Conversion of a Fleeting Open‐Shell Iron Nitride into an Iron Nitrosyl

Abstract Terminal metal nitrides have been proposed as key intermediates in a series of pivotal chemical transformations. However, exploring the chemical activity of transient tetragonal iron(V) nitrides is largely impeded by their facile dimerization in fluid solutions. Herein, in situ EPR and Mössbauer investigations are presented of unprecedented oxygenation of a paramagnetic iron(V) nitrido intermediate, [FeVN(cyclam‐ac)]+ (2, cyclam‐ac−=1,4,8,11‐tetraazacyclotetradecane‐1‐acetate anion), yielding an iron nitrosyl complex, [Fe(NO)(cyclam‐ac)]+ (3). Further theoretical studies suggest that during the reaction a closed‐shell singlet O atom is transferred to 2. Consequently, the N−O bond formation does not follow a radical coupling mechanism proposed for the N−N bond formation but is accomplished by three mutual electron‐transfer pathways between 2 and the O atom donor, thanks to the ambiphilic nature of 2.

. Further theoretical studies suggest that during the reaction ac losed-shell singlet Oa tom is transferred to 2.Consequently,the N À Obond formation does not follow aradical coupling mechanism proposed for the N À Nb ond formation but is accomplished by three mutual electron-transfer pathways between 2 and the Oa tom donor, thanks to the ambiphilic nature of 2.
Itisoffundamental importance to understand the structureactivity relationship of terminal metal nitrides (N 3À )b ecause of their involvement in N 2 reduction [1] and NH 3 oxidation processes. [2] Among all transition metals,iron nitrides attract particular attention owing to their biological relevance. [1a,b] To date,aplethora of iron nitrides in various iron oxidation states have been synthesized, all featuring either tetragonal or trigonal symmetry. [3] Because most trigonal iron(IV/V) nitrides are isolable,p resently the majority of reactivity investigations focus on these species,w hich indeed exhibit diversified reactivity. [3,4] Forinstance,treating such complexes with reducing agents was found to afford ammonia, [5] and they are typically capable of initiating nitrogen atom transfer to arange of nucleophiles. [5c, 6] In contrast, all tetragonal iron(V/ VI) nitrides are highly unstable and have to be prepared invariably under cryogenic conditions. [3] Forexample,influid solutions,[ Fe V N(cyclam-ac)] + (2,c yclam-ac À = 1,4,8,11-tetraazacyclotetradecane-1-acetate anion) undergoes facile dimerization to eventually release N 2 . [7] Then itrido ligands of tetragonal low-spin iron(V) nitrides feature considerable radical character,b ecause their singly occupied molecular orbital (SOMO) is one of the two FeÀN p*molecular orbitals (MOs) formed by the antibonding combinations of the Fe d xz/yz and Np x/y atomic orbitals (Scheme 1). [8] In contrast, the SOMO of the corresponding trigonal species is either of the Fe d x 2 Ày 2 or the Fe d xy based MOs,b oth perpendicular to the FeÀNi nteraction. [9] Consequently,t rigonal iron(V) nitrides are often more stable than their tetragonal analogues. Thefleeting nature of the latter poses achallenge to explore their reactivity,a nd mass spectrometry [10] or time-resolved spectroscopy has to be relied upon. [11] Using these methods, tetragonal iron nitrides were shown to activate CÀHand C=C bonds of organic substrates, [10,11] similar to their trigonal congeners. [12] Complex 2 having an S = 1/2 ground state [13] exhibits an unusual electron paramagnetic resonance (EPR) spectrum with as lightly asymmetric zero-crossing derivative signal around g ? = 1.7 and ab road negative peak at g k = 1. [8] Recently,t hese highly anisotropic g values were shown to be au nique signature of tetragonal low-spin iron(V) nitrides. [8] Thus,EPR can be used as asensitive and efficient tool to detect such transient intermediates,w hich opens up anew way to probe their chemical activity.Herein, we present in situ spectroscopic investigations of oxygenation of 2 to yield the corresponding nitrosyl species (Scheme 2b). Note that chemisorbed nitrogen atoms on metal surfaces are key intermediates of selective NH 3 oxidation for industrial NO production. [2] To the best of our knowledge,conversion of an open-shell metal nitride into am etal nitrosyl is unprecedented. Only oxygenation of diamagnetic 4d (ruthenium(IV/ VI) [14] )a nd 5d (osmium(VI) [15] and iridium(III/V) [16] )m etal nitrides has been reported (Scheme 2a), and, more importantly,the underlying mechanism remains poorly understood.
