Synthesis, Redox and Spectroscopic Properties of Pterin of Molybdenum Cofactors.

Pterins are bicyclic heterocycles that are found widely across Nature and are involved in a variety of biological functions. Notably, pterins are found at the core of molybdenum cofactor (Moco) containing enzymes in the molybdopterin (MPT) ligand that coordinates molybdenum and facilitates cofactor activity. Pterins are diverse and can be widely functionalized to tune their properties. Herein, the general methods of synthesis, redox and spectroscopic properties of pterin are discussed to provide more insight into pterin chemistry and their importance to biological systems.

S K-edge XAS of CuII, CuI, and ZnII oxidized Dithiolene complexes: Covalent contributions to structure and the Jahn-Teller effect.

Reduced dithiolene ligands are bound to high valent Mo centers in the active site of the oxotransferase family of enzymes. Related model complexes have been studied with great insight by Prof. Holm and his colleagues. This study focuses on the other limit of dithiolene chemistry: an investigation of the 2-electron oxidized dithiolene bound to low-valent late transition metal (TM) ions (ZnII, CuI, and CuII). The bonding descriptions of the oxidized dithiolene [N,N-dimethyl piperazine 2,3-dithione (Me2Dt0)] complexes are probed using S K-edge X-ray absorption spectroscopy (XAS) and the results are correlated to density functional theory (DFT) calculations. These experimentally supported calculations are then extended to explain the different geometric structures of the three complexes. The ZnII(Me2Dt0)2 complex has only ligand-ligand repulsion so it is stabilized at the D2d symmetry limit. The CuI(Me2Dt0)2 complex has additional weak backbonding thus distorts somewhat from D2d toward D2h symmetry. The CuII(Me2Dt0)2 complex has a strong σ donor bond that leads to both a large Jahn-Teller stabilization to D2h and an additional covalent contribution to the geometry. The combined strong stabilization results in the square planar, D2h structure. This study quantifies the competition between the ligand-ligand repulsion and the change in electronic structures in determining the final geometric structures of the oxidized dithiolene complexes, and provides quantitative insights into the Jahn-Teller stabilization energy and its origin.

Interligand communication in a metal mediated LL'CT system - a case study.

A series of oxo-Mo(iv) complexes, [MoO(Dt2-)(Dt0)] (where Dt2- = benzene-1,2-dithiol (bdt), toluene-3,4-dithiol (tdt), quinoxaline-2,3-dithiol (qdt), or 3,6-dichloro-benzene-1,2-dithiol (bdtCl2); Dt0 = N,N'-dimethylpiperazine-2,3-dithione (Me2Dt0) or N,N'-diisopropylpiperazine-2,3-dithione ( i Pr2Dt0)), possessing a fully oxidized and a fully reduced dithiolene ligand have been synthesized and characterized. The assigned oxidation states of coordinated dithiolene ligands are supported with spectral and crystallographic data. The molecular structure of [MoO(tdt)( i Pr2Dt0)] (6) demonstrates a large ligand fold angle of 62.6° along the S⋯S vector of the Dt0 ligand. The electronic structure of this system is probed by density functional theory (DFT) calculations. The HOMO is largely localized on the Dt2- ligand while virtual orbitals are mostly Mo and Dt0 in character. Modeling the electronic spectrum of 6 with time dependent (TD) DFT calculations attributes the intense low energy transition at ∼18 000 cm-1 to a ligand-to-ligand charge transfer (LL'CT). The electron density difference map (EDDM) for the low energy transition depicts the electron rich Dt2- ligand donating charge density to the redox-active orbitals of the electron deficient Dt0 ligand. Electronic communication between dithiolene ligands is facilitated by a Mo-monooxo center and distortion about its primary coordination sphere.

Dithione, the antipodal redox partner of ene-1,2-dithiol ligands and their metal complexes.

Defining the oxidation state of the central atom in a coordination compound is fundamental in understanding the electronic structure and provides a starting point for elucidating molecular properties. The presence of non-innocent ligand(s) can obscure the oxidation state of the central atom as the ligand contribution to the electronic structure is difficult to ascertain. Redox-active ligands, such as dithiolene ligands, are well known non-innocent ligands that can exist in both a fully reduced (Dt2-) and fully oxidized (Dt0) states. Complexes containing the fully oxidized dithione state of the ligand are uncommon and only a few have been completely characterized. Dithione ligands are of interest due to their electron-deficient nature and ability to act as an electron acceptor for more electron-rich moieties, such as other dithiolene ligands or metal centers. This article focuses the syntheses, structures, and metal coordination, particularly coordination compounds, of dithione ligands. Various examples of mono, bis, and tris dithione complexes are discussed.

Syntheses, spectroscopic, redox, and structural properties of homoleptic Iron(III/II) dithione complexes.

Two sets of FeIII/II dithione complexes [FeII( i Pr2Dt0)3][PF6]2 ([1][PF6]2), [FeII(Me2Dt0)3][PF6]2 ([2][PF6]2), and [FeIII( i Pr2Dt0)3][PF6]3 ([3][PF6]3), [FeIII(Me2Dt0)3][PF6]3 ([4][PF6]3), and compound [FeIII( i Pr2Dt0)3][FeCl4][PF]2 ([3][FeCl4][PF6]2) were synthesized from N,N'-diisopropyl piperazine-2,3-dithione ( i Pr2Dt0) and N,N'-dimethyl piperazine-2,3-dithione (Me2Dt0) ligands. Complexes [1][PF6]2-[4][PF6]3 have been characterized by NMR, IR, and UV-visible spectroscopies, and by electrochemistry. The molecular structures of [2][PF6]2 and [3][FeCl4][PF6]2 have been determined by X-ray crystallography. Complexes [2][PF6]2 and [3][FeCl4][PF6]2 both crystallized in the monoclinic space group P21/n. Both complexes exhibit distorted octahedral geometry and the three coordinated ligands in each complex exhibit different dithione folding. Complexes [1][PF6]2-[4][PF6]3 exhibit a single FeIII/II based couple and three quasi-reversible ligand-based redox couples. The electronic spectra of [1][PF6]2-[4][PF6]3 show intense MLCT bands that indicate strong mixing between metal and ligand orbitals. DFT calculations were used to provide a framework for understanding the electronic origin of their redox chemistry and spectroscopic features.