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Multimetallic half-sandwich complexes of the paramagnetic lanthanides gadolinium, terbium, dysprosium, holmium, and erbium as well as of diamagnetic yttrium are readily accessible via AlMe 4 /halogenido exchange reactions of Cp*Ln(AlMe 4 ) 2 with the mild halogenido transfer reagents Me 3 SiI and Me 3 GeX (Cp* = C 5 Me 5 ; X = Br, Cl). Depending on the rare-earth-metal center and halogenido ion size, complexes with distinct structural motifs and nuclearities are obtained, including dimeric compounds [Cp*Ln(AlMe 4 )(μ 2 -Cl)] 2 for the smaller metal centers Ln = Ho, Er, iodido-bridged tetranuclear rings [Cp*Ln(μ 2 -I) 2 ] 4 (Ln = Y, Tb, Dy, Ho, Er), and a heterobimetallic tetramethylaluminato-bridged gadolinium cluster [Cp* 4 Gd 4 I 7 (AlMe 4 )] 2 . The tetranuclear dysprosium complex [Cp*Dy(μ 2 -I) 2 ] 4 shows single-molecule magnet (SMM) behavior in zero applied field with an effective energy barrier of 164(10) cm -1 .

The employment of the [1+1] condensation approach for the preparation of new macrocyclic scaffolds (LN6 and LN3O3) towards high-performance Dy(III) single-molecule magnets (SMMs) with pseudo-D 6h symmetry is described. Engineering of the macrocycles denticity from LN6 to LN3O3 leads to a mononuclear SMM with a large U eff value of 1300 K. The experimental results are supported by ab initio calculations, which indicate relaxation of the magnetization via the second-excited state.

Reduction of hexaazatrinaphthylene (HAN) and its hexamethyl derivative with [Cp* 2 Sm(THF) 2 ] or [Cp* 2 Yb(OEt 2 )] produces [(Cp* 2 Ln) 3 (R 6 HAN)] (Ln = Sm, Yb; R = H, Me), where the heterocyclic ligand forms as a trianion. The magnetism and electronic structure of these compounds reflect unusual multiconfigurational character within the reduced ligand but not the lanthanide ions.

The potassium silole K 2 [SiC 4 -2,5-(SiMe 3 ) 2 -3,4-Ph 2 ] reacts with [M(η 8 -COT)(THF) 4 ][BPh 4 ] (M=Er, Y; COT=cyclo-octatetraenyl) in THF to give products that feature unprecedented insertion of the nucleophilic silicon centre into a carbon-oxygen bond of THF. The structure of the major product, [(μ-η 8  : η 8 -COT)M(μ-L 1 )K] ∞ (1 M ), consists of polymeric chains of sandwich complexes, where the spiro-bicyclic silapyran ligand [C 4 H 8 OSiC 4 (SiMe 3 ) 2 Ph 2 ] 2- (L 1 ) coordinates to potassium via the oxygen. The minor product [(μ-η 8  : η 8 -COT)M(μ-L 1 )K(THF)] 2 (2 M ) features coordination of the silapyran to the rare-earth metal. In forming 1 M and 2 M , silole insertion into THF only occurs in the presence of potassium and the rare-earth metal, highlighting the importance of bimetallic synergy. The lower nucleophilicity of germanium(II) leads to contrasting reactivity of the potassium germole K 2 [GeC 4 -2,5-(SiMe 3 ) 2 -3,4-Me 2 ] towards [M(η 8 -COT)(THF) 4 ][BPh 4 ], with intact transfer of the germole occurring to give the coordination polymers [{η 5 -GeC 4 (SiMe 3 ) 2 Me 2 }M(η 8 -COT)K] ∞ (3 M ). Despite the differences in reactivity induced by the group 14 heteroatom, the single-molecule magnet properties of 1 Er , 2 Er and 3 Er are similar, with thermally activated relaxation occurring via the first-excited Kramers doublet, subject to effective energy barriers of 122, 80 and 91 cm -1 , respectively. Compound 1 Er is also analysed by high-frequency dynamic magnetic susceptibility measurements up to 10 6  Hz., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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.

