A Redox‐Active Heterobimetallic N‐Heterocyclic Carbene Based on a Bis(imino)pyrazine Ligand Scaffold

Abstract A new redox‐active N‐heterocyclic carbene (NHC) architecture is obtained using N‐methylated pyrazinediimine iron complexes as precursors. The new species exhibit strong π‐accepting/σ‐donating properties and are able to ligate two metal centres simultaneously. The redox activity was demonstrated by the reversible chemical oxidation of a heterobimetallic Fe0/RhI example, which affords an isolable ligand‐based radical cation. The reversible redox process was then applied in the catalytic hydrosilylation of 4,4′‐difluorobenzophenone, where the reaction rate could be reversibly controlled as a function of the catalyst oxidation state. The new NHC exhibits high electrophilicity and nucleophilicity, which was demonstrated in the reversible activation of alcohols and amines. The electronic structure of the resulting complexes was investigated through various spectroscopic and computational methods.


Introduction
N-heterocyclic carbenes (NHCs) have had as ignificant impact on the field of organometallic chemistry and of homogenous catalysis,w here they have become widespread ligands for am yriad of transformations.K ey to their success was the ability to accommodate alarge number of transition metals,aswell as facile tunability of steric bulk and electronic s-donating/p-accepting properties.T he parametrisation of these properties through the buried volume (%V bur )o rt he Tolman Electronic Parameter (TEP) has provided useful tools for the design of powerful new catalytic systems, [1] allowing chemists to choose the most suitable candidates from ap lethora of possibilities. [2] In the instances where af acile and significant change in electronic properties is desired, whilst keeping the steric environment unaltered, ac onvenient strategy is the installation of ar edox switch. Redox activity is typically achieved through an organic (e.g. naphthoquinone) [3] or organometallic (most commonly sandwich-type structures,f or example,f errocene) [4] redox-active fragment. In the latter case,t he Fe II /Fe III reversible couple enables the modulation of the NHC electronic properties upon reversible oxidation of the ferrocene backbone, [5] which in turn expands their catalytic scope,i nc omparison to classical NHCs. [6] Moreover,t heir ambiphilicity makes them excellent tools for small molecule activation, [7] and enables them to ligate av ariety of metals,e xpanding therefore the tool-box available for redox-switch catalysis. [4][5][6] Nevertheless, the prevalence of ferrocene as ar edox-switch narrows the potential-window needed to be applied for the redox chemistry to occur,making them largely dependent on those of the Fc 0 /Fc + (Fc = ferrocene) couple.H erein, we wish to introduce an ew,n on-ferrocene based redox-active carbene architecture,w hich makes use of aF e 0 -ligated pyrazinediimine ligand (P Pz DI), where both the iron centre and the ligand framework can be involved in the redox activity. [8] We envisaged that the new ligand scaffold would offer the following advantages: (a) as the formally Fe 0 centre in PDI/ P Pz DI-type ligands (PDI = pyridinediimine,P Pz DI = pyrazinediimine) is more easily oxidised than the Fe II centre in ferrocene,m ilder oxidation conditions would allow access to the oxidised form [9] (b) while in ferrocene,t he iron centre is more reluctant to engage in reactivity,i ron-PDI complexes display very rich chemistry ranging from catalysis to small molecule activation, making them the systems of choice for ac onsiderable number of transformations [10] (c) the P Pz DIligand is itself redox active through the reversible reduction of the imine functionality or ligand core,t herefore allowing access to more redox states.Herein, we wish to communicate the synthesis of the new iron-P Pz DI NHC-like precursors,the redox chemistry of the corresponding rhodium complexes and examples of reversible alcohol and amine activation at the in situ generated carbene centre.

