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The Nature of the Heavy Alkaline Earth Metal-Hydrogen Bond: Synthesis, Structure, and Reactivity of a Cationic Strontium Hydride Cluster Debabrata Mukherjee, Thomas Höllerhage, Valeri Leich, Thomas P. Spaniol, Ulli Englert, Laurent Maron, and Jun Okuda J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13796 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018
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The Nature of the Heavy Alkaline Earth Metal–Hydrogen Bond: Synthesis, Structure, and Reactivity of a Cationic Strontium Hydride Cluster Debabrata Mukherjee,a Thomas Höllerhage,a Valeri Leich,a,† Thomas P. Spaniol,a Ulli Englert,*,a Laurent Maron,*,b and Jun Okuda*,a a
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, Aachen 52056, Germany. Université de Toulouse et CNRS, INSA, UPS, CNRS; UMR 5215 LPCNO, 135 avenue de Rangueil, 31077 Toulouse, France. KEYWORDS. Strontium, Hydride, Cationic cluster, H2 activation, Hydride-Hydroxide Co-crystallization, Ketyl. b
ABSTRACT: The molecular strontium hydride [(Me3TACD)3Sr3(3-H)2][SiPh3] (2) was isolated as the dark red benzene solvate 2•C6H6 in 69% yield from the reaction of [Sr(SiPh3)2(thf)3] (1') with (Me3TACD)H (1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane). This reaction can be considered as a redox process, with the Brønsted acidic amine proton in (Me3TACD)H transformed into the hydride by the anion [SiPh3]. Trace amounts of water resulted in the formation of [(Me3TACD)3Sr3(3-H)(3-OH)][SiPh3] (2*), which co-crystallized with 2. Single-crystal X-ray diffraction of 2 revealed a substitutional disorder of a bridging hydride with a hydroxide ligand. Hydride complex 2 was also obtained by hydrogenolysis of [(Me3TACD)Sr(SiPh3)] (3), although pure 3 proved difficult to isolate. In the presence of a twofold excess of (Me3TACD)H, the reaction with disilyl 1' gave [(Me3TACD)SiPh3] (4). Complex 2 underwent facile H/D exchange with D2 (1 bar), with the anion [SiPh3] decomposing concurrently. In the reaction of 2 with 1,1-diphenylethylene (DPE), the anion [SiPh3] was added to the C=C bond in DPE to give [(Me3TACD)3Sr3H2][Ph2CCH2SiPh3] (5), whereas the cationic cluster [(Me3TACD)3Sr3H2]+ remained unchanged. 9-Fluorenone underwent one-electron reduction with 2 to give the paramagnetic ketyl complex [{(Me3TACD)H}Sr(OC13H8•)2(thf)2] (6). These strontium compounds are structurally similar to the lighter calcium congeners, but more reactive, in particular with regard to fast H/D exchange and [SiPh3] anion decomposition. DFT studies on the cationic hydride clusters suggest a more pronounced covalent character for strontium compared to calcium. Disilyl 1, strontium diketyl 6, and the calcium congener of 6, [{(Me3TACD)H}Ca(OC13H8·)2] (10), were also characterized by X-ray diffraction.
Introduction Among the saline hydrides of alkaline earth metals, MgH2 has gained importance as a reversible hydrogen storage material with a relatively high hydrogen content of 7.7 weight-%.1 Currently, both the particle-size thermodynamics2 and releaseand-uptake kinetics3 still pose challenges on the way to practical mobile application.4 Although the gravimetric density decreases going down group 2, the lattice energy of saline [MH2]∞ decreases from Be to Ba5 and a polarization effect between the soft hydride anion and the softer metal appears.6 The recent surge in interest in the chemistry of molecular magnesium7 and calcium5,8 hydrides is contributing to the understanding of the nature of group 2 metal–hydride interactions in general. The large size and high MH bond polarity6e,g play a significant role in the behavior of the hydride anion in molecular systems of the heavier alkaline metals. In addition, molecular hydrides of nontoxic alkaline earths are gaining importance in homogeneous catalysis.8b,9 The catalytic behavior of the heavier congeners could resemble that of transition metals.8g [SrH2]∞ as a highly air- and moisture-sensitive colorless solid has been known for more than 100 years.10 The crystal structure of [SrD2]∞ was established by single-crystal X-ray11 and neutron diffraction.12 Mixed hydrides of strontium with s-, d-, and f-
block metals have also been studied.13 The first material to show H transport is also based on strontium.6,14 Recently, the neutral strontium hydride cluster [(PMDTA)3Sr6H9{N(SiMe3)2}3] (PMDTA = N,N,N',N",N"-pentamethyldiethylenetriamine) containing H–H interactions was reported as well as the dimeric strontium hydride [(LAd)SrH]2 [LAd = RC{N(Dipp)}{N(Ar†)}] (Dipp = 2,6-diisopropylphenyl; Ar† = C6H2{C(H)Ph2}2iPr2,6,4; R = 1-adamantyl].15 The barium hydride dimer [(TpAd,iPr)Ba(-H)]2 [TpAd,iPr = hydrotris(3-adamantyl-5isopropyl-pyrazolyl)borate], stabilized by a bulky scorpionate ligand,16 and the heptanuclear hydrido silazide cluster [Ba7H7{N(SiMe3)2}7] have appeared recently.17 We report here on the synthesis, characterization, and reactivity of a cationic strontium hydride cluster [(Me3TACD)3Sr3(3-H)2][SiPh3]. The ligand Me3TACD [(Me3TACD)H = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane] can stabilize molecular magnesium and calcium hydrides of diverse nuclearity and structural motifs (Chart 1). While magnesium gives rise to a range of compounds varying from mononuclear [{Me3TACD(AliBu3)}MgH]18 to large cationic magnesium hydride clusters [(Me3TACD)6Mg13H18][X]2 (X = [AlEt4], [AlnBu4], [B{3,5-(CF3)2C6H3}4]),19 calcium invariably forms a trinuclear cationic cluster [(Me3TACD)3Ca3H2][X] (X = SiPh3, SiPh3H2,
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[N(SiMe3)(SiPh3)], [(SiHPh2)CHPh]).8c,d Structural characterization of [(Me3TACD)3Sr3(3-H)2][SiPh3] revealed a cocrystal where one species contains a bridging hydroxide instead of a hydrido ligand. This has implications for molecular hydrides in general. Chart 1. Me3TACD-supported molecular magnesium and calcium hydrides.
