Samarium Polystibides Derived from Highly Activated Nanoscale Antimony

Extension and Fusion of Cyclic Polyantimony Units

The synthesis and characterization of a series of polyantimony anionic clusters are reported. The products [(NbCp)2Sb10]2-, [MSb13]3- (M = Ru/Fe), and [MSb15]3- (M = Ru/Fe) were isolated as either K(18-crown-6) or K([2.2.2]-crypt) salts. The Sb10 ring contained in the [(NbCp)2Sb10]2- cluster can be viewed as an extension of two envelope-like cyclo-Sb5 units and represents by far the largest monocyclic all-antimony species. The clusters [MSb13]3- and [MSb15]3- (M = Ru/Fe) illustrate the variability of crown-like Sb8 ring motifs and reveal the fusion of different antimony fragments featuring unique Sb-Sb chain-like units. The reported synthetic approaches involve the fabrication of a variety of distinctive polyantimony anionic clusters, enhancing our understanding of the coordination chemistry of heavier group 15 elements.

f-Element Zintl Chemistry: Actinide-Mediated Dehydrocoupling of H2Sb1- Affords the Trithorium and Triuranium Undeca-Antimontriide Zintl Clusters [{An(TrenTIPS)}3(μ3-Sb11)] (An = Th, U; TrenTIPS = {N(CH2CH2NSiiPr3)3}3-)

Reaction of the cesium antimonide complex [Cs(18C6)2][SbH2] (1, 18C6 = 18-crown-6 ether) with the triamidoamine actinide separated ion pairs [An(TrenTIPS)(L)][BPh4] (TrenTIPS = {N(CH2CH2NSiiPr3)3}3-; An/L = Th/DME (2Th); U/THF (2U)) affords the triactinide undeca-antimontriide Zintl clusters [{An(TrenTIPS)}3(μ3-Sb11)] (An = Th (3Th), U (3U)) by dehydrocoupling. Clusters 3Th and 3U provide two new examples of the Sb113- Zintl trianion and are unprecedented examples of molecular Sb113- being coordinated to anything since all previous reports featured isolated Sb113- Zintl trianions in separated ion quadruple formulations with noncoordinating cations. Quantum chemical calculations describe dominant ionic An-Sb interactions in 3Th and 3U, though the data suggest that the latter exhibits slightly more covalent An-Sb linkages than the former. Complexes 3Th and 3U have been characterized by single crystal X-ray diffraction, NMR, IR, and UV/vis/NIR spectroscopies, elemental analysis, and quantum chemical calculations.

Thorium(IV)-antimony complexes exhibiting single, double, and triple polar covalent metal-metal bonds

There is continued burgeoning interest in metal-metal multiple bonding to further our understanding of chemical bonding across the periodic table. However, although polar covalent metal-metal multiple bonding is well known for the d and p blocks, it is relatively underdeveloped for actinides. Homometallic examples are found in spectroscopic or fullerene-confined species, and heterometallic variants exhibiting a polar covalent σ bond supplemented by up to two dative π bonds are more prevalent. Hence, securing polar covalent actinide double and triple metal-metal bonds under normal experimental conditions has been a fundamental target. Here we exploit the protonolysis and dehydrocoupling chemistry of the parent dihydrogen-antimonide anion, to report one-, two- and three-fold thorium-antimony bonds, thus introducing polar covalent actinide-metal multiple bonding under normal experimental conditions between some of the heaviest ions in the periodic table with little or no bulky-substituent protection at the antimony centre. This provides fundamental insights into heavy element multiple bonding, in particular the tension between orbital-energy-driven and overlap-driven covalency for the actinides in a relativistic regime.

