Article pubs.acs.org/Organometallics
Observation and Characterization of CH3CH2−MH, (CH2)2−MH2, and CH3−CMH3 Prepared in Reactions of Ethane with Laser-Ablated Group 6 Metal Atoms Han-Gook Cho† and Lester Andrews*,‡ †
Department of Chemistry, Incheon National University, 119 Academy-ro, Yonsu-gu, Incheon, 22012, South Korea Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States
‡
S Supporting Information *
ABSTRACT: Reactions of laser-ablated group 6 metal atoms with ethane have been carried out, and the products identified in argon matrix IR spectra on the basis of deuterium shifts, correlation with computation results, and previous related works. Mo and W generated the insertion, metallacyclopropane, and ethylidyne complexes [CH3CH2−MH, (CH2)2−MH2, and CH3CMoH3], whereas Cr gave only the insertion product. The higher oxidation-state complex from ethane becomes relatively more stable in the heavier metal system, parallel to those from methane. The unobserved ethylidene CH3CHMH2 is energetically higher than (CH2)2−MH2, and the energy barrier from CH3CH2−MH to the ethylidene is also substantially higher than that for the observed cyclic product. NBO calculations indicate that the C−M bonds of the group 6 metal ethylidenes and ethylidynes are true double and triple bonds.
■
only the insertion complex.10 Reactions of Mo and W with methane produced the CH3−MH, CH2MH2, and HC MH3 molecules, but Cr gave only the first insertion complex.11 The group 6 metal and methane reaction products provide a starting point for the new molecules to expect with ethane. In this paper, we report the primary products in reactions of group 6 metal atoms with ethane. The generation of these newly identified products (insertion, metallacyclopropane, and ethylidyne molecules) is in line with our electronic structure calculations and previous work with methane.
INTRODUCTION Efficient alkane activation via C−H bond insertion, which can transform the enormous hydrocarbon resources into more valuable products, is a long-standing quest in chemical research.1 Many synthetic reactions and catalytic processes are also based upon bond insertion and the following rearrangements.2 Recent investigations have demonstrated that laser-ablated metal atoms are effective C−H(X) bond insertion agents for methane and methyl halides, and the following H(X)-migration also leads to generation of high oxidation-state products, methylidene and methylidynes (e.g., CH2MH2 and HCMH3).3 The journal Organometallics opened this year with a special issue on hydrocarbon chemistry. The introductory articles focused on the dramatic impact on the earth of burning hydrocarbons and the importance of C−H bond activation chemistry to mitigate hydrocarbons.4 Previous investigations have shown that metal atoms, including coinage metals and actinides, are reactive with methane and halomethanes.3 These new complexes, small versions of the larger complexes, often show agostic distortions, which are similar to those observed from their larger relatives, and photochemistry, including reversible photoisomerizations. The small sizes of these new complexes have allowed theoretical approaches to investigate the origins of the structural properties and reaction paths.3,5 Activation of ethane has also drawn attention. Group 4 metals have generated CH3CH2−MH, (CH2)2−MH2, and CH2CH-ZrH3 in reactions with ethane,6 which contrast the products CH3−MH, CH2MH2, and HCMH3 identified from methane.7 Th and U produce similar products,8 whereas Re and Os generate only ethylidynes (CH3CMH3),9 and Pt © XXXX American Chemical Society
■
EXPERIMENTAL AND COMPUTATIONAL METHODS
