A Cornucopia of Iridium Nitrogen Compounds Produced from Laser‐Ablated Iridium Atoms and Dinitrogen

Abstract The reaction of laser‐ablated iridium atoms with dinitrogen molecules and nitrogen atoms yield several neutral and ionic iridium dinitrogen complexes such as Ir(N2), Ir(N2)+, Ir(N2)2, Ir(N2)2 −, IrNNIr, as well as the nitrido complexes IrN, Ir(N)2 and IrIrN. These reaction products were deposited in solid neon, argon and nitrogen matrices and characterized by their infrared spectra. Assignments of vibrational bands are supported by ab initio and first principle calculations as well as 14/15N isotope substitution experiments. The structural and electronic properties of the new dinitrogen and nitrido iridium complexes are discussed. While the formation of the elusive dinitrido complex Ir(N)2 was observed in a subsequent reaction of IrN with N atoms within the cryogenic solid matrices, the threefold coordinated iridium trinitride Ir(N)3 could not be observed so far.


Introduction
Molecular complexes combining nitrogen and platinum group metals (PGM), such as dinitrogen complexes L m M(N 2 ) n and polynitrido metal complexes L m M(N) n have recently attracted much attention. [1] Molecular dinitrogen complexes are of vivid interesti nn itrogen fixation and reduction since 1966, when the first iridium dinitrogen complex was published, shortly after the first transition metal dinitrogen complexes [Ru(NH 3 ) 5 N 2 ]X 2 with X = Br À ,I À and BF 4 À werer eported in 1965. [2] The activation and weakening of the strong triple bond in the N 2 molecule is facilitated by p-back-bonding from orthogonal d xz and d yz or even po rbitals into the antibonding p*-orbitals of the N 2 ligand. [1a, 2c, 3] This effect is readily observable spectroscopically by ar ed-shift of the NÀNs tretching mode compared to free dinitrogen in the IR spectra.A ll binary PGM dinitrogen complexes,e xcept those of iridium, werei nvestigated experimentally using matrix isolationt echniques, where metal atoms are generatedb yt hermale vaporation or laser ablation forR u, [4] Rh, [5] Pd, [6] Re, [7] Os, [4] and Pt. [6b, 8] By these methods homoleptic dinitrogen complexes M(N 2 ) n can be prepared, which allow the investigation of metal-nitrogen bondingi nteractions independent of the influence of other li-gands and thus give important insighti nto the bonding properties and mechanisms of dinitrogen activation.
Polynitrido metal complexes have recently attracted attention as the nitrido ligand facilitates high oxidation states. Examplesa re the group 6c omplexes NM + VI F 3 (with M = Cr,M o and W) [9] and the more recently predicted but so far unknown NIr + IX O 3 . [1b] The concept of "formal oxidation states" is ap opular and important methodo fc ounting and assigning electrons to chemical elements in molecular and solid-states tructures. [10] In recent years the range of compounds in high and unusual formal oxidation states has been expanded experimentallya s well as theoretically.T he so far highest experimentally attained formal oxidation state across all chemical elements is + IX of iridium in the [IrO 4 ] + cation. [11] It was generated in the gas phase and detectedu sing infrared photodissociations pectroscopy after it was predicted theoretically. [12] But also compounds with iridium in the oxidation states of + VI and + VIII are scarce:I r + VI F 6 ,I rO 3 ,I r + VI (h 2 -O 2 )(O) 2 and Ir + VIII (O) 4 are the only experimentallyk nown examples. [13] Nitrogen is the third most electronegative elementa nd with af ormal oxidation number of À3i tc an increaset he formal oxidation state of the metal centerb yt hree units, while occupying only as ingle coordination site. The problem associatedw ith the N 3À ligand is that, compared to F À and O 2À ,i ti sm ore easily oxidized by strongo xidizing metal centers, especially in complexes bearing metals in high oxidation states.S everal binary transition metal nitrides were previously prepared by the reaction of laser-ablated metal atoms with pure dinitrogen or dinitrogen diluted in rare gases, and subsequent deposition on ac old matrix support. Although this method mainly yields metal dinitrogen complexes,a lso molecular mono-andd initrides of the platinum group metals,s uch as RuN and Ru(N) 2 , [4] RhN and Rh(N) 2 , [5b] OsN and Os(N) 2 , [4] and PtN [8c] were formed as well. So far,t he only known binary molecular iridium nitrogen compound is the IrN molecule, first produced by laser ablation of iridium atoms in the presence of NH 3 and characterizedb yo ptical/Stark spectroscopy. [14] Subsequently,i ts spectral and bonding properties were studied further experimentally and theoretically. [15] Furthermore, high-pressure materials of the composition Ir 2 N, Ir(N) 2 and Ir(N) 3 ,r espectively,a re potentially (super) hard materials and their structural, electronic and mechanical properties were previously investigated theoretically [16] and experimentally. [17] These materials however contain quasi-molecular N 2 2À or N 2 4À units rather than N 3À . [18] We have carriedo ut reactions of laser-ablated iridium atoms with dinitrogen molecules and studied the reaction products by matrix-isolation IR spectroscopy.T he photodecomposition of N 2 molecules and the formationo fNatoms inducedb y plasma radiation in the laser-ablation process should also facilitate the formation of molecular binary iridium nitrides up to Ir + IX (N) 3 .T hese molecular binary iridium nitrides will allow to gauge the ability of the Na tom to oxidize the iridium metal center and to investigate the nature of the chemical bonding independentoft he influenceo fo ther ligands.

