Tuning the redox profile of the 6,6'-biazulenic platform through functionalization along its molecular axis.

The E1/2 potential associated with reduction of the linearly-functionalized 6,6'-biazulenic scaffold is accurately correlated to the combined σp Hammett parameters of the substituents over >600 mV range. X-ray crystallographic analysis of the 2,2'-dichloro-substituted derivative revealed unexpectedly short C-Cl bond distances, along with other metric changes, suggesting a non-trivial cycloheptafulvalene-like structural contribution.

Homoleptic complexes of isocyano- and diisocyanobiazulenes with a 12-electron, ligand-based redox capacity.

Oligo- and polyazulenes are attractive π-conjugated building blocks in designing advanced functional materials. Herein, we demonstrate that anchoring one or both isocyanide termini of the redox non-innocent 2,2'-diisocyano-6,6'-biazulenic π-linker (1) to the redox-active [Cr(CO)5] moiety provided a convenient intramolecular redox reference for unambiguously establishing that the 6,6'-biazulenic scaffold undergoes a reversible one-step 2e- reduction governed by reduction potential compression/inversion. Treatment of bis(η6-naphthalene)chromium(0) with six equiv. of 2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene (6) or [(OC)5Cr(η1-2,2'-diisocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (11) afforded homoleptic Cr(0) complexes 13 and 14 with a 12e- (per molecule) ligand-based reduction capacity at mild E1/2 of -1.29 V and -1.15 V vs. Cp2Fe0/+, respectively. The overall reversible redox capacity varies from 15e- for the mononuclear complex 13 to 21e- for the heptanuclear complex 14. The latter "nanocomplex" has a diameter of ca. 5 nm and features seven Cr(0) centers interlinked with six 2,2'-diisocyano-6,6'-biazulenic bridges. The X-ray structure of [(OC)5Cr(2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (7) indicated a 43.5° interplanar angle between the two azulenic moieties. Self-assembly of 11 on a Au(111) substrate afforded an organometallic monolayer film of 11 featuring approximately upright orientation of the 2,2'-diisocyano-6,6'-biazulenic linkers, as evidenced by ellipsometric measurements and the RAIR signature of the C4v-symmetric [(-NC)Cr(CO)5] infrared reporter within 11. Remarkably, comparing the FTIR spectrum of 11 in solution with the RAIR spectrum of 11 adsorbed on Au(111) suggested electronic coupling at a ca. 2 nm distance between the Cr(0) and Au atoms linked by the 2,2'-diisocyano-6,6'-biazulene bridge.

Tuning π-Acceptor/σ-Donor Ratio of the 2-Isocyanoazulene Ligand: Non-Fluorinated Rival of Pentafluorophenyl Isocyanide and Trifluorovinyl Isocyanide Discovered.

Isocyanoazulenes (CNAz) constitute a relatively new class of isocyanoarenes that offers rich structural and electronic diversification of the organic isocyanide ligand platform. This article considers a series of 2-isocyano-1,3-X2-azulene ligands (X = H, Me, CO2Et, Br, and CN) and the corresponding zero-valent complexes thereof, [(OC)5Cr(2-isocyano-1,3-X2-azulene)]. Air- and thermally stable, X-ray structurally characterized 2-isocyano-1,3-dimethylazulene may be viewed as a non-benzenoid aromatic congener of 2,6-dimethyphenyl isocyanide (2,6-xylyl isocyanide), a longtime "workhorse" aryl isocyanide ligand in coordination chemistry. Single crystal X-ray crystallographic {Cr-CNAz bond distances}, cyclic voltametric {E1/2(Cr0/1+)}, 13C NMR {δ(13CN), δ(13CO)}, UV-vis {dπ(Cr) → pπ*(CNAz) Metal-to-Ligand Charge Transfer}, and FTIR {νN≡C, νC≡O, kC≡O} analyses of the [(OC)5Cr(2-isocyano-1,3-X2-azulene)] complexes provided a multifaceted, quantitative assessment of the π-acceptor/σ-donor characteristics of the above five 2-isocyanoazulenes. In particular, the following inverse linear relationships were documented: δ(13COtrans) vs. δ(13CN), δ(13COcis) vs. δ(13CN), and δ(13COtrans) vs. kC≡O,trans force constant. Remarkably, the net electron withdrawing capability of the 2-isocyano-1,3-dicyanoazulene ligand rivals those of perfluorinated isocyanides CNC6F5 and CNC2F3.

Positional Isomers of Isocyanoazulenes as Axial Ligands Coordinated to Ruthenium(II) Tetraphenylporphyrin: Fine-Tuning Redox and Optical Profiles.

