Hydrocarbon-soluble, hexaanionic fulleride complexes of magnesium

Fullerene C60 reacts with dimagnesium(i) compounds LMgMgL, where L is a monoanionic β-diketiminate ligand, to contact ion complexes [(LMg)nC60], where n is predominantly 2, 4 or 6.


Introduction
Dimagnesium(I) compounds LMgMgL, where L is a sterically demanding monoanionic ligand, have been employed as soluble, selective, strong, stoichiometric and safe reducing agents towards a range of organic, organometallic and inorganic substrates. 1 They have, for example, been employed as reductants to afford compounds with magnesium(I), 2 zinc(I) and zinc(0), 3 aluminium(I), 4 and germanium(0) centres, 5 as well as reduced anthracene to its dianion, benzophenone to the ketyl radical, 6 reversibly added to the C]C bonds of selected alkenes, 7 and aer activation reductively coupled CO. 8 Thus, they can be regarded as strong to very strong reducing agents, 9 although the reduction potentials of these reagents have not yet been experimentally determined due to decomposition reactions. 1a Reactions of dimagnesium(I) compounds towards substrates with a known series of reduction potentials could give some indication about the reducing strength of these complexes.
Buckminsterfullerene, C 60 , shows ve degenerate orbitals at the HOMO level of h u symmetry, and three degenerate orbitals at the LUMO (t 1u ) and LUMO+1 (t 1g ) level, respectively. [10][11][12] Fullerenes are generally difficult to oxidise but are readily reduced multiple times. 10, 13 In line with having a triply degenerate LUMO level, C 60 shows six reduction waves from cyclic voltammetry experiments, for example giving E 1/2 of À1.03, À1.44, À1.94, À2.42, À2.91, and À3.28 V relative to Fc 0/+ (approximately À0.38 V vs. SCE or À0.62 V vs. SHE) 14 in toluene/ acetonitrile, 15 with almost equidistant potentials (ca. 0.45 V) between reduction steps. 13 A large number of materials involving fullerene anions (fullerides, fullerenides) have been studied and those with polyanionic fullerides are typically paired with electropositive metals such as alkali metals, alkaline earth metals or selected lanthanoids. 16,17 Alkali metals have especially been used to afford phases such as M 6 C 60 (M ¼ alkali metal) and even some of composition M 12 C 60 , and others with differing metal-to-fullerene ratios. 13,16,17 The C 60 6À anion is a symmetric and diamagnetic species due to the fully lled C 60 LUMO (t 1u ) level. For the rare C 60 12À , the former LUMO+1 (t 1g ) level is lled, 10,13,16,17 although complete charge transfer from the metal to the fulleride cannot be certain in some materials. Some reduced fullerene states offer the possibility of different spin states, e.g. a singlet versus a triplet state for C 60 2À . 12,13 A non-symmetric occupation of these orbital levels can effect a Jahn-Teller distortion in the C 60 framework and this has been observed for all C 60 nÀ ions with 1 # n # 5. This is even the case for n ¼ 3, which could be expected to prefer a symmetric structure with a degenerate t 1u level in a quartet state, but instead forms a slightly distorted species with a lower doublet spin state. 13,16 The M 3 C 60 class materials have received considerable attention because some phases with M ¼ K, Rb, Cs are superconductors. 16,17 Other fulleride materials with s-block metal ions can show structures with C-C bonded polymeric C 60 units that demonstrate superionic conductivity and are of interest for battery materials. 18 In addition to insoluble solid state materials, many well-dened exohedral 19 metal complexes involving metal coordination to neutral fullerenes, fullerides with low charges or fullerene derivatives are known predominantly with transition metals. 20 Soluble alkali metal complexes with polyanionic fullerides have been prepared and structurally characterised, typically using potassium, rubidium or caesium metal involving solution state chemistry in coordinating solvents and, in some cases, the addition of donor ligands such as crown ethers, cryptands etc. [21][22][23] These experiments formed well-dened isolated complexes with fulleride anions up to C 60 4À . 21,22 Furthermore, potassium complexes of C 60 5À have been studied in liquid ammonia 24 and the reaction of lithium metal with C 60 in THF afforded solutions showing a sharp 13 C NMR signal for the C 60 6À anion. 25 In this work we report on the facile formation of soluble fulleride complexes from reactions of dimeric magnesium(I) compounds LMgMgL with C 60 .  (Fig. S41-S46 †), have shown a similar course to those described above for 1c with comparable dominating species 2 (although 2a was not observed), 3 and 4 observed in solution. The different composition in 4a compared to 2b-d as a nal product is likely due to the large steric demand of the Dip group in the series, vide infra. In situ preparations of 2-4 oen afforded high conversions in solution with a single dominant species only that were characterised by NMR spectroscopy. The formation of these complexes were in some instances accompanied by dark precipitates, especially for products with low solubility. These reactions have been found to be considerably faster in toluene compared with benzene, due to the higher solubility of C 60 in toluene. 29 The relatively low solubility of crystalline C 60 , especially in benzene, appears to be one of the main limiting factors for the reaction rates. Finely grinding the fullerene crystals and using sufficiently large solvent volumes accelerate product formation.
