Reductive Carbonylation of Oxorhenium Hydrides Induced by Lewis

Reductive Carbonylation of Oxorhenium Hydrides Induced by Lewis Acids. Nikola S. ... John J. Kiernicki , Selena L. Staun , Matthias Zeller , and Suzan...
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Reductive Carbonylation of Oxorhenium Hydrides Induced by Lewis Acids Nikola S. Lambic, Cassandra P. Lilly, Leanna K. Robbins, Roger D. Sommer, and Elon A. Ison* Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: Several oxorhenium hydride complexes with chelating diamidopyridine (DAP), diamidoamine (DAAm), and 2-mercaptoethyl sulfide (SSS) groups have been isolated and characterized. Adduct formation is observed when the DAP complex 1a is treated with the Lewis acid B(C6F5)3. However, treatment of 1a,b with B(C6F5)3 or BF3·OEt2 in the presence of CO results in reduction of the metal center by four electrons from Re(V) to Re(I).

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INTRODUCTION Transition-metal hydrides have been shown to play a significant role in reactions related to energy storage and energy utilization.1,2 These species have been proposed as valuable intermediates in many catalytic reactions.1a,3 For example, insertions of olefins,4 CO2,2j,5 or isocyanides6 lead to the corresponding alkyl, formate, or iminoformyl ligands that can be further functionalized to incorporate small-molecule building blocks into C2+ organic products. Several air-stable oxorhenium hydride complexes, with a variety of ligand frameworks, are depicted in Chart 1. The

catalysis. The Toste and Abu-Omar groups have demonstrated the intermediacy of these species in hydrosilylation reactions.8 Although the intermediacy of these complexes has been confirmed and their solid-state structures obtained, further studies are required to clarify the role of these species in catalysis. In order to gain further insights into reactivity of oxorhenium hydrides, we have synthesized a series of complexes with diamidoamine (DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5, mesitylene), diamidopyridine (DAP = (2,6- bis((mesitylamino)methyl)pyridine), and SSS (SSS = 2-mercaptoethyl sulfide) chelating ligands with differing steric and electronic demands. Recently our laboratory showed that (O)Re(DAAm)X or (O)Re(DAP)X, where X = methyl, acyl, Cl, that contain terminal oxos are ambiphilic: i.e., they can react with both Lewis acids and bases.9 Given the renewed interest in oxorhenium hydride complexes, we were interested in examining the reactivity of oxorhenium hydrides with Lewis acids in order to determine if coordination of the oxo ligand to a Lewis acid could affect the electronics of the hydride and, subsequently, alter its reactivity. In this paper we describe the synthesis of a series of oxorhenium hydrides that contain DAAm, DAP, and SSS ligands. We report adduct formation when DAP hydrides react with the Lewis acid B(C6F5)3 and examine the reactivity of these adducts with CO.

Chart 1. Examples of Oxorhenium Hydride Complexes

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RESULTS AND DISCUSSION Synthesis of Hydride Complexes. A series of oxorhenium hydride complexes were successfully synthesized and isolated in good yields via transmetalation of the Re−X (X = Cl, Br) precursors with tributyltin hydride (Bu3SnH). The conditions for synthesis are outlined in Scheme 1.

stability and ancillary ligand tolerance of oxorhenium hydrides allows for fine tuning of the steric and electronic properties of the complex.5c Further, the tuning of the metal center via π donation by the oxo ligand should affect the reactivity of highvalent rhenium hydrides and may contribute to high reactivity and catalysis. Several research groups have demonstrated the ability of oxorhenium(V) hydrides to activate small molecules such as CO, olefins, and aldehydes.7 More recently, rhenium hydrides have been proposed as intermediates in reduction © XXXX American Chemical Society

Received: May 13, 2016

A

DOI: 10.1021/acs.organomet.6b00393 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of Oxorhenium Hydrides

Scheme 2. Tentative Mechanism for the Formation of 3 via Oxygen Atom Transfer

disorder associated with the flexible methylene backbone of the diamidoamine ligand. The disorder is also observed in the crystal structure of these species (Supporting Information). X-ray Crystal Structure for 2. X-ray-quality crystals for 2 (Figure 1) were obtained by vapor diffusion of pentane into a

