CuI complexes with N,N',S,S' scorpionate ligands: evidence for dimer-monomer equilibria.

The heteroscorpionate N, N', S, S' donor ligands 4-methoxy-3,5-dimethyl-2-(3-(methylthio)-1-(3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)propyl)pyridine (L(a)) and 4-methoxy-3,5-dimethyl-2-(2-(methylthio)-1-(3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)ethyl)pyridine (L(b)) were prepared. The Cu(I) complexes [Cu(L(a))]2(BF4)2 (a2(BF4)2) and [Cu(L(b))]2(BF4)2 (b2(BF4)2) were synthesized and characterized by X-ray crystallography. Both compounds exhibit a dinuclear structure, presenting each Cu(I) center in a distorted N, N', S, S' tetrahedral environment. On the basis of nuclear magnetic resonance (NMR) and ESI-mass data, the presence of a mononuclear complex in equilibrium with the dimer was hypothesized for both complexes. The dimerization constants of the processes, 2a(+) = a2(2+) and 2b(+) = b2(2+) , were obtained by (1)H NMR dilution experiments (fast-exchange regime) in CD 3CN: log K(a2(2+)) = 3.55(6) and log K(b2(2+)) = 3.23(5) at 300 K. Thermodynamic parameters were determined by a van't Hoff analysis (280-310 K temperature range): DeltaH(0)(a2(2+)) = -12(1) kJ mol (-1), DeltaH(0)(b2(2+)) = -10(1) kJ mol(-1), DeltaS(0)(a2(2+)) = +27(4) kJ mol (-1), and DeltaS(0)(b2(2+)) = +28(4) kJ mol (-1). Pulsed gradient spin-echo (PGSE) NMR experiments provided the weighted-average hydrodynamic volume (VH) of the species present in CD 3CN solution at different copper concentrations (CCu). Nonlinear interpolation of VH as a function of C Cu for a dimer-monomer equilibrium led to the hydrodynamic volumes of both monomers (VH(0)(M)) and dimers (VH(0)(D)): VH(0)(a(+)) = 620(40) A(3), VH(0)(b(+)) = 550(10) A(3), VH(0)(a2(2+)) = 950(20) A(3), and VH(0)(b2(2+)) = 900(10) A(3). Cyclic voltammetry experiments performed in CH3CN and CH2Cl2 showed a quasi-reversible to irreversible behavior of the Cu(I)/Cu(II) redox couple for both complexes.

Mononuclear and polynuclear copper(I) complexes with a new N,N',S-donor ligand and with structural analogies to the copper thionein core.

The N,N',S-donor ligand 4-methoxy-3,5-dimethyl-2-((3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)methyl)pyridine (L) was prepared from 2-(chloromethyl)-4-methoxy-3,5-dimethylpyridine hydrochloride and 3-(2-(methylthio)phenyl)-1H-pyrazole. The Cu(I) complexes [Cu2(L)2CH3CN][Cu(L)CH3CN](BF4)3 (1), [Cu(L)PPh3]BF4 (2), and [Cu6(L)2(C6F5S)6] (3) were prepared and characterized by X-ray crystallography (PPh3=triphenylphosphine, C6F5S-=pentafluorothiophenolate). The unit cell of compound 1 consists of cocrystallized mononuclear and dinuclear entities in which all of the copper atoms exhibit distorted tetrahedral coordination. Compound 2 is monomeric with L bound in the kappa3-N,N',S mode and a PPh3 molecule that completes the coordination environment. Compound 2 presents a fluxional behavior in CDCl3 solution due to the boat inversion of the six-membered N,N' chelate ring (DeltaH=+43.6(3) kJ mol(-1), DeltaS=-16(1) J mol(-1) K(-1)). Crystallization of 3 in acetonitrile leads to a polynuclear structure that contains a CH3CN molecule coordinated to one of the copper atoms: [Cu6(L)2(C6F5S)6CH3CN] (3a). The core of 3a partially resembles a {Cu4S6} adamantane-like moiety, the only difference being that the Cu-NCCH3 interaction leads to the opening of the cluster by disrupting a Cu-Cu interaction. Part of this assembly is found in the yeast metallothionein copper(I)-cysteinate core whose crystal structure has recently been reported. Two additional [Cu(L)]+ peripheral moieties interact with the cluster by means of bridging thiolates. ESI-mass spectrometry, conductivity measurements, and 1H/19F pulsed gradient spin echo (PGSE) NMR experiments suggest that 3a dissociates in acetonitrile solution: 3a+CH3CN-->[Cu4(C6F5S)6]2-+2[Cu(L)CH3CN]+. The stability of the cluster with respect to the hypothetical mononuclear species, [Cu(L)(C6F5S)], is confirmed by DFT calculations (B3LYP), which illustrate the exergonic character of the reaction: 6[Cu(L')(C6H5S)]-->[Cu6(L')2(C6H5S)6]+4L' (DeltaG298=-58.6 kJ mol(-1), where L' and C6H5S- are simplified models for L and C6F5S-, respectively). The energetics pertinent to the ionic dissociation of the cluster in acetonitrile is computed using the polarizable continuum model (PCM) approach.

