Chiral metal complexes part 37. A new stereospecific N₄ tetradentate, N,N′-di(2-picolyl)-N′-methyl-2S-aminomethylpyrrolidine, S-picpyrrMe, and the crystal structures of its ternary Λ-α₁-[Con^III(S-picpyrrMe)(L)]²⁺ ions (LS-or R-alaninate)

A new linear N₄ tetradentate with terminal pyridyl groups, N,N′-di(2-picolyl)-N′-methyl-2-aminomethylpyrrolidine, S-picpyrrMe, has been synthesized via a six-step process in 19% yield from the naturally-occurring α-amino acid S-proline. The ligand stereospecifically adopts a λ-α topology on coordination to cobalt(III) when isolated as the dichloro complex. This configuration is retained on substitution of the chloro ligands by nitrite or R- or S-alaninate ions. The products were characterised by their electronic absorption and chiroptical properties and by ¹H NMR. The low-temperature crystallographic analyses of the R- and S-alaninate complexes are reported. λ- α₁-[Co(S-picpyrrMe)(R-ala)](ClO₄)₂ crystallises in the monoclinic space group P2₁ with a =10.888(4), b=10.243(4), c=12.365(6) Å,β=109.37(3)° and Z=2; λ-α₁-[Co(S-picpyrrMe)(S-ala)](ClO₄)₂· 1 2H₂O crystallises in the orthorhombic space group P2₁2₁2₁ with a =10.580(8), b= 14.391(8), c=18.078(7) Å and Z=4. The structures were refined by full-matrix least-squares procedures to R=0.046 for 1662 reflections and R=0.046 for 1771 reflections, respectively. In each case the amino acid coordinates as a N,O-bidentate, completing a CoN₅O octahedral coordination sphere with α₁ topology. The average CoN(pyridine) distance is 1.955(8) Å and the CoN(tertamine) distances are 1.996(8) and 1.979(8) Å for the methylated and pyrrolidine amines, respectively. The R-alaninate chelate ring is considerably flattened due to a steric interaction between its methyl group and a pyridyl ring of the tetradentate, whereas the S-alaninate has an unstrained puckered conformation.