Zirconium Hydride Complex Supported by a Tetradentate Carbon

Article pubs.acs.org/IC

Zirconium Hydride Complex Supported by a Tetradentate CarbonCentered Tripodal Tris(aryloxide) Ligand: Synthesis, Structure, and Reactivity Yusuke Nakanishi, Yutaka Ishida, and Hiroyuki Kawaguchi* Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan S Supporting Information *

ABSTRACT: A dizirconium trihydride complex supported by a tetradentate carbon-centered tris(aryloxide) ligand [{Na(dme)} 3 {(O 3 C)Zr} 2 (μ-H) 3 ] (2; [O 3 C] = [(3,5- t Bu 2 -2O−C6H2)3C]4−) was prepared by reacting [(O3C)Zr(thf)3] (1) with NaBHEt3 in toluene. Exposure of 2 in THF to CO2 (1 atm) resulted in facile insertion of CO2 into Zr−H bonds, yielding a formate complex [{Na(thf) 2}3 {(O3 C)Zr}2 (μO2CH)3] (3). Treatment of 2 with P4 in toluene led to formation of [Na(thf)5][{Na(thf)2}2{(O3C)Zr}2(μ-P3)] (4) and PH3, in which hydrogenation of P4 took place. Complex 2 reacted with Me3SiN3 to afford an azide-bridged cyclotrimer [{Na(thf)2}{(O3C)ZrN3(thf)}]3 (5) with concomitant liberation of Me3SiH. The molecular structures of these complexes 2−5 have been determined by X-ray diffraction analyses.

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reagents.10 A tripodal tris(aryloxide) ligand [O3]3− (tris(2-oxy3,5-di-tert-butylphenyl)methane) is among them.11 Two main features make this tris(aryloxide) a useful ligand. First, its three oxygen atoms generate rigid, facial tridentate coordination at a metal center. For example, the diniobium hydride complex having the [O3] ligand is found to react spontaneously with N2, yielding the nitride-bridging diniobium complex.12 The use of the robust, rigid [O3] scaffold enables further functionalization of this N2-derived nitride into ammonia, imides, isocyanate, and ureate.13 Second, the [O3] ligand is prone to cyclometalation of the bridgehead C−H bond. By utilizing the latter feature, we previously reported that the reaction of Zr(CH2Ph)4 with H3[O3] afforded [(O3C)Zr(thf)3] (1), in which the carboncentered tripodal tris(aryloxide) [O3C] ligand is bound to the metal center in a tetradentate manner.11b Here we present the synthesis and reactions of a dizirconium trihydride complex supported by the [O3C] ligand.

INTRODUCTION The chemistry of molecular metal hydride complexes continues to attract much interest due to their structural diversity and reactivity over a wide range of transformations.1,2 The reactivity of hydride complexes depends largely on the electronic and geometrical properties of the metal center. Therefore, the design of auxiliary ligand systems provides an attractive avenue to explore hydride complexes with novel properties. For early transition metals, hydride species have played a crucial role in studying olefin hydrogenation,3 polymerization,4 and CO reduction.2,5 While most of them contain the cyclopentadienyl ligand and its substituted derivatives, there is a highly reactive class of compounds incorporating O-donor ligands such as alkoxide,6 aryloxide, or siloxide ligands due to their coordinative unsaturation. Rothwell and co-workers reported that niobium and tantalum hydride species supported by bulky aryloxide ligands were implicated in catalytic hydrogenation of polycyclic aromatic hydrocarbons with good chemoselectivity and stereoselectivity.7 Wolczanski and co-workers have extensively studied the reactivity of tantalum siloxide hydride complexes, including conversion of CO to a μ-ethylidene ligand via hydrogenation, deoxygenation, and C−C coupling.8 Given the unique reactivity of O-ligated metal hydrides and the ease with which various substituents can be introduced onto the aryloxide ring, the aryloxide-based ligand set would be appropriate for the development of early transition metal hydride complexes. We have been investigating the synthesis and reactivity of early transition metal complexes stabilized by tris(aryloxide) ligands,9 and a series of their hydride derivatives were prepared by the reactions of chloride precursors with borohydride © XXXX American Chemical Society

