Metal-arene bonding

Mass spectral studies on bisarene chromium derivatives have shown that the strength of the metal arene bond increases when electron-... 

Only a few metal complexes of Sc, Y, La, and the lanthanides are known for oxidation states O and -f I , and their oxidation chemistry with halogen-containing compounds has not been explored extensively. The zerovalent bisarene complexes M(TTB)2 (M = Sc, Y, Gd, Dy, Ho, Er, Lu TTB = f/ -LS.S-t-BusCeHa) are oxidized by I2 to give MI3 and the free TTB ligand . These reactions have been utilized for calorimetric determination of metal-arene bond enthalpies in these complexes . 

The reaction of I2 with zerovalent bisarene complexes M(TTB)2 (M = Ti, Zr, Hf) and Ti(fj -toluene)2, to give MI4 and free arene ligand, was employed for the measurement of metal-arene bond enthalpies in much the same manner as for the lanthanide complexes (see above) . 

Incorporation of sterically demanding aryl substituents allows isolation of bis(arene)zirconium and hafnium complexes. The bond enthalpies of (7]6-(l,3,5-tBu)3C6H3)2M (M = Ti, 1 Zr, 2 Hf, 3) have been measured by iodinolytic bath calorimetry and values of 49(1), 64(3), and 67(4) kcal mol-1 have been determined for the respective metal-arene bond enthalpies (Scheme l).4 Computational studies establish that the major metal-arene bonding interaction is a 5-backbond formed from the overlap of metal dxz-yz and orbitals with the appropriate linear combination of arene p-orbitals. The observed increase in metal-arene bond strength is consistent with increased backbonding down the... 

Thermo chemical studies show the arene-Mo bond (68kcal mol in [(q -CgHg)Mo(CO)3] (40)) to be stronger than the arene-Cr bond (53 kcal mol" in [(q -CgHg)Cr(CO)3] (1)) [63,64]. Kinetically, however, the situation is reversed. The metal arene bond in the Mo complex 40 is far more labile than that in the Cr complex 1. In the absence of a Lewis base catalyst, arene exchange in (arene)Mo(CO)3 complexes is measurable at temperatures as low as 60 °C (com-... 

With the exception of the labile complexes of condensed aromatics, the metal-arene bond in (arene)Mn(CO)3 complexes is very robust. It is more difficult to cleave than that in the neutral arene complexes of chromium and molybdenum and requires strongly oxidative conditions such as DDQ in refluxing acetonitrile. 

The metal arene bond in (arene)FeCp complexes is photolabile [93], and this forms the basis of the use of these complexes as photoinitiators for cationic polymerizations of epoxides [94]. Arene exchange via this route has not been used extensively. On photolysis of (p-xylene)FeCp in CH2CI2 or acetone by a medium pressure Hg-arc or by bright sunlight , the complex undergoes arene exchange with hexamethylbenzene, paracyclophane and thiophene under mild conditions... 

The NMR spectrum of fra s-l,3-dimethylindane exhibits only six resonances despite the diastereotopic nature of the molecule. However, when complexed to the Cr(CO)8 moiety the NMR spectrum shows eleven resonances for the /j -indenyl ligand. Low temperature spectra (—80 Q shows slight broadening of the single carbonyl resonance suggesting a low barrier to metal-arene bond rotation (197). 

The NMR of rrring carbons and eleven resonances were observed for the A -/ro/ij-l,3-dimethylindane ligand. Only a single carbonyl resonance was observed, 235.0 ppm, even down to — 80°C which suggests that the barrier to rotation about the metal-arene bond is quite low even in this relatively crowded complex. Alternatively the Cr(CO)3 moiety may be non rigid. 

Thermochemical data for arene complexes are less abundant than for metallocenes. This is due in part to incomplete combustion of the compounds. The energies of dissociation of metal-arene bonds were obtained from mass spectra for most of the complexes. However, data obtained by this method are somewhat higher (by ca lOOkJmol" and even higher). In some cases, even the reverse order of the relative stabilities for a given series of compounds is obtained. Table 10.8 gives energies of dissociation of metal-arene bonds which are obtained by calorimetric methods. 

Arenes are usefril 7c-ligands of transition metals. The metal-arene bonding comprises both rr-donation from ligand to empty d-orbital of metal and rr-back-donation from filled metal d-orbital to 7i -orbital of ligand, and... 

The photochemical displacement of the arene is also observed for the complexes ( / -cp)(7 -arene)M (M = Fe, Ru). This reaction occurs from the photoactive a E ligand field excited state. A linear correlation exists between log (0/(l-0)) and o-p, the Hammett parameter for a series of complexes with chloro-and methyl-substituted arenes. The data indicate that a small amount of negative charge builds up at the arene in the transition state for the reaction that results in arene dissociation in these systems. The temperature dependence for the photodissociation of the arene ligand indicates that the metal-arene bond is almost broken in the a E excited state. When the analogous pentamethylcyclopentadienyl complexes (7 -cp )( / -arene)M (M = Fe, Ru) are photolyzed, the quantum yields for arene release are lower than those found for the unsubstituted compounds... 

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