Ionization of Cp2ZrMe2 and Lewis Bases by Methylaluminoxane: Computational Insights

The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2’-bipyridine (bipy) with a sheet model for the principal activator (MeAlO)₁₆(Me₃Al)₆ (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me₃Al prior to methide abstraction by 16,6 to form the ion-pairs [Me₂Al(κ²-L)][ 16,6 ] ( 5 : L=OMTS, 6 : L=bipy, [ 16,6 ]⁻=[(MeAlO)₁₆(Me₃Al)₆ Me]⁻) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp₂ZrMe₂ with 16,6 to form both neutral adducts 1 Cp₂ZrMe₂ ⋅ 16,6 and contact ion-pairs 4 and 7 , both with formula [Cp₂ZrMe][μ-Me(MeAlO)₁₆(Me₃Al)₆], featuring terminal and chelated MAO-anions, respectively was studied by DFT. The displacement of the anion with either excess Cp₂ZrMe₂ or Me₃Al was also studied, forming outer-sphere ion-pairs [(Cp₂ZrMe)₂μ-Me][ 16,6 ] ( 2 ) and [Cp₂Zr(μ-Me)₂AlMe₂][ 16,6 ] ( 3 ). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp₂ZrMe₂ and found to be in good agreement with the reported data and assignments. These studies confirm that 16,6 is a very suitable model for the activators present in MAO but highlight the difficulty in accurately calculating thermodynamic quantities for molecules in this size regime.     

Spectroscopic Studies of Synthetic Methylaluminoxane: Structure of Methylaluminoxane Activators

Hydrolysis of trimethylaluminum (Me₃Al) in polar solvents can be monitored by electrospray ionization mass spectrometry (ESI-MS) using the donor additive octamethyltrisiloxane [(Me₃SiO)₂SiMe₂, OMTS]. Using hydrated salts, hydrolytic methylaluminoxane (h-MAO) features different anion distributions, depending on the conditions of synthesis, and different activator contents as measured by NMR spectroscopy. Non-hydrolytic MAO was prepared using trimethylboroxine. The properties of this material, which contains incorporated boron, differ significantly from h-MAO. In the case of MAO prepared by direct hydrolysis, oligomeric anions are observed to rapidly form, and then more slowly evolve into a mixture dominated by an anion with m/z 1375 with formula [(MeAlO)₁₆(Me₃Al)₆Me]⁻. Theoretical calculations predict that sheet structures with composition (MeAlO)_n(Me₃Al)_m are favoured over other motifs for MAO in the size range suggested by the ESI-MS experiments. A possible precursor to the m/z 1375 anion is a local minimum based on the free energy released upon hydrolysis of Me₃Al.     

Mass Spectrometric Characterization of Methylaluminoxane‐Activated Metallocene Complexes

Electrospray‐ionization mass spectrometric studies of poly(methylaluminoxane) (MAO) in the presence of [Cp₂ZrMe₂], [Cp₂ZrMe(Cl)], and [Cp₂ZrCl₂] in fluorobenzene (PhF) solution are reported. The results demonstrate that alkylation and ionization are separate events that occur at competitive rates in a polar solvent. Furthermore, there are significant differences in ion‐pair speciation that result from the use of metallocene dichloride complexes in comparison to alkylated precursors at otherwise identical Al/Zr ratios. Finally, the counter anions that form are dependent on the choice of precursor and Al/Zr ratio; halogenated aluminoxane anions [(MeAlO)_x(Me₃Al)_y?z(Me₂AlCl)_zMe]^? (z=1, 2, 3…?) are observed using metal chloride complexes and under some conditions may predominate over their non‐halogenated precursors [(MeAlO)_x(Me₃Al)_yMe]^?. Specifically, this halogenation process appears selective for the anions that form in comparison to the neutral components of MAO. Only at very high Al/Zr ratios is the same “native” anion distribution observed when using [Cp₂ZrCl₂] when compared with [Cp₂ZrMe₂]. Together, the results suggest that the need for a large excess of MAO when using metallocene dichloride complexes is a reflection of competitive alkylation vs. ionization, the persistence of unreactive, homodinuclear ion pairs in the case of [Cp₂ZrCl₂], as well as a change in ion pairing resulting from modification of the anions formed at lower Al/Zr ratios. Models for neutral precursors and anions are examined computationally.     

