Mechanochemical and Solution Synthesis of the Tris(allyl) - MDPI

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May 27, 2017 - william.brennessel@rochester.edu .... π-bound structure (a) is 4.0 kcal·mol−1 lower in energy (AG◦) than the σ-bound geometry (b) [11].
inorganics Article

Symmetric Assembly of a Sterically Encumbered Allyl Complex: Mechanochemical and Solution Synthesis of the Tris(allyl)beryllate, K[BeA0 3] (A0 = 1,3-(SiMe3)2C3H3) Nicholas C. Boyde 1 , Nicholas R. Rightmire 1 , Timothy P. Hanusa 1, * and William W. Brennessel 2 1 2

*

Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA; [email protected] (N.C.B.); [email protected] (N.R.R.) Department of Chemistry, University of Rochester, Rochester, NY 14627, USA; [email protected] Correspondence: [email protected]

Academic Editor: Matthias Westerhausen Received: 23 April 2017; Accepted: 23 May 2017; Published: 27 May 2017

Abstract: The ball milling of beryllium chloride with two equivalents of the potassium salt of bis(1,3-trimethylsilyl)allyl anion, K[A0 ] (A0 = [1,3-(SiMe3 )2 C3 H3 ]), produces the tris(allyl)beryllate K[BeA’3 ] (1) rather than the expected neutral BeA’2 . The same product is obtained from reaction in hexanes; in contrast, although a similar reaction conducted in Et2 O was previously shown to produce the solvated species BeA’2 (OEt2 ), it can produce 1 if the reaction time is extended (16 h). The tris(allyl)beryllate is fluxional in solution, and displays the strongly downfield 9 Be NMR shift expected for a three-coordinate Be center (δ22.8 ppm). A single crystal X-ray structure reveals that the three allyl ligands are bound to beryllium in an arrangement with approximate C3 symmetry (Be–C (avg) = 1.805(10) Å), with the potassium cation engaging in cation–π interactions with the double bonds of the allyl ligands. Similar structures have previously been found in complexes of zinc and tin, i.e., M[M0 A0 3 L] (M0 = Zn, M = Li, Na, K; M0 = Sn, M = K; L = thf). Density functional theory (DFT) calculations indicate that the observed C3 -symmetric framework of the isolated anion ([BeA0 3 ]− ) is 20 kJ·mol−1 higher in energy than a C1 arrangement; the K+ counterion evidently plays a critical role in templating the final conformation. Keywords: allyl ligands; beryllium; coordination modes; mechanochemistry; X-ray diffraction; density functional theory calculations

1. Introduction The physical and chemical properties of first-row elements often differ appreciably from their second-row and heavier counterparts; for the group 2 metals, the outlier (”black sheep” [1]) designation belongs to beryllium. To a considerably greater extent than its heavier congeners, even magnesium, the small size of the Be2+ cation (0.27 Å for CN = 4; cf. 0.57 Å for Mg2+ ) [2] and its corresponding high charge/size ratio ensures its bonds will be strongly polarized and possess substantial covalent character. Not surprisingly, beryllium compounds with the same ligand sets commonly have different structures from those of the other, more electropositive alkaline earth (Ae) metals. The bis(trimethylsilyl)amides of Mg–Ba, for example, have a common dimeric bridged structure, [Ae(N(SiMe3 )(µ-N(SiMe3 )2 ]2 [3], whereas that of beryllium is a two-coordinate monomer [4]. Similarly, the bis(cyclopentadienyl) complex Cp2 Be has an η1 ,η5 -Cp structure [5] that is unlike that of the heavier metallocenes [6]. Investigation of these differences, and indeed research with all beryllium compounds, has traditionally been limited

Inorganics 2017, 5, 36; doi:10.3390/inorganics5020036

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because concernsits about toxicity [7], but serving that has as not prevented its compounds serving as useful has prevented compounds from benchmarks of steric and has not not of prevented its compounds from serving as useful useful benchmarks of the thefrom steric and electronic electronic benchmarks of the steric and electronic consequences of crowded metal environments [8–10]. consequences consequences of of crowded crowded metal metal environments environments [8–10]. [8–10]. consequences is theis stabilitystability of η1 - vs.of η3 -bonded ligands allyl in compounds of One of these consequences the ηη11-- vs. ηη33-bonded ligands One of ofthese these consequences isrelative the relative relative stability of vs. allyl -bonded allyl ligands in in 0 ]− (A0 = [1,3-(SiMe ) C −− (A′ highly electropositive metals. We found some time ago that the bulky allyl [A H ]) compounds of highly electropositive metals. We found some time ago that the bulky allyl [A′] 3 2 3(A′3= compounds of highly electropositive metals. We found some time ago that the bulky allyl [A′] = 1 -bonded A’ ligands1 in the solid · can be used to form the ether adduct BeA’ OEt , which displays η [1,3-(SiMe 2 ether 2 adduct [1,3-(SiMe33))22C C33H H33]) ]) can can be be used used to to form form the the ether adduct BeA’ BeA’22·OEt ·OEt22,, which which displays displays ηη1-bonded -bonded A’ A’ state [11]. The compound is fluxional in solution, and exhibits symmetric, “π-type”symmetric, bonding in“πits ligands in state The is in and ligands in the the solid solid state [11]. [11]. The compound compound is fluxional fluxional in solution, solution, and exhibits exhibits symmetric, “πNMR spectra (e.g., only one peak is observed for the SiMe groups). Density functional theory (DFT) type” bonding in its NMR spectra (e.g., only one peak is observed for the SiMe 3 groups). Density type” bonding in its NMR spectra (e.g., only one peak is3 observed for the SiMe3 groups). Density calculations suggested that a base-free Be(C SiH3 ) complex slightly more functional (DFT) calculations suggested aa H, base-free Be(C 33E H, 33)) complex 3 H3 E2 )that 2 (E = functional theory theory (DFT) calculations suggested that base-free Be(C33H Hwould E22))22 (E (Ebe== more H, SiH SiH complex stable with delocalized, π-type allyls than with monodentate, sigma-bonded ligands (Scheme 1). If so, would be more slightly more stable with delocalized, π-type allyls than with monodentate, sigmawould be more slightly more stable with delocalized, π-type allyls than with monodentate, sigmaberyllium allyls would join those of magnesium, in which monodentate allyl ligands are uniformly bonded bonded ligands ligands (Scheme (Scheme 1). 1). If If so, so, beryllium beryllium allyls allyls would would join join those those of of magnesium, magnesium, in in which which found in complexes that areare ether-solvated [12], but that in the that absence of ethers, cation–π monodentate allyl uniformly in are [12], but monodentate allyl ligands ligands are uniformly found found in complexes complexes that are ether-solvated ether-solvated [12],interactions but that that in in with the metal can create “slipped-π” bonding [13]. the absence of ethers, cation–π interactions with the metal can create “slipped-π” bonding [13]. the absence of ethers, cation–π interactions with the metal can create “slipped-π” bonding [13].

Scheme Scheme 1. 1. Optimized Optimized geometries geometries of of Be(1,3-(SiH Be(1,3-(SiH33))22C C33H H33))22.. At At the the B3PW91/aug-cc-pVDZ B3PW91/aug-cc-pVDZ level, level, the the ππScheme 1. Optimized geometries−1of Be(1,3-(SiH3 )2 C3 H3 )2 . At the B3PW91/aug-cc-pVDZ level, the −1 lower in energy (∆G°) than the σ-bound geometry (b) [11]. bound structure (a) is 4.0 kcal·mol in energy (∆G°) than the σ-bound geometry (b) [11]. bound structure (a) is 4.0 kcal·mol lower − 1 ◦ π-bound structure (a) is 4.0 kcal·mol lower in energy (∆G ) than the σ-bound geometry (b) [11].