Ablated Mo, W, and Cr atoms produced by a Nd:YAG laser from solid metal pieces (Johnson-Matthey) were codeposited with dilute mixtures (0.5−1.0%) of reagent vapor (C2H6, Matheson; C2D6, MSD Isotopes) in argon onto a CsI window cooled to 10 K using a closed-cycle refrigerator (Air Products HC-2). These methods have been described in detail with a diagram of the apparatus and a picture of a Zr ablation plume in progress.3,12 Briefly, the pulsed (10 ns) Nd:YAG laser 1064 nm fundamental (Spectra Physics, approximately 5−20 mJ/pulse after passing from two mirrors and through a 10 cm focal length lens and glass window into the vacuum chamber) is focused onto the rotating metal target, which makes a circular groove 5−10 mm in diameter and 0.2 mm wide and deep. The major ablated species are metal atoms along with a much smaller proportion of metal cations.12a As the picture of the ablation emission plume illustrates,3 some of the ablated atoms are in excited electronic states, although many of these are quenched by the condensing matrix, but some excited atoms survive to react during sample deposition, or they are excited by laser plume irradiation. The temperature generated by the Received: January 17, 2017
A
DOI: 10.1021/acs.organomet.7b00041 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Frequencies of Product Absorptions Observed from Reactions of Ethane with Mo in Excess Argona C2H6 i
m
y
C2 D 6
Obsdb
Harmc
Obsdb
1688.6 1065.0 578.3 1770.5 1757.6, 1753.4 804.2 1800.3 1795.3 1422.7 795.6, 787.3, 781.5 658.9 620.7
1804.2 (240) 1181.1 (37) 596.7 (42) 1855.0 (219) 1847.6 (329) 828.5 (34) 1894.9 (134) 1892.5 (306 × 2) 1457.8 (9 × 2) 846.3 (49 × 2) 667.9 (12 × 2) 656.3 (65) 580.2 (11)
1211.6 850.7
1264.7 1299.9 1296.3 572.1, 569.5
555.2d
Harmc
Description
1282.8 (122) 981.1 (11) 454.9 (29) 1315.9 (110) 1317.5 (169) 599.2 (20) 1385.1 (13) 1351.0 (158 × 2) 1049.6 (4 × 2) 602.0 (24 × 2) 504.2 (4 × 2) 441.0 (28) 568.0 (13)
Mo−H str. CH2 wag CMoH bend A1 MoH2 str. B1 MoH2 str. A1 MoH2 scis. A1 MoH3 str. E MoH3 str. E CH3 bend E MoH3 bend E MoH3 rock A1 MoH3 deform A1 C−Mo str.
−1
a
All frequencies are in cm . Stronger absorptions in a set are bold. Description gives major coordinate. bObserved in an argon matrix. cHarmonic frequency calculated with B3LYP/6-311++G(3df,3pd)/SDD. The numbers in parentheses are the computed absorption constant in (km/mol). d Tentative assignment. i, m, and y stand for insertion, metallacyclopropane, and methylidyne products, respectively.
Table 2. Frequencies of Product Absorptions Observed from Reactions of Ethane with W in Excess Argona C2H6 Obsd i
m
y
b
C2D6 Harm
c
1798.2 1162.1
1901.7 (173) 1195.7 (26)
593.1 1871.3 1849.2, 1844.9 1035.0, 1028.5 925.2 498.2 1888.1 1879.8 844.8 669.0
622.0 (16) 1927.0 (189) 1913.0 (246) 1078.0 (29) 966.7 (17) 509.6 (22) 1954.7 (137) 1941.2 (270 × 2) 835.6 (29 × 2) 683.0 (57)
b
Obsd
Harmc
Description
1288.3 961.4 822.1d 456.5 1340.4 1327.3
1348.9 (87) 995.6 (6) 866.8 (6) 473.1 (12) 1365.1 (95) 1359.2 (125) 1182.8 (16) 807.1 (3) 445.0 (22) 1381.0 (53) 1380.0 (139 × 2) 593.1 (14 × 2) 465.8 (19)
W−H str. CH2 wag C−C str. CWH bend A1 WH2 str. B1 WH2 str. A1 C−C str. B2 CH2 wag B2 WC2 str. A1 WH3 str. E WH3 str. E WH3 scis. A1 WH3 deform
1352.4 1351.2
a All frequencies are in cm−1. Stronger absorptions in a set are bold. Description gives major coordinate. bObserved in an argon matrix. cHarmonic frequency calculated with B3LYP/6-311++G(3df,3pd)/SDD. The numbers in parentheses are the computed absorption constant in (km/mol). d Tentative assignment. i, m, and t stand for insertion, metallacyclopropane, and methylidyne products, respectively.
