An Effective Route to Dinuclear Niobium and Tantalum Imido

Article pubs.acs.org/IC

An Effective Route to Dinuclear Niobium and Tantalum Imido Complexes Manuel Gómez, Cristina Hernández-Prieto, Avelino Martín, Miguel Mena, and Cristina Santamaría* Departamento de Química Orgánica y Química Inorgánica, Universidad de Alcalá, Campus Universitario, E-28805 Alcalá de Henares, Madrid, Spain

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S Supporting Information *

ABSTRACT: Thermal treatment of the trichloro complexes [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta) (Xyl = 2,6Me2C6H3) under vacuum affords the dinuclear imido species [MCl2(μ-Cl)(NR)py]2 (R = tBu, Xyl; M = Nb 1, 3; Ta 2, 4) with loss of pyridine. Complexes 1−4 can be easily transformed to the mononuclear starting materials [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta) upon reaction with pyridine. While reactions of compounds 1 and 2 with a series of alkylating reagents render the mononuclear peralkylated imido complexes [MR3(NtBu)] (R = Me, CH2Ph, CH2CMe3, CH2CMePh, CH2SiMe3), the analogous treatment with allylmagnesium chloride results in the formation of the dinuclear niobium(IV) derivative [(NtBu)(η3-C3H5)M(μ-C3H5)(μ-Cl)2M(NtBu)py2] (5). Additionally, the treatment of the starting materials 1 and 2 with the organosilicon reductant 1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene yields the pyridyl-bridged dinuclear derivatives [M2Cl2(μ-Cl)2(NtBu)2py2]2(μ-NC4H4N)2 (M = Nb 6, Ta 7). Controlled hydrolysis reaction of 1 and 2 affords the oxo chlorido-bridged products [MCl(μ-Cl)(NtBu)py]2(μ-O) (M = Nb 8, Ta 9) in a quantitative way, while the treatment of these latter with one more equivalent of pyridine led to complexes [MCl2(NtBu)py2]2(μO) (M = Nb 10, Ta 11). Structural study of these dinuclear imido derivatives has been also performed by X-ray crystallography.

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INTRODUCTION

In this paper we report that thermal treatment of the mononuclear imido derivatives [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta) produces the elimination of the pyridine molecule occupying the trans position to afford dinuclear [MCl2(μCl)(NR)py]2 complexes in high yield. A study of the reactivity of these species against a series of reagents in metathesis, reduction, and hydrolysis processes in order to increase the nuclearity of these systems is presented.

Transition-metal complexes containing the unsaturated MNR linkage were known decades ago. As ancillary ligand, the involvement of this multiply bonded functionality in reactions and applications has been extensively reviewed.1−9 Focusing our attention on niobium and tantalum, a variety of synthetic protocols afford imido complexes with nearly any organic substituent from available starting materials in high yield.1,10,11 A common method to prepare imido complexes of formula [MCl3(NR)L2] comprises the reaction of MCl5 with the corresponding amines NHRSiMe3 and nitrogen-, oxygen-, and sulfur-donor molecules (L) in hydrocarbon solvents.12−14 These compounds exhibit a pseudo-octahedral geometry with a six-coordinate metal center, where the imido ligand and one donor molecule (L) occupy axial positions. The trans imido ligand in these species labilizes the donor group located opposite to it, this latter being easily replaced in subsequent reactions.2,15 Moreover, the strong labilizing effect of the imido groups on the donor ligand trans to the MNR bond makes it possible to form dinuclear3 or even polynuclear complexes. For instance, our research group has reported a synthetic procedure for the construction of cube-type tetranuclear niobium and tantalum sulfide clusters by using the mononuclear imido compounds [MCl3(NR)(py)2] (M = Nb, Ta) and hexamethyldisilathiane (SiMe3)2S, as sulfur source.14 © 2017 American Chemical Society

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out under a dry argon atmosphere using Schlenk tube and cannula techniques or in a conventional argon-filled glovebox. Solvents were carefully refluxed over the appropriate drying agents and distilled prior to use: C6D6 and hexane (Na/K alloy), CDCl3 (CaH2), pyridine and C6D5N (P2O5), dichloromethane and CD2Cl2 (P2O5), tetrahydrofuran (Na/benzophenone), and toluene (Na). Starting materials [MCl3(NR)py2] (R = tBu,12,13 Xyl;14 M = Nb, Ta), organolithium reagents LiR (R = CH2CMe3,16 CH2CMe2Ph,17 CH2SiMe318), and organosilicon reductant 1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene19 were synthesized according to published procedures. Grignard reagents MgClR (R = Me, 3 M in thf; R = CH2Ph, 1 M in diethyl ether; R = C3H5, 2 M in thf) were purchased from Aldrich and were used without further purification. Microanalyses (C, H, N, S) were performed in a LECO CHNS-932 microanalyzer. Samples for IR Received: June 28, 2017 Published: September 13, 2017 11681

