Infrared Spectra of the Complexes Os← NCCH3, Re← NCCH3, CH3

Article pubs.acs.org/Organometallics

Infrared Spectra of the Complexes Os←NCCH3, Re←NCCH3, CH3− ReNC, CH2Re(H)NC, and CHRe(H)2NC and their Mn Counterparts Prepared by Reactions of Laser-Ablated Os, Re, and Mn Atoms with Acetonitrile in Excess Argon Han-Gook Cho† and Lester Andrews*,‡ †

Department of Chemistry, University of Incheon, 119 Academy-ro, Yonsu-gu, Incheon, 406-772, South Korea Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States

‡

S Supporting Information *

ABSTRACT: Acetonitrile forms primarily N-coordination complexes (M←NCCH3) with Os and Re metal atoms, but these metal atoms produce exclusively methylidyne complexes (HCMH2X) in similar previous reactions with small alkanes and halomethanes. The Os complex increases on visible photolysis and dissociates partially on UV irradiation without the generation of other new products, whereas the Re complex converts to other products (CH3−ReNC, CH2Re(H)NC, and CHRe(H)2NC) on photolysis. The primary formation of the N-coordination complex originates from its stability relative to that of the nitrile π-complex in these systems. The agostic interaction in the methylidene complex is apparently insignificant, and this rare observation of a methylidyne product with Re reflects that H migration in the isocyanide system is less favorable than those in the hydride and halide analogues. Experiments with Mn gave weaker counterpart product absorptions.

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showing that an electron-deficient species is first attracted by the N end1,2 lead to the proposition of reaction path (1).

INTRODUCTION The interactions of acetonitrile with Lewis acids, which often leads to adducts with its N end, have been an interesting topic.1,2 Its vibrational bands along the CCN axis provide valuable information about the interaction with electrondeficient species. This versatile organic solvent has also shown various photoisomerizations and fragmentations.3,4 Its large dipole moment and simple C3v structure along with CO2 laser line coincidence have made it a good subject for electrooptical studies.5 Direct activation of small alkanes and halomethanes and further conversions to high-oxidation-state products have been demonstrated with transition metals, including lanthanides and actinides.6 These organometallic species not only show interesting photochemistry and structures but also are small cousins of the larger complexes. The reaction products vary with the metal: complexes with higher oxidation state metals are more favored on going down the group column in the periodic table, and this tendency is most obvious for group 6−8 metals. In particular, Os and Re exclusively generate the methylidyne products (XCMX3, X = H, halogen) in reactions with small alkanes and halomethanes.7 Recently similar reactions of group 4−6 and early-actinide metals have been performed with acetonitrile, producing the Ncoordination, nitrile π, C−C insertion, methylidene, and methylidyne isocyanide complexes (M←NCCH3, η2-M(NC)CH3, CH3-MNC, CH2M(H)NC, and HCM(H)2NC, respectively).8−11 Observation of the isocyanide complexes with no trace of cyanide analogues8−11 and the previous results © 2012 American Chemical Society

M + CH3CN → M←NCCH3 → η2‐M(NC)CH3 → CH3MNC → CH 2M(H)NC → HCM(H)2 NC

(1)

The increasing preference for higher oxidation state products on going down the family column is also evident in acetonitrile reactions. The group 4−6 metal and actinide methylidenes (CH2M(H)NC) also show agostic distortions12 similar to those of hydride and halide analogues.8−11 However, the Ncoordination (η2-M(NC)CH3) and methylidyne (HCM(H)2NC) products have been rarely observed. Relatively weak absorptions of M←NCCH3 are observed in the Cr and Mo spectra, and HCM(H)2NC is tentatively identified only in the W system.10 This paper reports reactions of Os, Re, and Mn with acetonitrile in an argon matrix. The products are identified by isotopic (D and 13C) substitution and DFT computations. The major primary products are the complexes N-coordinated to Received: May 23, 2012 Published: August 14, 2012 6095

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Figure 1. Infrared spectra in the product absorption regions from reaction of the laser-ablated osmium atom with CH3CN in excess argon at 10 K: (a) Os and CH3CN (0.50% in argon) codeposited for 1 h; (b) as in (a) after visible (λ > 420 nm) irradiation; (c) as in (b) after UV (240−380 nm) irradiation; (d) as in (c) after visible irradiation; (e) as in (d) after annealing to 28 K. The product absorptions are marked with “n”, while P stands for precursor absorptions in the CH3CN matrix spectra. CH3NC, H2CCNH, CH2CN, and HCNC absorptions produced through precursor irradiation by the laser ablation plume are also indicated.

frequencies of products (Tables 1−3 and Tables S1−S11 (Supporting Information)) and their relative energies (Figures 8−10), and their structures (Figures 11−13) will be presented in turn. Os + CH3CN. The product absorption regions from reactions of laser-ablated Os atoms with acetonitrile isotopomers are shown in Figures 1−3. The absorptions of the isomerization and dissociation products of acetonitrile

the neutral atoms instead of the methylidynes, and they undergo conversions in subsequent photolysis and annealing.

