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 [(pyN4)Ni-Cl-Ni(pyN4)l(PF6)3 IpyN4 = 2,6-Bis(1′,3′-diamino-2′-methylprop-2′-yl)pyridine]

The architecture of the tetrapodal pentaamine ligand 2,6-bis(1′,3′-diamino-2′-methylprop-2′-yl)pyridine (pyN₄, 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 = OH₂, OClO₃, NCS, N₃, {Cl-Ni(pyN₄)}) 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)](PF₆)₃. This complex has an antiferromagnetically coupled ground state of total spin S_T = 0, as determined from variable-temperature magnetic susceptibility measurements.