Design and Synthesis of a New Binucleating Ligand via Cobalt-Promoted C-N Bond Fusion Reaction. Ligand Isolation and Its Coordination to Nickel, Palladium, and Platinum

A new polydentate bridging ligand, NH4C5N=NC6H4N(H)C5H4N (HL^2), is synthesized by the cobalt-mediated phenyl ring amination of coordinated NH4C5N=NC6H5. The green cobalt complex intermediate [Co(L2)2](ClO4), [1](ClO4), and the free ligand HL^2 were isolated and characterized. The X-ray structure of [H2L^2](ClO4) is reported. The ligand, upon deprotonation, behaves as a bridging ligand. It reacts with NiCl2·6H2O and Na2[PdCl4] to produce dimetallic complexes, [Ni2Cl2(L^2)2], 2, and [Pd2(L^2)2](ClO4)2, [3] (ClO4)2, respectively. X-ray structures of these two dimetallic complexes are reported. The structure of the dinickel complex, in particular, is unique. In this complex, the two deprotonated secondary amine nitrogens of the two [L^2]^- ligands bind to two nickel centers simultaneously forming a planar Ni2N2 arrangement. The complex [3](ClO4)2 is diamagnetic while the complex 2 is paramagnetic. The results of magnetic measurements on the dinickel complex in the temperature range 1.8-300 K are reported. The system can be described as a single spin S = 2 in the low-temperature range T J/k whereas at high temperatures, T J/k, it behaves as two independent spins S = 1.The reaction of [L^2]^- with K2[PtCl4], however, yielded a monometallic platinum complex, [PtCl3 (L^2)], 5, where the pyridyl nitrogen of the aminopyridyl function remained unused. The X-ray structure of the complex 4a is reported. The bond lengths along the ligand backbones in all the complexes indicate extensive (pi)-delocalization. Spectral data of the complexes are reported and compared.