In previous work, we found that one-electron oxidation of [UIV(TrenTIPS)(NH)]- to [UV(TrenTIPS)(NH)] resulted in disproportionation to [UIV(TrenTIPS)(NH2)] and [UVI(TrenTIPS)(N)]73,74. By contrast, the analogous one-electron oxidation of [UIV(TrenTIPS)(PH)]- results in the isolation of [UIV(TrenTIPS)(PH2)] and II51, but the latter can be viewed as dimerized [UVI(TrenTIPS)(P)] and hence that reaction also has the initial appearance of a uranium disproportionation reaction. However, a striking feature of the computed reaction profiles in this study is the dominance of the UIV oxidation state and the appearance of radical species rather than the all-electron paired high oxidation state U analogues. This is certainly consistent with HSAB theory where the soft P evidently does not stabilize high oxidation states of U. Thus, this work sheds light on three key aspects: (i) the redox chemistry is driven by the P and not U centres; (ii) the presence of open-shell radical species in these reactions, rather than closed-shell singlets, suggests that attempts to isolate high oxidation state phosphinidene and phosphide complexes of UV and UVI is inherently challenging75, which can again be traced back to HSAB drivers; (iii) the prevalence of radical intermediates is responsible for the formation of P-P catenated derivatives. Although high oxidation state U will have somewhat contracted valence orbitals, the P-anion valence orbitals are reasonably diffuse, and so we suggest that the prevalence of open-shell radical over closed-shell singlet species principally originates from poor energetic matching of U and P frontier orbitals rather than excessively poor spatial overlap. Overall, it is clear that there are common reaction steps in all the reactions studied here, but the precise diverged outcomes are controlled by the ancillary ligands, the radical nature of intermediates, and the phosphinidene substituent.
All manipulations were carried out under an inert atmosphere of dry N using Schlenk techniques or an MBraun UniLab glovebox. Solvents were dried by passage through activated alumina towers, or for benzene distilled from K, and degassed before use. Solvents were stored over K-mirrors except for ethers which were stored over activated 4 Å sieves. D-solvents were distilled from K, degassed by three freeze-pump-thaw cycles, and stored under N prior to use. ClSiMeBu was distilled from Mg, degassed by three freeze-pump-thaw cycles and stored under N. Magnesium powder was activated prior to use as described below. Sodium bis(trimethylsilyl)amide was recrystallised from a saturated pentane solution prior to use. Triphenylborane and anthracene were dried under dynamic vacuum (1 × 10 mbar) for 24 h prior to use. Elemental potassium was freed from oxides and washed with hexane to remove mineral oil prior to use. Other chemicals were purchased from commercial sources and used as received. Depleted UO was supplied by the National Nuclear Laboratory. The compounds [(Anthracene)Mg(THF)], PDBN-NMe (1) (PDBN = 7λ-phosphadibenzonorbornadiene), PDBN-H (2), PDBN-(BPh)Na(OEt), PDBN-Bu, KC, UI(1,4-dioxane), [U{N(SiMe)}], UCl, TrenLi(THF), [(Tren)U(Cl)], [(Tren)U(I)], [{[K(Toluene)][(Tren)U]}], [(Tren)U] (3), [(Ar*O)U] (5) were prepared by the modified procedures described below.
Single crystals were examined variously using either a Rigaku XtaLab Synergy or Rigaku FR-X diffractometer, each equipped with a HyPix 6000HE photon counting pixel array detector with mirror-monochromated Cu Kα (λ = 1.5418 Å) radiation. Intensities were integrated from a sphere of data recorded on narrow (0.5° (Synergy) or 1.0° (FR-X)) frames by ω rotation. Cell parameters were refined from the observed positions of all strong reflections in each data set. Gaussian grid face-indexed absorption corrections with a beam profile correction were applied. The structures were solved by dual methods using SHELXT and all non-hydrogen atoms were refined by full-matrix least-squares on all unique F values with anisotropic displacement parameters with exceptions noted in the respective cif files. Except where stated for P-H hydrogens, all hydrogen atoms were refined with constrained geometries and riding thermal parameters; U(H) was set at 1.2 (1.5 for methyl groups) times U of the parent atom. The largest features in final difference syntheses were close to heavy atoms and were of no chemical significance. CrysAlisPro was used for control and integration, and SHELXL and Olex2 were employed for structure refinement. ORTEP-3 and POV-Ray were employed for molecular graphics.
