β-Diketiminate Derivatives of Alkali Metals and Uranium

Article pubs.acs.org/Organometallics

β‑Diketiminate Derivatives of Alkali Metals and Uranium Ashley J. Wooles, William Lewis, Alexander J. Blake, and Stephen T. Liddle* School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K. S Supporting Information *

ABSTRACT: Treatment of [M(Bn)] (M = K, Cs) with [HC{C(But)NDipp}{C(But)NHDipp}] (LButH; Dipp = 2,6-diisopropylphenyl) afforded [K(LBut)(THF)n], 1, and [Cs(LBut)], 2, which were crystallized from hexane or benzene to afford [K(LBut)(THF)3], 1a, and [{Cs(LBut)}{Cs(LBut)(η3-C6D6)}·C6D6]∞, 2a, respectively. Complexes 1 and 2 were utilized in the preparation of the previously reported [U(LBut)Cl3], which was cleanly converted to [U(LBut)I3], 3, via reaction with an excess of Me3SiI. Attempts to prepare U(III) complexes incorporating the LBut ligand proved unsuccessful, but utilizing [HC{C(Me)NDipp}{C(Me)NHDipp}] (LMeH) led to the isolation of [U(LMe)I2(THF)2], 4, via the reaction of [K(LMe)] with [U(I)3(THF)4]. Complex 4 can be derivatized via reaction with [K(Cp*)] (Cp* = η5-C5Me5) or [K{N(SiMe3)2}] to afford [U(LMe)(Cp*)I], 5, and [U(LMe){N(SiMe3)2}I], 6, respectively. The reaction of 4 with two equivalents of [K{N(SiMe3)2}] did not afford [U(LMe){N(SiMe3)2}2] as expected, but instead led to the isolation of the U(IV) species [U{HC[C(Me)NDipp][C(CH2)NDipp]}{N(SiMe3)2}2], 7, via deprotonation of the LMe ligand. The reduction of 4 with KC8 in benzene afforded the diuranium inverse sandwich complex [{U(LMe)I}2(μ-η6:η6-C6H6)], 8, albeit in low yield. Complexes 1−8 have been characterized by single-crystal X-ray diffraction studies, by multielement NMR spectroscopy, and variously by FTIR spectroscopy, elemental analysis, UV/vis/NIR spectroscopy, and solution-state magnetic studies.

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INTRODUCTION Since the initial discovery of inverted sandwich diuranium arenebridged complexes by Cummins in 2000,1 many examples have now been reported.2−12 Of most relevance to this study is our recently reported preparation of [{U(BIPMTMSH)(I)}2(μ-η6:η6C6H5CH3)] (BIPMTMSH = HC(PPh2NSiMe3)2), prepared during our investigations of uranium-carbenes,13−17 which showed properties characteristic of single-molecule magnetism.5 Since this arene complex was isolated in a relatively low yield (20%), we targeted a different synthetic approach incorporating an alternative ancillary ligand in place of BIPMRH (R = trimethylsilyl, mesityl, 2,6-diisopropylphenyl), namely, [HC{C(R)NDipp}2]− (R = Me, But: Dipp = 2,6-diisopropylphenyl), which for brevity we abbreviate to LMe or LBut, respectively. This β-diketiminate ligand has similar steric demands to BIPMRH, has been reported to stabilize both inverse sandwich complexes and low-valent metal complexes,18,19 and does not contain redox-active P(V) centers, which may facilitate side reactions in the preparation of [{U(BIPMTMSH)(I)}2(μ-η6:η6-C6H5CH3)] under reducing conditions, leading to low yields. With this in mind we targeted [(LR)UI2(THF)n] as a potential synthetic precursor to diuranium arene inverse sandwich species. A range of U(V) and U(VI) uranyl β-diketiminate complexes have been published.20−23 However, of most relevance to this study are non-uranyl uranium β-diketiminate complexes, of which several have been reported. Lappert disclosed the mixed valence ion pair complex [{UCl(μ-Cl)(L)(NR)}2][UCl2(L)2]2 (L = HC{C(Ph)NSiMe3}2) in 1995,24 which exhibits a U(VI) dication and two U(III) anionic centers, with the U(III) center coordinated by one β-diketiminate ligand in a typical η2-N,N © XXXX American Chemical Society

