Reversible Electron Transfer Coupled to Spin Crossover in an Iron Coordination Salt in the Solid State

Abstract

Mössbauer spectroscopy of a series of bis(a-diimine)iron complexes has recently unequivocally shown that these distorted tetrahedral complexes with a triplet ground state contain a high-spin ferrous ion (S Fe = 2) and two monoanionic p radicals LC À (S rad = 1/2, Scheme 1), which are intramolecularly antiferromagnetically coupled to the iron ion to yield the observed S t = 1 ground state.[1] The electronic structure of these complexes cannot be described as species containing a low-valent iron(0) center (S Fe = 1, d 8 ) and two neutral adiimine ligands as proposed [2] in the literature. This [1] and other [3, 4] recent studies have clearly established that adiimines are noninnocent ligands with three accessible oxidation levels L 0 , LC À , and L 2À, which can be clearly discerned by their differing CÀC and CÀN bond lengths (Scheme 1).In an attempt to synthesize such [FeL 2 ] complexes with two different a-diimines (heteroleptic complexes), we prepared the extremely air-sensitive complex [Fe II (pda 2À )(dme) 2 ] (1; pda 2À = N,N'-bis(pentafluorophenyl)-o-phenylenediamido(2À) ligand, dme = dimethoxyethane). When 1 reacted with 1 equivalent of N,N'-bis(phenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene (dad 0 ), the EI mass spectrum indicated the presence of [Fe(pda)(dad)], but the isolated main product turned out to be black crystals of [Fe(dad) 3 ][Fe(pda) 2 ]·-(toluene) (2; Figure 1). When the ratio dad:1 was changed to 3:2, the yield of 2 increased to 43 %. As complex 2 possesses a very interesting temperature-dependent electronic structure (see below), its molecular structure was determined by singlecrystal X-ray crystallography at 120 and 270 K.[5]The low-temperature structure of 2 consists of a cationanion pair (Figure 2). Both the octahedral cation and the distorted tetrahedral anion contain one central iron ion; the charges of these ions are not specified at this point, because both dad and pda are noninnocent ligands. As shown in Scheme 1, their respective oxidation level can be determined by X-ray crystallography. The three dad ligands in 2 have long CÀC bonds (av 1.473(4) ) and short CÀN bonds (av [*] Dr.

How to Switch Spin‐Crossover Metal Complexes at Constant Room Temperature

“…The above examined one‐electron ligand‐based reduction proceeds in solution, and thus a redox‐induced SCO at room temperature was achieved in solution . Indications that a redox‐event at the ligand might trigger a SCO in the solid state were reported as well, albeit at lower temperatures . It is worth mentioning that numerous ligands in SCO metal complexes possess extended π‐systems with heteroatoms.…”

Characterization of Three Members of the Electron‐Transfer Series [Fe(pda)2]n (n=2−, 1−, 0) by Spectroscopy and Density Functional Theoretical Calculations [pda=Redox Non‐innocent Derivatives of N,N′‐Bis(pentafluorophenyl)‐o‐phenylenediamide(2−, 1.−, 0)]

“…[9] This salt possesses a complicated temperature-dependent electronic structure (Scheme 2). At low temperature, 3…”

Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical Complex

“…Examples of SCO in metal complexes containing radical ligands that are directly bound to the metal ions are rather rare, presumably due to difficulties in identifying suitable target molecules, and in preparing these complexes . One of us has initiated a research endeavor to synthesize such complexes and to study the influence of the coordinated ligand radical on the SCO properties of the metal ion.…”

“…[3,19,20] Examples of SCO in metal complexes containing radical ligandst hat are directly bound to the metal ions are rather rare, presumably due to difficulties in identifying suitable targetm olecules, and in preparing these complexes. [21][22][23][24][25][26][27][28] One of us has initiated ar esearch endeavor to synthesize such complexes and to study the influence of the coordinated ligand radicalo nt he SCO properties of the metal ion. Thus we succeeded in demonstratingt he occurrence of SCO in an octahedral Fe II bis(imino)acenaphthalene radicalc omplex [29] as well as in an octahedral Co II semiquinonate radicalcomplex.…”