In silico quest for stable phosphastannaallenes

A computational study at different levels of theory was performed for the not yet synthesized phosphastannaallenes >SnCP– in order to evaluate the strength of the SnC bond, the main postulated factor to stabilize such species, and the geometry in R₂SnCPR derivatives. The influence of the substituents with various electronic effects (H, Me, Ph, F, Cl, OMe, SiMe₃) at the Sn or P atoms of the SnCP unit on the SnC bond order was evaluated in the quest for a substituent that would stabilize the phosphastannaallenic unit. PC bond orders have also been calculated.     

Reaction of the RuTp(PR3)Cl fragment with alkynols: Formation of carbene,vinylidene, allenylidene,and carbyne complexes

The reaction of RuTp(COD)Cl (1) with PR₃ (PR₃ = PPh₂ⁱPr, PⁱPr₃, PPh₃) and propargylic alcohols HCCCPh₂OH, HCCCFc₂OH (Fc = ferrocenyl), and HCCC(Ph)MeOH has been studied.In the case of PR₃ = PPh₂ⁱPr, PⁱPr₃ and HCCCPh₂OH, the 3-hydroxyvinylidene complexes RuTp(PPh₂ⁱPr)(CCHC(Ph)₂OH)Cl (2a) and RuTp(PⁱPr₃)(CCHC(Ph₂)OH)Cl (2b) were isolated.With PR₃ = PPh₂ⁱPr and HCCCFc₂OH as well as with PR₃ = PPh₃ and HCCCPh₂OH dehydration takes place affording the allenylidene complexes RuTp(PPh₂ⁱPr)(CCCFc₂)Cl (3b) and RuTp(PPh₃)(CCCPh₂)Cl (3c).Similarly, with PPh₂ⁱPr and HCCC(Ph)MeOH rapid elimination of water results in the formation of the vinylvinylidene complex RuTp(PPh₂ⁱPr)(CCHC(Ph)CH₂)Cl (4).In contrast to the reactions of the RuTp(PR₃)Cl fragment with propargylic alcohols, with HCC(CH₂)_nOH (n = 2, 3, 4, 5) six-, and seven-membered cyclic oxycarbene complexes RuTp(PR₃)(C₄H₆O)Cl (5), RuTp(PR₃)(C₅H₈O)Cl (6), and RuTp(PR₃)(C₆H₁₀O)Cl (7) are obtained. On the other hand, with 1-ethynylcyclohexanol the vinylvinylidene complex RuTp(PPh₂ⁱPr)(CCHC₆H₉)Cl (8) is formed. The reaction of the allenylidene complexes 3a–c with acid has been investigated. Addition of CF₃COOH to a solution of 3a–c resulted in the reversible formation of the novel RuTp vinylcarbyne complexes [RuTp(PPh₂ⁱPr)(C–CHCPh₂)Cl]⁺ (9a), [RuTp(PPh₂ⁱPr)(C–CHCFc₂)Cl]⁺ (9b), and [RuTp(PPh₃)(C–CHCPh₂)Cl]⁺ (9c). The structures of 3a, 3b, and 5b have been determined by X-ray crystallography.     

A theoretical study on the alkene insertion step in Rh-Yanphos catalyzed hydroformylation

This paper studied the mechanism of the alkene insertion elementary step in the asymmetric hydroformylation （AHF） catalyzed by RhH（CO）2[（R,S）-Yanphos] using four alkene substrates （CH2=CH- Ph, CH2=CH-Ph-（p）-Me, CH2=CH-C（==O）OCH3 and CH2=CH-OC（=O）-Ph, abbreviated as A1-A4）. Interestingly, the equatorial vertical coordination mode （A mode） with respect to the Rh center was found for AI and A2 but not for A3 and A4, although the equatorial in-plane coordination mode （E mode） was found for A1 -A4. The relative energy of the E mode of the -q2-intermediates is lower than that of the A mode. In the alkene insertion step, Path 1 is more favorable than Path 2 for this system. As for AI and A2, there could be a transformation between 2eq and 2ax.     

