1,4-Dioxane Adducts of Grignard Reagents: Synthesis, Ether

Sep 18, 2009 - Exchange of the thf coligands in closely related [(thf)2Mg(Mes)2] for other Lewis bases such as 2,2′-bipyridine (bpy) and 2,4,6-trime...
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Organometallics 2009, 28, 5814–5820 DOI: 10.1021/om9000567

1,4-Dioxane Adducts of Grignard Reagents: Synthesis, Ether Fragmentation Reactions, and Structural Diversity of Grignard Reagent/1,4-Dioxane Complexes Jens Langer,* Sven Krieck, Reinald Fischer, Helmar G€ orls, Dirk Walther, and Matthias Westerhausen Institute of Inorganic and Analytical Chemistry, Friedrich-Schiller-Universit€ at Jena, August-Bebel-Strasse 2, D-07743 Jena, Germany Received January 25, 2009

The 1,4-dioxane precipitation method was employed to obtain solutions of R2Mg from Grignard reagents via precipitation of polymeric magnesium dihalides as dioxane adducts. The addition of a slight excess of dioxane resulted in partial cleavage of the initially formed [( μ-diox)MgR2]¥ polymer into smaller fragments of the type [(diox)nþ1(MgR2)n]. In addition to the polymeric compounds [( μ-diox)MgR2]¥, as found for [( μ-diox-O,O’)Mg(cyclo-C6H11)2]¥ (1), other oligomers such as [(diox)3(Mg{CH2Ph}2)2] (2) can be obtained from such solutions. In the system MesMgBr/1,4dioxane the compounds [( μ-diox)MgBr2]¥ and [( μ-diox)Mg(Mes)2]¥ are both sparingly soluble, offering the opportunity to remove most of these compounds from the reaction mixture and leaving the products of ether cleavage reactions as the major species in solution. Using this procedure, the new ether degradation products [(diox)Mg(Mes)(μ-OEt)]2 (3) and [(EtOCH(Me)CH(Me)OEt)Mg(Mes)2] (4) were isolated. Excess dioxane yields mononuclear [(diox)2Mg(Mes)2] (5). Exchange of the thf coligands in closely related [(thf)2Mg(Mes)2] for other Lewis bases such as 2,20 -bipyridine (bpy) and 2,4,6-trimethylphenyl (Mes) anions allowed the preparation of [(bpy)Mg(Mes)2] (6) and [(thf)4Li]þ[Mg(Mes)3]- (7). Molecular structures of all new compounds 2-7 are reported.

1. Introduction Organomagnesium compounds (Grignard reagents)1 are a powerful tool in organic and organometallic chemistry as well as catalysis.2-6 Their importance led to extensive investigations of their molecular structures in solution and in the solid state, their physical and chemical properties. Several review articles7-9 describe the structural principles of organomagnesium derivatives. The first investigation of the action of 1,4-dioxane (diox) on organomagnesium halides in solution dates back to 1929.10 Schlenk and Schlenk, Jr. found that Grignard reagents (RMgX) exist in equilibrium with the corresponding *To whom correspondence should be addressed. Fax: þ49 (0) 3641 948102. E-mail: [email protected]. (1) Grignard, V. C. R. Hebd. S eances Acad. Sci. 1900, 130, 1322–1325. (2) Bickelhaupt, F. J. Organomet. Chem. 1994, 475, 1–14. (3) Wakefield, B. J. Organomagnesium Methods in Organic Synthesis; Academic Press: London, 1995. (4) Richey, H. G., Ed. Grignard Reagents: New Developments; Wiley: Chichester, U.K., 2000. (5) Henderson, K. W.; Kerr, W. J. Chem. Eur. J. 2001, 7, 3430–3437. (6) Ila, H.; Baron, O.; Wagner, A. J.; Knochel, P. Chem. Commun. 2006, 583–593. (7) Markies, P. R.; Akkerman, O. S.; Bickelhaupt, F.; Smeets, W. J. J.; Spek, A. J. Adv. Organomet. Chem. 1991, 32, 147–226. (8) Holloway, C. E.; Melnik, M. J. Organomet. Chem. 1994, 465, 1–63. (9) Bickelhaupt F. In Grignard Reagents: New Developments; Richey, H. G., Ed.; Wiley: Chichester, U.K., 2000; Chapter 9, p 299-328. (10) Schlenk, W.; Schlenk, W., Jr. Ber. Dtsch. Chem. Ges. 1929, 62, 920–924. pubs.acs.org/Organometallics

Published on Web 09/18/2009

magnesium dihalides and diorganomagnesium species in diethyl ether (eq 1). Dioxane precipitated the Mg-X-containing species as polymeric dioxane adducts, leaving the R2Mg component in solution.

2RMgXhMgX2 þ R2 Mg

ð1Þ

The effect of different R groups and of the halide substituents on the Schlenk equilibrium was further investigated by Schlenk, Jr. and other groups.11,12 It was found that the addition of dioxane to solutions of Grignard reagents not only led to precipitation of MgX2 and RMgX as their dioxane adducts but also could shift the equilibrium to the right.13,14 This offered a simple and general approach to the preparation of halide-free diorganomagnesium compounds. Furthermore, the solvent and temperature dependence of the Schlenk equilibrium in the presence of dioxane was studied.15 Nowadays the dioxane precipitation method is a standard procedure to remove magnesium halides from their ether solutions. However, there have been very few structural investigations of dioxane adducts of organomagnesium compounds. The known examples showed that this bidentate cyclic ether (11) (12) (13) 1356. (14) (15)

Schlenk, W., Jr. Ber. Dtsch. Chem. Ges. 1931, 64, 734–736. Noller, C. R. J. Am. Chem. Soc. 1931, 53, 635–643. Noller, C. R.; White, W. R. J. Am. Chem. Soc. 1937, 59, 1354– Wright, G. F. J. Am. Chem. Soc. 1939, 61, 1152–1156. Cope, A. C. J. Am. Chem. Soc. 1935, 57, 2238–2240. r 2009 American Chemical Society

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can act as a monodentate ligand, as in the carborate [(diox)2Mg(2-Me-1,2-C2B10H10)2],16 and as a bridging ligand, as in polymeric [(diox)Mg(CH2tBu)2]¥17 and in the related molecular magnesium amide [(R2N)2Mg( μ-diox)Mg(NR2)2] with three-coordinate magnesium atoms (R = SiMe3).18 Herein we report new examples of the structural diversity of such dioxane adducts as well as a new approach to the investigation of the decomposition products typically formed in Grignard reactions based on the dioxane method.

