Michal Horáček

Comparison of the catalytic activity of MOFs and zeolites in Knoevenagel condensation

The catalytic behavior of metal–organic-frameworks (MOFs) CuBTC and FeBTC was investigated in Knoevenagel condensation of cyclohexane carbaldehyde and benzaldehyde with active methylene compounds and compared with zeolites BEA and TS-1. High yields were achieved over the CuBTC catalyst in the Knoevenagel condensation involving malonitrile, especially at a relatively low reaction temperature (80 °C); no leaching of the active phase was evidenced. In contrast, zeolites were not active under such reaction conditions. We propose an activation of malonitrile on a pair of adjacent Cu ions to explain the high catalytic activity of CuBTC with respect to conventional catalysts. Compared with CuBTC, zeolites exhibited usually lower selectivities, which is ascribed to a high acid strength of their active sites promoting consecutive reactions.

Substituent effects in cyclic voltammetry of titanocene dichlorides

Abstract Methyl-substituted titanocene dichlorides (C5H5−nMen)2TiCl2 (n=0–5), [C5Me4(SiMe3)]2TiCl2, (C5Me4Ph)2TiCl2 (Ph=phenyl), [C5Me4(FPh)]2TiCl2 (FPh=para-fluorophenyl), [C5Me4(CH2Ph)]2TiCl2 and ansa-compounds Me2Si(C5H4)2TiCl2 and Me2Si(C5Me4)2TiCl2 were investigated by means of cyclic voltammetry at a mercury electrode in tetrahydrofuran. The standard potential (E°1) of the first electron uptake shifts to more negative values proportionally to the number of methyl groups in the (C5H5−nMen)2TiCl2 (n=0–3) compounds, with an increment of 0.093 V per one methyl group. A decline from this linear dependence is observed for (C5HMe4)2TiCl2 and a positive shift for (C5Me5)2TiCl2. The [C5Me4(R)]2TiCl2 (R=SiMe3, Ph, FPh and CH2Ph) compounds show even larger positive shifts of E°1. These positive shifts can be brought about by a steric strain between the cyclopentadienyl ligands which lowers the dihedral angle between cyclopentadienyl ring planes (φ) and thus decreases energies of bent titanocene 1a1 and b2 LUMO orbitals. This opinion is corroborated by the voltammetry of ansa-compounds Me2Si(C5H4)2TiCl2 and Me2Si(C5Me4)2TiCl2, having a large and fixed angle φ. Their E°1 values are close to those of (C5H5)2TiCl2 and (C5HMe4)2TiCl2, respectively.

Reduction of Bis[η5-(ω-alkenyl)tetramethylcyclopentadienyl]titanium Dichlorides: An Efficient Synthesis of Long-Chainansa-Bridged Titanocene Dichlorides by Acidolysis of Cyclopentadienyl-Ring- Tethered Titanacyclopentanes

The reduction of symmetric, fully-substituted titanocene dichlorides bearing two pendant ω-alkenyl groups, [TiCl2(η5-C5Me4R)2], RCH(Me)CH=CH2 (1 a), (CH2)2CH=CH2 (1 b) and (CH2)3CH=CH2 (1 c), by magnesium in tetrahydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [TiIV{η1:η1:tlsb&endash;3%>η5:η5-C5Me4CH(Me)CH(Ti)CH2CH(CH2(Ti))CH(Me)C5Me4}] (2 a), [TiIV{η1:η1:η5:η5-C5Me4(CH2)2CH(Ti)(CH2)2CH(Ti)(CH2)2C5Me4}] (2 b) and [TiIV{η1:η1:η5:η5-C5Me4(CH2)2CH(Ti)CH(Me)CH(Me)CH(Ti)(CH2)2C5Me4}] (2 c), respectively, as the products of oxidative coupling of the double bonds across a titanocene intermediate. For the case of complex 1 c, a product of a double bond isomerisation is obtained owing to a preferred formation of five-membered titanacycles. The reaction of the titanacyclopentanes with PbCl2 recovers starting materials 1 a from 2 a and 1 b from 2 b, but complex 2 c affords, under the same conditions, an isomer of 1 c with a shifted carbon-carbon double bond, [TiCl2{η5-C5Me4(CH2CH2CH=CHMe)}2] (1 c′). The titanacycles 2 a-c can be opened by HCl to give ansa-titanocene dichlorides ansa-[{η5:η5-C5Me4CH(Me)CH2CH2CH(Me)CH(Me)C5Me4}TiCl2] (3 a), ansa-[{η5:η5-C5Me4(CH2)8C5Me4}TiCl2] (3 b), along with a minor product ansa-[{η5:η5-C5Me4CH2CH=CH(CH2)5C5Me4}TiCl2] (3 b′), and ansa-[{η5:η5-C5Me4(CH2)3CH(Me)CH(Me)CH=CHCH2C5Me4}TiCl2] (3 c), respectively, with the bridging aliphatic chain consisting of five (3 a) and eight (3 b, 3 b′ and 3 c) carbon atoms. The course of the acidolysis changes with the nature of the pendant group; while the cyclopentadienyl ring-linking carbon chains in 3 a and 3 b are fully saturated, compounds 3 c and 3 b′ contain one asymetrically placed carbon-carbon double bond, which evidently arises from the β-hydrogen elimination that follows the HCl addition.

