Jordi Ribas-Arino

Covalent mechanochemistry: theoretical concepts and computational tools with applications to molecular nanomechanics.

1. What is “Covalent Mechanochemistry” ? 5412 2. From Macroscopic Milling to Bond-Selective Manipulation 5415 2.

Understanding Covalent Mechanochemistry

The time is ripe: A general theoretical framework based on force-transformed potential energy surfaces rationalizes the intriguing results of recent experiments in the emerging field of covalent mechanochemistry.

Mechanochemical Transduction of Externally Applied Forces to Mechanophores

We present a theoretical study on the role played by aliphatic polymer chains in the transduction of external forces to mechanophores being at the heart of force spectroscopy and sonication experiments. Upon introducing a rigorous approach rooted in catastrophe theory, we demonstrate that the rupture force of a cis 1,2-disubstituted benzocyclobutene features a remarkable dependence, including odd-even effects, on the length of attached polymer chains. This unexpected finding is furthermore rationalized by establishing a correlation between the rupture force and local distortions of the mechanophore at its junctions with the transducing chains. The force-transformed Hessians unveil a surprising force-dependence of harmonic force constants associated with relevant structural parameters of these chains. Not only do our findings highlight the necessity of taking into account the polymer chains explicitly when thinking about mechanochemical manipulation, but they also announce the possibility of tuning the properties of mechanoresponsive polymers by tailoring the force-transducing chain molecules.

Self-assembly of an azido-bridged [NiII6] cluster featuring four fused defective cubanes.

The cluster [Ni6(H2L)2(HL1)2(N3)8].2C2H5OH.2H2O [1.2C2H5OH.2H2O], featuring four fused defective cubanes, has been obtained via azido-bridge-driven dimerization of two phenolate-centered trinuclear Ni3 fragments.

Designed Topology and Site-Selective Metal Composition in Tetranuclear [MM′⋅⋅⋅M′M] Linear Complexes

The ligand 1,3-bis[3-oxo-3-(2-hydroxyphenyl)propionyl]benzene (H(4)L), designed to align transition metals into tetranuclear linear molecules, reacts with M(II) salts (M=Ni, Co, Cu) to yield complexes with the expected [MMMM] topology. The novel complexes [Co(4)L(2)(py)(6)] (2; py=pyridine) and [Na(py)(2)][Cu(4)L(2)(py)(4)](ClO(4)) (3) have been crystallographically characterised. The metal sites in complexes 2 and 3, together with previously characterised [Ni(4)L(2)(py)(6)] (1), favour different coordination geometries. These have been exploited for the deliberate synthesis of the heterometallic complex [Cu(2)Ni(2)L(2)(py)(6)] (4). Complexes 1, 2, 3 and 4 exhibit antiferromagnetic interactions between pairs of metals within each cluster, leading to S=0 spin ground states, except for the latter cluster, which features two quasi-independent S=1/2 moieties within the molecule. Complex 4 gathers the structural and physical conditions, thus allowing it to be considered as prototype of a two-qbit quantum gate.

The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials

The neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is a prototype of molecule-based bistable materials. TTTA crystals undergo a first-order phase transition between their low-temperature diamagnetic and high-temperature paramagnetic phases, with a large hysteresis loop that encompasses room temperature. Here, based on ab initio molecular dynamics simulations and new X-ray measurements, we uncover that the regular stacking motif of the high-temperature polymorph is the result of a fast intra-stack pair-exchange dynamics, whereby TTTA radicals continually exchange the adjacent TTTA neighbour (upper or lower) with which they form an eclipsed dimer. Such unique dynamics, observed in the paramagnetic phase within the whole hysteresis loop, is the origin of a significant vibrational entropic gain in the low-temperature to high-temperature transition and thereby it plays a key role in driving the phase transition. This finding provides a new key concept that needs to be explored for the rational design of novel molecule-based bistable magnetic materials.