All Re−H complexes and their deuterated analogs were isolated and characterized using 1H and 13C NMR spectroscopy, FTIR spectroscopy, and elemental analysis. As is typical for metal hydrides, the rhenium hydride complexes are converted to their chloride precursors in chlorinated solvents. Similar behavior was observed by Mayer and co-workers, as heating the solution of Tp*Re(O)H(OTf) in chloroform resulted in hydride for chloride exchange, yielding Tp*Re(O)Cl(OTf).7b Ligand electronics play a significant role in the reduction of the metal center. Harsher conditions were required for the formation of 3 in comparison to 2 presumably because the chloride precursor for 3 is more electron rich. A tentative mechanism for the formation of 3 is depicted in Scheme 1. As reported in previous studies, the corresponding chloride precursor required initial reduction with PPh3 in the presence of Bu3SnH to form a putative ReIII(H) species, which yields 3 upon exposure to air (Scheme 2).10 By 1H NMR spectroscopy, the Re−H resonances for complexes 1a,b and 2−4 were observed at 6.03, 6.60, 6.80, and 8.40 ppm, respectively. The corresponding Re−D resonances for deuterated analogues 1a-d, 2-d, 3-d, and 4-d, respectively, were not observed in the 1H NMR spectra of these species. Other NMR spectral data are consistent with those for previously published structures bearing the same ligands.9−11 By FTIR, the Re−H stretch is observed at 2019 and 2031 cm−1 for complexes 1a and 4, respectively, and is manifested as a single band of weak intensity, which shifted to lower frequency in the deuterated complex. The new Re−D stretch overlaps with aromatic CC bonds in the complex and therefore could not be identified. In contrast, the Re−H stretches of compounds 2 and 3 exhibit two bands in the hydride region 2000−2025 cm−1. This behavior is attributed to the structural

Figure 1. X-ray crystal structure of 2. Thermal ellipsoids are at the 50% probability level. Selected bond lengths (Å) and angles (deg): Re1−O1, 1.689(6); Re1−H1, 1.88(8); Re1−N2, 1.962(4); Re1−N1, 2.191(6). N2−Re1−N2, 129.1(2); O1−Re1−N2, 115.37(13).

concentrated solution of the molecule in methylene chloride. The coordination environment exhibits substantial deviations from ideal trigonal-bipyramidal (tbp) and square-pyramidal (sp) geometries and can be viewed as either distorted tbp or sp. In order to describe the structure more accurately, the τ value described by Addison and co-workers12 was employed. The value obtained for 2 (0.28) supports the description of the coordination environment as distorted square pyramidal with the oxo ligand occupying the axial position. The angles N1− Re1−H1 (147°) and N1−Re1−N2 (129°) are both severely distorted from the ideal angles of 180°. For 2, the hydride ligand was located in the Fourier difference map and is tentatively assigned as Re1−H1 = 1.88(8) Å, which is within the normal range for other reported rhenium hydrides.4 Other bond lengths and angles are within normal ranges for other oxorhenium complexes. One of two independent molecules B

DOI: 10.1021/acs.organomet.6b00393 Organometallics XXXX, XXX, XXX−XXX

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Organometallics present in the unit cell is depicted in Figure 1. The other molecule contains similar bond lengths and bond angles but differs in that the methylene backbone is disordered (see the Supporting Information). Reactivity of Oxorhenium Hydrides. Reactivity with CO and Other Unsaturated Molecules. The DAP complexes 1 decompose in the presence of carbon monoxide and isocyanides and do not react with carbon dioxide or alkenes even at higher temperatures or under photolysis. Interesting reactivity is observed with the DAAm hydrides 2 and 3, and this will be the subject of a subsequent publication. However, the SSS complex 4 proved to be quite inert toward a wide variety of small molecules, including carbon monoxide, isocyanides, and unsaturated hydrocarbons. The lack of reactivity is best explained by the relatively weakly donating SSS ligand, which in turn results in a relatively strong and unreactive Re−H bond. Reactivity of 1 with Lewis Acids. Complex 1a forms a stable adduct with B(C6F5)3 through the rhenium−oxo bond. The Lewis acid/base adduct formed is the result of nucleophilic nature of the rhenium oxo (Scheme 3). Complex 5 was

Scheme 4. Synthesis of Complexes 6a,b

terminal CO ligands. In addition, a weak band at 3300 cm−1 was observed and has been attributed to the N−H stretch. Despite several attempts, an X-ray crystal structure for 6a could not be obtained. Similar spectral features were observed in the 1H NMR spectrum of 6b, including the additional coupling to the N−H bond of the protonated ligand, as well as the characteristic singlet resonance of the imine ligand (8.61 ppm). The broken symmetry of the molecule is best manifested by four separate multiplets for the isopropyl C−H fragment of the diisopropylphenyl group. Three carbonyl ligands were confirmed by FTIR spectroscopy (2049, 1946, and 1919 cm−1). In addition, signals for the N−H group (3276 cm−1) and a weak CN vibration of the imine moiety (1606 cm−1) were observed. X-ray Structure of 6b. X-ray-quality crystals for 6b (Figure 2) were obtained by vapor diffusion of pentane into a concentrated solution of the molecule in methylene chloride. The geometry around the metal center is pseudo-octahedral with two CO ligands in the axial positions and the DippDAP ligand and one CO ligand in the equatorial plane. Consistent

