二氧六环和格氏试剂络合物的结构研究

/OrganometallicsPublished on Web 09/18/2009r 2009American Chemical Society5814Organometallics 2009,28,5814–5820DOI:10.1021/om90005671,4-Dioxane Adducts of Grignard Reagents:Synthesis,Ether Fragmentation Reactions,and Structural Diversity of GrignardReagent/1,4-Dioxane ComplexesJens Langer,*Sven Krieck,Reinald Fischer,Helmar G €orls,Dirk Walther,and Matthias WesterhausenInstitute of Inorganic and Analytical Chemistry,Friedrich-Schiller-Universit €at Jena,August-Bebel-Strasse 2,D-07743Jena,GermanyReceived January 25,2009The 1,4-dioxane precipitation method was employed to obtain solutions of R 2Mg 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)MgR 2]¥polymer into smaller fragments of the type [(diox)n þ1(MgR 2)n ].In addition to the polymeric compounds [(μ-diox)MgR 2]¥,as found for [(μ-diox-O ,O’)Mg(cyclo-C 6H 11)2]¥(1),other oligomers such as [(diox)3(Mg{CH 2Ph}2)2](2)can be obtained from such solutions.In the system MesMgBr/1,4-dioxane the compounds [(μ-diox)MgBr 2]¥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 ing 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)4-Li]þ[Mg(Mes)3]-(7).Molecular structures of all new compounds 2-7are reported.1.IntroductionOrganomagnesium compounds (Grignard reagents)1are a powerful tool in organic and organometallic chemistry as well as catalysis.2-6Their importance led to extensive in-vestigations of their molecular structures in solution and in the solid state,their physical and chemical properties.Sev-eral review articles 7-9describe 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 1929.10Schlenk and Schlenk,Jr.found that Grignard rea-gents (RMgX)exist in equilibrium with the correspondingmagnesium dihalides and diorganomagnesium species in diethyl ether (eq 1).Dioxane precipitated the Mg -X-con-taining species as polymeric dioxane adducts,leaving the R 2Mg component in solution.2RMgX h MgX 2þR 2Mgð1ÞThe effect of different R groups and of the halide substit-uents on the Schlenk equilibrium was further investigated by Schlenk,Jr.and other groups.11,12It was found that the addition of dioxane to solutions of Grignard reagents not only led to precipitation of MgX 2and RMgX as their dioxane adducts but also could shift the equilibrium to the right.13,14This 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 stu-died.15Nowadays the dioxane precipitation method is a standard procedure to remove magnesium halides from their ether solutions.However,there have been very few structural investiga-tions of dioxane adducts of organomagnesium compounds.The known examples showed that this bidentate cyclic ether*To whom correspondence should be addressed.Fax:þ49(0)3641948102.E-mail:nger@uni-jena.de.(1)Grignard,V.C.R.Hebd.S eances Acad.Sci.1900,130,1322–1325.(2)Bickelhaupt,anomet.Chem.1994,475,1–14.(3)Wakefield,anomagnesium 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,mun.2006,583–593.(7)Markies,P.R.;Akkerman,O.S.;Bickelhaupt, F.;Smeets,W.J.J.;Spek,anomet.Chem.1991,32,147–226.(8)Holloway,C.E.;Melnik,anomet.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.(11)Schlenk,W.,Jr.Ber.Dtsch.Chem.Ges.1931,64,734–736.(12)Noller,C.R.J.Am.Chem.Soc.1931,53,635–643.(13)Noller,C.R.;White,W.R.J.Am.Chem.Soc.1937,59,1354–1356.(14)Wright,G.F.J.Am.Chem.Soc.1939,61,1152–1156.