Cyclic Tetranuclear Complex Research Articles

Chiral Octapalladium Chains Supported by Enantiopure P-Stereogenic Linear Tetraphosphines, (R,R)- and (S,S)-Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2.

Binuclear Pd(II) and Pt(II) complexes supported by rac-dpmppm (bis[(diphenylphosphinomethyl)phenylphosphino]methane) in a triply-bridged Z-form, [M2Cl4(rac-dpmppm)] (M = Pd (3a), Pt (3b)), readily reacted with 2,6-xylyl isocyanide (XylNC) in the presence of NH4PF6 to afford [M2Cl2(rac-dpmppm)(XylNC)2](PF6)2 (M = Pd (4a), Pt (4b)), in which each metal center accommodates one isocyanide ligand at the trans position to the inner P atom of dpmppm. Similarly, treatment of 3a and 3b with axially chiral (R/S)-1,1'-binaphthyl-2,2'-bisisocyanide (rac-Binac) in the presence of NH4OTf gave cyclic tetranuclear complexes, [{M2Cl2(rac-dpmppm)(rac-Binac)}2](OTf)4 (M = Pd (5), Pt (8)), where two {M2Cl2(rac-dpmppm)}2+ fragments are connected by two rac-Binac ligands through chirality sorting of (R*,R*)-dpmppm and (R*)-Binac. Complex 5 could be transformed into the halide exchanged tetranuclear complexes, [{Pd2X2(rac-dpmppm)(rac-Binac)}2](OTf)4 (X = Br (6), I (7)), to show that the rectangular arrangement of four Pd(II) ions is elongated by repulsive interaction between halide ligands. By using (R)- and (S)-Binac, enantiopure Pd4 complexes, [{Pd2Cl2((R*,R*)-dpmppm)((R*)-Binac)}2](OTf)4 (5RR/R and 5SS/S), were successfully isolated as pure crystalline forms, from which enantiopure (R,R)- and (S,S)-dpmppm were obtained by treatment with NaCN aqueous solution. Namely, optical resolution of rac-dpmppm was established through the tetranuclear Pd complexes, which is the first example for methylene-bridged polyphosphines, R2P(CH2PR)nCH2PR2 (n > 0). Furthermore, chiral octapalladium chains, [Pd8((R*,R*)-dpmppm)4(N≡CCH3)2](BF4)4 (2RR and 2SS), were synthesized by reacting enantiopure P-chiral dpmppm with [Pd2(CH3CN)6](BF4)2 and [Pd2(dba)3]·C6H6 and were characterized by spectroscopic and X-ray crystallographic analyses, to determine the absolute configurational structures. The Pd8 chains are the longest enantiopure chiral single-metal-atom chains structurally characterized, thus far, and the electronic structures were examined on the basis of DFT calculations of 2RR.

Luminescence homoleptic cyclic tetranuclear [Ln4(BTFA)10(μ3-OH)2(H2O)4] complexes (Ln = Eu or Tb)

Through self-assembly of the typical β-diketone ligand HBTFA (HBTFA = 4,4,4-trifluoro-1-phenyl-1,3-butanedione) with LnCl3·6H2O in the presence of Et3N, the six-tenth in the η2-Mode I and the four-tenth in the μ-(O)-η2-bridging Mode II of the ten BTFA− ligands, together with the involvement of two μ3-(OH)− anions, two μ1-coordinating H2O and two μ2-bridging H2O molecules, endow the formation of homoleptic cyclic tetranuclear complexes [Ln4(BTFA)10(μ3-OH)2(H2O)4] (Ln = La, 1; Eu, 2; Tb, 3 or Gd, 4), respectively. Moreover, attributing to effective ligand-to-Ln3+ transfer energy, color-purity red-light (ΦLEu = 20.1%) for complex 2 while few near-white-light (Φem = 6.4%) for complex 3 are exhibited, respectively, despite the (μ3-OH)−-and-H2O-induced OH-oscillators surrounded from the four Ln3+ ions.

