Nickel, Ruthenium, and Rhodium NCN-Pincer Complexes Featuring a Six-Membered N-Heterocyclic Carbene Central Moiety and Pyridyl Pendant Arms

Abstract

NCN pincer ligand precursor 1•HX (X = Cl, PF 6 ), having a six-membered N-heterocyclic carbene central moiety and pyridyl pendant arms, was prepared via N−C crosscoupling of 2-bromo-6-tert-butylpyridine with propane-1,3diamine, followed by ring closure with triethylorthoformate. The free ligand 1 was not accessible via deprotonation, yet its copper(I) complex 2 was prepared by reacting 1•HCl with CuI and potassium hexamethyldisilazide. It was used further as a transmetalation agent to prepare nickel(II) and ruthenium(II) dihalide NCN-pincer complexes 3a, 4a, and 4b, featuring a distorted square-pyramidal geometry at the metal, with the carbene ligand in the axial position. The rhodium(III) NCNpincer complex 6b was obtained from 2 and [Rh(cod)Cl] 2 via transmetalation followed by oxidation with Cu(I) in THF. The rhodium(III) NCC-pincer complex 9b was prepared under similar conditions in acetonitrile, where a spontaneous rollover cyclometalation occurred. Complex 9b has a distorted octahedral geometry, with the acetonitrile ligand trans to the carbene. Very short Ru−C NHC and Rh−C NHC bonds were measured in complexes 4a, 4b, and 9b. Halogen exchange was observed by 1 H and 13 C NMR spectroscopy in both ruthenium and rhodium systems during the transmetalation stage, and the purification was accomplished by pushing the respective equilibrium with excess halide.

Transition Metal Complexes Supported by N‐Heterocyclic Carbene‐Based Pincer Platforms: Synthesis, Reactivity and Applications

“…In this regard, Roesler's group described the synthesis of a series of complexes bearing 6‐membered NHCs flanked by two pyridine donors (Scheme 40). [56] Cu carbene complex 121 was first synthesized for the subsequent transmetalation. For the cases of Ni and Ru, five‐coordinated pincer complexes 122 a/b were obtained.…”

“…In analogy to the synthesis of symmetrical pincer complex 124 ( vide supra , Scheme 40) by Roesler et al ., the same group reported the preparation of unsymmetrical NCC‐type pincer complex 182 in CH 3 CN (Scheme 53). [56] The formation of 182 was proposed to involve the rollover cyclometallation of 123 . The solvent chosen is crucial, and changing the solvent to THF brings a very different reactivity pattern ( vide supra , Scheme 40).…”

“…In this regard, Roesler's group described the synthesis of a series of complexes bearing 6-membered NHCs flanked by two pyridine donors (Scheme 40). [56] Cu carbene complex 121 was first The metalation reactivity was found to be solvent-dependent. Changing the reaction media from THF to CH 3 CN will lead to a series of complex intermediates, resulting in a non-symmetric pincer complex (vide infra, Scheme 53).…”

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

“…After the first family of PCP complexes was synthetized by Shaw in 1976 [7], numerous research groups have applied this concept for almost all types of homogeneously catalyzed chemical reactions. A myriad of novel complexes with different pincer arms have been synthetized and characterized, including a vast family of carbene pincers [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], PNP [24][25][26][27][28][29], PNN [30,31], POP [32], PCP [33][34][35][36][37][38], SNS [39,40], NCN [41], NSiN [42], CNC [43], CNN [44], NNN [45], as well as sulfur- [46][47][48], silicon- [49,50], selenium- [51,52], and boron-functionalized [53,…”

A β‐Diketiminato‐Based Pincer‐Type Nickel(II) Complex: Synthesis and Catalytic Performance in the Cross‐Coupling of Aryl Fluorides with Aryl Grignard Reagents

“…In recent years, tridentate pincer‐type complexes have generated significant interest because the pincer‐type ligand stabilizes the metal complex and its properties can be tuned to achieve optimal reactivity . Naturally, extensive attention has been focused on the combination of pincer‐type ligands with nickel, and effective catalysts for cross‐coupling reactions have been reported . Aryl chlorides as electrophiles are effectively catalyzed by pincer‐type Ni complexes.…”