Are Methylaluminoxane Activators Sheets?

Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5-coordinate and 4-coordinate Al, are likely precursors to ion-pairs seen during the hydrolysis of trimethylaluminum (Me₃Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me]⁻) and [Me₂Al]⁺ abstraction, involving this donor, were studied by polarizable continuum model calculations in fluorobenzene (PhF) and o-difluorobenzene (DFB) media. These studies suggest that low MW, 5-coordinate sheets ionize by [Me₂Al]⁺ abstraction, while [Me]⁻ abstraction from Me₃Al-OMTS is the likely process for higher MW 4-coordinate sheets. Further, comparison of anion stabilities per mole of aluminoxane repeat unit (MeAlO)_n, suggest that anions such as [(MeAlO)₇(Me₃Al)₄Me]⁻=[ 7,4 ]⁻ are especially stable compared to higher homologues, even though their neutral precursors are unstable.     

Mechanism of olefin polymerization by a soluble zirconium catalyst

A mechanistic study has been carried out on the homogeneous olefin polymerization/oligomerization catalyst formed from Cp₂ZrMe₂ and methylaluminoxane, (MeAlO)_x, in toluene. Formal transfer of CH₃ from Zr to Al yields low concentrations of Cp₂ZrMe⁺ solvated by [(Me₂AlO)_y(MeAlO)_x−y]_y. The cationic Zr species initiates ethylene oligomerization by olefin coordination followed by insertion into the Zr–CH₃ bond. Chain transfer occurs by one of two competing pathways. The predominant one involves exchange of Cp₂Zr–P⁺ (P=growing ethylene oligomer) with Al–CH₃ to produce another Cp₂ZrMe⁺ initiator plus an Al-bound oligomer. Terminal Al–C bonds in the latter are ultimately cleaved on hydrolytic workup to produce materials with saturated end groups. Concomitant chain transfer occurs by sigma bond metathesis of Cp₂Zr–P⁺ with ethylene. Metathesis results in cleavage of the Zr–C bond of the growing oligomer to produce materials also having saturated end groups; and a new initiating species, Cp₂Zr-CHCH₂⁺. The two chain transfer pathways afford structurally different oligomers distinguishable by carbon number and end group structure. Oligomers derived from the Cp₂ZrMe⁺ channel are C_n (n=odd) alkanes; those derived from Cp₂Zr–CHCH₂⁺ are terminally mono-unsaturated C_n (n=even) alkenes. Chain transfer by beta hydride elimination is detectable but relatively insignificant under the conditions employed. Propylene and 1-hexene react similarly but beta hydride elimination is the predominant chain transfer step. The initial Zr-alkyl species produces a Cp₂ZrH⁺ complex that is the principle chain initiator. Chain transfer is fast relative to propagation and the products are low molecular weight oligomers.     

Real-time analysis of methylalumoxane formation

Methylalumoxane (MAO), a perennially useful activator for olefin polymerization precatalysts, is famously intractable to structural elucidation, consisting as it does of a complex mixture of oligomers generated from hydrolysis of pyrophoric trimethylaluminum (TMA). Electrospray ionization mass spectrometry (ESI-MS) is capable of studying those oligomers that become charged during the activation process. We have exploited that ability to probe the synthesis of MAO in real time, starting less than a minute after the mixing of H₂O and TMA and tracking the first half hour of reactivity. We find that the process does not involve an incremental build-up of oligomers; instead, oligomerization to species containing 12–15 aluminum atoms happens within a minute, with slower aggregation to higher molecular weight ions. The principal activated product of the benchtop synthesis is the same as that observed in industrial samples, namely [(MeAlO)₁₆(Me₃Al)₆Me]⁻, and we have computationally located a new sheet structure for this ion 94 kJ mol⁻¹ lower in Gibbs free energy than any previously calculated.

The activator methylaluminoxane is made by hydrolysis of trimethylaluminum. Analysis using ESI-MS reveals rapid formation of small oligomers is followed by slower aggregation to the larger precursors most capable of releasing [Me₂Al]⁺.     