Structural and Physical Investigations of Coordination Complexes Involving Pyridylethylenedithio–Tetrathiafulvalene Ligands Decorated with Cyanoethylsulfanyl and Cyanoethylselanyl Moieties

The two new ligands 4,5-bis(2-cyanoethylthio)-4′,5′-(4-pyridylethylene-1,2-dithio)tetrathiafulvalene (L1) and 4,5-bis(2-cyanoethylseleno)-4′,5′-(4-pyridylethylene-1,2-dithio)tetrathiafulvalene (L2) have been elaborated and used to synthesize five new coordination complexes of transition-metal ions, namely, [Mn2(hfac)4(L1)2]2 (1; hfac– = 1,1,1,5,5,5-hexafluoroacetylacetonate), [Cd(hfac)2(L1)2]·2CHCl3 (2), [Cu(hfac)2(L1)2] (3), [Co(hfac)2(L1)2]·2C6H14 (4) and [Mn(hfac)2(L2)2]·2C6H14 (5), which were characterized by single-crystal X-ray diffraction. Compounds 2–5 were identified as mononuclear complexes with different crystal packing depending on the nature of the metal centre. On the contrary, compound 1 was identified as a cyclic tetranuclear complex in which L1 is coordinated to the MnII ions through two different pyridyl and nitrile functionalizations. The photophysical properties of the ligands L1 and L2 were studied in solution and rationalized with time-dependent density functional theory calculations.

FeFe]-Hydrogenase H-Cluster Mimics with Unique Planar μ-(SCH2 )2 ER2 Linkers (E=Ge and Sn).

Analogues of the [2Fe-2S] subcluster of hydrogenase enzymes in which the central group of the three-atom chain linker between the sulfur atoms is replaced by GeR2 and SnR2 groups are studied. The six-membered FeSCECS rings in these complexes (E=Ge or Sn) adopt an unusual conformation with nearly co-planar SCECS atoms perpendicular to the Fe-Fe core. Computational modelling traces this result to the steric interaction of the Me groups with the axial carbonyls of the Fe2 (CO)6 cluster and low torsional strain for GeMe2 and SnMe2 moieties owing to the long C-Ge and C-Sn bonds. Gas-phase photoelectron spectroscopy of these complexes shows a shift of ionization potentials to lower energies with substantial sulfur orbital character and, as supported by the computations, an increase in sulfur character in the predominantly metal-metal bonding HOMO. Cyclic voltammetry reveals that the complexes follow an ECE-type reduction mechanism (E=electron transfer and C=chemical process) in the absence of acid and catalysis of proton reduction in the presence of acid. Two cyclic tetranuclear complexes featuring the sulfur atoms of two Fe2 S2 (CO)6 cores bridged by CH2 SnR2 CH2 , R=Me, Ph, linkers were also obtained and characterized.

A cyano-bridged hetero-tetranuclear [Sm2(o-phen)2(DMF)6(H2O)2(μ-CN)4Fe2(CN)8] · 5H2O · CH3OH: synthesis, structure, Mössbauer spectrum, and magnetism

Two new hetero-tetranuclear complexes, [Sm2(o-phen)2(DMF)6(H2O)2(µ-CN)4Fe2(CN)8]·;5H2O·;CH3OH (1) and [Sm2(o-phen)2(DMF)6(H2O)2(µ-CN)4Co2(CN)8]·;5H2O (2), have been prepared from reaction of SmCl3·;6H2O, K3[Fe(CN)6]·;3H2O or K3[Co(CN)6], and o-phen in methanol/DMF, and characterized. The structure of 1 consists of a cyano-bridged discrete cyclic tetranuclear complex in which the Sm(III) and Fe(III) centers are linked by four CN groups. Mössbauer spectrum of 57Fe indicates that both Fe(III) atoms in 1 have the same low-spin (S = 1/2) electronic ground state. From comparison of the magnetic data of 1 and 2, at low temperature for 1 indicates weak ferromagnetic coupling between Sm(III) and Fe(III).

