Synthesis, Structural Elucidation, and Diffusion-Ordered NMR Studies

Jul 9, 2012 - School of Chemistry, Newcastle University, Newcastle upon Tyne, U.K. NE1 7RU ... alkyl ligand CH2SiMe3 in the presence of a variety of L...
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Synthesis, Structural Elucidation, and Diffusion-Ordered NMR Studies of Homoleptic Alkyllithium Magnesiates: Donor-Controlled Structural Variations in Mixed-Metal Chemistry ́ ́ lvarez,§ Eva Hevia,*,† Alan R. Kennedy,† Jan Klett,† Sharon E. Baillie,† William Clegg,‡ Pablo Garcıa-A and Luca Russo‡ †

WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, U.K. G1 1XL School of Chemistry, Newcastle University, Newcastle upon Tyne, U.K. NE1 7RU § Departamento de Quı ́mica Orgánica e Inorgánica-IUQOEM, Universidad de Oviedo-CSIC, E-33071 Oviedo, Spain ‡

S Supporting Information *

ABSTRACT: This paper presents the synthesis and characterization of new homoleptic lithium magnesiate reagents incorporating the silyl-substituted alkyl ligand CH2SiMe3 in the presence of a variety of Lewis base donors, namely tetrahydrofuran (THF), 1,4-dioxane, N,N,N′,N′-tetramethylethylenediamine (TMEDA), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA). The constitution of these bimetallic compounds has been assessed in both the solid state and solution using a combination of X-ray crystallographic studies and multinuclear NMR spectroscopy, including 1 H diffusion-ordered (1H-DOSY) NMR experiments. These studies highlight the major role played by the donor molecule in controlling the structure of the complexes as well as the wide structural diversity available for these mixed-metal species ranging from discrete molecules, as found for [(PMDETA)LiMg(CH2SiMe3)3] (6), to more complex supramolecular arrangements, as in the 1D-polymeric chain [{(THF)LiMg(CH2SiMe3)3}∞] (2) or in the stoichiometrically distinct dioxane solvates [{(dioxane)2LiMgR3}∞] (3) and [{(dioxane)Li2Mg2R6}∞] (4). Furthermore, these studies have also revealed that in some cases the donor molecule can promote a redistribution process, as shown for the reaction of triorganomagnesiate [LiMg(CH2SiMe3)3] (1) with 1 molar equiv of TMEDA, which led to the formation of lithium-rich tetraorganomagnesiate [(TMEDA)Li2Mg(CH2SiMe3)4] (5) along with Mg(CH2SiMe3)2. The formation of the unprecedented cationic lithium magnesiate [{(PMDETA)2Li2Mg(CH2SiMe3)3}+{Mg3(CH2SiMe3)6(OCH2SiMe3)}−] (7) is also described, by the controlled exposure to oxygen of the monomeric compound [(PMDETA)LiMg(CH2SiMe3)3] (6).

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available using single-metal bases.4 Outperforming classical organolithium or Grignard reagents, homoleptic alkyllithium magnesiates (which depending on their stoichiometry can be grouped as triorganomagnesiates LiMgR3 or tetraorganomagnesiates Li2MgR4) have also been established as useful reagents, finding extensive applications in a myriad of fundamental organic transformations such as nucleophilic alkylation,5 magnesium−halogen exchange,6 and deprotonation (C−H bond breaking) reactions,7 to name a few. Surprisingly, it should be noted that in these seminal reactivity studies the constitution of the active mixed lithium−magnesium organometallic species (usually prepared in situ) remains unknown in most cases. In addition, resembling the behavior of classical organolithium reagents,8 some reports have highlighted that the overall performance of alkali-metal magnesiates in organic transformations can be finely tuned by incorporating within the

INTRODUCTION Among the vast catalog of polar organometallic reagents, lithium magnesiates have emerged as a selective and versatile family of compounds by delivering new chemistry which cannot be replicated by either organolithium or organomagnesium reagents on their own.1 These bimetallic reagents, the early examples of which can be credited to Wittig some 60 years ago,2 have witnessed a recent resurgence in popularity due to their synergic chemical profiles, which can include a superior reactivity and functional group tolerance, combined with selectivity patterns distinct to those exhibited by their monometallic components. Among the important breakthroughs in this area was the development of the Turbo Grignard reagents “RMgX·LiCl”, whose enhanced nucleophilicity allows access to highly functionalized Grignard reagents via Mg−halogen exchange reactions3 or the use of heterobimetallic amides in alkali-metal-mediated magnesiation (AMMMg) processes to facilitate in certain cases the regioselective deprotonation of organic substrates in remote positions, not © 2012 American Chemical Society

Received: May 30, 2012 Published: July 9, 2012 5131

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Scheme 1. Reactivity of 1 with Selected Lewis Basesa

a

Legend: (i) THF; (ii) dioxane (1 equiv); (iii) dioxane (0.25 equiv); (iv) TMEDA; (v) PMDETA.

raphy with NMR spectroscopic studies, including 1H-DOSY NMR experiments. These studies highlight the broad structural diversity available for these mixed-metal species, ranging from discrete molecules to more complex supramolecular arrangements, and provide new insights into the fundamental correlations existing between the structures in the solid state and the true aggregation of these species in solution, which can be key in understanding the unique reactivities reported for these species. Furthermore, the controlled exposure to oxygen of the PMDETA-solvated derivative [(PMDETA)LiMg(CH2SiMe3)3] (6; PMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine) leads to the isolation of a novel cationic lithium magnesiate which is unprecedented in mixed-metal chemistry.

reaction solution variable amounts of Lewis donors (such as TMEDA (N,N,N′,N′-tetramethylethylenediamine) and THF),7d,f,9 hinting at an effect of the latter in modifying their degree of aggregation and/or constitution. Furthermore, within the context of mixed-metal chemistry, the unique (synergic) reactivity of “ate” reagents has been linked to the presence of bridging ligands that, by bringing both metals into close proximity to each other, enables an effective metal−ligand− metal′ communication.1d,e,10 Surprisingly, despite the growing amount of interest that alkali-metal magnesiates are attracting and their numerous synthetic applications, there is still a considerable lack of knowledge about their constitutions, either in the solid state or more importantly in solution, as well as about the role that different solvents or donor molecules may play in controlling the structure/reactivity relationships of this class of organometallic reagents. Thus, underlining this underdevelopment, a search of the current CSD11 reveals that only four homoleptic alkyl alkali-metal magnesiates have been structurally characterized.12 Starting to fill this important gap in mixed-metal chemistry, we have recently crystallographically confirmed the structure of the first donor-free alkali-metal tris(alkyl)magnesiate in [{NaMg(CH2SiMe3)3}∞] and estimated its aggregation in solution using 1H diffusion-ordered NMR spectroscopy (1H-DOSY).12c Building on this previous work, herein we extend our investigations to the synthesis of a new series of homoleptic alkyllithium magnesiates by systematically studying the cocomplexation reactions of [LiCH2SiMe3] with [Mg(CH2SiMe3)2] in the presence of a variety of donor molecules with different hapticities and coordinative properties. Thus, the effect that these Lewis bases impose on the structures in the solid-state and solution behavior of these lithium magnesiates has been assessed by combining X-ray crystallog-

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RESULTS AND DISCUSSION Syntheses. On the basis of previous reports in alkali-metal chemistry which show that group 1 alkyl complexes containing silyl-substituted methyl groups exhibit enhanced stabilities in comparison to more conventional carbon-based alkyl groups,13 we chose the monosilyl CH2SiMe3 group as the anionic ligand for our study. First we studied the cocomplexation reaction of equimolar amounts of both monometallic alkyls LiR and MgR2 (R = CH2SiMe3) using noncoordinating hexane as the solvent. Gently heating the resulting suspension afforded a colorless solution, suggesting the formation of a mixed-metal complex, since on its own MgR2 is completely insoluble in this solvent.14,15 Slow cooling of this solution resulted in the deposition of colorless needle crystals of lithium magnesiate [LiMg(CH2SiMe3)3] (1) in 96% yield (see the Experimental Section and Scheme 1). The Lewis base donors tetrahydrofuran (THF), 1,4-dioxane, TMEDA, and PMDETA were each added to 1 to study their coordination effects (Scheme 1). Adducts 5132

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Figure 1. Section of polymeric 2 demonstrating propagation and showing selected atom labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms, disorder in THF, and disorder in one TMS group have been omitted for clarity.