Glutathione and N-acetylcysteinylglycine: protonation and Zn2+ complexation.

The speciation study of the Zn(2+)/glutathione (GSH, H(3)G) and Zn(2+)/N-acetylcysteinylglycine (NAcCG, H(2)L) was performed in aqueous solution by means of potentiometry and ESI mass spectrometry. The ligand N-acetylcysteinylglycine was synthesized by protection/activation strategies. (1)H NMR data for the Zn(2+)/NAcCG system at different pH were also collected, to gain insight in the coordination modes for the ligand. The information collected for the NAcCG model ligand were used to propose the structure in solution for the Zn(2+)/GSH complexes. Dinuclear complexes of GSH with Zn(2+), which have never been proposed previously in the literature, were identified in solution and a model of their structure was proposed. Moreover, the Zn(2+) promoted deprotonation of the cysteinyl peptidic NH with formation of five membered (S,N(Cys)(-)) chelating rings was evidenced. The speciation study of the ternary Zn(2+)/GSH/NAcCG system was also performed, showing that the Zn(2+) does not bind preferentially to GSH in presence of NAcCG. The (1)H NMR protonation studies of both GSH and NAcCG were also performed, and a novel proton dissociation microconstant calculation procedure has been proposed and applied to GSH equilibria.

A new chiral N,N',O-donor heteroscorpionate ligand. Structures of Ni2+, Cu2+, Zn2+ complexes and study of solution equilibria by means of 1H NMR/UV-vis titrations and EXSY NMR spectroscopy.

The N,N',O-heteroscorpionate ligand 1-(4-methoxy-3,5-dimethyl-pyridin-2-yl)-2-methyl-1-pyrazol-1-yl-propan-2-ol (LOH) was prepared in two high-yield steps. Complexes [M(LOH)2][MCl4] (M2+ = Cu2+ and Zn2+) and [M(LOH)2]Cl2 (M2+ = Ni2+ and Cu2+) were prepared and characterized by X-ray crystallography. The speciation in solution (methanol:water 95:5) of the M2+/LOH systems was investigated by means of spectrophotometric (Ni2+ and Cu2+) and 1H NMR (Zn2+) titrations. The beta1 and beta2 global formation constants for the [M(LOH)]2+ and [M(LOH)2]2+ species were obtained and are in agreement with the Irving-Williams series: Ni2+< Cu2+> Zn2+. The Zn2+/LOH system was studied by means of quantitative 1H-1H EXSY spectroscopy (300 K, mixing time = 0.2-0.8 s), which allows the description of the equilibria occurring between five octahedral [Zn(LOH)2]2+ structural isomers and tetrahedral [Zn(LOH)Cl]Cl species. Exchange constants kijex and associated rate constants kij suggest that two types of interconversion occur: octahedral-octahedral (faster) and octahedral-tetrahedral (slower). DFT calculations (B3LYP/6-311+G(d)) were employed to evaluate the relative stability of the [Zn(LOH)2]2+ isomers, which are comparable for the five complexes with a maximum energy difference of 6.3 kJ/mol.

Thioamido coordination in a thioxo-1,2,4-triazole copper(II) complex enhances nonapoptotic programmed cell death associated with copper accumulation and oxidative stress in human cancer cells.

