Oliver Schuster

Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared,7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

Efficient Electronic Communication of Two Ruthenium Centers through a Rigid Ditopic N-Heterocyclic Carbene Linker

A 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 Ru(II)/Ru(III) species and the fully oxidized Ru(III)/Ru(III) 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.

Preparation of tris(azolyl)phosphine gold(I) complexes: digold(I) coordination and variation in solid state intermolecular interactions

Various tris(azolyl)phosphines R3P (R = 1-methylimidazol-2-yl, thiazol-2-yl, 4-methylthiazol-2-yl or 4,5-dimethylthiazol-2-yl) (1a–d) were utilised to prepare complexes of the type R3PAuCl (2a–d). The donor strength of the nitrogen atoms was assessed with natural-abundance 15N{1H} NMR of 1a–c and 2a–c. The chloride of 2c could be successfully substituted by the anions BzS– and NCS–. Further utilisation of the imine nitrogens of the tris(azole)phosphines to coordinate additional AuI centres was successful only for 2a where treatment with 3 mole equivalents of C6F5Au(tht) (tht = tetrahydrothiophene) afforded bis(pentafluorophenyl)-µ-[tris(1-methylimidazol-2-yl)phosphine-κ2P,N]digold(I) (4). A hydrolysis product consisting of two bis(1-methylimidazol-2-yl)phosphinite ligands bridging a Au24+ centre and further coordination to two AuC6F5 moieties (5) was formed during this reaction. The crystal and molecular structures were determined of compounds 1d, 2a–d and 3b. Intriguingly, 2b and 2c crystallise in a total of seven polymorphs and solvates exhibiting different modes of intermolecular association. Compound 2b crystallises in three polymorphs; two of them and the solvate 2b·0.5CH2Cl2 exhibit aurophilic interaction while the third one is stabilised by a short Au⋯Cl interaction of 3.2660(9) A. In 2c, one polymorph exhibits a strong aurophilic interaction of 3.0393(4) A, but the other, as well as the solvate 2c·thf, lack such contacts. Product 2b is the first simple gold compound known to have both Au⋯Au or Au⋯Cl contacts in different crystals. Calculations at the B3LYP and MP2 levels of theory using quasi-relativistic basis sets show that for 2b2 the Au⋯Cl interaction is between 2.6 and 12.2 kJ mol–1 greater than the Au⋯Au interaction, depending on the level of theory and basis set. This contrasts with a model (PH3AuCl)2 dimer, where the Au⋯Au interaction is found to be stronger.

Titanoxycarbene complexes of Ti(IV) with O- and N-donor ligands

Heterobimetallic complexes of the general formula [{(OC)5MC(Me)OTi(OiPr)2}(µ-OiPr)]2 with M = Cr (1), W (2) were synthesized by reacting (OC)5MC(Me)OLi (M = Cr, W) with ClTi(OiPr)3. X-Ray crystallography revealed dimeric molecular structures based on four-membered Ti2O2-rings for both compounds. Most intriguingly crystals of 1 virtually “shattered” upon cooling below −60 °C. Treatment of 1 and 2 with an excess of pyridine did not effect symmetric cleavage of the dimers, but afforded the unique rearrangement products [(OC)5MC(Me)O]2Ti(OiPr)2(C5H5N)2 with M = Cr (3) and W (4). The isomorphous structures, determined by means of X-ray diffraction, are only the second examples of an unrestrained titanium center coordinated by four OR and two N-donor ligands. Complexes 3 and 4 were also accessible from Cl2Ti(OiPr)2(C5H5N)2 (5) and (OC)5MC(Me)OLi. Owing to the inherent dynamic behaviour of all complexes, low-temperature NMR studies were undertaken. The formation of nanoparticles upon thermolysis of complex 2 lends support to the notion that the presented (or related) compounds could be useful as single source precursors for nano-sized mixed metal oxides or oxo nitrides.