Publications by authors named "Frank W Heinemann"

172 Publications

A Pair of Cobalt(III/IV) Terminal Imido Complexes.

Angew Chem Int Ed Engl 2021 Apr 13. Epub 2021 Apr 13.

Universität des Saarlandes: Universitat des Saarlandes, Chemistry, GERMANY.

Reaction of the cobalt(I) complex [(TIMMN mes )Co I ](BPh 4 ) ( 2 ) (TIMMN mes = t ris -[2-(3-mesityl- im idazolin-2-ylidene)- m ethyl]-ami n e) with 1-adamantyl azide yields the cobalt(III) imido complex [(TIMMN mes )Co III (NAd)](BPh 4 ) ( 3 ) with concomitant release of dinitro-gen. The N -anchor in diamagnetic 3 features an unusual, planar tertiary amine, which results from repulsive electro-static interaction with the cobalt ion's filled d ( z 2 ) orbital and negative hyperconjugation with the neighboring methylene groups. One-electron oxidation of 3 with [FeCp 2 ][OTf] provides access to the rare, high valent cobalt(IV) imido complex [(TIMMN mes )Co IV -(NAd)](OTf) 2 ( 4 ). Both complexes 3 and 4 were fully characterized. Despite a half-life of less than one hour at room temperature, 4 could be isolated at low temperatures in analytically pure form. Single-crystal X-ray diffractometry (SC-XRD) and EPR spectroscopy corroborate the compound's molecular structure and its d 5 low-spin, S = ½, electron configuration. A compu-tational analysis of 4 suggests high covalency within the Co IV =NAd bond with non-negligible spin density located at the imido moiety, which translates into substantial triplet nitrene character.
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http://dx.doi.org/10.1002/anie.202103170DOI Listing
April 2021

A Fluorescence-Detected Coordination-Induced Spin State Switch.

J Am Chem Soc 2021 Mar 26;143(9):3466-3480. Epub 2021 Feb 26.

Inorganic Chemistry IV, University of Bayreuth, Universitätsstraße 30, NW I, 95447 Bayreuth, Germany.

The response of the spin state to variation of the coordination number (CISSS) is a promising and viable approach to smart sensor materials, yet it suffers to date from insensitive detection. Herein, we present the synthetic access to a family of planar nickel(II) complexes, whose CISSS is sensitively followed by means of fluorescence detection. For this purpose, nickel(II) complexes with four phenazine-based Schiff base-like ligands were synthesized and characterized through solution-phase spectroscopy (NMR and UV-vis), solid-state structure analysis (single-crystal XRD), and extended theoretical modeling. All of them reveal CISSS in solution through axial ligating a range of N- and O-donors. CISSS correlates nicely with the basicity of the axial ligand and the substitution-dependent acidity of the nickel(II) coordination site. Remarkably, three out of the four nickel(II) complexes are fluorescent in noncoordinating solvents but are fluorescence-silent in the presence of axial ligands such as pyridine. As these complexes are rare examples of fluorescent nickel(II) complexes, the photophysical properties with a coordination number of 4 were studied in detail, including temperature-dependent lifetime and quantum yield determinations. Most importantly, fluorescence quenching upon adding axial ligands allows a "black or white", i.e. digital, sensoring of spin state alternation. Our studies of fluorescence-detected CISSS (FD-CISSS) revealed that absorption-based CISSS and FD-CISSS are super proportional with respect to the pyridine concentration: FD-CISSS features a higher sensitivity. Overall, our findings indicate a favored ligation of these nickel(II) complexes in the excited state in comparison to the ground state.
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http://dx.doi.org/10.1021/jacs.0c12568DOI Listing
March 2021

Cobalt Diazo-Compounds: From Nitrilimide to Isocyanoamide via a Diazomethanediide Fleeting Intermediate.

Angew Chem Int Ed Engl 2021 May 8;60(20):11138-11142. Epub 2021 Apr 8.

Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Inorganic Chemistry, Egerlandstrasse 1, 91058, Erlangen, Germany.

Lithium trimethylsilyldiazomethanide and a cobalt (II) precursor with an N-anchored tris-NHC (TIMEN ) ligand provide access to the cobalt nitrilimide 1. Complex 1 was structurally characterized by single-crystal X-ray diffractometry (SC-XRD) and its electronic structure was examined in detail, including EPR spectroscopy, SQUID magnetometry and computational analyses. The desilylation of the C-(trimethylsilyl)nitrilimide reveals a transient complex with an elusive diazomethanediide ligand, which substitutes one of the mesitylene rings of the ancillary ligand through C-N bond cleavage. This transformation results in the cyclometalated cobalt(II) complex 2, featuring a rare isocyanoamido-κ-C ligand.
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http://dx.doi.org/10.1002/anie.202016539DOI Listing
May 2021

Ligand Tailoring Toward an Air-Stable Iron(V) Nitrido Complex.

J Am Chem Soc 2021 Jan 12;143(3):1458-1465. Epub 2021 Jan 12.

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Egerlandstraße 1, 91058 Erlangen, Germany.

