Grignard reagents Discovered by Victor Grignard at the university of Lyon in 1900, compounds with empirical formula RMgX (R = carbanion, X = Cl, Br, I) are known as Grignard reagents. They are widely used in organic synthesis. Grignard reagents are a common source of carbanion equivalents, which can be used to perform nucleophilic addition, substitution, transmetalation, and metal-halogen exchange reactions. The first crystal structure of Grignard reagents was reported by Guggenberger and Rundle in 1964, from a crystalline EtMgBr(THF)2 (Et = ethyl, THF =
tetrahydrofuran). The Mg-C bond length was found to be 2.15(2) Å, which is about the sum of
covalent radii of magnesium (141(7) pm) and carbon (76(1) pm at sp3
hybridization). Grignard reagents are dynamic in solution. The R and X groups are exchanged between magnesium centers. Via the
Schlenk equilibrium, RMgX, MgR2, and MgX2 equilibrate as well. These equilibria are relevant to the reactivity of Grignard reagents.
Magnesium alkyls, alkynyls, and aryls Dialkylmagnesium is a fundamental type of organomagnesium compound. Such compounds can be prepared from Grignard reagents, via precipitation of magnesium halide. Solid state dialkylmagnesium forms one-dimensional chains via Mg-C-Mg
3c-2e bonds, and the Mg-C bond length is 2.24(3) Å in
dimethylmagnesium (Me2Mg)
n, which is about 10 pm longer than the terminal alkyl-Mg bonds (e.g. 2.15(2) Å in EtMgBr(THF)2). Dialkylmagnesium compounds can prepared by treating magnesium hydride with
alkenes: : Many simple homoleptic organomagnesium species are known. Examples include , and . Illustrating the use of
salt metathesis reaction as a synthesis route, the
phenylene complex was prepared from and
magnesium bromide: : Illustrative of an alkynyl ("acetylide") complex is . Such species are relatively easily generated reflecting the diminished basicity of the "acetylide anion" relative to the alkyl carbanions.
Carbomagnesiation is the addition of C-Mg bonds across bonds. The process typically employs a catalyst and proceeds via the intermediacy of vinyl-Mg species: :
Mixed metal derivatives Being electron-rich, diorganomagnesium compounds function as ligands. With alkaline metals, they forms a variety of "
ate complexes". In this way very simple compounds can be prepared such as , featuring tetrabenzylmagnesium bound via two bridging methyl ligands to a Li(tmeda)+ center. This style of work often utilizes
tetramethylethylenediamine (tmeda), an aprotic bidentate ligand that has a high affinity for alkali and alkaline earth metals. Treating dimethylmagnesium with
trimethylaluminium gives the neutral . Similarly, treating dimethylmagnesium-tmeda with the nickel(0) ethylene complex gives the neutral , with displacement of one ethylene ligand. Subsequent work led to the isolation of the soluble derivative [(C14H10)Mg(THF)3]. According to
X-ray crystallography, the Mg-C9 and Mg-C10 distances are 2.225(1) Å]. The structural results show that magnesium anthracene can be treated as an magnesium alkyl. It is a particularly versatile
reagent. Derivatives of magnesium anthracene have been described. In terms of its reactivity, [(C14H10)Mg(THF)3] behaves as the equivalent of [C14H10]2- . The two negative charges localized on C9 and C10. It thus act as nucleophile to give functionalized anthracene or
9,10-dihydroanthracene derivatives. Magnesium anthracene reacts with arylphosphinous chlorides to give dibenzo-7λ3-phosphanorbornadiene (RPC14H10), which can be used as
phosphinidene transfer reagent. ==
N-heterocyclic carbene complexes==