The thioamido function of [CuCl2(1H)]Cl (2) (1=4-amino-1,4-dihydro-3-(2-pyridyl)-5-thioxo-1,2,4-triazole), a cytotoxic copper complex, was converted into thioether moieties, leading to the synthesis of [CuCl2(3)]2 (4) and [CuCl2(5)] (6) (3=6-methyl-3-pyridin-2-yl-7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazine; 5=4-amino-5-ethylthio-3-(2-pyridyl)-1,2,4-triazole). These complexes were structurally characterized, and their stability constants, along with their biological activity, were determined. 4 and 6 were slightly less stable and significantly less active than 2. However, as 2, both complexes induced nonapoptotic vacuolar cell death. Copper uptake, investigated in both 2-sensitive and -insensitive cell types, was markedly higher in sensitive cells where it was associated with an increase in oxidized glutathione. These data suggest that the thioamido function enhances the cytotoxicity of copper complexes in cancer cells promoting the accumulation of the metal and its interaction with cell thiols.

Cu(I) dinuclear complexes with tripodal ligands vs monodentate donors: triphenylphosphine, thiourea, and pyridine. A 1H NMR titration study.

Complexes [PPh3Cu(Tr(Mes,Me))] (1), [PPh3Cu(Tr(Me,o-Py))] (2), and [PPh3Cu(Br(Mes)pz(o-Py))] (3) (Tr(Mes,Me) = hydrotris[1,4-dihydro-3-methyl-4-mesityl-5-thioxo-1,2,4-triazolyl]borate; Tr(Me,o-Py) = hydrotris[1,4-dihydro-4-methyl-3-(2-pyridyl)-5-thioxo-1,2,4-triazolyl]borate; Br(Mes)pz(o-Py) = hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]borate; PPh3 = triphenylphosphine) were synthesized by the reaction of dinuclear complexes [Cu(Tr(Mes,Me))]2, [Cu(Tr(Me,o-Py))]2, [Cu(Br(Mes)pz(o-Py))]2, and PPh3. 1-3 were characterized by 1H, 13C, and 31P NMR spectroscopy and ESI-mass spectrometry. Crystal structure analyses were performed for 1 and 2. Both complexes crystallize in the triclinic P space group with the metal in a slightly distorted tetrahedral geometry (S3P coordination) bound by a kappa3-S3 ligand and a PPh3 molecule. The solution molecular structures were investigated by means of variable-temperature (210-310 K, CDCl3, 1-2; 200-310 K, CD2Cl2, 3) and NOESY NMR spectroscopy. The solution structures of 1-2 are in accordance with the X-ray structures, and the complexes do not exhibit fluxional behavior. On the other hand, 3 is subject to an equilibrium between two species with a coalescing temperature of approximately 260 K. DFT geometry optimizations suggest that the major species of 3 consists of the Br(Mes)pz(o-Py) ligand bound to Cu(I) in the kappa3-S2H fashion with two C=S groups and a [Cu...H-B] interaction. A PPh3 completes the copper coordination (S2HP coordination). The complex [TuCu(Tr(Mes,Me))] (4) (Tu = thiourea) was crystallized using an excess of Tu with respect to [Cu(Tr(Me,2-Py))]2 (approximately a 6:1 ratio). The metal adopts a distorted tetrahedral geometry with an overall S3H coordination determined by the bound kappa3-S2H ligand (two C=S groups and a [B-H...Cu] interaction) and by a Tu. The reactivity of dinuclear complexes [Cu(Tr(Mes,Me))]2, [Cu(Tr(Me,o-Py))]2, and [Cu(Br(Mes)pz(o-Py))]2 with monodentate ligands was investigated by means of NMR titrations with PPh3, Tu. and pyridine (Py), and formation constants for the adducts [DCu(L)] (D = monodentate donor, L = tripodal ligand) were determined.

Synthesis, solution equilibria and antiproliferative activity of copper(II) aminomethyltriazole and aminomethylthioxotriazoline complexes.

