Aluminum–Boron Bond Formation by Boron Ester Oxidative Addition at an Alumanyl Anion

The potassium diamidoalumanyl, [K{Al(SiNDipp)}]2 (SiNDipp = {CH2SiMe2NDipp}2), reacts with the terminal B–O bonds of pinacolato boron esters, ROBpin (R = Me, i-Pr), and B(OMe)3 to provide potsassium (alkoxy)borylaluminate derivatives, [K{Al(SiNDipp)(OR)(Bpin)}]n (R = Me, n = 2; R = i-Pr, n = ∞) and [K{Al(SiNDipp)(OMe)(B(OMe)2)}]∞, comprising Al–B σ bonds. An initial assay of the reactivity of these species with the heteroallene molecules, N,N′-diisopropylcarbodiimide and CO2, highlights the kinetic inaccessibility of their Al–B bonds; only decomposition at high temperature is observed with the carbodiimide, whereas CO2 preferentially inserts into the Al–O bond of [K{Al(SiNDipp)(OMe)(Bpin)}]2 to provide a dimeric methyl carbonate species. Treatment of the acyclic dimethoxyboryl species, however, successfully liberates a terminal alumaboronic ester featuring trigonal N2Al–BO2 coordination environments at both boron and aluminum.


Single Crystal X-ray Diffraction Analysis
Single Crystal X-ray diffraction data for compounds 15 -20 were collected on a SuperNova EosS2 diffractometer using CuKα (λ = 1.54184Å) radiation.The crystals were maintained at 150 K during data collection.Using Olex2, 1 the structures were solved with the olex2.solve 2 structure solution program or ShelXT and refined with the ShelXL 3 refinement package using Least Squares minimisation.Noteworthy points follow, and where disorder has been modelled, both distance and ADP restraints have been employed, on merit, in these regions to assist convergence.
In the structure of 15, the asymmetric unit contains a monomer (which contributes to a dimer the remainder of which is completed via inversion symmetry) plus a region of solvent.While the main feature is entirely ordered, the electron density pertaining to the guest solvent was noted to be very smeared in the direction of the structural channels, along c, in which it is located.As such, this was optimally treated using the solvent mask algorithm available in Olex 2, and allowance made for one molecule of hexane, in the motif formula, as presented.
A monomer which propagates to generate 1-D polymers in the gross structure plus half of a molecule of benzene (straddling a crystallographic inversion centre) constitute the asymmetric unit of 16.The Hydrogen atoms attached to C26, C38 and C38 were located and refined at a distance of 0.98 Å from the relevant parent atom.Disorder (55:45) pertaining to the carbons in the borane ligand was readily modelled, as was 65:35 disorder of the solvent moiety.Distance and ADP restraints were employed, on merit, in each disorder region to assist convergence.
In 17, the asymmetric unit is constituted by a monomer.This moiety the basis for a gross structure that is dominated by 1-D polymers parallel to the b axis.The hydrogen atoms attached to C33 were located and refined freely, subject to being located 0.98 Å from the parent atom.
The asymmetric unit constitutes half of a dimer in the structure of 18.The remainder of the molecule arises by virtue of crystallographic inversion symmetry.While all hydrogens were included at calculated positions, those with potential contact to the potassium centres were refined with free Uiso values in order to afford additional credibility regarding their locations.
The asymmetric unit in 19 contains half of a dimer (the remainder of which arises by virtue of crystallographic inversion symmetry) and one and a half molecules of hexane.Disorder prevailed in both the main feature and in the solvent.In the former, the Bpin carbons were modelled to take account of a 50:50 split over two proximate sites.The solvent presented as one complete molecule of hexane which did lend itself to some exceedingly restrained disorder treatment and a separate region of extremely smeared electron density.Ultimately, both areas were addressed using the solvent-mask algorithm in Olex-2, and an appropriate allowance made in the formula as presented.The crystal was refined subject to being a racemic twin and this may contribute to the location of the three largest residual electron-density peaks, in a manner that might suggest disorder of O1, O2 and O3.Modelling of this putative disorder was rigorously explored, but it was ultimately abandoned as concomitant disorder of the aluminium and potassium centres (in order to achieve a chemically sensible model) did not prove accessible.In trials, the maximum level of oxygen disorder refined to approximately 10% and if this was credible, one would expect observation of a minor component for both Al1 and K1! Otherwise, this experiment has rendered a very credible structural characterisation of this material.
There are two molecules present in the asymmetric unit of 20.The largest residual electron density peak is an artefact, and most likely reflects the presence of a small passenger on the main crystal.A twin integration only served to degrade the data, because of the very low diffracting ability of the second component.As such, the model presented herein is based on a data reduction that recognises only the major sample component.Refinement has also taken account of racemic twinning in this crystal structure.

Figure S16: 1 H
Figure S16: 1 H NMR (400 MHz, 298 K, d6-benzene) spectrum of the reaction mixture of N,N'-diisopropylcarbodiimide and 15 after being being kept at 110 o C for one hour.

Figure S26 :
Figure S26: Space filling representation (Mercury) of the structure of compound 20, illustrating the steric protection afforded the aluminum (pink) to boron (yellow) bond.

Table S2 :
Crystal data and structure refinement for compounds 19 and 20.