Tuning of Supramolecular Architectures of l‐Valine‐Containing Dicyanoplatinum(II) 2,2′‐Bipyridine Complexes by Metal–Metal, π–π Stacking,and Hydrogen‐Bonding Interactions

A series of newly synthesized dicyanoplatinum(II) 2,2′‐bipyridine complexes exhibits self‐assembly properties in solution after the incorporation of the l ‐valine amino units appended with various hydrophobic motifs. These l ‐valine‐derived substituents were found to have critical control over the aggregation behaviors of the complexes in the solution state. On one hand, one of the complexes was found to exhibit interesting circularly polarized luminescence (CPL) signals at low temperature due to the formation of chiral spherical aggregates in the temperature‐dependent studies. On the other hand, systematic transformation from less uniform aggregates to well‐defined fibrous and rod‐like structures via Pt???Pt and π–π stacking interactions has also been observed in the mixed‐solvent studies. These changes were monitored by UV/Vis absorption, emission, circular dichroism (CD), and CPL spectroscopies, and morphologies were studied by electron microscopy.     

The Role of Substituent Effects in Tuning Metallophilic Interactions and Emission Energy of Bis‐4‐(2‐pyridyl)‐1,2,3‐triazolatoplatinum(II) Complexes

The photoluminescence spectra of a series of 5‐substituted pyridyl‐1,2,3‐triazolato Pt^II homoleptic complexes show weak emission tunability (ranging from λ=397–408 nm) in dilute (10^?6 M ) ethanolic solutions at the monomer level and strong tunability in concentrated solutions (10^?4 M ) and thin films (ranging from λ=487–625 nm) from dimeric excited states (excimers). The results of density functional calculations (PBE0) attribute this “turn‐on” sensitivity and intensity in the excimer to strong Pt–Pt metallophilic interactions and a change in the excited‐state character from singlet metal‐to‐ligand charge transfer (¹MLCT) to singlet metal‐metal‐to‐ligand charge transfer (¹MMLCT) emissions in agreement with lifetime measurements.     

trans‐Dichloro(dimethyl sulfide‐κS)(pyridine‐κN)platinum(II)

The structure of the title compound, [PtCl₂(C₅H₅N)(C₂H₆S)], consists of discrete mol­ecules in which the Pt‐atom coordination is slightly distorted square planar. The Cl atoms are trans to each other, with a Cl—Pt—Cl angle of 176.60 (7)°. The pyridine ligand is rotated 64.5 (2)° from the Pt square plane and one of the Pt—Cl bonds essentially bisects the C—S—C angle of the di­methyl sulfide ligand. In the crystal structure, there are extensive weak C—H⋯Cl interactions, the shortest of which connects mol­ecules into centrosymmetric dimers. A comparison of the structural trans influence on Pt—S and Pt—­N distances for PtS(CH₃)₂ and Pt(pyridine) fragments, respectively, in square‐planar Pt^II complexes is presented.     

Conjugated Polyelectrolyte‐Induced Self‐Assembly of Alkynylplatinum(II) 2,6‐Bis(benzimidazol‐2′‐yl)pyridine Complexes

