Transition metal complexes of the novel tridentate di-2-pyridylmethanamine (dipa)

The first row transition metal complexes [M(dipa)2]n+ (dipa = di-2-pyridylmethanamine, M = Fe(III), Fe(II), Co(III), Ni(II) and Cu(II)) have been synthesized in generally high yields. Stability constants for the labile Co(II), Ni(II), and Cu(II) complexes have been determined by potentiometric titrations and are as follows (log K1, log K2): 7.63 (1), 6.57 (1); 8.55 (1), 8.25 (1); and 8.89 (1), 7.94 (1). A rather large degree of steric strain within the complexes is evident from the X-ray crystal structures of rac-[Fe(dipa)2](ClO4)2.2H2O (monoclinic, P2(1)/n, a = 9.392 (3) angstrom, b = 16.749 (3) angstrom, c = 17.131 (14) angstrom, beta = 96.62 (5)-degrees, Z = 4) rac-[Co(dipa)2](ClO4)3 (orthorhombic, Pcca, a = 17.574 (5) angstrom, b = 8.568 (2) angstrom, c = 19.159 (3) angstrom, Z = 4), meso-[Ni(dipa)2](S2O6).4H2O (triclinic, P1BAR, a = 7.903 (3) angstrom, b = 9.810 (3) angstrom c = 10.900 (2) angstrom, alpha = 113.10 (3)-degrees, beta = 106.01 (3)-degrees, gamma = 96.93 (4)-degrees, Z = 1), and meso- [Cu(dipa)2] (ClO4)2 (monoclinic, P2(1)/n, a = 7.964 (3) angstrom, b = 10.1 54 (2) angstrom, c = 16.842 (4) angstrom, beta = 100.12 (4)-degrees, Z = 2). Molecular mechanics calculations indicate that the formation of rac-[Co(dipa)2]3+ in preference to the meso isomer is dominant, and this is in agreement with experimental results based on NMR spectra of equilibrated solutions. In the case of Cu(II), formation of the meso isomer is not selective, and this was confirmed by the successful Cu(II)-directed template condensation of the rac isomer with formaldehyde and nitroethane. Calculations also indicate that the rac isomer of the Ni(II) complex is more stable than the crystallized meso isomer.