( 2-Pyridyloxy ) silanes as Ligands in Transition Metal Coordination Chemistry

Proceeding our initial studies of compounds with formally dative TM→Si bonds (TM = Ni, Pd, Pt), which feature a paddlewheel arrangement of four (N,S) or (N,N) bridging ligands around the TM–Si axis, the current study shows that the (N,O)-bidentate ligand 2-pyridyloxy (pyO) is also capable of bridging systems with TM→Si bonds (shown for TM = Pd, Cu). Reactions of MeSi(pyO)3 with [PdCl2(NCMe)2] and CuCl afforded the compounds MeSi(μ-pyO)4PdCl (1) and MeSi(μ-pyO)3CuCl (2), respectively. In the latter case, some crystals of the Cu(II) compound MeSi(μ-pyO)4CuCl (3) were obtained as a byproduct. Analogous reactions of Si(pyO)4, in the presence of HpyO, with [PdCl2(NCMe)2] and CuCl2, afforded the compounds [(HpyO)Si(μ-pyO)4PdCl]Cl (4), (HpyO)2Si[(μ-pyO)2PdCl2]2 (5), and (HpyO)2Si[(μ-pyO)2CuCl2]2 (6), respectively. Compounds 1–6 and the starting silanes MeSi(pyO)3 and Si(pyO)4 were characterized by single-crystal X-ray diffraction analyses and, with exception of the paramagnetic compounds 3 and 6, with NMR spectroscopy. Compound 2 features a pentacoordinate Si atom, the Si atoms of the other complexes are hexacoordinate. Whereas compounds 1–4 feature a TM→Si bond each, the Si atoms of compounds 5 and 6 are situated in an O6 coordination sphere, while the TMCl2 groups are coordinated to pyridine moieties in the periphery of the molecule. The TM–Si interatomic distances in compounds 1–4 are close to the sum of the covalent radii (1 and 4) or at least significantly shorter than the sum of the van-der-Waals radii (2 and 3). The latter indicates a noticeably weaker interaction for TM = Cu. For the series 1, 2, and 3, all of which feature the Me–Si motif trans-disposed to the TM→Si bond, the dependence of the TM→Si interaction on the nature of TM (Pd(II), Cu(I), and Cu(II)) was analyzed using quantum chemical calculations, that is, the natural localized molecular orbitals (NLMO) analyses, the non-covalent interaction (NCI) descriptor, Wiberg bond order (WBO), and topological characteristics of the bond critical points using the atoms in molecules (AIM) approach.


Introduction
The coordination number of tetravalent silicon can easily be enhanced (up to five or six) with the aid of monodentate or chelating ligands [1][2][3][4].In some of our studies, we have also shown that late transition metals (Ni(II), Pd(II), and Pt(II)) may serve the role of a lone pair donor at hexacoordinate silicon, for example, in I and II (Chart 1) [5][6][7][8].That kind of complexes with silicon as a lone pair acceptor in the coordination sphere of a transition metal (TM) thus complements TM silicon complexes with, e.g., silylene ligands, in which the Si atom is the formal lone pair donor, such as III, IV, and V [9][10][11][12][13], and the silyl complexes, in which the TM-Si bond is one out of four bonds to a tetravalent Si atom (e.g., VI, VII, VIII, and IX) [14][15][16][17][18].In complex IX and some other compounds with group 9 metals [19][20][21][22], the 2-pyridyloxy (pyO) ligand was successfully utilized for stabilizing the TM-Si bond by forming two bridges over the heterodinuclear core.Some other compounds have been reported, in which one pyO ligand bridges the TM-Si bond (e.g., X, and some others) [23][24][25].In all of these Si(µ-pyO)TM compounds, the ambidentate pyO ligand is bound to silicon via the Si-O bond, while the softer Lewis base (pyridine N atom) is TM bound.This structural motif should enable access to a new class of paddlewheel-shaped complexes with formally dative TM→Si bonds, in which the Si atom may carry a sterically more demanding group or alkyl group, because of the rather poor steric demand of the surrounding donor atoms (i.e., O atoms), whereas in the previously reported TM→Si paddlewheel complexes (such as I and II), the donor atom situation at silicon merely allowed for the presence of a small electronegative H-acceptor moiety (i.e., a halide).
Inorganics 2018, 6, x FOR PEER REVIEW 2 of 19 a tetravalent Si atom (e.g., VI, VII, VIII, and IX) [14][15][16][17][18].In complex IX and some other compounds with group 9 metals [19][20][21][22], the 2-pyridyloxy (pyO) ligand was successfully utilized for stabilizing the TM-Si bond by forming two bridges over the heterodinuclear core.Some other compounds have been reported, in which one pyO ligand bridges the TM-Si bond (e.g., X, and some others) [23][24][25].In all of these Si(µ-pyO)TM compounds, the ambidentate pyO ligand is bound to silicon via the Si-O bond, while the softer Lewis base (pyridine N atom) is TM bound.This structural motif should enable access to a new class of paddlewheel-shaped complexes with formally dative TM→Si bonds, in which the Si atom may carry a sterically more demanding group or alkyl group, because of the rather poor steric demand of the surrounding donor atoms (i.e., O atoms), whereas in the previously reported TM→Si paddlewheel complexes (such as I and II), the donor atom situation at silicon merely allowed for the presence of a small electronegative H-acceptor moiety (i.e., a halide).

Syntheses and Characterization of Silanes MeSi(pyO)3 and Si(pyO)4
For the starting materials, we synthesized MeSi(pyO)3 via triethylamine supported reaction of MeSiCl3 and 2-hydroxypyridine, and Si(pyO)4, via transsilylation, with the preceding synthesis of Me3Si(pyO), which is known in the literature [26] (Scheme 1).Both of the silanes formed crystals suitable for single-crystal X-ray diffraction analysis (Figure 1 and Table 1).In both of the compounds, the Si atom is essentially tetracoordinate and the coordination sphere may be described as (4 + 3) in MeSi(pyO)3 and (4 + 4) in Si(pyO)4, because of the pyridine N atoms, which are capping the faces of the tetrahedral coordination spheres from distances close to the sum of the van-der-Waals radii (ranging between 2.91 and 3.03 Å).This tetracoordination of Si in Si(pyO)4 is in contrast to the Si hexacoordination in the thio analog Si(pyS) 4 [27] (and other pyS-bearing Si compounds [28]), in which two of the 2-mercaptopyridyl ligands form four-membered chelates, and in complexes of the N-oxide of the pyO system, which forms five-membered chelates [29,30].

