Chiral Teropyrenes: Synthesis, Structure, and Spectroscopic Studies.

We present the inaugural synthesis of a chiral teropyrene achieved through a four-fold alkyne benzannulation catalyzed by InCl3, resulting in good yields. The product underwent thorough characterization using FT-Raman and FT-IR spectroscopies, demonstrating a close agreement with calculated spectra. X-ray crystallographic analysis unveiled a notable twist in the molecule's backbone, with an end-to-end twist angle of 51°, consistent with computational predictions. Experimentally determined enantiomeric inversion barriers revealed a significant energy barrier of 23 kcal/mol, facilitating the isolation of enantiomers for analysis by circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopies. These findings mark significant strides in the synthesis and characterization of chiral teropyrenes, offering insights into their structural and spectroscopic properties.

Partial Deoxygenative CO Homocoupling by a Diiron Complex.

One route to address climate change is converting carbon dioxide to synthetic carbon-neutral fuels. Whereas carbon dioxide to CO conversion has precedent in homo- and heterogeneous catalysis, deoxygenative coupling of CO to products with C-C bonds-as in liquid fuels-remains challenging. Here, we report coupling of two CO molecules by a diiron complex. Reduction of Fe2 (CO)2 L (2), where L2- is a bis(ß-diketiminate) cyclophane, gives [K(THF)5 ][Fe2 (CO)2 L] (3), which undergoes silylation to Fe2 (CO)(COSiMe3 )L (4). Subsequent C-OSiMe3 bond cleavage and C=C bond formation occurs upon reduction of 4, yielding Fe2 (µ-CCO)L. CO derived ligands in this series mediate weak exchange interactions with the ketenylidene affording the smallest J value, with changes to local metal ion spin states and coupling schemes (ferro- vs. antiferromagnetism) based on DFT calculations, Mössbauer and EPR spectroscopy. Finally, reaction of 5 with KEt3 BH or methanol releases the C2 O2- ligand with retention of the diiron core.

Dinitrogen Insertion and Cleavage by a Metal-Metal Bonded Tricobalt(I) Cluster.

Reduction of a tricobalt(II) tri(bromide) cluster supported by a tris(ß-diketiminate) cyclophane results in halide loss, ligand compression, and metal-metal bond formation to yield a 48-electron CoI3 cluster, Co3LEt/Me (2). Upon reaction of 2 with dinitrogen, all metal-metal bonds are broken, steric conflicts are relaxed, and dinitrogen is incorporated within the internal cavity to yield a formally (µ3-η1:η2:η1-dinitrogen)tricobalt(I) complex, 3. Broken symmetry DFT calculations (PBE0/def2-tzvp/D3) support an N-N bond order of 2.1 in the bound N2 with the calculated N-N stretching frequency (1743 cm-1) comparable to the experimental value (1752 cm-1). Reduction of 3 under Ar in the presence of Me3SiBr results in N2 scission with tris(trimethylsilyl)amine afforded in good yield.

Access to Metal Centers and Fluxional Hydride Coordination Integral for CO2 Insertion into [Fe3(µ-H)3]3+ Clusters.

CO2 insertion into tri(µ-hydrido)triiron(II) clusters ligated by a tris(ß-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO2 approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (Fe3H3L2, 4) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener Fe3H3L1 (2), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO2 coordination to an Fe center in the mechanism. The estimated hydricity (50 kcal/mol) based on observed H/D exchange with BD3 requires Fe-O bond formation in the product to offset an endergonic CO2 insertion. µ3-hydride coordination is noted to lower the activation barrier for the first CO2 insertion event in DFT calculations.

Synthetic Factors Governing Access to Tris(ß-diketimine) Cyclophanes versus Tripodal Tri-ß-aminoenones.

Tris(ß-diketimine) cyclophanes are an important ligand class for investigating cooperative multimetallic interactions of bioinorganic clusters. Discussed herein are the synthetic factors governing access to tris(ß-diketimine) cyclophanes versus tripodal tri-ß-aminoenones. Cyclophanes bearing Me, Et, and MeO cap substituents and ß-Me, Et, or Ph arm substituents are obtained, and a modified condensation method produced α-Me ß-Me cyclophane. These operationally simple procedures produce the ligands in gram quantities and in 22-94% yields.

Evaluating Metal Ion Identity on Catalytic Silylation of Dinitrogen Using a Series of Trimetallic Complexes.