The aim of the present study was the synthesis, the determination of formation constants, and the evaluation of the antiproliferative activity of two copper(II) complexes formed with triazole-type ligands. The synthesis of the unsymmetrical triazole ligand 4-amino-3-aminomethyl-5-methyl-1,2,4-triazole (L1), and its copper(II) complex is reported. The ligand was prepared by functionalization of the carboxylate function of tert-butyloxycarbonyl (BOC) protected glycine O-methyl ester. All intermediates and final products were isolated and characterized with IR, 1H NMR, and elemental analysis. X-ray structures of the ligand as a sulfate salt ((H2L1)2SO4.H2O) and the copper(II) complex [CuCl2(L1)(2)] are described. The ligand forms a (N,N) bidentate chelate with the amino group and one triazole nitrogen atom. The tetragonally distorted octahedral coordination of Cu(II) results from two axially coordinated chloride ions. Protonation constants for L1 and speciation of the Cu(II)/L1 system were determined in 0.1 M aqueous KCl solution at 25 degrees C. Complexes formed in solution were also characterized by visible spectrophotometry. Ligand substitution competition between L1 and glycine has also been studied using potentiometric titrations. Antiproliferative activities of ([CuCl2(L1)2]) and [CuCl2(H2L2)]Cl, where HL2 is the 5-thioxo analog of L1, against human tumor cell lines HT1080 and HT29 as well as normal human fibroblasts (HF) are presented along with the antiproliferative activities of L1, CuCl2, and cisplatin. Activity of these two complexes are discussed and compared with the activity of analogous compounds reported in the literature which contain pyridyl groups in place of the aminomethyl group. In particular, it is suggested that a lypophilic residue such as a pyridyl group is important for antiproliferative activity of this class of compounds.

Synthesis, structure, and electrochemical properties of copper(I) complexes with S/N homoscorpionate and heteroscorpionate ligands.

Dinuclear Cu(I) complexes with bifunctionalized homoscorpionate ligands, hydrotris(thioxotriazolyl)borato [Li(Tr(Me,o)(-)(Py)) (1) and Li(Tr(Mes,Me)) (2)], and the heteroscorpionate ligand hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]borato [K(Br(Mes)pz(o)(-)(Py))] (3) were synthesized and crystallographically characterized. The complexes [Cu(Tr(Mes,Me))](2) (4) and [Cu(Tr(Me,o)(-)(Py))](2) (5) exhibit a similar coordination geometry where every metal is surrounded by three thioxo groups in a trigonal arrangement. The presence of a [B-H...Cu] three-center-two-electron interaction in both compounds causes the overall coordination to become tetrahedrally distorted (S(3)H coordination for each metal). The complex [Cu(Br(Mes)pz(o)(-)(Py))](2) (6) presents a trigonal geometry in which the metals interact with two thioxo groups and a bridging pyrazolyl nitrogen atom. A weak contact with a pyridine nitrogen atom completes the coordination of the metals (S(2)N,N' coordination for each metal). [Cu(Tr(Mes,Me))](2), [Cu(Tr(Me,o)(-)(Py))](2), and [Cu(Br(Mes)pz(o)(-)(Py))](2) exhibit fluxional behavior in solution as evidenced by variable-temperature NMR spectroscopy, and for 5 and 6 two species in equilibrium [in the ratio 2/1 for 5 (CDCl(3)) and 3/2 for 6 (CD(2)Cl(2))] are distinguishable in the (1)H NMR spectra at 270 K. 2D-NOESY spectra recorded at 270 K assisted in the attribution of solution molecular geometries for each isomer of 5 and 6. The free energy of activation (DeltaG()(Tc)) was determined for both equilibria from the evaluation of the coalescence temperature. DFT calculations were performed to describe plausible molecular geometry for the minor isomer of 5 and 6 and to propose a possible mechanism of interconversion between major and minor isomers. Cyclic voltammograms were recorded in CH(2)Cl(2) (3 and 6) or CH(2)Cl(2)/CH(3)CN (1/1, v/v) (2, 4, and 5) solutions using 0.1 M TBAHFP or TBAOTf as supporting electrolytes. [Cu(Tr(Mes,Me))](2), [Cu(Tr(Me,o)(-)(Py))](2), and [Cu(Br(Mes)pz(o)(-)(Py))](2) exhibit a quasi-reversible Cu(I)/Cu(II) redox behavior with E(pa) = +719 mV and E(pc) = +538 mV for 4, E(pa) = +636 mV and E(pc) = -316 mV for 5, and E(pa) = +418 mV and E(pc) = -319 mV for 6.

