Heteroatom substitution effects in spin crossover dinuclear complexes.

We probe the effect of heteroatom substitution on the spin crossover (SCO) properties of dinuclear materials of the type [Fe-2(NCX)(4)(R-trz)(5)]S (X = S, Se; S = solvent; R-trz = (E)-N-(furan-2-ylmethylene)-4H-1,2,4-triazol-4-amine (furtrz); (E)-N-(thiophen-2-ylmethylene)-4H-1,2,4-triazole-4-amine (thtrz)). For the furtrz family ([Fe-2(NCX)(4)(furtrz)(5)]furtrzMeOH; X = S (furtrz-S) and X = Se (furtrz-Se)) gradual and incomplete one-step SCO transitions are observed (furtrz-S (T-1/2 = 172 K) and furtrz-Se (T-1/2 = 205 K)) and a structural evolution from [HS-HS] to [HS-LS] per dinuclear species. Contrasting this, within the thtrz family ([Fe-2(NCX)(4)(thtrz)(5)]4MeOH; X = S (thtrz-S) and X = Se (thtrz-Se)) more varied SCO transitions are observed, with thtrz-S being SCO-inactive (high spin) and thtrz-Se showing a rare complete two-step SCO transition (T-1/2(1,T-2) = 170, 200 K) in which the Fe-II sites transition from [HS-HS] to [HS-LS] to [LS-LS] per dinuclear unit with no long range ordering of spin-states at the intermediate plateau. Detailed structure-function analyses have been conducted within this growing dinuclear family to rationalise these diverse spin-switching properties.