cocomplexation reaction suggests that its formation is quantitative (no other organometallic species were detected in solution). Solid-State Structures. Although the parent magnesiate 1 could be isolated as colorless crystals, they appear to be too reactive to be analyzed by X-ray crystallography. Thus, even when low temperatures were employed (123 K), the crystals decomposed in the X-ray beam before the data collection could be completed. The THF-solvated triorganomagnesiate 2 exhibits a polymeric structure (Figure 1) made up of {(THF)LiMgR3} units, where two of the alkyl groups act as bridges between Li and Mg to form a four-membered [LiCMgC] ring. The remaining alkyl group on Mg bonds to a Li center of a neighboring unit through its methylene anion, giving rise to a 1D chain structure which allows each Li atom to attain further coordinative stabilization. Surprisingly, in contrast with other alkali-metal magnesiates which also contain THF as part of their constitution,9a,18 in 2 this Lewis base chooses to coordinate exclusively to magnesium; furthermore, this preferred Mg− O(THF) interaction is not only present in the solid-state structure of 2 but also appears to be retained in deuterated benzene solutions, as indicated by 1H−7Li HOESY NMR experiments (vide infra), reflecting the strong Lewis acidity of Mg in comparison with Li. In the extended structure of 2 (Figure 1), the THF molecules and the SiMe3 groups (from the alkyl groups which connect the monomeric units) adopt an alternating disposition across the Li···Mg···C vector. Unfortunately a large amount of motion between the metal sites and the THF molecules in 2 (and/or unresolved disorder) adversely affects the precision of this structure and therefore prevents discussion of any geometrical parameters, although its connectivity is definite. To the best of our knowledge, 2 constitutes the first example of a homoleptic lithium magnesiate with a polymeric structure to be structurally defined; furthermore, the propagation of this structure occurs exclusively through the formation of Li−C electron-deficient bonds with a neighboring unit, which is extremely rare in mixed-metal chemistry.19 Thus, this type of bonding is reminiscent of that described in the solvent-free sodium tris(alkyl) magnesiate [{NaMgR3}∞] (R = CH2SiMe3) which features an unprecedented 2D honeycomb network structure made up exclusively of metal−carbon bonds.12c,20

[{(THF)LiMgR 3 } ∞ ] (2), [{(dioxane) 2 LiMgR 3 } ∞ ] (3), [(TMEDA)2Li2MgR4] (5), and [(PMDETA)LiMgR3] (6) were formed from the addition of 1 molar equiv of the relevant donor to the homoleptic trialkyl magnesiate [LiMgR3] (1), whereas [{(dioxane)Li2Mg2R6}∞] (4) was made via the addition of a substoichiometric 0.25 equiv of 1,4-dioxane (Scheme 1). All donor-solvated compounds were characterized by multinuclear NMR spectroscopy, and their structures were elucidated by X-ray crystallography, which confirmed their bimetallic constitution and revealed the changes inferred in the structure and constitution of the magnesiates depending on the donor molecule present in the bimetallic species. Compound 2 was isolated as colorless crystals in a poor 11% yield due to a great extent by its excellent solubility in hexane at room temperature (this yield could be improved to 93% by concentration of the filtrate and storage at −30 °C). Dioxane adducts 3 and 4 were obtained in 30% and 50% yields, respectively. Taking into account their constitution (3 contains two molecules of dioxane, whereas 4 contains 0.5 equiv of dioxane per {LiMgR3} fragment) and the stoichiometries employed for their preparation (1 and 0.25 equiv of dioxane for 3 and 4, respectively), a maximum yield of 50% could be expected for both species. In contrast with the triorganomagnesiate formulation {LiMgR3} exhibited by 2−4 and 6 (vide infra), the addition of TMEDA to 1 induces a change in its formulation, affording the lithium-rich (or “higher-order” magnesiate) species16 [(TMEDA)2Li2MgR4] (5) along with the formation of MgR2. The formation of 5 (which could only be isolated in 23% yield) could be rationalized in terms of an equilibrium process in solution where the putative triorganomagnesiate [(TMEDA)LiMgR3] undergoes a redistribution to a mixture of 5 and MgR2, as depicted in eq 1. Redistributions of this type are known to occur in mixed-metal chemistry17 and can be favored by the preferred crystallization of one of its components.12c Compound 5 can be prepared in a rational stoichiometric manner from a 2/2/1 TMEDA/MgR2/LiR mixture in hexane, which allowed its isolation in a higher 56% yield. 2[(TMEDA)LiMgR 3] ⇌ [(TMEDA)2 Li 2MgR4] + MgR 2 (1)

The PMDETA compound 6 is an oil at room temperature, which precludes an accurate determination of its isolated yield. Notwithstanding, NMR analysis of the crude product of the 5133

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Figure 2. Section of polymeric 3 showing propagation and selected atom labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Li1−C4 = 2.268(3), Li1−C18 = 2.281(3), Mg4−C4 = 2.229(2), Mg4−C18 = 2.2322(16), Mg4−C6 = 2.1494(16), Li1−O2 1.977(3), Li1−O4 = 2.011(3), Mg4−O3 = 2.1289(12); Li1−C4−Mg4 = 73.912(3), Li1−C18−Mg4 = 74.292(3), C4−Mg4−C18 = 106.840(3), O4−Li1−O2 = 102.078(2), C6−Mg4−O3 = 99.429(2).

Figure 3. Section of polymeric 4 showing selected atom labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Li1−C13 = 2.2033(1), Li1−C9 = 2.1392(1), Li1−O1 = 1.978(3), Li2− O2 = 1.936(3), Mg1−C5 = 2.242(2), Mg1−C13 = 2.229(2), Mg1−C9 = 2.240(2), Mg2−C1 = 2.231(2), Li1−C13−Mg1 = 73.574(3), Li1−C9− Mg1 = 74.575(3), O1−Li1−C13 = 124.717(3), O1−Li1−C9 = 115.973(4).

Switching to the related oxygen-donor dioxane, possessing two oxygen atoms at positions 1 and 4, precludes this bidentate donor operating in a chelating fashion, but it is well-known to favor the formation of polymeric structures, acting as a bridging ligand in main-group chemistry.21,22 The reaction of 1 with (i) an equimolar amount and (ii) 0.25 equiv of dioxane led to the isolation of the complexes [{(dioxane)2LiMgR3}∞] (3) and [{(dioxane)Li2Mg2R6}∞] (4), respectively, structures of which were determined by X-ray crystallography (Figures 2 and 3). Displaying a polymeric arrangement (Figure 2), 3 contains the same basic {LiMgR3} organometallic core as 2, forming four-membered [LiCMgC] rings; however, now the polymeric chain is constructed via Li−O and Mg−O dative bonding with dioxane molecules alternately bridging the Li of one {LiMgR3} monomer to the Li of the next unit (R 3 MgLi−O(CH2CH2)2O−LiMgR3) and then bridging Mg to Mg in a similar fashion, resulting in a “head-to-head”, “tail-to-tail” repeating pattern. Thus, in contrast with 2, the remaining alkyl group of the {LiMgR3} unit takes no part in the chain propagation but binds exclusively to Mg. The structure is completed by an additional molecule of dioxane bonded terminally to lithium. Three distinct types of dioxane coordination modes are observed within the asymmetric unit of 3 (two molecules act as bridges propagating the polymeric

structure, with one of them connecting Li atoms, another connecting Mg atoms, and a third type terminally bonded to Li). Within the asymmetric unit both Mg and Li exhibit distorted-tetrahedral geometries (bond angles covering the ranges 99.66(6)−122.45(8) and 101.96(12)−116.90(14)° for Mg and Li, respectively). As expected, the Mg−C bonds of the bridging alkyl groups are elongated (mean Mg−C bond length 2.231 Å) in comparison to that of the terminal type (Mg4−C6 = 2.1494(16) Å). A search of the CSD revealed that the polymeric structure of 3 is unprecedented in magnesiate chemistry, constituting the first example of an alkali-metal magnesiate solvated by dioxane, although a close precedent can be found in the mixed lithium−gallium complex [(dioxane)3Li2Ga2R8] (R = CH2SiMe3),23 which has a dimeric structure where the two {(dioxane)LiGaR4} units are connected by a bridging dioxane molecule acting as a 1,4oxygen donor to the lithium atoms.24 In contrast with 3, the addition of a substoichiometric amount of dioxane (0.25 equiv) allowed the isolation of the polymer [{(dioxane)Li2Mg2R6}∞] (4). This polymer comprises dimeric trialkyl magnesiate [{LiMgR3}2] units linked through bridging dioxane molecules that solvate the lithium atoms (Figure 3). The magnesium atoms in 4 adopt a distortedtetrahedral geometry made up of four carbon atoms that form a 5134

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molecule of TMEDA.29 The contact ion pair structure of 5 contrasts with those reported for related TMEDA-solvated alkyl magnesiates, which display solvent-separated structures.12a,30 Unfortunately, all samples of 5 were weakly diffracting and the resulting data set thus gives only a low-resolution model that is used merely to confirm connectivity; hence, a discussion of geometric parameters must be waived. Deaggregation to a monomeric arrangement of the triorganomagnesiate {LiMgR3} was accomplished using the tridentate nitrogen donor PMDETA (Figure 5). Featuring a

