Heterobimetallic Nitrido Complexes of Group 8 Metalloporphyrins

Apr 21, 2017 - Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People's Republic of ...
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Heterobimetallic Nitrido Complexes of Group 8 Metalloporphyrins Wai-Man Cheung, Wai-Hang Chiu, Matthew de Vere-Tucker, Herman H.-Y. Sung, Ian D. Williams, and Wa-Hung Leung* Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China S Supporting Information *

ABSTRACT: Heterobimetallic nitrido porphyrin complexes with the [(L)(por)M−N−M′(LOEt)Cl2] formula {por2− = 5,10,15,20-tetraphenylporphyrin (TPP2−) or 5,10,15,20-tetra(ptolyl)porphyrin (TTP2−) dianion; LOEt− = [Co(η5-C5H5){P(O)(OEt)2}3]−; M = Fe, Ru, or Os; M′ = Ru or Os; L = H2O or pyridine} have been synthesized, and their electrochemistry has been studied. Treatment of trans-[Fe(TPP)(py)2] (py = pyridine) with Ru(VI) nitride [Ru(LOEt)(N)Cl2] (1) afforded Fe/Ru μ-nitrido complex [(py)(TPP)Fe(μ-N)Ru(LOEt)Cl2] (2). Similarly, Fe/Os analogue [(py)(TPP)Fe(μ-N)Os(LOEt)Cl2] (3) was obtained from trans-[Fe(TPP)(py)2] and [Os(LOEt)(N)Cl2]. However, no reaction was found between trans-[Fe(TPP)(py)2] and [Re(LOEt)(N)Cl(PPh3)]. Treatment of trans-[M(TPP)(CO)(EtOH)] with 1 afforded μ-nitrido complexes [(H2O)(TPP)M(μ-N)Ru(LOEt)Cl2] [M = Ru (4a) or Os (5)]. TTP analogue [(H2O)(TTP)Ru(μ-N)Ru(LOEt)Cl2] (4b) was prepared similarly from trans-[Ru(TTP)(CO)(EtOH)] and 1. Reaction of [(H2O)(por)M(μ-N)M(LOEt)Cl2] with pyridine gave adducts [(py)(por)M(μ-N)Ru(LOEt)Cl2] [por = TTP, and M = Ru (6); por = TPP, and M = Os (7)]. The diamagnetism and short (por)M−N(nitride) distances in 2 [Fe−N, 1.683(3) Å] and 4b [Ru−N, 1.743(3) Å] are indicative of the MIVNM′IV bonding description. The cyclic voltammograms of the Fe/Ru (2) and Ru/Ru (4b) complexes in CH2Cl2 displayed oxidation couples at approximately +0.29 and +0.35 V versus Fc+/0 (Fc = ferrocene) that are tentatively ascribed to the oxidation of the {LOEtRu} and {Ru(TTP)} moieties, respectively, whereas the Fe/Os (3) and Os/Ru (5) complexes exhibited Os-centered oxidation at approximately −0.06 and +0.05 V versus Fc+/0, respectively. The crystal structures of 2 and 4b have been determined.

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INTRODUCTION The nitrido (N3−) group is an electronically flexible bridging ligand that, depending upon the extent of π-electron delocalization, can bind to two metal ions in an either symmetric or unsymmetric fashion to give a linear (I−III) or bent (IV) M−N−M′ bridge (Scheme 1).1 Our interest in bimetallic

porphyrin (por) dianion]. The cation radical species [(mCBA)(TPP)FeIV−N−FeIVO(TPP•+)] (m-CBA = 3-chlorobenzoate) generated by the oxidation of [Fe(TPP)]2(μ-N) (TPP2− = tetraphenylporphyrin dianion) with m-chloroperoxybenzoic acid at low temperatures has been characterized spectroscopically.5 The nitrido-bridged diiron complexes were found to be more active as oxidation catalysts than the oxo and carbido analogues or mononuclear Fe(III) porphyrin/phthalocyanine complexes are, indicating that the μ-nitrido ligand plays a role in stabilizing the reactive high-valence diiron oxo species. Theoretical studies suggest that the μ-nitrido ligand in the diiron complexes may act as a charge reservoir and lower the SOMO energy through strong π-stabilization, thus facilitating C−H oxidation by FeO.2,6−8 In an effort to explore the oxidation chemistry of bimetallic nitrido complexes, we sought to synthesize μ-nitrido complexes of group 8 metalloporphyrins. Dinuclear nitrido porphyrin complexes are generally prepared by photolysis/thermolysis of the azido precursors [M(por)N3] (por2− = porphyrin dianion).9

Scheme 1. Bonding Modes of Nitrido-Bridged Bimetallic Complexes (ref 1)

nitrido porphyrin complexes is stimulated by the reports that nitrido-bridged diiron phthalocyanine and porphyrin complexes that contain symmetric Fe−N−Fe units are capable of catalyzing the C−H oxidation of hydrocarbons, including methane, and dehalogenation of aromatics with H2O2 under mild conditions.2−4 The active species for the diiron-catalyzed oxidations are believed to be high-valence diiron(IV) oxo species, [(L)FeIV−N−FeIVO(L•+)] [L2− = phthalocyanine (pc) or © 2017 American Chemical Society

Received: February 3, 2017 Published: April 21, 2017 5680

DOI: 10.1021/acs.inorgchem.7b00281 Inorg. Chem. 2017, 56, 5680−5687

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Inorganic Chemistry Scheme 2. Ru μ-Nitrido Complexes Derived from 1

