Synthesis and Characterization of an Amidinate-Stabilized

NOTE pubs.acs.org/Organometallics

Synthesis and Characterization of an Amidinate-Stabilized Siladithiocarboxylate and Its Germanium(II) Complex Shu-Hua Zhang,† Hong-Wei Xi,‡ Kok Hwa Lim,‡ and Cheuk-Wai So*,† †

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore ‡ Division of Chemical and Biomolecular Engineering, School of Chemical and Biomedical Engineering, Nanyang Technological University, 637459 Singapore

bS Supporting Information ABSTRACT: The synthesis and characterization of the amidinate-stabilized potassium siladithiocarboxylate [{LSi(S)2}K (THF)2]2 (2, L = PhC(NBut)2) are described. Compound 2 was synthesized by the reaction of [LSiCl] (1) with two equivalents of elemental sulfur and KC8 in THF. X-ray crystallography and DFT calculations show that the Si
S bond lengths in 2 are intermediate between the SidS double and Si
S single bond lengths. There is a weak electronic delocalization along the “Si(S)2” skeleton. Compound 2 underwent a salt elimination reaction with GeCl2 3 dioxane to form [{LSi(S)2}2Ge:] (3), which contains two “LSi(S)2
” moieties chelating to the germanium(II) atom in an anisobidentate manner.

C

arboxylates A (Scheme 1) are one of the most significant functional groups in chemistry, biochemistry, and material sciences. It is well-known that the C
O bond lengths of a carboxylate are equal and the negative charge is delocalized on the carboxylate skeleton. Carboxylates can act as a nucleophile in various organic reactions. Dithiocarboxylates B and dithiocarbamates C, in which two oxygen atoms in the carboxyl group are replaced by sulfur atoms, have been synthesized. Their coordination chemistry toward main-group and transition metals were investigated extensively.1 They can stabilize metal complexes with a variety of oxidation states and coordination geometries. In contrast, the siladichalcogenocarboxylates [RSi(E)2]
(D, R = supporting substituent, E = chalcogen) are still unknown. Recently, Driess et al. synthesized successfully the silathiocarboxylic acid
base adduct [{HC(CMeNAr)2}Si(S)OH(dmap)] (Ar = 2,6-Pri2C6H3), which reacted with AlMe3 to form the aluminum silathiocarboxylate [{HC(CMeNAr)2}Si(S)OAlMe2(dmap)].2 However, the reaction of [HC{CMeNAr}2Si(S)OH (dmap)] with MeLi or LiN(SiMe3)2 cannot afford the corresponding anion. Roesky et al. showed that the reaction of [LGeCl(μ-S)]2 (L = PhC(NBut)2) with two equivalents of KC8 in THF formed the germadithiocarboxylate [{LGe(S)2}K(μ-THF)]2.3 In contrast, the lighter congener, siladithiocarboxylate, is still unknown. The coordination chemistry of heavier group 14 analogues of dichalcogenocarboxylate toward main-group and transition metals is also rare. Herein, we report the synthesis and characterization of an amidinate-stabilized siladithiocarboxylate, [{LSi(S)2}K(THF)2]2 (2), and a germanium(II) siladithiocarboxylate, [{LSi(S)2}2Ge:] (3). The novel siladithiocarboxylate [{LSi(S)2}K(THF)2]2 (2) was synthesized by the reaction of [LSiCl] (1)4 with two equivalents of r 2011 American Chemical Society

Scheme 1. Anions A
D

elemental sulfur and KC8 in THF (Scheme 2). The reaction was stirred for 2 h. The reaction mixture was filtered, and volatiles of the filtrate were removed in vacuo. The crude product was then characterized by 1H and 29Si NMR spectroscopy in THF-d8. The spectra show a mixture of 2 (major product), [LSi(S)Cl]5 (minor product), and unidentified compounds (minor product). The crude product was extracted with THF. After filtration and concentration, only compound 2 was afforded as colorless crystals. An attempt to isolate [LSi(S)Cl]5 and the unidentified compounds from the mother liquor by recrystallization was unsuccessful. Roesky et al. showed that the germadithiocarboxylate [{LGe(S)2}K (μ-THF)]2 was synthesized by the reaction of [LGeCl(μ-S)]2 with two equivalents of KC8 in THF.3 In this regard, the treatment of [LSi(S)Cl]5 with two equivalents of KC8 in THF was performed. A mixture of 2 (major product) and the disilylene [LSi-SiL] (minor product)6 was afforded in the reaction, which has been confirmed by NMR spectroscopy. Received: April 2, 2011 Published: June 07, 2011 3686