H, Si{H}, and P spectra were recorded on a Bruker 400 MHz spectrometer operating at 400, 79, and 162 MHz, respectively; chemical shifts are quoted in ppm and are relative to TMS (H, Si), and 85% HPO (P), respectively. ATR-IR spectra were recorded on a Bruker Alpha spectrometer with a Platinum-ATR module in the glovebox. Raman spectra were recorded on a Horiba XploRA Plus Raman microscope with a 638 nm laser with a power of 1.5 mW. The power was adjusted using a power filter for each complex to inhibit sample decomposition. UV/Vis/NIR spectra were recorded on a Perkin Elmer Lambda 750 spectrometer. Data were collected in a 1 mm path-length cuvette and were run versus the appropriate solvent. Variable-temperature magnetic moment data were recorded in an applied direct current (DC) field of 0.5 Tesla on a Quantum Design MPMS3 superconducting quantum interference device magnetometer using recrystallized powdered samples. Measurements were performed in dc scan mode using 40 mm scan length and 6 s scan time. Samples were carefully checked for purity and data reproducibility between independently prepared batches. Samples were crushed with a mortar and pestle under an argon atmosphere and immobilized in an eicosane matrix within 400 MHz Wilmad borosilicate NMR tubes to prevent sample reorientation during measurements. The tube was flame-sealed under dynamic vacuum (1 × 10 mbar) to a length of ~3 cm and mounted in the centre of a drinking straw, with the straw fixed to the end of an MPMS 3 sample rod. Care was taken to ensure complete thermalization of the sample before each data point was measured by employing delays at each temperature point as well as a slow cooling rate (5 K/min from 300 to 100 K; 2.5 K/min from 100 to 50 K; 1 K/min from 50 to 1.8 K). The sample was held at 2 K for 30 min before isothermal magnetization measurements to account for slow thermal equilibration of the sample. Diamagnetic corrections were applied using tabulated Pascal constants. Measurements were corrected for the effect of the blank sample holders (flame sealed Wilmad NMR tube and straw) and eicosane matrix. CHN microanalyses were carried out on a Flash 2000 elemental analyser.
Prior to use, the magnesium powder needs to be activated. Under an argon atmosphere, Mg powder (2.40 g, 100 mmol) was heated to 250 °C under vacuum ( ~ 1 × 10 mbar) for 4 hours. The reaction vessel was then allowed to cool to room temperature, before THF (300 mL) was added along with a few drops of 1,2-dibromoethane. The resultant suspension was stirred for 12 hours, before solid anthracene (21.36 g, 120.00 mmol) was added in a portion-wise manner using a solid-addition funnel. The reaction mixture was then stirred for four days, during which time there was precipitation of an orange solid which was separated by filtration. Volatiles were then removed in vacuo before the solid was washed with THF (4 × 50 mL) and dried in vacuo to afford [(Anthracene)Mg(THF)] as an orange solid, which was used without further purification. Yield: 36.12 g, 86%. The poor solubility of [(Anthracene)Mg(THF)] in hydrocarbon, arene, and ethereal solvents precluded the acquisition of NMR spectroscopic data. ATR-IR ν/cm: 3029 (m), 2948 (m), 2887 (m), 1568 (m), 1456 (s), 1434 (w), 1362 (s), 1245 (s), 1199 (w), 1173 (s), 1143 (w), 1103 (s), 1020 (s), 915 (m), 872 (s), 839 (w), 806 (s), 778 (m), 754 (s), 714 (s), 674 (w), 571 (m), 428 (s).
MeNPCl (5.00 g, 34.26 mmol) was dissolved in THF (300 mL) and cooled to -78 °C. To this, [(Anthracene)Mg(THF)] (14.35 g, 34.26 mmol) was added in a portion-wise manner with vigorous stirring and a 20-minute delay between each portion added (approx. 8 portions in total). Over the course of the addition, there was a colour change to orange and then pale yellow. The reaction was stirred at -78 °C for four hours, before being allowed to warm to room temperature and volatiles removed in vacuo to afford a pale-yellow residue. Toluene (200 mL) was added the mixture and slurried for 10 min before being filtered through a Celite-padded coarse porosity frit to yield a yellow solution. Volatiles were then removed in vacuo before soluble residues were extracted through the addition of DCM (100 mL). The resultant suspension was stored at -30 °C for 6 h, resulting in the precipitation of unwanted side-products. The mixture was filtered, and volatiles were then removed in vacuo to obtain a yellow solid. Soluble residues were extracted with EtO (60 mL) and filtered to yield a yellow solution, which was stored at -30 °C for 24 hours to afford 1 as a pale yellow crystalline solid. Yield: 3.13 g, 36%. H NMR (400 MHz, CD, 298 K): δ (ppm) 2.23 (d, J = 7.5 Hz, 6H, NCH), 4.12 (d, J = 13.0 Hz, 2H, CH), 6.83 (m, 2H, Ar-H), 7.03 (m, 2H, Ar-H), 7.09 (m, 2H, Ar-H), 7.28 (m, 2H, Ar-H). ATR-IR ν/cm: 3059 (w), 3012 (w), 2973 (w), 2919 (w), 2886 (w), 2841 (w), 2795 (w), 1465 (w), 1448 (s), 1408 (w), 1262 (s), 1192 (w), 1179 (w), 1155 (m), 1104 (w), 1072 (s), 1056 (w), 1017 (w), 965 (s), 882 (s), 789 (s), 756 (s), 744 (s), 724 (s), 679 (m), 668 (m), 621 (w), 603 (s), 575 (w), 514 (s), 473 (w), 454 (w), 434 (w), 416 (w).