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RESULTS AND DISCUSSION Addition of 1.5 equiv of NaBHEt3 to 1 in toluene led to a gradual color change form pale yellow to yellow and formation of a hydride-bridged dizirconium complex. Removal of volatiles under vacuum and recrystallization of the residue by toluene/ DME provided yellow crystals of [{Na(dme)}3{(O3C)Zr}2(μH)3] (2) in 44% yield (Scheme 1). When the amount of NaBHEt3 was increased, the similar results were obtained, and Received: January 28, 2016

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DOI: 10.1021/acs.inorgchem.6b00233 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

ligand and three bridging hydride ligands. The geometry of the zirconium center is best described as monocapped octahedral with the bridgehead carbon atom of the [O3C] ligand occupying the capping position. The Zr−C and Zr−O distances (average 2.361 and 2.077 Å) are comparable to those of the THF adduct 1. The molecule contains three [Na(dme)]+ moieties in a tight ion-pair form. Two sodium atoms bind to two aryloxide oxygen atoms and one hydride, while the other binds to one aryloxide oxygen atom and one hydride. The Zr···Zr separation of 3.2766(6) Å is significantly longer than those reported for dizirconium complexes with three μ2-bridging hydride ligands (3.1050(6)−3.1630(10) Å).10c,14 This elongation may be due to association of sodium cations at hydrides. The solid-state structure of 2 is not consistent with an averaged 3-fold symmetry observed in solution by NMR spectroscopy. This is likely a result of sodium cation exchange in solution on the NMR time scale. Complex 2 appears to be a NaH adduct formed via interaction of the hydride with the zirconium centers and complexation of the sodium by the aryloxide groups. However, attempts at preparing 2 by the reaction of 1 with NaH were unsuccessful, resulting in recovery of the starting THF complex 1. In contrast, Floriani and co-workers reported that the zirconium porphyrinogen complex acts as a bifunctional acid− base system and reacts with NaH to generate the NaH adduct soluble in hydrocarbon solution.15 The reactivity of 2 with a series of small molecules including N2, CO, CO2, and white phosphorus (P4) was surveyed (Scheme 2). Complex 2 was found to be unreactive toward N2, and the reaction of 2 with CO yielded intractable products that have so far resisted characterization. However, exposure of 2 to 1 atm of CO2 led to complete consumption of the starting hydride complex and clean formation of a single product, as indicated by 1H NMR spectroscopy. On a preparative scale, the product was obtained as colorless crystals in 45% isolated yield after recrystallization from THF/pentane. An X-ray diffraction analysis indicated that this product is a dizirconium tris(formate) complex [{Na(thf)2}3{(O3C)Zr}2(μ-O2CH)3] (3) as a result of CO2 insertion into Zr−H bonds. The IR spectrum of 3 displays a broad absorption at 1613 cm−1 for the μ-formate moieties. The molecular structure of 3 is shown in Figure 2. The two zirconium centers are linked by one μ2:η1,η2-formate and two μ2:η1,η1-formates. One zirconium center (Zr(1)) adopts a eight-coordinate dodecahedral geometry, while the other (Zr(2)) is seven-coordinate, monocapped octahedral in geometry. Aside from having a different geometry, the Z−C and Zr−O(aryloxide) distances of these [(O3C)Zr] moieties are similar. The Zr···Zr separation of 4.390(1) Å is much longer than that of its precursor 2 to accommodate three formates between two zirconium centers. The bridging Zr(1)−O(10) distance of the μ2:η1,η2-formate (2.661(7) Å) is longer than the other Zr−O(formate) distances (2.161(5)−2.436(6) Å). The Zr−O distances of the μ2:η1,η1-formates in 3 (average 2.259 Å) are comparable to those reported for zirconium formate complexes.16 Complex 3 occurs in a tight ion-pair form, with each sodium cation completing its coordination through one formate oxygen atom, two aryloxide oxygen atoms, and two THF molecules. The solution behavior of 3 is fluxional and temperature dependent. The variable-temperature 1H NMR spectra in THFd8 indicate that the formate groups undergo mutual site exchange. At 293 K, the formate groups are observed as two