A new supporting ligand, -[2-(3-mesityl-idazol-2-ylidene)ethyl]amie (TIMMN), was developed and utilized to isolate an air-stable iron(V) complex bearing a terminal nitrido ligand, which was synthesized by one-electron oxidation from the iron(IV) precursor. Single-crystal X-ray diffraction analyses of both complexes reveal that the metal-centered oxidation is escorted by iron nitride (Fe≡N) bond elongation, which in turn is accompanied by the accommodation of the high-valence iron center closer to the equatorial plane of a trigonal bipyramid. This contrasts with the previous observation of the only other literature-known Fe(IV)≡N/Fe(V)≡N redox pair, namely, [PhB(Im)FeN]. On the basis of Fe Mössbauer, EPR, and UV/vis electronic absorption spectroscopy as well as quantum chemical calculations, we identified the lesser degree of pyramidalization around the iron atom, the Jahn-Teller distortion, and the resulting nature of the SOMO to be the decisive factors at play.
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http://dx.doi.org/10.1021/jacs.0c11141DOI Listing
January 2021

A valence tautomeric cobalt-dioxolene complex with an anchoring group for prospective chemical grafting to metal oxides.

Dalton Trans 2020 Dec;49(48):17532-17536

Friedrich-Alexander University Erlangen-Nürnberg (FAU), Department of Chemistry and Pharmacy, Egerlandstr. 1, 91058, Erlangen, Germany.

Here, we synthesized a valence tautomeric cobalt-dioxolene complex featuring a protected anchoring group. At room temperature, the complex reveals a nearly pure low-spin-Co(iii)-catecholate state in the solid state, but a nearly pure high-spin-Co(ii)-semiquinonate state in toluene solution. Thermal switchability of the complex in solution and in the solid state is investigated.
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http://dx.doi.org/10.1039/d0dt03771kDOI Listing
December 2020

Oxidative Addition of Water, Alcohols, and Amines in Palladium Catalysis.

Angew Chem Int Ed Engl 2020 Nov 21;59(47):21088-21095. Epub 2020 Sep 21.

Inorganic Chemistry: Coordination Chemistry, Saarland University, Campus, Geb. C4.1, 66123, Saarbrücken, Germany.

The homolytic cleavage of O-H and N-H or weak C-H bonds is a key elementary step in redox catalysis, but is thought to be unfeasible for palladium. In stark contrast, reported here is the room temperature and reversible oxidative addition of water, isopropanol, hexafluoroisopropanol, phenol, and aniline to a palladium(0) complex with a cyclic (alkyl)(amino)carbene (CAAC) and a labile pyridino ligand, as is also the case in popular N-heterocyclic carbene (NHC) palladium(II) precatalysts. The oxidative addition of protic solvents or adventitious water switches the chemoselectivity in catalysis with alkynes through activation of the terminal C-H bond. Most salient, the homolytic activation of alcohols and amines allows atom-efficient, additive-free cross-coupling and transfer hydrogenation under mild reaction conditions with usually unreactive, yet desirable reagents, including esters and bis(pinacolato)diboron.
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http://dx.doi.org/10.1002/anie.202008350DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7692900PMC
November 2020

Electronic Structure and Magnetic Properties of a Titanium(II) Coordination Complex.

Inorg Chem 2020 May 11;59(9):6187-6201. Epub 2020 Apr 11.

Department of Biological, Physical and Health Sciences, Roosevelt University, Chicago, Illinois 60605, United States.

Stable coordination complexes of Ti (3d) are relatively uncommon, but are of interest as synthons for low oxidation state titanium complexes for application as potential catalysts and reagents for organic synthesis. Specifically, high-spin Ti ions supported by redox-inactive ligands are still quite rare due to the reducing power of this soft ion. Among such Ti complexes is -[TiCl(tmeda)], where tmeda = ,,','-tetramethylethane-1,2-diamine. This complex was first reported by Gambarotta and co-workers almost 30 years ago, but it was not spectroscopically characterized and theoretical investigation by quantum chemical theory (QCT) was not feasible at that time. As part of our interest in low oxidation state early transition metal complexes, we have revisited this complex and report a modified synthesis and a low temperature (100 K) crystal structure that differs slightly from that originally reported at ambient temperature. We have used magnetometry, high-frequency and -field EPR (HFEPR), and variable-temperature variable-field magnetic circular dichroism (VTVH-MCD) spectroscopies to characterize -[TiCl(tmeda)]. These techniques yield the following = 1 spin Hamiltonian parameters for the complex: = -5.23(1) cm, = -0.88(1) cm, (/ = 0.17), = [1.86(1), 1.94(2), 1.77(1)]. This information, in combination with electronic transitions from MCD, was used as input for both classical ligand-field theory (LFT) and detailed QCT studies, the latter including both density functional theory (DFT) and methods. These computational methods are seldom applied to paramagnetic early transition metal complexes, particularly those with > 1/2. Our studies provide a complete picture of the electronic structure of this complex that can be put into context with the few other high-spin and mononuclear Ti species characterized to date.
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http://dx.doi.org/10.1021/acs.inorgchem.0c00311DOI Listing
May 2020

An Iron Pincer Complex in Four Oxidation States.

Inorg Chem 2020 Apr 9;59(8):5632-5645. Epub 2020 Apr 9.

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany.