In the strict absence of light, a solution of di-iso-butylaluminum hydride (1 M in hexane, 20.00 mL, 20.00 mmol) was added to a stirring solution of 1 (2.00 g, 8.00 mmol) in toluene (10 mL) at -78 °C. The mixture was allowed to warm to room temperature, during which time there was the precipitation of a solid resulting in the formation of a milky-white suspension. After two hours, hexane (50 mL) was added resulting in the rapid precipitation of more solid. The reaction mixture was carefully filtered to isolate the precipitate, and removal of volatiles in vacuo afforded 2 as a white solid. Yield: 1.48 g, 88%. H NMR (400 MHz, CD, 298 K): δ (ppm) 3.86 (d, J = 14.2 Hz, 2H, CH), 5.41 (d, J = 162.0 Hz, 1H, PH), 6.72 (m, 2H, Ar-H), 6.85 (m, 2H, Ar-H), 6.97 (m, 2H, Ar-H), 7.10 (m, 2H, Ar-H). ATR-IR ν/cm: 3050 (w), 3013 (w), 2999 (w), 2236 (P-H, s), 1463 (w), 1447 (s), 1182 (m), 1169 (m), 1151 (s), 1107 (w), 1093 (s), 1054 (s), 1013 (s), 998 (w), 981 (w), 956 (w), 937 (w), 906 (w), 881 (s), 766 (s), 736 (s), 725 (m), 699 (s), 629 (s), 595 (s), 492 (s), 474 (s), 432 (m).
In the strict absence of light, a solution of sodium bis(trimethylsilyl)amide (0.92 g, 5.00 mmol) in diethyl ether (10 mL) was added to a stirring solution of 2 (1.26 g, 5.00 mmol) and triphenylborane (1.21 g, 5.00 mmol) in diethyl ether (30 mL) at -78 °C. The mixture was allowed to warm to room temperature, during which time there was the precipitation of a solid resulting in the formation of an off-white suspension. After one hour, the reaction mixture was carefully filtered to isolate the precipitate, and removal of volatiles in vacuo afforded an off-white solid which was washed with EtO (2 × 5 mL) to yield PDBN-(BPh)Na(OEt) as a colourless solid. Yield: 2.43 g, 78%. H NMR (400 MHz, CD, 298 K): δ (ppm) 0.96 (t, 12H, OEt-CH), 3.10 (q, 8H, OEt-CH), 3.76 (d, J = 12.8 Hz, 2H, CH), 6.44-6.49 (m, 4H, Ar-H), 6.64 (m, 2H, Ar-H), 6.89-6.96 (m, 5H, Ar-H), 7.05 (m, 6H, Ar-H), 7.23 (d, 6H, Ar-H). ATR-IR ν/cm: 3058 (br, m), 2972 (br, m), 2930 (w), 2866 (br, w), 1581 (m), 1479 (m), 1464 (w), 1448 (s), 1427 (m), 1383 (m), 1351 (w), 1303 (w), 1264 (w), 1176 (w), 1152 (m), 1083 (s), 1031 (w), 927 (w), 843 (w), 782 (s), 763 (w), 702 (s), 649 (m), 629 (m), 601 (m), 573 (w), 507 (s).
BuPCl (1.58 g, 10.00 mmol) was dissolved in THF (100 mL) and cooled to -78 °C. To this, [(Anthracene)Mg(THF)] (4.18 g, 10.00 mmol) was added in a portion-wise manner with vigorous stirring and a 20-minute delay between each portion added (~4 portions in total). Over the course of the addition, there was a colour change to orange and then yellow/green. The reaction was stirred at -78 °C for two hours, before being allowed to warm to room temperature and volatiles removed in vacuo to afford a pale-yellow residue. Toluene (100 mL) was added the mixture and slurried for 10 minutes before being filtered through a Celite-padded coarse porosity frit to yield a yellow solution. Volatiles were then removed in vacuo before soluble residues were extracted through the addition of DCM (50 mL). The resultant suspension was stored at -30 °C for 6 h, resulting in the precipitation of unwanted side-products. The mixture was filtered, and volatiles were then removed in vacuo to obtain a yellow solid. Soluble residues were extracted with hexane (60 mL) and filtered to yield a yellow solution, which was concentrated (approx. 15 mL) and stored at -30 °C for 24 h to afford PDBN-Bu as a colourless crystalline solid. Yield: 0.43 g, 16%. H NMR (400 MHz, CD, 298 K): δ (ppm) 0.77 (d, NCH, 9H), 3.90 (d, 2H, CH), 6.74-6.76 (m, 2H, Ar-H), 6.92-6.96 (m, 4H, Ar-H), 7.18-7.20 (m, 2H, Ar-H). ATR-IR ν/cm: 3048 (w), 1620 (m), 1533 (w), 1448 (s), 1314 (m), 1271 (w), 1181 (w), 1146 (m), 1119 (w), 1073 (w), 1017 (w), 997 (m), 967 (w), 955 (s), 906 (w), 881 (s), 789 (w), 757 (w), 736 (w), 722 (s), 680 (w), 666 (w), 602 (m), 514 (m), 472 (s), 464 (w), 440 (w), 425 (w), 414 (w).
