Three- and Four-Coordinate Homoleptic Iron(I)–NHC Complexes

Article pubs.acs.org/Organometallics

Three- and Four-Coordinate Homoleptic Iron(I)−NHC Complexes: Synthesis and Characterization Zhenwu Ouyang,† Yinshan Meng,‡ Jun Cheng,† Jie Xiao,† Song Gao,‡ and Liang Deng*,† †

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China ‡ Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The use of the monodentate N-heterocyclic carbene (NHC) 1,3,4,5-tetramethylimidazol-2-ylidene (IMe2Me2) and its N,N-diethyl and N,N-diisopropyl analogues IEt2Me2 and IPr2Me2 enabled the preparation of the fourcoo rd inate ho moleptic iron(I)−NHC complexes [(NHC)4Fe][BPh4] (NHC = IMe2Me2, 1; IEt2Me2, 2; IPr2Me2, 3) and the first three-coordinate homoleptic iron(I)−NHC complex, [(IPr2Me2)3Fe][BPh4] (5). X-ray crystallographic studies revealed the tetrahedral coordination geometries of the cations in 1 and 2, the square-planar geometry in 3, and the trigonal-planar geometry in 5. 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements suggested that 1, 2, and 5 are high-spin (S = 3/2) iron(I) species and that 3 is a low-spin (S = 1/2) complex. 1H NMR and UV−vis−NIR spectroscopies revealed the existence of tetrahedral−square-planar isomerization (for 2 and 3) and NHC dissociation (for 3) in solutions of 2 and 3.

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Noting the strong σ-donating nature and tunable steric property of persistent carbene ligands,11 we have employed them to stabilize low-coordinate iron(I) complexes.10c,d We found that the controlled reduction of ferrous halides in the presence of bulky N-aryl carbene ligands can produce threecoordinate iron(I)−carbene complexes (NHC)2FeX (NHC = IMes, Me2-cAAC, cyIDep, sIDep; X = Cl, Br). Further halogenabstraction reactions of these iron(I) halide complexes by NaBArF4 resulted in the preparation of the two-coordinate homoleptic iron(I)−NHC complexes [(NHC)2Fe][BArF4] (NHC = Me2-cAAC, cyIDep, sIDep) (A in Chart 1).10c,d

INTRODUCTION Iron(I) complexes have attracted great interest because of their interesting chemical and magnetic properties that have led to useful applications in small-molecule activation1 and singlemolecule magnets.2 Compared with iron(II) and iron(III) complexes, iron(I) complexes are much less abundant. The isolable iron(I) complexes are mostly six- and five-coordinate complexes bearing π-accepting ligands, e.g., CO,3 arene,4 and phosphine.5 Under the strong ligand fields, this type of iron(I) complex generally adopts a low-spin electronic configuration (S = 1/2), wherein the π-accepting ligands incur iron(I)-to-ligand back-donation, lowering the energy of the occupied antibonding molecular orbitals. Iron(I) complexes with coordination numbers of four and three are also known. Compared with their congeners with high coordination numbers, lowcoordinate iron(I) complexes are usually high-spin (S = 3/2). The high electron occupation of their low-energy antibonding orbitals results in a low ligand-field stabilization energy. Thus, chelating ligands, e.g., tris(pyrazolyl)borate,6 tris(phosphine)borate,7 and β-diketiminate,8 are commonly employed to decrease the ligand lability. In many cases, additional ancillary π-accepting ligands are also used.6,7,8b Recently, several twocoordinate high-spin iron(I) complexes bearing bulky alkyl,2 amide,9 and persistent carbene ligands9c,10 have been reported. For the stabilization of these unique two-coordinate high-spin iron(I) species, kinetic stabilization exerted by the bulky monodentate ligands plays an important role. Notably, among all of the reported structurally well-defined iron(I) complexes, two-coordinate species are the only examples of homoleptic iron(I) complexes.9a,b,10 © XXXX American Chemical Society

Chart 1. Schematic Representations of Homoleptic Iron(I)− NHC Species with Different Coordination Geometries and the Designations for the NHCs

Special Issue: Organometallics in Asia Received: January 20, 2016

A

DOI: 10.1021/acs.organomet.6b00047 Organometallics XXXX, XXX, XXX−XXX

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Organometallics The structures of these complexes were established by singlecrystal X-ray diffraction studies. Their 57Fe Mö ssbauer parameters are characteristic of the low-coordinate, high-spin (S = 3/2) iron(I) species. Prompted by this progress, we then targeted the pursuit of homoleptic iron(I)−NHC complexes with higher coordination numbers. Herein we report the synthesis and characterization of three- and four-coordinate homoleptic iron(I) complexes by using the N-alkyl Nheterocyclic carbenes (NHCs) IPr 2 Me 2 , IEt 2 Me 2 , and IMe2Me2 as ligands (B−D in Chart 1).