Scheme 1. Synthesis of 2

further incorporation of hydrides into the metal coordination sphere was not observed. The 1H and 13C{1H} NMR spectra of 2 are consistent with the molecule possessing high symmetry in solution. Salient spectroscopic features of 2 include the observation of two tert-butyl environments, indicating that all six aryloxide groups are equivalent. A signal at δ 3.83, which integrates as 1.5 H per [O3C] ligand, is assigned to the hydride ligands. The 1H NMR spectroscopy shows that 2 is stable in a benzene solution for several weeks at room temperature. The solid-state molecular structure of 2 was determined by X-ray diffraction, which revealed the dizirconium nature of the compound (Figure 1). The bridging hydride atoms were located in the difference Fourier map, but were not refined. Each zirconium center is seven-coordinate with one κ4-[O3C]

Figure 1. Structure of 2 with thermal ellipsoids set at 50% probability level. All hydrogen atoms bound to carbon atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Zr(1)···Zr(2) 3.2766(6), Zr(1)−O(1) 2.045(3), Zr(1)−O(2) 2.077(3), Zr(1)−O(3) 2.074(3), Zr(1)−C(1) 2.377(3), Zr(2)−O(4) 2.092(3), Zr(2)−O(5) 2.089(3), Zr(2)−O(6) 2.086(3), Zr(2)−C(2) 2.345(4), O(1)−Zr(1)−O(2) 120.85(11), O(2)−Zr(1)−O(3) 96.77(11), O(3)−Zr(1)−O(1) 121.59(11), O(4)−Zr(2)−O(5) 128.95(11), O(5)−Zr(2)−O(6) 99.42(11), O(6)−Zr(2)−O(4) 109.97(11), Zr(1)−H(1) 2.18(4), Zr(1)−H(2) 2.17(5), Zr(1)−H(3) 2.15(3), Zr(2)−H(1) 2.12(4), Zr(2)−H(2) 2.16(4), Zr(2)−H(3) 2.12(3), Na(1)−H(3) 2.44, Na(2)−H(2) 2.58, and Na(3)−H(1) 2.33. B

DOI: 10.1021/acs.inorgchem.6b00233 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Reactions of 2 with CO2, Me3SiN3, and P4

resonances at δ 1.27 and 1.38 do not significantly change with temperature. On cooling the sample to 233 K, the formate resonance at δ 8.77 and the aryl resonances further broaden, but the low-temperature limiting spectrum is not observed. On warming the sample to 333 K, the formate resonances coalesce into a singlet at δ 8.68, and the spectrum sharpens considerably. Coordination and functionalization of P4 by metal complexes have been extensively investigated,17 while reports on hydrogenation of P4 with metal hydride complexes have been limited. For example, Peruzzini and Vizza reported that treatment of [(triphos)MH3] (M = Rh, Ir; triphos = MeC(CH2PPh2)3) with P4 resulted in formation of [(triphos)M(η1,η2-P4H)], which subsequently reacted with H2 to produce [(triphos)M(η3-P3)] and PH3.18 Metallocene hydrides [Cp2MoH2] and [Cp2TaH3] were observed to undergo fragmentation and hydrogenation of P4, yielding the diphosphene complexes [Cp2Mo(η2-P2H2)]19 and [Cp2TaH(η2-P2H2)],20 respectively. Chirik and co-workers found that treatment of [Cp*(C5H4tBu)ZrH2] with P4 did not lead to the diphosphene complex but instead to [{Cp*(C5H4tBu)Zr}2(μ2:η2,η2-P4H2)].21 With an interest in forming a P−H bond, we sought hydrogenation of P4 with 2. The reaction of P4 with 2 was found to be solvent dependent. Treatment of 2 with P4 in THF resulted in formation of the THF adduct 1 along with unidentified insoluble P-containing products. However, when the reaction was carried out in toluene at room temperature, a brown powder of [Na(thf)5][{Na(thf)2}2{(O3C)Zr}2(μ-cyclo-P3)] (4) precipitated from solution. Monitoring the reaction in C6D6 by 31P NMR spectroscopy revealed formation of PH3, which is observed as a quartet at δ −244 with 1JPH = 185 Hz.18 During the course of the reaction, the dizirconium trihydride complex 2 underwent complete hydrogenation of one phosphorus atom of the P4 molecule, yielding the cyclo-P3 complex 4 and PH3.22 Complex

