Cationic Magnesium π–Arene Complexes - Organometallics (ACS

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Cationic Magnesium π−Arene Complexes Jürgen Pahl, Alexander Friedrich, Holger Elsen, and Sjoerd Harder* Inorganic and Organometallic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany

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S Supporting Information *

ABSTRACT: Reaction of the trityl cation in [Ph3C+][B(C6F5)4−] with the n-butyl anion in (BDI)MgnBu led to ßhydride abstraction and formation of Ph3CH, 1-butene, and [(BDI)Mg+][B(C6F5)4−] (1) (BDI = CH[C(CH3)N-Dipp]2; Dipp = 2,6-diisopropylphenyl). The “naked” Mg center in 1 is weakly bound to B(C6F5)4− through two Mg···F interactions. Addition of arenes to 1 gave strongly bound cationic magnesium π−arene complexes (BDI)Mg+·arene in good yields arene = benzene (94%), toluene (74%), m-xylene (82%), and mesitylene (63%). 1,2,4,5-Tetramethylbenzene is too bulky to give a coordination complex. Crystal structures of these π-arene complexes show η3−arene−Mg interactions for the smaller arenes (benzene, toluene and m-xylene). In each case, the coordination sphere was filled by an additional Mg···F interaction. For mesitylene, η6-coordination was found, leaving no space at the metal for supplementary Mg···F interaction. Dissolved in C6D5Br, all arene complexes are in association−dissociation equilibrium: (BDI)Mg+ + arene ⇄ (BDI)Mg+·arene. For the most strongly bound mesitylene ligand, a decoalescence of the 1H NMR resonances was reached at −20 °C, each species giving separate signals. From the temperature dependency of this equilibrium, the following thermodynamic parameters have been deduced: ΔH0 = −6.9 kcal·mol−1 and ΔS0 = −28.2 cal·mol−1· K−1. DFT calculations reveal that the Mg···arene bonding is mainly electrostatic of nature with only little charge transfer from arene to Mg2+ (ca. 0.05 e) and a slight polarization of π-electron density toward the metal. Substitution of benzene in (BDI)Mg+·C6H6 for mesitylene is exothermic by −11.7 kcal/mol; however, including the B(C6F5)4− counterion in the calculation gave an energy gain of −2.2 kcal/mol. This clearly demonstrates that weakly coordinating anions can affect these Mg···arene interactions substantially.

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INTRODUCTION The isolation of bis(benzene)chromium by Fischer and Hafner in 1955 started a thriving era of transition metal π-arene chemistry that also in modern chemistry finds many applications.1,2 The mainly covalent bonding in such sandwich complexes relies on symmetry matching of the metal’s dorbitals with those on the arene ligands. In sharp contrast, sblock metal π-arene complexes are much less researched. This is inherent to their considerably weaker, mainly electrostatic, metal−arene bond which makes such fleeting complexes highly dynamic and more challenging to study. However, being responsible for ion transport in K+ channels or for the functioning of many biological systems, the importance of such cation π-interactions is indisputable.3,4 The alkali metal cation···benzene bond energy decreases with increasing ion size (Li+ > Na+ > K+) suggesting that such interactions may be more relevant for the smaller cations. However, since the metal cation···water bond energies decrease even more rapidly along this row, it was realized that the larger (softer) metal cations in fact favor the more extended π-ligands over hard ligands like water.5 A Cambridge Crystallographic Database search on s-block metal benzene interactions gave the following statistics Li(5), Na(4), K(21), © XXXX American Chemical Society

Rb(1), Cs(3), Be(0), Mg(0), Ca(0), Sr(0), Ba(3), clearly demonstrating favorable C6H6 interactions with the softer larger metals.6 These simple statistics are clearly biased by the less intensive research on the more exotic heavier elements Rb, Cs, Sr, and Ba, but the crystal structure of a monomeric alkylcesium complex (I in Figure 1), in which the largest (nonradioactive) metal Cs is solvated by three η6-coordinated C 6 H 6 ligands, 7 demonstrates the importance of such interactions for the larger s-block metals (a similar compound is known for K).8 The preference of large cations for π-ligating ligands is underscored by the fact that for solvation of Ba2+ C6H6 easily competes with highly polar donors like THF, as demonstrated by crystallization of the Ba···benzene complexes in the presence of THF (e.g., II in Figure 1).9,10 Our longstanding interest in such weak noncovalent cation π-interactions is documented by investigations on s-block metal···alkene interactions,11 for which even less precedence exists.12 Thorough understanding of the bonding character and strength of such interactions is crucial to the rapidly developing area of early main group metal catalysis.13 These even weaker Received: July 13, 2018

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DOI: 10.1021/acs.organomet.8b00489 Organometallics XXXX, XXX, XXX−XXX

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unsupported Mg···alkyne coordination in (BDI)Mg+·EtC CEt (III in Figure 1) that crystallized as its B(C6F5)4− salt (BDI = CH[C(CH3)N-Dipp]2; Dipp = 2,6-diisopropylphenyl).15 The same report presented the first metal···benzene bonds for the lighter metals Mg2+ and Ca2+ in the crystal structures of (BDI)M+·C6H6 (M = Mg2+ and Ca2+). Especially the π-ligation of benzene to Mg2+ was found to be surprisingly strong, and also evidence for such interactions in solution have been presented. The recently observed nucleophilic addition of a Ca-Et functionality to benzene, giving a Ca−H species and ethylbenzene, is proposed to profit from such an activating Ca2+···benzene interaction, giving credit to the importance of the Lewis acidic component in group 2 metal chemistry. Herein we extend our investigations on Mg−arene interactions. We describe in detail their challenging syntheses and purification, the many pitfalls and possible side reactions, the structures of a range of Mg arene complexes with increasing ligand bulk, and theoretical investigations on the nature of these interactions.

