Biradicals are molecules consisting of two unpaired electrons in two nearly degenerate non-bonding molecular orbitals (NBMOs). They play a central role in bond breaking and formation processes and ...are usually very short-lived species under standard laboratory conditions. By replacing the carbon-based skeletons of archetypal organic biradicals by main group elements, intriguing mimics of otherwise only transient species are isolable, which can then be characterized by generally applied methods for stable molecules. Nonetheless, the price to pay for gaining stability is the reduction of the biradical character, which makes their designation as biradicaloids more appropriate. Recent advances in the syntheses and characterization of main group element biradicaloids as well as their chemical reactivity and theoretical investigations served as topic of this review article.
The field of multimetallic catalysis is rapidly developing and some multimetallic complexes catalyze organic transformations to yield the desired products in more efficient ways owing to the combined ...action of different metals in a cooperative fashion. This Concept article describes the recent advances of cooperative catalysis playing in multimetallic systems such as homo‐multimetallic complexes with short metal‐metal distances, homo‐multimetallic complexes with long metal‐metal distances, hetero‐multimetallic complexes and metallocene‐based multimetallic complexes with special attention towards redox‐switchable catalysis. Examples are illustrated in which the use of multimetallic complexes show clear enhancement of catalytic outcomes when compared with the sum of their corresponding mononuclear counterparts. Furthermore, several examples are discussed showing the effects of electronic communication in cooperative systems.
Teamwork: In this Concept article the authors discuss selected examples of cooperativity in multimetallic systems. Homobimetallic systems with short and long metal‐metal distances are considered, and the interplay of the different metals in several heterometallic complexes is reviewed. Cooperativity in metallocene‐based systems and their utilization in redox‐switchable catalysis is also discussed.
Homoleptic carbonyl radical cations are a textbook family of complexes hitherto unknown in the condensed phase, leaving their properties and applications fundamentally unexplored. Here we report on ...two stable 17-electron Cr(CO)
salts that were synthesized by oxidation of Cr(CO)
with NO
Al(OR
)
(R
= C(CF
)
)) in CH
Cl
and with removal of NO gas. Longer reaction times led to NO/CO ligand exchange and formation of the thermodynamically more stable 18-electron species Cr(CO)
(NO)
, which belongs to the family of heteroleptic chromium carbonyl/nitrosyl cations. All salts were fully characterized (IR, Raman, EPR, NMR, scXRD, pXRD, magnetics) and are stable at room temperature under inert conditions over months. The facile synthesis of these species enables the thorough investigation of their properties and applications to a broad scientific community.
We herein report on new synthetic strategies for the preparation of pyridine and imidazole substituted 2,2’‐dihalo biphenyls. These structures are pre‐ligands suitable for the preparation of ...respective stannoles. The latter can successfully be transmetalated to KAuCl4 forming non‐palindromic (C^C^D)AuIII pincer complexes featuring a lateral pyridine (D=N) or N‐heterocyclic carbene (NHC, D=C’) donor. The latter is the first report on a pincer complex with two formally anionic sp2 and one carbenic carbon donor. The (C^C^D)AuIII complexes show intense phosphorescence in solution at room temperature. We discuss the developed multistep strategy and touch upon synthetic challenges. The prepared complexes have been fully characterized including X‐ray diffraction analysis. The gold(III) complexes’ photophysical properties have been investigated by absorption and emission spectroscopy as well as quantum chemical calculations on the quasi‐relativistic two‐component TD‐DFT and GW/Bethe–Salpeter level including spin–orbit coupling. Thus, we shed light on the electronic influence of the non‐palindromic pincer ligand and reveal non‐radiative relaxation pathways of the different ligands employed.
Capture the gold(fish)! Donor‐substituted dihalobiphenyls are suitable pre‐ligands for the preparation of luminescent, non‐palindromic (C^C^D)AuIII complexes with a lateral pyridine or carbene donor. Like an anglerfish's rod enmeshes its prey, the lateral donor tunes the gold‘s electronic properties which is hold by the rigid biphenyl mouth. Synthesis, spectroscopy and quantum chemistry draw a comprehensive picture about the new complexes prepared.
The synthesis, structure, and full characterization of a redox‐switchable germylene based on a 3ferrocenophane ligand arrangement, Fc(NMes)2Ge (4), is presented. The mesityl (Mes)‐substituted title ...compound is readily available from Fc(NHMes)2 (2) and Ge{N(SiMe3)2}2, or from the dilithiated, highly air‐ and moisture‐sensitive compound Fc(NLiMes)2⋅3 Et2O (3) and GeCl2. Cyclic voltammetry studies are provided for 4, confirming the above‐mentioned view of a redox‐switchable germylene metalloligand. Although several 1:1 RhI and IrI complexes of 4 (5–7) are cleanly formed in solution, all attempts to isolate them in pure form failed due to stability problems. However, crystalline solids of Mo(κ1Ge‐4)2(CO)4 (8) and W(κ1Ge‐4)2(CO)4 (9) were isolated and fully characterized by common spectroscopic techniques (8 by X‐ray diffraction). DFT calculations were performed on a series of model compounds to elucidate a conceivable interplay between the metal atoms in neutral and cationic bimetallic complexes of the type Rh(κ1E‐qE)(CO)2Cl0/+ (qE=Fc(NPh)2E with E=C, Si, Ge). The bonding characteristics of the coordinated Fc‐based metalloligands (qE/qE+) are strongly affected upon in silico oxidation of the calculated complexes. The calculated Tolman electronic parameter (TEP) significantly increases by approximately 20 cm−1 (E=C) to 25 cm−1 (E=Si, Ge) upon oxidation. The change in the ligand‐donating abilities upon oxidation can mainly be attributed to Coulombic effects, whereas an orbital‐based interaction appears to have only a minor influence.
