Triply Bonded Pancake π-Dimers Stabilized by Tetravalent Actinides.

Aromatic π-stacking is a weakly attractive, noncovalent interaction often found in biological macromolecules and synthetic supramolecular chemistry. The weak nondirectional nature of π-stacking can present challenges in the design of materials owing to their weak, nondirectional nature. However, when aromatic π-systems contain an unpaired electron, stronger attraction involving face-to-face π-orbital overlap is possible, resulting in covalent so-called "pancake" bonds. Two-electron, multicenter single pancake bonds are well known, whereas four-electron double pancake bonds are rare. Higher-order pancake bonds have been predicted, but experimental systems are unknown. Here, we show that six-electron triple pancake bonds can be synthesized by a 3-fold reduction of hexaazatrinaphthylene (HAN) and subsequent stacking of the [HAN]3- triradicals. Our analysis reveals a multicenter covalent triple pancake bond consisting of a σ-orbital and two equivalent π-orbitals. An electrostatic stabilizing role is established for the tetravalent thorium and uranium ions in these systems. We also show that the electronic absorption spectrum of the triple pancake bonds closely matches computational predictions, providing experimental verification of these unique interactions. The discovery of conductivity in thin films of triply bonded π-dimers presents new opportunities for the discovery of single-component molecular conductors and other spin-based molecular materials.

Identification of Oxidation State +1 in a Molecular Uranium Complex.

The concept of oxidation state plays a fundamentally important role in defining the chemistry of the elements. In the f block of the periodic table, well-known oxidation states in compounds of the lanthanides include 0, +2, +3 and +4, and oxidation states for the actinides range from +7 to +2. Oxidation state +1 is conspicuous by its absence from the f-block elements. Here we show that the uranium(II) metallocene [U(η5-C5iPr5)2] and the uranium(III) metallocene [IU(η5-C5iPr5)2] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η5-C5iPr5)2]- (1) (C5iPr5 = pentaisopropylcyclopentadienyl) as the salt of [K(2.2.2-cryptand)]+. An X-ray crystallographic study revealed that 1 has a bent metallocene structure, and theoretical studies and magnetic measurements confirmed that the electronic ground state of uranium(I) adopts a 5f3(7s/6dz2)1(6dx2-y2/6dxy)1 configuration. The metal-ligand bonding in 1 consists of contributions from uranium 5f, 6d, and 7s orbitals, with the 6d orbitals engaging in weak but non-negligible covalent interactions. Identification of the oxidation state +1 for uranium expands the range of isolable oxidation states for the f-block elements and potentially signposts a synthetic route to this elusive species for other actinides and the lanthanides.

Discovery of a Dysprosium Metallocene Single-Molecule Magnet with Two High-Temperature Orbach Processes.

Magnetic bistability in single-molecule magnets (SMMs) is a potential basis for new types of nanoscale information storage material. The standard model for thermally activated relaxation of the magnetization in SMMs is based on the occurrence of a single Orbach process. Here, we show that incorporating a phosphorus atom into the framework of the dysprosium metallocene [(CpiPr5)Dy(CpPEt4)]+[B(C6F5)4]- (CpiPr5 is penta-isopropylcyclopentadienyl, CpPEt4 is tetraethylphospholyl) leads to the occurrence of two distinct high-temperature Orbach processes, with energy barriers of 1410(10) and 747(7) cm-1, respectively. These barriers provide experimental evidence for two different spin-phonon coupling regimes, which we explain with the aid of ab initio calculations. The strong and highly axial crystal field in this SMM also allows magnetic hysteresis to be observed up to 70 K, using a scan rate of 25 Oe s-1. In characterizing this SMM, we show that a conventional Debye model and consideration of rotational contributions to the spin-phonon interaction are insufficient to explain the observed phenomena.

Dominance of Cyclobutadienyl Over Cyclopentadienyl in the Crystal Field Splitting in Dysprosium Single-Molecule Magnets.

