Article pubs.acs.org/Organometallics
29
Si NMR Spectra of Silicon-Containing Uranium Complexes
Cory J. Windorff and William J. Evans* Department of Chemistry, University of California, Irvine, California 92697-2025, United States S Supporting Information *
ABSTRACT: 29Si NMR spectra have been recorded for a series of uranium complexes containing silicon, and the data have been combined with results in the literature to determine if any trends exist between chemical shift and structure, ligand type, or oxidation state. Data on 52 paramagnetic inorganic and organometallic uranium complexes are presented. The survey reveals that, although there is some overlap in the range of shifts of U4+ complexes versus U3+ complexes, in general U3+ species have shifts more negative than those of their U4+ analogues. The single U2+ example has the most negative shift of all at −322 ppm at 170 K. With only a few exceptions, U4+ complexes have shifts between 0 and −150 ppm (vs SiMe4), whereas U3+ complexes resonate between −120 and −250 ppm. The small data set on U5+ species exhibits a broad 250 ppm range centered near 40 ppm. The data also show that aromatic ligands such as cyclopentadienide, cyclooctatetraenide, and the pentalene dianion exhibit chemical shifts less negative than those of other types of ligands.
■
INTRODUCTION Si NMR spectroscopy has been used extensively to characterize many types of silicon-containing molecules and materials.1−7 Correlations of 29Si NMR chemical shifts with charge, substitution pattern, ring size, coordination number, and π interactions have been made.2−6 Typically, the range of shifts for a given structural type is large and the ranges overlap as the structural type is varied, but trends can be discerned when comparing mean chemical shifts for each structural type. This has provided an additional analytical criterion for examining new silicon-containing compounds. One area of chemistry for which few 29Si NMR data have been reported is that of silicon-containing complexes of uranium. It is understandable that only a few reports of 29Si NMR spectra of uranium compounds are given in the literature,8−23 since complexes of uranium in the +2, +3, +4, and +5 oxidation states are paramagnetic. To date, these data serve only as a fingerprint of a specialized type for these compounds. We recently synthesized several series of silicon-containing uranium complexes24−28 that provide the opportunity to collect a substantial amount of 29Si NMR data on closely related complexes. We wondered if the 29Si NMR spectra would reveal any correlations between the chemical shift and any aspect of the composition, structure, or oxidation state of the uranium
complexes. We were particularly interested in correlations with oxidation state, since the 3.62 and 3.58 μB calculated free ion magnetic moments for U3+ and U4+, respectively, are too close to be distinguished by room-temperature magnetic susceptibility measurements. Moreover, in practice, the room-temperature magnetic moments of complexes of U3+ and U4+ exhibit broad ranges that overlap considerably and variable-temperature SQUID analysis is necessary to accurately assess the oxidation state.29−37 To explore the utility of 29Si NMR spectroscopy in uranium chemistry, we obtained NMR data on our recently synthesized complexes, as well as other previously reported inorganic and organometallic uranium complexes with silicon-containing ligands. We also scanned the literature to find 29Si NMR data already reported on uranium complexes.8−23 The data obtained so far show some interesting preliminary trends. We report this information now to encourage a broader collection of data of this type to determine how powerful this method of analysis can become in the area of uranium chemistry.