Table 3. Frequencies of Product Absorptions Observed from Reactions of Ethane with Cr in Excess Argona C2H6 Obsdb i
1640.0, 1635.2 1130.5 529.9 501.0d
C2D6 Harmc 1663.4 1155.7 566.3 453.8
Obsdb
(233) (51) (73) (3)
1166.7 930.0
Harmc 1188.5 956.5 439.8 398.6
(121) (22) (47) (4)
Description Cr−H str. CH2 wag CCrH bend C−Cr str.
a All frequencies are in cm−1. Description gives major coordinate. bObserved in an argon matrix. cHarmonic frequency calculated with B3LYP/6311++G(3df,3pd). The numbers in parentheses are the computed absorption constant in (km/mol). dTentative assignment. i stands for insertion product.
focused laser on the sample surface is way above the vaporization point of the metal and is on the order of 3000 K, depending mostly on the pulse width and sharpness of the laser focus: most of this energy, also, is absorbed by the matrix as it forms [no ablation occurs using an unfocused laser beam]. After deposition, infrared spectra of products formed were recorded at a resolution of 0.5 cm−1 using a Nicolet 550 spectrometer with an MCT-B detector. Matrix samples were next irradiated with a mercury arc street lamp (175 W) with the globe removed (λ > 220 nm), with or without an optical filter for 20 min, and subsequently warmed and recooled to 10 K (annealed), and more
spectra were recorded. The activation energies of the matrix reactions we investigate are mostly below 200 kJ/mol, which can be overcome by the excited metal atom or photon energy we provide, considering that a 500 nm photon corresponds to 240 kJ/mol. Theoretical calculations for possible reaction products were performed using the Gaussian 09 package.13 In all instances, the B3LYP hybrid density functional14 was employed, and carbon, hydrogen, and chromium atoms were given the large 6-311+ +G(3df,3pd) Gaussian basis.15 Heavier molybdenum and tungsten atoms were treated with the SDD pseudopotential and basis sets.16a B
DOI: 10.1021/acs.organomet.7b00041 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 1. IR spectra in the product absorption regions for laser-ablated Mo atoms codeposited with C2H6 in excess argon at 10 K and their variation: (a) Mo + 0.50% C2H6 in Ar codeposited for 1 h. (b−i) As (a) after photolysis with λ > 420 nm, 240 < λ < 380 nm, λ > 420 nm, 240 < λ < 380 nm, λ > 420 nm, and 240 < λ < 380 nm and annealing to 20 and 28 K in sequence. i, m, and y denote product absorption groups (insertion, metallacyclopropane, and methylidyne complexes), and c and p stand for common and precursor absorptions.
Figure 2. IR spectra in the regions 1950−1750, 1180−1130, and 1050−900 cm−1 for laser-ablated W atoms codeposited with C2H6 in excess argon at 10 K and their variation: (a) W + 1.0% C2H6 in Ar codeposited for 1 h. (b−h) As (a) after photolysis with λ > 420 nm, 240 < λ < 380 nm, λ > 420 nm, 240 < λ < 380 nm, λ > 420 nm, and 240 < λ < 380 nm and annealing to 28 K in sequence. i, m, and y denote product absorption groups (insertion, metallacyclopropane, and methylidyne complexes), and c and p stand for common and precursor absorptions. These B3LYP calculations predict harmonic vibrational frequencies which are in reasonable agreement with observed anharmonic frequencies for transition-metal compounds: the observed frequencies are 0.92−1.00 times the computed values for most of our previously investigated transition-metal products.6−10 Recent investigation of agreement between B3LYP calculated harmonic frequencies for small molecules finds the observed values to be 0.96 (scale factor) of the observed values.17 In our case, there is a small red matrix shift which also contributes to the observed frequencies being slightly lower than the computed values. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. Separate calculations with BPW9116b−d were also carried out to supplement the B3LYP results. In the calculation of the binding energy of a metal complex, the zero-point energy is included.
product vibrational characteristics and their variations upon photolysis and annealing as well as the product structures are given in Tables 1−3 and Figures 1−7 and S1−S3. The observed frequencies are compared with the calculated values in Tables S1−S16. Mo + ethane. Figure 1 shows the product absorption regions of the matrix IR spectra from reactions of laser-ablated Mo atoms with C2H6, and Figure S1 shows those with C2D6. The observed new product absorptions are marked with i, m, or y (i, m, and y for insertion, metallacyclopropane, and ethylidyne complexes) on the basis of intensity variation on photolysis and annealing. The i absorptions were most visible in the deposition spectrum, almost disappeared on visible (λ > 420 nm) irradiation, slightly recovered on UV (380 < λ < 240 nm) irradiation, and disappeared again in the second visible irradiation. The i absorptions became nearly invisible after two cycles of visible and UV irradiation.
