Regioselective Ring Opening Reactions of Pyridine N-Oxide

Sep 1, 2017 - Zhang, Waddell, Tiedemann, Padilla, Mei, Chen, and Carrow. 2018 140 (28), pp 8841–8850. Abstract: Group 10 metal catalysts have shown ...
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Regioselective Ring Opening Reactions of Pyridine N‑Oxide Analogues by Magnesium Hydride Complexes Hongyan Xie,†,‡ Xinli Liu,† and Dongmei Cui*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China S Supporting Information *

ABSTRACT: The stoichiometric reactions of phosphinimino-amino (PIA)-supported magnesium hydride complex 1, [L1MgH]2 (L1 = (2,6-iPr2-C6H3)NC(Me)CHP(Cy2)N(2,6-Me2-C6H3)), with pyridine N-oxide and 2-phenylpyridine N-oxide afforded 2,4-pentadiene-1-oximate complex 2 and 5-phenyl-2,4-pentadiene-1-oximate complex 3, respectively. The reaction of 1 with 2-methylpyridine N-oxide showed a unique regioselectivity to produce 2,4-hexadiene-1-oximate 4a in toluene and 3,5hexadiene-2-oximate 4b in THF, respectively. Treatment of β-diketiminato (BDI)-supported magnesium hydride complex 5, [L2MgH]2 (L2 = (2,6-iPr2-C6H3)NC(Me)CHC(Me)N(2,6-iPr2-C6H3)), with quinoline N-oxide gave 1,2-dihydroquinoline type product 6, while treatment of complex 5 with 2-methylpyridine N-oxide either in toluene or THF afforded 1-methyl-2,4pentadiene-1-oximate complex 7 as the only product. All these complexes were fully characterized by NMR spectroscopy and Xray diffraction analyses, and mechanism researches were conducted to understand the ring-opening reaction of pyridine N-oxide.

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complexes.21−25 It is noteworthy that regio-specificity of 1,4addition process was observed for the α,β-unsaturated ester reduction reaction.24 The 1,2- and 1,4-regioselective addition processes have been observed depending on the different substituted groups on pyridine in the reduction reactions of pyridine by magnesium hydride complexes.22 Recently, an interesting single-electron transfer reaction of BDI-supported magnesium hydride complexes with 2,2,6,6-tetramethyl-1piperidinyloxy (TEMPO) compounds has been observed.26 As a structural analogue of TEMPO (considering a similar electron-withdrawing behavior of O atom on the N−O bond), pyridine N-oxide and its derivatives, which are widely used in organic synthesis,27 have seldom been employed to react with organometallic complexes. The only precedent revealed that Th and U alkyl complexes promoted the ortho sp2 C−H bond activation of pyridine N-oxide.28,29 We have disclosed the synthesis of phosphinimino-amino (PIA)-supported magnesium hydride complex 1.30 In this contribution, we want to investigate the correlative reactions of

INTRODUCTION Recently, organo-magnesium hydride complexes, especially LMgH type, have attracted great attention for their better thermal stability and solubility in organic solvents than its precursor MgH2.1−5 To date, ancillary ligands, such as βdiketiminato (BDI), 6 − 8 tris(pyrazolyl)methanide, 9 Ph2PCH2PPh2(flu) (flu = fluorene),10 and tetraazacyclododecane,11,12 have been employed for the preparation of LMgH type magnesium hydride complexes, while magnesium hydrides bearing other ligands tend to form polyhydride aggregates13−17 because of the nature of highly polarized magnesium hydrogen bonds. LMgH type magnesium hydride complexes exhibit abundant and unique reactivities with small molecules. For instance, the unsaturated bonds in azide, azobenzene, carbodiimide, and carbon dioxide compounds can be easily reduced by magnesium hydride complexes.7,8,11,12 In particular, magnesium hydride complexes have also been demonstrated to be effective catalysts for the hydrosilylation and hydroboration reactions of imine, aldehyde, and ketone compounds.18−20 In some cases, the conjugated CC bonds of the α,β-unsaturated esters or even pyridine rings can be reduced to enolate or unconjugated amido-magnesium species by magnesium hydride © XXXX American Chemical Society

Received: July 10, 2017

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DOI: 10.1021/acs.organomet.7b00517 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of Compounds 2 and 3