Scheme 2. Preparation Route for 5

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RESULTS AND DISCUSSION Synthesis. The preparation of the four-coordinate N-alkylsubstituted NHC−iron(I) complexes employs the sequential reaction protocol shown in Scheme 1. Treatment of [FeScheme 1. Preparation Route for 1−3 Solid-State Properties. The compositions of these iron(I)−NHC complexes were confirmed by combustion analyses (C, H, and N). Their molecular structures were established by single-crystal X-ray diffraction studies. Further characterization by 57Fe Mössbauer spectroscopy and SQUID magnetometry indicated the common high-spin ground state of 1, 2, and 5 and the low-spin ground state of 3. The crystal structures of the iron(I)−NHC complexes consist of well-separated univalent cations and anions. The structures of the cations in 1−3 and 5 are shown in Figure 1. The key interatomic distances and angles within the cations are compiled in Table 1. The iron centers in the cations of 1 and 2, [(IMe2Me2)4Fe]+ and [(IEt2Me2)4Fe]+, respectively, show similar tetrahedral coordination geometries with comparable Fe−C(carbene) distances and C(carbene)−Fe−C(carbene) angles (Table 1). Their average Fe−C(carbene) distances (2.047(2) and 2.058(4) Å for 1 and 2, respectively) are close to those of tetrahedral iron(I) complexes with NHC ligation, e.g., [ Ph B ( M e s I m ) 3 F e ( η 2 - C 8 H 1 4 ) ] ( 2 . 0 6 8 ( 3 ) Å ) 1 4 a n d [(IPr2Me2)2Fe(PMe3)2][BArF4] (2.080(6) Å),15 and are much shorter than those of tetrahedral iron(II)−NHC complexes, e.g., [(IPr2Me2)2FeCl2] (2.133(2) Å)16 and [CH2(tBuNHC)2FeBr2] (2.127(6) Å).17 The short Fe−C(carbene) distances in the iron(I) complexes are probably related to the enhanced d → π* back-donation in low-valent iron−NHC compounds. Different from 1 and 2, the iron center in the cation of 3, [(IPr2Me2)4Fe]+, has a square-planar coordination geometry, resembling those of its cobalt(I) analogues [(NHC) 4 Co]+ (NHC = IMe 2 Me 2 , IEt 2 Me 2 , IPr2Me2).18 Its average Fe−C(carbene) distance (1.996(2) Å) is shorter than those of 1 and 2 by 0.06 and 0.05 Å, respectively, consistent with its low-spin electronic configuration (vide infra). In accord with the larger atomic radius of iron versus cobalt, the Fe−C(carbene) bonds in 3 are longer than those in [(IPr2Me2)4Co]+ (1.970(2) Å on average).18 Complex 5 represents a rare example of three-coordinate homoleptic transition-metal−NHC complexes after the Ni(0)− NHC complexes [(IMe2H2)3Ni]19 and [(TIMEN)Ni]20 and the Rh(I)−NHC complex [(IBioxMe4)3Rh]+.21 However, the nickel and rhodium complexes are diamagnetic. The trigonalplanar FeC3 core in [(IPr2Me2)3Fe]+ shows trigonal-pyramidal distortion with the iron center sitting ca. 0.26 Å above the C(carbene)−C(carbene)−C(carbene) plane and the differentiated C(carbene)−Fe−C(carbene) angles (104.7(2), 121.4(2), and 129.1(2)°). Its average Fe−C(carbene) distance (2.059(7) Å) is comparable to those of 1 and 2 and longer than

