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

Ni-II, Cu-II, Zn-II and Fe-II complexes of edampda(2-) (N,N'-bis(pyridylmet hyl)ethylenediamine-N,N'-diacetate) were prepared by the spontaneous chelat ion of a labile salt of each ion with the H(2)edampda in aqueous solution ( with the Fe-II complex under N-2). edampda(2-) is designed to have an inter mediate 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 degrees C as shown by its ligand H-1 NMR resonances app earing at low field (ortho pyridyl ring protons at 118 ppm, pyridylmethyl C H2 protons at 57 ppm, coordinated glycinato protons at 44 ppm) as broadened features. The differential pulse voltammogram of [Fe-II(edampda)] in solut ion is consistent with seven-coordinate species [Fe-II(edampda)(H2O)] (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)(H2O)](2-) complex. As anticipated from ligand fie ld factors the (II/III) reduction couple is intermediate in the series [Fe- II(edta)(H2O)](2-); 0.14 V < [Fe(edampda)(H2O)]; 0.44 V < [Fe(tpen)](2+); 0 .84 V. Admission of NO to [Fe(edampda)(H2O)] solutions causes a loss of the 0.44 V wave and the appearance of a new wave at 1.08 V in forming [Fe(edam pda)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 pyri dyl ring donors for the S = 3/2 species but only on NO for the S = 1/2 spec ies. A spin-polarized {Fe(III), NO- triplet} species and low-spin {Fe(I), N O+} entity account for the spin-crossover mixture. The assignments are cons istent 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 stre ngth of edampda(2)- relative to edta(4-) and tpen was obtained from the (3) A(2g) <-- 3T(1g) (F) transition energies of the (NiL)-L-II series: [Ni(edta )](2-) (17 094 cm(-1)) < [Ni(edampda)] (18 587 cm(-1)) < [Ni(tpen)](2+) (19 531 cm(-1)); the E-2(g) <-- T-2(2g) transitions for the (CuL)-L-II complex es: [Cu(edta)](2-) (13 605 cm(-1)) < [Cu-II(edampda)] (15 432 cm(-1)); and the E-1/2 values of the Ni(II)/Ni(III) redox couples: [Ni(edta)](2-) (1.29 V) < [Ni(edampda)] (1.32 V) < [Ni(tpen)](2+) (greater than or equal to 1.49 V). The [Zn-II(edampda)] complex is predominantly octahedrally-coordinated as shown by stereochemically rigid and coordinated glycinato and pyridylme thyl donors exhibiting AB quartets in the H-1 NMR spectrum appearing at 4.2 9 and 3.33 ppm, respectively. Evidence for a trace amount of a [Zn-II(edampda)] complex having a dissocia ted, pendant pyridyl arm is also observed; the species is most probably a p seudo-trigonal bypyramidal structure based on the EPR spectrum of [Cu-II(ed ampda)] (reported elsewhere) which also exhibits features characteristic of distortion from tetragonal symmetry toward a distorted trigonal bipyramida l structure. (C) 1999 Elsevier Science S.A. All rights reserved.