Syntheses and Characterization of Silanes MeSi(pyO) 3 and Si(pyO) 4
For the starting materials, we synthesized MeSi(pyO) 3 via triethylamine supported reaction of MeSiCl 3 and 2-hydroxypyridine, and Si(pyO) 4 , via transsilylation, with the preceding synthesis of Me 3 Si(pyO), which is known in the literature [26] (Scheme 1).Both of the silanes formed crystals suitable for single-crystal X-ray diffraction analysis (Figure 1 and Table 1).In both of the compounds, the Si atom is essentially tetracoordinate and the coordination sphere may be described as (4 + 3) in MeSi(pyO) 3 and (4 + 4) in Si(pyO) 4 , because of the pyridine N atoms, which are capping the faces of the tetrahedral coordination spheres from distances close to the sum of the van-der-Waals radii (ranging between 2.91 and 3.03 Å).This tetracoordination of Si in Si(pyO) 4 is in contrast to the Si hexacoordination in the thio analog Si(pyS) 4 [27] (and other pyS-bearing Si compounds [28]), in which two of the 2-mercaptopyridyl ligands form four-membered chelates, and in complexes of the N-oxide of the pyO system, which forms five-membered chelates [29,30].The 29 Si NMR shifts (MeSi(pyO)3 in CDCl3: −46.5;Si(pyO)4 in solid state: −87.9, in CDCl3: −97.2) are in support of tetracoordination, as they are similar to (and even more downfield shifted than) the 29 Si NMR shifts of the related phenoxysilanes MeSi(OPh)3 (−54.0)[31] and Si(OPh)4 (−101.1)[32], respectively.Some effect of the three-or four-fold capped coordination spheres is evident from the bond angles of the Si atoms, which exhibit notable deviations from the ideal tetrahedral angle (in MeSi(pyO)3, O1-Si1-O3 97.39 (5)° and O2-Si1-O3 114.68 (5)°; in Si(pyO)4, two sets of O-Si-O angles of 101.42(9)° and 113.64(5)°).In general, the Si tetracoordination in these silanes, in combination with the vacant pyridine N atoms, should be favorable for complex formation with additional metal atoms.The same feature, tetracoordinate Si atom and vacant additional donor atoms, has also been encountered with methimazolylsilanes [5,33] and 7-azaindolylsilanes [8], which turned out to be suitable starting materials for compounds such as I and II.

Choice of Metals: Pd(II) and Cu(I)
In reactions with 7-azaindolylsilanes, Pd(II) already proved to be a suitable candidate for forming paddlewheel-complexes with TM→Si bonds and TM-bound pyridine moieties (e.g., in II).In addition to d 8 systems, other electron rich TMs may also be capable of forming complexes with TM→Si bonds, as shown by Bourissou et al. for the Au(I)→Si system (e.g., XI, Chart 2), in which the d 10 metal is the electron pair donor [34][35][36].Furthermore, for compounds XII (M = Cu, Ag, Au),  The 29 Si NMR shifts (MeSi(pyO)3 in CDCl3: −46.5;Si(pyO)4 in solid state: −87.9, in CDCl3: −97.2) are in support of tetracoordination, as they are similar to (and even more downfield shifted than) the 29 Si NMR shifts of the related phenoxysilanes MeSi(OPh)3 (−54.0)[31] and Si(OPh)4 (−101.1)[32], respectively.Some effect of the three-or four-fold capped coordination spheres is evident from the bond angles of the Si atoms, which exhibit notable deviations from the ideal tetrahedral angle (in MeSi(pyO)3, O1-Si1-O3 97.39(5)° and O2-Si1-O3 114.68(5)°; in Si(pyO)4, two sets of O-Si-O angles of 101.42(9)° and 113.64(5)°).In general, the Si tetracoordination in these silanes, in combination with the vacant pyridine N atoms, should be favorable for complex formation with additional metal atoms.The same feature, tetracoordinate Si atom and vacant additional donor atoms, has also been encountered with methimazolylsilanes [5,33] and 7-azaindolylsilanes [8], which turned out to be suitable starting materials for compounds such as I and II.

Choice of Metals: Pd(II) and Cu(I)
In reactions with 7-azaindolylsilanes, Pd(II) already proved to be a suitable candidate for forming paddlewheel-complexes with TM→Si bonds and TM-bound pyridine moieties (e.g., in II).In addition to d 8 systems, other electron rich TMs may also be capable of forming complexes with TM→Si bonds, as shown by Bourissou et al. for the Au(I)→Si system (e.g., XI, Chart 2), in which the d 10 metal is the electron pair donor [34][35][36].Furthermore, for compounds XII (M = Cu, Ag, Au), The 29 Si NMR shifts (MeSi(pyO) 3 in CDCl 3 : −46.5;Si(pyO) 4 in solid state: −87.9, in CDCl 3 : −97.2) are in support of tetracoordination, as they are similar to (and even more downfield shifted than) the 29 Si NMR shifts of the related phenoxysilanes MeSi(OPh) 3 (−54.0)[31] and Si(OPh) 4 (−101.1)[32], respectively.Some effect of the three-or four-fold capped coordination spheres is evident from the bond angles of the Si atoms, which exhibit notable deviations from the ideal tetrahedral angle (in MeSi(pyO) 3 , O1-Si1-O3 97.39 (5) • and O2-Si1-O3 114.68 (5) • ; in Si(pyO) 4 , two sets of O-Si-O angles of 101.42 (9) • and 113.64(5) • ).In general, the Si tetracoordination in these silanes, in combination with the vacant pyridine N atoms, should be favorable for complex formation with additional metal atoms.The same feature, tetracoordinate Si atom and vacant additional donor atoms, has also been encountered with methimazolylsilanes [5,33] and 7-azaindolylsilanes [8], which turned out to be suitable starting materials for compounds such as I and II.

Choice of Metals: Pd(II) and Cu(I)
In reactions with 7-azaindolylsilanes, Pd(II) already proved to be a suitable candidate for forming paddlewheel-complexes with TM→Si bonds and TM-bound pyridine moieties (e.g., in II).In addition to d 8 systems, other electron rich TMs may also be capable of forming complexes with TM→Si bonds, as shown by Bourissou et al. for the Au(I)→Si system (e.g., XI, Chart 2), in which the d 10 metal is the electron pair donor [34][35][36].Furthermore, for compounds XII (M = Cu, Ag, Au), Kameo et al. have shown that Cu(I) and Ag(I) are weaker donors [37].In this context, we need to note that a previous study by Bourissou et al. [38] revealed an interaction between the Si-Si σ-bond electron pair of a disilane (as donor) and Cu(I) (as acceptor) (XIII).Similar systems (with rather weak d 10 metal-silicon interaction) have been investigated for Ni(0), Pd(0), and Pt(0) by Grobe et al. (e.g., XIV) [39][40][41].Whereas Au(I), Ni(0), Pd(0), and Pt(0) are more susceptible to phosphine ligands, Cu(I) represents a d 10 system likely to bind to more than one or two N-donor ligands, and thus we included Cu(I) (as CuCl) in our investigations.[37].In this context, we need to note that a previous study by Bourissou et al. [38] revealed an interaction between the Si-Si σ-bond electron pair of a disilane (as donor) and Cu(I) (as acceptor) (XIII).Similar systems (with rather weak d 10 metal-silicon interaction) have been investigated for Ni(0), Pd(0), and Pt(0) by Grobe et al. (e.g., XIV) [39][40][41].Whereas Au(I), Ni(0), Pd(0), and Pt(0) are more susceptible to phosphine ligands, Cu(I) represents a d 10 system likely to bind to more than one or two N-donor ligands, and thus we included Cu(I) (as CuCl) in our investigations.
Chart 2. Selected metal-silicon complexes with donor-acceptor-interactions between the Si atom of a tetravalent silane and a d 10 metal atom.

