The influence of the spin state of nearest neighbours on the spin transition of a given centre has been modelled with density functional theory methods for the linear oligomeric Fe(ii) complexes of 4-amino-1,2,4-triazole ligands. The calculated parameter Hcoop which is equal to the difference in the LS → HS spin transition energy for a centre with two LS and two HS spin nearest neighbours, respectively, was found to be always positive, varying from 15 to 35 kJ mol(-1) depending on the applied model and exchange-correlation functional. On the other hand Hcoop was found to be of negligible value for a model of a linear Fe(ii) complex of the more flexible alkylene-bis-tetrazole ligands. This corresponds well to the observed cooperative behaviour of the spin transition for the complexes of triazole derivatives and the gradual transition for the above bis-tetrazole ligands. The analysis of the bond distances in the optimised structures points towards elongation of the Fe-N bonds of the iron centre upon changing of the spin of the neighbours from low-spin to high-spin. This effect is related to the rigid nature of the bridging triazole ligand and is assumed to be the primary reason for the observed cooperativity. The presence of the next-to-nearest neighbour effects has been inferred. The effect of the softening of the Fe-N bonds in the LS centre distorted by the presence of the HS neighbours is also observed and its entropic effects are discussed.

The LytB/IspH protein catalyzes the last step of the methylerythritol phosphate (MEP) pathway which is used for the biosynthesis of essential terpenoids in most pathogenic bacteria. Therefore, the MEP pathway is a target for the development of new antimicrobial agents as it is essential for microorganisms, yet absent in humans. Substrate‐free LytB has a special [4Fe‐4S]2+ cluster with a yet unsolved structure. This motivated us to use synchrotron‐based nuclear resonance vibrational spectroscopy (NRVS) in combination with quantum chemical‐molecular mechanical (QM/MM) calculations to gain more insight into the structure of substrate‐free LytB. The apical iron atom of the [4Fe‐4S]2+ is clearly linked to three water molecules. We additionally present NRVS data of LytB bound to its natural substrate, (E)‐4‐hydroxy‐3‐methylbut‐2‐en‐1‐yl diphosphate (HMBPP) and to the inhibitors (E)‐4‐amino‐3‐methylbut‐2‐en‐1‐yl diphosphate and (E)‐4‐mercapto‐3‐methylbut‐2‐en‐1‐yl diphosphate.

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(βAlatrz)3](BF4)2⋅2 H2O (1⋅2 H2O), whose precursor βAlatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a β-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), (57)Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two-step spin crossover (T1/2(↓) = 230 K and T1/2(↑) = 235 K, and T1/2(↓) = 172 K and T1/2(↑) = 188 K, respectively) is registered for the first time for a 1,2,4-triazole-based Fe(II) 1D coordination polymer. The two-step SCO configuration is observed in a 1:2 ratio of low-spin/high-spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1⋅2 H2O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low-spin and high-spin states of 1⋅2 H2O. Vibrational spectra of 1⋅2 H2O reveal that the BF4(-) anions and water molecules play no significant role on the vibrational properties of the [Fe(βAlatrz)3](2+) polymeric chains, although non-coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(βAlatrz)3](BF4)2 (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated.

The vibrational properties of the cationic spin crossover (SCO) coordination polymers [{Fe(4-amino-1,2,4-triazole)(3)}(+2)](n) containing the anions chlorine, methanosulfonate and 1-naphthalenesulfonate have been studied via nuclear inelastic scattering of synchrotron radiation (NIS) as well as by Raman spectroscopy. Although the different anions have a strong influence on the spin crossover temperature, they have little effect on the positions of the spin marker bands in the NIS and Raman spectra. By comparing the line positions of the NIS spin marker bands with those observed by Raman spectroscopy, it has been possible to distinguish vibrations symmetry (A(u) or A(g)) because modes of A(u) and A(g) symmetries are NIS active, but only the A(g) modes are Raman active. The normal mode analysis of charge compensated cationic pentameric and hexameric model structures which have been obtained by density functional calculations reproduces the experimentally observed mode frequencies and the geometry optimization reproduces iron-ligand distances reported for these and related SCO coordination complexes. The effect of charge compensation appears to be independent of the choice of the functional and the basis set which shows that DFT calculations using B3LYP in conjunction with the basis set CEP-31G are a time effective approach in order to study vibrational properties of Fe(II) SCO compounds.

The vibrational properties of the trimeric iron complex [Fe(3)(4-(2'-hydroxy-ethyl)-1,2,4-triazole)(6)(H(2)O)(6)](CF(3)SO(3))(6) which serves as a model of the 1D iron coordination polymers based on 1,2,4-triazoles have been investigated by nuclear inelastic scattering of synchrotron radiation (NIS), as well as by Raman and infrared (IR) spectroscopy. The system reveals a soft spin crossover involving only the central iron atom with its FeN(6) core, while the terminal FeN(3)O(3) units show no spin transition. The NIS spectra of the central low-spin isomer exhibit a number of marker bands in the 350-450 cm(-1) region which have not been detected in the Raman spectra. The density functional theory (DFT) calculations allowed the assignment of these bands to Fe-N bending and stretching modes. A characteristic high-spin marker mode has been identified and discriminated from the iron-ligand modes of the terminal iron atoms. This characteristic central Fe-N mode has been observed experimentally at 245 cm(-1) and theoretically at 255 cm(-1). Contrary to mononuclear centrosymmetric Fe complexes, some of the symmetric vibrations of the trimeric complex involving iron movements are observed by NIS. Furthermore the DFT calculations displayed the importance of the coulombic repulsion between metal ions for the geometry and stability of a given spin isomer.