Scheme 3. Synthesis of Lewis Acid/Base Adduct 5

previously reported13 and characterized by 1H, 13C, and 19F NMR spectroscopy, elemental analysis, and X-ray crystallography. The altered nature of the Re−H bond in complex 5 resulting from the coordination of B(C6F5)3 is best manifested in the 1H NMR spectrum. The Re−H resonance of complex 5 is shifted downfield significantly from 6.03 ppm for 1 to 11.83 ppm for 5 in benzene. The corresponding reaction with the isopropylphenyl analogue 1b did not result in adduct formation, presumably because of the increased sterics of the ligand. Reactivity of Complexes 1a,b with CO in the Presence of Lewis Acids. In the presence of the Lewis acids B(C6F5)3 and BF3·OEt2, 1 reacts with carbon monoxide to afford the rhenium species 6 (Scheme 4). The proposed structure for 6a (Scheme 4) is consistent with NMR and IR data. The methylene protons in 1a appear as two doublets at 5.6 and 5.4 ppm, while the methylene protons in 6a appear as one doublet of doublets (4.1 ppm) and one doublet (4.3 ppm). Several 2D NMR techniques, (COSY, HMBC, and H−H NOE) were also used to characterize 6a. A doublet observed in the 1H NMR spectrum of 6a at 6.4 ppm is assigned to the N− H group and is coupled both through space (NOE) and through bond (COSY) to the methylene proton oriented syn to the oxo. In addition, a 1H−13C HSQC experiment indicates that the signal at 6.4 ppm is not coupled to any of the carbons, which provides further evidence that the diamidopyridine ligand is protonated. The 19F NMR spectrum is consistent with the presence of a B(C6F5)4 anion as well as C6F5H as a byproduct. Three separate CO stretches were observed in the FTIR spectrum (2053, 1946, and 1919 cm−1), which are indicative of

Figure 2. X-ray crystal structure of 6b. Thermal ellipsoids are at the 50% probability level. Hydrogens are omitted for clarity. Selected bond lengths (Å): Re1−C1, 2.000(4); Re1−C2, 1.913(4); Re1−C3, 1.986(3); C1−O1, 1.133(4); C2−O2, 1.160(5); C3−O3, 1.143(4); Re1−N1, 2.236(3); Re1−N3, 2.134(3); C10−N3, 1.293(4); C4−N1, 1.513(4); H1−F1, 2.127. C

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2.000 and 1.986 Å), while the equatorial Re−carbonyl had a shorter bond length of 1.939 Å (experimental 1.913 Å). The nature of the frontier molecular orbitals was investigated. As indicated in Figure 3, the highest occupied

with the NMR and FTIR data, one arm of the diamidopyridine ligand was fully protonated (amine) while the other arm was oxidized (imine).10a,14 This is evidenced by the significantly shorter C10−N3 bond (1.293 Å) of the imine fragment in comparison to the elongated C4−N1 bond (1.513 Å) of the protonated amine. The molecule was isolated with BF4 as a counterion, which is hydrogen-bound to the fluorine atom of the protonated amine. The two axial CO ligands have similar rhenium−carbon bond lengths (Re−C1, 2.00 Å; Re−C3, 1.99 Å). However, these bond lengths are significantly longer than that to the CO ligand in the equatorial plane (Re−C2, 1.91 Å). Mechanism of Reduction. A tentative mechanism for the reduction of 1 involves the initial migration of the hydride ligand to the metal oxo to form the Re(III) hydroxyl intermediate 7 (Scheme 5).15 Deprotonation of one of the

Figure 3. DFT (M06) calculated frontier molecular orbitals of 6b. Kohn−Sham orbitals for HOMO and LUMO are depicted using an isocontour value of 0.065.

