Molecular Architecture Based on Metal Triangles Derived from 2,2‘-Bipyrazine (Bpz) and EnMII(M = Pt, Pd)

Abstract

The molecular triangle [{enPt(bpz-N4,N4‘)}3]6+ (en = ethylenediamine; bpz = 2,2‘-bipyrazine) has been crystallized as a C2-symmetric species (1), as a compound of approximate C3 symmetry, and as a mixture of both forms (1b). The two triangles differ in their topologies, their Pt−Pt distances, and their anion binding properties. While for the C2 form insertion of a single PF6- anion in the central cavity is seen in 1b, the C3 forms of 1a and 1b incorporate either two different anions simultaneously, NO3- and ClO4- (1a), or only a single PF6- (1b). Anion inclusion also occurs in solution as detected by 1H NMR spectroscopy. The molecular triangles 1−1b are the kinetic reaction products of enPtII 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-cis geometry of the bpz ligands in the triangle (C3 form) can be locked by chelation of three enPdII to the N1,N1‘ sites. Hexanuclear [{enPt(N4,N4‘-bpz-N1,N1‘)Pden}3]12+ (3) has been reported by us before. Now the Pt6 analogue 4, the Pd6 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 C3 type can be fused by Ag+ cations via N1,N1‘ positions to afford the higher-nuclearity compounds Pt6Ag2 (7) and Pt6Ag3 (8). The latter encapsulates an additional Ag+ and five NO3- anions in its interior (8a). X-ray crystal structures for 1a, 1b, 2a, 4, 5, 6, 7, and 8a are reported.