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

[Pt(2,2'-bpy)(1-MeC-N3)(2)](NO3)(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)(2)Pd(en)](NO3)(2)center dot 5H(2)O (2) with head-head arranged 1-methylctosinato (1-MeC-) ligands and Pd being coordinated to two exocyclic N4H(-) positions. Addition of AgNO3 to a solution of 2 leads to formation of a pentanuclear chain compound [{Pt(2,2'-bpy)(1-MeC-)(2)Pd(en)}(2)Ag]NO3)(5)center dot 14H(2)O (5) in which Ag+ cross-links two cations of 2 via the four available 02 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)](NO3)(4)center dot 10H(2)O (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)(NO3)](NO3)(3)center dot 5H(2)O (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 H-1 NMR spectrum.