Variable-Temperature Studies of Laser-Initiated 5T2 → 1A1 Intersystem Crossing in Spin-Crossover Complexes: Empirical Correlations between Activation Parameters and Ligand Structure in a Series of Polypyridyl Ferrous Complexes

Abstract

Results are presented from a variable-temperature solution-phase laser photolysis study of the 5T2 → 1A1 intersystem crossing in a series of related complexes: [Fe(tpen)](ClO4)2, [Fe(tppn)](ClO4)2, [Fe(tptn)](ClO4)2, [Fe(t-tpchxn)](ClO4)2, and [Fe(dpa)2](ClO4)2. The hexadentate ligands are formed with four 2-pyridylmethyl arms attached to ethylenediamine (tpen), 1,2-diaminopropylene (tppn), 1,3-diaminopropylene (tptn), or trans-1,2-diaminocyclohexane (t-tpchxn). The dpa ligand is a tridentate analogue of these ligands, namely, bis(2-pyridylmethyl)amine. The complex [Fe(mtpen)](ClO4)2·2/3H2O, where mtpen is the same as tpen except one of the pyridyl rings has a 6-methyl substituent, crystallizes in the space group C2/c, which at 173 K has a unit cell with a = 41.390(13) Å, b = 9.5239(16) Å, c = 24.016(6) Å, β = 108.24(3)°, and Z = 12. Refinement with 2844 observed [F > 5.0σ(F)] reflections gave R = 0.075 and Rw = 0.076. The complex [Fe(tppn)](ClO4)2·2/3H2O crystallizes in the space group P21/n, which at 296 K has a unit cell with a = 12.979(4) Å, b = 12.624(4) Å, c = 19.475(6) Å, β = 108.17(2)°, and Z = 4. Refinement with 2357 observed [F > 5.0σ(F)] reflections gave R = 0.1198 and Rw = 0.1141. The mtpen complex is a high-spin FeII complex at all temperatures (4.2−400 K). The hydrated tpen complex is a spin-crossover complex with the temperature where there are 50% high-spin complexes (T1/2) is ≃ 385 K, the hydrated tppn complex is also spin-crossover with a higher T1/2 value, and the hydrated tptn complex is low spin up to 400 K. The present crystallographic results, together with previously reported structural results for the tpen complex at two temperatures, are used to show that the conversion from low spin to high spin leads to an increase in the trigonal twist of these distorted octahedral complexes. The influence of this variation in trigonal twist on the rate of 5T2 → 1A1 intersystem crossing is examined with variable-temperature laser-flash photolysis. Data collected for the tpen complex in MeOH in the 190−294 K range give a linear Arrhenius plot with an activation energy of Ea = 767 ± 22 cm-1 and a preexponential term of A = (1.35 ± 0.2) × 109 s-1. The tppn complex gives similar results of Ea = 771 ± 17 cm-1 and A = (1.45 ± 0.2) × 109 s-1. At 294 K the rate (k-1) for the 5T2 → 1A1 intersystem crossing is 2.87 × 107 s-1 for the tpen complex, and 3.21 × 107 s-1 for the tppn complex. On the other hand the tptn complex has k-1 = 6.25 × 108 s-1 at 295 K as measured with a picosecond spectrometer, and together with nanosecond data measured in the 186−210 K range gives an Arrhenius activation energy of Ea = 777 ± 50 cm-1 with A = (2.6 ± 0.8) × 1010 s-1. The bis(tridentate) complex [Fe(dpa)2](ClO4)2 in MeOH is found to give k-1 = 4.59 × 107 s-1 at 282 K and with the 191−282 K data gives Arrhenius values of Ea = 339 ± 13 cm-1 and A = (2.5 ± 0.25) × 108 s-1. The terpyridine complex [Fe(terpy)2](ClO4)2 is found to have k-1 = 1.0 × 108 s-1 in MeOH at 239 K, and the analysis of 190−239 K data gives Ea = 532 ± 36 cm-1 and A = (2.4 ± 0.4) × 109 s-1. Previous studies have shown that the greater the trigonal twist, the lower in energy is the 3T1 state which facilitates the spin−orbit interaction between the 5T2 high-spin and 1A1 low-spin states. It is suggested that the trigonal twist is a vibrational coordinate strongly coupled to the 5T2 → 1A1 intersystem crossing. It is additionally shown that the data are consistent with a model wherein the “intrinsic” rate of 5T2 → 1A1 intersystem crossing, as gauged by the preexponential term, is a function of how far along the reaction coordinate a complex proceeds.