Intramolecular aminoalkene hydroamination mediated by a tethered bis(ureate)zirconium complex: computational perusal of various pathways for aminoalkene activation

The present study comprehensively explores alternative mechanistic pathways for the intramolecular hydroamination of the prototype 2,2-dimethyl-5-penten-1-amine aminoalkene (1) by bis(ureate)Zr(IV)(NMe(2))(2)(HNMe(2)) (2), which proceeds through a Zr(IV)(NHR)(2) intermediate using density functional theory (DFT) calculations. The classical stepwise σ-insertive mechanism that includes insertion of the C═C double bond into the Zr-N amido σ bond followed by Zr-C alkyl-bond aminolysis has been compared with a single-step pathway for amidoalkene → cycloamine conversion through a concerted amino proton transfer associated with N-C ring closure. Noncompetitive kinetics for reversible σ-insertive cyclization, together with the incompatibility of a turnover-limiting insertion step with observed pronounced primary kinetic isotope effects (KIEs), strongly militates against the operation of a σ-insertive mechanism. A noninsertive pathway evolving through an ordered six-center transition-state structure describing N-C bond formation at an axial Zr-N amido σ bond triggered by concurrent proton transfer from an equatorially bound substrate molecule onto the adjacent olefin-carbon center is found to prevail energetically. The proton-triggered noninsertive cyclization commencing from a catalytically relevant Zr(IV)(NHR)(2)(NH(2)R) substrate adduct is strongly downhill, followed by product expulsion via dissociative amine exchange. The assessed effective barrier compares reasonably well with the previously determined Eyring parameters, and the computationally estimated primary KIEs match the observed values pleasingly well. The present study reveals a comparable strength of substrate and product binding in relevant seven-coordinate intermediates, together with a rapid equilibrium between related primary and secondary amido species, which favors the former, as unique features of the studied catalyst. Thus, in line with experimental observations, competitive product inhibition can be discarded. On the basis of all of these findings, it is suggested that a Zr(NHR)(2)(substrate) intermediate corresponds to the catalyst resting state at high substrate concentrations, while it becomes a Zr(NHR)(2)(cycloamine) species when the product concentration is high or with the addition of excess 2-methylpiperidine, and this ambivalent behavior explains the observed operation of two distinct kinetic regimes, depending upon the extent of the reaction.     

Insertion, isomerization, and cascade reactivity of the tethered silylalkyl uranium metallocene (η(5)-C5Me4SiMe2CH2-κC)2U

Investigation of the insertion reactivity of the tethered silylalkyl complex (η(5)-C(5)Me(4)SiMe(2)CH(2)-κC)(2)U (1) has led to a series of new reactions for U-C bonds. Elemental sulfur reacts with 1 by inserting two sulfur atoms into each of the U-C bonds to form the bis(tethered alkyl disulfide) complex (η(5):η(2)-C(5)Me(4)SiMe(2)CH(2)S(2))(2)U (2). The bulky substrate N,N'-diisopropylcarbodiimide, (i)PrN═C═N(i)Pr, inserts into only one of the U-C bonds of 1 to produce the mixed-tether complex (η(5)-C(5)Me(4)SiMe(2)CH(2)-κC)U[η(5)-C(5)Me(4)SiMe(2)CH(2)C((i)PrN)(2)-κ(2)N,N'] (3). Carbon monoxide did not exclusively undergo a simple insertion into the U-C bond of 3 but instead formed {μ-[η(5)-C(5)Me(4)SiMe(2)CH(2)C(═N(i)Pr)O-κ(2)O,N]U[OC(C(5)Me(4)SiMe(2)CH(2))CN((i)Pr)-κ(2)O,N](2) (4) in a cascade of reactions that formally includes U-C bond cleavage, C-N bond cleavage of the amidinate ligand, alkyl or silyl migration, U-O, C-C, and C-N bond formations, and CO insertion. The reaction of 3 with isoelectronic tert-butyl isocyanide led to insertion of the substrate into the U-C bond, but with a rearrangement of the amidinate ligand binding mode from κ(2) to κ(1) to form [η(5):η(2)-C(5)Me(4)SiMe(2)CH(2)C(═N(t)Bu)]U[η(5)-C(5)Me(4)SiMe(2)CH(2)C(═N(i)Pr)N((i)Pr)-κN] (5). The product of double insertion of (t)BuN≡C into the U-C bonds of 1, namely [η(5):η(2)-C(5)Me(4)SiMe(2)CH(2)C(═N(t)Bu)](2)U (6), was found to undergo an unusual thermal rearrangement that formally involves C-H bond activation, C-C bond cleavage, and C-C bond coupling to form the first formimidoyl actinide complex, [η(5):η(5):η(3)-(t)BuNC(CH(2)SiMe(2)C(5)Me(4))(CHSiMe(2)C(5)Me(4))]U(η(2)-HC═N(t)Bu) (7).     

