Inorganica Chimica Acta 455 (2017) 112–117

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Inorganica Chimica Acta

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Research paper

Synthesis and reactivity of silyl iron hydride via SiAH bond activation

http://dx.doi.org/10.1016/j.ica.2016.10.0170020-1693/� 2016 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: hjsun@sdu.edu.cn (H. Sun).

Yaomin Shi a, Xiaoyan Li a, Tingting Zheng a,b, Benjing Xue a, Shumiao Zhang a, Hongjian Sun a,⇑, Olaf Fuhr c,Dieter Fenske c

a School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Shanda Nanlu 27,250199 Jinan, PR ChinabDepartment of Chemistry, Capital Normal University, 100037 Beijing, PR Chinac Institut für Nanotechnologie (INT) und Karlsruher Nano-Micro-Facility (KNMF), Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344Eggenstein-Leopoldshafen, Germany

a r t i c l e i n f o

Article history:Received 9 September 2016Received in revised form 10 October 2016Accepted 11 October 2016Available online 12 October 2016

Keywords:SiAH bond activationSilyl iron hydrideSilazane ligand

a b s t r a c t

The preligand pyridine-2-amino(methyl)-dimethylsilane (1) was used to synthesize silyl iron hydride.Iron(II) hydride FeH(N(jN-C5H4N)Me(SiMe2))(PMe3)3 (2) was obtained through the reaction of 1 andFe(PMe3)4 via the activation of one SiAH bond and the coordination of nitrogen atom of pyridine. Thesubstitution reaction of complex 2 with haloalkane (CH3I and EtBr) delivered complexes FeX(N(jN-C5H4 N)Me(SiMe2))(PMe3)3 (X = I (3); Br (4)). Complex 2 reacting with acetylacetone resulted in the for-mation of Fe(acac)(N(jN-C5H4N)Me(SiMe2))(PMe3)3 (5). However, the insertion products Fe(g3-OCOH)(N(jN-C5H4N)Me(SiMe2))(PMe3)3 (6) and Fe(g3-Ph-NCOH)(N(jN-C5H4N)Me(SiMe2))(PMe3)3 (7) wereformed through the insertion of the unsaturated bonds into FeAH bond. Complex 6 could also be synthe-sized through the substitution reaction of 2with HCOOH. The molecular structures of complexes 2 and 4–7 were determined by single crystal X-ray diffraction.

� 2016 Elsevier B.V. All rights reserved.

1. Introduction

In the past 20 years, chemists have paid more and more atten-tion to silyl transition-metal (TM) complexes due to their interest-ing and important properties. Silyl transition-metal complexescould be obtained via activation or oxidative addition of SiAH tothe metal center. It has been confirmed that silyl transition-metalcomplexes as catalysts can be used in many TM-catalyzed reac-tions [1,2].

Silazane compounds (R3Si)nN(R’)3�n have shown a lot of appli-cations from materials in industry to organic chemical processes[3–5]. They can be used as preligands in the synthesis of silylTM-complexes. Recently, Smart and co-workers reported mononu-clear and dinuclear Ru complexes with different equivalents ofsilazane [6]. These complexes show versatile kinds of RuAHASibonds. Turculet and coworkers obtained PSiP pincer Ru complexvia SiAH bond activation [7] and found that ammonia NAH bondactivation could be realized by a [PSiP]Ir complex [8]. Recently,Turculet reported synthesis and characterization of five-coordinate16-electron Ru(II) complexes supported by tridentate bis(phos-phino)silyl ligation [9] and explored the reduction of CO2 to CH4

with tertiary silanes catalyzed by Pt and Pd silyl pincer complexes[10]. Similarly, PCP pincer transition metal complexes could also beprepared via CAH bond activation [11].

Comparing with the precious metals such as Ru, Pt, Pd and etc,we should focus more on cheaper metals, such as iron, cobalt andnickel. Recently, Nishiyama obtained silyl NCN-pincer iron com-plexes via oxidative addition of a CASi bond of the organosilaneprecursor [12]. Sun developed a new method to the direct synthe-sis of FeASi-bonded complexes by the reaction of cyclopentadieneswith pentacarbonyliron in the presence of hydrosilanes [13]. Dengsynthesized silyl iron complex by sequential iron-mediated ben-zylic CAH bond activation and silylation reactions [14]. Tatsumiprepared coordinatively unsaturated half-sandwich iron-silyl com-plexes with a NHC carbene as supporting ligands [15]. Hydrido ironcomplexes could also be used in both dehydrogenative and addi-tion reactions. Beller presented a highly active hydrido iron cata-lyst supported by phosphine ligand which could catalyze thedehydrogenation of formic acid [16]. Lately, the cyclometalatediron(II) hydrido complexes were demonstrated to be efficient cat-alysts for the dehydrogenation of formic acid [17]. Hydrido ironcomplexes could be applied in not only hydrosilylation of aldehy-des and ketones but also the reductive dehydration of amides tonitriles [18]. However, no reports on the hydrido iron complexeswith silazane as preligands have been found. Therefore, in this