Versatile ligand behavior of hydrotris(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate. Syntheses and crystal structures of Cu(I) and Bi(III) complexes.

Preparations of copper(I) and bismuth(III) complexes of hydrotris(4-ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate (Tr(Et,Me)) are described. These complexes have been characterized by means of spectroscopy and microanalysis. Molecular structures of [Cu(Tr(Et,Me))](2) x 2.5CH(3)CN x 0.5H(2)O (3a) and [Bi(Tr(Et,Me))(2)]NO(3) x 2CHCl(3) (4a) have been determined by single-crystal X-ray diffraction. In the centrosymmetric dimeric copper(I) complex, Tr(Et,Me) acts in the k(3)S,S',H:kS' ' coordination mode. The metal is found in a distorted trigonal geometry as the ligand exhibits an "S(3)-inverted" conformation at the boron center so that a weak [B-H.Cu] agostic interaction renders the overall coordination of the (3 + 1) type. On the other hand, in the bismuth complex, Tr(Et,Me) presents the k(3)S,S',S' ' coordination mode and the "S(3)-normal" conformation. The metal is found in a regular octahedral geometry bound by six thioxo groups of two ligands. Species distributions in solution have been studied using electrospray ionization mass spectrometry upon dissolution of 3a and 4a crystals in acetonitrile. Monomeric and polynuclear copper(I) complexes with different M:L ratios are present in solution, while for 4a only the monomeric species is present.

Synthesis and molecular structure of the dihydrobis(thioxotriazolinyl)borato complexes of zinc(II), bismuth(III), and nickel(II). M...H-B interaction studied by Ab initio calculations.

Reacting the heterocycle 5-thioxo-1,4-dihydro-4-ethyl-3-methyl-1,2,4-triazole (thioxotriazoline) with sodium tetrahydroborate in the molar ratio of approximately 2:1 at 130 degrees C provides the new ligand dihydrobis(thioxotriazolinyl)borato, [Bt(Et,Me)](-), as its sodium salt. The neutral complexes of this anionic ligand with zinc(II), bismuth(III), and nickel(II) have been synthesized and characterized by X-ray crystallography. In every complex, the ligand is coordinated to the metal in the S(2) mode, generating eight-membered chelate rings. The bismuth and nickel complexes exhibit two M.H-B interactions responsible for the dodecahedral and octahedral geometries, respectively. For the zinc complex, the trigonal-bipyramidal coordination is achieved with an apical Zn.H-B interaction. The crystal structures for the three complexes are described, and ab initio calculations on Bi(III), Ni(II), and Zn(II) compounds have been performed in order to assess the nature of the M.H-B interaction and its role for the definition of the molecular geometries.

Synthesis, molecular structure, solution equilibrium, and antiproliferative activity of thioxotriazoline and thioxotriazole complexes of copper II and palladium II.

Preparations of copper(II) and palladium(II) complexes of 4-amino-5-methylthio-3-(2-pyridyl)-1,2,4-triazole (L(1)) and the copper(II) complex of 1,4-dihydro-4-amino-3-(2-pyridyl)-5-thioxo-1,2,4-triazole (HL) are described. These complexes have been characterized by means of spectroscopy and microanalysis. Molecular structures of HL (1), [CuCl(2)(H(2)L)]Cl.2H(2)O (2a), cis-[CuCl(2)(L(1))] (3), and cis-[PdCl(2)(L(1))] (4) have been determined by single-crystal X-ray diffraction. The HL ligand acts as a N,S bidentate through the thioxo moiety and the exo-amino group whilst the ligand L(1) forms N,N coordination complexes through the pyridine and triazole nitrogen atoms. Speciation in solution of the systems Cu/HL and Cu/L(1) have been determined by means of potentiometry and spectrophotometry as well as for the Cu/L(1)/A (HA=glycine) system in order to determine species present at physiological pH. Antiproliferative activity of these complexes and their ligands was evaluated, using the AlamarBlue Assay, on normal human fibroblasts (HF) and human fibrosarcoma tumor (HT1080) cells. The copper compounds cis-[CuCl(2)(H(2)L)]Cl and cis-[CuCl(2)(L(1))] exerted significant antiproliferative activity of both normal and neoplastic cells; although dose-response experiments revealed that the HT1080 cell line was more sensitive to the tested drugs than normal fibroblasts.