High-yield syntheses of the lanthanide dinitrogen complexes [(Cp 2 ttt M) 2 (μ-1,2-N 2 )] ( 1 M , M = Gd, Tb, Dy; Cp ttt = 1,2,4-C 5 t Bu 3 H 2 ), in which the [N 2 ] 2- ligands solely adopt the rare end-on or 1,2-bridging mode, are reported. The bulk of the tert -butyl substituents and the smaller radii of gadolinium, terbium, and dysprosium preclude formation of the side-on dinitrogen bonding mode on steric grounds. Elongation of the nitrogen-nitrogen bond relative to N 2 is observed in 1 M , and their Raman spectra show a major absorption consistent with N═N double bonds. Computational analysis of 1 Gd identifies that the local symmetry of the metallocene units lifts the degeneracy of two 5d π orbitals, leading to differing overlap with the π* orbitals of [N 2 ] 2- , a consequence of which is that the dinitrogen ligand occupies a singlet ground state. Magnetic measurements reveal antiferromagnetic exchange in 1 M and single-molecule magnet (SMM) behavior in 1 Dy . Ab initio calculations show that the magnetic easy axis in the ground doublets of 1 Tb and 1 Dy align with the {M-N═N-M} connectivity, in contrast to the usual scenario in dysprosium metallocene SMMs, where the axis passes through the cyclopentadienyl ligands. The [N 2 ] 2- ligands in 1 M allow these compounds to be regarded as two-electron reducing agents, serving as synthons for divalent gadolinium, terbium, and dysprosium. Proof of principle for this concept is obtained in the reactions of 1 M with 2,2'-bipyridyl (bipy) to give [Cp 2 ttt M(κ 2 -bipy)] ( 2 M , M = Gd, Tb, Dy), in which the lanthanide is ligated by a bipy radical anion, with strong metal-ligand direct exchange coupling.

The synthesis of acylpyrazolone salts and their complexes of main group elements, transition metals, lanthanides, and actinides are described and characterized inter alia by means of single-crystal X-ray crystallography, NMR, and IR spectroscopies. The complexes consist of two, three, or four acylprazolone ligands bound to the metal atom, resulting in a structurally diverse set of coordination complexes with (distorted) octahedral, pentagonal-bipyramidal, or antiprismatic arrangements. Several complexes proved to be polymeric in the solid state including heterobimetallic sodium/lanthanide coordination polymers. A selection of the polymeric compounds was analyzed via TG/DTA measurements to establish their stability. The ligands, in turn, were readily synthesized in good yields from commercially available hydrazine hydrochloride salts. These findings demonstrate that acylpyrazolone ligands can form complexes with metals of varying ionic radii, highlighted by their utility in other areas such as analytical and metal organic framework chemistry.

The first germole-ligated single-molecule magnets are reported, with contrasting properties found for the near-linear sandwich complexes [(η 8 -COT)Ln(η 5 -Cp Ge ] - , where Ln=Dy (1 Dy ) or Er (1 Er ), COT is cyclo-octatetraenyl and Cp Ge is [GeC 4 -2,5-(SiMe 3 ) 2 -3,4-Me 2 ] 2- . Whereas 1 Er has an energy barrier of 120(1) cm -1 in zero applied field and open hysteresis loops up to 10 K, the relaxation in 1 Dy is characterized by quantum tunneling within the ground state., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The structure and magnetic properties of the bimetallic borohydride-bridged dysprosocenium compound [{(η 5 -Cp ttt )(η 5 -Cp Me4t )Dy} 2 (μ:κ 2 :κ 2 -BH 4 )] + [B(C 6 F 5 ) 4 ] - ([ 3 Dy ][B(C 6 F 5 ) 4 ]) are reported along with the solution-phase dynamics of the isostructural yttrium and lutetium analogues (Cp ttt is 1,2,4-tri( tert -butyl)cyclopentadienyl, Cp Me4t is tetramethyl( tert -butyl)cyclopentadienyl). The synthesis of [ 3 M ][B(C 6 F 5 ) 4 ] was accomplished in the 2:1 stoichiometric reactions of [(η 5 -Cp ttt )(η 5 -Cp Me4t )Dy(BH 4 )] ( 2 M ) with [CPh 3 ][B(C 6 F 5 ) 4 ], with the metallocenes 2 M obtained from reactions of the half-sandwich complexes [(η 5 -Cp ttt )M(BH 4 ) 2 (THF)] ( 1 M ) (M = Y, Dy, Lu) with NaCp Me4t . Crystallographic studies show significant lengthening of the M···B distance on moving through the series 1 M , 2 M , and 3 M , with essentially linear {M···B···M} bridges in 3 M . Multinuclear NMR spectroscopy indicates restricted rotation of the Cp ttt ligands in 3 Y and 3 Lu in solution. The single-molecule magnet (SMM) properties of [ 3 M ][B(C 6 F 5 ) 4 ] are characterized by Raman and Orbach processes, with an effective barrier of 533(18) cm -1 and relaxation via the second-excited Kramers doublet. Although quantum tunneling of the magnetization (QTM) was not observed for [ 3 M ][B(C 6 F 5 ) 4 ], it was, surprisingly, found in its magnetically dilute version, which has a very similar barrier of U eff = 499(21) cm -1 . Consistent with this observation, slightly wider openings of the magnetic hysteresis loop at 2 K are found for [ 3 M ][B(C 6 F 5 ) 4 ] but not for the diluted analogue. The dynamic magnetic properties of the dysprosium SMMs and the role of exchange interactions in 3 Dy are interpreted with the aid of multireference ab initio calculations.