DOI: 10.1021/acs.inorgchem.7b01652 Inorg. Chem. 2017, 56, 11681−11687

Article

Inorganic Chemistry

(m, 2H, CH2 allyl), 1.40 (s, 9H, Me3CN), 1.14 (s, 9H, Me3CN). 13 C{1H} NMR (125 MHz, C6D6): δ = 152.1, 151.3 (Co py), 137.3, 136.0 (Cp py), 123.9, 123.8 (Cm py), 122.6, 40.0 (CH2CHCH2), 79.7, 69.8, 63.3, 58.4, (CH2CHCH2), not detected (Me3CN), 30.7 overlapping (Me 3 CN). Elemental analysis (%) calcd for C24H38Cl2N4Nb2 (639.29): C, 45.09; H, 5.99; N 8.76. Found: C, 45.76; H, 5.47; N 6.92. Partial loss of coordinated pyridine precluded a better elemental analysis of compound 5. Preparation of [M4Cl4(μ-Cl)4(NtBu)4py4{μ-(NC4H4N)2}] (M = Nb 6, Ta 7). A solution of [MCl2(μ-Cl)(NtBu)py]2 (0.30 g, M = Nb 1 0.43 mmol, Ta 2 0.34 mmol), 1,4-bis(trimethylsilyl)-1,4-diaza-2,5cyclohexadiene (M = Nb 0.10 g, 0.43 mmol; Ta 0.08 g, 0.34 mmol), and 30−40 mL of dichloromethane was prepared in a 100 mL Schlenk vessel. The solutions were left stirring at room temperature for 24 h, turning the color from pale yellow to blue-violet, and after 2 days at −20 °C a precipitate was observed. The solid was isolated by filtration, washed with hexane (3 × 10 mL), and dried in vacuum. 6: yield 0.26 g (86%). IR (KBr, cm−1): v ̅ = 2970 (s), 2924 (w), 2892 (m), 1665 (w), 1605 (s), 1488 (w), 1445 (s), 1359 (m), 1238 (vs). 1H NMR (500 MHz, CD2Cl2): δ = 8.99, 8.81 (m, 8H, Ho py), 7.96, 7.87 (m, 4H, Hp py), 7.24, 7.16 (m, 8H, Hm py), 7.43 (br, 8H, 1,4-diaza-2,5cyclohexadiene), 1.42 (s, 36H, Me3CN). 13C{1H} NMR (125 MHz, CD2Cl2): δ = 152.9, 151.5 (Co py), 140.5, 138.9 (Cp py), 129.3, 128.5 (Cm py), 125.0, 124.6 (1,4-diaza-2,5-cyclohexadiene), not detected (Me3CN), 29.4 (Me3CN). Elemental analysis (%) calcd for C44H64Cl8N12Nb4 (1416.31): C, 37.31; H, 4.55; N 11.87. Found: C, 36.97; H, 5.36; N 11.25. 7: yield 0.28 g (92%). IR (KBr, cm−1): v ̅ = 2968 (s), 2923 (m), 2891 (vs), 1665 (w), 1636 (w), 1604 (s), 1487 (vs), 1358 (m), 1280 (s). 1H NMR (500 MHz, CD2Cl2): δ = 8.98, 8.90 (m, 8H, Ho py), 7.94, 7.87 (m, 4H, Hp py), 7.22, 7.14 (m, 8H, Hm py), 7.43 (m, 8H, 1,4-diaza-2,5-cyclohexadiene), 1.33 (s, 36H, Me3CN). 