(tmeda)Cl2]2 (tmeda = N,N,N′,N′-tetramethyl-1,2-ethylenediamine) with 8 equiv of the free NHC ligands IMe2Me2, IEt2Me2, and IPr2Me2 in tetrahydrofuran (THF) gave yellowish-brown mixtures that were then treated with 2 equiv of NaBPh4. After removal of NaCl by filtration, the filtrates were subjected to reduction with 2 equiv of KC8 at −25 °C. Further workup of the resulting mixtures afforded the iron(I)−NHC complexes [(IMe2Me2)4Fe][BPh4] (1), [(IEt2Me2)4Fe][BPh4] (2), and [(IPr2Me2)4Fe][BPh4] (3) as orange, orange, and green solids, respectively, in moderate yields (35−86%). The formation of these tetrakis(NHC)iron(I) complexes seemed to involve the generation of tris(NHC)iron(II) chloride intermediates prior to the reduction step.12 This assumption was confirmed by the isolation of [(IPr2Me2)3FeCl][BPh4] (4) in 94% yield from the reaction of [Fe(tmeda)Cl2]2 with 6 equiv of IPr2Me2 and 2 equiv of NaBPh4 in THF (Scheme 2). Complex 4 is unreactive toward NaBPh4 even in the presence 1 equiv of IPr2Me2. However, its reaction with equimolar amounts of KC8 and IPr2Me2 produced the tetrakis(NHC)iron(I) complex 3 in 80% yield (Scheme 2). The reaction of 3 with 1.5 equiv of bis(pinacolato)diboron (Pin2B2) as an NHC-scavenging reagent in THF gave the threecoordinate homoleptic iron(I)−NHC complex [(IPr2Me2)3Fe][BPh4] (5) as a blue crystalline solid in 67% yield (Scheme 2).13 Attempts to prepare 5 via the reaction of 4 with 1 equiv of KC8 at room temperature or −78 °C gave mixtures of 3 and 5 with the former being predominant. On the other hand, application of the carbene-scavenging method to 1 and 2 resulted in the formation of intractable mixtures, which signifies the importance of the NHC’s steric property in stabilizing three-coordinate homoleptic iron(I)−NHC species. B

DOI: 10.1021/acs.organomet.6b00047 Organometallics XXXX, XXX, XXX−XXX

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Figure 1. Structures of the cations in (left to right) 1−3 and 5, showing 30% probability ellipsoids. Hydrogen atoms have been omitted for clarity.

(PMe3)2][BArF4] (0.58 mm/s).15 The ΔEQ values (0.17 and 0.82 mm/s for 1 and 2, respectively) are small, which is probably induced by the prevailing Fe···H−CHRN (R = H, Me) interactions in these iron(I)−NHC complexes. The effect of secondary metal−ligand interactions on ΔEQ was noticed in Power’s two-coordinate iron(II)−amido complexes25 and, more recently, Walter’s cyclopentadienyliron(II)−NHC complexes.26 The zero-field 57Fe Mö ssbauer spectrum of 3 measured at 80 K is heavily broadened (Figure S4). Fortunately, the 200 K spectrum features a well-separated quadrupole doublet that can be fitted with δ = 0.36 mm/s and ΔEQ = 1.93 mm/s. The δ value lies between those of the lowspin square-planar iron(I) complex [(nacnac)Fe(CO)2] (0.18 mm/s)27 and the fully reduced iron−porphyrin species [Fe(TPP)]2− (0.48 mm/s).28 The latter compound was assigned the formula of low-spin [FeI(TPP3−)]2−. The spectrum of 5 has δ and ΔEQ values of 0.63 and 2.52 mm/s, respectively. The δ value is close to those of 1, 2, and the threecoordinate bis(NHC)iron(I) halide species [(NHC)2FeX] (ca. 0.67 mm/s),10c,d indicative of their common high-spin iron(I) nature. The temperature-dependent magnetic susceptibilities of the iron(I)−NHC complexes were measured on polycrystalline solid samples under a direct-current (dc) field of 1 kOe. As shown in Figure 3, the μeff values of 1, 2, and 5 at 300 K are 4.63, 5.10, and 5.94μB, respectively, which are larger than the spin-only value for S = 3/2 ions (3.87μB), whereas that of 3 is 3.31μB, which is much smaller than those of 1, 2, and 5 but larger than the spin-only value for S = 1/2 ions (1.73μB). With decreasing temperature, the μeff values of 1 and 2 remain nearly constant down to 100 K while those of 3 and 5 decrease slowly, ultimately reaching 3.32, 3.61, 2.25, and 3.90μB at 2 K for 1−3 and 5, respectively. The observed large magnetic moments of these iron(I)−NHC complexes are likely due to an

Table 1. Selected Bond Lengths and Angles of 1−3 and 5 from X-ray Diffraction Studies and Their 57Fe Mössbauer Spectroscopic Data Fe−C (Å)

C−N (Å)a C−Fe−C (deg)

δ (mm/s) ΔEQ (mm/s)

1

2

3

2.064(2) 2.045(2) 2.042(2) 2.037(2) 1.366(3) to 1.376(3) 98.9(1) to 117.1(1)

2.063(4) 2.062(4) 2.055(4) 2.052(4) 1.366(3) to 1.376(3) 100.4(2) to 114.5(2)

2.008(2) 1.998(2) 1.997(2) 1.981(2) 1.373(3) to 1.386(3) 89.5(1) to 91.1(1)