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EXPERIMENTAL AND COMPUTATIONAL METHODS

Laser-ablated Os, Re, and Mn atoms were reacted with acetonitrile isotopomers (CH3CN, CD3CN, and 13CH313CN) in excess argon during condensation at 8 K using a closed-cycle refrigerator (Air Products Displex). These methods have been described in detail in previous publications.13 Reagent gas mixtures are typically 0.5% in argon. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused onto the rotating metal targets (Johnson-Matthey) using 5−10 mJ/pulse. After the initial reaction, infrared spectra were recorded at 0.5 cm−1 resolution using a Nicolet 550 spectrometer with a Hg−Cd−Te range B detector. Then samples were irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed using a combination of optical filters or annealed to allow further reagent diffusion. This lamp intensity is probably not strong enough for multiphoton processes in these experiments. In order to provide support for the assignment of new experimental frequencies and to correlate with related works,7−12,14 density functional theory (DFT) calculations were performed using the Gaussian 09 package,15 the B3LYP density functional,16 the allelectron 6-311++G(3df,3pd) basis sets for H, C, N, and Mn,17 and the SDD pseudopotential and basis sets18 for the heavy Os and Re metals in order to provide vibrational frequencies for anticipated reaction products. Geometries were fully relaxed during optimization, and the optimized geometries and transition-state structures were confirmed by vibrational analysis. The BPW9119 functional was also employed to complement the B3LYP results. The vibrational frequencies were calculated analytically, and zero-point energy is included in the calculation of binding and reaction energies. Previous investigations have shown that DFT calculated harmonic frequencies are usually slightly higher than observed frequencies,6−13,20 and they provide useful predictions for the infrared spectra of new molecules.

Figure 2. Infrared spectra in the product absorption regions from reaction of the laser-ablated osmium atom with CD3CN in excess argon at 10 K: (a) Os and CD3CN (0.50% in argon) codeposited for 1 h; (b) as in (a) after visible (λ > 420 nm) irradiation; (c) as in (b) after UV (240−380 nm) irradiation; (d) as in (c) after visible irradiation; (e) as in (d) after annealing to 28 K. The product absorptions are marked with “n”, while P and c designate the precursor and common absorptions in the CD3CN matrix spectra. CD3NC, D2CCND, and CD2CN absorptions are also indicated. Bands labeled c are common for this precursor and different laser-ablated metal experiments.

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RESULTS AND DISCUSSION Reactions of laser-ablated Os, Re, and Mn atoms with acetonitrile have been investigated, and infrared spectra (Figures 1−7), the observed and predicted vibrational 6096

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Figure 3. Infrared spectra in the product absorption regions from reaction of the laser-ablated osmium atom with 13CH313CN in excess argon at 10 K: (a) Os and 13CH313CN (0.50% in argon) codeposited for 1 h; (b) as in (a) after visible (λ > 420 nm) irradiation; (c) as in (b) after UV (240−380 nm) irradiation; (d) as in (c) after visible irradiation; (e) as in (d) after UV irradiation; (f) as in (e) after annealing to 28 K. The product absorptions are marked with “n”, while P and c designate the precursor and common absorptions in the 13CH313CN matrix spectra. 13CH3N13C, H213C13CNH, and 13CH213CN absorptions are also indicated. Bands labeled c are common for this precursor and different laser-ablated metal experiments.

