[N,N′-Di-tert-butyl-P,P-diphenylphosphinimidic Amidato-κN,κN′]chlorosilicon-κSi-tetracarbonyliron

Abstract

The title complex {[Ph₂P(tBuN)₂](Cl)Si:->Fe(CO)₄} (2) was synthesized via the reaction of chlorosilylene [Ph₂P(tBuN)₂]SiCl (1), supported by an iminophosphonamide ligand with Fe(CO)₅ in THF. The molecular structure of 2 was fully characterized by NMR (¹H, ¹³C, ²⁹Si, and ³¹P) and IR spectroscopies, as well as single-crystal X-ray diffraction (SCXRD) analysis. In the SCXRD analysis of 2, the silylene ligand was located in the axial positions of the coordination sphere of the central iron atom and other sites were occupied by carbonyl ligands.

1. Introduction

The chemistry of stable divalent silicon(II) species, silylenes, is currently one of the most intensively researched areas of basic silicon chemistry [1,2,3]. In this context, a series of Lewis-base-stabilized silylenes can serve as an ancillary ligand in the coordination chemistry because they can behave as a strong nucleophile [4,5,6]. In particular, the coordination chemistry of silylenes, supported by N,N′-chelating amidinate ligands, has developed dramatically over the past few years. Indeed, numerous transition metal complexes utilizing amidinato–silylene ligands have been investigated [7,8]. The resulting complexes were found to be capable of acting as catalysts for transforming organic molecules [5,6] or activating inactive small molecules [9,10]. Among them, amidinato–silylene–iron complexes are particularly attractive candidates for catalysis due to the low cost and high natural abundance of iron, as well as their non-toxic and biocompatible properties [11,12,13,14,15,16,17,18,19,20].

Our group has been engaged in the chemistry of p-block elements supported by N,N′-chelating iminophosphonamides [R₂P(NR′)₂] [21,22,23,24,25,26]. In particular, chlorosilylenes [Ph₂P(NR)₂SiCl] (R = ^tBu and 2,6-ⁱPr₂C₆H₃) show the unprecedented complexation ability of transition metals and unique nucleophile behavior due to their strong σ-donating properties [21,22,23,26]. In this contribution, we present the synthesis and structural characterization of silylene–tetracarbonyliron(0) complex {[Ph₂P(tBuN)₂](Cl)Si:->Fe(CO)₄}, wherein the silylene(II) center is stabilized by an N,N′-di-tert-butyliminophosphonamide ligand.

2. Results and Discussion

The complexation of chlorosilylene 1 with Fe(CO)₅ proceeded readily at room temperature in THF to afford the corresponding silylene–tetracarbonyliron(0) complex 2 as yellow crystals in 69% yield with the release of a molecule of CO (Scheme 1).

Single crystals of 2 were obtained from a saturated THF solution at −10 °C. The molecular structure of 2 was unequivocally determined using single-crystal X-ray diffraction (SCXRD) analysis. The ORTEP is depicted in Figure 1, and selected bond lengths and angles are summarized in

Table 1. Selected bond lengths [Å] and bond angles [°].

Bond Lengths	[Å]	Bond Angles	[°]
Si1–Fe1	2.2675(6)	Si1–Fe1–C21	174.28(6)
Si1–Cl1	2.1161(7)	Si1–Fe1–C22	86.99(6)
Si1–N1	1.8001(15)	Si1–Fe1–C23	86.12(6)
Si1–N2	1.8078(15)	Si1–Fe1–C24	91.12(6)
Fe1–C21	1.791(2)	C22–Fe1–C23	127.39(9)
Fe1–C22	1.784(2)	C23–Fe1–C24	111.64(9)
Fe1–C23	1.774(2)	C22–Fe1–C24	120.59(9)
Fe1–C24	1.788(2)	Cl1–Si1–N1	104.37(6)
		Cl1–Si1–N2	104.82(5)
		N1–Si1–N2	80.31(7)