Y. Shi et al. / Inorganica Chimica Acta 455 (2017) 112–117 113

paper we investigated the synthesis and properties of silyl ironhydride with silazane as preligand via SiAH activation. The chem-ical reactivity of silyl iron hydride was also explored.

2. Experimental section

2.1. General procedures and materials

Standard Schlenk techniques were performed to exclusion of airand moisture in all materials. Solvents were distilled from Na/ben-zophenone under nitrogen atmosphere and well dried. Accordingto previous reported literature procedures, pyridine-2-amine(methyl)-dimethylsilane [6], Fe(PMe3)4 [19] were prepared. BrukerALPHA FT-IR instrument recorded the infrared spectra (4000–400 cm�1). NMR spectra were recorded on a Bruker Avance 300and 400 MHz spectrometers at room temperature. Elemental anal-yses were measured by an Elementar Vario ELIII instrument. Undernitrogen condition, melting points were performed in capillariessealed and were uncorrected.

2.2. Synthesis of 2

After combining pyridine-2-amino(methyl)dimethylsilane(0.42 g, 2.53 mmol) in 40 mL of THF with Fe(PMe3)4 (1.0 g,2.78 mmol) in 30 mL of THF the mixed solution was stirred at35 �C for 5 days. Then the mixture turned dark red. After removalof the volatiles under reduced pressure, the residue was extractedwith pentane. Complex 2 was isolated as red crystals from pentaneat �20 �C. Yield: 0.96 g (83%). Dec. >78.2 �C. Anal. Calc. for C17H41-FeN2P3Si (450.37 g mol�1): C, 45.34; H, 9.18; N, 6.22. Found: C,45.74; H, 8.90; N, 6.41. IR (Nujol, mull, 4000–400 cm�1): 1806 v(FeAH), 1598 v(C@C), 935 q(PMe3); 1H NMR (300 MHz, C6D6,300 K, ppm): d �12.58 (ddd, 2J(PH) = 27, 42 and 48 Hz, 1H, FeAH),0.57 (s, 6H, SiCH3), 0.96–0.97 (d, 2J(PH) = 3 Hz, 9H, PCH3), 1.03–1.05(d, 2J(PH) = 6 Hz, 9H, PCH3), 1.09–1.10 (d, 2J(PH) = 3 Hz, 9H, PCH3),2.6 (s, 3H, NCH3), 5.6–5.71 (m, 2H, Py-H), 6.71–6.76(m, 1H, Py-H), 7.96 (s, 1H, Py-H); 31P NMR (121 MHz, C6D6, 300 K, ppm): d40.4 (m, 1P, PMe3), 18.2 (m, 1P, PMe3), 7.7 (m, 1P, PMe3); 13CNMR (75 MHz, C6D6, 300 K, ppm): d Caliphatic: 10.1, 12.5, 23.8,25.5, 27.9, 31.5, Caromatic: 104.1, 107.7, 134.0, 168.5.

2.3. Synthesis of 3

MeI (0.12 g, 0.83 mmol) was added to a solution of complex 2(0.375 g, 0.83 mmol) in 60 mL of THF at 0 �C. The mixture was stir-red for 13 h at room temperature. After removal of the volatilesunder reduced pressure, the residue was extracted with pentane.Complex 3 was isolated as red crystals at �20 �C. Yield: 0.29 g(61%). Dec. >77.1 �C. Anal. Calc. for C17H40FeIN2P3Si(576.27 g mol�1): C, 35.43; H, 7.00; N, 4.86. Found: C, 35.78; H,7.38; N, 5.01. IR (Nujol, mull, 4000–400 cm�1): 1597 v(C@C), 947q(PMe3); 1H NMR (300 MHz, C6D6, 300 K, ppm): d 0.54 (s, 6H,SiCH3), 1.46 (br s, 27H, PCH3), 2.57 (s, 3H, NCH3), 5.83 (d, 2J(HH)= 6 Hz, 1H, Py-H), 6.05 (m, 1H, Py-H), 6.95 (m, 1H, Py-H), 10.11(m, 1H, Py-H); 31P NMR (121 MHz, C6D6, 300 K, ppm): d 5.6(m, 2P, PMe3), 17.6 (m, 1P, PMe3).

