Metal fragment exchange from a molybdaborane to a tungstaborane

Reaction of the molybdaborane arachno-2-[Mo(η-C₅H₅)(η⁵:η¹-C₅H₄)B₄H₇] (I) with the electron-rich molecule [W(PMe₃)₃H₆] at 60 °C for 12 h in toluene gives the novel tungstaborane nido-2-W(PMe₃)₃H₂B₄H₇[Mo(η-C₅H₅)(η⁵:η¹-C₅H₄)H₂] (II) in 60% yield. The reaction is almost quantitative when followed by NMR. This is a rare example of metal fragment exchange within a metallaborane cage. The molybdenum atom is retained in the molecule via a σ-bond between the substituted cyclopentadienyl ring and a basal boron atom in the metallaborane cluster.     

Formation of ferraboranes from pentaborane(9) or BH3 · thf and an electron-rich cyclopentadienyl iron phosphine hydride

[(η⁵-C₅R₅)Fe(PMe₃)₂H] (R = H, Me) can be made in good yields in a simple one-pot reaction between FeCl₂, PMe₃, C₅R₅H (R = H, Me) and Na/Hg in thf. Reaction of [(η⁵-C₅H₅)Fe(PMe₃)₂H] with pentaborane(9) gives the known metallaborane [(η⁵-C₅H₅)-nido-2-FeB₅H₁₀] (1) in improved yield as well as the new metallaboranes [(η-C₅H₅)-nido-2-FeB₅H₈{μ-5,6-Fe(η⁵-C₅H₅)(PMe₃)(μ-6,7-H)}] (2), [(η-C₅H₅)(PMe₃)-arachno-2-FeB₃H₈] (3), [(η⁵-C₅H₅)₂-capped-nido-2,3-Fe₂B₄H₈] (4), [(η⁵-C₅H₅)-nido-2-FeB₄H₇(PMe₃)] (5) and [(η⁵-C₅H₅)-nido-2-FeB₅H₈(PMe₃)] (6). Reaction of [(η⁵-C₅Me₅)Fe(PMe₃)₂H] with pentaborane(9) gives predominantly [(η⁵-C₅Me₅)-nido-2-FeB₅H₁₀] (7) and [(η⁵-C₅Me₅)(PMe₃)-arachno-2-FeB₃H₈] (8). Reaction of [(η⁵-C₅H₅)Fe(PMe₃)₂H] with 2 equiv. of BH₃ · thf gives low yields of ferrocene and compound 3. Compound 7 thermally isomerises to the apical isomer [(η⁵-C₅H₅)-nido-2-FeB₅H₁₀] (9) in low yield. Compounds 1 and 7 deprotonate cleanly in the presence of KH at the unique B-H-B bridge to give [(η⁵-C₅H₅)-nido-2-FeB₅H₉⁻][K⁺] (10) and [(η⁵-C₅Me₅)-nido-2-FeB₅H₉⁻][K⁺] (11) respectively, whilst 6 deprotonates more slowly at one of two equivalent B-H-B bridges to give the fluxional anion [(η⁵-C₅H₅)-nido-2-FeB₅H₇(PMe₃)⁻] (12).     

Reaction of nido-1,2-(CpRuH)2B3H7 with ethynylferrocene to yield new metallacarboranes

Addition of ethynylferrocene to nido-1,2-(Cp^∗RuH)₂B₃H₇ (1) at ambient temperature leads to nido-1,2-(Cp^∗Ru)₂(1,5-μ-C{Fc}Me)B₃H₇ (2, 3) and closo-4-Fc-1,2-(Cp^∗RuH)₂-4,6-C₂B₂H₃ (4). Compounds 2 and 3 represent a pair of geometric isomers, nido-species in which the regiochemistry of the alkyne reduction conforms to the Markovnikoff rule. Compound 4 is an octahedral structure in which the inserted alkyne is on an open face of the closo cluster.     