Titanium-catalyzed head-to-tail dimerization of tert-butylacetylene. Crystal structures of [(C5HMe4)2Ti(μ-H)2Mg(THF)(μ-Cl)]2 (THF-tetrahydrofuran) and (C5HMe4)2TiOCMe3

Abstract tert-Butylacetylene (TBUA) is readily dimerized exclusively to 2,4-di-tert-butyl-1-buten-3-yne, head-to-tail dimer (HTTD), in the presence of the (η5-C5HMe4)2TiCl2/Mg/THF system. The ESR investigation revealed the formation of Ti–Mg hydride complexes [(η5-C5HMe4)2Ti(μ-H)2Mg(THF)(μ-Cl)]2 (2) and [(η5-C5HMe4)2Ti(μ-H)2]2Mg (3) in the absence of TBUA and a tweezer-type complex, probably [(η5-C5HMe4)2Ti(η1-CCCMe3)2]− [Mg(THF)Cl]+ (4) in its presence. The catalytic system as well as complexes 2–4 were deactivated by presumably tert-butanol contained in TBUA to give (η5-C5HMe4)2TiOCMe3 (5). Purification of TBUA improved the turnover by up to 8.8×103 mol TBUA per 1 mol Ti using complex 3 as a catalyst, however, complex 5 remained the only observable product of deactivation. The crystal structures of 2 and 5 were determined by X-ray diffraction analysis.

Syntheses and structures of doubly tucked-in titanocene complexes with tetramethyl(aryl)cyclopentadienyl ligands

Titanocene–bis(trimethylsilyl)ethyne complexes [Ti(η5-C5Me4R)2(η2-Me3SiCCSiMe3)], where R=benzyl (Bz, 1a), phenyl (Ph, 1b) and p-fluorophenyl (FPh, 1c), thermolyse at 150–160°C to give products of double CH activation [Ti(η5-C5Me4Bz){η3:η4-C5Me3(CH2)(CHPh)}] (2a), [Ti(η5-C5Me4Bz){η3:η4-C5Me2Bz(CH2)2}] (2a′), [Ti(η5-C5Me4Ph){η3:η4-C5Me2Ph(CH2)2}] (2b), and [Ti(η5-C5Me4FPh){η3:η4-C5Me2FPh(CH2)2}] (2c). In the presence of 2,2,7,7-tetramethylocta-3,5-diyne (TMOD) the thermolysis affords analogous doubly tucked-in compounds bearing one η3:η4-allyldiene and one η5-C5Me4R ligand having TMOD attached by its C-3 and C-6 carbon atoms to the vicinal methylene groups adjacent to the substituent R (R=Bz (3a), Ph (3b), and FPh (3c)). Compound 3a is smoothly converted into air-stable titanocene dichloride [TiCl2{η5-C5Me2Bz(CH2CH(t-Bu)CHCHCH(t-Bu)CH2)}(η5-C5Me4Bz)] (4a) by a reaction with hydrogen chloride. Yields in both series of doubly tucked-in complexes decrease in the order of substituents: Bz≫Ph>FPh. Crystal structures of 1c, 2a, 2b, and 3b have been determined.

