Structure and physical properties of trigonal monopyramidal iron(II), cobalt(II), nickel(II), and zinc(II) complexes

A series of trigonal monopyramidal complexes of the tripodal ligand tris((N-rert-butylcarbamoyl)methyl)aminato, [1(But)](3-), have been synthesized and characterized. The structures of [Co1(But)](-), [Zn1(But)](-), and [Ni1(But)](-) confirm that trigonal monopyramidal coordination geometry occurs in these complexes where the three amidate nitrogens are arranged in the trigonal plane and the amine nitrogen is bonded apically to the metal ions. The solid-state structures of [Co1(But)](-), [Zn1(But)](-), and [Ni1(But)](-) are nearly identical indicating that the trigonal ligand [1(But)](3-) enforces the trigonal monopyramidal structure in these metal ions. Crystal data: K[Co1(But)].0.5DMF crystallizes in the monoclinic space group C2/c, with cell dimensions a = 18.844(4) Angstrom, b = 9.809(3) Angstrom, c = 28.715(13) Angstrom, beta = 102.70 degrees, and Z = 8; (NEt4)[Zn1(But)].THF crystallizes in the monoclinic space group P2(1)/c, with cell dimensions a = 13.244(3) Angstrom, b = 11.285(5) Angstrom, c = 25.625(3) Angstrom, beta = 104.45(1)degrees and Z = 4. The H-1 NMR spectrum of the diamagnetic [Zn1(But)](-) also suggests that the complex retains its C-3 symmetry in solution. Room-temperature magnetic susceptibility measurements show that [Fe1(But)](-), [Co1(But)](-), and [Ni1(But)](-) are high spin. The cyclic voltammetry of [Co1(But)](-) and [Ni1(But)](-) at a glassy carbon surface and at a scan rate of 100 mV s(-1) shows quasi-reversible one electron oxidation at E-1/2 = 0.77 (Delta E-p = 93 mV, i(pc)i(pa)(-1) = 0.69) and 0.56 (Delta E-p = 75 mV, i(pc)i(pa)(-1) = 0.79) V versus SCE, respectively. However, at slower scan rates these redox processes become irreversible and attempts to isolate the oxidized products at room temperature were unsuccessful. The chemical oxidation of [Ni1(But)](-) with [Fe(bpy)(3)](3-) in 1:1 propionitrile-DMF mixture at -75 degrees C generated a EPR-active species (77 K, g(1) = 2.29, g(2) = 2.16, g(3) = 2.03, a(3) = 20 G) assigned to a Ni(III) complex with rhombic symmetry. [Fe1(But)](-) shows one irreversible oxidation (E-p,E-a = 0.05 V versus SCE) under the same conditions. These results are consistent with [1(But)](3-) being able to stabilize trigonal monopyramidal complexes of low-valent metal ions.