Crystal structures of 2-bromo-1,1,1,3,3,3-hexa­methyl-2-(tri­methyl­sil­yl)tris­ilane and 2-bromo-1,1,1,3,3,3-hexa­isopropyl-2-(triiso­propyl­sil­yl)tri­silane

Gulotty, E.M.; Staples, R.J.; Biros, S.M.; Gaspar, P.P.; Rath, N.P.; Winchester, W.R.

doi:10.1107/S2056989018009696

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Volume 74| Part 8| August 2018| Pages 1142-1146

https://doi.org/10.1107/S2056989018009696

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Crystal structures of 2-bromo-1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilane and 2-bromo-1,1,1,3,3,3-hexaisopropyl-2-(triisopropylsilyl)trisilane

Eva M. Gulotty,^a Richard J. Staples,^b Shannon M. Biros,^a Peter P. Gaspar,^c Nigam P. Rath ^d and William R. Winchester ^a ^*

^aDepartment of Chemistry, Grand Valley State University, 1 Campus Dr., Allendale, MI 49401, USA, ^bCenter for Crystallographic Research, Department of Chemistry, Michigan State, University, 578 S. Shaw Lane, East Lansing, MI, USA, ^cDepartment of Chemistry, Washington University, St. Louis, MO 63130-4899, USA, and ^dDepartment of Chemistry, University of Missouri St. Louis, St. Louis, MO 63121-4499, USA
^*Correspondence e-mail: winchesr@gvsu.edu

Edited by H. Stoeckli-Evans, University of Neuchâtel, Switzerland (Received 29 June 2018; accepted 6 July 2018; online 24 July 2018)

The synthesis and crystal structures of two tris(trialkylsilyl)silyl bromide compounds, C₉H₂₇BrSi₄ (I, HypSiBr) and C₂₇H₆₃BrSi₄ (II, TipSiBr), are described. Compound I was prepared in 85% yield by free-radical bromination of 1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilane using bromobutane and 2,2′-azobis(2-methylpropionitrile) as a radical initiator at 333 K. The molecule possesses threefold rotational symmetry, with the central Si atom and the Br atom being located on the threefold rotation axis. The Si—Br bond distance is 2.2990 (12) Å and the Si—Si bond lengths are 2.3477 (8) Å. The Br—Si—Si bond angles are 104.83 (3)° and the Si—Si—Si bond angles are 113.69 (2)°, reflecting the steric hindrance inherent in the three trimethylsilyl groups attached to the central Si atom. Compound II was prepared in 55% yield by free-radical bromination of 1,1,1,3,3,3-hexaisopropyl-2-(triisopropylsilyl)trisilane using N-bromosuccinimide and 2,2′-azobis(2-methylpropionitrile) as a radical initiator at 353 K. Here the Si—Br bond length is 2.3185 (7) Å and the Si—Si bond lengths range from 2.443 (1) to 2.4628 (9) Å. The Br—Si—Si bond angles range from 98.44 (3) to 103.77 (3)°, indicating steric hindrance between the three triisopropylsilyl groups.

Keywords: crystal structure; HypSiBr; TipSiBr; supersilyl bromide; steric hindrance; intramolecular C-H⋯Br hydrogen bonding.

CCDC references: 1854536; 1854535

1. Chemical context

The steric and electronic effects of the tris(trimethylsilyl)silane group have been exploited for the synthesis and study of a variety of reactive centers including silylenes (Wendel et al., 2017 ) and silylanions (Kayser et al., 2002 ; Mechtler et al., 2004 ; Zirngast et al., 2008 ; Marschner, 2015 ). This sterically hindered group has been shown to lead to lower coordination by solvent when it is attached to organolithium compounds (Feil & Harder, 2003 ). It has also been used in organic synthesis to produce highly stereoselective aldol reactions leading to unique reactivity (Gati & Yamamoto, 2016 ). For this research we prepared tris(trimethylsilyl)silylbromide (HypSiBr) as a precursor to vinyltris(trimethylsilyl)silane. The even bulkier tris(triisopropylsilyl)silylbromide (TipSiBr) was prepared as a potential precursor to methoxytris(triisopropylsilyl)silane. Herein, we report on the crystal structures of these two sterically hindered silylbromides 2-bromo-1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilane (I), and 2-bromo-1,1,1,3,3,3-hexaisopropyl-2-(triisopropylsilyl)trisilane (II).

2. Structural commentary

The molecular structure of compound I (HypSiBr), is shown in Fig. 1, and selected geometrical parameters are given in Table 1. The asymmetric unit is composed of one trimethylsilyl group, with the central silicon atom Si1 and the bromine atom Br1 lying on a threefold rotation axis. This supersilylbromide crystallized in the cubic space group Pa $[\overline{3}]$ with a central 4-coordinate silicon atom, Si1, that deviates slightly from an ideal tetrahedron due to the steric bulk of the attached trimethylsilyl (TMS) groups. The τ₄ descriptor for fourfold coordination around Si1 is 0.94 (where, for extreme forms, τ₄ = 0.00 for square-planar, 1.00 for tetrahedral and 0.85 for trigonal–pyramidal; Yang et al., 2007 ). Interestingly, the τ₄ descriptor for fourfold coordination around the TMS atom Si2 is 0.99, which demonstrates an ideal tetrahedral geometry around this silicon atom. The Si2—Si1—Si2^i,ii bond angle is 113.69 (2)° while the Br1—Si1—Si2 bond angle is 104.83 (3)°, indicating that the trimethylsilyl groups are forced away from one another. The Si1—Br1 bond length is 2.2990 (12) Å. As for Si2, the C—Si2—C bond angles range from 107.1 (2) to 110.55 (17)°, while the C—Si2—Si1 bond angles range from 108.61 (10) to 110.16 (11)°.