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RESULTS AND DISCUSSION Syntheses. Although there are various examples of cationic alkaline earth metal (Ae) complexes, the highly Lewis acidic Ae metal is always stabilized by external Lewis bases (THF, pyridine) or highly chelating pendant donor arms.16 We recently described the first donor-free cationic Ae complexes.15

Figure 1. Formulas I−III.

metal···alkene interactions can be a requirement for CC double bond activation and subsequent functionalization.14 Very recently, we described synthesis and structure of the first

Scheme 1. General Synthesis of (BDI)Mg+ Complexes and an Overview of Observed Side/Decomposition Reactionsa

a

Crystal structures of 2−5 are shown in Figure 2, and crystal structures of complexes 6−9 can be found in the Supporting Information. B

DOI: 10.1021/acs.organomet.8b00489 Organometallics XXXX, XXX, XXX−XXX

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Organometallics The complex [(BDI)Mg+][B(C6F5)4−] (1) was prepared by reaction of [Ph3C+][B(C6F5)4−] with (BDI)MgR (R = nPr or nBu) in a chlorobenzene solution (see Scheme 1 for an overview). This reaction proceeds via a β-hydride abstraction, and besides the respective terminal alkene H2CCH−R′ (R′ = Me or Et) and Ph3CH, the solvent-free ion-pair [(BDI)Mg+][B(C6F5)4−] (1) is formed in quantitative conversion. The abstraction of the β-hydrogen of n-alkyl ligands bound to Lewis acidic metals (e.g., AlEt3) by addition of [Ph3C+][B(C6F5)4−] is well-known.17 Methyl substituents (e.g., in (BDI)Al(Me)218 or (BDI)Sc(Me)219) are abstracted directly, and H3CCPh3 is formed. In order to isolate the cationic Mg products in good yield, in situ formation of the magnesium alkyl precursor (BDI)MgR (R = nPr or nBu) and direct reaction with [Ph3C+][B(C6F5)4−] should be avoided due to the formation of unwanted side products. Therefore, all precursor complexes were isolated and purified by crystallization prior to use. The conversion of [(BDI)Mg(nBu)]2 into its cation by addition of [Ph3C+][B(C6F5)4−] proceeds very fast and is finished in less than 1 min as observed by a color change from the initially strongly yellow/orange trityl cation solution to almost colorless. Alternatively, it is also possible to synthesize the solvent-free ion-pair [(BDI)Mg+][B(C6F5)4−] (1) in a salt metathesis reaction by suspending [(BDI)MgI]2 and [Ag+][B(C6F5)4−] in bromobenzene. Immediate precipitation of AgI in form of a yellow solid led to complete conversion to give the cationic complex within approximately 5 min. The salt metathesis route by reacting equimolar amounts of [(BDI)MgBr]2 and [Li+][B(C6F5)4−], however, did not give any reaction. Even heating this suspension in a chlorobenzene/ benzene mixture to 80 °C for 18 h failed to give the desired product. An elegant synthetic pathway, which is similar to the method of Liu et al. for synthesis of [(BDI)Sr+·(pyridine)3][H2N{B(C6F5)3}2−],16c led us previously to the Ca derivative [(BDI)Ca+·(C6H6)][B(C6F5)4−].15 This was achieved by directly reacting [BDI-H2+][B(C6F5)4−], obtained by reaction of Jutzi’s acid20 [H+(Et2O)2][B(C6F5)4−] with BDI-H,21 with the strong base dibenzylcalcium Ca(p-tBu-benzyl)2.22 The same synthetic procedure, however, did not work for the preparation of cationic Mg complexes from commercially available diorganomagnesium precursors. Addition of either Mg(nBu)2 or MgPh2 to a chlorobenzene/benzene solution of [BDI-H2+][B(C6F5)4−] gave in both cases a mixture of various unidentifiable products. As discussed previously, the purification of [(BDI)Mg+][B(C6F5)4−] (1) by crystallization was found to be difficult because of the tendency of these ion pairs to form clathrates, and therefore, crystalline 1 could only be obtained in 12% yield. Addition of a weakly coordinating aromatic ligand like benzene, however, led to a much better yield (94%) of crystalline [(BDI)Mg+·C6H6][B(C6F5)4−] (2). Increasing the bulk of the arene ligand using toluene, m-xylene and mesitylene gave the respective complexes in good yield: 3 (74%), 4 (82%), and 5 (63%). All attempts to isolate a durene (1,2,4,5tetramethylbenzene) coordinated (BDI)Mg+ complex were not successful and led to [(BDI)Mg+][B(C6F5)4−] instead. Therefore, the synthesis of (BDI)Mg+·arene complexes is currently limited to mesitylene. Some side and/or decomposition products that occurred during optimization of the synthetic procedures for 1 and 2 in Scheme 1 could be characterized by X-ray diffraction. Although we have been able to trap and identify these decomposition products (6−9), we have not been able to develop reliable

synthetic pathways to isolate these compounds in good yields, neither starting with 1 or with 2 for which decomposition essentially gives the same products. Therefore, the characterization of these compounds is limited to a crystal structure determination. The identity of these species, however, nicely demonstrates the various decomposition routes. Lessons learned from their identification are valuable for handling and treating these highly reactive cationic complexes. Thus, we were able to identify the first step of the undesired hydrolysis of 1 which gave an insoluble dicationic species of the composition [(BDI-H)Mg(OH)+]2·[B(C6F5)4−]2 (6) (Scheme 1). This complex could also be obtained by stoichiometric reaction of 2 with H2O; however, it could only be isolated in minimal crystalline yields. At a first glance it is surprising that protonation of 2 by H2O proceeds at the nucleophilic backbone C instead of at the Mg−N bond. The significant basicity of the backbone C in cationic (BDI)Ae+ complexes was previously shown by serendipitous oxidation of the BDI ligand in a cationic Ca complex.16c The undesired oxidation of 2 by traces of O2 to give the Mg complex 7 is directly related to this decomposition route. The latter Mg complex is very similar to its analogue Ca complex which crystallized with one additional THF ligand per metal.16c It is likely formed via insertion of O2 between the backbone C and Mg to give a C-OO-metal peroxo intermediate, a compound class for which precedence exists.23 Subsequently, this reacts further with another equivalent of (BDI)Mg+ to give the observed C−O−Mg species 7. The reactivity with O2 and H2O already highlights the very high sensitivity of these cationic (BDI)Ae+ complexes. It is crucial to consider not only rigorous drying and degassing of solvents but also the choice of solvent. The solvent should be polar, unreactive, and weakly- or noncoordinating. Whereas the highly polar but weakly coordinating solvent dichloromethane is often an excellent solvent choice, cationic (BDI)Ae+ complexes are much too reactive, and contact with CH2Cl2 led to a variety of decomposition products. From solutions of 2 in a mixture of chlorobenzene and dichloromethane, it was possible to isolate a minute amount of colorless crystals of compound 8. Complex 9 was isolated similarly from bromobenzene/dichloromethane solutions. Again, minimal yields only allow for single crystal structure determinations. These products, however, are useful snapshots that illustrate the versatile reactivity of (BDI)Mg+ with CH2Cl2. All reactivity is centered on the BDI backbone C atom which either functions as a Brønsted base that deprotonates CH2Cl2 or acts as a nucleophile for SN2 attack. The first reaction explains the protonated backbone C, whereas the second reaction is in agreement with functionalization of the BDI ligand with a CH2Cl-substituent. Both reactions lead to generation of the chloride anions Cl− that bridge the Mg cations in 8 and 9. Based on these observations any contact with CH2Cl2 has to be avoided, and our solvent choice is restricted to the polar but weakly coordinating solvents C6H5Cl and C6H5Br. In contrast to cationic BDI calcium and strontium complexes,16c our magnesium complexes are also stable in THF solution for prolonged times. However, in order to isolate the (BDI)Mg+· arene complexes, THF should be avoided at all times. Reactions of 1 or 2 with H2O, O2, or CH2Cl2 may alternatively be described as “Frustrated Lewis Pair” reactivity in which the reagents react with (BDI)Mg+ by bridging the Lewis-basic nonmetal bound backbone C and a Lewis-acidic Mg center. C

