The Isocyanoarene Motif in Organometallic Crystal Engineering and New Azulene-Based Organometallics

Abstract

Isocyanides (:CN-R) and their metal complexes play an important role in many areas including synthetic organic chemistry, catalysis, diagnostic medicine, as well as surface, polymer, and materials sciences. The rich chemistry of isocyanides stems from the tunability of the molecular and electronic structure through variations of the substituent R. Isocyanides represent a rather versatile class of ligands and can accommodate metal ions in both high and low oxidation states upon complexation. Isocyanoarene derivatives have been shown to be effective in the design of charge transport materials (e.g., molecular wires). In addition, di- and other polyisocyanoarenes have been employed as building blocks in the coordination chemistry of polynuclear organometallics. In the past decade, Barybin et al. have developed the chemistry of electron-rich compounds and materials that incorporate isocyanoarene ligands featuring nonbenzenoid aromatic azulenyl and η5-cyclopendadienyl substituents. In this Dissertation, isocyanide-terminated benzenoid and nonbenzenoid arenes as well as the organometallic complexes thereof are discussed. Chapter I constitutes a review of recent developments in the chemistry of isocyanoarenes as ligands in low-valent organometallics. Particular emphases are placed on (1) isocyanometalates (isocyanide complexes of metals in negative oxidation states), (2) low-coordinate complexes of extremely bulky isocyanoarenes, and (3) the chemistry of nonbenzenoid isocyanoarenes. The first part of Chapter II is dedicated to the chemistry of an unusual supramolecular charge-transfer ensemble ([Cp2Co]2[{(OC)5V}2(μ-CNC6Me4NC)])∞ (Cp = cyclopentadienyl) held together via synergistic π-stacking and contact-ion interactions. This three-dimensional, porous framework features channels capable of housing linear molecules such as acetonitrile, carbon dioxide, etc., and offers new opportunities in organometallic crystal engineering. The second part of Chapter II describes preliminary studies on the interaction of the novel 2-isocyano-1,3-dimethylazulene ligand with sub-valent metal ions (e.g., Co(I-) and Fe(II-)). Chapter III of this Dissertation describes the syntheses and coordination chemistry of the polar, linear 2-isocyano-1,3-diethoxycarbonyl-2',6-biazulene ligand and related species. Detailed electrochemical and spectroscopic studies of these novel ligands and their low-valent homoleptic complexes shed light on electron delocalization between the azulenic/biazulenic π-systems and electron-rich metal ions mediated by the isocyanide junction. In Chapter IV, synthetic studies toward a family of azulene-based metal-organic frameworks are described. Two-dimensional, rectangular metal-organic frameworks were formed by bridging {Cp*ClIr(III)} corner fragments (Cp* = pentamethylcyclopentadienyl) with either asymmetric 2,6-diisocyanoazulenic or symmetric 2,2'-diisocyano-6,6'-biazulenic ditopic edge units. 16-Electron metal carbonyl units, namely [Cr(CO)5], were employed as end caps to control orientation of the molecular dipole of 2,6-diisocyano-1,3-diethoxycarbonylazulene within tetrametallic molecular frameworks.