Assignment of these series of complexes was conducted by recording 13 Fig. 2 and the ESI. † These molecular structures show the coordination of six ( Ar nacnac)Mg fragments around a central C 60 core with various Mg/C 60 coordination modes. The ( Ar nacnac)Mg coordination to C 60 including their relative positions is presented in colour-coded format in Fig. 3 highlighting h 2 , h 5 , and h 6 coordination modes. Compounds 2b 0 and 2c 0 each crystallised with a full molecule in the asymmetric unit. The most sterically crowded derivative 2b 0 demonstrates that the six ( Ar nacnac)Mg units completely wrap-in the C 60 core, see the space-lling model in Fig. 2 and S60 † for that of 2c 0 . In 2b 0 , all six Mg centres coordinate in a h 5 fashion to ve-membered rings of the fullerene. These ve-membered rings are all adjacent to each other, i.e. only separated by one [6,6] C-C bond each, and form a chiral "ribbon" on the C 60 surface, see Fig. 3. Solvate 2c 0 shows four h 5 Mg/C 60 interactions to ve-membered carbon rings and two h 2 interactions to [5,6] C-C bonds. Two of these Mg centres coordinate to adjacent ve-membered rings. Complex 2c 00 crystallised with two full independent molecules in the asymmetric unit showing identical Mg/C 60 coordination modes. 2c 00 shows four Mg centres coordinate h 5 to vemembered rings, two of these being adjacent to each other, and two coordinate in an h 6 fashion to six-membered carbon  rings. The least sterically shielded derivative 2d 0 crystallised with half a molecule in the asymmetric unit and shows two h 6 , h 5 and h 2 coordination modes each on opposite ends of the C 60 core (Fig. 3). The structure of 2d 0 shows a near-perfect octahedral coordination arrangement of six ( Xyl nacnac)Mg fragments around the central C 60 unit, with orthogonal arrangements of ( Xyl nacnac)Mg ligand planes with respect to neighbouring ligand planes ( Fig. 2 and S63 †).
