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.