Sn Bond Length Research Articles

Conformational study of the electronic interactions and nitric oxide release potential of new S‑nitrosothiols esters derivatives of ibuprofen, naproxen and phenyl acids substituted (SNO-ESTERS): Synthesis, infrared spectroscopy analysis and theoretical calculations.

The conformational study on the new S‑nitrosothiols esters (SNO-ESTERS): para-substituted (X = H, OMe, Cl and NO2) S‑nitrosothiol derivatives 2‑methyl‑2‑(sulfanyl)propyl phenylacetates (R1), 2‑(4‑isobutylphenyl)propanoate (ibuprofen, R2), and 2‑(4‑isobutylphenyl)propanoate of 2‑methyl‑2‑(nitrososulfanyl)propyl (naproxen, R3) was performed using infrared spectroscopy (IR) in solvents with increasing polarity (CCl4, CH3Cl, and CH3CN), and theoretical calculations, to determine the preferential conformer and the potential of these compounds to release nitric oxide (NO). S‑Nitrosothiols were synthesized by esterification reactions, using chlorides of the corresponding carboxylic acids, with good yields (~60%). IR results showed that these compounds presented only one conformation, and the experimental data were supported by the theoretical results obtained by density functional theory (DFT) calculations using the 6311+G (2df, 2p) basis set. The calculations revealed that all S‑nitrosothiols presented one preferential anticlinal (ac) geometric conformation, which agrees with the data obtained experimentally in CCl4. These conformers are stabilized by intramolecular hydrogen bonds. Examination of the geometry with regard to the RSNO group revealed that these compounds are preferentially in the trans (anti) conformation. The calculation of the orbital interactions using the Natural Bond Orbital (NBO) method showed that the nO(NO) → σ(SN)∗ hyper-conjugative interaction increases the SN bond length. The strong nS → π(NO)∗ interaction and electronic delocalization induces a partial π character to the SN bond. The weak σSN bond indicates strong delocalization of the electron pair in O (NO) by the nO(NO) → σ(SN)∗ interaction, thereby increasing the capacity of NO release from SNO-ESTERS.

Binäre Lanthan‐Stannide mit Sn:La‐Verhältnissen nahe 1:1 – Synthesen, Kristallstrukturen, Chemische Bindung

AbstractFour binary lanthanum stannides close to the 1:1 ratio of Sn:La were synthesized from mixtures of the elements. The structures of the compounds have been determined by means of single‐crystal X‐ray data. The low temperature (α) form of LaSn (CrB‐type, orthorhombic, space group Cmcm, a = 476.33(6), b = 1191.1(2), c = 440.89(6) pm, Z = 4, R1 = 0.0247), crystallizes with the CrB‐type. The structure exhibits planar tin zigzag chains with a Sn–Sn bond length of 299.1 pm. In contrast to the electron precise Zintl compounds of the alkaline earth elements, additional La–Sn bonding contributions become apparent from the results of band structure calculations. In the somewhat tin‐richer region, the new compound La3Sn4 (orthorhombic, space group Cmcm, a = 451.45(4), b = 1190.44(9), c = 1583.8(2) pm, Z = 4, R1 = 0.0674), crystallizing with the Er3Ge4 structure type, exhibits Sn3 segments of the zigzag chains of α‐LaSn together with a further Sn atom in a square planar Sn coordination with increased Sn–Sn bond lengths. In the Lanthanum‐richer region, La11Sn10 (tetragonal, space group I4/mmm, a = 1208.98(5), c = 1816.60(9) pm, Z = 4, R1 = 0.0325) forms the undistorted tetragonal Ho11Ge10 structure type. Its structure, which contains isolated Sn atoms, [Sn2] dumbbells and planar [Sn4] rings is related to the high temperature (β) form of LaSn. The structure of β‐LaSn (space group Cmmm, a = 1766.97(6), b = 1768.28(5), c = 1194.32(3) pm, Z = 60, R1 = 0.0453), which forms a singular structure type, can be derived from that of La11Sn10 by the removal of thin slabs. Due to the different stacking of the remaining layers, planar [Sn4] chain segments and linear [Sn–Sn–Sn] anions are formed as additional structural elements. The chemical bonding (Sn–Sn covalent bonding, Sn–La contributions) is discussed on the basis of the simple Zintl concept and the results of FP‐LAPW calculations (density of states, band structure, valence electron densities and electron localization function).