29
© 2014 American Chemical Society
Received: May 14, 2014 Published: July 15, 2014 3786
dx.doi.org/10.1021/om500512q | Organometallics 2014, 33, 3786−3791
Organometallics
■
Article
RESULTS AND DISCUSSION
Data Collection. 29Si NMR spectroscopy is typically more challenging than 1H or 13C NMR because of low natural abundance (4.67%), a low negative gyromagnetic ratio (−5.3146 × 107 rad T−1 s−1), low sensitivity (4 × 10−4 relative to 1H), and long relaxation times.1−6 There is also an inherent resonance from the glass in the NMR tube and probe that extends from −80 to −130 ppm and has the potential to obstruct signals. Due to these considerations, concentrated samples were needed (≥20 mg/0.5 mL) and data were collected with an acquisition time of 0.5 s and a t1 value of ≥2 s. A reviewer suggested a t1 value of 1 s should be adequate for paramagnetic samples and we have confirmed that spectra with acceptable signal to noise ratios can be obtained in less time with this shorter t1 value. Initial experiments were conducted with a large window, from 200 to −400 ppm, since it was uncertain how much the paramagnetic uranium would shift the signal. Each experiment was followed by a second experiment with a smaller window, typically 250 ppm. Data collection required 1−3 h per sample with the 2 s t1 value. Typical examples of spectra are given in the Supporting Information (Figures S1−S9). All samples in this study used C6D6 as the solvent, except for (C 5 Me 4 SiMe 3 )MgCl(THF), [K(THF) x ][(C 5 Me 4 SiMe 3 ) 2 UMe2], [K(2.2.2-crypt)][(C5H4SiMe3)3UH], and [K(2.2.2crypt)][(C5H4SiMe3)3U], which are insoluble in C6D6 and were measured in either C4H8O or C4D8O. Previous studies have shown that the solvent does not have a large effect on 29Si NMR shifts.2,4 Several samples were examined in both C6D6 and C4H8O (THF) to test this generalization with uraniumcontaining species, and a similarly small solvent effect was again observed: complex (shift in C6D6, shift in C4H8O in ppm) (C5Me4SiMe3)2UCl2 (−51, −50); (C5Me4SiMe3)2UMeCl (−67, −66); (C5Me4SiMe3)2UMe2 (−84, −84); [C5H3(SiMe3)2]3U, (−162, −163); (C5H4SiMe3)3U, (−165, −158). Exceptions to this solvent independence were observed in the case of a solvated cation such as [K(THF)6]{U[N(SiMe3)2]4}, which has chemical shifts of −192 and −142 ppm in C6D6 and C4H8O, respectively. All newly reported data were collected at room temperature except for the U 2 + complex [K(2.2.2-cryptand)][(C5H4SiMe3)3U],23 which has limited stability in solution at room temperature. To obtain 29Si NMR data on this complex, it was synthesized at low temperature in an NMR tube. The chemical shift of this complex as a function of 1/T is plotted in Figure 1. The data follow a typical Curie−Weiss dependence on temperature. 29 Si NMR Data. The most common types of siliconcontaining ligands in the uranium complexes are the bulky trimethylsilyl-substituted amide and alkyl ligands [N(SiMe3)2]− and [CH(SiMe3)2]− and trimethylsilyl-substituted cyclopentadienyl groups (C 5 Me 4 SiMe 3 ) − , (C 5 H 4 SiMe 3 ) − , and [C5H 3(SiMe3 )2 ]−. The prevalence of the trimethylsilyl substituent in this study provides some uniformity in the immediate environment of the silicon. Table 1 shows 29Si NMR data collected in this study on inorganic and organometallic U3+ and U4+ complexes with silicon-containing ligands, along with other data found in the literature. A single example of U2+ is also given in Table 1. Data on U5+ and U6+ complexes are given in Table 2. For reference, data on free ligands and derivatives are shown in Table 3. The data are arranged according to decreasing chemical shift in
Figure 1. Plot of chemical shift (δ) vs T−1 for [K(2.2.2-crypt)][(C5H4SiMe3)3U].