Figure 1. Molecular structure of [Sr(SiPh3)2(thf)4] (1). Displacement parameters are shown at a 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances [Å]: Sr1Si1 3.2745(7), Sr1O1 2.5171(17), Sr1O2 2.5690(17). Selected bond angles (°): Si1Sr1O1 93.48(5), Si1Sr1O1' 86.52(5), Si1Sr1O2 89.76(4), Si1Sr1O2' 90.24(4).
Results and Discussion Synthesis and Structure Salt metathesis between [SrI2(thf)] and [K(SiPh3)(thf)] in a 1:2 ratio in THF at room temperature provided bis(triphenylsilyl)strontium [Sr(SiPh3)2(thf)4] (1) in 78% yield as a yellow powder (Scheme 1).20 Complex 1 is soluble in THF and the solution is stable for several days at room temperature. It is poorly soluble in benzene and toluene and decomposes readily. Scheme . Synthesis of [Sr(SiPh3)2(thf)4] (1).
The 1H NMR spectrum of 1 in THF-d8 shows the characteristic resonances of SiPh3 and coordinated thf ligands. X-ray diffraction revealed a mononuclear structure (Figure 1) with crystallographic inversion symmetry. The strontium center is octahedrally coordinated by four thf and two silyl ligands with a SrSi bond distance of 3.2745(7) Å. Heavier group 2 metal silyls are rare.20-21 The related strontium compounds [Sr(SiR3)2(THF)3] and [Sr(SiR3)2(TMEDA)(thf)] (R = SiMe3; TMEDA = N,N,N',N'-tetramethylethylenediamine) show similar SrSi distances of 3.167(1)–3.224(1) Å and 3.195(1)– 3.251(1) Å, respectively.21a
Prolonged drying of 1 removed one THF molecule, leading to the composition [Sr(SiPh3)2(thf)3] (1'), as shown by the 1H NMR spectrum in THF-d8. The 29Si{1H} resonance of 1 and of 1' in THF-d8 appears at –9.76 ppm. In subsequent reactions, solid 1' was used, because this compound has a consistent composition. Heating a 1:1 mixture of 1' and (Me3TACD)H in benzene at 60 °C for 24 h in a Teflon-sealed glass ampule gave dark red crystals of [(Me3TACD)3Sr3H2][SiPh3]·C6H6 (2•C6H6) in 69% yield (Scheme 2). Fast reaction of 1' with (Me3TACD)H prevented 1' from decomposing in benzene. Magnetic stirring was avoided to obtain large-sized crystals for easier separation. The co-crystallized benzene molecule could not be removed in vacuo. The stoichiometry can be rationalized by assuming three steps (Scheme 2). Three equiv of (Me3TACD)H and 1' initially reacted to form three equiv each of [(Me3TACD)Sr(SiPh3)] (3) and Ph3SiH (step 1). Two equiv of those reacted further to give two equiv each of hexaphenyldisilane (Ph3SiSiPh3) and a putative strontium hydride “[(Me3TACD)SrH]” (step 2), which was captured by the third equiv of 3 to give 2 (step 3). The byproducts Ph3SiH and Ph3SiSiPh3 were identified by NMR spectroscopy. The overall reaction is a redox process, since the Brønsted acidic amine proton in (Me3TACD)H acted as an indirect source for the hydrides. The silicon atom in the [SiPh3] anion has a formal oxidation state of +2. Five out of six [SiPh3]anions from three equiv of 1' give a total of six electrons that reduce three NH protons in (Me3TACD)H to three hydrides. Two of those hydrides end up in the cation [(Me3TACD)3Sr3H2]+, while the third one is in the byproduct Ph3SiH (formal oxidation state of silicon +4). The residual four oxidized silicon atoms end up in two equiv of Ph3SiSiPh3 (formal oxidation state of silicon +3). Scheme 2. One-pot synthesis of 2·C6H6 from (Me3TACD)H and 1'.
