A reversible NO complex of [Fe~(II)(edampda)] and the [Ni~(II)(edampda)] analogue (edampda~(2-)=N,N'-bis(pyridylmethyl)ethylenediamine-N,N'-diacetate)

摘要

Ni~(II), Cu~(II), Zn~(II) and Fe~(II) complexes of edampda~(2-) (N,N'-bis(pyridylmethyl)ethylenediamine-N,N'-diacetate) were prepared by the spontaneous chelation of a labile salt of each ion with the H_2edampda in aqueous solution (with the Fe~(II) complex under N_2). edampda~(2-) is designed to have an intermadiate ligand field stronger than edta~(4-), but less than tpen (N,N,N',N'-tetrakis(pyridylmethyl)ethylenediamine). [Fe~(II)(edampda)] is a high-spin (S = 2) complex at 25 deg C as shown by its ligand ~1H NMR resonances appearing at low field (ortho pyridyl ring protons at 118 ppm, pyridylmethyl CH_2 protons at 57 ppm, coordinated glycinato protons at 44 ppm) as broadened features. The differential pulse voltammogram of [Fe~(II)(edampda)] in solution is consistent with seven-coordinate species [Fe~(II)(edampda)(H_2O)] (E_(1/2) = 0.44 V) and [Fe~(II)(edampda)Cl]~- (E_(1/2) = 0.37 V) which are analogous to the known [Fe~(II)(edta)(H_2O)]~(2-) complex. As anticipated from ligand field factors the (II/III) reduction couple is intermediate in the series [Fe~(II)(edta)(H_2O)]~(2-); 0.14 V < [Fe(edampda)(H_2O)]; 0.44 V< [Fe(tpen)]~(2+); 0.84 V. Admission of NO to [Fe(edampda)(H_2O)] solutions causes a loss of the 0.44 V wave and the appearance of a new wave at 1.08 V in forming [Fe(edampda)NO]. The process is reversed by Ar purging. [Fe(edampda)NO] in a 50:50 ethanol-water glass exhibits an EPR spectrum at 106 K which has components of an S = 3/2 species (g = 4.16, 3.44, 2.0) and a second prominent S = 1/2 species (g = 2.04). N-shf features reveal a three-line, A_N = 41G, contact for the S = 1/2 species and a five line, A_N = 65G, contact for the S = 3/2 species suggesting of delocalization of spin on to the two adjacent pyridyl ring donors for the S = 3/2 species but only on NO for the S = 1/2 species. A spin-polarized {Fe(III), NO~-, triplet} species and low-spin {Fe(I), NO~+} entity account for the spin-crossover mixture. The assignments are consistent with the known S = 3/2 [Fe(edta)(NO)]~(2-) and S = 1/2 [Fe(tpen)(NO)]~(2+) species. Further support for the intermediacy of the ligand field strength of edampda~(2-) relative to edta~(4-) and tpen was obtained from the ~3A_(2g) implied by ~3T_(1g) (F) transition energies of the Ni~(II)L complexes: [Ni(edta)]~(2-) (17094 cm~(-1)) < [Ni(edampda)] (18 587 cm~(-1)) < [Ni(tpen)]~(2+) (19 531 cm~(-1)); the ~2E_g implied by ~2T _(2g) transitions for the Cu~(II)L redox couples: [Ni(edta)]~(2-) (1.29 V) < [Ni(edampda)] (1.32 V) < [Ni(tpen)]~(2+) (>= 1.49 V). The [Zn~(II)(edampda)] complex is predominantly octahedrally-coordinated as shown by stereochemically rigid and coordinated glycinato and pyridylmethyl donors exhibiting AB quartets in the ~1H NMR spectrum appearing at 4.29 and 3.33 ppm, respectively. Evidence for a tract amount of a [Zn~(II)(edampda)] complex having a dissociated, pendant pyridyl arm is also observed; the species is most probably a pseudo-trigonal bypyramidal structure based on the EPR spectrum of [Cu~(II)(edampda)] (reported elsewhere) which also exhibits features characteristic of distortion from tetragonal symmetry toward a distorted trigonal bipyramidal structure.