Table 1
Selected bond lengths (Å), bond angles (°) and the fourfold coordination descriptor, τ₄,^a for compounds I (HypSiBr), II (TipSiBr), III (HypSiCl) and IV (TipSiH)

Compound	I (HypSiBr)	II (TipSiBr)	III (HypSiCl)^b	IV (TipSiH)^c
Si1—X^d	2.2990 (12)	2.3185 (7)	2.1248 (9)	1.608 (1)
Si1—Sin^e	2.3477 (8)	2.4430 (10), 2.4448 (10), 2.4628 (9)	2.3406 (6)	2.405 (1)
Si2—Si1—Sin^f	113.69 (2)	115.02 (4), 116.55 (3), 116.59 (4)	113.13 (2)	117.9 (1)
Si2—Si1—X^d	104.83 (3)	98.44 (3) to 103.77 (3)	105.51 (2)	98.3 (1)
τ₄ of Si1	0.94	0.90	0.95	0.88
Notes: (a) Yang et al. (2007); (b) Kuzora et al. (2009); (c) X-ray data (Gaspar et al., 1999); (d) X = Br for I and II, Cl for III, and H for IV; (e) n = 2 for I, III and IV, and 2, 3 and 4 for II; (f) n = 2ⁱ and 2ⁱⁱ for I, III and IV [symmetry codes: (i) = z, x, y; (ii) = y, z, x], and 2, 3 and 4 for II.

Figure 1
A view of the molecular structure of compound I, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. Unlabelled atoms are related to the labelled atoms by threefold rotation symmetry [symmetry codes: (i) z, x, y; (ii) y, z, x].

The asymmetric unit of compound II (TipSiBr), is shown in Fig. 2, and selected geometrical parameters are given in Table 1. This compound crystallized in the triclinic space group P $[\overline{1}]$ with a central four-coordinate silicon atom, Si1, that deviates from the ideal tetrahedron as shown from its τ₄ descriptor for fourfold coordination of 0.90. The Br1—Si1—Si2/Si3/Si4 bond angles range from 98.44 (3) to 103.77 (3)°, and the Si1—Br1 bond distance is 2.3185 (7) Å, which is longer than that of compound I [2.2990 (12) Å]. The τ₄ descriptor values for atoms Si2, Si3 and Si4 (the silicon atoms of the triisopropylsilyl groups) are 0.96, 0.97 and 0.95, respectively, indicating that their coordination geometry is closest to an ideal tetrahedron.

Figure 2
A view of the molecular structure of compound II, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level, and all hydrogen atoms have been omitted for clarity.

3. Supramolecular features

There are no significant intermolecular contacts, other than weak van der Waals interactions, present in the crystals of compounds I or II. Compound II, however, contains four intramolecular C—H⋯Br hydrogen bonds (Table 2, Fig. 3). These hydrogen bonds contain D⋯A distances that range from from 3.584 (3) to 3.726 (3) Å, and D—H⋯A bond angles that range from 131 to 155°.

Table 2
Hydrogen-bond geometry (Å, °) for II

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C5—H5B⋯Br1	0.98	2.80	3.711 (3)	155
C16—H16⋯Br1	1.00	2.84	3.584 (3)	131
C23—H23A⋯Br1	0.98	2.87	3.685 (3)	142
C24—H24C⋯Br1	0.98	2.93	3.726 (3)	139

Figure 3
Intramolecular C—H⋯Br hydrogen bonds (blue dashed lines; see Table 2

) present in compound II. For clarity, only the hydrogen atoms involved in a hydrogen bonding are shown.

4. Database survey

The Cambridge Structural Database (CSD, version 5.39, February 2018; Groom et al., 2016 ) contains 1398 structures containing a Si₃Si group. Of these, there are only 42 structures where the central silicon atom is bonded directly to a halogen.

Of particular interest to this work is the structure of tris(trimethylsilyl)chlorosilane (III, HypSiCl) [CSD refcode QULWEA; Kuzora et al., 2009 ], the isotypic chloro derivative of compound I, and the structure of (iPr₃Si)₃SiH (IV, TipSiH), isotypic with compounds I and III. The analysis of IV by both X-ray and neutron diffraction has been described by Gaspar et al. (1999 ). Table 1 contains pertinent bond lengths and bond angles for compounds I, II, III (HypSiCl) and IV (TipSiH).

For compounds I and III the Si—X bond lengths follow the expected trend with the Si1—Cl bond length of QULWEA at 2.1248 (9) Å compared to the Si1—Br1 bond length of 2.2990 (12) Å for compound I. The Si1—Si2 bond length of the bromo derivative I reported here is 2.3477 (8) Å, which is slightly longer than the Si1—Si2 bond length of the chloro derivative at 2.3406 (6) Å. The central silicon atom of the chloro derivative appears less sterically hindered with an Si2—Si1—Cl1 bond angle of 105.508 (18)° and Si2—Si1—Si2^i,ii bond angles of 113.126°, versus a smaller Si2—Si1—Br1 bond angle of 104.83 (3)° and a larger Si2—Si1—Si2^i,ii bond angle of 113.69 (2)° for compound I [symmetry codes: (i) z, x, y; (ii) y, z, x]. The protio derivative (HypSiH) is a liquid at room temperature, and the structure of the iodo derivative (HypSiI) has not been deposited in the CSD.

The X-ray data for compound IV (TipSiH) was not found in the CSD, but the journal article (Gaspar et al., 1999) contains all pertinent structural data to allow for a comparison with (iPr₃Si)₃SiBr, viz. compound II (TipSiBr). Like compounds I and III, compound IV crystallizes in the cubic space group Pa $[\overline{3}]$ , and the molecule possesses threefold rotation symmetry. The presence of a small hydrogen atom bonded to the central silicon atom Si1 allows the three (ⁱPr₃)Si– groups to push further away from one another, resulting in Si2—Si1—Si2^i,ii bond angles of 117.9 (1)° and Si2^i,ii—Si1—H bond angles of 98.3 (1)° [symmetry codes: (i) z, x, y; (ii) y, z, x]. In II, the corresponding Si—Si—Si bond angles range from 115.02 (4) to 116.59 (4)° and the Si—Si—Br bond angles vary from 98.44 (3) to 103.77 (3)°.