The synthesis and characterization of a series of homoleptic iron complexes [Fe(OCO)] supported by the tridentate bis-aryloxide benzimidazolin-2-ylidene pincer ligand OCO () is presented. While the reaction of 2 equiv of free ligand with a ferrous iron precursor leads to the isolation of the coordination polymer [Fe(OCOK)] (), treatment of with ferric iron salts allows for the synthesis and isolation of the mononuclear, octahedral bis-pincer compound K[Fe(OCO)] () and its crown-ether derivative [K(18c6)(THF)][Fe(OCO)] (). Electrochemical studies of suggested stable products upon further one- and two-electron oxidation. Hence, treatment of with 1 equiv of AgPF yields the charge-neutral species [Fe(OCO)] (). Similarly, the cationic complex [Fe(OCO)]PF () is obtained by addition of 2 equiv of AgPF. The characterization of complexes , , and reveals iron-centered reduction and oxidation processes; thus, preserving the dianionic, closed-shell structure of both coordinated OCO pincer chelates, . This implies a stabilization of a highly Lewis acidic iron(IV) center by four phenolate anions rather than charge distribution across the ligand framework with a lower formal oxidation state at iron. Notably, the overall charge-neutral iron(IV) complex undergoes reductive elimination of the pincer ligand, providing a metal-free compound that can be described as a spirocyclic imidazolone ketal (). In contrast, the ligand-metal bonds in , formally an iron(V) complex, are considerably covalent, rendering the assignment of its oxidation state challenging, if not impossible. All compounds are fully characterized, and the complexes' electronic structures were studied with a variety of spectroscopic and computational methods, including single-crystal X-ray diffraction (SC-XRD), X-band electron paramagnetic resonance (EPR), and zero-field Fe Mössbauer spectroscopy, variable-field and variable-temperature superconducting quantum interference device (SQUID) magnetization measurements, and multi-reference (NEVPT2/CASSCF) as well as density functional theory (DFT) studies. Taken altogether, the electronic structure of is best described as an iron(IV) center antiferromagnetically coupled to a ligand-centered radical.
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http://dx.doi.org/10.1021/acs.inorgchem.0c00355DOI Listing
April 2020

Werner-Type Complexes of Uranium(III) and (IV).

Inorg Chem 2020 Feb 24;59(4):2443-2449. Epub 2020 Jan 24.

Inorganic Chemistry, Department of Chemistry and Pharmacy , Friedrich-Alexander-University Erlangen-Nürnberg (FAU) , Erlangen 91058 , Germany.

Transmetalation of the β-diketiminate salt [M][nacnac] (M = Na or K; nacnac = {PhNC(CH)}CH) with UI(THF) resulted in the formation of the homoleptic, octahedral complex [U(nacnac)] (). Green colored was fully characterized by a solid-state X-ray diffraction analysis and a combination of UV/vis/NIR, NMR, and EPR spectroscopic studies as well as solid-state SQUID magnetization studies and density functional theory calculations. Electrochemical studies of revealed this species to possess two anodic waves for the U(III/IV) and U(IV/V) redox couples, with the former being chemically accessible. Using mild oxidants, such as [CoCp][PF] or [FeCp][Al{OC(CF)}], yields the discrete salts (A = PF, Al{OC(CF)}), whereas the anion exchange of with NaBPh yields .
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http://dx.doi.org/10.1021/acs.inorgchem.9b03229DOI Listing
February 2020

Reversible Shifting of a Chemical Equilibrium by Light: The Case of Keto-Enol Tautomerism of a β-Ketoester.

Org Lett 2020 01 7;22(2):604-609. Epub 2020 Jan 7.

Department of Chemistry and Pharmacy , Friedrich-Alexander University Erlangen-Nürnberg (FAU) , Egerlandstrasse 1 , 91058 Erlangen , Germany.

Manipulating the equilibrium between a ketone and an enol by light opens up ample opportunities in material chemistry and photopharmacology. By incorporating β-ketoester into the ethene bridge of a photoactive diarylethene, we achieved reversible light-induced tautomerization to give thermally stable enol. In a pristine state, the tautomeric equilibrium is almost completely shifted toward the ketone. Photocyclization of diarylethene results in a new equilibrium containing a significant fraction of the enol tautomer.
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http://dx.doi.org/10.1021/acs.orglett.9b04376DOI Listing
January 2020

Dispersion Forces Drive the Formation of Uranium-Alkane Adducts.

J Am Chem Soc 2020 Jan 13;142(4):1864-1870. Epub 2020 Jan 13.

Department of Molecular Theory and Spectroscopy , Max-Planck Institute for Kohlenforschung , Kaiser Wilhelm-Platz-1 , 45470 Mülheim-an-der-Ruhr , Germany.

