A DFT study of SiH(4) activation by Cp(2)LnH

Early Lanthanide(III) Ate Complexes Featuring Ln-Si Bonds (Ln = La, Ce): Synthesis, Structural Characterization, and Bonding Analysis

While research on lanthanide (Ln) complexes with silyl ligands is receiving growing attention, significantly unbalanced efforts have been devoted to different Ln elements. In comparison with the intense investigations on Ln elements such as Sm and Yb, the chemistry of silyl lanthanum and cerium complexes is much slower to develop, and no solid-state structure of a silyl lanthanum complex has been reported so far. In this research, four types of ate complexes, including [(DME)₃Li][Cp₃LnSi(H)Mes₂], [(18-crown-6)K][Cp₃LnSi(CH₃)Ph₂], [(DME)₃Li][Cp₃LnSiPh₃], and [(12-crown-4)₂Na] [Cp₃LnSi(Ph)₂Si(H)Ph₂] (Ln = La, Ce), were synthesized by reacting [(DME)₃Na][Cp₃La(μ-Cl)LaCp₃] or Cp₃Ce(THF) with alkali metal silanides. All of the synthesized silyl Ln ate complexes were structurally characterized. La-Si bond lengths are in a range of 3.1733(4)-3.1897(10) Å, and the calculated formal shortness ratios of the La-Si bonds (1.071.08) are comparable to those in the reported silyl complexes having other Ln metal centers. The Ce-Si bond lengths (3.1415(6)-3.1705(9) Å) are within the typical range of reported silyl cerium ate complexes. ²⁹Si solid-state NMR measurements on the diamagnetic silyl lanthanum complexes were conducted, and large one-bond hyperfine splitting constants arising from = 7/2) were resolved. Computational studies on these silyl lanthanum and cerium complexes suggested the polarized covalent feature of the Ln-Si bonds, which is in line with the measured large ¹J_¹³⁹La-Si splitting constants.

Hydrosilane σ-Adduct Intermediates in an Adaptive Zinc-Catalyzed Cross-dehydrocoupling of Si-H and O-H Bonds

Three-coordinate Ph BOXMe 2 ZnR (Ph BOXMe 2 =phenyl-(4,4-dimethyl-oxazolinato; R=Me: 2 a, Et: 2 b) catalyzes the dehydrocoupling of primary or secondary silanes and alcohols to give silyl ethers and hydrogen, with high turnover numbers (TON; up to 107 ) under solvent-free conditions. Primary and secondary silanes react with small, medium, and large alcohols to give various degrees of substitution, from mono- to tri-alkoxylation, whereas tri-substituted silanes do not react with MeOH under these conditions. The effect of coordinative unsaturation on the behavior of the Zn catalyst is revealed through a dramatic variation of both rate law and experimental rate constants, which depend on the concentrations of both the alcohol and hydrosilane reactants. That is, the catalyst adapts its mechanism to access the most facile and efficient conversion. In particular, either alcohol or hydrosilane binds to the open coordination site on the Ph BOXMe 2 ZnOR catalyst to form a Ph BOXMe 2 ZnOR(HOR) complex under one set of conditions or an unprecedented σ-adduct Ph BOXMe 2 ZnOR(H-SiR'3 ) under other conditions. Saturation kinetics provide evidence for the latter species, in support of the hypothesis that σ-bond metathesis reactions involving four-centered electrocyclic 2σ-2σ transition states are preceded by σ-adducts.

Rare-Earth-Silyl ate-Complexes Opening a Door to Selective Manipulations

The reactions of a number of rare-earth (RE) trichlorides and an oligosilanylene diide containing a siloxane unit in the backbone in DME are described. The formed products of the type [(DME)4·K][(DME)·RE(Cl)2{Si(SiMe3)2SiMe2}2O] (RE = Y, La, Ce, Pr, Sm, Tb, Dy, and Er) are disilylated dichloro metalate complexes and include the first examples of Si-La and Si-Pr compounds as well as the first structurally characterized example of a Si-Dy complex. A most intriguing aspect of the synthesis of these complexes is that they offer entry into a systematic study of the still largely unexplored field of silyl RE complexes by the possibility of ligand exchange reactions under preservation of the Si-RE interaction. This was demonstrated by the conversion of [(DME)4·K][(DME)·RE(Cl)2{Si(SiMe3)2SiMe2}2O] to [(DME)4·K][Cp2Y{Si(SiMe3)2SiMe2}2O].