D and 13C counterparts at 886.7 and 953.2 cm−1 (H/D and 12/ 13 frequency ratios of 1.098 and 1.021). On the basis of its frequency and substantial 13C shift, this is most probably the C−C stretching band. We assign these product absorptions to the C−N and C−C stretching bands of the Os N-coordination product (Os←NCCH3). The third strong product absorption is observed at 2926.0 cm−1, and its D and 13C counterparts appeared at 2110.5 and 2922.6 cm−1 (H/D and 12/13 frequency ratios of 1.386 and 1.001). The large D shift leads to an assignment to the CH3 symmetric stretching mode. The observed frequencies are in good agreement with the DFT frequencies computed for the N-coordination product (Os←NCCH3) as shown in Table S1 (Supporting Information) (the observed frequencies are 0.950, 0.984, and 0.968 of the B3LYP values and the observed H/D ratios, 1.001, 1.098, and 1.386, are consistent with the predicted values 1.000, 1.094, and 1.394), and the observed three n absorptions are the strongest three bands calculated for the Ncoordination complex. The observed product absorptions substantiate formation of the Os N-coordination complex with acetonitrile. The predicted frequencies of other plausible products are also given in Tables S2−S5 (Supporting Information), although none of them are observed in this study. Exclusive formation of the N-coordination complex is not found in our previous studies of reactions of transition-metal atoms with acetonitrile.8−11 Only relatively weak n absorptions have been recently observed from Cr and Mo reactions with acetonitrile because of the formation of further products.10 While the electron-rich N end of acetonitrile is believed to first attract the electron-deficient metal atom to form the Ncoordination complex in our matrix isolation experiements, previous studies using standard synthetic methods show that the nickel(0) fragment [(dippe)Ni] reacts with a variety of aromatic nitriles to form the nitrile π complex, which undergoes C−C insertion and H migration to generate more stable products.21

(CH2CNH, CH2NCH, CH3NC, H2CCN, and H2CNC)3,4 are also observed due to the laser-plume radiation during ablation and will not be discussed further. The Os product spectrum is relatively simple, with only one set of relatively strong, broad product absorptions (labeled “n” for the N-coordination complex). The n absorptions are strong in the original spectra after codeposition. Photolysis was done in the order of visible (λ > 420 nm), UV (240−380 nm), and visible irradiations. These absorptions increase ∼20%, decrease ∼50%, and again increase ∼20% on visible, UV, and visible irradiations, respectively. They increase an additional ∼50% on the early annealing and later decrease on higher temperature annealings. No other considerable product absorptions are observed in the original deposition spectrum and during the photolysis and annealing process. The strongest product absorption at 2180.8 cm−1 shifts to 2179.2 and to 2131.1 cm−1, respectively, on deuteration and on 13 C substitution (Table 1). Its frequency is ∼130 cm−1 above the N−C stretching bands of the isocyanide products previously observed from transition-metal reactions with acetonitrile, while it is still lower than the nitrile stretching frequency of 2256 cm−1 for free acetonitrile.8−11 These small D and large 13C shifts suggest that it is a nitrile C−N stretching mode. Another n absorption is observed at 973.4 cm−1 with its Table 1. Frequencies of Product Absorptions (n) Observed from Reactions of Os with Acetonitrile Isotopomers in Excess Argona CH3CN

CD3CN

2930.2, 2926.0 2180.8 973.4

2113.4, 2110.5 2179.2 886.7

13

CH313CN

2926.5, 2922.6 2131.1 953.2

description A1 CH3 asym str A1 CN str A1 C−C str

All frequencies are in cm−1. Stronger absorptions in a set are given in boldface. The description gives the major vibrational coordinate. a

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Figure 4. Infrared spectra in the product absorption regions from reaction of the laser-ablated rhenium atom with CH3CN in excess argon at 10 K (the 2035−2015 cm−1 region is expanded to show more clearly the product bands in this important region): (a) Re and CH3CN (0.50% in argon) codeposited for 1 h; (b) as in (a) after visible (λ > 420 nm) irradiation; (c) as in (b) after UV (240−380 nm) irradiation; (d) as in (c) after full arc (λ > 220 nm) irradiation. n, i, m, and y stand for the product absorption groups, while P and c designate the CH3CN precursor and common absorptions in the matrix spectra. The H2CCNH, HCNCH2, CH2CN, and CH2NC absorptions produced by laser ablation plume photolysis of CH3CN are also indicated. Bands labeled c are common for the CH3CN precursor and different laser-ablated metal experiments.