(15)Cope,A.C.J.Am.Chem.Soc.1935,57,2238–2240.Article Organometallics,Vol.28,No.19,20095815 can act as a monodentate ligand,as in the carborate[(diox)2-Mg(2-Me-1,2-C2B10H10)2],16and as a bridging ligand,as inpolymeric[(diox)Mg(CH2t Bu)2]¥17and in the related mole-cular magnesium amide[(R2N)2Mg(μ-diox)Mg(NR2)2]withthree-coordinate magnesium atoms(R=SiMe3).18Herein we report new examples of the structural diversityof such dioxane adducts as well as a new approach to theinvestigation of the decomposition products typicallyformed in Grignard reactions based on the dioxane method.2.Results and Discussion2.1.Synthesis and Structures of Multinuclear Organomag-nesium Compounds.While the addition of1,4-dioxane todiethyl ether solutions of Grignard reagents results in pre-cipitation of[(μ-diox)MgX2]¥and[(μ-diox)Mg(R)X]¥,10,11[(μ-diox)MgR2]¥also precipitates,when a1:1dioxane tomagnesium ratio is used.16,17The addition of further dioxaneleads to partial cleavage of the[(μ-diox)MgR2]¥polymerchain into smaller fragments,making the R2Mg componentsoluble while the dihalide species remain insoluble.If[(μ-diox)Mg(R)X]¥has a higher solubility than[(μ-diox)Mg-X2]¥in the presence of an excess of dioxane,it dispropor-tionates via the Schlenk equilibrium to insoluble magnesiumdihalide and soluble[(diox)nþ1(MgR2)n].13The amount ofdioxane necessary to achieve solubility of[(diox)nþ1(MgR2)n]species strongly depends on the substituent R.With a largeexcess of dioxane monomeric compounds should be obser-ved,as reported in the case of[(diox)2Mg(2-Me-1,2-C2B10-H10)2].16Cooling diethyl ether solutions of[(diox)nþ1(MgR2)n]often results in re-formation of polymeric[(diox)MgR2]¥by liberation of dioxane from oligomeric or monomericspecies.This behavior enables the isolation of crystallineproducts of the type[(diox)MgR2]¥.Crystals of[(μ-diox-O,O0)Mg(cyclo-C6H11)2]¥(1)were obtained in this way.Achain structure with bridging dioxane molecules was found.The crystal structure determination was hampered due to thevery small crystals of poor quality that were obtained.However,the connectivity that was established is shownin Figure1.Similar structures also have been observed for[(μ-diox-O,O0)Mg(CH2t Bu)2]¥,17[(μ-diox-O,O0)MgPh2]¥,19and[(μ-diox-O,O0)MgEt2]¥.20Diethyl ether solutions of the dibenzylmagnesium dioxa-nate2tend to oversaturate when cooled,and no crystal-lization 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 dioxanate2shows a different structure in the solid state,an example of the struc-tural diversity of such dioxane adducts.In[(diox)Bz2Mg-(μ-diox-O,O0)Mg(diox)Bz2](2;Bz=benzyl,CH2Ph)both coordination modes of1,4-dioxane are realized:in this di-nuclear 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 brid-ging1,4-dioxane-O,O0molecule.Each magnesium atom completes its tetrahedral coordination sphere with another dioxane ligand.The molecular structure and numbering scheme are shown in Figure2.The magnesium atoms bind to the methylene carbon atoms of the benzyl substituents with no short contacts to theπ-systems of the phenyl groups. Thus,the benzyl unit behaves as an alkyl group without charge delocalization from the carbanionic CH2unit into the phenyl group.The Mg-C-C Ph angles are in the expected range of alkylmagnesium derivatives.7-9Similar dioxane-bridged dinulclear magnesium com-pounds have been observed before,but in those cases addi-tional coordination sites for dioxane have been blocked by either bulky substituents18or chelating ligands,21thus pre-ventingpolymerization.Figure1.Part of the chainlike structure of[(μ-diox-O,O0)Mg-(c Hex)2]¥(1).Symmetry-related atoms are marked with the letters A(-xþ1,y,-z-1/2)and B(-xþ2,y,-zþ1/2).Figure2.