Multinuclear rare-earth metal coordination clusters bridged by oxo/hydroxo units have emerged from being lab curiosities to become a readily accessible class of compounds. New synthetic strategies for accessing these compounds on reasonable scales have been established. Clusters of different sizes have been investigated in terms of their magnetic and photophysical properties. Some preliminary biological applications have also been established.

OH functionalized triarylphosphines were used to assemble zirconocene-based metalloligands with phosphine donor sites in varying positions. These complexes were subsequently treated with different gold precursors to obtain early–late heterometallic compounds in which the metal atoms exhibit different intermetallic distances. All compounds were fully investigated by spectroscopic techniques, photoluminescence measurements and single crystal X-ray diffraction. Quantum chemical calculations were also performed. Some compounds show bright emission even at room temperature with quantum yields of up to 19 % (excitation at 350 nm). Furthermore, the reactivity of dimethyl zirconocene derivatives towards gold complexes was investigated, revealing simultaneous ligand exchange and transmetallation reactions.

Reduction of [Cp*Fe(η⁵-As₅)] with [Cp′′₂Sm(thf)] (Cp′′=η⁵-1,3-(tBu)₂C₅H₃) under various conditions led to [(Cp′′₂Sm)(μ,η⁴:η⁴-As₄)(Cp*Fe)] and [(Cp′′₂Sm)₂As₇(Cp*Fe)]. Both compounds are the first polyarsenides of the rare-earth metals. [(Cp′′₂Sm)(μ,η⁴:η⁴-As₄)(Cp*Fe)] is also the first d/f-triple decker sandwich complex with a purely inorganic planar middle deck. The central As₄²⁻ unit is isolobal with the 6π-aromatic cyclobutadiene dianion (CH)₄²⁻. [(Cp′′₂Sm)₂As₇(Cp*Fe)] contains an As₇³⁻ cage, which has a norbornadiene-like structure with two short As−As bonds in the scaffold. DFT calculations confirm all the structural observations. The As−As bond order inside the cyclo As₄ ligand in [(Cp′′₂Sm)(μ,η⁴:η⁴-As₄)(Cp*Fe)] was estimated to be in between an As−As single bond and a formally aromatic As₄²⁻ system.

Die Reduktion von [Cp*Fe(η⁵-As₅)] mit [Cp′′₂Sm(thf)] (Cp′′=η⁵-1,3-(tBu)₂C₅H₃) führt unter verschiedenen Reaktionsbedingungen zu [(Cp′′₂Sm)(μ,η⁴:η⁴-As₄)(Cp*Fe)] und [(Cp′′₂Sm)₂As₇(Cp*Fe)]. Bei beiden Verbindungen handelt es sich um die ersten Polyarsenide der Seltenerdmetalle. [(Cp′′₂Sm)(μ,η⁴:η⁴-As₄)(Cp*Fe)] ist zudem der erste d/f-Tripeldeckersandwichkomplex mit einem planaren rein anorganischen Mitteldeck. Die zentrale As₄²⁻-Einheit ist isolobal zum 6π-aromatischen Cyclobutadiendianion (CH)₄²⁻. [(Cp′′₂Sm)₂As₇(Cp*Fe)] enthält einen As₇³⁻-Käfig, der eine norbornadienartige Struktur mit zwei kurzen As-As-Bindungen hat. DFT-Rechnungen bestätigen die strukturellen Beobachtungen. Abschätzungen ergaben, dass die As-As-Bindungsordnung innerhalb des cyclo-As₄-Liganden in [(Cp′′₂Sm)(μ,η⁴:η⁴-As₄)(Cp*Fe)] zwischen einer As-As-Einfachbindung und der eines formal aromatischen As₄²⁻-Systems liegt.

Diphenylphosphinoethyl-functionalised imidazolium salts and their silver–carbene complexes were used to synthesise a series of di- and trinuclear gold complexes through ligand exchange and transmetallation, respectively. Besides a few positively charged macrocyclic compounds with different anions (both with and without activation of the carbene function), we were able to obtain neutral polynuclear complexes by varying the gold precursor. The synthesised gold complexes show a variety of photophysical properties, including bright white photoluminescence at ambient temperature.

