New templating strategies with salen scaffolds (Salen=N,N'-bis(salicylidene)ethylenediamine dianion)

Templated approaches towards selective organic synthesis is a common feature in nature in which nucleic acid templated synthesis plays a crucial role in various fundamental biological processes. The key feature that allows control over the amazing selectivity found in natural processes is evidently the effective molarity of the reaction partners that is mediated by the macromolecular templation event. An ongoing challenge within many chemical sciences is to exploit similar templating principles and make use of synthetic systems that are designed for specific chemical conversions. Here, we describe the recent developments that involve (metallo)salen scaffolds that are used for diverse templating events (salen=N,N'-bis(salicylidene)ethylenediamine dianion).     

Synthese und Kristallstruktur eines μ-Methylen-μ-hydrido-dialanats [R2Al(μ-CH2)(μ-H)AlR2]− (R = CH(SiMe3)2)

Synthesis and Crystal Structure of a μ-Methylene-μ-hydrido-dialanate [R₂Al(μ-CH₂)(μ-H)AlR₂]^? (R = CH(SiMe₃)₂) tert-Butyl lithium reacts with the recently synthesized methylene bridged dialuminium compound [(Me₃Si)₂CH]₂Al? CH₂? Al[CH(SiMe₃)₂]₂ 2 in the presence of TMEDA under β-elimination; the thereby formed hydride anion is bound in a chelating manner by both unsaturated aluminium atoms forming a 3c–2e–Al? H? Al bond. The crystal structure of the product shows two independent molecules differing only slightly in bond lengths and angles, but significantly in conformation. While one of the Al₂CH heterocycles deviates little from planarity with a rough C₂ symmetry for the whole anion, the other one is folded with an angle of 21.1° and the arrangement of the substituents is best described by C_s symmetry.     

Periphere Anbindung von Quecksilber(II)-iodid an dreikernige Molybdän-Schwefel-Dithiophosphinatocluster: [Mo3S4(R2PS2)4HgI2] (R = Et,Pr)

Peripheral Bonding of Mercury(II) Iodide to Trinuclear Molybdenum-Sulfur-Dithiophosphinato Clusters: [Mo₃S₄(R₂PS₂)₄HgI₂] (R = Et, Pr) Reaction of Mo₃S₄(R₂PS₂)₄ 1 (a : R = Et, b : R = Pr) with HgI₂ in THF yields the diamagnetic title complexes [Mo₃S₄(R₂PS₂)₄HgI₂] 3 . The crystal structure of [ 3a (H₂O)] · 2 CH₂Cl₂ shows the complexes to consist of a triangular array of Mo atoms which are bridged by μ₂? S atoms and capped by a μ₃? S atom. Each of the Mo atoms is chelated by a dithiophosphinato ligand Et₂PS₂^? and in addition two Mo atoms are bridged by a Et₂PS₂^? ligand while the H₂O molecule is bonded weakly to the third Mo atom. Thus, all Mo atoms reveal a distorted octahedral coordination sphere. HgI₂ is ?peripherally”? bonded to the cluster via two S atoms, one of which belongs to a chelating ligand and the other one to the bridging ligand. Space group P1 , lattice constants a = 12.157(2), b = 15.284(3), c = 16.049(3) Å, α = 115.56(1), β = 107.35(1), and γ = 94.62(1)°; Z = 2, d_calc = 2.23 mg/mm³; 4 236 observed reflections, R = 0.068. In organic solvents complexes 3 are strong electrolytes. VT-³¹P NMR data suggest a stepwise dissociation of 3 with formation of [Mo₃S₄(R₂PS₂)₃] ⁺[(R₂PS₂)HgI₂]^? and elimination of the bridging ligand from the cluster.     

A two-state computational investigation of methane C--H and ethane C--C oxidative addition to [CpM(PH3)]n+ (M = Co, Rh, Ir; n = 0, 1)

Reductive elimination of methane from methyl hydride half-sandwich phosphane complexes of the Group 9 metals has been investigated by DFT calculations on the model system [CpM(PH(3))(CH(3))(H)] (M = Co, Rh, Ir). For each metal, the unsaturated product has a triplet ground state; thus, spin crossover occurs during the reaction. All relevant stationary points on the two potential energy surfaces (PES) and the minimum energy crossing point (MECP) were optimized. Spin crossover occurs very near the sigma-CH(4) complex local minimum for the Co system, whereas the heavier Rh and Ir systems remain in the singlet state until the CH(4) molecule is almost completely expelled from the metal coordination sphere. No local sigma-CH(4) minimum was found for the Ir system. The energetic profiles agree with the nonexistence of the Co(III) methyl hydride complex and with the greater thermal stability of the Ir complex relative to the Rh complex. Reductive elimination of methane from the related oxidized complexes [CpM(PH(3))(CH(3))(H)](+) (M = Rh, Ir) proceeds entirely on the spin doublet PES, because the 15-electron [CpM(PH(3))](+) products have a doublet ground state. This process is thermodynamically favored by about 25 kcal mol(-1) relative to the corresponding neutral system. It is essentially barrierless for the Rh system and has a relatively small barrier (ca. 7.5 kcal mol(-1)) for the Ir system. In both cases, the reaction involves a sigma-CH(4) intermediate. Reductive elimination of ethane from [CpM(PH(3))(CH(3))(2)](+) (M = Rh, Ir) shows a similar thermodynamic profile, but is kinetically quite different from methane elimination from [CpM(PH(3))(CH(3))(H)](+): the reductive elimination barrier is much greater and does not involve a sigma-complex intermediate. The large difference in the calculated activation barriers (ca. 12.0 and ca. 30.5 kcal mol(-1) for the Rh and Ir systems, respectively) agrees with the experimental observation, for related systems, of oxidatively induced ethane elimination when M = Rh, whereas the related Ir systems prefer to decompose by alternative pathways.     