Experimental and Computational Methods
Matrix-isolation experiments 14 N 2 (99.999 %, Linde) and 15 N 2 (98 + atom %, Campro) were premixed with neon or argon (both 99.999 %, Linde) in as tainlesssteel cylinder.T he mixing vessel was connected to as tainless-steel vacuum line connected to as elf-made matrix chamber by as tainless-steel capillary.T he gas mixture was then co-deposited with laser-ablated iridium atoms onto aC sI window (argon and dinitrogen matrices) or onto ag old plated copper mirror (neon matrices) and cooled to 4Kby using ac losed-cycle helium cryostat (Sumitomo Heavy Industries, RDK-205D) inside the vacuum chamber.F or the laser-ablation, the 1064 nm fundamental of aN d:YAGl aser (Continuum, Minilite II, 10 Hz repetition rate, 35-50 mJ pulse À1 )w as focused onto ar otating iridium metal target through ah ole in the cold window.I nfrared spectra were recorded on aB ruker Vertex 70 spectrometer purged with dry air (argon and dinitrogen matrices) or aB ruker Vertex 80v with evacuated optical path (neon matrices) at 0.5 cm À1 resolution in the region 4000-430 cm À1 by using a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. Far-IR (FIR) spectra were recorded at ar esolution of 0.5 cm À1 at the Bruker Vertex 80v equipped with aF IR multilayer mylar beam-splitter (680-30 cm À1 ), aC sI window (> 180 cm À1 ), and al iquid helium cooled bolometer.T he matrix samples were irradiated by am ercury arc streetlamp (Osram HQL 250) with the outer globe removed. Wavelength selective irradiations in the visible spectrum were realized with OSRAM LEDs with typical powers between 5a nd 10 watts.

Electronic-structure calculations
Density functional theory (DFT) calculations were performed using the TURBOMOLE 7.0.1 program package [19] employing the GGA exchange-correlation density functional BP86 [20] with the polarized quadruple-x basis set def2-QZVP [21] which applies the Stuttgart-Dresden effective core potential for iridium. [22] All Coupled Cluster Single Double and perturbative Triple excitations (CCSD(T)) combined with Dunning's augmented correlation consistent polarized triple-x basis sets aug-cc-pVTZ for nitrogen, [23] and aug-cc-pVTZ-PP combined with the ECP60MDF effective core potential for iridi-um [24] were performed using the CFOUR 2.00beta software. [25] State-averaged complete active space self-consistent field (SA-CASSCF) calculations combined with Dunning's correlation consistent polarized valence triple-x basis sets cc-pVTZ [26] and cc-pVTZ-PP [24] for nitrogen and iridium and the effective core potential (ECP60MDF) for iridium were carried out for iridium dinitride using the Molpro 2019 software. [27] The active space was chosen to consist of the molecular orbitals formed by the 2p(N), 5d(Ir) and 6s(Ir) atomic orbitals, yielding 15 electrons in 12 molecular orbitals. One calculation for each spin multiplicity,d oublet, quartet and sextet was carried out employing the state-averaging formalism in C 2v point group symmetry,i ncluding two states of each state symmetry (A 1 ,B 1 ,B 2 and A 2 ), resulting in eight states with equal weights of 0.125. Harmonic vibrational frequency calculations were carried out for all optimized structures analytically (BP86) or numerically (CCSD(T)). The decomposition pathways of Ir(N) 2 and Ir(N) 3 were analyzed by optimizing the geometries of the nitrides, the complexes formed by the rearrangement, and the transition states connecting both minima using the BP86 exchange-correlation density functional with the application of the zeroth-order regular relativistic approximation (ZORA) [28] combined with the adapted version of the def2 basis set ZORA-def2-TZVPP for nitrogen and the segmented all-electron relativistic contracted SARC-ZORA-TZVPP for iridium [29] as implemented in ORCA 4.1.2. [30] Additionally,t he meta-GGA M06-L exchange correlation density functional [31] was used for calculating the energy barriers associated with the decompositions of Ir(N) 2 and Ir(N) 3 .T he NBO and AIM analyses were carried out using wavefunctions obtained at the BP86/def2-QZVP level of theory using NBO 7.0 [31] and Multiwfn 3.5, [33] respectively.B ecause of the multitudes of combinations and the rapidly increasing computational challenges, compounds of the formula Ir x N y ,w ith y and x greater than two are not explicitly considered.

Results and Discussion
Laser-ablated iridium atoms werer eacted with dilutedd initrogen in avacuum chamber and the reaction products weresubsequently deposited on am atrix support under cryogenic conditions and studied using IR spectroscopy.T he experimental details are presented in the experimental section. The obtained products can be separated in two different sets:d initrogen and nitrido complexes. The NÀNstretching vibrationso fthe dinitrogen complexes occur in the regionf rom 2350-1850 cm À1 , and the IrÀNs tretching vibrations of dinitrogen and nitrido complexes in the region below 1150 cm À1 (Table 1). The main absorptions that appeared in the NÀNs tretching region of the IR spectra are located at 2270.  (Table 2a nd Figure 2, trace d). In the 14 N 2 matricesaband appeared at 2221.7 cm À1 ,a ccompanied by am atrix site at 2214.3 cm À1 (Figure 2), which is probably associated with clusters of Ir x (N 2 ) y .
