Judith Baumgartner

Coordination Chemistry of Cyclic Disilylated Stannylenes and Plumbylenes to Group 4 Metallocenes

Reduction of group 4 metallocene dichlorides with magnesium in the presence of cyclic disilylated stannylene or plumbylene phosphine adducts yielded the respective metallocene tetrylene phosphine complexes. Under the same conditions the use of the respective dimerized stannylene or plumbylene gave metallocene ditetrylene complexes. A computational analysis of these reactions revealed for all investigated compounds multiple-bonded character for the M–E(II) linkage, which can be rationalized in the case of the monotetrylene complex with the classical σ-donor/π-acceptor interaction. The strength of the M–E(II) bond increases descending group 4 and decreases going from Sn to its heavier congener Pb. The weakness of the Ti–E(II) bonds is caused by the significantly reduced ability of the titanium atom for d–p π-back-bonding.

Dispersion Energy Enforced Dimerization of a Cyclic Disilylated Plumbylene

By reaction of 1,4-dipotassio-1,1,4,4-tetrakis(trimethylsilyl)tetramethyltetrasilane with PbBr2 in the presence of triethylphosphine a base adduct of a cyclic disilylated plumbylene could be obtained. Phosphine abstraction with B(C6F5)3 led to formation of a base-free plumbylene dimer, which features an unexpected single donor–acceptor PbPb bond. The results of density functional computations at the M06-2X and B3LYP level of theory indicate that the dominating interactions which hold the plumbylene subunits together and which define its actual molecular structure are attracting van der Waals forces between the two large and polarizable plumbylene subunits.

A cyclic disilylated stannylene: synthesis, dimerization, and adduct formation.

Reaction of 1,4-dipotassio-1,1,4,4-tetrakis(trimethylsilyl)tetramethyltetrasilane with [(Me3Si)2N]2Sn led to the formation of an endocyclic distannene via the dimerization of a transient stannylene. In the presence of strong donor molecules such as PEt3, the stannylene could be trapped as adduct. Reaction of the PEt3 derivative with B(C6F5)3 gave rise to the formation of the stannylene B(C6F5)3 adduct.

Oxo-molybdenum and oxo-tungsten complexes of Schiff bases relevant to molybdoenzymes

A series of octahedral dioxomolybdenum(VI) complexes of the type [MoO(2)L(2)] {L = 4-Ar-pent-2-en-ol; L(i-Pr2Ph) with Ar = 2,6-diisopropylphenyl (1); L(Me2Ph) with Ar = 2,6-dimethylphenyl (2), L(MePh) with Ar = 2-methylphenyl (3) and with Ar = phenyl (4)} and dioxotungsten(VI) compounds [WO(2)L(2)] {L(i-Pr2Ph) (5); L(Me2Ph) (6) and L(MePh) (7)} with Schiff bases have been synthesized as models for oxotransferases. Spectroscopic characterization in solution shows with the sterically encumbered ligands L(i-Pr2)Ph and L(Me2)Ph isomerically pure products whereas the ligand with only one substituent in ortho position at the aromatic ring L(MePh) revealed a dynamic mixture of three isomers as confirmed by variable temperature NMR spectroscopy. Single crystal X-ray diffraction analyses of compounds 1, 2, and 4 and showed them to be in the N,N-trans conformation consistent with the larger steric demand at nitrogen. Oxygen atom transfer (OAT) properties towards trimethylphosphine were investigated leading to the isolation of two mononuclear molybdenum(IV) compounds [MoO(PMe(3))(L(Me2Ph))(2)] (8) and [MoO(PMe(3))(L(MePh))(2)] (9) as confirmed by spectroscopic and crystallographic means. The kinetics of OAT between complex [MoO(2)(L(Me2Ph))(2)] (2) and PMe(3) was investigated by UV/Vis spectroscopy under pseudo-first-order conditions revealing single-step reactions with Eyring values of DeltaH(double dagger) = +60.79 kJ mol(-1) and DeltaS(double dagger) = -112 J mol(-1) K(-1) and a first-order dependence of phosphine consistent with a slow nucleophilic attack of the phosphine showing the octahedral geometries of this system to be unfavorable for OAT. Compound 1 showed no OAT reactivity towards PMe(3) emphasizing the influence of sterical properties. Furthermore, the reactivity of the reduced compounds [MoO(PMe(3))(L(Me2Ph))(2)] (8) and [MoO(PMe(3))(L(MePh))(2)] (9) towards molecular oxygen was investigated leading, in the case of 8, to the substitution of PMe(3) by O(2) under formation of the peroxo compound [MoO(O(2))(L(Me2Ph))(2)] (10). In contrast, the analogous reaction employing 9 led to oxidation forming the dioxo compound [MoO(2)(L(MePh))(2)] (3).