5. Synthesis and crystallization

Compound I: Tris(trimethylsilyl)silane (2.0 g, 8.0 mmol) was added to an oven-dried nitrogen-flushed 250 ml Schlenk flask with a stir-bar. Bromobutane (2.0 g, 14.6 mmol) was filtered through a plug of silica gel in a Pasteur pipette and was transferred into the Schlenk flask. AIBN [2,2-azobis(2-methylpropionitrile); 20 mg] was then added to the flask, and the reaction was heated to 333 K using an oil bath and then heating was stopped. After stirring the reaction overnight at room temperature, GC–MS analysis of a sample indicated incomplete reaction and more AIBN (11 mg) was added to the flask. The reaction was heated once more to 333 K for 1 h. Analysis by GC–MS now indicated that the reaction was complete. The flask was placed in a freezer at 243 K and colourless block-like crystals of I formed overnight. Removal of the solvent in vacuo yielded 2.2 g (85%). ¹H NMR (300 MHz, chloroform-d) δ 0.24 (s, 27H); ¹³C NMR (75 MHz, chloroform-d) δ −0.51 ppm; GC–MS: 11.24 min, m/z = 328, base peak: 73.

Compound II: Tris(triisopropylsilyl)silane (110 mg, 0.22 mmol) was dissolved in freshly distilled benzene (10 ml) along with NBS (45 mg) and AIBN (2 mg, initiator). The mixture was heated using an oil bath at 333 K for 30 min, when GC–MS analysis indicated that no reaction had occurred. At this point the solution was heated with a heat gun until the reaction mixture turned slightly yellow. The yellow colour dissipated in less than 1 min. Analysis of the reaction mixture by ¹H NMR indicated that only 60% of the starting material had been consumed. An additional amount of NBS (N-bromosuccinimide; 20 mg) was added to the reaction flask, and the solution was again heated with a heat gun. The product was isolated by removing the solvent in vacuo and extracting the product from the crude reaction mixture with pentane. The pentane solution was filtered through glass wool, concentrated and weighed (135 mg). Analysis of the product with ¹H NMR indicated this was 90% pure. The product was further purified by dissolving this solid in 1 ml pentane, cooling to 195 K and isolating the colourless needle-like crystals of II by removing the solvent with a syringe, washing with pentane and drying in vacuo (yield 62 mg, 55%). ¹H NMR (300 MHz, C₆D₆) δ 1.34 (d, J = 7.3 Hz, 54H), 1.66 (heptet, J = 7.4 Hz, 9H); ¹³C NMR (75 MHz, chloroform-d) δ 16.4, 21.6; HRMS for C₁₇H₆₃BrSi₄ calculated 535.2642 (M − C₃H₇), found 535.2641.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. For both compounds the hydrogen atoms were placed in calculated positions and refined using a riding model: C—H = 0.98-1.00 Å with U_iso(H) = 1.5U_eq(C-methyl) and 1.2U_eq(C) for other H atoms.

Table 3
Experimental details

	I	II
Crystal data
Chemical formula	C₉H₂₇BrSi₄	C₂₇H₆₃BrSi₄
M_r	327.57	580.04
Crystal system, space group	Cubic, Pa $[\overline{3}]$	Triclinic, P $[\overline{1}]$
Temperature (K)	173	173
a, b, c (Å)	15.6497 (19), 15.6497 (19), 15.6497 (19)	8.4412 (4), 11.1336 (6), 18.8477 (10)
α, β, γ (°)	90, 90, 90	92.565 (4), 90.527 (4), 108.718 (4)
V (Å³)	3832.8 (14)	1675.44 (15)
Z	8	2
Radiation type	Mo Kα	Mo Kα
μ (mm⁻¹)	2.37	1.38
Crystal size (mm)	0.45 × 0.24 × 0.14	0.38 × 0.10 × 0.02

Data collection
Diffractometer	Bruker APEXII CCD	Bruker SMART APEX CCD area detector
Absorption correction	Multi-scan (SADABS; Bruker, 2014 )	Multi-scan (SADABS; Bruker, 2014)
T_min, T_max	0.571, 0.745	0.554, 0.674
No. of measured, independent and observed [I > 2σ(I)] reflections	11038, 1174, 953	21505, 6628, 4752
R_int	0.046	0.057
(sin θ/λ)_max (Å⁻¹)	0.601	0.622

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.028, 0.075, 1.05	0.037, 0.075, 1.01
No. of reflections	1174	6628
No. of parameters	46	307
H-atom treatment	H-atom parameters constrained	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.31, −0.16	0.36, −0.28
Computer programs: APEX2, SMART and SAINT (Bruker, 2014), SHELXS97 and SHELXTL (Sheldrick, 2008 ), SHELXT2013 (Sheldrick, 2015a ), SHELXL2017 (Sheldrick, 2015b ), OLEX2 (Dolomanov et al., 2009 ; Bourhis et al., 2015 ) and CrystalMaker (Palmer, 2007 ).

Supporting information

CCDC references: 1854536; 1854535

Crystal structure: contains datablocks I, II, Global. DOI: https://doi.org/10.1107/S2056989018009696/su5451sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989018009696/su5451Isup2.hkl

Structure factors: contains datablock II. DOI: https://doi.org/10.1107/S2056989018009696/su5451IIsup3.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2056989018009696/su5451Isup4.cml

Supporting information file. DOI: https://doi.org/10.1107/S2056989018009696/su5451IIsup5.cml

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2014) for (I); SMART (Bruker, 2014) for (II). For both structures, cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014). Program(s) used to solve structure: SHELXS97 (Sheldrick, 2008) for (I); SHELXT2013 (Sheldrick, 2015a) for (II). Program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015b) for (I); SHELXL2014 (Sheldrick, 2015b) for (II). Molecular graphics: OLEX2 (Dolomanov et al., 2009; Bourhis et al., 2015) for (I); SHELXTL (Sheldrick, 2008) for (II). Software used to prepare material for publication: CrystalMaker (Palmer, 2007) for (I); SHELXTL (Sheldrick, 2008) for (II).

