Heiko Jacobsen

A Possible Mechanism for Enantioselectivity in the Chiral Epoxidation of Olefins with [Mn(salen)] Catalysts

The origin of enantioselectivity in the Jacobsen–Katsuki reaction has been investigated by applying density functional calculations in combination with molecular mechanics methodologies. The calculations suggest that a high enantiomeric excess is connected to three specific features: 1) a chiral diimine bridge, which induces folding of the salen ligand(H2salen=bis(salicylidene)ethylenediamine), and hence the formation of a chiral pocket; 2) bulky groups at the 3,3′-positions of the salen ligand, which cause a preferential approach from the side of the aromatic rings; and 3) π conjugation of the olefinic double bond, which confers regioselectivity and, consequently, enantioselectivity. In combination with experimental studies, the model also provides a rationale for the decrease in ee values when one of these components is missing.

Radical Intermediates in the Jacobsen – Katsuki Epoxidation

Insight into the controversial mechanism of the Mn - salen-catalyzed epoxidation of olefins is provided in a theoretical study based on density functional theory. The calculations suggest that radical species A, but not manganaoxetanes B, are likely candidates for viable intermediates.

On the Accuracy of DFT Methods in Reproducing Ligand Substitution Energies for Transition Metal Complexes in Solution: The Role of Dispersive Interactions

The performance of a series of density functionals when tested on the prediction of the phosphane substitution energy of transition metal complexes is evaluated. The complexes Fe-BDA and Ru-COD (BDA=benzylideneacetone, COD=cyclooctadiene) serve as reference systems, and calculated values are compared with the experimental values in THF as obtained from calorimetry. Results clearly indicate that functionals specifically developed to include dispersion interactions usually outperform other functionals when BDA or COD substitution is considered. However, when phosphanes of different sizes are compared, functionals including dispersion interactions, at odd with experimental evidence, predict that larger phosphanes bind more strongly than smaller phosphanes, while functionals not including dispersion interaction reproduce the experimental trends with reasonable accuracy. In case of the DFT-D functionals, inclusion of a cut-off distance on the dispersive term resolves this issue, and results in a rather robust behavior whatever ligand substitution reaction is considered.

Activation of Hydrogen by Palladium(0): Formation of the Mononuclear Dihydride Complex trans‐[Pd(H)2(IPr)(PCy3)]

An even split: In sharp contrast with the general behavior of Pd(0) complexes, [Pd(IPr)(PCy(3))] is able to activate the H-H bond. The resulting trans-[Pd(H)(2)(IPr)(PCy(3))] is the first isolated mononuclear dihydride palladium compound. Its formation is supported by multinuclear NMR spectroscopy, density functional calculations, and X-ray diffraction studies. The stability and reactivity of this new species are examined.

PROBING REGIOSELECTIVE INTERMOLECULAR HYDROGEN BONDING TO RE(CO)H2(NO)(PR3)2 COMPLEXES BY NMR TITRATION AND EQUILIBRIUM NMR METHODOLOGIES

The interaction of acidic alcohols with the title compound is tuned by the ligand framework of the transition metal complex (see picture). The phosphorus donor dictates the site of attack, whereas the ligand trans to the hydride determines the strength of the dihydrogen bond.

Remarkable low symmetry hydrogen bonding network in the structure of ReCl2(NCMe)(NO)(PMe3)2

Abstract The title compound 1 crystallizes in the space group P21/a with Z=44, being the largest Z value for a transition metal complex observed up to now. Responsible for the unusual high number of independent molecules in the asymmetric unit is a dense network of hydrogen bonds, which do not follow the general rule of lowest entropy in crystals. Methyl groups serve exclusively as H donor groups, leading to hydrogen bonds of the type CH⋯O and CH⋯Cl. The putative appearance of bond-stretch isomers within one single crystal could be traced to a hidden disorder phenomenon. Density functional calculations provided further evidence for this conclusion.

