Effective and Reversible Carbon Dioxide Insertion into Cerium Pyrazolates

Abstract

The homoleptic pyrazolate complexes [CeIII4(Me2pz)12] and [CeIV(Me2pz)4]2 quantitatively insert CO2 to give [CeIII4(Me2pz⋅CO2)12] and [CeIV(Me2pz⋅CO2)4], respectively (Me2pz=3,5‐dimethylpyrazolato). This process is reversible for both complexes, as observed by in situ IR and NMR spectroscopy in solution and by TGA in the solid state. By adjusting the molar ratio, one molecule of CO2 per [CeIV(Me2pz)4] complex could be inserted to give trimetallic [Ce3(Me2pz)9(Me2pz⋅CO2)3(thf)]. Both the cerous and ceric insertion products catalyze the formation of cyclic carbonates from epoxides and CO2 under mild conditions. In the absence of epoxide, the ceric catalyst is prone to reduction by the co‐catalyst tetra‐n‐butylammonium bromide (TBAB).

Recent Advances in the Chemistry of Metal Carbamates

“…The CO2 pre-activation occurs through the preliminary incorporation of the CO2 reactant in a carbamato ligand, followed by transfer to the organic substrate and ready restoring of the carbamato unit guaranteed by external CO2. A similar pathway was recently proposed by Bayer et al, studying the CO2/epoxide coupling by means of dimethylpirazolate cerium amides and cerium carbamates [134]. Interestingly, [Fe(O 2 CNR 2 ) 3 ] (R = Et, i Pr, Bn) revealed a promising catalytic activity for the production of cyclic carbonates from CO 2 and epoxides [324].…”

“…Since then, the M(NR 2 ) n /CO 2 route has been successfully employed in the preparation of homoleptic complexes of the main group metals [42,123], early transition elements [124][125][126][127][128][129][130][131], uranium(IV) and thorium(IV) [132]. In 2010, Kennedy et al applied this methodology to obtain Li + , Na + and K + 2,2,6,6-tetramethylpiperidine (TMP) carbamates [133] [134]. These compounds are peculiar in that they contain only ligands of the same type (hence they are 'homoleptic') but one nitrogen atom of the pyrazole ring is also involved in coordination (vide infra).…”

“…Unusual coordination environments have been recognized for cerium 3,5-dimetylpyrazole (Me 2 pz) carbamates, also due to the chelation of the metal center by the non-carbonatated pyrazole nitrogen [134]…”

“…The scenario changed over the last 15 years, as several carbamato complexes obtained by this methodology were reported, and especially organometallic species. These include late d-block metals such as iridium [190,191], nickel [192][193][194][195][196][197], palladium [198,199], gold [200] and zinc [201][202][203][204], f-block metals cerium [134] and uranium [205][206][207][208][209] and p-block metals tin [210] and gallium [211,212]. Furthermore, in situ-formed amide complex of Zn(II) and alkyl-amide of Mg(II) were mixed under carbon dioxide atmosphere to afford a heterobimetallic Zn/Mg alkyl-carbamato derivative [213].…”

“…In 2010, Kennedy et al applied this methodology to obtain Li + , Na + and K + 2,2,6,6-tetramethylpiperidine (TMP) carbamates [ 133 ]. A few years ago, the same method was applied to synthesize homoleptic cerium carbamates [Ce 3 (O 2 CMe 2 pz) 3 ] 4 and [Ce(O 2 CMe 2 pz) 4 ] (Me 2 pz = 3,5-dimethylpyrazole) from the respective amides [Ce(Me 2 pz) 3 ] 4 and [Ce(Me 2 pz) 4 ] 2 ( Scheme 16 a) [ 134 ]. These compounds are peculiar in that they contain only ligands of the same type (hence they are ‘homoleptic’) but one nitrogen atom of the pyrazole ring is also involved in coordination (vide infra).…”