We report catalytic silylation of dinitrogen to tris(trimethylsilyl)amine by a series of trinuclear first row transition metal complexes (M = Cr, Mn, Fe, Co, Ni) housed in our tris(ß-diketiminate) cyclophane (L 3- ). Yields are expectedly dependent on metal ion type ranging from 14 to 199 equiv NH4 +/complex after protonolysis for the Mn to Co congeners, respectively. For the series of complexes, the number of turnovers trend observed is Co > Fe > Cr > Ni > Mn, consistent with prior reports of greater efficacy of Co over Fe in other ligand systems for this reaction.

Synthesis, Structure, Photophysical Properties, and Photostability of Benzodipyrenes.

This work explores the syntheses, structures, photophysical properties, and photostability of benzodipyrenes (BDPs). BDPs were synthesized through an InCl3 -AgNTf2 -catalyzed, four-fold alkyne benzannulation reaction. The structures of BDP 4 a and its corresponding endoperoxide product were unambiguously confirmed by X-ray crystallography. The BDPs reported here can also be recognized as peri- and cata-benzannulated pentacenes with a non-functionalized central ring. Unlike the previous reported pentacene-based polycyclic aromatic hydrocarbons, the absorbances of the BDPs were blueshifted by ca. 40 nm relative to pentacene, even after extension of π-conjugation. The newly synthesized BDP products exhibit relatively good stability with half-lives as high as 4612 min in THF.

Chalcogen Impact on Covalency within Molecular [Cu3(µ3-E)]3+ Clusters (E = O, S, Se): A Synthetic, Spectroscopic, and Computational Study.

Reaction of the tricopper(I)-dinitrogen tris(ß-diketiminate) cyclophane, Cu3(N2)L, with O-atom-transfer reagents or elemental Se affords the oxido-bridged tricopper complex Cu3(µ3-O)L (2) or the corresponding Cu3(µ3-Se)L (4), respectively. For 2 and 4, incorporation of the bridging chalcogen donor was supported by electrospray ionization mass spectrometry and K-edge X-ray absorption spectroscopy (XAS) data. Cu L2,3-edge X-ray absorption data quantify 49.5% Cu 3d character in the lowest unoccupied molecular orbital of 2, with Cu 3d participation decreasing to 33.0% in 4 and 40.8% in the related sulfide cluster Cu3(µ3-S)L (3). Multiedge XAS and UV/visible/near-IR spectra are employed to benchmark density functional theory calculations, which describe the copper-chalcogen interactions as highly covalent across the series of [Cu3(µ-E)]3+ clusters. This result highlights that the metal-ligand covalency is not reserved for more formally oxidized metal centers (i.e., CuIII + O2- vs CuII + O-) but rather is a significant contributor even at more typical ligand-field cases (i.e., Cu3II/II/I + E2-). This bonding is reminiscent of that observed in p-block elements rather than in early-transition-metal complexes.

Highly Regioselective Domino Benzannulation Reaction of Buta-1,3-diynes To Construct Irregular Nanographenes.

The properties of nanographenes can be tuned by changing their shapes, therefore the development of new methods suitable for the synthesis of various nanographenes is highly desirable. Described herein is an intramolecular InCl3 /AgNTf2 -catalyzed regioselective domino benzannulation reaction of buta-1,3-diynes to build irregular nanographenes. Different nanographene compounds were easily obtained in moderate to high yields through careful design of the precursor compounds. This new domino reaction was successfully applied to a fourfold alkyne benzannulation of dimethoxy-1,1'-binaphthalene derivatives to arrive at novel chiral butterfly ligand precursors. The regioselectivity of the benzannulation reaction was unambiguously confirmed by X-ray crystallography. Moreover, this new method enables us to synthesize different nanographene isomers and study their optical properties as a function of shape.

Chiral Peropyrene: Synthesis, Structure, and Properties.

Herein we describe the synthesis, structure, and properties of chiral peropyrenes. Using p-terphenyl-2,2″,6,6″-tetrayne derivatives as precursors, chiral peropyrenes were formed after a 4-fold alkyne cyclization reaction promoted by triflic acid. Due to the repulsion of the two aryl substituents within the same bay region, the chiral peropyrene adopts a twisted backbone with an end-to-end twist angle of 28° that was unambiguously confirmed by X-ray crystallographic analysis. The chiral peropyrene products absorb and emit in the green region of the UV-visible spectrum. Circular dichroism spectroscopy shows strong Cotton effects (Δε = ±100 M-1 cm-1 at 300 nm). The Raman data shows the expected D-band along with a split G-band that is due to longitudinal and transversal G modes. This data corresponds well with the simulated Raman spectra of chiral peropyrenes. The chiral peropyrene products also display circularly polarized luminescence. The cyclization reaction mechanism and the enantiomeric composition of the peropyrene products are explained using DFT calculations. The inversion barrier for racemization was determined experimentally to be 29 kcal/mol and is supported by quantum mechanical calculations.