The cationic zinc triple-decker complex [Zn₂Cp*₃]⁺[BAr^F₄]⁻ (BAr^F₄=B(3,5-(CF₃)₂C₆H₃)₄) exhibits catalytic activity in intra- and intermolecular hydroamination reactions in the absence of a cocatalyst. These hydroaminations presumably proceed through the activation of the CC multiple bond of the alkene or alkyne by a highly electrophilic zinc species, which is formed upon elimination of the Cp* ligands. The reaction of [Zn₂Cp*₃]⁺[BAr^F₄]⁻ with phenylacetylene gives the hydrocarbonation product (Cp*)(Ph)CCH₂, which might be formed via a similar reaction pathway. Additionally, several other structurally well-defined cationic zinc organyls have been examined as precatalysts for intermolecular hydroamination reactions without the addition of a cocatalyst. These studies reveal that the highest activity is achieved in the absence of any donor ligands. The neutral complex [ZnCp^2S₂] (Cp^2S=C₅Me₄(CH₂)₂SMe) shows a remarkably high catalytic activity in the presence of a Brønsted acid.

A new family of pentadecanuclear coordination cluster compounds (from now on simply referred to as clusters) [{Ln₁₅(OH)₂₀(PepCO₂)₁₀(DBM)₁₀Cl}Cl₄] (PepCO₂=2-[{3-(((tert-butoxycarbonyl)amino)methyl)benzyl}amino]acetate, DBM=dibenzoylmethanide) with Ln=Y and Dy was obtained by using the cell-penetrating peptoid (CPPo) monomer PepCO₂H and dibenzoylmethane (DBMH) as supporting ligands. The combination of an inorganic cluster core with an organic cell-penetrating peptoid in the coordination sphere resulted in a core component {Ln₁₅(μ₃-OH)₂₀Cl}²⁴⁺ (Ln=Y, Dy), which consists of five vertex-sharing heterocubane {Ln₄(μ₃-OH)₄}⁸⁺ units that assemble to give a pentagonal cyclic structure with one Cl atom located in the middle of the pentagon. The solid-state structures of both clusters were established by single-crystal X-ray crystallography. MS (ESI) experiments suggest that the cluster core is robust and maintained in solution. Pulsed gradient spin echo (PGSE) NMR diffusion measurements were carried out on the diamagnetic yttrium compound and confirmed the stability of the cluster in its dicationic form [{Y₁₅(μ₃-OH)₂₀(PepCO₂)₁₀(DBM)₁₀Cl}Cl₂]²⁺. The investigation of both static (dc) and dynamic (ac) magnetic properties in the dysprosium cluster revealed a slow relaxation of magnetization, indicative of single-molecule magnet (SMM) behavior below 8 K. Furthermore, the χT product as a function of temperature for the dysprosium cluster gave evidence that this is a ferromagnetically coupled compound below 11 K.

Invited for the cover of this issue are the groups of Yanhua Lan, Peter W. Roesky, and Stefan Bräse at the Karlsruhe Institute of Technology, and Ignacio Fernández at the University of Almería. The image depicts seals on a North Sea beach. Seals are known to be very skilled at balancing and other tricks, whereas our skills include crystallizing new compounds and further examining their properties. Read the full text of the article at 10.1002/chem.201405569.

Ion-like ethylzinc(II) compounds with weakly coordinating aluminates [Al(OR^F)₄]⁻ and [(R^FO)₃Al-F-Al(OR^F)₃]⁻ (R^F=C(CF₃)₃) were synthesized in a one-pot reaction and fully characterized by single-crystal X-ray diffraction, NMR and vibrational spectroscopy, and by quantum chemical calculations. The catalytic activity of ion-like Et-Zn[Al(OR^F)₄] in intermolecular hydroamination and in the unusual double hydroamination of anilines and alkynes was investigated. Favorable performance was also found in comparison to the Et₂Zn/ [PhNMe₂H]⁺[B(C₆F₅)₄]⁻ system generated in situ at lower catalyst loadings of 2.5 mol %.

