Supramolecular Isomerism of 2,2′-Bipyrazine Complexes with cis-(NH3)2PtII: Ligand Rotational State and Sequential Orientation Determine the 3D Shape of Metallacycles

• Published in: Chemistry : a European journal 2011-09, Vol.17 (38), p.10771-10780
• Main Authors: Galstyan, Anzhela, Shen, Wei-Zheng, Freisinger, Eva, Alkam, Hussein, Hiller, Wolf, Sanz Miguel, Pablo J., Schürmann, Markus, Lippert, Bernhard
• Format: Article
• Language: English
• Subjects: anions; Chemistry; host-guest systems; Ligands; nitrogen heterocycles; platinum; self-assembly
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201101381

2,2′‐Bipyrazine (2,2′‐bpz) reacts with cis‐(NH3)2PtII in water to give a variety of products, several of which were isolated and characterized by X‐ray analysis: cis‐[Pt(NH3)2(2,2′‐bpz‐N4)2](NO3)2⋅3 H2O (1), [{cis‐Pt(NH3)2(2,2′‐bpz‐N4,N4′)}3](PF6)5NO3⋅7 H2O (2 a), [{cis‐Pt(NH3)2(2,2′‐bpz‐N4,N4′)}3](BF4)2(SiF6)2⋅15 H2O (2 b), and [{cis‐Pt(NH3)2(2,2′‐bpz‐N4,N4′)}4](SO4)4⋅22 H2O (3). In 1, 2 b, and 3 the 2,2′‐bpz ligands adopt approximately C2h symmetries, hence the two pyrazine halves are in trans orientation, whereas in 2 a all three 2,2′‐bpz bridges are approximately C2v symmetric, with the pyrazine halves cis to each other. The topologies of the two triangular complexes 2 a and 2 b are consequently distinctly different, but nevertheless both cations act as hosts for anions. In 2 a a PF6− and a NO3− anion are associated simultaneously with the +6 cation, whereas in 2 b it is a BF4− anion and a water molecule, which are trapped in its cavity. There is no anion inclusion in case of the metallasquare 3. In principle, 3 can exist in a large number of stereoisomers, depending on the rotational states of the bridging 2,2′‐bpz ligands. Isolation of a single rotamer form of 3 with C2h symmetric 2,2′‐bpz ligands and an overall meso form is proposed to be a consequence of a highly efficient self‐assembly process that starts from the precursor 1 and reaction with two cis‐(NH3)2PtII units. This process leads to the isolated rotamer of 3 regardless of whether two cations 1 in head–head form react with two cis‐(NH3)2PtII, or whether the Δ enantiomer of the chiral head–tail form of 1 combines with its Λ enantiomer through two cis‐(NH3)2PtII entities. Shape shifters: The rotational states (cis or trans) of the flexible bridging 2,2′‐bipyrazine ligands, their sequence, and occasionally encapsulated anions determine the overall shapes of tri‐ and tetranuclear metallacycles that contain cis‐(NH3)2PtII entities (see figure).