Molecular Thorium Trihydrido Clusters Stabilized by Cyclopentadienyl Ligands

Hydrogenolysis of alkyl‐substituted cyclopentadienyl (Cp^R) ligated thorium tribenzyl complexes [(Cp^R)Th(p‐CH₂‐C₆H₄‐Me)₃] ( 1 – 6 ) afforded the first examples of molecular thorium trihydrido complexes [(Cp^R)Th(μ‐H)₃]_n (Cp^R=C₅H₂(^tBu)₃ or C₅H₂(SiMe₃)₃, n=5; C₅Me₄SiMe₃, n=6; C₅Me₅, n=7; C₅Me₄H, n=8; 7 – 10 and 12 ) and [(Cp^#)₁₂Th₁₃H₄₀] (Cp^#=C₅H₄SiMe₃; 13 ). The nuclearity of the metal hydride clusters depends on the steric profile of the cyclopentadienyl ligands. The hydrogenolysis intermediate, tetra‐nuclear octahydrido thorium dibenzylidene complex [(Cp^ttt)Th(μ‐H)₂]₄(μ‐p‐CH‐C₆H₄‐Me)₂ (Cp^ttt=C₅H₂(^tBu)₃) ( 11 ) was also isolated. All of the complexes were characterized by NMR spectroscopy and single‐crystal X‐ray analysis. Hydride positions in [(Cp^Me4)Th(μ‐H)₃]₈ (Cp^Me4=C₅Me₄H) were further precisely confirmed by single‐crystal neutron diffraction. DFT calculations strengthen the experimental assignment of the hydride positions in the complexes 7 to 12 .     

Reaction of group 13 sulfido cubanes with dimethylzirconocene

The reaction of Cp₂ZrMe₂ with the aluminum- and gallium-sulfido cubane compounds [( ¹ Bu)M(₃-S)]₄ (M = Al, Ga), has been followed by NMR spectroscopy. Cleavage of the M₄S₄ core occurs resulting in abstraction of a monomeric ( ¹ Bu)M(S) moiety and yielding Cp₂Zr(-S)(-Me)Al( ¹ Bu)Me (1) and [Cp₂Zr(/gm-S)]₂,[Ga( ¹ Bu)Me₂]₂ (3), respectively. The remaining ( ¹ Bu)₃M₃S₃ fragment reacts further with Cp₂ZrMe₂ to give [(Cp₂Zr)M₃(₃-S)₃ ( ¹ Bu)₃Me₂], M = Al (2), Ga (4). The molecular structure of [(Cp₂Zr)Ga₃,(₃-S)₃('Bu)₃Me₂] (4) has been confirmed by X-ray crystallography. All these compounds subsequently decompose to [Cp₂Zr(-S)]₂ and M( ¹ Bu)Me₂. The structure of compound 3 is discussed with respect to the decreased propensity of gallium, as compared to aluminum, to form 3-center 2-electron bridging bonds. Crystal data for [(Cp₂Zr)Ga₃(₃-S)₃( ¹ Bu)₃Me₂] (4): monoclinic, P2₁/n,a = 10.585(2),b = 17.970(4),c = 16.418(3) A, = 101.00(3)°, R = 0.0402, R _w = 0.0402.     

Reaktionen von Cyclostibanen (RSb)n [R = (Me3Si)2CH,n = 3; Me3CCH2, n = 4, 5] mit den Übergangsmetallcarbonyl‐Komplexen [W(CO)5(thf)], [CpxMn(CO)2(thf)], [CpxCr(CO)3]2 und [Co2(CO)8]; Cpx = MeC5H4

Reactions of Cyclostibanes, (RSb)_n [R = (Me₃Si)₂CH, n = 3; Me₃CCH₂, n = 4, 5] with the Transition Metal Carbonyl Complexes [W(CO)₅(thf)], [Cp^xMn(CO)₂(thf)], [Cp^xCr(CO)₃]₂, and [Co₂(CO)₈]; Cp^x = MeC₅H₄ (RSb)₃ [R = (Me₃Si)₂CH] reacts with [W(CO)₅(thf)], [Cp^xMn(CO)₂(thf)], or [Co₂(CO)₈] to give [(RSb)₃W(CO)₅] ( 1 ), [RSb{Mn(CO)₂Cp^x}₂] ( 2 ) or [RSbCo(CO)₃]₂ ( 3 ). The reaction of (R′Sb)_n (n = 4, 5; R′ = Me₃CCH₂) with [Cp^xCr(CO)₃]₂ leads to [(R′Sb)₄{Cr(CO)₂Cp^x}₂] ( 4 ); Cp^x = MeC₅H₄, thf = Tetrahydrofuran.     