2.4. Synthesis of 4

EtBr (0.15 g, 1.37 mmol) was added to a solution of complex 2(0.61 g, 1.35 mmol) in 70 mL of THF at 0 �C. The mixture was stir-red for 15 h at room temperature. After removal of the volatilesunder reduced pressure, the residue was extracted with pentane.Complex 4 was isolated as block red crystals at �20 �C. Yield:0.44 g (59%). Dec. >89.7 �C. Anal. Calc. for C17H40BrFeN2P3Si

(529.27 g mol�1): C, 38.58; H, 7.62; N, 5.29. Found: C, 39.13; H,7.40; N, 4.79. IR (Nujol, mull, 4000–400 cm�1): 1598 v(C@C), 946q(PMe3); 1H NMR (300 MHz, C6D6, 300 K, ppm): d 0.50 (s, 6H,SiCH3), 1.34 (br s, 27H, PCH3), 2.59 (s, 3H, NCH3), 5.88 (s, 1H, Py-H), 6.13 (s, 1H, Py-H), 6.98 (s, 1H, Py-H), 9.88 (s, 1H, Py-H). 31PNMR (121 MHz, C6D6, 300 K, ppm): d 10.6 (m, 2P, PMe3), 20.4 (m,1P, PMe3). 13C NMR (75 MHz, C6D6, 300 K, ppm): d Caliphatic: 9.8,31.6, Caromatic: 105.2, 111.2, 135.6, 156.3, 169.1.

2.5. Synthesis of 5

Hacac (0.12 g, 1.2 mmol) was added to a solution of complex 2(0.54 g, 1.2 mmol) in 70 ml of THF at 0 �C. The mixture was stirredfor 2 days at room temperature. After removal of the volatilesunder reduced pressure, the residue was extracted with pentane.Complex 5 was isolated as red block crystals at �20 �C. Yield:0.40 g (71%). Dec. >118.3 �C. Anal. Calc. for C19H38FeN2O2P2Si(472.42 g mol�1): C, 48.30; H, 8.11; N, 5.93. Found: C, 48.60; H,8.28; N, 5.62. IR (Nujol, mull, 4000–400 cm�1): 1597 v(C@C), 930q(PMe3). 1H NMR (300 MHz, C6D6, 300 K, ppm): d 0.26 (s, 3H,SiCH3), 0.92 (s, 3H, SiCH3), 0.95 (m, 9H, PCH3), 1.26 (d, 2J(PH)= 6 Hz, 9H, PCH3), 1.61 (s, 3H, CH3CO), 2.12 (s, 3H, CH3CO), 2.90(s, 3H, NCH3), 5.28 (s, 1H, COCHCO), 6.13 (d, 2J(HH) = 9 Hz, 1H,Py-H), 6.29 (m, 1H, Py-H), 6.31 (m, 1H, Py-H), 8.21 (m, 1H, Py-H).31P NMR (121 MHz, C6D6, 300 K, ppm): d 40.2 (d, 2J(PP) = 49.6 Hz,1P, PMe3), 43.5 (d, 2J(PP) = 49.6 Hz, 1P, PMe3). 13C NMR (75 MHz,C6D6, 300 K, ppm): d Caliphatic: 0.1, 4.6, 17.4, 16.2, 26.3, 26.7, 28.4,30.0, Caromatic: 96.6, 103.4, 107.6, 134.2, 148.5, 166.9, 183.3, 185.3.

