Syntheses, Structural Aspects, Solution Behavior, and Catalytic Utility of Cyclopalladated N,N′,N″‑Triarylguanidines [κ2(C,N))Pd(Pyrazole)2X] (X = Br, OC(O)CF3, and PF6) in Suzuki–Miyaura Coupling Reactions of Aryl Bromides

The reactions of six-membered cyclopalladated N,N′,N″-triarylguanidines [κ2(C,N)­Pd­(μ-X)]2 (3–7) with 2 equiv of pyrazole (pzH) and 3,5-dimethylpyrazole (3,5-dmpzH) in CH2Cl2 at RT for 24 h afforded a new class of cyclopalladated N,N′,N″-triarylguanidines, [κ2(C,N)­Pd­(PzH)2X] (Ar = 2-MeC6H4; X = Br; PzH = 3,5-dmpzH (8), pzH (9); X = OC­(O)­CF3; PzH = 3,5-dmpzH (10); Ar = 2-(MeO)­C6H4; X = Br; PzH = 3,5-dmpzH (11); OC­(O)­CF3 (12); Ar = Ph; X = OC­(O)­CF3; PzH = 3,5-dmpzH (13)) in good yields. The reaction of 8 with NH4PF6 in CH2Cl2 at RT for 24 h afforded [κ2(C,N)­Pd­(3,5-dmpzH)2(PF6)] (14) in 83% yield. Complexes 8–14 were characterized by elemental analyses, IR, NMR (1H, 13C, and 19F) spectroscopic techniques, and conductivity measurements. Molecular structures of six-membered cyclopalladated N,N′,N″-triphenylguanidine [κ2(C,N)­Pd­(μ-OC­(O)­CF3)]2·PhMe (7·PhMe), 8, 10, 12, and 14 were determined by single-crystal X-ray diffraction, which revealed transoid in-in (7·PhMe), β-out (10 and 14), and α-out (12) conformations. The eight-membered “[Pd­(κ1N-PzH)2X]” rings in 8, 10, 12, and 14 that consist of a pair of N–H···X hydrogen bonds are hitherto unknown in palladacyclic chemistry. VT 1H NMR (CD2Cl2, 400 MHz) spectra of 10 revealed the presence of a mixture of three species, while VT 19F NMR (CD2Cl2, 376.2 MHz) spectra revealed the presence of a mixture of seven species at 213 K. The presence of more than one solution species of 10 was explained by invoking the presence of a mixture of conformers and intermediates formed between a pair of related conformers. The catalytic efficacy of 8–14 in Suzuki–Miyaura coupling reactions of various aryl bromides with PhB­(OH)2 was evaluated, and these cyclopalladated guanidines showed a wide substrate scope in the coupling reactions with low catalyst loadings (1.00, 0.1, and 0.01 mol %) under mild reaction conditions.