1,721,018 research outputs found
Mechanistic Insights into Superoxide Dismutation Driven by Dinuclear Manganese Complexes: The Role of the Mn2-Core
No full textThe dinuclear Mn2(II,II)L2-core (HL = 2-{[[di(2-pyridyl)methyl](methyl)amino]-methyl}phenol) has been recently reported to be the most active dual superoxide dismutase (SOD) and catalase (CAT) functional analogue, enabling cascade detoxification of the superoxide radical anion. Here, we investigated the mechanism of catalytic O2•- decomposition by two stereoisomers with the Mn2(II,II)L2-core, Mn2L2Ac and Mn2L2, in order to (i) precisely determine the catalytic SOD activity of the complexes, (ii) characterize the key intermediates involved in the dismutation process, and (iii) discriminate between single- and di-Mn center catalysis in relation to the configuration of the Mn2-core. The conclusions drawn from low-temperature mass spectrometry, stopped-flow kinetics, cyclic voltammetry, water exchange 17O nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) analyses were supported by the structural characterization and quantum chemical analysis of the proposed reaction intermediates. This study allows us to determine kcat for Mn2L2Ac and Mn2L2 (4.6 × 107 and 2.2 × 107 M-1 s-1, respectively, in 3-(N-morpholino)propanesulfonic acid (MOPS) at pH = 7.4) and detect the key intermediates involved in the catalytic cycle driven by these Mn2-SOD mimics, highlighting the formation of a side-on η2-Mn2(III,II)-peroxo, as an initial intermediate. The effects of the Mn2(II,II)-core configuration on the SOD activity were discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Iminreduktion mit Hauptgruppenmetallkatalysatoren
No full textThe efficient catalytic reduction of imines with phenylsilane is achieved by using the potassium, calcium and strontium based catalysts [(DMAT)K(THF)]ꚙ, (DMAT)2Ca·(THF)2 and (DMAT)2Sr·(THF)2 (DMAT=2-dimethylamino-α-trimethylsilylbenzyl). Eight different aldimines and the ketimine Ph2C=NPh could be successfully reduced by PhSiH3 at temperatures between 25–60 °C with catalyst loadings down to 2.5 mol%. Also, simple amides like KN(SiMe3)2 or Ae[N(SiMe3)2]2 (Ae=Ca, Sr, Ba) catalyze this reaction. Activities increase with metal size. For most substrates the activity increases along the row K<Ca<Sr<Ba. Fastest conversion was found for imines with alkyl substituents at N and aryl rings at C, for example, PhC(H)=NtBu, while tBuC(H)=NtBu or PhC(H)=NPh react much slower. Reasonable functional group tolerance is observed. The proposed metal hydride mechanism is supported by stoichiometric reactions using a catalyst model system, isolation of intermediates and DFT calculations.
Imine-to-amine conversion with catalytic instead of stoichiometric quantities of LiAlH4 is demonstrated (85 °C, catalyst loading ≥ 2.5 mol%, pressure ≥ 1 bar). The effects of temperature, pressure, solvent, and catalyst modifications, as well as the substrate scope are discussed. Experimental investigations and preliminary DFT calculations suggest that the catalytically active species is generated in situ: LiAlH4 + Ph(H)C=NtBu → LiAlH2[N(tBu)CH2Ph]2. A cooperative mechanism in which Li and Al both play a prominent role is proposed.
Commercially available LiAlH4 can be used in catalytic quantities in the hydrogenation of imines to amines with hydrogen gas. Combined experimental and theoretical investigations give deeper insight in the mechanism and identifies the most likely catalytic cycle. Activity is lost when Li in LiAlH4 is exchanged for the heavier alkali metals Na or K. Exchanging Al for B or Ga also led to dramatically reduced activities. This indicates a heterobimetallic mechanism in which synergy between Li and Al is important. Potential intermediates on the catalytic pathway have been isolated from reactions of MAlH4 (M = Li, Na, K) and different imines. Depending on the imine, products of double, triple or quadruple imine insertion have been structurally characterized. Prolonged reaction of LiAlH4 with PhC(H)=NtBu led to a side-reaction and gave the double insertion product LiAlH2[N]2 ([N] = N(tBu)CH2Ph) which at higher temperature reacts further by double ortho-metallation in the Ph ring. A DFT study led to a number of conclusions. The most likely catalyst for hydrogenation of PhC(H)=NtBu with LiAlH4 is LiAlH2[N]2. Insertion of a third imine via a heterobimetallic transition state has a barrier of +23.2 kcal/mol (ΔH). The rate-determining step is hydrogenolysis of LiAlH[N]3 with H2 with a barrier of +29.2 kcal/mol. In agreement with experiment, replacing Li for Na (or K) and Al for B (or Ga) led to higher calculated barriers. Also, the AlH4ˉ anion showed very high barriers. Calculations support the experimentally observed effects of the imine substituents at C and N: the lowest barriers are calculated for imines with aryl-substituents at C and alkyl-substituents at N.
