Arenechromiumtricarbonyl complexes of silyl(germyl)(stannyl)- and silyl(germyl)(plumbyl)methanes including unexpected formation of arenechromiumtricarbonyldimethylsilanol, (η⁶-C₆H₅)Cr(CO)₃SiMe₂OH

Abstract

Treatment of PhMe₂SiCH₂GeMe₃ (1) with t-BuLi followed by addition of Me₃ECl, E = Sn, Pb, results in the formation of phenylsilyl(germyl)stannyl- and phenylsilyl(germyl)plumbyl-methanes, PhMe₂Si(Me₃Ge)(EMe₃)CH, E = Sn (2), Pb (3). The thermal reaction of 1, 2 and 3 with Cr(CO)₆ yields the corresponding aryl-Cr(CO)₃ analogs, {(η⁶-C₆H₅)Cr(CO)₃}Me₂Si(Me₃Ge)CH₂ (4) and {(η⁶-C₆H₅)Cr(CO)₃}Me₂Si(Me₃Ge)(EMe₃)CH, E = Sn (5), Pb (6). The thermal treatment of 2 with Cr(CO)₆ in wet THF/di-n-butyl ether mixture results in the formation of the arenechromiumtricarbonyl silanol {(η⁶-C₆H₅)Cr(CO)₃}Me₂SiOH (7) which exhibits amphiphilic character, forming H-bonded chains in the solid state in a head–to-head arrangement of the areneCr(CO)₃ units.

1. Introduction

Di- and tri-substituted Group 14 homoleptic methyl groups, R₂CH and R₃C, R = R´₃Si, R´₃Ge, R´₃Sn, are routinely used as stabilizing groups to isolate kinetically labile methylenes and dimetallenes of Group 14 elements [1, 2]. They are also versatile reagents in synthetic chemistry [3, 4] and for example, functionalized allylic silanes have been synthesized in high yield from the reaction of tris(trimethylsilyl)methyl borane (Me₃Si)₃CBH₂ with alkenyl lithium reagent [4]. However, related heteroleptic systems with differing group 14 groups have received only scant attention [5–7]. We and Shimizu group reported the synthesis of mixed group 14 methanes, (Me₃Si)(Me₃Ge) (Me₃Sn)CH [5, 6]. In continuation of such studies, we report the synthesis and characterization of arenechromiumtricarbonyl complexes of the mixed group 14 methanes, (PhMe₂Si)(Me₃Ge)(Me₃E)CH (E = Sn, Pb). A crystal structure of the silanol, (η⁶-C₆H₅)Cr(CO)₃SiMe₂OH, obtained from the thermal reaction of {η⁶-C₆H₅)Me₂Si}(Me₃Ge)(Me₃Sn)CH with Cr(CO)₆ in wet THF / butyl ether mixture, is also reported.

2. Results and discussion

Treatment of phenyldimethylsilyl(trimethylgermyl)methane, PhMe₂SiCH₂GeMe₃, (1) with t-BuLi in a 4:1 THF/HMPA mixture at −78 °C led to the formation of [PhMe₂SiCHGeMe₃]⁻Li⁺. Subsequent quenching with Me₃ECl (E = Sn, Pb) resulted in mixed-trisubstituted methanes, (PhMe₂Si)(Me₃Ge)(Me₃E)CH (E = Sn, Pb) (E = Sn (2); Pb (3)) after hydrolytic workup in 72% and 40% yields respectively, as colorless oils. If the metallation reaction is performed with n-BuLi, only poor yields of 2 and 3 were obtained. The new compounds were characterized by NMR spectroscopy, Table 1. The ¹H and ¹³C NMR of 2 and 3 exhibited two resonances for the diastereotopic methyl protons and carbons of the SiMe₂ groups. The related ¹¹⁹Sn NMR and ²⁰⁷Pb NMR were significantly shifted to low field compared to Me₄Sn and Me₄Pb, i.e. 14 ppm and 63 ppm, respectively.

Table 1.

Selected ²⁹Si, ¹¹⁹Sn and ²⁰⁷Pb NMR shifts (δ ppm) for 1–6.

