Pyrazole Molecules Research Articles

Hydrogen bonding in the crystal structure of the molecular salt of pyrazole-pyrazolium picrate.

The asymmetric unit of the title organic salt [systematic name: 1H-pyrazol-2-ium 2,4,6-tri-nitro-phenolate-1H-pyrazole (1/1)], H(C3H4N2)2 (+)·C6H2N3O7 (-), consists of one picrate anion and one hydrogen-bonded dimer of a pyrazolium monocation. The H atom involved in the dimer N-H⋯N hydrogen bond is disordered over both symmetry-unique pyrazole mol-ecules with occupancies of 0.52 (5) and 0.48 (5). In the crystal, the component ions are linked into chains along [100] by two different bifurcated N-H⋯(O,O) hydrogen bonds. In addition, weak C-H⋯O hydrogen bonds link inversion-related chains, forming columns along [100].

Crystal structure of 6-amino-4-(3-bromo-4-meth-oxy-phen-yl)-3-methyl-2,4-di-hydro-pyrano[2,3-c]pyrazole-5-carbo-nitrile dimethyl sulfoxide monosolvate.

In the pyrazole mol­ecule of the title solvate, C15H13BrN4O2·C2H6OS, the dihedral angle between the benzene ring and the mean plane of the di­hydro­pyrano[2,3-c]pyrazole ring system [r.m.s deviation = 0.031 (2) Å] is 86.71 (14)°. In the crystal, the pyrazole mol­ecules are linked by N—H⋯N hydrogen bonds, forming a layer parallel to (10-1). The pyrazole and dimethyl sulfoxide mol­ecules are connected by an N—H⋯O hydrogen bond.

ChemInform Abstract: Anticancer Activity of Pyrazole via Different Biological Mechanisms

AbstractReview: 109 refs.

A systematic investigation of cooperativity between two types of hydrogen bonding in the nonlinear clusters of an aromatic molecule: Pyrazole

Crystalline pyrazole consists of nonlinear chains of an aromatic molecule. It includes two independent molecules which in turn causes two different types of hydrogen bonds (HBs). These two types of HBs with slight differences in their NH⋯N geometries can be considered as interesting ones in the recent studies of cooperativity between different HBs. These HBs are investigated in several pyrazole clusters by electronic structure calculations. Parameters such as structure, binding energy, charge transfer, chemical shielding and electric field gradient (EFG) parameters calculated at the second order Moller–Plesset perturbation (MP2) and density functional (DF) levels of theory. Both the basis set superposition error (BSSE) and zero point vibrational energy (ZPVE) corrections on the cooperativity enhancement were considered. Changes of different properties of clusters against crystal size were investigated by proposed diagrams fitted to a logarithmic function which renders their extrema in the crystal limit. In each cluster, pyrazole molecules for which their parameters are more affected by cooperativity enhancement were explored employing these fitted diagrams. Most calculated energetic and spectroscopic parameters were in good linear correlations with both the structural parameters and charge transfer along HB (qn(N2)→σ∗(N1–H1)). These correlations in the cases of nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) parameters, were explained in the terms of natural charges of bonding (σ(N1H1)) and antibonding (σ*(N1H1)) orbitals. Organizing calculated data for mental clusters with similar molecules and HB types produced better regression values in all linear correlations. According to the experimental CQ of N(2) in solid state and zero charge transfer in the gas phase, the value of charge transfer in the crystalline pyrazole and gas phase value of CQ of N(2) were assessed, respectively. Diagrams of the structural parameters against either crystal size or HB cooperativity propose that experimental structure in the case of both hydrogen and heavy nuclei positions is in doubt and should be revisited.

