Matrix Infrared Spectra of Manganese and Iron Isocyanide Complexes

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Matrix Infrared Spectra of Manganese and Iron Isocyanide Complexes Xiuting Chen, Qingnuan Li, Lester Andrews, and Yu Gong J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09241 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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The Journal of Physical Chemistry

Matrix Infrared Spectra of Manganese and Iron Isocyanide Complexes Xiuting Chen1,2, Qingnuan Li1, Lester Andrews3 and Yu Gong1,3* 1 Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Department of Chemistry, University of Virginia, Charlottesville, VA, 22904-4319, United States Abstract: Mono and diisocyanide complexes of manganese and iron were prepared via the reactions of laser ablated manganese and iron atoms with (CN)2 in an argon matrix. Product identifications were carried out based on the characteristic infrared absorptions from isotopically labeled (CN)2 experiments as compared with computed values for both cyanides and isocyanides. Manganese atoms reacted with (CN)2 to produce Mn(NC)2 upon λ>220 nm irradiation, during which MnNC was formed mainly as a result of the photo-induced decomposition of Mn(NC)2. Similar reaction products FeNC and Fe(NC)2 were formed during the reactions of Fe and (CN)2. All the product molecules together with the unobserved cyanide isomers were predicted to have linear geometries at the B3LYP level of theory. The cyanide complexes of manganese and iron were computed to be more stable than the isocyanide isomers with energy differences between 0.4 and 4 kcal/mol at the CCSD(T) level. Although manganese and iron cyanide molecules are slightly more stable according to the theory, no absorption can be assigned to these isomers in the region above the isocyanides possibly due to their low infrared intensities. 1

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Introduction Middle transition metal cyanide ions such as ferricyanide and ferrocyanide have been known for a long time.1 Their well-characterized coordination structures and chemical properties make them classic examples of transition metal coordination complexes in the textbook.2 However, our understanding of the coordination chemistry of the corresponding neutral molecules is way behind due to the lack of experimental and theoretical results. In recent years, neutral triatomic molecules containing Fe, N and C have been the focus of a number of studies due to their importance in the interstellar medium. In addition, the element iron lies in the middle position for first row transition metals where the relative stability of cyanide and isocyanide switches.3 The FeNC molecule was first detected by laser fluorescence excitation spectroscopy, and a sextet ground state was assumed for this most stable isomer.4 A recent rotational spectroscopic study revealed that the ground state FeCN (4∆) molecule is more stable than ground state FeNC (6∆) by about 1.9 kcal/mol.5,6 Based on the spin-orbit patterns, a quartet ground state was established for FeCN and the results of FeNC were about the same as those from fluorescence measurements. Theoretical investigations using multireference and restricted open-shell singlereference methods predicted a 6∆ ground state for both FeCN and FeNC.7-9 The energy difference between the two isomers was around 1 kcal/mol, and the level of theory determined which isomer was slightly more stable. A very recent ab initio study employing coupled cluster theory up to full quadruple excitations with large basis set and multireference configuration interaction provided a 4∆ ground state 3



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FeCN and 6∆ ground state FeNC. However, the computed adiabatic transition energy was in poor agreement with the experiment and the ground state of FeNC changed when extra additive corrections were included.10 The controversial electronic structure of FeCN and FeNC was not completely resolved even at this very expensive and sophisticated level of theory. For Fe(CN)n (n=2-6) with higher coordination numbers, theoretical calculations predicted that the cyanide complex is more stable although the Fe(CN)2 and Fe(NC)2 isomers are very close in energy.11 In contrast to studies on the iron cyanide and isocyanide complexes, the manganese analogs have been barely investigated. Density functional theory and ab intio calculations revealed that MnNC was more stable than MnCN and both isomers possessed 7Σ+ ground states.3 No experimental result is available regarding the spectroscopic properties of binary manganese cyanide and isocyanide molecules. Our recent investigations have demonstrated that metal isocyanide molecules can be readily prepared via the reactions of metal atoms and (CN)2, and the calculations show that cyanides have more than 100 cm-1 higher frequencies and much weaker infrared intensities than the isocyanides.12,13 In this paper, we report the formation of manganese and iron isocyanide molecules via the reactions of laser-ablated iron and manganese atoms with (CN)2 in solid argon. Mono and diisocyanide molecules of manganese and iron with nitrogen bound CN ligands were identified based on the experimental vibrational frequencies as well as theoretical calculations. Experimental and theoretical methods The details on the experimental apparatus for investigating the reactions of 4


