Electron-Deficient Imidazolium Substituted Cp Ligands and their Ru Complexes

Ruthenium complexes bearing terpyridyl ligands of distinct donor-acceptor configuration for solar energy conversion

A novel series of heteroleptic ruthenium complexes employing a tbutyl-functionalized terpyridine (tpy) as a donor and tpy functionalized with up to three carboxylic acid groups as acceptors are prepared. Structural tailoring of these complexes helped not only to tune the photo-excited-state lifetimes but also aided in accumulating electron density through the anchoring groups. Wider separation between the triplet metal-to-ligand charge transfer (3MLCT) state and the metal-centered states is achieved while maintaining a reasonable barrier between the S0 ground and the 3MLCT states. This electronic state alignment facilitated the electron injection into TiO2 photoanodes from the excited states of complexes. The structural configuration comprising a tbutyl-tpy donor and three carboxylate-functionalized tpy acceptors around the ruthenium center ensured faster interfacial electron transfer (IET) and charge dissipation into the TiO2 clusters to longer diffusion lengths. This work presents a facile synthetic route to access these push-pull complexes and evaluates their use as molecular photosensitizers via a combined experimental and computational approach. Quantum dynamics simulation results demonstrate the extent of vectorial electron transfer from these complexes into the TiO2 cluster.

Carbene-Functionalized Bulky-Cyclopentadiene Rings

A series of significantly bulky mono- and di-substituted cyclic alkyl-amino carbene (cAAC)- functionalized cyclopentadiene ring (Cp) compounds were synthesized. The functionalization of the Cp ring with cAAC ligands makes them significantly bulkier, while retaining their ligation properties. These compounds display interesting fluorescence properties. In these compounds, intra-molecular charge transfer is observed from electron-rich carbene to electron-deficient cyclopentadiene unit. These high yielding compounds have been characterized by X-ray single-crystal diffraction and their emission properties have been studied. Rotational conformers (via C-C bond rotation) play a pivotal role with possible different extent of intramolecular charge-transfer (ICT) from carbene to cyclopentadiene ring. Variable temperature-dependent NMR studies were performed along with NOSY, COSY and different 2D NMR techniques to estimate the energy barriers and 1,5-Hydrogen shift.

Revisiting Hafner's Azapentalenes: The Chemistry of 1,3-Bis(dimethylamino)-2-azapentalene

Stable azaheterocyclic derivatives of pentalene have been reported by the group of Hafner in the 1970s. However, these structures remained of low interest until recently, when they started to be investigated in the context of organic light-emitting diodes' (OLEDs') development. Herein, we revisit the synthesis of stable azapentalene derivative 1,3-bis(dimethylamino)-2-azapentalene and further explore its properties both computationally and experimentally. Beyond the reproduction and optimization of some previously reported transformations, such as formylation and amine substitution, the available scope of reactions was expanded with azo-coupling, selective halogenations, and cross-coupling reactions.

IZCp and PZCp: Redox Non-innocent Cyclopentadienyl Ligands as Electron Reservoirs for Sandwich Complexes

A long-sustained effort of systematic steric and electronic modification of cyclopentadienyl (Cp) ligands has enabled them to find wide-ranging, valuable applications. Herein, we present two novel Cp ligands: imidazolium- and pyrrolinium-substituted zwitterionic Cps (IZCp and PZCp), whose key utility is redox non-innocence─the ability to participate cooperatively with the metal center in redox reactions. Through the simple metalation of ZCps, the Cr(0) and Mo(0) half-sandwich complexes (IZCp)Cr(CO)3, (PZCp)Cr(CO)3, (IZCp)Mo(CO)3, and (PZCp)Mo(CO)3, respectively, as well as the Ru(II) sandwich complexes [(IZCp)RuCp]PF6 and [(PZCp)RuCp]PF6 were prepared. The sandwich complexes were fully characterized and showed by cyclic voltammetry reversible one-electron reduction at E1/2 potentials ranging from -1.7 to -2.7 V vs Fc/Fc+. These values are unusually low and have not been observed with other Cp ligands due to the instability of the reduced complexes. Density functional theory (DFT) calculations for the reduced sandwich derivatives with IZCp and PZCp showed their spin densities to be highly delocalized over their ZCp ligand moieties (70-90%). Electron paramagnetic resonance (EPR) analysis of the isolated K[(PZCp)Mo(CO)3] and (PZCp)RuCp also indicated a high degree of ligand-localized radical character. Thus, the IZCp and PZCp ligands act as electron reservoirs to sustain these sandwich complexes in highly reduced states. At the same time, the CO stretching frequencies of K[(PZCp)Mo(CO)3]: νCO 1871, 1748, and 1699 cm-1, rank the [PZCp]- ligand as the strongest electron-donating Cp ligand among the reported CpMo(CO)3 derivatives, whose νCO > 1746 cm-1. In addition, these redox non-innocent Cps were obtained in high yields and found to be practically air- and moisture-stable, unlike typical Cps.

