Bis[μ-N-(tert-butyl­dimethyl­silyl)-N-(pyridin-2-ylmeth­yl)amido]­bis­[methyl­cobalt(II)]

Malassa, A.; Agthe, C.; Görls, H.; Westerhausen, M.

doi:10.1107/S1600536812032321

metal-organic compounds

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ISSN: 2056-9890

Volume 68| Part 9| September 2012| Pages m1167-m1168

doi:10.1107/S1600536812032321

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Bis[μ-N-(tert-butyldimethylsilyl)-N-(pyridin-2-ylmethyl)amido]bis[methylcobalt(II)]

Astrid Malassa,^a Christine Agthe,^a Helmar Görls ^a and Matthias Westerhausen ^a ^*

^aInstitut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität, Jena, Humboldt-Strasse 8, 07743 Jena, Germany
^*Correspondence e-mail: m.we@uni-jena.de

(Received 6 July 2012; accepted 16 July 2012; online 11 August 2012)

The green title complex, [Co₂(CH₃)₂(C₁₂H₂₁N₂Si)₂], was obtained from bis{[μ-N-tert-butyldimethylsilyl-N-(pyridin-2-ylmethyl)amido]chloridocobalt(II)} and methyllithium in diethyl ether at 195 K via a metathesis reaction. The dimeric cobalt(II) complex exhibits a crystallographic center of inversion in the middle of the Co₂N₂ ring (average Co—N = 2.050 Å). The Co^II atom shows a distorted tetrahedral coordination sphere. The exocyclic Co—N bond length to the pyridyl group shows a similar value of 2.045 (4) Å. The exocyclic methyl group has a rather long Co—C bond length of 2.019 (5) Å.

Related literature

The metathetical conversion of a cobalt chloride functionality into a methyl cobalt fragment via the reaction with methyllithium was reported earlier for tetra-coordinate cobalt(II) complexes bound to three additional aza-bases, see: Au-Yeung et al. (2007 ); Bowman et al. (2010 ); Humphries et al. (2005 ); Kleigrewe et al. (2005 ), Wallenhorst et al. (2008 ). The synthesis of dialkyl cobalt complexes succeeds starting from hexa-coordinate [(L)₄CoCl₂] with L being a pyridyl base, see: Milani et al. (2003 ); Zhu et al. (2010 ). The coordination number of the final cobalt(II) complexes depends on intramolecular steric strain yielding hexa-coordinate [(bpy)₂CoMe₂] (bpy = 2,2′-bipyridine) and tetra-coordinate [(py)₂CoR₂] (R = CH₂C(Me₂)Ph). The formation of para-tolylcobalt complexes was reported by Zhu & Budzelaar (2010 ) who proposed a radical mechanism.

Experimental

Crystal data

[Co₂(CH₃)₂(C₁₂H₂₁N₂Si)₂]
M_r = 590.72
Triclinic, $[P \overline 1]$
a = 8.4751 (8) Å
b = 9.8055 (12) Å
c = 10.6130 (6) Å
α = 72.837 (6)°
β = 83.450 (6)°
γ = 69.216 (6)°
V = 787.81 (13) Å³
Z = 1
Mo Kα radiation
μ = 1.15 mm⁻¹
T = 183 K
0.06 × 0.06 × 0.04 mm

Data collection

Nonius KappaCCD diffractometer
5417 measured reflections
3551 independent reflections
1685 reflections with I > 2σ(I)
R_int = 0.074

Refinement

R[F² > 2σ(F²)] = 0.061
wR(F²) = 0.129
S = 0.92
3551 reflections
160 parameters
H-atom parameters constrained
Δρ_max = 0.39 e Å⁻³
Δρ_min = −0.39 e Å⁻³

Data collection: COLLECT (Nonius, 1998 ); cell refinement: DENZO (Otwinowski & Minor, 1997 ); data reduction: DENZO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL/PC (Sheldrick, 2008); software used to prepare material for publication: SHELXL97.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536812032321/im2393sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536812032321/im2393Isup2.hkl

