Coordination and Homologation of CO at Al(I): Mechanism and Chain Growth, Branching, Isomerization, and Reduction

Homologation of carbon monoxide is central to the heterogeneous Fischer–Tropsch process for the production of hydrocarbon fuels. C–C bond formation has been modeled by homogeneous systems, with [CnOn]2– fragments (n = 2–6) formed by two-electron reduction being commonly encountered. Here, we show that four- or six-electron reduction of CO can be accomplished by the use of anionic aluminum(I) (“aluminyl”) compounds to give both topologically linear and branched C4/C6 chains. We show that the mechanism for homologation relies on the highly electron-rich nature of the aluminyl reagent and on an unusual mode of interaction of the CO molecule, which behaves primarily as a Z-type ligand in initial adduct formation. The formation of [C6O6]4– from [C4O4]4– shows for the first time a solution-phase CO homologation process that brings about chain branching via complete C–O bond cleavage, while a comparison of the linear [C4O4]4– system with the [C4O4]6– congener formed under more reducing conditions models the net conversion of C–O bonds to C–C bonds in the presence of additional reductants.


Crystallography:
Single-crystal X-ray diffraction data for all compounds were collected on an Oxford Diffraction/Agilent SuperNova diffractometer equipped with a 135 mm Atlas CCD area detector. Crystals were selected under Paratone-N oil, mounted on MiTeGen Micromount loops and quench-cooled using an Oxford Cryosystems open flow N2 cooling device. S3 Data were collected at 150 K using mirror monochromated Cu Kα radiation (λ = 1.5418 Å; Oxford Diffraction Supernova). Data collected were processed using the CrysAlisPro package, including unit cell parameter refinement and inter-frame scaling (which was carried out using SCALE3 ABSPACK within CrysAlisPro). S4 Equivalent reflections were merged and diffraction patterns processed with the CrysAlisPro suite. S4 Structures were solved ab initio from the integrated intensities using SHELXT S5 and refined on F2 using SHELXL S6 with the graphical interface OLEX2 S7 or XSeed S8 . Selected crystallographic data are summarised in Table S1, and full details are given in the supplementary deposited CIF files (CCDC 2114859-2114865 and 2118134). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. S15  Table S1/contd: Crystallographic data for CO homologation and related compounds.       S24 Figure S22: Molecular structure of 5 in the solid state as determined by single crystal diffraction. Thermal ellipsoids have been set at 50% probability. Hydrogen atoms have been omitted and selected groups are shown in wireframe for clarity.

Computational details:
All computational work reported here was carried out using density functional theory (DFT). Geometry optimisations for model compounds A -F and toluene were performed with Gaussian16 (Revision C.01) S11 and for model compounds G -J, R, P, TS1-TS4, I1 -I3 and carbon monoxide using Orca (Revision 5.0.2). S12 Calculations were performed for model systems with simplified ligands to reduce computational cost. In all cases the tBu groups on the xanthene backbone were replaced by methyl substituents. Further, the Dipp groups were replaced with Ph groups for compounds G -J, or with ortho-Xylyl groups for compounds R, P, TS1 -TS4, I1 -I3.

Different isomers of the [C 4 O 4 ] 2and [C 4 O 4 ] 4containing compounds
A -E were initially scoped using the B3LYP exchange correlation functional, S13-S16 with Def-SVP basis sets, S17,S18 and Grimme's empirical dispersion correction (GD3BJ). S19,S20 Integration grid accuracy was set at ultrafine. The nature of stationary points found (minima) was confirmed by full frequency calculations. Further geometry optimizations were performed for synthetically viable species A, E and F using the B3LYP exchange correlation functional and Def-TZVP basis set , with Grimme's empirical dispersion correction (GD3BJ).

S29
The mechanism for the formation of P ( Figure S24) is proposed to proceed through initial binding of CO in a bent fashion via the C atom (I1) with a second CO binding in reverse orientation via the O atom (I2). An isomerisation then occurs (TS3) to a carbene (I3) which can then dimerize to form P, the observed homologation product. Dimerization (TS4) is the rate determining step with a moderate high barrier due to the large steric bulk and electrostatic repulsion of the anionic fragments. This moderate high barrier is consistent with the experimental findings, that heating is required and low yields are observed. Through the course of this mechanistic investigation a range of binding modes were considered ( Figure S25), with the bent CO binding (H) being the lowest in energy for the initial binding of CO. Attempts to coordinate CO to the vacant p-orbital on Al in a linear fashion did not converge to a stable structure. When binding two CO molecules, an energetic minimum was found featuring two Al bound C atoms (I2'). However, we propose that the isomeric system I2 is the relevant intermediate in the transformation from R to P with I2' being an unproductive intermediate. The conversion between I1 and I2' is somewhat reversible due to the moderate barrier and small ΔG, and as such the conversion of I2 to I2' is proposed to proceed via I1 not via direct rotational interconversion, as the barrier (TS2B) is significantly higher in energy. Binding of an isocarbonyl ligand OC to form I2 is energetically unfavourable with respect to I2'; however onward isomerisation to carbene system I3 is near barrier-less with I3 lying downhill of both R and I2'. ETS-NOCV (using the implementation in multiwfn wavefunction analysis package) S25 was used to further investigate the binding of the two CO molecules in I2 and I2'. This analysis revealed that the binding of CO involves two main interactions; the dominant interaction involves the CO π* symmetry orbitals and the lone pair on the aluminium to form a multicentre bond ( Figure S26). This bonding motif was also found to support reverse binding of CO in I2 with only a modest decrease in the NOCV pair energy.

Figure S25
: Proposed mechanism for CO homologation at B3LYP/def2-TZVP//B3LYP/def2-SVP with solvation modelled with CPCM(Benzene). Pathway in blue representing lowest energy pathway with reverse binding of CO (I2); pathway in red in a representing the prior binding of a second CO through carbon (I2') then isomerization to I2

S31
To investigate the mechanism of CO activation different binding modes of one or two CO molecules to the aluminium centre were calculated (G -J) using the using the B3LYP exchange correlation functional, S12-S15 with 6-31G(d) Pople basis sets S21,22 and DFT-D4 dispersion. S23 Further mechanistic work (R, P, TS1-TS4, I1 -I3) was performed using the B3LYP exchange correlation functional, S12-S15 with Def-SVP basis sets, S16 DFT-D4 dispersion and LR-CPCM (benzene) solvent modelling, S24 with single point energies being performed with the Def-TZVP basis set. The transition state structures (saddle points) were confirmed by having a single imaginary frequency.