Division of Chemistry and Chemical Engineering , California Institute of Technology , Pasadena , California 91125 , United States.
J Am Chem Soc. 2019 May 22;141(20):8116-8127. doi: 10.1021/jacs.8b12082. Epub 2019 May 14.
Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N-to-NH conversion (NRR) mediated by synthetic iron complexes and relatedly as intermediates of NRR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR), and also of further downstream intermediates (e.g., Fe(NNH)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N species, supported by a bulky trisphosphinoborane ligand (P), generates an S = 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The H-hyperfine for PFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom ( a = 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH)] in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH)] intermediate of NRR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N-to-NH conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated NRR in a synthetic system.
铁氮二烯物种（Fe(NNH)）被提议为合成铁配合物介导的催化 N 到 NH 转化（NRR）的最早中间体，并且作为氮酶酶的 NRR 中间体。然而，由于其高度不稳定性，直接鉴定这些铁物种，无论是在催化过程中还是独立于催化过程中，都证明具有挑战性。更稳定的硅烷化重氮类似物 Fe(NNSiR)的分离，以及进一步下游中间体（例如，Fe(NNH)）的分离，尽管如此，表明 Fe(NNH)是合成系统中质子化的关键第一个中间体。在此，我们表明，由大体积三膦硼烷配体（P）支持的末端结合的 Fe-N 物种的低温质子化，生成可通过连续波（CW）和脉冲 EPR 技术检测和表征的 S = 1/2 末端 Fe(NNH)物种。源自所提出的 ENDOR 研究的 PFe(NNH)的 H 超精细值对于远程结合的 H 原子（a = 16.5 MHz）是诊断性的。在存在额外酸的情况下，Fe(NNH)物种进一步演变为阳离子[Fe(NNH)]，后者与以前由远不那么复杂的铁三（膦）硼烷催化剂介导的 NRR 中表征的[Fe(NNH)]中间体有关。虽然在目前空间位阻非常拥挤的系统中抑制了催化作用，但仍可以证明 N 到 NH 的转化。这些观察结果总体上增加了支持 Fe(NNH)作为合成系统中 Fe 介导的 NRR 的最早中间体的核心作用的观点。
