inorganics
Article Reduction of Bromo- and Iodo-2,6-bis(diphenylphosphanylmethyl)benzene with Magnesium and Calcium
Alexander Koch, Sven Krieck, Helmar Görls and Matthias Westerhausen *
Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University of Jena, 07745 Jena, Germany; [email protected] (A.K.); [email protected] (S.K.); [email protected] (H.G.) * Correspondence: [email protected]; Tel.: +49-3641-9-48110
Academic Editor: Lee J. Higham Received: 1 November 2016; Accepted: 25 November 2016; Published: 1 December 2016
Abstract: Arylmagnesium and -calcium reagents are easily accessible; however, ether degradation processes limit storability, especially of the calcium-based heavy Grignard reagents. Ortho-bound substituents with phosphanyl donor sites usually block available coordination sites and stabilize such complexes. The reaction of bromo-2,6-bis(diphenylphosphanylmethyl)benzene (1a) with magnesium in tetrahydrofuran yields [Mg{C6H3-2,6-(CH2PPh2)2}2](2) after recrystallization from 1,2-dimethoxyethane. However, the similarly performed reduction of bromo- (1a) and iodo-2,6-bis(diphenylphosphanylmethyl)benzene (1b) with calcium leads to ether cleavage and subsequent degradation products. α-Deprotonation of tetrahydrofuran (THF) yields 1,3-bis(diphenylphosphanylmethyl)benzene. Furthermore, the insoluble THF adducts of dimeric calcium diphenylphosphinate halides, [(thf)3Ca(X)(µ-O2PPh2)]2 [X = Br (3a), I (3b)], precipitate verifying ether decomposition and cleavage of P–C bonds. Ether adducts of calcium halides (such as [(dme)2(thf)CaBr2](4)) form, supporting the initial Grignard reaction and a subsequent Schlenk-type dismutation reaction.
Keywords: Grignard reagent; arylmagnesium halide; phosphinates; calcium phosphinates; direct synthesis; ether degradation; calcium bromide
1. Introduction The Grignard reaction is a widely used synthesis of organomagnesium compounds for diverse applications such as metalation of H-acidic compounds (magnesiation), group transfer (salt metathesis reactions), addition to, e.g., ketones and aldehydes (synthesis of alcohols), and C–C coupling reactions (Kumada cross coupling). The insertion of magnesium into a carbon–halogen bond can be accompanied by ether cleavage with the initial α- or β-deprotonation step or a radical mechanism [1,2]. The organocalcium halides (heavy Grignard reagents) tend much more to degrade ethers and only selected arylcalcium [3–10] and trimethylsilylmethylcalcium complexes [11] are easily prepared via the direct synthesis. Enhancement of steric demand of the aryl groups does not necessarily enhance the stability of the arylcalcium halides. Bulky substituents such as tert-butyl groups in ortho-position destabilize the arylcalcium halides significantly, finally leading to 2,5-dimethyl-2,5-bis[3,5-di(tert-butyl)phenyl]hexane [7,10]. In order to stabilize magnesium- and calcium-based Grignard reagents, ortho-substituents with Lewis basic donor atoms such as phosphanes in the side arms could repel intramolecular side reactions and ether degradation by blocking of the coordination sites at the alkaline earth cations. One would expect that soft phosphorus bases bind less strongly to the rather hard magnesium and calcium cations than hard oxygen and nitrogen bases. In addition, phosphanes can only act as pure σ-donors. Due to
Inorganics 2016, 4, 39; doi:10.3390/inorganics4040039 www.mdpi.com/journal/inorganics Inorganics 2016, 4, 39 2 of 12
can onlyInorganics act as 2016pure, 4, 39σ-donors. Due to this fact the phosphane bases are commonly incorporated2 of 12 into the anions leading to multidentate anions. Nevertheless, a variety of phosphane complexes of magnesium (see e.g., [12–16]) and calcium (see e.g., [15–19]) have been isolated and structurally this fact the phosphane bases are commonly incorporated into the anions leading to multidentate studied. anions.In order Nevertheless, to stabilize a variety magnesium- of phosphane and complexes calcium-based of magnesium Grignard (see e.g.,reagents, [12–16]) phosphanylmethyl and calcium substituents(see e.g.,in ortho-position [15–19]) have been of isolated the arylalkaline and structurally earth studied. metal In halides order to stabilizeseem to magnesium- be a suitable and choice. Müller andcalcium-based coworkersGrignard alreadyreagents, investigated phosphanylmethyl solvent-free substituents in[Li-C ortho-position6H3-2,6-(CH of the2PMe arylalkaline2)2]2 (A) and earth metal halides seem to be a suitable choice. Müller and coworkers already investigated solvent-free [Mg{C6H3-2,6-(CH2PMe2)2}2] (B, Scheme 1; Mg–P 276.1(1) and 277.0(1) pm, Mg–C 221.6(1) pm) with [Li-C H -2,6-(CH PMe ) ] (A) and [Mg{C H -2,6-(CH PMe ) } ](B, Scheme1; Mg–P 276.1(1) and ortho-bound6 3 dimethylphosphanylmethyl2 2 2 2 6 3 side 2 arms2 2 2 [14]. The bite of the 277.0(1) pm, Mg–C 221.6(1) pm) with ortho-bound dimethylphosphanylmethyl side arms [14]. The bite 2,6-bis(dimethyl-phosphanylmethyl)of the 2,6-bis(dimethyl-phosphanylmethyl)phenylphenyl base is base flexible is flexible and, and, hence, this this ligand ligand is able is to able to encapsulateencapsulate cations cations with withvarious various radii. radii.
SchemeScheme 1. Representation 1. Representation of of[Li-C [Li-C6H6H33-2,6-(CH-2,6-(CH22PMePMe2)2)]22]2(A (A); and); and [Mg{C [Mg{C6H3-2,6-(CH6H3-2,6-(CH2PMe22)PMe2}2](B2)).2}2] (B).
