Heteroleptic Alkyl and Amide Iminoanilide Alkaline Earth and Divalent Rare Earth Complexes for the Catalysis of Hydrophosphination and (Cyclo)Hydroamination Reactions

[{N^N}M(X)(thf)_n] alkyl (X=CH(SiMe₃)₂) and amide (X=N(SiMe₃)₂) complexes of alkaline earths (M=Ca, Sr, Ba) and divalent rare earths (Yb^II and Eu^II) bearing an iminoanilide ligand ({N^N}^?) are presented. Remarkably, these complexes proved to be kinetically stable in solution. X‐ray diffraction studies allowed us to establish size–structure trends. Except for one case of oxidation with [{N^N}Yb^II{N(SiMe₃)₂}(thf)], all these complexes are stable under the catalytic conditions and constitute effective precatalysts for the cyclohydroamination of terminal aminoalkenes and the intermolecular hydroamination and intermolecular hydrophosphination of activated alkenes. Metals with equal sizes across alkaline earth and rare earth families display almost identical apparent catalytic activity and selectivity. Hydrocarbyl complexes are much better catalyst precursors than their amido analogues. In the case of cyclohydroamination, the apparent activity decreases with metal size: Ca>Sr>Ba, and the kinetic rate law agrees with R_CHA=k[precatalyst]¹[aminoalkene]¹. The intermolecular hydroamination and hydrophosphination of styrene are anti‐Markovnikov regiospecific. In both cases, the apparent activity increases with the ionic radius (Ca<Sr<Ba) but the rate laws are different, and obey R_HA=k[styrene]¹[amine]¹[precatalyst]¹ and R_HP=k[styrene]¹[HPPh₂]⁰[precatalyst]¹, respectively. Mechanisms compatible with the rate laws and kinetic isotopic effects are proposed. [{N^N}Ba{N(SiMe₃)₂}(thf)₂] ( 3 ) and [{N^N}Ba{CH(SiMe₃)₂}(thf)₂] ( 10 ) are the first efficient Ba‐based precatalysts for intermolecular hydroamination and hydrophosphination, and display activity values that are above those reported so far. The potential of the precatalysts for C? N and C? P bond formation is detailed and a rare cyclohydroamination–intermolecular hydroamination “domino” sequence is presented.     

Alkaline Earth Metallocenes Coordinated with Ester Pendants: Synthesis,Structural Characterization,and Application in Metathesis Reactions

A variety of ester‐substituted cyclopentadiene derivatives have been synthesized by one‐pot reactions of 1,4‐dilithio‐1,3‐butadienes, CO, and acid chlorides. Direct deprotonation of the ester‐substituted cyclopentadienes with Ae[N(SiMe₃)₂]₂ (Ae=Ca, Sr, Ba) efficiently generated members of a new class of heavier alkaline earth (Ca, Sr, Ba) metallocenes in good to excellent yields. Single‐crystal X‐ray structural analysis demonstrated that these heavier alkaline earth metallocenes incorporated two intramolecularly coordinated ester pendants and multiply‐substituted cyclopentadienyl ligands. The corresponding transition metal metallocenes, such as ferrocene derivatives and half‐sandwich cyclopentadienyl tricarbonylrhenium complexes, could be generated highly efficiently by metathesis reactions. The multiply‐substituted cyclopentadiene ligands bearing an ester pendant, and the corresponding heavier alkaline earth and transition‐metal metallocenes, may have further applications in coordination chemistry, organometallic chemistry, and organic synthesis.     

Computational Mechanistic Elucidation of the Intramolecular Aminoalkene Hydroamination Catalysed by Iminoanilide Alkaline‐Earth Compounds