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It is believed that the photolysis/thermolysis of [M(por)N3] generates a reactive terminal nitrido species, [M(por)(N)], that couples with [M(por)]+ to yield [M(por)]2(μ-N). For example, the photolysis of [Fe(TPP)(N3)] affords [Fe(TPP)]2(μ-N)10 that can be oxidized to the diiron(IV) species [{FeIV(TPP)}2(μ-N)]+.11 Related heterobimetallic nitrido complexes have been prepared similarly by coupling of MN with metalloporphyrins/phthalocyanines.9,12 Dinuclear nitrido complexes have also been invoked as intermediates in inter-metal N atom transfer of nitrido-metalloporphyrins.13 Although terminal nitrido complexes of Ru and Os porphyrins are welldocumented,14,15 dinuclear Ru and Os μ-nitrido porphyrin complexes are rare.16 Very few heterobimetallic nitrido complexes of Ru porphyrins and phthalocyanines, e.g., [(OEP)(NO)Ru(μ-N)OsO3] (OEP2− = octaethylporphyrin dianion),9,17,18 have been synthesized and structurally characterized. In this work, the Ru(VI) nitride [Ru(LOEt )(N)Cl2 ] [1 (Scheme 2)] supported by the Kläui tripodal ligand LOEt− {LOEt− = [Co(η5-C5H5){P(O)(OEt)2}3]−} was used as the starting material for heterodinuclear nitrido complexes because our previous work has shown that 1 displays electrophilic reactivity toward a variety of nucleophiles, including hydride and aryl compounds.19 In addition, 1 can function as an innersphere two-electron oxidant and reacts with organoruthenium(II) complexes to give diruthenium nitrido complexes. For example, reactions of 1 with [RuII(η6-p-cymene)Cl2]2 and [RuII(η5-indenyl)2] afforded tetra- and trinuclear μ-nitrido complexes V and VI, respectively (Scheme 2), that possess RuIVNRuIV bridges.20,21 On the other hand, treatment of 1 with [Ru(H)(CO)Cl(PCy3)2] (Cy = cyclohexyl) gave a RuVIN−RuII complex, VII (Scheme 2),20 presumably because the electron-withdrawing carbonyl ligand inhibits the oxidation of the Ru(II) hydride. Encouraged by the successes in isolation of Ru μ-nitrido complexes V and VI, we sought to synthesize dinuclear nitrido porphyrin complexes derived from 1. Herein, we describe the synthesis and structures of heterometallic μ-nitrido complexes [(L)(TPP)M(μ-N)Ru(LOEt)Cl2] (M = Fe, Ru, or Os; L = H2O or pyridine) by reactions of 1 with group 8 metalloporphyrins. The electrochemistry of the μ-nitrido porphyrin complexes has been investigated using cyclic voltammetry.

EXPERIMENTAL SECTION

General Considerations. All manipulations were performed under nitrogen using standard Schlenk techniques. Solvents were purified by standard procedures and distilled prior to use. NMR spectra were recorded on a Bruker AV 400 spectrometer operating at 400, 376.5, and 162 MHz for 1H, 19F, and 31P, respectively. Chemical shifts (δ, parts per million) were reported with reference to SiMe4 (1H and 13C), CF3C6H5 (19F), and 85% H3PO4 (31P). Infrared spectra (KBr) were recorded on a PerkinElmer 16 PC FT-IR spectrophotometer. Cyclic voltammetry was performed with a CH Instruments model 600D potentiostat. The working and reference electrodes were glassy carbon and Ag/AgNO3 (0.1 M in acetonitrile) electrodes, respectively. Potentials were reported with reference to the ferrocenium/ferrocene (Fc+/0) couple. Ultraviolet−visible (UV−vis) spectra were recorded on a PerkinElmer LAMBDA 900 UV/vis/NIR spectrometer. Elemental analyses were performed by Medac Ltd. (Surrey, U.K.). Porphyrin complexes trans-[Fe(TPP)(py)2],22 trans-[Ru(por)(CO)(EtOH)] (por = TPP or TTP),23 and trans-[Os(TPP)(CO)(EtOH)]24 were prepared according to literature methods. Nitrido complexes [M′(LOEt)(N)Cl2] [M′ = Ru (1)19 or Os25] were synthesized as described elsewhere. The atom labeling scheme for the TPP and TTP ligands is shown in Scheme 3. UV−visible spectral data of bimetallic μ-nitrido porphyrin complexes are listed in Table 1.

Scheme 3. Atom Labeling Scheme for the TPP and TTP Ligands

Syntheses. [(py)(TPP)Fe(μ-N)Ru(LOEt)Cl2] (2). To a solution of trans-[Fe(TPP)(py)2] (80 mg, 0.10 mmol) in tetrahydrofuran (THF) (10 mL) was added 1 equiv of 1 (72 mg, 0.10 mmol). The reaction mixture was stirred at room temperature for 3 h. The solvent was 5681