dx.doi.org/10.1021/om200285u | Organometallics 2011, 30, 3686–3689

Organometallics

NOTE

Scheme 2. Synthesis of 2

Figure 1. Molecular structure of 2 with thermal ellipsoids at the 50% probability level. Disordered solvent molecule and hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Si(1)
S(1) 2.0302(5), Si(1)
S(2) 2.0243(5), S(1)
K(1) 3.2648(5), S(1)
K(1A) 3.3046(5), S(2)
K(1) 3.3449(5), S(2)
K(1A) 3.2783(5), Si(1)
N(1) 1.8502(11), Si(1)
N(2) 1.8472(11), C(5)
N(1) 1.3350(16), C(5)
N(2) 1.3387(16), K(1)
O(1) 2.7307(12), K(1)
O(2) 2.7482(13), S(1)
Si(1)
S(2) 118.82(2), Si(1)
S(1)
K(1) 83.30(2), Si(1)
S(2)
K(1) 81.29(2), Si(1)
S(1)
K(1A) 83.24(2), Si(1)
S(2)
K(1A) 84.03(2), S(1)
K(1)
S(2) 63.73(1), S(1)
K(1)
S(1A) 114.38(1), S(2)
K(1)
S(2A) 114.99(1), N(1)
Si(1)
S(1) 113.55(4), N(1)
Si(1)
S(2) 115.19(4), N(2)
Si(1)
S(1) 114.45(4), N(2)
Si(1)
S(2) 114.84(4), Si(1)
N(1)
C(5) 91.35(8), N(1)
C(5)
N(2) 106.46(11), C(5)
N(2)
Si(1) 91.36(8), N(1)
Si(1)
N(2) 70.80(5).

Moreover, [LSi(S)Cl]5 did not react with Na2S in THF to form [{LSi(S)2}Na]. Compound 2 is stable in solution or the solid state at room temperature under an inert atmosphere. It is soluble in THF only. The 1H and 13C{1H} NMR spectra of 2 in THF-d8 display resonances due to the amidinate ligand. The 29Si{1H} NMR spectrum of 2 in THF-d8 exhibits one singlet at δ 18.4 ppm, which shows a downfield shift compared with that of [LSi(S)SBut] (δ 1.57 ppm)7 and [LSi(S)Cl] (δ
17.5 ppm).5 The molecular structure of 2 is shown in Figure 1. The molecular structure of 2 is different from that of [{LGe(S)2}K(μ-THF)]2,3 in which the potassium atoms lie on the germadithiocarboxylate plane and two THF molecules are bridged between the potassium atoms. Compound 2 crystallized as a dimer in which two [LSi(S)2]
moieties chelate to the potassium atoms K(1) and K(1A) in a headto-head manner. The K(1) and K(1A) atoms are displaced from the S(1)
Si(1)
S(2)
S(1A)
Si(1A)
S(2A) least-squares plane by 1.779 Å. Each potassium atom is coordinated with two THF molecules. The geometries around the S(1) and S(2) atoms are trigonal pyramidal. The amidinate ligands are bonded in a N,N0 chelate fashion to the Si atoms, which adopt a tetrahedral geometry. The Si(1)
N(1)
C(5)
N(2) ring is planar and almost perpendicular to the S(1)
Si(1)
S(2)
S(1A)
Si(1A)
S(2A) least-squares plane (dihedral angle: 89.5°). The molecular structure of 2 is similar to that of [{(4-MeC6H4)C(S)2}K]2,8 except that the Si atoms in 2 are four-coordinated. The Si(1)
S(1) (2.0302(5) Å) and Si(1)
S(2) (2.0243(5) Å) bonds are almost identical. The Si
S bond lengths in 2 are

