Direct Spectroscopic Evidence of Orbital Magnetism of Essentially Free Ion Magnitude in a Rigorously Linear Two- Coordinate High-Spin Fe (II) Compound W. M. Reiff, a A. M. LaPointe, b and C. E. Schulz c a Department of Chemistry, Northeastern University, Boston MA 02115, USA, b Symyx Technologies Inc., Santa Clara, CA 95051, USA, and c Department of Physics, Knox College, Galesburg IL, 61401, USA,

Substantial first order orbital contributions are important to the ground state magnetic behavior of certain first transition series metal ions in six coordination. Thus nominal O h symmetry ground states such as 2 T 2g (Ti 3+, low-spin Fe 3+ ), 5 T 2g (high-spin Fe 2 + and Co 3+ ), 4 T 1g (high-spin Co 2+ ) and 3 T 1 (Ni 2+ in T d ) are well established examples. In practice, such orbital angular momentum is largely quenched by any of low symmetry ligand field components, crystal packing effects or more fundamentally, the linear Jahn-Teller Effect (vide infra: top half of slide number (6)). The limit of such quenching effects is so-called spin-only magnetism as illustrated for high-spin Fe 2+ in the following slide, (3). Kahn, O. Molecular Magnetism,VCH-Wiley:New York,1993, Jahn, H. A.; Teller, E. Proc.Roy. Soc., 1937, A161, One can plausibly assume that complexes having minimal regular ligation, e.g. rigorously linear two coordination should afford maximal orbital effects, and depending on the overall symmetry essentially those of the corresponding free gas phase ion (slide (4), high-spin Fe 2+ ). Significantly, in this (linear) situation, there is no requirement for a Jahn-Teller distortion effect, slide (6) bottom half. Higher order non-linear terms in the vibronic coupling interaction can lead to distortion ( bending in the present context ) and lift orbital degeneracy. However, bulky, stereochemically demanding ligands that stabilize coordinative unsaturation are precisely those that vitiate bending effects in the solid state. One sees that complex 1, slide (7), has all of these characteristics leading to true orbital ground state degeneracy or quasi degeneracy, slide (8)) and extraordinary orbital magnetism. Mössbauer spectroscopy uniquely highlights this magnetism in terms of the highly symmetry sensitive orbital contribution, H L, to H internal.

J Classically and depending on the symmetry, increasing coordination number enhances the overall ability of ligands to impede the free orbital circulation of electrons and leads to a decrease in L

Fe 2+ exhibits the largest free ion magnetic moment of the first transition series metal ions. For real compounds that are highly coordinatively unsaturated (2 or 3 coordinate), this is a particularly propitious situation in the context of directly investigating orbital angular momentum effects. An isotope specific spectroscopic technique such as the Mossbauer Effect can more readily discern these effects in terms of the contributions to H internal as opposed to classical bulk powder susceptibility methods.

High density near random powder average SQUID data. These data indicate very large first order orbital angular momentum for complex 1, slide 7, and close to the free gas phase ion value of slide 4. The characteristic temperature dependence of  is typical of depopulation of spin-orbit states for <0, with  ~1 and no distortion. Figgis, B. N.; Lewis, J.; Mabbs, F. E.; Webb, G. A.: J. Chem.Soc (A)(1967), The spin-only value of  for a fictitious S= 3 cation is 6.93 . Thus, the present orbital effect is more or less equivalent to adding ~ two full spins to the magnetism of Fe 2+, i.e. S= 4/2 6/2, arguably a very large effect.

The above will generally lead to partial if not complete loss of first order orbital angular momentum.

Bis(tris(trimethylsilyl)methyl)Fe(II) C------Fe------C=180.0° Fe Fe ~9Å DIHEDRAL ANGLE = 60°, Ideal Staggered D 3d Symmetry 1 LaPointe, A. M.; Inorg. Chimica. Acta, 2003, 345,

CRYSTAL FIELD SPLITTING DIAGRAM FOR IDEAL LINEAR TWO COORDINATION WITH HOMOLEPTIC LIGATION IN D 3d SYMMETRY The ground (E g ) orbital doublet wave functions leads to the maximum orbital angular momentum for d n configurations in view of the m l values of the spherical harmonics linearly combined to produce the real d xy and d x 2 -y 2 pair. The degeneracy of this pair in combination with the C 8 symmetry element (slide 8) connecting them is essential to manifestation of the orbital momentum. More classically, for rigorously linear two coordination there are no in - plane ligands (only axial) to impede the orbital circulation of the electrons.

L L L L C8C8 (45  ) The C 8 symmetry element transforms d xy into d x 2 -y 2 with which it is degenerate Sketches of Four of the Five Real Spherical Harmonics for L=2

The orbital ground state of 1 in local D  h symmetery is 5  g. 5  g ( l  s) various spin–orbit states. The ground spin-orbit state is the “pure” M j = ± 4 spin-orbit doublet. Since,  m j = ±8 for this doublet, transitions within this doublet are highly forbidden and lead to slow relaxation. Hence, we expect 1 to be ESR silent and exhibit hyperfine splitting in zero external field as shown in the following two slides.  ~0.4 mm/s  E~ 1.2 mm/s

Complex 1 undergoes gradual slow relaxation and Zeeman splitting over the range 293 to 47 K. However, the broad resonances and trace rapidly relaxing paramagnetic phase suggest that we are near but not quite at the infinitely slow relaxation limit at 47 K. Analysis of the obvious quadrupole shift (s 1 -s 2 ) of the spectrum indicates that H internal relaxes parallel to the C 3 axis of 1 as expected.

{  m I = 0 ( )} At 4.2K, 1 is clearly at the slow relaxation limit and exhibits, by far, the largest value for the internal hyperfine field ever observed for iron! H 0 = 0 Reiff, W. M.; LaPointe, A. M.; Witten, E., J. Am. Chem. Soc. 2004,126, * ** The  m I = 0 transitions have sin 2  dependence (  = the angle between Eγ and H int ), Their intensities will approach zero with increasing application of a longitudinal field, see the next slide.

A relatively small longitudinal external field (0.5 T) largely polarizes the sample magnetizaton parallel to E γ as expected for a simple paramagnet. For even larger applied fields (to 9 T) H eff increases confirming a dominant orbital contribution to H nternal * * *

6 4 6 { -LINEAR THE INTEGERS TO THE UPPER LEFT OF THE FIRST THREE ENTRIES ARE THE COORDINATION NUMBERS OF THE IRON CHROMPHORES FOR THESE COMPOUNDS.

Recall that, H int = H f +H d +H l where H f is the Fermi contact term (~ T/spin for iron) and H d the usually small dipolar term that can be positve or negative and that is neglected herein. Hence from the result of slide 12: 152 T ~ -50 T + ~0 + H L implying H L is ~200 T. This is a remarkable result for two sigma bonding, non-delocalizing, hard Lewis base carbanion ligands in the rigorously linear geometry of complex 1, slide 7. The results of this work also imply that for appropriate geometry real compounds can exhibit magnetic properties very close to that of the corresponding free ion. Conclusion the results of the preceding table are entirely consistent with the comments of this narrative. Clearly, the absence of the Jahn- and Renner-Teller effects for the linear coordination of two bulky ligands in the solid state is shown to have an enormous effect on the magnitude of internal hyperfine field primarily through the increased magnitude of the orbital contribution.