Exotic Dimers Part One: The Case of the Vanishing Hyperfine Field

1. Exotic Dimers Part One: The Case of the Vanishing Hyperfine Field W. M. Reiffa, J.S. Millerb, J.. H. Zhangc and D. O’Hared aDepartment of Chemistry, Northeastern University, Boston, MA, 02115 USA, bDepartment of Chemistry, University of Utah, Salt Lake City, UT 84112 USA, cDepartment of Chemistry, Xavier University, New Orleans LA 70125 USA, dDepartment of Inorganic Chemistry, Oxford University, Oxford OX1 3QR England

2. So Called Simple Dimers-Numerous investigations of the magnetism of simple dimers have proven invaluable to our greater understanding of transition metal magnetic behavior in general. However, there are those who would say that this area of research, while interesting, is largely finished in the context of observing truly new phenomena or at least novelties, i.e., “there is nothing much new under the sun here.” In this vignette, we briefly present results that clearly deflate the foregoing assertion particularly for materials that we can arguably classify under the rubric of molecular magnetism. The specific system we shall discuss is based on the deca-methylferrocenium cation, [Fe(Cp*)2]+. This species is highly amenable to study using nuclear gamma resonance (Mössbauer Effect) spectroscopy owing to the fact that its ambient temperature spectrum is the least complicated possible, ~ a sharp singlet owing to zero or very small quadrupole effects. This cation contains low-spin (S=1/2) Fe(III) corresponding to a nominal 2E ground state. Its orbital magnetism is not fully quenched and hence large orbital contributions to hyperfine fields arising from either long range magnetic order or slow paramagnetic relaxation are expected, i.e. Hn ~ 40 to 45T for a system with nevertheless only one unpaired electron! The preponderance of data for the dimer system we discuss (type C next slide) indicates that it exhibits intra-dimer ferromagnetism (J>0) and positive axial zero field splitting, D. Such dimers (versus J<0) are difficult to study (1) using susceptibility techniques owing to the fact that the  and  versus T variation are often not that different than for single ion paramagnetic systems with varying degrees of zero-field splitting. Also, a signature maximum in  vs T is typically not observed for the case of a J >0 dimer. We shall see that a combination of zero and applied field Mössbauer spectroscopy unambiguously confirm a J >0 dimer whose ST=1 ground state has D>0 leading to the unprecedented disappearance of all magnetic hyperfine effects at very low T. (1) J.Comarmond, P.Plumere, J.M. Lehn Y.Agnus, R. Louis, R. Weiss,O.Kahn.I.M.Badarau. J.Am.Chem.Soc.1982,104, 6330-6340.

3. Donor-Acceptor Charge Transfer Structure Archetypes S= ½ donor acceptor cation anion units {} A B C (2) J.S. Miller, J. C. Calabrese, and D. A. Dixon, J. Phys. Chem., 1991, 95, 3139-3148. Types A and B are parallel chains thathave been extensively investigated in the contexts of their long range order and molecular magnetism.Type C has received little attention save for its syntheses and structure determination and theory (2).The {D+D+,S=1/2 S=1/2}ferrocenium dimers are magnetically “isolated” from each other by spin singlet, closed shell, polycyano di-anionic spacers.Their dianionic nature is incontrovertibly confirmed by Fe57 Mössbauer spectroscopy in conjunction with the 2:1 stoichiometry determined from X-ray study.