Replacing a monoanionic cyclopentadienyl (Cp) ligand in dysprosium single-molecule magnets (SMMs) with a dianionic cyclobutadienyl (Cb) ligand in the sandwich complexes [(η4 -Cb'''')Dy(η5 -C5 Me4 t Bu)(BH4 )]- (1), [(η4 -Cb'''')Dy(η8 -Pn† )K(THF)] (2) and [(η4 -Cb'''')Dy(η8 -Pn† )]- (3) leads to larger energy barriers to magnetization reversal (Cb''''=C4 (SiMe3 )4 , Pn† =1,4-di(tri-isopropylsilyl)pentalenyl). Short distances to the Cb'''' ligands and longer distances to the Cp ligands in 1-3 are consistent with the crystal field splitting being dominated by the former. Theoretical analysis shows that the magnetic axes in the ground Kramers doublets of 1-3 are oriented towards the Cb'''' ligands. The theoretical axiality parameter and the relative axiality parameter Z and Zrel are introduced to facilitate comparisons of the SMM performance of 1-3 with a benchmark SMM. Increases in Z and Zrel when Cb''' replaces Cp signposts a route to SMMs with properties that could surpass leading systems.

Two-Coordinate, Nonlinear Vanadium(II) and Chromium(II) Complexes of the Silylamide Ligand-N(SiMePh2)2: Characterization and Confirmation of Orbitally Quenched Magnetic Moments in Complexes with Sub-d5 Electron Configurations.

The two-coordinate metal amide complexes V{N(SiMePh2)2}2 (1) and Cr{N(SiMe2Ph)2}2 (2) were synthesized by reaction of two equivalents of LiN(SiMePh2)2 with VI2(THF)4 or CrCl2(THF)2 in n-hexane. Their crystal structures showed that they have bent coordination, N-V-N = 137.0(4)°, N-Cr-N = 139.19(5)°, at the metals. The vanadium complex (1) displayed no tendency to isomerize as previously observed for some V(II) amido complexes. Curie fits of SQUID magnetic measurements afforded magnetic moments of 3.36 (1) and 4.68 (2) µB, consistent with high-spin configurations. These values are lower than the spin-only values of 3.88 and 4.90 µB expected for d3 and d4 complexes, suggesting a significant unquenched orbital angular momentum contribution to the overall moment, which is lower as a result of the positive spin-orbit coupling constants.

Radical-Bridged Ln4 Metallocene Complexes with Strong Magnetic Coupling and a Large Coercive Field.

Inducing magnetic coupling between 4f elements is an ongoing challenge. To overcome this formidable difficulty, we incorporate highly delocalized tetrazinyl radicals, which strongly couple with f-block metallocenes to form discrete tetranuclear complexes. Synthesis, structure, and magnetic properties of two tetranuclear [(Cp*2 Ln)4 (tz. )4 ]⋅3(C6 H6 ) (Cp*=pentamethylcyclopentadienyl; tz=1,2,4,5-tetrazine; Ln=Dy, Gd) complexes are reported. An in-depth examination of their magnetic properties through magnetic susceptibility measurements as well as computational studies support a highly sought-after radical-induced "giant-spin" model. Strong exchange interactions between the LnIII ions and tz. radicals lead to a strong magnet-like behaviour in this molecular magnet with a large coercive field of 30 kOe.

Carbonyl Back-Bonding Influencing the Rate of Quantum Tunnelling in a Dysprosium Metallocene Single-Molecule Magnet.

The isocarbonyl-ligated metallocene coordination polymers [Cp*2M(µ-OC)W(Cp)(CO)(µ-CO)]∞ were synthesized with M = Gd (1, L = THF) and Dy (2, no L). In a zero direct-current field, the dysprosium version 2 was found to be a single-molecule magnet (SMM), with analysis of the dynamic magnetic susceptibility data revealing that the axial metallocene coordination environment leads to a large anisotropy barrier of 557(18) cm-1 and a fast quantum-tunnelling rate of ∼3.7 ms. Theoretical analysis of two truncated versions of 2, [Cp*2Dy{(µ-OC)W(Cp)(CO)2}2]- (2a), and [Cp*2Dy(OC)2]+ (2b), in which the effects of electron correlation outside the 4f orbital space were studied, revealed that tungsten-to-carbonyl back-donation plays an important role in determining the strength of the competing equatorial field at dysprosium and, hence, the dynamic magnetic properties. The finding that a classical organo-transition-metal bonding scenario can be used as an indirect way of tuning the rate of quantum tunnelling potentially provides an alternative chemical strategy for utilizing the fast magnetic relaxation properties of SMMs.

Isolation of a Perfectly Linear Uranium(II) Metallocene.