Cyclobutadienyl complexes of the f-elements are a relatively new yet poorly understood class of sandwich and half-sandwich organometallic compounds. We now describe cyclobutadienyl transfer reactions of the magnesium reagent [(η 4 -Cb'''')Mg(THF) 3 ] ( 1 ), where Cb'''' is tetrakis(trimethylsilyl)cyclobutadienyl, toward thorium(IV) and uranium(IV) tetrachlorides. The 1:1 stoichiometric reactions between 1 and AnCl 4 proceed with intact transfer of Cb'''' to give the half-sandwich complexes [(η 4 -Cb'''')AnCl(μ-Cl) 3 Mg(THF) 3 ] (An = Th, 2 ; An = U, 3 ). Using a 2:1 reaction stoichiometry produces [Mg 2 Cl 3 (THF) 6 ][(η 4 -Cb'''')An(η 3 -C 4 H(SiMe 3 ) 3 -κ-(CH 2 SiMe 2 )(Cl)] (An = Th, [Mg 2 Cl 3 (THF) 6 ][ 4 ]; An = U [Mg 2 Cl 3 (THF) 6 ][ 5 ]), in which one Cb'''' ligand has undergone cyclometalation of a trimethylsilyl group, resulting in the formation of an An-C σ-bond, protonation of the four-membered ring, and an η 3 -allylic interaction with the actinide. Complex solution-phase dynamics are observed with multinuclear nuclear magnetic resonance spectroscopy for both sandwich complexes. A computational analysis of the reaction mechanism leading to the formation of 4 and 5 indicates that the cyclobutadienyl ligands undergo C-H activation across the actinide center.

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(η 5 -C 5 i Pr 5 ) 2 ] and the uranium(III) metallocene [IU(η 5 -C 5 i Pr 5 ) 2 ] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η 5 -C 5 i Pr 5 ) 2 ] - ( 1 ) (C 5 i Pr 5 = 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 5f 3 (7s/6d z 2 ) 1 (6d x 2 - y 2 /6d xy ) 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.

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 [(Cp i Pr5 )Dy(Cp PEt4 )] + [B(C 6 F 5 ) 4 ] - (Cp i Pr5 is penta-isopropylcyclopentadienyl, Cp PEt4 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.

Replacing a monoanionic cyclopentadienyl (Cp) ligand in dysprosium single-molecule magnets (SMMs) with a dianionic cyclobutadienyl (Cb) ligand in the sandwich complexes [(η 4 -Cb'''')Dy(η 5 -C 5 Me 4 t Bu)(BH 4 )] - (1), [(η 4 -Cb'''')Dy(η 8 -Pn † )K(THF)] (2) and [(η 4 -Cb'''')Dy(η 8 -Pn † )] - (3) leads to larger energy barriers to magnetization reversal (Cb''''=C 4 (SiMe 3 ) 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 Z rel are introduced to facilitate comparisons of the SMM performance of 1-3 with a benchmark SMM. Increases in Z and Z rel when Cb''' replaces Cp signposts a route to SMMs with properties that could surpass leading systems., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The dimetallic fulvalene-bridged dysprosium complex [{Dy(Cp*)(μ-BH4)}2(Fvtttt)] (1, Cp* = C5Me5) is converted into the trimetallic borohydride-bridged species [{Dy(Cp*)(Fvtttt)}2Dy(μ-BH4)3] (2). In turn, 2 is reacted with a silylium electrophile to give [{Dy(Cp*)(μ-BH4)(Fvtttt)}2Dy][B(C6F5)4] ([3][B(C6F5)3]), the first trimetallic dysprosocenium cation. Compound [3][B(C6F5)3] can also be formed directly from 1 by adding two equivalents of the electrophile. A three-fold enhancement in the effective energy barrier from 2 to 3 is observed and interpreted with the aid of ab initio calculations.

Competing Interests: Conflict of interest The authors declare that they have no conflict of interest.

Addition of the bipyridyl-embedded cycloparaphenylene nanohoop bipy[9]CPP to [Fe{H 2 B(pyz) 2 }] (pyz=pyrazolyl) produces the distorted octahedral complex [Fe(bipy[9]CPP){H 2 B(pyz) 2 } 2 ] (1). The molecular structure of 1 shows that the nanohoop ligand contains a non-planar bipy unit. Magnetic susceptibility measurements indicate spin-crossover (SCO) behaviour with a T 1/2 of 130 K, lower than that of 160 K observed with the related compound [Fe(bipy){H 2 B(pyz) 2 } 2 ] (2), which contains a conventional bipy ligand. A computational study of 1 and 2 reveals that the curvature of the nanohoop leads to the different SCO properties, suggesting that the SCO behaviour of iron(II) can be tuned by varying the size and diameter of the nanohoop., (© 2020 Wiley-VCH GmbH.)