Self-assembly in dinuclear, cyclic homotetranuclear and cyclic heterotetranuclear complexes with a picolylhydrazone ligand

Reactions among Cu(ClO 4) 2 · 6H 2O, Cu(acac) 2/VO(acac) 2 and 3-methoxysalicylaldehyde Picoloylhydrazone in different solvents give three complexes, [Cu 2L(acac)(H 2O) 2]ClO 4 ( 1), [Cu 4L 2(acac) 2(py) 2](ClO 4) 2 ( 2) and (VO 2) 2L 2Cu 2(acac) 2 ( 3) (acac = acetyl acetonate and py = pyridine). There is an extended 2D structure in complex 1 constructed by hydrogen bonds between the binuclear complex cation and the ClO 4 − anion, and an extended 1D structure in complex 2 constructed by weak π⋯π stacking interactions between neighboring cyclic tetranuclear complex molecules. Complex 3 is the first oxovanadium–copper complex with a bridging oxo oxygen atom between the V atom and the Cu atom. The solid-state photoluminescent properties of the three title complexes have been studied. There is an antiferromagnetic interaction in 1.

Design and Non‐Covalent DNA Binding of Platinum(II) Metallacalix[4]arenes

A set of cyclic tetranuclear complexes of the metallacalix[4]arene type with formula [{Pt(en)(L)}(4)](4+) (en=ethylenediamine; 2: LH=5-chloro-2-hydroxypyrimidine (5-Cl-Hpymo); 3: LH=5-bromo-2-hydroxypyrimidine (5-Br-Hpymo); 4: LH=5-iodo-2-hydroxypyrimidine (5-I-Hpymo)) have been obtained from the reaction between cis-protected square-planar [Pt(en)(H(2)O)(2)](2+) metal entities and LH in aqueous media. Additionally, the binding properties of 2, 3, 4 and their congener [{Pt(en)(L)}(4)](4+) (1: LH=2-hydroxypyrimidine (Hpymo)) with calf thymus-DNA (ct-DNA) have been studied by using different techniques including circular and linear dichroism (CD and LD, respectively) and UV-visible absorbance spectroscopies, gel electrophoresis, fluorescence competitive-binding studies and atomic force microscopy (AFM). The results are consistent with significant non-covalent interactions taking place between the polynuclear cyclic species and ct-DNA. Moreover, gel electrophoresis, linear dichroism titrations and AFM images of ct-DNA with metallacalixarenes show ct-DNA coiling at low metallacalixarene concentrations and upon subsequent increments in metallacalixarene concentration ct-DNA can be seen to uncoil with concomitant formation of long and inflexible ct-DNA structures.

Hydrothermal syntheses, structures and properties of three cyclic tetranuclear complexes and one 1D chain complex with 3,5-dinitrosalicylate

The hydrothermal reactions of Co(OAc) 2 · 4H 2O, 3,5-dinitrosalicylate (3,5-(NO 2) 2sal) and 2,2′-bipyridine (2,2′-bipy) with different reaction periods give metallamacrocycles 1 and 2 with the same chemical formula Co 4(2,2′-bipy) 4{3,5-(NO 2) 2sal} 4. Replacing Co(OAc) 2 · 4H 2O with Zn(NO 3) 2 · 6H 2O, using the same synthetic procedures, results in the formation of compound [Zn(2,2′-bipy){3,5-(NO 2) 2sal}] n ( 3) with a 1D chain structure and the metallamacrocycle compound Zn 4(2,2′-bipy) 4{3,5-(NO 2) 2sal} 4 ( 4). Compounds 1 and 2 crystallize as two different polymorphs of cyclic tetranuclear compounds. Compounds 3 and 4 are polymorphic too. The compounds 1 and 4 are isomorphous. The weak coordination interactions have significant influence on the spacial orientations of the 3,5-(NO 2) 2sal ligand, and may affect the crystallization processes. There are antiferromagnetic interactions in the cyclic tetranuclear cobalt(II) compounds 1 and 2. Compound 3 exhibits weak fluorescent emission in the solid state at room temperature.