Group 4 metallocene complexes of disilenes, digermenes, and a silagermene.

Reactions of 1,2-dipotassiotetrakis(trimethylsilyl)disilane with group 4 metallocene dichlorides lead to the formation of the respective metallocene 1,1,2,2-tetrakis(trimethylsilyl)disilene complexes. While the disilene titanocene complex could be structurally characterized, the zirconocene and hafnocene compounds, which are believed to possess some degree of bis-[bis(trimethylsilyl)silylene] character, can only be isolated in substance as the respective trimethylphosphane adducts. Analogous metallocene 1,1,2,2-tetrakis(trimethylsilyl)digermene complexes and a tetrakis(trimethylsilyl)silagermene complex were prepared. Instead of metallocene 1,1,2,2-tetrakis(trimethylsilyl)distannene complexes, four-membered rings composed of a metallocene and three bis(trimethylsilyl)stannylene units were obtained.

Replacement of an Oxo by an Imido Group in Oxotransferase Model Compounds: Influence on the Oxygen Atom Transfer

Treatment of [MoO(N-t-Bu)Cl(2)(dme)] (dme = dimethoxyethane) with 2 equiv of the potassium salts of Schiff base ligands of the type KArNC(CH(3))CHC(CH(3))O afforded oxo imido molybdenum(VI) compounds [MoO(N-t-Bu)L(2)] {1, with Ar = phenyl (L(Ph)), 2 with Ar = 2-tolyl (L(MePh)), 3 with Ar = 2,6-dimethylphenyl (L(Me2Ph)) and 4 with Ar = 2,6-diisopropylphenyl (L(iPr2Ph))}. We have also prepared related bisimido complexes [Mo(N-t-Bu)(2)L(2) (5 with L = L(Ph), 6 with L = L(MePh), and 7 with L = L(Me2Ph)) by treatment of [Mo(N-t-Bu)(2)Cl(2)(dme)] with 2 equiv of the potassium salt of the respective ligand. 1, 3, 5, and 6 were characterized via single crystal X-ray diffraction. The oxo imido complexes exhibit oxygen atom transfer (OAT) reactivity toward trimethyl phosphine. Kinetic data were obtained for 1 and 3 by UV/vis spectroscopy revealing decreased OAT reactivity in comparison to related dioxo complexes with the same Schiff base ligands and decreased reactivity of 1 versus 3. Cyclic voltammetry was used to probe the electronic situation at the molybdenum center showing reversible reduction waves for 3 and [MoO(2)(L(Me2Ph))(2)] at comparable potentials while 1 exhibits a significant lower potential. Density functional theory (DFT) calculations showed a higher electron density on oxygen in the oxo imido complexes.

Oxorhenium(V) complexes with ketiminato ligands: coordination chemistry and epoxidation of cyclooctene.