Alkaline earth (Ae) metal complexes with the alanate anion AlH4− have been prepared by salt metathesis between NaAlH4 and AeCl2 in THF and could be isolated as Mg(AlH4)2·(THF)4, Ca(AlH4)2·(THF)4, and Sr(AlH4)2·(THF)5. The previously reported crystal structure of the Mg alanate complex shows bonding of AlH4− with one bridging hydride, H3Al-(μ-H)-Mg, while the Ca and Sr alanates show a combination of H3Al-(μ-H)-Ae and H2Al-(μ-H)2-Ae bridging. The heteroleptic β-diketiminate complexes (DIPPBDI)Mg(AlH4)·THF and (DIPPBDI)Ca(AlH4)·(THF)2 have been prepared by reaction of the corresponding Ae hydride complexes with AlH3·(THF)2 [DIPPBDI = DIPP-NC(Me)C(H)C(Me)N-DIPP, where DIPP = 2,6-diisopropylphenyl]. Crystal structures show H2Al-(μ-H)2-Ae bridging. The Ca complex decomposes at room temperature by reduction of the β-diketiminate anion. Density functional theory calculations (B3PW91/def2tzvpp) show that the formation of Ae(AlH4)2 from AeH2 and AlH3 is exothermic by ΔH (kilocalories per mole): Be, −68.8; Mg, −66.1; Ca, −95.4; Sr, −100.9; Ba, −112.3. Calculations of NPA charges on LiAlH4 and the Ae alanate complexes (Ae = Mg, Ca, or Sr) show that these are highly ionic salts in which the charge on AlH4− of approximately −0.95 is hardly dependent on the countercation. Compared to LiAlH4, the Ae alanates are very efficient catalysts for imine hydrogenation, clearly extending the substrate scope. In addition to aldimines RC(H)=NR′ (R/R′ = Ph/tBu, tBu/tBu, nPr/tBu, or Ph/Ph), ketimine PhC(Me)=NtBu could be reduced. The salt [Bu4N+][AlH4−] is catalytically not active, which shows that the s-block metal is crucial. The highest activities were found for the heterobimetallic Ca and Sr alanates
Aktivierung und Hydrierung von Alkenen mit Erdalkalimetallen
Full text linkThis work describes advances in the field of hydrogenation catalysis and hydrogen isotope exchange with alkaline earth metals. Further emphasis is given to early main group metal-alkene bonding and the synthesis of large decanuclear group 2 metal hydride clusters.Diese Dissertation beschreibt Fortschritte auf dem Gebiet der Hydrierungskatalyse und des Wasserstoffisotopenaustauschs mit Erdalkalimetallen. Weitere Schwerpunkte sind die Synthese neuartiger Hauptgruppenmetall-Alken-Komplexe sowie großer dekanuklearer Erdalkalimetall-Hydridcluster
Auf dem Weg zu funktionalen Eisenkomplexen: Elektronenspeicher, Chromophore, Magnete und Aktivierung kleiner Moleküle
No full textThe base metal iron holds many advantages over precious metals, as it is abundant, compar-
atively cheap, and biocompatible. Therefore, it is only reasonable to search for application
of iron in those fields of coordination chemistry that are usually dominated by the less
abundant 4d and 5d metals. Further, the prevalence of iron in naturally occurring enzymes
already teaches researchers how this base metal is indeed capable of performing the most
challenging transformations, activating and even breaking the most stable chemical bonds.
While this has been an immense area of bioinorganic research over the past decades, and
outstanding discoveries have been made in understanding the processes in enzymes, many
structures, transformations, and exact origins of the observed high catalytic activities re-
main still a matter of scientific debate. The same is true for small molecule activation that
requires the careful design of synthetic complexes to achieve the desired reactivities, and
yet, the outcome can still be surprising.
In light of this, the mixed carbene-phenolate, bis(aryloxide) benzimidazolin-2-ylidene pin-
cer ligand (OCO)2– was introduced to iron chemistry and the resulting homo- and het-
eroleptic iron complexes now provide a fruitful landscape from the stable [Fe(OCO)2] com-
plexes to the reactive, coordinatively unsaturated [Fe(OCO)] iron pincer motifs. Addition-
ally, the N-anchored tris(carbene) chelate tris-[(3-mesitylimidazol-2-ylidene)methyl]amine
(TIMMNMes) was employed to stabilize low oxidation states of iron – not only closing the
gap in a series of iron complexes in eight oxidation states – but further establishing a reactive
platform with a very adaptable cavity for small to medium size molecule binding. The ho-
moleptic iron complexes [Fe(OCO)2] and, additionally, iron complexes of very simple amide
ligands in [M(TMP)2], (M = Fe or Co, TMP = tetramethylpiperidine anion) were probed for
application in photo- and magnetochemistry, respectively. In the case of [Fe(OCO)2], iron
complexes are envisioned to replace the textbook example, [Ru(bipy)3]2+ in the future, once
the excited-state lifetimes can be sufficiently prolonged in iron coordination compounds.
