Synthesis and structures of mono and binuclear nickel(II) thiolate complexes of a dicompartmental pseudo-macrocycle with N(imine)2S2 and N(oxime)2S2 metal-binding sites

The reaction of [Ni(pftp)] [pftp = N,N-propane-1,3-diyl-(6-formyl-4-methyliminatothiophenolato)] with hydroxylamine hydrochloride in the presence potassium acetate in MeOH resulted in the formation of the complex [Ni(LH₂)] [L = N,N-propane-1,3-diyl-(4-methyl-2-methyliminato-6-methyloxime-thiophenolato)] in good yield. A single crystal X-ray diffraction structural determination showed a mononuclear nickel(II) complex with the new acyclic ligand LH₂ that had been functionalised with two oxime groups containing an empty N(oxime)₂S₂ pocket to which another metal ion could be added. A further reaction of [Ni(LH₂)] with NiCl₂·6H₂O, triethylamine and ammonium hexafluorophosphate in MeOH gave a dark red product that yielded red crystals of [Ni₂(LH)]PF₆·DMF via slow recrystallisation from a DMF/PrⁱOH solvent mixture. A single crystal X-ray diffraction study of these crystals confirmed the presence of a dinuclear nickel(II) complex linked by a dithiolato-bridge. Both nickel(II) ions exhibited square-planar geometry where the metal centres are coordinated in two distinct cis-S₂N(imine)₂ and cis-S₂N(oxime)₂ binding sites provided by the new dicompartmental oxime/thiolate-containing ligand LH.     

Antitumor activity of phenylene bridged binuclear bis(imino-quinolyl)palladium(II) and platinum(II) complexes

Antitumor effects of a known bis(imino-quinolyl)palladium(II) complex 1 and its newly synthesized platinum(II) analogue 2 were evaluated against human breast (MCF-7) and human colon (HT-29) cancer cell lines. The complexes gave cytotoxicity profiles that were better than the reference drug cisplatin. The highest cytotoxic activities were pronounced in complex 2 across the two examined cancer cell lines. Both compounds represent potential active drugs based on bimetallic complexes.     

Syntheses and characterization of mono- and di-N-hydroxyethylated tetraaza macrocycles containing eight C-methyl groups and their nickel(II) and copper(II) complexes

New partially N-hydroxyethylated 14-membered tetraaza macrocycles 1,8-bis(2-hydroxyethyl)-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L²) and 1-(2-hydroxyethyl))-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L³) have been synthesized selectively by the one-step reaction of 2,5,5,7,9,12,12,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L¹) with 2-hydroxyethyl bromide. The complexes [NiL³]²⁺, [CuL²]²⁺, and [CuL³]²⁺ have been prepared and characterized. The complex [CuL²](ClO₄)₂ has a square-pyramidal coordination geometry with one apical oxygen atom; only one of the two hydroxyethyl groups is coordinated to the metal ion. Electronic absorption spectra of [CuL³](ClO₄)₂ containing one hydroxyethyl pendant arm indicate that the geometry is similar to that of [CuL²](ClO₄)₂. Unexpectedly, the nickel(II) complex [NiL³](ClO₄)₂ has a severely distorted trigonal bipyramidal coordination geometry with the oxygen atom of the pendant arm at the equatorial position. The Ni---O bond distance of the nickel(II) complex is shorter, or not longer, than the Ni---N bond distances. The ligand in [CuL²]²⁺ is in the RRSS (trans-III) configuration, as usual, whereas that in [NiL³]²⁺ has the RRRR (trans-V) conformation. The coordination geometry and properties of [NiL³]²⁺ are quite different from those reported for other related nickel(II) complexes containing one functional pendant arm.     

