Reactions of [FeL2(CH3CN)2](PF6)2 (L = N-pyrimid-2-ylimidazol-ylidene) with N-, P-, O-, and S-donors and its catalytic activity

Science Bulletin, Jul 2012

Reactions of [FeL2(CH3CN)2]2+ (L = N-pyrimid-2-ylimidazolylidene) with various N-, P-, O-, and S-donors were investigated. By replacing the labile acetonitrile, various iron-NHC complexes containing additional N-, P-, O-, and S-ligands were prepared. All the iron-NHC complexes were fully characterized by NMR spectroscopy and X-ray crystallography. [FeL2(CH3CN)2]2+ could efficiently catalyze the coupling reactions of various Grignard reagents with heteroaryl bromides or chlorides.

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Reactions of [FeL2(CH3CN)2](PF6)2 (L = N-pyrimid-2-ylimidazol-ylidene) with N-, P-, O-, and S-donors and its catalytic activity

ZHANG Yin 0 1 2 LIU Bin 0 1 WU HuaYue 1 2 CHEN WanZhi ) 0 1 0 Department of Chemistry, Zhejiang University , Xixi Campus, Hangzhou 310028, China 1 iron, N-heterocyclic carbene, C-C coupling, X-ray diffraction 2 Faculty of Chemistry & Material Engineering, Wenzhou University , Wenzhou 325027, China Reactions of [FeL2(CH3CN)2]2+ (L = N-pyrimid-2-ylimidazolylidene) with various N-, P-, O-, and S-donors were investigated. By replacing the labile acetonitrile, various iron-NHC complexes containing additional N-, P-, O-, and S-ligands were prepared. All the iron-NHC complexes were fully characterized by NMR spectroscopy and X-ray crystallography. [FeL2(CH3CN)2]2+ could efficiently catalyze the coupling reactions of various Grignard reagents with heteroaryl bromides or chlorides. - Because iron is cheap, nontoxic, and environmentally friendly, iron complexes are important catalysts. Interest in the catalytic applications of iron in organic synthesis has increased in recent years [16]. In earlier studies, we found that functionalized N-heterocyclic carbenes (NHCs) were good ligands in homogeneous catalysis because of strong donating abilities [711]. Fe-NHCs could be used as catalysts to develop cheap and green catalytic organic transformations for industrial applications. Some Fe-NHC complexes have been used in polymerization of olefins and CC and C-heteroatom bond formation reactions. Grubbs and coworkers [11] reported the catalytic activity of welldefined [(NHC)2FeX2] complexes in atom transfer radical polymerization. Iron(II) complexes of functionalized NHC were found to catalyze ring-opening polymerization of -caprolactone [12]. Iron-SIPr (saturated 1,3-bis(2,6diisopropylphenyl) imidazolylidene) fluoride could catalyze selective biaryl coupling [13]. A half-sandwich iron-NHC complex was reported as an efficient catalyst for CH bond activation/borylation of furans and thiophenes [14,15], hydrogen transfer reactions [16], and hydrosilylation of carbonyl derivatives [17]. Several iron complexes, including mononuclear complexes containing monodentate [1820] or polydentate NHC ligands [2125], iron clusters [2628], and polymeric iron complexes [29], have been recently reported. However, compared to noble metal-NHC complexes [30,31], the organometallic chemistry of iron-NHC