Figure 5. Infrared spectra in the product absorption regions from reaction of the laser-ablated rhenium atom with CD3CN in excess argon at 7 K: (a) Re and CD3CN (0.50% in argon) codeposited for 1 h; (b) as in (a) after visible (λ > 420 nm) and UV (240−380 nm) irradiation; (c) as in (b) after visible irradiation; (d) as in (c) after UV irradiation. n, m, and y stand for the product absorption groups, while P and c designate the precursor and common absorptions in the CD3CN matrix spectra. DCNCD2, CD2NC, and water residue absorptions are also indicated. Bands labeled c are common for this precursor and different laser-ablated metal experiments.

It is interesting that, unlike the case for our previously studied systems,8−11 the Os N-coordination complex is more stable than the nitrile π complex and is energetically comparable to the insertion complex in the reaction path. The Os←NCCH3, η2-Os(NC)CH3, CH3OsNC, CH2Os(H)NC, and HCOs(H)2NC products are 131, 82, 131, 142, and 213 kJ/mol more stable (lower in energy) in their triplet, quintet, triplet, triplet, and singlet ground states, respectively, than the reactants (Os(5D) + CH3CN). Once the stable Os Ncoordination complex is formed, further reactions during codeposition and photolysis afterward appear to be less favorable than in the other metal cases. Re + Acetonitrile. Figures 4−6 show reaction product spectra from laser-ablated Re atoms with acetonitrile isotopomers and their intensity variation on photolysis and annealing. Unlike the Os case, four sets of new product absorptions are observed and are labeled n, m, y, and i (for Ncoordination, methylidene, methylidyne, and insertion complexes, respectively) on the basis of intensity variation during photolysis and annealing of the deposited samples.

The n absorptions are predominantly strong and broad in the original deposition spectra, similar to the Os case, but they disappear on the first visible photolysis and never reappear. The m absorptions are weak in the original deposition spectra but increase ∼20, ∼100, and ∼100% on visible, UV, and full arc (λ > 220 nm) irradiation and further increase in the following visible irradiation. The y absorptions are originally barely discernible but dramatically increase (more than triple) on UV irradiation, decrease to almost half the maximum intensity on the following visible irradiation, and increase on the next UV irradiation. The i absorptions, on the other hand, emerge on UV irradiation but decrease on the following full arc and UV irradiations. The observed product absorption frequencies are given in Table 2 and compared with the DFT computed values in Tables S6−S10 (Supporting Information) for the plausible reaction products. The strong, broad n absorption observed at 2100 cm−1 shifts to 2092 and 2049 cm−1 on deuteration and 13C substitution. The frequency is ∼80 cm−1 lower than that of the Os case but is still higher than the typical isonitrile stretching frequencies (∼2050 cm−1), and the 13C shift of 51 cm−1 is also relatively 6098

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unreasonably long Re···N bond and thereby a much higher nitrile stretching frequency, 2363 cm−1. The BPW91 results also reproduce the predominantly high nitrile stretching absorption intensity, while the B3LYP intensity is unusually weak. It is clear that the B3LYP functional simply does not provide a reliable interaction energy for this Re complex and likely not for the Mn analogue as well. Accordingly, we will illustrate the BPW91 structures for the n complexes in the structure figures below. MP2 calculations for the N-coordination complex (Re← NCCH3), on the other hand, yields a C−N stretching frequency of 2224 cm−1 and an Re···N bond length of 3.079 Å.22 This Re N-coordination complex offers a testing ground for theoretical methods. The results vary greatly with the methods. With disappearance of the n absorption on visible irradiation, other product absorptions emerge. The 1921.2 cm−1 m absorption is accompanied by a D counterpart at 1378.9 cm−1 (H/D isotopic frequency ratio of 1.393). The 13C counterpart is observed as a shoulder absorption at 1921 cm−1. This frequency can be compared with previously observed Re− H stretching modes of 2037.3, 1985.0, and 1646.4 cm−1 for ReH4, ReH, and ReH223 and observations of 1946.4, 1809.9, and 1804.0 cm−1 for HCReH3, which have comparable H/D isotopic frequency ratios.7a The single strong Re−H stretching absorption is indicative of a product with a Re−H bond formed through H migration from C to Re in the Re + CH3CN system and is in line with previous studies;8−11 the most probable such species in reaction path (1) is CH2Re(H)NC. The observed frequencies agree reasonably well with the DFT computed frequencies, which show substantial coupling with the N−C stretching modes (Table S9 (Supporting Information)). Another m absorption is observed at 639.0 cm−1, and its weak 13C counterpart shows a negligible frequency shift, while the D counterpart is too weak to observe in the lower frequency region. This weak band is assigned to the Re−H inplane bending mode. On the other hand, the strong N−C stretching band is most probably covered by the very strong CH3CN precursor absorption at 2030−2060 cm−1. However, there is apparently no coupling between this unobserved NC stretching mode and the Re−H stretching mode with the common heavy Re atom in between, as the present H/D isotopic frequency ratio (1.393) is almost exactly the same as the ReH, ReH2, and ReH4 values (1.394, 1.392, 1.396, respectively).23 Thus, we assign the two observed m absorptions along with their D and 13C counterparts to CH2Re(H)NC. The strong y absorption observed at 2027.9 cm−1 is accompanied by D and 13C counterparts at 2033.5 and 1994.0 cm−1, and the frequency position, small D shift, and large 13C shift indicate an N−C stretching mode. Especially, the small blue shift observed on deuterium substitution (5.6 cm−1) indicates coupling with the strong Re−H stretching mode on the blue side of the N−C stretching band. Removal of this coupling in the Re−D stretching mode by shifting it much lower to 1378.9 cm−1 leads to a blue shift of the N−C stretching band. Our DFT computations predict a higher symmetric ReH2 stretching frequency in HCRe(H)2NC relative to the NC stretching frequency (58 and 99 cm−1 higher in the B3LYP and BPW91 calculations, respectively), predict the two unobserved weak ReH2 stretching modes to have 33 or 35/553 (6%) of the infrared intensity (B3LYP) of the strongest NC stretching mode, and reproduce the blue

