Bis-N-Heterocyclic Carbene (NHC) Stabilized η⁶-Arene Iron(0) Complexes: Synthesis, Structure, Reactivity, and Catalytic Activity

Abstract

Reaction of FeCl2 with the chelating bis-N-heterocyclic carbene (NHC) bis-(N-Dipp-imidazole-2-ylidene)methylene (abbreviated {((Dipp)C:)2CH2}) (Dipp = 2,6-di-isopropylphenyl) affords the complex [FeCl2{((Dipp)C:)2CH2}] (1) in high yield. Reduction of complex 1 with excess KC8 with a 10-fold molar excess of PMe3 affords the Fe(II) complex [FeH{((Dipp)C:)2CH2}(PMe3)(η(2)-PMe2CH2)] (2) as a mixture of three stereoisomers. Complex 2, the first example of any iron(II) complex bearing mutually an NHC and PMe3 ligand, is likely obtained from the in situ, reductively generated 16 VE Fe(0) complex, [Fe{((Dipp)C:)2CH2}(PMe3)2] (2'), following intramolecular C-H activation of one of the phosphorus-bound CH3 groups. Complex 2 is unstable in aromatic solvents and forms, via a novel synthetic transformation involving intramolecular reductive elimination and concomitant PMe3 elimination, the Fe (0) arene complex [Fe{((Dipp)C:)2CH2}(η(6)-C6D6)] (4-d6) in C6D6. Complex 4-d6 represents the first example of an NHC stabilized iron (0) arene complex. The transformation from 2 to 4-d6 can be accelerated at higher temperature and at 60 °C forms immediately. Alternatively, the reduction of 1 in the presence of toluene or benzene affords the complexes [Fe{((Dipp)C:)2CH2}(η(6)-C7H8)] (3) and [Fe{((Dipp)C:)2CH2}(η(6)-C6H6)] (4), selectively and in good yields. DFT calculations characterizing the bonding situation in 3 and 4 reveal similar energies of the HOMO and LUMO orbitals, with the LUMO orbital of both complexes located on the Dipp rings of the bis-NHC. The HOMO orbital reflects a π-back-bonding interaction between the Fe(0) center and the chelating NHC ligand, while the HOMO-1 is associated with the arene interaction with the Fe(0) site. The calculations do not suggest any noninnocence of the coordinated arene in either complex. Moreover, the (57)Fe Mössbauer spectrum of 4 at 80K exhibits parameters (δ = 0.43 mm·s(-1); ΔEQ = 1.37 mm·s(-1)) which are consistent with a five-coordinate Fe(0) system, rendering 3 and 4 the first examples of well-defined authentic Fe(0)-η(6)-arene complexes of the type [Fe(η(6)-arene)L2] (L = η(1 or 2) neutral ligand, mono or bidentate). Some reactivitiy studies of 3 are also reported: The reaction of 3 with excess CO selectively yields the five-coordinate piano-stool complex [Fe{((Dipp)C:)2CH2}(CO)3] (6) in near quantitative yields, while the reaction of complex 3 with C6D6 under heating affords by toluene elimination 4-d6. The catalytic ability of 4 was also investigated with respect to amide reduction to amines, for a variety of substrates using Ph2SiH2 as a hydride source. In all cases good to excellent yields to the corresponding amines were obtained. The use of 4 as a precatalyst represents the first example of a well-defined Fe(0) complex to effect this catalytic process.

Formation and Properties of Poly(Ionic Liquid)–Carbene Nanogels Containing Individually Stabilized Silver Species

“…Through the combined effects of the PIL and NHCs, greater levels of stability/chelation on both the atomic and nanometer scales may be achieved for the development of high‐performing and reusable catalysts. Such is the case for the iron‐based catalyst developed by Driess and co‐workers, in which the chelating NHC ligand allows the unusually high stabilization and synthesis of an η 6 ‐arene iron(0) complex for the catalytic reduction of amines. Previously, we have demonstrated that the hydrogen‐evolution reaction is enhanced when using atomically dispersed silver in a carbon‐nitride matrix and further illustrates the need for designing new, single‐atom‐stabilized systems…”

Amine synthesis via transition metal homogeneous catalysed hydrosilylation

“…of diphenylsilane at 70 °C in THF for 24 h. 76 While secondary and tertiary amides were successfully reduced to corresponding amines, the reduction of primary amides to primary amines is still a challenging task in iron catalysis field. …”

Synthesis of BINAM‐Based Chiral Di‐1,2,3‐triazolylidene Complexes and Application of the Di‐NHC RhI Catalyst in Enantioselective Hydrosilylation

“…[30,37] Given the resemblance of ligand 3 to these systems, we decided to probe the activity of complex (R)-5 for this application. Complexes based on various metals, [53][54][55][56][57] but especially rhodium, [9,18,58,59] are known to facilitate hydrosilylation. The conditions required for this reaction are milder than most hydrogenation reactions [52] and, in addition, the bulky nature of silanes compared with H 2 may lead to improved enantioselectivities.…”