Spin State Variations and Spin‐Crossover in Diiron(II) Complexes of Bis(pentadentate) Pyrazolate‐Based Ligands. Issue 14 (28th June 2018)

• Record Type: Journal Article
• Authors: Schober, Anne; Demeshko, Serhiy; Meyer, Franc
• Is Part Of: Zeitschrift für anorganische und allgemeine Chemie. Volume 644:Issue 14(2018)
• Page Start: 719
• Page End: 728
• Publication Date: 2018-06-28
• Subjects: Iron complexes -- Iron -- Pyrazolate ligands -- Bridging ligands -- Dinuclear complexes -- Spin‐crossover; Chemistry, Inorganic -- Periodicals; Chemistry -- Periodicals; 540
• DOI: 10.1002/zaac.201800148
• Languages: English
• ISSNs: 0044-2313

Abstract

Abstract : Compartmental pyrazolato‐bridged ligands [L Py ] − and [L Im ] −, each providing two {N5 } binding sites but differing in the terminal N‐donors (pyridine vs. imidazole), were used to prepare a set of diferrous complexes; the ligand HL Im is new and its synthesis is reported. Complexes [L Py Fe2 (MeCN)2 ](OTf)3, (1 OTf ), [L Py Fe2 (MeCN)2 ](BF4 )3 (1 BF4 ), [L Im Fe{Fe(MeCN)}](OTf)3 (2 OTf ), and [L Im Fe{Fe(MeCN)}](BF4 )3 (2 BF4 ) were characterized by single‐crystal X‐ray diffraction, revealing a MeCN coligand at both (1 OTf, 1 BF4 ) or at only one (2 OTf, 2 BF4 ) of the two accessible Fe coordination sites that are directed into the bimetallic cleft. Magnetic measurements (SQUID) and 57 Fe Mößbauer spectroscopy indicate low‐spin states for the six‐coordinate Fe II ions in1 OTf and1 BF4 that are based on [L Py ] −, but two high‐spin Fe II ions in2 BF4 that is based on [L Im ] −, in agreement with structural data. Complex2 OTf shows abrupt spin‐crossover (SCO) from a mixed high‐spin/low‐spin to an all high‐spin state at T ½ = 264 K, which appears to be correlated with the Fe–N≡C(Me) bonding angle. SCO, though gradual, is observed for2 OTf also in solution ( T ½ = 273 K in MeCN, 248 K in EtCN), which was studied by Mössbauer spectroscopy of frozen solution samples, variable temperature UV/Vis spectroscopy and SQUID magnetometry; the latter methods consistently gave thermodynamic parameters Δ H = 24.2 kJ· mol –1, Δ S = 89 J· mol –1 · K –1 for the SCO of2 OTf in MeCN, and Δ H = 19.1 kJ· mol –1, Δ S = 77 J· mol –1 · K –1 in EtCN. Abstract :