Figure 6. Infrared spectra in the product absorption regions from reaction of the laser-ablated rhenium atom with 13CH313CN in excess argon at 7 K: (a) Re and 13CH313CN (0.50% in argon) codeposited for 1 h; (b) as in (a) after visible (λ > 420 nm) irradiation; (c) as in (b) after UV (240−380 nm) irradiation; (d) as in (c) after annealing to 28 K. n, i, m, and y stand for the product absorption groups, while P and c designate the precursor and common absorptions in the 13CH313CN matrix spectra. The m absorption at 1921.6 cm−1 is overlapped with a common absorption. 13CH213NC absorptions are also indicated. Bands labeled c are common for this precursor and different laser-ablated metal experiments.

Table 2. Frequencies of Product Absorptions Observed from Reactions of Re with Acetonitrile Isotopomers in Excess Argona product

CH3CN

CD3CN

n

2100 covered 3105.0 2087.8 2031.7 2027.9 628.8b 606.9b 1922.4, 1921.2 639.0 2022.9b

2092 778 2355.0 1505.0 1459.9 2033.5 447.6 455.2 1380.6, 1378.9

y

m i

13

CH313CN 2049 covered covered 2083.8b 1994.0 599.6b 1921.6c 638.9 1985.7b

description A′ C−N str A′ C−C str A′ C−H str A′ ReH2 sym str A″ ReH2 asym str A′ C−N str A″ ReH2 twist A′ ReH2 wag A′ Re−H str A′ Re−H ip bend A′ NC str

All frequencies are in cm−1. Stronger absorptions in a set are given in boldface. The description gives the major coordinate. bTentative assignment. cOverlapped with a common band. a

larger than those for the isonitrile stretching bands.8−11 Parallel to the Os case, we assign it to the nitrile stretching mode of the N-coordination complex (Re←NCCH3). Unfortunately, the regions expected for the second and third strongest n absorptions (∼1400 and 880 cm−1) are covered by precursor absorptions in the CH3CN and 13CH313CN spectra; another n absorption is observed at 778 cm−1 in the CD3CN spectrum, and it is assigned to the C−C stretching mode. The observed frequencies of the isotopomers are compared with the DFT frequencies in Table S6 (Supporting Information). It is noticed that the observed values are in very good agreement with the BPW91 nitrile stretching frequencies of 2143.8, 2143.0, and 2089.4 cm−1 for Re← NCCH3 and its deuterated and 13C substituted isotopomers and the C−C stretching frequency of 816 cm−1 for Re← NCCD3. The B3LYP computation, however, yields an 6099