intermediate between the
Si(S)
(1.984(8) Å) and Si
SBut (2.131(7) Å) bond lengths in [LSi(S)SBut].7 They are shorter than typical Si
S single bond lengths (average 2.14 Å), but longer than the SidS double bond length in [SdSi{C(SiMe3)2CH2CH2C(SiMe3)2}] (1.9575(7) Å).9 They are comparable with those in the base-stabilized silanethiones reported by research groups of Driess (1.9854(9)
2.006(1) Å) and Corriu (2.013(3) Å).10 The results show that the Si
S bonds in 2 could have some double-bond character or zwitterionic Siþ
S
bonding character.11 The S(1)
K(1) (3.2648(5) Å) and S(2)
K(1) (3.3449(5) Å) bonds are comparable to those in [{(4-MeC6H4)C(S)2}K]2 (3.2972(9), 3.354(1) Å).8 The C
N bond lengths (C(5)
N(1): 1.3350(16) Å, C(5)
N(2): 1.3387(16) Å) are approximately between the C
N double and C
N(sp2) single bond lengths. This geometry shows considerable delocalization throughout the NCN backbone of the ligand. In order to understand the bonding nature in compound 2, a simple derivative, 2A (Figure S1, see the Supporting Information), was investigated by theoretical calculations.12 The calculated structural parameters (N
Si: 1.905 Å, Si
S: 2.038 Å, S
K: 3.308 Å) are in good agreement with the crystallographic data. The Wiberg bond index13 (WBI of the S
K bonds: 0.0379, 0.0387) indicates that the S
K interactions are essentially electrostatic in nature. The natural-bond-orbital (NBO)14 analysis shows that there are three lone pair (LP) orbitals on each S atom (Table S1). Together with the calculated NPA charges of compound 2A (Si: 1.24; S:
0.86,
0.87; K: 0.91), it is suggested that the bonding in the “{LSi(S)2}K” moiety in 2A is 3687

dx.doi.org/10.1021/om200285u |Organometallics 2011, 30, 3686–3689

Organometallics

NOTE

Scheme 3. Compound 2A and E

Scheme 4. Synthesis of 3

consistent with the structure E (Scheme 3). LP(2) on each S atom is delocalized into the σ* orbital of another Si
S bond by n
σ* hyperconjugation (second-order perturbation stabilizing energy: S(1) 8.30, S(2) 8.68 kcal mol
1, Table S2). Therefore, there is a weak electronic delocalization along the “Si(S)2” skeleton. LP(2) and LP(3) on each S atom are stabilized by forming n
σ* hyperconjugation with the σ* orbitals of two Si
N bonds, respectively (Figure S2). The sum of second-order perturbation stabilizing energies due to the n
σ* hyperconjugation is considerably high (S(1): 9.15 þ 9.30 þ 11.80 þ 11.63 = 41.88 kcal mol
1; S(2): 9.55 þ 9.40 þ 12.09 þ 12.26 = 43.30 kcal mol
1). The strong n
σ* hyperconjugation leads to short Si
S bonds. The Si
S bonds (2.038 Å) in 2A are shorter than the Si
S single bond (2.167 Å) in HS
SiH3. The results are consistent with the Wiberg bond index of the Si
S bonds (WBI: 1.34, 1.35) in 2A such that the Si
S bonds have some multiple-bond character. The salt elimination of 2 with one equivalent of GeCl2 3 dioxane in THF afforded the germanium(II) siladithiocarboxylate [{LSi(S)2}2Ge:] (3, Scheme 4). It is noteworthy that germanium(II) dithiocarboxylate or dithiocarbamate complexes are scarcely found. Compound 3 was isolated as a highly air- and moisture-sensitive colorless crystalline solid that is slightly soluble in THF and CH2Cl2. 3 is stable in solution or the solid state at room temperature under an inert atmosphere. The 1H and 13 C{1H} NMR spectra of 3 display resonances due to the amidinate ligand. The 29Si{1H} NMR spectrum of 3 exhibits one singlet at δ 15.2 ppm, which shows an upfield shift compared with that of 2. The molecular structure of 3 is shown in Figure 2. Two “LSi(S)2
” moieties chelate to the germanium atom in an anisobidentate manner (Ge(1)
S(1): 2.7835(4) Å, Ge(1)
S(2): 2.3963(4) Å). The Ge(1) atom adopts a distorted seesaw geometry. The S(1) and S(1A) atoms are at the axial positions (S(1)
Ge(1)
S(1A): 163.91(2)°), and the stereochemically active lone pair of electrons is at the equatorial position. The Ge
S bonds in 3 are comparable with those in [Me2ClGe(S2CNMe2)] (2.254(1), 2.896(1) Å) and [Me2Ge(S2CNMe2)2] (2.275(2), 3.078(2), 2.281(2), 3.158(2) Å), in which the dithiocarbamate ligand coordinates to the GeIV center in an anisobidentate manner.15 The amidinate ligands in 3 are bonded in a N,N0 -chelate fashion to the Si atoms, which adopt a tetrahedral

