AreneMoCO Complexes
Arene tricarbonyl molybdenum complexes are yellow, often crystalline compounds. They are weakly air-sensitive in the solid state and have to be stored under inert atmosphere and out of light. They are best purified by crystallization. In solution, they are unstable to air. The trait that has most hampered development of the use of (arene)Mo(CO)3 complexes in organic synthesis, however, is the lability of the arene metal bond. Lewis basic solvents such as THF, DMF, DM-SO, acetone and acetonitrile rapidly displace benzene in (benzene)Mo(CO)3. This lability of the arene-Mo bond, while making handling difficult, holds promise for the catalytic use of this class of compounds.
Synthesis of (Arene)Mo(CO)3 Complexes
The direct synthesis of (arene)Mo(CO)3 complexes from arene and Mo(CO)6 is much more limited than for chromium (Scheme 4) [11, 51]. The long reaction times at elevated temperature (e.g., ten days for (benzene)Mo(CO)3) and the high sensitivity to oxygen often results in low yields for substituted arenes. While (benzene)Mo(CO)3 (40) has been reportedly obtained in near quantitative yield, the yield was based on liberated CO rather than isolated complex [11]. In the author's laboratory, an isolated yield of 50% is more realistic for this procedure. The reaction time can be shortened by reacting Mo(CO)6 in benzene in the presence of pyridine in an autoclave [52]. Toma and coworkers have described a different procedure that uses a double condenser system, and decalin plus ethylformate as solvent [53]. With a bath temperature of 240 °C this cuts the preparation time of the aniline complex 42 to 1 h (55% yield) (Scheme 4). In the authors laboratory the method is used routinely for the synthesis of complex 40 (18 h, 60% yield).
Alternatively ligand substitution in Mo(CO)3L3 complexes can be used and a particularly useful method is the use of Mo(CO)3py3 and BF3-OEt2 in the presence of an excess arene (Scheme 5) [58-60]. Other sources of Mo(CO)3 fragments that have been used in the synthesis of (arene)Mo(CO)3 complexes are Mo(CO)3(diglyme) [61], and Mo(CO)3(DMF)3 [62].
Thermo chemical studies show the arene-Mo bond (68 kcal mol-1 in [(h6-C6H6)Mo(CO)3] (40)) to be stronger than the arene-Cr bond (53 kcal mol-1 in [(h6-C6H6)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-
Scheme 4
Scheme 5 Synthesis of (Arene)Mo(CO)3 complexes from Mo(CO)3py3, arene, and BF3 • OEt2
Scheme 6
pared to >150 °C in arene Cr complexes). In pioneering work, Muetterties determined equilibria of arene exchange in (arene)Mo(CO)3 complexes (arene=ben-zene, toluene, xylene, mesitylene, ...). The acetone catalyzed reactions proceed at room temperature and equilibria are reached in less than three days. Stabilities of the arene complexes increases with the degree of Me substitution [65].
This lability can be used preparatively for functionalized arenes as shown by the examples in Scheme 6 [66, 67].
Arene Decomplexation from Mo
Given the lability of the Mo-arene bond, the removal of the activating group Mo(CO)3 is very easy and is readily achieved by the addition of an excess of a coordinating solvent (MeCN, THF) or another Lewis base (PR3, NR3) to a solution of the arene complex.
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