Sky-blue Tp*WOCl₂ has been synthesized from the high-yielding reaction of Tp*WO₂Cl with boron trichloride in refluxing toluene. Dark-red Tp*WOI₂ was prepared via thermal decarbonylation followed by aerial oxidation of Tp*WI(CO)₃ in acetonitrile. From these precursors, an extensive series of mononuclear tungstenyl complexes, Tp*WOXY [X = Cl-, Y = OPh-, SPh-; X = Y = OPh-, 2-(n-propyl)phenolate (PP-), SPh-, SePh-; XY = toluene-3,4-dithiolate (tdt²−), quinoxaline-2,3-dithiolate (qdt²-), benzene-1,2-diselenolate (bds²-); Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate], was prepared by metathesis with the respective alkali-metal salt of X-/XY²- or (NHEt₃)₂(qdt). The complexes were characterized by microanalysis, mass spectrometry, electrochemistry, IR, electron paramagnetic resonance (EPR), and electronic absorption spectroscopies, and X-ray crystallography (for X = Y = OPh-, PP-, SPh-; XY = bds²-). The six-coordinate, distorted-octahedral tungsten centers are coordinated by terminal oxo [W≡O = 1.689(6)-1.704(3) Å], tridentate Tp*, and monodentate or bidentate O/S/Se-donor ligands. Spin Hamiltonian parameters derived from the simulation of fluid-solution X-band EPR spectra revealed that the soft-donor S/Se ligand complexes had larger g values and smaller 183W hyperfine coupling constants than the less covalent hard-donor O/Cl species. The former showed low-energy ligand-to-metal charge-transfer bands in the near-IR region of their electronic absorption spectra. These oxotungsten(V) complexes display lower reduction potentials than their molybdenum counterparts, underscoring the preference of tungsten for higher oxidation states. Furthermore, the protonation of the pyrazine nitrogen atoms of the qdt²- ligand has been examined by spectroelectrochemistry; the product of the one-electron reduction of [Tp*WO(qdtH)]+ revealed usually intense low-energy bands.

The reactions of TpiPrMoVIO2Cl with salicylanilides and NEt3 produce cis-TpiPrMoVIO2(2-OC6H4CONHR) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate, R = Ph, 4-C6H4Cl, 4-C6H4OMe). The N-methyl complex, TpiPrMoO2{2-OC6H4CON(Me)Ph}, is similarly prepared. Reduction of the amido complexes by cobaltocene produces green, EPR-active compounds, [CoCp2][TpiPrMoVO2(2-OC6H4CONHR)], that exhibit strong, low energy, ν(MoO2) IR bands at 895 and 790 cm−1 (cf. 935 and 900 cm−1 for the Mo(VI) analogues). The X-ray structures of all seven complexes have been determined. In each case, the Mo center exhibits a distorted octahedral coordination geometry defined by mutually cis oxo and phenolate ligands and a tridentate fac-TpiPr ligand. The Mo(V) anions exhibit greater Mo═O distances (av. 1.738 Å vs 1.695 Å) and O═Mo═O angles (av. 112.4° vs 102.9°) than their Mo(VI) counterparts, indicative of the presence of a three-center (MoO2), π* semioccupied molecular orbital in these d1 complexes. The amido Mo(VI) and Mo(V) complexes exhibit an intramolecular hydrogen-bond between the NH and Ophenolate atoms. Protonation of [CoCp2][TpiPrMoVO2(2-OC6H4CONHR)] by lutidinium tetrafluoroborate is quantitative and produces EPR-active, cis-(hydroxo)oxo-Mo(V) complexes, TpiPrMoVO(OH)(2-OC6H4CONHR), related to the low pH Mo(V) forms of sulfite oxidase.

A series of bis-salicylidene based N₂S₂ copper macrocycles were prepared, structurally characterised and subjected to electrochemical analysis. The aim was to investigate the effects of length of polymethylene chains between either the imine donors or the sulfur donors on redox state and potential of the metal. The complexes structurally characterised had either distorted square planar or tetrahedral geometries depending on their oxidation state (Cu²+ or Cu+, respectively), and the N–(CH₂)n–N bridge was found to be most critical moiety in determining the redox potential and oxidation state of the copper macrocycles, with relatively little change in these properties caused by lengthening the S–(CH₂)n–S bridge from two to three carbons. In fact, a weakness was observed in the complexes at the sulfur donor, as further lengthening of the S–(CH₂)n–S methylene bridge to four carbons caused fission of the carbon–sulfur bond to give dimeric rings and supramolecular assemblies. Cu+ complexes could be oxidised to Cu²+ by tert-butylhydroperoxide, with a corresponding change in the spectrophotometric properties, and likewise Cu²+ complexes could be reduced to Cu+ by treatment with β-mercaptoethylamine. However, repeated redox cycles appeared to compromise the stability of the macrocycles, most probably by a competing oxidation of the ligand. Thus the copper N₂S₂ macrocycles show potential as redox sensors, but further development is required to improve their performance in a biochemical environment.

