Suzanne C. Bart

[2π+2π] Cycloaddition of Isocyanates to Uranium(IV) Imido Complexes for the Synthesis of U(IV) κ2-Ureato Compounds

Co-reporter:Caleb J. Tatebe;Matthias Zeller

Inorganic Chemistry February 20, 2017 Volume 56(Issue 4) pp:1956-1965

Publication Date(Web):February 6, 2017

DOI:10.1021/acs.inorgchem.6b02547

A new family of uranium(IV) imido complexes of the form Tp*2U(NR) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; R = benzyl (Bn), para-tolyl (p-Tol), 2,6-diethylphenyl (detp), and 2,6-diisopropylphenyl (dipp)) have been generated by bibenzyl extrusion from Tp*2UBn. Tp*2U(NBn), Tp*2U(Np-Tol), and Tp*2U(Ndetp), along with previously reported Tp*2U(NPh) and Tp*2U(NAd) (Ad = 1-adamantyl), readily undergo [2π+2π] cycloaddition with isocyanates and isothiocyanates to generate κ2-ureato and κ2-thioureato derivatives, respectively. These new uranium(IV) complexes were characterized via multinuclear NMR, vibrational and electronic absorption spectroscopies, and, where possible, X-ray crystallography. The steric demands of the ligands were quantitatively assessed using computational modeling, and it was shown that cycloaddition only occurs for imido species where ligands occupy 90% or less of the coordination sphere.

The Vibrancy and Variety of Modern f-Element Organometallic Chemistry

Co-reporter:Suzanne C. Bart and Eric J. Schelter

Organometallics December 11, 2017 Volume 36(Issue 23) pp:4507-4507

Publication Date(Web):December 11, 2017

DOI:10.1021/acs.organomet.7b00829

Neodymium(III) Complexes Capable of Multi-Electron Redox Chemistry

Co-reporter:Ezra J. Coughlin;Dr. Matthias Zeller; Suzanne C. Bart

Angewandte Chemie 2017 Volume 129(Issue 40) pp:12310-12313

Publication Date(Web):2017/09/25

DOI:10.1002/ange.201705423

AbstractA family of neodymium complexes featuring a redox-active ligand in three different oxidation states has been synthesized, including the iminoquinone (L0) derivative, (dippiq)2NdI3 (1-iq), the iminosemiquinone (L1−) compound, (dippisq)2NdI(THF) (1-isq), and the amidophenolate (L2−) [K(THF)2][(dippap)2Nd(THF)2] (1-ap) and [K(18-crown-6)][(dippap)2Nd(THF)2] (1-ap crown) species. Full spectroscopic and structural characterization of each derivative established the +3 neodymium oxidation state with redox chemistry occurring at the ligand rather than the neodymium center. Oxidation with elemental chalcogens showed the reversible nature of the ligand-mediated reduction process, forming the iminosemiquinone metallocycles, [K(18-crown-6)][(dippisq)2Nd(S5)] (2-isq crown) and [K(18-crown-6)(THF)][(dippisq)2Nd(Se5)] (3-isq crown), which are characterized to contain a 6-membered twist-boat ring.

Facile Reductive Silylation of UO22+ to Uranium(IV) Chloride

Co-reporter:John J. Kiernicki;Dr. Matthias Zeller; Suzanne C. Bart

Angewandte Chemie International Edition 2017 Volume 56(Issue 4) pp:1097-1100

Publication Date(Web):2017/01/19

DOI:10.1002/anie.201609838

AbstractGeneral reductive silylation of the UO22+ cation occurs readily in a one-pot, two-step stoichiometric reaction at room temperature to form uranium(IV) siloxides. Addition of two equivalents of an alkylating reagent to UO2X2(L)2 (X=Cl, Br, I, OTf; L=triphenylphosphine oxide, 2,2′-bipyridyl) followed by two equivalents of a silyl (pseudo)halide, R3Si-X (R=aryl, alkyl, H; X=Cl, Br, I, OTf, SPh), cleanly affords (R3SiO)2UX2(L)2 in high yields. Support is included for the key step in the process, reduction of UVI to UV. This procedure is applicable to a wide range of commercially available uranyl salts, silyl halides, and alkylating reagents. Under this protocol, one equivalent of SiCl4 or two equivalents of Me2SiCl2 results in direct conversion of the uranyl to uranium(IV) tetrachloride. Full spectroscopic and structural characterization of the siloxide products is reported.

Neodymium(III) Complexes Capable of Multi-Electron Redox Chemistry

Co-reporter:Ezra J. Coughlin;Dr. Matthias Zeller; Suzanne C. Bart

Angewandte Chemie International Edition 2017 Volume 56(Issue 40) pp:12142-12145

Publication Date(Web):2017/09/25

DOI:10.1002/anie.201705423

AbstractA family of neodymium complexes featuring a redox-active ligand in three different oxidation states has been synthesized, including the iminoquinone (L0) derivative, (dippiq)2NdI3 (1-iq), the iminosemiquinone (L1−) compound, (dippisq)2NdI(THF) (1-isq), and the amidophenolate (L2−) [K(THF)2][(dippap)2Nd(THF)2] (1-ap) and [K(18-crown-6)][(dippap)2Nd(THF)2] (1-ap crown) species. Full spectroscopic and structural characterization of each derivative established the +3 neodymium oxidation state with redox chemistry occurring at the ligand rather than the neodymium center. Oxidation with elemental chalcogens showed the reversible nature of the ligand-mediated reduction process, forming the iminosemiquinone metallocycles, [K(18-crown-6)][(dippisq)2Nd(S5)] (2-isq crown) and [K(18-crown-6)(THF)][(dippisq)2Nd(Se5)] (3-isq crown), which are characterized to contain a 6-membered twist-boat ring.

A Uranium(IV) Triamide Species with Brønsted Basic Ligand Character: Metal–Ligand Cooperativity in the f Block

Co-reporter:John J. Kiernicki, Selena L. Staun, Matthias Zeller, and Suzanne C. Bart

Organometallics 2017 Volume 36(Issue 3) pp:

Publication Date(Web):January 12, 2017

DOI:10.1021/acs.organomet.6b00861

Deprotonation of the tridentate triamine ligand H3N3Mes ((2,4,6-Me3C6H2N(H)CH2CH2)2NH) with 2 equiv of KCH2Ph followed by treatment with 1 equiv of UCl4 afforded the diamidoamine uranium complex (THF)2UCl2(HN3Mes) (1-THF). This species was further derivatized with either O═PPh3 or KCp* to generate (Ph3PO)UCl2(HN3Mes) (1-OPPh3) or Cp*UCl(HN3Mes) (2-Cl), respectively. Deprotonation of 2-Cl with nBuLi furnished the uranium(IV) triamido compound Cp*U(N3Mes-LiCl(THF)2) (3-LiCl), which is stabilized by the presence of LiCl. 3-LiCl reacts readily with alcohols and thiols, including HOPh, HSPh, and HOtBu, to furnish the respective products Cp*U(OPh)(HN3Mes) (2-OPh), Cp*U(SPh)(HN3Mes) (2-SPh), and Cp*U(OtBu)(HN3Mes) (2-OtBu), which show cooperative addition of the H–E (E = O, S) bond across the U–N bond, serving to regenerate the diamidoamine ligand. Similar cooperative addition was noted for 3-LiCl with benzophenone, furnishing Cp*U(N3Mes-OCPh2) (3-OCPh2), which features new U–O and N–C bonds. The Brønsted basicity of the central nitrogen of 3-LiCl was illustrated by addition of PhOAc, which favored α-carbon deprotonation over nucleophilic attack at the carbonyl. All species were subject to a complete spectroscopic and crystallographic analysis, confirming that the reactivity of 3-LiCl in fact involves cooperation from the triamido ligand and uranium center.