f-Element heavy pnictogen chemistry

The coordination and organometallic chemistry of the f-elements, that is group 3, lanthanide, and actinide ions, supported by nitrogen ligands, e.g. amides, imides, and nitrides, has become well developed over many decades. In contrast, the corresponding f-element chemisty with the heavier pnictogen analogues phosphorus, arsenic, antimony, and bismuth has remained significantly underdeveloped, due largely to a lack of suitable synthetic methodologies and also the inherent hard(f-element)-soft(heavier pnictogen) acid-base mismatch, but has begun to flourish in recent years. Here, we review complexes containing chemical bonds between the f-elements and heavy pnictogens from phosphorus to bismuth that spans five decades of endeavour. We focus on complexes whose identity has been unambiguously established by structural authentication by single-crystal X-ray diffraction with respect to their synthesis, characterisation, bonding, and reactivity, in order to provide a representative overview of this burgeoning area. By highlighting that much has been achieved but that there is still much to do this review aims to inspire, focus and guide future efforts in this area.

Divalent metallocenes of the lanthanides - a guideline to properties and reactivity

Since the discovery in the early 1980s, the soluble divalent metallocenes of lanthanides have become a steadily growing field in organometallic chemistry. The predominant part of the investigation has been performed with samarium, europium, and ytterbium, whereas only a few reports dealing with other rare earth elements were disclosed. Reactions of these metallocenes can be divided into two major categories: (1) formation of Lewis acid-base complexes, in which the oxidation state remains +II; and (2) single electron transfer (SET) reductions with the ultimate formation of Ln(III) complexes. Due to the increasing reducing character from Eu(II) over Yb(II) to Sm(II), the plethora of literature concerning redox reactions revolves around the metallocenes of Sm and Yb. In addition, a few reactivity studies on Nd(II), Dy(II) and mainly Tm(II) metallocenes were published. These compounds are even stronger reducing agents but significantly more difficult to handle. In most cases, the metals are ligated by the versatile pentamethylcyclopentadienyl ligand: (C₅Me₅). Other cyclopentadienyl ligands are fully covered but only discussed in detail, if the ligand causes differences in synthesis or reactivity. Thus, the focus lays on three compounds: [(C₅Me₅)₂Sm], [(C₅Me₅)₂Eu] and [(C₅Me₅)₂Yb] and their solvates. We discuss the synthesis and physical properties of divalent lanthanide metallocenes first, followed by an overview of the reactivity rendering the full potential of these versatile reactants.

Formation of a cyclooctatetraenylsamarium(III) inverse sandwich that ring-opens tetrahydrofuran

Reductive trapping of the cyclooctatetraenyl dianion (COT^2-) by treatment of [Sm(DippForm)₂(thf)₂] (DippFormH = N,N'-bis(2,6-diisopropylphenyl)formamidine; thf = tetrahydrofuran) with 1,3,5,7-cyclooctatetraene (C₈H₈) in toluene yields an inverse sandwich dinuclear complex [Sm₂(DippForm)₄(COT)] (1), but [Sm(DippForm)(COT)(thf)₂] (2) and [Sm(DippForm)₂(O-C₄H₈-DippForm)(thf)] (3) in thf, and 1 yields 2 and 3 on treatment with thf.

Tetramethylcyclopentadienyl Samarium(II) Metallocene Chemistry: Isolation of a Bimetallic Sm(II)/Sm(II) Complex

The salt metathesis reaction between one equivalent of SmI2(THF)2 and two equivalents of K(C5Me4H) in THF afforded single crystals of the unusual, toluene-soluble, and asymmetric bimetallic Sm(II)/Sm(II) complex, (C5Me4H)2SmII(μ-η3:η5-C5Me4H)SmII(C5Me4H)(THF)2, instead of the expected product, (C5Me4H)2SmII(THF)2. The toluene-insoluble products of this reaction can be worked up in 1,2-dimethoxyethane (DME) to provide X-ray quality crystals of the monomeric Sm(II) metallocene, (C5Me4H)2SmII(DME). (C5Me4H)2SmII(DME) can also be synthesized directly by the reaction between one equivalent of SmI2(THF)2 and two equivalents of K(C5Me4H) in neat DME. The isolation and characterization of the bimetallic Sm(II)/Sm(II) complex provides supporting evidence for the possible oligomerization that may occur during the synthesis of Sm(II) complexes with cyclopentadienyl ligands that are less sterically bulky and less solubilizing than (C5Me5)1−.

