The borylamino-diborata-allyl anion

Reactions of β-diketiminato alkaline earth alkyldiboranate derivatives [(BDI)Ae{pinBB(R)pin}] (BDI = HC{(Me)CNDipp}2; Dipp = 2,6-i-Pr2C6H3; Ae = Mg, R = n-Bu or Ae = Ca, R = n-hexyl) with t-BuNC provide access to the respective group 2 derivatives of unprecedented diborata-allyl, {(pinB)2CNBpin(t-Bu)}−, anions. Although the necessary mode of B–C bond cleavage implicated in these transformations could not be elucidated, further studies of the reactivity of magnesium triboranates toward isonitriles delivered a more general and rational synthetic access to analogous anionic moieties. Extending this latter reactivity to a less symmetric triboranate variant also provided an isomeric Mg–C-bonded dibora-alkyl species and sufficient experimental insight to prompt theoretical evaluation of this reactivity. DFT calculations, thus, support a reaction pathway predicated on initial RNC attack at a peripheral boron centre and the intermediacy of such dibora-alkyl intermediates.


Crystallographic Details
Single Crystal X-ray diffraction data were collected on a SuperNova, EosS2 diffractometer using CuKα (λ = 1.54184Å) radiation, except for 8 which were collected on an Xcalibur, EosS2 diffractometer using MoKα (λ = 0.71073 Å) radiation.Samples were universally maintained at 150 K during data collection.Using Olex2, 4 the structures were solved with the olex2.solve 5 structure solution program or ShelXT and refined with the ShelXL 6 refinement package using Least−Squares minimisation.
In the structure of 3, the asymmetric unit comprises one molecule of the magnesium complex and a region of solvent.The isopropyl group in the main feature, based on C24, was modelled to take account of 55:45 disorder while C36-C41 were treated for a 75:25 split.The solvent moiety is a toluene molecule with half site-occupancy, that is disordered with itself, about a crystallographic inversion centre.Distance, planarity and ADP restraints were used in the disordered regions (on merit) to attain a chemically sensible convergence.
The asymmetric unit in 4 was host to one molecule of toluene in addition to one molecule of the calcium complex.The solvent was disordered in a 50:50 ratio over two very proximate sites, and was successfully refined with the addition of distance and ADP restraints.
One molecule of benzene in addition to one molecule of the magnesium complex constitute the asymmetric unit in 7.
In 8, the asymmetric unit plays host to one molecule of the magnesium complex and half of a molecule of hexane.The latter lies proximate to a crystallographic inversion centre which serves to generate the remainder.The solvent content was modelled to take account of 85:15 disorder with the inclusion of appropriate distance and ADP restraints. -S19-

Computational Details
DFT calculations were run with Gaussian 16 (C.01). 7The Mg centres were described with the Stuttgart RECPs and associated basis sets, 8 and the 6-31G** basis set was used for all other atoms (BS1). 9Initial BP86 optimisations were performed using the 'grid = ultrafine' option, 10 with all stationary points being fully characterised via analytical frequency calculations as minima or transition states (all positive eigenvalues or one imaginary eigenvalue respectively).
All energies were recomputed with a larger basis set featuring 6-311++G** basis sets on all atoms (BS2).Corrections for the effect of benzene (ε = 2.2706) solvent were run using the polarisable continuum model and BS1. 11Single-point dispersion corrections to the BP86 results employed Grimme's D3 parameter set with Becke-Johnson damping as implemented in Gaussian. 12Please note that the structure and energetics of I has already been reported previously. 1

Breakdown of energy contributions
The following table details the evolution of the relative energies as the successive corrections to the initial SCF energy are included.

Table S1 :
Crystal Data and Structural Refinement for Compounds 3

Table S2 .
Terms used are: Relative energies (kcal mol -1 ) for computed structures.Data in bold are those used in the main text.All energies are quoted relative to I at 0.0 kcal/mol.       /     /    +        Free energy profile (calculated with DFT at the BP86-D3BJ(PCM=Benzene)/BS2//BS2586/BS1 level of theory, energies in kcal mol -1 ) of Figure S25.The HOMO of compound 3, which predominantly features B-C-B -bonding character, computed at the BP86-D3BJ/6-311++G**//BP86/BS1 level of theory.Mg B B C N Figure S26.t BuNC to I to form PM. Inset: Relative difference in free energies between Vneo (crystalographically observed) and Pneo.