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.

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 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.

Reactivity studies of oxo-Mo(IV) complexes, Tp iPr MoO{2-OC₆H4₄C(O)R-ᴋ²O,O'} (R = Me, Et, OMe, OEt, OPh, NHPh), containing chelated hydrogen-bond donor/acceptor phenolate ligands are reported.

The reactions of TpiPrMoO(SR)(NCMe) (TpiPr = hydrotris(3-isopropylpyrazolyl)borate) with propylene sulfide in toluene result in the formation of the diamagnetic, isovalent Mo(V) complex, [TpiPrMoVO]2(μ-S)(μ-S2). This complex and its previously reported μ-oxo analog, [TpiPrMoVO]2(μ-O)(μ-S2), react with cobaltocene to produce one-electron-reduced, mixed-valent complexes, [CoCp2][{TpiPrMoIV,VO}2(μ-E)(μ-S2)] (E = S or O, respectively). All complexes have been isolated and characterized by microanalysis, mass spectrometry, IR and 1H NMR or EPR spectroscopies, and X-ray crystallography. Neutral [TpiPrMoVO]2(μ-S)(μ-S2) exhibits a pseudo-C2 symmetric structure, with distorted octahedral anti oxo-Mo(IV) centers coordinated by TpiPr and linked by μ-sulfido and μ-disulfido ligands. A similar structure is adopted by the anion in mixed-valent [CoCp2][{TpiPrMoIV,VO}2(μ-S)(μ-S2)]; this compound adopts a hexagonal, supramolecular structure with columns of tight ion-pairs with (μ-S2)...CoCp2+ interactions, interconnected through weaker (μ-S2)...CoCp2+ contacts to three neighboring columns. The structure contains large interstitial voids filled with lattice solvent molecules. EPR investigation of the mixed-valent complexes gave rise to unusually broad signals with no evident hyperfine splitting. The synthesis and characterization of a number of cis-dioxo-Mo(VI) precursors are also reported.

The synthesis and structure of a homologous series of cationic N₂S₂ copper(I) Schiff base complexes constructed using o-tert-butylthiobenzaldehyde and a series of terminal diamines (ethane, propane, butane) are reported. The complexes differ only in the length of the methylene chain between the imine groups. This simple modification forces the copper centre to shift geometry from a planar (1,2-diaminoethane) to a more distorted tetrahedral motif (1,4-diaminobutane). The redox potentials of the three cations were measured using cyclic voltammetry in donor (acetonitrile) and non-donor solvents (dichloromethane). The S–Cu–N angles for each complex are correlated against the respective redox potential allowing an analysis of the geometric impact on the redox potential in soft copper centres. The redox potential is observed to increase as the metal centre moves from a planar towards a tetrahedral motif. Comparing this data with the reported structures of the blue copper proteins (rusticyanin and plastocyanin) allows an assessment of the contribution of the geometry of the metal binding site to the operating potential of these proteins to be made.