Predictable Electronic Tuning By Choice of Azine Substituent in Five Iron(II) Triazoles: Redox Properties and DFT Calculations

Abstract

Five new mononuclear iron(II) tris‐ligand complexes, and four solvatomorphs, have been made from the azine‐substituted 1,2,4‐triazole ligands (Lazine): [FeII(Lpyridazine)3](BF4)2 (1), [FeII(Lpyrazine)3](BF4)2 (2), [FeII(Lpyridine)3](BF4)2 (3), [FeII(L2pyrimidine)3](BF4)2 (4), and [FeII(L4pyrimidine)3](BF4)2 (5). Single‐crystal XRD and solid‐state magnetometry reveal that all of them are low‐spin (LS) iron(II), except for solvatomorph 5⋅4 H2O. Evans method NMR studies in CD2Cl2, (CD3)2CO and CD3CN show that all are LS in these solvents, except 5 in CD2Cl2 (consistent with L4pyrimidine imposing the weakest field). Cyclic voltammetry in CH3CN vs. Ag/0.01 m AgNO3 reveals an, at best quasi‐reversible, FeIII/II redox process, with Epa increasing from 0.69 to 0.99 V as the azine changes: pyridine< pyridazine<2‐pyrimidine<4‐pyrimidine< pyrazine. The observed Epa values correlate linearly with the DFT calculated HOMO energies for the LS complexes.

Extension of Azine-Triazole Synthesis to Azole-Triazoles Reduces Ligand Field, Leading to Spin Crossover in Tris-L Fe(II)

“…The [Fe II (L) 2 (NCE) 2 ] complexes of the R dpt /R at ligands (Figure , bottom left; E = −BH 3 , −S, −Se) are often SCO-active in both solution and the solid state . In contrast, the tris-L complexes, [Fe II (L) 3 ]­(BF 4 ) 2 , of these azine-triazole ligands are consistently LS, − a finding which extends to include the recently reported pairs of dinuclear helicates, [Fe II 2 ( L 2/4pym‑meta ) 3 ]­(BF 4 ) 4 and tetranuclear cages [Fe II 4 ( L 2/4pym‑meta ) 6 ]­(BF 4 ) 8 of new m - and p -phenylene-linked ditopic R at ligands (Figure , middle), all four of which are LS . While these helicates and cages also feature tris-R at binding pockets (which had always generated LS complexes as noted above), it had been hoped that the ligand field would be reduced sufficiently by the potential ligand strain inherent in linking these binding pockets together across the ends of the helicates or along the edges of the cages and by the use of 2/4-pyrimidine-triazole pockets (pyrimidine has the weakest ligand field of the diazines). ,, This was not found to be the case; therefore, in order to develop access to SCO-active supramolecular assemblies of such ligands, the ligand field imposed upon tris-ligand coordination to iron­(II) must be reduced further.…”

Quantitative and Chemically Intuitive Evaluation of the Nature of M−L Bonds in Paramagnetic Compounds: Application of EDA‐NOCV Theory to Spin Crossover Complexes

“…In contrast to the SCO‐active [Fe II ( L azine ) 2 (NCBH 3 ) 2 ] family, [5k] the related family of [Fe II ( L azine ) 3 ](BF 4 ) 2 complexes are all LS, [16] which implies that the replacement of two NCBH 3 − anions by one bidentate L azine ligand increases the ligand field experienced by the iron(II) centre. The Δ E orb,σ+π values (Figure 8) for [Fe( L azine ) 2 (NCBH 3 ) 2 ] (−335 kcal mol −1 ) and [Fe( L azine ) 3 ] 2+ (−368 kcal mol −1 ) show that replacement of 2x NCBH 3 − by one L azine leads to an increase in the stabilization (ΔΔ E orb,σ+π ) of −33 kcal mol −1 (Figure 8), which is consistent with the experimental observation that the [Fe( L azine ) 2 (NCBH 3 ) 2 ] family are SCO‐active whereas the [Fe( L azine ) 3 ] 2+ family are solely LS.…”

“…Using this protocol all of the calculated structures, for both the LS and HS complexes, are in good agreement with the available experimental X‐ray crystallographic data for the LS and HS states of the [Fe II ( L pyridine ) 2 (NCBH 3 ) 2 ] complex [19] (Table S3). The [Fe II ( L azine ) 3 ](BF 4 ) 2 complexes had been previously optimized by using the same protocol [16] . These sets of optimized structures were then used in single‐point calculations for the subsequent EDA‐NOCV analyses performed using the ADF program package (version 2018.106; please note that the ADF version used does not allow the inclusion of solvent effects when performing EDA‐NOCV) at the BP86‐D3(BJ)/TZ2P level of theory [20]…”

“… The two families of complexes studied here: a) five SCO‐active complexes, [Fe II ( L azine ) 2 (NCBH 3 ) 2 ], shown in order of increasing T 1/2 in CDCl 3 solution as a function of the azine, that is, position of the uncoordinated N (red): absent ( L pyridine ); or present in the 2‐position ( L 2pyrimidine ), 3‐position ( L pyrazine ), 4‐position ( L 4pyrimidine ), or 5‐position ( L pyridazine ) [5k] and b) five LS [Fe II ( L azine ) 3 ](BF 4 ) 2 complexes [16] …”

“…Finally, the optimized EDA‐NOCV protocol developed for the SCO‐active [Fe II ( L azine ) 2 (NCBH 3 ) 2 ] complexes was then used for the closely related [Fe II ( L azine ) 3 ](BF 4 ) 2 family of LS complexes, [16] where our calculations showed that three L azine ligands produce a stronger octahedral ligand field than a combination of 2 L azine +2 NCBH 3 , which is in line with experimental findings.…”