A comprehensive computational exploration of plausible alternative mechanistic pathways for the intramolecular hydroamination (HA) of aminoalkenes by a recently reported class of kinetically stabilised iminoanilide alkaline‐earth silylamido compounds [{N^N}Ae{N(SiMe₃)₂} ? (thf)_n] ({N^N}=iminoanilide; Ae=Ca, Sr, Ba) is presented. On the one hand, a proton‐assisted concerted N?C/C?H bond‐forming pathway to afford the cycloamine in a single step can be invoked and on the other hand, a stepwise σ‐insertive pathway that involves a fast, reversible migratory olefin 1,2‐insertion step linked to a less rapid, irreversible metal?C azacycle tether σ‐bond aminolysis. Notably, these alternative mechanistic avenues are equally consistent with reported key experimental features. The present study, which employs a thoroughly benchmarked and reliable DFT methodology, supports the prevailing mechanism to be a stepwise σ‐insertive pathway that sees an initial conversion of the {N^N}Ae silylamido into the catalytically competent {N^N}Ae amidoalkene compound and involves thereafter facile and reversible insertive N?C bond‐forming ring closure, linked to irreversible intramolecular Ae?C tether σ‐bond aminolysis at the transient {N^N}Ae alkyl intermediate. Turnover‐limiting protonolysis accounts for the substantial primary kinetic isotope effect observed; its DFT‐derived barrier satisfactorily matches the empirically determined Eyring parameter and predicts the decrease in rate observed across the series Ca>Sr>Ba correctly. Non‐competitive kinetic demands militate against the operation of the concerted proton‐assisted pathway, which describes N?C bond‐forming ring closure triggered by concomitant amino proton delivery at the C?C linkage evolving through a multi‐centre TS structure. Valuable insights into the catalytic structure–activity relationships are unveiled by a detailed comparison of [{N^N}Ae(NHR)] catalysts. Moreover, the intriguingly opposite trends in reactivity observed in intramolecular (Ca>Sr>Ba) and intermolecular (Ca<Sr<Ba) HA catalysis for the studied family of iminoanilide alkaline‐earth amido catalysts are rationalised.     

Charge‐Separated and Molecular Heterobimetallic Rare Earth–Rare Earth and Alkaline Earth–Rare Earth Aryloxo Complexes Featuring Intramolecular Metal–π‐arene Interactions

A new family in town! Treatment of a rare‐earth metal (Ln) and either a potential divalent rare‐earth metal (Ln′) or an alkaline earth metal (Ae) with 2,6‐diphenylphenol (HOdpp) at elevated temperatures (200–250 °C) afforded heterobimetallic aryloxo complexes (see figure). Both a charge‐separated species, [(Ln′/Ae)₂(Odpp)₃][Ln(Odpp)₄], and a neutral species, [AeEu(Odpp)₄], were obtained and crystallographically characterised.

     

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

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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.     

Metal‐to‐Ligand Alkyl Migration Inducing Carbon–Sulfur Bond Cleavage in Dialkyl Yttrium Complexes Supported by Thiazole‐Containing Amidopyridinate Ligands: Synthesis,Characterization, and Catalytic Activity in the Intramolecular Hydroamination Reaction

Neutral Y^III dialkyl complexes supported by tridentate N^?,N,N monoanionic methylthiazole– or benzothiazole–amidopyridinate ligands have been prepared and completely characterized. Studies on their stability in solution revealed progressive rearrangement of the coordination sphere in the benzothiazole‐containing system through an unprecedented metal‐to‐ligand alkyl migration and subsequent thiazole ring opening. Attempts to synthesize hydrido species from the dialkyl precursor led to the generation of a dimeric yttrium species stabilized by a trianionic N^?,N,N^?,S^? ligand as the result of metal‐to‐ligand hydride migration with chemoselective thiazole ring opening and subsequent dimerization through intermolecular addition of the residual Y?H group to the imino fragment of a second equivalent of the ring‐opened intermediate. DFT calculations were used to elucidate the thermodynamics and kinetics of the process, in support of the experimental evidence. Finally, all isolated yttrium complexes, especially their cationic forms prepared by activation with the Lewis acid Ph₃C⁺[B(C₆F₅)₄]^?, were found to be good candidate catalysts for intramolecular hydroamination/cyclization reactions. Their catalytic performance with a number of primary and secondary amino alkenes was assessed.     

σ‐Insertive Mechanism versus Concerted Non‐insertive Mechanism in the Intramolecular Hydroamination of Aminoalkenes Catalyzed by Phenoxyamine Magnesium Complexes: A Synthetic and Computational Study

