Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence.

Recently reported manganese(I) complexes with chelating arylisocyanide ligands exhibit luminescent metal-to-ligand charge-transfer (MLCT) excited states, similar to ruthenium(II) polypyridine complexes with the same d6 valence electron configuration used for many different applications in photophysics and photochemistry. However, chelating arylisocyanide ligands require substantial synthetic effort, and therefore it seemed attractive to explore the possibility of using more readily accessible monodentate arylisocyanides instead. Here, we synthesized the new Mn(I) complex [Mn(CNdippPhOMe2)6]PF6 with the known ligand CNdippPhOMe2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide. This complex was investigated by NMR spectroscopy, single-crystal structure analysis, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) measurements, IR spectroscopy supported by density functional theory (DFT) calculations, cyclic voltammetry, and time-resolved as well as steady-state UV-vis absorption spectroscopy. The key finding is that the new Mn(I) complex is nonluminescent and instead undergoes arylisocyanide ligand loss during continuous visible laser irradiation into ligand-centered and charge-transfer absorption bands, presumably owed to the population of dissociative d-d excited states. Thus, it seems that chelating bi- or tridentate binding motifs are essential for obtaining emissive MLCT excited states in manganese(I) arylisocyanides. Our work contributes to understanding the basic properties of photoactive first-row transition metal complexes and could help advance the search for alternatives to precious metal-based luminophores, photocatalysts, and sensors.

Spin Crossover in a Cobalt Complex on Ag(111).

The Co-based complex [Co(H2 B(pz)(pypz))2 ] (py=pyridine, pz=pyrazole) deposited on Ag(111) was investigated with scanning tunneling microscopy at ≈5 K. Due to a bis(tridentate) coordination sphere the molecules aggregate mainly into tetramers. Individual complexes in these tetramers undergo reversible transitions between two states with characteristic image contrasts when current is passed through them or one of their neighbors. Two molecules exhibit this bistability while the other two molecules are stable. The transition rates vary linearly with the tunneling current and exhibit an intriguing dependence on the bias voltage and its polarity. We interpret the states as being due to S=1 /2 and 3 /2 spin states of the Co2+ complex. The image contrast and the orders-of-magnitude variations of the switching yields can be tentatively understood from the calculated orbital structures of the two spin states, thus providing first insights into the mechanism of electron-induced excited spin-state trapping (ELIESST).

Electron-Induced Spin-Crossover in Self-Assembled Tetramers.

The spin crossover compound Fe(H2B(pyrazole)(pyridylpyrazole))2 was investigated in detail on Ag(111) with scanning tunneling microscopy (STM). A large fraction of the deposited molecules condenses into gridlike tetramers. Two molecules of each tetramer may be converted between two states by current injection. We attribute this effect to a spin transition. This interpretation is supported by control experiments on the analogous, magnetically passive Zn compound that forms virtually identical tetramers but exhibits no switching. The switching yields were studied for various electron energies, and the resulting values exceed those reported from other SCO systems by 2 orders of magnitude. The other two molecules of a tetramer were immutable. However, they may be used as contacts for current injection that leads to conversion of one of their neighbors. This "remote" switching is fairly efficient with yields reduced by only one to two orders of magnitude compared to direct excitation of a switchable molecule. We present a model of the tetramer structure that reproduces key observations from the experiments. In particular, sterical blocking prevents spin crossover of two molecules of a tetramer. Density functional theory calculations show that the model indeed represents a minimum energy structure. They also reproduce STM images and corroborate a remote-switching mechanism that is based on electron transfer between molecules.

Crystal structure of bis-{(3,5-di-methyl-pyrazol-1-yl)di-hydro-[3-(pyridin-2-yl)pyrazol-1-yl]-borato}iron(II).

The structure determination of [Fe(C13H15BN5)2] was undertaken as part of a project on the modification of the recently published spin-crossover (SCO) complex [Fe{H2B(pz)(pypz)}2] (pz = pyrazole, pypz = pyridyl-pyrazole). To this end, a new ligand was synthesized in which two additional methyl groups are present. Its reaction with iron tri-fluoro-methane-sulfonate led to a pure sample of the title compound, as proven by X-ray powder diffraction. The asymmetric unit consists of one complex mol-ecule in a general position. The FeII atom is coordinated by two tridentate N-binding {H2B(3,5-(CH3)2-pz)(pypz)}- ligands. The Fe-N bond lengths range between 2.1222 (13) and 2.3255 (15) Å, compatible with FeII in the high-spin state, which was also confirmed by magnetic measurements. Other than a very weak C-H⋯N non-classical hydrogen bond linking individual mol-ecules into rows extending parallel to [010], there are no remarkable inter-molecular inter-actions.