Synthesis and evaluation of some bioactive compounds having oxygen and nitrogen heteroatom

Some new 3,4–disubstituted isocoumarins were synthesized having bioactive pyrazole molecule at 3rd position of isocoumarin moiety (5a,b), from isocoumarin -3- carboxylic acid hydrazide (4a,b) followed by cyclization with acetyl acetone. A series of isocoumarin derivative having Schiff base as lateral side chain at 3rd position of isocoumarin moiety were also synthesized (7a,b), by condensing isocoumarin acid hydrazide and benzaldehyde derivative followed by dehydration. The chemical structures of all the compounds were determined by analytical and spectral method. The lead compounds were screened for antimicrobial and analgesic activities. The article presents the synthesis and characterization of different isocoumarin derivatives from Isocoumarin-3-carboxylic acid hydrazide. The compounds were also screened for their antimicrobial and analgesic activities.

Self-assembly of 4-aryl-1H-pyrazoles as a novel platform for luminescent supramolecular columnar liquid crystals

Supramolecular liquid crystals containing the unprecedented 4-aryl-1H-pyrazole unit are reported. This moiety is able to self-assemble by H-bonding to give columnar mesophases and also display luminescent properties in the visible region. The molecular structures consist of a polyalkoxybenzamide group substituted at the nitrogen atom by the 4-(3,5-dimethyl-1H-pyrazol-4-yl)phenyl unit. A family of compounds was synthesized to evaluate the supramolecular structures and the mesomorphic properties. The 3,4,5-tri-n-decyloxybenzamide derivative is a non-discoid molecule that displays a hexagonal columnar mesophase and a rectangular columnar mesophase. Powder X-ray diffraction studies show that these molecules are able to self-assemble in columns, and that a columnar stratum of the hexagonal columnar mesophase contains five molecules on average. A model for the columnar organization is proposed in which a self-assembled aggregate is formed by an antiparallel arrangement of the molecules partially interdigitated and interacts by hydrogen bonds. The amide group was changed by an ester group, in order to asses the role of the amide and pyrazole groups in the self-assembly. The ester compound also exhibited a columnar mesophase with similar cell parameters. This means that, together with the tapered shape of the molecule, the 1H-pyrazole moiety, and not the amide group, is essential for the aggregation leading to the mesomorphism. In addition, the single crystal structure of a model compound N-(4-(3,5-dimethyl-1H-pyrazol-4-yl)phenyl)benzamide was solved. The structure was a methanol solvate in which each pyrazole is engaged in hydrogen bonding to form a dimer through methanol bridges. The pyrazole molecules stack along the a axis and interact by hydrogen bonding through the amide groups to yield parallel infinite chains. Benzamide compounds are luminescent in the solid state and in the liquid crystalline state, in the blue region of the visible spectrum.

Nature of Magnetic Interactions in 3D {[MII(pyrazole)4]2[NbIV(CN)8]·4H2O}n (M = Mn, Fe, Co, Ni) Molecular Magnets

The self-assembly of [Nb(IV)(CN)(8)](4-) with different 3d metal centers in an aqueous solution and an excess of pyrazole resulted in the formation of four 3D isostructural compounds {[M(II)(pyrazole)(4)](2)[Nb(IV)(CN)(8)].4H(2)O}(n), where M(II) = Mn, Fe, Co, and Ni for 1-4, respectively. All four assemblies crystallize in the same I4(1)/a space group and show identical cyanido-bridged structures decorated with pyrazole molecules coordinated to M(II) centers. All four compounds show also long-range magnetic ordering below 24, 8, 6, and 13 K, respectively. A thorough analysis of the structural and magnetic data utilizing the molecular field model has allowed for an estimation of the values of coupling constants J(M-Nb) attributed to the one type of M(II)-NC-Nb(IV) linkage existing in 1-4. The J(M-Nb) values increase monotonically from -6.8 for 1 through -3.1 for 2 and +3.5 for 3, to +8.1 cm(-1) for 4 and are strongly correlated with the number of unpaired electrons on the M(II) metal center. Average orbital contributions to the total exchange coupling constants J(M-Nb) have also been identified and calculated: antiferromagnetic J(AF) = -21.6 cm(-1) originating from the d(xy), d(xz), and d(yz) orbitals of M(II) and ferromagnetic J(F) = +15.4 cm(-1) originating from d(z(2)) and d(x(2)-y(2)) orbitals of M(II). Antiferromagnetic interaction is successively weakened in the 1-4 row with each additional electron on the t(2g) level, which results in a change of the sign of J(M-Nb) and the nature of long-range magnetic ordering from ferrimagnetic in 1 and 2 to ferromagnetic in 3 and 4.