We assumedWe assumed that thatthe thephosphanyl phosphanyl bases bases might bebe suitable suitable donor donor sites sites in organocalcium in organocalcium chemistry. Due to the larger ionic radius of Ca2+ compared to Mg2+, the softer cation Ca2+ should chemistry. Due to the larger ionic radius of Ca2+ compared to Mg2+, the softer cation Ca2+ should prefer the softer phosphorus-based Lewis bases, enabling the isolation of ether-free diarylcalcium. prefer theFor softer comparison phosphorus-based reasons we intended Lewis tobases, prepare enabling the magnesium the isolation and calcium of ether-free complexes diarylcalcium. with For comparisonbis(phosphanylmethyl)-substituted reasons we intended phenyl to prepare groups. the magnesium and calcium complexes with bis(phosphanylmethyl)-substituted phenyl groups. 2. Results
2. Results2.1. Reduction of Bromo-2,6-bis(diphosphanylmethyl)benzene with Magnesium For the reduction of sterically shielded bromo-2,6-bis(diphenylphosphanylmethyl)benzene (1a), 2.1. Reductiona 1:1 mixtureof Bromo-2,6-Bis(diphosphanylmethyl)benzene of Rieke magnesium and magnesium turnings with wasMagnesium used in THF. This suspension For wasthe heatedreduction under of reflux, sterically and iodineshielded crystals brom hado-2,6-bis(diphen to be added to maintainylphosphanylmethyl)benzene the reduction reaction. (1a), Monitoring of the reaction by 31P{1H} NMR spectroscopy showed four resonances at δ = −10.6, a 1:1 mixture−13.2, of− 15.5,Rieke and magnesium−22.1 ppm (Figureand magnesium1). The first twoturnings signals was can beused assigned in THF. to 2,6-bis(diphenyl This suspension was heated underphosphanylmethyl)benzene reflux, and iodine and crystals1a, respectively. had to At be 4 ◦Cadded an amorphous to maintain precipitate the formedreduction which reaction. 31 1 ◦ Monitoringwas of dissolved the reaction with 1,2-dimethoxyethane. by P{ H} NMR spectroscopy Repeated cooling showed to 4 fourC yielded resonances [(thf)2MgBr at δ 2=] − and10.6, −13.2, −15.5, andcrystalline −22.1 bis[2,6-bis(diphenylphosphanylmethyl)phenyl]magnesiumppm (Figure 1). The first two signals can be (2assigned) with a chemical to 2,6-bis(diphenyl- shift of δ 31 1 − phosphanylmethyl)benzene( P{ H}) = 15.6 ppm. Additionand 1a, ofrespectively. 1,4-dioxane shifted At 4 the °CSchlenk an amorphousequilibrium toward precipitate the homoleptic formed which derivatives MgR2 and MgBr2 due to quantitative precipitation of [(diox)MgBr2]∞. The resonance at was dissolvedδ = −22.1 with vanished 1,2-dimethoxyethane. and complex 2 was isolated Repeated with moderate cooling yieldto 4 (Equation °C yielded (1)). The [(thf) presence2MgBr2] and crystallineof significantbis[2,6-bis(diphenylph amounts of 2,6-bis(diphenylphosphanylmethyl)benzeneosphanylmethyl)phenyl]magnesium underlines (2) with the a relevance chemical of shift of δ(31P{1H})ether = degradation,−15.6 ppm. especially Addition because of drastic1,4-dioxane reaction conditionsshifted (suchthe asSchlenk boiling THF)equilibrium are required toward to the Grignard Grignard homolepticexpedite derivatives the MgRreaction.2 and ItMgBr is remarkable2 due to that quantitative the reagentprecipitation2 crystallizes of [(diox)MgBr from ethereal 2]∞. The resonancesolvents at δ = without −22.1 vanished ligated ether and molecules, complex even 2 was in the isolated presence ofwith bidentate moderate 1,2-dimethoxyethane. yield (Equation (1)). The presence of significant amounts of 2,6-bis(diphenylphosphanylmethyl)benzene underlines the relevance of ether degradation, especially because drastic reaction conditions (such as boiling THF) are required to expedite the Grignard reaction. It is remarkable that the Grignard reagent 2 crystallizes from ethereal solvents without ligated ether molecules, even in the presence of bidentate 1,2-dimethoxyethane.
(1)
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can only act as pure σ-donors. Due to this fact the phosphane bases are commonly incorporated into the anions leading to multidentate anions. Nevertheless, a variety of phosphane complexes of magnesium (see e.g., [12–16]) and calcium (see e.g., [15–19]) have been isolated and structurally studied. In order to stabilize magnesium- and calcium-based Grignard reagents, phosphanylmethyl substituents in ortho-position of the arylalkaline earth metal halides seem to be a suitable choice. Müller and coworkers already investigated solvent-free [Li-C6H3-2,6-(CH2PMe2)2]2 (A) and [Mg{C6H3-2,6-(CH2PMe2)2}2] (B, Scheme 1; Mg–P 276.1(1) and 277.0(1) pm, Mg–C 221.6(1) pm) with ortho-bound dimethylphosphanylmethyl side arms [14]. The bite of the 2,6-bis(dimethyl-phosphanylmethyl)phenyl base is flexible and, hence, this ligand is able to encapsulate cations with various radii.
Scheme 1. Representation of [Li-C6H3-2,6-(CH2PMe2)2]2 (A); and [Mg{C6H3-2,6-(CH2PMe2)2}2] (B).
We assumed that the phosphanyl bases might be suitable donor sites in organocalcium chemistry. Due to the larger ionic radius of Ca2+ compared to Mg2+, the softer cation Ca2+ should prefer the softer phosphorus-based Lewis bases, enabling the isolation of ether-free diarylcalcium. For comparison reasons we intended to prepare the magnesium and calcium complexes with bis(phosphanylmethyl)-substituted phenyl groups.
2. Results
2.1. Reduction of Bromo-2,6-Bis(diphosphanylmethyl)benzene with Magnesium For the reduction of sterically shielded bromo-2,6-bis(diphenylphosphanylmethyl)benzene (1a), a 1:1 mixture of Rieke magnesium and magnesium turnings was used in THF. This suspension was heated under reflux, and iodine crystals had to be added to maintain the reduction reaction. Monitoring of the reaction by 31P{1H} NMR spectroscopy showed four resonances at δ = −10.6, −13.2, −15.5, and −22.1 ppm (Figure 1). The first two signals can be assigned to 2,6-bis(diphenyl- phosphanylmethyl)benzene and 1a, respectively. At 4 °C an amorphous precipitate formed which was dissolved with 1,2-dimethoxyethane. Repeated cooling to 4 °C yielded [(thf)2MgBr2] and crystalline bis[2,6-bis(diphenylphosphanylmethyl)phenyl]magnesium (2) with a chemical shift of δ(31P{1H}) = −15.6 ppm. Addition of 1,4-dioxane shifted the Schlenk equilibrium toward the homoleptic derivatives MgR2 and MgBr2 due to quantitative precipitation of [(diox)MgBr2]∞. The resonance at δ = −22.1 vanished and complex 2 was isolated with moderate yield (Equation (1)). The presence of significant amounts of 2,6-bis(diphenylphosphanylmethyl)benzene underlines the relevance of ether degradation, especially because drastic reaction conditions (such as boiling THF) are requiredInorganics to 2016expedite, 4, 39 the Grignard reaction. It is remarkable that the Grignard reagent 23 ofcrystallizes 12 from ethereal solvents without ligated ether molecules, even in the presence of bidentate 1,2-dimethoxyethane.