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(-xþ1,-y,-zþ1).The thermal ellipsoids represent a probability of40%;hydro-gen 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-Mg-C8=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).(16)Clegg,W.;Brown,D.A.;Bryan,S.J.;Wade,anomet. 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€a rtner,M.;Fischer,R.;Langer,J.;G€o rls,H.;Walther,D.;Westerhausen,M.Inorg.Chem.2007,46,5118–5124.(20)Fischer,R.;Walther,D.;Gebhardt,P.;G€o rls,anometal-lics2000,19,2532–2540.(21)Blackmore,I.J.;Gibson,V.C.;Hitchcock,P.B.;Rees,C.W.; Williams,D.J.;White,A.J.P.J.Am.Chem.Soc.2005,127,6012–6020.5816Organometallics,Vol.28,No.19,2009Langer et al. Compound2illustrates that other species in addition to thepolymer and the monomer do exist,making compounds of thetype[(diox)nþ1(MgR2)n]an interesting subject for the study ofself-organization with and without additional templates.2.2.Ether Scission by Mesityl Grignard Reagents.Inorganolithium chemistry ether cleavage reactions are rathercommon and well understood.22Depending on the substit-uents of the ether,R,β,and R0,β-elimination was observed.In the case of diethyl ether all three mechanisms would lead toethene and lithium ethoxide(eq2).Deuteration experimentshave shown thatβ-elimination is the dominating reactionpathway,23whereas THF is R-deprotonated and decomposesto ethene and the lithium enolate of acetaldehyde.24,25Themetalation of2,6-Me2-4-ClC6H2OH with tert-butyllithiumin the presence of dioxane led to cleavage of1,4-dioxane withformation of LiO(CH2)2OCH d CH2,which was crystallizedas hexanuclear[(Aryl-O)2(R-O)4Li6(diox)](Aryl=2,6-Me2-4-ClC6H2;R=CH2CH2OCH d CH2).26RLiþCH3CH2OCH2CH3f RHþH2C d CH2þLiOCH2CH3ð2ÞIn the case of organomagnesium reagents such etherfragmentation reactions are less pronounced during prepara-tion 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/1,4-dioxane(Mes=2,4,6-trimethylphenyl).In this case the com-pounds[(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 char-acterize two such products,[(diox)Mg(Mes)(μ-OEt)]2(3) and[(EtOCH(Me)CH(Me)OEt)Mg(Mes)2](4).Derivative3represents an expected diethyl ether degrada-tion product.Its molecular structure and numbering scheme are shown in Figure3.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;27however,the origin of the ethoxide anion in this compound is not quite clear.28Mulvey and co-workers reported the formation of the closely related[(thf)Mg(Bu t)(μ-OBu t)]2with an alkoxide ligand arising from oxygen insertion into the metal-carbon bond.29 Other comparable alkoxide-or phenoxide-bridged dimeric organomagnesium compounds30,31are also known,but in none of these cases does the alkoxide stem from ether cleavage reactions.Crystalline4was obtained after reduction of the volume of the mother liquor and cooling of the residual solution which contained the coligand2,pound4is the first well-defined metal complex containing this biden-tate ligand,even though ether cleavage reactions have been studied for decades.However,the formation of2,3-diethoxy-butane was observed earlier duringγ-irradiation of diethyl ether32,33as well as in Grignard reactions of magnesium with a substituted bromocyclopropane in diethyl ether.34,35In agreement with these references,the formation of the neutral coligand2,3-diethoxybutane of4suggests a radical mechan-ism 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 chelating2,3-diethoxybutane then gives mononuclear4. 