A series of hybrid uranocenes consisting of uranium(iv) sandwiched between cyclobutadienyl (Cb) and cyclo-octatetraenyl (COT) ligands has been synthesized, structurally characterized and studied computationally. The dimetallic species [(η 4 -Cb'''')(η 8 -COT)U(μ:η 2 :η 8 -COT)U(THF)(η 4 -Cb'''')] ( 1 ) forms concomitantly with, and can be separated from, monometallic [(η 4 -Cb'''')U(THF)(η 8 -COT)] ( 2 ) (Cb'''' = 1,2,3,4-tetrakis(trimethylsilyl)cyclobutadienyl, COT = cyclo-octatetraenyl). In toluene solution at room temperature, 1 dissociates into 2 and the unsolvated uranocene [(η 4 -Cb'''')U(η 8 -COT)] ( 3 ). By applying a high vacuum, both 1 and 2 can be converted directly into 3 . Using bulky silyl substituents on the COT ligand allowed isolation of base-free [(η 4 -Cb'''')U{η 8 -1,4-( i Pr 3 Si) 2 C 8 H 6 }] ( 4 ), with compounds 3 and 4 being new members of the bis(annulene) family of actinocenes and the first to contain a cyclobutadienyl ligand. Computational studies show that the bonding in the hybrid uranocenes 3 and 4 has non-negligible covalency. New insight into actinocene bonding is provided by the complementary interactions of the different ligands with uranium, whereby the 6d orbitals interact most strongly with the cyclobutadienyl ligand and the 5f orbitals do so with the COT ligands. The redox-neutral activation of diethyl ether by [(η 4 -Cb'''')U(η 8 -C 8 H 8 )] is also described and represents a uranium-cyclobutadienyl cooperative process, potentially forming the basis of further small-molecule activation chemistry., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The dinucleating fulvalenyl ligand [1,1',3,3'-(C 5 t Bu 2 H 2 ) 2 ] 2- (Fv tttt ) was used to synthesize the dimetallic dysprosocenium cation [{Dy(η 5 -Cp*)} 2 (μ-BH 4 )(η 5 :η 5 -Fv tttt )] + ( 3 ) as the salt of [B(C 6 F 5 ) 4 ] - (Cp* = C 5 Me 5 ). Compound [ 3 ][B(C 6 F 5 ) 4 ] was obtained using a method in which the double half-sandwich complex [{Dy(BH 4 ) 2 (THF)} 2 (Fv tttt )] ( 1 ) was reacted with KCp* to give the double metallocene [{Dy(Cp*)(μ-BH 4 )} 2 (Fv tttt )] ( 2 ), followed by removal of a bridging borohydride ligand upon addition of [(Et 3 Si) 2 (μ-H)][B(C 6 F 5 ) 4 ]. The dimetallic fulvalenyl complexes 1-3 give rise to single-molecule magnet (SMM) behaviour in zero applied field, with the effective energy barriers of 154(15) cm -1 , 252(4) cm -1 and 384(18) cm -1 , respectively, revealing a significant improvement in performance across the series. The magnetic properties are interpreted with the aid of ab initio calculations, which show substantial increases in the axiality of the crystal field from 1 to 2 to 3 as a consequence of the increasingly dominant role of the Fv tttt and Cp* ligands, with the barrier height and hysteresis properties being attenuated by the equatorial borohydride ligands. The experimental and theoretical results described in this study furnish a blueprint for the design and synthesis of poly-cationic dysprosocenium SMMs with properties that may surpass those of benchmark systems., (This journal is © The Royal Society of Chemistry 2020.)

Intact transfer of the cyclobutadienyl ligand [C 4 (SiMe 3 ) 4 ] 2- to yttrium and dysprosium (M) produces the half-sandwich complexes [M{η 4 -C 4 (SiMe 3 ) 4 }(BH 4 ) 2 (THF)] - as coordination polymers with bridging sodium or potassium ions. The dysprosium versions are single-molecule magnets (SMMs) with energy barriers of 371(7) and 357(4) cm -1 , respectively. The pristine cyclobutadienyl ligands provide a strong axial crystal field that enhances the SMM properties relative to related cyclopentadienyl compounds.

Addition of Lewis acidic [Cp* 2 M] + (M = Y, Gd) to the dinitrosyliron complexes (DNICs) [(NacNac Ar )Fe(NO) 2 ] - (Ar = mesityl, 2,6-diisopropylphenyl) results in formation of the isonitrosyl-bridged DNICs [(Cp*) 2 M(μ-ON) 2 Fe(NacNac Ar )]. When Ar = 2,6-diisopropylphenyl, coupling of the NO ligands and release of N 2 O occur. Two factors contribute to this previously unobserved DNIC reactivity mode. First, the oxophilic rare-earth elements drive the formation of isonitrosyl bonds, forcing the DNIC nitrogen atoms into proximity. Second, the bulky substituents further squeeze the DNIC, which ultimately overcomes the barrier to NO coupling, demonstrating that N 2 O elimination can occur from a single iron center.

