Bis‑<i>N</i>‑Heterocyclic\nCarbene (NHC) Stabilized η⁶‑Arene Iron(0)\nComplexes: Synthesis, Structure, Reactivity, and Catalytic Activity

Abstract

Reaction of FeCl₂ with\nthe chelating bis-<i>N</i>-heterocyclic carbene (NHC) bis-(<i>N</i>-Dipp-imidazole-2-ylidene)­methylene\n(abbreviated {(^DippC:)₂CH₂}) (Dipp\n= 2,6-di-isopropylphenyl) affords the complex [FeCl₂{(^DippC:)₂CH₂}] (<b>1</b>) in high\nyield. Reduction of complex <b>1</b> with excess KC₈ with a 10-fold molar excess of PMe₃ affords the Fe­(II)\ncomplex [FeH­{(^DippC:)₂CH₂}­(PMe₃)­(η²-PMe₂CH₂)] (<b>2</b>) as a mixture of three stereoisomers. Complex <b>2</b>, the first example of any iron­(II) complex bearing mutually an NHC\nand PMe₃ ligand, is likely obtained from the <i>in\nsitu</i>, reductively generated 16 VE Fe(0) complex, [Fe­{(^DippC:)₂CH₂}­(PMe₃)₂] (<b>2</b>′), following intramolecular C–H activation\nof one of the phosphorus-bound CH₃ groups. Complex <b>2</b> is unstable in aromatic solvents and forms, <i>via</i> a novel synthetic transformation involving intramolecular reductive\nelimination and concomitant PMe₃ elimination, the Fe (0)\narene complex [Fe­{(^DippC:)₂CH₂}­(η⁶-C₆D₆)] (<b>4-<i>d</i></b>_<b>6</b>) in C₆D_6. Complex <b>4-<i>d</i></b>_<b>6</b> represents the\nfirst example of an NHC stabilized iron (0) arene complex. The transformation\nfrom <b>2</b> to <b>4-<i>d</i></b>_<b>6</b> can be accelerated at higher temperature and at 60\n°C forms immediately. Alternatively, the reduction of <b>1</b> in the presence of toluene or benzene affords the complexes [Fe­{(^DippC:)₂CH₂}­(η⁶-C₇H₈)] (<b>3</b>) and [Fe­{(^DippC:)₂CH₂}­(η⁶-C₆H₆)] (<b>4</b>), selectively and in good yields. DFT calculations\ncharacterizing the bonding situation in <b>3</b> and <b>4</b> reveal similar energies of the HOMO and LUMO orbitals, with the\nLUMO orbital of both complexes located on the Dipp rings of the bis-NHC.\nThe HOMO orbital reflects a π-back-bonding interaction between\nthe Fe(0) center and the chelating NHC ligand, while the HOMO-1 is\nassociated with the arene interaction with the Fe(0) site. The calculations\ndo not suggest any noninnocence of the coordinated arene in either\ncomplex. Moreover, the ⁵⁷Fe Mössbauer spectrum of <b>4</b> at 80K exhibits parameters (δ = 0.43 mm·s^–1; Δ<i>E</i>_Q = 1.37 mm·s^–1) which are consistent with a five-coordinate Fe(0)\nsystem, rendering <b>3</b> and <b>4</b> the first examples\nof well-defined authentic Fe(0)-η⁶-arene complexes\nof the type [Fe­(η⁶-arene)­L₂] (L = η^{1 or 2} neutral ligand, mono or bidentate). Some reactivitiy\nstudies of <b>3</b> are also reported: The reaction of <b>3</b> with excess CO selectively yields the five-coordinate piano-stool\ncomplex [Fe­{(^DippC:)₂CH₂}­(CO)₃] (<b>6</b>) in near quantitative yields, while the\nreaction of complex <b>3</b> with C₆D₆ under heating affords by toluene elimination <b>4-d</b>_<b>6.</b> The catalytic ability of <b>4</b> was\nalso investigated with respect to amide reduction to amines, for a\nvariety of substrates using Ph₂SiH₂ as a hydride\nsource. In all cases good to excellent yields to the corresponding\namines were obtained. The use of <b>4</b> as a precatalyst represents\nthe first example of a well-defined Fe(0) complex to effect this catalytic\nprocess.