direct spectroscopic evidence of orbital magnetism of essentially free ion magnitude in a rigorously...

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

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. Schulzc

aDepartment of Chemistry, Northeastern University, Boston MA 02115, USA, bSymyx Technologies Inc., Santa Clara, CA 95051, USA, and cDepartment of Physics, Knox

College, Galesburg IL, 61401, USA, [email protected]


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 Oh symmetry ground states such as 2T2g (Ti3+, low-spin Fe3+), 5T2g(high-spin Fe2+ and Co3+), 4T1g(high-spin Co2+) and 3T1 (Ni2+in Td) 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 Fe2+ in the following slide, (3).

Kahn, O. Molecular Magnetism,VCH-Wiley:New York,1993, 31-52

Jahn, H. A.; Teller, E. Proc.Roy. Soc., 1937, A161, 220-226.

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 Fe2+). 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, HL, to Hinternal.


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


Fe2+ 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 Hinternal 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), 442-447.

6.71

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 Fe2+, 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 D3d Symmetry

1

LaPointe, A. M.; Inorg. Chimica. Acta, 2003, 345, 359-362


CRYSTAL FIELD SPLITTING DIAGRAM FOR IDEAL LINEAR TWO COORDINATION WITH HOMOLEPTIC LIGATION

IN D3d SYMMETRY

The ground (Eg) orbital doublet wave functions leads to the maximum orbital angular momentum for dn configurations in view of the ml values of the spherical harmonics linearly combined to produce the real dxy and dx

2-y

2 pair. The degeneracy of this pair in combination with the C8 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

C8 (45)

The C8 symmetry element transforms dxy into dx2-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 Dh symmetery is 5g.5g ( ls) various spin–orbit states.The ground spin-orbit state is the “pure” Mj= ± 4 spin-orbit doublet.Since, mj = ±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/sE~ 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 (s1-s2) of the spectrum indicates that Hinternal relaxes parallel to the C3 axis of 1 as expected.


{mI = 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!

H0 = 0

Reiff, W. M.; LaPointe, A. M.; Witten, E., J. Am. Chem. Soc. 2004,126, 10206-10207.

*

**

The mI = 0 transitions have sin2 dependence ( = the angle between Eγ and Hint), 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 Eγ as expected for a simple paramagnet. For even larger applied fields (to 9 T) Heff increases confirming a dominant orbital contribution to Hnternal

*

*

*


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, Hint = Hf+Hd+Hl where Hf is the Fermi contact term (~ -12.5 T/spin for iron) and Hd 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 + HL implying HL 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.

direct spectroscopic evidence of orbital magnetism of essentially free ion magnitude in a rigorously...

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