Water‐soluble cationic alkynylplatinum(II) 2,6‐bis(benzimidazol‐2′‐yl)pyridine (bzimpy) complexes have been demonstrated to undergo supramolecular assembly with anionic polyelectrolytes in aqueous buffer solution. Metal–metal‐to‐ligand charge transfer (MMLCT) absorptions and triplet MMLCT (³MMLCT) emissions have been found in UV/Vis absorption and emission spectra of the electrostatic assembly of the complexes with non‐conjugated polyelectrolytes, driven by Pt???Pt and π–π interactions among the complex molecules. Interestingly, the two‐component ensemble formed by [Pt(bzimpy‐Et){C?CC₆H₄(CH₂NMe₃‐4)}]Cl₂ ( 1 ) with para‐linked conjugated polyelectrolyte (CPE), PPE‐SO₃^?, shows significantly different photophysical properties from that of the ensemble formed by 1 with meta‐linked CPE, mPPE‐Ala. The helical conformation of mPPE‐Ala allows the formation of strong mPPE‐Ala– 1 aggregates with Pt???Pt, electrostatic, and π–π interactions, as revealed by the large Stern–Volmer constant at low concentrations of 1 . Together with the reasonably large Förster radius, large HOMO–LUMO gap and high triplet state energy of mPPE‐Ala to minimize both photo‐induced charge transfer (PCT) and Dexter triplet energy back‐transfer (TEBT) quenching of the emission of 1 , efficient Förster resonance energy transfer (FRET) from mPPE‐Ala to aggregated 1 molecules and strong ³MMLCT emission have been found, while the less strong PPE‐SO₃^?– 1 aggregates and probably more efficient PCT and Dexter TEBT quenching would account for the lack of ³MMLCT emission in the PPE‐SO₃^?– 1 ensemble.     

Anionic polymerization of 2,7‐di‐t‐butyldibenzofulvene: Synthesis,structure, and photophysical properties of the oligomers with a π‐stacked conformation

2,7‐Di‐t‐butyldibenzofulvene (tBu₂DBF), a bulky dibenzofulvene derivative, was polymerized using n‐butyllithium as initiator in tetrahydrofuran at ?78 °C and in toluene at 0 °C. tBu₂DBF afforded mainly oligomers up to trimer even at [monomer]₀/[initiator]₀ = 20 ([monomer]₀ = 0.2 M) at ?78 °C and 0 °C, indicating that this monomer is much less reactive than dibenzofulvene (DBF), its parent compound. The reaction at the same [monomer]₀/[initiator]₀ ratio at an elevated [monomer]₀ gave a small amount of insoluble polymer. The oligomers indicated a hypochromic effect in the absorption spectra and only monomer emission in the fluorescent spectra. The conformation of the trimer and the dimer was examined by means of NOESY NMR spectra and semiempirical calculations. In the trimer conformation, the fluorene moieties of the central and the initiation‐side monomeric units were found to be closely stacked on top of each other, while the termination‐side monomeric unit appeared to be in a faster conformational dynamics compared with the other monomeric units. Although the dimer seemed to have a relatively flexible conformation, a π‐stacked structure appeared to be involved in the conformational dynamics to show hypochromicity. The results of this study suggest that the reported intramolecular excited dimer (excimer) formation of the poly‐ and oligo(DBF)s [J Am Chem Soc 2003, 125, 15474] is based on a slight, local conformational change upon photo absorption, leading to a closer π‐stacked alignment of two neighboring fluorene units than that in the ground state. Such a local conformational transition may be difficult for the tBu₂DBF trimer because of steric repulsion of the t‐butyl groups. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 561–572, 2006     

Sulfur‐Assisted Intermolecular Triphenylphosphine Dissociation of Thiocarbamoyl Ligand in Platinum(II). X‐ray Structure of Complex [Pt(PPh3)2(η1‐SCNMe2)(Cl)]

Treatment of Pt(PPh₃)₄ with N,N‐dimethylthiocarbamoyl chloride, Me₂NC(=S)Cl, in dichloromethane at ?20 °C processes the oxidative addition reaction to produce platinum complex [Pt(PPh₃)₂(η¹‐SCNMe₂)(Cl)], 2 with releasing two triphenylphosphine molecules. The ³¹P{¹H} NMR spectra of complex 2 shows the dissociation of the triphenylphosphine ligand to form diplatinum complex [Pt(PPh₃)Cl]₂(μ,η²‐SCNMe₂)₂, 3 in which the two SCNMe₂ ligands coordinated through carbon to one metal center and bridging the other metal through sulfur. Complex 2 is characterized by X‐ray diffraction analysis.     

