Structure of trans-dichloro(ethylene)(pyrazole)platinum(II)

Substituent Control of Hydrogen Bonding in Palladium(II)−Pyrazole Complexes

“…However, we have been very interested in the rarer coordination mode shown in A − D , because complexes of this type are expected to show an interplay between the metal center and the N−H moiety ( A and B ) or between the metal center and the uncoordinated, basic N ( C and D ). Examples of ligands to which a metal-coordinated pyrazole can intramolecularly donate a hydrogen bond include halide, hydroxide, or peroxo species. ,− Fascinating structures such as H are also known. ,, As for the acidity of metal-coordinated pyrazoles, the measured p K a of pyrazole complexes of Cr III (NH 3 ) 5 and Co III (NH 3 ) 5 , and Ru III (NH 3 ) 5 , , are in the range 5.98−7.21, similar to that of protonated imidazole, but much lower than the p K a of pyrazole itself (14.2) . One report appearing in 1998, as we began our work, showed the potential difficulties of maintaining structures such as A − D under basic conditions: Cu(II) complexes of type A undergo deprotonation and dimerization (forming bridging structures similar to E ) rather than catalyze phosphate ester hydrolysis …”

“…Examples of ligands to which a metal-coordinated pyrazole can intramolecularly donate a hydrogen bond include halide, hydroxide, or peroxo species. 2, [45][46][47][48][49][50][51][52][53][54][55][56] Fascinating structures such as H are also known. 53,57,58 As for the acidity of metalcoordinated pyrazoles, the measured pK a of pyrazole complexes of Cr III (NH 3 ) 5 and Co III (NH 3 ) 5 , and Ru III (NH 3 ) 5 , 59,60 are in the range 5.98-7.21, similar to that of protonated imidazole, but much lower than the pK a of pyrazole itself (14.2).…”

Stereoselective formation and catalytic activity of hydrido(acylphosphane)(chlorido)(pyrazole)rhodium(iii) complexes. Experimental and DFT studies

“…The N⋯Cl distances, between 3.017 Å and 3.067 Å, for all the three complexes are as expected for moderately strong hydrogen bonds, however the N-H⋯Cl angles, between 128(1) and 140(1)°, are in the lower range of the corresponding values. 7,32 Nevertheless, the importance of the hydrogen bonding can also be assessed by the dihedral angle between two planes, that of the pyrazole ring (including the N-H bond) and the plane defined by the coordinated N atom of pyrazole, chlorido, and their trans atoms to the metal and the metal, which should be close to 0°. 8 In 2-Hpz these angles are very small, 4.8 and 6.6°f or the planes involving the N4 and N2 atoms, respectively, indicating that the N-H vector is directed in order to maximize the intramolecular hydrogen bonding.…”

“…6 Intramolecular hydrogen bonds are important in establishing the conformation of the complexes. 7 Halide, hydroxide or peroxo species are known as hydrogenbond acceptors towards metal-coordinated pyrazoles 8 and the presence of strong hydrogen bonds between pyrazole and pyrazolate moieties in late transition metal complexes has also been reported. 9 Therefore, we thought it interesting to prepare acylrhodium derivatives containing pyrazoles, which could form hydrogen bonds involving the oxygen atom of the acyl group and behave as catalysts for the hydrolysis of amineboranes to release hydrogen.…”