Results and Discussion
Deprotonation of the iron-based methylpyrazinium complex 1·[I] in the presence of aweakly nucleophilic base such as KO t Bu is accompanied by arapid colour change from brown to purple.The NMR spectroscopic data suggest the loss of the C 2v symmetry in solution, and the formation of adduct 3 through aformal nucleophilic attack on the a-C (Scheme 1). Increasing the steric bulk of the base,b ye mploying KN-(SiMe 3 ) 2 and even Li(OEt) 2 NCy t Bu [11] does not prevent the addition of the poor nucleophiles even when the reaction is conducted at À40 8 8C, and the corresponding adducts 4 and 5 could be observed by 1 HNMR spectroscopy in [D 6 ]benzene or [D 8 ]THF solutions,suggesting also that the pyrazine core is dearomatised (vide infra). While stable in solution for at least 24 h, complexes, 3-5 cannot be isolated as solids:removal of the respective solvent under vacuum, followed by re-dissolving the reaction mixture in the same NMR solvent shows acomplex mixture of species.T he reactivity pattern suggests that the addition of the weakly nucleophilic base is reversible, and points towards an unstable NHC 2 intermediate,g enerated upon subjecting the reaction mixture to high vacuum. [12] Formal reductive elimination from NHC derivatives on steric grounds is documented in the literature,a nd in the case of 3-5 we assume that the steric bulk of the added base is the driving force for regenerating the free carbene. [13] We would like to point out, that while analysing the degradation of 2,w ecould not observe any evidence for aW anzlick-type dimerization;t he resulting reaction mixture likely contains paramagnetic species as judged by the broad signals observed by 1 HNMR spectroscopy.
Thereactivity of 2 with very poor nucleophiles testifies to the strong electrophilic character of the transient carbene species.I no rder to further assess this characteristic experimentally, 4 was treated with elemental selenium, which allowed the clean conversion to the selenourea derivative 6 (Scheme 2). Interestingly,even though 4 bears aformally Fe 0 centre,n oo xidation of the metal was observed, and the resulting complex 6 exhibits well-resolved NMR resonances, typical for ad iamagnetic compound. Selenium NMR chemical shifts of selenoureas are an established method to measure the p-acidity of NHCs. [1,14] In this respect, ameasured 77 Se NMR signal located at 535 ppm, places 2 significantly downfield compared to the established IPr (d Se = 87 ppm) and SIPr congeners (d Se = 181 ppm), [1] and between Bertrands5memberd ring cyclic(alkyl)(amino)carbenes (5-cAAC) (d Se = 492 ppm) and bicyclic(alkyl)(amino)carbenes BICAAC (d Se = 645 ppm), [15] ac haracteristic that corroborates well with the high electrophilicity observed experimentally. [16] To further explore the electronic properties of 2,wehave synthesised the corresponding Rh complexes.A sd escribed previously,the hexamethyldisilazane (HMDS) adduct proved to be agood precursor for 2,and reacting afreshly prepared solution of 4 with either [Rh(COD)Cl] 2 or [Rh(CO) 2 Cl] 2 readily afforded the heterobimetallic Fe/Rh complexes 7 and 8 (Scheme 2). [17] TheR h-carbonyl complex 8 could also be generated by placing 7 under one atmosphere of carbon monoxide.T he facile COD displacement testifies to the strong trans-effect exerted by the carbene ligand towards the RhÀCOD bond, reducing the energetic barrier for the ligand exchange reaction. Both 7 and 8 display well-resolved NMR spectra, suggesting that the closed-shell singlet state of the formally Fe 0 centre is not changed by the introduction of as econd metal such as rhodium. Particularly informative is the 13 CNMR chemical shift of the rhodium ligated carbene Catom, which exhibits ac haracteristic low field doublet (in 7: d C = 204.4 ppm, 1 J RhC = 42 Hz;i n8: d C = 185.5 ppm, 1 J RhC = 37.5 Hz). 