Complementary spectra werea lso recorded in solid argon (Figures S1 and S2), andi nt he FIR region using neon as matrix host ( Figure S3). Af ull list of absorptionsf ound in argon matrices are given in Ta ble 3. The bands centered at 2144.7, 2110.6, 2087.6, 1004.1, 848.2 cm À1 were assigned and marked in the Figures S1 and S2 to Ir(N 2 ) 2 ,I r x (N 2 ), Ir(N 2 ), IrIrN and Ir(N) 2 ,r espectively.B ands obtained in the FIR region are shown in Figure S3. They are due to the three 14/15 Ni sotopologues of Ir(N 2 ) 2 embedded in solid neon andl ocated at 402.8, 397.4 and 393.1 cm À1 ,r espectively.O ptimized structures of the above-mentioned dinitrogen and nitrido complexes of iridium were obtained at the DFT and CCSD(T) levelso ft heory and depicted in Figure 6. Computed harmonic frequencies of the reaction products are summarized in Ta ble S1, and computed reaction enthalpies relatedt ot he formation of the observed reaction products are listed in Ta ble 4. In the following the infrared spectra and the annealing and photolysis behavior of the reaction products, as well as our computational results are discussed, startingw ith the dinitrogen iridium complexes. Table 1. Infrared absorptions (cm À1 )and 14 N 2 / 15 N 2 isotopic ratios obtained from the reactiono fl aser-ablated iridium atoms co-deposited with dinitrogend iluted in neon at 4-5 K. 14

Ir(N 2 )
The band observed at 2097.4 cm À1 in neon doped with 0.5 % 14 N 2 with aw eaker matrixs itea t2 099.8 cm À1 is assigned to the Ir(N 2 )complex ( Figure 1). This band is unaffected by broadband irradiation and grows upon annealing, while the sharpm atrix site at 2099.8cm À1 overtakes the initially stronger band at 2097.4 cm À1 .T he 15 Nc ounterpart exhibits the same behavior upon irradiation andi sl ocated at 2027.6 and 2030.0 cm À1 , giving an isotopic frequency ratio of 1.0344 typical for NÀN stretching modes. In the mixed 14 N 2 and 15 N 2 isotopic experiment the band at 2097.4 cm À1 is interfered by as tronger band associated with Ir( 14 N 2 )( 15 N 2 ). However,d ue to the sharp, distinctive band shape of the matrix site at 2099.8 cm À1 after annealing, the weaker Ir(N 2 )b and clearly stands out ( Figure 1e). The spectrum does not show any band relatedt oas crambled 14 N/ 15 Ns peciesi nt he mixed 14 N 2 + 15 N 2 experiment, thus, the characteristic doublet isotope pattern indicates ac arrierb earing as ingle N 2 unit. Thec orresponding absorption in the argon matrix is red-shifted by 9.8 cm À1 relative to neon and the same isotopic ratio is found ( Figure S1). Due to the formation of the higher coordinated species Ir(N 2 ) 2 ,t he intensity of the Ir(N 2 )a bsorption band decreases with increasing amount of N 2 and is absent in the neat dinitrogen spectrum.T he assignmentsa re supported by harmonic frequency calculations at the DFTand CCSD(T) level of theory.I na nalogyw ith RhNN, DFT and CCSD(T) calculations on Ir(N 2 )r esult in al inear C 1v point group symmetry (Figure6)f or the 2 D electronic ground  . Infrared spectra in the 1150-1050cm À1 and 875-820 cm À1 regions of the reaction products of laser-ablated iridium atomsw ith 14 N 2 (a), 15 N 2 (b) as well as a1:1 mixture of 14 N 2 and 15 N 2 (c). Bands due to iridiumn itrogen compoundsa nd some selected 14/15 Ni sotope patternsa re indicated. Bands markedwith an asterisk exhibit no isotopic shifta nd remained unassigned.
The NÀNs tretching band of the cationic species Ir(N 2 ) + appeared red-shifted by 172 at 2270.3 cm À1 and is observed in all experiments using N 2 /Ne mixtures as well as in neat N 2 ,i n which the band is blue-shifted by 1.0 cm À1 (Figure 1). Selective irradiations using LED light sources of l = 656, 455, 405 and 365 nm did not affect the absorption intensity.H owever,f ull arc irradiation depleted, and annealing of the dinitrogen matrix to 30 K, destroyed the band entirely.T he NÀNs tretch of the Ir( 15 N 2 ) + isotopologuei sl ocated at 2194.8 cm À1 ,r esulting in an isotopicr atio of 1.0344, typical for modes involving two nitrogen atoms. As for the neutral species, no additional bands could be assigned to this speciesi nt he 1:1 14 N 2 / 15 N 2 mixed isotope experiment, implying the presence of as ingle dinitrogen unit. Computational resultsa tt he DFT and CCSD(T) levels of theory support the assignment further.T he calculated harmonic frequencies are at 2212 and 2286 cm À1 ,r espectively.F or [Ir(N 2 )] + a 3 D electronic ground state was found with one electron removed from an on-bonding s type molecular orbital. Compared to the neutral species the electronic ground state of the cation was computed to be 848 kJ mol À1 higher in energy.

Ir(N 2 ) 2
The band centered at 2154.0cm À1 with am atrix site at 2158.0 cm À1 obtained in solid neon doped with 0.5 % 14 N 2 and shown in Figure 2r emained unaffected by annealing but decreasedd ramatically upon irradiation with LED light of l = 455 nm. Annealing after photolysis increasedt he intensity of the initially weaker matrix site at 2158.0 cm À1 .T he 15 Nc ounterparts at 2082.4 and 2086.1 cm À1 result in an isotopicr atio of 1.0344. Increasing the amount of N 2 in the solid Ne matrices strongly increases the intensity of the band and it is red-shifted by 4.0 cm À1 in neat N 2 (Figure 2). The mixed 14 N 2 and 15 N 2 spectrum displays ac haracteristic pattern for linear Ir(N 2 ) 2 consisting of the three antisymmetric NÀNs tretching modes of ( 14 N 2 )Ir( 14 N 2 ), ( 14 N 2 )Ir( 15 N 2 )a nd ( 15 N 2 )Ir( 15 N 2 )a t2 154.0, 2100.3 and 2082.4 cm À1 ,r espectively.A dditionally,t he symmetric NÀN stretching mode of the ( 14 N 2 )Ir( 15 N 2 )s peciesb ecomes IR active due to lower point group symmetry C 1v and is found at 2194.6 cm À1 .F igure 1f shows the 14 N/ 15 Ni sotope pattern of Ir(N 2 ) 2 arising from a1 :1 mixture of 14 N 2 and 14 N 2 (10 %i nN e), shown in ad ifference spectrum obtained by subtracting the spectra after and prior to selective photolysis with LED light of l = 455 nm. In the FIR spectrum shown in Figure S3 a 1:2:1triplet 14/15 N 2 isotope pattern of Ir(N 2 ) 2 was also observed at 402.8, 397.4 and 393.1 cm À1 originating from a1 :1 mixture of 14 N 2 and 14 N 2 in solid neon whichr esults in an isotopic ratio [a] BP86/ZORA-def2-TZVPP(N)/SARC-ZORA-TZVPP(Ir).