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Compound 2•C6H6 is insoluble in aliphatic and aromatic hydrocarbons; in THF solution, it is stable at room temperature, decomposing after 16 h at 60 °C. The 1H NMR spectrum in THF-d8 agrees with a ratio Me3TACD/H/SiPh3/C6H6 of 3:2:1:1. The chemically equivalent hydrides appear as a sharp singlet at 5.20 ppm. Two characteristic NMe singlets of Me3TACD appear at 2.53 and 2.18 ppm, while the methylene protons exhibit multiple broad signals in the range 2.87–1.73 ppm. The [SiPh3] anion shows three multiplets in the range 7.35– 6.66 ppm for the ortho-, meta-, and para-Ph protons in a 2:2:1 ratio. The 29Si{1H} resonance of the [SiPh3] anion was recorded at 8.1 ppm.8d Recrystallization of 2•C6H6 from THF at 30 °C replaced benzene with a THF molecule to give the composition as 2•THF. In comparison, the neutral hydride cluster [(PMDTA)3Sr6H9{N(SiMe3)2}3] shows three SrH signals in the region 4.48–5.85 ppm in a 1:4:4 ratio15a and a broad singlet at 4.91 ppm is detected for [(LAd)SrH]2.15b X-ray diffraction of 2•C6H6 revealed an ion pair with a C3 symmetric cation [(Me3TACD)3Sr3H2]+ and a weakly coordinating anion [SiPh3] (Figure 2). One equivalent of benzene is co-crystallized within the lattice. Both hydrides (H1 and H2) could be located by a Fourier difference map and are refined in their positions. The [Sr3H2]4+ core forms a trigonal bipyramid with the hydrides equally shared by the three Sr atoms. One of the three Me3TACD ligands is disordered, but this could be modeled with split positions in the refinement. The strontium atoms are bridged by the Me3TACDamido nitrogen atoms, resulting in an almost coplanar Sr3N3 ring. The hydrides H1 and H2 are located 1.37(4) Å and 1.49(4) Å away from that plane, respectively, and are 2.85(4) Å apart from each other. The SrN distances vary between 2.568(2) and 2.743(4) Å. If the Sr∙∙∙Sr distances of 3.5689(6)-3.5846(6) Å are taken into account, each Sr center acquires a formal coordination number of nine. The SrH bonds are in the range 2.43(4)–2.60(4) Å. In [(PMDTA)3Sr6H9{N(SiMe3)2}3], the Sr∙∙∙Sr, SrH, and H∙∙∙H distances are in the ranges 3.7520(7)–3.8777(8) Å, 2.38(4)– 2.78(8) Å, and 2.57(9)–3.14(6) Å, respectively.15a The SrH bonds in [(LAd)SrH]2 are 2.38(3) and 2.43(3) Å long.15b
Figure 2. Structure of 2•C6H6. Displacement parameters are shown at 50% probability level. The co-crystallized benzene molecule and all hydrogens except those of the Sr3H2 fragment are omitted for clarity. One of the three Me3TACD ligands is disordered; only the majority conformer is shown. Selected interatomic distances (Å): Sr1N1 2.614(3); N1Sr2 2.568(3); Sr2N5 2.615(3); N5Sr3 2.610(3); Sr3N9 2.612(3); N9Sr1 2.618(3); Sr1∙∙∙Sr2 3.5689(6) Å; Sr2∙∙∙Sr3 3.5846(6); Sr3∙∙∙Sr1 3.5790(6); Sr1H1 2.54 (4); Sr2H1 2.43(4); Sr3H1 2.47(4); Sr1H2 2.60(4); Sr2H2 2.54(4); Sr3H2 2.50(4); Selected bond angles (°): Sr1Sr2Sr3 60.042(12); Sr2Sr3Sr1 59.761(11); Sr2Sr1Sr3 60.197(10).
Diffraction experiments on several batches showed that 2 cocrystallizes with the isostructural compound [(Me3TACD)3Sr3(3-H)(3-OH)][SiPh3] (2*), where a hydrido ligand is replaced by a hydroxide. Figure 3 (left) visualizes the alternative bridging moieties above the Sr3N3 ring. A spacefilling model on the right-hand side of Figure 3 shows that the arrangement of the three NNNN macrocycles Me3TACD leaves enough space for a hydroxide ligand to coordinate as the upper vertex of the central bipyramid. Thus 2 and 2* share the same steric requirements and shape but differ significantly with respect to electron density: the partially occupied site associated with the hydroxide O atom in 2* can be assigned unambiguously in a difference Fourier synthesis. Electron density maps comprising the hydride and hydroxide vertices in the co-crystal and additional details concerning the refinement of 2* are given in the supporting information.
Figure 3. Structure of the molecular cation in the hydride– hydroxide co-crystal 2*. Left: alternate hydride–hydroxide bridging within the central core. Right: Space-filling model; the position of the hydroxide “O2” (red) is marked.
Alternative occupancy of a hydride site by oxide or hydroxide is not unprecedented. In the realm of inorganic solids, Kobayashi et al. found hydride/oxide disorder or wellordered separate sites for both anions in the oxohydride La2LiHO3, depending on the crystal phase.14 As for
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coordination compounds, single-crystal X-ray diffraction allowed the detection of substitutional hydride versus hydroxide disorder in a mononuclear magnesium -diketiminate complex.22 Jones et al. have identified post-synthetic hydrolysis in their crystals as the hydroxide source, whereas in our system the formation of 2* instead of 2 can be associated with trace moisture and is thus suppressed by thorough silylation of the surface of the glass equipment. The observation in this work demonstrates that structural investigations of hydrides by X-ray diffraction have to be performed with great care. In view of the very different electron density of the coordinating atoms H or O, even a minor amount of a cocrystallized hydroxide species can lead to misinterpretation of the diffraction pattern by de facto characterizing the byproduct as a molecular hydride. Isolation of the heteroleptic compound 3 proved difficult. High reactivity toward the byproduct Ph3SiH required rapid separation that was not possible due to the high solubility of Ph3SiH in aromatic solvents. Reaction conditions employed so far gave erratic and inconsistent results. Reasonably pure 3 was isolated on a single occasion from toluene in 14% yield as a light yellow solid (Scheme 3). Scheme . Synthesis of [(Me3TACD)Sr(SiPh3)] (3) and [(Me3TACD)SiPh3] (4) from (Me3TACD)H and 1'.