complexes has not been well explored. We [32] recently reported that iron-NHC complexes can be prepared from iron powder with imidazolium salts or an Ag-NHC complex in air. Alternatively, iron-NHC complexes can be electrochemically prepared using an imidazolium salt as the supporting electrolyte [33]. These synthetic procedures allow for scaling the preparation of iron-NHC complexes, and thus offer more opportunity for careful investigation of the reactivity and application of iron-NHC complexes. In this paper, we report the reactions of [FeL2(CH3CN)2]2+ (L=N-pyrimid-2-ylimidazolylidene) with N-, P-, O-, and S-donors and the structural characterization of resulting iron(II)-NHC complexes. Experimental All the chemicals were obtained from commercial suppliers The Author(s) 2012. This article is published with open access at Springerlink.com and used without further purification. [FeL2(CH3CN)2]2+ (1, L=N-pyrimid-2-ylimidazolylidene) was prepared according to an established procedure [32]. Elemental analyses were performed on a Flash EA1112 instrument (Thermo Fisher Scientific, Waltham, MA, USA). 1H, 13C and 31P NMR spectra were recorded on Bruker (Billerica, MA, USA) Avance-400 (400 MHz) spectrometer. Chemical shifts ( ) are expressed in ppm downfield to tetramethylsilane at = 0, and coupling constants (J ) are expressed in Hz. Synthesis of [FeL2(Ph3PO)2](PF6)2 (2) A yellowish-red solution of 1 (38 mg, 0.05 mmol) in 4 mL of acetonitrile and triphenylphosphine (26 mg, 0.1 mmol) was stirred at 60C for 10 h in air. Most of 1 was recovered. However, further crystallization of the filtrate gave a few dark red single crystals suitable for X-ray diffraction. Spectroscopic characterization was not performed. Synthesis of [FeL2(dppe)](PF6)2 (3 and 3) A yellowish-red solution of 1 (38 mg, 0.05 mmol) in 4 mL of acetonitrile and 1,2-bis(diphenylphosphino)ethane (20 mg, 0.05 mmol) was stirred at 40C for 12 h in air. After concentrating to approximately 2 mL, addition of 20 mL of diethyl ether afforded a red solid which was collected, washed with diethyl ether, and dried in vacuo. Yield: 44 mg (83%). Recrystallization from CH3CN-diethyl ether gave 3 and 3 as dark red and red crystalline solids which were separated by hand. For 3: 1H NMR (400 MHz, acetone-d6): 9.00 (dd, J = 4.8, 1.6 Hz, pyrimidyl, 1H), 8.64 (dd, J = 4.8, 1.6 Hz, pyrimidyl, 1H), 8.55 (d, J = 6.0 Hz, pyrimidyl, 1H), 8.28, 8.26 (both d, J = 2.2 Hz, imidazolylidene, each 1H), 7.75 (d, J = 6.0 Hz, pyrimidyl, 1H), 7.72 (d, J = 2.4 Hz, imidazolylidene, 1H), 7.687.64 (m, Ph, 2H), 7.537.39 (m, pyrimidyl and Ph, 9H), 7.317.24 (m, Ph, 3H), 7.157.00 (m, pyrimidyl, imidazolylidene and Ph, 7H), 6.76 (t, J = 8.8 Hz, Ph, 2H), 4.33 (dtd, J = 41.6, 15.0, and 4.4 Hz, CH2, 1H), 4.10 (dtd, J = 41.6, 15.0, and 4.4 Hz, CH2, 1H), 3.47 (s, CH3, 3H), 3.433.38 (m, CH2, 1H), 3.353.29 (m, CH2, 1H), 2.58 (s, CH3, 3H). 31P{1H} NMR (162 MHz, DMSO-d6): 74.0, 57.4 (both d, JPP = 39 Hz). 