Mechanistic investigations of stoichiometric and catalytic Pt-mediated oxidative functionalization at a proximal boron center

The focus of the work detailed in this dissertation is the investigation of mechanism and catalytic applications of Pt complexes supported by novel anionic di(2-pyridyl)borate ligands. It was found that oxidation of Me,MeBPy2-supported PtII complexes bearing no hydrocarbyl complexes directly generated dimethyl ether in quantitative yields, with one methyl originating from the MeB fragment. We also found that increasing formal charge on the metal center renders related complexes reluctant to undergo oxidation. Based on a proposed mechanism involving a transient PtIV-Me complex, we set out to develop a series of modified R,RBPy2 ligands to prevent such oxidatively induced hydrocarbyl transfer. We found that the strategy of replacing one hydrocarbyl (Me) group in the dmdpb ligand by methoxo (OMe) was not sufficient in completely preventing degradation of the borate center. However, derived mono- and di-hydrocarbyl PtII complexes could still be easily oxidized under aerobic conditions. Interestingly, oxidation products corresponding to both B-to-PtIV methyl migration and ligand retention were observed. We focused our attention to a unique 1,5-cyclooctanediylBPy2 ligand, which, we presumed, would prevent hydrocarbyl migration due to the rigid structure imposed by the bicyclic framework. The derived PtIVMe3 complex was found to exhibit `enhanced' BC-H agostic stabilization of the penta-coordinate PtIV center. Oxidation of derived PtII complexes results in hydride migration from the B-CH fragment onto the PtIV center, led to the formation of a series of (MeO),(MeO)BPy2 supported Pt complexes, and unanticipated C-C and C=C coupling at the borate center. The (MeO),(MeO)BPy2 ligand proved to be the first example of anionic facially chelating borate ligand capable of resisting oxidative degradation. The derived PtIV(Ph)2(OH) can be used for catalytic aerobic oxidation of NaBH(OMe)3 and NaBH4, with TOFs of 178/h and 216/h respectively. This may be of particular interest from the perspective of a direct-borohydride-fuel-cell (DBFC). We also found that the PtIV(Ph)2(OH) complex could be used as a catalyst to oxidize isopropanol to acetone under aerobic conditions with a TON of 3.8 after 56h at 80 °C. A mechanism involving selective hydride migration from a B-bound isopropoxy fragment to the PtIV center was proposed

Functionalization and solubilization of BN nanotubes by interaction with Lewis bases

By interaction with a trialkylamine or trialkylphosphine, BN nanotubes can be dispersed in a hydrocarbon medium with retention of the nanotube structure

Charge density analysis of two proton transfer complexes: understanding hydrogen bonding and determination of in-crystal dipole moments

An experimental charge density study has been carried out on proton-transfer complexes exhibiting nonlinear optical (NLO) properties-melaminium tartrate monohydrate and L-asparaginium picrate employing high-resolution X-ray diffraction at 100 K. Both the complexes crystallize in non-centric space group P21 and the structures exhibit interesting patterns of N-H...O and O-H...O hydrogen bonding. Experimental determination of the dipole moment (μ) for the asymmetric unit reveals that for both the crystals, there is a large cooperative enhancement in the crystalline μ arising essentially due to hydrogen bond mediated charge transfer between the melaminium ion and the L-tartrate in one case, between the Lasparaginium ion and the picrate in the other complex. We have additionally performed theoretical calculations at the density functional theory (DFT) level to understand the origin of enhancement of the dipole moments in the two systems

Semiconductor to metal transition in SWNTs caused by interaction with gold and platinum nanoparticles

Single-walled carbon nanotubes (SWNTs) have been coated with gold and platinum nanoparticles either by microwave treatment or by the click reaction and the Raman spectra of these SWNT-metal nanoparticle composites have been investigated. Analysis of the G bands in the Raman spectra shows an increase in the proportion of metallic SWNTs on attachment with metal nanoparticles. This conclusion is also supported by the changes observed in the RBM bands. Ab-initio calculations reveal that semiconductor-metal transition occurs in SWNTs due to Columbic charge transfer between the metal nanoparticles and the semiconducting SWNTs.Comment: 16 pages, 5 figure

Role of dipolar interactions in fine-tuning the linear and nonlinear optical responses in porphyrins

The variation in the ground-state dipole moment, polarizability and the 1st hyperpolarizability for non-chelated porphyrins are studied with increase in the inter-dipolar angles between the pyrrole rings. The different dipolar orientations are realized through twisting of one of the ring along the porphyrin plane. Computations performed on both conformational and configurational isomers of porphyins lead to a variety of structures. For the conformational distortions, the dipole-moment (μ), the polarizability (α) and the 1st hyperpolarizabilities (β) increase with increase in the distortions. This is understood on the basis of a simple excitonic picture wherein the splitting due to dipolar interactions reduces with increase in the distortions which effectively reduces the optical gap for the system. The computations reveal that dihedral twists within the conventional porphyrin structure provide strategies to design molecules with enhanced linear and nonlinear response functions

