Complexes of a Bulky Bis(carbene)borate Ligand - American

Jun 26, 2014 - and Jeremy M. Smith*. ,† ... Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United S...
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Low-Coordinate Iron(II) Complexes of a Bulky Bis(carbene)borate Ligand Wei-Tsung Lee,† Ie-Rang Jeon,‡ Song Xu,† Diane A. Dickie,§ and Jeremy M. Smith*,† †

Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Department of Chemistry and Chemical Biology, The University of New Mexico, 300 Terrace Street NE, Albuquerque, New Mexico 87131, United States ‡

S Supporting Information *

ABSTRACT: The bulky bis(carbene)borate ligand H2B(tBuIm)2− allows for the synthesis of three- and four-coordinate iron(II) complexes, including heteroleptic H2B(tBuIm)2FeN(TMS)2 and homoleptic [H2B(tBuIm)2]2Fe. The magnetic properties of these coordinatively unsaturated complexes have been characterized by SQUID magnetometry, but no evidence of single-molecule magnet behavior is observed, despite large negative uniaxial zero field splitting. The three-coordinate complex H2B(tBuIm)2FeN(TMS)2 serves as a precursor for the synthesis of the four-coordinate mixed carbene complex H2B(tBuIm)2(iPr2Im)FeCl, which has a coordination environment similar to that found in tris(carbene)borate iron(II) chloride complexes. Despite this similarity, attempts to prepare the corresponding iron(IV) nitride were unsuccessful, suggesting that subtle structural factors are critical to stabilizing this species.

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INTRODUCTION Low-coordinate transition-metal complexes have attracted attention for their unusual bonding and reactivity. For example, the three-coordinate iron(II) dipyrrinato complexes have been shown to catalyze intra- and intermolecular C−H bond amination,1 while reduction of three-coordinate iron(II) βdiketiminate complexes leads to N2 activation and cleavage.2 More recently, low-coordinate iron(II) complexes have been shown to have interesting magnetic behavior in which the large axial zero-field splitting D resulting from the unusual coordination number leads to single-molecule-magnet (SMM) behavior at low temperatures.3 Very bulky, monoanionic ligands allow three-coordinate homoleptic Fe(II) complexes to be prepared: e.g., [Fe(SC6H22,4,6-tBu3)3]− and [Fe{N(SiMe3)2)3]−.4,5 The reactivity of these complexes tends to involve wholesale changes to the ligand environment, frequently accompanied by increased coordination numbers. This problem can often be overcome through the use of suitably bulky bidentate ligands that allow heteroleptic three-coordinate complexes to be isolated. Since this structural motif allows the third ligand to be selectively transformed, new ligands can readily be introduced to modify the properties of the metal center while maintaining the threecoordinate environment. In the case of divalent iron, a number of bulky bidentate ligands have been shown to stabilize threecoordinate complexes (Chart 1). The most widely used ligands for this purpose are the versatile β-diketiminates, where a variety of ancillary ligands can be installed, including alkoxido, alkyl, amido, aryl, halides, hydrazido, hydrido, phosphine, sulfide, etc. Other ligands that provide a similar coordination © 2014 American Chemical Society

Chart 1. Ligands That Stabilize Three-Coordinate Heteroleptic Iron(II) Complexes

environment are also shown in Chart 1,1,6 although the coordination chemistry of these ligands has been less well explored. We noted that bulky bis(pyrazolyl)borates are among the ligands that support three-coordinate iron(II).6g Given our interest in the topologically related bis(carbene)borate ligands, this result piqued our attention, suggesting that suitably bulky bis(carbene)borates may likewise stabilize three-coordinate ferrous complexes, but with a ligand field considerably stronger than that for the other ligands shown in Chart 1. In this paper, we report on the synthesis of low-coordinate iron(II) complexes containing the bis(3-tert-butylimidazol-2-ylidene)borate ligand. With a suitably bulky ancillary ligand, a threeSpecial Issue: Catalytic and Organometallic Chemistry of EarthAbundant Metals Received: April 21, 2014 Published: June 26, 2014 5654

