Article pubs.acs.org/IC
An Unusual Cobalt Azide Adduct That Produces a Nitrene Species for Carbon−Hydrogen Insertion Chemistry Lauren N. Grant, Maria E. Carroll, Patrick J. Carroll, and Daniel J. Mindiola* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: A family of Co(II) complexes supported by the bulky, dianionic bis(pyrrolyl)pyridine pincer ligand pyrr2py [pyrr2py2− = 3,5-tBu2-bis(pyrrolyl)pyridine] are reported in this work. These compounds include 1-OEt2, 1·toluene, and 1-N3Ad (Ad = 1-adamantyl), the latter which is prepared via addition of N3Ad to 1-OEt2 [1 = (pyrr2py)Co]. While complexes 1-OEt2 and 1-N3Ad are fourcoordinate systems having a Co(II) ion confined in a cis-divacant octahedral geometry, complex 1·toluene possesses a Co(II) ion in a T-shaped environment where the toluene is interstitial and intercalated between two (pyrr2py)Co molecules. Complex 1-N3Ad is notable in that the organic azide binds to the metal through γ-N in a κ1 fashion. Photolysis of 1-N3Ad results in N2 extrusion and formation of C−H insertion product [(pyrrpypyrrNHAd)Co] (2). We propose complex 2 form via insertion of the nitrene (NAd) into one tBu C−H bond, thus resulting in a pincer ligand having a pendant secondary amine. Complexes 1-OEt2, 1· toluene, and 1-N3Ad and C−H insertion product 2 have been structurally characterized, and in the case of 1-OEt2, we also present electrochemical data.
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INTRODUCTION
Late 3d transition metal imido complexes have received significant attention recently given their role in group transfer catalysis, in particular, their ability to conduct amination and aziridination reactions. These species are sufficiently reactive to perform nitrene insertion reactions to even the less reactive “non-functionalized” hydrocarbon C−H bonds.1−14 As a result, there is a great interest in harnessing these reactive groups for the purpose of hydrocarbon conversion. Our group recently reported the synthesis of a low-spin (S = 0) Fe(IV) imido complex, supported by a bulky, dianionic bis(pyrrolyl)pyridine pincer ligand [pyrr2py2− = 3,5-tBu2bis(pyrrolyl)pyridine], namely (pyrr2py)FeNAd (Ad = 1adamantyl).15,16 The meridional nature of this pincer ligand coupled with the strong σ-donating pyrrole groups constrains the geometry of the metal ion to an unusual cis-divacant octahedron. The ability of this ligand scaffold to support highvalent Fe(IV) imidos, which showed remarkable inertness toward C−H bonds and olefins, led us to pursue a more electron rich Co(IV) system. Shifting to Co, one would expect this species to contain an unpaired electron in an MN π* (dyz) orbital (Figure 1), thus rendering this ligand “iminyl”-like. As demonstrated by Holland, Betley, and co-workers,5,7 the presence of an unpaired electron in their FeNR motif (imidyl group) offers diverse reactivity toward C−H bonds. Warren and co-workers have also observed C−H amination in even more electron rich transition metal nitrenes.10 As a result, we sought to extend the use of the pincer ligand pyrr2py for the synthesis of an electron rich cobalt imido species having imidyl character (Figure 1) and even attempt the isolation of a low© XXXX American Chemical Society
Figure 1. d-Orbital splitting diagram for complex (pyrr2py)FeNAd and a hypothetical complex (pyrr2py)CoNR in a cis-divacant octahedral geometry.