The Synthesis and Characterization of Highly Fluorescent Polycyclic Azaborine Chromophores.

Six new heteroaromatic polycyclic azaborine chromophores were designed, synthesized, and investigated as easily tunable high-luminescent organic materials. The impact of the nitrogen-boron-hydroxy (N-BOH) unit in the azaborines was investigated by comparison with their N-carbonyl analogs. Insertion of the N-B(OH)-C unit into heteroaromatic polycyclic compounds resulted in strong visible absorption and sharp fluorescence with efficient quantum yields. The solid-state fluorescence of the heteroaromatic polycyclic compounds displayed a large Stokes shift compared to being in solution. The large Stokes shifts observed offset the self-quench effect in the solid state.

Pyrenes, Peropyrenes, and Teropyrenes: Synthesis, Structures, and Photophysical Properties.

The design of a relatively simple and efficient method to extend the π-conjugation of readily available aromatics in one-dimension is of significant value. In this paper, pyrenes, peropyrenes, and teropyrenes were synthesized through a double or quadruple benzannulation reaction of alkynes promoted by Brønsted acid. This novel method does not involve cyclodehydrogenation (oxidative aryl-aryl coupling) to arrive at the newly incorporated large arene moieties. All of the target compounds were synthesized in moderate to good yields and were fully characterized with the structures unambiguously confirmed by X-ray crystallography. As expected, photophysical characterization clearly shows increasing red-shifts as a function of extended conjugation within the fused ring systems.

Subtle Modulation of Cu4X4L2 Phosphine Cluster Cores Leads to Changes in Luminescence.

A series of Cu4X4(PPh2py)2 compounds (X = Cl (1), Br (2), I (3), PPh2py = 2-(diphenylphosphino)pyridine) were prepared and characterized using X-ray crystallography, NMR, UV-vis, and luminescence spectroscopy. The copper chloride and bromide clusters have Cu4X4 octahedral cores while the copper iodide clusters contain an unprecedented butterfly shaped core. Crystallization of the copper bromide and iodide clusters from the appropriate solvent produced the solvates 2·2CH2Cl2, 2·2CHCl3, and 3·0.5CH2Cl2 where the presence of the lattice solvate influences the overall structural properties. Using TD-DFT calculations, the emission was assigned to a mixed metal- and halide-to-ligand charge transfer, (M + X)LCT. Subtle differences in the copper core geometry and µ-halide bonding perturb the emissions of these copper(I) halide clusters.

Luminescent Mechanochromism in a Gold(I)-Copper(I) N-Heterocyclic Carbene Complex.

The silver(I) species [Ag(benzim(CH2py)2)2]PF6 (1) was prepared by refluxing the ligand precursor [H(benzim(CH2py)2)2]PF6 with Ag2O and aqueous sodium hydroxide in dichloromethane. Simple transmetalation of 1 with tetrahydrothiophenegold(I) chloride forms the gold(I) analogue [Au(benzim(CH2py)2)2]PF6 (2). The addition of 2 equiv of [Cu(NCCH3)4]PF6 to 2 in acetonitrile produces a blue-luminescent, trimetallic complex, [AuCu2(benzim(CH2py)2)2(NCCH3)4](PF6)3·2CH3CN (3·2CH3CN). When blue-luminescent 3·2CH3CN is exposed to air, the complex loses four acetonitrile molecules, and the emission of the desolvated complex (4) appears aquamarine. Crystallization of 4 from different solvents produces the complexes [AuCu2(benzim(CH2py)2)2](PF6)3 (5) and [AuCu2(benzim(CH2py)2)2(NCCH2CH3)2](PF6)3 (6). Upon grinding, both 3·2CH3CN and 4 exhibit mechanochromic transformations to a yellow-luminescent powder (ground-4). The reversible mechanochromic transformation of 3·2CH3CN to ground-4 is a crystalline-to-amorphous conversion accompanied by partial desolvation. The luminescent mechanochromism of 4 to ground-4 is an "amorphous-to-amorphous" process and does not require solvent loss. In addition to their mechanochromic properties, both 3·2CH3CN and 4 exhibit luminescent thermochromism through desolvation to form a weak luminescent powder (7).

An Air- and Water-Tolerant Zinc Hydride Cluster That Reacts Selectively With CO2.