0.57b 0.17b

0.57b 0.82b

0.36c 1.93c

5 2.074(7) 2.064(7) 2.040(6) 1.345(10) to 1.360(8) 104.7(2) 121.3(2) 129.1(2) 0.63b 2.52b

a c

Range of the C(carbene)−N distances. bData for the 80 K spectrum. Data for the 200 K spectrum.

that of the two-coordinate iron(I)−NHC compound [(cyIDep)2Fe][BArF4] (1.996(7) Å).10d Compared with those of three-coordinate iron(II)−NHC species, e.g., [(IPr2Me2)Fe(Mes)2] (2.125(3) Å),22 [(IPr)Fe(NHDipp)2] (IPr = 1,3bis(2′,6′-diisopropylphenyl)imidazol-2-ylidene) (2.142(2) Å),23 and [(IPr)Fe(CH2Ph)2] (2.122(2) Å),24 the Fe− C(carbene) distances in 5 are substantially shorter, which again can be attributed to the enhanced d → π* back-donation in the low-valent iron−NHC compounds. The zero-field 57Fe Mössbauer spectra of 1, 2, and 5 at 80 K and 3 at 200 K are shown in Figure 2, and the fitted isomer shifts (δ) and quadrupole splittings (ΔEQ) are included in Table 1. The δ values of the tetrahedral iron(I) complexes 1 and 2 (0.57 and 0.57 mm/s, respectively) are comparable to those of the high-spin tetrahedral iron(I) species [PhB(CH2PiPr2)3Fe(PMe3)] (0.57 mm/s)7b and [(IPr2Me2)2Fe-

Figure 2. Zero-field 57Fe Mössbauer spectra of 1 at 80 K, 2 at 80 K, 3 at 200 K, and 5 at 80 K. The data (dots) and fits (solid lines) are shown. The fitting parameters are given in Table 1. C

DOI: 10.1021/acs.organomet.6b00047 Organometallics XXXX, XXX, XXX−XXX

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Figure 4. 1H NMR spectra of 1 (300 MHz at 21 °C), 2 (300 MHz at 21 °C), 3 (400 MHz at 21 °C), and 5 (400 MHz at 22 °C) in THF-d8.

Figure 3. Temperature-dependent magnetic susceptibilities of 1−3 and 5 in the solid state (Hdc = 1 kOe).

[(IPr2Me2)4Fe]+. On the basis of these results, the equilibria shown in Scheme 3 are proposed for 2 and 3. In THF, both of

unquenched orbital momentum contribution that is known in four-coordinate cobalt(II) and three-coordinate iron(I) complexes. For example, [Ph4P]2[Co(SPh)4]29 has a roomtemperature magnetic moment of around 5.0μ B , and [(DippNCtBu)2CH)Fe(η2-HCCPh)]30 has a value of 4.7μB. Taken together, the magnetic susceptibility, Mössbauer, and structural data collectively point to an S = 3/2 ground state for the tetrahedral iron(I) complexes 1 and 2 and the trigonalplanar iron(I) complex 5 and an S = 1/2 ground state for the square-planar iron(I) complex 3. Solution Properties. Complexes 1, 3, and 5 are soluble in THF and insoluble in toluene and n-hexane. Complex 2, however, has poor solubility in THF once it crystallizes out from the mother liquor. The colors of the THF solutions of 1 and 5 are consistent with those of the solids (orange and blue, respectively). In contrast, a yellowish-green solution of 2 and a greenish-blue solution of 3 were developed when the corresponding orange and yellowish-green solids were dissolved in THF. These phenomena suggest that the cations in 1 and 5 might keep their structures in the solution phase while those of 2 and 3 could undergo further conversions in solution. To probe the identity of these iron(I) species in solution, their 1 H NMR and electronic spectra were recorded. The 1H NMR spectra of 1 and 5 in THF-d8 (Figure 4) show two paramagnetically shifted peaks (+19.47 and −20.02 ppm for 1 and +27.75 and −12.83 ppm for 5) and no peaks arising from free NHCs, suggesting that the corresponding tetrahedral and trigonal-planar geometries of [(IMe2Me2)4Fe]+ and [(IPr2Me2)3Fe]+ are kept in solution. The poor solubility of 2 in THF-d8 results in a poor signal-to-noise ratio in the spectrum, but four paramagnetically shifted peaks at +15.33, +8.76, +1.55, and −14.35 ppm (rather than just two peaks) can be noticed. The number of peaks and the absence of free carbene signals hint at the existence of a tetrahedral−squareplanar equilibrium for the cation [(IEt2Me2)4Fe]+ in THF. The 1 H NMR spectrum of 3 in THF-d8 (Figure 4) is more complicated, featuring six paramagnetically shifted signals at +27.73, +12.08, +2.35, −6.47, −12.84, and −59.42 ppm and also the resonances of the free IPr2Me2 ligand (+4.16, +2.03, and +1.40 ppm). The resonances at +27.73 and −12.84 ppm indicate the presence of 5 in the THF solution of 3. The left four paramagnetically shifted peaks might correspond to the mixture of the tetrahedral and square-planar isomers of