A DFT Quantum‐Chemical Study of Ion‐Pair Formation for the Catalyst Cp2ZrMe2 /MAO

A process of ion‐pair formation in the system Cp₂ZrMe₂/methylaluminoxane (MAO) has been studied by means of density functional theory quantum‐chemical calculations for MAOs with different structures and reactive sites. An interaction of Cp₂ZrMe₂ with a MAO of the composition (AlMeO)₆ results in the formation of a stable molecular complex of the type Al₅Me₆O₅Al(Me)O–Zr(Me)Cp₂ with an equilibrium distance r(Zr–O) of 2.15 Å. The interaction of Cp₂ZrMe₂ with “true” MAO of the composition (Al₈Me₁₂O₆) proceeds with a tri‐coordinated aluminum atom in the active site (OAlMe₂) and yields the strongly polarized molecular complex or the μ‐Me‐bridged contact ion pair ( d ) [Cp₂(Me)Zr(μMe)Al≡MAO] with the distances r(Zr–μMe) = 2.38 Å and r(Al–μMe) = 2.28 Å. The following interaction of the μ‐Me contact ion pair ( d ) with AlMe₃ results in a formation of the trimethylaluminum (TMA)‐separated ion pair ( e ) [Cp₂Zr(μMe)₂AlMe₂]⁺–[MeMAO]^– with r[Zr–(MeMAO)] equal to 4.58 Å. The calculated composition and structure of ion pairs ( d ) and ( e ) are consistent with the ¹³C NMR data for the species detected in the Cp₂ZrMe₂/MAO system. An interaction of the TMA‐separated ion pair ( e ) with ethylene results in the substitution of AlMe₃ by C₂H₄ in a cationic part of the ion pair ( e ), and the following ethylene insertion into the Zr–Me bond. This reaction leads to formation of ion pair ( f ) of the composition [Cp₂ZrCH₂CH₂CH₃]⁺–[Me‐MAO]^– named as the propyl‐separated ion pair. Ion pair ( f ) exhibits distance r[Zr–(MeMAO)] = 3.88 Å and strong C_γ‐agostic interaction of the propyl group with the Zr atom. We suppose this propyl‐separated ion pair ( f ) to be an active center for olefin polymerization.     

Unsaturation in binuclear iron carbonyl complexes of the split (3 + 2) five-electron donor hydrocarbon ligand bicyclo[3.2.1]octa-2,6-dien-4-yl: Role of agostic hydrogen atoms

The experimentally known split (3 + 2) five-electron donor bicyclo[3.2.1]octa-2,6-dien-4-yl (bcod) ligand provides a flexible alternative to the rigid planar cyclopentadienyl (Cp) ligand. In this connection, the structures and energetics of the binuclear iron carbonyl complexes (bcod)₂Fe₂(CO)_n (n = 4, 3, 2, 1) have been investigated by density functional theory for comparison with the corresponding Cp₂Fe₂(CO)_n derivatives. The cis and trans doubly CO-bridged (bcod)₂Fe₂(μ-CO)₂(CO)₂ structures are the lowest energy tetracarbonyl structures, similar to the Cp₂Fe₂(CO)₄ system. However, an unbridged (bcod)₂Fe₂(CO)₄ isomer lies only ~1 kcal/mol in energy above the doubly bridged isomers. The flexibility of the bcod ligand leads to low-energy singlet and triplet spin state structures with agostic hydrogen atoms for the unsaturated (bcod)₂Fe₂(CO)_n (n = 3, 2, 1) systems. Analogous structures are not found in the corresponding Cp₂Fe₂(CO)_n systems with the rigid Cp ligand. Such structures, effectively involving donation of an electron pair from an olefinic C-H bond to an iron atom through three-center two-electron C-H-Fe bonding, are energetically competitive with isomeric structures with metal-metal multiple bonds.     