Figure 2. Structure of 3 with thermal ellipsoids at the 50% probability level. All tert-butyl groups and hydrogen atoms except formyl moieties are omitted for clarity. Selected bond lengths [Å] and angles [°]: Zr(1)···Zr(2) 4.390, Zr(1)−O(1) 2.080(5), Zr(1)−O(2) 2.083(5), Zr(1)−O(3) 2.096(5), Zr(1)−O(7) 2.338(5), Zr(1)−O(9) 2.431(6), Zr(1)−O(10) 2.661(7), Zr(1)−O(11) 2.161(5), Zr(1)−C(1) 2.390(6), Zr(2)−O(4) 2.057(5), Zr(2)−O(5) 2.061(5), Zr(2)−O(6) 2.080(5), Zr(2)−O(8) 2.205(5), Zr(2)−O(10) 2.267(6), Zr(2)−O(12) 2.331(5), Zr(2)−C(2) 2.373(6), O(1)−Zr(1)−O(2) 140.89(19), O(2)−Zr(1)−O(3) 96.1(2), O(3)−Zr(1)−O(1) 92.8(2), O(4)−Zr(2)−O(5) 110.7(2), O(5)− Zr(2)−O(6) 93.1(2), O(6)−Zr(2)−O(4) 133.2(2).

broad resonances at δ 8.77 and 8.63 for 2 and 1H per dimer, and the aryl protons appear as three broad resonances at δ 6.78 for 6H, δ 7.43 for 4H, and 7.63 for 2H each. The tert-butyl C

DOI: 10.1021/acs.inorgchem.6b00233 Inorg. Chem. XXXX, XXX, XXX−XXX

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ring of 4 is assigned a formal charge of 3−. The anionic dizirconium part further incorporates two [Na(thf)2]+ cations, with each binding to one phosphorus atom of the P3 ring and two aryloxide oxygen atoms. To further probe the reactivity of 2, the reaction with Me3SiN3 in THF was examined. Metathesis reaction between 2 and Me3SiN3 proceeded with formation of a white precipitate, presumably NaN3. The release of Me3SiH during the reaction was observed by 1H NMR spectroscopy. Centrifugation and removal of volatile materials under reduced pressure afforded a yellow powder of [{Na(thf)2}{(O3C)ZrN3(thf)}]3 (5) in 78% yield. Complex 1 was not found to react with NaN3, and metathesis reaction of hydride species with Me3SiN3 offers an alternative synthesis route to azide complexes.29 The solid-state structure of 5 was determined by X-ray crystallography on a crystalline sample obtained from a solution of THF layered with pentane. Complex 5 crystallizes as a cyclotrimer in which three [(O3C)Zr(thf)] moieties are bridged by three azide ligands in the 1,3-fashion (Figure 4). There is a

