Solvent-Dependent Switch of Ligand Donor Ability and Catalytic Activity of Ruthenium(II) Complexes Containing Pyridinylidene Amide (PYA) N-Heterocyclic Carbene Hybrid Ligands

Abstract

Chelating ligands incorporating both N-[1-alkylpyridin-4(1H)-ylidene]amide (PYA) and N-heterocyclic carbene (NHC) donor sites were prepared and used for the synthesis of ruthenium(II) complexes. Cyclic voltammetry, NMR, and UV–vis spectroscopy of the complexes indicate a solvent-dependent contribution of the limiting resonance structures associated with the ligand in solution. The neutral pyridylidene imine structure is more pronounced in apolar solvents (CH2Cl2), while the mesoionic pyridinium amide form is predominant in polar solvents (MeOH, DMSO). The distinct electronic properties of these hybrid PYA-NHC ligands in different solvents have a direct influence on the catalytic activity of the ruthenium center, e.g., in the dehydrogenation of benzyl alcohol to benzaldehyde. The activity in different solvents qualitatively correlates with the solvent permittivity.

Adaptive N-Mesoionic Ligands Anchored to a Triazolylidene for Ruthenium-Mediated (De)Hydrogenation Catalysis

“…35 Under standard transfer hydrogenation conditions using iPrOH as dihydrogen source and solvent, complex 5 displayed good catalytic activity and reached 92% conversion of the starting material to the alcohol after 24 h (Table 1, entry 1; see also Figure S3). This result is a significant improvement when compared to previously reported complexes containing PYA imidazolylidene ligands, 20 probably due to the enhanced stability of the complex toward basic degradation. Indeed, no precipitate was observed at later stages of the reaction that might indicate decomposition of the complex, and in fact it was possible to recover complex 5 after catalytic runs, as confirmed by NMR spectroscopy.…”

“…28,29 In the pyridyl heterocycle, the C4− C5 (1.406(2) Å) and C4−C8 bonds (1.405(2) Å) are significantly longer than the bonds linking C6−C5 (1.376(2) Å) and C7−C8 (1.367(2) Å), similar to the corresponding bond lengths in related PYA imidazolylidene complexes. 20 This partial double-bond localization indicates a predominance of the neutral resonance structure (cf. Figure 1a left) in the solid state with less contribution from a delocalized aromatic system comprising a mesoionic ground state (cf.…”

Optimization of Synthetically Versatile Pyridylidene Amide Ligands for Efficient Iridium‐Catalyzed Water Oxidation

“…Cyclometalation was demonstrated for all the complexes by the loss of one proton signal in the aromatic region of the 1 HNMR spectrum.T he structures of complex 8b and 8c were unequivocally confirmed by single-crystal X-ray diffraction analysis ( Figure 3). [19,20,23] Electrochemical analysiso fc omplexes 8a-c by CV in CH 2 Cl 2 revealed af ully reversible oxidation at E 1/2 =+0.64 V( vs. SCE) for all complexes 8a-c,w hich is identical to the potentialm ea-Scheme1.Synthesis of IrCp*PYA-type complexes 8a-c with different alkyl chains. [19] The pyridyl C a ÀC b bonds are consistently shorter (average 1.35 )t han the C b ÀC g bonds (1.41 ), confirming substantial contributiono ft he neutrald iene-type resonance structure in the solid state, in agreement with previous studies (see Ta ble S1 in the Supporting Information).…”

Normal and Abnormal N-Heterocyclic Carbene Ligands

“…2.12). 97 This comparison suggests that the carbenic resonance form in 62 may not be very pronounced. Similarly, the ruthenium complexes 62 and 63 have been prepared containing a tridentate terpyridine-derived ligand set featuring either a 2,2 0 :6 0 ,4 00 connectivity pattern or a 2,2 0 :6 0 ,3 00 linkage ( Fig.…”