■
RESULTS AND DISCUSSION Reactions of laser-ablated group 6 metal atoms with ethane in condensing argon have been investigated. The observed C
DOI: 10.1021/acs.organomet.7b00041 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 3. IR spectra in the product absorption region for laser-ablated Cr atoms codeposited with C2H6 in excess argon at 10 K and their variation: (a) Cr + 1.0% C2H6 in Ar codeposited for 1 h. (b−e) As (a) after photolysis with λ > 420 nm, 240 < λ < 380 nm, and λ > 220 nm and annealing to 28 K in sequence. i denotes product (insertion complex) absorptions, and c and p stand for common and precursor absorptions. w stands for water residue absorptions.
Figure 5. B3LYP structures of plausible products from reaction of W with ethane. The bond lengths and angles are in Å and degrees. The insertion, metallacyclopropane, and ethylidyne products [CH3CH2− WH, (CH2)2−WH2, and CH3CWH3] have been observed in the IR matrix spectra, but the energetically higher ethylidene product (CH2CHWH2) has not.
UV irradiation. The y absorptions became eventually the strongest product bands in the Mo + ethane spectra, and they first sharpened up in the process of annealing and later gradually decreased. The observed photoreversible variations in the intensities of the product absorptions suggest that the product responsible for the i absorptions converts to those responsible for the m and y absorptions on visible irradiation and vice versa in UV irradiation. Similar photoreversibility was observed previously in the matrix reaction of Mo with CH4; CH3−MoH converted to CH2MoH2 and HCMoH3 on visible irradiation via Hmigration from C to Mo, and vice versa on UV irradiation via H-migration from Mo to C.11 Spectral assignments have been carried out on the basis of spectral variation by deuteration, correlation with the computed values, and previous studies on reactions of metal atoms with
Figure 4. B3LYP structures of plausible products from reaction of Mo with ethane. The bond lengths and angles are in Å and degrees. The insertion, metallacyclopropane, and ethylidyne products [CH3CH2− MoH, (CH2)2−MoH2, and CH3CMoH3] have been observed in the IR matrix spectra, but the higher energy ethylidene product (CH2CHMoH2) has not. TS1 and TS2 are the structures of the transition states from CH3CH3−MoH to CH3CHMoH2 and (CH2)2−MoH2 shown in Figure 7.
The m absorptions were relatively weak in the original deposition spectra. They repeatedly decreased and increased on visible and UV irradiation. The y absorptions dramatically increased in the first cycle of visible and UV irradiation, and they then increased and decreased continuously on visible and D
DOI: 10.1021/acs.organomet.7b00041 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
symmetric and antisymmetric stretching modes of Mo metallacyclopropane dihydride [(CH2)2−MoH2]. The D counterpart of the antisymmetric stretching absorption is observed at 1264.7 cm−1 (the symmetric stretching absorption is probably overlapped). The H/D ratio of 1.390 is appropriate for the metal−hydrogen stretching modes. These MoH2 stretching frequencies are 21.1 and 23.9 cm−1 lower than those arising from previously provided CH2MoH2 at 1791.6 and 1781.5 cm−1.11 The weak m absorption at 804.2 cm−1 is designated to the MoH2 scissoring mode of (CH2)2−MoH2 (Tables 1 and S2). The previous studies of transition-metal reactions with methane and methyl halides have shown that the first-produced insertion complexes (CH3−MZ, Z = H or halogen) convert to the methylidene product (CH2MHz) via H-migration from C to M by photon