Cytotoxic palladium(II) complexes of 8-aminoquinoline derivatives and the interaction with human serum albumin

Palladium(II) complexes are potential antitumor metallodrugs for their chemical resemblance to platinum(II) complexes. Two palladium(II) complexes (1 and 2) in the formula of [PdLⁿCl] [L¹ = N-(tert-butoxycarbonyl)-l-methionine-N′-8-quinolylamide, L² = L-alanine-N′-8-quinolylamide] have been synthesized accordingly. The structures of the complexes were fully characterized by X-ray crystallography. The palladium(II) center in 1 is coordinated by two N atoms and an S atom from L¹ with one chloride anion as the leaving group; while that in 2 is coordinated by three N atoms from L² with one chloride anion as the leaving group. The interaction between complex 1 and human serum albumin (HSA) has been investigated using fluorescence and circular dichroism spectroscopies. The complex seems to react with HSA chiefly through hydrophobic and electrostatic interactions, and it does not alter the α-helical nature of HSA. The cytotoxicity of these complexes has been tested against the human cervical cancer (HeLa), human mammary cancer (MCF-7), and human lung cancer (A-549) cell lines. Complex 1 displays a cytotoxic activity comparable to that of cisplatin, but complex 2 is less active than cisplatin.     

Synthesis and crystal structures of trans-dichlorobis{tri(2-methylphenyl)stibine}palladium(II) and trans-dibromobis{tri(2-methylphenyl)stibine}palladium(II)

Tri(2-methylphenyl)stibine reacts with [PdX₂(cod)] or PdX₂ (X = Cl or Br) to give the trans-[PdX₂{Sb(2-Me- C₆H₄)₃}₂]. The single crystal X-ray structures of trans-[PdX₂{Sb(2-Me-C₆H₄)₃}₂] (X = Cl or Br) revealed that the palladium atom is a square planar environment with mutually trans-Sb(2-methylphenyl)₃ ligands. Iodine analogous of complex can also be prepared from trans metallation reaction with sodium iodine.     

Complexation of lead(II) by nicotinamide (nia): Crystal structure of polymeric Pb(nia)(NO3)2

Lead(II) nitrate complex with nicotinamide has been synthesized and characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR, UV-Vis spectroscopy and X-ray crystallography. The Pb²⁺ centres linked through nicotinamide and nitrate(V) ligands form a two-dimensional polymeric structure defining a double layer and each layer is also stabilized by hydrogen bonds of N-H?O and C-H?O type.     

Coordination chemistry of acrylamide 6: Formation and structural characterization of [Fe(O-OC(NH2)CHCH2)6][Fe2OCl6]

The new acrylamide iron(II)/iron(III) complex [Fe(O-OC(NH₂)CHCH₂)₆][Fe₂OCl₆] (1) was obtained by the reaction of a mixture of anhydrous FeCl₂ and anhydrous FeCl₃ with acrylamide (molar ratio 1:2:6) in 98% pure commercial nitromethane under nitrogen atmosphere. According to an X-ray structural analysis, the acrylamide ligands in the cation are coordinated via the amide-oxygen atoms. The formation of the (μ-oxo)bis[trichloroferrate(III)]²⁻ anion presumably resulted from partial hydrolysis of FeCl₃ or [FeCl₄]⁻ by small amounts of water in the nitromethane and/or by the nitromethane itself.     

Cobalt(III) complexes with linear hexadentate N6 or N4S2 donor set atoms providing pyridyl-amide-amine/thioether coordination

Two new cobalt(III) complexes of symmetric hexadentate ligand with N₆ [1,10-bis(2-picolinamide)-4,7-diazadecane (pycdpnen)] and N₄S₂ [1,8-bis(2-picolinamide)-3,6-dithiaoctane (pycdadt)] donor set atoms have been synthesized as perchlorate salts and characterized by spectroscopic methods. All two ligands with strong-field pyridylcarboxamido N donor stabilize Co(III) as demonstrated by the facile oxidation of the cobalt center. The structures of [Co(pycdpnenH₋₂)](ClO₄) (1) and [Co(pycdadtH₋₂)](ClO₄) · H₂O (2) investigated by COSY, HMBC, HMQC and NOESY NMR studies show that compounds 1 and 2 have the same geometrical configuration. The X-ray analysis reveals that complex 2 crystallizes in a orthorhombic space group Pccn. The cation [Co(pycdadtH₋₂)]⁺ is distorted octahedral with the two pyridyl groups in cis position.     

Reactivity of palladium(II) methyl complexes towards CO2: formation of carbonate complexes

Treatment of a benzene solution of (tmeda)PdMe₂ or (dppe)PdMe₂ with carbon dioxide gives the corresponding methyl bicarbonate complex, (L-L)PdMe(O₂COH). These were characterised by NMR spectroscopy and elemental analysis. Under strictly dry conditions no reaction was observed. Recrystallisation of the tmeda bicarbonate complex from acetone yields the corresponding η²-carbonate complex, which was characterised by X-ray crystallography. The reaction probably proceeds through attack by free carbonic acid on the dimethyl complex.     