DOI: 10.1021/acs.organomet.8b00489 Organometallics XXXX, XXX, XXX−XXX

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Figure 2. Solid-state structures of (a) [(BDI)Mg+·C6H6][B(C6F5)4−], (b) [(BDI)Mg+·toluene] [B(C6F5)4−], (c) [(BDI)Mg+·m-xylene][B(C6F5)4−], and (d) [(BDI)Mg+·mesitylene][B(C6F5)4−]. Bond lengths and angles have been summarized in Table 1.

Table 1. Overview of Distances (Å) and Angles (deg) of the Solid-State Structures of the Naked Cation [(BDI)Mg+][B(C6F5)4−] and Its Respective Arene Adducts complex

1

2

3

4

5

arene ligand

none

benzene

toluene

m-xylene

mesitylene

Mg···C30 Mg···C31 Mg···C32 Mg···C33 Mg···C34 Mg···C35 Mg···arene/plane C−N−C

118.62(15) 120.93(14) 2.029(1) 2.056(1) 1.9787(16) 1.9928(15)

2.3673(17) 2.6858(18) 2.8101(19) 2.3537(13) 120.18(12) 120.10(12) 2.0463(9) 1.9852(13) 1.9943(13)

2.4602(19) 2.5552(18) 2.8150(19) 2.3842(12) 120.45(13) 119.41(13) 2.0952(10) 1.9975(14) 2.0008(14)

2.403(4) 2.652(4) 2.663(4) 2.357(3) 121.4(3) 119.6(3) 2.159(2) 1.996(3) 1.997(3)

2.5790(15) 2.6321(16) 2.6988(16) 2.6257(17) 2.6058(18) 2.5325(17) 2.2017(8) 116.42(12) 117.28(13) 2.0092(13) 2.0118(14)

Mg···F1 Mg···F6 Mg−N

The key step to the isolation of ether and amine-free cationic (BDI)Mg+ complexes in good yields was not so much improving the reaction conditions but the addition of nonpolar but potentially π-ligating cosolvents such as arenes or an alkyne (EtCCEt). As shown very recently, the highly reactive (BDI)Mg+ species can also be trapped by coordination of the otherwise inert silyl ether Me3SiOSiMe3.24 The silyl ether ligand in the latter is coordinated so weakly that this complex,

which can be obtained in a relatively high yield of 74%, also can be used as a precursor for the synthesis of the ligand-free (BDI)Mg+ complex. This substantially improved the yield for [(BDI)Mg+][B(C6F5)4−] from 12% to 72%. Since the crystallization/isolation methods for all of the cationic complexes are unique, each synthesis had to be painstakingly optimized. Given the challenge to isolate and purify such cationic complexes in reproducible high yields, we like to D

DOI: 10.1021/acs.organomet.8b00489 Organometallics XXXX, XXX, XXX−XXX

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noncoordinating anion. This allows for a symmetric η6coordination of the mesitylene ligand (Mg−C = 2.5325(17) − 2.6988(16) Å). Although all individual Mg−C bond distances are relatively long, the distance between Mg and the arene plane is with 2.2017(8) Å significantly shorter than in the other structures. The symmetric η6-coordination of the bulky mesitylene ligand clearly shows that (BDI)Mg+ has ample free space for ligand coordination. The strength of the mesitylene···Mg coordination is demonstrated by the fact that the Dipp substituents are pushed backward as can be seen from the C−N−C angles in the BDI ligand that in comparison to those in the other structures are squeezed significantly (Table 1). This steric pressure is underscored by the slightly but significantly elongated Mg−N bond distances (Table 1). At the same time the mesitylene Me groups bend away from the metal: the centroid−C−CH3 angles vary from 174.8° to 176.4° (average 175.5°) which means an out-of-plane bending of nearly 5°. Increasing the number of Me substituents on the arene clearly has two effects. It increases the donor strength of the arene ligand, as demonstrated by a shorter Mg···arene contact, but it also leads to steric pressure, which hinders Mg··· arene bonding. The latter factor seems to be dominating for durene in which case Mg···arene coordination was not established. Structures in Solution. Previous NMR investigations on crystals of [(BDI)Mg+·C6H6][B(C6F5)4−] in C6D5Br solution showed a slight highfield shift of the 1H NMR resonance for the coordinated C6H6 ligand.15 Our earlier studies also revealed that the benzene complex is in equilibrium with the naked cationic Mg species and free benzene. The 1H NMR of [(BDI)Mg+·toluene][B(C6F5)4−] in the noncoordinating solvent C6D5Br gave a broad signal at 2.02 ppm for the toluene CH3 group which is highfield shifted in respect of the signal for free toluene in this solvent (2.18 ppm). The 1H NMR resonance for the m-xylene CH3 groups in [(BDI)Mg+· m-xylene][B(C6F5)4−] is similarly shifted to highfield and is observed as a broad signal at 2.04 ppm (free m-xylene: 2.16 ppm). Upon dilution of [(BDI)Mg+·m-xylene][B(C6F5)4−] in C6D5Br, the 1H−NMR signal at 2.04 ppm shifts gradually to 2.16 ppm indicating a m-xylene dissociation-association equilibrium (Figure S9). Such an equilibrium can be observed especially well in the 1H NMR spectrum of [(BDI)Mg+· mesitylene][B(C6F5)4−] at room temperature. The signals for the BDI and mesitylene ligands are broadened significantly (Figure S10). Upon cooling to −20 °C, however, two sets of signals can be observed for both, the BDI and mesitylene ligands (also in the 13C NMR spectrum). Exchange between bound and free mesitylene has been confirmed by a 1H EXSY NMR spectrum (Figure S18). The 1H NMR resonances of one of the two species present are almost identical to those for [(BDI)Mg+][B(C6F5)4−], whereas the mesitylene CH3 signals for the other [(BDI)Mg+·mesitylene][B(C6F5)4−] species are strongly shifted upfield to 1.44 ppm in contrast to 2.17 ppm for free mesitylene. The highfield shift for the coordinated mesitylene ligand is in agreement to that observed for mesitylene coordinated to a highly Lewis acidic cationic Sc complex: the mesitylene ligand in [(BDI)ScMe+·mesitylene][B(C6F5)4−] shows a similar η6-coordination and a 1H NMR chemical shift of 1.36 ppm for the mesitylene CH3 groups.26 Upon heating the solution of the Mg-mesitylene complex to 50 °C, coalescence into one set of signals is observed giving further support for the equilibrium: (BDI)Mg+ + mesitylene ⇄ (BDI)Mg+·mesitylene. From the temperature dependency of