A 250 mL round-bottomed Schlenk flask was charged with reagent grade ( > 99.9%) graphite (3.55 g, 818.4 mmol) and dried under dynamic vacuum (1 × 10 mbar) at 100 °C for 4 h. In an argon-filled glovebox, freshly cut potassium metal (1.45 g, 102.3 mmol) is added. The mixture is then heated under an argon atmosphere with a blowtorch whilst agitating causing the potassium metal to melt and intercalation to occur. Continue heating until the mixture has completely changed colour from black to bronze, which will be for ~3 h. Once the reaction is complete, allow to cool to room temperature. Yield: 5.0 g, 99%.
A 500 mL Young's ampoule equipped with a PTFE stirrer bar was charged with uranium turnings (12.18 g, 51.17 mmol). To this, 1,4-dioxane ( ~ 200 mL) was added and the mixture cooled to 0 °C before solid I (19.48 g, 76.75 mmol; 1.5 equivalents per U) was added in a portion-wise manner. Note: The addition of I is exothermic so I should be added slowly to control the reaction. After the addition of I was complete, the reaction mixture appeared red in colour. The ampoule was then sealed and the reaction mixture vigorously stirred at room temperature for 7 days. During this time, the red colour dissipated, and a deep purple/blue colour formed with concomitant deposition of solid. After the reaction was complete (no obvious U metal turnings are left), the reaction was concentrated to half volume in vacuo and then EtO (50 mL) added to precipitate the product. The purple/blue solid was collected by filtration through a coarse porosity frit, washed with EtO (2 × 25 mL), and dried in vacuo to afford [UI(1,4-dioxane)] as a purple/blue solid. Yield: 38.05 g, 99%. H NMR (400 MHz, CD, 298 K): δ 3.35 (s, 6H, 1,4-dioxane -CH) (ppm). ATR-IR ν/cm: 2929 (w), 1449 (m), 1433 (w), 1371 (w), 1297 (s), 1256 (s), 1121 (m), 1093 (s), 1057 (s), 1039 (w), 888 (m), 855 (w), 840 (s), 813 (w), 765 (w), 611 (s).
A 250 mL Young's ampoule equipped with a PTFE stirrer bar was charged with a solid mixture of [UI(1,4-dioxane)] (7.50 g, 10 mmol) and [NaN(SiMe)] (5.5 g, 30 mmol). Note: Do not use [KN(SiMe)] as this will lead to an increased formation of [U(I){N(SiMe)}] as a by-product. The mixture was cooled to -78 °C, then THF (100 mL) was added, and the suspension was allowed to warm slowly to room temperature, during which time there was a colour change from dark blue to dark red/purple. Volatiles were removed in vacuo to afford a red/purple residue. Pentane (100 mL) was added and the mixture slurried for 10 min before being filtered through a Celite-padded coarse porosity frit into a 250 mL round bottom Schlenk flask. The resultant filtrate was stored at -30 °C for 24 h, resulting in the precipitation of unwanted NaI and [U(I){N(SiMe)}] by-products. The filtrate is then allowed to warm to room temperature before being filtered to yield a clear red/purple solution. This step was repeated once more, before volatiles were removed in vacuo to afford a red/purple residue which was washed with cold (0 °C) SiMe (2 × 20 mL). The resultant solid was dried in vacuo to afford 1 as a dark purple solid. Yield: 4.50 g, 62.6%. H NMR (400 MHz, CD, 298 K): δ (ppm) -11.31 (s, 54H, Si(CH)). ATR-IR ν/cm: 2951 (s), 2897 (w), 1438 (br, w), 1244 (s), 985 (s), 854 (w), 823 (s), 808 (w), 756 (s), 676 (m), 656 (m), 594 (s).
A 1000 mL round-bottomed flask was charged with UO (23.54 g, 82.22 mmol) and hexachloropropene (250 mL). The flask was equipped with two condensers stacked on top of one another, and the flask placed under an inert gas supply. The mixture was heated carefully to reflux, which was accompanied by a violent exotherm and the liberation of a dark brown gas. The flask was lifted away from the heating mantle to allow the exotherm to subside before heating was resumed. Note: this moderation of the exotherm step may be needed to be conducted multiple times. The reaction mixture was then left to gently reflux for 16 hours. During which time, UCl precipitates from solution as a green solid. The mixture was cooled to room temperature, and the reaction mixture was carefully filtered away from the green solid before washing with DCM (3 × 150 mL). Removal of volatiles in vacuo afforded UCl as a free-flowing green powder, which was used without further purification. Yield: 28.01 g, 90%.