Oligonuclear nickel(II) complexes sustained by intermolecular hydrogen-bonding

Reaction of Ni(OAc)₂ with the symmetric `end-off' compartmental proligand 2,6-[N,N^′-bis(2-hydroxy-phenylmethyl)-N,N^′-bis(2-pyridylmethyl)aminomethyl]-4-methylphenol (H₃L) in the presence of NaPF₆ has been found to generate a homotetranuclear nickel(II) complex [(Ni₄HL)(L)(OAc)₂(H₂O)₂(HOAc)₂]PF₆. The crystal structure of the complex reveals that the complex is donor asymmetric and that the extended supra-ligand periphery is maintained by a tight hydrogen-bond between two pendant phenol/phenoxy groups of adjacent ligands and by further tight hydrogen-bonds between coordinated acetic acid molecules and the remaining pendant phenols of the ligand, generating a double acid salt of the type [CH₃COO?H?LH?L?H?OOCCH₃]⁵⁻. Reaction of H₃L with Ni(OAc)₂ and NaClO₄ in methanol gave the complex [Ni₂(HL)(OAc)₂(OH₂)₂][ClO₄]. The structure was determined by X-ray diffraction and showed that the complex exists as a dimer promoted by intermolecular hydrogen-bonding.     

Synthesis, structures, and magnetic properties of dicopper(II) and dinickel(II) complexes with a new bis(macrocycle), 7,7-(2-hydoxypropane-1,3-diyl)bis{3,7,11,17-tetraazabicyclo[11.3.1]- heptadeca-1(17),13,15-triene}

A new bis(macrocycle) ligand, 7,7^′-(2-hydoxypropane-1,3-diyl)-bis{3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene} (HL), and its dicopper(II) ([Cu₂(HL)Cl₂](NO₃)₂ · 4H₂O (4a), [Cu₂(HL)I₂]I₂ · H₂O (4b)) and dinickel(II) ([Ni₂(L)(OH₂)](ClO₄)₃ (5a), [Ni₂(L)(OH₂)]I₃ · 2H₂O (5b), [Ni₂(L)N₃](N₃)₂ · 7H₂O (5c)) complexes have been synthesized. The alkoxide bridged face-to-face structure of the dinickel(II) complex 5c has been revealed by X-ray crystallography, as well as the “half-opened clamshell” form of the bis(macrocyclic) dicopper(II) complex 4b. Variable temperature magnetic susceptibility studies have indicated that there exists intramolecular antiferromagnetic coupling (J=−33.8 cm⁻¹ (5a), −32.5 cm⁻¹ (5b), and −29.7 cm⁻¹ (5c)) between the two nickel(II) ions in the nickel(II) complexes.     

Synthesis of two configurational isomers of a 14-membered tetraaza macrocycle bearing N-CH2CH2CONH2 pendent arms and their copper(II) complexes: Crystal structures of the complexes

Two isomers of 1,8-bis(N-carbamoylethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L²) bearing two N-CH₂CH₂CONH₂ groups, C-meso-L² and C-racemic-L², have been prepared and characterized. Each isomer reacts with Cu(II) ion to form a five-coordinate complex, [Cu(C-meso-L²)](ClO₄)₂ (1) or [Cu(C-racemic-L²)](ClO₄)₂ (2), in which only one pendent amide group is coordinated to the metal ion. The crystal structure of 1 · CH₃CN shows that the complex possesses trans-III-type N-configuration and has a slightly distorted square-pyramidal coordination geometry with a relatively long axial Cu-O (N-CH₂CH₂CONH₂) bond (2.207(3) Å). On the other hand, 2 exhibits trans-V configuration and has a slightly distorted trigonal bipyramidal coordination geometry with a very short equatorial Cu-O (N-CH₂CH₂CONH₂) bond (2.007(3) Å); the Cu-O distance is distinctly shorter than the Cu-N distances (2.062(4)-2.090(4) Å). The complex 1 exhibits a d-d transition band at approximately 565 nm, whereas the band for 2 is observed at approximately 770 nm.     

