Structural Trends in Alkaline Earth and Rare Earth Metal 3,5‐Diisopropylpyrazolates

Organomagnesia: Reversibly High Carbon Dioxide Uptake by Magnesium Pyrazolates

A series of new pyrazolate and mixed pyrazolate/pyrazole magnesium complexes is described and their reactivity toward carbon dioxide is examined. The dimeric complex [Mg(pzt Bu, t Bu)2]2 inserts CO2 instantly and quantitatively forming the tetrameric complex [Mg(CO2·pzt Bu, t Bu)2]4 and monomeric donor-stabilized [Mg(CO2·pzt Bu, t Bu)2(thf)2]. Complexes of the type [Mgx(pzR,R)2 x(HpzR,R)y]n (R = iPr, tBu) engage in similar insertion reactions involving dissociation of the carbamic acid HOOCpzR,R. Even solid polymeric derivatives [Mg(pzR,R)2]n (R = Me, H) react instantaneously and exhaustively with CO2, the resulting [Mg(CO2·pz)2]m featuring a CO2 capacity of 35.7 wt% (8.2 mmol g-1). All described magnesium pyrazolates display completely reversible CO2 uptake in solution and in the solid state, respectively, as monitored via VT 1H NMR and in situ FTIR spectroscopy as well as thermogravimetric analysis. Fluorinated [Mg2(pzCF3,CF3)4(thf)3] does not yield any isolable CO2 insertion product but exhibits the highest activity in the catalytic transformation of epoxides and CO2 to cyclic carbonates.

Reduction chemistry yields stable and soluble divalent lanthanide tris(pyrazolyl)borate complexes

Reduction of the heteroleptic Ln(III) precursors [Ln(Tp)₂(OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with either an aluminyl(I) anion or KC₈ yielded the adduct-free homoleptic Ln(II) complexes dimeric 1-Eu [{Eu(Tp)(μ-κ¹:η⁵-Tp)}₂] and monomeric 1-Yb [Yb(Tp)₂]. Complexes 1-Ln have good solubility and stability in both non-coordinating and coordinating solvents. Reaction of 1-Ln with 2 Ph₃PO yielded 1-Ln(OPPh3)2. All complexes are intensely coloured and 1-Eu is photoluminescent. The electronic absorption data show the 4f-5d electronic transitions in Ln(II). Single-crystal X-ray diffraction data reveal first μ-κ¹:η⁵-coordination mode of the unsubstituted Tp ligand to lanthanides in 1-Eu.

Europium is Different: Solvent and Ligand Effects on Oxidation State Outcomes and C-F Activation in Reactions Between Europium Metal and Pentafluorophenylsilver

Unique outcomes have emerged from the redox transmetallation/ protolysis (RTP) reactions of europium metal with [Ag(C₆ F₅ )(py)] (py=pyridine) and pyrazoles (RR'pzH). In pyridine, a solvent not normally used for RTP reactions, the products were mainly Eu^II complexes, [Eu(RR'pz)₂ (py)₄ ] (RR'pz=3,5-diphenylpyrazolate (Ph₂ pz) 1; 3-(2-thienyl)-5-trifluoromethylpyrazolate (ttfpz) 2; 3-methyl-5-phenylpyrazolate (PhMepz) 3). However, use of 3,5-di-tert-butylpyrazole (tBu₂ pzH) gave trivalent [Eu(tBu₂ pz)₃ (py)₂ ] 4, whereas the bulkier N,N'-bis(2,6-difluorophenyl)formamidine (DFFormH) gave divalent [Eu(DFForm)₂ (py)₃ ] 5. In tetrahydrofuran (thf), the usual solvent for RTP reactions, C-F activation was observed for the first time with [Ag(C₆ F₅ )(py)] in such reactions. Thus trivalent [{Eu₂ (Ph₂ pz)₄ (py)₄ (thf)₂ (μ-F)₂ }{Eu₂ (Ph₂ pz)₄ (py)₂ (thf)₄ (μ-F)₂ }] (6), [Eu₂ (ttfpz)₄ (py)₂ (dme)₂ (μ-F)₂ ] (7), [Eu₂ (tBu₂ pz)₄ (dme)₂ (μ-F)₂ ] (8) were obtained from the appropriate pyrazoles, the last two after crystallization from 1,2-dimethoxyethane (dme). Surprisingly 3,5-dimethylpyrazole (Me₂ pzH) gave the divalent cage [Eu₆ (Me₂ pz)₁₀ (thf)₆ (μ-F)₂ ] (9). This has a compact ovoid core held together by bridging fluoride, thf, and pyrazolate ligands, the last including the rare μ₄ -1η⁵ (N₂ C₃ ): 2η² (N,N'): 3κ(N): 4κ(N') pyrazolate binding mode. With the bulky N,N'-bis(2,6-diisopropylphenyl)formamidine (DippFormH), which often favours C-F activation in RTP reactions, neither oxidation to Eu^III nor C-F activation was observed and [Eu(DippForm)₂ (thf)₂ ] (10) was isolated. By contrast, Eu reacted with Bi(C₆ F₅ )₃ and Ph₂ pzH or tBu₂ pzH in thf without C-F activation, to give [Eu(Ph₂ pz)₂ (thf)₄ ] (11) and [Eu(tBu₂ pz)₃ (thf)₂ ] (12) respectively, the oxidation state outcomes corresponding to that for use of [Ag(C₆ F₅ )(py)] in pyridine.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

