Hypervalent Silicon Compounds by Coordination of Diphosphine-Silanes to Gold

A Theoretical Investigation of Novel Sila- and Germa-Spirocyclic Imines and Their Relevance for Electron-Transporting Materials and Drug Discovery

A new class of spirocyclic imines (SCIs) has been theoretically investigated by applying a variety of quantum chemical methods and basis sets. The uniqueness of these compounds is depicted by various peculiarities, e.g., the incidence of planar six-membered rings each with two imine groups (two π bonds) and the incorporation of the isosteres carbon, silicon, or germanium spiro centers. Additional peculiarities of these novel SCIs are mirrored by their three-dimensionality, the simultaneous occurrence of nucleophilic and electrophilic centers, and the cross-hyperconjugative (spiro-conjugation) interactions, which provoke charge mobility along the spirocyclic scaffold. Substitution of SCIs with strong electron-withdrawing substituents, like the cyano group or fluorine, enhances their docking capability and impacts their reactivity and charge mobility. To gain thorough knowledge about the molecular properties of these SCIs, their structures have been optimized and various quantum chemical concepts and models were applied, e.g., full NBO analysis and the frontier molecular orbitals (FMOs) theory (HOMO-LUMO energy gap) and the chemical reactivity descriptors derived from them. For the assessment of the charge density distribution along the SCI framework, additional complementary quantum chemical methods were used, e.g., molecular electrostatic potential (MESP) and Bader's QTAIM. Additionally, using the aromaticity index NICS (nuclear independent chemical shift) and other criteria, it could be shown that the investigated cross-hyperconjugated sila and germa SCIs are spiro-aromatics of the Heilbronner Craig-type Möbius aromaticity.

ortho-Phenylene-bridged phosphorus/silicon Lewis pairs

A series of five ortho-phenylene-bridged phosphorus-silicon Lewis pairs was synthesized, with phosphorus bearing isopropyl groups while the substituents at the silicon atom vary (-CH₃, -Cl, -F). Possible interactions between Lewis acid and base were investigated both experimentally (NMR, XRD) and theoretically to determine the influence of the different substituents. Calculated ortho-interaction energies (OIEs) show a stabilizing interactions between the acidic and basic units which were also found for the meta- and para-interaction energies (MIEs and PIEs, respectively), indicating stabilization resulting not from direct acid-base interaction but from electronic interactions through the ring. Further spectroscopic (NMR, XRD) and theoretical (NBO, QTAIM, SAPT) investigations confirmed the absence of direct interactions between silicon and phoshorus.

A trigonal coordination of Au(I) phosphane complexes stabilized by O-H ⋯ X (X = Cl-, Br-, I-) interactions

In this work, we show that intramolecular hydrogen bonding can be used to stabilize tri-coordinated phosphane-gold(I) complexes. Two molecular structures of 2-(diphenylphosphino)benzoic acid (L) coordinated to a gold(I) atom were determined by single-crystal X-ray diffraction. The linear L-Au-Br shows a standard linear coordination and dimerizes via hydrogen bonds of the carboxylic acid. Upon addition of two additional phosphane ligands the complex [L3Au]X is formed which is stabilized by three intramolecular -C(O)O-H … X hydrogen bonds as proven by the X-ray structure of the respective chlorido-complex. X-ray powder diffractograms suggest the same structure also for X- = Br- and I-.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00706-021-02843-2.

Fluorosilane Activation by Pd/Ni→Si-F→Lewis Acid Interaction: An Entry to Catalytic Sila-Negishi Coupling

A new mode of bond activation involving M→Z interactions is disclosed. Coordination to transition metals as σ-acceptor ligands was found to enable the activation of fluorosilanes, opening the way to the first transition-metal-catalyzed Si-F bond activation. Using phosphines as directing groups, sila-Negishi couplings were developed by combining Pd and Ni complexes with external Lewis acids such as MgBr₂. Several key catalytic intermediates have been authenticated spectroscopically and crystallographically. Combined with DFT calculations, all data support cooperative activation of the fluorosilane via Pd/Ni→Si-F→Lewis acid interaction with conversion of the Z-type fluorosilane ligand into an X-type silyl moiety.

Semi-bridging σ-silyls as Z-type ligands

The reactions of SiHPh(NCH₂PPh₂)₂C₆H₄-1,2 with zerovalent group 10 reagents afford the homoleptic bimetallic complexes [M₂{μ-κ³-Si,P,P′-SiPh(CH₂PPh₂)₂C₆H₄}₂] (M = Ni, Pd, Pt) in which the M–M bond is unsymmetrically bridged by two σ-silyl groups.

(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.