Strontium complex 3 is highly soluble in THF and was characterized by 1H and 13C{1H} NMR spectroscopy. Exceptionally broad resonances for the ligand Me3TACD suggest dynamic behavior or fast Schlenk-type ligand redistribution. The two NMe singlets appear at 2.33 and 2.04 ppm, while the methylene resonances are in the range 3.15– 2.07 ppm. Sharp phenyl resonances appear in the normal range. The 29Si{1H} NMR signal at 7.7 ppm is close to that of the free anion [SiPh3] in 2 ( 8.1 ppm). Strontium disilyl 1' reacted with a twofold excess of (Me3TACD)H in THF within 1 h at room temperature to give [(Me3TACD)SiPh3] (4) instead of the homoleptic [Sr(Me3TACD)2] (Scheme 3). A small amount (~15%) of 2 and some greyish precipitate, presumably [SrH2]∞, were also formed. An in situ NMR spectroscopic analysis in THF-d8 suggested the initial formation of [Sr(Me3TACD)2] and Ph3SiH in a 1:2 ratio; these reacted further to afford 4 by SrNamido/HSi bond metathesis. The homoleptic [Sr(Me3TACD)2] could not be isolated. SiN bond formation mediated by group 2 metals including strontium is common.9g,23 This pathway is apparently inactive during the synthesis of 2 (Scheme 2).
Reactivity Strontium complex 3 reacted with H2 (1 bar) in THF-d8 to give 2 and Ph3SiH within 2 h at room temperature (Scheme 4). Strontium complex 2-d2 prepared in situ from 3 and D2 (1 bar) in THF (Scheme 4) showed an SrD resonance at 5.23 ppm in
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the 2H{1H} NMR spectrum. The synthesis of hydrides by hydrogenolysis of MC or MN bonds is well established for group 3 and rare earth metals,24 but much less common for the more electropositive early main group metals.8b,d,e,g,25 The increasingly polar MSi bonds for the heavier congeners reflect similarities with “frustrated” Lewis pairs (FLP).26 Cooperative donation from(H2) to the Lewis acidic (Sr)+ in tandem with the Lewis basic interaction from (Si) to *(H2) could promote H2 activation.27 This bond polarization hypothesis agrees with the low reactivity of [(Me3TACD)Mg(SiPh3)] toward H2.28 Scheme . Formation of 2/2-d2 from 3 and H2/D2.
Leaving the hydrogenolysis reaction mixture overnight promoted the anion decomposition in 2, which also occurred when isolated 2•THF was treated with H2 (1 bar) in THF-d8. The in situ NMR spectroscopic analysis showed partial degradation of the [SiPh3]anion within 12 h, while resonances of the hypervalent [SiH2Ph3] (SiH 5.96 ppm),29 Ph3SiH (SiH 5.44 ppm), and a trace amount of [SiHPh2] (SiH 4.49 ppm)8d were all evident. Benzene ( 7.30 ppm) also formed. After a week, signals of [SiPh3], [SiH2Ph3], [SiHPh2], and SiHPh3 disappeared. Several new, low-intensity aromatic signals appeared in the range 6.56–8.13 ppm and more benzene formed. During this time, the [(Me3TACD)3Sr3H2]+ cation appeared to be unaffected. Treating 2•THF with D2 (1 bar) in THF-d8 provided more insights. Apart from anion degradation and benzene formation, the SrH signal at 5.21 ppm decreased with time, with concomitant formation of H2 ( 4.54 ppm) and HD ( 4.51 ppm, 1 JHD = 42.7 Hz), while the resonances of Me3TACD remained unchanged. The SrH signal nearly disappeared after one week. On the basis of these observations with D2, the following mechanism is proposed (Scheme 5). The cationic hydride 2 initially reacts with D2 to give a neutral strontium hydride [(Me3TACD)3Sr3H2D] (A) and Ph3SiD. Since A is probably unstable (as suggested by DFT calculations, see Supporting Information), it rearranges immediately by H/D migration to form the cationic hydride cluster [(Me3TACD)3Sr3H2xDx][SiHxD2xPh3] (x = 0, 1) (B). The hypervalent silicate anion in B is presumably unstable and reductively eliminates either benzene PhHxD1x to give [(Me3TACD)3Sr3H2xDx][SiHxD1xPh2] (x = 0, 1) (C) or dihydrogen HxD2-x (x = 0, 1), leading to [(Me3TACD)3Sr3H2xDx][SiPh3] (D). The scrambling continues to eventually give the cation [(Me3TACD)3Sr3D2]+ and a mixture of silyl anions along with more benzene. The cluster cation plays a vital role as mediator by accepting and releasing hydrides/deuterides. Degradation of silicon-containing anions is well-documented.30 [(Me6TREN)K(SiPh3)] (Me6TREN = tris{2-(dimethylamino)ethyl}amine) reacted with H2 (1 bar) at room temperature by stepwise hydrogenolytic cleavage of the SiPh bonds to finally give -KSiH3 under elimination of benzene and free Me6TREN.29b Scheme . Proposed mechanism for anion degradation of 2 in the presence of H2/D2.
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Journal of the American Chemical Society involving an NMe against an NH unit in the (Me3TACD)H ligand. The attachment of two THF molecules increases the coordination at the strontium center to eight, leading to a distorted square-antiprismatic geometry. The SrN bond lengths of 2.879(2) Å are slightly longer than those of 2•C6H6. The SrOketyl and SrOTHF bond lengths are 2.4428(17) and 2.658(2) Å, respectively. The ketyl fragments are planar (the sum of the bond angles around C7 = 360°) with nonlinear coordination to the metal center (bond angle O1SrO1' = 124.50(9)°), most likely for steric reasons. The bond length within the ketyl unit COketyl of 1.285(3) Å reflects a bond order between one and two. To the best of our knowledge, 6 is the first example of a crystallographically authenticated strontium ketyl derivative. The exact reaction stoichiometry remains obscure; 6 was the only isolated product. A fivefold excess of 9-fluorenone gave 6 in 42% yield based on the hydride 2•C6H6 (Scheme 6). Complex 6 is insoluble in common organic solvents and was not further characterized. Attempted transfer hydrogenation from 1,3-cyclohexadiene failed, likely due to this insolubility.
