Solution Spin Crossover Versus Speciation Effects: A Cautionary Tale
Sriram Sundaresan, Sally Brooker
Abstract
Two acyclic tetradentate Schiff base ligands, H L X–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 (H L X–OH )(NC E ) 2 ], with three different NCE co-ligands ( E = BH 3, 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 NC E co-ligand, (c) solvent employed ( P′ ), and (d) potentially also by speciation effects. In CD 3 CN, for the pair of NC E = NC BH 3 complexes, when X = H, the complex is practically LS (extrapolated T 1/2 ∼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, Δ T 1/2 = 78 K) or S (361 > 342 K, Δ T 1/2 = 19 K). For the family of three X = Br complexes, the change of E from BH 3 (373 K) to Se (351 K) to S (342 K) leads to an overall Δ T 1/2 (apparent) = 31 K, whereas the decreases are far more pronounced in the X = H family (BH 3 ∼624 > Se 429 > S 361 K). Changing the solvent used from CD 3 CN to (CD 3 ) 2 CO and CD 3 NO 2, for [Fe II (H L Br–OH )(NCE) 2 ] with either E = BH 3 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 = BH 3 gave T 1/2 300 < 373 < 451 K, Δ T 1/2 = 151 K; E = S gave T 1/2 288 < 342 < 427 K, Δ 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 (H L X–OH )(NCE) 2 ] where X = H or Br. The 15 N NMR chemical shifts of the nitrogen atom in the three NCE co-ligands (direct measurement) show modest correlations ( R 2 = 0.74 for L H–OH family and 0.80 for L Br–OH family) with the apparent T 1/2 values of the corresponding complexes.