The synthesis of zinc(II) complexes of 2-(2-mercaptophenyl)-iminophenol by electrochemical cleavage of a disulfide bond; the crystal structure of {(2, 2′-bipyridine)[2-(2-mercaptophenyl)iminophenoxy]}zinc(II)

Heteroligand α-Diimine-Zn(II) Complexes with O,N,O'- and O,N,S-Donor Redox-Active Schiff Bases: Synthesis, Structure and Electrochemical Properties

A number of novel heteroligand Zn(II) complexes (1-8) of the general type (Lⁿ)Zn(NN) containing O,N,O'-, O,N,S-donor redox-active Schiff bases and neutral N,N'-chelating ligands (NN) were synthesized. The target Schiff bases LⁿH₂ were obtained as a result of the condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with substituted o-aminophenols or o-aminothiophenol. These ligands with combination with 2,2'-bipyridine, 1,10-phenanthroline, and neocuproine are able to form stable complexes upon coordination with zinc(II) ion. The molecular structures of complexes 4∙H₂O, 6, and 8 in crystal state were determined by means of single-crystal X-ray analysis. In the prepared complexes, the redox-active Schiff bases are in the form of doubly deprotonated dianions and act as chelating tridentate ligands. Complexes 6 and 8 possess a strongly distorted pentacoordinate geometry while 4∙H₂O is hexacoordinate and contains water molecule coordinated to the central zinc atom. The electrochemical properties of zinc(II) complexes were studied by the cyclic voltammetry. For the studied complexes, O,N,O'- or O,N,S-donor Schiff base ligands are predominantly involved in electrochemical transformations in the anodic region, while the N,N'-coordinated neutral nitrogen donor ligands demonstrate the electrochemical activity in the cathode potential range. A feature of complexes 5 and 8 with sterically hindered tert-butyl groups is the possibility of the formation of relatively stable monocation and monoanion forms under electrochemical conditions. The values of the energy gap between the boundary redox orbitals were determined by electrochemical and spectral methods. The parameters obtained in the first case vary from 1.97 to 2.42 eV, while the optical bang gap reaches 2.87 eV.

Titanium(IV) complexes of disulfide-linked Schiff bases

With the goal of preparing Ti(IV) complexes bearing a sulfur-based redox function of possible use in electrocatalytic oxidations of alcohols at electrode surfaces, a series of seven 2,2'-dithiodianiline Schiff-base derivatives, including two new variations, were tested in reactions with Ti(OR)(4) (R = (i)Pr, (t)Bu). Instead of the expected dimetallic products of general formula [LTi(OR)(2)](2), mononuclear species LTi(OR)(2) were obtained, confirmed by crystallographic determinations to have an unprecedented, symmetrical, and macrocyclic arrangement with four-point binding to the metal center and with the disulfide moieties remaining uncoordinated. Cyclic voltammetry performed in CH(2)Cl(2) displayed oxidations at potentials useful for fuel cells (+1.1-1.5 V vs Ag/AgCl), but despite the uncoordinated disulfide moieties, the complexes were reticent to engage in reduction processes.

Experimental evidence for the noninnocence of o-aminothiophenolates: coordination chemistry of o-iminothionebenzosemiquinonate(1-) pi-radicals with Ni(II), Pd(II), Pt(II)

The ligand 2-mercapto-3,5-di-tert-butylaniline, H[L(AP)], an o-aminothiophenol, reacts with metal(II) salts of Ni and Pd in CH3CN or C2H5OH in the presence of NEt3 under strictly anaerobic conditions with formation of beige to yellow cis-[M(II)(L(AP))2] (M = Ni (1), Pd (2)) where (L(AP))1- represents the o-aminothiophenolate(1-) form. The crystal structure of cis-[Pd(II)(L(AP))2][HN(C2H5)3][CH3CO2] has been determined by X-ray crystallography. In the presence of air the same reaction produces dark blue solutions from which mixtures of the neutral complexes trans/cis-[M(II)(L(ISQ))2] (M = Ni (1a/1b), Pd (2a/2b), and Pt (3a/3b)) have been isolated as dark blue-black solid materials. By using HPLC the mixture of 3a/3b has been separated into pure samples of 3a and 3b, respectively; (L(ISQ))1- represents the o-iminothionebenzosemiquinonate(1-) pi-radical. The structures of 1a.dmf and 3a.CH2Cl2 have also been determined. All compounds are square-planar and diamagnetic. 1H NMR spectroscopy established the cis <==> trans equilibrium of 1a/1b, 2a/2b, and 3a/3b in CH2Cl2 solution where the isomerization rate is very fast for the Ni, intermediate for the Pd, and very slow for the Pt species. It is shown that the electronic structures of 1a/1b, 2a/2b, 3a, and 3b are best described as diradicals with a singlet ground state. The spectro- and electrochemistries of all complexes display the usual full electron transfer series where the monocation, the neutral species, the mono- and dianions have been spectroscopically characterized. X-band EPR spectra of the monocations [1a/1b]+ and [3a]+ support the assignment of an oxidation-state distribution as predominantly [M(II)(L(ISQ))(L(IBQ))]+ where (L(IBQ))0 represents the o-iminothionequinone level. In contrast, the EPR spectra of the monoanions [1a/1b]- and [3a]- indicate an [M(II)(L(ISQ))(L(AP)-H)]- distribution but with a significant contribution of the [M(I)(L(ISQ))(2)]- resonance hybrid; (L(AP)-H)2- represents the o-imidothiophenolato(2-) oxidation level. Analysis of the geometric features of 120 published structures of complexes containing ligands of the o-aminothiophenolate type show that high precision X-ray crystallography allows to discern the differing protonation and oxidation levels of these ligands. o-Aminothiophenolates are unequivocally shown to be noninnocent ligands; the (L(ISQ))1- radical form is quite prevalent in coordination compounds and the electronic structure of a number of published complexes must be reconsidered.

