A Uranium(IV) Triamide Species with Brønsted Basic Ligand

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A Uranium(IV) Triamide Species with Brønsted Basic Ligand Character: Metal−Ligand Cooperativity in the f Block John J. Kiernicki, Selena L. Staun, Matthias Zeller, and Suzanne C. Bart* H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Deprotonation of the tridentate triamine ligand H3N3Mes ((2,4,6Me3C6H2N(H)CH2CH2)2NH) with 2 equiv of KCH2Ph followed by treatment with 1 equiv of UCl4 afforded the diamidoamine uranium complex (THF)2UCl2(HN3Mes) (1THF). 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. 3LiCl 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.

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INTRODUCTION The importance of metal−ligand cooperativity has been established in a variety of catalytic transformations.1−9 Bifunctional ligands, those that participate in bond-forming or bond-breaking reactions simultaneously with the metal center,3 have risen to the forefront of this field, most notably for their ability to cleave strong bonds.4 Catalysts that feature this capability have widespread use, particularly in systems aimed at transfer hydrogenation,5 hydrogenation of polar multiple bonds,1 and dihydrogen activation.6,7 Cooperative metal− ligand cleavage of heterolytic bonds has also been established.2,8,9 Popular ligands to induce metal−ligand cooperativity include those featuring an amide functionality, which are effective proton acceptors, converting the amide to an amine during the course of a reaction. Thus, attractive choices to study metal− ligand cooperativity are the tridentate diamidoamine10−24 ligands popularized by Schrock, as deprotonation of the central nitrogen gives the triamido21,24−26 form of the ligand. While this triamido form is reminiscent of the related tripodal triamidoamine family, which supports unparalleled chemistry at both transition metals27 and those in the f block,28 the advantage of the tridentate ligand lies in the coordinative unsaturation it imparts to metal centers. We have recently demonstrated the utility of tridentate anionic ligands in small-molecule activation mediated by uranium with the synthesis of Cp*U(MesPDIMe)(THF) (MesPDIMe = 2,6-((Mes)NCMe)2-C5H3N, Mes = 2,4,6trimethylphenyl), which contains a redox-active pyridine(diimine) chelate that is reduced by three electrons.29 This © XXXX American Chemical Society

hard trianionic ligand, along with the uranium(IV) center, is easily oxidized in the presence of organoazides,29 diazenes,30 and N-methylmorpholine N-oxide31 to generate a family of high-valent uranium species featuring multiple bonds. Cp*U(MesPDIMe)(THF) also performs C−C bond formation in the form of pinacol coupling, which occurs upon addition of aldehydic substrates.32 As an alternative to redox cooperativity, we sought to explore small-molecule activation using Schrock’s diethylene triamine derived redox-innocent ligand H3N3Mes ((2,4,6-Me3C6H2N(H)CH2CH2)2NH), which has mesityl substituents on the terminal nitrogens, giving it steric properties similar to those of Mes PDIMe.21 Schrock uses this ligand in its diamido form to support organozirconium catalysts capable of 1-hexene polymerization;21 however, during the course of our studies, we generated a uranium(IV) species with the triamido form of the ligand, where the central nitrogen has a buildup of negative charge, causing it to act as a Brønsted base for the heterolytic cleavage of polarized small molecules. Herein, we report the synthesis of a new family containing tridentate H3N3Mes, where both the diamidoamine (HN3Mes) and triamido (N3Mes) forms of the ligand exhibit cooperative reactivity of one of the metal− amide bonds toward alcohols, thiols, and carbonylated substrates. Characterization using a variety of spectroscopic techniques and X-ray crystallography is also discussed to highlight this rare example of metal−ligand cooperativity for the actinides. Received: November 16, 2016

