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Chiral Induction in the Inner-Sphere Reduction of [Co(ox) 3 ] 3 - by [Co(2,2,2-tet)(OH 2 ) 2 ] 2+ and Derivatives

1996; American Chemical Society; Volume: 35; Issue: 10 Linguagem: Inglês

10.1021/ic951403g

1520-510X

Eileen C. Sheu, Maoyu Shang, A. Graham Lappin,

Metal complexes synthesis and properties

The reductions of [Co(ox)3]3- by [Co(2,2,2-tet)(OH2)2]2+ and derivatives are rapid inner-sphere electron transfer reactions which proceed through a doubly chelated oxalate bridge. The second-order rate constants for the reactions are 1.6 × 104 M-1 s-1 and 3.6 × 104 M-1 s-1 respectively for [Co(2,2,2-tet)(OH2)2]2+ and [Co(2,3,2-tet)(OH2)2]2+. Hydrolysis of the complexes inhibits the reaction and reflects the need to form a chelate bridge in the electron transfer precursor complex. The primary products in the reaction with [Co(2,2,2-tet)(OH2)2]2+ are Λ,Δ-cis-β(SR,RS)-[Co(2,2,2-tet)(ox)]+ (15%) and Λ,Δ-cis-β(SS,RR)-[Co(2,2,2-tet)(ox)]+ (85%) and they have been characterized by X-ray crystallography. Λ,Δ-cis-β(SR,RS)-[Co(2,2,2-tet)(ox)]ClO4 crystallizes in Pna21 with Z = 4, a = 15.126(1) Å, b = 7.9468(4) Å, c = 11.603(1) Å, and R = 0.021 for 1882 reflections. Λ,Δ-cis-β(SS,RR)-[Co(2,2,2-tet)(ox)]Cl·3H2O crystallizes in P1̄, with Z = 2, a = 7.636(3) Å, b = 8.468(2) Å, c = 13.456(3) Å, α = 107.45(2)°, β = 102.00(3)°, γ = 99.62(3)°, and R = 0.045 for 1784 reflections. A single cis-β product, Λ,Δ-cis-β(SS,RR)-[Co(2,3,2-tet)(ox)]+ is formed with the [Co(2,3,2-tet)(OH2)2]2+ as reductant, and reaction with [Co(5(R,S)-Me-2,2,2-tet)(OH2)2]2+ gives three products, all with the cis-β geometry. Two of these products have been characterized by X-ray crystallography. Λ,Δ-cis-β(SR,RS)-exo-[Co(5(S,R)-Me-2,2,2-tet)(ox)](ClO4)·2NaClO4·H2O crystallizes in P1̄, with Z = 2, a = 8.796(1) Å, b = 11.479(1) Å, c = 12.632(2) Å α = 107.59(1)°, β = 90.21(1)°, γ = 106.43(1)°, and R = 0.049 for 2790 reflections. Λ,Δ-cis-β(SS,RR)-endo-[Co(5(R,S)-Me-2,2,2-tet)(ox)]Cl·2H2O crystallizes in P1̄, with Z = 2, a = 8.2143(9) Å, b = 8.2572(9) Å, c = 11.755(1) Å, α = 80.845(9), β = 84.621(9)°, γ = 75.912(9)°, and R = 0.040 for 2527 reflections. When optically active Δ-[Co(ox)3]3- is used, chiral induction is observed in the products. The sense of the induction is dependent on the configuration of the polydentate amine ligand. When it is bound as cis-β(SS,RR), a Δ-product results. When it is bound as cis-β(SR,RS), a Λ-product results. The induction is small, typically <10% enantiomeric excess, but can be correlated with the fluxionality of the amine.