Single-crystal cryogenic X-ray diffraction at 6 K, electron paramagnetic resonance spectroscopy, and correlated electronic structure calculations are combined to shed light on the nature of the metal-tris(aryloxide) and η-H, C metal-alkane interactions in the [((ArO)tacn)U(cy-C6)]·(cy-C6) adduct. An analysis of the ligand field experienced by the uranium center using ab initio ligand field theory in combination with the angular overlap model yields rather unusual U-O and U-N bonding parameters for the metal-tris(aryloxide) interaction. These parameters are incompatible with the concept of σ and π metal-ligand overlap. For that reason, it is deduced that metal-ligand bonding in the [((ArO)tacn)U] moiety is predominantly ionic. The bonding interaction within the [((ArO)tacn)U] moiety is shown to be dispersive in nature and essentially supported by the upper-rim Bu groups of the (ArO)tacn ligand. Our findings indicate that the axial alkane molecule is held in place by the guest-host effect rather than direct metal-alkane ionic or covalent interactions.
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http://dx.doi.org/10.1021/jacs.9b10620DOI Listing
January 2020

Photochromic diarylethene ligands featuring 2-(imidazol-2-yl)pyridine coordination site and their iron(II) complexes.

Beilstein J Org Chem 2019 15;15:2428-2437. Epub 2019 Oct 15.

Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Egerlandstraße 1, 91058 Erlangen, Germany.

A new family of photochromic diarylethene-based ligands bearing a 2-(imidazol-2-yl)pyridine coordination unit has been developed. Four members of the new family have been synthesized. The photoactive ligands feature non-aromatic ethene bridges (cyclopentene, cyclopentenone, and cyclohexenone), as well as closely spaced photoactive and metal coordination sites aiming a strong impact of photocyclization on the electronic structure of the coordinated metal ion. The ligands with cyclopentenone and cyclohexenone bridges show good cycloreversion quantum yields of 0.20-0.32. The thermal stability of closed-ring isomers reveals half-lives of up to 20 days in solution at room temperature. The ligands were used to explore coordination chemistry with iron(II) targeting photoswitchable spin-crossover complexes. Unexpectedly, dinuclear and tetranuclear iron(II) complexes were obtained, which were thoroughly characterized by X-ray crystallography, magnetic measurements, and Mössbauer spectroscopy. The formation of multinuclear complexes is facilitated by two coordination sites of the diarylethene, acting as a bridging ligand. The bridging nature of the diarylethene in the complexes prevents photocyclization.
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http://dx.doi.org/10.3762/bjoc.15.235DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6808200PMC
October 2019

A Series of Iron Nitrosyl Complexes {Fe-NO} and a Fleeting {Fe-NO} Intermediate en Route to a Metalacyclic Iron Nitrosoalkane.

J Am Chem Soc 2019 10 17;141(43):17217-17235. Epub 2019 Oct 17.

Department of Chemistry and Pharmacy, Inorganic Chemistry , Friedrich-Alexander-University Erlangen-Nürnberg (FAU) , Egerlandstrasse 1 , D-91058 Erlangen , Germany.

Iron-nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark-Feltham introduced the {Fe-NO} notation. In this work, we succeeded in characterizing a series of {Fe-NO} complexes, including a reactive {Fe-NO} intermediate. All complexes were synthesized with the tris--heterocyclic carbene ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyl]amine (TIMEN), which is known to support iron in high and low oxidation states. Reaction of NOBF with [(TIMEN)Fe] resulted in formation of the {Fe-NO} compound [(TIMEN)Fe(NO)(CHCN)](BF) (). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe-NO} (-). Reduction of {Fe-NO} with Cs electride finally yields the highly reduced {Fe-NO} intermediate, key to formation of [Cs(crypt-222)][(TIMEN)Fe(NO)], () featuring a metalacyclic [Fe-(NO-NHC)] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as Fe Mössbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe-NO} complex was found via oxygenation of the nitride complex [(TIMEN)Fe(N)](BF). Surprisingly, the resulting {Fe-NO} species is electronically and structural similar to the [(TIMEN)Fe(N)] precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark-Feltham nomenclature to nitrido complexes, rendering [(TIMEN)Fe(N)] as a {Fe-N} species.
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http://dx.doi.org/10.1021/jacs.9b08053DOI Listing
October 2019

A Terminal Iron Nitrilimine Complex: Accessing the Terminal Nitride through Diazo N-N Bond Cleavage.

Angew Chem Int Ed Engl 2019 Dec 31;58(51):18547-18551. Epub 2019 Oct 31.

Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Department of Chemistry and Pharmacy, General and Inorganic Chemistry, Egerlandstrasse 1, 91058, Erlangen, Germany.

A novel method for the N-N bond cleavage of trimethylsilyl diazomethane is reported for the synthesis of terminal nitride complexes. The lithium salt of trimethylsilyl diazomethane was used to generate a rare terminal nitrilimine transition metal complex with partially occupied d-orbitals. This iron complex 2 was characterized by CHN combustion analysis, H and C NMR spectroscopic analysis, single-crystal X-ray crystallography, SQUID magnetometry, Fe Mössbauer spectroscopy, and computational analysis. The combined results suggest a high-spin d  (S=2) electronic configuration and an allenic structure of the nitrilimine ligand. Reduction of 2 results in release of the nitrilimine ligand and formation of the iron(I) complex 3, which was characterized by CHN combustion analysis, H NMR spectroscopic analysis, and single-crystal X-ray crystallography. Treatment of 2 with fluoride salts quantitatively yields the diamagnetic Fe nitride complex 4, with concomitant formation of cyanide and trimethylsilyl fluoride through N-N bond cleavage.
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http://dx.doi.org/10.1002/anie.201910428DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6916540PMC
December 2019

Phenanthroline-Based Molecular Switches for Prospective Chemical Grafting: A Synthetic Strategy and Its Application to Spin-Crossover Complexes.

