Axial additions in nickel(II) complexes of (5SR,7RS,12RS,14SR)-tetra-methyl-1,4,8,11-tetra-azacyclotetradecane (Lα) and crystal-structure analysis of the planar orange and octahedral violet perchlorate salts NiLα,(ClO4)2

Abstract

Reduction of the nickel(II) complex of C-meso-5,7,12,14-tetramethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene with NaBH₄ gives predominantly the nickel(II) complex of the title ligand, L_α. Both orange (1) and violet (2) isomers of NiL_α(ClO₄)₂ have been characterised and structures established by X-ray crystallography. Crystals of (1) are monoclinic, space group P2₁/c with two formula units in a unit cell of dimensions a= 8.244(1), b= 8.656(1), c= 16.701(1)Å, and β= 110.28(1)°. Crystals of (2) are triclinic, space group P with two formula units in a cell of dimensions a= 10.645(1), b= 8.298(1), c= 14.198(2)Å, α= 121.36(1), β= 94.61(1), and γ= 82.77(1)°. Both structures were solved by the heavy-atom method and refined by full-matrix least-squares calculations. For (1), R= 0.036 for 2 434 observed reflections; for (2). R= 0.043 for 1 924 observed reflections. Orange complex (1) contains ionic perchlorate with the closest Ni ⋯ O contact 2.808(5)Å. In the centrosymmetric square-planar cation the Ni–N distances are 1.964(3) and 1.974(3)Å. The purple complex (2) contains two independent discrete half molecules (2a) and (2b) lying on inversion centres in the asymmetric unit, both with axially bound perchlorate. Principal distances are Ni–O 2.235(3) and 2.221(2)Å and Ni–N 2.063(2)–2.081 (2)Å. The metal complexes have the trans III arrangement of the chiral nitrogen centres, with the four methyl groups equatorial. The complex [NiL_α]²⁺ undergoes axial additions with unidentate ligands (H₂O, Br^–, Cl^–, NCS^–, N₃^–, NO₂^–, O₂CMe^–) and trans-[NiLX₂]ⁿ⁺ have been prepared and characterised. Infrared and d–d spectra are reported. The equilibrium [NiL_α]²⁺+ 2H₂O ⇌[NiL_α(OH₂)₂]²⁺has been studied over a temperature range, yielding K= 5.06 at 25 °C with ΔH^⊖=–22.1 kJ mol^–1 and ΔS^⊖=–60.6 J K^–1 mol^–1.