2-Bromo-1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilane (I) top

Crystal data top

C₉H₂₇BrSi₄	Mo Kα radiation, λ = 0.71073 Å
M_r = 327.57	Cell parameters from 3863 reflections
Cubic, Pa3	θ = 2.3–25.3°
a = 15.6497 (19) Å	µ = 2.37 mm⁻¹
V = 3832.8 (14) Å³	T = 173 K
Z = 8	Block, colourless
F(000) = 1376	0.45 × 0.24 × 0.14 mm
D_x = 1.135 Mg m⁻³

Data collection top

Bruker APEXII CCD diffractometer	953 reflections with I > 2σ(I)
φ and ω scans	R_int = 0.046
Absorption correction: multi-scan (SADABS; Bruker, 2014)	θ_max = 25.3°, θ_min = 2.3°
T_min = 0.571, T_max = 0.745	h = −18→16
11038 measured reflections	k = −18→17
1174 independent reflections	l = −15→18

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)] = 0.028	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.075	H-atom parameters constrained
S = 1.05	w = 1/[σ²(F_o²) + (0.0349P)² + 1.4333P] where P = (F_o² + 2F_c²)/3
1174 reflections	(Δ/σ)_max = 0.002
46 parameters	Δρ_max = 0.31 e Å⁻³
0 restraints	Δρ_min = −0.16 e Å⁻³

Special details top

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Br1	0.74011 (2)	0.74011 (2)	0.74011 (2)	0.04337 (17)
Si1	0.65529 (4)	0.65529 (4)	0.65529 (4)	0.0327 (3)
Si2	0.69238 (5)	0.51472 (4)	0.69229 (5)	0.0473 (2)
C1	0.6249 (2)	0.43945 (18)	0.6286 (2)	0.0734 (10)
H1A	0.6331	0.4506	0.5675	0.110*
H1B	0.5646	0.4478	0.6434	0.110*
H1C	0.6415	0.3805	0.6414	0.110*
C2	0.8079 (2)	0.49575 (19)	0.6707 (3)	0.0943 (14)
H2A	0.8227	0.4370	0.6866	0.141*
H2B	0.8422	0.5358	0.7044	0.141*
H2C	0.8194	0.5044	0.6098	0.141*
C3	0.6752 (3)	0.49746 (19)	0.8090 (2)	0.0953 (13)
H3A	0.6142	0.5032	0.8222	0.143*
H3B	0.7076	0.5401	0.8414	0.143*
H3C	0.6946	0.4401	0.8247	0.143*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br1	0.04337 (17)	0.04337 (17)	0.04337 (17)	−0.00621 (10)	−0.00621 (10)	−0.00621 (10)
Si1	0.0327 (3)	0.0327 (3)	0.0327 (3)	−0.0008 (3)	−0.0008 (3)	−0.0008 (3)
Si2	0.0529 (5)	0.0317 (4)	0.0573 (5)	−0.0013 (3)	−0.0106 (4)	0.0008 (3)
C1	0.087 (2)	0.0426 (16)	0.091 (2)	−0.0092 (16)	−0.0304 (19)	−0.0051 (16)
C2	0.056 (2)	0.0494 (19)	0.178 (4)	0.0145 (16)	−0.007 (2)	0.001 (2)
C3	0.168 (4)	0.052 (2)	0.066 (2)	0.006 (2)	−0.019 (2)	0.0183 (17)

Geometric parameters (Å, º) top

Br1—Si1	2.2990 (12)	Si2—C1	1.870 (3)
Si1—Si2ⁱ	2.3478 (8)	Si2—C2	1.862 (3)
Si1—Si2	2.3477 (8)	Si2—C3	1.866 (3)
Si1—Si2ⁱⁱ	2.3478 (8)

Br1—Si1—Si2	104.83 (3)	C1—Si2—Si1	108.61 (10)
Br1—Si1—Si2ⁱⁱ	104.83 (3)	C2—Si2—Si1	110.16 (11)
Br1—Si1—Si2ⁱ	104.83 (3)	C2—Si2—C1	110.55 (17)
Si2—Si1—Si2ⁱⁱ	113.69 (2)	C2—Si2—C3	107.12 (19)
Si2—Si1—Si2ⁱ	113.69 (3)	C3—Si2—Si1	109.96 (10)
Si2ⁱ—Si1—Si2ⁱⁱ	113.69 (2)	C3—Si2—C1	110.45 (16)

Symmetry codes: (i) y, z, x; (ii) z, x, y.

2-Bromo-1,1,1,3,3,3-hexaisopropyl-2-(triisopropylsilyl)trisilane (II) top

Crystal data top

C₂₇H₆₃BrSi₄	Z = 2
M_r = 580.04	F(000) = 632
Triclinic, P1	D_x = 1.150 Mg m⁻³
a = 8.4412 (4) Å	Mo Kα radiation, λ = 0.71073 Å
b = 11.1336 (6) Å	Cell parameters from 8192 reflections
c = 18.8477 (10) Å	θ = 2.0–26.0°
α = 92.565 (4)°	µ = 1.38 mm⁻¹
β = 90.527 (4)°	T = 173 K
γ = 108.718 (4)°	Needles, colourless
V = 1675.44 (15) Å³	0.38 × 0.10 × 0.02 mm

Data collection top

Bruker SMART APEX CCD area detector diffractometer	6628 independent reflections
Radiation source: sealed tube	4752 reflections with I > 2σ(I)
Detector resolution: 8 pixels mm^-1	R_int = 0.057
ω and φ scans	θ_max = 26.3°, θ_min = 1.1°
Absorption correction: multi-scan (SADABS; Bruker, 2014)	h = −10→10
T_min = 0.554, T_max = 0.674	k = −13→13
21505 measured reflections	l = −23→23

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)] = 0.037	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.075	H-atom parameters constrained
S = 1.01	w = 1/[σ²(F_o²) + (0.0274P)² + 0.3128P] where P = (F_o² + 2F_c²)/3
6628 reflections	(Δ/σ)_max = 0.001
307 parameters	Δρ_max = 0.36 e Å⁻³
0 restraints	Δρ_min = −0.28 e Å⁻³