Recent contributions highlighted the use of octahedral, coordinatively saturated iron com-
plexes supported by strongly donating carbene ligands for the preparation of Fe(II)/Fe(III)
chromophores with long-lived excited states. For this purpose, the (OCO)2– ligand seemed
a promising candidate, due to the strongly donating carbene central atom, accompanied
by flanking phenolate units for strong binding to iron(II) and iron(III). First, the homolep-
tic iron complexes [(Fe(OCOK)2]n (1) and the one, two, and three-electron oxidized com-
pounds [Fe(OCOK)(OCO)] (2) / [K(18-crown-6)(THF)2][Fe(OCO)2] (3), [Fe(OCO)2] (4),
and [Fe(OCO)2]PF6 (5) were synthesized and fully characterized (Figure 7.1). While 1 is
present as a polymer bridged by potassium ions in the solid state, the (OCO)2– ligand is
suitable for stabilizing mid to high oxidation states of iron, as in 2: [FeIII(OCOK)(OCO)]0,
3: [FeIII(OCO)2]– and 4: [FeIV(OCO)2]0. The highest oxidized complex of the series, 5,
formally Fe(V), is however, best described as [FeIV(OCO)2–(OCO)·−]+. Therefore, the iso-
lation of complexes 1 to 5 presents a series of iron coordination compounds spanning a range
of four oxidation states. This rich redox chemistry of the [Fe(OCO)2] motif, stemming from
the redox-flexible iron ion coordinated by the redox-active phenolate units, is further studied
by electrochemical methods. The iron(IV) complex 4 is stable in the solid-state, but readily
undergoes two-electron reductive elimination in solution at room temperature to form the
spirocyclic imidazolone ketal 6. This reaction was studied by 1H NMR spectroscopy to de-
termine the kinetics of this unusual reactivity at iron. The electronic structure in 1, 3, and 4
is readily assigned to a d6, d5, and d4 electron configuration at the metal centers, supported
by closed-shell, dianionic ligands, as determined by the applied spectroscopic methods and
computational analyses. The combined structural, spectroscopic, and computational results
for 5 are consistent with the formulation of an overall Stot. = 3/2 system.
A thorough description of the excited state landscape in high-valent [Fe(OCO)]2–/0/+ com-
plexes was obtained by studying the compounds by femtosecond transient absoption spec-
troscopy (fsTAS) (conducted by Dr. Alejandro Cadranel, FAU). While it was found that
the (OCO)2– donor strength is not sufficient to prevent population of metal-centered ex-
cited states upon photo excitation and, hence, does not lead to prolonged excited-state
lifetimes larger than tens of picoseconds, important insights into the photo-active infrared
region were collected. The photo-induced intervalence charge-transfer (PIIVCT) as observed
for the complex’ excited states, and similar to the intervalence charge-transfer (IVCT) in
mixed-valent 5 ([FeIV(OCO)2–(OCO)·−]+), can serve as a fingerprint in inorganic photo-
chemistry with ligand-to-metal charge-transfer (LMCT) chromophores, e.g. Fe(III) low-spin
complexes.
Inspired by the utilization of relatively simple coordination compounds for complex physical
applications, linear [M(TMP)2] complexes, d6 [Fe(TMP)2] and d7 [Co-(TMP)2] (synthe-
sized by Dr. Alessandra Logallo and Dr. Lewis Maddock at the group of Prof. Dr. Eva
Hevia, University of Bern), were studied for their potentials as a single-molecule magnet
(SMM) by direct current (DC) and alternating current (AC) SQUID magnetization mea-
surements. SMMs are in principle tiny nano-switches that could be used in storage devices.
However, the correlation between the molecular and electronic structure and the magnetic
properties of earth abundant 3d transition metals is an ongoing topic in current inorganic
chemistry. It is often difficult to predict the magnetic relaxation behavior even from com-
bined experimental and computational analysis of potentially suitable complexes. Therefore,
extensive screening is necessary, which requires sophisticated equipment. Hence, a better
understanding of these relaxation processes or their combination is of great importance.
While the [M(TMP)2] complexes do show slow magnetic relaxation (at fields of 0.1 T for
[Fe(TMP)2] and zero-applied field for [Co(TMP)2]), the experimentally determined en-
ergy reversal barriers Ueff. for magnetization relaxation is expected to be too low in both
cases for efficient use in SMM applications. Nevertheless, the finding that the typical obser-
vation of quantum tunneling of the magnetization (QTM) being diminished in non-integer
spin system, yet is observed for d7 [Co(TMP)2] at zero-applied field, can be used to
contribute to the better understanding of decay mechanisms of the magnetization and con-
comitantly, how larger Ueff. in transition metal SMMs could be obtained. Further, the study
of the magnetic relaxation behavior of the TMP systems is now part of the relatively small
library of linear 3d transition metal complexes that show potential SMM characteristics.
The (OCO)2– ligand was further utilized to synthesize two coordinatively unsaturated iron
complexes, [(OCO)Fe(MeCN)]2 (7), and [(OCO)Fe(py)3]OTf (OTf = trifluoromethanesul-
fonate) (8). Complexes 7 and 8 were fully characterized both in the solid state and in
solution, where large differences in their coordination chemistry under the respective con-
ditions became apparent. These dynamics, resulting from the open coordination sphere
next to the mer coordinating (OCO)2– pincer unit, were found to depend largely on the
donor strength of the solvent used for recrystallization or for the preparation of respective
solutions of 7 or 8. Complex 7 is present as a dimeric structure [(OCO)Fe(MeCN)]2 in
the solid state, when recrystallized from MeCN, as unambiguously confirmed by its solid-
state structure (Figure 7.1). Upon studying THF solutions of 7 by diffusion-ordered NMR
spectroscopy (DOSY), the determined molecular weight in solution corresponds to the for-
mulation as a [(OCO)Fe(THF)] complex. Since solid-state dimer 7dimer readily opens up in
solutions of coordinating solvents, e.g. THF, resulting in the monomer [(OCO)Fe(THF)],
complex 7monomer is expected to offer a platform for reactivity, which is controlled by the
(OCO)2– pincer ligand. In contrast to iron(II) complex 7, iron(III) complex 8 is a monomer
in both solutions of coordinating solvents and in the solid state (Figure 7.1).