Scheme 5. Proposed Mechanism for Reductive Carbonylation16

molecular orbital (HOMO) mostly consists of the metalcentered dxy orbital that has been primarily stabilized through back-bonding to the π* orbital of the carbonyl ligand lying in the xy plane. The LUMO is ligand centered and includes delocalization into the pyridine ring and the imine moiety. The presence of three CO ligands modulates the energy levels of the acceptor orbitals according to the following diagram given in Figure 4. The lowest energy d orbital is assigned as dyz and is stabilized by π back-bonding to all three carbonyl ligands. Similarly, backbonding is observed with the dxz orbital, to the two axial CO antibonding orbitals. Finally, the highest occupied metal orbital

CH2 arms of the diamidopyridine ligand by the hydroxyl ligand followed by the reaction with 2 equiv of CO results in the Re(I) species 8. A proton is then transferred from the H2O·BF3 followed by counterion exchange of the methylene group of the diamidopyridine ligand to give 6. This mechanism of this transformation has not been investigated fully; however, several hydroxyborane species were identified by 19F NMR spectroscopy. Orbital Analysis of 6b. Rhenium tricarbonyl species have garnered much interest in the past few years due to their potential for solar energy conversion.17 Kubiak and co-workers were also able to utilize the Re(I) complexes as electrocatalysts for CO 2 hydrogenation to formate and its reductive disproportionation to CO.18 To gain a better understanding of the electronic structure of the rhenium tricarbonyl species 6, DFT calculations on 6b were performed. Structure optimizations were carried out using the M0619 functional with the 631G (d,p)20 basis set on light atoms and the SDD21+f basis set on Re.22 The calculated structure is in good agreement with the crystal structure of 6b. For example, a similar elongation of the axial Re−C bonds was observed by DFT; the axial CO bond lengths were calculated to be 1.994 and 2.012 Å (experimental

Figure 4. DFT calculated metal-centered d orbitals and their relative energies. Single-point energy calculations were carried out using a triple-ζ 6-311G++(d,p) basis set on light atoms and an SDD+f basis set on rhenium. Kohn−Sham orbitals for HOMO, HOMO-2, and HOMO-6 are depicted using the isocontour value of 0.065. D

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Organometallics is dxy, which is stabilized by back-bonding to the CO ligand in the equatorial plane. The axial carbonyls in this case cannot achieve orbital overlap with the dxy orbital; thus, this orbital is the least stabilized. Therefore, the relative energy levels of the d orbitals are as follows: dyz< dxz< dxy. Electronic Absorption Spectra. Analogous rhenium(I) complexes of the type (L)Re(CO)3X have been previously shown to have charge transfer features in the visible spectrum.23 These transitions have been assigned as metal to ligand charge transfers (MLCT).23 Furthermore, long emission lifetimes from the 3MLCT state have been reported for similar rhenium compounds.17b−e,24 In order to investigate the nature of the excited state in our (NNN)Re(CO)3 cation, we have carried out experimental and time-dependent density functional theory (TD-DFT) studies. The experimentally obtained and computationally predicted electronic absorption spectra for 6b are shown in Figure 5.

Figure 6. Natural transition orbitals (NTOs) calculated for complex 6b. The dominant natural transition orbital pairs for the singlet state excitations at 501 and 433 nm are depicted. The excitation wavelengths, as well as the oscillator strengths ( f), are given. HONTO denotes the highest occupied natural transition orbital and LUNTO the lowest unoccupied natural transition orbital. NTOs are displayed with an isocontour value of 0.065. The associated eigenvalues λ are 0.99 and 0.95, respectively.

complex and facilitates its reduction. Recently the effect of Lewis acids on the redox activity of complexes with oxo functionalities has been recognized.26 This type of reduction with CO led to the isolation of new low-valent rhenium tricarbonyl complexes whose electronic structures were evaluated by TD/DFT.

Figure 5. Experimental electronic absorption spectrum of complex 6b in methylene chloride solution (red) and DFT calculated spectrum (black). The two peak maxima are shown. The TD-DFT spectrum was fit with a Lorentzian line shape with the half-width at half-maximum (hwhm) equal to 25 cm−1.

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The observed featureless absorptions at 470 and 417 nm (red) are assigned as charge transfer bands and correlate well with the TD-DFT predicted absorptions at 501 and 433 nm (black), respectively. The nature of these transitions was further investigated by natural transition orbital (NTO) analysis.25 The NTOs associated with each transition are shown in Figure 6. As indicated by the NTO plots in Figure 6, transitions at both 501 and 433 nm can be characterized as MLCT in nature. The transition at 501 nm is characterized as charge transfer from the intermediate-energy HOMO-2 orbital (Figure 4) to the LUMO. The intense transition calculated at 433 nm has similar features and is described as an MLCT from the HOMO6 orbital to the ligand-centered LUMO.

