A Stable Homoleptic Organometallic Iron(IV) Complex

Abstract A homoleptic organometallic FeIV complex that is stable in both solution and in the solid state at ambient conditions has been synthesized and isolated as [Fe(phtmeimb)2](PF6)2 (phtmeimb=[phenyl(tris(3‐methylimidazolin‐2‐ylidene))borate]−). This FeIV N‐heterocyclic carbene (NHC) complex was characterized by 1H NMR, HR‐MS, elemental analysis, scXRD analysis, electrochemistry, Mößbauer spectroscopy, and magnetic susceptibility. The two latter techniques unequivocally demonstrate that [Fe(phtmeimb)2](PF6)2 is a triplet FeIV low‐spin S=1 complex in the ground state, in agreement with quantum chemical calculations. The electronic absorption spectrum of [Fe(phtmeimb)2](PF6)2 in acetonitrile shows an intense absorption band in the red and near IR, due to LMCT (ligand‐to‐metal charge transfer) excitation. For the first time the excited state dynamics of a FeIV complex was studied and revealed a ≈0.8 ps lifetime of the 3LMCT excited state of [Fe(phtmeimb)2](PF6)2 in acetonitrile.

High-valent iron complexes, both heme-a nd non-heme,f unction as active key intermediates in various biological catalytic cycles and important organic transformations. [1] This has inspired the developmento fs ynthetic chemistry of Fe IV complexes. [1,2] Several heteroleptic high-valent Fe IV complexes have been reported, where the high oxidation state is stabilized by terminal p-donating auxiliary ligands( PDALs) such as oxide,n itride, imide, isocyanide,a nd ketimide,w ith variouss tabilities. [3][4][5][6][7] In particular, Fe IV -oxo complexes have been found capable of various oxidative transformations. [8] Examples of Fe IV coordination compounds withouts tabilizing PDALs are scarce however.B esides FeF 4 ,t hat could be isolated in am atrix, [9] there are only few examples of more traditional Werner complexesusing for example, electron-donating dithiocarbamate ligandst os tabilize the iron centeri no xidations tate Fe IV , [10][11][12] or Fe IV complexes containing multidentate macrocyclic tetraamide ligands, [13] aF e IV cyclam-azidec omplex [14] and aF e IV cyanocomplex based on at ridentate imino-ligand, [15] the two latter electrochemically generated and studied in situ. To date, the exceptional example of an entirely stable Fe IV complex belongs to the class of coordination cage compounds; Fe IV hexahydrazide clathrochelate. [16] This compound shows infinite stabilityi n both aqueousa nd non-aqueous solutions as well as in the solid state, unique to synthetic Fe IV complexes.
[Fe(phtmeimb) 2 ](PF 6 ) 2 is stable in the solid state exposed to air as well as in acetonitrile solution for days at ambient temperature. In addition, the crystals of [Fe(phtmeimb) 2 ](PF 6 ) 2 ,a re stable when washed with water,h owever slow decomposition was observed when water was added to asolution of the complex in acetonitrile.
The FeÀCb ond lengths are 1.99 to 2.01 ,w hich are close to that of the Fe III congener (1.96 to 2.01 ). [21] Similarly,t he C-Fe-C bite angles (87.38 to 87.78)f or [Fe(phtmeimb) 2 ](PF 6 ) 2 are similar to the Fe III congener (86.28 to 87.68). [21] Thus, [Fe-(phtmeimb) 2 ](PF 6 ) 2 exhibits ac lose to perfect octahedral geometry that is virtually identical to its Fe III salt (details in section S4, Supporting Information). This indicates that very little structural re-organization energy is needed when alteringb etween the Fe IV /Fe III oxidation states in [Fe(phtmeimb) 2 ], thus facilitating rapid electron transfer processes involving this couple. The very similar FeÀCd istances between the Fe IV and Fe III oxidation states, as well as the FeÀCd istances themselves, have also been observed for ah eteroleptic macrocyclic tetra-NHC complex in the two oxidations tates, as Fe IV =Oa nd Fe III -O-Fe III complexes. [25b] We suggestt hat the very minor difference in FeÀCb ond lengths is due to the rigidity of the tridentate phtmeimb ligand and/or the covalent nature of this bond, where the influence of the formal charge differences on iron is of little importance for this bond length in the latter case.