checkCIF report

Comment top

Au-Yeung et al. (2007) performed a metathetical ligand substitution reaction at (tmeda)cobalt(II) 2,6-dimethylphenyl-N-trimethylsilylamide chloride (tmeda = tetramethylethylenediamine) with methyllithium in toluene. Whereas in this complex the cobalt(II) adopts a distorted tetrahedral coordination sphere, severe distortions were observed using tridentate aza-Lewis bases (Bowman et al., 2010; Humphries et al., 2005; Kleigrewe et al., 2005; Wallenhorst et al., 2008). Treatment of tetrakis(pyridine)cobalt(II) dichloride with trimethylsilylmethyllithium or 2-methyl-2-phenylpropyllithium in n-pentane yielded [(py)₂CoR₂] (R = CH₂SiMe₃)₂, CH₂C(Me₂)Ph), respectively, with tetra-coordinate cobalt centers (Zhu et al., 2010). Less bulky methyl groups allowed the formation of [(bpy)₂CoMe₂] with a hexa-coordinate cobalt atom in a slightly distorted octahedral environment (Milani et al., 2003). Contrary to these procedures, a radical mechanism was discussed by Zhu & Budzelaar (2010) for the formation of para-tolyl-cobalt complexes. Whereas all of these cobalt(II) complexes represent mononuclear derivatives, the reaction of bis[N-(pyidin-2-ylmethyl)-N-(tert-butyldimethylsilyl)amido cobalt(II) chloride] with methyllithium in tetrahydrofuran (THF) yielded the centrosymmetric dinuclear title compound 1 with a central planar Co₂N₂ ring.

Related literature top

The metathetical conversion of a cobalt chloride functionality into a methyl cobalt fragment via the reaction with LiR was reported earlier for tetra-coordinate cobalt(II) complexes bound to three additional aza-bases, see: Au-Yeung et al. (2007); Bowman et al. (2010); Humphries et al. (2005); Kleigrewe et al. (2005), Wallenhorst et al. (2008). The synthesis of dialkyl cobalt complexes succeeds starting from hexa-coordinate [(L)₄CoCl₂] with L being a pyridyl base, see: Milani et al. (2003); Zhu et al. (2010). The coordination number of the final cobalt(II) complexes depends on intramolecular steric strain yielding hexa-coordinate [(bpy)₂CoMe₂] (bpy = 2,2'-bipyridine) and tetra-coordinate [(py)₂CoR₂] (R = CH₂C(Me₂)Ph). The formation of para-tolylcobalt complexes was reported by Zhu & Budzelaar (2010) who proposed a radical mechanism.

Experimental top

Bis{chlorido-[N-(pyidin-2-ylmethyl)-N-(tert-butyldimethylsilyl)amido]cobalt(II)} (0.84 g, 1.32 mmol) was dissolved in 15 ml of THF and this solution cooled to -78 °C. Then 1.7 ml (2,72 mmol) of a 1,6M methyllithium solution in diethyl ether was added dropwise. A brown reaction solution formed which was warmed to ambient temperature and stirred for an additional hour. Thereafter all volatile materials were removed and the residue dried in vacuo. This residue was extracted with 15 ml of n-hexane. The volume of this solution was reduced to third of the original volume and cooled to -20 °C. Within several hours green rod-like crystals of 1 precipitated. Yield: 0.21 g (0.36 mmol, 27%).

Computing details top

Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL/PC (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top

	Fig. 1. Molecular structure and numbering scheme of the title compound 1; Displacement ellipsoids are at the 40% probability level. H atoms are neglected for clarity reasons. Symmetry-related atoms are marked with the letter i [symmetry code: (i) -x + 1, -y + 1, -z + 1].
	Fig. 2. Packing of the molecules by short ring-interactions (distance between the centroids of the aromatic rings 3.689 (3) Å).

Bis[µ-N-(tert-butyldimethylsilyl)-N-(pyridin-2- ylmethyl)amido]bis[methylcobalt(II)] top

Crystal data top

[Co₂(CH₃)₂(C₁₂H₂₁N₂Si)₂]	Z = 1
M_r = 590.72	F(000) = 314
Triclinic, P1	D_x = 1.245 Mg m⁻³
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 8.4751 (8) Å	Cell parameters from 5417 reflections
b = 9.8055 (12) Å	θ = 3.3–27.5°
c = 10.6130 (6) Å	µ = 1.15 mm⁻¹
α = 72.837 (6)°	T = 183 K
β = 83.450 (6)°	Prism, green
γ = 69.216 (6)°	0.06 × 0.06 × 0.04 mm
V = 787.81 (13) Å³

Data collection top

Nonius KappaCCD diffractometer	1685 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	R_int = 0.074
Graphite monochromator	θ_max = 27.5°, θ_min = 3.3°
phi– + ω–scan	h = −10→9
5417 measured reflections	k = −10→12
3551 independent reflections	l = −13→13

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.061	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.129	H-atom parameters constrained
S = 0.92	w = 1/[σ²(F_o²) + (0.0369P)²] where P = (F_o² + 2F_c²)/3
3551 reflections	(Δ/σ)_max < 0.001
160 parameters	Δρ_max = 0.39 e Å⁻³
0 restraints	Δρ_min = −0.39 e Å⁻³

