Highly basic alkyl-substituted bis(benzhydryl) CaII and YbII complexes with β-CH–M agostic interactions

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Hydrocarbon-soluble calcium hydride: a "worker-bee" in calcium chemistry

The reactivity of the hydrocarbon-soluble calcium hydride complex [{CaH(dipp-nacnac)(thf)}(2)] (1; dipp-nacnac=CH{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)) with a large variety of substrates has been investigated. Addition of 1 to C=O and C=N functionalities gave easy access to calcium alkoxide and amide complexes. Similarly, reduction of the C[triple chemical bond]N bond in a cyanide or an isocyanide resulted in the first calcium aldimide complexes [Ca{N=C(H)R}(dipp-nacnac)] and [Ca{C(H)=NR}(dipp-nacnac)], respectively. Complexation of 1 with borane or alane Lewis acids gave the borates and alanates as contact ion pairs. In reaction with epoxides, nucleophilic ring-opening is observed as the major reaction. The high reactivity of hydrocarbon-soluble 1 with most functional groups contrasts strongly with that of insoluble CaH(2), which is essentially inert and is used as a common drying agent. Crystal structures of the following products are presented: [{Ca{OC(H)Ph(2)}(dipp-nacnac)}(2)], [{Ca{N=C(H)Ph}(dipp-nacnac)}(2)], [{Ca{C(H)=NC(Me)(2)CH(2)C(Me)(3)}(dipp-nacnac)}(2)], [{Ca{C(H)=NCy}(dipp-nacnac)}(2)], [Ca(dipp-nacnac)(thf)](+)[H(2)BC(8)H(14)](-) and [{Ca(OCy)(dipp-nacnac)}(2)]. The generally smooth and clean conversions of 1 with a variety of substrates and the stability of most intermediates against ligand exchange make 1 a valuable key precursor in the syntheses of a wide variety of beta-diketiminate calcium complexes.     

Analogy of the coordination chemistry of alkaline earth metal and lanthanide Ln(2+) ions: the isostructural zoo of mixed metal cages [IM(OtBu)4{Li(thf)}4(OH)] (M=Ca, Sr, Ba, Eu), [MM'6(OPh)8(thf)6] (M=Ca, Sr, Ba, Sm, Eu, M'=Li, Na), and their derivatives with 1,2-dimethoxyethane

As previously shown, alkali and alkaline earth metal iodides in nonaqueous, aprotic solvents behave like transition metal halides, forming cis- and trans-dihalides with various neutral O-donor ligands. These compounds can be used as precursors for the synthesis of new mixed alkali/alkaline earth metal aggregates. We show here that Ln2+ ions form isostructural cluster compounds. Thus, with LiOtBu, 50% of the initial iodide can be replaced in MI2, M=Ca, Sr, Ba, Eu, to generate the mixed-metal alkoxide aggregates [IM(OtBu)4{Li(thf)}4(OH)], for which the M--OH contacts were investigated by theoretical methods. With M'OPh (M'=Li, Na), a new mixed-metal aryloxide cluster type [MM'6(OPh)8(thf)6] is obtained for M=Ca, Sr, Ba, Sm, Eu. Their stability versus DME (DME=1,2-dimethoxyethane) as bidentate ligand is studied.     

CaII,YbII and SmII Bis(Amido) Complexes Coordinated by NHC Ligands: Efficient Catalysts for Highly Regio- and Chemoselective Consecutive Hydrophosphinations with PH3

The first example of intermolecular hydrophosphination of styrene, 2-vinylpyridine and phenylacetylene with PH₃ catalyzed by bis-(amido) complexes [(Me₃Si)₂N]₂M(NHC)₂ (M=Ca, Yb, Sm) coordinated by NHC ligands is described. The reactions of styrene with PH₃ proceed under mild conditions in quantitative yields to afford only anti-Markovnikov product and allow for the chemoselective synthesis of primary, secondary and tertiary phosphines. Addition of phenylacetylene to PH₃ regardless the initial molar substrates ratio results in the exclusive formation of a tertiary tris-(Z-styryl)-phosphine. Crucial effect of the Lewis base coordinated to the metal ion in precatalyst on catalytic activity in styrene hydrophosphination with PH₃ was demonstrated. Free NHCs were also found to be able to promote addition of PH₃ to styrene, however they provide much lower reaction rates compared to the metal complexes.     

