Tunable trimers: using temperature and pressure to control luminescent emission in gold(I) pyrazolate-based trimers

Ligand-Directed Metalation of a Gold Pyrazolate Cluster

Solid "[AuL]" (HL = 3-[pyrid-2-yl]-5-tertbutyl-1H-pyrazole) can be crystallized as cyclic [Au₃(μ-L)₃] and [Au₄(μ-L)₄] clusters from different solvents. The crystalline tetramer contains a square Au₄ core with an HT:TH:TH:HT arrangement of ligand substituents, which preorganizes the cluster to chelate to additional metal ions via its pendant pyridyl groups. The addition of 0.5 equiv of AgBF₄ to [AuL] yields [Ag₂Au₄(μ₃-L)₄][BF₄]₂, where two edges of the Au₄ square are spanned by Ag⁺ ions via metallophilic Ag···Au contacts. Treatment of [AuL] with [Cu(NCMe)₄]PF₆ affords the metalloligand helicate [Cu₂Au₂(μ-L)₄][PF₆]₂, via oxidation of the copper and partial fragmentation of the cluster.

Modulating the Extent of Anisotropic Cuprophilicity via High Pressure with Piezochromic Luminescence Sensitization

Metallophilicity has been widely studied as a fundamental supramolecular interaction. However, the extent and directionality thereof remain controversial. A major obstacle lies in the difficulty to separately control the geometry and chemical composition. Herein, we address this challenge by modulating metallophilicity with mechanical pressure. Using a multinuclear Cu(I) complex as model system, we report anomalous anisotropies of (supra)molecular structures, vibrations, and interaction energies upon isotropic compression as well as concomitant (essentially turn-on) piezochromic luminescence enhancement with ∼10³ modulation. The in situ characterizations indicate opposite behaviors of contact distances and cuprophilic interactions for intermolecular vs intramolecular Cu-Cu pairs under pressure. Theoretical calculations break down the attractive and repulsive forces associated with cuprophilicity, its spontaneous 4p-3d hybridization origin, and direction-dependent interaction strength. The use of isotropic mechanical force reveals the intrinsic anisotropy of metallophilicity in multinuclear systems.

Crystal engineering of aurophilic supramolecular architectures and coordination polymers based on butterfly-like Copper-dicyanoaurate complexes: vapochromism, P-T behaviour and multi-metallic cocrystal formation

Using the equilibrium properties of CuII in the presence of the chelating ligand and the characteristics of the dicyanoaurate anion, we were able to obtain a family of 10 bimetallic...

Luminescent polymorphic aggregates of trinuclear Cu(I)-pyrazolate tuned by intertrimeric CuNPy weak coordination bonds

Five luminescent polymorphic aggregates of trinuclear Cu(i)-pyrazolate, namely [anti-Cu3L3]2 (1), [syn-Cu3L3·C2H5OH]2 (2), [anti-Cu3L3·C2H5OH]n (3), [anti-Cu3L3·0.5C7H8]n (4) and [syn-Cu3L3·C8H10]n (5) (HL = 4-(pyridin-4-ylthio)-3,5-dimethyl-1H-pyrazole), were reported. The trimeric Cu3L3 fragments present syn- and anti-conformations dependent on the dangled direction of 4-pyridyl groups on the two sides of the Cu3Pz3 plane (Pz = pyrazolate). Intertrimeric NPyCu weak coordination bonds associate these Cu3L3 fragments together to form dimeric or polymeric structures, which are further stabilized by crystallized solvent molecules or intertrimeric CuCu interactions. The solvated complexes (3-5) may be transformed into the unsolvated complex 1 by evacuation of the crystallized solvents upon heating. All these complexes emit from green to yellow under UV irradiation, which originated from the triplet excited states of metal to ligand charge transfer (3MLCT) mixed with intertrimeric CuCu interactions. This work provides a novel kind of supramolecular aggregate based on Cu3Pz3 beyond the classical π-acidbase adducts and metallophilicity-dependent dimers/oligomers.

Pressure-induced phosphorescence enhancement and piezochromism of a carbazole-based cyclic trinuclear Cu(i) complex

Interest in piezochromic luminescence has increased in recent decades, even though it is mostly limited to pure organic compounds and fluorescence. In this work, a Cu3Pz3 (Cu3, Pz: pyrazolate) cyclic trinuclear complex (CTC) with two different crystalline polymorphs, namely 1a and 1b, was synthesized. The CTC consists of two functional moieties: carbazole (Cz) chromophore and Cu3 units. In crystals of 1a, discrete Cz-Cu3-Cu3-Cz stacking was found, showing abnormal pressure-induced phosphorescence enhancement (PIPE), which was 12 times stronger at 2.23 GPa compared to under ambient conditions. This novel observation is ascribed to cooperation between heavy-atom effects (i.e., from Cu atoms) and metal-ligand charge-transfer promotion. The infinite π-π stacking of Cz motifs was observed in 1b and it exhibited good piezochromism as the pressure increased. This work demonstrates a new concept in the design of piezochromic materials to achieve PIPE via combining organic chromophores and metal-organic phosphorescence emitters.

