Fixation of Carbon Dioxide by Oxalic Amidinato Magnesium Complexes: Structures and Reactions of Trimetallic Magnesium Carbamato and Related Complexes

Abstract

The reaction of oxalic amidines R1–N=C(NHR2)–C(NHR2)=N–R1 with CH3MgX followed by uptake of CO2 results in the formation of the trimeric carbamato complexes [R1–N=C(NR2–COO)–C(NR2COO)=N–R1]3Mg3(THF)6 (2a: R1 = R2 = Ph; 2b: R1 = R2 = p‐tolyl) as the thermodynamically stable final products of the reaction. Their X‐ray crystal structures show that the three metal centres are in a linear arrangement. The central magnesium ion is octahedrally surrounded by six O‐donor atoms of the μ2‐carbamato bridges, while both peripheral magnesium ions are facially coordinated by three O‐donor atoms of the carbamato groups and three THF molecules. This coordination sphere can be considered as a structural model for the active centre in the ribulose‐1,5‐bisphosphate carboxylase/oxygenase enzyme. Compound 2a reacts with ZnCl2 or CoBr2, with CO2 elimination, to form dimeric complexes of the type [X2M(oxalamidinato)MX2][Mg(DMF)6] (M = Zn, Co; X = Cl, Br). X‐ray crystal structure analyses show that the d‐metals are tetrahedrally coordinated. The magnesium‐bromide‐containing intermediates in the formation of 2a and 2b are able to transfer CO2 to acetophenone, thus simulating the CO2 activation step in enzymatic biotin‐dependent carboxylation reactions.

Exposing the hidden complexity of stoichiometric and catalytic metathesis reactions by elucidation of Mg-Zn hybrids

“…Several examples of other mixed-metal compounds containing either one of these two ions, separately but never together, have been reported (23)(24)(25)(26). However, surprisingly a search of the Cambridge Crystallographic Database revealed that the 1∶2, Mg∶Zn stoichiometry found in compound 3 is extremely rare across the whole of mixed magnesium-zinc chemistry, with only three such compounds structurally characterized to date (27)(28)(29). The formation of 3 can be rationalised when the classical Schlenk equilibrium of Grignard reagents is taken into consideration (Scheme 2).…”

Structurally simple complexes of CO2

“…82,83 Others have drawn comparisons between these complexes and those that may be generated in the magnesium-containing active site of the enzyme Rubisco (Ribulose-1,5-biphosphate carboxylase/oxygenase) during the biochemical fixation of CO2. 84 Various binding modes have been identified in Mg-carbamato complexes, 85,86 some of which are shown in Figure 5. The C-N bond lengths and O-C-O bond angles generally remain comparable to those in Hbonded carbamate salts and zwitterions, suggesting that these species are no less stable.…”

Carbon dioxide and related heterocumulenes at zinc and lithium cations: bioinspired reactions and principles

“…The reaction of tetraphenyloxalic amidine (H 2 A) with two equivalents of CH 3 MgBr followed by addition of gaseous CO 2 afforded the trimeric carbamato complexes 21 (Fig. 69 The X-ray crystal structure shows a trimeric structure in which three magnesium centres are connected by six l 2 -carbamato bridges according to the formal composition [Mg(CO 2 ) 2 (A)(THF) 2 ] 3 . 69 The X-ray crystal structure shows a trimeric structure in which three magnesium centres are connected by six l 2 -carbamato bridges according to the formal composition [Mg(CO 2 ) 2 (A)(THF) 2 ] 3 .…”