fashion, while the second ligand is coordinated in an unusual η3-(N,C,C′)-azaallyl mode. This unusual bonding mode was also shown by Kiplinger in 2004 in the U(III) complex [U(LMe)2I], which also exhibits one ligand bound in a η2-N,N fashion with a second ligand bound in a η3-(N,C,C′)-azaallyl mode.25 Very recently,26 Kiplinger reported a range of U(IV) and Th(IV) complexes incorporating either the LMe or LBut β-diketiminate ligand frameworks, which has prompted us to disclose our work. Kiplinger reported the preparation of [U(LMe)Cl3(THF)] and [U(LBut)Cl3] by the straightforward salt metathesis reactions of UCl4 with [K(LMe)] and [K(LBut)], respectively, and also reported the preparation of Th(IV) bromide and iodide analogues by similar methodologies.26 With prior reports in mind we anticipated that our target complexes [U(LR)I2(THF)n] could be prepared by the salt metathesis reactions of [U(I)3(THF)4] with [K(LMe)] or [K(LBut)].

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RESULTS AND DISCUSSION Preparation of (LBut) Ligand Transfer Reagents. The preparation of the ligand transfer reagents [K(LMe)]27−29 and [K(LBut)]30 has been reported previously along with a structural characterization of [K(LMe)].27 However, despite the solid-state structures of [Li(LBut)(S)] (S = THF, Et2O) and [{Na(LBut)}2] being reported previously,30−32 no satisfactory single-crystal X-ray diffraction studies of [K(LBut)] have been reported. In 2006 Winter reported the structure of [K(LBut)(THF)3], where the potassium is bound to the ligand by one nitrogen atom and Received: May 17, 2013

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dx.doi.org/10.1021/om400435b | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

[{M(LBut)(S)}n] (M = Li, S = THF, Et2O, n = 1; M = Na, S = none, n = 2) and [K(LMe)], where the LR ligand is bound to each metal center in a η2-N,N fashion.27,30−32 The change in coordination mode is likely due to the combination of high steric demands of both the LBut ligand and potassium destabilizing the η2-N,N coordination mode as the open “wedge” required between the N-Dipp groups would be large, causing greater steric repulsion between the two But groups. The observed coordination mode orientates the two bulky But groups in a trans geometry, which reduces the steric crowding in the complex. While one of the THF molecules in [K(LBut)(THF)3] exhibits positional disorder and was refined isotropically, the mean K1−O distances in [K(LBut)(THF)3] can be reliably determined as 2.628(3) Ǻ , which is well within the sum of the respective covalent radii (2.69 Å)34 and shorter than the mean reported K−O bond distance of 2.816 Å in the CSD.35 The K1−N2 distance of 2.725(3) Å is within the sum of covalent radii of potassium and nitrogen (2.74 Å)34 and toward the short end of the range of previously reported K−N bond distances (2.306−4.565 Å), suggesting a significant interaction is present.35 A potassium η2-Dipp interaction is suggested by the K1···C24 and K1···C29 distances of 3.075(4) and 3.308(4) Å, respectively, while the K1−C23 distance of 3.405(4) Å and K−HMe distance of 2.707(4) Å suggest an agostic-type interaction may be present between a methyl C−H bond and potassium. These distances are similar to corresponding K−C and K−H distances for agostic-type interactions reported in [K{Si(SiMe3)3}]2, [{K(η6-Ar)2}M{N(SiMe3)2}3] (M = Mg; Ar = benzene, toluene, p-xylene), and [Y{μ-η5:η1-ArNC(CH3) CHC(CH2)NAr}2K(DME)2].36−38 We have previously reported that in cases where potassium ligand transfer reagents do not furnish the expected salt elimination reactions, a heavier group 1 (Rb or Cs) ligand transfer reagent can successfully afford the desired complex.39 This was the case in the attempted reaction of [{K(BIPMTMS)H}2] with [LaI3(THF)4], which did not proceed smoothly, but utilizing [Cs(BIPMTMSH)] allowed for the clean isolation of [La(BIPMTMSH)I2(THF)2] in good yield.39 With this in mind we targeted [Cs(LBut)] as a potential synthetic precursor to the

coordinated by three THF molecules, but a meaningful assessment of the bonding parameters could not be made due to the low quality of the diffraction data.33 Herein we report an improved preparation of [K(LBut)(THF)n], 1, by the reaction of LButH with [K(Bn)] (Bn = CH2Ph), which reacts smoothly at room temperature with no forcing conditions required (Scheme 1). During routine workup we obtained a Scheme 1. Preparation of 1−2a

crop of yellow crystals of [K(LBut)(THF)3], 1a, suitable for single-crystal X-ray diffraction studies and were able to obtain data of a suitable quality to analyze the molecular geometry of this complex. The structure of 1a is shown in Figure 1 with selected bond lengths and angles in Table 1. The potassium center is coordinated to the β-diketiminate ligand through one nitrogen atom and an η2-interaction to a N-Dipp group, with three THF molecules completing the coordination sphere of the potassium. The coordination mode of potassium to a single nitrogen rather than two is in contrast to the situation in