Chlorido[hydridotris(pyrazol-1-yl-κN)borato](1H-pyrazole-κN)(triphenyl-phosphine-κP)ruthenium(II).

In the title compound, [Ru(C9H10BN6)Cl(C3H4N2)(C18H15P)], the RuII atom is coordinated by an N,N′,N′′-tridentate hydrido­trispyrazolylborate (Tp) ligand, a pyrazole (HPz) mol­ecule, a chloride ion and a triphenyl­phosphine ligand, resulting in a distorted RuClPN4 octa­hedral coordination for the metal ion: the tridentate N atoms occupy one octa­hedral face and the Cl and P atoms are cis. One of the phenyl rings is disordered over two orientations in a 0.547 (10):0.453 (10) ratio, and a weak intra­molecular N—H⋯Cl hydrogen bond generates an S(5) ring.

Catena-Poly[[[[3-(2-pyridyl)-1H-pyrazole]manganese(II)]-μ-oxalato] sesquihydrate

In the title compound, {[Mn(C2O4)(C8H7N3)]·1.5H2O}n, the MnII ion is chelated by two O,O′-bidentate oxalate ions and an N,N′-bidentate 3-(2-pyrid­yl)pyrazole mol­ecule, resulting in a distorted cis-MnN2O4 octa­hedral geometry for the metal ion. The bridging oxalate ions generate wave-like polymeric chains propagating in [001]. The packing is consolidated by N—H⋯O and O—H⋯O hydrogen bonds. One of the water O atoms lies on a crystallographic twofold axis.

Catena-Poly[[[[3-(2-pyridyl)-1H-pyrazole]nickel(II)]-μ-oxalato] sesquihydrate

In the title compound, {[Ni(C2O4)(C8H7N3)]·1.5H2O}n, both unique NiII ions are chelated by an O,O′-bidentate oxalate ion and an N,N′-bidentate 3-(2-pyrid­yl)pyrazole mol­ecule. A second, symmetry-generated, oxalate ion completes a distorted cis-NiN2O4 octa­hedral geometry for both metal centres. The bridging oxalate ions result in two distinct wave-like polymeric chains propagating in [100]. The packing is consolidated by N—H⋯O and O—H⋯O hydrogen bonds. The crystal studied was found to be an inversion twin.

Cu2+‐Mediated Nucleophilic Addition of Different Nucleophiles to Dicyanamide – Synthesis, Structures, and Magnetic Properties of a Family of Mononuclear, Trinuclear, Hexanuclear, and Polymeric Copper(II) Complexes