Reduction of the uranium(III) metallocene [(η 5 -C 5 i Pr 5 ) 2 UI] (1) with potassium graphite produces the "second-generation" uranocene [(η 5 -C 5 i Pr 5 ) 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 5f 3 6d 1 . 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The 1 : 1 reactions of uranium(iv) tetrakis(borohydride) with the sodium and potassium salts of the cyclobutadienyl anion [C 4 (SiMe 3 ) 4 ] 2- (Cb'''') produce the half-sandwich complexes [Na(12-crown-4) 2 ][U(η 4 -Cb'''')(BH 4 ) 3 ] and [U(η 4 -Cb'''')(μ-BH 4 ) 3 {K(THF) 2 }] 2 . In the 1 : 2 reaction of U(BH 4 ) 4 with Na 2 Cb'''', formation of [U(η 4 -Cb'''')(η 3 -C 4 H(SiMe 3 ) 3 -κ-(CH 2 SiMe 2 )(BH 4 ))] - reveals that a Cb'''' ligand undergoes an intramolecular deprotonation, resulting in an allyl/tuck-in bonding mode. A computational study reveals that the uranium-Cb'''' bonding has an appreciable covalent component with contributions from the uranium 5f and 6d orbitals.

The reaction of the uranium(III) complex [U(η 8 -Pn †† )(η 5 -Cp*)] ( 1 ) (Pn †† = C 8 H 4 (1,4-Si i Pr 3 ) 2 , Cp* = C 5 Me 5 ) with ethene at atmospheric pressure produces the ethene-bridged diuranium complex [{(η 8 -Pn †† )(η 5 -Cp*)U} 2 (μ-η 2 :η 2 -C 2 H 4 )] ( 2 ). A computational analysis of 2 revealed that coordination of ethene to uranium reduces the carbon-carbon bond order from 2 to a value consistent with a single bond, with a concomitant change in the formal uranium oxidation state from +3 in 1 to +4 in 2 . Furthermore, the uranium-ethene bonding in 2 is of the δ type, with the dominant uranium contribution being from f-d hybrid orbitals. Complex 2 reacts with hydrogen to produce ethane and reform 1 , leading to the discovery that complex 1 also catalyzes the hydrogenation of ethene under ambient conditions.

The isocarbonyl-ligated metallocene coordination polymers [Cp* 2 M(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* 2 Dy{(μ-OC)W(Cp)(CO) 2 } 2 ] - ( 2a ), and [Cp* 2 Dy(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.

Innovative synthetic coordination and, increasingly, organometallic chemistry are at the heart of advances in molecular magnetism. Smart ligand design is essential for implementing controlled modifications to the electronic structure and magnetic properties of transition metal and f-element compounds, and many important recent developments use nontraditional ligands based on low-coordinate main group elements to drive the field forward. This review charts progress in molecular magnetism from the perspective of ligands in which the donor atoms range from low-coordinate 2p elements-particularly carbon but also boron and nitrogen-to the heavier p-block elements such as phosphorus, arsenic, antimony, and even bismuth. Emphasis is placed on the role played by novel main group ligands in addressing magnetic anisotropy of transition metal and f-element compounds, which underpins the development of single-molecule magnets (SMMs), a family of magnetic materials that can retain magnetization in the absence of a magnetic field below a blocking temperature. Nontraditional p-block donor atoms, with their relatively diffuse valence orbitals and more diverse bonding characteristics, also introduce scope for tuning the spin-orbit coupling properties and metal-ligand covalency in molecular magnets, which has implications in areas such as magnetic exchange coupling and spin crossover phenomena. The chemistry encompasses transition metals, lanthanides, and actinides and describes recently discovered molecular magnets that can be regarded, currently, as defining the state of the art. This review identifies that main group chemistry at the interface molecular magnetism is an area with huge potential to deliver new types of molecular magnets with previously unseen properties and applications.

Abstraction of iodide from [(η 5 -C 5 i Pr 5 ) 2 UI] (1) produced the cationic uranium(III) metallocene [(η 5 -C 5 i Pr 5 ) 2 U] + (2) as a salt of [B(C 6 F 5 ) 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Single-molecule magnets (SMMs) containing only one metal center may represent the lower size limit for molecule-based magnetic information storage materials. Their current drawback is that all SMMs require liquid-helium cooling to show magnetic memory effects. We now report a chemical strategy to access the dysprosium metallocene cation [(Cp i Pr5 )Dy(Cp*)] + (Cp i Pr5 , penta-iso-propylcyclopentadienyl; Cp *, pentamethylcyclopentadienyl), which displays magnetic hysteresis above liquid-nitrogen temperatures. An effective energy barrier to reversal of the magnetization of U eff = 1541 wave number is also measured. The magnetic blocking temperature of T B = 80 kelvin for this cation overcomes an essential barrier toward the development of nanomagnet devices that function at practical temperatures., (Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The potassium cyclobutadienyl [K 2 {η 4 -C 4 (SiMe 3 ) 4 }] (1) reacts with MCl 3 (THF) 3.5 (M=Y, Dy) to give the first rare-earth cyclobutadienyl complexes, that is, the complex anions [M{η 4 -C 4 (SiMe 3 ) 4 }{η 4 -C 4 (SiMe 3 ) 3 -κ-(CH 2 SiMe 2 }] 2- , (2 M ), as their dipotassium salts. The tuck-in alkyl ligand in 2 M is thought to form through deprotonation of the "squarocene" complexes [M{η 4 -C 4 (SiMe 3 ) 4 } 2 ] - by 1. Complex 2 Dy is a single-molecule magnet, but with prominent quantum tunneling. An anisotropy barrier of 323(22) cm -1 was determined for 2 Dy in an applied field of 1 kOe, and magnetic hysteresis loops were observed up to 7 K., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Structurally unique double-shelled 4f and 3d-4f supramolecular grids were constructed and assembled into 3D hydrogen-bonded organic frameworks (HOFs), with the 3d-4f heterometallic HOF showing single-molecule magnet (SMM) behaviour.

