The sigma amine-borane complexes Rh(L1)(η
:η
-H
B⋅NRH
)OTf (L1=2,6-bis-1-(2,6-diisopropylphenylimino)ethylpyridine, R=Me, Et,
Pr) are described, alongside Rh(L1)(NMeH
)OTf. Using R=Me as a ...pre-catalyst (1 mol %) the dehydropolymerization of H
B ⋅ NMeH
gives H
BNMeH
selectively. Added NMeH
, or the direct use of Rh(L1)(NMeH
)OTf, is required for initiation of catalysis, which is suggested to operate through the formation of a neutral hydride complex, Rh(L1)H. The formation of small (1-5 nm) nanoparticles is observed at the end of catalysis, but studies are ambiguous as to whether the catalysis is solely nanoparticle promoted or if there is a molecular homogeneous component. Rh(L1)(NMeH
)OTf is shown to operate at 0.025 mol % loadings on a 2 g scale of H
B ⋅ NMeH
to give polyaminoborane H
BNMeH
M
=30,900 g/mol, Ð=1.8 that can be purified to a low residual Rh (6 μg/g). Addition of NaN(SiMe
)
to H
BNMeH
results in selective depolymerization to form the eee-isomer of N,N,N-trimethylcyclotriborazane H
BNMeH
: the chemical repurposing of a main-group polymer.
A cobalt σ‐alkane complex, Co(Cy2P(CH2)4PCy2)(norbornane)BArF4, was synthesized by a single‐crystal to single‐crystal solid/gas hydrogenation from a norbornadiene precursor, and its structure was ...determined by X‐ray crystallography. Magnetic data show this complex to be a triplet. Periodic DFT and electronic structure analyses revealed weak C−H→Co σ‐interactions, augmented by dispersive stabilization between the alkane ligand and the anion microenvironment. The calculations are most consistent with a η1:η1‐alkane binding mode.
A cobalt σ‐alkane complex was synthesized by a single‐crystal to single‐crystal solid/gas hydrogenation from a norbornadiene precursor, and its structure was determined by X‐ray crystallography. Periodic DFT and electronic structure analyses revealed weak C−H→Co σ‐interactions, augmented by dispersive stabilization between the alkane ligand and the anion microenvironment.
Reversible encapsulation of CH2Cl2 or Xe in a non‐porous solid‐state molecular organometallic framework of Rh(Cy2PCH2PCy2)(NBD)BArF4 occurs in single‐crystal to single‐crystal transformations. These ...processes are probed by solid‐state NMR spectroscopy, including 129Xe SSNMR. Non‐covalent interactions with the ‐CF3 groups, and hydrophobic channels formed, of BArF4− anions are shown to be important, and thus have similarity to the transport of substrates and products to and from the active site in metalloenzymes.
Access control: Reversible encapsulation of CH2Cl2 or Xe in a non‐porous solid‐state molecular organometallic (SMOM) framework of Rh(Cy2PCH2PCy2)(NBD)BArF4 occurs in single‐crystal to single‐crystal transformations. The process is similar to the transport of substrates and products to and from the active site in metalloenzymes.
A modified, convenient, preparation of solvent-free, anhydrous, Li+, Na+ and K+ salts of the ubiquitous BArF4- anion is reported, that involves a simple additional recrystallisation step. Anhydrous ...NaBArF4, KBArF4, and Li(H2O)BArF4, were characterised by single-crystal X-ray diffraction.
This study reports the first structural elucidation of β-diketiminate anions (BDI
−
), known for strong coordination, in their unbound form within caesium complexes. β-Diketiminate caesium salts ...(BDICs) were synthesised, and upon the addition of Lewis donor ligands, free BDI
−
anions and donor-solvated Cs
+
cations were observed. Notably, the liberated BDI
−
anions exhibited an unprecedented dynamic
cisoid
-
transoid
exchange in solution.
Setting β-diketiminate ligands free: Unveiling the unbound dynamic nature of free-form β-diketiminate anions.
