Reports: AC3

44200-AC3 Investigation of Polynuclear Transition Metal Complexes with Spin Transition

Catalina Achim, Carnegie Mellon University

The current quest for increased capacity of information storage devices has fuelled intense research of molecule-based materials that can be bistable, i.e. that have two different states accessible within the same domain of environmental conditions. Numerous mononuclear coordination complexes of Fe(II) can adopt two different d-electron configurations, and consequently, two different spin states. Some of these complexes manifest bistability underscored by cooperativity based on strong intermolecular interactions exerted in solid state. 

Our research is focused on the investigation of molecules that can undergo spin transitions and contain multiple Fe(II) ions, and in which the transition can be affected by intramolecular interactions between the metal ions.  One of the type of molecules from this category that we study is that of pentanuclear, cyanide-bridged clusters {[M(tmphen)2]3[M'(CN)6]2} in which M and M' are transition metal ions, and tmphen is a bidentate 3,4,7,8-tetramethyl-1,10-phenanthroline.  The design of these clusters was inspired by Prussian Blue and is based on the use of tmphen, which acts as “capping” ligand that favors the formation of molecule-based materials over infinite, 3-D ones.  The clusters have a trigonal bipyramid core with three [M(tmphen)2(CN)2] ions in the equatorial positions and two [M'(CN)6] ions situated in the axial positions.  The complexes are prepared in the laboratory of our collaborator, Professor Kim Dunbar at Texas A&M.  In past research, we studied clusters in which M/M' are Zn/Cr, Zn/Fe, Fe/Fe, Fe/Co, Co/Fe, and Fe/Cr and have shown

– the manifestation of a charge transfer induced spin transition (CTIST) in Co/Fe clusters

– a spin transition between high spin (HS) and low spin (LS) FeII in the Fe/Fe and Fe/Co clusters

– the first direct, experimental observation of uncompensated spin density at diamagnetic FeII metal ions that bridge the paramagnetic CrIII in the Fe/Cr clusters. 

The CTIST involves formal intramolecular electron transfer and spin state changes at the metal ions within a molecule or material and creates the potential for the control of the magnetic and optical properties by changes in temperature or with irradiation.  The first report of such a phenomenon was made in 1996 and since then only several molecule-based materials, including the {[Co(tmphen)2]3[Fe(CN)6]2} cluster we studied, were shown to display this property. 

In the last two years we have investigated the new cyanide-bridged {[Fe(tmphen)2]3[Os(CN)6]2} and {[Fe(tmphen)2]3[Ru(CN)6]2} cluster by structural, spectroscopic and magnetic methods.   Magnetic susceptibility studies of both clusters showed an increase in their magnetic moment with the increasing temperature, which is indicative of a change in the spin state, and possibly the oxidation state of the Fe and Ru/Os ions.  X-ray crystallography studies of the {[Fe(tmphen)2]3[Os(CN)6]2} cluster at several temperatures between 30 K and room temperature showed an increase of ~0.2 Å in the Fe-N bond lengths with the increasing temperature that could be due to a transition between a LS and a HS configuration at the FeII sites or to CTIST from LS FeII-CoIII to HS FeIII-CoII.  A search of the Cambridge Structural Database (September 2009) showed that there are no examples of HS FeIII complexes with four imine ligands and two N-coordinated cyanide or thiocyanide ligands, and thus no benchmarks of HS FeIII-N bond lengths to which we could compare the corresponding bond lengths {[Fe(tmphen)2]3[Os(CN)6]2} cluster at high temperature.

To investigate directly the oxidation and spin state of the Fe sites of the cluster, we conducted variable temperature, variable field Mössbauer studies.  These studies showed that at low temperature the Fe/Os cluster has a LS FeII/ OsIII configuration, and that at high temperature there is no HS FeII being formed, which excludes the possibility that a classical LS-to-HS transition is induced by temperature at the FeII sites of the cluster.  Therefore the increase in the magnetic susceptibility with the temperature must be due to a CTIST from OsIII(S = 1/2)-LS FeII(0) to OsII(S = 0)-HS FeIII(S = 5/2).  EPR studies conducted in collaboration with Professor Doros Petasis from Allegheny College corroborated the information obtained from Mössbauer about the low temperature charge distribution in the Fe/Os cluster and IR spectroscopy studies of the cluster by the group of Jan Musfeldt from University of Tennessee supported the conclusion that a CTIST takes place in the cluster.

To our knowledge, the {[Fe(tmphen)2]3[Os(CN)6]2} cluster:

(1) is only the second example of cluster that contains the hexacyanoosmate(III) anion as a building block,

(2) contains at high temperature the first example HS FeIII ion with a coordination of four imines and two cyanides or isothiocyanides, and

(3) exhibits an unprecedented type of reversible, temperature-induced CTIST centered at room temperature from LS FeII-OsIII to HS FeIII-OsII,  the first time that the LS FeII becomes HS FeIII, thus undergoing the largest possible change in spin of DS = 5/2.

These unique results represent a valuable addition to the relatively small literature on CTIST phenomenon.  A manuscript reporting these results is complete and will be submitted to Angewandte Chemie in October 2009.