Compounds of the 16-Membered Macrocyclic Tetraamine (4RS,12RS)-2,2,4,10,10,12-Hexamethyl-1,5,9,13-tetraaza-cyclohexadecane with Cobalt(II), Nickel(II), Copper(II) and Zinc(II); Structural and Molecular Mechanics Studies

Abstract

The cyclic tetraamine compound (4RS,12RS)-(2,2,4,10,10,12-hexamethyl-1,5,9,13-tetraazacyclohexadecane) nickel(II) perchlorate, [Ni(L¹ )] (ClO₄)₂, has been prepared by reduction of the imine functions of (2,4,4,10,12,12-hexamethyl-1,5,9,13-tetraazacyclohexadeca-1,9-diene)nickel(II) by catalytic hydrogenation, or by NaBH₄ in acetonitrile/methanol. The ligand L ¹ was isolated, after reaction with acid, as the salt [(L ¹ )H₄](ClO₄)Cl₃·H₂O. Compounds with cobalt(II), copper(II), and zinc(II) were prepared from this, and the structures of [M(L ¹ )] (ClO₄)₂, M = Co, Ni and Cu, and [Cu(L ¹ )] [ZnCl₄], determined by X-ray diffraction: [Co(C₁₈H₄₀N₄)] (ClO₄)₂, orthorhombic, Pn2₁/a, a 8.644(3), b 14.869(8), c 19.541(7) Å, Z 4, R₁ 0.062 for 99 reflections with I > 2s(I); [Ni(C₁₈H₄₀N₄)] (ClO₄)₂, monoclinic, C2/c, a 11.780(4), b 12.580(4), c 16.930(3) Å, b 90.14(4)˚, Z 4, R₁ 0.067 for 1644 reflections with I > 2s(I); [Cu(C₁₈H₄₀N₄)] (ClO₄)₂, monoclinic, C2/c, a 11.508(7), b 13.17(2), c 16.83(1) Å, b 92.37(5)˚, Z 4, R₁ 0.114 for 2366 reflections with I > 2s(I); [Cu(C₁₈H₄₀N₄)] [ZnCl₄], monoclinic, P2₁/n, a 9.392(3), b 14.915(6), c 17.591(7) Å, b 95.94(3)˚, Z 4, R₁ 0.041 for 2301 reflections with I > 2s(I). All have the C-rac, 1RS,4SR,5RS,9RS,12RS,13SR, configuration with N–H groups alternating on opposite sides of the molecular ‘plane’ around the ring. All have flattened tetrahedral coordination geometry, with the quartet ground state cobalt(II) cation most twisted and the copper(II) tetrachlorozincate, perchlorate and singlet ground state nickel(II) complex cations progressively more flattened. The macrocycles have overall saddle-like conformations with chelate rings tilted alternately to opposite sides of the molecular ‘plane’. The compounds all have both trimethyl-substituted chelate rings in chair conformations. The unsubstituted chelate rings both have chair conformations for the cobalt(II) compound, both have twist-boat conformations for the nickel(II) and copper(II) perchlorate salts and have one chair and one twist-boat conformation for the copper(II) tetrachlorozincate salt. Mean M–N distances (Å) and trans-N–M–N angles {N(1)–M–N(9), N(5)–M–N(13) (degrees)} are: Co, 1.99, 130.3(5), 115.5(5); Ni, 1.93, 163.6(4), 155.8(4); Cu (ClO₄), 1.99, 155.0(6), 143.8(7); Cu (ZnCl₄) 2.00, 146.6(2), 135.0(2). The influence of axial interactions with counter ions, hydrogen bonding and other lattice interactions on the structures adopted are discussed. Strain energies for the possible configurations/conformations of coordinated L 1 have been calculated by molecular mechanics and related to the observed structures.