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Preparation of [H2B(tBuImH)][H2B(tBuIm)FeCl2] (5). To a stirred solution of H2B(tBuIm)FeN(TMS)2 (100 mg, 0.21 mmol) in toluene (2 mL) at ambient temperature under an N2 atmosphere was added a suspension of 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (90 mg, 0.21 mmol) in toluene (2 mL). The resulting slurry was stirred for 2 days at ambient temperature. After filtration, the filtrate was dried in vacuo to yield a brown oil, which was extracted with n-pentane to afford a tan solid (23 mg, 21%). Crystals suitable for X-ray diffraction were grown from a concentrated toluene solution at −30 °C. 1H NMR (400 MHz, C6D6, δ): 47.3, 27.4, 18.2, 4.48. HRMS (ESI): calcd for C14H24BCl2FeN4 m/z 385.9290 ([M]−), found 385.9306. We have been unable to obtain this complex sufficiently pure for elemental analysis. Magnetic Measurements. Magnetic measurements of 1 were performed on polycrystalline samples sealed in a polyethylene bag under a dinitrogen atmosphere. For 2, microcrystalline samples were loaded into a quartz tube and coated with eicosane in a glovebox. The quartz tube was fixed to a sealable hose adapter, evacuated on a Schlenk line outside of the glovebox while frozen in liquid N2, and then flame-sealed. All data were collected using a Quantum Design MPMS-XL SQUID magnetometer from 1.8 to 300 K at applied dc fields ranging from 0 to +7 T. ac magnetic susceptibility data were collected under an ac field of 4 Oe, oscillating at frequencies in the range 1−1500 Hz. dc susceptibility data were corrected for diamagnetic contributions from the sample holder and for the core diamagnetism of each sample (estimated using Pascal’s constants).9 M(H) curves, constructed from data collected from 0 to 4 T at 100 K, confirmed the absence of ferromagnetic impurities. The coherence of the collected data was checked across the different measurements. Solid Angle Calculations. Solid angle calculations were performed using the program Solid-G.10 The ligand coordinates used in the calculations were taken from the X-ray crystal structure data of 4 and PhB(tBuIm)3FeCl. The metal−ligand bond lengths were set at a distance of 2.28 Å. In cases where there was more than one molecule in the asymmetric unit, solid angles were calculated for each molecule, and the average value is reported. For the truncated calculations, ORTEP-3 (version 2.02) was used to define a sphere of enclosure (4 Å, centered on Fe), and the resulting coordinates were used for solid angle calculations.