1,3-Dipolar cycloaddition of diaryldiazomethanes across N-ethoxy-carbonyl-N-(2,2,2-trichloroethylidene)amine and reactivity of the resulting 2-azabutadienes towards thiolates and cyclic amides

1,3-dipolar cycloaddition of diaryldiazomethanes Ar₂CN₂ across Cl₃C–CHN–CO₂Et 1 yields Δ³-1,2,4-triazolines 2. Thermolysis of 2 leads, via transient azomethine ylides 3, to diaryldichloroazabutadienes [Ar(Ar')CN–CHCCl₂] 4. Treatment of 4a (Ar = Ar' = C₆H₅) and 4c (Ar = Ar' = p-ClC₆H₄) with NaSR in DMF yields 2-azabutadienes [Ar₂CN–C(H)C(SR)₂] 5. In contrast, nucleophilic attack of NaStBu on 4 affords azadienic dithioethers [Ar₂CN–C(S^tBu)C(H)(S^tBu)] (7a Ar = C₆H₅; 7b Ar' = p-ClC₆H₄). The reaction of 4a with NaSEt conducted in neat EtSH produces [Ph₂CN–C(H)(SEt)–CCl₂H] 8, which after dehydrochloration by NaOMe and subsequent addition of NaSEt is converted to [Ph₂CN–C(SEt)C(H)(SEt)] 7c. Upon the reaction of 4c with NaSⁱPr, the intermediate dithioether [(p-ClC₆H₄)₂CN–CHC(SⁱPr)₂] 5k is converted to tetrakisthioether [(p-ⁱPrSC₆H₄)₂CN–CHC(SⁱPr)₂] 6. Treatment of 4a with the sodium salt of piperidine leads to [Ph₂CN–CHC(NC₅H₁₀)₂] 10. The coordination of 6 on CuBr affords the macrocyclic dinuclear Cu(I) complex 11. The crystal structures of 5i, 7a,b, 10 and 11 have been determined by X-ray diffraction.     

Nitrogen- and oxygen-bridged bidentate phosphaalkene ligands

An overview is given on synthesis and structures of new bidentate phosphaalkene ligands [(RMe₂Si)₂CP]₂E (E = O, NR, N^?) and (RMe₂Si)₂CPN(R′)PR′′₂. Exceptional properties of these ligands, extending beyond predictable properties of phosphaalkenes are: (i) the NSi bond cleavage of [(iPrMe₂Si)₂CP]₂NSiMe₃ with Au^I and Rh^I chloro complexes under mild conditions leading to binuclear complexes of the 6π-delocalised imidobisphosphaalkene anion [(iPrMe₂Si)₂CP]₂N^?, and (ii) the chlorotropic formation of molecular 1:2 Pd^II and Pt^II metallochloroylid complexes with novel ylid-type ligands [(RMe₂Si)₂CP(Cl)N(R)PR₂]^?, and the transformation of a P-platina-P-chloroylid complex into a C-platina phosphaalkene by intramolecular chlorosilane elimination. Properties of the heavier congeners [(RMe₂Si)₂CP]₂E (E = S, Se, Te, PR, P^?, As^?) and (RMe₂Si)₂CPEPR′′₂ (E = S, Se, Te) are also described.     

Reactions of α-phenylethynyl-trans-β-styryl complexes with isonitriles: hemi-labile alkyne coordination

Treatment of the complex [Ru{C(CCPh)CHPh}Cl(CO)(PPh₃)₂] (1) with one equivalent of CNR(R =^tBu, C₆H₃Me₂-2,6) gives [Ru{C(CCPh)CHPh}Cl(CNR)(CO)(PPh₃)₂]. Addition of a further equivalent of isonitrile and [NH₄]PF₆ leads to the salts [Ru{C(CCPh)CHPh}Cl(CNR)₂(CO)(PPh₃)₂]PF₆ and the mixed species [Ru{C(CCPh) CHPh}(CO)(CN^tBu)(CNC₆H₃Me₂-2,6)(PPh₃)₂]PF₆. The related [Ru{C(CCPh)CHPh}(CN^t(CO)₂     

Hemilability and nonrigidity in metal complexes of bidentate P,PS donor ligands

Hemilability and nonrigidity in a series of mixed P,PS donor ligands has been studied in the complexes [Pd(P,PS)Cl₂], [Pd(η³-C₃H₅)(P,PS)][SbF₆], and [Rh(cod)(P,PS)][SbF₆] (P,PS = Ph₂P-Q-P(S)Ph₂). The effect of bite angle, the rigidity of the ligand backbone, and the role of the ancillary ligands are discussed.     