Compd	29Si	119Sn	207Pb
1	−2.98
2	−1.72	14.1
3	−0.56		63.2
4	1.13
5	2.29	12.5
6	3.45		59.3

The thermal reactions of 1, 2 and 3 with Cr(CO)₆ in a refluxing 7:1 di-n-butylether /THF solvent mixture resulted in the formation of yellow crystalline chromium tricarbonyl complexes, 4, 5 and 6 in moderate yields, equation 1.

(1a)

(1b)

The yellow complexes slowly oxidize in the solid state, whereas in solution the decomposition is more rapid; their spectral and analytical data are reported in the experimental section. As expected the NMR spectra of the chromium complexes 4–6 exhibited an upfield displacement of chemical shifts of the aromatic protons and carbons of ~ 2 ppm and ~ 30–40 ppm, respectively [8,9]. No significant change in the chemical shifts for the ¹¹⁹Sn and ²⁰⁷Pb resonances of the arenechromiumtricarbonyl complexes 5 and 6 were observed compared to those of the precursors 2 and 3.

Our attempts to deprotonate complex 5 with LDA (lithium diisopropamide) and subsequent quenching with Me₃PbBr which would have allowed access to arenechromiumtricarbonyl methane with four different group 14 elements, catorcane molecule [5], have not, thus far, been successful.

2.1. Hydrolysis Reaction

Thermal treatment of 2 with Cr(CO)₆, in a wet n-butyl ether/THF solution mixture at 140 °C for three days produced the unexpected arenechromiumtricarbonyl silanol 7 as well as complex 4 in an approximately 1:2 ratio, equation 2.

(2)

Presumably, product 7 derives from the hydrolytic cleavage of the Si-CH bond at high temperature, a reaction that would also produce Me₃SnCH₂GeMe₃. Indeed the NMR spectral analysis of the liquid obtained after removing the butyl ether did exhibit the expected ¹¹⁹Sn resonance at 13.3 ppm assigned to Me₃SnCH₂GeMe₃ [7e]. Complex 7 was characterized by NMR spectroscopy and recrystallization from toluene afforded yellow crystals suitable for X-ray crystallography, vide infra.

In addition to the cleavage of the Si-CH bond, there is significant cleavage of the Sn-CH bond to produce the chromium complex 4, i.e. we obtained two unexpected arenechromiumtricarbonyl products at the same time, without formation of the expected product 5. It is not, however, clear at this time whether the cleavage reactions occur prior to, or subsequent to, chromiumtricarbonyl complexation. The cleavage of the Sn-CH₂ bond in Me₃SnCH₂EMe₃ (E = Si, Ge) under mild conditions by electrophiles and acids has been described previously [10]. The Me₃E group appears to activate the Sn-CH₂ toward such reactions, thus in our system the presence of water can be expected to result in such Sn-CH bond cleavage. The cleavage of the Si-CH₂ bond in the Me₃SnCH₂SiMe₃ was not observed [10] and suggests that the equivalent bond in our system, Si-CH, is activated by the presence of the (η⁶-C₆H₅)Cr(CO)₃) group. The fact that there appears to be no Ge-CH cleavage supports this suggestion and is currently being tested. It is well documented that the benzylic carbon is activated by the chromiumtricarbonyl species to produce stable benzylic cations [11]. The electrochemical studies of the (organosilyl)arenechromium tricarbonyl complexes also suggests the stability of the oxidized (organosilyl)arenechromiumtricarbonyl compounds which is solvent and substituent depended [12].