Reduction of the uranium(III) metallocene [(η5 -C5 i Pr5 )2 UI] (1) with potassium graphite produces the "second-generation" uranocene [(η5 -C5 i Pr5 )2 U] (2), which contains uranium in the formal divalent oxidation state. The geometry of 2 is that of a perfectly linear bis(cyclopentadienyl) sandwich complex, with the ground-state valence electron configuration of uranium(II) revealed by electronic spectroscopy and density functional theory to be 5f3 6d1 . Appreciable covalent contributions to the metal-ligand bonds were determined from a computational study of 2, including participation from the uranium 5f and 6d orbitals. Whereas three unpaired electrons in 2 occupy orbitals with essentially pure 5f character, the fourth electron resides in an orbital defined by strong 7s-6d z 2 mixing.

Uranocenium: Synthesis, Structure, and Chemical Bonding.

Abstraction of iodide from [(η5 -C5 i Pr5 )2 UI] (1) produced the cationic uranium(III) metallocene [(η5 -C5 i Pr5 )2 U]+ (2) as a salt of [B(C6 F5 )4 ]- . The structure of 2 consists of unsymmetrically bonded cyclopentadienyl ligands and a bending angle of 167.82° at uranium. Analysis of the bonding in 2 showed that the uranium 5f orbitals are strongly split and mixed with the ligand orbitals, thus leading to non-negligible covalent contributions to the bonding. Investigation of the dynamic magnetic properties of 2 revealed that the 5f covalency leads to partially quenched anisotropy and fast magnetic relaxation in zero applied magnetic field. Application of a magnetic field leads to dominant relaxation by a Raman process.

Nickel as a Lewis Base in a T-Shaped Nickel(0) Germylene Complex Incorporating a Flexible Bis(NHC) Ligand.

Flexible, chelating bis(NHC) ligand 2, able to accommodate both cis- and trans-coordination modes, was used to synthesize (2)Ni(η2 -cod), 3. In reaction with GeCl2 , it produced (2)NiGeCl2 , 4, featuring a T-shaped Ni0 and a pyramidal Ge center. Complex 4 could also be prepared from [(2)GeCl]Cl, 5, and Ni(cod)2 , in a reaction that formally involved Ni-Ge transmetalation, followed by coordination of the extruded GeCl2 moiety to Ni. A computational analysis showed that 4 possesses considerable multiconfigurational character and the Ni→Ge bond is formed through σ-donation from the Ni 4s, 4p, and 3d orbitals to Ge. (NHC)2 Ni(cod) complexes 9 and 10, as well as (NHC)2 GeCl2 derivative 11, incorporating ligands that cannot accommodate a wide bite angle, failed to produce isolable Ni-Ge complexes. The isolation of (2)Ni(η2 -Py), 12, provides further evidence for the reluctance of the (2)Ni0 fragment to act as a σ-Lewis acid.

Rare-Earth Cyclobutadienyl Sandwich Complexes: Synthesis, Structure and Dynamic Magnetic Properties.

The potassium cyclobutadienyl [K2 {η4 -C4 (SiMe3 )4 }] (1) reacts with MCl3 (THF)3.5 (M=Y, Dy) to give the first rare-earth cyclobutadienyl complexes, that is, the complex anions [M{η4 -C4 (SiMe3 )4 }{η4 -C4 (SiMe3 )3 -κ-(CH2 SiMe2 }]2- , (2M ), as their dipotassium salts. The tuck-in alkyl ligand in 2M is thought to form through deprotonation of the "squarocene" complexes [M{η4 -C4 (SiMe3 )4 }2 ]- by 1. Complex 2Dy is a single-molecule magnet, but with prominent quantum tunneling. An anisotropy barrier of 323(22) cm-1 was determined for 2Dy in an applied field of 1 kOe, and magnetic hysteresis loops were observed up to 7 K.

Effects of Remote Ligand Substituents on the Structures, Spectroscopic, and Magnetic Properties of Two-Coordinate Transition-Metal Thiolate Complexes.