energy.4 Matrix reactions of group 4 metals and actinides with ethane have revealed that metallacyclopropanes [(CH2)2−MH2] were produced from CH3CH2−MH via β-H-migration and cyclization instead of α-H-migration.6,8 αH-migration would have generated CH3CHMH2, which is energetically higher than the metallacyclopropane [e.g., CH3− CHTiH2 is 88 kJ/mol higher in energy than (CH2)2− TiH2].6 In this Mo system, the metallacyclopropane is 29.6 kJ more stable than the methylidene. The y absorptions in the Mo spectra were the strongest after photolysis. Those at 1800.3 and 1795.3 cm−1 in the Mo−H stretching region are assigned to the A1 and E MoH3 stretching modes of CH3CMoH3 (C3v symmetry) with the D counterparts at 1299.9 and 1296.3 cm−1 (H/D ratios of both 1.385). The observed hydrogen stretching frequencies of the ethylidyne product are compared with those reported for HCMoH3 (1839.7, 1836.4, and 1830.0 cm−1). The other y absorptions at 1422.7, 787.3, 658.9, and 620.7 cm−1 are assigned to the E CH3 bending, E MoH3 bending, E MoH3 rocking, and A1 MoH3 deformation modes of the Mo ethylidyne trihydride (CH3−CMoH3) on the basis of good correlation with the predicted values (Tables 1 and S3), substantiating generation of the ethylidyne complex. The y absorption at 555.2 cm−1 in C2D6 spectra is tentatively assigned to the A1 C−Mo stretching mode. The Mo−H stretching frequencies of the observed ethylidyne products are ∼40 cm−1 lower than those of HCMoH3, in line with the previously studied other transition-metal analogues.6−10 The hyperconjugation of the methyl group evidently lowers the M−H stretching frequencies, probably by supplying more electron density to the metal center (e.g., Mulliken charges of +1.17 and +1.45 for CH3CMoH3 and HCMoH3), weakening the M−H bonds (e.g., M−H bond lengths of 1.707 and 1.703 Å for CH3CMoH3 and HC MoH3). This trihydrido Mo product is nearly isoergic (only 0.6 kJ/ mol higher) with the metallacyclopropane. The CH3CH2− MoH, CH3−CHMoH2, (CH2)2−MoH2, and CH3−C MoH3 molecules in their ground quintet, triplet, triplet, and singlet states are −18.5, +7.3, −22.3, and −21.7 kJ/mol higher in energy than the reactants [Mo(7S) + C2H6], respectively, and their first excited triplet, quintet, quintet, and triplet states are 63, 101, 35, and 120 kJ/mol higher in energy than the reactants. The computed vibrational characteristics for their ground states are compared with the observed values in Tables S1−4. Energetically higher CH3CHMoH2 has not been detected in this study, whose strong symmetric and antisymmetric MoH2 stretching absorptions would be ∼50
Figure 6. B3LYP structures of plausible products from reaction of Cr with ethane. The bond lengths and angles are in Å and degrees. Only the insertion product (CH3CH2−CrH) has been observed in the IR matrix spectra while the metallacyclopropane, ethylidyne, and ethylidene products [(CH2)2−CrH2, CH3CCrH3, and CH3CH CrH2] have not been observed.
Figure 7. Energy diagram for isomerization of CH3CH2−MoH to (CH2)2−MoH2. The energy barrier for the metallacyclopropane (68 kJ/mol) is considerably lower than that to the ethylidene complex (165 kJ/mol). Qi and T stand for quintet and triplet electronic states. The insertion, metallacyclopropane, and ethylidyne products [CH3CH2−MoH, (CH2)2−MoH2, and CH3CMoH3] have been observed in the IR matrix spectra, but the higher energy ethylidene product (CH2CHMoH2) has not. The numbers are the energies of the products in kJ/mol relative to CH3CH2−MoH(Qi). The structures of TS1 and TS2 are shown in Figure 4 with those of other plausible products.