Structure of Ce1−xSnxO2 and its relation to oxygen storage property from first-principles analysis

CeO(2)-SnO(2) solid solution has been reported to possess high oxygen storage/release property which possibly originates from local structural distortion. We have performed first-principles based density functional calculations of Ce(1-x)Sn(x)O(2) structure (x=0, 0.25, 0.5, 1) to understand its structural stability in fluorite in comparison to rutile structure of the other end-member SnO(2), and studied the local structural distortion induced by the dopant Sn ion. Analysis of relative energies of fluorite and rutile phases of CeO(2), SnO(2), and Ce(1-x)Sn(x)O(2) indicates that fluorite structure is the most stable for Ce(1-x)Sn(x)O(2) solid solution. An analysis of local structural distortions reflected in phonon dispersion show that SnO(2) in fluorite structure is highly unstable while CeO(2) in rutile structure is only weakly unstable. Thus, Sn in Ce(1-x)Sn(x)O(2)-fluorite structure is associated with high local structural distortion whereas Ce in Ce(1-x)Sn(x)O(2)-rutile structure, if formed, will show only marginal local distortion. Determination of M-O (M=Ce or Sn) bond lengths and analysis of Born effective charges for the optimized structure of Ce(1-x)Sn(x)O(2) show that local coordination of these cations changes from ideal eightfold coordination expected of fluorite lattice to 4+4 coordination, leading to generation of long and short Ce-O and Sn-O bonds in the doped structure. Bond valence analyses for all ions show the presence of oxygen with bond valence approximately 1.84. These weakly bonded oxygen ions are relevant for enhanced oxygen storage/release properties observed in Ce(1-x)Sn(x)O(2) solid solution.

Substituent effects in ditetrel alkyne analogues: multiple vs. single bonded isomers

The synthesis and characterization of a series of digermynes and distannynes stabilized by terphenyl ligands are described. The ligands are based on the Ar′ (Ar′ = C6H3-2,6(C6H3-2,6-iPr2)2) or Ar* (Ar* = C6H3-2,6(C6H2-2,4,6-iPr3)2) platforms which were modified at the meta or para positions of their central aryl rings to yield 4-X-Ar′ (4-X-Ar′ = 4-X-C6H2-2,6(C6H3-2,6-iPr2)2, X = H, F, Cl, OMe, tBu, SiMe3, GeMe3) and 3,5-iPr2-Ar′ or Ar* and 3,5-iPr2-Ar*. The compounds were synthesized by reduction of the terphenyl germanium(II) or tin(II) halide precursors with a variety of reducing agents. The precursors were obtained by the reaction of one equivalent of the lithium terphenyl with GeCl2 dioxane or SnCl2. For germanium, their X-ray crystal structures showed them to be either Ge–Ge bonded dimers with trans-pyramidal geometries or V-shaped monomers. In contrast, the terphenyl tin halides had no tin–tin bonding but existed either as halide bridged dimers or V-shaped monomers. Reduction with a variety of reducing agents afforded the digermynes ArGeGeAr (Ar = 4-Cl-Ar′, 4-SiMe3-Ar′ or 3,5-iPr2-Ar*) or the distannynes ArSnSnAr (Ar = 4-F-Ar′, 4-Cl-Ar′, 4-MeO-Ar′, 4-tBu-Ar′, 4-SiMe3-Ar′, 4-GeMe3-Ar′, 3,5-iPr2-Ar′, 3,5-iPr2-Ar*), which were characterized structurally and spectroscopically. The digermynes display planar trans-bent core geometries with Ge–Ge distances near 2.26 Å and bending angles near 128° consistent with Ge–Ge multiple bonding. In contrast, the distannynes had either multiple bonded geometries with Sn–Sn distances that averaged 2.65 Å and an average bending angle near 123.8°, or single bonded geometries with a Sn–Sn bond length near 3.06 Å and a bending angle near 98°. The 3,5-iPr2-Ar*SnSnAr*-3,5-iPr2 species had an intermediate structure with a longer multiple bond near 2.73 Å and a variable torsion angle (14–28°) between the tin coordination planes. Mössbauer data for the multiple and single bonded species displayed similar isomer shifts but had different quadrupole splittings.