Inorg Chem 2020 Mar 11;59(5):2659-2666. Epub 2019 Sep 11.

Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Egerlandstraße 1, 91058 Erlangen, Germany.

1,10-Phenanthroline represents a well-known versatile ligand system finding many applications in chemistry, biology, and material science. The properties and thus the use of these molecules are determined by coordinating metal ions and ligand substituents. Advanced ligand systems that, for instance, feature simultaneously an integrated photochrome and a surface anchoring group require the introduction of several differing substituents and the synthesis of asymmetric derivatives. In spite of a long history of the ligand system-and to our great surprise-a general synthetic approach allowing the introduction of differing substituents at positions (3,8) and (5,6) of 1,10-phenanthroline is not known. Here, we present a general approach for the synthesis of such phenanthrolines. The approach is used to integrate a diarylethene photochrome into a functionalized phenanthroline and thus to synthesize a novel photoswitchable phenanthroline and a corresponding spin-crossover molecular photoswitch. The functionality of both the ligand and its iron(II) complex at room temperature has been demonstrated. The importance of this work for chemical grafting of molecular switches based on phenanthrolines is emphasized.
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http://dx.doi.org/10.1021/acs.inorgchem.9b01424DOI Listing
March 2020

A complete series of uranium(iv) complexes with terminal hydrochalcogenido (EH) and chalcogenido (E) ligands E = O, S, Se, Te.

Dalton Trans 2019 Aug 5;48(29):10853-10864. Epub 2019 Apr 5.

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany.

We here report the synthesis and characterization of a complete series of terminal hydrochalcogenido, U-EH, and chalcogenido uranium(iv) complexes, U≡E (with E = O, S, Se, Te), supported by the (ArOH)tacn (1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane) ligand system. Reaction of HE with the trivalent precursor [((ArO)tacn)U] (1) yields the corresponding uranium(iv) hydrochalcogenido complexes [((ArO)tacn)U(EH)] (2). Subsequent deprotonation of the terminal hydrochalcogenido species with KN(SiMe), in the presence of 2.2.2-cryptand, gives access to the uranium(iv) complexes with terminal chalcogenido ligands [K(2.2.2-crypt)][((ArO)tacn)U≡E] (3). In order to study the influence of the varying terminal chalogenido ligands on the overall molecular and electronic structure, all complexes were studied by single-crystal X-ray diffractometry, UV/vis/NIR, electronic absorption, and IR vibrational spectroscopy as well as SQUID magnetometry and computational analyses (DFT, MO, NBO).
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http://dx.doi.org/10.1039/c9dt00530gDOI Listing
August 2019

Cyaarside (CAs ) and 1,3-Diarsaallendiide (AsCAs ) Ligands Coordinated to Uranium and Generated via Activation of the Arsaethynolate Ligand (OCAs ).

Angew Chem Int Ed Engl 2019 Feb 9;58(6):1679-1683. Epub 2019 Jan 9.

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-University of Erlangen-Nürnberg (FAU), Egerlandstrasse 1, 91058, Erlangen, Germany.

Reaction of the trivalent uranium complex [(( ArO) N)U(DME)] with one molar equiv [Na(OCAs)(dioxane) ], in the presence of 2.2.2-crypt, yields [Na(2.2.2-crypt)][{(( ArO) N)U (THF)}(μ-O){(( ArO) N)U (CAs)}] (1), the first example of a coordinated η -cyaarside ligand (CAs ). Formation of the terminal CAs is promoted by the highly reducing, oxophilic U precursor [(( ArO) N)U(DME)] and proceeds through reductive C-O bond cleavage of the bound arsaethynolate anion, OCAs . If two equiv of OCAs react with the U precursor, the binuclear, μ-oxo-bridged U complex [Na(2.2.2-crypt)] [{(( ArO) N)U } (μ-O)(μ-AsCAs)] (2), comprising the hitherto unknown μ:η ,η -coordinated (AsCAs) ligand, is isolated. The mechanistic pathway to 2 involves the decarbonylation of a dimeric intermediate formed in the reaction of 1 with OCAs . An alternative pathway to complex 2 is by conversion of 1 via addition of one further equiv of OCAs .
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http://dx.doi.org/10.1002/anie.201811332DOI Listing
February 2019

An Isolable Terminal Imido Complex of Palladium and Catalytic Implications.

Angew Chem Int Ed Engl 2018 Dec 8;57(49):16228-16232. Epub 2018 Nov 8.

Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058, Erlangen, Germany.

Herein, we report the isolation and a reactivity study of the first example of an elusive palladium(II) terminal imido complex. This scaffold is an alleged key intermediate for various catalytic processes, including the amination of C-H bonds. We demonstrate facile nitrene transfer with H-H, C-H, N-H, and O-H bonds and elucidate its role in catalysis. The high reactivity is due to the population of the antibonding highest occupied molecular orbital (HOMO), which results in unique charge separation within the closed-shell imido functionality. Hence, N atom transfer is not necessarily associated with the high valency of the metal (Pd , Pd ) or the open-shell character of a nitrene as commonly inferred.
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http://dx.doi.org/10.1002/anie.201809152DOI Listing
December 2018

Synthesis and Reactivity of Low-Valent f-Element Iodide Complexes with Neutral Iminophosphorane Ligands.

