Chiral metal complexes. 29. A crystallographic study of the stereoselective hydrogenation of 1,2-dehydroproline coordinated to cobalt(III)

The crystal structure of the complex Λ-β₂-[Co(R,R-picchxn)(pro-2H)]Cl·ClO ₄·H₂O (I) where R,R-picchxn is N,N′-bis(2-picolyl)-1R,2R-diaminocyclohexane and pro-2H is the 1,2-dehydroprolinate anion) has been determined. The complex crystallises in the orthorhombic space group P2₁2₁2₁, with a = 8.063(5), b = 15.320(9), c = 21.043(11) Å and Z and 4. The structure was refined by full-matrix least-squares methods to R = 0.049 for 2501 non-zero reflexions. The coordinated dehydroproline iminoacid is closely planar, and the structure suggest that the CN double bond would be equally accessible to a reacting species approaching from either side of the ring. The crystal structure of the two products obtained after hydrogenation of I, i.e. Λ-β₂-[Co(R,R-picchxn)(R-pro)](ClO₄) ₂ (II) and Λ-β₂-[CoR,R-picchxn)(S-pro)](ClO₄) ₂·H₂O (III), have also been determined by similar means. Complex II is monoclinic, space group P2₁ with a = 9.385(3), b = 15.066(5), c = 1.4925(7) Å, β = 110.79°, Z = 2, and was refined to R = 0.029 for 2650 non-zero reflexions. Crystals of III are trigonal, space group P3₂21 with a = 11.417(2), c = 38.586(7) Å, Z = 6, and was refine to R = 0.039 for 2686 non-zero reflexions. The molecular geometry of the Co^III(R,R-picchxn) fragment is essentially the same in each structure. However, upon hydrogenation of I the iminoacid CN bond increases by 0.22 Å in conjunction with the expected lack of planarity of the aminoacid pyrolidine and chelate rings. Short non-bonded HH contacts that are produced in the hydrogenation products II and III suggest that III would be the more sterically hindered. Comparisons are made between these structures and those of models computed for Λ-β₁-pro analogues.