The "Metallo"-Diels-Alder Reactions: Examining the Metalloid Behavior of Germanimines.

Addition of MesN3 (Mes=2,4,6-Me3 C6 H2 ) to germylene [(NONtBu )Ge] (NONtBu =O(SiMe2 NtBu)2 ) (1) gives germanimine, [(NONtBu )Ge=NMes] (2). Compound 2 behaves as a metalloid, showing reactivity reminiscent of both transition metal-imido complexes, undergoing [2+2] addition with heterocumulenes and protic sources, as well as an activated diene, undergoing a [4+2] cycloaddition, or "metallo"-Diels-Alder, reaction. In the latter case, the diene includes the Ge=N bond and π-system of the Mes substituent, which is reactive towards dienophiles including benzaldehyde, benzophenone, styrene, and phenylacetylene.

Reduction vs. Addition: The Reaction of an Aluminyl Anion with 1,3,5,7-Cyclooctatetraene.

The potassium aluminyl complex K[Al(NONAr )] (NON=NONAr =[O(SiMe2 NAr)2 ]2- , Ar=2,6-iPr2 C6 H3 ) reacts with 1,3,5,7-cyclooctatetraene (COT) to give K[Al(NONAr )(COT)]. The COT-ligand is present in the asymmetric unit as a planar µ2 -η2 :η8 -bridge between Al and K, with additional K⋅⋅⋅π-aryl interactions to neighboring molecules that generate a helical chain. DFT calculations indicate significant aromatic character, consistent with reduction to [COT]2- . Addition of 18-crown-6 causes a rearrangement of the C8 -carbocycle to form the isomeric 9-aluminabicyclo[4.2.1]nona-2,4,7-triene anion.

Halogenated Meroditerpenoids from a South Pacific Collection of the Red Alga Callophycus serratus.

A detailed examination of the red alga Callophycus serratus collected in Tonga led to the isolation of six new halogenated meroditerpenoids: callophycol C (1), callophycoic acid I (2), iodocallophycols E (3) and F (4), iodocallophycoic acid B (5), and callophycoic acid J (6). Of these, compounds 3-5 are new iodinated additions to the growing family of Callophycus meroditerpenoids. The relative configurations of compounds 1-6 were deduced by analyses of 1D NOE data and 1H-1H scalar coupling constants, and 3-6 are proposed to differ from the closely related compounds reported in the literature, iodocallophycoic acid A and iodocallophycols A-D. Iodocallophycol E (3) exhibited moderate cytotoxicity against the promyelocytic leukemia cell line HL-60 with an IC50 value of 6.0 µM.

Indyllithium and the Indyl Anion [InL]- : Heavy Analogues of N-Heterocyclic Carbenes.

Reduction of the indate complex In(NONAr )(µ-Cl)2 Li(OEt2 )2 (NONAr =[O(SiMe2 NAr)2 ]2- ; Ar=2,6-iPr2 C6 H3 ) with sodium generates the InII diindane species [In(NONAr )]2 . Further reduction with a mixture of potassium and [2.2.2]crypt affords the InI N-heterocyclic indyl anion [In(NONAr )]- , which crystallizes with a non-contacted [K([2.2.2]crypt)]+ cation. The indyl anion can also be isolated as the indyllithium compound In(NONAr )(Li{THF}3 ), which contains an In-Li bond. Density functional theory calculations show that the HOMO of the indyl anion is a metal-centred lone pair, and preliminary reactivity studies confirm its nucleophilic behaviour.

Bismuth(III) Complex of the [S4]•- Radical Anion: Dimer Formation via Pancake Bonds.

Reaction of bismuth(II) compounds with sulfur gives mixtures of [Bi(NONR)]2(µ2-Sn) (NONR = [O(SiMe2NR)2]2-). Examples for n = 1 and 3 have been crystallographically verified for R = 2,6-iPr2C6H3 (Dipp) and R = tBu, and the pentasulfide (n = 5) for R = Dipp. The corresponding product from reaction with the new Bi(II) radical Bi(NONAr‡)• (Ar‡ = C6H2(CHPh2)2-tBu-2,6,4) exists as the dimer [Bi(NONAr‡)(S4)]2, with π*(SOMO)-π*(SOMO) interactions linking the sulfur chains through trans-antarafacial pancake bonds.

Choosing the right precursor for thermal decomposition solution-phase synthesis of iron nanoparticles: tunable dissociation energies of ferrocene derivatives.