Crystal structure of bis(5-bromo-1,10-phenanthroline-κ2 N,N')bis-[di-hydro-bis-(pyrazol-1-yl)borato-κ2 N 2,N 2']iron(II) toluene disolvate.

The structure determination of the title compound was undertaken as part of a project on the modification and synthesis of new spin-crossover (SCO) compounds based on octa-hedral FeII bis-(pyrazol-yl)borate complexes. In the course of these investigations, the compound [Fe(C6H8BN4)2(C12H7BrN2)] was synthesized, for which magnetic measurements revealed an incomplete spin-crossover behaviour. Crystallization of this compound from toluene led to the formation of crystals of the toluene disolvate, [Fe(C6H8N4B)2(C12H7N2Br)]·2C7H8. Its asymmetric unit comprises two discrete metal complex mol-ecules and two toluene solvent mol-ecules. One of the latter is severely disordered and its contribution to the diffracted intensities was removed using the SQUEEZE routine [Spek (2015 ▸). Acta Cryst. C71, 9-18]. In each complex mol-ecule, the FeII cation is coordinated by the two N atoms of a 5-bromo-1,10-phenanthroline ligand and by two pairs of N atoms of chelating di-hydro-bis(pyrazol-1-yl)borate ligands in the form of a slightly distorted octa-hedron. The discrete complexes are arranged in columns along the a-axis direction with the toluene solvate mol-ecules located between the columns. The 5-bromo-1,10-phenanthroline ligands of neighbouring columns are approximately parallel and are slightly shifted relative to each other, indicating π-π inter-actions.

Spin Transition of an Iron(II) Organoborate Complex in Different Polymorphs and in Vacuum-Deposited Thin Films: Influence of Cooperativity.

Two polymorphic modifications (1-I and 1-II) of the new spin crossover (SCO) complex [Fe{H2B(pz)(pypz)}2] (pz = pyrazole, pypz = pyridylpyrazole; 1) were prepared and investigated by differential scanning calorimetry (DSC), magnetic measurements, Mößbauer, vibrational, and absorption spectroscopy as well as single-crystal and X-ray powder diffraction. DSC measurements reveal that upon heating the thermodynamically metastable form 1-II to ∼178 °C it transforms into 1-I in an exothermic reaction, which proves that these modifications are related by monotropism. Both forms show thermal SCO with T1/2 values of 390 K (1-II) and 270 K (1-I). An analysis of the crystal structures of 1-II and the corresponding Zn(II) (2) and Co(II) (3) complexes that are isotypic with 1-I reveals that form II consists of dimers coupled by strong intramolecular π···π interactions, which is not the case for 1-I. In agreement with these findings, investigations of thin films of 1, where significant π···π interactions should be absent, reveal SCO behavior similar to that of 1-I. These results underscore the importance of cooperativity for the spin-transition behavior of this class of complexes.

Spin-crossover behavior of bis[dihydrobis(4-methylpyrazol-1-yl-borate)]-(2,2'-bipyridine)iron and analogous complexes in the bulk and in thin films: Elucidating the influence of π-π-interactions on the type of spin transition.

The new complex [Fe(H2B(4-CH3-pz)2)2(bipy)] (1, pz = pyrazole, bipy = 2,2'-bipyridine) is synthesized and investigated by temperature-dependent magnetic measurements, Mößbauer, electronic absorption and vibrational spectroscopy as well as single crystal x-ray diffraction. In the crystal structure of 1 the complexes are pairwise linked to dimers by intermolecular π-π interactions between their bipyridine ligands, with a shortest intradimer distance between two neighboring pyridine rings of 3.575 Å. Analysis of the crystal structures of related iron(II) bis(dihydrobis(pyrazoyl)borate) complexes reveals that most of them contain similar dimers, and that at short π-π intra-dimer distances the complexes are locked in the high-spin state whereas at long distances complete thermal spin crossover (SCO) is observed. Compound 1 with an intermediate π-π intra-dimer distance shows incomplete SCO in the bulk but complete SCO in vacuum-deposited thin films where intermolecular interactions are absent. The implications of this remarkable structure-property relationship are discussed.

Effect of ligand methylation on the spin-switching properties of surface-supported spin-crossover molecules.

