Revital Cohen

Metal–ligand cooperation in the trans addition of dihydrogen to a pincer Ir(i) complex: A DFT study

DFT calculations on the hydrogenation of a (PNP)Ir(I) complex, to give the trans--rather then the cis--dihydride isomer, show that the reaction proceeds via a deprotonation/protonation of the ligand arm with concomitant dearomatization/aromatization of the pyridine core. Thus, the actual H(2) activation step occurs by an Ir(III) complex and not by the Ir(I) starting complex, as supported by experimental observations. This ligand participation allows for products that would otherwise be inaccessible. In addition, trace amounts of water, which are likely to be present in the solvent, facilitate proton transfer reaction steps.

Stable aromatic dianion in water.

Perylene diimide (PDI) bearing polyethylene glycol substituents at the imide positions was reduced in water with sodium dithionite to produce an aromatic dianion. The latter is stable for months in deoxygenated aqueous solutions, in contrast to all known aromatic dianions which readily react with water. Such stability is due to extensive electron delocalization and the aromatic character of the dianion, as evidenced by spectroscopic and theoretical studies. The dianion reacts with oxygen to restore the parent neutral compound, which can be reduced again in an inert atmosphere with sodium dithionite to give the dianion. Such reversible charging renders PDIs useful for controlled electron storage and release in aqueous media. Simple preparation of the dianion, reversible charging, high photoredox power, and stability in water can lead to development of new photofunctional and electron transfer systems in the aqueous phase.

Single molecule electron transport junctions: charging and geometric effects on conductance.

A p-benzenedithiolate (BDT) molecule covalently bonded between two gold electrodes has become one of the model systems utilized for investigating molecular transport junctions. The plethora of papers published on the BDT system has led to varying conclusions with respect to both the mechanism and the magnitude of transport. Conductance variations have been attributed to difficulty in calculating charge transfer to the molecule, inability to locate the Fermi energy accurately, geometric dispersion, and stochastic switching. Here we compare results obtained using two transport codes, TRANSIESTA-C and HUCKEL-IV, to show that upon Au-S bond lengthening, the calculated low bias conductance initially increases by up to a factor of 30. This increase in highest occupied molecular orbital (HOMO) mediated conductance is attributed to charging of the terminal sulfur atom and a corresponding decrease in the energy gap between the Fermi level and the HOMO. Addition of a single Au atom to each terminal of the extended BDT molecule is shown to add four molecular states near the Fermi energy, which may explain the varying results reported in the literature.

Effect of CO on the oxidative addition of arene C-H bonds by cationic rhodium complexes.

Sequential addition of CO molecules to cationic aryl-hydrido Rh(III) complexes of phosphine-based (PCP) pincer ligands was found to lead first to C-H reductive elimination and then to C-H oxidative addition, thereby demonstrating a dual role of CO. DFT calculations indicate that the oxidative addition reaction is directly promoted by CO, in contrast to the commonly accepted view that CO hinders such reactions. This intriguing effect was traced to repulsive pi interactions along the aryl-Rh-CO axis, which are augmented by the initially added CO ligand (due to antibonding interactions between occupied Rh d(pi) orbitals and occupied pi orbitals of both CO and the arene moiety), but counteracted by the second CO ligand (due to significant pi back-donation). These repulsive interactions were themselves linked to significant weakening of the pi-acceptor character of CO in the positively charged rhodium complexes, which is concurrent with an enhanced sigma-donating capability. Replacement of the phosphine ligands by an analogous phosphinite-based (POCOP) pincer ligand led to significant changes in reactivity, whereby addition of CO did not result in C-H reductive elimination, but yielded relatively stable mono- and dicarbonyl aryl-hydrido POCOP-Rh(III) complexes. DFT calculations showed that the stability of these complexes arises from the higher electrophilicity of the POCOP ligand, relative to PCP, which leads to partial reduction of the excessive pi-electron density along the aryl-Rh-CO axis. Finally, comparison between the effects of CO and acetonitrile on C-H oxidative addition revealed that they exhibit similar reactivity, despite their markedly different electronic properties. However, DFT calculations indicate that the two ligands operate by different mechanisms.

