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Article

Hexacoordinate Silicon Compounds with a Dianionic Tetradentate (N,N′,N′,N)-Chelating Ligand

by
Daniela Gerlach
1,
Erica Brendler
2 and
Jörg Wagler
1,*
1
Institut für Anorganische Chemie, TU Bergakademie Freiberg, D-09596 Freiberg, Germany
2
Institut für Analytische Chemie, TU Bergakademie Freiberg, D-09596 Freiberg, Germany
*
Author to whom correspondence should be addressed.
Inorganics 2016, 4(2), 8; https://doi.org/10.3390/inorganics4020008
Submission received: 17 March 2016 / Revised: 6 April 2016 / Accepted: 7 April 2016 / Published: 14 April 2016
(This article belongs to the Special Issue Traversing the Boundaries of Inorganic Chemistry)
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Abstract

:
In the context of our systematic investigations of penta- and hexacoordinate silicon compounds, which included dianionic tri- (O,N,O′; O,N,N′) and tetradentate (O,N,N,O; O,N,N′,O′) chelators, we have now explored silicon coordination chemistry with a dianionic tetradentate (N,N′,N′,N) chelator. The ligand [o-phenylene-bis(pyrrole-2-carbaldimine), H2L] was obtained by condensation of o-phenylenediamine and pyrrole-2-carbaldehyde and subsequently silylated with chlorotrimethylsilane/triethylamine. Transsilylation of this ligand precursor (Me3Si)2L with chlorosilanes SiCl4, PhSiCl3, Ph2SiCl2, (Anis)2SiCl2 and (4-Me2N-C6H4)PhSiCl2 afforded the hexacoordinate Si complexes LSiCl2, LSiPhCl, LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4), respectively (Anis = anisyl = 4-methoxyphenyl). 29Si NMR spectroscopy and, for LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4), single-crystal X-ray diffraction confirm hexacoordination of the Si atoms. The molecular structures of LSiCl2 and LSiPhCl were elucidated by computational methods. Despite the two different N donor sites (pyrrole N, X-type donor; imine N, L-type donor), charge delocalization within the ligand backbone results in compounds with four similar Si–N bonds. Charge distribution within the whole molecules was analyzed by calculating the Natural Charges (NCs). Although these five compounds carry electronically different monodentate substituents, their constituents reveal rather narrow ranges of their charges (Si atoms: +2.10–+2.22; monodentate substituents: −0.54–−0.56; L2−: −1.02–−1.11).

Graphical Abstract

1. Introduction

The coordination number of tetravalent silicon can easily be enhanced (up to five or six) with the aid of monodentate or chelating ligands. Whereas the former preferentially bind to Si atoms that carry strongly electron withdrawing groups (e.g., formation of pyridine adducts of halosilanes, Scheme 1, A, [1,2,3]), the latter offer greater opportunities of creating five- and six-coordinate silicon compounds even in case of the absence of halides from the silicon coordination sphere (e.g., pentacoordinate silicon with SiC5 coordination sphere, B [4,5,6]; and hexacoordinate silicon with a tetradentate chelator and two Si–CH3 groups, C [7]). For various reasons, such as activation of Si–X bonds by silicon hypercoordination [8,9,10,11,12,13,14,15,16], exploring special electronic/optical properties arising from the higher coordination number of silicon in combination with selected ligands [17,18,19,20] or the aim of creating and exploring hitherto unusual Si coordination compounds, e.g., with transition metals [21,22,23,24,25,26,27,28,29] or very soft Lewis bases in their ligand sphere [30,31,32,33,34,35,36,37,38], silicon coordination chemistry continues to be an attractive research field, reflected by frequently published research articles [39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58] and reviews [59,60,61,62,63]. Thus, in the context of our systematic investigations of penta- and hexacoordinate silicon compounds with dianionic tri-(O,N,O′; O,N,N′) [20,64,65,66,67,68,69,70,71,72] and tetradentate (O,N,N,O; O,N,N′,O′) [7,8,9,10,11,18,19,73,74,75,76,77] chelators, we have now explored silicon coordination chemistry with a dianionic tetradentate (N,N′,N′,N) chelator. Thus far, macrocyclic π-conjugated (porphyrine- and phthalocyanine-type) ligands [78,79,80,81] have been explored in silicon coordination chemistry (e.g., D [81]), and our study aims at building a bridge between these cyclic (N,N,N,N)-chelators with essentially chemically equivalent donor atoms and “open chain” type chelators such as salen-type (O,N,N,O)-ligands (like the tetradentate ligand in C).

2. Results and Discussion

2.1. Syntheses

Based on our experience with pyrrolide as anionic anchoring group in chelating ligands for Si coordination chemistry [20,64,82,83,84], we studied the syntheses and molecular structures of silicon compounds with a pyrrole-2-carbaldimine functionalized (N,N′,N′,N)-chelating dianionic tetradentate ligand. This ligand, H2L, has already been reported in the literature [85] and was synthesized by condensation of o-phenylenediamine with two equivalents of pyrrole-2-carbaldehyde (Scheme 2). Prior to reacting with the chlorosilanes to be chelated upon substitution of two Cl atoms [SiCl4, PhSiCl3, Ph2SiCl2, (Anis)2SiCl2 and (4-Me2N-C6H4)PhSiCl2], H2L was converted into its bis(trimethylsilyl) derivative (Me3Si)2L, as its conversion with further chlorosilanes (in a transsilylation reaction) produces chlorotrimethylsilane as a liquid and volatile byproduct, which allows for easy separation from the target complex. Whereas SiCl4, PhSiCl3, and Ph2SiCl2 were commercially available, (Anis)2SiCl2 and (4-Me2N-C6H4)PhSiCl2 were prepared from SiCl4 and PhSiCl3, respectively, and suitable Grignard reagents (see Experimental Section).
Complexes LSiCl2, LSiPhCl, LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4) were obtained as orange solids that exhibited poor solubility in various organic solvents. Therefore, the emphasis of characterization is put on solid-state methods (29Si CP/MAS NMR spectroscopy, single-crystal X-ray diffraction) and computational methods.