Oligodimethylsilane Initiators for Photochemical Vulcanization of Siloxane Rubber

Photolytic vulcanization of siloxane rubber films in the presence of trimethylsiloxy-substituted di- and trisilanes, oligodimethylsilanosiloxanes (Me₂SiO) _m (SiMe₂) _n , Me(Me₂SiO) _m (SiMe₂) _n Me, oligodimethylsilanes Me(Me₂Si) _n Me, and volatile pyrolysis products of polydimethylsilane was studied.     

Synthesis and Spectroscopic Characterisation of a New Class of Heterobimetallic Zirconocene(IV) Compounds

Reactions of Cp₂ZrCl₂ with homometallic complexes of aluminium containing one residual hydroxy group Al(OGO)(OGOH) and Al(L)(OGOH) [where G=G¹=CMe₂CMe₂ (1a); G=G²=CMe₂CH₂CHMe (1b); G= G³=CMe₂CH₂CH₂CMe₂ (1c) and L=L¹=OC₆H₄CH=NCH₂CH₂O, G=G¹ (2a); L=L¹, G=G² (2b); L=L¹, G=G³ (2c); L=L²=OC₁₀H₆CH=NCH₂CH₂O, G=G¹ (2d); L=L², G=G² (2e); L=L², G=G³ (2f)] in THF using Et₃N as HCl acceptor affords novel heterobimetallic compounds of the types Al(OGO)₂Zr(Cl)Cp₂ and Al(L)(OGO)Zr(Cl)Cp₂, respectively. All of these derivatives have been characterised by elemental analyses, molecular weight measurements, and spectroscopic [IR, NMR (¹H and ²⁷Al)] studies.     

Synthesis and characterization of heterobimetallic mixed-ligand complexes containing Zr(μ-OPri)2Al bridging units

Interesting varieties of heterobimetallic mixed-ligand complexes [Zr{M(OPrⁱ) _n }₂ (L)] (where M = Al, n = 4, L = OC₆H₄CH = NCH₂CH₂O (1); M = Nb, n = 6, L = OC₆H₄CH = NCH₂CH₂O (2); M = Al, n = 4, L = OC₁₀H₆CH = NCH₂CH₂O (3); M = Nb, n = 6, L = OC₁₀H₆CH = NCH₂CH₂O (4)), [Zr{Al(OPrⁱ)₄}₂Cl(OAr)] (where Ar = C₆H₃Me₂-2,5 (5); Ar = C₆H₂Me-4-Bu₂-2,6 (6), [Zr{Al(OPrⁱ)₄}₂(OAr)₂] (where Ar = C₆H₃Me₂-2,5 (7); Ar = C₆H₂Me-4-Bu₂-2,6 (8), [Zr{Al(OPrⁱ)₄}₃(OAr)] (where Ar = C₆H₃Me₂-2,5 (9); Ar = C₆H₃Me₂-2,6 (10), [ZrAl(OPrⁱ)_7-n (ON=CMe₂) _n ] (where n = 4 (11); n = 7 (12), [ZrAl₂(OPrⁱ)_10-n (ON=CMe₂) _n ] (where n = 4 (13); n = 6 (14); n = 10 (15) and [Zr{Al(OPrⁱ)₄}₂{ON=CMe(R)} _n Cl_2–n] [where n = 1, R = Me (16); n = 2, R = Me (17); n = 1, R = Et (18); n = 2, R = Et (19)] have been prepared either by the salt elimination method or by alkoxide-ligand exchange. All of these heterobimetallic complexes have been characterized by elemental analyses, molecular weight measurements, and spectroscopic (I.r., ¹H-, and ²⁷Al- n.m.r.) studies.     