15 NNMR chemical shifts for 6-8 suggest that the pyrazine ring retains aromaticity (d N (NMe) = 170.6 ppm in 6, 176.1 in 7,171.4 in 8,similar to 145.5 in [1]·I). [18] Theaverage value of the Rh-carbonyl stretching frequencies (n av Rh-CO )i s generally used to measure the overall donor capabilities of NHCs.D ichloromethane solutions of compound 8 exhibit three main CO stretching frequencies,a saresult of the overlapping between the Fe-CO and Rh-CO vibrational modes. [1,2] Based on comparison with our previous compounds and DFT calculations (vide infra), we could assign the stretching frequencies located at 2074 and 1994 cm À1 to the Rh-CO fragments (n av Rh-CO = 2034 cm À1 ,T EP = 2047 cm À1 ). Thea verage value suggests that despite the high electrophilicity, 2 has high electron donating capacity-higher than the standard imidazolin-2-ylidene and imidazolidin-2-ylidene derivatives,but lower than certain cyclic alkyl amino carbenes (cAACs). Nevertheless,t aken together,t he spectroscopic data suggest that 2 has an ambiphilic character. [1,19] The ambiphilicity can be rationalised in the unique combination of ap yrazine ring and aF e(CO) 2 fragment. While the diazine ring is p-acidic,t he iron-ligated nitrogen atom is unable to stabilise the carbene formally empty p p orbital as it is already engaged in interaction with the iron centre (vide infra). To the best of our knowledge,using apyrazine ring as ascaffold for the generation of NHCs to give well-defined metal complexes is so far unreported. [20] Themolecular structures of 7 and 8 could be determined by single crystal X-ray diffraction ( Figure 1). [21] Themolecular structure of 7 reveals that the pyrazine ring retains its aromaticity [22] and is essentially flat. TheN -Ca-C angle is nevertheless compressed (]114.29 (14)8 8 in 7,c ompared to Scheme 2. NHC 2 was generated in situ as described in Scheme 1a nd its yield was assumed 35 %. Conditions:a )Se(2equiv),T HF, 1h, quant. (NMR) b) [Rh(COD)Cl] 2 (0.5 equiv), THF, 2h,9 0% (isolated). c) [Rh(CO) 2 Cl] 2 (0.5 equiv), THF, 2h,96% (isolated).
119.03 (18) in 1·[I] [8] ). Them agnitude of the angle compression is similar to the one in the analogous isoquinolin-1ylidine rhodium(I) complexes. [23] Thediffraction data quality obtained for 8 precluded us from discussing the metric parameters;h owever, the connectivity could be established unambiguously.
In order to probe the redox-switchability of the isolated heterobimetallic complexes,w eh ave initially performed cyclic voltammetry of 7 and 8. Both complexes exhibit one quasi-reversible redox wave at very similar peak potentials (À0.6 Vin7,and À0.54 Vin8,vs. Fc/Fc + ), which we assign to the reversible redox chemistry of the formally Fe 0 centre.This is followed by two irreversible oxidation waves,w hich we tentatively assign to the oxidation of the rhodium centre. [24] Interestingly,u nlike the diaminocarbene[3]ferrocenophanes described by Bielawski and Siemeling, [5a,b] where as witch from COD to CO on the rhodium centre had am arked influence on the ferrocene Fe II /Fe III redox potentials (of ca. 370 mV), the same variation of substituents appears to have negligible influence on the redox potentials of the Fe 0 /Fe I couple.T he marked cathodic shift in diaminocarbene- [3]ferrocenophane is explained by through space Fe···Rh interactions,which would be absent in 7 and 8.Nevertheless, the attenuated shift in 7 and 8 could also be explained by CO/ COD ligand scrambling between the two metal centres in 7, which is facilitated by the labilisation of the FeÀCO bond as ar esult of the oxidation of the iron centre,e ffectively reducing the amount of backbonding interactions.
In order to verify the reversibility observed by cyclic voltammetry,w eh ave treated 8 with af errocenium salt ([Fc][BArF 24 ]) which gave rise to aparamagnetic compound 9 (Scheme 3) displaying amagnetic susceptibility characteristic for one unpaired electron (m eff = 1.91 m B ). In contrast to the diaminocarbene [3]ferrocenophanes system described by Bielawski, [5a] the oxidised 9 product is stable enough to be isolated, even though we could not obtain X-ray quality crystals despite numerous attempts.D ichloromethane solutions of 9 display three main IR stretching frequencies,a s ar esult of the overlapping between the Fe-CO (2007 and 1948 cm À1 )a nd Rh-CO (2081 and 2007 cm À1 )v ibrational modes.T hese values are shifted to higher frequencies compared to the parent compound 8,inline with the reduced amount of backbonding expected for 9. Overall, the IR data suggest asignificant alteration of the electronic properties of 9 (n av Rh-CO = 2044 cm À1 ,T EP = 2055 cm À1 )c ompared to 8 (n av Rh-CO = 2034 cm À1 ,T EP = 2047 cm À1 ), with the overall donating properties of 9 being similar to the established IMes and IPr NHCs.This variation in the TEP is comparable to the values reported for other redox-switchable NHCs. [4] Additionally,w ec ould prove that this chemical oxidation is fully reversible:treatment of 9 with one equivalent of cobaltocene cleanly regenerates 8 alongside [CoCp 2 ][BArF 24 ](Scheme 3).