13C{1H} NMR (100 MHz, acetone-d6): 198.6 (dd, JC-P = 23.6, 21.2 Hz, FeC), 196.3 (dd, JC-P =16.4, 12.3 Hz, FeC), 163.9, 163.8, 161.2, 160.3, 159.8, 158.9, 158.0, 135.7128.7 (Ph), 120.2, 119.9, 119.0, 118.9, 116.7, 37.5, 36.2, 26.7 (dd, JC-P = 25.8, 12.6 Hz), 24.4 (dd, JC-P = 26.4, 11.8 Hz). Anal. Calcd. (%) for C42H40F12FeN8PCH3CNH2O: C, 47.03; H, 4.04; N, 11.22. Found (%): C, 46.75; H, 3.81; N, 11.27. For 3: 1H NMR (400 MHz, acetone-d6): 8.96 (dd, J = 5.0, 1.4 Hz, pyrimidyl, 2H), 8.34 (dd, J = 5.0, 1.4 Hz, pyrimidyl, 2H), 8.19 (d, J = 1.8 Hz, imidazolylidene, 2H), 7.49, (t, J = 5.0, pyrimidyl, 2H), 7.467.40 (m, Ph, 2H), 7.367.35 (m, Ph, 2H), 7.397.32 (m, Ph, 8H), 7.257.22 (m, Ph, 4H), 7.20 (d, J = 1.8 Hz, imidazolylidene, 2H), 7.006.96 (m, Ph, 4H), 4.03 (dd, J = 26.8, 9.6 Hz, CH2, 2H), 3.15 (d, J = 8.4 Hz, CH2, 2H), 3.1 (s, CH3, 6H). 31P{1H} NMR (162 MHz, acetone-d6): 63.9 (d, JPP = 26.7 Hz). 13C{1H} NMR (100 MHz, acetone-d6): 200.1 (t, JCP = 19.1 Hz, FeC), 166.4, 161.8, 160.2, 132.4, 131.7, 130.2130.0 (Ph), 129.4, 129.3, 129.2, 121.5, 120.2, 39.8, 27.5 (t, JCP = 19.8 Hz). Anal. Calcd. (%) for C42H40F12FeN8P4: C, 47.39; H, 3.79; N, 10.53. Found (%): C, 47.31; H, 3.86; N, 10.44. Synthesis of [FeL2(en)](PF6)2 (4) Compound 4 was prepared from 1 (38 mg, 0.05 mmol) and ethane-1,2-diamine (10 L, 0.05 mmol) in 4 mL of acetonitrile in a similar manner to 3. Yield: 33 mg (91%). Recrystallization from CH3CN/CH2Cl2-diethyl ether gave dark red crystals. 1H NMR (400 MHz, DMSO-d6): 8.84 (dd, J = 5.2, 1.6 Hz, pyrimidyl, 2H), 8.77 (dd, J = 5.2, 1.6 Hz, pyrimidyl, 2H), 8.27, 7.48 (both d, J = 2.0 Hz, imidazolylidene, each 2H), 7.46 (d, J = 5.2 Hz, pyrimidyl, 2H), 4.88 (d, J = 10.8 Hz, NH2, 2H), 4.18 (d, J = 4.8 Hz, NH2, 2H), 2.90, 2.35 (both br, CH2, each 2H), 2.72 (s, CH3, 6H). 13C{1H} NMR (100 MHz, DMSO-d6): 205.3 (FeC), 164.2, 160.7, 157.7, 128.7, 118.1, 117.5, 43.2, 35.9. Anal. Calcd. (%) for C18H24F12FeN10P20.5CH3CN0.5CH2Cl2: C, 29.68; H, 3.38; N, 18.63. Found (%): C, 30.08; H, 3.30; N, 18.60. Synthesis of [FeL2(pda)](PF6)2 (5) Compound 5 was prepared from 1 (76 mg, 0.1 mmol) and propane-1,3-diamine (50 L) in a similar manner to 3. Yield: 69 mg (93%). Recrystallization from CH3CN/CH2Cl2diethyl ether gave dark red crystals. 1H NMR (400 MHz, DMSO-d6): 8.97 (d, J = 5.6 Hz, pyrimidyl, 2H), 8.86 (d, J = 5.6 Hz, pyrimidyl, 2H), 8.23, 7.41 (both d, J = 2.2 Hz, imidazolylidene, each 2H), 7.50 (t, J = 5.6 Hz, pyrimidyl, 2H), 4.54, 3.92 (both t, J = 6.5 Hz, NH2, each 2H), 2.87, 2.68, 1.58 (all unresolved m, CH2, each 2H), 2.67 (s, CH3, 6H). 13C{1H} NMR (100 MHz, acetone-d6): 203.1 (FeC), 165.0, 158.0, 128.3, 118.6, 118.4, 117.5, 40.1, 39.8, 35.0, 27.1. Anal. Calcd. (%) for C19H26F12FeN10P21.5CH3CN 0.5CH2Cl2: C, 32.01; H, 3.76; N, 19.08. Found (%): C, 32.47; H, 3.65; N, 18.78. Synthesis of [FeL2(phen)](PF6)2 (6 and 6) Compounds 6 and 6 were prepared from 1 (38 mg, 0.05 mmol) and 1,10-phenanthroline (9 mg, 0.05 mmol) in a similar manner to 3. Yield: 24 mg (57%). Recrystallization from CH3CN-diethyl ether gave two isomers of crystals that were red and dark red, respectively. For 6: 1H NMR (400 MHz, DMSO-d6): 8.92 (d, J = 4.8 Hz, pyrimidyl, 2H), 8.68 (d, J = 7.6 Hz, phenanthroline, 2H), 8.53 (s, phenanthroline, 2H), 8.28, 7.70 (both s, imidazolylidene, each 2H), 8.13 (d, J=4.8 Hz, pyrimidyl, 2H), 7.737.67 (m, phenanthroline, 4H), 7.40 (t, J=4.8 Hz, pyrimidyl, 2H), 2.64 (s, CH3, 6H). 