Shashi Bhushan Sinha

Kinematic and mechanical profile of the self-actuation of thermosalient crystal twins of 1,2,4,5-tetrabromobenzene: a molecular crystalline analogue of a bimetallic strip.

A paradigm shift from hard to flexible, organic-based optoelectronics requires fast and reversible mechanical response from actuating materials that are used for conversion of heat or light into mechanical motion. As the limits in the response times of polymer-based actuating materials are reached, which are inherent to the less-than-optimal coupling between the light/heat and mechanical energy in them, a conceptually new approach to mechanical actuation is required to leapfrog the performance of organic actuators. Herein, we explore single crystals of 1,2,4,5-tetrabromobenzene (TBB) as actuating elements and establish relations between their kinematic profile and mechanical properties. Centimeter-size acicular crystals of TBB are the only naturally twinned crystals out of about a dozen known materials that exhibit the thermosalient effect-an extremely rare and visually impressive crystal locomotion. When taken over a phase transition, crystals of this material store mechanical strain and are rapidly self-actuated to sudden jumps to release the internal strain, leaping up to several centimeters. To establish the structural basis for this colossal crystal motility, we investigated the mechanical profile of the crystals from macroscale, in response to externally induced deformation under microscope, to nanoscale, by using nanoindentation. Kinematic analysis based on high-speed recordings of over 200 twinned TBB crystals exposed to directional or nondirectional heating unraveled that the crystal locomotion is a kinematically complex phenomenon that includes at least six kinematic effects. The nanoscale tests confirm the highly elastic nature, with an elastic deformation recovery (60%) that is far superior to those of molecular crystals reported earlier. This property appears to be critical for accumulation of stress required for crystal jumping. Twinned crystals of TBB exposed to moderate directional heating behave as all-organic analogue of a bimetallic strip, where the lattice misfit between the two crystal components drives reversible deformation of the crystal.

A Stable Coordination Complex of Rh(IV) in an N,O-Donor Environment.

We describe facial and meridional isomers of [Rh(III)(pyalk)3], as well as meridional [Rh(IV)(pyalk)3](+) {pyalk =2-(2-pyridyl)-2-propanoate}, the first coordination complex in an N,O-donor environment to show a clean, reversible Rh(III/IV) redox couple and to have a stable Rh(IV) form, which we characterize by EPR and UV-visible spectroscopy as well as X-ray crystallography. The unprecedented stability of the Rh(IV) species is ascribed to the exceptional donor strength of the ligands, their oxidation resistance, and the meridional coordination geometry.

Synthesis and Characterization of Iridium(V) Coordination Complexes With an N,O-Donor Organic Ligand

We have prepared and fully characterized two isomers of [IrIV (dpyp)2 ] (dpyp=meso-2,4-di(2-pyridinyl)-2,4-pentanediolate). These complexes can cleanly oxidize to [IrV (dpyp)2 ]+ , which to our knowledge represent the first mononuclear coordination complexes of IrV in an N,O-donor environment. One isomer has been fully characterized in the IrV state, including by X-ray crystallography, XPS, and DFT calculations, all of which confirm metal-centered oxidation. The unprecedented stability of these IrV complexes is ascribed to the exceptional donor strength of the ligands, their resistance to oxidative degradation, and the presence of four highly donor alkoxide groups in a plane, which breaks the degeneracy of the d-orbitals and favors oxidation.

Synthesis of pyridine-alkoxide ligands for formation of polynuclear complexes

We have prepared and characterized a series of novel polydentate N,O-donor ligands derived from our well-studied ligand 2-(2-pyridinyl)-2-propanol (pyalkH), having the general formula Me{C(OH)(2-py)CH2}nH, where n = 2 or 3. Like pyalkH, these analogues bind via N and O with deprotonation at the latter, thus extending the strongly donor pyridine-alkoxide chelation power of pyalkH to polydentate forms. The greater denticity allows for more effective binding and polynuclear cluster formation with first-row transition metals. Several stable alkoxo-bridged polynuclear clusters of these ligands with Mn, Cu, Co and Ni have been prepared; all reported ligands and complexes have been characterized, including by X-ray crystallography. We report a one-step synthesis of these ligands, alongside pyalkH, on a multi-gram scale from inexpensive starting materials. We have also developed a new scalable procedure for the isolation of pyalkH that avoids the need for chromatography, making large-scale production of this ligand commercially viable.

Co(II), a catalyst for selective conversion of phenyl rings to carboxylic acid groups

An inexpensive protocol for the conversion of –C6H4R into –COOH groups using Co(II)–Oxone mixture as the catalytic system is described. A series of substrates containing substituted and non-substituted phenyl groups could be selectively converted into carboxylic acids. Initial mechanistic data have been provided.

A Dinuclear Iridium(V,V) Oxo-Bridged Complex Characterized Using a Bulk Electrolysis Technique for Crystallizing Highly Oxidizing Compounds

We report a general method for the preparation and crystallization of highly oxidized metal complexes that are difficult to prepare and handle by more conventional means. This method improves typical bulk electrolysis and crystallization conditions for these reactive species by substituting oxidation-prone organic electrolytes and precipitants with oxidation-resistant compounds. Specifically, we find that CsPF6 is an effective inert electrolyte in acetonitrile, and appears to have general applicability to electrochemical studies in this solvent. Likewise, CCl4 is not only an oxidation-resistant precipitant for crystallization from MeCN but it also enters the lattice. In this way, we synthesized and characterized an Ir(V,V) mono-μ-oxo dimer which only forms at a very high potential (1.9 V vs NHE). This compound, having the highest isolated oxidation state in this redox-active system, cannot be formed chemically. DFT calculations show that the oxidation is centered on the Ir-O-Ir core and facilitated by strong electron-donation from the pyalk (2-(2-pyridinyl)-2-propanolate) ligand. TD-DFT simulations of the UV-visible spectrum reveal that its royal blue color arises from electron excitations with mixed LMCT and Laporte-allowed d-d character. We have also crystallographically characterized a related monomeric Ir(V) complex, similarly prepared by oxidizing a previously reported Ir(IV) compound at 1.7 V, underscoring the general applicability of this method.