[MgCMgC] planar four-membered ring (sum of angles 359.7°) which is orthogonal to two adjacent [LiCMgC] rings (sum of angles 358.6°). While this core motif of three fused fourmembered rings, comprising a tetranuclear alkali-metal···Mg···Mg···alkali-metal skeleton, has previously been reported,18d,25 this is the first example for a homoleptic alkyl species. The Mg−C distances in 4 (mean 2.236 Å) are comparable to the bridging Mg−C bond lengths in 3 (mean 2.231 Å). Unlike 3, in 4 each lithium center is bonded to a single molecule of dioxane and two bridging alkyl groups. This translates into a contraction of the Li−C bond lengths (average of 2.171 Å in 4 versus 2.275 Å in 3) as well as the presence of a medium-long electrostatic interaction of each Li with a methyl belonging to the SiMe3 group of one of the alkyl ligands (Li1···C10 = 2.613(5); Li2···C20 = 2.534(4) Å). These secondary (but significant) Li···Me contacts26 result in the pyramidalization of the {LiCCO} coordination (the sums of the C−Li−C and C− Li−O angles are 348.05 and 347.27° for Li1 and Li2, respectively). Thus, these results show that, in contrast to the case for the THF-solvated polymer 2, which propagates uniquely through the alkyl group, structures 3 and 4 polymerize through the bidentate donor nature of dioxane which, acting as a bridge, links the [LiMgR 3 ] and [{LiMgR 3 } 2 ] units together, respectively. When a substoichiometric amount of this Lewis base is employed, a longer fragment of the organometallic oligomer is trapped, allowing a greater insight into the constitution of the unsolvated species 1, the structure of which is still elusive.27 When nitrogen is used as a donor atom, the chelating bidentate ligand TMEDA not only changes the aggregation to produce a monomer (though trinuclear) but, even more significantly, alters the stoichiometry of the mixed-metal species, resulting in the tetraalkyl magnesiate [(TMEDA)2Li2MgR4] (5) (Figure 4). Exhibiting the classical “Weiss motif”1b previously found in [(TMEDA)2Li2MgMe4],12b,28 5 can be envisaged as two heterometallic {LiCMgC} rings fused through the tetrahedrally C4-coordinated Mg center, giving rise to a linear Li···Mg···Li arrangement where each lithium completes its coordination sphere by bonding to a chelating

Figure 5. Asymmetric unit of 6 showing selected atom labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms and minor disorder in PMDETA are omitted for clarity. Selected bond lengths (Å) and angles (deg): Li1−C2 = 2.326(3), Mg1−C3 = 2.1531(16), Mg1−C1 = 2.1556(16), Mg1−C2 = 2.1930(16); Li1−C2−Mg1 = 149.419(2), C2−Mg1−C1 = 119.298(2), C2−Mg1−C3 = 117.897(1), C1−Mg1−C3 = 122.121(2).

novel open structural motif where both metals are connected by a single alkyl bridge, [(PMDETA)LiMgR3] (6) can be envisaged as an intermediate between a solvent-separated ion pair arrangement9a,31 and the closed four-membered-ring structure [MINMgX] (X = N, C) found in other contact ion pair amido-based alkali-metal magnesiates,9a,18c,26a,29b,32 as suggested by the long intermetallic Li···Mg distance (4.360(3) Å) in comparison with that found in 3 (2.714(3) Å). Furthermore, a close comparison of the M−C bond lengths of 6 shows that the bridging alkyl group binds more strongly to Mg than to Li (Mg1−C2 = 2.1930(16) Å vs Li1−C2 = 2.326(3) Å), and in fact there is very little variation between this Mg−C distance and those found for the terminal R groups (Mg1−C1 = 2.1556(16) Å, Mg1−C3 = 2.1531(16) Å). The elongated Li−C contact in 6 is in contrast with those found in the related {LiMgR3} variants 3 and 4 (mean Li−C bond lengths 2.275 and 2.171 Å). The distorted-tetrahedral Li atom of 6 completes its coordination by bonding to the three nitrogen atoms of PMDETA, in a fashion similar to that reported for monomeric [(PMDETA)LiCH 2 SiMe 3 ]. 33 Although this open {LiCMgC} arrangement (as evidenced by the wide Li1C3Mg1 angle (149.419(2)°) and the long nonbonding Li···C distances (Li1···C1 = 5.878(3) Å, Li1···C3 = 5.422(3) Å)) has no precedent in magnesiate chemistry, it is reminiscent of some structures recently reported for lithium amidozincates34 and the open dimer motif found in TMEDA-

Figure 4. Asymmetric unit of 5 showing selected atom labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. 5135

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Scheme 2. Formation of 7 via Oxygen Insertion

pseudocubane structure with a missing vertex.37 Although several alkyl-alkoxo magnesium compounds have been structurally characterized,38 the structure of this anionic fragment is unprecedented.39 Even more unusual is the constitution of the cationic component of 7 {(PMDETA)2Li2MgR3}+, which is completely unique within mixedmetal chemistry, representing, as far as we can ascertain, the first example of a cationic lithium magnesiate. Exhibiting a trinuclear Li···Mg···Li chain where the metals are connected by two alkyl groups, with a molecule of PMDETA solvating each Li and a terminal alkyl group bonded to Mg, this cationic fragment can be interpreted as being an adduct of the triorganomagnesiate 6 and one {(PMDETA)Li}+ cation. In fact, 6 and the cation present in 7 share several structural features; thus, a close inspection of their metal−carbon distances shows they are almost identical (average Li−C and Mg−C lengths 2.353 and 2.165 Å in 7 vs 2.326 and 2.167 Å in 6), with both Li and Mg metals exhibiting very similar coordination environments. Furthermore, as previously discussed for 6, significantly obtuse Li−C−Mg angles (Li1−C7− Mg11 = 149.884(2)°, Li2−C2−Mg11 = 156.455(2)°) are observed for 7 and the Li−C bonds are significantly longer (Li1−C7 = 2.341(4) Å, Li2−C2 = 2.366(4) Å) than those found in the related lithium magnesiates 3 and 4. The synthesis of 7 could be reproduced by deliberately exposing 6 to O2 for 30 min (using a drying tube),40 with crystals isolated in 34% yield which were confirmed by 1H NMR spectroscopy to be 7 (Scheme 2). The formation of 7 is in contrast with recent precedents on controlled exposure to oxygen of alkali-metal magnesiates which have led to the isolation of inverse crown ether complexes,1b,41 macrocyclic cationic structures hosting oxygen or alkoxide anions in their core. A plausible explanation for this difference could be the presence of the tridentate donor PMDETA which, by solvating the lithium atoms, limits their coordination requirement to a single alkyl group, precluding the closure of a cyclic structure and favoring instead a linear arrangement such as that in 7. The synthesis of cationic mixedmetal fragments such as that present in 7 could be of potential importance for the design of new catalytic systems, bearing in mind the high activity that cationic alkyl magnesium compounds exhibit in, for example, ring-opening polymerization reactions42 as well as recent reports1 which have

solvated alkali-metal 2,2,6,6-tetramethylpiperidide (TMP) compounds.35 In an attempted repeat synthesis of 6, large crystals formed readily in the freezer that, unlike those of 6, did not rapidly melt at room temperature. Analysis of these crystals by X-ray crystallography revealed a fortuitous inclusion of traces of oxygen, resulting in the unexpected formation of the alkylalkoxo Mg-rich lithium magnesiate36 7, which has the extraordinary ion pair composition [{(PMDETA)2Li2Mg(CH2SiMe3)3}+{Mg3(CH2SiMe3)6(OCH2SiMe3)}−] (Scheme 2 and Figure 6). In the trinuclear anion, the magnesium centers are connected through three μ2-alkyl ligands and one μ3-alkoxide OCH2SiMe3 group, resulting from oxygen insertion into a Mg−C bond. The anionic cluster is completed by three terminal alkyl groups, each bonded to one Mg, giving rise to a

Figure 6. Cation (left) and anion (right) of 7 showing selected atom labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Li1−C7 = 2.341(4), Li2−C2 = 2.366(4), Mg11−C21 = 2.131(2), Mg11−C2 = 2.1818(19), Mg11−C7 = 2.183(2), Mg6−O1 = 2.0478(21), Mg7−O1 = 2.0586(14), Mg10−O1 = 2.0447(13); Li1−C7−Mg11 = 149.884(2), Li2−C2−Mg11 = 156.455(2), Mg6−O1−C4 = 140.423(3), Mg6−O1−Mg7 = 91.908(2), Mg7−O1−Mg10 = 90.880(2), Mg6−O1−Mg10 = 92.382(2). 5136

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Table 1. Selected Chemical Shifts (ppm) in the 1H, 13C, and 7Li NMR Spectra of LiR and Compounds 1−7 in Deuterated Benzene Solutions compd

δ(1H) (CH2)

LiR [LiMgR3] (1) [{(THF)LiMgR3}∞] (2) [{(dioxane)2LiMgR3}∞] (3) [{(dioxane)Li2Mg2R6}∞] (4) [(TMEDA)2Li2MgR4] (5) [(PMDETA)LiMgR3] (6) [{(PMDETA)2Li2MgR3}+{Mg3(R)6(OR)}−] (7)

−1.92 −1.36 −1.31 −1.51 −1.47 −1.99 −1.31 −1.11(br)

δ(13C) (CH2)

δ(7Li)