Synthesis and Spectral Properties of 4‐(2,5‐Dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO)

An interesting flourophore, 4‐(2,5‐dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) was synthesized by mixing an equivalent molar quantity of hippuric acid and 2,5‐dimethoxybenzaldehyde in acetic anhydride in the presence of anhydrous sodium acetate. The absorption and fluorescence characteristics of 4‐(2,5‐dimethoxy‐phenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) were investigated in different solvents. DMPO dye exhibits red shift in both absorption and emission spectra as solvent polarity increases, indicating change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMPO gave strong excimer like emission at 630 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B‐type class of Steven's Classification. DMPO displayed fluorescence quenching by triethylamine via nonemissive exciplex formation.     

Luminescence Color Tuning of PtII Complexes and a Kinetic Study of Trimer Formation in the Photoexcited State

We investigated the luminescence properties and color tuning of [Pt(dpb)Cl] (dpbH=1,3‐di(2‐pyridyl)benzene) and its analogues. An almost blue emission was obtained for the complex [Pt(Fmdpb)CN] (FmdpbH=4‐fluoro‐1,3‐di(4‐methyl‐2‐pyridyl)benzene), modified by the introduction of ?F and ?CH₃ groups to the dpb ligand and the substitution of ?Cl by ?CN. As the concentration of the solution was increased, the color of the emission varied from blue to white to orange. The color change resulted from a monomer–excimer equilibrium in the excited state. A broad emission spectrum around 620 nm was clearly detected along with a structured monomer emission around 500 nm. Upon further increases in concentration, another broad peak appeared in the longer wavelength region of the spectrum. We assigned the near‐infrared band to the emission from an excited trimer generated by the reaction of the excimer with the ground‐state monomer. The emission lifetimes of the monomer, dimer, and trimer were evaluated as τ_M=12.8 μs, τ_D=2.13 μs, and τ_T=0.68 μs, respectively, which were sufficiently long to allow association with another Pt^II complex and dissociation into a lower order aggregate. Based on equilibrium constants determined from a kinetic study, the formation of the excimer and the excited trimer were concluded to be exothermic processes, with ΔG*_D=?24.5 kJ mol^?1 and ΔG*_T=?20.4 kJ mol^?1 respectively, at 300 K.     

cis‐Dichloro(dimethyl sulfoxide‐S)(2‐methoxypyridine‐N)platinum(II)

The title complex, [PtCl₂(C₆H₇NO)(C₂H₆OS)], exhibits square‐planar geometry. The plane of the pyridine ring makes a dihedral angle of 67.2 (3)° with the square plane of the metal center. The S—O bond is nearly aligned with the adjacent Pt—N bond, leaving the methyl groups of the di­methyl sulfoxide ligand to stagger the Pt—Cl bond.     

Yellow Circularly Polarized Luminescence from C1‐Symmetrical Copper(I) Complexes

The synthesis of chiral C₁‐symmetrical copper(I) complexes supported by chiral carbene ligands is described. These complexes are yellow emitters with modest quantum yields. Circularly polarized luminescence (CPL) spectra show a polarized emission band with dissymmetry factors |g_lum|=1.2×10^?3. These complexes are the first reported examples of molecular copper(I) complexes exhibiting circularly polarized luminescence. In contrast with most CPL‐emitting molecules, which possess either helical or axial chirality, the results presented show that simple chiral architectures are suitable for CPL emission and unlock new synthetic possibilities.     

