Deconvolution of appearance potential spectra

Soft x-ray appearance potential spectra from solids carry information on the local density of unoccupied states in the surface region of the sample. The intensity of the emitted characteristic radiation is, according to a simple model, proportional to a convolution product of the transition densities of the incoming electron and the ionized core electron. In a first approximation, the two transition densities are proportional to the density of states in the unoccupied part of the conduction band. Attempts to retrieve the density of states from measured spectra must rely on deconvolution procedures. In this paper, we introduce a new simple method. It is essentially a stabilized version of the Hagstrum-Becker method which appears to work quickly and reliably. As an example, the method is applied to an ironL _III appearance potential spectru. Comparison of the results with a theoretical calculation of the density of states and with measured Bremsstrahlungsisochromats obtained by Turtle and Liefeld strongly supports the self-convolution model in this case.     

Vibrational spectroscopy and normal-mode analysis of Fe(II) octaethylporphyrin

The normal-mode spectrum for the four-coordinated heme compound Fe(II) octaethylporphyrin, Fe(OEP), has been determined by refining force constants to the experimental Fe vibrational density of states measured with nuclear resonance vibrational spectroscopy (NRVS). Convergence of the calculated spectrum to the data was achieved by first imposing D4 symmetry on the model structure as well as the force constants, progressively including different internal coordinates of motion, then allowing the true Ci (or S2) point group symmetry of the C(i)1 Fe(OEP) crystal structure. The NRVS-refined normal modes are in good agreement with Raman and IR spectra at high frequencies. Prior density functional theory predictions for a model porphyrin are similar to the core modes computed with the best-fit force field, but significant differences between D4 and Ci modes underline the sensitivity of porphyrin Fe normal modes to structural details. Some differences between the Ci best fit and the NRVS data can be attributed to intermolecular contacts not included in the normal-mode analysis.     

Combined Photoredox/Enzymatic C−H Benzylic Hydroxylations

Chemical transformations that install heteroatoms into C?H bonds are of significant interest because they streamline the construction of value‐added small molecules. Direct C?H oxyfunctionalization, or the one step conversion of a C?H bond to a C?O bond, could be a highly enabling transformation due to the prevalence of the resulting enantioenriched alcohols in pharmaceuticals and natural products,. Here we report a single‐flask photoredox/enzymatic process for direct C?H hydroxylation that proceeds with broad reactivity, chemoselectivity and enantioselectivity. This unified strategy advances general photoredox and enzymatic catalysis synergy and enables chemoenzymatic processes for powerful and selective oxidative transformations.     

Access to the Bis‐benzene Cobalt(I) Sandwich Cation and its Derivatives: Synthons for a “Naked” Cobalt(I) Source?

The synthesis and structural characterization of the hitherto unknown parent Co(bz)₂⁺ (bz=benzene) complex and several of its derivatives are described. Their synthesis starts either from a CoCO₅⁺ salt, or directly from Co₂(CO)₈ and a Ag⁺ salt. Stability and solubility of these complexes was achieved by using the weakly coordinating anions (WCAs) [Al(OR^F)₄]^? and [F{Al(OR^F)₃}₂]^? {R^F=C(CF₃)₃} and the solvent ortho‐difluorobenzene (o‐DFB). The magnetic properties of Co(bz)₂⁺ were measured and compared in the condensed and gas phases. The weakly bound Co(o‐dfb)₂⁺ salts are of particular interest for the preparation of further Co^I salts, for example, the structurally characterized low‐coordinate 12 valence electron Co(P^tBu₃)₂⁺ and Co(NHC)₂⁺ salts.     

Protonation of Homoenolate Equivalents Generated by N-Heterocyclic Carbenes

Homoenolate equivalents are generated by Lewis basic N-heterocyclic carbene catalysts and then protonated to generate efficiently saturated esters from unsaturated aldehydes. This reactivity is extended to the generation of β-acylvinyl anions from alkynyl aldehydes. The asymmetric protonation of a homoenolate equivalent generated from a β,β-disubstituted aldehyde can be accomplished with a chiral N-heterocyclic carbene.     