2.2. Molecular Structures

The starting material (4-Me2N-C6H4)PhSiCl2 was obtained as a crystalline solid, therefore we determined its molecular structure in the solid state by single-crystal X-ray diffraction (Figure 1, Table 1). Because of the different substituents, the Si coordination sphere is distorted tetrahedral, with the C–Si–C angle (117.3°) significantly exceeding the tetrahedral angle. Interestingly, the Si1–C1 bond to the 4-dimethylaminophenyl group is slightly shorter than the Si1–C9 bond to the phenyl group.
The diaryl substituted silicon compounds LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4) crystallized from the reaction mixture (as THF solvates) and their molecular structures were thus determined by single-crystal X-ray diffraction analyses (Figure 2, Table 1). Selected bond lengths are listed in Table 2. As LSiCl2 and LSiPhCl were obtained as fine powders, even during slow formation in very dilute solutions of starting materials, and our attempts of growing crystals from other solvents failed, the molecular structures of LSiCl2, LSiPhCl and LSiPh2 were optimized (gas phase) at the DFT MPW1PW91/6-311G(d,p) level of theory (Figure 3). Compound LSiPh2 thus serves as our internal reference for the reliability of the computed molecular structure as an experimentally determined structure of this compound is available for comparison. Selected bond lengths of these molecules are also listed in Table 2.
In the solid-state compounds LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4) exhibit trans-disposed Si–C bonds, which appears reasonable because of the rigidity of the tetradentate chelator. Therefore, the molecular structures of LSiCl2, LSiPhCl and LSiPh2 were optimized starting from a model with trans-disposed monodentate substituents. Direct comparison of the bond lengths found for LSiPh2 in the solid state and predicted by computational methods shows satisfactory agreement, even though the computational analysis appears to underestimate the bond strength to the tetradentate ligand (it predicts longer Si–N bonds) and the weakening of the Si–C bonds resulting therefrom. Thus, we only interpret the trends observed in the optimized molecular structures rather than interpreting the absolute bond length values.
Both in the series of crystallographically determined molecular structures and in the series of computed structures we find systematic shortening of the Si–N bonds upon lowering the electron releasing power or increasing the electron withdrawing power of the monodentate substituents. Even though the experimental data confirm slightly longer Si–Nimine bonds for LSiPh2, whereas computational analysis predicts slightly longer Si–Npyrrole bonds, these two kinds of bonds exhibit only marginal bond length differences (despite their formally different character as dative bond from an imine N atom and a covalent bond from a pyrrole N atom). Thus, the tetradentate ligand under investigation reveals nearly perfect delocalization of its anionic charges over the chemically different nitrogen donor atoms (Scheme 3). In this regard, its imine-pyrrolide donor moiety, at least in hexacoordinate Si complexes, reflects the intermediate situation of what we had encountered with the pentacoordinate Si complexes E and F [64] (Scheme 3), each of which prefers one of the resonance contributions, depending on the situation of the imine and pyrrolide donor sites within the distorted trigonal bipyramidal coordination sphere (axial vs. equatorial positions). In this regard, compound LSiPh2 is related to (tetraphenylporphyrine)SiPh2 [79], which also has a delocalized π-system and similar Si–N bond lengths. The Si–C bonds in LSiPh2 (1.98 Å), however, are slightly longer than those in the porphyrine complex (1.95 Å), whereas the Si–N bonds (1.89, 1.91 Å) are shorter than in the porphyrine complex (1.97 Å). We attribute this difference (shorter Si–N bonds in LSiPh2 and pronounced Si–C bond lengthening resulting therefrom) to the smaller chelate size, i.e., five-membered rings in LSiPh2, whereas in porphyrin complexes six-membered rings are encountered.
Moreover, for the two complexes bearing two different monodentate substituents (LSiPh(4-Me2N-C6H4) and LSiPhCl) we observe pronounced bond strengthening of the Si–C bond to the more electron releasing group, whereas the bond to the trans-disposed rather electron withdrawing group (Ph in LSiPh(4-Me2N-C6H4) , Cl in LSiPhCl) is significantly weakened relative to the corresponding bonds in the symmetrically substituted complexes LSiPh2 and LSiCl2, respectively.

2.3. 29Si NMR

Compounds LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4) exhibited sufficient solubility in DMSO for NMR spectroscopic investigation. Their 29Si NMR shifts (δ −165.3, −165.1, and −164.4 ppm, respectively) are very similar to one another and are characteristic of hexacoordinate Si complexes. Moreover, for LSiPh2, LSiPhCl and LSiCl2 solid-state 29Si NMR (cross-polarization magic angle spinning, CP/MAS), spectra were recorded and isotropic chemical shifts δiso characteristic of hexacoordinate Si were recorded (−164.3, −145.2, and −175.3, respectively). Comparison of the 29Si NMR shift of LSiCl2 with the 29Si NMR shifts of porphyrine-SiCl2 complexes (ca. −220 ppm [78]) reveals a clear difference between these two classes of silicon compounds on the NMR spectroscopic level. Comparison of the solution state and solid-state 29Si NMR shift of LSiPh2 indicates that the hexacoordination found in the solid state is retained in solution, hence, dynamic equilibria between hexa- and tetracoordinate Si complexes, which would shift the 29Si resonance to lower field, do not play a significant role. Interestingly, for LSiPhCl we observe a surprisingly lowfield shifted 29Si signal, although stepwise substitution of Ph for Cl should cause upfield shifts of the 29Si signal, as shown for the series (oxinate)2SiPh2 (δiso −137 ppm), (oxinate)2SiPhCl (δiso −152 ppm), (oxinate)2SiCl2 (δiso −159 ppm) [86]. We attribute the downfield shift observed for LSiPhCl to a partial transition towards pentacoordinate Si in terms of Si–Cl bond lengthening (towards ionic Si–Cl bond dissociation). Kost et al. [87] have already reported that ionic dissociation of Si–Cl bonds is favored upon replacing an SiCl2 moiety of a hexacoordinate Si complex by an SiCl(alkyl) moiety. Apparently, a phenyl substituent can serve the same purpose of supporting this ionic dissociation, and the Si–Cl bond in the optimized molecular structure of LSiPhCl (2.29 Å, vide supra) also hints at significant Si–Cl bond lengthening (with respect to the Si–Cl bonds of LSiCl2, 2.23 Å), supported by shortening of the trans-disposed Si–C bond (1.95 Å) with respect to the Si–C bonds in LSiPh2 (1.97 Å). For the (oxinate)2SiPhnCl2−n system this Si–Cl bond lengthening has not been observed (Si–Cl 2.19 Å for n = 0, 2.20 Å for n = 1).
As for compound LSiPh2, both solid-state 29Si NMR data and crystallographically determined molecular structure are available, we have determined the chemical shift anisotropy (CSA) tensor both from CP/MAS spinning side bands (Figure 4) and quantum chemical calculations based on the solid-state molecular structure (atomic coordinates from the crystal structure were used, only coordinates of H atoms were subject to optimization prior to calculating the 29Si CSA tensor). As the computationally obtained data are in good agreement with the experimental values (Table 3), they allow for assignment of the directions of the CSA tensor principal components within the molecule (Figure 4).
In accord with the CSA data of a somewhat related complex, (O,N,N,O)SiPh2 with trans-disposed Si–C bonds and an equatorial dianionic tetradentate salen-type ligand [7] (the same ligand as for compound C in Scheme 1), pronounced shielding (direction δ33) is found along the C–Si–C axis, whereas components δ11 and δ22 are located in the idealized plane of the tetradentate chelating ligand. Interestingly, the change of the tetradentate chelator exerts similar influence on all principal components, hence, the span Ω of the CSA tensor is essentially the same and even the skew κ is hardly altered. In this regard, we find that the directional electronic effects of the (N,N′,N′,N) ligand used in this study on the 29Si NMR shift are very similar to those of salen type (O,N,N,O) ligands.