highlight a few methods that worked especially well and discuss these in detail. The complex [(BDI)Mg+][B(C6F5)4−] (1) was best crystallized by adding an unpolar solvent (pentane/hexane) directly to the oily viscous residue that is obtained after removing all volatiles from the reaction mixture. This method has the disadvantage that the crystals are covered in a similar oily substance and further purification by repeated washing procedures generally resulted in considerably lower yields. The benzene-stabilized complex [(BDI)Mg+·C6H6][B(C6F5)4−] (2) was isolated by a procedure similar to the synthesis of [(BDI)Al(Me)+][B(C6F5)4−] by Radzewich et al. where the clathrate/oil is washed repeatedly with pentane/ hexane until a powder is obtained.25 This procedure works also for the toluene coordinated complex [(BDI)Mg+·toluene][B(C6F5)4−] (3), yet we could find another, more promising procedure. It is possible to directly crystallize the complex from a toluene/hexane clathrate by carefully initiating crystallization using a spatula to scratch the glass wall. Also the adducts with larger arenes (m-xylene and mesitylene) could be directly crystallized from a clathrate of pure m-xylene (or mesitylene) leading to [(BDI)Mg+·m-xylene][B(C6F5)4−] (4) in 82% yield or [(BDI)Mg+·mesitylene][B(C6F5)4−] (5) in a yield of 63%. For the larger arenes this works very well and in some cases almost instant crystallization was observed. Although applicable to all arene coordinated (BDI)Mg+ complexes, the smaller arenes benzene and toluene generally gave lower yields [(BDI)Mg + ·C 6 H 6 ][B(C 6 F5 ) 4 −] (86%) and [(BDI)Mg + · toluene][B(C6F5)4−] (65%). Crystal Structures. The size of the open coordination sphere around the Mg center in (BDI)Mg+ can be quantified by comparison of the [(BDI)Mg+·arene][B(C6F5)4−] structures with arenes of increasing bulk: benzene < toluene < mxylene < mesitylene (Figure 2, Table 1). The solid-state structures of [(BDI)Mg+·toluene][B(C6F5)4−] and [(BDI)Mg+·m-xylene][B(C6F5)4−] are structurally similar to that of [(BDI)Mg+·C6H6][B(C6F5)4−]. All three show the same strong η3-arene···Mg bonding and one additional Mg···F contact to the weakly coordinating [B(C6F5)4−] anion. The shortest Mg···C distances are with 2.4602(19) Å in the toluene complex and 2.403(4) Å in the m-xylene complex slightly longer than the very short Mg···C contact of 2.3673(17) Å in [(BDI)Mg+·C6H6][B(C6F5)4−]. However, the distances between Mg and the plane of the aromatic ring are in all three structures almost identical (Table 1). The most striking difference between the crystal structures with ligating benzene, toluene, and m-xylene ligands is the Mg···F distance which steadily increases from 2.0463(9) Å (benzene) to 2.0952(10) Å (toluene) and 2.159(2) Å (m-xylene). This effect is not only due to the increased sterics of the arene ligands but there is likely also an electronic contribution. Although the Mg···arene bond should be regarded as a mainly electrostatic interaction, DFT calculations on (BDI)Mg+·benzene show a small arene → Mg charge transfer of 0.051 e.15 The increasing number of Me substituents at the ligating arene also increase its donor capability, thus reducing the Lewis acidity of the (BDI)Mg+· arene fragment along the row benzene > toluene > m-xylene > mesitylene. Although the anion [B(C6F5)4−] cannot simply be neglected, the large influence of arene bulk and electronics on the Mg···F distances demonstrates that the Mg···anion interaction is clearly very weak. This trend is continued in the structure of [(BDI)Mg+· mesitylene][B(C6F5)4−] in which the [B(C6F5)4−] is a truly E

DOI: 10.1021/acs.organomet.8b00489 Organometallics XXXX, XXX, XXX−XXX

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the experimentally determined values of ΔH0 = −6.9 kcal· mol−1 and ΔS0 = −28.2 cal·mol−1·K−1, except for ΔS which is generally overestimated in calculations.27 To obtain further insight into the role of the weakly coordinating anion B(C6F5)4− we studied the exchange of the benzene ligand in [(BDI)Mg+·C6H6][B(C6F5)4−] (where B(C6F5)4− is coordinated) with mesitylene forming [(BDI)Mg+·mesitylene][B(C6F5)4−] (where B(C6F5)4− is not coordinated). Not considering the anion, benzene-mesitylene exchange was found to be exothermic by −11.7 kcal·mol−1 (vide supra). Including the anion gave a much smaller preference for mesitylene coordination: ΔE = −2.2 kcal· mol−1. Benzene···Mg coordination may be less strong than mesitylene···Mg coordination but the supplementary B(C6F5)4−···Mg coordination observed in the complex with the smaller benzene ligand provides an additional energy gain of ca. 10.5 kcal/mol. These calculations show that incorporation of the non- or weakly coordinating anion is important.