Reactions of methyl-substituted titanocene–bis(trimethylsilyl)acetylene complexes with acetone azine: crystal structures of (η5:η1-C5HMe3CH2CMe2NH)2Ti and (C5Me5)2Ti(NCMe2)

Abstract The titanocene–bis(trimethylsilyl)acetylene (BTMSA) complexes Cp′2Ti[η2-C2(SiMe3)2] (Cp′C5H5−nMen; n=0–5) react with acetone azine Me2CNNCMe2 (AA) in two different ways depending on the number of Me substituents at the Cp′ ligands (n). For n=0–2, BTMSA is replaced by AA, which then undergoes an oxidative addition accompanied by a proton transfer, affording titana-2-isopropyl-4-methyl-2,3-diazacyclopent-3-ene complexes A0–A2. For n=3–5, replacement of the acetylene is followed by the splitting of AA to give either (C5Me5)2Ti(III)(NCMe2) (C5) or the Ti(IV) (Cp′A)2Ti complexes B3 and B4 for n=3 and 4, respectively. The intramolecularly bridging Cp′A ligand 1-(η5-2,3,5-trimethylcyclopentadienide)-2-(η1-amide)-2,2′-dimethylethane arises from a formal insertion of the CN bond of isopropylidene amide into a CH bond of one methyl group of the cyclopentadienyl ligands before or after splitting of AA. Crystal structures of C5 and (η5:η1-C5HMe3CH2CMe2NH)2Ti(IV) (B4) were determined.

Cyclic voltammetry of methyl- and trimethylsilyl-substituted zirconocene dichlorides

Abstract Redox properties of the substituted zirconocene dichlorides: (C 5 H 5− n Me n ) 2 ZrCl 2 ( n =0–5), [C 5 H 5− n (SiMe 3 ) n ] 2 ZrCl 2 ( n =0–3), [C 5 Me 4 (SiMe 3 )](C 5 HMe 4 )ZrCl 2 and two ansa -analogues Me 2 Si(C 5 H 4 ) 2 ZrCl 2 and Me 2 Si[C 5 H 2 (SiMe 3 ) 2 ] 2 ZrCl 2 were investigated by cyclic voltammetry on a mercury electrode in tetrahydrofuran. In the (C 5 H 5− n Me n ) 2 ZrCl 2 ( n =0–4) compounds, standard electrode potential ( E °) of the one-electron uptake shifts to more negative values by 0.071 V per one methyl group. A deviation from this linear dependence to a less negative E ° is observed for (C 5 Me 5 ) 2 ZrCl 2 . This effect is attributed to the steric hindrance between rotating C 5 Me 5 ligands which tends to decrease the angle between the cyclopentadienyl ring planes ( φ ) and consequently, the energy difference between MO frontier orbitals. In the trimethylsilylated compounds, the net effect of SiMe 3 is negligible, giving virtually the same value of E ° for n =0–3. In μ-SiMe 2 -bridged ansa -compounds the difference in E ° of 147 mV corresponds to the negative shift of 37 mV per one SiMe 3 group. Owing to the rigid angle φ , this shift can be tentatively accounted for the electronic effect of the SiMe 3 groups. In the non- ansa -compounds, the negative shift due to the electronic effect of SiMe 3 groups is assumed to be roughly compensated by a positive shift resulting from a sterically controlled diminution of φ .

Syntheses and properties of some exo,exo-bis(isodicyclopentadienyl)titanium low-valent complexes

Abstract The paramagnetic compounds exo,exo-bis(η5-isodicyclopentadienyl)chlorotitanium(III) (3) and its analogue with trimethylsilyl-substituted isodicyclopentadienide (isodiCp) ligand (4), and the similar pair of diamagnetic exo,exo-bis(isodicyclopentadienyl)[η2-bis(trimethylsilyl)ethyne]titanium(II) complexes 5 and 6 were obtained by common reduction procedures from exo,exo-bis(isodicyclopentadienyl)titanium(IV) dichloride (1) and exo,exo-bis[η5-2-(trimethylsilyl)isodicyclopentadienyl]titanium(IV) dichloride (2), respectively. As indicated by ESR spectroscopy compound 3 is a dimer in the solid state and in frozen toluene glass but monomeric in toluene solution. Compound 4 is monomeric in solution as well as in the solid state. As judged from the red shift of the ν(CC) vibration, compound 6 binds bis(trimethylsilyl)ethyne more strongly than compound 5. A comparison of their wavenumbers with those of the [Ti(C5H5−nMen)2(η2-Me3SiCCSiMe3)] (n=2–5) complexes shows that Lewis acidity of the central titanium atom decreases in the order of ligands 1,3-dimethylcyclopentadienyl>isodiCp∼1,2,3-trimethylcyclopentadienyl>(trimethylsilyl)isodiCp∼tetramethylcyclopentadienyl. The crystal structure of the most bulky complex 6 shows a bis-lateral (anti) conformation of the isodiCp ligands with the π-coordinated five-membered rings nearly eclipsed.