The molecular structures 2c 0 and 2d 0 show the highest bond precisions in the series, well-ordered fulleride units and thus a more detailed analysis is described here. The Mg-N distances (2.006Å mean in 2c 0 , 2.001Å mean in 2d 0 ) are short, and for example comparable to those of recently reported cationic [( Dip nacnac)Mg$arene] + complexes, 30 and those in [( Dip nacnac) MgPh] with a three-coordinate Mg centre. 31 The shortest Mg-N distances [e.g. Mg(3)-N(47) 1.9805(12)Å and Mg(3)-N(51) 1.9867(12)Å in 2d 0 ] are those for Mg centres involved in h 2 ( Ar nacnac)Mg/C 60 interactions, which supports anticipated ionic Mg/C 60 bonding interactions, vide infra. The Mg/C distances also strongly depend on the coordination mode. For 2d 0 , the h 6 [2.074Å to centroid; Mg-C range: 2.4218 (14) [5,6] bonds (1.444Å mean in 2c 0 , 1.441Å mean in 2d 0 ) are slightly longer than the [6,6] bonds (1.424Å mean in 2c 0 , 1.423Å mean in 2d 0 ); the latter are elongated compared to those in free C 60 . 10,13 However, an overlapping range for individual bonds has been found, for example 1.431(2)-1.456(2)Å plus two outliers for [5,6] bonds, and 1.416(2)-1.441(2)Å for [6,6] bonds in 2c 0 . The two [5,6] outliers are somewhat longer and are the two C-C bonds that show h 2 -coordination to Mg centres [1.469(2)Å to Mg6 and 1.471(2)Å to Mg5]. Similarly, the longest [5,6] bonds in 2d 0 [2 Â 1.4556 (19)Å] are involved in coordinating to Mg centres. The slight lengthening of C-C bonds of fragments that coordinate to Mg centres in comparison to similar uncoordinated fragments is also observed in the mean values of C-C bonds in ve-membered carbon rings with (1.448Å) and without (1.439Å) h 5 Mg coordination in 2c 0 . In 2c 0 two Mg centres (Mg1, Mg2) coordinate to two ve-membered rings that are only separated by one [6,6] bond and lead to a relatively short Mg/Mg separation of ca. 5.69Å. Similar values are found for related motifs in the other molecular structures of 2.  3) ]. The Mg-N bonds (1.975Å average for 3a 0 and 3a 00 ) are short. No close C 60 /C 60 interactions are found in the packing of 3a 0 /3a 00 . In contrast, while the molecular structure of 3b 0 shows similar bond lengths to those found for 3a 0 /3a 00 , the two ( Dep nacnac)Mg units are coordinated to the C 60 core in a more acute fashion as illustrated by comparison between the Mg/  [6,6] and [5,6] C-C bonds (pink). C 60-centre /Mg angles in 3b 0 (ca. 79 ) and 3a 0 /3a 00 (ca. 126 ). This allows for some relatively close C 60 /C 60 interactions from pstacked one-dimensional fulleride chains along the b-axis, arising from short intermolecular C/C interactions (ca. 3.4Å) between slightly offset co-planar ve-membered carbon rings from neighbouring C 60 2À moieties. The fulleride C-C bond lengths in 3a 0 /3a 00 do show a wide range of distances, but also a comparatively low bond precision that does not allow a detailed analysis with respect to differences between, for example, [6,6] and [5,6] bonds, coordination between Mg and fulleride unit or Jahn-Teller distortion. Analysis of the distances between fulleride carbon atoms to the fullerene centre in the C 60 2À complex with the highest data quality, 3a 00 , revealed that 58 of them all lie within 0.02-0.03Å of the mean value (3.54Å) and no clear and signicant pattern for a fulleride distortion emerged. Jahn-Teller distortion in fullerides is relatively small, but can in some cases be observable by structural studies. 13,32 We did, however, observe that the two carbon centres that terminally coordinate the Mg centres are signicantly more pyramidalized, slightly protrude from the fulleride "sphere" and show C/C 60-centroid distances that are 0.09-0.10Å longer than the mean from the remaining 58 distances. Also, the fulleride C-C bond lengths in 3b 0 cannot be analysed due to poor ordering and low bond precision. Complex [{( Dip nacnac)Mg} 4 C 60 ] 4a crystallised with half a molecule in the asymmetric unit and suffers from fulleride disorder, meaning that the Mg/C 60 coordination modes cannot be commented on. The four ( Dip nacnac)Mg units are approximately arranged in a "square planar" fashion around the C 60 core, albeit with signicant tetrahedral distortion. This results in two small (93 mean) and one large (155 mean) Mg/ C 60-centre /Mg angle for each Mg centre (see Fig. S70 †).