Crystal-packing-induced antiferromagnetic interactions of metallocenes: cyanonickelocenes, -cobaltocenes, and -ferrocenes

The cyano-substituted metallocenes [M(C5H4CN)2] (M=Fe, 1; Co, 2; Ni 3) and [M(C5Me5)(C5H4CN)] (M=Fe, 4; Co, 5; Ni, 6) were synthesized in yields up to 58 % by treating K(C5H4CN) or Tl(C5H4CN) with suitable transition-metal precursors. Cyclic voltammetry indicated that the oxidation and reduction potentials of all the cyanometallocenes were shifted to positive values by up to 0.8 V. Single-crystal X-ray structure analysis showed that 1 had eclipsed ligands, formed planes in the lattice, and--unlike usual metallocenes--lined up in stacks perpendicular to these planes. Powder X-ray studies established that 1 and 2 are isotypic. The 1H and 13C NMR spectra were recorded for all the new compounds. Signal shifts of up to delta=1500 ppm were recorded for the paramagnetic molecules 2 and 3 and were, at a given temperature, strikingly different for solution and solid-state spectra. These results pointed to antiferromagnetic interactions as a consequence of molecular ordering in the lattice, as confirmed by magnetic measurements. The temperature-dependent susceptibilities were reproduced by Heisenberg spin-chain models (H=-J sum n- 1 i=1 SiSi+1), thus yielding J=-28.3 and -10.3 cm(-1) for 2 and 3, respectively, whereas J=-11.8 cm(-1) was obtained for 3 from the Ising spin-chain model. In accordance with molecular orbital (MO) considerations, much spin density was found to be delocalized not only on the cyclopentadienyl ligand but also the cyano substituents. The magnetic interaction was interpreted as a Heitler-London spin exchange and was analyzed based on how the interaction depends on the singly occupied MOs and the shift of parallel metallocenes relative to each other.     

Reversible NO motion in crystalline [Fe(Porph)(1-MeIm)(NO)] derivatives

The synthesis, characterization, and X-ray structures of three low-spin (nitrosyl)iron(II) tetraarylporphyrinates, [Fe(TpXPP)(NO)(1-MeIm)], where X = F (in a triclinic and a monoclinic form) and OCH(3) are reported. All three molecules, at 100 K, have a single orientation of NO. These structures are the first examples of ordered NO's in [Fe(Porph)(NO)(1-MeIm)] complexes. The three new derivatives have similar structural features including a previously unnoted "bowing" of the N(NO)-Fe-N(Im) angle caused by a concerted tilting of the axial Fe-N(NO) and Fe-N(Im) bonds. Structural features such as the displacement of Fe out of the mean porphyrin plane toward NO, tilting of the Fe-N(NO) bond off the heme normal, and the asymmetry of the Fe-N(por) bonds further strengthen and confirm observations from earlier studies. The [Fe(TpXPP)(NO)(1-MeIm)] complexes were also studied at temperatures between 125 and 350 K to investigate temperature-dependent variations and trends in the coordination group geometry. At varying temperatures (above 150 K), all three derivatives display a second orientation of the NO ligand. The population and depopulation of this second orientation are thermally driven, with no apparent hysteresis. Crystal packing appears to be the significant feature in defining the order/disorder of the NO ligand. The length of the bond trans to NO, Fe-N(Im), was also found to be sensitive to temperature variation. The Fe-N(Im) bond length increases with increased temperature, whereas no other bonds change appreciably. The temperature-dependent Fe-N(Im) bond length change and cell volume changes are consistent with a "soft" Fe-N(Im) bond. Variable-temperature measurements show that the N-O stretching frequency changes with the Fe-N(Im) bond length. Temperature-dependent changes in the Fe-NIm bond length and N-O stretching frequency were also found to be completely reversible with no apparent hysteresis.     

Cooperative Catalysis and Activation with N‐Heterocyclic Carbenes

N‐Heterocyclic carbene (NHC) catalysis has emerged as a powerful stratagem in organic synthesis to construct complex molecules primarily by polarity reversal (umpolung) approaches. These unique Lewis bases have been used to generate acyl anions, enolates, and homoenolates in catalytic fashion. Recently, a new strategy has emerged that dramatically expands the synthetic utility of carbene catalysis by leveraging additional activation modes: cooperative catalysis. The careful selection and balance of cocatalysts have led to enhanced reactivity, increased yields, and improved stereoselectivity. In certain cases, these catalytic additives have changed the regioselectivity or diastereoselectivity. This Minireview highlights new advances in NHC cooperative catalysis and surveys the evolution of this field.