A platform technology for the creation of spatially resolved surfaces encoded with a monolayer consisting of different metal complexes was developed. The concept entails the light-triggered activation of a self- assembled monolayer (SAM) of UV-labile anchors, that is, phenacylsulfides, and the subsequent cycloaddition of selected diene-functionalized metal complexes at defined areas on the surface. The synthesis and characterization of the metal complexes for the UV-light assisted anchoring on the surface and a detailed study of a short-chain oligomer model system in solution confirm the high efficiency of the photoreaction. The hybrid materials obtained by this concept can potentially be utilized for the design of highly valuable catalytic or (opto-)electronic devices.

Transition-metal carbene complexes have been known for about 50 years and widely applied as reagents and catalysts in organic transformations. In contrast, the carbene chemistry of the rare-earth metals is much less developed, but has attracted the research interest in the recent years. In this field rare-earth-metal alkylidene, especially methylidene, compounds are an emerging class of compounds with a high synthetic potential for organometallic chemistry and maybe in the future also for organic chemistry.

Übergangsmetallcarbene sind bereits seit 50 Jahren bekannt und haben eine breite Anwendung als Reagentien und Katalysatoren in der organischen Synthese gefunden. Die Carbenchemie der Seltenerdmetalle ist dagegen noch recht wenig untersucht und erst in den letzten Jahren in den Fokus der Forschung gerückt. Vor allem Seltenerdmetallalkylidenverbindungen, insbesondere Methylidenkomplexe, sind eine aufstrebende Verbindungsklasse mit einem hohen Potential für die Organometallchemie und möglicherweise künftig für die organische Synthese.

The reaction of decamethylytterbocene [(η⁵-C₅Me₅)₂Yb(THF)₂] with SO₂ at low temperature gave two new compounds, namely, the Yb^III dithionite/sulfinate complex [{(η⁵-C₅Me₅)₂Yb(μ₃,1κ²O^1,3,2κ³O^2,2′,4-S₂O₄)}₂{(η⁵-C₅Me₅)Yb(μ,1κO,2κO′-C₅Me₅SO₂)}₂] (1) and the Yb^III dithionite complex [{(η⁵-C₅Me₅)₂Yb}₂(μ,1κ²O^1,3,2κ²O^2,4-S₂O₄)] (2). After extraction of 1, the mixture was heated to give the dinuclear tetrasulfinate complex [{(η⁵-C₅Me₅)Yb}₂(μ,κO,κO’-C₅Me₅SO₂)₄] (3 a). In contrast, from the reaction of [(η⁵-C₅Me₅)₂Eu(THF)₂] with SO₂ only the tetrasulfinate complex [{(η⁵-C₅Me₅)Eu}₂(μ,κO,κO’-C₅Me₅SO₂)₄] (3 b) was isolated. Two major reaction pathways were observed: 1) reductive coupling of two SO₂ molecules to form the dithionite anion S₂O₄²⁻; and 2) nucleophilic attack of one metallocene C₅Me₅ ligand on the sulfur atom of SO₂. The compounds presented are the first dithionite and sulfinate complexes of the f-elements.

Polymer diols are a class of polymeric building blocks of high interest for the synthesis of complex macromolecular edifices. Rare-earth borohydride complexes are known as efficient initiators for the ring-opening polymerization (ROP) of cyclic esters, directly affording α,ω-dihydroxy-telechelic polyesters. Here, were report the direct synthesis of poly(benzyl β-malolactonate) (PMLABe) diols, from the ROP of racemic (benzyl β-malolactonate) (rac-MLABe), a valuable and renewable monomer, initiated by the homoleptic [Ln(BH₄)₃(thf)₃] (Ln=La, Nd, and Sm) complexes. These initiators enabled the controlled ROP of this β-lactone, affording well-defined syndiotactic-enriched (P_r≈0.83) PMLABes (M_n up to 21 300 g mol⁻¹, Ð_M≈1.5) as evidenced by size exclusion chromatography, ¹H and ¹³C NMR spectroscopy, and MALDI-ToF mass spectrometry analyses. The first and second insertions of rac-MLABe, as assessed by DFT calculations, revealed more favorable stationary front-side than migratory back-side insertions, the thermodynamically and kinetically competitive ROP on two distinct arms with that on a one arm-only, and the thermodynamically slightly favored formation of syndiotactic-enriched PMLABes.