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DOI: 10.1021/acs.organomet.6b00861 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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the solution was concentrated to ca. 2 mL, layered with 10 mL of n pentane, and cooled to −35 °C to facilitate precipitation. After 16 h, the liquor was decanted and the yellow-brown solid dried in vacuo to afford (Ph3PO)UCl2(MesHN3) (0.270 g, 0.292 mmol, 91%). This sample was subjected to elemental analysis. Single X-ray-quality crystals were obtained from a concentrated THF/n-pentane solution (5/1) stored at −35 °C. Anal. Calcd for C40H46N3Cl2OPU: C, 51.95; H, 5.01; N, 4.54. Found: C, 52.91; H, 5.10; N, 3.92. 1H NMR (C6D6, 25 °C): δ −160.19 (73, 1H, NH), −49.53 (27, 2H, CH), −28.84 (54, 2H, CH), −7.84 (41, 6H, o-OPPh3), 3.85 (33, 6H, m-OPPh3), 4.77 (34, 3H, p-OPPh3), 9.31 (7, 6H, CH3), 19.40 (39, 6H, CH3), 20.81 (10, 2H × 2, Ar-CH), 25.20 (43, 6H, CH3), 49.95 (32, 2H, CH), 49.98 (33, 2H, CH). 31P NMR (C6D6, 25 °C): δ −25.23. IR (KBr): ν 3346 cm−1 (N−H). Synthesis of Cp*UCl(HN3Mes) (2-Cl). A 20 mL scintillation vial was charged with 0.863 g (1.092 mmol) of (THF)2UCl2(HN3Mes) and 15 mL of THF. With stirring, 0.190 g (1.090 mmol) of KCp* was added, resulting in a gradual color change from yellow to orange. After 3 h, volatiles were removed in vacuo. The mixture was extracted into 20 mL of diethyl ether, filtered through Celite, and dried to afford an orange powder (0.759 g, 1.017 mmol, 93%) assigned as Cp*UCl(HN3Mes). This sample was subjected to elemental analysis. Single Xray-quality crystals were obtained from a concentrated diethyl ether solution stored at −35 °C. Anal. Calcd for C32H46N3ClU: C, 51.51; H, 6.21; N, 5.63. Found: C, 51.00; H, 6.31; N, 5.33. 1H NMR (C6D6, 25 °C): δ −70.96 (39, 1H, NH), −9.53 (9, 6H, CH3), −5.20 (27, 2H, CH), 2.76 (4, 6H, CH3), 3.17 (8, 15H, Cp*), 5.00 (12, 6H, CH3), 5.43 (27, 2H, CH), 6.12 (8, 2H, CH), 11.47 (10, 2H, CH), 53.23 (28, 2H, CH), 55.07 (27, 2H, CH). IR (KBr): ν 3333 cm−1 (N−H). Synthesis of Cp*U(N3Mes-LiCl(THF)2) (3-LiCl). A 20 mL scintillation vial was charged with 0.445 g (0.596 mmol) of Cp*UCl(HN3Mes) and 10 mL of THF and frozen. In a separate vial, 0.235 mL (0.588 mmol, 2.5 M in hexane) of n-butyllithium was diluted with 3 mL of THF and frozen. After it was thawed, the n-butyllithium solution was added to the solution of Cp*UCl(HN3Mes) and stirred for 1 h. Following removal of volatiles, the crude sample was washed with cold n-pentane (−35 °C, 3 × 10 mL) to afford a light brown-red solid (0.372 g, 0.415 mmol, 70%) assigned as Cp*U(N3Mes-LiCl(THF)2). This sample was subjected to elemental analysis. Single X-ray-quality crystals were obtained by slow diffusion of n-pentane into a concentrated diethyl ether solution at −35 °C. Anal. Calcd for C36H64N3OLiClU: C, 52.39; H, 6.60; N, 5.09. Found: C, 52.78; H, 6.63; N, 5.51. 1H NMR (C6D6, 25 °C): δ −52.85 (16, 6H, Ar-CH3), −25.68 (18, 6H, Ar-CH3), −5.62 (27, 2H, Ar-CH), −1.75 (9, 6H, ArCH3), 0.13 (12, 2H, Ar-CH), 1.44 (42, 8H, THF−CH2), 3.84 (70, 8H, THF-CH2), 13.06 (11, 15H, Cp*-CH3), 40.29 (32, 2H, CH), 40.50 (28, 2H, CH), 86.23 (35, 2H, CH), 91.72 (36, 2H, CH). Synthesis of Cp*U(OPh)(HN3Mes) (2-OPh). A 20 mL scintillation vial was charged with 0.145 g (0.162 mmol) of Cp*U(N3MesLiCl(THF)2) and 5 mL of toluene. With stirring, 0.015 g (0.159 mmol) of phenol dissolved in 1 mL of toluene was added dropwise. After 1 h, volatiles were removed in vacuo. The mixture was extracted into diethyl ether, filtered through Celite, and dried to afford a yellowbrown solid (0.123 g, 0.153 mmol, 95%) assigned as Cp*U(OPh)(HN3Mes). This sample was subjected to elemental analysis. Single Xray-quality crystals were obtained from a concentrated n-pentane/ hexamethyldisiloxane (2/1) solution at −35 °C. Anal. Calcd for C38H52N3OU: C, 56.71; H, 6.51; N, 5.22. Found: C, 56.40; H, 6.68; N, 5.78. 1H NMR (C6D6, 25 °C): δ −87.26 (30, 1H, NH), −16.83 (8, 6H, Ar-CH3), −16.52 (21, 2H, CH), −14.49 (24, 2H, CH), −1.85 (4, 15H, Cp*-CH3), 0.26 (3, 6H, Ar-CH3), 1.29 (6, 2H, Ar-CH), 10.73 (5, 2H, Ar-CH), 15.63 (t, J = 8, 1H, p-Ph-CH), 15.84 (d, J = 11, 2H, o-PhCH), 17.72 (t, J = 6, m-Ph-CH), 29.14 (24, 2H, CH), 32.48 (16, 2H, CH), 33.80 (12, 6H, Ar-CH3). IR (KBr): ν 3355 cm−1 (N−H). Synthesis of Cp*U(SPh)(HN3Mes) (2-SPh). A 20 mL scintillation vial was charged with 0.100 g (0.112 mmol) of Cp*U(N3MesLiCl(THF)2) and 5 mL of toluene. With stirring, 0.0114 mL (0.112 mmol) of thiophenol was added. After 1 h, the solution was filtered over Celite, and volatiles were removed in vacuo to afford a red-brown solid (0.087 g, 0.106 mmol, 97%) assigned as Cp*U(SPh)(HN3Mes).

EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive manipulations were performed using standard Schlenk techniques or in an MBraun inert-atmosphere drybox with an atmosphere of purified nitrogen. The MBraun drybox was equipped with a cold well designed for freezing samples in liquid nitrogen as well as two −35 °C freezers for cooling samples and crystallizations. Solvents for sensitive manipulations were dried and deoxygenated using literature procedures with a Seca solvent purification system.33 Benzene-d6 was purchased from Cambridge Isotope Laboratories, dried with molecular sieves and sodium, and degassed by three freeze−pump−thaw cycles. n-Butyllithium (2.5 M in hexane), benzophenone, benzophenone-d10, phenol, thiophenol, tert-butyl alcohol, phenyl acetate, and triphenylphosphine oxide were purchased from commercial sources. H3N3Mes,21 benzylpotassium,34 and UCl435 were prepared according to literature procedures. KCp* was synthesized by deprotonation of 1,2,3,4,5pentamethylcyclopentadiene (Norquay Technology, Inc.) with potassium hydride in THF. 1 H NMR spectra were recorded on a Varian Inova 300 spectrometer operating at 299.992 MHz. All chemical shifts are reported relative to the peak for SiMe4, using 1H (residual) chemical shifts of the solvent as a secondary standard. The spectra for paramagnetic molecules were obtained by using an acquisition time of 0.5 s; thus, the peak widths reported have an error of ±2 Hz. For paramagnetic molecules, the 1H NMR data are reported with the chemical shift, followed by the peak width at half height in hertz, the integration value, and, where possible, the peak assignment. Elemental analyses were performed by either the UIUC Microanalysis Laboratory or Complete Analysis Laboratories, Inc. Electronic absorption spectroscopic measurements were recorded at ambient temperature in sealed 1 cm quartz cuvettes with either a Cary 6000i UV−vis−NIR spectrophotometer or Jasco V-6700 spectrophotometer. Infrared spectra were recorded using a Thermo Nicolet iS5 FT-IR spectrometer. Samples were diluted into dry KBr and recorded as pellets. Single crystals of 1-THF, 1-OPPh3, 3-LiCl, and 2-OtBu suitable for X-ray diffraction were coated with poly(isobutylene) oil in a glovebox and quickly transferred to the goniometer head of a Nonius KappaCCD diffractometer equipped with a graphite crystal, incident beam monochromator. Preliminary examination and data collection were performed with Mo Kα radiation (λ = 0.71073 Å). In a similar fashion, single crystals of 2-Cl, 2-SPh, 2-OPh, and 3-OCPh2 suitable for X-ray diffraction were transferred to the goniometer head of a Rigaku Rapid II image plate diffractometer equipped with a MicroMax002+ high-intensity copper X-ray source with confocal optics. Preliminary examination and data collection were performed with Cu Kα radiation (λ = 1.54184 Å). Additional details can be found in the Supporting Information. Synthesis of (THF)2UCl2(HN3Mes) (1-THF). A 20 mL scintillation vial was charged with 0.716 g (2.109 mmol) of H3N3Mes and 5 mL of THF and frozen. A separate vial was charged with 0.550 g (4.223 mmol) of benzylpotassium and 5 mL of THF and frozen. After it was thawed, the benzylpotassium solution was added dropwise to the H3N3Mes solution with stirring and warmed to room temperature. After 1 h, the solution was added dropwise to a separate vial containing UCl4 (0.800 g, 2.106 mmol) dissolved in 10 mL of THF. The solution was stirred for 3 h, and volatiles were removed in vacuo. The mixture was extracted into toluene, filtered through Celite, and dried. The resulting solid was washed with 10 mL of n-pentane and dried to afford a mustard yellow solid (1.240 g, 1.568 mmol, 74%) assigned as (THF)2UCl2(HN3Mes). Single X-ray-quality crystals were obtained by slow diffusion of n-pentane into a concentrated THF solution at −35 °C. Elemental analysis of this species was unsuccessful due to the issues described in the text and is consistent with the 1H NMR analysis (Figures S8 and S9 in the Supporting Information). Synthesis of (Ph3PO)UCl2(HN3Mes) (1-OPPh3). A 20 mL scintillation vial was charged with 0.255 g (0.323 mmol) of (THF)2UCl2(HN3Mes) and 5 mL of toluene. With stirring, 0.090 g (0.323 mmol) of triphenylphosphine oxide was added. After 15 min, B

DOI: 10.1021/acs.organomet.6b00861 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Metalation of H3N3Mes and Synthesis of 3-LiCl

Figure 1. Molecular structures (from left to right) of 1-THF, 1-OPPh3, 2-Cl, and 3-LiCl displayed with 30% probability ellipsoids. Hydrogen atoms not attached to nitrogens as well as 2,4,6-triphenylmethyl substituents have been omitted for clarity. Found, C, 60.16; H, 6.85; N, 4.89. 1H NMR (C6D6, 25 °C) δ −26.51 (d, J = 11, 2H, CH), −24.85 (d, J = 9, 2H, CH), −3.14 (3, 15H, Cp*− CH3), −0.52 (7, 6H, Ar-CH3), 2.80 (3, 6H, Ar-CH3), 7.00 (33, 4H, oPh), 7.07 (40, 2H, p-Ph), 7.58 (42, 4H, m-Ph), 7.85 (6, 2H, Mes-CH), 11.84 (6, 2H, Mes-CH), 16.72 (104, 2H, CH), 17.90 (d, J = 9, 2H, CH), 25.39 (10, 6H, Ar-CH3). Synthesis of Cp*U(OPh)(HN3Mes) (2-OPh) from 3-LiCl and Phenyl Acetate. A 20 mL scintillation vial was charged with 0.245 g (0.273 mmol) of Cp*U(N3Mes-LiCl(THF)2) and 10 mL of toluene. With stirring, 0.035 mL (0.276 mmol) of phenyl acetate was added, resulting in a gradual color change from red-orange to brown-orange. After 1 h, volatiles were removed in vacuo. The mixture was extracted into diethyl ether (15 mL), filtered through Celite, and dried to afford an orange-brown solid (0.215 g, 0.267 mmol, 98%) identified as Cp*U(OPh)(HN3Mes).

