Markus Hermann

Low coordinate germanium(II) and tin(II) hydride complexes: efficient catalysts for the hydroboration of carbonyl compounds.

This study details the first use of well-defined low-valent p-block metal hydrides as catalysts in organic synthesis. That is, the bulky, two-coordinate germanium(II) and tin(II) hydride complexes, L(†)(H)M: (M = Ge or Sn, L(†) = -N(Ar(†))(SiPr(i)3), Ar(†) = C6H2{C(H)Ph2}2Pr(i)-2,6,4), are shown to act as efficient catalysts for the hydroboration (with HBpin, pin = pinacolato) of a variety of unactivated, and sometimes very bulky, carbonyl compounds. Catalyst loadings as low as 0.05 mol % are required to achieve quantitative conversions, with turnover frequencies in excess of 13 300 h(-1) in some cases. This activity rivals that of currently available catalysts used for such reactions.

The Facile Reduction of Carbon Dioxide to Carbon Monoxide with an Amido‐Digermyne

Taking the fizz out: A digermyne compound with a Ge-Ge single bond has been shown to quantitatively reduce CO(2) to CO at temperatures as low as -40 °C. The mechanism of this unprecedented reaction has been probed by spectroscopic and computational techniques and involves a metastable intermediate (see picture; Ar*=C(6) H(2){C(H)Ph(2)}(2)Me-2,6,4).

One-electron-mediated rearrangements of 2,3-disiladicarbene.

A disiladicarbene, (Cy-cAAC)2Si2 (2), was synthesized by reduction of Cy-cAAC:SiCl4 adduct with KC8. The dark-colored compound 2 is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190 °C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a Si═Si double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that 2 can quasi-reversibly accept an electron to produce a very reactive radical anion, 2(•-), as an intermediate species. Thus, reduction of 2 with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of 2(•-).

The boron–boron triple bond in NHC→BB←NHC

Quantum chemical calculations of the compound B2(NHCMe)2 and a thorough examination of the electronic structure with an energy decomposition analysis provide strong evidence for the appearance of boron–boron triple bond character. This holds for the model compound and for the isolated diboryne B2(NHCR)2 of Braunschweig which has an even slightly shorter B–B bond. The bonding situation in the molecule is best described in terms of NHCMe→B2←NHCMe donor–acceptor interactions and concomitant π-backdonation NHCMe←B2→NHCMe which weakens the B–B bond, but the essential features of a triple bond are preserved. An appropriate formula which depicts both interactions is the sketch NHCMe⇄BB⇄NHCMe. Calculations of the stretching force constants FBB which take molecules that have genuine single, double and triple bonds as references suggest that the effective bond order of B2(NHCMe)2 has the value of 2.34. The suggestion by Koppe and Schnockel that the strength of the boron–boron bond in B2(NHCH)2 is only between a single and a double bond is repudiated. It misleadingly takes the force constant FBB of OBBO as the reference value for a B–B single bond which ignores π bonding contributions. The alleged similarity between the B–O bonds in OBBO and the B–C bonds in B2(NHCMe)2 is a mistaken application of the principle of isolable relationship.

Stabilization of a cobalt-cobalt bond by two cyclic alkyl amino carbenes.

(Me2-cAAC:)2Co2 (2, where Me2-cAAC: = cyclic alkyl amino carbene, :C(CH2)(CMe2)2N-2,6-iPr2C6H3)) was synthesized via the reduction of precursor (Me2-cAAC:Co(II)(μ-Cl)Cl)2 (1) with KC8. 2 contains two cobalt atoms in the formal oxidation state zero. Magnetic measurement revealed that 2 has a singlet spin ground state S = 0. The cyclic voltammogram of 2 exhibits both one-electron oxidation and reduction, indicating the possible synthesis of stable species containing 2(•-) and 2(•+) ions. The latter was synthesized via reduction of 1 with required equivalents of KC8 and characterized as [(Me2-cAAC:)2Co2](•+)OTf(-) (2(•+)OTf(-)). Electron paramagnetic resonance spectroscopy of 2(•+) reveals the coupling of the electron spin with 2 equiv (59)Co isotopes, leading to a (Co(0.5))2 state. The experimental Co1-Co2 bond distances are 2.6550(6) and 2.4610(6) Å for 2 and 2(•+)OTf(-), respectively. Theoretical investigation revealed that both 2 and 2(•+)OTf(-) possess a Co-Co bond with an average value of 2.585 Å. A slight increase of the Co-Co bond length in 2 is more likely to be caused by the strong π-accepting property of cAAC. 2(•+) is only 0.8 kcal/mol higher in energy than the energy minimum. The shortening of the Co-Co bond of 2(•+) is caused by intermolecular interactions.

