Diiron Aminocarbyne Complexes with NCE− Ligands (E = O, S, Se)

Diiron μ-aminocarbyne complexes [Fe2Cp2(NCMe)(CO)(μ-CO){μ-CN(Me)(R)}]CF3SO3 (R = Xyl, [1aNCMe]CF3SO3; R = Me, [1bNCMe]CF3SO3; R = Cy, [1cNCMe]CF3SO3; R = CH2Ph, [1dNCMe]CF3SO3), freshly prepared from tricarbonyl precursors [1a–d]CF3SO3, reacted with NaOCN (in acetone) and NBu4SCN (in dichloromethane) to give [Fe2Cp2(kN-NCO)(CO)(μ-CO){μ-CN(Me)(R)}] (R = Xyl, 2a; Me, 2b; Cy, 2c) and [Fe2Cp2(kN-NCS)(CO)(μ-CO){μ-CN(Me)(CH2Ph)}], 3 in 67–81% yields via substitution of the acetonitrile ligand. The reaction of [1aNCMe–1cNCMe]CF3SO3 with KSeCN in THF at reflux temperature led to the cyanide complexes [Fe2Cp2(CN)(CO)(μ-CO){μ-CNMe(R)}], 6a–c (45–67%). When the reaction of [1aNCMe]CF3SO3 with KSeCN was performed in acetone at room temperature, subsequent careful chromatography allowed the separation of moderate amounts of [Fe2Cp2(kSe-SeCN)(CO)(μ-CO){μ-CN(Me)(Xyl)}], 4a, and [Fe2Cp2(kN-NCSe)(CO)(μ-CO){μ-CN(Me)(Xyl)}], 5a. All products were fully characterized by elemental analysis, IR, and multinuclear NMR spectroscopy; moreover, the molecular structure of trans-6b was ascertained by single crystal X-ray diffraction. DFT calculations were carried out to shed light on the coordination mode and stability of the {NCSe-} fragment.

Notwithstanding, the metal coordination of the {NCSe} − group might be unstable, resulting in decomposition into the cyanide ion and selenium atom; this reaction was previously reported with copper(II) acetate [24,25] and was exploited for catalytic purposes in cyanation reactions. Selenocyanate to cyanide degradation was also observed in human Here, we extended the chemistry of diiron aminocarbyne complexes with pseudohalide ligands, including reactions with a selenocyanate source. Structural and thermodynamic aspects were elucidated by means of DFT calculations.