The phenoxyamine magnesium complexes [{ONN}MgCH₂Ph] ( 4 a : {ONN}=2,4‐tBu₂‐6‐(CH₂NMeCH₂CH₂NMe₂)C₆H₂O^?; 4 b : {ONN}=4‐tBu‐2‐(CH₂NMeCH₂CH₂NMe₂)‐6‐(SiPh₃)C₆H₂O^?) have been prepared and investigated with respect to their catalytic activity in the intramolecular hydroamination of aminoalkenes. The sterically more shielded triphenylsilyl‐substituted complex 4 b exhibits better thermal stability and higher catalytic activity. Kinetic investigations using complex 4 b in the cyclisation of 1‐allylcyclohexyl)methylamine ( 5 b ), respectively, 2,2‐dimethylpent‐4‐en‐1‐amine ( 5 c ), reveal a first‐order rate dependence on substrate and catalyst concentration. A significant primary kinetic isotope effect of 3.9±0.2 in the cyclisation of 5 b suggests significant N?H bond disruption in the rate‐determining transition state. The stoichiometric reaction of 4 b with 5 c revealed that at least two substrate molecules are required per magnesium centre to facilitate cyclisation. The reaction mechanism was further scrutinized computationally by examination of two rivalling mechanistic pathways. One scenario involves a coordinated amine molecule assisting in a concerted non‐insertive N?C ring closure with concurrent amino proton transfer from the amine onto the olefin, effectively combining the insertion and protonolysis step to a single step. The alternative mechanistic scenario involves a reversible olefin insertion step followed by rate‐determining protonolysis. DFT reveals that a proton‐assisted concerted N?C/C?H bond‐forming pathway is energetically prohibitive in comparison to the kinetically less demanding σ‐insertive pathway (ΔΔG^≠=5.6 kcal mol^?1). Thus, the σ‐insertive pathway is likely traversed exclusively. The DFT predicted total barrier of 23.1 kcal mol^?1 (relative to the {ONN}Mg pyrrolide catalyst resting state) for magnesium?alkyl bond aminolysis matches the experimentally determined Eyring parameter (ΔG^≠=24.1(±0.6) kcal mol^?1 (298 K)) gratifyingly well.     

Polymerization of Polar Monomers Using Cyclopentadienyl Complexes of the Alkaline Earth Metals

Tetramethylcyclopentadienyl complexes of magnesium, calcium and strontium are efficient catalysts at −78 °C for the solution polymerization of polar monomers such as alkyl methacrylates in the presence of only small amounts of triethylaluminium as cocatalyst. The molecular mass of the resulting polymers is predictable on the monomer/catalyst ratio. The polymerization reaction follows a first order kinetics with respect to the initial catalyst concentration. Block-copolymers are easily available by sequential dosage of the monomer. A living polymerization mechanism is proposed involving activation of the catalyst by alkylation and subsequent insertion of coordinated monomers.     

Phenalenyl‐Based Organozinc Catalysts for Intramolecular Hydroamination Reactions: A Combined Catalytic,Kinetic, and Mechanistic Investigation of the Catalytic Cycle

Herein, we report the synthesis and characterization of two organozinc complexes that contain symmetrical phenalenyl (PLY)‐based N,N‐ligands. The reactions of phenalenyl‐based ligands with ZnMe₂ led to the formation of organozinc complexes [N(Me),N(Me)‐PLY]ZnMe ( 1 ) and [N(iPr),N(iPr)‐PLY]ZnMe ( 2 ) under the evolution of methane. Both complexes ( 1 and 2 ) were characterized by NMR spectroscopy and elemental analysis. The solid‐state structures of complexes 1 and 2 were determined by single‐crystal X‐ray crystallography. Complexes 1 and 2 were used as catalysts for the intramolecular hydroamination of unactivated primary and secondary aminoalkenes. A combined approach of NMR spectroscopy and DFT calculations was utilized to obtain better insight into the mechanistic features of the zinc‐catalyzed hydroamination reactions. The progress of the catalysis for primary and secondary aminoalkene substrates with catalyst 2 was investigated by detailed kinetic studies, including kinetic isotope effect measurements. These results suggested pseudo‐first‐order kinetics for both primary and secondary aminoalkene activation processes. Eyring and Arrhenius analyses for the cyclization of a model secondary aminoalkene substrate afforded ΔH^≠=11.3 kcal mol^?1, ΔS^≠=?35.75 cal K^?1 mol^?1, and E_a=11.68 kcal mol^?1. Complex 2 exhibited much‐higher catalytic activity than complex 1 under identical reaction conditions. The in situ NMR experiments supported the formation of a catalytically active zinc cation and the DFT calculations showed that more active catalyst 2 generated a more stable cation. The stability of the catalytically active zinc cation was further supported by an in situ recycling procedure, thereby confirming the retention of catalytic activity of compound 2 for successive catalytic cycles. The DFT calculations showed that the preferred pathway for the zinc‐catalyzed hydroamination reactions is alkene activation rather than the alternative amine‐activation pathway. A detailed investigation with DFT methods emphasized that the remarkably higher catalytic efficiency of catalyst 2 originated from its superior stability and the facile formation of its cation compared to that derived from catalyst 1 .     

