Marisa J. Monreal

Synthesis and Characterization of Three-Coordinate Ni(III)-Imide Complexes

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

Beyond CH Activation with Uranium: A Cascade of Reactions Mediated by a Uranium Dialkyl Complex†

The ring opening of aromatic N heterocycles has been restricted to a few examples involving transition metals, such as niobium, tantalum, titanium, scandium, yttrium, and rhenium; strong actinide–oxygen bonds could also drive the ring opening of pyridine N-oxides. Our interest in the reactions of electrophilic alkyl complexes 12] supported by a ferrocene (fc) diamide ligand with aromatic N heterocycles prompted us to investigate the reactivity of the uranium dibenzyl complex 1-(CH2Ph)2 ((NN )U(CH2Ph)2; NN = 1,1’-fc(NSitBuMe2)2) [11, 16] with imidazoles. It has been reported that actinide complexes are more reactive than analogous Group IV complexes towards aromatic N heterocycles, and that C H activation occurs from neutral dialkyl complexes as opposed to the cationic alkyl complexes of group IV metals. We reasoned that diamide ligands may enhance the reactivity of the actinide complexes, since such ligands are known to support metal centers that are more electrophilic than those in metallocenes. This characteristic becomes important when aromatic N heterocycles are involved, because these substrates tend to be strong Lewis bases and shut down further reactivity at the metal center. The uranium dibenzyl complex 1-(CH2Ph)2 features two alkyl ligands; therefore, we became interested in finding reactions that would involve two C H activation events. We anticipated that this complex would show different reactivity from that reported by Guram and Jordan for a related zirconium complex ([Cp2ZrMe(THF)] + did not engage in C H activation with the imidazole ring; only coordination was observed). The ability to undergo two C H activation reactions is unique to uranium. It has been reported that both alkyl ligands of a uranium dialkyl complex react with the C H bonds of terminal acetylenes; however, reactions that involve two sp-hybridized C H bonds are not known. One reason for this lack of reactivity may be the fact that the acidity of C(sp) H bonds is lower than that of C(sp) H bonds: the pKa value of phenyl acetylene is 23.2, [22] whereas the pKa value of 1-methylimidazole is 33.1 (experimental; [23] calculated: pKa 35.1; the pKa value of 1-methylbenzimidazole is 32.5). Herein, we report a novel double C H activation followed by the C C coupling, ring opening, and migratory insertion of imidazoles; these reactions are uniquely promoted by 1-(CH2Ph)2 and represent the first examples of the cleavage of aromatic N heterocycles by actinide complexes without the involvement of oxygen atoms or redox processes. The reaction between 1-(CH2Ph)2 and 2 equivalents of 1methylimidazole occurred at room temperature (Scheme 1). The H NMR spectrum was indicative of a symmetrical

Reactions of Aromatic Heterocycles with Uranium Alkyl Complexes

The reactivity of a uranium dibenzyl complex supported by a ferrocenediamide ligand toward aromatic N-heterocycles was investigated. The uranium dibenzyl complex reacted with 1 equiv of pyridine or picoline to give the respective ortho-metalated product. In turn, the eta(2)-N,C-pyridyl complexes reacted with benzoxazole or benzothiazole, leading to a cascade of transformations that involves C-C coupling of the two heterocyclic rings, ring opening, and alkyl transfer. A similar cascade was previously reported for 1-methylimidazole. The present results provide support for the mechanism proposed earlier and attest to the generality of the reaction cascade described.

The riches of uranium

Uranium is best known, and feared, for its involvement in nuclear energy. Marisa J. Monreal and Paula L. Diaconescu take a look at how its unique combination of properties is now increasingly attracting the attention of chemists.