PD Dr. habil. Moritz Schmidt

Head Chemistry of the f-elements
Phone: +49 351 260 3156
+49 351 260 2536

Latest Publications of the "Chemistry of the f-elements" department

Triply Bonded Pancake π-Dimers Stabilized by Tetravalent Actinides

Barluzzi, L.; Ogilvie, S. P.; Dalton, A. B.; Kaden, P.; Gericke, R.; Mansikkamäki, A.; Giblin, S. R.; Layfield, R. A.

Aromatic π-stacking is a weakly attractive, non-covalent interaction often found in biological macromolecules and synthetic supramolecular chemistry. The weak non-directional nature of π-stacking can present challenges in the design of materials owing to their weak, non-directional nature. However, when aromatic π-systems contain an unpaired electron, stronger attraction involving face-to-face π-orbital overlap is possible, resulting in covalent so-called ‘pancake’ bonds. Two-electron, multicentre single pancake bonds are well-known whereas four-electron double pancake bonds are rare. Higherorder
pancake bonds have been predicted, but experimental systems are unknown. Here, we show that six-electron triple pancake bonds can be synthesized by threefold reduction of hexaazatrinaphthylene (HAN) and subsequent stacking of the [HAN]³‾ tri-radicals. Our analysis reveals a multicentre covalent triple pancake bond consisting of a π-orbital and two equivalent π-orbitals. An electrostatic stabilizing role is established for tetravalent thorium and uranium ions in these systems. We also show that the electronic absorption spectrum of the triple pancake bonds closely matches computational predictions, providing experimental verification of these unique interactions. The discovery of conductivity in thin films of the triply bonded π-dimers presents new opportunities for the discovery of single-component molecular conductors and other spinbased molecular materials.

Keywords: actinides; triply bonded pancake; stabilization; magnetism; EPR; quantum chemistry

  • Open Access Logo Journal of the American Chemical Society 146(2024)6, 4234-4241
    Online First (2024) DOI: 10.1021/jacs.3c13914

The Hexacoordinate Si Complex SiCl₄(4-Azidopyridine)₂—Crystallographic Characterization of Two Conformers and Probing the Influence of SiCl₄-Complexation on a Click Reaction with Phenylacetylen

Riedel, S.; Gerwig, M.; Gerlach, D.; Brendler, E.; Gericke, R.; Kroke, E.; Wagler, J.

4-Azidopyridine (1) and SiCl₄ react with the formation of the hexacoordinate silicon complex SiCl₄(4-azidopyridine)₂ (2). Upon dissolving in warm chloroform, the complex dissociates into the constituents 1 and SiCl₄ and forms back upon cooling. Depending on the cooling, two different crystalline modifications of 2 were obtained, which feature two different trans-conformers. Slow cooling to room temperature afforded conformer 2′, which features coplanar pyridine rings. Rapid cooling to −39 °C afforded crystals of conformer 2′′, in which the planes of the pyridine ligands are nearly orthogonal to one another. Whereas 2′ resembles the molecular arrangement of various other known SiX₄(pyridine)₂ (X = halide) complexes, 2′′ represents the first crystallographically confirmed example of a SiX₄(pyridine)₂ complex in this conformation. Conformers 2′ and 2′′ were studied with ¹³C and ²⁹Si solid state NMR spectroscopy. Their differences in ²⁹Si chemical shift anisotropy, as well as energetic differences, were further investigated with computational analyses. In spite of the similar stabilities of the two conformers as isolated molecules, the crystal packing of 2′′ is less stable, and its crystallization is interpreted as a kinetically controlled effect of seed formation. (3+2)-cycloaddition of 1 and phenylacetylene in toluene at 110 °C yields a mixture of 1-(4-pyridyl)-4-phenyl-1,2,3-triazole (1,4-3) and 1-(4-pyridyl)-5-phenyl-1,2,3-triazole (1,5-3) in approximate 1:2 molar ratio. The crystal structures of the two isomers were determined via X-ray diffraction. In chloroform (at 60 °C), this reaction is slow (less than 2% conversion within 4 h), but the presence of SiCl₄ enhanced the rate of the reaction slightly, and it shifted the triazole isomer ratio to ca. 1:6 in favor of 1,5-3.