Reactive Solubilization of Heterometallic Clusters by Treatment of (TrBi3 )2- Anions (Tr=Ga, In, Tl) with [Mn{N(SiMe3 )2 }2 ]

Lowering the charge of Zintl anions by (element-)organic substituents allows their use as sources of (semi)metal nanostructures in common organic solvents, as realized for group 15 anions or Ge₉ ^4- and Sn₉ ^4- . We developed a new strategy for other anions, using low-coordinate 3d metal complexes as electrophiles. [K(crypt-222)]⁺ salts of (TrBi₃ )^2- anions dissolved in situ in Et₂ O and/or THF when reacted with [Mn(hmds)₂ ]. Work-up afforded soluble [K(crypt-222)]⁺ salts of [{(hmds)₂ Mn}₂ (TlBi₃ )]^2- (in 1), [{(hmds)₂ Mn}₂ (Bi₂ )]^2- (in 2), and [{(hmds)Mn}₄ (Bi₂ )₂ ]^2- (in 3) (crypt-222=4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane; Tr=Ga, In, Tl; hmds=N(SiMe₃ )₂ ), representing rare cases of Zintl clusters with open-shell metal atoms. 1 comprises the first coordination compound of the (TlBi₃ )^2- anion, 2 features a diamond-shaped {Pn₂ M₂ } unit, and 3 is a mixed-valent Mn^I /Mn^II compound. The uncommon electronic structures in 1-3 and magnetic coupling were studied by comprehensive DFT calculations.

Synthesis of polyantimony ligand complexes starting from Cp*4Sb4

The reactivity of Cp*₄Sb₄ (1) towards ionic compounds and transition metal complexes with labile ligands was investigated in order to synthesize polyantimony ligand complexes. The silver salts [Ag][X] and the metalate Na[Cp*Mo(CO)₃] were primarily used, leading in the raction with Cp*₄Sb₄ to the formation of [Cp*₂Sb][X] (X = TEF (2a), FAL (2b)), [(Cp*Mo(CO)₃)₃(μ₃-Sb₃)] (3) and [Cp*Mo(CO)₂(η³-Sb₃)] (4), respectively. The reaction of 1 with the transition metal complexes [(Cp'''M)₂(tol)] leads to a degradation of the original Sb₄ unit and to the formation of [(Cp'''M)₄(μ₃-Sb)₄] (M = Ni (5); Co (6)). Towards [Cp^RFe(CO)₂]₂, substitutions on the antimony atoms were observed to give [{Cp^RFe(CO)₂}₄(μ₄-Sb₄)] (Cp^R = Cp'' (7a), Cp''' (7b)). All complexes were characterized by XRD and spectroscopic methods.

Reductively disilylated N-heterocycles as versatile organosilicon reagents

The reductively disilylated N-heterocyclic systems 1,4-bis(trimethylsilyl)-1-aza-2,5-cyclohexadiene (1Si), 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (2Si) and its methyl derivatives (3Si and 4Si), and 1,1'-bis(trimethylsilyl)-4,4'-bipyridinylidene (5Si) are proficient organosilicon reagents owing to their low first vertical ionization potentials and the heterophilicity of the polarized N-Si bonds. These have prompted their reactivity as two-electron reductants or reductive silylations. These reactions benefit from the concomitant rearomatization of the N-heterocycles and liberation of trimethylsilyl halides or (Me₃Si)₂O, which are mostly volatile or easily removable byproducts. In this perspective, we have discussed the utilization of these reductively disilylated N-heterocyclic systems as versatile reagents in the salt-free reduction of transition metals (A) and main-group halides (B), in organic transformations (C) and in materials syntheses (D).

