Accessing Sodium Ferrate Complexes Containing Neutral and Anionic N-Heterocyclic Carbene Ligands : Structural , Synthetic and Magnetic Insights

This study reports the synthesis, single crystal X-ray crystallographic, NMR spectroscopic and magnetic characterization of a series of sodium ferrates using bis(amide) Fe(HMDS)2 as a precursor (HMDS= 1,1,1,3,3,3-hexamethyldisilazide). Reaction with sodium reagents NaHMDS and NaCH2SiMe3 in hexane afforded donor-solvent free sodium ferrates [{NaFe(HMDS)3}∞] (1) and [{NaFe(HMDS)2(CH2SiMe3)}∞] (2) respectively which exhibit contacted ion pair structures, giving rise to new polymeric chain arrangements made up of a combination of inter and intramolecular NaMe(HMDS) electrostatic interactions. Addition of the unsaturated NHC IPr (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) to 1 and 2 caused deaggregation of their polymeric structures to form discrete NHC-stabilized solvent-separated ion pairs [Na(IPr)2][Fe(HMDS)3] (3) and [(THF)3•NaIPr][Fe(HMDS)2CH2SiMe3] (4), where in both cases, the NHC ligand coordinates preferentially to Na. Contrastingly, when IPr is sequentially reacted with the single-metal reagents NaCH2SiMe3 and Fe(HMDS)2, novel heteroleptic ferrate (THF)3Na[:C{[N(2,6-Pr2C6H3)]2CHCFe(HMDS)2}] (5) is obtained. This contains an anionic NHC ligand acting as an unsymmetrical bridge between the two metals, coordinating through its abnormal C4 position to Fe and its normal C2 position to Na. The formation of 5 can be described as an indirect ferration process where IPr is first metallated at C4 position by the polar sodium alkyl reagent which in turn undergoes transmetallation to the more electronegative Fe(HMDS)2 fragment. Treatment of 5 with one molar equivalent of methyltriflate (MeOTf) led to the isolation and structural elucidation of neutral abnormal NHC (aNHC) tricoordinate iron complex [CH3C{[N(2,6-iPr2C6H3)]2CHCFe(HMDS)2}] (6) with the subsequent elimination of NaOTf, disclosing the selectivity of complex 5 to react

with the subsequent elimination of NaOTf, disclosing the selectivity of complex 5 to react with this electrophile via its C2 position, leaving its Fe-C4 and Fe-N bonds intact. The magnetic susceptibility properties of compounds 1-6 have been examined. This study revealed a drastic change of magnetic susceptibility in replacing a pure -donor from an idealized trigonal coordination environment by an NHC donating character.

Introduction
Over the past decade the chemistry of s-block heterobimetallic (ate) reagents, which combine metals of markedly different polarities has developed at a remarkable pace, with the realization of their synergic chemical profiles, which cannot be replicated by their singlemetal components. 1 Switching on cooperative effects, these ates, usually made by pairing an alkali-metal with either Mg or Zn, can display enhanced reactivities, unique selectivities and superior functional group tolerance to traditional polar organometallic reagents such as organolithium and Grignard reagents. 2 Finding widespread applications in a myriad of organic transformations, some of these multicomponent systems have emerged as versatile and potent deprotonating reagents, allowing direct magnesiation or zincation of a wide range of aromatic substrates. 3 Isolation of key organometallic intermediates involved in these transformations has provided new insights into how these bimetallic systems operate as recently shown for the unprecedented meta-meta' dimagnesiation of N,N-dimethylaniline where the supramolecular structure of the mixed-metal base templates the regioselectivity of the deprotonation process. 4 Extension of some of these studies to transition metal systems, replacing the low polarity s-block metal by another divalent metal such as Mn(II), Cr(II) or Fe(II) have already hinted at the potential of these systems to display related synergic chemistry. 5 Focusing on Fe(II) ferrate complexes, direct ferration of substituted aromatic substrates has been reported using mixed lithium-iron bases which contain the bulky amide group TMP (TMP= 2,2,6,6-tetramethylpiperidide). 6,7 Interestingly in some cases the in situ generated functionalized aryl ferrate intermediates can subsequently undergo Ni-catalyzed cross-coupling reactions with organic halides. 6 In closely related work, Mulvey has shown that structurally defined sodium ferrate [(TMEDA)NaFe(TMP)(CH2SiMe3)2] (TMEDA= N,N,N',N'-tetramethylethylenediamine) promotes the regioselective two-fold ferration of benzene at the sterically-optimal 1-and 4-positions affording a unique iron-host inversecrown complex. 5a More recently Bedford has elegantly disclosed the key involvement of homoleptic aryl ferrates in iron-catalyzed Kumada coupling processes. 8 Surprisingly, despite their synthetic relevance and the increasing attention that organoiron chemistry is currently being paid, 9 the number of structurally defined alkali-metal ferrates still remains scarce. 10 Running in parallel to this research has been that of iron complexes containing N-heterocyclic carbene ligands (NHC's), 11 which have found numerous applications in catalytic transformations, including C-C and C-N bond formation processes. 12 Although in many cases the nature of the active iron species implicated in these transformations has not been made forthcoming, the involvement of low-coordinate Fe NHC-complexes has been postulated, 13 which have sparked widespread interest in the synthesis and reactivity of this particular type of compound. 11,14 Merging these two evolving fields in synthesis, namely cooperative bimetallics and NHC-Fe chemistry, together, here we report our findings on the synthesis of a new series of NHCstabilized sodium ferrates containing the unsaturated carbene IPr (IPr = 1,3-bis-(2,6diisopropylphenyl)imidazol-2-ylidene). Combining X-ray crystallography and spectroscopic studies with SQUID magnetization investigations, we assess the constitution and reactivity of this family of complexes, unveiling a method that grants access to a three-coordinate abnormal-NHC Fe complex. with the sodium silanide (THF)2Na(SitBu3) and co-formation of disilane tBu3Si-SitBu3. 22 Having a mean value of 2.497 Å, the Na-N distances in 1 show a noticeable variation from its 1 H NMR spectrum shows a broad signal at -4.72 ppm for the SiMe3 groups, which is moved drastically upfield from the corresponding resonance found for the Fe(HMDS)2 precursor in the same deuterated solvent (at 60.27 ppm). The solution-phase magnetic moment of 1 was found to be 4.72 µB (determined by the Evans method), 30 which is close to the expected value (4.90 µB) for a high-spin (S = 2) Fe(II) centre. 31 The 1 H NMR spectrum of 2 displayed two distinct SiMe3 resonances easily assignable for the HMDS groups (at -8.57

