Otto S. Akkerman

X-ray structural analyses of organomagnesium compounds

Publisher Summary This chapter discusses the X–ray structural analyses of organomagnesium compounds. Organomagnesium compounds are well-established tools in organic chemistry. Compared to the long history of the synthetic applications of organomagnesium compounds, the investigation of their structure and bonding covers a much shorter period. By the use of gas-phase electron diffraction (GED), it is possible to determine the structure of molecules in the gas phase. A major drawback of GED is that only small molecules can be resolved completely; for larger molecules, the degrees of freedom must be limited during the calculations by assuming and optimizing values for many of the structural parameters. In almost all structure determinations of simple Grignard or diorganylmagnesium compounds, the magnesium atoms are surrounded by four ligands. In the absence of coordinating solvents such as ethers, most organomagnesium compounds tend to form polymeric structures. Solid and volatile organomagnesium compounds can be slowly sublimed in a static high vacuum. On the symmetrization of di-Grignard reagents to the analogous halogen-free compounds, both polymeric and cyclic species may be expected. Although the tetrahedral coordination geometry is preferred by organomagnesium compounds, many examples have been structurally characterized, which show deviating coordination numbers. Increased coordination numbers for magnesium are also found for an ethylmagnesium-chloride/magnesium-chloride adduct. Fascinating structures can be obtained on the complexation of organomagnesium compounds with polyether ligands.

Synthese von 1,3-Dimetallacyclobutanen mit dem Methylen-di-Grignardreagens

Abstract An improved preparation of the methylene-di-Grignard reagent CH2(MgBr)2 (2) is described 2 is applied as a synthon for the preparation of 1,3-dimetallacyclobutanes (1) in a two step sequence. First,, two molar equivalents of 2 and one molar equivalent of a dichlorometallocene Cp2MCl2 (3) (M = Ti, Zr, Hf) are combined to form a 1,3-di-Grignard reagent Cp2M(CH2MgBr)2 (4) which with a metal dichloride L2M′Cl2 (L2M′ = Cp2Ti, Cp2Zr, Cp2Hf, Me2Si, Me2Ge, Me2Sn) gives 1. The 1H and 13C NMR spectra of 1 show interesting trends which are briefly discussed.

The coordination modes of simple diarylmagnesium species: Some representative X-ray crystal structures

The coordination modes of simple diarylmagnesium species have been investigated by crystal structure studies of representation compounds. In the case of diphenylmagnesium (1), both the solvated monomeric complex Ph2Mg·[THF]2 (1a) and the solvent-free compound [Ph2Mg]n (1b) were characterized. A polymeric structure was found for 1b, with tetrahedrally coordinated magnesium atoms interconnected by μ,η1-bridging phenyl groups to form linear chains. Bis(p-tolyl)magnesium (3) crystallized in a remarkable double structure that contained dimeric and monomeric units in a 1:2 ratio. In the dimer, two aryl groups are μ,η1-bridging.

An investigation of the di-Grignard approach to metallabenzocyclobutenes of group 14

Abstract The reactions of 1,2-dihydro-1-magnesabenzocyclobutene (5) with dichlorodimethylsilane (12a), dichlorodimethylgermane (12b) and dichlorodimethylstannane (12c) are reported; 1,2-dihydro-1,1,dimethyl-1-silabenzocyclobutene (14a) and 1,2-dihydro-1,1-dimethyl-1-germabenzocyclobutene (14b) were formed in high yields, but the tin analogue 14c was not obtained. Eight-membered ring species, the dimers 17 and 18, were isolated for all three metals. Other products gave useful indications of the probable course of these interesting and complex reactions.

A direct synthesis of 1,3-bis(bromomagnesio)propane

Abstract Careful addition of 1,3-dibromopropane to magnesium in Et 2 O yields 1,3-bis(bromomagnesio)propane which is purified via “magnesacyclobutane” . Reactions of with H 2 O, CO 2 , HgBr 2 and Me 3 SnCl are reported. MgBr 2 catalyses the decomposition of to allylmagnesium bromide .

