Mass spectrometric and theoretical study on the formation of uranyl hydride from uranyl carboxylate

Formation and Structure of Gas-Phase Lanthanide(III) Cyanobenzyne Complex (η2-4-CNC6H3)LnCl2-, Obtained via Both the Single- and Dual-Ligand Strategies

The lanthanide(III) cyanobenzyne complexes (η2-4-CNC6H3)LnCl2- (Ln = La-Lu except Eu; Pm was not examined) were generated in the gas phase using an electrospray ionization mass spectrometry coupled with collision-induced dissociation (CID) technique. For all lanthanides except Sm, Eu, and Yb, (4-CNC6H3)LnCl2- can be generated either via a single-ligand strategy through consecutive CO2 and HCl losses of (4-CNC6H4CO2)LnCl3- or via a dual-ligand strategy through successive CO2/C6H5CN or 4-CNC6H4CO2H and CO2 losses of (4-CNC6H4CO2)2LnCl2-. For Sm and Yb, although only reduction products LnCl3- were formed upon CID of (4-CNC6H4CO2)LnCl3-, (4-CNC6H3)LnCl2- were obtained via the dual-ligand strategy without the appearances of other products. CID of (4-CNC6H4CO2)EuCl3- and (4-CNC6H4CO2)2EuCl2- gave EuCl3- and the cyanophenyl complex (4-CNC6H4)EuCl2-, respectively, in both of which the +III oxidation state of Eu was reduced to +II. Density functional theory (DFT) calculations reveal that (4-CNC6H3)LnCl2- are formally described as Ln(III) cyanobenzyne complexes, (η2-4-CNC6H3)LnCl2-, with the dianionic cyanobenzyne ligand (4-CNC6H32-) coordinating to the Ln(III) centers through two Ln-C σ bonds, which is in accordance with their reactivities toward water. Benzyne and substituted benzyne complexes (XC6H3)LuCl2- (X = H, 3-CN, 4-F, 4-Cl, and 4-CH3) were also synthesized in the gas phase via the single- and dual-ligand strategies.

Gas-phase synthesis of [OU-X]+ (X = Cl, Br and I) from a UO22+ precursor using ion-molecule reactions and an [OUCH]+ intermediate

Difficulty in the preparation of gas-phase ions that include U in middle oxidation states(III,IV) have hampered efforts to investigate intrinsic structure, bonding and reactivity of model species. Our group has used preparative tandem mass spectrometry (PTMS) to synthesize a gas-phase U-methylidyne species, [OUCH]+, by elimination of CO from [UO2(CCH)]+ [M. J. van Stipdonk, I. J. Tatosian, A. C. Iacovino, A. R. Bubas, L. Metzler, M. C. Sherman and A. Somogyi, J. Am. Soc. Mass Spectrom., 2019, 30, 796-805], which has been used as an intermediate to create products such as [OUN]+ and [OUS]+ by ion-molecule reactions. Here, we investigated the reactions of [OUCH]+ with a range of alkyl halides to determine whether the methylidyne is a also a useful intermediate for production and study of the oxy-halide ions [OUX]+, where X = Cl, Br and I, formally U(IV) species for which intrinsic reactivity data is relatively scarce. Our experiments demonstrate that [OUX]+ is the dominant product ion generated by reaction [OUCH]+ with neutral regents such as CH3Cl, CH3CH2Br and CH2CHCH2I.

Organolanthanide Complexes Containing Ln-CH3 σ-bonds: Unexpectedly Similar Hydrolysis Rates for Trivalent and Tetravalent Organocerium

We report the gas-phase preparation, isolation, and reactivity of a series of organolanthanides featuring the Ln-CH3 bond. The complexes are formed by decarboxylating anionic lanthanide acetates to form trivalent [LnIII(CH3)(CH3CO2)3]- (Ln = La, Ce, Pr, Nd, Sm, Tb, Tm, Yb, Lu), divalent [EuII(CH3)(CH3CO2)2]-, and the first examples of tetravalent organocerium complexes featuring CeIV-Calkyl σ-bonds: [CeIV(O)(CH3)(CH3CO2)2]- and [CeIV(O)(CH3)(NO3)2]-. Attempts to isolate PrIV-CH3 and TbIV-CH3 were unsuccessful; however, fragmentation patterns reveal that the oxidation of LnIII to a LnIV-oxo-acetate complex is more favorable for Ln = Pr than for Ln = Tb. The rate of Ln-CH3 hydrolysis is a measure of bond stability, and it decreases from LaIII-CH3 to LuIII-CH3, with increasing steric crowding for smaller Ln stabilizing the harder Ln-CH3 bond against hydrolysis. [EuII(CH3)(CH3CO2)2]- engages in a much faster hydrolysis versus LnIII-CH3. The surprising observation of similar hydrolysis rates for CeIV-CH3 and CeIII-CH3 is discussed with respect to sterics, the oxo ligand, and bond covalency in σ-bonded organolanthanides.