Reactions of MeSi(pyO)3 with [PdCl2(NCMe)2] and CuCl
The reactions of MeSi(pyO)3 and [PdCl2(NCMe)2] (in 1:1 molar ratio) in chloroform proceeded with the partial dissolution of [PdCl2(NCMe)2] and the formation of a clear (slightly yellow, almost colorless) solution, from which the crystals of the chloroform solvate of compound 1 formed within one day (Scheme 2).The formal loss of Cl and the addition of a fourth pyO-bridge indicate ligand scrambling in the course of this reaction.The addition of excess MeSi(pyO)3 (as sacrificial pyO source) eventually led to the complete dissolution of [PdCl2(NCMe)2] and the formation of crystals of compound 1 • 2 CHCl3 in good yield.From such a crystal, the molecular structure of 1 was determined using X-ray diffraction analysis (Figure 2 and Table 1).In principle, the molecule has paddlewheel architecture with four pyO ligands attached to Si (via Si-O bonds) and Pd (via Pd-N bonds) of a Me-Si-Pd-Cl axis.The idealized planes of the pyO ligands are slightly tilted against the Si-Pd axis (Pd-Si-O-C torsion angles ranging between 34.6(3)° and 26.3(3)°), and the axial angles (C21-Si1-Pd1 and Si1-Pd1-Cl1) exhibit some deviation from linearity (177.88(12)° and 177.36(3)°, respectively).The Si and Pd atoms are displaced from the O4 and N4 least-squares planes, respectively (into opposite directions), by 0.252(1) and 0.165(1) Å, respectively.The Pd-Si bond (2.6268(2) Å) is slightly longer than in the methimazolyl bridged paddlewheel complexes (where the Pd-Si bond lengths in the range 2.53-2.60Å were observed) [7], and slightly shorter than in the 7-azaindolyl bridged paddlewheels [8].The Si-C bond (1.853(4) Å) is only marginally longer than in the starting silane MeSi(pyO)3 (1.837(2) Å), thus hinting at only a weak Pd→Si lone pair donor action.At hexacoordinate silicon with trans-disposed stronger donor (e.g., another hydrocarbyl group [42,43], 2-pyridinethiolato N atom [28] or 8-oxyquinolinyl N atom [44,45]), one would expect a Si-C bond lengthening beyond 1.90 Å.The Pd-Cl bond (2.891(1) Å), however, is unexpectedly long, thus hinting at ionic dissociation.This is supported by four H•••Cl contacts with the pyO-H 6 atoms.The 29 Si NMR shift of compound 1 (-116.9ppm in CDCl3) is notably more upfield with respect to MeSi(pyO)3 and Si(pyO)4, thus indicating the hypercoordination of the Si atom.This 29 Si NMR shift, however, may be representative of either penta-or hexacoordinate silicon, and therefore the role of the sixth donor moiety (Pd→Si) requires further elucidation (vide infra).

Reactions of MeSi(pyO) 3 with [PdCl 2 (NCMe) 2 ] and CuCl
The reactions of MeSi(pyO) 3 and [PdCl 2 (NCMe) 2 ] (in 1:1 molar ratio) in chloroform proceeded with the partial dissolution of [PdCl 2 (NCMe) 2 ] and the formation of a clear (slightly yellow, almost colorless) solution, from which the crystals of the chloroform solvate of compound 1 formed within one day (Scheme 2).The formal loss of Cl and the addition of a fourth pyO-bridge indicate ligand scrambling in the course of this reaction.The addition of excess MeSi(pyO) 3 (as sacrificial pyO source) eventually led to the complete dissolution of [PdCl 2 (NCMe) 2 ] and the formation of crystals of compound 1 • 2 CHCl 3 in good yield.From such a crystal, the molecular structure of 1 was determined using X-ray diffraction analysis (Figure 2 and Table 1).In principle, the molecule has paddlewheel architecture with four pyO ligands attached to Si (via Si-O bonds) and Pd (via Pd-N bonds) of a Me-Si-Pd-Cl axis.The idealized planes of the pyO ligands are slightly tilted against the Si-Pd axis (Pd-Si-O-C torsion angles ranging between 34.6(3) • and 26.3(3) • ), and the axial angles (C21-Si1-Pd1 and Si1-Pd1-Cl1) exhibit some deviation from linearity (177.88 (12) • and 177.36(3) • , respectively).The Si and Pd atoms are displaced from the O 4 and N 4 least-squares planes, respectively (into opposite directions), by 0.252(1) and 0.165(1) Å, respectively.The Pd-Si bond (2.6268(2) Å) is slightly longer than in the methimazolyl bridged paddlewheel complexes (where the Pd-Si bond lengths in the range 2.53-2.60Å were observed) [7], and slightly shorter than in the 7-azaindolyl bridged paddlewheels [8].The Si-C bond (1.853(4) Å) is only marginally longer than in the starting silane MeSi(pyO) 3 (1.837(2)Å), thus hinting at only a weak Pd→Si lone pair donor action.At hexacoordinate silicon with trans-disposed stronger donor (e.g., another hydrocarbyl group [42,43], 2-pyridinethiolato N atom [28] or 8-oxyquinolinyl N atom [44,45]), one would expect a Si-C bond lengthening beyond 1.90 Å.The Pd-Cl bond (2.891(1) Å), however, is unexpectedly long, thus hinting at ionic dissociation.This is supported by four H•••Cl contacts with the pyO-H 6 atoms.The 29 Si NMR shift of compound 1 (−116.9ppm in CDCl 3 ) is notably more upfield with respect to MeSi(pyO) 3 and Si(pyO) 4 , thus indicating the hypercoordination of the Si atom.This 29 Si NMR shift, however, may be representative of either penta-or hexacoordinate silicon, and therefore the role of the sixth donor moiety (Pd→Si) requires further elucidation (vide infra).
Inorganics 2018, 6, x FOR PEER REVIEW 4 of 19 Kameo et al. have shown that Cu(I) and Ag(I) are weaker donors [37].In this context, we need to note that a previous study by Bourissou et al. [38] revealed an interaction between the Si-Si σ-bond electron pair of a disilane (as donor) and Cu(I) (as acceptor) (XIII).Similar systems (with rather weak d 10 metal-silicon interaction) have been investigated for Ni(0), Pd(0), and Pt(0) by Grobe et al. (e.g., XIV) [39][40][41].Whereas Au(I), Ni(0), Pd(0), and Pt(0) are more susceptible to phosphine ligands, Cu(I) represents a d 10 system likely to bind to more than one or two N-donor ligands, and thus we included Cu(I) (as CuCl) in our investigations.
Chart 2. Selected metal-silicon complexes with donor-acceptor-interactions between the Si atom of a tetravalent silane and a d 10 metal atom.