Ion pair charge-transfer complexes between anionic and cationic metal-dithiolenes [M(II) = Pd, Pt].

New [M(R(2)pipdt)(2)](BF(4))(2) salts [R(2)pipdt = N,N'-dialkyl-piperazine-2,3-dithione; M = Pd(II), R = Me and M = Pt(II), R = Me, Et, Pr(i)] bearing redox-active cationic dithiolene complexes have been prepared and characterized. These cations react with the redox-active [M(mnt)(2)](2-) [M = Pd(II), Pt(II); mnt = maleonitrile-2,3-dithiolate] anionic dithiolenes to form salts describable as ion pair charge-transfer complexes. X-ray crystallographic studies have shown that [M(Me(2)pipdt)(2)][M(mnt)(2)] complexes, with M = Pd(II) and Pt(II), are isomorphous. Crystal data of the Pt salt (3a): triclinic, Ponemacr; (No. 2); Z = 1; T = 293(2) K; a = 6.784(7) A, b = 8.460(6) A, c = 13.510(5) A, alpha = 100.63(2) degrees, beta = 104.04(2) degrees, gamma = 96.90(2) degrees; R1 = 0.0691 [wR2 = 0.2187 (all data)]. Structural data show that approximately square-planar [Pt(Me(2)pipdt)(2)] dications and regular square-planar [Pt(mnt)(2)] dianions form an infinite anion-cation one-dimensional stack along axis a with a Pt...Pt a/2 distance of 3.392 A and a Pt...Pt...Pt angle of 180 degrees. Anions and cations arrange themselves face-to-face so as to take on a staggered arrangement. These salts exhibit strong absorptions in the visible-near-infrared region assigned to ion pair charge-transfer transitions. A relation between the optical and thermal electron transfer in the solid state is obtained using a "Marcus-Hush model", and a solid-state electrical conductivity in agreement with expectations is observed. Vibrational spectroscopy is in agreement with the existence of charge-transfer interactions between the cationic and anionic components of the salts.

A New Type of Doubly Silylamido-Bridged Cyclopentadienyl Group 4 Metal Complexes.

Doubly bridged di(silyl-η-amido)cyclopentadienyltitanium and -zirconium complexes and their related cations as the [(PhCH2 )B(C6 F5 )3 ]- salts have been isolated (see structure of the Ti derivative). The neutral benzylzirconium complex was a very efficient catalyst in the presence of methylaluminoxane for producing high molecular weight polyethylene and ethylene-1-hexene copolymers.

Structure and Bonding of Diiodine Adducts of the Sulfur-Rich Donors 1,3-Dithiacyclohexane-2-thione (ptc) and 4,5-Ethylenedithio-1,3-dithiole-2-thione (ttb).

The reactions of I(2) with ptc and ttb (title ligands) have been investigated in CHCl(3) solution at different temperatures by spectrophotometry. A least-squares method procedure provided evidence for the formation of the 1:1 adducts. Crystals of the latter have been analyzed by X-ray diffraction methods (both monoclinic, P2(1)/c; ptc.I(2), a = 8.691(6) Å, b = 9.010(6) Å, c = 13.237(5) Å, beta = 103.43(2) degrees, Z = 4, R = 0.0305; ttb.I(2), a = 12.090(6) Å, b = 6.433(5) Å, c = 15.731(6) Å, beta = 99.30(2) degrees, Z = 4, R = 0.0419). Both structures show that the thionic sulfur (in any case a CS(3) group inserted in a ring) is bound almost collinearly with the diiodine molecule. The d(S-I) separations are 2.755(2) and 2.805(3) Å in the ptc.I(2) and ttb.I(2) adducts, respectively, while d(I-I) is practically the same (2.812(2) Å). An evident stereochemical difference is that the S-I-I moiety is nearly coplanar with the CS(3) group in ptc.I(2) while it is upright in ttb.I(2). However, the feature is not expected to cause a major electronic difference. In order to reproduce the structural features, different ab initio approaches have been attempted, with the best results being obtained with the density functional method (DFT). Despite the S-I distances which are slightly longer than the experimental ones (by ca. 0.25 Å), the distribution of filled and empty frontier molecular orbitals (MOs) allows a good interpretation of the visible spectra. Also a rationalization of the sigma electronic density distributed over the three centers S-I-I has been attempted by qualitative MO theory (EHMO method). Provided the good agreement with the higher level calculations, the perturbation theory arguments highlight the variable sp hybridization at the central iodine atom as the electronic factor of importance. The strength of the donor (D) affects significantly the redistribution of six electrons over four atomic orbitals, and the classic model is revised as a four-orbital/six-electron one. Thus, it is pointed out that a major four-electron repulsion is exerted over the D-I or the I-I linkages with major consequences for their respective lengths.