spin Co(V) imido system [(pyrr2py)CoNR]+ that would be isoelectronic to our previously reported (pyrr2py)FeNAd. A limited number of Co imido complexes have been reported or proposed, and the majority of such species consist of lowspin Co(III) centers that are relatively stable.17−22 A few Received: May 6, 2016
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DOI: 10.1021/acs.inorgchem.6b01114 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
dryness, and the resulting purple-brown residue was extracted into 60 mL of diethyl ether. Cooling the solution to −35 °C overnight resulted in the formation of red-purple needles: yield 82% (530 mg, 0.94 mmol); 1H NMR (25 °C, 400 MHz, C6D6) δ 135.3 (s, 2 H, Pyrr-H), 82.2 (s, 1 H, Py-Hmeta), 55.0 [s, 4 H, O(CH2CH3)2], 15.5 (s, 18 H, PyPyrr2-tBu), −0.67 [s, 6 H, O(CH2CH3)2], −9.39 (s, 18 H, PyPyrr2-tBu), −23.3 (s, 1 H, s, Py-Hpara); μeff = 4.3 μB (C6D6, 298 K, Evans’ method). Multiple attempts to conduct satisfactorily elemental analysis failed. Synthesis of (pyrr2py)Co·Toluene (1·toluene). To a 20 mL scintillation vial containing a 5 mL green toluene solution of Co[N(SiMe3)2]2 (260 mg, 0.686 mmol), which was cooled to −35 °C, was added a 3 mL toluene solution of H2pyrr2py (300 mg, 0.692 mmol). The solution was warmed to room temperature, turning a deep red color, and was stirred for 16 h. The volatiles were removed in vacuo. The product was recrystallized from a 7 mL toluene solution at −35 °C overnight: yield 67% (270 mg, 0.463 mmol). Single dark red crystals of 1·toluene were grown by cooling a concentrated pentane solution to −35 °C: 1H NMR (25 °C, 400 MHz, C6D6) δ 202.7 (s, 2 H, Pyrr-H), 89.9 (s, 1 H, Py-Hmeta), 35.5 (s, 18 H, PyPyrr2-tBu), 6.78− 6.37 (m, 5 H, Tol-Ar-H), 2.77 (s, 3 H, Tol-CH3), 0.74 (brs, 18 H, PyPyrr2-tBu), −51.1 (s, 1 H, s, Py-Hpara); μeff = 4.6 μB (C6D6, 298 K, Evans’ method). Anal. Calcd for 1·toluene, CoN3C36H49: C, 74.2; H, 8.48; N, 7.21. Found: C, 71.52; H, 7.99; N, 7.13. Synthesis of (pyrr2py)Co(N3Ad) (1-N3Ad). To a 20 mL scintillation vial containing a 15 mL green toluene solution of Co(PyPyrr2)(OEt2) (500 mg, 0.885 mmol) was added a 5 mL toluene solution of 1-azido-1-adamantane, N3Ad (157 mg, 0.885 mmol). No notable color change occurred upon addition, and the solution was stirred for 20 h. Toluene was removed in vacuo, leaving a green powder. The product, 1-N3Ad, was recrystallized from a 12 mL pentane solution overnight at −35 °C: yield 67% (400 mg, 0.599 mmol). Single crystals suitable for X-ray diffraction were grown from a concentrated pentane solution at −35 °C: 1H NMR (25 °C, 500 MHz, C6D6) δ 134.8 (s, 2 H, Pyrr-H), 82.2 (s, 2 H, Py-Hmeta), 16.7 (s, 18 H, PyPyrr2-tBu), −0.97 (s, 3 H, NCHCH2), −2.05 (s, 6 H, NCHCH2), −6.72 (s, 18 H, PyPyrr2-tBu), −10.9 (s, 6 H, NCHCH2CHCH2), −24.2 (s, 1 H, s, Py-Hpara); μeff = 4.7 μB (C6D6, 298 K, Evans’ method); IR (toluene) νN3 = 2117, 2090 cm−1. Anal. Calcd for 1N3Ad, CoN6C39H56: C, 70.14; H, 8.45; N, 12.58. Found: C, 69.40; H, 8.05; N, 11.06. Photolysis of (1)-N3Ad To Form N Insertion Product [(pyrrpypyrrNHAd)Co] (2). In a J-young NMR tube, a 2 mL C6D6 solution of 1-N3Ad (50 mg, 0.075 mmol) was photolyzed with a 300 V xenon lamp for 1 h. The conversion was monitored by 1H NMR spectroscopy in a J-young tube. The J-young tube was brought into a glovebox, where the contents were transferred to a 20 mL scintillation vial and concentrated to dryness under vacuum. The resulting green residue was extracted into 3 mL of pentane and cooled to −35 °C. Green microcrystalline material formed after 2 days at −35 °C: yield 83% (40 mg, 0.0625 mmol). Single crystals suitable for X-ray diffraction were grown from a concentrated