Bobadova-Parvanova Petia, Wang Qingfang, Quinonero-Santiago David, Morokuma Keiji, Musaev Djamaladdin G
Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
J Am Chem Soc. 2006 Sep 6;128(35):11391-403. doi: 10.1021/ja057937q.
The mechanisms of dinitrogen hydrogenation by two different complexes--(eta(5)-C(5)Me(4)H)(2)Zr(mu(2),eta(2),eta(2)-N(2)), synthesized by Chirik and co-workers [Nature 2004, 427, 527], and {[P(2)N(2)]Zr}(2)(mu(2),eta(2),eta(2)-N(2)), where P(2)N(2) = PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh, synthesized by Fryzuk and co-workers [Science 1997, 275, 1445]--are compared with density functional theory calculations. The former complex is experimentally known to be capable of adding more than one H(2) molecule to the side-on coordinated N(2) molecule, while the latter does not add more than one H(2). We have shown that the observed difference in the reactivity of these dizirconium complexes is caused by the fact that the former ligand environment is more rigid than the latter. As a result, the addition of the first H(2) molecule leads to two different products: a non-H-bridged intermediate for the Chirik-type complex and a H-bridged intermediate for the Fryzuk-type complex. The non-H-bridged intermediate requires a smaller energy barrier for the second H(2) addition than the H-bridged intermediate. We have also examined the effect of different numbers of methyl substituents in (eta(5)-C(5)Me(n)H(5)(-)(n))(2)Zr(mu(2),eta(2),eta(2)-N(2)) for n = 0, 4, and 5 (n = 5 is hypothetical) and (eta(5)-C(5)H(2)-1,2,4-Me(3))(eta(5)-C(5)Me(5))(2)Zr(mu(2),eta(2),eta(2)-N(2)) and have shown that all complexes of this type would follow a similar H(2) addition mechanism. We have also performed an extensive analysis on the factors (side-on coordination of N(2) to two Zr centers, availability of the frontier orbitals with appropriate symmetry, and inflexibility of the catalyst ligand environment) that are required for successful hydrogenation of the coordinated dinitrogen.
通过密度泛函理论计算，比较了由Chirik及其同事[《自然》2004年，第427卷，第527页]合成的两种不同配合物——[(η⁵-C₅Me₄H)₂Zr]₂(μ₂,η₂,η₂-N₂)和由Fryzuk及其同事[《科学》1997年，第275卷，第1445页]合成的{[P₂N₂]Zr}₂(μ₂,η₂,η₂-N₂)（其中P₂N₂ = PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh）对二氮加氢的机理。实验已知前一种配合物能够向侧面配位的N₂分子添加不止一个H₂分子，而后一种配合物添加的H₂分子不超过一个。我们已经表明，这些二锆配合物反应活性的观察差异是由于前一种配体环境比后一种更刚性这一事实导致的。结果，添加第一个H₂分子会产生两种不同的产物：Chirik型配合物的非H桥连中间体和Fryzuk型配合物的H桥连中间体。非H桥连中间体进行第二次H₂添加所需的能垒比H桥连中间体小。我们还研究了[(η⁵-C₅MeₙH₅⁻ₙ)₂Zr]₂(μ₂,η₂,η₂-N₂)中n = 0、4和5（n = 5为假设情况）以及[(η⁵-C₅H₂-1,2,4-Me₃)(η⁵-C₅Me₅)₂Zr]₂(μ₂,η₂,η₂-N₂)中不同甲基取代基数目的影响，并表明这类所有配合物都将遵循类似的H₂添加机理。我们还对配位二氮成功加氢所需的因素（N₂与两个Zr中心的侧面配位、具有适当对称性的前沿轨道的可用性以及催化剂配体环境的刚性）进行了广泛分析。
