Three-Coordinate Iron(II) N-Heterocyclic Carbene Alkyl Complexes

Communication pubs.acs.org/Organometallics

Three-Coordinate Iron(II) N-Heterocyclic Carbene Alkyl Complexes Andreas A. Danopoulos,*,† Pierre Braunstein,*,† Marcel Wesolek,† Kirill Yu. Monakhov,† Pierre Rabu,*,‡ and Vincent Robert§ †

Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 Rue Blaise Pascal, F-67081 Strasbourg, France ‡ Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS) (UMR 7504 CNRS), Université de Strasbourg, 23 Rue du Loess, F-67034 Strasbourg, France § Laboratoire de Chimie Quantique, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 1 Rue Blaise Pascal, F-67008 Strasbourg, France S Supporting Information *

ABSTRACT: Mononuclear alkyl N-heterocyclic carbene complexes of the type Fe(NHC)R2, NHC = SIPri, IPri, R = CH2SiMe3, CH2C6H5, were prepared by the alkylation of “FeCl2(THF)1.5” with one equivalent per Fe of MgR2, in the presence of the NHC. Alkylation with 0.5 equiv of MgR2 under carefully controlled conditions gave Fe(NHC)(CH2SiMe3)Cl, NHC = SIPri, IPri. Magnetic and computational studies and structural determination by X-ray diffraction reveal three-coordinate high-spin (S = 2) alkyl complexes.

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intensified due to their potential in applications as singlemolecule magnets.35 Recently, iron catalytic systems for the alkenyl−aryl11 and aryl−aryl36 cross-coupling have been described, formed in situ from the reaction of a Fe source (iron halide), an excess of a nucleophilic organometallic reagent (usually Grignard), and a bulky imidazolinium salt, the latter obviously acting as a source of NHC ligand. Computational studies on the possible nature of the catalytic species supported a mono-NHC Fe complex or mono-NHC−Fe−Mg heterobimetallic assembly.36 In a study of the rational synthesis of the complexes Fe(NHC)2R2, NHC = 2,5-diethyl-3,4-dimethylimidazol-1ylidene, 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene, R = Me, CH2SiMe3, the attempted reaction of Fe(NHC)2Cl2 with MesMgBr (Mes = mesityl) led to the unusual, three-coordinate complex Fe(NHC)(Mes)2; the authors ascribed its formation to a combination of the size of the mesityl ligand and the lability of the NHC on FeII.27 Other four-coordinate Fe NHC halide complexes featuring chelating ligands have appeared very recently but not their alkyl or aryl derivatives.37,38 We have been interested in the organometallic chemistry of Fe supported by NHC ligands,34,39−41 and herein we report preliminary results on simple “underligated” Fe NHC alkyls, which are of fundamental interest and could serve as models for catalytically active iron centers. Initial experiments showed that the reaction of SIPri and IPri with “FeCl2(THF)1.5” in THF gave almost quantitative yields (by 1H NMR) of the colorless, air-sensitive paramagnetic

omogeneous catalysis with iron complexes is currently under intense study due to the “green” credentials and the low cost of iron1 (“iron-age of catalysis”).2−16 In the majority of the cases the nature of the active catalytic species considered to involve “spectator” ligand(s) and reactive Fe−C, Fe−Si, or Fe−H bonds is speculative. Information on the coordination number, the oxidation and spin states of the iron, and their changes during catalysis is not available; likewise the possible formation of heterometallic aggregates and the occurrence of intraligand redox processes (ligand noninnocence) have been postulated but rarely established. In the majority of reactive or catalytically relevant complexes, the metal is stabilized by cyclopentadienyl, carbonyl, phosphine, bulky β-diketiminate (nacnac), and mixed donor tripodal amido- or silylphosphine ligands, although the use of Nheterocyclic carbenes is now showing an emerging potential. FeL2R2 and, even more so, FeLR2 (L= neutral 2 e− donor, R = alkyl, aryl, hydride) are very interesting, electronically unsaturated “open-shell” complexes (14 and 12 e−, respectively) lying on the borderline between Werner-type complexes and closed-shell 18 e− organometallics. The four-coordinate, mainly tetrahedral complexes have been studied in more detail (L = neutral N-donor,17−22 P-donor,23,24 O-donor25); in contrast, mononuclear three-coordinate species are very rare and exclusively include σ-aryl ligands19,22,26−29 with the exception of Fe(nacnac)R, R = alkyl, which constitute the only family of three-coordinate Fe(II) monoalkyls and exhibit remarkable electronic structures and reactivity.30−32 Threecoordinate Fe amides have also been reported.33,34 Interest in three-coordinate high-spin Fe complexes has been lately © 2012 American Chemical Society

