Donatella Armentano

Field‐Induced Hysteresis and Quantum Tunneling of the Magnetization in a Mononuclear Manganese(III) Complex

High-nuclearity complexes of transition-metal ions have been of special interest during the last two decades owing to the possibility of observing slow magnetic relaxation effects at the molecular level. These molecular nanomagnets have potential applications as new high-density magnetic memories and quantum-computing devices in the field of molecular spintronics. The first example of a discrete molecule exhibiting hysteresis and quantum tunneling of the magnetization was the mixed-valent dodecanuclear manganese(III,IV) complex [Mn12O12(CH3CO2)16(H2O)4]. [3] Since then, a plethora of both homoand heterovalent, manganese-based molecular nanomagnets of varying metal oxidation states (i.e., Mn, Mn and/or Mn) have been reported, with nuclearities from up to [Mn84] down to the smaller [Mn III 2] species. [4] However, to our knowledge, there are no examples of mononuclear manganese complexes exhibiting the slow magnetic relaxation effects typical of molecular nanomagnets, referred to as single-ion magnet (SIMs). This is somewhat puzzling, since several SIMs of other highly anisotropic first-row transition metals (i.e., Co and Fe) have been recently reported which has rekindled the debate in the field of singlemolecular magnetism. The six-coordinated octahedral high-spin d Mn ion (S = 2) has an orbitally degenerate Eg ground electronic term that is split by the Jahn–Teller effect into A1g and B1g orbital singlet low-lying states. Owing to the large mixing between them, second-order spin-orbit coupling (SOC) effects are ultimately responsible for the occurrence of a large axial magnetic anisotropy whose sign depends on the ground state, that is, on the nature of the axial tetragonal distortion. For an axially elongated octahedral Mn environment, negative D values are expected that can potentially lead to a large energy barrier for the magnetization reversal between the two lowest MS = 2 states. To provide this type of geometry and obtain manganese(III)-based SIMs, planar tetradentate chelating ligands with strong donor groups are a well-suited choice. Herein, we report a complete study on the synthesis, structural characterization, spectroscopic and magnetic properties, and theoretical calculations of Ph4P[Mn(opbaCl2)(py)2] (1) [H4opbaCl2 = N,N’-3,4-dichloro-o-phenylenebis(oxamic acid), py = pyridine, and Ph4P + = tetraphenylphosphonium cation]. Complex 1 is the first example of a mononuclear manganese(III) complex exhibiting a field-induced slow magnetic relaxation behavior, thus increasing the number of first-row transition-metal-ion SIMs. Complex 1 was obtained as well-formed deep brown cubic prisms by slow evaporation of a methanol/pyridine (1:4 v/v) solution of its tetramethylammonium salt in the presence of an excess of Ph4PCl (see Supporting Information). It crystallizes in the P21/c space group of the monoclinic system (Table S1, Supporting Information). The crystal structure of 1 consists of mononuclear manganese(III) complex anions, [Mn(opbaCl2)(py)2] (Figure 1), which are well separated from each other due to the presence of the bulky tetraphenylphosphonium countercations (Figure S1, Supporting Information). The manganese atom of 1 has a tetragonally elongated octahedral coordination geometry which is typical of the Jahn–Teller distorted d Mn ion. The equatorial plane is formed by two amidate nitrogen and two carboxylate oxygen atoms from the opbaCl2 ligand, while the axial positions are occupied by two pyridine nitrogen atoms (Figure 1a). The planar opbaCl2 ligand adopts a tetradentate coordination

Synthesis, crystal structures and magnetic properties of cyanide- and phenolate-bridged [MIIINiII]2 tetranuclear complexes (M = Fe and Cr)

