Thermally Two-stepped Spin Transitions Induced by Intramolecular Electron Transfers in a Cyanide-bridged Molecular Square

Abstract

A cyanide-bridged molecular square, [Co 2 Fe 2 (CN) 6 (tp*) 2 -(dtbbpy) 4 ](PF 6 ) 2 ¢2MeOH (1) (tp* = hydrotris(3,5-dimethylpyrazol-1-yl) borate and dtbbpy = 4,4¤-di-tert-butyl-2,2¤-bipyridine) was prepared by the reaction of (Bu 4 N) [Fe(CN) 3 (tp*)] with Co(CF 3 SO 3 ) 2 ¢6H 2 O and dtbbpy in methanol. Variable temperature X-ray structural analyses, magnetic and spectroscopic data revealed that 1 exhibited novel thermal two-stepped spin transitions induced by intramolecular electron transfers.Prussian blue analogs (PBAs) are 3-D bulk materials with face-centered cubic structures, where metal ions with various electronic and oxidation states are bridged by cyanide ions. Cyanide ions mediate electronic and magnetic interactions between metal ions, some of which allow PBA to exhibit fruitful physical properties, such as high T c magnets, spincrossover, linkage isomerism, and ferroelectric properties. 3 On the other hand, cyanide-bridged multinuclear complexes have core structures similar to a constituent unit of PBA, and such complexes have potential to show specific molecular functions, such as single-molecular magnetism, multistepped spin-crossover, and multistepped redox behavior. 4Dunbar and co-workers have reported the first observation of gradual CTIST behavior in a discrete molecule, that is, a cyanide-bridged pentanuclear FeCo cluster with a trigonalbipyramidal core. 5 Although there are few reports on other complexes exhibiting one-step CTISTs, 6 to the best of our knowledge, there have been no reports of the two-stepped CTIST behavior in both bulk and discrete molecular systems. We report here a novel cyanide-bridged molecular square of [Co 2 Fe 2 (CN) 6 (tp*) 2 (dtbbpy) 4 ](PF 6 ) 2 ¢2MeOH (1) (tp* = hydrotris(3,5-dimethylpyrazol-1-yl) borate, dtbbpy = 4,4¤-di-tert-butyl-2,2¤-bipyridine), exhibiting a two-stepped CTIST.1 was prepared by the reaction of (Bu 4 N)[Fe III (CN) 3 (tp*)] with Co II (CF 3 SO 3 ) 2 ¢6H 2 O and dtbbpy in methanol. 7 X-ray crystal structure analysis for 1 was performed at 100 K and an ORTEP diagram of the complex cation is depicted in Figure 1. 8 1 has a square-shaped macrocyclic core, in which Fe and Co ions are alternately bridged by cyanide ions. 1 crystallized in a monoclinic space group C2/c and the asymmetric unit is composed of half of the complex cation, [(tp*)Fe(CN) 3 Co-(dtbbpy) 2 ]. The Fe ions are coordinated by tridentate tp* with facial configuration and the remaining coordination sites are occupied by three cyanide carbon atoms.The two bidentate dtbbpy ligands coordinate to the Co ion, and bridging cyanide ions in cis-positions are linked to the neighboring [(tp*)Fe] units. The coordination bond lengths about the Co ion at 100 K are in the range of 1.892(7) 1.944(6) ¡, characteristic of LS Co(III) ions. The average coordination bond length around the Fe centers is 1.959(7) ¡, which corresponds to a LS Fe(II) or LS Fe(III) ion. Although the assignments of oxidation states of LS Fe(II) and Fe(III) ions are difficult only from the structural data, Mössbauer measu...

Redox‐Assisted Self‐Assembly of a Water‐Soluble Cyanido‐Bridged Mixed Valence {CoIII/FeII}2 Square

“…The latter, in unbuffered aqueous solution, shows two redox waves with close potentials; the two iron centers (590 and 710 mV) are not equivalent, despite having very similar reduction potentials (see Figure b, solid line). The separation of 50–60 mV between the two anodic and cathodic peaks confirms one electron processes, and the separation between the two signals (120 mV) produces a comproportionation equilibrium constant of 1×10 2 , comparatively small when compared to the values for similar mixed valence complexes . The positive shifts of the potential with respect to free hexacyanidoferrate are consistent with inductive effects of the Co III moieties attached to the center.…”

“…The separation of 50-60 mV between the two anodic and cathodic peaks confirmso ne electron processes, and the separation between the two signals (120 mV) produces ac omproportionation equilibrium constanto f1 10 2 ,c omparatively small when compared to the values for similar mixed valencec omplexes. [12,48,66] The positive shifts of the potential with respect to free hexacyanidoferrate [67] are consistent with inductive effectso ft he Co III moieties attached to the center.I nterestingly,w hen the same electrochemical experiment is carried out in 2 m HClO 4 ,t he two signals of the iron center coalesce with acathodic-anodic signalseparationofapproximately 100 mV at 815 and 910 mV (Figure 1b,d ashed line). It is clear that, on sequential protonation of the terminal cyanidol igands, the two iron centers become formally equivalent owing to positive induction caused by the change to Fe II -CNH groups,and the comproportionation equilibrium constant is drastically reduced.…”

“…[16,[28][29][30][31][32][33][34][35][36][37] The establishmento fs tructure-property relationships for the latter,a sw ell as their theoretical treatment, are facilitated by the fact that the structure is finite and the metal-coordination environments are well defined. In particular,an umber of interesting examples of Prussian blue square complexes {Fe 2 Co 2 }h ave been reported, [34,[38][39][40][41][42][43][44][45][46][47][48] because they usually exhibit interesting switchable opticala nd magnetic properties arisingf rom electron-transfer intramolecular processes.…”

Cyanide‐Bridged Molecular Squares – The Building Units of Prussian Blue

“…However, the Oshio group recently reported the first example of an ETCST-active complex that showed two-step behavior, and a metastable intermediate state. [49] An X-ray structural analyses conducted at temperatures corresponding to the IM and HT phases showed that the crystallographic space groups remained the same (C2/c), but that the bonding distances around the metal ions changed markedly. In the HT phase the Co and Fe ions had average bonding distances of 2.113 and 1.964 Å, respectively, suggesting complete ETCST from the LT phase to the HT phase.…”

Electric‐Field Control of Magnetic and Polarizability Properties of Cyanide‐Bridged Fe–Co Clusters

“…The observed broad thermal hysteresis of 18 K reveals the first‐order phase transition associated with the electron‐transfer process. Another important result was reported by Oshio and co‐workers, who used the compound {[(Tp*)Fe III (CN) 3 ] 2 [Co II (dtbbpy) 2 ] 2 }[PF 6 ] 2 · 2MeOH (dtbbpy = 4,4′‐di‐ tert ‐butyl‐2,2′‐bipyridine) to demonstrate the possible transfer of the electron‐transfer phenomenon and the associated properties from the solid state to solution , . Recently, the dinuclear cyanide‐bridged [{(Tp)Fe(CN) 3 }{Co‐(PY5Me 2 )}][CF 3 SO 3 ] (Tp = tripyrazolylborate) complex was obtained and demonstrated to exhibit charge‐transfer‐induced electron transfer .…”