On the basis of early studies of Franz Hein (1892−1976) on (poly)phenylchromium(III) compounds of the type Ph x CrX 3−x (L) n (1 ≤ x ≤ 3) and polyphenylchromate(III) derivatives of the type (L) n M x−3 CrPh x (3 ≤ x ≤ 6) with L being an ether like tetrahydrofuran (thf), 1,2-dimethoxyethane (dme), or tetrahydropyran (thp), we reinvestigated the coordination chemistry depending on the s-block metal of the phenyl-transfer reagent (Li, Mg, Ca) and the solvent (denticity, donor strength). Thus, the following compounds have been prepared, and their molecular structures determined (the number of the complex relates to the number of σ-bound phenyl groups): [mer-I 3 Cr(thf) 3 ], [trans-Cl 2 CrPh(thf) 3 ] (1-Cl), [trans-I 2 CrPh(thf) 3 ] (1-I), [(thf) 5 MgCl] + [{Ph 2 Cr(thf)} 2 (μ-Cl) 3 ] − ((2-Cl) 2 •MgCl 2 ), [fac-Ph 2 ICr(thf) 3 ] (2-I), [fac-Ph 3 Cr(thf) 3 ] (3), [Li(dme) 3 ] + [Ph 4 Cr(dme)] − (4-Li), [PhMg(dme) 2 (thf)] + [Ph 4 Cr(dme)] − (4-Mg), [{(dme)Li} 2 CrPh 5 ] (5-Li), [Li(dme) 3 ] + [{(dme)Li}CrPh 5 ] − (5-Li′), [{(dme)(thf)Ca(μ-Ph) 3 } 2 Cr] + [Ph 4 Cr(thf)] − (5-Ca), [Li(thf) 4 ] + [{(thf)Li(μ-Ph) 3 } 2 Cr] − (6-Li), [Li(thp) 4 ] + [{(thp)Li(μ-Ph) 3 } 2 Cr] − (6-Li′), and [Li(dme) 3 ] + [{(thf)Li(μ-Ph) 3 } 2 Cr] − (6-Li″). Especially the hexaphenylchromates(III) with lithium counterions are very reactive, and the ether degradation product [{(thf) 3 Li 2 O}CrPh 3 ] 2 (7) or other side products like [Li(thp) 4 ] + [{(thp)Li} 2 (μ-Ph) 4 (μ-C 12 H 8 )Cr] − ( 8) with a biphenyl-2,2′-diide ligand are observed. In phenylchromium(III) complexes, the octahedral environment is preferred. Highly nucleophilic phenyl reagents (like phenyl-lithium) are required to prepare hexaphenylchromate(III) complexes, whereas an excess of diphenyl calcium is not sufficient to synthesize pure derivatives with the general composition of Ca 3 (CrPh 6 ) 2 . Furthermore, hexaphenylchromate(III) ions are only stable if the ipso-carbon atoms of the phenyl groups are bridged by electropositive cations (like lithium).

Systematic variation of the 1,4-dioxane (dx) concentration during the precipitation of sparingly soluble [MgBr 2 (dx) 2 ] from ethereal Grignard solutions of RMgBr has allowed the structural investigation of crystallized [R 2 Mg(dx) n ] (n = 1, 1.5, 2, and 3), which form during this dioxane method, depending on the bulkiness of R. The numbering of the complexes explored in this study is based on the number n of dioxane molecules per magnesium atom, followed by the substituent R; an apostrophe denotes coordination polymers. The following derivatives were studied by X-ray crystal-structure determination and NMR spectroscopy :(1.5-oTol), and [(Me 3 Si-CC) 2 Mg(dx) 1.5 ] 1 (1.5'-C 2 SiMe 3 ); n = 2: [tBu 2 Mg(dx) 2 ] (2-tBu) and [oTol 2 Mg(dx) 2 ] (2-oTol); n = 3: [Ph 2 Mg(dx) 3 ] (3-Ph). In the structure types 1', 1.5, and 2, the magnesium atom exhibits the coordination number 4, whereas pentacoordinate metal atoms are observed in types 3 and 1.5'. The structure type 2' is realized for [(Ph-C C) 2 Mg(dx) 2 ] 1 (2'-C 2 Ph), [MgCl 2 (dx) 2 ] 1 (2'-Cl), and [MgBr 2 (dx) 2 ] 1 (2'-Br) with hexacoordinate metal atoms. The solubility of the dioxane adducts in common organic solvents strongly depends on the degree of aggregation with the solubility decreasing from molecular to strand to layer structures.[a] Dr.Scheme 2. Dioxane method to shift the Schlenk equilibrium of organylmagnesium halides toward soluble [R 2 Mg(dx) n ] and insoluble [MgX 2 (dx) 2 ] 1 by substitution of the Lewis base L (e.g., diethyl ether or thf) by 1,4-dioxane (dx).Scheme 3. Structural diversity of hitherto known 1,4-dioxane adducts of diorganylmagnesium complexes authenticated by X-ray crystal-structure determinations.

