Matrix Infrared Spectra of Insertion and Metallacyclopropane Complexes

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX-XXX

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Matrix Infrared Spectra of Insertion and Metallacyclopropane Complexes [CH3CH2−MH and (CH2)2−MH2] Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Ethane Published as part of The Journal of Physical Chemistry virtual special issue “W. Lester S. Andrews Festschrift”. Han-Gook Cho*,†,‡ and Lester Andrews‡ †

Department of Chemistry, Incheon National University, 119 Academy-ro, Yeonsu-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: CH3CH2−MH and (CH2)2−MH2 were identified in the matrix IR spectra from reactions of laser-ablated group 3 metal atoms with ethane, and they were characterized via theoretical investigations. The observed products are the most stable in the proposed reaction path. Because of the small number of valence electrons, the group 3 metal high oxidation-state complexes are less stable. The C−C insertion product [(CH3)2M], which was predicted to be more stable than the observed ones, was not observed probably because of the high energy barrier and a likely slower rate for insertion into one C−C bond than one of six C−H bonds. The C−C bond of the metallacyclopropanes is the shortest among the early transition-metal analogues, and its stretching frequencies are the highest, revealing the weakest interaction between the metal dihydride and ethylidene groups. The undetected ethylidene is not agostic, parallel to the previously examined methylidene.

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INTRODUCTION Transition metals are effective alkane activation agents, and following H-migrations lead to the formation of small high oxidation-state products.1−8 These recent results are linked to the possible conversion of vast natural hydrocarbon resources into more valuable products,9−11 and the chemistry involved in the reactions and the unusual product structures offer means to understand better the characteristics of their larger ligated relatives.12−14 These small organometallic complexes allow various theoretical approaches for investigation for their reaction paths, photochemical activities, and origins of their interesting molecular conformations.7,8,15−18 The products identified in the matrix IR spectra from reactions of methane with transition metals (CH3−MH, CH2 MH2, HCMH3, CH3−M, CH3−MH−, and HCMH3−) are appropriate for a reaction path (CH4 + M → CH3−MH → CH2MH2 → HCMH3) with possible electron capture and dissociation along the way.7,8 The reaction of ethane is similar but more complex (C2H6 + M → CH3CH2−MH → (CH2)2− MH2 → CH2CH−MH3 → CH3CMH3).19−28 In addition to the main reaction path, the insertion complex can rearrange to ethylidene (CH3CH2−MH → CH3CHMH2), and C−C bond insertion [C2H6 + M → (CH3)2−M] might occur. Anion and fragmented products have also been observed.20,22 The previous studies of ethane reactions have shown that the observed products, which are usually the most stable constituents in the reaction path, are largely dependent on the reacting transition metal.7,8,19−28 The preference for the © XXXX American Chemical Society

higher oxidation-state complexes (e.g., ethylidynes, CH3C MH3) observed in heavier metal systems in the middle of the dblock of the periodic table gradually fades away with moving to its right and left sides.7,8,19−28 The relative stability of a product among the plausible ones is largely affected by the number of available valence electrons of the metal atom. The primary products of a group 4 metal are (CH3)2−MH2 and CH2CH− MH3; those of heavy group 6−8 metals are CH3CMH3, and a group 5 metal forms the insertion product as well as the anionic ethylidyne (CH3CMH3−).19−21,23,25 The early actinides are chemically similar to the group 4 metals.28 In this study we performed group 3 metal reactions with ethane and identified the products in the matrix IR spectra on the basis of deuterium shifts, theoretical predictions, and related previous results. The insertion and metallacyclopropane complexes were the primary products, and the interesting product structures and reaction path were characterized. The C−C bonds of the group 3 metallacyclopropanes have strong π character with computed 1.46 natural bond orders.29

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EXPERIMENTAL AND COMPUTATIONAL METHODS Laser-ablated Sc, Y, and La atoms (Johnson-Matthey) produced by a Nd:YAG laser were codeposited with dilute mixtures Received: July 10, 2017 Revised: October 6, 2017 Published: October 19, 2017 A

