Unsupported Mg–Alkene Bonding

Abstract The first intermolecular early main group metal–alkene complexes were isolated. This was enabled by using highly Lewis acidic Mg centers in the Lewis base‐free cations (MeBDI)Mg+ and (tBuBDI)Mg+ with B(C6F5)4 − counterions (MeBDI=CH[C(CH3)N(DIPP)]2, tBuBDI=CH[C(tBu)N(DIPP)]2, DIPP=2,6‐diisopropylphenyl). Coordination complexes with various mono‐ and bis‐alkene ligands, typically used in transition metal chemistry, were structurally characterized for 1,3‐divinyltetramethyldisiloxane, 1,5‐cyclooctadiene, cyclooctene, 1,3,5‐cycloheptatriene, 2,3‐dimethylbuta‐1,3‐diene, and 2‐ethyl‐1‐butene. In all cases, asymmetric Mg–alkene bonding with a short and a long Mg−C bond is observed. This asymmetry is most extreme for Mg–(H2C=CEt2) bonding. In bromobenzene solution, the Mg–alkene complexes are either dissociated or in a dissociation equilibrium. A DFT study and AIM analysis showed that the C=C bonds hardly change on coordination and there is very little alkene→Mg electron transfer. The Mg–alkene bonds are mainly electrostatic and should be described as Mg2+ ion‐induced dipole interactions.


Introduction
Neutral p ligandsa re common in transition metal chemistry. [1] In fact, the first organometallic compound isolated in pure form was ap latinume thylene complex. Commonly referred to as Zeise's salt, K 2 [PtCl 3 (C 2 H 4 )]·H 2 Ow as isolated already as early as 1827. [2,3] Although compositiona nd bonding mode were at that time not fully understood, this early pioneering work marks the beginning of metal-alkenec hemistry. [4] Detailed knowledge of its structure led to formulation of the Dewar-Chatt-Duncanson bonding model in which s bonding between the ethylene HOMO and an empty metal orbital depletes electron density in a p-bonding molecular orbital, while d!p*b ackbonding fills the antibonding molecularo rbital (Scheme 1a). Thisc ombined action results in activation and elongation of the ethylene C=Cbond from 1.336(1) (free ethylene) [5] to 1.375 (4) . [6] Comprehensive Raman studies on Zeise's salt suggests trong PtÀCb ondinga nd considerable weakening of the C=Cb ond, indicated by as hift of the C=C stretching frequency from 1653 cm À1 in free ethylene to 1243 cm À1 in the Pt complex. [7] Bond activation by metal-alkene coordination is therefore ac rucial first step in catalytic alkene conversion. [8][9][10][11][12] The most striking difference between the transition and main group metals is the inability of the latter to strongly bind p ligands. This has its origin in their lack of low-lying, partially filled, dorbitals. Despite this deficiency,metal-alkenecoordination is often proposed asafirst elementary step in alkaline earth (Ae) metal catalyzed alkene conversion. [13][14][15] Although this type of bondingi sv ery weaka nd not well understood, it is in some cases crucial for catalytic activity. [16] Early main group metalsa ctivate substrates only by metal Lewis acid···alkene interaction. Especially an asymmetric metal alkene coordination mode leads to polarization of the C=Cb ond, which induces the d + /dÀ charges eparation necessary for nucleophilic attack (Scheme 1b). [16] Proof for such weak interactions was mainly found by switchingo ff the detrimental effect of entropy loss, and therefore the vast majority of all main group metal-alkene interactions are of the intramolecular type. [17][18][19][20][21][22][23][24][25] The first unsupported metal-alkene bonding between the pblock metal Ga and 1,5-cyclooctadiene (cod) hasb een realized very recently in the homoleptic complex cation Ga(cod) 2 + . [26] Like in transition metal chemistry,t he chelating coordination mode of this bis-alkene certainly contributes to its stability.D espite the importance of s-block metal-alkene bonding in catalysis, unsupported metal-alkene bonding has so far not been described. We recently reported ac ationic b-diketiminate (BDI) Mg complex in which the metal center is