−2.21 −3.33

0.38 1.24 1.25 0.32 0.88 0.76 0.20

−3.19 −3.28

structure is not retained or a dynamic equilibrium is taking place which rapidly interconverts the dioxane molecules. Furthermore, inspection of the crystal structure of 3 shows two bridging and one terminal R group; however, the presence of only one set of signals at room temperature suggests a dynamic interconversion is occurring between the R groups. Thus, a variable-temperature 1H NMR experiment was undertaken which showed that, when the temperature is gradually reduced to 215 K, the resonance for the CH2 group decoalesces into two distinct signals, with an approximate 2:1 integration ratio at −1.52 and −1.64 ppm, respectively (see Figure S10 in the Supporting Information). This ratio supports the presence of a fast dynamic process taking place which exchanges the bridging and terminal alkyl groups.43 In addition, the 1H NMR spectrum recorded at this low temperature also shows a significant broadening of the dioxane resonance (Figure S10b). As mentioned earlier, tetraalkyl species 5 shows the most upfield resonance of the CH2 group due to the relative increase in charge on the anionic R groups; that is to say, 5 can be considered a MgR42− dianion in comparison to the MgR3− monoanion in compounds 1−4 and 6. All R groups are equivalent and display sharp signals. The TMEDA resonances are indicative of the molecule remaining coordinated in the solvent, as the hydrogen atoms of the ethylene bridge are found to be more upfield (at 1.65 ppm) than the methyl protons (at 2.02 ppm) of the ligand and when free this pattern is reversed (at 2.36 and 2.12 ppm for the CH2 and CH3 protons, respectively).44 A similar trend was found for the PMDETA ligand in 6 (1.73−1.95 ppm), although now the signals are also significantly broader and overlapped with those observed for nonsolvating PMDETA substantially more downfield (2.20− 2.59 ppm).45 The alkyl-alkoxo compound 7 was analyzed by 1H and 7Li NMR spectroscopy (see the Experimental Section and the Supporting Information). The most notable feature in the 1H NMR spectrum of 7 is the presence of a singlet at 3.93 ppm integrating to two hydrogen atoms and corresponding to the methylene group of the single alkoxy ligand MgOCH2SiMe3. Similarly to those in 6, the PMDETA resonances are broad and appear at even lower chemical shifts (1.95−1.63 ppm), whereas the region of the spectrum for the monosilyl group shows broad signals resulting from the overlapping of the different types of alkyl groups present in the molecule. Comparison of the NMR data obtained for 6 with that for 7 shows a downfield shift in the CH2 group of the alkyl ligand (from −1.31 ppm in 6 to −1.11 ppm in 7), as may be expected due to the Mg-rich nature of 7, where a large contribution is made from the anion. Diffusion-Ordered NMR Spectroscopy Studies. Considering the defining role that aggregation plays in modulating

advanced the understanding of how bimetallic systems can operate synergically. Solution NMR Spectroscopic Studies. Complementing their solid-state characterization, all the new compounds have also been examined in C6D6 solution using multinuclear (1H, 13 C, 7Li) NMR spectroscopy (see Table 1 for key chemical shifts). For compounds 1−6 a single set of resonances for the alkyl ligand CH2SiMe3 was observed. Taking into account the different structures observed by X-ray crystallography, where in many cases different types of alkyl groups are present, this indicates that rapid exchange of the alkyl positions or even the formation of solvent-separated ion-pair structures occurs in deuterated benzene solutions. Regarding the chemical shifts observed for the monosilyl group,the TMEDA adduct 5, which displays a tetraalkyl [Li2MgR4] core, shows a resonance for M− CH2 considerably upfield (at −1.99 ppm) from those observed in the species displaying a trialkyl [LiMgR3] core (1−4, 6) (ranging from −1.31 to −1.51 ppm; Table 1). Indeed, at −1.99 ppm these hydrogen atoms are even more shielded than those in the homometallic LiR species (−1.91 ppm) (no comparison can be made with MgR2, which, due to its polymeric structure, is insoluble in deuterated benzene). In contrast, the carbon signal in the 13C{1H} spectra associated with this M−CH2 does not show a significant difference between the tetraalkyl and trialkyl species, with 5 resonating at −3.19 ppm, a chemical shift between those observed for unsolvated species 1 (−2.21 ppm) and the THF adduct 2 (−3.33 ppm). Having stated that, we should note that due to the reduced solubility of dioxanesolvated 3 and 4 and the rapid decomposition of 7 in C6D6 solutions, fewer data are available overall for comparison of the 13 C{1H} chemical shifts. A comparison of the resonances belonging to the donor ligands in the 1H NMR spectra of 2−6 with those observed for the free Lewis bases in deuterated benzene solutions suggest that the donors remain coordinated to the metal centers in solution. Thus, 1H NMR analysis of 2 showed two multiplets at 1.15 and 3.40 ppm for CH2 and OCH2 of the THF ligand, respectively, which appear at chemical shifts significantly different from those observed for free THF (1.40 and 3.57 ppm, respectively). Furthermore, 1H-DOSY experiments confirmed that THF and the monosilyl group are part of the same species in solution, since the hydrogen atoms from both units display essentially the same diffusion coefficient (vide infra). Similarly, the dioxane molecules in 3 and 4 display a resonance different from that of the free dioxane (moving upfield when coordinated to a metal, with OCH2 resonances at 3.30 and 3.21 ppm for 3 and 4, respectively in comparison to 3.36 ppm in free dioxane), but despite the presence of three distinct dioxane molecules in the solid-state structure of 3, only one singlet is observed, suggesting that either the polymeric 5137

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Information). In this regard, it must be noted that electrostatic π interactions between alkali metals and neutral arene ligands are well-known in organometallic chemistry,49 with several examples being structurally characterized.50 In order to ascertain if the THF ligand remains attached to Mg (as in the solid state) or fluctuates to Li in solution, a heteronuclear Overhauser effect spectroscopy (HOESY)51 NMR experiment was performed to detect “through-space” connectivities between nonbonded nuclei. In C6D6 solution the Li atoms in 2 show a strong interaction with the protons of the CH2 group and weaker interactions with the methyl groups attached to silicon (see Figure S7 in the Supporting Information); however, no cross-peaks are observed between the THF hydrogen atoms and Li, suggesting that no interaction is taking place between Li and THF and that the donor is possibly still bound to Mg. Thus, collectively these studies appear to indicate that the polymeric structure of 2 is not retained in benzene solution and deaggregation to a monomeric (dinuclear) arrangement appears to be more likely to occur, where the Lewis base THF remains attached to the more electronegative Mg and with Li attaining further coordination by π-engaging with the deuterated solvent as well as probably forming weak electrostatic interactions with the nearby SiMe3 groups of the R ligands. Turning to the dioxane-solvated polymers 3 and 4, 1HDOSY experiments were carried out using the same internal reference standards as described above. In contrast with 2, the 1 H-DOSY spectra of 3 show significantly different diffusion coefficients for the triorganomagnesiate [LiMgR3] fragment (D = 7.86 × 10−10 m2 s−1) and dioxane (D = 1.23 × 10−9 m2 s−1) (see Figure S25 in the Supporting Information). Furthermore, the FW inferred for dioxane in solution (201 g mol−1) is substantially larger than the FW of free dioxane (88.1 g mol−1).52 These results, coupled with the different chemical shifts observed for the dioxane protons in the 1H NMR spectrum of 3 in comparison with that found for free dioxane, indicate that in solution the dioxane molecules partially dissociate from the {LiMgR3} unit. A plausible situation could be the dissociation of the bridging dioxane molecules which propagate the polymeric structure of 3, which would give rise to the formation of monomeric {(dioxane)LiMgR3} aggregates, as similarly described for 2. A comparison of the estimated FW of the {LiMgR3} unit (393 g mol−1) by the 1HDOSY experiment shows a good agreement with this solution scenario ({(dioxane)LiMgR3}, 381 g mol−1, −3% error from the predicted FW value); however, it must be noted that this dynamic equilibrium can give an artificial estimate of the molecular weight of the R-containing molecules, as the value obtained for the diffusion coefficient D will be the average of the diffusion coefficients of the individual species in which R is reasonably involved (as for example in [(dioxane)LiMgR3], [(dioxane)2LiMgR3], or LiMgR3).53 With respect to dioxane-solvated polymer 4, 1H-DOSY experiments revealed deaggregation to two distinct sizes of dioxane-containing lithium magnesiate species in solution (species A and B, Figure 8). However, although in the same size range of 1,2,3,4-tetraphenylnaphthalene, due to the overlap of the signals between these species the FW values for the individual components in solution cannot be ascertained (Figure 8). Switching to deuterated toluene as a solvent, crystals of 4 were dissolved in it to allow for low-temperature analysis of the solution. At 295 K an expected ratio of 4:27:6 hydrogen atoms was observed for the dioxane, methyl, and

the reactivity of organometallic compounds as well as the fact that crystal structures of many of these species do not necessarily correlate with their constitution in solution, we performed 1H NMR diffusion-ordered spectroscopy experiments (1H-DOSY) of polymeric magnesiates 2−4, to gain insight into their structures and aggregation in C6D6 solutions. This method has been previously employed to estimate the molecular size and aggregation of organometallic compounds46 as well as to detect dynamic processes.47 Thus, Williard has shown that, by using simple hydrocarbons as references, accurate hydrodynamic dimensions of several organolithium compounds can be established.48 In our study we chose 1,2,3,4tetraphenylnaphthalene (TPhN), 1-phenylnaphthalene (PhN), and tetramethylsilane (TMS) as internal standards, as they exhibit good solubility in benzene with minimal overlapping of signals and are inert to magnesiates 2−4. The 1H NMR spectrum of a mixture of 2 with these standards in C6D6 at 27 °C clearly showed that all the components separate in the diffusion dimension according to their increasing diffusion coefficients D (Figure 7), confirming that THF and the

Figure 7. 1H-DOSY NMR spectrum of 2 and the standards TPhN, PhN and TMS in C6D6 at 298 K (some traces of grease are also observed).

