Metal Pyrazolates Research Articles

Conversion of Metal Pyrazolate/(Hydr)oxide Clusters into Nanojars: Solution vs Solid-State Structure and Magnetism.

Nanojars are a class of anion binding and extraction agents composed of a series of [Cu(μ-OH)(μ-pz)]n (pz = pyrazolate; n = 26-36) supramolecular metal-organic complexes. In contrast to other anion binding agents amenable to liquid-liquid extraction, nanojars only form by self-assembly around the target anion, and guest-free nanojar hosts cannot be isolated. An extraordinary binding strength toward highly hydrophilic anions such as carbonate and sulfate was demonstrated by the inability of Ba2+ ions to precipitate the corresponding insoluble barium salts from nanojars. Herein, we provide an additional proof for the superior robustness of the nanojar framework based on competition experiments with other transition metal pyrazolate/(hydr)oxide complexes. In addition to the mass spectrometric characterization, we present variable-temperature nuclear magnetic resonance studies with an emphasis on the influence of the paramagnetic Cu2+ centers on 1H hyperfine shifts, along with X-ray crystallographic analysis of two polymorphs of (MePh3P)2[CO3⊂{Cu(OH)(pz)}27], including the highest (cubic) symmetry nanojar crystal lattice obtained to date as well as magnetism studies for the first time. Furthermore, we provide evidence for the first molybdate-incarcerating nanojars, [MoO4⊂{Cu(μ-OH)(μ-pz)}n]2- (n = 28, 31-33), formed by rearrangement from [MoVI8O12(μ-O)9(μ-pz)6(pzH)6·3pzH] in the presence of Cu2+ ions.

Carbon dioxide affinity (“carboxophilicity”) of trivalent light metal pyrazolates

Group 3 and 13 light metal pyrazolates reversibly insert carbon dioxide. The inverse correlation of their catalytic activity and carboxophilicity is striking.

Filling the gap with a bulky diaryl boron group: fluorinated and non-fluorinated copper pyrazolates fitted with a dimesityl boron moiety on the backbone.

Successful synthesis has been reported of 4-Mes2B-3,5-(CF3)2PzH and 4-Mes2B-3,5-(CF3)2PzH bearing sterically demanding diarylboron moieties at the pyrazole ring 4-position, and their corresponding copper(I) pyrazolate complexes. They show visible blue photoluminescence in solution. The X-ray crystal structures revealed that the fluorinated {[4-BMes2-3,5-(CF3)2Pz]Cu}3 crystallizes as discrete trinuclear molecules whereas as the non-fluorinated {[4-BMes2-3,5-(CH3)2Pz]Cu}3 forms dimers of trimers with two close inter-trimer Cu⋯Cu separations. The solid {[4-BMes2-3,5-(CF3)2Pz]Cu}3 featuring a sterically confined Cu3N6 core displays bright blue phosphorescence while {[4-BMes2-3,5-(CH3)2Pz]Cu}3, which is a dimer of a trimer, is a red phosphor at room temperature. This work illustrates the modulation of photo-physical properties of metal pyrazolates by adjusting the supporting ligand steric features and introducing secondary diarylboron luminophores. Computational analysis of the structures and photophysical properties of copper complexes are also presented.

The use of DOSY experiments to determine the solution structures of coinage metal pyrazolates: The case of {[3,5-(CF3 )2 Pz]Ag}3.

A series of DOSY experiments have been carried out to determine the solution stoichiometry of silver(I) 3,5-bis (trifluoromethyl)pyrazolate species. This compound exists as a trimer in the solid state (n = 3) but in solutions of chlorinated solvents, the DOSY data suggest the presence of a mixture of solvent stabilized monomer (n = 1) and dimer (n = 2) in equilibrium. Different approximations have been used including the Stokes-Einstein and the Stokes-Einstein-Gierer-Wirtz equations. Some methodological problems are discussed.

Coinage metal metallacycles involving a fluorinated 3,5-diarylpyrazolate

Photoluminescent, trinuclear, coinage metal pyrazolates have been isolated using a fluorinated diaryl-pyrazolate.

