Cooperative dihydrogen activation at a Na( I ) 2 / Mg( I ) 2 ensemble

[{SiN Dipp }MgNa] 2 ({SiN


Na 3s
ð Þ interactions between the frontier MOs of both H 2 and the tetrametallic core of The binding and cleavage of the strong (436 kJ mol À1 ) H-H s-bond of dihydrogen at mid and late transition metal centres provided a fulcrum for both a fundamental understanding of d-block complex reactivity and the development of numerous catalytic and stoichiometric processes. 1,2This chemistry is enabled by the narrowly spread manifold of nd valence orbital energies involved in the synergic activation of the H 2 molecule (Fig. 1a).
As articulated in Power's influential review of 2010, 3 typical s-and p-block compounds do not generally display the modest separation in valence orbital energies (r4 eV) required to effect similar interactions.5][6][7] Although semantics might identify frustrated Lewis pairs (FLPs) as a separate sub-category of main group chemistry, 8,9 a commonly considered mode of H 2 activation (Fig. 1d) attributes a significant degree of complementarity. 10From this viewpoint, therefore, the reactivity of an FLP toward dihydrogen may be rationalised in an analogous manner, albeit one in which the frontier orbitals are introduced as spatially separated basic (HOMO) and acidic (LUMO) components.
Inspired in part by the synthesis of 4, we have described the topologically-related, [{SiN Dipp }MgNa] 2 (6; [{SiN Dipp } = {CH 2 Si-Me 2 N(Dipp)} 2 ; Dipp = 2,6-i-Pr 2 C 6 H 3 )] (Fig. 2). 16Although 6 bears a resemblance to the previously reported species of Yang and co-workers (e.g.7), 17 the redox-innocence of the {SiN Dipp } ligands and the N-aryl encapsulation of the sodium cations provides significant electronic discrimination.This is reflected in the structure of 6 through an elongation of the Mg-Mg bond [3.2124(11)Å (6) versus 2.9370(18) Å ( 7)].Computational (NBO, QTAIM) analysis also identified a degree of electronic cooperativity between the magnesium and sodium centres.This manifests as a pronounced yellow colour arising from an absorption at 409 nm (3.0 eV) attributed to a transition between the Mg-Mg s-bond (HOMO), arising from overlap of the magnesium 3s wavefunctions, and a LUMO largely represented by an out-ofphase combination of the sodium 3s atomic orbitals (Fig. 3a). 18he behaviour of 6 supports an interpretation of its bonding as a tetrametallic ensemble.Most strikingly, treatment with non-reducible bases such as THF (Fig. 3b) results in the selective extrusion of metallic sodium and oxidation of the Mg(I) centres to the more conventional Mg(II) state. 18Although these processes are also characterised by a structural reorganisation of the chelated diamide spectator ligand to form the macrocyclic species 8, computational studies indicated that intramolecular electron transfer is expedited via the Mg(I)derived HOMO and Na(I)-derived LUMO.The narrow separation in energy between these frontier orbitals (ca. 3 eV) is more reminiscent of the low oxidation state p-block species (typically 2-3 eV) depicted in Fig. 1b and c than previously reported Mg-Mg bonded derivatives (Z4 eV). 12With these observations in mind, therefore, it was speculated that the alkali metalcentred frontier orbitals of 6 may also facilitate a cooperative reaction with H 2 .
Although treatment of a degassed benzene solution of 6 with H 2 (2 bar) provided no evidence of an observable reaction over 12 hours at room (ca. 25 1C) temperature, monitoring by 1 H NMR spectroscopy indicated conversion to a predominant new compound (9) was induced after heating at 40 1C for 3 days.This process occurred with the formation of a black metallic precipitate.Compound 9 was isolated as colourless single crystals from hexane at room temperature.The resultant X-ray diffraction analysis (Fig. 4) revealed that compound 9 is a centrosymmetric heterobimetallic hydride species.The hydride ligands of 9 were located and refined with a riding U iso value to be trigonally encapsulated by a still chelated diamidomagnesium centre and two sodium cations.The sodium atoms are bound in a Z 6 -fashion by each of the Dipp substituents of the chelated {SiN Dipp } ligands, but are differentiated by their interactions with an additional diamide dianion that now adopts a {Na2-m-k 1 -N,m-k 1 -N 0 -Na2 0 } bridging mode reminiscent of those to Mg in the macrocyclic species 8. 