Structural and Luminescent Properties of Homoleptic Silver(I), Gold(I), and Palladium(II) Complexes with nNHC-tzNHC Heteroditopic Carbene Ligands

Abstract

Novel silver(I), gold(I), and palladium(II) complexes were synthesized with bidentate heteroditopic carbene ligands that combine an imidazol-2-ylidene ( n NHC) with a 1,2,3-triazol-5-ylidene ( tz NHC) connected by a propylene bridge. The silver(I) and gold(I) complexes were dinuclear species, [M 2 ( n NHC- tz NHC) 2 ](PF 6 ) 2 (M = Ag or Au), with the two bidentate ligands bridging the metal centers, whereas in the palladium(II) complex [Pd( n NHC- tz NHC) 2 ](PF 6 ) 2 , the two ligands were chelated on the same metal center. Because of the presence of two different carbene units, isomers were observed for the gold(I) and palladium(II) complexes. The molecular structures of the head-to-tail isomer for gold(I) complexes, with a twisted or folded- syn conformation of the bridge between the carbene units, were determined by X-ray diffraction analysis. The study was completed with a systematic structural investigation through density functional theory (DFT) calculations. For palladium(II) species, the head-to-head form was structurally characterized. The dinuclear gold(I) complexes were emissive in the solid state in the blue region (PLQY up to 8%); time-dependent density functional theory (abbreviated as TD-DFT) calculations disclosed that the absorption bands have metal-to-ligand-charge-transfer character and evidenced that the emission occurs from the T 1 level (phosphorescence).

Possible Synthetic Approaches for Heterobimetallic Complexes by Using nNHC/tzNHC Heteroditopic Carbene Ligands

“…The 13 C-NMR spectrum shows two carbene signals, at 184.0 for the C im -Au and 165.1 for the C tz -Ag; the chemical shift of these signals is comparable with those reported in the literature for imidazol-2-ylidene and 1,2,3-triazol-5-ylidene ligands coordinated to gold(I) [15,33,37] and silver(I) [38,39] centers respectively. Furthermore, similar chemical shifts were observed for the previously synthesized homobimetallic Au(I)-Au(I) and Ag(I)-Ag(I) complexes having an analogous ditopic n NHC- tz NHC ligand [28]. As expected the mesoionic carbene carbon (C tz ) has a lower chemical shift than the normal carbene (C im ), indicative of superior donor properties [40,41,42].…”

“…Complex 6 was therefore converted to complex 8 by using a strong methylating agent, the Meerwein salt Me 3 OBF 4 , in 2 h in acetonitrile solution (Scheme 4). Other methylating agents, like for example MeI do not afford any product; this is not surprising considering that also in the literature it is reported that strong alkylating agents should be used in the case of triazole rings [28].…”

“…1:1. In this case, two sets of signals are present in the 1 H-NMR spectra in a 1:1 ratio, associable to two constitutional isomers of the gold(I) complex; in particular, we have recently reported and commented on this regard, concluding that the two isomers are associable to the head-to-head or head-to-tail coordination of the heteroditopic ligand [28]. We have labeled the crude mixture of complexes isolated at the end of the synthesis 11/11′ with the apostrophe indicating the head-to-head coordination of the ligand.…”

“…We have recently reported an n NHC- tz NHC heteroditopic ligand with a propylene linker between the two carbene donors [28]; considering the synthetic procedure to obtain the ligand precursor, we embarked on the investigation of a rational stepwise synthetic procedure for the isolation of heterobimetallic complexes with this type of ligand. A very similar n NHC- tz NHC ligand (with a shorter linker between the carbene units) was used by Cowie et al in the synthesis of a Pd(II)/Rh(I) heterobimetallic complex in a stepwise metalation approach [29], exploiting the different acidic properties of the two azole rings [30].…”

Hydroalkoxylation of Terminal and Internal Alkynes Catalyzed by Dinuclear Gold(I) Complexes with Bridging Di(N-Heterocyclic Carbene) Ligands

“…An interesting feature of this class of compounds is the possible presence of intramolecular aurophilic interaction, usually supported by the bridging diNHC ligand [23,24]. When present, aurophilicity can deeply modify the properties of the system, promoting high quality photoemission properties [25][26][27][28][29], and imparting new reactivity of the complexes (e.g., in the oxidative addition of halogens, favoring the formation of gold(II) dinuclear complex with a covalent gold(II)-gold(II) bond) [30][31][32][33]. More recently, we have also started to investigate the possible influence of this interaction in promoting cooperative effects in the catalytic properties of dinuclear gold(I) complexes of general formula Au 2 Br 2 (µ-diNHC) [34].…”