DOI: 10.1021/acs.jpca.7b06785 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Table 1. Frequencies of Product Absorptions Observed from Reactions of Ethane with Sc, Y, and La in Excess Argon and Major Harmonic Frequencies Calculated Using B3LYPa C2H6 obsd Sc

i m

Y

i m

h La

i m

h

b

1436.6 408d 1451.8, 1446.3 1424.8, 1414.9 1402.3, 1395.3 1203.4, 1201.9 742.7 511.2, 493.2 1395.5 1419.3, 1412.2, 1410.5 1366.5, 1362.5, 1359.6 1190.8, 1189.1 798.0 696.7, 681.7 521.4, 507.0 1347.4, 1344.9, 1338.9 1168.5 1263.1 1303.8 1252.5 1201.0 497.0, 487.3 1238.1, 1225.9

C2D6 c

harm

obsd

1513.2 (496) 423.9 (123) 1519.5 (184) 1537.5 (364) 1483.8 (823) 1236.6 (109) 794.7 (41) 559.7 (138) 1450.4 (421) 1450.7 (466) 1406.6 (842) 1215.0 63) 832.5 (9) 755.9 (50) 547.0 (163)

b

1042.6 covered 1029.6, 1017.0 1009.3, 1006.9 940.2 605.2 999.4 1016.4, 1006.2 984.4, 983.0, 977.1 937.6

harmc

description

1081.9 (242) 383.5 (73) 1341.3 (53) 1091.7 (293) 1068.3 (431) 961.5 (30) 645.6 (21) 419.5 (61) 1031.6 (216) 1029.5 (241) 1004.1 (433) 957.8 (17) 635.3 (12) 610.4 (25) 399.4 (90)

Sc−H str C−Sc str A1 CH2 scis A1 ScH2 str B1 ScH2 str A1 C−C str B2 CH2 wag A1 ScH2 scis Y−H str A1 YH2 str B1 YH2 str A1 C−C str A1 CH2 wag B2 CH2 wag A1 YH2 scis Y−H str C−C str La−H str A1 CH2 scis A1 LaH2 str B1 LaH2 str A1 C−C str A1 LaH2 scis La−H str

967.8, 959.0 1337.2 (705) 1517.5 (15) 1346.7 (602) 1288.9 (1228) 1229.4 (118) 532.9 (199)

908.8 1282.2 931.2 898.3 916.4 886.0, 883.4

947.7 (320) 1331.7 (61) 951.0 (322) 917.2 (624) 963.2 (25) 382.8 (104)

All frequencies are in wavenumbers (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). The numbers in parentheses are the computed absorption intensities in (km/mol). i and m stand for insertion and metallacyclopropane products, and h denotes an unidentified higher-order product. a

Figure 1. IR spectra in the product absorption regions for laser-ablated Sc atoms codeposited with C2H6 in excess argon at 10 K and their variation: (a) Sc + 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 20 K in sequence. i and m denote product absorptions, and p stands for precursor absorptions. OScO− absorption (ref 56) is also indicated.

gold/cobalt vs chromel thermocouple temperature measurement), and more spectra were recorded. Theoretical calculations for possible reaction products were performed using the Gaussian 09 package.32 In all instances, the B3LYP hybrid density functional33,34 was used, and carbon, hydrogen, and scandium atoms employed the large 6-311+ +G(3df,3pd) Gaussian basis.35 Heavier yttrium and lanthanum atoms required the SDD pseudopotentials and basis sets.36−38 Such density functional theory (DFT) calculations predict vibrational frequencies with reasonable accuracy for transition-

(0.5−1.0%) of reagent vapor (C2H6, Matheson; C2D6, MSD Isotopes) in argon (Matheson) onto a CsI window cooled to 10 K using a closed-cycle refrigerator (Air Products HC-2). These methods have been described in detail elsewhere.30,31 After deposition, infrared spectra were recorded in the region of 4000−400 cm−1 at a resolution of 0.5 cm−1 using a Nicolet 550 spectrometer with an MCT-B detector. Matrix samples were next irradiated by a mercury arc street lamp (175 W) with the globe removed (λ > 220 nm) with or without optical filters for 20 min and subsequently warmed and recooled (annealed, B


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The Journal of Physical Chemistry A Table 2. Geometric Parameters and Physical Constants Calculated for the Plausible Productsa products

r(C−C)

r(C−M)

r(M−H)

∠(CMH)