2. Results and Discussion 2.1. Synthesis and Structures of Multinuclear Organomagnesium Compounds. While the addition of 1,4-dioxane to diethyl ether solutions of Grignard reagents results in precipitation of [( μ-diox)MgX2]¥ and [( μ-diox)Mg(R)X]¥,10,11 [( μ-diox)MgR2]¥ also precipitates, when a 1:1 dioxane to magnesium ratio is used.16,17 The addition of further dioxane leads to partial cleavage of the [( μ-diox)MgR2]¥ polymer chain into smaller fragments, making the R2Mg component soluble while the dihalide species remain insoluble. If [( μ-diox)Mg(R)X]¥ has a higher solubility than [( μ-diox)MgX2]¥ in the presence of an excess of dioxane, it disproportionates via the Schlenk equilibrium to insoluble magnesium dihalide and soluble [(diox)nþ1(MgR2)n].13 The amount of dioxane necessary to achieve solubility of [(diox)nþ1(MgR2)n] species strongly depends on the substituent R. With a large excess of dioxane monomeric compounds should be observed, as reported in the case of [(diox)2Mg(2-Me-1,2-C2B10H10)2].16 Cooling diethyl ether solutions of [(diox)nþ1(MgR2)n] often results in re-formation of polymeric [(diox)MgR2]¥ by liberation of dioxane from oligomeric or monomeric species. This behavior enables the isolation of crystalline products of the type [(diox)MgR2]¥. Crystals of [( μ-diox-O, O0 )Mg(cyclo-C6H11)2]¥ (1) were obtained in this way. A chain structure with bridging dioxane molecules was found. The crystal structure determination was hampered due to the very small crystals of poor quality that were obtained. However, the connectivity that was established is shown in Figure 1. Similar structures also have been observed for [( μ-diox-O,O0 )Mg(CH2tBu)2]¥,17 [( μ-diox-O,O0 )MgPh2]¥,19 and [( μ-diox-O,O0 )MgEt2]¥.20 Diethyl ether solutions of the dibenzylmagnesium dioxanate 2 tend to oversaturate when cooled, and no crystallization took place at low temperatures. Crystals of this compound slowly formed when a saturated solution in diethyl ether was allowed to stand at room temperature for a long time, but the isolated yields were low due to its high solubility under the applied conditions. Although it is closely related to the above-mentioned complexes, the dibenzylmagnesium dioxanate 2 shows a different structure in the solid state, an example of the structural diversity of such dioxane adducts. In [(diox)Bz2Mg( μ-diox-O,O0 )Mg(diox)Bz2] (2; Bz = benzyl, CH2Ph) both (16) Clegg, W.; Brown, D. A.; Bryan, S. J.; Wade, K. J. Organomet. Chem. 1987, 325, 39–46. (17) Parvez, M.; Pajerski, A. D.; Richey, H. G. Acta Crystallogr. 1988, C44, 1212–1215. (18) Her, T. Y.; Chang, C. C.; Lee, G. H.; Peng, S. M.; Wang, Y. J. Chin. Chem. Soc. 1993, 40, 315–317. (19) G€ artner, M.; Fischer, R.; Langer, J.; G€ orls, H.; Walther, D.; Westerhausen, M. Inorg. Chem. 2007, 46, 5118–5124. (20) Fischer, R.; Walther, D.; Gebhardt, P.; G€ orls, H. Organometallics 2000, 19, 2532–2540.

Figure 1. Part of the chainlike structure of [(μ-diox-O,O0 )Mg(cHex)2]¥ (1). Symmetry-related atoms are marked with the letters A (-x þ 1, y, -z - 1/2) and B (-x þ 2, y, -z þ 1/2).

Figure 2. Molecular structure and numbering scheme of [(diox)Bz2Mg(μ-diox-O,O0 )Mg(diox)Bz2] (2). Symmetry-related atoms are marked with the letter A (-x þ 1, -y, -z þ 1). The thermal ellipsoids represent a probability of 40%; hydrogen atoms are not shown for clarity reasons. Selected bond lengths (A˚): Mg-C1=2.156(2), Mg-C8=2.155(2), Mg-O1= 2.051(2), Mg-O3 = 2.059(2), C1-C2 = 1.476(3), C2-C3 = 1.400(3), C2-C7 = 1.401(3), C8-C9 = 1.475(3), C9-C10 = 1.403(3), C9-C14 = 1.408(3). Selected angles (deg): C1-MgC8 = 123.75(9), C1-Mg-O1 = 112.05(8), C1-Mg-O3 = 105.54(8), C8-Mg-O1 = 111.31(8), C8-Mg-O3 = 108.23(8), O1-Mg-O3 = 90.39(6), Mg-C1-C2 = 112.7(1), Mg-C8 = 108.5(1).

coordination modes of 1,4-dioxane are realized: in this dinuclear complex, which is the smallest representative of the general formula [(diox)nþ1(MgR2)n] (n=2) in addition to the monomer, the magnesium atoms are connected via a bridging 1,4-dioxane-O,O0 molecule. Each magnesium atom completes its tetrahedral coordination sphere with another dioxane ligand. The molecular structure and numbering scheme are shown in Figure 2. The magnesium atoms bind to the methylene carbon atoms of the benzyl substituents with no short contacts to the π-systems of the phenyl groups. Thus, the benzyl unit behaves as an alkyl group without charge delocalization from the carbanionic CH2 unit into the phenyl group. The Mg-C-CPh angles are in the expected range of alkylmagnesium derivatives.7-9 Similar dioxane-bridged dinulclear magnesium compounds have been observed before, but in those cases additional coordination sites for dioxane have been blocked by either bulky substituents18 or chelating ligands,21 thus preventing polymerization. (21) Blackmore, I. J.; Gibson, V. C.; Hitchcock, P. B.; Rees, C. W.; Williams, D. J.; White, A. J. P. J. Am. Chem. Soc. 2005, 127, 6012–6020.