(1) (1)
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Figure 1. 31FigureP{1H} 1.NMR31P{1H} spectrum NMR spectrum of the of theGrignardGrignard reaction of bromo-2,6-bis(diphenylphosphanylof bromo-2,6-bis(diphenylphosphanyl- methyl)benzene (1a, RBr) with magnesium in tetrahydrofuran (see text, R = C6H3-2,6-(CH2PPh2)2; methyl)benzene (1a, RBr)◦ with magnesium in tetrahydrofuran (see text, R = C6H3-2,6-(CH2PPh2)2; 162 162 MHz, 25 C; [D6]benzene was added to the reaction solution for the NMR measurement). MHz, 25 °C; [D6]benzene was added to the reaction solution for the NMR measurement). The molecular structures and atom numbering schemes of 1a and 2 are depicted in Figures2 and3 , The molecularrespectively. structures In bromo-2,6-bis(diphenylphosphanylmethyl)benzene and atom numbering schemes of (1a1a) and the phosphanyl 2 are depicted side arms in Figures 2 and 3, respectively.are turned In outside bromo-2,6-bis(diphenylphosphanylmethyl)benzene and hydrogen atoms of the methylene moieties (at C7 and C8)(1a point) the towardphosphanyl side the bromine atom. The P–C bond lengths to the sp3 hybridized methylene groups C7 and C8 are arms are turnedslightly outside larger than and the hydrogen P–C values atoms to the phenyl of the groups methylene with sp moieties2 hybridized (at ipso-carbon C7 and C8) atoms. point toward the bromineThe atom. phosphorus The P–C atoms bond are in lengths pyramidal to environments the sp3 hybridized with anglesums methylene of 300.4◦ andgroups 304.4 ◦C7for and C8 are ◦ slightly largerP1 andthan P2, the respectively. P–C valu Thees C2–C1–C6to the phenyl bond angle groups of the with ipso-carbon sp2 hybridized atom (123.4(2) ipso-carbon) is slightly atoms. The ◦ phosphoruslarger atoms than are 120 indue pyramidal to the positive environments charge on C1. Thewith umpolung angle sums at this carbonof 300.4° via substitution and 304.4° of for P1 and bromine by magnesium leads to narrower angles at the ipso-carbon atoms in complex 2 (C2–C1–C6 P2, respectively.115.8(2) The◦ and C2–C1–C6 C34–C33–C38 bond 115.6(2) angle◦) due of to electrostaticthe ipso-carbon repulsion atom between (123.4(2)°) the negatively is slightly charged larger than 120° due to electronthe positive pair and charge the neighboring on C1. bondsThe umpolung to the ortho-carbon at this atoms. carbon The via Mg1–C substitution bond lengths of of bromine by magnesium219.3(3) leads to and narrower 218.3(3) pm angles lie in the at expectedthe ipso-carbon range as also atoms observed in complex for other 2 organomagnesium (C2–C1–C6 115.8(2)° and complexes [20–24]. The magnesium atom is rather small for the bite of the anionic ligand. Therefore, C34–C33–C38each 115.6(2)°) phosphanyl-substituted due to electrostatic aryl group additionally repulsion forms between a strong the Mg–P negatively bond (Mg1–P1 charged 269.2(1), electron pair and the neighboringMg1–P4 268.9(1) bonds pm) to and the a significantlyortho-carbon longer atoms. bond (Mg1–P2 The Mg1–C 296.1(1), bond Mg1–P3 lengths 286.3(1) of pm). 219.3(3) and 218.3(3) pmThe lie formation in the ofexpected coordinative range Mg–P bondsas also increases observed the angle for sums other at the phosphorusorganomagnesium atoms, taking complexes only the carbon atoms into account (P1: 311.1◦, P2: 308.7◦, P3: 309.3◦, P4: 311.5◦). Despite the [20–24]. The magnesium atom is rather small for the bite of the anionic ligand. Therefore, each attraction between the magnesium ion and the phosphanyl bases, the P–CCH2–Cortho bond angles are phosphanyl-substitutedstill larger than the aryl tetrahedral group angle additionally due to intramolecular forms repulsiona strong at the Mg–P periphery bond of the (Mg1–P1 complex. 269.2(1), Mg1–P4 268.9(1)This repulsion pm) and is also a thesignificantly reason for the peculiarlonger coordination bond (Mg1–P2 behavior 296.1(1), with drastically Mg1–P3 different 286.3(1) Mg–P pm). The formation ofbonds coordinative because the dimethylphosphanylmethyl-substitutedMg–P bonds increases the angle derivative sums atB (seethe Scheme phosphorus1) exhibits atoms, a taking symmetric coordination mode [14]. only the carbon atoms into account (P1: 311.1°, P2: 308.7°, P3: 309.3°, P4: 311.5°). Despite the attraction between the magnesium ion and the phosphanyl bases, the P–CCH2–Cortho bond angles are still larger than the tetrahedral angle due to intramolecular repulsion at the periphery of the complex. This repulsion is also the reason for the peculiar coordination behavior with drastically different Mg–P bonds because the dimethylphosphanylmethyl-substituted derivative B (see Scheme 1) exhibits a symmetric coordination mode [14].
Figure 2. Molecular structure and atom numbering scheme of bromo-2,6-bis(diphenylphosphanyl- methyl)benzene (1a). The ellipsoids represent a probability of 30%; H atoms are shown with arbitrary radii. Selected bond lengths (pm): Br1–C1 191.8(3), P1–C8 186.1(3), P1–C9 184.2(3), P1–C15 183.6(3), P2–C7 186.5(3), P2–C21 183.1(3), P2–C27 184.0(3); angles (°): C8–P1–C9 99.4(1), C8–P1–C15 101.0(1), C9–P1–C15 100.1(1), C7–P2–C21 103.5(1), C7–P2–C27 102.2(1), C21–P2–C27 102.2(1), Br1–C1–C2 117.9(2), Br1–C1–C6 118.7(2), C2–C1–C6 123.4(2), C2–C7–P2 112.3(2), C6–C8–P1 110.8(2).
Inorganics 2016, 4, 39 3 of 12
Figure 1. 31P{1H} NMR spectrum of the Grignard reaction of bromo-2,6-bis(diphenylphosphanyl-
methyl)benzene (1a, RBr) with magnesium in tetrahydrofuran (see text, R = C6H3-2,6-(CH2PPh2)2; 162
MHz, 25 °C; [D6]benzene was added to the reaction solution for the NMR measurement).
The molecular structures and atom numbering schemes of 1a and 2 are depicted in Figures 2 and 3, respectively. In bromo-2,6-bis(diphenylphosphanylmethyl)benzene (1a) the phosphanyl side arms are turned outside and hydrogen atoms of the methylene moieties (at C7 and C8) point toward the bromine atom. The P–C bond lengths to the sp3 hybridized methylene groups C7 and C8 are slightly larger than the P–C values to the phenyl groups with sp2 hybridized ipso-carbon atoms. The phosphorus atoms are in pyramidal environments with angle sums of 300.4° and 304.4° for P1 and P2, respectively. The C2–C1–C6 bond angle of the ipso-carbon atom (123.4(2)°) is slightly larger than 120° due to the positive charge on C1. The umpolung at this carbon via substitution of bromine by magnesium leads to narrower angles at the ipso-carbon atoms in complex 2 (C2–C1–C6 115.8(2)° and C34–C33–C38 115.6(2)°) due to electrostatic repulsion between the negatively charged electron pair and the neighboring bonds to the ortho-carbon atoms. The Mg1–C bond lengths of 219.3(3) and 218.3(3) pm lie in the expected range as also observed for other organomagnesium complexes [20–24]. The magnesium atom is rather small for the bite of the anionic ligand. Therefore, each phosphanyl-substituted aryl group additionally forms a strong Mg–P bond (Mg1–P1 269.2(1), Mg1–P4 268.9(1) pm) and a significantly longer bond (Mg1–P2 296.1(1), Mg1–P3 286.3(1) pm). The formation of coordinative Mg–P bonds increases the angle sums at the phosphorus atoms, taking only the carbon atoms into account (P1: 311.1°, P2: 308.7°, P3: 309.3°, P4: 311.5°). Despite the attraction between the magnesium ion and the phosphanyl bases, the P–CCH2–Cortho bond angles are still larger than the tetrahedral angle due to intramolecular repulsion at the periphery of the complex. This repulsion is also the reason for the peculiar coordination behavior with drastically Inorganicsdifferent2016, 4 Mg–P, 39 bonds because the dimethylphosphanylmethyl-substituted derivative B (see Scheme4 of 12 1) exhibits a symmetric coordination mode [14].