DOI: 10.1021/acs.inorgchem.7b00281 Inorg. Chem. 2017, 56, 5680−5687

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Inorganic Chemistry

3.12−3.40 (m, 8H, OCH2), 4.36 (s, 5H, Cp), 7.66 (m, 12H, Hm and Hp), 8.12 (d, 3JHH = 6.8 Hz, 4H, Ho), 8.23 (d, 3JHH = 7.6 Hz, 4H, Ho), 8.80 (s, 8H, Hβ); 31P{1H} NMR (CDCl3) δ 109.45−116.49 (m). Anal. Calcd for C61H65Cl2CoN5O10OsP3Ru·0.5CH2Cl2: C, 46.61; H, 4.20; N, 4.42. Found: C, 46.52; H, 3.95; N, 4.51. [(py)(TTP)Ru(μ-N)Ru(LOEt)Cl2] (6). To a solution of 4b (50 mg, 0.033 mmol) in THF (10 mL) was added 1.2 equiv of pyridine (3 μL, 0.04 mmol). The reaction mixture was stirred at room temperature for 3 h. The solvent was pumped off, and the residue was washed with hexanes. Recrystallization from CH2Cl2/Et2O/hexanes afforded purple crystals: yield 46 mg, 89%; 1H NMR (CDCl3) δ 0.76 (t, 3JHH = 7.2 Hz, 6H, CH3), 0.82 (t, 3JHH = 6.8 Hz, 6H, CH3), 0.88 (m, 2H, py), 1.03 (t, 3JHH = 6.8 Hz, 6H, CH3), 2.24 (m, 2H, OCH2), 2.34 (m, 2H, OCH2), 2.67 (s, 12H, CH3), 3.18−3.46 (m, 8H, OCH2), 4.40 (s, 5H, Cp), 4.94 (t, 3JHH = 6.0 Hz, 2H, py), 5.93 (t, 3JHH = 8.0 Hz, 1H, py), 7.46 (m, 8H, Hm), 7.89 (d, 3JHH = 7.2 Hz, 4H, Ho), 8.27 (d, 3JHH = 7.2 Hz, 4H, Ho), 8.81 (s, 8H, Hβ); 31P{1H} NMR (CDCl3) δ 113.66− 117.19 (m). Anal. Calcd for C70H76Cl2CoN6O9P3Ru2·0.5CH2Cl2: C, 52.50; H, 4.81; N, 5.21. Found: C, 52.41; H, 4.61; N, 5.07. [(py)(TPP)Os(μ-N)Ru(LOEt)Cl2] (7). This compound was prepared using a procedure similar to that used for 7 using 5 (50 mg, 0.032 mmol) in place of 4b. Recrystallization from CH2Cl2/Et2O/hexanes afforded purple crystals: yield 43 mg, 83%; 1H NMR (CDCl3) δ 0.77 (t, 3JHH = 7.2 Hz, 6H, CH3), 0.83 (t, 3JHH = 6.8 Hz, 6H, CH3), 1.00 (d, 3JHH = 6.8 Hz, 2H, py), 1.04 (t, 3JHH = 7.2 Hz, 6H, CH3), 2.29 (m, 2H, OCH2), 2.41 (m, 2H, OCH2), 3.21−3.56 (m, 8H, OCH2), 4.42 (s, 5H, Cp), 5.05 (t, 3JHH = 6.8 Hz, 2H, py), 6.04 (t, 3JHH = 6.0 Hz, 1H, py), 7.69 (m, 12H, Hm and Hp), 8.01 (d, 3JHH = 6.8 Hz, 4H, Ho), 8.41 (d, 3JHH = 6.8 Hz, 4H, Ho), 8.77 (s, 8H, Hβ); 31P{1H} NMR (CDCl3) δ 109.53−116.67 (m). Anal. Calcd for C66H68Cl2CoN6O9OsP3Ru· 0.5CH2Cl2: C, 48.53; H, 4.23; N, 5.11. Found: C, 48.56; H, 4.18; N, 4.98. X-ray Crystallography. Crystallographic data and experimental details for complexes 2 and 4b are summarized in the Supporting Information. Intensity data were collected on a Bruker SMART APEX 1000 CCD diffractometer using graphite-monochromated Mo Kα radiation. The collected frames were processed with the software SAINT. Structures were determined by direct methods and refined by full-matrix least squares on F2 using the SHELXTL software package.26,27 Atomic positions of non-hydrogen atoms were refined with anisotropic parameters and with suitable restraints. Disordered atoms were refined isotropically. Hydrogen atoms were generated geometrically and allowed to ride on their respective parent carbon atoms before the final cycle of least-squares refinement. CCDC files 1497204 and 1497205 contain the supplementary crystallography data for complexes 2 and 4b, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif.

Table 1. Ultraviolet−Visible Spectral Data for Bimetallic μ-Nitrido Porphyrin Complexes in CH2Cl2 complex

λmax (nm) (log ε)

[(py)(TPP)Fe(μ-N)Ru(LOEt)Cl2] (2) [(py)(TPP)Fe(μ-N)Os(LOEt)Cl2] (3) [(H2O)(TPP)Ru(μ-N)Ru(LOEt)Cl2] (4a) [(H2O)(TTP)Ru(μ-N)Ru(LOEt)Cl2] (4b) [(H2O)(TPP)Os(μ-N)Ru(LOEt)Cl2] (5) [(py)(TTP)Ru(μ-N)Ru(LOEt)Cl2] (6) [(py)(TPP)Os(μ-N)Ru(LOEt)Cl2] (7)

392 (4.91), 420 (4.88), 528 (4.16) 418 (4.45), 564 (3.96) 410 sh (4.76), 432 (4.83), 551 (4.19) 414 sh (4.97), 431 (5.03), 549 (4.39) 430 (5.17), 545 (4.30) 442 (4.84), 556 (4.17), 592 (3.66) 434 (5.70), 550 (4.86)