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Compound 2 illustrates that other species in addition to the polymer and the monomer do exist, making compounds of the type [(diox)nþ1(MgR2)n] an interesting subject for the study of self-organization with and without additional templates. 2.2. Ether Scission by Mesityl Grignard Reagents. In organolithium chemistry ether cleavage reactions are rather common and well understood.22 Depending on the substituents of the ether, R, β, and R0 ,β-elimination was observed. In the case of diethyl ether all three mechanisms would lead to ethene and lithium ethoxide (eq 2). Deuteration experiments have shown that β-elimination is the dominating reaction pathway,23 whereas THF is R-deprotonated and decomposes to ethene and the lithium enolate of acetaldehyde.24,25 The metalation of 2,6-Me2-4-ClC6H2OH with tert-butyllithium in the presence of dioxane led to cleavage of 1,4-dioxane with formation of LiO(CH2)2OCHdCH2, which was crystallized as hexanuclear [(Aryl-O)2(R-O)4Li6(diox)] (Aryl = 2,6-Me24-ClC6H2; R = CH2CH2OCHdCH2).26

RLi þ CH3 CH2 OCH2 CH3 f RH þ H2 CdCH2 þ LiOCH2 CH3

ð2Þ

In the case of organomagnesium reagents such ether fragmentation reactions are less pronounced during preparation and storage, making the isolation of cleavage products more difficult in presence of a huge excess of Grignard reagent. Adaptation of the protocol described above in order to address this challenge led us to the system MesMgBr/1,4dioxane (Mes=2,4,6-trimethylphenyl). In this case the compounds [( μ-diox)MgBr2]¥ and [( μ-diox)Mg(Mes)2]¥ are both only sparingly soluble under typical conditions, giving an opportunity to remove most of the components of the Schlenk equilibrium from the reaction mixture and leaving the ether degradation products the major species in solution. This procedure allowed us to crystallize, isolate, and characterize two such products, [(diox)Mg(Mes)( μ-OEt)]2 (3) and [(EtOCH(Me)CH(Me)OEt)Mg(Mes)2] (4). Derivative 3 represents an expected diethyl ether degradation product. Its molecular structure and numbering scheme are shown in Figure 3. The ethoxide groups occupy bridging positions between the two magnesium atoms. Despite their bridging position, the ethoxide ligands show Mg-O bonds shorter than those of the coordinated ether molecules due to electrostatic attraction between the magnesium cation and the ethoxide anions. A comparable structure was observed for [{(Me3Si)2N}Mg( μ-OEt)(thf)]2;27 however, the origin of the ethoxide anion in this compound is not quite clear.28 (22) Maercker, A. Angew. Chem. 1987, 99, 1002-1019; Angew. Chem., Int. Ed. Engl. 1987, 26, 972-989. (23) Maercker, A.; Demuth, W. Angew. Chem. 1973, 85, 90-92; Angew. Chem., Int. Ed. Engl. 1973, 12, 75-76. (24) Rembaum, A.; Siao, S.-P.; Indictor, N. J. Polym. Sci. 1962, 56, S17–S19. (25) Bates, R. B.; Kroposki, L. M.; Potter, D. E. J. Org. Chem. 1972, 37, 560–562. (26) Randazzo, J.; Morris, J. J.; Rood, J. A.; Noll, B. C.; Henderson, K. W. Inorg. Chem. Commun. 2008, 11, 1270–1272. (27) Yang, K.-C.; Chang, C.-C.; Huang, J.-Y.; Lin, C.-C.; Lee, G.-H.; Wang, Y.; Chiang, M. Y. J. Organomet. Chem. 2002, 648, 176–187. (28) The synthesis of [{(Me3Si)2N}Mg( μ-OEt)(thf )]2 as described in ref 22 is somewhat unclear. The reaction equation of MgEt2 with HN(SiMe3)2 in THF shows the formation of the diethyl ether complex [{(Me3Si)2N}Mg( μ-OEt)(OEt2)]2. However, the molecular structure clearly displays the THF adduct [{(Me3Si)2N}Mg( μ-OEt)(thf )]2. Therefore, the origin of the bridging ethanolate group remains unclear, because it could stem from ether cleavage reactions or from insertion of oxygen into a Mg-Et unit of starting diethylmagnesium.

Figure 3. Molecular structure and numbering scheme of [(diox)Mg(Mes)(μ-OEt]2 (3). Symmetry-related atoms (-x þ 1, -y þ 2, -z) are marked with the letter A. The thermal ellipsoids represent a probability of 40%; H atoms are neglected for clarity reasons. Selected bond lengths (A˚): Mg1-C3 = 2.159(3), Mg1O1=1.961(2), Mg1-O1A=1.966(2), Mg1-O2=2.057(2). Selected angles (deg): Mg1-C3-C4 = 120.8(2), Mg1-C3-C8 = 124.0(2), C4-C3-C8 = 115.2(2), Mg1-O1-Mg1A = 96.75(9), C3-Mg1-O1 = 127.5(1), C3-Mg1-O1A = 130.5(1), C3Mg1-O2 = 104.6(1), O2-Mg-O1 = 104.26(9), O2-Mg1O1A = 102.5(1), O1-Mg1-O1A = 83.25(9).