13C{1H} NMR (125 MHz, CD2Cl2): δ = 152.9, 151.5 (Co py), 140.4, 138.8 (Cp py), 129.0, 128.2 (Cm py), 125.3, 124.8 (1,4diaza-2,5-cyclohexadiene), not detected (Me 3 CN), 31.2 (Me3CN). Elemental analysis (%) calcd for C44H64Cl8N12Ta4 (1768.47): C, 29.88; H, 3.65; N 9.50. Found: C, 29.05; H, 4.17; N 10.04. Preparation of [MCl(μ-Cl)(NtBu)py]2(μ-O) (M = Nb 8, Ta 9). To a toluene (50 mL) solution of [MCl2(μ-Cl)(NtBu)py]2 (0.20 g; M = Nb 1 0.29 mmol, Ta 2 0.23 mmol) placed in a 100 mL Schlenk at room temperature was added D2O (M = Nb; 5.23 μL, 0.29 mmol; M = Ta 4.15 μL, 0.23 mmol). The solutions were left stirring for 2 h and then were settled and filtered. The white solids were dried in vacuum. 8: yield 0.16 g (90%). IR (KBr, cm−1): v ̅ = 3230 (m), 2966 (vs), 2895 (vs), 2805 (vs), 1609 (s), 1535 (s), 1513 (s), 1484 (s), 1403 (s), 1374 (s), 1303 (s), 1218 (s), 882 (br), 747 (s), 676 (s), 451 (s). 1H NMR (500 MHz, CDCl3): δ = 8.82 (br, 4H, Ho py), 8.40 (m, 2H, Hp py), 7.92 (br, 4H, Hm py), 1.45 (s, 18H, Me3CN). 13C{1H} NMR (125 MHz, CDCl3): δ = signals not observed for the pyridine ligands, 52.9 (Me3CN), 27.8 (Me3CN). Elemental analysis (%) calcd for C18H28Cl4N4ONb2 (644.07): C, 33.57; H, 4.38; N 8.70. Found: C, 33.46; H, 4.55; N 8.13. 10: yield 0.18 g (95%). IR (KBr, cm−1): v ̅ = 3228 (w), 2980 (vs), 2896 (s), 2798 (m), 2776 (m), 1609 (m), 1537 (m), 1512 (m), 1486 (s), 1402 (s), 1376 (s), 1304 (m), 1220 (s), 887 (br), 752 (m), 679 (m), 609 (m), 452 (m). 1H NMR (500 MHz, CDCl3): δ = 8.80 (br, 4H, Ho py), 8.44 (m, 2H, Hp py), 7.96 (br, 4H, Hm py), 1.45 (s, 18H, Me3CN). The low solubility of this compound precluded a 13C NMR spectrum. Elemental analysis (%) calcd for C18H28Cl4N4OTa2 (820.15): C, 26.36; H, 3.44; N 6.83. Found: C, 26.55; H, 3.52; N 6.51. Crystal Structure Determination of Complexes 1, 4, 5, 7, 10, 11, and [Ta(CH2CMe3)3(NtBu)]. Crystals of 1 and 4 were grown in an NMR tube by heating and slow cooling of saturated chloroform-d1 solutions. Crystals of 5, 7, and [Ta(CH2CMe3)3(NtBu)] were obtained by slow cooling at −20 °C of the corresponding toluene solutions. On the other hand, crystals of 10 or 11 were obtained by slow cooling the toluene crude reaction selecting the crystals from the obtained mixture. In all cases, crystals were removed from the Schlenks and covered with a layer of a viscous perfluoropolyether (FomblinY). A suitable crystal was selected with the aid of a microscope, mounted

