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Acta Cryst. (2007). E63, m2477    [ doi:10.1107/S1600536807033922 ]

A polymeric mixed-metal hexamethyldisilazide

J. J. Morris, B. C. Noll and K. W. Henderson

Abstract top

The title compound, catena-poly[[[mu]2-bis(trimethylsilyl)amido-[kappa]2N:N]-lithium(I)-[[mu]2-bis(trimethylsilyl)amido-[kappa]2N:N]potassium], [LiK(C6H18NSi2)2]n or [{(Me3Si)2NLi}{(Me3Si)2NK}][infinity], is a polymeric one-dimensional chain composed of alternating lithium and potassium centers bridged through the amide N atoms of the hexamethyldisilazide anions. The repeat unit is centrosymmetric, with K on a center of symmetry, Li on a twofold rotation axis, and the complete hexamethydisilazide anion in the asymmetric unit. All of the methyl C atoms in the hexamethyldisilazide anion are disordered over two sites, with the site occupancies refined to 0.63 (2) for the major component of one trimethylsilyl group and 0.54 (3) for the major component of the second trimethylsilyl group.

Comment top

The title complex was synthesized during the attempted synthesis of mixed alkali metal hexamethyldisilazide complexes coordinated by ferrocene (Morris et al., 2007). The asymmetric unit contains a hexamethyldisilazide (HMDS) anion that coordinates to both a lithium and potassium through the nitrogen center (Fig. 1). The nitrogen has an approximately tetrahedral coordination geometry, bonding to two silicons, a potassium and a lithium. Symmetry expansion gives a polymeric one-dimensional chain composed of alternating lithium and potassium cations, each coordinated to two bridging HMDS anions (Fig. 2). The one-dimensional chain polymer adopts a zigzag conformation with a K—N—Li angle of 107.80 (9)°, which is slightly larger than the Na—N—Na angle of 102.0° seen in the isostructural sodium hexamethyldisilazide complex (Grüning & Atwood, 1977). The potassium has a crystallographically required linear N—K—N coordination environment with a K—N distance of 2.861 (1) Å, while the lithium has a near linear N—Li—N angle of 176.4 (2)° and a Li—N distance of 1.935 (1). In addition, the potassium has short K—C agostic interactions of 3.136 (13) Å for K1—C4 and 3.336 (13) Å for K1—C2. The structure is notable in forming a chain polymer rather than a LiN2K heterodimeric ring, as found for the THF solvate [{(Me3Si)2N}2LiK}(THF)3] (Williard & Nichols, 1991).

Related literature top

The isostructural one-dimensional chain polymer sodium hexamethyldisilazide (Grüning & Atwood, 1977), the dimeric mixed lithium–potassium hexamethyldisilazide solvated by THF (Williard & Nichols, 1991), and the related homometallic ferrocene solvates (Morris et al., 2007) have been reported.

Experimental top

All experimental manipulations were performed under a purified nitrogen atmosphere using standard Schlenk techniques. n-Butyllithium was purchased from Aldrich and standardized prior to use. KHMDS was purchased from Aldrich and used as received. Toluene was dried immediately before use by passage through columns of copper-based catalyst and alumina and stored over 4 Å molecular sieves. Ferrocene was purchased from Aldrich and dried by sublimation. Hexamethyldisilazane was purchased from Fluka and distilled over CaH2 prior to use. NMR spectroscopy data were recorded on a Varian Unity Plus 300 spectrometer at 298 K. For the synthesis of the title compound, n-BuLi (2.5 mmol of a 1.6 M solution in hexane) was added to a stirred solution of hexamethyldisilazane (2.5 mmol) in toluene (6.0 ml) at 273 K. KHMDS (2.5 mmol) and ferrocene (2.5 mmol) were added to give an orange solution. The solution was then cooled to 263 K, which gave colourless crystals within 24 h. 1H NMR (300 MHz, Toluene-d8, 298 K): δ 0.15 (s, Si(CH3)3).

Refinement top

H atoms were placed in calculated positions and allowed to ride on their respective parent atom. The isotropic methyl hydrogen thermal parameters were set to 1.5Ueq(C). All of the methyl carbons in the hexamethyldisilazide anion are disordered over two sites, with the site occupancy refined to 0.63 (2) for the major component of the first trimethylsilyl group (C1—C3) and 0.54 (3) for the major component of the second trimethylsilyl group (C4—C6). In addition, all Si—C bonds were restrained to be equal using the SADI restraint (30 total restraints).

