2,2'-Bipyridine is a neutral heterocyclic N-donor ligand used in chemistry. Bipyridine ligands are widely used as metal chelating ligands due to their strong redox stability and ease of functionalisation. Since bipyridine is a neutral ligand, it forms charged complexes with metal cations for the design and synthesis of metal-bipyridine complexes.

Synthesis of (S,S)-Bolm's ligand (2,2'-bipyridine-α,α'-1-t-butyl-diol)

An efficient and practical method was developed for the synthesis of C2-symmetric 2,2'-bipyridine-α,α'-1-t-butyl-diol ligands. The disclosed synthesis involves the Ullmann homocoupling of a keto bromo-pyridine under NiCl2/Zn/PPh3 conditions, followed by the stereoselective double hydrogen transfer to the obtained 2,2'-bipyridine-diketone using RuII Noyori-Ikariya catalysts. This approach allowed the successful synthesis of 2,2'-bipyridine-α,α'-1-t-butyl-diol, i.e (S,S)-Bolm's ligand, with a quantitative yield and an excellent stereoselectivity (ee > 99.5%, de > 99.5%), with an overall yield of 69% from easily accessible starting materials.

Changes in 2,2'-Bipyridine upon coordination with metal ions (Ru²⁺)

The chemical reactivity of 4,4'-dichloro-2,2'-bipyridine (bipy-Cl2) changes profoundly upon coordination to a [Ru]2+ center. When not coordinated to [Ru]2+, bipy-Cl2 is relatively unreactive toward nucleophiles; but when coordinated to [Ru]2+, the chlorine atoms become susceptible to nucleophilic displacement. The X-ray structures of bipy-Cl2, cis-dichlorobis (4,4'-dichloro-2,2'-bipyridine)rhodium (III) hexafluorophosphate [Rh](PF6), and tris(4,4‘-dichloro-2,2‘-bipyridine)ruthenium (II) hexafluorophosphate [Ru](PF6)2 reveal that the carbon-chlorine bond lengths do not change substantially upon coordination to the rhodium or ruthenium centers – implying that the carbon-chlorine bond strengths also do not change substantially. B3LYP calculations reveal that the standard enthalpy of activation (ΔH°≠) for the nucleophilic substitution of the chlorine atom in [Ru (bipy)2{bipy-Cl}]2+(bipy?=?2,2'-bipyridine; bipy-Cl?=?4-chloro-2,2‘-bipyridine) by OCH3? is 46.7?kJ?mol?1, while the calculated ΔH°≠ value for the nucleophilic substitution of the chlorine atom in free bipy-Cl by OCH3? is 72.8?kJ?mol?1. However, the B3LYP calculations of the ΔH°≠ values for the nucleophilic displacement of the chlorine atom in the cis and trans isomers of [Ru (bipy) (2,2'-biphenyl){bipy-Cl}], which are neutral complexes, are 76.0 and 73.8?kJ?mol?1 respectively – comparable to that for the reaction involving free bipy-Cl. Hence, the calculations suggest that the overall positive charge of the complex is primarily responsible for lowering the activation barrier to nucleophilic substitution in coordinated chloro-bipyridines.

Applications

A periodic mesoporous organosilica (PMO) containing 2,2′-bipyridine groups (BPy-PMO) has been shown to have a unique pore wall structure in which the 2,2′-bipyridine groups are densely and regularly arranged. The 2,2′-bipyridine moiety on the surface can act as a chelating ligand to form metal complexes, thereby creating molecularly defined catalytic sites on the surface of the material. The photochemical properties of the Ru complexes were tuned by incorporating electron-donating or electron-taking functional groups on BPy-PMO, and efficient photocatalytic water oxidation was achieved. The number of reaction turnovers based on the number of Ru complexes was increased to 20, which is higher than the previous value obtained from the PMO system used as a water oxidation photocatalyst.