[b] M06-L/ZORA-def2-TZVPP(N)/SARC-ZORA-TZVPP(Ir). of 1.0247. Comparing our assignmentst ot he frequencies calculated at the DFT level of theory there is av ery good agreement for the antisymmetric NÀNs tretching mode at 2149 cm À1 and an isotopic ratio of 1.0349.T he symmetric NÀN stretching mode in Ir( 14 N 2 )( 15 N 2 )i sc alculated to be centered at 2182 cm À1 and the position of the antisymmetricI rÀNs tretching mode at 439 cm À1 ,l eading to an isotopic ratio of 1.0267. In analogy to the Ir(N 2 )c omplex, DFT andC CSD(T) calculationso n Ir(N 2 ) 2 find a 2 D g ground state (D 1h point group symmetry) having an unpaired electron located in ad egenerate d g molecular orbital. The HOMO!LUMO( 1 d g !2p u )e xcitation gives rise to the lowest quartet state 4 P u , which is 246 kJ mol À1 higher in energy than the electronic ground state.
Ir(N 2 ) 2 À À As trong band observed at 1955.8 cm À1 in neat 14 N 2 shown in Figures 2a nd S7 decreases completely on annealing and is unaffected by broadband irradiation. The corresponding absorption in solid neon doped with 10 % 14 N 2 at 1956.4 cm À1 lead to ar elated 15 Ni sotopologue absorption at 1890.3 cm À1 (Figure S7). To gether with as tronger band at 1912.9 and aw eak band at 1988.1 cm À1 which appeared in the 1:1 14 N 2 / 15 N 2 spectrum a 14/15 Ni sotope pattern similart ot hat of Ir(N 2 ) 2 is observed.T herefore, the band is assigned to the anionic complex Ir(N 2 ) 2 À .T he assignment is supported by DFTc alculations, which predict a 1 A 1 singlet ground state of C 2v point group symmetry and infrareda bsorptions at 1988, 1940 and 1921 cm À1 for the antisymmetric NÀNs tretching modes of Ir( 14 N 2 )( 14 N 2 ) À ,I r( 14 N 2 )( 15 N 2 ) À and Ir( 15 N 2 )( 15 N 2 ) À .T he experimental and calculated isotopic ratios are 1.0344 (in neatN 2 ), 1.0350 (in solid Ne)a nd 1.0349 (DFTc alc.) and are in very good agreement for the less interacting neon matrix. For ab ent structure of Ir(NN) 2 À and unliket he case of al inear Ir(N 2 ) 2 ,t he symmetric NÀNs tretching mode is IR active. However,t his band is not observed in the experiment, probablyb ecause of its low intensity,w hich is calculated to be 50 times lower than that of the antisymmetric mode. In the Ir( 14 N 2 )( 15 N 2 ) À isotopologue the intensity ratio of the symmetric and antisymmetric modes change to 1:4, and hence, the symmetric NÀNs tretching combination can be observed at 2031 cm À1 in the neat1 :1 14 N 2 / 15 N 2 spectrum.C ompared to the neutral complex the anion Ir(NN) 2 À is 224 kJ mol À1 lower in energy at the DFT level of theory,w hich corresponds to the adiabatic electron affinity of Ir(N 2 ) 2 .

IrNNIr
Laser-ablated iridium atoms co-depositedw ith 10 % 14 N 2 in neon give raise to ab and at 786.5 cm À1 whichi su naffected by annealingt o1 2Kandv anishes after 10 min of irradiation with l = 455 nm (Figure 4). The same response waso bserved for a band at 761.6 cm À1 under the same conditions using 15 N 2 .T he isotopict riplet observed at 786.5, 774.1 and7 61.6 cm À1 in a 1:1mixture of 14 N 2 and 15 N 2 indicates an IrÀNs tretching mode involving two equivalent nitrogen atoms. The intensity pattern of 1:2:1s uggestst he presence of the isotopologue containing both isotopes, 14 n and 15 NB ased on the very good agreement of the band positions, isotopicp attern and isotopic ratio of the antisymmetric IrÀNstretching mode obtained by our quantum-chemical calculations, the band wasa ssigned to IrNNIr. This dimer could probably be formed by an oxidative coupling of two IrN molecules (Equation 1): The observed isotopic ratios in solid neon and solid dinitrogen are 1.0327 and 1.0331, which are in very good agreement with the calculated DFT value of 1.0324. The calculated absorptions are 782, 769 and 757 cm À1 for the Ir 14 N 14 NIr,Ir 14 N 15 NIra nd Ir 15 N 15 NIr isotopologues, respectively.T he electronic ground state is found to be at riplet 3 S u + ,w ith the two unpaired electrons located at each of the metal centers in degenerated molecular orbitals of d x 2 Ày 2 -a nd d xy -character,r eminiscent of the Ir(N 2 )c omplex.T he NÀNs tretching mode is IR inactivea nd calculated to be centered at 2081 cm À1 at the DFT level of theory.