Anion‐Dependent Structural Diversity in Silver(I) Complexes of 4‐Amino‐3,5‐diisopropyl‐1,2,4‐triazole

AbstractReactions of silver salts with 4‐amino‐3,5‐diisopropyl‐1,2,4‐triazole (L) afford dinuclear complex [Ag2L2(NO3)2(CH3CN)], triangular complex [Ag3L3](CF3SO3)3, and cyclic tetranuclear complexes [Ag4L6](BF4)4 and [Ag4L6](ClO4)4. On the basis of the X‐ray analyses, anions are found to play an important role in determining the structures. Through coordination/noncoordination, anions can impart their influence on the coordination geometry of Ag atoms and the angles of N–Ag–N, which seems to be the main factor controlling the nuclearity of the reaction products in this system. 1H NMR and MS measurements suggest that these complexes probably retain their structural integrities in CH3CN solution. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Infinite zig-zag and cyclic-tetranuclear isomeric imidazolate-bridged polynuclear copper(II) complexes: Magnetic properties, catalytic activity and electrospray mass and tandem mass spectrometry characterization

Two mononuclear copper(II) complexes 1 and 2 with the unsymmetrical tridentate ligands 2- and 4-[((imidazol-2-ylmethylidene)amino)ethyl]pyridine have been prepared. In alkaline solution, deprotonation of the imidazole moiety in 1 and 2 promotes self-assembly, which yielded two structurally different species. Depending on the binding site in the imidazole ring, a polymeric complex with an infinite zig-zag-chain 3, or a cyclic-tetranuclear complex 4 is formed, as shown by spectroscopic and spectrometric analysis. Herein, structural characterization of these isomeric polynuclear complexes was performed by electrospray mass (ESI-MS) and tandem mass spectrometric experiments (ESI-MS/MS). Each isomer was shown to be stable in methanolic solutions and to display unique mass spectra with characteristic multiply charged molecular and fragment ions, corroborating previous data by EPR measurements. Magnetic data in the solid state fit a typical curve for an one-dimensional infinite regular chain system, with J = −(32.4 ± 1.2) cm −1 and g = 2.03 for 3, and that of a cyclic-tetranuclear structure with J = −(55.5 ± 0.4) cm −1 and g = 2.29 for 4. In the oxidation of 3,5-di- tert-butylcatechol (3,5-DTBC) by molecular oxygen, both complexes were shown to act as efficient catalysts, exhibiting very similar ratios: k cat/ K M = 9.12 × 10 6 mol −1 dm 3 min −1 for 3 and 8.73 × 10 6 mol −1 dm 3 min −1 for 4. These similar ratios indicate that interactions between the metal centres in 3 or 4 and the substrate in solution occur predominantly at the outside of the catalyst framework.

Synthesis and magnetic properties of heterometal cyclic tetranuclear complexes [Cu(II)LM(II)(hfac)]2 (M(II) = Zn, Cu, Ni, Co, Fe, Mn; H3L = 1-(2-hydroxybenzamido)-2-((2-hydroxy-3-methoxybenzylidene)amino)ethane; Hhfac = hexafluoroacetylacetone).