Rhenium(V) oxo complexes of the type [ReOX(L)(2)] (1-7; X = Cl, Br) containing beta-ketiminate ligands (L = CH(3)C(O)CH(2)C(NAr)CH(3): Ar = Ph (APOH), 2-MePh (MPOH), 2,6-Me(2)Ph (DPOH), 2,6-(i)Pr(2)Ph (DiPOH)) have been prepared by reaction of [ReOX(3)(OPPh(3))(SMe(2))] (X = Cl, Br) with the lithium salts of the corresponding ligands. All compounds have been spectroscopically characterized, showing [ReOX(DiPO)(2)] (X = Cl (1), Br (5)), [ReOX(DPO)(2)] (X = Cl (2), Br (6)), and [ReOX(APO)(2)] (X = Cl (4), Br (7)) to be isomerically pure, in contrast to complex [ReOCl(MPO)(2)] (3), which exhibits a mixture of isomers. Compounds 2, 3, 5, and 7 were crystallographically characterized, showing similar octahedral coordination spheres with trans O horizontal lineRe-O and cis O horizontal lineRe-Cl bonds. However, the coordination of the nitrogen atoms vs each other is found to be cis or trans. Compounds 2 and 5 showed a trans-N,N configuration, for compound 3 both isomers (trans-N,N 3 and cis-N,N 3) were structurally characterized, and 7 gave a cis-N,N configuration. Compounds 1-6 are catalyst precursors for the epoxidation of cis-cyclooctene with 3 equiv of tert-butyl hydroperoxide (TBHP). Yields of the formed epoxide were up to 55% with all precursors, except with 2 and 6, where only up to 13% of epoxide was obtained under analogous conditions.

Coordination of non-stabilized germylenes, stannylenes, and plumbylenes to transition metals

Abstract Complexes of transition metals with heavy analogs of carbenes (tetrylenes) as ligands have been studied now for some 40 years. The current review attempts to provide an overview about complexes with non-stabilized (having no π-donating substituents) germylenes, stannylenes, and plumbylenes. Complexes are known for groups 4–11. For groups 6–10 not only examples of monodentate tetrylene ligands, but also of bridging ones are known. While this review covers almost 200 complexes, the field in general has been approached only very selectively and real attempts for systematic studies are very scarce. Although some isolated reports exist which deal with the reactivity of the tetrylene complexes most of the so far published work concentrates on synthesis and characterization.

Coordination Chemistry of Disilylated Germylenes with Group 4 Metallocenes.

Reaction of the PEt3 adduct of a disilylated five-membered cyclic germylene with group 4 metallocene dichlorides in the presence of magnesium led to the formation of the respective germylene metallocene phosphine complexes of titanium, zirconium, and hafnium. Attempts to react the related NHC adduct of a disilylated four-membered cyclic germylene under the same conditions with Cp2TiCl2 did not give the expected germylene NHC titanocene complex. This complex was, however, obtained in the reaction of Cp2Ti(btmsa) with the NHC germylene adduct. A computational analysis of the structure of the group 4 metallocene germylene complexes revealed the multiple-bond character of the M–Ge(II) linkage, which can be rationalized with the classical σ-donor/π-acceptor interaction. The strength of the M–Ge(II) bond increases descending group 4.

Formation of Formal Disilene Fluoride Adducts

Reactions of trisilylated silylfluorides with potassium tert-butoxide were found to give disilylated fluorosilylenoids. The latter undergo a self-condensation reaction, leading to the formation of beta-fluorodisilanyl anions, which may also be considered as fluoride adducts of disilenes. The stereochemical outcome of this self-condensation depends on the bulkiness of the silyl substituents. Thus, mixtures of diastereomers are obtained in some cases. Reaction of a disilene adduct with magnesium bromide triggers metal fluoride elimination and formation of the respective tetrasilylated disilene. Attempts to exchange one silyl group of the starting material for a methyl, phenyl, or tert-butyl group led to a different course of reaction for the methyl and phenyl cases. The tert-butyl-substituted example gave the expected disilene adduct, but it was not the main product.