Tantalum(iv) pyrazolate: new wine in the old wineskin

Homoleptic binuclear Ta(iv) pyrazolate [Ta₂(pz)₈] with Ta–Ta distance of 3.1895(7) Å was obtained in a reaction of Ta(NMe₂)₅ with unsubstituted pyrazole Hpz.

A Computational Study of Metallacycles Formed by Pyrazolate Ligands and the Coinage Metals M = Cu(I), Ag(I) and Au(I): (pzM)n for n = 2, 3, 4, 5 and 6. Comparison with Structures Reported in the Cambridge Crystallographic Data Center (CCDC)

The structures reported in the Cambridge Structural Database (CSD) for neutral metallacycles formed by coinage metals in their valence (I) (cations) and pyrazolate anions were examined. Depending on the metal, dimers and trimers are the most common but some larger rings have also been reported, although some of the larger structures are not devoid of ambiguity. M06-2x calculations were carried out on simplified structures (without C-substituents on the pyrazolate rings) in order to facilitate a comparison with the reported X-ray structures (geometries and energies). The problems of stability of the different ring sizes were also analyzed.

Coinage-Metal-Based Cyclic Trinuclear Complexes with Metal-Metal Interactions: Theories to Experiments and Structures to Functions

Among the d¹⁰ coinage metal complexes, cyclic trinuclear complexes (CTCs) or trinuclear metallocycles with intratrimer metal-metal interactions are fascinating and important metal-organic or organometallic π-acids/bases. Each CTC of characteristic planar or near-planar trimetal nine-membered rings consists of Au(I)/Ag(I)/Cu(I) cations that linearly coordinate with N and/or C atoms in ditopic anionic bridging ligands. Since the first discovery of Au(I) CTC in the 1970s, research of CTCs has involved several fundamental areas, including noncovalent and metallophilic interaction, excimer/exciplex, acid-base chemistry, metalloaromaticity, supramolecular assemblies, and host/guest chemistry. These allow CTCs to be embraced in a wide range of innovative potential applications that include chemical sensing, semiconducting, gas and liquid adsorption/separation, catalysis, full-color display, and solid-state lighting. This review aims to provide a historic and comprehensive summary on CTCs and their extension to higher nuclearity complexes and coordination polymers from the perspectives of synthesis, structure, theoretical insight, and potential applications.

Unequivocal Experimental Evidence of the Relationship between Emission Energies and Aurophilic Interactions

In this paper, we describe experimental evidence of a change in the emission energy as a function of the Au-Au distance. We have employed a luminescent complex exhibiting an aurophilic interaction, which is weak enough to allow its length to be modified by external pressure but rigid enough to confer structural stability on the complex. By determining the crystal structures and emission characteristics over a range of pressures, we have identified an exponential relationship between the energy of the emitted light and the metal-metal distances under pressure. This result can be indirectly related to the repulsive branch of the fitted function representing the energy of the system in the ground state at different Au-Au distances. The relativistic nature of gold appears to play an important role in the behavior of this complex.

Remarkable Aurophilicity and Photoluminescence Thermochromism in a Homoleptic Cyclic Trinuclear Gold(I) Imidazolate Complex

A new aurophilically-bonded cyclic trinuclear gold(I) complex, tris[μ₂-(1-ethylimidazolato-N³,C²)gold(I)] ([Au₃(EtIm)₃], 1), has been synthesized and characterized by temperature-dependent crystallographic and photophysical investigations. The crystal packing of 1 reveals two independent molecules in the unit cell, signifying two distinct pairs of dimer-of-trimer units convened by pairwise intermolecular Au···Au interactions of 3.0662(3) and 3.1407(3) Å at 100 K, representing the shortest pairwise intermolecular aurophilic interactions among all cyclic trimetallic gold(I) complexes to date. Remarkably, crystals of 1 exhibit gigantic photoluminescence thermochromism of 10164 cm^-1-from violet to red!-attributed to internal conversion between a higher-energy (T₂ → S₀; λ_max ∼409 nm) and lower-energy (T₁ → S₀; λ_max ∼700 nm) phosphorescent band below and above 200 K, respectively, likely representing an excited-state phase change.

Observation of a re-entrant phase transition in the molecular complex tris(μ2-3,5-diiso-propyl-1,2,4-triazolato-κ2N1:N2)trigold(I) under high pressure