Reactions of titanocene-bis(trimethylsilyl)ethyne complexes with diethynylsilane derivatives

Titanocene complexes [Ti(η 5 -C 5 H 5− n Me n ) 2 (η 2 -Me 3 SiCCSiMe 3 )] ( n =0, 4 and 5) react uniformly with siladiynes R 2 2 Si(CCR 1 ) 2 , where R 1 =Ph, and R 2 =Ph or Me, at elevated temperature in hydrocarbon solvents to give the corresponding silacyclobutene-annelated titanacyclobutene complexes, 3-bis(η 5 -cyclopentadienyl)titana-6-diorganylsilabicyclo[2.2.0]hexa-1(2),4(5)-dienes, [(η 5 -C 5 H 5− n Me n ) 2 Ti{R 1 2 C 4 (SiR 2 2 )}]. Products arising from [Ti(η 5 -C 5 H 5− n Me n ) 2 (η 2 -Me 3 SiCCSiMe 3 )] ( n =0, 2 (1,3-isomer), 4 and 5) and Me 2 Si(CCCMe 3 ) 2 vary with n : the non-methylated titanocene complex affords a mixture of an analogous silacyclobutene-annelated titanacyclobutene and [{Ti(η 5 -C 5 H 5 ) 2 } 2 {μ-η(3–5):η(4–6)-Me 3 CCCCCCMe 3 }], the permethylated titanocene precursor gives mainly the allyldiene complex [Ti(η 5 -C 5 Me 5 )(η 3 :η 4 -C 5 Me 3 (CH 2 ) 2 )] while no titanium-containing product could be isolated for n= 4. The reaction of [Ti(η 5 -1,3-C 5 H 3 Me 2 ) 2 (η 2 -Me 3 SiCCSiMe 3 )] with Me 2 Si(CCCMe 3 ) 2 , however, cleanly affords the expected silacyclobutene–titanacyclobutene complex. All complexes were studied by spectral methods and solid-state structure of two representatives, [(η 5 -C 5 Me 5 ) 2 Ti{Ph 2 C 4 (SiMe 2 )}] and [(η 5 -1,3-C 5 H 3 Me 2 ) 2 Ti{(Me 3 C) 2 C 4 (SiMe 2 )}] was determined by single-crystal X-ray diffraction.

Post-synthesis incorporation of Al into germanosilicate ITH zeolites: the influence of treatment conditions on the acidic properties and catalytic behavior in tetrahydropyranylation

Post-synthesis alumination of germanosilicate medium-pore ITH zeolites was shown to be an effective procedure for tuning their acidity. Treatment of ITH zeolites synthesized with different chemical compositions (i.e. Si/Ge = 2.5, 4.4 and 5.8) with aqueous Al(NO3)3 solution led to the formation of strong Bronsted and Lewis acid sites and an increasing fraction of ultramicro- and meso-pores in Ge-rich ITH samples (Si/Ge = 2.5 and 4.4). The concentration of Al incorporated into the framework increases with decreasing Si/Ge ratio of the parent ITH. The increasing temperature of alumination from 80 to 175 °C (HT conditions) resulted in (1) a 1.5–2-fold increase in the concentration of Bronsted acid sites formed and (2) a decreasing fraction of framework Al atoms detectable with base probe molecules (i.e. pyridine, 2,6-di-tert-butylpyridine), i.e. an increased concentration of the “inner” acid sites. The activity of prepared Al-substituted ITH zeolites in tetrahydropyranylation of alcohols is enhanced with increasing amount of accessible acid sites in bulky crystals (e.g. alumination at lower temperature) or with increasing total concentration of acid centres within tiny ITH crystals (e.g. alumination under HT conditions). This trend became more prominent with increasing kinetic diameter of the substrate molecules under investigation (methanol < 1-propanol < 1-hexanol).