AbstractFive new copper(II) complexes have been isolated from the reaction system of CuII/N(CN)2–/HNu (HNu = pyrazole or MeOH) by Cu2+‐mediated in situ nucleophilic additions of methanol or pyrazole to dicyanamide; they are [Cu(HdcadMeOH)2](ClO4)2·2H2O (3), [Cu(dca)2(3‐nic)2]n·(HdcaMeOH)2n (4), [Cu(dcapz)(dcadMeOH)] (5), [Cu3(dcadpz)2(Hpz)2(MeCN)2](ClO4)2(NO3)2 (6), and [Cu6(dcadpz‐2H)2(μ‐pz)6] (7) [HdcadMeOH = bis(methoxycarbimido)amine, HdcaMeOH = (methoxycarbimido)cyanoamine, dcadMeOH– = bis(methoxycarbimido)aminato, dcapz– = (pyrazolecarbimido)cyanoaminato, dcadpz– = bis(pyrazolecarbimido)aminato, Hpz = pyrazole, 3‐nic = nicotinamide]. X‐ray crystallography revealed that 3 and 5 are mononuclear complexes, 6 is a trinuclear complex, 7 is a cyclic hexanuclear cluster, whereas 4 is composed of neutral one‐dimensional [Cu(μ1,5‐dca)2(3‐nic)2]n coordination chains with HdcaMeOH guest molecules located between these chains. In the five complexes, the 1:1 and 1:2 nucleophilic addition reactions occurred between dca– and methanol or pyrazole molecules under controlled conditions. In complexes 3, 5, 6, and 7, the addition products generated in situ act as bridging or chelating ligands, whereas in compound 4 the addition product acts as the guest molecule. The magnetic properties of complexes 6 and 7 were studied by variable‐temperature magnetic susceptibility and magnetization measurements. In 6, significant ferromagnetic coupling exists between the three S = 1/2 spins within the Cu3 unit through the dcadpz– bridges, whereas in 7, strong antiferromagnetic couplings exist between the neighboring copper ions through the [dcadpz‐2H]3– and pz– bridges with an intratrimer exchange interaction of –250.6 K and an intertrimer coupling interaction of –69.1 K. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Transformations of cobalt trimethylacetate complexes with pyrazole upon oxidation by air in THF

Synthetic routes to oxidation of cobalt atoms in cobalt trimethylacetate complexes containing coordinated pyrazole molecules have been considered.

Condensation of Rare Earth Pyrazolates: The Dimeric, Trimeric and Polymeric Units [Gd2(Pz)6(PzH)4], [Nd3(Pz)9(PzH)2] and

Solvent free high-temperature oxidations of rare earth metals with the heterocycle pyrazole as well as in low to non-coordinating solvents were investigated to isolate intermediate stages between monomeric and polymeric pyrazolates of the lanthanides. Reaction conditions were tuned according to simultaneous DTA/TG and temperature dependent X-ray powder diffraction experiments on known monomeric and polymeric pyrazolates, that gave rise to the idea that further structure intermediates could be isolated. Reactions in 1,2,3,4-tetrahydroquinoline gave the dimeric complex [Gd2(Pz)6(PzH)4](PzH)(Tech) (1) as well as the triangular complex [Nd3(Pz)9(PzH)2](PzH)(Tech)2 (2). The solvent free melt synthesis resulted in a new polymeric form of (3) (pyrazole, PzH = C3H3NNH; pyrazolate anion, Pz− = C3H3NN−; 1,2,3,4-tetrahydroquinoline, Tech = C9H13N). All three compounds contain coordinating pyrazolate amide groups and pyrazole molecules the latter decreasing in numbers upon condensation of the building units. According to simultaneous DTA/TG/MS investigations the condensation process can be identified with the release of pyrazole molecules.1 consists of dimeric molecules containing trivalent gadolinium with a C.N. of eight. The two gadolinium atoms show different coordination polyhedra. Only σ coordination and bridging is found for 1. 2 consists of trimeric molecules containing trivalent neodymium. The neodymium atoms also exhibit different coordination polyhedra with C.N.s of eight and nine. Both π and σ coordination is found for 2, the π coordinating pyrazolate ligands acting as lids of the triangular units. Topological analysis of the electron localization function (ELF) for 2 calculated at the scalar-relativistic DFT level reveals only weakly covalent π donor η5-Pz–Nd interactions compared to the stronger covalent σ donor Pz–Nd interactions. The topological analysis of both, the ELF and the electron density reveals no significant differences of the respective charges of the Nd atoms. 3 exhibits a one-dimensional chain structure with EuII and a C.N of ten. It can thus be addressed the β form of the referring formula with a new arrangement of the coordinating ligands. Like the α form 3 shows σ and π coordination of pyrazole and pyrazolate ligands. Simultaneous DTA/TG analysis reveals that the low-temperature α form shows a phase transition into the β form between 110 °C and 130 °C. The three compounds were investigated by low-temperature single crystal X-ray analysis, Mid IR and Far IR spectroscopy.

