Synthesis of Molybdenum Nitrido Complexes for Triple-Bond Metathesis of Alkynes and Nitriles

Complexes of the type NMo(OR)3 (R = tertiary alkyl, tertiary silyl, bulky aryl) have been synthesized in the search for molybdenum-based nitrile–alkyne cross-metathesis (NACM) catalysts. Protonolysis of known NMo(NMe2)3 led to the formation of NMo(O-2,6-iPr2C6H3)3(NHMe2) (12), NMo(OSiPh3)3(NHMe2) (5-NHMe2), and NMo(OCPh2Me)3(NHMe2) (17-NHMe2). The X-ray structure of 12 revealed an NHMe2 ligand bound cis to the nitrido ligand, while 5-NHMe2 possessed an NHMe2 bound trans to the nitride ligand. Consequently, 17-NHMe2 readily lost its amine ligand to form NMo(OCPh2Me)3 (17), while 12 and 5-NHMe2 retained their amine ligands in solution. Starting from bulkier tris-anilide complexes, NMo(N[R]Ar)3 (R = isopropyl, tert-butyl; Ar = 3,5-dimethylphenyl) allowed for the formation of base-free complexes NMo(OSiPh3)3 (5) and NMo(OSiPh2tBu)3 (16). Achievement of a NACM cycle requires the nitride complex to react with alkynes to form alkylidyne complexes; therefore the alkyne cross-metathesis (ACM) activity of the complexes was tested. Complex 5 was found to be an efficient catalyst for the ACM of 1-phenyl-1-butyne at room temperature. Complexes 12 and 5-NHMe2 were also active for ACM at 75 °C, while 17-NHMe2 and 16 did not show ACM activity. Only 5 proved to be active for the NACM of anisonitrile, which is a reactive substrate in NACM catalyzed by tungsten. NACM with 5 required a reaction temperature of 180 °C in order to initiate the requisite alkylidyne-to-nitride conversion, with slightly more than two turnovers achieved prior to catalyst deactivation. Known molybdenum nitrido complexes were screened for NACM activity under similar conditions, and only NMo(OSiPh3)3(py) (5-py) displayed any trace of NACM activity.

为开发钼基腈-炔交叉复分解（nitrile–alkyne cross-metathesis, NACM）催化剂，我们合成了通式为N≡Mo(OR)₃（R为叔烷基、叔硅基、位阻芳烃）的配合物。

对已知的N≡Mo(NMe₂)₃进行质子解反应，得到了N≡Mo(O-2,6-iPr₂C₆H₃)₃(NHMe₂)（12）、N≡Mo(OSiPh₃)₃(NHMe₂)（5-NHMe₂）以及N≡Mo(OCPh₂Me)₃(NHMe₂)（17-NHMe₂）。X射线晶体结构（X-ray structure）表征显示，配合物12中的NHMe₂配体与氮配体呈顺式配位，而5-NHMe₂中的NHMe₂配体则与氮配体呈反式配位。据此，17-NHMe₂可轻易脱去胺配体生成N≡Mo(OCPh₂Me)₃（17），而12与5-NHMe₂在溶液中仍可保留其胺配体。

从位阻更大的三苯胺基配合物N≡Mo(N[R]Ar)₃（R为异丙基、叔丁基；Ar为3,5-二甲基苯基）出发，我们成功制备了无碱配合物N≡Mo(OSiPh₃)₃（5）与N≡Mo(OSiPh₂tBu)₃（16）。

实现NACM催化循环需要氮化物配合物与炔烃反应生成卡拜配合物，因此我们对各配合物的炔烃交叉复分解（alkyne cross-metathesis, ACM）活性进行了测试。实验发现，配合物5在室温下可作为1-苯基-1-丁炔交叉复分解的高效催化剂。配合物12与5-NHMe₂在75 ℃下同样具备ACM催化活性，而17-NHMe₂与16未表现出ACM活性。

仅配合物5对茴香腈的NACM反应具有催化活性——茴香腈是钨基NACM催化体系中的活性底物。使用配合物5催化的NACM反应需在180 ℃下启动所需的卡拜-氮化物转化过程，在催化剂失活前仅实现了略多于2个催化周转数。

我们在相似条件下对已知钼基氮化物配合物的NACM活性进行了筛选，仅N≡Mo(OSiPh₃)₃(py)（5-py）表现出微弱的NACM催化活性。