Azulene is an attractive building block for molecular electronics design owing to the intrinsic charge separation within its sp2-carbon scaffold. In this context, the structure and molecular organization in self-assembled monolayers (SAMs) of unsubstituted and nitrile-functionalized azulenethiolates on Au(111) substrates were studied by a variety of complementary spectroscopic techniques. The molecule of 2-mercapto-6-cyanoazulene was specifically designed to be addressable within the core hole clock approach in the general framework of resonant Auger electron spectroscopy. The azulenic SAMs described herein were documented to be well-defined and densely packed, with their individual molecular constituents oriented upright with respect to the Au surface, but with considerable tilt and twist. The electron transfer (ET) properties of these azulenic SAMs were found to be comparable to those of analogous naphthalene-based monolayers, which suggests that the charge separation and the related dipole moment have a minor effect on dynamics of ET through the molecular framework. The characteristic time for the latter process, triggered by the [N 1s]π1* excitation and occurring along the nitrile–backbone–substrate pathway, was estimated at 23 ± 4 fs.

Mercapto (–SH) and isocyano (–NC) terminated conducting π-linkers are often employed in the ever-growing quest for organoelectronic materials. While such systems typically involve symmetric dimercapto or diisocyano anchoring of the organic bridge, this article introduces the chemistry of a linear azulenic π-linker equipped with one mercapto and one isocyano terminus. The 2-isocyano-6-mercaptoazulene platform was efficiently accessed from 2-amino-6-bromo-1,3-diethoxycarbonylazulene in four steps. The 2-NC end of this 2,6-azulenic motif was anchrored to the [Cr(CO)5] fragment prior to formation of its 6-SH terminus. Metalation of the 6-SH end of [(OC)5Cr(η1-2-isocyano-1,3-diethoxycarbonyl-6-mercaptoazulene)] (7) with Ph3PAuCl, under basic conditions, afforded X-ray structurally characterized heterobimetallic Cr0/AuI ensemble [(OC)5Cr(μ-η1:η1-2-isocyano-1,3-diethoxycarbonyl-6-azulenylthiolate)AuPPh3] (8). Analysis of the 13C NMR chemical shifts for the [(NC)Cr(CO)5] core in a series of the related complexes [(OC)5Cr(2-isocyano-6-X-1,3-diethoxy-carbonylazulene)] (X = –NC, Br, H, SH, SCH2CH2CO2CH2CH3, SAuPPh3) unveiled remarkably consistent inverse-linear correlations δ(13COtrans) vs. δ(13CN) and δ(13COcis) vs. δ(13CN) that appear to hold well beyond the above 2-isocyanoazulenic series to include complexes [(OC)5Cr(CNR)] containing strongly electron-withdrawing substituents R, such as CF3, CFClCF2Cl, C2F3, and C6F5. In addition to functioning as a sensitive 13C NMR handle, the essentially C4v-symmetric [(–NC)Cr(CO)5] moiety proved to be an informative, remote, νNC/νCO infrared reporter in probing chemisorption of 7 on the Au(111) surface.

Two new heterotrinuclear Fe–Ru–Fe complexes of ruthenium(II) tetraphenylporphyrin axially coordinated with a pair of isocyanoferrocene ((FcNC)2RuTPP, 1) or 1,1′-diisocyanoferrocene (([C5H4NC]2Fe)2RuTPP, 2) ligands [Fc = ferrocenyl, TPP = 5,10,15,20-tetraphenylporphyrinato(2−) anion] were synthesized and characterized by UV–vis, magnetic circular dichroism, NMR, and FTIR spectroscopies as well as by electrospray ionization mass spectrometry and single-crystal X-ray diffraction. Isolation of insoluble polymeric {([C5H4NC]2Fe)RuTPP}n molecular wires (3) was also achieved for the first time. The redox properties of the new trinuclear complexes 1 and 2 were probed using electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation methods and correlated to those of the bis(tert-butylisocyano)ruthenium(II) tetraphenylporphyrin reference compound, (t-BuNC)2RuTPP (4). In all cases, the first oxidation process was attributed to the reversible oxidation of the RuII center. The second and third reversible oxidation processes in 1 are separated by ∼100 mV and were assigned to two single-electron FeII/FeIII couples, suggesting a weak long-range iron–iron coupling in this complex. Electrochemical data acquired for 2 are complicated by the interaction between the axial η1-1,1′-diisocyanoferrocene ligand and the electrode surface as well as by axial ligand dissociation in solution. Spectroelectrochemical and chemical oxidation methods were used to elucidate the spectroscopic signatures of the [1]n+, [2]n+, and [4]n+ species in solution. DFT and time-dependent DFT calculations aided in correlating the spectroscopic and redox properties of complexes 1, 2, and 4 with their electronic structures.