Received: March 23, 2012 Published: May 23, 2012 4102

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[Fe(NHC)Cl(μ-Cl)]2, NHC = SIPri 1a; NHC = IPri 1b (Scheme 1). Scheme 1. Summary of the Complexes Describeda

Figure 1. Molecular structure of 3a. Thermal ellipsoids are at the 30% level; methyl groups of the DiPP are omitted. Selected bond lengths (Å) and angles (deg): Fe1−C1 2.177(3); Fe1−C28 2.056(2); N1−C1 1.336(2); C28−Fe1−C1 118.56(7); C28−Fe1−C28′ 122.89(14). a

Legend: (i) 1 equiv of NHC in THF; (ii) 1 equiv of SIPri or IPri followed by 1.2 equiv of MgR2 in THF; (iii) 1equiv of SIPri or IPri followed by 0.6 equiv of MgR2 in THF; (iv) 1.2 equiv of MgR2 in THF.

The structure of 1a features a centrosymmetric chloridebridged dimer with Fe centers in a distorted tetrahedral coordination geometry (see Figure S1 in the Supporting Information).34 Interestingly, [Fe(SIPri)Cl3][SIPriH], 2, was obtained from FeCl2 sources and SIPri in THF when proton sources were introduced into the system either adventitiously or intentionally (see Figure S2 in the Supporting Information). Three-coordinate Fe(NHC)R2 (R = CH2SiMe3, NHC = SIPri, 3a; R = CH2SiMe3, NHC = IPri, 3b; R = CH2C6H5, NHC = SIPri, 4a; R = CH2C6H5, NHC = IPri, 4b) were prepared in good yields by the reaction of “FeCl2(THF)1.5” in THF/dioxane with NHC and excess Mg(CH2SiMe3)2 or Mg(CH2C6H5)2(THF)2, respectively. Careful temperature control during and after the addition of the dialkyl magnesium improves product yields and eliminates undesired metal reduction. Complex 3a could also be prepared in reduced yields from 1a and 1 equiv of Mg(CH2SiMe3)2. Attempts to prepare 3b and 4a,b from 1a and 1b have not been studied further yet. The dialkyls were isolated as extremely air sensitive, off-white (3a, 3b) or orange (4a, 4b) crystalline materials. In toluene solution at room temperature or in the solid state they were paramagnetic. Given the paramagnetism, the peaks in their 1H NMR spectra were shifted and broadened but mostly observable;42,43 tentative assignments were possible in certain cases, aided by comparison of the spectra of the SIPri and IPri analogues and by integration. The structures of 3a and 4b are shown in Figures 1 and 2, respectively. In both dialkyls the metal is in a strictly planar, distorted trigonal geometry (sum of coordination angles at Fe 359.7− 360.0°) with some variation of the interligand angles, possibly reflecting the steric differences of the alkyl groups. Of particular interest is the magnitude of the dihedral angle between the coordination plane and the heterocyclic ring (in 3a: ca. 16.4°; in 4b: ca. 22.5°), which may have implications on the nature of Fe−NHC bonding. The Fe−CNHC bond distances show significant variation (in 3a: 2.177(3) Å; in 4b: 2.112(2) Å), in contrast to the Fe−Calkyl bond distances, which are virtually equal (in 3a, 2.056(2) Å; in 4b, 2.063(3) and 2.065(2) Å) and consistent with the literature values for the Fe nacnac complexes.30 To the best of our knowledge, 3 and 4 constitute the first examples of three-coordinate Fe complexes with two