∠(CCM)

q(M)b

symc

ΔEd

CH3CH2−ScH (CH2)2−ScH2 CH2CH−ScH3 CH3CH−ScH2 (CH3)2−Sc CH3CH2−YH (CH2)2−YH2 CH2CH−YH3 CH3CH−YH2 (CH3)2−Y CH3CH2−LaH (CH2)2−LaH2 CH2CH−LaH3 CH3CH−LaH2 (CH3)2−La

1.533 1.394 1.284 1.495 3.692 1.537 1.407 1.297 1.499 3.863 1.527 1.398 1.329 1.500 3.967

2.168 2.367 2.340 2.143 2.178 2.331 2.533 2.509 2.317 2.335 2.423 2.706 2.445 2.427 2.439

1.825 1.847, 1.847 1.832, 1.832, 1.928 1.680, 1.841

116.8 117.9, 117.9 106.5, 106.5, 98.4 118.0, 120.0

1.972 2.004, 2.004 2.001, 2.001, 2.082 2.000, 2.001

110.1 118.9, 118.9 111.9, 111.9, 92.1 114.4

2.096 2.132, 2.132 2.156, 2.156, 2.213 2.121, 2.124

110.7 119.3 113.9, 113.9, 95.5 110.2, 111.1

118.5 72.9 86.3 131.5 32.0 116.1 74.2 91.2 132.3 34.2 94.1 75.0 94.3 130.7 35.6

0.74 0.92 0.85 0.92 0.92 1.04 1.39 1.45 1.30 1.48 0.75 1.40 1.43 1.42 0.76

Cs C2v Cs Cs C2v C1 C2v Cs C1 C2v C1 C2v Cs C1 C2v

22 11 230 97 −43 −64 −67 130 14 −127 −106 −96 109 −17 −164

a

Calculated with B3LYP/6-311++G(3df,3pd)/SDD. The full electron basis set was employed for Sc. The bond length, bond angle, charge, and energy are in angstroms, degrees, e, and kilojoules per mole, respectively. bMulliken charge. cMolecular symmetry. dEnergy relative to the reactants [M(2D) + C2H6].

metal compounds.19−21,39−41 Geometries were fully relaxed during optimization, and the optimized geometries were confirmed by vibrational analysis. Separate calculations with BPW9142,43 were employed to supplement the B3LYP results. In the calculation of binding energy of a metal complex, the zero-point energy is included.

intensity variation in the process of photolysis and annealing. The i absorption was the strongest in the original deposition spectrum and remained so even after photolysis and annealing. They increased slightly on irradiation with λ > 420 nm but more than doubled on irradiation with 240 < λ < 380 nm, and they increased further on photolysis with λ > 220 nm (more than triple in total). The m absorptions were barely visible in the original deposition spectrum. Irradiation with λ > 420 nm slightly increased the intensities, but UV irradiation (240 < λ < 380 nm) more than doubled the m absorptions, and the full arc with λ > 220 nm increased them a little more. The strong i absorption at 1436.6 cm−1 overlapped with a precursor absorption was observed at 1439.0 cm−1, and the product band increased over the parent band on UV irradiation: on deuteration this strong absorption shifted to 1042.6 cm−1 clearly below the parent absorption (H/D frequency ratio 1.378). They are compared with the Sc−H stretching frequency of 1487.7 and 1079.1 cm−1 for ScH and its D counterpart (H/D ratio 1.379).44,45 Previously prepared CH3−ScH had its Sc−H band at 1469.0 cm−1 and the deuterated isotopomer at 1055.9 cm−1 with an H/D ratio of 1.391.46 We assign the 1436.6 cm−1 i absorption to the Sc−H stretching mode of CH3CH2−ScH on the basis of good correlation with the calculated values (Tables 3 and S1) and earlier results concerning Sc−H stretching frequencies.46 Ethyl substitution for methyl lowers the Sc−H stretching frequency by 32.4 cm−1. Another i absorption at 408 cm−1 in the noisy region near the observation limit may be due to the C−Sc stretching mode. The other vibrational bands of CH3CH2−ScH were too weak to observe as shown by the calculated intensities for all of its vibrational modes in Table S1. Assignment of the strong 1436.6 cm−1 band to the Sc−H stretching mode of the C−H insertion product is confirmed by DFT calculations. The observed/calculated frequency ratios using the B3LYP functional are 0.949 for the Sc−H and 0.964 for the Sc−D modes and slightly higher at 0.963 and 0.978, respectively, using the lower calculated BPW91 frequencies. The agreement is excellent, and a higher ratio for the more harmonic Sc−D mode is consistent with the fact that the Sc−D mode is more harmonic (lower in the potential well).