the equilibrium constants we determined the following approximate thermodynamic parameters: ΔH0 = −6.9 kcal· mol−1 and ΔS0 = −28.2 cal·mol−1·K−1 (Figure S32). Addition of up to three equivalents of C6H6 to [(BDI)Mg·mesitylene][B(C6F5)4−] only replaced the mesitylene coordination partly which is surprising when considering the steric repulsion of the BDI ligand and mesitylene. However, the increase of electron density in the π-system (+I−effects) as well as the missing stabilization from B(C6F5)4− compared with [(BDI)Mg+· C6H6][B(C6F5)4−] have to be taken in account when describing this interaction. Theoretical Investigations. In an earlier report, we already presented calculations on the (BDI)Mg+·C6H6 cation and its B(C6F5)4− complex. As an addition to this work, the structures of the free cation (BDI)Mg+·mesitylene and [(BDI)Mg+·mesitylene][B(C6F5)4−] were optimized by Density Functional Theory (DFT) at the ωB97XD/6-31+G** level of theory and energies were calculated at the ωB97XD/6311+G** level. This method includes correction for dispersion using Grimme’s D2 method. The optimized structure for (BDI)Mg+·mesitylene fits that of the cation in the crystal structure of [(BDI)Mg+· mesitylene][B(C6F5)4−] in which the anion is truly noncoordinating. The average Mg−C(mesitylene) distance of 2.619 Å compares well with the X-ray data (average: 2.612 Å). The methyl substituents of the mesitylene are slightly bent out of the arene plane away from the metal (average centroid−C− CH3 angle: 175.3°) as also observed in the crystal structure (average centroid−C−CH3 angle: 175.5°). This could be related to sterics (repulsion between BDI and mesitylene) but may also be explained by polarization of the π-electron density toward the metal (vide supra). Coordination of benzene to the naked cation (BDI)Mg+ was found to be exothermic by ΔE = −36.1 kcal/mol. Mesitylene coordination was calculated to be stronger: ΔE = −47.8 kcal/mol. This means that exchange of the benzene ligand in (BDI)Mg+·C6H6 for mesitylene is exothermic by −11.7 kcal/mol. Considering the larger steric bulk of mesitylene, this is a surprising finding. It may, however, be explained by electronic effects: σ-donating Me substituents make the mesitylene ring more electron rich and therefore a stronger donor. From the NBO charge analysis it is evident that the Mg···arene interactions are predominantly of electrostatic nature. The charge on mesitylene in its Mg complex (+ 0.046) is similar than that on benzene (+0.051) and indicates only minor electron transfer. Mesitylene···Mg coordination affects the C−C ring bond distances in mesitylene only marginally: an elongation of ca. 0.01 Å is observed. Also the QTAIM analysis demonstrates that changes in the electron density and bond ellipticity at the bond critical points or changes in the delocalization indices are very small (see ESI). There is a slight polarization of the π-electron density toward the metal (Figure S33). The optimized structure of [(BDI)Mg+·mesitylene][B(C6F5)4−] with full inclusion of [B(C6F5)4−] fits also well with its solid state structure. It should be noted that B(C6F5)4− is here truly noncoordinating and therefore there are likely many more minima of more or less equal energy at the potential surface. The calculated thermodynamic parameters for the equilibrium [(BDI)Mg+][B(C6F5)4−] + mesitylene ⇄ [(BDI)Mg+·mesitylene][B(C6F5)4−] are the following: ΔE =

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CONCLUSIONS In the present study, we elaborated the details of the synthetic methods and possible side reactions for the challenging syntheses and purifications of (BDI)Mg+·arene complexes. We also found that increasing the size of the arene by Me substitution (benzene−toluene−xylene−mesitylene) led to a change in Mg−arene bonding from η3 (for the smaller arenes) to η6 (for mesitylene). This is due to the fact that smaller arenes leave free coordination space at the metal for Mg···F interactions with B(C6F5)4−. The latter interactions increase in length upon increasing the bulk of the arene ligand. Larger arenes like durene are too bulky to coordinate and leave the Mg metal “naked”. NMR studies reveal that these complexes also exist in solution; however, there is clear evidence for dissociation−association equilibria. DFT calculations demonstrate that metal−arene bonding is mainly electrostatic and there are very little changes in the electronics of the bound arene ligand. A slight polarization of π-electron density toward the metal augments these electrostatic interactions. Despite its larger size, there is a preference of mesitylene over benzene coordination. This can be attributed to the inductive effect of three Me groups which create a higher electron density in the mesitylene ring. The calculational study on these weak interactions also shows that reliable energy values can only be obtained by including the anion B(C6F5)4−, which in some cases can be weakly coordinating. Generation of such cationic complexes with more weakly coordinating anions would enhance the metal-arene bond energies. A slight increase in the bulk of the BDI ligand may also avoid coordination of the fluorinated anion, increasing the Lewis acidity of the metal center. It is likely that these interactions play a large role in the activation of arenes for further functionalization, a topic that is currently under investigation.

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EXPERIMENTAL SECTION

General Experimental Procedures. All experiments were conducted under an inert nitrogen atmosphere using standard Schlenk and glovebox techniques (MBraun, Labmaster SP). All solvents were degassed with nitrogen, dried over activated aluminum oxide (Solvent Purification System: Pure Solv 400−4−MD, Innovative Technology) and stored over 3 Å molecular sieves. Chlorobenzene and bromobenzene were dried over calcium hydride, distilled under N2 atmosphere and stored over molecular sieves 3 Å. m-Xylene and mesitylene were degassed with nitrogen (three freeze−

−

−8.96 kcal·mol 1, ΔH0 = −6.51 kcal·mol−1, ΔS0 = −47.12 cal· mol−1·K−1, ΔG0 = +7.53 kcal·mol−1. This fits quite well with F

DOI: 10.1021/acs.organomet.8b00489 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

