Alessandro Prescimone

A Mixed‐Valence Manganese Cubane Trapped by Inequivalent Trilacunary Polyoxometalate Ligands

The title compound contains an embedded mixed-valence {Mn5O6} cubane core, which is structurally similar to the active site in photosystem II. Solid-, solution-, and gas-phase studies indicate the presence of three lacunary Keggin fragments, thereby giving insight into the complex solution chemistry of plenary POM fragments.

1D chains of Mn-6 single-molecule magnets

Use of the dicarboxylates iso-phthalate and succinate in the preparation of [Mn(6)] SMMs links the hexametallic units into 1D chains.

[Mn6] under Pressure: A Combined Crystallographic and Magnetic Study†

Folding under pressure: Crystallographic studies on a Mn6 single-molecule magnet under high pressure conditions show the drastic structural changes of the magnetic core (see picture, Mn purple, O red, N blue), which impact on the magnetic properties of ferromagnetic exchange between the metal atoms will be in booster weaker, and under extremely high pressure, a transition to antiferromagnetic behavior.

Enhancing U-eff in oxime-bridged [Mn(6)(III)Ln(2)(III)] hexagonal prisms

The first 3d-4f clusters built using derivatised salicylaldoximes (R-saoH(2)) describe unusual hexagonal prisms. Replacement of the paramagnetic Gd(III) ions with diamagnetic Ln(III) ions allows for a more thorough understanding of the magnetic properties, whilst replacement with Tb(III) doubles U(eff).

Constructing clusters with enhanced magnetic properties by assembling and distorting Mn3 building blocks

The reaction of Mn(ClO4)(2).6H2O with Naphth-saoH2 (Naphth-saoH2=2-hydroxy-1-napthaldoxime) in pyridine (py) forms the complex [MnIII3O(Naphth-sao)3(py)3](ClO4).0.5py (.0.5py) in very good yields. Reaction of with NaO2CPh in EtOH produces the complex [MnIII6O2(Naphth-sao)6(O2CPh)2(EtOH)6].[MnIII6O2(Naphth-sao)6(O2CPh)2(EtOH)4].2.5Et2O.0.5H2O (.2.5Et2O.0.5H2O). Further reaction of complex with 1 equivalent of both NaN3 and Mn(ClO4)(2).6H2O in MeOH produces the complex [MnII2MnIII6O2(Naphth-sao)6(N3)6(MeOH)8].10MeOH (.10MeOH) that displays an S approximately 0 ground state. Ligand substitution of Naphth-saoH2 with Me-saoH2 in CH2Cl2-MeOH for the latter complex (Me-saoH2= 2-hydroxyphenylethanone oxime) forms the complex [MnII2MnIII6O2(Me-sao)6(N3)6(MeOH)8].10MeOH (.10MeOH) that displays an S=7 ground state with Ueff= 44.6 K. In all four complexes the main building block is the triangular {MnIII3O(R-sao)3} unit (R=Naphth for , and ; R=Me for ). The ligand substitution in triggers a structural distortion in the [Mn6] sub-core as observed by the increased (Mn-N-O-Mn) torsion angles in , switching the interactions from antiferro- to ferromagnetic, dramatically changing the ground-state of the octanuclear complexes from S=0 to 7.

Studies on bifunctional Fe(ii)-triazole spin crossover nanoparticles: time-dependent luminescence, surface grafting and the effect of a silica shell and hydrostatic pressure on the magnetic properties

Pure and silica wrapped Fe(II)-triazole (FeHTrz) spin-crossover (SCO) nanoparticles have been prepared following a water-in-oil synthetic procedure. The size and shape can be tuned by controlling the Fe(II) and triazole concentrations in the aqueous phase. The magnetic properties of these nanoparticles are strongly affected by the presence of a silica shell embedding the nanostructured FeHTrz polymer. Whereas bare FeHTrz nanoparticles exhibit abrupt and cooperative spin transition with 24–35K-wide thermal hysteresis loops, for the silica derivates the hysteresis width increases up to 37–42 K. This probes the efficiency of the silica shell to promote interparticle interactions and enhance cooperativity effects. Tomographic studies of the FeHTrz@SiO2 nanoparticles reveal a core–shell structure with the pure FeHTrz polymer wrapped into a thin shell of pure silica. Taking advantage of the chemical properties of the silica shell, these hybrid nanoparticles were coated with a dansyl derivate fluorophore whose luminescence properties can be adjusted by the spin state of the SCO polymer. Time-dependent luminescence studies reveal the existence of a non-radiative energy transfer (Forster type) between the organic fluorophore and the Fe(II)-low spin ions. These nanoparticles have also been functionalized with thiol groups allowing them to be deposited onto a gold surface in a controlled manner.