A series of heterometal cyclic tetranuclear complexes [Cu(II)LM(II)(hfac)](2) (M(II) = Zn (1), Cu (2), Ni (3), Co (4), Fe(5), and Mn (6)) have been synthesized by the assembly reaction of K[CuL] and [M(II)(hfac)(2)(H(2)O)(2)] with a 1:1 mole ratio in methanol, where H(3)L = 1-(2-hydroxybenzamido)-2-((2-hydroxy-3-methoxybenzylidene)amino)ethane and Hhfac = hexafluoroacetylacetone. The crystal structures of 2, 4, and [Cu(II)LMn(II)(acac)](2) (6a) (Hacac = acetylacetone) were determined by single-crystal X-ray analyses. Each complex has a cyclic tetranuclear Cu(II)(2)M(II)(2) structure, in which the Cu(II) complex functions as a "bridging ligand complex", and the Cu(II) and M(II) ions are alternately arrayed. One side of the planar Cu(II) complex coordinates to one M(II) ion at the two phenoxo and the methoxy oxygen atoms, and the opposite side of the Cu(II) complex coordinates to another M(II) ion at the amido oxygen atom. The temperature-dependent magnetic susceptibilities revealed spin states of S(M) = 0, 1/2, 1, 3/2, 2, and 5/2 for the Zn(II), Cu(II), Ni(II), Co(II), Fe(II), and Mn(II) ions, respectively. Satisfactory fittings to the observed magnetic susceptibility data were obtained by assuming a rectangular arrangement with two different g-factors for the Cu(II) and M(II) ions, two different isotropic magnetic exchange interactions, J(1) and J(2), between the Cu(II) and M(II) ions, and a zero-field splitting term for the M(II) ion. In all cases, the antiferromagnetic coupling constants were found for both exchange interactions suggesting nonzero spin ground states with S(T) = 2/S(M) - S(Cu)/, which were confirmed by the analysis of the field-dependent magnetization measurements.

Mononuclear to Tetranuclear Structural Transformation in Vanadyl Complexes of 3-Hydroxypyridine-2-carboxylic Acid (H2hpic)

Abstract Reaction of vanadyl sulfate with 3-hydroxypyridine-2-carboxylic acid (H2hpic) affords two chargeless complexes, [VO(Hhpic-O,O)(Hhpic-O,N)(H2O)]·3H2O (1) and unprecedented cyclic tetranuclear complex [(VO)4(μ-(hpic-O,O′,N))(H2O)4]·8H2O (2), which were characterized by X-ray crystallographic analysis. The unique structural transformation between complexes 1 and 2 was found.

New mononuclear, cyclic tetranuclear, and 1-D helical-chain Cu(II) complexes formed by metal-assisted hydrolysis of 3,6-di-2-pyridyl-1,2,4,5-tetrazine (DPTZ): crystal structures and magnetic properties.

The reactions of 3,6-di-2-pyridyl-1,2,4,5-tetrazine (DPTZ) with different Cu(II) salts generate two new ligands, 2,5-bis(2-pyridyl)-1,3,4-oxodiazole (L(1)) and N,N'-bis(alpha-hydroxyl-2-pyridyl)ketazine (H(2)L(2)), from the metal-assisted hydrolysis of DPTZ, and form three new complexes: a mononuclear complex [Cu(L(1))(2)(H(2)O)(2)] .2ClO(4) (1), a linear coordination polymer [Cu(L(1))(NO(3))(2)](8) (2), and a cyclic tetranuclear complex [Cu(4)(L(2))(2)(Im)(2)(NO(3))(4)(H(2)O)(2)] (3) (Im = imidazole). Crystal data for 1: space group P2(1)/n with a = 10.339(3) A, b = 10.974(2) A, c = 13.618(4) A, beta = 103.24(1) degrees, and Z = 2. Crystal data for 2: space group C2/c with a = 13.9299(14) A, b = 9.2275(9) A, c = 12.1865(13) A, beta = 111.248(2) degrees, and Z = 4. Crystal data for 3: space group P2(1)/n with a = 9.3422(14) A, b = 15.987(2) A, c = 13.963(2) A, beta = 108.587(3) degrees, and Z = 2. L(1) acts as a bidentate chelating ligand in 1 and as a bis-bidentate chelating ligand in 2 with the shortest intramolecular Cu...Cu distance of 6.093 A. L(2) is a hexadentate ligand to bridge four Cu(II) ions, forming an interesting neutral cyclic tetranuclear complex 3 with Cu...Cu distances varying from 4.484 to 9.370 A. The mechanism of the metal assisted hydrolysis of DPTZ is discussed in detail. Magnetic susceptibility measurements indicate that 2 shows weak ferromagnetic interaction (J = 2.85 cm(-1)) along the 1-D helical chain, and that 3 displays weak antiferromagnetic interaction (J = -1.19 cm(-1) for the N-N bridge) and ferromagnetic interaction (j = 0.11 cm(-1) for the O-C=N bridge) between the adjacent Cu(II) ions.

