H. Tom Dieck

Photoelectron spectra and molecular properties : XXIII. Photoelectron spectra of silicon-substituted ylidic systems☆

Abstract The photoelectron (PE) spectra of the simplest phosphorus ylide (CH 3 ) 3 PCH 2 and several C-silyl-substituted derivatives have been measured, and the influence of this substitution on the frontier orbitals is discussed. Experimental values for PC π-ionization and CNDO/2 calculations on these systems show that “silyl stabilization” is not the consequence of a π-energy lowering but of a decrease of the MO coefficient at the ylidic, quasi-anionic carbon atom. The PE spectra of the ylides are compared with those of isoelectronic systems, which contain N or O instead of C, and Si instead of P. σ(SiC) ionizations are shown to be rather dependent on the other groups attached to silicon, and to vary over a remarkable 2 eV range.

Zur komplexchemie von 4-zentren-π-systemen : VIII. ESR-untersuchungen an heterobutadien-chelaten. I. Kalium-(diazabutadien-tetracarbonyl-metallate)

Abstract Glyoxal- and diacetyl-bis(imine)metal tetracarbonyls (RNCR′CR′NR)M(CO) 4 (M = Cr, Mo, W) are reduced with potassium in DME to give paramagnetic mono-anions. Their high-resolutior, ESR spectra show the hyperfine splittings of the magnetically active ligand atoms as well as the satellites of the metal isotopes 53 Cr, 95,97 Mo and 183 W. The coupling constants are discussed with respect to π-bonding mechanisms.

ESR metallorganischer radikale—IV1 : Sterische effekte bei 1,4-diazabutadienchelaten

Zusammenfassung 1,4-Diazabutadiene, zweizahnige Liganden des Typs R 2 NCR 1 CR 1 NR 2 (R 1 H; R 2 t.-C 4 H 9 , i-C 3 H 7 , i-C 4 H 9 ), bilden ebenso wie entsprechende Molybdantetracarbonylkomplexe (R 1 H, CH 3 ; R 2 CH 3 , CH 2 C 6 H 5 , i-C 3 H 7 , c-C 6 H 11 , c-C 3 H 5 ) mit Kalium stabile paramagnetische Monoanionen. Deren hochaufgeloste ESR-Spektren erlauben den Vergleich der Konformationen des Substituenten R 2 im freien und komplexgebundenen Liganden. Seine Beweglichkeit wird als Funktion von R 1 und der Metallcarbonylgruppen im Hinblick auf sterische Ligandenqualitaten diskutiert.

Zur komplexchemie von vierzentren-π-systemen VII. 1-azabutadien-komplexe des molybdäns

Abstract 1-Azabutadienes, R 1 NCHCHCHR 2 , substitute the acetonitrile in (CH 3 CN) 2 Mo(PBu 3 ) 2 (CO) 2 under mild conditions and yield 1-azabutadiene π-complexes, which can be protonated reversibly. IR spectroscopic data of butadiene-, 1-heterobutadieneand 1,4-diheterobutadiene-molybdenum dicarbonyls as well as electron spectroscopic data on butadiene complexes and their aza analogues are compared. ab]1-Azabutadiene, R 1 NCHCHCHR 2 , substituieren das Acetonitril in (CH 3 CN) 2 Mo(PBu 3 ) 2 (CO) 2 unter milden Bedingungen und geben 1-Azabutadien-π-Komplexe, die reversibel protonierbar sind. IR-spektroskopische Daten von Butadien-, 1-Heterobutadien- und 1,4-Diheterobutadien-molybdan-dicarbonylen sowie elektronen-spektroskopische Daten von Butadienkomplexen und ihren Aza-analogen werden verglichen.