coordinate complex can be isolated; however, the bis(carbene)borate ligand is insufficiently bulky to prevent the formation of higher coordination numbers. Nonetheless, the three-coordinate complex serves as a useful precursor to the formation of a four-coordinate complex containing two types of carbene donors.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were performed under a nitrogen atmosphere by standard Schlenk techniques or in an M. Braun Labmaster glovebox. Glassware was dried at 150 °C overnight. Diethyl ether, n-pentane, tetrahydrofuran, and toluene were purified by the Glass Contour solvent purification system. Deuterated benzene was first dried with CaH2, then over Na/benzophenone, and then with vacuum transfer into a storage container. Before use, an aliquot of each solvent was tested with 1 drop of a solution of sodium benzophenone ketyl in THF. All reagents were purchased from commercial vendors and used as received. H2B(tBuImH)I and Fe[N(TMS)2]2 were prepared according to literature procedures.7,8 1 H NMR data were recorded on a Varian Unity 400 spectrometer (400 MHz) at 22 °C. Resonances in the 1H NMR spectra are referenced either to residual C6D5H at δ 7.16 ppm or to C4D7HO at δ 3.58 ppm. Infrared spectra were recorded on a Perkin-Elmer Spectrum Two FTIR instrument. High-resolution mass spectra were collected using a MAT 95 XP mass spectrometer (Thermo Electron Corp.). Elemental analysis was conducted by Midwest Microlab, LLC (Indianapolis, IN). Preparation of [H2B(tBuIm)]2Fe (1). To a stirred slurry of H2B(tBuImH)I (100 mg, 0.30 mmol) in toluene (3 mL) at ambient temperature under an N2 atmosphere was added a suspension of LDA (64 mg, 0.60 mmol) in toluene (2 mL) over 1 h. Solid FeCl2·1.5THF (35 mg, 0.15 mmol) was added, and the resulting slurry was stirred for 18 h at ambient temperature. Volatiles were removed under reduced pressure, the residue was extracted with toluene, and the extract was filtered through Celite. The filtrate was dried in vacuo to yield a white solid (73 mg, 85%). Crystals suitable for X-ray diffraction were grown from a concentrated diethyl ether solution at −30 °C. 1H NMR (400 MHz, C6D6, δ): 95 (2H, BH), 25 (2H, ArH), 22 (18H, C(CH3)3), 17 (2H, ArH). IR (THF, cm−1): 2346, 2353 (w, B−H). Anal. Calcd for C28H48B2FeN8·2LiCl·H2O: C, 49.68; H, 7.44; N, 16.55. Found: C, 49.96; H, 7.29; N, 16.28. HRMS (ESI): calcd for C28H47B2FeN8 m/z 573.3459 ([M − H]+), found 573.3476. Preparation of H2B(tBuIm)FeN(TMS)2 (2). To a stirred slurry of H2B(tBuImH)I (100 mg, 0.30 mmol) in toluene (3 mL) at ambient temperature under an N2 atmosphere was added a suspension of LDA (28 mg, 0.26 mmol) in toluene (2 mL) over 1 h. The reaction mixture was added to a solution of Fe[N(TMS)2]2 (107 mg, 0.28 mmol) in toluene (2 mL). The resulting slurry was stirred for 18 h at ambient temperature. Volatiles were removed under reduced pressure, the residue was extracted with toluene, and the extract was filtered through Celite. The filtrate was dried in vacuo to yield a white solid (82 mg, 66%). Crystals suitable for X-ray diffraction were grown from a concentrated toluene solution at −30 °C. 1H NMR (400 MHz, C6D6, δ): 126 (2H, BH), 89 (2H, ArH), 42 (2H, ArH), 1.9 (18H, Si(CH3)3), −40 (18H, C(CH3)3). IR (THF, cm−1): 2381 (w, B−H). Despite repeated attempts, we have been unable to obtain satisfactory elemental analysis data for this complex. Preparation of H2B(tBuIm)Fe(DiiPrIm)Cl (4). To a stirred solution of H2B(tBuIm)FeN(TMS)2 (100 mg, 0.21 mmol) in toluene (2 mL) at ambient temperature under an N2 atmosphere was added a suspension of 1,3-diisopropylimidazolium chloride (40 mg, 0.21 mmol) in toluene (2 mL). The resulting slurry was stirred for 2 days at ambient temperature. After filtration, the filtrate was dried in vacuo to yield a pale yellow solid (56 mg, 53%). Crystals suitable for X-ray diffraction were grown from a concentrated toluene solution at −30 °C. 1H NMR (400 MHz, C4D8O, δ): 143, 52, 33, 29, 27, 14, 6.5. IR (THF, cm−1): 2383 (w, B−H). HRMS (ESI): calcd for C23H40BFeN6 m/z 467.2757 ([M − Cl]+), found 467.2783. Anal. Calcd for C23H40BClFeN6·LiCl·1.5THF: C, 53.32; H, 8.02; N, 12.86. Found: C, 53.39; H, 8.42; N, 13.09.

Scheme 1. Synthesis of Complexes 1 and 2

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RESULTS AND DISCUSSION Syntheses of Three- and Four-Coordinate Complexes. The homoleptic complex 1 (Scheme 1) can be prepared as the sole reaction product when the bis(carbene)borate ligand is generated using LDA as the base. The molecular structure of 1 was determined by single-crystal X-ray diffraction, showing a distorted tetrahedral iron center bound to two bis(carbene)borate ligands (Figure 1). The Fe−C bond lengths in 1 (2.111(4) and 2.129(3) Å) compare well with those observed in other high-spin four-coordinate NHC iron(II) complexes. Interestingly, although a similar structure was observed for the related bis(carbene)methane complexes [H2C(RIm)2]2Fe2+, the bite angle C−Fe−C (99.12(13)°) in 1 is significantly larger and the α angle (2.3(3)°) smaller than those in the related complexes, in which the two NHC donors are linked by a neutral methylene backbone (86.1(2)−91.1(3) and 23.50− 39.36°, respectively; Figure S1, Supporting Information).11−14 The bite and α angles both strongly depend on the backbone 5655