pentane solution at −35 °C: 1H NMR (25 °C, 400 MHz, C6D6) δ 128.5 (s, 1 H, Pyrr-H), 94.3, 90.8, 88.9 (s, 1 H ea, Pyrr-H, Py-Hmeta), 50.5, 50.4 (s, 6 H ea, Ad-H), 21.8 (s, 9 H, PyPyrr2-tBu), 5.7 (s, 9 H, PyPyrr2-tBu), −16.3 [s, 6 H, PyPyrr2-C(CH3)2], −19.8 (s, 3 H, Ad-H), −21.2 (s, 9 H, PyPyrr2-tBu), −26.6 (s, 2 H, PyPyrr2-CCH2), −44.5 (s, 1 H, Py-Hmeta), −92.85 (s, 1 H, NH); μeff = 3.9 μB (C6D6, 298 K, Evans’ method); IR (toluene) 2089 cm−1. Anal. Calcd for CoN4C39H56: C, 73.21; H, 8.82; N, 8.76. Multiple attempts to obtain satisfactorily elemental analysis failed given the thermal instability of 2.
exceptions to this trend include work by Theopold and Betley, in which Co(III) imido complexes access high-spin states under certain conditions (imidyl character), therefore resulting in insertion into a peripheral C−H bond of the supporting ligand.7,11 More recently, Deng and co-workers reported the first examples of Co(IV) and even Co(V) bis-imido complexes.13 When heated, their Co(IV) species was found to engage in intramolecular C−H amination of the supporting NHC ligand.13 We report here the synthesis of new Co complexes supported by the bis(pyrrolyl)pyridine pincer ligand pyrr2py. Attempts to prepare a Co−imido species from an organic azide reduction resulted instead in isolation of a rare example of an “arrested” N2 group on the organic azide, which is bound to Co(II) through the γ-nitrogen atom. Under photolytic conditions, the azide group does extrude N2 to generate a transient Co−imido that immediately inserts into a C−H bond of the ligand. Complete characterization, including electrochemical studies of the Co(II) precursors as well as the product resulting from nitrene insertion into a C−H bond of a tert-butyl group on the pincer ligand, is presented and discussed.
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EXPERIMENTAL DETAILS
General Procedures. Unless otherwise stated, all operations were performed in an M. Braun Lab Master double drybox under an atmosphere of purified dinitrogen or using high-vacuum standard Schlenk techniques under an argon atmosphere. Benzene, diethyl ether (Et2O), pentane, tetrahydrofuran (THF), and toluene were purchased from Fisher Scientific, with the exception of Et2O, which was purchased from Sigma-Aldrich. Solvents were sparged with argon for 20 min and dried using a two-column solvent purification system in which columns designated for benzene, pentane, and toluene were packed with Q5 and alumina and columns designated for Et2O were packed with alumina. Deuterated benzene was purchased from Cambridge Isotope Laboratories (CIL), sparged with argon for 20 min, and dried using a two-column solvent purification system packed with Q5. All solvents were transferred into a drybox and stored over 4 Å sieves. All sieves were heated to 200 °C under vacuum overnight prior to use. Celite for filtrations was also heated to 200 °C under vacuum overnight prior to use. Co[N(SiMe3)2]2 and H2pyrr2py were prepared following the reported literature procedures.15,23 All other chemicals were purchased from Strem Chemicals, Aldrich, or Alfa Aesar. 1H NMR spectra were recorded on Bruker 500 or 400 MHz NMR spectrometers.24 Synthesis of (pyrr2py)Co(OEt2) (1)-OEt2 via Salt Metathesis (method A). To a 20 mL scintillation vial containing a 3 mL yellow THF solution of H2pyrr2py (500 mg, 1.15 mmol) was added dropwise a 3 mL THF solution of K[N(SiMe3)2] (460 mg, 2.30 mmol). After being stirred for 2 h at room temperature, the solution was added dropwise to a blue slurry of CoCl2 (149 mg, 1.15 mmol) in 5 mL of THF, causing the color to change to dark brown green. The solution was stirred for 20 h, and then THF