Ruthenium(II) DMSO complexes with CˆC* cyclometalated phenylimidazol NHC ligands

Unravelling the kinetics of electro- and photochemical S → O linkage isomerization in Ru(II)-NHC-DMSO complexes utilised for photoinduced substitution reactions

Our recently reported Ru(III)-NHC complexes 1a and 1b were utilized as suitable precursors to prepare new Ru(II)-NHC-(DMSO)2 complexes 2a and 2b. Complexes 2a and 2b reacted with 2,2'-bipyridine to give complexes 3a and 3b, respectively, with substitution of only one of the DMSO ligands. All new complexes were characterized using various spectroscopic techniques and the molecular structures of 2a and 3a were determined using single-crystal X-ray diffraction technique. Complexes 2a, 2b, 3a, and 3b showed the S → O linkage isomerization of the DMSO ligand upon oxidation of the Ru centre from +II to +III, as confirmed by the thermodynamic and kinetic data obtained from cyclic voltammetry experiments. It was observed that in the bisdimethylsulfoxide complexes 2a and 2b, only one DMSO ligand isomerized, which was further corroborated by the computational studies performed to optimize the geometry of the possible linkage isomers of complexes 2a and 3a in +2 and +3 oxidation states, whereas complexes 3a and 3b showed a high preference for the O-bound isomer in the Ru(III) redox state. The role of NHC in stabilizing the mixed isomer in complexes 2a and 2b and preventing the isomerization of both DMSO ligands coordinated to the Ru centre was studied; moreover, NHC provided good solvent compatibility for photochemical S → O isomerization in all the complexes. Taking advantages of the photoinduced linkage isomerization in 2a and 2b, the synthesis of 3a and 3b was revisited and performed using 2a and 2b, respectively, following a photoinduced substitution reaction in the presence of 2,2'-bipyridine. The kinetics of the reversion from the O-bound to S-bound isomer was found to follow the DMSO-assisted intermolecular S → O isomerization pathway.

Recent Advances in the Syntheses and Catalytic Applications of Homonuclear Ru-, Rh-, and Ir-Complexes of CNHC ^C Cyclometalated Ligands

In the past few decades, chemistry of cyclometalated species has gained momentum with increased applications in several areas of scientific developments. Cyclometalation reactions result in the formation of stable metallacycles through the generation of metal-carbon covalent bonds by activating the unreactive Csp² -H or Csp³ -H bonds. The extra stability gained by the formation of metallacycles enhances their applicability scopes especially in the area of homogeneous catalysis. In the recent research development in this area, NHC ligands (strong σ-donor and generally, weak π-acceptor) have been found to be one of the most suitable candidates for the intramolecular C-H activation process which leads to the cyclometalated species. The growth in the area of cyclometalation chemistry that started in the late 20^th century is still continuing and in the past few decades, various examples of NHC derived transition metal-based cyclometalated complexes came into the picture. As covering all the reported literatures in this area (includes mainly late transition metals) will exceed the limits of minireview, we restricted ourselves to the recent (2015 - May 2021) examples of the most common Ru-, Rh-, and Ir-based C_NHC ^C cyclometalated complexes and their applications in various homogeneous catalytic conversions such as transfer hydrogenation, amidation, oxidation of alcohols, annulations, and so forth.

Complementing Pyridine-2,6-bis(oxazoline) with Cyclometalated N-Heterocyclic Carbene for Asymmetric Ruthenium Catalysis

A strategy for expanding the utility of chiral pyridine-2,6-bis(oxazoline) (pybox) ligands for asymmetric transition metal catalysis is introduced by adding a bidentate ligand to modulate the electronic properties and asymmetric induction. Specifically, a ruthenium(II) pybox fragment is combined with a cyclometalated N-heterocyclic carbene (NHC) ligand to generate catalysts for enantioselective transition metal nitrenoid chemistry, including ring contraction to chiral 2H-azirines (up to 97 % ee with 2000 TON) and enantioselective C(sp3 )-H aminations (up to 97 % ee with 50 TON).

Hexene hydrogenation catalysed by the complex monohydrid complexes: A DFT study of associated vs dissociated pathways

A DFT study was conducted to elucidate the mechanism of hexene hydrogenation catalysed by a series of ruthenium (II) monohydride catalysts: RuH(CO)(Cl)(LL') where L and L' represent C(cyclohexyl), Me (methyl) and IMe (N, N '-bis (mesityl) imidazole-2-ylidene). This investigation explores the feasibility of two different proposed mechanisms: the first describes the dissociated pathway and exploits a single phosphine complex. The second is the associative one and uses a two phosphines complex. The detailed pathways have been explored for the catalyst model with L = L' = Me. Three possibilities have been supported for the dissociative route. Pathway (A) begins with a phosphine release. The initial addition of hexane or a dihydrogen molecule on the ruthenium catalyst generates the pathways (B) and (C), respectively. Pathways (B) and (C) merge with the pathway (A) before and after the first proton transfer, respectively. Activation energies in the first hydrogen migration (the key-step of the mechanism) are close. Therefore, both mechanisms (A) and (B) are possible but the former is more probable. The substitution of the catalyst model RuHCl(CO)(PMe₃)₂ by the real catalysts RuHCl(CO)(PCy₃)₂ or RuHCl(CO)(IMes)(PMe₃) shows no significant influence on the energetic barriers of hexene hydrogenation mechanism. The energy profile of the first hydrogen migration for the catalyst RuHCl(CO)(PCy₃)₂ is characteristic of a concerted asynchronous mechanism while our calculation led to two separated synchronous steps when the model catalyst is used. The associative pathway (D) integrates the two experimentally detected intermediates and generates activation energies close to those of dissociative pathways (A) and (B). The rationale to explain the experimentally detected species is achieved by considering the four proposed mechanisms where they occur simultaneously and with different rates (ie. The dissociative mechanism has the highest rate).

Synthesis, characterization and cytotoxic activity of cationic half-sandwich Ru(ii) complexes stabilized by iminophosphorane N,N,S and N,N,Se tridentate ligands

New Ru(ii) half-sandwich complexes stabilized by tridentate iminophosphorane N,N,S or N,N,Se ligands show potent cytotoxic activity with high selectivity.