This sample was subjected to elemental analysis. Single X-ray-quality crystals were obtained from a concentrated diethyl ether solution stored at −35 °C. Anal. Calcd for C38H52N3SU: C, 55.60; H, 6.38; N, 5.12. Found: C, 55.39; H, 5.85; N, 5.86. 1H NMR (C6D6, 25 °C): δ −136.95 (24, 1H, NH), −20.24 (141, 2H), −12.80 (d, J = 9, 2H oSPh), −12.68 (23, 2H), −6.03 (5, 6H, CH3), −2.92 (t, J = 6, 1H, pSPh), −2.13 (t, J = 8, 2H, m-SPh), 0.60 (3, 15H, Cp*), 5.46 (3, 6H, CH3), 9.56 (14, 2H), 18.89 (33, 6H, CH3), 26.24 (6, 6H, CH3), 54.80 (165, 2H, CH), 57.84 (18, 2H, CH). IR (KBr): ν 3355 cm−1 (N−H). Synthesis of Cp*U(O t Bu)(HN 3 Mes ) (2-O t Bu). A 20 mL scintillation vial was charged with 0.225 g (0.251 mmol) of Cp*U(N3Mes-LiCl(THF)2) and 8 mL of toluene. With stirring, 0.018 g (0.243 mmol) of tert-butyl alcohol dissolved in 1 mL of toluene was added dropwise over 1 min. After 16 h, volatiles were removed in vacuo. The mixture was extracted into diethyl ether, filtered through Celite, and dried. The resulting orange solid was washed with 5 mL of cold n-pentane (−35 °C) and dried to afford a solid (0.152 g, 0.194 mmol, 77%) assigned as Cp*U(OtBu)(HN3Mes). This sample was subjected to elemental analysis. Single X-ray-quality crystals were obtained by diffusion of n-pentane into a concentrated diethyl ether solution at −35 °C. Anal. Calcd for C36H56N3OU: C, 55.09; H, 7.19; N, 5.35. Found, C, 58.99; H, 6.82; N, 4.83. 1H NMR (C6D6, 25 °C): δ −88.51 (41, 1H, NH), −28.71 (14, 6H, CH3), −26.85 (24, 2H, CH), −25.48 (23, 2H, CH), −7.40 (d, J = 10, 2H, CH), −4.87 (6, 2H, ArCH), −3.41 (6, 15H, Cp*), −0.79 (4, 6H, CH3), 12.90 (5, 2H, ArCH), 13.36 (d, J = 12, 2H, CH), 44.92 (8, 9H, C(CH3)3), 49.45 (11, 6H, CH3). IR (KBr): ν 3354 cm−1 (N−H). Synthesis of Cp*U(N3Mes-OCPh 2) (3-OCPh2). A 20 mL scintillation vial was charged with 0.100 g (0.112 mmol) of Cp*U(N3Mes-LiCl(THF)2) and 5 mL of toluene and chilled to −35 °C. With stirring, 0.020 g (0.110 mmol) of benzophenone was added. After 1 h, the solution was filtered over Celite, and volatiles were removed in vacuo to afford an orange-brown powder (0.093 g, 0.104 mmol, 93%) assigned as Cp*U(N3Mes-OCPh2). This sample was subjected to elemental analysis. Single X-ray-quality crystals were obtained from a concentrated n-pentane/diethyl ether solution stored at −35 °C. Anal. Calcd for C45H56N3OU: C, 60.59; H, 6.21; N, 4.71.

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RESULTS AND DISCUSSION Synthesis of a Trianionic N3Mes-Bound Uranium Complex. Initial efforts in these studies were focused on metalation of the redox-innocent H3N3Mes ligand with UCl4, which was accomplished by first deprotonating H3N3Mes 21 with 2 equiv of KCH2Ph in situ, affording a mustard yellow powder (Scheme 1). Previous work on group IV metals shows that metalation can be achieved using H3N3Mes with metal alkyl13 and amide compounds.21 Analysis of the uranium product by 1 H NMR spectroscopy confirmed a paramagnetic species, suggesting installation of the triamine ligand; however, the broadness and variability of the resonances between samples precluded absolute assignment (see spectroscopic data in Figures S8 and S9 in the Supporting Information). Analysis by infrared spectroscopy revealed an N−H moiety, consistent with the formation of UCl2(HN3Mes). Importantly, performing the metalation using 3 equiv of KCH2Ph did not give the triamido derivative. C

DOI: 10.1021/acs.organomet.6b00861 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Metrical Parameters for 1-THF, 1-OPPh3, 2-Cl, 3-LiCl, 2-OPh, 2-SPh, 2-OtBu, and 3-OCPh2 bond (Å) or angle (deg)

1-THF (X = Cl)

1-OPPh3 (X = Cl)

2-Cl (X = Cl)

3-LiCl (X = Cl)

2-OPh (X = OPh)

2-SPh (X = SPh)

