Molybdenum dimers

Finally, the implication is made that the sigma ionization is also in the region of the pi ionization for the molybdenum dimers. We have recently carried out an examination of the pi ionization of M02(02CCH3)i+ in which we observe clear vibrational fine structure across the band. This observation is not expected if two different ionizations, the sigma and pi, are overlapped in the same envelope. Thus, I do not see any evidence at this stage that the sigma ionization is in the region of the pi ionization for these complexes. 

The tetracarboxylatodimolybdenum dimers that result from the reaction of molybdenum hexacarbonyl with a wide variety of carboxylic acids are the initial source of quadruply bound molybdenum units for many further chemical investigations. The initial isolation of Mo2(02CC6H5)4 (2), Mo2(02CCH3)4 (14), and the higher alkyl carboxylate analogues (235) by Wilkinson et al. established a reactivity pattern that has been successfully exploited to prepare many related molybdenum dimers. In addition to those carboxylates enumerated above, a series of substituted aromatic carboxylate derivatives has been prepared by refluxing the appropriate acid with molybdenum hexacarbonyl in diglyme (155). 

The extensive series of known quadruply bound molybdenum dimers has also been the subject of detailed experimental investigations to unravel the electronic properties characterizing excited states in these Mo2+ derivatives. Consideration of the Mo2Xj spectra is presented first since the Mo2(02CR)4 compounds exhibit significantly different features in their electronic spectra. 

Raman spectra of quadruply bound molybdenum dimers show a much greater frequency variation for the metal- metal stretching mode than is found in related rhenium compounds. The Mo2X - anions and other halo derivatives are considered first, then a review of tetracarboxylatodimolybdenum vibrational data is given, and finally comparisons for various molybdenum dimers are presented. 

Cyclohexene oxidation in the presence of the molybdenum complex, [C5Hr)Mo(CO)3]2, gave two major products at low conversion VI and VII nearly 1 1 mole ratio, Table V. The ketone, VIII, was formed in very low yield in contrast to oxidations using the iron complex. This reaction is far more selective than the oxidation of cyclohexene in the presence of Mo02(acac)2 reported by Gould and Rado (24). When a cyclohexene solution of V was exposed to [CsHsMk COJs] at 70°C, VI and VII were formed in approximately equimolar amounts (Table VI). These data show that the molybdenum complex efficiently catalyzes the epoxidation of cyclohexene by V before the allylic hydroperoxide decomposes substantially. Reaction 16 represents the predominant course of cyclohexene oxidation in the presence of cyclopentadienyltricarbonyl molybdenum dimer. 

Upon warming the clear yellow solution to — 20 °C, it turns red and deposits dark purple crystals of the molybdenum dimer 7 in essentially quantitative yields, and the accompanying mother liquor also affords an equimolar amount of the (cyclohexadiene)iron dimer 6a,... 

The molybdenum dimer 547 can be converted, by alkylation, to a cation (548), which can be transformed into an anion (549) by two-electron chemical reduction. Treatment of 547 with excess methyllithium leads, after work-up in air, to the bis (/z-methylthiolate) 550, which can also be prepared by addition of methyllithium to 548. The addition of vinylmag-nesium bromide to 548 yields 551. The anion reacts with alkynes, activated olefines, and an allene to form 552, 553, and 554, respectively (305). 

A molybdenum dimer with a Mo-M triple bond is prepared by the high temperature reaction of pentamethylcyclopentadiene with Mo(CO)6 ... 

Chisholm has reported diacids that can connect two tungsten (or molybdenum) dimers, to make tetramers. RH Cayton, MH Chisholm, JC Huffman, EB Lobkovsky. J Am Chem Soc 113 8709-8721, 1991. 

Redox processes are fairly common in the formation of Z —CO— complexes of transition metals, and an example is given in Eq. (9). In this reaction, titanium is oxidized from the + 2 to the +3 state, thus becoming a better Lewis acid, and the molybdenum dimer is reductively cleaved, thus developing Z —CO— donor character (59). A characteristic low-frequency Z —CO— band is observed in the IR spectrum, and a crystal structure is available. A proposed mechanism for the redox process, based on CO mediated electron transfer, is discussed in Section IV,C. 

The Mo-Mo atomic distance of the molybdenum dimer species was reported to depend on the sulfidation temperature. With Mo(CO)e/NaY (Si/Al =... 

McGinnis RN, Ryan TR, McCarley RE (1978) Reactions of activated quadruply bonded molybdenum dimers. Formation of new rectangular clusters containing the tetrametal analog of cyclobutadiyne. J Am Chem Soc 100 7900-7902... 

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