Formoxyboranes as hydroborane surrogates for the catalytic reduction of carbonyls through transfer hydroboration.

A new class of Lewis base stabilized formoxyboranes demonstrates the feasibility of catalytic transfer hydroboration. In the presence of a ruthenium catalyst, they have shown broad applicability for reducing carbonyl compounds. Various borylated alcohols are obtained in high selectivity and yields up to 99%, tolerating several functional groups. Computational studies enabled to propose a mechanism for this transformation, revealing the role of the ruthenium catalyst and the absence of hydroborane intermediates.

Synthesis of sydnonimines from sydnones and their use for bioorthogonal release of isocyanates in cells.

In this article, we report the synthesis of sydnonimines from sydnones and their use as dipoles for fast click-and-release reactions. The process relies on nucleophilic aromatic substitution of aliphatic and aromatic amines with triflated sydnones. This new methodology allowed the preparation of functionalised sydnonimine probes that are otherwise difficult to prepare. These probes were then used to release a drug and a fluorescent aromatic isocyanate inside living cells.

Fluorophosphoniums as Lewis acids in organometallic catalysis: application to the carbonylation of ß-lactones.

We describe the synthesis and characterisation of four organic Lewis acids based on fluorophosphoniums, with tetracarbonyl cobaltate as the counter-anion: [R3PF]+[Co(CO)4]- (with R = o-Tol, Cy, iPr, and tBu). Their catalytic activity was investigated for the carbonylation of ß-lactones to succinic anhydrides. In the presence of [tBu3PF]+[Co(CO)4]- IV (3 mol%), 90% of succinic anhydride was afforded from ß-propiolactone after 16 h at 80 °C, at a very mild pressure of 2 bar of carbon monoxide. Our study sets the first example of the use of a main-group cation as a Lewis acidic partner in the cobalt-catalyzed carbonylation of ß-lactones.

Exploration of the Copper-Catalyzed Sydnone and Sydnonimine-Alkyne Cycloaddition Reactions by High-Throughput Experimentation.

The reactivity of sydnones and sydnonimines toward terminal alkynes under copper catalysis has been explored using High-Throughput-Experimentation. A large panel of ligands and reaction conditions have been tested to optimize the copper-catalyzed sydnone click reaction discovered by our group ten years ago. This screening approach led to the identification of new ligands, which boosted the catalytic properties of copper and allowed the discovery of a new copper-catalyzed click-and-release reaction involving sydnonimines. This reaction allowed chemoselective ligation of terminal alkynes with sydnonimines and, simultaneously, the release of an isocyanate fragment molecule that can be used for further transformations.

Cooperative Spin-State Switching and Vapochromism of Mononuclear Ni(II) Complexes by Pyridine Coordination/Decoordination.

Two mononuclear Ni(II) complexes (1 and 2) have been found to display color changes upon coordination/decoordination of pyridine, resulting in their structural transformation between square-planar and octahedral geometries as well as a change in their spin state. Compound 1 changes between red (1r) and yellow (1y) upon exposure to or elimination of pyridine, while 2 undergoes a two-step transformation, changing orange 2o (S = 0) ⇄ gray 2g' (S = 1) → yellow 2y' (S = 1) depending on the reaction time. The first step (2o → 2g') takes less than 45 min, which is significantly faster than the previously reported reaction time of 1 day for a Ni(II) complex/pyridine vapor system. Compound 2o reacting with pyridine can be easily prepared by dispersing 2g in methanol instead of annealing at high temperatures (130 °C), which can be applied to develop chemical sensors for pyridine utilizing color changes and/or magnetic switching.

Metal-Free Catalytic Hydrogenolysis of Chlorosilanes into Hydrosilanes with "Inverse" Frustrated Lewis Pairs.

The challenging metal-free catalytic hydrogenolysis of silyl chlorides to hydrosilanes is unlocked by using an inverse frustrated Lewis pair (FLP), combining a mild Lewis acid (Cy2 BCl) and a strong phosphazene base (BTPP) in mild conditions (10 bar of H2 , r. t.). In the presence of a stoichiometric amount of the base, the hydrosilanes R3 SiH (R=Me, Et, Ph) are generated in moderate to high yields (up to 95 %) from their chlorinated counterparts. A selective formation of the valuable difunctional monohydride Me2 SiHCl is also obtained from Me2 SiCl2 . A mechanism is proposed based on stoichiometric experiments and DFT calculations; it highlights the critical role of borohydride species generated by the heterolytic splitting of H2 .