Transfer of polyantimony units

Transfer reagents are useful tools in chemistry to access metastable compounds. The reaction of [Cp′′₂ZrCl₂] with KSb(SiMe₃)₂ leads to the formation of the novel polyantimony triple decker complex [(Cp′′Zr)₂(μ,η^1:1:1:1:1:1-Sb₆)] (1, Cp′′ = 1,3-di-tertbutyl-cyclopentadienyl), containing a chair-like Sb₆⁶⁻ ligand. Compound 1 represents a valuable transfer reagent to form novel antimony ligand complexes. Thus, the reaction of 1 with Cp^R-substituted transition metal halides of nickel, cobalt and iron leads to the formation of a variety of novel Sb_n ligand complexes, such as the cubane-like compounds [(Cp′′′Ni)₄(μ₃-Sb)₄] (2) and [(Cp′′′Co)₄(μ₃-Sb₄)] (3a) or the complexes [(Cp^BnCo)₃(μ₃-Sb)₂] (4) and [(Cp′′′Fe)₃(μ₃-Sb)₂] (5), representing a trigonal bipyramidal structure. Moreover, beside the transfer of Sb₁ units, also the complete entity can be transferred as seen in the iron complex [(Cp′′′Fe)₃(μ_3,η^4:4:4-Sb₆)] (6). DFT calculations shed light on the bonding situation of the products.

The synthesis and characterization of the unique polyantimony complex [(Cp′′Zr)₂(μ,η^1:1:1:1:1:1-Sb₆)] (1) is described. Compound 1 was used as antimony source to transfer Sb_n units to late transition metal fragments [Cp^RM] (M = Fe, Co, Ni).

Reactivity of mono- and divalent aluminium compounds towards group 15 nanoparticles

Herein, we present a novel approach towards organometallic group 13/15-compounds, i.e. the reaction of nanoparticular arsenic and antimony with low-valent aluminium species. The reaction of the two-electron reducing agent [AlCp*]₄ (Cp* = C₅Me₅) with arsenic nanoparticles gave rise to a mixture of two unprecedented deca- and dodecanuclear Al-As clusters. In contrast, the analogous transformation with nanoparticular antimony yielded the already known Al-Sb compound [(AlCp*)₃Sb₂]. Additionally, two different dialanes [AlCp*X]₂ (X = Br, I) were employed as one-electron reducing agents, forming calix like coordination compounds upon reaction with nano arsenic. The isolated species significantly enlarge the accessible structural variety of molecular group 13/15 compounds, highlighting the exceptional utility and reactivity of nanoscale group 15 precursors.

(thf)2Ln(Ge9{Si(SiMe3)3}3)2 (Ln = Eu, Sm): the first coordination of metalloid germanium clusters to lanthanides

We report the synthesis, structure and magnetic properties of the first rare earth complexes of metalloid group 14 clusters [(thf)2Ln(Ge9Hyp3)2] (Ln = Eu, Sm, Hyp = Si(SiMe3)3). X-Ray crystallographic analysis and DFT calculations reveal a novel η2-coordination mode of the Ge9Hyp3 units and a slight distortion of the Ge9 cage.

d/f-Polypnictides Derived by Non-Classical Ln2+ Compounds: Synthesis, Small Molecule Activation and Optical Properties

Reduction chemistry induced by divalent lanthanides has been primarily focused on samarium so far. In light of the rich physical properties of the lanthanides, this limitation to one element is a drawback. Since molecular divalent compounds of almost all lanthanides have been available for some time, we used one known and two new non‐classical reducing agents of the early lanthanides to establish a sophisticated reduction chemistry. As a result, six new d/f‐polyphosphides or d/f‐polyarsenides, [K(18‐crown‐6)] [Cp′′₂Ln(E₅)FeCp*] (Ln=La, Ce, Nd; E=P, As) were obtained. Their reactivity was studied by activation of P₄, resulting in a selective expansion of the P₅ rings. The obtained compounds [K(18‐crown‐6)] [Cp′′₂Ln(P₇)FeCp*] (Ln=La, Nd) are the first examples of an activation of P₄ by a f‐element‐polypnictide complex. Additionally, the first systematic femtosecond (fs)‐spectroscopy investigations of d/f‐polypnictides are presented to showcase the advantages of having access to a broader series of lanthanide compounds.