Mulvey and co-workers reported the formation of the closely related [(thf)Mg(But)( μ-OBut)]2 with an alkoxide ligand arising from oxygen insertion into the metal-carbon bond.29 Other comparable alkoxide- or phenoxide-bridged dimeric organomagnesium compounds30,31 are also known, but in none of these cases does the alkoxide stem from ether cleavage reactions. Crystalline 4 was obtained after reduction of the volume of the mother liquor and cooling of the residual solution which contained the coligand 2,3-diethoxybutane. Compound 4 is the first well-defined metal complex containing this bidentate ligand, even though ether cleavage reactions have been studied for decades. However, the formation of 2,3-diethoxybutane was observed earlier during γ-irradiation of diethyl ether32,33 as well as in Grignard reactions of magnesium with a substituted bromocyclopropane in diethyl ether.34,35 In agreement with these references, the formation of the neutral coligand 2,3-diethoxybutane of 4 suggests a radical mechanism involving the abstraction of an R-hydrogen atom of Et2O and recombination of two such radicals. Replacement of two dioxane molecules from [(diox)nþ1(MgMes2)n] (n = 1) by chelating 2,3-diethoxybutane then gives mononuclear 4. 2.3. Further Structural Investigations of Mesitylmagnesium Compounds. In addition to compounds 3 and 4, mononuclear [(diox)2Mg(Mes)2] (5) was isolated from the very concentrated mother liquor that now contained a high concentration of dioxane. Mononuclear [(diox)2Mg(Mes)2] (5) crystallized with two crystallographically independent molecules which are distinguished by the letters A and B. (29) Conway, B.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; Weatherstone, S. Dalton Trans. 2005, 1532–1544. (30) Squiller, E. P.; Whittle, R. R.; Richey, H. G., Jr. Organometallics 1985, 4, 1154–1157. (31) Zhang, D.; Kawaguchi, H. Organometallics 2006, 25, 5506–5509. (32) Ng, M. K. M.; Freeman, G. R. J. Am. Chem. Soc. 1965, 87, 1635– 1639. (33) Ng, M. K. M.; Freeman, G. R. J. Am. Chem. Soc. 1965, 87, 1639– 1643. (34) Hamdouchi, C.; Topolski, M.; Goedken, V.; Walborsky, H. M. J. Org. Chem. 1993, 58, 3148–3155. (35) Garst, J. F.; Lawrence, K. E.; Batlaw, R.; Boone, J. R.; Ungvary, F. Inorg. Chim. Acta 1994, 222, 365–375.

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Figure 4. Molecular structure of molecule A of [(EtOCH(Me)CH(Me)OEt-O,O0 )Mg(Mes)2] (4). The thermal ellipsoids represent a probability of 40%; H atoms are not drawn for clarity reasons. Selected bond lengths (A˚): MgA-C1A = 2.165(5), MgA-C10A = 2.166(4), MgA-O1A = 2.114(3), MgAO2A = 2.100(3). Selected angles (deg): MgA-C1A-C2A = 126.3(3), MgA-C1A-C6A = 118.3(3), C2A-C1A-C6A = 115.4(4), MgA-C10A-C11A = 119.1(3), MgA-C10A-C15A = 125.2(3), C11A-C10A-C15A = 115.6(4).

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Figure 6. Molecular structure of [(bpy)Mg(Mes)2] (6). The thermal ellipsoids represent a probability of 40%; H atoms are neglected for clarity reasons. Selected bond lengths (A˚): Mg1-C1 = 2.177(2), Mg1-C10 = 2.169(2), Mg1-N1 = 2.166(2), Mg1-N2 = 2.181(2). Selected angles (deg): Mg1-C1-C2 = 125.3(2), Mg1-C1-C6 = 119.8(1), C2-C1-C6 = 114.9(2), Mg1-C10-C11=118.5(1), Mg1-C10-C15=125.9(1), C11C10-C15 = 115.6(2).

The molecular structure and numbering scheme of molecule A are shown in Figure 5. The structural parameters are very similar to those of [(thf)2Mg(Mes)2].36 According to Seidel and B€ urger37 the addition of 2,20 bipyridine (bpy) to an Et2O solution of Mes2Mg at low temperatures yielded [(bpy)Mg(Mes)2] (6), which decomposes slowly at room temperature. Due to its instability in solution, crystallization was found to be rather challenging and had to be performed at low temperature (-78 C). The molecular structure and numbering scheme of 6 are shown in Figure 6.

To the best of our knowledge, compound 6 is the first structurally characterized 2,20 -bipyridine magnesium complex containing σ-bonded alkyl or aryl groups. This demonstrates the high reactivity of such adducts, normally resulting in decomposition products containing (bpy-)Mg species.38 Table 1 summarizes selected structural parameters of the mononuclear dimesitylmagnesium compounds 4-6 and the related THF adduct. In all of these complexes the magnesium atoms are in distorted-tetrahedral environments. The 1,4-dioxane ligand is slightly bulkier than the THF molecule, leading to a smaller C-Mg-C angle in 5 in comparison to that in [(thf)2Mg(Mes)2]. On the one hand, bidentate ligands with a small bite (intraligand O 3 3 3 O or N 3 3 3 N distances) lead to small O-Mg-O- and N-Mg-N angles. On the other hand, this narrow angle allows an increase in the C-Mg-C angle. The influence of the bulkiness of the neutral coligands on the Mg-C bond lengths is very small. However, less bulky aryl substituents allow shorter Mg-C bonds, as observed in [(thf)2Mg(Ph)2] (Mg-C = 2.127(4) A˚, Mg-O = 2.030(4) A˚),39 [(thf)2Mg(C6H4-4-Me)2] (Mg-C=2.126(7) and 2.132(8) A˚, Mg-O= 2.031(6) and 2.050(5) A˚),39 and [( μ-diox)Mg(Ph)2]¥ (Mg-C= 2.135(2) A˚, Mg-O = 2.081(2) and 2.061(2) A˚).19 In addition to the neutral mesitylmagnesium compounds described above, the ionic ate complex [(thf)4Li]þ[Mg(Mes)3]- (7) was prepared as described by Seidel and B€ urger37 and its molecular structure was determined (Figure 7). The [Mg(Mes)3]- anion contains a three-coordinate magnesium atom with an average Mg-C bond length of 2.168 A˚. Despite the smaller coordination number of magnesium, similar Mg-C bond lengths are observed as described above for mononuclear complexes with tetracoordinate Mg atoms, because steric and electrostatic repulsion between the mesityl groups does not allow a shorter Mg-C bond.

(36) Waggoner, K. M.; Power, P. P. Organometallics 1992, 11, 3209– 3214. (37) Seidel, W.; B€ urger, I. Z. Anorg. Allg. Chem. 1978, 447, 195–198.

(38) Kaim, W. Chem. Ber. 1981, 114, 3789–3800. (39) Markies, P. R.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Smeets, W. J. J.; van der Sluis, P.; Spek, A. L. J. Organomet. Chem. 1990, 393, 315–331.

