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

Abstract

A new polydentate bridging ligand, NH₄C₅N=NC₆H₄N(H)C₅H₄N (HL²), 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 HL² were isolated and characterized. The X-ray structure of [H₂L²](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(L²)₂], 2, and [Pd₂(L²)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 [L²]^- 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 [L²]^- with K₂[PtCl₄], however, yielded a monometallic platinum complex, [PtCl₃(L²)], 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 π -delocalization. Spectral data of the complexes are reported and compared.