IrN
Av ery weak band at 1102.8 cm À1 which was not observed in the initially formed solid 14 N 2 deposit but grew in upon annealing to 35 Ka nd was destroyed by broadband irradiation, returned on subsequent annealing to 35 K ( Figure 4). Thec orresponding absorption in 14 N 2 doped neon is blue-shifted to 1111.1 cm À1 and the 15 Nc ounterparts were found at 1076.4 and 1066.9 cm À1 in neon and 15 N 2 ,r espectively,w hile no band due to am ixed 14/15 Ni sotopologue occurredi ne xperiments using a1 :1 mixture of 14 N 2 and 15 N 2 ( Figure 5). The assignment of this band to IrN is supported by ap reviousF ouriert ransform emission spectroscopics tudy of Ram and Bernath, [15a] in which ag round-state fundamental Ir-N stretchingf requency of 1113.6 cm À1 for 193 Ir 14 Nw as reported, revealing reasonable matrix shifts of À2.5 and À10.8 cm À1 for neon and solid dinitrogen. For the sake of completeness, DFT and CCSD(T) calculations were carried out and the results are listed in Ta bleS1. The annealing behavior of IrN suggestsatemperature induced mobility of Nr adicals reacting with iridium atoms to IrN (DH = À612 kJ mol À1 ).

IrIrN
An intense band at 1002.2 cm À1 in solid 14 N 2 matrices and their 15 Nc ounterparts at 970.7 cm À1 grow in upon annealing to 35 K and remains unaffected by broadband irradiation (Figure 4). The much weakerb ands located at 1004.4 and 972.8 cm À1 in solid neon doped with 10 %N 2 depicted in Figure 3s howt he same behavior and in experiments using a 14 N 2 / 15 N 2 mixture no additional band due to am ixed 14/15 Ni sotopologueo ccurred, suggesting an IrÀNs tretching mode with as ingle nitrogen atom involved. With IrN already assignedt oaband centered about 100 cm À1 blue-shifted, the carriero ft his unknown band could be Ir(N)(N 2 ), ad initrogen complex of IrN, or IrIrN.Calculations at the DFT level of theory predict af airly strong N N stretching mode for Ir(N)(N 2 )a t2 110cm À1 ,m uch highert han the Ir Ns tretching mode at 1085 cm À1 ,h owever,n os uch N Nb and could be identified in the spectrum and hence,r uled out an assignment to Ir(N)(N 2 ). On the other side, infrared absorptions computed at the DFT level of theory support the assignment of the band at 1004.4 in solid neon to IrIrN, with calculated harmonic frequencies of 1054 and 1021 cm À1 for IrIr 14 N and IrIr 15 N, respectively,a ccountingf or an isotopic ratio of 1.0323, which is very close to the experimental ones of 1.0325 in solid dinitrogen and neon. In analogy to IrN, the observed behavior upon annealing suggests af ormation by mobilizing nitrogen radicals which react with Ir 2 units present in the matrix. This is furthers upported by ac omputed reaction enthalpy for the formationo fI rIrN from Na toms and the iridium dimer obtained at the DFT level of theory of DH = À508 kJ mol À1 (Table 4). The electronic ground state of the bent IrIrN structure in the C s point group symmetry is ad oublet 2 A' state.

Ir(N) 2
Av ery weak band, located at 853.5 cm À1 in solid neon doped with 0.5% 14 N 2 and unaffected by broadband irradiations, presents ad oublet pattern in neon doped with 0.5 %o fa1:1mixture of 14 N 2 and 15 N 2 at 853.5 and 827.7 cm À1 (Figure 3). The picture changes when pure dinitrogen is used as matrix host: besides as light blue-shift of these bands to 857.1 and 831.2 cm À1 ,a ni ntermediate band at 842.6 cm À1 appears, yielding at riplet pattern with an intensity ratio of about 1:2:1 ( Figure 5). The observed triplet pattern is consistent with the involvement of two equivalentn itrogen atoms in an antisymmetric IrÀNs tretching mode, such as in iridium dinitride, Ir(N) 2 . This assignment is supported by the fact that the intermediate band belonging to the 14 NIr 15 Ni sotopologue is red-shifted 1.6 cm À1 from the center, indicating ac oupling between the symmetric andt he anti-symmetric IrÀNs tretching modes, which in the lower point group symmetry C s have the same a' symmetry.T he different patternso bservedi ns olid neon and pure dinitrogen matrices can be explained by differentr eaction mechanisms leadingt oI r(N) 2 :W hile ad irecti nsertion of iridium atoms into ad initrogen bond is proposed in nitrogen doped neon mixtures (Equation 2a), the reaction of IrN with N atoms preferentially occurred in solidd initrogen matrices (Equation 2b).
Ir þ N 2 ! IrðNÞ 2 DH ¼þ20 kJ mol À1 ð2aÞ While the reaction enthalpy of the direct insertion is slightly positive on the DFT level of theory,t he high temperature of laser ablated iridium atoms can overcome this barrier andcryogenic conditions prevent the spontaneous eliminationo faN 2 unit. In contrastt oo smium,s pontaneous insertion into the NN triple bond at cryogenicc onditions is not observed. [4] Harmonic frequencies obtained by calculations on the DFT level of theory are in very good agreement,r esulting in antisymmetric b 2 stretching frequencies of 869, 853 and 842 cm À1 for Ir( 14 N) 2 , Ir( 14 N)( 15 N) and Ir( 15 N) 2 .T he symmetric a 1 IrÀ 14 Ns tretching mode was not observed and calculated to be located at 1027 cm À1 having an intensity less than 4% of the antisymmetric one. From the band positions of the antisymmetrics tretching modes in the isotopes ubstitution experiment an estimate of the upperl imit of the N-Ir-N bond angle can be estimated to 1308, [34] which is in agreement with the calculated angle of 1128 for the C 2v ( 2 B 1 )e lectronic ground state geometry.

Ir(N) 3
No band could be assigned to iridium trinitride, althoughw e observed nitrogen atom mobility in the formationo fI r(N) 2, and the thirdaddition of anitrogen atom wascalculated to be exothermic( DH = À243 kJ mol À1 ,T able 4). However,T able S1 showst hat the integrated intensity of the IR activeI r ÀN stretching absorption in the observable region, the degenerate e' mode, is calculated to be 1.4 km mol À1 ,w hich is about 3% of the calculated integrated intensity of the corresponding very weakb and assigned to Ir(N) 2 .T he amount of iridium trinitride formed according to that mechanismw ould certainly be very low.

