Heteroleptic iron(ii) complexes of chiral 2,6-bis(oxazolin-2-yl)-pyridine (PyBox) and 2,6-bis(thiazolin-2-yl)pyridine ligands – the interplay of two different ligands on the metal ion spin sate

Abstract

Complexation of Fe[ClO4]2·6H2O by 1 equiv 2,6-bis((4S)-4-phenyl-4,5-dihydrooxazol-2-yl)pyridine ((S)-L1Ph) and 2,6-bis((4R)-4-phenyl-4,5-dihydrothiazol-2-yl)pyridine ((R)-L2Ph) cleanly affords [Fe((S)-L1Ph)((R)-L2Ph)][ClO4]2; [Fe((R)-L1iPr)((S)-L2iPr)][ClO4]2 (L1iPr = 2,6-bis(4-isopropyl-4,5-dihydrooxazol-2-yl)pyridine; L2iPr = 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine) was prepared by a similar route. The compounds exhibit...

Selective Pathway toward Heteroleptic Spin-Crossover Iron(II) Complexes with Pyridine-Based N-Donor Ligands

“…Our failed attempts to obtain a heteroleptic iron­(II) complex by a simple mixing of 3-bpp and tpy ligands with an iron­(II) salt, which successfully produced their cobalt­(II) analogues or other heteroleptic complexes after proper purification, − ,− ,, prompted us to devise an alternative synthetic pathway. A ligand substitution is often the method of choice; , however, it is not the best one for the weak-field 3-bpp ligands .…”

“…A new synthetic procedure is devised to selectively produce previously elusive heteroleptic iron­(II) complexes with the N -donor ligands from two popular familiesterpyridines (tpy) and bis­(pyrazol-3-yl)­pyridines (3-bpp)that stabilize the opposite spin states of the metal ion. This approach accommodates a wider scope of substrates than the simple mixing of ligands and a metal salt often used for this purpose − ,− ,, or the ligand substitution . As compared to other synthetic pathways, − the stepwise introduction of the ligands proceeds selectively because of a good solubility of the intermediate pentacoordinate complex with the triflate ligands; the latter does not have to be isolated from the reaction.…”

“…The proposed ligand design with ortho-functionalized N -aryl groups (Scheme ) afforded the first SCO-enabling N , N ′-disubstituted 3-bpp suitable for tailoring SCO behavior in heteroleptic complexes. The latter typically contain at least one ligand known to induce an SCO in homoleptic complexes. − ,− The effect of the co-ligand on the SCO behavior can be screened, even in a mixture of paramagnetic products, , by various approaches − of variable-temperature NMR spectroscopy. , Among others, they allowed identifying the SCO activity in heteroleptic cobalt­(II) complexes obtained by simply mixing 3-bpp and tpy ligands …”

“…As previously suggested, a heteroleptic complex is a kinetic product, while a homoleptic complex is thermodynamically favored. Although the former is often obtained − ,− ,, by mixing of two ligands and a metal salt followed by purification, this method can only be applied if the heteroleptic complex is the major product, and hence, there is no thermodynamic control of the reaction. Otherwise, another approach is needed that provides the kinetic control, such as the ligand substitution , or a stepwise introduction of the ligands into the coordination sphere of the metal ion through the formation of an intermediate pentacoordinate complex with chloride or acetylacetonate ligands. − Both these approaches are, however, not suitable for 3-bpp as a weak-field ligand that produces low-soluble pentacoordinate complexes …”

Improving spin crossover characteristics in heteroleptic [FeIII(qsal-5-I)(qsal-5-OMe)]A complexes

“…For this reason, interest in exploring heteroleptic systems is growing. [12][13][14][15][16][17][18][19][20][21] These systems allow the combination of ligands with different functional groups, that can subtly alter or even improve the SCO features. In some cases, these systems show intermediate properties, such as the T 1/2 as found in [Fe(qsal-Cl)(qsal-Br)]A•sol, (A = NCS − , solv = MeOH; PF 6 − , BPh 4 − , sol = DCM, OTf − , sol = 0.5MeOH).…”