The (iminophosphoranyl)(thiophosphoranyl)methane zinc complexes [{(PPh₂=NSiMe₃)(PPh₂=S)CH₂}ZnX₂] (X = Cl, I) have been obtained by the reaction of {CH₂(PPh₂=NSiMe₃)(PPh₂=S)} with the corresponding zinc dihalides ZnCl₂ and ZnI₂. The (iminophosphoranyl)(thiophosphoranyl)methane ligand coordinates as a bidentate ligand through the nitrogen and sulfur atoms to the zinc atom to form a six-membered metallacycle (N–P–C–P–S–Zn). The reaction of {CH₂(PPh₂=NSiMe₃)(PPh₂=S)} with [Zn{N(SiMe₃)₂}₂] and ZnPh₂ resulted in the (iminophosphoranyl)(thiophosphoranyl)methanide complexes [{(PPh₂=NSiMe₃)(PPh₂=S)CH}Zn{N(SiMe₃)₂}] and [{(PPh₂=NSiMe₃)(PPh₂=S)CH}ZnPh], respectively. In addition to the coordination of the phosphinimine nitrogen and sulfur atoms to the zinc atom, a long contact between the methine carbon atom of the P–C–P bridge and the zinc atom was observed in these complexes. The solid-state structures of all the new compounds were established by single-crystal X-ray diffraction.

Treatment of N,N′-bis(4carboxysalicylidene)ethylenediamine (H₄L), with MnCl₂⋅(H₂O)₄, and Ln(NO₃)₃⋅(H₂O)_m (Ln=Nd, Eu, Gd, Dy, Tb), in the presence of N,N-dimethylformamide (DMF)/pyridine at elevated temperature resulted (after work up) in the formation of 1D coordination polymers {[Ln₂(MnLCl)₂(NO₃)₂(dmf)₅]⋅4 DMF}_n (1–5). In these coordination polymers the rare earth ions are connected through carboxylate groups from Mn–salen units in a 1D chain structure. Thus, the Mn–salen complex acts as a “metalloligand” with open coordination sites. All compounds were used as catalysts in the liquid-phase epoxidation of trans-stilbene with molecular oxygen, which resulted in the formation of stilbene oxide. Since the choice of the lanthanide had virtually no influence on the performance of the catalyst, only the manganese–gadolinium was studied in detail. The influence of solvent, catalyst concentration, reaction temperature, oxidant, and oxidant flow rate on conversion, yield, and selectivity was analyzed. A conversion of up to 70 %, the formation of 61 % stilbene oxide (88 % selectivity), and a TON of 84 were observed after 24 h. A hot filtration test confirmed that the reaction is mainly catalyzed through a heterogeneous pathway, although a minor contribution of homogeneous species could not be completely excluded. The catalyst could be reused without significant loss of activity.

Treatment of (η⁶-benzoic acid)tricarbonylchromium withlead(II) acetate in the presence of trans-1,2-bis(4-pyridyl)ethene (bpe) afforded the one-dimensional coordination polymer [Pb{(η⁶-C₆H₅COO)Cr(CO)₃}₂bpe]_n (1). In contrast, the reaction of (η⁶-benzoic acid)tricarbonylchromium with lead(II) acetate in the presence of 1,10-phenanthroline (1,10-phen) or 2,2′-bipyridine (2,2′-bipy) gave the lead(II) compounds [Pb{(η⁶-C₆H₅COO)Cr(CO)₃}₂(1,10-phen)] (2) and [Pb{(η⁶-C₆H₅COO)Cr(CO)₃}₂(2,2′-bipy)] (3). The solid-state structures of all compounds were determined by single-crystal X-ray diffraction.