Figure 1. Molecular structure of 1a with selective atom labeling. Displacement ellipsoids are drawn at 50% probability, and hydrogen atoms and minor disordered components are omitted for clarity. B

dx.doi.org/10.1021/om400435b | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1−10 1 K1−O1 K1−O2 K1−O3 K1−N2 K1···C24 N1−C2−C1 C2−C1−C19 C1−C19−N2 N2−K1−O1 N2−K1−O2 K1−N2−C24 2 Cs1−C12 Cs1−C13 Cs1−C14 Cs1−C15 Cs1−C16 Cs1−C17 Cs1−C19 Cs1−C46 Cs1−C59 Cs1−C64 Cs1−C66 Cs1−C69 Cs1−C80 Cs1−C81 Cs1−C82 Cs1−N4 C36−C42−N4 Cs1−N4−C59 Cs1−C12−N1 Cs2−N3−C37 3·C6D6 U1−I1 U1−I2 U1−I3 U1−N1 U1−N2 U1−C1 U1−C2 U1−C19 N1−U1−I2 N1−U1−I3 I1−U1−I2 I1−U1−I3 4 U1−I1 U1−I2 U1−N1 U1−N2 U1−O1 U1−O2 O1−U1−N1 O1−U1−N2 O2−U1−N1 O2−U1−N2 I2−U1−O1

2.637(3) 2.624(3) 2.623(3) 2.725(3) 3.075(4) 121.8(4) 134.5(4) 121.7(4) 115.45(11) 117.69(11) 90.2(2)

N1−C2 C1−C2 C1−C19 N2−C19 K1···C29 K1···C23 C19−N2−K1 O1−K1−O2 O1−K1−O3 O2−K1−O3 N2−K1−O3

1.321(5) 1.417(5) 1.420(5) 1.340(5) 3.308(4) 3.405(4) 148.9(3) 84.79(11) 96.60(11) 108.83(11) 124.49(11)

3.526(6) 3.482(6) 3.467(6) 3.495(6) 3.478(7) 3.504(6) 3.759(7) 2.909(7) 3.256(6) 3.655(7) 3.949(7) 4.078(7) 3.651(8) 3.516(8) 3.647(8) 3.022(5) 121.9(6) 86.3(3) 112.9(3) 161.1(4)

Cs2−C24 Cs2−C25 Cs2−C26 Cs2−C27 Cs2−C28 Cs2−C29 Cs2−C45 Cs2−C47 Cs2−C48 Cs2−C49 Cs2−C50 Cs2−C51 Cs2−C52 Cs2−C55 Cs2−N2 Cs2−N3 N3−C37−C36 C37−C36−C42 N1−C2−C1 C2−C1−C7 C1−C7−N2

3.268(6) 3.417(6) 3.578(7) 3.668(6) 3.572(6) 3.391(6) 3.739(7) 3.256(6) 3.477(6) 3.672(6) 3.714(6) 3.560(6) 3.356(6) 3.356(7) 3.655(5) 3.607(5) 123.6(6) 137.9(6) 121.7(5) 136.7(6) 123.7(6)

2.9935(6) 2.9550(6) 2.9627(6) 2.282(3) 2.409(3) 2.750(4) 2.991(3) 2.710(4) 122.86(7) 129.03(7) 93.40(2) 84.589(13)

N1−C19 C1−C19 C1−C2 N2−C2 U1−N2−C2 N2−U1−I1 N2−U1−I2 N2−U1−I3 U1−N1−C19 I2−U1−I3 N1−U1−N2 N1−U1−I1

1.365(5) 1.397(5) 1.465(5) 1.301(4) 103.4(2) 173.00(7) 92.74(7) 90.41(7) 92.5(2) 106.864(12) 77.62(10) 101.76(7)