was removed in vacuo. The resulting brown solid was extracted into pentane and filtered through a pad of Celite. The filtrate was concentrated to dryness, and the resulting purple-brown residue was extracted into 60 mL of diethyl ether and stirred for 12 h at room temperature. Red-purple needles formed when the solution was cooled to −35 °C overnight: yield 39% (250 mg, 0.45 mmol). Synthesis of (pyrr2py)Co(OEt2) (1)-Et2O via Protonolysis (method B). To a 20 mL scintillation vial containing a 10 mL green diethyl ether solution of Co[N(SiMe3)2]2(THF) (521 mg, 1.15 mmol) cooled to −35 °C was added a 5 mL diethyl ether solution of H2pyrr2py (500 mg, 1.15 mmol). No notable color change occurred upon addition of the ligand. The solution was warmed to room temperature and then allowed to stir for 18 h. Volatiles were removed in vacuo. The resulting brown solid was extracted into pentane and filtered through a pad of Celite. The filtrate was concentrated to
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RESULTS AND DISCUSSION To access a Co(II) precursor, a slurry of CoCl2 in THF was treated with [K2(pyrr2py)],15 which was generated in situ by treatment of a THF solution of pyrr2pyH2 with K[N(SiMe3)2] that had been stirred for 2 h prior to metalation. Upon addition of the ligand salt to the CoCl2 slurry, a gradual color change from blue to brown green was observed, and after 20 h, removal B
DOI: 10.1021/acs.inorgchem.6b01114 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 1. Synthesis of Complex 1-OEt2 by Two Independent Routes
position, therefore rendering the geometry at Co more in accord with a cis-divacant octahedron (τ4 = 0.77). This geometry is strikingly similar to that observed for the analogous Fe complex [(pyrr2py)Fe(OEt2)] (τ4 = 0.80), in addition to similar metrical parameters for the bound Et2O ligand: Fe− OEt2O, 2.1103(19) Å; Co−OEt2O, 2.027(3) Å. To illustrate the similarity in their molecular geometry (based on bonds and angles), an overlay of the two structures is presented in Figure 3, where 1-OEt2 is colored blue and the Fe analogue is colored
of THF under reduced pressure resulted in a dark solid. After the solid was extracted into pentane and filtered and the filtrate dried, dark purple crystals of complex [(pyrr2py)Co(OEt2)] (1)-OEt2 could be obtained in 39% yield by extracting the solid with Et2O, filtering, and then cooling the filtrate to −35 °C overnight (Scheme 1). Alternatively, slow addition of H2pyrr2py to a diethyl ether solution of [Co(N{SiMe3}2)2]23 resulted in the formation of 1-OEt2 but with an improved yield of 82% after recrystallization from diethyl ether (Scheme 1). The 1H NMR spectrum of 1 in C6D6 features seven paramagnetically shifted resonances ranging from 135 to −23 ppm. The number of NMR spectroscopic signals is consistent with a Cs symmetric system akin to that observed for the analogue [(pyrr2py)Fe(OEt2)].16 A room-temperature magnetic susceptibility measurement in C6D6 reveals a μeff of 4.3 μB, consistent with a high-spin Co(II) (S = 3/2) complex. Single crystals of X-ray quality were grown by cooling a concentrated diethyl ether solution of 1-OEt2 to −35 °C over the course of 2 days. Complex 1-OEt2 crystallizes in orthorhombic space group Pbcm, which is analogous to that of the Fe complex, [(pyrr2py)Fe(OEt2)].16 Not surprisingly, the unit cell is also nearly identical to that of [(pyrr2py)Fe(OEt2)]. In the solid state structure shown in Figure 2, the Co(II) center is four-coordinate with a meridionally bound pyrr2py ligand. A diethyl ether molecule is bound in a cis
Figure 3. Overlay of 1-OEt2 (blue) and [(pyrr2py)Fe(OEt2)] (red) for qualitative comparison of their geometries.