2,6-Dimercapto-1,3-diethoxycarbonylazulene, a rare example of an unsymmetrical dimercaptoarene, was efficiently synthesized via two routes from common azulenic precursors. The SH-termini of this linear nonbenzenoid dimercaptan exhibit markedly different acidities, which permitted exclusively regioselective metallation of its 6-SH end with the [PPh3PAuI] fragment to afford the only mononuclear adduct (10) of a dimercaptoarene known to date. Subsequent metallation of the 2-SH terminus of 10 with another equivalent of [PPh3PAuI] gave the corresponding dinuclear AuI complex featuring the 2,6-azulenedithiolate linker. The complex constitutes a unique unsymmetrical platform for probing the exchange of gold-bound thiolates within a single compound.

A new trinuclear iron(II) complex involving two isocyanoferrocene ligands axially coordinated to iron(II) phthalocyanine, (FcNC)2FePc [Fc = ferrocenyl; Pc = phthalocyaninato(2−) anion], was isolated and characterized using a variety of spectroscopic methods as well as single-crystal X-ray diffraction. The redox behavior of the above molecular wire was investigated through electrochemical, spectroelectrochemical, and chemical oxidation approaches and compared to that of the bis(tert-butylisocyano)iron(II) phthalocyanine reference compound, (t-BuNC)2FePc. For both complexes, the first oxidation involves the phthalocyanine ligand and results in the formation of a red phthalocyanine cation-radical-centered [(RNC)2FePc]+ species, as evidenced by their UV–vis and electron paramagnetic resonance spectra. Despite the ∼11.5 Å distance between the isocyanoferrocene iron centers, the second and third oxidation potentials for (FcNC)2FePc are separated by ∼80 mV, which is indicative of a weak long-range metal–metal coupling in this system. Spectroscopic signatures of the mixed-valence [(FcNC)2FePc]2+ dication were obtained using spectroelectrochemical and chemical oxidation approaches. These experimentally assessed characteristics were also correlated with the electronic structure, redox properties, and spectroscopic signatures predicted by density functional theory (DFT) and time-dependent DFT analyses.

Three synthetic routes to the unusual supramolecular complex ([Cp2Co]2[{(OC)5V}2(μ-1,4-CNC6Me4NC)])∞, which was crystallographically characterized, are presented. The dianion [{(OC)5V}2(μ-1,4-CNC6Me4NC)]2− constitutes the first subvalent organometallics featuring a diisocyanoarene linker.

Synthesis and self-assembly of structurally related mercapto- and isocyanoazulenes, including novel 2-mercapto-1,3-dicyanoazulene (4) and 2-isocyano-1,3-dicyanoazulene (5), are reported. Exposing 5 adsorbed on Au(111) to a solution of 4 displaces the isocyanoazulene monolayer with that of the mercaptoazulene as judged by νCN signatures of these films.

The key step in accessing the title species (5), the first nonbenzenoid diisocyanobiaryl, involved an unexpected homocoupling of a 6-bromoazulene derivative. The reversible 2e− reduction of 5 was addressed electrochemically and computationally. The shifts in energies of the S0→S1 and S0→S2 transitions for a series of related 6,6′-biazulenyl derivatives correlate with the e−-donating/-withdrawing strength of their 2,2′-substituents but follow opposite trends. Species 5 adsorbs end-on (η1) to the Au(111) surface via one of its −NC groups to form a 2-nm-thick film. In addition, bimetallic coordination of 5’s −NC termini can be readily achieved.

This review is focused on the emerging chemistry of nonbenzenoid aromatic isocyanides, a relatively new family of aryl isocyanide molecules. Two types of systems are discussed: (1) isocyanoazulenes, for which five archetypal isomeric structures may be envisioned, and (2) η5-stabilized isocyanocyclopentadienides. So far, the latter are represented by isocyanoferrocene, 1,1′-diisocyanoferrocene, and isocyanocymantrene. In addition, the synthesis and chemistry of the linear 2,6-diisocyanoazulene motif, including regioselective installation and complexation of its –NC termini with controlled orientation of the azulenic dipole, are described. Self-assembly of nonbenzenoid aryl isocyanides and diisocyanides on gold(1 1 1) surfaces is reviewed as well.

