Methyl transfers. 15. CoIPc- as a nucleophile and leaving group

Abstract

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.

X-ray Structures and Homolysis of Some Alkylcobalt(III) Phthalocyanine Complexes

“…The activation enthalpies for the methyl system are at least 6 kcal/mol higher than those determined for S N 2 methyl transfers from CH 3 CoPc, while the activation entropies for the two reactions are not very different; this does not allow for an easy distinction of the displacement and homolytic processes. 9 The previously determined activation parameters for the thermolysis of the methyl system under aerobic conditions 21 are generally in agreement with the BDEs obtained in this study in the presence of TEMPO. Apparently, in dilute solutions of organocobalt substrate, alkyl radical trapping by O 2 is quite effective.…”

“…For the methyl-cobalt system, apart from BDE values for the homolysis of MeCbl 18 and its popular model, CH 3 Co(DH) 2 (py), 19 and some kinetic data on CH 3 Co[TC-(4,4)], 5 there is probably only one report on the homolysis of σ-bonded methylcobalt(III) porphyrinic model 20 and one regarding the thermolysis of CH 3 Co III Pc under aerobic conditions. 21 A variety of alkylcobalt model complexes have been synthesized and extensively studied. The phthalocyanine model of MeCbl is unique in its outstanding methyl donor ability.…”

“…In addition, β-hydrogen-containing alkyls, such as ethyl, could have been avoided because of possible in-cage β-H elimination. For the methyl−cobalt system, apart from BDE values for the homolysis of MeCbl and its popular model, CH 3 Co(DH) 2 (py), and some kinetic data on CH 3 Co[TC-(4,4)], there is probably only one report on the homolysis of σ-bonded methylcobalt(III) porphyrinic model 20 and one regarding the thermolysis of CH 3 Co III Pc under aerobic conditions …”

Axial Bonding Capabilities of Square Planar d8-ML4Complexes. Theoretical Study and Structural Correlations

“…According to the theoretical discussion above, one might expect square planar complexes to act as nucleophiles through the z 2 electron pair, and the addition of an apical ligand should enhance its nucleophilicity. The nucleophilic character of square planar complexes in a number of reactions has been proposed by several authors for compounds of Os(0), 16 Rh-(I), 17,18 Co(I), [19][20][21] and Pt(II). 22 Furthermore, the nucleophilicity of reduced cobaloximes has been found to depend on the nature of the axial ligand, 19 and the addition of a fifth ligand (iodide, [23][24][25][26][27] amines, 24 or acetate 25 ) to square planar complexes has been reported to enhance their nucleophilicity.…”

Methyl transfer from MeCo(III)Pc to thiophenoxide

“…In an earlier study' it was shown that Co(1)Pc-(1-) was a good leaving group from MeCo(II1)Pc (3) [the notation is that in Ref. 1 in which the number corresponds to the formal oxidation state: 1-= Co(1)Pc-, 2 = Co(II)Pc, 3 = MeCo(III)Pc] in the solvent dimethylacetamide (DMA) with the nucleophiles I -, Br-and CN-. Cobalt complexes both as Co(1) and Co(1I) are leaving groups in cases in which the nucleophile is also a cobalt complex,273 and reactions with thiolate nucleophiles have often been studied with alkyl cobalt complexes with dimethylglyoxime and related complexing agents and also vitamin B12.…”