Figure 5. Molecular structure of molecule A of [(diox)2Mg(Mes)2] (5). The thermal ellipsoids represent a probability of 40%; H atoms are omitted for clarity reasons. Selected bond lengths (A˚): MgA-C1A = 2.169(2), MgA-C10A = 2.167(3), MgA-O1A=2.107(2), MgA-O3A=2.084(2). Selected angles (deg): MgA-C1A-C2A = 127.7(2), MgA-C1A-C6A = 116.9(2), C2A-C1A-C6A = 115.0(2), MgA-C10A-C11A = 127.3(2), MgA-C10A-C15A = 117.1(2), C11A-C10A-C15A = 115.4(2).

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Table 1. Comparison of Selected Bond Lengths (A˚) and Angles (deg) of [(thf)2Mg(Mes)2],28 Molecule A of [(EtOCH(Me)CH(Me)OEt)Mg(Mes)2] (4), Molecule A of [(diox)2Mg(Mes)2] (5), and [(bpy)Mg(Mes)2] (6) [(thf)2Mg(Mes)2]

4

5

6

Mg1-C1 Mg1-C10 Mg1-O/N

2.182(3) 2.165(3) 2.067(3) 2.079(3)

2.165(5) 2.166(4) 2.114(3) 2.100(3)

2.169(2) 2.167(3) 2.107(2) 2.084(2)

2.177(2) 2.169(2) 2.166(2) 2.181(2)

C1-Mg-C10 C1-Mg-O/N

118.8(1) 105.5(1) 121.8(1) 118.3(1) 100.9(1) 88.4(1)

121.2(2) 120.9(2) 104.7(1) 105.8(2) 121.1(2) 75.3(1)

117.55(9) 103.14(8) 121.80(9) 122.89(9) 101.23(8) 87.90(8)

122.68(7) 104.68(7) 117.85(7) 120.99(7) 106.43(6) 75.28(6)

C10-Mg-O/N O/N-Mg-O/N

Scheme 1. Reactivity Diagram of “MesMgBr”

Regardless of the coligands, the mesityl group shows characteristic distortions. The C-C-C angle at the ipso carbon atom is rather narrow in all compounds discussed above. Repulsive forces between the free electron pair with the anionic charge at the ipso carbon atom and the neighboring C-C bonds enforce a reduction of the endocyclic C-C-C bond angles. This feature is characteristic not only for magnesium derivatives but for all aryl compounds with electropositive metals (i.e., aryl metal derivatives with largely ionic metal-carbon bonds). The chemistry of dimesitylmagnesium discussed above is summarized in Scheme 1.

3. Conclusion

Figure 7. Molecular structure and numbering scheme of solvent-separated [(thf)4Li]þ[Mg(Mes)3]- (7). The thermal ellipsoids represent a probability of 40%; H atoms are omitted for clarity reasons. Selected bond lengths (A˚): Mg-C1 = 2.168(2), Mg1-C10=2.169(2), Mg1-C19=2.166(2), Li1-O1=1.910(5), Li1-O2 = 1.938(5), Li1-O3 = 1.929(2), Li1-O4 = 1.905(5). Selected angles (deg): Mg1-C1-C2 = 121.3(2), Mg1-C1-C6 = 123.2(2), C2-C1-C6 = 115.5(2), Mg1-C10-C11 = 121.8(2), Mg1-C10-C15=122.1(2), C11-C10-C15=116.0(2), Mg1C19-C20 = 125.0(2), Mg1-C19-C24 = 119.4(2), C20-C19C24 = 115.4(2).

This compound crystallizes isomorphous to the manganese derivative [(thf)4Li]þ[Mn(Mes)3]-.40 Larger aryl groups such as 2,4,6-triisopropylphenyl substituents lead to the magnesiate [Mg(C6H2-2,4,6-iPr3)3]-, with longer and differing Mg-C bonds of 2.249(4), 2.206(4), and 2.147(4) A˚ and strong distortions (C-Mg-C angles vary between 105.0(1) and 131.2(2)).36 Smaller aryl groups allow larger coordination numbers at the magnesium atom. Thus, a triphenylmagnesiate anion dimerizes, yielding [Ph2Mg( μ-Ph)2MgPh2]2- anions,41 or this moiety completes its coordination sphere via addition of a Lewis base such as THF, as in [(thf)MgPh3]-,42 or via addition of another phenyl group, as in [MgPh4]2-.40 Very bulky aryl groups are able to stabilize even two-coordinate magnesium atoms, as in [Mg(C6H22,4,6-tBu3)2] (Mg-C = 2.118(3) and 2.120(3) A˚).43 (40) Bartlett, R. A.; Olmstead, M. M.; Power, P. P.; Shoner, S. C. Organometallics 1988, 7, 1801–1806. (41) Thoennes, D.; Weiss, E. Chem. Ber. 1978, 111, 3726–3731. (42) Pajerski, A. D.; Kushlan, D. M.; Parvez, M.; Richey, H. G. Organometallics 2006, 25, 1206–1212. (43) Wehmschulte, R. J.; Power, P. P. Organometallics 1995, 14, 3264–3267.

In this study we have investigated the structural diversity of dioxane adducts of diorganomagnesium compounds of the general formula [(diox)nþ1(MgR2)n]. The compounds isolated range from monomeric [(diox)2Mg(Mes)2] (5) and dimeric [(diox)Bz2Mg( μ-diox-O,O0 )Mg(diox)Bz2] (2) to polymeric [( μ-diox-O,O0 )Mg(cyclo-C6H11)2]¥ (1). Additionally, the dioxane method was used to investigate diethyl ether fragmentation during the synthesis and storage of MesMgBr in diethyl ether. The two products obtained, [(diox)Mg(Mes)( μ-OEt)]2 (3) and [(EtOCH(Me)CH(Me)OEt)Mg(Mes)2] (4), suggest that radical processes as well as elimination reactions play a significant role in diethyl ether cleavage by mesitylmagnesium species.