Results and Discussion
The diiron aminocarbyne complexes [1a-d]CF3SO3 were prepared from commercially available [Fe2Cp2(CO)4] following the appropriate literature procedures (see Experimental). To evaluate the influence of the aminocarbyne ligand on the substitution reactions herein discussed, we considered a series of R substituents bearing different steric and electronic properties (Scheme 2). In fact, previous findings indicated that R may The reactions of [1a NCMe -1c NCMe ]CF 3 SO 3 with sodium cyanate were performed in acetone at room temperature and afforded the neutral complexes 2a-c in 67-81% yields directly as cis isomers, in alignment with the major stability of this configuration that is usually exhibited by complexes based on the [Fe 2 (CO) 2 Cp 2 ] frame [36]. Thus, the IR and NMR spectra of the previously reported complex 2b (see Scheme 1) matched the literature data [39]. The IR spectra of 2a and 2c (in CH 2 Cl 2 , 2300-1500 cm −1 spectral region) were similar to that of 2b, consisting of three bands related to the coordinated cyanate ligand and the terminal and bridging carbonyl ligands (e.g., at 2242, 1986, and 1818 cm −1 , respectively, in the case of 2a). The major stability of N-coordination (compared to O-coordination) of the {NCO} moiety was as expected for a low-valent iron center [45][46][47]; otherwise, the preference for O-coordination of potential Nand O-donors is commonly observed with high-valent metal complexes [48][49][50]. Note that O-coordination towards the Fe I center in related systems has been rarely observed and only as part of a coordination by means of multidentate hydrocarbyl ligands [51,52]. In 2c, the band due to the µ-CN moiety fell at 1540 cm −1 , in alignment with some double bond character (vide infra). In 2b, the corresponding band was at 1578 cm −1 .
The NMR spectra of 2b displayed one set of resonances, while the NMR spectra of 2a and 2c contained two sets of resonances ascribable to the α and β isomers, differing in the orientation of the N-substituents with respect to the cyanate ligand. This kind of isomerism was previously reported for diiron and diruthenium complexes of the type [M 2 Cp 2 (L)(CO)(µ-CO){µ-CN(Me)(R)}] 0/+ with R = Me [36,37]. The α isomer (R pointing to NCO) was slightly prevalent with respect to the β isomer (R pointing to terminal CO) in the case of 2c (α/β ratio = 1.3), whereas the bulkier xylyl group made the α form much more favorable in 2a (α/β ratio = 4.5). Notably, that rotation of the amine group around the carbyne-nitrogen axis was inhibited due to the substantial C=N double bond character [36,37]. Ongoing from a to b, the N-Me resonance shifted to high fields by ca. 0.3 ppm in the 1 H NMR spectra. The salient 13 C NMR feature was provided by the carbyne carbon, resonating within the 329-338 ppm range [36,53].
Motivated by our interest in the chemistry of carbamato species [54][55][56] and based on the documented reactivity of isocyanates [12,57,58], we tested the reactivity of 2a,c with a range of alcohols and amines, but no reaction occurred even under high-temperature conditions. The substantial inertness of the isocyanate ligand in 2a was observed even towards strong electrophiles (i.e., methyl triflate and trimethylsilyl triflate).
The reaction of the benzyl-aminocarbyne complex [1d NCMe ]CF 3 SO 3 with tetrabutylammonium thiocyanate was conducted in dichloromethane at room temperature and led to 3, which was finally isolated in 80% yield. The IR spectrum of 3 (in CH 2 Cl 2 ) displayed carbonyl absorptions at 1970 and 1810 cm −1 , which were indicative of the trans configuration of the Cp ligands. Moreover, the {NCS} group was N-coordinated to the iron center, on account of an infrared absorption at 2114 cm −1 . This value was very close to that previously detected in other {Fe-NCS} species [2,15,39]. The NMR spectra of 3 pointed out the occurrence of α/β isomerism, with the α isomer prevailing. The same behavior was observed when related methyl-and benzyl-aminocarbyne complexes were used (Scheme 1) [39].
The reactivity of the diiron aminocarbyne complexes with a selenium compound was investigated for the first time by allowing [1a NCMe -1c NCMe ]CF 3 SO 3 to react with potassium selenocyanate in acetone. When these reactions were conducted at room temperature, complicated mixtures of products were afforded, including modest amounts of 6a-c. Careful alumina chromatography on the mixture arising from [1a NCMe ]CF 3 SO 3 allowed the separation of three components, which were spectroscopically analyzed and thus identified as complexes 4a (15% yield), 5a (13%), and 6a (48%). Complexes 4a and 5a comprised Seand N-coordinated selenocyanate ligands, respectively, while 6a was a cyanide adduct. The infrared absorption for the pseudohalide ligand was detected at 2113 (4a) and 2109 cm −1 (5a); in general, the stretching vibration of a metal-coordinated {SeCN} group occurred at higher frequencies when it was Se-coordinated rather than N-coordinated [21,[59][60][61][62]. The 77 Se NMR spectra of 4a and 5a clearly pointed out the different coordinations of the {SeCN} moiety. Thus, two signals were recognized in the 77 Se spectrum of 4a, at −232.5 (major) and −246.9 ppm (minor) [63,64], while the 77 Se spectrum of 5a displayed a unique signal at −340.2 ppm. This picture was consistent with the literature data reported for other complexes and the general trend whereby 77 Se NMR shielding increases from {M-SeCN} ongoing to {M-NCSe} [65]. The 1 H and 13 C NMR spectra of 4a displayed two sets of resonances, attributed to cis and trans isomers, while the 1 H and 13 C NMR spectra of 5a closely resembled those related to the homologous complex featuring a cis arrangement of the Cp rings and an N-coordinated NCS ligand [39]. In the 13 C NMR spectrum of 4a, the resonance for the selenocyanate ligand occurred above 128.7 ppm, whereas the iso-selenocyanate resonated at 108.4 ppm in the 13 C NMR spectrum of 5a [63,66,67]. The signal for the aminocarbyne carbon was upfield shifted in 5a (340.4 ppm) compared to 4a (345.4 ppm in the trans isomer), suggesting a different degree of back-donation from the diiron backbone to the carbyne in 4a and 5a [36]. Formation of cis/trans mixtures is believed to be consequent to rotation around the Fe-Fe bond (Adams-Cotton mechanism) [68,69], which is operative during the nitrile substitution process. In the majority of the cases, trans isomers based on the Fe 2 Cp 2 (CO) x (x = 2, 3) scaffold are kinetic and less thermodynamically favored products, which might be observed due to a combination of electronic and steric effects [36,37,39,70]. In the case of 4a and 5a, stability studies revealed that the trans to cis route was not viable in boiling THF solution, whereas formation of cyanide complexes was observed (vide infra). Evidence for the formation of (iso)selenocyanate complexes (IR spectroscopy) was supplied by the room temperature reactions of [1b,c NCMe ]CF 3 SO 3 with KSeCN, but attempts to isolate and characterize the products failed.
With the aim of elucidating structural aspects, DFT calculations were carried out on the products obtained from the reaction of [1a NCMe ] + with selenocyanate, taking into account the spectroscopic outcomes. Views of the most stable cis and trans isomers of 4a and of the most stable cis isomer of 5a are provided in Figure 1; all structures exhibited an α arrangement of the substituents on the aminocarbyne moiety with respect to the seleniumcontaining ligand. Selected computed bond lengths are summarized in the caption of Figure 1. The Se-coordination of the selenocyanate anion was meaningfully bent, with computed Fe2-Se-C angles between 107 • and 114 • . On the other hand, the alternative N-coordination was almost linear, with the computed Fe2-N-C angle in cis-5a around 180 • . The C-N distance in selenocyanate was scarcely affected by the coordination mode, while on the other hand, the C-Se bond was elongated by more than 0.03 Å when the bonding to iron occurred with the selenium atom. A comparison of the computed bond lengths between cis-4a and cis-5a revealed that the Fe2-C bond lengths were negligibly affected by the selenocyanate coordination mode. In accordance with the experimental outcomes, the ν CN stretching of selenocyanate was predicted at slightly higher wavenumbers for the 4a isomers (unscaled values 2322 cm −1 for cis-4a and 2316 cm −1 for trans-4a) with respect to 5a (unscaled value 2314 cm −1 ). From a thermodynamic point of view, cis-4a was less stable than cis-5a by about 7.7 kcal mol −1 , and the Gibbs energy difference between trans-4a and cis-4a was about 4.7 kcal mol −1 in favor of the cis isomer. The relative Gibbs energy values, therefore, indicated that the reaction of [1a NCMe ] + with selenocyanate afforded a mixture of kinetic products and that conversion of one product into another could take place as promoted by alumina during chromatography. The preference for Nrather than Se-coordination was computationally established for the mono iron systems [3].
Complex 6a displayed an infrared band at 2090 cm −1 , accounting for iron N-coordinated cyanide receiving a significant back-donation [71][72][73]; the carbonyl ligands manifested themselves as two IR bands at 1959 and 1808 cm −1 , suggesting the trans configuration of the Cp ligands [74]. In fact, the cis isomer of 6a was previously synthesized from the room temperature reaction of [1a-NCMe] with NBu 4 CN [75], and its IR spectrum in the same conditions (CH 2 Cl 2 solution) consisted of three absorptions at 2091 (C≡N), 1982 (CO), and 1804 cm −1 (CO). The 1 H NMR spectrum of 6a revealed the presence of a minor amount of the cis isomer (<15%).
It appeared that the isolation of 6a from the reaction of [2a NCMe ]CF 3 SO 3 with KSeCN at room temperature was the result of the preliminary coordination of selenocyanate, followed by a rearrangement giving 6a and releasing one atom of selenium. To confirm this hypothesis, we performed the same reaction in tetrahydrofuran at reflux temperature; in this condition, 6a was the only isolated product accompanied by the formation of a black solid (presumably elemental selenium). The elimination of selenium probably followed activation of the Se-C bond, therefore compounds of type 4a were most likely involved in the reaction. The computed Gibbs free energy variation for the reaction trans-4a → trans-6a + 1/8 Se 8 was negative by 8.0 kcal mol −1 , in alignment with a thermodynamically favorable process. Reactants and products are depicted in Figure 2. Complex 6a displayed an infrared band at 2090 cm −1 , accounting for iron N-coordinated cyanide receiving a significant back-donation [71][72][73]; the carbonyl ligands manifested themselves as two IR bands at 1959 and 1808 cm −1 , suggesting the trans configuration of the Cp ligands [74]. In fact, the cis isomer of 6a was previously synthesized from the room temperature reaction of [1a-NCMe] with NBu4CN [75], and its IR spectrum in the same conditions (CH2Cl2 solution) consisted of three absorptions at 2091 (C≡N), 1982  lowed by a rearrangement giving 6a and releasing one atom of selenium. To confirm this hypothesis, we performed the same reaction in tetrahydrofuran at reflux temperature; in this condition, 6a was the only isolated product accompanied by the formation of a black solid (presumably elemental selenium). The elimination of selenium probably followed activation of the Se-C bond, therefore compounds of type 4a were most likely involved in the reaction. The computed Gibbs free energy variation for the reaction trans-4a → trans-6a + 1/8 Se8 was negative by 8.0 kcal mol −1 , in alignment with a thermodynamically favorable process. Reactants and products are depicted in Figure 2.  Similarly, the thermal reactions of [2b,c NCMe ]CF3SO3 with KSeCN provided a direct route to the cyanide complexes 6b-c (53-67% yields). These results confirmed the low thermal stability of the selenocyanate complexes with respect to the corresponding cyanide derivatives. The IR spectra of 6b-c were quite similar to that of 6a, thus suggesting the predominance of trans species. The NMR spectra of 6b-c displayed two sets of resonances. These were attributed to trans and cis isomers in the case of 6b, with the former largely prevalent (83%), and to  and  isomers in the case of 6c (isomer ratio 1.3). The diagnostic 13 C resonance for the cyanide ligand occurred within the range 139.7-141.4 ppm. Former computational studies highlighted the higher stability of the trans isomer of 6b with respect to the corresponding cis isomer [76].
The X-ray structure of trans-6b was determined by single crystal X-ray diffraction ( Figure 3). This represented a very rare case of the [Fe2Cp2(L)(CO)(µ-CO){µ-CN(Me)(R)}] n (L = mono-anionic ligand, n = 0; L = neutral ligand, n = 1+) complex possessing a trans geometry. The overall structure and bonding parameters were very similar to those reported for [Fe2Cp2(NCS)(CO)(µ-CO){µ-CN(Me)2}] [39]. The Fe(2)-C(4) interaction Similarly, the thermal reactions of [2b,c NCMe ]CF 3 SO 3 with KSeCN provided a direct route to the cyanide complexes 6b-c (53-67% yields). These results confirmed the low thermal stability of the selenocyanate complexes with respect to the corresponding cyanide derivatives. The IR spectra of 6b-c were quite similar to that of 6a, thus suggesting the predominance of trans species. The NMR spectra of 6b-c displayed two sets of resonances. These were attributed to trans and cis isomers in the case of 6b, with the former largely prevalent (83%), and to α and β isomers in the case of 6c (isomer ratio 1.3). The diagnostic 13 C resonance for the cyanide ligand occurred within the range 139.7-141.4 ppm. Former computational studies highlighted the higher stability of the trans isomer of 6b with respect to the corresponding cis isomer [76].