The Chemistry of New Nitrosyltungsten Complexes with Pyridyl-Functionalized Phosphane Ligands

The coordination chemistry of pyridylphosphanes, such as 2-(6-tert-butylpyridyl)diphenylphosphane (Ph2P-tert-Bupy) (6) and 2-(6-tert-butylpyridyl)dimethylphosphane (Me2P-tert-Bupy) (7) towards a number of nitrosyltungsten complexes is reported. Displacement of the loosely coordinated MeCN from [W(CH3CN)3(CO)2(NO)][BF4] led to the following cationic compounds incorporating mono- and bidentate coordinated phosphane ligands: cis,cis-[W(CO)2(NO)(Ph2PR)(η2-Ph2PR)][BF4], [R = 2-pyridyl (9a), 2-picolyl (11)], cis,cis-[W(CO)(NO)(η2-Ph2Ppy)2][BF4] (20), trans,trans-[W(CO)(NO)(η2-Ph2Ppy)2][BPh4] (21), fac-[W(CO)2(NO)(Me2Ppy)3][BF4] (16), fac-[W(CO)2(NO)(Me2P-tert-Bupy)3][BF4] (18), cis,cis-[W(CO)2(NO)(Me2Ppy)(η2-Me2Ppy)][BF4] (22), and cis,cis-[W(CO)2(CH3CN)(NO)(Me2P-tert-Bupy)2][BF4] (23a). The cationic complex cis,mer-[W(CO)3(NO)(Ph2P-tert-Bupy)2][PF6] (14) has been prepared by nitrosylation of cis/trans-W(CO)4(Ph2P-tert-Bupy)2 (13). Reactions of 9a, 11, 14, 16, and 18 with hydride transfer reagents afforded trans,trans-HW(CO)2(NO)(Ph2Ppy)2 (10), trans,trans-HW(CO)2(NO)(Ph2Ppic)2 (12), trans,trans-HW(CO)2(NO)(Ph2-tBupy)2 (15), cis/trans-HW(CO)2(NO)(Me2Ppy)2 (17), and cis/trans-HW(CO)2(NO)(Me2P-tert-Bupy)2 (19), respectively. Reactivity experiments with acetic acid, hydroiodic acid, carbon dioxide, and acetylenedicarboxylic acid were performed, and were found to afford trans-W(CO)(NO)(Ph2Ppy)2(η2-CH3CO2) (24), trans,trans-IW(CO)2(NO)(Ph2Ppy)2 (25), trans-W(HCO2)(CO)2(NO)(Ph2Ppy)2 (26), and trans-W{η2-(Z)-C(CO2Me)=CH[C(O)OMe]}(CO)(NO)(Ph2Ppic)2 (27), respectively. The influence of the pyridyl substituent in 10 was probed by a comparative H/D exchange experiment in which 10 and the analogous complex HW(CO)2(NO)(PPh3)2 were treated with MeOD. The deuterated complex trans,trans-WD(CO)2(NO)(Ph2Ppy)2 (28) could be isolated. The structures of 9a, 11, 14, and 20 have been determined by single-crystal X-ray diffraction analysis.

Comparison of Characteristic Structural Features among the Triade of Tris(cyclopentadienyl)(Group‐4 Metal) Complex Cations: a Combined Theoretical and Experimental Study

A density functional theory computational chemistry study has revealed a fundamental structural difference between [Ti(Cp)3]+ and its congeners [Zr(Cp)3]+ and [Hf(Cp)3]+/(Cp=cyclopentadienyl). Whereas the latter two are found to contain three uniformely η5-coordinated Cp ligands (3η5-structural type), [Ti(Cp)3]+ is shown to prefer a 2η5η2 structure. [Ti(Cp)3]+[B(C6F5)3(Me)]− (10⋅[B(C6F5)3(Me)]−) was experimentally generated by treatment of [Ti(Cp)3(Me)] (7a) with B(C6F5)3 (Scheme 3). Low-temperature 1H-NMR spectroscopy in CDFCl2 (143 K, 600 MHz; Fig. 8) showed a splitting of the Cp resonance into five lines in a 2 : 5 : 2 : 5 : 1 ratio which would be in accord with the theoretically predicted 2η5η2-type structure of [Ti(Cp)3]+. The precursor [Ti(Cp)3(Me)] (7a) exhibits two 1H-NMR Cp resonances in a 10 : 5 ratio in CD2Cl2 at 223 K. Treatment of [HfCl(Cp)2(Me)] (6c) with sodium cyclopentadienide gave [Hf(Cp)3(Me)] (7c) (Scheme 1). Its reaction with B(C6F5)3 furnished the salt [Hf(Cp)3]+[B(C6F5)3(Me)]− (8⋅[B(C6F5)3(Me)]−), which reacted with tert-butyl isocyanide to give the cationic complex [Hf(Cp)3(C=N−CMe3)]+ (9a; with counterion [B(C6F5)3(Me)]− (Scheme 2). Complex cation 9a was characterized by X-ray diffraction (Fig. 7). Its Hf(Cp3) moiety is of the 3η5-type. The structure is distorted trigonal-pyramidal with an average D−Hf−D angle of 118.8° and an average D−Hf−C(1) angle of 96.5° (D denotes the centroids of the Cp rings; Table 6). Cation 9a is a typical d0-isocyanide complex exhibiting structural parameters of the C≡N−CMe3 group (d(C(1)−N(2))=1.146 (5) A; IR: v˜(C≡N) 2211 cm−1) very similar to free uncomplexed isonitrile. Analogous treatment of 8 with carbon monoxide yielded the carbonyl (d0-group-4-metal) complex [Hf(Cp)3(CO)]+ (9b; with counterion [B(C6F5)3(Me)]−) (Scheme 2) that was also characterized by X-ray crystal-structure analysis (Fig. 6). Complex 9b is also of the 3η5-structural type, similar to the peviously described cationic complex [Zr(Cp)3(CO)]+, and exhibits properties of the CO ligand (d(C−O)=1.11 (2) A; IR: v˜(C≡O) 2137 cm−1) very similar to the free carbon monoxide molecule.

