Anthony W. Addison

Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane]copper(II) perchlorate

The linear quadridentate N2S2 donor ligand 1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane (bmdhp) forms mono- and di-hydrate 1 : 1 copper(II) complexes which are significantly more stable toward autoreduction than those of the non-methylated analogue. The deep green monohydrate of the perchlorate salt crystallises as the mononuclear aqua-complex, [Cu(bmdhp)(OH2)][ClO4]2, in the monoclinic space group P21/n, with Z= 4, a= 18.459(3), b= 10.362(2), c= 16.365(3)A, and β= 117.14(1)°. The structure was solved and refined by standard Patterson, Fourier, and least-squares techniques to R= 0.047 and R′= 0.075 for 3 343 independent reflections with l > 2σ(l). The compound consists of [Cu(bmdhp)(OH2)]2+ ions and ClO4– counter ions. The co-ordination around copper is intermediate between trigonal bipyramidal and square pyramidal, with Cu–N distances of 1.950(4) and 1.997(4)A, Cu–O(water) 2.225(4)A, and Cu–S 2.328(1) and 2.337(1)A. In the solid state, the perchlorate dihydrates co-ordination sphere may be a topoisomer of the monohydrates. A new angular structural parameter, τ, is defined and proposed as an index of trigonality, as a general descriptor of five-co-ordinate centric molecules. By this criterion, the irregular co-ordination geometry of [Cu(bmdhp)(OH2)]2+ in the solid state is described as being 48% along the pathway of distortion from square pyramidal toward trigonal bipyramidal. In the electronic spectrum of the complex, assignment is made of the S(thioether)→ Cu charge-transfer bands by comparison with those of the colourless complex Zn(bmdhp)(OH)(ClO4). E.s.r. and ligand-field spectra show that the copper(II) compounds adopt a tetragonal structure in donor solvents.

Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C

Abstract Caution should be exercised when one is comparing the redox potentials of complexes measured in CH 3 CN solutions versus different reference electrodes. A critical review of conversion constants amongst various reference electrodes reported in the literature reveals that in most cases the comparisons of redox potential values are far from accurate. From measurements of relationships amongst various common reference electrodes in CH 3 CN solutions, conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C are proposed, along with a convention for reporting redox potential values.

Is ligand topology an influence on the redox potentials of copper complexes

Abstract The known redox potentials of selected Cu 2+ /Cu + couples complexed with N-, S- and O-donor ligands are empirically correlated with various characteristics of the ligands. Closed (macrocyclic) ligand topology has little significant effect on the E 1/2 values, but certain branched (tripodal) ligands show a marked influence on the complexes’ redox potentials, generally raising E 1/2 by a few hundred millivolts. Comparison of the empirically computed E 1/2 for the [Cu- (1,4,7-trithiacyclononane)] 2+/+ couple with an observed value (+0.465 V versus SCE) suggests that the effect of tripod ligands on E 1/2 is related to the three- fold pseudosymmetry of the donor atoms’ dispositions.

Pentacoordinate copper complexes of nitrogen-sulfur donors: structural chemistry of two complexes of bis(2-(2-benzimidazolyl)ethyl) sulfide with the sulfur alternatively in equatorial and axial coordination modes

Resolution structurale des deux complexes [Cu(BBES) (MeOH) (OH 2 )] (ClO 4 ) 2 et [Cu(BBES) (acoc)] PF 6 •H 2 O•EtOH, BBES=bis-(N-methylbenzimidazolyl-2)-1,5 oxo-3 pentane. Spectres RPE montrant que les complexes gardent leurs structures deformees en solution

Spectroscopic and redox studies of some copper(II) complexes with biomimetic donor atoms: implications for protein copper centres

The redox chemistry and e.s.r. spectra of a number of copper(II) complexes with nitrogen- and sulphur-donor ligands have been examined, including complexes with 2,2′-bi-imidazole, histamine, and cyclic and acyclic saturated amine and thioether ligands. The tetrahedral CuIIS4 centre generated by γ-irradiation of single crystals of a thioacetamide complex of CuI has a low value of |A∥|. Tetrahedral distortion of otherwise tetragonal copper(II) centres increases g∥ and decreases |A∥| for N-, S-, and O-donor ligands, and is a more effective source of reduction of |A∥| than charge effects. The quotient g∥/|A∥| appears to be a convenient empirical index of distortion of the donor set from planar toward tetrahedral, indicating that most type I copper in proteins is tetrahedrally, rather than tetragonally, co-ordinated.

Complexes of structural analogues of terpyridyl with iron and zinc; the x-ray crystal structure of bis[2,6-bis(benzimidazol2-yl)pyridine]iron(II) trifluoromethylsulphonate bis-ethanol solvate

Abstract Complexes of iron(II) and zinc(II) with a series of tridentate ligands of the type [2,6-bis(benz-X-azol-2-yl)pyridine] (X = imid, thi, ox) have been prepared. The crystal structure of [Fe(2,6-bis(benzimidazol-2-yl)pyridine)2](CF3SO3)2·2C2H5OH shows the iron(II) to be low spin with a meridional octahedral coordination by two tridentate ligands. 57Fe Mossbauer spectra of the complexes [FeL2]2+ show strong dependence of ligand strength on the non-coordinated heteroatom, X, of the ligand: benzimidazole ligands give low spin complexes, benzoxazoles give high spin complexes and benzthiazoles show spin-crossover properties. Derivatization of the imidazole nitrogen has only a small effect on the ligand strength. The complexes [FeLX2] (X = anion) are all high spin. Partial dissociation is observed in solvents with a strong affinity for iron(II); the iron(II) complexes show metal-to-ligand charge transfer transitions whose intensity depends strongly on the spin state of the iron.

