Unsymmetrical Ligand Backbone Drives Site-Selective Assembly of Heterometallic Complexes
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Heterometallic catalysts harness cooperative interactions between distinct metal centers to enable unique reactivity; however, their development remains limited by inherent synthetic challenges associated with their selective assembly. Here, we report that 2,7-di(pyridin-2-yl)imidazo[1,2-a]pyrimidine (dpip) enables the one-step, site-selective formation of the heterometallic complexes [Ni2Cu2(dpip)2(µ–OH)2(O2CCF3)5(H2O)2][O2CCF3] (NiCu), [Ni2Cu(dpip)(µ–OH)(O2CCF3)5(H2O)] (2NiCu), and [Co2Cu(dpip)(µ–OH)(O2CCF3)5(MeCN)] (2CoCu). Metal redistribution experiments, in which the homometallic complexes [Ni3(dpip)(µ–OH)(O2CCF3)5(MeCN)(H2O)] (3Ni) and [Cu2(dpip)(µ–OH)(O2CCF3)3] (2Cu) were treated with Cu(O2CCF3)2·H2O and Ni(O2CCF3)2·4H2O, afforded the heterometallic complexes NiCu and 2NiCu, consistent with reversible metal exchange and thermodynamically controlled assembly. Comparative experiments using a symmetric 1,8-naphthyridine-based ligand yielded an unselective mixture of complexes, demonstrating that the imidazopyrimidine backbone is critical for directing selective heterometallic assembly. Quantum-chemical calculations were used to construct thermodynamic cycles describing metal exchange reactions and the relative affinity of each metal center for each binding site within dpip. The resulting Gibbs energies (ΔG298) indicated that the heterometallic complexes are thermodynamically more stable than their homometallic counterparts. Natural Bond Orbital (NBO) deletion analyses attributed the site selectivity in the heterometallic complexes to stronger copper(II) interactions at the more electron-donating imidazole-containing binding site. Consistent with this electronic differentiation, synthesis of the monometallic complex [Pd(dpip)(O2CCF3)2] showed selective coordination at the more electron-donating site. Together, these results demonstrate that imidazopyrimidine-based ligands provide a platform for the controlled assembly of heterometallic complexes.
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DOI: 10.19061/iochem-bd-6-539
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Manuscript title: Unsymmetrical Ligand Backbone Drives Site-Selective Assembly of Heterometallic Complexes