4. Experimental Section 4.1. General Considerations. All manipulations were carried out under an argon atmosphere using standard Schlenk techniques. The solvents were dried according to common procedures and distilled under argon; deuterated solvents were dried over sodium, degassed, and saturated with argon. The yields given are not optimized. The 1H and 13C{1H} NMR spectra were obtained on a Bruker AC 400 MHz spectrometer. Mass spectra were obtained on a Finnigan MAT SSQ 710 system, and IR measurements were carried out using a Perkin-Elmer System 2000 FTIR. The IR spectra were taken as Nujol mulls between KBr windows. Melting and decomposition points were measured with a Reichert-Jung apparatus, type 302102, and are uncorrected. The magnesium content was determined by complexometric titration of a hydrolyzed aliquot (after treatment with HNO3) with 0.05 M EDTA using Eriochrome Black T as the indicator.44 The starting materials mesitylmagnesium bromide in THF or Et2O and [(thf)2Mg(Mes)2] were prepared according (or analogously (44) M€ uller, G.-O. Lehr- und U¨bungsbuch der anorganisch-analytischen Chemie, 7th ed.; Verlag Harri Deutsch: Frankfurt am Main, 1992; Vol. 3 (Quantitativ-anorganisches Praktikum).

Article in the case of diethyl ether) to literature procedures.37 Benzylmagnesium chloride in diethyl ether was purchased from Aldrich. 4.2. Synthesis of [( μ-diox-O,O0 )Mg(cyclo-C6H11)2]¥ (1). Dioxane (14 mL, 0.164 mol) was added dropwise with rapid stirring to a solution of [(cyclo-C6H11)MgBr] freshly prepared from magnesium (3.0 g, 0.123 mol) and cyclohexyl bromide (16.3 g, 0.1 mol) in ether (100 mL). The resulting white suspension was allowed to stand overnight and filtered afterward. The white residue was washed with ether (20 mL) and discarded. The combined ether solutions were stored at -20 C for 3 days, yielding colorless crystals which were isolated by filtration and dried in vacuo. Yield: 5.8 g (42%). Mp: >270 C. Anal. Calcd for C16H30MgO2 (278.72 g mol-1): Mg, 8.5. Found: Mg, 8.7. 1H NMR (400.25 MHz, 25 C, [D6]benzene/[D8]THF (3:1)): δ 0.14 (2H, m, HC1), 1.48-3.19 (20H, m, H2,20 -H4), 3.38 (8H, s, CH2O, diox). 13C{1H} NMR (50.32 MHz, 25 C, [D6]benzene/ [D8]THF (3:1)): δ 25.9 (2C, C1), 29.7 (2C, C4), 32.3 (4C, C3,30 ), 35.9 (4C, C2,20 ), 66.9 (12C, CH2O, dx). MS (EI, m/z [%]): 88 (diox) [10], 82 (C6H10) [85], 67 (C5H7) [40], 57 (C4H9) [100], 44 (C2H2O) [60]. IR (Nujol, KBr, ν, cm-1): 2917, vs (br); 2798, vs; 2756, m; 2360, w; 1659, w; 1631, w; 1548, w; 1454, vs; 1376, s; 1299, m; 1257, s; 1151, m; 1106, s; 1074, vs; 1040, m; 1022, m; 965, m; 893, s; 860, vs; 830, m; 791, w; 721, w; 615, s. Suitable crystals for X-ray diffraction experiments were obtained directly from the reaction mixture. 4.3. Synthesis of [(diox)Bz2Mg( μ-diox-O,O0 )Mg(diox)Bz2] (2). Dioxane (15 mL, 0.176 mol) was added dropwise with rapid stirring to a solution of [BzMgCl] in ether (80 mL, 0.9 M). The resulting white suspension was allowed to stand overnight and filtered afterward. The white residue was washed with ether (20 mL) and discarded. The combined ether solutions were stored at room temperature for 3 months. The colorless crystals that had formed were isolated by filtration and dried in vacuo. Yield: 1.31 g (1.93 mmol, 11%). Decomposition above 43 C. Anal. Calcd for C40H52Mg2O6 (677.47 g mol-1): Mg, 7.2. Found: Mg, 7.1. 1H NMR (400.25 MHz, 25 C, [D6]benzene/ [D8]THF (3:1)): δ 1.59 (8H, s, PhCH2), 3.32 (24H, s, CH2O, dx), 6.54 (4H, t, 3J=7.2 Hz, p-H), 6.89 (8H, dt, 3J=7.2 Hz, m,m0 -H), 6.96 (8H, t, 3J = 7.2 Hz, o,o0 -H). 13C{1H} NMR (100.65 MHz, 25 C, [D6]benzene/[D8]THF (3:1)): δ 23.1 (4C, PhCH2), 67.6 (12C, OCH2, diox), 116.1 (4C, p-C), 123.6 (8C, m-C), 129.2 (8C, o-C), 157.2 (4C, i-C). MS (EI, m/z [%]): 91 (C7H7) [100], 88 (diox) [30], 79 (C6H7) [55], 77 (C6H5) [40]. IR (Nujol, KBr, ν, cm-1): 2918, vs (b); 2854, vs; 1937, w; 1711,w; 1585, s; 1477, s; 1453, vs; 1409, w; 1377, s; 1299, m; 1256, s; 1208, s; 1174, m; 1122, s; 1105, m; 1071, m; 1061, m; 1044, m; 1026, m; 998, w; 912, s; 895, m; 873, vs; 827, m; 713, s; 750, s; 727, m; 703, s; 617, m; 577, m; 551, m; 521, m. The crystals obtained as described above were suitable for X-ray diffraction experiments. 4.4. Synthesis of [(diox)Mg(Mes)( μ-OEt)]2 (3), [(EtOCH(Me)CH(Me)OEt)Mg(Mes)2] (4), and [(diox)2Mg(Mes)2] (5). Dioxane (5.1 mL, 60 mmol) was added dropwise with rapid stirring to a freshly prepared solution of [MesMgBr] in diethyl ether (50 mL, 0.8 M). The resulting off-white suspension was allowed to stand overnight and was diluted afterward with additional ether (40 mL). Filtration gave 50 mL of a pale yellow solution, while the rest of the solvent remained in the voluminous residue. This solution was stored for 3 days at 0 C, and a small amount of precipitate that had formed was removed by filtration. Then the volume of the mother liquor was reduced to 10 mL by evaporation of the solvent in vacuo. Colorless crystals of 4 (110 mg, 0.27 mmol, 2.7%) were obtained from this solution at 0 C overnight and isolated by decantation. The mother liquor was further concentrated to 4 mL and stored again at 0 C for 1 week. The newly formed crystals were isolated by filtration and dried in vacuo. Yield: 450 mg; mixture of 3 (190 mg, 0.36 mmol, 3.6%) and 5 (260 mg, 0.59 mmol, 3.0%) as judged by NMR. Data for 3 are as follows. 1H NMR (200.13 MHz, 25 C, [D8]THF): δ 1.19 (6H, t, 3JH-H = 7.0 Hz, CH3 OEt), 2.16 (6H, s, p-CH3), 2.41