The role of H bonding and dipole-dipole interactions on the electrical polarizations and charge mobilities in linear arrays of urea, thiourea, and their derivatives

Computational studies using density functional theory are carried out on linear chains of urea, N,N'-dimethyl urea and N,N,N',N'-tetramethyl urea, and their sulfur analogs, viz., thiourea, N,N'-dimethyl thiourea and N,N,N',N'-tetramethyl thiourea with varying chain length, to understand the effect of hydrogen bonding and dipolar interactions on the optoelectronic response properties of such linear aggregates. While molecules of urea, N,N'-dimethyl urea, and the corresponding sulfur analogs, thiourea, N,N'-dimethyl thiourea, are stabilized in linear chains by hydrogen bonding, the molecules of N,N,N',N'-tetramethyl urea and N,N,N',N'-tetramethyl thiourea in the linear chains are stabilized by purely dipolar interactions. To understand the contributions of electrostatic and polarization effects on such intermolecular interactions, we study the effect of an external electric field on the intermolecular interactions in these systems. We find that the strength of hydrogen bonding increases while that of dipolar interactions decreases with increase in external field strength. We account for such findings by decomposing the interaction terms into charge-transfer and electrostatic interaction terms. The effects of these interactions on the linear and nonlinear optical properties together with transport properties such as carrier mobilities are estimated to understand their suitability for device applications

Dehydrogenation of Dimethylamine–Borane Catalyzed by Half-Sandwich Ir and Rh Complexes: Mechanism and the Role of Cp* Noninnocence

Half-sandwich Cp*Rh^III complexes (Cp* = η⁵-1,2,3,4,5-pentamethylcyclopentadienyl) supported by 2,2′-bipyridine or 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine catalyze dehydrogenation of dimethylamine–borane (Me₂NH·BH₃) to produce H₂ and dimethylamino–borane dimer (Me₂NBH₂)₂ with turnovers of 2200. The Ir^III analogues, on the other hand, display dramatically poorer catalytic activity. Mechanistic inferences drawn from stoichiometric reactions and DFT calculations suggest noninnocent involvement of the Cp* moiety as a proton shuttle

Facile Styrene Formation from Ethylene and a Phenylplatinum(II) Complex Leading to an Observable Platinum(II) Hydride

A new 2-(di-<i>tert</i>-butylphosphanyl)­benzenesulfonate-supported phenylplatinum­(II) complex instantaneously but reversibly binds ethylene at room temperature. Direct and rapid styrene formation at room temperature via insertion of the Pt^II-bound ethylene into the Pt^II–Ph fragment followed by β-hydride elimination results in the formation of a solution-stable Pt^II–H complex. The Pt^II–H fragment is resistant toward protonolysis by acetic acid. Oxidation of the Pt^II–H fragment by excess Cu^II(OTf)₂ leads to an inorganic Pt^II complex incapable of C–H activation

Dehydrogenation of Dimethylamine–Borane Catalyzed by Half-Sandwich Ir and Rh Complexes: Mechanism and the Role of Cp* Noninnocence

Half-sandwich Cp*Rh^III complexes (Cp* = η⁵-1,2,3,4,5-pentamethylcyclopentadienyl) supported by 2,2′-bipyridine or 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine catalyze dehydrogenation of dimethylamine–borane (Me₂NH·BH₃) to produce H₂ and dimethylamino–borane dimer (Me₂NBH₂)₂ with turnovers of 2200. The Ir^III analogues, on the other hand, display dramatically poorer catalytic activity. Mechanistic inferences drawn from stoichiometric reactions and DFT calculations suggest noninnocent involvement of the Cp* moiety as a proton shuttle

Exclusive Csp³–Csp³ vs Csp²–Csp³ Reductive Elimination from Pt^IV Governed by Ligand Constraints

Selective reductive elimination of ethane (Csp³-Csp³ RE) was observed following bromide abstraction and subsequent thermolysis of a Pt^IV complex bearing both Csp³- and Csp²-hybridized hydrocarbyl ligands. Through a comparative experimental and theoretical study with two other Pt^IV complexes featuring greater conformational flexibility of the ligand scaffold, we show that the rigidity of a meridionally coordinating ligand raises the barrier for Csp²-Csp³ RE, resulting in unprecedented reactivity