unsolvated,o nly showingM g···F interactions with the weakly coordinating anion (WCA) B(C 6 F 5 ) 4 À (I in Scheme 2). [27] We demonstrated its considerable Lewis acidity by isolation of the first unsupported complexes Mg···O(SiMe 3 ) 2 (II) [28] andM g···(3-hexyne)( III) [27,29] . We also reportedaseries of cationic Mg and Ca p-arene complexes IV. [27,30] Hill et al. simultaneously published similar complexes V [31] that contain the less coordinating Krossinga nion Al[OC(CF 3 ) 3 ] 4 À and are devoid of metal···anion interactions. Cation-anion interactions can also be avoided by increasing the bulk of the BDI ligand.R eplacing the Me groups in the BDI ligand backbone with tBu substituents led to complete cleavage of cation-anion contacts (VI). [32] Krossinge tal. recently described Ae 2 + (hexamethylbenzene) ions (Ae = Ca, Sr,B a) stabilized by weakly coordinating aluminate anions [33] and most recently introduced ac atalytically active dicationic ansa-arene Sr 2 + complex. [34] Althought he interaction between Ae metal cationsa nd electron-rich p-arene or p-alkyne ligandsh as been comprehensively investigated, there is currently no information on unsupported Ae metal-alkenec omplexes. Considering their important role as intermediates in catalysis, we now fill this gap by reporting av ariety of Mg complexes with intermolecular bisand mono-alkene ligands, discussm etal-alkene bonding, and provideaDFT study that gives insight into their relative bond energies.

Results and Discussion
Synthesis Synthesis of Mg complexes with weakly bound alkene ligands is challenging, and certainr equirements should be taken into consideration. The solventw ill competew ith aw eak Mg···alkene interaction. Therefore, not only ethereal solvents buta lso aromatic solvents such as benzene or toluenes hould be avoided, and insteadh alogenated solvents must be used. During the course of our investigations we found that chlorobenzene is preferable to the more polar and more strongly coordinating fluorobenzene. As has been shown by Hill and co-workers,f luoroarenes can indeed tightly bind to ( Me BDI)Ae + . [31] Due to competition between Mg···solvent and Mg···alkene interactions, it is also advisablet ou se al arge excess of the alkene.A nother factor to be considered is competition between Mg···alkene bondinga nd cation-anioni nteractions. Although the B(C 6 F 5 ) 4 À anion is only weakly coordinating, short Mg···F interactions may interrupt Mg···alkene bonding. We recently reported that increased steric bulk in the backbone of the BDI ligand fully impedes the Mg···B(C 6 F 5 ) 4 À interaction, which even allowed for isolationo facomplexw ithw eaklyc oordinatingc hlorobenzene (VI). [32] Therefore, ourfirst effortst oisolate Mg alkene complexes concentrated on usingthe strong Lewisacid( tBu BDI)Mg + .
Since dienes such as divinylsiloxane or cod gave stable Mgalkene complexes in which only one of the alkene bonds directly interacts with the metal center,also alkene complexation with the less bulky b-diketiminate ligand Me BDI was attempted.

Reactiono f( Me BDI)MgnBu with[ Ph 3 C + ][B(C 6 F 5 ) 4
À ]i nc hlorobenzene led to in situ formation of the cation-anion pair The much more open coordination sphere in ( Me BDI)Mg + allowedf or isolation of an Mg-alkene complexw ith ac helating cod ligand (4,S cheme 3). Interestingly,t he asymmetric unit of the crystal structure of 4 contains five independent( Me BDI)Mg + (h 4 -cod) cations, whichc an be divided into two types of structures, that is, those with an additional Mg···F contact (4a)o rt hose without cation-anion interaction (4b). The smaller b-diketiminate ligand also enabled isolation of complexes with cht (5), dmbd (6), and eb (7), that is, ligandst hat could not form complexes with the bulkier ( tBu BDI)Mg + .T he Mg···alkene interaction is in all cases accompanied by ac ation-anion contact.
The most important bond lengths and interactions are summarizedi nT able 1.
