Facially coordinating triamine ligands with a cyclic backbone: some structure-stability correlations.

Metal complex formation of the two cyclic triamines 6-methyl-1,4-diazepan-6-amine (MeL(a)) and all-cis-2,4,6-trimethylcyclohexane-1,3,5-triamine (Me(3)tach) was studied. The structure of the free ligands (H(x)MeL(a))(x+) and H(x)Me(3)tach(x+) (0 ≤ x ≤ 3) was investigated by pH-dependent NMR spectroscopy and X-ray diffraction experiments. The crystal structure of (H(2)Me(3)tach)(p-O(3)S-C(6)H(4)-CH(3))(2) showed a chair conformation with axial nitrogen atoms for the doubly protonated species. In contrast to a previous report, Me(3)tach was found to be a stronger base than the parent cis-cyclohexane-1,3,5-triamine (tach); pK(a)-values of H(3)Me(3)tach(3+) (25 °C, 0.1 M KCl): 5.2, 7.4, 11.2. The crystal structures of (H(3)MeL(a))(BiCl(6))·2H(2)O and (H(3)MeL(a))(ClO(4))Cl(2) exhibited two distinct twisted chair conformations of the seven membered diazepane ring. [Co(MeL(a))(2)](3+) (cis: 1(3+), trans: 2(3+)), trans-[Fe(MeL(a))(2)](3+) (3(3+)), [(MeL(a))ClCd(µ(2)-Cl)](2) (4), trans-[Cu(MeL(a))(2)](2+) (5(2+)), and [Cu(HMeL(a))Br(3)] (6) were characterized by single crystal X-ray analysis of 1(ClO(4))(3)·H(2)O, 2Br(3)·H(2)O, 3(ClO(4))(3)·0.8MeCN·0.2MeOH, 4, 5Br(2)·0.5MeOH, and 6·H(2)O. Formation constants and redox potentials of MeL(a) complexes were determined by potentiometric, spectrophotometric, and cyclovoltammetric measurements. The stability of [M(II)(MeL(a))](2+)-complexes is low. In comparison to the parent 1,4-diazepan-6-amine (L(a)), it is only slightly enhanced. In analogy to L(a), MeL(a) exhibited a pronounced tendency for forming protonated species such as [M(II)(HMeL(a))](3+) or [M(II)(MeL(a))(HMeL(a))](3+) (see 6 as an example). In contrast to MeL(a), Me(3)tach forms [M(II)L](2+) complexes (M = Cu, Zn) of very high stability, and the coordination behavior corresponds mainly to an "all-or-nothing" process. Molecular mechanics calculations showed that the low stability of L(a) and MeL(a) complexes is mainly due to a large amount of torsional strain within the pure chair conformation of the diazepane ring, required for tridentate coordination. This behavior is quite contrary to Me(3)tach and tacn (tacn =1,4,7-triazacyclononane), where the main portion of strain is already preformed in the free ligand, and the amount, generated upon complex formation, is comparably low.

Spin-orbit-induced anomalous pH-dependence in (1)H NMR spectra of Co(III) amine complexes: a diagnostic tool for structure elucidation.

The pH-dependent (1)H NMR characteristics of a series of Co(III)-(polyamin)-aqua and Co(III)-(polyamin)-(polyalcohol) complexes, [Co(tach)(ino-kappa(3)-O(1,3,5))](3+) (1(3+)), [Co(tach)(ino-kappa(3)-Omicron(1,2,6))](3+) (2(3+)), [Co(tach)(taci-kappa-Nu(1)-kappa(2)-O(2,6))](3+) (3(3+)), [Co(ditame)(H(2)O)](3+) (4(3+)), and [Co(tren)(H(2)O)(2)](3+) (5(3+)), were studied in D(2)O by means of titration experiments (tach = all-cis-cyclohexane-1,3,5-triamine, ino = cis-inositol, taci = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol, tren = tris(2-aminoethyl)amine, ditame = 2,2,6,6-tetrakis-(aminomethyl)-4-aza-heptane). A characteristic shift was observed for H(-C) hydrogen atoms in the alpha-position of a coordinated amino group upon deprotonation of a coordinated oxygen donor. For a cis-H-C-N-Co-O-H arrangement, deprotonation of the oxygen donor resulted in an additional shielding (shift to lower frequency) of the H(-C) proton, whereas for a trans-H-C-N-Co-O-H arrangement, deprotonation resulted in a deshielding (shift to higher frequency). The effect appears to be of rather general nature: it is observed for primary (1(3+)-5(3+)), secondary (4(3+)), and tertiary (5(3+)) amino groups, and for the deprotonation of an alcohol (1(3+)-3(3+)) or a water (4(3+), 5(3+)) ligand. Spin-orbit-corrected density functional calculations show that the high-frequency deprotonation shift for the trans-position is largely caused by a differential cobalt-centered spin-orbit effect on the hydrogen nuclear shielding. This effect is conformation dependent due to a Karplus-type behavior of the spin-orbit-induced Fermi-contact shift and thus only significant for an approximately antiperiplanar H-C-N-Co arrangement. The differential spin-orbit contribution to the deprotonation shift in the trans-position arises from the much larger spin-orbit shift for the protonated than for the deprotonated state. This is in turn due to a trans-effect of the deprotonated (hydroxo or alkoxo) ligand, which weakens the trans Co-N bond and thereby interrupts the Fermi-contact mechanism for transfer of the spin-orbit-induced spin polarization to the hydrogen nucleus in question. The unexpectedly large long-range spin-orbit effects found here for 3d metal complexes are traced back to small energy denominators in the perturbation theoretical expressions of the spin-orbit shifts.

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.

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.