2.3.Further Structural Investigations of Mesitylmagne-sium Compounds.In addition to compounds3and4,mono-nuclear[(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 andB. Figure3.Molecular structure and numbering scheme of [(diox)Mg(Mes)(μ-OEt]2(3).Symmetry-related atoms(-xþ1, -yþ2,-z)are marked with the letter A.The thermal ellipsoids represent a probability of40%;H atoms are neglected for clarity reasons.Selected bond lengths(A):Mg1-C3=2.159(3),Mg1-O1=1.961(2),Mg1-O1A=1.966(2),Mg1-O2=2.057(2).Selec-ted 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),C3-Mg1-O2=104.6(1),O2-Mg-O1=104.26(9),O2-Mg1-O1A=102.5(1),O1-Mg1-O1A=83.25(9).(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,.Chem.1972, 37,560–562.(26)Randazzo,J.;Morris,J.J.;Rood,J.A.;Noll,B.C.;Henderson, mun.2008,11,1270–1272.(27)Yang,K.-C.;Chang,C.-C.;Huang,J.-Y.;Lin,C.-C.;Lee,G.-H.; Wang,Y.;Chiang,anomet.Chem.2002,648,176–187. (28)The synthesis of[{(Me3Si)2N}Mg(μ-OEt)(thf)]2as described in ref22is somewhat unclear.The reaction equation of MgEt2with HN(SiMe3)2in 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.There-fore,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.(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.,anometallics 1985,4,1154–1157.(31)Zhang,D.;Kawaguchi,anometallics2006,25,5506–5509.(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. .Chem.1993,58,3148–3155.(35)Garst,J.F.;Lawrence,K.E.;Batlaw,R.;Boone,J.R.;Ungv a ry,F.Inorg.Chim.Acta1994,222,365–375.Article Organometallics,Vol.28,No.19,20095817The 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].36According to Seidel and B €urger 37the addition of 2,20-bipyridine (bpy)to an Et 2O solution of Mes 2Mg at low tem-peratures 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 6are shown in Figure 6.To the best of our knowledge,compound 6is the firststructurally characterized 2,20-bipyridine magnesium com-plex containing σ-bonded alkyl or aryl groups.This de-monstrates the high reactivity of such adducts,normally resulting in decomposition products containing (bpy -)Mg species.38Table 1summarizes selected structural parameters of the mononuclear dimesitylmagnesium compounds 4-6and the related THF adduct.In all of these complexes the magne-sium 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 5in comparison to that in [(thf)2Mg(Mes)2].On the one hand,bidentate ligands with a small bite (intraligand O 333O or N 333N 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 bulki-ness 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-(C 6H 4-4-Me)2](Mg -C=2.126(7)and 2.132(8)A,Mg -O=2.031(6)and 2.050(5)A),39and [(μ-diox)Mg(Ph)2]¥(Mg -C=2.135(2)A,Mg -O =2.081(2)and 2.061(2)A ).19In addition to the neutral mesitylmagnesium com-pounds described above,the ionic ate complex [(thf)4Li]þ-[Mg(Mes)3]-(7)was prepared as described by Seidel and B €urger 37and its molecular structure was determined (Figure 7).The [Mg(Mes)3]-anion contains a three-coordinate mag-nesium atom with an average Mg -C bond length of 2.168A.