Diverse Reactivity of ECp* (E = Al, Ga) toward Low-Coordinate Transition Metal Amides [TM(N(SiMe₃)₂)₂] (TM = Fe, Co, Zn): Insertion, Cp* Transfer, and Orthometalation

The reactivity of the carbenoid group 13 metal ligands ECp* (E = Al, Ga) toward low valent transition metal complexes [TM­(btsa)₂] (TM = Fe, Co, Zn; btsa = bis­(trimethylsilyl)­amide) was investigated, revealing entirely different reaction patterns for E = Al and Ga. Treatment of [Co­(btsa)₂] with AlCp* yields [Cp*Co­(μ-H)­(Al­(κ²-(CH₂SiMe₂)­NSiMe₃)­(btsa))] (<b>1</b>) featuring an unusual heterometallic bicyclic structure that results from the insertion of AlCp* into the TM–N bond with concomitant ligand rearrangement including C−H activation at one amide ligand. For [Fe­(btsa)₂], complete ligand exchange gives FeCp*₂, irrespective of the employed stoichiometric ratio of the reactants. In contrast, treatment of [TM­(btsa)₂] (TM = Fe, Co) with GaCp* forms the 1:1 and 1:2 adducts [(GaCp*)­Co­(btsa)₂] (<b>2</b>) and [(GaCp*)₂Fe­(btsa)₂] (<b>3</b>), respectively. The tendency of AlCp* to undergo Cp* transfer to the TM center appears to be dependent on the nature of the TM center: For [Zn­(btsa)₂], no Cp* transfer is observed on reaction with AlCp*; instead, the insertion product [Zn­(Al­(η²-Cp*)­(btsa))₂] (<b>4</b>) is formed. In the reaction of [Co­(btsa)₂] with the trivalent [Cp*AlH₂], transfer of the amide ligands without further ligand rearrangement is observed, leading to [Co­(μ-H)₄(Al­(η²-Cp*)­(btsa))₂] (<b>5</b>)

Large Thermal Hysteresis for Iron(II) Spin Crossover Complexes with <i>N</i>‑(Pyrid-4-yl)isonicotinamide

A new series of iron­(II) 1D coordination polymers with the general formula [FeL1­(pina)]·<i>x</i>solvent with L1 being a tetradentate N₂O₂^2– coordinating Schiff-base-like ligand [([3,3′]-[1,2-phenylenebis­(iminomethylidyne)]­bis­(2,4-pentanedionato)­(2-)-<i>N</i>,<i>N</i>′,<i>O</i>²,<i>O</i>²′], and pina being a bridging axial ligand <i>N</i>-(pyrid-4-yl)­isonicotinamide, are discussed. The X-ray crystal structure of [FeL1­(pina)]·2MeOH was solved for the low-spin state. The compound crystallizes in the monoclinic space group <i>P</i>2₁<i>/c</i>, and the analysis of the crystal packing reveals the formation of a hydrogen bond network where additional methanol molecules are included. Different magnetic properties are observed for the seven samples analyzed, depending on the nature of the included solvent molecules. The widest hysteresis loop is observed for a fine crystalline sample of composition [FeL1­(pina)]·<i>x</i>H₂O/MeOH. The 88 K wide thermal hysteresis loop (<i>T</i>_1/2↑ = 328 K and <i>T</i>_1/2↓ = 240 K) is centered around room temperature and can be repeated without of a loss of the spin transition properties. For the single crystals of [FeL1­(pina)]·2MeOH, a 51 K wide hysteresis loop is observed (<i>T</i>_1/2↑ = 296 K and <i>T</i>_1/2↓ = 245 K) that is also stable for several cycles. For a powder sample of [FeL1­(pina)]·0.5H₂O·0.5MeOH a cooperative spin transition with a 46 K wide hysteresis loop around room temperature is observed (<i>T</i>_1/2↑ = 321 K and <i>T</i>_1/2↓ = 275 K). This compound was further investigated using Mössbauer spectroscopy and DSC. Both methods reveal that, in the cooling mode, the spin transition is accompanied by a phase transition while in the heating mode a loss of the included methanol is observed that leads to a loss of the spin transition properties. These results show that the pina ligand was used successfully in a crystal-engineering-like approach to generate 1D coordination polymers and improve their spin crossover properties

Large Thermal Hysteresis for Iron(II) Spin Crossover Complexes with <i>N</i>‑(Pyrid-4-yl)isonicotinamide

A new series of iron­(II) 1D coordination polymers with the general formula [FeL1­(pina)]·<i>x</i>solvent with L1 being a tetradentate N₂O₂^2– coordinating Schiff-base-like ligand [([3,3′]-[1,2-phenylenebis­(iminomethylidyne)]­bis­(2,4-pentanedionato)­(2-)-<i>N</i>,<i>N</i>′,<i>O</i>²,<i>O</i>²′], and pina being a bridging axial ligand <i>N</i>-(pyrid-4-yl)­isonicotinamide, are discussed. The X-ray crystal structure of [FeL1­(pina)]·2MeOH was solved for the low-spin state. The compound crystallizes in the monoclinic space group <i>P</i>2₁<i>/c</i>, and the analysis of the crystal packing reveals the formation of a hydrogen bond network where additional methanol molecules are included. Different magnetic properties are observed for the seven samples analyzed, depending on the nature of the included solvent molecules. The widest hysteresis loop is observed for a fine crystalline sample of composition [FeL1­(pina)]·<i>x</i>H₂O/MeOH. The 88 K wide thermal hysteresis loop (<i>T</i>_1/2↑ = 328 K and <i>T</i>_1/2↓ = 240 K) is centered around room temperature and can be repeated without of a loss of the spin transition properties. For the single crystals of [FeL1­(pina)]·2MeOH, a 51 K wide hysteresis loop is observed (<i>T</i>_1/2↑ = 296 K and <i>T</i>_1/2↓ = 245 K) that is also stable for several cycles. For a powder sample of [FeL1­(pina)]·0.5H₂O·0.5MeOH a cooperative spin transition with a 46 K wide hysteresis loop around room temperature is observed (<i>T</i>_1/2↑ = 321 K and <i>T</i>_1/2↓ = 275 K). This compound was further investigated using Mössbauer spectroscopy and DSC. Both methods reveal that, in the cooling mode, the spin transition is accompanied by a phase transition while in the heating mode a loss of the included methanol is observed that leads to a loss of the spin transition properties. These results show that the pina ligand was used successfully in a crystal-engineering-like approach to generate 1D coordination polymers and improve their spin crossover properties