C6D6 at 293−303 K, except where noted. For convenience, the data are presented as a graph of the 29Si resonances in Figure 2. Correlations of chemical shift and the distance of silicon from uranium in the solid state X-ray crystal structures were also examined. These are presented in the Supporting Information, since the plots show no consistent correlations (Figure S10). For example, for U4+ complexes, a scatter plot is observed (Figure S11). For U3+ complexes, 4 compounds with close U···Si distances have larger shifts than 13 compounds with longer U···Si distances, but within these two groups there is no correlation of shift and the U···Si distance in the solidstate structure (Figure S12). Correlations were also examined within a ligand set (Figures S13 and S14). Three U3+ complexes of [N(SiMe3)2]− show a correlation (Figure S15), as do four U5+ complexes of Tren ligands (Figure S16), but two pairs of U4+ complexes with similar U···Si distances have shifts that vary by 100 ppm (Figure S13). A correlation for 4 U4+ complexes of cyclopentadienyl, cyclooctatetraenyl, and pentalenyl ligands looks better than that for 11 U3+ complexes of the same types of ligands (Figure S14), but we are reluctant to overinterpret these correlations with so few data. Perhaps more insight can be gained with these plots as more data are added. The difficulty of finding correlations in the NMR shifts of paramagnetic species has been pointed out.3 Competing factors can affect the shift (e.g., contact and pseudocontact effects) and distances and geometries in solution may not reflect data obtained in the solid state. Ligand Effects. A ligand-based trend discernible from the data in Table 1 is that uranium complexes with ligands such as triamidoamine (Tren) and amide exhibit chemical shifts more negative than those with aromatic ligands such as cyclopentadienide, cyclooctatetraenide, and the pentalene dianion. This is exemplified by the U3+ complexes [C8H6(SiMe3)2]U(C5Me5), and [C5H3(SiMe3)]3U, which resonate at −145 and −162 ppm, respectively,12 whereas (TrenTIPS)U and U[N(SiMe3)2]3 resonate at −178 and −219 ppm, respectively.17 Comparison of the −247 ppm shift for (C5Me5)2U[N(SiMe3)2] versus the −158 ppm shift of (C5Me5)2U[CH(SiMe3)2] shows the amide to resonate at a shift more negative than that for the corresponding alkyl. The order of the resonances in these two complexes is reversed from what is observed in the free ligands. For example, CH2(SiMe3)2 resonates at −0.12 ppm, whereas HN(SiMe3)2 resonates at 1.95 ppm. The data show that cyclopentadienide ligands resonate at shifts less negative than those for cyclooctatetraenyl ligands (C8H6R2-1,4)2− (R = SiMe3, SiiPr3,). For example, three of the 3787
dx.doi.org/10.1021/om500512q | Organometallics 2014, 33, 3786−3791
Organometallics
Article
Table 1. 29Si NMR Chemical Shifts of Un+ Complexes (n = 2−4) Si chem shift δ (ppm)b
Table 2. 29Si NMR Chemical Shifts of Un+ Complexes (n = 5, 6)
ref
compounda
n
Si chem shift δ (ppm)b
64c 36 21 13 −11 −11 −41 −51 −67 5; −74
23 20 20 17 14 19 14 this work this work 19
5 5 6 6 5
160 152 −0.69 −8.28 −13
6 5 5 5 5
−21.70 −37 −58 −71 −90c
4 4 4 4 4 4 4
−84 −74; −83; −152 −90 −97 −101; −134c −108; −185 −108; −143c
this work this work 19 19 8 9 12
(ROUO)2(L) (PhMe2SiOUO)2(L) (BIPM)UOCl2 (BIPM)U(NMes)(O)(DMAP)2 (TrenTIPS)U(μ-N)[μ-Na(15-crown5)] (TrenTIPS)UN (TrenTIPS)UNAd (TrenTIPS)UNR (TrenTIPS)UO [(C8H6R2)(C5Me5)U]2(μ-η2:η2C4O4)
4
c
−113
12
4
−115c
10
3 4 3 4 3 3 3 4 3 3 3 3 3 3 3 3 3 3 3
−116 −121 −124 (C8H6R′2)c −127 −133c −137c −145d −149 −154e −154c −155 −158 −159c −162 −165 (−158e) −173 (−295c,h) −176c −178 −137; −181; −198 −192 (−142e) −206; −231 −219 −237c −247 −255 −322f,i
29
compounda
n
{U[OSi(Mes)3]3}2(μ-η2:η2-N2) [(TrenTIPS)U(THF)][BPh4] (TrenTIPS)U(PH2) (TrenTIPS)UCl [κ2-C5H4N(2-NR)]3UI (TrenTIPS)UN3 [κ2-C5H4N(2-NR)]3UCl (C5Me4R)2UCl2 (C5Me4R)2UMeCl [κ5-(NHCMe2SiiPr2NCH2CH2) N(CH2CH2NR′)2]U (C5Me4R)2UMe2 U(NR2)2(κ2-CH2SiMe2NR) (TrenTIPS)UNH2 (TrenTIPS)UF (C8H4R′2)2U [(C8H4R′2)(C5Me5)U]2(μ-η2:η2-N2) [(C8H6R2)(C5Me4R)U]2(μη1:η1-13C2O2) [(C8H6R′2)(C5Me4Et)U]2(μη1:η1-13C2O2) [(C8H6R′2)(C5Me5)U]2(μη1:η1-13C2O2) (C8H6R′2) U(κ3-TpMe2) [K(2.2.2-crypt)][(TrenTIPS)U(PH)] (C8H6R′2)(C5Me4R)U U(NR2)4 (C8H6R′2)(C5Me4iPr)U (C8H6R′2)(C5Me4Et)U (C8H6R2)(C5Me5)U [(TrenDMSB)U]2(μ-η1:η1-C2O2) [K(2.2.2-crypt)][(C5H4R)3UH] (C8H6R2)(C5Me4R)U (C5Me4R)3U (C5Me5)2U(CHR2) (C8H4R′2)U(κ3-TpMe2) (C5H3R2)3U (C5H4R)3U (C8H4R′2)(C5Me5)U (C8H6R2)(C5Me5)U(THF) (TrenTIPS)U (C5Me4R)2U(CHR2)
4 4 4 4 4 4 4 4 4 4
[K(THF)6][U(NR2)4] (C5Me4R)2U(NR2) U(NR2)3 [K(THF)x][(C5Me4R)2UMe2] (C5Me5)2U(NR2) [(η5:η1-C5Me4SiMe2CH2)2U] [K(2.2.2-crypt)][(C5H4R)3U]
3 3 3 3 3 4 2
c, g
29
19 19 17, 19 18 12
a
Abbreviations: R = SiMe3, R′ = SiiPr3, L = Pacman-like binucleating Schiff-base calixpyrrole macrocycle, BIPM = bis(iminophosphorano)methanediide = C(PPh2NSiMe3)2, Mes = mesityl, DMAP = 4dimethylaminopyridine, TrenTIPS = N(CH2CH2NSiiPr3)3, Ad = adamantyl. bIn C6D6 unless noted otherwise. cIn C7D8.