Keywords: chemical shift anisotropy; (3+2)-cycloaddition; Hirshfeld surface analysis; hypercoordination; packing efficiency; polymorphism; silicon; triazole

Lewis Acid-Base Adducts of α-Amino Acid-Derived Silaheterocycles and N-Methylimidazole

Seidel, A.; Gericke, R.; Kutzner, B.; Wagler, J.

In chloroform solution, the reaction of bis(tert-butylamino)dimethylsilane ((tBuNH)₂SiMe₂) and an α-amino acid (α-amino isobutyric acid, H₂Aib; D-phenylglycine, H₂Phg; L-valine, H₂Val) in the presence of N-methylimidazole (NMI) gave rise to the formation of the pentacoordinate silicon complexes (Aib)SiMe₂-NMI, (Phg)SiMe₂-NMI and (Val)SiMe₂-NMI, respectively. Therein, the amino acid building block was a di-anionic bidentate chelator at the silicon atom. In solution, the complexes were involved in rapid coordination–dissociation equilibria between the pentacoordinate Si complex (e.g., (Aib)SiMe₂-NMI) and its constituents NMI and a five-membered silaheterocycle (e.g., (Aib)SiMe₂), as shown by ²⁹Si NMR spectroscopy. The energetics of the Lewis acid-base adduct formation and the competing solvation of the NMI molecule by chloroform were assessed with the aid of computational methods. In CDCl₃ solution, deuteration of the silaheterocycle NH group proceeded rapidly, with more than 50% conversion within two days. Upon cooling to -44 °C, the chloroform solvates of the adducts (Aib)SiMe₂-NMI and (Phg)SiMe₂-NMI crystallized from their parent solutions and allowed for their single-crystal X-ray diffraction analyses. In both cases, the Si atom was situated in a distorted trigonal bipyramidal coordination sphere with equatorial Si–C bonds and an equatorial Si–N bond (the one of the silaheterocycle). The axial positions were occupied by a carboxylate O atom of the silaheterocycle and the NMI ligand’s donor-N-atom.

Keywords: bidentate ligands; deuterium transfer; hypercoordination; quantum chemical calculations; silicon; X-ray diffraction

Pd–Si complexes of the type ClPd(μ²-pyO)₄SiR (R = Me, Ph, Bn, Allyl, κO-(pyO)PdCl(η³-allyl); pyO = pyridine-2-olate): The influence of substituent R on the Pd–Si bond

Wagler, J.; Gericke, R.

The reactions of organosiliconpyridine-2-olates (pyridyl-2-oxysilanes) RSi(pyO)₃ (pyO = pyridine-2-olate, R = Me (1a), Ph (1b), Bn (1c) and Allyl (1d)) and [PdCl₂(NCMe)₂] in chloroform afforded the hexacoordinate silicon complexes RSi(μ²-pyO)₄PdCl (R = Me (2a), Ph (2b), Bn (2c) and Allyl (2d), respectively), which feature a Pd–Si bond, in which the Pd atom is the formal lone pair donor toward Si. The new compounds 2b, 2c, 2d were characterized with multi-nuclear NMR spectroscopy and elemental analysis. The effect of the Si-bound substituent R on the trans-disposed Pd–Si bond was studied by single-crystal X-ray diffraction and computational analyses (e.g., Natural Localized Molecular Orbitals, NLMO; topological analyses of the electron density at the bond critical point with Quantum Theory of Atoms-In-Molecules, QTAIM). A structurally related byproduct, (η³-allyl) ClPd(pyO)Si(μ²-pyO)₄PdCl 2d’, which formed along with target product 2d and features an Si–O bond trans to Pd–Si, was included in this systematic study. Another byproduct from the synthesis of 2d, the pentanuclear complex ClPd(μ²-pyO)₂Si(μ²-pyO)₂Pd(μ²-pyO)₂Si(μ²-pyO)₂PdCl (compound 3) was characterized crystallographically. This compound features pentacoordinate Si atoms within trigonal–bipyramidal Si(O₄Pd) coordination spheres with equatorial Pd–Si bonds to the terminal Pd atoms. The Pd–Si bond situation in this compound was elucidated with the aid of computational analyses. QTAIM analyses of 3 in conjunction with a model compound PdSi4, which features two silyl groups and two silylene ligands, indicate topological properties of the electron density at the Pd–Si bond critical point which are similar to Pd–Si bonds of silyl and silylene compounds. The latter exhibit greater similarity, which indicates features of a Pd←Si bond. In contrast, NLMO analyses of 3 identify a polar covalent Pd–Si bond with predominant Pd contribution (formal Pd→Si donation).