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 -Cbond.Figure 4.Molecular structure of molecule A of [(EtOCH(Me)-CH(Me)OEt-O ,O 0)Mg(Mes)2](4).The thermal ellipsoids repre-sent a probability of 40%;H atoms are not drawn for clarityreasons.Selected bond lengths (A):MgA -C1A =2.165(5),MgA -C10A = 2.166(4),MgA -O1A = 2.114(3),MgA -O2A =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).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 bondlengths (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).Figure 6.Molecular structure of [(bpy)Mg(Mes)2](6).The thermal ellipsoids represent a probability of 40%;H atoms areneglected 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),C11-C10-C15=115.6(2).(36)Waggoner,K.M.;Power,anometallics 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,anomet.Chem.1990,393,315–331.5818Organometallics,Vol.28,No.19,2009Langer et al.This compound crystallizes isomorphous to the manganese derivative [(thf)4Li]þ[Mn(Mes)3]-.40Larger aryl groups such as 2,4,6-triisopropylphenyl substituents lead to the magnesi-ate [Mg(C 6H 2-2,4,6-i Pr 3)3]-,with longer and differingMg -C bonds of 2.249(4),2.206(4),and 2.147(4)Aand strong distortions (C -Mg -C angles vary between 105.0(1)and 131.2(2)°).36Smaller aryl groups allow larger coordina-tion numbers at the magnesium atom.Thus,a triphenyl-magnesiate anion dimerizes,yielding [Ph 2Mg(μ-Ph)2Mg-Ph 2]2-anions,41or this moiety completes its coordination sphere via addition of a Lewis base such as THF,as in [(thf)MgPh 3]-,42or via addition of another phenyl group,as in [MgPh 4]2-.40Very bulky aryl groups are able to stabilize even two-coordinate magnesium atoms,as in [Mg(C 6H 2-2,4,6-t Bu 3)2](Mg -C =2.118(3)and 2.120(3)A).43Regardless 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 neighbor-ing 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.ConclusionIn this study we have investigated the structural diversity of dioxane adducts of diorganomagnesium compounds of the general formula [(diox)n þ1(MgR 2)n ].The compounds isolated range from monomeric [(diox)2Mg(Mes)2](5)and dimeric [(diox)Bz 2Mg(μ-diox-O ,O 0)Mg(diox)Bz 2](2)to poly-meric [(μ-diox-O ,O 0)Mg(cyclo -C 6H 11)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 elimina-tion reactions play a significant role in diethyl ether cleavage by mesitylmagnesium species.4.Experimental Section4.1.General Considerations.All manipulations were carried out under an argon atmosphere using standard Schlenk techni-ques.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 400MHz spectrometer.Mass spectra were obtained on a Finnigan MAT SSQ 710system,and IR measurements were carried out using a Perkin-Elmer System 2000FTIR.The IR spectra were taken as Nujol mulls between KBr windows.Melting and decomposition points were mea-sured 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 HNO 3)with 0.05M EDTA using Eriochrome Black T as the indicator.44The starting materials mesitylmagnesium bromide in THF or Et 2O and [(thf)2Mg(Mes)2]were prepared according (or analogouslyTable parison of Selected Bond Lengths (A)and Angles (deg )of [(thf )2Mg (Mes )2],28Molecule 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]456Mg1-C1 2.182(3) 2.165(5) 2.169(2) 2.177(2)Mg1-C10 2.165(3) 2.166(4) 2.167(3) 2.169(2)Mg1-O/N 2.067(3) 2.114(3) 2.107(2) 2.166(2)2.079(3) 2.100(3) 2.084(2) 2.181(2)C1-Mg -C10118.8(1)121.2(2)117.55(9)122.68(7)C1-Mg -O/N 105.5(1)120.9(2)103.14(8)104.68(7)121.8(1)104.7(1)121.80(9)117.85(7)C10-Mg -O/N 