The discovery of materials capable of storing magnetic information at the level of single molecules and even single atoms has fueled renewed interest in the slow magnetic relaxation properties of single-molecule magnets (SMMs). The lanthanide elements, especially dysprosium, continue to play a pivotal role in the development of potential nanoscale applications of SMMs, including, for example, in molecular spintronics and quantum computing. Aside from their fundamentally fascinating physics, the realization of functional materials based on SMMs requires significant scientific and technical challenges to be overcome. In particular, extremely low temperatures are needed to observe slow magnetic relaxation, and while many SMMs possess a measurable energy barrier to reversal of the magnetization ( U eff ), very few such materials display the important properties of magnetic hysteresis with remanence and coercivity. Werner-type coordination chemistry has been the dominant method used in the synthesis of lanthanide SMMs, and most of our knowledge and understanding of these materials is built on the many important contributions based on this approach. In contrast, lanthanide organometallic chemistry and lanthanide magnetochemistry have effectively evolved along separate lines, hence our goal was to promote a new direction in single-molecule magnetism by uniting the nonclassical organometallic synthetic approach with the traditionally distinct field of molecular magnetism. Over the last several years, our work on SMMs has focused on obtaining a detailed understanding of why magnetic materials based on the dysprosium metallocene cation building block {Cp 2 Dy} + display slow magnetic relaxation. Specifically, we aspired to control the SMM properties using novel coordination chemistry in a way that hinges on key considerations, such as the strength and the symmetry of the crystal field. In establishing that the two cyclopentadienyl ligands combine to provide a strongly axial crystal field, we were able to propose a robust magneto-structural correlation for understanding the properties of dysprosium metallocene SMMs. In doing so, a blueprint was established that allows U eff and the magnetic blocking temperature ( T B ) to be improved in a well-defined way. Although experimental discoveries with SMMs occur more rapidly than quantitative theory can (currently) process and explain, a clear message emanating from the literature is that a combination of the two approaches is most effective. In this Account, we summarize the main findings from our own work on dysprosium metallocene SMMs, and consider them in the light of related experimental studies and theoretical interpretations of related materials reported by other protagonists. In doing so, we aim to contribute to the nascent and healthy debate on the nature of spin dynamics in SMMs and allied molecular nanomagnets, which will be crucial for the further advancement of this vibrant research field.

The Suzuki-Miyaura (SM) reaction is one of the most important methods for C-C bond formation in chemical synthesis. In this communication, we show for the first time that the low toxicity, inexpensive element zinc is able to catalyze SM reactions. The cross-coupling of benzyl bromides with aryl borates is catalyzed by ZnBr 2 , in a process that is free from added ligand, and is compatible with a range of functionalized benzyl bromides and arylboronic acid pinacol esters. Initial mechanistic investigations indicate that the selective in situ formation of triaryl zincates is crucial to promote selective cross-coupling reactivity, which is facilitated by employing an arylborate of optimal nucleophilicity., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis and structural characterization of two benzoquinone-bridged dinuclear rare-earth complexes [BQ(MCl 2 ·THF 3 ) 2 ] (BQ = 2,5-bisoxide-1,4-benzoquinone; M = Y (1), Dy (2)) are described. Of these reported metal complexes, the dysprosium analogue 2 is the first discrete bridged dinuclear lanthanide complex in which both metal centres reside in pentagonal bipyramidal environments. Interestingly, both complexes undergo significant thermal expansion upon heating from 120 K to 293 K as illustrated by single-crystal X-ray and powder diffraction experiments. AC magnetic susceptibility measurements reveal that 2 does not show the slow relation of magnetization in zero dc field. The absent of single-molecule behaviour in 2 arises from the rotation of the principal magnetic axis as compared to the pseudo-C 5 axis of the pentagonal bipyramidal environment as suggested by ab initio calculations. The cyclic voltammetry and chemical reduction experiments demonstrated that complexes 1 and 2 can be reduced to radical species containing [BQ 3 ˙ - ]. This study establishes efficient synthetic strategy to make bridged redox-active multinuclear lanthanide complexes with a pentagonal bipyramidal coordination environment that are potential precursors for single-molecule magnets.