Gas-phase formation of Grignard-type organolanthanide (III) ions RLnCl3 - : The influences of lanthanide center and hydrocarbyl group

RATIONALE

Compared with organomagnesium compounds (Grignard reagents), the Grignard-type organolanthanides (III) exhibit several utilizable differences in reactivity. However, the fundamental understanding of Grignard-type organolanthanides (III) is still in its infancy. Decarboxylation of metal carboxylate ions is an effective method to obtain organometallic ions that are well suited for gas-phase investigation using electrospray ionization (ESI) mass spectrometry in combination with density functional theory (DFT) calculations.

METHODS

The (RCO₂ )LnCl₃ ^- (R = CH₃ , Ln = La-Lu except Pm; Ln = La, R = CH₃ CH₂ , CH₂ CH, HCC, C₆ H₅ , and C₆ H₁₁ ) precursor ions were produced in the gas phase via ESI of LnCl₃ and RCO₂ H or RCO₂ Na mixtures in methanol. Collision-induced dissociation (CID) was employed to examine whether the Grignard-type organolanthanide (III) ions RLnCl₃ ^- can be obtained via decarboxylation of lanthanide chloride carboxylate ions (RCO₂ )LnCl₃ ^- . DFT calculations can be used to determine the influences of lanthanide center and hydrocarbyl group on the formation of RLnCl₃ ^- .

RESULTS

When R = CH₃ , CID of (CH₃ CO₂ )LnCl₃ ^- (Ln = La-Lu except Pm) yielded decarboxylation products (CH₃ )LnCl₃ ^- and reduction products LnCl₃ ·^- with a variation in the relative intensity ratio of (CH₃ )LnCl₃ ^- /LnCl₃ ·^- . The trend is as follows: (CH₃ )EuCl₃ ^- /EuCl₃ ·^-  < (CH₃ )YbCl₃ ^- /YbCl₃ ·^-  ≈ (CH₃ )SmCl₃ ^- /SmCl₃ ·^-  < other (CH₃ )LnCl₃ ^- /LnCl₃ ·^- , which complies with the trend of Ln (III)/Ln (II) reduction potentials in general. When Ln = La and hydrocarbyl groups were varied as CH₃ CH₂ , CH₂ CH, HCC, C₆ H₅ , and C₆ H₁₁ , the fragmentation behaviors of these (RCO₂ )LaCl₃ ^- precursor ions were diverse. Except for (C₆ H₁₁ CO₂ )LaCl₃ ^- , the four remaining (RCO₂ )LaCl₃ ^- (R = CH₃ CH₂ , CH₂ CH, HCC, and C₆ H₅ ) ions all underwent decarboxylation to yield RLaCl₃ ^- . (CH₂ CH)LaCl₃ ^- and especially (CH₃ CH₂ )LaCl₃ ^- are prone to undergo β-hydride transfer to form LaHCl₃ ^- , whereas (HCC)LaCl₃ ^- and (C₆ H₅ )LaCl₃ ^- are not. A minor reduction product, LaCl₃ ·^- , was formed via C₆ H₅ radical loss of (C₆ H₅ )LaCl₃ ^- . The relative intensities of RLaCl₃ ^- compared to (RCO₂ )LaCl₃ ^- decrease as follows: HCC > CH₂ CH > C₆ H₅  > CH₃  > CH₃ CH₂  >> C₆ H₁₁ (not visible).

CONCLUSION

A series of Grignard-type organolanthanide (III) ions RLnCl₃ ^- (R = CH₃ , Ln = La-Lu except Pm; Ln = La, R = CH₃ CH₂ , CH₂ CH, HCC, and C₆ H₅ ) were produced from (RCO₂ )LnCl₃ ^- via CO₂ loss, whereas (C₆ H₁₁ )LaCl₃ ^- did not. The experimental and theoretical results suggest that the reduction potentials of Ln (III)/Ln (II) couples as well as the bulkiness and hybridization of hydrocarbyl groups play important roles in promoting or limiting the formation of RLnCl₃ ^- via decarboxylation of (RCO₂ )LnCl₃ ^- .