Synthesis of Unprecedented 4d/4f-Polypnictogens

A series of 4d/4f-polyarsenides, -polyarsines and -polystibines was obtained by reduction of the Mo-pnictide precursor complexes [{Cpt Mo(CO)2 }2 (μ,η2:2 -E2 )] (E=As, Sb; Cpt =tBu substituted cyclopentadienyl) with two different divalent samarocenes [Cp*2 Sm] and [(CpMe4nPr )2 Sm]. For the reductive conversion of the Mo-stibide only one product was isolated, featuring a planar tetrastibacyclobutadiene moiety as an unprecedented ligand for organometallic compounds. For the corresponding Mo-arsenide a tetraarsacyclobutadiene and a second species with a side-on coordinated As2 2- anion was isolated. The latter can be considered as reaction intermediate for the formation of the tetraarsacyclobutadiene.

Unprecedented icosahedral clusters built of polyantimony: from single [Ni0.5@{Sb6Ni6(CO)8}]4− and [Ni@{Sb7Ni5(CO)6}]3− to the Sb84−-linked dimer [(Sb8){Sb7Ni5(CO)4}2]6−

First icosahedral clusters built of polyantimony were prepared from the reactions of K2ZnSb and Ni(CO)2(PPh3)2. Sb84−-linked dimer provides a new way of thinking for the synthesis of hybrid clusters datively coordinated by polydentate Zintl anions.

Bismuthanylstibanes

Thermally-robust bismuthanylstibanes are prepared in a one-step, high yield reaction, providing the first examples of neutral Bi-Sb σ-bonds in the solid state. DFT calculations indicate that the bis(silylamino)naphthalene scaffold is well-suited for supporting otherwise labile bonds. The reaction chemistry of the Bi-Sb bond is debuted by showing fission using NH₃BH₃ and insertion of a sulfur atom, the latter providing the first example of a Bi-S-Sb motif.

Iron β-diiminate complexes with As2-, As4- and As8-ligands

Depending on the sterics of the flanking group at the β-diiminate ligand at Fe(i) a tetramerisation or dimerisation of As₄ units is observed; for the latter case the ligand conformation was influenced by the crystallization temperature.

Frontiers in the solution-phase chemistry of homoatomic group 15 Zintl clusters

Recent developments in the solution-phase chemistry of polypnictogen Zintl cluster are discussed, including the preparation of new clusters, wet synthetic methods, and their subsequent small molecule activations.

3d–4f heterometallic complexes by the reduction of transition metal carbonyls with bulky LnII amidinates

Heterometallic lanthanide-transition metal carbonyl complexes [Sm₂–Co₂], [Yb–Co], and [Sm₂–Fe₃] have been synthesized by redox reactions between bulky amidinate stabilized divalent Ln and TM carbonyl complexes.

Dysprosium complexes bearing unsupported DyIII-GeII/SnII metal-metal bonds as single-ion magnets

Two dysprosium complexes bearing unsupported Dy-Ge/Sn metal-metal bonds are reported here, wherein the Dy-Ge and Dy-Sn bonds both contain relatively large covalency. The complexes exhibit slow relaxation of magnetization at zero field with energy barriers of 485 and 620 K, respectively, and the blocking temperature of 6 K.

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.

Main Group Chemistry at the Interface with Molecular Magnetism

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.