Mimicking the electronic structure of endohedral triangular lanthanide clusters in “free” from host carbon cages metallofullerenes uncovers a peculiar reactivity pattern

Highly reduced [Cp₃Ln₃(μ₂-H)₃]^?/0 (Ln = La or Lu; Cp = C₅H₅^?) clusters “free” from host carbon cages were used as models to mimic the electronic structure of La₃@C₁₁₀ and Lu₃@C₈₀ EMFs. DFT calculations revealed that these clusters “unshielded” from host carbon cages are highly reactive, disrupting strong single H–H, H–X (X = F, Cl, Br, and I) and double O=O bonds and descending the inert N≡N triple bond up to a single N–N bond, yielding stable bicapped trinuclear [Cp₃Ln₃(μ₂-H)₃(μ₃-H)₂]^?, [Cp₃Ln₃(μ₂-H)₃(μ₃-H)(μ₃-X)]^?, [Cp₃Ln₃(μ₂-H)₃(μ₃-O)₂]^?, and [Cp₃Ln₃(μ₂-H)₃(μ₃-N)₂]^? clusters. The calculated thermodynamics of the reactions revealed an unprecedented reactivity pattern inherent to multimetallic cooperative effect on nonclassic oxidative addition reactions which proceed by electrophilic attack of the oxidative addition (oxad) substrates at the center of the highly reduced triangular trilanthanide Ln₃ rings accompanied by “penetration” of the ring plane that cuts the strong bonds. The [Cp₃Ln₃(μ₂-H)₃]^?/0 (Ln = La or Lu) clusters, mimicking also the electronic structure of La₃@C₁₁₀ and Lu₃@C₈₀ EMFs, easily capture hydrogen, nitrogen, oxygen, and halogen atoms to yield monocapped trimetallic [Cp₃Ln₃(μ₂-H)₃(μ₃-X)] (X = H, N, O, F, Cl, Br, and I) clusters. The molecular and electronic structures of the “free” from the cage monocapped trimetallic clusters are thoroughly discussed.     

Aluminum‐poor hexacarbalane structures: The transition from localized organoaluminum structures to delocalized polyhedra

The series of hexacarbalanes C₆Al_n–6Me_n (n = 7–11) represent a progression from localized organoaluminum structures to delocalized polyhedral structures en route to experimentally known 13‐ and 14‐vertex hexacarbalanes such as (AlMe)₈(CCH₂Ph)₅(µ₄ H), (AlMe)₈(CCH₂Ph)₅(CCPh), [R₄N⁺]₂[(AlH)₈(CR)₆], and (AlNMe₃)₂(AlR)₆(CR)₆. In this connection, the lowest energy seven‐vertex C₆AlMe₇ structure has a tetrahapto benzene ring with the four Al C(cage) bonding interactions required to give the aluminum the favored octet configuration. Related eight‐vertex C₆Al₂Me₈ structures are found with a benzene ring bound to an Al₂ unit with a short AlAl distance of ∼2.55 Å suggesting a formal double bond. However, the lowest energy C₆Al₂Me₈ structure has a dialuminacyclobutene unit fused to a tricyclohexane unit through an Al₂ edge. Other relatively low‐energy C₆AlMe₇ and C₆Al₂Me₈ structures consist of a six‐carbon hexatriene chain either forming a seven‐membered C₆Al ring in the seven‐vertex structure or acting as a “flyover” between an Al₂ unit. The lowest energy nine‐vertex hexacarbalane C₆Al₃Me₉ has two separate C₃ units bridged by both an Al₂ pair and a single aluminum atom. Higher energy C₆Al₃Me₉ hexacarbalanes contain a pentadienyl chain and an isolated carbon atom with an imbedded bonded Al₃ triangle. The low‐energy 10‐vertex C₆Al₄Me₁₀ structures have a central Al₄ butterfly with nonbonding distances between the wingtips ranging from 3.35 to 3.91 Å. The lowest energy 11‐vertex C₆Al₅Me₁₁ structure has a central Al₄ quadrilateral with a diagonal bridged by the fifth aluminum atom. Higher energy C₆Al₅Me₁₁ structures have an edge rather than a diagonal of the central Al₄ quadrilateral bridged by the fifth aluminum atom.     