Stable pyrazolate-based metal-organic frameworks for drug delivery

To explore materials stable in alkaline solution, a linear ligands 1,4-benzenedi(4′-pyrazolyl), in which the pyrazolate group was considered as a soft Lewis base, was used to reacted with two relative soft Lewis acid Ni(II) and Zn(II) metal ions, to constructed stable MOFs structure. Since the metal pyrazolate clusters resemble with metal carboxylate clusters performing the same connection mode during self-assembly, the obtained materials PFC-6 and PFC-7 exhibit same topologies with classic carboxylate-based MOFs UiO-66 and MIL-53, respectively. Due to the strong coordination bond between soft Lewis base and soft Lewis acid, PFC-6 and PFC-7 exhibit high porosity and excellent stability from pH = 2 to pH = 13. Inspired by these features, PFC-6 was used as a drug carrier and the DCF drug uptake/release behavior were investigated. This work presented here demonstrates that the pyrazolate-based ligands are capable of constructing MOFs structure possessing same topologies with carboxylate ligands but high stability in alkaline solution.

Supramolecular assembly of group 11 phosphorescent metal complexes for chemosensors of alcohol derivatives

We report on systematic study on vapochromic sensing of ethanol by using phosphorescent trinuclear metal pyrazolate complexes with supramolecular assembly of weak intermolecular metal-metal interactions using 4-(3,5-dimethoxybenzyl)-3,5-dimethyl pyrazole ligand (1) and group 11 metal ions (Cu(I), Ag(I), Au(I)). Upon excitation at 284, the resulting complexes showed emission bands with a peak centered at 616, 473 and 612 nm for 2(Cu), 2(Ag) and 2(Au), respectively. Chemosensor 2(Cu) showed positive response to ethanol vapors in 5 mins by blue-shifting its emission band from 616 to 555 nm and emitting bright orange to green. Otherwise 2(Au) gave shifting from its emission band centered at 612 to 587 nm with Δλ of 25 nm (41%) and color changes from red-orange to light green-orange while 2(Ag) showed quenching in its original emission intensity at 473 nm in 40% with color changes from dark green to less emissive. These results demonstrate that sensing capability of chemosensor 2(Cu) with suitable molecular design of ligand and metal ion in the complex is due to the formation of a weak intermolecular hydrogen bonding interaction of O atom at the methoxy of the benzyl ring with the OH of the vapors at the outside of the molecules.

The Role of Weak Interactions in Strong Intermolecular M···Cl Complexes of Coinage Metal Pyrazolates: Spectroscopic and DFT Study.

The nondestructive reversible complexation of the macrocyclic group 11 metal pyrazolates {[3,5-(CF3)2Pz]M}3 (M = Cu(I), Ag(I)) to the halogen atom X = Cl, Br of η(3)-allyliron tricarbonyl halides (η(3)-2-R-C3H4)Fe(CO)3X is revealed by the variable-temperature spectroscopic (IR, NMR) study combined with density functional theory calculations. The composition of all complexes at room temperature is determined as 1:1. In the case of the [AgL]3 macrocycle, complexes 1:2 are observed at low temperature (<260 K). The complex's stability depends on the substituents in the allyl fragment and halide ligand as well as on the metal atom (Ag(I), Cu(I)) in the macrocycle. For bulky substituents (Me and Ph) the endo/exo equilibrium of the parent (η(3)-2-R-C3H4)Fe(CO)3X shifts upon the complex formation in favor of the exo isomer due to additional noncovalent interactions of the substituent with macrocycle.

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.

Crowning of Coinage Metal Pyrazolates: Double-Decker Homo- and Heteronuclear Complexes with Synergic Emissive Properties

A new pyrazole ligand with flexible thioether chelate arms was synthesized and was used to obtain an unprecedented class of hexanuclear coinage metal complexes of general formula [MM'L]3Y3 (M, M' = Cu, Ag, Au; Y = OTf, BF4). Three of them were characterized by X-ray crystallography, namely, homometallic [Ag2L]3(OTf)3 and [Ag2L]3(BF4)3 as well as heterometallic [CuAgL]3(OTf)3, revealing that the classical [M(μ-pz)]3 core is crowned by a second deck of S-bound M' ions. Depending on the solvent, these oligonuclear systems undergo rapid dynamics and show cation-anion aggregation in solution, which has been investigated by DOSY and temperature dependent NMR spectroscopy. Preliminary luminescence data for selected hexametallic [MM'L]3Y3 complexes show that the combination of ligand-directed intramolecular and supramolecular d(10) metal ion interactions in the solid state gives rise to synergic emissive properties that allow for a selective addressing of different emission wavelengths.