The following general and more specific observations can be made: 1) The Mg centerincomplexes with the bulkier b-diketiminate ligand ( tBu BDI) can only interact with one alkene bond. This is exemplified by the mono-alkene coordination of divinylsiloxane and cod. 2) Complexes with the smaller b-diketiminate ligand ( Me BDI) are generally stabilized by one additional Mg···F interaction with the borate anion and can accommodate up to two alkenes in the Mg coordination sphere.T his is illustrated by the crystal structures of the Mg-codc omplex 4 and the Mg-cht complex 5,i nw hich the Mg center is too small to interact with all three alkenes. Note that in the latter the two remote alkene bonds in cht coordinate with Mg but one alkene is merely bound h 1 with al ong contact distance of 2.844 (2) .C oordination of neighboring alkenes is appa-rentlyu nfavorable,w hich explainst hat for dmbd only mono-alkene insteado fd iene coordination is observed( the cis isomer of dmbd is only 2kcal mol À1 higheri ne nergy than its trans isomer). [38] Also the recently isolated cationic Au complex [(tBu 3 P)Au + (h 2 -dmbd)][SbF 6 À ]s hows ap lanar trans conformation of dmbda nd binding of the metal centert oonly one of the C=Cbonds. [39] 3) In all cases, asymmetricM g···alkene bonding is observed; that is, one of the Mg-C contacts is generally significantly shorter than the other.As imilar preference for asymmetric alkene coordinationw as found in structures with intramoleculara lkene coordination. [24] Interestingly,i nt he crystal structure of [( Me BDI)Mg + (cod)][B(C 6 F 5 ) 4 À ]( 4), the asymmetric unit contains five independentm olecules with aw ide range of different Mg···C interactions. There is not only a difference in alkene coordinationb ut also in Mg···F bonding, which is absent in two of these molecules. This enormousf luctuation in as et of identical molecules originates only from crystal packing effects and clearly shows that the Mg-alkene bond should be considered weak and very dynamic. 4) The most extreme asymmetry in Mg-alkene coordination is found in complexes with the cations( Me BDI)Mg + (dmbd) ( (5) ). While in the latter,t he longer Mg···C distance should be considered very weak, the shorter 2.338(2) contact is hitherto, to the best of our knowledge, the shortestM g···C interaction observed to an eutral p ligand and even shorter than previously reported intramo-lecularM g···C(alkene) interactions (range:2 .55-2.85 ). [19,20,24,25] It is also shorter than the shortestM g···C contact in alkyne complex III ()2.399(2) or benzene adduct IV (2.367 (2) ). [27] It is comparable to the MgÀCd istanceo f2 .304 (8) in Cp 2 Mg, which has as trong electrostatic interaction between Mg 2 + and the Cp À p ligands. [40] DFT calculations confirm that strong coordinationo fM gt o the least alkylated olefin carbon atom is indeed most favorable (vide infra). 5) As noted previously,t he metal-siloxane coordination in ( 1)i sr are and usual- Complex 1234a 4b 567 Mg···F bond [a] no Mg···Fb ond [ ly only observed in silacrown-metal complexes, in which metal complexation is facilitated by the cooperative effect of multiple metal-siloxane interactions. [41,42] We reported the first metal-O(SiMe 3 ) 2 coordination in the form of ac ationic Mg complex (II). The currentd ivinylsiloxanec omplex 1 shows, apart from unusualM g-alkene coordination,a lso as econd example of intermolecular Mg-siloxane coordination. The somewhat longerM g ÀOb ond length of 2.049 (1) [44] Solution studies The crystal structures of the Mg-alkene complexes in the solid state are subject to crystal packing effects. The variety of differ-ent structuresp resent in the unit cell of cod complex 4 already hints at highlyd ynamic solution behavior.A ll complexes dissolve in deuterated bromobenzene and were studied by NMR spectroscopy.A lkene coordination typically resultsi nashift of the alkene signals compared to the free alkenes. Significant broadening of the alkene signals, which waso bservedi ns ome cases, indicates ligand exchange equilibria:( BDI)Mg + (alkene)Q(BDI)Mg + + alkene. Bromobenzene solutionso ft he Mg-alkene complexes with the bulkier tBu BDI ligand (1-3)a ll give 1 HNMR spectra in which the alkene signals are sharp and coincide with those of the free alkenes. This demonstrates that thesec omplexes are mainly dissociated in bromobenzene.