Materials and Methods
Reactants and solvents were purchased from Alfa Aesar, Merck, Strem, or TCI Chemicals and were of the highest purity available. Diiron complexes [1a-d]CF3SO3 were prepared according to the literature [78,79]. Reactions were conducted under N2 atmosphere using standard Schlenk techniques. Products were stored in air once isolated. Dichloromethane and tetrahydrofuran were dried using the solvent purification system mBraun MB SPS5, while acetonitrile was distilled from CaH2. IR spectra of solutions were recorded using a CaF2 liquid transmission cell (2300-1500 cm −1 ) on a Perkin Elmer Spectrum 100 FTIR spectrometer. IR spectra were processed with Spectragryph software [80]. 1 H, 13 C, and 77 Se NMR spectra were recorded at 298 K on a Jeol JNM-ECZ500R instrument equipped with a Royal HFX Broadband probe. Chemical shifts (expressed in parts per million) were referenced to the residual solvent peak in 1 H and 13 C NMR spectra [81] and to an external standard (Me2Se) in 77 Se NMR spectra. NMR spectra were assigned with the assistance of 1 H-13 C (gs-HSQC and gs-HMBC) correlation experiments [82]. NMR signals due to secondary isomeric forms (where it is possible to detect them) are italicized. Elemental analyses were performed using a Vario MICRO cube instrument (Elementar).