Mixed chloride–phosphine complexes of the dirhenium core: 9. The first mixed monodentate phosphine complex, 1,2,7,8-Re2Cl4(PMe2Ph)3(PEt2H)

The reduction of the Re₂ ⁵⁺ core in 1,2,7-Re₂Cl₅(PR₃)₃ molecules, followed by addition of 1 equiv. of a different phosphine ligand, PR₃ ′, allows the preparation of the mixed monodentate phosphine compounds of the Re₂ ⁴⁺ type, namely 1,2,7,8-Re₂Cl₄(PR₃)₃(PR₃ ′). The stereochemistry of the starting materials dictates the stereochemistry of the final products. The one-electron reduction of the 1,2,7-isomer of Re₂Cl₅(PMe₂Ph)₃ with KC₈ to the corresponding anion, [1,2,7-Re₂Cl₅(PMe₂Ph)₃]⁻ (1), followed by non-redox substitution of one chloride ion by one diethylphosphine, PEt₂H, afforded the first mixed monodentate phosphine compound of the dirhenium(II) core, Re₂Cl₄(PMe₂Ph)₃(PEt₂H) (2), in good yield. Crystal structure determination as well as other physical methods and elemental analysis unambiguously confirmed the formation of 2. The related system 1,2,7-Re₂Cl₅(PMe₃)₃---Co(C₅H₅)₂---PEt₂H leads to several products, one of which is 1,2,7,8-Re₂Cl₄(PMe₃)₃(PEt₂H) (3).     

Copper complexes with new oxaaza-pendant-armed macrocyclic ligands: X-ray crystal structure of a macrocyclic copper(II) complex

The synthesis of new oxaaza macrocyclic ligands (2-4) derived from O¹,O⁷-bis(2-formylphenyl)-1,4,7-trioxaheptane and functionalized tris(2-aminoethyl)amine are described. Mononuclear copper(II) complexes were isolated in the reaction of the corresponding macrocyclic ligand and copper(II) perchlorate. The structure of the [Cu(2)](ClO₄)₂ complex was determined by X-ray diffraction analysis. The copper(II) ion is five-coordinated by all N₅ donor atoms, efficiently encapsulated by the amine terminal pendant-arm, with a trigonal-bipyramidal geometry. The complexes are further characterized by UV-Vis, IR and EPR studies. The electronic reflectance spectra evidence that the coordination geometry for the Cu(II) complexes is trigonal-bipyramidal with the ligands 1 and 2 or distorted square-pyramidal with the ligands 3 and 4. The electronic spectra in MeCN solutions are different from those in the solid state, which suggest that some structural modification may occur in solution. The EPR spectrum of powder samples of the copper complex with 2 presents axial symmetry with hyperfine split at g_// with the copper nuclei (I = 3/2), which is characteristic of weakly exchange coupled extended systems. The EPR parameters (g_// = 2.230, A_// = 156 × 10⁻⁴ cm⁻¹ and g_⊥ = 2.085) indicate a d_x²-y² ground state. The EPR spectra of the complexes with ligands 3 and 4 show EPR spectra with a poorly resolved hyperfine structure at g_//. In contrast, the complex with ligand 2 shows no hyperfine split and a line shape which was simulated assuming rhombic g-tensor (g₁ = 2.030, g₂ = 2.115 and g₃ = 2.190).     

Kinetic studies on the reversible ring opening-closure reaction of the triazine ligand and structural properties of palladium(II) complexes

