Density functional theory has been applied to the investigation of the reductive cleavage mechanism of methylcobalamin (MeCbl). In the reductive cleavage of MeCbl, the Co-C bond is cleaved homolytically, and formation of the anion radical ([MeCbl]*-) reduces the dissociation energy by approximately 50%. Such dissociation energy lowering in [MeCbl]*- arises from the involvement of two electronic states: the initial state, which is formed upon electron addition, has dominant pi*corrin character, but when the Co-C bond is stretched the unpaired electron moves to the sigma*Co-C state, and the final cleavage involves the three-electron (sigma)2(sigma*)1 bond. The pi*corrin-sigma*Co-C states crossing does not take place at the equilibrium geometry of [MeCbl]*- but only when the Co-C bond is stretched to 2.3 A. In contrast to the neutral cofactor, the most energetically efficient cleavage of the Co-C bond is from the base-off form. The analysis of thermodynamic and kinetic data provides a rationale as to why Co-C cleavage in reduced form requires prior departure of the axial base. Finally, the possible connection of present work to B12 enzymatic catalysis and the involvement of anion-radical-like [MeCbl]*- species in relevant methyl transfer reactions is discussed.

CH3CoIIIPc (Pc = dianion of phthalocyanine) has been characterized by equilibrium studies of its trans axial ligation and cyclic voltammetry as a relatively "electron poor" model of methylcobalamin, which in noncoordinating solvents persists as a five-coordinate complex. Axial base (N-donors, PBu3, SCN-, weakly binding O-donors) inhibition of methyl transfer from CH3CoIIIPc shows that the reaction proceeds via the reactive five-coordinate species, even in coordinating solvents. The virtual inactivity of six-coordinate CH3CoIIIPc(L) complexes provides a reference point for important biological processes.

Many complexes of cobalt in the + 1 oxidation state are excellent nucleophiles. The complex with phthalocyanine, Pcz-, is an example; the rates of reaction of complex (CdPc)-, 1-, with Me1 and other alkylating agents in dimethylacetamide (DMA) solution are reported. In contrast to previous studies with other Co(1) complexes, the rate of reaction of the methylated product, (MeCoII'Pc, 3) with I-is also readily measured, allowing a kinetic evaluation of the equilibriumSimilarly, B r also reacts with 3 reversibly, and the rate and equilibrium constants are reported. Thus 1-is also a good leaving group. With CN, 3 gives complex MeCo*IIPcCNreversibly and slowly gives MeCN in a reaction zero order in CN-. The fast identity rate constants for I-attack on Me1 as well as that for B r attack on Me1 are given. Two different paths to the identity methyl transfer rate constant, kcdo, for the 1-+ 3 reaction with use of the Marcus equation gave kcdo = 4.4 M-I s-1 from the 3 + I-data but kcdo = 0.088 M-I s-1 from the 3 + B r data. This discrepancy, which is outside of experimental error, constitutes a deviation from the Marcus treatment; it is discussed.

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Density functional theory (DFT) has been applied to the analysis of the structural and electronic properties of the alkyl-cobalt(III) phthalocyanine complexes, [CoIIIPc]-R (Pc = phthalocyanine, R = Me or Et), and their pyridine adducts. The BP86/6-31G(d) level of theory shows good reliability for the optimized axial bond lengths and bond dissociation energies (BDEs). The mechanism of the reductive cleavage was probed for the [CoIIIPc]-Me complex which is known as a highly effective methyl group donor. In the present analysis, which follows a recent study on the reductive Co-C bond cleavage in methylcobalamin (J. Phys. Chem. B 2007, 111, 7638-7645), it is demonstrated that addition of an electron and formation of the pi-anion radical [CoIII(Pc*)]-Me- significantly lowers the energetic barrier required for homolytic Co-C bond dissociation. Such BDE lowering in [CoIII(Pc*)]-Me- arises from the involvement of two electronic states: upon electron addition, a quasi-degenerate pi*Pc state is initially formed, but when the cobalt-carbon bond is stretched, the unpaired electron moves to a sigma*Co-C state and the final cleavage involves the three-electron (sigma)2(sigma*)1 bond. As in corrin complexes, the pi*Pc-sigma*Co-C states crossing does not take place at the equilibrium geometry of [CoIII(Pc*)]-Me- but only when the Co-C bond is stretched to approximately 2.3 A. The DFT computed Co-C BDE of 23.3 kcal/mol in the one-electron-reduced phthalocyanine species, [CoIII(Pc*)]-Me-, is lowered by approximately 37% compared to the neutral Py-[CoIIIPc]-Me complex where BDE = 36.8 kcal/mol. A similar comparison for the corrin-containing complexes shows that a DFT computed BDE of 20.4 kcal/mol for [CoIII(corrin*)]-Me leads to approximately 45% bond strength reduction, in comparison to 37.0 kcal/mol for Im-[CoIII(corrin)]-Me+. These results suggest some preference by the alkylcorrinoids for the reductive cleavage mechanism.