Materials and Methods
Reactants and solvents were purchased from Alfa Aesar, Merck, Strem, or TCI Chemicals and were of the highest purity available. Diiron complexes [1a-d]CF 3 SO 3 were prepared according to the literature [78,79]. Reactions were conducted under N 2 atmosphere using standard Schlenk techniques. Products were stored in air once isolated. Dichloromethane and tetrahydrofuran were dried using the solvent purification system mBraun MB SPS5, while acetonitrile was distilled from CaH 2 . IR spectra of solutions were recorded using a CaF 2 liquid transmission cell (2300-1500 cm −1 ) on a Perkin Elmer Spectrum 100 FTIR spectrometer. IR spectra were processed with Spectragryph software [80]. 1 H, 13 C, and 77 Se NMR spectra were recorded at 298 K on a Jeol JNM-ECZ500R instrument equipped with a Royal HFX Broadband probe. Chemical shifts (expressed in parts per million) were referenced to the residual solvent peak in 1 H and 13 C NMR spectra [81] and to an external standard (Me 2 Se) in 77 Se NMR spectra. NMR spectra were assigned with the assistance of 1 H-13 C (gs-HSQC and gs-HMBC) correlation experiments [82]. NMR signals due to secondary isomeric forms (where it is possible to detect them) are italicized. Elemental analyses were performed using a Vario MICRO cube instrument (Elementar).