spectroscopy were prepared as KBr pellets and recorded on the PerkinElmer IR-FT Frontier spectrophotometer (4000−400 cm−1). 1 H and 13C NMR spectra were obtained by using Varian NMR system spectrometers, Unity-300 Plus, Mercury-VX, and Unity-500, and are reported with reference to solvent resonances. 1H−13C gHSQC were recorded using the Unity-500 MHz NMR spectrometer operating at 25 °C. General Procedure for the Synthesis of [MCl2(μ-Cl)(NR)py]2 (R = tBu, Xyl; M = Nb 1, 3; Ta 2, 4). [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta) were placed into a 2.0 cm × 5.0 cm glass vial in a glovebox, and the vial was mounted into a sublimation drying tube of a Büchi B-580 glass oven. The system was evacuated to ca. 0.1 mmHg and then heated for 12 h. A sublimated solid is observed on the top of the tube along with a dark brown residue at the bottom of the glass vial. On cooling of the oven, the sublimated solid was treated in the glovebox, washed with cold hexane (2 × 10 mL), and dried in vacuum. On the other hand, the identification of the residue was not feasible due to the nonvolatility and insolubility in most of the common solvents. Preparation of [MCl2(μ-Cl)(NtBu)py]2 (M = Nb 1, Ta 2). [MCl3(NtBu)py2] (2.50 g; M = Nb 1, 5.83 mmol; Ta 2, 4.84 mmol) were heated at 170 °C for 12 h to yield bright yellow (1) or pale yellow (2) solids. 1: yield 1.67 g (82%). IR (KBr, cm−1): v ̅ = 3084 (w), 3058 (w), 2974 (s), 2926 (m), 2892 (m), 1609 (s), 1487 (vs), 1446 (s), 1361 (vs), 1232 (s). 1H NMR (500 MHz, CDCl3): δ = 9.03 (br, 4H, Ho py), 7.87 (m, 2H, Hp py), 7.42 (m, 4H, Hm py), 1.41 (s, 18H, Me3CN). 13C{1H} NMR (125 MHz, CDCl3): δ = 153.0 (Co py), 139.9 (Cp py), 124.6 (Cm py), 54.2 (Me3CN), 28.3 (Me3CN). Elemental analysis (%) calcd for C18H28Cl6N4Nb2 (698.97): C, 30.93; H, 4.04; N 8.02. Found: C, 31.60; H, 3.97; N 8.25. 2: yield 1.76 g (83%). IR (KBr, cm−1): v ̅ = 3123 (w), 3088 (w), 3060 (w), 2972 (s), 2925 (m), 2891 (vs), 1613 (s), 1490 (vs), 1448 (s), 1361 (vs), 1267 (s). 1H NMR (500 MHz, CDCl3): δ = 9.11 (br, 4H, Ho py), 7.92 (m, 2H, Hp py), 7.53 (m, 4H, Hm py), 1.30 (s, 18H, Me3CN). 13C{1H} NMR (125 MHz, CDCl3): δ = 153.1 (Co py), 140.1 (Cp py), 125.1 (Cm py), 77.2 (Me3CN), 31.7 (Me3CN). Elemental analysis (%) calcd for C18H28Cl6N4Ta2 (875.05): C, 24.71; H, 3.23; N 6.40. Found: C, 24.19; H, 3.00; N 6.30. Preparation of [MCl2(μ-Cl)(NXyl)py]2 (M = Nb 3, Ta 4). [MCl3(NXyl)(py)2] (2.00 g; M = Nb 1, 4.20 mmol; Ta 2, 3.54 mmol) were heated at 140 °C for 12 h to yield dark purple (3) or salmon (4) solids. 3: yield 1.33 g (80%). IR (KBr, cm−1): v ̅ = 3071 (m), 2917 (m), 1608 (vs), 1446 (vs), 1300 (s), 1223 (s). 1H NMR (500 MHz, CDCl3): δ = 8.99 (br, 4H, Ho py), 8.00−6.70 (overlapping signals, 12H, py and 2,6-Me2C6H3N), 2.62 (s, 12H, 2,6-Me2C6H3N). 13 C{1H} NMR (125 MHz, CDCl3): δ = 152.4−124.6 (py and 2,6Me2C6H3N), 19.0 (2,6-Me2C6H3N). Elemental analysis (%) calcd for C26H28Cl6N4Nb2 (795.06): C, 39.28; H, 3.55; N 7.05. Found: C, 39.25; H, 3.43; N 7.12. 4: yield 1.55 g (90%). IR (KBr, cm−1): v ̅ = 3000 (w), 2943 (m), 2924 (m), 1610 (vs), 1447 (vs), 1329 (vs), 1223 (s). 1H NMR (500 MHz, CDCl3): δ = 9.09 (m, 4H, Ho py), 8.00−6.60 (overlapping signals, 12H, py and 2,6-Me2C6H3N), 2.63 (s, 12H, 2,6-Me2C6H3N). 13C{1H} NMR (125 MHz, CDCl3): δ = 152.4− 125.4 (py and 2,6-Me2C6H3N), 18.8 (2,6-Me2C6H3N). Elemental analysis (%) calcd for C26H28Cl6N4Ta2 (971.14): C, 32.16; H, 2.91; N 5.77. Found: C, 32.56; H, 3.04; N 5.97. Preparation of [Nb2(μ-C3H5)(η3-C3H5)(μ-Cl)2(NtBu)2py2] (5). A hexane (50 mL) solution of 1 (1.00 g, 1.43 mmol) was placed in a 100 mL Schlenk vessel with stirring at 0 °C, and a solution of allylmagnesium chloride 2 M in thf (1.91 mL, 5.72 mmol) was added. The pale-yellow solution turned green and finally red. The solution was filtered after being stirred at room temperature for 24 h, the solvent was removed in vacuum, and the resulting red solid was washed with hexane (3 × 10 mL) and dried in vacuum (0.58 g, 64%). IR (KBr, cm−1): v ̅ = 2966 (s), 2942 (vs), 2921 (w), 1605 (vs), 1486 (s), 1446 (s), 1355 (vs), 1258 (s). 1H NMR (500 MHz, C6D6): δ = 9.01 (br, 2H, Ho py), 8.98 (br, 2H, Ho py), 6.60 (m, 1H, Hp py), 6.55 (m, 1H, Hp py), 6.30 (m, 2H, Hm py), 6.25 (m, 2H, Hm py), 6.18 (m, 1H, CH allyl), 2.47 (m, 1H, CH allyl), 4.58, 3.84 (m, 2H, CH2 allyl), 4.50, 2.56 (m, 2H, CH2 allyl), 4.32, 3.20 (m, 2H, CH2 allyl), 3.61, 1.29 11682