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laser-ablated manganese and iron atoms with (CN)2 in excess argon at 4 K have been described previously.14,15 The Nd:YAG laser fundamental (Continuum II, 1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a freshly cleaned manganese or iron target mounted on a rotating rod. Laser-ablated metal atoms were codeposited with argon (research grade) containing 1% (CN)2 gas prepared in this laboratory. We employed thermal decomposition of AgCN at 360-380︒C until a constant pressure was reached in a stainless steel vacuum line following earlier work.12,13 The product gas was condensed at 77 K and evacuated before use. Isotopic reagents were synthesized by thermal decomposition of Ag13CN and AgC15N, which were prepared via the reactions of K13CN and KC15N (99% enriched, Cambridge Isotopic Laboratories) and silver nitrate. FTIR spectra were recorded at 0.5 cm-1 resolution on a Nicolet iS50 FTIR instrument with a HgCdTe range A detector. Matrix samples were annealed at different temperatures and cooled back to 4 K for spectral acquisition. Selected samples were subjected to uv-vis photolysis by a medium-pressure mercury arc street lamp (Philips, 175W) with the outer globe removed. Density functional theory (DFT) calculations were performed using the Gaussian 09 program.16 The hybrid B3LYP density functional was employed in our calculations,17,18 and the 6-311+G(d) basis set was used for all the elements.19-22 Harmonic vibrational frequencies at the B3LYP level were obtained analytically at the optimized structures, and zero-point vibrational energies were derived. The single point energies of all the structures optimized at the B3LYP level of theory were 5



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calculated using the CCSD(T) method with the same basis sets.23 Results and discussion The infrared spectra from the reactions of laser-ablated manganese atoms and (CN)2 in excess argon are shown in Figure 1. The intense absorption at 2054.2 cm-1 due to CNCN was observed upon sample deposition.24 Both sample annealing and uv-vis irradiation (>220 nm) have little effect on its infrared intensity. Very weak absorptions due to CNNC and CN appeared in the spectra as well.25,26 All these three molecules were common products upon reactions of (CN)2 and laser-ablated metal atoms.12,13 In addition to these well characterized species, new product absorptions were observed when the sample was subjected to uv-vis irradiation, during which a broad band ranging from 2055 to 2065 cm-1 appeared. Further sample annealing to 30K sharpened two bands at 2058.2 and 2064.4 cm-1, both of which decreased upon annealing to 35K.

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Figure 1. Infrared spectra in the product absorption regions from reaction of the laser-ablated manganese atoms with (CN)2 in excess argon at 4 K. (a) Mn and 1% (CN)2 co-deposited for 1 h, (b) after annealing to 25 K, (c) after full arc (λ > 220 nm) irradiation, (d) after annealing to 30 K, (e) after annealing to 35 K.

Figure 2 shows the infrared spectra from the reactions of laser ablated iron atoms and (CN)2 in solid argon, which exhibits similar features to the spectra from the manganese and (CN)2 reactions. The broad feature on the left side of the CNCN peak sharpened upon sample annealing and two peaks at 2058.5 and 2060.2 cm-1 were resolved. A sharp absorption at 2052.6 cm-1 appeared on the right side of the CNCN peak, and it sharpened as well when the sample was annealed to higher temperature.

Figure 2. Infrared spectra in the product absorption regions from reaction of the 7



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laser-ablated iron atoms with (CN)2 in excess argon at 4 K. (a) Fe and 1% (CN)2 co-deposited for 1 h, (b) after annealing to 25 K, (c) after full arc (λ > 220 nm) irradiation, (d) after annealing to 30 K, (e) after annealing to 35 K.

These experiments were repeated using isotopically labeled (13CN)2 and (C15N)2 samples to help assign the absorptions of the new reaction products. Mixed isotopic samples such as (12CN)2 + (13CN)2, (12CN)2 + NC13CN + (13CN)2 and (C14N)2 + NCC15N + (C15N)2 were employed to identify the stoichiometries of the new reaction products. The infrared spectra from the reactions of manganese and iron with isotopically labeled (CN)2 are shown in Figures 3 and 4, and the observed product absorptions are listed in Table 1.