Application of the Redox-Transmetalation Procedure to Access Divalent Lanthanide and Alkaline-Earth NHC Complexes**

Divalent lanthanide and alkaline-earth complexes supported by N-heterocyclic carbene (NHC) ligands have been accessed by redox-transmetalation between air-stable NHC-AgI complexes and the corresponding metals. By using the small ligand 1,3-dimethylimidazol-2-ylidene (IMe), two series of isostructural complexes were obtained: the tetra-NHC complexes [LnI₂(IMe)₄] (Ln=Eu and Sm) and the bis-NHC complexes [MI₂(IMe)₂(THF)₂] (M=Yb, Ca and Sr). In the former, distortions in the NHC coordination were found to originate from intermolecular repulsions in the solid state. Application of the redox-transmetalation strategy with the bulkier 1,3-dimesitylimidazol-2-ylidene (IMes) ligand yielded [SrI₂(IMes)(THF)₃], while using a similar procedure with Ca metal led to [CaI₂(THF)₄] and uncoordinated IMes. DFT calculations were performed to rationalise the selective formation of the bis-NHC adduct in [SrI₂(IMe)₂(THF)₂] and the tetra-NHC adduct in [SmI₂(IMe)₄]. Since the results in the gas phase point towards preferential formation of the tetra-NHC complexes for both metal centres, the differences between both arrangements are a result of solid-state effects such as slightly different packing forces.     

Heavy alkaline earth metal pyrazolates: synthetic pathways, structural trends, and comparison with divalent lanthanoids

Two series of heavy alkaline earth metal pyrazolates, [M(Ph(2)pz)(2)(thf)(4)] 1 a-c (Ph(2)pz=3,5-diphenylpyrazolate, M=Ca, Sr, Ba; THF=tetrahydrofuran) and [M(Ph(2)pz)(2)(dme)(n)] (M=Ca, 2 a, Sr, 2 b, n=2; M=Ba, 2 c, n=3; DME=1,2-dimethoxyethane) have been prepared by redox transmetallation/ligand exchange utilizing Hg(C(6)F(5))(2). Compounds 1 a and 2 b were also obtained by redox transmetallation with Tl(Ph(2)pz). Alternatively, direct reaction of the alkaline earth metals with 3,5-diphenylpyrazole at elevated temperatures under solventless conditions yielded compounds 1 a-c and 2 a-c upon extraction with THF or DME. By contrast, [M(Me(2)pz)(2)(Me(2)pzH)(4)] 3 a-c (M=Ca, Sr, Ba; Me(2)pzH=3,5-dimethylpyrazole) were prepared by protolysis of [M[N(SiMe(3))(2)](2)(thf)(2)] (M=Ca, Sr, Ba) with Me(2)pzH in THF and by direct metallation with Me(2)pzH in liquid NH(3)/THF. Compounds 1 a-c and 2 a-c display eta(2)-bonded pyrazolate ligands, while 3 a,b exhibit eta(1)-coordination. Complexes 1 a-c have transoid Ph(2)pz ligands and an overall coordination number of eight with a switch from mutually coplanar Ph(2)pz ligands in 1 a,b to perpendicular in 1 c. In eight coordinate 2 a,b the pyrazolate ligands are cisoid, whilst 2 c has an additional DME ligand and a metal coordination number of ten. By contrast, 3 a,b have octahedral geometry with four eta(1)-Me(2)pzH donors, which are hydrogen-bonded to the uncoordinated nitrogen atoms of the two trans Me(2)pz ligands. The application of synthetic routes initially developed for the preparation of lanthanoid pyrazolates provides detailed insight into the similarities and differences between the two groups of metals and structures of their complexes.     