Article

Inorganic Chemistry on a cryoloop, and immediately placed in the low-temperature nitrogen stream of the diffractometer. The intensity data sets were collected at 200 K on a Bruker-Nonius KappaCCD diffractometer equipped with an Oxford Cryostream 700 unit. Crystallographic data for all complexes are presented in Table S1. The structures were solved, by using the WINGX package,20 by direct methods (SHELXS-2013 for complexes 1, 5, 10, and [Ta(CH2CMe3)3(NtBu)])21 or intrinsic phasing (SHELXT-2014 for 4, 7, and 11)22 and refined by least-squares against F2 (SHELXL2014).23 Solid structure of [Ta(CH2CMe3)3(NtBu)] showed two crystallographically independent molecules per asymmetric unit. All the hydrogen atoms were positioned geometrically and refined by using a riding model. All non-hydrogen atoms were refined anisotropically. Crystals diffracted weakly, and only data collections up to θ ≈ 25° could be used for crystals of 5, 7, 11, and [Ta(CH2CMe3)3(NtBu)]. Crystals of 4, 7, and 11 crystallized with a huge number of toluene solvent molecules, but it was not possible to get sensible chemical models for them. The Squeeze24 procedure of the PLATON package was employed to remove the contribution of that electronic density to the structure factors, obtaining solvent accessible volumes in the range 21−34% of the unit cell volume. EADP constraints, to carbon atoms from the bridging allyl fragment in 5, and SIMU restraints, to the pyridine carbon atoms, were used to suppress the alerts for large displacement parameter in checkcif.

Figure 1. Molecular structure of 1. Thermal ellipsoids are at 50% probability. Selected lengths (Å) and angles (deg): Nb1···Nb1a 4.145(1), Nb1−Cl1 2.347(1), Nb1−Cl2 2.377(1), Nb1−Cl3 2.482(1), Nb1−Cl3a 2.816(1), Nb1−N1 2.286(3), Nb1−N2 1.729(4), Nb1−N2−C21 176.0(4), N2−Nb1−Cl3a 172.4(1).

Å); all of them are substantially longer than the corresponding values involving the terminal chlorine atoms (1 2.36(2) Å, 4 2.35(2) Å). These features are in accordance with the structural trans effect of the imide ligands previously mentioned. The metal−nitrogen bond lengths (1 1.729(4) Å, 4 1.764(8) Å) along with the metal−nitrogen−carbon angles (1 176.0(4)°, 4 177.7(7)°) of the imido ligands are consistent with a metal− nitrogen triple bond. Despite the change in the imido substituent or in the metal center, the overall structural features about the coordination sphere of 1 and 4 are quite similar (Figure S1). Additionally, the geometrical parameters within the [M2(μ-Cl)2} unit of 1 and 4 are analogous to those found in known dinuclear imido group 5 species.25−27 Compounds 1 and 2 are discretely soluble in the usual solvents such as benzene, toluene, or chloroform and scarcely soluble in hexane, while otherwise compounds 3 and 4 are hardly soluble in the mentioned solvents. In order to move forward with our study, we next investigated the reactivity of the soluble complexes 1 and 2 with several alkylating reagents in the corresponding metathesis reactions. Thus, we could be able to obtain a series of suitable dinuclear substrates that would provide valuable insights about cooperative effects between adjacent metal centers. Treatment of 1 or 2 with 4 equiv of LiR (R = CH2SiMe3, CH2CMe3, CH2CMe2Ph) and MgClR (R = Me, C3H5) at room temperature in toluene allowed us to detect by NMR spectroscopy and/or isolate the mononuclear trialkyl complexes [MR3(NtBu)] (R = Me, CH2Ph, CH2CMe3, CH2CMe2Ph, CH2SiMe3), previously reported,28 together with the starting materials 1 and 2. X-ray diffraction analysis of complex [Ta(CH2CMe3)3(NtBu)] allowed us to confirm the structural formulation of these species (see the Supporting Information for full details). In contrast, the reaction of 1 with allylmagnesium chloride does not break the double chloro bridge, and it does reduce the metal centers and strengthen the dinuclear system by a metal− metal bond, as it is outlined in Scheme 2. The treatment of [NbCl2(μ-Cl)(NtBu)py]2 (1) with allylmagnesium chloride in hexane at room temperature in 1:4 ratio resulted in the formation of the reduced niobium species [Nb2(μ-C3H5)(η3C3H5)(μ-Cl)2(NtBu)2py2] (5) in 64% yield, while 2 afforded a mixture likely due to the dinuclear imido [Ta(C3H5)2(μCl)(NtBu)py]2 and the corresponding complex analogous to 5,

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RESULTS AND DISCUSSION Thermal treatment of [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta) under vacuum at 140 °C (R = Xyl) or 170 °C (R = tBu) afforded the dinuclear species [MCl2(μ-Cl)(NR)py]2 (R = tBu, Xyl; M = Nb 1, 3; Ta 2, 4) as barely soluble solids in high yields (80−90%) (Scheme 1). The 1H NMR spectrum of complexes Scheme 1. Formation of [MCl2(μ-Cl)(NR)py]2 (1−4)