Trinuclear Copper(I) and Silver(I) Adducts of 4-Chloro-3,5-bis(trifluoromethyl)pyrazolate and 4-Bromo-3,5-bis(trifluoromethyl)pyrazolate

Syntheses, structural, and photoluminescence properties of {[4-X-3,5-(CF3)2Pz]M}3 (X = Cl or Br, M = Cu or Ag) containing a heavier halide at the pyrazolyl ring 4-positions are reported. The Cu2O and Ag2O react with [4-Cl-3,5-(CF3)2Pz]H or [4-Br-3,5-(CF3)2Pz]H to form the corresponding metal pyrazolates, which are trinuclear adducts of the type {[4-X-3,5-(CF3)2Pz]M}3 with a nine-membered M3N6 metallacyclic core. They also feature relatively short M···Cl or M···Br intertrimer separations (∼ 3.6 Å) leading to supramolecular aggregates in the solid state. Distinct from the 4-H analogues {[3,5-(CF3)2Pz]M}3, none of the four complexes described herein exhibits short intertrimer metal-metal interactions (as closest such M···M separations are at a distance greater than 5.0 Å). The {[4-X-3,5-(CF3)2Pz]M}3 adducts exhibit bright photoluminescence even at room temperature. The photophysical data suggest that the {[4-X-3,5-(CF3)2Pz]Cu}3 complexes emit from an associative excited state, and the drastic Stokes shift suggests a significant change to the ground state structure of the trinuclear moiety and/or intermolecular interactions upon photoexcitation. The {[4-X-3,5-(CF3)2Pz]Ag}3 complexes emit from a ligand-centered excited state affected by silver and the heavier halogens. Thin films of {[4-X-3,5-(CF3)2Pz]Cu}3 trimers are promising for volatile organic compound (VOC) sensor applications as they exhibit luminescence color change upon exposure to vapors of benzene and its alkylated derivatives.

A New Dimension in Cyclic Coinage Metal Pyrazolates: Decoration with a Second Ring of Coinage Metals Supported by Inter-ring Metallophilic Interactions

When pyrazolate ligands with thioether chelate arms are used in cyclic coinage metal pyrazolates [Au(μ-pz)](n), the inner gold ring can be framed with an outer silver ring to give novel heterometallic double-crowned complexes [AuAg(μ-L(x))(BF(4))](4). They feature short intramolecular in-plane Ag-Au interactions, are stable as octanuclear species in solution, and show promising luminescence properties.

Silver Pyrazolates as Coordination-Polymer Luminescent Metallomesogens

Silver pyrazolates with columnar liquid-crystal phases that are stable at room temperature have been prepared by reaction of silver nitrate with 3,5-diarylpyrazolates. The complexes consist of open-chain oligomers, despite the fact that the most common structural type for homoleptic coinage metal pyrazolates is the trimeric metallacycle [M(μ-pz)](3). The special characteristics of silver in forming reversible metal-ligand bonds in solution, evidenced experimentally, leads to supramolecular organizations in which the silver cations promote self-organization of the nonmesomorphic pyrazolates into helical 1D polymers that exhibit columnar mesophases. The materials are readily soluble in common organic solvents and are liquid-crystalline over a broader temperature range than their gold counterparts, which are known to form discrete cyclic trinuclear species. Thin films of the silver complexes show luminescence at room temperature. The compounds described here are the first examples of luminescent metallomesogens formed by a main-chain coordination polymer.

Coordination chemistry in the solid state: synthesis and interconversion of pyrazolium salts, pyrazole complexes, and pyrazolate MOFs

Solid pyrazole reacts with HCl gas to form pyrazolium chloride [H2pz]Cl, which reacts in the solid state, under grinding, with metal chlorides MCl2 (M = Co, Zn, Cu) to form the pyrazolium tetrachlorometallate salts [H2pz]2[MCl4] (M = Co 1, Zn 3, Cu 5). Salt 5 cannot be made in solution, and upon standing at room temperature spontaneously emits HCl to give the coordination compound [CuCl2(Hpz)2] (6). Compounds 1 and 3 do not exhibit this behaviour, but can be ground together with bases such as KOH or K2CO3 to effect the elimination of HCl and afford their respective [MCl2(Hpz)2] compounds (M = Co 2, Zn 4). 2, 4 and 6 can also be synthesised in the solid-state by direct reaction of the appropriate metal chloride with pyrazole, or by reaction of a basic metal salt such as the carbonate or hydroxide with pyrazolium chloride. 4 and 6 {and their nickel analogue [NiCl2(Hpz)2]} can be ground with a further two equivalents of base to make the known polymeric metal pyrazolates [M(pz)2]n (M = Ni 7, Cu 8, Zn 9); the same reaction appears to work for the cobalt analogue 2, but the presumed product [Co(pz)2]n10 then decomposes by oxidation. The imidazolate complexes [M(im)2] (M = Ni, 11; Cu, 12; Zn, 13; Co, 14) were similarly prepared by grinding the appropriate [M(Him)2Cl2] precursor with KOH.