2.6. Synthesis of 6

(a) A sample of complex 2 (0.50 g, 1.11 mmol) in 70 mL of THFwas stirred under a bar of CO2 at room temperature for 20 h. Allvolatiles were removed under vacuo. The residue was extractedwith pentane. Complex 6 (0.376 g, 0.90 mmol) was isolated asred crystals at �20 �C in 81% yield. (b) Complex 2 (0.5 g,1.11 mmol) in 40 mL of THF was combined with HCOOH (0.05 g,1.12 mmol) in 20 mL of THF for 12 h. After workup complex 6was isolated as red crystals at �20 �C. Yield: 0.28 g (61%). Dec.>121.5 �C. Anal. Calc. for C15H32FeN2O2P2Si (418.31 g mol�1): C,43.07; H, 7.71; N, 6.70. Found: C, 43.26; H, 7.79; N, 6.47. IR (Nujol,mull, 4000–400 cm�1): 1600 v(C@C), 943 q(PMe3); 1H NMR(300 MHz, C6D6, 300 K, ppm): d 0.28 (s, 3H, SiCH3), 0.71 (s, 3H,SiCH3), 0.71 (s, 9H, PCH3), 1.26 (s, 9H, PCH3), 2.71 (s, 3H, NCH3),5.99 (m, 1H, Py-H), 6.21 (s, 1H, Py-H), 7.03 (s, 1H, Py-H), 8.09 (s,1H, FeOCHO), 8.63 (m, 1H, Py-H). 31P NMR (121 MHz, C6D6,300 K, ppm): d 34.3 (d, 2J(PP) = 49.61 Hz, PMe3), 49.5 (d, 2J(PP)= 49.61 Hz, PMe3). 13C NMR (75 MHz, C6D6, 300 K, ppm): d Caliphatic:1.4, 4.9, 7.4, 16.7, 17.7, 20.3, 31.8, Caromatic: 111.7, 123.3, 136.2,150.2, 165.9, 169.3.

2.7. Synthesis of 7

PhN = C@O (0.13 g, 1.1 mmol) was added to a solution of com-plex 2 (0.54 g, 1.2 mmol) in 60 mL of THF at 0 �C. The mixturewas stirred for 8 h at room temperature. After removal of the vola-tiles under reduced pressure, the residue was extracted with pen-tane. Complex 7 was isolated as red needle crystals at �20 �C.Yield: 0.49 g (90%) Dec. >109.8 �C. Anal. Calc. for C21H37FeN3OP2Si(493.42 g mol�1): C, 51.12; H, 7.56; N, 8.51. Found: C, 51.51; H,7.72; N, 8.69. IR (Nujol, mull, 4000–400 cm�1): 1600 v(C@C), 933q(PMe3). 1H NMR (300 MHz, C6D6, 300 K, ppm): d 0.16 (s, 3H,SiCH3), 0.37 (s, 3H, SiCH3), 0.68 (d, 2J(PH) = 9 Hz, 9H, PCH3), 1.11(d, 2J(PH) = 6 Hz, PCH3, 9H), 2.75 (s, 3H, NCH3), 5.98 (d, 2J(PH)= 6 Hz, 1H, Ar-H), 6.08 (m, 1H, Ar-H), 6.49 (m, 2H, Ar-H), 6.71 (m,1H, Ar-H), 6.89 (m, 3H, Ar-H), 8.07 (s, 1H, FeOCHNPh), 8.61 (m,

Fig. 2. Molecular structure of 2, selected distances (Å) and angles (�): Fe1–H50 1.39(5), Fe1–N2 2.061(3), Fe1–Si1 2.246(1), Fe1–P2 2.170(1), Fe1–P3 2.224(1), Fe1–P12.249(1), N1–Si1 1.809(3); Si1–Fe1–P1 147.79(5), P2–Fe1–N2 171.51(1), H50–Fe1–P3 173(2), N1–Si1–Fe1 99.30(1), C13–N2–Fe1 121.72(2), C13–N1–Si1 117.7(2), N1–C13–N2 116.1(3), N2–Fe1–H50 88(2), P2–Fe1–Si1 89.74(4), P2–Fe1–P1 95.25(4),N2–Fe1–P1 90.1(1), N2–Fe1–Si1 82.3(1). H50–Fe1–P2 86(2), P2–Fe1–P3 95.18(4),P3–Fe1–N2 89.24(9), H50–Fe1–Si1 71(2), Si1–Fe1–P3 101.77(4), P3–Fe1–P1 109.40(5), P1–Fe1–H50 77(2).

114 Y. Shi et al. / Inorganica Chimica Acta 455 (2017) 112–117

1H, Ar-H). 31P NMR (121 MHz, C6D6, 300 K, ppm): d 35.5 (d, 2J(PP)= 49 Hz, 1P, PMe3), 45.5 (d, 2J(PP) = 49 Hz, 1P, PMe3). 13C NMR(75 MHz, C6D6, 300 K, ppm): d Caliphatic: 4.9, 7.3, 17.7, 20.3, 31.8,Caromatic: 104.7, 106.2, 111.7, 113.8, 122.0, 123.6, 135.9, 146.8,150.5, 165.6, 169.9.