A Pd(II) complex containing didentate triazine ligand L¹ (2-(2-methylphenyl)-3-(2-pyridyl)-2,3-dihydronaphtho[2,1-e][1,2,4]triazine) [PdCl₂(L¹)] (1) was prepared, and the structure was determined by X-ray crystallography. The absorption spectrum of complex 1 in dichloromethane changed gradually with isosbestic points when methanol was added to the solution, and [PdCl(L²)] (2) (L² = N-[methoxy(2-pyridyl)methyl]-1-(2-methylphenylazo)-2-naphthylamide) was obtained from the resulting solution after the reaction was completed. Addition of hydrogen chloride to the solution of complex 2 led to the recovery of complex 1. Thus, a reversible ring opening and closure reaction of the triazine ligand was observed. The progress of the ring opening reaction was monitored by observing the absorbance changes at several wavelengths of the visible spectra as a function of the concentration of methanol. The absorbance plots were fitted successfully with a mechanism that includes the consumption of two methanol molecules and the release of HCl, whose concentration is equivalent to that of the produced Pd complex . In dichloromethane with a low dielectric constant, the polar HCl molecule will be stabilized by forming an adduct with methanol. The equilibrium constant was determined as at T = 25.0 °C. The kinetics of the reaction of [PdCl₂(L¹)] with methanol was investigated by monitoring the absorbance change of the reacting solution with time. We obtained rate constant values of k₁ = (2.40 ± 0.07) × 10⁻³ s⁻¹ and k₂ = (5.8 ± 0.1) × 10⁻³ M⁻¹ s⁻¹ at T = 25.0 °C on the observed pseudo-first-order rate constant of the forward reaction, k_f = k₁ + k₂ [CH₃OH].     

Synthesis, crystal structure and magnetic properties of a novel complex Ni(NIT2-thz)3(ClO4)2

A novel Ni(II)-nitronyl nitroxide-substituted thiazole complex, Ni(NIT2-thz)₃(ClO₄)₂ (NIT2-thz = 2-(2′-thiazole)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) was synthesized and characterized structurally and magnetically. It crystallizes in the monoclinic space group P2₁c. The metal coordination sphere is fully occupied by three chelating nitroxide ligands, showing a distorted octahedral geometry. The complex molecules were connected as 1-D chain structure by the intermolecular interaction. Magnetic studies show that antiferromagnetic couplings occurred between the nickel(II) ion and the organic radicals, and ferromagnetic between the adjacent molecules.     

Synthesis, characterization and structural studies of new palladium(II) complexes including non-symmetric phosphorus ylides

The reaction of the non-symmetric phosphorus ylides, Ph₂P(CH₂)_nPPh₂C(H)C(O)PhR [Y₁-Y₄: n = 1, R = Cl, Br, NO₂, OCH₃ and Y₅-Y₈: n = 2, R = Cl, Br, NO, OCH₃] with dichloro(1,5-cyclooctadiene)palladium(II) in dichloromethane under mild conditions afford the monomeric P-C chelated complexes, [(Y)PdCl₂] (Y = Y₁-Y₈). These complexes were fully characterized by elemental analysis and spectroscopic techniques such as IR, ¹H, ³¹P, and ¹³C NMR. In addition, the identity of complexes [(Y₅)PdCl₂] (1b) and [(Y₈)PdCl₂] (4b) was unequivocally determined by single crystal X-diffraction techniques, both structures consisting of six-membered rings formed by coordination of the ligands through the phosphine group and the ylidic carbon atom to the metal center. The coordination geometry around the Pd atoms in both these complexes be defined as slightly distorted square planar. Furthermore, their electrochemical behavior was also investigated by cyclic voltammeters, thus the cyclic voltammetry of complex [(Y₁)PdCl₂], in dichloromethane solution with Pt electrode, shows that the redox reaction of the pair Pd(II)/Pd(0) is irreversible with the cathodic peak potential at −1.08 V versus Ag wire.     

Synthesis and characterization of carbamoyl methyl pyrazole(CMPz) compounds of palladium(II) chloride: The crystal and molecular structure of [PdCl2{C3H3N2CH2CONBu2}2]

Carbamoyl methyl pyrazole compound of palladium(II) chloride of the type [PdCl₂L₂] (where L =  C₅H₇N₂CH₂CON(C₄H₉)₂, C₅H₇N₂CH₂CON(ⁱC₄H₉)₂, C₃H₃N₂CH₂CON(C₄H₉)₂, or C₃H₃N₂CH₂CON(ⁱC₄H₉)₂) has been synthesized and characterized by IR and ¹H NMR spectroscopy. The structure of the compound [PdCl₂{C₃H₃N₂CH₂CONⁱBu₂}₂] has been determined by single crystal X-ray diffraction and shows that the ligands are bonded through the soft pyrazolyl nitrogen atom to the palladium(II) chloride in a trans disposition.     