The study reports an advance in designing copper-based redox sensing MRI contrast agents. Although the data demonstrate that copper(II) complexes are not able to compete with lanthanoids species in terms of contrast, the redox-dependent switch between diamagnetic copper(I) and paramagnetic copper(II) yields a novel redox-sensitive contrast moiety with potential for reversibility.

The X-ray crystal structures of (N,N'-bis-(o-amidobenzilidene)-1,3-diaminopropane)nickel (Niambpr), (N,N'-bis-(o-amidobenzilidene)-1,4-diaminobutane)nickel (Niambut), (N,N'-bis-(o-thiobenzilidene)-1,4-diaminobutane)nickel(II) (Nitsalbut), {bis-acetonitrile-(N,N'-bis-(o-aminobenzyl)-1,2-diaminoethane)}nickel(II) tetrafluoroborate [Ni(H₄amben)(MeCN)₂] [BF₄]₂, {bis-O-acetato-(N,N'-bis-(o-aminobenzyl)-1,2-diaminoethane)}nickel(II) [Ni(H₄amben)(OAc)₂ · H₂O] and {bis-O-acetato-(N,N'-bis-(o-aminobenzyl)-1,3-diaminopropane)}nickel(II) [Ni(H₄ambpr)(OAc)₂] are presented. These structures complete the structural characterisation of the simple unsubstituted Schiff's base complexes with N₄ and N₂S₂ donor sets and allow us to assess the effects of donor groups and polymethylene chain length on the coordination geometries of nickel(II). The hydrogenated N4 complexes offer an insight into the effects of increased flexibility and character of the internal nitrogen donors. Unlike the parent N4 imine species the hydrogenated amine species do not deprotonate at the peripheral nitrogen donors and do not seem to be restricted to the meridial plane of the nickel.

The title tetradentate Schiff base complex (systematic name: {2,2'-[propane-1,3-diylbis(nitrilomethylidyne)]benzenethiolato- κ⁴S,N,N',S'}nickel(II) 1,4-dioxane solvate), [Ni(C₁₇H₁₆-N₂S₂)]·C₄H₈O₂, contains an Ni atom coordinated within a tetrahedrally distorted planar N₂S₂ environment, with average Ni-N and N-S bond lengths of 1.922 (1) and 2.167 (1) Å, respectively.

The oxo-MoIV complexes formed in the reactions of cis-TpiPrMoVIO2(OAr-R) [TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate, –OAr-R = phenolate derivative] complexes with PEt3 or PEt2Ph in acetonitrile depend on the nature of the potential hydrogen-bonding group (R) incorporated into the phenolate ligand. Green, diamagnetic, oxo(phosphoryl)-MoIV complexes, TpiPrMoIVO(OAr-R)(OPR'3), are produced when R is absent or is a non-coordinating group such as 2-OMe and 3-NEt2; six-coordinate TpiPrMoO(OC6H4OMe-2)(OPEt3) was structurally characterized and exhibits a distorted octahedral geometry typical of such species. When R is a carbonyl functionality, complete oxygen atom transfer leads to green or purple, diamagnetic, chelate complexes of the type, TpiPrMoIVO(OAr-R-κ2O,O'). The R = 2-COEt, 2-CO2Me and 2-CO2Ph derivatives exhibit six-coordinate, distorted-octahedral structures possessing fac TpiPr, terminal oxo and bidentate O,O'-donor –OAr-R ligands. Where R is an amido functionality, CONHPh, the complexes, TpiPrMoIVO(OC6H4CONHPh-2-κ2O,O')·OPR'3 (R'3 = Et3, Et2Ph), are isolated. Here, the six-coordinate, distorted-octahedral complex forms an intermolecular NH···OPR'3 hydrogen bond to the lattice OPR'3 molecule. Thus, facile chelation of potential hydrogen-bonding phenolate ligands suppresses hydrogen-bond-stabilized aquation or hydroxylation cis to the oxo group in these MoIV complexes.