methane and methyl halides.6−11 The observed i absorption at 1688.6 cm−1 is assigned to the Mo−H stretching mode of the insertion complex (CH3CH2−MoH) with its D counterparts at 1211.6 cm−1 (Table 1 and S1). The H/D ratio 1.394 is appropriate for the vibrational mode. The Mo−H stretching frequency is compared with 1728.0 cm−1 previously reported for CH3−MoH.11 Another i absorption at 1065.0 cm−1 is designated to the CH2 wagging mode with its D counterpart at 850.7 cm−1. A weaker i absorption at 578.3 cm−1 is assigned to the CMoH bending mode without observation of the D counterpart. The strong m absorptions at 1770.5 and 1757.6 cm−1 (with a site absorption at 1753.4 cm−1) are assigned to the MoH2 E
DOI: 10.1021/acs.organomet.7b00041 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics cm−1 apart (Table S4). The methylidyne is relatively less stable in the Mo + CH4 system; CH3−MoH is 49 kJ/mol more stable than HCMoH3.11 The methyl group apparently stabilizes the C−Mo triple bond. W + ethane. The product spectra from reactions of laserablated tungsten atoms with ethane and deuterated isotopomer are shown in Figures 2 and S2. Parallel to the Mo case described above, the product absorptions are marked with i, m, and y depending the intensity variation on photolysis and annealing. The y absorptions are the strongest in the W spectra (even in the original deposition spectra) and the i absorptions are weaker, consistent with the general trend that the higher oxidation-state product of a heavier metal in a family group is more favored.4 The i absorptions increase on uv irradiation and decrease on visible irradiation, similar to the above-described Mo case. The m and y absorptions increase first on both visible and uv irradiation and later only slight increase on each photolysis. The frequencies of the i absorptions correlate well with the predicted values and are assigned accordingly as shown in Table 2. The i absorptions at 1798.2, 1162.1, and 593.1 cm−1 are 0.946, 0972, and 0.954 of the B3LYP frequencies for the W−H stretching, CH2 wagging, and CWH bending modes of CH3CH2−WH. The D counterparts were observed at 1288.3, 961.4, and 456.5 cm−1, whose H/D ratios 1.396, 1.209, and 1.299 were appropriate for the vibrational modes. The observed W−H stretching frequency is also compared with 1822.2 cm−1 for that of CH3−WH. The i absorption at 822.1 cm−1 in the C2D6 spectra was tentatively designated to the C−C stretching mode without observation of the H counterpart. The strongest m absorptions at 1871.3 and 1844.9 cm−1, which are accompanied by their D counterparts at 1340.4 and 1327.3 cm−1 (H/D counterparts 1.396 and 1.390), are assigned to the symmetric and antisymmetric WH2 stretching modes of (CH2)2−WH2 on the basis of its frequencies and good correlation with the predicted values of 1927.0 and 1913.0 cm−1. These WH2 stretching frequencies are compared with 1864.5 and 1817.2 cm−1 observed from CH2WH2. The weaker m absorptions at 1035.0, 925.2, and 498.2 cm−1 are designated to the C−C stretching, CH2 wagging, and WC2 antisymmetric stretching modes of the W metallacyclopropane. The strong y absorptions in the W−H stretching region were observed at 1888.1 and 1879.8 cm−1 and assigned to the A1 and E WH3 stretching modes of CH3−CWH3. They are in good correlation with the predicted values (0.966 and 0.968 of the predicted values of 1954.7 and 1941.2 cm−1). The D counterparts were observed at 1352.4 and 1352.2 cm−1, showing appropriate H/D ratios of 1.396 and 1.391. These W−H stretching frequencies of CH3−CWH3 are compared with those previously observed at 1896.3, 1893.1, and 1888.3 cm−1 from HCWH3.11 The y absorptions observed at 844.8 and 669.0 cm−1 are assigned to the E WH3 scissoring and A1 WH3 deformation modes. The other vibrational bands are predicted to be too weak to observe as shown in Table S7. The W−H stretching absorptions marked y are the strongest product absorptions in the W + C2H6 spectra, and CH3−C WH3 is also the most stable among the plausible products. CH3CH2−WH, CH3CHWH2, (CH2)2−WH2, and CH3− CWH3 in their quintet, triplet, triplet, and singlet ground states are −114, −147, −161, and −206 kJ/mol higher in energy than the reactants [W(5D) + C2H6], respectively, and their first excited triplet, singlet, quintet, and triplet states −53, −80, −73, and −33 kJ/mol higher in energy than the reactants.
