Varying Acidity of Aqua Ligands in Dependence on the Microenvironment in Mononucleobase (nb) Complexes of Type cis- and trans-[Pt(NH₃)₂(nb)(H₂O)]ⁿ⁺

Abstract

Aqua ligands in mixed aqua/nucleobase metal complexes are potential sites of acid-base catalysis and/or, when present as hydroxo ligands, can directly be involved in hydrolysis reactions. pKa values of close to 7 are consequently of particular interest and potential significance. Here we report on the differential acidity of aqua complexes in model nucleobase (nb) complexes of cis- and trans-[Pt(NH3)2 (nb)(H2O)]n+ and discuss reasons as to why the nb in cis complexes influences the pKa (pKa 4.8-7.0), whereas in trans complexes the pKa values are rather constant (pKa approximately 5.2-5.3). The results of DFT calculations of a series of mono(nucleobase) complexes derived from cis-Pt(NH3)2 are critically examined with regard to the role of exocyclic groups of nucleobases in stabilizing aqua/hydroxo ligands through intracomplex hydrogen bond formation. This applies in particular to the exocyclic amino groups of nucleobases, for which gas-phase calculations suggest that they may act as H bond acceptors in certain cases, yet in the condensed phase this appears not to be the case.

Synthesis, Characterization, Thermodynamic and Kinetic Properties of a New Series of Dinuclear PtII Complexes

“…[9] Finally, the lower pK a value for the 8-ane and 10-ane complexes than for the Pt(dach) complex could be also a result of a stabilization of the hydroxo ligand by intramolecular hydrogen bond formation with the amino group of the diaminoalkane chain as it was reported for similar systems. [10] To gain deeper insight into the structural details of the unspoiled Pt centres, we performed DFT (BP86/LACVP*) calculations on mononuclear model aqua and hydroxo complexes (model-OH 2 and model-OH). As already observed before, [8] hydroxo complexes can be stabilized by intramolecular hydrogen bonds, which is not possible in the case of model-OH 2 , as depicted in Figure 6.…”

Ligand‐pKa Shifts through Metals: Potential Relevance to Ribozyme Chemistry

“…As a consequence, it is to be expected that these co-ligands, hence the microenvironment of the metal-bound H 2 O molecule, will have an influence on the aqua-group acidity, too. We have recently studied this aspect by applying mixed ternary complexes of Pt II , containing ammonia ligands, model nucleobases, and a H 2 O molecule [53]. When comparing the pK a values of the H 2 O ligand in complexes of general composition cis-or…”

(N7)-Platination and its effect on (N1)H-acidification in nucleoside phosphate derivatives

“…Although differences in the trans influence of the am(m)ine ligands are expected to also lead to differences in pK a values [75], we doubt that the trans influences of NH 3 and cyclohexylamine are sufficiently different to account for this finding. At least as far as the acidities of the two aqua ligands in cis-[(NH 3 )(cyclohexylamine)Pt(OH 2 ) 2 ] 2+ are concerned, no major difference with cis-[(NH 3 ) 2 Pt(OH 2 ) 2 ] 2+ can be seen [76].…”