Organometallic coordination compounds in general and metallocenes in particular are convenient precursors for the synthesis of metal nanoparticles through thermal decomposition. The strength of the interaction between the metal ion and its ligands determines the conditions under which decomposition occurs, most importantly the range of temperatures and pressures at which a given compound is useful as a precursor. We show that a comprehensive analysis of all individual contributions to the ligand metal interactions that establishes the nature of the interaction can be used to select compounds that are tuned to a specific dissociation energy with advantageous properties under experimental conditions. To this end, we apply the Morokuma-Ziegler-Energy Decomposition Analysis (MZ-EDA) to a series of ferrocene analogues using high-level density functional theory (DFT). We find that asymmetrically substituted ferrocene derivatives are unlikely to be useful as precursors because of the large energy required to remove the second cyclopentadienyl-derivative from the central iron atom. However, we are able to establish that symmetrically substituted chloroferrocenes exhibit a wide range of relatively low bond dissociation energies for both dissociation steps and are hence good candidates for the synthesis of highly mono-disperse iron nanoparticles.

Accurate prediction of the optical rotation and NMR properties for highly flexible chiral natural products.

Despite advances in electronic structure theory the theoretical prediction of spectroscopic properties remains a computational challenge. This is especially true for natural products that exhibit very large conformational freedom and hence need to be sampled over many different accessible conformations. We report a strategy, which is able to predict NMR chemical shifts and more elusive properties like the optical rotation with great precision, through step-wise incremental increases of the conformational degrees of freedom. The application of this method is demonstrated for 3-epi-xestoaminol C, a chiral natural compound with a long, linear alkyl chain of 14 carbon atoms. Experimental NMR and [α]D values are reported to validate the results of the density functional theory calculations.

Bi-P Bond Homolysis as a Route to Reduced Bismuth Compounds and Reversible Activation of P4.

Bismuth diphenylphosphanides Bi(NONR )(PPh2 ) (NONR =[O(SiMe2 NR)2 ], R=tBu, 2,6-iPr2 C6 H3 , Aryl) undergo facile decomposition via single-electron processes to form reduced Bi and P species. The corresponding derivatives Bi(NONR )(PCy2 ) are stable. Reaction of the isolated BiII radical . Bi(NONAr ) with white phosphorus (P4 ) proceeds with the reversible and selective activation of a single P-P bond to afford the bimetallic µ,η1:1 -bicyclo[1.1.0]tetraphosphabutane compound.

Isolation and Characterization of a Bismuth(II) Radical.

More than 80 years after Paneth's report of dimethyl bismuth, the first monomeric Bi(II) radical that is stable in the solid state has been isolated and characterized. Reduction of the diamidobismuth(III) chloride Bi(NON(Ar))Cl (NON(Ar)=[O(SiMe2NAr)2](2-); Ar=2,6-iPr2C6H3) with magnesium affords the Bi(II) radical ˙Bi(NON(Ar)). X-ray crystallographic measurements are consistent with a two-coordinate bismuth in the +2 oxidation state with no short intermolecular contacts, and solid-state SQUID magnetic measurements indicate a paramagnetic compound with a single unpaired electron. EPR and density functional calculations show a metal-centered radical with >90% spin density in a p-type orbital on bismuth.

Accurate density functional theory description of binding constants and NMR chemical shifts of weakly interacting complexes of C60 with corannulene-based molecular bowls.

Density functional calculations on "catch and release" complexes of C60 with corannulene derived molecular bowls show that computationally obtained (1) H nuclear magnetic resonance (NMR) chemical shifts can be used as a reliable predictor of binding constants. A wide range of functionals was benchmarked against accurate ab initio calculations to ensure a credible representation of the weak forces that dominate the interactions in these systems. The most reliable density functional theory (DFT) results were then calibrated using experimentally observed NMR data. Careful analysis and comparison of a wide range of commonly used density functionals shows that the explicit inclusion of dispersion corrections is currently the only reliable way to accurately describe the systems investigated in our study. Moreover, we are able to show that the B97-D and ωB97X-D functionals are not only able to reproduce ab initio benchmark calculations, but they do so accurately with a moderately sized basis sets and without the problems of numerical integration we encountered with other functionals in this study.

13C NMR analysis of 3,6-dihydro-2H-pyrans: assignment of remote stereochemistry using axial shielding effects.

The rational analysis of (13)C NMR axial shielding effects has enabled the assignment of remote relative stereochemistry in 3,6-oxygen-substituted 3,6-dihydro-2H-pyrans. Comparison of the (13)C NMR shifts of equivalent centers in cis- and trans-substituted 3,6-dihydro-2H-pyrans allows the relative configuration at the C3 and C6 positions to be defined in diastereoisomeric mixtures. Density functional calculations were used to validate this method and assess the conformational bias present in the ring system. Ultimately, the coupling of computational chemistry with this (13)C NMR-based method provided a reliable and convenient method for stereochemical assignment of a single diastereomer. This approach provides a facile and complementary alternative to the practices previously employed for determining the relative configuration in 3,6-dihydro-2H-pyrans.

