Dipole-Moment Modulation in New Incommensurate Ferrocene

Despite 70 years of research on metallocenes and their applications, there are still unresolved regions in its phase diagram of the prototypic sandwich compound, ferrocene Fe²⁺[C₅H₅]^-₂ (FeCp₂), and its molecular 5-fold symmetry cannot be reconciled with the dielectric response of this crystal. We found a new phase I″ of ferrocene, which reveals the relationships between the molecular conformation, intermolecular interactions, and electric permittivity of this compound. Between 172.8 and 163.5 K, the conformational disorder of ferrocene molecules transforms into the incommensurate modulation. The structure of phase I″ is described in the (3+2)-dimensional superspace, where the molecular conformations, rotations and inclinations of the Cp rings, molecular tilts, and displacements of the Fe²⁺ cations, as well as the CH···π bonds in the crystal environment, are modulated. These geometric changes combine into the FeCp₂ bending distortion, breaking the 5-fold symmetry and generating waves of molecular dipole moments with their amplitudes approaching 4 × 10^-30 C·m.

Influence of Solvent in Crystal Engineering: A Significant Change to the Order-Disorder Transition in Ferrocene

We present the structure of a novel solvate adduct formed by dissolving ferrocene, FeCp₂, in hexafluorobenzene, C₆F₆. This adduct demonstrates the remarkably strong interactions between the five-membered aromatic rings of FeCp₂ and the six-membered aromatic ring of C₆F₆. These molecular interactions are sufficiently strong and anisotropic to change the temperature of the order-disorder transition of the ferrocene molecule from below ca. 164 K to RT. No solvate adduct could be formed between benzene and FeCp₂. These observations will be of particular relevance to the crystal engineering community, whose goal is the design of solids with bespoke properties.

Polar Metallocenes

Crystalline polar metallocenes are potentially useful active materials as piezoelectrics, ferroelectrics, and multiferroics. Within density functional theory (DFT), we computed structural properties, energy differences for various phases, molecular configurations, and magnetic states, computed polarizations for different polar crystal structures, and computed dipole moments for the constituent molecules with a Wannier function analysis. Of the systems studied, Mn₂(C₉H₉N)₂ is the most promising as a multiferroic material, since the ground state is both polar and ferromagnetic. We found that the predicted crystalline polarizations are 30–40% higher than the values that would be obtained from the dipole moments of the isolated constituent molecules, due to the local effects of the self-consistent internal electric field, indicating high polarizabilities.

Triethylphosphine as a molecular gear - phase transitions in ferrocenyl-acetylide-gold(I)

A sequence of two discontinuous phase transitions, occurring just above 125 K and 148 K, has been observed for a ferrocenyl-acetylide-gold(I) complex with triethylphosphine, structure (1), by means of a multi-temperature single-crystal X-ray diffraction technique. Three distinct phases have been identified. The high-temperature α and low-temperature γ phases share the same space group Pbca, whereas the intermediate β phase is in the Pb2₁a subgroup of Pbca. In all phases molecules of (1) form well defined double layers, with PEt₃ groups interlocking in planes perpendicular to c. On the molecular level, both phase transitions involve almost uniquely a conformational change of triethylphosphine: a gear-like rotation around the P-Au axis and concerted flips of the ethyl moieties. The mechanism of these transitions may be imagined as initiated by a rotation of a single PEt₃ group in a double layer (a single gear movement), followed by adjacent phosphines adjusting their conformations as a result of steric strain. The structural changes underlying phase transitions are sequential, occurring layer-wise, the γ→β transition involving approximately every other layer in the crystal lattice, the β→α yielding a total conformation change. The sequence of phase transitions results in a noticeable contraction of the crystal cell volume.

Deformation twinning of ferrocene crystals assisted by the rotational mobility of cyclopentadienyl rings

A mechanical load lets ferrocene crystals become twinned through the rotational mobility of cyclopentadienyl rings.