On the Unexpected Stability of the Dianion of Perylene Diimide in Water-A Computational Study

It was recently reported (Shirman, J. Phys. Chem. B, 2008, 112, 8855) that the stable dianion of perylene diimide can be prepared in water. Herein, a computational study (using DFT at the M06-2X/6-31++G** level of theory) of this species is presented. It is shown that this dianion is aromatic and that its reaction with water is highly endergonic. The primary cause for this is the stabilization provided by the enhanced aromaticity of the dianion relative to its neutral counterpart. Comparison with other aromatic dianions is also presented.

A Pincer‐Type Anionic Platinum(0) Complex

Pincer-type complexes constitute a large family of compounds that have attracted much recent interest. Among these compounds, aryl-anchored, d pincer complexes of the type [M(LCL’)] (M=Ni, Pd, Pt; L= neutral ligand such as phosphine, amine, dialkyl sulfide) are a major group that plays important roles in organometallic reactions and mechanisms, catalysis, and in the design of new materials. In contrast, and to our knowledge, no complexes of this type with the metal in the zero oxidation state have been prepared. Such d [M(LCL’)] complexes with neutral L ligands and an “anionic” aryl anchor would be anionic, and would be expected to possess distinctly different properties to neutral d (M) complexes. We chose to utilize bulky bis-chelating pincer-type ligands in this study as they have been shown to be effective in stabilizing reactive species and have led to unusual complexes. We have recently shown that reduction of PCP-type Pd complexes [Pd(X)C6H3(CH2PiPr2)2] (X=Cl, trifluoroacetate) with sodium metal results in collapse of the pincer system, leading to formation of the diamagnetic binuclear complex [Pd{C6H3(CH2PiPr2)2}2Pd], which contains a 14electron linear Pd moiety and a completely nonplanar “butterfly”-type 16-electron Pd moiety. Oxidation of the binuclear complex, or its reaction with organic halides, regenerates the original mononuclear framework. To avoid collapse of the pincer system upon reduction we decided to use a PCP-Pt complex with the hope that the more diffuse nature of the Pt orbitals (compared with Pd) might stabilize the reduced metal center. In addition, increasing the steric bulk of the pincer phosphine ligand might protect the reduced metal center against intermolecular reactions and might lead to a rare monometallic Pt anionic complex. We are aware of only one monometallic anionic Pt complex, namely [Pt(Me2NCS2)(PEt3)] , which was generated in situ at 78 8C by proton abstraction from the Pt hydride complex [PtH(Me2NCS2)(PEt3)]. [4] This complex was not isolated but was trapped with a variety of electrophiles to give a range of Pt complexes. Herein we report the preparation, characterization, and computational study of the first thermally stable, monometallic anionic Pt complex. The reactivity of this electron-rich, 16-electron PCP-type anionic Pt complex shows that it is a Brønsted base and an effective electron-transfer reagent that is capable of C F activation under exceedingly mild conditions. Reduction of the bulky PCP-Pt complex 1 (Scheme 1) with sodium in dry [D8]THF at room temperature overnight led to a dramatic color change from colorless to dark red. A

The Impact of Weak C-H···Rh Interactions on the Structure and Reactivity of trans-[Rh(CO)2(phosphine)2]+ : An Experimental and Theoretical Examination

The crystal structure of the new cationic Rh(I) complex trans-[Rh(CO)(2)(L)(2)]BF(4) (L=alpha(2)-(diisopropylphosphino)isodurene) was found to exhibit a nonlinear OC-Rh-CO fragment and weak intramolecular C-H...Rh interactions. These interactions, which have also been shown to occur in solution, have been examined by density functional theory calculations and found to be inextricably linked to the presence of the distorted OC-Rh-CO fragment. This linkage has also been demonstrated by comparison with a highly similar Rh(I) complex, in which these C-H...Rh interactions are absent. Furthermore, the presence of these weak interactions has been shown to have a significant effect on the reactivity of the metal center.