N(CHCHNH) (12 mL, 80.42 mmol) was dissolved in THF (50 mL). BuLi (2.5 M, 100 mL, 250.00 mmol) was added dropwise at -78 °C, warmed to room temperature and the mixture stirred for 6 h. The solution was then cooled to -78 °C, and ClSiMeBu (37.6 g, 250.00 mmol) was added in a portion-wise manner and the solution stirred at room temperature for 16 hours. Removal of volatiles in vacuo resulted in a pale-yellow sticky solid. The product was extracted with pentane (2 ×80 mL), and the solution was filtered from the LiCl precipitate. BuLi (2.5 M, 100 mL, 250.00 mmol) was added dropwise at -78 °C, warmed to room temperature and the solution was stirred for 6 h at room temperature. Removal of volatiles in vacuo resulted in an off-white solid which was washed with cold pentane (2 × 20 mL) to yield TrenLi as a white powder. Colourless crystals of TrenLi were grown from a concentrated solution in hexane stored at -30 °C. Yield: 36.06 g, 62%. H NMR (400 MHz, CD, 298 K): δ (ppm) 0.16 (s, 18H, SiBu(CH)), 1.11 (s, 27H, SiMe(C(CH)), 1.36 (m, 12H, THF-CH), 2.37 (t, 6H, NCHCH), 3.17 (t, 6H, NCHCH), 3.52 (m, 12H, THF-CH). ATR-IR ν/cm: 2924 (m), 2879 (m), 2846 (m), 2820 (w), 1467 (m), 1384 (w), 1340 (m), 1270 (w), 1236 (s), 1144 (w), 1084 (s), 1058 (m), 1035 (m), 1004 (m), 950 (w), 935 (s), 900 (m), 818 (s), 762 (s), 647 (s), 593 (w), 566 (w), 542 (w), 517 (w), 440 (w), 420 (w).
A solution of TrenLi(THF) (7.23 g, 10 mmol) in THF (50 mL) was added dropwise to a stirring solution of UCl (3.80 g, 10 mmol) in THF (80 mL) at -78 °C. The mixture was allowed to warm to room temperature before stirring for 16 h. Removal of volatiles in vacuo resulted in a brown solid. Soluble residues were extracted in hot toluene (100 mL) and the solution was filtered from the LiCl precipitate. Removal of volatiles in vacuo resulted in a pale-brown solid which was washed with hexane (2 × 10 mL) to yield [(Tren)U(Cl)] as a brown solid. Green crystals of [(Tren)U(Cl)] were grown from a concentrated solution in toluene stored at -30 °C for 24 h. Yield: 6.42 g, 85%. H NMR (400 MHz, CD, 298 K): δ (ppm) -23.36 (s, 6H, NCHCH), 6.08 (s, 18H, SiBu(CH)), 6.65 (s, 27H, SiMe(C(CH)), 7.79 (s, 6H, NCHCH). ATR-IR ν/cm: 2949 (s), 2925 (s), 2878 (m), 2851 (s), 1463 (s), 1387 (w), 1359 (w), 1334 (w), 1248 (s), 1140 (w), 1071 (w), 1058 (s), 1021 (m), 1005 (m), 922 (s), 897 (s), 825 (s), 796 (m), 770 (s), 740 (w), 704 (s), 659 (s), 560 (s), 456 (s), 435 (w).
A Schlenk flask equipped with a PTFE-coated stirrer bar was charged with solid [(Tren)U(Cl)] (3.30 g, 4.4 mmol). To this, pentane was added ( ~ 40 mL) with stirring to form a clear solution to which MeSiI (2 mL, 14 mmol) was added all at once resulting in a colour change to light brown. The resultant reaction mixture was stirred for 48 h, during which time there was deposition of a brown solid. The suspension was filtered, and the resultant solid was dried in vacuo to afford a brown powder, which was washed with pentane (3 × 25 mL). The resultant solid was dried in vacuo for one hour to afford [(Tren)U(I)] as a light brown powder. Yield: 2.32 g, 63%. H NMR (400 MHz, CD, 298 K): δ (ppm) -32.48 (s, 6H, NCHCH), 6.32 (s, 6H, NCHCH), 9.54 (s, 27H, SiMe(C(CH)), 11.20 (s, 18H, SiBu(CH)). ATR-IR ν/cm: 2949 (s), 2924 (s), 2879 (m), 2851 (s), 1464 (s), 1387 (w), 1359 (m), 1332 (w), 1252 (s), 1141 (s), 1059 (s), 1021 (m), 922 (s), 896 (s), 825 (s), 797 (m), 773 (s), 739 (m), 698 (s), 659 (s), 564 (s), 459 (s), 440 (w).
Under an atmosphere of argon, a potassium mirror (20-fold excess relative to U) was formed within a 250 mL Young's ampoule which was then charged with a glass-coated stirrer bar. To this, a slurry of [(Tren)U(I)] (4.25 g, 5.00 mmol) in toluene (80 mL) was added and the mixture stirred vigorously for 48 hours. The suspension was then filtered, volatiles removed in vacuo, and the green solid dried for two hours. Full consumption of [(Tren)U(I)] was confirmed by H NMR spectroscopy. The solid was then washed with pentane (2 × 10 mL) and dried in vacuo for one hour to yield [{[K(Toluene)][(Tren)U]}] as a green powder. Note: performing this reaction under a dinitrogen atmosphere will result in partial conversion to the diuranium-N-complex, [{U(Tren)}(μ-η:η-N)]. Yield: 2.48 g, 58%. H NMR (400 MHz, CD, 298 K): δ (ppm) -87.96 (s), -54.96 (s), -45.80 (s), -43.41 (s), -38.45 (s), -28.70 (s), -23.39 (s), -6.29 (s), -2.82 (s), 0.88 (s), 1.24 (s), 20.20 (s), 24.79 (s), 26.48 (s), 29.76 (s), 34.61 (s), 35.84 (s), 152.76 (s), 160.11 (s).