Complex 2 was generated by bulk photolysis of [Fe III -(N 3 )(cyclam-ac)](PF 6 )( 1)i nf rozen solutions with ay ield of almost 100 %, as revealed by EPR investigations (Figure 1a). [8] Upon thawing, 2 was found to decay rapidly via dimerization. [7] To intercept this pathway,alarge excess of oxygen atom transfer (OAT)agents was mixed with 1 prior to freezing and illumination. During photolysis,the formation of 2 and its reaction with the OATreactant in the solid matrix were monitored by repeated spectroscopic measurements of the frozen samples.
After one-hour irradiation of the frozen reaction mixture of 1mm of 1 and 100 equiv of isopropyl 2-iodoxybenzoate (IBX-ester) in CH 3 CN,t he EPR spectrum revealed emergence of an ew nearly isotropic signal centered at g % 2 attributed to new species 3,a long with some unreacted 2 ( Figure 1b). Numerical integration showed that the total spin concentration remained unchanged and the relative amount of 3:2 was ca. 1:9. Upon thawing, the sample in acold EtOH bath (À40 8 8C) and immediately refreezing it, 3 elicited awellresolved hyperfine structure,c oncurrent with complete disappearance of the broad signal of 2 (Figure 1c,d ). Remarkably,the EPR features of 3 match exactly those published for [Fe(NO)(cyclam-ac)] + , [17] astructurally and spectroscopically well-characterized S = 1/2 {Fe-NO} 7 complex. [18] Control experiments demonstrated that IBX-ester is photostable under the experimental conditions.S pin quantifications revealed that the thawing process led to an approximate ten-fold decrease in the total spin concentration, and that the maximal yield of 3 is only 10 %relative to 1.Wesurmised that once formed, the majority of 3 converted into an EPR-silent species in the thawed reaction mixture.
To identify the EPR-silent product, the same reaction was performed with 50 % 57 Fe-enriched 1 (5 mm)i nC H 3 CN.T he 80 Kzero-field Mçssbauer spectrum of the frozen photolyzed reaction mixture is dominated by ah ighly asymmetric and broad doublet known for 2 (Figure 2a,i somer shift d = 0.00 mm s À1 and quadrupole splitting j DE Q j= 1.72 mm s À1 ). [7] Unfortunately,the Mçssbauer spectrum of 3 (d = 0.28 mm s À1 and j DE Q j= 0.85 mm s À1 )was also found to have asimilar line shape. [17] Consequently,the overlap of the Mçssbauer features of 2 and 3 renders quantification of 3 difficult. Careful simulation analyses indicated that before thawing the sample   (4), the one-electron oxidized product of 3,b ased on its Mçssbauer parameters reported earlier. [17] However,direct generation of 4 from the reaction of 2 with IBX-ester is unlikely to happen (see below). More importantly,t reating independently prepared 3 with alarge excess of IBX-ester was found to afford 4 (Supporting Information, Figure S8). All observations thus suggested that newly formed 3 was oxidized to 4 by unreacted IBX-ester in the reaction mixture.H ence,t he yield of conversion of 2 to 3 must exceed 90 %.
TheI Rs pectrum of the final product generated by the same reaction using 15 N-14 N-14 N À as the starting material showed two bands at 1901 and 1864 cm À1 with nearly identical intensity (Figure 3a). In comparison with the N À Ostretching frequency (1903 cm À1 )r eported for 4-14 NO, [17] the two features can be unambiguously assigned to [4][5][6][7][8][9][10][11][12][13][14] NO, respectively,b ecause the measured 14 N/ 15 Ni sotope shift of 37 cm À1 is in good agreement with that (35 cm À1 )predicted by the harmonic oscillator approximation. TheE SI-MS spectrum of the reaction product also revealed that the overlapping isotope patterns of  NO have similar intensity (Figure 3b). These findings demonstrated that the N atom in the NO ligand of 3 and 4 originates from the azido ligand of 1.
