Elsevier

Polyhedron

Volume 26, Issue 8, 21 May 2007, Pages 1669-1676
Polyhedron

Synthesis and structural characterisation of a series of cobalt complexes of N-appended anthracenyl cyclam

https://doi.org/10.1016/j.poly.2006.12.003Get rights and content

Abstract

The synthesis and structural characterisation of a series of cobalt complexes of 1-(anthracen-9-ylmethyl)-1,4,8,11-tetraazacyclotetradecane (hereafter L2) is described. The complexes, of the type trans-[CoL2X2]+ in which X = Cl, NCS and NO2- were synthesised for their use in photoactivated ligand release studies. X-ray crystal structures show that the macrocycle in all three complexes of the series adopts a trans-(III) configuration. Both thiocyanato ligands were found to be N-bound in trans-[CoL2(NCS)2]+ while trans-[CoL2(NO2)2]+ contained both a nitro and nitrito ligand in the one complex. The synthesis of the complex cis-[CoL2(NO2)2]+ from sodium hexanitrocobaltate(III) is also described. This complex also contains both a nitro and nitrito ligand in the one complex, with the cyclam in a cis-(V) arrangement about the metal centre.

Graphical abstract

Geometric and linkage isomerism is a feature of a series of anthracenylcyclam cobalt(III) complexes which have been synthesised for future use in photochemical studies.

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Introduction

A considerable range of systems has been developed which utilise the electron and energy transfer processes occurring in linked donor–acceptor molecules for particular applications. The scope of this work encompasses the use of transition metal complexes and a number of potential uses of metal-based systems are being realised. Fabrizzi’s group have described several molecular systems in which a fluorescent chromophore has been covalently linked to a cyclic or open quadridentate ligand [1], [2], [3], [4], [5], [6], [7]. Such systems can be used for a variety of applications such as molecular thermometers [1], molecular switches of fluorescence [2], [3], [4] or sensitive metal ion sensors [5], [6], [7] when triggered by complexation with a transition metal ion. De Rosa et al. describe a range of Cr(III) cyclam complexes in which pendant chromophores could serve as sensitisers for metal centred photoreactions [8], [9]. Complexes involving a coordinated nitrite ligand were found to be capable of photoactivated nitric oxide generation [9]. Compounds of this type could enable delivery of the biologically important NO within a well-defined volume when coupled with a focussed excitation source [9]. We are interested in similar donor–acceptor complexes containing a Co(III) metal centre coupled to a range of ligands for future use in photoactivated ligand release studies. Towards this end we have previously described the synthesis and characterisation of the ligand 6-(anthracen-9-ylmethyl)-1,4,8,11-tetraazacyclotetradecane (hereafter L1) in which an anthracene moiety is appended to the apical carbon of the cyclam framework by a methylene bridge (Fig. 1) [10]. Subsequent metallation with cobalt was also described to yield the complex trans-[CoL1Cl2]+ and some of its photochemical properties were examined. Excitation of a solution of the complex in DMF led to spectral changes consistent with substitutions at the Co(III) centre [10].

With a view to further developing these ideas, and to better understand the photochemistry of donor–acceptor complexes containing cobalt(III) as the acceptor group, we have prepared a series of cobalt(III) complexes of 1-(anthracen-9-ylmethyl)-1,4,8,11-tetraazacyclotetradecane (hereafter L2) in which anthracene (the donor) is appended to a nitrogen atom of the cyclam macrocycle (Fig. 1). The complexes are of the type trans-[CoL2X2]+ in which X=Cl-, NCS, and NO2-. These anionic ligands were chosen to cover a broad range of the spectrochemical series and it is these ligands that may be lost following a photoinduced process, with the cobalt atom remaining bound to the cyclam macrocycle. Investigation of complexes of L2 will also allow comparison of the photochemical properties of these N-appended complexes with those of their C-appended analogues derived from L1. In this paper we describe the synthesis and structural characterisation of a series of complexes derived from L2. The preparation and structure of the complex cis-[CoL2(NO2)2]+, which contains one N-bonded and one O-bonded nitrite ligand is also described.

Section snippets

Synthesis

The synthesis of L2 was accomplished by the N-alkylation of cyclam with 9-chloromethyl-anthracene following the method described by Fabrizzi et al. [7]. The synthetic route to the series of complexes trans-[CoL2X2]+ (X=Cl-, NCS and NO2-) is summarised in Scheme 1.

The complex trans-[CoL2Cl2]+ was prepared by the addition of cobalt(II) chloride to L2 in methanol. Subsequent addition of hydrochloric acid and aerial oxidation led to formation of the trans-dichloro cobalt(III) complex which was

Conclusion

In this work we have demonstrated the coordinating ability of the ligand L2 by preparing a series of cobalt(III) complexes of the form [CoL2X2]+. In the ‘simple’ complex trans-[Co(cyc)(NO2)2]+ both nitro ligands are N-bound [15], so we were somewhat surprised to observe linkage isomerism in the solid state for both the cis- and trans-isomers of [CoL2(NO2)2]+. There are several examples of mononuclear complexes containing both a nitro and nitrito ligand in the one molecule for complexes of

Instrumentation

ESI-MS experiments were performed using an ion trap (Finnigan Mat) LC-Q spectrometer on dilute solutions of the complexes in methanol. 1H NMR spectra were measured in d-chloroform, d3-acetonitrile and d4-methanol on a Unity Plus 400 spectrometer. Chemical shifts (δ, positive downfield) are given in ppm (solvent reference) and J values in Hz. Aromatic hydrogens are labelled according to IUPAC conventions for anthracene (see supplementary information). Absorption spectra were collected using a

Acknowledgement

We thank the Australian Research Council for financial support of this work.

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