4. Specific polycyanide*** radical (mono-)anions of interest in the present context in their corresponding closed shell dianionic form as stabilized in 2:1 polymers (Type C) Trans-hexacyano-butadiene Hexacyano-Trimethylenecyclopropane *** ***

5. The diagram and results on the three Power Point slides subsequent to this refer to the specific trans–hexacyano-butadiene 2:1 charge transfer polymer (hereafter 1) shown below. 1

6. (A) is the usual Bleaney-Bower (spin only) equation for  ( powder average). In (B) and (C), the principal susceptibilities take account of axial zero field splitting, D. There are four possibilities for equivalent metals in a simple dimer: 1. J -, D-; 2. J-, D+; 3, J+, D – and 4.J+,D+.It will be seen that only case 4 can result in the unprecedented temperature dependence of the Mössbauer spectrum of 1 shown on the next slide for realistic values of J and D T (A) *Note that mStotal = ± 1 is a slowly relaxing Kramers doublet since mStotal = ± 2 and is highly forbidden! Best fit for  vs T : 2J ~+ 180K D~+6.5K (B) (C) T * T T No consideration is given to non-axial zero field splitting, E, owing to the absence of detailed ESR results for 1 and the likelyhood that E is small or zero in view of the cationssymmetry. T

7. There are three distinct regions of interest below: A) At ~293K and above, 1 is a rapidly relaxing paramagnet; B) below 293K to ~ 10K, 1 undergoes slow (intradimer) paramagnetic relaxation and reaches the infinitely slow relaxation limit with two hyperfine patterns evident (C) Below 4K, the intensity of the hyperfine patterns of 1 approaches zero leading to singlet reminiscent of that at 293K but corresponding to the true non-magnetic mS (total) = 0 ground state of the dimer. (A less compressed velocity scale was used at 4.2K)* (1) * H0=0 at all T (mm/s)

8. A relatively small transverse field induces a slowly relaxing background superimposed on the singlet expected for the non-magnetic ground state. This indicates that at 0.52 K in a magnetic field, there is some population of the lower energy Zeeman level (mStotal = -1) of the ± 1 doublet which is at ~6 K in zero field relative to the ground state. The next slide shows the profound effect of transverse field ~ 3.6 times that used below. 1

9. H0 ~ 15 kG induces slow paramagnetic relaxation at essentially the infinitely slow limit and a spectrum reminiscent of that in H0 = 0 at 10.4 Ksave for the fact that the sample magnetization is (as expected for H0 tranverse) now strongly polarized normal to Eγ.This is evidenced by amplification of the mI =0 transitions*.There are clearly two hyperfine patterns as seen (e.g. in zero field at 10.4 K) with Hnvalues of ~40 and ~ 45 T, i.e. differing by ~ 5 T. (1) Residual Rapidly Relaxing Phase * *

10. Conclusion-Arguably, Mössbauer spectroscopy discerns novel aspects of the electronic/magnetic behavior of the present complex dimers that are simply not accessible by other investigative techniques one can envision, at least not in such a facile manner. An intriguing question remains. What is the origin of the multiple nuclear Zeeman patterns observed for the present dimer/dianion pair polymer and for others (not discussed herein) and yet not observed for still others? We speculate here, but not without sound precedence. It is well known that in the solid state at ambient temperature ferrocene, Fe(Cp)2, corresponds to a staggered (D5d symmetry) ground state conformation while ruthenocene has eclipsed D5h. Furthermore X-ray and neutron diffraction studies confirm that (neutral) ferrocene exhibits structural phase transformations between these symmetry extremes as a function of temperature. On the other hand, similar investigations (2) show that the Fe(Cp*)+2 cation in the …. D+D+A2-….polymer series of this vignette is close to either the D5h or D5d symmetry extreme depending the specific A2- dianion present. Hence it is appropriate to suggest that the cation conformation in the latter {[Fe(Cp*)2]+}2 series can undergo analogous changes with T. In fact, such behavior has been definitively demonstrated (3) for the cation of [Fe(Cp)2]+ [FeCl4]-., ~ D5d (298K) ~D5h(213K). In view of the sensitivity of the orbital contribution, HL, to symmetry, it is not difficult to imagine the latter process being reflected in the observation of two Zeeman patterns whose internal fields differ by ~ 5T and where the process does not go to completion thus resulting in different intensities for such patterns. (3) F. A. Cotton, L.M. Daniels, and I. Pascual, Acta Cryst.(1998). C54, 1575-1578.