From a cycloheptatrienylzirconium allyl complex to a cycloheptatrienylzirconium imidazolin-2-iminato "pogo stick" complex with imido-type reactivity

The reaction of the cycloheptatrienylzirconium half-sandwich complex [(η(7)-C(7)H(7))ZrCl(tmeda)] (1) (tmeda = N,N,N',N'-tetramethylethylenediamine) with Li(Im(Dipp)N), generated from bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im(Dipp)NH) with methyllithium, yields the imidazolin-2-iminato complex [(η(7)-C(7)H(7))Zr(Im(Dipp)N)(tmeda)] (2). The corresponding tmeda-free complex [(η(7)-C(7)H(7))Zr(Im(Dipp)N)] (5) can be synthesized via the 1,3-bis(trimethylsilyl)allyl complex [(η(7)-C(7)H(7))Zr{η(3)-C(3)H(3)(TMS)(2)}(THF)] (3; TMS = SiMe(3)), which undergoes an acid-base reaction with Im(Dipp)NH to form 5 and 1,3-bis(trimethylsilyl)propene. 5 exhibits an unusual one-legged piano stool ("pogo stick") geometry with a particularly short Zr-N bond of 1.997(2) ?. Addition of 2,6-dimethylphenyl or tert-butyl isocyanide affords the complexes [(η(7)-C(7)H(7))Zr(Im(Dipp)N)(CNR)] (R = o-Xy, 6; R = t-Bu, 7), while the reaction with 2,6-dimethylphenyl isocyanate results in a [2 + 2] cycloaddition to form the ureato(1-) complex [(η(7)-C(7)H(7))Zr{Im(Dipp)N(C═O)N-o-Xy}] (8). 5 can also act as an initiator for the ring-opening polymerization of ε-caprolactone. These reactivity patterns together with density functional theory calculations reveal a marked similarity of the bonding in imidazolin-2-iminato and conventional imido transition-metal complexes.     

Cycloaddition reactions of allenylphosphonates and related allenes with dialkyl acetylenedicarboxylates, 1,3-diphenylisobenzofuran, and anthracene

Cycloaddition reactions of allenylphosphonates [(RO)(2)P(O)[(R(1))C═C═CR(2)(2)] with dialkyl acetylenedicarboxylates, 1,3-diphenylisobenzofuran, and anthracene have been investigated and compared with those of allenoates [(EtO(2)C)RC═C═CH(2)] and allenylphosphine oxides [Ph(2)P(O)(R(1))C═C═CR(2)(2)] in selected cases. Allenylphosphonates (RO)(2)P(O)(Ar)C═C═CH(2) with an α-aryl group preferentially undergo [4 + 2] cycloaddition with DMAD/DEAD under thermal activation, but in addition to the expected 1:1 (allene: DMAD) product, the reaction also leads to 1:2 as well as 2:1 products that were not reported before. When an extra vinyl group is present at the γ-carbon of allenylphosphonate [e.g., (OCH(2)CMe(2)CH(2)O)P(O)(Ph)C═C═CH(C═CHMe)], [4 + 2] cycloaddition takes place utilizing either the vinylic or the aryl end, but additionally a novel cyclization wherein complete opening of the [β,γ] carbon-carbon double bond of the allene is realized. In contrast to these, the reaction of allenylphosphonate (OCH(2)CMe(2)CH(2)O)P(O)(H)C═C═CMe(2) possessing a terminal ═CMe(2) group with DMAD occurs by both [2 + 2] cycloaddition and ene reaction. While the reaction of ═CH(2) terminal allenylphosphonates as well as allenylphosphine oxides with 1,3-diphenylisobenzofuran afforded preferentially endo-[4 + 2] cycloaddition products via [α,β] attack, the analogous allenoates [(EtO(2)C)RC═C═CH(2)] underwent exo-[4 + 2] cyclization. Under similar conditions, allenylphosphonates with a terminal ═CR(2) group gave only [β,γ]-cycloaddition products. An unusual ring-opening of a [4 + 2] cycloaddition product followed by ring-closing via [4 + 4] cycloaddition, as revealed by (31)P NMR spectroscopy, is reported. Anthracene reacted in a manner similar to 1,3-diphenylisobenzofuran, albeit with lower reactivity. Key products, including a set of exo- and endo- [4 + 2] cycloaddition products, have been characterized by single crystal X-ray crystallography.     

