Advances in bis(N‐heterocyclic carbene) chemistry: new classes of structurally dynamic materials

Abstract

A major challenge in the pursuit of dynamic materials is the dichotomy between reversible structure and electronic conjugation. Guided by the precedence for N‐heterocyclic carbenes (NHCs) to dimerize, couple with electrophiles and bind to transition metals, we proposed that linearly opposed bis(NHC)s could, in a similar manner, afford unsaturated homopolymers, alternating copolymers, and metallopolymers. To realize these goals, we developed synthetic methods for accessing ditopic NHCs and investigated their propensities to undergo homopolymerization via dimerization and copolymerization via coupling with ditopic electrophiles or divalent transition metals. The materials obtained from these reactions were of relatively high molecular weight, exhibited electronic properties that were consistent with extensively delocalized systems, and in many cases, were found to be thermally reversible. In addition to expanding the scope of carbene chemistry, these polymers represent significant advancements towards the realization of reversible, conjugated polymers. Copyright © 2012 John Wiley & Sons, Ltd.

Pyracene‐Linked Bis‐Imidazolylidene Complexes of Palladium and Some Catalytic Benefits Produced by Bimetallic Catalysts

“…The solvent was pumped off to yield a yellow solid. Suitable crystals for Xray analysis were obtained after slow evaporation of a CH 2 194.8, 148.4, 142.3, 136.6, 132.1, 131.8, 131.6, 129.7, 129.2, 128.2, 126.3, 123.6, 116.4, 74.4, 30.6, 27.0, 25.4, 24.9 General procedure for the acylation of aryl halides: MS (1 g), tetrabutyl-A C H T U N G T R E N N U N G ammonium bromide (16 mg, 0.05 mmol), catalyst (1-2 %), aryl halide (0.5 mmol), hydrocinnamaldehyde (0.080 mg, 0.6 mmol), pyrrolidine (0.071 g, 1 mmol), and DMF ( 2 mL) were added to a 50 mL high-pressure Schlenk tube. The mixture was stirred and heated at 115 8C for 16 h. The yields of the reaction were calculated by GC and using anisole as internal standard.…”

“…[1] An important class of bisA C H T U N G T R E N N U N G (NHCs) features two facially opposed carbenes that inhibit chelation, thereby facilitating structures in which the ligand is bridging two transition metals that can be comprised in a discrete molecule or in a polymeric system. [2] Some representative examples of this type of bisA C H T U N G T R E N N U N G (NHCs) include triazolylidenes, [3] benzobis(imidazolylidenes), [4] oxanthrobis(imidazolylidenes), [4b] quinobis-A C H T U N G T R E N N U N G (imidazolylidenes), [4d, e] bis(imidazolinylidenes), [5] and our recently described pyracenebis(imidazolylidene). [6] Such types of biscarbenes, especially those linked by rigid p-conjugated systems, enable electronic communication between the carbene units, which can allow for interesting applications in molecular electronic devices.…”

Synthesis and structural investigation of R2Si (R = Me, Ph) bridged di-N-heterocyclic carbenes

“…8 Interesting poly-NHCs featuring two or more carbene functionalities have been reported; however, a majority of them is linked via the nitrogen atom of the imidazol ring. [9][10][11][12][13][14] In addition to their applications as ligands in catalysis, 9 such poly-NHCs have shown their poten-tial as building-blocks to design a new class of structurally dynamic materials. 14 Among available methods of fine-tuning, functionalization of NHCs impacts the properties as well as allows access to a new class of NHCs.…”

“…[9][10][11][12][13][14] In addition to their applications as ligands in catalysis, 9 such poly-NHCs have shown their poten-tial as building-blocks to design a new class of structurally dynamic materials. 14 Among available methods of fine-tuning, functionalization of NHCs impacts the properties as well as allows access to a new class of NHCs. [15][16][17][18][19][20][21][22] The prospect of direct access to functionalized NHCs from an NHC, with p-block moieties in particular, is enormous as it enables the synthesis of new ligands.…”

Triphenylene‐Based Tris(N‐Heterocyclic Carbene) Ligand: Unexpected Catalytic Benefits

“…While most of these carbenes refer to Janus‐type (facially opposed) bis(NHC)s,3a, d–f, 5 the only known rigid tris(NHC)s with geometrically isolated carbenes is restricted to the example reported by Williams and Bielawski in 2010 ( 1 ; Figure 1). 6 This tris(NHC) holds potential in reticular chemistry and may serve for accessing an assortment of functional organic and metal–organic frameworks, but lacks an extended π‐conjugated system (the π delocalization is disrupted by the presence of the triptycene core), which limits its utility in the formation of structurally dynamic materials 4a,b. With these precedents in mind, we sought to obtain a star‐shaped tris(NHC) in which the three carbenes are connected by a π‐delocalized polyaromatic system derived from a triphenylene core.…”