Alkali Metal Triphenyl- and Trihydridosilanides Stabilized by a Macrocyclic Polyamine Ligand

Potassium silanide [KSiH₃]_∞ contains 4.2 wt % of hydrogen and has been intensely studied as hydrogen storage material. The macrocyclic ligand Me₄TACD (1,4,7,10‐tetramethyl‐1,4,7,10‐tetraaminocyclododecane, L) stabilizes the full range of triphenylsilyl complexes [(L)MSiPh₃]_n (M=Li–Cs), which react with H₂ or PhSiH₃ to form molecular [(L)MSiH₃]_n that can be isolated in soluble form and fully characterized.

New perspectives in organolanthanide chemistry from redox to bond metathesis: insights from theory

A fifteen year contribution of computational studies carried out in close synergy with experiments is summarized.

Grafting of lanthanide complexes on silica surfaces dehydroxylated at 200 °C: a theoretical investigation

The grafting reaction of lanthanide silylamide complexes has been studied, in the framework of the DFT, highlighting the different grafting modes on a silica surface dehydroxylated at 200 °C.

Effect of secondary ligands' size on energy transfer and electroluminescent efficiencies for a series of europium(III) complexes, a density functional theory study

In this paper, a quantum chemistry method was used to investigate the effect of different sizes of substituted phenanthrolines on absorption, energy transfer, and the electroluminescent performance of a series of Eu(TTA)(3)L (L = [1,10] phenanthroline (Phen), Pyrazino[2,3-f][1,10]phenanthroline (PyPhen), 2-methylprrazino[2,3-f][1,10]phenanthroline(MPP), dipyrido[3,2-a:2',3'-c]phenazine(DPPz), 11-methyldipyrido[3,2-a:2',3'-c]phenazine(MDPz), 11.12-dimethyldipyrido[3,2-a:2',3'-c]phenazine(DDPz), and benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (BDPz)) complexes. Absorption spectra calculations show that different sizes of secondary ligands have different effects on transition characters, intensities, and absorption peak positions. The larger secondary ligands DPPz, MDPz, DDPz and BDPz lead to incomplete energy transfer from the triplet states of ligands to the (5)D(0) of the Eu(3+) ion compared with smaller ones (PyPhen and MPP) due to their lower S(1) or T(1) state energy levels than that of TTA or (5)D(0) of Eu(3+). "Small polaron" stabilization energy (SPE) results reveal that electron trapping is the dominant electroluminescence (EL) mechanism in these materials due to their lower LUMO energies than 4,4'-N,N'-dicarbazolebiphenyl (CBP). Reorganization energy (l) values show that these materials have better electron than hole transporting properties. In addition, the reasons for the origin of the 500 nm emission in Eu-PyPhen- and Eu-MPP-based OLED devices were investigated, and we suppose this emission may result from 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), not from Alq(3).

Theoretical prediction of O–H, Si–H, and Si–C σ-bond activation reactions by titanium(IV)–imido complex

The O–H σ-bond activation of methanol, the Si–H σ-bond activation of silane, and the Si–C σ-bond activation of methylsilane by titanium(IV)–imido complex (Me3SiO)2Ti(NSiMe3) were theoretically investigated with DFT and MP2 to MP4(SDQ) methods. The O–H σ-bond activation of methanol occurs with small activation barrier (Ea) of 7.1 (14.6) kcal/mol and large exothermicity (Eexo) of 65.8 (61.4) kcal/mol to afford (Me3SiO)2Ti(OCH3)[NH(SiMe3)], indicating that the O–H σ-bond activation occurs easier than the C–H σ-bond activation (Ea = 14.6 (21.5) kcal/mol and Eexo = 22.7 (16.5) kcal/mol), where DFT- and MP4(SDQ)-calculated values are presented without and in parenthesis hereafter. Though the OCH3 group becomes anionic and the H atom becomes proton-like in this activation reaction, population changes more moderately occur than those of the C–H σ-bond activation. This is because the H–OCH3 bond is already polarized in methanol. In the Si–H σ-bond activation, two reaction courses were investigated; in one course, the product is (Me3SiO)2Ti(SiH3)[NH(SiMe3)] in which the H atom and the SiH3 group are bound to the N atom and the Ti center, respectively, while in the other course the product is (Me3SiO)2Ti(H)[N(SiH3)(SiMe3)] in which the H atom and the SiH3 group are bound to the Ti center and the imido N atom, respectively. Though the former reaction occurs with small Ea value and large exothermicity, the latter reaction occurs easier with further smaller Ea value of 2.6 (4.3) kcal/mol and larger Eexo value of 32.5 (34.1) kcal/mol than those of the former reaction. This is because the Ti–H bond energy is much larger than the Ti–SiH3 one. The Si–C σ-bond activation occurs with moderate activation barrier of 19.1 (18.6) kcal/mol and considerably large exothermicity of 33.9 (37.7) kcal/mol. Based on these results, we wish to propose the theoretical prediction that the titanium(IV)–imido complex is useful for O–H, Si–H, and Si–C σ-bond activation reactions.