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RESULTS AND DISCUSSION Reactions of laser-ablated group 3 metal atoms with ethane in condensing argon were investigated. The observed product vibrational characteristics and their spectral variations upon photolysis and annealing as well as the product structures and energies are given in Table 1 and Figures 1−11. They are compared with the calculated values in Tables S1−S15. The key geometric parameters are summarized in Table 2. Sc + Ethane. Two groups of product absorptions marked “i” and “m” (indicating, respectively, insertion and metallacyclopropane complexes) are observed in the Sc + C2H6 spectra (Table 1 and Figures 1 and 2) on the basis of their

Figure 2. IR spectra in the product absorption regions for laser-ablated Sc atoms codeposited with C2D6 in excess argon at 10 K and their variation: (a) Sc + 1.0% C2D6 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 20 K in sequence. i and m denote product absorptions, and p stands for precursor absorption. C


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Figure 3. IR spectra in the product absorption regions for laser-ablated Y atoms codeposited with C2H6 in excess argon at 10 K and their variation: (a) Y + 1.0% C2H6 in Ar codeposited for 1 h. (b−g) As (b) after photolysis with λ > 420 nm and 240 < λ < 380 nm, annealing to 20 K, and photolysis with λ > 420 nm and 240 < λ < 380 nm, and annealing to 28 K in sequence. i, m, and h denote product absorptions, and p stands for precursor absorption. OYO− and C2H5 absorptions (refs 57 and 52) are also indicated.

The m absorptions at 1414.9 and 1402.3 cm−1 (with site absorptions at 1424.8 and 1395.3 cm−1) in Figure 1 have their D counterparts at 1017.0 and 1009.3 cm−1 (H/D ratio 1.391 and 1.389; with site absorptions at 1029.6 and 1006.9 cm−1). We assign them to the ScH2 stretching modes of (CH2)2− ScH2. The other observed m absorptions at 1446.3, 1201.9, 742.7, and 493.2 cm−1 are assigned to the A1 CH2 scissoring, C−C stretching, B2 CH2 wagging, and A1 ScH2 scissoring modes on the basis of reasonable correlation with the predicted values (Tables 1 and S2). These observed bands are the strongest expected from the smallest metal cyclic complex as shown in Table S2. The two A1 symmetry modes at 1414.9 and 1446.3 cm−1 no doubt interact, but the observed/calculated frequency ratio for the bending mode 1446.3/1519.5 = 0.951 (Table 3) is reasonable. The observed/calculated frequency ratios for the two Sc−H2 stretching modes, 0.920, 0.945, are compared with those for the two ScD2 stretching modes 0.932, 0.945. These ratios are probably due to the symmetric mode coupling described above, and it precludes drawing any useful information from them. The observed C−H insertion and metallacyclic products are also considerably more stable than the undetected vinyl tryhydrido and ethylidene complexes; CH 3 CH 2 −ScH, (CH2)2−ScH2, CH2CH−ScH3, and CH3CH−ScH2 are 22, 11, 230, and 97 kJ/mol higher in energy than the reactants [Sc(2D) + C2H6]. Formation of the above products requires the reaction of excited Sc atoms in the ablation process or in subsequent photolysis, and there must be a barrier to decomposition back to the reagents. The Sc ethylidene (CH3CH−ScH2) would show not only the ScH2 stretching absorptions but also the moderately strong ScH2 scissoring band expected at ∼595 cm−1 (Table S4) apart from the absorptions of the identified products, displaying concerted intensity variation on photolysis and annealing. They were not observed in this study. The group 3 metal C−C bond insertion complex [(CH3)2− M] was not observed, parallel to the earlier results of transitionmetal reactions with ethane.19−28 The undetected C−C bond insertion complex [(CH3)2−Sc] is actually lower in energy (−43 kJ/mol) than the observed products. The high energy barrier and the low absorption intensities (Table S5) probably prevent generation of a detectable amount of this elusive product.19−28 In addition, there is probably a kinetic effect

owing to a much higher cross section for insertion into one of six surrounding C−H bonds over the single C−C bond. The A1 and B2 CH3 deformation bands expected near 1090 cm−1 with medium intensity were not observed. This indicates that insertion of the C−C bond by the metal atom is harder than insertion into the more exposed six C−H bonds. Y + Ethane. The product spectra from reactions of laserablated yttrium atoms with ethane and deuterated ethane are shown in Figures 3 and 4, and their frequencies are listed in