The The coordinated coordinated ether ether in in BeA’ BeA’22·OEt ·OEt22 proved proved impossible impossible to to remove remove without without destroying destroying the the The coordinated ether in BeA’ OEt proved impossible to remove without destroying the · 2 2 complex [11], and thus we investigated mechanochemical methods of synthesis as a means to bypass complex [11], and thus we investigated mechanochemical methods of synthesis as a means to bypass complex and thus we investigated mechanochemical methods neutral of synthesis as a was means bypass the ethereal solvents [14]. below, complex the use use of of[11], ethereal solvents [14]. As As detailed detailed below, an an unsolvated unsolvated neutral complex was not nottoisolated isolated the use of ethereal solvents [14]. As detailed below, an unsolvated neutral complex was not isolated via via via this this route, route, and and the the beryllate beryllate anion anion that that was was produced produced instead instead has has structural structural parallels parallels with with this route, and the beryllate anion thatof instead structural with previously previously described -ate Zn [15] [16]. In of species, metal previously described -ate complexes complexes ofwas Znproduced [15] and and Sn Sn [16]. has In all all of these theseparallels species, the the alkali alkali metal described -ate complexes of Zn [15] and Sn [16]. In all of these species, the alkali metal counterion, usually + + + + + + counterion, counterion, usually usually K K but but sometimes sometimes Na Na and and Li Li ,, appears appears to to play play aa critical critical role role in in the the assembly assembly of of + K sometimes Na+ and Li+ , appears to play a critical role in the assembly of the symmetric complexes. the symmetric complexes. thebut symmetric complexes. 2. Results Results and Discussion Discussion 2. 2. Results and and Discussion 2.1. Solid-State Synthesis 2.1. 2.1. Solid-State Solid-State Synthesis Synthesis The reaction of BeCl2 and K[A’] was conducted mechanochemically with a planetary ball mill, The of K[A’] was mechanochemically with planetary ball The reaction reaction of BeCl BeCl22 and and was conducted conducted mechanochemically with aa ratios planetary ball mill, mill, followed by an extraction withK[A’] hexanes. Initial investigations used 2:1 molar of BeCl 2 and followed by an extraction with hexanes. Initial investigations used 2:1 molar ratios of BeCl 22 and K[A’], followed by an extraction with hexanes. Initial investigations used 2:1 molar ratios of BeCl and K[A’], K[A’], based on the assumption that the product formed would be BeA’2 (Equation (1)). Although the based the assumption that the product formed would based on onare theoff-white assumption thatand thewhite product formed would be be BeA’ BeA’22 (Equation (Equation (1)). (1)). Although Although the the reagents (K[A’]) (BeCl 2 ), the ground reaction mixture (15 min/600 rpm) is reagents are off-white (K[A’]) and white (BeCl 22), the ground reaction mixture (15 min/600 rpm) is reagents are off-white (K[A’]) and white (BeCl ), the ground reaction mixture (15 min/600 rpm) orange. Extraction with hexanes, followed by filtration, yielded an orange filtrate and ultimately is a orange. Extraction with hexanes, followed by filtration, yielded an orange filtrate and ultimately aa orange. Extraction with hexanes, followed by filtration, yielded an orange filtrate and ultimately dark orange, highly air-sensitive solid (1) on drying. dark dark orange, orange, highly highly air-sensitive air-sensitive solid solid (1) (1) on on drying. drying.

(1) (1) (1) A single crystal analysis (described below) revealed that 1 is the potassium tris(allyl)beryllate, A single crystal analysis (described below) revealed that 1 is the potassium tris(allyl)beryllate, A 3single 1 is the potassium tris(allyl)beryllate, forms in of that 2:1 of used optimum K[BeA’ 3]. ]. This Thiscrystal forms analysis in spite spite (described of the the fact fact below) that the therevealed 2:1 ratio ratiothat of reagents reagents used is is not not optimum for for its its K[BeA’ K[BeA’ ]. This forms in spite of the fact that the 2:1 ratio of reagents used is not optimum forand its 3 production. As production. As detailed detailed below, below, conducting conducting the the reaction reaction with with 1:1 1:1 and and 3:1 3:1 molar molar ratios ratios of of K[A’] K[A’] and production. As detailed below, the reaction with 1:1It 3:1 molarthat ratios ofexcess K[A’] and BeClis BeCl 11 as sole hexane-extractable product. is the halide BeCl22 still still yields yields as the the soleconducting hexane-extractable product. Itand is possible possible that the excess halide is2 still yields 1 as the sole hexane-extractable product. It is possible that the excess halide is captured 2− 2− 2− 2− captured captured in in the the form form of of polyhalide polyhalide anions anions such such as as [BeCl [BeCl44]] or or [Be [Be22Cl Cl66]] [17], [17], although although these these have have not not been been definitively definitively identified. identified. 22 K[A’] K[A’] ++ BeCl BeCl22

BeA’ BeA’22 ++ 22 KCl KCl (expected) (expected)

