Capped square antiprismatic structure

X-ray crystal structures have been determined for a series of [/f(L10 lH)] chelates, defining nine-coordinate capped square antiprismatic (CSAP) or inverted capped square antiprismatic structures. The acetate arms of the L10 or DOTA ligand are arranged in a propeller-like fashion above the basal plane made up of the four N donor atoms, which encompass the... 

Figure 1. The structure of c o%o-[(PMe2Ph)2(l-NiBgFlyCl2-2,4)] showing the bi-capped-square antiprismatic arrangement of the NiB cluster, which is analo-...

X-Ray crystallographic analysis used alongside solution phase techniques such as NMR, is invaluable in the design of selective anion receptors as it can be used to define the solid-state structure of the anion-bound complex. Several X-ray structures of anion-bound complexes of [Ln.la]3+ have been defined recently (acetate, lactate, citrate, alanine, glycine, methionine, serine and threonine) [8,21,22], The complexes adopt a monocapped square antiprismatic structure with one base comprising of four N atoms of the macrocycle and the other base containing three O atoms of the pendent arms. The latter base is completed by a carboxylate O donor of the anion, which binds in a bidentate manner and simultaneously caps this base by O (acetate), OH (a-hydroxy acids) or NH2 (a-amino acids). The nature of the donor atom in the capping (or axial) position correlates well with the observed NMR shift in the solution phase. 

Clusters containing a more close-packed metal framework, viz. mono-capped square antiprismatic, are exemplified by [Rh9E(CO)2i] (E = P, " As). The structure is schematically shown in Fig. 16 and it is strictly related to the trans-... 

The interpretation of the observed proton and C PCS led to the conclusion that the two structural isomers are enantiomeric pairs of diastereoisomers differing in the octadentate ligand conformation, namely, capped square antiprismatic (SA or M) and twisted capped square antipris-matic (TSA or m) geometries, respectively (Figure 5). " The Ln + coordination polyhedral of SA and TSA have two opposite parallel faces occupied by the ligand ring nitrogen... 

The (2,2,2) cryptand is flexible enough to include all the hetero-atoms in the coordination polyhedron and to allow the additional bonding of one bidentate anion, either perchlorate or nitrate. Thus all the 1 1, 4 3, and 2 1 cryptates for which a crystal structure has been reported contain a [RX (2,2,2)] " cation, X = CIO4 or NOJ, and n = 1 or 2. The coordination number is 12 for La(III) and 10 for Nd(III), Sm(III), and Eu(III). The latter ions have a distorted capped-square antiprismatic geometry, two N atoms capping the two square faces. The 2 1 Nd and Sm cryptates are isostructural and their asymmetric unit contains 11-coordinate [R(N03)5(H20)] anions. The water molecule makes weak hydrogen bonds to different anions. 

Key Colour % indicates preparation but no report of colour) mp/°C (na indicates value not reported) coordination 9 ttp = tricapped trigonal prismatic 8 d = dodecahedral 8 sa = square antiprismatic 8 btp = bicapped trigonal prismatic 8,7 = mixed 8- and 7-coordination (SrBr2 structure) 7 cc = capped octahedral 7 pbp = pentagonal bipyramidal 6 o = octahedral 6 och = octahedral chain, 6 ol = octahedral layered. 

The structures are known for a number of these compounds thus, of the eight-coordinate compounds, Eu(acac)3(phen) is square antiprismatic, Eu(dpm)3(py)2 and Nd(tta)3(Ph3PO)2 are dodecahedral seven-coordinate Lu(dpm)3(3-picoline) is a face-capped trigonal prism and Eu(dpm)3(quinuclidine) a capped octahedron. Nine-coordination is found in Eu(dpm)3(terpy) and also in Pr(facam)3(DME)3(facam = 3-trifluoroacetyl-D-camphorate). 

---