Expected and unconventional Ag+ binding modes in heteronuclear Pt,Ag coordination polymers derived from trans-[Pt(methylamine) 2(pyrazole)2]2+

Abstract

The combination of AgNO3 and trans-[Pt(ma)2(Hpz) 2]2+ (1; ma = methylamine; Hpz = neutral pyrazole) in water yields mixed Pt,Ag coordination polymers of different stoichiometries, depending on the ratio between Ag and Pt, as well as the pH. The products that were isolated and X-ray structurally characterized display both conventional (Ag+ coordination to pyrazole-N) and unconventional Ag+ binding modes (η1 and η2 binding to C atoms of the pyrazole/pyrazolate ligands; Pt → Ag dative bonds). Specifically, in trans-[Pt(ma)2(Hpz)2]Ag 2(NO3)4 (2) and in trans-[Pt(ma) 2(pz)2]2Ag3(NO3) 3 (4), silver ions bind to C4 positions of Hpz (2) and pz- (4) ligands in η1 fashions, with Ag-C distances of 2.574(4) and 2.643(16) Å, respectively. In 4 there is additional cross-linking by a second Ag+ of N2 sites of adjacent pz- rings, further reinforced by weak dative bonds from Pt to Ag. Ag-N coordination to both a neutral Hpz and an anionic pz- ligand is observed in trans-[Pt(ma)2(pz)2]Ag2(Hpz) 2(NO3)2 (5), with individual trinuclear PtAg2 entities associated through weak η2 contacts that involve the C3 and C4 positions of the neutral Hpz ligands. As in 4, intramolecular Pt-Ag distances of 3.1374(6) Å suggest weak dative bond interactions between Pt and Ag. The acidities of the two Hpz ligands in 1 are distinctly different (pKa values of 7.25 and 9.08 in H2O), thereby suggesting a stabilization of the monodeprotonated species trans-[Pt(ma)(Hpz)(pz)]+ (1c) in solution, probably through intermolecular hydrogen-bond formation between Hpz and pz- ligands. Not (only) the available second-ring N atom of pyrazole and pyrazolate ligands in complexes of PtII appears to be the preferred Ag+ binding site in moderately acidic aqueous solution, but rather the C4 atom and/or the C4,C5 double bond. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.