Structural, magnetic and related attributes of some oximate-bridged tetranuclear nickel(ii) rhombs and a dinuclear congenerElectronic supplementary information (ESI) available: mass spectra, χT vs. T, response of magnetic properties, low-lying spin levels and UV-VIS data. See http://www.rsc.org/suppdata/dt/b3/b300539a/

New oximate-bridged tetranuclear nickel(II) complexes of compositions {Ni(Dien)}2(μ3-OH)2{Ni2(Moda)4}(ClO4)2·Solv (Solv = H2O, 1a; Solv = 2CH3NO2, 1b; Solv = 2H2O·2C4H8O2, 1c), {Ni(Sdien)}2(μ3-OH)2{Ni2(Moda)4}(ClO4)2·H2O (2), {Ni(Odien)}2(μ3-OH)2{Ni2(Moda)4}(ClO4)2·0.6H2O (3), {Ni(Dien)}2(μ3-OH)2{Ni2(Inaf)4}(ClO4)2·CH3NO2 (4) and {Ni(Odien)}2(μ3-OH)2{Ni2(Inaf)4}(ClO4)2·2NaClO4·2H2O (5) and the dinuclear complex (Ni{Odien})2(Moda)2(ClO4)2 (6) have been prepared (Dien = 1,5-diamino-3-azapentane, Odien = 1,5-diamino-3-oxapentane, Sdien = 1,5-diamino-3-thiapentane, ModaH = butane-2,3-dione monooxime, InafH = phenylglyoxaldoxime). X-ray examination revealed similar structures for 1a, 1b, 1c, 2 and 3, with a rhombic “chair” (out-of-phase “butterfly”) arrangement of the four nickel(II) atoms and two hydroxo-bridges. The complexes were examined by magnetochemistry, UV-VIS spectroscopy and voltammetry. 1–3 display antiferromagnetic coupling of the central with the terminal nickel(II) atoms and ferromagnetic spin alignment between the central nickel(II) atoms. In contrast, all the spin exchanges in 4 and 5 were found to be antiferromagnetic, as is the coupling in dinuclear 6.

A Tetrameric Nickel(II) “Chair” with both Antiferromagnetic Internal Coupling and Ferromagnetic Spin Alignment

Interest in polynuclear complexes of the 3d metals has been stimulated by the search for new magnetic materials[1, 2] and by demonstration of the occurrence of oligonuclear metal centers in proteins such as urease.[3] Of the relatively small number of reported tetranuclear complexes of S 1 nickel( ) of known structure, the majority have a hemicubaneor TMbutterfly-∫ rather than a squarelike core.[4] Of these molecules, just one is entirely antiferromagnetic,[5] while the remainder entail purely ferromagnetic interactions amongst the nickel( ) ions.[6±10] Oximes have shown promise as bridging ligands for the preparation of polynuclear complexes.[11, 12] The reaction of 1,4,7-triazaheptane (diethylenetriamine, Dien) with the monooxime of 2,3-butanedione (ModaH) in the presence of NiII ions, instead of yielding the anticipated Schiff base derivative, gave the tetranuclear NiII compound 1 (dark brown crystals; C4H8O2 1,4-dioxane), containing uncondensed but coordinated ketone and amine groups. Figure 1a shows the structure of the the cation of 1, while Figure 1b highlights its NiII core.

Structural aspects of the bis(2,2′-dipicolylamine)iron(II) cation

The title complex crystallized as the BF 4 − salt in the orthorhombic space group Fdd 2, with unit cell parameters a = 21.749 (σ = 0.004), b = 27.788(0.006), c = 8.900(0.002) A. The iron(II) atom has slightly distorted octahedral coordination, in which the ligands are bound in a cis-facial configuration. The electronic properties of this brightly coloured low-spin complex are briefly discussed in relationship to the determined structure, and commentary is made upon the relationship between the lengths of FeN(aliphatic) and FeN(heteroaromatic) bonds in sexacoordinate iron(II) complexes and the spin state of the iron.

The First Example of a True “Turnbull’s Blue” Family Compound with Trapped Iron Oxidation States

The first compound of a true Turnbull′s Blue family [i.e. an iron(II) hexacyanoferrate(III)] complex with valence-trapped states of iron − [Fe([9]ane-S3)2]3[Fe(CN)6]2·2H2O (Ukrainian Red, UR) was synthesized. It crystallizes in the trigonal space group P3, a = 12.038(8) A, b = 12.038(8) A, c = 14.342(7) A, α = 90.00°, β = 90.00°, γ = 120.00°. The unit cell of UR consists of separate [Fe([9]ane-S3)2]2+ cations, [Fe(CN)6]3− anions and 5.3 molecules of water. Encapsulation of the FeII in the homoleptic thioether environment plays the dual role of stabilizing FeII and protecting the central metal ion from direct bridging with hexacyanoferrate(III) anions. The absence of such interactions and trapping of iron oxidation states in the cations and anions was demonstrated by X-ray diffraction and magnetic susceptometry. Magnetic measurements are consistent with diamagnetic FeII inside the macrocyclic moieties and S = 1/2 for FeIII in the hexacyanoferrate(III) anions.