Self-Assembled Molecular Gear: A 4:1 Complex of Rh(III)Cl Tetraarylporphyrin and Tetra(p‑pyridyl)cavitand

The components of a 4:1 mixture of Rh­(III)Cl tetrakis­(4-methyl­phenyl)­porphyrin 1 and a bowl-shaped tetra­(4-pyridyl)­cavitand 4 self-assemble into a 4:1 complex 1₄•4 via Rh–pyridyl axial coordination bonds. The single-crystal X-ray diffraction analysis and variable-temperature (VT) ¹H NMR study of 1₄•4 indicated that 1₄•4 behaves as a quadruple interlocking gear with an inner space, wherein (i) four subunits-1 are gear wheels and four p-pyridyl groups in subunit-4 are axes of gear wheels, (ii) one subunit-1 and two adjacent subunits-1 interlock with one another cooperatively, and (iii) four subunits-1 in 1₄•4 rotate quickly at 298 K on the NMR time scale. Together, the extremely strong porphyrin-Rh–pyridyl axial coordination bond, the rigidity of the methylene-bridge cavitand as a scaffold of the pyridyl axes, and the cruciform arrangement of the interdigitating p-tolyl groups as the teeth moiety of the gear wheels in the assembling 1₄-unit make 1₄•4 function as a quadruple interlocking gear in solution. The gear function of 1₄•4 was also supported by the rotation behaviors of other 4:1 complexes: 2₄•4 and 3₄•4 obtained from Rh­(III)Cl tetrakis­[4-(4-methyl­phenyl)­phenyl]­porphyrin 2 or Rh­(III)Cl tetrakis­(3,5-dialkoxy­phenyl)­porphyrin 3 and 4 also served as quadruple interlocking gears, whereas 1₄•5 obtained from 1 and tetrakis­[4-(4-pyridyl)­phenyl]­cavitand 5 did not behave as a gear. The results of activation parameters (ΔH^⧧, ΔS^⧧, and ΔG^⧧) obtained from Eyring plots based on line-shape analysis of the VT ¹H NMR spectra of 1₄•4, 2₄•4, and 3₄•4 also support the interlocking rotation (geared coupled rotation) mechanism.