Molecular Architectures Derived from Metal Ions and the Flexible 3,3′-Bipyridine Ligand: Unexpected Dimer with Hg(II)

The flexible ditopic ligand 3,3′-bipyridine (3,3′-bpy) has been reacted with a series of transition metal species (Ag+, Hg2+, cis-a2M2+ (a = NH3 or a2 = en; M = Pt, Pd), trans-a2Pt2+ (a = NH3)) in an attempt to produce discrete cyclic constructs. While Ag+ gave a polymeric structure {[Ag(3,3′-bpy)](ClO4) · H2O}n (1), with all other metal entities cyclic structures were formed. Interestingly, Hg(CH3COO)2 produced a dinuclear complex [Hg(3,3′-bpy)(CH3COO)2]2 · 3H2O (2), in which the two 3,3′-bpy ligands adopt a cis-orientation of the coordinating pyridyl entities. With cis-(NH3)2Pt2+, a cyclic complex 4 was isolated in crystalline form which, according to HRMS, is a trimer. With trans-(NH3)2Pt2+, different species are formed according to 1H NMR spectroscopy, the nature of which was not established.


Introduction
The "molecular library" concept has proven highly efficient in designing discrete supramolecular metal complexes by combining di-or multitopic metal entities with rigid dior multitopic ligands [1,2]. It is less straightforward if ligands are flexible and can adopt, in principle, different rotamer states. In its simplest form, this is the case when two N-heterocyclic ligands are connected via a C-C bond. Examples are, among others, 2, 2 -bipyridine (2, 2 -bpy), 3, 3 -bipyridine (3, 3 -bpy) and, 2, 2 -bipyrazine (2, 2 -bpz) (Scheme 1). While 2, 2 -bpy, in the overwhelming number of structures, acts as a chelating ligand with the two ring N atoms in a cis-orientation, there are also rare cases of 2, 2 -bpy adopting a bridging mode, hence being in a transconfiguration or half-way between cis and trans- [3]. It depends on the conformation of the ligand and the geometry of the metal, what kind of construct/s is/are formed. With 2, 2 -bpz, we have studied this question in more detail and have characterized a number of discrete molecular entities, which include a flat triangular structure, 3D triangular entities of different shapes (prism, vase), as well as a tetranuclear open box [4,5]. In all these cases the N4/N4 positions are involved in metal coordination, occasionally complemented by addition of metal chelation via N1/N1 , and influenced by counter anions.
In principle, 3, 3 -bipyridine (3, 3 -bpy) metal complexes should be able to reveal analogous topologies as 2, 2 -bpz, with the advantage of higher basicities of the N donor atoms ( Figure 1). There are several reports in the literature on polymeric structures containing cis- [6] and in particular trans-arranged 3, 3 -bpy ligands [7], yet none with a discrete molecular metallacycle. The only related examples are those of trinuclear cycles containing three cis-a 2 M II units (a 2 = diamine; M = Pd or Pt) and three 4,7-phenanthroline ligands, which can be considered rigid analogous of 3, 3bpy ligands with the two pyridine entities fixed in a cisorientation [8,9]. Our interest in discrete cationic metallacycles stems, among others, from their potential of interacting noncovalently with DNA [10] or particular DNA secondary structures such as DNA quadruplexes [11], as well as their ability to act as hosts for anions [12,13]. In the present study, we have employed different transition metal ions and metal entities which previously have been shown by others and ourselves to produce discrete cyclic complexes, namely Ag(I), Hg(II), enPd(II), cis-(NH 3 ) 2 Pt(II) as well as trans-a 2 Pt(II) (a = NH 3 ) [14][15][16].

X-Ray Crystal Structure
Determination. X-ray crystal data for 1 and 2 (Table 1) were recorded at 150 K with an Xcalibur diffractometer equipped with an area detector and graphite monochromated Mo Kα radiation (0.71073Å). Data reduction was done with the CrysAlisPro software [21]. Both structures were solved by direct methods and refined by full-matrix least-squares methods based on F 2 using SHELXL-97 [22]. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms (including water molecules) were positioned geometrically and refined with isotropic displacement parameters according to the riding model. All calculations were performed using the SHELXL-97 and WinGX programs [22,23]. CCDC 763713 and 763714 contain the crystallographic data for compounds 1 and 2.
The crystal structure of the dinuclear species [Hg(3,3'bpy)(CH 3 COO) 2 ] 2 · 3H 2 O (2) is given in Figure 5. Unlike in 1, in 2 the 3, 3 -bpy ligands adopt a cis-conformation of the two pyridyl rings, with a twist angle of 30.4(2) • , and act as bridges between two mercury centers. The coordination geometry of the Hg ion (Table 3) Table 3. Both 3, 3 -bpy ligands and their bonded mercury atoms are almost coplanar with a tendency towards a boat conformation (distance from Hg1 to the plane defined by N1a, N1b, N1a , N1b is 0.58Å).

Complexes with enPd II and cis-(NH
The high resolution MS of a sample of 4 was carried out and confirmed a triangular structure (see (II) or (IV) in Figure 1). The mass spectrum displayed peaks due to [M-(PF 6  Without exception, the spectra displayed time-dependent changes, but within 2-3 d at 50 • C, constant spectra were obtained. Even then, however, resonances due to multiple products were present. In the case of a large excess of ligand over Pt (10 : 1), the spectrum reveals the presence of a major species attributed to trans-[Pt(NH 3 ) 2 (3, 3 -bpy) 2 ] 2+ and excess 3, 3 -bpy (Figure 7). The resonances of the free 3, 3 -bpy (L) were unambiguously identified by adding solid 3, 3 -bpy to the NMR sample. The two sets of pyridine resonances of the coordinated 3, 3 -bpy ligands of the 1 : 2 complex are assigned on the basis of their relative intensities. What strikes is that the H2 and H4 resonances of the free ligand are very much broadened (cf. Figure 2(a)) and that H2, H4, and H6 are upfield shifted by ca. 0.2, 0.08, and 0.14 ppm, respectively. As these shifts cannot be interpreted with a pD effect, we propose that the presence of the 1 : 2-Pt complex has an effect on the rotamer equilibrium of the free ligand. Consistent with this proposal, the two resonances closest to the C3-C3 bond, hence H2 and H4, become quite broad. Stacking interactions between free and coordinated 3, 3 -bpy could possibly account for this feature. The 1 H NMR spectrum of a 1 : 1 mixture of trans-[Pt(NH 3 ) 2 (D 2 O) 2 ] 2+ and 3, 3 -bpy displays four H2 (H2 ) singlets of different relative intensities at lowest field, and at least for the H6 (H6 ) resonances also four components can be differentiated. Free 3, 3 -bpy is not detectable. It is obvious that the self-assembly process of trans-(NH 3 ) 2 Pt II and 3, 3bpy does not lead to a preferred single product, unlike in the case of enPd II and cis-(NH 3 ) 2 Pt II .

Summary
The flexible ditopic ligand 3, 3 -bipyridine forms with Hg(CH 3 COO) 2    represents the smallest possible entity of any cyclic complex. It appears that the opening of the N1a-Hg-N1b angle to ca. 115 • allows the dinuclear to be formed. A similar structure, with the two 3, 3 -bpy ligands approximately coplanar, is not to be expected for cis-a 2 Pt II with its 90 • bonding angle. Consequently, 4 is a cyclic trinuclear compound. On the NMR time scale, 2 is kinetically labile in aqueous solution, but 4 is inert. We plan to further study 4 with regard to its host-guest chemistry and its noncovalent interaction with DNA.