Mono- and Dinuclear Copper(ii) and Iron(iii) Complexes of a Tetradentate Bispidine-diacetate Ligand

Abstract

The synthesis of a new tetradentate bispidine ligand (LH₂ = 2,2′-(1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonan-3,7-diyl)diacetic acid), containing two tertiary amine and two carboxylic groups, is reported along with the preparation and characterization of the corresponding Cu(ii) and Fe(iii) complexes. The mononuclear [LCu(OH₂)]·4H₂O (1) complex contains a five-coordinate Cu(ii) centre, which adopt a square pyramidal geometry with the four donor atoms of the ligand (N₂O₂) occupying the equatorial plane and a water molecule occupying the axial position. An axial electron paramagnetic resonance (EPR) signature is observed for 1 (g_x = 2.054, g_y = 2.050, g_z = 2.234; A_x = 18 × 10^–4 cm^–1, A_y = 20 × 10^–4 cm^–1, A_z = 188 × 10^–4 cm^–1) in frozen methanolic solution (0.1 mM). Dimerization of 1 in concentrated solution (10 mM) was observed by EPR spectroscopy (g_∥ = 2.24, g_⊥ = 2.07, A_∥ = 195 × 10^–4 cm^–1, and A_⊥ = 12 × 10^–4 cm^–1 for each Cu centre). The structure of the dimeric species [LCu(OH₂)]₂ (1b) was determined by a combination of molecular mechanics with the simulation of the EPR spectrum (MM-EPR). The dimer has each Cu(ii) centre coordinated by the two amines and one carboxylate of one ligand (L), while the other carboxylate bridges to the second Cu(ii) centre; each coordination sphere is completed by an axial water ligand, with the Cu···Cu distance 5.5 Å (relative orientation from EPR simulation: α = 60°, β = 0°, γ = 25°). The aqueous reaction between the tetradentate ligand (L) and Fe(ii) leads to the formation of an oxo-bridged diiron(iii) complex, [LFe-(μ-O)-FeL] (2), with a Fe–O–Fe angle of 180° (d_Fe···Fe = 3.516 Å), as revealed by X-ray crystallography. The Mössbauer spectrum of 2 consists of one quadrupole doublet with an isomer shift (δ) of 0.37 mm s^–1 and a quadrupole splitting (ΔE_Q) of 0.73 mm s^–1, which is consistent with S = 5/2 Fe(iii) centres. Variable-temperature magnetic susceptibility measurements show the presence of intramolecular antiferromagnetic interactions between the two Fe(iii) centres, with an exchange coupling constant J of –91(3) cm^–1 (H = –2JS₁·S₂).