Role of Donor and Secondary Interactions in the Structures and Thermal Properties of Alkaline-Earth and Rare-Earth Metal Pyrazolates

The addition of neutral coligands to reduce the aggregation and improve the volatility of potential heavy alkaline-earth metal chemical vapor deposition (CVD) precursors has typically resulted in liberation of the coligand upon heating. A new series of dinuclear alkaline-earth and rare-earth metal pyrazolates, bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)calcium] (1), bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)strontium] (2), and bis[bis(3,5-di-tert-butylpyrazolato)bis(tetrahydrofuran)barium] (3), have been obtained from our previous donor-free oligonuclear complexes [{M(3,5-tBu2pz)2}n] (5, M = Ca, n = 3; 6, M = Sr, n = 4; 7, M = Ba, n = 6) by treatment with tetrahydrofuran (THF). Compounds 1-3, as well as the europium analogue bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)europium(II)] (4), can also be prepared by direct reaction of the metals and pyrazole in THF and anhydrous liquid ammonia. Recrystallization from hexane led to single crystals of 2-4, while the powder diffraction pattern of 1 revealed it to be isostructural with the previously published bis[bis(3,5-di-tert-butylpyrazolato)(tetrahydrofuran)ytterbium(II)] (8), providing important insight into differences and similarities between the two groups of metals. Detailed structural analysis of the compounds reveals secondary interactions including pi-bonding and agostic interactions, which are considered essential in stabilizing the metal complexes. The direct comparison of structural features and thermal properties (as evaluated by thermogravimetric analysis and sublimation studies) of the donor-free oligonuclear and the donor-containing dinuclear species offers a better understanding of the role of donors and secondary interactions.

Intertrimer and Intratrimer Metallophilic and Excimeric Bonding in the Ground and Phosphorescent States of Trinuclear Coinage Metal Pyrazolates: A Computational Study

The interactions present in cyclic trinuclear coinage metal pyrazolates were studied computationally. Cuprophilic interaction was found to bind the singlet ground state of the dimer of trimers [[Cu(Pz)](3)](2), overcoming electrostatic repulsion. The large variation in intertrimer separations found in the literature for coinage metal pyrazolates is consistent with the relatively weak metallophilic interaction. The emissive triplet excited-state geometry of [[M(Pz)](3)](2) is predicted by density functional calculations to show major geometric distortion caused by Jahn-Teller instability and excimeric M-M bonding. Large calculated Stokes' shifts, which are also observed for experimental models, are consistent with significant excited-state distortions for these materials. The major finding derived from the present study is that the intertrimer M...M contraction in the emissive T(1) state is much more than the intratrimer contraction in all [[M(Pz)](3)](2) models, giving rise to a lower T(1) --> S(0) phosphorescence energy in these models than in analogous monomer-of-trimer models. The observations made here point to a great potential for rationally tuning the emission properties of trinuclear coinage metal complexes through choice of the metal and ligands.

X-RAY POWDER DIFFRACTION CHARACTERIZATION OF POLYMERIC METAL DIAZOLATES

ABSTRACT Polymeric metal diazolates typically appear as insoluble and intractable powders, the structure of which could only be retrieved by the extensive use of ab-initio X-ray powder diffraction (XRPD) methods from conventional laboratory data. A number of selected examples from the metal pyrazolate, imidazolate, pyrimidin-2-olate and pyrimidin-4-olate classes are presented, highlighting the specific crystallochemical properties, material functionality and methodological aspects of the structure determination process. Linear and helical one-dimensional polymers, layered systems and three-dimensional networks are described, with particular emphasis on polymorphism and on the thermal, optical, magnetic and sorption properties. A brief outline of the method, as it has been tailored in our laboratories during the last decade, is also offered.

Heavy alkaline earth metal pyrazolates: synthetic pathways, structural trends, and comparison with divalent lanthanoids.

