Rotaxane synthesis exploiting the M(i)/M(iii) redox couple

Abstract

In the context of advancing the use of metal-based building blocks for the construction of mechanically interlocked molecules, we herein describe the preparation of late transition metal containing [2]rotaxanes (1). Capture and subsequent retention of the interlocked assemblies are achieved by the formation of robust and bulky complexes of rhodium(III) and iridium(III) through hydrogenation of readily accessible rhodium(I) and iridium(I) complexes [M(COD)(PPh₃)₂][BAr^F₄] (M = Rh, 2a; Ir, 2b) and reaction with a bipyridyl terminated [2]pseudorotaxane (3·db24c8). This work was underpinned by detailed mechanistic studies examining the hydrogenation of 1 : 1 mixtures of 2 and bipy in CH₂Cl₂, which proceeds with disparate rates to afford [M(bipy)H₂(PPh₃)₂][BAr^F₄] (M = Rh, 4a[BAr^F₄], t = 18 h @ 50 °C; Ir, 4b[BAr^F₄], t < 5 min @ RT) in CH₂Cl₂ (1 atm H₂). These rates are reconciled by (a) the inherently slower reaction of 2a with H₂ compared to that of the third row congener 2b, and (b) the competing and irreversible reaction of 2a with bipy, leading to a very slow hydrogenation pathway, involving rate-limiting substitution of COD by PPh₃. On the basis of this information, operationally convenient and mild conditions (CH₂Cl₂, RT, 1 atm H₂, t ≤ 2 h) were developed for the preparation of 1, involving in the case of rhodium-based 1a pre-hydrogenation of 2a to form [Rh(PPh₃)₂]₂[BAr^F₄]₂ (8) before reaction with 3·db24c8. In addition to comprehensive spectroscopic characterisation of 1, the structure of iridium-based 1b was elucidated in the solid-state using X-ray diffraction.