2.4. Light Sensitivity

For diorganosilicon complexes with tetradentate salen-type ligands we have reported migration of one of the Si-bound substituents to an imine C-atom (with formation of pentacoordinate Si complexes) upon irradiation with UV (Scheme 4, left) [9,11,77], and we have observed similar behavior for silacycloalkanes with two (O,N) bidentate oxinate ligands (oxinate)2Si(CH2)n (n = 3–6) [90]. The herein reported hexacoordinate diorgano Si complexes LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4) are also sensitive to light and have thus been synthesized in the dark. Upon exposure to visible light or upon irradiation with UV the initially orange compounds turn dark red. Deliberate exposure of THF solutions of LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4) to UV thus resulted in the formation of dark red solutions. In contrast to our previous studies of complexes with (O,N) and (O,N,N,O) chelated diorgano silicon compounds, UV-induced rearrangement of the herein reported compounds of the type LSiRR′ seems to produce complex mixtures of polymeric products rather than affording well-defined monomolecular complexes. This was concluded from the absence of detectable 29Si NMR signals from solutions of the product mixtures. The same observations were made under less vigorous conditions, i.e., upon stirring THF solutions of these complexes in Schenk tubes out of regular glassware exposed to the visible light of the laboratory environment. Nonetheless, rearrangement of one Si-bound aryl group with migration to an imine C atom (according to Scheme 4, right) should be thermodynamically allowed, as we could show by quantum chemical calculations. The proposed rearrangement products (LRSiR′) are thermodynamically more stable by about 35 kcal·mol−1. Atomic coordinates of the optimized molecular structures of compounds LSiRR′ and LRSiR′ can be found in the supporting information.

2.5. Charge Distribution

The different combinations of Si-bound monodentate substituents in the herein discussed complexes gave rise to different Si–N separations within the tetradentate (N,N′,N′,N)-chelate and we found the (expected) general trend of shortening of Si–N bonds upon enhancing the electron withdrawing capabilities of the monodentate Si-bound substituents. In order to shed some light on these electrostatic interaction, we have employed quantum chemical calculations of the Natural Charges (NCs) of the atoms of all herein discussed complexes LSiRR′. A comparison of the sum of NCs for the molecular fragments L, Si, R and R′ is listed in Table 4. Interestingly, the NC of the silicon atom hardly changes upon alteration of the Si-bound monodentate substituents. Replacing aryl substituents by Cl atoms causes lowering of the Si atom’s positive NC (especially for LSiCl2). Comparison of the Cl atoms’ NCs in LSiPhCl and LSiCl2 indicates that in the latter the Cl atoms carry a less pronounced negative charge (which we attribute to the shorter Si–Cl bond). Thus, we attribute the lowering of the Si atom’s NC in LSiCl2 to charge compensation effects caused by the closer proximity of Si and its monodentate substituents. In the same manner, the NCs of the tetradentate ligands L become less negative as the Si–N bonds are getting shorter (i.e., from LSi(aryl)2 via LSiPh to LSiCl2). Inter-ligand push-pull effects (i.e., lowering of one ligand’s charge while enhancing the other ligand’s charge) are not observed. This is most obvious for LSiPh(4-Me2N-C6H4), in which the two electronically different aryl groups still carry similar NCs, despite the electron releasing NMe2 substituent. The slightly less pronounced NC of the 4-Me2N-C6H4 group can again be interpreted as a result of slightly better charge compensation caused by the shorter Si–C bond, whereas the NC of the trans-disposed Ph group does not change (with respect to the NCs of the aryl groups in LSiPh2 and LSi(Anis)2).