General Procedure
A solution of [1a-c]CF 3 SO 3 (ca 0.4 mmol) in MeCN (20 mL) was treated with Me 3 NO·2H 2 O (1.1 eq.) and the resulting mixture was stirred for 1 h, during which progressive color darkening was observed. The conversion of the starting material into the acetonitrile adduct [1 NCMe ] + was checked by IR spectroscopy, as is routine for this type of reaction [75]. Volatiles were removed under vacuum to give a brown residue, which Molecules 2023, 28, 3251 9 of 18 was dissolved in deaerated acetone (30 mL), and NaOCN (3.0 eq.) was added to this solution. The resulting mixture was stirred for 18 h at room temperature and then the solvent was evaporated under reduced pressure. The resulting solid was dissolved in the minimum volume of CH 2 Cl 2 and this solution was charged on an alumina column. Impurities were separated using neat CH 2 Cl 2 and neat THF as eluents, and then a brown fraction corresponding to 2a-c was collected with MeCN. The solvent was removed under reduced pressure, and the residue was suspended in hexane for 24 h. After filtration, the separated solid was dried under vacuum. [

General Procedure
A solution of [1a-c]CF3SO3 (ca 0.4 mmol) in MeCN (20 mL) was treated with Me3NO·2H2O (1.1 eq.) and the resulting mixture was stirred for 1 h, during which progressive color darkening was observed. The conversion of the starting material into the acetonitrile adduct [1 NCMe ] + was checked by IR spectroscopy, as is routine for this type of reaction [75]. Volatiles were removed under vacuum to give a brown residue, which was dissolved in deaerated acetone (30 mL), and NaOCN (3.0 eq.) was added to this solution. The resulting mixture was stirred for 18 h at room temperature and then the solvent was evaporated under reduced pressure. The resulting solid was dissolved in the minimum volume of CH2Cl2 and this solution was charged on an alumina column. Impurities were separated using neat CH2Cl2 and neat THF as eluents, and then a brown fraction corresponding to 2a-c was collected with MeCN. The solvent was removed under reduced pressure, and the residue was suspended in hexane for 24 h. After filtration, the separated solid was dried under vacuum. [