Figure 3. Infrared spectra in the product absorption regions from reaction of the laser-ablated manganese atoms with isotopically substituted (CN)2 in excess argon at 8


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4 K. Spectra were taken after full arc (λ > 220 nm) irradiation followed by annealing to 30 K (a) 0.25% (C14N)2 + 0.5% NCC15N + 0.25% (C15N)2, (b) 1% (C15N)2, (c) 1% (CN)2, (d) 0.8% (13CN)2, (e) 0.7% (12CN)2 + 0.3% (13CN)2, (f) 0.25% (12CN)2 + 0.5% NC13CN + 0.25% (13CN)2. The asterisks denote the absorptions of CNCN and its isotopomers.

Figure 4. Infrared spectra in the product absorption regions from reaction of the laser-ablated iron atoms with isotopically substituted (CN)2 in excess argon at 4 K. Spectra were taken after full arc (λ > 220 nm) irradiation followed by annealing to 30 K (a) 0.25% (C14N)2 + 0.5% NCC15N + 0.25% (C15N)2, (b) 1% (C15N)2, (c) 1% (CN)2, (d) 1% (13CN)2, (e) 0.8% (12CN)2 + 0.8% (13CN)2, (f) 0.25% (12CN)2 + 0.5% NC13CN + 0.25% (13CN)2. The asterisks denote the absorptions of CNCN and its isotopomers.

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Table 1. Infrared Absorptions (cm−1) Observed for Products from the Reactions of Manganese and Iron with (CN)2 in Excess Argon (12CN)2

(13CN)2

(12CN)2+(13CN)2

(12CN)2+NC13CN+(13CN)2

(C15N)2

(C14N)2+NCC15N+(C15N)2

MnNC

2058.2

2017.8 a

2058.2, 2017.8 a

2058.0, 2017.6 a

2024.9

2058.0, 2024.6 a

Mn(NC)2

2064.4

2023.8

2064.4, 2023.8

2066.3,2064.8,

2031.0

2066.3, 2064.8,

2025.6,2023.8

2032.4, 2031.2

FeNC

2052.6

2012.7

2052.6, 2012.7

2052.6, 2012.7

2019.1

2052.6, 2019.1

Fe(NC)2

2058.5b

2018.4

2058.5, 2018.4

2060.1, 2058.5,

2025.1

2060.1, 2058.5, 2026.6, 2025.1 a

2020.1, 2018.4 a

-1

overlap with CNCN isotopomer bands. b matrix site absorption at 2060.2 cm

MnNC. The 2058.2 cm-1 absorption shifted to 2017.8 and to 2024.9 cm-1 in the experiments with (13CN)2 and (C15N)2 samples (Figure 3). The

12

C/13C and

14

N/15N

isotopic frequency ratios of 1.0200 and 1.0164 calculated from these observed frequencies are consistent with values for the C-N stretching mode of metal isocyanide molecules.12,13 When manganese atoms reacted with scrambled (12CN)2 + NC13CN + (13CN)2 and (C14N)2 + NCC15N + (C15N)2 samples, two sets of doublets at 2058.0 and 2017.6 cm-1 as well as 2058.0 and 2024.6 cm-1 were observed, respectively, suggesting that only one CN moiety is involved in the absorber of the 2058.2 cm-1 band. Since this new band is located in a region where metal isocyandes were commonly observed,12,13,27 we assign the 2058.2 cm-1 absorption to the triatomic MnNC molecule. 10


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FeNC.

