Solution Spin Crossover Versus Speciation Effects: A Cautionary Tale

Abstract

Two acyclic tetradentate Schiff base ligands, HL 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, [FeII(HL X–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 = NCBH 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, ΔT 1/2 = 78 K) or S (361 > 342 K, ΔT 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 ΔT 1/2(apparent) = 31 K, whereas the decreases are far more pronounced in the X = H family (BH3 ∼624 > Se 429 > S 361 K). Changing the solvent used from CD3CN to (CD3)2CO and CD3NO2, for [FeII(HL Br–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 , ΔT 1/2 = 151 K; E = S gave T 1/2 288 < 342 < 427 K, ΔT 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, [FeII(HL X–OH )(NCE)2] where X = H or Br. The 15N 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.

Iron(ii) complexes of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine and related ligands with annelated distal heterocyclic donors

“…The fitted thermodynamic data are typical for iron( ii ) complexes of N-donor ligands, 55,95,96 with no apparent contributions from other solution equilibria. 97,98 In contrast, 3′[BF 4 ] 2 ·H 2 O and 4[BF 4 ] 2 remain high-spin over the liquid range of the solvent.…”

Two-dimensional spin-crossover coordination polymers based on the 1,1,2,2-tetra(pyridin-4-yl)ethene ligand

“…26–30 The SCO temperature can be accurately controlled by ligand-field modulation (NCS < NCSe < NCBH 3 ) in both solid and solution states. 31–35 In the past two decades, a new class of (4,4)-connected SCO-CPs, with the formula [FeL 2 (NCX) 2 ]·solv, have been developed utilizing bidentate N-donor pillars. 36–40 The guest-accessible channels within the 2D skeleton enable various host–guest interactions mediated by H-bonding or π⋯π stacking, which enriches the toolkits for synergistic regulation via solvent molecules.…”

Synergetic spin singlet–quintet switching and luminescence in mononuclear Fe(ii) 1,3,4-oxadiazole tetradentate chelates with NCBH3 co-ligand

“…In the process of tuning the ligand field strength through ligand design, three NCE co-ligands, where E = BH 3 , Se and S, are commonly studied to tune the ligand field strength and are more commonly used in the solid state 23–26 and also used rarely in solution. 27,28 Tuning of the T 1/2 in the solid state is often governed by anions, lattice solvent effects and packing effects due to π-π interactions and hydrogen bonding interactions (also mediated by anions or lattice solvents) which makes it hard to enable predictable tuning. 29–33 These effects can be both advantageous (increase in cooperativity, thus more abrupt spin transition with hysteresis) and disadvantageous (SCO is dependent on these interactions and the complex is locked in one spin state) to the SCO phenomenon in the solid state.…”

“…26,30,34,35 In solution, packing effects are absent and the ligand field has the main influence, and some good linear correlations with different parameters such as the Hammett constants, 36 solvent polarity index ( P ′), 37,38 alkyl chain lengths, 37 choice of NCEs, and coordination ability parameter ( a TM ) are achieved. 28 The main drawback with the solution spin crossover studies is the speciation. The ligand systems most commonly studied in the SCO literature are bidentate and tend to speciate.…”

“…Moving to more robust ligands with higher denticity (chelate effect) might aid in reducing the speciation problem, but can never be ruled out as known from some recent solution studies. 28…”