General Procedure
A solution of [1a-c]CF3SO3 (ca 0.4 mmol) in MeCN (20 mL) was treated Me3NO·2H2O (1.1 eq.) and the resulting mixture was stirred for 1 h, during whic gressive color darkening was observed. The conversion of the starting material in acetonitrile adduct [1 NCMe ] + was checked by IR spectroscopy, as is routine for this t reaction [75]. Volatiles were removed under vacuum to give a brown residue, whic dissolved in deaerated acetone (30 mL), and NaOCN (3.0 eq.) was added to this so The resulting mixture was stirred for 18 h at room temperature and then the solve evaporated under reduced pressure. The resulting solid was dissolved in the min volume of CH2Cl2 and this solution was charged on an alumina column. Impuritie separated using neat CH2Cl2 and neat THF as eluents, and then a brown fraction sponding to 2a-c was collected with MeCN. The solvent was removed under re pressure, and the residue was suspended in hexane for 24 h. After filtration, the sep solid was dried under vacuum.
[   reaction [75]. Volatiles were removed under vacuum to give a brown residue, w dissolved in deaerated acetone (30 mL), and NaOCN (3.0 eq.) was added to this The resulting mixture was stirred for 18 h at room temperature and then the so evaporated under reduced pressure. The resulting solid was dissolved in the m volume of CH2Cl2 and this solution was charged on an alumina column. Impur separated using neat CH2Cl2 and neat THF as eluents, and then a brown fracti sponding to 2a-c was collected with MeCN. The solvent was removed under pressure, and the residue was suspended in hexane for 24 h. After filtration, the s solid was dried under vacuum. (Figure 4).

Me3NO·2H2O
(1.1 eq.) and the res gressive color darkening was obse acetonitrile adduct [1 NCMe ] + was ch reaction [75]. Volatiles were remov dissolved in deaerated acetone (30 The resulting mixture was stirred f evaporated under reduced pressu volume of CH2Cl2 and this solution separated using neat CH2Cl2 and n sponding to 2a-c was collected w pressure, and the residue was susp solid was dried under vacuum. [Fe2Cp2(kN-NCO)(CO)(µ-CO    separated using neat CH2Cl2 and neat THF as eluents, and then a brown fraction corresponding to 2a-c was collected with MeCN. The solvent was removed under reduced pressure, and the residue was suspended in hexane for 24 h. After filtration, the separated solid was dried under vacuum.     Figure 7). [ (Figure 6).   A mixture of [1d NCMe ]CF 3 SO 3 , freshly generated from [1d]CF 3 SO 3 (90 mg, 0.148 mmol) according to the procedure above, and NBu 4 SCN (220 mg, 0.733 mmol) in CH 2 Cl 2 (12 mL) was stirred for 3 h at room temperature. The final solution was directly charged on an alumina column, and elution with neat dichloromethane afforded the fraction corresponding to the title product. Thus, volatiles were removed under vacuum to give an orange solid. Yield