Computing details top

Data collection: APEX2 (Bruker, 2006); cell refinement: APEX2 and SAINT (Bruker, 2006); data reduction: SAINT and XPREP (Sheldrick, 2003); program(s) used to solve structure: XS in SHELXTL (Sheldrick, 2001); program(s) used to refine structure: XL in SHELXTL; molecular graphics: XP in SHELXTL; software used to prepare material for publication: XCIF in SHELXTL.

Figures top
[Figure 1] Fig. 1. Asymmetric unit showing 50% probability displacement ellipsoids. Only the the methyl groups of the major component of the disordered hexamethyldisilazide anion are shown for clarity.
[Figure 2] Fig. 2. Section of the polymeric zigzag chain of the title compound highlighting the alternating lithium and potassium metal sites.
catena-poly[[µ2-bis(trimethylsilyl)amido-κ2N:N]-lithium(I)-\ [µ2-bis(trimethylsilyl)amido-κ2N:N]-potassium] top
Crystal data top
[LiK(C6H18N1Si2)2]F000 = 800
Mr = 366.83Dx = 1.030 Mg m−3
Monoclinic, C2/cMo Kα radiation
λ = 0.71073 Å
Hall symbol: -C 2ycCell parameters from 5346 reflections
a = 11.3154 (3) Åθ = 2.4–25.6º
b = 17.5465 (5) ŵ = 0.42 mm−1
c = 12.6528 (5) ÅT = 100 (2) K
β = 109.6570 (10)ºParallelepiped, colourless
V = 2365.76 (13) Å30.32 × 0.30 × 0.24 mm
Z = 4
Data collection top
Bruker SMART APEX II CCD
diffractometer		2265 independent reflections
Radiation source: fine-focus sealed tube1943 reflections with I > 2σ(I)
Monochromator: graphiteRint = 0.029
Detector resolution: 8.33 pixels mm-1θmax = 25.8º
T = 100(2) Kθmin = 2.2º
ω and φ scansh = −13→13
Absorption correction: multi-scan
(SADABS; Sheldrick, 2004)		k = −21→16
Tmin = 0.86, Tmax = 0.90l = −15→15
24282 measured reflections
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.029H-atom parameters constrained
wR(F2) = 0.083  w = 1/[σ2(Fo2) + (0.0436P)2 + 0.9001P]
where P = (Fo2 + 2Fc2)/3
S = 1.08(Δ/σ)max < 0.001
2265 reflectionsΔρmax = 0.23 e Å−3
161 parametersΔρmin = −0.23 e Å−3
30 restraintsExtinction correction: none
Primary atom site location: structure-invariant direct methods
Crystal data top
[LiK(C6H18N1Si2)2]V = 2365.76 (13) Å3
Mr = 366.83Z = 4
Monoclinic, C2/cMo Kα
a = 11.3154 (3) ŵ = 0.42 mm−1
b = 17.5465 (5) ÅT = 100 (2) K
c = 12.6528 (5) Å0.32 × 0.30 × 0.24 mm
β = 109.6570 (10)º
Data collection top
Bruker SMART APEX II CCD
diffractometer		2265 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2004)		1943 reflections with I > 2σ(I)
Tmin = 0.86, Tmax = 0.90Rint = 0.029
24282 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.02930 restraints