EXPERIMENTAL SECTION

General Considerations. Complexes (O)Re(DAAm)Cl, (O)Re(MesDAAm)Cl, (O)Re(DAP)Cl, and (O)Re(SSS)Br were previously reported;11c,27 all other reagents were purchased from commercial resources and used as received. B(C6F5)3 was purchased from Strem Chemicals and sublimed prior to use. 1H, 13C, and 19F NMR spectra were obtained on 300 or 400 MHz spectrometers at room temperature. Chemical shifts are listed in parts per million (ppm) and referenced to their residual protons or carbons of the deuterated solvents, respectively. All reactions were run under an inert atmosphere with dry solvents unless otherwise noted. FTIR spectra were obtained in KBr thin films. Elemental analyses were performed by Atlantic Micro Laboratories, Inc. (MesDAP)Re(O)H (1a). In a 25 mL scintillation vial, (MesDAP)Re(O)Cl (0.164 mmol, 100 mg) was dissolved in ∼15 mL of THF or acetonitrile. Tributyltin hydride (0.821 mmol, 0.220 mL) was added to the reaction mixture. The reaction mixture was stirred for 16 h, after which the solvent was removed under reduced pressure. The resulting red residue was dissolved in a minimal amount of CH2Cl2. Addition of excess pentane resulted in a red precipitate. Filtration led to 93.0% yield of the red powder Re(O)H(DAP). 1H NMR (CD2Cl2): δ 8.06 (t, J = 7.9 Hz, 1H, NC2H2CH), 7.67 (d, J = 7.7 Hz, 2H, NC2H2CH), 6.88 (s, 2H, Mes-meta-H), 6.82 (s, 2H, Mes-meta-H), 6.03 (s, 1H, ReH), 5.64 (d, J = 20.2 Hz, 2H, MesNCH2), 5.40 (d, J = 19.9 Hz, 2H, MesNCH2), 2.43 (s, 6H, Mes-CH3), 2.28 (s, 6H, Mes-CH3), 1.71 (s, 6H, Mes-CH3). 13C NMR (CD2Cl2): δ 169.43, 159.63, 141.70, 135.88,

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CONCLUSIONS The synthesis of several oxorhenium(V) hydride complexes with chelating diamidopyridine (DAP), diamidoamine (DAAm), and 2-mercaptoethyl sulfide (SSS) is described. Adduct formation is observed when the DAP complex 1a is treated with the Lewis acid B(C6F5)3. However, treatment of 1a,b with B(C6F5)3 or BF3·OEt2 in the presence of CO results in reduction of the metal center by four electrons from Re(V) to Re(I). Thus, the binding of the oxo ligand to a Lewis acid appears to change the nature of the oxorhenium hydride E