Generation and capture of methyl(vinyl)silanone

Methyl(vinyl)dichlorosilane reacts with DMSO in the presence of hexamethyldisiloxane to give the corresponding linear oligosiloxanes of the general formula Me₃Si(OSiMeVin) _n OSiMe₃ (n=1–6) as well as MeSi(OSiMe₃)₃ and Me₃Si(MeOSiVin) _m OSi(OSiMe₃)(Me)OSiMe₃ (m=1–2). The same reaction in the presence of chlorotrimethylsilane results in oligomers of the general formula Me₃Si(OSiMeVin) _n Cl (n=1–3). A possible scheme of their formation is discussed. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1614–1616, August, 1998.     

Selective Functionalization of P4 by Metal‐Mediated CP Bond Formation

A new and selective one‐step synthesis was developed for the first activation stage of white phosphorus by organic radicals. The reactions of NaCp^R with P₄ in the presence of CuX or FeBr₃ leads to the clean formation of organic substituted P₄ butterfly compounds Cp^R₂P₄ (Cp^R: Cp^BIG=C₅(4‐nBuC₆H₄)₅ ( 1 a ), Cp′′′=C₅H₂tBu₃ ( 1 b ), Cp*=C₅Me₅ ( 1 c ) und Cp^4iPr=C₅HiPr₄ ( 1 d )). The reaction proceeds via the activation of P₄ by Cp^R radicals mediated by transition metals. The newly formed organic derivatives of P₄ have been comprehensively characterized by NMR spectroscopy and X‐ray crystallography.     

Generation and capture of alkylchlorosilanones

Alkyltrichlorosilanes react with DMSO (molar ratio 1 : 1 0 °C) to give cyclic oligoalkylchlorosiloxanes of the general formula [R(Cl)SiO] _n (where R=Me or Et;n=3–6). With an excess of alkyltrichlorosilane (2: 1), linear oligoalkylchlorosiloxanes Cl[R(Cl)SiO] _m SiCl₂R (where R=Me or Et;m=1–5) are also formed. In the presence of hexamethyldisiloxane (molar ratio Cl₃SiR : DMSO: (Me₃Si)₂O=1:1:2, 20 °C), the reaction products are both cyclic and linear oligoalkyl(trimethylsilyloxy)siloxanes [R(Me₃SiO)SiO] _n (n=3–5) and Me₃Si[OSi(OSiMe₃)R] _m OSiMe₃ (m=1–3), respectively. The reaction of DMSO with trichloro(vinyl)silane and hexamethyldisiloxane occurs in a similar manner. A plausible scheme of formation of the final products via intermediate alkylchlorosilanones RClSi=O and alkyl(trimethylsilyloxy)silanones is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 361–364, February, 2000.     

Fine Tuning of Metallaborane Geometries: Chemistry of Metallaboranes of Early Transition Metals Derived from Metal Halides and Monoborane Reagents

Reaction of [Cp_nMCl_4?x] (M=V: n=x=2; M=Nb: n=1, x=0) or [Cp*TaCl₄] (Cp=η⁵‐C₅H₅, Cp*=η⁵‐C₅Me₅), with [LiBH₄?thf] at ?70 °C followed by thermolysis at 85 °C in the presence of [BH₃?thf] yielded the hydrogen‐rich metallaboranes [(CpM)₂(B₂H₆)₂] ( 1 : M=V; 2 : M = Nb) and [(Cp*Ta)₂(B₂H₆)₂] ( 3 ) in modest to high yields. Complexes 1 and 3 are the first structurally characterized compounds with a metal–metal bond bridged by two hexahydroborate (B₂H₆) groups forming a symmetrical complex. Addition of [BH₃?thf] to 3 results in formation of a metallaborane [(Cp*Ta)₂B₄H₈(μ‐BH₄)] ( 4 ) containing a tetrahydroborate ligand, [BH₄]^?, bound exo to the bicapped tetrahedral cage [(Cp*Ta)₂B₄H₈] by two Ta‐H‐B bridge bonds. The interesting structural feature of 4 is the coordination of the bridging tetrahydroborate group, which has two B? H bonds coordinated to the tantalum atoms. All these new metallaboranes have been characterized by mass, ¹H, ¹¹B, and ¹³C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of 1 – 4 .