2-OtBu (X = OtBu)

3-OCPh2 (X = OCPh2)

U1−N1 U1−N2 U1−N3 U1−X1 U1−Ct U1−X1−C33 τ value

2.251(2) 2.550(2) 2.260(3) 2.6751(7)

2.215(3) 2.528(3) 2.231(3) 2.7038(9)

2.264(9) 2.543(10) 2.260(11) 2.674(3) 2.523

2.307(6) 2.316(6) 2.302(6) 2.7414(16) 2.536 0.783

2.258(5) 2.574(5) 2.257(5) 2.7237(16) 2.545 115.2(2) 1.005

2.288(5) 2.595(6) 2.316(5) 2.065(5) 2.561 178.2(4) 0.378

2.271(8) 2.595(9) 2.267(9) 2.171(7) 2.502

1.021

2.290(11) 2.561(13) 2.290(14) 2.147(9) 2.536 165.6(10) 0.987

0.745

as an orange powder in high yield (93%). Spectroscopic analysis of 2-Cl supported formation of the desired product, with a large resonance noted at 3.17 ppm by 1H NMR spectroscopy for the Cp* protons and an upfield-shifted resonance for the N−H proton. The assignment of 2-Cl was confirmed by X-ray diffraction analysis of single crystals obtained from a concentrated diethyl ether solution stored at −35 °C. Refinement of the data revealed a pseudo-trigonal-bipyramidal (τ = 1.021)38 uranium chloride species capped by an η5-Cp* ligand (U1−Ct = 2.523 Å) (Figure 1 and Table 1). Similar to the case for 1-THF and 1-OPPh3, the dianionic HN3Mes ligand displays two short uranium−nitrogen contacts (U1−N1 = 2.264(9) Å; U1−N3 = 2.260(11) Å) as well as a single long contact to the central nitrogen (U1−N2 = 2.543(10) Å). Structurally, 2-Cl is reminiscent of the redox-active ligand complexes CpXUI(MesPDIMe)32 (CpX = Cp*, CpP (CpP = 1,1-dimethylbenzylcyclopentadienyl)) that have been established to contain closedshell pyridine(diimine) dianions. As previously noted, while addition of 3 equiv of KCH2Ph to H3N3Mes did not afford triple deprotonation, isolation of the N3Mes trianion was accessible by deprotonation of 2-Cl with alkyllithium reagents.24 Addition of 1 equiv of nbutyllithium to a THF solution of 2-Cl resulted only in a slight darkening of the solution. Notably, workup of the reaction mixture in nonpolar solvents did not cause precipitation of the LiCl byproduct, leading to the assignment as Cp*U(N3Mes-LiCl(THF)2) (3LiCl). Surprisingly, attempted deprotonation of 2-Cl with NaCH2SiMe3 or KCH2Ph led to intractable products or no reaction, suggesting that the LiCl byproduct imparts stability to the resultant species. Successful synthesis of 3-LiCl was evidenced by infrared spectroscopy, which revealed the absence of an N−H absorbance. The 1H NMR spectrum revealed a paramagnetically shifted Cs-symmetric spectrum with resonances ranging from −52.85 to 91.72 ppm, with a large shift for the Cp*-CH3 resonances to 13.06 ppm. Absolute structural confirmation of 3-LiCl was achieved by X-ray crystallography of single crystals obtained by slow diffusion of n-pentane into a concentrated diethyl ether solution at −35 °C. Refinement of the data revealed a pseudo-trigonal-bipyramidal (τ = 0.783) N3Mes -chelated uranium complex bound by an η5-Cp* anion (U1−Ct = 2.536 Å) (Figure 1 and Table 1). As predicted, the uranium coordination sphere is saturated by LiCl. The U−Cl distance (U1−Cl1 = 2.7414(16) Å) is longer than that in the precursor 2-Cl, suggestive of a weaker interaction. The lithium cation is coordinated to the central N3Mes nitrogen (N2−Li1 = 2.089(14) Å) as well as to two THF molecules. Following the third deprotonation, the N3Mes ligand chelates the uranium center with three similar anionic uranium−nitrogen interactions (U1−N av = 2.308 Å). 3-LiCl is reminiscent of [Li-

Absolute structural assignment was achieved by single-crystal X-ray diffraction of yellow crystals obtained by slow diffusion of n-pentane into a concentrated THF solution at −35 °C. Refinement of the data confirmed the predicted product as the THF adduct (THF)2UCl2(HN3Mes) (1-THF), featuring a dianionic HN3Mes ligand derived from double deprotonation (Figure 1 and Table 1). The central nitrogen atom, containing the lone hydrogen, possesses a dative interaction with uranium (U1−N2 = 2.550(2) Å), while the flanking nitrogen atoms bind to uranium with much shorter U−N distances (U1−N1 = 2.251(2) Å; U1−N3 = 2.260(3) Å) consistent with anionic character.36 The chloride ions are nearly trans (155.19(2)°) with U−Cl distances (U1−Cl1 = 2.6751(7) Å; U1−Cl2 = 2.7048(8) Å) similar to other uranium(IV) species.37 The U− O distances (U1−O1 = 2.646(2) Å; U1−O2 = 2.603(2) Å) show weakly coordinated THF molecules, which likely accounts for the broad/inconsistent 1H NMR spectra. To probe this hypothesis for 1-THF, substitution of the THF ligands for a rigid, better coordinating Lewis base was achieved by addition of 1 equiv of triphenylphosphine oxide, which afforded (Ph3PO)UCl2(HN3Mes) (1-OPPh3) in high yield (eq 1). As desired, the OPPh3 adduct gave a sharper and readily

