Activation of O-H and C-O Bonds in Water and Methanol by a Vanadium-Bound Thiyl Radical.

The reaction of [V(PS3")]- (1) (PS3"=[P(C6 H3 -3-Me3 Si-2-S)3 ]3- ) with H2 O led to the formation of [VIV (PS3")(PS2"SH )]- (2) (PS2"SH =[P(C6 H3 -3-Me3 Si-2-S)2 (C6 H3 -3-Me3 Si-2-SH)]2- ), indicating a hydrogen atom transfer from H2 O to a bound thiolate in 1. Furthermore, the reaction of 1 with CH3 OH gave the generation of complexes 2 and 3, [VIV (PS3")(PS2"SCH3 )]- (PS2"SCH3 =[P(C6 H3 -3-Me3 Si-2-S)2 (C6 H3 -3-Me3 Si-2-SCH3 )]2- ), implying that C-O and O-H bonds are cleaved by 1. Quantum mechanical calculations were performed to provide the mechanistic understanding for the reactivity of 1 with water. A key transition state with a lower kinetic barrier is identified. It involves an O-H bond cleavage by a dissociated thiyl radical with an interaction between an OH group and a neighboring bound sulfur donor. To our knowledge, the reactivity of 1 represents a new mode for water activation conducted through cooperation between a metal-stabilized thiyl radical and a neighboring thiolato donor.

Redox Interconversion of Non-Oxido Vanadium Complexes Accompanied by Acid-Base Chemistry of Thiol and Thiolate.

The redox nature of the non-oxido vanadium sulfur center is associated with several biological systems such as vanadium nitrogenase, the reduction of vanadium ion in ascidians, and the function of amavadin, which is a vanadium(IV) natural product contained in Amanita mushrooms. But the related chemistry is less explored and understood compared to oxido vanadium species due to the oxophilic character of high valent vanadium ions. Herein, we present a class of non-oxido vanadium thiolate complexes, [VIII(PS2″SH)2]- (1) (PS2″SH = [P(C6H3-3-Me3Si-2-S)2(C6H3-3-Me3Si-2-SH)]2-), [VIV(PS3″)(PS2″SH)]- (2) (PS3″ = [P(C6H3-3-Me3Si-2-S)3]3-), [V(PS3″)2]- (3), [V(PS3″)(PS2″SH)] (4), and [VIV(PS3*)2]2- (5a) (PS3* = [P(C6H3-3-Ph-2-S)3]3-), and study their interconversion through the redox and acid-base reactions. Complex 1 consists of a six-coordinate octahedral vanadium center; complexes 2 and 4 are seven-coordinate with distorted capped trigonal prismatic geometry. Vanadium centers of 3 and 5a are both eight-coordinate; the former adopts ideal dodecahedral geometry, but the latter is better viewed as a distorted square antiprism. Complex 1 is oxidized to complex 2 and then to complex 3 with dioxygen. Each one-electron oxidation process is accompanied by the deprotonation of unbound thiol to bound thiolate. Complex 3 is also produced from complex 2 through stepwise addition of Fe(Cp)2+/n-BuLi, or in the reverse order. The formation of 2 from 3 is achieved in the order of adding Co(Cp)2 and acid or, as with the previous complex, inversely. Notably, the reduction of complex 2 to complex 1 accompanying the protonation of bound thiolate to unbound thiol only occurs with the presence of both Co(Cp)2 and acid, indicating a cooperative effect between the metal-centered reduction and bound thiolate protonation. The conversions among these complexes are observed with ESI-MS and UV-vis-NIR spectroscopies. The work demonstrates two-electron redox interconversion in these complexes mediated by transformations between unbound thiol and bound thiolate.

A non-oxo methanolate-bridged divanadium(IV) complex with tris(2-sulfanidylphenyl)phosphane ligands: synthesis, structural characterization and magnetic investigation.