Inorganics 2017, 5, 36

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inInorganics the form of 5,polyhalide anions such as [BeCl4 ]2− or [Be2 Cl6 ]2− [17], although these have not been 2017, 36 3 of 11 definitively identified. 2.2. Synthesis Inorganics 2017, 5,in36Solution 3 of 11 2.2. Synthesis in Solution The reaction of K[A’] and BeCl2 was also examined in solution, using diethyl ether and hexanes. 2.2. Synthesis in Solution 2.2. Synthesis in Solution TheseThe results are summarized in Table 1.also Previous reactions with diethyl ether involved stirring for reaction of K[A’] and BeCl 2 was examined in solution, using diethyl ether and hexanes. reaction of K[A’] and BeCl also examined inasolution, using diethyl ether and hexanes. 2 was 1. 2 h The atThe room temperature, which formed BeA’ 2 ·OEt 2 from 2:1 reaction (#5); Schlenk equilibrium was These results are summarized in Table Previous reactions with diethyl ether involved stirring for reaction of K[A’] and BeCl2 was also examined in solution, using diethyl ether and hexanes. These results are summarized in Table 1. Previous reactions with diethyl ether involved stirring for observed in temperature, a are 1:1 summarized mixturewhich that in was allowed to2·OEt react for one hour (#4). When theinvolved 2:1 equilibrium reaction in Et 2O 2 h at room formed 2 reactions from a 2:1 reaction (#5);ether Schlenk was These results Table 1.BeA’ Previous with diethyl stirring for nics 2017, 5, 36 3 of Schlenk 11 2 is h at room temperature, which BeA’ ·OEt a 2:1 reaction (#5); equilibrium wasin 2 to 2 from for 16 h,formed however, the is observed (#6)Schlenk exclusively. Reaction observed intotemperature, aproceed 1:1 mixture that was allowed react for of one hour (#4). When the 2:1 equilibrium reaction in Et 2O 2 hallowed at room which formed BeA’ 2formation ·OEt 2 from a 12:1 reaction (#5); was observed in a 1:1 mixture that wasreactions, allowed toinreact for one hour (#4). When the 2:1 reaction in Et2 O hexanes mimics the solid-state that 1 is the exclusively detected organoberyllium is allowed to proceed for 16 h, however, the formation of 1 is observed (#6) exclusively. Reaction in a 1:1 mixture that was allowed to react for one hour (#4). When the 2:1 reaction in Et2in O Synthesis in Solutionisobserved allowed to proceed for 16 h, however, theLonger formation of 1 isand observed (#6) exclusively. Reaction in product from a 1:1the reaction 1reactions, h (#7). reactions a higher ratio of K[A’] to BeCl2 (e.g., hexanes mimics solid-state that 1 isofthe detected organoberyllium is allowed to proceed for 16after h, however, theinformation 1 isexclusively observed (#6) exclusively. Reaction in hexanes mimics thealso solid-state reactions, in that 1 is the exclusively detected organoberyllium product The reaction of K[A’] 2 was examined solution, using diethyl ether and hexanes. 3:1)and doBeCl not change this outcome. product from a 1:1 reaction afterin1reactions, h (#7). Longer reactions and a higher ratio of K[A’] to BeCl2 (e.g., hexanes mimics the solid-state in that 1 is the exclusively detected organoberyllium from a 1:1 reaction after 1 h (#7). Longer reactions and a higher ratio of K[A’] to BeCl2 (e.g., 3:1) do not e results are summarized in Table 1. Previous reactions with diethyl ether involved stirring for 3:1) do not change outcome. product from a 1:1 this reaction after 1 h (#7). Longer reactions and a higher ratio of K[A’] to BeCl2 (e.g., change this outcome. t room temperature, which formed 2·OEt2of from 2:1 reaction (#5); Schlenk was as molar ratios. Table 1.BeA’ Summary K[A’]a and BeCl2 reactions; amountsequilibrium of reagents given 3:1) do not change this outcome. rved in a 1:1 mixture that was allowed to react one hour When amounts the 2:1 reaction in given Et2O as molar ratios. Table 1. Summary offor K[A’] and BeCl(#4). 2 reactions; of reagents a K[A’]:BeCl Medium Time (#6)amounts Organoberyllium Product(s) Yield (%) b Table 1. Summary of K[A’]of and BeCl of reagents given ratios. 2 reactions; owed to proceed for 16No. h, however, the2 formation 1 is observed exclusively. Reaction inasasmolar Table 1. Summary of K[A’] and BeCl 2 reactions; amounts of reagents given molar ratios. a No. K[A’]:BeCl 2 Medium Time Organoberyllium Yield 1 1:1 15 min detected K[BeA’3]Product(s) 97(%) b nes mimics the solid-state reactions, in that 1 is the exclusively organoberyllium a b No. K[A’]:BeCl Medium Time Organoberyllium Product(s) Yield (%) a b 2 2 Medium Time Yield 12 1:1 15 min ratio ofOrganoberyllium ]Product(s) 97 2:1 Longer 21(%) uct from a 1:1 reactionNo. after 1K[A’]:BeCl h (#7). reactions and a higher K[A’] toK[BeA’ BeCl23(e.g., 15 min K[BeA’3K[BeA’ ] 97 1:1 15 min 33] 97 2:11:1 21 do not change this outcome. 25 312 1 3:1

o.

1

2

15 min K[BeA’ ] BeA’3]2 + BeCl 21 2:1 151min 21 3K[BeA’ 25c,d 342 2 3:1 h 2A’BeCl n/a 1:12:1 Et2O Table 1. Summary of34K[A’] and BeCl 2 reactions; amounts of reagents given as molar ratios. 15 min K[BeA’ ] BeA’ 25 c 3K[BeA’ 1512min 3]2 2+ BeCl 25c,d 3:1 h 2A’BeCl n/a 1:1 Et2O BeA’ 2·OEt 77 5 3 2:13:1 c,d 4 1:1 Et O 1 h n/a 2A’BeCl  BeA’ + BeCl c,d 2 2 BeA’ 12 hh Product(s) 2A’BeCl n/a 1:1 Time a BeA’ 2·OEt 77 16 K[BeA’ 3b]2 2+ BeCl 98c 645 2:1 Et22OOrganoberyllium K[A’]:BeCl2 Medium Yield (%) 5 2:1 Et2 O 2h BeA’2 ·OEt2 77 c 21 hh 3] 2·OEt 77 K[BeA’ 3] 2 98 675 6 2:12:115 min hexanes Et2O 24c 1:1 1:1 K[BeA’ Et 16 h16 K[BeA’BeA’ 98 2O 3 ] 97 16 3] 98 67= ball 2:11:115 Et2O 7 milling 1K[BeA’ hfor K[BeA’3K[BeA’ ] 21 been a 24 1:1 hexanes 1 hhmechanical at min 600 rpm. The symbol milling has proposed 24 in ref. [14]; 2:1 3]

3

3:1

4

1:1

5

2:1

6

2:1

7

1:1

b

c Ref. [11]; d Products 9Be NMR, a 7= ball 3] ref. 24 b 1:1limiting hexanes 1 hmechanical K[BeA’ Unrecrystallized; reagent taken into account; observed with at 600 600rpm. rpm. symbol for milling has were been proposed in ref. [14]; = ballmilling milling at TheThe symbol for mechanical milling has been proposed in [14]; b Unrecrystallized;

a

15 min K[BeA’3] 25 c Ref. [11]; d Products were observed with 9 Be NMR, and were not isolated. limiting reagent taken into account;taken c Ref. [11]; d Products were observed with 9Be NMR,b aUnrecrystallized; and were not isolated. limiting reagent into account; = ball milling at 600 rpm. The symbol for mechanical milling has 1h 2A’BeCl BeA’2 + BeCl n/a c,dbeen proposed in ref. [14]; Et2O c d and were not isolated. Unrecrystallized; limiting account; Ref. [11]; Products observed with 9Be NMR, c 2 h reagent taken intoBeA’ 2·OEt2 77 were Et2O 2.3. NMR Spectroscopy 2.3. NMR Spectroscopy and were not isolated. 16 h K[BeA’3] 98 Et2O