Investigation of the pine green crystals obtained from a pyridine/benzene mixture by single-
crystal X-ray diffraction (sc-XRD) revealed a [(OCO)Fe(py)3]OTf structure. Analysis of the
bond metrics indicated a low-spin configuration of the central iron ion, which was further
confirmed by 57Fe Mössbauer and EPR spectroscopy, where the latter was performed in
pyridine solutions. Therefore, a similar coordination geometry as observed in the solid-state
structure of 8 is expected (Figure 7.1). Dissolving 8 in benzene or toluene, the color change
to ink blue already marks a change in the electronic structure of the complex. While no
single-crystals could be obtained from toluene or benzene solutions, the EPR spectrum of
8 in toluene solutions clearly assigns a high-spin configuration. Similar to 7, trivalent 8
readily adapts to different environments and towards different substrates (as illustrates by
the different solvents). Therefore, both complexes 7 and 8 fulfill the typical demands for
pincer supported metals – the stabilization of the metal center by a rigid, mer-coordinated,
tridentate ligand, combined with an open coordination sphere for further reactivity and mark
the successful introduction of mono(OCO)2–-supported complexes to iron pincer chemistry.
Biomimetic chemistry of 3d base metals is at the heart of inorganic chemistry research, since
a lot of enzymes use these readily available and non-toxic metals to perform reactions that
need harsh conditions when conducted synthetically, at ambient pressure and temperature.
The solid-state form of [(OCO)Fe(MeCN)]2 (7) structurally resembles the resting state of
methane monooxygenase, the enzyme that converts methane into methanol by selectively
breaking one of the 105 kcal mol–1 C–H bonds using O2 as the active oxidant. With this
in mind, the reactivity of 7 towards dioxygen was investigated. Performing this reaction
at room temperature and without the addition of any other substrates, the oxygen atom
transfer product [(OCO)Fe(μ-O)(O(C=O)O)Fe] (9) was obtained. In diferric 9, one of the
former N-heterocyclic carbene ligands was oxidized to yield a urea motif. At low temper-
atures (–80°C), the intermediate of this transformation can be observed, and is sufficiently
stable for detailed characterization by a variety of inorganic spectroscopic methods. It was
found that 7 activates dioxygen to form a [Fe2IV(μ-O)2] diamond core in the transient com-
plex 10. Intermediate 10 is stable for at least ten hours at –80°C, which allows for its
thorough spectroscopic investigations. Combined with extensive computational modeling of
plausible intermediate structures, these studies support a mechanism that is reminiscent of
the oxygen uptake in diiron enzymes, such as in soluble methane monooxygenase and ri-
bonucleotide reductase. The spectroscopic and computational investigations show that the
bis-phenolate carbene ligand (OCO)2– is capable of stabilizing a high valent [Fe2IV(μ-O)2]
diamond core structure as found in intermediate Q (Figure 7.2). This demonstrates the
ability of 7 to break the dioxygen bond and fully reduce the O2 unit without the addition
of protons, electrons or any other additional reagents forming the diamond core motif di-
rectly from an iron(II) complex and O2. Notably, despite the plethora of synthesized model
compounds to date, the direct conversion of gaseous dioxygen remains scarce, and usually
oxygen atom transfer reagents or additives have to be employed to generate the high valent
iron oxo species. The decay of 10 was studied by electronic absorption spectroscopy. The
conversion of 10 to 9 follows pseudo-zeroth order kinetics pointing towards a pre-equilibrium
in solution.
While 10 was found to be inactive in C–H bond activation with 9,10-dihydroanthracene,
triphenylmethane, fluorene, and 1,3-cyclohexadiene at –50°C, it reacts with easily oxidizable
substrates, such as phosphines and aldehydes at –50 °C in THF solution. The demonstrated
lack of reactivity of the [Fe2IV(μ-O)2] core with C–H bonds provides strong evidence to the
experimental and DFT-derived theory that the [Fe2IV(μ-O)2] diamond core in Q in sMMO
is kinetically inert and needs to isomerize to more reactive terminal iron(IV) oxo cores for
attacking the strong C–H bonds in methane (Figure 7.3). The detailed characterization of
10 by various spectroscopic techniques, helps to establish the spectroscopic markers for the
biologically relevant [Fe2IV(μ-O)2] cores, which may aid in their detection in future studies
dealing with the reaction mechanism of iron-containing catalytic systems in chemistry and
biology.
The tripodal tris(carbene) ligand family (PhB(ImMes)3)– (phenyl-tris(1-mesitylimidazol-
2-ylidene)borate), TIMEMe (tris((3-methyl-imidazol-2-ylidene)methyl)ethane), TIMENMes
(tris(2-(3-mesityl-imidazol-2-ylidene)ethyl)amine), and the latest member TIMMNMes (tris
(2-(3-mesityl-imidazol-2-ylidene)methyl)amine) (Chart 7.1) has proven to be sufficiently
flexible to host iron in various oxidation states. These range from iron(0) to iron(VII) and
the corresponding complexes thus are able to activate a number of small molecules from their
low valent states or stabilize biologically relevant, high valent intermediates. The former
represents an important first step for transforming inert small molecules into more complex
products.