A Heterometallic (NiII–CuII) Decanuclear Cluster Containing Two Distorted Cubane-like Pentanuclear Cores: Synthesis, Structure, and Magnetic Properties

A new heterometallic Ni(II)-Cu(II) decanuclear cluster, {[Ni(4)Cu(6)(μ-OH(2))(2)(dpkO(2))(8)(OAc)(4)(H(2)O)(4)]·2CH(3)OH·17H(2)O} (1), has been synthesized by self-assembly of the constituent metal ions and the precursor di-2-pyridylketone (dpk) of multinucleating ligand dpkO(2)(2-) and is structurally characterized. The cluster 1 is formed by the union of two symmetry-related distorted cubane-like pentanuclear cores. A magnetic study of 1 reveals strong antiferromagnetic interactions operating through the Ni-O-Ni pathway, which is independent of the assumption D = 0 or D ≠ 0. The pentanuclear cores are ferromagnetically coupled, as supported by density functional theory calculations.

A 3D Cu(II) coordination framework with μ4-/μ2-oxalato anions and a bent dipyridyl coligand: unique zeolite-type NiP2 topological network and magnetic properties.

The reaction of copper(II) nitrate, oxamide, and an angular bridging ligand 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-bpo) under hydrothermal conditions affords a 3D pillared-layer coordination framework {[Cu(2)(4-bpo)(ox)(2)](H(2)O)(4)}(n) (1) (ox = oxalate), featuring the unique zeolite-type NiP(2) network and interesting properties.

Density-functional study of two Fe4-based single-molecule magnets

We present the results of our all-electron density-functional calculations on the electronic structure and magnetic anisotropy energy of the [Fe4(OMe)6(dpm)6] and [Fe4(thme)2(dpm)6] molecular clusters, which are experimentally found to behave as single-molecule magnets. The calculated magnetic anisotropy energy barriers are 2.65 and 15.8 K, respectively, which agree with the experimental data. We also present a density-functional study on the effect of the structure distortions on the magnetic anisotropy of the [Fe(H2O)6]3+ complex. This study, together with an analysis of the projected anisotropies of each iron ion in both molecular clusters, allows us to qualitatively understand why the magnetic anisotropy energy (MAE) barrier of the second single-molecule magnet (SMM) is larger than the MAE of the first SMM.

Structure and Magnetic Interactions in the Organic-Based Ferromagnet Decamethylferrocenium Tetracyanoethenide, [FeCp*2]•+[TCNE]•−

The structures of three temperature-dependent polymorphs of solvent-free decamethylferrocenium tetracyanoethenide, [FeCp*(2)][TCNE], are determined from high-resolution synchrotron powder diffraction data. [FeCp*(2)][TCNE] is the first organic-based ferromagnetic material to be synthesized and is known to have two structural phase transitions at 249 and 282 K. The low-temperature phase, which exhibits spontaneous ferromagnetic order below 4.8 K, was determined at 12 K. At that temperature, it has monoclinic space group P2(1)/c [a = 9.6637(4) A, b = 14.1217(5) A, c = 18.6256(7) A, beta = 113.231(2) degrees, Z = 4] and consists of parallel chains of alternating [Fe(C(5)Me(5))(2)](*+) and [TCNE](*-) ions, with an intrachain Fe...Fe distance of 10.45 A. Structures of the intermediate and ambient temperature phases, also studied here, are characterized by increasing disorder. At 250 K, the unit cell space group is P2(1)/m [a = 9.7100(3) A, b = 14.4926(4) A, c = 9.4997(3) A, beta = 113.153(1) degrees, Z = 2]. At ambient temperature, the lattice, albeit quite disordered, belongs to the orthorhombic space group Cmcm [a = 10.629(1) A, b = 16.128(1) A, c = 14.593(1) A, Z = 4]. Nearest-neighbor magnetic interactions were evaluated for the 12 K structure by CASSCF and CASSCF/MCQDPT calculations (a methodology similar to the CASPT2 method). Similar trends are observed in computations with and without inclusion of spin-orbit coupling. The strongest are two intrachain [FeCp*(2)](*-)...[TCNE](*-) interactions (ferromagnetic with values of approximately 45 and approximately 29 cm(-1)), although weaker, nonnegligible, ferro- or antiferromagnetic interchain interactions of less than +/-0.2 cm(-1) are also present. Magnetic interactions that lead to ordering are therefore three-dimensional, despite the vastly different intra- and interchain coupling strengths.