monosilyl groups present in 2 belong to the same sized species, as the cross points for both ligand resonances are aligned in the second dimension (average D value [7.6(1)] × 10−10 m2 s−1). A correlation between log D and log FW (FW = molecular weight) of the linear least-squares fit to the internal standards can be established (log D = −0.6636 log FW − 7.3888; r = 0.9976; Figure S23 in the Supporting Information), and therefore by interpolating the value of log D for 2 in this calibration curve, an approximate value of the molecular weight of the structure in solution can be estimated. This was estimated to be 410 g mol−1. Analysis of these data suggests that the polymeric constitution of 2 in the solid state is not retained in solution, as the error associated with much smaller aggregates such as a dimer [{LiMgR3(THF)}2] (730 g mol−1) or a trimer [{LiMgR3(THF)}3] (1095 g mol−1) is quite high (44 and 63%, respectively; see Figure S24 in the Supporting Information). Therefore, it appears that a substantial deaggregation of the 1D polymer occurs in solution, as better correlations are observed for monomeric [LiMgR3(THF)] (−13% error) and for [LiMgR3(THF).C6D6], in which lithium is further solvated by a molecule of deuterated solvent (9% error from the predicted FW; Figure S24 in the Supporting 5138

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constitutions of these new mixed-metal species in solution, compounds 2−7 were characterized by 1H, 13C, and 7Li NMR, with the aid of 1H-DOSY NMR, which revealed that the supramolecular structures of compounds 2−4 are not retained in solution, breaking into smaller monomeric aggregates, where the interaction with the Lewis donor is maintained at least in part. Collectively these results show the intricate chemistry that lithium magnesiates can display in solution and shed new light on the effect that donor solvents can have on the structure/ constitution of these intriguing bimetallic reagents.

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EXPERIMENTAL SECTION

General Considerations. All reactions were performed under a protective argon atmosphere using standard Schlenk techniques. Hexane, toluene, and THF were dried by heating to reflux over sodium benzophenone ketyl and distilled under nitrogen prior to use. Mg(CH 2 SiMe 3 ) 2 was prepared from the Grignard reagent (Me3SiCH2)MgCl by manipulation of the Schlenk equilibrium via the dioxane precipitation method. The resultant off-white solid was purified via sublimation at 175 °C (10−2 Torr) to furnish pure Mg(CH2SiMe3)2. LiCH2SiMe3 was purchased from Sigma Aldrich Chemicals and used as received. PMDETA, TMEDA, and 1,4-dioxane were distilled over CaH2 prior to use. NMR spectra were recorded on a Bruker DPX400 MHz spectrometer, operating at 400.13 MHz for 1 H, 100.62 MHz for 13C, and 155.50 MHz for 7Li. Elemental analyses were attempted using a Perkin-Elmer 2400 elemental analyzer; however, due to the extreme air sensitivity of the compounds satisfactory analyses could not be obtained. Crystal Structure Determinations of 2−7. Measurements were made at 123 or 150 K on Oxford Diffraction (now Agilent Technologies) Gemini S and Gemini A Ultra diffractometers using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) and ω scans. Cell parameters were refined from the observed positions of all strong reflections. Intensities were corrected semiempirically for absorption, on the basis of symmetry-equivalent and repeated reflections. The structures were solved by direct methods and refined on F2 values for all unique data. Table S1 (Supporting Information) gives further details. All non-hydrogen atoms were refined anisotropically, except for a few atoms in disordered groups, and most H atoms were constrained with a riding model (except for CH2 groups bonded to more than one metal center, for which appropriate soft restraints were used as required); U(H) was set at 1.2 (1.5 for methyl groups) times Ueq for the parent atom. Programs were Oxford Diffraction CrysAlisPro and SHELX/SHELXTL for structure solution, refinement, and molecular graphics.54 Synthesis of [LiMg(CH 2SiMe 3 ) 3] (1). To a solution of LiCH2SiMe3 (1 mL of a 1 M solution in hexane, 1 mmol) in hexane (15 mL) was added Mg(CH2SiMe3)2 (0.20 g, 1 mmol), and the resulting suspension was heated gently, affording a clear solution. Slow cooling resulted in the formation of clear, colorless crystals (0.28 g, yield 96%). 1H NMR (400.13 MHz, 298 K, 0.135 M in C6D6): δ −1.36 (6H, s, SiCH2), 0.25 (27H, s, Si(CH3)3). 13C{1H} NMR (100.62 MHz, 298 K, C6D6): δ −2.21 (SiCH2), 4.16 (Si(CH3)3). 7Li NMR (155.50 MHz, 298 K, C6D6): δ 0.38. Synthesis of [{(THF)LiMg(CH2SiMe3)3}∞] (2). To a solution of LiCH2SiMe3 (1 mL of a 1 M solution in hexane, 1 mmol) in hexane (15 mL) was added Mg(CH2SiMe3)2 (0.20 g, 1 mmol), and the resulting suspension was stirred for 1 h, affording a clear solution. THF (0.08 mL, 1 mmol) was then added, and the clear solution was frozen in liquid nitrogen before transfer to the freezer (−30 °C). Clear, colorless crystals formed after 18 h and were isolated (0.04 g, yield 11%). 1H NMR (400.13 MHz, 298 K, C6D6): δ −1.31 (6H, s, SiCH2), 0.35 (27H, s, Si(CH3)3), 1.15 (4H, m, CH2, THF), 3.40 (4H, m, OCH2, THF). 13C{1H} NMR (100.62 MHz, 298 K, C6D6): δ −3.33 (SiCH2), 4.37 (Si(CH3)3), 25.02 (CH2, THF), 68.90 (OCH2, THF). 7 Li NMR (155.50 MHz, 298 K, 0.110 M in C6D6): δ 1.24. Synthesis of [{(dioxane)2LiMg(CH2SiMe3)3}∞] (3). To a solution of LiCH2SiMe3 (2 mL of a 1 M solution in hexane, 2 mmol) in hexane

Figure 8. 1H-DOSY NMR spectrum of 4 and the standards TPhN, PhN, and TMS in C6D6 at 298 K.

methylene bridge hydrogen atoms, respectively; however, on cooling to 270 K the proportion of dioxane falls to 1:27:6, indicating that an equilibrium is taking place which is driven by the precipitation of a dioxane-containing species. Unfortunately a low-temperature DOSY experiment could not be carried out, due to the lower solubility of the product, suggesting aggregation is taking place at this lower temperature.

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CONCLUSION Highlighting the structural diversity of alkali-metal magnesiates, this systematic study of the cocomplexation reactions of homometallic alkyls LiR and MgR2 in the presence of several Lewis bases, including TMEDA and THF, two of the most commonly used donor molecules in organic synthesis for the activation of organometallic reagents such as BuLi and RMgX, has revealed that the final outcome of these reactions is strongly influenced by the donor ligand employed. Thus, for oxygen donors such as THF and dioxane, the formation of polymeric chains 2−4 is observed, although two distinct types of supramolecular arrangements are seen for each donor. For THF adduct 2, an unusual structure is obtained, with THF bound to Mg rather than Li, giving rise to an infinite chain held together by intermolecular Mg−C−Li electron-deficient bonds. In contrast, the dioxane-containing polymers 3 and 4 are formed as a consequence of the ability of this donor to function as a 1,4-bridge through its two oxygen atoms, linking {LiMgR3} or {LiMgR3}2 units to afford 3 and 4, respectively. Employing the bidentate and tridentate N-donor ligands TMEDA and PMDETA allows the formation of discrete molecular structures, although they exert markedly different effects on the constitution/structure of the lithium magnesiate. TMEDA induces a redistribution process of the mixed-metal precursor [LiMgR3] (1) to form the higher order tetraorganomagnesiate [(TMEDA)2Li2MgR4] (5) with MgR2 coproduct. On the other hand, PMDETA is found to form the monomeric complex [(PMDETA)LiMgR3] (6), which, exhibiting an unusual open structural motif with both metals connected by a single bridging alkyl group, can be envisaged as an intermediate between a solvent-separated ion pair and a contact ion pair species. Monomer 6, with its open ring arrangement, is extremely sensitive and can react with traces of O2 to yield the novel [{(PMDETA) 2 Li 2 Mg(CH 2 SiMe 3 ) 3 } + {Mg 3 (CH 2 SiMe 3 ) 6 (OCH2SiMe3)}−] (7), which constitutes the first example of a cationic lithium magnesiate species. Shedding light on the 5139

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0.45 (91H, overlapping m, Si(CH3)3), 1.80 (43H, overlapping m, NCH2, N(CH3)2, and N(CH3) PMDETA), 3.93 (2H, s, OCH2). 7Li NMR (155.50 MHz, 298 K, C6D6): δ 0.20. The relatively low solubility of 7 in C6D6 solution precluded the collection of a meaningful 13C{1H} NMR spectrum.