Reactions of MeSi(pyO)3 with [PdCl2(NCMe)2] and CuCl
The reactions of MeSi(pyO)3 and [PdCl2(NCMe)2] (in 1:1 molar ratio) in chloroform proceeded with the partial dissolution of [PdCl2(NCMe)2] and the formation of a clear (slightly yellow, almost colorless) solution, from which the crystals of the chloroform solvate of compound 1 formed within one day (Scheme 2).The formal loss of Cl and the addition of a fourth pyO-bridge indicate ligand scrambling in the course of this reaction.The addition of excess MeSi(pyO)3 (as sacrificial pyO source) eventually led to the complete dissolution of [PdCl2(NCMe)2] and the formation of crystals of compound 1 • 2 CHCl3 in good yield.From such a crystal, the molecular structure of 1 was determined using X-ray diffraction analysis (Figure 2 and Table 1).In principle, the molecule has paddlewheel architecture with four pyO ligands attached to Si (via Si-O bonds) and Pd (via Pd-N bonds) of a Me-Si-Pd-Cl axis.The idealized planes of the pyO ligands are slightly tilted against the Si-Pd axis (Pd-Si-O-C torsion angles ranging between 34.6(3)° and 26.3(3)°), and the axial angles (C21-Si1-Pd1 and Si1-Pd1-Cl1) exhibit some deviation from linearity (177.88(12)° and 177.36(3)°, respectively).The Si and Pd atoms are displaced from the O4 and N4 least-squares planes, respectively (into opposite directions), by 0.252(1) and 0.165(1) Å, respectively.The Pd-Si bond (2.6268(2) Å) is slightly longer than in the methimazolyl bridged paddlewheel complexes (where the Pd-Si bond lengths in the range 2.53-2.60Å were observed) [7], and slightly shorter than in the 7-azaindolyl bridged paddlewheels [8].The Si-C bond (1.853(4) Å) is only marginally longer than in the starting silane MeSi(pyO)3 (1.837(2) Å), thus hinting at only a weak Pd→Si lone pair donor action.At hexacoordinate silicon with trans-disposed stronger donor (e.g., another hydrocarbyl group [42,43], 2-pyridinethiolato N atom [28] or 8-oxyquinolinyl N atom [44,45]), one would expect a Si-C bond lengthening beyond 1.90 Å.The Pd-Cl bond (2.891(1) Å), however, is unexpectedly long, thus hinting at ionic dissociation.This is supported by four H•••Cl contacts with the pyO-H 6 atoms.The 29 Si NMR shift of compound 1 (-116.9ppm in CDCl3) is notably more upfield with respect to MeSi(pyO)3 and Si(pyO)4, thus indicating the hypercoordination of the Si atom.This 29 Si NMR shift, however, may be representative of either penta-or hexacoordinate silicon, and therefore the role of the sixth donor moiety (Pd→Si) requires further elucidation (vide infra).The reaction of MeSi(pyO) 3 with CuCl (Scheme 2) proceeds in the expected straightforward manner in a 1:1 molar ratio, i.e., CuCl dissolves in chloroform and in tetrahydrofuran (THF) in the presence of one mol equivalent of MeSi(pyO) 3 , to afford an almost colorless (slightly greenish, by traces of Cu(II)) solution.Also, whereas the starting material produces a 29 Si NMR signal at −46.5 ppm (in CDCl 3 ), the resonance is shifted upfield for the solutions of MeSi(pyO) 3 with CuCl (−49.6 ppm for the THF solution, −64.1 ppm in CDCl 3 ).From the THF solution, some colorless crystals of the expected product 2 formed, which were suitable for single-crystal X-ray diffraction analysis (Figure 2 and Table 1).The crystal structure is comprised of two independent molecules that exhibit similar conformation, that is, a propeller with a Cl-Cu-Si-CH 3 axis and three pyO-bridges (bound to Si via Si-O bonds and to Cu via Cu-N bonds).The Cu•••Si separation is rather long (ca.3.2 Å), but it is shorter than in complex XII (M = Cu, Cu•••Si 3.48 Å) [37], and the effect of Cu(I) on the Si coordination sphere is evident from the flattening of the SiO 3 pyramidal base (sum of angles ca.337 • ).Furthermore, the 29 Si NMR shift of this solid is significantly upfield with respect to the starting silane (−70.0 and −71.4 ppm for the two crystallographically independent Si sites).In the CDCl 3 solution, compound 2 produces one set of broad 1 H NMR signals for the pyO moieties.Thus, this 1 H NMR signal broadening and the less pronounced upfield shift of the 29 Si NMR signal in the solution indicate conformational changes, such as coordination equilibria between isomers MeSi(µ-pyO) 3 CuCl and Me(κO-pyO) 2 Si(µ-pyO)CuCl, the latter with a weaker or absent Si•••Cu interaction.Upon repeated opening/closing of the Schlenk flasks with solutions of 2 in THF or chloroform (e.g., for drawing NMR samples), some blue crystals of the related copper(II) compound 3 formed (Scheme 2) as solvent free crystals (from THF) and chloroform solvate (from the chloroform solution).The crystal structures of both compounds were determined using single-crystal X-ray diffraction (Table 2), and the molecular structure of 3 in the solvent free crystals (Figure 2) is included in the discussion as a representative example.This molecule combines features of both compounds 1 and 2, as the Si•••Cu separation is rather long (2.92 Å), thus being similar to compound 2, but the axis of this paddlewheel complex is bridged by four pyO ligands.The Si-C bond length is intermediate between those of 1 and 2, but the Si-O bonds are almost as long as in 1, presumably as a result of the trans-arrangement of the Si-O bonds, but with a somewhat greater deviation of the O-Si-O axes from linearity in 3 (by ca. 25 • ).The Cu-N bond lengths in 3 are similar to those in 2, and we attribute this similarity to a combination of two antagonist effects, that is, bond shortening by a higher oxidation state of the Cu atom and bond lengthening by trans-arrangement along the N-Cu-N axes.Deviations of the O-Si-O and N-Cu-N axes from linearity as well as the rather long Si•••Cu separations are combined with displacement of Si and Cu atoms from the O 4 and N 4 least-squares planes, respectively, into opposite directions by 0.379(1) and 0.371(1) Å, respectively.Thus, the coordination spheres of both the Si and Cu atoms are best described as square-pyramidal.Unfortunately, the deliberate synthesis of larger amounts of 3 (by deliberate exposure of solutions of 2 to air) failed, and therefore we have not been able to isolate pure 3 for further spectroscopic or other investigation.Nonetheless, this compound represents a welcome link between compounds 1 and 2, and therefore we performed computational analyses of the electronic situations in 1, 2, and 3.For the following investigations, we used the crystallographically determined molecular structures of 1, 2, and 3 (Figure 2) as a starting point, followed by the optimization of the H atom positions for the isolated molecules in the gas phase.
Natural localized molecular orbital (NLMO) analyses were performed using DFT-(RO)B3LYP functional with an SDD basis set for Pd and Cu, 6-311+G(d) for C, Cl, H, N, O, and Si (for details see experimental section).Figure 3 shows the NLMOs identified for the TM→Si donor-acceptor σ-interaction, and Table 3 lists the selected features of these NLMOs.In all three of the cases, the NLMO analysis treated this interaction as a donation of a mainly TM localized lone pair into a vacant orbital at Si.The latter has a σ*(Si-Me) character, which reasons its rather poor acceptor qualities.Thus, in contrast to Pd→Si-Cl systems such as I and II, which feature Pd/Si contributions of 84%/12% [6] and 83%/15% [8], respectively, the corresponding σ-donor electron pair in 1 is more TM localized (91% Pd contribution).In Cu-Si complexes 2 and 3, the corresponding lone pair is even more metal localized (ca.98%), and has less than a 1% Si contribution.Thus, it resembles a d(z 2 ) orbital more closely.Regardless of the different oxidation states of the Cu atoms and the different Cu•••Si separations, the characteristics of the Cu→Si NLMOs of these two compounds are surprisingly similar.As it was shown for the related Pd and Ni systems that TM→Si interactions of related complexes from first and second row TMs may be very similar (83% Ni, 13% Si contribution to the Ni→Si NLMO in the Ni-analog of compound I) [6], we attribute the poor donicity of Cu in compounds 2 and 3 to the enhanced effective nuclear charge (group 11 instead of group 10 element) rather than to Cu being a first row transition metal.As the TM-Si interactions in compounds 1 and especially in 2 and 3 appear to be of an electrostatic/polar nature rather than covalent, the non-covalent interactions (NCI) descriptor of these compounds was analyzed (Figure 4).In compound 1, a strong electrostatic non-covalent interaction between Pd and Si is detected, represented by a deep blue disc-or toroid-shaped area of the NCI along the Pd-Si bond.This feature is similar to the NCI along the Pd-Cl bond in this compound (and similar to the Cu-Cl bonds in compounds 2 and 3).In sharp contrast, the light blue color of the NCI encountered along the Cu-Si paths in 2 and 3 hints at significantly weaker interactions, and their nature seems to be more closely related to the polar interactions of the pyO 6 -hydrogen atoms with the metal bound chloride (i.e., weak hydrogen contacts).
In order to quantify these non-covalent interactions (TM•••Si vs. C-H•••Cl), topological analyses of the electron density distributions in compounds 1, 2, and 3 were performed using the atoms in molecules (AIM) approach.This AIM analysis of the wave function detected the bond critical points (BCPs) between Si and TM (TM = Pd, Cu) in all three of the complexes.Some of their features are listed in Table 4.For compounds 2 and 3, the electron density ρ(rb) and the positive Laplacian ∇ 2 ρ(rb) are of low magnitude, and the ratio |V(rb)|/G(rb) slightly above 1 is indicative of an intermediate closed shell interaction with pronounced ionic contribution [46] (in accordance with the low Wiberg bond order (WBO) [47]).The |V(rb)|/G(rb) ratio in complex 1 is slightly greater than 2, and in combination with the negative Laplacian, it is indicative of a strong polarized shared shell interaction [48][49][50].According to Espinosa et al. [51], the ratio H(rb)/ρ(rb) can be utilized as a covalence degree parameter (for systems where d < dcov, |V(rb)| > G(rb), H(rb) < 0), the greater magnitude of which indicates the stronger atom-atom interaction (leading to an order of increasing strength 2 < 3 < 1).For the series under investigation, the analysis of the estimated interaction energies (Eint) [52,53]  Furthermore, for the paramagnetic compound 3, this analysis afforded a Mulliken spin density distribution of 65.9% Cu-localization and 6.9-7.3%, located at each of the four nitrogen atoms.This is in accordance with the ligand field theory, which would assign the unpaired electron of a d 9 system  As the TM-Si interactions in compounds 1 and especially in 2 and 3 appear to be of an electrostatic/polar nature rather than covalent, the non-covalent interactions (NCI) descriptor of these compounds was analyzed (Figure 4).In compound 1, a strong electrostatic non-covalent interaction between Pd and Si is detected, represented by a deep blue disc-or toroid-shaped area of the NCI along the Pd-Si bond.This feature is similar to the NCI along the Pd-Cl bond in this compound (and similar to the Cu-Cl bonds in compounds 2 and 3).In sharp contrast, the light blue color of the NCI encountered along the Cu-Si paths in 2 and 3 hints at significantly weaker interactions, and their nature seems to be more closely related to the polar interactions of the pyO 6 -hydrogen atoms with the metal bound chloride (i.e., weak hydrogen contacts).
In order to quantify these non-covalent interactions (TM•••Si vs. C-H•••Cl), topological analyses of the electron density distributions in compounds 1, 2, and 3 were performed using the atoms in molecules (AIM) approach.This AIM analysis of the wave function detected the bond critical points (BCPs) between Si and TM (TM = Pd, Cu) in all three of the complexes.Some of their features are listed in Table 4.For compounds 2 and 3, the electron density ρ(r b ) and the positive Laplacian ∇ 2 ρ(r b ) are of low magnitude, and the ratio |V(r b )|/G(r b ) slightly above 1 is indicative of an intermediate closed shell interaction with pronounced ionic contribution [46] (in accordance with the low Wiberg bond order (WBO) [47]).The |V(r b )|/G(r b ) ratio in complex 1 is slightly greater than 2, and in combination with the negative Laplacian, it is indicative of a strong polarized shared shell interaction [48][49][50].According to Espinosa et al. [51], the ratio H(r b )/ρ(r b ) can be utilized as a covalence degree parameter (for systems where d < d cov , |V(r b )| > G(r b ), H(r b ) < 0), the greater magnitude of which indicates the stronger atom-atom interaction (leading to an order of increasing strength 2 < 3 < 1).For the series under investigation, the analysis of the estimated interaction energies (E int ) [52,53] yields an order of increasing E int (TM-Si) in compounds 2 ∼ = 3 < 1.The estimated E int and the ratio H(r b )/ρ(r b ) indicate a slightly stronger interaction in 3 relative to 2. In accordance with the NCI, the Cu•••Si interactions in 2 and 3 are indeed similar to the C-H•••Cl contacts in these molecules in terms of energetics.Whereas the E int (Cu•••Si) in 2 and 3 were estimated to −2.3 and −3.7 kcal•mol −1 , respectively, the same approach of the analysis of V(r b ) at the BCPs the C-H•••Cl contacts in 1, 2, and 3 yielded an average E int of −1.90, −1.65, and −2.02 kcal mol −1 , respectively.Furthermore, for the paramagnetic compound 3, this analysis afforded a Mulliken spin density distribution of 65.9% Cu-localization and 6.9-7.3%, located at each of the four nitrogen atoms.This is in accordance with the ligand field theory, which would assign the unpaired electron of a d 9 system in the square pyramidal coordination sphere to the d(x 2 −y 2 ) orbital, thus providing a d(z 2 )-located lone pair for potential donor-acceptor interactions perpendicular to the Cu(II)N 4 plane in this particular case of compound 3.

