Solution Spin Crossover Versus Speciation Effects: A Cautionary Tale

Two acyclic tetradentate Schiff base ligands, HLX-OH (X = H and Br), were synthesised by 2:1 condensation of either 2-pyridinecarboxaldehyde or 5-bromo-2-pyridinecarboxaldehyde and 1,3-diamino-2-propanol and then used to prepare six mononuclear complexes, [Fe-II(HLX-OH)(NCE)(2)], with three different NCE co-ligands (E = BH3, Se, and S). The apparent solution spin crossover switching temperature, T-1/2, of these 6 complexes, determined by Evans method NMR studies, is tuned by several factors: (a) substituent X present at the 5 position of the pyridine ring of the ligand, (b) E present in the NCE co-ligand, (c) solvent employed (P'), and (d) potentially also by speciation effects. In CD3CN, for the pair of NCE = NCBH3 complexes, when X = H, the complex is practically LS (extrapolated T-1/2 similar to 624 K), whereas when X = Br, it is far lower (373 K), which implies a higher field strength when X = H than when it is Br. The same trend, X = H results in a higher apparent T-1/2 than X = Br, is seen for the other two pairs of complexes, with E = Se (429 > 351 K, Delta T-1/2 = 78 K) or S (361 > 342 K, Delta T-1/2 = 19 K). For the family of three X = Br complexes, the change of E from BH3 (373 K) to Se (351 K) to S (342 K) leads to an overall Delta T-1/2(apparent) = 31 K, whereas the decreases are far more pronounced in the X = H family (BH3 similar to 624 > Se 429 > S 361 K). Changing the solvent used from CD3CN to (CD3)(2)CO and CD3NO2, for [Fe-II(HLBr-OH)(NCE)(2)] with either E = BH3 or S, revealed excellent, and very similar, positive linear correlations (R-2 = 0.99) of increasing solvent polarity index P' (from 5 to 7) with increasing apparent T-1/2 of the complex (E = BH3 gave T-1/2 300 < 373 < 451 K, Delta T-1/2 = 151 K; E = S gave T-1/2 288 < 342 < 427 K, Delta T-1/2 = 147 K). Several other solvent parameters were also correlated with the apparent T-1/2 of these complexes (R-2 = 0.74-0.96). Excellent linear correlations (R-2 = 0.99) are also obtained with the coordination ability (a(TM)) of the three NCE co-ligands with the apparent T-1/2 in both families of compounds, [Fe-II(HLX-OH)(NCE)(2)] where X = H or Br. The N-15 NMR chemical shifts of the nitrogen atom in the three NCE co-ligands (direct measurement) show modest correlations (R-2 = 0.74 for LH-OH family and 0.80 for LBr-OH family) with the apparent T-1/2 values of the corresponding complexes.

Automated summary

Two acyclic tetradentate Schiff base ligands, HLX-OH (X = H and Br), were synthesized and used to prepare six mononuclear complexes. The solution spin crossover switching temperature, T-1/2, of these complexes is influenced by factors such as substituent X, co-ligand E, solvent, and speciation effects. The polarity of the solvent and the coordination ability of the co-ligands are correlated with the T-1/2 values of the complexes.