Tridentate Complexes of Group 10 Bearing Bis-Aryloxide N-Heterocyclic Carbene Ligands: Synthesis, Structural, Spectroscopic, and Computational Characterization

Abstract

A series of group 10 complexes featuring chelating tridentate bis-aryloxide N-heterocyclic carbenes were synthesized and characterized by using different techniques. Ni(II), Pd(II), and Pt(II) complexes were isolated in good yields by straightforward direct metalation of the corresponding benzimidazolium or imidazolium precursors in a one-pot procedure. All of the compounds were fully characterized, including single-crystal X-ray diffractometric determination for three of the derivatives. In the solid state, the complexes adopt a typical square-planar coordination geometry around the platinum atom, sizably distorted in order to comply with the geometrical constraints imposed by the bis-aryloxide N-heterocyclic carbene ligand. For platinum and palladium derivatives, a joint experimental and theoretical characterization was performed in order to study the optical properties of the newly prepared complexes by means of electronic absorption and steady-state and time-resolved photophysical techniques as well as density functional theory (DFT) and time-dependent DFT in both vacuum and solvent. When the temperature was lowered to 77 K in frozen glassy matrix, three platinum complexes showed broad and featureless, yet weak, photoluminescence in the green region of the visible spectrum with excited-state lifetimes on the order of a few microseconds. On the basis of joint experimental and computational findings and literature on platinum complexes, such emission was assigned to a triplet-manifold metal−ligand-to-ligand charge transfer (3MLLCT) transition.

From betaines to anionic N-heterocyclic carbenes. Borane, gold, rhodium, and nickel complexes starting from an imidazoliumphenolate and its carbene tautomer

“…The Ni–O bond lengths were determined to be 187.60(13) pm and 190.11(13) pm, whereas the Ni–C carbene bonds have lengths between 184.06(19) pm and 184.82(19) pm, respectively. These values correspond to literature-known bond lengths of cis arranged bidentate ligands around Ni [42–44], but, as expected, they differ from those of [(PEt 3 )(Ph)Ni(imidazo[1,5- a ]quinolin-9-olate-1-ylidene)] [45] as well as Ni complexes with tridentate ligands [46–47]. …”

Tridentate Complexes of Palladium(II) and Platinum(II) Bearing bis‐Aryloxide Triazole Ligands: A Joint Experimental and Theoretical Investigation

“…In this respect, some of us recently reported on the efficient use of bis ‐anionic tridentate N N N ligands based on coordinating 1,2,4‐triazolate moieties for the preparation of highly emissive platinum complexes –. Very recently, we also investigated the synthesis and photophysical properties of a series of group 10 derivatives bearing tridentate O^C^O‐based ligands derived by the combination of both N ‐heterocyclic carbene (NHC) and chelating aryloxide moieties and the comparison with the corresponding tetradentate O^C^C^O‐based complexes provided …”

“…Luminescent complexes have attracted growing attention in the recent past due to their potential application as active species in optoelectronics, [1] bio-imaging, [2] sensing, [2c, 3] solarenergy conversion [4] and photo-catalysis. [5] Amongst all different complexes, those based on second and third row transition metals such as octahedral d [6] Ir III ,R u II and Re I as well as squareplanar d [8] Pt II showedt he most interesting photophysical features so far in terms of PLQY,e mission color modulation and quantum efficiencyw hen employed as active species in solidstate devices. [1,3,6] Generally,t heir great efficiency stems from the sizeable spin-orbit coupling (SOC) effect exerted by the heavy atom that allows mixing of both singlet and triplet manifolds and, consequently,p opulation of formally spin-forbidden triplet-manifold excited states through intersystem crossing( ISC) processes with nearly unity efficiency.H ence, radiativec oupling of the lowest-lying triplete xcited state with the singlet ground state becomes highly allowed yieldingt o phosphorescence.…”

“…[7a-c] Very recently, we also investigated the synthesis and photophysical properties of as eries of group 10 derivatives bearingt ridentate O^C^O-based ligands derived by the combination of both Nheterocyclic carbene (NHC) and chelating aryloxide moieties and the comparison with the corresponding tetradentateÔC^C^Obased complexesp rovided. [8] The 4-[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]benzoic acid compound (also called deferasirox, Scheme 1) has been shown to efficiently bind to iron ions and is now used as an orally active iron chelator in order to reduce chronic iron overload in particulari np atients who are receiving long-termb lood transfusions. [10] Beside their biomedical applications, we anticipated that such molecular structure would be interesting for the development of group 10 metal complexes for use in material science.…”

“…Herein, we report on the easy access of robust group 10 complexes bearing bis-aryloxide triazole ligandsa sn ovel O^N^O chelating motifs. [8] As eries of Pd and Pt complexes with various substituents on the triazole moiety were synthesized in as traightforwardm anner and fully characterized. The photophysical properties of the most promising luminescent platinum complexes were also studied.…”