Mössbauer Investigation of the Photoexcited Spin States and Crystal Structure Analysis of the Spin-Crossover Dinuclear Complex [{Fe(bt)(NCS)2}2bpym] (bt=2,2′-Bithiazoline, bpym=2,2′-Bipyrimidine)

The crystal structure of the complex [{Fe(bt)(NCS)2}2bpym] (1) (bt=2,2′‐bithiazoline, bpym=2,2′‐bipyrimidine) has been solved at 293, 240, 175 and 30 K. At all four temperatures the crystal remains in the P$\bar 1$ space group with a=8.7601(17), b=9.450(2), c=12.089(3) Å, α=72.77(2), β=79.150(19), γ=66.392(18)°, V=873.1(4) Å3 (data for 293 K structure). The structure consists of centrosymmetric dinuclear units in which each iron(II) atom is coordinated by two NCS− ions in the cis position and two nitrogen atoms of the bridging bpym ligand, with the remaining positions occupied by the peripheral bt ligand. The iron atom is in a severely distorted octahedral FeN6 environment. The average FeN bond length of 2.15(9) Å indicates that compound 1 is in the high‐spin state (HS–HS) at 293 K. Crystal structure determinations at 240, 175 and 30 K gave a cell comparable to that seen at 293 K, but reduced in volume. At 30 K, the average FeN distance is 1.958(4) Å, showing that the structure is clearly low spin (LS–LS). At 175 K the average FeN bond length of 2.052(11) Å suggests that there is an intermediate phase. Mössbauer investigations of the light‐induced excited spin state trapping (LIESST) effect (λ=514 nm, 25 mW cm−2) in 1 (4.2 K, Hext=50 kOe) show that the excited spin states correspond to the HS–HS and HS–LS pairs. The dynamics of the relaxation of the photoexcited states studied at 4.2 K and Hext=50 kOe demonstrate that HS–HS pairs revert with time to both HS–LS and LS–LS configurations. The HS–LS photoexcited pairs relax with time back to the ground LS–LS configuration. Complex [{Fe0.15Zn0.85(bt)(NCS)2}2bpym] (2) exhibits a continuous spin transition centred around 158 K in contrast to the two‐step transition observed for 1. The different spin‐crossover behaviour observed for 2 is due to the decrease of cooperativity (intermolecular interactions) imposed by the matrix of ZnII ions. This clearly demonstrates the role of the intermolecular interactions in the stabilization of the HS–LS intermediate state in 1. Crossover to the other spin: The crystal structure determination of the two‐step spin‐crossover system [{Fe(bt)(NCS)2}2bpym] (shown here) at 293, 240, 175 and 30 K is reported. The nature of the metastable states and the dynamics of the relaxation after the light‐induced excited spin state trapping (LIESST) effect (λ=514 nm) in the two‐step dinuclear spin‐crossover complexes [{Fe(bt)(NCS)2}2bpym] and [{Fe0.15Zn0.85(bt)(NCS)2}2bpym] are also discussed