π,π-conjugated structure. The Mg−O bond lengths (1.999(3) and 2.012(4) Å) fall in the reasonable range of those reported in the literature.31 The reaction of 1 with a phenyl-substituted pyridine N-oxide under the same conditions afforded pale yellow crystals of complex 3 (Scheme 1). In the 1H NMR of 3 (Figure S7), the doublet resonance at δ 7.64 ppm (3JH−H = 9.6 Hz) indicates the existence of one proton adjacent to the N atom of the oximate group, which comes from the magnesium hydride species. The molecular structure of complex 3 was further characterized by single-crystal X-ray diffraction as depicted in Figure 1b, which adopts a triclinic space group P1̅ analogous to that of complex 2. The phenyl group is located in the ε position of the newly generated oximate group, indicating that the ring-opening takes place at the NC bond. Regioselective Reaction of 1 with 2-Methyl-Pyridine N-Oxide. A toluene suspension of complex 1 was treated with 1 equiv of 2-methylpyridine N-oxide at room temperature for 5 h. The reaction mixture became clear, and corresponding oximate complex 4a was isolated as a pale yellow crystalline solid (68%) (Scheme 2). The 1H NMR spectrum (Figure S9) shows a doublet at δ 7.46 ppm (3JH−H = 10.0 Hz) attributed to the proton adjacent to the oximate N atom as in complex 2, suggesting the ring-opening of the pyridine N-oxide. The doublet resonance at δ 1.86 ppm (3JH−H = 6.6 Hz) is assigned to the methyl protons CH3 neighboring to a methine group, indicating that the ring-opening of the pyridine N-oxide cleaves the NC bond adjacent to the methyl substituent. The alkenyl protons in the oximate segments are observed at δ 5.83 (βproton), 6.48 (γ-proton), 5.99 (δ-proton), and 5.73 (ε-proton) ppm. Therefore, complex 4a is a 1,2-addition product of Mg−H on the pyridine N-oxide. The single crystal structural analysis confirmed that complex 4a has the same monoclinic space group P2(1)/c, adopting a dimeric structure as shown in Figure 2. The methyl group is located at the terminal carbon of the 2,4-hexadiene-1-oximate. When the reaction of 1 and 2-methylpyridine N-oxide was performed in THF, suprisingly, a 1,6-addition product of complex 4b was obtained as a white powder, which gave an NMR spectrum (Figure S12) different from that of 1,2-addition product 4a. The chemical shifts of the alkenyl protons are close to those in 2, except that there is no resonance at approximate δ 7.46−7.47 ppm indicating no proton is located on the carbon atom adjacent to the oximate N atom while the signal for the methyl protons appears at δ 1.62 ppm as a singlet peak without splitting; this means that there are two protons located on the ε-position carbon atom and methyl group connects with the oximate N atom. Alternatively, the ring-opening of methyl-

pyridine N-oxide with magnesium hydride complex 1. The ringopening reactions of pyridine N-oxide analogues catalyzed by PIA-supported magnesium hydride 1 afford the corresponding magnesium-based oximate complexes. In particular, novel regioselectivity of the reaction between 2-methylpyridine N-oxide and 1 in different solvents is first observed. Further researches provide a reasonable explanation for the mechanistic pathway and the regio-selectivity.

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RESULTS AND DISCUSSION Reactivity of 1 with Pyridine N-Oxide. The synthesis of phosphinimino-amino magnesium hydride complex 1 supported by the anionic PIA ligand ((2,6-iPr2-C6H3)NC(Me)CHP(Cy2)N(2,6-Me2-C6H3))− was according to our previous work.30 Treatment of 1 with pyridine N-oxide in toluene or THF at room temperature for 5 h afforded corresponding 2,4pentadiene-1-oximate magnesium complex 2 (Scheme 1) in a medium yield (65%). Complex 2 was soluble in THF and chloroform but sparingly soluble in benzene. In the 1H NMR spectrum of complex 2 (Figure S5), the proton adjacent to N resonates at δ 7.47 ppm (3JH−H = 8.0 Hz), while the other alkene moieties of the newly formed oximate are observed as multiplets at 6.04 (β- and γ-protons), 6.70 (δ-proton) and 5.21 (ε-protons) ppm (Chart 1). The methine proton CH-PCy2 in Chart 1. α, β, γ, δ, and ε Positions on the Oximate Group

the PIA ligand gives a doublet resonance at δ 2.93 ppm (2JP−H = 19.7 Hz) due to coupling with phosphorus atom. In the 31P NMR spectrum, the oximate complex shows a singlet resonance at δ 35.39 ppm (Figure S6), suggesting the symmetric structure of complex 2. The single crystals of 2 were obtained from a mixture of toluene and THF solution at −30 °C. X-ray diffraction analysis revealed that complex 2 (Figure 1a) adopts a monoclinic space group P2(1)/c with a dimeric structure where the two magnesium ions are bridged by two μ2-O atoms of the oximates. Each magnesium ion is coordinated by a PIA ligand via an N,N-bidentate chelating mode, adopting a twisted tetrahedral geometry with the two N and two O atoms occupying the apexes and the magnesium ion siting in tetrahedron center. All the atoms of the newly formed 2,4pentadiene-1-oximate are coplanar, consistent with the highly B

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Figure 1. Molecular structures: (a) complex 2; (b) complex 3. Hydrogen atoms had been omitted for clarity (thermal ellipsoids at the 35% probability level). Selected bond lengths (Å) and angles (deg) for 2 and [3]: Mg1−N2 2.064(4) [2.036(3)], Mg1−N1 2.094(4) [2.069(3)], Mg1··· Mg1i 3.146(3) [3.158(2)], Mg1−O1 1.999(3) [2.025(3)], Mg1−O1i 2.012(4) [1.987(2)], O1−N3 1.383(5) [1.388(4)], N2−P1 1.614(4) [1.636(3)], N2−Mg1−N1 97.80(17) [100.19(11)], O1−Mg1−O1i 76.65(16) [76.19(11)], Mg1−O1−Mg1i 103.35(16) [103.81(11)]. Symmetry transformations used to generate equivalent atoms: (′) −x+1, −y+1, and −z+1 for 2 and (′) −x+1, −y+1, and −z for 3.