3.0921(5) 3.1210(5) 2.444(5) 2.448(6) 2.569(5) 2.591(5) 95.90(17) 171.20(17) 170.29(17) 94.75(17) 77.46(10)

N2−C2 C1−C2 C1−C4 N1−C4 I1−U1−I2 N1−U1−N2 O1−U1−O2 I1−U1−O1 I1−U1−O2 I1−U1−N1 I1−U1−N2

1.341(9) 1.390(11) 1.381(10) 1.337(9) 153.029(14) 75.56(18) 93.75(15) 83.09(10) 84.71(10) 95.43(12) 95.43(12)

4 I2−U1−O2 I2−U1−N1 C4−N1−U1 5 U1−N1 U1−N2 U1−C30 U1−C31 U1−C32 U1−C33 U1−C34 U1−I2 N2−U1−N1 6 U1−I1 U1−N1 U1−N2 U1−N3 I1−U1−N1 I1−U1−N2 I1−U1−N3 C2−C1−C16 U1−N1−C2 U1−N2−C16 7·0.5C6H14 U1−N1 U1−N2 U1−N3 U1−N4 C2−C3 C16−C17 N1−C2−C3 N1−C2−C1 C3−C2−C1 C2−C1−C16 C1−C16−C17 C1−C16−N2 C17−C16−N2 8·2C7H8 U1−I1 U1−N1 U1−N2 U1−C30 U1−C31 U1−C32 U1−C30a U1−C31a U1−C32a C3−C4 U1−N2−C2 N2−C2−C3 C2−C3−C4 C3−C4−N1 I1−U1−N1 C31−C30−C32a C30−C31−C32 C30a−C31a−C32a

target complex [U(LBut)I2(THF)n] in case 1 did not react with [U(I)3(THF)4] as expected.

78.17(10) 104.93(12) 127.6(4)

I2−U1−N2 C2−N2−U1

106.53(13) 128.0(5)

2.410(2) 2.400(2) 2.770(3) 2.798(3) 2.798(3) 2.761(3) 2.735(3) 3.0612(3) 80.47(7)

N1−C2 C1−C2 C1−C16 N2−C16 N2−U1−I2 N1−U1−I2 U1−N1−C2 U1−N2−C16

1.332(3) 1.411(4) 1.409(4) 1.344(4) 106.82(5) 107.99(5) 111.56(17) 111.22(17)

3.0786(4) 2.355(4) 2.385(4) 2.305(4) 104.31(10) 107.47(10) 121.47(11) 132.2(5) 102.7(3) 102.6(3)

C1−C2 N1−C2 N2−C16 C1−C16 N2−U1−N3 N1−U1−N3 N1−U1−N2 N2−C16−C1 N1−C2−C1

1.402(8) 1.346(7) 1.341(7) 1.418(8) 121.65(15) 113.33(15) 79.85(14) 122.7(5) 123.2(5)

2.275(3) 2.283(3) 2.290(3) 2.274(3) 1.513(5) 1.348(5) 116.8(3) 123.4(3) 119.6(3) 132.8(3) 117.8(3) 116.7(3) 125.4(4)

N2−C16 C1−C16 C1−C2 N1−C2 N1−U1−N2 N1−U1−N3 N1−U1−N4 N2−U1−N3 N2−U1−N4 N3−U1−N4 C2−N1−U1 C16−N2−U1

1.406(5) 1.484(5) 1.356(5) 1.408(5) 92.27(11) 105.79(11) 116.24(11) 113.46(11) 101.48(11) 123.30(11) 97.4(2) 110.4(2)

3.0454(6) 2.368(6) 2.356(6) 2.539(8) 2.550(8) 2.568(8) 2.587(8) 2.586(8) 2.574(8) 1.399(12) 116.0(5) 123.7(7) 130.7(7) 125.2(7) 102.17(15) 120.3(8) 120.3(9) 120.3(9)

C2−N2 C2−C3 C30−C31 C31−C32 C32−C30a C32a−C30 C30a−C31a C31a−C32a N1−C4 U1−N1−C4 U1−C30−C31 U1−C31−C32 U1−C32−C30a N2−U1−N1 I1−U1−N2 C31−C32−C30a C32−C30a−C31a C31a−C32a−C30

1.330(11) 1.437(11) 1.445(15) 1.431(13) 1.464(14) 1.464(14) 1.445(15) 1.431(13) 1.336(11) 115.3(5) 73.9(5) 74.5(5) 72.0(5) 80.7(2) 100.67(15) 119.3(9) 120.3(8) 119.3(9)

The reaction of [Cs(Bn)] with LButH in THF afforded, following workup, [Cs(LBut)], 2, as a yellow solid in 76% yield C

dx.doi.org/10.1021/om400435b | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Figure 2. Section of the polymeric structure of 2 with selective atom labeling and displacement ellipsoids drawn at 50% probability. Hydrogen atoms and lattice solvent are omitted for clarity.