2.3.The NMR Spectroscopy 1H NMR The1 H NMRspectrum spectrumofof11displays displaysresonances resonancestypical typicalofofaa“π-bound” “π-bound”A’A’ligand, ligand,with witha atriplet triplet 24 hexanes 1h K[BeA’3] 2.3. NMR Spectroscopy representing , a broad resonance (ν = 39 Hz, presumably an unresolved doublet) representing (β) , a broad resonance (ν 1/2 = 39 Hz, presumably an unresolved doublet) representing representing H The 1HH NMR spectrum of 1 displays resonances typical of a “π-bound” A’ ligand, with a triplet 1/2 (β) = ball milling at 600 rpm. The1 symbol for mechanical milling has been proposed in ref. [14]; b the HH H(γ) ,of and asinglet singlet the two equivalent trimethylsilyl groups (Figure 1). theequivalent equivalent (α) and H , and two equivalent trimethylsilyl (Figure The (β) , aspectrum broad resonance (ν1/2 for = for 39the Hz, presumably unresolved doublet) representing representing H The H NMR 1 adisplays resonances typical of aan “π-bound” A’groups ligand, with a1). triplet (α) (γ) crystallized; limiting reagent taken into account; c Ref. [11]; d Products were observed with 9Be NMR, The of aHsymmetric spectrum even when σ-bound ligands expected isrepresenting consistent appearance ofHsuch abroad symmetric when σ-bound ligands are are expected is consistent theappearance equivalent H (γ)resonance , and spectrum a singlet two equivalent trimethylsilyl groups (Figure 1).with The (β)(α) ,such aand (ν1/2 for =even 39the Hz, presumably an unresolved doublet) representing were not isolated. with a high degree of fluxionality, as was also observed in the σ-bound complex BeA’ O)1).[11]. aappearance high degree of(α)fluxionality, asspectrum alsofor observed in the σ-bound complex BeA’ 2·(Et O) 2[11]. The of such a symmetric even when σ-bound ligands are expected is consistent with the equivalent H and H(γ), and awas singlet the two equivalent trimethylsilyl groups (Figure 2 ·2(Et The triplet resonance theallyl allylligands, ligands, atδ6.97, δ6.97, shifted downfield from ofof BeA’ ·2consistent (Et (δ6.53); triplet resonance the isis shifted downfield from that BeA’ ·(Et 2O) (δ6.53); high degree of of fluxionality, asspectrum was at also observed in the σ-bound complex BeA’ 22·(Et 2O) [11]. The of such aof symmetric even when σ-bound ligands arethat expected is with 2 O) NMR Spectroscopy aappearance the resonance atat δ3.19 isisslightly upfield (cf. BeA’ ·downfield (Et O)). The NMR chemical shifts for 1 are resonance δ3.19 slightly upfield (cf.δ3.33 δ3.33 in BeA’ 2·(Et 2 O)). The NMR chemical shifts for 1 are triplet resonance of the allyl ligands, δ6.97, isin shifted from that of BeA’ 2 ·(Et 2 O) (δ6.53); athe high degree of fluxionality, as was at also observed in 2the σ-bound complex BeA’ 2 ·(Et 2 O) [11]. The 2 The 1H NMR spectrum of 1 displays resonances typical of a “π-bound” A’ ligand, with a triplet inin line with those observed for other M[M’A’ ] complexes (Table 2). In particular, the NMR shifts of 3]iscomplexes (Table InNMR particular, the2·(Et NMR shifts of line with those observed for other M[M’A’ the resonance at δ3.19 slightly upfield (cf. 3δ3.33 in BeA’downfield 2·(Et 2O)).2). The shifts 1 are triplet resonance of theisallyl ligands, at δ6.97, shifted from thatchemical of BeA’ 2O)for (δ6.53); resonance (νat1/2 =observed 39sensitive Hz, presumably an unresolved doublet) representing esenting H(β), a broad the ligands are sensitive both totothe identity of central divalent metal and totothat alkali allyl ligands are both the identity the divalent metal and thatofof alkali 3] complexes (Table 2). InNMR particular, the NMR shifts of in line with those for other M[M’A’ theallyl resonance δ3.19 is slightly upfield (cf. δ3.33 inofthe BeA’ 2central ·(Et 2O)). The chemical shifts for 1 are quivalent H(α) andmetal H (γ), and a singlet for the two equivalent trimethylsilyl groups (Figure 1). The counterion, evidence that the compounds exist as contact ion pairs in solution. Compound 1 metal counterion, evidence that the compounds exist as contact ion pairs in solution. Compound 1 the allyl ligands are sensitive both to the identity of the central divalent metal and to that of alkali in line with those observed for other M[M’A’3] complexes (Table 2). In particular, the NMR shifts of + arance of such a symmetric spectrum even when σ-bound ligands are expected is consistent with + and the greatest similarities, which may reflect their having the same (K(K)1) andK[ZnA’ K[ZnA’ ] share the greatest similarities, which may reflect their having the same counterion metal counterion, evidence that thetocompounds exist as contact ion pairsmetal in solution. Compound the allyl ligands are sensitive both the identity of the central divalent andcounterion to that of alkali 3 ]3share h degree of fluxionality, as was alsoofevidence observed in the complex BeA’ 2·(Et 2O) [18]. The and central metals similar electronegativity (χ (1.57); Zn (1.65)) and central metals of similar electronegativity (χBeBe (1.57); Zn (1.65)) [18]. K[ZnA’ 3] share the greatest similarities, which may their having the same counterion (K+1) metal counterion, that theσ-bound compounds exist asreflect contact ion[11]. pairs in solution. Compound 9 Be et resonance of the and allylJohn ligands, at δ6.97, is shifted downfield from that of BeA’ 2·(Et2O) (δ6.53); 9Be and co-workers have demonstrated that NMR chemical shift values can be diagnostic for John and co-workers have demonstrated that NMR chemical shift values can be diagnostic central metals similar electronegativity (χ may (1.57); Zn (1.65)) [18]. the same counterion (K+) K[ZnA’ 3] shareof the greatest similarities, which reflect their having esonance at δ3.19 is slightly upfield (cf. δ3.33 in BeA’ 2 ·(Et 2 O)). The NMR chemical shifts for 1complexes arevalues 9 coordination numbers solution [19]. Typically, organoberyllium with low formal coordination for coordination numbers in electronegativity solution [19]. Typically, organoberyllium with formal John and co-workers have demonstrated that NMR chemical shift can below diagnostic and central metals ofinsimilar (χ Be Be (1.57); Zncomplexes (1.65)) [18]. 3 ] complexes (Table 2). In particular, the NMR shifts of ne with those observed for other M[M’A’ 9 numbers, such BeMe · Et O (coordination number 3, δ20.8 ppm in Et O), are observed well downfield coordination numbers, such as BeMe 2 ·Et 2 O (coordination number 3, δ20.8 ppm in Et 2 O), are observed for coordination numbers in solution [19]. Typically, organoberyllium complexes with low formal John andas co-workers have demonstrated that Be NMR chemical shift values can be diagnostic 2 2 2 9 llyl ligands are sensitive both to the identity of the central divalent metal and to that of alkali 9 ofwell 0 ppm. BeA’ · (Et O) has a Be chemical shift of δ18.2 ppm, which is consistent with a three-coordinate downfield of2numbers 0 ppm. BeA’ 2BeMe ·(Et2O) has Be chemical shift of δ18.2 ppm, which consistent with coordination such 2·Et 2Oa(coordination number 3, δ20.8 ppm in Etis 2with O), are observed for coordination inassolution [19]. Typically, organoberyllium complexes low formal 2numbers, l counterion, evidence that the compounds as contact ion pairs solution. Compound 1which 9Be geometry in solution It exist should be noted, that the correlation between coordination number a three-coordinate geometry solution It in should be noted, however, that theare correlation well downfield of 0[11]. ppm. BeA’ 2BeMe ·(Et 2O) has a[11]. chemical shift of δ18.2 ppm, consistent with coordination numbers, such asin 2·Et 2Ohowever, (coordination number 3, δ20.8 ppm in Etis 2O), observed +) 9 Be chemical K[ZnA’3] share theand greatest similarities, which may reflect their having the same counterion (K 9Becan 9 shift is not exact, and be strongly influenced by the electronic properties of theby between coordination number and chemical shift is not exact, and can be strongly influenced a three-coordinate geometry in solution [11]. It should be noted, however, that the correlation well downfield of 0 ppm. BeA’2·(Et2O) has a Be chemical shift of δ18.2 ppm, which is consistent with central metals of similar electronegativity (χ Be (1.57); Zn (1.65)) [18]. 9 ligands. The 2-coordinate complex beryllium bis(N,N´-bis(2,6-diisopropylphenyl)-1,3,2-diazaborolyl), for the electronic properties of the ligands. The Itshift 2-coordinate complex between coordination number and Be chemical is notbe exact, andhowever, canberyllium be strongly influenced by a three-coordinate geometry in solution [11]. should noted, thatbis(N,N´-bis(2,6the correlation 9Be NMR chemical shift values can be diagnostic John and co-workers have demonstrated that of 9Be example, has an extreme downfield shift ofchemical δ44 ppm [20], whereas the 2-ccordinate displays diisopropylphenyl)-1,3,2-diazaborolyl), for example, has an exact, extreme downfield shift bis(N,N´-bis(2,6of influenced δ44 [20], the electronic properties the ligands. The shift 2-coordinate complex between coordination number and is not and canberyllium be Be(N(SiMe strongly by 3 )2 ppm 9 Bein 9 Be oordination numbers solution [19]. Typically, organoberyllium complexes with low formal 9Be a the NMR shift at δ12.3 ppm [4]. Nevertheless, the of 1 is at δ22.8 ppm, which to our knowledge is whereas the 2-ccordinate Be(N(SiMe 3 ) 2 displays a NMR shift at δ12.3 ppm [4]. Nevertheless, the diisopropylphenyl)-1,3,2-diazaborolyl), for example, has an extreme downfield shift bis(N,N´-bis(2,6of δ44 ppm [20], electronic properties of the ligands. The 2-coordinate complex beryllium 9Be dination numbers,the such as1BeMe ·Et2shift O ppm, (coordination 3, example, δ20.8appm inspecies. Et2extreme O), are 9is of isthe at 22-ccordinate δ22.8 which tonumber our most positive shift yetwere reported for a threemost positive yet reported for three-coordinate DFT methods used toppm predict whereas Be(N(SiMe 3a )2knowledge displays Bethe NMR shift atobserved δ12.3 ppm [4]. Nevertheless, the diisopropylphenyl)-1,3,2-diazaborolyl), for has an downfield shift of δ44 [20], 9Be chemical shift of δ18.2 ppm, which9 is consistent with 9 Be chemical downfield of 0 ppm. BeA’ 2species. O) has appm, 9coordinate DFT methods used to predict the Beshift chemical shift value of 1 (B3LYP-D3/6the shift value of 1 (B3LYP-D3/6-311+G(2d,p)//B3LYP-D3/6-31G(d)). It was calculated Be of 12·(Et isthe at δ22.8 which towere our most positive shift yet reported for a threewhereas 2-ccordinate Be(N(SiMe 3)2knowledge displays a 9isBethe NMR at δ12.3 ppm [4]. Nevertheless, the ree-coordinate geometry in solution [11]. It should be noted, however, that the correlation 9 9311+G(2d,p)//B3LYP-D3/6-31G(d)). It was calculated at δ25.9 ppm, in reasonable agreement with the coordinate species. DFT methods used to predict the Be chemical shift yet value of 1 (B3LYP-D3/6Be of 1 is at δ22.8 ppm, which towere our knowledge is the most positive shift reported for a three9Be chemical shift is not exact, een coordination number and and can be strongly influenced by 2+ 9Be 2)used 4] calculated with an isotropic shielding constant ofagreement 108.98 ppm). observed (referenced to [Be(OH 311+G(2d,p)//B3LYP-D3/6-31G(d)). It was atthe δ25.9 ppm, in reasonable with the coordinatevalue species. DFT methods were to predict chemical shift value of 1 (B3LYP-D3/6electronic properties of the ligands. The 2-coordinate complex beryllium bis(N,N´-bis(2,62+ with an 2 ) 4 ] isotropic shielding constant of 108.98 ppm). observed value (referenced to [Be(OH 311+G(2d,p)//B3LYP-D3/6-31G(d)). It was calculated at δ25.9 ppm, in reasonable agreement with the propylphenyl)-1,3,2-diazaborolyl), for example,tohas an extreme downfield shift of δ44 ppm [20], 2)4]2+ with an isotropic shielding constant of 108.98 ppm). observed value (referenced [Be(OH reas the 2-ccordinate Be(N(SiMe3)2 displays a 9Be NMR shift at δ12.3 ppm [4]. Nevertheless, the f 1 is at δ22.8 ppm, which to our knowledge is the most positive shift yet reported for a three-