Synthesis and characterization of hydrotris(3-phenylpyrazolyl)borate ligands on low-valent uranium

Co-reporter:Sara A. Johnson, Caleb J. Tatebe, Stephanie Gonzalez, Matthias Zeller, Suzanne C. Bart

Polyhedron 2017 Volume 125(Volume 125) pp:

Publication Date(Web):29 March 2017

DOI:10.1016/j.poly.2016.09.024

Treating both UI3(THF)4 and UI4(dioxane)2 with one equivalent of potassium hydrotris(3-phenylpyrazolyl)borate, KTpPh, resulted in the formation of new low-valent uranium derivatives, TpPhUI2(THF)2 (1) and TpPhUI3 (2), respectively. Spectroscopic characterization of these species shows that the TpPh ligand is 1,2-borotropically shifted, causing one phenyl group to point towards the B–H rather than the uranium center. Derivatization of these materials was possible via salt metathesis reactions, and in each case, the borotropically shifted phenyl group is maintained. Treating trivalent 1 with one equivalent of potassium phenoxide, KOPh, afforded trivalent TpPhU(OPh)I(THF)2 (1-OPh), whereas, a uranium(IV) derivative, TpPhUI2(Phpz)(THF) (2-Phpz), was synthesized by treating 2 with potassium 3-phenylpyrazolide, KPhpz. All compounds were characterized by 1H and 11B NMR, infrared, and electronic absorption spectroscopies, and where possible, X-ray crystallography.The synthesis and characterization of low valent uranium compounds with a tridentate tris(3-phenylpyrazolyl)borate ligand are reported. The scorpianate ligand features a unique 1,2-borotropic shift as a means of relieving steric pressure around the congested uranium metal center.Download high-res image (79KB)Download full-size image

Examining the Effects of Ligand Variation on the Electronic Structure of Uranium Bis(imido) Species

Co-reporter:John J. Kiernicki, Maryline G. Ferrier, Juan S. Lezama Pacheco, Henry S. La Pierre, Benjamin W. Stein, Matthias Zeller, Stosh A. Kozimor, and Suzanne C. Bart

Journal of the American Chemical Society 2016 Volume 138(Issue 42) pp:13941-13951

Publication Date(Web):October 12, 2016

DOI:10.1021/jacs.6b06989

Arylazide and diazene activation by highly reduced uranium(IV) complexes bearing trianionic redox-active pyridine(diimine) ligands, [CpPU(MesPDIMe)]2 (1-CpP), Cp*U(MesPDIMe)(THF) (1-Cp*) (CpP = 1-(7,7-dimethylbenzyl)cyclopentadienide; Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienide), and Cp*U(tBu-MesPDIMe) (THF) (1-tBu) (2,6-((Mes)N═CMe)2-p-R-C5H2N, Mes = 2,4,6-trimethylphenyl; R = H, MesPDIMe; R = C(CH3)3, tBu-MesPDIMe), has been investigated. While 1-Cp* and 1-CpP readily reduce N3R (R = Ph, p-tolyl) to form trans-bis(imido) species, CpPU(NAr)2(MesPDIMe) (Ar = Ph, 2-CpP; Ar = p-Tol, 3-CpP) and Cp*U(NPh)2(MesPDIMe) (2-Cp*), only 1-Cp* can cleave diazene N═N double bonds to form the same product. Complexes 2-Cp*, 2-CpP, and 3-CpP are uranium(V) trans-bis(imido) species supported by neutral [MesPDIMe]0 ligands formed by complete oxidation of [MesPDIMe]3– ligands of 1-CpP and 1-Cp*. Variation of the arylimido substituent in 2-Cp* from phenyl to p-tolyl, forming Cp*U(NTol)2(MesPDIMe) (3-Cp*), changes the electronic structure, generating a uranium(VI) ion with a monoanionic pyridine(diimine) radical. The tert-butyl-substituted analogue, Cp*U(NTol)2(tBu-MesPDIMe) (3-tBu), displays the same electronic structure. Oxidation of the ligand radical in 3-Cp* and 3-tBu by Ag(I) forms cationic uranium(VI) [Cp*U(NTol)2(MesPDIMe)][SbF6] (4-Cp*) and [Cp*U(NTol)2(tBu-MesPDIMe)][SbF6] (4-tBu), respectively, as confirmed by metrical parameters. Conversely, oxidation of pentavalent 2-Cp* with AgSbF6 affords cationic [Cp*U(NPh)2(MesPDIMe)][SbF6] (5-Cp*) from a metal-based U(V)/U(VI) oxidation. All complexes have been characterized by multidimensional NMR spectroscopy with assignments confirmed by electronic absorption spectroscopy. The effective nuclear charge at uranium has been probed using X-ray absorption spectroscopy, while structural parameters of 1-CpP, 3-Cp*, 3-tBu, 4-Cp*, 4-tBu, and 5-Cp* have been elucidated by X-ray crystallography.

Elucidating the Mechanism of Uranium Mediated Diazene N═N Bond Cleavage

Co-reporter:John J. Kiernicki, Robert F. Higgins, Steven J. Kraft, Matthias Zeller, Matthew P. Shores, and Suzanne C. Bart

Inorganic Chemistry 2016 Volume 55(Issue 22) pp:11854-11866

Publication Date(Web):November 2, 2016

DOI:10.1021/acs.inorgchem.6b01922

Investigation into the reactivity of reduced uranium species toward diazenes has revealed key intermediates in the four-electron cleavage of azobenzene. Trivalent Tp*2U(CH2Ph) (1a) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) and Tp*2U(2,2′-bpy) (1b) both perform the two-electron reduction of diazenes affording η2-hydrazido complexes Tp*2U(AzBz) (2-AzBz) (AzBz = azobenzene) and Tp*2U(BCC) (2-BCC) (BCC = benzo[c]cinnoline) in contrast to precursors of the bis(Cp*) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide) ligand framework. The four-electron cleavage of diazenes to give trans-bis(imido) species was possible by using Cp*U(MesPDIMe)(THF) (3) (MesPDIMe = 2,6-((Mes)N═CMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), which is supported by a highly reduced trianionic chelate that undergoes electron transfer. This proceeds via concerted addition at a single uranium center supported by both a crossover experiment and through addition of an asymmetrically substituted diazene, Ph-N═N-Tol. Further investigation of 3 and its substituted analogue, Cp*U(tBu-MesPDIMe)(THF) (3-tBu) (tBu-MesPDIMe = 2,6-((Mes)N═CMe)2-p-C(CH3)3-C5H2N), with benzo[c]cinnoline, revealed that the four-electron cleavage occurs first by a single electron reduction of the diazene with the redox chemistry performed solely at the redox-active pyridine(diimine) to form dimeric [Cp*U(BCC)(MesHPDIMe)]2 (5) and Cp*U(BCC)(tBu-MesPDIMe) (6). While a transient pyridine(diimine) triplet diradical in the formation of 5 results in H atom abstraction and p-pyridine coupling, the tert-butyl moiety in 6 allows for electronic rearrangement to occur, precluding deleterious pyridine-radical coupling. The monomeric analogue of 5, Cp*U(BCC)(MesPDIMe) (7), was synthesized via salt metathesis from Cp*UI(MesPDIMe) (3-I). All complexes have been characterized by 1H NMR and electronic absorption spectroscopies, X-ray diffraction, and, where pertinent, EPR spectroscopy. Further, the electronic structures of 3-I, 5, and 7 have been investigated by SQUID magnetometry.

Reductive silylation of Cp*UO2(MesPDIMe) promoted by Lewis bases

Co-reporter:J. J. Kiernicki, J. S. Harwood, P. E. Fanwick and S. C. Bart

Dalton Transactions 2016 vol. 45(Issue 7) pp:3111-3119

Publication Date(Web):18 Jan 2016

DOI:10.1039/C5DT04776E

Functionalization of the uranyl moiety (UO22+) in Cp*UO2(MesPDIMe) (1-PDI) (MesPDIMe = 2,6-((Mes)NCMe)2C5H3N; Mes = 2,4,6-triphenylmethyl), which bears a reduced, monoanionic pyridine(diimine) ligand, is reported. Silylating reagents, R3Si-X (R = Me, X = Cl, I, OTf, SPh; R = Ph, X = Cl), effectively add across the strong OUO bonds in the presence of the Lewis base, OPPh3, generating products of the form (R3SiO)2UX2(OPPh3)2 (R = Me, X = I (2-OPPh3), Cl (3-OPPh3), SPh (5-OPPh3), OTf (6-OPPh3); R = Ph, X = Cl (4-OPPh3)). During this transformation, reduction to uranium(IV) occurs with loss of (Cp*)2 and MesPDIMe, each of which acts as a one-electron source. In the reaction, the Lewis base serves to activate the silyl halide, generating a more electrophilic silyl group, as determined by 29Si NMR spectroscopy, that undergoes facile transfer to the oxo groups. Complete U–O bond scission was accomplished by treating the uranium(IV) disiloxide compounds with additional silylating reagent, forming the family (Ph3PO)2UX4. All compounds were characterized by 1H NMR, infrared, and electronic absorption spectroscopies. X-ray crystallographic characterization was used to elucidate the structures of 2-OPPh3, 4-OPPh3, 5-OPPh3, and 6-OPPh3.