removed in vacuo, and the residue was chromatographed over silica gel using a CH2Cl2/ethyl acetate [2:1 (v/v)] eluent. The product was isolated as a reddish orange band. Recrystallization from CH2Cl2/ Et2O/hexanes afforded purple crystals: yield 77 mg, 55%; 1H NMR (CDCl3) δ 0.76 (t, 3JHH = 7.2 Hz, 6H, CH3), 0.89 (overlapping t and m, 3JHH = 7.2 Hz, 8H, CH3 and py), 1.05 (t, 3JHH = 7.2 Hz, 6H, CH3), 2.17 (m, 2H, OCH2), 2.36 (m, 2H, OCH2), 3.38 (m, 8H, OCH2), 4.42 (s, 5H, Cp), 5.15 (m, 2H, py), 6.11 (m, 1H, py), 7.63 (m, 12H, Hm and Hp), 7.93 (m, 4H, Ho), 8.37 (m, 4H, Ho), 8.83 (s, 8H, Hβ); 31P{1H} NMR (CDCl3) δ 114.32−118.59 (m). Anal. Calcd for C66H68Cl2CoFeN6O9P3Ru·0.5CH2Cl2: C, 52.85; H, 4.60; N, 5.56. Found: C, 52.56; H, 4.73; N, 5.34. [(py)(TPP)Fe(μ-N)Os(LOEt)Cl2] (3). This compound was prepared using a procedure similar to that used for 2 using [Os(LOEt)(N)Cl2] (81 mg, 0.10 mmol) in place of 1. The product was purified by silica gel chromatography using a CH2Cl2/ethyl acetate [5:1 (v/v)] eluent and recrystallized from THF/Et2O/hexanes as purple crystals: yield 77 mg, 55%; 1H NMR (C6D6) δ 0.88−1.11 (m, 18H, CH3), 1.60 (m, 2H, py), 3.76−4.02 (m, 12H, OCH2), 4.03−4.30 (m, 4H, OCH2), 4.21 (m, 2H, py), 4.49 (m, 5H, Cp), 4.98 (m, 1H, py), 7.49 (m, 12H, Hm and Hp), 8.47 (m, 8H, Ho), 9.08 (s, 8H, Hβ); 31P{1H} NMR (C6D6) δ 110.37−123.75 (m). Anal. Calcd for C66H68Cl2CoFeN6O9OsP3· C4H8O: C, 51.57; H, 4.70; N, 5.16. Found: C, 51.45; H, 4.71; N, 4.99. [(H2O)(por)Ru(μ-N)Ru(LOEt)Cl2] [por = TPP (4a) or TTP (4b)]. To a solution of trans-[Ru(por)(CO)(EtOH)] (79 mg for TPP and 84 mg for TTP, 0.10 mmol) in CH2Cl2 (10 mL) was added 1 equiv of 1 (72 mg, 0.10 mmol). The reaction mixture was stirred at room temperature for 3 h. The solvent was pumped off, and the residue was chromatographed over silica gel using a CH2Cl2/ethyl acetate [2:1 (v/v)] eluent, and the reddish orange band was collected. Recrystallization from CH2Cl2/Et2O/hexanes afforded red crystals. 4a: yield 80 mg, 55%; 1H NMR (CDCl3) δ 0.73 (t, 3JHH = 7.2 Hz, 6H, CH3), 0.80 (t, 3JHH = 7.2 Hz, 6H, CH3), 1.03 (t, 3JHH = 7.2 Hz, 6H, CH3), 2.26 (m, 2H, OCH2), 2.34 (m, 2H, OCH2), 3.17−3.31 (m, 8H, OCH2), 4.40 (s, 5H, Cp), 7.73 (m, 8H, Hm and Hp), 8.19 (d, 3 JHH = 7.2 Hz, 4H, Ho), 8.38 (d, 3JHH = 7.2 Hz, Ho), 8.90 (s, 8H, Hβ); 31 1 P{ H} NMR (CDCl3) δ 111.78−117.43 (m). Anal. Calcd for C61H65Cl2CoN5O10P3Ru2·CH2Cl2·H2O: C, 47.86; H, 4.47; N, 4.50. Found: C, 48.09; H, 4.92; N, 4.44. 4b: yield 101 mg, 67%; 1H NMR (CDCl3) δ 0.79 (t, 3JHH = 7.2 Hz, 6H, CH3), 1.02 (t, 3JHH = 7.2 Hz, 6H, CH3), 1.11 (t, 3JHH = 7.2 Hz, 6H, CH3), 2.21 (m, 2H, OCH2), 2.31 (m, 2H, OCH2), 2.69 (s, 12H, CH3), 3.16−3.44 (m, 8H, OCH2), 4.38 (s, 5H, Cp), 7.50 (t, 3JHH = 6.8 Hz, 8H, Hm), 8.04 (d, 3JHH = 7.6 Hz, 4H, Ho), 8.26 (d, 3JHH = 7.6 Hz, Ho), 8.90 (s, 8H, Hβ); 31P{1H} NMR (CDCl3) δ 111.66− 117.19 (m). Anal. Calcd for C65H73Cl2CoN5O10P3Ru2: C, 51.73; H, 4.88; N, 4.64. Found: C, 51.66; H, 5.28; N, 4.21. [(H2O)(TPP)Os(μ-N)Ru(LOEt)Cl2] (5). This compound was prepared using a procedure similar to that used for 4a using trans-[Os(TPP)(CO)(EtOH)] (88 mg, 0.10 mmol) in place of trans-[Ru(TPP)(CO)(EtOH)]. Recrystallization from CH2Cl2/Et2O/hexanes afforded purple crystals: yield 66 mg, 43%; 1H NMR (CDCl3) δ 0.70 (t, 3JHH = 7.2 Hz, 6H, CH3), 0.76 (t, 3JHH = 7.2 Hz, 6H, CH3), 1.01 (t, 3JHH = 7.2 Hz, 6H, CH3), 2.22 (m, 2H, OCH2), 2.31 (m, 2H, OCH2),

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RESULTS AND DISCUSSION Fe/M (M = Ru or Os) Complexes. Treatment of trans[Fe(TPP)(py)2] with 1 afforded the air-stable bimetallic Fe/Ru nitrido complex [(py)(TPP)Fe(μ-N)Ru(L OEt )Cl 2 ] (2) (Scheme 4). Complex 2 is diamagnetic and exhibited sharp 1 H NMR spectral signals that can be assigned to the TPP, Scheme 4. Syntheses of Fe/M (M = Ru or Os) μ-Nitrido Complexes

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Inorganic Chemistry LOEt−, and pyridine ligands. The UV−vis spectrum of 2 in CH2Cl2 showed the Soret band with absorption maxima at 392 and 400 nm that are similar to that of diiron(IV) complex [{FeIV(TPP)}2(μ-N)](SbCl6) (398 nm).10 The diamagnetism and observed short Fe−N(nitride) distance (vide inf ra) in 2 are consistent with the FeIVNRuIV bonding description [type I (Scheme 1)]. Reported isoelectronic diiron(IV) nitrido complexes such as [(TPP)Fe(μ-N)Fe(Pc)](I5)18 that possess FeIVNFeIV linkages are also diamagnetic. Similarly, treatment of trans-[Fe(TPP)(py)2] with Os(VI) nitride [Os(LOEt)(N)Cl2] afforded Fe/Os complex [(py)(TPP)Fe(μ-N)Os(LOEt)Cl2] (3), which, according to a preliminary X-ray diffraction study, also contains an FeNOs unit (see the Supporting Information). Unfortunately, we have not been able to refine the structure of 3 satisfactorily because of the poor quality of the crystal. As opposed to 2, 3 is somewhat air-sensitive in solution, as evidenced by NMR spectroscopy. After exposure to air for 1 h, the sharp 1H NMR spectral signals of 3 in C6D6 became very broad and ill-defined, suggesting that a paramagnetic species was formed. We were not able to isolate and characterize the air oxidation product of 3. In contrast with the Ru(VI) and Os(VI) analogues, Re(V) nitride [(LOEt)Re(N)(PPh3)Cl] does not react with trans-[Fe(TPP)(py)2].25 This can be rationalized by the poor interaction between the nucleophilic Re(V) nitride25 and the electron-rich Fe(II)(TPP) complex. By contrast, the electrophilic Ru(VI) and Os(VI) nitrides that can be viewed as π-acceptor metalloligands (vide inf ra) bind to Fe(TPP) tightly through π back-donation. M/Ru (M = Ru or Os) Complexes. Treatment of trans[M(TPP)(CO)(EtOH)] with 1 afforded μ-nitrido complexes [(H2O)(TPP)M(μ-N)Ru(LOEt)Cl2] [M = Ru (4a) or Os (5)] (Scheme 5) that were isolated as air-stable purple solids. The

