Conformational Flexibility of the Square-Pyramidal Coordination Cap in a Series of Octahedral Nickel(II) Pentaamine Complexes - Magnetochemical Characterization of the Singly μ-Cl-Bridged Nickel(II) Dimer [(pyN_4)Ni-Cl-Ni(pyN_4)](PF_6)_3 [pyN_4 = 2,

摘要

The architecture of the tetrapodal pentaamine ligand 2,6-bis(1',3'-diamino-2'-methylprop-2'-yl)pyridine (pyN_4, 1) allows it to coordinate to nickel(II) as a square pyramidal coordination cap. The pyridine nitrogen atom occupies an apical position of the coordination octahedron, while four equivalent pendent primary amino groups occupy the equatorial positions, with a sixth coordination site remaining for a monodentate ligand. Exchange of this ligand is facile, and a series of complexes [(1)NiX]~(n+) (X = OH_2, OClO_3, NCS, N_3, {Cl-Ni(pyN_4)}) has been prepared and characterised by elemental analysis, IR and UV/Vis spectroscopies (as applicable), and X-ray structure determination. While the solid state structures show varying degrees of distortion of the ligand cap 1 from C_(2v) symmetry, a polynucleating coordination mode has not been observed. The ligand enables the synthesis of dinuclear nickel(II) complexes containing a single bridging ligand, as exemplified by the singly μ-chloro bridged complex [(1)Ni-Cl-Ni(1)](PF_6)_3. This complex has an antiferromagnetically coupled ground state of total spin S_T = 0, as determined from variable-temperature magnetic susceptibility measurements.