Special details top

Refinement. All H atoms were positioned geometrically and refined using a riding model with C—H = 0.95–0.99 Å and wAith U_iso(H) = 1.2 (1.5 for methyl groups) times U_eq(C).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Br1	1.06221 (3)	0.19737 (3)	0.24790 (2)	0.02768 (9)
Si1	0.77723 (8)	0.16573 (6)	0.25116 (3)	0.01926 (16)
Si2	0.72101 (8)	0.26469 (7)	0.14487 (4)	0.02137 (17)
Si3	0.67829 (9)	−0.06634 (7)	0.24791 (4)	0.02310 (17)
Si4	0.74993 (9)	0.27556 (7)	0.36447 (4)	0.02177 (17)
C1	0.8269 (3)	0.4443 (2)	0.15761 (13)	0.0249 (6)
H1	0.7549	0.4757	0.1905	0.030*
C2	1.0018 (3)	0.4866 (3)	0.19270 (15)	0.0349 (7)
H2A	1.0777	0.4575	0.1626	0.052*
H2B	0.9962	0.4498	0.2393	0.052*
H2C	1.0435	0.5794	0.1987	0.052*
C3	0.8371 (4)	0.5176 (3)	0.08928 (14)	0.0335 (7)
H3A	0.8753	0.6091	0.1015	0.050*
H3B	0.7262	0.4931	0.0660	0.050*
H3C	0.9162	0.4972	0.0570	0.050*
C4	0.7935 (3)	0.1980 (3)	0.06084 (12)	0.0266 (6)
H4	0.7464	0.1034	0.0618	0.032*
C5	0.9832 (3)	0.2301 (3)	0.05683 (14)	0.0371 (7)
H5A	1.0101	0.1830	0.0161	0.056*
H5B	1.0268	0.2065	0.1006	0.056*
H5C	1.0345	0.3214	0.0513	0.056*
C6	0.7282 (4)	0.2347 (3)	−0.00855 (13)	0.0388 (7)
H6A	0.7814	0.3257	−0.0146	0.058*
H6B	0.6067	0.2155	−0.0066	0.058*
H6C	0.7549	0.1862	−0.0487	0.058*
C7	0.4836 (3)	0.2270 (2)	0.13810 (14)	0.0287 (6)
H7	0.4439	0.2136	0.1879	0.034*
C8	0.4264 (4)	0.3368 (3)	0.11280 (17)	0.0429 (8)
H8A	0.4536	0.3500	0.0627	0.064*
H8B	0.4841	0.4146	0.1414	0.064*
H8C	0.3053	0.3155	0.1182	0.064*
C9	0.3878 (3)	0.1029 (3)	0.09537 (15)	0.0394 (7)
H9A	0.2692	0.0772	0.1073	0.059*
H9B	0.4342	0.0360	0.1073	0.059*
H9C	0.3991	0.1169	0.0444	0.059*
C10	0.6700 (3)	−0.1257 (3)	0.15145 (13)	0.0304 (6)
H10	0.6105	−0.0769	0.1245	0.036*
C11	0.8446 (4)	−0.0949 (3)	0.11986 (15)	0.0402 (8)
H11A	0.9038	−0.1481	0.1406	0.060*
H11B	0.9078	−0.0052	0.1305	0.060*
H11C	0.8337	−0.1118	0.0683	0.060*
C12	0.5697 (4)	−0.2671 (3)	0.13666 (15)	0.0423 (8)
H12A	0.5798	−0.2910	0.0866	0.063*
H12B	0.4517	−0.2814	0.1470	0.063*
H12C	0.6138	−0.3189	0.1669	0.063*
C13	0.4641 (3)	−0.1251 (2)	0.28895 (14)	0.0278 (6)
H13	0.4741	−0.0754	0.3352	0.033*
C14	0.3289 (4)	−0.0951 (3)	0.24525 (16)	0.0426 (8)
H14A	0.3107	−0.1440	0.1997	0.064*
H14B	0.3651	−0.0041	0.2367	0.064*
H14C	0.2244	−0.1178	0.2714	0.064*
C15	0.4016 (4)	−0.2660 (3)	0.30713 (16)	0.0402 (7)
H15A	0.2967	−0.2836	0.3327	0.060*
H15B	0.4859	−0.2842	0.3371	0.060*
H15C	0.3827	−0.3196	0.2632	0.060*
C16	0.8392 (3)	−0.1171 (2)	0.29934 (14)	0.0292 (6)
H16	0.9513	−0.0608	0.2850	0.035*
C17	0.8383 (4)	−0.2537 (3)	0.28398 (15)	0.0383 (7)
H17A	0.9229	−0.2705	0.3143	0.057*
H17B	0.8637	−0.2649	0.2340	0.057*
H17C	0.7277	−0.3129	0.2938	0.057*
C18	0.8317 (4)	−0.0933 (3)	0.38032 (15)	0.0461 (8)
H18A	0.7258	−0.1494	0.3976	0.069*
H18B	0.8392	−0.0047	0.3909	0.069*
H18C	0.9252	−0.1109	0.4039	0.069*
C19	0.5967 (3)	0.1584 (2)	0.42272 (13)	0.0290 (6)
H19	0.6217	0.0766	0.4166	0.035*
C20	0.4130 (4)	0.1287 (3)	0.40018 (18)	0.0467 (8)
H20A	0.3450	0.0522	0.4228	0.070*
H20B	0.4012	0.1145	0.3484	0.070*
H20C	0.3751	0.2003	0.4149	0.070*
C21	0.6171 (4)	0.1952 (3)	0.50277 (14)	0.0486 (9)
H21A	0.5929	0.2749	0.5118	0.073*
H21B	0.7321	0.2064	0.5185	0.073*
H21C	0.5392	0.1280	0.5290	0.073*
