O-Alkylated Derivatives of 1,3,5-Triamino-1,3,5-trideoxy-cis-inositol: Triamine Ligands with Unexpectedly High Affinity toward Divalent Transition- and d(10)-Metal Ions.

Abstract

The ligands all-cis-2,4,6-trimethoxycyclohexane-1,3,5-triamine (tmca) and all-cis-2,4,6-tribenzoxycyclohexane-1,3,5-triamine (tbca) were prepared almost quantitatively by using [Ni(taci)(2)](2+) (taci = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol) as precursor, where Ni(2+) acted as a very efficient protecting group for the nitrogen donors. The structure of tmca in solution was investigated by NMR spectroscopy. A strongly solvent-dependent conformational equilibrium was observed. In CD(3)CN, a chair conformation with three axial amino groups formed exclusively, whereas in D(2)O, the conformation with three equatorial amino groups predominated. This effect, as well as conformational changes in the course of stepwise protonation, is discussed in terms of hydrogen bonding effects. The crystal structure of H(3)tmca(3+) exhibits a chair conformation with three equatorial ammonium groups and three axial methoxy groups. The trihydrochloride hydrate crystallizes in the monoclinic space group P2(1)/n, a = 11.057(5) Å, b = 9.960(6) Å, c = 14.671(6) Å, beta = 93.79(3) degrees, Z = 4 for C(9)Cl(3)H(26)N(3)O(4). A variety of bis complexes [M(tmca)(2)](2+) (M = Ni, Cu, Zn, Cd) and [M(tbca)(2)](2+) (M = Ni, Cu) were prepared and they were isolated as solid, crystalline trinitrate or trichloride salts. Crystal data: [Ni(tmca)(2)](NO(3))(2).4H(2)O, triclinic, space group P&onemacr;, a = 8.919(11) Å, b = 9.293(9) Å, c = 9.942(11) Å, alpha = 96.73(9) degrees, beta = 100.66(9) degrees, gamma = 101.95(9) degrees, Z = 1 for C(18)H(50)N(8)NiO(16); [Cu(tmca)(2)](NO(3))(2), tetragonal, space group P&fourmacr;2(1)c, a = 13.017(6) Å, c = 15.985(10) Å, Z = 4 for C(18)CuH(42)N(8)O(12); [Ni(tbca)(2)](NO(3))(2).MeCN.H(2)O, monoclinic, space group P2(1)/c, a = 12.930(8) Å, b = 19.324(10) Å, c = 22.724(14) Å, beta = 97.21(5) degrees, Z = 4 for C(56)H(71)N(9)NiO(13). The formation constants of [M(tmca)](2+) and [M(tmca)(2)](2+) were determined by means of a series of potentiometric titration experiments. A comparison of the taci complexes with the corresponding tmca complexes revealed an unexpected increase of stability for the latter. This increase is more than 2 orders of magnitude for the 1:1 complexes and about 5 orders of magnitudes for the 1:2 complexes. Possible reasons for this unexpected increase in stability are discussed.