Alkaline‐Earth‐Promoted CO Homologation and Reductive Catalysis

Abstract

Reaction between a b-diketiminato magnesium hydride and carbon monoxide results in the isolation of adimeric cis-enediolate species through the reductive coupling of two CO molecules.Under catalytic conditions with PhSiH 3 , an observable magnesium formyl species may be intercepted for the mild reductive cleavage of the CO triple bond.The deoxygenative conversion of carbon monoxide to hydrocarbon fuels and lower oxygenates is typically achieved through heterogeneous catalytic methods.[1] Although Fischer-Tropsch (F-T) chemistry and related processes have been successfully implemented for some eighty years to produce aS chultz-Flory distribution of hydrocarbons,o nly limited success has been achieved with well-defined homogeneous systems,t ypically at CO pressures in excess of 1000 atm.[2] More recent activity has targeted the synthesis of putative F-T intermediates including,and most pertinent to the current work, enediolates such as the zirconium and cerium species I-III,which were prepared through treatment of the relevant metal hydride with CO.[3] Our own research efforts have focused on the development of ah omogeneous catalytic chemistry for complexes derived from the inexpensive and environmentally benign alkaline-earth elements, [4] in particular magnesium and calcium;r espectively the eighth and fifth most abundant elements in the lithosphere.I nt his contribution we demonstrate that similar CO homologation reactivity may be achieved through exposure of the bdiketiminato magnesium hydride (IV) [5] to one atmosphere of CO and that this and other alkaline earth species enable the highly selective catalytic reduction of the carbon monoxide molecule under similarly mild reaction conditions.Compound IV reacted under one atmosphere of CO at room temperature to provide as ingle new species (1) 12 CO resulted in no simplification of the AA'XX' spin system even at elevated temperatures indicating that the formation of compound 1 is irreversible.T hese spectral data are closely comparable to those reported for compounds I-III and are thus similarly indicative of the reductive coupling of two molecules of CO to form ad inuclear magnesium enediolate species [Eq. (1)]. [3] These deductions were confirmed through as ubsequent X-ray structural analysis,the results of which are illustrated in Figure 1, performed on as elected single crystal of compound 1 grown from as aturated n-pentane solution. While the C30-C31 distance [1.327(3) ] is clearly indicative of the formation of aC =Cdouble bond and a cis-enediolate similar to that observed in both compounds II and III,the ligand does not display ad irectly analogous symmetrical bridging disposition. Rather, the planar cis-enediolate adopts an asymmetric bridging mode in which one four-coordinate pseudo-tetrahedral magnesium (Mg1) center is bound through chelation of both O1 [Mg1-O1 1.92(1) ] and O2 [Mg1-O2 2.01 (1) ]. This latter bond is slightly elongated as O2 is also bound to the further trigonal Mg2 center [Mg2-O2 1.88(1) ].V ariable-temperature 1 HNMR st...

Magnesium hydrides bearing sterically demanding amidinate ligands: synthesis, reactivity and catalytic application

“…4 A has been widely employed in catalysis, either used directly or formed in situ, for reactions such as the hydroboration of aldehydes and ketones and the reduction of CO with phenylsilane. 5,6 The isolation of terminal magnesium hydride species has also been possible through the use of ligand systems with high coordination numbers ( Fig. 1: B and C).…”

Magnesium-stabilised transition metal formyl complexes: structures, bonding, and ethenediolate formation

“…Complete NBO data can be found in the ESI (Tables S5 and S6) The spectroscopic data are consistent with that reported for related magnesium ethenediolate complexes by Hill and Jones. 43,44 The hydride bridging the two magnesium centres appears as a singlet resonance at d H ¼ 2.72 ppm. In the solid state, the key ethenediolate ligand spans chromium and magnesium centre.…”

“…31 The metal formyl was observed below À50 C and not isolated. More recently, a number of main group and transition metal systems have been reported for ethenediolate formation, [40][41][42][43][44] in no case has a metal formyl intermediate been isolated, rather DFT calculations support their involvement in the mechanism for C-C bond formation.…”

Carbon–Carbon Bond Formation from CO and H– The Role of Metal Formyl Intermediates

“…Specifically, the oxymethylene complex Andersen and co-workers reported the reaction of monomeric [Ce(Cp')2H] (18; Cp' = 1,2,4trimethylcyclopentadienyl) with CO yielded the corresponding cis-ethenediolate complex 21 (Scheme 6). [39] Conducting the reaction in n-pentane solution led to precipitation of the bridging In 2015 Hill and co-workers [40] and Jones, Stasch, Maron and co-workers [41] simultaneously reported the reaction of a -diketiminate stabilised magnesium hydride reagent with CO energy barrier to give the bridged oxymethylene complex 33 (H ‡ = -15.8 kcal mol -1 ). A second CO insertion and subsequent 1,2-H shift yielded the identified ethenediolate complex through an overall exergonic reaction (H° = -82.9 kcal mol -1 from free 28 and CO).…”