A one-pot transformation of bis(2-isocyano-3-methylphenyl)ethane affords gram quantities of 8,16,24,32-tetraisocyano[2.2.2.2]metacyclophane (3). The solid state structure of 3 is remarkably close to the lowest energy conformation found on the potential energy landscape for 3 by DFT. In solution, the structure of metacyclophane 3 is mobile but can be locked in a rectangular gauche−anti−gauche−anti conformation by coordination of the isocyanide substituents to the [W(CO)5] units to give [M]4(μ4-η1:η1:η1:η1-3) (5). The tetranuclear [M]4(μ4-η1:η1:η1:η1-3) motif featured in crystallographically characterized 5 may be present in several insoluble complexes of 3 previously described as mononuclear η4 species. A self-assembled monolayer of metacyclophane 3 is formed upon exposing a solution of 3 to the gold(111) surface with no precautions to exclude air or light. The monolayer nature of the film was confirmed by optical ellipsometry. The isocyanide stretching band for 3 shifts from 2119 cm−1 in solution to 2175 cm−1 upon chemisorption to metallic gold. The FTIR spectrum of the film indicates interaction of 3 with the gold surface via all four of its isocyanide anchors. No gold-facilitated oxidation of the −NC junctions was detected under ambient conditions. The energy cost associated with accessing the conformations of 3 suitable for μ4-η1:η1:η1:η1 interaction of the molecule with the Au(111) surface is under 8 kcal/mol, a value that can be easily offset by formation of a gold−isocyanide bond. Two different μ4-η1:η1:η1:η1 coordination arrangements of 3 with respect to gold atoms on the (111) face of the fcc Au lattice are suggested.

Synthesis and self-assembly of structurally related mercapto- and isocyanoazulenes, including novel 2-mercapto-1,3-dicyanoazulene (4) and 2-isocyano-1,3-dicyanoazulene (5), are reported. Exposing 5 adsorbed on Au(111) to a solution of 4 displaces the isocyanoazulene monolayer with that of the mercaptoazulene as judged by νCN signatures of these films.

2,6-Dimercapto-1,3-diethoxycarbonylazulene, a rare example of an unsymmetrical dimercaptoarene, was efficiently synthesized via two routes from common azulenic precursors. The SH-termini of this linear nonbenzenoid dimercaptan exhibit markedly different acidities, which permitted exclusively regioselective metallation of its 6-SH end with the [PPh3PAuI] fragment to afford the only mononuclear adduct (10) of a dimercaptoarene known to date. Subsequent metallation of the 2-SH terminus of 10 with another equivalent of [PPh3PAuI] gave the corresponding dinuclear AuI complex featuring the 2,6-azulenedithiolate linker. The complex constitutes a unique unsymmetrical platform for probing the exchange of gold-bound thiolates within a single compound.

Three synthetic routes to the unusual supramolecular complex ([Cp2Co]2[{(OC)5V}2(μ-1,4-CNC6Me4NC)])∞, which was crystallographically characterized, are presented. The dianion [{(OC)5V}2(μ-1,4-CNC6Me4NC)]2− constitutes the first subvalent organometallics featuring a diisocyanoarene linker.

Mercapto (–SH) and isocyano (–NC) terminated conducting π-linkers are often employed in the ever-growing quest for organoelectronic materials. While such systems typically involve symmetric dimercapto or diisocyano anchoring of the organic bridge, this article introduces the chemistry of a linear azulenic π-linker equipped with one mercapto and one isocyano terminus. The 2-isocyano-6-mercaptoazulene platform was efficiently accessed from 2-amino-6-bromo-1,3-diethoxycarbonylazulene in four steps. The 2-NC end of this 2,6-azulenic motif was anchrored to the [Cr(CO)5] fragment prior to formation of its 6-SH terminus. Metalation of the 6-SH end of [(OC)5Cr(η1-2-isocyano-1,3-diethoxycarbonyl-6-mercaptoazulene)] (7) with Ph3PAuCl, under basic conditions, afforded X-ray structurally characterized heterobimetallic Cr0/AuI ensemble [(OC)5Cr(μ-η1:η1-2-isocyano-1,3-diethoxycarbonyl-6-azulenylthiolate)AuPPh3] (8). Analysis of the 13C NMR chemical shifts for the [(NC)Cr(CO)5] core in a series of the related complexes [(OC)5Cr(2-isocyano-6-X-1,3-diethoxy-carbonylazulene)] (X = –NC, Br, H, SH, SCH2CH2CO2CH2CH3, SAuPPh3) unveiled remarkably consistent inverse-linear correlations δ(13COtrans) vs. δ(13CN) and δ(13COcis) vs. δ(13CN) that appear to hold well beyond the above 2-isocyanoazulenic series to include complexes [(OC)5Cr(CNR)] containing strongly electron-withdrawing substituents R, such as CF3, CFClCF2Cl, C2F3, and C6F5. In addition to functioning as a sensitive 13C NMR handle, the essentially C4v-symmetric [(–NC)Cr(CO)5] moiety proved to be an informative, remote, νNC/νCO infrared reporter in probing chemisorption of 7 on the Au(111) surface.