Mono- and tetra-nuclear copper complexes bearing bis(imino)phenoxide derived ligands: catalytic evaluation for benzene oxidation and ROP of epsilon-caprolactone

Complexes of the type [Cu(L)2] (1) and [Cu4L2(μ4−O)(OAc)4] (2) have been obtained from the reaction of the phenoxydiimine 1,3-(2,6-R22C6H3N=CH)2-5-R1C6H2OH-2 (LH) (where R1 = Me, tBu, Cl; R2 = Me, iPr) with copper(II) acetate [Cu(OAc)2]; changing the molar ratio of the reactants affords 10 differing amounts of 1 or 2. Reaction of the parent dialdehyde [1,3-(CHO)2-5-MeC6H2OH-2] with [Cu(OAc)2] in the presence of Et3N afforded, following work-up, a polymeric chain (3) comprising {[Cu2(OAc)4]OAc}n, HNEt3 and MeCN. The crystal structures of 1 (R1 = Me, R2 = iPr 1a; R1 = Cl, R2 = iPr 1b), 2 (R1 = Me, R2 = Me 2a; R1 = Me, R2 = iPr 2b; R1 = tBu, R2 = Me 2c; R1 = Cl, R2 = Me 2d; R1 = Cl, R2 = iPr 2e; R1 = tBu, R2 =iPr 2f) and 3 are reported (synchrotron radiation was necessary for 3). The 15 magnetic properties of the cluster 2b are presented. Complexes of type 2 and 3 were screened for the ring opening polymerization (ROP) of ε-caprolactone, with or without benzyl alcohol present, under a variety of conditions, however only trace polymer was isolated. The electrochemistry of all complexes was also investigated, together with their ability to catalyze benzene oxidation (using hydrogen peroxide); although low conversions were observed, the tetra-nuclear complexes exhibited excellent selectivity

Vanadyl sulfates: molecular structure, magnetism and electrochemical activity

Reaction of differing amounts of vanadyl sulfate with p-tert-butylthiacalix[4]areneH4 and base allows access to the vanadyl-sulfate species [NEt4]4[(VO)4(μ3-OH)4(SO4)4]·½H2O (1), [HNEt3]5[(VO)5(μ3-O)4(SO4)4]·4MeCN (2·4MeCN) and [NEt4]2[(VO)6(O)2(SO4)4(OMe)(OH2)]·MeCN (3·MeCN). Similar use of p-tert-butylsulfonylcalix[4]areneH4, p-tert-butylcalix[8]areneH8 or p-tert-butylhexahomotrioxacalix[3]areneH3 led to the isolation of [HNEt3]2[H2NEt2]2{[VO(OMe)]2p-tert-butylcalix[8-SO2]areneH2} (4), [HNEt3]2[V(O)2p-tert-butylcalix[8]areneH5] (5) and [HNEt3]2[VIV2VV4O11(OMe)8] (6), respectively. Dc magnetic susceptibility measurements were performed on powdered microcrystalline samples of 1–3 in the T = 300–2 K temperature range. Preliminary screening for electrochemical water oxidation revealed some activity for 2 with turnover frequency (TOF) and number (TON) of 2.2 × 10−4 s−1 and 6.44 × 10−6 (mmol O2/mmol cat.), respectively. The compound 3 showed an improved electrochemical activity in the presence of water. This is related to the increased number and the rate of electrons exchanged during oxidation of V4+ species, facilitated by protons generated in the water discharge process

Self-Assembly of the Hexabromorhenate(IV) Anion with Protonated Benzotriazoles: X‑ray Structure and Magnetic Properties

Two novel Re^IV compounds of formulas [HBTA]₂­[Re^IVBr₆] (<b>1</b>) and [HMEBTA]₂­[Re^IVBr₆] (<b>2</b>) [BTA = 1<i>H</i>-benzotriazole and MEBTA = 1-(methoxy­methyl)-1<i>H</i>-benzotriazole] have been synthesized and magneto-structurally characterized. <b>1</b> and <b>2</b> crystallize in the triclinic system with space group <i>P</i>1̅. In both compounds, the rhenium ion is six-coordinate, bonded to six bromo ligands in a regular octahedral geometry. Short Re^IV–Br···Br–Re^IV contacts, π–π stacking, and H-bonding interactions occur in the crystal lattice of both <b>1</b> and <b>2</b>, generating novel supramolecular structures based on the Re^IV. The different dispositions of the cations and the intermolecular Br···Br contacts in <b>1</b> and <b>2</b> play an important structure–property role, with the magnetic properties of <b>1</b> and <b>2</b> revealing a significant antiferromagnetic coupling between Re^IV ions through intermolecular Br···Br interactions. In <b>1</b>, these interactions account for a maximum in the magnetic susceptibility at ca. 10 K