The anion [SiPh3] in 2 reacted with 1,1-diphenylethylene (DPE) in THF-d8 at room temperature to give [(Me3TACD)3Sr3H2][Ph2CCH2SiPh3] (5) as a dark red solid. (Scheme 6). The hydrides remained untouched even in the presence of excess DPE, indicating the kinetic stability of [(Me3TACD)3Sr3H2]+. Scheme . Reactivity of 2 toward DPE and 9-fluorenone.
Figure 4. Molecular structure of 6. Displacement parameters are shown at the 50% probability level. Hydrogen atoms except for NH in (Me3TACD)H are omitted for clarity. Selected bond distances (Å): SrN1 2.829(2); SrN2 2.829(2); SrOketyl 2.4428(17); SrOthf 2.658(2); O1–C7 1.285(3). Selected bond angles (°): O1Sr1O1' 124.50(9); O2Sr1O2' 122.79(12); Sr1O1C7 160.40(18); O1C7C8 126.7(2); C8C7C19 105.9(2); O1C7C19 127.4(2).
Interestingly, 2 reacted with the more electrophilic 9-fluorenone in THF with gas evolution and an instant color change from yellow to black. Black microcrystals were deposited within a few hours. In situ NMR spectroscopic analysis in THF-d8 suggested the formation of some NMR-silent species, since Ph3SiH and H2 were the only recognizable byproducts. SQUID measurement has confirmed the paramagnetism of complex 6 with χmT = 0.66 cm3 K mol–1 at 290 K. This value is slightly below the value 0.75 cm3 K mol–1 expected for two non-interacting, free electrons and distinctly larger than for a single electron. X-ray diffraction identified the composition of the black crystals as [{(Me3TACD)H}Sr(OC13H8·)2(THF)2] (6), with crystallographic C2 symmetry, a neutral (Me3TACD)H ligand, and two fluorenone ketyl radical anions (Figure 4). The crystallographically imposed C2 symmetry leads to disorder
Group 2 metal ketyls are synthetically valuable,31 but because they are highly reactive, they are difficult to isolate; therefore, crystallographically characterized examples are limited.32 [(Dippnacnac)Mg(OCPh2•)(DMAP)] (Dippnacnac = [{(2,6i Pr2C6H3)NCMe}2CH]; DMAP = 4-dimethylaminopyridine) was obtained by reacting the low-valent magnesium complex [{(Dippnacnac)Mg}2] with benzophenone in a 1:2 ratio.32a [Ca(OCPh2•)2(HMPA)3], [Ca(OC13H8•)2(HMPA)2(thf)2], and [Ca(OC13H8•)2(HMPA)3(thf)] (HMPA = hexamethylphosphoramide) were prepared by directly oxidizing the calcium metal with 2 equiv of the corresponding ketone.32b,c The mechanism for the formation of 6 remains obscure. Single-electron transfer from SrH bonds can be the initiation step, as in the reduction of aromatic ketones by [CpMgH(THF)]233 and Ph3SnH.34 The highly electropositive Sr2+ retains its oxidation state. A radical process may also be initiated by the anion [SiPh 3].
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Discussion and DFT Analysis The strontium and homologous calcium complexes show similarities as well as differences. Bis(triphenylsilyl)calcium [Ca(SiPh3)2(thf)4] (7) is isostructural to 1, but less reactive.8d Unlike 1, the calcium complex 7 dissolves in benzene without decomposing.8d Likewise, the heteroleptic [(Me3TACD)Ca(SiPh3)] (8) is less reactive toward Ph3SiH than 3 is.8d Moreover, 8 is insoluble in benzene and was conveniently isolated in quantitative yield by filtration.8d In contrast to 3, the 1 H and 13C{1H} NMR signals of the calcium compound 8 are sharp.8d In previous work, reaction of 8 with dihydrogen provided the homologous cationic calcium hydride cluster [(Me3TACD)3Ca3H2][SiPh3]·2THF (9•2THF) in quantitative yield.8d Reaction of 7 with (Me3TACD)H gave the calcium hydride 9•C6H6 as a proof of the generality of this direct route. Compounds 2 and 9 are isostructural.8d The MN, M∙∙∙M, and MH distances are only slightly shorter for M = Ca.8d In the 1H NMR spectrum, the CaH resonance ( 4.00 ppm) in 9 appears at higher field than the SrH resonance in 2 ( 5.21 ppm).8d Anion degradation by H2/D2 was slower in 9 than in 2, only occurring at 60 °C.8d The cationic dicalcium trihydride complex [(Me4TACD)2Ca2H3][SiPh3] (Me4TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) underwent H/D exchange in the presence of D2 (1 bar), but without degradation of the [SiPh3] anion.8e It apparently involves a different mechanism without a hypervalent silicate anion.8e Black single crystals of [{(Me3TACD)H}Ca(OC13H8·)2] (10) were similarly isolated from [(Me3TACD)3Ca3H2][H2SiPh3] and 9-fluorenone, indicating that a similar one-electron reduction process prevails regardless of the anion. Calcium ketyl 10 does not contain a coordinated THF molecule, likely due to the smaller size of calcium (see SI). The geometry of the [(Me3TACD)3Sr3H2]+ cation was optimized at the DFT level by using the B3PW91-D3BJ functional. The optimized geometry is in excellent agreement with the experimental one. The Sr∙∙∙Sr distances are well reproduced (3.55 Å vs. 3.58 Å experimentally), as are the bridging SrN distances (2.56/2.59 Å vs. 2.56/2.61 Å). In the same way, the 3 coordination of the two hydrides was also found computationally and the average SrH distance is reproduced (2.50 vs. 2.54 Å). Scrutinizing the molecular orbitals revealed a strongly bonding orbital that describes the SrH interaction of the hydrogen with the three strontium centers (Figure 5). This bond is strongly polarized toward the hydrogen (87%) and involves a hybrid s(11.5%)-p(34%)d(54.5%) orbital at the strontium centers. Results at the NBO level of theory consistently reveal SrH bonds delocalized over the three strontium atoms. The Wiberg bond index (WBI) 0.27 for SrH can be compared with 0.20 for SrN(bridging). A WBI of 0.24 for the Sr∙∙∙Sr interaction is in line with the contribution from the three strontium centers (Figure 5). For comparison, the calcium congener was optimized at the same level of theory. The results are close to those found for strontium. The WBI for CaH is 0.26, whereas that for Ca∙∙∙Ca is only 0.19. In spite of a similar three-center bonding orbital HOMO-1, the calcium atoms contribute less. This indicates a slightly higher covalent character of the Sr∙∙∙H bonds in [(Me3TACD)3Sr3H2]+ than of the Ca∙∙∙H interactions in [(Me3TACD)3Ca3H2]+. Finally, the putative neutral trinuclear hydride cluster [(Me3TACD)3Sr3H3] (A in Scheme 5) was considered and its geometry optimized. A stable structure was located on the Potential Energy Surface, but the H coordination
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to [(Me3TACD)3Sr3H2]+ is endothermic by 56.8 kcal/mol. This is attributed to the lower stability of top coordination of the hydride with respect to the μ3–bridging ones found in [(Me3TACD)3Sr3H2]+.