(m, 2H, tol-ArH), 4.87 (s, 1H, CCHC), 2.68 (hept, 3JHH = 6.8 Hz, 4H, CHMe2), 2.02 (br, 6H, tol-CH3), 1.53 (s, 6H, CCH3), 1.02 (m, 24H, CHCH3). 13C NMR (151 MHz, C6D5Br, 298 K) δ 173.3 (s, NC(CH3)), 149.1 (br d, 1JCF = 243 Hz, B(C6F5)4), 142.3 (s, ArC), 142.2 (s, ArC), 138.1 (br t, 1JCF = 246 Hz, B(C6F5)4), 131.9 (s, ArC), 130.3 (s, ArC), 128.9 (s, ArC), 127.7 (s, ArC), 127.2 (s, ArC), 125.2 (s, ArC), 97.0 (s, CCHC), 29.1 (s, CHMe2), 24.7 (s, NC(CH3)), 24.4 (s, CHCH3), 21.8 (s, tol-CH3). 19F NMR (565 MHz, C6D5Br, 298 K) δ −131.0 (d, 3JFF = 19 Hz, 8F, o-CF), −160.4 (t, 3JFF = 21 Hz, 4F, p-CF), −165.3 (t, 3JFF = 21 Hz, 8F, m-CF). 11B NMR (193 MHz, C 6 D 5 Br, 298 K) δ −15.6 (s, B(C 6 F 5 ) 4 ). Anal. Calcd for C67H57BF20MgN2: C 61.65, H 4.40, N 2.15. Found: C 61.44, H 4.31, N 2.03. Synthesis of [(BDI)Mg+·m-Xylene][B(C6F5)4−] (4). [(BDI)MgnBu]2 (0.0893 g, 0.0895 mmol) was dissolved in a mixture of chlorobenzene (0.9 mL) and m-xylene (0.1 mL). Addition of [Ph3C+][B(C6F5)4−] (0.150 g, 0.163 mmol) resulted in a red−brown solution which changed color to a clear solution within 1 min. After filtration, all volatiles were removed in vacuo giving the crude product as a semisolid orange oil. Subsequent addition of m-xylene (0.5 mL) resulted in immediate crystallization. The crystals were isolated, washed with hexane (3 × 1 mL), and dried in vacuo to give [(BDI)Mg+·m-xylene][B(C6F5)4−] in 82% yield (164 mg). The crystal structure revealed that complex 4 cocrystallized with one molecule of chlorobenzene. 1 H NMR (C6D5Br, 600 MHz, 298 K): δ 7.18 (t, 3JHH = 7.7 Hz, 2H, ArH), 7.05 (d, 3JHH = 7.7 Hz, 4H, ArH), 6.84 (br. m, 4H, xyl-ArH), 4.94 (s, 1H, CCHC), 2.74 (sept., 3JHH = 6.9 Hz, 4H, CH(CH3)2), 2.04 (br. s, 6H, xyl−CH3), 1.57 (s, 6H, NCCH3), 1.05 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2), 0.98 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2) ppm; signals for cocrystallized chlorobenzene have not been assigned or integrated. 13C NMR (C6D5Br, 151 MHz, 298 K): δ 173.6 (s, CCHC), 150.8 (d, JCF = 244 Hz, B(C6F5)4), 149.1 (d, JCF = 244 Hz, B(C6F5)4),142.4 (s, ArC), 141.9 (s, ArC), 137.3 (d, JCF = 244 Hz, B(C6F5)4), 127.7 (s, ArC), 125.2 (s, ArC), 121.4 (s, ArC), 97.1 (s, CCHC), 29.1 (s, CH(CH3)2), 24.7 (s, CH(CH3)2), 24.6 (s, CH(CH3)2), 24.4 (s, NCCH3), 21.9 (s, xyl-CH3) ppm. 19F NMR (C6D5Br, 565 MHz, 298 K): δ −165.5 (8F, o-ArF), −160.4 (4F, pArF), −131.5 (8F, m-ArF) ppm. 11B NMR (C6D5Br, 193 MHz, 298 K): δ −16.0 (s, B(C6F5)4) ppm. Anal. Calcd for C67H56BClF20MgN2: C 60.07%, H 4.21%, N 2.09%. Found: C 60.82%, H 4.43%, N 2.12%. Although the C value is outside the range viewed as establishing analytical purity, it is provided to illustrate the best value obtained to date. Synthesis of [(BDI)Mg+·Mesitylene][B(C6F5)4−] (5). [(BDI)MgnBu]2 (0.0927 g, 0.0929 mmol) and [Ph3C+][B(C6F5)4−] (0.156 g, 0.169 mmol) were dissolved in chlorobenzene (1.0 mL). The brownish solution was stirred until it became colorless (30 s), and subsequently, all volatiles were removed in vacuo. Mesitylene (1.0 mL) was added to the yellow oil which formed two liquid phases. After 10 days crystals of 5 formed. Both phases were removed and the colorless crystals were washed with 1 × C6H5Cl (0.1 mL), 1 × mesitylene (0.2 mL), and hexane (0.3 mL) before drying in vacuo. The product was isolated with a yield of 63% (0.1315 mg, 0.1061 mmol). 1 H NMR (400 MHz, C6D5Br, 333 K) δ 7.20−7.14 (m, 2H, ArH), 7.08−7.03 (m, 4H, ArH), 6.67 (s, 3H, Mes-ArH), 4.97 (s, 1H, CCHC), 2.74 (hept, 3JHH = 7.0 Hz, 4H, CHMe2), 2.12 (s, 9H, MesCH3), 1.60 (s, 6H, CCH3), 1.06 (d, 3JHH = 7.0 Hz, 12H, CHCH3), 0.93 (d, 3JHH = 7.0 Hz, 12H, CHCH3). 13C NMR (101 MHz, C6D5Br, 333 K) δ 173.8 (s, NC(CH3)), 149.1 (br d, 1JCF = 243 Hz, B(C6F5)4), 142.4 (s, ArC), 141.9 (s, ArC), 139.1 (br d, 1JCF = 244 Hz, B(C6F5)4), 138.2 (s, Mes-CCH3 (only observed in HMBC)), 137.4 (br d, 1JCF = 243 Hz, B(C6F5)4), 127.7 (s, ArC), 127.6 (s, Mes-ArC), 125.1 (s, ArC), 97.2 (s, CCHC), 29.2 (s, CHMe2), 24.6 (s, NC(CH3)), 24.5 (s, CHCH3), 24.4 (s, CHCH3), 21.7 (s, Mes-CH3). 19F NMR (376 MHz, C6D5Br, 333 K) δ −131.6 (d, 3JFF = 18 Hz, 8F, o-CF), −159.5 (t, 3JFF = 21 Hz, 4F, p-CF), −164.5 (t, 3JFF = 20 Hz, 8F, m-CF). 11B NMR (193 MHz, C6D5Br) δ −15.6 (s, B(C6F5)4).