Figure 2. Molecular structure of 3 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ge(1)
S(1) 2.7835(4), Ge(1)
S(2) 2.3963(4), Si(1)
S(1) 2.0182(5), Si(1)
S(2) 2.0706(6), Si(1)
N(1) 1.8226(13), Si(1)
N(2) 1.8286(14), C(5)
N(1) 1.334(2), C(5)
N(2) 1.3462(18), S(1)
Ge(1)
S(1A) 163.91(2), S(2)
Ge(1)
S(2A) 100.36(2), S(1)
Ge(1)
S(2) 81.56(1), Si(1)
S(2)
Ge(1) 87.53(2), Si(1)
S(1)
Ge(1) 78.61(2), S(1)
Si(1)
S(2) 112.30(3), N(1)
Si(1)
S(1) 119.31(5), N(1)
Si(1)
S(2) 113.78(5), N(1)
Si(1)
N(2) 72.05(6), Si(1)
N(1)
C(5) 91.06(9), N(1)
C(5)
N(2) 106.48(13), C(5)
N(2)
Si(1) 90.41(10).

geometry. The Si(1)
S(1) (2.0182(5) Å) and Si(1)
S(2) (2.0706(6) Å) bonds in 3 are similar to those in 2. In conclusion, the first example of the amidinate-stabilized siladithiocarboxylate anion, [{LSi(S)2}K(THF)2]2 (2), was synthesized successfully by the reaction of [LSiCl] (1) with two equivalents of elemental sulfur and KC8 in THF. X-ray crystallography and DFT calculations show that the Si
S bond lengths in 2 are intermediate between the SidS double and Si
S single bond lengths. There is a weak electronic delocalization along the “Si(S)2” skeleton. Compound 2 underwent a salt elimination reaction with GeCl2 3 dioxane to form [{LSi(S)2}2Ge:] (3), which contains two “LSi(S)2
” moieties chelating to the germanium(II) atom in an anisobidentate manner.

