Sterically controlled reductive oligomerisations of CO by activated magnesium(i) compounds: deltate vs. ethenediolate formation

An extremely bulky, symmetrical three-coordinate magnesium(i) complex, [{(TCHPNacnac)Mg}2] (TCHPNacnac = [{(TCHP)NCMe}2CH]−, TCHP = 2,4,6-tricyclohexylphenyl) has been prepared and shown to have an extremely long Mg–Mg bond (3.021(1) Å) for such a complex. It was shown not to react with either DMAP (4-dimethylaminopyridine) or CO. Three unsymmetrical 1 : 1 DMAP adducts of less bulky Mg–Mg bonded species have been prepared, viz. [(ArNacnac)Mg–Mg(DMAP)(ArNacnac)] (ArNacnac = [(ArNCMe)2CH]− Ar = 2,6-xylyl (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), and their reactivity toward CO explored. Like the previously reported bulkier complex, [(DipNacnac)Mg–Mg(DMAP)(DipNacnac)] (Dip = 2,6-diisopropylphenyl), [(DepNacnac)Mg–Mg(DMAP)(DepNacnac)] reductively trimerises CO to give a rare example of a deltate complex, [{(DepNacnac)Mg(μ-C3O3)Mg(DMAP)(DepNacnac)}2]. In contrast, the two smaller adduct complexes react with only two CO molecules, ultimately giving unusual ethenediolate complexes [{(ArNacnac)Mg{μ-OC(H) 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 C(DMAP−H)O}Mg(ArNacnac)}2] (Ar = Xyl or Mes). DFT calculations show the latter reactions to proceed via reductive dimerizations of CO, and subsequent intramolecular C–H activation of Mg-ligated DMAP by “zig–zag” [C2O2]2− fragments of reaction intermediates. Calculations also suggest that magnesium deltate complexes are kinetic products in these reactions, while the magnesium ethenediolates are thermodynamic products. This study shows that subtle changes to the bulk of the reacting 1 : 1 DMAP–magnesium(i) adduct complexes can lead to fine steric control over the products arising from their CO reductive oligomerisations. Furthermore, it is found that the more activated nature of the adduct complexes, relative to their symmetrical, three-coordinate counterparts, [{(ArNacnac)Mg}2], likely derives more from the polarisation of the Mg–Mg bonds of the former, than the elongated nature of those bonds.


General considerations.
All manipulations were carried out using standard Schlenk and glove box techniques under an atmosphere of high purity dinitrogen. Pentane and diethyl ether were distilled over Na/K alloy (50:50), while hexane, cyclohexane, toluene and THF were distilled over molten potassium. 1 H and 13 C{ 1 H} NMR spectra were recorded on Bruker Avance III 400 or Bruker Avance III 600 spectrometers and were referenced to the resonances of the solvent used or external SiMe4. Mass spectra were collected using an Agilent Technologies 5975D inert MSD with a solid-state probe.

Comments on variable temperature 1 H NMR spectroscopic studies of DMAP adduct complexes 6 and 8.
Similar to the previous report on 4, 5 variable temperature NMR spectroscopic studies of the adducts 6 and 8 revealed fluxional behavior, which is believed to arise from rapid "hopping" of the DMAP ligand between the two Mg centers. This is rapid on the NMR timescale at room temperature, as evidenced by the presence of one set of -diketiminate signals in their spectra.
Cooling d8-toluene solutions of 6 and 8 leads to their 1 H NMR spectra resolving to exhibit two sets of ligand -diketiminate signals, typically at temperatures below -20 °C.

X-Ray Crystallographic Studies
Crystals suitable for X-ray structural determination were mounted in silicone oil.
Crystallographic measurements were made using either an Rigaku Xtalab Synergy Dualflex diffractometer with a graphite monochromator with Mo K radiation ( = 0.71073 Å) or Cu K radiation (1.54180 Å); or the MX2 beamline of the Australian Synchrotron ( = 0.71090 Å). The software package Blu-Ice 6 was used for synchrotron data acquisition, while the program XDS 7 was employed for synchrotron data reduction. All structures were solved by direct methods and compounds. 3 Compound 10 crystallised with 4 molecules of heavily disordered THF in the asymmetric unit (8 THFs/molecule of 10). All attempts to model this disorder were S27 unsatisfactory. As a result, the SQUEEZE program 9 was used to remove their contribution to the structure factors. The final refinement of the structure included the contribution of the THF molecules to the empirical formula and F(000). The relatively high R1 and wR2values for the crystal structures of 9 and 10 are due to weak diffraction data above  angles of 23°. Despite this, the molecular connectivities of the compounds are unambiguous, and their presented metrical parameters are reliable within the calculated esd values. Crystal data, details of data collections and refinements for all structures can be found in their CIF files and are summarized in Table S1.  (10) 12.4858 (2) 18.7194 (2) 17.9190 (6) 15.480 (3) 15.1058 (2) b (Å) 26.32970 (10) 12.0704 (2) 11.31290 (10) 19.6004 (9) 15.114 (3) 19.0783 (2) c (Å) 25.8355 (2) 31.1338 (2) 26.0645 (3) 20.9467 (7) 27.505 (6)

Computational Studies
Geometry optimizations were performed using Gaussian09 suite of programs 10 using the Becke's 3-parameter hybrid functional, 11 combined with the non-local correlation functional provided by Perdew/Wang. 12 The 6-311+G(d) all-electron basis set was used for the magnesium atoms and the 6-31G(d,p) for the remaining atoms. 13 All stationary points have been identified for minimum (Nimag=0) or transition states (Nimag=1). Intrinsic Reaction Paths (IRPs) 14 were traced from the various transition structures to obtain the connected intermediates.