Phosphonium-substituted Diphosphaindenylide (PPI): Exploration of Biradical Character and Ligand Properties

Starting from C6 H4 (PCl2 )2 and the TMS-substituted ylide (TMS)2 C=PR3 (TMS=trimethylsilyl, R=p-tolyl), the phosphonium-substituted diphosphaindenylide PPI was prepared in two steps. CASSCF calculations as well as the reactivity toward diphenyl acetylene suggest a notable biradical character in PPI. Reaction with [Cr(CO)3 (MeCN)3 ] affords the complex [Cr(CO)3 (η5 -PPI)] (5). This complex was employed to explore the ligand properties of PPI, which demonstrates considerable potential through the combination of strong metal-ligand interactions and the possibility of a pronounced indenyl effect.

Energy Decomposition Analysis Coupled with Natural Orbitals for Chemical Valence and Nucleus-Independent Chemical Shift Analysis of Bonding, Stability, and Aromaticity of Functionalized Fulvenes: A Bonding Insight

The Donor base ligand-stabilized cyclopentadienyl-carbene compounds L-C5H4 (L = H2C, aAAC; (CO2Me)2C, Py; aNHC, NHC, PPh3; SNHC; aAAC = acyclic alkyl(amino) carbene, aNHC = acyclic N-hetero cyclic carbene, NHC = cyclic N-hetero cyclic carbene, SNHC = saturated N-hetero cyclic carbene, Py = pyridine) (1a-1d, 2a-2c, 3) have been theoretically investigated by energy decomposition analysis coupled with natural orbitals for chemical valence calculation. Among all these compounds, aNHC=C5H4 (2a) and Ph3P=C5H4 (2c) had been reported five decades ago. The bonding analysis of compounds with the general formula L=C5H4 (1a-1d) [L = (H2C, aAAC, (CO2Me)2C, Py] showed that they possess one electron-sharing σ bond and electron-sharing π bond between L and C5H4 neutral fragments in their triplet states as expected. Interestingly, the bonding scenarios have completely changed for L = aNHC, NHC, PPh3, SNHC. The aNHC analogue (2a) prefers to form one electron-sharing σ bond (CL-CC5H4) and dative π bond (CL ← CC5H4) between cationic (aNHC)+ and anionic C5H4 - fragments in their doublet states. Similar bonding scenarios have been observed for NHC (2b) and PPh3 (2c) (PL-CC5H4, PL ← CC5H4) analogues. In contrast, the SNHC and C5H4 neutral fragments of SNHC=C5H4 (3) prefer to form a dative σ bond (CSNHC → CC5H4) and a dative π bond (CSNHC ← CC5H4) in their singlet states. The pyridine analogue 1d is quite different from 2c from the bonding and aromaticity point of view. The nucleus-independent chemical shifts of all the abovementioned species (1-3) corresponding to aromaticity have been computed using the gauge-independent atomic orbital approach.

Electron-Deficient Imidazolium Substituted Cp Ligands and their Ru Complexes

The synthesis of electron-poor mono-, di- and tri(imidazolium)-substituted Cp-ylides is presented and their electronic properties are discussed based on NMR spectroscopy, X-ray structure analyses, electrochemical investigations and DFT calculations as well as by their reactivity toward [Ru(CH3 CN)3 Cp*](PF6 ). With mono- and di(imidazolium)-substituted cyclopentadienides the respective monocationic and dicationic ruthenocences are formed (X-ray), whereas tri(imidazolium) cyclopentadienides are too electron-poor to form the ruthenocenes. Cyclic voltammetric analysis of the ruthenocenes shows reversible oxidation at a potential that increases with every additional electron-withdrawing imidazolium substituent at the Cp ligand by 0.53-0.55 V in an electrolyte based on a weakly coordinating anion. A reversible oxidation can be observed for the free 1,3-disubstituted ligand as well.