Theoretical Prediction of the Electronic Properties of Bidentate Ligands (HEP2) and Synthesis of Bis(<i>N</i>-heterocyclic carbene) Silver and Palladium Complexes

A theoretical approach based on density functional theory (DFT) was developed to determine the electronic Huynh parameter for bidentate ligands (HEP2). The ¹³C chemical shift of the carbene carbon atom in the 1,3-diisopropylbenzimidazolin-2-ylidene ligand (ⁱPr₂-bimy) was calculated within the model complexes [PdBr(L₂)(ⁱPr₂-bimy)]⁺, where L₂ represents a bidentate ligand. Strong correlations were observed between the calculated and experimental chemical shifts for both <i>N</i>-donor and <i>C</i>-donor L₂ ligands, with <i>R</i>² values of 0.9699 and 0.9926, respectively. This DFT approach demonstrates the potential for estimating the HEP2 parameter of bidentate ligands of these types. This method was applied to estimate the HEP2 values for three bis(<i>N</i>-heterocyclic carbene) ligands containing carboxylate groups, (diNHC^R)^2-, which are explored as ligands in this study. These NHC ligands were derived from three precursor compounds: 1-(carboxymethyl)-3-((3-(carboxymethyl)-1<i>H</i>-imidazol-3-ium-1-yl)methyl)-1<i>H</i>-imidazol-3-ium bromide (<b>1a</b>), 1-(1-carboxyethyl)-3-((3-(1-carboxyethyl)-1<i>H</i>-imidazol-3-ium-1-yl) methyl)-1<i>H</i>-imidazol-3-ium bromide (<b>1b</b>), and 2-(1-(2-(1-(carboxymethyl)-1<i>H</i>-imidazol-3-ium-3-yl)ethyl)-1<i>H</i>-imidazol-3-ium-3-yl)acetate bromide (<b>1c</b>). They were synthesized and characterized using IR, NMR, and single-crystal X-ray crystallography (for <b>1a</b> and <b>1c</b>). The interaction of <b>1a</b> and <b>1c</b> with Ag₂O in the presence of aqueous sodium hydroxide produced the complexes [Na₂(H₂O)₈][Ag₂(di_CH2NHC^H)₂] (<b>2a</b>) and [Na₂(H₂O)₈][Ag₂(di_C2H4NHC^H)₂] (<b>2c</b>), respectively. Complex <b>2a</b> was structurally characterized, revealing a binuclear structure in which the bis-carbene ligand adopts a bridging μ-κ¹(C),κ¹(C') coordination. Furthermore, <b>2a</b> exhibited reactivity in transmetalation, allowing the synthesis of complex [Na₂(H₂O)₁₀][Pd(di_CH2NHC^H)₂] (<b>3a</b>) through a reaction with palladium acetate. Complex <b>3a</b>, which was also structurally characterized, consists of a mononuclear tetracarbene species where each bis-carbene ligand coordinates in a bidentate κ²(C,C') fashion.