Scheme 2. Preparation of Complexes 4a and 4b

Figure 2. Molecular structure of complex 4a (thermal ellipsoids at the 35% probability level). Hydrogen atoms had been omitted for clarity. Selected bond lengths (Å) and angles (deg): Mg1−N2 2.049(4), Mg1−N1 2.092(4), Mg1···Mg1i 3.194(3), Mg1−O1 1.994(4), Mg1−O1i 2.027(4), O1−N3 1.356(5), N2−P1 1.629(4), N2−Mg1−N1 97.25(16), O1−Mg1−O1i 74.79(17), Mg1−O1−Mg1i 105.21(17). Symmetry transformations used to generate equivalent atoms: (′) −x+2, −y, −z+2.

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Organometallics Scheme 3. Preparation of Complexes 6 and 7

Figure 3. Molecular structure of complex 6 (thermal ellipsoids at the 35% probability level). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Mg1−N2 2.058(3), Mg1−N1 2.079(3), Mg1−N3 2.451(3), Mg1−O1 1.915(2), Mg1−O(2) 1.970(2), O1−N3 1.435(4), O2−N4 1.330(3), N3−C30 1.460(4), N2−Mg1−N1 91.96(11), O1−Mg1−O2 110.59(11), O1−Mg1−N3 35.78(10).

substituted pyridine N-oxide takes place via NC bond that is away from the methyl group, in contrast to those in complexes 3 and 4a. In correspondence, the 31P NMR spectrum (Figure S13) shows that the singlet resonance at δ 36.23 ppm shifts downfield compared with that of 4a (δ 35.24 ppm, Figure S10). Although we tried several times to achieve suitable single crystals of 4b, only powder or polycrystalline samples were obtained. Nevertheless, based on the above NMR spectrum analysis, it is reasonable to suppose that the newly formed oximate is 3,5-hexadiene-2-oximate. Mechanistic Insights into the Regioselective RingOpening Reactions. To recognize the detailed mechanism of the reaction between LMgH and pyridine N-oxide derivatives, isolation of the reaction intermediate is a crucial step. However, the intermediate of the reaction between 1 and pyridine Noxide is too labile to be isolated. Thereby, quinoline N-oxide was selected as its phenyl ring may stabilize the corresponding intermediate. When magnesium hydride complex 5 [BDIMgH]2 (BDI = (2,6-iPr2-C6H3)NC(Me)CHC(Me)N(2,6-iPr2C6H3))6,32 was reacted with 2 equiv of quinoline N-oxide, 1,2dihydroquinoline complex 6 was obtained as brown powder in

a high yield (90%) (Scheme 3). X-ray diffraction analysis revealed that complex 6 is monomeric (Figure 3), where the Mg2+ ion is coordinated by a 1,2-dihydrolized quinoline Noxide moiety via the μ2 -O:κ1-N mode and by a neutral quinoline N-oxide via theκ1-O mode as well as by the bidentate κ1-N:κ1-N BDI ligand. The N(3)−C(30) bond length is 1.460(4) Å, much longer than the N(4)−C(39) double bond length (1.333(4) Å), indicating the CN double bond is reduced by the magnesium hydride. This is consistent with the 1 H NMR spectrum of complex 6 (Figure S14), where the resonance of Mg-H disappears and the newly formed sp3 CH2 resonates at δ 3.61 ppm. Complex 6 provides a reference for elucidating the mechanistic pathway for the formation of the oximate complexes. The 1,2-insertion of the NC bond into the Mg−H bond might take place prior to the ring-opening, and the intermediate then isomerizes under the driving force of the potentially greater electron delocalization to form oximate.33,34 Then, reaction of complex 5 with 2-methylpyridine N-oxide in toluene or THF gave the same product, 3,5-hexadiene-2oximate complex 7 (Scheme 3), as proved by their similar 1H D

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Figure 4. Variable-temperature 31P NMR of hydride complex 1 in THF-d8 in the range of 253−313 K.

intermediate elucidate the mechanistic pathway: First, the NC bond in the pyridine ring is inserted into Mg−H bond to form dihydropyridine intermediate; then, ring-opening reaction of the intermediate proceeds to afford oximate product with greater delocalization of electron. The steric hindrance change of 1 in different solvents, which brings different coordinating and/or inserting mode of 2-methylpyridine N-oxide, is supposed to play a crucial role for the regio-selectivity.

