Synthesis and crystal structure of nickel(II) complexes of macrobicyclic ligands: identification and electron-transfer reactions of the corresponding nickel(III) complexes in solution

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Abstract
The macrobicyclic complex [NiL1(ClO4)]ClO4(L1= 15-oxa-1,5,8,12-tetraazabicyclo[10.5.2]nonadecane) and the monocyclic precursor [NiL2(NCMe)][ClO4]2, [L2=N,N′-bis(3-aminopropyl)-1-oxa-4,7-diazacyclononane] have been prepared and their crystal structures determined: [NiL1(ClO4)]ClO4, space group P21/n, a= 13.580(6), b= 15.400(8), c= 10.418(4)Å, β= 97.91(2)°, Z= 4, R= 0.067, R′= 0.069; [NiL2(NCMe)][ClO4]2, monoclinic, space group Cc, a= 9.182(1), b= 19.859(3), c= 12.903(2)Å, β= 100.99(1)°, Z= 4, R= 0.063, R′= 0.065. In each of these species the complexed cation is six-co-ordinate with the ether oxygen co-ordinated ‘axially’ at distances of 2.109(6) and 2.189(12)Å, respectively. However, the equatorial co-ordination is significantly different. In [NiL1(ClO4)]+ the nickel arm is coplanar with the four nitrogen donors of the 14-membered macrocyclic ring. In [NiL2(NCMe)]2+ one pendant arm is axial (trans to the ether oxygen) with an acetonitrile molecule in the equatorial plane. Coplanarity of the equatorial donor set is not realized in this cation. The sixth position in [NiL1(ClO4)]+ is occupied by a weakly bound perchlorate ion [Ni–O 2.373(7)Å]. The corresponding nickel(III) complexes may be generated by Co3+(aq) oxidation in aqueous media or NO+ in non-aqueous media. Electron spin resonance measurements are consistent with an octahedral d7 on with the spectra obtained being sensitive to the nature of the nuclei in the sixth site. The kinetics of reduction of the nickel(III) macrobicyclic ligand complex and its thiatetraaza and pentaaza analogues with two outer-sphere electron-transfer reagents has been studied. Rapid electron self-exchange rate constants have been evaluated for both the aqua and chloro species [NiL(X)]n+(X = H2O, n= 3; X = Cl, n= 2), with a sensitivity to the nature of the axial ligands being observed.

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