A tetracobalt(ii) cluster with a two vertex truncated dicubane topology endogenously supported by carboxylate-based (2-pyridyl)methylamine ligands: magneto-structural and DFT studies

Abstract

A reaction between CoCl₂ and L³-(CO₂⁻)₂ (2 : 1 stoichiometry) in CH₃OH affords a discrete complex [Co^II_4-{L³-(CO₂⁻)₂}₂(μ₃-OCH₃)₂(CH₃OH)₂(H₂O)₂Cl₂] (1) [L³-(CO₂⁻)₂ = 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)]. The structure of 1 reveals two terminal mononuclear Co^II{L³-(CO₂⁻)₂}Cl units connected by a dimeric Co^II₂(μ₃-OCH₃)₂(CH₃OH)₂(H₂O) unit present in the centre through two methoxo (μ₃-OCH₃)- and two carboxylate (μ-1,1-OCO-) bridges affording a tetranuclear coordination cluster of Co(II) with a defective dicubane topology. In 1, Co1 (terminal) has distorted octahedral Co^IIN₂O₃Cl and the central Co2 has Co^IIO₆ coordination. Such coordination arrangements afford the observed topology. Variable-temperature magnetic studies reveal anti-ferromagnetic coupling in 1. Three magnetic exchange interactions (one anti-ferromagnetic and two ferromagnetic: J₁ = +3.3 cm⁻¹ (Co⋯Co 3.176 Å; μ-1,1-OCO- and μ₃-OCH₃ bridges), J₂ = −2.5 cm⁻¹ (Co⋯Co 3.228 Å; μ-1-OCO- and μ₃-OCH₃ bridges) and J₃ = +10.6 cm⁻¹ (Co⋯Co 3.084 Å; two μ₃-OCH₃ bridges)) have been identified, with the inclusion of the orbital reduction parameter (α = Aκ = 1.38), spin–orbit coupling (λ = −158 cm⁻¹) and axial distortion (energy gap Δ = −975 cm⁻¹ between singlet and doublet levels), rationalized by density functional theory (DFT) calculations.