Results and Discussion
ppm integrating for 36H) and for the monosilyl ligand (at 13.51 ppm integrating for 9H) (see Supporting Information for details). [32][33] Reactivity studies using the NHC, IPr. Next we investigated the reactivity of sodium ferrates with unsaturated NHC IPr, finding that treating hexane solutions of each bimetallic compound with equimolar amounts of this carbene led to a significant color change (from green to light brown) and the formation of insoluble products. Addition of fluorobenzene in the case of 1 and THF for 2 afforded crystals of the NHC-stabilized sodium ferrates   We next attempted the synthesis of a ferrate complex containing an anionic NHC 38   X-ray crystallographic studies established the contacted ion pair structure of 5 ( Figure 5) where the carbene has now been incorporated into the ferrate scaffold acting as an anionic ligand, coordinating through its normal C2 position to Na [Na-C2 2.510 (4)  IPr is not attached to a paramagnetic Fe(II) centre.
The formation of 5 can be rationalized in terms of a stepwise indirect ferration process.
Initially IPr is deprotonated at its C4 position by the polar organosodium reagent to form Na + IPr − , which in turn undergoes transmetallation with the more electronegative iron bis(amide). Although the white powder obtained by reacting IPr with NaCH2SiMe3 cannot be characterized spectroscopically due to its complete lack of solubility in organic solvents such as THF or toluene, the isolation of 5 provides compelling proof that the metallation of the NHC has occurred. 43 C4-deprotonation of unsaturated NHCs constitutes one of the main synthetic routes to access anionic (or ditopic) NHCs. 38 The molecular structure of 6 was established by X-ray crystallographic studies ( Figure 6).

Magnetic studies
The electronic structure of the Fe(II) centres in 1-6 was studied through bulk magnetization measurements. Thus, molar paramagnetic susceptibility ( M) data were collected on microcrystalline samples in the 2 to 300 K temperature range, under a constant magnetic field of 0.5 T (0.1 T in the case of 3), in the warming mode. The results are represented in Figure 7 in form of MT vs T curves. In all cases, the MT product at 300 K is slightly higher than the  The results from the fitting are collected in Table 1. For compounds 1 and 3, the best fits reveal the presence of axial zero-field splitting (D values of +7.5 and +8.3 cm 1 , respectively, and E = 0) with anisotropic g values where gx = gy > gz. These parameters are in accordance with the consistency criterion derived from perturbation theory defined as D = 0.5 (gz gx), 53 taking a value of the spin-orbit coupling parameter for Fe(II) close to the that of the free ion ( 102 cm 1 ). 54  In contrast to this, the fits for compounds 5 and 6 led to negative D values ( 15.5 and 17.7 cm 1 , respectively) with the appearance of a rhombic zero-field splitting parameter, E (±4.2 and ±0.7 cm 1 ). The latter is the natural consequence of the deviation from the idealized trigonal environment around the metal ion resulting from the presence of NHC ligands, which introduce a new interaction with the metal d orbitals due to donating character of the heterocycles. In fact, this not only generates rhombicity but changes the sign of the anisotropy, turning axial instead of easy plane. A drastic change in anisotropy of 3d metals as a result of changes to the -donor properties of ligands as observed here, has been previously predicted theoretically. 56 The anisotropy to the g factor (Table 1)

Conclusions
The new solvent-free sodium ferrates 1 and 2 have been synthesized straightforwardly by cocomplexation of Fe(HMDS)2 with the sodium reagents Na(HMDS) and NaCH2SiMe3 respectively. The complicated polymeric arrangements of 1 and 2 can be broken down by introducing unsaturated carbene IPr to form the discrete NHC-separated ion pair ferrate 3 and partially NHC-separated (THF is also needed) ion pair 4 respectively. In these complexes the IPr neutral donor coordinates preferentially to the Na atom, while more Lewis acidic Fe is coordinated exclusively to anionic ligand sets. Interestingly, 3 and 4 were also obtained when the NHC complex [IPrFe(HMDS)2] was treated with Na(HMDS) and NaCH2SiMe3 respectively. Contrastingly, sequentially reacting IPr with NaCH2SiMe3 then

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By comparing two alternative bimetallic approaches, two different types of sodium ferrate complexes have been realized. Reaction of NHC IPr with mixed Na/Fe bimetallic compounds, gives donor-acceptor coordination products. Contrastingly, illustrating how metal pairs can also work in an stepwise synergistic manner, sequential treatment of IPr with a sodium alkyl followed by the addition of an iron bis(amide) affords a novel ferrate containing a anionic NHC ligand, resulting from an indirect ferration of process.