Reactions of nido-1,2-(Cp*RuH)2B3H7 with RCCR′ (R, R′=H, Ph; Me, Me) to yield novel metallacarboranes

Addition of the internal alkyne, 2-butyne, to nido-1,2-(Cp*RuH)₂B₃H₇ (1) at ambient temperature produces nido-1,2-(Cp*Ru)₂(μ-H)(μ-BH₂)-4,5-Me₂-4,5-C₂B₂H₄ (2), nido-1,2-(Cp*RuH)₂-4,5-Me₂-4,5-C₂B₂H₄ (3), and nido-1,2-(Cp*RuH)₂-4-Et-4,5-C₂B₂H₅ (4), in parallel paths. On heating, 2, which contains a novel exo-polyhedral borane ligand, is converted into closo-1,2-(Cp*RuH)₂-4,5-Me₂-4,5-C₂B₃H₃ (5) and nido-1,6-(Cp*Ru)₂-4,5-Me₂-4,5-C₂B₂H₆ (6) the latter being a framework isomer of 3. Heating 2 with 2-butyne generates nido-1,2-(Cp*RuH)₂-3-{CMeCMeB(CMeCHMe)₂}-4,5-Me₂-4,5-C₂B₂H₃ (7) in which the exo-polyhedral borane is triply hydroborated to generate a boron bound ---CMeCMeB(CMeCHMe)₂ cluster substituent. Along with 3, 4, 5, 6, and 7, the reaction of 1 with 2-butyne at 85 °C gives closo-1,7-(Cp*Ru)₂-2,3,4,5-Me₄-6-(CHMeCH₂Me)-2,3,4,5-C₄B (8). Reaction of 1 with the terminal alkyne, phenylacetylene, at ambient temperature permits the isolation of nido-1,2-(Cp*Ru)₂(μ-H)(μ-CHCH₂Ph)B₃H₆ (9) and nido-1,2-(Cp*Ru)₂(μ-H)(μ-BH₂)-3-(CH₂)₂Ph-4-Ph-4,5-C₂B₂H₄ (11). The former contains a Ru---B edge-bridging alkylidene fragment generated by hydrometallation on the cluster framework whereas the latter contains an exo-polyhedral borane like that of 2. Thermolysis of 11 results in loss of hydrogen and the formation of closo-1,2-(Cp*RuH)₂-3-(CH₂)₂Ph-4-Ph-4,5-C₂B₃H₃ (12).     

Reactions of pentaborane(9) with electron-rich molybdenum and tungsten phosphine polyhydrides

Reaction of [W(PMe₂Ph)₃H₆] with pentaborane(9) gives nido-2-[W(PMe₂Ph)₃H₂B₄H₈] (1) as well as nido-2-[W(PMe₂Ph)₃HB₅H₁₀] (2). The crystal structure of (2) has been determined. Compound (2) has a novel metallaborane structure containing an edge-bridging {BH₃} group between the tungsten atom and one of the basal boron atoms in a “nido-WB₄” pyramid. Reaction of [W(PMe₃)₄(η²-CH₂PMe₂)H] with pentaborane(9) gives nido-2-[W(PMe₃)₃H₂B₄H₈] (3) whilst reaction of [Mo(L)₄H₄] with pentaborane(9) gives nido-2-[Mo(L)₃H₂B₄H₈] [L = PMe₃ (4), PMe₂Ph (5)]. Treatment of [Mo(PMe₃)₄H₄] with excess BH₃ · thf gives the known borohydride [Mo(PMe₃)₄H(η²-BH₄)].     

Functionalization of the macropolyhedral borate anion cluster [B22H22]: isolation and characterization of the OH and OEt derivatives

Preparation and characterization of the first derivatives of the fused macropolyhedral anion [B₂₂H₂₂]²⁻ are reported. The species [B₂₂H₂₁OH]²⁻ (1) and [B₂₂H₂₁OEt]²⁻ (2) are obtained from workup of the products of the reaction between HgBr₂ and [NBzlEt₃]₂[B₂₂H₂₂]; a cluster involving the conjoining of a closo-B₁₂ icosahedron with a nido-B₁₀ cluster. Washing the products with ethanol followed by thin-layer chromatography allows the isolation of 1 and 2, reproducibly, in yields of 27 and 20%, respectively. The species were characterized by NMR spectroscopy, elemental analysis and X-ray diffraction studies. The crystal structure determinations of the two species identify novel features. Apparently the influence of the O atoms in the ions [B₂₂H₂₁OH]²⁻ and [B₂₂H₂₁OEt]²⁻ results in the lengthening of what was a gunwale B---B connection adjacent to the junction of the two cages such that the distances are 2.180 and 2.230 Å, respectively. These latter are longer than the corresponding distance in the parent species [B₂₂H₂₂]²⁻, which is 2.09 Å; quite long for a normal B---B distance. Thus it is assumed that these B atoms, in 1 and 2, one of which bears the substituent, are not bonded to each other.     

