Metalloradical Chemistry of Cobalt(II) Porphyrins. The Syntheses, Structure, and Reactivity of Triphenyltin(II)- and Trihalomethylcobalt(III) Octaethylporphyrin

Abstract

Stannanes R3SnH (R = n-Bu, Ph) reacted with CoIII(OEP)CH3 or CoII(OEP) to afford CoIII(OEP)SnR3 and CH4 or H2, respectively. CoIII(OEP)SnR3 was more efficiently prepared by reaction of CoI(OEP)- with R3SnCl. CoIII(OEP)SnPh3, C54H59CoN4Sn, crystallized in the triclinic space group P1̄ (Z = 2) with unit cell dimensions a = 12.124(5) Å, b = 14.700(5) Å, c = 15.221(7) Å, α = 109.56(4)°, β = 91.44(5)°, γ = 113.27(1)°, and V = 2308.4(1.8) Å3 at 295(2) K. The structure resembled that of five-coordinate alkylcobalt(III) porphyrin complexes with a square-pyramidal Co atom displaced 0.077 Å out of the porphyrin plane toward Sn and a Co−Sn bond length of 2.510(2) Å. The bond dissociation energy of the Co−Sn bond was considerably larger than that of the Co−C bond in alkylcobalt(III) porphyrin complexes. CoIII(OEP)SnPh3 was air stable in solution and decomposed by homolysis slowly at 120 °C in toluene. The Co−Sn bond was rapidly cleaved, though, when oxidized by I2 or by electrochemical means. In contrast, the Co−C bonds in CoIII(OEP)CX3 (X = Cl, Br, I) were substantially weaker than in the Co−C bond in alkylcobalt(III) porphyrin complexes and weakened progressively with heavier halogens. CoIII(OEP)CX3 complexes were prepared by reaction of CoI(OEP)- with CX4 (X = Cl, Br) or by reaction of CoII(OEP) with CBrCl3 or CX4 (X = Br, I). The reaction of CoI(OEP)- with CX4 (X = Cl, Br, I) also afforded small amounts of CoIII(OEP)CHX2 complexes, which were obtained in greater yield by reaction of CoI(OEP)- with CHX3. The substitution of one hydrogen for a halogen stabilized the CoIII(OEP)CHX2 complexes relative to the corresponding CoIII(OEP)CX3 complexes.

Reaction of cobalt porphycene with hydride reagents: spectroscopic detection of Co–H porphycene species and formation of Co–SnR3 porphycene species

“…The reaction profile is summarized in Scheme 4. The formation of the Co(III)-SnR 3 complex has also been reported for cobalt porphyrin 2-Co(II) [55]. The analogy between Co(II) porphyrin and porphycene regarding the reactivities toward trialkyltin radical suggests that both porphyrinoids would be appropriate catalysts for hydrostannylation.…”

“…The Bu 3 Sn-H bond is capable of undergoing a net homolytic cleavage (to afford Bu 3 Sn • and H • via a sequential electronproton transfer: Bu 3 SnH → Bu 3 SnH •+ + e -→ Bu 3 Sn • + H • (≡ H + + e -)) as well as the heterolytic cleavage (Bu 3 SnH → Bu 3 Sn + + H -) [55]. Consequently, another possibility for production of the Co(II) species is the reaction of the hydride species with excess Bu 3 SnH through homolytic cleavage of the Sn-H/Co-H bonds.…”

“…In the reaction of cobalt porphyrin with tributyltin hydride, the formation of the Co-Sn complex, which is believed to be important species in catalytic hydrostannylation, has been reported [50,[53][54][55]. The generation of the Co-Sn complex in porphycene is also demonstrated in the present work and the choice of the reaction pathway to reach the Co-H and Co-Sn complexes is discussed in context of the characteristics of the porphyrinoid ligands, the metal oxidation states and the hydride donors.…”

Synthesis, Characterization, and Reactivity Studies of para-Substituted Triaryltin Cobaloximes

“…The equatorial Co-N bonds for Co III (OEP)SnPh3 are slightly longer (1.962 -1.975 Å) than (4-tBuPy)Co(DH)2Sn(C6H4-p-tolyl)3. 26 The axial Co-N bond for the 4-t-BuPy is 2.063(3) Å. This is similar to the bond distances in vitamin B12 (1.97 -2.06 Å) and alkyl cobaloxime (~2.04 Å), but is shorter than the axial Co-N bond in coenzyme B12 (2.24 Å).…”

“…Stolzenberg has done extensive research involving the synthesis, characterization, and reactivity studies of organocobalt porphyrins. [25][26][27][28][29] These organocobalt porphyrins can also be used as models for vitamin B12. where E = Si, Ge, Sn, and Pb.…”