13C{1H} NMR (100 MHz, DMSO-d6): 203.5 (FeC), 167.8, 159.7, 154.6, 148.4, 136.0, 129.9, 129.5, 128.0, 125.7, 120.7, 118.8, 35.3. Anal. Calcd. (%) for C28H24F12FeN10P20.5Et2OH2O: C, 40.38; H, 3.39; N, 15.70. Found (%): C, 39.95; H, 2.90; N, 15.63. For 6: 1H NMR (400 MHz, DMSO-d6): 8.4 (t, J = 6.8 Hz, pyrimidyl, 3H), 8.62 (d, J = 8.4 Hz, pyrimidyl, 1H), 8.59, 8.41, 7.84, 7.49 (all s, imidazolylidene, each 1H), 8.38 (d, J = 9.2 Hz, pyrimidyl, 1H), 8.27 (d, J = 8.8 Hz, pyrimidyl, 1H), 8.037.98 (m, phenanthroline, 3H), 7.80 (d, J = 6.0 Hz, phenanthroline, 1H), 7.63 (t, J = 6.8 Hz, phenanthroline, 1H), 7.37 (d, J = 5.6 Hz, phenanthroline, 1H), 7.32 (t, J = 5.8 Hz, phenanthroline, 1H), 7.28 (t, J = 10.8 Hz, phenan throline, 1H), 3.26, 2.47 (all s, CH3, each 3H). 13C{1H} NMR (100 MHz, DMSO-d6): 204.6 (FeC), 203.1 (FeC), 163.6, 161.5, 160.1, 159.7, 159.5, 159.0, 158.5, 153.5, 148.5, 147.5, 137.6, 136.8, 130.5, 130.0, 129.9, 129.5, 128.1, 126.9, 126.2, 120.7, 119.8, 119.5, 118.5, 118.4, 36.1, 35.1. Anal. Calcd. (%) for C28H24F12FeN10P20.5CH3CN: C, 40.18; H, 2.97; N, 16.97. Found (%): C, 40.58; H, 3.08; N, 16.42. Synthesis of [FeL2(acac)](PF6) (7) A solution of 1 (38 mg, 0.05 mmol) in 4 mL of acetonitrile was added to acetylacetonate (6 L, 0.06 mmol) and K2CO3 (8.3 mg, 0.06 mmol) at 40C. After stirring for 5 h, the solution was concentrated to approximately 2 mL. Addition of diethyl ether (20 mL) afforded a dark red solid which was collected, washed with water and diethyl ether, and dried in vacuo. Yield: 24 mg (77%). Recrystallization from CH3CN-diethyl ether gave dark red crystals suitable for X-ray diffraction. 1H NMR (400 MHz, acetone-d6): 8.86 (dd, J = 5.0, 2.4 Hz, pyrimidyl, 2H), 8.87 (dd, J = 5.0, 2.4 Hz, pyrimidyl, 2H), 8.25, 7.37 (d, J = 2.0 Hz, imidazolylidene, each 2H), 7.47 (t, J = 5.0 Hz, pyrimidyl, 2H), 5.45 (s, CH, 1H), 2.96 (s, CH3, 6H), 1.86 (s, CH3, 6H). 13C{1H} NMR (100 MHz, acetone-d6): 207.5 (FeC), 189.2, 163.6, 160.4, 157.4, 127.8, 118.0, 117.7, 98.6, 35.6, 27.9. Anal. Calcd. (%) for C21H23F6FeN8O2P: C, 40.53; H, 4.05; N, 18.01. Found (%): C, 40.42; H, 3.69; N, 17.79. Synthesis of [FeL2(pyridine-2-thiolate)](PF6) (8) Compound 8 was prepared from 1 (36 mg, 0.05 mmol), pyridine-2-thiol (5.5 mg, 0.05 mmol), and K2CO3 (8.3 mg, 0.06 mmol) in 4 mL of acetonitrile using the same procedure as for 7. Yield: 15 mg (48%). Recrystallization from CH3CN-diethyl ether gave dark red crystals suitable for X-ray diffraction. 1H NMR (400 MHz, DMSO-d6): 9.80 (dd, J = 5.6, 2.2 Hz, pyrimidyl, 1H), 9.04 (dd, J = 5.0, 2.2 Hz, pyrimidyl, 1H), 8.61 (dd, J = 5.0 Hz, 2.2 Hz, pyrimidyl, 1H), 8.47, 8.41, 7.70, 7.62 (all d, J = 2.4 Hz, imidazolylidene, each 1H), 7.83 (dd, J = 5.6, 2.2 Hz, pyrimidyl, 1H), 7.75 (t, J = 5.6 Hz, pyrimidyl 1H), 7.21 (dt, J = 6.8, 1.6 Hz, pyridyl, 1H), 7.01 (t, J = 5.0 Hz, pyrimidyl, 1H), 6.64 (d, J = 4.8 Hz, pyridyl, 1H), 6.54 (d, J = 8.4 Hz, pyridyl, 1H), 6.45 (ddd, J = 7.2, 5.2, 1.6 Hz, pyridyl, 1H), 3.19, 3.09 (both s, CH3, each 3H). 