Synthesis of spiro[bicyclo[2.2.1]heptane-2,2′-furan]-3-amines via stereoselective cycloadditions of trimethylenemethane to (1S,3EZ,4R)-3-arylimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones

Stereoselective [3+2] cycloadditions of trimethylenemethane (TMM) to the exocyclic CO and CN double bonds of (1S,3EZ,4R)-3-arylimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones gave the corresponding spiro[bicyclo[2.2.1]heptane-2,2′-furan] and spiro[bicyclo[2.2.1]heptane-3,2′-pyrrolidine] derivatives. Further stereoselective reductions of the CN or CO bond in these cycloadducts furnished new chiral amines, diamines, and a new aminoalcohol. All cycloadditions and reductions of the CN double bonds took place from the less hindered endo-face of the (1S,3EZ,4R)-3-arylimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones, exclusively, thus giving the corresponding products in 100% de. The structures were determined by NMR, NOESY spectroscopy, and by X-ray diffraction.     

A convenient route to six-membered heterocyclic carbene complexes: Reactions of aminoallenylidene complexes with 1,3-bidentate nucleophiles

Pentacarbonyl dimethylamino(methoxy)allenylidene complexes of chromium and tungsten, [(CO)₅MCCC(NMe₂)OMe] (M = Cr (1a), W (1b)), react with 1,3-bidentate nucleophiles such as amidines and guanidine, H₂N–C(NH)R (R = Ph, C₆H₄NH₂-4, C₆H₄NO₂-3, NH₂), by displacing the methoxy substituent to give exclusively dimethylamino(imino)-allenylidene complexes, [(CO)₅MCCC{NC(NH₂)R}NMe₂] (2a–5a, 2b). Treatment of the chromium complexes 2a–5a with catalytic amounts of hydrochloric acid or HBF₄ gives rise to an intramolecular cyclization. Addition of the terminal NH₂ substituent to the C_α–C_β bond of the allenylidene chain affords pyrimidinylidene complexes 6–9 in high yield. In contrast to the chromium complexes 2a–5a, the corresponding tungsten complex 2b could not be induced to cyclize due to the lower electrophilicity of the α-carbon atom in 2b. The dimethylamino(phenyl)allenylidene complex [(CO)₅CrCCC(NMe₂)Ph] (10) reacts with benzamidine or guanidine similarly to 1a. However, the second reaction step – cyclization to give pyrimidinylidene complexes – proceeds much faster. Therefore, the formation of an imino(phenyl)allenylidene complex as an intermediate is established only by IR spectroscopy. The analogous reaction of 10 with 3-amino-5-methylpyrazole affords, via a formal [3+3]-cycloaddition, a pyrazolo[1,5a]pyrimidinylidene complex 13. Compound 13 is obtained as two isomers differing in the relative position of the N-bound proton (1H or 4H). The related reaction of 10 with thioacetamide yields a thiazinylidene complex and additionally an alkenyl(amino)carbene complex.     

1,1-Bis(trifluoromethyl)butadiene-1,3—A new fluorinated building block

Although 1,1-bis(trifluoromethyl)butadiene-1,3 (1) reacts with dimethylamine with selective formation of 1,4-adduct [trans-(CF₃)₂CHCHCHCH₂N(CH₃)₂], halogenation of 1 proceeds with predominant formation (>92%) of 1,2-adducts (CF₃)₂CCHCHXCH₂X (X = Cl or Br). Electrophilic conjugated addition of “ClF” or “BrF” to 1 proceeds exclusively with the formation of 1,2-adducts (CF₃)₂CCHCHFCH₂X (major) and (CF₃)₂CCHCHXCH₂F (X = Cl or Br). Difluorocarbene adds selectively to CHCH₂ moiety of 1 forming thermally stable vinylcyclopropane. In Diels-Alder reaction with linear or cyclic dienes (butadienes, cyclopentadiene, cyclohexadiene-1,3) and quadricyclane compound 1 behaves as dienophile providing for the reaction electron-deficient CHCH₂ bond. The relative rate of cycloaddition of 1 and other fluoroolefins to quadricyclane, measured by high temperature NMR, indicates that (CF₃)₂CCH acts as very strong electron-withdrawing substituent. Synthetic utility of products based on 1 was also demonstrated.     