Two isomeric ruthenium(II)/5,10,15,20-tetraphenylporphyrin complexes featuring axially coordinated redox-active, low-optical gap 2- or 6-isocyanoazulene ligands have been isolated and characterized by NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopic methods, high-resolution mass spectrometry, and single-crystal X-ray crystallography. The UV-vis and MCD spectra support the presence of the low-energy, azulene-centered transitions in the Q band region of the porphyrin chromophore. The first coordination sphere in new L2RuTPP complexes reflects compressed tetragonal geometry. The redox properties of the new compounds were assessed by electrochemical and spectroelectrochemical means and correlated with the electronic structures predicted by density functional theory and CASSCF calculations. Both experimental and theoretical data are consistent with the first two reduction processes involving the axial azulenic ligands, whereas the oxidation profile (in the direction of increasing potential) is exerted by the ruthenium ion, the porphyrin core, and the axial azulenic moieties.

First π-linker featuring mercapto and isocyano anchoring groups within the same molecule: Synthesis, heterobimetallic complexation and self-assembly on Au(111).

Mercapto (-SH) and isocyano (-N≡C) 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-N≡C 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 = -N≡C, 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 asensitive 13C NMR handle, the essentially C4v-symmetric [(-NC)Cr(CO)5] moiety proved to be an informative, remote, νN≡C/νC≡O infrared reporter in probing chemisorption of 7 on the Au(111) surface.

Probing Electronic Communications in Heterotrinuclear Fe-Ru-Fe Molecular Wires Formed by Ruthenium(II) Tetraphenylporphyrin and Isocyanoferrocene or 1,1'-Diisocyanoferrocene Ligands.

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 Ru(II) center. The second and third reversible oxidation processes in 1 are separated by ∼100 mV and were assigned to two single-electron Fe(II)/Fe(III) 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 η(1)-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.

Probing the electronic properties of a trinuclear molecular wire involving isocyanoferrocene and iron(II) phthalocyanine motifs.

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 Šdistance 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.

Diisocyanoarene-linked pentacarbonylvanadate(I-) ions as building blocks in a supramolecular charge-transfer framework assembled through noncovalent π-π and contact ion interactions.

Three synthetic routes to the unusual supramolecular complex ([Cp(2)Co](2)[{(OC)(5)V}(2)(µ-1,4-CNC(6)Me(4)NC)])(∞), which was crystallographically characterized, are presented. The dianion [{(OC)(5)V}(2)(µ-1,4-CNC(6)Me(4)NC)](2-) constitutes the first subvalent organometallics featuring a diisocyanoarene linker.

Ancillary nitrile substituents as convenient IR spectroscopic reporters for self-assembly of mercapto- and isocyanoazulenes on Au(111).

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 ν(C≡N) signatures of these films.

Stereochemically controlled asymmetric 1,2-reduction of enones mediated by a chiral sulfoxide moiety and a lanthanum(III) ion.

Enantiomerically pure (Z)-ß-sulfinyl allylic alcohols of either handedness can be readily prepared from (Z)-ß-sulfinyl enones using NaBH(4) or DIBAL reductants in the presence of LaCl(3) as a chelating agent. A chiral sulfoxide auxiliary induces the remote 1,2-asymmetric reduction (1,4-induction) to afford various chiral allylic alcohols in high yields with excellent stereoselectivities (up to 100% de).

Linear 6,6'-biazulenyl framework featuring isocyanide termini: synthesis, structure, redox behavior, complexation, and self-assembly on Au(111).

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 S(0)→S(1) and S(0)→S(2) 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 (η(1)) 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.

Molecular, electronic structure and spectroscopic properties of MM quadruply bonded units supported by trans-6-carboethoxy-2-carboxylatoazulene ligands.

The reaction between M(2)(TiPB)(4) (M = Mo, W) where TiPB = 2,4,6-triisopropylbenzoate and 6-carboethoxy-2-azulenecarboxylic acid (2 equiv.) in toluene leads to the formation of complexes M(2)(TiPB)(2)(6-carboethoxy-2-azulenecarboxylate)(2). Compound (M = Mo) is blue and compound (M = W) is green. Both are air sensitive, hydrocarbon soluble species that gave the corresponding molecular ions in their mass spectra (MALDI-TOF). They show metal based oxidations and ligand based reductions. Electronic structure calculations (DFT and time dependent DFT) indicate that the two azulene carboxylate pi systems are coupled by their interactions with the M(2)delta orbitals. Their intense colors arise from M(2)delta to azulene pi* electronic transitions. While compound exhibits weak emission at approximately 900 nm, no emission has been detected for . Both and have been studied by fs and ns transient absorption spectroscopy. The X-ray analysis of the molecular structure of in the solid state confirmed the paddlewheel nature of its W(2)(O(2)C)(4) core and the trans orientation of the ligands.