A solution of [1a-c]CF3SO3
(ca 0.4 mmol) in MeCN (20 mL) was treated with Me3NO·2H2O (1.1 eq.) and the resulting mixture was stirred for 1 h, during which progressive color darkening was observed. The conversion of the starting material into the acetonitrile adduct [1 NCMe ] + was checked by IR spectroscopy, as is routine for this type of reaction [75]. Volatiles were removed under vacuum to give a brown residue, which was dissolved in deaerated acetone (30 mL), and NaOCN (3.0 eq.) was added to this solution. The resulting mixture was stirred for 18 h at room temperature and then the solvent was evaporated under reduced pressure. The resulting solid was dissolved in the minimum volume of CH2Cl2 and this solution was charged on an alumina column. Impurities were separated using neat CH2Cl2 and neat THF as eluents, and then a brown fraction corresponding to 2a-c was collected with MeCN. The solvent was removed under reduced pressure, and the residue was suspended in hexane for 24 h. After filtration, the separated solid was dried under vacuum.       . Volatiles were evaporated under reduced pressure, hence a solution of the residue in the minimum volume of dichloromethane was charged on an alumina column. Neat CH 2 Cl 2 allowed the separation of a light green fraction corresponding to 4a, while CH 2 Cl 2 /THF (9/1 v/v) mixture was used to collect a red fraction corresponding to 5a. After removing impurities with THF, elution with MeCN/MeOH (9/1 v/v) led to the separation of a dark green fraction corresponding to 6a. For each fraction, the solvent was removed under reduced pressure, and the residue was suspended in hexane for 24 h. After filtration, the solid product was dried under vacuum.
[   2112 m (SeCN), 2068 w, 1970 vs-br (CO), 1812 s-br (µ-CO)]. Volatiles were evaporated under reduced pressure, hence a solution of the residue in the minimum volume of dichloromethane was charged on an alumina column. Neat CH2Cl2 allowed the separation of a light green fraction corresponding to 4a, while CH2Cl2/THF (9/1 v/v) mixture was used to collect a red fraction corresponding to 5a. After removing impurities with THF, elution with MeCN/MeOH (9/1 v/v) led to the separation of a dark green fraction corresponding to 6a. For each fraction, the solvent was removed under reduced pressure, and the residue was suspended in hexane for 24 h. After filtration, the solid product was dried under vacuum.     2112 m (SeCN), 2068 w, 1970 vs-br (CO), 1812 s-br (µ-CO)]. Volati were evaporated under reduced pressure, hence a solution of the residue in the minimu volume of dichloromethane was charged on an alumina column. Neat CH2Cl2 allowed separation of a light green fraction corresponding to 4a, while CH2Cl2/THF (9/1 v/v) m ture was used to collect a red fraction corresponding to 5a. After removing impurities w THF, elution with MeCN/MeOH (9/1 v/v) led to the separation of a dark green fract corresponding to 6a. For each fraction, the solvent was removed under reduced pressu and the residue was suspended in hexane for 24 h. After filtration, the solid product w dried under vacuum.

General Procedure
(1.1 eq.) and the res gressive color darkening was obse acetonitrile adduct [1 NCMe ] + was ch reaction [75]. Volatiles were remov dissolved in deaerated acetone (30 The resulting mixture was stirred evaporated under reduced pressu volume of CH2Cl2 and this solutio separated using neat CH2Cl2 and sponding to 2a-c was collected w pressure, and the residue was susp solid was dried under vacuum. [Fe2Cp2(kN-NCO)(CO)(µ-CO

General Procedure
The acetonitrile adduct was prepared from [1b,c]CF3SO3 as described above, the was dissolved in deaerated acetone (30 mL), and KSeCN (3.0 eq.) was added to this so tion. The resulting mixture was stirred for 16 h at room temperature. The volatiles w evaporated under reduced pressure, thus the residue was dissolved in dichlorometh and this solution was charged on an alumina column. Neat CH2Cl2 allowed the separat of impurities, and then CH2Cl2/THF (2/1 v/v) mixture was used to collect a brown fracti Elution with MeCN/MeOH (9/1 v/v) led to the separation of a dark green fraction co sponding to 6b-c. For each fraction, the solvent was removed under reduced pressu and the residue was suspended in hexane for 24 h. After filtration, isolated products ( brown solid and green solid) were dried under vacuum. separated using neat CH2Cl2 and n sponding to 2a-c was collected w pressure, and the residue was susp solid was dried under vacuum.
[Fe2Cp2(kN-NCO)(CO)(µ-CO  gressive color darkening was observed. T acetonitrile adduct [1 NCMe ] + was checked b reaction [75]. Volatiles were removed und dissolved in deaerated acetone (30 mL), an The resulting mixture was stirred for 18 h evaporated under reduced pressure. The volume of CH2Cl2 and this solution was ch separated using neat CH2Cl2 and neat TH sponding to 2a-c was collected with MeC pressure, and the residue was suspended i solid was dried under vacuum.