Scheme 3. Proposed Equilibria in the THF Solutions of 2 and 3 at Room Temperature

these four-coordinate iron(I)−NHC species could form equilibria between their tetrahedral and square-planar isomers. In addition, the NHC dissociation/complexation processes could occur in the solution of 3. The proposed equilibria are further supported by the absorption spectra of their THF solutions. As shown in Figure 5, the spectrum of 1 in THF features two near-infrared absorption bands at 1058 and 1413 nm with extinction coefficients (ε) of ca. 1500 M−1 cm−1. These bands are tentatively assigned to the ligand-field transitions of the highspin tetrahedral species [(IMe2Me2)4Fe]+. The absorption spectrum of 2 exhibits similar absorptions at 1105 and 1484 nm, but their extinction coefficients (ca. 500 M−1 cm−1) are smaller. Moreover, a characteristic absorption band centered at 707 nm, which might arise from the ligand-field transitions of the low-spin square-planar species [(IEt2Me2)4Fe]+, can also be seen in the spectrum of 2. The spectra of 3 and 5 share common absorption bands at 582, 975, and 1240 nm assignable to the ligand-field transitions of high-spin [(IPr2Me2)3Fe]+. Despite the similarity, a rising tail near 700 nm is observed in the spectrum of 3, hinting at the existence of the low-spin square-planar species [(IPr2Me2)4Fe]+. The lack of ligand-field transitions of the high-spin tetrahedral [(IPr2Me2)4Fe]+ species D

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coordinate homoleptic iron(I)−NHC complex [(IPr2Me2)3Fe][BPh4] (5). Single-crystal X-ray diffraction studies established the solid-state structures of these iron(I)− NHC complexes as tetrahedral [(NHC)4Fe]+ for 1 and 2, square-planar [(NHC) 4 Fe] + for 3, and trigonal-planar [(NHC)3Fe]+ for 5. 57Fe Mö ssbauer spectroscopy and SQUID magnetometry measurements suggested the high-spin natures of 1, 2, and 5 and the low-spin nature of 3. 1H NMR and UV−vis−NIR spectroscopic characterization revealed that complexes 1 and 5 can keep their corresponding tetrahedral and trigonal-planar geometries in THF at room temperature, the solution of 2 forms a tetrahedral−square-planar equilibrium, and the solution of 3 exhibits not only the tetrahedral− square-planar equilibrium but also the conversion to 5 upon NHC dissociation. This study and our recent report on the two-coordinate iron(I)−NHC complexes complete the synthesis of the series of homoleptic iron(I)−NHC complexes with coordination numbers of 4, 3, and 2, which showcases the unparalleled ability NHCs to stabilize low-coordinate iron(I) species compared with other monodentate ligands.

Figure 5. Absorption spectra of 1 (black), 2 (red), 3 (green), and 5 (blue) in the UV−vis−NIR region measured in THF at room temperature.

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in the spectrum of 3 is likely due to the pronounced NHC dissociation process in the dilute THF solution, as the concentration of the THF solution for the absorption spectrum measurement was ca. 2 mmol/L, which was much lower than that for the 1H NMR measurement (ca. 50 mmol/L). In accord with this explanation, we noted that the addition of free IPr2Me2 (10 or 20 equiv) to the solution of 5 led to decreasing and increasing intensities of the absorption peaks at 582 and 700 nm, respectively (Figure S8). Moreover, the addition of free IMe2Me2 (10 or 100 equiv) to the solution of 3 resulted in the formation of 1 (Figure S9). Tetrahedral−square-planar isomerization has been observed for late 3d metal complexes of cobalt(II),31 nickel(II),32 and copper(II).33 We recently found that the iron(II) aryl complex [(IPr2Me2)2Fe(Ar)2] can also undergo similar geometric isomerization in solution.34 In addition to these, we have shown herein that such a geometrical change can also occur at a four-coordinate iron(I) center. The readiness of these tetrakis(NHC)iron(I) complexes to undergo the isomerization forms a sharp contrast to their cobalt(I) and cobalt(II) analogues, which have stable square-planar geometries.18 The difference should be related to the lower ligand-field stabilization energy of iron(I) compared with cobalt(I) and cobalt(II). The transformation of 3 to 5 indicates the lability of IPr2Me2 in [(IPr2Me2)4Fe]+. As NHC dissociation from iron(II)−NHC complexes usually entails a sterically crowded ligand environment35 and no NHC dissociation was observed on 1 and 2, it is conceivable that the differentiated steric property of IPr2Me2 versus the other two NHCs is the cause of the NHC dissociation. Steric effects should also contribute the difference between [(IMe2Me2)4Fe]+ and its IEt2Me2 and IPr2Me2 analogues. For the former ion, its NHC ligand with the least sterically demanding nature could incur the least steric repulsion among the ligands, endowing the tetrahedral species higher stability.