Kinetic studies of the reduction of hexaaquacobalt(III) perchlorate by a cobaloxime, [Co(dmgBF2)2(H2O)2]

The reaction of with Co(dmgBF₂)₂(H₂O)₂ in 1.0 M HClO₄/LiClO₄ was found to be first-order in both reactants and the [H⁺] dependence of the second-order rate constant is given by k_2obs = b/[H⁺], b at 25 °C is 9.23 ± 0.14 × 10² s⁻¹. The [H⁺] dependence at lower temperatures shows some saturation effect that allowed an estimate of the hydrolysis constant for as K_a = 9.5 × 10⁻³ M at 10 and 15 °C. Marcus theory and the known self-exchange rate constant for Co(OH₂)₅OH^2+/+ were used to estimate an electron self-exchange rate constant of k₂₂ = 1.7 × 10⁻⁴ M⁻¹ s⁻¹ for .     

Bis(acetylacetonato)ruthenium(II) complexes containing bulky tertiary phosphines. Formation and redox behaviour of Ru(acac)2 (PR3) (R = Pr, Cy) complexes with ethene, carbon monoxide, and bridging dinitrogen

Reaction of cis-[Ru(acac)₂(η²-C₈H₁₄)₂] (1) (acac = acetylacetonato) with two equivalents of PⁱPr₃ in THF at −25 °C gives trans-[Ru(acac)₂(PⁱPr₃)₂], trans-3, which rapidly isomerizes to cis-3 at room temperature. The poorly soluble complex [Ru(acac)₂(PCy₃)₂] (4), which is isolated similarly from cis-[Ru(acac)₂(η²-C₂H₄)₂] (2) and PCy₃, appears to exist in the cis-configuration in solution according to NMR data, although an X-ray diffraction study of a single crystal shows the presence of trans-4. In benzene or toluene 2 reacts with PⁱPr₃ or PCy₃ to give exclusively cis-[Ru(acac)₂(η²-C₂H₄)(L)] [L = PⁱPr₃ (5), PCy₃ (6)], whereas in THF species believed to be either square pyramidal [Ru(acac)₂L], with apical L, or the corresponding THF adducts, can be detected by ³¹P NMR spectroscopy. Complexes 3-6 react with CO (1 bar) giving trans-[Ru(acac)₂(CO)(L)] [L = PⁱPr₃ (trans-8), PCy₃ (trans-9)], which are converted irreversibly into the cis-isomers in refluxing benzene. Complex 5 scavenges traces of dinitrogen from industrial grade dihydrogen giving a bridging dinitrogen complex, cis-[{Ru(acac)₂(PⁱPr₃)} ₂(μ-N₂)] (10). The structures of cis-3, trans-4, 5, 6 and 10 · C₆H₁₄ have been determined by single-crystal X-ray diffraction. Complexes trans- and cis-3, 5, 6, cis-8, and trans- and cis-9 each show fully reversible one-electron oxidation by cyclic voltammetry in CH₂Cl₂ at −50 °C with E_1/2(Ru^3+/2+) values spanning −0.14 to +0.92 V (versus Ag/AgCl), whereas for the vinylidene complexes [Ru(acac)₂ (CCHR)(PⁱPr₃)] [R = SiMe₃ (11), Ph (12)] the process is irreversible at potentials of +0.75 and +0.62 V, respectively. The trend in potentials reflects the order of expected π-acceptor ability of the ligands: PⁱPr₃, PCy₃ <C ₂H₄ < CCHR < CO. The UV-Vis spectrum of the thermally unstable, electrogenerated Ru^III-ethene cation 6⁺ has been observed at −50 °C. Cyclic voltammetry of the μ-dinitrogen complex 10 shows two, fully reversible processes in CH₂Cl₂ at −50 °C at +0.30 and +0.90 V (versus Ag/AgCl) corresponding to the formation of 10⁺ (Ru^II,III) and 10²⁺ (Ru^III,III). The former, generated electrochemically at −50 °C, shows a band in the near IR at ca. 8900 cm⁻¹ (w_1/2 ca. 3700 cm⁻¹) consistent with the presence of a valence delocalized system. The comproportionation constant for the equilibrium 10 + 10²⁺ ? 2 10⁺ at 223 K is estimated as 10^13.6.