Dinitrogen Splitting and Functionalization in the Coordination Sphere of Rhenium

[ReCl₃(PPh₃)₂(NCMe)] reacts with pincer ligand HN­(CH₂CH₂P<i>t</i>Bu₂)₂ (<i>H</i>PNP) to five coordinate rhenium­(III) complex [ReCl₂(PNP)]. This compound cleaves N₂ upon reduction to give rhenium­(V) nitride [Re­(N)­Cl­(PNP)], as the first example in the coordination sphere of Re. Functionalization of the nitride ligand derived from N₂ is demonstrated by selective C–N bond formation with MeOTf

Four- and Five-Coordinate Osmium(IV) Nitrides and Imides: Circumventing the “Nitrido Wall”

Osmium nitride chemistry is dominated by osmium­(VI) in octahedral or square-pyramidal coordination. The stability of the d² configuration and preference of the strong σ- and π-donor nitride for apical coordination is in line with the Gray–Ballhausen bonding model. In contrast, low-valent osmium­(IV) or other d⁴ nitrides are rare and have only been reported with lower coordination numbers (CN ≤ 4), thereby avoiding π-bonding conflicts of the nitride ligand with the electron-rich metal center. We here report the synthesis of the square-planar osmium­(IV) nitride [Os^IVN­(PNP)] (PNP = N­(CHCHP<i>t</i>Bu₂)₂). From there, a square-pyramidal isonitrile adduct could be isolated, which surprisingly features basal nitride coordination. Analysis of this five-coordinate d⁴ nitride shows an unusual binding mode of the isonitrile ligand, which explains the preference of the weakest σ-donor and strongest π-acceptor isonitrile for apical coordination

Rh-Mediated Carbene Polymerization: from Multistep Catalyst Activation to Alcohol-Mediated Chain-Transfer

Rh-mediated polymerization of carbenes gives access to new highly substituted and stereoregular polymers. While this reaction is of interest for the synthesis of syndiotactic polymers that are functionalized at every carbon atom of the polymer backbone, the catalyst activation, chain-initiation, and chain-termination processes were so far poorly understood. In this publication we present new information about these processes on the basis of detailed end-group analyses, dilution-kinetic studies, and a comparison of the activity of well-defined catalysts containing a preformed Rh–C bond. All data point toward complex catalyst activation processes under the applied reaction conditions. The use of well-defined Rh^I(cod)-alkyl, aryl, and allyl complexes does <i>not</i> lead to better initiation efficiencies or higher polymer yields. MALDI-ToF MS of the oligomeric fractions indicates that during the incubation time of the reaction, the precatalysts are first transformed into oligomer forming species with a suppressed tendency toward β-hydrogen elimination, and accordingly a shift to saturated oligomeric chains that are terminated by protonolysis. Further catalyst modifications lead to a shift from atactic oligomerization to stereoregular high molecular weight polymerization activity. Dilution-kinetic studies reveal that under diluted conditions two different active species operate that differ largely in their chain-termination behavior. Analysis of the reaction products by MALDI-ToF MS also allows conclusions about chain-initiation and chain-termination. Chain-initiation can occur by insertion of a preformed carbene into a Rh-ligand or Rh-hydride bond or by (internal or external) nucleophilic attack of water and/or alcohol on a Rh-carbene moiety. Chain-termination takes place mainly by (nucleophilic) protonolysis involving water or alcohols, while β-H elimination plays only a minor role and is only observed for the shorter oligomers. The detection of ethoxy and hydroxyl end-groups demonstrates the importance of trace amounts of water and ethanol toward chain-initiation. Alcohols further function as a chain-transfer agent, and increasing the alcohol concentration accelerates the chain-transfer process (which remains however relatively slow compared to chain-propagation). On the basis of the chemical properties of the alcohols, we propose a chain-transfer mechanism involving nucleophilic attack of the alcohol (nucleophilic, σ-bond metathesis type, protonolysis). This further allows us to draw some (careful) new conclusions about the oxidation state of the actual polymerization species

Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex

A comprehensive mechanistic study of N₂ activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl₂(PNP)] is presented (PNP^– = N­(CH₂CH₂P<i>t</i>Bu₂)₂^–). Low-temperature studies using chemical reductants enabled full characterization of the N₂-bridged intermediate [{(PNP)­ClRe}₂(N₂)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re­(N)­Cl­(PNP)]. This first example of molecular electrochemical N₂ splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N₂ activation to form the N₂-bridged intermediate. CV data was acquired under Ar and N₂, and with varying chloride concentration, rhenium concentration, and N₂ pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N₂ activation that proceeds via initial reduction to Re^II, N₂ binding, chloride dissociation, and further reduction to Re^I before formation of the N₂-bridged, dinuclear intermediate by comproportionation with the Re^III precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N₂ splitting in the tetragonal frameworks enforced by rigid pincer ligands

Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex

A comprehensive mechanistic study of N₂ activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl₂(PNP)] is presented (PNP^– = N­(CH₂CH₂P<i>t</i>Bu₂)₂^–). Low-temperature studies using chemical reductants enabled full characterization of the N₂-bridged intermediate [{(PNP)­ClRe}₂(N₂)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re­(N)­Cl­(PNP)]. This first example of molecular electrochemical N₂ splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N₂ activation to form the N₂-bridged intermediate. CV data was acquired under Ar and N₂, and with varying chloride concentration, rhenium concentration, and N₂ pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N₂ activation that proceeds via initial reduction to Re^II, N₂ binding, chloride dissociation, and further reduction to Re^I before formation of the N₂-bridged, dinuclear intermediate by comproportionation with the Re^III precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N₂ splitting in the tetragonal frameworks enforced by rigid pincer ligands

Amplified Vibrational Circular Dichroism as a Probe of Local Biomolecular Structure

We show that the VCD signal intensities of amino acids and oligopeptides can be enhanced by up to 2 orders of magnitude by coupling them to a paramagnetic metal ion. If the redox state of the metal ion is changed from paramagnetic to diamagnetic the VCD amplification vanishes completely. From this observation and from complementary quantum-chemical calculations we conclude that the observed VCD amplification finds its origin in vibronic coupling with low-lying electronic states. We find that the enhancement factor is strongly mode dependent and that it is determined by the distance between the oscillator and the paramagnetic metal ion. This localized character of the VCD amplification provides a unique tool to specifically probe the local structure surrounding a paramagnetic ion and to zoom in on such local structure within larger biomolecular systems