Priyabrata Banerjee

Electronic Structure of the [Tris(dithiolene)chromium]z(z=0,1―, 2―, 3―) Electron Transfer Series and Their Manganese(IV) Analogues. An X-ray Absorption Spectroscopic and Density Functional Theoretical Study

From the reaction mixture of 3,6-dichlorobenzene-1,2-dithiol, H(2)(Cl(2)-bdt), [CrCl(3)(thf)(3)], and NEt(3) in tetrahydrofuran (thf) in the presence of air, dark green crystals of [N(n-Bu)(4)](2)[Cr(Cl(2)-bdt)(3)] (S = 1) (1) were isolated upon addition of [N(n-Bu)(4)]Br. Oxidation of the AsPh(4)(+) salt of 1 with [Fc]PF(6) yielded microcrystals of [AsPh(4)][Cr(Cl(2)-bdt)(3)] (S = (1)/(2)) (2) whereas the reduction of 1 with sodium amalgam produced light green crystals of [N-(n-Bu)(4)](3)[Cr(Cl(2)-bdt)(3)].thf (S = (3)/(2)) (3). The corresponding maleonitriledithiolato complexes [PPh(4)](2)[Cr(mnt)(3)] (S = 1) (4) and [PPh(4)](3)[Cr(mnt)(3)] (S = (3)/(2)) (5) have been synthesized. Isoelectronic manganese complexes of 3 and 5, namely, [NEt(4)](2)[Mn(Cl(2)-bdt)(3)] (S = (3)/(2)) (6) and [PPh(4)](2)[Mn(mnt)(3)] (S = (3)/(2)) (7), have also been prepared. Complexes 1, 6, and 7 have been characterized by single crystal X-ray crystallography. Complexes 1-7 have been electrochemically studied and their UV-vis and electron paramagnetic resonance spectra (EPR) have been recorded; magnetic properties have been elucidated by temperature-dependent susceptibility measurements. It is shown by chromium K-edge and sulfur K-edge X-ray absorption spectroscopy (XAS) that the oxidation state of the central Cr ion in each compound is the same (+III, d(3)) and that all one-electron redox processes are ligand-based, involving one, two, or three ligand pi radical monoanions. Complexes 6 and 7 possess a Mn(IV) ion with three dianionic ligands. The results have been corroborated by broken symmetry (BS) density functional theoretical (DFT) calculations by using the B3LYP functional. Time-dependent DFT calculations have been performed to calculate the metal and sulfur K-pre-edges. It is suggested that the neutral complexes [Cr(dithiolene)(3)](0) S = 0 possess octahedral rather than trigonal prismatic CrS(6) polyhedra. Three ligand pi radicals (S(rad) = (1)/(2)) couple antiferromagnetically to the central Cr(III) ion (d(3)) yielding the observed diamagnetic ground state. It is established that the four members of the [Cr(dithiolene)(3)](z) (z = 0, 1-, 2-, 3-) electron transfer series are related by ligand-based one-electron transfer processes; for each of the four members it is shown that they contain a central Cr(III) (d(3)) ion, and the CrS(6) polyhedron is a (distorted) octahedron.

Monoanionic molybdenum and tungsten tris(dithiolene) complexes: a multifrequency EPR study.