Nanotubule inclusion in the channels formed by a six-fold interpenetrated, triperiodic framework.

When reacted together with uranyl ions under solvo-hydrothermal conditions, a bis(pyridiniumcarboxylate) zwitterion (L) and tricarballylic acid (H3tca) give the complex [NH4]2[UO2(L)2][UO2(tca)]4·2H2O (1). The two ligands are segregated into different units, an anionic nanotubule for tca3- and a six-fold interpenetrated cationic framework with lvt topology for L. The entangled framework defines large channels which contain the square-profile nanotubules. Complex 1 has a photoluminescence quantum yield of 19% and its emission spectrum shows the superposition of the signals due to the two independent species.

Woven, Polycatenated, or Cage Structures: Effect of Modulation of Ligand Curvature in Heteroleptic Uranyl Ion Complexes.

Combining the flexible zwitterionic dicarboxylate 4,4'-bis(2-carboxylatoethyl)-4,4'-bipyridinium (L) and the anionic dicarboxylate ligands isophthalate (ipht2-) and 1,2-, 1,3-, or 1,4-phenylenediacetate (1,2-, 1,3-, and 1,4-pda2-), of varying shape and curvature, has allowed isolation of five uranyl ion complexes by synthesis under solvo-hydrothermal conditions. [(UO2)2(L)(ipht)2] (1) and [(UO2)2(L)(1,2-pda)2]·2H2O (2) have the same stoichiometry, and both crystallize as monoperiodic coordination polymers containing two uranyl-(anionic carboxylate) strands united by L linkers into a wide ribbon, all ligands being in the divergent conformation. Complex 3, [(UO2)2(L)(1,3-pda)2]·0.5CH3CN, with the same stoichiometry but ligands in a convergent conformation, is a discrete, binuclear species which is the first example of a heteroleptic uranyl carboxylate coordination cage. With all ligands in a divergent conformation, [(UO2)2(L)(1,4-pda)(1,4-pdaH)2] (4) crystallizes as a sinuous and thread-like monoperiodic polymer; two families of chains run along different directions and are woven into diperiodic layers. Modification of the synthetic conditions leads to [(UO2)4(LH)2(1,4-pda)5]·H2O·2CH3CN (5), a monoperiodic polymer based on tetranuclear (UO2)4(1,4-pda)4 rings; intrachain hydrogen bonding of the terminal LH+ ligands results in diperiodic network formation through parallel polycatenation involving the tetranuclear rings and the LH+ rods. Complexes 1-3 and 5 are emissive, with complex 2 having the highest photoluminescence quantum yield (19%), and their spectra show the maxima positions usual for tris-κ2O,O'-chelated uranyl cations.

Chiral perturbation on single benzene-based fluorophores: A structure/(chir)optical activity relationship study.

In this paper, we describe the synthesis and the (chir)optical properties of a novel series of circularly polarized luminescent emitters. These molecules involve a compact single benzene-based donor-acceptor fluorophore composed of two cyclic alkylamines as electron donors and a phthalonitrile moiety as electron acceptor linked to a configurationally stable BINOL acting as a chiral perturbation unit. These new compounds display fair quantum yields (up to 66%) with emission maxima around 500 nm in toluene solutions, and the study of their chiroptical properties has shown that the cyclic alkylamine's ring size affects significantly the luminescence dissymmetry factors, reaching 2.2 × 10-3 for the larger cyclic alkylamine moieties.

Flexible Aliphatic Diammonioacetates as Zwitterionic Ligands in UO22+ Complexes: Diverse Topologies and Interpenetrated Structures.