3. Experimental Section

3.1. General Considerations

Chemicals commercially available were used as received without further purification. 4-Bromoanisole and 4-bromo-N,N-dimethylaniline were stored over activated molecular sieves (3 Å) for at least 7 days. THF, diethyl ether, toluene and triethylamine were distilled from sodium benzophenone. Triethylamine, DMSO and chloroform were stored over activated molecular sieves (3 Å) and THF, diethyl ether and toluene were stored over sodium wire under argon atmosphere. All reactions were carried out under an atmosphere of dry argon utilizing standard Schlenk techniques. H2L was synthesized according to a literature procedure [85]. Solution NMR spectra (1H, 13C, 29Si) were recorded on a Bruker DPX 400 MHz spectrometer (Me4Si as internal standard). 29Si (CP/MAS) NMR spectra were recorded on a Bruker Avance 400 WB spectrometer with 7 mm zirconia (ZrO2) rotors and KelF inserts. Elemental analyses were performed on an Elementar Vario MICRO cube. Single-crystal X-ray diffraction data were collected on a Bruker APEX2 CCD (LSiPh2·THF) and a STOE IPDS-2T diffractometer ((4-Me2N-C6H4)PhSiCl2, [LSi(Anis)2]2·THF, LSiPh(4-Me2N-C6H4)·THF) using Mo Kα-radiation. The structures were solved by direct methods using SHELXS-97 and refined with the full-matrix least-squares methods of F2 against all reflections with SHELXL-97 [91,92,93]. All non-hydrogen atoms were anisotropically refined. Hydrogen atoms were isotropically refined in idealized position (riding model). Graphics of molecular structures were generated with ORTEP-3 [94] and POV-Ray 3.62 [95]. CCDC 999945 (LSi(Anis)2·0.5·THF), 999946 (LSiPh2·THF), 999947 (Ph(4-Me2N-C6H4)SiCl2) and 999948 (LSiPh(4-Me2N-C6H4)·THF) contain the supplementary crystal data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Computational analyses were performed with Gaussian 09 [96]. All molecules were optimized using the DFT-MPW1PW91 [97,98,99,100] functional and the 6-311G(d,p) [101,102] basis set. The single-point energies and NBO analyses [103,104,105,106,107,108,109] have been calculated at the MP2/cc-pVTZ level [110,111,112,113,114,115,116,117,118,119]. Color graphics (Figures S1–S9) and atomic coordinates (Tables S1–S9) of the optimized molecular structures are available in the supplementary material. Prior to calculations of 29Si NMR shifts (for the molecular structures obtained by X-ray crystallography) the positions of the H atoms were optimized (using the DFT-MPW1PW91 functional and the 6-311G(d,p) basis set), and the reference molecule SiMe4 was (fully) optimized at the same level of theory as the target molecule. The NMR calculation of the molecules and the reference molecule were obtained with DFT-B3LYP/6-311G(d,p) [120].