4 has poor solubility in hydrocarbon solvents, benzene, and toluene, because of its ion-pair formulation (vide infra), but it is soluble and stable in polar solvents such as THF or pyridine. Analytically pure samples of 4 could be isolated as golden brown crystals in 30% yield by recrystallization from THF. Complex 4 displays the 1H NMR spectrum in THF-d8 analogous to that of 2, which is also consistent with high symmetry. The 31P NMR spectrum of 4 in THF-d8 exhibits a sharp singlet at δ −282.1, indicating that all phosphorus atoms of the cyclo-P3 moiety are equivalent on the NMR time scale. This chemical shift is comparable to those for the previously reported Pd(μ-cyclo-P3)Pd complexes.23 The solid-state molecular structure of 4 has been established by X-ray diffraction analysis. Two crystallographically independent ion pairs possessing similar geometries exist in the asymmetric unit. For conciseness, one of them is discussed. In each ion pair, one discrete [Na(thf)5]+ cation is partnered with the [{Na(thf)2}2{(O3C)Zr}2(μ-cyclo-P3)]− anionic complex shown in Figure 3. This anion part displays an inverted

Figure 3. Structure of the anion part of 4 with thermal ellipsoids at the 50% probability level. Only one of two independent moleucles in the asymmetric units is shown. All tert-butyl groups and hydrogen atoms are omitted for clarity. Selected bond length [Å] and angles [°] for molecule A: Zr(1)···Zr(2) 4.873, Zr(1)−P(1) 2.802(3), Zr(1)−P(2) 2.751(3), Zr(1)−P(3) 2.715(3), Zr(1)−O(1) 2.077(6), Zr(1)−O(2) 2.017(6), Zr(1)−O(3) 2.040(6), Zr(1)−C(1) 2.399(8), Zr(2)−P(1) 2.734(3), Zr(2)−P(2) 2.728(3), Zr(2)−P(3) 2.797(3), Zr(2)−O(4) 2.039(6), Zr(2)−O(5) 2.068(7), Zr(2)−O(6) 2.071(6), Zr(2)−C(2) 2.361(8), P(1)−P(2) 2.229(4), P(2)−P(3) 2.229(4), P(3)−P(1) 2.220(4), Na(1)−P(1) 3.239(5), Na(2)−P(3) 3.212(5), O(1)−Zr(1)−O(2) 120.0(3), O(2)−Zr(1)−O(3) 125.0(3), O(3)−Zr(1)−O(1) 95.7(3), O(4)−Zr(2)−O(5) 125.6(3), O(5)−Zr(2)−O(6) 96.7(3), O(6)−Zr(2)−O(4) 118.9(3), P(1)− P(2)−P(3) 59.72(11), P(2)−P(3)−P(1) 60.14(12), P(3)−P(1)− P(2) 60.14(12).

Figure 4. Structure of 5 with thermal ellipsoids at the 50% probability level. All tert-butyl groups and hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Zr(1)−N(1) 2.327(4), Zr(1)−N(3)′ 2.435(4), Zr(1)−O(1) 2.052(3), Zr(1)−O(2) 2.026(3), Zr(1)−O(3) 2.060(3), Zr(1)−C(1) 2.389(4), Zr(1)−O(4) 2.395(3), Na(1)−N(3)′ 2.555(4), Na(1)−−O(1) 2.543(4), Na(1)−O(3) 2.592(4), N(1)−N(2) 1.174(5), N(2)−N(3) 1.168(5), O(1)−Zr(1)−O(2) 127.47(13), O(2)−Zr(1)−O(3) 115.79(13), O(3)−Zr(1)−O(1) 95.23(13), Zr(1)−N(1)−N(2) 154.7(3), Zr(1)−N(3)′−N(2)′ 135.7(3), N(1)−N(2)−N(3) 178.9(5).

