Ground State Spin Values Research Articles

Hexacyanometalate molecular chemistry: trinuclear CrNi2 complexes; micro-SQUID magnetisation studies of intermolecular interactions

Three different CrNi2 complexes were synthesised. They differ from each other by the nature of the terminal ligand and of the counter anion: [Cr(CN)4{CNNi(tetren)}2]Cl, that crystallises in two different crystallographic systems and [Cr(CN)4{CNNi(dienpy2)}2](ClO4). The ground state spin value is 7/2 for the three systems (ferromagnetic interaction between chromium(III) and nickel(II) ions). The magnetisation of the three CrNi2 complexes was measured using an array of micro-SQUIDs in a temperature range between 0.04 and 7 K, under a magnetic field up to ±1 T. The three samples present a three-dimensional magnetic ordering. The correlation between the structure and the intermolecular coupling is analysed in terms of steric hindrances of the terminal ligand, orientation of the molecules in the unit cell (canted structure) and crystal symmetry.

Hexacyanometalate molecular chemistry: heptanuclear heterobimetallic complexes; control of the ground spin state.

Following a bottom-up approach to nanomaterials, we present a rational synthetic route from hexacyanometalates [M(CN)(6)](3-) (M=Cr(III), Co(III)) cores to well-defined heptanuclear complexes. By changing the nature of the metallic cations and using a localised orbital model it is possible to control and to tune the ground state spin value. Thus, with M=Cr(III), d(3), S=3/2, three heptanuclear species were built and characterised by mass spectrometry in solution, by single-crystal X-ray diffraction and by powder magnetic susceptibility measurements, [Cr(III)(CNbondM'L(n))(6)](9+) (M'=Cu(II), Ni(II), Mn(II), L(n)=polydentate ligand), showing spin ground states S(G)=9/2 [Cu(II)], with ferromagnetic interactions J(Cr,Cu)=+45 cm(-1), S(G)=15/2 [Ni(II)] and J(Cr,Ni)=+17.3 cm(-1), S(G)=27/2 [Mn(II)], with an antiferromagnetic interaction J(Cr,Mn)=-9 cm(-1), (interaction Hamiltonian H=-J(Cr,M) [S(Cr)Sigma(i)S(M)(i)], i=1-6). With M=Co(III), d(6), S=0, the heptanuclear analogues [Co(III)(CN-M'L(n))(6)](9+) (M'=Cu(II), Ni(II), Mn(II)) were similarly synthesised and studied. They present a singlet ground state and allow us to evaluate the weak antiferromagnetic coupling constant between two next-nearest neighbours M'-Co-M'.

Polynuclear complexes of the pendent-arm ligand 1,4,7-tris(acetophenoneoxime)-1,4,7-triazacyclononane.

The ligand 1,4,7-tris(acetophenoneoxime)-1,4,7-triazacyclononane (H(3)L) has been synthesized and its coordination properties toward Cu(II), Ni(II), Co(II), and Mn(II) in the presence of air have been investigated. Copper(II) yields a mononuclear complex, [Cu(H(2)L)](ClO(4)) (1), cobalt(II) and manganese(II) ions yield mixed-valence Co(III)(2)Co(II) (2a) and Mn(II)(2)Mn(III) (4) complexes, whereas nickel(II) produces a tetranuclear [Ni(4)(HL)(3)](2+) (3) complex. The complexes have been structurally, magnetochemically, and spectroscopically characterized. Complex 3, a planar trigonal-shaped tetranuclear Ni(II) species, exhibits irregular spin-ladder. Variable-temperature (2-290 K) magnetic susceptibility analysis of 3 demonstrates antiferromagnetic exchange interactions (J = -13.4 cm(-1)) between the neighboring Ni(II) ions, which lead to the ground-state S(t) = 2.0 owing to the topology of the spin-carriers in 3. A bulk ferromaganetic interaction (J = +2 cm(-1)) is prevailing between the neighboring high-spin Mn(II) and high-spin Mn(III) ions leading to a ground state of S(t) = 7.0 for 4. The large ground-state spin value of S(t) = 7.0 has been confirmed by magnetization measurements at applied magnetic fields of 1, 4 and 7 T. A bridging monomethyl carbonato ligand formation occurs through an efficient CO(2) uptake from air in methanolic solutions containing a base in the case of complex 4.