Salt-Free Reduction of Transition Metal Complexes by Bis(trimethylsilyl)cyclohexadiene, -dihydropyrazine, and -4,4'-bipyridinylidene Derivatives

Chemical reduction of transition metals provides the corresponding low-valent transition metal species as a key step for generating catalytically active species in metal-assisted organic transformations and is a fundamental unit reaction for preparing organometallic complexes. A variety of metal-based reductants, such as metal powders and organometallic reagents of alkali and alkaline-earth metals, have been developed to date to access low-valent metal species. During the reduction, however, reductant-derived metal salts are formed as reaction waste, some of which often interact with the reactive low-valent metal center, thereby disrupting the catalytic performance and hampering the isolation of organometallic complexes as a result of salt coordination to the coordinatively unsaturated vacant and active sites and the formation of thermally unstable ate complexes. In this Account, we emphasize the synthetic utility and versatility of organic reductants containing two trimethylsilyl groups, i.e., 1,4-bis(trimethylsilyl)cyclohexa-2,5-diene (1a) and its methyl derivative (1b), 1,4-bis(trimethylsilyl)dihydropyrazine (2a) and its dimethyl (2b) and tetramethyl (2c) derivatives, and 1,1'-bis(trimethylsilyl)-4,4'-bipyridinylidene (3), leading to the reduction of various kinds of metal compounds in a salt-free fashion by release of two electrons together with the coproduction of easily removable (hetero)aromatics and trimethylsilyl derivatives from these organic reductants 1-3. When homoleptic chlorides of group 5 and 6 metals are treated with 1a and 1b, in situ-generated highly reactive low-valent metal species react with redox-active molecules such as ethylene, α-diimines, and α-diketones to produce metallacyclopentane, (ene-diamido)metal, and (ene-diolato)metal complexes, respectively. The advantage of the salt-free protocol is further exemplified in the low-valent titanocene-catalyzed Reformatsky-type reaction when 2c is used as a reductant: the yield of the product using the organosilicon reductant is higher than that when manganese powder is used as the reductant for the catalytic Reformatsky-type reaction of ethyl 2-bromoisobutyrate and its derivatives with various aldehydes. Moreover, when halides, carboxylates, and acetylacetonate compounds of late transition metals and main-group elements are treated with the organosilicon reductant 2c, metal(0) particles are smoothly precipitated under mild conditions. Among them, metallic nickel(0) nanoparticles are applicable to reductive biaryl formation and reductive cross-coupling of aryl halides/aryl aldehydes. In addition, reduction of the heterogeneous catalysts on a solid supporting matrix was also achieved by this salt-free reduction method; volatile byproducts are easily removed from the catalyst surface without suppressing the catalytic performance. Thus, the salt-free reduction strategy is a very powerful synthetic method that can be extended to various metals throughout the periodic table.

Samarium Polyarsenides Derived from Nanoscale Arsenic

Zintl phases of arsenic and molecular compounds containing Zintl-type polyarsenide ions are of fundamental interest in basic and applied sciences. Unfortunately, the most obvious and reactive arsenic source for the preparation of defined molecular polyarsenide compounds, yellow arsenic As₄ , is very inconvenient to prepare and neither storable in pure form nor easy to handle. Herein, we present the synthesis and reactivity of elemental As⁰ nanoparticles (As⁰ _Nano , d=7.2±1.8 nm), which were successfully utilized as a reactive arsenic source in reductive f-element chemistry. Starting from [Cp*₂ Sm] (Cp*=η⁵ -C₅ Me₅ ), the samarium polyarsenide complexes [(Cp*₂ Sm)₂ (μ-η² :η² -As₂ )] and [(Cp*₂ Sm)₄ As₈ ] were obtained from As⁰ _nano , thereby generating the largest molecular polyarsenide of the f-elements and circumventing the use of As₄ in preparative chemistry.

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- , (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{η⁴ -C₄ (SiMe₃ )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.