Reactions of Si(pyO)4 with [PdCl2(NCMe)2] and CuCl2
In analogy to the formation of I and II from the respective silane Si(L)4 (L = bridging ligand) and [PdCl2(NCMe)2], Si(pyO)4 should be capable of forming paddlewheel complexes of the type ClSi(µ-pyO)4TMCl upon reaction with TMCl2, or suitable complexes thereof.Thus, we aimed at synthesizing ClSi(µ-pyO)4PdCl (4') through the reaction of [PdCl2(NCMe)2] and Si(pyO)4 in chloroform (Scheme 3).As the main product, a beige solid of very poor solubility formed.In the dispersion of this fine solid in chloroform, some coarse crystals (beige, almost colorless) formed in the course of some days of storage at room temperature.Single-crystal X-ray diffraction analysis (Table 2, Figure 5) revealed the identity of this compound as the HpyO-adduct of the intended product (compound 4, Scheme 3).Presumably, the intended product 4' had formed initially and then reacted further with traces of free HpyO.The deliberate synthesis of 4 by reacting [PdCl2(NCMe)2], Si(pyO)4, and HpyO in a 1:1:1 molar ratio in chloroform, eventually afforded this compound in good yield.The molecular structure of the cation [ClPd(µ-pyO)4Si(HpyO)] + of 4 resembles the paddlewheel architecture of compound 1, and notable differences that arise from the different substituent at Si (trans to Pd) are the following: The Pd-Si bond is noticeably shorter (2.50 Å) because of the more electronegative Si-bound substituent.As a consequence, the Si atom is less