3.2. Syntheses

Ph(4-Me2N-C6H4)SiCl2. Magnesium turnings (13.5 g, 0.557 mol) in THF (40 mL) were activated with a small amount of iodine at room temperature. To this mixture a small amounts (ca. 2 mL) of a solution of p-bromo-N,N-dimethylaniline (22.2 g, 0.185 mol) in THF (20 mL) was added via dropping funnel. The remaining aniline solution was then diluted further with THF (80 mL) and added dropwise. After completion of addition of p-bromo-N,N-dimethylaniline solution (about 60 min), the reaction mixture was stirred for another 5 h at room temperature. After storage at room temperature over night the Grignard reagent was transferred via cannula into a dropping funnel and the residual magnesium was washed with THF (3 × 30 mL) and dried in vacuo (converted Magnesium 2.97 g, 0.122 mol, 66% p-bromo-N,N-dimethylaniline reacted). The Grignard reagent was added dropwise (within 3 h) to a solution of phenyltrichlorsilane (25.3 g, 0.120 mol) and diethyl ether (60 mL) under vigorous stirring. The mixture was stored at room temperature for 5 d, whereupon the solvent was removed under reduced pressure (condensation into a cold trap). The residue was refluxed in toluene (80 mL) for 2 h and then cooled to 0 °C. The solid precipitate was filtered off and washed with toluene (5 × 20 mL). From the combined filtrate and washings the volatiles were condensed in a cold trap under reduced pressure. The residue was distilled at 3.8–4.5 × 10−2 Torr (b.p. = 190–192 °C). The liquid product solidified at room temperature within 8 h. Yield: 9.27 g (31.3 mmol, 17% based on the p-bromo-N,N-dimethylaniline used). Elemental analysis for C14H15Cl2NSi (296.26 g·mol−1): C, 56.76%; H, 5.10; N, 4.73; found C, 53.61%; H, 5.45%; N, 4.25%. The composition found indicates complete reaction with water during sample preparation, it fits the formula [Ph(p-HMe2N-C6H4)SiClOH]Cl. Calculated for C14H17Cl2NOSi (314.28 g·mol−1): C, 53.50%; H, 5.45%; N, 4.46. 1H NMR (400.1 MHz, CDCl3): δ (ppm) 3.00 (s, 6H, –CH3), 6.71–6.72 (m, 2H, aryl), 7.40–7.48 (mm, 3H, aryl), 7.57–7.59 (m, 2H, aryl), 7.75–7.77 (m, 2H, aryl); 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) 47.3 (–NMe2), 111.4, 115.9, 128.2, 131.4, 133.0, 134.1, 135.4, 152.4 (aryl); 29Si{1H} NMR (79.5 MHz, CDCl3): δ (ppm) 6.2.
(Anis)2SiCl2. (With reference to a literature method [121]): Magnesium turnings (10.6 g, 0.437 mol) in 30 mL diethyl ether were activated with a small amount of iodine. To this mixture only 1–2 mL of the total amount of p-bromoanisole (27.0 g, 0.144 mol) was added. The remaining p-bromoanisole was dissolved in diethyl ether (60 mL) and added dropwise to the vigorously stirring mixture (within 1.5 h). Thereafter, more diethyl ether (50 mL) was added and the mixture was stirred under reflux for 45 min and then cooled to room temperature. The Grignard reagent solution was transferred into a dropping funnel (via cannula), the residual magnesium was washed with diethyl ether (4 × 10 mL) and the washings were added to the Grignard solution. The residual magnesium was then washed with THF (40 mL) and dried in vacuum to find a weight difference of converted Magnesium of 3.48 g, 0.143 mol, 99% according to the p-bromoanisole used. The Grignard reagent was added dropwise (within 2 h) to a vigorously stirring solution of SiCl4 (12.2 g, 0.0716 mol) and diethyl ether (20 mL) at 0 °C. Thereafter, this solution was stored at room temperature overnight (10 h), whereupon the solvent was removed under reduced pressure (condensation into a cold trap). The white residue was stirred with hexane (150 mL), filtered off and washed with hexane (40 mL). From the filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap) and the residual colorless oil was distilled under reduced pressure to yield (Anis)SiCl3 (6.53 g, 27.0 mmol, 19%) at 70 °C, 3.3 × 10−2 Torr and (Anis)2SiCl2 (2.93 g, 9.36 mmol, 13%) at 160–161 °C, 1.8–2.0 × 10−2 Torr. (Anis)SiCl3: 1H NMR (400 MHz, CDCl3): δ (ppm) 3.45 (s, 6H, –OCH3), 6.99 (d, J = 8.6 Hz, 2H, aryl), 7.73 (d, J = 8.6 Hz, 2H, aryl). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) 55.3 (–CH3), 114.2, 122.5, 135.1, 163.1 (aryl); 29Si{1H} NMR (79,5 MHz, CDCl3): δ (ppm) −1.1. (Anis)2SiCl2: 1H NMR (400 MHz, CDCl3): δ (ppm) 3.84 (s, 6H, –OCH3), 6.97 (d, J = 8.70 Hz, 4H, aryl), 7.67 (d, J = 8.70 Hz, 4H, aryl). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) 55.2 (–CH3), 114.0, 123.3, 135.9, 162.3 (aryl). 29Si{1H} NMR (79.5 MHz, CDCl3): δ (ppm) 5.9.
LSiCl2. A solution of H2L (0.60 g, 2.29 mmol) and triethylamine (0.93 g, 9.21 mmol) in THF (7.5 mL) was stirred at room temperature, and chlorotrimethylsilane (0.60 g, 5.53 mmol) in THF (2.5 mL) was added dropwise via syringe. The yellow suspension thus obtained was heated to and stirred at 50 °C (3 h) and cooled to room temperature. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (3 mL). From the combined filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap), the orange residue was dissolved in chloroform (5 mL) and a solution of SiCl4 (0.41 g, 2.41 mmol) in chloroform (2 mL) was added dropwise via syringe. Within few seconds a precipitate formed. The mixture was stored at 6 °C (2 d), whereupon the solid was filtered off, washed with chloroform (0.5 mL) and dried in vacuum. Yield: 0.90 g (2.09 mmol, 82%) of LSiCl2·CHCl3. This compound is basically insoluble in common solvents such as hexane, diethyl ether, THF, toluene and DMSO. Elemental analysis for C17H13N4SiCl5 (478.66 g·mol−1): C, 42.66%; H, 2.74; N, 11.70%; found C, 42.72; H, 3.17%; N, 11.70%. 29Si{1H} CP/MAS NMR (79.5 MHz): δiso (ppm) −175.3.