As the formal oxidation of the iron centre has am arked effect on the stretching frequencies of the Rh-CO unit, we wondered whether the resulting unpaired electron resides on the iron centre,oritisdelocalised over the entire conjugated system. As aF e( or Rh) based radical would have as pecific EPR signature compared to an organic radical, 9 was investigated by X-band CW-EPR spectroscopy.T he data, recorded at 6K(Scheme 3, right), reveals apattern characteristic for aligand centred radical (S = 1 = 2 ), with no distinguishable hyperfine structure and with very small ga nisotropy, centred at g = 2.0080 (g e = 2.0023). These data suggest reduced spin density on the iron centre and are consistent with significant spin delocalisation. As ignal possessing the same characteristics could also be observed at room temperature in THF solutions,albeit with reduced intensity (see the supporting information).
In order to ascertain whether the radical delocalisation over the entire ligand scaffold is ar esult of the carbene functionality,wehave prepared [(P Pz DI)Fe(CO     contrast to 9,the X-band CW-EPR spectrum of 10 recorded at 6Kdisplays ar hombic signal (Figure 2, right), and the fit of the data yielded the following g values: g min = 1.997, g mid = 2.044, g max = 2.124, consistent with aS= 1 = 2 compound. Theg anisotropy indicates that the singly occupied molecular orbital (SOMO) is iron-based rather than ligand-based. The characteristics of 10 are similar to the pyridine-based Fe I analogue,[(PDI)Fe(CO) 2 ][BArF 24 ]r eported by Chirik. [9] Catalytic Redox Switchability We have shown that the oxidation of 8 significantly alters the electronic properties of the resulting species,a sd emonstrated by IR and EPR spectroscopy.Asthe oxidised species 9 is stable and isolable while undergoing reduction cleanly to regenerate compound 8 (Scheme 3), we wondered whether we could employ the reversible change in electronic properties in redox-switch catalysis.For demonstrating the change in reactivity between the reduced and oxidised forms,w eh ave investigated the hydrosilylation reaction of 4,4'-difluorobenzophenone in the presence of Ph 2 SiH 2 .The choice of substrate was made in order to facilitate reaction monitoring by NMR ( 19 F) and IR (C = Os tretching of the ketone,S i-H wagging mode of the silane) spectroscopy.I no rder to minimise the induction time,the more labile COD substituted [FeRh] precatalyst 7 was chosen. Initial catalytic runs were performed in the presence of neutral (7)a nd the in situ-oxidised analogue (7 + )r espectively (Scheme 5). Under the same reaction conditions,w hile full conversion of 4,4-difluorobenzophenone in the presence of 7 was observed after 12 hours, as ignificant acceleration of the reaction rate was observed when 7 + was used as ac atalyst, with full conversion after 2.5 hours being noted by NMR and IR spectroscopy.A comparison of the reaction rate constants extracted through the initial rates method indicates that the reaction catalysed by the oxidised species is one order of magnitude faster. [25] As we envisage that arhodium hydride is the catalytically active species,r educed electron density on the rhodium centre would enhance the hydridic character and facilitate the subsequent insertion step. [26,27] Taking the significant difference in the reaction rate between the oxidised and reduced forms into account, we sought to demonstrate that the change between the two kinetic regimes was possible through in situ oxidation and reduction. Conducting the catalytic hydrosilylation reaction in ac ell fitted with an IR probe allowed us to detect rapid changes in the kinetic profile as afunction of external stimuli. 24 ]a nd CoCp 2 successfully alters the reaction rate by one order of magnitude.Inline with the stability of both the neutral (7)and oxidised species (7 + ), the temporal control was depicted through three "on/off" cycles,w hich could be performed without apparent loss of catalytic activity. [25,28] These results demonstrate that PDIligands can be used as redox switches in NHC chemistry,and therefore join the family of more established redox-switches such as metallocenes and quinones. [29] Reactivity at the Carbene Centre As detailed above,reacting 1·[I] with bulky alkoxides and amides readily affords the corresponding alcohol and amine adducts 3-5,w hich act as carbene precursors,b ut cannot be isolated as solids due to the kinetic lability of the bases employed. In order to get more insight in the structure and reactivity of carbene 2,t he structure and reactivity of these adducts would provide valuable information. We therefore envisaged that replacing the bulky amine or alcohol fragments with OMe would increase the stability of the resulting adducts.R eacting derivative 3 with methanol affords indeed the OMe substituted derivative 11 in quantitative yield, alongside t BuOH as observed by 1 HNMR spectroscopy (Scheme 6).