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Organometallics

2H, -N−CH2−), 2.29 (s, 6H, Mes-CH3), 2.27 (s, 6H, Mes-CH3), 2.01 (s, 6H, Mes-CH3). 13C NMR (CD2Cl2): δ 160.11, 135.62, 133.53, 131.81, 128.83, 68.77, 68.18, 49.57, 20.53, 18.93. Anal. Calcd for C23H34N3ORe: C, 49.80; N, 7.57; H, 6.18. Found: C, 50.07; N, 7.29; H, 6.21). IR (FTIR): 2000 and 2024 cm−1. (MesDAAm)Re(O)D was prepared in the same manner by with Bu3SnD. (SSS)Re(O)H (4). In a 25 mL scintillation vial 300 mg (0.691 mmol) of (SSS)Re(O)Br was dissolved in THF (15 mL) in a nitrogen-filled glovebox. Tributyltin hydride (0.279 mL, 1.38 mmol) was slowly added, and the reaction mixture was stirred for 0.5 h. The reaction mixture was then concentrated in vacuo. Excess hexanes (30 mL) was added to the concentrated mixture, and the resulting powder was filtered, washed with ether, and dried under vacuum. (SSS)Re(O)H (183.1 mg, 0.515 mmol) was obtained as a black powder (74% yield). 1H NMR (CDCl3): δ 8.40 (s, 1H, Re-H), 4.16 (m, 2H, −SCH2−), 3.83 (m, 2H, −S-CH2−), 2.84 (m, 2H, −S-CH2−), 1.60 (m, 2H, −S-CH2−). 13C NMR (CDCl3): δ 46.16, 47.25. Anal. Calcd for C4H9S3ORe: C, 13.51; H, 2.55; S, 27.05. Found: C, 13.72; H, 2.68; S, 27.27. IR (FTIR, KBr pellet, cm−1): ν(Re−H) 2030, ν(Re−O) 969. (SSS)Re(O)D was prepared in the same manner with Bu3SnD. [(DAPH)Re(CO)3][B(C6F5)4] (6a). Under an N2 atmosphere, (DAP)Re(O)H (5.0 mg, 0.0087 mmol) and tris(pentafluorophenyl) orane (6.7 mg, 0.013 mmol) were placed in an oven-dried J. Young tube. Deuterated benzene (0.2 mL) was added. The reaction mixture was then subjected to three freeze−pump−thaw cycles to remove N2 and pressurized with CO (5 psi). The reaction was monitored by 1H NMR spectroscopy. The compound was not isolated because of its high solubility in a majority of solvents. Satisfactory elemental analysis could not be obtained for this molecule. 1H NMR (C6D6): δ 7.77 (t, J = 7.6 Hz, 1H, NC2H2CH), 7.46 (1H, −C(H)N), 7.43 (d, J = 7.5 Hz, 1H, NC2H2CH), 7.03 (d, J = 7.5 Hz, 1H, NC2H2CH), 6.91 (s, 1H, Mes-aromatic), 6.85 (s, 1H, Mes-aromatic), 6.82 (s, 1H, Mesaromatic), 6.64 (s, 1H, Mes-aromatic), 6.43 (bd, 1H, N-H), 4.29 (d, J = 18.5 Hz, 1H, MesN(H)CH2), 4.14 (dd, J = 18.5 Hz, J = 8.1 Hz, 1H, MesNCH2), 2.32 (s, 3H, Mes-methyl), 2.30 (s, 3H, Mes-Methyl), 2.26 (s, 3H, Mes-methyl), 2.20 (s, 3H, Mes-methyl), 2.10 (s, 3H, Mesmethyl), 1.96 (s, 3H, Mes-methyl). 19F NMR (C6D6,): δ −133.6 (m, 2F, ortho), −162.8 (t, 1F, para), −166.6 (t, 2F, meta). IR (FTIR, cm−1): ν(CO) 2053, 1953, 1927, ν(N−H) 3300, ν(CN) 1606. [(DippDAPH)Re(CO)3][ BF4] (6b). Under an N2 atmosphere in a 50 mL pressure vessel, (DippDAP)Re(O)H (50 mg, 0.08 mmol) was dissolved in 10 mL of toluene. To the resulting red solution was then added 18.6 μL (0.15 mmol, 2 equiv) of BF3·OEt2. The reaction mixture was immediately removed from the glovebox, pressurized with 60 psi of CO, and stirred for 1 h. Addition of excess pentane resulted in precipitation of an orange solid, which was collected on a filter frit and dried in vacuo. The compound was isolated as a solid (50.5 mg, 82% yield). Vapor diffusion of pentane into concentrated solution of CH2Cl2 resulted in crystals suitable for X-ray analysis. 1H NMR (CH2Cl2, δ): 8.61 (s, 1H, −C(H)N) 8.37 (overlapping m, 2H, pyridine para, meta H), 7.99 (d, 1H, J = 8 Hz, pyridine meta H), 7.56 (bs, 1H, N-H), 7.38−7.19 (m, 6H, Ph rings), 5.48 (dd, J = 18 Hz, 7.6 Hz, 1H, −CH2N), 5.48 (dd, J = 18 Hz, 7.6 Hz, 1H, −CH2N), 5.26 (dd, J = 18.2 Hz, 3.6 Hz, 1H, −CH2N), 3.07 (m, 1H, iPr), 2.79 (m, 1H, iPr), 2.70 (m, 1H, iPr), 2.53 (m, 1H, iPr), 1.42−1.15 (overlapping d, 24H, iPr-CH3). 19F NMR (CH2Cl2, δ): −151.9 (BF4 anion). IR (FTIR, cm−1): ν(CO) 2049, 1946, 1919, ν(N−H) 3276, ν(CN) 1606. Anal. Calcd for C34H41BF4N3O4Re·H2O: C, 49.16; H, 5.22; N, 5.06. Found: C, 48.94; H, 5.12; N, 5.14. Computational Methods. Computations were performed on clusters provided by the NC State Office of Information Technology High Performance Computing (HPC). Theoretical calculations have been carried out using the Gaussian 0928 implementation of M0619 density functional theory. All geometry optimizations were carried out in the gas phase using tight convergence criteria (“opt = tight”) and pruned ultrafine grids (“Int = ultrafine”). The basis set for rhenium was the small-core (311111,22111,411) → [6s5p3d] Stuttgart− Dresden basis set and relativistic effective core potential (RECP) combination (SDD)21 with an additional f polarization function.22 The 6-31G(d,p)20 basis set was used for all other atoms. Cartesian d