18 While Na1/Na1 1 engage via further polyhapto interactions with C31-C36 (and C31 1 -C36 1 ) comprising the Dipp substituents of the bridging dianion, the coordination spheres of Na2/Na2 1 are completed by N3/N3 1 .The Mg1-N1 [1.9795( 13) Å] and Mg1-N2 [1.9711( 14) Å] bonds of 9 are significantly shorter than the Mg-N distances observed in 6 (avg.2.08 Å), 16 consistent with the oxidation of Mg(I) to Mg(II).While several amido-derived Na/Mg hydrides have been reported to result from either b-C-H elimination or metal amide/Si-H metathesis, 19 compound 9 is the first such species in which the hydride ligands arise from the direct activation of dihydrogen.Although its presence could not be identified by 1 H NMR spectroscopy in d 6 -benzene solution, dihydrogen was confirmed as the source of the hydride ligands of 9 by performance of a further reaction of 6 with D 2 .This latter process provided similar observations and resulted in the isolation of 9-d 2 , which was characterised by a singlet signal at d 4.16 ppm as the sole observable resonance in its 2 H NMR spectrum in benzene.
Acid digestion and quantitative analysis by ICP-OES revealed that the solid residue deposited during the reaction of 6 with H 2 comprised magnesium as the sole constituent s-block metal  with levels of sodium below the detection limit of the technique (see ESI †).Furthermore, the quantity of magnesium was consistent with the reaction stoichiometry shown in eqn (1), which we surmise results from the formal disproportionation of the Mg(I) centres of compound 6.
DFT calculations (BP86-D3BJ/BS2(benzene)//BP86/BS1 level of theory, see the ESI † for full details) were performed to assess the kinetics of H 2 addition to 6 (denoted as I in the computational study) and the structure of the resulting H 2 adduct.Initial H 2 addition was identified to take place via TS(I-II) and a barrier of +18.5 kcal mol À1 to form II (+14.1 kcal mol À1 ).Subsequent H 2 reorientation via a low-lying saddle point, TS(II-III) (+13.6 kcal mol À1 ), affords a more stable adduct, III (+11.0 kcal mol À1 ), in which a terminus of the H 2 molecule is directed towards the Mg-Mg s bond (Fig. 5a). 20BO-based donor-acceptor interaction analysis of III reveals two appreciable interactions between H 2 and the tetrametallic Mg 2 Na 2 unit; a s MgÀMg !s Ã HÀH interaction (DE (2) = 9.2 kcal mol À1 ; Fig. 4b) supplemented by a subtle but still significant engagement via s HÀH !n Ã
While we cannot yet discount alternative polarised metathesis or radical-based pathways, 11,14 these deductions infer that the frontier orbital interactions invoked in the initial coordination of H 2 to 6 bear some analogy to those of the generalised d-and p-block-derived systems depicted in Fig. 1.The semiheterogeneous nature of the Mg(0) extrusion process and the structural complexity of 9, however, dictate that the onward process does not lend itself to further prudent mechanistic analysis.
These combined experimental and theoretical results imply that the H 2 activation process invoked through its interaction with 6 may be rationalised as a consequence of the frontier orbitals arising from the low oxidation state assembly of the two dissimilar s-block elements (Fig. 3a).This perspective serves to further discriminate the chemistry of the low oxidation state heterobimetallic species, 6, from that so far deduced for the previously described Mg-Mg bonded Mg(I) derivatives (e.g. Similarly, the conceptual framework provided by the intermediate III signposts a potential ability to manipulate the frontier orbitals of related systems toward even more challenging small molecule transformations through the  templated assembly of further low oxidation state arrays of dissimilar s-block element centres.We are continuing to explore these possibilities with a broader scope of complex types, metal identities and small molecule substrates.
HYL performed the synthesis and characterisation of the new compounds reported.MSH and CLM conceptualised the study and finalised the manuscript for submission.SEN and BLM performed the computational analysis and MFM finalised the X-ray diffraction analysis of 9 for publication.

Fig. 3
Fig. 3 (a) Representations of the calculated frontier molecular orbitals of 6; (b) reactivity of compound 6 toward THF to form 8 and elemental sodium.