(η6-p-Cymene)bis(trichloridostannyl)(triethoxyphosphine-κP)ruthenium(II)

In the title complex, [RuSn2(C10H14)Cl6(C6H15O3P)], the Ru—Sn bond lengths [2.5619 (3) and 2.5669 (3) Å] are about 0.3 Å shorter than the sum of the covalent Ru and Sn radii (1.46 + 1.39 = 2.85 Å), in line with other structurally characterized arene ruthenium trichlorido­stannyl derivatives. The Ru(II) atom is surrounded by a para-cymene, a triethylphosphite and two trichloridostannyl ligands in a typical piano-stool coordination.

Natural bond orbital analysis of some S-nitrosothiols biological molecules

Theoretical study of several S-nitrosothiols biological molecules has been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods with 6-31G(d,p) basis set. Geometries obtained from DFT calculations were used to perform natural bond orbital (NBO) analysis. It is noted that the weakness in the SN sigma bond is due to nO1σ delocalization and is responsible for the longer SN bond length in S-nitrosothiols. It is also noted that decreased occupancy of the localized σSN orbital in the idealized Lewis structure or increased occupancy of σ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital of σSN bond orbital. In addition, the charge transfer energy decreases with the increasing of the Hammett constants of substituent groups, and the partial charge distribution on the skeletal atoms shows that the electrostatic repulsion or attraction between atoms can give a significant contribution to the intramolecular and intermolecular interaction. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

{2,6‐(Me2NCH2)2C6H3}Sn]2: An Intramolecularly Coordinated Diorganodistannyne

Not only steric protection by bulky substituents but also intramolecular N→Sn coordination makes possible the isolation and characterization of dimeric organotin(I) compounds such as [{2,6-(Me2NCH2)2C6H3}Sn]2 (see structure), which according to a crystallographic study exhibits a SnSn bond length of 2.9712(12) Å.

Synthesis, Structural Characterization, and Biological Studies of Six- and Five-Coordinate Organotin(IV) Complexes with the Thioamides 2-Mercaptobenzothiazole, 5-Chloro-2-mercaptobenzothiazole, and 2-Mercaptobenzoxazole

Organotin(IV) complexes with the formulas [(C6H5)3Sn(mbzt)] (1), [(C6H5)3Sn(cmbzt)] (3), and [(C6H5)2Sn(cmbzt)2] (4) (Hmbzt = 2-mercaptobenzothiazole and Hcmbzt = 5-chloro-2-mercaptobenzothiazole) have been synthesized and characterized by elemental analysis; FT-IR, Raman, 1H, 13C, and 119Sn NMR, and Mössbauer spectroscopic techniques; and X-ray crystallography at various temperatures. The crystal structures of complexes 1, 3, and 4 were determined by X-ray diffraction at room temperature [295(1) or 293(2) K]. The complexes [(C6H5)3Sn(mbzo)] (2) and [(n-C4H9)2Sn(cmbzt)2] (5) (Hmbzo = 2-mercaptobenzoxazole) were synthesized by new improved methods, and their structures were determined at low temperature [100(1) K] and compared to those solved at room temperature. Comparison with {(CH3)2Sn(cmbzt)2]} (6), already reported, was also attempted. The influence of temperature on the geometry of the complexes is discussed. In the cases of complexes 1-3, three carbon atoms from phenyl groups and one sulfur atom and one nitrogen atom from thione ligands form a tetrahedrally distorted trigonal-bipyramidal geometry around the five-coordinate tin(IV) ion. In complexes 4-6, two carbon atoms from aryl groups and two sulfur atoms and two nitrogen atoms from thione ligands form a distorted tetrahedral geometry, tending toward octahedral, around the six-coordinate tin(IV) ions, with trans-C2, cis-N2, and cis-S2 configurations. Although the C-Sn and S-Sn bond distances are found to be constant in compounds 1-6, their N-Sn bond lengths vary significantly (from 2.635 to 3.078 A), with the longer distances found in the cases of five-coordinate complexes 1-3.

