Iron complexes fluxionality

Access to alkyl substituted derivatives of the homotropylidene complexes is provided via the >j4-tropone iron complex by reaction with diazoalkanes, followed by mild thermolysis of the 3+2 py razoline adduct to give the corresponding homotropone complexes (equation 149)217,218. The 8,8-dimethyl derivative was used as starting material for the preparation of the fluxional ( 5-2,8,8-trimethylbicyclo[5.1.0]octa-2,4-dienylium)Fe(CO)3 cation complex219. More recently (homotropone)Fe(CO)3 was used for the synthesis of unique chiral 1,2-homoheptafulvene iron complexes220 221. 

Many of the bridged silylene complexes show complex fluxional behavior or slow isomerization between different isomeric forms. An example is the iron-silylene complex shown in Scheme 11, in which the proposed mechanism is illustrated. A full discussion appears in Ogino and Tobita s review338. 

The iron complex of 1,2-cycloheptadiene was synthesized to study the relationship between 312 and a valence isomeric form (316) that has been referred to as the allyl cation form (Scheme 39).119 Because of ring strain, the latter had been predicted by EHMO calculations to lie rather close in energy to an allene ground state.126 The NMR of 312 confirmed the allene form as the ground state and fluxionality experiments on the terminal allene... 

A wealth of new Sn NMR data has become available for transition metal complexes with M-Sn bonds (Table 9 see also Fig. 2). Many of these complexes possess a rigid structure as compared to the NMR time scale. Others are fluxional, and " Sn NMR parameters can be used to understand the dynamic processes. An example is the iron complex cis-[Fe(CO)4(SnMe3)2], in which the magnitude of the geminal coupling constant V( Sn, Sn) does not change under slow or fast exchange conditions. This... 

The reaction of methyl tropyl ether-iron tricarbonyl with acid yields a tropylium complex, in which the ring formally contains an uncoordinated double bond (188), but is actually an example of a fluxional molecule. 

A variety of synthetic routes to monoene and polyene tri-fluorophosphine-transition metal complexes have been devised. Direct photochemically induced reaction of a metal-PF3 complex with an activated alkene or diene (method A) has proved useful only for iron, the products being either [Fe(PF3)4(alkene)J or [Fe(PF3)3(diene)] (194). Mixed carbonyl-trifluorophosphine complexes of the type [Fe(PF3)x(CO)3 x(diene)] result from either thermal or photochemical reactions of dieneiron carbonyl complexes and PF3 (52, 53) (method B). The compounds are fluxional. 

Isomerization of safrole and eugenol is reported to occur when these alkenes are irradiated in the presence of iron carbonyls. Co-ordinatively unsaturated iron carbonyls are first formed and subsequently give iron carbonyl-alkene complexes whose alkene ligands are then isomerized. A further reported photoinduced isomerization involves Fe(fnb-Ti -cot)(CO)3 (cot = cyclo-octatetraene), which on irradiation in a CH4 matrix at 12 K is converted into Fe(cfiair-Ti -cot)(CO)3 via Fe(T -cot)(CO)3 as intermediate. The results of these studies are related to the fluxional behaviour of Fe(i7 -cot)(CO)3 in solution. ... 

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