sandwich-type structure, where two [(O3C)Zr] moieties are bridged by a cyclo-P3 unit which is η3-bound to each zirconium center in a staggered orientation with the Zr−P distances ranging from 2.715(3) to 2.802(3) Å. This μ2 :η 3 ,η 3 coordination mode for the cyclo-P3 ligand is found in [(C5H3tBu2)2Th(μ-cyclo-P3)Th(Cl)(C5H3tBu2)2]24 and a series of M2(μ-cyclo-P3) complexes supported by the triphos ligands (M = Fe, Co, Rh, Ir, Ni, Pd, Pt).23,25 The average P−P distance of 4 (2.226 Å) are typical of P−P single bonds26 and slightly longer than those found in [(nacnac)V(Ntolyl2)]2(μ2-η3:η2cyclo-P3) (2.1658(10)−2.2155(9) Å)27 and reported other cyclo-P3 complexes.23−25,28 Each zirconium metal center adopts a distorted monocapped octahedral geometry with the average Zr−C and Zr−O distances of 2.380 and 2.052 Å. Hence the P3

crystallographically imposed 3-fold axis in the center of a (ZrN3)3 core, and the zirconium metals lie only 0.33 Å from the planes defined by the azide ligands. Each seven-coordinate zirconium is at the center of a distorted monocapped octahedron, with the Zr−C and Zr−O(aryloxide) distances (2.389(4) and average 2.046 Å, respectively) being comparable to those found in zirconium [O3C] complexes 1−4. The Zr−N3 functionalities are bent at 154.7(3) and 135.7(3) ° (Zr(1)−N(1)−N(2) and Zr(1)−N(3)−N(2)), and the Zr−N distances of 2.327(4) and 2.435(4) Å are consistent with the Zr−N single bonds.30 The azide ligands are almost linear with D

DOI: 10.1021/acs.inorgchem.6b00233 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the N(1)−N(2)−N(3) angle of 178.9(5) °, and the N−N distances of the azide ligands (1.174(5), 1.168(5) Å) are within the typical range.31−36 Similar cyclic structures with 1,3-briding azide ligands are found in azide complexes of Al,32 Ni,33 U,34 La,35 Y,36 and Sm,36 whereas 5 provides a rare example of zirconium azide complexes.37 Complex 5 contains three sodium metals and occurs in a tight ion-pair form. Each sodium within the [Na(thf)2] units binds to one azide nitorgen atom and two aryloxide oxygen atoms. The IR spectrum of 5 exhibits a strong absorption band at 2115 cm−1 due to the asymmetric N−N−N stretch.31 NMR spectroscopic characterization of this azide complex in THF-d8 indicates that 5 is highly symmetric in solution.

