inorganics Article (2-Pyridyloxy)silanes as Ligands in Transition Metal Coordination Chemistry Lisa Ehrlich 1, Robert Gericke 1 , Erica Brendler 2 and Jörg Wagler 1,* 1 Institut für Anorganische Chemie, TU Bergakademie Freiberg, D-09596 Freiberg, Germany; [email protected] (L.E.); [email protected] (R.G.) 2 Institut für Analytische Chemie, TU Bergakademie Freiberg, D-09596 Freiberg, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-3731-39-4343 Received: 29 September 2018; Accepted: 26 October 2018; Published: 31 October 2018 Abstract: 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. Keywords: AIM; DFT; intermetallic bond; 29Si NMR spectroscopy; X-ray diffraction 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 [1–4]. 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 (Chart1)[ 5–8]. 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 [9–13], 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)[14–18]. In complex IX and some other compounds with group Inorganics 2018, 6, 119; doi:10.3390/inorganics6040119 www.mdpi.com/journal/inorganics Inorganics 2018,, 6,, 119x FOR PEER REVIEW 22 of of 19 a tetravalent Si atom (e.g., VI, VII, VIII, and IX) [14–18]. In complex IX and some other compounds 9 metals [19–22], the 2-pyridyloxy (pyO) ligand was successfully utilized for stabilizing the TM–Si with group 9 metals [19–22], the 2-pyridyloxy (pyO) ligand was successfully utilized for stabilizing bond by forming two bridges over the heterodinuclear core. Some other compounds have been the TM–Si bond by forming two bridges over the heterodinuclear core. Some other compounds have reported, in which one pyO ligand bridges the TM–Si bond (e.g., X, and some others) [23–25]. In all of been reported, in which one pyO ligand bridges the TM–Si bond (e.g., X, and some others) [23–25]. these Si(µ-pyO)TM compounds, the ambidentate pyO ligand is bound to silicon via the Si–O bond, In all of these Si(µ-pyO)TM compounds, the ambidentate pyO ligand is bound to silicon via the Si– while the softer Lewis base (pyridine N atom) is TM bound. This structural motif should enable access O bond, while the softer Lewis base (pyridine N atom) is TM bound. This structural motif should to a new class of paddlewheel-shaped complexes with formally dative TM!Si bonds, in which the Si enable access to a new class of paddlewheel-shaped complexes with formally dative TM→Si bonds, atom may carry a sterically more demanding group or alkyl group, because of the rather poor steric in which the Si atom may carry a sterically more demanding group or alkyl group, because of the demand of the surrounding donor atoms (i.e., O atoms), whereas in the previously reported TM!Si rather poor steric demand of the surrounding donor atoms (i.e., O atoms), whereas in the previously I II paddlewheelreported TM→ complexesSi paddlewheel (such as complexesand ), the(such donor as atomI and situation II), the donor at silicon atom merely situation allowed at forsilicon the presencemerely allowed of a small for electronegativethe presence of H-acceptora small electronegative moiety (i.e., H-acceptor a halide). moiety (i.e., a halide). Chart 1. Selected metal–silicon complexes. 