’ EXPERIMENTAL SECTION General Procedure. All manipulations were carried out under an inert atmosphere of argon gas using standard Schlenk techniques. THF was dried over and distilled over Na/K alloy prior to use. 1 was prepared as described in the literature.4 The 1H, 13C, and 29Si NMR spectra were recorded on a JEOL ECA 400 spectrometer. The chemical shifts δ are relative to SiMe4 for 1H, 13C, and 29Si. Elemental analyses were performed by the Division of Chemistry and Biological Chemistry, Nanyang Technological University. Melting points were measured in sealed glass tubes and were not corrected. [{LSi(S)2}K(THF)2]2 (2). THF (10 mL) was added to a mixture of 1 (0.58 g, 1.98 mmol), S8 (0.13 g, 0.51 mmol), and KC8 (0.60 g, 4.44 mmol) at room temperature. The resulting red solution was stirred for 2 h and then filtered. The red filtrate was concentrated to afford 2 as colorless block crystals. Yield: 0.16 g (14.9%). Mp: 340.4 °C (dec). Anal. Found: C, 54.25; H, 7.41; N, 5.27. Calcd for C46H78K2N4O4S4Si2: C, 54.51; H, 7.76; N, 5.53. 1H NMR (395.9 MHz, THF-d8, 22.0 °C): δ 1.26 (s, 36H, But), 1.73
1.79 (m, 32H, THF), 3.60
3.63 (m, 32H, THF), 7.43
7.51 (m, 10H, Ph). 13C{1H} NMR (100.5 MHz, THF-d8, 22.3 °C): δ 24.5 (THF), 31.8 (CMe3), 55.0 (CMe3), 66.7 (THF), 128.6, 129.7, 130.4, 133.9 (Ph), 169.9 ppm (NCN). 29Si{1H} NMR (79.4 MHz, THF-d8, 22.3 °C): δ 18.4 ppm. [{LSi(S)2}2Ge:] (3). 2 (0.11 g, 0.10 mmol) in THF (10 mL) was added dropwise to GeCl2 3 dioxane (0.023 g, 0.11 mmol) in THF (5 mL) at 0 °C. The resulting white suspension was raised to ambient temperature 3688

dx.doi.org/10.1021/om200285u |Organometallics 2011, 30, 3686–3689

Organometallics

NOTE

Table 1. Crystallographic Data for Compounds 2 and 3 2

3

formula

C50H86K2N4O5S4Si2

C30H46GeN4S4Si2

M

1085.85

719.72

color

colorless

colorless

cryst syst

triclinic

monoclinic

space group

P1

C2/c

a/Å

11.1594(8)

24.1519(4)

b/Å

11.4396(8)

8.7472(1)

c/Å R/deg

12.4599(9) 87.130(3)

17.9561(3) 90

β/deg

72.379(4)

110.329(1)

γ/deg

74.688(3)

90

V /Å3

1461.40(18)

3557.15(9)

Z

1

4

dcalcd/g cm
3

1.234

1.344

μ/mm
1

0.391

1.190

F(000) cryst size/mm

584 0.40  0.30  0.30

1512 0.30  0.10  0.02

index range


16 e h e 16


38 e h e 38


16 e k e 16


13 e k e 12


18 e l e 18


28 e l e 28

no. of reflns collected

30 020

34 833

R1, wR2 (I > 2(σ)I)

0.0350, 0.1023

0.0352, 0.0852

R1, wR2 (all data)

0.0471, 0.1230

0.0638, 0.1053

goodness of fit., F2 no. of data/restraints/params

1.128 9322/171/376

1.094 7539/0/192

largest diff peak, hole/e Å
3

1.128,
0.490

0.819,
0.910

and then stirred overnight. The white suspension was filtered and concentrated to afford 3 as colorless crystals. Yield: 0.052 g (73.0%). X-ray quality crystals were grown from a mixture of THF/CH2Cl2. Mp: 250.0 °C (dec). Anal. Found: C, 49.94; H, 6.40; N, 7.71. Calcd for C30H46GeN4S4Si2: C, 50.08; H, 6.45; N, 7.79. 1H NMR (399.5 MHz, CDCl3, 23.5 °C): δ 1.19 (s, 18H, But), 1.30 (s, 18H, But), 7.38
7.51 (m, 10H, Ph). 13C{1H} NMR (100.5 MHz, CDCl3, 23.9 °C): δ 31.3 (CMe3), 31.5 (CMe3), 55.0 (CMe3), 56.6 (CMe3), 127.9, 128.1, 128.5, 129.9, 131.6, 131.9 (Ph), 171.3 ppm (NCN). 29Si{1H} NMR (78.7 MHz, CDCl3, 22.0 °C): δ 15.2 ppm. X-ray Data Collection and Structural Refinement. Intensity data for compounds 2 and 3 were collected using a Bruker APEX II diffractometer. The crystals of 2 and 3 were measured at 103(2) K. The structures were solved by direct phase determination (SHELXS-97) and refined for all data by full-matrix least-squares methods on F2.16 All non-hydrogen atoms were subjected to anisotropic refinement. The hydrogen atoms were generated geometrically and allowed to ride on their respective parent atoms; they were assigned appropriate isotopic thermal parameters and included in the structure-factor calculations. The X-ray crystallographic data are summarized in Table 1. CCDC816368 (2) and -816369 (3) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallography Data Center via ww.ccdc.cam.ac.uk/data_request/cif.