tetra(p-tolyl)porphyrin (TTP) analogue [(H2O)(TTP)Ru(μN)Ru(LOEt)Cl2] (4b) that can crystallize more easily has been prepared from 1 and trans-[Ru(TTP)(CO)(EtOH)]. The structure of 4b has been unambiguously established by X-ray crystallography (vide inf ra). The observed diamagnetism and short Ru−N(nitride) distance in 4b (vide inf ra) are consistent with the RuIVNRuIV formulation. Although the crystal structure of 5 has not been determined, we believe that the diamagnetic Os/Ru complex also possesses a RuIVNOsIV linkage. The formation of complexes 4a and 5 probably involves the binding of the Ru(VI) nitride to M(II)(TPP) (M = Ru or Os), electron transfer through the nitrido bridge, and oxidative decarbonylation. NMR and UV−vis spectroscopy indicated that 1 reacted with trans-[Ru(TPP)(CO)(EtOH)] very rapidly in CDCl3 at room temperature to yield 4a as the sole product. The reaction between 1 and trans-[Os(TPP)(CO)(EtOH)] is slower, presumably because of the stronger Os−CO bond, and has been monitored by UV−vis spectroscopy (Figure 1). The

Scheme 5. Syntheses of M/Ru (M = Ru or Os) μ-Nitrido Complexes Figure 1. UV−vis spectral change for the reaction of trans-[Os(TPP)(CO)(EtOH)] with 1 in CH2Cl2 at room temperature at 2 min time intervals. Concentrations of both reactants were 3.9 × 10−3 mM.

observation of well-defined isosbestic points at ∼422, ∼508, and ∼576 nm indicates that it is a clean reaction without the involvement of any long-lived intermediate(s). The 1H NMR spectrum of the reaction mixture of 1 and trans-[Os(TPP)(CO)(EtOH)] in CDCl3 displayed signals that can be attributed to 1, trans-[Os(TPP)(CO)(EtOH)], and 5 only; no intermediate was observed during the reaction. Previously, Mayer and co-workers suggested that because of the low-lying nitrido-based LUMO of the M−N π-bonding, electrophilic nitrido complexes of late transition metals (e.g., Os) can behave like π-acceptor ligands (cf. CO).31 Therefore, the formation of 4a and 5 can be viewed as the substitution of the carbonyl ligand in [M(TPP)(CO)] with the strong π-accepting metalloligand 1, with concomitant two-electron oxidation of M(II) and decarbonylation. Also, this reaction is reminiscent of the oxidation of [RuII(por)(CO)(EtOH)] with nitrene precursors such as RN3 (R = aryl) and PhINTs (Ts = tosyl) to give bis(imido) complexes [(por)RuVI(NR)2],32 presumably via transient Ru(IV) monoimido intermediates, “[(por)RuIVNR]”. The aqua ligands in 4b and 5 are labile and can be easily substituted with Lewis bases. For example, treatment of 4b and 5 with pyridine yielded adducts [(py)(TTP)Ru(μ-N)Ru(LOEt)Cl2] (6)

1

H NMR spectra of 4a and 5 displayed sharp signals in the normal region, indicative of diamagnetic behavior. The pyrrolic protons appeared as sharp singlets at δ 8.90 and 8.80, respectively, which are typical for diamagnetic Ru(IV)28,29 and Os(IV)30 porphyrins. The UV−vis spectrum of 4a displayed the Soret band centered at 414 nm that is lower in energy than that for Ru(IV) μ-oxo dimer [RuIV(TPP)(OMe)]2(μ-O) (398 nm).23 Unfortunately, we were not able to obtain quality crystals of 4a for structure determination. Therefore, the 5683

DOI: 10.1021/acs.inorgchem.7b00281 Inorg. Chem. 2017, 56, 5680−5687

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Inorganic Chemistry

chelating N donor ligands, e.g., [{FeIV(bpb)(CN)}2(μ-N)]− [1.662(4) and 1.656(4) Å; bpb2− = 1,2-bis(pyridine-2-carboxamido)benzenate(2-)], 33 [{LFe IV (nadiol)} 2 (μ-N)]PF6 [1.69(1) Å; L = 1,4,7-trimethyl-1,4,7-triazacyclononane; nadiol2− = naphthalene-2,3-diolate],34 and [{L(Cl4cat)FeIV}2(μ-N)]Br [1.703(1) Å; Cl4cat2− = tetrachlorocatecholate].35 The Fe−nitride distance in 2 is, however, significantly shorter than that in [PhB(MesIm)3FeII−NVV(Mes)3] [1.953(7) Å; PhB(MesIm) = a tripodal tris(carbene)borate ligand; Mes = mesityl], in which the Fe−nitride bond is formulated as a single bond.36 The short Ru−N and Fe−N distances in 2 and its observed diamagnetism are indicative of the FeIVNRuIV bonding description [type I (Scheme 1)]. The molecular structure of 4b that features an approximately linear Ru−N−Ru unit [∠Ru−N−Ru = 174.11(18)°] is shown in Figure 3. Selected bond lengths and angles are listed in Table 3. The (LOEt)Ru−N distance in 4b of 1.716(3) Å is similar to that in 2 (vide inf ra) and consistent with a RuN bond. The (TPP)Ru−N(nitride) distance in 4b of 1.743(3) Å is shorter than those in the Ru(VI) nitrido {e.g., 1.656(5) Å in [RuVI{3,4,5-MeO-TPP}(N)(OH)], where 3,4,5-MeO-TPP = tetrakis(3,4,5-trimethoxyphenyl)porphyrin dianion}15 and imido {e.g., 1.808(4) and 1.806(4) Å in [RuVI(TPP)(NR)2], where R = 3,5-bis(trifluoromethyl)phenyl} complexes, but shorter than those in the Ru(IV) amido porphyrins {e.g., 1.956(7) Å in [Ru(TTP)(NHR)2], where R = 4-chlorophenyl},29 indicative of double-bond character. The observed short Ru−N bond distances in 4b together with its diamagnetism are suggestive of the RuIVNRuIV bonding description featuring antiferromagnetic coupling between the two d4 Ru(IV) centers. Electrochemistry. The electrochemistry of the heterobimetallic nitrido porphyrin complexes has been studied using cyclic voltammetry (see Figures S1−S6), and the formal potentials (E1/2) are summarized in Table 4. The cyclic voltammogram of Fe/Ru complex 2 displayed redox waves at +0.29,38 +0.77, and +1.20 V. The couple at +0.77 V can be ascribed to the porphyrin ring oxidation because similar potentials have been found for reported Fe(TPP) complexes.37 The origin of