Reactions of Si(pyO) 4 with [PdCl 2 (NCMe) 2 ] and CuCl 2
In analogy to the formation of I and II from the respective silane Si(L) 4 (L = bridging ligand) and [PdCl 2 (NCMe) 2 ], Si(pyO) 4 should be capable of forming paddlewheel complexes of the type ClSi(µ-pyO) 4 TMCl upon reaction with TMCl 2 , or suitable complexes thereof.Thus, we aimed at synthesizing ClSi(µ-pyO) 4 PdCl (4') through the reaction of [PdCl 2 (NCMe) 2 ] and Si(pyO) 4 in chloroform (Scheme 3).As the main product, a beige solid of very poor solubility formed.In the dispersion of this fine solid in chloroform, some coarse crystals (beige, almost colorless) formed in the course of some days of storage at room temperature.Single-crystal X-ray diffraction analysis (Table 2, Figure 5) revealed the identity of this compound as the HpyO-adduct of the intended product (compound 4, Scheme 3).Presumably, the intended product 4' had formed initially and then reacted further with traces of free HpyO.The deliberate synthesis of 4 by reacting [PdCl 2 (NCMe) 2 ], Si(pyO) 4 , and HpyO in a 1:1:1 molar ratio in chloroform, eventually afforded this compound in good yield.The molecular structure of the cation [ClPd(µ-pyO) 4 Si(HpyO)] + of 4 resembles the paddlewheel architecture of compound 1, and notable differences that arise from the different substituent at Si (trans to Pd) are the following: The Pd-Si bond is noticeably shorter (2.50 Å) because of the more electronegative Si-bound substituent.As a consequence, the Si atom is less displaced from the O 4 least-squares plane (by 0.101(1) Å), whereas the displacement of the Pd atom from the N 4 plane (into the opposite direction, by 0.167(1) Å) is not altered.Interestingly, the Si1-O5 bond (trans to Pd) is significantly shorter than the equatorial Si-O bonds, thus hinting at still rather poor Pd→Si donor action.In spite of the NH group of the trans-Pd-Si located HpyO moiety, the C-O bonds of all five of the pyO and HpyO ligands are very similar, ranging between 1.313(2) and 1.325(2) Å.The 29 Si NMR shift of this compound (δ 29 Si −147.9 ppm in CD 2 Cl 2 ) is in accordance with the hexacoordination of the Si atom, and the 1 H and 13 C NMR spectra feature two sets of pyO-signals in a 4:1 intensity ratio, reflecting the four bridging and the dangling pyO moieties, respectively, and the retention of this molecular architecture in solution.Upon harvesting the crystalline solid 4 • 4 CHCl3, some yellow crystals of another compound formed in the filtrate.Using single-crystal X-ray diffraction analysis, they were identified as the chloroform solvate 5 • 6 CHCl3 of the complex 5 (Scheme 4).The deliberate synthesis of this complex, by using [PdCl2(NCMe)2], Si(pyO)4, and HpyO in a 2:1:2 molar ratio in chloroform, afforded compound 5 in good yield.Interestingly, the crystals that were initially formed during the synthesis consisted of a different solvate (5 • 8 CHCl3), and upon filtration, some more crystals of solvate 5 • 2 CHCl3 formed in the filtrate.The elemental analysis of the final product upon drying was in agreement with the composition of the solvate 5 • 2 CHCl3.Even though all three solvates (5 • 8, 6, 2 CHCl3, respectively) were characterized crystallographically (Table 5), only the molecular structure of 5 in the solvate 5 • 6 CHCl3 is discussed as a representative example (Figure 6), as there are only marginal differences between the molecular conformations in the three different solvates.
The Si atom of compound 5 is, in the crystal structures, located on a center of inversion, thus the trans angles are 180°, and the very similar Si-O bond lengths and cis angles close to 90° (maximum  Upon harvesting the crystalline solid 4 • 4 CHCl3, some yellow crystals of another compound formed in the filtrate.Using single-crystal X-ray diffraction analysis, they were identified as the chloroform solvate 5 • 6 CHCl3 of the complex 5 (Scheme 4).The deliberate synthesis of this complex, by using [PdCl2(NCMe)2], Si(pyO)4, and HpyO in a 2:1:2 molar ratio in chloroform, afforded compound 5 in good yield.Interestingly, the crystals that were initially formed during the synthesis consisted of a different solvate (5 • 8 CHCl3), and upon filtration, some more crystals of solvate 5 • 2 CHCl3 formed in the filtrate.The elemental analysis of the final product upon drying was in agreement with the composition of the solvate 5 • 2 CHCl3.Even though all three solvates (5 • 8, 6, 2 CHCl3, respectively) were characterized crystallographically (Table 5), only the molecular structure of 5 in the solvate 5 • 6 CHCl3 is discussed as a representative example (Figure 6), as there are only marginal differences between the molecular conformations in the three different solvates.
The Si atom of compound 5 is, in the crystal structures, located on a center of inversion, thus the trans angles are 180°, and the very similar Si-O bond lengths and cis angles close to 90° (maximum Upon harvesting the crystalline solid 4 • 4 CHCl 3 , some yellow crystals of another compound formed in the filtrate.Using single-crystal X-ray diffraction analysis, they were identified as the chloroform solvate 5 • 6 CHCl 3 of the complex 5 (Scheme 4).The deliberate synthesis of this complex, by using [PdCl 2 (NCMe) 2 ], Si(pyO) 4 , and HpyO in a 2:1:2 molar ratio in chloroform, afforded compound 5 in good yield.Interestingly, the crystals that were initially formed during the synthesis consisted of a different solvate (5 • 8 CHCl 3 ), and upon filtration, some more crystals of solvate 5 • 2 CHCl 3 formed in the filtrate.The elemental analysis of the final product upon drying was in agreement with the composition of the solvate 5 • 2 CHCl 3 .Even though all three solvates (5 • 8, 6, 2 CHCl 3 , respectively) were characterized crystallographically (Table 5), only the molecular structure of 5 in the solvate 5 • 6 CHCl 3 is discussed as a representative example (Figure 6), as there are only marginal differences between the molecular conformations in the three different solvates.
The Si atom of compound 5 is, in the crystal structures, located on a center of inversion, thus the trans angles are 180       In an attempt at synthesizing a paddlewheel complex of the composition ClCu(µ-pyO) 4 SiCl, in comparison to the attempted synthesis of 4' (Scheme 3, left), anhydrous CuCl 2 and Si(pyO) 4 were dispersed in chloroform.Whereas most of the reactants remained unchanged, the solution phase became blue and some blue crystals formed upon storage within one week.The crystals were identified as compound 6 by single-crystal X-ray diffraction analysis.Apparently, traces of HpyO in the sample gave rise to the formation of this Cu-analog of compound 5. Deliberate synthesis, by using CuCl 2 , Si(pyO) 4 , and HpyO in a 2:1:2 molar ratio in chloroform, afforded compound 6 in good yield (Scheme 4).Because of the very poor solubility and the paramagnetic Cu(II) sites in this compound, NMR spectroscopic characterization was no option, and therefore we only discuss the molecular structure of this complex.The central parts of the molecule, that is, the bond lengths and angles of the hexacoordinate Si atom as well as the equatorial arrangement of two Si(µ-pyO) 2 M clamps and two axially situated HpyO ligands, which establish N-H•••O hydrogen bridges to adjacent pyO oxygen atoms, are very similar to the arrangement in compound 5.The noteworthy difference is associated with the Cu(II) coordination sphere, which is distorted and intermediate between the tetrahedral and square planar.Then sum of the cis angles of Cu (367.9(2)• ) deviates significantly from planarity.The N-Cu-N and Cl-Cu-Cl angles in particular are wider than 90 • , and the angle between the CuN 2 and CuCl 2 planes is 30.7(2)• .Thus, in spite of the notable distortion, this coordination sphere is still closer to the square rather than tetrahedral.