Water oxidation intermediates applied to catalysis: benzyl alcohol oxidation

Four distinct intermediates, Ru(IV)═O(2+), Ru(IV)(OH)(3+), Ru(V)═O(3+), and Ru(V)(OO)(3+), formed by oxidation of the catalyst [Ru(Mebimpy)(4,4'-((HO)(2)OPCH(2))(2)bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) and 4,4'-((HO)(2)OPCH(2))(2)bpy = 4,4'-bismethylenephosphonato-2,2'-bipyridine] on nanoITO (1-PO(3)H(2)) have been identified and utilized for electrocatalytic benzyl alcohol oxidation. Significant catalytic rate enhancements are observed for Ru(V)(OO)(3+) (~3000) and Ru(IV)(OH)(3+) (~2000) compared to Ru(IV)═O(2+). The appearance of an intermediate for Ru(IV)═O(2+) as the oxidant supports an O-atom insertion mechanism, and H/D kinetic isotope effects support net hydride-transfer oxidations for Ru(IV)(OH)(3+) and Ru(V)(OO)(3+). These results illustrate the importance of multiple reactive intermediates under catalytic water oxidation conditions and possible control of electrocatalytic reactivity on modified electrode surfaces.     

Expanding molecular transition metal cubane clusters of the form [M4(μ3)-O)4]12+: syntheses, spectroscopic and structural characterizations of molecules M4(μ3-O)4(O2P(Bn)2)4(O4), M = V(V) and W(V)

Oxidizing the trimer V(3)(μ(3)-O)(O(2))(μ(2)-O(2)P(Bn)(2))(6)(H(2)O) in the presence of excess (t)BuOOH results in V(4)(μ(3)-O)(4)(μ(2)-O(2)P(Bn)(2))(4)(O(4)) and heating W(CO)(6) and bis(benzyl)phosphinic acid in 1:1 EtOH/THF at 120 °C produces W(4)(μ(3)-O)(4)(μ(2)-O(2)P(Bn)(2))(4)(O(4)).     

Reaction of zirconium hydrocarbyls with carbon monoxide

Carbon monoxide reacts with tetrahydrocarbylzirconium compounds, (RCH₂)₄Zr, where R = phenyl or vinyl to give a polymeric species containing ZrOC bonds, which on hydrolysis yields products consistent with successive insertion of CO and ZrC(O)CH₂R into the ZrC bonds.     

Nonhydride mechanism of metal-catalyzed hydrosilylation

A 1:1:1 reaction between complex (Tp)(ArN═)Mo(H)(PMe(3)) (3), silane PhSiD(3), and carbonyl substrate established that hydrosilylation catalyzed by 3 is not accompanied by deuterium incorporation into the hydride position of the catalyst, thus ruling out the conventional hydride mechanism based on carbonyl insertion into the M-H bond. An analogous result was observed for the catalysis by (O═)(PhMe(2)SiO)Re(PPh(3))(2)(I)(H) and (Ph(3)PCuH)(6).     

