Braulio Rodríguez-Molina

Anisochronous Dynamics in a Crystalline Array of Steroidal Molecular Rotors: Evidence of Correlated Motion within 1D Helical Domains

We describe the solid-state dynamics of a molecular rotator (2) consisting of a p-phenylene rotor flanked by two ethynyl steroidal moieties that act as a stator. Single-crystal X-ray diffraction analysis of polymorph I revealed a packing motif containing 1D columns of nested rotors arranged in helical arrays (space group P3(2)) with the central phenylenes disordered over two sites related by an 85° rotation about their 1,4-axes. Unexpected line shapes in quadrupolar-echo (2)H NMR measurements between 155 and 296 K for the same polymorph with a deuterated phenylene isotopologue (2-d(4)) were simulated by trajectories involving fast (>10(8) s(-1)) 180° rotation (twofold flips) in each of the two rotationally disordered sites and slower exchange (2 × 10(4) to 1.5 × 10(6) s(-1)) between them. A negative activation entropy and a low enthalpic barrier for the slower 85° exchange are interpreted in terms of highly correlated processes within the 1D helical domains.

Amphidynamic Crystals of a Steroidal Bicyclo[2.2.2]octane Rotor: A High Symmetry Group That Rotates Faster than Smaller Methyl and Methoxy Groups

The synthesis, crystallization, single crystal X-ray structure, and solid state dynamics of molecular rotor 3 provided with a high symmetry order and relatively cylindrical bicyclo[2.2.2]octane (BCO) rotator linked to mestranol fragments were investigated in this work. By use of solid state (13)C NMR, three rotating fragments were identified within the molecule: the BCO, the C19 methoxy and the C18 methyl groups. To determine the dynamics of the BCO group in crystals of 3 by variable temperature (1)H spin-lattice relaxation (VT (1)H T1), we determined the (1)H T1 contributions from the methoxy group C19 by carrying out measurements with the methoxy-deuterated isotopologue rotor 3-d6. The contributions from the quaternary methyl group C18 were estimated by considering the differences between the VT (1)H T1 of mestranol 8 and methoxy-deuterated mestranol 8-d3. From these studies it was determined that the BCO rotator in 3 has an activation energy of only 1.15 kcal mol(-1), with a barrier for site exchange that is smaller than those of methyl (E(a) = 1.35 kcal mol(-1)) and methoxy groups (E(a) = 1.92 kcal mol(-1)), despite their smaller moments of inertia and surface areas.

Rotation of a Bulky Triptycene in the Solid State: Toward Engineered Nanoscale Artificial Molecular Machines

We report the design and dynamics of a solid-state molecular rotor with a large triptycene rotator. With a cross-section and surface area that are 2 and 3 times larger than those of the phenylene rotators previously studied in the solid state, it is expected that van der Waals forces and steric hindrance will render the motion of the larger triptycene more difficult. To address this challenge, we used a rigid and shape-persistent stator in a dendritic structure that reaches ca. 3.6 nm in length. Using variable-temperature solid-state (2)H NMR spectroscopy, we determined a symmetric three-fold rotational potential with a barrier of 10.2 kcal/mol and a pre-exponential factor of 1.1 × 10(10) s(-1), which correspond to ca. 4600 Brownian jumps per second in the solid state at 300 K.

Synthesis, Characterization, and Rotational Dynamics of Crystalline Molecular Compasses with N-Heterocyclic Rotators

We describe in this paper the synthesis, crystallization, and solid-state NMR dynamics of molecules intended to emulate the structure and function of macroscopic compasses. The desired structures consist of polar pyridine (2) and pyridazine (3) groups as well as their corresponding N-oxides (2O and 3O), each linked axially to two bulky triphenyl methyl groups by 1,4-triple bonds. The structures are such that the central polar heterocycles may rotate about the dialkyne axle while being sterically shielded by the two trityl groups. In addition to the synthesis of samples with natural isotopic abundance, we describe the preparation of 2-d(2) and 3-d(30), one labeled with deuteria in the pyridine rotator and the other fully deuterated in the two trityl groups in the stator. Crystal structures of 2 and 3 revealed packing motifs analogous to those previously reported for samples prepared with substituted phenylene rotators. While solid-state NMR measurements by (13)C CPMAS NMR revealed insufficient chemical shift dispersion for a dynamic characterization of the rotation of compounds 2 and 3 (including 3-d(30)), the use of quadrupolar echo (2)H NMR methods with 2-d(2) revealed a rotational site exchange with a barrier of ca. 8.5 kcal/mol (35.5 kJ mol(-1)) for the pyridine group in 2.

Macrocyclic Molecular Rotors with Bridged Steroidal Frameworks

In this work, we describe the synthesis and solid-state dynamics of isomeric molecular rotors 7E and 7Z, consisting of two androstane steroidal frameworks linked by the D rings by triple bonds at their C17 positions to a 1,4-phenylene rotator. They are also linked by the A rings by an alkenyl diester bridge to restrict the conformational flexibility of the molecules and reduce the number of potential crystalline arrays. The analysis of the resulting molecular structures and packing motifs offered insights of the internal dynamics that were later elucidated by means of line shape analyses of the spectral features obtained through variable-temperature solid-state (13)C NMR; such analysis revealed rotations in the solid state occurring at kilohertz frequency at room temperature.