Figure 2. Molecular structure of 4b. Thermal ellipsoids are at the 30% level. Selected bond lengths (Å) and angles (deg): C1−Fe1 2.122(2); C35−Fe1 2.063(3); C28−Fe1 2.065(2); C35−Fe1−C28 116.67(12); C28−Fe1−C1 119.88(9); C35−Fe1−C1 123.09(10); C36−C35−Fe1 109.99(19); C29−C28−Fe1 114.06(17).

terminal metal−alkyl σ-bonds; all other reported examples comprise bulky terminal aryls19,26−28 or alkyls bridging two metal centers.20,22,44 Refinement of the synthetic methodology (see Supporting Information) enabled the selective preparation of the remarkable three-coordinate Fe(NHC)Cl(CH2SiMe3) (NHC = SIPri, 5a; IPri, 5b). The off-white, extremely air sensitive complexes 5a and 5b were characterized by analytical and spectroscopic methods. The structure of 5a (Figure 3) represents a unique example of a monomeric three-coordinate alkyl chloride of iron, featuring distorted trigonal-planar geometry at Fe (sum of coordination angles at Fe is 359.9°)

Figure 3. Molecular structure of 5a. Thermal ellipsoids are at the 30% level. Selected bond lengths (Å) and angles (deg): C1−Fe1 2.127(4), Cl1−Fe1 2.2372(14), C28−Fe1 2.028(5), C28−Fe1−C1 119.58(17), C28−Fe1−Cl1 118.77(14),C1−Fe−Cl1 121.56(11). 4103

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Fe(II) ions, with interaction of the ground state with low-lying excited states. In view of the rarity of the three-coordinate geometry in Fe(II) complexes and the well-established σ-donor properties of the ligand sets, further insight into the electronic structure of the representative complexes of the type 3 and 5 was sought by computational methods. In particular, we addressed the following points: (i) the energy differences of possible spin multiplicities (S = 0, 1, 2). The two methodologies employed (relativistic density functional calculations at the GGA-BP86 and hybrid-B3LYP levels using the spin-unrestricted formalism and ab initio CASCCF, the latter to dispose of DFT limitations when applied to “open-shell” electronic configurations) produce computed structures in good agreement with the experimental and invariably S = 2 ground states. (ii) The shape and the energetic splitting of the d-type MOs (see Figure 5). In

and Fe−CNHC and Fe−Calkyl bond lengths very similar to those of 3a and 4b. Interestingly, the dihedral angle between the coordination plane and the heterocyclic ring is now ca. 85°. The origin of this conformation is under investigation. Complexes 3 and 4 decompose without melting at ca. 220 °C, while in aromatic hydrocarbon solvents they also show remarkable stability (3a is stable at 125 °C in C7D8 for 24 h, while for 4a and 4b the t1/2 in C6D6 at 70 °C is ca. 24 h). Furthermore, they lend themselves to the study of the solution dynamic behavior of NHCs on a three-coordinate Fe center. Preliminary work in this direction showed that they are very labile species, although mechanistic details are currently under investigation. For example, mixing 4a in C6D6 with IPri at room temperature resulted in fast formation of a mixture of 4a and 4b, while mixing of 3a and 4b in C6D6 resulted in the scrambling of the NHC ligands and fast formation of a mixture of 3a, 3b, 4a, and 4b. Furthermore, the dialkyls 3a/4a and 3b/ 4b react at room temperature very fast with excess Me3SiCH2Cl in C6D6 to give 1a or 1b, respectively, and with Me3SiCH2Br to give the bromo analogue of 1a or 1b. Complexes 5a and 5b are also thermally robust (unchanged at 125 °C in C7D8 for at least 12 h) and react with excess Mg(CH2SiMe3)2 in C6D6 to 3a and 3b (no metal reduction or bimetallic or -ato complex formation). Complexes 5a and 5b are also formed as products of disproportionation between 1a or 1b and equimolecular amounts of 3a and 3b in C6D6, respectively. Representative magnetic studies performed with 3b and 5a are summarized in Figure 4, which shows moments decreasing

Figure 4. Temperature variation of the χT for compounds 3b and 5a measured at 1000 Oe. The red full curves correspond to the fit of the data using the axial anisotropy model given as Supporting Information.

