Paramagnetic carbene complexes

A one-electron oxidation of the Ru(II) imidazolylidene complex 42, generating the paramagnetic carbene complex 43, has been noted (75) ... 

Paramagnetic carbene complexes of Fe(I) and Cr(I) are obtained by oxidation of Fe(0) and Cr(0) complexes, respectively, using silver tetrafluoroborate. ... 

Oxidation of carbene complexes leads either to their decomposition with evolution of organic molecules or to formation of paramagnetic carbene complexes ... 

Mncp(CO)2(CPh)] [BCl4 ] reacts with rf-butylisonitrile to form a paramagnetic carbene complex (cf. Ref. m, Table 5.8) ... 

Paramagnetic carbene complexes of Cr obtained, e.g., by the reaction sequence in equation (26) are stablest with the chelating phosphine, dppe. E.s.r. 

Paramagnetic Carbene Complexes with Group 14 Elements... 

An electrophilic metal-carbene complex intermediate has been proposed in the cyclopropanation reaction, whereas a paramagnetic copper nitrene species, which behaves as an electrophilic, nitrogen-centered radical, is proposed as the intermediate for the aziridination reaction.45 [Cu(Tp GF3 2)(C2H4)] is a good aziridination catalyst, readily converting a variety of olefins into the corresponding A-tosyl aziridines.46... 

Recently, FT-IR and theoretical analysis of iron porphyrin carbene complexes revealed a possible paramagnetic intermediate carbene state during the reaction of ethyl diazoacetate with Fe(TPP). Focusing on the C = O stretching frequency in the carbene iron complex, a decrease of the value is observed, in comparison with diamagnetic carbene complexes since the radical on the C(carbene) delocalizes over the a-carbonyl group [78]. All isolated axial iron carbene complexes are, however, diamagnetic. 

Spectroscopy C NMR is a very valuable technique for detecting carbene complexes because the carbene carbon is very deshielded and resonates at —200-400 ppm to low field of TMS. It is tempting to ascribe this deshieiding to the d character of the carbene carbon, but as we shall see, Schrock carbenes, which are in character at carbon, show similar shifts. In fact, the shift is probably a result of the existence of low energy electronic excited states for the complex, which leads to a large paramagnetic contribution to the shift. A proton substituent at the carbene carbon resonates from +10 to +208. 

In addition to Fukuzumi s early contribution, NHC-muonium radical 169 was first observed by Percival and Clybume using muon spin resonance (pSR) spectroscopy (which is similar to electron spin resonance (FSR) spectroscopy) in 2003 (Figure 5.19). The muonium (Mu = [p" e ]) could be used as a hydrogen atom (H-) equivalent and readily reacted with carbenes and carbene-main group complexes to obtain paramagnetic complexes vide infra). Unlike the aforementioned carbene-acyl radicals, 169 was not a stable... 

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