The kinetics of the reaction of O,NC,H,C(L)(R)NO, (R = Me, Et, Pri, NNPE, NNPP, or MNNPP, respectively; L = H, D) with 1,8-diazabicyclo[5.4.O]undec-7-ene (DBU) in acetonitrile (MeCN) are reported. The nature of the product indicates that substantial dissociation into free ions occurs. The usefulness of Benesi-Hildebrand relationship for distinguishing between ion pairs and ions of the product is discussed in detail. The reaction shows l o w activation enthalpy value AHS = 15.4, 17.8, and 19.9 kJ mol-' and large negative entropies of activation ASS = -131, -134, and -147 J mol-' K-' for N N PE, N N PP, and M N N PP respectively. The kinetic isotope effects k H / k D (I 2.5, 12.4, and 12.3) are large, showing no variation, the more sterically hindered the substrate. The values of the isotope effects exerted on the activation parameters indicate the contribution of a tunnelling effect QH/QD = 1.35 at 25 "C.The influence of water o n the kinetics is also examined and discussed with respect t o reliability of kinetic measurements of such reactions systems.' 3-1 ho wever the problem is far from resolved. In view of

WLODZIMIERZ GALEZOWSKI and ARNOLD JARCZEWSKI. Can. J. Chem. 68, 2242 (1990).The kinetics of the reaction of (R = Me, Et, i-Pr; NPNE, NPNP, MNPNP respectively; L is H or D) with 1,s-diazabicyclo[5.4.O]undec-7-ene (DBU) base in tetrahydrofuran (THF) and chlorobenzene (CB) solvents are reported. The products of these proton transfer reactions are ion pairs absorbing at A,,,, = 460-480 nm. The equilibrium constants in THF were K :~'~ = 2060,560, 3.7 and in CB K ? ,~'~ = 500, 220,4.7 mol-I dm3 for NPNE, NPNP, MNPNP respectively. The thermodynamic parameters of the reactions are also quoted. The substrate reacts with DBU in both THF and CB solvents in a normal second-order proton transfer reaction. In the case of deuteron transfer, isotopic D/H exchange is much faster than internal return. The reactions show low values of enthalpy of activation AH' = 14.3, 18. I , 24.2 and 13.0, 15.1, 18.6 kJ mol-' for NPNE, NPNP, and MNPNP inTHF and CB respectively, and large negative entropies of activation -AS' = 141, 139, 146; 140, 146, 160 J mol-I deg-I for the same sequence of substrates and solvents. The kinetic isotope effects are large, (kH/kD)ZOaC = 12.2, 13.0, 10.1; 12.9, 12.0, 10.2 for the above sequence of substrates and solvents, and show no difference with changes in either steric hindrance of the C-acids or polarity of the solvents.Key words: proton transfer, kinetic isotope effect.WLODZIMIERZ GALEZOWSKI et ARNOLD JARCZEWSKI. Can. J. Chem. 68, 2242 (1990).On a examint la cinttique des reactions du t-

Transfer of the cobalt-bound methyl in MeCo(II1)Pc to thiophenoxide ion was studied (HzPc is the planar macrocyclic phthalocyanine; the cobalt is held in the center in this plane). In dimethylacetamide solution, the reaction is rapid, requiring stopped Pow for the kinetics, and yielding MeSPh and Co(1)Pc-in good yield. The kinetics are not simple second order, but instead approach a constant rate at high [PhS-1 , attributed to the reversible formation of an inert complex with PhS-occupying the vacant octahedral site in MeCo(lII)Pc, on the other side of the phthalocyanine plane from the methyl group. The kinetics allow the estimation of the equilibrium constant, K, and the &2 rate constant, k, which at 25 O C have values of ca. 9.4 X 103 I mol-' and 1.8 X lo4 I mol-', respectively.Although these values are rough, the ratio k / K is 6rm at 1.91 f 0.02 s-'; this is the limit of the rate at high [PhS-1. An alternative mechanism, which is entirely consistent with the kinetics, involves a rate-determining homolysis of the Co-S bond of the same complex. The mechanism is not favored because the product yields are high for a radical combination process and alternative chain processes are kinetically unacceptable. Further, the rate constant is about what would be expected from the reactivity of other nucleophiles in &2 reactions. Further arguments in favor of the &2 mechanism are presented. This transfer of the methyl group from Co to S is part of the possible analogy to the vitamin B1z-promoted methionine synthesis in nature. The other step in the biological, enzymatic process is the transfer of methyl from the nitrogen of N-methyltetrahydrofolate to cobalt. An attempt to model this with the very reactive N-methyl-2,6-dichloropyridinium ion was unsuccessful; the reaction took an entirely different course, presumably initiated by electron transfer, but leading to substantial loss of CI-from the pyridine. No more than 0.5% methyl transfer took place. This system does mimic well the complete natural enzymatic process.