Novel open-framework architectures in lanthanide phosphonates

Two novel three-dimensional lanthanide coordination polymers have been prepared hydrothermally with the phosphonic acid N,N′N,N′-piperazine bis(methylenephosphonic acid), H2O3PCH2N(C2H4)2NCH2PO3H2 (LH4). The structures of Gd2(LH2)3⋅3H2O (I) and Nd2(LH2)3⋅9H2O (II) have been characterised by single crystal X-ray techniques. One-dimensional ‘lanthanide-phosphate’ chains are a key feature in both structures, although there are major structural differences between the chains, with (I) displaying octahedral GdO6 coordination and (II) showing eight-coordinate NdO8 polyhedra. In each case, three-dimensional connectivity is completed by coordination of the phosphonate group resulting in open framework structures encapsulating loosely bound water molecules. Isostructural Y3+ and Yb3+ analogues of (I) have been prepared, suggesting that cation size is a key factor in controlling the differing reaction products. In the case of Y2(LH2)3⋅5H2O, isostructural to (I), it is shown that the extra-framework water molecules may be removed reversibly without framework collapse. Structural relationships to other known lanthanide phosphonates are discussed.

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