Structure and dynamics of dinuclear zirconium(IV) complexes

We have determined by X-ray crystallography the structures of three dinuclear zirconium(IV) complexes containing the heptadentate ligand dhpta (where H(5)dhpta = 1,3-diamino-2-propanol-N,N,N',N'-tetraacetic acid, 1) and different countercations: K(2)[Zr(2)(dhpta)(2)].5H(2)O (2.5H(2)O), Na(2)[Zr(2)(dhpta)(2)].7H(2)O.C(2)H(5)OH (3.7H(2)O.C(2)H(5)OH), and Cs(2)[Zr(2)(dhpta)(2)].H(5)O(2).Cl.4H(2)O (4.H(5)O(2).Cl.4H(2)O). In the K(I) complex 2, crystallized from water, the two Zr(IV) ions are 3.5973(4) A apart and bridged via two alkoxo groups (average Zr-O 2.165 A). Each Zr(IV) is eight-coordinate and also bound to two N atoms (average Zr-N 2.448 A), and four carboxylate O atoms (average Zr-O 2.148 A). The two dhpta ligands in the dinuclear unit have different conformations. One face of the complex contains an array of 14 oxygen atoms and interacts strongly with the two K(I) ions, one of which is 6-coordinate, the other 8-coordinate, which are 3.922(4) A apart and bridged by a carboxylate O and by two water molecules. The structures of the dinuclear anion [Zr(2)(dhpta)(2)](2-) in the Na(I) complex 3 and in the Cs(I) complex 4 are essentially identical to that found in complex 2, although the alkali metal ions coordinate differently to the oxygen-rich face. All Zr(IV) ions have a distorted triangulated dodecahedral geometry. Although the crystal structure of complex 2 does not indicate the presence of acidic protons, in 4 an [H(5)O(2)](+) unit is strongly H-bonded to an oxygen atom of a coordinated carboxylate group. 1D and 2D (1)H and (13)C NMR spectroscopic and potentiometric studies reveal two deprotonations with pK(a) values of 9.0 and 10.0. At low pH, two carboxylate groups appear to undergo protonation accompanied by chelate ring-opening, and the complex exhibits dynamic fluxional behavior in which the two magnetically nonequivalent dhpta ligands exchange at a rate of 11 s(-1) at pH 3.30, 298 K, as determined from 2D EXSY NMR studies. Ligand interchange is not observed at high pH (>11). The same crystals of complex 2 were obtained from solutions at pH 3 or 12. The dynamic configurational change is therefore mediated by the aqueous solvent.     

Reactivity of TCNE or TCNQ derivatives of quinonoid zwitterions: platinum-induced HCN elimination vs oxidative-addition

The reaction of the functional, zwitterionic quinonoid molecule (6E)-4-(butylamino)-6-(butyliminio)-3-oxo-2-(1,1,2,2-tetracyanoethyl)cyclohexa-1,4-dien-1-olate, [C(6)H-2-{C(CN)(2)C(CN)(2)H}]-4,6-(···NH n-Bu)(2)-1,3(···O)(2) (2), which has been previously prepared by regioselective insertion of TCNE into the C-H bond adjacent to the C···O bonds of the zwitterionic benzoquinone monoimine (6E)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate, C(6)H(2)-4,6-(···NHn-Bu)(2)-1,3-(···O)(2) (1), with 2 equiv of [Pt(C(2)H(4))(PPh(3))(2)], afforded the Pt(0) complex [Pt(PPh(3))(2)(4)] (6) (4 = 2-HCN; (6E)-4-(butylamino)-6-(butyliminio)-3-oxo-2-(1,2,2-tricyanoethenyl)cyclohexa-1,4-dien-1-olate), in which a tricyanoethenyl moiety is π-bonded to the metal. A metal-induced HCN elimination reaction has thus taken place. The same complex was obtained directly by the reaction of 1 equiv of the Pt(0) complex [Pt(C(2)H(4))(PPh(3))(2)] with the olefinic ligand [C(6)H-2-{C(CN)═C(CN)(2)}]-4,6-(···NHn-Bu)(2)-1,3-(···O)(2)) (4), previously obtained by the reaction of 2 with NEt(3) in THF. A similar reactivity pattern was observed between 2 and 2 equiv of the Pd(0) precursor [Pd(dba)(2)] in the presence of dppe, which led to [Pd(dppe)(4)] (7), which was also directly obtained from 4 and 1 equiv [Pd(dba)(2)]/dppe. In contrast to the behavior of the TCNE derivative 2, the reaction of the TCNQ derivative (6E)-4-(butylamino)-6-(butyliminio)-2-(dicyano(4-(dicyanomethyl)phenyl)methyl)-3-oxocyclohexa-1,4-dien-1-olate, [C(6)H-2-{C(CN)(2)p-C(6)H(4)C(CN)(2)H}]-4,6-(···NHn-Bu)(2)-1,3-(···O)(2)) (3), with 2 equiv of [Pt(C(2)H(4))(PPh(3))(2)] led to formal oxidative-addition of the C-H bond of the C(CN)(2)H moiety to give the Pt(II) hydride complex trans-[PtH(PPh(3))(2){N═C═C(CN)p-C(6)H(4)C(CN)(2)-2-[C(6)H-4,6-(···NHn-Bu)(2)-1,3-(···O)(2))}] (8). The molecular structures of 3, 4, 6·0.5(H(2)O), and 8·3(CH(2)Cl(2)) have been determined by single-crystal X-ray diffraction.     