Exploratory studies into 3<i>d</i>/4<i>f</i> cluster formation with fully bridge-substituted calix[4]arenes^*

<p>Calix[4]arenes are extremely versatile ligands that are capable of supporting the formation of a wide variety of polymetallic clusters of paramagnetic metal ions. One can exert influence over cluster formation through alteration of the calix[4]arene framework and subsequent ‘expansion’ of the lower-rim polyphenolic binding site. The present contribution investigates cluster formation with calix[4]arenes substituted at all four methylene bridge positions with furan moieties. Two known cluster types have been isolated with this ligand, the structures of which lend insight into factors that may ultimately preclude the formation of mixed-metal species.</p

Effect of Protonated Organic Cations and Anion−π Interactions on the Magnetic Behavior of Hexabromorhenate(IV) Salts

Two novel Re^IV compounds of formula (Hbpym)₂[Re^IVBr₆]·4H₂O (<b>1</b>) and (H₄biim)­[Re^IVBr₆]·4H₂O (<b>2</b>) [Hbpym⁺ = 2,2′-bipyrimidinium cation and H₄biim²⁺ = 2,2′-biimidazolium dication] have been prepared and magnetostructurally characterized. <b>1</b> and <b>2</b> exhibit distinct crystal packing, and the presence of weak intermolecular contacts, such as Re–Br···Br–Re (<b>1</b> and <b>2</b>), Re–Br···(H₂O)···Br–Re (<b>1</b> and <b>2</b>), and Re–Br···π···Br–Re (<b>2</b>), lead to different magnetic behaviors. While <b>1</b> is antiferromagnetic, <b>2</b> is a ferromagnetic compound and indeed the first example of ferromagnetic salt based on the hexabromorhenate­(IV) anion. These results suggest a straightforward synthetic route to the preparation of new ferromagnetically coupled Re^IV compounds

Chiral Single-Chain Magnet: Helically Stacked [Mn^III₂Cu^II] Triangles

The one-dimensional complex [Mn^III₂Cu^II(μ₃-O)­(Cl-sao)₃(EtOH)₂]·EtOH (Mn₂Cu) was obtained by the metal replacement reaction of the trinuclear manganese complex (Et₃NH)­[Mn^III₃(μ₃-O)­Cl₂(Cl-sao)₃(MeOH)₂(H₂O)₂] with [Cu­(acac)₂]. The Mn₂Cu chain exhibits single-chain-magnet behavior with finite-size effects due to its large magnetic anisotropy

Synthesis, Structure, and Magnetism of a Family of Heterometallic {Cu₂Ln₇} and {Cu₄Ln₁₂} (Ln = Gd, Tb, and Dy) Complexes: The Gd Analogues Exhibiting a Large Magnetocaloric Effect

The syntheses, structures, and magnetic properties of two heterometallic Cu^II–Ln^III (Ln^III = Gd, Tb, and Dy) families, utilizing triethanolamine and carboxylate ligands, are reported. The first structural motif displays a nonanuclear {Cu^II₂Ln^III₇} metallic core, while the second reveals a hexadecanuclear {Cu^II₄Ln^III₁₂} core. The differing nuclearities of the two families stem from the choice of carboxylic acid used in the synthesis. Magnetic studies show that the most impressive features are displayed by the {Cu^II₂Gd^III₇} and {Cu^II₄Gd^III₁₂} complexes, which display a large magnetocaloric effect, with entropy changes −Δ<i>S</i>_m = 34.6 and 33.0 J kg^–1 K^–1 at <i>T</i> = 2.7 and 2.9 K, respectively, for a 9 T applied field change. It is also found that the {Cu^II₄Dy^III₁₂} complex displays single-molecule magnet behavior, with an anisotropy barrier to magnetization reversal of 10.1 K

