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

The self-assembly of [W(CN)8]4– with cis-[CoIII(N4)Cl(OH2)]2+{N4= trien = triethylenetetraamine and tren = tris(2-aminoethylene)amine} in aqueous solution generates supramolecular assemblies: K2[CoIII(trien)]2[WIV(CN)8]2·8H2O·EtOH (1) and K2[CoIII(tren)]2[WIV(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-[CoIII(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=ka[Co(N4)]2[W(CN)8] +kb[Co(N4)][W(CN)8]+k–b[Co(N4)] where ka= 4.89 × 102 dm6 mol–2 s–1, H= 46 kJ mol–1, S=–38 J K–1 mol–1; kb= 2.14 × 10–1 dm3 mol–1 s–1, H= 121 kJ mol–1, S=+149 J K–1 mol–1; k–b= 0.66 × 10–3 dm3 mol–1 s–1, H= 68 kJ mol–1, S=–77 J K–1 mol–1(T= 25 °C, pH = 2 (HCl + KCl), I= 0.5 mol dm–3). The ka pathway corresponds to the ion-triplet {CoIII,WIV,CoIII} formation followed by rate-determining outer-sphere electron transfer, subsequent fast substitution by [W(CN)8]4– and spontaneous back electron transfer to give Co2IIIW2IV, whereas the kb pathway is consistent with the rate-determining reversible cyano bridged dimer formation within an ion-pair {CoIII,WIV} followed by inner-sphere electron transfer, fast aggregation and spontaneous back electron transfer to give the final Co2IIIW2IV product.