NMR spectra (Figures S15 and S16). This result indicated that the steric bulkiness between employed complexes 1 and 5 rather than the reaction media influenced the product structure and alternatively the regioselectivity. Therefore, complex 1 was dissolved in THF-d8, and we monitored the structural variation under various temperatures by NMR spectrum technique (Figure 4 for 31P NMR and Figure S17 for 1H NMR). 31P NMR showed two singlet resonances at 33.2 (major) and 34.4 (minor) ppm, which can be ascribed to the dimeric complex 1 (major) and its THF-solvated monomeric counterpart 1·THF (minor), respectively, indicating partial disaggregation of 1 in THF (Scheme 4). As expected, the amount of 1·THF increases

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EXPERIMENTAL SECTION

General Methods. All manipulations were performed under a nitrogen atmosphere using standard Schlenk techniques or an MBraun glovebox. All solvents were purified from an MBraun SPS system. Samples of magnesium complexes for NMR spectroscopic measurements were prepared in the glovebox by use of NMR tubes sealed by paraffin film. 1H, 31P, and 13C NMR spectra were recorded on a Bruker AV400 (FT, 400 MHz for 1H; 100 MHz for 13C; 162 MHz for 31P) spectrometer. Elemental analysis was performed at National Analytical Research Centre of Changchun Institute of Applied Chemistry (CIAC). BDI magnesium hydride [L2MgH]2 (5) was synthesized according to the literature.6,32 Phenylsilane was dried over CaH2 under stirring for 24 h and distilled under reduced pressure before use. Pyridine N-oxide was purchased from Sigma-Aldrich and used as received. 2-Methylpyridine N-oxide, 2-phenylpyridine N-oxide, and quinoline N-oxide were synthesized according to the literature.35−37 The synthesis of ligand HL was according to our previous report.30,38 Single Crystal X-ray Diffraction Studies. Crystals for X-ray analysis were obtained as described in the preparations. The crystals were manipulated in a glovebox. Data collections were performed at −88.5 °C on a Bruker SMART APEX diffractometer with a CCD area detector, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The determination of crystal class and unit cell parameters was carried out by the SMART program package.39 The raw frame data were processed using SAINT and SADABS to yield the reflection data file.40 The structures were solved by using the SHELXTL program.41 Refinement was performed on F2 anisotropically for all non-hydrogen atoms by the full-matrix least-squares method. The hydrogen atoms were placed at calculated positions and were included in the structure calculations without further refinement of the parameters. Crystallographic data can also be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif using accession codes CCDC-1561357 (2), 1561358 (3), 1561359 (4a), and 1561360 (6).

Scheme 4. Chemical Equilibrium of Complex 1 and Its THF Adduct 1·THF in THF

with the temperature rising from 253 to 313 K, indicating the presence of chemical equilibrium between the dimeric and monomeric structures. When THF solvent was used as the reaction media, less sterically hindered complex 1·THF has a greater ability to incoporate a pyridine N-oxide molecule than bulky dimeric 1, thereby allowing it to form, via different coordination and insertion mode, larger ring-opening product 4b after isomerization (Scheme 5).

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CONCLUSION We have demonstrated the reactions of magnesium hydride complexes with pyridine N-oxide analogues to afford the corresponding oximate complexes. Strikingly, the reaction of complex 1 with 2-methylpyridine N-oxide shows a unique regio-selectivity in different solvents. The isolation of E

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Scheme 5. Possible Mechanism for the Regioselectivity of the Reaction of 1 and 2-Methyl-pyridine N-Oxide in Toluene and THF