Organometallics, Vol. 28, No. 19, 2009

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(12H, s, o-CH3), 3.54 (16H, s, OCH2 diox), 3.93 (4H, q, 3JH-H = 7.0 Hz, CH2 OEt), 6.61 (4H, s, m-CH). 13C{1H} NMR (50.33 MHz, 25 C, [D8]THF): δ 21.4 (2C, p-CH3), 22.2 (2C, CH3 OEt), 28.3 (4C, o-CH3), 67.7 (8C, OCH2 diox), 124.8 (4C, m-CH), 133.7 (2C, p-CH), 147.3 (4C, o-C), 162.3 (2C, i-C). Data for 4 are as follows. 1H NMR (200.13 MHz, 25 C, [D8]THF): δ 0.97 (6H, m, CHCH3), 1.07 (6H, t, 3JH-H =7.0 Hz, CH3 OEt), 2.07 (6H, s, p-CH3), 2.26 (12H, s, o-CH3), 3.3-3.5 (6H, m, CH2 OEt þ CHCH3), 6.48 (4H, s, m-CH). 13C{1H} NMR (50.33 MHz, 25 C, [D8]THF): δ 14.3 (2C, CHCH3), 16.0 (2C, CH3 OEt), 21.4 (2C, p-CH3), 27.6 (4C, o-CH3), 64.9 (2C, OCH2), 77.5 (2C, CHCH3), 125.0 (4C, m-CH), 133.1 (2C, p-CH), 141.3 (4C, o-C), 165.2 (2C, i-C). Data for 5 are as follows. 1H NMR (200.13 MHz, 25 C, [D8]THF): δ 2.14 (6H, s, p-CH3), 2.33 (12H, s, oCH3), 3.54 (16H, s, OCH2 diox), 6.56 (4H, s, m-CH). 13C{1H} NMR (50.33 MHz, 25 C, [D8]THF): δ 21.4 (2C, p-CH3), 27.7 (4C, o-CH3), 67.7 (8C, OCH2 diox), 125.0 (4C, m-CH), 133.2 (2C, p-CH), 147.1 (4C, o-C), 165.3 (2C, i-C). The NMR data for 3 and 5 were obtained from an isolated mixture of these compounds. The signals were assigned on the basis of twodimensional NMR experiments (H,H-COSY, HMBC, HSQC) as well as their different signal intensities. The crystals of 3-5 obtained directly from the reaction mixture were suitable for X-ray diffraction measurements. Alternatively, crystals of 5 3 (toluene) in extremely low yields were obtained by extraction of the voluminous precipitate formed during the dioxane addition (see above) with a hot mixture (60 C) of toluene and dioxane (4:1) and subsequent cooling of the resulting solution to -40 C. 4.5. Synthesis of [(bpy)Mg(Mes)2] (6). Solid [(thf)2Mg(Mes)2] (0.52 g, 1.28 mmol) was suspended in Et2O (15 mL) and a solution of 2,20 -bipyridine (0.20 g, 1.28 mmol) in Et2O (5 mL) was added dropwise at -40 C. Stirring of the mixture was continued for 4 h at -20 C, after which yellow microcrystals had formed (the mother liquor turned dark red). Separation, washing with Et2O (4 mL), and drying in vacuo yielded 0.46 g (1.10 mmol, 86%) of pyrophoric 6. Decomposition above 150 C. Anal. Calcd for C28H30MgN2 (418.88 g mol-1): Mg, 5.8. Found: 5.5. 1H NMR (400.25 MHz, 25 C, [D8]THF): δ 1.31 (6H, s, p-CH3), 2.23 (12H, s, o-CH3), 6.75 (4H, s, m-CH), 7.46 (2H, t, 3 JH-H = 7.6 Hz, H4,40 -bpy), 7.77 (2H, d, 3JH-H = 7.6 Hz, H5,50 bpy), 8.49 (2H, br, H6,60 -bpy), 8.63 (2H, br, H3,30 -bpy). 13C{1H} NMR (100.65 MHz, 25 C, [D8]THF): δ 21.3 (2C, p-CH3), 27.3 (4C, o-CH3), 121.3 (2C, C6,60 -bpy), 127.5 (4C, m-CH), 128.2(2C, p-CH), 128.9 (2C, C5,50 -bpy), 129.5 (2C, C4,40 -bpy), 138.0 (4C, o-C), 149.9 (2C, C3,30 -bpy), 157.0 (2C, C1,10 -bpy), 172.1 (2C, i-C). MS (EI-, m/z [%]): 121 (MesH) [100], 105 (C8H9þ) [30], 77 (py) [38]. Single crystals suitable for X-ray crystallographic measurements were obtained by recrystallization of 6 in a mixture of diethyl ether and tetrahydrofuran (10:1) at -78 C. 4.6. Synthesis of [Li(thf)4][Mg(Mes)3] (7). Solid [(thf)2Mg(Mes)2] (0.65 g, 1.60 mmol) was dissolved in THF (5 mL), and LiMes (1.96 mL, 1.60 mmol, 0.814 M) in THF was added at 0 C. After the mixture was stirred at room temperature for 12 h, Et2O (10 mL) was added and the yellow solution was filtered. Cooling of the filtrate to -40 C led to precipitation of colorless crystals. Separation, washing with n-pentane (10 mL), and drying in vacuo yielded 0.72 g (1.06 mmol, 66%) of [Li(thf)4][Mg(Mes)3] (7). Decomposition above 134 C. 1H NMR (400.25 MHz, 25 C, [D8]THF): δ 1.78 (16H, m, CH2, thf), 2.08 (9H, s, p-CH3), 2.35 (18H, s, o-CH3), 3.61 (16H, m, CH2O, thf), 6.40 (6H, s, m-H). 13C{1H} NMR (100.65 MHz, 25 C, [D8]THF): δ 21.6 (3C, p-CH3), 25.2 (8C, CH2, thf), 27.9 (6C, o-CH3), 67.5 (6C, CH2O, thf), 123.8(6C, m-C), 131.2 (3C, p-C), 147.0 (6C, o-C), 172.1 (3C, i-C). 7Li NMR (155.55 MHz, 25 C, [D8]THF): δ -0.82. MS (ESI-, m/z [%]): 381 (MgMes3) [15], 353 (MgMes3 - 2Me) [25], 339 (MgMes3 - 3Me) [30]. IR (Nujol, KBr, ν, cm-1): 2924, vs (br); 2361, w; 1608, m; 1588, w; 1460, s; 1376, s; 1310, w; 1254, w; 1208, w; 1045, m; 887, m; 834, m; 721, w; 687, m; 548, m. Crystals suitable for X-ray diffraction experiments were obtained directly from the reaction mixture.