Keywords: Ambidentate ligands; Hypercoordination; NMR spectroscopy; Palladium; Silicon; Single-crystal X-ray diffraction; Topological analysis

Ge–Cu-Complexes Ph(pyO)Ge(μ²-pyO)₂CuCl and PhGe(μ²-pyO)₄CuCl - Representatives of Cu(I)→Ge(IV) and Cu(II)→Ge(IV) Dative Bond Systems

Wagler, J.; Gericke, R.

Phenylgermaniumpyridine-2-olate PhGe(pyO)₃ (compound 1Ge) and CuCl react with the formation of the heteronuclear complex Ph(pyO)Ge(μ²-pyO)₂CuCl (2Ge’) rather than forming the expected compound PhGe(μ²-pyO)₃CuCl (2Ge). Single-point calculations (at the B2T-PLYP level) of the optimized molecular structures confirmed the relative stability of isomer 2Ge’ over 2Ge and, for the related silicon congeners, the relative stability of 2Si over 2Si’. Decomposition of a solution of 2Ge’ upon access to air provided access to some crystals of the copper(II) compound PhGe(μ²-pyO)₄CuCl (3Ge). Compounds 2Ge’ and 3Ge were characterized by single-crystal X-ray diffraction analyses, and the Ge–Cu bonds in these compounds were analyzed with the aid of quantum chemical calculations, e.g., Natural Bond Orbital analyses (NBO), Non-Covalent Interactions descriptor (NCI), and topology of the electron density at bond critical point using Quantum Theory of Atoms-In-Molecules (QTAIM) in conjunction with the related silicon compounds PhSi(μ²-pyO)₃CuCl (2Si), PhSi(μ²-pyO)₄CuCl (3Si), as well as the potential isomers Ph(pyO)Si(μ²-pyO)₂CuCl (2Si’) and PhGe(μ²-pyO)₃CuCl (2Ge). Pronounced Cu→Ge (over Cu→Si) lone pair donation was found for the Cu(I) compounds, whereas in Cu(II) compounds 3Si and 3Ge, this σ-donation is less pronounced and only marginally enhanced in 3Ge over 3Si.

Keywords: atoms-in-molecules; copper; dimetallic complexes; NLMO; single-crystal X-ray diffraction analyses

Formation of Heterobimetallic Complexes by Addition of d10-Metal Ions to [(Me3P)xM(2-C6F4PPh2)2] (x = 1, 2; M = Ni and Pt): A Synthetic and Computational Study of Metallophilic Interactions

Gericke, R.; Bennett, M. A.; Privér, S.; Bhargava, S.

