Synthesis, Coordination, and Spectroscopic Properties of 2,2′-Bipyridyldimesitylpalladium(II) and 6-Mesityl-2,2′-bipyridyldimesitylpalladium(II)

Abstract

The synthetic route to the dimesitylpalladium(II) complex [(bpy)PdMes2] (1) (Mes = mesityl = 2,4,6‐trimethyl phenyl) does not only give the desired compound but also the 6‐mesityl‐2,2′bipyridyldimesitylpalladium [(6‐Mes‐bpy)PdMes2] (2) complex and the free ligand 6,6′‐dimesityl‐2,2′‐bipyridine in reasonable yields. Single crystals of 2 were examined by X‐Ray diffraction. The compound reveals a sterically crowded molecular structure. An intramolecular π‐stacking interaction was found between the mesityl substituent on the bipyridine ligand and the adjacent mesityl ligand. The electrochemical behaviour of 1 and 2 together with a related compound was examined at various temperatures showing two reversible reduction reactions and reversible one‐electron oxidation steps at low temperatures. The latter are assigned to PdII/PdIII couples.

The Electrochemical Behaviour of Organonickel Complexes: Mono‐, Di‐ and Trivalent Nickel

“…The precursor complex [(PPh 3 ) 2 Ni(Mes)Br] exhibits a far higher potential than the corresponding diimine complexes and the Fmes co‐ligand shifts the oxidation potentials to higher values. At lower temperatures the dimesitylnickel complexes exhibit some degree of reversibility for the first oxidation wave, which is in agreement with observations for related platinum(II)33 or palladium(II)34 complexes where sterically shielding mesityl ligands have allowed the observation of trivalent states. However, the latter exhibit reversible waves already at 298 K, whereas the nickel analogues are not fully reversible even at 213 K. The phosphane complex [(PPh 3 ) 2 Ni(Mes)Br] shows irreversible oxidation behaviour even at 213 K. A remarkable difference is observed for the oxidation waves of the Fmes derivatives.…”

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

“…Similar behaviour is observed for the protoligands HC ^N^N 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][44][45][46][48][49][50][51][52][53][57][58][59][60][61][62][63] we thus assume that the lowest unoccupied molecular orbital (LUMO) is centred largely on the pyridine-thiazole/-benzothiazole moiety.…”

“…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).…”

“…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).…”

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

“…Neither for the ligands nor for the neutral Pd II complexes [( L 1 – 6 )PdCl 2 ], were oxidation waves found in the accessible range (+1.2 V). The complex [(bpy)Pd(Mes)Cl] shows an irreversible oxidation wave at 0.8 V, which is attributed to a Pd II /Pd III couple , , . In view of the strongly electron‐donating nature of the mesityl coligand in this latter complex, the Pd II /Pd III couple of the other complexes is probably high, above +1 V. Therefore, the irreversible oxidation waves observed for [(R′terpy)PdCl]Cl (R′ = H or SMe) must be attributed to the oxidation of the chloride anions of these two compounds (Figure , right).…”