Structural studies of trans-N2S2 copper macrocycles
Graphical abstract
The dynamic behaviour of two related trans-N2S2 copper macrocycles are reported in relation to their use as redox sensors and models of blue copper proteins. Two processes are studied namely the boat chair conformational change of the alkyl moieties within the macrocyclic backbone (52.1 kJ mol−1) and the energy required to move the ring from a distorted planar into to a tetrahedral geometry (26.2 kJ/mol−1). The view obtained is of a ligand which is able to re-organise to suit the prevailing conditions.
Introduction
Cis- and trans-disposed N2S2 bis-benzo macrocycles (Fig. 1) are well known and are of great value as models of the metal binding site in blue copper proteins (BCP) [1], [2], [3], [4], [5], [6]. The ability of the copper centre in BCP’s to shuttle electrons efficiently is of great interest as the mechanism proposed demands that the reorganisational energy of the binding site during electron transfer be low [7], [8], [9]. Indeed DFT molecular orbital calculations on model complexes suggest that the energy required is between 62 and 90 kJ mol−1 [8], [9]. The behaviour of the copper motif in BCP’s indicates that it should be possible to build copper complexes that are stable in both their uni- and di-valent forms and furthermore, that can be tuned to reduce and oxidise at a specific potential (0.2–0.8 V). This hypothesis leads to the suggestion that CuN2S2 complexes could be developed as biological redox sensors. We have recently reported on a series of cis-N2S2 imine macrocyclic copper complexes, including their related structural and electrochemical features [10]. To develop this work further, we have turned our attention to the related tetradentate trans-N2S2 complexes (Fig. 1), in order to determine what effect changing the positions of the donor atoms has on the redox behaviour of the copper centre and the conformation of the macrocyclic ring.
Section snippets
Experimental
All experiments were carried out using standard apparatus and commercially available chemicals except for [Cu(CH3CN)4] BF4 which was prepared by literature methods [11]. Solvents were used as supplied, unless stated as ‘dry’ in which case they were distilled prior to use. 1H and 13C NMR data were acquired at ambient temperature on Bruker DPX or AVANCE NMR spectrometers operating at a proton resonance frequency of 400.13 MHz. All spectra were referenced internally to the residual proton resonance
Results and discussion
The complexes of interest, ([Cu(I) trans-(N-en-S)2] and [Cu(I) trans-(N-pr-S)2)]), have been prepared (Scheme 1) previously by Martin et al. as triflate salts [5]. However, we favour the tetrafluoroborate salts in our studies [10]. This trivial alteration can have some interesting consequences. Indeed the trans-(N-en-S)2 species was retrieved here in the Cu(II) form (Fig. 2, pace Martin et al. [5]). Comparing the disposition of the macrocycle in the two forms provides an illustrative view of
Acknowledgements
KDT and JR would like to thank the University of Strathclyde, WestCHEM and BBSRC (BBS/B/01553) for funding. KDT gratefully acknowledges the technical assistance of Mr. Craig Irving at the University of Strathclyde WestCHEM NMR Facility in the collection of these NMR data. We wish to thank the EPSRC National Crystallographic Service, University of Southampton, for X-ray data collection.
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