Cyanide-Bridged Iron−Copper Molecular Squares with Doublet and Quintet Spin Ground States

Reactions of [FeII(CN)2(bpy)2] and [FeIII(CN)2(bpy)2](PF6) with [CuII(bpy)(CH3OH)2](PF6)2 in methanol yielded cyanide-bridged cyclic tetranuclear complexes of [FeII2CuII2(μ-CN)4(bpy)6](PF6)4·2H2O·4CHCl3 (1) and [FeIII2CuII2(μ-CN)4(bpy)6](PF6)6·4CH3CN·2CHCl3 (2), respectively. In the squares of 1 and 2, the Fe2+/3+ (low-spin) and Cu2+ ions are alternately bridged by the cyanide ions, the carbon atoms of which coordinate to the iron ions. Variable-temperature magnetic susceptibility studies of complexes revealed that the Cu2+ ions in 1 are magnetically isolated. In the square of 2, the adjacent Fe3+ and Cu2+ ions were ferromagnetically coupled through the cyanide bridges with a J1 value of +6.3(1) cm-1 and the weaker antiferromagnetic interactions between the orthogonal Fe3+···Fe3+ and Cu2+···Cu2+ pairs are operative (J2 = −3.1(1) cm-1). The propagation of the ferromagnetic interaction in 2 can be understood by the orthogonal magnetic orbitals of the low-spin Fe3+ (dπ) and Cu2+ (dσ) ions. Cyclic voltammogra...

Ni3Se(o-C6H4{CH2Se}2)3]2- und [Ni4(SeiC3H7)8], die ersten höherkernigen NickeI(II)-Komplexe mit vollständiger Selen-Ligandensphäre [Ni3Se(o-C6H4{CH2Se}2)3]2- and [Ni4(SeiC3H7)8], the First Polynuclear Nickel(II) Complexes with Complete Selenium Ligand Spheres

Reaction of nickel(II) chloride with disodium ο-xylenediselenolate in methanol yields the trinuclear mixed selenide-selenolate complex anion [Ni3Se(ο-C6H4{CH2Se}2)3]2- (3) which was isolated both as tetramethylammonium and mixed sodium/tetraethylammonium salt of formula [Me4N]2[Ni3Se(ο-C6H4{CH2Se}2)3] · MeOH (1) and [Et4N]3Na[Ni3Se-(ο-C6H4{CH2Se}2)3]2- · 3MeOH · 3H2O (2), respectively. Crystals of 1 are triclinic, space group P1̄, a = 9.065(2), b = 13.281(3); c = 18.019(4) Å , α = 92.81(2), β = 97.55(2), γ = 105.09(2)° and Z = 2. 2 crystallizes in the rhom bohedral space group R3c with a = 20.305(5), c = 41.709(9) Å and Z = 6. The structures were refined to R = 0.0705 (1) and 0.0794 (2), respectively. Both compounds contain the complex anion [Ni3Se- (ο-C6H4{CH2Se}2)3]2- (3), which possesses the same principal structural features as the corresponding thiolato derivatives. Reaction of nickel(II) chloride with sodium iso-propane selenolate in methanol leads to the cyclic tetranuclear complex [Ni4(SeiC3H7)8] (4), which crystallizes in the monoclinic space group P2/n with a = 13.161(3), b = 10.464(2), c = 14.603(2) Å , β = 94.42(1)° and Z = 2. The structure refinement converged to R = 0.0541. 4 is isostructural with the corresponding thiolato compound. The introduction of selenium instead of sulfur ligands reduces the midpoint potential of the reversible metal-centered oxidation from +595 to +285 mV.