Table 3. 29Si NMR Chemical Shifts of Diamagnetic Derivatives of Ligands 29
compounda RCl R2CHCl K2[C8H4R′2] Li3(TrenTIPS) HNR2 C5Me4RH CH2R2 HC[SiMe2N(Li)Ar]3 HC[SiMe2N(Li)Ar′]3(OEt2)2 HC[SiMe2N(Li)Ar]3(OEt2) K[C5H4N(2-NR)](THF)0.25 Li[C5H4N(2-NR)](THF)2 C8H6(SiPh3)2 (C5Me4R)MgCl(THF) (RNPPh2)2CC(H)Ph (RNPPh2)2CC(H)R″ K(C5H4R) K(C5H3R2) (RNPPh2)2CCPh2 KNR2 bis(trimethylsiloxy)furanone bis(dimethylphenylsiloxy)furanone
11 20 12 this work 12 12 12 16 this work 12 this work this work 11 this work this work 9, 13 12 17 this work this this this this this this this
ref 22 22 15 21 19
work work work work work work work
Si chem shift δ (ppm)b
30.11 4.40/3.43 4.19 2.60 1.95−2.22c 1.32 0.50/−0.12 −0.56 −4.01 −6.06 −8.52 −8.55 −9.9d −11.75e −12.02; −12.51 −12.47; −13.36 −14.43 −14.59 −17.22 −20.49 −21.82 −21.94
ref 2, this work 2, this work 8 17 2, this work this work 2, this work 38 38 38 14 14 27 this work 15 39 this work this work 40 this work 16 16
a
R = SiMe3, R′ = SiiPr3, R″ = anthracene, C14H9, TrenTIPS = N(CH2CH2NSiiPr3)3, Ar = 3,5-Me2C6H3, Ar′ = 4-MeC6H4 bIn C6D6 unless noted otherwise. cIn C6H6. dIn CDCl3. eIn C4H8O.
[C5H3(SiMe3)2]3U, and (C5H4SiMe3)3U resonate at −155, −162, and −165 ppm, respectively. Oxidation State Analysis. As can be seen in Table 1 and Figure 2, there is a general trend of U3+ complexes occurring at values more negative than those for U4+ complexes. The range of most of the U4+ complexes in Table 1 is between 0 and −150 ppm. In contrast, most of the U3+ complexes have resonances between −120 and −250 ppm. The low-temperature spectrum of the U2+ example had the most negative chemical shift of all, −322 ppm at 170 K, but extrapolation of the data in Figure 1 gives an estimated shift of −216 ppm at 298 K, at the low end
a
Abbreviations: R = SiMe 3 , R′ = Si i Pr 3 , Tren TIPS = N(CH2CH2NSiiPr3)3, TrenDMSB = N(CH2CH2NSiMe2tBu)3, TpMe2 = hydridotris(3,5-dimethylpyrazolyl)borate. bIn C6D6 unless noted otherwise. cIn C7D8. dIn C6D12. eIn C4H8O. fIn C4D8O. gAt 328 K. h At 201 K. iAt 170 K.
U3+ complexes containing the [(C8H6R2-1,4)](C5Me4R′)U (R = SiiPr3, R′ = iPr, Et; R = SiMe3, R′ = Me) structure resonate at −133, −137, and −145 ppm, respectively,12 whereas the tris(cyclopentadienyl) complexes , (C 5 Me 4 SiMe 3 ) 3 U, 3788
dx.doi.org/10.1021/om500512q | Organometallics 2014, 33, 3786−3791
Organometallics
Article
exhibit more negative shifts. The lower the oxidation state of the uranium in the series +4, +3, and +2, the lower the 29Si NMR chemical shift.
