Now showing 1 - 4 of 4
- PublicationPyridine-derived N-heterocyclic carbenes: An experimental and theoretical evaluation of the bonding in and reactivity of selected normal and abnormal complexes of Ni(II) and Pd(II)We report a thorough investigation of a series of isomeric complexes with the general formula trans-(pyridylidene)M(PPh3)2Cl (M = Pd, Ni). For the first time, a systematic comparison of normal, abnormal and remote bonding modes is presented. X-ray structural and 13C NMR data indicate the importance of carbenoid mesomeric contributions in their compound class. The catalytic performance of the palladium complexes, trans-(pyridylidene)Pd(PPh3)2Cl, as precursors in Suzuki-Miyaura-type cross-coupling suggests a correlation of remote bonding with catalyst robustness and thus effectivity. When metal precursors M(PPh3)4 are reacted with 2,4-dichloro pyridinium salts, preferential formation of remote carbene complexes occurs and indications are that electronic parameters rather than steric influences are responsible for the observed selectivity. Calculations at the BP86/TZ2P level of theory support interpretation of the results.
214Scopus© Citations 60
- PublicationEfficient Electronic Communication of Two Ruthenium Centers through a Rigid Ditopic N-Heterocyclic Carbene LinkerA ditopic benzobis(carbene) ligand precursor was prepared that contained a chelating pyridyl moiety to ensure co-planarity of the carbene ligand and the coordination plane of a bound octahedral metal center. Bimetallic ruthenium complexes comprising this ditopic ligand [L4Ru-C,N-bbi-C,N-RuL4] were obtained by a transmetalation methodology (C,N-bbi-C,N=benzobis(N-pyridyl-N′-methyl-imidazolylidene). The two metal centers are electronically decoupled when the ruthenium is in a pseudotetrahedral geometry imparted by a cymene spectator ligand (L4=[(cym)Cl]). Ligand exchange of the Cl−/cymene ligands for two bipyridine or four MeCN ligands induced a change of the coordination geometry to octahedral. As a consequence, the ruthenium centers, separated through space by more than 10 Å, become electronically coupled, which is evidenced by two distinctly different metal-centered oxidation processes that are separated by 134 mV (L4=[(bpy)2]; bpy=2,2′-bipyridine) and 244 mV (L4=[(MeCN)4]), respectively. Hush analysis of the intervalence charge-transfer bands in the mixed-valent species indicates substantial valence delocalization in both complexes (delocalization parameter Γ=0.41 and 0.37 in the bpy and MeCN complexes, respectively). Spectroelectrochemical measurements further indicated that the mixed-valent RuII/RuIII species and the fully oxidized RuIII/RuIII complexes gradually decompose when bound to MeCN ligands, whereas the bpy spectators significantly enhance the stability. These results demonstrate the efficiency of carbenes and, in particular, of the bbi ligand scaffold for mediating electron transfer and for the fabrication of molecular redox switches. Moreover, the relevance of spectator ligands is emphasized for tailoring the degree of electronic communication through the benzobis(carbene) linker.
404Scopus© Citations 33
- PublicationA new, mild one-pot synthesis of iodinated heterocycles as suitable precursors for N-heterocyclic carbene complexesThe use of I2/AgOAc in dichloromethane constitutes a cheap, mild, and efficient method for the selective iodination of a variety of heterocycles. In a number of cases, this method provides superior yields than other literature methods and affords iodo-functionalized heterocycles that are suitable precursors for carbene complexes.
756Scopus© Citations 12
- PublicationThe Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active MaterialsThe application of N-heterocyclic carbene complexes as active sites in materials other than catalysis has been remarkably scarce. Inspired by the — often misleading — ‘carbene’ label, which implies a substantial degree of M=C π bonding, we have been interested in evaluating the potential of N-heterocylclic carbene complexes as building blocks for constructing electronically active materials. Electron mobility via the metal-carbon bond has been investigated in monometallic imidazol-2-ylidene complexes and subsequently expanded to polymetallic systems. In particular, ditopic benzobisimidazolium-derived ligands have been explored for the fabrication of bimetallic molecular switches and main-chain conjugated organometallic polymers. Electrochemical analyses have allowed for quantifying the degree of electronic coupling between the metal sites and for identifying the key parameters that govern the intermetallic communication.
636Scopus© Citations 34