Structures and Properties of Dinitrosyl Iron and Cobalt Complexes Ligated by

: Two dinitrosyl iron and cobalt complexes [Fe(NO) 2 (L1”)](BF 4 ) and [Co(NO) 2 (L1”)](BF 4 ) are synthesized and characterized, supported by a less hindered bidentate nitrogen ligand bis(3,5-diisopropyl-1-pyrazolyl)methane (denoted as L1”), are surprisingly stable under argon atmosphere. X-ray structural analysis shows a distorted tetrahedral geometry. Spectroscopic and structural parameters of the dinitrosyl iron and cobalt complexes are consistent with the previous reported {Fe(NO) 2 } 9 and {Co(NO) 2 } 10 . Two N–O and M–N(O) stretching frequencies and their magnetic properties are also consistent with the above electronic structural assignments. We explored the dioxygen reactivities of the obtained dinitrosyl complexes. Moreover, the related [FeCl 2 (L1”)], [Co(NO 3 ) 2 (L1”)], and [Co(NO 2 ) 2 (L1”)] complexes are also characterized in detail. this an ether solution mL) of iodine (1.08 g, 4.26 mmol) was added dropwise over 1 h. About 200 cm 3 of NO was passed through the solution, and the mixture was stirred at room temperature for 1 h, which was then stirred for another 1 h. After the reaction, the solvent was removed under vacuum. Dark brown crystals were obtained by sublimation of the complex. mL) was added and the mixture was stirred at 0 ◦ C for 30 min. The brown solution was ﬁltered o ﬀ using Celite. The solvent was removed under vacuum. Recrystallization from tetrahydrofuran / n -heptane at − 30 ◦ C gave dark brown crystals. Single crystals suitable for X-ray di ﬀ raction were obtained by slow recrystallization under the same experimental conditions.

Moreover, NO itself is a redox active molecule [17,18]. Therefore, it is very difficult to define its oxidation state. Due to the complicated electronic structures of transition-metal complexes with NO, the oxidation state of the metal NO unit is typically indicated using the Enemark−Feltham notation, in which the metal nitrosyl is denoted by {M(NO) x } n . Here, x is the number of nitrosyl ligand(s) and n is the sum of the metal-d and NO-π* electrons [19].
In our previous works, a series of first row transition metal complexes with NO, [M(NO)(L3)] (M = Fe, Co, Ni, and Cu), were prepared, structurally characterized and ligated by the same hydrotris(pyrazolyl)borate coligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3 − , Scheme 1 left) [32][33][34][35]. Our synthetic strategy is to use exactly the same hindered supporting ligand to make four coordinate mono-nitrosyl transition metal complexes to directly compare their electronic properties and reactivities. However, by using this hindered tridentate ligand, we do not synthesize any dinitrosyl complexes as a stable form. The reaction of the iron complex [Fe(NO)(L3)] with NO gas, we found the formation of DNIC as [Fe(NO) 2 (L3)] with two N-O stretching vibrations at 1805 and 1732 cm −1 in its IR spectrum. However, this obtained DNIC is not stable, [Fe(NO)(L3)] is easily recovered upon drying the obtained brown colored solution in vacuo [32]. Therefore, the detailed structure and properties of the DNIC [Fe(NO) 2 (L3)] cannot be characterized in detail. Thus, we explored a less hindered bidentate nitrogen-containing ligand to stabilize DNICs. stabilize DNICs.

Synthesis
In our previous synthetic method of [Fe(NO)(L3)], we used [Fe(NO) 2 (µ-I)] 2 as an iron and NO sources. In this work, we followed the same method to synthesize the dinitrosyl iron complex.