wR(F2) = 0.083H-atom parameters constrained
S = 1.08Δρmax = 0.23 e Å−3
2265 reflectionsΔρmin = −0.23 e Å−3
161 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2sigma(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R– factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Li10.00000.1461 (2)0.25000.0508 (9)
K10.25000.25000.50000.0795 (2)
N10.18132 (10)0.14957 (6)0.31084 (9)0.0352 (3)
Si10.23253 (4)0.06732 (2)0.37876 (4)0.04687 (15)
Si20.22947 (4)0.20084 (3)0.22140 (4)0.04769 (15)
C10.1043 (5)0.0361 (7)0.4356 (9)0.075 (2)0.64 (2)
H1A0.08750.07720.48110.112*0.64 (2)
H1B0.1319−0.00950.48220.112*0.64 (2)
H1C0.02770.02460.37290.112*0.64 (2)
C1'0.1023 (8)0.0058 (10)0.3916 (18)0.077 (4)0.36 (2)
H1D0.05140.03510.42640.115*0.36 (2)
H1E0.1380−0.03870.43840.115*0.36 (2)
H1F0.0495−0.01130.31690.115*0.36 (2)
C20.3699 (8)0.0751 (7)0.5133 (5)0.061 (2)0.64 (2)
H2A0.43640.10540.50010.092*0.64 (2)
H2B0.40190.02400.53890.092*0.64 (2)
H2C0.34280.09990.57070.092*0.64 (2)
C2'0.3637 (17)0.0856 (15)0.5143 (13)0.092 (7)0.36 (2)
H2D0.43610.10670.49820.138*0.36 (2)
H2E0.38780.03760.55570.138*0.36 (2)
H2F0.33570.12200.55970.138*0.36 (2)
C30.2750 (14)−0.0137 (6)0.3029 (11)0.098 (3)0.64 (2)
H3A0.2043−0.02510.23470.147*0.64 (2)
H3B0.2945−0.05870.35160.147*0.64 (2)
H3C0.34850.00020.28260.147*0.64 (2)
C3'0.303 (2)0.0040 (10)0.2957 (15)0.082 (4)0.36 (2)
H3D0.24570.00060.21810.124*0.36 (2)
H3E0.3168−0.04700.32930.124*0.36 (2)
H3F0.38330.02540.29660.124*0.36 (2)
C40.3468 (10)0.2758 (6)0.2978 (11)0.059 (2)0.54 (3)
H4A0.30650.31210.33380.088*0.54 (3)
H4B0.37720.30290.24420.088*0.54 (3)
H4C0.41770.25140.35500.088*0.54 (3)
C4'0.3276 (17)0.2872 (8)0.2749 (16)0.089 (5)0.46 (3)
H4D0.28080.32340.30470.133*0.46 (3)
H4E0.34860.31110.21350.133*0.46 (3)
H4F0.40500.27240.33470.133*0.46 (3)
C50.0906 (7)0.2606 (5)0.1331 (9)0.071 (2)0.54 (3)
H5A0.05270.28650.18240.107*0.54 (3)
H5B0.02820.22740.08090.107*0.54 (3)
H5C0.11960.29850.09050.107*0.54 (3)
C5'0.0863 (8)0.2267 (16)0.0976 (14)0.111 (6)0.46 (3)
H5D0.04030.18020.06530.167*0.46 (3)
H5E0.11260.25280.04080.167*0.46 (3)
H5F0.03190.26030.12260.167*0.46 (3)
C60.3043 (16)0.1554 (8)0.1259 (12)0.078 (3)0.54 (3)
H6A0.38300.13090.17080.117*0.54 (3)
H6B0.32170.19440.07770.117*0.54 (3)
H6C0.24740.11700.07920.117*0.54 (3)
C6'0.3196 (17)0.1392 (10)0.1535 (16)0.092 (4)0.46 (3)
H6D0.40020.12500.20940.138*0.46 (3)
H6E0.33440.16740.09230.138*0.46 (3)
H6F0.27110.09300.12350.138*0.46 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Li10.0303 (17)0.063 (2)0.057 (2)0.0000.0117 (15)0.000