Figure 5. Spatial representation of UHF (unrestricted Hartree−Fock)generated α MOs with dominant Fe d-type character of the simplified structures 3asimplified and 5asimplified. The lowest lying MO shown is doubly occupied, corresponding to the depicted α and a closely similar β counterpart (not shown), each singly occupied.

gradually from 4.25 and 3.65 K emu mol−1 at 300 K to 1.7 and 1.4 K emu mol−1 at 2 K, respectively. The high-temperature values correspond to effective moments μeff = 5.8 and 5.4 μB for 3b and 5a, respectively, and fall at the high end of the range found in the literature for trigonal S = 2 FeII ions in the solid state.35,45,46 In a trigonal coordination environment with C2v symmetry and 3d electronic configuration (a1)2(a2)1(b2)1(a1)1(b1)1, i.e., (dz2)2(dyz)1(dxz)1(dx2−y2)1(dxy)1, the presence of four unpaired electrons leads to the S = 2 spin state. The orbital moment can be partially unquenched by the crystal field, which explains the higher than the spin-only value of the effective moment.35 Yet, isolated trigonal FeII ions exhibit quite strong zero-field splitting (ZFS) of the |±MS⟩ spin states, stabilizing the |±2⟩ doublet.47 ZFS is responsible for the decrease of the χT products observed in Figure 4, the population of the highest spin states decreasing at lower temperatures. The evaluation of the ZFS parameters and the magnetization versus field curves recorded at low temperatures are given as Supporting Information (Figure S3). Our findings are consistent with a paramagnetic behavior expected for isolated

this case information on the strength of the ligand field in these unusual complexes was obtained by performing unrestricted Hartree−Fock (UHF) calculations for simplified analogues of 3 and 5 (3simplified and 5simplified, respectively). Results show that, at the level of theory employed, the ligand field splitting (as measured by the energy difference Δ in Figure 5) is comparable for 3asimplified and 5asimplified (cf. magnetic measurements), but, remarkably, higher in the latter. Details of the computational approaches and results are given in the Supporting Information. In conclusion, this preliminary work shows that the bulky NHC ligands SIPri and IPri stabilize mononuclear coordinatively unsaturated three-coordinate alkyl complexes, opening the way for future studies of their bonding and reactivity of relevance to catalysis and other organometallic transformations. The lability of the NHC and the unusual thermal stability may be of relevance to the use of such complexes in catalysis or for modeling catalytically active sites. The magnetic studies point to interesting and tunable behavior. 4104

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After the submission of this communication, an article describing the synthesis and structure of analogues of 1 appeared in this journal;48 NHC alkyl complexes were not described in that paper.

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ASSOCIATED CONTENT

S Supporting Information *

Text and figures giving experimental details and characterization data of the complexes reported, crystallographic data for 1a, 2, 3a, 4b, and 5a, details of fitting of magnetic data, and details of computed structures and energetics. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]; pierre. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the CNRS, the Université de Strasbourg, and the Ministère de l’Enseignement Supérieur et de la Recherche (Paris). A.A.D. is grateful to the Région Alsace, the Département du Bas-Rhin, and the Communauté Urbaine de Strasbourg for the award of a Gutenberg Excellence Chair (2010−2011). K.Y.M. is grateful to the DFG for a postdoctoral fellowship. We also thank Dr. L. Brelot and Mrs. C. Bailly for assistance in crystallography and the University of Strasbourg High-Performance Computing Center for the provision of computational facilities.

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REFERENCES

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