Thermodynamic Evaluation of Aromatic CH/π Interactions and Rotational Entropy in a Molecular Rotor

A molecular rotor built with a stator formed by two rigid 9β-mestranol units having a 90° bent angle linked to a central phenylene rotator has an ideal structure to examine aromatic CH/π interactions. Energies and populations of the multiple solution conformations from quantum-mechanical calculations and molecular dynamics simulations were combined with variable-temperature (VT) (1)H NMR data to establish the enthalpy of this interaction and the entropy associated with rotation about a single bond. Rotational dynamics in the solid state were determined via VT cross-polarization magic-angle spinning (13)C NMR spectroscopy.

Synthesis and solid state characterization of molecular rotors with steroidal stators: ethisterone and norethisterone

In this article we describe the synthesis and dynamic behavior of two new molecular rotors with 1,4-diethynylphenylene rotators axially linked to two conformationally rigid steroidal norethisterone acetate or ethisterone frames. The resulting 1,4-bis(19-nor-17alpha-ethynyltestosterone-17beta-acetate)benzene (1) and 1,4-bis(17alpha-ethynyltestosterone)benzene (2) were fully characterized in solution and in the solid state, and the rotational dynamics of the central phenylene were explored with the help of (13)C NMR with cross polarization and magic angle spinning (CPMAS), and with quadrupolar echo variable temperature (VT) (2)H NMR in the case of 1. Splitting of signals from the aromatic ring on the (13)C CPMAS NMR and a broad quadrupolar spin echo (2)H spectrum of polycrystalline samples indicated that the rotation of the central aromatic ring in these compounds was limited at ambient temperature in the solid state. Variable temperature (2)H NMR experiments at 350 K in the case of 1-d(4) suggested a 2-fold rotational exchange with upper frequency limit of ca. 10 kHz. Single crystal X-ray analysis of this compound revealed that a crowded environment around the prospective phenylene rotator is responsible of the restricted rotation in the solid state.

X-Ray crystallographic and spectroscopic properties of eight Schiff bases as evidence of the proton transfer reaction. Role of the intermolecular hydrogen bond

A spectroscopic study of several ortho-hydroxy Schiff bases was carried out, and the corresponding crystal structures were analyzed in order to identify their characteristic hydrogen bonding patterns. The X-ray analysis showed that the enol (O–H⋯N) tautomer is the most stable in compounds 1–3 whereas the keto (N–H⋯O) form is preferred in compounds 4–7. The specific intermolecular O–H⋯O hydrogen bonding interactions that control the supramolecular arrangement of each tautomer are discussed. Additionally, a complete characterization of the polycrystalline samples was attained using solid-state NMR and IR experiments. Solution VT NMR and UV-visible experiments were also used to obtain valuable insights about the nature and stability of the tautomers.

Crystalline arrays of molecular rotors with TIPS-trityl and phenolic-trityl stators using phenylene, 1,2-difluorophenylene and pyridine rotators

In this work, we describe the synthesis and solid-state characterization of a series of molecular rotors with tri-isopropylsilyloxy-substituted (TIPS) trityl stators axially linked to 1,4-diethynylphenylene, 3,6-diethynyl-1,2-difluorophenylene and 2,5-diethynylpyridine rotators to produce 1,4-bis[(3,3-diphenyl-3-(3′-(tri-isopropylsilyloxy)-phenyl)-prop-1-yn-1-yl)]benzene (1), 1,4-bis[(3,3-diphenyl-3-(3′-(tri-isopropylsilyloxy)-phenyl)-prop-1-yn-1-yl)]-2,3-difluorobenzene (2) and 2,5-bis[(3,3-diphenyl-3-(3′-tri-isopropylsilyloxy)-phenyl)-prop-1-yn-1-yl)]pyridine (3). The subsequent removal of the TIPS protecting group led to their corresponding hydroxyl-substituted trityl derivatives (4) and (5). TIPS- and HO-substituted stators are involved in different inter- and intramolecular interactions (hydrogen bonding, phenyl embraces, C–H–π interactions) that give rise to isomorphic packing motifs that constrained the rotational dynamics in the solid-state to the slow exchange regime.

Synthesis and Evaluation of Molecular Rotors with Large and Bulky tert-Butyldiphenylsilyloxy-Substituted Trityl Stators

The search for voluminous stators that may accommodate large rotator units and speed rotational dynamics in the solid state led us to investigate a simple and efficient method for the synthesis of molecular rotors with tert-butyldiphenylsilyl-protected (TBDPS) triphenylmethyl stators. Additionally, solid state characterization of these systems with two-, four-, and six-TBDPS groups provided us with a description of their crystallinity and thermal stability. Among them, molecular rotor 7c with the largest and most symmetric stator resulting from six peripheral silyl groups showed the best tendency to crystallize, and the study of its isotopologue 7c-d(4) by solid state (2)H NMR revealed a 2-fold motion of the 1,4-diethynylphenylene-d(4) rotator in the kHz regime.