The first-row transition-metal(II) dithiolates M(SAriPr4)2 [AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2; M = Cr (1), Mn (3), Fe (4), Co (5), Ni (6), and Zn (7)] and Cr(SArMe6)2 [2; ArMe6 = C6H3-2,6-(C6H2-2,4,6-Me3)2] and the ligand-transfer reagent (NaSAriPr4)2 (8) are described. In contrast to their M(SAriPr6)2 (M = Cr, Mn, Fe, Co, Ni, and Zn; AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2) congeners, which differ from 1 and 3-6 in having p-isopropyl groups on the flanking aryl rings of the terphenyl substituents, compounds 1 and 4-6 display highly bent coordination geometries with S-M-S angles of 109.802(2)° (1), 120.2828(3)° (4), 91.730(3)° (5), and 92.68(2)° (6) as well as relatively close metal-flanking aryl ring η6 interactions with metal-centroid distances of 2.11477(6) Å (1), 1.97188(3) Å (2), 2.15269(6) Å (4), 1.62058(9) Å (5), and 1.724(8) Å (6). However, the d5 (Mn) and d10 (Zn) complexes 3 and 7 display linear or near-linear coordination with no close metal-ligand distances. The nonlinear geometries of 1 and 4-6 also contrast with those of their AriPr4-substituted alkoxo and amido congeners, which have strictly linear coordination. Complexes 1-7 were synthesized by the reaction of the lithium or sodium thiolate salt with the metal dihalide or, in the case of 3, by the reaction of the thiol with the amido complex Mn[N(SiMe3)2]2. All compounds were characterized by electronic spectroscopy, X-ray crystallography, and magnetic measurements using Evans' method and SQUID magnetometry. It was concluded that, despite the large bulk of the AriPr4 substituents, the absence of p-isopropyl groups on the flanking rings of the ligand permits close secondary metal-flanking ring distances. The compounds are characterized by more intense colors and display magnetic moments that are generally lower than the spin-only values, in agreement with the covalent character of the close metal-flanking ring η6 interactions.

Mono- and Bis(imidazolidinium ethynyl) Cations and Reduction of the Latter To Give an Extended Bis-1,4-([3]Cumulene)-p-carboquinoid System.

An extended π-system containing two [3]cumulene fragments separated by a p-carboquinoid and stabilized by two capping N-heterocyclic carbenes (NHCs) has been prepared. Mono- and bis(imidazolidinium ethynyl) cations have also been synthesized from the reaction of an NHC with phenylethynyl bromide or 1,4-bis(bromoethynyl)benzene. Cyclic voltammetry coupled with synthetic and structural studies showed that the dication is readily reduced to a neutral, singlet bis-1,4-([3]cumulene)-p-carboquinoid as a result of the π-accepting properties of the capping NHCs.

Dynamic Magnetic and Optical Insight into a High Performance Pentagonal Bipyramidal DyIII Single-Ion Magnet.

The pentagonal bipyramidal single-ion magnets (SIMs) are among the most attractive prototypes of high-performance single-molecule magnets (SMMs). Here, a fluorescence-active phosphine oxide ligand CyPh2 PO (=cyclohexyl(diphenyl)phosphine oxide) was introduced into [Dy(CyPh2 PO)2 (H2 O)5 ]Br3 ⋅2 (CyPh2 PO)⋅EtOH⋅3 H2 O, and combined dynamic magnetic measurement, optical characterization, ab initio calculation, and magneto-optical correlation of this high-performance pseudo-D5h DyIII SIM with large Ueff (508(2) K) and high magnetic hysteresis temperature (19 K) were performed. This work provides a deeper insight into the rational design of promising molecular magnets.

Interplay of spin-dependent delocalization and magnetic anisotropy in the ground and excited states of [Gd2@C78]- and [Gd2@C80].

The magnetic properties and electronic structure of the ground and excited states of two recently characterized endohedral metallo-fullerenes, [Gd2@C78]- (1) and [Gd2@C80]- (2), have been studied by theoretical methods. The systems can be considered as [Gd2]5+ dimers encapsulated in a fullerene cage with the fifteen unpaired electrons ferromagnetically coupled into an S = 15/2 high-spin configuration in the ground state. The microscopic mechanisms governing the Gd-Gd interactions leading to the ferromagnetic ground state are examined by a combination of density functional and ab initio calculations and the full energy spectrum of the ground and lowest excited states is constructed by means of ab initio model Hamiltonians. The ground state is characterized by strong electron delocalization bordering on a σ type one-electron covalent bond and minor zero-field splitting (ZFS) that is successfully described as a second order spin-orbit coupling effect. We have shown that the observed ferromagnetic interaction originates from Hund's rule coupling and not from the conventional double exchange mechanism. The calculated ZFS parameters of 1 and 2 in their optimized geometries are in qualitative agreement with experimental EPR results. The higher excited states display less electron delocalization, but at the same time they possess unquenched first-order angular momentum. This leads to strong spin-orbit coupling and highly anisotropic energy spectrum. The analysis of the excited states presented here constitutes the first detailed study of the effects of spin-dependent delocalization in the presence of first order orbital angular momentum and the obtained results can be applied to other mixed valence lanthanide systems.

