Design of Novel Electron-Rich Organometallic Frameworks Involving Metal-Isocyanide Junctions

Abstract

Aromatic diisocyanides are extensively employed as building blocks in coordination and surface chemistry as charge transport mediators for nanotechnology applications. The first diisocyanoarene-bridged bimetallate system, [{(CO)5V}2(μ-CNC6Me4NC)]2-, was synthesized from V(CO)6 through a neutral [(CO)5V]2(μ-CNC6Me4NC) intermediate, or, less conveniently, directly from subvalent [V(CO)6-. In the solid state, the bimetallic dianions [{(CO)5V}2(μ-CNC6Me4NC)]2- undergo regular π-stacking aggregation and exhibit strong contact ion association with cobaltocenium counter-ions. This results in an unusual electron-rich polymeric supramolecular ensemble held together exclusively via non-covalent interactions. The chemistry of a second vanadium(-I) bimetallate featuring the 1,1'-diisocyanoferrocene bridge is described as well. Synthesis of 2,2'-diisocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene involving Pd-catalyzed coupling of monoazulenic precursors was developed. The single crystal X-ray analysis of this nearly 2 nm-long linear ditopic diisocyanobiazulene linker revealed an interplanar angle of 67° between the two azulene rings. The two-electron reduction of this diisocyanobiazulene is perfectly reversible, at least on the electrochemical scale, and occurs in a single step at an unusually mild potential of ca. 1.0 V vs. ferrocene/ferrocenium couple. Generation of a closed-shell biazulenic dianion accompanied by a substantial decrease in the interplanar angle between the two azulenic halves of this novel ligand is strongly suggested. Mono- and bimetallic complexation of the 2,2'-diisocyano-6,6'-biazulene allowed assessment of the change in the metal-to-diisocyanobiazulene charge transfer energy upon binucleation of the linker by electronic spectroscopy. In addition, efficient regioselective mono-isocyanide functionalization of the perfectly planar, symmetric 2,2'-biazulene and initial organometallic chemistry of the resulting long nonbenzenoid aryl isocyanide is described. The first examples of biazulenic self-assembled monolayers (SAMs) on a metal surface were prepared and characterized by FTIR and ellipsometric techniques. The organic substrates feature symmetric linear 2,2'- or 6,6'-biazulenic motif functionalized with one or two isocyanide "alligator clips". In all cases, terminal upright η1 coordination to gold is realized. The gold-isocyanide junction in these biazulenic films appears to be much less prone to gold-assisted oxidation of isocyanide to isocyanate under ambient conditions compared to SAMs of many benzenoid aryl isocyanides on Au(111). The kinetic stability of adsorbed benzenoid aryl isocyanides on gold can be substantially improved by linking several coordinated isocyanoarene moieties together. This is demonstrated by the adsorption of 8,16,24,32-tetraisocyano[2.2.2.2]metacyclophane on the gold(111) surface via all four of its isocyanide groups to form an air-stable SAM that features approximately upright orientation of the aryl isocyanide units with respect to the surfaces as evidenced by FTIR and optical ellipsometry measurements. The energetic gain upon chemisorption of the tetraisocyanide to the gold surface in the μ4-η1: η1: η1: η1 fashion substantially outweighs the cost of the metacyclophane conformational change required for such coordination. Thus, the isocyanide groups function as effective two-atom long anchors for self-assembling stable monolayers of cyclophanes on metallic gold and the strategy developed herein should be of general utility.