C22	0.9557 (3)	0.3395 (2)	0.41777 (13)	0.0286 (6)
H22	0.9246	0.3756	0.4631	0.034*
C23	1.0335 (4)	0.2414 (3)	0.44175 (15)	0.0397 (7)
H23A	1.0703	0.2024	0.4002	0.059*
H23B	0.9504	0.1757	0.4669	0.059*
H23C	1.1299	0.2831	0.4736	0.059*
C24	1.0893 (3)	0.4526 (3)	0.38677 (14)	0.0364 (7)
H24A	1.1806	0.4887	0.4218	0.055*
H24B	1.0390	0.5175	0.3750	0.055*
H24C	1.1334	0.4236	0.3437	0.055*
C25	0.6831 (3)	0.4212 (2)	0.34999 (13)	0.0258 (6)
H25	0.7695	0.4766	0.3192	0.031*
C26	0.6873 (4)	0.5007 (3)	0.41963 (14)	0.0450 (8)
H26A	0.6597	0.5772	0.4092	0.067*
H26B	0.7994	0.5251	0.4417	0.067*
H26C	0.6053	0.4502	0.4522	0.067*
C27	0.5152 (4)	0.3988 (3)	0.31189 (15)	0.0392 (7)
H27A	0.4243	0.3572	0.3434	0.059*
H27B	0.5082	0.3445	0.2688	0.059*
H27C	0.5050	0.4803	0.2990	0.059*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br1	0.02088 (14)	0.03279 (18)	0.02927 (15)	0.00846 (11)	0.00155 (11)	0.00149 (12)
Si1	0.0195 (4)	0.0190 (4)	0.0191 (4)	0.0060 (3)	0.0012 (3)	0.0007 (3)
Si2	0.0215 (4)	0.0220 (4)	0.0204 (4)	0.0066 (3)	0.0003 (3)	0.0017 (3)
Si3	0.0256 (4)	0.0192 (4)	0.0247 (4)	0.0076 (3)	0.0019 (3)	0.0009 (3)
Si4	0.0255 (4)	0.0205 (4)	0.0195 (4)	0.0075 (3)	0.0028 (3)	0.0008 (3)
C1	0.0272 (14)	0.0233 (15)	0.0229 (14)	0.0060 (12)	0.0038 (11)	0.0043 (11)
C2	0.0326 (16)	0.0281 (17)	0.0391 (17)	0.0031 (13)	−0.0053 (13)	0.0016 (13)
C3	0.0416 (18)	0.0255 (16)	0.0336 (16)	0.0103 (13)	0.0054 (13)	0.0078 (13)
C4	0.0325 (15)	0.0280 (16)	0.0199 (13)	0.0110 (12)	−0.0005 (11)	−0.0002 (11)
C5	0.0398 (18)	0.049 (2)	0.0266 (15)	0.0205 (15)	0.0080 (13)	0.0018 (14)
C6	0.0447 (18)	0.047 (2)	0.0228 (15)	0.0122 (15)	−0.0018 (13)	−0.0007 (14)
C7	0.0234 (15)	0.0325 (17)	0.0306 (15)	0.0091 (12)	−0.0023 (12)	0.0033 (13)
C8	0.0280 (16)	0.044 (2)	0.059 (2)	0.0136 (14)	−0.0057 (15)	0.0083 (16)
C9	0.0287 (16)	0.0406 (19)	0.0425 (18)	0.0022 (14)	−0.0059 (13)	0.0037 (14)
C10	0.0392 (17)	0.0255 (16)	0.0284 (15)	0.0138 (13)	0.0009 (13)	−0.0026 (12)
C11	0.053 (2)	0.0409 (19)	0.0339 (16)	0.0245 (15)	0.0101 (15)	0.0006 (14)
C12	0.055 (2)	0.0358 (19)	0.0371 (17)	0.0182 (16)	−0.0091 (15)	−0.0096 (14)
C13	0.0281 (15)	0.0207 (15)	0.0328 (15)	0.0053 (12)	0.0030 (12)	0.0029 (12)
C14	0.0303 (17)	0.044 (2)	0.052 (2)	0.0098 (14)	0.0021 (14)	0.0088 (15)
C15	0.0404 (18)	0.0301 (17)	0.0447 (18)	0.0030 (14)	0.0061 (14)	0.0067 (14)
C16	0.0312 (15)	0.0220 (16)	0.0371 (16)	0.0119 (12)	−0.0010 (12)	0.0045 (12)
C17	0.0431 (18)	0.0310 (18)	0.0456 (18)	0.0180 (14)	0.0007 (15)	0.0072 (14)
C18	0.062 (2)	0.042 (2)	0.0410 (18)	0.0268 (17)	−0.0133 (16)	−0.0041 (15)
C19	0.0364 (16)	0.0218 (16)	0.0287 (15)	0.0086 (12)	0.0091 (12)	0.0038 (12)
C20	0.0369 (18)	0.0338 (19)	0.069 (2)	0.0089 (14)	0.0210 (16)	0.0101 (16)
C21	0.075 (2)	0.036 (2)	0.0324 (17)	0.0132 (17)	0.0229 (16)	0.0069 (14)
C22	0.0327 (16)	0.0310 (17)	0.0212 (14)	0.0097 (13)	−0.0035 (12)	−0.0036 (12)
C23	0.0491 (19)	0.042 (2)	0.0312 (16)	0.0197 (15)	−0.0109 (14)	−0.0033 (14)
C24	0.0348 (17)	0.0362 (18)	0.0322 (16)	0.0041 (14)	−0.0043 (13)	−0.0040 (13)
C25	0.0347 (16)	0.0231 (15)	0.0211 (13)	0.0117 (12)	0.0018 (12)	−0.0006 (11)
C26	0.077 (2)	0.0380 (19)	0.0302 (16)	0.0336 (17)	−0.0023 (16)	−0.0064 (14)
C27	0.0388 (18)	0.0384 (19)	0.0464 (18)	0.0213 (14)	−0.0010 (14)	0.0007 (15)