A Dysprosium Metallocene Single-Molecule Magnet Functioning at the Axial Limit.

Abstraction of a chloride ligand from the dysprosium metallocene [(Cpttt )2 DyCl] (1Dy Cpttt =1,2,4-tri(tert-butyl)cyclopentadienide) by the triethylsilylium cation produces the first base-free rare-earth metallocenium cation [(Cpttt )2 Dy]+ (2Dy ) as a salt of the non-coordinating [B(C6 F5 )4 ]- anion. Magnetic measurements reveal that [2Dy ][B(C6 F5 )4 ] is an SMM with a record anisotropy barrier up to 1277 cm-1 (1837 K) in zero field and a record magnetic blocking temperature of 60 K, including hysteresis with coercivity. The exceptional magnetic axiality of 2Dy is further highlighted by computational studies, which reveal this system to be the first lanthanide SMM in which all low-lying Kramers doublets correspond to a well-defined MJ value, with no significant mixing even in the higher doublets.

Coordination Complexes of a Neutral 1,2,4-Benzotriazinyl Radical Ligand: Synthesis, Molecular and Electronic Structures, and Magnetic Properties.

A series of d-block metal complexes of the recently reported coordinating neutral radical ligand 1-phenyl-3-(pyrid-2-yl)-1,4-dihydro-1,2,4-benzotriazin-4-yl (1) was synthesized. The investigated systems contain the benzotriazinyl radical 1 coordinated to a divalent metal cation, Mn(II) , Fe(II) , Co(II) , or Ni(II) , with 1,1,1,5,5,5-hexafluoroacetylacetonato (hfac) as the auxiliary ligand of choice. The synthesized complexes were fully characterized by single-crystal X-ray diffraction, magnetic susceptibility measurements, and electronic structure calculations. The complexes [Mn(1)(hfac)2 ] and [Fe(1)(hfac)2 ] displayed antiferromagnetic coupling between the unpaired electrons of the ligand and the metal cation, whereas the interaction was found to be ferromagnetic in the analogous Ni(II) complex [Ni(1)(hfac)2 ]. The magnetic properties of the complex [Co(1)(hfac)2 ] were difficult to interpret owing to significant spin-orbit coupling inherent to octahedral high-spin Co(II) metal ion. As a whole, the reported data clearly demonstrated the favorable coordinating properties of the radical 1, which, together with its stability and structural tunability, make it an excellent new building block for establishing more complex metal-radical architectures with interesting magnetic properties.

The Instability of Ni{N(SiMe3 )2 }2 : A Fifty Year Old Transition Metal Silylamide Mystery.

The characterization of the unstable Ni(II) bis(silylamide) Ni{N(SiMe3 )2 }2 (1), its THF complex Ni{N(SiMe3 )2 }2 (THF) (2), and the stable bis(pyridine) derivative trans-Ni{N(SiMe3 )2 }2 (py)2 (3), is described. Both 1 and 2 decompose at ca. 25 °C to a tetrameric Ni(I) species, [Ni{N(SiMe3 )2 }]4 (4), also obtainable from LiN(SiMe3 )2 and NiCl2 (DME). Experimental and computational data indicate that the instability of 1 is likely due to ease of reduction of Ni(II) to Ni(I) and the stabilization of 4 through dispersion forces.

Reversible complexation of ethylene by a silylene under ambient conditions.

Treatment of toluene solutions of the silylenes Si(SAr(Me6))2 (Ar(Me6) = C6H3-2,6(C6H2-2,4,6-Me3)2, 1) or Si(SAr(Pr(i)4))2 (Ar(Pr(i)4) = C6H3-2,6(C6H3-2,6-Pr(i)2)2, 2) with excess ethylene gas affords the siliranes (Ar(Me6)S)2SiCH2CH2 (3) or (Ar(Pr(i)4)S)2SiCH2CH2 (4). Silirane 4 evolves ethylene spontaneously at room temperature in toluene solution. A Van't Hoff analysis by variable-temperature (1)H NMR spectroscopy showed that ΔG(assn) = -24.9(2.5) kJ mol(-1) for 4. A computational study of the reaction mechanism using a model silylene Si(SPh)2 (Ph = C6H5) was in harmony with the Van't Hoff analysis, yielding ΔG(assn) = -24 kJ mol(-1) and an activation energy ΔG(‡) = 54 kJ mol(-1).