A naturally-occurring new lead-based halocuprate(I)

Abstract

Pb2Cu(OH)2I3 is a new type of halocuprate(I) that is a framework of alternating [Pb4(OH)4]4+ and [Cu2I6]4− units. The structure has been determined in orthorhombic space group Fddd to R1=0.037, wR2=0.057, GoF=1.016. Unit cell parameters are a=16.7082(9)Å, b=20.8465(15)Å, c=21.0159(14)Å, V=7320.0(8)Å3 (Z=32). There is no synthetic counterpart. The structure is based upon a cubane-like Pb4(OH)4 nucleus that is coordinated to sixteen iodide ions. Cu+ ions are inserted into pairs of adjacent edge-sharing tetrahedral sites in the iodide motif to form [Cu2I6]4- groups. The Raman spectrum of Pb2Cu(OH)2I3 has two O-H stretching modes and as such is consistent with space group Fddd, with two non-equivalent OH groups, rather than the related space group I41/acd which has only one non-equivalent OH group. Consideration of the 18-electron rule implies that there is a Cu=Cu double bond, which may be consistent with the short Cu…Cu distance of 2.78Å, although the dearth of published data on the interpretation of Cu…Cu distances in halocuprate(I) compounds does not allow a clear-cut interpretation of this interatomic distance. The orthorhombic structure is compared with that of the synthetic halocuprate(I) compound Pb2Cu(OH)2BrI2 with space group I41/acd and having chains of corner-linked CuI4 tetrahedra rather than isolated Cu2I6 pairs. The paired motif found in Pb2Cu(OH)2I3 cannot be achieved in space group I41/acd and, conversely, the chain motif cannot be achieved in space group Fddd. As such, the space group defines either a chain or an isolated-pair motif. The existence of Pb2Cu(OH)2I3 suggests a new class of inorganic halocuprate(I)s based upon the Pb4(OH)4 group.