FigureFigure 2. Molecular 2. Molecular structure structure and and atom atom numberingnumbering sch schemeeme of of bromo-2,6-bis(diphenylphosphanyl- bromo-2,6-bis(diphenylphosphanyl methyl)benzene (1a). The ellipsoids represent a probability of 30%; H atoms are shown with arbitrary methyl)benzene (1a). The ellipsoids represent a probability of 30%; H atoms are shown with arbitrary radii. Selected bond lengths (pm): Br1–C1 191.8(3), P1–C8 186.1(3), P1–C9 184.2(3), P1–C15 183.6(3), radii. Selected bond lengths (pm): Br1–C1 191.8(3), P1–C8 186.1(3), P1–C9 184.2(3), P1–C15 183.6(3), P2–C7 186.5(3), P2–C21 183.1(3), P2–C27 184.0(3); angles (°):◦ C8–P1–C9 99.4(1), C8–P1–C15 101.0(1), P2–C7C9–P1–C15 186.5(3), P2–C21100.1(1), 183.1(3),C7–P2–C21 P2–C27 103.5(1), 184.0(3); C7–P2–C27 angles 102.2(1), ( ): C8–P1–C9 C21–P2–C27 99.4(1), 102.2(1), C8–P1–C15 Br1–C1–C2 101.0(1), C9–P1–C15117.9(2), 100.1(1),Br1–C1–C6 C7–P2–C21 118.7(2), C2–C1–C6 103.5(1), 123.4( C7–P2–C272), C2–C7–P2 102.2(1), 112.3(2), C21–P2–C27 C6–C8–P1 110.8(2). 102.2(1), Br1–C1–C2 Inorganics 2016, 4, 39 4 of 12 117.9(2), Br1–C1–C6 118.7(2), C2–C1–C6 123.4(2), C2–C7–P2 112.3(2), C6–C8–P1 110.8(2).
Figure 3. Figure Molecular3. Molecular structure structure and and atomatom numberingnumbering scheme scheme ofof bis(2,6-bis(diphenylphosphanyl- bis(2,6-bis(diphenylphosphanyl methyl)phenyl)magnesiummethyl)phenyl)magnesium (2). (2 The). The ellipsoids ellipsoids represent represent a a probability probability ofof 30%;30%; H atoms atoms are are omitted omitted for the sakefor ofthe clarity. sake of Selected clarity. Selected bond lengths bond le (pm):ngths Mg1–C1 (pm): Mg1–C1 219.3(3), 219.3(3), Mg1–C33 Mg1–C33 218.3(3), 218.3(3), Mg1–P1 Mg1–P1 269.2(1), Mg1–P2269.2(1), 296.1(1), Mg1–P2 Mg1–P3 296.1(1), 286.3(1), Mg1–P3 Mg1–P4 286.3(1), 268.9(1), Mg1–P4 268.9(1), P1–C7 183.9(3), P1–C7 183.9(3), P1–C8 P1–C8 183.3(3), 183.3(3), P1–C14 P1–C14 182.7(3), P2–C20182.7(3), 185.1(3), P2–C20 P2–C21 185.1(3), 182.5(3), P2–C21 P2–C27 182.5(3), 183.3(3), P2–C27 P3–C39183.3(3), P3–C39 184.5(3), 184.5(3), P3–C40 P3–C40 183.1(3), 183.1(3), P3–C46 P3–C46 182.3(3), P4–C52182.3(3), 184.3(3), P4–C52 P4–C53 184.3(3), 182.4(3), P4–C53 P4–C59 182.4(3), 183.1(3); P4–C59 angles 183.1(3); (◦): angles C1–Mg1–C33 (°): C1–Mg1–C33 129.2(1), 129.2(1), C1–Mg1–P1 76.88(7),C1–Mg1–P1 C1–Mg1–P2 76.88(7), 72.63(7), C1–Mg1–P2 C1–Mg1–P3 72.63(7), C1–M 89.57(7),g1–P3 C1–Mg–P4 89.57(7), C1–Mg–P4 143.91(7), 143.91(7), C33–Mg1–P1 C33–Mg1–P1 145.56(8), C33–Mg1–P2145.56(8),88.14(7), C33–Mg1–P2 C33–Mg1–P3 88.14(7), C33–Mg1–P3 73.32(7), C33–Mg1–P4 73.32(7), C33–Mg1–P4 76.65(7), P1–Mg1–P2 76.65(7), P1–Mg1–P2 124.40(4), 124.40(4), P1–Mg1–P3 P1–Mg1–P3 86.58(3), P1–Mg1–P4 93.55(3), P2–Mg1–P3 137.24(4), P2–Mg1–P4 85.40(3), P3–Mg1–P4 86.58(3), P1–Mg1–P4 93.55(3), P2–Mg1–P3 137.24(4), P2–Mg1–P4 85.40(3), P3–Mg1–P4 124.98(4), 124.98(4), C2–C1–C6 115.8(2), C34–C33–C38 115.6(2). C2–C1–C6 115.8(2), C34–C33–C38 115.6(2). 2.2. Reduction of 2,6-Bis(diphosphanylmethyl)phenyl Halides with Calcium 2.2. Reduction of 2,6-bis(diphosphanylmethyl)phenyl Halides with Calcium Due to the fact that the bite of the 2,6-bis(diphenylphosphanylmethyl)phenyl group is too large Dueto symmetrically to the fact that bind the to biteMg2+ ofions, the larger 2,6-bis(diphenylphosphanylmethyl)phenyl alkaline earth ions might be more adequate group for this is tooaryllarge to symmetricallysubstituent. Therefore, bind to Mgbromo-2,6-bi2+ ions, largers(diphenylphosphanyl alkaline earthmethyl)benzene ions might be more(1a) and adequate activated for this aryl substituent.calcium powder Therefore, were stirred bromo-2,6-bis(diphenylphosphanylmethyl)benzene at room temperature in THF for six hours. In order to ( 1aaccelerate) and activated the calciumreduction powder reaction, were a stirred few crystals at room of iodine temperature were adde ind. THF Filtration for six and hours. storage In of order the filtrate to accelerate at 4 °C the reductionyielded reaction, crystals of a fewthe calcium crystals diphenylphosphinate of iodine were added. bromide, Filtration [(thf)3Ca(Br)(µ-O and storage2PPh2)]2 of (3a the). Rather filtrate at poor crystal quality and heavily disordered thf ligands limit the discussion of the molecular 4 ◦C yielded crystals of the calcium diphenylphosphinate bromide, [(thf) Ca(Br)(µ-O PPh )] (3a). structure and only a motif was refined, which is shown in Figure 4. The3 connectivity2 can2 be2 Ratherelucidated; poor crystal however, quality a discussion and heavily of the disordered structuralthf parameters ligands limitcan only the br discussioniefly be offered of the for molecular the structureinner andfragment. only aThe motif calcium was atoms refined, are whichin distorted is shown octahedral in Figure environments4. The connectivity with a Ca1–Br1 can be elucidated;distance however, of 296.8(2) a pm. discussion The diphenylphosphinate of the structural anions parameters occupy can bridging only brieflypositions be leading offered to for a the innerdinuclear fragment. complex The calcium with a atomscentral areeight-membered in distorted octahedralCa2P2O4 ring environments containing a crystallographic with a Ca1–Br1 center distance of 296.8(2)of symmetry. pm. The The diphenylphosphinate Ca1–O1A/2 bonds of anions225.5(6) occupyand 221.6(5) bridging pm are positions rather short leading due toto astrong dinuclear complexelectrostatic with a central attractions. eight-membered The P1–O1/2–Ca1A/1 Ca2P2O4 bondring containingangles of 142.3(3)° a crystallographic (O1) and 175.3(4)° center of(O2) symmetry. are large and differ significantly, supporting mainly non-directional ionic interactions between calcium and the anions.