Under an atmosphere of argon, a Schlenk flask was charged with a glass-coated stirrer bar and a solid mixture of [(Tren)U(Cl)] (0.75 g, 1 mmol) and potassium graphite (0.4 g, 3 mmol, 3 eq.). At -40 °C, hexane (20 mL) was added and the reaction mixture stirred cold before being allowed to warm slowly to room temperature resulting in the formation of a dark brown/purple suspension, which was stirred for a further 36 h. The suspension was then filtered, volatiles removed in vacuo, and the dark purple solid dried for 2 h. Full conversion to the [(Tren)U] species was confirmed by H NMR spectroscopy. Under an atmosphere of argon, pentane (2 mL) was added to form a very dark purple solution which was stored at -30 °C for 3 days to yield dark-purple crystals of [(Tren)U]. Note: performing this reaction under a dinitrogen atmosphere will result in partial conversion to the diuranium-N-complex, [{U(Tren)}(μ-η:η-N)]. Yield: 0.25 g, 65%. H NMR (400 MHz, CD, 298 K): δ (ppm) -37.01 (s, 6H, NCHCH), -1.55 (s, 18H, SiBu(CH)), 9.81 (s, 27H, SiMe(C(CH)), 21.64 (s, 6H, NCHCH). ATR-IR ν/cm: 2949 (w), 2923 (m), 2878 (m), 2849 (w), 2830 (m), 1462 (s), 1441 (w), 1386 (w), 1357 (w), 1344 (w), 1247 (s), 1122 (m), 1097 (w), 1061 (s), 1021 (w), 1004 (w), 924 (s), 822 (w), 799 (s), 769 (s), 737 (w), 700 (m), 660 (w), 648 (m), 585 (w), 555 (s), 511 (w), 442 (s).
At -78 °C, a pale-yellow solution of 1 (0.13 g, 0.5 mmol) in toluene (5 mL) was added to a dark red purple solution of 3 (0.36 g, 0.5 mmol) in toluene (10 mL), and then the mixture was slowly warmed to room temperature. After being stirred for 24 h, the mixture turned into a dark red brown solution, and removal of volatiles in vacuo gave a brown solid residue. Pentane (5 mL) was added to the residue to afford a white slurry which was stored at -30 °C for 6 h to ensure all the anthracene and unreacted 1 precipitated out of the red brown solution. After filtration, the filtrate was concentrated to ~2 mL and stored at -30 °C for 24 h, giving 4 as dark brown crystals. The crystalline solid was isolated by decanting the mother liquor before being washed with cold SiMe (2 × 1 mL), and then dried in vacuo. Yield: 0.13 g, 32%. H NMR of the crude product indicated the amide species 4 was the main product, and there was no sign of a diuranium-diphosphorus complex; this might be due to the diphosphorus ligand being too reactive to be stabilized by the less sterically demanding Tren ligand environment. Moreover, by P NMR spectroscopy there was no evidence for P formation in the crude products, so the fate of PDBN P atom remains unknown. In addition, 3 does not react with PDBN-P(BPh)Na(OEt) or PDBN-PBu, reflecting the more reactive nature of PDBNP-H/-NMe as phosphinidene group transfer reagents. It should be noted that 4 can also be prepared by the reaction of [(Tren)U(I)] with LiNMe via a salt metathesis method: At -78 °C, a colourless solution of LiNMe (0.025 g, 0.5 mmol) in THF (5 mL) was added to a brown solution of [(Tren)U(I)] (0.42 g, 0.5 mmol) in THF (20 mL), and then the mixture was slowly warmed up to room temperature. After being stirred for 24 h, the mixture turned into a brown solution, and removal of volatiles in vacuo gave a brown solid residue. Pentane (15 mL) was added to the residue to afford a brown slurry which was filtered, and the filtrate was concentrated to ~4 mL and stored at -30 °C for 24 h, yielding 4 as dark brown crystals. The crystalline solid was isolated by decanting the mother liquor before being dried in vacuo. Yield: 0.25 g, 65%. Anal. Calcd for CHNSiU: C, 40.66; H, 8.27; N, 9.12%. Found: C, 40.57; H, 8.12; N, 9.25%. H NMR (400 MHz, CD, 298 K): δ (ppm) -24.48 (s, 18H, SiBu(CH)), -11.79 (s, 27H, SiMe(C(CH)), 8.55 (s, 6H, N(CH)), 59.01 (s, 6H, NCHCH), 100.61 (s, 6H, NCHCH). Si{H} NMR (79 MHz, CD, 298 K): δ (ppm) - 192.59 (s). ATR-IR v/cm: 2950 (m), 2922 (m), 2849 (m), 2816 (m), 2761 (w), 1460 (m), 1384 (w), 1356 (w), 1331 (w), 1248 (m), 1136 (w), 1056 (m), 1024 (w), 923 (s), 891 (m), 820 (s), 767 (s), 706 (s), 653 (m), 566 (w), 493 (m), 452 (w).