Surface organobarium and organomagnesium chemistry on periodic mesoporous silica MCM-41: convergent and sequential approaches traced by molecular models

The alkaline earth metal alkyl complexes [Ba(AlEt(4))(2)](n) and Mg(AlMe(4))(2) were directly grafted onto periodic mesoporous silica MCM-41, which had been dehydroxylated at 270 °C (specific surface area a(s): 1023 m(2) g(-1); pore volume V(p): 1.08 cm(3) g(-1); main pore diameter 3.4 nm). Alternatively, barium alkyl surface species were generated by sequential grafting of MCM-41 with Ba[N(SiHMe(2))(2)](2)(thf)(4) and AlEt(3) to yield the hybrid material AlEt(3)@Ba[N(SiHMe(2))(2)](2)(thf)(4)@MCM-41. For a better understanding of the surface chemistry, AlEt(3)@MCM-41 was also accessed. All hybrid materials were analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, elemental analysis, nitrogen physisorption, and solid-state NMR spectroscopy; this clearly revealed distinct surface chemistry for the alkylaluminate-treated materials [Ba(AlEt(4))(2)]@MCM-41 and Mg(AlMe(4))(2)@MCM-41. In an attempt to mimic the surface chemistry, the organometallic precursors were treated with HOSi(OtBu)(3). The reaction of equimolar amounts of {Ba[N(SiHMe(2))(2)](2)}(n) and HOSi(OtBu)(3) produced a mixed silylamido/siloxide cluster of Ba(3)[OSi(OtBu)(3)](3)[N(SiHMe(2))(2)](3) with bridging-only siloxide ligands as well as one bridging and two terminal silylamido ligands. The Schlenk equilibrium was found to govern the [Ba(AlEt(4))(2)](n)-HOSi(OtBu)(3) and Mg(AlMe(4))(2)-HOSi(OtBu)(3) reactions, leading to the isolation of complexes of [Ba(AlEt(4))(2) (toluene)](2) and Mg[OSi(OtBu)(3))](2)(AlMe(3))(2), respectively. Allowing for a donor-induced cleavage of Mg(AlMe(4))(2), the reaction of [MgMe(2)] with one or two equivalents of HOSi(OtBu)(3) was studied. While putative Mg[OSi(OtBu)(3)](Me) and Mg[OSi(OtBu)(3)](2) could not be crystallized from the reaction mixtures, cluster complexes Mg(5)(O)[OSi(OtBu)(3)](5)Me(3) and Mg(4)(OH)(2)[OSi(OtBu)(3)](6) could be unambiguously identified by X-ray crystallography.     

Heteroleptic Silylamido Phenolate Complexes of Calcium and the Larger Alkaline Earth Metals: β‐Agostic Ae⋅⋅⋅Si?H Stabilization and Activity in the Ring‐Opening Polymerization of L‐Lactide