The combined analysis of the molecular structures of 2-4 shows that many Mg/C 60 coordination modes (h 1 , h 2 , h 5 , h 6 ) are possible and that all fulleride fragments (carbon atoms, [6,6] bonds, [5,6] bonds, ve-membered rings, six-membered rings) can be involved in bonding to a metal. This, together with bond distance considerations, suggest ionic Mg/C 60 interactions, i.e. that 2-4 are coordination complexes of n [( Ar nacnac)Mg] + with central C 60 nÀ ions. These can be regarded as "inverse coordination compounds" 33 where cationic LMg + "ligands" coordinate to a central C 60 nÀ "superatom". 34 The molecular structure 2d 0 shows a near-perfect octahedral coordination mode of six [( Xyl nacnac)Mg] + around the C 60 6À unit with an orthogonal arrangements of ligand planes to those of their neighbours. This arrangement is comparable in overall structure to, for example, those in the transition metal complexes M(NMe 2 ) 6 (M ¼ Mo, W) with inverted charges. 35 Other coordination geometries can be regarded as distorted octahedral (2), distorted square-planar (4a) and bent (3). Mg} 2 ] 1b. This is in line with the highly crowded ligand sphere in the molecular structure of 2b. Recording spectra of 2b immediately aer dissolution showed the highest concentration of 2b, followed by growing resonances for 5b, then 4b ( Fig. S6-S11 †). Trace quantities of 1b were also found in these spectra though these do not account for all the ( Dep nacnac)Mg fragments removed from 2b. For comparison, [{( Ar nacnac) Mg} 2 ] 1 forms equilibria with selected alkenes and alkynes such as 1,1-diphenylethene to novel 1,2-dimagnesioethane species, 7 and a related equilibrium process may be in operation for 2b, plus follow-on reactivity forming 5b, 4b, 1b and other species. 1 H NMR spectra for [{( Ar nacnac)Mg} 2 C 60 ] 3 (Fig. S2, S16, S18, S31 and S37 †) and [{( Ar nacnac)Mg} 4 C 60 ] 4 ( Fig. S4 ] 4b, were recorded at À80 C in deuterated toluene (Fig. S12-S15, S20 and S21 †) and all support symmetric solution behaviour at this temperature. 1 H NMR data show as expected one ligand backbone CH resonance for each complex, though broader resonances indicate a more hindered rotation of the ethyl groups in the ligand sphere and/or a reduced solubility. The low temperature 13 C { 1 H} NMR spectra for 2b, 3b and 4b all show a fulleride resonance that is essentially unchanged with respect to its room temperature chemical shi. Also, the sharp appearance of the 13 C{ 1 H} NMR fulleride singlet of 3b and the broad signal shape for 4b are virtually unchanged when compared to their room temperature spectra and support highly uxional behaviour at this temperature. The 13 C{ 1 H} NMR fulleride resonance of 2b, however, is broadened or starts to split into individual resonances (Fig. S13 †). This could have several reasons and could signify splitting of fulleride resonances between different coordination modes and/or coordinated versus uncoordinated carbon centres amplied by the extreme ligand crowding in the complex, or simply result from the low solubility under these experimental conditions. The 13 C NMR fulleride data found for complexes 2 is very close to those reported for M 6 C 60 (M ¼ alkali metal cations) solid state materials. 13,16,36 The data deviates signicantly for complexes 3 (by ca. 25-30 ppm) and 4 (by ca. 15-20 ppm) compared to those reported for related solid state materials and soluble ionic fulleride complexes in coordinating solvents. These generally show 13 C NMR chemical shis for C 60 2À and C 60 4À species of approximately 180-185 ppm; species with C 60 À and C 60 3À ions can show even further downeld shis. 13,24b In general it has been suggested that a larger downeld shi can be associated with a higher paramagnetism in fullerides, 13 and that species with the same number of unpaired electrons may show 13 C NMR chemical shi in a similar region. 13,16 Comparisons with 13 C NMR chemical shis available for solid state materials may be complicated due to contributions from the conduction electrons (Knight shis) and magnetic coupling between fulleride anions with close contacts in the solid state. 