Treatment of the bis(chelate) complexes trans-[M(κ2-2-C6F4PPh2)2] (trans-1M; M = Ni, Pt) and cis-[Pt(κ2-2-C6F4PPh2)2] (cis-1Pt) with equimolar amounts or excess of PMe3 solution gave complexes of the type [(Me3P)xM(2-C6F4PPh2)2] (x = 2: 2Ma, 2Mb x = 1: 3Ma, 3Mb; M = Ni, Pt). The reactivity of complexes of the type 2M and 3M towards monovalent coinage metal ions (M’ = Cu, Ag, Au) was investigated next to the reaction of 1M towards [AuCl(PMe3)]. Four different complex types [(Me3P)2M(µ-2-C6F4PPh2)2M’Cl] (5MM’; M = Ni, Pt; M’ = Cu, Ag, Au), [(Me3P)M(κ2-2-C6F4PPh2)(µ-2-C6F4PPh2)M’Cl]x (x = 1: 6MM’; M = Pt; M’ = Cu, Au; x = 2: 6PtAg), head-to-tail-[(Me3P)ClM(µ-2-C6F4PPh2)2M’] (7MM’; M = Ni, Pt; M’ = Au) and head-to-head-[(Me3P)ClM(µ-2-C6F4PPh2)2M’] (8MM’; M = Ni, Pt; M’ = Cu, Ag, Au) were observed. Single crystal X-ray analyses of complexes 5-8 reveal short metal-metal separations (2.7124(3) – 3.3287(7) Å) suggestive of attractive metal-metal interactions. Quantum chemical calculations (AIM, ELF, NCI, NBO) gave theoretical support that the interaction characteristics reach from pure attractive non-covalent to electron-shared (covalent) character.

Keywords: heterobimetallic; metallophilic interactions; single-crystal X-ray diffraction; NMR spectroscopy; quantum chemical calculations

Evidence for a High-Valent Iron-Fluoride That Mediates Oxidative C(sp3)‐H Fluorination

Panda, C.; Anny-Nzekwue, O.; Doyle, L. M.; Gericke, R.; McDonald, A. R.

(Fe(II)(NCCH₃)(NTB))(OTf)₂ (NTB = tris(2- benzimidazoylmethyl)amine, OTf = trifluoromethanesulfonate) was reacted with difluoro(phenyl)-λ³-iodane (PhIF₂) in the presence of a variety of saturated hydrocarbons, resulting in the oxidative fluorination of the hydrocarbons in moderate-to-good yields. Kinetic and product analysis point towards a hydrogen atom transfer oxidation prior to fluorine radical rebound to form the fluorinated product. The combined evidence supports the formation of a formally Fe(IV)(F)₂ oxidant that performs hydrogen atom transfer followed by the formation of a dimeric μ-F−(Fe(III))₂ product that is a plausible fluorine atom transfer rebound reagent. This approach mimics the heme paradigm for hydrocarbon hydroxylation, opening up avenues for oxidative hydrocarbon halogenation.

Keywords: high-valent iron; fluorination; hydrogen atom transfer; proton coupled electron transfer; fluorine atom transfer

Synthesis and Characterization of a Masked Terminal Nickel-Oxide Complex

Heim, P.; Spedalotto, G.; Lovisari, M.; Gericke, R.; O’Brien, J.; Farquhar, E. R.; McDonald, A. R.

In exploring terminal nickel-oxo complexes, postulated to be the active oxidant in natural and non-natural oxidation reactions, we report the synthesis of the pseudo-trigonal bipyramidal Ni(II) complexes (K)[Ni(II)(LPh)(DMF)] (1[DMF]) and (NMe₄)₂[Ni(II)(LPh)(OAc)] (1[OAc]) (LPh = 2,2’,2’’-nitrilo-tris-(N-phenylacetamide); DMF = N,N-dimethylformamide; OAc = acetate). Both complexes were characterized using NMR, FTIR, ESI-MS, and X-ray crystallography, showing the LPh ligand to bind in a tetradentate fashion, together with an ancillary donor. The reaction of 1[OAc] with peroxyphenyl acetic acid (PPAA) resulted in the formation of [(LPh)Ni(III)-O-H···OAc]²-, 2, that displays many of the characteristics of a terminal Ni=O species. 2 was characterized by UV-Vis, EPR, and XAS spectroscopies and ESI-MS. 2 decayed to yield a Ni(II)-phenolate complex 3 (through aromatic electrophilic substitution) that was characterized by NMR, FTIR, ESI-MS, and X-ray crystallography. 2 was capable of hydroxylation of hydrocarbons and epoxidation of olefins, as well as oxygen atom transfer oxidation of phosphines at exceptional rates. While the oxo-wall remains standing, this complex represents an excellent example of a masked metal-oxide that displays all of the properties expected of the ever elusive terminal M=O beyond the oxo-wall.