2.8. Single crystal X-ray analysis

Diffraction data were collected on a STOE Stadi Vari (2, 4–6), ora STOE IPDS2T (7) diffractometer using graphite-monochromatedMo Ka radiation (k = 0.71073 Å). Using Olex2 [20], the structureswere solved with the ShelXS [21] structure solution program usingDirect Methods and refined with the ShelXL [22] refinement pack-age using Least Squares minimization. Non-hydrogen atoms wererefined with anisotropic displacement factors. Hydrogen atomsbinding to carbon atoms were added on idealized positions; Hatoms binding to metal atoms were refined freely. Crystallographicdata for the structures reported in this paper have been depositedwith the Cambridge Crystallographic Data Centre as supplemen-tary publication No. CCDC – 1412194 (2), 1412195 (4), 1434291(5), 1412196 (6) and 1434292 (7). Copies of the data can beobtained free of charge from www.ccdc.cam.ac.uk/conts/retriev-ing.html.

3. Results and discussion

3.1. Synthesis of hydrido iron(II) complex 2

Compound 1was treated with Fe(PMe3)4 in THF, the color of themixture changed from yellow to dark red slowly. Complex 2 wasobtained as red cubic crystals at �20 �C (Eq. (1)).

N NMe

SiH Me

Me

Fe(PMe3)4N N

Me

SiMe

MeFeMe3P

Me3P

H

PMe31 2

+THF

35 oC ð1Þ

Complex 2 was generated through the SiAH bond activation atthe iron(0) atom. In the IR spectrum of complex 2, a strong absorp-tion at 1806 cm�1 for FeAH bond was observed. The characteristichydrido signal of complex 2 in the NMR spectrum was monitoredat �12.61 ppm as a ddd (doublets of doublets of doublets) peakand the H-P coupling constants are 2JPH = 27, 42, 48 Hz (Fig. 1),respectively. The existence of three kinds of phosphine ligandswere confirmed by the resonances at 40.35, 18.28, 7.71 ppm inthe 31P NMR spectrum of complex 2.

A hexa-coordinated octahedral molecular configuration (Fig. 2)of complex 2 was confirmed by X-ray crystallography. The ironcenter and [P1P2Si1N2] are in the equatorial plane with[H50–Fe1–P3 = 173(2)�] in the axial direction. The iron atom is

Fig. 1. Hydrido resonances of complex 2.

coordinated with a five-membered metallacycle. The order of thebond lengths Fe1–P1 (2.249(1) Å) > Fe1–P3 (2.224(1) Å > Fe1–P2(2.170(1) Å is explained by the order of trans-influence Si > H > N.The hydrido hydrogen atom (H50) was located from the X-raydiffraction data. The bond length Fe1–H50 (1.39(5) Å) is slightlyshorter than a normal FeAH bond (�1.42 Å) [23].

3.2. Substitutions of hydrido ligand of 2 by halogen ligands

As substitution products complexes 3 and 4 were obtainedthrough the reaction of complex 2 with MeI and EtBr in THFrespectively via elimination of alkane (Eq. (2)).

N NMe

SiMe

MeFeMe3P

Me3P

H

PMe3

RX N NMe

SiMe

MeFeX

Me3P

Me3P

PMe3

RH+THF

+

2 3 and 43 4

R = Me EtX = I Br

ð2ÞComplexes 3 and 4 are typical metal halides [24–27] and well

characterized by IR and NMR. In the 1H NMR spectra of complexes3 and 4, the three PMe3 ligands were recorded at 1.46 ppm (3) and1.34 ppm (4) respectively as broad singlet. Two 31P NMR reso-nances (17.62 and 5.55 ppm (3); 20.36 and 10.64 ppm (4)) wereassigned to two kinds of the phosphine ligand in the integral ratioof 2:1. This also indicates a mer-orientation of the three phosphineligands. The structure (Fig. 3) of complex 4 as a hexa-coordinateoctahedral geometry was further confirmed by single crystal X-ray diffraction analysis. Comparing the structure of 4 with that of2, the Br atom in 4 is located at the trans-position opposite to thesilyl group while the hydrido hydrogen atom in 2 is situated atthe trans-position opposite to one phosphine ligand. This rear-rangement during the ligand substitution is attributed to thestrong trans-influence of the Si atom. The iron atom is located atthe center of the complex and four atoms [N1 P1 P2 and P3] arein the equatorial plane with Si1–Fe1–Br1 (178.43(3) Å) in the axialposition. The sum of the bond angles (P2–Fe1–P3 = 94.40(3)�;