assignable Cs-symmetric paramagnetic 1H NMR spectrum. All resonances appeared within the range of −160.19 to 49.98 ppm, with the most upfield signal attributed to the N−H proton. The Cs symmetry of the species in solution suggests that the central nitrogen atom of the HN3Mes ligand is static on the NMR time scale. For comparison to 1-THF, X-ray analysis of single crystals of 1-OPPh3 obtained from a concentrated THF/n-pentane solution (5/1) stored at −35 °C was also performed (Figure 1 and Table 1). Refinement of the data for 1-OPPh3 showed the same structural trends as for 1-THF, albeit with slightly shorter U−N distances for the meridional κ3-HN3 Mes ligand. As expected, the OPPh3 displays a significantly shorter U−O contact (U1−O1 = 2.354(2) Å) in comparison to its THF counterparts in 1-THF, thus highlighting its robust solution behavior. While 1-OPPh3 provided a useful spectroscopic handle, the lability of the THF ligands in 1-THF proved this material to be a versatile synthon. Treating a 1-THF solution with 1 equiv of KCp* caused a color change from yellow to orange. Isolation and workup of the product afforded Cp*UCl(HN3Mes) (2-Cl) D

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Organometallics (THF)2Cl]2[Li]2[U(NHtBu)6], reported by Hayton and coworkers, which is a uranium(IV) amide species that also displays LiCl incorporation, despite attempts to remove it.39 Electronic absorption spectroscopy for 1-THF, 1-OPPh3, 2Cl, and 3-LiCl is in agreement with the presence of a tetravalent uranium in each case (Figure 2).

Single-crystal X-ray diffraction studies were performed on 2OPh (obtained from a concentrated n-pentane/hexamethyldisiloxane (2/1) solution at −35 °C) and 2-SPh (obtained from a concentrated diethyl ether solution at −35 °C) to confirm their assignments and assess their structural parameters (Figure 3

Figure 2. Electronic absorption spectra (THF, ambient temperature) of 1-THF (green), 1-OPPh3 (red), 2-Cl (blue), and 3-LiCl (orange) recorded from 280 to 1800 nm. Solvent overtones are present from 1670 to 1760 nm.

Each complex displays sharp absorptions of weak intensity (ca. 5−100 M−1 cm−1) throughout the near-infrared region and into the visible region, signifying a uranium 5f2 electronic configuration.40,41 In the visible region, the complexes show weak shoulders in the 420 nm region, consistent with the observed light orange-yellow appearance. Reactivity of Cp*U(N3Mes-LiCl(THF)2) (3-LiCl). In order to assess the character of the basal amide in 3-LiCl, its reactivity toward polar organic substrates was investigated without further purification to remove the LiCl. We hypothesized this would be possible on the basis of the rich oxidation chemistry observed previously for [Li(THF)2Cl]2[Li]2[U(NHtBu)6], which occurs along with extrusion of LiCl.39 Addition of either phenol or thiophenol to a toluene solution of 3-LiCl results in expulsion of LiCl and protonation of the central N3Mes nitrogen, as confirmed by infrared spectroscopy. 1 H NMR analysis of the products reveal Cs-symmetric, paramagnetic molecules, each containing 14 resonances consistent with the assignment as Cp*U(EPh)(HN3Mes) (E = O, 2-OPh; E = S, 2-SPh) (eq 2). For 2-OPh, resonances range

Figure 3. Molecular structures (clockwise from top left) of 2-OPh, 2SPh, 2-OtBu, and 3-OCPh2 displayed with 30% probability ellipsoids. Hydrogen atoms not attached to nitrogens as well as 2,4,6triphenylmethyl substituents have been omitted for clarity.

and Table 1). Refinement of the data for 2-OPh revealed a pseudo-trigonal-bipyramidal uranium phenoxide complex capped by an η5-Cp* (U1−Ct = 2.536 Å) (τ = 0.987). Upon single protonation, the HN3Mes ligand resumes its dianionic chelation to uranium with two short uranium−nitrogen contacts (U1− N1 = 2.290(11) Å; U1−N3 = 2.290(14) Å) and a single dative contact (U1−N2 = 2.561(13) Å). The phenoxide ligand displays a typical interaction with uranium(IV) (U1−O1 = 2.147(9) Å), while the U1−O1−C33 angle of 165.6(10)° is similar to those observed for the uranium(IV) phenoxide series Cp*2U(O-2,6-iPr2C6H3)X (X = Me, F, N3) of 163.2(4), 165.4(6), and 165.2(4)°, respectively.42 Inspection of the metrical parameters of 2-SPh reveals similar trends (τ = 1.005) with two shorter (U1−N1 = 2.258(5) Å; U1−N3 = 2.257(5) Å) and one longer (U1−N2 = 2.574(5) Å) uranium−nitrogen distance. The U−S distance of 2.7237(16) Å is on the order of other cyclopentadienyl uranium(IV) thiophenoxide compounds, including (CpMe4)2U(SPh)2 (2.6845(7), 2.6967(7) Å)43 and Cp*2U(SPh)CH3 (2.7060(14) Å).42 The U1−S1− C33 angle of 115.2(2)° is significantly smaller than that in 2OPh and is consistent with the trends observed in the series Cp*2U(EPh)2 (E = S, Se, Te).44,45 While amine protonation by alcohols and thiols is common in f-block chemistry,46 the cooperative reactivity of HOPh and HSPh noted for 3-LiCl is significant, as the triamidoamine ligand remains coordinated to the uranium. Such examples are

from −87.26 to 33.80 ppm, with the re-emergence of the N−H proton at the most upfield shifted position. A large singlet attributed to the Cp*-CH3 appears at −1.85 ppm. The analogous sulfur species 2-SPh has these resonances shifted to −136.95 and 0.60 ppm, respectively. E