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deuterium shift in the NC mode (B3LYP and BPW91 values of 5.4 and 3.8 cm−1) owing to this mode mixing (Table S10 (Supporting Information)). Finally, a weak y absorption at 3105.0 cm−1 with a D counterpart at 2355.0 cm−1 is attributed to the C−H stretching mode. The two weak y absorptions in the low-frequency region at 628.8 and 606.9 cm−1 are tentatively assigned to the ReH2 twist and wagging modes. The observed y absorptions, particularly the strong CN absorption and its unusual blue shift on deuterium substitution, support formation of the Re methylidyne product (HCRe(H)2NC). The i absorption at 2022.9 cm−1 exhibits a 13C counterpart at 1985.6 cm−1 with a weak D counterpart at 2020.2 cm−1. These weak bands are assigned to the N−C stretching mode of the insertion complex (CH3−ReNC) on the basis of its frequency position, very small D and sizable 13C shifts, and consistency with the DFT calculated frequencies. Parallel to the Os case, generation of the Re N-coordination complex is consistent with its energy being lower than that of the nitrile π complex. The Re←NCCH3, η2-Re(NC)-CH3, CH3ReNC, CH2Re(H)NC, and HCRe(H)2NC complexes in their sextet, quartet, sextet, quartet, and doublet ground states are 0, 15, −57, −62, and −102 kJ/mol higher than the reactants (Re(6S) + CH3CN). The BPW91 energy of Re←NCCH3 is 6 kJ/mol more stable than the reactants. The energy difference between the N-coordination and nitrile π complexes (15 kJ/mol) is smaller than that in the Os case (49 kJ/mol). Generation of the N-coordination and methylidyne complexes with the same metal atom reaction has not been observed, because either or both of them are higher than other products along the reaction pathway in previously investigated metal systems (pathway (1)).8−11 Mn + Acetonitrile. Product absorptions for the analogous reactions with Mn are illustrated in Figure 7 for the diagnostic 2000−2100 cm−1 region. New absorptions labeled i, n, and m are observed at 2080.1, 2063.4, and 2060.0 cm−1 after codeposition with natural isotopic CH3CN (black spectra) and at 2041.5, 2022.7, and 2019.2 cm−1 after codeposition with enriched isotopic 13CH313CN (blue spectra), and i not observed, 2063.4, and 2060.0 cm−1 after codeposition with CD3CN (red spectra) in excess argon. The 2080.1 and 2060.0 cm−1 bands decreased with UV and more with full arc irradiation, but the 2063.4 cm−1 band was not altered with this treatment. Ultraviolet irradiation increased an absorption at 2069.9 cm−1, labeled iA, for the acetonitrile complex with species i, which increased markedly on annealing to 28 K. Isotopic counterpart absorptions given in Table 3 exhibited the same band profile and behavior on irradiation with the mercury arc lamp and on annealing. Similar DFT calculations were done for the analogous product molecules, and the energy profile is shown in Figure 10, including the metastable Mn 8P atomic state, and the product structures are illustrated in Figure 13. Reactions. Exclusive initial formation of the N-coordination complex in reactions of Os and Re with acetonitrile is traced to the stability of the product relative to η2-M(NC)CH3, which is the next constituent in reaction path (1). Figures 8−10 show the energies of the plausible products in reaction path (1) relative to the reactants. After formation of the N-coordination complex (M←NCCH3), its conversion to η2-M(NC)CH3 in the cryogenic matrix is expected to be less favorable than in the previously studied group 4−6 and actinide systems, where the π

Figure 7. Infrared spectra in the product absorption N−C stretching region from reactions of laser-ablated manganese atoms with 12 CH312CN, 13CH313CN, and CD3CN in excess argon at 7 K: (a) Mn and CH3CN (0.50% in argon) codeposited for 1 h; (b) as in (a) after visible (λ > 420 nm) irradiation; (c) as in (b) after UV (240−380 nm) irradiation; (d) as in (c) after full arc (λ > 220 nm) irradiation; (e) as in (d) after annealing to 28 K (black spectra). The i, n, iA, and m labels represent the product absorption groups, while P and c designate the precursor and common absorptions in the CH3CN matrix spectra for this precursor and different laser-ablated metal experiments. Spectra f−j are analogous for 13CH313CN, and spectra (blue spectra), and spectra k−o are analogous for CD3CN (red spectra). The H2CCNH absorptions produced by laser plume irradiation are also indicated.

