Mononuclear Phenolate Diamine Zinc Hydride Complexes and Their Reactions With CO2

The synthesis, characterization, and zinc coordination chemistry of the three proligands 2-tert-butyl-4-[tert-butyl (1)/methoxy (2)/nitro (3)]-6-{[(2′-dimethylaminoethyl)methylamino]methyl}phenol are described. Each of the ligands was reacted with diethylzinc to yield zinc ethyl complexes 4–6; these complexes were subsequently reacted with phenylsilanol to yield zinc siloxide complexes 7–9. Finally, the zinc siloxide complexes were reacted with phenylsilane to produce the three new zinc hydride complexes 10–12. The new complexes 4–12 have been fully characterized by NMR spectroscopy, mass spectrometry, and elemental analyses. The structures of the zinc hydride complexes have been probed using VT-NMR spectroscopy and X-ray diffraction experiments. These data indicate that the complexes exhibit mononuclear structures at 298 K, both in the solid state and in solution (d8-toluene). At 203 K, the NMR signals broaden, consistent with an equilibrium between the mononuclear and dinuclear bis(μ-hydrido) complexes. All three zinc hydride complexes react rapidly and quantitatively with carbon dioxide, at 298 K and 1 bar of pressure over 20 min, to form the new zinc formate complexes 13–15. The zinc formate complexes have been analyzed by NMR spectroscopy and VT-NMR studies, which reveal a temperature-dependent monomer–dimer equilibrium that is dominated by the mononuclear species at 298 K.


DFT Calculations
The structure of 10 as a dimer was calculated and optimized. It was shown to have a structure as illustrated in Fig. S6.

Figure S6
: Proposed structure of the dimeric form of 10 (present at reduced temperature) which was used for the DFT calculations.
All ab initio computations were carried out using the Gaussian 09 package. 1 The structure was optimised using the B3LYP functionals, 2 with no symmetry constraints.
The basis sets used here were 3-21G*. 3 Frequency calculations were carried out on these optimised geometries at the corresponding levels and there were no imaginary frequencies.
The X-ray crystal structure of 10 The Zn-H hydrogen atom in the structure of 10 was found from a ΔF map and refined freely. Before the inclusion of this hydrogen atom, the five largest residual electron density peaks in the ΔF map were 0.80, 0.29, 0.25, 0.24 and 0.22 eÅ -3 , and it was the largest of these that was assigned as the Zn-H hydrogen atom position.

DOSY NMR Spectroscopy
The experimental hydrodynamic radii were calculated from the VT-DOSY NMR spectra, measured in d 8 -toluene at 193 K and 298 K. The hydrodynamic radii were calculated using the Stokes-Einstein equation (Equation S1) and the diffusion constant was measured directly from the spectra.
The diffusion coefficient for each experiment was determined using NMRtec software (full details given in experimental section) and the viscosity of toluene at different temperatures determined by interpolation using referenced data. 5 The viscosity of toluene at 233 K was 1.79 mPa.s and for 298 K it was 0.55 mPa.s. The diffusion constants for 10 determined from the DOSY experiments were 7.90 x 10 -9 m 2 s -1 at 233 K and 8.33 x 10 -10 m 2 s -1 at 298 K.
The radius of the monomer was determined from the crystal structure of 10 by measuring the structure along the longest distance (H to H), found to be 5.11 Å. The DOSY experiment at 298 K determined the radius to be 4.75 Å. The radius of the dimer was determined from the calculated DFT structure described above, by measuring the structure along the longest distance (H to H), found to be 9.68 Å. The DOSY experiment at 233 K determined the radius to be 12.09 Å. The DOSY spectra at each temperature are shown below ( Figure S7 and S8).

In situ ATR-IR Spectroscopy
The reactor was loaded in a glove box with toluene (1.0 mL) and compound 11 (36 mg) and sealed under an atmosphere of nitrogen. To initiate the reaction, the gas handling system was purged with CO 2 and pressurised. The reactor inlet valve was opened to allow the CO 2 into the reactor, and then closed after the pressure had been observed at 2 bar to allow the reaction to progress in a sealed system.

Reaction of 11 with CO 2
The concentration of 14 was determined as 0 M at time zero and it was assumed that all 11 was converted to 14 with no side products formed (as evidenced from 1 H, 13 C{ 1 H} NMR and IR studies). The formate peak was identified and the point 1589 cm -1 taken for all the concentration studies (Fig. S9). cm -1 as the reference.

9
The rate was determined from the initial (5-15 %) reaction points using the initial rates method and shown to be 0.033 Mmin -1 (Fig. S10). The dissolution of CO 2 was tested in toluene, under identical conditions to the insertion reaction (Fig. S11). The ATR-IR absorbance was calibrated to the CO 2 concentration, following a procedure outlined by Falk and Miller. 6 Figure S11: Plot of the change in concentration vs. time for the dissolution of CO 2 into toluene.
Using the initial rates method, the rate of CO 2 dissolution into toluene was 0.034 Mmin -1 (Fig. S12).
Figure S12: Initial concentration vs. time plot for CO 2 dissolution in toluene, with linear regression analysis.