A [3]Rotaxane Containing {Ti₇Ga} Rings Linking Cu^II: Synthesis, Structure, and Spectroscopic Studies

We report the synthesis of extended hybrid organic-inorganic rotaxanes based on (pyCH₂NH₂CH₂CH₂py)[Ti^IV ₇Ga^IIIO₈(O₂C^tBu)₁₆] (1) and (pyCH₂NH₂CH₂py)[Ti^IV ₇Ga^IIIO₈(O₂C^tBu)₁₆] (3) building blocks, containing an anionic macrocycle and doubly pyridyl-terminated secondary ammonium threads. Reaction of 1 and 3 with [Cu(hfac)₂] (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate) gives {[Cu(hfac)₂]₃[1]₂} (2) and {[Cu(hfac)₂]₂[3]} (4), respectively, which are characterized by single-crystal X-ray crystallography. The products are controlled by the differing steric demands imposed by the two arms of the thread. The structure of 2 is an extended [3]rotaxane, with two molecules of 1 binding to a central [Cu(hfac)₂] molecule via the longer arm of the thread, and two [Cu(hfac)₂] units terminating the structure at the shorter arm. The structure of 4, where both arms of the thread are short, is an extended [2]rotaxane terminated by two molecules of [Cu(hfac)₂]. The Cu…Cu…Cu fragment in 2 is linear, with the central Cu lying on an inversion center and with a separation between the terminal copper ions of 31.3 Å. Double Electron-Electron Resonance (DEER; also known as PELDOR) spectroscopy proves that the [3]rotaxane structure remains intact in solution, detecting both terminal…center and terminal…terminal Cu…Cu interactions, with orientation-selective measurements demonstrating that there is a rearrangement of the structure at the terminal positions on dissolution.