The factors governing the stability and the reactivity towards cyclic esters of heteroleptic complexes of the large alkaline earth metals (Ae) have been probed. The synthesis and stability of a family of heteroleptic silylamido and alkoxide complexes of calcium [{LOⁱ}Ca? Nu(thf)_n] supported by mono‐anionic amino ether phenolate ligands (i=1, {LO¹}^?=4‐(tert‐butyl)‐2,6‐bis(morpholinomethyl)phenolate, Nu^?=N(SiMe₂H)₂^?, n=0, 4 ; i=2, {LO²}^?=2,4‐di‐tert‐butyl‐6‐{[2‐(methoxymethyl)pyrrolidin‐1‐yl]methyl}phenolate, Nu^?=N(SiMe₂H)₂^?, n=0, 5 ; i=4, {LO⁴}^?=2‐{[bis(2‐methoxyethyl)amino]methyl}‐4,6‐di‐tert‐butylphenolate, Nu^?=N(SiMe₂H)₂^?, n=1, 6 ; Nu^?=HC?CCH₂O^?, n=0, 7 ) and those of the related [{LO³}Ae? N(SiMe₂H)₂] ({LO³}^?=2‐[(1,4,7,10‐tetraoxa‐13‐azacyclopentadecan‐13‐yl)methyl]‐4,6‐di‐tert‐butylphenolate Ae=Ca, 1 ; Sr, 2 ; Ba, 3 ) have been investigated. The molecular structures of 1 , 2 , [( 4 )₂], 6 , and [( 7 )₂] have been determined by X‐ray diffraction. These highlight Ae???H? Si internal β‐agostic interactions, which play a key role in the stabilization of [{LOⁱ}Ae? N(SiMe₂H)₂] complexes against ligand redistribution reactions, in contrast to regular [{LOⁱ}Ae? N(SiMe₃)₂]. Pulse‐gradient spin‐echo (PGSE) NMR measurements showed that 1 , 4 , 6 , and 7 are monomeric in solution. Complexes 1 – 7 mediate the ring‐opening polymerization (ROP) of L ‐lactide highly efficiently, converting up to 5000 equivalents of monomer at 25 °C in a controlled fashion. In the immortal ROP performed with up to 100 equivalents of exogenous 9‐anthracenylmethanol or benzyl or propargyl alcohols as a transfer agent, the activity of the catalyst increased with the size of the metal ( 1 < 2 < 3 ). For Ca‐based complexes, the enhanced electron‐donating ability of the ancillary ligand favored catalyst activity ( 1 > 6 > 4 ≈ 5 ). The nature of the alcohol had little effect over the activity of the binary catalyst system 1 /ROH; in all cases, both the control and end‐group fidelity were excellent. In the living ROP of L ‐LA, the HC?CCH₂O^? initiating group (as in 7 ) proved superior to N(SiMe₂H)₂^? or N(SiMe₃)₂^? (as in 6 or [{LO⁴}Ca? N(SiMe₃)₂] ( B ), respectively).     

Monodisperse REPO4 (RE = Yb, Gd, Y) hollow microspheres covered with nanothorns as affinity probes for selectively capturing and labeling phosphopeptides

Rare-earth phosphate microspheres with unique structures were developed as affinity probes for the selective capture and tagging of phosphopeptides. Prickly REPO(4) (RE = Yb, Gd, Y) monodisperse microspheres, that have hollow structures, low densities, high specific surface areas, and large adsorptive capacities were prepared by an ion-exchange method. The elemental compositions and crystal structures of these affinity probes were confirmed by energy-dispersive spectroscopy (EDS), powder X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy. The morphologies of these compounds were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen-adsorption isotherms. The potential ability of these microspheres for selectively capturing and labeling target biological molecules was evaluated by using protein-digestion analysis and a real sample as well as by comparison with the widely used TiO(2) affinity microspheres. These results show that these porous rare-earth phosphate microspheres are highly promising probes for the rapid purification and recognition of phosphopeptides.     

Luminescent Nitridophosphates CaP2N4:Eu2+, SrP2N4:Eu2+, BaP2N4:Eu2+, and BaSr2P6N12:Eu2+

