An Anthyridine-Based Pentanitrogen Donor Switches from Mono- to Tetradentate with Pd(II) Ions

Treatment of 5-phenyl-2,8-bis(2-pyridinyl)anthyridine (L) with (PPh3)2PdCl2 or (dppe)PdCl2 in the presence of a silver salt resulted in the formation of [trans-(PPh3)2PdLCl](BF4) (1a), [trans-(PPh3)2PdL(MeCN)](BF4)2 (1b), [trans-(PPh3)2PdLCl](PF6) (1c) ,or [cis-(dppe)PdLCl](BF4) (4), respectively. The ligand L in these complexes acts as a monodentate ligand with N(10) of anthyridine binding to the metal center. In the presence of PPh3, dinuclear complexes [Pd2L(CH3CN)2Cl2](BF4)2 (2) and [Pd2L(CH3CN)4](BF4)4 (3) readily underwent dechelation to yield 1a,b, respectively, whereas the reaction of 2 with dppe gave 4. On the other hand, treatment of 1a and 4 with S8 in the presence of a sufficient amount of palladium ions provided the corresponding dinuclear complex 2. Furthermore, this kind of substitution is also applicable with the Pd-Me complex [Pd2L(CH3CN)2Me2](BF4)2 (5), which could be prepared from complexation of L with 2 equiv of [(COD)Pd(CH3CN)Me](BF4). Thus, [trans-(PPh3)2PdLMe](BF4) (7) was obtained by the reaction of 5 with PPh3. However, the reaction of PPh3 with [Pd2L(CH3CN)2(MeCO)2](BF4)2 (6), a CO insertion product of 5, gave a messy result. The catalytic activity of these complexes in the Suzuki–Miyaura coupling of aryl halide with arylboronic acid under a CO atmosphere was investigated. Crystal structures of 1c, 4, 5, and 7 are reported to confirm their structural details. This work demonstrates the novelty of L as a hypodentate ligand toward palladium ions.