Complex

δ H(α)/H(γ)

δ H(β)

δ SiMe3

M–C (σ) Å

M’…C(olefin) Å

Ref.

Li[ZnA’3]

6.46

3.50

0.15

2.117(3)

2.745(4), 2.268(3)

[15]

Na[ZnA’3]

7.59

4.00

0.16

2.103(3)

2.857(3), 2.567(3)

6.97

3.19

0.20

1.805(10)

3.153(7), 2.940(7)

7.05

3.42

0.23

2.068(4)

3.205(3), 2.945(3)

6.43

4.42

0.42, 0.23

2.344(7)

3.201(7), 3.164(8), 3.065(8)

K[BeA’3] Inorganics 2017, 5, 36 K[ZnA’3] K(thf)[SnA’3]

a

b

b

[15] this work 4 of 11 [15] [16]

2+ with an a Two b Distance(s) at δ25.9 ppm, in reasonable agreement observed (referenced to [Be(OH resonances are observed for the with SiMe3the groups, as the value A’ ligands are not fluxional; 2 )4 ] isotropic shielding constant of disorder. 108.98 ppm). affected by crystallographic

Figure 1. 11H NMR spectrum (500 Mhz) of isolated 1, recorded in C6D6. The starred peaks represent Figure 1. H NMR spectrum (500 Mhz) of isolated 1, recorded in C6 D6 . The starred peaks represent impurities: δ7.15 (residual protons of C6D6); δ0.9 and δ1.3, residual hexanes. impurities: δ7.15 (residual protons of C6 D6 ); δ0.9 and δ1.3, residual hexanes.

2.4. Solid State Structure

Table 2. 1 H NMR shifts (ppm) and bond distances in M[M’A’3 ] complexes.