The interaction of Lewis acidic boron derivatives with Re(CO)5−nH(PMe3)n complexes

Abstract Lewis acid adducts of the hydrides cis - and trans -Re(CO)(PMe 3 ) 4 H ( 1 ) and ( 2 ), mer -Re(CO) 2 (PMe 3 ) 3 H ( 3 ), fac -Re(CO) 2 (PMe 3 ) 3 H ( 4 ) and trans -Re(CO) 3 (PMe 3 ) 2 H ( 5 ) were studied with BH 3 and 9-borabicyclo[3,3,1] norbonane (BBNH). Using BH 3 ·THF and (BBNH) 2 1 and 2 afforded Re(CO)(PMe 3 ) 3 (η 2 -BH 4 ) ( 6 ) and Re(CO)(PMe 3 ) 3 (η 2 -BBNH 2 ) ( 7 ) as stable and isolable products. VT IR studies established for the reaction to 7 that BBNH first attaches in a pre-equilibrium to the O CO atom of 1 or 2 . At higher temperatures ReH adduct formation occurs with instantaneous transformation to 7 and elimination of PMe 3 ·BBNH. In a similar way, the hydrides 3 and 4 were converted with BH 3 ·THF and (BBNH) 2 to yield the stable complexes Re(CO) 2 (PMe 3 ) 2 (η 2 -BH 4 ) ( 8 ) and Re(CO) 2 (PMe 3 ) 2 (η 2 -BBNH 2 ) ( 9 ). The intermediacy of the η 1 -BH 4 adducts mer -/fac-Re(CO) 2 (PMe 3 ) 3 (η 1 -BH 4 ) was confirmed by VT 1 H, 31 P NMR and VT IR experiments. The conversion of 5 with BH 3 ·THF led to equilibria with adducts at the O CO terminus in trans position to H and with H Re as revealed by VT IR studies. Temperature dependent 31 P equilibrium studies allowed to calculate Δ H =−4.9 kcal mol −1 and Δ S =+0.034 e.u. for this reaction. These adducts could not be isolated. Compound 5 does not react with (BBNH) 2 even at elevated temperatures. DFT calculations were carried out to support the structures of the BH 3 adducts of 5 . In addition a vibrational analysis helped to unravel the IR band assignments of the involved compounds. DFT calculations on 8 confirmed its C 2 v structure. X-ray diffraction studies were carried out on single crystals of 6 and 7 .

Ligand influences on hydrogen migration in transition metal hydrido–carbyne complexes

Abstract Density Functional BP86 calculations are reported for the interconversion reactions of WH(CH)(CO)(PH3)3 and WH(CH)(CO)2(PH3)2 to the corresponding five-coordinated carbene complexes W(CH2)(CO)(PH3)3 and W(CH2)(CO)2(PH3)2, respectively. Transition states have been calculated to be 9.9 and 0.7 kcal mol−1 higher in energy than the carbynes. A comparison with the isoelectronic osmium system OsHCl2(CMe)(PH3)2 with a transition state for interconversion 27.2 kcal mol−1 above the carbyne leads to the conclusion that additional π-acceptors competing with the carbyne or carbene ligand for back donation facilitate the interconversion reaction, whereas additional π-donors undergoing synergic push–pull interactions impede the interconversion reaction.