Inorg Chem 2018 Aug 6;57(15):9230-9240. Epub 2018 Jul 6.

LCM, Ecole Polytechnique, CNRS , Université Paris-Saclay , F-91128 Palaiseau Cedex , France.

The coordination and reactivity of simple iodide salts of low-valent f elements [YbI, SmI, TmI, and UI(THF), where THF = tetrahydrofuran] with iminophosphorane (RP═NR') ligands are reported. The studied chelates were observed to adapt their geometry and effectively bind divalent ytterbium and samarium centers, as well as the trivalent uranium cation. The reactivity of the ytterbium adducts with benzophenone was found to be dependent on the steric demand of the supporting iminophosphorane ligand. In particular, a rare example of a stable charge-separated ketyl radical species is reported with ytterbium. Additionally, divalent thulium was observed to induce a reductive coupling at the ligand's central pyridine ring.
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http://dx.doi.org/10.1021/acs.inorgchem.8b01259DOI Listing
August 2018

Oxa[7]superhelicene: A π-Extended Helical Chromophore Based on Hexa-peri-hexabenzocoronenes.

Angew Chem Int Ed Engl 2018 05 17;57(20):5938-5942. Epub 2018 Apr 17.

Department Chemie und Pharmazie & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg, Nikolaus-Fiebiger-Strasse 10, 91058, Erlangen, Germany.

A novel π-extended "superhelicene" based on hexa-peri-hexabenzocoronenes (HBCs) has been synthesized by an efficient four-step synthetic procedure starting from diphenyl ether. Comprehensive structural analysis of the helicene was performed by NMR spectroscopy and mass spectrometry measurements together with X-ray analysis. Physicochemical analysis of the superhelicene and suitable HBC references revealed it had outstanding fluorescent features with quantum yields of over 80 %.
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http://dx.doi.org/10.1002/anie.201800585DOI Listing
May 2018

The role of uranium-arene bonding in HO reduction catalysis.

Nat Chem 2018 03 11;10(3):259-267. Epub 2017 Dec 11.

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstrasse 1, D-91058 Erlangen, Germany.

The reactivity of uranium compounds towards small molecules typically occurs through stoichiometric rather than catalytic processes. Examples of uranium catalysts reacting with water are particularly scarce, because stable uranyl groups form that preclude the recovery of the uranium compound. Recently, however, an arene-anchored, electron-rich uranium complex has been shown to facilitate the electrocatalytic formation of H from HO. Here, we present the precise role of uranium-arene δ bonding in intermediates of the catalytic cycle, as well as details of the atypical two-electron oxidative addition of HO to the trivalent uranium catalyst. Both aspects were explored by synthesizing mid- and high-valent uranium-oxo intermediates and by performing comparative studies with a structurally related complex that cannot engage in δ bonding. The redox activity of the arene anchor and a covalent δ-bonding interaction with the uranium ion during H formation were supported by density functional theory analysis. Detailed insight into this catalytic system may inspire the design of ligands for new uranium catalysts.
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http://dx.doi.org/10.1038/nchem.2899DOI Listing
March 2018

Synthesis, Characterization, and Properties of Iron(II) Spin-Crossover Molecular Photoswitches Functioning at Room Temperature.

Inorg Chem 2017 Nov;56(21):13174-13186

Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg , Egerlandstrasse 1, 91058 Erlangen, Germany.

Spin-crossover molecular switches [Fe(HB(pz))L] (L = novel phenanthroline-based ligands featuring photochromic diarylethene units; pz = 1-pyrazolyl) were synthesized and thoroughly characterized by variable-temperature X-ray crystallography, Mössbauer spectroscopy, and magnetic measurements. The effect of substituents introduced into the phenanthroline backbone (L2) and into the photochromic diarylethene unit (L3) on photophysical properties of metal-free ligands and spin-crossover iron(II) complexes 2 and 3, respectively, were investigated in detail. Both ligands and complexes could be switched with light in solution at room temperature. The photocyclization of 2 was accompanied by a high-spin to low-spin photoconversion determined at 19%. The closed-ring isomers of L3 and 3 reveal the lifetimes in the range of minutes, whereas those of L2 and 2 are thermally stable for days in solutions at room temperature. The reversibility of the photoswitching can be improved by avoiding the photostationary states. Prospective introduction of anchoring groups to the phenanthroline backbone might allow the construction of chemisorbed self-assembled monolayers of spin-crossover species switchable with light at room temperature.
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http://dx.doi.org/10.1021/acs.inorgchem.7b01952DOI Listing
November 2017

Synthesis and characterization of uranium(iv) tetrachloro complexes in bis-pyrazolylpyridine ligand environments.

Dalton Trans 2017 Oct;46(40):13811-13823

Inorganic Chemistry, Department of Chemistry and Pharmacy, Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Egerlandstr. 1, 91058 Erlangen, Germany.