The binuclear complex NiII2L(H2O)2(ClO4)2(1) and the neutral tetranuclear bimetallic compounds [{M(III)(phen)(CN)4}2{NiII2L(H2O)2}].2CH3CN with M=Fe (2) and Cr (3)[H2L=11,23-dimethyl-3,7,15,19-tetraazatricyclo[19.3.1.1(9,13)]hexacosa-2,7,9,11,13(26),14,19,21(25),22,24-decaene-25,26-diol] have been synthesized and the structures of and determined by single crystal X-ray diffraction. and are isostructural compounds whose structure is made up of centrosymmetric binuclear cations [Ni2(L)(H2O)2]2+ and two peripheral [M(phen)(CN)4]- anions [M=Fe (2) and Cr (3)] acting as monodentate ligands towards the nickel atoms through one of their four cyanide nitrogen atoms. The environment of the metal atoms in 2 and 3 is six-coordinated: two phen-nitrogen and four cyanide-carbon atoms at the iron and chromium atoms and a water molecule, one cyanide-nitrogen and two phenolate-oxygens and two imine-nitrogens from the binucleating ligand L2- at the nickel atom build distorted octahedral surroundings. The values of the FeNi and CrNi separations through the single cyanide bridge are 5.058(1) and 5.174(2)A respectively, whereas the Ni-Ni distances across the double phenolate bridge are 3.098(2)(2) and 3.101(1) A (3). The magnetic properties of have been investigated in the temperature range 1.9-290 K. The magnetic behaviour of corresponds to that of an antiferromagnetically coupled nickel(II) dimer with J=-61.0(1) cm-1, the Hamiltonian being defined as H=-J S(A).S(B). An overall antiferromagnetic behaviour is observed for and with a low-lying singlet spin state. The values of the intramolecular magnetic couplings are J(Fe-Ni)=+17.4(1) cm-1 and J(Ni-Ni(a))=-44.4(1) cm-1 for and J(Cr-Ni)=+11.8(1) cm-1 and J(Ni-Ni(a))=-44.6(1) cm-1 for [H=-J(M-Ni)(S(M).S(Ni)+S(Ma).S(Nia))-J(Ni-Nia)S(Ni)S(Nia)]. Theoretical calculations using methods based on density functional theory (DFT) have been employed on in order to analyze the efficiency of the exchange pathways involved and also to substantiate the exchange coupling parameters.

Heterotetranuclear Oxalato-Bridged ReIV3MII (M = Mn, Fe, Co, Ni, Cu) Complexes: A New Example of a Single-Molecule Magnet (M = Ni)

The use of the mononuclear species (NBu(4))(2)[Re(IV)Cl(4)(ox)] (NBu(4)(+) = tetra-n-butylammonium cation; ox = oxalate dianion) as a ligand toward fully solvated divalent first-row transition-metal ions affords the tetranuclear complexes (NBu(4))(4)[{Re(IV)Cl(4)(mu-ox)}(3)M(II)] with M = Mn (1), Fe (2), Co (3), Ni (4), and Cu (5). Their structure is made up of discrete [{ReCl(4)(mu-ox)}(3)M](4-) anions and bulky NBu(4)(+) cations. The complexes 2-5 crystallize in the triclinic system with space group P1; 2 and 5 as well as 3 and 4 are isostructural. The Re and M atoms exhibit somewhat distorted ReCl(4)O(2) and MO(6) octahedral surroundings, with the oxalate groups adopting the bis-bidentate bridging mode. Magnetic susceptibility measurements on polycrystalline samples of 1-5 in the temperature range 1.9-300 K show the occurrence of intramolecular antiferromagnetic [J = -1.30 cm(-1) (1)] and ferromagnetic couplings [J = +1.62 (2), +3.0 (3), +16.3 (4), and +4.64 cm(-1) (5)], with the Hamiltonian being defined as H = -J[S(M)(S(Re1) + S(Re2) + S(Re3))]. Compound 4 is the first example of an oxalato-bridged heterometallic species that behaves as a single-molecule magnet with a ground-state spin S = (11)/(2) and D = -0.8(1) cm(-1), as shown by the study of its static and dynamic magnetic properties and a high-frequency electron paramagnetic resonance study on polycrystalline samples together with detailed micro-SQUID measurements on single crystals.

2,2'-Bipyrimidine- and 2,3-bis(2-pyridyl)pyrazine-containing manganese(II) compounds: structural and magnetic properties