The reduction of the divalent precursor [(TIMMNMes)Fe(Cl)]PF6 (11-PF6) to the iron(0)
complex [(TIMMNMes)Fe(CO)3] (12) and iron(I) complexes [(TIMMNMes)Fe(L)]+ (13) (with
L = free site, η1-N2, CO, or py) demonstrates that the tris(carbene) chelate TIMMNMes is
able to stabilize eight iron oxidation states within the very same ligand framework. This
is achieved through the high steric and electronic flexibility of the ligand. Complexes 11,
12, and 13 differ in oxidation and spin states, which is possible by the variation of the
Fe–N anchor and the Fe–C carbene distances, the out-of-plane-shifts of the iron ion, the
size of the axial cavity and even the number of coordinated carbene arms. The low valent
and reducing nature of complex 13 is illustrated by the slight activation of N2 and CO,
and the reduction of benzophenone by 13-N2 to give the iron(II) benzophenone radical
anion [(TIMMNMes)Fe(OCPh2)]BArF4 (14). The present study of low valent 13 showcases
the adaptability of the axial cavity that can host simple ligands, such as Cl– , small, di-
atomic linear molecules, such as N2 and CO, heterocycles, such as pyridine, and molecules
as large as benzophenone. It can therefore be concluded that the tris(carbene) chelate is
both capable of stabilizing super-oxidized iron nitrides, and equally suitable to support the
iron center in its low oxidation states 0 and +1. Further, the iron(I) complex binds inert
molecules such as N2 and CO, the crucial first step in small molecule activation chemistry.
Interesting parallels were found in the reactivity of the TIMMN-iron complexes in high and
low oxidation states, respectively. While low valent 12 coordinates additional π-acceptor
ligands (CO in 12), the iron(VI) and iron(VII) nitride complexes bind an additional σ-donor
(F– ). Both compounds “balance”, i.e. increase or reduce, their oxidation states by forming
the ligand-activated, metallacyclic divalent [(TIMMNMes*)Fe(py)]+ (15) and pentavalent
[(TIMMNNMes)Fe(NMes)]3+ (16), triggered by H atom abstraction in monovalent 13-N2 or
electrophilic attack, followed by 1,2- methyl and H+ shift as well as HF elimination within
the superoxidized Fe(VII) complex. Likewise, the iron center bounces back to an Fe(II)
state in 15 and a very similar structure in 16, regardless of the – formally – much higher
oxidation state. The study of the low-oxidation states of the [Fe(TIMMNMes)] complexes,
in combination with their high valent counterparts, allows for a unique comparison of (elec-
tronic) structure/reactivity correlation within the very same core structure, of one metal
supported by one ligand system, spanning eight oxidation states (Chart 7.2)
Towards Alkaline Earth Metal Complexes with Superbulky Amide Ligands and Highly Potent Alkene and Arene Hydrogenation Catalysts Based on Abundant Metals
Full text linkDissertationsschrift zur Erlangung des Doktorgrades Dr. rer. nat
New Calcium Hydride Complexes: Syntheses, Structures and Reactivities
Full text linkZusammenfassung
Die Aufgabe, die Chemie von Calciumhydriden zu erweitern, war ein sehr hoch gestecktes Ziel. Nichtsdestotrotz erwies sich die schon bekannte Struktur [(DIPPnacanac-CaH)2(THF)]2 als sehr hilfreich, um diese Aufgabe zu vollbringen. Seit den Anfängen dieser Chemie war klar, dass die Sterik des Ligandsystems eine entscheidende Eigenschaft war, um die Schlenk Gleichgewichte zu unterdrücken. Das Ersetzen von -diketiminaten durch Amidinate ergab die erwünschten Ergebnisse, jedoch erst nach einer überlegten Planung der Reaktionsbedingungen und einer sorgfältige Einführung von bestimmten Gruppen ins Ligandsystem. Da die gewählte Syntheseroute zur Darstellung der Hydride auf Phenylsilan basierte, machte dies eine Analyse der Stabilität der Ca-N(SiMe3)2 Vorstufen erforderlich, welche deutlich stabiler als die korrespondierenden Hydride sind. Unter gewißen Bedingungen war es nicht mal möglich diese als stabile Produkte zu isolieren.
Tabelle 1: die N-Substituenten (blau) scheinen, den großten Effekt auf die Stabilität der Komplexe zu haben. Die “Backbonegruppen” (grün) dagegen erwiesen sich als perfekt, um feine Änderungen und Anpassungen vorzunehmen.
Ca-N(SiMe3)2 N J J J J J Stabilität
Ca-H Stabilität N N N N J J
Milde Reaktionsbedingungen und nicht koordinierende Lösungsmittel spielen eine sehr entscheidende Rolle in der Synthese dieser Calciumhydridverbindungen, die sonst in THF oder Et2O unmöglich wäre, da sie das Schlenk Gleichgewicht begünstigen. Die dimerischen Produkte (tBuAmDIPP-CaH)2 und (AdAmDIPP-CaH)2 wurden aus diesem Grund nur aus Hexan und bei -30°C erhalten: hohe Temperaturen führen entweder zur Zersetzung oder zu einer enormen Verringerung der Ausbeuten. Literatur unbekannte trimerische Calciumhydridstrukturen wurden ebenfalls in guten Ausbeuten erhalten. Da solche Strukturen sich als zu unlöslich und zu unstabil in Lösung erwiesen, war keine NMR spektroskopische Charakterisierung möglich.