Starting from dimethyl 2,6-pyridinedicarboxylate (3), four pyridine-bridged bispyrazole ligands 1a-1d were generated in a two- or three-step synthesis sequence and further treated with UCl to yield the corresponding novel mononuclear uranium(iv) complexes [U(R'''L)(Cl)] (2a-2d). Compounds 2a-2d were characterized by a variety of spectroscopic and physical methods (e.g. UV/Vis, SQUID, CV, etc.), corroborating the +4 oxidation state in 2a-2d. Single-crystal X-ray structure analyses revealed that 2a·2THF crystallizes in the orthorhombic space group Pbca, 2b·0.8THF·0.2EtO in the monoclinic Sohncke space group P2, 2c·0.25EtO in the monoclinic one P2/c, and finally 2d·0.5THF in the orthorhombic Sohncke space group P222. In the solid state, complexes 2a-2d possess a distorted pentagonal-bipyramidal coordination sphere at the U centers and an out-of-plane shift (d) of up to 1.12 Å, which can be explained by an increased steric pressure on the metal ions at the binding sites of the chelating ligands 1c and 1d. Finally, by combination of different 1D and 2D NMR experiments, the H and C resonances can be unequivocally assigned in the corresponding paramagnetic NMR spectra of 2a-2d.
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http://dx.doi.org/10.1039/c7dt02947kDOI Listing
October 2017

Protonation of Ferrocene: A Low-Temperature X-ray Diffraction Study of [Cp FeH](PF ) Reveals an Iron-Bound Hydrido Ligand.

Angew Chem Int Ed Engl 2017 10 19;56(43):13372-13376. Epub 2017 Sep 19.

Friedrich-Alexander-Universität Erlangen-Nürnberg, Department für Chemie und Pharmazie, Anorganische Chemie, Egerlandstrasse 1, 91058, Erlangen, Germany.

Ferrocene, Cp Fe, is quantitatively protonated in a mixture of liquid HF/PF to yield [Cp FeH](PF ), which was characterized by H/ C NMR and Fe Mössbauer spectroscopy as well as single-crystal X-ray diffraction analysis. X-ray diffraction analysis at 100 K revealed a disordered, iron-coordinated hydrido ligand, which was unambiguously located by aspherical atom refinement at 100 K, and by analyzing the non-disordered crystal structure at 30 K, revealing a non-agostic structure.
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http://dx.doi.org/10.1002/anie.201704854DOI Listing
October 2017

Synthesis of Rationally Halogenated Buckybowls by Chemoselective Aromatic C-F Bond Activation.

Angew Chem Int Ed Engl 2017 04 24;56(17):4834-4838. Epub 2017 Mar 24.

Department of Organic Chemistry, Friedrich Alexander University Erlangen-Nuremberg, Henkestrasse 42, 91054, Erlangen, Germany.

Halogenated buckybowls or bowl-shaped polycyclic aromatic hydrocarbons (BS-PAHs) are key building blocks for the "bottom-up" synthesis of various carbon-based nanomaterials with outstanding potential in different fields of technology. The current state of the art provides quite a limited number of synthetic pathways to BS-PAHs; moreover, none of these approaches show high selectivity and tolerance of functional groups. Herein we demonstrate an effective route to BS-PAHs that includes directed intramolecular aryl-aryl coupling through C-F bond activation. The coupling conditions were found to be completely tolerant toward aromatic C-Br and C-Cl bonds, thus allowing the facile synthesis of rationally halogenated buckybowls with an unprecedented level of selectivity. This finding opens the way to functionalized BS-PAH systems that cannot be obtained by alternative methods.
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http://dx.doi.org/10.1002/anie.201700814DOI Listing
April 2017

Uranium Tetrakis-Aryloxide Derivatives Supported by Tetraazacyclododecane: Synthesis of Air-Stable, Coordinatively-Unsaturated U(IV) and U(V) Complexes.

Inorg Chem 2017 Mar 16;56(6):3201-3206. Epub 2016 Nov 16.

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg , Egerlandstraße 1, 91058Erlangen, Germany.

We present the synthesis, characterization, and one-electron oxidation of two uranium(IV) complexes, coordinated to the cyclen-anchored tetrakis(aryloxide) ligands tetrakis-hydroxybenzyl-1,4,7,10 tetraazacyclododecane, (ArOH)cyclen; R = Bu, Me. The new uranium(IV) and (V) complexes exhibit an eight-coordinate, tetragonal ligand environment, effecting exceptional stability of the coordinatively unsaturated uranium compounds. Cyclic voltammetry studies reveal redox events ranging from tri- to hexavalent species, covering an electrochemical window of ∼4 V.
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http://dx.doi.org/10.1021/acs.inorgchem.6b02123DOI Listing
March 2017

Molecular and Electronic Structures of Eight-Coordinate Uranium Bipyridine Complexes: A Rare Example of a Bipy Ligand Coordinated to a U Ion.

Inorg Chem 2017 Mar 14;56(5):2792-2800. Epub 2017 Feb 14.

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg (FAU) , Egerlandstrasse 1, 91058 Erlangen, Germany.