1−4 in CDCl3 displayed a singlet resonance at δ 1.41 (1) and 1.30 (2) corresponding to the tBu groups and at δ 2.66 (3) and 2.64 (4) due to the methyl groups of the arylimido moiety. Additionally, one set of resonances assignable to one pyridine molecule per metal was observed based on the 1:1 molar ratio of the tBu or Xyl groups and the pyridine ligand. These spectroscopic data are in agreement with the dinuclear nature of 1−4, established by the single crystal X-ray diffraction studies of 1 and 4. Moreover, when samples of pure compounds 1−4 were dissolved in pyridine at room temperature, they underwent conversion to the mononuclear species [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta). The solid-state structure of complex 1 is depicted in Figure 1 along with a selection of interatomic distances and angles, while the analogous molecular structure of 4 can be found in the Supporting Information. All of them exhibit a dinuclear structure with two [MCl2(NR)py] units bridged by two chlorine atoms. The geometry about each metal center exhibits significant deviation from the ideal octahedral geometry, with angles metal−terminal ligands in the range 77.2(1)−98.6(1)° in 1 (77.5(1)−98.3(3) in 4). The central [M2(μ-Cl)2] cores possess two M−Cl bond distances (1 2.816(1) Å, 4 2.728(2) Å) clearly longer than the other two (1 2.482(1) Å, 4 2.451(2) 11683


Article

Inorganic Chemistry

The distances C2−C3 (1.51(2) Å) and C1−C2 (1.24(2) Å) are typical of single bond in the first case, and in agreement with the existence of a double bond in the second.29 Moreover, the distance Nb2−C3 (2.17(1) Å) corresponds to an usual single bond, while Nb1−C1 (2.28(1) Å) and Nb1−C2 (2.418(1) Å) are in the range of niobium(III)−olefin complexes.30,31 The distance between the two niobium atoms (3.214(1) Å) in 5 is longer than those of other chloridobridged dinuclear complexes containing a Nb−Nb single bond,32−37 but it is considerably shorter than those found for similar chlorido-bridged dinuclear niobium(V) derivatives (4.096−4.161 Å)25,27 confirming that, together with the diamagnetic behavior of 5 in solution, there is an interaction between both niobium atoms, that could be interpreted as a dative NbV←:NbIII better than a NbIV−NbIV covalent single bond. In view of these results and the possibility to study the reduction of these systems, we focused our investigation on the reactions of the chlorido-bridged dinuclear species [MCl2(μCl)(NtBu)py]2 (M = Nb 1, Ta 2) toward reductants. For our purposes, we first chose sodium amalgam Na/Hg and Mg as reducing reagents, but although 1 and 2 reacted with them, an intractable mixture was obtained and no further isolation and characterization was possible. In order to use weaker reductants, we decided to use the organosilicon reagent 1,4bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene due to its success in reducing high-valent early transition metal complexes.38,39 Thus, treatment of dichloromethane solutions of 1 or 2 with two equivalents of the reducing reagent, at room temperature, proceeds to give the metal(V) imido complexes [M2Cl2(μ-Cl)2(NtBu)2py2]2(μ-NC4H4N)2 (M = Nb 6, Ta 7), as blue and blue-purple solids, respectively, in high yields along with the formation of Me3SiCl (Scheme 3). These compounds

Scheme 2. Reaction of 1 with Allylmagnesium Chloride

[Ta2(μ-C3H5)(η3-C3H5)(μ-Cl)2(NtBu)2py2]. Attempts to obtain the individual compounds in pure form by separation of the mixture were unsuccessful. The 1H NMR spectrum of complex 5 showed two singlet resonances at δ 1.40 and 1.14 due to two NtBu groups, two sets of inequivalent allyl substituents each bearing diastereotopic methylene groups, and two sets of signals assignable to two pyridine molecules. After slow crystallization from toluene at −20 °C, dark red crystals of 5 suitable for X-ray diffraction study were isolated. The asymmetric structure of this complex is shown in Figure 2

Figure 2. Molecular structure of 5. Thermal ellipsoids are at 50% probability. Selected lengths (Å) and angles (deg): Nb1−Nb2 3.214(1), Nb1−Cl1 2.653(3), Nb1−Cl2 2.482(3), Nb1−N1 1.758(8), Nb1−N3 2.316(9), Nb1−N4 2.298(9), Nb1−C1 2.28(1), Nb1−C2 2.418(1), Nb2−Cl1 2.800(3), Nb2−Cl2 2.524(3), Nb2−N2 1.74(1), Nb2−C2 2.534(1), Nb2−C3 2.17(1), Nb2−C4 2.38(1), Nb2−C5 2.36(1), Nb2−C6 2.43(1), N1−C7 1.45(1), N2−C11 1.46(2), C1−C2 1.24(2), C2−C3 1.51(2), C4−C5 1.37(2), C5−C6 1.33(2), Cl2−Nb1−Cl1 78.7(1), N1−Nb1−Cl1 168.6(3), Cl2−Nb2− Cl1 75.3(1), N2−Nb2−Cl1 167.6(3), Nb1−Cl1−Nb2 72.2(1), Nb1− Cl2−Nb2 79.9(1), C7−N1−Nb1 168.8(8), C11−N2−Nb2 177(1), C1−C2−C3 133(1), C6−C5−C4 130(2).

