Two‐ and Three‐Spin Hybrid Inorganic‐Organic [2]Rotaxanes Containing Metallated Salen Groups

Mono- and bis-salen functionalised [2]rotaxanes have been synthesised from the esterification of [2]rotaxanes containing phenol-terminated threads (salen = N,N′-bis(salicylidene)ethylenediamine). The [2]rotaxanes have general formula [RH][Cr₇NiF₈(O₂C^tBu)₁₆], where [RH]⁺ is a thread with a central secondary ammonium site that templates a [Cr₇NiF₈(O₂C^tBu)₁₆]⁻ ring. The threads are terminated at one or both ends by carboxylic acid functionalised salen groups. The {M(salen)} groups can be free-base [M = (H+)2] or metallated [M = Cu²+, Ni²⁺, (VO)²⁺]. The [2]rotaxanes have been characterised by single crystal XRD and solid- and solution-state EPR spectroscopy. Where two paramagnetic M ions are involved [M = Cu²⁺ and/or (VO)²⁺] the [2]rotaxanes contain three electron spin S = ½ centres, since the {Cr₇Ni} ring has an S = ½ ground state which is well isolated at low temperatures. These three-spin [2]rotaxanes have been characterised in solution by pulsed dipolar EPR spectroscopies (DEER, also known as PELDOR, and RIDME). The M···M and M···{Cr₇Ni} interactions measured are consistent with dipolar interactions and also with the distances from single crystal XRD.