Solid-State119Sn NMR and Mössbauer Spectroscopy of “Distannynes”: Evidence for Large Structural Differences in the Crystalline Phase

The "distannynes" Ar'SnSnAr' (Ar' = C6H3-2,6(C6H3-2,6-Pr(i)2)2) and ArSnSnAr (Ar = C6H3-2,6(C6H2-2,4,6-Pr(i)3)2) were examined by solid-state (119)Sn NMR and Mössbauer spectroscopy. The two compounds display substantially different spectroscopic parameters, while differing only in the absence (Ar'SnSnAr') or presence (ArSnSnAr) of a para-Pr(i) group in the flanking aryl rings of their terphenyl substituents. The spectroscopic differences can be interpreted in terms of a more trans-bent geometry and a longer Sn-Sn bond for ArSnSnAr in comparison to the wider Sn-Sn-C angle (125.24(7) degrees ) and shorter Sn-Sn bond length (2.6675(4)A) determined from the crystal structure of Ar'SnSnAr'. The differences are consistent with previously published calculations by Nagase and Takagi for ArSnSnAr.

An Ab Initio Study and NBO Analysis of the Stability and Conformational Properties of Hexakis(trimethylelementhyl)benzene (Element = C, Si, Ge, and Sn)

Ab initio molecular orbital and density functional theory were used to investigate energetic and structural properties of the various conformations of hexa-tertbutylbenzene (1), hexakis(trimethylsilyl)benzene (2), hexakis (trimethylgermyl)benzene (3), and hexakis(trimethylstannyl)benzene (4). HF/3-21G//HF/3-21G and B3LYP/3-21G//HF/3-21G results revealed that the Twist-Boat (TB) conformer of compound 1 is more stable than the 1-Chair (C), 1-Boat (B), and 1-Planar (P) conformers. B3LYP/3-21G//HF/3-21G results show that the 1- TB conformer is more stable than 1- C, 1- B, and 1- P conformers of about 1.13, 4.34, and 99.94 kcal mol−1 , respectively. Contrary to the stability order of compound 1 conformers, the C conformer of compounds 2–4 is more stable than TB, B, and P conformations, as calculated by B3LYP/3-21G//HF/3-21G and HF/3-21G//HF/3-21G levels of theory. The energy gap between the C and P conformers in compounds 1–4 is decreased in the following order: ΔE(4: C, P) < ΔE (3: C, P) < ΔE(2: C, P) < ΔE (1: C, P). This fact can be explained in terms of the increase of C aromatic -M (M═C, Si, Ge, and Sn) bond lengths and the decrease of steric (van der Waals) repulsions in the previously discussed compounds. For compounds 1–3, the calculations were also performed at the B3LYP/ 6-31G*//HF/3-21G level of theory. However, the comparison showed that the results at B3LYP/3-21G//HF/3-21G methods correlated well with those obtained at the B3LYP/6-31G*// HF/6-31G method. Further, NBO analysis revealed that in compounds 1–4, the resonance energy associated with the σM-C1 to σ*C2-C3 delocalization is 5.20, 9.68, 11.15, and 12.27 kcal mol−1, respectively. These resonance energy values could explain the easiness of the ring flipping processes of C, B, and TB conformers of compounds 4 to 1. Also, the NBO results showed that by an increase of the σM-C1 → σ *C2-C3 resonance energies in compounds 1–4, the σM-C1 bonding orbital occupancies decrease. This fact could fairly explain the increase of the Caryl-M bond length from compound 1 to 4. The NBO results are also in good agreement with the calculated energy barriers for the ring flipping of the chair conformations in compounds 1–4, as calculated by B3LYP and HF methods.