The structure of 1 was determined from single crystal X-ray diffraction. In the solid state, 1 Complex δ H(α) /H δ H(β) withδσ-bound SiMe3 Ref. in M–C (σ) Å and aM’ ···C(olefin)cation Å exhibits approximate C3(γ) -symmetry, A’ ligands potassium engaging b b cation–π with the three bonds of the allyls. It is isostructural with the previously Li[ZnA’interactions 6.46 3.50 double0.15 [15] 2.117(3) 2.745(4), 2.268(3) 3] Na[ZnA’ 0.16 2.103(3) 2.857(3), 3] reported M[ZnA’ 3] 7.59 (M = Li, Na,4.00 K) and K(thf)[SnA’ 3] complexes [15,16]. The 2.567(3) beryllium center[15] is in a K[BeA’ ] 6.97 3.19 0.20 1.805(10) 3.153(7), 2.940(7) this work 3 nearly planar trigonal environment (sum of C–Be–C´ angles = 357.7°) (Figure 2). K[ZnA’3 ] 7.05 3.42 0.23 2.068(4) 3.205(3), 2.945(3) [15] The average Be–C Åa has 2.344(7) few direct3.201(7), points 3.164(8), of comparison other K(thf)[SnA’ 6.43 distance 4.42of 1.805(10) 0.42, 0.23 3.065(8) with[16] 3] − complex, the other being molecules, as 1 is only the second crystallographically characterized [BeR 3 ] a Two resonances are observed for the SiMe groups, as the A’ ligands are not fluxional; b Distance(s) affected by 3 lithium tri-tert-butylberyllate [21]. The latter’s Be center, like that in 1, is in a nearly perfectly planar crystallographic disorder. trigonal environment (sum of C–Be–C angles = 359.9°). In the solid state, however, tri-tertbutylberyllate is a dimer, [Li{Be(t-C4H9)3}]2, with some corresponding distortions in the Be–C bond 2.4. Solid State Structure lengths; Be–C distances range from 1.812(4) Å to 1.864(4) Å, averaging to 1.843(6) Å. The Be–C length The structure of 1 was determined from single crystal X-ray diffraction. In the solid state, 1 exhibits in 1 is indistinguishable from the Be–Ccarbene length of 1.807(4) Å in the [Ph2Be(IPr)] (IPr = 1,3-bis(2,6approximate C3 -symmetry, with σ-bound A’ ligands and a potassium cation engaging in cation–π di-isopropylphenyl)-imidazol-2-ylidene) complex, which also has a three-coordinate Be center [22]. interactions with the three double bonds of the allyls. It is isostructural with the previously reported The anionic methyl groups in [Ph2Be(IPr)] are at a noticeably shorter distance, however (1.751(6) Å, M[ZnA’ ] (M = Li, Na, K) and K(thf)[SnA’3 ] complexes [15,16]. The beryllium center is in a nearly ave.). A 3similar relationship between the Be–C carbene and Be–CH3 bond lengths exists in the related planar trigonal environment (sum of C–Be–C´ angles = 357.7◦ ) (Figure 2). [Me2Be(IPr)] [23] and [Me2Be(IMes)] (IMes = N,N’-bis(2,4,6-trimethylphenyl)imidazol-1-ylidene) The average Be–C distance of 1.805(10) Å has few direct points of comparison with other molecules, complexes [1]. A comparison of the Be–C length in 1 could also − be made with the Be–C distance of as 1 is only the second crystallographically characterized [BeR ] complex, the other being lithium 1.84 Å in lithium tetramethylberyllate, Li2[BeMe4], although the3bond distance would be expected to tri-tert-butylberyllate [21]. The latter’s Be center, like that in 1, is in a nearly perfectly planar trigonal be slightly longer in the latter owing to the higher coordination number of beryllium and the greater environment (sum of C–Be–C angles = 359.9◦ ). In the solid state, however, tri-tert-butylberyllate is a negative charge [24]. dimer, [Li{Be(t-C4 H9 )3 }]2 , with some corresponding distortions in the Be–C bond lengths; Be–C distances range from 1.812(4) Å to 1.864(4) Å, averaging to 1.843(6) Å. The Be–C length in 1 is indistinguishable from the Be–Ccarbene length of 1.807(4) Å in the [Ph2 Be(IPr)] (IPr = 1,3-bis(2,6di-isopropylphenyl)-imidazol-2-ylidene) complex, which also has a three-coordinate Be center [22]. The anionic methyl groups in [Ph2 Be(IPr)] are at a noticeably shorter distance, however (1.751(6) Å, ave.). A similar relationship between the Be–Ccarbene and Be–CH3 bond lengths exists in the related [Me2 Be(IPr)] [23] and [Me2 Be(IMes)] (IMes = N,N’-bis(2,4,6-trimethylphenyl)imidazol-1-ylidene) complexes [1]. A comparison of the Be–C length in 1 could also be made with the Be–C distance of

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1.84 Å in lithium tetramethylberyllate, Li2 [BeMe4 ], although the bond distance would be expected to be slightly longer in the latter owing to the higher coordination number of beryllium and the greater Inorganics 2017, 5, 36 5 of 11 negative charge [24].

Figure 2. Thermal ellipsoid plotofof1,1,illustrating illustrating the used in the text.text. Ellipsoids Figure 2. Thermal ellipsoid plot the numbering numberingscheme scheme used in the Ellipsoids are drawn at the 50%level, level,and andfor for clarity, clarity, hydrogen removed fromfrom the the are drawn at the 50% hydrogen atoms atomshave havebeen been removed trimethylsilyl groups. Selected bond distances (Å) and angles (deg): Be1–C1, 1.795(6); Be1–C10, 1.810(6); trimethylsilyl groups. Selected bond distances (Å) and angles (deg): Be1–C1, 1.795(6); Be1–C10, Be1–C19, 1.811(6); C(2)–C(3), 1.351(5); C(11)–C(12), 1.350(5); C(20)–C(21), 1.358(5); K(1)–C(2), 3.138(4); 1.810(6); Be1–C19, 1.811(6); C(2)–C(3), 1.351(5); C(11)–C(12), 1.350(5); C(20)–C(21), 1.358(5); K(1)–C(2), K(1)–C(3), 2.940(4); K(1)–C(11), 3.206(4); K(1)–C(12), 2.943(4); K(1)–C(20), 3.114(4); K(1)–C(21), 2.938(4); 3.138(4); K(1)–C(3), 2.940(4); K(1)–C(11), 3.206(4); K(1)–C(12), 2.943(4); K(1)–C(20), 3.114(4); K(1)– C(1)–Be(1)–C(10), 119.1(3); C(1)–Be(1)–C(19), 119.0(3); C(10)–Be(1)–C(19), 119.4(3). C(21), 2.938(4); C(1)–Be(1)–C(10), 119.1(3); C(1)–Be(1)–C(19), 119.0(3); C(10)–Be(1)–C(19), 119.4(3).

The C–C and C=C bonds in the alkyl groups in 1 are localized at 1.475(5) Å and 1.353(9) Å, The C–C and C=C bonds in the alkyl groups in 1 are localized at 1.475(5) Å and 1.353(9) Å, respectively. The K+ ···C(olefin) contacts average 3.153(7) Å and 2.940(7) Å to the carbon atoms β +… contacts average Å andatom, 2.940(7) Å to the carbon atoms β (C2, respectively. (C2, C11, The and K C20)C(olefin) and γ (C3, C12, and C21) to3.153(7) the beryllium respectively. These distances + C11, are and C20) and γ (C3, C12, and C21) to the beryllium atom, respectively. These distances comparable to, but slightly shorter than, the range of K ···C contacts found in the related zincate are structureto, (3.205(3) Å and shorter 2.945(3) than, Å, respectively), which the shorter bonds inzincate 1. comparable but slightly the range of K+…reflects C contacts foundM–C in the (α) related The distance between and K (3.59 Å) is long enough to rule out significant metal-metal interactions. structure (3.205(3) Å andBe2.945(3) Å, respectively), which reflects the shorter M–C(α) bonds in 1. The

distance between Be and K (3.59 Å) is long enough to rule out significant metal-metal interactions. 2.5. Computational Investigations

