Halide perovskites have attracted great attention in the fields of photovoltaics, LEDs, lasers, and most recently photocatalysis, owing to their unique optoelectronic properties. The all‐inorganic ...halide perovskite CsPbBr3/TiO2 composite material catalyzes selective benzyl alcohol oxidation to benzaldehyde under visible‐light illumination. The catalyst, which is prepared by a facile wet‐impregnation method, shows very good selectivity towards benzaldehyde (>99 % at 50 % conversion). Action spectra and electron spin resonance (ESR) studies reveal that photoexcited electrons formed within CsPbBr3 upon visible‐light illumination take part in the reaction via reduction of oxygen to form superoxide radicals. The detailed post‐catalysis characterization by UV/Vis and X‐ray photoelectron spectroscopy, X‐ray diffraction, and high‐angle annular dark‐field scanning transmission electron microscopy studies further demonstrated the good stability of CsPbBr3 in terms of morphology and crystal structure under the reaction conditions. This study sheds light on promising new photocatalytic applications of halide perovskites.
Remain in light: Halide perovskites are promising for diverse optoelectronic applications. Recently, these materials have also entered the field of photocatalysis. A CsPbBr3/TiO2 composite catalyzes the selective oxidation of benzyl alcohol to benzaldehyde under visible‐light illumination. Spectroscopic studies show that photoexcited electrons from the CsPbBr3 conduction band participate in the reaction through formation of superoxide radicals.
Reaction of the CoI complex (TIMMNmes)CoI(PF6) (1) (TIMMNmes=tris‐2‐(3‐mesityl‐imidazolin‐2‐ylidene)‐methylamine) with mesityl azide yields the CoIII imide (TIMMNmes)CoIII(NMes)(PF6) (2). Oxidation ...of 2 with FeCp2(PF6) provides access to a rare CoIII imidyl (TIMMNmes)Co(NMes)(PF6)2 (3). Single‐crystal X‐ray diffractometry and EPR spectroscopy confirm the molecular structure of 3 and its S=1/2
ground state. ENDOR, X‐ray absorption spectroscopy and computational analyses indicate a ligand‐based oxidation; thus, an imidyl‐radical electronic structure for 3. Migratory insertion of one ancillary NHC to the imido ligand in 2 gives the CoI N‐heterocyclic imine (4) within 12 h. Conversely, it takes merely 0.5 h for 3 to transform to the CoII congener (5). The migratory insertion in 2 occurs via a nucleophilic attack of the imido ligand at the NHC to give 4, whereas in 3, a nucleophilic attack of the NHC at the electrophilic imidyl ligand yields 5. The reactivity shunt upon oxidation of 2 to 3 confirms an umpolung of the imido ligand.
Straightforward access to a CoIII terminal imidyl radical complex was provided by one‐electron oxidation of a CoIII terminal imido precursor. Oxidation of the CoIII terminal imido to its imidyl redox isomer facilitates intramolecular migratory insertion reactions of the imido with an NHC ligand by a switch of mechanism through umpolung of the imido ligand.
The divinyldiarsene radical cations {(NHC)C(Ph)}As2(GaCl4) (NHC=IPr: C{(NDipp)CH}2 3; SIPr: C{(NDipp)CH2}2 4; Dipp=2,6‐iPr2C6H3) and dications {(NHC)C(Ph)}As2(GaCl4)2 (NHC=IPr 5; SIPr 6) are readily ...accessible as crystalline solids on sequential one‐electron oxidation of the corresponding divinyldiarsenes {(NHC)C(Ph)}As2 (NHC=IPr 1; SIPr 2) with GaCl3. Compounds 3–6 have been characterized by X‐ray diffraction, cyclic voltammetry, EPR/NMR spectroscopy, and UV/vis absorption spectroscopy as well as DFT calculations. The sequential removal of one electron from the HOMO, that is mainly the As−As π‐bond, of 1 and 2 leads to successive elongation of the As=As bond and contraction of the C−As bonds from 1/2→3/4→5/6. The UV/vis spectrum of 3 and 4 each exhibits a strong absorption in the visible region associated with SOMO‐related transitions. The EPR spectrum of 3 and 4 each shows a broadened septet owing to coupling of the unpaired electron with two 75As (I=3/2) nuclei.