2. Results and Discussion 2.1. Syntheses and Characterization of Silanes MeSi(pyO)3 andand Si(pyO) 4 For the starting materials, we synthesizedsynthesized MeSi(pyO)MeSi(pyO)3 via triethylamine supported reaction of MeSiCl3 and 2-hydroxypyridine, and Si(pyO) 44,, via via transsilylation, transsilylation, with with the the preceding synthesis of Me3Si(pyO),Si(pyO), which which is is known in the literature [[26]26] (Scheme1 1).). BothBoth ofof thethe silanessilanes formedformed crystalscrystals suitable forfor single-crystal single-crystal X-ray X-ray diffraction diffraction analysis analysis (Figure (Figure1 and Table1 and1). InTable both 1). of theIn compounds,both of the thecompounds, Si atom is the essentially Si atom is tetracoordinate essentially tetracoordinate and the coordination and the coordi spherenation may besphere described may be as described (4 + 3) in MeSi(pyO)as (4 + 3) in3 MeSi(pyO)and (4 + 4)3 and in Si(pyO) (4 + 4) 4in, because Si(pyO) of4, because the pyridine of the N pyridine atoms, whichN atoms, are which capping are thecapping faces ofthe the faces tetrahedral of the coordinationtetrahedral coordination spheres from distancesspheres from close todistances the sum close of the to van-der-Waals the sum of radiithe (rangingvan-der-Waals between radii 2.91 (ranging and 3.03 between Å). This 2.91 tetracoordination and 3.03 Å). This of tetracoordination Si in Si(pyO)4 is of in Si contrast in Si(pyO) to the4 is in Si hexacoordinationcontrast to the Si in hexacoordination the thio analog Si(pyS) in the4 [ 27thio] (and analog other Si(pyS) pyS-bearing4 [27] Si (and compounds other pyS-bearing [28]), in which Si twocompounds of the 2-mercaptopyridyl [28]), in which two ligands of the form2-mercapto four-memberedpyridyl ligands chelates, form and four-membered in complexes of chelates, the N-oxide and ofin thecomplexes pyO system, of the whichN-oxide forms of the five-membered pyO system, which chelates forms [29,30 five-membered]. chelates [29,30]. Inorganics 2018, 6, 119 3 of 19 InorganicsInorganics 2018, 6 2018, x FOR, 6, x PEERFOR PEER REVIEW REVIEW 3 of3 19 of 19 Scheme 1. Syntheses of starting materials MeSi(pyO)3 and Si(pyO)4. SchemeScheme 1. 1.Syntheses Syntheses of ofstarting starting materialsmaterials MeSi(pyO) 33 andand Si(pyO) Si(pyO)4. 4. Figure 1. Molecular structures of MeSi(pyO)3 and Si(pyO)4 in the crystal; thermal displacement Figure 1. Molecular structures of MeSi(pyO)3 and Si(pyO)4 in the crystal; thermal displacement ellipsoidsellipsoids are drawn are drawn at the at the 50% 50% probability probability level; level; H atoms H atoms are omitted are omitted for clarity for and clarity selected and atoms selected atoms areare labeled.labeled. Because Because of the of special the special crystallographic crystallographic position of position the Si atom of of the Si(pyO) Si atom4 in the of solid Si(pyO) (S4 4 in theFigure solidsymmetry), (1.S 4Molecularsymmetry), the asymmetricstructures the asymmetric unitof MeSi(pyO) consists unit of consists3 ¼and of theSi(pyO) ofmolecule.1/44 ofin thetheThe molecule. crystal;asterisked thermal labels The asterisked indicatedisplacement thelabels indicateellipsoidssymmetry the symmetryare drawn equivalents. at equivalents. the 50% Selected probability Selected interatomic level; interatomic dist H ancesatoms distances (Å) are andomitted (Å)angles andfor (deg) clarity angles are and (deg)as selectedfollows, are as atomsfor follows, are labeled.MeSi(pyO) Because3: Si1–O1 of the 1.645(1), special Si1–O2 crystallographic 1.638(1), Si1–O3 position 1.652(1), of the Si1–C16 Si atom 1.837(2), of Si(pyO) Si1···N14 in the 2.998(1), solid (S4 for MeSi(pyO)3: Si1–O1 1.645(1), Si1–O2 1.638(1), Si1–O3 1.652(1), Si1–C16 1.837(2), Si1···N1 2.998(1), Si1···N2 3.028(1), Si1···N3 2.920(1), O1–Si1–O2 114.68(5), O2–Si1–O3 112.74(5), O1–Si1–O3 97.77(5), Si1symmetry),···N2 3.028(1), the Si1asymmetric···N3 2.920(1), unit consists O1–Si1–O2 of ¼ 114.68(5),of the molecule.