New properties in old systems: cooperative electric order in ferrocene and ammonia-borane

After half a century of studies on ferrocene and ammonia-borane, it is very exciting to observe that these materials still hide interesting properties not described before. Here we report dielectric transitions associated with the phase transition.

Phase transition of [2,2]-paracyclophane--an end to an apparently endless story

In this contribution, the solid-state low-temperature phase structure of [2,2]-paracyclophane is unambiguously characterised by single-crystal X-ray analysis. Additionally, a heat capacity measurement was undertaken, which proves the existence of a λ-type phase transition at 45 K, a transition that is connected with the formation of a secondary Cp/T feature at 60 K. The low-temperature phase (<45 K) crystallises in the lower symmetry space group P4n2, whereas the high-temperature phase (>60 K) crystallises in space group P4(2)/mnm. This proves what has been postulated both by experimental and theoretical chemists but has repeatedly been dismissed as speculation many times.

Muon Implantation of Metallocenes: Ferrocene

Muon Spin Relaxation and Avoided Level Crossing (ALC) measurements of ferrocene are reported. The main features observed are five high field resonances in the ALC spectrum at about 3.26, 2.44, 2.04, 1.19 and 1.17 T, for the low-temperature phase at 18 K. The high-temperature phase at 295 K shows that only the last feature shifted down to about 0.49 T and a muon spin relaxation peak at about 0.106 T which approaches zero field when reaching the phase transition temperature of 164 K. A model involving three muoniated radicals, two with muonium addition to the cyclopentadienyl ring and the other to the metal atom, is postulated to rationalise these observations. A theoretical treatment involving spin-orbit coupling is found to be required to understand the Fe-Mu adduct, where an interesting interplay between the ferrocene ring dynamics and the spin-orbit coupling of the unpaired electron is shown to be important. The limiting temperature above which the full effect of spin-orbit interaction is observable in the muSR spectra of ferrocene was estimated to be 584 K. Correlation time for the ring rotation dynamics of the Fe-Mu radical at this temperature is 3.2 ps. Estimated electron g values and the changes in zero-field splittings for this temperature range are also reported.

Mössbauer spectroscopy of bucky ferrocenes: lattice dynamics and motional anisotropy of the metal atom

Temperature-dependent (57)Fe Mössbauer effect spectroscopy has been used to elucidate the metal atom dynamics in three neutral and two cationic bucky ferrocenes. For the three diamagnetic complexes Fe(C(60)H(5))Cp (1), Fe(C(60)Me(5))Cp (2), and Fe(C(60)Ph(5))Cp (3), the metal atom motion is anisotropic and the temperature dependence of the mean-square amplitude of vibration of the metal atom at a number of temperatures is reported. The Mössbauer lattice temperatures have been determined and compared to the parent ferrocene (6). The synthesis and X-ray crystal structure of 3 have been determined at 153(2) K, and the (1)H and (13)C NMR spectra have been recorded. The cationic complexes derived from 2 and 3 show spin-lattice relaxation. The relaxation rate appears insensitive to the nearest-neighbor environment of the metal atom in this pair.

The crystal and molecular structure of tetrakis(trifluoromethyl)cyclopentadienone- π -cyclopentadienyl cobalt

(CF 3 ) 4 C 5 O. C 5 H 5 . Co crystallizes in a monoclinic cell: a = 25·39 Å, b = 8·68 Å, c = 14·40 Å, β = 94·8°; Z = 8, C 2/ c . A least squares analysis of three-dimensional precession data has reduced the discrepancy index to 0·088, cobalt-carbon distances being reported with an e. s. d. of 0·023 Å and other bond lengths with an e. s. d. of 0·032 Å. A system of σ - and π -bonding from the cyclopentadienone ligand to the cobalt atom is evident from the molecular conformation, a comparison being made with the stereochemistry and bonding in other organometallic complexes. Other features of the molecular and crystal structure are discussed in relation to features of the non-bonded atomic interactions.