red. The only notable differences that cause these two geometries not to be 100% superimposable are the differing orientations of Et groups in the bound Et2O. As a note, the overall geometry of 1-OEt2 is slightly distorted from a perfect cis-divacant octahedron, because the Co center is 0.55 Å deviated along the plane defined by the three pyrr2py nitrogen atoms. To understand the redox properties of 1-OEt2, we examined electrochemical data of this complex using the [NnBu4][PF6] electrolyte in 1,2-difluorobenzene. As shown in Figure 4, the CV data shows an irreversible cathodic wave at a very negative potential of approximately −2.70 V. Scanning anodically, however, one can easily observe two reversible one-electron waves at 0.38 and 0.10 V (vs the FeCp20/+ redox couple at 0.0 V and used as an internal standard). We observed that there are two reversible waves in the CV of equal intensity in the absence of FeCp2, but upon addition of FeCp2, the intensity of the wave at 0.10 V significantly increases because of the overlap of this wave with ferrocene. Despite being formally Co(II), complex 1OEt2 should be readily oxidized by one and two electrons based on the two anodic waves shown in Figure 4. Unfortunately, chemical oxidations of 1-OEt2 by one-electron oxidants have so far been unfruitful. Attempts to remove the Et2O ligand in 1-OEt2 were unsuccessful even when a solution was heated in toluene or other solvents at 80 °C for hours. However, the diethyl ether ligand in 1-OEt2 can be gradually removed with AlMe3
Figure 2. Solid state structures (top) of complexes 1-OEt2 (left) and 1·toluene displaying thermal ellipsoids at the 50% probability level and the unit cell (bottom) of complex 1·toluene showing the stacking of toluene between two molecules of 1 (oriented along the a axis). C
DOI: 10.1021/acs.inorgchem.6b01114 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
the pyrrolyl ligand, the toluene methyl interacts equally with both pockets created by the tBu groups, resulting in disorder of the toluene molecule. Given this unique stacking interaction, we tantalizingly refer to complex 1·toluene as a threecoordinate Co(II) complex32−35 where the metal ion is Tshaped (N2−Co−N2′, 165.59°; N1−Co−N2, 80.80°). We next examined the reactivity of 1-OEt2 with 1-adamantyl azide, N3Ad. Treatment of a toluene solution of 1-OEt2 with 1 equiv of N3Ad at room temperature did not produce a significant color change, and no effervescence was observed. The solution remained dark green, and after the mixture had been stirred for 20 h and worked up, a new material could be crystallized from pentane in 67% yield. We proposed this species to be the azide adduct [(pyrr2py)Co(N3Ad)] (1)-N3Ad (Scheme 2). While the room-temperature solution magnetic study by the method of Evans reveals 1-N3Ad to possess a μeff of 4.7 μB, in accord with this species being a high-spin Co(II) (S = 3/2) complex, the 1H NMR spectrum of this new complex exhibits eight new paramagnetically shifted resonances (δ −24 to 125). To determine the degree of reduction of the azide moiety as well as its binding mode, X-ray diffraction single quality crystals were grown from a concentrated pentane solution at −35 °C, and a solid state structural diagram is shown in Figure 5. The molecular structure reveals a four-
Figure 4. CV data of complex 1 (anodic scan) in 1,2-difluorobenzene with [NnBu4][PF6] as the electrolyte (100 mM). This sample was run in the absence of FeCp20/+ to feature both the reversible feature at 0.38 V and a second reversible wave at 0.10 V. The concentration of complex 1-OEt2 was 1.5 mM, and the scan rate was 250 mV/s.
