Solution Spin Crossover Versus Speciation Effects: A Cautionary Tale.

Two acyclic tetradentate Schiff base ligands, H ( = 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(H)(NC)], with three different NCE co-ligands ( = BH, Se, and S). The apparent solution spin crossover switching temperature, , of these 6 complexes, determined by Evans method NMR studies, is tuned by several factors: (a) substituent present at the 5 position of the pyridine ring of the ligand, (b) present in the NC co-ligand, (c) solvent employed (), and (d) potentially also by speciation effects. In CDCN, for the pair of NC = NC complexes, when = H, the complex is practically LS (extrapolated ∼624 K), whereas when = Br, it is far lower (373 K), which implies a higher field strength when = H than when it is Br. The same trend, = H results in a higher apparent than = Br, is seen for the other two pairs of complexes, with = Se (429 > 351 K, Δ = 78 K) or S (361 > 342 K, Δ = 19 K). For the family of three = Br complexes, the change of from BH (373 K) to Se (351 K) to S (342 K) leads to an overall Δ(apparent) = 31 K, whereas the decreases are far more pronounced in the = H family (BH ∼624 > Se 429 > S 361 K). Changing the solvent used from CDCN to (CD)CO and CDNO, for [Fe(H)(NCE)] with either = BH or S, revealed excellent, and very similar, positive linear correlations ( = 0.99) of increasing solvent polarity index (from 5 to 7) with increasing apparent of the complex ( = BH gave 300 < 373 < 451 K , Δ = 151 K; = S gave 288 < 342 < 427 K, Δ = 147 K). Several other solvent parameters were also correlated with the apparent of these complexes ( = 0.74-0.96). Excellent linear correlations ( = 0.99) are also obtained with the coordination ability () of the three NCE co-ligands with the apparent in both families of compounds, [Fe(H)(NCE)] where X = H or Br. The N NMR chemical shifts of the nitrogen atom in the three NCE co-ligands (direct measurement) show modest correlations ( = 0.74 for family and 0.80 for family) with the apparent values of the corresponding complexes.