■
EXPERIMENTAL SECTION
All manipulations and syntheses described below were conducted with the rigorous exclusion of air and water using standard Schlenk line and glovebox techniques under an argon or dinitrogen atmosphere. Solvents were sparged with UHP argon and dried by passage through columns containing Q-5 and molecular sieves prior to use. Deuterated NMR solvents were dried over sodium benzophenone ketyl or sodium/potassium alloy, degassed by three freeze−pump−thaw cycles, and vacuum-transferred before use. 1H, 13C, and 29Si NMR spectra were recorded on a Bruker GN 500 MHz spectrometer operating at 499.3, 125.6, and 99.2 MHz, respectively, at 298 K unless otherwise stated. 1H NMR spectra were referenced internally to residual protio solvent resonances, 13C NMR spectra were referenced internally to solvent, and 29Si NMR spectra were referenced externally to SiMe4. The following compounds were prepared following literature procedures: C5Me4SiMe3MgCl(THF),24 (C5Me4SiMe3)2UMe2,24 ( η 5 : η 1 - C 5 M e 4 S iM e 2 C H 2 ) 2 U , 2 4 ( C 5 M e 4 S i M e 3 ) 2 U M e 2 K , 2 5 (C 5 Me 4 SiMe 3 ) 3 U, 2 5 (C 5 H 4 SiMe 3 ) 3 U, 2 8 [K(2.2.2-crypt)][(C5H4SiMe3)3UH],28 (C5Me5)2U[N(SiMe3)2],41 [(C5Me5)2U][(μPh2)BPh2],42 KU[N(SiMe3)2]4,37 U[N(SiMe3)2]4,37 and U[N(SiMe3)2]2(κ2-CH2SiMe2NSiMe3).37 The following compounds were prepared following the literature, but with the exceptions noted: (C5Me4SiMe3)2UMeCl was made analogously to (C5Me4H)UMeCl,43 (C5Me4SiMe3)2UCl224 was recrystallized from toluene at −35 °C after filtration, [(C5Me4SiMe3)2U][(μ-Ph2)BPh2]25 was washed with methylcyclohexane and hexane and then extracted in benzene, and U[N(SiMe3)2]344 was sublimed under vacuum at 10−7 Torr at 80 °C for 6 h before use. (C 5 Me 4 SiMe 3 ) 2 U[N(SiMe 3 ) 2 ] and (C5Me4SiMe3)2U[CH(SiMe3)2] were prepared following the method of (C5Me5)2U[N(SiMe3)2].41 The syntheses of (C5Me4SiMe3)2U[N(SiMe3)2] and (C5Me4SiMe3)2U[CH(SiMe3)2] can be found in the Supporting Information. C5Me4SiMe3H.45 As a variation from the literature method, Me3SiCl (0.83 g, 7.6 mmol) in toluene (10 mL) was added dropwise over 5 min to a stirred suspension of KC5Me4H (1.23 g, 7.67 mmol). The solution was stirred for 48 h, and the solvent was removed under reduced pressure. Hexane (60 mL) was added to the reaction mixture, which was then filtered on a 60 mL medium-porosity frit and washed with 2 × 10 mL of hexane. The hexane was removed under reduced pressure to yield C5Me4SiMe3H as a colorless liquid (0.850 g, 57%). 1 H NMR45 (C6D6): δ 2.67 (s, 1H, C5Me4SiMe3H), 1.90 (s, 6H, C5Me4SiMe3H), 1.79 (s, 6H, C5Me4SiMe3H), −0.04 (s, 9H, C5Me4SiMe3H). 13C NMR (C6D6): δ 135.4 (C5Me4SiMe3), 133.0 (C5Me4SiMe3), 55.3 (i-(C5Me4SiMe3), 11.3 (C5Me4SiMe3), −1.6 (C5Me4SiMe3). 29Si NMR (C6D6): δ 1.32 (C5Me4SiMe3). (C5Me5)2U[CH(SiMe3)2].46 This complex was prepared following the method of (C5Me5)U[N(SiMe3)2]41 rather than the literature synthesis. LiCH(SiMe3)2 (37 mg, 0.22 mmol) was added to a stirred slurry of [(C5Me5)2U][(μ-Ph2)BPh2] (170 mg, 0.206 mmol) in benzene (5 mL). After 20 h, an aliquot was drawn for NMR spectroscopy to ensure the reaction had gone to completion, following which white solids (presumably LiBPh 4 ) were removed by centrifugation and the solvent was removed under reduced pressure to yield (C5Me5)2U[CH(SiMe3)2] as a dark brown solid (104 mg, 75%). The product was confirmed by 1H NMR spectroscopy.46 29Si NMR (C6D6): δ −158 [CH(SiMe3)2]. [C5H3(SiMe3)2]3U.47 This synthesis is an alternative to that given in the literature. KC5H3(SiMe3)2 (321 mg, 1.29 mmol) was tapped into a stirred slurry of UI3 (264 mg, 0.427 mmol) in Et2O (10 mL). An immediate color change from purple to green was observed, and the volume was increased to 20 mL. After 1 h, white solids (presumably KI) were removed by centrifugation and the solvent was removed under reduced pressure. The product was dissolved in 15 mL of hexane and the solution stirred for 4 h. More white solids (presumably KI) were removed by centrifugation, and the solvent was removed