A solution of [U{N(SiMe)}] (3.60 g, 5.00 mmol) in hexane (20 mL) was added dropwise to a stirring solution of Ar*OH (3.10 g, 15.00 mmol, Ar* = 2,6-BuCH) in hexane (20 mL) at -78 °C. The mixture was allowed to warm to room temperature before stirring for 16 hours, resulting in the formation of a dark green/black suspension. The suspension was then filtered and volatiles removed in vacuo, resulting in a dark green solid, which was washed with hexane (2 × 5 mL) to yield 5 as a dark green/black solid. Yield: 3.08 g, 72%. H NMR (400 MHz, CD, 298 K): δ (ppm) -6.09 (s, 54 H, Ar-Bu), 13.69 (s, 3H, para-Ar-H), 16.61 (s, 6H, meta-Ar-H). ATR-IR ν/cm: 3065 (w), 2951 (br, m), 2908 (w), 1582 (m), 1457 (m), 1407 (s), 1385 (m), 1354 (m), 1263 (w), 1232 (s), 1194 (m), 1154 (w), 1123 (m), 1097 (s), 922 (w), 882 (w), 862 (s), 817 (s), 797 (w), 746 (s), 653 (s), 594 (m), 545 (s), 451 (s).
At -78 °C, toluene (30 mL) was added to the solid mixture of 5 (0.43 g, 1.00 mmol) and 1 (0.13 g, 0.50 mmol), and then the mixture was warmed to room temperature. After being stirred for 24 h at room temperature, the mixture turned into a yellow brown solution, from which volatiles were removed in vacuo to afford a brown solid residue. The anthracene by-product was removed by sublimation (80 °C, 2.0 × 10 mbar) and the remaining residue extracted with pentane (5 mL) and filtered, resulting in a brown filtrate which was concentrated to ~3 mL. Storing this brown filtrate at -30 °C for 24 h produced red crystals of 6 suitable for single-crystal X-ray diffraction studies, but the crystalline samples were mixed with a brown solid (putatively assigned as [(Ar*O)U(NMe)] product) which could not be separated due to each having essentially identical solubilities in common solvents such as pentane, EtO, and toluene. Attempts to manually separate the two solids were unsuccessful due to the small size of the crystals.
At -78 °C, a dark purple solution of 3 (0.73 g, 1.00 mmol) in toluene (10 mL) was added to a colourless solution of 2 (0.23 g, 1.10 mmol) in toluene (10 mL), and then the mixture was warmed up to room temperature. After being stirred for 30 min at room temperature, the mixture turned into a bright red solution, and removal of volatiles in vacuo gave a dark red solid residue. The anthracene by-product was removed by sublimation (80 °C, 2.0 × 10 mbar) and the remaining residue was extracted with pentane (5 mL) and filtered, resulting a bright red filtrate which was concentrated to ~2 mL and stored at -30 °C for 24 h, yielding 8 as dark red crystals that were suitable for a single-crystal X-ray diffraction studies. The crystalline solid was isolated by decanting the mother liquor, washing with cold SiMe (2 × 1 mL), and then drying in vacuo. Yield: 0.18 g, 36% (by U content, maximum 66% yield as the H NMR of the crude product showed that the ratio of 8:7 = 2:1). Note: adding a solution of 2 in toluene to that of 3 in toluene does not affect the reaction outcome and extending the reaction time longer than 2 hours leads to the decomposition of the diphosphene product resulting in a lower yield. Anal. Calcd for CHNPSiU(pentane): C, 38.13; H, 7.73; N, 7.41%. Found: C, 37.96; H, 7.82; N, 7.34%. H NMR (400 MHz, CD, 298 K): δ (ppm) -147.57 (s, 2H, HPPH), -17.93 (s, 12H, NCHCH), -4.45 to -3.10 (s, br, 36H, SiBu(CH)), 2.40 (s, 12H, NCHCH), 5.86 (s, 54H, SiMe(C(CH). P NMR (162 MHz, CD, 298 K): δ (ppm) 1065.44 (s, br, due to its broad nature, the P-H coupling was not observed). Si{H} NMR (79 MHz, CD, 298 K): δ (ppm) -42.43 (s). ATR-IR v/cm: 2952 (m), 2924 (m), 2878 (m), 2847 (m), 2248 (P-H stretch, w), 1469 (m), 1387 (w), 1359 (w), 1331 (w), 1244 (m), 1145 (w), 1049 (s), 1023 (m), 925 (s), 891 (m), 825 (s), 770 (s), 709 (s), 656 (s), 570 (m), 450 (m). Raman v/cm: 2895 (w), 2842(w), 1465 (w), 1238 (w), 841 (w), 586 (w), 567 (w), 448 (s). Complex 7 was isolated from the reaction mixture by the following work-up: the above insoluble residue was extracted with THF (2 mL) and filtered, affording a dark red brown filtrate which was stored at -30 °C for 2 days, yielding a small crop of dark red brown crystals of 7. The crystalline solid was isolated by decanting the mother liquor, washing with pentane (2 × 1 mL), and then drying in vacuo. Yield: 0.03 g, 12% (by U content, maximum 34% yield). It should be noted that 7 can also be prepared by the reaction of a known cyclometallated U species [{[K(toluene)][(Tren)U]}] with 2. At -78 °C, toluene (20 mL) was