Metal-Organic Frameworks as Functional, Porous Materials

The research presented in this thesis investigates the use of metal carboxylates as permanently porous materials called metal-organic frameworks (MOFs). The project has focused on three broad areas of study, each which strives to develop a further understanding of this class of materials.

The first topic is concerned with the synthesis and structural characterization of MOFs. Our group and others have found that the reaction of metal salts with carboxylic acids in polar solvents at elevated temperatures often leads the formation of crystalline MOF materials that can be examined by single crystal X-ray diffraction. Specifically, Chapter 2 reports on some of the first examples of magnesium MOFs, constructed from formate or aryldicarboxylate ligands. The magnesium formate MOF, [Mg3(O2CH)6] was found to be a permanently porous 3-D material capable of selective uptake and exchange of small molecules.

Once the synthesis and structures of some of these materials was known, their physical properties were studied. The magnesium formate MOF, [Mg3(O2CH)6], was found to be permanently porous and able to reversibly adsorb both N2 and H2 gas. Furthermore, the material was also capable of taking up a variety of organic molecules to form new inclusion compounds that were characterized by XRD studies. Size exclusion was shown for cyclohexane and larger molecules.

Chapters 3, 5, and 6 attempt to build off of the synthetic findings reported in Chapter 2. Specifically, the ability of these materials to take up guest molecules is expanded by the attempted synthesis of porous, homochiral MOFs using enantiopure carboxylic acids in the synthesis. It was found that under the appropriate synthetic conditions, both L-tartaric acid and (+)-camphoric acid were robust linkers for the formation of homochiral MOFs. Of the compounds synthesized, the most interesting were the set of compounds, [Zn2(Cam)2(bipy)ÌÄ'¡3DMF] and [Zn2(Cam)2(apyr)ÌÄ'¡2DMF]. These compounds formed isoreticular cubic networks in which the pore size was dependent on the size of the linker molecule (bipy or apyr).

Additonally, the compounds [Zn2(Cam)2(bipy)ÌÄ'¡3DMF] and [Zn2(Cam)2(apyr)ÌÄ'¡2DMF] were found to be capable of guest exchange. Due to their chiral nature, these materials were screened for the enanatioselective separation of racemic alcohols. No selectivity was seen with either MOF, likely owing to factors such as large pore size and disorder in the chiral camphorate ligand. [Zn2(Cam)2(bipy)ÌÄ'¡3DMF] contained large voids and preliminary studies showed that free-radical polymerization of methylmethacrylate could take place within the channels of the material. The amino group of the apyr ligand in [Zn2(Cam)2(apyr)ÌÄ'¡2DMF] was able to be functionaled with acetaldehyde by treatment of the porous MOF with the bulk organic reagent.

A further area of study detailed in this work deals with a central question in MOF chemistry, concerning the assembly process of these extended materials from solution. Chapter 3 reveals that the trimeric species Mg2(HCam)3+, the SBU for the formation of the MOF [Mg2(Hcam)3.3H2O].NO3.MeCN, can be identified using ESI-MS on the the reaction solution prior to crystallization. Further studies showed that the addition of chelating additives led to new solid-state structures and new ions in the mass spectrum, indicating that the Mg2(HCam)3+ ion is likely present in solution prior to MOF formation. Chapter 4 discusses extension of these ESI-MS studies on various other MOF and organometallic systems.

Finally, Chapter 7 discusses the synthesis and structures of magnesium imides. These compounds were originally investigated for use as SBUs in network synthesis. This strategy proved to be unsuccessful, as the compounds form molecular clusters in the solid state. The coordination chemistry and computational studies regarding the adopted aggregation state is detailed.