The 2052.6 cm-1 absorption in Figure 2 is assigned to the C-N stretch of

FeNC which appears in the same region as MnNC. This band shifted to 2012.7 and 2019.1 cm-1 upon 13C and 15N substitutions (Figure 4) with 12C/13C and 14N/15N ratios of 1.0198 and 1.0166, which are nearly the same as the above ratios observed for MnNC, and typical for the C-N stretches of metal isocyanides.12,13 The absence of intermediate absorption with either (12CN)2 + NC13CN + (13CN)2 or (C14N)2 + NCC15N + (C15N)2 sample suggests the involvement of one CN moiety in this mode. As described above for MnNC, the observed isotopic frequency ratios are much closer to the values computed for FeNC than for FeCN. So again the isocyanide isomer is the preferred assignment in the argon matrix. Mn(NC)2. The new peak at 2064.4 cm-1 exhibited

12

C/13C and

14

N/15N ratios of

1.0201 and 1.0164, both of which are characteristic of a MnN-C stretch as compared to computed isotopic frequency ratios for Mn(CN)2 and Mn(NC)2 in Table 2, and its position just 6.2 cm-1 above this mode for MnNC. In the experiment with mixed (12CN)2 + (13CN)2 sample, the infrared spectrum is the sum of the spectra from pure (12CN)2 and (13CN)2 samples, suggesting that only one (CN)2 molecule is involved in this vibrational mode. When manganese reacted with statistical (12CN)2 + NC13CN + (13CN)2, however, a quartet was observed at 2066.3, 2064.8, 2025.6 and 2023.8 cm-1 (Figure 3, trace f), and two of these (2066.3 and 2025.6 cm-1) were not present in the spectra with mixed (12CN)2 and (13CN)2 isotopic samples. The observation of the intermediate band at 2025.6 cm-1 indicates the presence of two equivalent CN moieties in the absorber of the 2064.4 cm-1 band, which is also consistent with the 11



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results from the scrambled (C14N)2 + NCC15N + (C15N)2 experiment. Considering the position of this new band, we assign it to the antisymmetric stretching vibrational mode of the Mn(NC)2 molecule. The absence of the symmetric mode in the experiments with pure isotopically labeled samples suggests that Mn(NC)2 is a linear molecule, and the 2025.6 cm-1 absorption together with the additional band at 2066.3 cm-1 are due to the two distinct C-N stretches of the Mn(N12C)(N13C) isotopomer. Fe(NC)2 .Following the example of Mn(NC)2, the peak at 2058.5 cm-1 is assigned to the antisymmetric C-N stretch of the Fe(NC)2 molecule which exhibited characteristic 12

C/13C and

14

N/15N ratios of 1.0199 and 1.0165. The 2060.2 cm-1 peak that is

partially overlapped with the 2058.5 cm-1 band has very close isotopic shift to the 2060.2 cm-1 absorption, indicating it should arise from the same mode of the product molecule trapped in different matrix site, which is common in matrix infrared spectra. No intermediate absorption was observed in the experiment with (12CN)2 + (13CN)2 sample while two sets of triplet at 2062.1, 2060.1, 2058.2 and 2021.4, 2020.1, 2018.4 cm-1 were observed when scrambled (12CN)2 + NC13CN + (13CN)2 was used (Figure 4, trace f). Note that the intensities of the 2060.1 and 2020.1 cm-1 bands are higher than those of 2058.2 and 2018.4 cm-1, in contrast to the relative intensities observed in Figure 2. In addition, the 2062.1 and 2021.4 cm-1 absorptions were not observed in the spectra from the reactions of iron and pure (12CN)2/(13CN)2 samples. The unique spectral features observed with scrambled (12CN)2 + NC13CN + (13CN)2 sample suggest the two sets of triplet are actually two sets of quartet with the matrix site bands at 2062.1 and 2021.4 cm-1 covered by the 2060.2 and 2020.1 cm-1 absorptions, 12


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and two equivalent CN moieties should be involved. Similar to the manganese case, Fe(NC)2 should possess a linear structure as well, and the intensity of the infrared inactive symmetric C-N stretch is significantly enhanced in the Fe(N12C)(N13C) isotopomer. To further support our experimental assignments, DFT calculations were performed on the mono and diisocyanide molecules of manganese and iron. For comparison, the corresponding cyanide isomers were also investigated (Table 2). The calculation results at the B3LYP level of theory reveal that both MnCN and MnNC possess 7Σ+ linear ground states with other spin states being less stable by at least 16 kcal/mol. The ground state manganese isocyanide was predicted to be slightly more stable than the cyanide by 1.1 kcal/mol. In order to confirm the relative stability of the two isomers, single point CCSD(T) calculations were carried out on the optimized geometries obtained at B3LYP level of theory. A very small energy difference of 0.4 kcal/mol was given with MnCN being lower in energy, which is different from the previous calculation results where MnNC was only 0.12 kcal/mol more stable than MnCN.3 The computed C-N bond length of MnCN is 0.018 Å shorter than that of MnNC, and the Mn-C and Mn-N distances are 2.102 and 1.984 Å (Figure 5), very close to the values reported previously.3 Although the energy difference between the two isomers is small, the difference in their infrared spectra are large enough for the identifications of the new product. Our recent investigations of the uranium and thorium isocyanide complexes revealed that the position of the C-N stretch can serve as fingerprint in identifying whether the product is cyanide or isocyanide.12,13 13



