Probing the Chemistry, Electronic Structure and Redox Energetics in Organometallic Pentavalent Uranium Complexes

Abstract

A series of organometallic pentavalent uranium complexes of the general formula (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(Y) (Y = monoanionic, non-halide ligand) have been prepared using a variety of routes. Utilizing the direct oxidation of (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(THF) (2) with the appropriate copper(I) salt yielded the triflate (Y = OTf (OSO(2)CF(3)), 11), thiolate (Y = SPh, 12), and acetylide (Y = C[triple bond]CPh, 13) complexes, while a salt metathesis route between the U(V)-imido (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(I) (10) and various alkali salts gave the diphenylamide (Y = NPh(2), 14), aryloxide (Y = OPh, 15), alkyl (Y = Me, 16), and aryl (Y = Ph, 17) complexes. Paired with 13, the isolation of 16 and 17 shows that U(V) can support the full range of carbon anions (sp, sp(2), and sp(3)), and these are, to the best of our knowledge, the first examples of pentavalent uranium complexes with anionic carbon moieties other than carbocyclic (C(5)R(5), C(7)H(7), C(8)H(8)) ligands. Finally, both protonolysis and insertion pathways afforded the U(V)-imido ketimide complex (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(N=CPh(2)) (18). The complexes have been isolated in good yield and characterized using various combinations of (1)H NMR spectroscopy, elemental analysis, mass spectrometry, single crystal X-ray diffraction, cyclic voltammetry, UV-visible-NIR absorption spectroscopy, and magnetic susceptibility measurements. All (C(5)Me(5))(2)U(=N-Ar)(X) (X = F, Cl, Br, I) and (C(5)Me(5))(2)U(=N-Ar)(Y) complexes exhibit U(VI)/U(V) and U(V)/U(IV) redox couples by voltammetry. The potential separation between these couples remains essentially constant at approximately 1.50 V, but both processes shift in tandem in potential by approximately 700 mV across the series of X/Y ligands. No significant differences between mu(eff) values or temperature dependencies in the magnetic susceptibility were observed for these complexes regardless of the identity of the ancillary X/Y ligand. However, an excellent linear correlation was observed between the chemical shift values of C(5)Me(5) ligand protons in the (1)H NMR spectra and the oxidation potentials of (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(X/Y), suggesting that there is a common origin, overall sigma-/pi-donation from the ancillary X/Y ligand to the metal, contributing to both observables. Combined, these data confer the following trend in increasing sigma/pi-donating ability of the X/Y ligand to the U(V) metal center: OTf < I < Br < Cl < SPh < C[triple bond]CPh < F < [OPh approximately Me approximately Ph] << NPh(2) < N=CPh(2). These (C(5)Me(5))(2)U(=N-2,6-(i)Pr(2)-C(6)H(3))(X/Y) complexes also show distinct hallmarks of a covalent bonding interaction between the metal and the imide ligand that is modulated to varying degrees by the interaction between the X/Y ancillary ligand and the U(V) metal center. These signatures of covalency include stabilization of multiple metal oxidations states [U(VI), U(V), and U(IV)] and enhanced intensities in the intraconfig...

Synthesis, structure, spectroscopy and redox energetics of a series of uranium(IV) mixed-ligand metallocene complexes

“…F. In effect, the better the p-donor, the more electron-rich the uranium center, which results in a larger shielding and an upfield shift of the auxiliary ligand protons. Similar trends have been observed for other paramagnetic trivalent [40], tetravalent [41,42] and pentavalent [15,16,43,44] uranium systems. Not surprisingly, the data in Table 1 also demonstrate that the chemical shift of the Si-Me group engaged in the agostic UÁ Á ÁH-C interaction with the uranium center is sensitive to the nature of the (X) group and moves downfield with variation of the halide/pseudohalide from I $ Br !…”

“…The intensity in these bands is on average $2-3 times less than that found in (C 5 Me 5 ) 2 U IV (ketimide) 2 complexes for which multiple-bond character is observed in the U-N bonds [12,21,22,24]. Thus, consistent with our maturing description of these f-f spectral intensities as markers for metal-ligand covalent bonding, these new spectral data for the (C 5 Me 5 ) 2 U[N(SiMe 3 ) 2 ](X) complexes indicate that they possess less covalent bonding than found in the U IV ketimides (or U IV and U V imides), but greater than that seen in simple U IV halide or alkyl/aryl complexes [12,15,16,21,22,24,52].…”

“…The first pertains to the electron-donating ability of the ancillary (X) ligand. Previous studies have clearly shown that the redox potentials for the uranium metal-based redox couples in bent-metallocene complexes are very sensitive to the electrostatic perturbation induced at the metal by the ancillary ligand(s) [12,15,16,21,22]. In particular, the more strongly electron-donating ancillary ligands destabilize the U IV /U III reduction process, shifting it to more negative potential values.…”

“…Optical spectroscopy of the series of complexes 1-6 is dominated by the (C 5 During the course of these studies, it became evident that the amide mixed-ligand framework provided the opportunity to examine a new homologous structural series from which we can draw new inferences regarding uranium metal-ligand bonding. This falls within the broader context of our continuing efforts to map out trends in the redox energetics and electronic structure of metallocene complexes of the early actinides and correlate these trends with structural chemistry [12][13][14][15][16]. In particular, the amide complexes represent an important end-member along the nitrogenous ligand series: imide (A) [17][18][19][20], ketimide (B) [21][22][23][24], hydrazonato (C) [2,[25][26][27], amide (D) [8,9,28], as illustrated in Fig.…”

Quantifying the Electron Donor and Acceptor Abilities of the Ketimide Ligands in M(N═CtBu2)4 (M = V, Nb, Ta)

“…9 In addition, Kiplinger and co-workers reported the formation of a rare U(V) organometallic, Cp* 2 U(NDipp)(N=CPh 2 ) (Dipp = 2,6- i Pr 2 C 6 H 3 ), stabilized by inclusion of the ketimide ligand. 2 The ability of the ketimide ligand to stabilize high oxidation states likely stems from the orbitals available for the metal-ketimide interaction. DFT calculations on Cr(N=C t Bu 2 ) 4 and Co(N=C t Bu 2 ) 4 reveal that the ketimide ligand is a strong σ- and π-donor, and, as such, should be adept at satisfying the 4+ charge of an M(IV) ion.…”