Calculation of the Vibrationally Resolved,Circularly Polarized Luminescence of d‐Camphorquinone and (S,S)‐trans‐β‐Hydrindanone

Circularly polarized luminescence (CPL), the differential emission of left‐ and right‐handed circularly polarized light from a molecule, is modeled by using time‐dependent density functional theory. Calculations of the CPL spectra for the first electronic excited states of d‐camphorquinone and (S,S)‐trans‐β‐hydrindanone under the Franck–Condon approximation and using various functionals are presented, as well as calculations of absorption, emission, and circular dichroism spectra. The functionals B3LYP, BHLYP, and CAM‐B3LYP are employed, along with the TZVP and aug‐cc‐pVDZ Gaussian‐type basis sets. For the lowest‐energy transitions, all functionals and basis sets perform comparably, with the long‐range‐corrected CAM‐B3LYP better reproducing the excitation energy of camphorquinone but leading to a blue shift with respect to experiment for hydrindanone. The vibrationally resolved spectra of camphorquinone are very well reproduced in terms of peak location, widths, shapes, and intensities. The spectra of hydrindanone are well reproduced in terms of overall envelope shape and width, as well as the lack of prominent vibrational structure in the emission and CPL spectra. Overall the simulated spectra compare well with experiment, and reproduce the band shapes, emission red shifts, and presence or absence of visible vibrational fine structure.     

Luminescent Benzoquinolate‐Isocyanide Platinum(II) Complexes: Effect of Pt⋅⋅⋅Pt and π⋅⋅⋅π Interactions on their Photophysical Properties

The neutral compounds [Pt(bzq)(CN)(CNR)] (R=tBu ( 1 ), Xyl ( 2 ), 2‐Np ( 3 ); bzq= benzoquinolate, Xyl=2,6‐dimethylphenyl, 2‐Np=2‐napthyl) were isolated as the pure isomers with a trans‐C_bzq,CNR configuration, as confirmed by ¹³C{¹H} NMR spectroscopy in the isotopically marked [Pt(bzq)(¹³CN)(CNR)] (R=tBu ( 1′ ), Xyl ( 2′ ), 2‐Np ( 3′ )) derivatives (δ¹³C_CN≈110 ppm; ¹J(Pt,¹³C)≈1425 Hz]. By contrast, complex [Pt(bzq)(C≡CPh)(CNXyl)] ( 4 ) with a trans‐N_bzq,CNR configuration, has been selectively isolated from [Pt(bzq)Cl(CNXyl)] (trans‐N_bzq,CNR) using Sonogashira conditions. X‐ray diffraction studies reveal that while 1 adopts a columnar‐stacked chain structure with Pt–Pt distances of 3.371(1) Å and significant π???π interactions (3.262 Å), complex 2 forms dimers supported only by short Pt???Pt (3.370(1) Å) interactions. In complex 4 the packing is directed by weak bzq???Xyl and bzq???C≡E (C, N) interactions. In solid state at room temperature, compounds 1 and 2 both show a bright red emission (?=42.1 % 1 , 57.6 % 2 ). Luminescence properties in the solid state at 77 K and concentration‐dependent emission studies in CH₂Cl₂ at 298 K and at 77 K are also reported for 1 , 1·CHCl3 , 2 , 2' , 2·CHCl3 , 3 , 4 .     

Synthesen und NMR‐Untersuchungen von Salzen mit den neuen Anionen [PtXn(CF3)6‐n]2— (n = 0 ‐ 5, X = F,OH, Cl,CN) und die Kristallstrukturanalyse von K2[(CF3)2F2Pt(μ‐OH)2PtF2(CF3)2]·2H2O