and [(py)(TPP)Os(μ-N)Ru(L OEt )Cl 2 ] (7), respectively (Scheme 5). Crystal Structures. The molecular structure of 2 that features an approximately linear Fe−N−Ru unit [∠Ru−N−Ru = 172.7(2)°] is shown in Figure 2. Selected bond lengths and

Figure 2. Molecular structure of 2. The ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for the sake of clarity.

angles are listed in Table 2. The (LOEt)Ru−N bond [1.704(3) Å] is significantly longer than the RuN bond in 1 [1.573(6) Å]19 but comparable to the RuN double bonds in [{(LOEt)RuIVCl2}2N]− [1.718(4) and 1.737(4) Å].20 The Fe−N(nitride) distance in 2 of 1.683(3) Å is longer than those in diiron(IV) nitrido porphyrins, e.g., [{FeIV(TTP)}2N](SbCl6) [1.6280(7) Å] and [(THF)(TPP)FeIV(μ-N)FeIV(pc)H2O)]+ [1.63(2) and 1.65(2) Å],10 but similar to those in related complexes with

Table 2. Selected Bond Lengths (angstroms) and Angles (degrees) of 2 Ru(1)−N(6) Ru(1)−Cl(1) Ru(1)−Cl(2) Ru(1)−O(7) Ru(1)−O(8) Ru(1)−O(9) Fe(1)−N(6)−Ru(1) Cl(1)−Ru(1)−Cl(2) Cl(1)−Ru(1)−O(7) Cl(1)−Ru(1)−O(8) Cl(1)−Ru(1)−O(9) Cl(1)−Ru(1)−N(6) Cl(2)−Ru(1)−O(7) Cl(2)−Ru(1)−O(8) Cl(2)−Ru(1)−O(9) Cl(2)−Ru(1)−N(6) O(7)−Ru(1)−O(8) O(7)−Ru(1)−O(9) O(7)−Ru(1)−N(6) O(8)−Ru(1)−O(9) O(8)−Ru(1)−N(6) O(9)−Ru(1)−N(6)

1.704(3) 2.3368(9) 2.3260(9) 2.083(2) 2.124(3) 2.066(3) 172.7(2) 90.77(3) 174.03(7) 86.67(7) 90.48(8) 94.47(10) 90.44(8) 83.72(8) 168.35(8) 95.54(10) 87.64(10) 87.17(10) 91.23(12) 84.80(11) 178.65(12) 95.91(12)

Fe(1)−N(6) Fe(1)−N(1) Fe(1)−N(2) Fe(1)−N(3) Fe(1)−N(4) Fe(1)−N(5) N(1)−Fe(1)−N(2) N(1)−Fe(1)−N(3) N(1)−Fe(1)−N(4) N(1)−Fe(1)−N(5) N(1)−Fe(1)−N(6) N(2)−Fe(1)−N(3) N(2)−Fe(1)−N(4) N(2)−Fe(1)−N(5) N(2)−Fe(1)−N(6) N(3)−Fe(1)−N(4) N(3)−Fe(1)−N(5) N(3)−Fe(1)−N(6) N(4)−Fe(1)−N(5) N(4)−Fe(1)−N(6) N(5)−Fe(1)−N(6)

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1.683(3) 2.009(3) 2.000(3) 1.989(3) 1.990(3) 2.135(3) 89.83(13) 173.75(12) 89.34(13) 88.35(12) 97.24(13) 89.56(13) 175.64(12) 87.88(12) 94.25(13) 90.80(12) 85.41(12) 89.01(14) 87.82(12) 90.10(13) 174.01(14)

DOI: 10.1021/acs.inorgchem.7b00281 Inorg. Chem. 2017, 56, 5680−5687

Article

Inorganic Chemistry

Table 4. Formal Potentials (E1/2) of Bimetallic μ-Nitrido Porphyrin Complexesa complex

E1/2 (V vs Fc+/0)

[(py)(TPP)Fe(μ-N)Ru(LOEt)Cl2] (2) [(py)(TPP)Fe(μ-N)Os(LOEt)Cl2] (3) [(H2O)(TPP)Ru(μ-N)Ru(LOEt)Cl2] (4a) [(H2O)(TTP)Ru(μ-N)Ru(LOEt)Cl2] (4b) [(H2O)(TPP)Os(μ-N)Ru(LOEt)Cl2] (5) [(py)(TTP)Ru(μ-N)Ru(LOEt)Cl2] (6) [(py)(TPP)Os(μ-N)Ru(LOEt)Cl2] (7)

+0.29,b +0.77,c +1.20d −0.06, +0.37, +0.62 +0.38, +0.79 +0.35, +0.80 +0.05 +0.32, +0.83c +0.07, +1.00d

a

Potentials were measured at a glassy carbon electrode in CH2Cl2 solutions with 0.2 M [n-Bu4N]PF6 as a supporting electrolyte. The scan rate was 100 mV s−1. ΔEp ∼ 100 mV. bOverlapping couples, average E1/2 value. cQuasi-reversible (ΔEp ∼ 160−170 mV). d Irreverisble, Epa value.