N,N,N',N'-Tetramethylethane-1,2-diammonioacetate (L1) and N,N,N',N'-tetramethylpropane-1,3-diammonioacetate (L2) are two flexible zwitterionic dicarboxylates which have been used as ligands for the uranyl ion, 12 complexes having been obtained from their coupling to diverse anions, mostly anionic polycarboxylates, or oxo, hydroxo and chlorido donors. The protonated zwitterion is a simple counterion in [H2L1][UO2(2,6-pydc)2] (1), where 2,6-pydc2- is 2,6-pyridinedicarboxylate, but it is deprotonated and coordinated in all the other complexes. [(UO2)2(L2)(2,4-pydcH)4] (2), where 2,4-pydc2- is 2,4-pyridinedicarboxylate, is a discrete, binuclear complex due to the terminal nature of the partially deprotonated anionic ligands. [(UO2)2(L1)(ipht)2]·4H2O (3) and [(UO2)2(L1)(pda)2] (4), where ipht2- and pda2- are isophthalate and 1,4-phenylenediacetate, are monoperiodic coordination polymers in which central L1 bridges connect two lateral strands. Oxalate anions (ox2-) generated in situ give [(UO2)2(L1)(ox)2] (5) a diperiodic network with the hcb topology. [(UO2)2(L2)(ipht)2]·H2O (6) differs from 3 in being a diperiodic network with the V2O5 topological type. [(UO2)2(L1)(2,5-pydc)2]·4H2O (7), where 2,5-pydc2- is 2,5-pyridinedicarboxylate, is a hcb network with a square-wave profile, while [(UO2)2(L1)(dnhpa)2] (8), where dnhpa2- is 3,5-dinitro-2-hydroxyphenoxyacetate, formed in situ from 1,2-phenylenedioxydiacetic acid, has the same topology but a strongly corrugated shape leading to interdigitation of layers. (2R,3R,4S,5S)-Tetrahydrofurantetracarboxylic acid (thftcH4) is only partially deprotonated in [(UO2)3(L1)(thftcH)2(H2O)] (9), which crystallizes as a diperiodic polymer with the fes topology. [(UO2)2Cl2(L1)3][(UO2Cl3)2(L1)] (10) is an ionic compound in which discrete, binuclear anions cross the cells of the cationic hcb network. 2,5-Thiophenediacetate (tdc2-) is peculiar in promoting self-sorting of the ligands in the ionic complex [(UO2)5(L1)7(tdc)(H2O)][(UO2)2(tdc)3]4·CH3CN·12H2O (11), which is the first example of heterointerpenetration in uranyl chemistry, involving a triperiodic, cationic framework and diperiodic, anionic hcb networks. Finally, [(UO2)7(O)3(OH)4.3Cl2.7(L2)2]Cl·7H2O (12) crystallizes as a 2-fold interpenetrated, triperiodic framework in which chlorouranate undulating monoperiodic subunits are bridged by the L2 ligands. Complexes 1, 2, 3, and 7 are emissive with photoluminescence quantum yields in the range of 8-24%, and their solid-state emission spectra show the usual dependence on number and nature of donor atoms.

Fast and Bioorthogonal Release of Isocyanates in Living Cells from Iminosydnones and Cycloalkynes.

Bioorthogonal click-and-release reactions are powerful tools for chemical biology, allowing, for example, the selective release of drugs in biological media, including inside animals. Here, we developed two new families of iminosydnone mesoionic reactants that allow a bioorthogonal release of electrophilic species under physiological conditions. Their synthesis and reactivities as dipoles in cycloaddition reactions with strained alkynes have been studied in detail. Whereas the impact of the pH on the reaction kinetics was demonstrated experimentally, theoretical calculations suggest that the newly designed dipoles display reduced resonance stabilization energies compared to previously described iminosydnones, explaining their higher reactivity. These mesoionic compounds react smoothly with cycloalkynes under physiological, copper-free reaction conditions to form a click pyrazole product together with a released alkyl- or aryl-isocyanate. With rate constants up to 1000 M-1 s-1, this click-and-release reaction is among the fastest described to date and represents the first bioorthogonal process allowing the release of isocyanate electrophiles inside living cells, offering interesting perspectives in chemical biology.

Electrocatalytic CO2 Reduction with a Binuclear Bis-Terpyridine Pyrazole-Bridged Cobalt Complex.

A pyrazole-based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO2 ) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV-vis spectro-electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro-electrochemical studies under CO2 and CO atmosphere are consistent with a reduced CO-containing dicobalt complex which results from the electroreduction of CO2 . In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single-site mechanism with up to 94 % selectivity towards CO formation when 1.47 M TFE were present, at -1.35 V vs. Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50 %) are attributed to the generation of CO-containing species formed during the electrocatalytic process, which inhibit the reduction of CO2 .