These materials are available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Academic Research Fund Tier 1 (C.-W.S: RG 47/08; K.H.L.: RG 28/07). The authors thank Dr. Y. Li for X-ray crystallography. ’ REFERENCES (1) For recent reviews, see: (a) Kano, N.; Kawashima, T. Top. Curr. Chem. 2005, 251, 141. (b) Heard, P. J. Prog. Inorg. Chem. 2005, 53, 1. (c) Hogarth, G. Prog. Inorg. Chem. 2005, 53, 71. (2) Xiong, Y.; Yao, S.; Driess, M. Angew. Chem., Int. Ed. 2010, 49, 6642. (3) Sen, S. S.; Ghadwal, R. S.; Kratzert, D.; Stern, D.; Roesky, H. W.; Stalke, D. Organometallics 2011, 30, 1030. (4) So, C.-W.; Roesky, H. W.; Magull, J.; Oswald, R. B. Angew. Chem., Int. Ed. 2006, 45, 3948. (5) Zhang, S.-H.; Yeong, H.-X.; So, C.-W. Chem.—Eur. J. 2011, 17, 3490. (6) Sen, S. S.; Jana, A.; Roesky, H. W.; Schulzke, C. Angew. Chem., Int. Ed. 2009, 48, 8536. (7) So, C.-W.; Roesky, H. W.; Oswald, R. B.; Pal, A.; Jones, P. G. Dalton Trans. 2007, 5241. (8) Kato, S.; Kitaoka, N.; Niyomura, O.; Kitoh, Y.; Kanda, T.; Ebihara, M. Inorg. Chem. 1999, 38, 496. (9) Iwamoto, T.; Sato, K.; Ishida, S.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 2006, 128, 16914. (10) (a) Yao, S.; Xiong, Y.; Driess, M. Chem.—Eur. J. 2010, 16, 1281. (b) Meltzer, A.; Inoue, S.; Pr€asang, C.; Driess, M. J. Am. Chem. Soc. 2010, 132, 3038. (c) Yao, S.; Xiong, Y.; Driess, M. Organometallics 2011, 30, 1748. (d) Asay, M.; Jones, C.; Driess, M. Chem. Rev. 2011, 111, 354. (e) Arya, P.; Boyer, J.; Carre, F.; Corriu, R.; Lanneau, G.; Lapasset, J.; Perrot, M.; Priou, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 1016. (11) Theoretical calculations show that the SidS bond in a basestabilized silanethione has some double-bond character or zwitterionic Siþ-S
bonding character: Epping, J. D.; Yao, S.; Karni, M.; Apeloig, Y.; Driess, M. J. Am. Chem. Soc. 2010, 132, 5443. (12) For the details of theoretical calculations and references, see the Supporting Information. (13) Wiberg, K. B. Tetrahedron 1968, 24, 1083. (14) Weinhold, F.; Landis, C. R. In Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective; Cambridge University Press, 2005. (15) (a) Chadha, R. K.; Drake, J. E.; Sarkar, A. B. Inorg. Chem. 1984, 23, 4769. (b) Chadha, R. K.; Drake, J. E.; Sarkar, A. B. Inorg. Chem. 1986, 25, 2201. (16) Sheldrick, G. M. SHELXL-97; Universit€at G€ottingen: G€ottingen, Germany, 1997.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures and tables giving selected calculation results of 2A. CIF files giving X-ray data of 2 and 3. 3689

dx.doi.org/10.1021/om200285u |Organometallics 2011, 30, 3686–3689