Synthesis and Tunability of Abnormal 1,2,3-Triazolylidene Palladium and Rhodium Complexes

Abstract

Palladation of N3-alkylated 1,2,3-triazolium salts with Pd(OAc) 2 afforded a μ 2 -I 2 bridged bimetallic complex [Pd(trz)I 2 ] 2 and monometallic bis(carbene) complexes Pd(trz) 2 I 2 as a mixture of trans and cis isomers (trz = 1,2,3-triazol-5-ylidene). Addition of excess halide or modification of the palladation procedure from direct functionalization to a transmetalation sequence involving a silver intermediate allowed for chemoselective formation of the bis(carbene) complex, while subsequent anion metathesis with NaI produced the monometallic bis(carbene) complexes exclusively. Modification of the wingtip group had little influence on the metalation to palladium or rhodium(I) via transmetalation. According to NMR analysis using δ C and 1 J Rh-C , subtle but noticeable tunability of the metal electronic properties was identified. In addition, phenyl wingtip groups as N-substituents in the triazolylidene ligands were susceptible to cyclopalladation in the presence of NaOAc and are thus not chemically inert.

Application of 1,2,3-triazolylidenes as versatile NHC-type ligands: synthesis, properties, and application in catalysis and beyond

“…One-pot syntheses of triazoles from an aniline starting material have been reported, 17,18 along with 10 syntheses utilizing alkyl azide where these azides are not isolated prior to the CuAAC process. 11,[19][20][21][22][23][24] Subsequent alkylation of the triazole with a suitable alkylating agent is generally selective and yields the 1,3,4-substituted triazolium salt (Scheme 1). Although triazolylidenes are a new class of NHC ligands, the variety of ligands that have been synthesised via the CuACC reaction is substantial.…”

“…In most cases the formation of the silver carbene is surmised, using NMR spectroscopy, by the disappearance of the low field 19,36,37 Both forms exist as syn and anti conformers. 36 However, performing the carbene transfer in the presence of NaI selectively yields the dimeric palladium species, 22, with bridging iodide groups.…”

“…However, thermally induced direct metallation with Pd(OAc) 2 has been demonstrated. 19 Scheme 6 Preparation of a series of free triazolylidenes and coordination 25 to ruthenium(II) and iridium(I). afford complexes 24 and 25, respectively.…”

“…In support of this proposal, the NMR 65 characteristics of the CO ligand trans to the carbene in rhodium complexes 37 are weakly dependent on the electronic properties of the wingtip groups. 19 An increase of the coupling constant ( 1 J RhC ) is observed when less electron-donating aryl wingtip groups are present, whereas alkyl substituents result in smaller 70 coupling constants. …”

Tunable single-site ruthenium catalysts for efficient water oxidation

“…The exact chemical shift is dependent on the substitutent at nitrogen (δ C 174.1, 172.9, 172.4, and 174.0 for 2a-2d, respectively), pointing to a moderate tunability of the electron density at this carbon via wingtip substitution. 15 Chelation of the pyridine unit in solution is supported by the highfield resonance of the proton in the pyridine ortho position, which shifted from δ H 8.4 in 1 to 9.4. The corresponding dicationic acetonitrile complexes 3 displayed similar spectroscopic characteristics for the bidentate triazolylidene ligand.…”