Nitridophosphates MP₂N₄:Eu²⁺ (M=Ca, Sr, Ba) and BaSr₂P₆N₁₂:Eu²⁺ have been synthesized at elevated pressures and 1100–1300 °C starting from the corresponding azides and P₃N₅ with EuCl₂ as dopant. Addition of NH₄Cl as mineralizer allowed for the growth of single crystals. This led to the successful structure elucidation of a highly condensed nitridophosphate from single‐crystal X‐ray diffraction data (CaP₂N₄:Eu²⁺ (P6₃, no. 173), a=16.847(2), c=7.8592(16) Å, V=1931.7(6) ų, Z=24, 2033 observed reflections, 176 refined parameters, wR₂=0.096). Upon excitation by UV light, luminescence due to parity‐allowed 4f⁶(⁷F)5d¹→4f⁷(⁸S_7/2) transition was observed in the orange (CaP₂N₄:Eu²⁺, λ_max=575 nm), green (SrP₂N₄:Eu²⁺, λ_max=529 nm), and blue regions of the visible spectrum (BaSr₂P₆N₁₂:Eu²⁺ and BaP₂N₄:Eu²⁺, λ_max=450 and 460 nm, respectively). Thus, the emission wavelength decreases with increasing ionic radius of the alkaline‐earth ions. The corresponding full width at half maximum values (2240–2460 cm^?1) are comparable to those of other known Eu²⁺‐doped (oxo)nitrides emitting in the same region of the visible spectrum. Following recently described quaternary Ba₃P₅N₁₀Br:Eu²⁺, this investigation represents the first report on the luminescence of Eu²⁺‐doped ternary nitridophosphates. Similarly to nitridosilicates and related oxonitrides, Eu²⁺‐doped nitridophosphates may have the potential to be further developed into efficient light‐emitting diode phosphors.     

Cyclohydroamination of Aminoalkenes Catalyzed by Disilazide Alkaline‐Earth Metal Complexes: Reactivity Patterns and Deactivation Pathways

The behavior of the first aminophenolate catalysts of the large alkaline earth metals (Ae) [(LOⁱ)AeN(SiMe₂R)₂(thf)_x] (i=1–4; Ae=Ca, Sr, Ba; R=H, Me; x=0–2) for the cyclohydroamination of terminal aminoalkenes is discussed. The complexes [(BDI)AeN(SiMe₂H)₂(thf)_x] (Ae=Ca, Sr, Ba, x=1–2; (BDI)H=H₂C[C(Me)N‐2,6‐(iPr)₂C₆H₃]₂)) and [(BDI)CaN(SiMe₃)₂(thf)] supported by the β‐diketiminate (BDI)^? ligand have also been employed for comparative and mechanistic considerations. The catalytic performances decrease in the order Ca>Sr?Ba, which is the opposite trend to that previously observed during the intermolecular hydroamination of activated alkenes catalyzed by the same alkaline‐earth metal complexes. Catalyst efficacy increases when the chelating and donating ability of the aminophenolate ligands decreases. For given metals and ancillary scaffolds, disilazide catalysts that incorporate the N(SiMe₃)₂^? amido group outclass their congeners containing the N(SiMe₂H)₂^? amide owing to the lower basicity of the N(SiMe₂H)₂^? with respect to the N(SiMe₃)₂^? group, and also because Ae–N(SiMe₂H)₂ catalysts suffer from irreversible deactivation through the dehydrogenative coupling of amine and hydrosilane moieties. This deactivation process takes place at 25 °C in the case of [(LOⁱ)AeN(SiMe₂H)₂(thf)_x] phenolate complexes and occurs even with the related [(BDI)AeN(SiMe₂H)₂(thf)_x] complex, albeit under conditions harsher than those required for effective cyclohydroamination catalysis. A mechanistic scenario for cyclohydroamination catalyzed by [(LX)AeN(SiMe₂H)₂(thf)_x] complexes ((LX)^?=(LOⁱ)^? or (BDI)^?) is proposed. Although beneficial for the synthesis of Ae heteroleptic complexes able to resist deleterious Schlenk‐type equilibria, the use of the N(SiMe₂H)₂^? is prejudicial to catalytic activity in the case of catalyzed transformations that involve reactive amine (and potentially other) substrates. Mechanistic and kinetic investigations further illustrate the interplay between the catalytic activity, operative mechanism, and identity of the metal, ancillary ligand, and amido group. These studies suggest that the widely accepted mechanism for cyclohydroamination reactions cannot be extended systematically to all alkaline‐earth catalysts. The [(BDI)CaN(SiMe₂H)₂{H₂NCH₂C(CH₃)₂CH₂CH?CH₂}₂] complex, the first Ca–aminoalkene adduct structurally characterized, was prepared quantitatively and essentially behaves like [(BDI)CaN(SiMe₂H)(thf)], thus serving as a model compound for mechanistic studies, as illustrated during stoichiometric reactions monitored by ¹H NMR spectroscopy.     