(15 mL) was added Mg(CH2SiMe3)2 (0.40 g, 2 mmol), and the resulting suspension was stirred for 1 h, affording a clear solution. 1,4Dioxane (0.17 mL, 2 mmol) was added dropwise, resulting in a white suspension which was stirred for 30 min. The volume was reduced in vacuo to approximately 5 mL. Addition of toluene (2 mL) and gentle heating gave a clear solution, which was left to cool slowly. Small, colorless crystals were isolated (0.28 g, yield 30%). 1H NMR (400.13 MHz, 298 K, 0.085 M in C6D6): δ −1.51 (6H, s, SiCH2), 0.35 (27H, s, Si(CH3)3), 3.30 (16H, s, OCH2, dioxane). 7Li NMR (155.50 MHz, 298 K, C6D6): δ 1.25. 13C{1H} NMR could not be collected due to poor solubility. Synthesis of [{(dioxane)Li2Mg2(CH2SiMe3)6}∞] (4). To a solution of LiCH2SiMe3 (2 mL of a 1 M solution in hexane, 2 mmol) in hexane (15 mL) was added Mg(CH2SiMe3)2 (0.40 g, 2 mmol), and the resulting suspension was stirred for 1 h, affording a clear solution. 1,4-Dioxane (43 μL via microsyringe, 0.5 mmol) was added dropwise, resulting in the formation of a white precipitate which was stirred for 18 h. Addition of toluene (4 mL) and gentle heating gave a clear solution, which was left to cool slowly. Small, colorless crystals were isolated (0.34 g, yield 50%). 1H NMR (400.13 MHz, 298 K, 0.120 M in C6D6): δ −1.47 (s, 6H, SiCH2), 0.28 (s, 27H, Si(CH3)3), 3.21 (s, 4H, OCH2, dioxane). 7Li NMR (155.50 MHz, 298 K, C6D6): δ 0.32 (s). The relatively low solubility and stability of 4 over prolonged periods of time in C6D6 solutions precluded the collection of its 13C{1H} NMR spectrum. Synthesis of [(TMEDA)2Li2Mg(CH2SiMe3)4] (5). To a solution of LiCH2SiMe3 (1 mL of a 1 M solution in hexane, 1 mmol) in hexane (15 mL) was added Mg(CH2SiMe3)2 (0.20 g, 1 mmol), and the resulting suspension was stirred for 30 min, affording an almost clear solution. TMEDA (0.15 mL, 1 mmol) was then added, giving a clear solution which was transferred to the freezer (−30 °C). After 18 h colorless crystals suitable for X-ray diffraction were deposited (0.14 g, 23%, which could be increased to 56% when prepared with the stoichiometry LiR/2MgR2/2TMEDA). 1H NMR (400.13 MHz, 298 K, 0.065 M in C6D6): δ −1.99 (2H, s, SiCH2), 0.46 (9H, s, Si(CH3)3), 1.65 (2H, s, NCH2, TMEDA), 2.02 (6H, s N(CH3)2, TMEDA). 13 C{1H} NMR (100.62 MHz, 298 K, C6D6): δ −3.19 (SiCH2), 5.71 (Si(CH3)3), 46.41 (CH3, TMEDA), 56.91 (CH2, TMEDA). 7Li NMR (155.50 MHz, 298 K, C6D6): δ 0.82. Synthesis of [(PMDETA)LiMg(CH2SiMe3)3] (6). To a solution of LiCH2SiMe3 (1 mL of a 1 M solution in hexane, 1 mmol) in hexane (15 mL) was added Mg(CH2SiMe3)2 (0.20 g, 1 mmol), and the resulting suspension was stirred for 1 h, affording a clear solution. PMDETA (0.21 mL, 1 mmol) was then added, giving a slightly cloudy solution. Gentle heating gave a clear solution, and the Schlenk tube was transferred to the freezer (−30 °C). After 4 days colorless crystals suitable for X-ray diffraction were deposited. Due to the fast formation of an oil at room temperature, an accurate yield has not been obtained; however, the NMR of the oil showed that no other species were present, indicating that the formation of 6 is quantitative. 1H NMR (400.13 MHz, 298 K, C6D6): δ −1.31 (6H, s, SiCH2), 0.42 (27H, s, Si(CH3)3), 1.73−1.94 (23H, br overlapping m, PMDETA). 13C{1H} NMR (100.62 MHz, 298 K, C6D6): δ −3.28 (SiCH2), 5.39 (Si(CH3)3), 44.91 (CH3, PMDETA), 45.83 ((CH3)2, PMDETA), 53.14 (CH2, PMDETA), 57.04 (CH2, PMDETA). 7Li NMR (155.50 MHz, 298 K, C6D6): δ 0.76. Synthesis of [{(PMDETA)2Li2Mg(CH2SiMe3)3}+{Mg3(CH2SiMe3)6(OCH2SiMe3)}−] (7). To a solution of LiCH2SiMe3 (1 mL of a 1 M solution in hexane, 1 mmol) in hexane (15 mL) was added Mg(CH2SiMe3)2 (0.40 g, 2 mmol), and the resulting suspension was stirred for 1 h, affording a clear solution. PMDETA (0.21 mL, 1 mmol) was then added, giving a slightly cloudy solution. A drying tube was fitted to the Schlenk tube for 30 min. Gentle heating gave a clear solution with a yellow oil deposited at the bottom of the Schlenk tube. A drying tube containing CaCl2 was fitted to the Schlenk tube to allow oxygen but not water to enter the system. After 30 min at room temperature, the drying tube was removed and the Schlenk tube was transferred to the freezer (−30 °C) overnight. Colorless crystals formed (0.23 g, yield 34%). 1H NMR (400.13 MHz, 298 K, 0.045 M in C6D6): δ −1.11 (18H, overlapping m, br, SiCH2),

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ASSOCIATED CONTENT

S Supporting Information *

Tables, figures, and CIF files giving crystallographic results, details of 1H DOSY NMR studies, and NMR spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the EPSRC, the Royal Society (University Research Fellowship to E.H.), the 7th European Community Framework Programme (Marie Curie Intra European Fellowship to P.G.A.), and the Carnegie Trust for the Universities of Scotland (postgraduate Carnegie scholarship to S.E.B.) for the generous sponsorship of this research. We also thank Professor R. E. Mulvey for helpful discussions.

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REFERENCES

(1) For selected references see: (a) Knochel, P.; Gavryushin, A.; Brade, K., The Chemistry of Organomagnesium Compounds; Wiley: Chichester, U.K., 2008. (b) Mulvey, R. E. Chem. Commun. 2001, 1049. (c) Mulvey, R. E. Organometallics 2006, 25, 1060. (d) Mulvey, R. E. Acc. Chem. Res. 2009, 42, 743. (e) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem., Int. Ed. 2007, 46, 3802. (2) Wittig, G.; Meyer, F. J.; Lange, G. Justus Liebegs, Ann. Chem. 1951, 571, 167. (3) For key references see: (a) Krasovskiy, A; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 3333. (b) Wunderlich, S. H.; Rohbogner, C. J.; Unsinn, A.; Knochel, P. Org. Process Res. Dev. 2010, 14, 339. (c) Haag, B.; Mosrin, M.; Ila, H.; Malakhov, V.; Knochel, P. Angew. Chem., Int. Ed. 2011, 50, 9794. (4) (a) Andrikopoulos, P. C.; Armstrong, D. R.; Clegg, W.; Gilfillan, C. J.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; O’Hara, C. T.; Parkinson, J. A.; Tooke, D. M. J. Am. Chem. Soc. 2004, 126, 116121. (b) Andrikopoulos, P. C.; Armstrong, D. R.; Graham, D. V.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; O’Hara, C. T.; Talmard, C. Angew. Chem., Int. Ed. 2005, 44, 3459. (c) Hevia, E.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E.; Sherrington, D. C. Angew. Chem., Int. Ed. 2005, 44, 68. (d) Andrikopoulos, P. C.; Armstrong, D. R.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E. Organometallics 2006, 25, 2415. (5) (a) Hatano, M.; Matsumura, T.; Ishihara, T. Org. Lett. 2005, 7, 573. (b) Sonsnicki, J. G. Tetrahedron Lett. 2005, 46, 4295. (c) Sonsnicki, J. G. Tetrahedron Lett. 2006, 47, 6809. (d) Sosnicki, J. G.; Struk, L. Synlett 2009, 1812. (6) (a) Iida, T.; Wada, T.; Tomimoto, K.; Mase, T. Tetrahedron Lett. 2001, 42, 4841. (b) Inoue, A.; Kitagawa, H.; Shinokubo, H.; Oshima, K. J. Org. Chem. 2001, 66, 4333. (c) Dumouchel, S.; Mongin, F.; Trecourt, F.; Queguiner, G. Tetrahedron Lett. 2003, 44, 2033. (d) Kitagawa, K.; Inoue, A.; Shinokubo, H.; Oshima, K. Angew. Chem., Int. Ed. 2000, 39, 2481. (7) (a) Yasuda, M.; Ide, M.; Matsumoto, Y.; Nakata, M. Synlett 1997, 899. (b) Awad, H.; Mongin, F.; Trecourt, F.; Queguiner, G.; Marsais, F.; Blanco, F.; Abarca, B.; Ballesteros, R. Tetrahedron Lett. 2004, 45, 6697. (c) Bayh, O.; Awad, H.; Mongin, F.; Hoarau, C.; Trecourt, F.; Queguiner, G.; Marsais, F.; Blanco, F.; Abarca, B.; Ballestros, R. Tetrahedron 2005, 61, 4779. (d) Bayh, O.; Awad, H.; Mongin, F.; Hoarau, C.; Trecourt, F.; Queguiner, G.; Marsais, F.; Blanco, F.; 5140

