Suranjan Bhanja Choudhury

Synthesis and structure of binuclear thioazobenzene pallada-cycles and their reaction with m-chloroperbenzoic acid: Formation and structure of dinuclear azophenolates

Abstract The cyclopalladation of 1,3-bis(2-thioazobenzene)propane (L 1 H 2 ) and its methyl substituted derivative L 2 H 2 affords complexes of the type LPd 2 Cl 2 (where L represents L 1 or L 2 which have been characterised by spectroscopy and X-ray crystallography. The crystal structure of L 1 Pd 2 C 2 has revealed that each azobenzene fragment along with its thioether sulphur acts in the tridentate (C,N,S) fashion and the fourth coordination position is occupied by a chloride ion. The complexes are thus of type [Pd(C,N,S)Cl] 2 , with a Pd ⋯ Pd contact of 5.420(1) A. The metallated carbon atom exerts a strong trans influence on the PdS bond. The reaction of LPd 2 Cl 2 with am -chloroperbenzoic acid leads to smooth oxygen insertion into both the PdC bonds to give excellent yields of dinuclear azophenolato complexes o -LPd 2 Cl 2 having the coordination sphere [Pd(O,N,S)Cl] 2 . The crystal structure of o -L 1 Pd 2 Cl 2 is similar to that of L 1 Pd 2 Cl 2 but with a somewhat longer Pd ⋯ Pd distance of 5.890(1) A. The insertion reaction has a negative entropy of activation, in keeping with an associative transition state for the electrophilic incorporation of oxygen. Reduction of the azophenolato complexes with hydrazine hydrate affords the free azophenols ( o -LH 2 ) in high yields. Thus organometallic route to o -LH 2 from LH 2 is provided.

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.

An X-ray absorption spectroscopic structural investigation of the nickel site in Escherichia coli NikA protein

The results of an X-ray absorption spectroscopic study of the structure of the Ni site in the periplasmic Ni-binding protein NikA are presented. These studies demonstrate that the Ni site is 6 or 7-coordinate, with the ligand environment composed of 6 or 7 O- or N-donor ligands with bonds averaging 2.06(2) A in length. Unlike UreE, another Ni-binding protein, the Ni ligands in NikA are not largely imidazole ligands from histidine residues but are more likely derived from aspartate and glutamate carboxylate side chains.

Manganese(IV)–amide binding: structural characterisation and redox stability of a hexadentate complex

The hexadentate diamide ligand H4L(1), formed from ethyl salicylate and triethylenetetramine, affords MnL·4H2O which has a trans–cis–cis MnIVN2(amide)N2(amine)O2(phenol) co-ordination sphere and a low MnIV/MnIII reduction potential.

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.

A novel zwitterionic ortho-metallated ruthenium(II) phenolate

Decarbonylative metallation of the dialdehyde 2b by [Ru(PPh3)3Cl2] affords the unusual complex [Ru(MeL)(CO)(PPh3)2(Cl)]4 which has been characterised by X-ray crystallography and in which the RuIIP2C2ClO coordination unit bears a single net negative charge; the Ru(MeL)(3, R = Me) fragment incorporates a four-membered C,O-chelated phenolato function with a neighbouring monoprotonated azomethine moiety.

Binding of bi- and tri-valent nickel by azophenolates incorporating thioether/ether donor sites

Nickel(II) complexes of tri- and hexa-dentate ligands in which the donors are azo nitrogen, phenolic oxygen and thioether sulfur or ether oxygen have been isolated. Structure determination of three complexes has established the presence of distorted octahedral NiN2O2X2(X = S or O) co-ordination spheres. In dichloromethane solution the nickel(III)–nickel(II) couple is electrochemically observable for the thioether complexes with E½ in the range 0.65–0.85 V vs. saturated calomel electrode. The blueviolet nickel(III) species can be quantitatively electrogenerated in solution. Upon freezing (77 K) axial EPR spectra (g∥≈ 2.18, g⊥≈ 2.06) compatible with the uncommon hole configuration (dx2–y2)1 are observed. The ether complexes are more difficult to oxidise and the nickel(III) species are not tractable. This is consistent with the higher strength of nickel–thioether compared to –ether binding.