Intermolecular Hydroamination of Vinylarenes by Iminoanilide Alkaline‐Earth Catalysts: A Computational Scrutiny of Mechanistic Pathways

A thorough computational exploration of the mechanistic intricacies of the intermolecular hydroamination (HA) of vinylarenes by a recently reported class of kinetically stabilised iminoanilide [{N^N}Ae{N(SiMe₃)₂} ? (THF)_n] alkaline‐earth amido compounds (Ae=Ca, Sr, Ba) is presented. Two distinct mechanistic pathways for catalytic HA mediated by alkaline‐earth and rare‐earth compounds have emerged over the years that account equally well for the specific features of the process. On one hand, a concerted proton‐assisted pathway to deliver the amine product in a single step can be invoked and, on the other, a stepwise σ‐insertive pathway that comprises a rapid, reversible migratory olefin insertion step linked to a less facile, irreversible Ae?C alkyl bond aminolysis. The results of the study presented herein, which employed a heavily benchmarked and reliable DFT methodology, supports a stepwise σ‐insertive pathway that involves fast and reversible migratory C?C bond insertion into the polar Ae?N pyrrolido σ bond. This proceeds with strict 2,1 regioselectivity via a highly polarised four‐centre transition state (TS) structure, linked to irreversible intramolecular Ae?C bond aminolysis of the alkaline‐earth alkyl intermediate as the energetically favourable mechanism. Turnover‐limiting aminolysis is consistent with the significant KIE measured; the DFT‐derived effective barrier matches the Eyring parameter empirically determined for the best‐performing {N^N}Ba(NR₂) catalyst gratifyingly well. It also predicts the observed trend in reactivity (Ca<Sr<Ba) correctly and the computationally estimated primary KIE is close to the observed values. Non‐competitive kinetic demands militate against the operation of the alternative concerted proton‐assisted pathway, which describes N?C bond formation triggered by concomitant amino proton transfer at the C?C linkage via a multi‐centre TS structure. A detailed comparison of {N^N}Ae(NR₂) catalysts revealed that the variation in the Ae?pyrrolido bond strength together with the degree of protection of the alkaline earth by a sterically encumbering iminoanilide ligand scaffold not only profoundly influences the performance in HA catalysis, but also the likelihood of traversing rival mechanistic pathways.     

A very high yielding and facile alkaline earth metals homogeneous catalysis of Biginelli reaction: an improved protocol

Abstract

A high yielding and facile Biginelli reaction for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones and thiones, using inexpensive and available alkaline earth metal chlorides (MgCl₂, CaCl₂, SrCl₂ and BaCl₂) and acetic acid as the solvent in a homogeneous catalytic reaction, is reported.     

A Highly Lewis Acidic Strontium ansa-Arene Complex for Lewis Acid Catalysis and Isobutylene Polymerization

The potential of a dicationic strontium ansa-arene complex for Lewis acid catalysis has been explored. The key to its synthesis was a simple salt metathesis from SrI₂ and 2 Ag[Al(OR^F)₄], giving the base-free strontium-perfluoroalkoxyaluminate Sr[Al(OR^F)₄]₂ (OR^F=OC(CF₃)₃). Addition of an ansa-arene yielded the highly Lewis acidic, dicationic strontium ansa-arene complex. In preliminary experiments, the complex was successfully applied as a catalyst in CO₂-reduction to CH₄ and a surprisingly controlled isobutylene polymerization reaction.     

Catching gaseous SO2 in cone-type lanthanide complexes: an unexpected coordination mode for SO2 in f-element chemistry

Easy come, easy go: the first molecular SO(2) complexes of the lanthanides (Ln=Sm, Eu) have been prepared. The compounds can reversibly coordinate gaseous SO(2). Concomitant with the addition and removal of SO(2), the color of the complexes changes reversibly. The structures of the SO(2) compounds could be confirmed in solution and in the solid state.