13 Comparisons to data from solid state materials or from species in coordinating solvents may in some cases be difficult due to possible signal averaging from exchange reactions between fullerides of different charge states. 10, 13 The fulleride complexes reported herein, at least those with higher charges such as 2 and 4, likely suppress direct C 60 nÀ /C 60 nÀ interactions; preventing close approach by the large organic ligand sphere as an outer separating layer (Fig. 2). This, together with the use of noncoordinating hydrocarbon solvents, will also likely suppress electron exchange reactions such as disproportionations. complexes with uneven numbers of n are inevitably paramagnetic species where issues such as signicant signal broadening and shiing of resonances in NMR spectra can be expected. For species with 2 # n # 4 several spin states are possible such as high and low spin congurations. 10,12, 13 We used Evan's method 37 to shed further light on the number of unpaired electrons and the dominating spin state for these species in solution (Fig. S43-S49 †). Although we could not yet study solutions of [{( Ar nacnac)Mg} 2 C 60 ] 3 or [{( Ar nacnac) Mg} 4 C 60 ] 4 that were completely free of impurities, and these fullerides may be in equilibrium with others of different charges as is known for fullerides in coordinating solvents, these experiments indicated a negligible to very low magnetic moment in solution (e.g. Fig. S47 and S48 †). This suggests that these species are dominated by a diamagnetic ground state and that all electrons are paired and/or antiferromagnetically coupled in an open shell singlet state. Previous studies on C 60 2À species suggest close singlet and triplet states that are partially populated; data for C 60 3À is consistent with a doublet state (typically described as a low spin species), and a triplet state is suggested for C 60 4À . 13,22,38 A multicongurational description for some species may be warranted and results in a complicated picture. The possibility that C 60 2À complexes 3 and C 60 4À complexes 4 are essentially diamagnetic suggests that these complexes could be in different electronic states to those of known ionic fulleride species, and requires further investigation. In contrast and for comparison, mixtures showing resonances for the tentatively assigned species [{( Dep nacnac) Mg} 5 C 60 ] 5b, which has to be a paramagnetic species, show a large relative paramagnetism from similar experiments in solution (Fig. S46a †). A diamagnetic state for 3 and 4 could furthermore explain the signicant difference in their 13 C NMR fulleride chemical shis to those of related reported materials with signicant upeld shi; especially the resonances for 3 which are shied by 25-30 ppm compared with known materials and are very close to those of diamagnetic C 60 6À in 2. The further downeld shi and fulleride signal broadening of C 60 4À species 4 could hint at a slightly higher average paramagnetic character despite only very small solution magnetic moments or hint at an open shell singlet state. In this context it is worth noting that the fulleride 13 C chemical shis and line shapes for 3b and 4b were essentially unchanged when data from room temperature and À80 C experiments were compared. All fulleride complexes observed herein are different shades of deep brown in solution. Complexes 2 show an orange-brown colour in solution, complexes 3 show a more green-brown colour. UV/Vis spectra of 2c (Fig. S1 †) recorded in hydrocarbon solution (n-hexane or toluene) show some absorption across the visible spectrum with an absorption peak at 428 nm (3 z 12 800 mol À1 dm 3 cm À1 ) and a rise in absorption towards the NIR region between 700 and 800 nm; the latter being the limit of the experiment. The absorption maximum is in line with an orange tint and the rise in absorption at longer wavelengths contributes to the overall brown colour.

