Olefin Isomerization by Iridium Pincer Catalysts. Experimental Evidence for an η(3)-Allyl Pathway and an Unconventional Mechanism Predicted by DFT Calculations

The isomerization of olefins by complexes of the pincer-ligated iridium species ((tBu)PCP)Ir ((tBu)PCP = κ(3)-C(6)H(3)-2,6-(CH(2)P(t)Bu(2))(2)) and ((tBu)POCOP)Ir ((tBu)POCOP = κ(3)-C(6)H(3)-2,6-(OP(t)Bu(2))(2)) has been investigated by computational and experimental methods. The corresponding dihydrides, (pincer)IrH(2), are known to hydrogenate olefins via initial Ir-H addition across the double bond. Such an addition is also the initial step in the mechanism most widely proposed for olefin isomerization (the "hydride addition pathway"); however, the results of kinetics experiments and DFT calculations (using both M06 and PBE functionals) indicate that this is not the operative pathway for isomerization in this case. Instead, (pincer)Ir(η(2)-olefin) species undergo isomerization via the formation of (pincer)Ir(η(3)-allyl)(H) intermediates; one example of such a species, ((tBu)POCOP)Ir(η(3)-propenyl)(H), was independently generated, spectroscopically characterized, and observed to convert to ((tBu)POCOP)Ir(η(2)-propene). Surprisingly, the DFT calculations indicate that the conversion of the η(2)-olefin complex to the η(3)-allyl hydride takes place via initial dissociation of the Ir-olefin π-bond to give a σ-complex of the allylic C-H bond; this intermediate then undergoes C-H bond oxidative cleavage to give an iridium η(1)-allyl hydride which "closes" to give the η(3)-allyl hydride. Subsequently, the η(3)-allyl group "opens" in the opposite sense to give a new η(1)-allyl (thus completing what is formally a 1,3 shift of Ir), which undergoes C-H elimination and π-coordination to give a coordinated olefin that has undergone double-bond migration.     

A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature M?ssbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.     

Customizable web service interface for light scattering simulation programs

Investigation of light scattering by simulations became increasingly important in recent years. Applications range from plasmonics to optical particle characterization. While there are hundreds of programs available, using them can be very time consuming: installation, configuration and operation often are laborious. As a result potential users of such software might be discouraged by the complexity of the necessary tasks. Not only ‘casual users’ – scientists who need light scattering simulation results for their daily work but who are not specialized on the topic – could benefit from tools to execute the calculations online via an easy to use web-interface accessed by a browser. In this case no local installation of the software is necessary, a user friendly interface helps learning and understanding the use of programs, while calculations can be done from anywhere and any time as long as a user is connected to the internet.This paper describes a newly developed interface that can be used for these purposes. It is made freely available to everyone and can be adapted to a variety of existing light scattering programs.     

Strange matter and big bang helium synthesis

Stable strange quark matter produced in the QCD phase transition in the early universe will trap neutrons and repel protons, thus reducing primordial helium production, Y_p. For reasonable values of Y_p, the radius of strange droplets must exceed 10^?6 cm if strange matter shall solve the dark-matter problem without spoiling Big Bang helium synthesis.     

Synthetic anthracyclinones-XXVII anthracyclinones by intramolecular marschalk reaction : Synthesis of the feudomycinones and rhodomycinones

1,4,5-Trihydroxy-9,10-anthraquinone is transformed regioselectively to the α-hydroxy dichlorides 18-21, which are cyclized to yield predominantely the naturally configurated 9,10-trans-diols 1,3,26, and 27 (80 to 96% d.e.). The monotrifluoroacetates 38-40 derived from the trans-diols are hydroxylated via bromination at C-7 to yield almost exclusively the 7,9-cis-9,10-trans-triols α₁-rhodomycinone (4), feudomycinone D (7), and 4-0-methyl-β-rhodomycinone (42). The feudomycinones A (5) and C (6) are obtained with less chemo- and stereoselectivity by hydroxylation of the l0 Deoxycompounds 33 and 34.     

A performance criterion for information theoretic data analysis

A previously unobserved electronic state in fluorobenzene appears as a two-photon resonance in the multiphoton resonance ionization spectrum. The band system is assigned to the 3s Rydberg ← b₂(π) HOMO transition with a quantum defect of 0.83 and an origin at 50 914 ± 15 cm^?1. Its correspondence to the benzene state characterized by Johnson in the same region assigns that state to E_1g 3s Rydberg.     

Castor quadruplorum

The busy beaver problem of Rado [6] is reexamined for the case of Turing machines given by quadruples rather than quintuples. Moreover several printing symbols are allowed. Some values of the corresponding beaver function are given and it is shown that this function for a fixed number of states and varying number of symbols is nonrecursive for three or more states and recursive for two states. As a byproduct we get that the minimal number of states in a universal Turing machine (quadruples) is three.     

Synthesis of ζ -pyrromycinone, 7-deoxyauramycinone,and 7-deoxyaklavinone via ketoester cyclizatiON 1)

7,10-O-Dimethyl- ζ -pyrromycinone (78) and the not naturally occurring 7-deoxyanthracyclinones 76 and 77 are prepared via base itinduced cyclization of the ketoesters 73 - 75, and stereoselectively hydroxylated to the 2,4-cis diol 83 and 85.