Zwitterionic and Anionic Polycarboxylates as Coligands in Uranyl Ion Complexes, and Their Influence on Periodicity and Topology.

The three zwitterionic di- and tricarboxylate ligands 1,1'-[(2,3,5,6-tetramethylbenzene-1,4-diyl)bis(methylene)]bis(pyridin-1-ium-4-carboxylate) (pL1), 1,1'-[(2,3,5,6-tetramethylbenzene-1,4-diyl)bis(methylene)]bis(pyridin-1-ium-3-carboxylate) (mL1), and 1,1',1″-[(2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene)]tris(pyridin-1-ium-4-carboxylate) (L2) have been used as ligands to synthesize a series of 15 uranyl ion complexes involving various anionic coligands, in most cases polycarboxylates. [(UO2)2(pL1)2(cbtc)(H2O)2]·10H2O (1, cbtc4- = cis,trans,cis-1,2,3,4-cyclobutanetetracarboxylate) is a discrete, dinuclear ring-shaped complex with a central cbtc4- pillar. While [UO2(pL1)(NO3)2] (2), [UO2(pL1)(OAc)2] (3), and [UO2(pL1)(HCOO)2] (4) are simple chains, [(UO2)2(mL1)(1,3-pda)2] (5, 1,3-pda2- = 1,3-phenylenediacetate) is a daisy chain and [UO2(pL1)(pdda)]3·10H2O (6, pdda2- = 1,2-phenylenedioxydiacetate) is a double-stranded, ribbon-like chain. Both [UO2(pL1)(pht)]·5H2O (7, pht2- = phthalate) and [(UO2)3(mL1)(pht)2(OH)2] (8) crystallize as diperiodic networks with the sql topology, the latter involving hydroxo-bridged trinuclear nodes. [(UO2)2(pL1)(c/t-1,3-chdc)2] (9, c/t-1,3-chdc2- = cis/trans-1,3-cyclohexanedicarboxylate) and [UO2(pL1)(t-1,4-chdc)]·1.5H2O (10, t-1,4-chdc2- = trans-1,4-cyclohexanedicarboxylate) are also diperiodic, with the V2O5 and sql topologies, respectively. Both [(UO2)2(mL1)(c/t-1,4-chdc)2] (11) and [(UO2)2(pL1)(1,2-pda)2] (12, 1,2-pda2- = 1,2-phenylenediacetate) crystallize as diperiodic networks with hcb topology, and they display threefold parallel interpenetration. [HL2][(UO2)3(L2)(adc)3]Br (13, adc2- = 1,3-adamantanedicarboxylate) contains a very corrugated hcb network with two different kinds of cells, and the uncoordinated HL2+ molecule associates with the coordinated L2 to form a capsule containing the bromide anion. [(UO2)2(pL1)(kpim)2] (14, kpim2- = 4-ketopimelate) is a three-periodic framework with pL1 molecules pillaring fes diperiodic subunits, whereas [(UO2)2(L2)2(t-1,4-chdc)](NO3)1.7Br0.3·6H2O (15), the only cationic complex in the series, is a triperiodic framework with dmc topology and t-1,4-chdc2- anions pillaring fes diperiodic subunits. Solid-state emission spectra and photoluminescence quantum yields are reported for all complexes.

Ni(2,2':6',2″-Terpyridine-4'-carboxylate)2 Zwitterions and Carboxylate Polyanions in Mixed-Ligand Uranyl Ion Complexes with a Wide Range of Topologies.