The redox‐switchable germylene Fc(NMes)2Ge, based on a 3ferrocenophane ligand arrangement, alongside some transition‐metal complexes are reported. The changes in the ligating properties upon oxidation were elucidated by DFT calculations on neutral and cationic bimetallic model complexes.
A straightforward access is provided to iron(II) complexes showing exceedingly slow spin-state interconversion by utilizing trigonal-prismatic directing ligands (L n ) of the extended-tripod type. A ...detailed analysis of the interrelations between complex structure (X-ray diffraction, density functional theory) and electronic character (SQUID magnetometry, Mössbauer spectroscopy, UV/vis spectroscopy) of the iron(II) center in mononuclear complexes FeL n reveals spin crossover to occur along a coupled breathing/torsion reaction coordinate, shuttling the complex between the octahedral low-spin state and the trigonal-prismatic high-spin state along Bailar’s trigonal twist pathway. We associate both the long spin-state lifetimes in the millisecond domain close to room temperature and the substantial barriers against thermal scrambling (E a ≈ 33 kJ mol–1, from Arrhenius analysis) with stereochemical constraints. In particular, the topology of the κ6N ligands controls the temporary and structural dynamics during spin crossover.
A stable cyclic (alkyl)(amino)carbene (CAAC) 1 inserts into the para‐CF bond of pentafluoropyridine, and after fluoride ion, the iminium‐pyridyl adduct 3+ was isolated. A cyclic voltammetry study ...shows a reversible three‐state redox system involving 3+, 3⋅, and 3−. The CAAC‐pyridyl radical 3⋅, obtained by reduction of 3+ with magnesium, has been spectroscopically and crystallographically characterized. In contrast to the lack of π communication between the CAAC and the pyridine units in cation 3+, the unpaired electron of 3⋅ is delocalized over an extended π system involving both heterocycles.
Presumed non‐innocent! The chemical reduction of a pyridyl‐substituted cyclic (alkyl)(amino)carbene (CAAC) iminium gave a highly stable organic radical. The neutral paramagnetic species was fully characterized; the unpaired electron is delocalized on both the CAAC and the pyridine heterocycles (see scheme).
A new C‐nucleoside structurally based on the hydroxyquinoline ligand was synthesized that is able to form stable pairs in DNA in both the absence and the presence of metal ions. The interactions ...between the metal centers in adjacent CuII‐mediated base pairs in DNA were probed by electron paramagnetic resonance (EPR) spectroscopy. The metal–metal distance falls into the range of previously reported values. Fluorescence studies with a donor–DNA–acceptor system indicate that photoinduced charge‐transfer processes across these metal‐ion‐mediated base pairs in DNA occur more efficiently than over natural base pairs.
Ace of base: DNA duplex stability can be significantly increased by coordinating CuII ions to hydroxyquinoline base pairs (metal‐mediated base pairing; see scheme). Fluorescence studies with a donor–DNA–acceptor system indicate that photoinduced charge‐transfer processes across these metal‐ion‐mediated base pairs in DNA occur more efficiently than over natural base pairs.
We report on the successful synthesis of Si5Mes6 (Mes = 2,4,6-trimethylphenyl), which consists of an archetypal 1.1.1 cluster core featuring two ligand-free, “inverted tetrahedral” bridgehead silicon ...atoms. The separation between the bridgehead Si atoms is much longer, and the bond strength much weaker, than usually observed for a regular Si−Si single bond. A detailed analysis of the electronic characteristics of Si5Mes6 reveals a low-lying excited triplet state, indicative of some biradical(oid) character. Reactivity studies provide evidence for both closed-shell and radical-type reactivity, confirming the unusual nature of the stretched silicon−silicon bond in this “nonclassical” molecule.
The magnetic properties of three spin‐crossover complexes, FeII(R‐bik)3(BF4)2·nH2O (1–3), based on bis(imidazolyl)ketone ligands, were investigated in solution and the solid state. Their properties ...were compared with those of the ketone‐free analogue, FeII(bim)3(OTf)2 (4). The alkyl and vinyl R groups have weak influence on the transition temperature, T1/2, in solution, while stronger differences are observed in the solid state, because different intermolecular interactions occur in 1–3. The spin‐state equilibria in solution were followed by SQUID magnetometry and the Evans NMR spectroscopy method. Interestingly, the equilibria can also be simply and efficiently probed by following the temperature dependence of an adequately chosen 1H chemical shift. Overall, these experiments give coherent results, with T1/2 located between 320 and 335 K, a narrow range, in comparison with the solid state. DFT calculations have allowed the rationalization of the magnetic differences. The molecular‐orbital and spin‐density calculations reveal that the presence of the C=O group between the imidazolyl units in the ligands of 1–3 leads to an extended aromatic system, an effective π‐acceptor effect, stabilizing the LS state and reducing the LS–HS gap, in comparison with 4.
A series of FeII spin‐crossover (SCO) complexes containing β‐diimine ligands were synthesized and characterized. The foregoing results particularly demonstrate the efficiency of variable‐temperature 1H NMR spectroscopic chemical shifts as a useful alternative for studying spin‐crossover equilibria in solution.
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