The vibrational characteristics calculated for their ground states are compared with the observed values in Tables S5−8. The stability of this W methylidyne reconfirms the tendency that the higher oxidation-state product becomes more favored with moving down the group column. HCWH3 and HCWH2X are also the most stable products in the formerly studied reactions of W with methane and methyl halides,11 and the W methylidyne products are the only products observed in reactions with di-, tri-, and tetrahalomethanes.4 Cr + ethane. The product absorptions observed from the Cr + C2H6 spectra (Figures 3 and S3) are all marked “i”. They increase continuously on photolysis and gradually decrease in annealing. The strongest product absorption observed at 1640.0 cm−1 (with a site absorption at 1635.2 cm−1) on the blue side of the water residue absorptions is designated as the Cr−H stretching mode of CH 3CH2 −CrH with its D counterpart at 1166.7 cm−1 (Table 3). These observed Cr−H stretching frequencies are compared with 1623.9 and 1172.3 cm−1 for CH3−CrH and 1650.9 and 1614.5 cm−1 for CrH2.11,18 The i absorptions observed at 1130.5 and 529.9 cm−1 are assigned to the CH2 wagging and CCrH bending modes of the Cr insertion product. A weak product absorption at 501.0 cm−1 is tentatively assigned to the C−Cr stretching mode. Similarly, the insertion complexes were the single primary products in matrix and gas phase reactions of the first row transition metals with small alkanes with the exception of the Sc and Ti cases, where both the insertion and methylidenes were observed.4,19 Unlike the reactions of Mo and W, only the insertion complex was observed in the previously studied reactions of Cr with the methane isotopomers.11 Computations also support the observed results; CH 3 CH 2 −CrH, CH 3 CHCrH 2 , (CH2)2−CrH2, and CH3CCrH3 in their ground quintet, triplet, triplet, and singlet states are 37.6, 192.1, 162.2, and 323.7 kJ/mol higher in energy than the reactants [Cr(7S) + C2H6], respectively, and their computed vibrational characteristics are shown in Tables S9−S13. The first excited triplet, quintet, and quintet states of CH3CH2−CrH, CH3CHCrH2, and (CH2)2−CrH2 are 201, 220, and 219 kJ/mol higher in energy than the reactants, but our attempts for optimization of CH3CCrH3 in its triplet state lead to the structure of CH3CHCrH2. Similarly only CH3−CrH was observed in reaction of Cr with CH4, and the insertion complex is the most stable among the plausible products in the first row transitionmetal systems. The kinetic energy of the laser-ablated Cr atoms can generate the insertion complex, but not the higher energy products. Structures. The B3LYP structures of the plausible products from reactions of group 6 metals and ethane are illustrated in Figures 4−6. The insertion complexes (CH3CH2−MH) all have C1 structures, whereas the dihydrido cyclic [(CH2)2− MH2] and ethylidyne (CH3CHCrH3) products have C2v and C3v structures, respectively. The undetected Mo and Cr ethylidenes have C1 structures, but the W analogue a Cs structure. While the observed Mo insertion, and metallacyclopropane and ethylidyne products are energetically comparable, the higher oxidation-state complex is more stable among the W complexes, but the high-oxidation-state analogues are less stable in the Cr system, as described above. The C−C bonds of the cyclic products (1.447−1.474 Å) are considerably shorter than those of the other products due to the extra pull by the bridging metal dihydride group, and the cyclic structure also strengthens the exocyclic C−H bonds. NBO20 analysis shows that the C−M bonds in these group 6 F
DOI: 10.1021/acs.organomet.7b00041 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
reactions with ethane originates from the high energy barriers toward the products. Formation of the cyclic product [(CH2)2MH2] via β-H-migration from C to M is kinetically more favored (eq 1).