reaction mixture in benzene without stir. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.21−7.16 (m, 4H, m-C6H3iPr2), 7.10−7.08 (m, 2H, pC6H3iPr2), 6.95−6,87 (m, 6H, C6H3Me2), 3.84 (s, 2H, MgH2Mg), 3.51 (sept, 4H, 3JH−H = 6.9 Hz, CH(CH3)2), 3.02 (d, 2JP−H = 23.2 Hz, 2H, PCH), 2.29 (m, 4H, ipso-Cy), 2.19 (s, 6H, NCCH3), 1.55 (s, 12H, C6H3(CH3)2), 1.33 (d, 3JH−H = 6.8 Hz, 12H, CH(CH3)2), 1.06 (m, 12H, CH(CH3)2), 2.12−0.97 (m, 40H, PCy2). 31P NMR (162 MHz, C6D6, 25 °C): δ 47.76 ppm (s). Anal. Calcd for C70H106Mg2N4P2: C, 75.46; H, 9.59; N, 5.03. Found: C, 75.29; H, 9.52; N, 5.07. Synthesis of PIA-Supported Magnesium 2,4-Pentadiene-1oximate (2). To a toluene suspension (5 mL) of hydride 1 (0.12 g, 0.11 mmol) was added dropwise pyridine N-oxide (21 mg, 0.22 mmol in 5 mL toluene) at room temperature. The mixture was stirred for 2 h at room temperature to afford a clear brown solution. Filtration, concentration of solvent to 0.5 mL, and cooling to −30 °C for 2 days gave colorless crystals suitable for X-ray analysis (91 mg, 65%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.48 (d, 2H, 3JHH = 8.0 Hz, ONCHCH), 7.11 (t, 2H, 3JHH = 7.4 Hz, p-C6H3iPr2), 7.11 (d, 4H, 3JHH = 7.4 Hz, m-C6H3iPr2), 6.70 (m, 2H, CHCHCH2), 6.56 (m, 6H, C6H3Me2), 6.04 (m, 4H, ONCH(CH)2CHCH2), 5.21 (m, 4H, CHCHCH2), 3.25 (sept, 4H, 3JH−H = 6.8 Hz, CH(CH3)2), 2.93 (d, 2 JP−H = 19.7 Hz, 2H, PCH), 2.12 (m, 4H, ipso-Cy), 1.71 (s, 12H, C6H3(CH3)2), 1.51 (s, 6H, NCCH3), 1.06 (d, 3JH−H = 6.7 Hz, 24H, CH(CH3)2), 1.85−0.82 (m, 40H, PCy2). 13C NMR (100 MHz, CDCl3, 25 °C) δ 171.34, 149.66, 148.17, 146.09, 144.85, 135.56, 135.51, 133.02, 130.72, 128.22, 128.20, 127.98, 127.76, 126.12, 124.00, 123.46, 123.31, 123.27, 121.38, 117.90, 77.36, 39.27, 38.69, 31.75, 28.19, 27.84, 27.65, 27.58, 27.46, 27.29, 27.17, 26.69, 26.52, 26.49, 25.06, 25.01, 24.93, 24.64, 24.06, 23.43, 23.30, 22.81, 19.92, 14.27. 31P NMR (162 MHz, CDCl3, 25 °C): δ 35.39 ppm (s). Anal. Calcd for C80H116Mg2N6O2P2: C, 73.66; H, 8.96; N, 6.44. Found: C, 73.99; H, 8.86; N, 6.32. Synthesis of PIA-Supported Magnesium 2,4-Pentadiene-5phenyl-1-oximate (3). To a toluene suspension (5 mL) of hydride 1 (0.111 g, 0.1 mmol) was added dropwise 2-methylpyridine N-oxide (31 mg, 0.2 mmol in 5 mL toluene) at room temperature. The mixture was stirred for 5 h at room temperature to afford a clear brown solution. Filtration, concentration of solvent to 0.5 mL, and cooling to −30 °C for 2 days gave yellow crystals suitable for X-ray analysis (0.103 g, 73%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.64 (d, J = 10.4 Hz, 1H, ONCHCH), 7.51 (d, J = 7.4 Hz, 2H, Ph-H), 7.35 (t, J = 7.7 Hz, 2H, Ph-H), 7.15 (m, 2H, Ph-H and ONCHCHCHCHCHPh), 7.06 (m, 2H, Ph-H), 6.58 (m, 4H, Ph-H and O N C H C H C H C H C H P h ) , 6 . 2 3 ( t , J = 1 1. 3 H z , 1H , ONCHCHCHCHCHPh), 6.01 (t, J = 9.4 Hz, 1H, ONCHCHCHCHCHPh), 3.31 (sept, 2H, J = 6.7 Hz, CH(CH3)2), 2.95 (d, J = 19.5 Hz, 2H, PCH), 2.13 (m, 2H, ipso-Cy), 1.73 (m, 9H, C6H3(CH3)2 and Cy-H), 1.57 (s, 3H, NCCH3), 1.55 (m, 2H, Cy-H),