118.3(1)105.8(2)122.89(9)120.99(7)100.9(1)121.1(2)101.23(8)106.43(6)O/N -Mg -O/N88.4(1)75.3(1)87.90(8)75.28(6)Figure 7.Molecular structure and numbering scheme of sol-vent-separated [(thf)4Li]þ[Mg(Mes)3]-(7).The thermal ellip-soids represent a probability of 40%;H atoms are omitted forclarity 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),Mg1-C19-C20=125.0(2),Mg1-C19-C24=119.4(2),C20-C19-C24=115.4(2).Scheme 1.Reactivity Diagram of“MesMgBr”(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,anometallics 2006,25,1206–1212.(43)Wehmschulte,R.J.;Power,anometallics 1995,14,3264–3267.(44)M €uller,G.-O.Lehr-und U ¨bungsbuch der anorganisch-analy-tischen Chemie ,7th ed.;Verlag Harri Deutsch:Frankfurt am Main,1992;V ol.3(Quantitativ-anorganisches Praktikum ).Article Organometallics,Vol.28,No.19,20095819in the case of diethyl ether)to literature procedures.37Benzyl-magnesium chloride in diethyl ether was purchased from Al-drich.4.2.Synthesis of[(μ-diox-O,O0)Mg(cyclo-C6H11)2]¥(1).Di-oxane(14mL,0.164mol)was added dropwise with rapid stirring to a solution of[(cyclo-C6H11)MgBr]freshly preparedfrom magnesium(3.0g,0.123mol)and cyclohexyl bromide (16.3g,0.1mol)in ether(100mL).The resulting white suspen-sion was allowed to stand overnight and filtered afterward.Thewhite residue was washed with ether(20mL)and discarded.The combined ether solutions were stored at-20°C for3days, yielding colorless crystals which were isolated by filtration and dried in vacuo.Yield:5.8g(42%).Mp:>270°C.Anal.Calcdfor C16H30MgO2(278.72g mol-1):Mg,8.5.Found:Mg,8.7.1H NMR(400.25MHz,25°C,[D6]benzene/[D8]THF(3:1)):δ0.14 (2H,m,H C1),1.48-3.19(20H,m,H2,20-H4),3.38(8H,s, C H2O,diox).13C{1H}NMR(50.32MHz,25°C,[D6]benzene/[D8]THF(3:1)):δ25.9(2C,C1),29.7(2C,C4),32.3(4C,C3,30), 35.9(4C,C2,20),66.9(12C,C H2O,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(15mL,0.176mol)was added dropwise with rapid stirring to a solution of[BzMgCl]in ether(80mL,0.9M).The resulting white suspension was allowed to stand overnight and filtered afterward.The white residue was washed with ether (20mL)and discarded.The combined ether solutions were stored at room temperature for3months.The colorless crystals that had formed were isolated by filtration and dried in vacuo. Yield:1.31g(1.93mmol,11%).Decomposition above43°C. Anal.Calcd for C40H52Mg2O6(677.47g mol-1):Mg,7.2. Found:Mg,7.1.1H NMR(400.25MHz,25°C,[D6]benzene/ [D8]THF(3:1)):δ1.59(8H,s,PhC H2),3.32(24H,s,C H2O,dx), 6.54(4H,t,3J=7.2Hz,p-H),6.89(8H,dt,3J=7.2Hz,m,m0-H), 6.96(8H,t,3J=7.2Hz,o,o0-H).13C{1H}NMR(100.65MHz, 25°C,[D6]benzene/[D8]THF(3:1)):δ23.1(4C,Ph C H2),67.6 (12C,O C H2,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).Diox-ane(5.1mL,60mmol)was added dropwise with rapid stirring to a freshly prepared solution of[MesMgBr]in diethyl ether (50mL,0.8M).The resulting off-white suspension was allowed to stand overnight and was diluted afterward with additional ether(40mL).Filtration gave50mL of a pale yellow solution, while the rest of the solvent remained in the voluminous residue. This solution was stored for3days at0°C,and a small amount of precipitate that had formed was removed by filtration.Then the volume of the mother liquor was reduced to10mL by eva-poration of the solvent in vacuo.Colorless crystals of4(110mg, 0.27mmol,2.7%)were obtained from this solution at0°C overnight and isolated by decantation.The mother liquor was further concentrated