Vanadium chemistry is of interest due its biological relevance and medical applications. In particular, the interactions of high-valent vanadium ions with sulfur-containing biologically important molecules, such as cysteine and glutathione, might be related to the redox conversion of vanadium in ascidians, the function of amavadin (a vanadium-containing anion) and the antidiabetic behaviour of vanadium compounds. A mechanistic understanding of these aspects is important. In an effort to investigate high-valent vanadium-sulfur chemistry, we have synthesized and characterized the non-oxo divanadium(IV) complex salt tetraphenylphosphonium tri-µ-methanolato-κ(6)O:O-bis({tris[2-sulfanidyl-3-(trimethylsilyl)phenyl]phosphane-κ(4)P,S,S',S''}vanadium(IV)) methanol disolvate, (C24H20P)[V(IV)2(µ-OCH3)3(C27H36PS3)2]·2CH3OH. Two V(IV) metal centres are bridged by three methanolate ligands, giving a C2-symmetric V2(µ-OMe)3 core structure. Each V(IV) centre adopts a monocapped trigonal antiprismatic geometry, with the P atom situated in the capping position and the three S atoms and three O atoms forming two triangular faces of the trigonal antiprism. The magnetic data indicate a paramagnetic nature of the salt, with an S = 1 spin state.

Reactivity of a Fe(III)-Bound Methoxide Supported with a Tris(thiolato)phosphine Ligand: Activation of C-Cl Bond in CH2Cl2 by Nucleophilic Attack of a Fe(III)-OCH3 Moiety.

Two mononuclear nonheme Fe(III) complexes, [PPh4][Fe(III)(PS3″)(OCH3)] (1) and [PPh4][Fe(III)(PS3″)(Cl)] (2), supported by a tris(benzenethiolato)phosphine derivative PS3″ (PS3″ = P(C6H3-3-Me3Si-2-S)3(3-)) have been synthesized and characterized. The structures resolved from X-ray crystallography show that Fe(III) centers in both complexes adopt distorted trigonal-bipyramidal geometry with a methoxide or a chloride binding in the axial position. The magnetic data for both are consistent with intermediate-spin Fe(III) centers with a C3 symmetry (S = 3/2 ground state). The bound methoxide in 1 is labile and can be replaced by a CH3CN molecule. The forming Fe(III)-CH3CN species can be further reduced by cobaltcene quantitatively to a stable Fe(II)-CH3CN complex, [Fe(PS3″)(CH3CN)](-). One-electron oxidation of 2 by ferrocenium gave a Fe(IV) analogue, [Fe(IV)(PS3″)(Cl)]. Importantly, the Fe(III)-OCH3 moiety in complex 1 acts as a strong nucleophile that activates the C-Cl bond in CH2Cl2, leading to the formation of complex 2 quantitatively. Complex 1 also reacts with other electrophiles, benzyl chloride and benzyl bromide, to generate Fe(III)-X species (X = Cl or Br). The reactions were investigated and monitored by UV-vis-NIR, NMR, and ESI-MS spectroscopies.

Non-oxido divanadium(IV) and divanadium(V) thiolate complexes with a new type of chalcogenide bridging motif.

In our effort to study vanadium chalcogenide chemistry, we have synthesized and characterized a class of non-oxido divanadium(IV) and divanadium(V) complexes with chalcogenide and dichalcogenide as bridges. All structures consist of a similar divanadium motif, in which two metal centers are bridged by one µ-chalcogenide and one µ-η(2):η(2)-dichalcogenide, forming a V2(µ-E)(µ-η(2):η(2)-E2) (E = S or Se) core structure. These compounds are [V(IV)2(PS3)2(µ-Se2)(µ-Se)][PPh4]2 (1), [V(V)2(PS3'')2(µ-Se2)(µ-Se)] (2), [V(V)2(PS3'')2(µ-S2)(µ-S)] (3a) and [V(V)2(PS3)2(µ-S2)(µ-S)] (3b) ([PS3](3-) = P(C6H4-2-S)3 and [PS3''](3-) = P(C6H3-3-SiMe3-2-S)3). Compound 1 exhibits diamagnetic behavior, indicating strong antiferromagnetic coupling between two d(1) centers. Compounds 2 and 3a-b have the highest oxidation states for vanadium ions (+5/+5) among those reported divanadium chalcogenide clusters. The work demonstrates that high-valent divanadium chalcogenide clusters can be obtained with the activation of elemental chalcogens by low-valent vanadium ions.