Dinitrogen complexes of iridium
The nature of the metal nitrogen bond in selected product molecules and in Ir(N) 3 will be discussed in terms of the relevant vibrational stretching modes as well as by analysis of the wavefunctions obtaineda tt he BP86/def2-QZVP level of theory. The coordination chemistry of the dinitrogen molecule is limited because it is ac omparatively poor s-donor,w eak p-acceptor and its lack of dipole moment. [35] The p-donation of the iridium center into the p*m olecular orbitals of the dinitrogen unit resultsi naweakening, or activation of the dinitrogen triple bond. The weakening of the NÀNb ond in dinitrogen complexes can be quantified experimentally by the red-shift of the NÀNs tretching mode in the IR spectrum,c omparing the NÀNb ondd istances and, theoretically,b ye xtracting information from the wavefunction. Several neutral PGM dinitrogen complexes have previouslyb een studied by matrix isolation spectroscopy. [4][5][6][7][8] Their experimental NÀNs tretching frequencies embedded in argon are given in Ta ble S2 andp rovide a solid basis for discussing the nature of bondingi ns uch homoleptic dinitrogen complexes.T he red-shift of the NÀNs tretching mode of Ir(N 2 )r elative to that in free dinitrogen (2327.1 cm À1 ,T able 3) is 240.3 cm À1 ,w hich is less than the one for the group 8m etal dinitride Os(N 2 )a nd greater than that for the group 10 analogueP t(N 2 ). This trend is consistent with a decreasing ability of late transitionm etals to donate electron density into the p*o rbitals of the coordinated N 2 moiety due to less MO overlap caused by larger bond distances and decreasingd -orbitale nergies. The same trend is observedw ith the corresponding first row transition metals. [1a] The electron density at the bond critical point (1 b )i namolecule can be taken as measureo fthec haracter of ab ond and its bond order. [36] The datap resented in Table S3 shows as ig-  decrease of 1 b (NN) going from Ir(N 2 )( 0.622) over Ir(N 2 ) 2 (0.592) down to IrNNIr( 0.579), indicating aw eakening of the corresponding NÀNb ond of the dinitrogen ligand within this series. This is also evidenti nt he minimums tructures shown in Figure 6, where the longest NÀNb ond lengths within this series of 115pmi se xhibitedi nt he binuclear complex IrNNIr. In contrast to the electron density at the bond criticalp oint (1 b ), which seems to be mainly affected by s donation from the N 2 ligand to the iridium center,t he slightly longer NÀN bond length in Ir(N 2 )( 113pm) compared to Ir(N 2 ) 2 (112 pm), is consistentw ith an increasing experimental NÀNs tretching frequencyf rom Ir(N 2 )( 2087.6 cm À1 )t oI r(N 2 ) 2 (2144.7 cm À1 ), and can most likelyb er ationalized by as tronger p backdonation from the iridium centert ot he N 2 ligand bonding in the Ir(N 2 ) complex. Weakly activatedN ÀNb ond lengths are typically less than 112pm, [35] placing Ir(N 2 )a nd Ir(N 2 ) 2 at the upper end of the scale for what is considered weakly activated. The slightly strongerN 2 activation in Ir(N 2 )c ompared to Ir(N 2 ) 2 is also supported by an NBOa nalysis, whichr esults in NPAb ond orders for the NÀNb onds in Ir(N 2 ), Ir(N 2 ) 2 ,a nd IrNNIr of 2.56, 2.64 and 2.51, respectively,a sw ell as by the shorter calculated IrÀN bond distance in Ir(N 2 )o f1 79 pm compared to 190 pm in Ir(N 2 ) 2 (Table S3).
Comparing the NÀNs tretching modes of the ions [Ir(N 2 )] + and [Ir(N 2 ) 2 ] À with those of their neutral counterparts, ab lueshift for the cation and ar ed-shift for the anion is observed, which is consistent with the calculatedc hanges in the corresponding NÀNb ond lengths (Table S3,F igure 6) and with the notion that oxidationo ft he metal center leads to al ower ability of p-back-donation, whiler eduction leads to an increase. [2c] In both cases, the addition or subtraction of an electron does not change the occupation number of the p-system, butl eads to an oxidation or reduction of the iridium center. Compared to as hift for the CÀOs tretching frequency in Ir(CO) + and Ir(CO) 2 À with respect to neutralI r(CO) of + 132 and À29 cm À1 , respectively, [37] the frequency shift for the isoelectronic dinitrogen complexes is with + 170 and À198 cm À1 significantly larger.T he highers ensitivity of the NÀNs tretching frequency upon oxidation or reduction of the metal centerc ompared to the CÀOf requency is another indication for the importance of p-back-bondinga st he most significant contribution to the IrÀ Nb ond strength. [2c] On the other side, the red-shift of the NÀN and CÀOs tretching frequencies in the neutral Ir(N 2 )a nd Ir(CO) complexes with respect to the free ligands is with 170 cm À1 (9.8 %) higher in Ir(N 2 )c ompared to 132 cm À1 (5.4 %) in Ir(CO). As pointed out by Pelikµna nd Boča, [2c] the larger red-shiftf or the NÀNs tretch does not indicate as tronger p-back-donation in the Ir(N 2 )c omplex,s ince both interactions, s-donation and p-acceptance lead to aw eakeningo ft he NÀNb ond, while in the Ir(CO) complex s-donation leads to an increase and p back-donation to ad ecrease in the CÀOb onds trength. Ta king the better s-donora bility of CO comparedt oN Ni nto account, [38] CO must be considered as tronger p-acceptor than the N 2 ligand.