at −0.06 V that is tentatively assigned as the Os(LOEt)-centered couple. The observation of a low oxidation potential explains why 3 is air-sensitive in solution. As opposed to that of 2, the oxidation of the Ru/Ru (4a) and Os/Ru (5) complexes with 1 equiv of [N(C6H4Br-4)3]SbCl6 resulted in significant changes in the M(TPP)-based absorption bands in the UV−vis spectra (Figures S9 and S10), indicating the redox events at +0.38 and +0.05 V, repectively, are associated with the {M(TPP)} moieties (M = Ru and Os). Upon coordination of pyridine to the Ru(TPP) moiety in 4a, the Ru(V/IV) potential is slightly decreased to +0.32 V (complex 6). However, the Os(V/IV) potential for 5 is very similar to that of its pyridine adduct, 7 (+0.07 V). Attempts to synthesize the one-electron oxidation product of 4a or 5 failed. Treatment of 4a in CH2Cl2 with 1 equiv of [N(C6H4Br-4)3]SbCl6 afforded a green oily material. We have not been able to crystallize this green species for further characterization.

Figure 3. Molecular structure of 4b. The ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for the sake of clarity.

the oxidation wave at +1.20 V for 2 is not clear at this stage. The redox event at +0.29 V is tentatively attributed to the oxidation of the {Ru(LOEt)} moiety in 2 because no appreciable change was found for the Fe(TPP)-based absorption bands in the UV−vis spectrum (see the Supporting Information) when 1 was reacted with 1 equiv of the one-electron oxidant [N(C6H4Br-4)3]SbCl6. By comparison, the RuV−RuIV/RuIV−RuIV potential for [{(LOEt)RuIVCl2}2N]− was determined to be +0.19 V.20 Recently, dirhodium μ-nitrido complex [{(PNP)Rh}2(μ-N)] [PNP− = 6-di(tert-butyl)phosphinomethyl-2,2′bipyridine] has been shown to possess predominant nitridyl radical character, i.e., N•2−.39 Therefore, given the extensive π-delocalization in the Fe−N−Ru unit, the involvement of the nitrido ligand in the redox event at +0.29 V for 2 cannot be ruled out. Similarly, Fe/Os complex 3 exhibited a redox couple

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CONCLUSIONS In summary, a series of heterometallic mixed-ligand (por)M− N−M′(LOEt) (M = Fe, Ru, or Os; M′ = Ru or Os) complexes

Table 3. Selected Bond Lengths (angstroms) and Angles (degrees) of 4b Ru(1)−N(5) Ru(1)−Cl(1) Ru(1)−Cl(2) Ru(1)−O(7) Ru(1)−O(8) Ru(1)−O(9) Ru(1)−N(5)−Ru(2) Cl(2)−Ru(1)−Cl(1) N(5)−Ru(1)−Cl(1) N(5)−Ru(1)−Cl(2) N(5)−Ru(1)−O(7) N(5)−Ru(1)−O(8) N(5)−Ru(1)−O(9) O(7)−Ru(1)−Cl(1) O(7)−Ru(1)−Cl(2) O(7)−Ru(1)−O(8) O(7)−Ru(1)−O(9) O(8)−Ru(1)−Cl(1) O(8)−Ru(1)−Cl(2) O(8)−Ru(1)−O(9) O(9)−Ru(1)−Cl(1) O(9)−Ru(1)−Cl(2)

1.716(3) 2.3571(8) 2.3541(9) 2.062(2) 2.086(2) 2.128(2) 174.11(18) 92.56(3) 93.02(9) 94.01(10) 93.76(11) 93.60(11) 178.42(11) 90.69(7) 171.40(7) 86.17(10) 86.66(10) 172.84(7) 89.68(8) 87.95(10) 85.44(7) 85.67(8)

Ru(2)−N(5) Ru(2)−N(1) Ru(2)−N(2) Ru(2)−N(3) Ru(2)−N(4) Ru(2)−O(10) N(1)−Ru(2)−N(2) N(1)−Ru(2)−N(4) N(1)−Ru(2)−O(10) N(2)−Ru(2)−N(4) N(2)−Ru(2)−O(10) N(3)−Ru(2)−N(1) N(3)−Ru(2)−N(2) N(3)−Ru(2)−N(4) N(3)−Ru(2)−O(10) N(4)−Ru(2)−O(10) N(5)−Ru(2)−N(1) N(5)−Ru(2)−N(2) N(5)−Ru(2)−N(3) N(5)−Ru(2)−N(4) N(5)−Ru(2)−O(10) C(31)−N(1)−Ru(2) 5685

1.743(3) 2.059(3) 2.060(3) 2.050(3) 2.062(3) 2.178(2) 89.98(11) 89.16(11) 85.86(10) 172.68(11) 86.05(10) 171.85(11) 89.36(11) 90.47(11) 86.00(10) 86.63(10) 97.59(12) 94.82(11) 90.56(12) 92.50(11) 176.44(12) 125.8(2) DOI: 10.1021/acs.inorgchem.7b00281 Inorg. Chem. 2017, 56, 5680−5687