Crystal data top

[Co₂(CH₃)₂(C₁₂H₂₁N₂Si)₂]	γ = 69.216 (6)°
M_r = 590.72	V = 787.81 (13) Å³
Triclinic, P1	Z = 1
a = 8.4751 (8) Å	Mo Kα radiation
b = 9.8055 (12) Å	µ = 1.15 mm⁻¹
c = 10.6130 (6) Å	T = 183 K
α = 72.837 (6)°	0.06 × 0.06 × 0.04 mm
β = 83.450 (6)°

Data collection top

Nonius KappaCCD diffractometer	1685 reflections with I > 2σ(I)
5417 measured reflections	R_int = 0.074
3551 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.061	0 restraints
wR(F²) = 0.129	H-atom parameters constrained
S = 0.92	Δρ_max = 0.39 e Å⁻³
3551 reflections	Δρ_min = −0.39 e Å⁻³
160 parameters

Special details top

Experimental. IR (in Nujol between KBr windows, cm^-1): = 1715 w, 1583 m, 1273 m, 1244 s, 1146 m, 1080 m, 1036 m, 1008 m, 889 m, 828 s, 770 m, 736 m. MS (DEI, rel. intensity in brackets): m/z = 501 ([M - CoMe₂]⁺, 11%), 165 ([Pyr-CH₂-NHSiMe₂]⁺, 100%). Elemental analysis (C₂₆H₄₈Co₂N₄Si₂, 590,72): calcd.: C 52.86, H 8.19, N 9.48; found: C 49.47, H 7.70, N 9.03 (the rather large deviations are caused by extreme sensitivity of the complex towards moisture and air; the low carbon value is a consequence of carbide and carbonate formation despite the fact that V₂O₅ was added prior to combustion).

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 > σ(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

	x	y	z	U_iso*/U_eq
Co1	0.50875 (8)	0.62393 (8)	0.52394 (5)	0.0360 (2)
Si1	0.36965 (16)	0.43232 (17)	0.77488 (11)	0.0373 (4)
N1	0.2891 (5)	0.7874 (4)	0.4452 (3)	0.0342 (10)
N2	0.3562 (4)	0.4948 (4)	0.6037 (3)	0.0306 (9)
C1	0.2668 (7)	0.9328 (6)	0.3756 (4)	0.0466 (14)
H1A	0.3599	0.9681	0.3636	0.056*
C2	0.1139 (7)	1.0311 (6)	0.3217 (4)	0.0524 (15)
H2A	0.1017	1.1325	0.2732	0.063*
C3	−0.0218 (7)	0.9803 (7)	0.3391 (5)	0.0570 (16)
H3A	−0.1275	1.0452	0.3003	0.068*
C4	−0.0012 (6)	0.8329 (6)	0.4142 (4)	0.0429 (13)
H4A	−0.0938	0.7967	0.4294	0.052*
C5	0.1560 (6)	0.7389 (6)	0.4668 (4)	0.0349 (12)
C6	0.1808 (5)	0.5827 (6)	0.5525 (4)	0.0387 (12)
H6A	0.1501	0.5256	0.5020	0.046*
H6B	0.1020	0.5892	0.6286	0.046*
C7	0.2682 (6)	0.5989 (6)	0.8446 (4)	0.0529 (15)
H7A	0.3243	0.6742	0.8080	0.079*
H7B	0.2788	0.5648	0.9408	0.079*
H7C	0.1485	0.6445	0.8213	0.079*
C8	0.5975 (6)	0.3468 (6)	0.8222 (4)	0.0504 (15)
H8A	0.6571	0.4165	0.7746	0.076*
H8B	0.6479	0.2504	0.7993	0.076*
H8C	0.6067	0.3288	0.9174	0.076*
C9	0.2621 (6)	0.2877 (6)	0.8564 (4)	0.0430 (13)
C10	0.0704 (6)	0.3488 (6)	0.8314 (5)	0.0594 (16)
H10A	0.0208	0.2719	0.8808	0.089*
H10B	0.0505	0.3722	0.7370	0.089*
H10C	0.0180	0.4411	0.8606	0.089*
C11	0.3389 (6)	0.1452 (6)	0.8085 (4)	0.0502 (14)
H11A	0.2790	0.0739	0.8499	0.075*
H11B	0.4584	0.0976	0.8324	0.075*
H11C	0.3288	0.1728	0.7125	0.075*
C12	0.2873 (7)	0.2393 (7)	1.0077 (4)	0.0700 (19)
H12A	0.2415	0.1575	1.0491	0.105*
H12B	0.2284	0.3263	1.0430	0.105*
H12C	0.4080	0.2034	1.0267	0.105*
C13	0.6200 (6)	0.7259 (7)	0.6082 (4)	0.0580 (16)
H13A	0.6269	0.8189	0.5442	0.087*
H13B	0.7339	0.6571	0.6363	0.087*
H13C	0.5532	0.7510	0.6850	0.087*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Co1	0.0333 (4)	0.0450 (5)	0.0363 (4)	−0.0195 (3)	0.0038 (3)	−0.0147 (3)
Si1	0.0355 (8)	0.0459 (10)	0.0285 (7)	−0.0122 (8)	0.0012 (5)	−0.0098 (6)
N1	0.039 (2)	0.033 (3)	0.033 (2)	−0.015 (2)	0.0103 (16)	−0.0130 (19)
N2	0.026 (2)	0.036 (3)	0.0285 (17)	−0.009 (2)	−0.0019 (15)	−0.0068 (16)
C1	0.051 (4)	0.038 (4)	0.050 (3)	−0.017 (3)	0.020 (3)	−0.015 (3)
C2	0.059 (4)	0.037 (4)	0.048 (3)	−0.010 (3)	0.011 (3)	−0.004 (3)
C3	0.051 (4)	0.050 (4)	0.054 (3)	0.000 (3)	−0.005 (3)	−0.010 (3)
C4	0.032 (3)	0.045 (4)	0.046 (3)	−0.009 (3)	0.001 (2)	−0.010 (3)
C5	0.037 (3)	0.041 (3)	0.024 (2)	−0.012 (3)	0.0019 (19)	−0.008 (2)
C6	0.038 (3)	0.044 (4)	0.037 (2)	−0.022 (3)	0.003 (2)	−0.007 (2)
C7	0.061 (4)	0.057 (4)	0.045 (3)	−0.018 (3)	0.003 (2)	−0.024 (3)
C8	0.045 (3)	0.070 (4)	0.038 (3)	−0.014 (3)	−0.007 (2)	−0.020 (3)
C9	0.044 (3)	0.042 (4)	0.029 (2)	−0.005 (3)	0.005 (2)	−0.003 (2)
C10	0.050 (4)	0.054 (4)	0.068 (3)	−0.022 (3)	0.016 (3)	−0.009 (3)
C11	0.048 (3)	0.042 (4)	0.056 (3)	−0.018 (3)	0.010 (2)	−0.007 (3)
C12	0.084 (4)	0.064 (5)	0.040 (3)	−0.019 (4)	0.001 (3)	0.009 (3)
C13	0.055 (4)	0.089 (5)	0.055 (3)	−0.047 (4)	0.013 (3)	−0.034 (3)