Closely-Related Zn^II₂Ln^III₂ Complexes (Ln^III = Gd, Yb) with Either Magnetic Refrigerant or Luminescent Single-Molecule Magnet Properties

The reaction of the compartmental ligand <i>N</i>,<i>N</i>′,<i>N</i>″-trimethyl-<i>N</i>,<i>N</i>″-bis­(2-hydroxy-3-methoxy-5-methylbenzyl)­diethylenetriamine (H₂L) with Zn­(NO₃)₂·6H₂O and subsequently with Ln­(NO₃)₃·5H₂O (Ln^III = Gd and Yb) and triethylamine in MeOH using a 1:1:1:1 molar ratio leads to the formation of the tetranuclear complexes {(μ₃-CO₃)₂[Zn­(μ-L)­Gd­(NO₃)]₂}·4CH₃OH (<b>1</b>) and­{(μ₃-CO₃)₂[Zn­(μ-L)­Yb­(H₂O)]₂}­(NO₃)₂·4CH₃OH (<b>2</b>). When the reaction was performed in the absence of triethylamine, the dinuclear compound [Zn­(μ-L)­(μ-NO₃)­Yb­(NO₃)₂] (<b>3</b>) is obtained. The structures of <b>1</b> and <b>2</b> consist of two diphenoxo-bridged Zn^II–Ln^III units connected by two carbonate bridging ligands. Within the dinuclear units, Zn^II and Ln^III ions occupy the N₃O₂ inner and the O₄ outer sites of the compartmental ligand, respectively. The remaining positions on the Ln^III ions are occupied by oxygen atoms belonging to the carbonate bridging groups, by a bidentate nitrate ion in <b>1</b>, and by a coordinated water molecule in <b>2</b>, leading to rather asymmetric GdO₉ and trigonal dodecahedron YbO₈ coordination spheres, respectively. Complex <b>3</b> is made of acetate–diphenoxo triply bridged Zn^IIYb^III dinuclear units, where the Yb^III exhibits a YbO₉ coordination environment. Variable-temperature magnetization measurements and heat capacity data demonstrate that <b>1</b> has a significant magneto–caloric effect, with a maximum value of −Δ<i>S</i>_m = 18.5 J kg^–1 K^–1 at <i>T</i> = 1.9 K and <b>B</b> = 7 T. Complexes <b>2</b> and <b>3</b> show slow relaxation of the magnetization and single-molecule magnet (SMM) behavior under an applied direct-current field of 1000 Oe. The fit of the high-temperature data to the Arrhenius equation affords an effective energy barrier for the reversal of the magnetization of 19.4(7) K with τ_o = 3.1 × 10^–6 s and 27.0(9) K with τ_o = 8.8 × 10^–7 s for <b>2</b> and <b>3</b>, respectively. However, the fit of the full range of temperature data indicates that the relaxation process could take place through a Raman-like process rather than through an activated Orbach process. The chromophoric L^2– ligand is able to act as an “antenna” group, sensitizing the near-infrared (NIR) Yb^III-based luminescence in complexes <b>2</b> and <b>3</b> through an intramolecular energy transfer to the excited states of the accepting Yb^III ion. These complexes show several bands in the 945–1050 nm region, corresponding to ²F_5/2→²F_7/2 transitions arising from the ligand field splitting of both multiplets. The observed luminescence lifetimes τ_obs are 0.515 and 10 μs for <b>2</b> and <b>3</b>, respectively. The shorter lifetime for <b>2</b> is due to the presence of one coordinated water molecule on the Yb^III center (and to a lesser extent noncoordinated water molecules), facilitating vibrational quenching via O–H oscillators. Therefore, complexes <b>2</b> and <b>3</b>, combining field-induced SMM behavior and NIR luminescence, can be considered to be dual magneto–luminescent materials