Inorganics 2016, 4, 39 5 of 12
The Ca1–O1A/2 bonds of 225.5(6) and 221.6(5) pm are rather short due to strong electrostatic attractions. The P1–O1/2–Ca1A/1 bond angles of 142.3(3)◦ (O1) and 175.3(4)◦ (O2) are large and differ significantly, supportingInorganics mainly 2016, 4, 39 non-directional ionic interactions between calcium and the anions. 5 of 12
Figure 4. Structural motif and atom numbering scheme of [(thf)3Ca(Br)(µ-O2PPh2)]2 (3a). Symmetry Figure 4. Structural motif and atom numbering scheme of [(thf)3Ca(Br)(µ-O2PPh2)]2 (3a). Symmetry relatedrelated atoms atoms are are marked marked with with the the letter letter “A”. “A”. TheThe ellipsoids represent represent a probability a probability of 30%; of 30%; the thf the thf ligands are drawn with arbitrary radii. H atoms and disordering of the thf ligands are not shown. ligands are drawn with arbitrary radii. H atoms and disordering of the thf ligands are not shown. Bond lengths of the inner core (pm): Ca1–Br1 296.8(2), Ca1–O1A 225.5(6), Ca1–O2 221.6(5), Ca1–O3 Bond lengths of the inner core (pm): Ca1–Br1 296.8(2), Ca1–O1A 225.5(6), Ca1–O2 221.6(5), Ca1–O3 234.9(7), Ca1–O4 237.6(7), Ca1–O5 238.8(5), P1–O1 150.4(5), P1–O2 150.3(5), P1–C1 180.6(9), P1–C7 234.9(7), Ca1–O4 237.6(7), Ca1–O5 238.8(5), P1–O1 150.4(5), P1–O2 150.3(5), P1–C1 180.6(9), P1–C7 180.0(8); angles (°): O1A–Ca1–O2 94.4(2), O1–P1–O2 119.1(3), P1–O1–Ca1A 142.3(3), P1–O2–Ca1 ◦ 180.0(8);175.3(4), angles Br1–Ca1–O1A ( ): O1A–Ca1–O2 174.0(1), Br1–Ca1–O2 94.4(2), O1–P1–O2 91.4(2). 119.1(3), P1–O1–Ca1A 142.3(3), P1–O2–Ca1 175.3(4), Br1–Ca1–O1A 174.0(1), Br1–Ca1–O2 91.4(2). After addition of 1,2-dimethoxyethane (DME) and storage in a refrigerator, another crop of Aftercrystals addition of [(dme) of2(thf)CaBr 1,2-dimethoxyethane2] (4) was obtained. (DME) The molecular and storage structure in aand refrigerator, numbering scheme another of cropC2 of symmetric 4 are depicted in Figure 5. The calcium atoms are in a distorted pentagonal bipyramidal crystals of [(dme)2(thf)CaBr2](4) was obtained. The molecular structure and numbering scheme of C2 environment with a Ca1–Br1 distance of 288.86(2) pm. Due to steric hindrance between the methoxy symmetric 4 are depicted in Figure5. The calcium atoms are in a distorted pentagonal bipyramidal groups of the dme ligands, the Ca1–O1 bond [249.0(1) pm] is slightly elongated compared to the environmentCa1–O3 distance with a Ca1–Br1to the thf distance ligand (238.0(1) of 288.86(2) pm). pm.The Duebulkiness to steric of these hindrance methoxy between groups theare methoxyalso groupsresponsible of the dme for the ligands, non-linearity the Ca1–O1 of the Br1–Ca1–Br1A bond (249.0(1) moiety pm) (171.07(2)°). is slightly elongated compared to the Ca1–O3 distanceThe product to thediversity thf ligand of the (238.0(1) reduction pm). of bromo-2,6-bis(diphenylphos The bulkiness of thesephanylmethyl)benzene methoxy groups are also responsible(1a) with for calcium the non-linearity hints toward of thefast Br1–Ca1–Br1Aether degradation moiety processes. (171.07(2) The◦ ).identification of calcium Thebromide product verifies diversity the initial of the formation reduction of ofthe bromo-2,6-bis(diphenylphosphanylmethyl)benzene heavy Grignard reagent Aryl–Ca–Br. This complex (1a) withdeprotonates calcium hints THF toward yielding fast 2,6-bi ethers(diphenylphosphanylmethyl)benzene degradation processes. The identification that has been of calcium observed bromide by 31 1 verifiesP{ theH} initialNMR formationspectroscopy. of theThe heavyisolationGrignard of [(thf)reagent3Ca(Br)(µ-O Aryl–Ca–Br.2PPh2)]2 (3a This) from complex these reduction deprotonates reactions proves that the P–C bond to the methylene unit has been cleaved, finally leading to the THF yielding 2,6-bis(diphenylphosphanylmethyl)benzene that has been observed by 31P{1H} NMR diphenylphosphinate anions. Earlier findings already demonstrated that oxide ions can be trapped spectroscopy. The isolation of [(thf) Ca(Br)(µ-O PPh )] (3a) from these reduction reactions proves that in calcium cage compounds in the3 absence of2 low-valent2 2 phosphorus species [25–28]. In summary, the P–Cthe bondvery reactive to the methylene calcium-based unit heavy has been Grignard cleaved, reagents finally quickly leading attack to the the diphenylphosphinate ether molecules, leading anions. Earlierto findingsarene and already diverse demonstratedether degradation that products. oxide ions Attempts can be to trapped isolate arylcalcium in calcium cagebromide compounds failed due in the absenceto the of fast low-valent decomposition phosphorus reactions. species [25–28]. In summary, the very reactive calcium-based heavy Grignard reagentsBased on quicklyour experience attack thethat etheriodoarenes molecules, react much leading more to arenereadily and than diverse bromoarenes ether degradation and in products.order Attemptsto accelerate to isolatethe formation arylcalcium of the bromide heavy Grignard failed due reagent, to the we fast reacted decomposition excess of activated reactions. Basedcalcium on with our iodo-2,6-bis(diphenylphosphanylmethyl)benzene experience that iodoarenes react much more (1b readily) in tetrahydrofuran than bromoarenes at room and in 31 1 ordertemperature. to accelerate A theP{ H} formation NMR spectrum, of the heavyrecordedGrignard after a reagent,few hours, we already reacted showed excess primarily of activated 2,6-bis(diphenylphosphanylmethyl)benzene. Storage of the reaction solution at −40 °C led to calcium with iodo-2,6-bis(diphenylphosphanylmethyl)benzene (1b) in tetrahydrofuran at room crystallization of [(thf)3Ca(I)(µ-O2PPh2)]2 (3b, Figure 6); a motif of this compound has been reported temperature. A 31P{1H} NMR spectrum, recorded after a few hours, already showed