Article

Inorganic Chemistry

Nefedov, S. E.; Sorokin, A. B. An N-bridged high-valent diiron−oxo species on a porphyrin platform that can oxidize methane. Nat. Chem. 2012, 4, 1024. (6) Silaghi-Dumitrescu, R.; Makarov, S. V.; Uta, M.-M.; Dereven’kov, I. A.; Stuzhin, P. A. Redox non-innocence of a nitrido bridge in a methane-activating dimer of iron phthalocyanine. New J. Chem. 2011, 35, 1140. (7) Colomban, C.; Kudrik, E. V.; Briois, V.; Shwarbrick, J. C.; Sorokin, A. B.; Afanasiev, P. X-ray absorption and emission spectroscopies of X-bridged diiron phthalocyanine complexes (FePc)2X (X = C, N, O) combined with DFT study of (FePc)2X and their high-valent diiron oxo complexes. Inorg. Chem. 2014, 53, 11517. (8) Ansari, M.; Vyas, N.; Ansari, A.; Rajaraman, G. Oxidation of methane by an N-bridged high-valent diiron−oxo species: electronic structure implications on the reactivity. Dalton Trans. 2015, 44, 15232. (9) Floris, B.; Donzello, M. P.; Ercolani, C. Single-atom bridged dinuclear metal complexes with emphasis on phthalocyanine systems. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Elsevier Science: San Diego, 2003; Vol. 18, pp 1−62, and references cited therein. (10) Summerville, D. A.; Cohen, I. A. Metal-metal interactions involving metalloporphyrins. III. Conversion of tetraphenylporphinatoiron(III) azide to an N-bridged hemin dimer. J. Am. Chem. Soc. 1976, 98, 1747. (11) Li, M.; Shang, M.; Ehlinger, N.; Schulz, C. E.; Scheidt, W. R. Molecular Structure of an Fe(IV) Species: {[Fe(TTP)]2N}SbCl6. Inorg. Chem. 2000, 39, 580. (12) (a) Ercolani, C.; Jubb, J.; Pennesi, G.; Russo, U.; Trigiante, G. (μ-Nitrido)((tetraphenylporphyrinato)iron)((phthalocyaninato)iron) and its Fe-Ru analogue: redox behavior and characterization of new Fe(IV)-containing species. X-ray crystal structure of [(THF) (TPP)Fe-N-FePc(H2O)](I5)·2THF. Inorg. Chem. 1995, 34, 2535. (b) Donzello, M. P.; Ercolani, C.; Kadish, K. M.; Ou, Z.; Russo, U. Synthesis, chemical-physical characterization, and redox properties of a new mixed-ligand heterobimetallic N-bridged dimer: (μ-nitrido)[((tetraphenylporphyrinato)manganese)((phthalocyaninato)iron)]. Inorg. Chem. 1998, 37, 3682. (c) Kudrik, E. V.; Afanasiev, P.; Sorokin, A. B. Synthesis and properties of FeIII-N = MnIV heterometallic complex with tetra-tert-butylphthalocyanine ligands. Makrogeterotsikly 2010, 3, 19. (13) (a) Woo, L. K.; Goll, J. G. Multielectron redox reactions between manganese porphyrins mediated by nitrogen atom transfer. J. Am. Chem. Soc. 1989, 111, 3755. (b) Woo, L. K. Intermetal oxygen, sulfur, selenium, and nitrogen atom transfer reactions. Chem. Rev. 1993, 93, 1125. (c) Bottomley, L. A.; Neely, F. L. The nitrogen atom transfer reactivity of nitridomanganese(V) porphyrins with chromium(III) porphyrins. J. Am. Chem. Soc. 1989, 111, 5955. (14) Antipas, A.; Buchler, J. W.; Gouterman, M.; Smith, P. D. Porphyrins. 39. Ammine and nitridoosmium porphyrins. Ligand effects on the electronic structure of osmium octaethylporphyrins. J. Am. Chem. Soc. 1980, 102, 198. (15) Leung, S. K.-Y.; Huang, J.-S.; Liang, J.-L.; Che, C.-M.; Zhou, Z.Y. Nitrido ruthenium porphyrins: Synthesis, characterization, and amination reactions with hydrocarbon or silyl enol ethers. Angew. Chem., Int. Ed. 2003, 42, 340. (16) Bottomley, L. A.; Tong, C.; Ding, Y.; Neely, F. L. Synthesis of homobinuclear μ-nitrido ruthenium and osmium porphyrins. 231st ACS National Meeting, Atlanta, March 2006, Abstract INORG 727. (17) Leung, W.-H.; Chim, J. L. C.; Lai, W.; Lam, L.; Wong, W.-T.; Chan, W.-H.; Yeung, C.-H. Heterobimetallic nitrido-bridged Ru(II)NOs(VIII) and Ru(II)NOs(VI) complexes containing ruthenium porphyrins. Inorg. Chim. Acta 1999, 290, 28. (18) (a) Rossi, G.; Gardini, M.; Pennesi, G.; Ercolani, C.; Goedken, V. L. ruthenium phthalocyanine chemistry: synthesis and properties of a mixed valence nitrido-bridged ruthenium phthalocyanine dimer. J. Chem. Soc., Dalton Trans. 1989, 193. (b) Bonomo, L.; Solari, E.; Scopelliti, R.; Floriani, C. Ruthenium nitrides: redox chemistry and

have been synthesized and characterized. This allows us to investigate systematically the effect of the metal center (M and M′) on the bonding and redox property of group 8 bimetallic nitrido porphyrin complexes. The observed diamagnetism and short metal−nitride bond distances in these bimetallic complexes are indicative of the MIVNM′IV bonding description. The aqua ligands in the Ru/Ru and Os/Ru complexes (4a/b and 5) are labile and can be easily replaced by Lewis bases such as pyridine. Cyclic voltammetry indicates that the M−N−M′ complexes display rich redox chemistry. Therefore, one would anticipate that higher-valence XM−N−M′ (X = oxo or imido) complexes can be accessible through oxo/imido transfer to M−N−M′ precursors. The isolation and characterization of such high-valence bimetallic nitrido complexes are underway.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00281. Crystallographic data and experimental details for 2 and 4b (Table S1), cyclic voltammograms, UV−vis spectral change for the oxidation with [N(C6H4Br-4)3]SbCl6, and 1 H NMR spectra of the complexes (PDF) CheckCIF/PLATON report (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wa-Hung Leung: 0000-0001-5961-6772 Notes

The authors declare no competing financial interest. M.d.V.-T. on study exchange from School of Chemistry, Cardiff University, Cardiff CF10 3AT, U.K.

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ACKNOWLEDGMENTS The support from the Hong Kong Research Grants Council (Project 16301815) and the Hong Kong University of Science and Technology is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00281 Inorg. Chem. 2017, 56, 5680−5687