Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover

Abstract

Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7tetrathiolate (TTFtt 2−) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt 2−. UV-vis-Near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt 2− π-manifold from the Fe(II)-centered spin-crossover. The TTFtt 2− centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character. Results and Discussion Synthesis and Structural Parameters Complex 1 was synthesized via reaction with the deprotected proligand 2,3,6,7-tetrakis(2cyanoethylthio)tetrathiafulvalene (TTFtt(C2H4CN)4) in good yield. Complex 1 was insoluble in all solvents we investigated which precluded detailed characterization but is pure as indicated by combustion analysis and behaves as a suitable synthon for subsequent chemistry. Complex 1 can be doubly oxidized with [Cp2Fe][BAr F 4] to form 2 which is more soluble, enabling common solution characterization including 1 H NMR and cyclic voltammetry measurements (Figure S1-S2). Oxidation from 1 to 2 could be ligandcentered (TTFtt 4− →TTFtt 2−), metal-centered (2 Fe(II)→2 Fe(III)), or some intermediate case, but the data acquired for 2 supports a TTFtt 2− structure arising from ligandcentered oxidation (Chart 1B, see below). Compound 2 was structurally characterized via singlecrystal X-ray diffraction (SXRD) at 293 K (2-HT; Figure S3) and 100 K (2-LT; Figure 1). In both structures TTFtt 2− is bridged between two TPA-capped Fe centers with two outer-sphere BAr F 4 − counter anions. The most striking difference between these temperatures is markedly longer Fe bond lengths in 2-HT. The Fe-Npyridine and Fe-Namine bond lengths in 2-LT are 1.958(6)-1.979(6) and 2.017(6) Å (Figure 1), respectively. These values are consistent with Fe-N bonds in other low-spin complexes with a Fe-TPA moiety. 16,17 In 2-HT, these bonds are 0.18-0.19 and 0.244(11) Å longer than their counterparts at 100 K, respectively, and are consistent with high-spin Fe-TPA complexes. The shorter Fe bonds at lower temperature indicate that 2 exhibits a temperature dependent spincrossover as observed in related compounds. 16,21

Understanding the factors governing the water oxidation reaction pathway of mononuclear and binuclear cobalt phthalocyanine catalysts

“…41 In this case, the existence of electronic interaction and delocalization between the halves of the Pc units in bi-CoPc makes the Co atoms interact with each other, and thus we think that bi-CoPc exhibited a racemization phenomenon. 41,42 Combining the results of XPS, we conclude that the chemical valence state of Co species in bi-CoPc are also +2. When immobilizing mono-CoPc or bi-CoPc molecules onto a carbon support, the hyperfine splitting phenomenon of Co 2+ is manifested in mono-CoPc/carbon (Fig.…”

Tuning spin-crossover transition temperatures in non-symmetrical homoleptic meridional/facial [Fe(didentate)3]2+complexes: what for and who cares about it?

“…Since precise chemical programming of spin state transitions are not accessible in solution, there is much less chance to propose some reliable design in solids, and major interest in the SCO community moved toward the systematic structural exploration of classes of ligands and complexes for designing addressable (via temperature, pressure, light) switches implemented in various materials (solids, films, nanoparticles, liquid crystals, etc.). 37,[48][49][50][51][52][53] The deliberate combination of SCO properties with other physical or chemical properties in a synergetic fashion in molecular entities, summarized under the term of multifunctionality, [54][55][56] remains in its infancy despite remarkable efforts focused on the modulation of magnetic communication 57,58 and luminescence. 59 For instance, the modulation of an emissive lanthanide by an adjacent SCO Fe II metallic site in an isolated dimetallic complex has been recently attempted ( Fig.…”

[2+2] Cycloadditions with an Iron Carbene: A Critical Step in Enyne Metathesis

“…We have been studying group 10 metal carbenes supported by [PC­(sp 2 )­P] ((bis­[2-(di-isopropyl­phosphino)­phenyl]­methane)) and discovered new reactivity patterns for late transition metal carbenes. − The rich reactivity observed for these carbenes − prompted us to expand our studies to iron − and probe the reactivity of the corresponding compounds. Herein, we report the synthesis of [{PC­(sp 2 )­P}­Fe­(N 2 )­(PMe 3 )] ( 1-N 2 -PMe 3 ) and its unprecedented [2+2] reactivity with alkynes to produce η 3 -vinyl carbene iron complexes, [{PC­(C Ph C­(sp 2 ) Ph }­Fe­(L)] ( 2-L , L = N 2 , PMe 3 , Figure E).…”