Table 3. Frequencies of Product Absorptions Observed from Reactions of Mn with Acetonitrile Isotopomers in Excess Argona product

CH3CN

CD3CN

i n iA m

2080.1 2063.4 2069.6, 2070.6b 2060.0

not obsd 2063.4 2069.6, 2070.5b 2060.0

13

CH313CN

2041.5 2022.7 2069.6, 2070.6b 2019.2

description A′ A′ A′ A′

C−N C−N C−N C−N

str str str str

All frequencies are in cm−1. The description gives the major coordinate. bSecond frequency due to matrix site which increases and sharpens on 28 K annealing. a

complexes are mostly more stable than or at least comparable to the N-coordination complexes. In the Os system, Os←NCCH 3 does not undergo conversions along reaction path (1) during photolysis and annealing. Instead, decrease of the product absorptions without emerging new bands indicates that the N-coordination complex dissociates with UV irradiation. The partial recovery on visible irradiation and further increase in annealing, on the other hand, reveal that the visible photon and matrix phonon energies drive the metal atom and acetonitrile to approach and form the Ncoordination complex in the matrix. UV

Os←NCCH3 XoooooooooY Os‐‐‐NCCH3 vis, anneal

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Figure 8. Energies of the plausible Os products in the proposed reaction path (1) relative to the reactants (Os(5D) + CH3CN). Only Os←NCCH3 is observed in the matrix IR spectra. On UV photolysis, the Os←NCCH3 absorptions decrease, but they recover on following visible photolysis and increased further in annealing. Probably the reaction does not proceed further due to the higher energy π complex next to Os←NCCH3 in the reaction path (see text). S, T, and Qi indicate singlet, triplet, and quintet states, respectively.

Figure 9. Energies of the plausible Re products in the proposed reaction path (1) relative to the reactants (Re(6S) + CH3CN). Only Re←NCCH3 was observed in the original matrix IR spectra, but it dissociates and CH3−ReNC, CH2Re(H)NC, and HCRe(H)2NC were produced on photolysis afterward. The nitrile π complex (η2-Re(NC)CH3) is not observed probably due to its higher energy (see text). D, Q, and Sx indicate doublet, quartet, and sextet electronic states, respectively.

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Figure 10. Energies of the plausible Mn products in the proposed reaction path (1) relative to the reactants (Mn(6S) + CH3CN). The Mn←NCCH3 complex, CH3−MnNC, and CH2Mn(H)NC were observed in the original matrix IR spectra. D, Q, and Sx indicate doublet, quartet, and sextet electronic states, respectively. The metastable Mn 8P state is placed above the ground 6S state from experimental energies.6c

Figure 11. B3LYP calculated structures of the plausible Os products using the 6-311++G(3df,3pd) basis sets for H, C, and N and the SDD pseudopotential and basis set for Os. Bond distances are in Å and angles in deg. Os←NCCH3 has a near-C3v structure, η2-Os(NC)CH3, CH3OsNC, and HCRe(H)2NC have Cs structures, and CH2Os(H)NC has a C1 structure. The agostic distortion of CH2Os(H)NC is insignificant.

rare.8−11 Only in the W + acetonitrile product spectra are weak absorptions tentatively assigned to the methylidyne complex (HCW(H)2NC), while it is predicted to be the most stable product in reaction path (1). While both Mn and Re have a 6S5/2 ground state, Re tends to utilize more d character for bonding. Previous studies reveal that Mn contributes less d character to the C−M and M−H bonds in MHCCH (17 and 13%) than Re (44 and 38%),

Vanishing of the n absorptions on photolysis with the emergence of other product absorptions in the Re spectra reveals easier dissociation of Re←NCCH3 by a photon and undergoing reactions following reaction path (1). Parallel to the previous Re results from reactions with small alkanes and halomethanes,7 the methylidyne (HCRe(H)2NC) is the most stable product. Observation of the methylidyne product from transition-metal reactions with acetonitrile has been 6102

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Figure 12. B3LYP calculated structures of the plausible Re products using the 6-311++G(3df,3pd) basis sets for H, C, and N and the SDD pseudopotential and basis set for Re. Bond distances are in Å and angles in deg. The BPW91 structure of Re←NCCH3 is shown, which better reproduces the observed results. η2-Re(NC)CH3, CH3ReNC, and HCRe(H)2NC possess Cs structures, and CH2Re(H)NC has a C1 structure. CH3ReNC and CH2Re(H)NC are close to C3v and Cs structures, respectively. CH2Re(H)NC does not show an agostic distortion as do the group 4−6 and actinide analogues.

