Cyanide-bridged decanuclear cobalt-iron cage

: A cyanide-bridged decanuclear [Co 6 Fe 4 ] cluster was synthesized by one-pot reaction, and the magnetic properties and electronic configuration were investigated. The complex displayed thermally-controlled electron-transfer-coupled spin transition (ETCST) behavior between Co(III)Low-Spin-NC-Fe(II)Low-Spin and Co(II)High-Spin-NC-Fe(III)Low-Spin states as confirmed by single crystal X-ray, magnetic and Mössbauer analyses. Functional molecular materials have great promise as inno-vative chemical compounds for the development of future electronic devices and medical products. 1 The controlled construction of highly organized molecular architectures is a necessity if species exhibiting desirable characteristics are to be reliably isolated. Synthetic techniques in supramolecular chemistry employ a variety of chemical bonds and molecular interactions, the organization of which depend on self-assembly. 2 Various molecular architectures have been constructed using ligands with strictly defined coordination modes in conjunction with the flexible coordination envi-ronments of transition metal ions. Cage-like compounds, 3 grid complexes, 4 metal wires, 5 rotaxanes, 6 and catenanes 7 are prime examples of self-assembled species, and show varied physical properties. Studies on cage compounds have fo-cused on the templating and molecular interactions of guest molecules captured in the cavity, 8 while grid complexes have attracted a great deal of attention as molecular units for nanoscale electric devices due to the controllability of their electronic states. 9 Metal wires can exhibit interesting metal-metal bonds resulting from careful molecular design, 10 and the

