AmericanChemicalSociety.com

In the second year studying of the molecular mechanism(s) for binding small molecules to network solids we have:

• Characterized and figured out the mechanism for the polymerization of nitrobenzene by aluminum chloride
• Determined the phase diagram and characterized important intermediates to understand the hydration of zinc chloride.

This work, as well as prior support from this grant is the primary basis of the doctoral dissertation of Robert J. Wilcox who defended on November 4, 2009.  Multiple chapters of this thesis will be rewritten as full paper manuscripts in the coming months.

1.  Polymerization of nitro benzene.

In our continuing study of the reactivity of CuAlCl₄ with olefins, carbonyls and aromatics, we explored its reactivity with nitrobenzene.  In all of the other systems examined, the copper I center exhibited Lewis acidic behavior, accepting ligand electron density into its low-lying, empty s/p-orbital.  Nitrobenzene, being an electron deficient aromatic molecule was expected to test the limits of the separation capability of this material.  Instead of forming an adduct with CuAlCl₄ it was found that under solvothermal reaction conditions, the aluminum chloride catalyzed the dehydro oligomer/polymerization of nitrobenzene to form a luminescent, paramagnetic nitroxo oligomer/polymer.  In fact the copper played no role in this reaction.    Both by column chromatography or differential solvent separation, three soluble and one insoluble fractions were isolated.  Mass spectral analysis of each fraction demonstrates that the resulting oligomers/polymers consist of a biphenyl repeat that is bridged by nitoxo units; with each fraction exhibiting a nearly equivalent mass spectrum.  The three solvent isolable fractions, red, purple, and blue suggest different conjugation lengths of each fraction.  We propose that the red (shorter conjugation length) oligomers result from nitroxo bridges between monomers on opposite sides of the biphenyl link  whereas the longer conjugation length results from nitroxo bridges across monomers on the same side of the biphenyl moiety (see blue brackets on scheme at right).   

2.  Understanding the Hydration of Zinc Chloride.

The hydration of zinc chloride has proved to be an important model reaction for the study of the reactivity of small molecules with crystalline solids, as well as reactivity of small molecules with other condensed phase media; specifically liquids.  As noted last year, using the environmentally controlled microbalance (ECMB), constructed as part of this project, we discovered that depending on the reaction conditions zinc chloride could sorb up to twenty or more equivalents of water per zinc. With careful environmental control of the ECMB system we were able to carefully prepare samples with varied ZnCl₂:H₂O ratios which were then examined by differential scanning calorimetry, infrared spectroscopy, Raman spectroscopy and synchrotron diffraction.  Together these provided data to map out the phase diagram for this system, correcting that previously published in 1905.  Most importantly, using strategies that we have developed for crystallization under conditions where nucleation is slow and growth is fast, we were able to in-situ grow single crystals of the previously elusive three equivalent adduct phase. Most importantly, using strategies that we have developed for crystallization of congruently melting phases under conditions where nucleation is slow and growth is fast, we were able to in-situ grow single crystals of the previously elusive three equivalent adduct phase. This crystal structure provides the key to understanding critical aspects of zinc chloride hydration.  Specifically, the crystal structure [Zn(OH­₂)₆][ZnCl₄],  demonstrates a complete partitioning of the water and chloride ligands to two distinct zinc atoms.  These complex ions are then packed with a CsCl salt-type arrangement.  That complete phase segregation of the chlorides and water’s takes place is consistent with the slightly larger Zn-Cl bond strength than the Zn-O bond.  This trend in relative bond strength holds unless the system is made more basic, at which point hydroxide ligation becomes competitive with chloride ligation.  The ZnCl₄^2- anion has a distinctive signature in Raman spectroscopy, with the primary symmetric stretch appearing at about 285cm^-1.  Interestingly this peak is clearly observe even up to dilutions of 1 mole percent ZnCl₂ to 99 mole percent H₂O.  Armed with the knowledge that the only half of the zinc cations in zinc chloride can from hydrated zinc cations, the other half being ZnCl₄^2-, we are finally able to interpret the structural data obtained by pair distribution function analysis of synchrotron scattering experiments.  Features that had previously been interpreted to be zinc/chloride/hydrate polymers in fact must be assigned to higher hydration spheres.  Combining all of our spectroscopic and diffraction data, we now have evidence for up to three distinct hydration spheres around the hydrated Zn²⁺ cation, [Zn(OH₂)₆(OH₂)₁₂(OH₂)₂₄]²⁺ which even in the liquid state pack with the ZnCl₄^2- anions in a CsCl or NaCl salt-like packing arrangement.  Thus, a 1.3m aqueous solution of zinc chloride in fact must be described as a molten salt.