Mechanism of copper(I)-catalyzed allylic alkylation of phosphorothioate esters: influence of the leaving group on α regioselectivity.

The mechanism of Cu(I) -catalyzed allylic alkylation and the influence of the leaving groups (OPiv, SPiv, Cl, SPO(OiPr)2 ; Piv: pivavloyl) on the regioselectivity of the reaction have been explored by using density functional theory (DFT). A comprehensive comparison of many possible reaction pathways shows that [(iPr)2 Cu](-) prefers to bind first oxidatively to the double bond of the allylic substrate at the anti position with respect to the leaving group, and this is followed by dissociation of the leaving group. If the leaving group is not taken into account, the reaction then undergoes an isomerization and a reductive elimination process to give the α- or γ-selective product. If OPiv, SPiv, Cl, or SPO(OiPr)2 groups are present, the optimal route for the formation of both α- and γ-substituted products changes from the stepwise elimination to the direct process, in which the leaving group plays a stabilizing role for the reactant and destabilizes the transition state. The differences to the energy barrier for the α- and γ-substituted products are 2.75 kcal mol(-1) with SPO(OiPr)2 , 2.44 kcal mol(-1) with SPiv, 2.33 kcal mol(-1) with OPiv, and 1.98 kcal mol(-1) with Cl, respectively; these values show that α regioselectivity in the allylic alkylation follows a SPO(OiPr)2 >SPiv>OPiv>Cl trend, which is in satisfactory agreement with the experimental findings. This trend mainly originates in the differences between the attractive electrostatic forces and the repelling steric interactions of the SPO(OiPr)2 , SPiv, OPiv, and Cl groups on the Cu group.

Studies of the H-D exchange mechanism of malonganenone B.

Malonganenone B (1) exhibits an unusual H-D exchange of a formyl proton when in deuteric-NMR solvents. Synthetic and kinetic investigations were made to probe the mechanism of this exchange, which appears to occur via an uncommon and transient amine-amide NHC intermediate.

Packed to the rafters: filling up C60 with rare gas atoms.

How many rare gas atoms can be placed into a fullerene cage until the pressure becomes large enough to break the C(60) framework? The answer given by density functional and ab initio computations is surprising and underlines the high stability of this unique carbon structure.

Kinetic and thermodynamic stability of the group 13 trihydrides.

The kinetic and thermodynamic stabilities of the group 13 hydrides EH(3) (E = B, Al, Ga, In, Tl, E113) are investigated by relativistic density functional and wave function based theories. The unimolecular decomposition of EH(3) --> EH + H(2) becomes energetically more favorable going down the Group 13 elements, with the H(2)-abstraction of InH(3), TlH(3), and (E113)H(3) (E113: element with nuclear charge 113) being exothermic. In accordance with the Hammond-Leffler postulate, the activation barrier for the dissociation process decreases accordingly going down the group 13 elements in the periodic table shifting to an early transition state, with activation energies ranging from 88.4 kcal/mol for BH(3) to 41.3 kcal/mol for TlH(3) and only 21.6 kcal/mol for (E113)H(3) at the scalar relativistic coupled cluster level of theory. For both TlH(3) and (E113)H(3) we investigated spin-orbit effects using Dirac-Hartree-Fock and second-order Møller-Plesset theory to account for electron correlation. For (E113)H, spin-orbit coupling results in a chemically inert closed 7p(1/2)-shell, thus reducing the stability of the higher oxidation state even further. We also investigated the known organothallium compound Tl(CH(3))(3), which is thermodynamically unstable similar to TlH(3), but kinetically very stable with an activation barrier of 57.1 kcal/mol.

Reduction of organic azides by indyl-anions. Isolation and reactivity studies of indium-nitrogen multiple bonds.

The synthesis of a new potassium-indyl complex, K[In(NONAr)] (NONAr = [O(SiMe2NAr)2]2-, Ar = 2,6-iPr2C6H3) and its reactivity with organic azides RN3 is reported. When R = 2,6-bis(diphenylmethyl)-4- t Bu-phenyl, a dianionic alkyl-amide ligand is formed via C-H activation across a transient In-Nimide bond. Reducing the size of the R-group to 2,4,6-trimethylphenyl (mesityl, Mes) enables oxidation of the indium and elimination of dinitrogen to afford the imide species, K[In(NONAr)(NMes)]. The anion contains a short In-Nimide bond, shown computationally to contain appreciable multiple bond character. Reaction of isolated imides with an additional equivalent of azide (R = Mes, SiMe3) generates tetrazenido-indium compounds K[In(NONAr){κ-N,N'-N4(Mes)(R)-1,4}], shown by X-ray crystallography to contain planar InN4 heterocycles in the anion.