The 57 Fe Mçßbauer spectrum of [Fe(phtmeimb) 2 ](PF 6 ) 2 is shown in Figure 3a). The isomer shift d and electric quadru-   23(1)a nd 3.04(1) mm s À1 ,r espectively,t he former in the same range as for complexes 1-3,a nd the latter in the same range as for complex 4, [18] and differ from the doublet at 87 Ko ft he Fe III congener,[ Fe(phtmeimb) 2 ](PF 6 ), À0.09 and 1.54 mm s À1 ,r espectively. [21] The combination of an unusual large D-value and an egative d-value fort he doublet supports, that this pattern emanatesf rom Fe IV tripletl ow spin S = 1i naquasi-octahedral coordination (Section S5, Supporting Information). [27,28] The magnetic susceptibility and magnetization data for [Fe-(phtmeimb) 2 ](PF 6 ) 2 are reportedi nF igure3b. The distinct nesting of the magnetization curves (Figure 3b,i nsert) differs from the response of the Fe III precursor and clearly demonstrates the system to have an effective S > 1 = 2 with as ignificant zero field splitting. The formulation of the complex as al ow-spin Fe IV is corroborated by these magnetic data (Section S6, Supporting Information). From the magnetization data, as izeable ZFS of D % 22 cm À1 was deduced. EPR of [Fe(phtmeimb) 2 ](PF 6 ) 2 , generated electrochemically from the Fe III congener,d oes not show anyE PR signal at X-band frequencies, in either perpen-dicular or parallel mode (SectionS7, Supporting Information). This is explained by the magnitude and likelyp ositive sign of the determined d-value which indeedp recludes detection of EPR signals at any accessible frequency.
The electronic absorption spectrum of [Fe(phtmeimb) 2 ](PF 6 ) 2 in deaerated acetonitrile (Figure 4) is dominated by abroad,intense absorption band peaking at 715 nm (e = 6850 m À1 cm À1 ) with as houlder around 810 nm, in excellent agreement with the reported spectrum obtained upon electrochemical oneelectron oxidation of the Fe III precursor [Fe(phtmeimb) 2 ](PF 6 ). [21] Based on electrochemical potentials of the Fe IV/III couple and ligand oxidation, the low energy absorption band of the oxidized complex was previously attributed to aL MCT transition. [21] This assignment can now be supported form the experimental data of the isolated complex and computational data (vide infra) that the transition occurs from the triplet ground state ( 3 GS) with a( t 2g 4 )e lectronic configurationt oa 3 LMCT state (t 2g 5 p L 1 ).
In previous studies of complexes [Fe(btz) 3 ] 3 + and [Fe(btz) 3 ] 2 + we showedt hat the same NHC ligand set furnishes both the Fe III and Fe II oxidations tates with exceptional lifetimes (hundreds of picoseconds) of their 2 LMCT and 3 MLCT excited states, respectively. [19,21] With the relative photostability of [Fe-(phtmeimb) 2 ](PF 6 ) 2 in acetonitrile solution( SectionS8, Supporting Information), we have the opportunity to compare the excited state dynamics following LMCT excitation of aF e IV complex to the recently reported record2 .0 ns lifetimeo ft he 2 LMCT state of aF ec omplex, featured by the Fe III congener. Transient absorption spectra following 800 nm excitation of [Fe(phtmeimb) 2 ](PF 6 ) 2 are shown in Figure 5. The pronounced ground state bleach (GSB), peaking at 715 nm, and the excited state absorption (ESA) at wavelengths below around 600 nm are readily rationalizedi nt erms of the spectral differences arising from the Fe IV to Fe III reduction. The additional excited state absorption at wavelengths above around9 00 nm can be attributed to the oxidation of the phtmeimb ligand in analogy to the spectrumo ft he LMCT excited state of [Fe-(phtmeimb) 2 ](PF 6 )t hat involves the same ligand oxidation. [21]   Quantumc hemical calculations of different relaxeds pin states corroborate the naturea nd structure of the ground state as at riplet state. Figure 6s hows the calculated ground state spin density of the 3 [Fe(phtmeimb) 2 ] 2 + complex, which, togetherw ith the calculatedM ulliken spin density on the iron for this state (Table 1) supports an assignment of the ground state as at riplet Fe IV complex with (t 2g ) 4 character,w ith some admixing of the frontier molecular orbitals with NHC-p contributions.
The synthesized and isolated [Fe(phtmeimb) 2 ](PF 6 ) 2 complex, is stable in the solid state and acetonitrile solution at ambient conditions. The facile tunability of the NHC ligand systems provide potential for stabilizationo fh igh-valent metal complexes in general.W eh ave built on Fehlhammer's initial observation, [24] and developed an organometallic high-valent Fe IV NHC complex withouta dditional stabilizing p-donating ligands, [4a,c, 5b, 25a,b,d] stable at ambient conditions, utilizing the strongly s-donating mono-anionic facial tris-NHC scorpionate ligand phtmeimb developedb yS mith. [23] The excited state dynamics of homoleptic [Fe(phtmeimb) 2 ](PF 6 ) 2 was studied, constitutingt he first of its kind study of aF e IV complex. We observedt he fundamentald ifferencei ne xcited state lifetimes for Fe III -2 LMCT (2 ns, 2.1 eV) vs. Fe IV -3 LMCT (0.8 ps, < 1.5 eV) states, which deserves further investigation beyondt he scope of the present paper.M oreover,w hile advanced designs and studies are needed for ab etter understanding of Fe IV NHC complexes, modifications that aim at further prolonging the 3 LMCT state through subtle tuning of the energy levels are also in progress. Finally,h aving access to stable Fe IV species, with long lived 3 LMCT state that can takep art in redox/photocatalytic processes/cycles, is an appealing area for future exploration.  Keywords: high-valent iron · N-heterocyclic carbenes · organometallic complexes · photophysics · transient absorption spectroscopy