Syntheses
MeSi(pyO) 3 .A Schlenk flask with magnetic stirring bar was charged with 2-hydroxypyridine (1.50 g, 15.6 mmol), evacuated, and set under Ar atmosphere prior to adding THF (50 mL) and triethylamine (1.89 g, 18.7 mmol), with stirring to afford a colorless solution.With continuous stirring at room temperature, methyltrichlorosilane (0.82 g, 5.5 mmol) was added dropwise via syringe, while the simultaneous formation of a white precipitate (Et 3 NHCl) was observed.Upon the complete addition of the silane stirring at room temperature was continued for 1 h, whereupon the hydrochloride precipitate was filtered off and washed with THF (2 × 5 mL).From the combined filtrate and washings, the solvent was removed under reduced pressure (condensed into a cold trap) to afford a crystalline residue, which was dissolved in hot THF (5 mL) and filtered prior to the addition of hexane (10 mL), and was stored at 6 • C. In the course of three days, colorless crystals of the product formed, which were separated by decantation (while cold) and were dried in vacuo (yield 0.90 g (2.7 mmol, 50%)).The crystals were suitable for X-ray diffraction analysis.Si(pyO) 4 .In a Schlenk flask with magnetic stirring bar 2-trimethylsiloxypyridine (3.00 g, 18.0 mmol) was dissolved in chloroform (4 mL), whereupon SiCl 4 (0.82 g, 4.8 mmol) was added (via syringe) with stirring.The resultant solution was heated with stirring under reflux for 2 h, to afford a white precipitate of the product.The mixture was allowed to attain room temperature, the solid was filtered, washed with chloroform (2 × 5 mL), and dried in vacuo.The yield was 1.08 g (2.67 mmol, 57%).The single-crystals suitable for X-ray diffraction analysis were obtained by recrystallization in THF.ClPd(µ-pyO) 4 SiMe • 2 CHCl 3 (complex 1 • 2 CHCl 3 ).A Schlenk flask was charged with a magnetic stirring bar, MeSi(pyO) 3 (0.40 g, 1.2 mmol) and [PdCl 2 (NCMe) 2 ] (0.16 g, 0.62 mmol), evacuated, and set under Ar atmosphere prior to adding chloroform (3 mL).Upon brief stirring at room temperature (within two minutes), the starting materials dissolved completely to afford a light-yellow solution.Immediately after dissolution, the stirring was stopped and the solution was stored undisturbed at room temperature.In the course of one day, colorless crystals of the product formed, but the solution was stored for another week in order to complete crystallization, whereupon the crystals were separated from the supernatant by decantation and then briefly dried in vacuo.A single-crystal suitable for X-ray diffraction analysis was taken out of the mother liquor.The yield was 0.31 g (0.39 mmol, 65%).The elemental analysis for C 23  ClCu(µ-pyO) 3 SiMe (complex 2).Procedure A: A Schlenk flask was charged with a magnetic stirring bar, MeSi(pyO) 3 (0.44 g, 1.4 mmol) and CuCl (0.14 g, 1.4 mmol), evacuated, and set under Ar atmosphere prior to adding THF (1.5 mL).Upon brief stirring at room temperature (within five minutes), the starting materials dissolved completely to afford a light turquoise (almost colorless) solution. 29Si NMR spectroscopic analysis of this solution (with D 2 O capillary used as lock) revealed one signal (at −49.6 ppm).In the course of one day, colorless crystals of the product formed, whereupon the crystals were separated from the supernatant by decantation, and then briefly dried in vacuo.A single-crystal suitable for X-ray diffraction analysis was taken out of the mother liquor.The yield was 0.12 g (0.28 mmol, 20%).The elemental analysis for C 16 H 15 ClCuN 3 O 3 Si (424.39 g•mol −1 ) was as follows: C, 45.28; H, 3.56; N, 9.90; found C, 43.44; H, 3.67; N, 9.44.The composition found indicates the presence of ca.4% of "inert" materials (free of C, H, N), such as CuCl, as C, H, and N were found in the expected ratio. 29 Procedure B: A Schlenk flask was charged with a magnetic stirring bar, MeSi(pyO) 3 (0.30 g, 0.92 mmol) and CuCl (0.09 g, 0.92 mmol), evacuated, and set under Ar atmosphere prior to adding CDCl 3 (1.0 mL).Upon brief stirring at room temperature (within five minutes), the starting materials dissolved completely to afford a light turquoise (almost colorless) solution, which was used for the 1 H and 29 Si NMR spectroscopic analysis.Compound 2 did not crystallize from this solution. 1H NMR obtained from a mixture of Si(pyO) 4 (0.10 g, 0.25 mmol) and CuCl 2 (0.03 g, 0.025 mmol), which was layered with chloroform (1 mL) and stored undisturbed at room temperature for one week.

Conclusions
Two different 2-pyridyloxysilanes, MeSi(pyO) 3 and Si(pyO) 4 , proved to be suitable starting materials for the syntheses of heteronuclear complexes, in which the ambidentate pyO ligand binds to Si via Si-O, and to transition metals via TM-N bonds.The chelation of the Si atom by this ligand has not been encountered, thus the pyO nitrogen atoms are readily available for complex formation.
As shown for a d 8 and a d 10 system (with formation of ClPd(µ-pyO) 4 SiMe 1 and ClCu(µ-pyO) 3 SiMe 2, respectively), pyridyloxysilanes may support the formation of different paddlewheel structures, that is, with four and three bridging ligands, respectively.The pyO buttresses do not force Si and TM into close proximity, as it is evident from the different Pd-Si and Cu-Si separations in 1 and 2, respectively.A higher coordination of the Si atom by the transition metal site still depends on the donicity of the TM site and the acceptor qualities of Si.As shown in the current study, the Si-Me moiety trans to an electron rich TM (Pd(II) in this particular case) causes weaker TM-Si interactions with respect to previously reported complexes with related TM→Si-Cl motif.Furthermore, group 11 metals (Cu(I) and Cu(II)) were shown to be significantly weaker lone pair donors toward Si(IV) than group 10 metals (e.g., Pd(II) and, according to previous studies, Ni(II) [6,7]).A systematic comparison of this series was enabled by the unintended formation of the decomposition product ClCu(µ-pyO) 4 SiMe 3. To our knowledge, compound 3 represents the first complex with a Cu(II)-Si(IV) bond (or at least with such a short (2.9 Å) Cu(II)•••Si(IV) separation).As shown by the quantum chemical calculations, the Cu•••Si interactions in compounds 2 and 3 are slightly stronger than the C-H•••Cl hydrogen contacts encountered in the same molecules.
The reactions of Si(pyO) 4 with [PdCl 2 (NCMe) 2 ] and CuCl 2 , in the presence of 2-hydroxypyridine HpyO, gave rise to the formation of an entirely different class of hypercoordinate Si-complexes of the type (HpyO) 2 Si[(µ-pyO) 2 TMCl 2 ] 2 , 5 and 6, respectively.As the coordination spheres about TM vary between square planar and distorted tetrahedral in compounds 5 and 6, respectively, this kind of complex architecture may turn out to be suitable for the complexation of various further TMX 2 moieties (X = halide, pseudo-halide etc.).