Syntheses, X-ray Crystal Structures, and Spectroscopic Properties of New Nickel Dithiolenes and Related Compounds.

The direct addition of nickel powder to the reaction mixtures of 1,3-dialkyl-4,5-dioxoimidazolidine-2-thione (1) with the thionation Lawesson reagent produces [Ni(II)(R(2)timdt(-))(2)] (R(2)timdt = 1,3-dialkylmidazolidine-2,4,5-trithione). These complexes belong to a new class of nickel-dithiolenes, showing remarkably high absorption (epsilon approximately 80 000 dm(3) mol(-)(1) cm(-)(1), lambda approximately 1000 nm) in the near-infrared region (near-IR), accompanied by high photochemical stability that makes these complexes promising near-IR dyes. In the absence of nickel the reaction yields generally the compounds 4,5,6,7-tetrathiocino[1,2-b:3,4-b']diimidazolyl-1,3,8,10-tetraalkane-2,9-dithione (2) instead of the expected 1,3-dialkylimidazolidine-2,4,5-trithione. However, with bulky substituents on the nitrogen atoms, well-characterized reaction products have not been obtained until now. Only in the R = Pr(i) case, the new tetrathiocino isomer (4,5,9,10-tetrathiocino[1,2-b:5,6-b']diimidazolyl-1,3,6,8-tetraisopropane-2,7-dithione (3) in trace amounts and the bis(1,3-diisopropyl-2-thioxoimidazolin-4-yl) disulfide (4) were isolated from the reaction with the Lawesson reagent and P(4)S(10), respectively. Reaction of 5 with different amounts of I(2) leads to a variety of products, and among them the following derivatives have been characterized: [Ni(II)(Pr(i)(2)timdt(-))(2)].2I(2) (6) a neutral adduct in which each I(2) molecule interacts with each peripheral thione sulfur atom of 5; [Ni(II)(Pr(i)(2)timdt(-))(2)].2I(2).(1)/(2)I(2) (7), which differs from 6 with the presence of half diiodine as a guest; with a larger diiodine excess (starting from a 1:10 molar ratio) a partial oxidation of 5 is achieved and [Ni(II)(Pr(i)(2)timdt(-))(2)] [Ni(II)(I)(2)(Pr(i)(2)timdt)(2)].5I(2) (8) is formed, a ligand mixed-valence compound in which the square-planar complex 5 and the octahedral complex [Ni(II)(I)(2)(Pr(i)(2)timdt)(2)] (the ligand in neutral form) are bound by diiodine molecules in such a way that a sequence of 12 iodine atoms (S.I(2).I(2).I(-).I(2).I(-).I(2).I(2).S) is formed. Spectroscopic and X-ray diffractometric studies of 3 (monoclinic, space group P2(1)/n, a = 13.104(6) Å, b = 15.091(6) Å, c = 6.067(7) Å, beta = 103.00(2) degrees, Z = 2), 4 (monoclinic, space group C2/c, a = 18.717(5) Å, b = 8.846(5) Å, c = 14.475(5) Å, beta = 97.78(2) degrees, Z = 4), 5 (orthorhombic, space group Pna2(1), a = 18.806(5) Å, b = 5.628(7) Å, c = 23.665(5) Å, Z = 4), 7 (triclinic, space group P&onemacr;, a = 9.126(7) Å, b = 9.153(7) Å, c = 12.924(6) Å, alpha = 82.11(2), beta = 70.14(2), gamma = 72.51(2) degrees, Z = 1), and 8 (monoclinic, space group C2/c, a = 21.971(6) Å, b = 28.081(5) Å, c = 12.207(8) Å, beta = 93.57(2) degrees, Z = 4) are given.