Structure
The

Reactivity Toward Dioxygen
Both  Figure S19). These new peaks appearances at 1549 and 1287 cm −1 are also detected by the dioxygen reaction in the solid state ( Figure S20). This suggests that the same product was formed in both solution and solid states. As similar to the dinitrosyl iron complex The paramagnetic 1 H-NMR spectrum of [FeCl 2 (L1")] shows resonances in the range from 52.5 to −7.24 ppm ( Figure S15). In general, the protons closest to the paramagnetic metal ion suffer the largest chemical shift and line broadening [44,45]. In this [FeCl 2 (L1")], the fourth position proton of the pyrazolyl ring is observed to have the largest downfield shift to 52.5 ppm (integration of 2 protons). The 1 H-NMR spectra of [Co(κ 2 -O 2 NO) 2 (L1")] and [Co(κ 2 -O 2 N) 2 (L1")] show paramagnetic shifts in the range from 49.6 to −1.03 ppm and from 46.3 to −7.05 ppm, respectively (Figures S16 and S17).  Figures S16, S17, S23 and S24). From these results, the accurate identification of the oxidation products is very difficult in this research. We need more detailed research to decide what happens in this dioxygen reaction. In [Fe(NO) 2 (L1")](BF 4 ), the reaction rate is faster than the rate in the related dinitrosyl cobalt complex. However, we cannot also decide the dioxygen reaction products in this research.  Figures S16 and S17, S23 and S24). From these results, the accurate identification of the oxidation products is very difficult in this research. We need more detailed research to decide what happens in this dioxygen reaction. In [Fe(NO)2(L1")](BF4), the reaction rate is faster than the rate in the related dinitrosyl cobalt complex. However, we cannot also decide the dioxygen reaction products in this research.

Material and General Techniques
The preparation and handling of all the complexes was performed under an argon atmosphere

Material and General Techniques
The preparation and handling of all the complexes was performed under an argon atmosphere using standard Schlenk tube techniques. Dichloromethane was purified by distillation from phosphorous pentoxide under an argon atmosphere [47]. Diethyl ether, tetrahydrofuran (THF) and n-heptane were distilled from sodium benzophenone ketyl under an argon atmosphere. Super-dehydrated acetone and methanol were purchased from Wako Pure Chemical Ind. Ltd. and deoxygenated by purging with argon gas. NO gas was purchased from TOKAI Holdings and purified by passing through a column filled with solid NaOH. Deuterated solvents in NMR experiments and 15 N 18 O gas in IR/far-IR experiments were obtained from Cambridge Isotope Laboratories, Inc and Shoko Co. Ltd., respectively. [Fe(NO) 2 (µ-I)] 2 and [Fe( 15 N 18 O) 2 (µ-I)] 2 and were prepared as described [32,48,49]. Other reagents are commercially available and were used without further purification. L1" (bis(3,5-diisopropyl-1-pyrazolyl)methane) was prepared as reported previously [36].

Instrumentation
Infrared (IR) and far-IR spectra were recorded on KBr pellets (4000-400 cm −1 ) using a JASCO FT/IR-6300 spectrometer (JASCO, Tokyo, Japan) and CsI pellets (650-150 cm −1 ) using a JASCO FT/IR-6200 spectrometer (JASCO, Tokyo, Japan), respectively. Abbreviations used in the description of vibrational data are as follows: vs, very strong; s, strong; m, medium; w, weak. IR spectra of solution samples were obtained in thin-layer solution cells equipped with KBr windows on the same instrument. Electronic absorption (UV-Vis) spectra (dichloromethane solution and solid 240-1040 nm) were recorded on a JASCO V-570 spectrophotometer (JASCO, Tokyo, Japan). Diffuse reflectance (DR) spectra were obtained with the JASCO V-570 spectrophotometer equipped with an integrating sphere apparatus (JASCO ISN-470) using fine powder samples. 1 H-NMR (500 MHz) spectra were obtained on a Bruker AVANCE-500 NMR spectrometer (Bruker Japan, Yokohama, Japan) at room temperature (298 K). Chemical shifts were reported as δ values relative to solvent residual signals and an internal standard (tetramethylsilane). The multiplicity of each signal is designated by the following abbreviations: s, singlet; d, doublet; sept, septet; br; broad. EPR spectra were recorded on a JEOL JES-RE2X EPR spectrometer (JEOL, Tokyo, Japan) in frozen dichloromethane solution at low temperature (−130 K) in quartz tubes (diameter 5 mm) with liquid-nitrogen temperature controller JEOL DVT UNIT. The elemental analyses (C, H, and N) were performed by the Center for Instrumental Analysis at Ibaraki University.