The reaction of [Zn3Cl3L], in which L(3-) is a tris(ß-diketiminate) cyclophane, with K(sBu)3BH afforded [Zn3(µ-H)3L] (2), as confirmed by NMR spectroscopy, NOESY, and X-ray crystallography. The complex 2 was air-stable and unreactive towards water, methanol, and other substrates (e.g., nitriles) at room temperature over 24 h but reacted with CO2 (ca. 1 atm) to generate [Zn3(µ-H)2(µ-1,1-O2CH)] (3). In contrast, [Zn3(OH)3L] (4) was found to be unreactive toward CO2 over the course of several days at 90 °C.

Modulation of luminescence by subtle anion-cation and anion-π interactions in a trigonal Au(I)···Cu(I) complex.

The trigonally coordinated [AuCu(PPh(2)py)(3)](BF(4))(2) (1) crystallizes in two polymorphs and a pseudopolymorph, each of which contains a trigonally coordinated cation with short Au(I)-Cu(I) separations of ∼2.7 Å. Under UV illumination, these crystals luminesce different colors ranging from blue to yellow. The structures of these cations are nearly superimposable, and the primary difference resides in the relative placement of the anions and solvate molecules. As confirmed by time-dependent density functional theory calculations, it is these interactions that are responsible for the differential emission properties.

Luminescent copper(I) halide adducts of [Au(im(CH2py)2)2]PF6 exhibiting short Au(I)···Cu(I) separations and unusual semibridging NHC ligands.

The picolyl-substituted NHC complex [Au(im(CH(2)py)(2))(2)]PF(6) (1) reacts with two equivalents of copper(I) halides, affording compounds [Au(im(CH(2)py)(2))(2)(CuX)(2)]PF(6) (X = Cl, 2; Br, 3; I, 4). Each complex contains a nearly linearly coordinated [Au(NHC)(2)](+) center where the two picolyl groups on each im(CH(2)py)(2) ligand chelate a single copper atom. The Cu(I) center resides in a distorted tetrahedral environment and is coordinated to two pyridyl groups, a halide ion, and a gold metalloligand. The Au(I)-Cu(I) separations measure 2.7030(5), 2.6688(9), and 2.6786(10) Å for 2-4, respectively. Additionally, each Cu(I) center is further coordinated by a semibridging NHC ligand with short Cu-C separations of ~2.3 Å. In solution, these complexes dissociate the Cu(I) ion. In the solid state, 2-4 are photoluminescent with respective emission maxima of 512, 502, and 507 nm. The reaction of [Au(im(CH(2)py)(2))(2)]PF(6) with four equivalents of CuBr afforded the coordination polymer {[AuCu(2)Br(2)(im(CH(2)py)(2))(2)]Br·3CH(3)CN}(n) (5). This polymeric complex contains [Au(NHC)(2)](+) units interconnected by Cu(2)Br(2) dimers. In 5, the Au-Cu separations are long at 4.23 and 4.79 Å, while the Cu-Cu distance is considerably shorter at 2.9248(14) Å. In the solid state, 5 is photoluminescent with a broad band appearing at 533 nm.

Modulation of metal-metal separations in a series of Ag(I) and intensely blue photoluminescent Cu(I) NHC-bridged triangular clusters.