thaw cycles) and dried and stored over molecular sieves 3 Å. C6D6 and C6D5Br (99.6% D, Sigma-Aldrich) were dried over 3 Å molecular sieves. [Ph3C+][(C6F5)4B−] (Boulder Scientific) was used as received. BDI-H (BDI = HC{(Me)CN(2,6-iPr2C6H3)}2),21 [(BDI)MgnPr]2,15 [(BDI)MgnBu]2,28 [Li+][(C6F5)4B−],29 [Ag+][(C6F5)4B−]30 and [(BDI)Mg+·O(SiMe3)2][B(C6F5)4−]24 were synthesized according to a literature procedure. [(BDI)MgI]2 was synthesized according to a modified literature procedure.31 NMR spectra were recorded with a Bruker Avance III HD 400 MHz or a Bruker Avance III HD 600 MHz spectrometer. The spectra were referenced to the respective residual signals of the deuterated solvents. Elemental analysis was performed with a Euro EA 3000 (Euro Vector) analyzer. All crystal structures have been measured on a SuperNova (Agilent) diffractometer with dual Cu and Mo microfocus sources and an Atlas S2 detector. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. 1855765 (3), 1855766 (4), 1855767 (5), 1855768 (6), 1855769 (7), 1855770 (8), and 1855771 (9). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44)1223−336−033; E-mail: [email protected]). Synthesis of [(BDI)MgI]2. In a first step the diethyl ether adduct [(BDI)MgI·OEt2]2 was prepared by the route of Bonyhady et al.31 The product could be freed from ether by stirring the colorless crystals at 140 °C for 17 h under vacuum (5 × 10−2 mbar) which afforded the ether-free product as a white powder in quantitative yield. The spectroscopic data for the complex were identical to those previously reported.32 Alternative Synthesis of [(BDI)Mg+][B(C6F5)4−] via a Disiloxane Complex. [(BDI)Mg+·O(SiMe3)2][B(C6F5)4−] (0.0302 g, 0.0235 mmol) was dissolved in chlorobenzene (1.0 mL), and all volatiles were removed in vacuo at room temperature. The remaining oil was stirred with hexane (1.0 mL) until the product was obtained in the form of a white powder (0.0190 g, 0.0169 mmol, 72%). Alternative Synthesis of [(BDI)Mg+][B(C6F5)4−] via a Salt Metathesis Route with a Silver Salt. A suspension of [(BDI)MgI]2 (0.0082g, 0.0067 mmol) and [Ag+][B(C6F5)4−] (0.0102g, 0.0130 mmol) in C6D5Br (0.5 mL) was shaken for 5 min. The yellow precipitate that formed during the reaction was filtered off, and the solution showed quantitative conversion to the naked cation [(BDI)Mg+][B(C6F5)4−] for which the analytical data correspond to those reported earlier.15 This complex can be further converted to the arene complexes by addition of the respective arene and workup as given below. Alternative Synthesis of [(BDI)Mg+·C6H6][B(C6F5)4−] (2). [(BDI)MgnBu]2 (0.0880g, 0.0882 mmol) and [Ph3C+][B(C6F5)4−] (0.1500 g, 0.1626 mmol) were dissolved in a mixture of chlorobenzene (0.9 mL) and benzene (0.1 mL). The brownish solution was stirred until it became colorless (30 s), and subsequently, all volatiles were removed in vacuo resulting in a viscous orange oil. Addition of benzene (1 mL) resulted in the formation of a liquid clathrate and subsequently very fast crystallization of the desired product. The colorless crystals were filtered off and washed with benzene (2 × 0.5 mL) before drying in vacuo. The product was obtained in a yield of 86% (0.1670 g, 0.1393 mmol). The spectroscopic data for the complex were identical to those previously reported.15 Synthesis of [(BDI)Mg+·Toluene][B(C6F5)4−] (3). [(BDI)MgnBu]2 (0.0923 g, 0.0925 mmol) and [Ph3C+][B(C6F5)4−] (0.1509 g, 0.1636 mmol) were dissolved in a mixture of chlorobenzene (1.8 mL) and toluene (0.2 mL). The brownish solution was stirred until it became colorless (30 s), and subsequently, all volatiles were removed in vacuo. A 1:1 mixture of toluene/hexane (0.5 mL) was added resulting in two phases. The biphasic mixture was scratched with a spatula until first nuclei of crystallization could be observed. After the mixture was allowed to stand overnight, the resulting crystalline material was washed with a 1:1 mixture of toluene/hexane (5 × 1 mL) and dried in vacuo. The product was obtained as colorless crystals with one cocrystallized toluene molecule (0.1570 mg, 0.1203 mmol, 74%). 1 H NMR (400 MHz, C6D5Br, 298 K) δ 7.23−7.17 (m, 2H, ArH), 7.12−7.04 (m, 8H, ArH), 7.02−6.97 (m, 4H, tol-ArH), 6.97−6.94 G

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Organometallics H NMR (400 MHz, C6D5Br, 298 K) δ 7.18 (t, 3JHH = 7.7 Hz, 2H, ArH), 7.06 (d, 3JHH = 7.7 Hz, 4H, ArH), 6.68 (s, 3H, Mes-ArH), 4.93 (s, 1H, CCHC), 2.73 (br s, 4H, CHMe2), 2.14 (br s, 9H, Mes-CH3), 1.56 (s, 6H, CCH3), 1.05 (d, 3JHH = 6.7 Hz, 12H, CHCH3), 0.97− 0.83 (br m, 12H, CHCH3). 13C NMR (101 MHz, C6D5Br, 298 K) δ 173.5 (s, NC(CH3)), 149.1 (br d, 1JCF = 245 Hz, B(C6F5)4), 142.3 (s, ArC), 138.8 (br d, 1JCF = 250 Hz, B(C6F5)4), 137.2 (br d, 1JCF = 238 Hz, B(C6F5)4), 127.7 (s, ArC), 125.1 (s, ArC), 97.0 (s, CCHC), 29.1 (s, CHMe2), 24.5 (br s, NC(CH3))), 24.4 (s, CHCH3), 21.7 (s, MesCH3). (Signals for the two aromatic carbon atoms of mesitylene and one quaternary carbon of the ligand could not be observed.) 19F NMR (376 MHz, C6D5Br, 298 K) δ −131.4 (d, 3JFF = 20 Hz, 8F, oCF), −160.0 (t, 3JFF = 21 Hz, 4F, p-CF), −165.0 (t, 3JFF = 20 Hz, 8F, m-CF). 11B NMR (128 MHz, C6D5Br, 298 K) δ −15.7 (s, B(C6F5)4). At 253 K decoalescence of the signals is observed. Both species, the cation−anion complex [(BDI)Mg+][B(C6F5)4−] and its mesitylene complex [(BDI)Mg+·mesitylene][B(C6F5)4−], can be observed separately. Assignment of signals is given below. [(BDI)Mg+·Mesitylene][B(C6F5)4−]. 1H NMR (400 MHz, C6D5Br, 253 K) δ 7.22−7.15 (m, 2H, ArH), 7.13−7.04 (m, 4H, ArH), 6.65 (br s, 3H, Mes-ArH), 4.70 (s, 1H, CCHC), 2.52 (hept, 3JHH = 6.8 Hz, 4H, CHMe2), 1.44 (br s, 9H, Mes-CH3), 1.37 (s, 6H, CCH3), 1.18 (d, 3JHH = 6.8 Hz, 12H, CHCH3), 1.01 (d, 3JHH = 6.8 Hz, 12H, CHCH3). 13C NMR (101 MHz, C6D5Br, 253 K) δ 172.7 (s, NC(CH3)), 148.8 (br d, 1JCF = 240 Hz, B(C6F5)4), 144.7 (s, ArC − Mes), 141.6 (s, ArC), 141.2 (s, ArC), 138.5 (br d, 1JCF = 247 Hz, B(C6F5)4), 136.8 (br d, 1JCF = 250 Hz, B(C6F5)4), 128.3 (s, ArC − Mes), 127.7 (s, ArC), 125.1 (s, ArC), 96.5 (s, CCHC), 28.8 (s, CHMe2), 24.7 (s, NC(CH3))), 24.1−23.4 (m, CHCH3), 20.1 (s, Mes-CH3). [(BDI)Mg+][B(C6F5)4−] 1H NMR (400 MHz, C6D5Br, 253 K) δ 7.22−7.15 (m, 2H, ArH), 7.13−7.04 (m, 4H, ArH), 4.92 (s, 1H, CCHC), 2.78 (hept, 3JHH = 6.8 Hz, 4H, CHMe2), 1.54 (s, 6H, CCH3), 1.05 (d, 3JHH = 6.8 Hz, 12H, CHCH3), 0.90 (d, 3JHH = 6.8 Hz, 12H, CHCH3). 13C NMR (101 MHz, C6D5Br, 253 K) δ 173.2 (s, NC(CH3)), 148.8 (br d, 1JCF = 240 Hz, B(C6F5)4), 142.8 (s, ArC), 142.1 (s, ArC) 138.5 (br d, 1JCF = 247 Hz, B(C6F5)4), 136.8 (br d, 1 JCF = 250 Hz, B(C6F5)4), 127.3 (s, ArC), 124.9 (s, ArC), 96.7 (s, CCHC), 28.7 (s, CHMe2), 24.4 (s, NC(CH3))), 24.4 (s, CHCH3), 24.1−23.4 (m, CHCH3). Anal. Calcd for C62H53BF20MgN2: C 60.00, H 4.30, N 2.26. Found: C 60.22, H 4.54, N 2.20. Reaction of 2 with CH 2 Cl2 and Crystallization of the Decomposition Product 8. Dichloromethane (0.05 mL) was added to a colorless solution of [(BDI)Mg+·C6H6][B(C6F5)4−] (0.0113 g, 0.00942 mmol) in C6H5Cl (0.5 mL). After 3 days, a few colorless crystals suitable for X-ray diffraction could be obtained from the now yellow solution. The crystal structure is shown in the Supporting Information (Figures S25 and S26). Reaction of 2 with CH 2 Cl2 and Crystallization of the Decomposition Product 9. Dichloromethane (0.1 mL) was added to a colorless solution of [(BDI)Mg+·C6H6][B(C6F5)4−] (0.0149 g, 0.0124 mmol) in C6D5Br (0.5 mL). After 3 days, a few colorless crystals suitable for X-ray diffraction could be obtained from the now yellow solution. The crystal structure is shown in the Supporting Information (Figures S27 and S28). 1