The preparation, crystal structures and magnetic properties of four different manganese(II) compounds of formula [Mn(bipym)Cl2]n·2nH2O (1), [Mn2(dpp)2(H2O)2Cl4]·2H2O (2), [Mn(dpp)(H2O)2]n(ClO4)2n·1.5nH2O (3) and [Mn(dpp)(dca)2]n (4) [bipym = 2,2′-bipyrimidine, dpp = 2,3-bis(2-pyridyl)pyrazine and dca = dicyanamide anion] are reported. Compounds 1 and 3 are uniform chains of six-coordinated manganese(II) ions bridged by bis(bidentate) bipym (1) and dpp (3) ligands with two chloride groups (1) and two water molecules (3) in cis position. The electroneutrality in 3 is achieved by uncoordinated perchlorate anions. The manganese atom in 1 and 3 exhibits a distorted octahedral environment mainly due to the short bite angle at the bis(chelating) bipym [70.9(1)°] and dpp [72.6(2)°] ligands. The intrachain Mn⋯Mn separations are 6.164(1) A in 1 and 7.289(2) A in 3. Complex 2 is binuclear with a pair of chlorine atoms bridging the manganese atoms in a central Mn2Cl2 plane, the intramolecular Mn⋯Mn separation being 3.805(2) A. Each manganese atom in 2 is six-coordinated with two nitrogen atoms from a bidentate dpp, a water molecule and three chlorine atoms (one terminal and two bridging) building a distorted octahedral environment. Compound 4 is a rhombic layered compound where the manganese atoms of each sheet are linked by single μ-1,5-dca bridges with Mn⋯Mn separations of 6.909(1) and 7.674(1) A. Each manganese atom in 4 is in a distorted octahedral environment, being coordinated to six nitrogen atoms, four from dca ligands and two from a bidentate dpp molecule. Magnetic susceptibility measurements for 1–4 in the temperature range 1.9–290 K show the occurrence of significant magnetic interactions between the local high spin manganese(II) ions which are antiferromagnetic across bridging bipym (J = −1.19 cm−1, 1), dpp (J = −0.25 cm−1, 3) and dca (J = −0.26 cm−1, 4) and ferromagnetic through the double-μ-chloro bridge (J = +0.11 cm−1, 2).

New family of thiocyanate-bridged Re(IV)-SCN-M(II) (M = Ni, Co, Fe, and Mn) heterobimetallic compounds: synthesis, crystal structure, and magnetic properties.

The heterobimetallic complexes of formula [(Me(2)phen)(2)M(μ-NCS)Re(NCS)(5)]·CH(3)CN [Me(2)phen = 2,9-dimethyl-1,10-phenanthroline and M = Ni (1), Co (2), Fe (3), and Mn (4)] have been prepared, and their crystal structures have been determined by X-ray diffraction on single crystals. Compounds 1-4 crystallize in the monoclinic C2/c space group, and their structure consists of neutral [(Me(2)phen)(2)M(μ-NCS)Re(NCS)(5)] heterodinuclear units with a Re-SCN-M bridge. Each Re(IV) ion in this series is six-coordinated with one sulfur and five nitrogen atoms from six thiocyanate groups building a somewhat distorted octahedral environment, whereas the M(II) metal ions are five-coordinated with four nitrogen atoms from two bidentate Me(2)phen molecules and a nitrogen atom from the bridging thiocyanate describing distorted trigonal bipyramidal surroundings. The values of the Re···M separation through the thiocyanate bridge in 1-4 vary in the range 5.903(1)-6.117(3) Å. The magnetic properties of 1-4 as well as those of the parent mononuclear Re(IV) compounds (NBu(4))(2)[Re(NCS)(6)] (A1) (NBu(4)(+) = tetra-n-butylammonium cation) and [Zn(NO(3))(Me(2)phen)(2)](2)[Re(NCS)(5)(SCN)] (A2) were investigated in the temperature range 1.9-300 K. Weak antiferromagnetic interactions between the Re(IV) and M(II) ions across the bridging thiocyanate were found in 1-4 [J = -4.3 (1), -2.4 (2), -1.8 (3), and -1.2 cm(-1) (4), the Hamiltonian being defined as Ĥ = -JŜ(Re)·Ŝ(M)]. The magnetic behavior of A2 is that of a magnetically diluted Re(IV) complex with a large and positive value of the zero-field splitting for the ground level (D(Re) = +37.0 cm(-1)). In the case of A1, although its magnetic behavior is similar to that of A2 in the high-temperature range (D(Re) being +19.0 cm(-1)), it exhibits a weak ferromagnetism below 3.0 K with a canting angle of 1.3°.

The MOF-driven synthesis of supported palladium clusters with catalytic activity for carbene-mediated chemistry

The development of catalysts able to assist industrially important chemical processes is a topic of high importance. In view of the catalytic capabilities of small metal clusters, research efforts are being focused on the synthesis of novel catalysts bearing such active sites. Here we report a heterogeneous catalyst consisting of Pd4 clusters with mixed-valence 0/+1 oxidation states, stabilized and homogeneously organized within the walls of a metal-organic framework (MOF). The resulting solid catalyst outperforms state-of-the-art metal catalysts in carbene-mediated reactions of diazoacetates, with high yields (>90%) and turnover numbers (up to 100,000). In addition, the MOF-supported Pd4 clusters retain their catalytic activity in repeated batch and flow reactions (>20 cycles). Our findings demonstrate how this synthetic approach may now instruct the future design of heterogeneous catalysts with advantageous reaction capabilities for other important processes.

Solid-State Molecular Nanomagnet Inclusion into a Magnetic Metal-Organic Framework: Interplay of the Magnetic Properties.

