Debashis Adhikari

A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.

Structural, Spectroscopic, and Theoretical Elucidation of a Redox-Active Pincer-Type Ancillary Applied in Catalysis

Pincer-type ligands are believed to be very robust scaffolds that can support multifarious functionalities as well as highly reactive metal motifs applied in organometallic chemistry, especially in the realm of catalysis. In this paper, we describe the redox and, therefore, noninnocent behavior of a PNP (PNP- = N[2-P(CHMe2)2-4-methylphenyl]2) pincer ancillary bound to nickel. A combination of structural, spectroscopic, and theoretical techniques suggests that this type of framework can house an electron hole when coordinated to Ni(II).

Selective surface and near-surface modification of a noncatenated, catalytically active metal-organic framework material based on Mn(salen) struts.

From a combination of chiral Mn(salen) struts and the tetratopic ligand tetrakis(4-carboxyphenyl)benzene, a large-pore, noncatenated metal-organic framework (MOF) material, MnSO-MOF, has been synthesized. Following solvent exchange with hydrophobic CHCl(3), treatment of MnSO-MOF with aqueous H(2)O(2) allowed for the selective demetalation of Mn(salen) struts at and near the surface of the crystals. The resulting crystals displayed greatly enhanced size-selective catalysis compared to the as-synthesized material. Handling of the mechanically fragile MnSO-MOF crystals was greatly facilitated by activation with supercritical CO(2).

A computational study of the mechanism of the [(salen)Cr + DMAP]-catalyzed formation of cyclic carbonates from CO2 and epoxide.

Epoxide and CO2 coupling reactions catalyzed by (salen)Cr(III)Cl have been modeled computationally to contrast a monometallic vs. a bimetallic mechanism. A low-energy CO2 insertion step into the metal-alkoxide bond was located.

P=N bond formation via incomplete N-atom transfer from a ferrous amide precursor.

Incomplete N-atom transfer from Fe to P is observed when the ferrous amide complex (PNP)Fe(dbabh) (PNP-=N[2-P(iPr)2-4-methylphenyl]2, dbabh=2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene), prepared from salt metathesis of (PNP)FeCl and Li(dbabh), is thermolyzed at 70 degrees C over 48 h in C6D6. Several plausible reaction pathways resulting from the transformation of (PNP)Fe(dbabh) are discussed, including the possibility of an Fe(IV) nitride as an intermediate.

H2S Reactivity on Oxygen-Deficient Heterotrimetallic Cores: Cluster Fluxionality Simulates Dynamic Aspects of Surface Chemical Reactions.

Understanding the mechanistic aspects of heterogeneous reactions on supported metal catalysts is challenging due to several disparate factors, among which the dynamic nature of the surface is a major contributor. In this study, the dynamic aspect of a surface has been probed by choosing small metal clusters as illustrative models. Two anionic hetero-trimetallic clusters, namely, W2TcO6(-) and W2OsO6(-), were reacted with H2S gas to exhibit splitting of the gas molecule and complete oxygen-sulfur exchange in the metal core. During this atom-exchange process, the core exhibits remarkable fluxionality to augment a thiol proton migration from one metal center to another, as well as a rapid interchange of the terminal and bridging oxygens. The fluxional nature of the core is further evidenced by two oppositely oriented oxo groups working in concert to accomplish the proton transfer, upon introduction of sulfur inside the core. These fluxional processes in the small hetero-trimetallic cores closely resemble the dynamic nature of the surface in a heterogeneous reaction. Throughout the fluxional processes investigated in this study, two-state reactivity and multiple instances of spin crossover are observed in our computational studies. Interestingly, the neutral hetero-trimetallic cores can also undergo complete oxygen-sulfur exchange reaction with H2S. The investigated metal clusters are promising materials, since they not only can liberate dihydrogen from water (reported in J. Phys. Chem. A, 2014, 118, 11047) but also can completely strip the sulfur from environmentally hazardous H2S gas.

Hydroxyl migration in heterotrimetallic clusters: an assessment of fluxionality pathways.

Water splitting at the unsaturated metal center and subsequent hydroxyl migration are key steps toward successful H2 liberation from cheap and abundant water using transition metal cluster anions. In this report we initiate a theoretical study (DFT) to assess the efficacy of heterometallic cores instead of the widely studied and well established homometallic cores. To accomplish this goal, one tungsten center in W3O6(-) core has been replaced by different transition metals such as titanium, technetium, and osmium. Introduction of the heterometal makes the core asymmetric and electronically anisotropic. To evaluate the efficiency of these heterometallic cores, fluxionality pathways for hydroxyl migration have been studied in detail. We show that the cores W2TcO6(-) (2) and W2OsO6(-) (3) can exhibit fluxionality for hydroxyl migration and thus can potentially facilitate H2 liberation from H2O. Notably, a new class of low-energy structures generated upon oxide bridge opening process and subsequent structural rearrangement facilitates the hydroxyl migration event. To illustrate the heterometallic effect further, we show that previously inaccessible energy barriers for hydroxyl migration in a homometallic trimolybdenum core become energetically achievable when one of the metals is replaced by a 5d element osmium.