[Fe(NO) 2 (L1")](BF 4 )
To a solution of [Fe(NO) 2 (µ-I)] 2 0.0438 g (0.090 mmol) in dichloromethane (10 mL) was added L1" (0.0575 g, 0.182 mmol) in dichloromethane (10 mL). The resulting black green colored solution was stirred at 0 • C under an argon atmosphere for 2 h. After that, a solution containing 0.0380 g (0.195 mmol) of AgBF 4 in acetone (5 mL) was added dropwise and the mixture was stirred at 0 • C for 30 min. The green solution was filtered off using Celite. The solvent was removed under vacuum. Recrystallization from tetrahydrofuran/n-heptane at −30 • C gave dark green crystals. Single crystals suitable for X-ray diffraction were obtained by slow recrystallization under the same experimental conditions. Yield

[Co(NO) 2 (µ-I)] n
This complex was prepared using a modified version of the reported method [49]. To remove water, the starting cobalt powder 0.995 g (16.9 mmol) was settled in an oil bath at 150 • C for 6 h. After cooling to room temperature, acetone (23 mL) was added and the mixture was stirred for a few minutes. To this solution, an ether solution (34 mL) of iodine (1.08 g, 4.26 mmol) was added dropwise over 1 h. About 200 cm 3 of NO gas was passed through the solution, and the mixture was stirred at room temperature for 1 h, which was then stirred for another 1 h. After the reaction, the solvent was removed under vacuum. Dark brown crystals were obtained by sublimation of the complex.
Yield To a solution of [Co(NO) 2 (µ-I)] n (0.0370 g, 0.151 mmol) in dichloromethane (10 mL) L1" (0.0478 g, 0.151 mmol) in dichloromethane (10 mL) was added. The resulting dark brown colored solution was stirred at 0 • C under an argon atmosphere for 2 h. After that, a solution containing 0.0308 g (0.158 mmol) of AgBF 4 in acetone (5 mL) was added and the mixture was stirred at 0 • C for 30 min. The brown solution was filtered off using Celite. The solvent was removed under vacuum. Recrystallization from tetrahydrofuran/n-heptane at −30 • C gave dark brown crystals. Single crystals suitable for X-ray diffraction were obtained by slow recrystallization under the same experimental conditions. Yield: 81% (0.064 g, 0.123 mmol). Calcd for C 19   In a 50 mL Schlenk tube, [Fe(NO) 2 (L1")](BF 4 ) was dissolved in dichloromethane at room temperature in an argon atmosphere. The argon was then replaced with dioxygen gas and the solution was stirred at room temperature for 15 min. During this time, the color of the solution changed to yellow. After the reaction finished, the solvent was removed under vacuum. Then, the sample was used for IR measurement.

Reaction of [Fe(NO) 2 (L1")](BF 4 ) with O 2 in the Solid State
In a 50 mL Schlenk tube, [Fe(NO) 2 (L1")](BF 4 ) was added and the microcrystalline material was pulverized by stirring under an argon atmosphere. After that, the atmosphere was replaced with dioxygen gas. The color of the solid changed over time to yellow, which was further facilitated for 1 day. Then, the sample was used for IR measurement.

Reaction of [Co(NO) 2 (L1")](BF 4 ) with O 2 in Solution
In a 50 mL Schlenk tube, [Co(NO) 2 (L1")](BF 4 ) was dissolved in dichloromethane at room temperature in an argon atmosphere. The argon was then replaced with dioxygen gas and the solution was stirred at room temperature for 3 h. During this time, the color of the solution changed to purple. After the reaction finished, the solvent was removed under vacuum. Then, the sample was used for IR measurement.

Reaction of [Co(NO) 2 (L1")](BF 4 ) with O 2 in the Solid State
In a 50 mL Schlenk tube, [Co(NO) 2 (L1")](BF 4 ) was added and the microcrystalline material was pulverized by stirring under an argon atmosphere. After that, the atmosphere was replaced with dioxygen gas. The color of the solid changed over time to purple, which was further facilitated for 14 days. Then, the sample was used for IR measurement.