The zwitterionic complex formed by NiII and 2,2':6',2″-terpyridine-4'-carboxylate, Ni(tpyc)2, has been used as a coligand with a diverse group of polycarboxylates in uranyl ion complexes synthesized under solvo-hydrothermal conditions, thus giving a series of 14 mixed ligand, heterometallic compounds. Both [(UO2)2(c-1,2-chdc)Ni(tpyc)2(NO3)2]2·4CH3CN (1) and [(UO2)2(tdc)Ni(tpyc)2(NO3)2]2 (2), where c-1,2-chdc2- is cis-1,2-cyclohexanedicarboxylate and tdc2- is 2,5-thiophenedicarboxylate, display discrete U4Ni2 dinickelatetrauranacycles, a motif which is also found as part of a daisychain coordination polymer in [(UO2)4(bdc)3Ni2(tpyc)4(NO3)2]·2CH3CN·2H2O (3), where bdc2- is 1,4-benzenedicarboxylate. Similar U4Ni2 rings associate to form a nanotubular polymer in [(UO2)2(tca)Ni(tpyc)2(NO3)]·2CH3CN·2H2O (4), where tca3- is tricarballylate. [(UO2)2(1,2-pda) (1,2-pdaH)Ni(tpyc)2(NO3)]·CH3CN (5), where 1,2-pda2- is 1,2-phenylenediacetate, crystallizes as a meander-like chain in which each bent section can be seen as an open, semi-U4Ni2 ring. Oxalate (ox2-) gives [(UO2)2(ox)2Ni(tpyc)2] (6), a monoperiodic polymer containing smaller U4Ni rings, while 1,2,3-benzenetricarboxylate (1,2,3-btc3-) and citrate (citH3-) give [Ni(tpycH)(H2O)3][UO2(1,2,3-btc)]2·2H2O (7) and [UO2Ni2(tpyc)4][UO2(citH)]2 (8), two complexes with charge separation, the latter displaying one-periodic + two-periodic semi-interpenetration. [(UO2)2(btcH)Ni(tpyc)2(NO3)] (9) and [(UO2)2(cbtcH)Ni(tpyc)2(NO3)] (10), where btc4- and cbtc4- are 1,2,3,4-butanetetracarboxylate and cis,trans,cis-1,2,3,4-cyclobutanetetracarboxylate, respectively, are diperiodic networks with hcb topology, and [(UO2)2(ndc)Ni(tpyc)2(OH)(NO3)] (11), where ndc2- is 2,6-naphthalenedicarboxylate, is a sql network containing dinuclear nodes and involving 100-membered U10Ni4 metallacyclic units. U4Ni2 rings are found in the diperiodic polymer formed in [(UO2)4(t-R-1,2-chdc)4Ni2(tpyc)4] (12), where t-R-1,2-chdc2- is trans-R,R-1,2-cyclohexanedicarboxylate, the heavily puckered sheets being interlocked. 1,3-Phenylenediacetate (1,3-pda2-) gives a very thick diperiodic polymer with KIa topology, [(UO2)4(1,3-pda)4Ni2(tpyc)4]·CH3CN·2H2O (13). A triperiodic framework is formed with nitrilotriacetate (nta3-) in [(UO2)2(nta)2Ni2(tpyc)2] (14), where NiII is found in Ni(tpyc)2 units as well as in Ni(nta)24- moieties which both act as 4-coordinated nodes.

Metal-Free Catalytic Hydrogenolysis of Silyl Triflates and Halides into Hydrosilanes.

The metal-free catalytic hydrogenolysis of silyl triflates and halides (I, Br) to hydrosilanes is unlocked by using arylborane Lewis acids as catalysts. In the presence of a nitrogen base, the catalyst acts as a Frustrated Lewis Pair (FLP) able to split H2 and generate a boron hydride intermediate capable of reducing (pseudo)halosilanes. This metal-free organocatalytic system is competitive with metal-based catalysts and enables the formation of a variety of hydrosilanes at room temperature in high yields (>85 %) under a low pressure of H2 (≤10 bar).

Contrasting Networks and Entanglements in Uranyl Ion Complexes with Adipic and trans,trans-Muconic Acids.