134.23, 133.53, 129.12, 129.10, 117.05, 78.09, 20.94, 18.82, 18.41. Anal. Calcd for C25H30N3ORe: C, 52.25; N, 7.31, H, 5.26. Found: C, 52.18, N, 7.36, H, 5.32. IR (FTIR, cm−1): ν(Re−H) 2019 cm−1. (DAP)Re(O)D was prepared in the same manner with Bu3SnD. (DippDAP)Re(O)Cl. In a 250 mL round-bottom flask, (SMe2)(OPPh3)Re(O)Cl3 (450 mg, 0.693 mmol) and DippDAP ligand (1.5 equiv, 1.04 mmol) were dissolved in 100 mL of absolute EtOH. 2,6Lutidine (6.93 mmol, 0.8 mL) was added to the reaction mixture, which was stirred overnight at room temperature. Filtration afforded 151.6 mg (32% yield) of green powder. Vapor diffusion of pentane into a concentrated solution of methylene chloride resulted in crystals suitable for X-ray analysis. 1H NMR (CD2Cl2): δ 8.23 (t, J = 7.9 Hz, 1H, NC2H2CH), 7.87 (d, J = 7.7 Hz, 2H, NC2H2CH), 7.21−7.14 (m, 6H, aromatic), 5.60 (s, 4H, MesNCH2), 3.88 (m, 2H, iPr-methine), 2.28 (m, 2H, iPr-methine), 1.26 (dd, 12H, iPr-CH3), 1.02 (dd, 12H, 13 C NMR (CD2Cl2): δ 168.16, 153.94, 147.30, 146.31, iPr-CH3). 143.73, 126.26, 124.02, 123.61, 117.30, 82.40, 27.96, 27.38, 25.42, 25.03, 24.30. Anal. Calcd for C31H41ClN3ORe: C, 53.70; N, 6.06, H, 5.96. Found: C, 53.83, N, 6.07, H, 6.13. (DippDAP)Re(O)H (1b). In a 25 mL pressure vessel (DippDAP)Re(O)Cl (100 mg, 0.144 mmol) was dissolved in 5 mL of dry THF in the glovebox. Tributyltin hydride (0.72 mmol, 0.2 mL) was added, and the reaction mixture was stirred at 80 °C for 2 days. Upon cooling, solvent was removed under vacuum and the remaining residue was taken up in excess hexane to precipitate the product as a red powder, which was filtered and dried under vacuum (82.0 mg, 87% yield). 1H NMR (CD2Cl2): δ 8.15 (t, J = 7.9 Hz, 1H, NC2H2CH), 7.70 (d, J = 7.7 Hz, 2H, NC2H2CH), 7.29 (s, 1H, Re−H), 7.14−7.09 (m, 6H, aromatic), 5.63 (d, J = 20.2 Hz, 2H, −NCH2-Pyr), 5.52 (d, J = 20.2 Hz, 2H, −NCH2-Pyr), 3.86 (septet, J = 5.8 Hz, 2H, iPr-methine), 2.56 (septet, J = 5.8 Hz, 2H, iPr-methine), 1.31 (d, J = 7.7 Hz 6H, iPr-CH3), 1.18 (d, J = 7.7 Hz, 6H, iPr-CH3), 1.01 (d, J = 7.7 Hz, 6H, iPr-CH3), 1.00 (d, J = 7.7 Hz 6H, iPr-CH3). 13C NMR (CD2Cl2): δ 169.0, 159.17, 147.10, 144.52, 141.49, 125.50, 124.04, 123.70, 116.66, 80.87, 27.54, 27.27, 25.36, 25.29, 24.33, 24.12. FTIR (KBr pellet): ν(Re−H) 2034 cm−1. Anal. Calcd for C31H42N3ORe·0.5H2O: C, 55.75; H, 6.49, N, 6.29. Found: C, 55.97, H, 6.49, N, 6.26. (C6F5-DAAm)Re(O)H (2). In a small round-bottom flask (C6F5DAAm)Re(O)Cl (0.730 mmol, 500 mg) was dissolved in ∼20 mL of THF. Tributyltin hydride (1.46 mmol, 0.392 mL) was then added to the reaction mixture, and the resulting dark brown solution was stirred overnight at room temperature. Solvent was removed under vacuum, and the brown residue was dissolved in a minimal amount of methylene chloride. Addition of excess pentane resulted in a light gray precipitate, which was washed with hexanes and cold diethyl ether. Filtration afforded 256 mg of the gray powder (C6F5-DAAm)Re(O)H (54% yield). 1H NMR (CD2Cl2): δ 6.11 (br s, 1H, Re-H), 4.31 (ddd, J = 12.4 Hz, J = 8 Hz, J = 4.8 Hz, 2H, −N-CH2−), 3.65 (dt, J = 12.6 Hz, J = 5 Hz, J = 5 Hz, 2H, −N-CH2−), 3.38 (s, 3H, N-CH3), 3.12 (dt, J = 11.8 Hz, J = 4.6 Hz, J = 4.6 Hz, 2H, −N-CH2−), 2.90 (m, 2H, −NCH2−). 13C NMR (CD2Cl2): δ 244.21, 66.51, 66.16, 49.93. Anal. Calcd for C17H12F10N3ORe·H2O: C, 30.54; N, 6.29; H, 2.1. Found: C, 30.89; N, 6.33; H, 1.84. IR (FTIR, cm−1): ν(Re−H) 1968 and 1999 cm−1. (DAAm)Re(O)D was prepared using the same procedure with Bu3SnD. (MesDAAm)Re(O)H (3). In a 100 mL pressure vessel, (O)Re(DAAm)Cl (0.180 g, 0.305 mmol) and triphenylphosphine (0.961 g, 3.68 mmol) were dissolved in 25 mL of THF. Tributyltin hydride (3.05 mmol, 0.820 mL) was then added via syringe. The reaction mixture was placed in an 80 °C oil bath and heated for 2 days. Upon cooling, the solvent was removed under vacuum and the remaining residue was dissolved in a minimal amount of CH2Cl2. Addition of excess pentane resulted in a gray precipitate that was filtered and washed with excess hexanes and diethyl ether. Isolated yield: 0.144 g, 85%. Vapor diffusion of pentane into a concentrated solution of 3 in dicholoromethane afforded an X-ray-quality crystal. 1H NMR (CD2Cl2): δ 6.83 (s, 4H, Mes-H), 6.79 (s, 1H, Re-H), 4.27 (ddd, J = 12.7 Hz, J = 9.5 Hz, J = 4.6 Hz, 2H, −N-CH2−), 3.65 (ddd, J = 12.7 Hz, J = 5.8 Hz, J 3.1 Hz, 2H, −N-CH2−), 3.38 (s, 3H, N-CH3), 3.14 (m, 2H, −N-CH2−), 2.87 (ddd, J = 11.4 Hz, J = 9.5 Hz, J = 5.8 Hz, F

