Highly Substituted Tris(pyrazolyl)methane Ligands and Some Zinc Complexes Thereof

Abstract

Tris(pyrazoly1)methane ligands in which two or three of the pyrazole carbon atoms bear organic substituents (L1-L7) were synthesized from chloroform and the corresponding pyrazole under phase transfer conditions. Their behavior towards zinc salts was found to span the range from no reaction at all to hydrolytic destruction. One hydrolysis product isolated and structurally characterized was the perchlorate complex [ ( H P z~)~Z~-O C~O~] C~O~ ( l ) , other ones were the 2 : l complexes (HPz3),ZnBrz (2) and (HPz6),Zn(N03), (3, HPz" = substituted pyrazole). Zinc perchlorate and tris(trimethy1py-razoly1)methane (Lz) formed the octahedral binary complex [L~Zn](ClO,), (4) as evidenced by a structure determination.Zinc halides produced the 1 : 1 complexes L' . ZnBr, (5), L4 . ~~ ZnC1, (6), and L4 . ZnBr, ( 7 ) , which according to the structure determinations of 6 and 7 contain tetrahedral ZnNzHalz units with only bidentate tris(pyrazoly1)methane ligands. In contrast, the zinc nitrate complex L4 . Zn(N03), ( 8 ) was found to have an octahedral structure with monoand bidentate nitrate and tridentate L4. The bromide complex 7 was converted by silver perchlorate hydrate into the labile compound [L4.ZnBr]C104 (9) and then into the unstable product [L4.Zn-OHZ](C1O4), (lo), both presumed to contain zinc in a tetrahedral ZnN3Br or ZnN30 environment, respectively. The ease of hydrolytic self-destruction prevented the exploitation of the reactivity of 9 and 10 in analogy to that of the corresponding tris(pyrazoly1)borate zinc complexes.The coordination chemistry of zinc suffers from lability and low thermodynamic stabilities. As a result it is difficult to gain a reliable control over the access to or reactivity at specific coordination sites. One approach to reduce these problems consists of the use of encapsulating ligands. These types of ligands, typically tripodal chelators, have the property of occupying all but one or two of the coordination sites around the metal. Thus, the thermodynamic stability of the complexes is enhanced due to the chelate effect, and their reactivity is reduced to the functionality of the remaining coordination sites.Only a limited number of tripodal ligands has been used so far with the purpose of studying zinc complex reactivities, mostly with a view to modelling zinc-containing en- zymes. Among them are tris(imidazolyl)phosphanes['l, tris-(imidazolyl)methanols[21, tris(pyridylmethyl)amines[31, bis-(amin~ethyl)glycine[~], and bis(nier~aptoethyl)pyridines[~]. We contributed studies with tris(aminomethyl)ethane[6], cyclohe~anetriamines[~], tris(pyridyl)phosphane[8], and tris-(imida~olylmethyl)amines[~]. The most productive tripod ligands for zinc enzyme modelling, in our[lo~lll as well as in other hands [12], have been the tris(pyrazoly1)borates. The reason for their superior usability is the fact that their encapsulating properties can be enhanced by voluminous substituents in the 3-and 5-positions of the pyrazole rings.In order to exploit these favorable properties with uncharged ligands we set out...

Diacetylplatinum(II) and ‐platinum(IV) Complexes Bearing κ2‐ and κ3‐Coordinated Tris(pyrazolyl)methane Ligands: Investigations on the Synthesis, Fluxionality, and Reactivity in Relation to the Substitution Pattern of the Ligands

“…[24] In contrast, the methine proton of the dimethyl-substituted tris(pyrazolyl)methane ligand exhibits no dependence either on the oxidation state of the platinum center or on the type of counterion in cationic platinum(IV) complexes 3b, 5b, 6b, and 7b (Tables 3 and 4). The 1 H resonances in complexes 3b and 5b-7b and in the free ligand (δ H = 8.00 ppm) [25] were all found at about δ = 8 ppm. This might be due to steric protection of the methine proton by the methyl groups at the 5-position and/or the electron-donating effect of the methyl groups.…”

Coordination, organometallic and related chemistry of tris(pyrazolyl)methane ligands

“…23 This in turn then attacks the unsubstituted pyrazole ring positions resulting in ring expansion reactions. 24,25 In 1987, Elguero and co-workers developed an improved synthesis of tris(pyrazolyl)methane-type ligands. 26 This method involved reaction of the appropriate pyrazole with chloroform under liquid-liquid phase transfer conditions, using K 2 CO 3 as a base.…”

3{5}-tert-Butylpyrazole is a ditopic receptor for zinc(ii) halidesElectronic supplementary information (ESI) available: tabulated and plotted NMR data for 1–3 in the presence and absence of added NBun4X (X– = Cl–, Br–, I–, BF4–). See http://www.rsc.org/suppdata/cc/b2/b200551b/

“…These structures contrast with the literature complex [Zn(OClO 3 )L 3 ]ClO 4 (L = 3-tertbutyl-2,4,5,6-tetrahydrocyclopentapyrazole), whose pyrazole ligands do not form a C 3 -symmetric cavity and take part in intermolecular hydrogen bonds to both coordinated and noncoordinated perchlorate anions. 9 The 13 C NMR spectra of 1-3 in C 6 D 6 , CDCl 3 and CD 2 Cl 2 are sharp, and show the number of peaks expected for one Hpz tBu environment (see ESI †). However, in {CD 3 } 2 CO the spectra show broadened and/or unobservable resonances for the pyrazole C 3 and C 5 atoms.…”