Structures of Vinylstannane (Ethenylstannane) and Allylstannane (2-Propenylstannane) Determined by Gas-Phase Electron Diffraction and Quantum Chemical Calculations

Vinylstannane (H2CCHSnH3) and allylstannane (H2CCHCH2SnH3) have been synthesized, and their structures and conformational properties have been determined by ab initio and density functional theory calculations and gas electron diffraction. There is only one stable conformation of vinylstannane, where one of the Sn−H bonds is synperiplanar to the double bond. The most important structural parameter is the C(sp2)−Sn(IV) bond length, which is ra = 215.1(6) pm (re = 214.1(6) pm). The CCSn bond angle is 121.6(4)°. Uncertainties are estimated errors equal to 2.5 times the least-squares standard deviation and include uncertainty in the electron wavelength. Theoretical calculations indicate that there are two stable rotameric forms of allystannane. The CC−C−Sn chain of atoms is synperiplanar (dihedral angle = 0°) in the less stable form and anticlinal (dihedral angle ≈ 106° from synperiplanar) in the more stable rotamer. Theoretical calculations predict an energy difference between the two conformations of about ...

Ti−Sn, Zr−Sn, and Hf−Sn Heterodimetallic Complexes: Stabilizing Unsupported Group 4−Group 14 Metal−Metal Bonds

Reaction of the tripodal amido complexes [MeSi{SiMe2N(4-CH3C6H4)}3ZrCl] and [MeSi{SiMe2N(4-CH3C6H4)}3HfCl] with LiSnPh3 in toluene yielded the Zr−Sn and Hf−Sn complexes [MeSi{SiMe2N(4-CH3C6H4)}3ZrSnPh3] (1) and [MeSi{SiMe2N(4-CH3C6H4)}3HfSnPh3] (2), while attempts to synthesize the corresponding Ti−Sn complex resulted in the oxidative coupling of the stannate to give Ph3Sn−SnPh3 and unidentified titanium species. More stable group 4−group 14 complexes were obtained by reaction of the triamido stannate complex [MeSi{SiMe2N(4-CH3C6H4)}3SnLi(OEt2)] with the metallocene dichlorides [Cp2MCl2] of all three titanium group metals, giving [MeSi{SiMe2N(4-CH3C6H4)}3SnM(η5-C5H5)2(Cl)] (M = Ti, 3; Zr, 4; Hf, 5). X-ray diffraction studies of 4 and 5 established Zr−Sn and Hf−Sn bond lengths of 3.02313(17) and 2.9956(3) A, respectively.

Synthesis and structure of dinuclear [(2- tert-butyl-4,5,6-trimethylphenyl) 2Sn-Co(η 2-ethene)(η 5-Me 5C 5)] and its thermal conversion to trinuclear [{(2- tert-butyl-4,5,6-trimethylphenyl) 2Sn} 2Co(η 5-Me 5C 5)] (Co–Sn 2)

The title compounds 4 and 12 are prepared by reacting bis{(η 2-ethene)}(η 5-pentamethylcyclopentadienyl)cobalt(I) 3 with Weidenbruch’s stannylene bis[bis{6- tert-butyl-2,3,4-trimethylphenyl}tin] ( 1) employing different thermal reaction conditions. Compounds 4 and 12 are rare examples of transition metal complexes of 1. According to an X-ray crystal structure determination, 4 displays a half-sandwich structure with trigonal planar coordinated Co atom and a subvalent Sn(II) center. The Co–Sn bond length in 4 is 2.3926(4) Å and represents one of the shortest reported so far. A comparison of relevant bonding parameters of 4 and several related Co–Sn half-sandwich compounds containing low valent SnR 2 fragments reveals comparable steric and electronic effects of the stannylenes towards the 16 e {(η 2-ethene)(Me 5C 5)Co} fragment in these complexes. In contrast to the isostructural ethene/stannylene complex bis{methyl(trimethylsilyl)}stannio(η 2-ethene)(η 5-cyclopentadienyl)cobalt 6 containing Lappert’s bis(stannanediyl) 2, 4 is inert towards H 2O and does not activate water by an oxidative addition reaction to give a mixed hydroxo hydrido complex. At elevated temperatures, 4 reacts via ethene dissociation and ligand redistribution to form the trinuclear cobalt bis stannylene half sandwich complex Bis[bis{6- tert-butyl-2,3,4-trimethylphenyl}]stannio(η 5-pentamethylcyclopentadienyl)}cobalt(I) 12 in moderate yields.