Carboxylatodirhodium(II) complexes with orthometallated tris( p-methoxyphenyl)phosphine and their reactions with pyrazole and imidazole

Exchange reactions of the acetato bridges in [ cis-Rh 2(μ-OAc) 2{μ-(C 6H 3-4-OMe)P(C 6H 4-4-OMe) 2} 2(HOAc) 2] ([ 1(HOAc) 2]) for trifluoroacetato and propionato bridges give new dirhodium(II) complexes with two orthometallated molecules of tris( p-methoxyphenyl)phosphine (PMP): [ cis-Rh 2(μ-O 2CCF 3) 2{μ-(C 6H 3-4-OMe)P(C 6H 4-4-OMe) 2} 2(HO 2CCF 3) 2] ([ 2(HO 2CCF 3) 2]) and [ cis-Rh 2(μ-O 2CCH 2CH 3) 2{μ-(C 6H 3-4-OMe)P(C 6H 4-4-OMe) 2} 2(HO 2CCH 2CH 3) 2] · HO 2CCH 2CH 3 ([ 3(HO 2CEt) 2] · HO 2CEt), respectively. X-ray structures of the complexes [ 2(H 2O)(PrOH)] · PrOH and [ 3(HO 2CEt) 2] · HO 2CEt, as well as of pyrazole adducts [ 2(C 3H 4N 2) 2] and [ 3(C 3H 4N 2) 2] have been determined. The Rh–Rh distances in [ 2(C 3H 4N 2) 2] and [ 3(C 3H 4N 2) 2] are a little longer than those in [ 2(H 2O)(PrOH)] · PrOH and [ 3(HO 2CEt) 2] · HO 2CEt. Reactions of all these complexes with pyrazole and imidazole were investigated using UV–Vis spectroscopy. It has been found that pyrazole molecules are coordinated to [ 1], [ 2] and [ 3] complexes more strongly than imidazole. Reactions with pyrazole were also studied using NMR spectroscopy.

Synthesis of Sybstituted Isoxazoles and Pyrazoles Based on 1-Aryl-3,4,4-trichloro-3-buten-1-ones

3-Aryl-5-methylenehydroxyiminoisoxazoles were synthesized by reaction of 1-aryl-3,4,4-trichloro-3-buten-1-ones with hydroxylamine. Reactions of ketones with hydrazine provided pyrazole structures; therewith two pyrazole molecules underwent “coupling” affording 3-arylpyrazole-5-carbaldehyde azines.

Divalent Europium in 1 ∞[Eu(Pz)2(Pz-H)2]: The First Unsubstituted Pyrazolate and Cp Analogous Pyrazole Complex of a Rare Earth Element in One

Abstract Bright yellow crystals of 1 ∞[Eu(Pz)2(Pz-H)2] were obtained by the reaction of europium metal with a melt of pyrazole. According to single crystal X-ray analysis the compound exhibits a onedimensional chain structure including both unsubstituted pyrazolate anions as well as unsubstituted neutral pyrazole molecules as ligands. The latter are isoelectronic with the cyclopentadienyl anion and link two adjacent Europium(II) centers in an η1-σ -bridging as well as Cp analogous η5-π-binding mode, whilst the pyrazolate anions are N-η1-coordinating terminal ligands. In addition to the crystal structure, MIR, FarIR, Raman and UV/vis spectroscopic data are presented.