It has previously been suggested that the occurrence of C3 -symmetric M[M’A’3 L] (M’ = Zn, M = Li, 2.5. Computational Investigations Na, K; M’ = Sn, M = K; L = thf) complexes is the result of a templating effect of the associated alkali metal It has previously suggested the occurrence of C3-symmetric 3L] complexes (M’ = Zn, M = counterion [25]. Thebeen rationale for thisthat proposal is that the neutral MA’3 (M =M[M’A’ As, Sb, Bi) Li, Na, K; M’ = Sn, M = K; L = thf) complexes is the result of a templating effect of the associated alkali always occur in two diastereomeric forms, with R,R,R (equivalently, S,S,S) and R,R,S (or S,S,R) − metalarrangements counterion of [25]. rationale for this proposal is thatThe theanionic neutral MA’ (M = As, Sb, the The allyl ligands around the central element. [MA in Bi) 3 ´] 3 complexes, contrast, are always found in the C -symmetric R,R,R (or S,S,S) configuration, and it is not unreasonable complexes always occur in two diastereomeric forms, with R,R,R (equivalently, S,S,S) and R,R,S (or 3 to arrangements assume that theof counterion responsible for the S,S,R) the allyl isligands around thedifference. central element. The anionic [MA3´]− complexes, A DFT investigation was undertaken to explore possible this effect. The and geometry in contrast, are always found in the C3-symmetricthe R,R,R (or origins S,S,S) ofconfiguration, it is not − of the free [BeA’3 ] anion was optimized with calculations employing the dispersion-corrected APF-D unreasonable to assume that the counterion is responsible for the difference. functional [26]. Three confirmations were examined: the C3 -symmetric form (S,S,S) found in the X-ray A DFT investigation was undertaken to explore the possible origins of this effect. The geometry crystal structure of 1, a related S,S,S form with one A’ ligand rotated antiparallel to the other two of the free [BeA’3]− anion was optimized with calculations employing the dispersion-corrected APF(C1 symmetry), and a R,R,S form, also with one ligand antiparallel to the other two, derived from the D functional confirmations examined: structure [26]. of theThree neutral AlA’3 complexwere (Figure 3) [27]. the C3-symmetric form (S,S,S) found in the X-

ray crystal structure of 1, a related S,S,S form with one A’ ligand rotated antiparallel to the other two (C1 symmetry), and a R,R,S form, also with one ligand antiparallel to the other two, derived from the structure of the neutral AlA’3 complex (Figure 3) [27].

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Figure 3. Geometry optimized structures of [BeA’3]− anions: (a) as found in the crystal structure of 1; (b) related S,S,S form with one A’ ligand rotated (C1 symmetry); and (c) R,R,S form derived from the − anions: (a) as found in the crystal structure of 1; Figure 3. Figure 3. Geometry Geometry optimized structures structures of of [BeA’ [BeA’33]]− anions: (a) as found in the crystal structure of 1; structure of AlA’3. optimized (b) (b) related related S,S,S S,S,S form form with with one one A’ A’ ligand ligand rotated rotated (C (C11 symmetry); symmetry); and and (c) (c) R,R,S R,R,S form form derived derived from from the the structure of Not surprisingly, structure of AlA’ AlA’33.. the calculated structures possess similar average Be–C bond lengths, ranging

from 1.782 Å (the C3-symmetric form (Figure 3a)) to 1.788 Å (for the rotated S,S,S form (Figure 3b)). the calculated structures possess Not surprisingly, calculated structures possess similar Be–C bond−1 lengths, Energetically, the R,R,Sthe form is the most stable; the rotated S,S,S average form is 10.1 kJ·mol higher inranging energy −1 in form 3a)) to Å the rotated form (Figure 3b)). from (the C33-symmetric -symmetric form (Figurestill 3a))(20.3 to 1.788 1.788 Å (for (for the rotated S,S,S form (Figure 3b)). (∆G°),1.782 and Å the C3-symmetric form is (Figure higher kJ·mol ∆G°). TheS,S,S origin of these energy −1 − 1 Energetically, the R,R,S form is the most stable; the rotated S,S,S form is 10.1 kJ·mol higher in energy Energetically, form is the most stable; rotated S,S,S to form 10.1 kJ·amounts mol higher differences is the notR,R,S immediately obvious, but it the may be related theisrelative of interligand −11 in ∆G°). ◦ ), and the C − ◦ ).… (∆G°), C 3-symmetric form is higher higher still (20.3 kJ·mol The origin of (∆G still (20.3 kJ · mol in ∆G The origin of energy 3 congestion present. The low energy R,R,S form, for example, has no Me Me´ contactsthese less than 4.0 but relative amounts interligand differences is not immediately obvious, it may be related to the relative amounts of interligand Å, the sum of the van der Waals radii [18]. In contrast, the C3 symmetric form has multiple contacts congestion present. The energy form, for Me ···… Me´ contacts less than 4.04.0 Å, congestion present. Thelow low energy R,R,S form, forexample, example, hasnono Me Me´ less between methyl groups of less thanR,R,S 4.0 Å, including two ashas short as 3.76 Å. Atcontacts this level ofthan theory, the sum of the van der Waals radii [18]. In contrast, the C symmetric form has multiple contacts Å, the sum of the van der Waals radii [18]. In contrast, the C 3 symmetric form has multiple the energetics of the free anions do not provide a rationale3 for the exclusive formation of the S,S,S between asas short as as 3.76 Å. Å. At At thisthis level of theory, the methyl groups groupsof ofless lessthan than4.0 4.0Å,Å,including includingtwo two short 3.76 level of theory, form. methyl energetics of the do not a +rationale for exclusive formation of the S,S,S the energetics of free the anions free anions doprovide not provide rationale for the alters exclusive formation of theform. S,S,S Not surprisingly, incorporation of the K ionainto the the complex the relative stability of the + ion into the complex alters the relative stability of the Not surprisingly, incorporation of the K form. species. The optimized geometries of the C3-symmetric K[BeA’3] found in the X-ray crystal structure species. optimized geometries of calculated the C3 -symmetric in anions, the crystal structure of NotThe surprisingly, incorporation of the K+ ion intoK[BeA’ thetocomplex alters theX-ray relative stability of the 3 ] found of 1 and a related S,S,R form were similarly the isolated and are depicted in 1species. and a related S,S,R form were calculated similarly to the isolated anions, and are depicted in Figure 4. The optimized geometries of the C 3 -symmetric K[BeA’ 3 ] found in the X-ray crystal structure Figure 4. of 1 and a related S,S,R form were calculated similarly to the isolated anions, and are depicted in Figure 4.

Figure 4. 4. Geometry structures of of the the K[BeA’ K[BeA’3]] complex: complex: (a) (a) as Figure Geometry optimized optimized structures as found found in in the the crystal crystal structure structure 3 of 1; and (b) related S,S,R form. of 1; and (b) related S,S,R form. Figure 4. Geometry optimized structures of the K[BeA’3] complex: (a) as found in the crystal structure The C3-symmetric formform. is 6.1 kJ·mol−−11more stable than the S,S,R form. This is not a consequence of 1; and (b) related S,S,R