Multipoint anchoring of the [2.2.2.2]metacyclophane motif to a gold surface via self-assembly: coordination chemistry of a cyclic tetraisocyanide revisited.

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(mu 4-eta (1):eta (1):eta (1):eta (1)- 3) ( 5). The tetranuclear [M] 4(mu 4-eta (1):eta (1):eta (1):eta (1)- 3) motif featured in crystallographically characterized 5 may be present in several insoluble complexes of 3 previously described as mononuclear eta (4) 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 mu 4-eta (1):eta (1):eta (1):eta (1) 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 mu 4-eta (1):eta (1):eta (1):eta (1) coordination arrangements of 3 with respect to gold atoms on the (111) face of the fcc Au lattice are suggested.

Concerning the molecular and electronic structure of a tungsten-tungsten quadruply bonded complex supported by two 6-carboethoxy-2-carboxylatoazulene ligands.

The preparation and molecular structure of a W2(4+)-quadruply bonded complex is reported wherein two mutually trans azulene-2-carboxylato ligands are shown to be strongly coupled by ligand pi-M2delta-ligand pi conjugation.

Homoleptic isocyanidemetalates of 4d- and 5d-transition metals: [Nb(CNXyl)(6)](-), [Ta(CNXyl)(6)](-), and derivatives thereof.

Treatment of [M(CO)(6)](-), M = Nb, Ta, with Ag(+), I(2) or NO(+) in the presence of CNXyl provided [M(CNXyl)(7)](+), M(CNXyl)(6)I, or cis-[M(CNXyl)(4)(NO)(2)](+), which are isocyanide analogues of the unknown carbonyl complexes [M(CO)(7)](+), M(CO)(6)I, or cis-[M(CO)(4)(NO)(2)](+), respectively. Reduction of M(CNXyl)(6)I by cesium graphite gave the respective Cs[M(CNXyl)(6)], which have been structurally characterized and represent the first isolable homoleptic isocyanidemetalates for second or third row transition metals. Nitrosylation of [Ta(CNXyl)(6)](-) affords a rare example of a mononitrosyl tantalum complex, Ta(CNXyl)(5)NO, which is an isocyanide analogue of the unknown Ta(CO)(5)NO. This study emphasizes, inter alia, the remarkable versatility of the CNXyl ligand compared to CO in stabilizing various electronic environments at heavier group 5 metal centers.

Interaction of mono- and diisocyanoazulenes with gold surfaces: first examples of self-assembled monolayer films involving azulenic scaffolds.

The formation and properties of a new class of self-assembled monolayers (SAM) of aryl isocyanides and diisocyanides based on the nonbenzenoid azulenic framework have been investigated using FTIR spectroscopy and ellipsometry. Syntheses of several new members of the isocyanoazulene family, a recently established type of aryl isocyanides, are reported as well. The FTIR spectra for the isocyanoazulene derivatives absorbed on the gold surface indicate the terminal upright coordination of every isocyanoazulene molecule studied. In addition, the ellipsometric thicknesses have been measured and are consistent with those calculated for single monolayers of the isocyanides oriented along the surface normal. Unlike SAMs of some benzenoid aryl isocyanides, the nonbenzenoid isocyanoazule-based SAMs proved resistant to oxidation, oligomerization, and isomerization into the corresponding nitriles under ambient conditions, which is an important prerequisite to their future applications.

The 2,6-diisocyanoazulene motif: synthesis and efficient mono- and heterobimetallic complexation with controlled orientation of the azulenic dipole.