Geometric parameters (Å, º) top

Co1—C13	2.019 (5)	C6—H6B	0.9900
Co1—N2ⁱ	2.032 (3)	C7—H7A	0.9800
Co1—N1	2.045 (4)	C7—H7B	0.9800
Co1—N2	2.067 (4)	C7—H7C	0.9800
Co1—Co1ⁱ	2.6812 (14)	C8—H8A	0.9800
Si1—N2	1.741 (3)	C8—H8B	0.9800
Si1—C8	1.873 (4)	C8—H8C	0.9800
Si1—C7	1.877 (5)	C9—C11	1.528 (7)
Si1—C9	1.898 (5)	C9—C10	1.544 (6)
N1—C5	1.345 (5)	C9—C12	1.551 (6)
N1—C1	1.354 (6)	C10—H10A	0.9800
N2—C6	1.499 (5)	C10—H10B	0.9800
N2—Co1ⁱ	2.032 (3)	C10—H10C	0.9800
C1—C2	1.376 (6)	C11—H11A	0.9800
C1—H1A	0.9500	C11—H11B	0.9800
C2—C3	1.381 (7)	C11—H11C	0.9800
C2—H2A	0.9500	C12—H12A	0.9800
C3—C4	1.388 (7)	C12—H12B	0.9800
C3—H3A	0.9500	C12—H12C	0.9800
C4—C5	1.390 (6)	C13—H13A	0.9800
C4—H4A	0.9500	C13—H13B	0.9800
C5—C6	1.487 (6)	C13—H13C	0.9800
C6—H6A	0.9900