to4mL and stored again at0°C for1week. The newly formed crystals were isolated by filtration and dried in vacuo.Yield:450mg;mixture of3(190mg,0.36mmol,3.6%) and5(260mg,0.59mmol,3.0%)as judged by NMR.Data for3 are as follows.1H NMR(200.13MHz,25°C,[D8]THF):δ1.19 (6H,t,3J H-H=7.0Hz,C H3OEt),2.16(6H,s,p-C H3),2.41(12H,s,o-C H3),3.54(16H,s,OCH2diox),3.93(4H,q,3J H-H= 7.0Hz,C H2OEt),6.61(4H,s,m-C H).13C{1H}NMR(50.33 MHz,25°C,[D8]THF):δ21.4(2C,p-C H3),22.2(2C,C H3OEt), 28.3(4C,o-C H3),67.7(8C,O C H2diox),124.8(4C,m-C H), 133.7(2C,p-C H),147.3(4C,o-C),162.3(2C,i-C).Data for4are as follows.1H NMR(200.13MHz,25°C,[D8]THF):δ0.97(6H, m,CHC H3),1.07(6H,t,3J H-H=7.0Hz,C H3OEt),2.07(6H,s, p-C H3),2.26(12H,s,o-C H3),3.3-3.5(6H,m,C H2OEtþC H CH3),6.48(4H,s,m-C H).13C{1H}NMR(50.33MHz, 25°C,[D8]THF):δ14.3(2C,CH C H3),16.0(2C,CH3OEt),21.4(2C,p-C H3),27.6(4C,o-C H3),64.9(2C,O C H2),77.5(2C,C HCH3),125.0(4C,m-C H),133.1(2C,p-C H),141.3(4C,o-C), 165.2(2C,i-C).Data for5are as follows.1H NMR(200.13 MHz,25°C,[D8]THF):δ2.14(6H,s,p-C H3),2.33(12H,s,o-C H3),3.54(16H,s,OC H2diox),6.56(4H,s,m-C H).13C{1H} NMR(50.33MHz,25°C,[D8]THF):δ21.4(2C,p-C H3),27.7 (4C,o-C H3),67.7(8C,O C H2diox),125.0(4C,m-C H),133.2 (2C,p-C H),147.1(4C,o-C),165.3(2C,i-C).The NMR data for 3and5were obtained from an isolated mixture of these compounds.The signals were assigned on the basis of two-dimensional NMR experiments(H,H-COSY,HMBC,HSQC) as well as their different signal intensities.The crystals of3-5 obtained directly from the reaction mixture were suitable for X-ray diffraction measurements.Alternatively,crystals of 53(toluene)in extremely low yields were obtained by extraction of the voluminous precipitate formed during the dioxane addi-tion(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] (0.52g,1.28mmol)was suspended in Et2O(15mL)and a solution of2,20-bipyridine(0.20g,1.28mmol)in Et2O(5mL) was added dropwise at-40°C.Stirring of the mixture was continued for4h at-20°C,after which yellow microcrystals had formed(the mother liquor turned dark red).Separation, washing with Et2O(4mL),and drying in vacuo yielded0.46g (1.10mmol,86%)of pyrophoric6.Decomposition above150°C. Anal.Calcd for C28H30MgN2(418.88g mol-1):Mg,5.8.Found: 5.5.1H NMR(400.25MHz,25°C,[D8]THF):δ1.31(6H,s, p-C H3),2.23(12H,s,o-C H3),6.75(4H,s,m-C H),7.46(2H,t, 3JH-H=7.6Hz,H4,40-bpy),7.77(2H,d,3J H-H=7.6Hz,H5,50-bpy),8.49(2H,br,H6,60-bpy),8.63(2H,br,H3,30-bpy).13C{1H} NMR(100.65MHz,25°C,[D8]THF):δ21.3(2C,p-C H3),27.3 (4C,o-C H3),121.3(2C,C6,60-bpy),127.5(4C,m-C H),128.2(2C, p-C H),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 measure-ments were obtained by recrystallization of6in 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](0.65g,1.60mmol)was dissolved in THF(5mL),and LiMes(1.96mL,1.60mmol,0.814M)in THF was added at0°C.After the mixture was stirred at room temperature for12h,Et2O(10mL)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(10mL),and drying in vacuo yielded0.72g (1.06mmol,66%)of[Li(thf)4][Mg(Mes)3](7).Decomposition above134°C.1H NMR(400.25MHz,25°C,[D8]THF):δ1.78 (16H,m,C H2,thf),2.08(9H,s,p-C H3),2.35(18H,s,o-C H3),3.61 (16H,m,C H2O,thf),6.40(6H,s,m-H).13C{1H}NMR(100.65 MHz,25°C,[D8]THF):δ21.6(3C,p-C H3),25.2(8C,C H2,thf), 27.9(6C,o-C H3),67.5(6C,C H2O,thf),123.8(6C,m-C),131.2(3C, p-C),147.0(6C,o-C),172.1(3C,i-C).7Li NMR(155.55MHz, 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.。