Abstract
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, [FeII2L(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: gamma(HS)(298 K) = 0-0.81. Variable temperature H-1 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 [Fe2L3](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 AgNO3/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 Delta(o), establishing: (a) the ligand field strength order of the ligands: 4-imidazole (11 420) similar to 1-methyl-4-imidazole (11 430) < 2-imidazole (11 505) similar to 4-oxazole (11 516) < 4-thiazole (11 804 cm(-1)), (b) that Delta(o) ([(Ni2L3)-L-II](4+)) strongly correlates (R-2 = 0.87) with T-1/2 ([FeII2L(3)](4+)), and (c) interestingly that Delta(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).