Fig. 4. Molecular structure of 5, selected distances and (Å) and angles (�): Fe1–O12.005(1), Fe1–O2 2.044(1), Fe1–N2 2.048(2), Fe1–P1 2.1721(5), Fe1–P2 2.1823(5),Fe1–Si1 2.2338(5), O1–C7 1.269(2), O2–C10 1.273(2), C7–C9 1.397(3), C9–C101.398(3), Si1–N1 1.802(2); Si1–Fe1–O2 173.38(4), O1–Fe1–P1 171.76(4), N2–Fe1–P2 174.79(5), N2–Fe1–O1 82.87(6), O1–Fe1–P2 92.03(4), P2–Fe1–P1 96.21(2), P1–Fe1–N2 88.89(5), N2–Fe1–O2 91.54(6), O2–Fe1–P2 89.46(4), P2–Fe1–Si1 94.32(2),Si1–Fe1–N2 84.22(4), Si1–Fe1–O1 85.39(4), O1–Fe1–O2 89.04(5), O2–Fe1–P1 90.86(4), P1–Fe1–Si1 94.13(2).

Fig. 3. Molecular structure of 4, selected distances (Å) and angles (�): Fe1–Si12.2455(7), Fe1–P3 2.2209(7), Br1–Fe1 2.6148(4), Fe1–N1 2.056(2), Fe1–P2 2.2840(8), Fe1–P1 2.2765(7); Si1–Fe1–Br1 178.43(3), N1–Fe1–P3 178.01(7), P2–Fe1–P1169.89(3), N2–Si1–Fe1 98.82(8), C1–N1–Fe1 120.8(2), Si1–Fe1–P2 90.11(3), P2–Fe1–Br1 91.33(2), Br1–Fe1–P1 89.83(2), P1–Fe1–Si1 88.65(3), Si1–Fe1–P3 97.50(3),P3–Fe1–Br1 83.02(2), Br1–Fe1–N1 95.60(7), N1–Fe1–Si1 83.91(7), P2–Fe1–P3 94.40(3), P3–Fe1–P1 95.71(3), P1–Fe1–N1 85.71(7), N1–Fe1–P2 84.18(7).

Y. Shi et al. / Inorganica Chimica Acta 455 (2017) 112–117 115

P3–Fe1–P1 = 95.71(3)�; P1–Fe1–N1 = 85.71(7)�; N1–Fe1–P2 84.18(7)�) is 360.00�. The equatorial plane, the chelate ring and pyridylgroup are almost coplanar. The extraordinarily long Fe1–Br1 bond(2.6148(4) Å) [28] is caused by the strong trans-influence of the Siatom.

3.3. Substitution of hydrido ligand of 2 by acetylacetone

Acetylacetone (Hacac) as a bidentate preligand has been widelyused in many transition-metal complexes. In some cases, the acidichydrogen atom of acetylacetone reacts with the metal hydridewith evolution of H2 [29,30]. Complex 5 was isolated from thereaction of complex 2 with Hacac (Eq. (3)).

N NMe

SiMe

MeFeMe3P

Me3P

H

PMe3

NNMe

SiMeMe

FeMe3P PMe3

O

O

Me

Me

Me Me

O O+

THF+

2 5

H2

ð3ÞIn the 1H NMR spectrum of complex 5, a peak at d 5.28 ppm was

assigned to the resonance of H atom of (CO-CH-CO) from the acety-lacetone. This shift is downfield in comparison with the previousreports (d 4.87 ppm) [31].

The octahedral molecular structure of complex 5was confirmedby the X-ray structural analysis (Fig. 4). Similar to our earlyreported cobalt complex, [Co(acac)(PMe3)2((C6H5)CHNH(C10H6))][31], the two oxygen atoms of the acetylacetone anion coordinateto iron center to form a six-membered chelate ring. Five atoms [Fe1Si1 P1 O1 O2] are in the equatorial plane with N2–Fe1–P2 (174.79(5)�) as the axial direction. The distance of Fe1–O1 (2.005(1) Å) islittle shorter than the Fe1–O2 distance (2.044(7) Å) because PMe3has the weaker trans-influence than that of silyl group.

