Dinitrogen Complexation and Extent of NN Activation within the Group 6 “End-On-Bridged” Dinuclear Complexes, {(η⁵-C₅Me₅)M[N(i-Pr)C(Me)N(i-Pr)]}₂(μ-η¹:η¹-N₂) (M = Mo and W)

Chemical reduction of Cp*M[N(i-Pr)C(Me)N(i-Pr)]Cl3 (Cp* = η5-C5Me5) (1, M = Mo) and (2, M = W) using 0.5% NaHg in THF provided excellent yields of the diamagnetic dinuclear end-on-bridged dinitrogen complexes {Cp*M[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2) (6, M = Mo) and (8, M = W), respectively. Chemical reduction of Cp*Mo[N(i-Pr)C(NMe2)N(i-Pr)]Cl2 (4) with 3 equiv of KC8 in THF similarly yielded diamagnetic {Cp*Mo[N(i-Pr)C(NMe2)N(i-Pr)]}2(μ-η1:η1-N2) (7). Single-crystal X-ray analyses of 7 and 8 confirmed the dinuclear end-on-bridged μ-η1:η1-N2 coordination mode and the solid-state molecular structures of these compounds provided d(NN) values of 1.267(2) and 1.277(8) Å for 7 and 8, respectively. Based on a comparison of 15N NMR spectra for 15N2 (99%)-labeled 6 and 15N2 (99%)-labeled 8, as well as similarities in chemical reactivity, a dinuclear μ-η1:η1-N2 structure for 6 is further proposed. For comparison with a first-row metal derivative, chemical reduction of Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]Cl2 (9) with KC8 in THF was conducted to provide {Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2) (10) for which a d(NN) value of 1.270(2) Å was obtained through X-ray crystallography. Compounds 6−8 were all found to be thermally robust in toluene solution up to temperatures of at least 100 °C, and 6 and 8 were determined to be inert toward the addition of H2 or H3SiPh under a variety of conditions. Single-crystal X-ray analysis of meso-{Cp*Mo(H)[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2) (meso-11), which was serendipitously isolated as a product of attempted alkylation of Cp*Mo[N(i-Pr)C(Me)N(i-Pr)]Cl2 (3) with 2 equiv of n-butyllithium, revealed a smaller d(NN) value of 1.189(4) Å that is consistent with two Mo(IV,d2) centers connected by a bridging diazenido, [μ-N2]2−, moiety. Moreover, meso-11 was found to undergo clean dehydrogenation in solution at 50 °C to provide 6 via a first-order process. Chemical oxidation of 8 with an excess of PbCl2 in toluene solution at 25 °C provided a 1:1 mixture of rac- and meso-{Cp*W(Cl)[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2) (12); both isomers of which provided solid-state structures through X-ray analyses that are consistent with an electronic configuration comprised of two W(IV,d2) centers linked through a bridging [N2]2− group [cf. for rac-12, d(NN) = 1.206(9) Å, and for meso-12, d(NN) = 1.192(3) Å]. Finally, treatment of 6 and 8 with either 4 equiv of CNAr (Ar = 