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CONCLUSIONS

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EXPERIMENTAL SECTION

Synthesis of [{Na(thf)2}3{(O3C)Zr}2(μ-O2CH)3] (3). A reaction tube was charged with a solution of 2 (19.0 mg, 0.011 mmol) in THF (0.5 mL). The solution was freeze−pump−thaw degassed, and then CO2 was admitted to the tube. The yellow solution was warmed to room temperature and turned colorless. The solvent was removed in vacuo, and the residue was recrystallized from THF/pentane to afford 3 as colorless needles in 45% yield (9.4 mg, 4.6 μmol). 1H NMR (500 MHz, THF-d8, 333 K, δ/ppm): 1.27 (s, 54H, tBu), 1.38 (s, 54H, tBu), 6.84 (s, 6H, Ar), 7.52 (brs, 6H, Ar), 8.68 (brs, 3H, −OCHO−). 13 C{1H} NMR (125 MHz, THF-d8, rt, δ/ppm): 30.2, 31.6 (CMe3), 33.9, 34.5 (CMe3), 118.3, 126.4, 137.5, 165.1 (Ar). Resonances due to formyl, bridgehead, and two aryl carbons could not be positively identified due to broad, overlapping resonances. IR (KBr pellet): 1613 cm−1 (μ-O2CH). Anal. Calcd. (%) for C113H171Na3O18Zr2: C, 65.60; H, 8.24. Found: C, 65.70; H, 8.24. Synthesis of [Na(thf)5][{Na(thf)2}2{(O3C)Zr}2(μ-cyclo-P3)] (4). To a solution of 2 (53.0 mg, 0.030 mmol) in toluene (5 mL) was added P4 (3.5 mg, 0.035 mmol) forming a reddish yellow slurry. After the mixture was stirred at room temperature for 12 h, the brown precipitate was isolated by centrifugation and dissolved in 3 mL of THF. Storing the solution at −30 °C give 4 as golden brown microcrystals in 30% (20.0 mg, 8.9 μmol). 1H NMR (500 MHz, THFd8, rt, δ/ppm): 1.26 (s, 54H, tBu), 1.45 (s, 54H, tBu), 1.75 (THF), 3.59 (THF), 6.78 (d, 4JHH = 2.1 Hz, 6H, Ar), 7.62 (d, 4JHH = 2.1 Hz, 6H, Ar). 31P NMR (202 MHz, THF-d8, rt, δ/ppm): −282.1 (μ-cycloP3). The 13C{1H} NMR spectrum was impossible to acquire because of the low solubility of 4. Anal. Calcd. (%) for C122H192Na3O15P3Zr2: C, 65.32; H, 8.63. Found: C, 65.05; H, 8.37. Synthesis of [{Na(thf)2}{(O3C)ZrN3(thf)}]3 (5). To a solution of 2 (24.0 mg, 0.013 mmol) in THF (5 mL) was added Me3SiN3 (8.0 μL, 0.057 mmol). The solution was stirred at room temperature for 6 h, forming a cloudy, pale yellow solution. After centrifugation to remove an insoluble material, the supernatant was evaporated to dryness. The residue was washed with 5 mL of pentane and dried in vacuo to a give light yellow powder of 5 in 78% yield (20 mg, 6.7 μmol). 1H NMR (500 MHz, THF-d8, rt, δ/ppm): 1.27 (s, 81H, tBu), 1.40 (s, 81H, tBu), 6.77 (s, 4JHH = 9H, Ar), 7.48 (s, 9H, Ar). 13C{1H} NMR (125 MHz, THF-d8, rt, δ/ppm): 30.1, 31.8 (CMe3), 33.9, 34.4 (CMe3), 71.8 (Ar3CZr), 118.0, 126.4, 131.2, 136.4, 148.0, 165.7 (Ar). IR (KBr pellet): 2115 cm−1 (μ-N3). Anal. Calcd. (%) for C165H252O18N9Na3Zr3: C, 66.23; H, 8.49; N, 4.21. Found: C 66.25; H, 8.10; N, 4.51. X-ray Crystallography. Single crystals were immerced in immersion oil on micromount and transferred to a Rigaku Varimax with Saturn system equipped a Rigaku GNNP low temperature device. Data were collected under a cold nitrogen stream at 123 K using graphite-monochromated Mo Kα radiation (λ = 0.71075 Å). Equivalent reflections were merged, and the images were processed with the Crystalclear (Rigaku) program. Empirical absorption corrections were applied. All structures were solved by direct method using SHELXS-9738 and refined by full-matrix least-squares method on F2 for all data using SHELXL-2013.39 All hydrogen atoms except Zr−H hydrogen atoms of complex 2 were placed at their geometrically calculated positions. For 2, three tert-butyl groups, two DME molecules, and one toluene molecule were disordered. Three Zr−H hydrogen atoms were found in the Fourier map and located, but not refined. For 3, five tert-butyl groups, two THF molecules and one pentane molecule were disordered. No hydrogen atom was put on the minor component of disordered THF molecule. For 4, two crystallographically independent molecules were present in the unit cell. The unit cell contains five THF molecules as crystal solvents. Three tert-butyl groups and 11 THF molecules were disordered. No hydrogen atom was put on the minor component of two of disordered THF molecules. Moreover, some residual electron density was difficult to model, and therefore the program SQUEEZE was used to remove the contribution of the electron density in the solvent region from the intensity data. A void space contains 160 electrons per unit cell, which could be attributed to distorted THF molecules (one molecule in the asymmetric unit). Single crystals of 5 exhibited weak reflections at the high angle region.