Photoelectron spectra of group V compounds. IX [1]. The relative perfluoroalkyl substituent effect

Abstract The gas phase He I p.e. spectra of the series (CF 3 ) 3 E (E = N, P, As, Sb) and (CF 3 ) n AsH 3−n are discussed and compared to those of their methylated analogs. The perfluoroalkyl effect — synonymous with large σ-electron withdrawal — causes a nearly parallel stabilization of all m.o. energies as compared with the respective alkyl counterparts. This property can be used, systematically and generally, as a valuable aid for a more precise interpretation of the p.e. spectra of the parent methyl derivatives. Band superposition due to interfering substituent m.o. energies is diminished without destroying the original orbital energy sequence found for the methyl compounds. Overlapping bands from the ionisation of σ- and π-type skeletal molecular orbitals can be resolved on replacing alkyl by perfluoroalkyl groups. Substituent effects including R = CF 3 , Hal, SiR′ 3 and H are surveyed for the series ER 3 (E = N, P).

Pallada- and platinacycles—III. Synthesis of 3,4- dialkylidene-platinacyclopentanes and their metallo-ene type reaction with dioxygen. X-ray structure of a 2,3-dioxa-platinacycloheptane (dad)CH2

Abstract The reaction of bis(dibenzylideneacetone)platinum with a diazadiene (dad = 2,6-Me2-Ph-dad, 2,6-iPr2-Ph-dad) and 1,1-dimethylallene gives the 3,4-bis(isopropylidene)platinacyclopentanes (dad PtCH 2 C(CMe 2 )C(CMe 2 )C H2 (6) and (7). The metallacyclopentane rings are non-planar and rigid. Under air or oxygen the violet complexes 6 and 7 are slowly and quantitatively converted to green complexes 8 and 9, respectively. The dioxygen does not insert directly into the PtC bond but, instead, reacts in a metallo-ene type manner to give the dioxaplatinacycloheptanes (dad) PtOOCMe 2 C(CH 2 )C(CMe 2 )C H2, as revealed by NMR and X-ray diffraction studies. In complex 8 the platinum atom shows square-planar coordination. The seven-membered ring (with an almost planar CαPtOO fragment) is folded in such a way that the two exo-alkylidene groups are not in conjugation. The conformation is also rigid in solution. The disposition of the methyl groups proves the unexpected allylic inversion which occurs with the incorporation of dioxygen.

Dinuclear 2,2′-biallyl nickel complexes containing square-planar nickel(I): crystal structure of μ(η3;3-(CH2)2CC(CH2)2)-trans- [Ni(dad)]2 (dad = biacetylbis(2,6-dimethylphenylimine))

Diazadiene nickel butadiene complexes react with allene in a 1 : 1 ratio to give paramagnetic complexes 2, of composition (dad)Ni(C3H4). A single-crystal structure determination in the case of 2a has revealed the presence of a planar bridging tetramethyleneethane (2,2′-biallyl) ligand to which two (dad)Ni fragments are coordinated on opposite sides. The complex can be regarded as containing two independent nickel(I) centres, each having a square-planar coordination geometry.

Diazadiene-Controlled C-C Coupling Reactions on Palladium and Iron

Transition metal centers can act as electrophiles or nucleophiles and also transfer electrons to or accept electrons from substrates. In addition the metals can act as simple templates and thus impose geometrical factors on two or more substrates. All of these features can be monitored by additional ligands. While for stoichiometric metal centered reactions drastic conditions and changes in the energy content of the metal compound are possible the energetic hypersurface for a good catalytic reaction must be rather smooth relative to the temperature. Since many substrates such as olefines, carbon monoxide or hydrogen are kinetically inert it seems unprobable that stable metal compounds are catalysts per se. Many of the activation steps in homogeneous catalysis therefore correspond to raising the total system to the ground level of the catalytic energy hypersurface. For a catalytic reaction in the mechanistic sense such a surface must not necessarily have a descend (almost degenerate catalyes such as olefin metathesis, H/D-scrambling, some olefin isomerizations), for a productive catalysis there should of course be a declining slope with the reaction coordinate. In the following paragraphs reactions involving the metal center activation, substrate binding and transformation, and product release will be discussed with many new examples from the catalytic chemistry of palladium and iron systems, in which diazadienes (DAD) constitute the control ligands [1].