Andreas W. Ehlers

Geometry optimizations in the zero order regular approximation for relativistic effects

Analytical expressions are derived for the evaluation of energy gradients in the zeroth order regular approximation (ZORA) to the Dirac equation. The electrostatic shift approximation is used to avoid gauge dependence problems. Comparison is made to the quasirelativistic Pauli method, the limitations of which are highlighted. The structures and first metal-carbonyl bond dissociation energies for the transition metal complexes W(CO)6, Os(CO)5, and Pt(CO)4 are calculated, and basis set effects are investigated.

Performance of the OPBE exchange-correlation functional

In a recent evaluation of density functional theory (DFT) functionals OPBE, which combines Handys optimized exchange (OPTX) with the PBE correlation, was shown to correctly predict the spin states (singlet through sextet) of seven different iron complexes (2004, J. Phys. Chem. A, 108, 5479). The present study provides a further test of OPBE as well as that of the OPerdew and OLYP functionals, in which OPTX is combined with the Perdew and LYP correlations, respectively. These three are compared to other pure DFT functionals for their performance in calculating the atomization energies for the G2-set of up to 148 molecules, six reaction barriers of SN2 reactions, geometry optimizations of 19 small molecules and four metallocenes, and zero-point vibrational energies for 13 small molecules. OPBE performs exceptional well in all cases.

Geminal phosphorus/aluminum-based frustrated lewis pairs: C−H versus C≡C activation and CO2 fixation.

Catch it! Geminal phosphorus/aluminum-based frustrated Lewis pairs (FLPs) are easily obtained by hydroalumination of alkynylphosphines. These FLPs can activate terminal acetylenes by two competitive pathways, which were analyzed by DFT calculations, and they can bind carbon dioxide reversibly. Therefore, alongside polyfluorinated boranes, alanes are also ideal Lewis acids for FLP chemistry.

Preorganized frustrated Lewis pairs.

Geminal frustrated Lewis pairs (FLPs) are expected to exhibit increased reactivity when the donor and acceptor sites are perfectly aligned. This is shown for reactions of the nonfluorinated FLP tBu(2)PCH(2)BPh(2) with H(2), CO(2), and isocyanates and supported computationally.

Stereomutation of Pentavalent Compounds. Validating the Berry Pseudorotation, Redressing the Ugi’s Turnstile Rotation, and Revealing the Two- and Three-Gated Turnstiles

A general reaction mechanism describes the qualitative change in chemical topology along the reaction pathway. On the basis of this principle, we present a method to characterize intramolecular substituent permutation in pentavalent compounds. A full description of the geometry around five-coordinate atoms using internal coordinates enables the analysis of the structural changes along the stereomutational intrinsic reaction coordinate. The fluxional behavior of experimentally known pentavalent phosphoranes, silicates, and transition-metal complexes has been investigated by density functional theory calculations, and three principal mechanisms have been identified: Berry pseudorotation, threefold cyclic permutation, and half-twist axial-equatorial interchange. The frequently cited turnstile rotation is shown to be equivalent to the Berry pseudorotation. In combination with graph theory, this approach provides a means to systematically investigate the stereomutation of pentavalent molecules and potentially identify hitherto-unknown mechanisms.

On the electronic impact of abnormal C4-bonding in N-heterocyclic carbene complexes.

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition-reductive elimination sequences. These reactivities as well as analytical investigations using X-ray diffraction and X-ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4-bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H(2) activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.

A Significant but Constrained Geometry Pt→Al Interaction: Fixation of CO2 and CS2, Activation of H2 and PhCONH2.