Geometric parameters (Å, º) top

Br1—Si1	2.3185 (7)	C12—H12B	0.9800
Si1—Si2	2.4430 (10)	C12—H12C	0.9800
Si1—Si3	2.4448 (10)	C13—C14	1.531 (4)
Si1—Si4	2.4628 (9)	C13—C15	1.541 (4)
Si2—C4	1.908 (2)	C13—H13	1.0000
Si2—C7	1.913 (3)	C14—H14A	0.9800
Si2—C1	1.914 (3)	C14—H14B	0.9800
Si3—C10	1.899 (3)	C14—H14C	0.9800
Si3—C13	1.899 (3)	C15—H15A	0.9800
Si3—C16	1.903 (3)	C15—H15B	0.9800
Si4—C22	1.909 (3)	C15—H15C	0.9800
Si4—C25	1.910 (3)	C16—C17	1.533 (4)
Si4—C19	1.911 (3)	C16—C18	1.543 (4)
C1—C2	1.532 (3)	C16—H16	1.0000
C1—C3	1.543 (4)	C17—H17A	0.9800
C1—H1	1.0000	C17—H17B	0.9800
C2—H2A	0.9800	C17—H17C	0.9800
C2—H2B	0.9800	C18—H18A	0.9800
C2—H2C	0.9800	C18—H18B	0.9800
C3—H3A	0.9800	C18—H18C	0.9800
C3—H3B	0.9800	C19—C20	1.530 (4)
C3—H3C	0.9800	C19—C21	1.539 (4)
C4—C5	1.528 (4)	C19—H19	1.0000
C4—C6	1.536 (4)	C20—H20A	0.9800
C4—H4	1.0000	C20—H20B	0.9800
C5—H5A	0.9800	C20—H20C	0.9800
C5—H5B	0.9800	C21—H21A	0.9800
C5—H5C	0.9800	C21—H21B	0.9800
C6—H6A	0.9800	C21—H21C	0.9800
C6—H6B	0.9800	C22—C23	1.525 (4)
C6—H6C	0.9800	C22—C24	1.538 (4)
C7—C9	1.543 (4)	C22—H22	1.0000
C7—C8	1.544 (4)	C23—H23A	0.9800
C7—H7	1.0000	C23—H23B	0.9800
C8—H8A	0.9800	C23—H23C	0.9800
C8—H8B	0.9800	C24—H24A	0.9800
C8—H8C	0.9800	C24—H24B	0.9800
C9—H9A	0.9800	C24—H24C	0.9800
C9—H9B	0.9800	C25—C27	1.525 (4)
C9—H9C	0.9800	C25—C26	1.543 (3)
C10—C11	1.534 (4)	C25—H25	1.0000
C10—C12	1.541 (4)	C26—H26A	0.9800
C10—H10	1.0000	C26—H26B	0.9800
C11—H11A	0.9800	C26—H26C	0.9800
C11—H11B	0.9800	C27—H27A	0.9800
C11—H11C	0.9800	C27—H27B	0.9800
C12—H12A	0.9800	C27—H27C	0.9800

Br1—Si1—Si2	103.77 (3)	C10—C12—H12C	109.5
Br1—Si1—Si3	98.44 (3)	H12A—C12—H12C	109.5
Si2—Si1—Si3	116.55 (3)	H12B—C12—H12C	109.5
Br1—Si1—Si4	102.65 (3)	C14—C13—C15	109.5 (2)
Si2—Si1—Si4	115.02 (4)	C14—C13—Si3	112.65 (19)
Si3—Si1—Si4	116.59 (4)	C15—C13—Si3	116.31 (18)
C4—Si2—C7	108.70 (11)	C14—C13—H13	105.9
C4—Si2—C1	111.49 (12)	C15—C13—H13	105.9
C7—Si2—C1	109.69 (11)	Si3—C13—H13	105.9
C4—Si2—Si1	112.06 (9)	C13—C14—H14A	109.5
C7—Si2—Si1	106.40 (9)	C13—C14—H14B	109.5
C1—Si2—Si1	108.35 (8)	H14A—C14—H14B	109.5
C10—Si3—C13	111.22 (12)	C13—C14—H14C	109.5
C10—Si3—C16	109.44 (12)	H14A—C14—H14C	109.5
C13—Si3—C16	111.50 (12)	H14B—C14—H14C	109.5
C10—Si3—Si1	107.75 (9)	C13—C15—H15A	109.5
C13—Si3—Si1	109.78 (8)	C13—C15—H15B	109.5
C16—Si3—Si1	106.98 (9)	H15A—C15—H15B	109.5
C22—Si4—C25	104.78 (12)	C13—C15—H15C	109.5
C22—Si4—C19	106.42 (12)	H15A—C15—H15C	109.5
C25—Si4—C19	111.59 (12)	H15B—C15—H15C	109.5
C22—Si4—Si1	112.92 (8)	C17—C16—C18	108.9 (2)
C25—Si4—Si1	111.74 (8)	C17—C16—Si3	116.29 (19)
C19—Si4—Si1	109.24 (8)	C18—C16—Si3	112.56 (18)
C2—C1—C3	107.7 (2)	C17—C16—H16	106.1
C2—C1—Si2	115.52 (18)	C18—C16—H16	106.1
C3—C1—Si2	114.54 (18)	Si3—C16—H16	106.1
C2—C1—H1	106.1	C16—C17—H17A	109.5
C3—C1—H1	106.1	C16—C17—H17B	109.5
Si2—C1—H1	106.1	H17A—C17—H17B	109.5
C1—C2—H2A	109.5	C16—C17—H17C	109.5
C1—C2—H2B	109.5	H17A—C17—H17C	109.5
H2A—C2—H2B	109.5	H17B—C17—H17C	109.5
C1—C2—H2C	109.5	C16—C18—H18A	109.5
H2A—C2—H2C	109.5	C16—C18—H18B	109.5
H2B—C2—H2C	109.5	H18A—C18—H18B	109.5
C1—C3—H3A	109.5	C16—C18—H18C	109.5
C1—C3—H3B	109.5	H18A—C18—H18C	109.5
H3A—C3—H3B	109.5	H18B—C18—H18C	109.5
C1—C3—H3C	109.5	C20—C19—C21	109.0 (2)
H3A—C3—H3C	109.5	C20—C19—Si4	113.92 (19)
H3B—C3—H3C	109.5	C21—C19—Si4	114.62 (19)
C5—C4—C6	108.5 (2)	C20—C19—H19	106.2
C5—C4—Si2	114.13 (17)	C21—C19—H19	106.2
C6—C4—Si2	114.24 (19)	Si4—C19—H19	106.2
C5—C4—H4	106.5	C19—C20—H20A	109.5
C6—C4—H4	106.5	C19—C20—H20B	109.5
Si2—C4—H4	106.5	H20A—C20—H20B	109.5
C4—C5—H5A	109.5	C19—C20—H20C	109.5
C4—C5—H5B	109.5	H20A—C20—H20C	109.5
H5A—C5—H5B	109.5	H20B—C20—H20C	109.5
C4—C5—H5C	109.5	C19—C21—H21A	109.5
H5A—C5—H5C	109.5	C19—C21—H21B	109.5
H5B—C5—H5C	109.5	H21A—C21—H21B	109.5
C4—C6—H6A	109.5	C19—C21—H21C	109.5
C4—C6—H6B	109.5	H21A—C21—H21C	109.5
H6A—C6—H6B	109.5	H21B—C21—H21C	109.5
C4—C6—H6C	109.5	C23—C22—C24	110.6 (2)
H6A—C6—H6C	109.5	C23—C22—Si4	116.47 (19)
H6B—C6—H6C	109.5	C24—C22—Si4	115.61 (18)
C9—C7—C8	109.7 (2)	C23—C22—H22	104.1
C9—C7—Si2	115.43 (18)	C24—C22—H22	104.1
C8—C7—Si2	114.49 (19)	Si4—C22—H22	104.1
C9—C7—H7	105.4	C22—C23—H23A	109.5
C8—C7—H7	105.4	C22—C23—H23B	109.5
Si2—C7—H7	105.4	H23A—C23—H23B	109.5
C7—C8—H8A	109.5	C22—C23—H23C	109.5
C7—C8—H8B	109.5	H23A—C23—H23C	109.5
H8A—C8—H8B	109.5	H23B—C23—H23C	109.5
C7—C8—H8C	109.5	C22—C24—H24A	109.5
H8A—C8—H8C	109.5	C22—C24—H24B	109.5
H8B—C8—H8C	109.5	H24A—C24—H24B	109.5
C7—C9—H9A	109.5	C22—C24—H24C	109.5
C7—C9—H9B	109.5	H24A—C24—H24C	109.5
H9A—C9—H9B	109.5	H24B—C24—H24C	109.5
C7—C9—H9C	109.5	C27—C25—C26	108.5 (2)
H9A—C9—H9C	109.5	C27—C25—Si4	117.05 (18)
H9B—C9—H9C	109.5	C26—C25—Si4	112.35 (18)
C11—C10—C12	110.3 (2)	C27—C25—H25	106.0
C11—C10—Si3	112.34 (19)	C26—C25—H25	106.0
C12—C10—Si3	115.08 (19)	Si4—C25—H25	106.0
C11—C10—H10	106.1	C25—C26—H26A	109.5
C12—C10—H10	106.1	C25—C26—H26B	109.5
Si3—C10—H10	106.1	H26A—C26—H26B	109.5
C10—C11—H11A	109.5	C25—C26—H26C	109.5
C10—C11—H11B	109.5	H26A—C26—H26C	109.5
H11A—C11—H11B	109.5	H26B—C26—H26C	109.5
C10—C11—H11C	109.5	C25—C27—H27A	109.5
H11A—C11—H11C	109.5	C25—C27—H27B	109.5
H11B—C11—H11C	109.5	H27A—C27—H27B	109.5
C10—C12—H12A	109.5	C25—C27—H27C	109.5
C10—C12—H12B	109.5	H27A—C27—H27C	109.5
H12A—C12—H12B	109.5	H27B—C27—H27C	109.5