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Frequency calculations on the ground state MnNC molecule gave a C-N stretch at 2101.0 cm-1, about 124 cm-1 lower than that of ground state MnCN at 2225.5 cm-1. The observed band at 2058.2 cm-1 is in excellent agreement with the calculated C-N stretch of MnNC while the MnCN frequency is too high to match the experimental frequency. Calculations on the C-N stretches of MnN13C and Mn15NC gave two frequencies at 2058.8 and 2066.9 cm-1 which are about 120 cm-1 lower than those of Mn13CN and MnC15N (Supporting Information). The calculated

12

C/13C and

14

N/15N

isotopic frequency ratios of 1.0205 and 1.0165 for MnNC also agree well with the observed values of 1.0200 and 1.0164. As shown in Table 2, the

12

C/13C ratio

calculated for MnCN (1.0222) is higher than the observed value while the

14

N/15N

ratio (1.0149) is lower (The central atom bonded to the metal supports a higher isotopic frequency ratio than the diatomic CN molecule, observed ratios 1.0211 and 1.0153 for

12

CN/13CN and C14N/C15N in solid argon, because the observed mode

involves antisymmetric character and more motion of the central atom and less motion of the terminal atom for the triatomic.) This comparison of observed isotopic frequency ratios with the diatomic values further supports our assignment to the linear 7 +

Σ MnNC molecule, which has a higher 14N/15N ratio, 1.0164, than the diatomic CN

molecule, 1.0153. In addition to the C-N stretching vibrational mode, two other modes, namely Mn-N/C stretching and doubly degenerate bending modes, were predicted at 383.5 and 151.3 cm-1 for MnCN as well as 444.5 and 108.1 cm-1 for MnNC. The calculated Mn-C stretch is about twice as intense as the C-N stretch of MnCN while the intensity of Mn-N stretch is 1/4 of that for MnN-C. This is another 14


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spectroscopic difference between monoisocyanide and monocyanide. Unfortunately, their band positions are beyond the spectral limit of our instrument.

Table 2. Comparisons between Calculated (DFT/B3LYP) Cyanide/Isocyanide and Observed Product Absorptions (cm−1) and Isotopic Frequency Ratios Computed from Frequencies 12

C/13C

Freq Molecule

MnCN

2225.4

MnNC

2101.0

Mn(CN)2

2234.8

FeCN

N/15N

Mode Calcd

Mn(NC)2

14

C-N str.

2092.4

Obsd

Calcd 1.0222

2058.2

1.0205

2064.4

1.0202

2094.5

Fe(CN)2

2231.8

Fe(NC)2

2086.9

1.0200

1.0204

1.0201

1.0201

1.0165

1.0164

1.0166

1.0164

1.0149 1.0198

1.0226 2058.5

Obsd

1.0148

1.0224 2052.6

Calcd 1.0149

1.0226

2225.7

FeNC

Obsd

1.0166

1.0166

1.0147 1.0199

1.0167

1.0165

Different from the manganese case, triatomic FeCN and FeNC molecules have been the subject of extensive theoretical and experimental studies.3-11 Although the ground states of FeCN and FeNC were determined to be 4∆ and 6∆ respectively by spectroscopic methods,5,6 computational studies were still unable to provide satisfactory results on the electronic structures of both isomers even using high very expensive and time consuming multireference configuration interaction and couple 15



