Rubidium and caesium aluminyls: synthesis, structures and reactivity in C–H bond activation of benzene†

Expanding knowledge of low valent aluminium chemistry, rubidium and caesium aluminyls are reported to complete the group 1 (Li-Cs) set of metal aluminyls. Both compounds crystallize as a contacted dimeric pair supported by M⋯π(arene) interactions with a pronounced twist between aluminyl units. Density functional theory calculations show symmetrical bonding between the M and Al atoms, with an Al centred lone-pair donating into vacant Rb and Cs orbitals. Interestingly, despite their structural similarity the Cs aluminyl enables C-H bond activation of benzene, but not the Rb aluminyl reflecting the importance of the alkali metal in these heterobimetallic systems.

A recent review highlighted the growing recognition of alkali metal mediation, AMM, that underpins a diversity of applications in main group organometallic chemistry. AMM can be defined as chemical transformations that cannot occur at all or cannot take place efficiently, without intervention of an alkali metal. 1 Often AMM is manifest in bimetallic formulations that contain an alkali metal partnered by a second metal, the unique properties of which stem from cooperative effects between the two metals. 2,3 One emerging area that is benefitting from AMM is low valent aluminium chemistry, specifically the new class of aluminyl anions, [Al{L} n ] À ({L} n = bidentate, dianionic ancillary ligand framework). 4 Access to these highly reactive compounds is principally achieved via reduction of a suitably supported aluminium(III) precursor using potassium metal 5,6 or KC 8 , 7 while syntheses from Al(II) [8][9][10] and Al(I) 11 starting materials are also known. In most cases the potassium plays a dual role in this process, acting as both an effective reagent for the reduction of Al(III) to Al(I) and as an integral stabilising component of the potassium aluminyl product, K[Al{L} n ].
Aluminyls may be classified according to their structure in a manner that is correlated with the nature and extent of MÁÁÁAl interactions (M = Li, Na, K). This is illustrated for the [Al(NON Dipp )] À system, where three distinct structural types are known for the potassium salts (Fig. 1). 12 Reduction of the aluminium(III) iodide Al(NON Dipp )I with potassium metal affords the donor-solvent free contacted dimeric pair (CDP) I. 5 This structure type is common for aluminyls in which arylsubstituents are present that allow KÁ Á Áp(arene) interactions. [5][6][7]11 Recent work has shown that the dimeric structure of I can be cleaved using TMEDA to afford a monomeric ionic pair (MIP) II, a structure related to that observed for a dialkyl-substituted aluminyl. 8 Furthermore, encapsulation of the potassium cation in I using [2.2.2]cryptand affords the separated ion pair (SIP) III, 10,12 representing a rare example of this motif in aluminyl chemistry where no interaction exists between aluminium and potassium. 9,10,13 Far from being superficial, these distinct structural types can impart profound differences on the chemical reactivity of aluminyls, demonstrating the importance of AMM in this area. This is best illustrated with the contrasting reactivity of the xanthene supported system [Al( Xanth NON Dipp )] À ( Xanth NON Dipp = [4,5-(NDipp) 2 -2,7-tBu 2 -9,9-Me 2 -xanthene] 2À ) with benzene. It was shown that the potassium CDP thermally activated a C-H bond, 7 whereas the corresponding [K(2.2.2)crypt] + SIP reversibly cleaved a C-C bond of the aromatic ring to afford the sevenmembered AlC 6 H 6 metallacycle. 13 We have recently extended this field to show that the [Al(NON Dipp )] À aluminyl anion can be accessed directly from reduction of Al(III) iodide using lithium and sodium metal. 14 The products isolated from non-coordinating solvent exist as 'slipped' CDPs (Scheme 1, IV-Li and IV-Na), with DFT studies confirming only one bond from aluminium to the alkali metal. Furthermore, we demonstrated that addition of Et 2 O cleaved the CDP to afford the corresponding solvated MIPs V-Li and V-Na, containing unsupported Al-Li and Al-Na bonds. 15,16 In the knowledge that systematic studies spanning the whole of group 1 (Li-Cs) are still relatively rare, 17 and that often substitution of one alkali metal for another can have a profound effect on structure and reactivity, in this contribution we complete the series of alkali metal aluminyls (Li-Cs), with the first report of rubidium and caesium aluminyls, [M{Al(NON Dipp )}] 2 .