Syntheses and NMR Spectroscopic Ivestigations of Salts containing the Novel Anions [PtX_n(CF₃)_6‐n]^2— (n = 0 ‐ 5, X = F, OH, Cl, CN) and Crystal Structure of K₂[(CF₃)₂F₂Pt(μ‐OH)₂PtF₂(CF₃)₂]·2H₂O The first syntheses of trifluoromethyl‐complexes of platinum through fluorination of cyanoplatinates are reported. The fluorination of tetracyanoplatinates(II), K₂[Pt(CN)₄], and hexacyanoplatinates(IV), K₂[Pt(CN)₆], with ClF in anhydrous HF leads after working up of the products to K₂[(CF₃)₂F₂Pt(μ‐OH)₂PtF₂(CF₃)₂]·2H₂O. The structure of the salt is determined by a X‐ray structure analysis, P2₁/c (Nr. 14), a = 11.391(2), b = 11.565(2), c = 13.391(3)Å, β = 90.32(3)°, Z = 4, R₁ = 0.0326 (I > 2σ(I)). The reaction of [Bu₄N]₂[Pt(CN)₄] with ClF in CH₂Cl₂ generates mainly cis‐[Bu₄N]₂[PtCl₂(CF₃)₄] and fac‐[Bu₄N]₂[PtCl₃(CF₃)₃], but in contrast that of [Bu₄N]₂[Pt(CN)₆] with ClF in CH₂Cl₂ results cis‐[Bu₄N]₂[PtX₂(CF₃)₄], [Bu₄N]₂[PtX(CF₃)₅] (X = F, Cl) and [Bu₄N]₂[Pt(CF₃)₆]. In the products [Bu₄N]₂[PtX_n(CF₃)_6‐n] (X = F, Cl, n = 0—3) it is possibel to exchange the fluoro‐ligands into chloro‐ and cyano‐ligands by treatment with (CH₃)₃SiCl und (CH₃)₃SiCN at 50 °C. With continuing warming the trifluoromethyl‐ligands are exchanged by chloro‐ and cyano‐ligands, while as intermediates CF₂Cl and CF₂CN ligands are formed. The identity of the new trifluoromethyl‐platinates is proved by ¹⁹⁵Pt‐ and ¹⁹F‐NMR‐spectroscopy.     

cis‐Dichloridobis{dimethyl[3‐(9‐phosphabicyclo[3.3.1]non‐9‐yl)propyl]amine‐κP}platinum(II)

The title compound, [PtCl₂(C₁₃H₂₆NP)₂], is a rare example of a sterically bulky ligand adopting a cis geometry in a square‐planar complex. It crystallizes on a twofold rotation axis which bisects the Pt centre and the P—Pt—P′ and Cl—Pt—Cl′ angles. The ligand exhibits a random packing disorder in the N,N‐dimethylpropylamine substituent, with the two orientations refining to occupancies of 0.404 (15) and 0.596 (15). Weak intermolecular interactions between a Cl and a H atom of the ligand of a neighbouring molecule result in extended chains along the a axis. The effective cone angle for the dimethyl[3‐(9‐phosphabicyclo[3.3.1]non‐9‐yl)propyl]amine (Phoban[3.3.1]‐C₃NMe₂) ligand was determined as being in the range 160–181°, depending on the choice of atoms used in the calculations.     

cis‐Di­phenyl­bis(1,3,5‐tri­aza‐7‐phospha­adamantane‐κP)platinum(II)

The structure of the title compound, [Pt(C₆H₅)₂(C₆H₁₂N₃P)₂] or [Pt(Ph)₂(PTA)₂] (where Ph is phenyl and PTA is 1,3,5‐tri­aza‐7‐phosphaadamantane), is discussed. Selected geom­etric parameters are: Pt—P = 2.2888 (16) and 2.2944 (17) Å, Pt—C = 2.052 (5) and 2.064 (6) Å, C—Pt—C = 84.6 (2)° and P—Pt—P = 99.28 (6)°. The effective cone angle for the PTA ligands was calculated as 113°.     