Figure 2. Graphical representation of the U4+/3+ data, where purple lines represent the shift of a U4+ complex and yellow lines represent U3+ complexes. Not shown are the resonances at positive chemical shifts for {U[OSi(Mes)3]3}2(μ-η2:η2-N2) at 64 ppm,23 [TrenTIPSU(THF)][BPh4] at 36 ppm,20 (TrenTIPS)U(PH2) at 21 ppm,20 (TrenTIPS)UCl at 13 ppm,17 and one of the resonances for [κ5(NHCMe2SiiPr2NCH2CH2)N(CH2CH2NR′)2]U at 5 ppm.19
of the U3+ range. This calculated room-temperature resonance is significantly shifted from that of the neutral (C5H4SiMe3)3U(THF) compound (at −159 ppm) and is more negative than those for most cyclopentadienyl complexes. All of these shifts are substantially more negative than the +30 to −25 ppm range found for the ligands in diamagnetic compounds such as C5Me4SiMe3H, K[C5H3(SiMe3)2], and KN(SiMe3)2 and the three examples of U6+ compounds in Tables 2 and 3. The U5+ compounds, on the other hand, exhibit the widest range of shifts: +160 to −90 ppm. However, to date there have only been seven examples for which 29Si NMR data on U5+ compounds are reported.12,17−19,22 The ranges for most U4+ and U3+ complexes do not overlap, but there are some exceptions. The two U4+ complexes that have resonances more negative than those for the other examples each contain cyclometalated silicon-containing ligands. In the U4+ complex (η5:η1-C5Me4SiMe2CH2)2U, in which the only silicon is in the ring of a cyclometalated ligand, a chemical shift at −255 ppm is observed. U[N(SiMe3)2]2(κ2CH2SiMe2NSiMe3) has three observable resonances at −74, −83, and −152 ppm. The latter resonance is outside the normal range and is attributed to the silicon in the cyclometalated ring. 29Si NMR shifts in cyclic silanes are known to be sensitive to ring size and ring strain;1−7 therefore, there is precedence for these silicon resonances to be shifted outside the expected range.
■
CONCLUSION Recently obtained 29Si NMR spectra on uranium complexes containing silicon and a review of the known literature has revealed some general trends among chemical shift, composition, and oxidation states that should prove to be useful for further studies. These trends should not be considered definitive at this stage, since 29Si NMR shifts are often variable and data on only 52 paramagnetic uranium complexes are collected here. Therefore, the following correlations should be further tested in future studies. Complexes containing nonaromatic ligands resonate at shifts lower than those with aromatic ligands. Complexes containing ligands with silicon bound to nitrogen resonate at values more negative than those with silicon bound to carbon. Silicon in cyclic ligands tends to 3789
dx.doi.org/10.1021/om500512q | Organometallics 2014, 33, 3786−3791
Organometallics
Article
under reduced pressure to give [C5H3(SiMe3)2]3U as a green solid (262 mg, 70%). The product was confirmed by 1 H NMR spectroscopy.47 29Si NMR (C6D6): δ −162 (C5H3(SiMe3)2). Small-Scale Synthesis of [K(2.2.2-crypt)][(C5H4SiMe3)3U]28 for Low-Temperature NMR Studies. (C5H4SiMe3)3U (85 mg, 0.13 mmol) and 2.2.2-cryptand (59 mg, 0.16 mmol) were dissolved in 1 mL of d8-THF to form a red-brown solution and placed into a −35 °C freezer for 2 h along with a pipet packed with ∼45 mg of KC8, an NMR tube, and a scintillation vial filled with hexane. Once removed from the freezer, the NMR tube was placed in the hexane bath and the KC8 column was placed above the NMR tube. The red-brown solution was quickly added to the column and pushed through to yield a very dark green solution. The NMR tube was capped, removed from the glovebox, sealed with Parafilm, frozen in liquid nitrogen, and taken to the NMR facility. The NMR spectrometer was locked and shimmed using pure d8-THF at low temperature. The sample was thawed in water and placed into the precooled probe. 1H and 29Si NMR spectra were recorded at 170 K, and spectra were then taken from 183 to 233 K in 10 K increments.