Synthesis, Characterization, and Crystal Structures of New Dications Bearing the -Se-Se- Bridge.

Starting from 1,3-dimethyl-4-imidazoline-2-selone (1), 1,2-bis(2-selenoxo-3-methyl-4-imidazolinyl-2-)ethane (3) and 1,3-dimethylimidazolidine-2-selone (4), the following six compounds, [(C(5)H(8)N(2)Se-)(2)](2+).2Br(-) (I), [(C(5)H(8)N(2)Se-)(2)](2+).2I(-) (II), [(C(5)H(8)N(2)Se-)(2)](2+).Cl(-).I(3)(-) (III) [(C(5)H(10)N(2)Se-)(2)](2+).Br(-).IBr(2)(-) (IV), [(C(5)H(7)N(2)Se-)(2)](2+).I(3)(-).(1)/(2)I(4)(-) (V) and [(C(5)H(7)N(2)Se-)(2)](2+).2I(-).CH(3)CN (VI), in which the selenium compounds are oxidized to dications bearing the uncommon -Se-Se- bridge, have been prepared, and I-V crystallographically characterized. I and III were obtained by reacting 1 with IBr and ICl respectively, while II was obtained by reduction of previously described hypervalent selenium compound of 1 (5) bearing the I-Se-I group with elemental tellurium. These three compounds contain the same [(C(5)H(8)N(2)Se-)(2)](2+) dication balanced by two bromides in I, two iodides in II, and Cl(-) and I(3)(-) in III. However, on the basis of the Se-Cl bond length of 2.778(5) Å, III can also be considered as formed by the [(C(5)H(8)N(2)Se-)(2)Cl](+) cation, with I(3)(-) as counterion. Similarly to III, compound IV, which was obtained by reacting 4 with IBr, can be considered as formed by [(C(5)H(10)N(2)Se-)(2)Br](+) cations and IBr(2)(-) anions. As in II, compound V has been prepared by reduction of the hypervalent selenium compound of 3 (6) bearing two I-Se-I groups with elemental tellurium. In V, the [(C(5)H(7)N(2)Se-)(2)](2+) cation is balanced by I(3)(-) and half I(4)(2-) anions. The structural data show that all the cations are very similar, with Se-Se bond lengths ranging from 2.409(2) to 2.440(2) Å. FT-IR and FT-Raman spectra of I-VI allow one to identify two bands around 230 +/- 10 and 193 +/- 5 cm(-1) that are common to all compounds. These bands are generally strong in the FT-Raman and weak in the FT-IR spectra and should contain a contribution of the nu(Se-Se) stretching vibration. The spectra are also in good agreement with the structural features of the polyhalide anions present in the crystals. Crystallographic data are as follows: I is monoclinic, space group P2(1), with a = 9.849(6) Å, b = 11.298(5) Å, c = 7.862(6) Å, beta = 106.44(2) degrees, Z = 2, and R = 0.0362; II is monoclinic, space group P2(1), with a = 8.063(6) Å, b = 11.535(5) Å, c = 10.280(5) Å, beta = 107.13(2) degrees, Z = 2, and R = 0.0429, III is monoclinic, space group P2(1)/n, with a = 10.431(7) Å, b = 18.073(5) Å, c = 11.223(6) Å, beta = 100.76(2) degrees, Z = 4, and R = 0.0490; IV is monoclinic, space group P2(1)/n, with a = 10.298(5) Å, b = 18.428(7) Å, c = 11.475(6) Å, beta = 104.10(4) degrees, Z = 4, and R = 0.0300; V is triclinic, space group P&onemacr;, with a = 7.456(6) Å, b = 11.988(5) Å, c = 12.508(5) Å, alpha = 79.32(2) degrees, beta = 85.49(2) degrees, gamma = 80.62(2) degrees, Z = 2, and R = 0.0340.