Sequential addition of [Fc][BArF
Unlike 3-5, 11 could be isolated as as olid and recrystallised from pentane and its structure could be determined by single crystal X-ray analysis (Figure 4, top). NMR data are consistent with aformal MeOH addition to Ca,which induces dearomatisation of the pyrazine ring, evident in the upfield shift of the d N (NMe) to 94.8, when compared to the 145.5 value measured for 1·[I].T he imine character of the C 19 = N 2 bond adjacent to the MeOH is also diminished, as shown by the significant upfield shift 15   TheC Os tretches determined by solid state IR spectroscopy are also responsive to the alteration of the electronic environment and shift to lower frequencies (1949, 1882 cm À1 in 11), as expected for am ore electron rich system, which enhances backbonding from Fe to the carbonyl ligands.
Theu nconventional way of synthesising 11,t hrough af ormal alkoxide exchange from 3 in the presence of methanol has prompted us to investigate whether this exchange reaction is general. Tr eating 3, 4 or 11 with isopropanol or benzylic alcohol affords the corresponding alcohol exchange products 12 and 13 (Scheme 6). Similarly, reaction between 11 and benzylamine afforded the corresponding amine derivative 14,w ith the liberation of methanol. Even upon using an excess (2-3 equiv) of the corresponding alcohols or amines,the conversion to 13 and 14 was around 75 %and 66 %respectively,suggesting an equilibrium reaction. [31] Full conversion in the case of 12 and 13 could however be achieved when starting directly form 3.M ethoxide and isopropoxide derivatives 11 and 12 are stable and could be isolated as solids,w hereas benzylic alcohol and amine derivatives 13 and 14 show clear signs of decomposition upon solvent removal. While the fate of the iron species could not be hitherto elucidated, in the case of the benzylic alcohol adduct 13,b enzaldehyde formation could be observed by 1 HNMR spectroscopy and GC-MS.T he formation of benzaldehyde could be explained by a1,3-hydride shift followed by a-elimination. [32] Computational Chemistry To get more insight in the electronic structure of the carbene 2 and its rhodium complexes,w eh ave performed DFT calculations at aB 3LYP/def2-TZVP(-f) level of theory. [33] Forc omplexes 1, 7 and 10,g eometry optimisations produced features which were in good agreement with the metric data from X-ray crystallography,aswell as vibrational data from IR spectroscopy (See the supporting information for details). Potential ligand redox non-innocence for complexes 2, 7, 8 and 11 was investigated through brokensymmetry (BS) calculations,w here all strategies employed converged to the same BS (0,0) (i.e.c losed-shell) solution, similar to the one observed for the starting material 1·[I],a s well as (P Pz DI)Fe(CO) 2 . [8] Thed ata therefore suggest aF e 0 metal centre supported by an eutral ligand, in line with the diamagnetism observed for the investigated complexes.W e have then proceeded by examining the frontier molecular orbitals (MOs) ( Figure 5). In the case of the free carbene 2, the HOMO and the LUMO are both energetically and morphologically similar to the ones calculated for (P Pz DI)Fe-(CO) 2 ,w ith an arrow HOMO-LUMO separation (2.60 eV), most likely due to the extended p-conjugation over the

