1,3-Dimethyl-4-(diphenylphosphino)imidazolium Triflate: A Functionalized Ionic Liquid with Ambivalent Coordination Capability

Abstract

Simply using 1-methylimidazole as the starting material, the functionalized ionic liquid 1,3-dimethyl-4-(diphenylphosphino)imidazolium triflate ([3]OTf) has been prepared, from which the corresponding 4-phosphinosubstituted N-heterocyclic carbene 4 was generated. The coordination capability of 3 and 4 toward a variety of transition-metal centers has been studied, as well as their electron-donor character through an evaluation of the TEP parameter. 3 behaves as a cationic phosphine ligand, whereas 4 shows a strong preference to coordinate to transition metals through the carbene carbon atom, leading to the obtention of mononuclear complexes with a free phosphine arm which act as metalloligands, allowing for the modular formation of homo-and heterodinuclear transition-metal complexes. ■ INTRODUCTION1,3-Dialkylimidazolium salts are currently receiving a great deal of attention, owing to their use as ionic liquids for novel media in organic synthesis and catalysis 1 and as precursors of Nheterocyclic carbenes (NHCs). 2 Recently, imidazolium salts containing functional groups have been used for task-specific applications, 3 such as separation of CO 2 from gas streams, 4 supports for organic synthesis, 5 extraction of metal ions, 6 construction of nanostructures, 7 and ion conductive materials. 8 Particularly, phosphino-functionalized imidazolium salts, most of them containing the phosphino group connected to a nitrogen atom of the imidazolium cycle through a carbon spacer, have been extensively employed in the formation of chelating or pincer NHC-phosphine ligands 9 and as ionophilic phosphines for ionic-liquid biphasic catalysis. 10 In recent pioneering works, the groups of Bertrand 11 and Gates 12 have described the synthesis of 4-phosphino-substituted 1,3-diarylimidazol-2-ylidenes and proved their suitability for the synthesis of several dimetallic transition-metal complexes. 13 Subsequently our group managed to isolate 4,5-diphosphinosubstituted imidazolium salts and use them for generating mixed diphosphine-NHC ligands. 14,15 In related studies, 4-phosphorylimidazolium and 4,5-bis(phosphoryl)imidazolium salts have also been prepared, some of them being remarkable examples of room-temperature ionic liquids, as well as their corresponding NHC-transition-metal complexes. 16 In this paper we describe the synthesis of the functionalized ionic liquid 1,3-dimethyl-4-(diphenylphosphino)imidazolium triflate ([3]OTf; Figure 1), which contains one of the simplest imidazolium scaffolds, and the study of its properties and coordination capabilities together with those of the corresponding NHC derivative. A comparison has been made with its isomer 1,3-dimethyl-2-(diphenylphosphino)imidazolium triflate ([2]OTf) as well as with the parent unsubstituted imidazolium salt ([1]OTf).

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

“…Similar to 509 , this and other bidentate carbon–phosphorus donor ligands were used for the preparation of various homo- and heterobimetallic late-transition metal complexes. , In addition, the abnormal analogue of the original NHC–phosphenium system [(IMe)­PPh 2 ] + (vide supra, Scheme ) was prepared by Ruiz and co-workers from [(IMe)­PPh 2 ]­OTf ( 512 ) by deprotonation with LiHMDS, which proceeded with formation of the NHC–phosphine 513 . Subsequent aqueous workup gave the 4-diphenylphosphanylimidazolium triflate 514 (Scheme ), and the coordination chemistry of both 513 and 514 was studied extensively . Treatment of NHC–phosphine 513 with PPh 2 Cl gave the cationic 1,3-diphenylphosphanyl imidazolium salt 515 , which could be converted into the NHC–diphosphine 516 and subsequently into the 4,5-diphosphanylimidazolium salt 517 following a closely related protocol.…”

“…Subsequent aqueous workup gave the 4-diphenylphosphanylimidazolium triflate 514 (Scheme 125), and the coordination chemistry of both 513 and 514 was studied extensively. 625 Treatment of NHC−phosphine 513 with PPh 2 Cl gave the cationic 1,3-diphenylphosphanyl imidazolium salt 515, which could be converted into the NHC−diphosphine 516 and subsequently into the 4,5-diphosphanylimidazolium salt 517 following a closely related protocol. These ligands were employed for the modular synthesis of mixed diphosphine− NHC heterometallic complexes, 626 and also monometallic complexes with the cationic diphosphine ligand 517 were prepared.…”

Functionalised N‐Heterocyclic Carbene Ligands in Bimetallic Architectures

“…[71] Additional work hasb een performed with N-Me substituted phosphinocarbenes as well (76-78). [72] In 2009, Gates and co-workersd iscovered 'abnormal' reactions of phosphaalkenesw ithN HCs. [73] This offered ar oute to NHCs functionalised on the backbonew ith ap hosphine group and coordination of two equivalents of AuCl was demonstrated (Scheme 10, 80).…”

Homo- and Heteropolynuclear Complexes Containing Bidentate Bridging 4-Phosphino-N-Heterocyclic Carbene Ligands

“…Additional examples include the 4-phosphino-substituted NHCs of Bertrand and co-workers, 4,5-diphosphino-substituted NHCs of Ruiz and co-workers, and 4-phosphanido-substituted NHCs of Streubel, Nyulaszi, and Gates . The incorporation of phosphorus at the backbone of an NHC has facilitated the synthesis of novel homo- and heteropolynuclear complexes (e.g., Chart , II and III ). ,,, …”