Computational studies
Density functional theory (DFT) studies at the pbe0/def2-svp level of theory on the model complex [{( Me nacnac)Mg} 6 C 60 ] 2Me ( Me nacnac ¼ HC(MeCNMe) 2 ), C 60 , C 60 6À and [( Me nacnac) Mg] + , were carried out to shed further light on the bonding situation of complexes 2b-d. Four isomers were optimised from core coordinates of X-ray solid state structures (Fig. S71-S74 †); 2Me-1 (using 2c 0 as a starting geometry), 2Me-2 (from 2b 0 ), 2Me-3 (from 2c 00 ) and 2Me-4 (from 2d 0 ). In each case, the [( Me nacnac) Mg] + groups underwent some repositioning in part due to the smaller steric demand of the ligand sphere in isomers of 2Me. Isomers 2Me-1 to 2Me-3 lie within 3 kcal mol À1 of one another and show a combination of h 5 and h 2 Mg coordination modes (Table S2 †), whereas isomer 2Me-4 is ca. +5 kcal mol À1 above the lowest isomer 2Me-3 and shows one terminal (h 1 ) Mg coordination mode not found in the solid state structures of 2. h 6 Mg coordination modes, as observed in structures 2c 00 and 2d 0 , were not found in isomers of 2Me. The fulleride C-C bond lengths in isomers of 2Me show variations within the [5,6] and [6,6] bonds and are affected by Mg coordination. Bond lengths for h 2 Mg coordinated C-C bonds are ca. 0.02Å longer for [5,6] bonds in 2Me-1, and ca. 0.04Å longer for [6,6] bonds in 2Me-4, compared to uncoordinated examples in the same respective optimised structure, and supports the trend found in the solid state structures of 2. Isomer 2Me-2 contains six ve-membered carbon rings coordinated to Mg centres and six not coordinated to Mg centres. The former C-C bond lengths (mean 1.445 A) are on average slightly longer than the latter ones (mean 1.438Å) and these values are highly comparable to those obtained from the mean measured bond lengths in 2c 0 , cf. 1.448Å and 1.439Å, respectively. The sum of the three C-C-C angles around the carbon atom with h 1 Mg coordination (ca. 341 ) in 2Me-4 is ca. 6 smaller than the mean from comparable uncoordinated carbon atoms in this isomer showing the trend towards pyramidalization and echoes the ndings for the carbon atoms with h 1 Mg coordination in the solid state structure of the C 60 2À species 3a 00 .
A partial molecular orbital diagram of 2Me-1 (Fig. 5) supports the formulation of a complex with a central C 60 6À ion.
The frontier orbital HOMO and LUMO levels of 2Me-1 are entirely C 60 based and largely resemble those of C 60 6À with three occupied (t 1u in Hückel theory) and three unoccupied (t 1g ) Fig. 5 Partial molecular orbital diagram for 2Me-1 (pbe0/def2-svp) showing selected energy levels and molecular orbitals. Orbitals for filled levels are shown in blue and red, and in pink and yellow for unoccupied orbitals (isovalue 0.02 e a.u. À3 ). molecular orbitals. 10-12 The HOMO-LUMO gap in 2Me-1 is small (1.64 eV, 158 kJ mol À1 , 37.8 kcal mol À1 ) and similar to that calculated for C 60 6À (1.78 eV) at the same DFT level (Fig. S77 †).
Below the HOMO level in 2Me-1 is a band of eleven orbitals that appear to originate from mixing of ve C 60 h u orbitals (the former C 60 HOMO level or the C 60 6À HOMO-1 level) and six [( Me nacnac)Mg] + HOMO orbitals (Fig. S78 †). The band of nine orbitals below are C 60 based and largely represent the four g g and ve h g orbitals that occur at almost identical energy in C 60 or C 60 6À , respectively (e.g. Fig. S76 and S77 †). Approximately 1 eV above the LUMO level of 2Me-1 starts a band of 14 orbitals that appear to originate from mixing eight C 60 based orbitals (5 h g and 3 t 2u ) with six [( Me nacnac)Mg] + LUMO+1 orbitals (Fig. S75 †). The gap between HOMO and LUMO+1 level approximately corresponds in energy to the UV absorption at 428 nm and the rise in absorption around 700-800 nm to the HOMO-LUMO gap. In relation to the relatively small HOMO-LUMO gap determined for 2Me-1 it is worth mentioning that the C 60 6À anion shows a signicantly higher aromatic character than C 60 with strong diatropic ring currents and a huge endohedral shielding as has been determined for 3 He@C 60 6À using 3 He NMR spectroscopy. 