Electronic and Structural Comparisons between Iron(II/III) and Ruthenium(II/III) Imide Analogs

Abstract

To examine structural and electronic differences between iron and ruthenium imido complexes, a series of compounds was prepared with different phosphine basal sets. The starting material for the ruthenium complexes was Ru(NAr/Ar*)(PMe 3 ) 3 (Ru1/Ru1*), where Ar = 2,6-( i Pr) 2 C 6 H 3 and Ar* = 2,4,6-( i Pr) 3 C 6 H 2 , which were prepared from cis-RuCl 2 (PMe 3 ) 4 and 2 equiv of LiNHAr/Ar*. The starting materials for the iron complexes were the analogous Fe(NAr/Ar*)(PMe 3 ) 3 species (Fe1/Fe1*), which were not isolated but could be generated in situ from FeCl 2 , PMe 3 , and LiNHAr/Ar*. With both iron and ruthenium, the PMe 3 starting materials underwent phosphine replacement with chelating ligands to give new group 8 imido complexes in the +2 oxidation state. Addition of 1,2-bis(diphenylphosphino)ethane (dppe) to M1/M1* gave Ru(NAr/Ar*)(PMe 3 )(dppe) and Fe(NAr/ Ar*)(PMe 3 )(dppe). Addition of 1,2-bis(dimethylphosphino)ethane (dmpe) provided Ru(NAr/Ar*)(dmpe) 2 . A triphos ligand, {P(Me) 2 CH 2 } 3 Si t Bu ( t P 3 ), was also examined. Addition of t P 3 to Fe1 provided Fe(NAr)( t P 3 ) (Fe4), but a similar reaction with Ru1 only gave intractable materials. Oxidation of Fe4 with AgSbF 6 gave {Fe(NAr)( t P 3 )} + SbF 6 − (Fe4a). Oxidation of Ru2 with AgSbF 6 gave the unstable cation {Ru(NAr)(PMe 3 )(dppe)} + , which dimerized in the presence of acetonitrile via C−C bond formation at the aryl group C4 positions, affording {Ru(NAr)(PMe 3 )(NCMe)(dppe)} 2 + . This suggested that there was substantial radical character in the imide π system on oxidation and that an aromatic group substituted at the 4-position might provide greater stability. The cations {Fe(NAr*)(PMe 3 )(dppe)} + (Fe2a*), {Ru(NAr*)(PMe 3 )(dppe)} + (Ru2a*), and Fe4a were examined by EPR spectroscopy, which suggested differences in electronic structure depending on the metal and ligand set. CASPT2 calculations on model systems for Ru2a* and Fe2a* suggested that the large differences in electronic structure are related to the energy gap between the π-antibonding HOMO and the π-bonding HOMO-1. Both the geometry of the phosphines, which is slightly different between the iron and ruthenium analogs, and the metal center seem to contribute to this energetic difference.

Between imide, imidyl and nitrene – an imido iron complex in two oxidation states

“…The potential is in line with those for the oxidation of imido iron(III) and imido iron(II) species (z À1.3 to À0.7 V). 16,17,33 Gratifyingly, chemical oxidation of (Fig. 3a) gave a c M T value of 3.3 cm 3 mol À1 K (m eff ¼ 5.14 m B ) at 210 K with D ¼ À3.5 cm À1 and g ¼ 2.12.…”

Catalytic Carbodiimide Guanylation by a Nucleophilic, High Spin Iron(II) Imido Complex

“…The solid-state molecular structure of 2 reveals an anionic iron imido complex in which the trigonal planar iron center is retained (sum of angles = 355°, Figure ). Reduction leads to elongation of the Fe–N bond (1.779(2) Å), which is over 0.1 Å longer than in those of four-coordinate ( S = 0) iron­(II) imido complexes, ,− despite 2 having a lower coordination number. The Fe–N distance is also longer than that in the two coordinate ( S = 2) iron­(II) imido complex [(IPr)­Fe­(NAr Trip )] (Ar Trip = 2,6-bis­(2′,4′,6′-triisopropyl­phenyl)-phenyl) (1.7151(16) Å) .…”

“…Interestingly, DippNH 2 can be clearly observed in the 1 H NMR spectrum of crystalline samples of 3 , suggesting the equilibrium: Using mesitylene as the internal standard, the equilibrium constant was determined by 1 H NMR spectroscopy to be K eq ≈ 8 × 10 –3 mol/L at room temperature. While common for early metal complexes, − the formation of an imido ligand by amine loss from the corresponding diamido complex is unusual for iron. , …”

“…While common for early metal complexes, 49−51 the formation of an imido ligand by amine loss from the corresponding diamido complex is unusual for iron. 39,52 The evidence for a nucleophilic imido ligand in 2 prompted us to investigate its activity toward substrates known to be active in [2 + 2] cycloaddition reactions. In an initial demonstration, we find that 2 reacts with equimolar i PrN CN i Pr to provide the yellow complex [Ph 2 B( t BuIm) 2 Fe-( i PrN) 2 CNDipp][K(18-C-6)THF 2 ] (4) in 92% yield (Scheme 2).…”

Multireference Methods are Realistic and Useful Tools for Modeling Catalysis

“…[33] That is, MR energies are computed as single-point calculations on structures optimized at the DFT level (and MR spin ladders may simultaneously be used to inform the choice of DFT functional used to generate the reaction coordinate). This approach has been applied to study catalysis by vanadium oxide [34] and bimetallic cobalt oxide clusters, [35] iron fluorides, [36] polyoxometalates, [37] transition metal organometallic complexes (e. g. MnN4py, [28c] [(Me 2 Py 2 TACN)FeN] 2 + , [38] porphyrins, [26a,33c,e,39] iron and ruthenium imido [40] and aquo complexes, [41] ruthenium diolefin diazadienes, [42] iridium-based systems, [43] and complexes featuring short metal-metal bonds such as Mn 2 (C 6 H 6 ) 2 [44] and N,N,N-tri(2-(2pyridylamino)ethyl)amine complexes [45] ), metal-organic frameworks (MIL-100(Fe), [10b] Fe 0.1 Mg 1.9 (dobdc) 2 , [10a] ZIF-8(Fe) [31] ), and iron-based zeolites (Fe-BEA*, [46] Fe-CHA, [30] Fe-FER, [46b] Fe-ZSM5). [46b] For any given study, a significant complication is the choice of orbitals to include in the active space.…”