supplementary materials

The title compound, (I), is the bromide analog of the previously published chloride (Hayoun et al., 2006) and iodide (Angle et al., 2007) derivatives.

There are no previous structural characterizations of [Pt(tpy)Br]⁺ (tpy=2,2':6',2"-terpyridine), but the interatomic distances within the [Pt(tpy)]²⁺ are unexceptional and unperturbed by the intermolecular interactions. According to the Cambridge Structural Database (Version 5.30, May 2009; Allen, 2002), the linear [AuBr₂]^- anion has been structurally characterized 32 times with an average Au—Br distance of 2.376 (3) Å and Br—Au—Br angle of 179.3 (2)°, with which the anions in (I) compare favorably. The structure of (I) is analogous to that of [Pt(tpy)Cl]⁺[AuCl₂]^-, with metallophilic interactions forming among two platinum(II) and one gold(I) centers to form {[Pt(tpy)Br]₂[AuBr₂]}⁺ cations (Figure 1). These cations also form metallophilic interactions among each other resulting in an infinite chain of {PtAuPt}_∞ metallophilic interactions along the b-axis with the remaining [AuBr₂]^- counteranion found outside of the chain (Figure 2). A solvent DMSO molecule was also found in the lattice. The bromide ligands are small enough to allow for metallophilic interactions between gold(I) and platinum(II) centers (Figure 1). No extended metallophilic chains exist in the iodide derivative, which exhibits only pairwise contacts between the cations and between the anions.

As seen in Table 1, the Pt(II)···Pt(II) distances in the bromide derivative are the shortest of all three halogenated species, at 3.4335 (6) Å. The chloride and iodide derivatives exhibit 3.4535 (7) and 3.5278 (3) Å Pt···Pt metallophilic distances, respectively. Evidently the bromide ligand promotes shorter Pt···Pt bonds than the chloride or iodide derivatives, consistent with expectations that more electron rich metal centers promote metallophilic interactions (Pyykkö, 1997). As bromide is softer and less electronegative than chloride, its adjacent platinum center is less electron deficient and bromide is small enough to allow stacking for metallophilic bonding. The gold-platinum distances increase slightly with halogen size from Cl to Br. The Au···Pt···Pt angle is more linear in the bromide derivative at 173.9°, compared to the chloride derivative with an angle of 165.1°. These distances and angles, along with other potentially interesting geometrical values, are collected in Table 1.

The structure of compound (I), therefore, completes a study of the [Pt(tpy)X][AuX₂] systems and demonstrates that the halide constituent in the [Pt(tpy)X]⁺ ion has a determining effect on the length of the Pt···Pt metallophilic contacts for steric and electronic reasons.