Formation of phosphino-substituted isocyanate by reaction of CO2 with group 2 complexes based on the (Me3Si)(i-Pr2P)NH ligand

The group 2 complexes [(Me(3)Si)(i-Pr(2)P)N](2)M(THF)(x) (M = Mg, x = 1; M = Ca/Sr, x = 2) as well as an unusual dimagnesium complex {[(Me(3)Si)(i-Pr(2)P)N](3)Mg}Mg(n-C(4)H(9)) have been prepared and characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction. Each complex was shown to react with CO(2) under extremely mild conditions (15 min, 1 atm, room temperature) to give the isocyanate (i-Pr)(2)P-N═C═O. The independent syntheses of (i-Pr)(2)P-N═C═O and the carbodiimide dimer [(i-Pr)(2)PNCNP(i-Pr)(2)](2) are also reported.     

Ln4(CH2)4 cubane-type rare-earth methylidene complexes consisting of "(C5Me4SiMe3)LnCH2" units (Ln = Tm, Lu)

Tetranuclear cubane-type rare-earth methylidene complexes consisting of four "Cp'LnCH(2)" units, [Cp'Ln(μ(3)-CH(2))](4) (4-Ln; Ln = Tm, Lu; Cp' = C(5)Me(4)SiMe(3)), have been obtained for the first time through CH(4) elimination from the well-defined polymethyl complexes [Cp'Ln(μ(2)-CH(3))(2)](3) (2-Ln) or mixed methyl/methylidene precursors such as [Cp'(3)Ln(3)(μ(2)-Me)(3)(μ(3)-Me)(μ(3)-CH(2))] (3-Ln). The reaction of the methylidene complex 4-Lu with benzophenone leads to C═O bond cleavage and C═C bond formation to give the cubane-type oxo complex [Cp'Lu(μ(3)-O)](4) and CH(2)═CPh(2), while the methyl/methylidene complex 3-Tm undergoes sequential methylidene addition to the C═O group and ortho C-H activation of the two phenyl groups of benzophenone to afford the bis(benzo-1,2-diyl)ethoxy-chelated trinuclear complex [Cp'(3)Tm(3)(μ(2)-Me)(3){(C(6)H(4))(2)C(O)Me}] (6-Tm).     

Donor-acceptor-stabilized silicon analogue of an acid anhydride

A stable silicon analogue of an acid anhydride {PhC(Bu(t)N)(2)}Si{═O·B(C(6)F(5))(3)}O-Si(H){═O·B(C(6)F(5))(3)}{(NBu(t))(HNBu(t))CPh} (4) with a O═Si-O-Si═O core has been prepared by treating monochlorosilylene PhC(Bu(t)N)(2)SiCl (1) with H(2)O·B(C(6)F(5))(3) in the presence of NHC (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Compound 4 has been characterized by elemental analysis and multinuclear NMR spectroscopic investigations. The molecular structure of 4 has been established by single-crystal X-ray diffraction studies, and DFT calculations support the experimental results.     

Facing unexpected reactivity paths with Zr(IV)-pyridylamido polymerization catalysts

This work provides original insights to the better understanding of the complex structure-activity relationship of Zr(IV)-pyridylamido-based olefin polymerization catalysts and highlights the importance of the metal-precursor choice (Zr(NMe(2))(4) vs. Zr(Bn)(4)) to prepare precatalysts of unambiguous identity. A temperature-controlled and reversible σ-bond metathesis/protonolysis reaction is found to take place on the Zr(IV)-amido complexes in the 298-383 K temperature range, changing the metal coordination sphere dramatically (from a five-coordinated tris-amido species stabilized by bidentate monoanionic {N,N(-)} ligands to a six-coordinated bis-amido-mono-amino complexes featured by tridentate dianionic {N(-),N,C(-)} ligands). Well-defined neutral Zr(IV)-pyridylamido complexes have been prepared from Zr(Bn)(4) as metal source. Their cationic derivatives [Zr(IV) N(-),N,C(-)}Bn](+)[B(C(6)F(5))(4)](-) have been successfully applied to the room-temperature polymerization of 1-hexene with productivities up to one order of magnitude higher than those reported for the related Hf(IV) state-of-the-art systems. Most importantly, a linear increase of the poly(1-hexene) M(n) values (30-250 kg mol(-1)) has been observed upon catalyst aging. According to that, the major active species (responsible for the increased M(n) polymer values) in the aged catalyst solution, has been identified.     

