k(r)) of these radical s are strongly solvent dependent. A homologous series of substituted b enzophenone electron accepters (A) with E-1/2 (A/A(-.)) ranging from - 1.21 to -2.26 V vs. SCE were chosen to study the electron transfer che mistry of the 19e- species in THF. Electron transfer rate constants (k (st)) were found to ranee from 4 x 10(4) M-1 s(-1) for reduction of th e least energetically favorable acceptor (cyclohexylphenyl-ketone, E-1 /2 = -2.26 V vs. SCE) up to 8 x 10(9) M-1 s(-1) for reduction of the m ost energetically favorable acceptor (decaflurobenzophenone, E-1/2 = - 1.21 V VS SCE). The data were interpreted within the context of the M arcus theory of electron transfer. The abnormally large total reorgani zation energy, lambda=70 kcal mol(-1) and slow derived self-exchange r ate constant, k(11) < 1 M-1 s(-1), reflect the large nuclear displacem ents associated with the 18e(-)/19e(-) couple. It is concluded that al though the 19e(-) solvent-W(CO)(5)(-.) radicals are thermodynamically ''super reducing agents'', capable in principle of single electron tra nsfers to a wide variety of substrates, they are kinetically incompete nt due to the enormous reorganization energy required to accommodate t he nineteenth electron. (C) 1998 Elsevier Science S.A.