Bild 1: Eine Reaktion die, direkt zur Isolierung des Trimers führt, kann nicht stattfinden. Sogar bei -30°C ist die Isolierung des zyklischen Calciumhydridtrimers in Et2O nicht möglich. Das beweist dass, zusammen mit den Reaktionsbedingungen, die richtige Auswahl des Lösungsmittels entscheidend ist.
239
New Calcium Hydride Complexes: Syntheses, Structures and Reactivities
Schließlich erwiesen sich die zahlreichen Koordinationsmöglichkeiten der Amidinaten als entscheidend für die Isolierung von neuartigen Calcium Hydriden: das Ligandsystem passt sich perfekt an viele Bedingungen an, und dadurch kann man die große Anzahl an neuen Strukturen erklären.
Der erste Schritt, der in Betracht gezogen wurde, um die Chemie des DIPPnacnac-CaH zu erweitern, war das Ersetzen von THF durch Et2O. Auch in diesem Fall waren die Reaktionsbedingungen äußerst wichtig, da nur unter milden Bedingungen und in nicht koordinierenden Lösungsmitteln die Isolierung des Hydrides erfolgen konnte. Das Hydrid ist nur stabil, wenn das Lösungsmittel an das Calcium koordinieren kann. Die Abwesenheit des Lösungsmittels führt zu einer schnellen Zersetzung in die homoleptischen Spezies. Da Et2O deutlich schwächer als THF an das Ca koordiniert, ist sofort klar, dass die Synthese unter milden bedingungen stattfinden muss.
Bild 2: Das Entfernen von Et2O unter Vollvakuum oder bei hohen Temperaturen führt ausschließlich zur Zerstezung. Die Koordination von Lösungsmitte an Ca ist erforderlich, um die benötigte Stabilität des Komplexes zu gewährleisten.
Et2O kann sehr einfach von polarer Lösungsmitteln, wie THF oder N-methylmorpholin verdrängt werden. Das letztgenannte führt zu einer dimerischen Struktur, die der originale [(DIPPnacanac- CaH)2(THF)]2 sehr ähnelt. Tertiäre Aminen weisen eine zu geringe Donorstärke auf, um an das Ca zu koordinieren, mit der einzigen Ausnahme des DABCO (1,4-Diazabicyclo[2.2.2]octan), das die Darstellung der ersten polymerischen Calciumhydridstruktur der Welt erlaubte. Trotz des nicht vorhandenen Dipolmoments, das Symmetriebedingt gleich null ist, können die Stickstoffe gleichzeitig zwei Ca koordinieren.
Bild 3: Die Ersetzung von Et2O durch DABCO erlaubte die Isolierung der ersten bekannten polymerischen Calciumhydridstruktur. Das Produkt ist jedoch sehr unlöslich in apolaren Lösungsmitteln, jedoch zeigt es eine außergewöhnliche Stabilität, auch an der Luft!
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New Calcium Hydride Complexes: Syntheses, Structures and Reactivities
Das Verhältnis dieser Komplexe in Lösung wurde durch NMR Studien untersucht, sowohl mit PGSE Messungen (Pulse Gradient Spin Echo Technik) als auch mit Temperaturabhängigen Messungen. Diese zeigten, dass die dimerische Form auch in Lösung beibehalten wird: [(DIPPnacanac- CaH)2(THF)]2 und [(DIPPnacanac-CaH)2(N-MeMo)]2 zeigten keine Verschiebung des Hydridsignales selbst über große Temperaturbereiche. (0.09 ppm für [(DIPPnacanac-CaH)2(THF)]2 und 0.18 ppm für [(DIPPnacanac-CaH)2(N-MeMo)]). Da die Reaktivität von [(DIPPnacanac-CaH)2(THF)]2 schon sorgfältig untersucht wurde, wurden nur die Reaktivität der lösungsmittelfreien (tBuAmDIPP-CaH)2 und(AdAmDIPP-CaH)2 untersucht.SubstratewieNHO(N-heterocyklischeOlefine)besitzeneinesehr polarisiserte C=CH2 Bindung, die hochreaktiv ist, und die die Isolierung eines neuartigen Calciumalkylkomplexes ermöglichte. Da solche Systeme meistens aromatisch sind, würde die Addition von Ca-H an die terminale C=CH2 Bindung den Verlust von Aromatizität bedeuten. In diesem besonderen Fall verhält sich das Hydrid als Base und deprotoniert den Ring, statt eine hochreaktiven Calciumalkylverbindung zu bilden. Das negativgeladene System weist einen stark ylidischen Charakter auf, und koordiniert dadurch zwei Ca gleichzeitig.
Bild 4: Reaktion von (tBuAmDIPP-CaH)2 mit NHO. Die unerwartete deprotonierung führte zur Isolierung der ersten negativegeladenen NHO Verbindung.
Die von Amidinatliganden stabilisierten Calciumhydridkomplexe reagieren sehr schnell, auch mit ungesättigten Kohlenwasserstoffe. Ein interessanter Fall ist die Reaktion mit Diphenylacetylen (Tolan), die zu einer stilbenischen Struktur führt. Das Stilben2- koordiniert gleichzeitig zwei Ca und ist der Kern eines dualen Reaktionsweges, da es sich sowohl wie eine starke Base als auch wie ein Reduktionsmittel verhält.