Supplementary Materials:
The following are available online at http://www.mdpi.com/2304-6740/6/4/119/s1: the crystallographic data of the compounds reported in this paper in CIF format.
Author Contributions: L.E. and J.W. conceived and designed the experiments; L.E. performed the experiments (syntheses); R.G. the computational analyses; J.W. the single-crystal X-ray diffraction analyses; E.B. the solid state NMR spectroscopic analyses; J.W. wrote the paper.
Funding: This research received no external funding.

Chart 2 .
Selected metal-silicon complexes with donor-acceptor-interactions between the Si atom of a tetravalent silane and a d 10 metal atom.

Figure 3 .
Figure 3. Natural localized molecular orbital (NLMO) representations of the TM→Si interaction in (from left) compounds 1, 2, and 3.For compound 3, the α-spin contribution is shown.NLMOs are depicted with an isosurface value of 0.02, the atom color code is consistent with Figure 2.
yields an order of increasing Eint(TM-Si) in compounds 2 ≅ 3 < 1.The estimated Eint and the ratio H(rb)/ρ(rb) indicate a slightly stronger interaction in 3 relative to 2. In accordance with the NCI, the Cu•••Si interactions in 2 and 3 are indeed similar to the C-H•••Cl contacts in these molecules in terms of energetics.Whereas the Eint(Cu•••Si) in 2 and 3 were estimated to −2.3 and −3.7 kcal•mol −1 , respectively, the same approach of the analysis of V(rb) at the BCPs the C-H•••Cl contacts in 1, 2, and 3 yielded an average Eint of −1.90, −1.65, and −2.02 kcal mol −1 , respectively.

Figure 3 .
Figure 3. Natural localized molecular orbital (NLMO) representations of the TM→Si interaction in (from left) compounds 1, 2, and 3.For compound 3, the α-spin contribution is shown.NLMOs are depicted with an isosurface value of 0.02, the atom color code is consistent with Figure 2.

Inorganics 2018, 6 ,
x FOR PEER REVIEW 9 of 19 in the square pyramidal coordination sphere to the d(x 2 −y 2 ) orbital, thus providing a d(z 2 )-located lone pair for potential donor-acceptor interactions perpendicular to the Cu(II)N4 plane in this particular case of compound 3.

Table 4 .
Selected features of the bond critical points (BCPs) of the TM-Si interaction in compounds 1, 2, and 3 derived from the topological analyses (AIM) of the wave function.

Inorganics 2018, 6 ,
x FOR PEER REVIEW 10 of 19 displaced from the O4 least-squares plane (by 0.101(1) Å), whereas the displacement of the Pd atom from the N4 plane (into the opposite direction, by 0.167(1) Å) is not altered.Interestingly, the Si1-O5 bond (trans to Pd) is significantly shorter than the equatorial Si-O bonds, thus hinting at still rather poor Pd→Si donor action.In spite of the NH group of the trans-Pd-Si located HpyO moiety, the C-O bonds of all five of the pyO and HpyO ligands are very similar, ranging between 1.313(2) and 1.325(2) Å. The29 Si NMR shift of this compound (δ 29 Si −147.9 ppm in CD2Cl2) is in accordance with the hexacoordination of the Si atom, and the 1 H and13 C NMR spectra feature two sets of pyO-signals in a 4:1 intensity ratio, reflecting the four bridging and the dangling pyO moieties, respectively, and the retention of this molecular architecture in solution.
• , and the very similar Si-O bond lengths and cis angles close to 90 • (maximum deviations of ca. 1 • ) furnish an Si atom almost perfect octahedrally coordinated by six pyO oxygen atoms.Two sets of mutually cis situated anionic pyO ligands, the four O atoms of which are located in one plane, act as (N,N)-chelate donors toward PdCl 2 , and the axial positions of the Si coordination sphere are occupied by the O atoms of HpyO, each of which establishes an N-H•••O contact to an adjacent pyO oxygen atom (as indicated in Scheme 4).In spite of the very similar Si-O bond lengths, the C-O bonds of the HpyO moieties (1.303(3) Å) are significantly shorter than the C-O bonds of the bridging pyO moieties (1.328(3) Å), and thus exhibit a pronounced double bond character, as expected for HpyO.In the solvate 5 • 8 CHCl 3 , these differences are even more pronounced with 1.296(3) vs. 1.331(3) and 1.336(3) Å.As expected, the Pd atoms are situated in a square planar coordination sphere (with a sum of cis angles of 360.3(1) • ).Compound 5 exhibited too poor of a solubility for solution NMR characterization, but the 29 Si cross-polarization magic-angle-spinning (CP/MAS) NMR spectroscopy of the solid unequivocally confirmed the hexacoordination of the central Si atom (δ 29 Si −190.2 ppm).This chemical shift is similar to other hexacoordinate Si complexes with SiO 6 coordination sphere (e.g., Si(acetylacetonate) 2 (salicylate) δ 29 Si −191.7 ppm) [54].Inorganics 2018, 6, x FOR PEER REVIEW 11 of 19 deviations of ca.1°) furnish an Si atom almost perfect octahedrally coordinated by six pyO oxygen atoms.Two sets of mutually cis situated anionic pyO ligands, the four O atoms of which are located in one plane, act as (N,N)-chelate donors toward PdCl2, and the axial positions of the Si coordination sphere are occupied by the O atoms of HpyO, each of which establishes an N-H•••O contact to an adjacent pyO oxygen atom (as indicated in Scheme 4).In spite of the very similar Si-O bond lengths, the C-O bonds of the HpyO moieties (1.303(3) Å) are significantly shorter than the C-O bonds of the bridging pyO moieties (1.328(3) Å), and thus exhibit a pronounced double bond character, as expected for HpyO.In the solvate 5 • 8 CHCl3, these differences are even more pronounced with 1.296(3) vs. 1.331(3) and 1.336(3) Å.As expected, the Pd atoms are situated in a square planar coordination sphere (with a sum of cis angles of 360.3(1)°).Compound 5 exhibited too poor of a solubility for solution NMR characterization, but the 29 Si cross-polarization magic-angle-spinning (CP/MAS) NMR spectroscopy of the solid unequivocally confirmed the hexacoordination of the central Si atom (δ 29 Si −190.2 ppm).This chemical shift is similar to other hexacoordinate Si complexes with SiO6 coordination sphere (e.g., Si(acetylacetonate)2(salicylate) δ 29 Si −191.7 ppm) [54].
Kameo et al. have shown that Cu(I) and Ag(I) are weaker donors

Table 3 .
Selected features of the natural localized molecular orbitals (NLMOs) of the TM→Si interaction in compounds 1, 2, and 3.

Table 3 .
Selected features of the natural localized molecular orbitals (NLMOs) of the TM→Si interaction in compounds 1, 2, and 3.