Oxalate networks

Hauser A, von Arx ME, Langford VS, Oetliker U, Kairouani S, Pillonnet A (2004) Photophysical Properties of Three-Dimensional Transition Metal Tris-Oxalate Network Structures. 241 65-96... 

The site symmetry of both the tris-oxalate and the tris-bipyridyl compexes is C3. Furthermore, the rather loose oxalate network is actively stabilized by the tris-bipyridyl complexes through electrostatic interactions from ir-overlap between oxalate and bipyridine along the trigonal axis. Selected crystallographic data including relevant bond lengths and angles are presented in Table 1. 

As already described, the counterion can template the formation of different oxalate frameworks. For example, the synthesis of a heterobimetallic 3D framework K" [Cr " (ox)3] was achieved with [Cu(trans[14]dien)] " (trans[14] dien = 5,7,7,12,14,14-hexamethyl-l,4,8,11-tetraazacyclotetradeca-4-11-diene) (66). While each and Cr atom is octahedrally coordinated, each Cu " " ion coordinates an oxalate oxygen atom in a monodentate fashion producing 21 X 9 A helical channels (Fig. 4). This generates a ths net (7) structure, while the previous 3D oxalate networks consist of chiral srs nets. Magnetic studies showed an antiferromagnetic interchain interaction between the copper and chromium centers. 

Photophysical Properties of Three-Dimensional Transition Metal Tris-Oxalate Network Structures... 

Keywords Oxalate networks [Cr(ox)3]3- [Cr(bpy)3]3+ 2E state Resonant energy transfer Phonon-assisted energy transfer Forster transfer Exchange interaction... 

The two transition metal complexes, [Cr(ox)3]3" and [Cr(bpy)3]3+ (ox=ox-alate, bpy=2,2 -bipyridine) depicted in Fig. 3a are well known chromophores in transition metal photochemistry and photophysics. In the three-dimensional oxalate network structure of composition [Cr(bpy)3][NaCr(ox)3]C104, the two can be combined in an unique manner [16]. The sodium ions, in fact, serve as glue in such a way that each oxalate ligand serves as bridging... 

Fig. 3 a The building blocks [Cr(ox)3]3 and [Cr(bpy)3]3+. b Space-filling model of the three-dimensional oxalate network (dark) encapsulating the tris-bipyridine cation (light) as in [NaCr(ox)3][Cr(bpy)3]C104... 

The corresponding curve in Fig. 9b yields an experimental value for n of 7. This also makes sense from a structural point of view. The nearest neighbour shell around a given donor chromophore in the three-dimensional oxalate network does indeed contain seven sites. However, as shown in Fig. 13, they are not all crystallographically equivalent. One of the seven, with a metal-metal distance RDA of only 6.1 A, sits on the same trigonal axis as the do-... 

This chapter is not so much meant to be a general review of the important topic of excitation energy transfer in coordination compounds, it is much more meant to show how unambiguous answers to basic questions can be obtained from experimental results by choosing a well-defined model system and state-of-the-art spectroscopy. The three-dimensional oxalate networks thus well and truly constitute a model system with sufficient chemical flexibility to investigate a given process with the required variation of the relevant parameters. 

The tris-bipyridine complexes on the other hand are encapsulated by the oxalate network. Thus in the co-doped systems a [Cr(bpy)3]3+ complex happening to sit in the first acceptor shell of a given donor is much closer to this donor than a [Cr(bpy)3]3+ complex sitting in the second shell, n-n overlap between ligand orbitals of the donor and an acceptor in the first shell ensure efficient energy transfer on the sub-microsecond timescale mediated by exchange interaction. Additionally, the relative orientation of donor and acceptor plays an important role for the n-n overlap. For acceptors further away, for which there is no exchange pathway, dipole-dipole interaction takes over. With a critical radius of the order of 11 A, this is much less efficient and the overall quantum efficiency is thus less than unity. 

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