Synthesis of the remarkably air- and thermally stable 2,6-diisocyano-1,3-diethoxycarbonylazulene linker from 2-amino-1,3-diethoxycarbonylazulene in 57% cumulative yield was developed. Incorporation of the ester "arms" in the design of this first diisocyanoazulene bridge permitted fully controlled stepwise installation and complexation of its isocyano junction groups. The -CO(2)Et arms in 2,6-diformamido-1,3-diethoxycarbonylazulene effectively suppress the rate of dehydration of its 2-NHCHO end relative to that of the 6-NHCHO end leading to practically exclusive formation of 6-isocyano-2-formamido-1,3-diethoxycarbonylazulene upon treatment of the above diformamide with an equimolar amount of POCl(3). This crystallographically characterized 6-isocyano-2-formamidoazulene derivative was employed to access mono- and heterobimetallic complexes of the 2,6-diisocyanoazulene scaffold with controlled orientation of the azulenic dipole. A complete series of monometallic, homobimetallic, and isomeric heterobimetallic ([M] = M(CO)(5), M = Cr and/or W) complexes of the 2,6-diisocyanoazulene motif was isolated and studied by a variety of techniques, including X-ray crystallography. The metal-to-bridge charge transfer in mono- and dinuclear adducts of 2,6-diisocyanoazulene, the assignment of which was corroborated by time-dependent density functional theory calculations, occurs at a dramatically lower energy as compared to the analogous systems featuring the 1,4-diisocyanobenzene scaffold. Moreover, the metal-to-diisocyanide charge transfer exhibits a substantially greater red shift upon binucleation of the mononuclear [M(CO)(5)] adducts of the nonbenzenoid 2,6-diisocyanoazulene linker versus the 1,4-diisocyanobenzene bridge.

The ferrous verdoheme-heme oxygenase complex is six-coordinate and low-spin.

A biosynthetic and enzymatic method was developed for the preparation of 13C-labeled verdoheme, which permits the 13C NMR spectroscopic characterization of this elusive intermediate in the heme oxidation path catalyzed by the enzyme heme oxygenase. The 13C NMR data indicate that the ferrous verdoheme complex of Neisseria meningitides heme oxygenase is hexacoordinate and diamagnetic, with a proximal histidine and likely a distal hydroxide as axial ligands. The coordination number and spin state of the ferrous verdoheme-heme oxygenase complex is in stark contrast to the pentacoordinate and paramagnetic nature of the heme-heme oxygenase complex and heme centers in general.

Long-range electronic coupling of MM quadruple bonds (M = Mo or W) via a 2,6-azulenedicarboxylate bridge.

The preparation of 2,6-azulenedicarboxylic acid (I) from its diester, 2-CO(2)(t)Bu-6-CO(2)-C(10)H(6) (II), is reported together with the crystal and molecular structure of the ester, II. From the reactions between the dicarboxylic acid I and the MM quadruply bonded complexes M(2)(O(2)C(t)Bu)(4), where M = Mo or W, the azulenedicarboxylate bridged complexes [M(2)(O(2)C(t)Bu)(3)](2)(mu-2,6-(CO(2))(2)-C(10)H(6)) have been isolated, III (M = Mo) and IV (M = W). The latter compounds provide examples of electronically coupled M(2) centers via a polar bridge. The compounds show intense electronic absorptions due to metal-to-bridge charge transfer. This occurs in the visible region of the spectrum for III (M = Mo) but in the near-IR for IV (M = W). One electron oxidation with Ag(+)PF(6)(-) in THF generates the radical cations III(+) and IV(+). By both UV-vis-NIR and EPR spectroscopy the molybdenum ion III(+) is shown to be valence trapped or Class II on the Robin and Day classification scheme. Electrochemical, UV-vis-NIR, and EPR spectroscopic data indicate that, in the tungsten complex ion IV(+), the single electron is delocalized over the two W(2) centers that are separated by a distance of ca. 13.6 A. Furthermore, from the hyperfine coupling to (183)W (I = (1)/(2)), the singly occupied highest molecular orbital is seen to be polarized toward one W(2) center in relationship to the other. Electronic structure calculations employing density functional theory indicate that the HOMO in compounds III and IV is an admixture of the two M(2) delta orbitals that is largely centered on the M(2) unit having proximity to the C(5) ring of the azulenedicarboxylate bridge. The energy of the highest occupied orbital of the bridge lies very close in energy to the M(2) delta orbitals. However, this orbital does not participate in electronic coupling by a hole transfer superexchange mechanism, and the electronic coupling in the radical cations of III and IV occurs by electron transfer through the bridge pi system.

First isocyanoazulene and its homoleptic complexes.

An efficient synthesis of remarkably stable 6-isocyanoazulene (CN6Az), the first nonbenzenoid organic isocyanide, is described. Its superb pi-accepting potential as a ligand was demonstrated through cyclic voltammetry studies on the binary complexes [Cr(CN6Az)6]0/+. The paramagnetic shift patterns for [Cr(CN6Az)6]+, the only azulenic pi-system studied by paramagnetic NMR, suggest that Cr(dpi) --> CN6Az(ppi*) electron delocalization involves both rings of the azulenyl substituents in [Cr(CN6Az)6]+. This conclusion is supported by density functional theory calculations on the complex [Cr(CN6Az)6][BF4].