Reactions of group 4 metallocene alkyne complexes with carbodiimides: experimental and theoretical studies of the structure and bonding of five-membered hetero-metallacycloallenes

The reaction of the low-valent metallocene(II) sources Cp(2)Ti(η(2)-Me(3)SiC(2)SiMe(3)) (7) and Cp(2)Zr(py)(η(2)-Me(3)SiC(2)SiMe(3)) (11, Cp = η(5)-cyclopentadienyl, py = pyridine) with carbodiimides RN═C═NR (R = Cy, i-Pr, p-Tol) leads to the formation of five membered hetero-metallacycloallenes Cp(2)M{Me(3)SiC═C═C[N(SiMe(3))(R)]-N(R)} (9M-R) (M = Ti, R = i-Pr; M = Zr, R = Cy, i-Pr, p-Tol). Elimination of the alkyne (as the hitherto known reactivity of titanocene and zirconocene alkyne complexes would suggest) was not observed. The molecular structures of the obtained complexes were confirmed by X-ray studies. Moreover, the structure and bonding of the complexes 9Zr-Cy and 9Zr-p-Tol was investigated by DFT calculations.     

S-oxygenation of thiocarbamides IV: Kinetics of oxidation of tetramethylthiourea by aqueous bromine and acidic bromate

The kinetics and mechanism of oxidation of tetramethylthiourea (TTTU) by bromine and acidic bromate has been studied in aqueous media. The kinetics of reaction of bromate with TTTU was characterized by an induction period followed by formation of bromine. The reaction stoichiometry was determined to be 4BrO(3)(-) + 3(R)(2)C═S + 3H(2)O → 4Br(-) + 3(R)(2)C═O + 3SO(4)(2-) + 6H(+). For the reaction of TTTU with bromine, a 4:1 stoichiometric ratio of bromine to TTTU was obtained with 4Br(2) + (R)(2)C═S + 5H(2)O → 8Br(-) + SO(4)(2-) + (R)(2)C═O + 10H(+). The oxidation pathway went through the formation of tetramethythiourea sulfenic acid as evidenced by the electrospray ionization mass spectrum of the dynamic reaction solution. This S-oxide was then oxidized to produce tetramethylurea and sulfate as final products of reaction. There was no evidence for the formation of the sulfinic and sulfonic acids in the oxidation pathway. This implicates the sulfoxylate anion as a precursor to formation of sulfate. In aerobic conditions, this anion can unleash a series of genotoxic reactive oxygen species which can explain TTTU's observed toxicity. A bimolecular rate constant of 5.33 ± 0.32 M(-1) s(-1) for the direct reaction of TTTU with bromine was obtained.     

Solvent-induced helix superstructure in achiral dumbbell-shaped hydrazine derivatives

We studied hydrogen-bonding assemblies in a series of dumbbell-shaped hydrazine derivatives, namely oxalyl N',N'-bis(3,4-dialkoxybenzoyl)-hydrazide (BFH-n, n = 4, 6, 8, 10) and oxalyl N',N'-dibenzoyl-hydrazide (FH-0). It has been demonstrated that NH-1 protons of BFH-n precipitated from tetrahydrofuran (THF) or dimethylformamide (DMF) were involved in intramolecular H-bonding to form 6-membered rings. Meanwhile, NH-2 protons of BFH-n precipitated from THF formed intermolecular hydrogen bonds with C═O groups of neighboring molecules, while NH-2 protons of BFH-n precipitated from DMF formed intermolecular hydrogen bonds with C═O group of neighboring DMF molecules. C═O, -CH(3), and -CH groups of DMF molecules participated in multiple intermolecular hydrogen bonds with the -N-H and -C═O groups of FH-0 molecules in single-crystals formed in DMF, leading to a double helix morphology with a pitch of 24.2 ? along the c direction. Both left- and right-handed helical micrometer-length ribbons with nonuniform helical pitches were observed in an achiral BFH-10 xerogel precipitated from DMF.     