Angewandte Chemie
Research Articles 19324 www.angewandte.org pyrazine ring and imine arms.Interestingly,the carbene based E s and E p* are the HOMOÀ1(stabilised by 0.68 eV compared to the HOMO) and LUMO + 1( destabilised by 0.50 eV compared to the LUMO). This distribution of molecular orbitals is similar to the one observed for the bisoxazolinebased NHC IBioxMe 4 , [34] dipyrido-annelated NHCs (dipiy), [35] and the recently reported 1,3-di(amino)oxyallyl pyrimidine-based NHC, [36] where,i ns ome cases,t he E s (HOMOÀ1) is stabilised by ca. 0.50 eV compared to the HOMO.I nt he case of Siemelingsn eopentyl-substituted diaminocarbene [3]ferrocenophane,t he carbene E s is in the HOMO (a typical situation for the vast majority of NHCs), whereas the carbene E p* is the LUMO + 2, while the LUMO and the LUMO + 1a re ferrocene-based (see the supporting information for afull MO diagram). [37] Thecalculated singlettriplet gap (DE ST )for 2 appears significantly narrow (37.3 kcal mol À1 ), however,t he magnitude is not directly comparable with the other reported NHCs,g iven the fact that in 2,t he carbene-based orbitals are not the frontier MOs.Anatural bond orbital (NBO) analysis [38] reveals that the s-symmetry carbene lone pair (42 %s ,5 8% pc haracter) is only weakly stabilised via hyperconjugation into the N 4 -C 1 and N 3 -C 18 s* MOs (see Scheme 2f or numbering), in line with the strong sdonating properties measured experimentally.Atoms N 4 -C 17 -C 18 form a3 c/4e bond, where,i nt he "best" Lewis structure, apartially occupied p-orbital located at C 18 interacts with the strongly polarised p-orbital of N 4 -C 17 (83 %N ,1 7% C). The p-type orbital located at C 18 is also strongly delocalised into the adjacent imine bond C 19 -N 2 ,aswell as the pyrazine N 4 -C 17 p*o rbitals (see supporting information for details). These data suggest that the carbene p-symmetry orbital is strongly delocalised over the entire diiminopyrazine ligand, and this stabilization is in line with the strong p-accepting properties determined experimentally.T he bonding picture strongly resembles the one in the 1,3-imidazol-4-ylidenes,w hich belong to the category of mesoionic N-heterocyclic carbenes (MICs). [39] Nevertheless,the extent of delocalization in MICs is reduced compared to 2,a nd therefore a p*m olecular orbital that satisfies the symmetry criteria in order to accept electron density from am etal centre is too high in energy. [40] Consequently,M ICs are quite poor p-acceptors,w hereas in the case of 2,amolecular orbital of appropriate symmetry is low in energy,accounting for the good p-accepting properties (see Figure 5, LUMO + 1).
In line with the MIC-like character of 2,f our resonance structures can be envisaged ( Figure 6). Taking the NBO analysis into account, the localised structures B and C have important contributions to the overall bonding picture.While representation of the type A is common for certain MICs,i t does not reflect well the electron distribution for 2.However, as the NBO analysis suggests astrongly delocalised structure, representation D depicts this feature in the best manner.I n line with the MIC-like character,formal charges are required for the representation of all limit structures.