10,11,25 The calculated Natural Bond Orbital charges for the isomers of 2Me show some variation of charges on C 60 carbon atoms that depend on nearby Mg coordination. In general, low hapticity Mg coordination leads to more localised anionic charge. For example, the terminal (h 1 ) carbon coordinated to Mg in 2Me-4 carries a charge of À0.41; its three direct neighbours only show an average of À0.06. In 2Me-3, the average charge of an h 2 coordinated carbon atom is ca. À0.28 and its four neighbours have a signicantly lower charge of ca. À0.02. Less polarisation of the central C 60 unit is found around h 5 coordination modes. This charge accumulation in Mg-coordinated fragments is accompanied by the previously discussed C-C bond lengthening and pyramidalization of carbon centres. For comparison, the expected average charges of 0.00 for C 60 and À0.10 for C 60 6À with very little to no variation are found. The Mg ions in 2Me show a charge of approximately +1.77, similar to that in [( Me nacnac)Mg] + . The sum of charges on C 60 of À5.8 in 2Me-3 is close to the ideal value of À6 for a wholly ionic system, and supports the view as a contact ion complex with a central C 60 6À ion coordinating to six [( Me nacnac)Mg] + ions. Similarly, the electrostatic potential visualises this situation (Fig. 6), and highlights the difference in potential between the highly anionic fulleride unit (red) and positive [( Me nacnac)Mg] + fragments (blue sections), especially the strongly positive Mg 2+ centres, that help balance and dissipate the accumulated charges by coordination. In order to further probe the nature of the interactions in complexes 2, we reacted [{( Mes nacnac)Mg} 6 C 60 ] 2c with six equivalents of [( Xyl nacnac)Li] and observed the very rapid formation of the magnesium(II) complex [( Mes nacnac)Mg( Xylnacnac)] 6, as suggested by 1 H, 7 Li and 13 C{ 1 H} NMR spectroscopy ( Fig. S52-S56 †), and the formation of an insoluble dark orange-brown precipitate, likely Li 6 C 60 . The rapid and facile breaking of the {( Mes nacnac)Mg} + /C 60 6À coordination bonds in this reaction is further support for the exible, ionic nature of these interactions and suggests that complexes 2 can serve as soluble sources of C 60 6À .

Conclusion
Dimagnesium(I) complexes LMgMgL, L ¼ Ar nacnac, react with C 60 to form a range of hydrocarbon-soluble fulleride complexes of the general formula [(LMg) n C 60 ] and can reduce C 60 up to its hexaanion if sterics permit. The combined experimental and computational studies for n ¼ 6 support the formulation as an inverse coordination complex with exible ionic LMg + to C 60 6À interactions, and a small HOMO-LUMO gap. The Mg/C 60 interactions can show a wide range of coordination modes that are easily distorted and represent relatively weak electrostatic interactions. The anions C 60 2À , C 60 4À and C 60 6À were found to be dominant species and were characterised in the solution and solid state. C 60 6À complexes 2 are as expected diamagnetic and indication for C 60 2À in complexes 3 and C 60 4À in complexes 4 suggest a dominating diamagnetic ground state. The ease of reducing C 60 to its hexaanion suggests that the reducing capabilities of LMgMgL compounds towards substrates are at least in the order of E 0 # À2.9 V (vs. SCE) or #À2.65 V (vs. SHE) and thus can be regarded as very strong reducing agents. 9 This can approximately be compared to the reported reduction potentials of Mg 0/I (Mg + + e À / Mg: E 0 ¼ À2.70 V vs. SHE; cf. Mg 2+ + 2 e À / Mg: E 0 ¼ À2.372 V vs. SHE) and Na metal (Na + + e À / Na: E 0 ¼ À2.71 V vs. SHE); 39 the latter can be used as a reducing agent in preparing LMgMgL complexes. In addition to thermodynamic considerations such as reduction potentials, kinetic factors, for example an appropriate set of sterics and a suitable mechanism, are important in the chemistry of LMgMgL complexes.

Conflicts of interest
The authors declare no conict of interest.