Synthesis, Characterization, and Stoichiometric U–O Bond Scission in Uranyl Species Supported by Pyridine(diimine) Ligand Radicals

Co-reporter:John J. Kiernicki; Dennis P. Cladis; Phillip E. Fanwick; Matthias Zeller

Journal of the American Chemical Society 2015 Volume 137(Issue 34) pp:11115-11125

Publication Date(Web):August 24, 2015

DOI:10.1021/jacs.5b06217

Two uranium(VI) uranyl compounds, Cp*UO2(MesPDIMe) (3) and Cp*UO2(tBu-MesPDIMe) (3-tBu) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide; MesPDIMe = 2,6-((Mes)N=CMe)2C5H3N; tBu-MesPDIMe = 2,6-((Mes)N=CMe)2-p-C(CH3)3C5H2N; Mes = 2,4,6-trimethylphenyl), have been synthesized by addition of N-methylmorpholine N-oxide to trianionic pyridine(diimine) uranium(IV) precursors, Cp*U(MesPDIMe)(THF) (1), Cp*U(MesPDIMe)(HMPA) (1-HMPA), and Cp*U(tBu-MesPDIMe)(THF) (1-tBu). These uranyl complexes contain singly reduced pyridine(diimine) ligands suggesting formation occurs via cooperative ligand/metal oxidation. Treating 3 or 3-tBu with stoichiometric equivalents of Me3SiI results in stepwise oxo silylation to form (Me3SiO)2UI2(MesPDIMe) (5) or (Me3SiO)UI2(tBu-MesPDIMe) (5-tBu), respectively. Additional equivalents result in full uranium–oxo bond scission and formation of UI4(1,4-dioxane)2 with extrusion of hexamethyldisiloxane. The uranium complexes have been characterized via multinuclear NMR, vibrational, and electronic absorption spectroscopies and, in some cases, X-ray crystallography.

Investigation of the Electronic Ground States for a Reduced Pyridine(diimine) Uranium Series: Evidence for a Ligand Tetraanion Stabilized by a Uranium Dimer

Co-reporter:Nickolas H. Anderson; Samuel O. Odoh; Ursula J. Williams; Andrew J. Lewis; Gregory L. Wagner; Juan Lezama Pacheco; Stosh A. Kozimor; Laura Gagliardi; Eric J. Schelter

Journal of the American Chemical Society 2015 Volume 137(Issue 14) pp:4690-4700

Publication Date(Web):April 1, 2015

DOI:10.1021/ja511867a

The electronic structures of a series of highly reduced uranium complexes bearing the redox-active pyridine(diimine) ligand, MesPDIMe (MesPDIMe = 2,6-(2,4,6-Me3-C6H2-N═CMe)2C5H3N) have been investigated. The complexes, (MesPDIMe)UI3(THF) (1), (MesPDIMe)UI2(THF)2 (2), [(MesPDIMe)UI]2 (3), and [(MesPDIMe)U(THF)]2 (4), were examined using electronic and X-ray absorption spectroscopies, magnetometry, and computational analyses. Taken together, these studies suggest that all members of the series contain uranium(IV) centers with 5f 2 configurations and reduced ligand frameworks, specifically [MesPDIMe]•/–, [MesPDIMe]2–, [MesPDIMe]3– and [MesPDIMe]4–, respectively. In the cases of 2, 3, and 4 no unpaired spin density was found on the ligands, indicating a singlet diradical ligand in monomeric 2 and ligand electron spin-pairing through dimerization in 3 and 4. Interaction energies, representing enthalpies of dimerization, of −116.0 and −144.4 kcal mol–1 were calculated using DFT for the monomers of 3 and 4, respectively, showing there is a large stabilization gained by dimerization through uranium–arene bonds. Highlighted in these studies is compound 4, bearing a previously unobserved pyridine(diimine) tetraanion, that was uniquely stabilized by backbonding between uranium cations and the η5-pyridyl ring.

Synthesis of low-valent uranium fluorides by C–F bond activation

Co-reporter:Christopher L. Clark, Jill J. Lockhart, Phillip E. Fanwick and Suzanne C. Bart

Chemical Communications 2015 vol. 51(Issue 74) pp:14084-14087

Publication Date(Web):07 Aug 2015

DOI:10.1039/C5CC05049A

The uranium(III) alkyl, Tp*2UCH2Ph (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), activates C–F bonds on a variety of fluorinated substrates. From these reactions two new uranium containing products, Tp*2UF and Tp*2UF2, were isolated and characterized by 1H, 13C, 11B NMR, infrared and electronic absorption spectroscopies, as well as X-ray crystallography. Formation of the uranium(III) or uranium(IV) product was found to be substrate dependent.

Spectroscopic and Structural Elucidation of Uranium Dioxophenoxazine Complexes

Co-reporter:Scott A. Pattenaude, Christopher S. Kuehner, Walter L. Dorfner, Eric J. Schelter, Phillip E. Fanwick, and Suzanne C. Bart

Inorganic Chemistry 2015 Volume 54(Issue 13) pp:6520-6527

Publication Date(Web):June 23, 2015

DOI:10.1021/acs.inorgchem.5b00855

Uranium derivatives of a redox-active, dioxophenoxazine ligand, (DOPOq)2UO2, (DOPOsq)UI2(THF)2, (DOPOcat)UI(THF)2, and Cp*U(DOPOcat)(THF)2 (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate), have been synthesized from U(VI) and U(III) starting materials. Full characterization of these species show uranium complexes bearing ligands in three different oxidation states. The electronic structures of these complexes have been explored using 1H NMR and electronic absorption spectroscopies, and where possible, X-ray crystallography and SQUID magnetometry.

Achievements in uranium alkyl chemistry: celebrating sixty years of synthetic pursuits

Co-reporter:Sara A. Johnson and Suzanne C. Bart

Dalton Transactions 2015 vol. 44(Issue 17) pp:7710-7726

Publication Date(Web):06 Oct 2014

DOI:10.1039/C4DT01621A

Organouranium complexes containing uranium–carbon σ-bonds have been highly sought after since the initial exploration of these complexes during the 1950s. Since this time, a variety of uranium starting materials have been developed and alkylating reagents used in order to generate such species. Trivalent uranium alkyl compounds have recently moved past using the bis(trimethylsilyl)methyl ligand with the use of larger ancillary hydrotris(pyrazolyl)borate ligands. The uranium(IV) congeners are dominated by cyclopentadienyl ligands, but recent developments have shown that amides, alkoxides, and phosphines are also suitable ligand frameworks for supporting such species. A family of uranium(IV) alkyls formed via cyclometallation and neutral homoleptics have also been described. Highly reactive uranium(V) and (VI) alkyl complexes have recently been synthesized at low temperatures. The representative studies highlighted herein have helped to pioneer the field of organouranium alkyl chemistry.

Investigation of Uranium Tris(imido) Complexes: Synthesis, Characterization, and Reduction Chemistry of [U(NDIPP)3(thf)3]

Co-reporter:Nickolas H. Anderson;Haolin Yin;John J. Kiernicki;Dr. Phillip E. Fanwick; Eric J. Schelter; Suzanne C. Bart

Angewandte Chemie 2015 Volume 127( Issue 32) pp:9518-9521

Publication Date(Web):

DOI:10.1002/ange.201503771

Abstract

Addition of KC₈ to trivalent [UI₃(thf)₄] in the presence of three equivalents of 2,6-diisopropylphenylazide (N₃DIPP) results in the formation of the hexavalent uranium tris(imido) complex [U(NDIPP)₃(thf)₃] (1) through a facile, single-step synthesis. The X-ray crystal structure shows an octahedral complex that adopts a facial orientation of the imido substituents. This structural trend is maintained during the single-electron reduction of 1 to form dimeric [U(NDIPP)₃{K(Et₂O)}]₂ (2). Variable-temperature/field magnetization studies of 2 show two independent U^V 5f ¹ centers, with no antiferromagnetic coupling present. Characterization of these complexes was accomplished using single-crystal X-ray diffraction, variable-temperature ¹H NMR spectroscopy, as well as IR and UV/Vis absorption spectroscopic studies.