Scheme 3. Reaction of Complexes 1 and 2 with 1,4Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene

together with a selection of distances and angles. The niobium atoms are bridged by two chlorine atoms and one CH2CHCH2 fragment. The Nb2 shows a distorted trigonal bipyramidal environment and completes the coordination sphere with one π allyl fragment and one tert-butylimido ligand, while Nb1 is bonded to two pyridine molecules and one tert-butylimido ligand in a distorted octahedral geometry. Such an asymmetrical structure is in good accordance with its 1H NMR spectrum. The near-linearity of the NbN-tBu systems suggest that the imido ligands act as a four-electron donor which is also reflected in the short Nb1−N1 and Nb2−N2 bond lengths (av 1.749(9) Å). The chlorine bridges are particularly unsymmetrical with distances Nb−Cl(1) (2.653(3), 2.800(3) Å) significantly longer than Nb−Cl(2) (2.482(3), 2.524(3) A), as a consequence of the structural trans effect exhibited by the tertbutylimido ligands over Cl1.

are soluble in aromatic (toluene and benzene), and chlorinated solvents (chloroform and dichloromethane), but show scarce solubility in hydrocarbon solvents (hexane or pentane). The molecular structures of 6 and 7 were unequivocally determined by a single-crystal X-ray diffraction study carried out for crystals of compound 7. Crystals of 7 proved to be useful for an X-ray structure analysis and revealed the tetranuclear structure of this complex, as shown in Figure 3, together with a selection of interatomic distances and angles. As it can be seen, two units [{TaCl(μCl)(NtBu)py}2] are linked by two 1,4-diaza-2,5-cyclohexadiene moieties. Each tantalum atom exhibits a pseudooctahedral environment comprising three chlorine atoms, a pyridine molecule, a tert-butylimido ligand, and a 1,4-diaza-2,5-cyclohexadiene fragment. It is important to note that [{TaCl(μCl)(NR)py}2] fragments keep the same structural parameters as those found for compound 4 (see Figure S1). 11684


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Scheme 4. Hydrolysis Reactions of the Imido Complexes

Although no X-ray structures could be obtained, it can be assumed that in compounds 8 and 9 the metal centers are in a distorted-octahedral disposition analogous to the structure reported for [{Nb(η5-C5H4SiMe3)Cl2}2(μ-O)(μ-Cl)2]44 or for [Ta(μ-Cl)(NMe2)3]2(μ-O).45 Such structural formulation with a pair of bridging chlorine atoms seems likely, and each metal center would achieve 16 valence electrons, in a similar fashion to that in the starting materials 1 and 2. In order to support the dinuclear nature of 8 and 9 and gain further insights into their structure, the reaction with pyridine was carried out (see Scheme 4). This reaction led us to prepare dimetallic oxo compounds, in contrast to the starting materials 1−4, where the dinuclear system did not remain intact in pyridine. A preliminary reaction monitored by 1H NMR spectroscopy revealed, after addition of two equivalents of pyridine to CDCl3 solutions of 8 or 9, the presence of a new single resonance for the tert-butylimido ligands. However, the low solubility of the products did not allow us to assign unambiguously the ratio tert-butylimido/pyridine signals in the new compounds 10 and 11. Fortunately, it was possible to isolate a few single crystals from the solution of the reaction. The X-ray analysis at 200 K revealed the molecular structure of the μ-oxo complexes [MCl2(NtBu)py2]2(μ-O) (M = Nb 10, Ta 11). However, the easy loss of pyridine in 10 or 11 leading to formation of compounds 8 and 9, respectively, precludes a preparative scale synthesis (see Scheme 4). The solid-state structure of complex 11 is depicted in Figure 4 along with a selection of interatomic distances and angles,

Figure 3. Molecular structure of 7. Thermal ellipsoids are at 50% probability. Selected lengths (Å) and angles (deg): Ta1···Ta2 4.099(2), Ta1−Cl1 2.400(4), Ta2−Cl2 2.394(4), Ta2−Cl12 2.513(4), Ta1−Cl12 2.725(4), Ta1−Cl21 2.497(4), Ta2−Cl21 2.727(4), Ta1−N1 1.73(1), Ta2−N2 1.76(1), Ta1−N3 2.02(1), Ta1−N11 2.32(1), Ta2−N21 2.32(1), Ta2−N4a 2.03(1), C31−C32 1.37(2), C31−N3 1.40(2), C32−N4 1.38(2), C33−C34 1.33(2), C33−N4 1.40(2), C34−N3 1.41(2), Ta1−N1−C1 177(1), C21− N2−Ta2 177(1), N1−Ta1−Cl12 167.9(4), N2−Ta2−Cl21 168.1(4).