Figure 5. 3D representation of the SrH bonding orbital.
Conclusion Calcium, strontium, and barium (rCN6 = 1.00 Å, 1.18 Å, 1.35 Å, respectively;Pauling = 1.00, 0.95, 0.89, respectively)35 and their compounds are usually considered ionic without showing major differences in reactivity. Yet small variations can be significant.23c,36 Despite close structural similarities, there are striking differences between strontium and calcium with regard to their silyl and hydride chemistry.23 The macrocyclic Me3TACD ligand provides a stable [M3H2]4+ core, inert toward DPE, but reactive toward the more electrophilic 9-fluorenone to give ketyl derivatives instead of alkoxides. H2 activation and H/D scrambling with the strontium hydrides are promising for catalysis with molecular dihydrogen. DFT analysis shows high contribution of d orbitals to the MH (M = Ca, Sr) bonds and higher covalency for the strontium cluster than for the calcium congener. Co-crystallization of the cation [(Me3TACD)3Sr3H2] with the hydroxide contaminant in [(Me3TACD)3Sr3(3-H)(3-OH)] demonstrated how easily a hydroxo unit may be mistaken for a hydride, even if only trace amounts are present. Because many hydrides of electropositive metals easily undergo hydrolysis to hydroxides, structural reports on molecular hydrides should be considered with great care.
EXPERIMENTAL SECTION General Remarks. All reactions were performed under a dry argon atmosphere using standard Schlenk techniques or under an argon atmosphere in a glovebox. Prior to use, glassware were dried overnight at 130 °C and solvents were dried, distilled, and degassed using standard methods. (Me3TACD)H37 and [K(SiPh3)(THF)]38 were prepared following literature procedures. Anhydrous SrI2, purchased from abcr, was recrystallized from THF to obtain [SrI2(THF)]. 1H, 13C{1H}, 29Si{1H}, 29Si-1H HSQC, and HMBC NMR spectra were recorded on a Bruker Avance-III spectrometer at ambient temperature. Chemical shifts (δ ppm) in the 1H and 13C{1H} NMR spectra were referenced to the residual signals of the deuterated solvents. IR spectra were recorded of samples prepared as KBr pellets on an AVATAR 360 FT-IR spectrometer. Elemental analyses were performed on an elementar vario EL machine. X-ray diffraction data for 1, 2•C6H6, 6, and 10 were collected on a Bruker APEX II diffractometer and are reported in a crystallographic information file (cif) accompanying this document.
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[Sr(SiPh3)2(THF)4] (1). A solution of [K(SiPh3)(THF)] (0.371 g, 1.001 mmol) in THF (5 mL) was added dropwise to a solution of [SrI2(thf)] (0.207 g, 0.501 mmol) in THF (5 mL). The dark yellow reaction mixture was stirred for 2 h at room temperature. A colorless precipitate was filtered off and the filtrate was concentrated to 2 mL under reduced pressure. A yellow solid was precipitated upon addition of n-pentane (15 mL). Washing the solid with n-pentane (3 × 5 mL), followed by drying under vacuum gave [Sr(SiPh3)2(THF)4] (1, 0.350 mg, 0.391 mmol) as a yellow powder in 78% yield. Crystals of 1 suitable for X-ray diffraction were grown from THF/n-pentane at 30 °C within 12 h. 1H NMR (400 MHz, THF-d8): δ 1.78 (m, 16H, THF), 3.63 (m, 16H, THF), 6.936.97 (m, 6H, para-Ph), 7.01-7.05 (m, 12H, meta-Ph), 7.36-7.38 (m, 12H, ortho-Ph). 13C{1H} NMR (100 MHz, THF-d8): δ 26.4 (THF), 68.3 (THF), 125.4 (para-Ph), 127.3 (meta-Ph), 137.0 (ortho-Ph), 154.2 (ipso-Ph). 29Si{1H} NMR (79.5 MHz, THF-d8): δ –9.8 (SiPh3). Drying 1 in vacuo for 4 h afforded [Sr(SiPh3)2(THF)3] (1') as shown by its 1H NMR spectrum. Anal. calc. for C48H54O3Si2Sr (1'): C, 70.07; H, 6.62; Found: C, 70.52; H, 7.40 %. [(Me3TACD)3Sr3H2][SiPh3]•C6H6 (2•C6H6). A solution of (Me3TACD)H (0.104 g, 0.485 mmol) in benzene (5 mL) was quickly added to solid 1' (0.400 g, 0.486 mmol). The resulting light orange solution was heated at 60 °C for 12 h in a Teflon-sealed glass ampule without magnetic stirring. During this time, red crystals precipitated from the reaction mixture. The crystals were collected after removal of the supernatant solution and were washed with benzene (3 × 5 mL) and n-pentane (3 × 5 mL). Drying in vacuum gave [(Me3TACD)3Sr3H2][SiPh3]•C6H6 (2•C6H6, 0.139 mg, 0.112 mmol) as red crystals in 69% yield. The same crystals were suitable for X-ray diffraction. NMR spectra and elemental analysis data were collected using a sample of 2•THF. 1H NMR (400.1 MHz, THF-d8): δ 7.37-7.33 (m, 6H, ortho-Ph), 6.86-6.80 (m, 6H, meta-Ph), 6.71-6.66 (m, 3H, para-Ph), 5.21 (s, 2H, SrH), 3.66-3.59 (m, 4H, THF), 3.22-3.07 (br, 8H, CH2), 2.94-2.81 (br, 12H, CH2), 2.77-2.65 (br, 12H, CH2), 2.53 (s, 18H, CH3), 2.39-2.23 (m, 16H, CH2), 2.19 (s, 9H, CH3), 1.82-1.74 (m, 4H, THF). 