Toward Controlling the Solid State Valence Tautomeric Interconversion Character by Solvation

Abstract

Crystals of [Co(diox)2(4-NO2-py)2] (1) and [Co(diox)2(4-CN-py)2](2) where diox are the o-Dioxolene 3,5-di-tbutylsemiquinonate (SQ •-)and/or 3,5-di-t-butylcatecholate (Cat 2-)ions, 4-NO2-py is 4-nitro-pyridine, 4-CN-py is 4-cyano-pyridine, are among the few known crystals presenting both thermal-induced and photoinduced ls-[M +3 (SQ •-)(Cat 2-)] hs-[M 2+ (SQ •-)2]valence tautomerism interconversion (VTI). In 2 the thermal-induced VTI is cooperative, characterizing an abrupt conversion and in 1 it is non-cooperative. In this work, crystals of [Co(diox)2(4-NO2-py)2]•benzene (1BZ), [Co(diox)2(4-NO2-py)2]•toluene (1TL), [Co(diox)2(4-CN-py)2]•benzene (2BZ) and [Co(diox)2(4-CN-py)2]•toluene (2TL) have been prepared and analyzed by single crystal X-ray diffraction in order to investigate how solvation modulates thermal-induced VTI. The solvate crystals, like the nonsolvated ones, present essentially reversible thermal-induced and photoinduced VTI. 1TL crystal presents the same monoclinic symmetry and the same intermolecular hydrogen-bonded network of 1 and both present a non-cooperative thermal-induced VTI. 1BZ crystal has triclinic symmetry and present a cooperative and abrupt VTI with a thermal hysteresis of ~30 K. In contrast to 2, thermal-induced VTI in 2BZ and 2TL is non-cooperative despite the fact that 2, 2BZ and 2TL crystals exhibit the same monoclinic symmetry and the same intermolecular hydrogen-bonded network. In 2BZ and 2TLbenzene and toluene molecules as well as the tbutyl groups of the o-dioxolene molecules convert gradually from being dynamically disordered at about 300 K to a static disorder state below 150 K. The layer separation distance of interacting [Co(diox)2(4-X-py)2], X=CN and NO2, molecules in all solvate crystals is ~15 Å whereas in the 2, which presents cooperative VTI, it is ~12 Å. An order-disorder component might account to the stabilization of the metastable hs-Co 2+ state in 2BZ and in 2TL but no disorder was found in the 1TL crystals. Therefore, the lack of cooperativeness in the thermal-induced VTI in these crystals seems to be due to the large distance between the layers of interacting molecules. Cooperativeness in the VTI of 1BZ crystal is likely to be related with the unique molecular bond scheme network that connects neighboring active [Co(diox)2(4-NO2-py)2] molecules through the o-Dioxolene oxygen atoms bonded directly to the Co ion.

Valence‐Tautomerism‐Driven Aromatic Nucleophilic Substitution in Cobalt‐Bound Tetrabromocatecholate Ligands – Influence of Positive Charge at the Ligand Backbone on Phenoxazinone Synthase Activity

“…The two aromatic rings in the 3,5,6‐tribromo‐4‐pyridiniumcatecholate ligand are mutually perpendicular, as suggested by the dihedral angle between the pyridyl and phenyl rings of 86.98°, which is usual for such organic molecules , . The Co–N(pyridine) and average Co–O(catecholate) bond lengths in 1 of 1.948(8) and 1.885(6) Å, respectively, are comparable with those in previously reported cobalt(III) catecholate complexes with similar coordination environments . The C–O and ring C–C (bearing catecholate –OH groups) bond lengths of the coordinated o ‐dioxolene ligand provide useful information regarding the oxidation state of the coordinated redox‐active dioxolene ligand.…”

“…1.34 Å). Similarly, the C–C bond lengths for the semiquinonate and catecholate states usually vary in the ranges 1.45–1.48 and 1.36–1.44 Å, respectively . In the present complex, the C–O and ring C–C bond lengths of 1.353(11) Å (average value) and 1.395(15) Å, respectively, indicate that the dioxolene ligand is in the catecholate state.…”

Use of Crown Ether Functions as Secondary Coordination Spheres for the Manipulation of Ligand–Metal Intramolecular Electron Transfer in Copper–Guanidine Complexes

“…[11] For example, variations in the number of unpaired electrons accompanied by this electron transfer couldb eu sed for the design of switchable magnetic devices, [12] and also allow the tuning of other important materialp roperties such as phase transitions. [13,14] In the past,e xamples for intramolecular ligand-metal electron transfer stimulated by physicalp arameters (temperature, [15][16][17][18] pressure or light irradiation [19][20][21] )w erer eported. [22] Moreover, chemists found ways to trigger ligand-metalI ET by (reversible) chemicalr eactions, for example, at ar emote part of the redoxactive ligand or at the co-ligands attached to the metal.…”

Solvent Control of Ligand–Metal Electron Transfer in Mononuclear Copper Complexes with Redox‐Active Bisguanidine Ligands

“…for copper containing enzymes especially galactose oxidase and copper amine oxidases). Particularly interesting are cases in which two or more energy minima of different electronic structure are obtained that could be interconverted by stimulated electron‐transfer between the metal and the ligand either by temperature changes or by light or pressure changes (especially in the case of Co complexes) The two minima that differ in the charge state of the ligand(s) and/or the metal oxidation number, are denoted valence tautomers or redox isomers. To ensure that structures that differ in their electron distribution are indeed isomers, a sufficiently high barrier for intramolecular electron transfer must be present.…”