We report a molecular crystal that exhibits four successive phase transitions under hydro-static pressure, driven by aurophilic interactions, with the ground-state structure re-emerging at high pressure. The effect of pressure on two polytypes of tris(μ2-3,5-diiso-propyl-1,2,4-triazolato-κ2N1:N2)trigold(I) (denoted Form-I and Form-II) has been analysed using luminescence spectroscopy, single-crystal X-ray diffraction and first-principles computation. A unique phase behaviour was observed in Form-I, with a complex sequence of phase transitions between 1 and 3.5 GPa. The ambient C2/c mother cell transforms to a P21/n phase above 1 GPa, followed by a P21/a phase above 2 GPa and a large-volume C2/c supercell at 2.70 GPa, with the previously observed P21/n phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure P21/n phases makes this a rare example of a re-entrant phase transformation. The phase behaviour has been characterized using detailed crystallographic theory and modelling, and rationalized in terms of molecular structural distortions. The dramatic changes in conformation are correlated with shifts of the luminescence maxima, from a band maximum at 14040 cm-1 at 2.40 GPa, decreasing steeply to 13550 cm-1 at 3 GPa. A similar study of Form-II displays more conventional crystallographic behaviour, indicating that the complex behaviour observed in Form-I is likely to be a direct consequence of the differences in crystal packing between the two polytypes.

Coinage Metal Pyrazolates [(3,5-(CF3)2Pz)M]3 (M = Au, Ag, Cu) as Buckycatchers

The synthesis and characterization of supramolecular assemblies {C60[M3]4}∞ consisting of C60 and coinage metal pyrazolates [M3] (i.e., [(3,5-(CF3)2Pz)M]3, where Pz = pyrazolate and M = Au, Ag, and Cu) are reported. {C60[Cu3]4}∞, {C60[Ag3]4}∞ and {C60[Au3]4}∞ form isomorphous crystals. The [M3] moieties adopt a concave conformation to complement the convex C60 surface. They exist as dimers of trimers (i.e., hexanuclear [M3]2 units) that are held together by three close M···M metallophilic interactions at 3.1580(17), 3.2046(7), and 3.2631(7) Å for copper, silver, and gold systems, respectively. The [M3]2 moieties surround each C60 in a tetrahedral fashion, while each [M3]2 is sandwiched by two C60 molecules to form a supramolecular 3D assembly.

Aurophilicity under pressure: a combined crystallographic and in situ spectroscopic study

High pressure crystallographic studies on [1,4-C₆H₄{PPh₂(AuCl)}₂] reveal the largest pressure-induced contraction of an aurophilic interaction observed for any Au(i) complex and further analysis confirms the presence of several types of intermolecular interactions.

Anion-, Solvent-, Temperature-, and Mechano-Responsive Photoluminescence in Gold(I) Diphosphine-Based Dimers

A series of [Au2 (nixantphos)2](X)2 (nixantphos=4,6-bis(diphenylphosphino)-phenoxazine; X=NO3, 1; CF3 COO, 2; CF3 SO3, 3; [Au(CN)2], 4; and BF4, 5) complexes that exhibit intriguing anion-switchable and stimuli-responsive luminescent photophysical properties have been synthesized and characterized. Depending on their anions, these complexes display yellow (3), orange (4 and 5), and red (1 and 2) emission colors. They exhibit reversible thermo-, mechano-, and vapochromic luminescence changes readily perceivable by the naked eye. Single-crystal X-ray studies show that the [Au2 (nixantphos)2](2+) cations with short intramolecular Au⋅⋅⋅Au interactions are involved as donors in an infinite N-H⋅⋅⋅X (X=O and N) hydrogen-bonded chain formation with CF3 COO(-) (2 C) and aurophilically linked [Au(CN)2](-) counterions (4 C). Both crystals show thermochromic luminescence; their room temperature red (2 C) and orange (4 C) emission turns into yellow upon cooling to 77 K. They also exhibit reversible mechanochromic luminescence by changing their emission color from red to dark (2 C), and orange to red (4 C). Compounds 1-5 also display reversible mechanochromic luminescence, altering their emission colors between orange (1) or red (2) to dark, as well as between yellow (3) or orange (4 and 5) to red. Detailed photophysical investigations and correlation with solid-state structural data established the significant role of NH⋅⋅⋅X interactions in the stimuli-responsive luminescent behavior.

Absorbate-induced piezochromism in a porous molecular crystal

Atmospherically stable porous frameworks and materials are interesting for heterogeneous solid-gas applications. One motivation is the direct and selective uptake of pollutant/hazardous gases, where the material produces a measurable response in the presence of the analyte. In this report, we present a combined experimental and theoretical rationalization for the piezochromic response of a robust and porous molecular crystal built from an extensively fluorinated trispyrazole. The electronic response of the material is directly determined by analyte uptake, which provokes a subtle lattice contraction and an observable bathochromic shift in the optical absorption onset. Selectivity for fluorinated absorbates is demonstrated, and toluene is also found to crystallize within the pore. Furthermore, we demonstrate the application of electronic structure calculations to predict a physicochemical response, providing the foundations for the design of electronically tunable porous solids with the chemical properties required for development of novel gas-uptake media.

A high-pressure crystallographic and magnetic study of Na5[Mn(l-tart)2]·12H2O (l-tart = l-tartrate)

The crystal structure and magnetic properties of the compound Na₅[Mn(l-tart)₂]·12H₂O (1, l-tart = l-tartrate) have been investigated over the pressure range 0.34–3.49 GPa. 1 retains its magnetic anisotropy even under pressure.