Beta-agostic silylamido and silyl-hydrido compounds of molybdenum and tungsten.

Reactions of bis(imido) compounds (RN)(2)Mo(PMe(3))(n) (n = 2, R = (t)Bu; n = 3, R =2,6-dimethylphenyl (Ar') and 2,6-diisopropylphenyl (Ar)) and (RN)(2)W(PMe(3))(3) (R = 2,6-dimethylphenyl and 2,6-diisopropylphenyl) with silanes afford four types of products: the beta-agostic silylamido compounds (RN)(eta(3)-RN-SiR'(2)-H...)MCl(PMe(3))(2) (M = Mo and W), mono(imides) (RN)MCl(2)(PMe(3))(3) (M = Mo and W), silyl hydride bis(imido) derivative (ArN)(2)W(PMe(3))(H)(SiMeCl(2)), and Si-Cl...W bridged product (ArN)(eta(2)-ArN-SiHMeCl-Cl...)WCl(PMe(3))(2). Reactions of molybdenum compounds (RN)(2)Mo(PMe(3))(m) (m = 2 or 3) with mono- and dichlorosilanes HSiCl(n)R'(3-n) (R' = Me, Ph; n = 1,2) afford mainly the agostic compounds (RN)(eta(3)-RN-SiR'(2)-H...)MoCl(PMe(3))(2), accompanied by small amounts of mono(imido) derivatives (RN)MoCl(2)(PMe(3))(3). In contrast, the latter compounds are the only transition metal products in reactions with HSiCl(3), the silicon co-product being the silanimine dimer (RNSiHCl)(2). The reaction of (ArN)(2)W(PMe(3))(3) with HSiCl(2)Me under continuous removal of PMe(3) affords the silyl hydride species (ArN)(2)W(PMe(3))(SiMeCl(2))H, characterized by NMR and X-ray diffraction. This product is stable in the absence of phosphine, but addition of catalytic amounts of PMe(3) causes a fast rearrangement into the Si-Cl...W bridged product (ArN)(eta(2)-ArN-SiHMeCl-Cl...)WCl(PMe(3))(2). The mechanism of silane addition to complexes (RN)(2)Mo(PMe(3))(n) has been elucidated by means of density functional theory calculations of model complexes (MeN)(2)Mo(PMe(3))(n) (n = 1-3). Complex (MeN)(2)Mo(PMe(3))(2) is found to be the most stable form. It undergoes facile silane-to-imido addition reactions that afford silylamido hydride complexes stabilized by additional Si...H interactions. The ease of this addition increases from HSiMe(2)Cl to HSiCl(3), consistent with experimental observations. The most stable final products of silane addition are the agostic complexes (MeN)(eta(3)-MeN-SiR(2)-H...)MoCl(PMe(3))(2) (R(2) = Me(2), MeCl, Cl(2)) and Cl-bridged silylamido complexes (MeN)(eta(2)-MeN-SiRH-Cl...)MoCl(PMe(3))(2) (R = Me or Cl). In the case of HSiMeCl(2) addition the former is the most stable, but for HSiCl(3) addition the latter is the preferred product. In all cases, the isomeric silyl hydride species (MeN)(2)Mo(PMe(3))(H)(SiClR(2)) are less stable by about 6 kcal mol(-1). Silane additions to the imido ligand of the tris(phosphine) (MeN)(2)Mo(PMe(3))(3) afford octahedral silylamido hydride derivatives. The different isomers of these addition products are destabilized relative to (MeN)(2)Mo(PMe(3))(3) only by 7-24 kcal mol(-1) (for the HSiMe(2)Cl additions), but since the starting tris(phosphine) is 14.8 kcal mol(-1) less stable than (MeN)(2)Mo(PMe(3))(2), silane addition to the latter is a more preferred pathway. A double phosphine dissociation pathway via the species (MeN)(2)Mo(PMe(3)) was ruled out because this complex is by 24.7 kcal mol(-1) less stable than (MeN)(2)Mo(PMe(3))(2)

A study of room-temperature LixMn1.5Ni0.5O4 solid solutions

Understanding the kinetic implication of solid-solution vs. biphasic reaction pathways is critical for the development of advanced intercalation electrode materials. Yet this has been a long-standing challenge in materials science due to the elusive metastable nature of solid solution phases. The present study reports the synthesis, isolation, and characterization of room-temperature Li(x)Mn(1.5)Ni(0.5)O(4) solid solutions. In situ XRD studies performed on pristine and chemically-delithiated, micron-sized single crystals reveal the thermal behavior of Li(x)Mn(1.5)Ni(0.5)O(4) (0 ≤ x ≤ 1) cathode material consisting of three cubic phases: LiMn(1.5)Ni(0.5)O(4) (Phase I), Li(0.5)Mn(1.5)Ni(0.5)O(4) (Phase II) and Mn(1.5)Ni(0.5)O(4) (Phase III). A phase diagram capturing the structural changes as functions of both temperature and Li content was established. The work not only demonstrates the possibility of synthesizing alternative electrode materials that are metastable in nature, but also enables in-depth evaluation on the physical, electrochemical and kinetic properties of transient intermediate phases and their role in battery electrode performance