In Figure 7t he frontier molecular orbitals of the p-system are shown for the neutral, linear dinitrogenc omplexes Ir(N 2 ), Ir(N 2 ) 2 ,and IrNNIr.Each of them is comprised of the degenerate 2p y and2 p z atomic orbitals of the nitrogen atoms and the 3d xy and 3d xz atomic orbitals of the iridium atoms involved. For IrNNIr with 12 p-electronst he first three of four pairs of the molecular p-orbitals are fully occupied (Figure 7, right). The first pair essentially forms the p-bonds of the NN unit, the second pair contains the corresponding NÀNa nti-bonding molecular orbitals of the first set, and the third pair of the p-bonding orbitals are IrÀNa nti-bondinga nd NÀNb onding. The electronic ground state of 3 S u + arises from two unpaired electrons residing in non-bonding molecular orbitals, essentially formed by the non-bonding iridium 3d yz atomico rbitals (not shown in Figure7). From isotopict riplet observed in a1 :1 mixture of 14 N 2 and 15 N 2 for the IrÀNs tretching vibration in the IR spectrum it has been concluded that IrNNIr is likely formed during matrixd epositionbythe coupling of two IrN units. Avery similar behavior was reported for the [N 2 {Ir(PNP)} 2 ]( PNP = N(CHCHPtBu 2 ) 2 )p incerc omplex, holding the same 12 electron IrNNIr p-system,w hich was observedt ob ef ormed in solution at room temperature by coupling of two terminal [IrN(PNP)] nitrido complexes. [1c] This coupling reaction can be viewed as the reverseo fs plitting ab ridging dinitrogenl igand into separate nitrido complexes,w hich recently was investigated for [NIr(PNP)] 2 n + (n = 0, 1, 2). [39] For these complexes reaction enthalpieso ft he coupling reaction of 2[NIr(PNP)] n + ,w ith 2n = 0, 1, and 2, are exotherm and calculated to DH = À510, À425 and À382 kJ mol À1 (D3BJ-PBE0(Cosmo (THF))/def2-TZVP//D3BJ-PBE0/def2-SVP) respectively. [39] In contrast to these results coupling of two IrN complexes,b are of any additional ligandsa nd under solvent-free conditions in an argon matrix, is significantly less exothermic with DH = À96 kJ mol À1 .T he lower reaction enthalpyf or the latter coupling reaction can be explained by the formation of two strong triple bonds in the Ir Nu nits.
The calculatedI r ÀNb ond lengthso fi ridium dinitride, Ir(N) 2 , in its 2 B 1 electronic ground state of 170 pm is considerably larger than the expected ones for at riple bond judged on the calculated bond lengths in iridium mononitride of 160 pm, which also corresponds well to the sum of the triple-bond covalent radii of iridium and nitrogen of 160 pm. [40] From the sum of reported double- [41] and triple-bond covalent radiiofi ridium and nitrogen of 160 and 175 pm, respectively,t he bond order in Ir(N) 2 can be estimated to be between at riple anda double bond, while the computed NPAb ond order is 2.06. The NPA( AIM) charges are significantly higherc ompared to diatomic Ir N, amountingt o0 .588 (0.896)a nd À0.294 (À0.448) for iridium and nitrogen, respectively.T he higher charges and lower covalentb ond order of the dinitride comparedt ot he mononitride suggest an Ir=Nd ouble bond in the former one, which is shortened due to ahigherionic character.The valence molecular orbitals and occupation numbers obtained from SA-CASSCF(15,12)/cc-pVTZ(-PP) calculations (for detailss ee Computational Details) for the 2 B 1 electronic ground state are depicted in Figure S4. These calculations revealalone pair (5a 1 ) at the iridium center,two s-bonding orbitals (6a 1 and 4b 2 ), two p-bonding orbitals (2b 1 and 1a 2 ), and their anti-bonding s* (8a 1 and 6b 2 )a nd p*c ounterparts (3b 1 and 2a 2 ). We note that one unpaired electron resides in the 3b 1 p*o rbital. Additionally,t wo essentially doubly occupied non-bonding orbitals (7a 1 and 5b 2 )r emaina tt he nitrogen ligands, andf inally,t here is a high-lying, low-occupied non-bonding orbital (9a 1 )w ith contributionsf rom the Ir(6s), Ir(5d x 2 Ày 2 ), Ir(6p z )a nd N(2p z )a tomic orbitals ( Figure S4). An effective bond order (EBO) of 1.56 can be estimated for the IrÀNb ond by counting the occupation numbers of the bonding-anda nti-bondingm olecular orbitals, neglectingt he slightly bonding characters of the nonbondingo rbitals 7a 1 and 5b 2 .F or the 2 B 1 electronic ground state the presence of nitrogen-centered unpaired electrons can be ruled out, however, an estimation of low-lying electronic states using SA-CASSCF(15,12)/cc-pVTZ(-PP) calculations show that the lowest lying quartet state is 84 kJ mol À1 and the lowest lying sextet state 252 kJ mol À1 higheri ne nergy than the electronic ground state ( Figure S5 and Ta ble S4). The most dominant configuration of the electronic ground state is a 1 6 b 1 3 b 2 4 a 2 2 with a weightof0.86 (74 %). Other contributionsare small and distributed over the whole expansion space.T his electron configuration can be described by the resonance Lewis structure shown in Scheme1,i nw hich an integral formal oxidation state cannotb ea ssigned to the iridium center ap riori.