Synthesis of (2,6-iPr2-C6H3)NHC(Me)CHP(Cy2)N(C6H32,6-Me2) (HL). To a THF (25 mL) solution of 5.05 g (12.2 mmol) of 2,6-iPr2-C6H3NC(Me)CH2PCy2 was dropwise added 2.15 g (14.6 mmol) of PhN3. N2 evolution commenced immediately, and the mixture was stirred at 25 °C for 24 h. Removal of THF under vacuum left a brown oily solid. The solid was washed with 30 mL of hexane, and the suspension then was stirred for 1 h at ambient temperature. Filtering, washing with 2 × 20 mL hexane, and evaporating the residual solvents afforded HL as pale yellow powder (5.72 g, 88%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.11−7.03 (m, 3H, C6H3iPr2), 6.93 (d, 2H, 3JH−H = 7.1 Hz, m-C6H3Me2), 6.58 (t, H, 3JH−H = 7.1 Hz, pC6H3Me2), 3.24 (d, 2JP−H = 13.7 Hz, 2H, PCH2), 2.72 (sept, 2H, 3JH−H = 6.6 Hz, CH(CH3)2), 2.29 (s, 6H, C6H3Me2), 1.77 (s, 3H, MeCCH2P), 1.14 (d, 12H, 3JH−H = 6.8 Hz, CH(CH3)2), 2.26−0.87 (m, 22H, PCy2). 31P NMR (162 MHz, CDCl3, 25 °C): δ 21.60 ppm (s). Anal. Calcd for C35H53N2P: C, 78.90; H, 10.03; N, 5.26. Found: C, 78.67; H, 10.07; N, 5.28. Synthesis of LMgnBu(THF). To a hexane solution (3 mL) of MgnBu2 (1.1 mL, 1 M, 1.1 mmol) was added dropwise HL (0.532 g, 1 mmol in 10 mL THF) at room temperature. The mixture was stirred for 12 h at room temperature to afford a clear pale yellow solution. Evaporation of solvent gave crystalline solids, which were washed with a small amount of hexane to remove impurities and dried in vacuo to give colorless crystals (0.555 g, 81%). Single crystals suitable for X-ray analysis were obtained from slow evaporation of the THF solution. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.19−7.02 (m, 3H, C6H3iPr2), 7.04 (d, 2H, 3JH−H = 7.4 Hz, m-C6H3Me2), 6.88 (t, 1H, 3JH−H = 7.4 Hz, pC6H3Me2), 3.67 (sept, 2H, 3JH−H = 6.8 Hz, CH(CH3)2), 3.18 (m, 4H, OCH2CH2), 3.05 (d, 2JP−H = 22.5 Hz, 1H, PCH), 2.62 (s, 6H, C6H3(CH3)2), 2.49 (m, 2H, ipso-Cy), 1.85 (s, 3H, NCCH3), 1.73− 1.64 (m, 2H, γ-nBu), 1.58−1.55 (m, 2H, β-nBu), 1.36 (d, 3JH−H = 7.1 Hz, 12H, CH(CH3)2), 1.15 (m, 4H, OCH2CH2), 1.02 (t, 3JH−H = 7.4, 3H, δ-nBu), 2.00−0.98 (m, 20H, PCy2), −0.23 (m, 2H, α-nBu). 13C NMR (100 MHz, C6D6, 25 °C): δ 171.57, 148.06, 147.60, 146.58, 144.40, 136.13, 136.08, 128.68, 128.66, 124.23, 123.84, 122.35, 122.32, 68.47, 56.76, 55.59, 40.17, 39.59, 33.88, 32.90, 28.18, 28.12, 27.94, 27.89, 27.87, 27.83, 27.41, 27.34, 27.31, 27.29, 27.14, 26.99, 26.85, 26.67, 26.19, 25.08, 24.99, 24.93, 24.83, 24.76, 24.64, 20.86, 20.72, 14.66, 9.08. 31P NMR (162 MHz, C6D6, 25 °C): δ 46.60 ppm (s). Anal. Calcd for C43H69MgN2OP: C, 75.36; H, 10.15; N, 4.09. Found: C, 75.21; H, 10.09; N, 4.12. Synthesis of Hydride [LMgH]2 (1). To a toluene solution (3 mL) of LMgnBu(THF) (0.21 g, 0.3 mmol) was added phenylsilane (0.05 g, 0.46 mmol in 2 mL toluene) at room temperature. The mixture was stirred for 12 h at room temperature to afford a suspension. Filtration and washing with hexane (3 × 5 mL) provided phosphinimino-amino magnesium hydride 1 as white powder (0.12 g, 71%). Single crystals suitable for X-ray analysis were obtained from slow evaporation of the F

