“…The pyrazole heterocycle forms metal complexes − in a wide variety of coordination modes, some of which are schematically illustrated in Chart . Bridging, anionic pyrazolates are very common (structures E and F ), as are the versatile tris(pyrazolyl)borates ( G ) and related systems. , Hundreds of poly(pyrazolyl)borate complexes ( G ) are known, because of the robust nature of the ligand and the ability to make derivatives of different steric and electronic properties. , Further, bridging pyrazolates ( E and F ) are useful in studying bimetallic reactivity and cooperativity and electronic communication between metals, to name a few properties of interest in coordination, bioinorganic, and organometallic chemistry. − ,− 1 Pyrazole Coordination Modes…”
Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.
“…The pyrazole heterocycle forms metal complexes − in a wide variety of coordination modes, some of which are schematically illustrated in Chart . Bridging, anionic pyrazolates are very common (structures E and F ), as are the versatile tris(pyrazolyl)borates ( G ) and related systems. , Hundreds of poly(pyrazolyl)borate complexes ( G ) are known, because of the robust nature of the ligand and the ability to make derivatives of different steric and electronic properties. , Further, bridging pyrazolates ( E and F ) are useful in studying bimetallic reactivity and cooperativity and electronic communication between metals, to name a few properties of interest in coordination, bioinorganic, and organometallic chemistry. − ,− 1 Pyrazole Coordination Modes…”
Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.
“…Metal complexation to the pyrazole heterocycle − could offer a straightforward way to couple the effect of a metal bound to one heterocyclic nitrogen with the ability of the N−H to donate a proton or hydrogen bond (see Chart , structures A and B ). The ability of metal-coordinated pyrazole to donate a hydrogen bond intramolecularly to a halide, hydroxide, carboxylate, or peroxo ligand is well-documented .…”
Syntheses of pyrazoles featuring a functionalized side chain attached to carbon 3 and varying alkyl and aryl substituents attached to carbon 5 are presented. Installation of R = methyl, isopropyl, tert-butyl, adamantyl, or phenyl groups at C5 is reported here, starting by coupling protected alkynols with acid chlorides RCOCl, forming alkynyl ketones, which are reacted with hydrazine to form the pyrazole nucleus. Alcohol deprotection and conversion to a chloride gave 5-substituted 3-(chloromethyl)- or 3-(2-chloroethyl)pyrazoles. This sequence can be done within 2 d on a 30 g scale in excellent overall yield. Through nucleophilic substitution reactions, the chlorides are useful precursors to other polyfunctional pyrazoles. In the work here, derivatives with side chains LCH(2)- and LCH(2)CH(2)- at C3 (L = thioether or phosphine) were made as ligands. The significance of the ligands made here is that by placing a ligating side chain on a ring carbon (C3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton will be available for hydrogen bonding, depending on the steric environment created by R at C5.
“…This property should make catalytic activity available at more benign potentials . However, cationic hydrocarbyl groups such as pyridinium or pyrylium bound to the metal via a M−C σ-bond are very rare. , Despite the large number of pyrylium compounds known, few σ-bonded organometallic pyrylium complexes have been prepared and investigated. , This oversight is surprising in light of the utility of pyrylium salts in organic synthesis and their applications as dyes, laser dyes, photosensitizers, and corrosion-inhibiting additives in paints. , They also have a rich redox chemistry, undergoing reduction to form stable radicals, dimers, or pyrans, depending on steric factors and on the reducing agent. The coupling of this redox chemistry to the recognized properties of transition-metal complexes has the potential to yield systems with useful physical and chemical properties.…”
A salt of a metal−pyrylium σ-complex, pentacarbonyl-3-(2-methoxy-5,6-diphenylpyrylium)manganese(I) tetrafluoroborate (1BF
4
), has been prepared by the action of Mn(CO)5BF4 on
methyl propiolate and diphenylacetylene. The pyrylium complex shows unusual resistance
to electrophilic cleavage of the Mn−C bond by H+ and a low aptitude for migratory insertion
reactions. The complex was studied by spectroscopic, voltammetric, and controlled potential
electrolysis methods. 1BF
4
displays two ligand-based one-electron reductions (E
1° = −1.33
V, E
pc,2 = −2.41 V vs Fc), the first of which is Nernstian. Its reactivity is enhanced by one-electron reduction, and it undergoes subsequent dimerization in THF (k
273K = 285 M-1 s-1)
and CH2Cl2 (k
273K = 260 M-1 s-1) and a CO dissociation equilibrium (K ≈ 1.4 × 105, k = 0.7
s-1). In the presence of triphenylphosphine, a slow electrocatalytic CO-substitution reaction
occurs for which a mechanism and rate constants have been determined by digital simulation.
When 1BF4 is reduced in the presence of HSnPh3 and a proton donor, another electrocatalytic
process results, presumably forming H2 at the electrode surface.
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