13C{1H} NMR (100 MHz, acetone-d6): 211.6 (FeC), 210.0 (FeC), 176.6, 164.8, 163.3, 162.3, 158.0, 155.8, 154.0, 152.7, 134.0, 128.2, 128.1, 124.6, 119.2, 118.2, 117.1, 117.0, 35.4, 35.0. Anal. Calcd. (%) for C21H20F6FeN9PS00.5H2O: C, 39.39; H, 3.31; N, 19.69. Found (%): C, 39.78; H, 3.03; N, 19.19. General procedure for Kumada reactions A Schlenk tube was charged with 1 (1.9 mg, 0.002 mmol), anhydrous THF (2 mL), and heteroaryl halide (1.0 mmol) under a N2 atmosphere. A solution of Grignard reagent (2.0 mL, 1.0 mol/L in THF) at room temperature was added to the solution of 1 in one portion with stirring. After 12 h, a saturated aqueous solution of NH4Cl was added to quench the reaction. The mixture was extracted with CH2Cl2 (3 10 mL), and the combined organic layer was dried over MgSO4. After removal of solvent, the crude product was purified by column chromatography on silica gel to afford the desired product. X-ray crystallography Single-crystal X-ray diffraction data from 2 were collected on a Siemens (Munich, Germany) Smart/CCD areadetector diffractometer, and from 3, 3, 4, 6, 7, and 8 on an Oxford Diffraction Gemini Ultra A diffractometer (Agilent Technologies, Santa Clara, CA, USA) with Mo K radiation (= 0.71073 ) and in -2 scan mode. Data from 5 were collected with Cu K radiation (= 1.54184 ). Unit-cell dimensions were obtained with least-squares refinement. Data collection and reduction were performed using SMART and SAINT software for 2 and using the Oxford Diffraction CrysAlisPro software for 38 [34,35]. The structures were solved by direct methods, and the nonhydrogen atoms were subjected to anisotropic refinement by full-matrix least squares on F2 using the SHELXTXL package [36]. Hydrogen atom positions for all of the structures were calculated and allowed to ride on their respective C atoms with CH distances of 0.930.97 and Uiso(H) = 1.21.5 Ueq(C). Disordered solvent could not be modeled successfully and was removed from the reflection data of 5 with SQUEEZE (solvent accessible void volume 459.0 3) [37]. Details are given in Table 1. Results and discussion [FeL2(CH3CN)2]2+ (1) was prepared by reacting Ag-Npyrimid-2-ylimidazolylidene complex with iron powder [32,33]. Reactions of 1 with various donors were performed at 40C, and the results are summarized in Scheme 1. Table 1 Summary of X-ray crystallographic data for complexes 24 and 68 Formula Fw Crystal system Space group a () b () c () () () () V (3) Z 0.0590, 0.1397 0.1039, 0.1758 0.0747, 0.2037 0.1296, 0.2202 0.0978, 0.2303 0.1527, 0.2750 0.0493, 0.1403 0.0631, 0.1473 0.0650, 0.1873 0.1120, 0.2034 0.0482, 0.1372 0.0605, 0.1454 0.0866, 0.2475 0.1064, 0.2722 Scheme 1 Syntheses for 26, 7, and 8. (i) PPh3, CH3CN, 60C; (ii) dppe, acetone, 40C; (iii) ethane-1,2-diamine, CH3CN, 40C; (iv) propane-1,3-diamine, acetone, 40C; (v) 1,10-phenanthroline, CH3CN, 40C; (vi) acetylacetone, K2CO3, 40C; (vii) pyridine-2-thiol, K2CO3, CH3CN, 40C. Reactions of [FeL2(CH3CN)2]2+ with phosphines Treatment of [FeL2(CH3CN)2]2+ with two equivalents of PPh3 in acetonitrile did not give the expected [FeL2(PPh3)2]2+, and the starting complex was recovered in up