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Organometallics

(t, J = 8.1 Hz, 1H, Ar−H), 6.82 (m, 2H, Ar−H), 6.72 (dd, J = 7.3, 1.5 Hz, 1H, Ar−H), 6.63 (d, J = 7.9 Hz, 1H, Ar−H), 6.50 (td, J = 7.3, 1.2 Hz, 1H, Ar−H), 6.27 (t, J = 7.1 Hz, 1H, Ar−H), 6.10 (dt, J = 9.7, 1.9 Hz, 1H, Ar−H), 5.29 (dt, J = 9.7, 3.9 Hz, 1H, Ar−H), 5.07 (s, 1H, NC(Me)CHC(Me)N), 3.61 (m, 6H, CH(CH3)2 and NCH2), 1.87 (s, 6H, NCCH3), 1.18 (m, 24H, CH(CH3)2). 13C NMR (100 MHz, C6D6) δ 168.35, 152.27, 146.09, 143.01, 136.44, 131.24, 130.18, 129.69, 129.34, 128.71, 128.57, 126.69, 125.70, 125.27, 125.07, 124.14, 124.04, 123.90, 123.71, 123.41, 122.26, 120.92, 119.85, 115.84, 111.57, 93.92, 67.84, 65.92, 55.28, 28.52, 28.40, 25.16, 25.09, 24.84, 24.62, 24.21, 23.79, 23.15, 21.44, 20.55, 15.61 ppm. Anal. Calcd for C47H56MgN4O2: C, 76.98; H, 7.70; N, 7.64. Found: C, 76.72; H, 7.65; N, 7.62. Synthesis of BDI-Supported Magnesium 3,5-Hexadiene-2oximate (7). To a toluene suspension (5 mL) of 5 (0.177 g, 0.2 mmol) was added dropwise 2-methylpyridine N-oxide (43 mg, 0.4 mmol in 8 mL toluene) at room temperature. The mixture was stirred for 5 h at room temperature to afford a clear yellow solution. Removal of toluene and washing with hexane 3 times afforded 7 as pale yellow powder (0.1958 mg, 89%). 1H NMR (400 MHz, CDCl3, 25 °C) δ 7.02 (m, 2H, C6H3iPr2), 6.96 (m, 2H, C6H3iPr2), 6.86 (m, 1H, CHCHCH2), 6.78 (m, 2H, C6H3iPr2), 6.13 (t, 1H, ONC(Me)(CH)2CHCH2), 5.23 (m, 2H, ONC(Me)CHCHCHCH2), 4.86 (s, 1H, CH), 3.27 (m, 2H, CH(CH3)2), 2.61 (m, 2H, CH(CH3)2), 2.06 (s, 3H, ONC(Me)CHCHCHCH2), 1.47 (s, 6H, NCMe), 1.27 (s, 6H, 3 JH−H = 6.9 Hz, CH(CH3)2), 1.05 (s, 6H, 3JH−H = 6.8 Hz, CH(CH3)2), 0.86 (s, 6H, 3JH−H = 6.8 Hz, CH(CH3)2), −0.16 (s, 6H, 3JH−H = 6.8 Hz, CH(CH3)2) ppm. Anal. Calcd for C70H98Mg2N6O2: C, 76.14; H, 8.95; N, 7.61. Found: C, 76.01; H, 8.93; N, 7.64.