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iron center ligated by one bis(carbene)borate and one silylamido ligand. The iron center adopts a trigonal-planar geometry, as quantified by the sum of angles (358.32(6)°) around the metal. To the best of our knowledge, this is the first example of a trigonal-planar iron(II) complex supported by a multidentate NHC ligand. The Fe−C bond lengths (2.109(2) Å) and bite angle C−Fe−C (98.80(7)°) of 2 are comparable to those in 3, although the α angle (23.30(14)°) is much larger. The bite angle is also comparable with those observed for three-coordinate iron(II) β-diketiminate amido complexes (92.67−96.38°).15 The 1H NMR spectrum of complex 2 is consistent with the three-coordinate geometry observed in the solid state. Thus, two resonances are observed for the imidazol-2-ylidene protons (δ 89 and 42 ppm) and a single resonance is observed for the tert-butyl groups (δ −40 ppm), consistent with a bidentate bis(carbene)borate ligand. In addition, resonances assigned to the BH2 group (δ 26 ppm) and the N(SiMe3)2− ligand (δ 1.9 ppm) are also observed. The BH2 group is observed in the IR spectrum (νB−H 2381 cm−1). The temperature dependence of the χT product of the isolated compound 2 is shown in Figure S3 (Supporting Information). The room-temperature χT product is 3.97 cm3 K/mol, which corresponds well with the expected value for a magnetically isolated high-spin iron(II) metal ion. In order to investigate the possibility of having significant zero-field splitting (ZFS), we collected low-temperature magnetization data at various applied dc fields between 1 and 7 T (Figure 3).

Figure 1. X-ray crystal structure of 1. Hydrogen atoms on carbon atoms and the lithium iodide salt are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Fe(1)−C(1), 2.129(3); Fe(1)−C(8), 2.111(4); C(1)−Fe(1)−C(8), 99.12(13).

linker length and the bulkiness of the R groups. In case of 1, the B−N bonds of the backbone are 0.1 Å shorter than the related C−N bonds in H2C(RIm)2 ligands, which opens up the N−B− N angle and consequently increases the ligand bite angle. A single set of paramagnetically shifted resonances is observed in the 1H NMR spectrum of 1, consistent with C2 symmetry in solution and therefore bidentate binding by the bis(carbene)borate ligands. Two resonances are assigned to the imidazol-2-ylidene groups (δ 25 and 17 ppm), a broad resonance at δ 95 ppm is assigned to the BH2 group, and a single and broad resonance at δ 22 ppm is assigned to the C(CH3)3 protons that are in close vicinity to the paramagnetic center. The BH2 group is observed in the IR spectrum (νB−H 2346 and 2353 cm−1). Variable-temperature dc magnetic susceptibility data of 1 were collected for a solid sample under an applied field of 10000 Oe (Figure S2, Supporting Information). The χT product at 300 K (4.70 cm3 K/mol) indicates the presence of noninteracting high-spin iron(II) centers in this compound. Variable-temperature magnetometry reveals that no spin state changes occur, despite the strong ligand field. The heteroleptic three-coordinate complex 2 can be prepared by an alternate synthetic route (Scheme 1). Specifically, 1 equiv of LDA is incubated with H2B(tBuImH)I, followed by the addition of 1 equiv of Fe[N(TMS)2]2. This synthetic procedure provides 2 as the only iron-containing product. Interestingly, complex 2 is not stable in THF solution, being gradually converted to the homoleptic complex 1. The molecular structure of 2 was determined by singlecrystal X-ray diffraction (Figure 2), revealing a three-coordinate

Figure 3. Low-temperature magnetization data for 2 collected under various applied dc fields. The black solid lines represent a fit to data.

