Synthesis and Structural Variation of Iron, Rhodium, Palladium, and Silver Complexes of a Chiral N-Heterocyclic Carbene−Phenoxyimine Hybrid Ligand

Metal complexes with oxygen-functionalized NHC ligands: synthesis and applications

Ligand design has met with considerable success with both categories of hybrid ligands, which are characterized by chemically different donor groups, and of N-heterocyclic carbenes (NHCs). Their spectacular development and diversity are attracting worldwide interest and offers almost unlimited diversity and potential in e.g. coordination/organometallic main group and transition metal chemistry, catalysis, medicinal chemistry and materials science. This review aims at providing a comprehensive update on a specific class of ligands that has enjoyed much attention in the past few years, at the intersection between the two categories mentioned above, that of hybrid NHC ligands in which the functionality associated with the carbene donor is of the oxygen-donor type. For each type of oxygen-donor present in such chelating (Section 1) or bridging (Section 2) hybrid ligands, we will examine the synthesis, structures and reactivity of their metal complexes and their applications, with a special focus on homogeneous catalysis (Section 3). Thus, hydrogenation, C-H bond activation, C-C, C-N, C-O bond formation, hydrolysis of silanes, oligomerization, polymerization, metathesis, hydrosilylation, C-C bond cleavage, acceptorless dehydrogenation, dehalogenation/hydrogen transfer, oxidation and reduction reactions will be successively presented in a tabular manner, to facilitate an overview and a rapid identification of the relevant publications describing which metals associated with a given oxygen functionality are most suitable. The literature coverage includes the year 2015.

Smart N-Heterocyclic Carbene Ligands in Catalysis

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term "smart" is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

N2S2 and N4S4 precursors to PS2 macrocycles and cyclic amidinium salts

The cyclo-condensation of 1R,2R-diaminocyclohexane with 2,2'-(ethane-1,2-diyldisulfanediyl)dibenzaldehyde gave the 1 : 1 addition compound chxn-(im)N2S2 in high yield. When the same condensation reaction was performed with 1R,3S-diamino-1,7,7-trimethylcyclopentane as the diamine, the 2 : 2 addition compound tmcp-(im)N4S4 was obtained selectively. Reduction of the diimines gave the saturated analogues chxn-N2S2 (1) and tmcp-N4S4 (3) the former of which could be phosphorylated with PhP(NMe2)2 to give the novel 13-membered macrocycle chxn-PS2, 2. Introduction of the phenylphosphine function proved stereoselective with a preference for the N(R)/N(S) configuration at the nitrogen atoms. The coordination chemistry of the novel phosphine has been explored with Cu(i) and Mo(0) through formation of the complexes Cu(2)I, 4, and Mo(CO)3(2), 5. Extension of the phosphorylation chemistry to tmcp-N4S4 (3) proved unsuccessful but ring closure reactions of both 1 and 3 with triethylorthoformate gave cyclic amidinium salts which are potential precursors to macrocyclic N-heterocyclic carbenes.

1-[6-(Hydroxymethyl)-2-pyridyl]-3-(2,4,6-trimethylbenzyl)-1H-imidazol-3-ium bromide

In the title compound, C₁₉H₂₂N₃O⁺·Br⁻, the imidazole ring is approximately coplanar with the pyridine ring [dihedral angle = 0.88 (13)°] and nearly perpendicular to the benzene ring [dihedral angle = 81.70 (13)°]. O—H⋯Br and C—H⋯Br hydrogen bonding helps to stabilize the crystal structure.