Hydrogen Bonding and Multiphonon Structure in One- and Two-Dimensional Polymeric Magnets*

We report a systematic investigation of the temperature dependent infrared vibrational spectra of a family of chemically related coordination polymeric magnets based upon two different bridging anions, fluoride (F$^-$) and bifluoride (HF$_2^-$), in copper-pyrazine complexes including Cu(HF$_2$)(pyz)$_2$BF$_4$, Cu(HF$_2$)(pyz)$_2$ClO$_4$, and CuF$_2$(H$_2$O)(pyz)). We compare our results with several one- and two-dimensional prototype materials including Cu(NO$_3$)$_2$(pyz) and Cu(ClO$_4$)(pyz) $_2$. Unusual low temperature hydrogen bonding, local structural transitions associated with stronger low-temperature hydrogen bonding, and striking multiphonon effects that derive from coupling of an infrared-active fundamental with strong Raman-active modes of the pyrazine building-block molecule are observed. Based on the spectroscopic evidence, these interactions are common to this family of coordination polymers and work to stabilize the low temperature magnetic state. Similar interactions are likely to be present in other molecule-based magnets.