metal ethylidene and ethylidyne complexes are true double and triple bonds (bond orders of 1.91, 1.94, and 1.90 for the Mo, W, and Cr ethylidenes and 2.84, 2.86, and 2.79 for the corresponding ethylidynes). Consequently, the C−M bonds of the ethylidynes (1.723, 1.750, and 1.597 Å for the Mo, W, and Cr ethylidynes) are noticeably shorter than those of the insertion, dihydrido cyclic, and ethylidene complexes (e.g., 2.121, 2.117, and 1.869 Å for the W products). The Mo and W ethylidenes are agostic ( λ > 240 nm, parallel to the interconversions in the previously investigated group 6 metal reactions with methane and methyl halides.11 Figure 1 shows that the photoconversions can be repeated many times in an argon matrix. The Mo insertion complex mostly converts to the high oxidation-state products on the first irradiation with λ > 420 nm and only slightly recovers on the following irradiation with 380 > λ > 240 nm.
Metal dihydride absorptions were weak in the original Mo + C2H6 deposition spectrum and after photolysis, similar to the group 4 metal cases, indicating that breaking of the C−M bonds of (CH2)2−MH2 to produce MH2 and ethylene is more difficult than the photoisomerizations via H-migration. Mo, W, and Cr with Ethyl Chloride. Complementary experiments were performed with ethyl chloride to see if a Cl substituent would affect the relative stabilities of the several products expected. The relative energies of the M + C2H5Cl products have a similar tendency as for ethane itself. The C 2 H 5 −MoCl, (CH 2 ) 2 −MoHCl, CH 3 CHMoHCl, and CH3CMoH3 are 227, 239, 208, and 230 kJ/mol more stable than their reactants. The C2H 5−WCl, (CH2) 2−WHCl, CH3CHWHCl, and CH3CWH3 are 317, 377, 353, and 416 kJ/mol more stable than their reactants. C2H5−CrCl, (CH2)2−CrHCl, CH3CHCrHCl, and CH3CCrH3 are 196, 74, 33, and −94 kJ/mol more stable than their respective reactants. The ethylidene is still somewhat higher in energy than the metallacyclopropane. G
DOI: 10.1021/acs.organomet.7b00041 Organometallics XXXX, XXX, XXX−XXX
Organometallics
■
ACKNOWLEDGMENTS This work was supported by an Incheon National University Research Grant in 2015.
Weak new absorptions are observed in the Mo experiments at 1826.7 and 1819.7 cm−1, which are ∼25 cm−1 higher than the 1800.3 and 1795.3 cm−1 bands assigned above to the Mo−H stretching modes for CH3CMoH3. Deuterium counterparts were observed at 1314.7 and 1303.2 cm−1 with frequency ratios 1.3894 and 1.3912. Calculations for the possible Mo reaction products are given in Tables S13−S16. The computed Mo−H stretching frequencies are also ∼25 cm−1 higher for CH3C MoH2Cl than for CH3CMoH3, which supports this assignment. One weak band was observed at 1781.1 cm−1 with a deuterium shift to 1280.3 cm−1 (ratio 1.3912) in the region for both (CH2)2−MoHCl and CH3CHMoHCl, and it is most likely due to the cyclo species, which is computed to be 31 kJ/ mol more stable than the ethylidene. No product absorptions were observed for W and Cr reacting with ethyl chloride.
■
CONCLUSIONS Reactions of laser-ablated group 6 metal atoms with ethane were carried out, and the products were identified in the matrix IR spectra on the basis of the isotopic shifts, correlation with theoretical predictions, and related previous results. CH3CH2− MH, (CH2)2−MH2, and CH3CMH3 have been identified in the Mo and W reactions while CH3CHMH2, which is energetically higher than the observed dihydrido cyclic product, has not. On the other hand, only the insertion product was observed in reactions of Cr, showing that the higher oxidationstate product was more favored in the heavier metal system. Photoisomerization between the observed Mo products is reversible, parallel to the previously investigated group 6 metal reactions with methane and methyl halides.11,21 The Mo and W ethylidenes (unobserved) are agostic, parallel to the group 4 and actinide analogues. The C−C bond length of a metallacyclopropane is shorter than those of the other products due to the extra pull by the bridging metal dihydride group. The C−M bond of an ethylidyne is noticeably short, consistent with the fact that it is a true triple bond.