Bild 5: Röntgenstruktur von [(tBuAmDIPP- Ca)2(SD)]. Die Elementarzelle des Kristalles ist fast identisch mit der entsprechenden Yb(II) Verbindung. Dies beweist die Ähnlichkeiten zwischen Ca2+ un Yb2+.
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New Calcium Hydride Complexes: Syntheses, Structures and Reactivities
[(tBuAmDIPP-Ca)2(SD)] kann I2 sehr schnell zu 2I- reduzieren, und Stilben wird als Nebenprodukt zusammen mit dem dimerischen [(tBuAmDIPP-CaI)2(THF)2]2 freigesetzt. In Gegenwart von H2 verhält es sich als Base und die Produkte der Reaktion sind (tBuAmDIPP-CaH)2 und 1,2-diphenylethan. Unter bestimmten Bedingungen (10mol %, 80°C und 6 bar H2) kann der Komplex auch Tolan katalytisch reduzieren, mit einer beinahen vollständigen Umsetzung nach 48 Stunden.
Bild 5: Katalytische Reduktion von Tolan zu 1,2-diphenylethan. Die Reaktion von [(tBuAmDIPP-Ca)2(SD)] mit I2 sollte unter milden Bedingungen durchgeführt warden, da hohen Temperaturen das Schlenk Gleichgewicht verstärken. Je länger die Reaktionstemperatur bei 0°C bleibt, desto höher ist die Ausbeute
Aktivierung kleiner Moleküle mit hochreaktiven Calcium(I)-Synthonen
No full textThis work describes the activation of small molecules with highly reactive calcium(I) synthons
Übergangsmetallkomplexe von N-geankerten N-heterozyklischen Carben-Liganden: Synthese, Charakterisierung und Reaktivität
Full text linkOur laboratory recently developed a nitrogen-anchored ligand series with donor functionalities ranging from tris(carbene) to tris(phenolate), including two new mixed ligands, (BIMPNMes,Ad,Me)– and (MIMPNMes,Ad,Me)2–, combining NHC carbene and phenolate donors. The new bis(carbene) mono(phenolate) ligand (BIMPNMes,Ad,Me)– was coordinated to manganese and the resulting complexes [(BIMPNMes,Ad,Me)MnII(Cl)], [(BIMPNMes,Ad,Me)MnII(N3)], and [(BIMPNMes,Ad,Me)MnII](BPh4), were synthesized and characterized in detail. SQUID magnetization studies confirmed high-spin ground states for all complexes. The molecular structures of the full complex series including the corresponding iron and cobalt complexes verified the desired ease of the steric strain compared to the tris(carbene) ligand TIMENR. Accordingly, the accessibility of the reactive metal center for small molecules is increased as intended; thus. promising a higher reactivity of the corresponding nitrido complexes. By photolysis of the cobalt(II) azido complex [(BIMPNMes,Ad,Me)CoII(N3)] at low temperatures (T = 10 K), the transient low-spin cobalt(IV) nitrido complex [(BIMPNMes,Ad,Me)CoIV(N)] was generated. This is the first example for a cobalt(IV) nitrido complex reported in the literature. At higher temperatures, the complex undergoes N-migratory insertion, yielding the stable cobalt(II) imino species [(NHBIMPNMes, Ad,Me)CoII](BPh4). It is a rare example of a trigonal pyramidal complex with four different donor ligands of a tetradentate chelate – an N-heterocyclic carbene, a phenolate, an imine, and an amine – binding to a high-spin cobalt(II) ion. This renders the complex chiral-at-metal. The reaction mechanism was studied by computational analysis and experimentally supported by CW X-band EPR spectroscopy studies. The N-migratory insertion product was isolated and fully characterized. Inspired by the tetracoordinate ligand, the tridentate analogue was synthesized. The flexible nitrogen-anchored bis(carbene) ligand (HBIMENMes) was coordinated to manganese, iron, and cobalt. The resulting chlorido complexes were characterized in detail as well. SQUID magnetization and zero-field 57Fe Mössbauer spectroscopy, where applicable, confirm high-spin ground states for all complexes in the solid state. The reactivity of cobalt(II) chlorido complex [(HBIMENMes)CoII(Cl)](PF6) towards a variety of reagents, including potassium, carbon monoxide, and sodium triethylborohydride was studied. The results are demonstrating the remarkable flexibility of the ligand to adopt trigonal, tetragonal, as well as square planar coordination geometries, the latter enforced by the deprotonation of the anchoring amine to the amide. Synthesis and photolysis experiments of the cobalt(II) azido complex [(HBIMENMes)CoII(N3)](PF6) at low temperatures (10 K) indicate the formation of a transient cobalt(IV) nitrido complex remarkably stable at liquid nitrogen temperature, but undergoes N-migratory insertion at higher temperatures. The reaction of the cobalt chlorido complex with potassium resulted in the cyclometallation of the metal center, generating [(Cyclo-BIMENMes)CoII]. The addition of one atmosphere of carbon monoxide to a solution of the cobalt chlorido complex led to the initial formation of the carbonyl complex [(HBIMENMes)CoII(Cl)(CO)](PF6). The hydrogen chloride adduct of the precursor complex, [(H2BIMENMes)CoII(Cl)2](PF6), was generated over time in a subsequent reaction. The subjection of the cobalt chlorido complex to two equivalents of sodium triethylborohydride yielded [(BIMENMes)CoII(H)]. To the best of our knowledge, this is the first cobalt(II) hydrido complex reported in the literature.Unser Labor entwickelte kürzlich eine stickstoff-geankerte Ligandenserie mit Donorfunktionalitäten von Tris(carben) bis Tris(phenolat), einschließlich zwei neuer gemischter Liganden, (BIMPNMes,Ad,Me)– und (MIMPNMes,Ad,Me)2–, in welchen NHC Carben und Phenolat-Donoren kombiniert werden. Der neue Bis(carben) Mono(phenolat)-Ligand (BIMPNMes,Ad,Me)– wurde an Mangan koordiniert und die resultierenden Komplexe [(BIMPNMes,Ad,Me)MnII(Cl)], [(BIMPNMes,Ad,Me)MnII(N3)] und [(BIMPNMes,Ad,Me)MnII](BPh4) wurden isoliert und detailliert charakterisiert. SQUID-magnetometrische Studien ergaben high-spin Grundzustände für alle Komplexe. Die molekularen Strukturen der vollständigen Komplexreihe mit den entsprechenden Eisen- und Kobaltkomplexen bestätigten die gewünschte Reduktion des sterischen Drucks gegenüber dem Tris(Carben) Liganden TIMENR. Dementsprechend wird die Zugänglichkeit des reaktiven Metallzentrums wie beabsichtigt erhöht, was eine höhere Reaktivität der korrespondierenden Nitridokomplexe verspricht. Durch die Photolyse des Kobalt(II)-Azidokomplexes [(BIMPNMes,Ad,Me)CoII(N3)] bei niedrigen Temperaturen (T = 10 K) wurde der flüchtige low-spin Kobalt(IV)- Nitridokomplex [(BIMPNMes,Ad,Me)CoIV(N)] erzeugt. Dies ist das erste Beispiel für einen Kobalt(IV)-Nitridokomplex, von dem in der Literatur berichtet wird. Bei höheren Temperaturen findet eine Insertionsreaktion des Nitrido-Stickstoffatoms in eine Metall-Carben-Bindung statt, was die stabile Kobalt(II) Iminospezies [(NH-BIMPNMes,Ad,Me)CoII](BPh4) ergibt. Dieser Komplex stellt ein seltenes Beispiel eines trigonal-pyramidalen Komplexes mit vier verschiedenen Donoren eines tetradentaten Chelatliganden dar (ein N-heterocyclisches Carben, ein Phenolat, ein Imin und ein Amin), wodurch der Komplex an seinem Metallzentrum chiral ist. Der Reaktionsmechanismus wurde durch theoretische computerchemische Analysen und experimentelle CW X-band EPRspektroskopische Studien unterstützt. Das Produkt der Insertionsreaktion wurde isoliert und voll charakterisiert. Angeregt durch den vierzähnigen Liganden wurde der analoge dreizähnige Ligand synthetisiert. Der flexible stickstoff-geankerte Bis(Carben)-Ligand (HBIMENMes) wurde an Mangan, Eisen und Kobalt koordiniert. Die erhaltenen Chloridkomplexe wurden ebenfalls detailreich charakterisiert. SQUIDmagnetochemische Studien und Nullfeld 57Fe Mössbauer-Spektroskopie, wenn anwendbar, bestätigten high-spin Grundzustände für alle Komplexe im Festkörper. Die Reaktivität des Kobalt(II)-Chloridokomplexes [(HBIMENMes)CoII(Cl)](PF6) gegenüber einer Vielzahl von Reagenzien, einschließlich Kalium, Kohlenmonoxid und Natriumtriethylborhydrid, wurde untersucht. Die Ergebnisse zeigen die bemerkenswerte Flexibilität des Liganden, trigonale, tetragonale sowie quadratisch planare Koordinationsgeometrien anzunehmen, wobei letztere durch die Deprotonierung des ankernden Amins zum Amid erzwungen wird. Synthese und Photolyseexperimente des Kobalt(II)-Azidokomplexes [(HBIMENMes)CoII(N3)](PF6) bei niedrigen Temperaturen (10 K) deuten auf die BIldung eines flüchtigen Kobalt(IV)-Nitridokomplexes hin, der bei 77 K bemerkenswert stabil ist und dessen Nitridoligand bei höheren Temperaturen insertiert. Die Reaktion des Kobalt(II)-Chloridokomplexes mit Kalium führte zur Cyclometallierung des Metallzentrum, wodurch [(Cyclo-BIMENMes)CoII] erzeugt wurde. Die Zugabe von Kohlenmonoxid zu einer Lösung des Kobalt(II)- Chloridokomplexes führte zur anfänglichen Bildung des Carbonylkomplexes [(HBIMENMes)CoII(Cl)(CO)](PF6), der im Laufe der Zeit in einer Folgereaktion das Chlorwasserstoff-Addukt des Vorläuferkomplexes, [(H2BIMENMes)CoII(Cl)2](PF6), erzeugt. Bei Zugabe von zwei Äquivalenten Natriumtriethylborhydrid zum Kobalt(II)-Chloridokomplex entsteht [(BIMENMes)CoII(H)], welcher nach unserem besten Wissen der erste in der Literatur beschriebene Kobalt(II)-Hydridokomplex ist
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