Palladium(II) Complexes of Ambidentate and Potentially Cyclometalating 5‐Aryl‐3‐(2′‐pyridyl)‐1,2,4‐triazine Ligands

Abstract

The 5-aryl-3-(2′-pyridiyl)-1,2,4-triazine ligands under study L 1 ; 5-(3-methoxyphenyl)-(PyTZ3Me-OPh) L 2 ; 5-(4-methoxyphenyl)-(PyTZ4MeOPh) L 3 ; 5-(4-trifluoromethylphenyl)-(PyTZ4CF 3 Ph) L 4 ; 5-(4-fluorophenyl)-(PyTZ4FPh) L 5 ; and tris-3,5,6-(2′-pyridyl)-1,2,4-triazine (Py 3 TZ) L 6 ] react with [(COD)PdCl 2 ] (COD = 1,5-cyclooctadiene) to form complexes [(L 1-6 )PdCl 2 ] with N,N bidentate binding ligands, also including the potentially N,N,N tridentate ligand L 6 . This was concluded from an in-depth NMR spectroscopic study of the new complexes and from comparison with Pd-terpy complexes [(R′ terpy)PdCl]Cl [R′terpy = 4′-R′-2,2′:6′,2′′-terpyridine; R′ = H or SMe], showing definite tridentate N,N,N coordination, and with the Pd-bpy complexes [(bpy)Pd(Mes)Cl] and [(bpy)PdCl 2 ], show-Eur.

Luminescent Pd(II) Complexes with Tridentate − Aryl‐pyridine‐(benzo)thiazole Ligands

“…Similar behaviour is observed for the protoligands H at potentials lying about 0.7 V lower than those corresponding to the complexes (Figures S6 to S17, Tables S2 and S3). Consistent with reports of similar complexes, [8,9,13,43–46,48–53,57–63] we thus assume that the lowest unoccupied molecular orbital (LUMO) is centred largely on the pyridine‐thiazole/‐benzothiazole moiety.…”

“…Electrochemical experiments were carried out on the series of Pd(II) complexes to assess the impact of the variation of the three groups in the C ^N^N ligands. Previous studies showed a good correlation between electrochemical data with absorption energies [2,4,12,13,31,38,40,43,[45][46][47][48][57][58][59][60][61][62][63] and DFT-calculated frontier orbitals and energies have been benchmarked with electrochemical data. [13,23,33,[45][46][47]63] At a first glance, all Pd(II) complexes showed a first reversible one-electron reduction wave at around À 1.7 V for the thiazole complexes and at À 1.6 V for the benzothiazole derivatives (Figure 1, Table 1).…”

“…Within the series of varying X coligands [Pd(ph(py)tz)X], with X = Cl, Br, I, the oxidation potentials decrease markedly from Cl to I (Table 1, Figures S11 to S13) as was recently also observed for the series [Pd(Phbpy)X] (HPhbpy = 6-(phenyl)-2,2'bipyridine). [63] Data of similar complexes [8,9,13,45,[48][49][50][59][60][61][62][63] suggest essentially metal (Pd II /Pd III ) based oxidation processes. However, the marked impact of the halide co-ligand and the sensitivity of the potentials to the solvent (THF vs. MeCN) suggest a contribution from the halide or the MÀ X bond to the oxidation processes.…”

“…The long wavelength absorptions of the Pd complexes are not strongly pronounced; they are broad, rather low in intensity and thus partially hidden behind the intense π-π*-related bands. Based on conclusions on similar complexes, [8,9,13,[44][45][46][47][48][49][50][57][58][59][60][61][62][63] they were tentatively assigned to transitions into metal(d Pd )-toligand(π*) charge transfer (MLCT) states. This is in line with the DFT-calculated LUMO and HOMO compositions (Figure S20).…”

C^N^N Coordination Accelerates the iClick Reaction of Square-Planar Palladium(II) and Platinum(II) Azido Complexes with Electron-Poor Alkynes and Enables Cycloaddition with Terminal Alkynes

“…6-Phenyl-2,2′-bipyridine (Hphbpy, 3 ) was selected as the C^N^N-chelating ligand and directly synthesized from 2,2′-bipyridine (bpy, 1 ) and phenyl lithium 2 in moderate yield but on a scale of 0.5 g (Scheme ). The reaction of Hphbpy 3 with [PdCl 2 (cod)] 4 (cod = 1,5-cyclooctadiene) at reflux for 18 h resulted in smooth metalation of the phenyl ring without the need for an external base. , However, an attempt to synthesize the analogous Pt­(II) complex using [PtCl 2 (cod)] failed, and thus, K 2 [PtCl 4 ] 5 was used as the starting material with good success. , The targeted chlorido compounds 6 and 7 were obtained in very good yield and easily converted to azido complexes 8 and 9 by refluxing with sodium azide in a mixture of water and acetone (Scheme ). The azido complexes 8 and 9 were then reacted at room temperature in a catalyst-free iClick reaction with a slight excess of electron-poor alkynes 4,4,4-trifluoro-2-butynoic acid ethyl ester (TFBE, 10 ) and dimethyl acetylenedicarboxylate (DMAD, 11 ) and methyl propiolate ( 12 ) to give the triazolato products 13–18 , with complexes 15 and 18 only prepared in a kinetic NMR tube experiment and not isolated.…”