Conformations and Threshold Rotational Mechanisms of C₅Ar₅ and C₅Ar₄X Molecular Propellers:  A Structure Correlation and Computational Study

Abstract

Crystallographically independent structures possessing persubstituted cyclopentadiene and -dienyl moieties were retrieved from the Cambridge Structural Database, and the torsional angles of selected diaryl frames presented in the form of conformational plots. Semiempirical calculations of the corresponding potential energy surfaces reproduced the conformational trends observed in the solid state. By the structure correlation principle, the internal oscillations of nearest (1,2-) and next-nearest (1,3-) aryl rings in all pseudopropeller subunits were found to be only partially correlated. These solid-state data sets, in combination with energetic predictions of the molecules C(5)Ph(5)(-) (1), C(5)Ph(4)H(-) (2), C(4)Ph(4)C=O (3), and C(4)Ph(4)CH(2) (4) reveal that a delayed n-ring-flip (where n = 4 and 5 in C(n)()Ar(n)()(-)(m)()X(m)()), which is otherwise unobservable via NMR spectroscopic methods, is the threshold rotational mechanism.

NMR Study of Hyperbranched Polyphenylenes from the AB2, (AB2 + AB) and (A2 + B3) Methods

“…Summary: The 1 H NMR and 13 C NMR spectra of hyperbranched polyphenylenes synthesised from AB 2 , (AB 2 þ AB) and (A 2 þ B 3 ) monomers (A: ethynyl group; B: cyclopentadienonyl group) were analysed with respect to the characteristic substructures of these polymers. The broad and overlapping NMR spectra were studied by a combination of 1D and 2D NMR techniques.…”

“…The broad and overlapping NMR spectra were studied by a combination of 1D and 2D NMR techniques. Furthermore, appropriate model compounds were synthesised, and their 1 H and 13 C NMR spectra were fully assigned. The signal assignments achieved allow to substantiate the different hyperbranched polyphenylene structures.…”

“…[11] They are examples for a type of molecular propellers composed of substituents arranged in a helical fashion about a planar polyatomic framework. [12,13] NMR techniques such as exchange spectroscopy (EXSY) [14] and variable-temperature (VT) experiments are valuable tools to prove the occurrence and to quantify the dynamics of intramolecular motions slow on the NMR time scale. Such studies on multi-n-rotor (n ¼ 5, 6 and 7) molecular propellers were reviewed recently.…”

Symmetry Breaking in NMR Spectroscopy: The Elucidation of Hidden Molecular Rearrangement Processes

“…Although increasing the ring size lengthens the radial distance of the external groups from the ring centre, this is more than compensated for by the diminishing value of ω. As a result, the peripheral rings find themselves in an increasingly crowded locale such that the twist angles for [C 5 Ph 5 ] − , C 6 Ph 6 and [C 7 Ph 7 ] + are approximately 50°, 70° and 80°, respectively [12,13]. These systems have been heavily investigated as molecular analogues of propellers or gears, although only those with an even number of blades would be capable of correlated conrotatory motion [14,15].…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…the molecular gear 3 (where R = alkyl DOI: 10.1039/b211499m or aryl, n = 5, 6, or 7, and ML x is an organometallic fragment) have been reviewed. 19 Novel cyclynes such as 4 have also been synthesised exhibiting new metallocenederived topologies. 20 New tetra-and pentacyclopropenylenecyclopentadienyl metallocenes have been prepared 21 and a general method reported for the synthesis of π-complexes of a diverse range of 1,2-azaborolyls.…”