The resulting plot of reduced magnetization reveals separation between a series of isofield curves, indicative of strong magnetic anisotropy. A fit to the data obtained using the MagProp program16 afforded axial and transverse zero field splitting parameters of D = −25.1 cm−1 and E = −4.5 cm−1, implying a theoretical anisotropic barrier of Ueff = 100.4 cm−1. The axial zero-field spltting parameter can be compared with D values measured for iron(II) complexes that show single-moleculemagnet behavior: e.g., [Fe(tpaR)]− (R = tBu, Mes, Ph; D = −48, −44, −26 cm−1, respectively)3c and Fe{N(SiMe3)2}2(PCy3) (D = −33 cm−1).3d,5 To check for the potential presence of slow relaxation of the magnetization, the variable frequency ac susceptibility data of this compound were collected at different temperatures between 1.8 and 5 K. As shown in Figure S4 (Supporting Information), no out-of-phase signal is observed down to 1.8 K, both at zero dc field and under an applied field of 1000 Oe. The absence of slow dynamics in the experimental

Figure 2. X-ray crystal structure of 2. Hydrogen atoms on carbon atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Fe(1)− C(1), 2.109(2); Fe(1)−N(3), 1.936(2); C(1)−Fe(1)−C(1#), 98.80(7). 5656

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regime might be due to the intermolecular dipolar interactions facilitating quantum tunneling, as commonly observed in mononuclear SMMs.3b,17 Scheme 2. Formation of Complex 3

Figure 4. X-ray crystal structure of 4. One of two molecules in the asymmetric unit is shown. Hydrogen atoms on carbon atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Fe(1)−C(1), 2.109(3); Fe(1)−C(8), 2.098(3); Fe(1)−C(15), 2.110(3); Fe(1)−Cl(1), 2.325(1); C(1)−Fe(1)−C(8), 97.19(11); C(1)−Fe(1)−Cl(1), 110.13(9); C(8)−Fe(1)−Cl(1), 113.43(9); C(15)−Fe(1)−Cl(1), 112.30(8).

Other attempts to prepare three-coordinate bis(carbene)borate complexes were not successful. In one instance, reaction of the in situ prepared bis(carbene)borate ligand (prepared using 2 equiv of LiN(TMS)2) with 1 equiv of FeX2(THF)x (X = Cl, Br) yielded the crystallographically characterized complex 3 as the major product along with other intractable species (Scheme 2 and Figure S5 (Supporting Information)). Despite 3 being a potential precursor to a three-coordinate complex by an intramolecular deprotonation reaction, heating this complex does not yield a three-coordinate complex but instead gives 1 as the only tractable product. Additional attempts to use 3 as a precursor to a three-coordinate complex have been stymied by the fact that its synthesis is not reproducible. An alternative attempt at preparing a three-coordinate complex by reaction of the boronium cation H2B(tBuImH)I with 1 equiv of Fe[N(TMS)2]2 yields unidentified oily product(s). Synthesis and Reactivity of a Four-Coordinate Mixed Carbene Donor Complex. The three-coordinate complex 2 serves as a platform for the synthesis of four-coordinate complexes with mixed carbene donors. For example, treating 2 with 1 equiv of 1,3-diisopropylimidazolium chloride yields the mixed carbene iron(II) complex 4 (Scheme 3).