Mechanisms of C-H bond activation: rich synergy between computation and experiment

Recent computational studies of C-H bond activation at late transition metal systems are discussed and processes where lone pair assistance via heteroatom co-ligands or carboxylates are highlighted as a particularly promising means of cleaving C-H bonds. The term 'ambiphilic metal ligand activation' (AMLA) is introduced to describe such reactions.

Cationic methyl complexes of the rare-earth metals: an experimental and computational study on synthesis, structure, and reactivity

Synthesis, structure, and reactivity of two families of rare-earth metal complexes containing discrete methyl cations [LnMe(2-x)(thf)n]((1+x)+) (x = 0, 1; thf = tetrahydrofuran) have been studied. As a synthetic equivalent for the elusive trimethyl complex [LnMe3], lithium methylates of the approximate composition [Li3LnMe6(thf)n] were prepared by treating rare-earth metal trichlorides [LnCl3(thf)n] with 6 equiv of methyllithium in diethyl ether. Heteronuclear complexes of the formula [Li3Ln2Me9L(n)] (Ln = Sc, Y, Tb; L = Et2O, thf) were isolated by crystallization from diethyl ether. Single crystal X-ray diffraction studies revealed a heterometallic aggregate of composition [Li3Ln2Me9(thf)n(Et2O)m] with a [LiLn2Me9](2-) core (Ln = Sc, Y, Tb). When tris(tetramethylaluminate) [Ln(AlMe4)3] (Ln = Y, Lu) was reacted with less than 1 equiv of [NR3H][BPh4], the dimethyl cations [LnMe2(thf)n][BPh4] were obtained. The coordination number as well as cis/trans isomer preference was studied by crystallographic and computational methods. Dicationic methyl complexes of the rare-earth metals of the formula [LnMe(thf)n][BAr4]2 (Ln = Sc, Y, La-Nd, Sm, Gd-Lu; Ar = Ph, C6H4F-4) were synthesized, by protonolysis of either the ate complex [Li3LnMe6(thf)n] (Ln = Sc, Y, Gd-Lu) or the tris(tetramethylaluminate) [Ln(AlMe4)3] (Ln = La-Nd, Sm, Dy, Gd) with ammonium borates [NR3H][BAr4] in thf. The number of coordinated thf ligands varied from n = 5 (Ln = Sc, Tm) to n = 6 (Ln = La, Y, Sm, Dy, Ho). The configuration of representative examples was determined by X-ray diffraction studies and confirmed by density-functional theory calculations. The highly polarized bonding between the methyl group and the rare-earth metal center results in the reactivity pattern dominated by the carbanionic character and the pronounced Lewis acidity: The dicationic methyl complex [YMe(thf)6](2+) inserted benzophenone as an electrophile to give the alkoxy complex [Y(OCMePh2)(thf)5](2+). Nucleophilic addition of the soft anion X(-) (X(-) = I(-), BH4(-)) led to the monocationic methyl complexes [YMe(X)(thf)5](+).

Versatile pathways for in situ polyolefin functionalization with heteroatoms: catalytic chain transfer

Chain-transfer processes represent highly effective chemical means to achieve selective, in situ d- and f-block-metal catalyzed functionalization of polyolefins. A diverse variety of electron-poor and electron-rich chain-transfer agents, including silanes, boranes, alanes, phosphines, and amines, effect efficient chain termination with concomitant carbon-heteroelement bond formation during single-site olefin-polymerization processes. High polymerization activities, control of polyolefin molecular weight and microstructure, and selective chain functionalization are all possible, with distinctly different mechanisms operative for the electron-poor and electron-rich reagents. A variety of metal centers (early transition metals, lanthanides, late transition metals) and single-site ancillary ligand arrays (metallocene, half-metallocene, non-metallocene) are able to mediate these selective chain-termination/functionalization processes.