Investigation of Uranium Tris(imido) Complexes: Synthesis, Characterization, and Reduction Chemistry of [U(NDIPP)3(thf)3]

Co-reporter:Nickolas H. Anderson;Haolin Yin;John J. Kiernicki;Dr. Phillip E. Fanwick; Eric J. Schelter; Suzanne C. Bart

Angewandte Chemie International Edition 2015 Volume 54( Issue 32) pp:9386-9389

Publication Date(Web):

DOI:10.1002/anie.201503771

Abstract

Addition of KC₈ to trivalent [UI₃(thf)₄] in the presence of three equivalents of 2,6-diisopropylphenylazide (N₃DIPP) results in the formation of the hexavalent uranium tris(imido) complex [U(NDIPP)₃(thf)₃] (1) through a facile, single-step synthesis. The X-ray crystal structure shows an octahedral complex that adopts a facial orientation of the imido substituents. This structural trend is maintained during the single-electron reduction of 1 to form dimeric [U(NDIPP)₃{K(Et₂O)}]₂ (2). Variable-temperature/field magnetization studies of 2 show two independent U^V 5f ¹ centers, with no antiferromagnetic coupling present. Characterization of these complexes was accomplished using single-crystal X-ray diffraction, variable-temperature ¹H NMR spectroscopy, as well as IR and UV/Vis absorption spectroscopic studies.

New Benzylpotassium Reagents and Their Utility for the Synthesis of Homoleptic Uranium(IV) Benzyl Derivatives

Co-reporter:Sara A. Johnson, John J. Kiernicki, Phillip E. Fanwick, and Suzanne C. Bart

Organometallics 2015 Volume 34(Issue 12) pp:2889-2895

Publication Date(Web):May 27, 2015

DOI:10.1021/acs.organomet.5b00212

A new family of benzylpotassium reagents, KBn′(1-Bn′) (Bn′ = p-iPrBn, p-tBuBn, p-NMe2Bn, p-SMeBn, m-OMeBn, o-OMeBn, 2-picolyl), was synthesized using a modified literature procedure and characterized by multinuclear NMR spectroscopy. Combining four equivalents of 1-Bn′ with UCl4 at low temperature in THF afforded the homoleptic uranium(IV) derivatives 2-Bn′ (2-p-iPr, 2-p-tBu, 2-p-NMe2, 2-p-SMe, 2-o-Picolyl, 2-m-OMe, 2-o-OMe). In addition to 1H NMR spectroscopic characterization, structural studies of five of these organouranium compounds (2-p-iPr, 2-p-tBu, 2-o-Picolyl, 2-m-OMe, 2-o-OMe) were performed, showing that in many cases the benzyl groups are coordinated in an η4-fashion, lending stability to these otherwise low-coordinate molecules. In the cases of U(o-OMeBn)4 (2-o-OMe) and U(2-picolyl)4 (2-o-Picolyl), heteroatom coordination to the uranium center is observed.

Utility of a redox-active pyridine(diimine) chelate in facilitating two electron oxidative addition chemistry at uranium

Co-reporter:John J. Kiernicki, Phillip E. Fanwick and Suzanne C. Bart

Chemical Communications 2014 vol. 50(Issue 60) pp:8189-8192

Publication Date(Web):13 Jun 2014

DOI:10.1039/C4CC03355H

Exposure of the uranium(IV) complex, CpPU(MesPDIMe) (1) (MesPDIMe = 2,6-((Mes)NCMe)2–C5H3N; Mes = 2,4,6-trimethylphenyl; CpP = 1-(7,7-dimethylbenzyl)cyclopentadienyl), which contains a [MesPDIMe]3− chelate, to I2, Cl2, PhSeCl, and PhEEPh (E = S, Se, Te) results in oxidative addition to form the uranium(IV) family, CpPU(XX′)(MesPDIMe) (X = X′ = I, Cl, EPh; X = SePh, X′ = Cl). Spectroscopic and structural studies support products with [MesPDIMe]1−, indicating the reducing equivalents derive from this redox-active chelate.

Trivalent Uranium Phenylchalcogenide Complexes: Exploring the Bonding and Reactivity with CS2 in the Tp*2UEPh Series (E = O, S, Se, Te)

Co-reporter:Ellen M. Matson, Andrew T. Breshears, John J. Kiernicki, Brian S. Newell, Phillip E. Fanwick, Matthew P. Shores, Justin R. Walensky, and Suzanne C. Bart

Inorganic Chemistry 2014 Volume 53(Issue 24) pp:12977-12985

Publication Date(Web):November 21, 2014

DOI:10.1021/ic5020658

The trivalent uranium phenylchalcogenide series, Tp*2UEPh (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, E = O (1), S (2), Se (3), Te (4)), has been synthesized to investigate the nature of the U–E bond. All compounds have been characterized by 1H NMR, infrared and electronic absorption spectroscopies, and in the case of 4, X-ray crystallography. Compound 4 was also studied by SQUID magnetometry. Computational studies establish Mulliken spin densities for the uranium centers ranging from 3.005 to 3.027 (B3LYP), consistent for uranium–chalcogenide bonds that are primarily ionic in nature, with a small covalent contribution. The reactivity of 2–4 toward carbon disulfide was also investigated and showed reversible CS2 insertion into the U(III)–E bond, forming Tp*2U(κ2-S2CEPh) (E = S (5), Se (6), Te (7)). Compound 5 was characterized crystallographically.

Multielectron C–O Bond Activation Mediated by a Family of Reduced Uranium Complexes

Co-reporter:John J. Kiernicki, Brian S. Newell, Ellen M. Matson, Nickolas H. Anderson, Phillip E. Fanwick, Matthew P. Shores, and Suzanne C. Bart

Inorganic Chemistry 2014 Volume 53(Issue 7) pp:3730-3741

Publication Date(Web):March 10, 2014

DOI:10.1021/ic500012x

A family of cyclopentadienyl uranium complexes supported by the redox-active pyridine(diimine) ligand, MesPDIMe (MesPDIMe = 2,6-((Mes)N═CMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), has been synthesized. Using either Cp* or CpP (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide, CpP = 1-(7,7-dimethylbenzyl)cyclopentadienide), uranium complexes of the type CpXUI2(MesPDIMe) (1-CpX; X = * or P), CpXUI(MesPDIMe) (2-CpX), and CpXU(MesPDIMe)(THF)n (3-CpX; *, n = 1; P, n = 0) were isolated and characterized. The series was generated via ligand centered reduction events; thus the extent of MesPDIMe reduction varies in each case, but the uranium(IV) oxidation state is maintained. Treating 2-CpX, which has a doubly reduced MesPDIMe, with furfural results in radical coupling between the substrate and MesPDIMe, leading to C–C bond formation to form CpXUI(MesPDIMe-CHOC4H3O) (4-CpX). Exposure of 3-Cp* and 3-CpP, which contain a triply reduced MesPDIMe ligand, to benzaldehyde and benzophenone, respectively, results in the corresponding pinacolate complexes Cp*U(O2C2Ph2H2)(MesPDIMe) (5-Cp*) and CpPU(O2C2Ph4)(MesPDIMe) (5-CpP). The reducing equivalents required for this coupling are derived solely from the redox-active ligand, rather than the uranium center. Complexes 1–5 have been characterized by 1H NMR and electronic absorption spectroscopies, and SQUID magnetometry was employed to confirm the mono(anionic) [MesPDIMe]− ligand in 1-CpP and 5-CpP. Structural parameters of complexes 1-CpP, 2-CpX, 4-Cp*, and 5-CpX have been elucidated by X-ray crystallography.

