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45586-AC3
Tunable Control of Magnetic Properties in Transition Metal Indenyl Complexes

Timothy P. Hanusa, Vanderbilt University

This research has as its goal the investigation of the synthesis, structures, and magnetic properties of (indenyl)metal complexes that display adjustable changes in spin state. Ultimately we aim to create new classes of magnetically active compounds, including charge-transfer molecular magnets that would be the foundation for data storage applications, and compounds whose spin state could control their behavior as synthetic reagents.

An extensive study of methylated bis(indenyl)Cr(II) complexes of Cr(II) has been completed. These compounds display spin states that are strongly dependent on the orientation of the indenyl ligands; i.e., compounds with staggered ligands are high spin, with four unpaired electrons (S = 2), whereas complexes with eclipsed ligands display thermally induced spin-crossover behavior. The origin of this behavior was identified as a consequence of particular symmetry interactions between the ligands and metals.

Although it was expected that the substitution of a large number of electron-donating methyl groups would favor low-spin complexes, it was a surprise to learn that their specific location on the indenyl ligands has a strong effect on the spin states; methyl groups on the six-membered benzo ring appear to be more effective in stabilizing a low-spin ground state than are those on the five-membered ring. Extensive density functional theory (DFT) calculations were conducted at the TPSSh/6-31+G(d) level, which traced the origin of this effect to the influence that substitution on the benzo ring has on the HOMO-2 (pi-3) orbital of the indenyl anion (see rightmost Figure below). The energy of the pi-3 orbital is raised by 0.30 eV relative to the unsubstituted anion. This ultimately has the effect of lowering the energy of the molecular HOMO, which favors the low-spin state. The tunability of the magnetic properties of bis(indenyl)metal complexes with site-specific ligand substitution has implications in the construction of charge transfer salts. It more broadly hints at the significant electronic (as distinct from steric) modifications that could be exploited with indenyl ligands that are used in polymerization catalysts.

We have made an extended push into manganese indenyl chemistry. The rationale for this is our previous isolation of the air-stable charge transfer salt, [(1,2,3-Me₃C₉H₄)₂Fe][DCNQ] (DCNQ = 2,3-dicyanonaphtho-1,4-quinone), which was found to be a magnetically ordered at low temperature. The incorporation of complexes with spin-crossover behavior into charge-transfer salt magnets could potentially induce new types of magnetic behavior, including thermal hysteresis. Cationic Mn(III) complexes, containing d⁴ metal centers, would be isoelectronic with neutral Cr(II) species, and might display analogous orientation-dependent spin crossover behavior. Bis(indenyl)Mn(II) compounds would be the logical precursors to the Mn(III) species.

Although the unsubstituted complex (C₉H₇)₂Mn is unknown, we have prepared and crystallographically characterized methylated versions, which can be directly compared to their chromium(II) counterparts. Initial investigations of their chemistry have been spectacular. For example, the yellow (2,4,7-Me₃C₉H₄)MnCl(thf) complex forms blue solutions when cooled under a nitrogen atmosphere; the blue color is discharged upon purging the solution with argon. All indications are that a dinitrogen complex is formed at low temperature, the first that we are aware of that involves Mn(II). The compound also reversibly binds dihydrogen, forming purple solutions at low temperature; a dihydrogen (or dihydride) complex is probably involved. The reaction with nitrogen is also displayed by the bis(indenyl)Mn complexes (2-MeC₉H₆)₂Mn and (2,4,7-Me₃C₉H₄)₂Mn. Despite an intriguing octomeric structure (see Figure below), the related (4,7-Me₂C₉H₅)₂Mn complex does not bind dinitrogen. It appears that some methylation of the five-membered ring (and possibly a spin state change) is critical for activating the (indenyl«)Mn complexes toward nitrogen/hydrogen binding.

We are continuing to accompany these synthetic, crystallographic, and magnetic studies with computational investigations, mainly employing DFT methods. Calculations will used to examine the comparative energy changes occurring on the binding of gases and in the Mn(II)/Mn(III) complexes incorporated into charge-transfer salts.

This research has put the PI into contact with the magnetic materials community, which has expanded his background in synthesis and structural characterization in new directions. It has also lead to an increasing involvement in computer modeling and calculations, and to a greater appreciation of the way in which computational results can illuminate experimental findings and suggest new directions in synthesis. It has lead to several presentations at regional and national meetings. The graduate student involved in the research has had to expand his repertoire of synthetic and computational knowledge, and to learn to communicate with scientists who are not in his immediate field of chemistry. He has presented his work at a national ACS meeting, and a second-year student that he is training will discuss the work at a regional ACS meeting later this year.

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