Abstraction of a chloride ligand from the dysprosium metallocene [(Cp ttt ) 2 DyCl] (1 Dy Cp ttt =1,2,4-tri(tert-butyl)cyclopentadienide) by the triethylsilylium cation produces the first base-free rare-earth metallocenium cation [(Cp ttt ) 2 Dy] + (2 Dy ) as a salt of the non-coordinating [B(C 6 F 5 ) 4 ] - anion. Magnetic measurements reveal that [2 Dy ][B(C 6 F 5 ) 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 2 Dy 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 M J value, with no significant mixing even in the higher doublets., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The stable metallocene-butyl complexes [(Cp Me ) 2 M( n Bu)] 2 (M = Y, Dy) were synthesized and their reactivity towards to ferrocene and bulky N-heterocyclic carbenes investigated. Selective mono-deprotonation of ferrocene and a benzylic methyl group of IMes were observed, whereas a control reaction of (Cp Me ) 3 M with IMes resulted in a normal-to-abnormal NHC rearrangement.

The synthesis, structure and magnetic properties of the indigo-bridged dilanthanide complexes [{(η 5 -Cp*) 2 Ln} 2 (μ-ind)] n- with Ln = Gd or Dy and n = 0, 1 or 2 are described. The gadolinium complexes with n = 0 and 2 show typically weak exchange coupling, whereas the complex bridged by the radical [ind] 3- ligand shows an unusually large coupling constant of J = -11 cm -1 (-2J formalism). The dysprosium complexes with n = 0 and 1 are single-molecule magnets in zero applied field, whereas the complex with n = 2 does not show slow magnetic relaxation.

Single-molecule magnets (SMMs) are coordination compounds that exhibit magnetic bistability below a characteristic blocking temperature. Research in this field continues to evolve from its fundamental foundations towards applications of SMMs in information storage and spintronic devices. Synthetic chemistry plays a crucial role in targeting the properties that could ultimately produce SMMs with technological potential. The ligands in SMMs are invariably based on non-metals; we now report a series of dysprosium SMMs (in addition to their magnetically dilute analogues embedded in yttrium matrices) that contain ligands with the metalloid element antimony as the donor atom, i.e. [(η 5 -Cp' 2 Dy){μ-Sb(H)Mes}] 3 ( 1-Dy ) and [(η 5 -Cp' 2 Dy) 3 {μ-(SbMes) 3 Sb}] ( 2-Dy ), which contain the stibinide ligand [Mes(H)Sb] - and the unusual Zintl-like ligand [Sb 4 Mes 3 ] 3- , respectively (Cp' = methylcyclopentadienyl; Mes = mesityl). The zero-field anisotropy barriers in 1-Dy and 2-Dy are U eff = 345 cm -1 and 270 cm -1 , respectively. Stabilization of the antimony-ligated SMMs is contingent upon careful control of reaction time and temperature. With longer reaction times and higher temperatures, the stibine pro-ligands are catalytically dehydrocoupled by the rare-earth precursor complexes. NMR spectroscopic studies of the yttrium-catalysed dehydrocoupling reactions reveal that 1-Y and 2-Y are formed during the catalytic cycle. By implication, 1-Dy and 2-Dy should also be catalytic intermediates, hence the nature of these complexes as SMMs in the solid-state and as catalysts in solution introduces a strategy whereby new molecular magnets can be identified by intercepting species formed during catalytic reactions.

The synthesis, structure and magnetic properties of the HAN-bridged tri-dysprosium complex [{(thd) 3 Dy} 3 HAN] (1) are described. The complex is an SMM that shows two relaxation processes owing to the presence of two geometrically distinct Dy 3+ sites in 1. Ab initio calculations reveal that the magnetic ground state of 1 is characterized by magnetic frustration.

The single-molecule magnet (SMM) properties of the isocarbonyl-ligated dysprosium metallocene [Cp*2 Dy{μ-(OC)2 FeCp}]2 (1Dy ), which contains a rhombus-shaped Dy2 Fe2 core, are described. Combining a strong axial [Cp*](-) ligand field with a weak equatorial field consisting of the isocarbonyl ligands leads to an anisotropy barrier of 662 cm(-1) in zero applied field. The dominant thermal relaxation pathways in 1Dy involves at least the fourth-excited Kramers doublet, thus demonstrating that prominent SMM behavior can be observed for dysprosium in low-symmetry environments., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The structural and bonding properties of a three-coordinate N-heterocyclic silyene (NHSi) complex of the iron(ii) amide [Fe{N(SiMe3)2}2] are reported. Computational studies reveal that dispersion forces between the amido SiMe3 substituents and the isopropyl substituents on the NHSi ligand significantly enhance the stability of the complex, along with Fe-to-Si π-backbonding.