One-pot synthesis of 2,4,5-trisubstituted oxazoles from N-acyl amino acids by a combination of cyclodehydration with N,N′-diisopropylcarbodiimide and Wittig olefination

By a combination of cyclodehydration of N-acyl amino acids with N,N′-diisopropylcarbodiimide (DIC) and non-classical Wittig olefination of the resultant 5(4H)-oxazolones with Ph₃PCHCN and Ph₃PCHCOOEt, 5-oxazoleacetonitriles and 5-oxazoleacetates were synthesized in one-pot in 41–85% and 57–70% yields, respectively.     

Understanding the reactivity of [WNAr(CH2tBu)2(CHtBu)] (Ar = 2,6-iPrC6H3) with silica partially dehydroxylated at low temperatures through a combined use of molecular and surface organometallic chemistry

Reaction of [WNAr(CH₂tBu)₂(CHtBu)] (Ar = 2,6-iPrC₆H₃) with silica partially dehydoxylated at 200 °C does not lead only to the expected bisgrafted [(SiO)₂WNAr(CHtBu)] species, but also surface reaction intermediates such as [(SiO)₂WNAr(CH₂tBu)₂]. All these species were characterized by infrared spectroscopy, 1D and 2D solid state NMR, elemental analysis and molecular models obtained by using silsesquioxanes. While a mixture of several surface species, the resulting material displays high activity in the olefin metathesis.     

Role of the intermolecular and intramolecular hydrogen bonding on the excited-state proton transfer behavior of 3-aminophthalimide (3AP) dimer

In this work, both the intermolecular and intramolecular hydrogen bonding of 3-aminophthalimide (3AP) dimer complex in the electronically excited state have been investigated theoretically using the time-dependent density functional theory (TDDFT) method. The calculated infrared spectrum of the hydrogen-bonded 3AP dimer complex for the S₁ state shows that the CO and H–N bonds involved in the intramolecular hydrogen bond C₃O₅?H₈–N₆ and intermolecular hydrogen bond C₁O₄?H_7′–N_2′ which are markedly red-shifted compared with those predicted for the ground state. The calculated length of the two hydrogen bonds C₃O₅?H₈–N₆ and C₁O₄?H_7′–N_2′ are significantly shorter in S₁ state than in the ground state. However, the bond lengths of the intramolecular hydrogen bond C_3′O₅?H_8′–N_6′ and intermolecular hydrogen bond C_1′O_4′?H₇–N₂ nearly unchanged upon electronic excitation to the S₁ state. Thus, the intramolecular hydrogen bond C₃O₅?H₈–N₆ and intermolecular hydrogen bond C₁O₄?H_7′–N_2′ of the hydrogen-bonded 3AP dimer complex are stronger in the electronically excited state than in the ground state. Moreover, it has been demonstrated that the excited-state proton transfer reaction is facilitated by the electronic excited-state hydrogen bond strengthening.     

Organometallic complexes for nonlinear optics. 41: Syntheses and quadratic NLO properties of 4-{4-(4-nitrophenyl)diazophenyl}ethynylphenylethynyl complexes

The syntheses of [Au(CC-4-C₆H₄CC-4-C₆H₄NN-4-C₆H₄NO₂)(PPh₃)] (3), trans-[Ru(CC-4-C₆H₄-CC-4-C₆H₄NN-4-C₆H₄NO₂)Cl(dppm)₂] (4), [Ru(CC-4-C₆H₄CC-4-C₆H₄NN-4-C₆H₄NO₂)(dppe)(η-C₅Me₅)] (5), and [Ni(CC-4-C₆H₄NN-4-C₆H₄NO₂)(PPh₃)(η-C₅H₅)] (6) are reported, together with a single-crystal X-ray diffraction study of 4. Quadratic nonlinearities for 3–6 and [Ru(CC-4-C₆H₄NO₂)(dppe)(η-C₅Me₅)] (7) have been determined at 1.064 μm and 1.300 μm by the hyper-Rayleigh scattering (HRS) technique, comparison to related complexes revealing that β values increase on introduction of azo group and π-system lengthening.     

