Iron Complexes of a Macrocyclic N-Heterocyclic Carbene/Pyridine Hybrid Ligand

Abstract

The tetradentate ligand system L 1 combines two Nheterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL 1 (MeCN) 2 ](PF 6 ) 2 (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (ΔH ⧧ = 15.1 kcal mol −1 , and ΔS ⧧ = −4.7 cal mol −1 K −1 ). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL 1 (CO)(MeCN)](PF 6 ) 2 (2) or neutral [FeL 1 (N 3 ) 2 ] (3). The strong ligand field of the NHC/pyridine hybrid ligand imparts low-spin states (S = 0) for all iron(II) complexes 1−3. Mossbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe−C NHC bonds in the basal plane constituted by the macrocyclic ligand L 1 . Oxidation of 1 has been monitored by UV−vis spectro-electro chemistry, and the resulting iron(III) complex [FeL 1 (MeCN) 2 ](PF 6 ) 3 (4) has been isolated after chemical oxidation. SQUID and Mossbauer data have shown an S = 1 / 2 ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL 1 } core are basically invariant with respect to changes in the metal ion's oxidation state. Density functional theory calculations support the experimental findings. The combined structural, spectroscopic, and electrochemical data for 1 with its {C 2 N 2 } hybrid ligand L 1 allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C 4 } tetracarbene ligand. N-Heterocyclic carbenes (NHCs) have attracted a great deal of attention over the past two decades. NHCs, most of them imidazol-2-ylidenes, are now established as a prime ligand class in organometallic chemistry, with numerous applications in homogeneous catalysis.

A trans-1,2 End-On Disulfide-Bridged Iron–Tetracarbene Dimer and Its Electronic Structure

“…However, the IR spectrum of a related structurally authenticated iron(II) complex with a macrocyclic N-heterocyclic carbene/pyridine hybrid ligand and two MeCN in axial positions showed no CN stretching mode despite being structurally very similar to 1. 22 DFT calculations of the IR spectra revealed the presence of a CN stretching band at 2231 cm −1 for 1 (close to the experimental value of 2258 cm −1 ) and at 2257 cm −1 for 2, but the signal intensity for 2 was calculated to be rather weak compared to 1 ( Figure S4). We therefore conclude that the coordination of certain ligands to the iron center, for example, Fe−S or Fe−N py , may effect vibrational quenching of the CN mode of coordinated MeCN in these systems.…”

“…22 We mention that comparable Mossbauer data are available for only one other complex with a trans-1,2-disulfide-bridge, namely, for the [Fe 2 (μ-S 2 )(P(o-C 6 H 4 S) 3 ) 2 ] 2− anion with tetradentate {S 3 P} capping ligands and five-coordinate iron sites showing δ = 0.35 mm/s and ΔE Q = 2.04 mm/s. 4 Although not explicitly assigned there, the isomer shift is more common for iron(III) in high-spin state; 23 in particular, it compares quite well with those of the tetrahedral ferric sites in typical iron− sulfur clusters with δ ≈ 0.3 mm/s.…”

Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

“…147 Our group has reported the synthesis and characterisation of a series of iron complexes supported by cyclic and acyclic tetradentate N-heterocyclic carbene ligands 71-74 ( Figure 12). [148][149][150][151] The octahedral coordination geometry of iron is completed by coordination of labile acetonitrile solvent molecules. For ligands 71a, 71b, 73, and 74 the iron complexes exhibit trans-labile sites, for 71c and 72b cis-labile sites as a consequence of the coordination mode of the respective tetradentate ligand.…”

Molecular Calcium Hydride: Dicalcium Trihydride Cation Stabilized by a Neutral NNNN‐Type Macrocyclic Ligand

“…[12] Thes tability of the cationic calcium hydride 3 was investigated by an NBO analysis using DFT calculations at the same level as for 2.The optimized structure compares well with the crystal structure.E xpectedly,n ob onding molecular orbitals connect the calcium atoms.S trong two-electron three-center bonds between the calcium and hydrogen atoms stabilize the calcium hydride complex (see the Supporting Information). [ [14] Scheme 2. Synthesis of cationic calcium trihydride 3.…”