Synthesis and Characterization of Alkynyl Complexes of Groups 1 and 2

The synthesis of N‐heterocyclic carbene adducts of alkynyl lithium and magnesium is achieved, and different degrees of association are observed. Reaction of strontium amide nacnacSrN(SiMe₃)₂(thf) (nacnac=CH(CMe2,6‐iPr₂C₆H₃N)₂) with PhC≡CH in THF yields the dimeric alkynyl complex [nacnacSr(thf)(μ‐C≡CPh)]₂ which shows an interesting coordination geometry around the metal center. The compound retains the THF molecules, unlike its lighter congener, even in hydrocarbon solvents.     

Chemistry of Diruthenium and Dirhodium Analogues of Pentaborane(9): Synthesis and Characterization of Metal N,S‐Heterocyclic Carbene and B‐Agostic Complexes

Building upon our earlier results on the synthesis of electron‐precise transition‐metal–boron complexes, we continue to investigate the reactivity of pentaborane(9) and tetraborane(10) analogues of ruthenium and rhodium towards thiazolyl and oxazolyl ligands. Thus, mild thermolysis of nido‐[(Cp*RuH)₂B₃H₇] ( 1 ) with 2‐mercaptobenzothiazole (2‐mbtz) and 2‐mercaptobenzoxazole (2‐mboz) led to the isolation of Cp*‐based (Cp*=η⁵‐C₅Me₅) borate complexes 5 a , b [Cp*RuBH₃L] ( 5 a : L=C₇H₄NS₂; 5 b : L=C₇H₄NOS)) and agostic complexes 7 a , b [Cp*RuBH₂(L)₂], ( 7 a : L=C₇H₄NS₂; 7 b : L=C₇H₄NOS). In a similar fashion, a rhodium analogue of pentaborane(9), nido‐[(Cp*Rh)₂B₃H₇] ( 2 ) yielded rhodaboratrane [Cp*RhBH(L)₂], 10 (L=C₇H₄NS₂). Interestingly, when the reaction was performed with an excess of 2‐mbtz, it led to the formation of the first structurally characterized N,S‐heterocyclic rhodium‐carbene complex [(Cp*Rh)(L₂)(1‐benzothiazol‐2‐ylidene)] ( 11 ) (L=C₇H₄NS₂). Furthermore, to evaluate the scope of this new route, we extended this chemistry towards the diruthenium analogue of tetraborane(10), arachno‐[(Cp*RuCO)₂B₂H₆] ( 3 ), in which the metal center possesses different ancillary ligands.     

离子色谱法同时分析中药中的碱金属与碱土金属离子

用离子色谱法简单快速地分析了中药中的阳离子。分析结果表明,川芎和酸枣仁中5种常见阳离子Na~+,NH~+_4,K~+,Mg~2+和Ca~2+的含量比例不同。各离子的检出限（S/N=3）为0.001～0.013mg／L,线性范围达3个数量级。     

Alkaline Earth Metals Activate N2 and CO in Cubic Complexes Just Like Transition Metals: A Conceptual Density Functional Theory and Energy Decomposition Analysis Study

Following the recent discovery of stable octa-coordinated alkaline earth metals with N₂ and CO, the role of group II metals in the catalytic reduction of these ligands by means of density functional theory (DFT) calculations and conceptual DFT-based reactivity indices is investigated. Cubic group IV and octahedral group VI transition metal complexes as well as the free ligands are computed for reference. The outer and most accessible atoms of N₂ and CO become much more nucleophilic and electrophilic in all complexes, relevant for N₂ fixation, as probed by the Fukui function and local softness. Within one row of the periodic table, the alkaline earth complexes often show the strongest activation. On the contrary, the electrostatic character is found to be virtually unaffected by complexation. Trends in the soft frontier orbital and hard electrostatic character are in agreement with calculated proton affinities and energy decomposition analyses of the protonated structures, demonstrating the dominance of the soft (HOMO–LUMO) orbital interactions.     