primarily earlier [29,30]. In analogy to 3a, the central feature is a centrosymmetric eight-membered Ca2P2O4 2,6-bis(diphenylphosphanylmethyl)benzene. Storage of the reaction solution at −40 ◦C led to ring with an average Ca–O distance of 224.3 pm. Due to a weaker electrostatic attraction, the Ca–Othf crystallizationbonds to the of tetrahydrofuran [(thf)3Ca(I)(µ-O ligands2PPh2 )]are2 ( 3belongated, Figure and6); aa motifmean ofvalue this of compound 238.7 pm is hasobserved. been reportedThe earlierrather [29,30 small]. In analogyCa–Othf distances to 3a, the support central featurelack of issteric a centrosymmetric strain in this compound. eight-membered The rather Ca2 Plarge2O4 ring with anP1–O1/2–Ca1/1A average Ca–O bond distance angles of 224.3of 165.2(3) pm. Due(O1) toand a weaker146.7(2)° electrostatic hint toward attraction, dominating the electrostatic Ca–Othf bonds to theinteractions tetrahydrofuran between ligands calcium are ions elongated and phosphinate and a mean anions value and of 238.7 negligible pm is covalent observed. contributions. The rather small Ca–O thf distances support lack of steric strain in this compound. The rather large P1–O1/2–Ca1/1A Inorganics 2016, 4, 39 6 of 12
◦ bondInorganicsInorganics angles 20162016 of,, 44 165.2(3),, 3939 (O1) and 146.7(2) hint toward dominating electrostatic interactions between66 ofof 1212 calcium ions and phosphinate anions and negligible covalent contributions. The phosphorus atom is in a distortedTheThe phosphorusphosphorus tetrahedral atomatom environment, isis inin aa distorteddistorted with the tetrahedtetrahed largestralral angle environment,environment, being O1–P1–O2 withwith thethe with largestlargest a value angleangle of 119.3(2) beingbeing ◦. TheO1–P1–O2O1–P1–O2 Ca1–I1 bond withwith length aa valuevalue of ofof 318.7(1) 119.3(2)°.119.3(2)°. pm TheThe lies Ca1–I1Ca1–I1 in the bondbond upper lengthlength region ofof as318.7(1)318.7(1) also observed pmpm lieslies inin for thethe ether upperupper adducts regionregion of phenylcalciumasas alsoalso observedobserved iodide forfor etherether and calcium adductsadducts diiodide ofof phenylcalciumphenylcalcium [31]. iodideiodide andand calciumcalcium diiodidediiodide [31].[31].
Figure 5. Molecular structure and atom numbering scheme of [(dme)2(thf)CaBr2](4). Symmetry related FigureFigure 5.5. MolecularMolecular structurestructure andand atomatom numberingnumbering schemescheme ofof [(dme)[(dme)22(thf)CaBr(thf)CaBr22]] ((44).). SymmetrySymmetry atoms are marked with the letter “A”. The ellipsoids represent a probability of 30%; H atoms are relatedrelated atomsatoms areare markedmarked withwith thethe letterletter “A”.“A”. TheThe ellipsoidsellipsoids representrepresent aa probabilityprobability ofof 30%;30%; HH atomsatoms neglected for clarity reasons. Selected bond lengths (pm): Ca1–Br1 288.86(2), Ca1–O1 249.0(1), Ca1–O2 areare neglectedneglected forfor clarityclarity reasons.reasons. SelectedSelected bondbond lengthslengths (pm):(pm): Ca1–Br1Ca1–Br1 288.86(2),288.86(2), Ca1–O1Ca1–O1 249.0(1),249.0(1), 243.4(1), Ca1–O3 238.0(1); angles (◦): Br1–Ca1–Br1A 171.07(2), Br1–Ca1–O1 80.03(3), Br1–Ca1–O2 Ca1–O2Ca1–O2 243.4(1),243.4(1), Ca1–O3Ca1–O3 238.0(1);238.0(1); anglesangles (°(°):): Br1–Ca1–Br1ABr1–Ca1–Br1A 171.07(2),171.07(2), Br1–Ca1–O1Br1–Ca1–O1 80.03(3),80.03(3), 94.07(3), Br1–Ca1–O3 85.535(8). Br1–Ca1–O2Br1–Ca1–O2 94.07(3),94.07(3), Br1–Ca1–O3Br1–Ca1–O3 85.535(8).85.535(8).
Figure 6. Molecular structure and atom numbering scheme of [(thf)3Ca(I)(µ-O2PPh2)]2 (3b). Symmetry FigureFigure 6.6. MolecularMolecular structurestructure andand atomatom numberingnumbering schemescheme ofof [(thf)[(thf)33Ca(I)(µ-OCa(I)(µ-O22PPhPPh22)])]22 ((3b3b).). relatedSymmetrySymmetry atoms relatedrelated are marked atomsatoms with areare markedmarked the letter withwith “A”. thethe The letterletter ellipsoids “A”.“A”. TheThe represent ellipsoidsellipsoids a probability rerepresentpresent aa of probabilityprobability 30%; H atoms ofof and30%;30%; disordering HH atomsatoms and ofand the disorderingdisordering thf ligands ofof are thethe not thfthf shown.ligandsligands are Bondare notnot lengths shown.shown. of BondBond the inner lengthslengths core ofof the (pm):the innerinner Ca1–I1 corecore 318.7(1),(pm):(pm): Ca1–O1Ca1–I1Ca1–I1 222.7(4), 318.7(1),318.7(1), Ca1–O2A Ca1–O1Ca1–O1 225.9(4),222.7(4),222.7(4), Ca1–O3 Ca1–O2ACa1–O2A 236.8(5), 225.9(225.9( Ca1–O44),4), Ca1–O3Ca1–O3 240.2(4), 236.8(5),236.8(5), Ca1–O5 Ca1–O4Ca1–O4 239.0(4), 240.2(4),240.2(4), P1–O1 Ca1–O5Ca1–O5 150.4(4), ◦ P1–O2239.0(4),239.0(4), 150.3(4), P1–O1P1–O1 P1–C1 150.4(4),150.4(4), 181.0(5), P1–O2P1–O2 P1–C7 150.3(4),150.3(4), 180.5(6); P1–C1P1–C1 angles 181.181.0(5),0(5), ( ): O1–Ca1–O2AP1–C7P1–C7 180.5(6);180.5(6); 92.4(1), anglesangles Ca1–O1–P1 (°):(°): O1–Ca1–O2AO1–Ca1–O2A 165.2(3), O1–P1–O292.4(1),92.4(1), 119.3(2),Ca1–O1–P1Ca1–O1–P1 P1–O2–Ca1A 165.2(3),165.2(3), 146.7(2),O1–P1–O2O1–P1–O2 I1–Ca1–O1 119.3(2)119.3(2),, 93.1(1), P1–O2–Ca1AP1–O2–Ca1A I1–Ca1–O2A 146.7(2),146.7(2), 174.2(1), I1–Ca1–O1I1–Ca1–O1 I1–Ca1–O3 93.1(1),93.1(1), 90.6(1), I1–Ca1–O4I1–Ca1–O2AI1–Ca1–O2A 87.8(1), 174.2(1),174.2(1), I1–Ca1–O5 I1–Ca1–O3I1–Ca1–O3 92.0(1), 90.6(1),90.6(1), O1–Ca1–O3 I1–Ca1–O4I1–Ca1–O4 92.8(2), 87.8(1),87.8(1), O1–Ca1–O4 I1–Ca1–O5I1–Ca1–O5 92.0(1), 175.8(2),92.0(1), O1–Ca1–O3O1–Ca1–O3 O1–Ca1–O5 92.8(2),92.8(2), 98.2(2), O2A–Ca1–O3O1–Ca1–O4O1–Ca1–O4 175.8(2), 91.2(2),175.8(2), O2A–Ca1–O4 O1–Ca1–O5O1–Ca1–O5 98.2(2),98.2(2), 86.9(2), O2A–CO2A–C O2A–Ca1–O5a1–O3a1–O3 91.2(2),91.2(2), 85.2(2), O2A–Ca1–O4O2A–Ca1–O4 O3–Ca1–O4 86.9(2),86.9(2), 83.1(2), O2A–Ca1–O5O2A–Ca1–O5 O3–Ca1–O5 168.6(2),85.2(2),85.2(2), O4–Ca1–O5 O3–Ca1–O4O3–Ca1–O4 85.8(2). 83.1(2),83.1(2), O3–Ca1–O5O3–Ca1–O5 168.6(2),168.6(2), O4–Ca1–O5O4–Ca1–O5 85.8(2).85.8(2).