■
(12) Tsoureas, N.; Summerscales, O. T.; Cloke, F. G. N.; Roe, S. M. Organometallics 2013, 32, 1353. (13) Tsoureas, N.; Kilpatrick, A. F. R.; Summerscales, O. T.; Nixon, J. F.; Cloke, F. G. N.; Hitchcock, P. B. Eur. J. Inorg. Chem. 2013, 2013, 4085. (14) King, D. M.; Lewis, W.; Liddle, S. T. Inorg. Chim. Acta 2012, 380, 167. (15) Mills, D. P.; Cooper, O. J.; Tuna, F.; McInnes, E. J. L.; Davies, E. S.; McMaster, J.; Moro, F.; Lewis, W.; Blake, A. J.; Liddle, S. T. J. Am. Chem. Soc. 2012, 134, 10047. (16) Gardner, B. M.; Stewart, J. C.; Davis, A. L.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 9265. (17) King, D. M.; Tuna, F.; McInnes, E. J.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Science 2012, 337, 717. (18) King, D. M.; Tuna, F.; McMaster, J.; Lewis, W.; Blake, A. J.; McInnes, E. J. L.; Liddle, S. T. Angew. Chem., Int. Ed. 2013, 52, 4921. (19) King, D. M.; Tuna, F.; McInnes, E. J.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Nat. Chem. 2013, 5, 482. (20) Gardner, B. M.; Balázs, G.; Scheer, M.; Tuna, F.; McInnes, E. J. L.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Angew. Chem., Int. Ed. 2014, 53, 4484. (21) Lu, E.; Cooper, O. J.; McMaster, J.; Tuna, F.; McInnes, E. J. L.; Lewis, W.; Blake, A. J.; Liddle, S. T. Angew. Chem., Int. Ed. 2014, 53, 6696. (22) Arnold, P. L.; Jones, G. M.; Odoh, S. O.; Schreckenbach, G.; Magnani, N.; Love, J. B. Nat. Chem. 2012, 4, 221. (23) Mansell, S. M.; Farnaby, J. H.; Germeroth, A. I.; Arnold, P. L. Organometallics 2013, 32, 4214. (24) Evans, W. J.; Siladke, N. A.; Ziller, J. W. Chem. Eur. J. 2010, 16, 796. (25) Siladke, N. A.; Ziller, J. W.; Evans, W. J. Z. Anorg. Allg. Chem. 2010, 636, 2347. (26) Siladke, N. A.; Ziller, J. W.; Evans, W. J. J. Am. Chem. Soc. 2011, 133, 3507. (27) Lorenz, V.; Schmiege, B. M.; Hrib, C. G.; Ziller, J. W.; Edelmann, A.; Blaurock, S.; Evans, W. J.; Edelmann, F. T. J. Am. Chem. Soc. 2011, 133, 1257. (28) MacDonald, M. R.; Fieser, M. E.; Bates, J. E.; Ziller, J. W.; Furche, F.; Evans, W. J. J. Am. Chem. Soc. 2013, 135, 13310. (29) Castro-Rodriguez, I.; Nakai, H.; Zakharov, L. N.; Rheingold, A. L.; Meyer, K. Science 2004, 305, 1757. (30) Castro-Rodriguez, I.; Meyer, K. Chem. Commun. 2006, 1353. (31) Lam, O. P.; Anthon, C.; Heinemann, F. W.; O’Connor, J. M.; Meyer, K. J. Am. Chem. Soc. 2008, 130, 6567. (32) Graves, C. R.; Vaughn, A. E.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2008, 47, 11879. (33) Graves, C. R.; Yang, P.; Kozimor, S. A.; Vaughn, A. E.; Clark, D. L.; Conradson, S. D.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Hay, P. J.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130, 5272. (34) Kraft, S. J.; Fanwick, P. E.; Bart, S. C. Inorg. Chem. 2010, 49, 1103. (35) Matson, E. M.; Forrest, W. P.; Fanwick, P. E.; Bart, S. C. Organometallics 2012, 31, 4467. (36) Fortier, S.; Brown, J. L.; Kaltsoyannis, N.; Wu, G.; Hayton, T. W. Inorg. Chem. 2012, 51, 1625. (37) Lewis, A. J.; Williams, U. J.; Carroll, P. J.; Schelter, E. J. Inorg. Chem. 2013, 52, 7326. (38) Patel, D.; Lewis, W.; Blake, A. J.; Liddle, S. T. Dalton Trans. 2010, 39, 6638. (39) Cooper, O. J.; Mills, D. P.; McMaster, J.; Moro, F.; Davies, E. S.; Lewis, W.; Blake, A. J.; Liddle, S. T. Angew. Chem. 2011, 123, 2431. (40) Cooper, O. J.; Mills, D. P.; Lewis, W.; Blake, A. J.; Liddle, S. T. Dalton Trans. 2014, DOI: 10.1039/C4DT00909F. (41) Evans, W. J.; Kozimor, S. A.; Ziller, J. W.; Kaltsoyannis, N. J. Am. Chem. Soc. 2004, 126, 14533.