. Complex 2 crystallized in the monoclinic space group P2₁/c with a molecule of THF per unit cell. The solvate THF molecules in 2 were disordered in two positions with the relative occupancy of 0.722(8) and 0.278(8). The SCXRD analysis of 2 reveals that the silylene ligand was located in the axial positions of the coordination sphere of the central iron atom and other sites were occupied by carbonyl ligands. The chlorine atom on the silicon atom and an equatorial carbonyl ligand were trans to one another with respect to the Si–Fe bond. The geometry of five-coordinated compounds can be evaluated using the angular structure parameter τ [τ = (β − α)/60] [27]. This parameter can be readily calculated using the two largest basal angles (α and β) around the central atom in the five-coordinated compound (Figure 2). Thus, τ = 1 had a trigonal bipyramidal structure with D_{3 h} symmetry, while the square pyramidal structure with C_4v symmetry had τ = 0. The calculated τ value for 2 was 0.78, indicating that the iron atom is defined as the somewhat distorted trigonal bipyramidal geometry. The silicon atom exhibited a distorted tetrahedral geometry; the sum of the bond angles around the Si1 atom (362.36°) was within the range of those of the corresponding silylene-ligated tetracarbonyliron(0) complexes [353.92–362.64°] [17,18,19]. Despite the strong σ-donor ability of 1, the Si1–Fe1 bond length [2.2675(6) Å] was slightly longer than that of the related amidinato–silylene–iron(0) complexes [2.237(7)–2.2777(5) Å] [17,18,19], probably due to the steric repulsion between the phenyl group on the phosphorus atom and the carbonyl ligand. Two Si–N bond lengths [1.8001(15) and 1.8078(15) Å] in the four-membered ring fell in the same range as those of the reported three-coordinated silylenes [1.8291(19)–1.8750(16) Å] [17,18,19]. The Si–Cl bond length in 2 was 2.1161(7) Å, which is slightly shorter than the corresponding bond length in the starting 1 [2.2291(11) Å] [21].

The structure of 2 was also confirmed following spectroscopic analyses. The ¹H NMR spectrum of 2 in C₆D₆ displayed a singlet signal assigned to the ^tBu groups at 1.22 ppm. The aromatic protons were observed as broad signals at 7.07 and 8.03 ppm. In the ¹³C{¹H} NMR spectrum of 2, the carbonyl ligands binding to the iron center were resonated at 217.0 ppm, which is comparable to those of the related amidinato–silylene–Fe(CO)₄ complexes (213.1–217.2 ppm) [17,18,19,20]. In the ³¹P{¹H} NMR of 2, a sharp singlet signal appeared at 46.8 ppm, which is quite similar to that of the starting 1 (46.6 ppm) [21]. The ²⁹Si{¹H} NMR spectrum of 2 showed a doublet signal via the ³¹P coupling of 11.9 Hz at 74.6 ppm, and shifts to a lower field compared to that of the starting 1 (59.8 ppm) [21] and falls in the range of those in the related amidinato–silylene–Fe(CO)₄ complexes (40.3–112.8 ppm) [17,18,19,20]. The absorptions due to carbonyl stretching vibrations in the IR spectrum were observed at 2029, 1944, and 1913 cm^–1, consistent with the results of SCXRD analysis exhibiting local C_{3 V} symmetry on the iron(0) site. All NMR (Figures S1–S4) and IR (Figure S5) spectra are in the Supplementary Materials.

3. Materials and Methods

3.1. General Considerations

Unless otherwise noted, all experiments were carried out under an argon atmosphere using standard Schlenk-line techniques or a UNICO glovebox. ¹H, ¹³C, ²⁹Si, and ³¹P NMR spectra were recorded on Bruker Avance-500 (500 MHz for ¹H, 126 MHz for ¹³C, 99 MHz for ²⁹Si, and 202 MHz for ³¹P) and Bruker Avance-400 (400 MHz for ¹H, 101 MHz for ¹³C, and 162 MHz for ³¹P) spectrometers, with C₆D₆ as the solvent at room temperature. The IR spectrum was recorded on a TENSOR II. All melting points were determined on a Mel-Temp capillary tube apparatus and were uncorrected. Elemental analyses were carried out at the Molecular Analysis and Life Science Center of Saitama University. All solvents were dried over 4A molecular sieves or potassium mirror before use. All materials were obtained from commercial suppliers and were used without further purification, except 1 [21], which was prepared according to the corresponding literature procedure.

