Correlations between ligand field Δ(o), spin crossover T(1/2) and redox potential E(pa) in a family of five dinuclear helicates

A family of five new bis-bidentate azole–triazole Rat ligands (1,3-bis(5-(azole)-4-isobutyl-4H-1,2,4-triazol-3-yl)benzene), varying in choice of azole (2-imidazole, 4-imidazole, 1-methyl-4-imidazole, 4-oxazole and 4-thiazole), and the corresponding family of spin-crossover (SCO) and redox active triply bridged dinuclear helicates, [Fe(II)(2)L(3)](4+), has been prepared and characterised. X-ray crystal structures show all five Fe(ii) helicates are low spin at 100 K. Importantly, DOSY NMR confirms the intactness of these SCO-active dinuclear helicates in D(3)-MeCN solution, regardless of HS fraction: γ(HS)(298 K) = 0–0.81. Variable temperature (1)H NMR Evans and UV-vis studies reveal that the helicates are SCO-active in MeCN solution. Indeed, the choice of azole in the Rat ligand used in [Fe(2)L(3)](4+) tunes: (a) solution SCO T(1/2) from 247 to 471 K, and (b) reversible redox potential, E(m)(Fe(II/III)), from 0.25 to 0.67 V for four helicates, whilst one has an irreversible redox process, E(pa) = 0.78 V, vs. 0.01 M AgNO(3)/Ag. For the four reversible redox systems, a strong correlation (R(2) = 0.99) is observed between T(1/2) and E(pa). Finally, the analogous Ni(ii) helicates have been prepared to obtain Δ(o), establishing: (a) the ligand field strength order of the ligands: 4-imidazole (11 420) ∼ 1-methyl-4-imidazole (11 430) < 2-imidazole (11 505) ∼ 4-oxazole (11 516) < 4-thiazole (11 804 cm(−1)), (b) that Δ(o) ([Ni(II)(2)L(3)](4+)) strongly correlates (R(2) = 0.87) with T(1/2) ([Fe(II)(2)L(3)](4+)), and (c) interestingly that Δ(o) strongly correlates (R(2) = 0.98) with E(pa) for the four helicates with reversible redox, so the stronger the ligand field strength, the harder it is to oxidise the Fe(ii) to Fe(iii).