LSiPhCl. A solution of H2L (0.50 g, 1.91 mmol) and triethylamine (0.58 g, 5.74 mmol) in THF (5 mL) was stirred at room temperature, and chlorotrimethylsilane (0.50 g, 4.61 mmol) in THF (2.5 mL) was added dropwise via syringe. The yellow suspension thus obtained was heated to and stirred at 50 °C (3 h) and cooled to room temperature. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (2 mL). From the combined filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap). The orange residue was dissolved in chloroform (5 mL) and a solution of phenyltrichlorsilane (0.42 g, 1.99 mmol) in THF (2.5 mL) was added dropwise via syringe. Within few seconds an orange precipitate formed. The mixture was stored at 6 °C (1 d), whereupon the solid product was filtered off, washed with THF (0.5 mL) and dried in vacuum. Yield: 0.49 g (1.04 mmol, 54%) of LSiClPh·THF. This compound is basically insoluble in common solvents such as hexane, diethyl ether, THF, toluene and DMSO. Elemental analysis for C26H25N4OSiCl (473.04 g·mol−1): C, 66.02%; H, 5.33%; N, 11.84%; found C, 65.98%; H, 5.51%; N, 11.79%. 29Si{1H} CP/MAS NMR (79.5 MHz): δiso (ppm) −145.2.
LSiPh2. A solution of H2L (2.00 g, 7.63 mmol) and triethylamine (3.85 g, 38.1 mmol) in THF (35 mL) was stirred at room temperature, and chlorotrimethylsilane (2.48 g, 22.9 mmol) in THF (5 mL) was added dropwise via syringe. The yellow suspension thus obtained was heated to and stirred at 50 °C (4 h) and cooled to room temperature. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (10 mL). From the combined filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap). The orange residue was dissolved in THF (10 mL). In the dark a solution of diphenyldichlorsilane (2.03 g, 8.02 mmol) in THF (3 mL) was added quickly (with stirring) and the solution was then stored at room temperature in the dark for 6 days. The orange crystals obtained were filtered off, washed with THF (5 mL) and briefly dried in vacuum. Yield: 2.86 g (5.56 mmol, 73%) of LSiPh2·THF. Elemental analysis for C32H30N4OSi (514.69 g·mol−1): C, 74.67%; H, 5.87%; N, 10.89%; found C, 74.14%; H, 5.56%; N, 10.81%. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 6.46–6.47 (m, 2H, aryl), 6.66–6.70 (m, 6H, aryl), 6.80–6.82 (m, 4H, aryl), 6.91–6.92 (m, 2H, aryl), 7.42–7.44 (m, 2H, aryl), 7.99–8.00 (m, 2H, aryl), 8.29 (s, 2H, aryl), 9.05 (s, 2H, –N=CH–); 13C{1H} NMR (100 MHz, DMSO-d6): δ (ppm) 115.7, 115.8, 118.9, 124.4, 126.3, 127.3, 132.0, 134.4, 134.6, 135.4, 141.6 (aryl), 160.4 ppm (–N=CH–); 29Si{1H} NMR (79.5 MHz, DMSO-d6): δ (ppm) −165.3.
LSi(Anis)2. A solution of H2L (0.80 g, 3.05 mmol) and triethylamine (1.00 g, 9.22 mmol) in THF (10 mL) was stirred at room temperature, and chlorotrimethylsilane (1.24 g, 12.3 mmol) in THF (4 mL) was added dropwise via syringe. The yellow suspension thus obtained was heated to and stirred at 50 °C (4 h) and cooled to room temperature. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (5 mL). From the combined filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap). The orange residue was dissolved in THF (7 mL). In the dark 2.04 g of a 50% solution of (Anis)2SiCl2 in hexane (corresponding to 1.02 g, 3.20 mmol) was added quickly with stirring and the solution was stored at room temperature in the dark for 6 days. The orange crystals obtained were filtered off, washed with THF (2 × 1.5 mL) and briefly dried in vacuum. Yield: 0.91 g (1.69 mmol, 55%) of LSi(Anis)2·0.5THF. This composition was concluded from the crystallographic data. 1H NMR data and elemental analysis suggest a somewhat higher THF content (0.7 THF). Elemental analysis for C32.8H31.6N4O2.7Si (553.11 g·mol−1): C, 71.22%; H, 5.76%; N, 10.13%; found C, 70.64%; H, 6.06%; N, 10.50%. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 3.45 (s, 6H, –O–CH3), 6.25–6.27 (m, 4H, aryl), 6.45–6.46 (m, 2H, aryl), 6.66–6.68 ppm (m, 4H, aryl), 6.91–6.92 (m, 2H, aryl), 7.42–7.43 (m, 2H, aryl), 7.98–8.00 (m, 2H, aryl), 8.21–8.22 (m, 2H, aryl), 9.04 (s, 2H, –N=CH–); 13C{1H} NMR (100 MHz, DMSO-d6): δ (ppm) 54.3 (–O–CH3), 111.8, 115.6, 115.7, 118.6, 127.3, 133.1, 134.2, 134.3, 135.3, 141.3, 151.3 (aryl), 156.1 (C=N); 29Si{1H} NMR (79.5 MHz, DMSO-d6): δ (ppm) −165.1.
LSiPh(4-Me2N-C6H4). A solution of H2L (0.70 g, 2.67 mmol) and triethylamine (1.08 g, 10.7 mmol) in THF (5 mL) was stirred at room temperature, and chlorotrimethylsilane (0.70 g, 6.45 mmol) in THF (2 mL) was added dropwise via syringe. The yellow suspension thus obtained was heated to and stirred at 50 °C (3 h) and cooled to room temperature. Thereafter, the triethylamine hydrochloride precipitate was filtered off and washed with THF (5 mL). From the combined filtrate and washings the solvent was removed under reduced pressure (condensation into a cold trap). The orange residue was dissolved in THF (5 mL). In the dark and with stirring a solution of Ph(4-Me2N-C6H4)SiCl2 (0.87 g, 2.94 mmol) in THF (5 mL) was added quickly. The mixture was stored at room temperature in the dark for 3 days. The orange crystals obtained were filtered off, washed with THF (2 mL) and briefly dried in vacuum. Yield: 0.64 g (1.15 mmol, 43%) of LSiPh(4-Me2N-C6H4)·THF. This composition was concluded from the crystallographic data. Elemental analysis suggests a somewhat lower THF content (0.9 THF), loss of solvent may have occurred during sample preparation. Elemental analysis for C33.6H34.2N5O0.9Si (550.55 g·mol−1): C, 73.30%; H, 6.26%; N, 12.72%; found C, 72.70%; H, 6.15%; N, 21.91%. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 2.59 (s, 6H, –NMe2), 6.10–6.12 (m, 2H, aryl), 6.44–6.90 (mm, 12H, aryl), 7.41–7.43 (m, 2H, aryl), 7.98–8.00 (m, 2H, aryl), 8.21–8.23 (m, 3H, aryl), 9.03 (s, 2H, –N=CH–). 13C{1H} NMR (100 MHz, DMSO-d6): δ (ppm) 40.1 (–NMe2), 111.3, 114.6, 115.9, 119.0, 124.3, 126.4, 126.8, 131.9, 133.0, 133.8, 134.4, 135.3, 138.8, 144.8, 147.6 (aryl), 160.1 (C=N). 29Si{1H} NMR (79.5 MHz, DMSO-d6): δ (ppm) −164.4.