Two-dimensional correlation infrared spectroscopy studies on the thermal-induced mesophase of 4-nitrobenzohydrazide derivative

Two-dimensional (2D) correlation infrared (IR) spectroscopy has been applied to explore the effect of hydrogen bondings (HBs) on the structure of mesophase in the dissymmetrical 4-nitrobenzohydrazide derivative, N-(4-cetyloxybenzoyl)-N′-(4′-nitrobenzoyl) hydrazine (C16-NO₂). The strength and species of HBs as well as the heat-induced structural variations in mesophase have been investigated. It has been found from 2D correlation IR spectroscopy that the sequential order of changes in the different functionalities in the course of liquid crystalline formation is that, firstly, the alkyl chain changes from the significant population of the trans conformation to the significant population of gauche conformation; then, the intermolecular HB between CO and NH groups is weakened, some even being broken, and consequently, the intermolecular distance is enlarged; finally, the skeleton of phenyl ring has enough space to change their conformation to weaken the π–π stacking interaction. In addition, besides a few free and some medium bonded NH and CO groups, strongly bonded NH and CO groups still predominantly exist in the mesophase.     

Observation of vinylidene emission in mixed phosphine/diimine complexes of Ru(II) at room temperature in solution

The mixed ruthenium(II) complexes trans-[RuCl₂(PPh₃)₂(bipy)] (1), trans-[RuCl₂(PPh₃)₂(Me₂bipy)](2), cis-[RuCl₂(dcype)(bipy)](3), cis-[RuCl₂(dcype)(Me₂bipy)](4) (PPh₃ = triphenylphosphine, dcype = 1,2-bis(dicyclohexylphosphino)ethane, bipy = 2,2′-bipyridine, Me₂bipy = 4,4′-dimethyl-2,2′-bipyridine) were used as precursors to synthesize the associated vinylidene complexes. The complexes [RuCl(CCHPh)(PPh₃)₂(bipy)]PF₆ (5), [RuCl(CCHPh)(PPh₃)₂(Me₂bipy)]PF₆ (6), [RuCl(CCHPh)(dcype)(bipy)]PF₆ (7), [RuCl(CCHPh)(dcype)(bipy)]PF₆ (8) were characterized and their spectral, electrochemical, photochemical and photophysical properties were examined. The emission assigned to the π–π1 excited state from the vinylidene ligand is irradiation wavelength (340, 400, 430 nm) and solvent (CH₂Cl₂, CH₃CN, EtOH/MeOH) dependent. The cyclic voltammograms of (6) and (7) show a reversible metal oxidation peak and two successive ligand reductions in the +1.5-(−0.64) V range. The reduction of the vinylidene leads to the formation of the acetylide complex, but due the hydrogen abstraction the process is irreversible. The studies described here suggest that for practical applications such as functional materials, nonlinear optics, building blocks and supramolecular photochemistry.     

Observation of vinylidene emission in mixed phosphine/diimine complexes of Ru(II) at room temperature in solution