Reducing hexaazatrinaphthylene (HAN) with potassium in the presence of 18-c-6 produces [{K(18-c-6)}HAN], which contains the S=1/2 radical [HAN](.-) . The [HAN](.-) radical can be transferred to the cobalt(II) amide [Co{N(SiMe3 )2 }2 ], forming [K(18-c-6)][(HAN){Co(N'')2 }3 ]; magnetic measurements on this compound reveal an S=4 spin system with strong cobalt-ligand antiferromagnetic exchange and J≈-290 cm(-1) (-2 J formalism). In contrast, the Co(II) centres in the unreduced analogue [(HAN){Co(N'')2 }3 ] are weakly coupled (J≈-4.4 cm(-1) ). The finding that [HAN](.-) can be synthesized as a stable salt and transferred to cobalt introduces potential new routes to magnetic materials based on strongly coupled, triangular HAN building blocks., (© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

The structures and magnetic properties of the arsenic- and selenium-ligated dysprosium single-molecule magnets (SMMs) [Cp' 3 Dy(AsH 2 Mes)] ( 3-Dy ), [(η 5 -Cp' 2 Dy){μ-As(H)Mes}] 3 ( 4-Dy ), [Li(thf) 4 ] 2 [(η 5 -Cp' 2 Dy) 3 (μ 3 -AsMes) 3 Li] ([Li(thf) 4 ] 2 [ 5-Dy ]), and [(η 5 -Cp' 2 Dy){μ-SeMes}] 3 ( 6-Dy ) are described. The arsenic-ligated complexes 4-Dy and 5-Dy are the first SMMs to feature ligands with metalloid elements as the donor atoms. The arsenide-ligated complex 4-Dy and the selenolate-ligated complex 6-Dy show large anisotropy barriers in the region of 250 cm -1 in zero d.c. field, increasing to 300 cm -1 upon 5% magnetic dilution. Theoretical studies reveal that thermal relaxation in these SMMs occurs via the second-excited Kramers' doublet. In contrast, the arsinidene-ligated SMM 5-Dy gives a much smaller barrier of 23 cm -1 , increasing to 35 cm -1 upon dilution. The field-dependence of the magnetization for 4-Dy and 5-Dy at 1.8 K show unusual plateaus around 10 kOe, which is due to the dominance of arsenic-mediated exchange over the dipolar exchange. The effects of the exchange interactions are more pronounced in 5-Dy , which is a consequence of a small but significant increase in the covalent contribution to the predominantly ionic dysprosium-arsenic bonds. Whereas the magnetically non-dilute dysprosium SMMs show only very narrow magnetization versus field hysteresis loops at 1.8 K, the impact of magnetic dilution is dramatic, with butterfly-shaped loops being observed up to 5.4 K in the case of 4-Dy . Our findings suggest that ligands with heavier p-block element donor atoms have considerable potential to be developed more widely for applications in molecular magnetism.

In the presence of stoichiometric or catalytic amounts of [M{N(SiMe3 )2 }2 ] (M=Fe, Co), N-heterocyclic carbenes (NHCs) react with primary phosphines to give a series of carbene phosphinidenes of the type (NHC)⋅PAr. The formation of (IMe4 )⋅PMes (Mes=mesityl) is also catalyzed by the phosphinidene-bridged complex [(IMe4 )2 Fe(μ-PMes)]2 , which provides evidence for metal-catalyzed phosphinidene transfer., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Three-electron reduction of hexaazatrinaphthylene (HAN) with a magnesium(I) reagent leads to [(HAN){Mg(nacnac)}3] (1), containing a [HAN](3-) ligand with a spin of S = 1/2. Ab initio calculations reveal that the [HAN](3-) ligand in 1 has a ground-state wave function with multiconfigurational properties, and can be described as a triradicaloid species with a small amount of open-shell doublet character.

Single-molecule magnets are a type of coordination compound that can retain magnetic information at low temperatures. Single-molecule magnets based on lanthanides have accounted for many important advances, including systems with very large energy barriers to reversal of the magnetization, and a di-terbium complex that displays magnetic hysteresis up to 14 K and shows strong coercivity. Ligand design is crucial for the development of new single-molecule magnets: organometallic chemistry presents possibilities for using unconventional ligands, particularly those with soft donor groups. Here we report dysprosium single-molecule magnets with neutral and anionic phosphorus donor ligands, and show that their properties change dramatically when varying the ligand from phosphine to phosphide to phosphinidene. A phosphide-ligated, trimetallic dysprosium single-molecule magnet relaxes via the second-excited Kramers' doublet, and, when doped into a diamagnetic matrix at the single-ion level, produces a large energy barrier of 256 cm(-1) and magnetic hysteresis up to 4.4 K.

Two dysprosium complexes, [(C5H4Me)2Dy(L(1))] (3) and [(L(1))Dy(μ-Cl)3{Li(tmeda)}]2 (4), with amino-functionalized pentadienyl ligands L(1) are described. Crystallographic studies of 3 and 4 show that the pendant amino group influences the pentadienyl conformation and the ligand hapticity. Electronic structure calculations reveal that L(1) has a strong influence on the orientation of the main magnetic axis of the ground Kramers doublets in 3 and 4.