C13—Si3—C10—C11	−172.26 (19)	C10—Si3—C13—C14	−52.9 (2)
C16—Si3—C10—C11	−48.6 (2)	C16—Si3—C13—C14	−175.32 (19)
Si1—Si3—C10—C11	67.4 (2)	Si1—Si3—C13—C14	66.3 (2)
C13—Si3—C10—C12	−44.9 (2)	C10—Si3—C13—C15	74.7 (2)
C16—Si3—C10—C12	78.8 (2)	C16—Si3—C13—C15	−47.8 (2)
Si1—Si3—C10—C12	−165.25 (18)	Si1—Si3—C13—C15	−166.17 (17)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C5—H5B···Br1	0.98	2.80	3.711 (3)	155
C16—H16···Br1	1.00	2.84	3.584 (3)	131
C23—H23A···Br1	0.98	2.87	3.685 (3)	142
C24—H24C···Br1	0.98	2.93	3.726 (3)	139

Selected bond lengths (Å), bond angles (°) and the fourfold coordination descriptor, τ₄,^a for compounds I (HypSiBr), II (TipSiBr), III (HypSiCl) and IV (TipSiH) top

Compound	I (HypSiBr)	II (TipSiBr)	III (HypSiCl)^b	IV (TipSiH)^c
Si1—X^d	2.2990 (12)	2.3185 (7)	2.1248 (9)	1.608 (1)
Si1—Sin^e	2.3477 (8)	2.4430 (10), 2.4448 (10), 2.4628 (9)	2.3406 (6)	2.405 (1)
Si2—Si1—Sin^f	113.69 (2)	115.02 (4), 116.55 (3), 116.59 (4)	113.13 (2)	117.9 (1)
Si2—Si1—X^d	104.83 (3)	98.44 (3) to 103.77 (3)	105.51 (2)	98.3 (1)
τ₄ of Si1	0.94	0.90	0.95	0.88

Notes: (a) Yang et al. (2007); (b) Kuzora et al. (2009); (c) X-ray data (Gaspar et al., 1999); (d) X = Br for I and II, Cl for III, and H for IV; (e) n = 2 for I, III and IV, and 2, 3 and 4 for II; (f) n = 2ⁱ and 2ⁱⁱ for I, III and IV [symmetry codes: (i) = z, x, y; (ii) = y, z, x], and 2, 3 and 4 for II.

Acknowledgements

The authors thank GVSU for financial support (Weldon Fund, CSCE) and Pfizer, Inc. for the donation of a Varian Inova 400 F T NMR. We also thank Jim Krikke for help with instrumentation at GVSU. The CCD-based X-ray diffractometers at Michigan State University were upgraded and/or replaced by departmental funds.

Funding information

Funding for this research was provided by: National Science Foundation, Division of Chemistry (grant Nos. CHE-9108130, CHE-9632897, CCLI CHE-0087655, MRI CHE-1725699); Department of Energy (grant No. DE-AC02-98CH10886).

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