Organolanthanide mediated catalytic cycles: a computational perspective

In this perspective the contribution of recent theoretical studies to our understanding of lanthanide (Ln) catalysis is explored. In general, the results of computational studies have proven consistent with available experimental evidence. Considerable success has been obtained in elucidating the mechanisms for C-H bond activation (sigma-bond metathesis in particular) and the addition of C-X bonds across an unsaturated functionality (and the hydroamination of alkenes in particular). Ln catalysts are computationally challenging because relativistic effects are important, and large ligands are required to restrict high coordination numbers, in addition to limiting facile redistribution processes. Thus, key technical issues relating to the computational investigation of organolanthanide complexes are discussed. Increasing computational resources have seen studies expand from the optimisation of simple molecules to the study of catalytic cycles where the Ln is coordinated by larger and more complex ligands. The ability of theoretical studies to complement experimental developments by supplying a deeper understanding of the mechanistic process is reviewed with emphasis on the elucidation of transition state structures, intermediates, spectator ligand coordination, and negative entropy steps. Recent computational investigations of the catalytic cycle for Ln mediated hydroamination are a focus, as these have provided substantial and detailed rationalisations for the regio- and stereo-selectivity of inter- and intra-molecular hydroamination. Examination of transition state geometries and electronic structure appears to offer insights that could be used to facilitate the rational design of new Ln-based catalysts.

Synthesis and coordination chemistry of the diamido-arsine ligand [NAsN] (NAsN = PhAs(CH2SiMe2NPh)2))

The preparation and characterization of the diamido-arsine ligand [NAsN]Li2(THF) (1) (where NAsN = PhAs(CH2SiMe2NPh)2), the protonated ligand precursor [NAsN]H2 (2), and its coordination chemistry with tantalum is presented. The complex [NAsN]TaMe3 (3) can be synthesized from 1 and TaMe3Cl2. Hydrogenation of 3 did not produce the desired tetrahydride ([NAsN]Ta)2(µ-H)4, instead, activation of Ta–N bonds in the complex produced a modest yield of the free ligand 2. In an attempt to understand the unusual reactivity of 3, a density functional theory investigation of the model complexes 'NAsN'Li2(OMe2)2 (4) and 'NAsN'TaMe3 (5) (where 'NAsN' = MeAs(CH2SiH2NMe)2) and their phosphine analogs 'NPN'Li2(OMe2) (7), 'NPN'TaMe3 (8), and a related niobium complex 'NPN'NbMe3 (10) (where 'NPN' = MeP(CH2SiH2NMe)2) was undertaken. The difference between the chemistry supported by the As and P ligands originates from the poor binding of As to Ta in these systems and is likely due to a mismatch of the soft As donor and the hard Ta(V) metal centre.Key words: arsine ligand, tridentate, tantalum, hydrogenation, DFT calculations.

Unique {H(SiR3)2}, (H2SiR3), H(HSiR3), and (H2)SiR3 Ligand Sets Supported by the {Fe(Cp)(L)} Platform (L=CO, PR3)

This work deals with the type and incidence of nonclassical Si--H and H--H interactions in a family of silylhydride complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(X)] (X=SiMe(n)Cl(3-n), H, Me, n=0-3) and [Fe(Cp)(Me(3)P)(SiMe(n)Cl(3-n))(2)H] (n=0-3). DFT calculations complemented by atom-in-molecule analysis and calculations of NMR hydrogen-silicon coupling constants revealed a surprising diversity of nonclassical Si--H and H--H interligand interactions. The compounds [Fe(Cp)(L)(SiMe(n)Cl(3-n))(2)H] (L=CO, PMe(3); n=0-3) exhibit an unusual distortion from the ideal piano-stool geometry in that the silyl ligands are strongly shifted toward the hydride and there is a strong trend towards flattening of the {FeSi(2)H} fragment. Such a distortion leads to short Si--H contacts (range 2.030-2.075 A) and large Mayer bond orders. A novel feature of these extended Si--H interactions is that they are rather insensitive towards the substitution at the silicon atom and the orientation of the silyl ligand relatively the Fe--H bond. NMR spectroscopy and bonding features of the related complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(Me)] (n=0-3) allow for their rationalization as usual eta(2)-Si--H silane sigma-complexes. The series of "dihydride" complexes [Fe(Cp)(OC)(SiMe(n)Cl(3-n))H(2)] (n=0-3) is different from the previous two families in that the type of interligand interactions strongly depends on the substitution on silicon. They can be classified either as usual dihydrogen complexes, for example, [Fe(Cp)(OC)(SiMe(2)Cl)(eta(2)-H(2))], or as compounds with nonclassical H--Si interactions, for example, [Fe(Cp)(OC)(H)(2)(SiMe(3))] (16). These nonclassical interligand interactions are characterized by increased negative J(H,Si) (e.g. -27.5 Hz) and increased J(H,H) (e.g. 67.7 Hz).