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Organometallics

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Abarca, B.; Ballestros, R. Tetrahedron 2005, 61, 4779. (e) Mongin, F.; Bucher, A.; Bazureau, J. P.; Bayh, O.; Awah, H.; Trecourt, F. Tetrahedron Lett. 2005, 46, 7989. (f) Bayh, O.; Awad, H.; Mongin, F.; Hoarau, C.; Bischoff, L.; Trecourt, F.; Queguiner, G.; Marsais, F.; Blanco, F.; Abarca, B.; Ballesteros, R. J. Org. Chem. 2005, 70, 5190. (8) (a) Clayden, J. Organolithiums: Selectivity for Synthesis; Pergamon, Elsevier Science Ltd.: Oxford, U.K., 2002. (b) Schlosser, M., Ed. Organometallics in Synthesis-A Manual, 2nd ed.; Wiley: Chichester, U.K., 2002. (c) Rathman, T. L.; Bailey, W. F. Org. Process Res. Dev. 2009, 13, 144. (9) (a) Graham, D. V.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; O’Hara, C. T.; Talmard, C. Chem. Commun. 2006, 417. (b) Armstrong, D. R.; Clegg, W.; Dale, S. H.; Graham, D. V.; Hevia, E.; Hogg, L. M.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E. Chem. Commun. 2007, 598. (c) Blair, V. L.; Carrella, L. M.; Clegg, W.; Conway, B.; Harrington, R. W.; Hogg, L. M.; Klett, J.; Mulvey, R. E.; Rentschler, E.; Russo, L. Angew. Chem., Int. Ed. 2008, 47, 6208. (10) For selected references see: (a) Merkel, S.; Stern, D.; Henn, J.; Stalke, D. Angew. Chem., Int. Ed. 2009, 48, 6350. (b) Armstrong, D. V.; Blair, V. L.; Clegg, W.; Dale, S. H.; Garcı ́a-Alvarez, J.; Honeyman, G. W.; Hevia, E.; Mulvey, R. E.; Russo, L. J. Am. Chem. Soc. 2010, 132, 9480. (c) Mulvey, R. E.; Armstrong, D. R.; Conway, B.; Crosbie, E.; Kennedy, A. R.; Robertson, S. D. Inorg. Chem. 2011, 50, 12241. (11) (a) Cambridge Structural Database v5.32 (November 2011); Cambridge Crystallographic Data Centre, Cambridge, U.K., 2011; (b) Allen, F. R. Acta Crystallogr., Sect. B 2002, 58, 380. (12) (a) Schubert, B.; Weiss, E. Chem. Ber. 1984, 117, 366. (b) Greiser, T.; Kopf, J.; Thoennes, D.; Weiss, E. Chem. Ber. 1981, 114, 209. (c) Baillie, S. E.; Clegg, W.; Garcı ́a-Alvarez, P.; Hevia, E.; Kennedy, A. R.; Klett, J.; Russo, L. Chem. Commun. 2011, 47, 388. (d) Andrikopoulos, P. C.; Armstrong, D. R.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; O’Hara, C. T. Chem. Commun. 2005, 1131. (13) The additional stability of these alkyl groups is mainly due to their lack of β-hydrogen atoms as well as their considerable steric bulk and electronic stabilization. See: (a) Davidson, P. J.; Lappert, M. F.; Pearce, R. Acc. Chem. Res. 1974, 7, 209. (b) Davidson, P. J.; Lappert, M. F.; Pearce, R. Chem. Rev. 1976, 76, 219. (14) Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Robertson, S. D. Eur. J. Inorg. Chem. 2011, 4675. (15) The increased solubility of magnesiate compounds in comparison to that of their homometallic components has been noted previously. See: Yorimitsu, H.; Oshima, K. The Chemistry of Organomagnesium Compounds; Wiley: Chichester, U.K., 2008. (16) , Conway, B.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E. Chem. Commun. 2007, 2864. (17) (a) Armstrong, D. R.; Dougan, C.; Graham, D. V.; Hevia, E.; Kennedy, A. R. Organometallics 2008, 27, 6063. (b) Chau, N. T. T.; Meyer, M.; Komagawa, S.; Chevallier, F.; Fort, Y.; Uchiyama, M.; Mongin, F.; Gros, P. C. Chem. Eur. J. 2010, 16, 12425. (c) Mobley, T. A.; Berger, S. Angew. Chem., Int. Ed. 1999, 38, 3070. (18) For selected references see: (a) Waggoner, K. M.; Power, P. P. Organometallics 1992, 11, 3209. (b) Forbes, G. C.; Kennedy, A. R.; Mulvey, R. E.; Rodger, P. J. A.; Rowlings, R. B. Dalton Trans. 2001, 1477. (c) Heitz, S.; Epping, J. D.; Driess, M. Chem. Mater. 2010, 22, 4563. (d) Armstrong, D. R.; Garcia-Alvarez, P.; Kennedy, A. R.; Mulvey, R. E.; Parkinson, J. A. Angew. Chem., Int. Ed. 2010, 49, 3185. (19) See the following examples in zincate chemistry: (a) Armstrong, D. R.; Herd, E.; Graham, D. V.; Hevia, E.; Kennedy, A. R.; Clegg, W.; Russo, L. Dalton Trans. 2008, 1323. (b) Armstrong, D. R.; Davies, R. P.; Linton, D. J.; Snaith, R.; Schooler, P.; Wheatley, A. E. H. Dalton Trans. 2001, 2838. (c) Niemeyer, M.; Power, P. P. Organometallics 1995, 14, 5488. (20) For a related example of a polymeric potassium tris(diisopropylamide) magnesiate see: Hevia, E.; Kenley, F. R.; Kennedy, A. R.; Mulvey, R. E.; Rowlings, R. B. Eur. J. Inorg. Chem. 2003, 3347. (21) For selected examples see: (a) Andrews, P. C.; Armstrong, D. R.; Baker, D. R.; Mulvey, R. E.; Clegg, W.; Horsburgh, L.; O’Neil, P. A.; Reed, D. Organometallics 1995, 14, 427. (b) MacDougall, D. M.; Morris, J. J.; Noll, B. C.; Henderson, K. W. Chem. Commun. 2005, 456.

(c) Morris, J. J.; Noll, B. C.; Henderson, K. W. Chem. Commun. 2007, 5191. (d) Gartner, M.; Görls, H.; Westerhausen, M. Acta Crystallogr., Sect. C 2007, 63, m2287. (e) Jaenschke, A.; Paap, J.; Behrens, U. Z. Anorg. Allg. Chem 2008, 634, 461. (22) For a recent report on the structural variations of Grignard reagents/dioxane complexes see: Langer, J.; Krieck, S.; Fischer, R.; Görls, H.; Walther, D.; Westerhausen, M. Organometallics 2009, 28, 5814. (23) Uhl, W.; Klinkhammer, K.-W.; Layh, M.; Massa, W. Chem. Ber. 1991, 124, 279. (24) For related structures in lithium aluminate chemistry see: (a) Schiefer, M.; Reddy, N. D.; Ahn, H. J.; Stasch, A.; Roesky, H. W.; Schlicker, A. G.; Schmidt, H. G.; Noltemeyer, M.; Vidovic, D. Inorg. Chem. 2003, 42, 4970. (b) Schiefer, M.; Hatop, H.; Roesky, H. W.; Schmidt, H. G.; Noltemeyer, M. Organometallics 2002, 21, 1300. (c) Garcı ́a-Alvarez, J.; Hevia, E.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. Chem. Commun. 2007, 2402. (25) (a) Hevia, E.; Henderson, K. W.; Kennedy, A. R.; Mulvey, R. E. Organometallics 2006, 25, 1778. (b) Hsueh, M.-L.; Ko, B.-T.; Athar, T.; Lin, C.-C.; Wu, T.-M.; Hsu, S.-F. Organometallics 2006, 25, 4144. (c) Cole, S. C.; Coles, M. P.; Hitchcock, P. B. Organometallics 2004, 23, 5159. (d) Garcı ́a-Alvarez, J.; Graham, D. V.; Hevia, E.; Kennedy, A. R.; Mulvey, R. E. Dalton Trans. 2008, 1481. (26) Stabilizing interactions of this type are common for alkali metals with low coordination numbers when polarized Siδ+−Meδ− groups are present in the structure. For selected examples see ref 12c and: (a) Kennedy, A. R.; Mulvey, R. E.; Rowlings, R. B. J. Am. Chem. Soc. 1998, 120, 7816. (b) Tatic, T.; Meindl, K.; Henn, J.; Pandey, S. K.; Stalke, D. Chem. Commun. 2010, 46, 4562. (c) Clegg, W.; Conway, B.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Russo, L. Eur. J. Inorg. Chem. 2011, 721. (27) A similar approach has been reported by Mulvey for the reaction of the sterically hindered 1,4-dimethylpiperazine with lithium anilide, which allowed the trapping of significant ladder fragments of Li(NHPh). See: Clegg, W.; Liddle, S. T.; Mulvey, R. E.; Robertson, A. Chem. Commun. 2000, 223. (28) Weiss, E. Angew. Chem., Int. Ed. 1993, 32, 1501. (29) Similar Li···Mg···Li linear arrangements have been found in the structures of mixed Li−Mg amido complexes: (a) Clegg, W.; Henderson, K. W.; Mulvey, R. E.; O’Neil, P. A. Chem. Commun. 1993, 969. (b) Clegg, W.; Henderson, K. W.; Mulvey, R. E.; O’Neil, P. A. Chem. Commun. 1994, 769. (30) Thoennes, D.; Weiss, E. Chem. Ber 1978, 111, 3726. (31) For selected examples see: (a) Forbes, G. C.; Kennedy, A. R.; Mulvey, R. E; Rodger, P. J. A. Chem. Commun. 2001, 1400. (b) Garcı ́aAlvarez, P.; Kennedy, A. R.; O’Hara, C. T.; Reilly, K.; Robertson, G. M. Dalton Trans. 2011, 40, 5332. (c) Hill, M. S.; Kociok-Köhn, G.; MacDougal, D. J. Inorg. Chem. 2011, 50, 5234. (32) For selected examples see: (a) Andrikopoulos, P. C.; Armstrong, D. C.; Kennedy, A. R.; Mulvey, R. E.; O’Hara, C. T.; Rowlings, R. B.; Weatherstone, S. Inorg. Chim. Acta 2007, 360, 1370. (b) Blair, V. L.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. Chem. Commun. 2008, 5426. (c) Kennedy, A. R.; O’Hara, C. T. Dalton Trans. 2008, 4975. (d) Campbell, R.; Conway, B.; Fairweather, G. S.; Garcia-Alvarez, P.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; O’Hara, C. T.; Robertson, G. M. Dalton Trans. 2010, 39, 4975. (33) Tatic, T.; Ott, H.; Stalke, D. Eur. J. Inorg. Chem. 2008, 3765. (34) (a) Armstrong, D. R.; Herd, E.; Graham., D. V.; Hevia, E.; Kennedy, A. R.; Clegg, W.; Russo, L. Dalton Trans. 2008, 1323. (b) Clegg, W.; Graham, D. V.; Herd, E.; Hevia, E.; Kennedy, A. R.; McCall, M. D.; Russo, L. Inorg. Chem. 2009, 48, 5320. (35) (a) Williard, P. G.; Liu, Q.-Y. J. Am. Chem. Soc. 1993, 115, 3380. (b) Armstrong, D. R.; Kennedy, A. R.; Mulvey, R. E.; Robertson, S. D. Chem. Eur. J. 2011, 17, 8820. (36) The 2:4 Li:Mg stoichiometry is out of line with other Li:Mg ratios previously reported.15 For another example of a Mg-rich lithium magnesiate with an unusual 1:4 Li:Mg ratio see: Henderson, K. W.; Mulvey, R. E.; Reinhard, F. B.; Clegg, W.; Horsburgh, L. J. Am. Chem. Soc. 1994, 116, 10777. 5141