The Exploitation of Versatile Building Blocks for the Self-Assembly of Novel Molecular Magnets

Using molecular building blocks to self-assemble lattices supporting long-range magnetic order is currently an active area of solid-state chemistry. Consequently, it is the realm of supramolecular chemistry that synthetic chemists are turning to in order to develop techniques for the synthesis of structurally well-defined supramolecular materials. In recent years we have investigated the versatility and usefulness of two classes of molecular building blocks, namely, tris-oxalato transition-metal (M. Pilkington and S. Decurtins, in “Magnetoscience—From Molecules to Materials,” Wiley–VCH, 2000), and octacyanometalate complexes (Pilkington and Decurtins, Chimia 54, 593 (2001)), for applications in the field of molecule-based magnets. Anionic, tris-chelated oxalato building blocks are able to build up two-dimensional honeycomb-layered structural motifs as well as three-dimensional decagon frameworks. The discrimination between the crystallization of the two- or three-dimensional structures relies on the choice of the templating counterions (Decurtins, Chimia 52, 539 (1998); Decurtins et al. Mol. Cryst. Liq. Cryst. 273, 167 (1995); New J. Chem. 117 (1998)). These structural types display a range of ferro, ferri, and antiferromagnetic properties (Pilkington and Decurtins, in “Magnetoscience—From Molecules to Materials”). Octacyanometalate building blocks self-assemble to afford two new classes of cyano-bridged compounds namely, molecular clusters and extended three dimensional networks (J. Larionova et al., Angew. Chem. Int. Ed. 39, 1605 (2000); Pilkington et al., in preparation). The molecular cluster with a MnII9MoV6 core has the highest ground state spin value, S=51/2, reported to-date (Larionova et al., Angew. Chem. Int. Ed. 39, 1605 (2000)). In the high-temperature regime, the magnetic properties are characterized by ferromagnetic intracluster coupling. In the magnetic range below 44 K, the magnetic cluster signature is lost as possibly a bulk behavior starts to emerge. The three-dimensional networks exhibit both paramagnetic and ferromagnetic behavior, since the magnetic properties of these materials directly reflect the electronic configuration of the metal ion incorporated into the octacyanometalate building blocks (Pilkington et al., in preparation). For both the oxalate- and cyanide-bridged materials, we are able to manipulate the magnetic properties of the supramolecular assemblies by tuning the electronic configurations of the metal ions incorporated into the appropriate molecular building blocks (Pilkington and Decurtins, in “Magnetoscience—From Molecules to Materials,” Chimia 54, 593 (2000)).

High-Spin Molecules: A Novel Cyano-Bridged MnMo Molecular Cluster with aS=51/2 Ground State and Ferromagnetic Intercluster Ordering at Low Temperatures

The world record in ground state spin value “S = 51/2” belongs to a ferromagnetically coupled, cyano-bridged pentadecanuclear Mn cluster (see picture). This molecule has 51 unpaired electrons. Magnetic dipole–dipole interactions together with exchange interactions resulting from the specific three dimensional crystal packing arrangement give rise to an additional ferromagnetic intercluster coupling below 44 K.

Mixed-Valent Heptairon Complex with a Ground-State Spin Value ofS=12/2 Constructed from a Triiron Cluster Ligand

The anionic triiron(III) cluster ligand [Fe(III)(3)(µ(3)-O)(bpca)(2)Cl(4)(EtO)(2)](-) (1; Hbpca=bis(2-pyridylcarbonyl)amine) was prepared as a building block for constructing larger metal assemblies. This "metal cluster complex ligand" was used in the synthesis of the mixed-valent heptairon complex [Fe(II)(1)(2)(EtOH)(2)], which has a ground-state spin value of S=12/2.

Mixed-Valent Heptairon Complex with a Ground-State Spin Value ofS=12/2 Constructed from a Triiron Cluster Ligand

Der anionische Trieisen(III)-Clusterligand [Fe(μ3-O)(bpca)2Cl4(EtO)2]− 1, (Hbpca=Bis(2-pyridylcarbonyl)amin) wurde als Baustein für die Konstruktion größerer Metallaggregate hergestellt. Dieser „Metallcluster-Komplexligand”︁ wurde in der Synthese des gemischtvalenten Heptaeisenkomplexes [FeII(1)2(EtOH)2] verwendet, dessen Grundzustandsspin S=12/2 beträgt.