Single-ion magnets (SIMs) are the smallest possible magnetic devices and are a controllable, bottom-up approach to nanoscale magnetism with potential applications in quantum computing and high-density information storage. In this work, we take advantage of the promising, but yet insufficiently explored, solid-state chemistry of metal-organic frameworks (MOFs) to report the single-crystal to single-crystal inclusion of such molecular nanomagnets within the pores of a magnetic MOF. The resulting host-guest supramolecular aggregate is used as a playground in the first in-depth study on the interplay between the internal magnetic field created by the long-range magnetic ordering of the structured MOF and the slow magnetic relaxation of the SIM.

New extended magnetic systems based on oxalate and iron(III) ions.

A series of oxalate-bridged iron(III) complexes have been synthesized by the reaction of FeCl 3 with oxalic acid (H 2ox) and XCl, where X is a substituted univalent ammonium or an alkaline cation. We have obtained basically two different types of compounds by varying the nature and the shape of the counterion, with the dimensionality of the resulting product being strongly influenced by the counterion. Three-dimensional (3D) networks of oxo- and oxalato-bridged iron(III) ions of the general formula {X 2[Fe 2O(ox) 2Cl 2]. pH 2O} n have been obtained for X = Li (+) ( 1), Na (+) ( 2), and K (+) ( 3) with p = 4 and X = MeNH 3 (+) ( 4), Me 2NH 2 (+) ( 5), and EtNH 3 (+) ( 6) with p = 2. Similar 3D hydroxo- and oxalato-bridged iron(III) networks of the formula {X[Fe 2(OH)(ox) 2Cl 2].2H 2O} n resulted for X = EtNH 3 (+) ( 7a) and PrNH 3 (+) ( 8). Compound 7a undergoes a solid-to-solid transformation, leading to a new species of the formula {(H 3O)(EtNH 3)[Fe 2O(ox) 2Cl 2].H 2O} n ( 7b). Chainlike compounds of the formula {X 2[Fe 2(ox) 2Cl 4]. pH 2O} n [X = Me 2NH 2 (+)( 9, p = 1), Me 3NH (+) ( 10, p = 2), and Me 4N (+) ( 11, p = 0)] have been obtained for the bulkier alkylammonium cations. Magnetic susceptibility measurements in the temperature range 1.9-295 K show the occurrence of weak ferromagnetic ordering due to spin canting in the 3D networks 1- 8, with the value of the critical temperature ( T c) varying with the cation in the range 26 K ( 2) to 70 K ( 8) without significant structural modifications. The last three one-dimensional compounds exhibit the typical behavior of antiferromagnetically coupled chains of interacting spin sextets [ J = -8.3 ( 9), -6.9 ( 10), and -8.4 ( 11) cm (-1) with H = - J summation operator i S i S i+1 ].

X-Ray structure of [ReCl4(μ-ox)Cu(pyim)2]: a new heterobimetallic ReIVCuII ferrimagnetic chain

A new heterobimetallic ReIVCuII compound has been prepared and its crystal structure determined by single-crystal X-ray diffraction; magnetic susceptibility measurements show that this compound behaves as a ferrimagnetic chain with significant antiferromagnetic interactions between Re(IV) and Cu(II) metal ions.

Oxalate and 2,2′-bipyrimidine as bis-chelating ligands in the honeycomb layered compound {[Fe2(bpym)(ox)2]·5H2O}n

The novel two-dimensional iron(II) compound of formula {[Fe2(bpym)(ox)2]·5H2O}n (1) [bpym = 2,2′-bipyrimidine and ox = oxalate dianion] is obtained by reaction of oxalic acid, iron(II) chloride and 2,2′-bipyrimidine in aqueous solution. The structure of 1 is made up of oxalato-bridged iron(II) chains cross-linked by bischelating bpym affording a honeycomb lattice. Variable-temperature magnetic susceptibility data of 1 show the occurrence of relatively large antiferromagnetic interactions between the high spin iron(II) ions separated by more than 5.5 A through bridging bpym [Jbpym = −4.0(2) cm−1] and ox [Jox = ca. −7.8(2) cm−1] ligands. These values compare well with those obtained in the iron(II) chain [Fe(dpa)(ox)]n (2) [Jox = −8.0(2) cm−1] (dpa = 2,2′-dipyridylamine) and dinuclear {[Fe(H2O)4]2(bpym)}(SO4)2·2H2O (3) (Jbpym = −3.4 cm−1) compounds where bischelating ox (1 and 2) and bpym (1 and 3) groups and the bidentate dpa (2) ligand are present.