Isolation of a uranium(III) benzophenone ketyl radical that displays redox-active ligand behaviour

Co-reporter:Ellen M. Matson, John J. Kiernicki, Nickolas H. Anderson, Phillip E. Fanwick and Suzanne C. Bart

Dalton Transactions 2014 vol. 43(Issue 48) pp:17885-17888

Publication Date(Web):24 Jul 2014

DOI:10.1039/C4DT01636J

The first uranium(III) charge separated ketyl radical complex, Tp*2U(OC·Ph2), has been isolated and characterized by infrared, 1H NMR, and electronic absorption spectroscopies, along with X-ray crystallography. Tp*2U(OC·Ph2) is a potent two-electron reductant towards N3Mes (Mes = 2,4,6-trimethylphenyl) and (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO), with reducing equivalents derived from the metal centre and the redox-active benzophenone.

Radical Reductive Elimination from Tetrabenzyluranium Mediated by an Iminoquinone Ligand

Co-reporter:Ellen M. Matson, Sebastian M. Franke, Nickolas H. Anderson, Timothy D. Cook, Phillip E. Fanwick, and Suzanne C. Bart

Organometallics 2014 Volume 33(Issue 8) pp:1964-1971

Publication Date(Web):April 7, 2014

DOI:10.1021/om4012104

Reductive elimination from U(CH2Ph)4 (1-Ph) mediated by 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (dippiq) was observed, resulting in the formation of (dippap)2U(CH2Ph)2(THF)2 (2) (dippap = 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)amido]phenolate) and bibenzyl. The crossover experiment with U(CD2C6D5)4 showed formation of bibenzyl-d7, indicating that reductive elimination occurs in a stepwise fashion via benzyl radical extrusion, presumably through an iminosemiquinone tris(benzyl) intermediate, (dippisq)U(CH2Ph)3. Synthesis of this intermediate was attempted by addition of the iminoquinone ligand to UI3(THF)4 to form (dippisq)UI3 (3), followed by alkylation with 3 equiv of benzylpotassium. However, this only resulted in the isolation of 2. Reduction of 3 with KC8 afforded the amidophenolate diiodide species (dippap)UI2(THF)2 (4), maintaining the tetravalent oxidation state of the uranium and reducing the ligand. Attempts at the formation of 2 via addition of 2 equiv of benzylpotassium to 4 resulted in decomposition. The uranium mono(alkyl) (dippap)UI(CH2Ph)(THF)2 (5) was observed upon addition of 1 equiv of benzylpotassium to 4. All products have been characterized by 1H NMR and electronic absorption spectroscopy. X-ray crystallography was employed to ascertain ligand reduction in 2, 3, and 5.

Multi-electron reduction facilitated by a trianionic pyridine(diimine) ligand

Co-reporter:Dennis P. Cladis, John J. Kiernicki, Phillip E. Fanwick and Suzanne C. Bart

Chemical Communications 2013 vol. 49(Issue 39) pp:4169-4171

Publication Date(Web):11 Dec 2012

DOI:10.1039/C2CC37193F

Reduction of Cp*2UI(THF) with KC8 in the presence of a pyridine(diimine) ligand (MesPDIMe) results in the formation of Cp*U(PDI)(THF), which features a triply reduced PDI ligand. This species performs four electron cleavage of azobenzene to generate the uranium bis(imido), Cp*(PDI)U(NPh)2.

Tris(phosphinoamide)-Supported Uranium–Cobalt Heterobimetallic Complexes Featuring Co → U Dative Interactions

Co-reporter:J. Wesley Napoline ; Steven J. Kraft ; Ellen M. Matson ; Phillip E. Fanwick ; Suzanne C. Bart ;Christine M. Thomas

Inorganic Chemistry 2013 Volume 52(Issue 20) pp:12170-12177

Publication Date(Web):October 10, 2013

DOI:10.1021/ic402343q

A series of tris- and tetrakis(phosphinoamide) U/Co complexes has been synthesized. The uranium precursors, (η2-Ph2PNiPr)4U (1), (η2-iPr2PNMes)4U (2), (η2-Ph2PNiPr)3UCl (3), and (η2-iPr2PNMes)3UI (4), were easily accessed via addition of the appropriate stoichiometric equivalents of [Ph2PNiPr]K or [iPr2PNMes]K to UCl4 or UI4(dioxane)2. Although the phosphinoamide ligands in 1 and 4 have been shown to coordinate to U in an η2-fashion in the solid state, the phosphines are sufficiently labile in solution to coordinate cobalt upon addition of CoI2, generating the heterobimetallic Co/U complexes ICo(Ph2PNiPr)3U[η2-Ph2PNiPr] (5), ICo(iPr2PNMes)3U[η2-(iPr2PNMes)] (6), ICo(Ph2PNiPr)3UI (7), and ICo(iPr2PNMes)3UI (8). Structural characterization of complexes 5 and 7 reveals reasonably short Co–U interatomic distances, with 7 exhibiting the shortest transition metal–uranium distance ever reported (2.874(3) Å). Complexes 7 and 8 were studied by cyclic voltammetry to examine the influence of the metal–metal interaction on the redox properties compared with both monometallic Co and heterobimetallic Co/Zr complexes. Theoretical studies are used to further elucidate the nature of the transition metal–actinide interaction.

“Oxidative Addition” of Halogens to Uranium(IV) Bis(amidophenolate) Complexes

Co-reporter:Ellen M. Matson, Stacey R. Opperwall, Phillip E. Fanwick, and Suzanne C. Bart

Inorganic Chemistry 2013 Volume 52(Issue 12) pp:7295-7304

Publication Date(Web):June 4, 2013

DOI:10.1021/ic4009812

A series of U(IV) complexes, (Rap)2U(THF)2 [R = tert-butyl (t-Bu) (1), adamantyl (Ad) (2), diisopropylphenyl (dipp) (3)], supported by the redox-active 4,6-di-tert-butyl-2-(R)amidophenolate ligand, have been synthesized by salt metathesis of 2 equiv of the alkali metal salt of the ligand, M2[Rap] [M = K (1 and 2), Na (3)], with UCl4. Exposure of these uranium complexes to 1 equiv of PhICl2 results in oxidative addition to uranium, forming the bis-(4,6-di-tert-butyl-2-(R)iminosemiquinone) ([Risq]1–) uranium(IV) dichloride dimer, [(Risq)2UCl]2(μ2-Cl)2 [R = t-Bu (4), Ad (5), dipp (6)]. The addition of iodine to 1 forms (tBuisq)2UI2(THF) (7), while the reactivity of I2 with 2 and 3 results in decomposition. Complexes 1–7 have been characterized by 1H NMR and electronic absorption spectroscopies. X-ray crystallography was employed to elucidate structural parameters of 2, 3, 5, and 7.

Synthesis of Terminal Uranium(IV) Disulfido and Diselenido Compounds by Activation of Elemental Sulfur and Selenium

Co-reporter:Ellen M. Matson;Mitchell D. Goshert;John J. Kiernicki;Brian S. Newell;Phillip E. Fanwick; Matthew P. Shores; Justin R. Walensky; Suzanne C. Bart

Chemistry - A European Journal 2013 Volume 19( Issue 48) pp:16176-16180

Publication Date(Web):

DOI:10.1002/chem.201303095

Exploring the Insertion Chemistry of Tetrabenzyluranium Using Carbonyls and Organoazides

Co-reporter:Steven J. Kraft, Phillip E. Fanwick, and Suzanne C. Bart

Organometallics 2013 Volume 32(Issue 11) pp:3279-3285

Publication Date(Web):May 28, 2013

DOI:10.1021/om400197j

The insertion chemistry of U(CH2C6H5)4 (1) was explored with acetone, benzophenone, mesityl azide, and 1-azidoadamantane. Using 2 equiv of acetone affords the double-insertion product U[OC(CH3)2(CH2C6H5)]2(CH2C6H5)2(THF)2 (2), while using 4 equiv results in the tri-inserted enolate product U[OC(CH3)2(CH2C6H5)]3[OC(CH3)CH2](THF)3 (3). Deuterium labeling experiments aided in the assignment of 2 and 3. With 4 equiv of benzophenone, insertion at all U–C bonds is noted, forming U[OC(C6H5)2(CH2C6H5)]4 (4) and the THF adduct U[OC(C6H5)2(CH2C6H5)]4(THF) (4-THF). Addition of 4 equiv of N3Mes to 1 forms the tetrakis(triazenido)uranium(IV) complex U[CH2(C6H5)NNN(Mes)-κ2N1,2][CH2(C6H5)NNN(Mes)-κ2N1,3]3 (5), while the same reaction with 1-azidoadamantane generates the uranium(VI) trans-bis(imido) complex U(NAd)2[CH2(C6H5)NNN(Ad)-κ2N1,3]2(THF) (6). All species were characterized by 1H NMR and infrared spectroscopy, with select examples being structurally characterized using single-crystal X-ray diffraction.