DOI: 10.1021/acs.organomet.6b00393 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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functions were used throughout: i.e., there are six angular basis functions per d function. All structures were fully optimized, and analytical frequency calculations were performed on all structures to ensure a zeroth-order saddle point (a local minimum). Energetics were calculated at 298.15 K with the 6-311++G(d,p) basis set for C, H, N, O, and F atoms and the SDD basis set with an added f polarization function for Re. Reported energies utilized analytical frequencies and the zero point corrections from the gas phase optimized geometries and included solvation corrections which were computed using the PCM method,29 with benzene as the solvent as implemented in Gaussian 09. Time-dependent DFT (TD-DFT)30 calculations were performed in the visible spectral region in benzene. Solvent effects were introduced via the PCM method. Natural transition orbital (NTO)25 analysis was carried out to better understand the results of TD-DFT calculations. The absorption spectra were simulated by convoluting the spectrum composed with a Lorentzian line shape with the half-width at halfmaximum (hwhm) equal to 25 cm−1. This hwhm was chosen to achieve the best match between the experimental and calculated spectra.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

CIF files. and XYZ files. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00393. X-ray experimental data for 2 and 6b, additional characterization data, details of the calculations, and the full Gaussian reference (PDF) X-ray experimental data for 2 and 6b (CIF) Cartesian coordinates for the calculated structures (XYZ)

Corresponding Author

*E-mail for E.A.I.: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge North Carolina State University and the National Science Foundation via the CAREER Award (CHE0955636) for funding.

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REFERENCES

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DOI: 10.1021/acs.organomet.6b00393 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.6b00393 Organometallics XXXX, XXX, XXX−XXX