Two series of heavy alkaline earth metal pyrazolates, [M(Ph(2)pz)(2)(thf)(4)] 1 a-c (Ph(2)pz=3,5-diphenylpyrazolate, M=Ca, Sr, Ba; THF=tetrahydrofuran) and [M(Ph(2)pz)(2)(dme)(n)] (M=Ca, 2 a, Sr, 2 b, n=2; M=Ba, 2 c, n=3; DME=1,2-dimethoxyethane) have been prepared by redox transmetallation/ligand exchange utilizing Hg(C(6)F(5))(2). Compounds 1 a and 2 b were also obtained by redox transmetallation with Tl(Ph(2)pz). Alternatively, direct reaction of the alkaline earth metals with 3,5-diphenylpyrazole at elevated temperatures under solventless conditions yielded compounds 1 a-c and 2 a-c upon extraction with THF or DME. By contrast, [M(Me(2)pz)(2)(Me(2)pzH)(4)] 3 a-c (M=Ca, Sr, Ba; Me(2)pzH=3,5-dimethylpyrazole) were prepared by protolysis of [M[N(SiMe(3))(2)](2)(thf)(2)] (M=Ca, Sr, Ba) with Me(2)pzH in THF and by direct metallation with Me(2)pzH in liquid NH(3)/THF. Compounds 1 a-c and 2 a-c display eta(2)-bonded pyrazolate ligands, while 3 a,b exhibit eta(1)-coordination. Complexes 1 a-c have transoid Ph(2)pz ligands and an overall coordination number of eight with a switch from mutually coplanar Ph(2)pz ligands in 1 a,b to perpendicular in 1 c. In eight coordinate 2 a,b the pyrazolate ligands are cisoid, whilst 2 c has an additional DME ligand and a metal coordination number of ten. By contrast, 3 a,b have octahedral geometry with four eta(1)-Me(2)pzH donors, which are hydrogen-bonded to the uncoordinated nitrogen atoms of the two trans Me(2)pz ligands. The application of synthetic routes initially developed for the preparation of lanthanoid pyrazolates provides detailed insight into the similarities and differences between the two groups of metals and structures of their complexes.

One-dimensional polymers containing strictly collinear metal ions: synthesis and XRPD characterization of homoleptic binary metal pyrazolates.

The synthesis of a number of 3d transition metal binary pyrazolates in microcrystalline form, thus suitable for a full XRPD characterization, has been pursued. The crystal and molecular structures of the Fe(pz)3, Co(pz)2, Co(pz)3, and Ni(pz)2 polymers, together with the few congeners reported in the recent literature, show that these species tend to afford highly crystalline materials where strictly collinear chains of metal atoms are present. Depending on the synthetic strategy used, Ni(pz)2 has been found to crystallize as two different alpha (orthorhombic) and beta (monoclinic) phases, possessing nearly identical intramolecular features. Data for each compound follow. Fe(pz)3: C9H9FeN6, hexagonal, P63/m, a = 9.1745(3) A, c = 7.2191(4) A, Z = 2. Co(pz)2: C6H6CoN4, orthorhombic, Ibam, a = 7.5239(5) A, b = 14.3461(9) A, c = 7.4331(5) A, Z = 4. Co(pz)3: C9H9CoN6, hexagonal, P63/m, a = 9.1966(3) A, c = 7.1051(3) A, Z = 2. Alpha-Ni(pz)2: C6H6N4Ni, orthorhombic, Cmcm, a = 16.6758(11) A, b = 6.4872(4) A, c = 6.9423(6) A, Z = 4. Beta-Ni(pz)2: C6H6N4Ni, monoclinic, P21/m, a = 9.967(2) A, b = 6.975(1) A, c = 6.016(1), A, beta = 98.50(1)degrees, Z = 2. The thermal stability and the detailed structural properties of these model compounds have been evaluated, in the light of the technologically relevant crystal phases (the well-known metal-diazolates showing reversible spin-crossover or spin-transition behavior) obtainable upon doping, magnetic dilution, and ring substitution (in the 4-position).

Zinc bis(pyrazolyl)silane complexes

The first two members of a new family of nitrogen-donor ligands, the bis(pyrazolyl)silanes Me 2Si(3,5-R 2pz) 2 (R=H, Me), have been readily synthesized in almost quantitative yield from Me 2SiCl 2 and the alkali metal pyrazolates M(3,5-R 2pz) (M=Li or Na). Both bidentate ligands, abbreviated Bps and Bps Me 2 , respectively, have been used to prepare the zinc derivatives LZnX 2 (L=Bps, Bps Me 2 ; X=Cl, Br, I), all of which have been characterized by a combination of analytical and spectroscopic techniques and, in the case of (Bps Me 2 )ZnI 2, also by single-crystal X-ray diffraction.