K10.0700 (4)0.0934 (5)0.0703 (4)−0.0042 (3)0.0174 (3)−0.0453 (4)
N10.0298 (6)0.0391 (6)0.0357 (6)0.0020 (5)0.0099 (5)0.0012 (5)
Si10.0353 (2)0.0437 (3)0.0515 (3)0.00070 (17)0.00125 (18)0.01018 (18)
Si20.0339 (2)0.0634 (3)0.0458 (3)−0.00183 (18)0.01350 (18)0.0130 (2)
C10.057 (2)0.066 (4)0.092 (4)−0.008 (2)0.012 (2)0.037 (3)
C1'0.060 (4)0.051 (6)0.106 (8)−0.014 (4)0.011 (4)0.028 (5)
C20.053 (3)0.070 (4)0.041 (3)0.010 (3)−0.010 (2)−0.001 (3)
C2'0.080 (9)0.085 (9)0.095 (11)0.045 (7)0.009 (7)0.042 (7)
C30.103 (5)0.045 (5)0.114 (5)0.018 (4)−0.006 (3)−0.022 (4)
C3'0.107 (10)0.030 (6)0.077 (5)0.022 (6)−0.013 (5)−0.007 (4)
C40.052 (3)0.044 (3)0.085 (4)−0.014 (3)0.030 (2)0.005 (3)
C4'0.107 (9)0.056 (5)0.124 (9)0.008 (4)0.068 (8)0.025 (5)
C50.046 (2)0.082 (4)0.076 (4)0.001 (2)0.006 (2)0.043 (3)
C5'0.068 (4)0.170 (13)0.089 (7)−0.001 (6)0.017 (4)0.085 (8)
C60.106 (5)0.095 (5)0.052 (5)−0.018 (4)0.053 (4)−0.009 (4)
C6'0.101 (7)0.134 (9)0.065 (7)−0.046 (6)0.060 (6)−0.021 (6)
Geometric parameters (Å, °) top
Li1—N11.9348 (11)C1'—H1E0.9800
Li1—N1i1.9348 (11)C1'—H1F0.9800
Li1—Si2i2.9032 (14)C2—H2A0.9800
Li1—Si1i2.933 (2)C2—H2B0.9800
Li1—K1i3.9133 (19)C2—H2C0.9800
K1—N12.8612 (11)C2'—H2D0.9800
K1—N1ii2.8612 (11)C2'—H2E0.9800
K1—C4ii3.136 (13)C2'—H2F0.9800
K1—C2'ii3.14 (3)C3—H3A0.9800
K1—C4'ii3.315 (19)C3—H3B0.9800
K1—C2ii3.336 (13)C3—H3C0.9800
K1—Si1ii3.5303 (5)C3'—H3D0.9800
K1—Si2ii3.5599 (5)C3'—H3E0.9800
K1—Li1ii3.9133 (19)C3'—H3F0.9800
N1—Si21.6738 (12)C4—H4A0.9800
N1—Si11.6798 (12)C4—H4B0.9800
Si1—C31.866 (5)C4—H4C0.9800
Si1—C1'1.878 (6)C4'—H4D0.9800
Si1—C2'1.880 (7)C4'—H4E0.9800
Si1—C3'1.881 (7)C4'—H4F0.9800
Si1—C21.884 (3)C5—H5A0.9800
Si1—C11.904 (4)C5—H5B0.9800
Si2—C4'1.866 (7)C5—H5C0.9800
Si2—C61.872 (5)C5'—H5D0.9800
Si2—C6'1.881 (6)C5'—H5E0.9800
Si2—C41.888 (5)C5'—H5F0.9800
Si2—C5'1.891 (6)C6—H6A0.9800
Si2—C51.907 (5)C6—H6B0.9800
C1—H1A0.9800C6—H6C0.9800
C1—H1B0.9800C6'—H6D0.9800
C1—H1C0.9800C6'—H6E0.9800
C1'—H1D0.9800C6'—H6F0.9800
N1—Li1—N1i176.4 (3)N1—Si2—C4111.2 (4)
N1—Li1—Si2i144.40 (12)C6—Si2—C4103.6 (7)
N1i—Li1—Si2i33.48 (5)N1—Si2—C5'107.8 (3)
N1—Li1—Si1i150.35 (18)C4'—Si2—C5'110.2 (7)
N1i—Li1—Si1i32.95 (6)C6'—Si2—C5'101.8 (8)
Si2i—Li1—Si1i63.407 (14)N1—Si2—C5107.5 (3)
N1—Li1—K1i133.50 (12)C6—Si2—C5108.6 (5)
N1i—Li1—K1i44.12 (6)C4—Si2—C5102.5 (5)
Si2i—Li1—K1i60.84 (4)Si1—C1—H1A109.5
Si1i—Li1—K1i60.069 (19)Si1—C1—H1B109.5
N1—K1—N1ii180.0Si1—C1—H1C109.5
N1—K1—C4ii121.49 (9)Si1—C1'—H1D109.5
N1ii—K1—C4ii58.51 (9)Si1—C1'—H1E109.5