Some applications of the methylene di-Grignard reagent for the synthesis of main group IV organometallic compounds

Abstract The di-Grignard reagent methylenedimagnesium dibromide, CH2(MgBr)2 (1), which can be prepared from dibromomethane and magnesium amalgam in diethyl ether/benzene 1/1, has been obtained in a pure form. Treatment of 1 with tetrahydrofuran gave insoluble 5″, a reagent with the approximate composition CH2Mg·CH2(MgBr)2. Both reagents were used for the synthesis of the dimetallomethanes (Me3M)2CH2 (M = Si, Ge, Sn) and (CH2(HgBr)2. Reaction of 1 or 5″ with dichlorodimethylgermane or dichlorodimethylstannane gave the polygermacycloalkanes ((Me2GeCH2)n; n = 2,3,4) or polystannacycloalkanes ((Me2SnCH2)n; n = 3,4), respectively, in useful yields (10 to 35%). In the germanium series, there is a pronounced tendency to form the smaller ring systems; in particular, the reaction of 5″ with Me2GeCl2 gave 30–35% of 1,1,3,3,-tetramethyl-1,3-digermacyclobutane. In contrast, the corresponding distannacyclobutane was not observed, but the eight-membered species (Me2MCH2)4 was formed more readily for M = Sn (9% yield) than for M = Ge (2% yield).

A general synthesis of metallacyclobutanes

Abstract Metallacycloalkanes containing four to six carbon atoms are easily synthesized from α,ω-dimetal-organic compounds and the appropriate metal dihalide. Metallacyclobutanes were not accessible in this way however, as the corresponding organometallic reagents were not easily available. However, our recent synthesis of di-Grignard reagents from 1,3-dibromopropanes allowed the development of a general synthesis of the title compounds. In addition to the synthesis in high yields of the (already known) germana- and titana-cyclobutanes, the method has also proved useful in the preparation of stanna-, hafna-, zircona- and vanada-cyclo butanes. The newly prepared compounds have been characterized by their NMR spectra (plus the ESR spectrum for the paramagnetic vanadium compound) and the products of their reaction with elemental bromine. Their behaviour as organometallic compounds has been derived from their reactions with cyclohexanone. The scope and limitations of the synthesis appear to be mainly determined by the stability of the 1,3-di-Grignard compounds, which is strongly dependent on the substitution pattern.

Complexation of diphenylzinc with simple ethers. Crystal structures of the complexes Ph2Zn · glyme and Ph2Zn · diglyme

Abstract The complexation behavior of diphenylzinc with a series of mono-, di- and tri-coordinating ethers has been investigated. Two stable crystalline 1 : 1 adducts, with ethyleneglycol dimethyl ether (glyme) ( 1 ) and diethyleneglycol dimethyl ether (diglyme) ( 2 ) were isolated and characterized by an X-ray diffraction study. Complex 1 has a normal tetra-coordinated zinc atom; 2 represents the first example of a complex penta-coordinated diorganozinc compound, and has a rather unusual geometry.

Intramolecularly coordinated arylmagnesium compounds: effects on the Schlenk equilibrium

Abstract A series of phenylmagnesium bromides (1, 3–8) with ortho-substutients capable of forming intra-molecular coordinative bonds along with the correspondng diarylmagnesium compounds (1a, 3a–6a, 8a) have been synthesized. The thermodynamic parameters ΔHs, and ΔSs for the Schlenk equilibria (2 ArMgBr &/rhar2; Ar2Mg + MgBr2) have been determined by variable temperature NMR spectroscopy. Crystal structures were obtained of 5,6,7,8,9-tetrahydrodibenz[d,g][1,6]oxamagnesecin (2a) and bis(2,6-di(methoxymethyl)phenyl)magnesium (4a). The extent of intramolecular coordination in these compounds as determined in the solid state, is used in the discussion of the influence of substituents on the Schlenk equilibrium parameters. Unusual penta- or hexa-coordination is encountered and explained as a consequence of intramolecular coordination.

Methylenedimagnesium dibromide as precursor of metal carbene complexes; preparation of titanacyclobutanes.

Abstract The reaction of CH 2 (MgBr) 2 with Cp 2 TiCl 2 gives a titanium methylene complex which, similar to Tebbes reagent, forms four-membered titana-heterocycles with alkenes and alkines.