Mixed-Valent Heptairon Complex with a Ground-State Spin Value of S=12/2 Constructed from a Triiron Cluster Ligand

Mixed-Valent Heptairon Complex with a Ground-State Spin Value of S=12/2 Constructed from a Triiron Cluster Ligand

( 6Li, 6He) reaction at 100 MeV/nucleon as a probe of spin-excitation strengths

For a selective excitation of spin-flip strength, ( 6Li, 6He) reactions can be a useful tool, because: a) 6Li and 6He have ground-state spin and parity values J π = 1 + and 0 +, respectively; b) 6He has no particle-stable excited state, thus effects from ejectile excitation can be avoided. The ( 6Li, 6He) experiments were performed at 100 MeV/nucleon, at which incident energy the reaction is expected to be single-step dominant. Through the analysis of the good-resolution ( 6Li, 6He) data on 13C, 26Mg, 27Al and 58Ni targets, the evidence for a proportional relationship between 0° cross sections and the corresponding Gamow-Teller transition strengths is examined. Suppression of non-spin flip excitations is discussed based on the hindrance of the excitation of the isobaric analog state.

The Case of Symmetry-Dependent Ground-State Spin Value in Ni(II) Clusters of High-Nuclearity. Crystal Structure and Magnetic Properties of a Pentanuclear and a Nonanuclear Ni(II) Clusters

The crystal structures and the magnetic properties of a pentanuclear (1) and a nonanuclear (2) Ni(II) clusters are described. In the former, crystallizing in the cubic I23 space group, the Ni(II) assembly is composed of a regular tetrahedral geometry of four metal ions centered on the fifth. The nonanuclear cluster crystallizes in the hexagonal crystallographic system -R3 space group - the Ni(II) assembly is composed of two regular tetrahedra, like that of the pentanuclear cluster, sharing a common apex. All metal ions in either cluster are six-coordinate. Variable-temperature magnetic susceptibility data show antiferromagnetic interactions for both clusters. Moreover, magnetization data are in line with uneven intermediate-spin ground states [S=1, S=2] and [S=2, S=3] for 1 and 2, respectively. A comparison of both the symmetry and the spin ground state of 1 to those of lower-symmetry pentanuclear Ni(II) clusters, reveals that the ground-state spin value increases with the cluster symmetry.

Tetranuclear and Octanuclear Manganese Carboxylate Clusters: Preparation and Reactivity of (NBu(n)(4))[Mn(4)O(2)(O(2)CPh)(9)(H(2)O)] and Synthesis of (NBu(n)(4))(2)[Mn(8)O(4)(O(2)CPh)(12)(Et(2)mal)(2)(H(2)O)(2)] with a "Linked-Butterfly" Structure.

The reaction of Mn(O2CPh)2·2H2O and PhCO2H in EtOH/MeCN with NBun4MnO4 gives (NBun4)[Mn4O2(O2CPh)9(H2O)] (4) in high yield (85−95%). Complex 4 crystallizes in monoclinic space group P21/c with the following unit cell parameters at −129 °C: a = 17.394(3) A, b = 19.040(3) A, c = 25.660(5) A, β = 103.51(1)°, V = 8262.7 A3, Z = 4; the structure was refined on F to R (Rw) = 9.11% (9.26%) using 4590 unique reflections with F > 2.33σ(F). The anion of 4 consists of a [Mn4(μ3-O)2]8+ core with a “butterfly” disposition of four MnIII atoms. In addition to seven bridging PhCO2- groups, there is a chelating PhCO2- group at one “wingtip” Mn atom and terminal PhCO2- and H2O groups at the other. Complex 4 is an excellent steppingstone to other [Mn4O2]-containing species. Treatment of 4 with 2,2-diethylmalonate (2 equiv) leads to isolation of (NBun4)2[Mn8O4(O2CPh)12(Et2mal)2(H2O)2] (5) in 45% yield after recrystallization. Complex 5 is mixed-valent (2MnII,6MnIII) and contains an [Mn8O4]14+ core that consists of two [Mn4O...

Ground-state variability in .mu.3-oxide trinuclear mixed-valence manganese complexes: spin frustration

A reexamination of bulk magnetic susceptibility data supplemented by variable-field magnetization and EPR studies definitively established the ground state of the mixed-valence complex [Mn II Mn III 2 O(O 2 CCH 3 ) 6 (py) 3 ](py) (1) as having S=3/2, in contrast to the S=1/2 ground state reported previously. This represents a rare example of a triangular μ 3 -oxide metal complex charaterized as having an intermediate spin ground state (i.e., ground-state spin value other than the minimum available to the system)