The first metallacarborane triple-decker complexes with a bridging borole ligand

The (borole)iodide complex [(η⁵-C₄H₄BPh)RhI]₄ reacts with the carborane anion [Carb′]⁻ (Carb′ = 9-SMe₂-7,8-C₂B₉H₁₀) giving (Carb′)Rh(η⁵-C₄H₄BPh) (2). Reactions of 2 with dicationic fragments [LM]²⁺ afford the μ-borole triple-decker complexes [(Carb′)Rh(μ-η⁵:η⁵-C₄H₄BPh)ML]²⁺ [LM = Cp^∗Ir (4), (Carb′)Rh (7)] or the arene-type complexes [(Carb′)Rh(μ-η⁵:η⁶-C₄H₄BPh)ML]²⁺ [LM = Cp^∗Rh (3), (Carb′)Ir (8)]. The structure of 4(BF₄)₂ was determined by X-ray diffraction.     

Structural assignment of organotin hydrides containing the oxazoline ligand

A series of triorganotin hydrides and diorganotin dihydrides containing the optically active 2-(4-isopropyl-2-oxazolinyl)-5-phenyl ligand have been characterized by means of the multinuclear low-temperature NMR investigations, the results of which are discussed. In the corresponding organotin hydrides values of the ¹J(¹H-^117/119Sn) couplings appeared to be temperature dependent, supporting an axial/equatorial position of the hydrogen attached to the tin.     

A new look at the nido-undecaborate system

The structure of the nido-undecaborate anion, [B₁₁H₁₄]⁻, has been re-examined because of what appear to be discrepancies that were observed between our determination of the structure of the anion in [(Cp₂Zr)₂B₅H₈][B₁₁H₁₄] (1) and previously published structures. The structure of 1 indicated the presence of two bridging H atoms and another pseudo-bridging one whereas those of a series of published structures indicate the presence of a plane of symmetry with two bridging H atoms and one endo-H atom. Thus, we undertook a series of structural determinations and also a computational study at the B3LYP/6-31++G(d,p) level. In addition to 1, the species studied included [NBnEt₃][B₁₁H₁₄] (2), [NBnEt₃][7-Br-nido-B₁₁H₁₃] (3) and [NBnEt₃][7-(η¹-dppm)-nido-B₁₁H₁₂] (4). Our structure of 2 indicated the presence of two bridging H atoms and an endo-hydrogen atom with some bridging character but that of 3 contained three bridging atoms. As expected the structure of 4 contains two bridging H atoms. Calculations of bond parameters fit well with the experimental data as do the ¹¹B NMR chemical shifts. The latter were calculated for the average of the two open face configurations, one with two bridging and one endo-hydrogen and the other with three bridging hydrogen atoms. The difference in energies for these two open face configurations is calculated to be 0.36 kJ/mol, which effectively suggests that the two structures are equally favored.     

Reactions of small boranes with ruthenium phosphine hydrides: Oligomerisation of monoborane-tetrahydrofuran to a nido-hexaruthenaborane

Reaction of [Ru(PPh₃)₄H₂] with BH₃ · thf at room temperature gives borane oligomerisation with the formation of the 6-vertex metallaborane nido-2-[Ru(PPh₃)₂(H)B₅H₁₀] (1). This cluster is also formed by reaction of [Ru(PPh₃)₄H₂] with nido-B₅H₉. Compound (1) is readily deprotonated by KH in thf at the unique basal B-H-B bridge to give (2). In contrast to [Ru(PPh₃)₄H₂] reaction of [cis-Ru(PMe₃)₄H₂] with BH₃ · thf gives initially the known borohydride [Ru(PMe₃)₃(H)(η²-BH₄)] which reacts with excess BH₃ · thf to give the 5-vertex metallaborane nido-2-[Ru(PMe₃)₃B₄H₈] (3). Reaction of [cis-Ru(PMe₃)₄H₂] with nido-B₅H₉ also gives (3) and nido-2-[Ru(PMe₃)₃B₉H₁₃] (4). [cis-Ru(PMe₃)₄H₂] is conveniently prepared in high yield in a one-pot synthesis by the sodium amalgam reduction of RuCl₃ · 3H₂O in thf with excess PMe₃ under dinitrogen.     