1.45 (m, 2H, Cy-H), 1.27 (m, 6H, Cy-H), 1.05 (m, 8H, Cy-H and CH(CH3)2), 0.98 (m, 3H, Cy-H), 0.89 (m, 8H, Cy-H and CH(CH3)2). 13C NMR (100 MHz, CDCl3, 25 °C) δ 13C NMR (101 MHz, CDCl3) δ 13C NMR (101 MHz, CDCl3) δ 171.53, 150.15, 146.22, 146.13, 144.86, 137.56, 135.67, 133.33, 130.38, 128.73, 128.14, 127.79, 126.78, 126.24, 125.05, 124.06, 123.47, 121.49, 121.47, 77.36, 39.13, 38.55, 31.76, 27.77, 27.72, 27.61, 27.52, 27.41, 27.22, 27.20, 27.09, 27.06, 26.41, 25.78, 25.19, 24.58, 22.82, 19.83, 14.28. 31P NMR (162 MHz, CDCl3, 25 °C): δ 35.56 ppm (s). Anal. Calcd for C92H124Mg2N6O2P2: C, 75.86; H, 8.58; N, 5.57. Found: C, 75.70; H, 8.60; N, 5.54. Synthesis of PIA-Supported Magnesium 2,4-Hexadiene-1oximate (4a). To a toluene suspension (5 mL) of hydride 1 (0.111 g, 0.1 mmol) was added dropwise 2-methylpyridine N-oxide (21.5 mg, 0.2 mmol in 5 mL toluene) at room temperature. The mixture was stirred for 5 h at room temperature to afford a clear brown solution. Filtration, concentration of solvent to 0.5 mL, and cooling to −30 °C for 2 days gave colorless crystals suitable for X-ray analysis (90 mg, 68%). 1H NMR (400 MHz, CDCl3, 25 °C) δ 7.46 (d, 2H, 3JHH = 10.0 Hz, ONCHCH), 7.12 (t, 2H, 3JHH = 7.1 Hz, p-C6H3iPr2), 7.04 (d, 4H, 3 JHH = 7.3 Hz, m-C6H3iPr2), 6.56 (m, 6H, C6H3Me2), 6.48 (dd, 2H, 3 JHH = 19.6, 6.9 Hz, ONCHCHCHCHCHCH3), 5.99 (t, 2H, 3JHH = 10.4 Hz, ONCHCHCHCHCHCH3), 5.83 (m, 2H, ONCHCHCHCHCHCH3), 5.73 (m, 2H, ONCHCHCHCHCHCH3), 3.27 (sept, 4H, 3JH−H = 6.7 Hz, CH(CH3)2), 2.92 (d, 2JP−H = 19.6 Hz, 2H, PCH), 2.12 (m, 4H, ipso-Cy), 1.86 (d, 6H, 3JH−H = 6.6 Hz, ONCHCHCHCHCHCH3), 1.71 (s, 12H, C6H3(CH3)2), 1.52 (s, 6H, NCCH3), 1.06 (d, 3JH−H = 6.7 Hz, 24H, CH(CH3)2), 1.81−0.84 (m, 40H, PCy2). 13C NMR (100 MHz, CDCl3, 25 °C) δ 171.07, 149.70, 148.18, 146.08, 144.75, 135.43, 130.26, 130.16, 129.03, 128.22, 128.07, 128.00, 125.30, 123.79, 123.27, 123.14, 121.19, 39.05, 38.47, 27.65, 27.52, 27.41, 27.29, 27.13, 27.01, 26.65, 26.49, 26.35, 24.87, 24.62, 24.43, 22.65, 21.45, 19.77, 18.32, 14.11. 31P NMR (162 MHz, CDCl3, 25 °C) δ 35.24 ppm (s). Anal. Calcd for C82H120Mg2N6O2P2: C, 73.92; H, 9.08; N, 6.31. Found: C, 73.59; H, 9.16; N, 6.37. Synthesis of PIA-Supported Magnesium 3,5-Hexadiene-2oximate (4b). To a THF suspension (5 mL) of hydride 1 (0.111 g, 0.1 mmol) was added dropwise 2-methylpyridine N-oxide (21.5 mg, 0.2 mmol in 5 mL THF) at room temperature. The mixture was stirred for 5 h at room temperature to afford a clear yellow solution. Removal of THF and washing with hexane for 3 times yielded 4b as white powder (0.115 mg, 87%). 1H NMR (400 MHz, CDCl3, 25 °C) δ 7.01 (m, 6H, C6H3iPr2), 6.79 (m, 2H, ONC(Me)CHCHCHCH2), 6.67 (m, 6H, C6H3iPr2), 5.89 (d, 2H, 3JHH = 11.2 Hz, ONC(Me)CHCHCHCH2), 5.71 (t, 2H, 3 JHH = 11.6 Hz, ONC(Me)CHCHCHCH2), 5.15 (t, 4H, 3 JHH = 13.7 Hz, ONC(Me)CHCHCHCH2), 3.33 (m, 4H, CH(CH3)2), 2.87 (d, 2H, 2JP−H = 19.9 Hz, PCH), 2.19 (m, 4H, ipso-Cy), 1.89 (s, 12H, C6H3(CH3)2), 1.62 (s, 6H, ONC(Me)CHCHCHCH2), 1.51 (s, 6H, NCCH3), 1.04 (d, 3JH−H = 6.1 Hz, 24H, CH(CH3)2), 1.97−0.84 (m, 40H, PCy2). 13C NMR (100 MHz, CDCl3, 25 °C) δ 172.86, 171.23, 170.82, 148.47, 144.61, 144.46, 137.83, 136.27, 135.80, 134.30, 131.08, 131.06, 129.68, 129.24, 129.04, 128.23, 128.04, 125.30, 123.87, 123.54, 123.18, 123.12, 121.55, 121.34, 119.54, 118.97, 118.67, 117.49, 60.12, 59.00, 31.59, 28.03, 27.92, 27.73, 27.62, 27.53, 27.39, 27.24, 27.13, 26.99, 26.93, 26.86, 26.80, 26.76, 26.71, 26.68, 26.58, 26.38, 25.99, 24.73, 24.65, 24.58, 24.44, 24.40, 23.90, 23.14, 22.66, 21.45, 21.22, 20.42, 20.36, 14.05. 31P NMR (162 MHz, CDCl3, 25 °C) δ 36.23 ppm (s). Anal. Calcd for C82H120Mg2N6O2P2: C, 73.92; H, 9.08; N, 6.31. Found: C, 73.75; H, 9.11; N, 6.33. Synthesis of BDI-Supported Magnesium Complex 6. To a toluene suspension (5 mL) of 5 (0.177 g, 0.2 mmol) was added dropwise quinoline N-oxide (0.116 g, 0.8 mmol in 5 mL toluene) at room temperature. The mixture was stirred for 5 h at room temperature to afford a dark brown solution. Removal of toluene and washing with hexane 3 times afforded 6 as brown powder (0.1958 mg, 89%). Single crystals for X-ray analysis were achieved from toluene solution. 1H NMR (400 MHz, C6D6, 25 °C) δ 9.04 (br, 1H, Ar−H), 8.57 (br, 1H, Ar−H), 7.16 (m, 6H, Ar−H), 7.02 (m, 2H, Ar−H), 6.92

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00517. NMR spectrum of complexes 2, 3, 4a, 4b, 6, and 7. (PDF) Accession Codes

CCDC 1561357−1561360 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86) 431 85262774. Tel: +86 431 85262773. ORCID

Dongmei Cui: 0000-0001-8372-5987 Author Contributions

H.X. and X.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by The National Natural Science Foundation of China for project Nos. 21361140371, 21574125.

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DOI: 10.1021/acs.organomet.7b00517 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00517 Organometallics XXXX, XXX, XXX−XXX