Unique and contrasting structures of homoleptic lanthanum(iii) and cerium(iii) 3,5-dimethylpyrazolates

Homoleptic [La(Me2pz)3]n (Me2pz = 3,5-dimethylpyrazolato) is a μ-η2:η5-Me2pz coordination polymer with 12-coordinate La atoms in an unusual η5:η5 Me2pz sandwich, whilst the cerium congener forms a molecular tetrametallic cage [Ce4(Me2pz)12] featuring six different Me2pz coordination modes.

Hg(ii) and Pd(ii) complexes with a new selenoether bridged biscarbene ligand: efficient mono- and bis-arylation of methyl acrylate with a pincer biscarbene Pd(ii) precatalyst

New selenoether (CSeC) bridged carbene complexes of Hg(ii) and Pd(ii) have been prepared; a pincer Pd(ii) complex showed catalytic activity in mono- and double-Heck reactions.

Reactivity of functionalized indoles with rare-earth metal amides. Synthesis, characterization and catalytic activity of rare-earth metal complexes incorporating indolyl ligands

The synthesized rare-earth metal amido complexes displayed different catalytic activity for intramolecular hydroamination of aminoalkenes.

Synthesis and photophysical properties of an Eu(II)-complex/PS blend: role of Ag nanoparticles in surface-enhanced luminescence

A novel Eu(II) complex with 2-ethylhexyl hydrogen 2-ethylhexyl phosphonate (EHHEHP or PC88A) was synthesized and blended with polystyrene polymer (PS). Both an independent complex and the Eu(II)/PS blend excited by near-UV light produced blue luminescence, arising from the 5d→ 4f transitions of Eu(II). Time-dependent density functional theory (TD-DFT) calculations on electronic structures of the complex molecule indicated that the absorbing and emitting center was associated with the (2)A(d(z(2))) state under the C(2) crystal field. We also synthesized silver nanoparticles (Ag NPs) with an average particle size of 4.48 nm (σ = 0.91 nm) using EHHEHP as a stabilizer. The effects of Ag NPs as a colloidal suspension and an interfacial layer on the luminescence intensity of the blend were investigated as functions of the concentration of Ag NPs and the thickness of the Ag NP layer, respectively. The critical concentration of the colloidal Ag NPs and the critical thickness of the interfacial Ag NP layer were ∼355 ppm and ∼0.16 μm, respectively. Under critical conditions, the Ag NPs increased the luminescence intensity by 4.4 times as a colloidal suspension in CH(2)Cl(2) and 2.2 times as an interfacial-layer state.

Developments in the Coordination Chemistry of Europium(II)

Recent advances in the coordination chemistry of Eu(2+) are reviewed. Common synthetic routes for generating discrete Eu(2+)-containing complexes reported since 2000 are summarized, followed by a description of the reactivity of these complexes and their applications in reduction chemistry, polymerization, luminescence, and as contrast agents for magnetic resonance imaging. Rapid development of the coordination chemistry of Eu(2+) has led to an upsurge in the utilization of Eu(2+)-containing complexes in synthetic chemistry, materials science, and medicine.

Pyrazoles and pyrazolides-flexible synthons in self-assembly

This Perspective summarises the chemistry of the pyrazole ring, and reviews the metal coordination modes adopted by 1H-pyrazoles and their anions. Pyrazolide anions are probably the most versatile ligands in coordination chemistry, with 20 different terminal or bridging coordination modes having been identified so far. Metal cluster compounds supported by pyrazolido ligation are surveyed, concentrating on those reported during the past ten years. Highlights include the wide structural diversity in apparently simple main group pyrazolides; luminescence and charge-transfer complexes in coinage metal pyrazolide clusters; the use of robust metal pyrazolide clusters to construct liquid crystals, supramolecular materials and metal-organic frameworks; and supramolecular complexes formed by pyrazolide-supported metallacrowns.