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cluster methods including up to full quadruple excitations.10 As pointed out previously, the quartet states of FeNC and FeCN possess significant multireference character.7 Our geometric optimizations and frequency calculations were therefore focused on the sextet. The B3LYP calculations reveal that both FeCN and FeNC possess linear geometries and 6∆ spin state with the latter being slightly more stable by 1.4 kcal/mol. Their relative stability switches based on the single point CCSD(T) calculations where FeCN was computed to be only 0.4 kcal/mol more stable than FeNC. As shown in Figure 5, the calculated Fe-C/N and C-N bond lengths of 6∆ FeNC and FeCN are very close to the high level calculation results.7,10 Other than the Fe-C bond length of FeCN, all the other geometric parameters of both molecules agree well with the experimental results (FeCN: 1.924 and 1.157 Å,5 FeNC: 2.01±0.05 and 1.03±0.08 Å.4). For the C-N stretching vibrational frequencies, the 6∆ FeNC molecule was predicted to absorb at 2094.5 cm-1 while the calculated band for FeCN was 2225.7 cm-1, in line with previous calculation results.3,7,10 The experimental frequency at 2052.6 cm-1 is better assigned to the C-N stretch of FeNC based on the calculated frequencies. In addition, the calculated isocyanide

12

C/13C and

14

N/15N isotopic

frequency ratios agree better with the experimental values than the calculated cyanide ratios (Table 2). Although the Fe-N stretch is out of our detection limit, it was reported to be 438 and 468 cm-1 based on the gas phase spectroscopic data,4,5 which are closer to our calculated frequency of 465.2 cm-1 for FeNC than the 401.2 cm-1 value computed for FeCN. Note that the 2052.6 cm-1 band is near the C-O stretch of the Fe(CO)+ cation in solid argon (2081.5 cm-1) which is isoelectronic with FeNC.28 It 16


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has been demonstrated that the bonding interactions between CN and manganese/iron are mainly electrostatic while covalent interactions only contribute about 25% to the total bonding,3 which is analogous to the case of corresponding metal carbonyl cations. In contrast, the C-O frequencies of neutral manganese and iron carbonyls are significantly lower due to the back donation from metal d orbital to π* orbital of CO,28,29 which is much weaker in the isocyanide case.3

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Figure 5. Optimized structures (bond lengths in angstrom units), electronic states, symmetries and energies (relative to cyanide, kcal/mol) for the linear manganese and iron cyanide/isocyanide at the B3LYP/6-311+G(d) level of theory. Relative energies at CCSD(T) level are in parenthesis.

Compared with the monocyanide/isocyanide molecules, studies on the dicyanide/isocyanide molecules are limited to the iron system using DFT and CCSD(T) methods.11 Our calculations reveal that the highest spin states with linear geometries are most stable for both isomers of iron and manganese. The computed C-N

bond

lengths of

diisocyanide/cyanide

are very close

to those of

monoisocyanide/cyanide (Figure 5). The 6Σ g+ Mn(CN)2 molecule was computed to be only 0.4 kcal/mol more stable than the 6Σg+ Mn(NC)2 molecule at B3LYP level, which is further confirmed by the energy difference of 3.9 kcal/mol from single point CCSD(T) calculations. Due to the linear geometry of both isomers, only the antisymmetric C-N stretch is infrared active. The antisymmetric C-N stretching vibrational frequencies for Mn(CN)2 and Mn(NC)2 were predicted at 2235.2 and 2092.4 cm-1. Apparently the observed frequency at 2064.4 cm-1 matches Mn(NC)2 much better, consistent with our experimental assignment. Although the symmetric mode is inactive, its infrared intensity is significantly increased when one of the CN groups is replaced by either

13

CN or C15N, which makes the two CN groups

essentially inequivalent. Frequency calculations on the Mn(N12C)(N13C) isotopomer reveal two infrared active C-N stretching modes at 2056.2 and 2099.5 cm-1 with 18