Reactions of In-Zn bonds with organic azides: products that result from hetero- and homo-bimetallic behaviour.

The indyl anion, K[In(NONDipp)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) reacts with group 12 compounds M(BDIR)Cl (M = Zn, Cd; BDI = [HC{C(Me)NR}2]-, R = 2,4,6-Me3C6H2 (Mes), Dipp) to afford the heterobimetallic compounds (NONDipp)In-M(BDIR) that contain the first In-Zn and In-Cd bonds. The reactivity of the In-Zn bonds towards organic azides, R'N3 (R' = Mes, Dipp, Ph) was investigated. (NONDipp)In-Zn(BDIMes) reduces MesN3via an isolable triazenide intermediate to generate the bridging imido compound, (NONDipp)In-(µ-NMes)-Zn(BDIMes). Similar reactivity is noted from early-late heterobimetallic complexes. Under the same conditions, PhN3 reacts to afford a product that contains a bridging tetraazenide ligand, which is formed from the formal (2 + 3)-cycloaddition of second azide to an indium-imido bond. However, increasing the bulk of the BDI-ligand in (NONDipp)In-Zn(BDIDipp) leads to reductive coupling of PhN3 to give the hexazene complex. This mode of reactivity is reminiscent of the reductive behaviour of homobimetallic compounds.

Unraveling aminophosphine redox mechanisms for glovebox-free InP quantum dot syntheses.

The synthesis of colloidal indium phosphide quantum dots (InP QDs) has always been plagued by difficulties arising from limited P3- sources. Being effectively restricted to the highly pyrophoric tris(trimethylsilyl) phosphine (TMS3P) creates complications for the average chemist and presents a significant risk for industrially scaled reactions. The adaptation of tris(dialkylamino) phosphines for these syntheses has garnered attention, as these new phosphines are much safer and can generate nanoparticles with competitive photoluminescence properties to those from (TMS)3P routes. Until now, the reaction mechanics of this precursor were elusive due to many experimental optimizations, such as the inclusion of a high concentration of zinc salts, being atypical of previous InP syntheses. Herein, we utilize density functional theory calculations to outline a logical reaction mechanism. The aminophosphine precursor is found to require activation by a zinc halide before undergoing a disproportionation reaction to self-reduce this P(iii) material to a P(-iii) source. We use this understanding to adapt this precursor for a two-pot nanoparticle synthesis in a noncoordinating solvent outside of glovebox conditions. This allowed us to generate spherical InP/ZnS nanoparticles possessing fluorescence quantum yields >55% and lifetimes as fast as 48 ns, with tunable emission according to varying zinc halide acidity. The development of high quality and efficient InP QDs with this safer aminophosphine in simple Schlenk environments will enable a broader range of researchers to synthesize these nontoxic materials for a variety of high-value applications.

Structure of Echivulgarine, a Pyrrolizidine Alkaloid Isolated from the Pollen of Echium vulgare.

1,2-Dehydropyrrolizidine alkaloids are common toxic metabolites isolated from plants within the Boraginaceae, in particular from the genera Heliotropium and Echium. Previous studies have deduced the structures of these often potent bioactives based upon mass spectrometric evidence, but these identifications have not established conclusive connectivity and configurational data. Herein, we describe the isolation and full structural characterization of echivulgarine, occurring in the pollen of Echium vulgare and correct the structure previously ascribed to the compound, using a comprehensive combination of both experimental and calculated nuclear magnetic resonance and electronic circular dichroism spectroscopic data.

How to choose a precursor for decomposition solution-phase synthesis: the case of iron nanoparticles.

The decomposition of organometallic compounds as precursors has revolutionized the synthesis of nanoparticles in solution. However, effective control of size and size distribution of iron nanoparticles has remained challenging due to the high reactivity of iron towards oxygen or oxygen-containing materials. Reported is a decomposition study that shows how metal to ligand bonding and symmetry of the compound can be manipulated to control the size and size distribution of iron nanoparticles in the 6-16 nm range. [Fe(η(5)-C6H3Me4)2] was found to be the optimal precursor with a narrow decomposition temperature range due to its symmetry and the low bond dissociation energy of the ligands from the Fe(ii) center. The precise control of nanoparticle size has enabled the tuning of magnetic properties from superparamagnetic to soft-ferromagnetic desirable for a wide range of biomedical applications.