The Mg-alkene complexes with the smaller Me BDI ligand are clearly more stable in bromobenzene solution. The cod 1 HNMR signals in (4)( 5.77 and 2.01 ppm) are shiftedf rom those of free cod in C 6 D 5 Br (5.53 and 2.24 ppm).T he relatively broad signals indicate an exchange equilibrium. On heating the sample to 55 8Cs harper signals could be obtained and signals shifted towards those for free cod. For the cht complex 5,s imilar observations have been made, and also this ligand is clearly partially bound in C 6 D 5 Br.I nc ontrast, the signals of dmbd and eb in complexes 6 and 7,w hich coincide with those of the free alkenes and are sharp, indicate dissociation.

Theoretical considerations
Interactions between the b-diketiminate Mg complexes and variousa lkenesw ere studied by DFT calculations (wB97XD/6-311 + G**//6-31 + G**, including corrections for dispersion by Grimme'sD 2m ethod). The Mg-alkene complexes 1-7 were fully optimized.I na ll cases the presence of the WCA B(C 6 F 5 ) 4 À was considered. Although the alkene ligandsi nt hese complexes are only looselyb ound and subject to facile deformation by crystal packing effects, there is as urprisingly good match between crystal and calculated structures (see Ta ble 1 for the mosti mportant bond lengths and interactions). Also the bond enthalpies ( Table 2) and NPAc harges ( Table S3 in the Supporting Information) werec alculated and Atoms in Molecules analyses (AIM) were performed. The following mainc onclusionscan be drawn.
1) Like in the crystal structures, the borate anion is not bound to Mg in complexes with the larger tBu BDI ligand (1-3). For 4-7,t hat is, complexes with the smaller Me BDI ligand,t he borate anion is in all cases bound to Mg by as ingle Mg···(h 1 )B(C 6 F 5 ) 4 interaction. In agreement with experiment, the longest Mg···F contacts are found in the Mg-cod complex 4, while Mg-cht complex 5 features an exceptionally short interaction.
2) The experimentally observed Mg-alkene coordination modes are fully reproduced by the calculations.F or example, the diene cod is chelating in the complex with the Me BDI ligand but only one alkene is bound in the complex with the bulkier( tBu BDI) ligand. The triene cht in 5 binds Mg with one short Mg-h 2 -alkene interaction (2.601(2)/2.692(2) ), while the second interaction should be categorized as an Mg-h 1 -alkene bond (2.844 (2) ). The diene dmbd in 6 can only bind Mg with one of its double bonds.
3) In all cases asymmetric Mg-alkene bonding is observed. The most extreme cases of asymmetricM g-alkene bonding are found in the complexes with dmbd ande bl igands( 6 and 7)a nd, although thesel igands are only loosely bound, the experimental solid-state values are reproduced surprisingly well by these gas-phase calculations. This important observation meanst hat the geometries of these complexes are mainly influenced by intramolecular forces and much lessb yi ntermolecular packing effects.
4) The C=Cb onds are in all cases hardly elongated by Mgalkenec oordination.C ompared with the free alkenes generally an elongation of 0.01-0.02 is observed. 5) Considering the complexation energies (Table 2) it is striking that alkene bondingt ot he more sterically congested [( tBu BDI)Mg + ][B(C 6 F 5 ) 4 À ]i ss ignificantly more exothermic than that to [( Me BDI)Mg + ][B(C 6 F 5 ) 4 À ]. Although this seems counterintuitive, it may be explained by the more facile cation-anion dissociation in the sterically congested complex. However, overestimation of the dispersion correctionc ould also contribute to this phenomenon. In the ( Me BDI)Mg + series of complexes, coordinationo fe bi sm ore exothermic than chelating cod complexation. Although this may seem surprising, it should be considered that the shortest Mg-C contact to cod (2.625 )i sl ongert han that to eb (2.399 ). Also the large asymmetry in the Mg-ebb ond and the concomitantC =C bond polarization (vide infra) contribute positively to the bond energy.C omparisono fDH and DG values shows especially for chelating ligands such as cod or divinylsiloxane large changes,s ignifying the considerable loss in entropyf or bidentate coordination.I ti sn oteworthy that alkene coordination can be more favorable than benzene coordination.E nergies [kcal mol À1 ]f or formation of the benzene complex [( Me BDI)Mg(benzene) + ][B(C 6 F 5 ) 4 À ]: DE À6.8, DH = À5.9, DG =+ 5.6.