One‐by‐one electron oxidation of diarsenes As2 featuring very efficient π‐donor N‐heterocyclic vinyl substituents with GaCl3 leads to the formation of radical cations As2+. and dications As2+ as crystalline solids. Experimental and computational studies revealed the delocalization of unpaired electron over the π‐conjugated CAs2C framework.
Stable N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, (IPr)(C6H4)(IPr) and (IPr)(C6H4)2(IPr) (4 and 5, respectively; IPr=C{N(2,6‐iPr2C6H3)}2CHCH), are reported. In a ...nickel‐catalyzed double carbenylation of 1,4‐Br2C6H4 and 4,4′‐Br2(C6H4)2 with IPr (1), (IPr)(C6H4)(IPr)(Br)2 (2) and (IPr)(C6H4)2(IPr)(Br)2 (3) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC8. Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet−triplet energy gap ΔES−T of 10.7 kcal mol−1, whereas 4 features more quinoidal character with a rather large ΔES−T of 25.6 kcal mol−1. In view of the low ΔES−T, 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character.
Couple up: N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, (iii) and (iv), are accessed as stable crystalline solids by two‐electron reduction of (i) and (ii) (n=1, 2) with KC8. Calculations indicate that (iv) has considerable diradical character (41 %), while a close‐shell singlet state is the ground state for (iii) and (iv). Dipp=2,6‐diisopropylphenyl.
Two‐fold C−C cross‐coupling of N‐heterocyclic carbenes NHCs; SIPr=C(NArCH2)2, 1; IPr=C(NArCH)2, 2; Me‐IPr=C(NArCMe)2, 3; Ar=2,6‐iPr2C6H3 with 4,4′′‐diiodo‐p‐terphenyl under Ni catalysis furnished ...(SIPr)(C6H4)3(SIPr)(I)2 (4), (IPr)(C6H4)3(IPr)(I)2 (5), and (Me‐IPr)(C6H4)3(Me‐IPr)(I)2 (6). Two‐electron reduction of 4–6 with KC8 readily afforded NHC analogues of Müller's hydrocarbon (MH), (SIPr)(C6H4)3(SIPr) (7), (IPr)(C6H4)3(IPr) (8), and (Me‐IPr)(C6H4)3(Me‐IPr) (9), respectively, as highly colored crystalline solids. Quantum chemical calculations suggested that the singlet ground state for 7–9 possesses a vertical singlet–triplet energy gap ΔES‐T of −7.24 to −7.60 kcal mol−1, which is significantly lower compared to that of the NHC analogues of Thiele's (TH) and Chichibabin's (CH) (18–38 kcal mol−1) hydrocarbons. Importantly, the calculated diradical character (y) of 7–9 (y≈0.6) is considerably higher compared to that of the related TH and CH (y=0.1–0.4), suggesting the open‐shell singlet character of 7–9.
Radical taming by NHCs. Crystalline N‐heterocyclic carbene (NHC) analogues of the highly reactive Müller's hydrocarbon (A), (NHC)(C6H4)3(NHC) (B) are reported NHC=C(NArCH2)2, SIPr; C(NArCH)2, IPr; C(NArCMe)2, Me‐IPr; Ar=2,6‐iPr2C6H3. They feature a singlet ground state with small singlet–triplet energy gaps (ΔES‐T≈7.4 kcal mol−1) along with an intermediate diradical character (y=65 %).