4. Conclusions

The silicon compounds LSiPh2, LSi(Anis)2, LSiPh(4-Me2N-C6H4), LSiPhCl and LSiCl2, with the dianionic tetradentate (N,N′,N′,N)-chelating ligand obtained from o-phenylenediamine and pyrrole-2-carbaldehyde, exhibit trans-disposed monodentate substituents and an almost planar arrangement of the tetradentate chelator. For LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4), this was concluded from the crystal structures, and for LSiPhCl and LSiCl2, the corresponding molecular structures were predicted by computational methods. With respect to this configurational feature, the herein presented complexes are related to both the group of hexacoordinate Si-complexes with salen-type (O,N,N,O)-ligands, which also exhibit trans-arrangement of their monodentate substituents, and to hexacoordinate Si-complexes with macrocyclic (N,N,N,N)-ligands, such as phthalocyanines and porphyrines. Moreover, compounds LSiPh2, LSi(Anis)2, LSiPh(4-Me2N-C6H4), LSiPhCl and LSiCl2 exhibit almost perfect charge delocalization within the pyrrolide/imine chelating moiety, which results in very similar Si–N bond lengths within one molecule. Whereas this structural feature makes them appear related to phthalocyanine and prophyrine complexes, the 29Si NMR characteristics (e.g., chemical shift range and orientation of the chemical shift anisotropy tensor of LSiPh2) are similar to those of Si-salen-complexes. Like their salen-chelated relatives, compounds such as LSiPh2 are light sensitive. In sharp contrast, upon irradiation they do not form distinct rearrangement products like the related salen-complexes (even though it would be thermodynamically allowed). Moreover, analysis of the development of bond lengths and natural charges upon altering the monodentate substituents R and R′ in compounds LSiRR′ indicates that enhanced electron withdrawing effects of R/R′ do not enhance charge separation between R/R′ and Si but lead to better charge compensation by bond shortening. This effect is observed for the tetradentate ligand as well. In the case of two different trans-disposed monodentate substituents (in LSiPh(4-Me2N-C6H4) and LSiPhCl), we have not found “push-pull effects” of charges (enhanced negative charge of one group at the expense of negative charge of the other trans-disposed group).