New chiral tin compounds containing the 2-(4-isopropyl-2-oxazolinyl)-5-phenyl ligand

A series of tri- and tetraorganotin compounds containing the optically active 2-(4-isopropyl-2-oxazolinyl)-5-phenyl ligand and tert-butyl, methyl and/or phenyl groups on the tin has been synthesized. All the novel compounds have been characterized, especially by means of the multinuclear NMR investigation, the results of which are discussed. The tin halides, as pairs of diastereoisomers in solution, crystallize in the form of one diastereoisomer. The single-crystal X-ray analysis of tin iodide 10a revealed pseudo-equatorial position of the tert-butyl group opposite to the isopropyl group. In the corresponding diastereomeric tin hydrides values of ¹J(¹H-^117/119Sn) differ significantly, suggesting a different pseudo-axial/equatorial position of the hydrogen atom.     

Reaction of Hexamethyldisilazane with Borane in Tetrahydrofuran

Hexamethyldisilazane 1 reacts with borane in tetrahydrofuran (THF? BH₃, 2 ) first by formation of an adduct (Me₃Si)₂NH? BH₃ ( 3 ), and then either to the N,N-bis-(trimethylsilyl)-μ-aminodiborane 5 or to the mixture of 5 and N-trimethylsilyl-μ-aminodiborane(6) 6 , depending on the reaction conditions. The compounds 5 and 6 can be quantitatively converted to the N,N′,N″-tris(trimethylsilyl)borazine 4 . Three intermediates can be identified, namely N,N-bis(trimethylsilyl)borane 7 , N,N-bis(trimethylsilyl)amino(N′-trimethylsilylamino)borane 8 and N-trimethylsilylaminoborane-trimer. All products and intermediates were characterized by multinuclear NMR spectroscopy, and coupling constant ¹J(²⁹Si, ¹⁵N) were measured from ²⁹Si NMR spectra by using the Hahn-echo-extended (HEED) INEPT pulse sequence.     

Dimesitylborane monomer-dimer equilibrium in solution, and the solid-state structure of the dimer by single crystal neutron and X-ray diffraction

Dimesitylborane dimer has been shown to exist in equilibrium with dimesitylborane monomer in solution. This equilibrium has been investigated by variable concentration and variable temperature multinuclear NMR spectroscopy and values for the dissociation constant, enthalpy and entropy of dissociation were found to be K_diss=(3.2±0.4)×10⁻³ M, ΔH=70 kJ mol⁻¹, and ΔS=212 J K⁻¹mol⁻¹, respectively. Ab initio methods have been used to investigate the gas-phase structures and energies of both monomer and dimer, and calculated ¹¹B-NMR shifts are also presented. The solid-state structure of dimesitylborane dimer has been investigated by single crystal X-ray diffraction at 100 K and the position of the bridging hydrogen atoms (B---H=1.340(2), 1.342(2) Å, H---B---H=92.46(14)°) has been determined accurately, for the first time, by single crystal neutron diffraction at 20 K.     

Further evidence of incorporation of chromium ions into the framework of silicoaluminophosphate molecular sieves

Three different molecular sieves were synthesised and characterized using³¹P and²⁷Al magic angle spinning nuclear magnetic resonance (³¹P and²⁷Al MAS NMR) spectroscopy and acidity measurement techniques. The synthesized solids were: a silicoaluminophosphate (SAPO-11) sample, a chromium-substituted silicoaluminophosphate (CrAPSO-11) sample and a chromium-supported SAPO-11 (Cr/SAPO-11) sample. Significant differences were observed between the CrAPSO-11 MAS NMR spectra and the spectra for the other two solids. The differences can be understood in terms of a different chemical environment for the Al(III) and P(V) ions in the molecular sieve framework, as a result of a different type of interaction, probably with substituted chromium ions in the framework. The acidity measurements were in agreement with the MAS NMR spectroscopy results, providing further evidence for the incorporation of chromium ions into the molecular sieve framework.     

Planar Iron Hydride Nanoclusters: Combined Spectroscopic and Theoretical Insights into Structures and Building Principles

The controlled assembly of well-defined planar nanoclusters from molecular precursors is synthetically challenging and often plagued by the predominant formation of 3D-structures and nanoparticles. Herein, we report planar iron hydride nanoclusters from reactions of main group element hydrides with iron(II) bis(hexamethyldisilazide). The structures and properties of isolated Fe₄, Fe₆, and Fe₇ nanoplatelets and calculated intermediates enable an unprecedented insight into the underlying building principle and growth mechanism of iron clusters, metal monolayers, and nanoparticles.     