(monitored by 1H NMR spectroscopy) in toluene to form a new complex that we infer to be the toluene species, (pyrr2py)Co·toluene, 1·toluene, based on 1H NMR spectroscopy. Formation of an arene complex, 1·toluene, is evident from the presence of a singlet at 2.77 ppm integrating for three H atoms or nuclei, in the expected region for the toluene CH3 group, as well as the presence of multiplets integrating for five Hs in the aromatic region. Interestingly, the arene in 1·toluene is stable to exchange with benzene when the complex is dissolved in C6D6 at room temperature. This was confirmed by an independent synthesis of the benzene complex, (pyrr2py)Co·benzene, which had a 1H NMR spectrum markedly different from that of 1·toluene in C6D6. Complex 1·toluene can be obtained in higher yield via treatment of Co[N(SiMe3)2]2 with pyrr2pyH2 in the absence of donor solvents. Accordingly, treatment of [Co(N{SiMe3}2)2] with H2pyrr2py in toluene afforded 1·toluene in 67% yield (Scheme 2). Akin to that of the diethyl ether adduct, 1-OEt2, the room-temperature magnetic susceptibility measurement of 1·toluene in C6D6 reveals a μeff of 4.6 μB, consistent with a high-spin Co(II) (S = 3/2) complex. Complex 1·toluene crystallizes in a different orthorhombic space group, Cmcm, where both the toluene and Co atoms are disordered (Figure 2). Surprisingly, the relatively long Co− Ctoluene distances [3.453(5)−4.539(3) Å] indicate minimal interaction between the metal center and toluene molecule, hence suggesting this interaction is governed more by packing forces than by orbital interactions.17,25−31 In fact, the packing diagram shows the toluene to be intercalated in a π-like fashion between two [(pyrr2py)Co] scaffolds (unit cell shown at the bottom of Figure 2). The methyl group of the intercalated toluene fits well into the space between the two tBu groups of the pyrrolyl moiety of the ligand. Because of the symmetry of
Figure 5. Solid state structure of azide adduct 1-N3Ad (left) and nitrene insertion product 2 (right) displaying thermal ellipsoids at the 50% probability level.
coordinate cobalt complex consisting of a meridionally coordinated pyrr2py ligand and an N3Ad ligand bound only through the γ-N atom. Akin to the Et2O ligand in 1-OEt2, the azide is coordinated orthogonal to the plane defined by [(pyrr2py)Co], hence rendering this species a distorted cisdivacant octahedron (τ4 = 0.70). Judging from the Co−Nazide [2.179(8) Å] and N−N [1.266(9) and 1.135(9) Å] distances of the azide, there is a minimal degree of reduction of the azide consistent with this species being a Co(II) complex having a dative N3Ad group. Additional characterization of this complex by infrared spectroscopy reveals a strong azide stretch at 2736
Scheme 2. Synthesis of 1·Toluene and Formation of Azide Adduct 1-N3Ad from 1-OEt2
D
DOI: 10.1021/acs.inorgchem.6b01114 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 3. Synthesis of Complex 2 via Photolysis of 1-N3Ad in Benzene
cm−1, slightly reduced when compared to the free N3Ad stretch at 2851 cm−1. Although transition metal azide adducts are often invoked in the mechanism of formation of imido complexes, such adducts have been isolated and characterized.36−47 Even less common are examples of an azide ligand bound through the γ-N atom, which have been computed or speculated only en route to the nitrene group in transition metals.41,43−46,48 Hence, examples of an azide adduct bound through the γ-N atom are quite rare and restricted to precious metals such Ir, Ag, Pd, and Au.36,43−45 Nitrene formation from organic azides has been proposed to occur via a [2+2] retrocycloaddition, [2+3] retrocycloadditions, or by a metal nitrene capture.38−42,47 Unlike the (pyrr2py)Fe(OEt2) species, which readily undergoes a two-electron oxidation with N3Ad to form a stable imido complex (pyrr2py)FeNAd,16 complex 1-N3Ad is remarkably stable as a solid or in solution. To the best of our knowledge, examples of 3d metals showing this type of azide coordination are unknown. Over the course of a week, however, C6D6 solutions of 1N3Ad gradually convert to a darker green color, from which a new, less symmetric product retaining both the pincer ligand and the adamantyl group, [(pyrrpypyrrNHAd)Co] (2), was formed based on the 1H NMR spectrum (Scheme 