We also examined the reaction of 5mm of 1 with 100 equiv of trimethylamine oxide (TMAO) in afrozen CH 3 CN/MeOH (1:1) solution. Ther eaction indeed generates 3 albeit with am uch lower yield (Scheme 2b,f or details,s ee the Supporting Information). Specifically,EPR and Mçssbauer measurements suggested that the yield of 3 is about 2% relative to 1, and the main product (93 %) is [Fe II (CH 3 CN)(cyclam-ac)] + (5), [7] thereby indicating that the OATp rocess with TMAO cannot compete with the dimerization of 2.
To gain deep insight into the reaction mechanism, we performed detailed computational investigations.T he reactions of 2 with IBX-ester and TMAOt og enerate 3 were predicted to be highly exothermic by 74.1 and 62.2 kcal mol À1 , respectively.I nc ontrast, the theoretical results revealed that the reaction of 2 with IBX-ester to yield 4 possesses am uch lower driving force (À15.7 kcal mol À1 ). Kinetically,t he computed potential energy surface (PES) for the OATp rocess with IBX-ester is always downhill (Supporting Information, Figure S9). Both factors thus render the direct generation of 4 from the reaction of 2 with IBX-ester improbable to occur. Unlike the reaction with IBX-ester,t he transformation with TMAOi sn early barrierless,y et with ap lateau in the PES upon TMAOapproaching the (FeN) 2+ core (Figure 4a). Note that the dimerization process of 2 features an even higher driving force (ca. À120 kcal mol À1 ), and the calculated PES shows ac onstant decrease in energy without ad iscernible barrier. [7] Therefore,c onsidering the reaction thermodynamics and kinetics,the OATreaction with IBX-ester has ahigher probability than that with TMAOt oc ompete with the selfdecay of 2,when alarge excess of the chosen OATagent was applied. This is why the different outcome was found for the reaction with TMAOand IBX-ester.
As shown in Figure 4c,a st he Oa tom of TMAO approaches the nitrido ligand of 2,t he O2pb ased lone pair of TMAOinthe FeÀN···O TAMO plane overlaps the unoccupied FeÀN p* xz MO,w hich forms the NO s bonding MO in the end. Theo ther lone pair perpendicular to the FeÀN···O TMAO plane interacts with the singly populated Fe À N p* yz orbital to generate the N À O p op and p* op MOs.W hen the N Fe ···O TMAO distance is shortened to 1.52 ,the O À N s bond (2.627 )in TMAOi se ssentially broken, thereby resulting in ad oubly occupied N2pb ased orbital and av acant O2pc entered orbital (Figure 4b). Theformer finally evolves to the lone pair  of trimethylamine,a nd the latter interacts with the doubly occupied Fe À Np seudo s-bonding orbital, leading to the inplane NO p bond. Theo ut-of-plane p interaction involves three electrons,b ut the unpaired electron in 2 resides in the in-plane NO p* ip MO rather than NO p* op as would be expected. As borne out from Figure 4c,the NO p* ip centered MO is stabilized by the bonding interaction between the NO p* ip and Fe d z 2 fragment orbitals,w hereas the NO p* ip based MO is an antibonding combination with respect to the FeÀNO interaction. Therefore,NOp* ip MO has lower energy than NO p* op . [17] In summary,during the reaction of 2 to 3,the OATagent essentially donates aclosed-shell singlet Oatom to 2,and the (FeN) 2+ unit functions not only as an electron acceptor but also an electron donor. Therefore,t he reaction does not follow aradical-coupling mechanism as postulated for the N 2 expulsion, [7,16] but reveals the ambiphilic nature of 2 found for related diamagnetic metal nitrides. [19] Consequently,t he synergetic orbital interactions accompanied by the mutual electron transfer between 2 and the O-atom donor build up N À Om ultiple bonds in ac oncerted yet asynchronous way without an intervening intermediate.O nt he basis of the above analysis,the oxygenation of closed-shell metal nitrides should follow as imilar mechanism. This work enriches the already diversified reactivity of high-valent iron nitrides and provides ad ifferent viable strategy for constructing multiple bonds for diatomic molecules.