A series of picolyl-substituted NHC-bridged triangular complexes of Ag(I) and Cu(I) were synthesized upon reaction of the corresponding ligand precursors, [Him(CH(2)py)(2)]BF(4) (1a), [Him(CH(2)py-3,4-(OMe)(2))(2)]BF(4) (1b), [Him(CH(2)py-3,5-Me(2)-4-OMe)(2)]BF(4) (1c), [Him(CH(2)py-6-COOMe)(2)]BF(4) (1d), and [H(S)im(CH(2)py)(2)]BF(4) (1e), with Ag(2)O and Cu(2)O, respectively. Complexes [Cu(3)(im(CH(2)py)(2))(3)](BF(4))(3) (2a), [Cu(3)(im(CH(2)py-3,4-(OMe)(2))(2))(3)](BF(4))(3) (2b), [Cu(3)(im(CH(2)py-3,5-Me(2)-4-OMe)(2))(3)](BF(4))(3), (2c), [Ag(3)(im(CH(2)py-3,4-(OMe)(2))(2))(3)](BF(4))(3), (3b), [Ag(3)(im(CH(2)py-3,5-Me(2)-4-OMe)(2))(3)](BF(4))(3) (3c), [Ag(3)(im(CH(2)py-6-COOMe)(2))(3)](BF(4))(3) (3d), and [Ag(3)((S)im(CH(2)py)(2))(3)](BF(4))(3) (3e) were easily prepared by this method. Complex 2e, [Cu(3)((S)im(CH(2)py)(2))(3)](BF(4))(3), was synthesized by a carbene-transfer reaction of 3e, [Ag(3)((S)im(CH(2)py)(2))(3)](BF(4))(3), with CuCl in acetonitrile. The ligand precursor 1d did not react with Cu(2)O. All complexes were fully characterized by NMR, UV-vis, and luminescence spectroscopies and high-resolution mass spectrometry. Complexes 2a-2c, 2e, and 3b-3e were additionally characterized by single-crystal X-ray diffraction. Each metal complex contains a nearly equilateral triangular M(3) core wrapped by three bridging NHC ligands. In 2a-2c and 2e, the Cu-Cu separations are short and range from 2.4907 to 2.5150 Å. In the corresponding Ag(I) system, the metal-metal separations range from 2.7226 to 2.8624 Å. The Cu(I)-containing species are intensely blue photoluminescent at room temperature both in solution and in the solid state. Upon UV excitation in CH(3)CN, complexes 2a-2c and 2e emit at 459, 427, 429, and 441 nm, whereas in the solid state, these bands move to 433, 429, 432, and 440 nm, respectively. As demonstrated by (1)H NMR spectroscopy, complexes 3b-3e are dynamic in solution and undergo a ligand dissociation process. Complexes 3b-3e are weakly photoemissive in the solid state.

Cleavage of cluster iron-sulfide bonds in cyclophane-coordinated FenSm complexes.

Reaction of the tri(µ-sulfido)triiron(iii) tris(ß-diketiminate) cyclophane complex, Fe3S3LEt/Me (1), or of the di(µ-sulfido)diiron(iii) complex Fe2S2HLEt/Me (5), with the related tri(bromide)triiron(ii) complex Fe3Br3LEt/Me (2) results in electron and ligand redistribution to yield the mixed-ligand multiiron complexes, including Fe3Br2SLEt/Me (3) and Fe2Br2SHLEt/Me (4). The cleavage and redistribution observed in these complexes is reminiscent of necessary Fe-S bond cleavage for substrate activation in nitrogenase enzymes, and provides a new perspective on the lability of Fe-S bonds in FeS clusters.

"On-off" Au(I)...Cu(I) interactions in a Au(NHC)2 luminescent vapochromic sensor.

Reaction of the Au(I) N-heterocyclic carbene (NHC) compound [Au(im(CH(2)py)(2))(2)]PF(6) with 2 equiv of [Cu(MeCN)(4)]PF(6) affords the tricationic compound [Au(im(CH(2)py)(2))(2)(Cu(MeCN)(2))(2)](PF(6))(3) (1), which exhibits blue luminescence (lambda(max) = 462 nm). Reaction of 1 with either liquid MeOH or MeOH vapor affords [Au(im(CH(2)py)(2))(2)(Cu(MeOH))(2)](PF(6))(3) (2), which produces green luminescence (lambda(max) = 520 nm) under UV excitation. The molecular structures of 1 x 2MeCN and 2 x 2MeOH.2Et(2)O were determined by single-crystal X-ray diffraction. Compound 1 contains a linearly coordinated [Au(NHC)(2)](+) core in which each picolyl side arm bridges a [Cu(MeCN)(2)](+) center. The Au...Cu separations are long at 4.596 A. Compound 2 exhibits two short Au...Cu interactions of 2.7195(7) A, with the Au(NHC)(2) core acting as an additional ligand toward each copper center to complete its tetrahedral coordination mode. Exposure of 2 to atmosphere produces a partial loss of MeOH accompanied by a luminescence color change to yellow (lambda(max) = 543 nm). The uptake and loss of MeOH vapor is rapid and reversible. Exposure of 2 to vacuum affords complete loss of MeOH, and the luminescence changes to yellow-orange (lambda(max) = 573 nm). Treatment of 2 with MeCN vapor regenerates 1. The interconversion of 1 and 2 was confirmed by powder X-ray diffraction. Compound 1 also reacts with acetone and H(2)O vapors, leading to species that produce yellow-orange (lambda(max) = 591 nm) and green (lambda(max) = 519 nm) emission, respectively. Compounds 1 and 2 are examples of molecular vapochromic materials that exhibit large changes in the emission though ligand substitution reactions between the solid complex and solvent vapors. The dramatic color change likely results from the "on-off" Au...Cu interactions induced by the ligand exchange reaction.