3). However, one can expedite the conversion of 1-N3Ad to 2 within 1 h in aromatic solvents using irradiation with a 300 V Xe light source. Thermolytic control experiments conducted independently in the absence of light demonstrated that this transformation is indeed photochemically promoted. When 1-N3Ad is being irradiated, complex 2 could be isolated from the mixture in 83% yield by recrystallization from pentane at −35 °C. The solution state IR spectrum of 2 exhibits a diagnostic stretch at 2089 cm−1 consistent with the presence of an N−H group. To conclusively establish the exact site of nitrene insertion or ligand activation, X-ray quality single crystals of 2 were grown from a concentrated solution of 2 in pentane at −35 °C. The molecular structure of 2 consists of a four-coordinate Co(II) center bound to the pincer ligand with a pendant secondary amine, in which the NAd group inserted into a C−H bond of a t Bu group of the pyrr2py ligand. The geometry of 2 is best described as a highly distorted square-planar complex, largely altered from the other complexes in this family that all favored cis-divacent octahedral geometries (N1−Co1−N4, 90.62°; N1−Co1−N3, 158.4°; N3−Co1−N4, 108.18°; τ4 = 0.37). Also of intrigue is the significant decrease in the magnetic moment of 2 (μeff = 3.9 μB) relative to that of the cis-divacent structures (vide supra; μeff = 4.3 or 4.7 μB), suggesting that the extent of spin−orbit coupling is likely affected by molecular geometry. Complex 2 is likely produced by N2 loss that forms a nitrene species (pyrr2py)CoNAd (A), which would then abstract a hydrogen atom from the proximal tert-butyl group and, then through radical recombination, forms a new C−N
bond. It is also quite possible that upon H atom abstraction, the radical recombination results in the formation of a Co−C bond, akin to systems reported by Theopold and Betley.11a,12 However, and in our case, one would expect Co(IV) to readily undergo reductive elimination resulting in C−N bond formation, which is what is observed in the case of complex 2.
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CONCLUSIONS In this study, we reported several new Co(II) synthons using a pincerlike ligand that enforces a meridional configuration. We have isolated and characterized a rare example of a cobalt azide adduct in which the γ-N binds to the metal center. This type of bonding mode has been proposed as an intermediate en route to imido complexes. Furthermore, the binding mode of the organic azide through the γ-N is a motif rarely observed in the literature and often reported as a transient intermediate with an arrested N2 or even as a transition state.48 The cis-divacant octahedral geometry observed in species such as 1-OEt2 and 1N3Ad is due to the meridional constraint imposed by the pincerlike pyrr2py ligand. This electron rich scaffold allows for the observation of two anodic oxidation waves (in the case of 1OEt2) and allows for a photochemical conversion to a transient Co(IV) nitrene (from 1-N3Ad) that ultimately inserts into an intramolecular tBu C−H bond. Given the unexpected presence of two reversible oxidation features in the CV, current endeavors are focused on one- and two-electron oxidants involving 1-OEt2 without activation of the peripheral ligand. Additionally, we are presently exploring what factors govern the bonding mode in Co−azide species 1-N3Ad, because this might shed some light on the mechanism of C−H amination or aziridination reactions.
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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.inorgchem.6b01114.
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NMR spectral data, electrochemical data, IR spectral data, and crystallographic data (PDF) Crystallographic data for complexes 1-Et2O, 1·toluene, 1N3Ad, and 2 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. E
DOI: 10.1021/acs.inorgchem.6b01114 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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ACKNOWLEDGMENTS For funding, we thank the University of Pennsylvania and the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (DEFG02-07ER15893).
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DOI: 10.1021/acs.inorgchem.6b01114 Inorg. Chem. XXXX, XXX, XXX−XXX