dx.doi.org/10.1021/om300477m | Organometallics 2012, 31, 5131−5142

Organometallics

Article

(50) (a) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. 1996, 35, 2150. (b) Fukawa, T.; Nakamoto, M.; Ya-Lee, V.; Sekiguchi, A. Organometallics 2004, 23, 2376. (51) (a) Yu, C.; Levy, G. C. J. Am. Chem. Soc. 1984, 106, 6533. (b) Bauer, W.; Clark, T.; Schleyer, R. J. Am. Chem. Soc. 1987, 109, 970. (52) For comparison a 1H DOSY NMR experiment on free dioxane with the internal standards was carried out, providing an estimated FW for dioxane of 89.34 g mol−1 (Figure S30 in the Supporting Information) (1% error with respect to DOSY size). (53) Armstrong, D. R.; Garcı ́a-Alvarez, P.; Kennedy, A. R.; Mulvey, R. E.; Robertson, S. D. Chem. Eur. J. 2011, 17, 6725. (54) (a) CrysAlisPro; Oxford Diffraction Ltd., Oxford, U.K., 2008. (b) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.

(37) Alternatively this anion can be envisaged as an anionic inverse crown ether complex comprising a six-membered {Mg3C3} ring hosting an alkoxide group in its core. (38) For selected examples, see: (a) Pajerski, A. D.; Squiller, E. P.; Parvez, M.; Whittle, R. R.; Richey, H. G., Jr. Organometallics 2005, 24, 809. (b) Henderson, K. W.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E.; Parkinson, J. A.; Sherrington, D. C. Dalton Trans. 2003, 1365. (c) Hevia, E.; Kennedy, A. R.; Mulvey, R. E.; Wheatherstone, S. Angew. Chem., Int. Ed. 2004, 43, 1709. (d) Gromada, J.; Mortreux, J.; Chenal, T.; Ziller, J. W.; Leising, F.; Carpentier, J. F. Chem.Eur. J. 2002, 8, 3773. (39) For a related example of a neutral homoleptic magnesium alkoxide with a missing-corner cubane structure, see: Zechmann, C. A.; Boyle, T. J.; Rodriguez, M. A.; Kemp, R. A. Inorg. Chim. Acta 2001, 319, 137. (40) It should be noted that if the reaction mixture was exposed to air for longer periods of time, an insoluble solid was obtained, which suggests the decomposition of 7. (41) (a) Kennedy, A. R.; Mulvey, R. E.; Rason, C. L.; Roberts, B. A.; Rowlings, R. B. Chem. Commun. 1999, 353. (b) Forbes, G. C.; Kennedy, A. R.; Mulvey, R. E.; Rowlings, R. B.; Clegg, W.; Liddle, S. T.; Wilson, C. C. Chem. Commun. 2000, 1759. The controlled exposure to oxygen of the related tris(alkyl) magnesiate [{NaMgR3}∞] leads to the isolation of an alkyl/alkoxide magnesium-sodium inverse crown complex. See: (c) Baillie, S. E.; Blair, V. L.; Hevia, E.; Kennedy, A. R. Acta Crystallogr., Sect. C 2011, 67, m249. (42) (a) Sarazin, Y.; Schormann, M.; Bochmann, M. Organometallics 2004, 23, 3296. (b) Ireland, B. J.; Wheaton, C. A.; Hayes, P. G. Organometallics 2010, 29, 1079. (43) A similar variable-temperature study was performed for 4 in deuterated toluene solutions, which showed that at lower temperatures a dioxane-solvated aggregate precipitates, causing a change in dioxane:CH2:SiMe3 integration ratio from 4:27:6 at room temperature to 1:27:6 at 270 K. (44) The relative positioning of the signals in the 1H NMR spectrum of TMEDA indicates whether the diamine is coordinated to the metal or isolated. See: Andrews, P. C.; Barnett, N. D. R.; Mulvey, R. E.; Clegg, W.; O’Neil, P. A. J. Organomet. Chem. 1996, 518, 85. (45) At room temperature only one set of signals is observed for the monosilyl ligands at −1.32 and 0.42 ppm for the CH2 and CH3 groups, respectively. Variable-temperature studies were attempted for 6; however, due to its limited stability in C6D6 solution, no meaningful data could be collected. (46) (a) Keresztes, I.; Williard, P. G. J. Am. Chem. Soc. 2000, 122, 10228. (b) Li, D.; Sun, C.; Liu, J.; Hopson, R.; Li, W.; Williard, P. G. J. Org. Chem. 2008, 73, 2373. (c) Merkel, S.; Stern, D.; Henn, J.; Stalke, D. Angew. Chem., Int. Ed. 2009, 48, 6350. (d) Granitzka, M.; Pöppler, A. C.; Schwarze, E. K.; Stern, D.; Schulz, T.; John, M.; Herbst-Irmer, R.; Pandey, S. K.; Stalke, D. J. Am. Chem. Soc. 2012, 134, 1344. (e) Garcı ́a-Alvarez, P.; Mulvey, R. E.; Parkinson, J. A. Angew. Chem., Int. Ed. 2011, 50, 9668. (f) Li, W.; Kaga, G.; Hopson, R.; Williard, P. G. ARKIVOC 2011, 5, 180. (g) Armstrong, D. R.; Clegg, W.; GarciaAlvarez, P.; McCall, M. D.; Nuttall, L.; Kennedy, A. R.; Russo, L.; Hevia, E. Chem. Eur. J. 2011, 17, 4470. (h) Kagan, G.; Li, W.; Hopson, R.; Williard, P. G. Org. Lett. 2010, 12, 520. (47) (a) Hassinen, A.; Moreels, I.; Donega, C. d. M.; Martins, J. C.; Hens, Z. J. Phys. Chem. Lett. 2010, 1, 2577. (b) Pöppler, A. C.; Meinholz, M. M.; Faßhuber, H.; Lange, A.; John, M.; Stalke, D. Organometallics 2012, 31, 42. (48) (a) Li, D.; Keresztes, I.; Hopson, R.; Willard, P. G. Acc. Chem. Res. 2009, 42, 270. (b) Kagan, G.; Li, W.; Li, D.; Hopson, R.; Williard, P. G. J. Am. Chem. Soc. 2011, 133, 6596. (c) Li, D.; Kagan, G.; Hopson, R.; Williard, P. G. J. Am. Chem. Soc. 2009, 131, 5627. (49) (a) Gokel, G. W.; De Wall, S. L.; Meadows, E. S. Eur. J. Org. Chem. 2000, 2967. (b) Gallagher, D. J.; Henderson, K. W.; Kennedy, A. R.; O’Hara, C. T.; Mulvey, R. E.; Rowlings, R. B. Chem. Commun. 2002, 376. 5142

dx.doi.org/10.1021/om300477m | Organometallics 2012, 31, 5131−5142