N1—K1—C2'ii121.81 (18)H1D—C1'—H1E109.5
N1ii—K1—C2'ii58.19 (18)Si1—C1'—H1F109.5
N1—K1—C4'ii121.43 (13)H1D—C1'—H1F109.5
N1ii—K1—C4'ii58.57 (13)H1E—C1'—H1F109.5
N1—K1—C2ii122.21 (8)Si1—C2—H2A109.5
N1ii—K1—C2ii57.79 (8)Si1—C2—H2B109.5
C4ii—K1—C2ii85.8 (3)Si1—C2—H2C109.5
N1—K1—Si1ii151.94 (2)Si1—C2'—H2D109.5
N1ii—K1—Si1ii28.06 (2)Si1—C2'—H2E109.5
C4ii—K1—Si1ii76.14 (15)H2D—C2'—H2E109.5
C2'ii—K1—Si1ii32.07 (14)Si1—C2'—H2F109.5
C4'ii—K1—Si1ii78.3 (2)H2D—C2'—H2F109.5
C2ii—K1—Si1ii31.69 (6)H2E—C2'—H2F109.5
N1—K1—Si2ii152.43 (2)Si1—C3—H3A109.5
N1ii—K1—Si2ii27.57 (2)Si1—C3—H3B109.5
C4ii—K1—Si2ii31.96 (9)Si1—C3—H3C109.5
C2'ii—K1—Si2ii73.9 (3)Si1—C3'—H3D109.5
C4'ii—K1—Si2ii31.23 (12)Si1—C3'—H3E109.5
C2ii—K1—Si2ii73.56 (13)H3D—C3'—H3E109.5
Si1ii—K1—Si2ii51.263 (10)Si1—C3'—H3F109.5
N1—K1—Li1ii151.92 (3)H3D—C3'—H3F109.5
N1ii—K1—Li1ii28.08 (3)H3E—C3'—H3F109.5
C4ii—K1—Li1ii76.34 (11)Si2—C4—H4A109.5
C2'ii—K1—Li1ii77.83 (16)Si2—C4—H4B109.5
C4'ii—K1—Li1ii74.08 (18)Si2—C4—H4C109.5
C2ii—K1—Li1ii77.48 (8)Si2—C4'—H4D109.5
Si1ii—K1—Li1ii46.06 (5)Si2—C4'—H4E109.5
Si2ii—K1—Li1ii45.415 (10)H4D—C4'—H4E109.5
Si2—N1—Si1132.29 (7)Si2—C4'—H4F109.5
Si2—N1—Li1106.91 (9)H4D—C4'—H4F109.5
Si1—N1—Li1108.27 (13)H4E—C4'—H4F109.5
Si2—N1—K1100.13 (5)Si2—C5—H5A109.5
Si1—N1—K198.70 (5)Si2—C5—H5B109.5
Li1—N1—K1107.80 (9)Si2—C5—H5C109.5
N1—Si1—C3119.7 (5)Si2—C5'—H5D109.5
N1—Si1—C1'113.1 (4)Si2—C5'—H5E109.5
N1—Si1—C2'110.5 (8)H5D—C5'—H5E109.5
C1'—Si1—C2'114.9 (10)Si2—C5'—H5F109.5
N1—Si1—C3'111.1 (7)H5D—C5'—H5F109.5
C1'—Si1—C3'101.6 (8)H5E—C5'—H5F109.5
C2'—Si1—C3'104.9 (10)Si2—C6—H6A109.5
N1—Si1—C2115.9 (4)Si2—C6—H6B109.5
C3—Si1—C2104.1 (6)Si2—C6—H6C109.5
N1—Si1—C1105.4 (2)Si2—C6'—H6D109.5
C3—Si1—C1109.6 (4)Si2—C6'—H6E109.5
C2—Si1—C1100.4 (5)H6D—C6'—H6E109.5
N1—Si2—C4'118.7 (6)Si2—C6'—H6F109.5
N1—Si2—C6121.9 (5)H6D—C6'—H6F109.5
N1—Si2—C6'110.4 (6)H6E—C6'—H6F109.5
C4'—Si2—C6'106.6 (9)
Symmetry codes: (i) −x, y, −z+1/2; (ii) −x+1/2, −y+1/2, −z+1.
Selected geometric parameters (Å, °) top
Li1—N11.9348 (11)N1—Si21.6738 (12)
K1—N12.8612 (11)N1—Si11.6798 (12)
N1—Li1—N1i176.4 (3)Si2—N1—Si1132.29 (7)
N1—K1—N1ii180.0Li1—N1—K1107.80 (9)
Symmetry codes: (i) −x, y, −z+1/2; (ii) −x+1/2, −y+1/2, −z+1.
Acknowledgements top

We gratefully acknowledge the Petroleum Research Fund (41716-AC3) and the University of Notre Dame for support. We also thank the National Science Foundation for instrument support (CHE-0443233).

references
References top

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