The mixed ruthenium(II) complexes trans-[RuCl₂(PPh₃)₂(bipy)] (1), trans-[RuCl₂(PPh₃)₂(Me₂bipy)](2), cis-[RuCl₂(dcype)(bipy)](3), cis-[RuCl₂(dcype)(Me₂bipy)](4) (PPh₃ = triphenylphosphine, dcype = 1,2-bis(dicyclohexylphosphino)ethane, bipy = 2,2′-bipyridine, Me₂bipy = 4,4′-dimethyl-2,2′-bipyridine) were used as precursors to synthesize the associated vinylidene complexes. The complexes [RuCl(CCHPh)(PPh₃)₂(bipy)]PF₆ (5), [RuCl(CCHPh)(PPh₃)₂(Me₂bipy)]PF₆ (6), [RuCl(CCHPh)(dcype)(bipy)]PF₆ (7), [RuCl(CCHPh)(dcype)(bipy)]PF₆ (8) were characterized and their spectral, electrochemical, photochemical and photophysical properties were examined. The emission assigned to the π–π1 excited state from the vinylidene ligand is irradiation wavelength (340, 400, 430 nm) and solvent (CH₂Cl₂, CH₃CN, EtOH/MeOH) dependent. The cyclic voltammograms of (6) and (7) show a reversible metal oxidation peak and two successive ligand reductions in the +1.5-(?0.64) V range. The reduction of the vinylidene leads to the formation of the acetylide complex, but due the hydrogen abstraction the process is irreversible. The studies described here suggest that for practical applications such as functional materials, nonlinear optics, building blocks and supramolecular photochemistry.     

The structure and two-dimensional correlation infrared spectroscopy study of a new 2D polyoxomolybdate complex containing β-octamolybdate linked up by potassium ions: (4,4′-Hbpy)2(K2Mo8O26)

A novel compound, (4,4′-Hbpy)₂(K₂Mo₈O₂₆) 1 (bpy = bipydine), was synthesized by hydrothermal method and characterized by X-ray single analysis, thermalgravimetric analysis, one-dimensional (1D) infrared spectroscopy and two-dimensional (2D) correlation infrared spectroscopy under thermal perturbation. In the compound 1, the [Mo₈O₂₆] units link to potassium ions to form layer structure, and the protonated 4,4′-bpy are linked to chains by hydrogen bonds. The 2D IR correlation spectroscopy study indicates that the intensity changes of MoO, NH and CC stretching vibration are sensitive to the temperature variation, and the intensity changes of asymmetry stretching vibration of the terminal MoO occur prior to that of terminal MoO linked by K atom. At the same time, the peaks of asymmetry stretching vibrations of the terminal MoO and the stretching vibrations of NH split into two peaks respectively in 2D IR correlation spectroscopy.     

Enthalpies of formation of small free radicals and stable intermediates: Interplay of experimental and theoretical values

A set of small radicals SiF, SiCl, F–CO, CN–O, O₃H, NO₃, CH₂NC, CF₃O, and O₃ exhibit pronounced discrepancies between different experimental as well as experimental and calculated values of the respective enthalpies of formation Δ_fH^o(298.15). For stable molecules, this quantity is well established and reliable values are available. However, for free radicals and other short-lived intermediates, the situation is not nearly as favorable. Consequently, critical evaluation of thermodynamic properties of free radicals is necessary, both originating from experiment and computation. Calculated enthalpies of formation for the above systems are based on the ab initio methods G3MP2B3 and CCSD(T)–CBS (W1U) for which mean absolute deviations are known.     

Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

The molecular structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) was determined by means of gas electron diffraction. The nozzle temperature was 185 °C. The results of MP2 and B3LYP calculations with the 6-31G⁷ basis set were used as supporting information. These calculations predicted that caffeine has only one conformer and some of the methyl groups perform low frequency internal rotation. The electron diffraction data were analyzed on this basis. The determined structural parameters (r_g and ∠_α) of caffeine are as follows: <r(NC)_ring> = 1.382(3) Å; r(CC) = 1.382(←) Å; r(CC) = 1.446(18) Å; r(CN) = 1.297(11) Å; <r(NC_methyl)> = 1.459(13) Å; <r(CO)> = 1.206(5) Å; <r(CH)> = 1.085(11) Å; ∠N₁C₂N₃ = 116.5(11)°; ∠N₃C₄C₅ = 121. 5(13)°; ∠C₄C₅C₆ = 122.9(10)°; ∠C₄C₅N₇ = 104.7(14)°; ∠N₉–C₄=C₅ = 111.6(10)°; <∠NCH_methyl> = 108.5(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrow in parentheses means that this parameter is bound to the preceding one.