Dual-Ligand Strategy for the Preparation of Gas-Phase Uranyl(VI) Benzyne Complexes from Uranyl(VI) Benzoates

The uranyl(VI) benzyne complex (η²-C₆H₄)UO₂Cl^- was prepared in the gas phase by electrospray ionization mass spectrometry coupled with collision-induced dissociation. It was formed via a dual-ligand strategy that requires the elimination of benzoic acid or benzene/CO₂ from the uranyl dibenzoate precursor (C₆H₅CO₂)₂UO₂Cl^-. This contrasts the known strategy for the formation of gas-phase benzyne complexes that would result from CO₂/HCl elimination from (C₆H₅CO₂)UO₂Cl₂^-, during which only one benzoate ligand is involved. Such dual-ligand strategy can be extended to the preparation of a series of methyl- and halo-substituted benzyne complexes of uranyl(VI). Density functional theory calculations at the B3LYP level reveal that the benzyne complex (η²-C₆H₄)UO₂Cl^- features a metallacyclopropene structure with the C₆H₄^2- ligand coordinated to uranium(VI) through two polarized U-C_benzyne σ bonds, in accordance with the reactivity test toward water. Dehydrochlorination of the benzyne complex (η²-C₆H₄)UO₂Cl^- from (C₆H₅)UO₂Cl₂^- that originates from decarboxylation of (C₆H₅CO₂)UO₂Cl₂^- with a single benzoate ligand is neither kinetically nor thermodynamically favorable than simple C₆H₅ radical loss to give U^VO₂Cl₂^-. This arises from the presence of an accessible V oxidation state for uranium and accounts for the necessity for the dual-ligand strategy in the preparation of uranyl(VI) benzyne complexes from uranyl benzoate precursors.

Influence of Metal Coordination on the Gas-Phase Chemistry of the Positional Isomers of Fluorobenzoate Complexes

The fragmentation behaviors of the o-, m-, and p-fluorobenzoate complexes of La³⁺, Ce³⁺, Fe³⁺, Cu²⁺, and UO₂²⁺ were investigated by electrospray ionization mass spectrometry, and the corresponding reaction mechanisms were explored by density functional theory (DFT) calculations. Fluoride transfer product La^IIIFCl₃^-/Ce^IIIFCl₃^- and decarboxylation product La^IIICl₃(C₆H₄F)^-/Ce^IIICl₃(C₆H₄F)^- were observed when the carboxylate precursors La^IIICl₃(C₆H₄FCO₂)^-/Ce^IIICl₃(C₆H₄FCO₂)^- were subjected to collision-induced dissociation. The variation in product ratios, which is not obvious in the meta and para cases, qualitatively follows the increasing overall energy barrier and reaction endothermicity of the two-step CO₂/C₆H₄ elimination mechanism, and this aligns with the increase in U-F distance in the ortho, meta, and para decarboxylation product isomers. In contrast, the mass spectra of Fe^IIICl₃(C₆H₄FCO₂)^-/Cu^IICl₂(C₆H₄FCO₂)^- are dominated by the reduction product FeCl₃^-/CuCl₂^- regardless of the fluorobenzoate isomer. DFT/B3LYP calculations show that the two-step CO₂/C₆H₄F elimination pathways are comparable in energy for all three positional isomers. It is energetically more favorable to give the reduction product than the fluoride transfer product, which is opposite to the lanthanum cases. Although the decarboxylation product was observed for all three U^VIO₂Cl₂(C₆H₄FCO₂)^- isomers, the ortho isomer behaves more similarly to La^IIICl₃(C₆H₄FCO₂)^-/Ce^IIICl₃(C₆H₄FCO₂)^- as evidenced by the formation of U^VIO₂FCl₂^-, and the appearance of U^VO₂Cl₂^- in the cases of the meta and para isomers indicates the similarity with Fe^IIICl₃(C₆H₄FCO₂)^-/Cu^IICl₂(C₆H₄FCO₂)^-. The shorter U-F distance in U^VIO₂Cl₂(o-C₆H₄F)^- causes the decrease in the fluoride transfer barrier and thus makes this process more favorable over o-C₆H₄F radical loss to give U^VO₂Cl₂^-.

Gas-phase synthesis and structure of thorium benzyne complexes

The thorium benzyne complex (η²-C₆H₄)ThCl₃^- was synthesized in the gas phase through consecutive decarboxylation and dehydrochlorination from the (C₆H₅CO₂)ThCl₄^- precursor upon collision-induced dissociation. Theoretical calculations suggest that (η²-C₆H₄)ThCl₃^- exhibits a metallacyclopropene structure with two polarized Th-C_benzyne σ bonds. This procedure can be generally extended to the synthesis of a wide range of gas-phase thorium benzyne complexes.

Discrimination and quantitation of halobenzoic acid positional isomers upon Th(IV) coordination by mass spectrometry

A fast and reliable mass spectrometry-based method has been developed to discriminate the positional isomers of o-, m- and p-C₆H₄XCO₂H (X = F, Cl and Br). This is based on the distinct fragmentation patterns of isomeric ThCl₄(C₆H₄XCO₂)^- ions generated by electrospray ionization of the solutions with C₆H₄XCO₂H isomers and ThCl₄. Moreover, the composition of these positional isomers can be conveniently quantified without any pre-treatment according to the proportion of gas-phase fragmentation products.