(Scheme 1). Due to the low solubility of 2 in d6-benzene, NMR experiments were performed in d8-THF, with the γ C−H atoms of the NCCCN framework resonating in the 1H and 13C NMR spectra at 3.82 and 86.02 ppm, respectively, which are both very similar to the corresponding resonances of 3.85 and 88.4 ppm reported for [K(LBut)].30 As resonances deriving from the C(CH3)3 and C(CH3)2 groups coincide at 0.95−1.34 ppm in the 1H NMR spectrum and between 20.99 and 28.65 ppm in the 13C{1H} NMR spectrum, it was not possible to unambiguously assign these resonances despite performing heteronuclear multiplequantum correlation (HMQC) experiments. To confirm the identity of 2, a single-crystal X-ray diffraction study was performed on yellow crystals of [{Cs(LBut)}{Cs(LBut)(η3-C6D6)}·C6D6]∞, 2a, grown from a concentrated d6benzene solution. The solid-state structure of 2a is shown in Figure 2 with selected bond lengths and angles in Table 1. Complex 2a is polymeric in the solid state and exhibits two distinct cesium environments with the ligand framework effectively bridging between the two cesium centers. Cs1 is coordinated by two N-Dipp groups from opposing ligands, in an η6-manner to one, with Cs1−C distances ranging from 3.467(6) to 3.526(6) Å and to the second Dipp group in an η2manner, with Cs1−C59 and Cs1−C64 distances of 3.256(6) and 3.655(7) Å, respectively. Agostic-type interactions between Cs and C−H bonds of both a But group and CH(Me)2 groups are also suggested by Cs1−C distances and Cs1−H distances in the ranges 3.759−4.078 Å and 3.236−3.464 Å, respectively. Completing the coordination sphere of Cs1 is a coordinated C6D6 molecule, which is bound to Cs1 in an η3-manner with Cs1−C distances ranging from 3.516(8) to 3.651(8) Å. Cs2 is coordinated by two N-Dipp groups from opposing ligands, each in an η6-manner, with Cs−C bond distances in the range 3.256(6)−3.714(6) Å for one interaction (Cs2−C(47−52)) and 3.268(6)−3.668(6) Å for the second interaction

(Cs2−C(24−29)). Completing the coordination sphere of Cs2 are two agostic-type interactions from three N-Dipp methyl C−H bonds, as exhibited by Cs2−C and Cs2−H distances ranging from 3.356 to 3.739 Ǻ and 3.099 to 3.300(4) Å. The Cs2−N3 and Cs2− N2 distances of 3.607(5) and 3.655(5) Å, respectively, are far outside the sum of the respective covalent radii (3.15 Å),34 suggesting only a weak interaction, if any, is present. The Cs−Caryl distances in 2 are comparable to the equivalent Cs−C distances in [{Cs(BIPMMes)}6] (3.457(6)−3.832(6) Å),40 while the Cs−C and Cs−H agostic-type interaction distances are similar to comparable interactions in [{CsSi(SiMe3)3}2(toluene)3] and [Cs(BIPMR)(S)n] (R = Ad or SiMe3, S = DME n = 2; R = Dipp, S = THF, n = 3)].36,39,40 The coordination of cesium to carbon rather than the nitrogens of the LBut ligand may reflect that Cs+ is a relatively soft metal favoring coordination to carbocyclic π-systems rather than the harder nitrogen atoms and is consistent with the HSAB principle. Preparation of (LBut)-Derived Uranium Complexes. The reactions of 1 or 2 with [U(I)3(THF)4] afforded, following filtration and workup, dark blue solids (Scheme 2). NMR spectroscopic studies performed on these solids were inconclusive, as the 1H NMR spectra exhibited very broad resonances, which precluded any meaningful assessment of the spectra. Crystallization of each from THF afforded crystals of [U(I)3(THF)4] in low yield (