to 80% yield. A few single crystals of [Fe(LO)2(Ph3PO)2]2+ (2) were obtained from some reactions. However, 2 was not obtained in all experiments. The structure of 2 is shown in Figure 1. The asymmetric unit consists of a half molecule with an inversion center at the Fe(II) ion. The two imidazolylidene and PPh3 were oxidized to imidazolone and PPh3O, respectively. The oxidation of coordinated imidazolylidene to imidazolone in air has been observed in some cases [33]. Such reactions might imply that the complex could activate molecular oxygen in air [38]. Each pair of the same donors was arranged in a trans fashion. Two Ph3PO molecules occupied the axial positions. The FeO bond distances between iron and Ph3PO or imidazolone were essentially the same. The ligand 1-methyl-3-(pyrimidin-2-yl)-1H-imidazol2(3H)-one was twisted forming a six-membered ring with Fe(II) ion in a boat conformation. Unlike PPh3, dppe reacted readily with 1 affording a mixture of 3 and 3 in a ratio of approximately 1/4. The two isomers could be easily separated by recrystallization from acetonitrile and diethyl ether. The 31P NMR spectrum of 3 showed two doublets at 74.0 and 57.4 with JPP = 39 Hz, and that of 3 showed a doublet with a coupling constant of 26.7 Hz. In the 13C NMR spectrum of 3, two double doublets were observed at 198.6 (JC-P = 23.6 and 21.2 Hz) and 196.3 (JC-P =16.4 and 12.3 Hz) because of coupling of the carbenic carbon with cis- and trans-arranged phosphines. By comparison, 3 showed only one triplet at 200.1 with JC-P = 19.1 Hz because of its coupling with two phosphines. The structures of 3 and 3 are given in Figure 2. Figure 1 ORTEP drawing of the cationic section of 2. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been removed for clarity. Selected bond distances (): Fe(1)O(1) 2.054(3), Fe(1)O(2) 2.063(3), and Fe(1)N(3) 2.102(4). Symmetry code: A x+2, y+1, z+1. Figure 2 ORTEP drawing of the cationic sections of 3 (a) and 3 (b). Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been removed for clarity. Selected bond distances () for 3: Fe(1)C(2) 1.954(5), Fe(1)C(10) 1.949(5), Fe(1)N(8) 2.027(4), Fe(1) N(4) 2.044(4), Fe(1)P(1) 2.270(2), and Fe(1)P(2) 2.302(2). For 3: Fe(1)C(1) 1.990(5), Fe(1)C(9) 1.969(5), Fe(1)N(3) 2.036(5), Fe(1)N(7) 2.020(5), Fe(1)P(1) 2.284(2), and Fe(1)P(2) 2.277(2). The two P-donors of dppe were crystallographically inequivalent and trans to one NHC and one pyrimidine group, respectively. In 3, the two imidazolylidenes were arranged trans to each other, and two phosphorous atoms were located trans to the two pyrimidines. The two planar 1-methyl-3-(pyrimidin-2-yl)imidazolylidenes were perpendicular to each other. The FeC and FeP bond distances were normal and consistent with those of known iron(II) complexes [39,40]. Reactions of [FeL2(CH3CN)2](PF6)2 with N-donors As described in Scheme 1, reaction of ethane-1,2-diamine with 1 readily led to the replacement of two coordinated acetonitriles, and produced [FeL2(en)](PF6)2 (4) as a dark red crystalline solid. [FeL2(pda)](PF6)2 (5) was obtained in a similar manner. The carbene carbon signals of 4 and 5 in their 