Gold(I) Complexes of the Geminal Phosphinoborane tBu2PCH2BPh2

In this work, we explored the coordination properties of the geminal phosphinoborane tBu₂PCH₂BPh₂ (2) toward different gold(I) precursors. The reaction of 2 with an equimolar amount of the sulfur-based complex (Me₂S)AuCl resulted in displacement of the SMe₂ ligand and formation of linear phosphine gold(I) chloride 3. Using an excess of ligand 2, bisligated complex 4 was formed and showed dynamic behavior at room temperature. Changing the gold(I) metal precursor to the phosphorus-based complex, (Ph₃P)AuCl impacted the coordination behavior of ligand 2. Namely, the reaction of ligand 2 with (Ph₃P)AuCl led to the heterolytic cleavage of the gold-chloride bond, which is favored over PPh₃ ligand displacement. To the best of our knowledge, 2 is the first example of a P/B-ambiphilic ligand capable of cleaving the gold-chloride bond. The coordination chemistry of 2 was further analyzed by density functional theory calculations.

Attempted synthesis of ortho-phenylene phosphino-tritylium cations

With the synthesis of ortho-phenylene phosphino-tritylium cations as an objective, we generated (2-lithiophenyl)diphenylphosphine and (2-lithiophenyl)di-isopropylphosphine and allowed these organolithium reagents to react with benzophenone. The resulting phosphino-triarylcarbinols were allowed to react with HBF₄ in the presence of trifluoroacetic anhydride in order to generate the corresponding cations. Instead of the targeted ortho-phenylene phosphino-tritylium, the cations formed in these reactions were identified as the four-membered phosphonium species 7,7-bis(phenyl)-8,8-bis(phenyl)-7-phosphoniabicyclo[4.2.0]octa-1,3,5-triene (3⁺) and 7,7-bis(phenyl)-8,8-bis(isopropyl)-7-phosphoniabicyclo[4.2.0]octa-1,3,5-triene (4⁺), which were both isolated as tetrafluoroborate salts. The structure of these compounds has been confirmed by X-ray analysis. These new cations are thermally unstable and isomerize into the corresponding 5,10-dihydro-5,5-diphenyl-10-phenyl-acridophosphinium (5⁺) and 5,10-dihydro-5,5-di(isopropyl)-10-phenyl-acridophosphinium (6⁺) as tetrafluoroborate salts. These reactions suggest the involvement of ortho-phenylene phosphino-tritylium cations, which would be obtained by dissociation of the R₃P⁺-CAr₃ bonds in 4⁺ and 5⁺This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

σ-Complexation as a strategy for designing copper-based light emitters

Introduction of weak σ-SiH coordination to a copper(i) center leads to a highly emissive copper complex to minimize geometry relaxation in MLCT excited states.

Nucleophilic T-Shaped (LXL)Au(I)-Pincer Complexes: Protonation and Alkylation

We report the synthesis and reactivity of unusual T-shaped (LXL)Au(I)-pincer complexes, based on a carbazole framework flanked by two mesoionic carbenes (MICs). In contrast to other Au(I) complexes, these complexes react with electrophiles. Protonation and alkylation occur either at the metal or the ligand, depending on steric factors. Of particular interest, protonation at gold leads to an unprecedented cationic Au(III) hydride, which gives a ¹H NMR resonance at δ -8.34 ppm. The reactivity of this "hydride", however, shows protic and not hydridic behavior.

Experimental and theoretical studies of Si–Cl and Ge–Cl σ-bond activation reactions by iridium-hydride

We report the iridium hydride-mediated Si–Cl and Ge–Cl σ-bond activation in a low-polarity toluene solution owing to diphosphine-chelation, in which the Si–Cl and Ge–Cl σ-bonds are readily cleaved through an S_N2-type pathway via the formation of a free chloride anion.

Novel zwitterionic complexes arising from the coordination of an ambiphilic phosphorus-aluminum ligand to gold

Coordination of Mes2PC(=CHPh)AltBu2 to metal chlorides has been studied. Bridging P→M-Cl→Al coordinations were observed with Rh and Pd fragments, while chloride abstraction systematically occurred with gold. The resulting zwitterionic complexes have been fully characterized and analyzed by DFT calculations.

Disilicon complexes with two hexacoordinate Si atoms: paddlewheel-shaped isomers with (ClN4 )Si-Si(S4 Cl) and (ClN2 S2 )Si-Si(S2 N2 Cl) skeletons

The reaction of 1-methyl-3-trimethylsilylimidazoline-2-thione with hexachlorodisilane proceeds toward substitution of four of the disilane Cl atoms during the formation of disilicon complexes with two neighboring hexacoordinate Si atoms. The N,S-bidentate methimazolide moieties adopt a buttressing role, thus forming paddlewheel-shaped complexes of the type ClSi(μ-mt)4 SiCl (mt=methimazolyl). Most interestingly, three isomers (i.e., with (ClN4 )SiSi(S4 Cl), (ClN3 S)SiSi(S3 NCl), and (ClN2 S2 )SiSi(S2 N2 Cl) skeletons as so-called (4,0), (3,1), and cis-(2,2) paddlewheels) were detected in solution by using (29) Si NMR spectroscopic analysis. Two of these isomers could be isolated as crystalline solids, thus allowing their molecular structures to be analyzed by using X-ray diffraction studies. In accord with time-dependent NMR spectroscopy, computational analyses proved the cis-(2,2) isomer with a (ClN2 S2 )SiSi(S2 N2 Cl) skeleton to be the most stable. The compounds presented herein are the first examples of crystallographically evidenced disilicon complexes with two SiSi-bonded octahedrally coordinated Si atoms and representatives of the still scarcely explored class of Si coordination compounds with sulfur donor atoms.