Shine bright or live long: substituent effects in [Cu(N^N)(P^P)]+-based light-emitting electrochemical cells where N^N is a 6-substituted 2,2'-bipyridine

We report [Cu(P^P)(N^N)][PF6] complexes with P^P = bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N^N = 6-methyl-2,2′-bipyridine (Mebpy), 6-ethyl-2,2′-bipyridine (Etbpy), 6,6′-dimethyl-2,2′-bipyridine (Me2bpy) or 6-phenyl-2,2′-bipyridine (Phbpy). The crystal structures of [Cu(POP)(Phbpy)][PF6]·Et2O, [Cu(POP)(Etbpy)][PF6]·Et2O, [Cu(xantphos)(Me2bpy)][PF6], [Cu(xantphos)(Mebpy)][PF6]·CH2Cl2·0.4Et2O, [Cu(xantphos)(Etbpy)][PF6]·CH2Cl2·1.5H2O and [Cu(xantphos)(Phbpy)][PF6] are described; each copper(I) centre is distorted tetrahedral. In the crystallographically determined structures, the N^N domain in [Cu(xantphos)(Phbpy)]+ and [Cu(POP)(Phbpy)]+ is rotated ∼180° with respect to its orientation in [Cu(xantphos)(Mebpy)]+, [Cu(POP)(Etbpy)]+ and [Cu(xantphos)(Etbpy)]+; in each complex containing xantphos, the xanthene ‘bowl’ retains the same conformation in the solid-state structures. The two conformers resulting from the 180° rotation of the N^N ligand were optimized at the B3LYP-D3/(6-31G**+LANL2DZ) level and are close in energy for each complex. Variable temperature NMR spectroscopy evidences the presence of two conformers of [Cu(xantphos)(Phbpy)]+ in solution which are related by inversion of the xanthene unit. The complexes exhibit MLCT absorption bands in the range 378 to 388 nm, and excitation into each MLCT band leads to yellow emissions. Photoluminescence quantum yields (PLQYs) increase from solution to thin-film and powder; the highest PLQYs are observed for powdered [Cu(xantphos)(Mebpy)][PF6] (34%), [Cu(xantphos)(Etbpy)][PF6] (37%) and [Cu(xantphos)(Me2bpy)][PF6] (37%) with lifetimes of 9.6–11 μs. Density functional theory calculations predict that the emitting triplet (T1) involves an electron transfer from the Cu–P^P environment to the N^N ligand and therefore shows a 3MLCT character. T1 is calculated to be ∼0.20 eV lower in energy than the first singlet excited state (S1). The [Cu(P^P)(N^N)][PF6] ionic transition-metal (iTMC) complexes were tested in light-emitting electrochemical cells (LECs). Turn-on times are fast, and the LEC with [Cu(xantphos)(Me2bpy)][PF6] achieves a maximum efficacy of 3.0 cd A−1 (luminance = 145 cd m−2) with a lifetime of 1 h; on going to the [Cu(xantphos)(Mebpy)][PF6]-based LEC, the lifetime exceeds 15 h but at the expense of the efficacy (1.9 cd A−1). The lifetimes of LECs containing [Cu(xantphos)(Etbpy)][PF6] and [Cu(POP)(Etbpy)][PF6] exceed 40 and 80 h respectively.

A Tris(diisocyanide)chromium(0) Complex Is a Luminescent Analog of Fe(2,2′-Bipyridine)32+

A meta-terphenyl unit was substituted with an isocyanide group on each of its two terminal aryls to afford a bidentate chelating ligand (CNtBuAr3NC) that is able to stabilize chromium in its zerovalent oxidation state. The homoleptic Cr(CNtBuAr3NC)3 complex luminesces in solution at room temperature, and its excited-state lifetime (2.2 ns in deaerated THF at 20 °C) is nearly 2 orders of magnitude longer than the current record lifetime for isoelectronic Fe(II) complexes, which are of significant interest as earth-abundant sensitizers in dye-sensitized solar cells. Due to its chelating ligands, Cr(CNtBuAr3NC)3 is more robust than Cr(0) complexes with carbonyl or monodentate isocyanides, manifesting in comparatively slow photodegradation. In the presence of excess anthracene in solution, efficient energy transfer and subsequent triplet-triplet annihilation upconversion is observed. With an excited-state oxidation potential of -2.43 V vs Fc+/Fc, the Cr(0) complex is a very strong photoreductant. The findings presented herein are relevant for replacement of precious metals in dye-sensitized solar cells and in luminescent devices by earth-abundant elements.

Two-dimensional frameworks built from Single-Molecule Magnets

Fine tuning the Mn/salicylaldoxime/trimesic acid reaction conditions leads to the formation of a regular 2D net held together by dative bonds and to a non-regular 2D net stabilised by both dative and hydrogen bonds. Both networks are built from [Mn6] Single-Molecule Magnets.

Pressure‐Driven Orbital Reorientations and Coordination‐Sphere Reconstructions in [CuF2(H2O)2(pyz)]

Successive reorientations of the Jahn-Teller axes associated with the Cu{sup II} ions accompany a series of pronounced structural transitions in the title compound, as is shown by X-ray crystallography and high-frequency EPR measurements. The second transition forces a dimerization involving two thirds of the Cu{sup II} sites due to ejection of one of the water molecules from the coordination sphere