The C3 -symmetric form is 6.1 kJ·mol more stable than the S,S,R form. This is not a consequence of closer K++…(C=C) distances, which are nearly the same (avg. 3.91 Å in the C3 form; 2.86 Å in the of closer K ···(C=C) distances, which are−1nearly the same (avg. 3.91 form. Å in the C3isform; 2.86 Å in the C3-symmetric is 6.1 kJ·mol more stable than the S,S,R not a lead consequence S,S,RThe arrangement). Theform asymmetric arrangement of the ligands in the S,S,RThis form does to closer S,S,R arrangement). The asymmetric arrangement of the ligands in the S,S,R form does lead to closer +…(C=C) distances, which are nearly the same (avg. 3.91 Å in the C3 form; 2.86 Å in the of closer K … interligand C C contacts in the allyl frameworks, however, as small as 3.37 Å, whereas there are no interligand C···C contacts in the allyl frameworks, however, as small as 3.37 Å, whereas there are S,S,R arrangement). The asymmetric arrangement of the ligands in the S,S,R form does lead to closer similar contacts less than 3.78 Å in the C 3 form. The somewhat greater stability of the C3 form, possibly no similar contacts less than 3.78 Å in the C3 form. The somewhat greater stability of the C3 form, interligand C… C contacts in crystal the allyl frameworks, however, to as the small as 3.37 appearance Å, whereas there no coupled with greater of packing, contribute exclusive of thatare form possibly coupled withease greater ease of crystal may packing, may contribute to the exclusive appearance of similar contacts less than 3.78 Å in the C3 form. The somewhat greater stability of the C3 form, possibly coupled with greater ease of crystal packing, may contribute to the exclusive appearance of that form

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thatthe form in thestructure. crystal structure. It isthat likely that a similar analysis forisostructural the isostructural Zn and in crystal It is likely a similar analysis holdsholds for the Zn and Sn Sn complexes. complexes. The failure to produce an unsolvated BeA’22 in the absence of a coordinating solvent (i.e., either mechanochemically or or in hexanes) was also examined computationally computationally with with the the aid of the Solid-G mechanochemically · program [28]. Both BeA’ Et O and 1 are found to have coordination sphere coverage program [28]. Both BeA’22·Et22 found to have coordination sphere coverage(G (G complex ) above complex 90% (i.e., 97.0% (Figure 5a) and 92.6% (Figure 5b), respectively). Although the coverage of the metal center in the hypothetical BeA’22 varies somewhat with the angle between the ligands, the minimum energy position depicted in Figure 5c (C22 symmetry) has only 78.7% coverage. It is not unreasonable bebe too coordinately unsaturated to be isolable, and and will to assume assume that thataamonomeric monomericBeA’ BeA’ 2 may too coordinately unsaturated to readily be readily isolable, 2 may bindbind an ethereal solvent molecule during synthesis, or, if that not available, an additional A’ ligand, will an ethereal solvent molecule during synthesis, or, ifisthat is not available, an additional A’ + + counterbalanced with a Kwith ion. ligand, counterbalanced a K ion.

(a)

(b)

(c)

Figure 5. Visualization Visualization of of the theextent extentof ofcoordination coordinationsphere spherecoverage coverage(G(Gcomplex)) of: of: (a) (a) BeA’ BeA’2··Et 2O (the Figure 5. 2 Et2 O (the complex coverage from the two allyls are assigned blue and green; that from the ether is in red); (b) 1 (all coverage from the two allyls are assigned blue and green; that from the ether is in red); (b) 1 (all three three allyls allyls are are in in blue); blue); and and (c) (c) BeA’ BeA’22,, using using optimized optimized coordinates coordinates (APF-D/6-311G(2d) (APF-D/6-311G(2d) (Be); (Be); 6-31G(d) 6-31G(d) (other (other atoms)) and the the program program Solid-G Solid-G[28]. [28]. The TheG Gcomplex value takes into account the net coverage; regions atoms)) and complex value takes into account the net coverage; regions of the coordination sphere where the projections of the ligands overlap are are counted counted only only once. once. of the coordination sphere where the projections of the ligands overlap

3. 3. Materials Materials and and Methods Methods General Considerations: Considerations: All General All syntheses syntheses were were conducted conducted under under rigorous rigorous exclusion exclusion of of air air and and moisture using Schlenk line and glovebox techniques. (NOTE: Beryllium salts are toxic and should moisture using Schlenk line and glovebox techniques. (NOTE: Beryllium salts are toxic and should be be handled appropriate protective equipment.) grinding was completed, jars were handled withwith appropriate protective equipment.) After After grinding was completed, the jarsthe were opened opened according to glovebox procedures to protect the compounds and to prevent exposure to according to glovebox procedures to protect the compounds and to prevent exposure to dust dust [10]. 1H) and carbon (13C) spectra were obtained on Bruker DRX-500 or DRX-400 [10]. Proton ( 1 13 Proton ( H) and carbon ( C) spectra were obtained on Bruker DRX-500 or DRX-400 spectrometers spectrometers (Karlsruhe, Germany), and to were referenced to residual of9C6D6. Beryllium (Karlsruhe, Germany), and were referenced residual resonances of C6 Dresonances 6 . Beryllium ( Be) spectra were 9Be) spectra were obtained on a Bruker DRX-500 at 70.2 MHz, and were referenced to BeSO4(aq). (obtained on a Bruker DRX-500 at 70.2 MHz, and were referenced to BeSO4 (aq). Combustion analysis Combustion analysis was performed by Tucson, ALS Environmental, Tucson, chloride AZ, USA. Beryllium chloride was performed by ALS Environmental, AZ, USA. Beryllium was purchased from was purchased from Strem, stored under an N 2 atmosphere and used as received. The K[A’] (A’ = Strem, stored under an N2 atmosphere and used as received. The K[A’] (A’ = 1,3-(SiMe3 )2 C3 H3 ) 1,3-(SiMe 3)2C3H3) reagent was synthesized as previously described [29,30]. Toluene, hexanes, and reagent was synthesized as previously described [29,30]. Toluene, hexanes, and diethyl ether were diethyl ether distilled nitrogen from potassium ketyl [31]. (C Deuterated distilled underwere nitrogen fromunder potassium benzophenone ketyl benzophenone [31]. Deuterated benzene 6 D6 ) was benzene (C 6D6) was distilled from Na/K (22/78) alloy prior to use. Stainless steel (440 grade) ball distilled from Na/K (22/78) alloy prior to use. Stainless steel (440 grade) ball bearings (6 mm,) were bearings (6 mm,) were thoroughly cleaned withprior hexanes andPlanetary acetone prior to use. milling thoroughly cleaned with hexanes and acetone to use. milling wasPlanetary performed with was performed with a Retsch model PM100 mill (Haan, Germany), 50 mL stainless steel grinding jar a Retsch model PM100 mill (Haan, Germany), 50 mL stainless steel grinding jar type C, and safety type C, and safety clamp for air-sensitive grinding. clamp for air-sensitive grinding. 3.1. 3.1. Mechanochemical Mechanochemical Synthesis Synthesis of of K[BeA’ K[BeA’33]] (1) (1) Solid BeCl BeCl22 (56.7 Solid (56.7 mg, mg, 0.71 0.71 mmol) mmol) and and K[A’] K[A’] (319 (319 mg, mg, 1.42 1.42 mmol) mmol) were were added added to to aa 50 50 mL mL stainless stainless steel grinding jar (type C). The jar was charged with stainless steel ball bearings (6 mm dia, 50 steel grinding jar (type C). The jar was charged with stainless steel ball bearings (6 mm dia, 50 count) count) and closed tightly tightly with with the the appropriate appropriate safety safety closer closer device device under under an an N N22 atmosphere. atmosphere. The reagents and closed The reagents were milled for 15 min at 600 rpm, resulting in a light orange solid. The product was extracted under an inert atmosphere with minimal hexanes (< 100 mL) and filtered through a medium porosity

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were milled for 15 min at 600 rpm, resulting in a light orange solid. The product was extracted under an inert atmosphere with minimal hexanes (