N-METHYLATED MACROBICYCLIC HEXAAMINES OF COPPER(II) AND NICKEL(II) - LARGE STERIC EFFECTS

Complexation of the penta-, hexa- and hepta-N-methylated derivatives o f the macrobicyclic hexaamine sar (3,6,10,13,16,19-hexaaza-bicyclo[6.6 .6]icosane) with nickel(II) and copper(II) is reported. Unlike their a nalogous hexadentate-coordinated [M(sar)](2+) complexes, the Cu(II) an d Ni(II) complexes of N-Me(5)sar, N-Me(6)sar and N-Me(7)sar(+) show th e ligands coordinating only as tetradentates, both in the solid state and in solution. The X-ray crystallographic analyses of [Ni(N-Me(5)sar )](ClO4)(2), orthorhombic, space group P2(1)2(1)2(1), a 9.915(2), b 10 .694(2), c 25.549(6) Angstrom, Z = 4; {[Ni(HN-Me(6)sar)](ClO4)(3).2H(2 )O}(3), triclinic, space group P $($) over bar$$ 1, a 14.160(2), b 17. 739(4), c 21.617(5) Angstrom, alpha 75.20(2), beta 89.72(2), gamma 68. 31(1)degrees, Z = 2; [Ni(N-Me(7)sar)](ClO4)(3), orthorhombic, space gr oup P2(1)2(1)2(1), a 10.752(2), b 13.410(2), c 21.643(3) Angstrom, Z = 4; [Cu(N-Me(5)sar)](ClO4)(2), orthorhombic, space group P2(1)2(1)2(1) , a 10.005(1), b 10.524(1), c 25.731(2) Angstrom, Z = 4 and [Cu(N-Me(7 )sar)](ClO4)(3).0.5H(2)O, orthorhombic, space group P2(1)2(1)2(1), a 1 1.337(2), b 12.802(2), c 22.170(2) Angstrom, Z = 4, are reported. The Ni(II). complexes exhibit marked solvent, temperature and pH-dependent electronic spectra as a result of high-spin/low-spin equilibria. Mole cular mechanics calculations indicate that hexadentate coordination of N-Me(6)sar to metal ions such as Cu(II) and Ni(II) will be unfavorabl e as a result of severe steric strain.