The Chemical Bond in C2

Quantum chemical calculations using the complete active space of the valence orbitals have been carried out for Hn CCHn (n=0-3) and N2. The quadratic force constants and the stretching potentials of Hn CCHn have been calculated at the CASSCF/cc-pVTZ level. The bond dissociation energies of the C-C bonds of C2 and HC≡CH were computed using explicitly correlated CASPT2-F12/cc-pVTZ-F12 wave functions. The bond dissociation energies and the force constants suggest that C2 has a weaker C-C bond than acetylene. The analysis of the CASSCF wavefunctions in conjunction with the effective bond orders of the multiple bonds shows that there are four bonding components in C2, while there are only three in acetylene and in N2. The bonding components in C2 consist of two weakly bonding σ bonds and two electron-sharing π bonds. The bonding situation in C2 can be described with the σ bonds in Be2 that are enforced by two π bonds. There is no single Lewis structure that adequately depicts the bonding situation in C2. The assignment of quadruple bonding in C2 is misleading, because the bond is weaker than the triple bond in HC≡CH.

Formation of a 1,4-Diamino-2,3-disila-1,3-butadiene Derivative

A 1,4-diamino-2,3-disila-1,3-butadiene derivative of composition (Me2-cAAC)2(Si2Cl2) (Me2-cAAC = :C(CMe2)2(CH2)N-2,6-iPr2C6H3) was synthesized by reduction of the Me2-cAAC:SiCl4 adduct with KC8. This compound is stable at 0 °C for 3 months in an inert atmosphere. Theoretical studies reveal that the silicon atoms exhibit pyramidal coordination, where the Cl-Si-Si-Cl dihedral angle is twisted by 43.3° (calcd 45.9°). The two silicon-carbon bonds are intermediates between single and double Si-C bonds due to twisting of the C-Si-Si-C dihedral angle (163.6°).

The [B3(NN)3]+ and [B3(CO)3]+ Complexes Featuring the Smallest π-Aromatic Species B3+

We report the spectroscopic identification of the [B3 (NN)3](+) and [B3 (CO)3](+) complexes, which feature the smallest π-aromatic system B3 (+). A quantum chemical bonding analysis shows that the adducts are mainly stabilized by L→[B3 L2 ](+) σ-donation.

Observation of Main-Group Tricarbonyls [B(CO)3] and [C(CO)3]+ Featuring a Tilted One-Electron Donor Carbonyl Ligand

A combined experimental and theoretical study on the main-group tricarbonyls [B(CO)3 ] in solid noble-gas matrices and [C(CO)3 ](+) in the gas phase is presented. The molecules are identified by comparing the experimental and theoretical IR spectra and the vibrational shifts of nuclear isotopes. Quantum chemical ab initio studies suggest that the two isoelectronic species possess a tilted η(1) (μ1 -CO)-bonded carbonyl ligand, which serves as an unprecedented one-electron donor ligand. Thus, the central atoms in both complexes still retain an 8-electron configuration. A thorough analysis of the bonding situation gives quantitative information about the donor and acceptor properties of the different carbonyl ligands. The linearly bonded CO ligands are classical two-electron donors that display classical σ-donation and π-back-donation following the Dewar-Chatt-Duncanson model. The tilted CO ligand is a formal one-electron donor that is bonded by σ-donation and π-back-donation that involves the singly occupied orbital of the radical fragments [B(CO)2 ] and [C(CO)2 ](+) .

Reaction Mechanisms of Small-Molecule Activation by Amidoditetrylynes R2N–EE–NR2 (E = Si, Ge, Sn)

The calculated reaction profiles using density functional theory at the BP86/TZVPP level for the reaction of small molecules with amidoditetrylynes R2N-EE-NR2 (E = Si, Ge, Sn) are discussed. Four projects are presented that feature the virtue of cooperation between theory and experiment. First, the calculated reaction paths for hydrogenation of the model systems (Me2N)EEL(NMe2) (E = Si, Ge, Sn), which possess E-E single bonds, are examined. The results for the germanium model systems are compared with hydrogenation of the real system L(†)GeGeL(†) where L(†) = NAr*(SiMe3) (Ar* = C6H2{C(H)Ph2}2Me-2,6,4). The second project introduced the multiply bonded amidodigermyne L(††)GeGeL(††), which carries the extremely bulky substituents L(††) = N(Ar(††))(SiPr(i)3), where Ar(††) = C6H2{C(H)Ph2}2Pr(i)-2,6,4. The theoretical reaction profile for dihydrogen addition to L(††)GeGeL(††) is discussed. Hydrogenation gives L(††)(H)GeGe(H)L(††) as the product, which is in equilibrium with the hydrido species Ge(H)L(††). The latter germanium hydride and tin homologue Sn(H)L(††) were found to be effective catalysts for hydroboration reactions, which is the topic of the third project. Finally, the calculated reaction course for the reduction of CO2 to CO with the amidodigermyne L(†)GeGeL(†) is discussed.