EXPERIMENTAL SECTION

General Procedures. All of the experiments referring to NHCs or iron complexes were performed under an atmosphere of dry dinitrogen with the rigid exclusion of air and moisture using standard Schlenk techniques or in a glovebox. Organic solvents were dried with a solvent purification system (Innovative Technology) and bubbled with dry N2 gas prior to use. 1H NMR spectra were recorded on an Agilent 300 or 400 MHz spectrometer. Chemical shifts are reported in units of parts per million with reference to the residual protons of the deuterated solvents. Elemental analysis was performed by the Analytical Laboratory of Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences. Absorption spectra were recorded with a Shimadzu UV-3600 UV−vis−NIR spectrophotometer. Preparation of [(IMe2Me2)4Fe][BPh4] (1). To a pale-green suspension of [Fe(tmeda)Cl2]236 (489 mg, 1.01 mmol) in THF (15 mL) was added IMe2Me237 (1.00 g, 8.05 mmol). The color of the mixture turned to brown. After the mixture was stirred for 1.5 h, NaBPh4 (688 mg, 2.01 mmol) was added. The mixture was stirred for 2 h and then filtered through diatomaceous earth. The filtrate was concentrated under vacuum to afford a brown residue. The residue was redissolved in THF (20 mL) to give a brown solution, which was kept at −25 °C for 4 h. To the cold mixture was added KC8 (313 mg, 2.32 mmol). The mixture was warmed to room temperature and further stirred for 2 h. After filtration through diatomaceous earth, the filtrate was concentrated under vacuum to give a yellowish-brown solid, which was washed with Et2O (20 mL) and cold THF (5 mL, −25 °C) to afford an orange solid. The orange solid was then dissolved in 3:1 THF/Et2O (40 mL) to give a red-orange solution. After the red-orange solution was allowed to stand at −25 °C for about a week, 1 was crystallized out as a red-orange crystalline solid (740 mg, 42% yield). The 1H NMR spectrum of this paramagnetic complex in THF-d8 displayed five characteristic peaks in the range from −20.02 to 19.47 ppm, which remained almost unchanged for 1 week at room temperature. 1H NMR (300 MHz, THF-d8, 21 °C): δ 19.47, 6.98, 6.60, 6.09, −20.02. Absorption spectrum (THF): λmax/nm (ε/M−1 cm−1) = 356 (7600), 1058 (1550), 1413 (1480). Anal. Calcd for C52H68BFeN8: C, 71.64; H, 7.86; N, 12.85%. Found: C, 71.91; H, 8.63; N, 12.35%. Preparation of [(IEt2Me2)4Fe][BPh4] (2). To a pale-green suspension of [Fe(tmeda)Cl2]2 (112 mg, 0.230 mmol) in THF (30 mL) was added IEt2Me237 (280 mg, 1.84 mmol). The color of the mixture turned to yellowish brown. After the mixture was stirred for 2 h, NaBPh4 (158 mg, 0.462 mmol) was added. The mixture was stirred for 2 h and then filtered through diatomaceous earth. The filtrate was concentrated under vacuum to afford a yellowish-brown residue. Dissolving the residue in THF (50 mL) gave a yellowish-brown

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CONCLUSION We have found that the reactions of [Fe(tmeda)Cl2]2 with free NHC ligands, NaBPh4, and KC8 furnish the four-coordinate homoleptic iron(I)−NHC complexes [(NHC)4Fe][BPh4] (NHC = IMe2Me2, 1; IEt2Me2, 2; IPr2Me2, 3). Upon the interaction with bis(pinacolato)diboron, the four-coordinate IPr2Me2 complex can be converted to a unique threeE