Synthesis and structural characterization of (THF) 3Na{(SnC 6H 3–2,6-Trip 2) 2} (Trip=C 6H 2-2,4,6- i-Pr 3): effects of cation–anion association on Sn–Sn multiple bonding

The synthesis and structural characterization of the contact ion-pair species (THF) 3Na{(SnC 6H 3–2,6-Trip 2) 2} ( 1) are described. The compound 1 was synthesized by the reduction of Sn(Cl)C 6H 3–2,6-Trip 2 with sodium anthracenide in THF. Its UV–vis and EPR spectra are very similar to those previously reported for the solvent separated ion pairs [K(THF) 6] [(SnC 6H 3–2,6-Trip 2) 2] and [K(dibenzo–18–crown–6)(THF) 3] [(SnC 6H 3–2,6-Trip 2) 2]. The X-ray crystal structure of 1 reveals an Sn–Sn distance 2.8107(13)Å and an Na–Sn bond length of 3.240(7)Å with C–Sn–Sn angles of ca. 98°. The Sn–Sn distance and tin geometry are very similar to those previously observed. These similarities lead to the conclusion that the Sn–Sn bonding is affected only in a minor way by sodium coordination. Crystal data at 130 K with CuK α ( λ=1.54178Å) radiation; 1, a=22.142(5), b=15.845(4), c=23.425(7)Å, β=107.83(2)°, Z=4, monoclinic, space group C2/ c, R1=0.0615 for 4872 ( I=2 σ( I)) data.

Metalloradical Chemistry of Cobalt(II) Porphyrins. The Syntheses, Structure, and Reactivity of Triphenyltin(II)- and Trihalomethylcobalt(III) Octaethylporphyrin

Stannanes R3SnH (R = n-Bu, Ph) reacted with CoIII(OEP)CH3 or CoII(OEP) to afford CoIII(OEP)SnR3 and CH4 or H2, respectively. CoIII(OEP)SnR3 was more efficiently prepared by reaction of CoI(OEP)- with R3SnCl. CoIII(OEP)SnPh3, C54H59CoN4Sn, crystallized in the triclinic space group P1̄ (Z = 2) with unit cell dimensions a = 12.124(5) Å, b = 14.700(5) Å, c = 15.221(7) Å, α = 109.56(4)°, β = 91.44(5)°, γ = 113.27(1)°, and V = 2308.4(1.8) Å3 at 295(2) K. The structure resembled that of five-coordinate alkylcobalt(III) porphyrin complexes with a square-pyramidal Co atom displaced 0.077 Å out of the porphyrin plane toward Sn and a Co−Sn bond length of 2.510(2) Å. The bond dissociation energy of the Co−Sn bond was considerably larger than that of the Co−C bond in alkylcobalt(III) porphyrin complexes. CoIII(OEP)SnPh3 was air stable in solution and decomposed by homolysis slowly at 120 °C in toluene. The Co−Sn bond was rapidly cleaved, though, when oxidized by I2 or by electrochemical means. In contrast, the Co−C bonds in CoIII(OEP)CX3 (X = Cl, Br, I) were substantially weaker than in the Co−C bond in alkylcobalt(III) porphyrin complexes and weakened progressively with heavier halogens. CoIII(OEP)CX3 complexes were prepared by reaction of CoI(OEP)- with CX4 (X = Cl, Br) or by reaction of CoII(OEP) with CBrCl3 or CX4 (X = Br, I). The reaction of CoI(OEP)- with CX4 (X = Cl, Br, I) also afforded small amounts of CoIII(OEP)CHX2 complexes, which were obtained in greater yield by reaction of CoI(OEP)- with CHX3. The substitution of one hydrogen for a halogen stabilized the CoIII(OEP)CHX2 complexes relative to the corresponding CoIII(OEP)CX3 complexes.