Chemistry related to cluster-borane analogues of the cyclopentadienide anion and ferrocene: New developments

A review on cluster-borane analogues of the cyclopentadienide anion (Cp) and ferrocene is presented. Analogues of Cp that have been so far isolated and characterised are the 11-vertex triheteroboranes of general structure [nido-E₃B₈H₈]⁻ (where E = CH or P and their combinations), the molecules of which contain an open pentagonal face. These anions were used as effective ligands for the preparation of “half- and full-sandwich” complexes [CpFeE₃B₈H₈] and [Fe(E₃B₈H₈)₂], respectively - analogues of ferrocene. Developments in this area of cluster-borane chemistry that include recent results in the synthesis and Fe-complexation reactions of 11-vertex tricarbaboranes (tricarbollides), phosphadicarbollides, and diphosphacarbollides are the subject of this work.     

Covalent incorporation of methyl red dyes into double-stranded DNA for their ordered clustering

An ordered dye cluster of Methyl Reds was formed in double-stranded DNA by hybridizing two complementary DNA-dye conjugates, each involving a Methyl Red moiety on a threoninol linker and a 1,3-propanediol spacer arranged alternately in the middle of the DNA sequence. In the duplex, Methyl Reds from each strand were axially stacked antiparallel to each other, as determined from NMR analysis. This clustering of Methyl Reds induced distinct changes in both UV/Vis and CD spectra. Single-stranded DNA-Methyl Red conjugates on D-threoninol linkers and (1,3-propanediol) spacers exhibited broad absorption spectra with lambda(max) at around 480 nm, and almost no CD was observed at around the absorption maximum of Methyl Red. However, as Methyl Reds were clustered by hybridization, lambda(max) shifted towards shorter wavelengths with respect to its monomeric transition. This hypsochromic shift increased as the number of Methyl Red molecules increased. Furthermore, a positive couplet was also strongly induced here. These dye clusters are H-aggregates, in which molecular excitons are coupled. The positive couplet demonstrates that the clusters on D-threoninol form a right-handed helix. In contrast, the induced CD became much weaker with Methyl Red on L-threoninol, which intrinsically prefers counterclockwise winding. Thus, mutual orientation of the stacked dye molecules was controlled by the chirality of the linker.     

Synthesis, characterization, and spectrophotometric studies of novel fluorescent arachno decaborane and nonaborane clusters containing aza-distyrylbenzene derivatives

Two aza-analogues of distyrylbenzene namely: 1,4-bis[β-(4-quinolyl)vinyl]benzene (PhQ) and 1,4-bis[β-(4-pyridyl)vinyl]benzene (PhPy) containing arachno-decaborane or arachno-nonaborane clusters have been isolated: 6,9-(PhQ)₂-arachno-B₁₀H₁₂ (1), N,N′-bis[9-Me₂S-arachno-B₁₀H₁₂-6-yl]PhQ (2), 6,9-(PhPy)₂-arachno-B₁₀H₁₂ (3), N,N′-bis[(9-Me₂S)-arachno-B₁₀H₁₂-6-yl]PhPy (4), N,N′-bis[arachno-B₉H₁₃-4-yl]PhQ (5), 4-PhQ-arachno-B₉H₁₃ (6), N,N′-bis[arachno-B₉H₁₃-4-yl]PhPy (7), and 4-PhPy-arachno-B₉H₁₃ (8). These boronated compounds were easily prepared from the displacement reactions of weaker ligand (SMe₂) of bis (dimethyl sulfide) arachno-decaborane(14) {6,9-(Me)₂SB₁₀H₁₂}or dimethyl sulfide-arachno-nonaborane {4-(Me)₂SB₉H₁₃} by the stronger bidentate ligands of PhQ or PhPy in ratio (1:2). The electronic interaction between decaborane or nonaborane arachno-type unit and the bonded pyridine units has been investigated by UV-Vis spectroscopy and by AM1 molecular orbital calculations. The resulting compounds undergo trans-cis photoisomerization upon excitation. The connection of boron clusters to PhQ and PhPy led to enhancing of the photoreactivity and decreasing of the fluorescence quantum yield of the products.