Synthesis and Protonation of an Encumbered Iron Tetraisocyanide Dianion

Reported here are synthetic studies probing highly reduced iron centers in an encumbering tetraisocyano ligand environment. Treatment of FeCl2 with sodium amalgam in the presence of 2 equiv of the m-terphenyl isocyanide CNArMes2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) produces the disodium tetraisocyanoferrate Na2[Fe­(CNArMes2)4]. Structural characterization of Na2[Fe­(CNArMes2)4] revealed a tight ion pair, with the Fe center adopting a tetrahedral coordination geometry consistent with a d10 metal center. Attempts to disrupt the cation–anion contacts in Na2[Fe­(CNArMes2)4] with cation-sequestration reagents lead to decomposition, except for the case of 18-crown-6, where a mononuclear complex featuring a dianionic 1-azabenz­[b]­azulene ligand was isolated in low yield. Formation of this 1-azabenz­[b]­azulene is rationalized to proceed by an aza-Büchner ring expansion of a CNArMes2 ligand mediated by a coordinatively unsaturated Fe center. Disodium tetraisocyanoferrate Na2[Fe­(CNArMes2)4] is readily protonated by trimethylsilanol (HOSiMe3) to produce the monohydride ferrate salt, Na­[HFe­(CNArMes2)4], the anionic portion of which serves as an isocyano analogue of the hydrido-tetracarbonyl metalate [HFe­(CO)4]−. Treatment of Na­[HFe­(CNArMes2)4] with methyl triflate (MeOTf; OTf = [O3SCF3]−) at low temperature in the presence of dinitrogen yields the five-coordinate Fe(0) complex Fe­(N2)­(CNArMes2)4. The formation of Fe­(N2)­(CNArMes2)4 in this reaction is consistent with the intermediacy of the neutral tetraisocyanide Fe­(CNArMes2)4. The decomposition of Fe­(N2)­(CNArMes2)4 to the dimeric complex [Fe­(η6-(Mes)-μ2-C-CNArMes)]2 and a seven-membered cyclic imine derived from a CNArMes2 ligand is presented and provides insight into the ability of CNArMes2 and related m-terphenyl isocyanides to stabilize zerovalent four-coordinate iron complexes in a strongly π-acidic ligand field.

本文报道了针对位阻型四异氰配体环境中高度还原铁中心的合成研究。在2当量间三联苯异氰配体CNArMes2（m-terphenyl isocyanide CNArMes2，ArMes2 = 2,6-二(2,4,6-三甲基苯基)苯基）存在下，以钠汞齐还原FeCl2，可制得四异氰合铁(0)酸钠Na2[Fe(CNArMes2)4]。对该配合物的结构表征显示其为紧密离子对，铁中心采取四面体配位几何构型，与d10电子构型金属中心的配位特征相符。尝试使用阳离子螯合试剂破坏Na2[Fe(CNArMes2)4]中的阴-阳离子相互作用，结果均导致配合物分解，唯有使用18-冠-6（18-crown-6）时例外：此时以低收率分离得到一种单核配合物，其配体为双阴离子型1-氮杂苯并[b]薁。该1-氮杂苯并[b]薁的生成可通过配位不饱和铁中心介导的CNArMes2配体的氮杂-Büchner环扩环反应（aza-Büchner ring expansion）得到合理解释。四异氰合铁(0)酸钠Na2[Fe(CNArMes2)4]可与三甲基硅醇（trimethylsilanol, HOSiMe3）发生质子化反应，生成一氢合铁(0)酸钠盐Na[HFe(CNArMes2)4]，其阴离子部分可视为氢合四羰基金属酸盐[HFe(CO)4]^−的异氰类似物。在低温及氮气氛围下，用三氟甲磺酸甲酯（methyl triflate, MeOTf；OTf 代表三氟甲磺氧基 [O3SCF3]^−）处理Na[HFe(CNArMes2)4]，可得到五配位零价铁配合物Fe(N2)(CNArMes2)4。该反应中Fe(N2)(CNArMes2)4的生成过程与中性四异氰合铁(0)配合物Fe(CNArMes2)4作为反应中间体的机制一致。本文还报道了Fe(N2)(CNArMes2)4分解为二聚体配合物[Fe(η6-(Mes)-μ2-C-CNArMes)]2以及由CNArMes2配体衍生得到的七元环亚胺的过程，该结果为理解CNArMes2及相关间三联苯异氰配体在强π酸性配体场中稳定零价四配位铁配合物的能力提供了重要研究视角。