Synthesis, Structure, and Thermolysis Mechanism of S-Alkoxythiazynes

Abstract S-Alkoxy-S,S-diarylthiazynes were prepared by two methods: the alkaline hydrolysis of S,S-diaryl-N-halosulfilimines in aqueous alcohols and the reaction of S,S-diaryl-S-fluorothiazynes with sodium alkoxides. The structure of S,S-diphenyl-S-propoxythiazyne was determined by an X-ray crystallographic analysis, which showed a short SN bond length of 1.441(3) Å. The thermolysis of S-alkoxythiazynes gave elimination products, which were identified as the corresponding carbonyl compounds and N-unsubstituted S,S-diarylsulfilimines. Kinetic experiments for the thermolysis of the S-alkoxy-S,S-diarylthiazynes were carried out. The first-order kinetic behavior, a large kinetic isotope effect (kH/kD = 6.1) using S,S-diphenyl-S-[1,1-2H2]propoxythiazyne, a negative activation entropy (ΔS‡ = −30 J K−1mol−1), and a negative Hammett ρ-value (ρ = −0.35) on the phenyl group were obtained, suggesting that the reaction proceeds via a concerted five-membered cyclic transition state. A deviation from the ideal concerted transition state is discussed in comparison with that for sulfoxides.

DFT- and post-HF-study on structure and electronic excitation of acyclic and cyclic sulfur diimides

A series of representative organic acyclic and cyclic sulfur diimides were studied by Hartree–Fock-plus-correlation ab initio quantum chemistry and by density functional theory using Becke's three-parameter functional along with the LYP functional. The widely used formula representation –NSN– for these compounds suggests octet expansion of sulfur. This is not confirmed by theory. Although d-polarization functions significantly improve the numerical results, sulfur d-orbitals are hardly occupied. The calculated electronic charge distribution derived by population analysis and by the atoms-in-molecules topological theory favors charge separation resulting in a more or less ylidic structure with –NS+–N−– and –N−=S+–N– resonance contributors. This structure does not exclude relatively short SN bond lengths. The characteristics of the bonds in the parent structures is preserved in some non-Kekule-type NSN heterocyles. Strong SN bond charge separations of organyl sulfur diimides are accompanied by short SN bond distances and narrow S0/T1 and S0/S1 energy gaps. The experimentally well known naphtho[1,8-cd][1,2,6]thiadiazine and the unknown 3,4-dimethylene-1,2,5-thiadiazole belong to this series. The calculations confirm that 1,2,3-thiadiazole is the diaza analog of thiophene rather than the vinylene-bridged sulfur diimide, while more complex heterocycles such as benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole take an intermediate position between classical and non-classical structures. Two closely related minimum structures are defined on the DFT Born–Oppenheimer energy surface of naphtho[1,8-cd;4,5-c′d′]bis[1,2,6]thiadiazine, but so far only one compound is experimentally known. The lowest energy structure is the quinoid form corresponding to the experimentally known compound. © 1997 John Wiley & Sons, Ltd.