13C NMR spectra showed only one singlet at 205.3 and 203.1, respectively. This illustrates that the two NHC moieties are magnetically equivalent. The structure of 4 is given in Figure 3. The iron was surrounded by two 1-methyl-3-(pyrimidin-2-yl)imidazolylidene ligands and one 1,2-ethylenediamine molecule. The two nitrogen atoms of ethylenediamine were trans to the NHC groups, and the two pyrimidines were trans to each other. The FeC and FeN bond distances were essentially the same as those in 1 [33]. Substitution of two acetonitriles of 1 by phenanthroline also gave a mixture of two isomers (6 and 6) of [FeL2(phen)](PF6)2 as red and dark-red crystalline solids. The 1H NMR spectrum of 6 showed only one set of resonance signals from 1-methyl-3-(pyrimidin-2-yl)imidazolylidene, and its carbene carbon showed only one singlet at 203.5 in the 13C spectrum. In the 1H and 13C spectra of 6, each aromatic proton and carbon showed independent resonance peaks. The 13C resonance peaks of the two carbenic carbons were observed at 204.6 and 203.1 for 6. The structure of 6 is given in Figure 4. The replacement of two acetonitriles resulted in a rearrangement of the coordination environment, and the two nitrogen atoms of phenanthroline Figure 3 ORTEP drawing of the cationic section of 4. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been removed for clarity. Selected bond distances (): Fe(1)C(1) 1.922(3), Fe(1) N(3) 1.996(3), and Fe(1)N(5) 2.061(3). Symmetry code: A x+1, y, z+1/2. Figure 4 ORTEP drawing of the cationic section of 6. Thermal ellipsoids are drawn at the 30% probability level. Selected bond distances (): Fe(1) C(1) 1.975(5), Fe(1)C(9) 1.966(5), Fe(1)N(3) 2.000(4), Fe(1)N(7) 2.011(4), Fe(1)N(9) 1.954(4), Fe(1)N(10) 1.970(4). were both trans to two pyrimidines. The two NHC groups were also arranged in a trans fashion. Two 1-methyl-3(pyrimidin-2-yl)imidazolylidene and one phenanthrolene were arranged perpendicular to each other. The FeC bond distances were slightly longer than those in 3 and 4 because of a large trans effect from the C-donors. Reactions of [FeL2(CH3CN)2](PF6)2 with O-donors Treatment of 1 with acetylacetone in the presence of K2CO3 afforded [FeL2(acac)](PF6) (7), which was isolated as a dark red crystalline solid. The 1H NMR spectrum of the crystals in acetone-d6 showed that two isomers existed in solution in a 2/3 ratio. However, only 7 could be crystallized from the solution. The structure of 7 is given in Figure 5. The compound can be viewed as a substitution product of 1, in which two acetonitrile molecules are replaced by an acetylacetonate. The FeO bond distances and the OFeO angle are not unusual [41]. Reactions of [FeL2(CH3CN)2](PF6)2 with O-donors Treatment of 1 with pyridine-2-thiol and K2CO3 gave [FeL2(pyridine-2-thiolate)](PF6) (8) as a dark red solid. In its 1H NMR spectrum, each proton of the aromatic rings showed an independent resonance peaks. The 13C NMR spectrum of 8 shows two peaks ascribed to two imidazolylidene carbons at 211.6 and 210.0. The structure of 8 is depicted in Figure 6. In the coordination sphere, two imidazolylidene groups