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relative intensity of 1.8:1 (Supporting Information). Together with the calculated antisymmetric stretching vibrational frequency of Mn(N13C)2 at 2050.9 cm-1, a quartet at 2099.5, 2092.4, 2056.2 and 2050.9 cm-1 should be observed with scrambled (12CN)2 + NC13CN + (13CN)2 sample based on DFT calculations, which is in excellent agreement with the quartet at 2066.3, 2064.8, 2025.6 and 2023.8 cm-1 observed in the experiment. If Mn(CN)2 were formed, a quartet at 2235.2, 2235.0, 2186.0 and 2185.7 cm-1 would be observed based on the calculations. Note that all these bands are too high to match the experimental frequencies, and the bands arising from Mn(12CN)(13CN) (2235.0 and 2185.7 cm-1) are slightly lower than those of Mn(12CN)2 and Mn(13CN)2, suggesting the observed 2064.4 cm-1 band cannot be assigned to the Mn(CN)2 molecule. A similar agreement is found between the calculated and observed frequencies for the Mn(NC)2 isotopomers formed from the reactions of manganese and (C14N)2 + NCC15N + (C15N)2. Finally, the calculated 14

12

C/13C and

N/15N ratios for the antisymmetric stretching vibrational frequency of Mn(NC)2 are

1.0202 and 1.0166, almost the same as the experimental values of 1.0201 and 1.0164 of the linear 6Σg+ Mn(NC)2 molecule. The computed isotopic frequency ratios for linear Mn(CN)2 are 1.0226 and 1.0148, which are not in as good agreement as those from linear Mn(NC)2. Geometry optimizations on other isomers such as Mn(NCCN), Mn(NCNC) and Mn(NC)(CN) were performed as well (supporting information). However, all of these species have two nonequivalent CN substituents, which require different isotopic splittings than observed for our Mn(NC)2 product. In addition, the vibrational frequencies and isotopic frequency ratios (12C/13C and 19


14

N/15N) of


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Mn(NCCN) and Mn(NCNC) do not match the experimental observations. For Fe(NC)2 and Fe(CN)2, our B3LYP calculations indicated both molecules possess linear 5∆g ground states, and the isocyanide isomer was predicted to be 0.1 kcal/mol lower in energy. In contrast, single point CCSD(T) calculations indicated Fe(CN)2 is more stable by 3.3 kcal/mol, consistent with the theoretical studies on the electronic structures of Fe(NC)2 and Fe(CN)2 by Redondo et al.11 The calculated C-N stretching vibrational frequencies are 2231.8 and 2231.9 cm-1 for Fe(CN)2 and 2086.9, 2103.7 cm-1 for Fe(NC)2, in which the 2231.9 and 2103.7 cm-1 bands due to symmetric C-N stretches are infrared inactive. Analogous to the Mn(NC)2 case, the calculated C-N stretching vibrational frequencies of Fe(NC)2, Fe(N12C)(N13C) and Fe(14NC)(15NC) as well as the corresponding isotopic frequency ratios are in better agreement with Fe(NC)2 values than with Fe(CN)2 results (Table 2), which supports our assignment of the 2058.5 cm-1 band to the linear 5∆g Fe(NC)2 molecule (Table 2). As shown in Figures 1 and 2, the absorptions due to Mn(NC)2 and Fe(NC)2 molecules increased only when the samples were subjected to λ >220 nm irradiation, suggesting it requires activation energy for the manganese and iron atoms to break the C-C bond of (CN)2 which has some double bond character.30 CCSD(T) calculations reveal highly exothermic reactions to produce Mn(NC)2 and Fe(NC)2 with 41.8 and 75.0 kcal/mol energy released upon their formation, respectively. For MnNC and FeNC, their increase upon uv-vis irradiation indicates they were most likely formed via the loss of one CN from the corresponding diisocyanides. The CN radical could either react with another CN radical to form NCCN or metal atom to form 20


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monoisocyanide, which explains why the CN absorption did not change in the experiment. Although the cyanide isomers were predicted to be more stable for both manganese and iron at the CCSD(T) level, no band in the region where cyanide would appear can be assigned to the cyanide molecule. It is possible that the cyanide molecules were formed in the experiments, but their very low infrared intensity (1/5-1/8 of isocyanide) makes it difficult for them to be detected. A very broad feature centered around 2233 cm-1 was produced after λ >220 nm irradiation of the sample containing manganese and (CN)2, and three weak but sharp peaks at 2251.2, 2247.4 and 2236.6 cm-1 appeared on the top of the broad band during subsequent sample annealing. All these absorptions exhibited larger 12C/13C ratios (~1.0278) and smaller 14