DFT- and post-HF-study on structure and electronic excitation of acyclic and cyclic sulfur diimides

A series of representative organic acyclic and cyclic sulfur diimides were studied by Hartree–Fock-plus-correlation ab initio quantum chemistry and by density functional theory using Becke's three-parameter functional along with the LYP functional. The widely used formula representation –NSN– for these compounds suggests octet expansion of sulfur. This is not confirmed by theory. Although d-polarization functions significantly improve the numerical results, sulfur d-orbitals are hardly occupied. The calculated electronic charge distribution derived by population analysis and by the atoms-in-molecules topological theory favors charge separation resulting in a more or less ylidic structure with –NS+–N−– and –N−=S+–N– resonance contributors. This structure does not exclude relatively short SN bond lengths. The characteristics of the bonds in the parent structures is preserved in some non-Kekulé-type NSN heterocyles. Strong SN bond charge separations of organyl sulfur diimides are accompanied by short SN bond distances and narrow S0/T1 and S0/S1 energy gaps. The experimentally well known naphtho[1,8-cd][1,2,6]thiadiazine and the unknown 3,4-dimethylene-1,2,5-thiadiazole belong to this series. The calculations confirm that 1,2,3-thiadiazole is the diaza analog of thiophene rather than the vinylene-bridged sulfur diimide, while more complex heterocycles such as benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole take an intermediate position between classical and non-classical structures. Two closely related minimum structures are defined on the DFT Born–Oppenheimer energy surface of naphtho[1,8-cd;4,5-c′d′]bis[1,2,6]thiadiazine, but so far only one compound is experimentally known. The lowest energy structure is the quinoid form corresponding to the experimentally known compound. © 1997 John Wiley & Sons, Ltd.

FLUORINATED CYCLOTHIAPHOSPHAZENES: SYNTHESIS, STRUCTURE AND REACTIVITY

Reaction of chlorocyclothiaphosphazenes NPCl2(NSOX)(2) (X = Cl, Ph) and (NPCl2)(2)NSOX (X = Cl, F, Ph) with KSO2F in bulk leads to fluorination at the phosphorus centers. The substitution reaction follows a geminal pattern. Reactions of trans-NPF2(NSOPh)(2) with Grignard reagents RMgX (R = Me, (t)Bu) show substitution of fluorine by alkyl groups to be a slow process. The more reactive PhMgBr gives an acceptable reaction rate and good yields. Organolithium reagents offer organo-substituted cyclothiaphosphazenes in low to moderate yields, depending on the reagent used. Crystals of cis-NPF2(NSOPh)(2) are monoclinic, space group P2(1)/n, with a = 13.665(7) Angstrom, b = 10.676(1) Angstrom, c = 9.897(3) Angstrom, beta = 90.55(3)degrees, V = 1443.8(9) Angstrom(3), and Z = 4. The final R and wR values are 0.034 and 0.043, respectively. The PN and SN bond lengths vary from 1.571(2) to 1.590(2) Angstrom.

Characterization of Pt–Sn/Sio2catalysts and the role of Sn in NO–hydrocarbon reactions

Pt–Sn/SiO2 catalysts prepared by selective chemical vapour deposition of Sn(CH3)4 to Pt particles supported on SiO2 have been characterized by extended X-ray absorption fine structure (EXAFS) at both Sn K-edge and Pt LIII-edge, FTIR, transmission electron microscopy (TEM), and H2 and CO adsorption measurements. Sn atoms were suggested to be located at the first layer of the bimetallic particles until Sn/Pt = 0.26, giving an Sn—Pt bond length of 0.263 nm and Sn—Sn bond length of 0.260 nm. A model structure of the Pt–Sn bimetallic layers was proposed. The Pt–Sn/SiO2 catalyst was remarkably active for NO dissociation, leaving deposited oxygen atoms at the surface. The deposited oxygen atoms were bonded to the Sn atoms to give Sn—O bonds of length 0.198 nm, which were active for hydrocarbon oxidation reactions. The reactivity of the SnOx layer was suggested to be promoted by the presence of Pt atoms. The catalytic NO–C3H8 and NO–CH4 reactions proceeded via NO dissociation and successive oxidation of hydrocarbons on Pt–Sn/SiO2. The reaction rate and selectivity varied with the Sn/Pt ratio. Sn addition increased the N2 selectivity from 50% for Pt/SiO2 to 100% for Pt–Sn/SiO2. N2 selectivity was related mainly to the oxygen deficiency in the SnOx layers.