Pyrazole, transition metal complexes

Of commercial interest are benzo- and other fused aromatic 1,2,3-diazaborine derivatives which have exhibited good antibacterial activity against a variety of microorganisms (155—157). The reaction of pyrazole or C-substituted pyrazoles with boranes yields the pyrazabole system, a class of exceptionally stable compounds. More than 70 species in this system have been reported and the subject comprehensively reviewed (158). These compounds have been used as ligands in transition-metal complexes (159). 

The synthesis of multimetallic transition metal complexes where the metals are held at specific distances from each other is an important objective because of their potential role in multimetal-centered catalysis in both biological and industrial reactions (1). Moreover, such systems, through cooperative electronic and/or steric effects between metal centers, might give rise to distinct reactivity patterns for both their stoichiometric and catalytic reactions, which are not available to their monometallic analogues (2). Of the ligands that are able to maintain the metal centers in close proximity, the pyrazolate ion (pz ) appears to be a particularly suitable candidate. Pyrazoles (Hpz ) are weak bases (3, 4) and behave as 2-monohapto ligands. 

Transition metal complexes containing the pyrazolate group as anionic noncoordinating ligand, although rare, have been reported (5). They are undoubtedly of lesser importance and will not be further discussed. 

In 1988 Lalor reported the synthesis and spectroscopic characterization of some new poly(benzo-triazolyl)borate salts (Scheme 8).180 The regiospecifity of the synthesis of I I B( Btz)4 differs from that of pyrazole/BH4 reaction in that B—N bond formation takes place in a manner that maximizes steric crowding at boron (i.e., at the triazole N(l) atoms). In 1989 Shiu describes some new metal carbonyls of HB(Btz)3.181 The synthesis and spectroscopic characterization of some first-row transition metal complexes was reported by Cecchi,182 whereas Hill described some new Rh183 and Ru184 carbonyl and isonitrile complexes. The formation of isonitrile derivatives was proposed to proceed through an associative mechanism involving an intermediate complex... 

Dimethylbis(pyrazol-1 -yl)- and bis(3,5-dimethylpyrazol-l-yl)-dimethylgallate ions act as bidentate chelating agents towards transition-metal ions." The [Me2B(pz)2] ion acts similarly, but the A1 and In analogues do not appear to form transition metal complexes. The crystal structure of (78), has been determined. The space group is Pljc. The whole molecule is in a pseudo-chair conformation, with two six-membered Ga—(N— N)2— Ni rings in boat conformations. The co-ordination at the Ni atom is planar." ... 

Luminescent ionic transition-metal complexes for light-emitting electrochemical cells (pyridine, phenanthroHne, pyrazole, imidazole, triazole derivatives as Hgands) 12AG(E)8178. 

The present contribution exemplifies a strategy for efficient and highly selective oxidation reactions of industrial interest directed toward organic synthesis, using catalytic systems based on transition metal complexes bearing tris(pyrazol-l-yl)methane-type scorpionates, RC(R pz)3 (pz = pyrazol-l-yl R = H or substituent at the methine carbon R = H or substituent at the pz ring Fig. 22.1). 

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

Pyrazole and its C-methyl derivatives acting as 2-monohaptopyrazoles in a neutral or slightly acidic medium give M(HPz) X, complexes where M is a transition metal, X is the counterion and m is the valence of the transition metal, usually 2. The number of pyrazole molecules, n, for a given metal depends on the nature of X and on the steric effects of the pyrazole substituents, especially those at position 3. Complexes of 3(5)-methylpyrazole with salts of a number of divalent metals involve the less hindered tautomer, the 5-methylpyrazole (209). With pyrazole and 4- or 5-monosubstituted pyrazoles M(HPz)6X2... 

Pyrazoles can be synthesized by thermal cycloreversion of adducts formed in the 1,3-dipolar cycloaddition of alkyldiazoacetates with norbornadiene. The rate of the primary process of cycloaddition is accelerated by iron pentacarbonyl (Scheme 88)155 a similar catalytic effect has been observed during the formation of ethyl 5-phenyl-A2-pyrazoline-3-carboxylate from cycloaddition of ethyl diazoacetate and styrene.155 Reactions of this type are catalyzed presumably because of coordination of one or both reactants to the transition metal, and a wider study of the effect of a variety of complexes on 1,3-dipolar cycloaddition processes would be valuable. 

Dipolar cycloaddition reactions are most commonly applied for the synthesis of five-membered heterocyclic compounds.86 87 [3+2] cycloaddition reactions of transition-metal propargyl complexes have been reviewed.88 Addition of diazomethane to carbene complexes (CO)5Cr= C(OEt)R results in cleavage of the M = C bond with formation of enol ethers H2C = C(OEt)R,3 89 but (l-alkynyl)carbene complexes undergo 1,3-dipolar cycloaddition reactions at the M = C as well as at the C=C bond. Compound lb (M = W, R = Ph) affords a mixture of pyrazole derivatives 61 and 62 with 1 eq diazomethane,90 but compound 62 is obtained as sole... 

Several d and / transition metals have been found to yield polymeric complexes with pyrazoles 14, 178). Reactions of these polymers with HFA produce monomeric species, HFA adding to the free nitrogen atom and the metal. Substitution of one carbon atom adjacent to nitrogen also results in degradation of the polymers. However, no reaction has been found when both a carbon atoms are sterically hindered 14). 

Binary pyrazolate complexes containing divalent metals bridged by two pz groups are known for many transition metals. The increasing interest for this class of complexes stems from the expectation that they may provide useful insights in the field of magnetostructural correlations as well as in multimetal centered catalysis. 

It is well known that hydride derivatives of transition metals can be prepared by hydrogen-transfer reactions from primary and secondary alcohols. Penta-methylcyclopentadienyl (/i-pyrazolate)-( t-hydrido) complexes of iridium and rhodium were obtained by using propan-2-ol as the hydride source (128, 150). 

The ligand hydrotris[3-(2/-thienyl)pyrazol-l-yl)]borate (= TpTn) is reported to have the second-lowest steric hindrance among the known poly(pyrazolyl)borates, also if a priori tris(pyrazolyl)borates with the 3-(2/-thienyl) substituent were expected to fit between those with 3-phenyl and those with 3-methyl substituents in terms of steric hindrance. TpTn forms octahedral [M(TpTn)2] complexes with first-row transition-metal ions, but fails to yield stable [MX(TpTn)j species, except with Zn(II).12... 

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