Red-black [TpiPr*MoVO]₂(μ-O)(μ-MoVIO4) (1, TpiPr* = hydrobis(3-isopropylpyrazolyl)(5-isopropylpyrazolyl)borate) has been isolated as a by-product in the synthesis of NEt4[TpiPrMo(CO)3] (TpiPr = hydrotris(3-isopropylpyrazolyl)borate) and characterized by spectroscopic and X-ray crystallographic techniques. The trinuclear, mixed-valence complex contains two distorted octahedral anti-TpiPr∗MoVO centers bridged by bent oxo (Mo-O-Mo av. 158.7°) and tetrahedral κO,κO'-molybdate ligands. The complex contains a six-membered, non-planar Mo3(μ-O)3 core and two 1,2-borotropically-shifted TpiPr* ligands (with the shifted pyrazolyl trans to MoV=O). Aerial decomposition of solid NEt4[TpiPrMo(CO)3] produces sky-blue, diamagnetic TpiPrMoO(iPrpz)(iPrpzH) (2, iPrpz⁻ = 3-isopropylpyrazolate, iPrpzH = 3-isopropyl-2H-pyrazole). Molecules of 2 feature a tridentate fac-TpiPr ligand and mutually cis terminal oxo (MoO = 1.665(2) Å) and monodentate iPrpz− and iPrpzH ligands. The latter are formed by B-N bond cleavage of TpiPr. The complex can also be synthesized by reacting NEt4[TpiPrMo(CO)3] with excess 3-isopropylpyrazole and dioxygen at 100 °C. Cleavage of the B-N bond(s) of TpiPr was also observed in the formation of TpiPrMoO(SPh)(iPrpzH) (3) as a by-product in the synthesis of TpiPrMoO₂(SPh). In the monohydrate, 3 exhibits a distorted octahedral geometry defined by a tridentate fac-TpiPr ligand and mutually cis terminal oxo (MoO = 1.676(3) Å) and monodentate SPh⁻ and iPrpzH ligands. The pyrazole β-NH group is observed to participate in a hydrogen-bond to the lattice water molecule. The complex can be synthesized in high yield by reducing TpiPrMoO₂(SPh) by HSPh or PPh3 in the presence of excess 3-isopropylpyrazole.

The complexes cis-TpiPrMoVIO2(OAr-R) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate, -OAr-R = hydrogen-bonding phenolate derivative) are formed upon reaction of TpiPrMoO2Cl, HOAr-R, and NEt3 in dichloromethane. The orange, diamagnetic, dioxo−Mo(VI) complexes exhibit strong ν(MoO2) IR bands at ca. 935 and 900 cm-1 and NMR spectra indicative of Cs symmetry. They undergo electrochemically reversible, one-electron reductions at potentials in the range −0.836 to −0.598 V vs SCE; the only exception is the 2-CO2Ph derivative, which exhibits an irreversible reduction at −0.924 V. The complexes display distorted octahedral geometries, with a cis arrangement of terminal oxo ligands and with d(Mo=O)av = 1.695 Å and (MoO2)av = 103.2°. The R groups of the 2-CHO and 2-NHCOMe derivatives are directed away from the oxo groups and into a cleft in the TpiPr ligand; these derivatives are characterized by Mo−O−Cipso angles of ca. 131° (conformation 1). The R group(s) in the 2-CO2Me and 2,3-(OMe)2 derivatives lie above the face of the three O-donor atoms (directed away from the TpiPr ligand) and the complexes display Mo−O−Cipso angles of 153.1(2) and 149.7(2)°, respectively (conformation 2). Conformations 1 and 2 are both observed in the positionally disordered 2-COMe and 2-COEt derivatives, the two conformers having Mo−O−Cipso angles of 130−140 and >150°, respectively. The 3-COMe and 3-NEt2 derivatives have substituents that project away from the TpiPr ligand and Mo−O−Cipso angles of 134.2(2) and 147.7(2)°, respectively. Many of the complexes exhibit fluxional behavior on the NMR time scale, consistent with the rapid interconversion of two conformers in solution.

The reactions of [V₂(μ-S₂)₂(S₂CNR₂)₄] (R = alkyl) with NOBF₄ produce highly-oxidised, sulfur-rich, V(IV/V) complexes, [V2(μ-S₂)₂(S₂CNR₂)₄]BF₄, that exhibit 15-line EPR spectra and structures consistent with Class III mixed-valence behaviour.