Figure 13. B3LYP calculated structures of the plausible Mn products using the 6-311++G(3df,3pd) all-electron basis sets for H, C, N, and Mn. Bond distances are in Å and angles in deg. The BPW91 structure of Mn←NCCH3 is shown, which better reproduces the observed results. CH3MnNC has a C3v structure and CH2Re(H)NC has a Cs structure. CH2Mn(H)NC does not show agostic distortion as do the group 4−6 analogues.

its multiplicity from the quintet atomic ground state on filling its d orbitals. The higher oxidation state complexes are preferred with going down a family group in the transition-metal series: HC WH3 was the primary product in the W + CH4 system, and the Os and Re analogues were exclusively generated in reactions with methane.6,7 In fact, the second- and third-row metals of groups 4−9 all generate methylidynes as one of the products in reactions with small alkanes or halomethanes either during codeposition or afterward on photolysis. This reveals that H(X) migration from C to M in the isocyanide complex to form a high-oxidation-state complex is more difficult than in the

leading to linear C−Mn−H and nonlinear C−Re−H moieties.14d Both Re and Os produce exclusively methylidyne complexes (XCMX3, X = H, X, CH3) in previous reactions,7 although Os is marginally more electronegative (2.2 vs 1.9 on the Pauling scale). These methylidynes also show highly distorted structures around the metal center, unlike the previously reported analogues of other transition metals. The Re insertion and N-coordination complexes (without a C−Re double or triple bond) would probably prefer the sextet ground state to preserve the half-filled d orbitals of Re ([Xe]4f145d56s2), whereas Os ([Xe]4f145d66s2) more easily lowers 6103

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Organometallics corresponding hydride and halide systems.8−11 The electronrich isocyanide group supplies more electron density to the empty d orbitals of the metal center, which in turn probably destabilizes the transition state for H migration. Molecular Structures. Figures 11−13 show the B3LYP structures of the plausible Os, Re, and Mn products along reaction path (1). Os←NCCH3 has essentially C3v symmetry, although the DFT computations indicate Cs symmetry, whereas η2-Os(NC)CH3, CH3OsNC, and HCOs(H)2NC have Cs structures. CH2Os(H)NC has a C1 structure, but unlike the previously studied group 4−6 metal and actinide methylidene isocyanides,8−11 the methylene group of CH2Os(H)NC shows only minor agostic distortion (∠HCOs = 114.2°, showing that the agostic interaction is much less significant). The methylene group, Os atom, and isocyanide group are in nearly the same plane, and the H atom bonded to Os sticks out above the plane (Φ(HCOsH) = 70.4°). While Re←NCCH3, η2-Re(NC)CH3, CH3ReNC, and HC Re(H)2NC possess Cs structures, CH2Re(H)NC has a nearCs structure, where the C, Re, H, and NC units are in the same plane and one of the methylene hydrogen atoms is above the plane and the other below (Φ(HCOsH) = 94.5°) as shown in Figure 12. The insertion complex (CH3ReNC) owns an unusual near-C3v structure. NBO analyses24 reveal that the carbon−metal bonds in the Os and Re methylidene and methylidyne complexes are double and triple bonds (natural bond orders of 1.982 and 2.703 for the Os products and 1.798 and 2.795 for the Re analogues, respectively).

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REFERENCES

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CONCLUSIONS Acetonitrile reactions with Os and Re generate primarily complexes coordinated to the metal atoms with its N end (M← NCCH3), unlike the reactions of small alkanes and halomethanes, which exclusively produce methylidyne complexes (XCMX3, X = H, halogen). Upon subsequent UV irradiation, the Os product partially dissociates without the formation of other new products, but it is later regenerated upon visible irradiation and produced more on annealing. In contrast, the Re←NCCH3 analogue mostly rearranges on photolysis, and the CH3ReNC, CH2Re(H)NC, and HCRe(H)2NC reaction products are generated. Primary formation of the Os and Re N-coordination complexes is consistent with their energies being lower than those of η2-M(NC)CH3, which is the next constituent in reaction path (1) proposed for transition-metal reactions with acetonitrile. Unlike the previously studied group 4−6 analogues, agostic distortion is insignificant in the Os and Re methylidene complexes. This unusual observation of the methylidyne products in reactions of acetonitrile reveals that H migration in the isocyanide system is more difficult than in the hydride and halide analogues. ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S14, giving computed frequencies. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We gratefully acknowledge financial support from National Science Foundation (U.S.) Grant CHE 03-52487 to L.A. and support from a Korea Research Foundation (KRF) grant funded by the Korean government (MEST) (No. 20090075428) and KISTI supercomputing center.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6104

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