Cooperative association of pyrazoles and phenols: A versatile binary systemElectronic supplementary information (ESI) available: Details for crystal structure determination and refinement and geometry of hydrogen bonding in structures. See http://www.rsc.org/suppdata/nj/b3/b317104c/

The ability of self-complementary H-bond donor/acceptor phenol (OH/O) and pyrazole (NH/N) molecules for cooperative association allowed the developing of a range of binary compounds, in which the molecular counterparts are integrated by hydrogen bonding. Complexes involving either monofunctional 3,5-dimethylpyrazole and phenol and polyfunctional 3,3′,5,5′-tetramethyl-4,4′-bipyrazole, hydroquinone and phloroglucinol are based on mixed phenol–pyrazole chains. They reveal common features allowing high level of control over the entire connectivity and dimensionality. Unprecedented NH⋯π pyrazole hydrogen bonding was essential for the structure of the resorcinol complex.

Infrared and ab initio studies of hydrogen bonding and proton transfer in the complexes formed by pyrazoles

The results of experimental studies and quantum mechanical calculations of vibrational spectra and structure of hydrogen bonded complexes formed by pyrazole (P) and 3,5-dimethylpyrazole (DMP) are presented. IR spectra of pyrazoles in solutions, gas phase, and solid state have been investigated in wide range of concentrations and temperatures. It has been found that in the gas phase both P and DMP reveal the equilibrium between monomers, dimers, and trimers. In solutions the equilibrium between monomers and trimers dominates, no bands, which can be attributed to dimers were detected. DMP retains the trimer structure in solid state, while in the case of pyrazole P, formation of the crystal provides another type of association. Geometrical and spectral characteristics of dimers and trimers, obtained by ab initio calculations, are presented and compared with experimental data. IR spectra of solutions containing P and DMP with a number of acids (acetic and trifluoroacetic acids, pentachlorophenol, HBr) have been studied in parallel with ab initio calculations. It has been found that pentachlorophenol forms with pyrazoles complexes with one strong hydrogen bond O–H⋯N, while NH pyrazole group remains unbonded. With carboxylic acids DMP forms 1:1 cyclic complexes with two hydrogen bonds. In the case of acetic acid, the complex in CH 2Cl 2 solution reveals molecular structure with OH⋯N and CO⋯HN bonds, in accordance with results of the calculations. For trifluoroacetic acid, the calculations predict the molecular structure to be energetically more stable in the case of the isolated binary complex (in gas phase), while the experimental spectrum of CH 2Cl 2 solution gives an evidence of the proton transfer with formation of the cyclic ionic pair with two NH +⋯O − bonds. The agreement with experimental results can be improved by taking into account the influence of environment in the framework of Onsager or Tomasi models. The shape of proton potential function of the complexes and medium effect on its parameters, resulted from experimental data and calculations, are discussed. It has been found that the number of potential minima and their relative depth depend strongly on the method of calculations and the basic set. Under excess of trifluoroacetic acid, the formation of 2:1 acid–DMP complex has been detected. Spectral characteristics and results of calculations point to the cyclic structure of this complex, which includes homoconjugated bis-trifluoroacetate anion and DMPH + cation. With HBr both studied pyrazoles were found to form ionic complexes including one or two pyrazole molecules per one acid molecule and correspondingly monocation or homoconjugated cation BHB +.

Pentanuclear pivalate Ni(II) and Co(II) clusters: modulation of molecular structures and magnetic properties