X-ray absorption spectroscopy investigations of the spin-state switching of spin-crossover (SCO) complexes adsorbed on a highly-oriented pyrolytic graphite (HOPG) surface have shown so far that HOPG is a promising candidate to realize applications such as spintronic devices because of the stability of SCO complexes on HOPG and the possibility of highly efficient thermal and light-induced spin-state switching. Herein, we present the spin switching of several Fe(II) SCO complexes adsorbed on an HOPG surface with particular emphasis on the thermally induced spin transition behaviour with respect to different structural modifications. The complexes of the type [Fe(bpz)2(L)] (bpz = dihydrobis(pyrazolyl)borate, L = 1,10-phenanthroline, 2,2'-bipyridine) and their methylated derivatives exhibit SCO in the solid state with some differences regarding cooperative effects. However, in the vacuum-deposited thick films on quartz, complete and more gradual spin transition behavior is observable via UV/vis spectroscopy. In contrast to that, all complexes show large differences upon direct contact with HOPG. Whereas the unmodified complexes show thermal and light-induced SCO, the addition of e.g. two or four methyl groups leads to a partial or a complete loss of the SCO on the surface. The angle-dependent measurement of the N K-edge compared to calculations indicates that the complete SCO and HS-locked molecules on the surface exhibit a similar preferential orientation, whereas complexes undergoing an incomplete SCO exhibit a random orientation on the surface. These results are discussed in the light of molecule-substrate interactions.

Electronic Structure, Vibrational Spectra, and Spin-Crossover Properties of Vacuum-Evaporable Iron(II) Bis(dihydrobis(pyrazolyl)borate) Complexes with Diimine Coligands. Origin of Giant Raman Features.

The vibrational properties of spin-crossover complexes [Fe(H2B(pz)2)2(L)] (pz = pyrazole) containing L = 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) ligands are investigated by temperature-dependent infrared and Raman spectroscopy. For comparison, the analogous cobalt(II) complexes [Co(H2B(pz)2)2(L)] (L = bipy and phen) and iron(II) compounds with L = 4,4'-dimethyl-2,2'-bipyridine and 4,7-dimethyl-1,10-phenanthroline coligands are studied. Highly intense, structured bands (giant Raman features, GRFs) are observed in the resonance Raman spectra of all Fe(II) complexes between 400 and 500 cm-1 at low temperatures in the HS state which, for the SCO complexes, is excited by the Raman laser. On the basis of magnetic field Mössbauer and saturation magnetization data electronic Raman effects are excluded to account for these features. Furthermore, detailed vibrational analysis also allows excluding a vibrational resonance Raman effect involving one of the modes of the individual complexes as a possible origin of the GRFs. Consequently, these features are attributed to coherent two-phonon excitation of metal-ligand stretching vibrations in molecular dimers coupled by π-π stacking interactions.

Crystal structure of bis-[di-hydro-bis-(pyrazol-1-yl)borato-κ2 N 2,N 2'](1,10-phenanthroline-κ2 N,N')zinc(II).

The asymmetric unit of the title compound, [Zn(C6H8N4B)2(C12H8N2)], comprises one half of a ZnII cation (site symmetry 2), one di-hydro-bis-(pyrazol-1-yl)borate ligand in a general position, and one half of a phenanthroline ligand, the other half being completed by twofold rotation symmetry. The ZnII cation is coordinated in form of a slightly distorted octa-hedron by the N atoms of a phenanthroline ligand and by two pairs of N atoms of symmetry-related di-hydro-bis-(pyrazol-1-yl)borate ligands. The discrete complexes are arranged into columns that elongate in the c-axis direction with a parallel alignment of the phenanthroline ligands, indicating weak π-π inter-actions.

Evolution of cooperativity in the spin transition of an iron(II) complex on a graphite surface.

Cooperative effects determine the spin-state bistability of spin-crossover molecules (SCMs). Herein, the ultimate scale limit at which cooperative spin switching becomes effective is investigated in a complex [Fe(H2B(pz)2)2(bipy)] deposited on a highly oriented pyrolytic graphite surface, using x-ray absorption spectroscopy. This system exhibits a complete thermal- and light-induced spin transition at thicknesses ranging from submonolayers to multilayers. On increasing the coverage from 0.35(4) to 10(1) monolayers, the width of the temperature-induced spin transition curve narrows significantly, evidencing the buildup of cooperative effects. While the molecules at the submonolayers exhibit an apparent anticooperative behavior, the multilayers starting from a double-layer exhibit a distinctly cooperative spin switching, with a free-molecule-like behavior indicated at around a monolayer. These observations will serve as useful guidelines in designing SCM-based devices.