Molecular architecture based on metal triangles derived from 2,2′-bipyrazine (Bpz) and EnMII (M = Pt, Pd)

The molecular triangle [{enPt(bpz-N4,N4')}(3)](6+) (en = ethylenediamine; bpz = 2,2'-bipyrazine) has been crystallized as a C-2-symmetric species (1), as a compound of approximate C-3 symmetry, and as a mixture of both forms (Ib). The two triangles differ in their topologies, their Pt-Pt distances, and their anion binding properties. While for the C-2 form insertion of a single PF6- anion in the central cavity is seen in Ib, the C-3 forms of 1a and 1b incorporate either two different anions simultaneously, NO3- and ClO4- (la), or only a single PF6- (1b). Anion inclusion also occurs in solution as detected by H-1 NMR spectroscopy. The molecular triangles l-lb are the kinetic reaction products of enPt(II) and bpz. The thermodynamic product is the chelate [enPt(bpz-N1,N1')](2+) (2a) that is obtained from 1 upon prolonged heating in water. The all-cia geometry of the bpz ligands in the triangle (Cg form) can be locked by chelation of three enPd(II) to the NI,N1' sites. Hexanuclear [{enPt(N4,N4'-bpz-N1,N1')Pden}(3)](12+) (3) has been reported by us before. Now the Pt-6 analogue 4, the Pd-6 analogue 5, and the isomer of 3 with the two sorts of metals inverted (6) have been isolated and structurally characterized. All four compounds (3-6), which crystallize as mixed NO3-, PF6- salts, have two different anions embedded in the cavity of the cations of +12 charge. Molecular triangles of the C-3 type can be fused by Ag+ cations via N1,N1' positions to afford the higher-nuclearity compounds Pt6Ag2 (7) and Pt6-Ag3 (8). The latter encapsulates an additional Ag+ and five NO3- anions in its interior (8a). X-ray crystal structures for la, Ib, 2a, 3, 5, 6, 7, and 8a are reported.