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) Inorganics 2016, 4, 39 2 of 12 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).