Application of unsymmetrical indirect covariance NMR methods to the computation of the (13)C <--> (15)N HSQC-IMPEACH and (13)C <--> (15)N HMBC-IMPEACH correlation spectra

Utilization of long-range (1)H--(15)N heteronuclear chemical shift correlation has continually grown in importance since the first applications were reported in 1995. More recently, indirect covariance NMR methods have been introduced followed by the development of unsymmetrical indirect covariance processing methods. The latter technique has been shown to allow the calculation of hyphenated 2D NMR data matrices from more readily acquired nonhyphenated 2D NMR spectra. We recently reported the use of unsymmetrical indirect covariance processing to combine (1)H--(13)C GHSQC and (1)H--(15)N GHMBC long-range spectra to yield a (13)C--(15)N HSQC-HMBC chemical shift correlation spectrum that could not be acquired in a reasonable period of time without resorting to (15)N-labeled molecules. We now report the unsymmetrical indirect covariance processing of (1)H--(13)C GHMBC and (1)H--(15)N IMPEACH spectra to afford a (13)C--(15)N HMBC-IMPEACH spectrum that has the potential to span as many as six to eight bonds. Correlations for carbon resonances long-range coupled to a protonated carbon in the (1)H--(13)C HMBC spectrum are transferred via the long-range (1)H--(15)N coupling pathway in the (1)H--(15)N IMPEACH spectrum to afford a much broader range of correlation possibilities in the (13)C--(15)N HMBC-IMPEACH correlation spectrum. The indole alkaloid vincamine is used as a model compound to illustrate the application of the method.     

Synthese und Kristallstruktur der Spiro-Verbindung [(i-Pr)2P(S)NSiMe3]2SnCl2

Synthesis and Crystal Structure of the Spirocycle [(i-Pr)₂P(S)NSiMe₃]₂SnCl₂ The reaction of (i-Pr)₂P(S)N(SiMe₃)₂ ( 1 ) with SnCl₄ in 2:1 ratio yields under elimination of ClSiMe₃ the four-membered spirocycle [(i-Pr)₂P(S)NSiMe₃]₂SnCl₂ ( 2 ). The molecular structure of 2 was investigated by an X-ray structure analysis. Compound 2 crystallises in the monoclinic space group P2₁, Z = 2, a = 938.1(1), b = 1 424.1(2), c = 1 207.2(1) pm, β = 110.59(1)°, R = 2.05% for 4 102 reflexions. Compound 2 is a spirocycle with two Sn? N? P? S-rings joined at tin. The two rings are in cis-position.     

{(MesGa)3[GaP(H)Mes](PMes)4}, Ein phosphorsubstituiertes Ga?P-Heterocuban

{(MesGa)₃[GaP(H)Mes](PMes)₄}, a Phosphorus-substituted Ga? P-Heterocubane A mixture of MesGaCl₂/GaCl₃ (ratio 3:1) reacts with 5 equivalents of MesPLi₂ in THF at ?78°C to the title compound {(MesGa)₃[GaP(H)Mes](PMes)₄} ( 1 ) by use of the “dilution principle”. 1 can be obtained in 30% yield. Recrystallization of 1 from DME and toluene, respectively, gives 1 · 0.5 DME and 1 · toluene. 1 was characterized by NMR-, IR-, and MS-techniques. According to the X-ray structure determination of 1 · toluene, 1 has a heterocubane structure, one corner of which is substituted with an P(H)Mes group.     

Electrodeposition mechanism of Ni?Mo?P alloy in the solution of ammoniac citrate

The induced codeposition mechanism of Mo, P and Ni from the solution of ammoniac citrate was studied by means of steady-state polarization, AC impedance and X-ray Photoelectron Spectroscopy (XPS). The result of electrochemical measurements proved that [NiCit(NHs)₂]- is the electro-active species of nickel, though nickel ions exist mainly as [NiCit(NH₃)₃]^? in ammoniac citrate. XPS experiments proved the existence of tetravalent molybdenum corresponding to MoO₂ on the surface of mme deposits. The intermediate product, MoO₂, WM probably reduced to Mo in the alloy deposit by atomic hydrogen adsorbed on the induced metal nickel. The reduction of H₂PO^?₂ occurs through two distinctive steps with PH₃ an an intermediate, which subsequently reacts with atomic hydrogen to form P in the alloy deposit. The electrodeposition mechanism was proposed in this paper.     

Self-supported chiral catalysts for heterogeneous enantioselective reactions

The development of heterogeneous chiral catalysts for enantioselective reactions is highly desirable in order to overcome some drawbacks of homogeneous catalysts. Different from the conventional approaches by using various types of supports or biphasic systems for the recovery and reuse of homogeneous catalysts, a conceptually new strategy for heterogenization of homogeneous chiral catalysts, that is, a "self-supporting" approach, has been developed to use homochiral metal-organic coordination polymers generated by the self-assembly of chiral multitopic ligands with metal ions, and thus obviates the use of any support. In this concept article, the success of this "self-supporting" strategy will be exemplified in heterogeneous catalysis of asymmetric carbonyl-ene, sulfoxidation, epoxidation, and asymmetric hydrogenation reactions.