One‐electron reduction of C2‐arylated 1,3‐imidazoli(ni)um salts (IPrAr)Br (Ar=Ph, 3 a; 4‐DMP, 3 b; 4‐DMP=4‐Me2NC6H4) and (SIPrAr)I (Ar=Ph, 4 a; 4‐Tol, 4 b) derived from classical NHCs ...(IPr=:C{N(2,6‐iPr2C6H3)}2CHCH, 1; SIPr=:C{N(2,6‐iPr2C6H3)}2CH2CH2, 2) gave radicals (IPrAr). (Ar=Ph, 5 a; 4‐DMP, 5 b) and (SIPrAr). (Ar=Ph, 6 a; 4‐Tol, 6 b). Each of 5 a,b and 6 a,b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid‐state characterization of NHC‐derived carbon‐centered radicals 6 a,b by single‐crystal X‐ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group.
Crystalline radicals (IPrAr). and (SIPrAr). derived from classical N‐heterocyclic carbenes (NHCs; IPr=:C{N(2,6‐iPr2C6H3)}2CHCH, SIPr=:C{N(2,6‐iPr2C6H3)}2CH2CH2) were synthesized by one‐electron reduction of the corresponding C2‐arylated 1,3‐imidazoli(ni)um cations (see scheme). Cyclic voltammetry, EPR and X‐ray diffraction studies, and DFT calculations emphasized the key role of the C2 substituent in the stability of the NHC‐derived radicals.
Herein, the first stable anions KSIPrBp (4 a‐K) and KIPrBp (4 b‐K) (SIPrBp=BpC{N(Dipp)CH2}2, IPrBp=BpC{N(Dipp)CH}2; Bp=4‐PhC6H4; Dipp=2,6‐iPr2C6H3) derived from classical N‐heterocyclic carbenes ...(NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a‐K and 4 b‐K are prepared by KC8 reduction of the neutral radicals SIPrBp (3 a) and IPrBp (3 b), respectively. The radicals 3 a and 3 b as well as Me‐IPrBp 3 c (Me‐IPrBp=BpC{N(Dipp)CMe}2) are accessible as crystalline solids on treatment of the respective 1,3‐imidazoli(ni)um bromides (SIPrBp)Br (2 a), (IPrBp)Br (2 b), and (Me−IPrBp)Br (2 c) with KC8. The cyclic voltammograms of 2 a–2 c exhibit two one‐electron reversible redox processes in −0.5 to −2.5 V region that correspond to the radicals 3 a–3 c and the anions (4 a–4 c)−. Computational calculations suggest a closed‐shell singlet ground state for (4 a–4 c)− with the singlet‐triplet energy gap of 17–24 kcal mol−1.
One‐electron reduction of the C2‐biphenylated 1,3‐imidazolinium cation (2 a+) affords the radical 3 a, which undergoes further 1e‐reduction to yield the anion 4 a−. In the solid‐state, 3 a is monomeric while 4 a‐K has a hexameric tubular structure. In addition to 3 a and 4 a‐K, synthesis, structures, and reactivity of stable radicals and anions based on unsaturated N‐heterocyclic carbene frameworks have been presented.
The light-induced Ni-L state of NiFe hydrogenases is well suited to investigate the identity of the amino acid base that functions as a proton acceptor in the hydrogen turnover cycle in this ...important class of enzymes. Density functional theory calculations have been performed on large models that include the complete NiFe center and parts of the second coordination sphere. Combined with experimental data, in particular from electron paramagnetic resonance and Fourier transform infrared (FTIR) spectroscopy, the calculations indicate that the hydride ion, which is located in the bridging position between nickel and iron in the Ni-C state, dissociates upon illumination as a proton and binds to a nearby thiolate base. Moreover, the formation of a functionally relevant nickel-iron bond upon dissociation of the hydride is unequivocally observed and is in full agreement with the observed g values, ligand hyperfine coupling constants, and FTIR stretching frequencies. This metal-metal bond can be protonated and thus functions like a base. The nickel-iron bond is important for all intermediates with an empty bridge in the catalytic cycle, and the electron pair that constitutes this bond thus plays a crucial role in the hydrogen evolution catalyzed by the enzyme.
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