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Organometallics

more linear U1−O1−C33 angle (178.2(4)°) and is situated nearly trans to the central HN3Mes amine (O1−U1−N2 = 149.47(19)°). In comparison to 2-OPh, the U−O distance is decreased by ca. 0.08 Å, which may contribute to the difference in geometry. While the central nitrogen of 3-LiCl displayed significant Brønsted base character, we sought to investigate whether it could also act as a nucleophile. To probe this, 1 equiv of benzophenone was added to a stirred toluene solution of 3LiCl. Following workup, the yellow-brown powder was analyzed by 1H NMR spectroscopy, which showed an analogous Cs-symmetric spectrum with resonances ranging from −26.51 to 25.39 ppm, including a Cp*-CH3 resonance at −3.14 ppm. Noticeably absent from this spectrum was the far upfield shifted N−H resonance characteristic of complexes 2. Infrared spectroscopy confirmed the absence of both N−H and CO moieties, leading to the assignment as Cp*U(N3MesOCPh2) (3-OCPh2), derived through reductive coupling of the substrate with the amide ligand (Scheme 3, right). Single crystals of 3-OCPh2 obtained from a concentrated npentane/diethyl ether solution stored at −35 °C were analyzed by X-ray diffraction (Figure 3 and Table 1). Refinement of the data revealed an η5-Cp*-capped (U1−Ct = 2.502 Å) uranium complex chelated through a new tripodal tetradentate ligand in a pseudo-trigonal-bipyramidal geometry (τ = 0.745). Two arms of the tripod, comprised of the −CH2CH2NMes fragments, show typical amide linkages to uranium (2.271(8) and 2.267(9) Å), while the alkoxide arm, −CPh2O, coordinates with a bond (U1−O1 = 2.171(7) Å) on the order of that for 2-OPh. Twoelectron reduction of the carbonyl fragment is evident through elongation of the carbon−oxygen bond (O1−C40 = 1.420(12) Å) in comparison to free benzophenone,52 as well as the formation of a new C−N single bond (1.544(13) Å). These C− N and O−C distances within the newly generated uranooxazetidine are significantly different from those of CO2 derived carbamate complexes (iPrPNP-CO2)FeH(CO) (iPrPNP-CO2 = OOCN(CH2CH2PiPr2)2)53 and (iPrPNP-CO2)W(NO)(CO)54 but most closely resemble those in [Co(15-TMC-CH2O)(OH)][ClO4] (TMC = tetramethylcyclam) synthesized via hydroxylation of a TMC-CH3 substituent.55 For actinides, ligand reductive coupling with carbonylated substrates has previously been observed for the radical-containing species Cp*2U(2,2′-bipyridyl)56 and Cp*UI(MesPDIMe).32 The formation of the metallacycle from 3-LiCl and benzophenone is reminiscent of the reactivity noted for highvalent metal imido species. For instance, Mountford reported the formation of the analogous equilibrium product with titanium(IV), generated from Cp*Ti (NTol)(PhC(NiPr)2) and 1 equiv of benzophenone at low temperature.57 Competition experiments between benzophenone and pyridine ruled out a simple Lewis base adduct, as did conversion to the μ-oxo species after 9 days. Similarly, Walter invoked the formation of

rare for the actinide elements, as protic substrates are typically used to metathesize ligands for specific targets. With a generic route established for the addition of polar H− E substrates across the uranium−amide bond, the reactivity of 3-LiCl was also investigated toward tert-butyl alcohol (Scheme 2). The reaction proceeded significantly more slowly than with Scheme 2. Proposed Pathway for the Formation of 2-OtBu from 3-LiCl

the HEPh substrates, which is surprising given that the electropositivity and oxophilicity of uranium are usually significant driving forces. However, the significantly higher pKa of tBuOH (32.2, DMSO) in comparison to both PhOH (18.0, DMSO) and PhSH (10.3, DMSO) is likely the origin of this dichotomy.47,48 Monitoring the reaction between 3-LiCl and tBuOH by 1H NMR spectroscopy revealed rapid conversion of 3-LiCl to 2-Cl, followed by gradual formation of Cp*U(OtBu)(HN3Mes) (2-OtBu). In the reaction, the observance of 2-Cl suggests that the LiCl in 3-LiCl does not dissociate to form a base-free analogue, Cp*U(N3Mes), which then performs σ bond metathesis; rather, stepwise protonolysis followed by anion metathesis appears to be the likely mechanism of formation (Scheme 2). This is further corroborated by Hayton, who reported the utility of LiOtBu to generate the uranium(IV) homoleptic tert-butoxide [Li(THF)]2[U(OtBu)6].49 Full characterization of 2-OtBu revealed a similar Cs -symmetric 1H NMR spectrum as 2-EPh with the N−H and Cp*-CH3 moieties located at −88.51 and −3.41 ppm, respectively. The tert-butoxide methyl substituents are shifted to 44.92 ppm, slightly upfield of the phenoxide resonances of 2OPh. Absolute structural confirmation of 2-OtBu was achieved through X-ray diffraction analysis of single crystals obtained by diffusion of n-pentane into a concentrated diethyl ether solution at −35 °C. Refinement of the data revealed the cyclopentadienyl uranium tert-butoxide compound is in a surprisingly different geometric arrangement in comparison to its −OPh and −SPh counterparts (Figure 3 and Table 1). The geometry about the uranium center is best described as distorted square pyramidal (τ = 0.378), with the apex defined as N1. The U1−O1 distance of 2.065(5) Å is on the order of those for other uranium(IV) tert-butoxide species, including Cp*2U(OtBu)Cl (2.016(8) Å)50 and Cp*2U(OtBu)SePh (2.029(6) Å).51 The strongly donating tert-butoxide displays a