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Accession Codes

CCDC 1855765−1855771 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sjoerd Harder: 0000-0002-3997-1440 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Mrs. C. Wronna (University of ErlangenNürnberg) for numerous CHN analyses and J. Schmidt Dr. C. Färber (University of Erlangen-Nürnberg) for assistance with the NMR analyses.

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REFERENCES

(1) (a) Fischer, E. O.; Hafner, W. Z. Di-benzol-chrom Ü ber Aromatenkomplexe von Metallen I. Z. Naturforsch., B: J. Chem. Sci. 1955, 10, 665. (b) Fischer, E. O.; Hafner, W. Ü ber Aromatenkomplexe von Metallen. III. Zur Darstellung des Di-benzol-chroms. Z. Anorg. Allg. Chem. 1956, 286, 146. (c) Seyferth, D. Bis(benzene)chromium. 2. Its Discovery by E. O. Fischer and W. Hafner and Subsequent Work by the Research Groups of E. O. Fischer, H. H. Zeiss, F. Hein, C. Elschenbroich, and Others. Organometallics 2002, 21, 2800−2820. (2) Pampaloni, G. Aromatic hydrocarbons as ligands. Recent advances in the synthesis, the reactivity and the applications of bis(η6-arene) complexes. Coord. Chem. Rev. 2010, 254, 402−419. (3) Ma, J. C.; Dougherty, D. A. The Cation−π Interaction,. Chem. Rev. 1997, 97, 1303−1324. (4) Mahadevi, A. S.; Sastry, G. N. The Cation−π Interaction: Its Role and Relevance in Chemistry, Biology, and Material Science,. Chem. Rev. 2013, 113, 2100−2138. (5) Dougherty, D. A. Cation−π Interactions in Chemistry and Biology: A New View of Benzene, Phe, Tyr and Trp. Science 1996, 271, 163−168. (6) Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. The Cambridge Structural Database,. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, 72, 171−179. (7) Eaborn, C.; Hitchcock, P. B.; Izod, K.; Smith, D. The Synthesis and Crystal Structures of RbC(SiMe3)3 and CsC(SiMe3)3·3.5 C6H6: A One-dimensional Ionic Solid and an Ionic Solid with a Molecular Structure. Angew. Chem., Int. Ed. Engl. 1995, 34, 687. (8) Klinkhammer, K. W. Tris(trimethylsilyl)silanides of the Heavier Alkali MetalsA Structural Study. Chem. - Eur. J. 1997, 3, 1418− 1430. (9) Bonomo, L.; Solari, E.; Scopelliti, R.; Floriani, C. The π Complexation of Alkali and Alkaline Earth Ions by the Use of mesoOctaalkylporphyrinogen and Aromatic Hydrocarbons,. Chem. - Eur. J. 2001, 7, 1322−1332. (10) Wiesinger, M.; Maitland, B.; Färber, C.; Ballmann, G.; Fischer, C.; Elsen, H.; Harder, S. Simple Access to the Heaviest Alkaline Earth Metal Hydride: A Strongly Reducing Hydrocarbon-Soluble Barium Hydride Cluster. Angew. Chem., Int. Ed. 2017, 56, 16654−16659. (11) Harder, S.; Lutz, M.; Obert, S. J. Crystal Structure of a SiliconBridged Anionic Sodocene Complex: Evidence for Alkene−Na+ πBonding. Organometallics 1999, 18, 1808−1810. (12) (a) Chi, Y.; Ranjan, S.; Chung, P.-W.; Liu, C.-S.; Peng, S.−M.; Lee, G.-H. Synthesis and characterization of two novel tetranuclear

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00489. NMR spectra complexes 3−5, details for the crystals structure determinations of 3−9, details for the determination of thermodynamic parameters from the association−dissociation equilibrium, and computational details (PDF) XYZ-coordinates (XYZ) H

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Organometallics

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DOI: 10.1021/acs.organomet.8b00489 Organometallics XXXX, XXX, XXX−XXX