added to the pre-cooled solid mixture of [{[K(toluene)][(Tren)U]}] (0.86 g, 0.50 mmol) and 2 (0.11 g, 0.50 mmol) and the mixture was allowed to warm up to room temperature. After being stirred for 24 hours, the mixture turned into a dark red solution. The reaction was filtered, and the dark filtrate was concentrated to ~5 mL and stored at -30 °C for 24 h, yielding 7 as dark brown crystals. The crystalline solid was isolated by decanting the mother liquor and drying in vacuo. Yield: 0.31 g, 42%. Anal. Calcd for CHNPSiU: C, 38.95; H, 7.83; N, 7.57%. Found: C, 39.00; H, 7.93; N, 7.49%. H NMR (400 MHz, CD, 298 K): δ (ppm) -154.14 (s, 1H, PH), -5.03 (s, 12H, NCHCH), -1.35 (s, 36H, SiBu(CH)), 2.87 (s, 54H, SiMe(C(CH) 5.88 (s, br, 12H, NCHCH). P NMR (162 MHz, CD, 298 K): δ (ppm) not observed due to its broad nature and the poor solubility of the complex in benzene or THF. Si{H} NMR (79 MHz, CD, 298 K): δ (ppm) -85.47 (s). ATR-IR v/cm: 2951 (m), 2924 (m), 2875 (m), 2841 (m), 2185 (br, PH), 1468 (m), 1445 (w), 1407 (w), 1388 (w), 1331 (w), 1247 (m), 1147 (w), 1058 (s), 1028 (w), 923 (s), 894 (m), 817 (s), 798 (s), 783 (s), 739 (s), 707 (s), 662 (s), 562 (m), 455 (s). Reliable UV/Vis/NIR spectra could not be obtained as dinuclear 7 is poorly soluble in common solvents such benzene, toluene or THF.
At -78 °C, toluene (30 mL) was added to the solid mixture of 5 (0.85 g, 1.00 mmol) and 2 (0.23 g, 1.10 mmol), and the mixture warmed up to room temperature with stirring. After 2 h the mixture turned into a red solution, and removal of volatiles in vacuo gave a red solid residue. The by-product anthracene was removed by sublimation (80 °C, 2.0 × 10 mbar) and the residue was extracted with toluene (10 mL) and filtered, resulting in a bright red filtrate which was concentrated to ~3 mL. Pentane ( ~ 3 mL) was layered on top of the filtrate and this was stored at -30 °C for 24 h, yielding 9 as dark red crystals that were suitable for single-crystal X-ray diffraction studies. The crystalline solid was isolated by decanting the mother liquor, washing with pentane (2 × 1 mL), and then drying in vacuo. Yield: 0.42 g, 47%. Anal. Calcd for CHOPU: C, 56.94; H, 7.28; N, 0%. Found: C, 56.60; H, 7.24; N, 0%. H NMR (400 MHz, CD, 298 K): δ (ppm) -175.99 (s, 2H, HPPH), -12.95 (s, 94H, Ar-Bu), -12.08 (s, 14H, Ar-Bu), 16.98 (s, 6H, para-Ar-H), 20.63 (s, 12H, meta-Ar-H). P NMR (162 MHz, CD, 298 K): δ (ppm) 844.40 (s, due to its broad nature, the P-H coupling was not observed). ATR-IR v/cm: 2954 (m), 2917 (m), 2864 (m), 2265 (P-H stretch, w), 1581 (w), 1459 (w), 1400 (s), 1359 (w), 1255 (w), 1179 (vs), 1111 (s), 930 (w), 852 (vs), 817 (s), 743 (s), 655 (s), 544 (w), 448 (w).
Calculations on 6, 7, 8, and 9 were performed using coordinates derived from their respective crystal structures as the starting points. No constraints were imposed on the structures during the geometry optimizations. The calculations were performed using the Amsterdam Density Functional (ADF) suite version 2017 with standard convergence criteria. The DFT geometry optimizations employed Slater type orbital (STO) triple-ζ-plus polarization all-electron basis sets (from the Dirac and ZORA/TZP database of the ADF suite). Scalar relativistic approaches (spin-orbit neglected) were used within the ZORA Hamiltonian for the inclusion of relativistic effects and the local density approximation (LDA) with the correlation potential due to Vosko et al was used in all of the calculations. Generalized gradient approximation (GGA) corrections were performed using the functionals of Becke and Perdew. Analytical frequency calculations were carried out within the ADF program. The Quantum Theory of Atoms in Molecules analysis was carried out within the ADF program. The ADF-GUI (ADFview) was used to prepare the three-dimensional plots of the electron density.
The optimization of three different spin states for uranium complexes were carried out by employing DFT hybrid functional (B3PW91) along with small core pseudopotential Stuttgart basis set for uranium, phosphorus atoms with additional polarization functions for phosphorus atoms. Pople basis sets (6-31 G**) were employed for the rest of the atoms. Frequency calculations were performed to locate minima for the optimized structures. All the calculations were performed using Gaussian 16 suite of programs. Dispersion corrections were included in our calculations by employing D3 version of Grimme's dispersion with Becke-Johnson damping.