2.2 X-ray structure of {(η⁶ –C₆H₅)Cr(CO)₃}Me₂SiOH (7)

The structure of compound 7, Figure 1, exhibits the classical η⁶-arene-ML₃ form of a piano stool with a planar benzene ring seat lying symmetrically above the π-bonded chromium atom with the three carbonyl groups oriented in a tripod fashion in opposite direction to the benzene ring [8, 9]. The structural parameters and selected bond angles and distances are provided in Table 2 and Table 3. The torsion angles C_ipso-ring centroid–Cr-C_n(O) (n = 7, 8 and 9 in Fig.1) are −168.40, +72.60 and −50.95 degrees respectively, the tripodal arrangement is about 12° from the ideally eclipsed conformation (torsion angles −60, +60 and +180 degrees) relative to the para and two ortho carbon atoms in the ring. The Si-C bond distances, in the range of 1.847–1.878 Ǻ, and a Si-O bond distance of 1.652(1) Ǻ are typical of organosilanols [11, 13–18]. The O-Si-C bond angles between 108.22–108.87° signify essential tetrahedral geometry and compare well with the reported values for its diphenyl analog, {(η⁶ –C₆H₅)Cr(CO)₃}Ph₂SiOH, 8 [11]. The hydrogen atom of the OH group of 7 is disordered over two positions with half occupancy at each site. The structure of 8 exhibits the gauche orientation of arenechromiumtricarbonyl with respect to the OH group, where as in 7 the orientation is anti. These two features accounts for the H-bonding (HB) patterns noted in 7, vide infra, which are largely absent in 8 presumably due to the steric reasons.

Figure 1.

Molecular structure of 7. Thermal ellipsoids set at 50% of probability, only one position for the disordered hydroxyl hydrogen is shown.

Table 2.

Structure Determination Summary of 7.

empirical formula	C11H12CrO4Si
crystal color	Yellow
crystal habit	Plate
crystal size/mm3	0.18 × 0.10 × 0.02
a/Å	29.902(3)
b/Å	10.9871(9)
c/Å	7.6276(6)
α /deg	90
β/deg	94.146(2)
γ/deg	90
volume/Å3	2499.4(4)
crystal system	Monoclinic
space group	C2/c
Z	8
Dc /g cm−3	1.532
μ /mm−1	1.01
absorption correction	Bruker SADABS program
temp/K	100
Wavelength	0.71073 Å
monochromator	Graphite
Diffractometer	Bruker SMART with Apex CCD
reflections collected	6967
independent reflections	2463 [R(int) = 0.0212]
structure solution technique	direct methods
structure solution program	SHELXS-97 (Sheldrick, 1990)
refinement technique	Full-matrix least-squares on F2
refinement program	SHELXS-97 (Sheldrick, 1997)
function minimized	Σ w(Fo2 - Fc2)2
goodness-of-fit on F2	1.072
R1, wR2 [I>2s(I)]	0.0312, 0.0791
R1, wR2 (all data)	0.0332, 0.0806

Table 3.

Selected Bond Lengths (Å) and Angles (degrees) for 7.

Cr-C1	2.2204(19)	Cr-C4	2.2074(19)
Cr-C9	1.857(2)	Si-O4	1.6518(14)
Si-C10	1.848(2)	Si-C11	1.853(2)
Si-C1	1.878(2)	O3-C9	1.149(3)
C1–C6	1.429(3)	C1–C2	1.421(3)
C2–C3	1.404(3)	C3–C4	1.402(3)
C4–C5	1.412(3)	C5–C6	1.402(3)
O4-Si-C10	108.79(9)	O4-Si-C11	108.28(10)
C10-Si-C11	111.35(12)	O4-Si-C1	108.22(8)
C10-Si-C1	108.68(10)	C11-Si-C1	111.44(10)

X-ray structures of a number of organometallic silanols, RR₂’SiOH (R = organometallic group), have been reported and the majority of them exhibit some type of hydrogen bonding dictated by the steric demand of the organometallic -group [13], Figure 2. Thus, discrete linear dimers [14, 15], four- membered cyclic dimers, [15, 16] and tetrameric species are known [16, 17] and we have reported that the related ferrocenyl dimethylsilanol, FcSiMe₂OH, 10 forms a cyclic H-bonded hexameric arrangement of SiOH groups [18] reminiscent of hexameric forms of water [19]. The organometallic silanols also form infinite chains depending upon the steric demands of the substituents on silicon [17b].

Figure 2.

Selected Examples for Organometallic Silanols 8–12.