Reaction of trivalent [((ArO)tacn)U] (1) with 2,2'-bipyridine (bipy) yields [((ArO)tacn)U(bipy)] (2) and subsequent reduction of 2 with KC in the presence of Kryptofix222 furnishes [K(2.2.2-crypt)][((ArO)tacn)U(bipy)] (3). Alternatively, complex 3 can be synthesized from 1 by addition of [K(bipy)] in the presence of the cryptand. New complexes 2 and 3 are characterized by a variety of spectroscopic, electrochemical, and magnetochemical methods, single-crystal X-ray diffraction, computational methods, and CHN elemental analysis. Structural analyses reveal a bipyridine radical (bipy) ligand in 2 and a dianionic (bipy) species in 3. Complex 3 represents a rare example of an isolated and unambiguously characterized bipy ligand coordinated to a uranium ion. The electronic structure assignments are supported by UV/vis/NIR and EPR spectroscopy, as well as SQUID magnetometry. The results of CASSCF calculations indicate multiconfigurational ground states for complexes 2 and 3. The electronic ground state for 2 consists of an open-shell doublet U(bipy) state (91%) and a closed-shell doublet U(bipy) state (9%). The almost degenerate multiconfigurational ground state for 3 was found to be composed of an open-shell singlet and pure triplet state 0.06 eV higher in energy, both resulting from the U(5f) (bipy) configuration.
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http://dx.doi.org/10.1021/acs.inorgchem.6b02954DOI Listing
March 2017

Liquid silver tris(perfluoroethyl)trifluorophosphate salts as new media for propene/propane separation.

Phys Chem Chem Phys 2016 Oct;18(40):28242-28253

Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Institute of Chemical Reaction Engineering, Egerlandstraße 3, 91058 Erlangen, Germany.

A series of silver tris(perfluoroethyl)trifluorophosphate (Ag[FAP]) complexes with various ligands (acetonitrile ACN, chloroacetonitrile Cl-ACN, acrylonitrile acryl-CN, pyridine py, ethylenediamine en and propene CH) have been synthesized starting from Ag[NO] and K[FAP] using three different routes. Physicochemical properties as well as crystal structures ([Ag(ACN)][FAP], [Ag(py)][FAP]) were determined and the suitability of such Ag salts for propene/propane separation processes was investigated. The investigated silver complexes exhibit either low melting points or form liquid complexes when contacted with gaseous propene at 30 °C. This makes them promising separation materials for both liquid membranes and absorber fluids due to their high silver content and significant propene capacity. Single (iGSC) and mixed (NMR) gas solubilities as well as diffusion coefficients (PFG-NMR) of propene and propane were determined to predict the theoretical selectivity of solubility, membrane selectivity, capacity and transport properties of the silver salts according to the solution diffusion model. A strong influence of the number and type of ligands on chemical complexation, physicochemical properties and separation performance has been observed.
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http://dx.doi.org/10.1039/c6cp05653aDOI Listing
October 2016

Uranium(iv) terminal hydrosulfido and sulfido complexes: insights into the nature of the uranium-sulfur bond.

Chem Sci 2016 Sep 10;7(9):5857-5866. Epub 2016 May 10.

Department of Chemistry and Pharmacy , Inorganic Chemistry , Friedrich-Alexander University Erlangen-Nürnberg , Egerlandstraße 1 , 91058 Erlangen , Germany . Email:

Herein, we report the synthesis and characterization of a series of terminal uranium(iv) hydrosulfido and sulfido complexes, supported by the hexadentate, tacn-based ligand framework (ArO)tacn (= trianion of 1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane). The hydrosulfido complex [((ArO)tacn)U-SH] () is obtained from the reaction of HS with the uranium(iii) starting material [((ArO)tacn)U] () in THF. Subsequent deprotonation with potassium bis(trimethylsilyl)amide yields the mononuclear uranium(iv) sulfido species in good yields. With the aid of dibenzo-18-crown-6 and 2.2.2-cryptand, it was possible to isolate a terminal sulfido species, capped by the potassium counter ion, and a "free" terminal sulfido species with a well separated cation/anion pair. Spectroscopic and computational analyses provided insights into the nature of the uranium-sulfur bond in these complexes.
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http://dx.doi.org/10.1039/c6sc00677aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6024247PMC
September 2016

Reductive disproportionation of nitric oxide mediated by low-valent uranium.

Chem Commun (Camb) 2016 Sep 15;52(72):10854-7. Epub 2016 Aug 15.

Friedrich-Alexander-University of Erlangen-Nürnberg (FAU), Department of Chemistry and Pharmacy, Inorganic Chemistry, Egerlandstrasse 1, 91058 Erlangen, Germany.

The reductive disproportionation of nitric oxide (1 atm) is mediated by the bulky U(III) aryloxide [U(III)(OAr(Ad,Ad,Me))3] ((Ad,Ad,Me)ArO = O-C6H2-2,6-Ad-4-Me) (1) to form the U(V) terminal oxo species [((Ad,Ad,Me)ArO)3U(V)(O)] (2) and N2O, as confirmed by single crystal X-ray diffraction and GC-MS measurements. The reaction is quantitative in the solid state. Mechanistic and theoretical studies of the reaction suggest that the N-N bond is formed by the coupling of an η(1)-O bound nitric oxide ligand with gaseous NO to give an η(1)-(N2O2)(1-) intermediate prior to the spontaneous extrusion of N2O to yield the U(V) terminal oxo species 2.
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http://dx.doi.org/10.1039/c6cc06095aDOI Listing
September 2016