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Light and Temperature Control of the Spin State of
A Magnetically Bistable Carbene
P. Costa, T. Lohmiller, I. Trosien, A. Savitsky, W. Lubitz, M. Fernandez-Oliva, E. Sanchez-Garcia, and W. Sander, J. Am. Chem. Soc. 138 (2016), 1622-1629.
Bis(p-methoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest energy singlet and triplet states. At 3 K, both states coexist indefinitely under these conditions. The carbene is investigated in argon matrices by IR, UV–vis, and X-band EPR spectroscopy and in MTHF glasses by W-band EPR and Q-band ENDOR spectroscopy. UV (365 nm) irradiation of the system results in formation of predominantly the triplet carbene, whereas visible (450 nm) light shifts the photostationary equilibrium toward the singlet state. Upon annealing at higher temperatures (>10 K), the triplet is converted to the singlet; however, cooling back to 3 K does not restore the triplet. Therefore, depending on matrix temperature and irradiation conditions, matrices containing predominantly the triplet or singlet carbene can be generated. Controlling the magnetic and chemical properties of carbenes by using light of different wavelengths might be of general interest for applications such as information storage and radical-initiated polymerization processes.
Switching the Spin State of Diphenylcarbene via Halogen Bonding
S. Henkel, P. Costa, L. Klute, P. Sokkar, M. Fernandez-Oliva, W. Thiel, E. Sanchez-Garcia, and W. Sander, J. Am. Chem. Soc. 138 (2016), 1689-1697.
The interactions between diphenylcarbene DPC and the halogen bond donors CF3I and CF3Br were investigated using matrix isolation spectroscopy (IR, UV–vis, and EPR) in combination with QM and QM/MM calculations. Both halogen bond donors CF3X form very strong complexes with the singlet state of DPC, but only weakly interact with triplet DPC. This results in a switching of the spin state of DPC, the singlet complexes becoming more stable than the triplet complexes. CF3I forms a second complex (type II) with DPC that is thermodynamically slightly more stable. Calculations predict that in this second complex the DPC···I distance is shorter than the F3C···I distance, whereas in the first (type I) complex the DPC···I distance is, as expected, longer. CF3Br only forms the type I complex. Upon irradiation I or Br, respectively, are transferred to the DPC carbene center and radical pairs are formed. Finally, on annealing, the formal C–X insertion product of DPC is observed. Thus, halogen bonding is a powerful new principle to control the spin state of reactive carbenes.
Triplet carbenes react with molecular oxygen with rates that approach diffusion control to carbonyl O-oxides, whereas triplet nitrenes react much slower. For investigating the reaction of phenylnitrene with O2, the nitrene was generated by flash vacuum thermolysis (FVT) of phenylazide and subsequently isolated in O2-doped matrices. FVT of the azide produces the nitrene in high yield and with only minor contaminations of the rearranged products that are frequently observed if the nitrene is produced by photolysis. The phenylnitrene was isolated in solid Ar, Xe, mixtures of these rare gases with O2, and even in pure solid O2. At temperatures between 30 and 35 K an extremely slow thermal reaction between the nitrene and O2 was observed, whereas at higher temperatures, solid Ar and O2 rapidly evaporate. Only O2-doped Xe matrices allowed us to anneal at temperatures above 40 K, and at these temperatures, the nitrene reacts with O2 to produce nitroso O-oxide mainly in its syn conformation. Upon visible light irradiation (450 nm), the nitroso oxide rapidly rearranges to nitrobenzene.
Soumya has won two Poster Awards for her poster "Matrix Isolation and Solvation studies of Diphenylmethyl Radical". Her poster was awarded at the 2015 GORDON RESEARCH CONFERENCE ON PHYSICAL ORGANIC CHEMISTRY in Holderness, New Hampshire (June 21-26) and at the EUROPEAN SYMPOSIUM ON ORGANIC REACTIVITY in Kiel, Germany (Aug 30 - Sep 04). Soumya presented the results of her research. She was able to generate the diphenylmethyl radical in argon matrix at 3 K and the benzhydryl cation upon irradiation with 308 nm. Further irradiation leads back to the formation of the precursor. She also studied the reversibility between the radical and cation species using infrared (IR) and UV-Vis spectrocopy.
Carbenes are among the few metal-free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H2 through a two-step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers, and are thus unreactive towards H2 at cryogenic temperatures. Here we show that 1-azulenylcarbene with a singlet ground state readily inserts into H2, and that QMT governs the insertion into both H2 and D2. This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.
The fluorenyl cation is a textbook example for a 4p antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4p antiaromatic five-membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible-light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.
“Antiaromatic compounds” is what chemists call a class of ring molecules which are extremely instable – the opposite of the highly stable aromatic molecules. Because they exist for mere split seconds, they can only be detected by extremely demanding, ultrafast methods. Together with colleagues from Max Planck Institute for Coal Research in Mülheim, researchers from the Cluster of Excellence RESOLV at Ruhr-Universität Bochum have succeeded in isolating the antiaromatic fluorenyl cation at extremely low temperatures in water ice. Thus, they were able to conduct a spectroscopic analysis for the very first time. Their report is published in “Angewandte Chemie”...(read more)
Diphenylcarbene (DPC) shows a triplet ground-state lying approximately 3 kcal/mol below the lowest singlet state. Under the conditions of matrix isolation at 25 K, DPC reacts with single water molecules embedded in solid argon and switches its ground state from triplet to singlet by forming a strong hydrogen bond. The complex between DPC and water is only metastable, and even at 3 K the carbene center slowly inserts into the OH bond of water to form benzhydryl alcohol via quantum chemical tunneling. Surprisingly, if DPC is generated in amorphous water ice at 3 K, it is protonated instantaneously to give the benzhydryl cation. Under these conditions, the benzhydryl cation is stable, and warming to temperatures above 50 K is required to produce benzhydryl alcohol. Thus, for the first time, a highly electrophilic and extremely reactive secondary carbenium ion can be isolated in a neutral, nucleophilic environment avoiding superacidic conditions.