In silanol 7, the hydrogen atom of the silanol group is disordered over two positions, with O4⋯O4B = 2.691 Ǻ, O-H41┄O4B = 159.6°, O⋯O4A = 2.708 Ǻ and O-H42⋯O4A = 154.3°). The disorder is due to the centrosymmetric structure. This disorder is also observed for {(η⁵-C₅Me₅)(CO)₂}FeCH₂SiMe₂OH (9) which crystallizes in the same spatial group C2/c generating the identical HB motif (Figure 3) [17b]. Both, compound 7 and 9, are amphiphiles which are packed in head-to head fashion with a HB chain running parallel to the c axis of the crystal structure; where the hydrophobic Cr(CO)₃ units are protecting the hydrophilic Si-OH groups (Figure 4). The chain formed is not linear; it represents a spiro[2.2]pentane molecule as shown in (Figure 5). The occurrence of the chain instead of a 2-D bilayer is due to the generation of another type of HB by the Cr(CO)₃ units through the carbonyl groups (C3-H3⋯O1, 2.54 Å; C5-H5⋯O2, 2.56 Å; C5-H5⋯O3, 2.54 Å), essentially identical interactions present in the crystal structure of 9 [17b, 20].

Figure 3.

Crystal packing of 7 exhibits H-bonding showing chain along the c axis illustrating head-to-head arrangement of the areneCr(CO)₃ units.

Figure 4.

View along the Si(OH) hydrogen-bonding chains illustrating the ribbon character of the extended system, Si blue, O red.

(For Online Version)

Figure 5.

View of the crystal packing through the c axis. Four chain motifs and the CO⋯H hydrogen bonds are shown.

3. Experimental

3.1 General procedures

All syntheses were performed under argon atmosphere using standard Schlenk techniques. Tetrahydrofuran and hexanes were distilled over sodium and benzophenone ketal. Hexamethylphoshoramide (HMPA) was distilled over calcium oxide. t-Butyllithium (1.7 M in pentane), Me₃SnCl and Cr(CO)₆ were purchased from Aldrich Chemicals; Me₃SiCH₂Cl and Me₃GeCl were purchased from Gelest; Me₃PbBr was obtained from the bromination of Me₄Pb [21], PhMe₂SiCH₂GeMe₃, 1 [7d], PhMe₂Si(Me₃Ge)(Me₃Sn)CH, 2 [5] were synthesized by the reported methods. NMR spectra were recorded on Bruker 300 MHz spectrometer using CDCl₃ for 2 and 3 and using C₆D₆ for 4–7. IR spectra were recorded on a Perkin-Elmer 1600 FT IR spectrophotometer. Elemental analyses were performed by Galbraith Laboratories.

3.2. Synthesis of PhMe₂Si(Me₃Ge)(Me₃Pb)CH, 3

A 250 mL Schlenk flask was charged with 20 mL of a THF solution containing 4.0 g (15 mmol) of PhMe₂SiCH₂GeMe₃, 1, and 5 mL of HMPA. The mixture was maintained at −78 °C and t-butyllithium (15.0 mmol) was added dropwise via syringe. The solution became yellow immediately and finally became orange and was stirred for 90 minutes between −78 to −70 °C. At this time a THF slurry of 4.98 g (15.0 mmol) of Me₃PbBr was added slowly at the same temperature. The color of the solution became bright yellow and finally greenish-black. The solution was permitted to warm to room temperature and stirred overnight. The reaction was hydrolyzed with 25mL of a 5% solution of NH₄Cl at 0 °C. The organic layer was extracted with 25 mL of hexanes and dried over MgSO₄. After filtration and removal of hexane on a rotary evaporator, the liquid was distilled through a column at 110–114 °C at 0.05 mm Hg to yield 3.1g (40 % yield) of 3.

3.2.1. Analytical and spectroscopic data for (PhMe₂Si)(GeMe₃)(PbMe₃)CH, 3

Anal. Calcd. (Found): C, 34.76 (35.05); H, 7.03 (6.93)

1H	0.076 (s, 9 H, GeMe3); 0.25 (s, 3H, SiMe), 0.26 (s, 3H, SiMe); 0.25 (s, 1H, J207Pb- H = 90.6 Hz, CH); 0.63 (s, 9H, 2J207Pb-H = 57.0 Hz, PbMe3); 7.25, 7.43(m, 5H, Ph)
13C	1.01 (1J Pb-C = 214.2 Hz, PbMe3); 1.16 (SiMe), 1.84 (3J Pb-C = 32.6 Hz, SiMe); 2.93 (3J Pb-C = 25.4 Hz, GeMe3); 5.02 (CH), 127.66, 128.52, 133.40, 142.50 (ipso)
207Pb	63.21
29Si	−0.56 (2J29Si-207Pb = 37.5 Hz)