Reaction of the geminal PAl ligand [Mes2PC(═CHPh)AltBu2] (1) with [Pt(PPh3)2(ethylene)] affords the T-shape Pt complex [(1)Pt(PPh3)] (2). X-ray diffraction analysis and DFT calculations reveal the presence of a significant Pt→Al interaction in 2, despite the strain associated with the four-membered cyclic structure. The Pt···Al distance is short [2.561(1) Å], the Al center is in a pyramidal environment [Σ(C-Al-C) = 346.6°], and the PCAl framework is strongly bent (98.3°). Release of the ring strain and formation of X→Al interactions (X = O, S, H) impart rich reactivity. Complex 2 reacts with CO2 to give the T-shape adduct 3 stabilized by an O→Al interaction, which is a rare example of a CO2 adduct of a group 10 metal and actually the first with η(1)-CO2 coordination. Reaction of 2 with CS2 affords the crystalline complex 4, in which the PPtP framework is bent, the CS2 molecule is η(2)-coordinated to Pt, and one S atom interacts with Al. The Pt complex 2 also smoothly reacts with H2 and benzamide PhCONH2 via oxidative addition of H-H and H-N bonds, respectively. The ensuing complexes 5 and 7 are stabilized by Pt-H→Al and Pt-NH-C(Ph) = O→Al bridging interactions, resulting in 5- and 7-membered metallacycles, respectively. DFT calculations have been performed in parallel with the experimental work. In particular, the mechanism of reaction of 2 with H2 has been thoroughly analyzed, and the role of the Lewis acid moiety has been delineated. These results generalize the concept of constrained geometry TM→LA interactions and demonstrate the ability of Al-based ambiphilic ligands to participate in TM/LA cooperative reactivity. They extend the scope of small molecule substrates prone to such cooperative activation and contribute to improve our knowledge of the underlying factors.

Functionalization of P4 Using a Lewis Acid Stabilized Bicyclo[1.1.0]tetraphosphabutane Anion

Reacting white phosphorus (P4 ) with sterically encumbered aryl lithium reagents (aryl=2,6-dimesitylphenyl or 2,4,6-tBu3 C6 H2 ) and B(C6 F5 )3 gives the unique, isolable Lewis acid stabilized bicyclo[1.1.0]tetraphosphabutane anion. Subsequent alkylation of the nucleophilic site of the RP4 anion gives access to non-symmetrical disubstituted bicyclic tetraphosphorus compounds. This novel method enables PC bond formation in a controlled fashion using white phosphorus as starting material.

The Homoleptic Sandwich Anion [Co(P2C2tBu2)2]−: A Versatile Building Block for Phosphaorganometallic Chemistry†

Phosphaalkynes (RC=P) are valuable starting materials for a wide range of low-coordinate phosphorus compounds.[1] Similar to alkynes,[2] these reactive, triply-bonded molecules oligomerize in the coordination sphere of transition metals to give diphosphacyclobutadienes, triphosphabenzenes, or higher oligomers.[1, 3] The reactions of phosphaalkynes with “vaporized” metal atoms are particularly intriguing, as illustrated by the reaction of cobalt atoms with tBuC=P which affords a mixture of three complexes (A–C) that are difficult to prepare by conventional synthetic methods.[4] Although fascinating compounds can be obtained by metal vapor (MV) synthesis,[5, 6] its applicability is limited due to the low product yields and the need for a special experimental setup.

The First Theoretical and Experimental Proof of Polythiocarbamatozinc(II) Complexes, Catalysts for Sulfur Vulcanization

The existence of polythiocar- bamatozinc complexes, species pre- sumed to be involved in catalyzing sulfur vulcanization, has been studied by com- putational and mass-spectrometric tech- niques. Density functional calculations reveal that the sulfuration energy of bis(trithiocarbamato)zinc(ii) is compara- ble to that of bis(phenyltrithiolato)- zinc(ii), a stable sulfurated complex. Interestingly, the analogous nonsym- metrically sulfurated complex (dithio- carbamato)(tetrathiocarbamato)zinc(ii) is energetically slightly more favorable. The calculations indicate that polythio- carbamatozinc complexes may indeed be involved in the sulfur vulcanization cascade, a conclusion corroborated by ensuing laser-desorption ionization (LDI) mass-spectrometric measure- ments on mixtures of dithiocarbamato- zinc(ii) complexes and elemental sulfur. These demonstrate the presence of poly- thiocarbamatozinc ions that are sulfu- rated with up to eight additional sulfur atoms.