Soma Karmakar

Thioether-ligated nickel. Synthesis, x-ray crystal structure and redox behaviour of complexes of hexadentate ligands incorporating thioether and triazene-1-oxide functions

Abstract The hexadentate ligands RN(O)NNHC6H4S(CH2)3SC6H4NHN(O)(NR (H2L1: R  Me; H2L2 : R  Prn; general abbreviation H2L) and their nickel(II) complexes, [NiL1] and [NiL2], have been synthesized. The X-ray structure of [NiL1] has revealed the presence of a severely distorted NiN2O2S2 coordination sphere with longer than normal NiS distances : 2.494(4) and 2.526(3) A. The low symmetry of the ligand field is reflected in large splittings of octahedral ν1 and ν2 bands. In dichloromethane solution the nickel(III)-nickel(II) redox couple is observable cyclic voltammetrically and the E 1 2 values are: [NiL1]  0.74 and [NiL2  0.75 V (vs SCE). Frozen (77 K) solutions of the electrogenerated nickel(III) species display rhombic EPR spectra: g1 = 2.195, g2 = 2.145, g3 = 2.038 for [NiL1+; g1 = 2.199, g2 = 2.138, g3 = 2.035 for [NiL2]+. The g1 and g2 parameters for each complex can be considered as split components of g⊥ of the idealized geometry affording the inequality g⊥ > g|, which corresponds to an effective axial elongation and (dz2)1 ground state.

Nickel complexes of tridentate ligands incorporating thioether and triazene-1-oxide functions. Synthesis, structure and metal redox

Abstract The reaction of nickel(II) acetate with RN(O)NNHC6H4SMe (HL1 : R = Me; HL2 : R = Prn; general abbreviation HL) in aqueous ethanol affords [NiL2] as brown crystalline solids. The X-ray structure of [NiL22] is reported. Each ligand acts in the tridentate meridional SNO fashion. The Ni(SNO)2 coordination sphere is severely distorted from octahedral geometry. The NiS distances, 2.519(2) and 2.549(2) A, constitute the longest nickel(II)thioether bonds reported so far. Due to low symmetry, the octahedral ν1 band of the complexes are split into components lying at ca 1400 and 1000 nm. The complexes display quasireversible cyclic voltammograms corresponding to the metal redox couple [NiIIIL2]+/[NiIIL2], E 1 2 ≈ 0.75 V (vs S.C.E.). Coulometrically generated [NiIIIL2]+ displays rhombic EPR spectra, the g values of the perpendicular components being larger than that of the parallel component corresponding to the (dz2)1 ground state.

Generation of manganese-(III) versus-(IV) complexes with a conjugated ONS donor set: controllingeffect of ligand substituents†

Manganese(IV) complexes, [MnL 2 ] [H 2 L = MeC(OH)CHCMeNN C(SH)SR (R = Me 1a or CH 2 Ph 1b)] and manganese(III) complexes, [Mn(O 2 CMe)L] 1c or [Mn(acac)L] 1d [acac = acetylacetonate; H 2 L = PhCH(OH)CPhNNC(SH)SCH 2 Ph] have been synthesized and characterized. The Schiff-base ligands which are derived from an aliphatic carbonyl function, favour the facile oxidation of manganese-(II) to -(IV) under ambient conditions. The structure determination of 1a showed that the molecule is octahedral with the two equivalent tridentate ligands spanned meridionally. The EPR spectrum of 1a with a strong but structured signal at g ≈ 4.0 and a weak one at g ≈ 2.0 implies a large zero-field splitting, but the spectral profile differed from an ideal axial form. All the complexes exhibited reversible or quasi-reversible Mn IV –Mn III redox couples in their cyclic voltammograms at potentials commensurate with the nature of the substituents in the appropriate ligands. A reasonable basis is suggested by which one may predict whether a particular ligand will stabilize manganese-(II), -(III) or -(IV) in an aerobic medium.

Azooximates of bi- and tri-valent nickel

The reaction of arylazooximes, RC(NOH)NNPh (HL R , R = Me or Ph), with nickel(II) acetate tetrahydrate in methanol under anaerobic conditions afforded [NiL R 3 ] - isolated as the NEt 4 + salt. One (L Ph ) - ligand in [NiL Ph 3 ] - underwent facile displacement by L–L ligands like 2,2′-bipyridine (bipy) furnishing [NiL Ph 2 (bipy)]. The Ni III –Ni II reduction potential of [NiL R 3 ] - in acetonitrile is ≈ 0.1 V vs. saturated calomel electrode. The trivalent complex [NiL R 3 ] was quantitatively isolated via constant-potential electrolysis at 0.3 V. The Ni IV –Ni III couple of the tris chelate was observed near 0.9 V, but the nickel(IV) complex could not be isolated in the solid state. The relatively low metal reduction potential allowing facile preparation of the stable [NiL R 3 ] system is attributed to the strong-field nature of the oximato-N atom. In going from [NiL Ph 3 ] - to [NiL Ph 2 (bipy)] the Ni III –Ni II reduction potential increases by ≈ 0.3 V showing that (L Ph ) - is a much better stabiliser of Ni III than is bipy. The crystal structures of [NEt 4 ][NiL Ph 3 ] and [NiL Ph 2 (bipy)] have been determined. The geometry of [NiL R 3 ] (S = ½) was studied with the help of its EPR spectrum (d z2 ground state) in the [CoL R 3 ] lattice. Both [NiL R 3 ] - and [NiL R 3 ] have exclusive meridional geometry consistent with steric and angular-overlap considerations. In [NiL Ph 2 (bipy)] the two anionic oximato functions are placed in mutually trans positions. The oximato-N ligand displays substantial trans influence. Thus in [NiL Ph 3 ] - the Ni–N (azo) bond lying trans to Ni–N (oxime) is ≈ 0.05 A longer than the other two mutually trans Ni–N (azo) bonds. The average Ni–N (azo) distance in [NiL Ph 2 (bipy)] is ≈ 0.04 A shorter than that in [NiL Ph 3 ] - because none of the Ni–N (azo) bonds in the former complex is subject to the trans influence of Ni–N (oxime). In both complexes the Ni–N (oxime) lengths are significantly shorter than the Ni–N (azo) lengths, consistent with stronger Ni–N (oxime) σ bonding which is also a reason behind the strong-field nature of the oximate ligand.