Carbene complexes fluxionality

Thermolysis of (cycloheptatrienylmethyl)carbene complexes 554 [toluene, 1-2 h, 80-100°C MLn = Cr(CO)5, W(CO)5] affords an equilibrium mixture of 4,5-homotropilidenes 555 and 556. According to the NMR data and the results of AMI calculations, the formation of isomer 556 (equation 218) is strongly favored277. This course of events was called intramolecular cyclopropanation , and it was shown that the equilibrium between the 4,5-homotropilidene complexes is significantly different from that of the metal-free ligands. By reaction of the latter (555 and 556) with bis(ethylene)rhodium 1,3-pentanedionate 557, the complexes 558 and 559 of both 4,5-homotropilidenes were obtained in a 1 3 ratio. These complexes are non-fluxional and are configurationally stable at room temperature (equation 219)277. 

AhHCl BulPCHzCHiCRCHiCH BuDlR = H(flux-ional) = CH3(not fluxional), olefinic and carbene complexes, and some new hydrides of iridium. 

Rapid cis-trans isomerization was detected in bridging diiron carbene complexes of type [Cp2Fe2(CO)2(/tA-CO)(/Lt-C(SMe)SR)] while in the bis(tricar-bonyliron) complex (25) the fluxional process concerned Fe(CO)3 moiety rotation, with one of the exo moieties rotating approximately three times faster than the other moiety. ... 

Treatment of the anion [ Mo (CNMe ) (CO) 2Cp ] with 1(0112)31 leads tc cis-MoI(CO)2[=C(CHq)3NMe]Cp rather than the anticipated trans species.This compound and other related carbene complexes undergo reduction with NaC2 QHg resulting in the carbene anions [M(C0)2(carbene)Cp] .The fulvene dianion [(C QHg) Mo(CO)3 2l reacts with I[CH2]3l to form the fluxional carbene (43) and with Mel to form the bis-methyl (Cj gHg ) [MoMe (CO) 3 ] 2 Thermolyis or photolysis results in evolution of propene. ... 

Transition-metal complexes of the more highly unsaturated seven-mem-bered allene 1,2,4,6-cycloheptatetraene have been isolated in two forms depending on the metal and its ligands an allene form (329), which is a complex of the parent cycloheptatetraene 327,134 and a carbene form (330/331), which is a complex of the controversial cycloheptatrienylidene 328.4135136 The carbene form corresponds to the allyl cation (320) that was suggested as an intermediate/transition state for fluxionality of 318. Scheme 42 lists all such complexes that have been prepared to date. The allene form is the ground state for all Pt(0) complexes (343, 344, 345, 347)82,83,130 137 138 and one dibenzannelated W(II) complex139 (346), whereas the carbene form is the ground state for all Fe(II)+ complexes (332-336),140-143 all Ru(II)+ complexes (337-340),144 one Pt(II)+ complex (342),137 and one W(0) complex (341).139... 

The delicate balance between dissociation and intramolecular pathways can be seen in the, albeit somewhat different, Tj -C Hg complex of Pt(PPh3)j. In the initial study, it was determined from saturation transfer and observation of scrambling of Pt isotopomers that the fluxionality was due to dissociation of Pt(PPhj)2 from the ring with AH = 26.8 kcal mol" and AS = 15.1 cal mol" K", determined from 60°C to 80°C. Subsequent work has confirmed the observations at 80 C, but shown that the process is entirely intramolecular at 60°C, possibly via a carbene intermediate. This study also showed that the bimetallic derivative (OC)3Mo(C7Hg)Pt(PPhj)2 shows fluxionality at lower temperature and proceeds by an intramolecular shift of Pt(PPhj)2. 

Pd2(dba)3 92 and 93 are generated by the reaction of Pd(OAc)2 with the ligand precursor. Aspects of their structure, fluxional behavior, and catalytic reactivity have been reported. The conformer interconversion for the cationic carbene-pyridine-carbene C-N-C) pincer 93 and the neutral C-C-C dicarbene Pd pincer 94 complexes were investigated, and it was found that, for cationic complexes, the choice of anion can affect the mechanism for... 

In 1991, Arduengo isolated one of the first NHC-Group 17 element adducts. The team found that NHCs cleanly reacted with iodopentafluoro-benzene to form hypervalent iodine complex 254 (Scheme 5.37). According to the NMR data, complex 254 was fluxional in solution, and in equilibrium with the free carbene and iodopentafluorobenzene. After several hours at room temperature, solutions of 254 were shown to undergo cleavage of the I-Cph bond, generating pentafluorobenzene and the 2-iodoimidazolium ion 255. 

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