Reactivity of the cyclic tetranuclear complex [Pd 4(μ- o-C 6H 4) 2(μ-O 2CMe) 4(μ-SPr i2) 2

Bridge splitting reactions of the cyclic tetranuclear complex, [Pd 4(μ-o-C 6H 4) 2(μ-O 2CMe) 4(μ-SPr i 2) 2], with neutral and anionic ligands afforded a variety of dinuclear palladium(II) complexes. In addition to the common one bridging o-phenylene ligand, the products contained different bridging ligands depending upon the ligand used. Dpm (Ph 2PCH 2PPh 2) or btmp (PhCH 2SCH 2PPh 2) acted as bidentate bridging ligands to give [Pd 2(μ-o- C 6H 4)(O 2CMe) 2(μ-L 2) 2] (L 2 = dpm, btmp). In contrast, Me 2bpy (4,4′-dimethyl-2,2′-bipyridyl) behaved as a bidentate chelating ligand to afford a cationic complex bearing the μ-acetato ligand, [Pd 2(μ- o-C 6H 4)(μ- O 2CMe)(Me 2bpy) 2](MeCO 2). In the case of Tl(acac), the acac ligand coordinated in a bidentate chelating fashion to yield a μ-sulfide complex, [Pd 2(μ- o-C 6H 4))(acac- O, O′) 2 (μ-SPr i 2)]. This complex further reacted with 3,5-lutidine to give [Pd 2(μ- o-C 6H 4))(acac- O, O′) 2(3,5-lut) 2 ]. Complexes obtained here were characterized by means of elemental analyses, NMR and IR spectroscopies.

Preparation and crystal structures of [AsPh4]4[Tc4N4O2(ox)6] and [AsPh4]2[TcO(ox)2(Hox)]·3H2O: technetium complexes containing quadridentate or unidentate oxalato ligands

The reaction of oxalic acid (H2ox) with [AsPh4][TcNCl4] and [NBu4][TcOCl4] in aqueous acetone yields crystals of [AsPh4]4[Tc4N4O2(ox)6](1) and, after precipitation with AsPh4Cl, [AsPh4]2[TcO(ox)2(Hox)]·3H2O (2) respectively. Complex (1) crystallises in the monoclinic space group P21/n with cell parameters a= 14.433(1), b= 13.229(1), c= 27.020(1)A, β= 92.90(1)°, and Z= 4. Refinement with data measured with Cu-Kα radiation converged at R= 0.069 for 3 076 observed reflections. The anion in (1) is a cyclic tetranuclear complex, [Tc4N4O2(ox)6]4–, with Ci point symmetry. Each technetium(VI) atom is co-ordinated by five oxygen atoms and one nitrogen atom to give a distorted octahedron. In each half of the anion, a quadridentate oxalato ligand forms a bridge between the two octahedra, each of which is also linked to an adjacent octahedron by the corner sharing of an oxo ligand. Two oxygen atoms of a bidentate oxalato ligand and a nitrido nitrogen complete the octahedral co-ordination in each polyhedron. The Tc ⋯ Tc spacings of 3.586(2) and 5.756(3)A preclude any Tc–Tc bonding. The monoclinic crystals of complex (2) belong to the space group P21/c with a= 16.495(3), b= 14.802(2), c= 21.805(4)A, β= 98.76(1)°, and Z= 4. Refinement with 5 099 observed data measured with Mo-Kα radiation converged at R= 0.059. The technetium(V) atom is co-ordinated by six oxygen atoms to give a distorted octahedron. The structure of the anion is unusual in that it contains a protonated unidentate oxalato ligand and that there is no appreciable lengthening of the Tc–O bond trans to the oxo ligand.