We have prepared digold(I) complexes with the rigid-backbone diphosphane ligands PhanePhos, xyl-PhanePhos, and Ph₂-GemPhos. All complexes were characterized by single-crystal X-ray diffraction, NMR, IR, Raman, and photoluminescence (PL) spectroscopy. [PhanePhos(AuCl)₂] and [GemPhos(AuCl)₂] show a very similar ligand scaffold, but different (aurophilic vs. nonaurophilic) intramolecular Au–Au distances. Absorption and PL spectra of both compounds are quite similar. Theoretical investigations reveal that the excited states are of different character (i.e., influenced by the Au–Au contacts). The respective transition energies, however, lie close to each other, thus resulting in similar experimental spectra. The gold atoms appear to produce a significant heavy-atom effect in the electronic relaxation dynamics. Thus, [PhanePhos(AuCl)₂] and [GemPhos(AuCl)₂] both show bright-green millisecond-long phosphorescence at low temperatures, although the latter is quenched at ambient temperature. [PhanePhos(AuCl)₂] and [GemPhos(AuCl)₂] were also used as catalysts in the intra- and intermolecular hydroamination of alkynes. Good to quantitative conversions on a reasonable time scale were observed. The overall performance of these catalysts in the studied reactions was similar, showing that Au–Au contacts do not have a major influence on the catalytic performance.

The treatment of the recently reported potassium salt (S)-N,N′-bis-(1-phenylethyl)benzamidinate ((S)-KPEBA) and its racemic isomer (rac-KPEBA) with anhydrous lanthanide trichlorides (Ln=Sm, Er, Yb, Lu) afforded mostly chiral complexes. The tris(amidinate) complex [{(S)-PEBA}₃Sm], bis(amidinate) complexes [{Ln(PEBA)₂(μ-Cl)}₂] (Ln=Sm, Er, Yb, Lu), and mono(amidinate) compounds [Ln(PEBA)(Cl)₂(thf)_n] (Ln=Sm, Yb, Lu) were isolated and structurally characterized. As a result of steric effects, the homoleptic 3:1 complexes of the smaller lanthanide atoms Yb and Lu were not accessible. Furthermore, chiral bis(amidinate)–amido complexes [{(S)-PEBA}₂Ln{N(SiMe₃)₂}] (Ln=Y, Lu) were synthesized by an amine-elimination reaction and salt metathesis. All of these chiral bis- and tris(amidinate) complexes had additional axial chirality and they all crystallized as diastereomerically pure compounds. By using rac-PEBA as a ligand, an achiral meso arrangement of the ligands was observed. The catalytic activities and enantioselectivities of [{(S)-PEBA}₂Ln{N(SiMe₃)₂}] (Ln=Y, Lu) were investigated in hydroamination/cyclization reactions. A clear dependence of the rate of reaction and enantioselectivity on the ionic radius was observed, which showed higher reaction rates but poorer enantioselectivities for the yttrium compound.

2D ¹H,⁸⁹Y heteronuclear shift correlation through scalar coupling has been applied to the chemical-shift determination of a set of yttrium complexes with various nuclearities. This method allowed the determination of ⁸⁹Y NMR data in a short period of time. Multinuclear NMR spectroscopy as function of temperature, PGSE NMR-diffusion experiments, heteronuclear NOE measurements, and X-ray crystallography were applied to determine the structures of [Y₅(OH)₅(L-Val)₄(Ph₂acac)₆] (1) (Ph₂acac=dibenzoylmethanide, L-Val=L-valine), [Y(2)(OTf)₃] (3), and [Y₂(4)(OTf)₅] (5) (2: [(S)P{N(Me)NC(H)Py}₃], 4: [B{N(Me)NC(H)Py}₄]⁻) in solution and in the solid state. The structures found in the solid state are retained in solution, where averaged structures were observed. NMR diffusion measurements helped us to understand the nuclearity of compounds 3 and 5 in solution. ¹H,¹⁹F HOESY and ¹⁹F,¹⁹F EXSY data revealed that the anions are specifically located in particular regions of space, which nicely correlated with the geometries found in the X-ray structures.