Adipic (hexane-1,6-dicarboxylic, adpH2) and trans,trans-muconic (trans,trans-hexa-2,4-diene-1,6-dicarboxylic, mucH2) acids have been reacted with uranyl cations under solvo-hydrothermal conditions, yielding nine homo- or heterometallic complexes displaying in their crystal structure the effects of the different flexibility of the ligands. The complexes [PPh4]2[(UO2)2(adp)3] (1) and [Ni(bipy)3][(UO2)2(muc)3]·5H2O (2), where bipy is 2,2'-bipyridine, crystallize as diperiodic networks with the hcb topology, the layers being strongly puckered or quasiplanar, respectively. Whereas [(UO2)2(adp)3Ni(cyclam)]·2H2O (3), where cyclam is 1,4,8,11-tetraazacyclotetradecane, crystallizes as a diperiodic network, [(UO2)2(muc)3Ni(cyclam)]·2H2O (4) is a triperiodic framework in which the NiII cations are introduced as pillars within a uranyl-muc2- framework with the mog topology. [UO2(adp)(HCOO)2Cu(R,S-Me6cyclam)]·2H2O (5), where R,S-Me6cyclam is 7(R),14(S)-5,5,7,12,12,14-hexamethylcyclam, is a diperiodic assembly with the sql topology, and it crystallizes together with [H2NMe2]2[(UO2)2(adp)3] (6), a highly corrugated hcb network with a square-wave profile, which displays 3-fold parallel interpenetration. In contrast, [(UO2)3(muc)2(O)2Cu(R,S-Me6cyclam)] (7) is a diperiodic assembly containing hexanuclear, µ3-oxido-bridged secondary building units which are the nodes of a network with the hxl topology. The two related complexes [PPh3Me]2[(UO2)2(adp)3]·4H2O (8) and [PPh3Me]2[(UO2)2(muc)3]·H2O (9) crystallize as hcb networks, but their different shapes, undulated or quasiplanar, respectively, result in different entanglements, 2-fold parallel interpenetration in 8 and 2-fold inclined 2D → 3D polycatenation in 9.

A Copper(I)-Catalyzed Sulfonylative Hiyama Cross-Coupling.

An air-tolerant Cu-catalyzed sulfonylative Hiyama cross-coupling reaction enabling the formation of diaryl sulfones is described. Starting from aryl silanes, DABSO and aryliodides, the reaction tolerates a large variety of polar functional groups (amines, ketones, esters, aldehydes). Control experiments coupled with DFT calculations shed light on the mechanism, characterized by the formation of a Cu(I)-sulfinate intermediate via fast insertion of a SO2 molecule.

Uranyl(VI) Triflate as Catalyst for the Meerwein-Ponndorf-Verley Reaction.

Catalytic transformation of oxygenated compounds is challenging in f-element chemistry due to the high oxophilicity of the f-block metals. We report here the first Meerwein-Ponndorf-Verley (MPV) reduction of carbonyl substrates with uranium-based catalysts, in particular from a series of uranyl(VI) compounds where [UO2(OTf)2] (1) displays the greatest efficiency (OTf = trifluoromethanesulfonate). [UO2(OTf)2] reduces a series of aromatic and aliphatic aldehydes and ketones into their corresponding alcohols with moderate to excellent yields, using iPrOH as a solvent and a reductant. The reaction proceeds under mild conditions (80 °C) with an optimized catalytic charge of 2.3 mol % and KOiPr as a cocatalyst. The reduction of aldehydes (1-10 h) is faster than that of ketones (>15 h). NMR investigations clearly evidence the formation of hemiacetal intermediates with aldehydes, while they are not formed with ketones.

Heterohelicenes through 1,3-Dipolar Cycloaddition of Sydnones with Arynes: Synthesis, Origins of Selectivity, and Application to pH-Triggered Chiroptical Switch with CPL Sign Reversal.

Regioselective access to heterohelicenes through the 1,3-dipolar cycloaddition of sydnones with arynes is described. Novel access to sydnones and poly(hetero)aromatic aryne precursors allowed the introduction of chemical diversity over multiple positions of the helical scaffolds. The origins of the unconventional regioselectivity during the cycloaddition steps was systematically investigated using density functional theory (DFT) calculations, unveiling the key features that control this reactivity, namely, face-to-face (π···π) or edge-to-face (C-H···π) interactions, primary orbital interactions and distortion from coplanarity in the transition structures (TSs) of the transformation. From the library of 24 derivatives synthesized, a pyridyl containing derivative displayed reversible, red-shifted, pH-triggered chiroptical switching properties, with CPL-sign reversal. It is found that protonation of the helicene causes a change of the angle between the electric and magnetic dipole moments related to the S1 → S0 transition, resulting in this rare case of reversible CPL sign inversion upon application of an external stimulus.