Self-assembly of tetranuclear {[Co(trien)]2[W(CN)8]2}2− or {[Co(tren)]2[W(CN)8]2}2− squares with alternating aliphatic tetramine Co(iii) and octacyanotungstate(iv) corners

Abstract

The self-assembly of [W(CN)₈]⁴⁻ with cis-[Co^III(N₄)Cl(OH₂)]²⁺ {N₄ = trien = triethylenetetraamine and tren = tris(2-aminoethylene)amine} in aqueous solution generates supramolecular assemblies: K₂[Co^III(trien)]₂[W^IV(CN)₈]₂·8H₂O·EtOH (1) and K₂[Co^III(tren)]₂[W^IV(CN)₈]₂·9H₂O (2). The crystal structure of 2 has been determined. The Co(III) and W(IV) centres linked alternatively by single cyano bridge afford {[Co(tren)]₂[W(CN)₈]₂}²⁻ square units joined by potassium ions into the bilayer structure. The products exhibit metal-to-metal charge-transfer (MMCT) transitions at 519 nm (1) and 512 nm (2) in aqueous solution. The self-assembly of [W(CN)₈]⁴⁻ and cis-[Co^III(trien)Cl(OH₂)]²⁺ into the tetranuclear square {[Co(trien)]₂[W(CN)₈]₂}²⁻ proceeds by two parallel reaction pathways with the rate law d{[Co(N₄)]₂[W(CN)₈]}/dt = k_a[Co(N₄)]²[W(CN)₈] + k_b[Co(N₄)][W(CN)₈] + k_−b[Co(N₄)] where k_a = 4.89 × 10² dm⁶ mol⁻² s⁻¹, ΔH^≠ = 46 kJ mol⁻¹, ΔS^≠ = −38 J K⁻¹ mol⁻¹; k_b = 2.14 × 10⁻¹ dm³ mol⁻¹ s⁻¹, ΔH^≠ = 121 kJ mol⁻¹, ΔS^≠ = +149 J K⁻¹ mol⁻¹; k_−b = 0.66 × 10⁻³ dm³ mol⁻¹ s⁻¹, ΔH^≠ = 68 kJ mol⁻¹, ΔS^≠ = −77 J K⁻¹ mol⁻¹ (T = 25 °C, pH = 2 (HCl + KCl), I = 0.5 mol dm⁻³). The k_a pathway corresponds to the ion-triplet {Co^III,W^IV,Co^III} formation followed by rate-determining outer-sphere electron transfer, subsequent fast substitution by [W(CN)₈]⁴⁻ and spontaneous back electron transfer to give Co₂^IIIW₂^IV, whereas the k_b pathway is consistent with the rate-determining reversible cyano bridged dimer formation within an ion-pair {Co^III,W^IV} followed by inner-sphere electron transfer, fast aggregation and spontaneous back electron transfer to give the final Co₂^IIIW₂^IV product.