X-ray Crystal Structure Determination
Crystal data and refinement parameters for two dinitrosyl transition-metal complexes [Fe(NO) 2 (L1")](BF 4 ) and [Co(NO) 2 (L1")](BF 4 ) were listed in Table 3 and those of the related [FeCl 2 (L1")], [Co(NO 3 ) 2 (L1")], and [Co(NO 2 ) 2 (L1")] are given in Table 4. All crystallographic data have been deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK and copies can be obtained on request, free of charge, by quoting the publication citation and the deposition numbers. The unit cell parameters were determined using CrystalClear from 18 images [50]. The crystal to detector distance was ca. 45 mm. Data were collected using 0.5 • intervals in ϕ and ω to a maximum 2θ value of 55.0 • . The highly redundant data sets were reduced using CrystalClear (Rigaku Oxford Diffraction) [50]. An empirical absorption correction was applied for each complex. Structures were solved by direct methods (SIR2008) [51]. For the refinements, the position of the metal ions and their first coordination sphere were located from a direct method E-map; other non-hydrogen atoms were found in alternating difference Fourier syntheses, and least-squares refinement cycles. During the final refinement cycles the temperature factors were refined anisotropically. Refinement was carried out by a full matrix least-squares method on F 2 . All calculations were performed with the CrystalStructure [52] crystallographic software package except for refinement, which was performed using SHELXL 2013 [53]. Hydrogen atoms were placed in calculated positions. Unit weightings were used. The absolute configurations of [[Fe(NO) 2 (L1")](BF 4 ) and [Co(NO) 2 (L1")](BF 4 ) were confirmed from the values of the Flack parameters (0.041(7) and 0.018(5)) refined using 1698 and 2482 Parsons' quotients pairs, respectively [54]. For [Co(NO) 2 (L1")](BF 4 ), PLATON SQUEEZE was used to account for severely disordered solvent molecules [55].

Conclusions
We synthesized the dinitrosyl iron and cobalt complexes, [Fe(NO) 2 (L1")](BF 4 ) and [Co(NO) 2 (L1")](BF 4 ). X-ray analysis shows that the coordination geometry is a distorted tetrahedral, which are surprisingly stable under argon atmosphere. IR spectrum of [Fe(NO) 2 (L1")](BF 4 ) exhibits symmetric ν s (N-O) and asymmetric ν as (N-O) frequencies at 1831 and 1759 cm −1 , respectively, which are similar to those for the other cationic {Fe(NO) 2 } 9 dinitrosyl complexes. Moreover, [Co(NO) 2 (L1")](BF 4 ) has symmetric ν s (N-O) and asymmetric ν as (N-O) bands at 1875 and 1798 cm −1 , respectively. These values are consistent with those of the other reported {Co(NO) 2 } 10 complexes. These symmetric and asymmetric ν(N-O) frequencies are shifted upon isotopic substitutions. We also characterized ν(M-N(O)) frequencies by far-IR spectroscopy. UV-vis and magnetic properties results are also consistent with the above assignments. Finally, we tried to the reactivity of both dinitrosyl complex with dioxygen. The related [Co(κ 2 -O 2 NO) 2 (L1")] and [Co(κ 2 -O 2 N) 2 (L1")] complexes are synthesized and characterized as authentic samples. Compared between the dioxygen reaction products and the related nitrato and nitrito complexes in their IR spectra, [Co(κ 2 -O 2 NO) 2 (L1")] might be obtained from both solution and solid states dioxygen reactions. However, 1 H-NMR spectra of the dioxygen products and [Co(κ 2 -O 2 NO) 2 (L1")] are clearly different. Therefore, we cannot conclude these dioxygen products. It is very different from our mononitrosyl [Fe(NO)(L3)] results. We are now investigating the reactivity with external substrates by mononitrosyl and dinitrosyl complexes and the detailed characterization of these dinitorosyl complexes to reveal their oxidation states.