ASSOCIATED CONTENT
S Supporting Information *
Details for the synthesis of additional compounds examined for the survey, sample 29Si NMR spectra, graphs of 29Si NMR chemical shifts vs U···Si distances by uranium oxidation state, graphs of 29Si NMR chemical shifts vs U···Si distances by ligand set, and a table of 29Si NMR chemical shifts and their U···Si distances. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail for W.J.E.:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank Dr. Philip R. Dennison for his helpful discussions and the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy (DE-SC0004739) for support.
■
REFERENCES
(1) Iggo, J. A. NMR Spectroscopy in Inorganic Chemistry; Oxford University Press: New York, 1999; Vol. 83. (2) Marsmann, H. NMR Basic Principles and Progress; SpringerVerlag: New York, 1981; Vol. 17. (3) Harris, R. K.; Mann, B. E. NMR and the Periodic Table; Academic Press: San Francisco, 1978. (4) Schraml, J. The Chemistry of Organic Silicon Compounds; Wiley: New York, 2001; Vol. 3. (5) Coleman, B. Chemically and Biochemically Important Elements; Academic Press: New York, 1983; Vol. 2. (6) Kennedy, J. D.; McFarlane, W. Multinuclear NMR; Plenum Publishing: New York, 1987. (7) Kupce, E.; Wrackmeyer, B. Advanced Application of NMR to Organometallic Chemistry; Wiley: New York, 1996. (8) Cloke, F. G. N.; Green, J. C.; Jardine, C. N. Organometallics 1999, 18, 1080. (9) Cloke, F. G. N.; Hitchcock, P. B. J. Am. Chem. Soc. 2002, 124, 9352. (10) Frey, A. S.; Cloke, F. G. N.; Hitchcock, P. B.; Day, I. J.; Green, J. C.; Aitken, G. J. Am. Chem. Soc. 2008, 130, 13816. (11) Farnaby, J. H.; Cloke, F. G. N.; Coles, M. P.; Green, J. C.; Aitken, G. C. R. Chim. 2010, 13, 812. 3790
dx.doi.org/10.1021/om500512q | Organometallics 2014, 33, 3786−3791
Organometallics
Article
(42) Evans, W. J.; Nyce, G. W.; Forrestal, K. J.; Ziller, J. W. Organometallics 2002, 21, 1050. (43) Evans, W. J.; Kozimor, S. A.; Hillman, W. R.; Ziller, J. W. Organometallics 2005, 24, 4676. (44) Andersen, R. A. Inorg. Chem. 1979, 18, 1507. (45) Horacek, M.; Gyepes, R.; Cisarova, I.; Polasek, M.; Varga, V.; Mach, K. Collect. Czech. Chem. Commun. 1996, 61, 1307. (46) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, C. S.; Vollmer, S. H.; Day, V. W. Organometallics 1982, 1, 170. (47) del Mar Conejo, M.; Parry, J. S.; Carmona, E.; Schultz, M.; Brennann, J. G.; Beshouri, S. M.; Andersen, R. A.; Rogers, R. D.; Coles, S.; Hursthouse, M. B. Chem. Eur. J. 1999, 5, 3000.
3791
dx.doi.org/10.1021/om500512q | Organometallics 2014, 33, 3786−3791