N/15N ratios (~1.0058) than those of manganese cyanides which possess similar

isotopic frequency ratios with isocyanides. Since they were not observed in the experiments when the sample was annealed right after sample deposition, it is most likely that they should be due to the association complexes of (CN)2 and Mn(NC)2. A broad band was also observed in the reactions of iron and (CN)2, and the 2254.1, 2251.5 and 2237.0 cm-1 absorptions are most likely due to the Fe(NC)2[(CN)2]x complexes which possess similar

12

C/13C and

14

N/15N frequency ratios with the

manganese analogs. Conclusions The reactions of manganese/iron atoms and (CN)2 were investigated by matrix isolation infrared spectroscopy and theoretical calculations. Laser-ablated manganese reacted with (CN)2 to form Mn(NC)2 upon λ>220 nm irradiation. The MnNC 21



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molecule was formed simultaneously most likely due to the photo-induced decomposition of Mn(NC)2. FeNC and Fe(NC)2 were formed via similar reactions of laser-ablated iron atoms and (CN)2. Both MnNC and MnCN molecules were predicted to have 7Σ+ ground state and linear geometry, and the MnCN molecule is 0.4 kcal/mol more stable at CCSD(T) level of thoery. For the linear Mn(NC)2 and Mn(CN)2 molecules, they were computed to have 6Σg+ ground state with the latter being lower in energy by 3.9 kcal/mol. Similar linear structures with 5∆g ground states were obtained for Fe(NC)2 and Fe(CN)2, and Fe(CN)2 was more stable by 3.3 kcal/mol based on the CCSD(T) results. For triatomic FeCN/FeNC, our calculations were focused on the sextet spin state since high level multireference configuration interaction and couple cluster calculations were still unable to provide satisfactory results on their structures, and the corresponding quartet system possesses significant multireference character. Single point CCSD(T) calculations reveal an energy difference of 0.4 kcal/mol with the linear 6∆ FeCN being slightly more stable. The calculated C-N stretching vibrational frequencies for all the cyanides are more than 100 cm-1 higher than those of isocyanides, and the latter agree well with the experimental values and their isotopic frequency ratios. Although manganese and iron cyanide molecules are computed to be more stable, no cyanide infrared absorptions were observed experimentally. It would be more likely to observe the cyanide products via the reactions of metal atoms and CN radical if cyanide were lower in energy. Breaking the C-C bond of (CN)2 requires activation energy from uv photolysis, which clearly form the isocyanides in the argon matrix. The cyanides have much 22


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weaker C-N stretching modes in the region above the isocyanides, and it would be more difficult to detect cyanides in the infrared spectra. Supporting Information: Complete citation for ref 16, calculated frequencies and intensities of M(CN)x and M(NC)x (M=Mn, Fe; x=1, 2) and their isotopomers. This material is available free of charge via the internet at http://pubs.acs.org. Acknowledgements This work was supported by the “Strategic Priority Research Program” and “Frontier Science Key Program” (Grant Nos. XDA02030000 and QYZDY-SSW-JSC016) (X.C., Q.L., Y.G.) of the Chinese Academy of Sciences, “Young Thousand Talented Program” (Y.G.), and retirement funds from TIAA (L.A.). References 1. Gail, E.; Gos, S.; Kulzer, R.; Lorösch, J.; Rubo, A.; Sauer, M.; Kellens, R.; Reddy, J.; Steier, N.; Hasenpusch, W. “Cyano Compounds, Inorganic”, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim October 2011. 2. Cotton, F. A.; Wilkinson, G. S.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, Wiley-Interscience, 6th edn., 1999. 3. Rayón, V. M.; Redondo, P.; Valdés, H.; Barrientos, C.; Largo, A. Cyanides and Isocyanides of First-Row Transition Metals: Molecular Structure, Bonding, and Isomerization Barriers. J. Phys. Chem. A 2007, 111, 6334-6344. 4. Jie, L.; Dagdigian, P. J. Observation of the FeNC Molecule by Laser Fluorescence Excitation Spectroscopy. J. Chem. Phys. 2001, 114, 2137-2143. 5. Flory, M. A.; Ziurys, L. M. Millimeter-Wave Rotational Spectroscopy of FeCN (X 23



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TOC Graphic

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Matrix Infrared Spectra of Manganese and Iron Isocyanide Complexes

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