(112(2)°) is smaller than that of in PhB(tBuIm)3FeCl (125(2)°).19 The difference between the coordination environments of iron in 4 and in the tris(carbene)borate complexes PhB(RIm)3FeCl (R = tBu, Mes) has been evaluated by continuous symmetry measurements.18 These measurements quantitatively determine the closeness of the molecular geometry to an idealized structure type. In the case of four-coordinate complexes, the deviation of a molecule from an idealized tetrahedron and square planar geometry is given by S(Td) and S(D4h), respectively. According to these measures, a perfect tetrahedron has S(Td) = 0 and S(D4h) = 33.3, while a perfect square plane has S(Td) = 33.3 and S(D4h) = 0. The continuous symmetry measurements for 4 (S(Td) = 0.63 and S(D4h) = 31.69) reveal that the iron center is closer to a tetrahedral environment than in the tris(carbene)borate complexes PhB(RIm)3FeCl (S(Td) = 3.50 and S(D4h) = 33.91 for R = tBu; S(Td) = 3.77 and S(D4h) = 33.24 for R = Mes). Thus, the symmetry measurements reveal that the rigidity of tris(carbene)borate ligands is important for stabilizing fourcoordinate metal centers in a nontetrahedral geometry. The 1H NMR spectrum of 4 shows resonances over the range +140 to +6 ppm. The number of resonances observed is in accord with the expectations of a Cs-symmetric complex and moreover is consistent with rapid rotation around the Fe−C bond of the monodentate NHC on the NMR time scale. Not all of the resonances can be unambiguously assigned; however, those due to the isopropyl (δ 6.5 ppm) and tert-butyl (δ 14 ppm) groups in 4 can be assigned on the basis of integration. The BH2 group is observed in the IR spectrum (νB−H 2383 cm−1). Reaction of 2 with bulkier imidazolium salts does not provide the same type of mixed carbene donor complexes. For example, treating 2 with 1 equiv of 1,3-bis(2,6-diisopropylphenyl)imidazolinium chloride provides the structurally characterized anionic complex 5 (Figure 5) as the only identifiable reaction product. The structure of 5 reveals a four-coordinate iron(II) center bound to two NHC donors and two chloride ligands. The bond lengths for Fe−C (2.115(2)−2.120(2) Å) and Fe− Cl (2.325(1)−2.336(1) Å) and bite angle (97.11(6)°) are comparable to those in 4. In contrast, reaction of 2 with 1 equiv of 1,3-di-tert-butylbenzimidazolium chloride results in the formation of the four-coordinate homoleptic complex 1 as the major product, as determined by 1H NMR spectroscopy.

Scheme 3. Synthesis of Complex 4

The structure of 4 reveals a four-coordinate iron(II) center bound to three NHC donors and one chloride ligand. These are the same ligand donors as in the tris(carbene)borate complexes PhB(RIm)3FeCl (R = tBu, Mes)19,20 (Figure 4); however, the geometries of these complexes differ from that observed in 4. The iron−carbon bond lengths in 4, i.e. Fe−Cmono (2.110(3) Å) and Fe−Cbis (2.098(3)−2.109(3) Å), are the same as those observed in tris(carbene)borate iron(II) halide complexes (Fe− C, 2.088−2.096 Å).19,20 In contrast, the Fe−Cl bond length in 4 is about 0.1 Å longer than those in the tris(carbene)borate complexes. Using the solid angle8 to measure the steric congestion at iron reveals that the crowding for complex 4 (53% coverage of a sphere) is similar to that for related tris(carbene)borate complexes (52% coverage of a sphere).20 Thus, it is likely that the longer Fe−Cl bond length in 4 stems from electronic factors. Furthermore, owing to the fact that the ligand framework is less rigid, the Fe center is much closer to the CCC plane in 4 (0.789(2) Å) than in tris(carbene)borate complexes (∼1 Å). Therefore, the average C−Fe−Cl angle in 4 5657

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ACKNOWLEDGMENTS Funding from the DOE-BES (DE-FG02-08ER15996) and Indiana University is gratefully acknowledged. J.M.S. is a Dreyfus Teacher-Scholar. David Harris is thanked for providing access to the SQUID magnetometer as well as for insightful discussions.

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Figure 5. X-ray crystal structure of 5. Cation and hydrogen atoms on carbon atoms are omitted for clarity. Thermal ellipsoids are shown at 50% probability. Selected bond lengths (Å) and angles (deg): Fe(1)− C(1,) 2.120(2); Fe(1)−C(8), 2.115(2); Fe(1)−Cl(1), 2.336(1); Fe(1)−Cl(2), 2.325(1); C(1)−Fe(1)−C(8), 97.11(6); C(1)− Fe(1)−Cl(1), 108.38(5); C(8)−Fe(1)−Cl(1), 110.45(4); Cl(2)− Fe(1)−Cl(1), 115.89(2).