Reaction of nickel cluster Ni9(HOOCCMe3)4(μ4-OH)3(μ3-OH)3(μn-OOCCMe3)12 (1) or cobalt pivalate polymer [Co(OH)n(OOCCMe3)2−n]x (7) with bis[3,5-(dimethylpyrazolyl)]-1,2,4,5-tetrazine (3) in MeCN solution was found as a result in the formation of isostructural pentanuclear clusters of composition M5(μ3-OH)2(μ-OOCCMe3)4(μ-N,N′,N″-3,5-Me2C3HN2C2(O)N4)4(MeCN)2 (M=Ni (4), Co (8)) with a planar spirane metal core. Reaction of cobalt complex 7 with 3 in benzene gives rise to a pentanuclear cluster Co5(μ3-OH)2(μ-OOCCMe3)4(μ-N,N′,N″-3,5-Me2C3HN2C2(O)N4)4(C5N2H8)2 (9), containing coordinated pyrazole molecules as terminating ligands instead of labile MeCN ligands in 8. The reaction of cluster 8 with 2,6-diaminopyridine (T=85°C) in MeCN results in the substitution of MeCN ligands and formation of a new cluster Co5(μ3-OH)2(μ-OOCCMe3)4(μ-N,N′,N″-3,5-Me2C3HN2C2(O)N4)4(C5H7N3)2 (10) with two 2,6-diaminopyridine ligands instead of MeCN molecules. The reactions of 4 and 8 with NiCl2·6H2O in MeCN/EtOH solution (3:1) give rise to isostructural ferromagnetic pentanuclear chloro-bridged clusters M5(μ3-OH)2(μ-CI)2(μ-OOCCMe3)2(μ-N,N′,N″-3,5-Me2C3HN2C2(O)N4)2(μ-N,N′,N″,O-3,5-Me2C3HN2C2(O)N4)2(MeCN)2 (M=Co (11), Ni (12)).The reaction of 8 with solid CoCl2·6H2O in MeCN results in new seven-nuclear ferromagnetic cluster Co5(μ3-OH)2(μ-CI)2(μ-OOCCMe3)2(μ-N,N′,N″-3,5-Me2C3HN2C2(O)N4)2(μ-N,N′,N″,O-3,5-Me2C3HN2C2(O)N4)2(MeCN)2[Co(CH3CN)CI2]2 (13). The molecular structures of 4, 8, 9, 10, 12 and 13 established by X-ray diffraction studies and magnetic behavior of new nickel and cobalt pivalate clusters are discussed.

An intramolecular N-H...(mu-H)Re2 dihydrogen bond and a novel mu 3-eta 2 coordination mode of the pyrazolate anion on a triangular cluster face.

The quantitative addition of pyrazole (Hpz) to the 44 valence-electron, triangular cluster anion [Re3(mu 3-H)-(mu-H)3(CO)9]- gives the novel unsaturated anion [Re3(mu-H)4(CO)9(Hpz)]- (1, 46 valence electrons), which contains a pyrazole molecule that is terminally coordinated on a cluster vertex. Solidstate X-ray and IR analyses reveal a rather weak hydrogen-bonding interaction between the NH proton and one of the hydrides bridging the opposite triangular cluster edge (delta H degree = -3.1 kcal mol-1 from the Iogansen equation). Both IR and NMR data indicate that such a proton-hydride interaction is maintained in the major conformer present in CD2Cl2, but also provide evidence of the presence of minor conformers of 1 in which the NH proton is involved in an intermolecular hydrogen bond with the solvent. The mu-H...HN bond length evaluated in solution through the T1 minimum value (2.07 A) and that determined in the solid state by X-ray diffraction (2.05 A) are in good agreement. NMR experiments show that, in acetone, intermolecular N-H...solvent interactions replace the intramolecular dihydrogen bond. At room temperature in CH2Cl2, the pyrazole ligand in 1 is labile and 1 slowly "disproportionates" to [Re3(mu 3-H)-(mu-H)3(CO)9]- and [Re3(mu-H)3(CO)9-(mu-eta 2-pz)(Hpz)]-, with H2 evolution. Slow H2 evolution also leads to the formation of the anion [Re3(mu-H)3-(CO)9(pz)]- (5), in which the pyrazolate anion adopts a novel mu 3-eta 2-coordination mode, as revealed by a single-crystal X-ray analysis. The analysis of the bond lengths indicates that the pyrazolate anion in 5 acts as a six-electron donor, with loss of the aromaticity. The formation of 5 from 1 is much faster in solvents with a high dielectric constant, such as acetone or DMF. Anion 5 was also obtained from the reaction of pyrazole with [Re3(mu-H)3(CO)9(mu 3-CH3)]- through the intermediate formation of two isomeric addition derivatives and following CH4 evolution.