Supramolecular isomerism of 2,2′-bipyrazine complexes with cis-(NH_3)_2Pt~(II): Ligand rotational state and sequential orientation determine the 3D shape of metallacycles

摘要

2,2′-Bipyrazine (2,2′-bpz) reacts with cis-(NH _3)_2Pt~(II) in water to give a variety of products, several of which were isolated and characterized by X-ray analysis: cis-[Pt(NH_3)_2(2,2′-bpz-N4)_2](NO _3)_2·3 H_2O (1), [{cis-Pt(NH _3)_2(2,2′-bpz-N4,N4′)}_3](PF _6)_5NO_3·7 H_2O (2 a), [{cis-Pt(NH3)2(2,2′-bpz-N4,N4′)} 3](BF4)2(SiF_6)_2· 15 H_2O (2 b), and [{cis-Pt(NH_3)_2(2,2′- bpz-N4,N4′)}_4](SO_4)_4·22 H _2O (3). In 1, 2 b, and 3 the 2,2′-bpz ligands adopt approximately C_(2h) symmetries, hence the two pyrazine halves are in trans orientation, whereas in 2 a all three 2,2′-bpz bridges are approximately C_(2v) 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 PF_6- and a NO_3- anion are associated simultaneously with the +6 cation, whereas in 2 b it is a BF_4- 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 C_(2h) 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-(NH_3)_2Pt~(II) units. This process leads to the isolated rotamer of 3 regardless of whether two cations 1 in head-head form react with two cis-(NH_3)_2Pt~(II), or whether the Δ enantiomer of the chiral head-tail form of 1 combines with its Λ enantiomer through two cis-(NH_3)_2Pt~(II) entities.