13C{1H}NMR (100.6 MHz, THF-d ): δ 162.2 (ipso-Ph), 137.3 8 (ortho-Ph), 126.2 (meta-Ph), 122.6 (para-Ph), 68.3 (THF), 59.2 (CH2), 54.7 (CH2), 54.0 (CH2), 51.8 (CH2), 44.6 (CH3), 42.3 (CH3), 26.4 (THF). 29Si{1H} NMR (80 MHz, THF-d8): δ 8.1 (SiPh3). Anal. calc. for C55H100N12OSiSr3 (2•THF): C, 53.43; H, 8.15; N, 13.59; Found: C, 52.83; H, 7.94; N, 13.83 %. [(Me3TACD)Sr(SiPh3)] (3). A solution of (Me3TACD)H (0.180 g, 0.840 mmol) in toluene (5 mL) was added dropwise using a metal cannula to a suspension of 1' (0.690 g, 0.839 mmol) in toluene (5 mL) at 78 °C while stirring with a glass stir bar. Warming the reaction mixture to room temperature gave a yellow solution with a small amount of light yellow solid. The solid was collected by filtration using a glass frit and was quickly washed with cold toluene (5 mL) and n-pentane (3 × 5 mL). Drying the solid under vacuum afforded [(Me3TACD)Sr(SiPh3)] (3, 0.065 g, 0.116 mmol) as a light yellow powder in 14% yield. 1H NMR (400 MHz, THFd8): δ 7.41-7.38 (m, 6H, ortho-Ph), 7.03-6.99 (m, 6H, meta-Ph), 6.93-6.88 (m, 3H, para-Ph), 3.15-3.04 (br, m, 4H, CH2), 2.77-2.71 (br, m, 4H, CH2), 2.43-2.25 (br, m, 4H, CH2), 2.33 (br, s, 6H, CH3) 2.16-2.07 (br, 4H, CH2), 2.04 (br, s, 3H, CH3). 13C{1H} NMR (100 MHz, THF-d8): δ 156.1 (ipso-Ph), 137.3 (ortho-Ph), 127.2 (metaPh), 125.0 (para-Ph), 63.8 (CH2), 58.2 (CH2), 56.7 (CH2), 52.7 (CH2), 47.5 (CH3), 44.9 (CH3). 29Si{1H} NMR (80 MHz, THF-d8): δ 7.7 (SiPh3).
[(Me3TACD)3Ca3H2][SiPh3]•C6H6. A solution of Me3TACD (0.086 g, 0.401 mmol) in benzene (5 mL) was added to solid [Ca(SiPh3)2(thf)4] (0.338 g, 0.399 mmol). The resulting yellow suspension was heated at 60 °C for 12 h in a Teflon-sealed glass ampule without stirring. Red crystals precipitated during this time. The crystals were collected and washed with benzene (3 × 5 mL) and n-pentane (3 × 5 mL). Drying in vacuum gave
[(Me3TACD)3Ca3H2][SiPh3]•C6H6 (0.37 g, 0.034 mmol) as red crystals in 26% yield. The NMR spectroscopic data recorded in THF-d8 were in agreement with those reported in the literature.8d [(Me3TACD)SiPh3] (4). THF solutions (3 mL each) of (Me3TACD)H (0.104 g, 0.485 mmol) and 1' (0.200 g, 0.243 mmol), cooled to −30 °C, were mixed together. After stirring for 1 h, the reaction mixture was evaporated to dryness. The residue was extracted with n-pentane. Cooling the n-pentane solution at 30 °C for 48 h gave colorless needles, which were collected and dried under vacuum to give [(Me3TACD)SiPh3] (4, 0.035 g, 0.077 mmol) in 16% yield. Since the NMR-scale reaction suggested a nearly quantitative yield of 4, low isolated yields are attributed to its high solubility in n-pentane. 1H NMR (400 MHz, THF-d8): δ 7.67-7.62 (m, 6H, ortho-Ph), 7.39-7.30 (m, 9H, meta- and para-Ph), 3.11 (t, 3J 3 HH = 6.52 Hz, 4H, CH2), 2.59 (t, JHH = 6.52 Hz, 4H, CH2), 2.512.46 (m, 4H. CH2), 2.39- 2.35 (m, 4H, CH2), 2.19 (s, 3H, CH3), 1.98 (s, 6H, CH3). 29Si {1H} NMR (80 MHz, THF-d8): δ 12.8 (SiPh3). [(Me3TACD)3Sr3H2][Ph2CCH2SiPh3] (5). A solution of 1,1diphenylethylene (0.007 g, 0.039 mmol) in THF (1mL) was added dropwise to a solution of 2•C6H6 (0.048 g, 0.039 mmol) in THF (2 mL). After 5 min, the volume was reduced to 1 mL and the solution layered with n-pentane (3 mL) and stored at −30 °C. Within 48 h, dark red crystals precipitated from the reaction mixture. The crystals were isolated, washed with n-pentane (3 × 2 mL), and dried under vacuum to give [(Me3TACD)3Sr3H2][Ph2CCH2SiPh3] (5, 0.051 mg, 0.038 mmol) as a dark red solid in 97% yield. 