Integrated Photoresponsive Alkaline Earth Metal Coordination Networks: Synthesis,Topology, Photochromism and Photoluminescence Investigation

Photoresponsive materials are a key part of the age of smart technology that have potential in a broad range of applications. Coordination networks (CNs) are widely used due to their designability and stability. In this work, three novel alkaline earth metal coordination networks (AEM-CNs): [Mg(CMNDI)(H₂O)₂], [Ca(CMNDI)(H₂O)₂]⋅H₂O, and [Sr(CMNDI)(H₂O)(DMF)] with fsl , cds , and scn topology nets were synthetized via N,N’-bis(carboxymethyl)-1,4,5,8-naphthalenediimide (H₂CMNDI); the scn net is not found in the Reticular Chemistry Structure Resource or ToposPro. The reusable and sensitive photochromic properties of the three CNs enable them to be used as secret inks or ultraviolet detectors. In addition, the CNs also exhibited reusable photoluminescent turn-off toward the drug molecules, balsalazide disodium (Bal.) and colchicine (Col.), with good limits of detection of 0.16 and 0.70 μM. To the best of our knowledge, this is the first study of a fluorescence sensor for Bal. Thus, the AEM-CNs provide a design idea for integrated photoresponsive materials that could be further improved in the near future by further study.     

Heteroleptic and Homoleptic Iron(III) Complexes with a Tris(N-Heterocyclic Carbene) Borate Ligand: Synthesis,Characterization, and Catalytic Application

Heteroleptic and homoleptic iron(III) complexes supported by a tris(N-heterocyclic carbene) borate ligand have been prepared and crystallographically characterized. The strong electron-donating character of the tris(carbene) donor was revealed by UV-vis absorption spectroscopy and electrochemical measurements combined with quantum chemical calculations. The catalytic activity of each complex was evaluated in cyclohexane oxidation reaction using meta-chloroperoxybenzoic acid (=mCPBA) as an oxidant, and both complexes show high catalytic activity and selectivity with TON=∼350 and A/(K+L)=8–10. Mechanistic studies suggested that radical-chain processes are involved in the reaction due to mCPBA acting as a one-electron oxidant, concomitant with the pathway of metal-based reactive species. Moreover, it was found that the homoleptic and heteroleptic complexes differed significantly in the involvement of metal-based active species, with the homoleptic complex exhibiting more metal-based reactions.     

Homoleptic Tris(Pyridyl Pyrazolate) IrIII Complexes: En Route to Highly Efficient Phosphorescent OLEDs

Treatment of the metal reagent IrCl₃ ? nH₂O with two equivalents of 2‐pyridyl pyrazole (N^N)H (3‐tert‐butyl‐5‐(2‐pyridyl) pyrazole, (bppz)H and 3‐trifluoromethyl‐5‐(2‐pyridyl) pyrazole, (fppz)H), afforded the isomeric Ir^III metal complexes with a general formula cis‐[Ir(bppz)₂Cl₂]H ( 2 a ), trans‐[Ir(bppz)₂Cl₂]H ( 3 a ), cis‐[Ir(fppz)₂Cl₂]H ( 2 b ), and trans‐[Ir(fppz)₂Cl₂]H ( 3 b ). Single‐crystal X‐ray diffraction studies on 2 b and 3 a revealed the coexistence of two pyrazolate chelates and two terminal chloride ligands on the coordination sphere. Subsequent reactivity studies confirmed their intermediacy to the preparation of homoleptic mer‐[Ir(bppz)₃] ( 1 a ) and mer‐[Ir(fppz)₃] ( 1 b ) that showed dual intraligand and ligand‐to‐ligand charge‐transfer phosphorescence at room temperature. To attain bright, room‐temperature phosphorescence further, we then synthesized two isoquinolinyl pyrazolate complexes, mer‐[Ir(bipz)₃] ( 4 a ) and mer‐[Ir(fipz)₃] ( 4 b ) ((bipz)H=3‐tert‐butyl‐5‐(1‐isoquinolyl) pyrazole and (fipz)H=3‐trifluoromethyl‐5‐(1‐isoquinolyl) pyrazole). Their orange luminescence is mainly attributed to the mixed MLCT/ππ* transition, and the quantum yields were as high as 86 ( 4 a ) and 50 % ( 4 b ) in degassed CH₂Cl₂ solution at RT. The organic light‐emitting diodes (OLEDs) were then fabricated by using 4 a as a dopant, giving orange luminescence with CIE_x,y=0.55, 0.45 (CIE_x,y=the 1931 Commission Internationale de L’Eclairage (x,y) coordinates) and peak efficiencies of 14.6 % photon/electron, 34.8 cd A^?1, 26.1 lm W^?1. The device data were then compared with the previously reported heteroleptic complex [Ir(dfpz)₂(bipz)] ( 5 ) ((dfpz)H=1‐(2,4‐difluorophenyl) pyrazole), revealing the possible effect of the bipz chelate and phosphor design on the overall electrophosphorescent performance, which can be understood by the differences in the carrier‐transport properties.