DOI: 10.1021/acs.organomet.6b00047 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

characteristic peaks in the range from −19.89 to 27.92 ppm. 1H NMR (400 MHz, THF-d8, 21 °C): δ 27.92, 11.72, 7.23, 6.80, 6.68, −19.89. Anal. Calcd for C57H80BClFeN6: C, 71.96; H, 8.48; N, 8.83%. Found: C, 71.97; H, 8.75; N, 8.86%. Preparation of [(IPr2Me2)3Fe][BPh4] (5). To a greenish blue solution of 3 (500 mg, 0.456 mmol) in THF (25 mL) was added bis(pinacolato)diboron (174 mg, 0.685 mmol) within 5 min. The color of the mixture turned to blue. After it was stirred for 2 h, the resulting mixture was concentrated under vacuum to give a mixture of blue and white solids. Successive washing of the mixture with Et2O (30 mL) and toluene (20 mL) removed the white solid. The blue residue was then dissolved in 5:1 THF/Et2O (10 mL) to give a blue solution, which was allowed to stand at −25 °C to facilitate crystallization. After 3 days, 5 was crystallized out as a blue crystalline solid (280 mg, 67% yield). The 1H NMR spectrum of this paramagnetic complex in THFd8 displayed five characteristic peaks in the range from −12.83 to 27.75 ppm, which remained almost unchanged for 7 days at room temperature. 1H NMR (400 MHz, THF-d8, 22 °C): δ 27.75, 6.96, 6.78, 6.61, −12.83. Absorption spectrum (THF): λmax/nm (ε/M−1 cm−1) = 344 (4300), 586 (1510), 1012 (180), 1272 (250). Anal. Calcd for C57H80BFeN6: C, 74.74; H, 8.80; N, 9.18%. Found: C, 74.32; H, 8.72; N, 9.61%. X-ray Structure Determination. Diffraction-quality crystals were obtained by freezing THF/Et2O solutions of 1, 3, and 5 at −25 °C, by diffusing n-hexane into a THF solution of 2 at room temperature, and by diffusing Et2O into a THF solution of 4 at room temperature. Crystals were coated with Paratone-N oil and mounted on a Bruker APEX CCD-based diffractometer equipped with an Oxford lowtemperature apparatus. Data were collected at 130(2) or 140(2) K. Cell parameters were retrieved with SMART software and refined using SAINT software on all reflections. Data integration was performed with SAINT, which corrects for Lorentz polarization and decay. Absorption corrections were applied using SADABS.38 Space groups were assigned unambiguously by analysis of symmetry and systematic absences as determined by XPREP. All of the structures were solved and refined using SHELXTL.39 Metal and firstcoordination-sphere atoms were located from direct-methods electron maps. Non-hydrogen atoms were found in alternating difference Fourier synthesis and least-squares refinement cycles and during the final cycles were refined anisotropically. Table S1 summarizes the crystal data and data collection and refinement parameters for these complexes. 57 Fe Mö ssbauer Spectroscopy. All of the solid samples for 57Fe Mössbauer spectroscopy were nonenriched samples of the as-isolated complexes. Each sample was loaded into a Delrin Mössbauer sample cup under liquid nitrogen. Low-temperature zero-field 57Fe Mössbauer measurements were performed using a SEE Co. MS4 Mössbauer spectrometer integrated with a Janis SVT-400T He/N2 cryostat for measurements at 80 or 200 K. Isomer shifts were determined relative to α-Fe at 298 K. All of the Mössbauer spectra were fit using the program WMoss (SEE Co.). Magnetic Susceptibility Measurements. Solid magnetic susceptibilities were measured using a Quantum Design MPMS-XL 5 magnetometer. The polycrystalline samples were fixed using Parafilm and sealed in quartz tubes. Corrections for the diamagnetic contributions from the sample holder, Parafilm, and sample were made.