The bond distances Ta−N(diazacyclohexadiene) (av 2.025(9) Å) are in the normal range observed for Ta−NR2 bonds,40−42 and, unlike what happens in coordinated pyridine, the C−C bonds are shorter than the N−C bonds in the diazacyclohexadiene ring. Based on the [{TaCl(μ-Cl)(NR)py}2] parameters and the coordination of diazacyclohexadiene to the tantalum centers, it can be concluded that the organosilicon reductant does not allow the reduction products of complexes 1 and 2 to be obtained. These results are in agreement with those found for Mashima and co-workers in the reaction of [NbCl 3(NtBu)py2] and the nonmethylated pyrazine-based reductant.39 The 1H and 13C NMR spectra in CDCl3 of 6 and 7 reveal the equivalence of the tert-butylimido ligands, with resonances quite similar to those found for the starting materials 1 and 2. Additionally, the resonances of the 1,4-diaza-2,5-cyclohexadiene fragments and the corresponding distinctive signals of the pyridine molecules could be assigned by 1H−13C gHSQC experiments. The hydrolysis of [MCl2(μ-Cl)(NtBu)py]2 (M = Nb 1, Ta 2) was also explored. Complexes 1 and 2 were found to react with D2O (1 equiv in toluene) to yield the oxo derivatives [MCl(μ-Cl)(NtBu)py]2(μ-O) (M = Nb 8, Ta 9) as white solids in good yields (90−95%) (see Scheme 4). These compounds are insoluble in hydrocarbon (toluene, hexane, benzene) and scarcely soluble in chlorinated solvents (chloroform or dichloromethane). The 1H NMR spectra in CDCl3 revealed one singlet resonance for the tert-butylimido ligands and one set of signals for the pyridine molecules in 1:1 ratio. These compounds were also characterized by infrared spectroscopy and microanalysis. The most remarkable feature of the IR spectra is the bands between 745 and 890 cm−1 in the spectral region of the M−O−M stretching vibrations.43

Figure 4. Molecular structure of 11. Thermal ellipsoids are at 50% probability. Selected bond lengths (Å) and angles (deg): Ta1−Cl1 2.396(5), Ta1−Cl2 2.448(5), Ta2−Cl3 2.442(5), Ta2−Cl4 2.400(5), Ta1−N11 2.46(2), Ta1−N12 2.26(2), Ta1−N13 1.76(2), Ta2−N21 2.43(2), Ta2−N22 2.32(2), Ta2−N23 1.76(2), Ta1−O1 1.91(1), Ta2−O1 1.93(1), Ta1−N13−C11 171(2), Ta2−N23−C31 173(2), Ta1−O1−Ta2 172.0(7). 11685


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while the molecular structure of 10 is included in the Supporting Information (Figure S2). The molecular structures of both compounds reveal two [MCl2(tBuN)py2] units rotated 90° with respect to each other and linked by a bridging oxo ligand. Each tantalum atom shows a distorted octahedral environment being coordinated to two terminal chlorine atoms, a pair of pyridine molecules, a tert-butylimido ligand, and a bridging oxo ligand. The coordination of the new pyridine molecules in 10 and 11 compensates electronically the loss of the two bridging chlorine atoms in 8 and 9. On the other hand, the central Ta−O−Ta angle of 172.0(7)° is nearly identical to that found for [Ta 2 Cl 2 (NMe 2 ) 6 (NHMe 2 ) 2 ](μ-O) 4 6 (174.3(3)°) or [{Ta(NMe2)(py)Cl3}2(μ-O)]47 (176.0(5)°) and, logically, much bigger than that found by Xue and cowokers for [Ta(μ-Cl)(NMe2)3]2(μ-O) (120.3(3)°).45

CONCLUSIONS We have developed a synthetic protocol to obtain a series of dinuclear imido species [MCl2(μ-Cl)(NR)py]2 (R = tBu, Xyl; M = Nb, Ta) from the mononuclear compounds [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta), reflecting the strong trans influence of the imido ligand over pyridine exchange. The rupture of the dinuclear system hampered the synthesis of the corresponding alkylated compounds, while the presence of unsaturated ligands, namely, allyl or diazacyclohexadiene, was able to preserve the dinuclear nature of such species. Furthermore, the controlled hydrolysis of the starting compounds yielded the dimetallic oxo complexes [MCl(μCl)(NtBu)py]2(μ-O) (M = Nb, Ta), which retain the pseudooctahedral geometry around each metal center. ASSOCIATED CONTENT

S Supporting Information *

The The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01652. Experimental data for the X-ray diffraction studies, ORTEP drawings and selected distances and angles, and NMR spectra (PDF) Accession Codes

CCDC 1558008−1558014 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cristina Santamaría: 0000-0003-2410-961X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work was provided by the Ministerio de Economiá y Competitividad (CTQ2013-44625-R) and the Universidad de Alcalá (CCG2016/EXP-009). C.H.-P. thanks the Universidad de Alcalá for a predoctoral fellowship. 11686


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