If we resort to MO theory and wavefunction analysis, spin populations can give insighti nto whiche xtend the unpaired electron is localized either on iridium, or at the nitridol igands. Mulliken and Loewdin population analysis yield spin populations of 0.40 and 0.47 at the iridium center, which meansthe ligands do not allow the definition of ac lear-cut integral oxidation state. The formal oxidation state lies between + Va nd + VI, with slightly more weight on the side of + V. The nitrido ligandsmust be considered as non-innocent. [42] Iridium trinitride is an intriguing compound since iridium would formally be considered in the oxidation state + IX, which so far was experimentally realized only for the cation [IrO 4 ] + ,a nd more recently claimed for the experimentally unknown nitrido compound NIrO 3 .A nother candidate for the oxidation state + IX could be Ir(N) 3 ,p rovidedt hat all 5d electrons from the valence shell of iridium can be formally assigned to the nitrogen ligands and no lone pair remains on the iridium atom. We have investigatedI r(N) 3 at the BP86/def2-QZVPl evel of theory and found ar egular D 3h structure with an IrÀNb ond length of 176 pm in the 1 A 1 ' ground electronic state ( Figure 6). We have furthera nalyzed the occupied molecular orbitalsa t the R-BP86/ZORA-def2-TZVPP(N)/SARC-ZORA-TZVPP(Ir) level of theory and depicted the valence molecular orbitals in Figure S6. These calculation reveals ad egeneratep air of s-bonding orbitals (4e')a sw ell as ad egenerate pair of p-bonding orbitals (1e"). In addition, seven ligand-centered lone pairs (4a 1 ', 2a 2 '',2 5e',M O's arising from the N(2s) orbitals are not shown in Figure S6) can be assigned and am etal centered d-orbital is attributed to the highest occupied MO (HOMO, 5a 1 '). We note that the lowest unoccupied MO (LUMO, 1a 2 ')i sanonbonding ligand-centered MO. Thus, our analysiss hows that the nitrido ligandsi nI r(N) 3 behave as non-innocent ligands as well, [42] meaningt hat an essentially non-bonding iridium (d z 2 )o rbital (5a 1 ')i sf illed by two electrons at the expense of al igand delocalizedn onbonding LUMO (1a 2 '). This bonding situation, which is consistent with af ormal oxidation state of + VII rather than + IX for the iridium atom in Ir(N) 3 ,c an be approximately described by the resonance Lewis structures shown in Scheme 2.
This electronic description is supported by the calculated AIM and NPAc harges shown in Table S3. While the nitrogen atomsi nI r(N) 3 adopt ac harge, whichi sv ery closet ot he one found in Ir(N) 2 ,t he charge at the iridium center raised to about 3/2 of those in Ir(N) 2 .
The most favored decomposition pathway for doublet Ir(N) 2 and singlet Ir(N) 3 is found to be the eliminationo fd initrogen, which is exothermic by DH = À20 and À388 kJ mol À1 for Ir(N) 2 and Ir(N) 3 ,r espectively.A ccording to our all-electron R-BP86/ ZORA-def2-TZVPP(N)/SARC-ZORA-TZVPP(Ir) calculation the lowest energy pathway for dinitrogen elimination proceed by cleavage of an IrÀNb ond and formation ad initrogen complex (Figure 8). The nitrido complexes are separated from their dini-trogen coordinated isomersb yabarrier of 244 and 44 kJ mol À1 for Ir(N) 2 and Ir(N) 3 ,r espectively.T he corresponding transition states on the quartet and triplet surfaces of Ir(N) 2 and Ir(N) 3 ,r espectively,h ave also been investigated, and found to be higher in energy with 257 and 114kJmol À1 above the respective minimum structures. According to these results the kinetic stabilityw ith respect to dinitrogen elimination of Ir(N) 3 is rather low,w hile Ir(N) 2 is kineticallys table and the isomeric dinitrogen complex Ir(N 2 )h as indeed been detected in the present study.

Conclusions
Laser-ablated iridium atoms were allowed to react with dinitrogen and nitrogen atoms formed from N 2 moleculesb yp lasma radiationa nd the products werei solated in solidn eon,a rgon and nitrogen matrices and identified by their infrared spectra. The assignments are supported by ab initio and first principle calculations as well as 14/15 Ni sotope substitution experiments. The neutral and ionic iridium dinitrogen complexes Ir(N 2 ), Ir(N 2 ) + ,I r(N 2 ) 2 ,I r(N 2 ) 2 À ,I rNNIr were formed and assigned by their characteristic N-N stretching frequencies at 2097.4, 2270.3, 2154.0, 1956.4a nd 786.5 cm À1 ,r espectively.I na ddition, the nitrido complexes IrN, Ir(N) 2 andI rIrN were observed and assigned to IrÀNs tretching bands centered at 1111.1, 853.5 and 1004.4 cm À1 ,r espectively.W hile Ir(N) 2 can be formed by a photo-rearrangement of the corresponding dinitrogen complex Ir(N 2 )o rf rom Na tomsa nd IrN, the latter process was deduced from 14/15 Ni sotopice xperiments.T he threefold coordinated iridium trinitride complex Ir(N) 3 was not be observed. The structurala nd electronicp roperties of the dinitrogen ligand in the N 2 complexes are discussed with respectt od initrogen activationu pon complexation. The largest dinitrogen activation was observed in the neutral, linear binuclear IrNNIr complexa nd in the anionic Ir(N 2 ) 2 À .A lso, the electronic structures of the nitrido complexes Ir(N) 2 and Ir(N) 3 werei nvestigated by DFT and ab initio calculations. The dinitride Ir(N) 2 adopts ab ents tructure in a 2 B 1 electronic ground state with one unpaired electron in ad elocalized p*m olecular orbital( 3b 1 )a nd an additional lone pair on the iridium center. Ir(N) 3 has a D 3h structure in the loweste nergy electronics tate in which al one pair can be attributed to an onbonding iridium centered 5d z 2 orbital( 5a 1 ')a nd af ormal oxidation state for iridium of + VII rather than + IX can be deduced. The loweste nergy decomposition pathway of thesen itrido complexes has been found computationally to proceedvia arearrangement to the isomeric dinitrogen complexes.