solution, which was cooled at −25 °C for 4 h. To the cold mixture was then added KC8 (75 mg, 0.555 mmol). The mixture was warmed to room temperature and stirred for 2 h, during the course of which the color of the mixture turned to yellowish green and an orange solid precipitated out. After filtration through diatomaceous earth, the filtrate was concentrated under vacuum to give a brown solid that was washed with Et2O (20 mL) and cold THF (15 mL, −25 °C) to afford 2 as an orange solid (160 mg, 35% yield). Single crystals of 2 suitable for X-ray studies were grown by diffusion of hexane to its THF solution at room temperature. The 1H NMR spectrum of this paramagnetic complex in THF-d8 displayed seven characteristic peaks in the range from −14.35 to 15.33 ppm, which remained almost unchanged for 3 days at room temperature. 1H NMR (300 MHz, THF-d8, 21 °C): δ 15.33, 8.76, 7.04, 6.56, 6.46, 1.55, −14.35. Absorption spectrum (THF): λmax/nm (ε/M−1 cm−1) = 340 (5500), 414 (3500), 479 (1530), 707 (820), 1105 (500), 1484 (370). Anal. Calcd for C60H84BFeN8: C, 73.23; H, 8.60; N, 11.39%. Found: C, 73.12; H, 8.80; N, 11.12%. Preparation of [(IPr2Me2)4Fe][BPh4] (3). Method A. To a palegreen suspension of [Fe(tmeda)Cl2]2 (500 mg, 1.03 mmol) in THF (20 mL) was added IPr2Me237 (1.48 g, 8.21 mmol). Lots of white solid precipitated from the mixture quickly. After the mixture was stirred for 1.5 h and NaBPh4 (704 mg, 2.06 mmol) was added, the resulting mixture was further stirred for 2 h, during the course of which the mixture turned into a pale-yellow solution. The resulting solution was filtered through diatomaceous earth and concentrated under vacuum to afford a pale-yellow solid. The yellow solid was redissolved in THF (20 mL) to give a yellow solution, which was cooled at −25 °C for 2 h. To the cold mixture was then added KC8 (306 mg, 2.26 mmol), and the mixture was warmed to room temperature and further stirred for 3 h, during the course of which the color of the mixture turned to bluegreen. The resulting mixture was filtered through diatomaceous earth and concentrated under vacuum to give a green solid. The green solid was then washed with n-hexane (20 mL) and Et2O (25 mL) to give 3 as a green solid (1.94 g, 86% yield). Single crystals of 3 suitable for Xray studies were easily obtained by freezing its THF/Et2O solution at −25 °C for 3 days. Method B. To a pale-yellowish-green suspension of 4 (400 mg, 0.420 mmol) in THF (20 mL) was added IPr2Me2 (76 mg, 0.422 mmol). The color of the mixture did not change obviously. After it was stirred for 1 h, the mixture was cooled at −25 °C for 3 h, and then KC8 (63 mg, 0.466 mmol) was added to it. The mixture was warmed to room temperature and stirred for 3 h, during the course of which the color of the mixture turned to blue-green. The resulting mixture was filtered through diatomaceous earth and concentrated under vacuum to give a green solid, which was washed with n-hexane (20 mL) and Et2O (15 mL). The green solid was redissolved in 5:1 THF/Et2O (25 mL) to give a blue-green solution. The solution was allowed to stand at −25 °C for about 3 days, leading to the crystallization of 3 as a green crystalline solid (370 mg, 80% yield). The 1H NMR spectrum of this paramagnetic complex in THF-d8 displayed 12 characteristic peaks in the range from −59.42 to 27.73 ppm, which remained almost unchanged for 7 days at room temperature. 1H NMR (400 MHz, THF-d8, 21 °C): δ 27.73, 12.08, 7.02, 6.75, 6.61, 4.16, 2.35, 2.03, 1.40, −6.47, −12.84, −59.42. Absorption spectrum (THF): λmax/nm (ε/M−1 cm−1) = 353 (4800), 477 (1100), 582 (1190), 712 (350), 975 (200), 1240 (230). Anal. Calcd for C68H100BFeN8: C, 74.50; H, 9.19; N, 10.22%. Found: C, 74.92; H, 9.49; N, 10.03%. Preparation of [(IPr2Me2)3FeCl][BPh4] (4). To a pale-green suspension of [Fe(tmeda)Cl2]2 (450 mg, 0.926 mmol) in THF (20 mL) was added IPr2Me2 (1.00 g, 5.55 mmol). Lots of white solid precipitated from the mixture quickly. After the mixture was stirred for 2 h, NaBPh4 (633 mg, 1.85 mmol) was added, and the mixture was further stirred for 6 h, during the course of which the mixture turned into a pale-yellow solution. The resulting solution was filtered through diatomaceous earth and concentrated under vacuum to afford 4 as a pale-yellow solid (1.66 g, 94% yield). Single crystals of 4 suitable for Xray studies were easily obtained by diffusion of Et2O into its THF solution at room temperature within several hours. The 1H NMR spectrum of this paramagnetic complex in THF-d8 displayed six

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00047. 57

Fe Mössbauer, NMR, and absorption spectra (PDF) Crystallographic data for 1−5 (CIF) F

DOI: 10.1021/acs.organomet.6b00047 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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*Tel.: 86-21-54925460. Fax: 86-21-54925460. E-mail: deng@ sioc.ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (21421091 and 21432001).

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DOI: 10.1021/acs.organomet.6b00047 Organometallics XXXX, XXX, XXX−XXX