1H NMR (400 MHz, THF-d8): δ 7.58-7.53 (m, 6H, ortho-SiPh3), 7.16-7.08 (m, 9H, meta- and para-SiPh3), 6.81 (d, 3JHH = 8.78 Hz, 4H, orthoPh1,1-DPE), 6.27 (dd, 3JHH = 6.78 Hz, 3JHH = 8.78 Hz, 4H, meta-Ph1,13 DPE), 5.43 (t, JHH = 6.78 Hz, 2H, para-Ph1,1-DPE), 5.21 (s, 2H, SrH), 3.22-3.07 (br, 6H, CH2), 2.94-2.81 (br, 12H, CH2), 2.77-2.65 (br, 12H, CH2), 2.72 (s, 2H, Ph2CCH2SiPh3), 2.53 (s, 18H, CH3), 2.392.23 (m, 18H, CH2), 2.19 (s, 9H, CH3). 13C{1H} NMR (100.6 MHz, THF-d8): δ 147.1 (Ph), 140.6 (Ph), 137.5 (Ph), 128.5 (Ph), 127.8 (Ph), 127.6 (Ph), 118.6 (Ph), 106.9 (Ph), 59.3 (CH2), 54.8 (CH2), 54.1 (CH2), 51.9 (CH2), 44.7 (CH3), 42.4 (CH3), 21.4 (Ph2CCH2SiPh3). 29Si{1H} NMR (80 MHz, THF-d8): δ 18.1 (SiPh3). Anal. calc. for C61H104N12SiSr3 (5): C, 58.06; H, 7.80; N, 12.50; Found: C, 57.76; H, 7.59; N, 12.61 %. [{(Me3TACD)H}Sr(OC13H8•)2(thf)2] (6). A solution of 2•C6H6 (0.040 g, 0.032 mmol) in THF (1 mL) was layered with a solution of 9-fluorenone (0.030 g, 0.166 mmol) in THF (1.5 mL). A color change from orange to black accompanied by gas evolution was observed. After 20 min, the mixture was layered with n-pentane (0.2 mL). Black crystals precipitated within 12 h at room temperature. The crystals were isolated, washed with n-pentane (3 × 1 mL), and dried under vacuum to give [{(Me3TACD)H}Sr(OC13H8•)2(thf)2] (6) (0.035 g, 0.043 mmol) as black crystals in 45% yield. These crystals were suitable for X-ray diffraction. Insolubility of 6 in common organic solvents prevented spectroscopic characterizations. Anal. calc. for C45H58N4O4Sr: C, 67.01; H, 7.25; N, 6.95. Found: C, 65.62; H, 7.61; N, 6.50 %. Reactions of 2 with D2 and H2. In J. Young-style NMR tubes, solutions of 2•THF (0.012 g, 0.010 mmol) in THF-d8 (0.4 mL) were degassed by three standard freeze–pump–thaw cycles and charged with H2 or D2 (1 bar). The reactions were monitored by 1H and 29Si NMR spectroscopy. Computational details. Geometry optimizations were performed with the Gaussian09 suite of programs (revision D.02)39 using the Becke’s 3-parameter hybrid functional,40 combined with the nonlocal correlation functional provided by Perdew and Wang.41 The 6-311+G(d) all-electron basis set was used for the strontium atom, and the 6-31G(d) set was used for the remaining atoms.42 We have also considered dispersion effects in the present study, in particular the third generation of Grimme’s dispersion corrections with the Becke-Johnson damping model43 on the B3PW91 geometries
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(single point calculations). All stationary points have been identified for minimum (Nimag = 0). Natural population analysis (NPA) was performed using Weinhold’s methodology.44 X-ray crystallography. X-ray diffraction data of 1, 2, 2*, 6, and 10 were collected at –173 °C on a Bruker D8 goniometer with an APEX CCD area-detector in ω-scan mode. Mo-K radiation (multilayer optics, = 0.71073 Å) from an Incoatec microsource was used. The SMART program package was used for the data collection and unit cell determination; processing of the raw frame data was performed using SAINT;45 absorption corrections were applied with SADABS.46 The structures were solved by direct methods using SIR-92.47 Additional details concerning the diffraction studies are provided in the Supporting Information.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Spectroscopic characterization of compounds 1-5, reaction of 2•THF with D2, and crystallographic details for compounds 1, 2, 2*, 6, and 10 (pdf). Crystallographic data for 1, 2, 2*, 6, and 10 (CIF). DFT analysis of 2, 2*, and A.
AUTHOR INFORMATION Corresponding Author *
[email protected] *
[email protected] *
[email protected] Present Addresses † (V.L.) Evonik Creavis GmbH, Marl, Germany.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We thank the Deutsche Forschungsgemeinschaft through the International Research Training Group “Selectivity in Chemo- and Biocatalysis” for financial support and the Alexander von Humboldt Foundation for a fellowship to D.M. We are grateful to Dr. J. van Leusen and Ms. C. Houben for the magnetic measurements.
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SYNOPSIS TOC The Nature of the Heavy Alkaline Earth Metal-Hydrogen Bond: Synthesis, Structure, and Reactivity of a Cationic Strontium Hydride Cluster
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