3.3. Synthesis of {(η⁶-C₆H₅)Cr(CO)₃Me₂Si}(Me₃Ge)(Me₃Pb)CH, 6

This synthesis is typical for all the new arenechromiumtricarbonyl complexes. A 100 mL round-bottom flask was charged with 0.7 g (1.35 mmol) of 3 and 0.3 g (1.36 mmol) of Cr(CO)₆ in 20 mL of n-butyl ether and 3 mL of THF. The mixture was heated in oil bath under nitrogen atmosphere at 90 °C for 24 hrs. The color of the solution turned gray due to the deposition of lead. After 24 hrs, the temperature of the oil bath was raised to 140 °C and mixture was heated at this temperature for 4 days. The reaction mixture was cooled to room temperature and filtered over Celite. Butyl ether was removed by flash distillation under vacuum and the yellow residue was extracted with 10 mL of a 2:1 mixture of hexane and toluene. Filtration and removal of the solvents produced yellow oil which solidified in the refrigerator to yield, 0.37 g (42 %) of 6, which was further recrystallized from a mixture of hexane and toluene.

3.3.1. Analytical and spectroscopic data for {(η⁶-C₆H₅)Cr(CO)₃Me₂Si} (GeMe₃)(PbMe₃)CH, 6

Anal. Calcd. (Found): C, 33.04 (33.09); H, 4.62 (4.53) m.p. 70–72°C

1H	0.043 (s, 1H, CH); 0.081 (s, 9 H, GeMe3); 0.29 (s, 3H SiMe), 0.31 (s, 3H, SiMe); 0.70 (s, 9H, J207Pb-H = 56.6 Hz, PbMe3); 4.36 (t, J = 6 Hz, 2H, Ph), 4.72 (t, J =6 Hz, 1H, Ph), 4.92 (d, J =7.5 Hz, 2H, Ph)
13C	0.88 (SiMe), 0.97 (SiMe); 1.50(1J Pb-C = 218.0 Hz, PbMe3); 3.10 (GeMe3), 5.40 (CH); 90.00, 95.50, 99.80 (Ph), 102.6 (ipso); 233.67 (CO)
207Pb	59.36
29Si	3.45 (2J29Si-207Pb = 44.5 Hz)

IR (ν CO) Hexane, cm⁻¹: 1977.1, 1911.4

3.4. Synthesis of (η⁶-C₆H₅)Cr(CO)₃SiMe₂OH, 7

A 100 mL round-bottom flask was charged with 1.74 g (4.04 mmol) of 2 and 0.891 g (4.04 mmol) of Cr(CO)₆ in 25 mL of n-butyl ether and 5 mL of wet THF. The mixture was treated as above for the synthesis of 6. After removal of butyl ether by flash distillation under vacuum; a solid yellow residue (mixture of 4 and 7) was obtained. The mixture was treated with 15 mL 2:1 mixture of hexanes and toluene, filtered and left in the refrigerator. The silanol 7 was crystallized from the solution. Recrystallization from toluene resulted in 0.29 g (25%) of 7. Evaporation of the solvents from the filtrate produced crude 4 which was recrystallized from hexane to yield 0.80 g (49%) of 4.

3.4.1. Analytical and spectroscopic data for (η⁶-C₆H₅)Cr(CO)₃SiMe₂OH, 7

Anal. Calcd. (Found): C, 45.82 (45.59); H, 4.19 (4.22)

1H	0.14 (s, 6H, SiMe2); 1.46 (1H, s, OH); 4.28 (t, J = 6 Hz, 2H, Ph), 4.62 (t, J = 6 Hz, 1H, Ph), 4.85 (d, J =6 Hz, 2H, Ph)
13C	−0.40 (SiMe2); 90.34, 95.26, 97.97 (ipso) 98.85 (Ph); 233.46 (CO)
29Si	7.12