Synthesis and Characterization of Zinc and Cadmium Compounds with Arenephosphinothiol Ligands. Crystal and Molecular Structures of [Cd(2){2-(Ph(2)PO)C(6)H(4)S}(4)], [Zn{2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)S}(2)], [Cd{2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}(2)(CH(3)OH)], and [Zn{PhPO(C(6)H(4)S-2)(2)}(bipy)]

The electrochemical oxidation of a metallic anode (zinc or cadmium) in an acetonitrile solution of a series of arenephosphinothiol ligands, 2-(Ph(2)P)C(6)H(4)SH, 2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)SH, 2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)SH, and PhP(C(6)H(4)SH-2)(2) [abbreviated RP-(SH)(x)(), x = 1 or 2], affords [M(RP-S)(2)] and [M(RP-S(2))], M = Zn, Cd. Adducts of several of these compounds with 1,10-phenanthroline and 2,2'-bipyridine have also been obtained by addition of these coligands to the electrolysis phase. The compounds obtained have been characterized by microanalysis, IR, UV-visible, FAB spectrometry and (1)H, (31)P NMR spectroscopic studies. The compounds, [Cd(2){2-(Ph(2)PO)C(6)H(4)S}(4)]CH(3)CN (1), [Zn{2-(Ph(2)P)-6-(Me(3)Si)C(6)H(3)S}(2)] (2), [Cd{2-(Ph(2)PO)-6-(Me(3)Si)C(6)H(3)S}(2)(CH(3)OH)] (3), and [Zn{PhPO(C(6)H(4)S-2)(2)}(bipy)] (4), have been also characterized by single-crystal X-ray diffraction. Compound 1 is binuclear with a {Cd(2)S(2)} core and distorted trigonal bipyramidal {CdO(2)S(3)} geometry about the Cd sites. Compounds 2, 3, and 4 are mononuclear with distorted tetrahedral {ZnP(2)S(2)}, distorted square pyramidal {CdO(3)S(2)}, and distorted trigonal bipyramidal {ZnON(2)S(2)} geometries, respectively. Crystal data: 1, C(42)H(37)N(3)O(2)P(2)S(2)Cd, triclinic, P&onemacr;, a = 13.5780(2) Å, b = 13.8505(2) Å, c = 13.9526(2) Å, alpha = 106.622(1) degrees, beta = 109.693(1) degrees, gamma = 107.137(1) degrees, V = 2133.30(5) Å(3), Z = 2, 9560 reflections, R = 0.0483; 2, C(46)H(50)N(2)P(2)S(2)Si(2)Zn, monoclinic, C2/c, a = 21.332(4) Å, b = 9.391(2) Å, c = 25.938(5) Å, beta = 113.84(3) degrees, V = 4753(2) Å(3), Z = 4, 2564 reflections, R = 0.0377; 3, C(43)H(48)CdO(3)P(2)S(2)Si(2), triclinic, P&onemacr;, a = 12.1237(4) Å, b = 14.0568(4) Å, c = 15.0938(2) Å, alpha = 70.836(2) degrees, beta = 83.410(2) degrees, gamma = 65.397(2) degrees, V = 2208.7(1) Å(3), Z = 2, 4936 reflections, R = 0.0738; 4, C(29)H(22)Cl(3)N(2)OP(2)S(2)Zn, triclinic, P&onemacr;, a = 8.9556(3) Å, b = 12.7911(4) Å, c = 14.0598(5) Å, alpha = 82.671(1) degrees, beta = 73.140(1) degrees, gamma = 74.113(1) degrees, V = 1480.44(9)(1) Å(3), Z = 2, 3820 reflections, R = 0.0511.