Different Rotamer States of Cytosine Nucleobases in Heteronuclear PtPd-, PtPd2, and Pt2Pd2Ag Complexes Derived from [Pt(2,2′-bpy)(1-MeC-N3)2]2+ (1-MeC = 1-Methylcytosine): First Examples of Species with Head–Head Oriented 1-MeC– Ligands

Abstract

[Pt(2,2′-bpy)(1-MeC-N3) 2](NO 3) 2 (1) (2,2′-bpy = 2,2′-bipyridine; 1-MeC = 1-methylcytosine) exists in water in an equilibrium of head-tail and head-head rotamers, with the former exceeding the latter by a factor of ca. 20 at room temperature. Nevertheless, 1 reacts with (en)Pd II (en = ethylenediamine) to give preferentially the dinuclear complex [Pt(2,2′-bpy)(1-MeC --N3,N4) 2Pd(en)](NO 3) 2·5H 2O (2) with head-head arranged 1-methylctosinato (1-MeC -) ligands and Pd being coordinated to two exocyclic N4H - positions. Addition of AgNO 3 to a solution of 2 leads to formation of a pentanuclear chain compound [{Pt(2,2′-bpy)(1-MeC -) 2Pd(en)} 2Ag](NO 3) 5·14H 2O (5) in which Ag + cross-links two cations of 2 via the four available O2 sites of the 1-MeC - ligands. 2 and 5 appear to be the first X-ray structurally characterized examples of di- and multinuclear complexes derived from a Pt II species with two cis-positioned cytosinato ligands adopting a head-head arrangement. (tmeda)Pd II (tmeda = N,N,N′,N′- tetramethylethylenediamine) and (2,2′-bpy)Pd II behave differently toward 1 in that in their derivatives the head-tail orientation of the 1-MeC - nucleobases is retained. In [Pt(2,2′-bpy)(1-MeC -) 2{Pd(2,2′-bpy)} 2](NO 3) 4·10H 2O (4), both (2,2′-bpy)Pd II entities are pairwise bonded to N4H - and O2 sites of the two 1-MeC - rings, whereas in [Pt(2,2′-bpy)(1-MeC -) 2{Pd(tmeda)} 2(NO 3)](NO 3) 3·5H 2O (3) only one of the two (tmeda)Pd II units is chelated to N4H - and O2. The second (tmeda)Pd II is monofunctionally attached to a single N4H - site. On the basis of these established binding patterns, ways to the formation of mixed Pt/Pd complexes and possible intermediates are proposed. The methylene protons of the en ligand in 2 are special in that they display two multiplets separated by 0.64 ppm in the 1H NMR spectrum. © 2012 American Chemical Society.