Abstract
The goals of this thesis are to study the reactions of the quintuply bonded dimolybdenum complex, Mo2[μ-η2-HC(N-2,6-iPr2C6H3)2]2 (1), with various nitriles and alkynes, from which [2+2+2] cycloaddition involving the Mo-Mo quintuple bond with interesting selectivity is found and possible mechanisms are discussed herein. When 1 reacts with two equivalents of isobutyronitrile, cyclohexanecarbonitrile, or pivalonitrile, intramolecular C-H bond activation is observed and [κ1-NC-R]2Mo2(H)[μ-κ2-HC(N-2,6-iPr2C6H3)(N-2-iPr-6-CH(κ1-CH2)CH3-C6H3)][μ-κ2-HC(N-2,6-iPr2C6H3)2] (R = iPr (2), Cy (3), tBu (4)) are isolated. In contrast, when 1 reacts with three equivalents of acetonitrile, [2+2+2] cycloaddition involving the Mo-Mo quintuple bond leads to the isolation of the cycloadduct [μ-κ1:η1-CH2CN][μ-κ2-NC(CH3)C(CH3)N]Mo2(H)[μ-κ2-HC(N-2,6- iPr2C6H3)2]2 (6). Not only does it generate an unusual head-to-tail [2+2+2] cycloaddtion, but also one molecule of acetonitrile coordinates to the Mo2 unit by an unprecedented coordination mode. Presumably, steric hindrance of alkyl nitriles is the main factor determining whether two equivalents of nitriles will undergo [2+2+2] cycloaddition with 1. Upon treatment with phenylacetylene, the CH2CN ligand of 6 is replaced “PhCC” to give [κ1-CCPh][μ-κ2-NC(CH3)C(CH3)N]Mo2(H)[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (7). Furthermore, reacting 1 with two equivalents of trimethylsilylformonitrile, we isolate [κ1-NC-TMS][κ1-NC]Mo2[μ-κ2-HC(N-2,6-iPr2C6H3)(N-2-iPr-6-CH(κ1-CH2) CH3-C6H3)][μ-κ2-HC(N-2,6-iPr2C6H3)2] (8), which not only shows intramolecular C-H bond activation, but also a swap of carbon and nitrogen atoms of the nitrile is observed. On the other hand, reaction of 1 with two equivalents of methyl-cyanoacetate leads to the isolation of the complex Mo2{μ-η2:η2-N(H)C[CHC(OMe)]O}2[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (9), which doesn’t undergo intramolecualr C-H bond activation, but a common coordination mode of nitrile to metal is observed. The reactions of 1 with internal and terminal alkynes lead to [2+2+2] cycloaddition. First, 1 undergoes a [2+2] cycloaddition reaction with one equivalent of 3-hexyne to give the four-membered ring complex Mo2(μ-η2-EtCCEt)[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (10). When 10 subsequently reacts with phenylacetylene, ethyl propiolate, or 1-pentyne, [4+2] six-membered ring complexes, Mo2[μ-κ2-5,6-Et2-3-RC4H][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R = Ph (11), COOEt (12), Pr (13)), are isolated. When Mo2(μ-κ2-MeCCPh)[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (14) reacts with trimethylsilylacetylene, 4-fluorophenylacetylene, 4-methoxyphenylacetylene, cyclopropylacetylene, cyclohexylacetylene, or 3-ethynylthiophene, [4+2] cycloaddition products can also be isolated along with their stereoisomers, Mo2[μ-κ2-6-Me-5-Ph-3-RC4H][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R = TMS (15), 4-FPh (18), 4-MeOPh (20), 2-thiophene(24)) and Mo2[μ-κ2-6-R-4-Me-3-PhC4H][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R = TMS (16), 4-FPh (17), 4-MeOPh (19), cyclopropyl (21), cyclohexyl (22), 2-thiophene (23)). The characterization of 11-13 suggests that the stereo orientation of terminal alkynes be affected by their steric hindrance. In contrast, the characterization of 15-24 shows that the stereo orientation of internal alkynes is controlled by electronic effect. In short, the electronic effect controls the stereo orientation of the first alkyne molecule and the steric effect controls the stereo orientation of the second one. Notably, when 14 or Mo2(μ-κ2-PhCCPh)[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (25) reacts with 1,6-heptadiyne, the [4+2] cycloaddition and the dimeric complexes {Mo2[μ-κ2-HC(N-2,6-iPr2C6H3)2](μ-κ2-4-Ph-3-PhC4H)}2-6-(CH2)3 (R = Ph (26), Me (27)) are isolated. It is worthy noting that reactions of 10 and internal alkynes and small nitrile result in three-component [2+2+2] cycloaddition as well. When 10 reacts with one equivalent of propionitrile or acetonitrile, [4+2] cycloaddition can be achieved and the pyridine-liked six-membered ring complexes Mo2[μ-κ2-NC(R)C(Et)C(Et)][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R = Et (30), Me (31)) can be obtained. Likewise, reactions of 14 with one equivalent of propionitrile and benzyacetionitrile gives the [4+2] cycloaddition products, Mo2[μ-κ2-NC(R)C(Ph)C(Me)][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R = Et (33), CH2Ph (35)). On the basis of the results above, we know that the electronic effect of nitriles dominates the high selectivity of the cycoaddition reactions. Finally, when 10 and 14 respectively react with two equivalents of acetonitrile, the cycloadducts [μ-κ1:η2-CH2CN][μ-κ2-NC(Me)C(R1)C(R2)]Mo2(H) [μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R1 = R2 = Et (32), R1 = Ph, R2 = Me (34)) are isolated, which are similar to 6.