C13—Co1—N2ⁱ	119.40 (17)	N2—C6—H6B	108.5
C13—Co1—N1	105.8 (2)	H6A—C6—H6B	107.5
N2ⁱ—Co1—N1	112.97 (13)	Si1—C7—H7A	109.5
C13—Co1—N2	130.97 (16)	Si1—C7—H7B	109.5
N2ⁱ—Co1—N2	98.30 (12)	H7A—C7—H7B	109.5
N1—Co1—N2	84.19 (15)	Si1—C7—H7C	109.5
C13—Co1—Co1ⁱ	151.34 (17)	H7A—C7—H7C	109.5
N2ⁱ—Co1—Co1ⁱ	49.71 (10)	H7B—C7—H7C	109.5
N1—Co1—Co1ⁱ	102.57 (11)	Si1—C8—H8A	109.5
N2—Co1—Co1ⁱ	48.59 (10)	Si1—C8—H8B	109.5
N2—Si1—C8	108.95 (18)	H8A—C8—H8B	109.5
N2—Si1—C7	109.2 (2)	Si1—C8—H8C	109.5
C8—Si1—C7	108.4 (2)	H8A—C8—H8C	109.5
N2—Si1—C9	114.74 (19)	H8B—C8—H8C	109.5
C8—Si1—C9	108.4 (2)	C11—C9—C10	107.8 (4)
C7—Si1—C9	107.0 (2)	C11—C9—C12	107.5 (4)
C5—N1—C1	119.0 (4)	C10—C9—C12	107.5 (4)
C5—N1—Co1	113.7 (3)	C11—C9—Si1	111.1 (3)
C1—N1—Co1	127.3 (3)	C10—C9—Si1	113.3 (3)
C6—N2—Si1	114.5 (2)	C12—C9—Si1	109.4 (3)
C6—N2—Co1ⁱ	109.3 (2)	C9—C10—H10A	109.5
Si1—N2—Co1ⁱ	125.9 (2)	C9—C10—H10B	109.5
C6—N2—Co1	108.8 (3)	H10A—C10—H10B	109.5
Si1—N2—Co1	111.27 (17)	C9—C10—H10C	109.5
Co1ⁱ—N2—Co1	81.70 (12)	H10A—C10—H10C	109.5
N1—C1—C2	122.1 (5)	H10B—C10—H10C	109.5
N1—C1—H1A	118.9	C9—C11—H11A	109.5
C2—C1—H1A	118.9	C9—C11—H11B	109.5
C1—C2—C3	119.1 (5)	H11A—C11—H11B	109.5
C1—C2—H2A	120.4	C9—C11—H11C	109.5
C3—C2—H2A	120.4	H11A—C11—H11C	109.5
C2—C3—C4	119.0 (5)	H11B—C11—H11C	109.5
C2—C3—H3A	120.5	C9—C12—H12A	109.5
C4—C3—H3A	120.5	C9—C12—H12B	109.5
C3—C4—C5	119.4 (5)	H12A—C12—H12B	109.5
C3—C4—H4A	120.3	C9—C12—H12C	109.5
C5—C4—H4A	120.3	H12A—C12—H12C	109.5
N1—C5—C4	121.3 (4)	H12B—C12—H12C	109.5
N1—C5—C6	117.8 (4)	Co1—C13—H13A	109.5
C4—C5—C6	120.8 (4)	Co1—C13—H13B	109.5
C5—C6—N2	115.1 (4)	H13A—C13—H13B	109.5
C5—C6—H6A	108.5	Co1—C13—H13C	109.5
N2—C6—H6A	108.5	H13A—C13—H13C	109.5
C5—C6—H6B	108.5	H13B—C13—H13C	109.5

Symmetry code: (i) −x+1, −y+1, −z+1.

Experimental details

Crystal data
Chemical formula	[Co₂(CH₃)₂(C₁₂H₂₁N₂Si)₂]
M_r	590.72
Crystal system, space group	Triclinic, P1
Temperature (K)	183
a, b, c (Å)	8.4751 (8), 9.8055 (12), 10.6130 (6)
α, β, γ (°)	72.837 (6), 83.450 (6), 69.216 (6)
V (Å³)	787.81 (13)
Z	1
Radiation type	Mo Kα
µ (mm⁻¹)	1.15
Crystal size (mm)	0.06 × 0.06 × 0.04

Data collection
Diffractometer	Nonius KappaCCD diffractometer
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	5417, 3551, 1685
R_int	0.074
(sin θ/λ)_max (Å⁻¹)	0.650

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.061, 0.129, 0.92
No. of reflections	3551
No. of parameters	160
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.39, −0.39

Computer programs: COLLECT (Nonius, 1998), DENZO (Otwinowski & Minor, 1997), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL/PC (Sheldrick, 2008).

Acknowledgements

We thank the Deutsche Forschungsgemeinschaft (DFG, Bonn–Bad Godesberg, Germany) for generous financial support. We also acknowledge funding from the Fonds der Chemischen Industrie (Frankfurt/Main, Germany).

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