To access the rubidium and caesium aluminyls, Al(NON Dipp )I was reduced in C 6 H 6 with RbC 8 and CsC 8 , respectively (Scheme 2). In each case, the initially colourless solution turned yellow after stirring at room temperature overnight. The reaction proceeded smoothly and the 1 H NMR spectra of the products from Rb (1) and Cs (2) reductions compared well to that of their lighter congeners [M{Al(NON Dipp )}] 2 (M = Li, Na, K), 5,14 showing a high-field singlet for the SiMe 2 groups (0.36 ppm) consistent with C 2h -symmetry ( Fig. S1 and S3, ESI †). As for all other aluminyl systems reported to date, no signals were observed in the 27 Al NMR spectra. The diffusion coefficients D of 1 (4.35 Â 10 À10 m 2 s À1 ) and 2 (5.60 Â 10 À10 m 2 s À1 ) obtained by 1 H diffusion-ordered NMR spectroscopy (DOSY, C 6 D 6 , 294 K) are in the same range as those of the Li (5.32 Â 10 À10 m 2 s À1 ), Na (4.60 Â 10 À10 m 2 s À1 ) and K (4.48 Â 10 À10 m 2 s À1 ) congeners. Both values are lower than that of the monomeric Al(III) iodide I (6.14 Â 10 À10 m 2 s À1 ) indicating that the CPD structure is retained in aromatic solvents.
Crystals suitable for X-ray crystallography were obtained by slowly cooling a saturated hexane solution from 60 1C to 5 1C overnight (1, M = Rb; 2, M = Cs). Both compounds crystallize as the centrosymmetric CDP ( Fig. 2 and Table 1)    This twisting also enables short contacts between the Al and the corresponding alkali metal, with mean values of 3.733 Å for Rb and for Cs 3.899 Å, which are both slightly larger than the sum of covalent radii (S cov (Al, Rb) = 3.41; S cov (Al, Cs) = 3.65). 19 Next we examined the nature of the bonding in 1 and 2 using DFT calculations and compared the results with known CDPs M 2 [Al(NON Dipp )] 2 (M = Li, Na, K). The bond parameters of the optimized structures were in good agreement with the X-ray diffraction data, with calculated twist angles y of 67.521 and 69.691 for the Rb and Cs CDPs, respectively. QTAIM analysis identified bond critical points (BCPs) (r(r) = 0.0081) between each Al and the alkali metal centres (Fig. 3), confirming the presence of four AlÁ Á ÁM bonding interactions in the twisted CDPs. This is consistent with [K{Al(NON Dipp )}] 2 and in contrast to M = Li and Na for which only one AlÁÁÁM BCP, albeit with higher covalent character, was present per Al. These bonding interactions in 1 and 2 are also implied from the Al-M Wiberg bond indices of 0.1327 (1) and 0.1430 (2) (Table S7, ESI †). As expected, and noted for the other members of the series, 14 the MÁÁÁAl bonding is essentially non-covalent in both CDPs, 23,24 with a lower degree of covalency for the heavier alkali metals (ÀG(r)/V(r) ratios: Cs 1.06, Rb 1.08, K 1.06, Na 0.98, Li 0.88). 25 The Laplacian of the electron densities for the Al 2 M 2 core of 1 and 2 indicate areas of charge accumulation at aluminium, suggesting the presence of lone-pairs of electrons located at each Al centre. Indeed, NBO analysis identifies two NBOs which possess dominant lone-pair character on each Al centre in 1 and 2, each with significantly more s-orbital character over p-orbital character (1:83.1% s, 16.9% p; 2:84.1% s, 15.9% p). These interact with vacant Rb and Cs NBOs in 1 and 2, respectively, where second-order perturbation energy analysis identifies moderate donor-acceptor interactions (DE (2) Rb-Al [1] E 10.2 kcal mol À1 , DE (2) Cs-Al [2] E 8.3 kcal mol À1 ). The natural atomic charges of the alkali metal atoms generally increase as the group is descended (Tables S3 and S4, ESI †), reflecting the lower electronegativity of the larger elements.
AMM is clearly present in the reaction between H 2 and [M{Al(NON Dipp )}] 2 (M = Li, Na, K) since hydrogenation (100 1C, 1.5 bar) proceeded in the order Li (t 1/2 = 1.5 days) c Na (t 1/2 = 6 days) 4 K (t 1/2 = 12 days). The hard lithium seems to be a perfect activator for H 2 , whereas descending the group the alkali metals become softer and thus reaction times increase.