Numerous Mo and W tris(dithiolene) complexes in varying redox states have been prepared and representative examples characterized crystallographically: [M(S(2)C(2)R(2))(3)](z) [M = Mo, R = Ph, z = 0 (1) or 1- (2); M = W, R = Ph, z = 0 (4) or 1- (5); R = CN, z = 2-, M = Mo (3) or W (6)]. Changes in dithiolene C-S and C-C bond lengths for 1 versus 2 and 4 versus 5 are indicative of ligand reduction. Trigonal twist angles (Θ) and dithiolene fold angles (α) increase and decrease, respectively, for 2 versus 1, 5 versus 4. Cyclic voltammetry reveals generally two reversible couples corresponding to 0/1- and 1-/2- reductions. The electronic structures of monoanionic molybdenum tris(dithiolene) complexes have been analyzed by multifrequency (S-, X-, Q-band) EPR spectroscopy. Spin-Hamiltonian parameters afforded by spectral simulation for each complex demonstrate the existence of two distinctive electronic structure types. The first is [Mo(IV)((A)L(3)(5-•))](1-) ((A)L = olefinic dithiolene, type A), which has the unpaired electron restricted to the tris(dithiolene) unit and is characterized by isotropic g-values and small molybdenum superhyperfine coupling. The second is formulated as [Mo(V)((B)L(3)(6-))](1-) ((B)L = aromatic dithiolene, type B) with spectra distinguished by a prominent g-anisotropy and hyperfine coupling consistent with the (d(z(2)))(1) paramagnet. The electronic structure disparity is also manifested in their electronic absorption spectra. The compound [W(bdt)(3)](1-) exhibits spin-Hamiltonian parameters similar to those of [Mo(bdt)(3)](1-) and thus is formulated as [W(V)((B)L(3)(6-))](1-). The EPR spectra of [W((A)L(3))](1-) display spin-Hamiltonian parameters that suggest their electronic structure is best represented by two resonance forms {[W(IV)((A)L(3)(5-•))](1-) ↔ [W(V)((A)L(3)(6-))](1-)}. The contrast with the corresponding [Mo(IV)((A)L(3)(5-•))](1-) complexes highlights tungstens preference for higher oxidation states.

Isolation and assessment of the molecular and electronic structures of azo-anion-radical complexes of chromium and molybdenum. Experimental and theoretical characterization of complete electron-transfer series.

The reaction of 3 equiv of the ligand 2-[(2-chlorophenyl)azo]pyridine (L(a)) or 2-[(4-chlorophenyl)azo]pyridine (L(b)) with 1 equiv of Cr(CO)(6) or Mo(CO)(6) in boiling n-octane afforded [Cr(L(a/b))(3)](0) (1a and 1b) and [Mo(L(a/b))(3)](0) (2a and 2b). The chemical oxidation reaction of these neutral complexes with I(2) in CH(2)Cl(2) provided access to air-stable one-electron-oxidized species as their triiodide (I(3)(-)) salts. The electronic structures of chromium and molybdenum centers coordinated by the three redox noninnocent ligands L(a/b) along with their redox partners have been elucidated by using a host of physical methods: X-ray crystallography, magnetic susceptibility measurements, nuclear magnetic resonance, cyclic voltammetry, absorption spectroscopy, electron paramagnetic resonance spectroscopy, and density functional theory. The four representative complexes, 1a, [1a]I(3), 2a, and [2a]I(3), have been characterized by X-ray crystallography. The results indicate a predominant azo-anion-radical description of the ligands in the neutral chromium(III) species, [Cr(III)(L(•-))(3)], affording a singlet ground state through strong metal-ligand antiferromagnetic coupling. All of the electrochemical processes are ligand-based; i.e., the half-filled (t(2g))(3) set of the Cr(III) d(3) ion remains unchanged throughout. The description of the molybdenum analogue is less clear-cut because mixing between metal- and ligand-based orbitals is more significant. On the basis of variations in net spin densities and orbital compositions, we argue that the oxidation events are again primarily ligand-based, although the electron density at the molybdenum center is clearly more variable than that at the chromium center in the corresponding series [1a](+), 1a, and [1a](-).

Synthesis of azoaromatic dyes via redox driven C-N bond fusion.

Two novel organic azo-dyes (2(+)) that feature an intense intramolecular charge transfer transition with end absorption reaching into the NIR region are introduced. Syntheses of these compounds were achieved by an unusual redox-driven C-N bond fusion of the tricyclo azo-aromatic compounds (1(+)). The compounds show reversible electro- as well as proton chromism. The results have generated further scope of research in the area of designed syntheses of functional azoaromatics.