IR (ν CO) Hexane, cm⁻¹: 1978.0, 1911.7

3.4.2. Analytical and spectroscopic data for {(η⁶-C₆H₅)Cr(CO)₃Me₂Si} (GeMe₃)CH₂, 4

Anal. Calcd. (Found): C, 44.70 (45.35); H, 5.50 (4.63), m.p. 79 °C

1H	−0.18 (s, 2H, CH2); 0.095 (s, 9 H, GeMe3); 0.16 (s, 6H, SiMe2); 4.35 (t, J = 6 Hz, 2H, Ph), 4.80 (t, J =6 Hz, 1H, Ph), 4.82 (d, J =6 Hz, 2H, Ph)
13C	−0.63 (SiMe2); 0.72(GeMe3); 2.09 (CH2); 90.57, 95.02, 99.09 (Ph), 100.93 (ipso); 233.69 (CO)
29Si	1.13

IR (ν CO) Hexane, cm⁻¹: 1977.6, 1911.4

3.5.1. Analytical and spectroscopic data for (PhMe₂Si)(GeMe₃)(SnMe₃)CH, 2

Anal. Calcd. (Found): C, 41.90 (42.2); H, 7.04 (6.93)

1H	−0.22 (s, 1H, 2J119Sn-H = 77.2 Hz, 2J117Sn-H = 74.0 Hz, CH); 0.05 (s, 9H, 2J119Sn-H = 51.0 Hz, 2J117Sn-H = 49.8 Hz, SnMe3); 0.16 (s, 9H, GeMe3); 0.33 (s, 3H, SiMe), 0.34 (s, 3H, SiMe); 7.32, 7.46 (m, 5H, Ph)
13C	−5.89 (1J 119Sn-C = 328.0 Hz, 1J 117Sn-C = 313.0 Hz, SnMe3); −2.82 (1J 119Sn-C = 221.7 Hz, CH); 1.20 (SiMe), 1.53 (SiMe); 2.74 (GeMe3); 127.6, 128.5, 133.40, 142.0 (ipso)
119Sn	14.1
29Si	−1.72 (2J29Si-119Sn = 25.8 Hz)

3.6.1. Analytical and spectroscopic data for {(η⁶-C₆H₅)Cr(CO)₃Me₂Si} (GeMe₃)(SnMe₃)CH, 5

Anal. Calcd. (Found): C, 38.21 (38.56); H, 5.34 (5.60), m.p 64 °C

1H	−0.65 (s, 1H, 2J119Sn-H = 78 Hz, CH); 0.038 (s, 9H, 2J119Sn-H = 51.6 Hz, 2J117Sn-H = 50.4 Hz, SnMe3); 0.087 (s, 9 H, GeMe3); 0.29 (s, 3H SiMe), 0.31 (s, 3H, SiMe); 4.33 (t, J = 6 Hz, 2H, Ph), 4.70 (t, J = 6 Hz, 1H, Ph), 4.89 (d, J = 6 Hz, 2H, Ph)
13C	−5.48 (SnMe3); −1.74(CH); 0.88 (SiMe), 0.99 (SiMe); 3.02(GeMe3); 89.92, 95.61, 99.95 (Ph), 102.1 (ipso); 233.68 (CO)
119Sn	12.51
29Si	2.29 (2J29Si-119Sn = 24.3 Hz)

IR (ν CO) Hexane, cm⁻¹: 1977.5, 1911.7

3.7. X-ray Crystal Structure for 7

A yellow plate of silanol 7 was used for the X-ray crystallographic analysis. The X-ray intensity data were measured at 100(2) K on a Bruker SMART APEX CCD area detector system equipped with a graphite monochromator and a MoKα fine-focus sealed tube (λ = 0.71073Å). The structure was solved and refined using the Bruker SHELXTL (Version 6.1) Software Package. A summary of the relevant crystallographic parameters for 7 is presented in Table 2, and selected bond lengths and angles are given in Table 3.

4. Conclusions

A convenient methodology for the synthesis of phenylsilyl(germyl)stannyl- and phenylsilyl(germyl)plumbyl-methanes, PhMe₂Si(Me₃Ge)(EMe₃)CH, (E = Sn, Pb) and their arenechromiumtricarbonyl-analogs is reported. The Sn-CH bond is labile in the presence of water, and the Si-CH₂ bond appears to be activated by the areneCr(CO)₃ substituent. The crystal structure of an arenechromiumtricarbonyl silanol exhibits intermolecular H-bonding forming chains along the c-axis with the head to head arrangement of areneCr(CO)₃ units which provide hydrophobic exterior to hydrophilic silanol groups.