Inorganics 2016, 4, 39 7 of 12
2.3. Theoretical View on Ether and Phosphane Complexes of Magnesium and Calcium Ions It remains puzzling that crystallization of the Grignard reagent bis(2,6-bis(diphenylphosphanyl methyl)phenyl)magnesium (2) from THF or from a mixture of THF and DME yielded solvent-free complexes. The Pearson concept of hard and soft acids and bases suggests that the hard magnesium ions prefer the hard ether bases rather than the soft phosphanes [32]. The structure of starting bromo-2,6-bis(diphenylphosphanylmethyl)benzene shows that the diphenylphosphanylmethyl side arms can turn to the side and open a coordination site at the metal center. However, entropic effects would favor intramolecular coordination of the phosphanyl moieties. In order to judge the preference of ether or phosphane coordination to magnesium ions, quantum chemical calculations, using the method B3LYP/6-311++G**, were undertaken with respect to Equation (2). The reaction enthalpies 2+ ∆H for these addition reactions are listed in Table1. The structures of the [(thf) 6M] ions are compared with those of selected X-ray structure determinations (M = Mg [33], Ca [34]) and show an excellent agreement. 2+ 2+ [(L)n M] + L ↔ (L)n+1 M (2)
−1 Table 1. Reaction enthalpies ∆H (kJ·mol ) for the addition of the Lewis base L (thf, PMe3) to the metal 2+ fragment [(L)nM] (M = Mg, Ca).
M, L n = 0 n = 1 n = 2 n = 3 n = 4 n = 5 Mg, thf −99 −87 −79 −78 −65 −23 Mg, −104 −82 −52 −37 +43 - a PMe3 Ca, thf −57 −47 −47 −46 −45 −28 Ca, PMe3 −37 −28 −27 −23 −2 −2 a 2+ A complex of the type [Mg(PMe3)6] is not stable and dissociates during optimization of the structure.
The coordination of the first thf and PMe3 molecule at the naked magnesium ion (n = 0) shows very similar energies. Due to the larger size of trimethylphosphane, the binding energies decrease much faster for larger coordination numbers and larger n values. According to these calculations, only four phosphane bases bind to the small Mg2+ cation, whereas a coordination number of six can be realized for the ether adduct. This finding is in agreement with the expectation that intramolecular repulsion between bulky ligands destabilizes complexes. The binding energies of the Lewis bases L to the larger alkaline earth ion Ca2+ are smaller due to larger Ca–L distances lowering the electrostatic attraction. Unexpectedly, the reaction enthalpies for the coordination of the softer phosphane are significantly smaller than for the binding of the hard thf base, however, hexa-coordinate calcium ions are feasible for thf and PMe3 adducts. Even if steric hindrance is absent (n = 0), the Ca–P binding 2+ 2+ enthalpy for [Ca-PMe3] is roughly a third smaller than for the ether adduct [Ca-thf] .
3. Discussion and Conclusions The reduction of bromo-2,6-bis(diphenylphosphanylmethyl)benzene (1a) in tetrahydrofuran requires the presence of activated magnesium, activation by iodine, and drastic reaction conditions due to the shielding of the C–Br functionality by the ortho-bound diphenylphosphanylmethyl side arms. In solution, the heteroleptic Grignard reagent Aryl–Mg–Br and homoleptic diarylmagnesium (2) coexist. In order to shift the Schlenk equilibrium toward the homoleptic complex 2, 1,4-dioxane was added to precipitate the magnesium bromide. Recrystallization from THF or THF/DME mixtures yielded ether-free 2. Intramolecular repulsion between the diphenylphosphanyl side arms enforces an asymmetric coordination behavior leading to significantly different Mg–P distances. A different reaction behavior was observed for the reduction of bromo- (1a) and iodo-2,6-bis(diphenylphosphanylmethyl)benzene (1b) with activated calcium in THF at room temperature. Heavy Grignard reagents of the type Aryl–Ca–Br or diarylcalcium were not accessible by this procedure, but ether degradation by arylcalcium species occurred immediately and was much Inorganics 2016, 4, 39 8 of 12 faster than the formation of these heavy Grignard-type reagents. The enormous reactivity also led to the cleavage of the P–C bonds to the methylene units and to oxidation reactions, finally yielding diphenylphosphinate anions. These results are in agreement with earlier studies that showed that methylation of phenylcalcium halide in 2,4,6-positions (mesitylcalcium halide) significantly enhances the reactivity, leading to faster ether degradation and also side reactions such as deprotonation of ortho-methyl groups (yielding benzyl derivatives) [35]. The quantum chemically-elucidated reaction enthalpies support the finding that ethers and phosphanes show comparable binding energies for magnesium complexes. In agreement with these findings, tetrahydrofuran and bidentate 1,2-dimethoxyethane were unable to replace the intramolecularly coordinated phosphanes in bis[2,6-bis(diphenylphosphanylmethyl)]magnesium (2), especially with additional consideration of entropic effects. A different situation can be envisioned for an isostructural calcium complex: On the one hand, ether binding is preferred compared to phosphane coordination, according to quantum chemical calculations. On the other hand, the Ca2+ ion is significantly more accessible to coordination of ether bases due to the larger size, thus easing intramolecular ether cleavage reactions via initial coordination and deprotonation steps. The preferred coordination of ethers rather than phosphanes can be understood by mainly electrostatic forces; due to significantly larger Ca–P distances the electrostatic attraction is smaller in comparison to calcium-ether interactions.
4. Materials and Methods
4.1. General All manipulations were carried out in an inert nitrogen atmosphere using standard Schlenk techniques. The solvents were dried according to standard procedures over KOH and subsequently distilled over sodium/benzophenone in a nitrogen atmosphere prior to use. Deuterated solvents were dried over sodium, degassed, and saturated with nitrogen. The yields given are not optimized. 1H, 13C{1H}, 31P, and 31P{1H} NMR spectra were recorded on Bruker AC 400 and AC 600 spectrometers (Bruker AXS GmbH, Karlsruhe, Germany). Chemical shifts are reported in parts per million relative to SiMe4 or 85% phosphoric acid as external standards. The residual signals of the deuterated solvents were used as internal standards for 1H and 13C{1H} NMR experiments. Commercially available chemicals were purchased from Sigma-Aldrich (Sigma-Aldrich Chemie GmbH, Munich, Germany) or Alfa Aesar (Thermo Fisher GmbH, Karlsruhe, Germany). Bromo-2,6-bis(diphenylphosphanylmethyl)benzene (1a)[36,37] and Rieke magnesium [38] were prepared according to literature procedures.