Carbon–Carbon Reductive Elimination from Homoleptic Uranium(IV) Alkyls Induced by Redox-Active Ligands

Co-reporter:Steven J. Kraft ; Phillip E. Fanwick

Journal of the American Chemical Society 2012 Volume 134(Issue 14) pp:6160-6168

Publication Date(Web):April 2, 2012

DOI:10.1021/ja209524u

The synthesis, characterization, and reactivity of the homoleptic uranium(IV) alkyls U(CH2C6H5)4 (1-Ph), U(CH2-p-CH3C6H4)4 (1-p-Me), and U(CH2-m-(CH3)2C6H3)4 (1-m-Me2) are reported. The addition of 4 equiv of K(CH2Ar) (Ar = Ph, p-CH3C6H4, m-(CH3)2C6H3) to UCl4 at −108 °C produces 1-Ph in good yields and 1-p-Me and 1-m-Me2 in moderate yields. Further characterization of 1-Ph by X-ray crystallography confirmed η4-coordination of each benzyl ligand to the uranium center. Magnetic studies produced an effective magnetic moment of 2.60 μB at 23 °C, which is consistent with a tetravalent uranium 5f 2 electronic configuration. Addition of 1 equiv of the redox-active α-diimine MesDABMe (MesDABMe = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)) to 1-Ph results in reductive elimination of 1 equiv of bibenzyl (PhCH2CH2Ph), affording (MesDABMe)U(CH2C6H5)2 (2-Ph). Treating an equimolar mixture of 1-Ph and 1-Ph-d28 with MesDABMe forms the products from monomolecular reductive elimination, 2-Ph, 2-Ph-d14, bibenzyl, and bibenzyl-d14. This is confirmed by 1H NMR spectroscopy and GC/MS analysis of both organometallic and organic products. Addition of 1 equiv of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1-Ph results in formation of the previously synthesized (dmpe)U(CH2C6H5)4 (3-Ph), indicating the redox-innocent chelating phosphine stabilizes the uranium center in 3-Ph and prevents reductive elimination of bibenzyl. Full characterization for 3-Ph, including X-ray crystallography, is reported.

Reductive heterocoupling mediated by Cp*2U(2,2′-bpy)

Co-reporter:Adil Mohammad, Dennis P. Cladis, William P. Forrest, Phillip E. Fanwick and Suzanne C. Bart

Chemical Communications 2012 vol. 48(Issue 11) pp:1671-1673

Publication Date(Web):25 Nov 2011

DOI:10.1039/C2CC16200H

Trivalent Cp*2U(2,2′-bpy) (2) (Cp* = C5Me5, 2,2′-bpy = 2,2′-bipyridine), which has a monoanionic bipyridine, was treated with p-tolualdehyde (a), furfuraldehyde (b), acetone (c), and benzophenone (d). Reduction of the CO bond followed by radical coupling with bipyridine forms the U(IV) derivatives [Cp*2U(2,2′-bpy)(OCRR′)] (3a–d).

Computational Insights into Uranium Complexes Supported by Redox-Active α-Diimine Ligands

Co-reporter:Giovanni Li Manni ; Justin R. Walensky ; Steven J. Kraft ; William P. Forrest ; Lisa M. Pérez ; Michael B. Hall ; Laura Gagliardi

Inorganic Chemistry 2012 Volume 51(Issue 4) pp:2058-2064

Publication Date(Web):January 30, 2012

DOI:10.1021/ic202522w

The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (MesDABMe)2U(THF) (1) and Cp2U(MesDABMe) (2) (MesDABMe = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum.

Synthesis of U(IV) imidos from Tp*2U(CH2Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) by extrusion of bibenzyl

Co-reporter:Ellen M. Matson, Marco G. Crestani, Phillip E. Fanwick and Suzanne C. Bart

Dalton Transactions 2012 vol. 41(Issue 26) pp:7952-7958

Publication Date(Web):04 Apr 2012

DOI:10.1039/C2DT12439D

Addition of organic azides, N3R (R = 2,4,6-trimethylphenyl (Mes), phenyl (Ph), 1-adamantyl (Ad)), to a solution of the uranium(III) alkyl complex, Tp*2U(CH2Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) (1), results in the formation of a family of uranium(IV) imido derivatives, Tp*2U(NR) (2-R). Notably, these complexes were synthesized in high yields by coupling of the benzyl groups to form bibenzyl. The uranium(IV) imido derivatives, 2-Mes, 2-Ph, and 2-Ad, were all characterized by both 1H NMR and IR spectroscopy, and 2-Mes and 2-Ad were also characterized by X-ray crystallography. In the molecular structure of 2-Mes, typical κ3-coordination of the Tp* ligands was observed; however in the case of 2-Ad, one pyrazole ring of a Tp* ligand has rotated away from the metal centre, forcing a κ2-coordination of the pyrazoles. This results in a uranium–hydrogen interaction with the Tp* B–H. Treating these imido complexes with para-tolualdehyde results in multiple bond metathesis, forming the terminal uranium(IV) oxo complex, Tp*2U(O), and the corresponding imine.

Diazoalkane Reduction for the Synthesis of Uranium Hydrazonido Complexes (Eur. J. Inorg. Chem. 33/2012)

Co-reporter:Ellen M. Matson;Phillip E. Fanwick

European Journal of Inorganic Chemistry 2012 Volume 2012( Issue 33) pp:

Publication Date(Web):

DOI:10.1002/ejic.201290106

Diazoalkane Reduction for the Synthesis of Uranium Hydrazonido Complexes

Co-reporter:Ellen M. Matson;Phillip E. Fanwick

European Journal of Inorganic Chemistry 2012 Volume 2012( Issue 33) pp:5471-5478

Publication Date(Web):

DOI:10.1002/ejic.201200606

Abstract

The reactivity of the uranium(III) alkyl Tp*₂UCH₂Ph (1) toward diazoalkanes is reported. Addition of 1 equiv. N₂CPh₂ produces 0.5 equiv. bibenzyl, along with Tp*₂U(N₂CPh₂) (2). This species is dynamic in solution at room temperature and rapidly interconverts between the η¹- and η² isomers as determined by variable-temperature ¹H NMR spectroscopy. X-ray crystallographic analysis at low temperature shows exclusively the η² isomer, which features a short U–N multiple bond analogous to an imido species. The η¹ isomer reacts quantitatively with aldehydes and ketones through multiple bond metathesis to produce Tp*₂U(O) and the corresponding ketazine. Treatment of 1 with N₂CHSiMe₃ generates 0.5 equiv. bibenzyl and the η¹ isomer Tp*₂U(N₂CHSiMe₃) (3). This species is unstable over the course of hours, and there is no spectroscopic evidence for the η² isomer. Tp*₂U(η¹-N₂CHSiMe₃) can be trapped by addition of phenylacetylene by a [2+2] cycloaddition to afford the uranium(IV) metallacycle Tp*₂U[(N-N = CHSiMe₃)CHCPh] (4). Crystallographic data for 4 are presented.

Use of Alkylsodium Reagents for the Synthesis of Trivalent Uranium Alkyl Complexes

Co-reporter:Ellen M. Matson, William P. Forrest, Phillip E. Fanwick, and Suzanne C. Bart

Organometallics 2012 Volume 31(Issue 12) pp:4467-4473

Publication Date(Web):June 1, 2012

DOI:10.1021/om3002763

A family of rare uranium(III) alkyl complexes, Tp*2UR (R = CH2SiMe3 (3-CH2SiMe3), CH3 (4-CH3), (CH2)3CH3 (5-(CH2)3CH3); Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by salt metathesis with alkylsodium reagents and Tp*2UI (2). All compounds were fully characterized using 1H NMR, infrared, and electronic absorption spectroscopies. Compounds 3-CH2SiMe3 and 4-CH3 were structurally characterized using X-ray crystallography and have U–C bond distances of 2.601(9) and 2.54(3) Å, respectively.