3,5-Me2C6H3) or an excess of CO in toluene provided excellent yields of Cp*M[N(i-Pr)C(Me)N(i-Pr)](CNAr)2 (13, M = Mo and 14, M = W) and Cp*M[N(i-Pr)C(Me)N(i-Pr)](CO)2 (15, M = Mo and 16, M = W), respectively. Single-crystal X-ray analyses of 13−16, along with observation of reduced IR vibrational νCN or νCO bond-stretching frequencies, provide strong support for the electron-rich character of the Cp*M[N(i-Pr)C(Me)N(i-Pr)] fragment that can engage in a high degree of back-donation with moderate to strong π-acceptors, such as N2, CNR, and CO. The collective results of this work are analyzed in terms of the possible steric and electronic factors that contribute to preferred mode of μ-N2 coordination and the extent of NN activation, including complete N−N bond scission, within the now completed experimentally-derived ligand-centered isostructural series of {Cp*M[N(i-Pr)C(Me)N(i-Pr)]}2(μ-N2) compounds where M = Ti, Zr, Hf, Ta, Mo, and W.以四氢呋喃（THF）为反应溶剂，使用0.5%钠汞齐（NaHg）对Cp*M[N(i-Pr)C(Me)N(i-Pr)]Cl3（其中Cp*为η5-五甲基环戊二烯基，化合物1：M=Mo，化合物2：M=W）进行化学还原，分别以优异产率得到抗磁性双核端基桥联二氮配合物{Cp*M[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2)（化合物6：M=Mo，化合物8：M=W）。以3当量石墨插层钾（KC8）在四氢呋喃中对Cp*Mo[N(i-Pr)C(NMe2)N(i-Pr)]Cl2（化合物4）进行化学还原，同样得到抗磁性配合物{Cp*Mo[N(i-Pr)C(NMe2)N(i-Pr)]}2(μ-η1:η1-N2)（化合物7）。对化合物7和8进行单晶X射线衍射分析，确认了其双核端基桥联μ-η1:η1-N2的配位模式，且固态分子结构测得其N-N键键长（d(NN)）分别为1.267(2) Å和1.277(8) Å。通过对比99%15N2标记的化合物6和8的氮-15核磁共振（15N NMR）谱图，结合二者相似的化学反应活性，进一步佐证了化合物6的双核μ-η1:η1-N2桥联结构。为与第一过渡系金属衍生物进行对比，以石墨插层钾在四氢呋喃中还原Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]Cl2（化合物9），得到{Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2)（化合物10），经X射线晶体学测得其N-N键键长为1.270(2) Å。实验发现，化合物6~8在甲苯溶液中至少100 ℃下仍具有良好的热稳定性；且在多种条件下，化合物6和8均不与氢气（H2）或苯基硅烷（H3SiPh）发生反应。意外分离得到的内消旋-{Cp*Mo(H)[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2)（meso-11），是尝试以2当量正丁基锂对Cp*Mo[N(i-Pr)C(Me)N(i-Pr)]Cl2（化合物3）进行烷基化的产物。对其进行单晶X射线衍射分析发现，其N-N键键长为1.189(4) Å，该数值与由两个Mo(IV,d2)中心通过桥联二氮二阴离子([μ-N2]2−)连接的电子结构相符。此外，研究发现meso-11在50 ℃溶液中以一级反应动力学过程发生洁净脱氢反应，转化为化合物6。在25 ℃甲苯溶液中，以过量氯化铅（PbCl2）对化合物8进行化学氧化，得到外消旋型与内消旋型{Cp*W(Cl)[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2)（12）的1:1混合物；通过X射线分析确定了两种异构体的固态结构，其电子构型均为两个W(IV,d2)中心通过桥联[N2]2−基团连接[其中外消旋体rac-12的d(NN)=1.206(9) Å，内消旋体meso-12的d(NN)=1.192(3) Å]。最终，分别以4当量芳基异氰酸酯（CNAr，Ar=3,5-二甲基苯基，即3,5-Me2C6H3）或过量一氧化碳（CO）在甲苯中处理化合物6和8，以优异产率得到Cp*M[N(i-Pr)C(Me)N(i-Pr)](CNAr)2（化合物13：M=Mo；化合物14：M=W）与Cp*M[N(i-Pr)C(Me)N(i-Pr)](CO)2（化合物15：M=Mo；化合物16：M=W）。对化合物13~16进行单晶X射线衍射分析，并结合其降低的红外振动νCN或νCO伸缩频率，有力证明了Cp*M[N(i-Pr)C(Me)N(i-Pr)]片段具有富电子特性，可与中等到强的π受体配体（如N2、异氰酸酯与CO）发生较强的反馈π键作用。本研究的全部结果，从空间位阻与电子效应两方面分析了影响μ-N2配位偏好模式与N≡N键活化程度（包括完全N-N键断裂）的可能因素，涉及目前已通过实验获得的配体中心同构系列{Cp*M[N(i-Pr)C(Me)N(i-Pr)]}2(μ-N2)（其中M=Ti、Zr、Hf、Ta、Mo与W）的相关研究。