The reaction of (benzoic acid)tricarbonylchromium [(η⁶-C₆H₅COOH)Cr(CO)₃] with cadmium acetate in methanol or DMF led to the formation of the dinuclear cadmium compounds [Cd₂{(η⁶-C₆H₅COO)Cr(CO)₃}₄(MeOH)₄] (1) or [Cd₂{(η⁶-C₆H₅COO)Cr(CO)₃}₄(DMF)₄] (2) containing four organometallic ligands with four methanol or DMF molecules coordinated to the cadmium atoms. The reaction of (benzoic acid)tricarbonylchromium [(η⁶-C₆H₅COOH)Cr(CO)₃] and cadmium acetate with 1,10-phenanthroline (1,10-phen) afforded the dinuclear cadmium complex [Cd₂{(η⁶-C₆H₅COO)Cr(CO)₃}₄(1,10-phen)₂] (3) with two 1,10-phenanthroline and four organometallic ligands. The reaction of (benzoic acid)tricarbonylchromium [(η⁶-C₆H₅COOH)Cr(CO)₃] with cadmium acetate in the presence of 4,4′-bipyridine (4,4′-bipy) led to the one-dimensional coordination polymer [Cd₂{(η⁶-C₆H₅COO)Cr(CO)₃}₄(4,4′-bipy)₂·3DMF]_n (4) containing a dinuclear cadmium complex as a repeating unit. The solid-state structures of all the compounds were determined by single-crystal X-ray diffraction.

Grafting of the bis(phosphinimino)methanide complex [La{CH(PPh₂NSiMe₃)₂}{N(SiHMe₂)₂}₂] (1) onto dehydroxylated silica (SiO₂-700) affords material 2. Full characterization of this material by infrared spectroscopy, elemental analysis, and multinuclear, multidimensional solid-state NMR spectroscopy showed formation of well-defined bis(phosphinimino)methanide amido surface species, [(≡SiO)La{CH(PPh₂NSiMe₃)₂}{N(SiHMe₂)₂}], along with [≡SiOSiHMe₂] sites.

The four different [(η⁶-arene)chromium tricarbonyl] complexes [(η⁶-C₆H₅CH₂COOH) Cr(CO)₃], [{η⁶-(1,2-C₆H₄(NH₂)COOEt)}Cr(CO)₃], [η⁶ -{2,6-C₆H₄(i-Pr)₂NH₂}Cr(CO)₃], and [{η⁶ -(1,4-C₆H₄(NH₂)CH₃)}Cr(CO)₃] were synthesized and structurally characterized by single crystal X-ray diffraction. All compounds form hydrogen bonds in the solid state, leading to dimeric or polymeric networks. The hydrogen bonds are an essential part of the network. The connectivity is influenced by the groups on which the protons are bound (COOH vs. NH₂) and by the steric demand of the ligand. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:294–300, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20679

The first zinc-catalyzed one-step synthesis of quaternary propargylamines with four different substituents is described. The domino hydroamination–alkyne addition reaction gives access to functionalized propargylamines under mild conditions.

A series of electronically and sterically modified β-diketiminate methylzinc complexes [{N,N′-bis(2,6-diisopropylphenyl)-β-diketiminato}methylzinc] (1), [{N,N′-bis(2-isopropylphenyl)-β-diketiminato}methylzinc] (2), [{N,N′-bis(2,4,6-trimethylphenyl)-β-diketiminato}methylzinc] (3), [{N,N′-bis(4-methoxyphenyl)-β-diketiminato}methylzinc] (4), a bis(β-diketiminato)zinc compound [bis{N,N′-bis(4-methoxyphenyl)-β-diketiminato}zinc] (5), and a β-diketiminate bis(trimethylsilylamido)zinc complex [{2-(1-cyclohexylimidazolidine-2-ylidene)-1-methylethylidene-(2,6-diisopropylphenyl)aminato}bis(trimethysilylamido)zinc] (6) were prepared and structurally characterized by single-crystal X-ray diffraction. All methylzinc compounds exhibit a trigonal-planar coordinated zinc atom, with the exception of polymeric compound 4, in which intermolecular coordination of the OMe group to the zinc atom additionally takes place. All zinc complexes were investigated as catalysts in the intramolecular hydroamination of nonactivated alkynes and show high catalytic activity in the presence of the cocatalyst [PhNMe₂H][SO₃CF₃]. The zinc complex 1 shows the highest catalytic activityin most of the cyclization reactions. In this context, it waspossible to isolate the intermediate, which was formed bythe reaction of the precatalyst 1 and the cocatalyst [PhNMe₂H][SO₃CF₃]. The resulting zinc catalyst [{N,N′-bis(2,6-diisopropylphenyl)-β-diketiminato}(trifluoromethanesulfonate)zinc]·thf (7) was structurally characterized by single-crystal X-ray diffraction. The zinc complex 7 show the same catalytic activity in the hydroamination when an equimolar mixture of the precatalyst 1 and the cocatalyst was used. It is supposed that the high steric bulk of the precatalyst 1 protects the Lewis acid zinc center from polar functional groups but allows interactions for the cyclization process.