6) The calculated NPAc harges show that bondingi nt he (BDI)Mg + cationsi sm ainly electrostatic (the charges on the Mg centers varyf rom + 1.81 to + 1.86 and those on the BDI ligands from À0.89 to À0.94). There is hardly any electron transfer from the alkenet ot he Mg 2 + cation:c harges on the alkene ligandsi nt he complexes vary from + 0.02 to + 0.05. This is in agreement with ap redominantly electrostatic interaction between ah ighly positively charged Mg 2 + cation and ap olarized alkene bond. AIM analyses show that the C=Cb ond in the alkene ligandsi so nly weakly affected by Mg coordination, as is evidentf rom minor changes in electron densities in the CÀC bond critical points, bond ellipticities, and delocalization indices (TableS4, Figures S55-61i nt he Supporting Information). 7) EarlierA IM studies have shown polarization of the C=C pelectron density towards the Ca tom closest to the metal atom (horizontal polarization)a nd towards the metal atom itself (vertical polarization). [24,25] The Mg-ebb ond in 7 is an extreme case of asymmetricM g-alkene bonding. AIM analysis of this complex shows tremendous horizontal and vertical polarization of the p-electron density (Figure 3). Strong horizontal polarization of Mg-boundH 2 C=CEt 2 is supported by the NPA chargeso nC H 2 (À0.70) and CEt 2 (+ 0.68) groups [c f. free eb: CH 2 (À0.43) andC Et 2 (+ 0.43)].A lthough the AIM parameters of the C=Cb ond are generally hardly affected by Mg-alkene coordination,M g-ebc oordination has ap ronouncede ffect on the C=Cb ond ellipticitiesa nd delocalization indices ( Table S4 in the Supporting Information).

Conclusion
The first crystal structures of unsupported magnesium alkene complexes have been presented. Complexation of neutral, nonpolara lkenes to Mg was accomplished by exploiting the very high Lewis acidity of the unsolvated b-diketiminate Mg cation (BDI)Mg + .S ince Mg-solvent interactions competew ith Mg-alkene bonding, weakly coordinating chlorobenzene was used as solvent. Also (BDI)Mg···(C 6 F 5 ) 4 B À interactions may hinder alkene coordination, but these cation-anion interactions can be minimized by choosing ab ulky BDI ligand, such as tBu BDI, that still leaves room for alkene coordination. Crystal structures of av ariety of Mg-alkene complexes show that the bulkier( tBu BDI)Mg + cation can only bind to one C=C bond, whilet he more accessible ( Me BDI)Mg + cation binds the diene cod in ac helating fashion. In all cases, asymmetricM galkene bondingi so bserved. Although it mayb ee xpected that such weak Mg···alkenei nteractions are prone to distortion by crystal packing effects, this is not the case. Calculated gasphase structures reproduce the experimental solid-state structures quite well. The geometry of Mg-alkene coordination is therefore mainly determined by intramolecular steric and electronic factors. Since the p-electron density in Et 2 C=CH 2 (eb) is strongly polarized towards the terminal CH 2 group by the selectron-donating Et substituents, this alkene shows highly asymmetricb onding. The complex could be regarded as having partial ( Me BDI)Mg-CH 2 -(Et) 2 C + character.A ll Mg-alkene bonds are quite labile and, when dissolved in deuterated bromobenzene, the complexes are either completely dissociated or in ad issociation equilibrium.
DFT calculations on the full complexes,t hat is, including the weakly coordinating (C 6 F 5 ) 4 B À anions, show that alkenes form stronger bonds to the bulkier ( tBu BDI)Mg + cation than to ( Me BDI)Mg + .T his is likely relatedt ot he fact that the smaller cation forms at ighter cation-anion pair.T his conclusion does not agree with solution studies, which show complete dissociation for alkene complexes with the bulkier tBu BDI ligand,w hile those with the Me BDI ligand could be detected. This discrepancy between gas-phase calculations and solution studies must be related to competition with solvent coordination. On coordination,t he C=Cb onds are hardly elongated, and also insignificant electron transfer to the metal is observed. The Mgalkene bonds are therefore mainly electrostatica nd should be described as Mg 2 + ion-induced dipole interactions, which are especially strongf or asymmetrica lkene coordination.A lthough much weaker than transition metal-alkene bonding,t hese first examples of unsupported s-block metal-alkene bonds clearly show that these interactions can also be of significance in early main group chemistry,e specially in the relatedf ield of catalysis.