Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Abstract

SummaryRh(CO)2Cl(NHC) complexes of dipyrido-annelated N-heterocyclic carbenes were prepared. From the C–H coupling constant of the respective imidazolium salts and the N–C–N angle of the N-heterocyclic carbene (NHC), a weaker σ-donor character than that of typical unsaturated NHCs is expected. However, the IR stretching frequencies of their Rh(CO)2Cl complexes suggest an electron-donor character even stronger than that of saturated NHCs. We ascribe this to the extremely weak π-acceptor character of the dipyrido-annelated NHCs caused by the conjugated 14 πe− system that thus allows for an enhanced Rh–CO backbonding. This extremely low π-acceptor ability is also corroborated by the 77Se NMR chemical shift of −55.8 ppm for the respective selenourea, the lowest value ever measured for imidazole derived selenoureas. DFT-calculations of the free carbene confirm the low σ-donor character by the fact that the σ-orbital of the carbene is the HOMO−1 that lies 0.58 eV below the HOMO which is located at the π-system. Natural population analysis reveals the lowest occupation of the pπ-orbital for the saturated carbene carbon atom and the highest for the pyrido-annelated carbene. Going from the free carbene to the Rh(CO)2Cl(NHC) complexes, the increase in occupancy of the complete π-system of the carbene ligand upon coordination is lowest for the pyrido-annelated carbene and highest for the saturated carbene.

A Redox‐Active Heterobimetallic N‐Heterocyclic Carbene Based on a Bis(imino)pyrazine Ligand Scaffold

“…In the case of the free carbene 2, the HOMO and the LUMO are both energetically and morphologically similar to the ones calculated for (P Pz DI)Fe-(CO) 2 ,w ith an arrow HOMO-LUMO separation (2.60 eV), most likely due to the extended p-conjugation over the pyrazine ring and imine arms.Interestingly,the carbene based E s and E p* are the HOMOÀ1(stabilised by 0.68 eV compared to the HOMO) and LUMO + 1( destabilised by 0.50 eV compared to the LUMO). This distribution of molecular orbitals is similar to the one observed for the bisoxazolinebased NHC IBioxMe 4 , [34] dipyrido-annelated NHCs (dipiy), [35] and the recently reported 1,3-di(amino)oxyallyl pyrimidine-based NHC, [36] where,i ns ome cases,t he E s (HOMOÀ1) is stabilised by ca. 0.50 eV compared to the HOMO.I nt he case of Siemelingsn eopentyl-substituted diaminocarbene [3]ferrocenophane,t he carbene E s is in the HOMO (a typical situation for the vast majority of NHCs), whereas the carbene E p* is the LUMO + 2, while the LUMO and the LUMO + 1a re ferrocene-based (see the supporting information for afull MO diagram).…”

A Theoretical Study on the Stability of PtL2 Complexes of Endohedral Fullerenes: The Influence of Encapsulated Ions, Cage Sizes, and Ligands

“…For fullerene isomers shown in Scheme 1, their symmetries and numbering schemes are in accordance with an approved classification. 41 It has to be noted that Hückel molecular orbital calculations indicate the 6:6 ring junctions at the poles of the molecules, which usually have the highest π bond orders (B) and then presumably should be the highest reactive sites. In other words, these sites having the highest π bond orders should be easily attacked (Scheme 1).…”

Synthesis, Antibacterial, Thrombolytic and Cytotoxic Evaluation of Substituted and Un-substituted Selenium-n-heterocyclic Carbene Adducts

“…Benzimidazolium based salts and their respective selenium adducts (5)(6)(7)(8)(9)(10)(11)(12) were investigated according to 10993-4 ISO standards by evaluating hemolytic potentials after exposing to blood of normal mouse using previously developed method with minor modi cation [21]. Blood samples of different concentrations were centrifuge for 5 min at 5000 rpm and were added in erythrocyte suspension which is prepared by adding PBS and incubate it for 30 minutes and after that centrifuged it for 5 min at 13000 rpm, at 540nm free hemoglobin from supernatant was investigated using spectrophotometer.…”

“…The synthesized compounds (1-8) were preliminary characterized by FTIR, some distinct spectral changes were observed prior and after the addition of selenium to benzimidazolium salt which could be the indication of successful synthesis. Only representative data of preligand (1,4), benzimidazolium salts (5, 6) And their corresponding selenium adducts (9,10) were shown in Figure S2 NMR spectra ( 1 H & 13 C) of compounds (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) were recorded in deuterated solvents (chloroform and DMSO) depending upon their organic and inorganic nature of solubilities. The 1 H NMR salts spectras (5-8) displayed that the acidic proton peak (NCHN) at 9-10 δ ppm vanished in selenium adducts (5-8) due to its replacement in adducts with selenium.…”