Atmospheric chemistry of CF3CF═CH2 and (Z)-CF3CF═CHF: Cl and NO3 rate coefficients, Cl reaction product yields, and thermochemical calculations

Rate coefficients, k, for the gas-phase reactions of Cl atoms and NO(3) radicals with 2,3,3,3-tetrafluoropropene, CF(3)CF═CH(2) (HFO-1234yf), and 1,2,3,3,3-pentafluoropropene, (Z)-CF(3)CF═CHF (HFO-1225ye), are reported. Cl-atom rate coefficients were measured in the fall-off region as a function of temperature (220-380 K) and pressure (50-630 Torr; N(2), O(2), and synthetic air) using a relative rate method. The measured rate coefficients are well represented by the fall-off parameters k(0)(T) = 6.5 × 10(-28) (T/300)(-6.9) cm(6) molecule(-2) s(-1) and k(∞)(T) = 7.7 × 10(-11) (T/300)(-0.65) cm(3) molecule(-1) s(-1) for CF(3)CF═CH(2) and k(0)(T) = 3 × 10(-27) (T/300)(-6.5) cm(6) molecule(-2) s(-1) and k(∞)(T) = 4.15 × 10(-11) (T/300)(-0.5) cm(3) molecule(-1) s(-1) for (Z)-CF(3)C═CHF with F(c) = 0.6. Reaction product yields were measured in the presence of O(2) to be (98 ± 7)% for CF(3)C(O)F and (61 ± 4)% for HC(O)Cl in the CF(3)CF═CH(2) reaction and (108 ± 8)% for CF(3)C(O)F and (112 ± 8)% for HC(O)F in the (Z)-CF(3)CF═CHF reaction, where the quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. NO(3) reaction rate coefficients were determined using absolute and relative rate methods. Absolute measurements yielded upper limits for both reactions between 233 and 353 K, while the relative rate measurements yielded k(3)(295 K) = (2.6 ± 0.25) × 10(-17) cm(3) molecule(-1) s(-1) and k(4)(295 K) = (4.2 ± 0.5) × 10(-18) cm(3) molecule(-1) s(-1) for CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF, respectively. The Cl-atom reaction with CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF leads to decreases in their atmospheric lifetimes and global warming potentials and formation of a chlorine-containing product, HC(O)Cl, for CF(3)CF═CH(2). The NO(3) reaction has been shown to have a negligible impact on the atmospheric lifetimes of CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF. The energetics for the reaction of Cl, NO(3), and OH with CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF in the presence of O(2) were investigated using density functional theory (DFT).     

13C NMR STUDY OF SUBSTITUENT EFFECTS IN 1,2,4-OXADIAZOLE AND 1,2,4-THIADIAZOLE DERIVATIVES

¹³C chemical shifts of C═N, C═O and C═S carbons of 3,4-disubstituted-1,2,4-oxadiazole-5-ones(thiones) and 3,4-disubstituted-1,2,4-thiadiazole-5-ones have been determined in CDCl₃ solution. Exceptionally good Hammett correlations of ¹³C NMR chemical shifts of these carbons with σ were obtained. The negative ρ values observed (inverse substituent effects) indicate π-polarization of the C═N, C═O and C═S bonds. As expected, the long distance C═O and C═S ¹³C chemical shifts were found less susceptible to substituent-induced electronic changes.     

Synthesis of a base-stabilized silanone-coordinated complex by oxygenation of a (silyl)(silylene)tungsten complex

Base-stabilized silanone complex Cp*(OC)(2)W(SiMe(3)){O═SiMes(2)(DMAP)} (2) was synthesized by the reaction of (silyl)(silylene)tungsten complex Cp*(OC)(2)W(SiMe(3))(═SiMes(2)) (1) with 1 equiv of pyridine-N-oxide (PNO) in the presence of 4-(dimethylamino)pyridine (DMAP). Further oxygenation of 2 with 3 equiv of PNO at 80 °C resulted in the formation of a W-O-Si-O-Si framework to give disiloxanoxy complex Cp*(O)(2)W{OSiMes(2)(OSiMe(3))} (3). Complex 3 was also obtained by the direct reaction of complex 1 with 4 equiv of PNO at 80 °C.