Michał Andrzejewski

Piezochromic Porous Metal–Organic Framework

Pressure changes the color of a new type of metal-organic porous hybrid material CoBbcDabcoH2O. It is built of Co2+ cations linked by 1,4-benzenedicabroxylate (Bdc) anions and 1,4-diazabicyclo[2.2.2]octane (Dabco) molecules into 2-D grid-like sheets, interconnected through OH···O bonds of water molecules to carboxylate H-acceptors. This first piezochromic MOF, stable in air and in many solvents, is an ideal ultraprecise sensor for pressure calibration. The color changes are due to strains generated by pressure in the highly asymmetric crystal field of Co2+ octahedral coordination, involving four different ligand types: a Dabco amine (twice), a monodentate carboxylate, a chelating carboxylate, and a water molecule. At 0.33 GPa/296 K and below 225 K/0.1 MPa a phase transition reduces the crystal symmetry from monoclinic to triclinic system and changes the conformation and orientation of linkers.

Remote halogen switch of amine hydrophilicity

Bromide and iodide anions switch hydrogen-bonding patterns in otherwise isostructural dimethanol solvates N-methyl-1,4-diazabicyclo[2.2.2]octanium bromide (dabcoCH3Br·2CH3OH) and analogous iodide (dabcoCH3I·2CH3OH), both synthesized in the high-pressure version of the Menshutkin reaction at 1.2 and 2.4 GPa, respectively. The magnitudes of the high pressure triggering these reactions correspond to identical molecular volumes of both solvates.

Reverse sequence of transitions in prototypic relaxor 1,4-diazabicyclo[2.2.2]octane

Two phase transitions consecutively reduce the crystal symmetry with rising temperature in 1,4-diazabicyclo[2.2.2]octane hydrobromide, [C6H13N2]+·Br−, dabcoHBr. Low-temperature prototypic hexagonal phase III, space group Pm2, Z = 1 above 458 K transforms into orientational states of orthorhombic phase II, space group Cmc21, Z = 4 and above 471 K to orthorhombic phase I, space group Pca21, Z = 8. The reverse transitions have been attributed to entropy and enthalpy contributions of the proton disordering, disproportionation of dabcoH+ cations into neutral dabco molecules and dabcoH2+2 dications at nanodomain walls, opposite propeller versus planar dabco conformations and conversions between NH+⋯N and NH+⋯Br− hydrogen bonds in dabcoHBr phase III, the NH+⋯N bonded relaxor material. All of the structural-disorder features in the NH+⋯N bonded aggregates are essential for the short-range polarization and giant dielectric response in dabcoHBr phase III. Despite the first-order type of the phase transitions, the lattice dimensions of phases I and II remain related to the prototypical relaxor phase III.

Piezochromic Topology Switch in a Coordination Polymer

Abrupt color changes coupled to a giant strain in the crystal of coordination polymer CoCl2bpp (bpp = 1,3-bis(4-pyridyl)propane) mark piezochromic reversible transformations at 1.93 GPa from blue phase α to green phase β and at 2.39 GPa to colorless phase γ. The clearly visible shape and color changes are ideal for calibrating discrete pressure magnitudes associated with these phase transitions. The crystal spectra have been measured and the structures have been determined in situ under pressure in a diamond-anvil cell. In phase α (of monoclinic space group P21/m) and phase β (orthorhombic space group Pnmm) the tetrahedral Co-coordination is stepwise modified within the 1D chain topology, but in phase γ (triclinic space group P1̅) the Co2+ cations become octahedrally coordinated and the polymer topology transforms to the 2D sheets.

Framework and coordination strain in two isostructural hybrid metal–organic perovskites

Monotonic compression of two iso-structural hybrid metal–organic perovskites [DMA]M(HCOO)3 (M = Co, Ni) has been connected to their structural distortions. The strongest effects of pressure occur in the tilts of the MO6 octahedra, M–O bond compression, angular distortions within the MO6 octahedra and small folding between the octahedra and formate units. The anisotropic stronger compression along the trigonal axis is consistent with NH⋯O bonding nearly perpendicular to this direction. Raman spectra indicate a first order solid-state phase transition between 3–5 GPa in both formates, which can be associated with the symmetry breaking induced by the ordering of the Dma cation.