Luminescence properties of soluble 2,2′,7,7′‐Tetrakis(2‐(9‐hexyl‐9H‐carbazol‐3‐yl)‐vinyl)‐9,9′‐spirobifluorene‐labeled dendrimers: Photoluminescence and electroluminescence

New light emitting dendrimers were synthesized by reacting 3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzoic acid or 3,5‐bis‐[3,5‐bis‐(3,5‐bis‐benzyloxy‐benzyloxy)‐benzyloxy]‐benzoic acid with a carbazolyl vinyl spirobifluorene moiety. A blue‐emitting core dye was encapsulated by multibenzyloxy dendrons, and two dendrimers having different densities of dendrons were prepared. Photoluminescence (PL) studies of the dendrimers demonstrated that at the higher density of benzyloxy dendrons, the featureless vibronic transitions were improved, causing lesser excimer emission. The similarity of the solution and solid emission spectra of the larger dendrimer, 10 , revealed the suppression of molecular aggregation in the solid film, which is attributed to the presence of the bulky benzyloxy dendrons. The electroluminescence spectra of multilayered devices made using 10 predominantly exhibited blue emissions; similar emission was observed in the PL spectra of its thin film. The multilayered devices made using 3 , 9 , and 10 showed luminances of 1021 cd m^?2 at 5 V, 916 cd m^?2 at 6 V, and 851 cd m^?2 at 6.5 V, respectively. The largest dendrimer, 10 , bearing a greater number of benzyloxy dendrons, exhibited a blue‐like emission with CIE 1931 chromaticity coordinates of x = 0.16 and y = 0.13, which is due to the influence of a higher shielding effect. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 501–514, 2008     

Poly(9‐undecyl‐9‐methyl‐fluorene) and poly(9‐pentadecyl‐9‐methyl‐fluorene): Synthesis,solution structure,and effect of side chain asymmetry on aggregation behavior

We report on solution aggregates and backbone conformation of poly(9‐undecyl‐9‐methyl‐fluorene) (PF1‐11) and poly(9‐pentadecyl‐9‐methyl‐fluorene) (PF1‐15), having two different side chains compared with poly(9,9‐dihexylfluorene) (PF6) and poly(9,9‐dioctylfluorene) (PF8) with two identical side chains. In the poor solvent methylcyclohexane (MCH), X‐ray scattering indicates that PF1‐11 and PF1‐15 appear as three‐dimensional aggregates (5–10 nm wide and thick), forming ribbon‐like agglomerates (correlation lengths of 100 nm). PF6 and PF8 appear as two‐dimensional aggregates (>10 nm wide and 2–3 nm thick) involving ribbon‐like agglomerates (correlation lengths much greater than 100 nm). Upon heating, all aggregates undergo a gel–sol transition which occurs at lower temperatures for PF1‐11 and PF1‐15 (<60°C) than for PF6 and PF8 (>80°C). In the good solvent toluene, PF1‐11 and PF1‐15 form networks of cylindrical particles. The mesh size and the cylinder radius are smaller in 24°C toluene (60 nm, 0.5 nm) than in 60°C MCH (300 nm, 1–2 nm). Nuclear magnetic resonance spectra in toluene‐d₈ together with density functional theory calculations suggest higher torsion angles between polymer repeat units for PF6, PF8, and PF1‐11 (less planar conformation) and a gauche arrangement of the dihedral angles between the bridge carbon atom and the side chain methylene groups in PF1‐15. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 826–837     

The amplified circularly polarized luminescence emission response of chiral 1,1′‐binaphthol‐based polymers via Zn(II)‐coordination fluorescence enhancement

Two kinds of chiral 1,1′‐binaphthol (BINOL)‐based polymer enantiomers were designed and synthesized by the polymerization of 5,5′‐((2,2′‐bis (octyloxy)‐[1,1′‐binaphthalene]‐3,3′‐diyl)bis(ethyne‐2,1‐diyl))bis(2‐hydroxybenzaldehyde) ( M1 ) with alkyl diamine ( M2 ) via nucleophilic addition–elimination reaction. The resulting chiral polymers can exhibit mirror image cotton effects either in the absence or in the presence of Zn²⁺ ion. Almost no fluorescence or circularly polarized luminescence (CPL) emission could be observed for two chiral BINOL‐based polymer enantiomers in the absence of Zn²⁺. Interestingly, the chiral polymers can show strong fluorescence and CPL response signals upon the addition of Zn²⁺, which can be attributed to Zn²⁺‐coordination fluorescence enhancement effect. This work can develop a new strategy on the design of the novel CPL materials via metal‐coordination reaction. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1282–1288     