The organomagnesium complexes [(thf) 2 Mg(μ-C 5 H 10 )] 2 (1), [(thf) 2 Mg(μ-C 4 H 8 )] ∞ (2), and [(thf) 2 Mg(μ-(C-(CH 3 ) 2 C 2 H 4 C(CH 3 ) 2 )] 2 (3) were prepared via direct synthesis from magnesium turnings and appropriate dichloroalkanes in tetrahydrofuran (THF). The aggregation degree in the solid state depends on the nature of the alkanediide. The THF solution of 3 shows a temperature-dependent equilibrium. The reactions of MgCl 2 (thf) 1.5 with 1,4-dilithiobutane yield the lithium magnesiates [Li(thf) 4 ] 2 [Mg 3 (C 4 H 8 ) 4 ] (4) and [{(tmeda)Li} 2 Mg(C 4 H 8 ) 2 ] (5) depending on the applied stoichiometry. The addition reaction of Ph 2 Mg(diox) (1,4-dioxane = diox) with 1,4-dilithiobutane leads to the formation of the heteroleptic magnesiate [{(tmeda)-Li} 2 MgPh 2 (C 4 H 8 )] (6), which shows in THF solution a ligand exchange (Schlenk-type) equilibrium with the homoleptic derivatives [{(thf) 2 Li} 2 Mg(C 4 H 8 ) 2 ] and [{(thf) 2 Li} 2 MgPh 4 ]. ■ INTRODUCTIONDianions are widely used reagents with a broad field of application in organic syntheses; however, common procedures avoid isolation and characterization of these intermediate derivatives. 1 As early as 1901 Tissier and Grignard reacted 1,2-dibromoethane with magnesium turnings in order to produce a vicinal di-Grignard reagent. 2 However, these reactions always proceeded with elimination of ethylene and formation of magnesium bromide. A few years later the reaction of 1,3-dibromopropane with magnesium yielded mainly cyclopropane in a Wurtz-type coupling reaction, but as a side reaction 1,6-hexanediyl-di(magnesium bromide) [hexamethylene di-(magnesium bromide)] was transferred into suberic acid (octanedioic acid) via 2-fold addition of carbon dioxide and water. 3 Generally, the direct synthesis of magnesium turnings with 1,ω-dihalogenated alkanes with more than three methylene units gave the corresponding di-Grignard reagents with high yields. However, the structures of these reagents remained unknown for a long time, and even in the recent textbook of Elschenbroich 4 these compounds were formulated as magnesiacycles, which resulted from a Schlenk-type equilibrium from BrMg-(CH 2 ) n+3 -MgBr (n = 1, 2, 3) after addition of dioxane (eq 1). Bickelhaupt and co-workers intensively investigated these magnesium 1,ω-alkanediides in solution and the solid state. 5 They proposed a monomer− dimer equilibrium in tetrahydrofuran for magnesiacyclohexane (eq 2) and a second Schlenk-type equilibrium for pentamethylene di(magnesium bromide) after addition of magnesium bromide (eq 1, n =2). 6 In the solid state, a centrosymmetric dimer of magnesiacyclohexane, 1,7-dimagnesiacyclododecane (Mg−C 213(1) and 215(1) pm), with very large C−Mg−C angles of 141.5(3)°was observed. 7 Bickelhaupt and co-workers explained the dimerization by entropic effects with the large C− Mg−C angle disfavoring small rings. On the contrary, magnesiacyclopentane (n = 1) was not detected in tetrahydrofuran solution, but this molecule completely dimerized to 1,6-dimagnesiacyclodecane. 8 In a...