Supplementary Materials

The following are available online at www.mdpi.com/2304-6740/4/2/8/s1, Figures S1–S9 and Tables S1–S9 containing graphical representations and Cartesian coordinates, respectively, of the optimized molecular structures of LSiPh2, LSi(Anis)2, LSiPh(4-Me2N-C6H4), LSiPhCl, LSiCl2, LPhSiPh, LAnisSiAnis, LPhSi(4-Me2N-C6H4) and L(4-Me2N-C6H4)SiPh.

Acknowledgments

We are grateful to Ute Groß for performing elemental analyses and to Beate Kutzner and Katrin Krupinski for solution NMR spectroscopic measurements.

Author Contributions

Daniela Gerlach and Jörg Wagler conceived and designed the experiments; Daniela Gerlach performed the experiments (syntheses) and computational analyses; Daniela Gerlach and Jörg Wagler performed the single-crystal X-ray diffraction analyses; Erica Brendler performed the solid-state NMR spectroscopic analyses; and Jörg Wagler wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Selected penta- and hexacoordinate silicon compounds with: monodentate ligands (A); chelating groups (B,C); and a macrocyclic (phthalocyanine) ligand (D).
Scheme 1. Selected penta- and hexacoordinate silicon compounds with: monodentate ligands (A); chelating groups (B,C); and a macrocyclic (phthalocyanine) ligand (D).
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Scheme 2. Syntheses of ligands H2L and (Me3Si)2L, silanes (Anis)2SiCl2 and(4-Me2N-C6H4)PhSiCl2 as well as complexes LSiCl2, LSiPhCl, LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4).
Scheme 2. Syntheses of ligands H2L and (Me3Si)2L, silanes (Anis)2SiCl2 and(4-Me2N-C6H4)PhSiCl2 as well as complexes LSiCl2, LSiPhCl, LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4).
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Figure 1. Molecular structure of (4-Me2N-C6H4)PhSiCl2 in the crystal, thermal displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): Si1–C1 1.841(2), Si1–C9 1.858(2), Si1–Cl1 2.083(1), Si1–Cl2 2.070(1); C1–Si1–C9 117.25(7), Cl1–Si1–Cl2 107.93(3).
Figure 1. Molecular structure of (4-Me2N-C6H4)PhSiCl2 in the crystal, thermal displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): Si1–C1 1.841(2), Si1–C9 1.858(2), Si1–Cl1 2.083(1), Si1–Cl2 2.070(1); C1–Si1–C9 117.25(7), Cl1–Si1–Cl2 107.93(3).
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Figure 2. Molecular structures of (from left) LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4) in their crystal structures (heteroatoms and Si-bound C atoms labeled, H-atoms, solvent molecules and second independent molecule of the asymmetric unit of LSiPh2 and LSi(Anis)2 are omitted).
Figure 2. Molecular structures of (from left) LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4) in their crystal structures (heteroatoms and Si-bound C atoms labeled, H-atoms, solvent molecules and second independent molecule of the asymmetric unit of LSiPh2 and LSi(Anis)2 are omitted).
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Figure 3. Molecular structures of LSiPh2, LSiPhCl and LSiCl2 optimized at the MPW1PW91/6-311G(d,p) level.
Figure 3. Molecular structures of LSiPh2, LSiPhCl and LSiCl2 optimized at the MPW1PW91/6-311G(d,p) level.
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Scheme 3. Resonance structures for the interpretation of the Si–N bonding situation in the hexacoordinate Si complexes discussed in this paper and two examples (E and F) of compounds that exhibit pronounced contributions of the one or the other mesomeric form.
Scheme 3. Resonance structures for the interpretation of the Si–N bonding situation in the hexacoordinate Si complexes discussed in this paper and two examples (E and F) of compounds that exhibit pronounced contributions of the one or the other mesomeric form.
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Figure 4. 29Si CP/MAS NMR spinning side band spectrum of LSiPh2 (νspin = 500 Hz) (left), and directions of the principal components δ11, δ22 and δ33 of the CSA tensor in the molecule (right).
Figure 4. 29Si CP/MAS NMR spinning side band spectrum of LSiPh2 (νspin = 500 Hz) (left), and directions of the principal components δ11, δ22 and δ33 of the CSA tensor in the molecule (right).
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Scheme 4. UV induced rearrangement of an Si-bound hydrocarbyl group in hexacoordinate Si complexes (migration to an imine C atom) observed for complexes of salen-type ligands (left) and calculated to be thermodynamically allowed for the herein reported diarylsilicon complexes LSiRR′ (right). Rearrangement energies calculated for the following combinations R,R′ are: Ph,Ph: −34.1 kcal·mol−1; Anis,Anis: −34.7 kcal·mol−1; (4-Me2N-C6H4),Ph: −35.6 kcal·mol−1; Ph,(4-Me2N-C6H4): −34.4 kcal·mol−1.
Scheme 4. UV induced rearrangement of an Si-bound hydrocarbyl group in hexacoordinate Si complexes (migration to an imine C atom) observed for complexes of salen-type ligands (left) and calculated to be thermodynamically allowed for the herein reported diarylsilicon complexes LSiRR′ (right). Rearrangement energies calculated for the following combinations R,R′ are: Ph,Ph: −34.1 kcal·mol−1; Anis,Anis: −34.7 kcal·mol−1; (4-Me2N-C6H4),Ph: −35.6 kcal·mol−1; Ph,(4-Me2N-C6H4): −34.4 kcal·mol−1.
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Table
Table 1. Crystallographic data from data collection and refinement for (4-Me2N-C6H4)PhSiCl2, LSiPh2·THF, LSi(Anis)2·0.5 THF and LSiPh(4-Me2N-C6H4)·THF.
Table 1. Crystallographic data from data collection and refinement for (4-Me2N-C6H4)PhSiCl2, LSiPh2·THF, LSi(Anis)2·0.5 THF and LSiPh(4-Me2N-C6H4)·THF.
Parameter(4-Me2N-C6H4) PhSiCl2LSiPh2·THF[LSi(Anis)2]2·THFLSiPh(4-Me2N-C6H4)·THF
FormulaC14H15Cl2NSiC32H30N4OSiC64H60N8O5Si2C34H35N5OSi
Mr296.26514.691077.38557.76
T (K)150(2)150(2)150(2)150(2)
λ (Å)0.710730.710730.710730.71073
Crystal systemmonoclinictriclinicmonoclinicmonoclinic
Space groupP21/cP-1P21/cCc
a (Å)16.6233(11)9.7135(3)14.3173(4)14.5651(7)
b (Å)7.5451(3)14.9827(4)18.2897(8)22.8720(8)
c (Å)12.2941(8)18.4932(4)21.3671(7)10.4529(5)
α (°)9082.491(1)9090
β (°)110.965(5)83.162(1)101.839(2)124.370(3)
γ (°)9088.353(1)9090
V (Å3)1439.90(15)2649.10(12)5476.1(3)2874.2(2)
Z4444
ρcalc (g·cm−1)1.3671.2901.3071.289
μMoKα (mm−1)0.5160.1220.1250.119
F(000)616108822721184
θmax(°), Rint30.0, 0.050027.0, 0.038428.0, 0.049630.0, 0.0350
Completeness100%98.6%99.9%99.9%
Reflns collected19,78745,81952,82327,745
Reflns unique420311,42013,1977879
Restraints0004
Parameters165685716377
GoF1.0441.0601.0471.052
R1, wR2 [I > 2σ (I)]0.0348, 0.07710.0463, 0.11400.0441, 0.10130.0331, 0.0800
R1, wR2 (all data)0.0535, 0.08410.0790, 0.12410.0744, 0.11290.0381, 0.0831
Largest peak/hole (e·Å−3)0.428, −0.3050.564, −0.4100.678, −0.5900.307, −0.190
Table
Table 2. Bond lengths (Å) in the Si coordination spheres of the hexacoordinate Si complexes discussed in this paper.
Table 2. Bond lengths (Å) in the Si coordination spheres of the hexacoordinate Si complexes discussed in this paper.
BondLSiPh(4-Me2N-C6H4)LSi(Anis)2 1LSiPh2 1LSiPh2 2LSiPhCl 2LSiCl2 2
Si–C1.994(1) 31.995(2)1.983(2)1.9671.949-
1.964(1) 41.974(2)1.977(2)1.967--
Si–Npyrrole1.900(1)1.904(1)1.882(2)1.9481.8771.851
1.890(1)1.896(1)1.890(1)1.9481.8771.852
Si–Nimine1.919(1)1.923(2)1.906(1)1.9331.9061.892
1.905(1)1.915(1)1.908(1)1.9331.9061.892
Si–Cl----2.2942.227
-----2.228
1 Data taken from one of the two crystallographically independent molecules (the molecule shown in Figure 2). The other molecule in the asymmetric unit exhibits very similar bond lengths; 2 from optimized molecular structures (see Figure 3); 3 C of the phenyl group; 4 C of the p-dimethylaminophenyl group.
Table
Table 3. Parameters of the 29Si NMR CSA tensor (isotropic chemical shift δiso; principal components δ11, δ22, δ33; span Ω; skew κ, according to the Herzfeld–Berger notation [88,89]) for LSiPh2 obtained from a CP/MAS spectrum (LSiPh2exp) and from quantum chemical calculations (LSiPh2calc) as well as experimentally obtained CSA tensor data of a hexacoordinate (O,N,N,O)SiPh2 complex of the chelating ligand shown in Scheme 1 for compound C.
Table 3. Parameters of the 29Si NMR CSA tensor (isotropic chemical shift δiso; principal components δ11, δ22, δ33; span Ω; skew κ, according to the Herzfeld–Berger notation [88,89]) for LSiPh2 obtained from a CP/MAS spectrum (LSiPh2exp) and from quantum chemical calculations (LSiPh2calc) as well as experimentally obtained CSA tensor data of a hexacoordinate (O,N,N,O)SiPh2 complex of the chelating ligand shown in Scheme 1 for compound C.
ParameterLSiPh2expLSiPh2calc(O,N,N,O)SiPh2
δiso (ppm)−164.3−166.9−177.7
δ11 (ppm)−147.3−149.9−162.8
δ22 (ppm)−164.6−168.1−174.3
δ33 (ppm)−181.0−182.9−196.0
Ω (ppm) 133.733.033.2
κ 2−0.03−0.110.31
1 Ω = δ11−δ33; 2 κ = 3(δ22−δiso)/Ω.
Table
Table 4. Natural Charges (NCs) of ligand L, Si and the monodentate substituents of the hexacoordinate Si complexes discussed in this paper.
Table 4. Natural Charges (NCs) of ligand L, Si and the monodentate substituents of the hexacoordinate Si complexes discussed in this paper.
MoietyLSiPh(4-Me2N-C6H4)LSi(Anis)2LSiPh2LSiPhClLSiCl2
Si2.2162.2092.2152.1862.102
L−1.110−1.107−1.100−1.051−1.023
R−0.559−0.556−0.557−0.562−0.539
(Ph)(Anis)(Ph)(Ph)(Cl)
R′−0.547−0.546−0.557−0.572−0.540
(4-Me2N-C6H4)(Anis)(Ph)(Cl)(Cl)

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Gerlach, D.; Brendler, E.; Wagler, J. Hexacoordinate Silicon Compounds with a Dianionic Tetradentate (N,N′,N′,N)-Chelating Ligand. Inorganics 2016, 4, 8. https://doi.org/10.3390/inorganics4020008

AMA Style

Gerlach D, Brendler E, Wagler J. Hexacoordinate Silicon Compounds with a Dianionic Tetradentate (N,N′,N′,N)-Chelating Ligand. Inorganics. 2016; 4(2):8. https://doi.org/10.3390/inorganics4020008

Chicago/Turabian Style

Gerlach, Daniela, Erica Brendler, and Jörg Wagler. 2016. "Hexacoordinate Silicon Compounds with a Dianionic Tetradentate (N,N′,N′,N)-Chelating Ligand" Inorganics 4, no. 2: 8. https://doi.org/10.3390/inorganics4020008

APA Style

Gerlach, D., Brendler, E., & Wagler, J. (2016). Hexacoordinate Silicon Compounds with a Dianionic Tetradentate (N,N′,N′,N)-Chelating Ligand. Inorganics, 4(2), 8. https://doi.org/10.3390/inorganics4020008

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