A series of dithienylethene (DTE) photoswitches with aza‐heteroaromatic cationic moieties is synthesized. The switches are characterized regarding their photochemical and photophysical properties in ...acetonitrile and in water. The efficiency of the switching and the photostationary state composition depend on the degree of π‐conjugation of the heteroaromatic systems. Thus, DTEs with acridinium‐derived moieties have very low quantum yields for the ring‐closing process, which is in contrast to switches with pyridinium and quinolinium moieties. All switches emit fluorescence in their open forms. The involved electronic transitions are traced back to an integrative picture including the DTE core and the cationic arms. The emission can be fine‐tuned by the π‐conjugation of the heteroaromatic cations, reaching the red spectral region for DTEs with acridinium moieties. On ring‐closing of the DTEs the fluorescence is not observable anymore. Theoretical calculations point to rather low‐lying energy levels of the highly conjugated ring‐closed DTEs, which would originate near‐infrared emission (> 1200 nm). The latter is predicted to be very weak due to the concurrent non‐radiative deactivation, according to the energy‐gap law. In essence, an ON–OFF fluorescence switching as the result of the electrocyclic ring‐closing reaction is observed.
Dithienylethenes are structurally integrated with heteroaromatic cations. The photoswitches in their ring‐open forms show fluorescence in the visible spectral range, while the ring‐closed isomers are non‐fluorescent. This leads to ON–OFF fluorescence switching with the emission output being fine‐tuned by the conjugation degree of the heteroaromatic units.
Reacting cesium fluoride with an equimolar n-hexane solution of lithium bis(trimethylsilyl)amide (LiHMDS) allows the isolation of CsHMDS (1) in 80% yield (after sublimation). This preparative route ...to 1 negates the need for pyrophoric Cs metal or organocesium reagents in its synthesis. If a 2:1 LiHMDS:CsF ratio is employed, the heterobimetallic polymer LiCs(HMDS)2∞ 2 was isolated (57% yield). By combining equimolar quantities of NaHMDS and CsHMDS in hexane/toluene toluene·NaCs(HMDS)∞ 3 was isolated (62% yield). Attempts to prepare the corresponding potassium-cesium amide failed and instead yielded the known monometallic polymer toluene·Cs(HMDS)∞ 4. With the aim of expanding the structural diversity of Cs(HMDS) species, 1 was reacted with several different Lewis basic donor molecules of varying denticity, namely, (R,R)-N,N,N′,N′-tetramethylcyclohexane-1,2-diamine (R,R)-TMCDA and N,N,N′,N′-tetramethylethylenediamine (TMEDA), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), tris2-(dimethylamino)ethylamine (Me6-TREN) and tris2-(2-methoxyethoxy)ethylamine (TMEEA). These reactions yielded dimeric donor·NaCs(HMDS)22 5–7 where donor is (R,R)-TMCDA, TMEDA and PMDETA respectively, the tetranuclear “open”-dimer {Me6-TREN·Cs(HMDS)}2{Cs(HMDS)}2 8 and the monomeric TMEEA·Cs(HMDS) 9. Complexes 2, 3, and 5–9 were characterized by X-ray crystallography and in solution by multinuclear NMR spectroscopy.
Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula ...Rh(Cy2PCH2CH2PCy2)(η n :η m -alkane)BArF 4 have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H–C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H–C interactions with the geminal C–H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of BArF 4− anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.
Carotenoids are lipophilic isoprenoid compounds synthesized by all photosynthetic organisms and some non-photosynthetic prokaryotes and fungi. With some notable exceptions, animals (including humans) ...do not produce carotenoids de novo but take them in their diets. In photosynthetic systems carotenoids are essential for photoprotection against excess light and contribute to light harvesting, but perhaps they are best known for their properties as natural pigments in the yellow to red range. Carotenoids can be associated to fatty acids, sugars, proteins, or other compounds that can change their physical and chemical properties and influence their biological roles. Furthermore, oxidative cleavage of carotenoids produces smaller molecules such as apocarotenoids, some of which are important pigments and volatile (aroma) compounds. Enzymatic breakage of carotenoids can also produce biologically active molecules in both plants (hormones, retrograde signals) and animals (retinoids). Both carotenoids and their enzymatic cleavage products are associated with other processes positively impacting human health. Carotenoids are widely used in the industry as food ingredients, feed additives, and supplements. This review, contributed by scientists of complementary disciplines related to carotenoid research, covers recent advances and provides a perspective on future directions on the subjects of carotenoid metabolism, biotechnology, and nutritional and health benefits.
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