3.2. Synthesis of Complex 2

In a Schlenk tube, to a THF (2 mL) solution of chlorosilylene 1 (140 mg, 0.36 mmol), Fe(CO)₅ (0.05 mL, 0.37 mmol) was added at ambient temperature. After stirring for 1 h, the reaction mixture was concentrated under reduced pressure, and the crude product was crystallized from a saturated THF solution at −10 °C to give 2 (138 mg, 69%) as yellow crystals. Mp. 141–142 °C (decomp.). ¹H NMR (400 MHz, 25 °C C₆D₆): δ = 1.22 (s, 18H, CH_3tBu), 7.07 (br, 6H, CH_Ar), 8.03 (br, 4H, CH_Ar). ¹³C{¹H} (100 MHz, 25 °C, C₆D₆): δ = 32.0 (d, J_CP = 5.6 Hz, CH_3tBu), 55.3 (C_tBu), 126.2 (d, J_CP = 99.3 Hz, C_Ar), 129.1 (d, J_CP = 11.3 Hz, CH_Ar), 129.1 (d, J_CP = 11.0 Hz, CH_Ar), 134.3 (d, J_CP = 2.9 Hz, CH_Ar), 134.5 (d, J_CP = 2.9 Hz, CH_Ar), 134.7 (d, J_CP = 12.1 Hz, CH_Ar), 134.8 (d, J_CP = 12.2 Hz, CH_Ar), 217.0 (CO). ³¹P{¹H} NMR (162 MHz, 25 °C, C₆D₆): δ = 46.6. ²⁹Si{¹H} NMR (99 MHz, 25 °C, C₆D₆): δ = 74.6 (d, J_SiP = 11.9 Hz). IR(KBr): ν_CO = 2029, 1944, 1913 cm^–1. Anal. Calculated for C₂₄H₂₈ClFeN₂O₄PSi: C, 51.58; H, 5.05; N, 5.01. Found: C, 51.84; H, 5.07; N, 4.90.

3.3. SCXRD Analysis of 2

A yellow single crystal of 2 was grown from a saturated THF solution at −10 °C. The intensity data were collected at 100 K on a Bruker SMART APEX II diffractometer employing graphite-monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved using direct methods (SHELXT) [29] and refined with full-matrix least-squares procedures on F² for all reflections (SHELXL) [30]. Hydrogen atoms were located by assuming the ideal geometry and were included in the structure calculations without any further refinement of the parameters. The structures of overlapped and disordered THF molecules were restrained to be identical to each other using DFIX (σ = 0.01), SAME (σ1 = 0.01, σ2 = 0.02,), and DELU (σ1 = 0.01, σ2 = 0.02) instructions.

Crystal data for C₂₈H₃₆ClFeN₂O₅PSi (2): M = 630.95 g mol⁻¹, monoclinic, P2₁/c, a = 9.3227(7), b = 27.786(2), c = 11.5041(9) Å, β = 94.5030(10)°, V = 2970.9(4) ų, Z = 4, D_x = 1.411 g cm⁻³, F(000) = 1320, and μ = 0.732 mm⁻¹. CCDC deposition number: 2192007.

4. Conclusions

We demonstrated the synthesis and spectroscopic characterization of the novel silylene–tetracarbonyliron(0) complex {[Ph₂P(tBuN)₂](Cl)Si:->Fe(CO)₄}. Further applications of N,N′-chelating silylenes are currently under investigation in our laboratory.