4.2. Synthesis of Iodo-2,6-bis(diphenylphosphanylmethyl)benzene (1b) Bromo-2,6-bis(diphenylphosphanylmethyl)benzene (1a) (5.71 g, 10.3 mmol) was suspended in a mixture of 50 mL of diethyl ether and 3 mL of TMEDA. At −78 ◦C, 6.78 mL of a 1.6 M n-butyllithium solution in hexane (10.5 mmol) were slowly added dropwise and the reaction mixture kept at this temperature for another hour. Thereafter, the suspension was slowly heated to room temperature, resulting in a clear solution. After stirring at room temperature for another hour, 2.88 g of iodine, dissolved in 50 mL of diethyl ether, were added dropwise. During stirring overnight, a pale yellow precipitate formed. This solid was collected, dissolved in dichloromethane and washed with aqueous sodium thiosulfate solution. The organic phase was separated and dried over anhydrous sodium sulfate. The solvent was removed in vacuo. The residue was washed with acetone, leaving 1.6 g of 1 ◦ 1b (25%). H NMR (400 MHz, CDCl3, 25 C): δ 7.40–7.30 (m, 8H), 7.30–7.20 (m, 12H), 6.63 (s, 1H), 13 1 ◦ 6.46–6.36 (m, 2H), 3.58 (s, 4H). C{ H} NMR (101 MHz, CDCl3, 25 C): δ 141.2 (dd, JPC = 7.3 and 1.7 Hz), 138.0 (d, JPC = 16.0 Hz), 133.2 (d, JPC = 19.1 Hz), 128.7 (s), 128.4 (d, JPC = 6.7 Hz), 127.9 (dd, JPC = 8.2 and 31 1 2.9 Hz), 126.9 (t, JPC = 1.7 Hz), 109.6 (t, JPC = 5.1 Hz), 43.0 (d, JPC = 17.2 Hz). P{ H} NMR (162 MHz, ◦ CDCl3, 25 C): δ −13.0. Inorganics 2016, 4, 39 9 of 12
4.3. Synthesis of Bis(2,6-bis(diphenylphosphanylmethyl)phenyl)Magnesium (2) Bromo-2,6-bis(diphenylphosphanylmethyl)benzene (1a) (3.0 g, 5.4 mmol), dissolved in 10 mL of THF, was added at once to a suspension of 100 mg of magnesium (1:1 ratio of Rieke magnesium and magnesium turnings, 6.7 mmol) in 10 mL of THF. This reaction was heated under reflux and a small amount of iodine was added. The reaction mixture was stirred overnight at room temperature. After removal of all solid materials, the filtrate was cooled to 5 ◦C. Then, 1,2-dimethoxyethane was ◦ added until all solids redissolved. Storage at 5 C led to crystallization of 2 and [(dme)2(thf)CaBr2](4). Acidimetric titration showed a conversion of 45%; therefore, 0.8 mL of 1,4-dioxane were added in order to shift the Schlenk equilibrium toward the homoleptic diarylmagnesium and magnesium bromide. Within one day, a cloudy suspension formed. The solid magnesium bromides were removed and the filtrate cooled to −20 ◦C, yielding 0.6 g of complex 2 (11%). The volume of the mother liquor was reduced to half of the original volume and cooled again to −20 ◦C, leading to another crop of crystals of 2 (1.2 g, 22%). Elemental analysis (C64H54P4Mg, 971.26): calcd.: C 79.14, H 5.60, found: C 79.27, 1 ◦ H 5.77; titration against EDTA: calcd.: Mg 2.50, found: Mg 2.7. H NMR (600 MHz, [D8]THF, 25 C): δ 7.36 (m, 8H), 7.17 (t, J = 7.4 Hz, 4H), 7.01 (t, J = 7.6 Hz, 8H), 6.81 (t, J = 7.4 Hz, 1H), 6.68 (d, J = 7.4 Hz, 13 1 ◦ 2H), 3.55 (s, 8H). C{ H} NMR (151 MHz, [D8]THF, 25 C): δ 172.0 (m, ipso-C), 147.0, 137.2, 133.6, 129.0, 31 1 ◦ 128.7, 126.3, 125.9, 40.4. P{ H} NMR (243 MHz, [D8]THF, 25 C): δ −15.7 (s).
4.4. X-ray Structure Determinations The intensity data for the compounds were collected on a Nonius KappaCCD diffractometer (Bruker AXS GmbH) using graphite-monochromated Mo-Kα radiation. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semi-empirical basis using multiple-scans [39–41]. The structures were solved by direct methods (SHELXS [42]) and refined 2 by full-matrix least squares techniques against Fo (SHELXL-97 [42,43] and SHELXL-2014 [44]). All hydrogen atoms bounded to the compounds 1a and 2 were located by difference Fourier synthesis and refined isotropically. All other hydrogen atoms were included at calculated positions with fixed thermal parameters. All non-hydrogen, non-disordered atoms were refined anisotropically [42–44]. The crystal of 2 was a partial-merohedral twin. The twin law was determined with PLATON [45] to (−1.0, 0.0, 0.0)/0.0, −1.0, 0.0/0.0, 0.0, 1.0). The contribution of the main component was refined to 0.926(1). The crystals of 3a were extremely thin and of low quality, resulting in a substandard data set; however, the structure is sufficient to show connectivity and geometry despite the high final R value. We will only publish the conformation of the molecule and the crystallographic data and will not deposit the data in the Cambridge Crystallographic Data Centre. Crystallographic data as well as structure solution and refinement details are summarized in Table S1. XP [46] and POV-Ray [47] were used for structure representations.
4.5. Quantum Chemical Calculations All molecular geometry optimizations were performed with the Gaussian09 [48] suite of programs at B3LYP/6-311++G** [49–52] level of theory. Vibrational analysis was performed at the same level of theory to confirm the nature of local structure minima. The basis set superposition errors were computed with the keyword “counterpoise”.
Supplementary Materials: The following are available online at: www.mdpi.com/2304-6740/4/4/39/s1, Table S1: Crystal data and refinement details for the X-ray structure determinations of the compounds 1a, 2, 3a, 3b, and 4. Cartesian coordinates of calculated structures. Crystallographic data (excluding structure factors) has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC-1509090 for 1a, CCDC-1509091 for 2, CCDC-1509092 for 3b, and CCDC-1509093 for 4. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (E-mail: [email protected]). Acknowledgments: We acknowledge the financial support by the Verband der Chemischen Industrie e.V. (FCI/VCI, Frankfurt/Main, Germany; fund No. 510259); Alexander Koch is grateful to the Verband der Chemischen Industrie e.V. (FCI/VCI) for a generous Ph.D grant. Inorganics 2016, 4, 39 10 of 12
Author Contributions: Alexander Koch, Sven Krieck, and Matthias Westerhausen conceived and designed the experiments; Alexander Koch performed the experiments; Alexander Koch, Sven Krieck, and Matthias Westerhausen analyzed the data; Helmar Görls carried out the X-ray crystal structure determinations and the interpretation of the crystal data; Alexander Koch performed all quantum chemical calculations; all authors contributed to the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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