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Table 2. Crystal Data and Refinement Details for the X-ray Structure Determinations 1 formula

C16H30MgO2 fw 278.71 T/C -90(2) cryst syst monoclinic space group P21/c a/A˚ 18.139(3) b/A˚ 7.785(1) c/A˚ 12.404(2) R/deg 90.00 β/deg 106.46(1) γ/deg 90.00 1679.8(4) V/A˚3 Z 4 1.102 F (g cm-3) -1 1.03 μ (cm ) no. of measd data 5129 no. of data with 2230 I > 2σ(I ) 3377/0.1030 no. of unique data/Rint wR2 (all data, on F2)a R1 (I > 2σ(I ))a sb resid dens/e A˚-3 Flack param CCDC No. not deposited

2

3

4

5 C26H38MgO4 438.87 -90(2) monoclinic P21 9.7105(5) 24.3161(13) 11.9075(4) 90.00 111.368(3) 90.00 2618.3(2) 4 1.113 0.94 17 130 6753

5 3 tol

6

7

C26H38MgO4 3 5 /8C7H8 496.46 -90(2) monoclinic P21/n 11.9417(2) 25.0853(3) 20.1698(3) 90.00 91.9630(10) 90.00 6038.54(15) 8 1.092 0.89 36 417 8563

C28H30MgN2 3 C4H10O 492.97 -90(2) monoclinic P21/n 11.6873(3) 14.1492(5) 18.4330(6) 90.00 105.589(2) 90.00 2936.06(16) 4 1.115 0.86 20 548 4356

[C16H32LiO4]þ[C27H33Mg]677.20 -90(2) orthorhombic P212121 15.1845(3) 16.4324(3) 17.2198(4) 90.00 90.00 90.00 4296.65(15) 4 1.047 0.77 30 397 6995

C40H52Mg2O6 677.44 -90(2) triclinic P1 7.9131(9) 9.8376(13) 12.2781(13) 96.614(6) 94.747(8) 102.338(7) 921.82(19) 1 1.220 1.1 5955 2594

C30H48Mg2O6 553.30 -90(2) rhombohedral R3 18.3120(9) 18.3120(9) 25.8739(9) 90.00 90.00 120.00 7513.9(6) 9 1.101 1.08 11 203 2138

C26H40MgO2 408.89 -90(2) triclinic P1 13.9143(5) 14.0687(5) 19.7294(6) 93.156(2) 97.783(2) 97.560(2) 3782.9(2) 6 1.077 0.88 25 903 9274

3969/0.0403

3374/0.0945

17 040/0.0434 10 844/0.0592 13 577/0.0492

6653/0.0444

9794/0.0475

0.1486

0.2124

0.3296

0.1675

0.1666

0.1328

0.2100

0.0528 0.0682 1.023 0.998 0.207/-0.296 0.164/-0.155

0.1019 0.0599 0.0673 0.0566 0.0572 0.986 1.005 1.017 1.013 1.018 0.868/-0.459 0.240/-0.224 0.647/-0.390 0.294/-0.279 0.177/-0.257 0.1(2) 0.1(3) 713880 713881 713882 713883 713884 713885 713886 P P P P P a Definition of the R indices: R1 = ( ||Fo| - |Fc||)/ |Fo|. wR2 = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2 with w-1 = σ2(Fo2) þ (aP)2. b s = { [w(Fo2 1/2 2 2 Fc ) ]/(No - Np)} .

4.7. Crystal Structure Determinations. The intensity data for the compounds were collected on a Nonius KappaCCD diffractometer using graphite-monochromated Mo KR radiation. Data were corrected for Lorentz and polarization effects but not for absorption effects.45,46 The structures were solved by direct methods (SHELXS)47 and refined by full-matrix least-squares techniques against Fo2 (SHELXL-97) (Table 2).48 All hydrogen atoms were included at calculated positions with fixed thermal parameters. All nondisordered non-hydrogen atoms were refined anisotropically.48 (45) COLLECT, Data Collection Software; Nonius BV, Delft, The Netherlands, 1998. (46) Otwinowski, Z.; Minor, W. Processing of X-Ray Diffraction Data Collected in Oscillation Mode. In Macromolecular Crystallography, Part A; Carter, C. W., Sweet, R. M., Eds.; Academic Press: San Diego, CA, 1997; Methods in Enzymology 276, pp 307-326. (47) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467–473. (48) Sheldrick, G. M. SHELXL-97 (Release 97-2); University of G€ ottingen, G€ ottingen, Germany, 1997.

Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (DFG, Bonn-Bad Godesberg, Germany). We also acknowledge financial support by the Fonds der Chemischen Industrie (Frankfurt/ Main, Germany). S.K. is grateful to the Verband der Chemischen Industrie (VCI/FCI) for a generous Ph.D. grant. We also thank Prof. D. Seyferth for helpful discussions. Supporting Information Available: CIF files giving data collection and refinement details as well as positional coordinates of all atoms. This material is available free of charge via the Internet at http://pubs.acs.org. In addition, crystallographic data (excluding structure factors) have also been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC713880 for 2, CCDC-713881 for 3, CCDC-713882 for 4, CCDC713883 for 5, CCDC-713884 for 5 3 (toluene), CCDC-713885 for 6, and CCDC-713886 for 7. Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (e-mail: [email protected]).