We have shown that the hydride complex 2 is readily accessible from the zirconium(IV) complex 1 with the labile THF ligand, and the [O3C] ligand system can be incorporated onto zirconium derivatives to generate seven- and eight-coordinate complexes. The hydride complex 2 was prepared by treatment of 1 with NaBHEt3 in toluene. The formation of 2 can be seen as a Lewis acid−base reaction to afford an “ate” complex. The ability of the [O3C] ligand to provide a platform for tripodal frameworks of transition metals as well as to function as a bridging ligand by the basic aryloxide oxygen atom helps to stabilize the zirconium/sodium hydride complex 2. Reactivity studies on 2 with CO2, P4, and Me3SiN3 revealed it to be a reactive species, which readily undergoes insertion and metathesis reactions.

General Considerations. All operations were performed under an inert atmosphere of argon using standard Schlenk techniques and in an MBraun glovebox. Pentane, toluene, THF, and DME were purchased from Kanto Chemical Co. and dried by passage through two columns of activated alumina and a Q-5 column. Benzene-d6 and THF-d8 were dried and degassed over a potassium mirror, vacuum transferred, and then stored in glovebox. NMR spectra were recorded on a JEOL ECX500 spectrometer. 1H NMR are reported with reference to solvent resonances of benzene-d6 and THF-d8 residual protons at δ = 7.15 and 3.62 ppm, respectively. 13C{1H} NMR were referenced to deuterated solvent peaks δ = 128 (C6D6) and 68.0 (THF-d8) ppm, respectively. 31 1 P{ H} NMR were referenced to 85% H3PO4 aq. (δ = 0) as external standard. Infrared spectra were recorded on a JASCO FT/IR-410 spectrometer. Elemental analyses (C, H, and N) were measured using an Elementer vario MICRO cube apparatus. Caution! P4 and PH3 are highly toxic and f lammable, and they should be handled with the utmost care. Synthesis of [{Na(dme)}3{(O3C)Zr}2(μ-H)3] (2). A solution of NaHBEt3 (1.0 M in toluene, 0.35 mL, 0.35 mmol) was added dropwise to a solution of 1 (215 mg, 0.231 mmol) in toluene (20 mL) at −98 °C. The mixture was warmed up to room temperature and stirred for 1 h, during which time a color of the solution gradually changed from pale yellow to yellow. After removal of all volatiles in vacuo, the resulting residue was dissolved in toluene (6 mL) containing a small amount of DME. Standing at room temperature afforded 2 as yellow crystals in 44% (91 mg, 0.051 mmol). 1H NMR (500 MHz, C6D6, rt, δ/ppm): 1.44 (s, 54H, tBu), 1.47 (s, 54H, tBu), 2.86 (s, 18H, DME), 3.00 (s, 12H, DME), 3.83 (s, 3H, Zr-H), 7.22 (s, 6H, Ar), 7.77 (s, 6H, Ar). 13C{1H} NMR (100 MHz, C6D6, rt, δ/ ppm): 31.0, 32.2 (CMe3), 34.5, 34.7 (CMe3), 58.9, 71.3 (DME), 84.1 (Ar3CZr), 119.9, 132.6, 140.2, 144.9, 164.1 (Ar). One aryl carbon resonance could not be observed because of overlapping resonances. Anal. Calcd. (%) for C98H153Na3O12Zr2·(C7H8)0.5: C, 66.96; H, 8.69. Found: C, 67.24; H, 8.35. E

DOI: 10.1021/acs.inorgchem.6b00233 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Four tert-butyl groups of one [(O3C)Zr] unit in 5 were disordered. Moreover, some residual electron density was difficult to model, and therefore the program SQUEEZE40 was used to remove the contribution of the electron density in the solvent region from the intensity data. A void space contains 2140 electrons per unit cell, which could be attributed to disordered pentane molecules (four molecules in the asymmetric unit).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00233. Crystallographic data in cif file format for 2, 3, 4, and 5 (CIF) Additional NMR spectroscopic data and crystallographic data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research [No. 15H00927 (Innovative Areas “Stimuli-responsive Chemical Species”) and No. 25288026] from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and by the Japan Science and Technology (ACT-C program).

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