Ligand Design, Coordination and Electrochemistry of Nonbenzenoid Aryl Isocyanides

Abstract

Abstract In this work, the strategy for accessing planar chiral isocyanides has been developed and the first member of this family has been isolated in the optically pure form and fully characterized. 2-Methyl-substituted isocyanoferrocene can be synthesized from commercial ferrocenylcarboxylic acid and S- or R-forms of valinol. Both pS and pR forms of this new organometallic isocyanide were isolated in excellent percent enantiomeric excess. The unambiguous assignment of the absolute configuration was obtained through the X-ray chrystallographic study of the PdI2 adduct with (pS)-1-isocyano-2-methylferrocene that contains two planar chiral isocyanoferrocene ligands bound to the PdII center in the trans fashion. Electrochemical analysis of the [Cr(CNR)6]0,1+,2+ (R = ferrocenyl and cymantrenyl) series indicated that the electronic influence of the ferrocenyl moiety within a ligand is similar to that of a hydrocarbon aryl substituent, rather than an alkyl group (e.g., methyl). In addition, this work has demonstrated that electronic (especially, the pi-acidity) tuning of the isocyanocyclopentadienide ligand can be accomplished within a substantial range by changing the nature of the transition metal fragment bound to its ring. A systematic electrochemical investigation of the first isocyanoazulene ligands and their electron-rich complexes has been accomplished. The electronic properties of azulene and its isocyanide derivatives have been investigated by electronic spectroscopy, cyclic voltammetry, and theoretical calculations. Through this work, the properties of the Frontier molecular orbitals of isocyanoazulenes were determined quantitatively. The previous qualitative theory predicting the effect of substituents on the magnitude of azulene's HOMO-LUMO gap (and, hence, its optical properties) popularized by Liu et al.81 does not hold in the case of the isocyanide substituent. The unexpected directions of the shifts of the S0 → S1 transition for 1- and 2-isocyanoazulenes have been fully rationalized by considering together the electronic spectra, electrochemical properties, and the time-dependant density functional theory structures of these species. Furthermore, the first comprehensive quantitative electrochemical assessment of electronic inhomogeneity of the azulenic scaffold has been perfomed by analyzing the redox properties of the complexes Cr(CNxAz)6 (Az = azulenyl' x = 1, 2, 4, 5, 6). The quantitative series of the donor/acceptor ratios of the isocyanoazulene ligands has been obtained. The first large gold macrocycle incorporating the redox-active ferrocene unit has been isolated and fully characterized. The 1,1'-diisocyanoferrocene ligand employed in this study undergoes eta-2 coordination to a pair of AuI ions under thermodynamic control, i.e., no oligomeric structures are produced at all. Formation of the cyclic product is facilitated by the near perfect match between the interplanar distance involving the five-membered rings of the diisocyanoferrocene ligand and the AuAu separation within the di-gold precursor that features an aurophilic interaction. Overall, the "parent" motifs to be incorporated in the potential novel mesogenic materials containing both ferrocene and azulene units have been established and the reasons for challenges in isolating gold(I) complexes with mixed isocyanide ligands have been uncovered.