Functional molecular materials have great promise as innovative chemical compounds for the development of future electronic devices and medical products. 1 The controlled construction of highly organized molecular architectures is a necessity if species exhibiting desirable characteristics are to be reliably isolated. Synthetic techniques in supramolecular chemistry employ a variety of chemical bonds and molecular interactions, the organization of which depend on selfassembly. 2 Various molecular architectures have been constructed using ligands with strictly defined coordination modes in conjunction with the flexible coordination environments of transition metal ions. Cage-like compounds, 3 grid complexes, 4 metal wires, 5 rotaxanes, 6 and catenanes 7 are prime examples of self-assembled species, and show varied physical properties. Studies on cage compounds have focused on the templating and molecular interactions of guest molecules captured in the cavity, 8 while grid complexes have attracted a great deal of attention as molecular units for nanoscale electric devices due to the controllability of their electronic states. 9 Metal wires can exhibit interesting metalmetal bonds resulting from careful molecular design, 10 and the dynamic behavior of rotaxanes and catenanes has been studied intensely with a view to their application in molecular machines. 11 The self-assembly of cage-like molecules has been thoroughly investigated by Fujita et al., who reported a M12L24 cage. 12 Similar transition metal cage systems have been reported, but the vast majority were constructed from the combination of rigid, 'pre-programmed' organic ligands and diamagnetic metal ions, such as Pd 2+ , Pt 2+ , Cd 2+ , and Zn 2+ . 3 We previously reported [Ni6Fe4] and [Co10] cage molecules, which were obtained by the self-assembly of chiral bidentate ligands with nickel or cobalt ions and hexacyano- Figure 1. The molecular (a) and core (b) structures of 1 at 100 K. The encapsulated tetraethylammonium cation is highlighted in yellow. Hydrogen atoms, counter anions outside the cage, and solvent molecules have been omitted for clarity. Color code: C, gray; N, light blue; Co II (HS), blue; Co III (LS), green; Fe II (LS), brown. metalate units. 13 The [Ni6Fe4] complexes show ferromagnetic interactions between nickel(II) and low spin iron(III) ions due to the orthogonality of their magnetic orbitals, and constitute a rare example of magnetic cage molecules. It is important for the development of future functional cage mole-cules to explore the physical properties and functional behavior accessible with such species.
In this work, we extended our study of cyanide-bridged cages to synthesize a [Co6Fe4] cage. Some cyanide bridged Co-Fe complexes are known to show electron-transfercoupled spin transition (ETCST) phenomena, and such species are expected to have applications in future nanoscale switching technologies. 14 (Figure 1). 15 Single crystal X-ray analysis reveals that 1 crystalized in the orthorhombic space group P212121, and has a cage structure, constructed from six [Co(L R )2] and four hexacyanoferrate units, within which one tetraethylammonium cation is encapsulated. The metal arrangement is adamantane-like in topology: very close to that of the reported [Ni6Fe4] cage molecules. 13 The cobalt ions have octahedral coordination geometries with four nitrogen donor atoms from two chiral bidentate ligands and two nitrogen donor atoms from two neighboring hexacyanoferrate units. All cobalt ions have -type conformations, stabilized by - stacking interactions between the phenyl moieties of the ligands in neighboring molecules. Considering the bond lengths around the cobalt centers and their distortion from perfect octahedral geometry at 100 K, three cobalt ions (Co1, Co2, and Co4) are +2 and high spin (dav 100 K(Co II -L) = 2.14 Å), while the other three (Co3, Co5 and Co6) are +3 and low-spin (dav 100 K(Co III -L) = 1.93 Å). Mössbauer data collected at 100 K suggest that all iron ions are +2 and low-spin (vide infra). The combined analyses suggest that compound 1 has a [Co II (HS)3Co III (LS)3Fe II (LS)4] electronic state at 100 K. Structural analysis at 291 K shows significant differences. Five cobalt ions (Co1-Co5) are divalent and high-spin (dav 291 K(Co II -L) = 2.12 Å), while one (Co6) remains trivalent and low-spin (dav 291 K(Co III -L) = 1.90 Å). X-ray structural analysis is of limited utility in differentiating between low spin Fe(II) and Fe(III) ions, but Mössbauer spectra at 300 K revealed their presence in a 1:1 ratio, indicating that two electron transfers lead to an electronic state of [Co II (HS)5Co III (LS)Fe II (LS)2Fe III (LS)2] at high temperature. The location of the Co(II) and Co(III) ions allow us to suggest the Fe(II) and Fe(III) positions based on charge repulsion considerations. Schematic representations of the metal ion positions are provided in Figure 2 (inset).
The magnetic susceptibility of a fresh sample of 1 was measured using a sealed quartz cell, and mT versus T plots are shown in Figure 2. Spin transition was observed around 250 K. At 300 K, the mT value of 12.42 emu mol -1 K is close to the expected value of 12.41 emu mol -1 K, estimated from the sum of the uncorrelated spins of five HS-Co II and two LS-Fe III ions (g = 2.21). Lowering the temperature caused the mT value to decrease, initially gradually, then more abruptly at around 250 K, before reaching a plateau below 180 K with an almost constant mT value of 6.83 emu mol -1 K, consistent with the value expected from three magnetically isolated HS-Co II ions (6.83 emu mol -1 K, g = 2.20). Below 100 K, the mT values decreased due to the orbital contribution of the Co(II) ions. The cooling and heating processes for the fresh sample were reversible to 300 K, and confirm the ETCST suggested by the structural data.   Table S1.
While over the relatively short time-scale of magnetic susceptibility measurements the phase transition was reversible, desolvation effects were apparent in the Mössbauer analyses. Measurements on a fresh sample of 1 conducted at 100 K revealed one doublet corresponding to LS-Fe(II) (IS = 0.0 mms -1 , EQ = 0.16 mms -1 ) consistent with the [Co II (HS)3Co III (LS)3Fe II (LS)4] assignment indicated by the susceptibility data (Figure 3). However, measurement at 300 K for one week caused irreversible transition to the high temperature phase. Solvent molecules in the crystal lattice can be removed by sample drying, and the formula of the dried sample was revealed to be (Et4N)2[{Co(L R )}6{Fe(CN)6}4](BF4) . 9H2O (1') by elemental analysis. The temperature dependent magnetic susceptibility of 1' is shown in Figure S1. The mT values increased from 1.8 to 300 K and no spin transition behavior was evident, with the [Co II (HS)5Co III (LS)Fe II (LS)2Fe III (LS)2] state stabilized across the full temperature range, a fact confirmed by Mössbauer spectra at 300 K and 100 K (Figure 3; for parameters see Table S1). Structural analysis of the fresh sample at 100 K indicates the existence of many hydrogen-bonding interactions between terminal cyanide groups of the cage and solvent molecules (MeOH and H2O). Hydrogen-bonded interactions affect the electronic states of hexacyanoferrate units by exerting an electron withdrawing effect that stabilizes the iron ions in their electron-rich divalent state. 14b,c Desolvation destabilizes the low temperature [Co II (HS)3Co III (LS)3Fe II (LS)4] phase, resulting in the loss of ETCST activity.
In summary, an ETCST-active decanuclear cobalt-iron cluster was synthesized by the reaction of a chiral bidentate ligand with a cobalt source and potassium hexacyanoferrate. Temperature modulation induced magnetic and structural transitions resulting from electron transfer between neighboring metal ions. Such magnetically switchable chiral cage molecules will be investigated for their selective host-guest behavior and specific magneto-optical functionalities.

ASSOCIATED CONTENT
Supporting Information X-ray crystallographic data in CIF format, and magnetic properties of dried sample 1'. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 982659 and 982660 contain the supplementary crystallographic data for 1 at 100 K and 291 K and can also be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Author Contributions
The manuscript was compiled from contributions by all authors.

Notes
The authors declare no competing financial interests.