Polyphenylchromium(III) organometallics with various phenylation degrees and stabilized by diverse Lewis bases with various donor strengths and denticity were investigated in order to better understand the formation of (η 6 -arene)chromium complexes according to the procedure of

One century ago, Franz Hein started the highly regarded arylchromium chemistry with his initial publication in January 1919. The formation of bis(π-arene)chromium compounds according to Hein's protocols has been performed with pure substrates of the type (Ar) 3−n CrCl n (L) x (n = 0, 1, 2; Ar = Ph, C 6 D 5 , p-F-C 6 H 4 , L = neutral Lewis bases with different base strength and denticity) as model compounds at magnesium halide-free conditions. Three main synthesis phases for the σ−π transfer of the phenyl groups have been recognized and simulated: Initial phase (excess of MgPh 2 ): Ph 3 Cr(thf) 3 •0.25 dx•(3-thf 3 , dx = 1,4-dioxane) eliminates tetrahydrofuran (THF) reversibly in C 6 D 6 , yielding bluish-green [Ph 3 Cr(thf) 2 ] (3thf 2 ) that reacts above 5 °C irreversibly to a black-brown solution of biphenyl, benzene, and "{(η 6 -Ph 2 )CrI(μ-η 6 -Ph)} 2 " (intermediate A). Ph 2 Mg reduces A to "(η 6 -Ph 2 )Cr 0 (η 6 -PhMgPh)" (intermediate B). Middle phase: Subsequent reactions of A: (i) Reduction to [(η 6 -C 6 H 6 )(η 6 -Ph 2 )Cr 0 ] (π-3) by THF, (ii) disproportionation to [(η 6 -Ph 2 ) 2 Cr 0 ] (π-4) and "[(η 6 -C 6 H 5 ) 2 Cr II ] n " that is reduced to "(η 6 -C 6 H 5 MgPh) 2 Cr 0 " by MgPh 2 , (iii) reductive coupling yielding [{(η 6 -Ph 2 )Cr} 2 (μ-η 6 :η 6 -Ph 2 )] ((π-3) 2 ), or (iv) decomposition to biphenyl and chromium. Labeling experiments with (D 5 C 6 ) 3 Cr(thf) 3 •0.25 dx ([D 15 ]3-thf 3 ) in C 6 D 6 and [fac-(p-F-C 6 H 4 ) 3 Cr(thf) 3 ] (3 F -thf 3 ) allow one to determine the origin of the hydrogen atom and the identification of the end products. Final phase: Deficit of Mg(Ar) 2 hinders formation of bis(π-arene) chromium complexes. "Ph 2 CrCl(thf) x " forms an equilibrium in THF solution with 3-thf 3 and PhCrCl 2 (thf) x (1-Cl). During heating of a solution of 3-thf 3 , CrCl 3 (thf) 3 , and H 2 O (3:1:1) in THF o-metalation of a phenyl group leads to formation of mixed-valent tetranuclear [Ph 4 Cr 4 (μ 4 -OH)(μ-Cl) 2 (μ 3 -Cl)(μ 3 -C 6 H 4 )(thf) 4 ] (4-Cr 4 ) besides biphenyl and benzene. The analogous reaction of [D 15 ]3-thf 3 , CrCl 3 (thf) 3 , and H 2 O yields [D 5 ]benzene, [D 10 ]biphenyl, and heptanuclear oxo-centered [Cr 7 II (μ-Cl) 10 (μ 4 -O) 2 (thf) 6 ] (0-Cr 7 ). Complexes of the type [(Ar)CrCl 2 (thf) n ] (Ar = Ph, Mes, p-F-C 6 H 4 ) show homolytic Cr−C bond cleavage, leading to chromium(II) chloride and biaryls.