Production of 5‐Hydroxymethylfurfural from Fructose in Ionic Liquid Efficiently Catalyzed by Cr(III)‐Al2O3 Catalyst

A series of metal‐Al₂O₃ catalysts were prepared simply by the conventional impregnation with Al₂O₃ and metal chlorides, which were applied to the dehydration of fructose to 5‐hydroxymethylfurfural (HMF). An agreeable HMF yield of 93.1% was achieved from fructose at mild conditions (100°C and 40 min) when employing Cr(III)‐Al₂O₃ as catalyst in 1‐butyl‐3‐methylimidazolium chloride ([Bmim]Cl). The Cr(III)‐Al₂O₃ catalyst was characterized via XRD, DRS and Raman spectra and the results clarified the interaction between the Cr(III) and the alumina support. Meanwhile, the reaction solvents ([Bmim]Cl) collected after 1^st reaction run and 5^th reaction run were analyzed by ICP‐OES and LC‐ITMS and the results confirmed that no Cr(III) ion was dropped off from the alumina support during the fructose dehydration. Notably, Cr(III)‐Al₂O₃ catalyst had an excellent catalytic performance for glucose and sucrose and the HMF yields were reached to 73.7% and 84.1% at 120°C for 60 min, respectively. Furthermore, the system of Cr(III)‐Al₂O₃ and [Bmim]Cl exhibited a constant stability and activity at 100°C for 40 min and a favorable HMF yield was maintained after ten recycles.     

Structures and Spectroscopy of some Adducts of Coinage Metal(I) Cyanides with ‘Tetrahedral’‐Unidentate‐N‐Bases

Crystallization of copper(I) cyanide from piperidine (‘pip’) solution yields an adduct of CuCN : pip (3 : 4) ratio, as established by a single crystal X‐ray structure determination, which also shows the complex to have a single‐stranded ···Cu(CN)Cu(CN)···spine (C,N scrambled), one‐third of the copper atoms carrying a pair of pip ligands, the others only one. Crystallization of silver(I) cyanide from piperidine (‘pip’) or cyclohexylamine (‘CyNH₂’) solutions yields adducts of the unusual AgX : unidentate‐N‐base (1 : 2) stoichiometry. The CyNH₂ adduct is, unusually for cyanide complexes of this type, mononuclear with a trigonal planar silver atom, [(NC)Ag(H₂NCy)₂], the AgCN component lying along the intersection of two crystallographic mirror planes which bisect and relate the H₂NCy ligands (Ag‐C, N 2.067(3), 2.335(2) Å; N‐Ag‐N, C 80·80(6), 139.60(4)°). In the pip adduct, the immediate silver atom environment is also three‐coordinate (Ag‐C; N, N 2.080(1); 2.288, 2.443(1) Å; N‐Ag‐N 88·34(4), N‐Ag‐C 144.47(4), 125.07(4), (Σ357.₉°) perturbed toward two‐coordination, but the silver atom environment geometry is further perturbed from planarity by the parallel approach of an inversion‐related molecule (Ag···C′ 2.926(1) Å (Ag···Ag′ 3.1842(2)°) forming a loose, albeit still discrete, dimer. Key features in the IR spectra of the above compounds and of AgCN : pip (1 : 1) and CuCN : CyNH₂ (2 : 3) are assigned and discussed in terms of the structures or of proposed structures in the case of the latter two adducts. The structure of [ClAg(pip)₃], adventitiously obtained, is also described (Ag‐Cl 2.471(3); Ag‐N 2.147(13), 2.188(7) (x2) Å; Cl‐Ag‐N 96.1(3), 98.5(2), N‐Ag‐N 116.3(2) (x2), 122.1(3)°).