Scheme 3. Reactivity of 3-LiCl with Phenyl Acetate (Left) and Benzophenone (Right)

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CONCLUSIONS In summary, we have reported a new uranium(IV) family of the diamidoamine ligand HN3Mes and its triamido counterpart N3Mes. The latter compound, 3-LiCl, acts as a Brønsted base, showing metal−ligand cooperative reactivity toward polarized small molecules, including HOPh, HSPh, HOtBu, OCPh2, and PhOAc. While it has been well-established for the metals of the d block, such cooperative behavior is less common for the actinides. Structural studies in this work are complemented by data obtained from multiple spectroscopic methods, all of which highlight the geometric and electronic similarities of both starting materials and products. The rare metal−ligand cooperative behavior observed here is noteworthy, given that the reactivity of uranium amides with polar substrates has been known for quite some time. By tethering the central Brønsted basic amide to the uranium with flanking donor amide arms, the molecule stays intact following reaction at that site. In certain cases, the reversibility of this cooperative activation process suggests that such a system could be appropriate for activation of strong bonds by the actinide elements by analogy to transition metals. Elements of the fblock would be advantageous for such chemistry, especially given their highly reducing, electropositive nature. Ongoing studies are aimed toward activation of additional substrates relevant to catalytic processes.

the analogous metallacycle as an intermediate in the conversion of [η5-1,2,4-(Me3C)3C5H2]2ThN(ptolyl) to the terminal thorium(IV) oxo species [η5-1,2,4-(Me3C)3C5H2]2ThO.58 Thus, isolation of 3-OCPh2 through an alternate synthetic pathway not involving metal−nitrogen multiple bonds has afforded a rare opportunity for structural characterization of an elusive species. With the nucleophilic character of the central nitrogen in 3LiCl established, we sought to determine its preference to act as a nucleophile or as a base. We envisioned the reactivity of 3LiCl with phenyl acetate as providing two potential outcomes: (1) nucleophilic attack at the carbonyl carbon or (2) deprotonation of the α-carbon. On addition of phenyl acetate to a toluene solution of 3-LiCl, 2-OPh is rapidly and cleanly generated, as confirmed by infrared and 1H NMR spectroscopy, suggesting the latter casedeprotonation of the α-carbon moiety. Repeating the reaction in a sealed NMR tube (C6D6) confirmed expulsion of ketene as the organic byproduct (Scheme 3, left). The analogous reaction with phenyl acetated3 showed formation of an N−D bond, as observed by 2H NMR spectroscopy (Figure S11 in the Supporting Information). The formation of ketene during the reaction is consistent with deprotonation of the α-carbon being favored over nucleophilic attack. As such, the secondary alkyl amide of the N3Mes ligand is best described as possessing Brønsted basic character over nucleophilic character. The reaction products derived from 3-LiCl were investigated by electronic absorption spectroscopy (Figure 4). As expected

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00861. Additional characterization details for all newly synthesized complexes, NMR spectra, and crystallographic data (PDF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF) X-ray crystallographic data (CIF)

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Figure 4. Electronic absorption spectra (THF, ambient temperature) of 2-OPh (green), 2-SPh (blue), 2-OtBu (orange), and 3-OCPh2 (red) recorded from 300 to 1800 nm. Solvent overtones are present from 1670 to 1760 nm.

AUTHOR INFORMATION

Corresponding Author

*E-mail for S.C.B.: [email protected]. ORCID

Suzanne C. Bart: 0000-0002-8918-9051 2

Notes

t

for a 5f electronic configuration, 2-OPh, 2-SPh, 2-O Bu, and 3-OCPh2 all display low intensity (ca. 25−125 M−1 cm−1) f−f transitions throughout the far-visible and near-infrared regions. Variation of X-type ligands (O vs S), molecular geometries as determined by X-ray crystallography (2-OPh vs 2-OtBu), or composition of the chelating ligand (2-OPh vs 3-OCPh2) did not have a profound effect on the observed transitions. This trend is further observed in the ultraviolet and visible regions, as each species is void of notable transitions at wavelengths greater than 300 nm. Thus, all of the products formed through the cooperative addition to 3-LiCl have analogous electronic structures, as gauged by spectroscopic and structural analyses.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, Heavy Elements Chemistry Program, of the U.S. Department of Energy through Grant DE-SC0008479.

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