As noted above, complex 4 features the same coordination environment as in previously reported tris(carbene)borate iron(II) chloride complexes,19,20 but in a different coordination geometry. This allows the impact of the coordination geometry on the reactivity of four-coordinate iron(II) complexes bound to three NHC donors to be investigated. We probed this difference through preliminary experiments aimed at preparing an iron(IV) nitride complex from complex 4. Photolysis of an in situ generated iron(II)azido compound (νNNN = 2066 cm−1) does not lead to any well-defined products.21 Additionally, the only tractable product obtained from the reaction of 4 with the nitrogen atom transfer reagent Li-bdabh22 is the homoleptic compound 1. Anthracene formation is observed in this experiment, suggesting that the high-valent iron nitride complex may transiently exist but is unstable under the reaction conditions.

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CONCLUSIONS The appropriate combination of bulky bis(carbene)borate and ancillary amido ligands allows for isolation of the threecoordinate iron(II) complex 2, a rare example of such a complex based on NHC ligands.23 This complex serves as a useful starting material for the synthesis of the four-coordinate mixed carbene complex 4. Despite the fact that this complex has the same ligand donors as tris(carbene)borate complexes, attempts to prepare the corresponding iron(IV) nitrido species were unsuccessful, providing evidence that the metal geometry plays a critical role in stabilizing high-valent iron−ligand multiple bonds.

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, figures, and CIF files giving details of the X-ray crystallographic data collection and crystallographic data for 1− 5 and additional NMR and magnetometry data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.M.S.: [email protected]. Notes

The authors declare no competing financial interest. 5658

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Organometallics

Article

Holland, P. L. Inorg. Chem. 2004, 43, 3306. (e) Panda, A.; Stender, M.; Wright, R. J.; Olmstead, M. M.; Klavins, P.; Power, P. P. Inorg. Chem. 2002, 41, 3909. (f) Cowley, R. E.; DeYonker, N. J.; Eckert, N. A.; Cundari, T. R.; DeBeer, S.; Bill, E.; Ottenwaelder, X.; Flaschenriem, C.; Holland, P. L. Inorg. Chem. 2010, 49, 6172. (16) The following Hamiltonian was used, where D and E are the axial and rhombic ZFS parameters, respectively:

⎛ 2 1 ⎞ 2 2 Ĥ = D⎜Sẑ − S(S + 1)⎟ + E(Sx̂ − Sŷ )) + gz μB HS ̂ ⎝ ⎠ 3 (17) (a) Zadrozny, J. M.; Long, J. R. J. Am. Chem. Soc. 2011, 133, 20732. (b) Jiang, S.-D.; Wang, B.-W.; Sun, H.-L.; Wang, Z.-M.; Gao, S. J. Am. Chem. Soc. 2011, 133, 4730. (c) Meihaus, K. R.; Reinhart, J. D.; Long, J. R. Inorg. Chem. 2011, 50, 8484. (18) (a) Casanova, D.; Alemany, P.; Bofill, J. M.; Alvarez, S. Chem. Eur. J. 2003, 9, 1281. (b) Llunell, M.; Casanova, D.; Cirera, J.; Alemany, P.; Alvarez, S. SHAPE (version 2.0); Universitat de Barcelona, Barcelona, Spain, 2010; http://www.ee.ub.es. (19) Nieto, I.; Ding, G.; Bontchev, R. P.; Wang, H.; Smith, J. M. J. Am. Chem. Soc. 2008, 130, 2716. (20) Scepaniak, J. J.; Fulton, M. D.; Bontchev, R. P.; Duesler, E. N.; Kirk, M. L.; Smith, J. M. J. Am. Chem. Soc. 2008, 130, 10515. (21) (a) Muñoz, S. B., III; Foster, W. K.; Lin, H.-J.; Margarit, C. G.; Dickie, D. A.; Smith, J. M. Inorg. Chem. 2012, 51, 12660. (b) The in situ generated azido compounds are obtained from the reaction of 4 with sodium azide. The disappearance of paramagnetic resonances was observed after 30 min of irradiation. (22) Li-dbabh (dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta2,5-diene). (23) Ingleson, M. J.; Layfield, R. A. Chem. Commun. 2012, 48, 3579.

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