Systematic angle variation in bis-monodentate ligands allowed the first non-templated synthesis of Pd₅L₁₀ rings. Across different Pd_nL_2n ring sizes, the nuclearity n is correlated to the ligand binding angle, with conformational strain being the decisive factor.

Abstract

We report a series of Pd(II)_nL_2n coordination rings for which nuclearity is controlled by the binding angle of the corresponding bis-monodentate bridging ligands. Judicious choice of the angle within a family of rather rigid ligands allowed for the first-time to synthesize a homoleptic five-membered Pd₅L₁₀ ring that does not require any template to form. We demonstrate that control over the ring size is maintained both in the solid-, solution-, and gas-phase. Two X-ray structures of five-membered rings from ligands with ideal angles (yielding a perfect pentagonal ring) vs. suboptimal angles (resulting in a highly distorted structure) illustrate the importance of the correct ligand geometry. A mathematical model for estimating the expected ring size based on the ligand angle was derived and DFT computations show that ring-strain is the major factor determining the assembly outcome.