Zerovalent arenes

Scheme 9.9 Total hydrogenation of monocyclic arene compounds by various zerovalent noble-metal nanoparticles.

Molybdenum trioxide, intercalation into, 12, 823 Molybdocenes, as anticancer agents, 1, 892 MOMNs, see Metal-organometallic coordination networks Monisocyanides, with silver(I) complexes, 2, 223 Monitoring methods, kinetic studies, 1, 513 Mono(acetylacetonate) complexes, with Ru and Os halfsandwich rf-arenes, 6, 523 tj2-Monoalkene monodentate ligands, with platinum divalent derivatives, 8, 617 tetravalent derivatives, 8, 625 theoretical studies, 8, 625 zerovalent derivatives, 8, 612... 

Laboratory in Oxford, and Geoffrey Ozin at the University of Toronto in the early 1970s. With the metal atom cocondensation technique (which as described in Chaps. 6 and 7 was also used to prepare a series of zerovalent arene and olefin metal complexes), they reported simultaneously that the elusive palladium and platinum tetracarbonyls, Pd(CO)4 and Pt(CO)4, as well as the coordinatively unsaturated fragments M(CO)3, M(CO)2, and M(CO) (M = Pd, Pt) were formed by cocondensation reactions of Pd and Pt atoms with CO in inert gas matrices at 4-10 K [119-122]. The comparison of the CO bond stretching force constants for Pd(CO)ra and Pt(CO)ra (n - 1-4) revealed that, in analogy to Ni(CO) , the most stable compounds were the tetracarbonyls. In a xenon matrix, Pd(CO)4 existed up to about 80 K [120]. Ozin s group as well as others... 

If the issue of n-complex formation with PMDA-ODA polyimide seems confused, the results of model compound studies have, in places, exacerbated the problem. Several research groups have turned to reactions of formally zerovalent chromium to model the chemistry of PMDA-ODA/Cr (58e, 60. 62) The studies assume that the chemistry of atomic Cr and of certain chromium carbonyl complexes is coextensive. Since atomic chromium has not been available as a benchtop reagent, popular sources of formally zerovalent chromium, disguised as Cr(CO)6 and (CH3CN)3Cr(CO)3, have been used. These compounds readily form (arene)Cr(CO)3 complexes (71). 

The correct identification of the sandwich structure of ferrocene led Fischer to consider the possibility of arenes acting as hexahapto 6VE ligands. By simple arithmetic, a neutral bis(arene) sandwich complex of a zerovalent Group 6 element, e.g. chromium (Figure 6.71), was anticipated, a line of reasoning which led Fischer to develop the synthesis of dibenzenechromium. His approach resulted in the comparatively general Fischer-Hafner synthesis (1955), which is applicable to many metals and arenes (devoid of Lewis-basic substituents, Figure 7.34). 

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) . 

In the specific case of scandium, mono- and mixed-valence species can also be isolated together with divalent complexes for instance, a Sc organometallic compoimd could be obtained imder relatively mild conditions. Finally, the author describes the few knovm zerovalent bis(arene) rare-earth complexes which have been obtained by co-condensation of arenes or heteroarenes with metal vapors. In his conclusion, F. Nief notes that the low-valence molecular chemistry of rare earths, which was once thought to be restricted to divalent samarium, europium, and ytterbium, has been extended to several other rare earths, as well as to lower valence oxidation states. It is the opinion of the author that this research area is likely to find fascinating developments in a near future. 

Molecules that have [ML/X Z ] classifications that are remote from the highly populated areas in an MLX plot are necessarily of considerable interest because of their uniqueness. For example, with respect to scandium chemistry, while the majority of compounds possess a ML/X3 classification (/ = 2-6), there are some very interesting examples of a zerovalent arene complexes ( j -ArH)2Sc e.g. ArH = C6H3Bu 3) which possess the MLg classification. This corresponds to a most unusual valence state for scandium, and the ability to isolate... 

The first authentic zerovalent arenelanthanoid complexes [l,3,5-(t-Bu)3C6H3]2Ln (Ln-Y, Gd) have been obtained by Cloke and coworkers in die cocondensation of vaporized metal with an excess of l,3,5-tri-(t-butyl)benzene at 77 K [63]. The products are isolated after the recrystallisation from pentane as deep purple crystals highly soluble in hydrocarbon solvents. They are stable at room temperature, and may be sublimed at ca. lOOT/lO " mbar with partial decomposition. Like all other arene derivatives of REM (Table IV.6 ) bis(arene) complexes are highly air and moisture sensitive. 

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