Rare-earth-metal borohydrides are known to be efficient catalysts for the polymerization of apolar and polar monomers. The bis-borohydrides [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln=Y, Lu) have been synthesized by two different synthetic routes. The lanthanum and the lutetium complexes were prepared from [Ln(BH₄)₃(THF)₃] and K{CH(PPh₂NSiMe₃)₂}, whereas the yttrium analogue was obtained from in situ prepared [{CH(PPh₂NSiMe₃)₂}YCl₂]₂ and NaBH₄. All new compounds were characterized by standard analytical/spectroscopic techniques, and the solid-state structures were established by single-crystal X-ray diffraction. The ring-opening polymerization (ROP) of ε-caprolactone initiated by [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln=Y, Lu) was studied. At 0 °C the molar mass distributions determined were the narrowest values (M̄_w/M̄_n=1.06–1.11) ever obtained for the ROP of ε-caprolactone initiated by rare-earth-metal borohydride species. DFT investigations of the reaction mechanism indicate that this type of complex reacts in an unprecedented manner with the first BH activation being achieved within two steps. This particularity has been attributed to the metallic fragment based on the natural bond order analysis.

The reaction of potassium 2,5-bis[N-(2,6-diisopropylphenyl)iminomethyl]pyrrolyl [(dip₂-pyr)K] with the borohydrides of the larger rare-earth metals, [Ln(BH₄)₃(thf)₃] (Ln=La, Nd), afforded the expected products [Ln(BH₄)₂(dip₂-pyr)(thf)₂]. As usual, the trisborohydrides reacted like pseudohalide compounds forming KBH₄ as a by-product. To compare the reactivity with the analogous halides, the dimeric neodymium complex [NdCl₂(dip₂-pyr)(thf)]₂ was prepared by reaction of [(dip₂-pyr)K] with anhydrous NdCl₃. Reaction of [(dip₂-pyr)K] with the borohydrides of the smaller rare-earth metals, [Sc(BH₄)₃(thf)₂] and [Lu(BH₄)₃(thf)₃], resulted in a redox reaction of the BH₄⁻ group with one of the Schiff base functions of the ligand. In the resulting products, [Ln(BH₄){(dip)(dip-BH₃)-pyr}(thf)₂] (Ln=Sc, Lu), a dinegatively charged ligand with a new amido function, a Schiff base, and the pyrrolyl function is bound to the metal atom. The by-product of the reaction of the BH₄⁻ anion with the Schiff base function (a BH₃ molecule) is trapped in a unique reaction mode in the coordination sphere of the metal complex. The BH₃ molecule coordinates in an η² fashion to the metal atom. The rare-earth-metal atoms are surrounded by the η²-coordinated BH₃ molecule, the η³-coordinated BH₄⁻ anion, two THF molecules, and the nitrogen atoms from the Schiff base and the pyrrolyl function. All new compounds were characterized by single-crystal X-ray diffraction. Low-temperature X-ray diffraction data at 6 K were collected to locate the hydrogen atoms of [Lu(BH₄){(dip)(dip-BH₃)-pyr}(thf)₂]. The (DIP₂-pyr)⁻ borohydride and chloride complexes of neodymium, [Nd(BH₄)₂(dip₂-pyr)(thf)₂] and [NdCl₂(dip₂-pyr)(thf)]₂, were also used as Ziegler–Natta catalysts for the polymerization of 1,3-butadiene to yield poly(cis-1,4-butadiene). Very high activities and good cis selectivities were observed by using each of these complexes as a catalyst in the presence of various cocatalyst mixtures.