3.4. Insertion of complex 2 with CO2

Complex 2 reacted with 1 mol of CO2 at room temperature for20 h and delivered the formate iron(II) complex 6 through theinsertion of the C@O double bond of CO2 into the FeAH bond of

complex 2 (Eq. (4)). Complex 6 was isolated as dark red crystalsin the yield of 81%.

N NMe

SiMe

MeFeMe3P

Me3P

H

PMe3

NNMe

SiMeMe

FeMe3P PMe3

O

O

CO2 or HCOOH+THF

2 6

ð4ÞComplex 6 was characterized by IR and NMR spectroscopy. The

two oxygen atoms of the formate ligand coordinate to the iron cen-ter and form a four-membered metallacycle in complex 6. Contrar-ily, only one of the oxygen atoms of the formate group coordinatesto the iron atom as a monodentate ligand in Field’s complex (trans-Fe(dmpe)2(OCHO)H) (dmpe = Me2PCH2CH2PMe2) [32]. Perhaps inthe case of 6 it is easier for PMe3 to dissociate from the iron centerthan dmpe. The resonance of the H atom of the g3-(OCHO) ligandin complex 6 is at d 8.09 ppm. In comparison with that (7.78 ppm)of Field’s complex [32] this is a downfield shift.

Based on the X-ray diffraction analysis, an octahedral coordina-tion geometry around the iron atom in complex 6 was confirmed(Fig. 5). P1–Fe1–N2 is identified as the axial direction with theangle of 175.85(4)�. Five atoms [Fe1, P2, Si1, O1 and O2] are inan equatorial plane. Fe1–O2 (2.216(1) Å) is remarkably longer thanFe1–O1 (2.086(1) Å) because silyl group has stronger trans-influ-ence than phosphine ligand. Meanwhile, C7–O1 (1.255(2) Å) isalmost equal to C7–O2 (1.256(3) Å). This implies that the p-elec-trons of the formate ligand are drastically delocalized.

It is noteworthy that complex 6 could also be obtained by theacid-base reaction of complex 2with equivalent amount of HCOOHat room temperature for 12 h in the yield of 61%. This process isalso a replacement of the hydrido ligand by formate anion.

Recently, Schneider and Beller reported dehydrogenation of for-mic acid using iron catalyst [33,16b]. In both of the catalytic reac-tions, formate iron complexes or g3-coordinated species which aresimilar with complex 6 worked as key intermediates in catalyticcycles. Inspired by these reports, we also explored the dehydro-genation of formic acid using hydrido iron complex 2 as catalystwith LiBF4 or NEt3 as additive at 50 �C. However, no conversion

Fig. 6. Molecular structure of 7, selected distances and (Å) and angles (�): Fe1–N12.06(2), Fe1–N2 2.008(2), Fe1–O1 2.197(2), Fe1–P1 2.1813(6), Fe1–P2 2.1684(5),Fe1–Si1 2.2248(6), N1–C7 1.298(3), C7–O1 1.271(3); N1–Fe1–P2 161.72(5), N2–Fe1–P1 176.14(5), O1–Fe1–Si1 164.25(4), N1–Fe1–O1 62.14(6), O1–Fe1–P2 101.33(4), P2–Fe1–Si1 93.96(2), Si1–Fe1–N1 102.18(5), N2–Fe1–N1 85.26(7), N1–Fe1–P190.94(5), P1–Fe1–P2 96.12(2), P2–Fe1–N2 87.72(5), N2–Fe1–O1 91.92(7), O1–Fe1–P1 86.90(5), P1–Fe1–Si1 95.28(2), Si1–Fe1–N2 84.88(5), C12–N2–Fe1 121.06(14),N3–Si1–Fe1 98.31(6), C7–N1–Fe1 92.64(13), C7–O1–Fe1 87.23(12), O1–C7–N1117.84(19).