Sensing of aqueous fluoride anions by cationic stibine-palladium complexes

Turn on the lantern! The stibine donor ligand of a cationic palladium complex acts as a Lewis acid and reacts with a fluoride anion to afford the corresponding fluorostiboranyl-palladium species (see scheme). Bindung of the fluoride anion to the antimony center induces a change in denticity of the triphosphine unit and leads to a bright-orange trigonal-bipyramidal d(8) lantern complex.

Discrete, solvent-free alkaline-earth metal cations: metal···fluorine interactions and ROP catalytic activity

Efficient protocols for the syntheses of well-defined, solvent-free cations of the large alkaline-earth (Ae) metals (Ca, Sr, Ba) and their smaller Zn and Mg analogues have been designed. The reaction of 2,4-di-tert-butyl-6-(morpholinomethyl)phenol ({LO(1)}H), 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenol ({LO(2)}H), 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO(3)}H), and 2-[(1,4,7,10-tetraoxa-13-azacyclo-pentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol ({RO(3)}H) with [H(OEt(2))(2)](+)[H(2)N{B(C(6)F(5))(3)}(2)](-) readily afforded the doubly acidic pro-ligands [{LO(1)}HH](+)[X](-) (1), [{LO(2)}HH](+)[X](-) (2), [{LO(3)}HH](+)[X](-) (3), and [{RO(3)}HH](+)[X](-) (4) ([X](-) = [H(2)N{B(C(6)F(5))(3)}(2)](-)). The addition of 2 to Ca[N(SiMe(3))(2)](2)(THF)(2) and Sr[N(SiMe(3))(2)](2)(THF)(2) yielded [{LO(2)}Ca(THF)(0.5)](+)[X](-) (5) and [{LO(2)}Sr(THF)](+)[X](-) (6), respectively. Alternatively, 5 could also be prepared upon treatment of {LO(2)}CaN(SiMe(3))(2) (7) with [H(OEt(2))(2)](+)[X](-). Complexes [{LO(3)}M](+)[X](-) (M = Zn, 8; Mg, 9; Ca, 10; Sr, 11; Ba, 12) and [{RO(3)}M](+)[X](-) (M = Zn, 13; Mg, 14; Ca, 15; Sr, 16; Ba, 17) were synthesized in high yields (70-90%) by reaction of 3 or 4 with the neutral precursors M[N(SiMe(3))(2)](2)(THF)(x) (M = Zn, Mg, x = 0; M = Ca, Sr, Ba, x = 2). All compounds were fully characterized by spectroscopic methods, and the solid-sate structures of compounds 1, 3, 7, 8, 13, 14, {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2) were determined by X-ray diffraction crystallography. Whereas the complexes are monomeric in the case of Zn and Mg, they form bimetallic cations in the case of Ca, Sr and Ba; there is no contact between the metal and the weakly coordinating anion. In all metal complexes, the multidentate ligand is κ(6)-coordinated to the metal. Strong intramolecular M···F secondary interactions between the metal and F atoms from the ancillary ligands are observed in the structures of {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2). VT (19)F{(1)H} NMR provided no direct evidence that these interactions are maintained in solution; nevertheless, significant Ae···F energies of stabilization of 25-26 (Ca, Ba) and 40 kcal·mol(-1) (Sr) were calculated by NBO analysis on DFT-optimized structures. The identity and integrity of the cationic complexes are preserved in solution in the presence of an excess of alcohol (BnOH, (i)PrOH) or L-lactide (L-LA). Efficient binary catalytic systems for the immortal ring-opening polymerization of L-LA (up to 3,000 equiv) are produced upon addition of an excess (5-50 equiv) of external protic nucleophilic agents (BnOH, (i)PrOH) to 8-12 or 13-17. PLLAs with M(n) up to 35,000 g·mol(-1) were produced in a very controlled fashion (M(w)/M(n) ≈ 1.10-1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg ≪ Zn < Ca < Sr ≈ Ba; also, Ae complexes supported by the aryloxide ligand are more active than their parents supported by the fluorinated alkoxide ancillary, possibly owing to the presence of Ae···F interactions in the latter case. The rate law -d[L-LA]/dt = k(p)·[L-LA](1.0)·[16](1.0)·[BnOH](1.0) was established by NMR kinetic investigations, with the corresponding activation parameters ΔH(++) = 14.8(5) kcal·mol(-1) and ΔS(++) = -7.6(2.0) cal·K(-1)·mol(-1). DFT calculations indicated that the observed order of catalytic activity matches an increase of the L-LA coordination energy onto the cationic metal centers with parallel decrease of the positive metal charge.