Structural and Electronic Analysis of Lanthanide Complexes: Reactivity May Not Necessarily Be Independent of the Identity of the Lanthanide Atom − A DFT Study

Density functional theory calculations were used to study a given complex for the whole series of lanthanide cations: [Ln(C3H5)Cp(OMe)] (1) [Ln = La (Z = 57)-Lu (Z = 71)], the radioactive lanthanide promethium (Z = 61) excepted. Contrary to the common assumptions, the calculations suggested a significant, albeit indirect, contribution of f electrons to bonding. Relativistic effects were considered in the calculations of the bonding energies, as well as in geometry optimizations in both spin-restricted and unrestricted formalisms. The unrestricted orbitals were finally used for the analysis of the charges and the composition of the frontier orbitals. It was confirmed that the ionic character was more pronounced for complexes of the late lanthanides.

DFT studies of the methyl exchange reaction between Cp2M–CH3or Cp*2M–CH3(Cp = C5H5, Cp* = C5Me5, M = Y, Sc, Ln) and CH4. Does M ionic radius control the reaction?

The activation energies for the methyl exchange reactions between Cp2M-CH3 and H-CH3 have been calculated for M = Sc, Y and representative metals of the lanthanide family (La, Ce, Sm, Ho, Yb and Lu) with DFT(B3PW91) calculations with large-core pseudopotentials for M. The sigma-bond metathesis reactions are calculated to have lower activation energies for early lanthanides than for late lanthanides and any of group 3 metals. The relative activation barriers are analyzed using the NBO charge distributions in the reactant and in the transition states. It is shown that the methane needs to be polarized in the transition state as H((+delta))-CH3((-delta)) by the reactant, because this sigma-bond metathesis is best viewed as heterolytic cleavage of methane, leading to a proton transfer between two methyl groups in the field of an electropositive M metal. Early lanthanides, which are involved in strongly ionic metal-ligands bonds are thus associated with the lowest activation energies. The ionic radius and the steric effects influence the relative rates of reaction for the complexes of Sc, Y and Lu. In agreement with earlier works of Sherer et al., the experimental reactivity trends found by Tilley are reproduced best with Cp*2M-CH3 (Cp* = C5Me5) rather than Cp2M-CH3 (Cp = C5H5) because the steric bulk of C5Me5 deactivates most the complex where the metal has the smallest ionic radius (Sc). While the steric effects and the influence of the metal ionic radius cannot be neglected, these factors are not the only ones involved in determining the activation barriers of the sigma-bond metathesis reaction.

Calcium, Strontium, Barium, and Ytterbium Complexes with Cyclooctatetraenyl or Cyclononatetraenyl Ligands1

Neutral triple-decker complexes of the heavy alkaline earth metals and ytterbium with tetraisopropylcyclopentadienide anions as terminal ligands and a cyclooctatetraene dianion as a middle deck have been synthesized from tetraisopropylcyclopentadienyl metal halide precursors and disodium cyclooctatetraenide. The pentaisopropylcyclopentadienyl analogue [{(C5iPr5)Yb}2(C8H8)] was prepared from ytterbium metal, cyclooctatetraene, and the free pentaisopropylcyclopentadienyl radical. X-ray crystal structure determinations for the barium and the calcium derivative show an almost linear arrangement of ring centers and metal atoms in both cases with metal-ring center distances of 2.33 A (Ca-Cp), 1.99/1.98 A (Ca-COT) and 2.71 A (Ba-Cp), 2.40 A (Ba-COT). The geometrical features of these molecules could be modeled quite accurately with density functional calculations. With potassium cyclononatetraenide, sparingly soluble bis(cyclononatetraenyl)barium could be prepared and characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. Cyclononatetraenyl(tetraisopropylcyclopentadienyl)barium was obtained from [(C5HiPr4)BaI(THF)2]2 and KC9H9 as a 1:1 mixture with octaisopropylbarocene. Density functional calculations predict sandwich structures with parallel rings and a 2.37 A Ba-ring distance for [Ba(C9H9)2] and a 174 bending with metal-ring distances of 2.72 A (Ba-Cp) and 2.35 A (Ba-CNT) for [(C5HiPr4)Ba(C9H9)]. All alkaline earth sandwich and triple-decker complexes mentioned above have been heated to 250 degrees C without decomposition and have been sublimed in oil pump vacuum.