New members of the [Mn₆/oxime] family and analogues with converging [Mn₃] planes

<p>The synthesis, structural, and magnetic characterization of five new members of the hexanuclear oximate [Mn^III₆] family are reported. All five clusters can be described with the general formula [Mn^III₆O₂(R-sao)₆(R′-CO₂)₂(sol)_x(H₂O)_y] (where R-saoH₂ = salicylaldoxime substituted at the oxime carbon with R = H, Me and Et; R′ = 1-naphthalene, 2-naphthalene, and 1-pyrene; sol = MeOH, EtOH, or MeCN; <i>x</i> = 0–4 and <i>y</i> = 0–4). More specifically, the reaction of Mn(ClO₄)₂·6H₂O with salicylaldoxime-like ligands and the appropriate carboxylic acid in alcoholic or MeCN solutions in the presence of base afforded complexes <b>1</b>–<b>5</b>: [Mn₆O₂(Me-sao)₆(1-naphth-CO₂)₂(H₂O)(MeCN)]·4MeCN (<b>1</b>·4MeCN); [Mn₆O₂(Me-sao)₆(2-naphth-CO₂)₂(H₂O)(MeCN)]·3MeCN·0.1H₂O (<b>2</b>·3MeCN·0.1H₂O); [Mn₆O₂(Et-sao)₆(2-naphth-CO₂)₂(EtOH)₄(H₂O)₂] (<b>3</b>); [Mn₆O₂(Et-sao)₆(2-naphth-CO₂)₂(MeOH)₆] (<b>4</b>) and [Mn₆O₂(sao)₆(1-pyrene-CO₂)₂(H₂O)₂(EtOH)₂]·6EtOH (<b>5</b>·6EtOH). Clusters <b>3</b>, <b>4,</b> and <b>5</b> display the usual [Mn₆/oximate] structural motif consisting of two [Mn₃O] subunits bridged by two O_oximate atoms from two R-sao²⁻ ligands to form the hexanuclear complex in which the two triangular [Mn₃] units are parallel to each other. On the contrary, clusters <b>1</b> and <b>2</b> display a highly distorted stacking arrangement of the two [Mn₃] subunits resulting in two converging planes, forming a novel motif in the [Mn₆] family. Investigation of the magnetic properties for all complexes reveal dominant antiferromagnetic interactions for <b>1</b>, <b>2,</b> and <b>5</b>, while <b>3</b> and <b>4</b> display dominant ferromagnetic interactions with a ground state of <i>S</i> = 12 for both clusters. Finally, <b>3</b> and <b>4</b> display single-molecule magnet behavior with <i>U</i>_eff = 63 and 36 K, respectively.</p

Dilution-Triggered SMM Behavior under Zero Field in a Luminescent Zn₂Dy₂ Tetranuclear Complex Incorporating Carbonato-Bridging Ligands Derived from Atmospheric CO₂ Fixation

The synthesis, structure, magnetic, and luminescence properties of the Zn₂Dy₂ tetranuclear complex of formula {(μ₃-CO₃)₂[Zn­(μ-L)­Dy­(NO₃)]₂}·4CH₃OH (<b>1</b>), where H₂L is the compartmental ligand <i>N</i>,<i>N</i>′,<i>N</i>″-trimethyl-<i>N</i>,<i>N</i>″-bis­(2-hydroxy-3-methoxy-5-methylbenzyl)­diethylenetriamine, are reported. The carbonate anions that bridge two Zn­(μ-L)­Dy units come from the atmospheric CO₂ fixation in a basic medium. Fast quantum tunneling relaxation of the magnetization (QTM) is very effective in this compound, so that single-molecule magnet (SMM) behavior is only observed in the presence of an applied dc field of 1000 Oe, which is able to partly suppress the QTM relaxation process. At variance, a 1:10 Dy:Y magnetic diluted sample, namely, <b>1′</b>, exhibits SMM behavior at zero applied direct-current (dc) field with about 3 times higher thermal energy barrier than that in <b>1</b> (<i>U</i>_eff = 68 K), thus demonstrating the important role of intermolecular dipolar interactions in favoring the fast QTM relaxation process. When a dc field of 1000 Oe is applied to <b>1′</b>, the QTM is almost fully suppressed, the reversal of the magnetization slightly slows, and <i>U</i>_eff increases to 78 K. The dilution results combined with micro-SQUID magnetization measurements clearly indicate that the SMM behavior comes from single-ion relaxation of the Dy³⁺ ions. Analysis of the relaxation data points out that a Raman relaxation process could significantly affect the Orbach relaxation process, reducing the thermal energy barrier <i>U</i>_eff for slow relaxation of the magnetization