General Procedure
The acetonitrile adduct was prepared from [1b,c]CF 3 SO 3 as described above, then it was dissolved in deaerated acetone (30 mL), and KSeCN (3.0 eq.) was added to this solution. The resulting mixture was stirred for 16 h at room temperature. The volatiles were evaporated under reduced pressure, thus the residue was dissolved in dichloromethane and this solution was charged on an alumina column. Neat CH 2 Cl 2 allowed the separation of impurities, and then CH 2 Cl 2 /THF (2/1 v/v) mixture was used to collect a brown fraction. Elution with MeCN/MeOH (9/1 v/v) led to the separation of a dark green fraction corresponding to 6b-c. For each fraction, the solvent was removed under reduced pressure, and the residue was suspended in hexane for 24 h. After filtration, isolated products (i.e., brown solid and green solid) were dried under vacuum. (1
When the reaction of [1b NCMe ]CF3SO3 (102 mg, 0.19 mmol) with KSeCN (122 mg, 0 mmol) was conducted in THF (or acetone) at reflux temperature, complete conversion the starting material into 6b was checked via IR spectroscopy after 4 h; 6b was isolated 53% yield after alumina chromatography.
[Fe2Cp2(CN)(CO)(µ-CO){µ-CNMe2}], 6b ( Figure 11).   The resulting mixture was stirred for 18 h at room temperatur evaporated under reduced pressure. The resulting solid was volume of CH2Cl2 and this solution was charged on an alumin separated using neat CH2Cl2 and neat THF as eluents, and th sponding to 2a-c was collected with MeCN. The solvent wa pressure, and the residue was suspended in hexane for 24 h. Af solid was dried under vacuum.

A solution of [1a-c]CF3SO3
(ca 0.4 mmol) in MeCN ( Me3NO·2H2O (1.1 eq.) and the resulting mixture was stirred gressive color darkening was observed. The conversion of the acetonitrile adduct [1 NCMe ] + was checked by IR spectroscopy, a reaction [75]. Volatiles were removed under vacuum to give a dissolved in deaerated acetone (30 mL), and NaOCN (3.0 eq.) The resulting mixture was stirred for 18 h at room temperatur evaporated under reduced pressure. The resulting solid was volume of CH2Cl2 and this solution was charged on an alumin separated using neat CH2Cl2 and neat THF as eluents, and th sponding to 2a-c was collected with MeCN. The solvent wa pressure, and the residue was suspended in hexane for 24 h. Af solid was dried under vacuum.

X-ray Crystallography
Crystal data and collection details for trans-6b are reported in Table 1. Data were recorded on a Bruker APEX II diffractometer equipped with a PHOTON2 detector using Mo-Kα radiation. The structures were solved by direct methods and refined by full-matrix least-squares based on all data using F 2 [83]. Hydrogen atoms were fixed at calculated positions and refined using a riding model.

Computational Details
Geometry optimizations were performed using the PBEh-3c method, which is a reparametrized version of PBE0 [84] (with 42% HF exchange) that uses a split-valence double-zeta basis set (def2-mSVP) [85,86] and adds three corrections considering dispersion, basis set superposition, and other basis set incompleteness effects [87][88][89]. The C-PCM implicit solvation model was added to PBEh-3c calculations, considering acetone as a continuous medium [90,91]. IR simulations were carried out using harmonic approximation, from which zero-point vibrational energies and thermal corrections (T = 298.15 K) were obtained [92]. The software used was ORCA version 5.0.3 [93].

Conclusions
New diiron aminocarbyne complexes with a terminal chalcogen-containing pseudohalide ligand were synthesized, and their stereochemistry and thermodynamic stability were investigated by IR and NMR spectroscopy and DFT calculations. Combined with previous findings, this work highlights that N-coordination generally prevails over the alternative coordination mode, although different kinetic products may be formed, and that the reactivity of the NCE − ligand increases along the sequence O (inertness) < S (electrophilic addition) < Se (chalcogen elimination). In particular, we provide clear evidence for the formation of diiron cyanide complexes from the fragmentation of the selenocyanate fragment.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28073251/s1, NMR spectra of products and DFT data. CCDC reference number 2225698 (trans-6b) contains the supplementary crystallographic data for the X-ray study reported in this paper.