Catalytic reduction of hydrazine to ammonia by a mononuclear iron(II) complex on a tris(thiolato)phosphine platform.

To provide the mechanistic information of nitrogenase at a molecular level, much effort has been made to develop synthetic metal complexes that have enzyme-like reactivity. Herein we obtain an iron(II) complex binding with a tris(thiolato)phosphine ligand, [P(Ph)4][Fe(PS3″)(CH3CN)] [1; PS3″ = P(C6H3-3-Me3Si-2-S)3(3-)] that catalyzes the reduction of hydrazine, an intermediate and a substrate of nitrogenase. The substrate- and product-bound adducts, [N(Bu)4][Fe(PS3″)(N2H4)] (2) and [N(Et)4][Fe(PS3″)(NH3)] (3), respectively, are also synthesized. This work provides the feasibility that the late stage of biological nitrogen fixation can be conducted at a single iron site with a sulfur-rich ligation environment.

Activation of dichloromethane by a V(III) thiolate complex: an example of S-based nucleophilic reactivity in an early transition metal thiolate.

A V(III) thiolate complex activated C-Cl bond in dichloromethane via S-based nucleophilic attack. The reaction products, a V(III)-Cl species (major one) and a V(IV) binding to a CH(2) containing ligand (minor one) were obtained. The work demonstrates sulfur donors in the early-transition metal thiolates having strong nucleophilic characteristics.

An eight-coordinate vanadium thiolate complex with charge delocalization between V(V)-thiolate and V(IV)-thiyl radical forms.

A six-coordinate oxovanadium(V) thiolate complex and an eight-coordinate non-oxovanadium thiolate complex, [PPh(4)][VO(PS3'')(OCH(3))] (1) and [NEt(4)][V(PS3'')(2)] (2) (PS3'' = P(C(6)H(3)-3-Me(3)Si-2-S)(3)(3-)), respectively, have been isolated and structurally characterized. The former belongs to a limited collection of oxovanadium(V) thiolate complexes. The latter has an unusual coordination number of eight. More importantly, its consensus electronic structure derived from its spectroscopic data should be considered as the resonance forms of V(V)-thiolate and V(IV)-thiyl radical species. This implies that V(IV)-thiyl radical can maintain a stable presence in biological systems.

Synthesis, dual fluorescence, and fluoroionophoric behavior of dipyridylaminomethylstilbenes.

The synthesis, dual fluorescence, and fluoroionophoric behavior of two donor-sigma spacer-acceptor (D-s-A) compounds, trans-4-(N,N-bis(2-pyridyl)amino)methylstilbene (1H) and trans-4-(N,N-bis(2-pyridyl)amino)methyl-4'-cyanostilbene (1CN), are reported and compared to that of trans-4-(N,N-bis(2-pyridyl)amino)methyl-4'-(N,N-dimethylamino)stilbene (1DPA). To gain insights into the dual fluorescence properties for 1H and 1CN in polar but not in nonpolar solvents, model compounds resulting from a replacement of the stilbene group by alkyl (2R) or xylyl (2X) groups or from a replacement of the dipyridylamino (dpa) group by dianisoleamino (3AA), diethylamino (3EE), methylanilino (3MP), or diphenylamino (3PP) groups also have been investigated. In addition to 1H and 1CN, all four compounds of 3 display dual fluorescence. The locally excited (LE) fluorescence mainly results from the stilbene group and the ICT fluorescence from the through-bond interactions between the amino donor and the stilbene acceptors. In the presence of transition metal ions such as Zn(II), Ni(II), Cu(II), and Cd(II), the ICT processes are switched from dpa (D) --> stilbene (A) in 1H and 1CN to stilbene (D) --> dpa/metal ion (A) in their complexes. Whereas the ICT states for the complexes are generally nonfluorescent, an exception was found for the case of 1H/Zn(II). As a result, substituent-dependent fluoroionophoric behavior has been demonstrated by 1H, 1CN, and 1DPA in response to Zn(II).