Functionalization of Carbon Dioxide and Carbon Disulfide Using a Stable Uranium(III) Alkyl Complex

Co-reporter:Ellen M. Matson ; William P. Forrest ; Phillip E. Fanwick

Journal of the American Chemical Society 2011 Volume 133(Issue 13) pp:4948-4954

Publication Date(Web):March 8, 2011

DOI:10.1021/ja110158s

A rare uranium(III) alkyl complex, Tp*2U(CH2Ph) (2) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by salt metathesis from Tp*2UI (1) and KCH2Ph and fully characterized using 1H NMR, infrared, and electronic absorption spectroscopies as well as X-ray crystallography. This complex has a uranium−carbon distance of 2.57(2) Å, which is comparable to other uranium alkyls reported. Treating this compound with either carbon dioxide or carbon disulfide results in insertion into the uranium−carbon bond to generate Tp*2U(κ2-O2CCH2Ph) (3) and Tp*2U(SC(S)CH2Ph) (4), respectively. These species, characterized spectroscopically and by X-ray crystallography, feature new carboxylate and dithiocarboxylate ligands. Analysis by electronic absorption spectroscopy supports the trivalent oxidation state of the uranium center in both of these derivatives. Addition of trimethylsilylhalides (Me3SiX; X = Cl, I) to 3 results in the release of the free silyl ester, Me3SiOC(O)CH2Ph, forming the initial uranium monohalide species, Tp*2UX, which can then be used over multiple cycles for the functionalization of carbon dioxide.

Synthesis, Characterization, and Multielectron Reduction Chemistry of Uranium Supported by Redox-Active α-Diimine Ligands

Co-reporter:Steven J. Kraft, Ursula J. Williams, Scott R. Daly, Eric J. Schelter, Stosh A. Kozimor, Kevin S. Boland, James M. Kikkawa, William P. Forrest, Christin N. Christensen, Daniel E. Schwarz, Phillip E. Fanwick, David L. Clark, Steve D. Conradson, and Suzanne C. Bart

Inorganic Chemistry 2011 Volume 50(Issue 20) pp:9838-9848

Publication Date(Web):July 15, 2011

DOI:10.1021/ic2002805

Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {MesDABMe = [ArN═C(Me)C(Me)═NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of MesDABMe, 3 equiv of KC8, and 1 equiv of UI3(THF)4 produced the bis(ligand) species (MesDABMe)2U(THF) (1). The metallocene derivative, Cp2U(MesDABMe) (2), was generated by the addition of an equimolar ratio of MesDABMe and KC8 to Cp3U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U LIII-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μB for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH3I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, (MesDABMe)(MesDABMe2)UI {3; MesDABMe2 = [ArN═C(Me)C(Me2)NAr]}, which was characterized by single-crystal X-ray diffraction and U M4- and M5-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone.

Formation of Trivalent U–C, U–N, and U–S Bonds and Their Reactivity toward Carbon Dioxide and Acetone

Co-reporter:Ellen M. Matson, Phillip E. Fanwick, and Suzanne C. Bart

Organometallics 2011 Volume 30(Issue 21) pp:5753-5762

Publication Date(Web):October 17, 2011

DOI:10.1021/om200612h

A family of uranium(III) complexes of the form Tp*2UX (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, X = CCPh (2-CCPh), CCSiMe3 (3-CCSiMe3), NHPh (4-NHPh), NHCH2Ph (5-NHCH2Ph), SPh (6-SPh)) have been synthesized in quantitative yields by protonation of the benzyl group Tp*2U(CH2Ph) (1-CH2Ph) with organic precursors. Full characterization of these complexes, which contain newly formed uranium–carbon, uranium–nitrogen, and uranium–sulfur bonds, was performed using 1H NMR and IR spectroscopies. These data along with the structural parameters of the uranium acetylide (2-CCPh) and uranium thiolate (6-SPh) species are reported. Treating the trivalent uranium acetylides and amides with 1 atm of carbon dioxide results in its insertion into the uranium–element bond to form the corresponding carboxylate or carbamate moiety in quantitative isolable yields. Reversible insertion of carbon dioxide (1 atm) into the uranium–sulfur bond of Tp*2U(SPh) (6-SPh) was also noted. The new family of CO2-inserted uranium(III) derivatives Tp*2UO2CX (X = CCPh (7-O2CCCPh), CCSiMe3 (8-O2CCCSiMe3), NHPh (9-O2CNHPh), NHCH2Ph (10-O2CNHCH2Ph), SPh (11-O2CSPh)) has been characterized spectroscopically and, in the case of 7-O2CCCPh, crystallographically. The newly formed U–O bonds were cleaved by addition of trimethylsilyl halides, generating the monohalide derivative. Attempts to insert acetone into the uranium–carbon bonds of 1-CH2Ph and 2-CCPh or the uranium–nitrogen bond in 4-NHPh resulted exclusively in protonation of the benzyl ligand and isomerization to form a rare enolate complex Tp*2U(OC(CH3)CH2) (12-OC(CH3)CH2). This complex has been fully characterized by 1H NMR and infrared spectroscopies as well as X-ray crystallography.

Crystallographic Evidence of a Base-Free Uranium(IV) Terminal Oxo Species

Co-reporter:Steven J. Kraft, Justin Walensky, Phillip E. Fanwick, Michael B. Hall and Suzanne C. Bart

Inorganic Chemistry 2010 Volume 49(Issue 17) pp:7620-7622

Publication Date(Web):July 30, 2010

DOI:10.1021/ic101136j

The uranium(IV) terminal oxo species Tp*2U(O) has been synthesized by oxygen-atom transfer from pyridine-N-oxide to Tp*2U(2,2′-bipyridine), a trivalent uranium species with a monoanionic bipyridine ligand. Full characterization of the oxo species using 1H NMR and IR spectroscopies, X-ray crystallography, and computational studies was performed.

Synthesis and Characterization of a Uranium(III) Complex Containing a Redox-Active 2,2′-Bipyridine Ligand

Co-reporter:Steven J. Kraft ; Phillip E. Fanwick

Inorganic Chemistry 2010 Volume 49(Issue 3) pp:1103-1110

Publication Date(Web):January 5, 2010

DOI:10.1021/ic902008w

Hydrotris(3,5-dimethylpyrazolyl)borate uranium(III) diiodide derivatives have been prepared as an entry into low-valent uranium chemistry with these ligands. The bis(tetrahydrofuran) adduct, Tp*UI2(THF)2 (1) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by addition of sodium hydrotris(3,5-dimethylpyrazolyl)borate (NaTp*) to an equivalent of UI3(THF)4. Addition of 2,2′-bipyridine (2,2′-bpy) to 1 displaced the THF molecules producing Tp*UI2(2,2′-bpy) (2). Both derivatives were characterized by 1H NMR and IR spectroscopies, magnetic measurements, and X-ray crystallography. Reduction of both species was attempted with two equivalents of potassium graphite. The reduction of 1 did not result in a clean product, but rather decomposition and ligand redistribution. However, compound 2 was reduced to form Tp*2U(2,2′-bpy), 3, which is composed of a uranium(III) ion with a radical monoanionic bipyridine ligand. This was confirmed by X-ray crystallography, which revealed distortions in the bond lengths of the bipyridine consistent with reduction. Further support was obtained by 1H NMR spectroscopy, which showed resonances shifted far upfield, consistent with radical character on the 2,2′-bipyridine ligand. Future studies will explore the reactivity of this compound as well as the consequences for redox-activity in the bipyridine ligand.