Rare-Earth Metal Cations Incorporated Silica Hybrid Nanoparticles Templated by Cylindrical Polymer Brushes

A novel template-directed approach based on core–shell cylindrical polymer brushes (CPBs) has been developed to prepare rare-earth metal cations (Ln³⁺) incorporated silica hybrid nanoparticles (NPs) with predictable dimensions. Tight chelation of Ln³⁺ ions in the core of the CPB template and a cross-linked silica layer deposited on the shell provide a very stable encapsulation of Ln³⁺ ions within the hybrid NPs and thus a high biocompatibility. As expected, the silica hybrid NPs obtain unique and diverse properties from the incorporated Ln³⁺ ions. That is, the hybrid NPs with Tb³⁺ or Eu³⁺ incorporation exhibit characteristic photoluminescence in visible light range, while the Gd³⁺- and Tb³⁺-containing hybrid NPs show paramagnetic behavior. Especially, the Gd³⁺-containing silica hybrid NPs show a remarkable longitudinal relaxation time (<i>T</i>₁) shortening effect as well as minimal cytotoxicity, suggesting the application potential of these NPs as effective magnetic resonance imaging (MRI) contrast agents. This novel template-directed approach succeeds in combining different functional centers via loading in situ mixed Ln³⁺ ions (e.g. Tb³⁺ and Gd³⁺) into individual CPBs resulting in multicomponent hybrid NPs, which possess both visible photoluminescence and <i>T</i>₁ contrast enhancement and can thus be applied as multimodal bioimaging probes

Spin-Crossover Iron(II) Coordination Polymer with Fluorescent Properties: Correlation between Emission Properties and Spin State

A spin-crossover coordination polymer [Fe­(L1)­(bipy)]_<i>n</i> (where L = a N₂O₂^2– coordinating Schiff base-like ligand bearing a phenazine fluorophore and bipy = 4,4′-bipyridine) was synthesized and exhibits a 48 K wide thermal hysteresis above room temperature (<i>T</i>_1/2↑ = 371 K and <i>T</i>_1/2↓ = 323 K) that is stable for several cycles. The spin transition was characterized using magnetic measurements, Mössbauer spectroscopy, and DSC measurements. <i>T</i>-dependent X-ray powder diffraction reveals a structural phase transition coupled with the spin transition phenomenon. The dimeric excerpt {(μ-bipy)­[FeL1­(MeOH)]₂}·​2MeOH of the coordination polymer chain crystallizes in the triclinic space group <i>P</i>1̅ and reveals that the packing of the molecules in the crystal is dominated by hydrogen bonds. Investigation of the emission properties of the complexes with regard to temperature shows that the spin crossover can be tracked by monitoring the emission spectra, since the emission color changes from greenish to a yellow color upon the low spin-to-high spin transition

Probing Interactions of N‑Donor Molecules with Open Metal Sites within Paramagnetic Cr-MIL-101: A Solid-State NMR Spectroscopic and Density Functional Theory Study

Understanding host–guest interactions is one of the key requirements for adjusting properties in metal–organic frameworks (MOFs). In particular, systems with coordinatively unsaturated Lewis acidic metal sites feature highly selective adsorption processes. This is attributed to strong interactions with Lewis basic guest molecules. Here we show that a combination of ¹³C MAS NMR spectroscopy with state-of-the-art density functional theory (DFT) calculations allows one to unravel the interactions of water, 2-aminopyridine, 3-aminopyridine, and diethylamine with the open metal sites in Cr-MIL-101. The ¹³C MAS NMR spectra, obtained with ultrafast magic-angle spinning, are well resolved, with resonances distributed over 1000 ppm. They present a clear signature for each guest at the open metal sites. Based on competition experiments this leads to the following binding preference: water < diethylamine ≈ 2-aminopyridine < 3-aminopyridine. Assignments were done by exploiting distance sum relations derived from spin–lattice relaxation data and ¹³C­{¹H} REDOR spectral editing. The experimental data were used to validate NMR shifts computed for the Cr-MIL-101 derivatives, which contain Cr₃O clusters with magnetically coupled metal centers. While both approaches provide an unequivocal assignment and the arrangement of the guests at the open metal sites, the NMR data offer additional information about the guest and framework dynamics. We expect that our strategy has the potential for probing the binding situation of adsorbate mixtures at the open metal sites of MOFs in general and thus accesses the microscopic interaction mechanisms for this important material class, which is essential for deriving structure–property relationships