Keywords: bioinorganic chemistry; high-valent nickel; hydrocarbon oxidation; oxo-wall; peracid activation

Aromatic and aliphatic hydrocarbon hydroxylation via a formally Ni(IV)=O oxidant

Heim, P.; Gericke, R.; Spedalotto, G.; Lovisari, M.; Farquhar, E. R.; McDonald, A. R.

The reaction of (NMe₄)₂[Ni(II)(LPh)(OAc)] (1[OAc], LPh = 2,2’,2’’-nitrilo-tris-(N-phenylacetamide); OAc = acetate) with 3-chloroperoxybenzoic acid (m-CPBA) resulted in the formation of a self-hydroxylated Ni(III)- phenolate complex, 2, where one of the phenyl groups of LPh underwent hydroxylation. 2 was characterised by UV-Vis, EPR, and XAS spectroscopies and ESI-MS. 2 decayed to yield a previously characterised Ni(II)-phenolate complex, 3. We postulate that self-hydroxylation was mediated by a formally Ni(IV)=O oxidant, formed from the reaction of 1[OAc] with m-CPBA, which undergoes electrophilic aromatic substitution to yield 2. This is supported by an analysis of the kinetic and thermodynamic properties of the reaction of 1[OAc] with m-CPBA. Addition of exogenous hydrocarbon substrates intercepted the selfhydroxylation process, producing hydroxylated products, providing further support for the formally Ni(IV)=O entity. This study demonstrates that the reaction between Ni(II) salts and m-CPBA can lead to potent metal-based oxidants, in contrast to recent studies demonstrating carboxyl radical is a radical free-chain reaction initiator in Ni(II)/m-CPBA hydrocarbon oxidation catalysis.

Keywords: hydrocarbon hydroxylation; nickel; XANES/EXAFS; electron paramagnetic resonance; quantum chemical calculation

Copper Complexes of Silicon Pyridine-2-olates RSi(pyO)₃ (R = Me, Ph, Bn, Allyl) and Ph₂Si(pyO)₂

Seidel, A.; Gericke, R.; Brendler, E.; Wagler, J.

The organosilicon pyridine-2-olates 1a–1d (RSi(pyO)₃, R = Me (a), Ph (b), Bn (c), Allyl (d); pyO = pyridine-2-olate) may serve as tripodal ligands toward CuCl with formation of complexes of the type RSi(μ²-pyO)₃CuCl (2a–2d). In addition, for R = Allyl, formation of the more stable isomer 2d′ (κO-pyO)Si(μ²-pyO)₂(μ²-Allyl)CuCl was observed. In the presence of dry air (as a source of oxygen), reactions of 1a–1d and CuCl afforded Cu(II) complexes RSi(μ²-pyO)₄CuCl (3a–3d); 3a–3c in good yield, and 3d only as a side product. Reaction of Ph₂Si(pyO)₂ (4) and CuCl in equimolar ratio afforded, depending on reaction conditions, a series of (CuCl)n-ladder-type oligonuclear Cu(I) complexes Ph₂Si(μ2-pyO)₂(CuCl)n(μ²-pyO)₂SiPh₂ (n = 2 (52), 3 (53), 4 (54)). In all of the above compounds, the pyO group is Si–O bound and, in the case of μ² coordination, Cu–N bound. All new compounds (1c, 1d, 2b, 2c, 2d, 2d′, 3b, 3c, 3d, 52, 53, 54) were characterized by single-crystal X-ray diffraction, and further characterization includes solution ¹H, ¹³C, ²⁹Si NMR spectroscopy (1c, 1d, 2b, 2c, 2d’, 53, 54), solid-state ²⁹Si (2b, 2c, 2d′, 53, 54) and ⁶³Cu NMR spectroscopy (2c, 2d′) as well as computational analyses of the isomerization of the couple 2d, 2d′.

Keywords: allyl complex; 2-hydroxypyridine; hypercoordination; 63Cu NMR spectroscopy; organosilanes; paddlewheel complex; quantum chemical calculations; X-ray diffraction