Reductive silylation of Cp*UO2(MesPDIMe) promoted by Lewis bases

Co-reporter:J. J. Kiernicki, J. S. Harwood, P. E. Fanwick and S. C. Bart

Dalton Transactions 2016 - vol. 45(Issue 7) pp:NaN3119-3119

Publication Date(Web):2016/01/18

DOI:10.1039/C5DT04776E

Functionalization of the uranyl moiety (UO22+) in Cp*UO2(MesPDIMe) (1-PDI) (MesPDIMe = 2,6-((Mes)NCMe)2C5H3N; Mes = 2,4,6-triphenylmethyl), which bears a reduced, monoanionic pyridine(diimine) ligand, is reported. Silylating reagents, R3Si-X (R = Me, X = Cl, I, OTf, SPh; R = Ph, X = Cl), effectively add across the strong OUO bonds in the presence of the Lewis base, OPPh3, generating products of the form (R3SiO)2UX2(OPPh3)2 (R = Me, X = I (2-OPPh3), Cl (3-OPPh3), SPh (5-OPPh3), OTf (6-OPPh3); R = Ph, X = Cl (4-OPPh3)). During this transformation, reduction to uranium(IV) occurs with loss of (Cp*)2 and MesPDIMe, each of which acts as a one-electron source. In the reaction, the Lewis base serves to activate the silyl halide, generating a more electrophilic silyl group, as determined by 29Si NMR spectroscopy, that undergoes facile transfer to the oxo groups. Complete U–O bond scission was accomplished by treating the uranium(IV) disiloxide compounds with additional silylating reagent, forming the family (Ph3PO)2UX4. All compounds were characterized by 1H NMR, infrared, and electronic absorption spectroscopies. X-ray crystallographic characterization was used to elucidate the structures of 2-OPPh3, 4-OPPh3, 5-OPPh3, and 6-OPPh3.

Synthesis of low-valent uranium fluorides by C–F bond activation

Co-reporter:Christopher L. Clark, Jill J. Lockhart, Phillip E. Fanwick and Suzanne C. Bart

Chemical Communications 2015 - vol. 51(Issue 74) pp:NaN14087-14087

Publication Date(Web):2015/08/07

DOI:10.1039/C5CC05049A

The uranium(III) alkyl, Tp*2UCH2Ph (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), activates C–F bonds on a variety of fluorinated substrates. From these reactions two new uranium containing products, Tp*2UF and Tp*2UF2, were isolated and characterized by 1H, 13C, 11B NMR, infrared and electronic absorption spectroscopies, as well as X-ray crystallography. Formation of the uranium(III) or uranium(IV) product was found to be substrate dependent.

Reductive heterocoupling mediated by Cp*2U(2,2′-bpy)

Co-reporter:Adil Mohammad, Dennis P. Cladis, William P. Forrest, Phillip E. Fanwick and Suzanne C. Bart

Chemical Communications 2012 - vol. 48(Issue 11) pp:NaN1673-1673

Publication Date(Web):2011/11/25

DOI:10.1039/C2CC16200H

Trivalent Cp*2U(2,2′-bpy) (2) (Cp* = C5Me5, 2,2′-bpy = 2,2′-bipyridine), which has a monoanionic bipyridine, was treated with p-tolualdehyde (a), furfuraldehyde (b), acetone (c), and benzophenone (d). Reduction of the CO bond followed by radical coupling with bipyridine forms the U(IV) derivatives [Cp*2U(2,2′-bpy)(OCRR′)] (3a–d).

Utility of a redox-active pyridine(diimine) chelate in facilitating two electron oxidative addition chemistry at uranium

Co-reporter:John J. Kiernicki, Phillip E. Fanwick and Suzanne C. Bart

Chemical Communications 2014 - vol. 50(Issue 60) pp:NaN8192-8192

Publication Date(Web):2014/06/13

DOI:10.1039/C4CC03355H

Exposure of the uranium(IV) complex, CpPU(MesPDIMe) (1) (MesPDIMe = 2,6-((Mes)NCMe)2–C5H3N; Mes = 2,4,6-trimethylphenyl; CpP = 1-(7,7-dimethylbenzyl)cyclopentadienyl), which contains a [MesPDIMe]3− chelate, to I2, Cl2, PhSeCl, and PhEEPh (E = S, Se, Te) results in oxidative addition to form the uranium(IV) family, CpPU(XX′)(MesPDIMe) (X = X′ = I, Cl, EPh; X = SePh, X′ = Cl). Spectroscopic and structural studies support products with [MesPDIMe]1−, indicating the reducing equivalents derive from this redox-active chelate.

Multi-electron reduction facilitated by a trianionic pyridine(diimine) ligand

Co-reporter:Dennis P. Cladis, John J. Kiernicki, Phillip E. Fanwick and Suzanne C. Bart

Chemical Communications 2013 - vol. 49(Issue 39) pp:NaN4171-4171

Publication Date(Web):2012/12/11

DOI:10.1039/C2CC37193F

Reduction of Cp*2UI(THF) with KC8 in the presence of a pyridine(diimine) ligand (MesPDIMe) results in the formation of Cp*U(PDI)(THF), which features a triply reduced PDI ligand. This species performs four electron cleavage of azobenzene to generate the uranium bis(imido), Cp*(PDI)U(NPh)2.

Achievements in uranium alkyl chemistry: celebrating sixty years of synthetic pursuits

Co-reporter:Sara A. Johnson and Suzanne C. Bart

Dalton Transactions 2015 - vol. 44(Issue 17) pp:NaN7726-7726

Publication Date(Web):2014/10/06

DOI:10.1039/C4DT01621A

Organouranium complexes containing uranium–carbon σ-bonds have been highly sought after since the initial exploration of these complexes during the 1950s. Since this time, a variety of uranium starting materials have been developed and alkylating reagents used in order to generate such species. Trivalent uranium alkyl compounds have recently moved past using the bis(trimethylsilyl)methyl ligand with the use of larger ancillary hydrotris(pyrazolyl)borate ligands. The uranium(IV) congeners are dominated by cyclopentadienyl ligands, but recent developments have shown that amides, alkoxides, and phosphines are also suitable ligand frameworks for supporting such species. A family of uranium(IV) alkyls formed via cyclometallation and neutral homoleptics have also been described. Highly reactive uranium(V) and (VI) alkyl complexes have recently been synthesized at low temperatures. The representative studies highlighted herein have helped to pioneer the field of organouranium alkyl chemistry.

Isolation of a uranium(III) benzophenone ketyl radical that displays redox-active ligand behaviour

Co-reporter:Ellen M. Matson, John J. Kiernicki, Nickolas H. Anderson, Phillip E. Fanwick and Suzanne C. Bart

Dalton Transactions 2014 - vol. 43(Issue 48) pp:NaN17888-17888

Publication Date(Web):2014/07/24

DOI:10.1039/C4DT01636J

The first uranium(III) charge separated ketyl radical complex, Tp*2U(OC·Ph2), has been isolated and characterized by infrared, 1H NMR, and electronic absorption spectroscopies, along with X-ray crystallography. Tp*2U(OC·Ph2) is a potent two-electron reductant towards N3Mes (Mes = 2,4,6-trimethylphenyl) and (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO), with reducing equivalents derived from the metal centre and the redox-active benzophenone.

Synthesis of U(IV) imidos from Tp*2U(CH2Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) by extrusion of bibenzyl

Co-reporter:Ellen M. Matson, Marco G. Crestani, Phillip E. Fanwick and Suzanne C. Bart

Dalton Transactions 2012 - vol. 41(Issue 26) pp:NaN7958-7958

Publication Date(Web):2012/04/04

DOI:10.1039/C2DT12439D

Addition of organic azides, N3R (R = 2,4,6-trimethylphenyl (Mes), phenyl (Ph), 1-adamantyl (Ad)), to a solution of the uranium(III) alkyl complex, Tp*2U(CH2Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) (1), results in the formation of a family of uranium(IV) imido derivatives, Tp*2U(NR) (2-R). Notably, these complexes were synthesized in high yields by coupling of the benzyl groups to form bibenzyl. The uranium(IV) imido derivatives, 2-Mes, 2-Ph, and 2-Ad, were all characterized by both 1H NMR and IR spectroscopy, and 2-Mes and 2-Ad were also characterized by X-ray crystallography. In the molecular structure of 2-Mes, typical κ3-coordination of the Tp* ligands was observed; however in the case of 2-Ad, one pyrazole ring of a Tp* ligand has rotated away from the metal centre, forcing a κ2-coordination of the pyrazoles. This results in a uranium–hydrogen interaction with the Tp* B–H. Treating these imido complexes with para-tolualdehyde results in multiple bond metathesis, forming the terminal uranium(IV) oxo complex, Tp*2U(O), and the corresponding imine.

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