Abstract
A series of cationic and neutral p-Br and p-NO2 pyridine substituted Eu(III) and Gd(III) coordination complexes serve as versatile synthetic intermediates. Nucleophilic aromatic substitution occurs readily at the para position under mild conditions, allowing C-N and C-C bond forming reactions to take place, permitting the introduction of azide, amino and alkynyl substituents. For Eu(III) complexes, this approach allows late stage tuning of absorption and emission spectral properties, exemplified by the lowering of the energy of an LMCT transition accompanied by a reduction in the Eu-Npy bond length. Additionally, these complexes provide direct access to the corresponding Eu(II) analogues. With the Gd(III) series, the nature of the p-substituent does not significantly change the EPR properties (linewidth, relaxation times), as required for their development as EPR spin probes that can be readily conjugated to biomolecules under mild conditions.
Highlights
The preparation of metal coordination complexes is normally undertaken using a convergent process, introducing the metal ion in the final step
We exemplify the synthetic versatility of the pÀ Br and pÀ NO2 pyridine lanthanide coordination complexes, [EuL1a-c], [LnL2a-2d] and [EuL3a] (Scheme 1), where displacement of the para substituent by a range of charged and uncharged nucleophiles is possible under mild conditions, (Scheme 2)
The stepwise synthesis of the ligand L1a was undertaken following standard methods reported for the parent complex, L1c, and details are given in the SI.[31,32]
Summary
Conditions, this divergent approach greatly increases the efficiency of the overall synthetic process. We exemplify the synthetic versatility of the pÀ Br and pÀ NO2 pyridine lanthanide coordination complexes, [EuL1a-c], [LnL2a-2d] and [EuL3a] (Scheme 1), where displacement of the para substituent by a range of charged and uncharged nucleophiles is possible under mild conditions, (Scheme 2). This late stage functionalisation strategy avoids the need for separate ligand syntheses (exemplified here, for comparison, in the rather laborious stepwise synthesis of the non-emissive complex, [EuL4]) (given in the Supporting Information), and allows the desired property of the complex to be imparted, or tuned, at the end of the synthetic pathway by a one-step structural transformation of the complex.
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