Fig. 5. Molecular structure of 6, selected distances (Å) and angles (�): Fe1–O1 2.086(1), Fe1–O2 2.216(1), Fe1–P2 2.1604(4), Fe1–P1 2.1916(5), Fe1–Si1 2.2166(5), Fe1–N2 2.021(1), O2–C7 1.256(2), O1–C7 1.255(2), Si1–N1 1.799(2); P1–Fe1–N2 175.85(4), O2–Fe1–Si1 157.60(4), P2–Fe1–O1 165.42(4), O2–C7–O1 122.19(13), O1–Fe1–N2 85.29(5), O1–Fe1–Si1 96.53(3), Si–Fe1–N2 84.65(4), Fe1–Si1–N1 98.85(5), Fe1–N2–C11 120.88(10), N2–C11–N1 116.78(14), Si1–N1–C11 118.77(11), O1–Fe1–O261.37(5), P2–Fe1–O2 105.68(4), O1–Fe1–Si1 96.53(3), P2–Fe1–Si1 95.754(17), P1–Fe1–O1 90.90(3), O1–Fe1–N2 85.29(5), P2–Fe1–N2 88.08(4), P2–Fe1–P1 96.00(2),P1–Fe1–O2 90.05(3), O2–Fe1–N2 89.55(5), N2–Fe1–Si1 84.65(4), Si1–Fe1–P194.174(19).

116 Y. Shi et al. / Inorganica Chimica Acta 455 (2017) 112–117

was found. The results might be explained by the extreme stabi-lization of complex 6 so that it cannot undergo b-H eliminationto regenerate the hydrido iron catalyst.

3.5. Insertion reaction of complex 6 with isocyanate

After complex 2 was combined with phenylisocyanate in THFfor 8 h, complex 7 as red crystals was obtained in the yield of71% (Eq. (5)).

N NMe

SiMe

MeFeMe3P

Me3P

H

PMe3

NNMe

SiMeMe

FeMe3P PMe3

N

O

PhN=C=OPh+

THF

2 7

ð5ÞIn the reaction, the phenylisocyanate inserted into the FeAH

bond of complex 2 and a chelate four-membered ring [FeNCO]was formed. Based upon the same reason with the insertion ofCO2 into the FeAH bond (Eq. (4)), after the insertion phenyliso-cyanate only as a monodentate ligand with O-coordination to theiron center was found in the Field’s complex [34]. The 31P NMRspectrum of complex 7 exhibiting two kinds of signals(d = 45.475 and 35.505 ppm) of PMe3 indicates that there are twochemically different phosphine ligands. The resonance of the Hatom of g3-(NCHO) is at d 8.07 ppm, similar to that of complex 6.

The molecular structure (Fig. 6) of 7 shows a hexa-coordinateoctahedral geometry with two cis-phosphine ligands. This is accor-dant with the signals of the NMR data. The four-membered chelatering is perpendicular to the plane formed by the five-memberedchelate ring and the pyridyl group. The axial bond angle N2–Fe1–P1 is 176.14(5)�. The sum of bond angles around the iron centerin the equatorial plane (N1–Fe1–O1 = 62.14(6)�; N1–Fe1–Si1 = 102.18(5)�; Si1–Fe1–P2 = 93.96(2)�; P2–Fe1–O1 = 101.33

(4)�) is 359.61�. This indicates that the five atoms [N1O1Fe1Si1P2]are almost in one plane.

It is widely known that hydrido metal complexes play a crucialrole in a variety of TM-catalyzed reactions [35–43]. Therefore,complex 2 was used as catalyst in a catalyst loading of 5 mol% toexplore the catalytic activity in the reduction of benzaldehyde with(EtO)3SiH as hydrogen source at 50 �C. Regrettably, no conversionwas confirmed. This might be caused by the difficult dissociationof the ligands in complex 2.

4. Conclusions

In summary, the preligand silazane pyridine-2-amino(methyl)dimethylsilane (1) was used to synthesize silyl iron hydride. Anew iron hydride FeH((C5H4N)NMe(SiMe2))(PMe3)3 (2) wasobtained through the reaction of Fe(PMe3)4 with 1. Through thereactions of complex 2 with MeI, EtBr and acetylacetone iron com-plexes 3–5 were isolated. The reactions of CO2 and phenyliso-cyanate with complex 2 afforded two insertion iron(II) products:a formate complex 6 and g3-(NCO) coordinated complex 7. Com-plex 6 could also be synthesized through the substitution reactionof 2 with HCOOH. The molecular structures of complexes 2, 4–7were determined by single crystal X-ray diffraction.

Acknowledgements

Support of this work by NSFC (Nos. 21572119/21272138) andBeijing Municipal Education Commission (No. 201510028008) isgratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.ica.2016.10.017.

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