Toward Electronic Materials Based on Metal Pincer-Type Complexes

There is currently a lot of interest in developing electrically conducting or semiconducting metal-organic frameworks (MOF\u27s), highly porous materials constructed by organic ligands bridging metal centers. Typically MOF\u27s are non-conducting and, moreover, they are susceptible to hydrolytic degradation. If hydrolytically stable and electrically conducting MOFs could be realized, then revolutionary new technologies could be envisioned. Currently, organic dicarboxylates are used as bridging organic ligands and one simple strategy to obtain the desired materials is to explore other ligand systems. Pincer ligands are organic compounds that are uninegative and bind metals in a tridentate, meridional fashion with two five-member chelate rings. There is intense contemporary interest in studying metal complexes of these pincer- or pincer-type ligands (variants with six-membered chelate rings) because they can exhibit remarkable stability and they can often promote unusual chemical transformations depending on the metal and any special properties of the ligand. Another attractive feature of pincer ligands for the purpose of developing conducting MOF\u27s is that certain classes are electrochemically non-innocent, and can readily accept or give away electrons at potentials that depend on the ligand\u27s constituents. This thesis describes investigations into metal complexes of new ligands that have either two pyrazolyl (pz) or one pz and one diphenylphospine flanking donor(s) attached to diarylamido anchor donors to give pincer-type derivatives with NNN- or NNP- donor sets, respectively. First, the preparation and reaction chemistry of (NNP)rhodium(I) complexes was investigated to determine their potential in catalytic chemistry. It was found by X-ray structural studies, NMR spectroscopic studies and DFT calculations that this ligand was hemilabile with rapid dissociation/association of the pyrazolyl arm. Next, the preparation and properties of [Ga(NNN)2](PF6) were thoroughly investigated experimentally and computationally. The complexes are hydrolytically stable. Moreover, electrochemical measurements show that the ligand is an electron donor, undergoing two sequential one-electron oxidations at potentials near 1.0 V vs Ag/AgCl. Spectroscopic studies verified that electronic communication occurs across a diamagnetic metal bridge and that the mono-oxidized species is a Robin-Day class IIA species. These results demonstrate that metal pincer complexes warrant further investigation as candidates for components of electrically conducting MOFs

Mixed Valent Metal Pincer Complexes and Reactivity of Metal Complexes of Extended Pincer Ligands

Historically, the study of mixed valence complexes has been critical for advancing our understanding of electron transfer processes in biological and abiological systems. The recent use of mixed valence complexes in electrochromic materials, and the promise of their use in future technological or molecular electronic applications, has spurred further interest in this class of compound. Previous studies by our research group have shown that gallium(III) or tricarbonylrhenium(I) complexes of pincer-type ligands with diarylamido anchors and either pyrazol-1-yl (pz) or diarylphosphino (PAr2) flanking donors are electroactive species with quasi reversible ligand-centered oxidations. Moreover, the one-electron oxidized derivative with pz flankers, [Ga(L)(L+)]2+ was found by both spectroscopic and electrochemical means to be a Robin Day Class II species with weak electronic communication occurring between pincers across the main group metal bridge. Cursory electrochemical studies suggested that stronger interactions occurred on replacing gallium(III) with other metal centers. This dissertation elaborates on these initial, prior, findings by describing more detailed synthetic protocol to various [M(L)2]n+ complexes where M = Ni, Co, Rh, Ir, n = 0-3 (depending on M), and where L has different organic groups decorating the periphery. Electrochemical measurements and in-depth spectroscopic analyses of oxidized and reduced forms of the complexes were used to better quantify the effects of metal and ligand substitution on their electronic properties including the extent of electronic communication in mixed valence derivatives. Another goal of the work was to prepare in multimetallic pincer complexes via both covalent and self-assembly approaches and study their electronic properties. Thus, the preparation and properties of [Re(CO)3]2(-L-L) with dinucleating pincers (L-L) is described. Initial successes and difficulties with the preparation and characterization of coordination networks based on these pincers and those with different Lewis donors at the para- aryl position are outlined next. Finally, ‘Extended Pincers’ (EP), ligands comprised of an N,N’-diarylformamidinate anchor with flanking pz and/or PAr2 ortho-aryl donors were prepared since they should support multimetallic complexes with unusual metal-metal bonds or reactivity due to proximity of the metal centers. Their group 1 and group 11 metal complexes may serve as useful reagents for future chemistry