were arranged trans to a pyrimidine and the sulfur atom. The FeS distance of 2.421(2) is relatively longer than those found in other iron(II) complexes with pyridine-2-thiolate. This could be because of a large trans effect for the NHC ligand [42,43]. PyS forms a Figure 5 ORTEP drawing of the cationic section of 7. Thermal ellipsoids are drawn at the 30% probability level. Selected bond distances (): Fe(1) C(10) 1.905(3), Fe(1)C(2) 1.912(3), Fe(1)N(1) 1.967(3), Fe(1)N(5) 1.980(3), Fe(1)O(2) 1.983(2), and Fe(1)O(1) 1.987(2). Figure 6 ORTEP drawing of the cationic section of 8. Thermal ellipsoids are drawn at the 30% probability level. Selected bond distances (): Fe(1) C(2) 1.908(6), Fe(1)C(10) 1.905(7), Fe(1)N(9) 1.955(5), Fe(1)N(7) 1.987(6), Fe(1)N(3) 2.033(5), and Fe(1)S(1) 2.421(2). Catalytic Kumada coupling reactions The catalytic efficiencies of iron salts in Kumada cross coupling reactions of Grignard reagents with aryl or alkyl halides have been demonstrated previously [13,4447]. However, investigations of Kumada coupling catalyzed by a welldefined iron-NHC complex are rare. We initially examined the coupling reactions of various Grignard reagents with aryl bromides or chlorides using an amount-of-substance fraction of 2.5% of 1. Unfortunately, the yields of desired biphenyls were poor and homocoupling products were the major products. Further studies showed that heteroaryl bromides could be smoothly coupled with ArMgBr in moderate yields at room temperature (Table 2). Decreasing the catalyst amount-of-substance fraction to 0.2% did not influence the yields significantly. Further decreases in the catalyst amount-of-substance fraction to 0.1% considerably decreased the yield. 2,6-Dibromopyridine could be doubly arylated, giving 2,6-diphenylpyridine in 52% yield. Under the same reaction conditions, 2-chloropyridine and 2-chloropyrimidine were much less efficient. 1d a) Reaction condition: heteroaryl chloride 1.0 mmol, Grignard reagent 2.0 mmol, THF 4 mL, 12 h, under N2. b) Isolated yield. c) 90C, 24 h. The reaction was carried out in toluene. Conclusion In summary, [FeL2(CH3CN)2]2+ (L = N-pyrimid-2-ylimidazolylidene) reacted with various N-, P-, O-, and S-donors to produce a number of iron-NHC complexes with ancillary heteroatom ligands. The structural characterization of these iron-NHC complexes provides new information for further design of new iron catalysts. [FeL2(CH3CN)2]2+ can efficiently catalyze the coupling reaction of various Grignard reagents with heteroaryl bromides or chlorides. Improvement of the catalytic behavior and application in other transformations of the iron-NHC complexes could be expected by tuning the NHC ligands. This work was supported by the National Natural Science Foundation of China (21072170). This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.


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Yin Zhang, Bin Liu, HuaYue Wu, WanZhi Chen. Reactions of [FeL2(CH3CN)2](PF6)2 (L = N-pyrimid-2-ylimidazol-ylidene) with N-, P-, O-, and S-donors and its catalytic activity, Science Bulletin, 2012, 2368-2376, DOI: 10.1007/s11434-012-5164-5