Jennifer J. Le Roy

Fine-tuning the Local Symmetry to Attain Record Blocking Temperature and Magnetic Remanence in a Single-Ion Magnet**

Remanence and coercivity are the basic characteristics of permanent magnets. They are also tightly correlated with the existence of long relaxation times of magnetization in a number of molecular complexes, called accordingly single-molecule magnets (SMMs). Up to now, hysteresis loops with large coercive fields have only been observed in polynuclear metal complexes and metal-radical SMMs. On the contrary, mononuclear complexes, called single-ion magnets (SIM), have shown hysteresis loops of butterfly/phonon bottleneck type, with negligible coercivity, and therefore with much shorter relaxation times of magnetization. A mononuclear Er(III) complex is presented with hysteresis loops having large coercive fields, achieving 7000 Oe at T=1.8 K and field variation as slow as 1 h for the entire cycle. The coercivity persists up to about 5 K, while the hysteresis loops persist to 12 K. Our finding shows that SIMs can be as efficient as polynuclear SMMs, thus opening new perspectives for their applications.

An Organometallic Sandwich Lanthanide Single-Ion Magnet with an Unusual Multiple Relaxation Mechanism

A dysprosium(III) sandwich complex, [Dy(III)(COT″)(2)Li(THF)(DME)], was synthesized using 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion (COT″). The complex behaves as a single-ion magnet and demonstrates unusual multiple relaxation modes. The observed relaxation pathways strongly depend on the applied static dc fields.

Synthesis, Electronic Structure, and Magnetism of (Ni(6-Mes) 2 ) + :A Two-Coordinate Nickel(I) Complex Stabilized by Bulky N‑Heterocyclic Carbenes

The two-coordinate cationic Ni(I) bis-N-heterocyclic carbene complex [Ni(6-Mes)2]Br (1) [6-Mes =1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene] has been structurally characterized and displays a highly linear geometry with a C-Ni-C angle of 179.27(13)°. Density functional theory calculations revealed that the five occupied metal-based orbitals are split in an approximate 2:1:2 pattern. Significant magnetic anisotropy results from this orbital degeneracy, leading to single-ion magnet (SIM) behavior.

An Organometallic Building Block Approach To Produce a Multidecker 4f Single-Molecule Magnet

An organometallic building block strategy was employed to investigate the magnetic properties of a Ln(III) organometallic single-ion magnet (SIM) and subsequent single-molecule magnet (SMM) after coupling two of the monomeric units. New homoleptic Dy(III)COT″2 and Ln(III)2COT″3 (Ln = Gd, Dy) complexes have been synthesized. DFT calculations of the bimetallic Dy(III) complex indicate strong metal-ligand covalency and uneven donation to the Dy(III) ions by the terminal and internal COT″(2-) (cyclooctatetraenide) rings that correlate with the respective bond distances. Interestingly, the studies also point to a weak covalent interaction between the metal centers, despite a large separation. The ac susceptibility data indicates that both Dy(III)COT″2 and Dy(III)2COT″3 act as an SIM and an SMM, respectively, with complex multiple relaxation mechanisms. Ab initio calculations reveal the direction of the magnetic anisotropic axis is not perpendicular to the planar COT″ rings for both Dy(III)COT″2 and Dy(III)2COT″3 complexes due to the presence of trimethylsilyl groups on the COT″ rings. If these bulky groups are removed, the calculations predict reorientation of the anisotropic axis can be achieved.

Isolation of a Hexanuclear Chromium Cluster with a Tetrahedral Hydridic Core and Its Catalytic Behavior for Ethylene Oligomerization

A chromium complex [2-(NHCH2PPh2)C5H4N]CrCl3·THF2 (1) of the ligand PyNHCH2PPh2 has been synthesized, characterized, and examined for its catalytic behavior toward ethylene oligomerization. When complex 1 was treated with (i-Bu)3Al, an unprecedented divalent polyhydride chromium cluster μ,κ(1),κ(2),κ(3)-N,N,P-{[2-(NCH2PPh2)C5H4N]Cr(μ-H)}4[(μ-Cl)Cr(μ-Cl)Al(i-Bu)2Cl]2 (2) was obtained. The complex contains a Cr4H4 core, which is expected to be diamagnetic, and which remains coordinated to two additional divalent high-spin Cr atoms via bridging interactions. Two aluminate residues remain bonded to the peripheral chromium atoms. The structure, magnetism, and electronic configuration are herein discussed.

Mononuclear, Dinuclear, and Trinuclear Iron Complexes Featuring a New Monoanionic SNS Thiolate Ligand

The new tridentate ligand, S(Me)N(H)S = 2-(2-methylthiophenyl)benzothiazolidine, prepared in a single step from commercial precursors in excellent yield, undergoes ring-opening on treatment with Fe(OTf)2 in the presence of base affording a trinuclear iron complex, [Fe3(μ2-S(Me)NS(-))4](OTf)2 (1) which is fully characterized by structural and spectroscopic methods. X-ray structural data reveal that 1 contains four S(Me)NS(-) ligands meridionally bound to two pseudooctahedral iron centers each bridged by two thiolates to a distorted tetrahedral central iron. The combined spectroscopic (UV-vis, Mössbauer, NMR), magnetic (solution and solid state), and computational (DFT) studies indicate that 1 includes a central, high-spin Fe(II) (S = 2) with two low-spin (S = 0) peripheral Fe(II) centers. Complex 1 reacts with excess PMePh2, CNxylyl (2,6-dimethylphenyl isocyanide), and P(OMe)3 in CH3CN to form diamagnetic, thiolate-bridged, dinuclear Fe(II) complexes {[Fe(μ-S(Me)NS(-))L2]2}(OTf)2 (2-4). These complexes are characterized by elemental analysis; (1)H NMR, IR, UV-vis, and Mössbauer spectroscopy; and single crystal X-ray diffraction. Interestingly, addition of excess P(OMe)3 to complex 1 in CH2Cl2 produces primarily the diamagnetic, mononuclear Fe(II) complex, {Fe(S(Me)NS(-))[P(OMe)3]3}(OTf) (5).

Chromium–Chromium Interaction in a Binuclear Mixed-Valent CrI–CrII Complex

A mixed-valent Cr(I)-Cr(II) binuclear complex, {κ(1),κ(2),κ(3)-N,P,P-cyclo[(Ph)PCH2N(CH2Ph)CH2]}2(CrCl2)[Cr(μ-Cl)(AlClMe2)]·4toluene (1), of a P2N2 cyclic ligand was obtained upon treatment of the chromium precursor with alkylaluminum. Complex 1 was accessible from either its trivalent or divalent precursors, and density functional theory calculations revealed the presence of only σ- and π-orbital interactions in the Cr-Cr bond.