In the case of the neutral metal complexes 7 and 8,t he HOMO and the LUMO are very similar to the ones of the free carbene 2,w hile the HOMOÀ1i saRh-based MO with ap redominant d-character.I nterestingly,t he energy of the HOMO (ligand and Fe based) is significantly influenced by the nature of the ligand on the rhodium atom, that is,t he HOMO is stabilised by 0.26 eV in the case of 8 (À5.12 eV), which contains the strong p-accepting CO ligands,compared to 7 (À4.86 eV). As expected, this stabilisation is even more pronounced for the rhodium-based HOMOÀ1( by 0.70 eV). An NBO analysis reveals ab onding situation very similar to the one described for 2.Additionally,the carbene lone pair is involved in a3 c/4e interaction with the trans RhÀCO bond, whereas the cis RhÀCO bond is involved in a3c/4e interaction with the chloride ligand. [41,42] In the case of the formal methanol addition product 11,NBO analysis reveals that the N 2 -C 19 -C 18 (numbering found in Figure 4) form a3 c/4e electron interaction, formally in as trong donor-acceptor interaction between ap artially filled po rbital at C 18 and the p*MOofthe N 2 -C 19 imine.This feature leads to partial loss of double bond character in of the N 2 -C 19 imine and ap artial gain in double bond character for C 19 -C 18 ,afeature also verified through NMR spectroscopy and X-ray crystallography (vide supra). [37] Lastly,w ew anted to address the discrepancyb etween distribution of the unpaired electron in 9 (ligand-based) and 10 (Fe-based) observed by EPR. [43] In the case of 10,t he SOMO was found to be aF e-based MO with ap redominant d-character,while the spin-density plot obtained from aMulliken-population analysis reveals 0.71 spin density on the iron centre ( Figure 6, right). This observation is in line with the rhombic signal obtained by EPR spectroscopy.O nt he other hand, for 9,t he qualitative MO diagram and Mulliken population analysis reveals that (i) the spin density is more delocalised over the entire ligand framework, with 0.56 spin density on the iron centre (Figure 7, left) and (ii) the SOMO (À8.70 eV) is stabilised by 0.21 eV compared to the rhodiumbased HOMO (À8.49 eV). Thee xtent of spin delocalisation on the ligand explains the low ganisotropy observed by EPR spectroscopy.

Conclusion
We have shown that P Pz DI-type systems are able to support abimetallic architecture,where the heterocyclic core is able to ligate both aN NN pincer-type chelating system, while simultaneously acting as an N-heterocyclic carbene, which can coordinate to as econd metal centre.F or the heterobimetallic Fe/Rh system studied herein, the NHC exhibits strong s-donating/p-accepting properties.T he carbene exhibits aspecial bonding situation, possessing both the characteristics specific to mesoionic carbenes,e xplaining the strong s-donating properties.F urthermore,t he system possesses low lying p*m olecular orbitals of the appropriate symmetry for backbonding,due to extensive delocalization of the p electrons over the P Pz DI system. These properties can be modulated through reversible chemical oxidation. As both oxidised and reduced forms are isolable,w ec ould directly compare the effect of oxidation on the carbene properties by spectroscopic and computational methods.T hese studies reveal that, in contrast to standard iron PDI chemistry,where oxidation takes place at the metal, in the case of the heterobimetallic complexes,t he unpaired electron is evenly distributed on the ligand core,t herefore impacting the electronic properties of both metals involved. Ther eversible modulation of electronic properties was then applied in the catalytic hydrosilylation reaction of 4,4'-difluorobenzophenone,where the oxidised species shows aten-fold increase in the reaction rate.W eh ave shown that the switch between aslow-rate regime and afast-rate regime could be achieved in situ through the addition of the appropriate external stimulus (oxidising or reducing agent) We have also demonstrated the ability of the NHC-type Fe-P Pz DI fragment to reversibly activate various alcohols and amines,l ikely through successively reversible formal oxidative addition/reductive elimination steps at the Ca.The formal oxidative addition of alcohols and amines is accompanied by the dearomatisation of the pyrazinium ring. This reactivity pattern is likely due to the ambiphilicity of the systems as ar esult of combining a pacidic pyrazine system with aF e(CO) 2 fragment which prevents the second nitrogen atom from engaging in the stabilisation of the carbene atom. It remains to be established if the redox-state dependent change in reaction rate or chemoselectivity of the bimetallic species is general, and can be fostered for the design of redox-switchable bimetallic catalysts.I ti sa lso to be expected that increasing the steric bulk of the substituents at the periphery of the carbene centre would allow the isolation of the free NHC.T hese directions are currently pursued in our laboratory.