Figure 4. IR spectra in the product absorption regions for laser-ablated Y atoms codeposited with C2D6 in excess argon at 10 K and their variation: (a) Y + 1.0% C2D6 in Ar codeposited for 1 h. (b−f) As (a) after photolysis with λ > 420 nm, 240 < λ < 380 nm, λ > 420 nm, and 240 < λ < 380 nm and annealing to 24 K in sequence. i, m, and h denote product absorptions, and p stands for precursor absorption.

Table 1 and compared with all of the calculated values for the plausible products in Tables S6−S10. The product absorptions are marked with i, m, and h (for C−H insertion, metallacyclopropane, and high-order complexes) depending upon the intensity variation on photolysis and annealing and correlation with the calculated values. The i absorptions were relatively strong in the original spectra, increased ∼20% on irradiation with λ > 420 nm, and another ∼20% with 380 > λ > 240 nm. In contrast, the m absorptions dramatically increased (more than fourfold) on irradiation with λ > 420 nm, becoming the most prominent product absorptions, and slightly decreased on following irradiation with 380 > λ > 240 nm. The h absorptions D


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The Journal of Physical Chemistry A increased in the process of photolysis and annealing and at higher reagent concentrations. The i absorption at 1395.5 cm−1 was accompanied by a D counterpart at 999.4 cm−1 (H/D ratio 1.396). We assign the former band to the Y−H stretching mode of the C−H bond insertion product (CH3CH2−YH). The observed frequencies are in good agreement with the calculated harmonic values (1450.4 and 1031.6 cm−1), where a substantial amount of the discrepancy is due to anharmonicity in the observed frequencies. These bands compare favorably with 1459.8 and 1397.8 cm−1 for YH2 and 1042.0 and 1003.2 cm−1 for YD2.44,45 The new i frequencies are also close to the Y−H stretching frequencies of previously studied CH3−YH and its D counterpart, 1409.7 and 1010.8 cm−1.46 The exchange of ethyl for methyl substitution (from CH3−YH to CH3CH2− YH) lowers the Y−H stretching frequency (by 14.2 cm−1), in line with the earlier investigations of ethane reactions with transition metals (e.g., the Nb−H stretching frequency decreases 24.2 cm−1 upon ethyl to methyl substitution).7,8,19−28 The other bands of CH3CH2−YH are unfortunately too weak to observe as shown in Table S1. The observed/calculated harmonic frequency ratios are 0.962 for the Y−H stretching mode of CH3CH2−YH and 0.969 for the Y−D stretching mode of CD3CD2−YD using the hybrid B3LYP functional. Since the BPW91 pure density functional gives slightly lower frequencies (Table S1) these ratios are slightly higher (0.972 and 0.979) but in the range expected for these two density functionals.40,41 The ratio for the Y−D mode is slightly higher than that for the corresponding Y−H mode, because the Y−D mode is more harmonic. The close agreement of these ratios confirms our assignments to the ethyl yttrium hydride molecule. The two strong m absorptions were observed at 1410.5 cm−1 (with matrix site absorptions at 1419.3 and 1412.2 cm−1) and at 1359.6 cm−1 (with site absorptions at 1366.5 and 1362.5 cm−1). Their D counterpart was observed at 1006.2 cm−1 (with a site absorption at 1018.4 cm−1) and 977 cm−1 (with site absorptions at 984.4 and 983.0 cm−1; H/D ratios of 1.402 and 1.391). These frequencies are also close to those of YH2 and YD2.44,45 The above absorptions are assigned to the YH2 stretching modes of the metallacyclopropane complex [(CH2)2−YH2] and its deuterium counterpart. The observed frequencies are even closer to the previously reported YH2 stretching frequencies of 1421.6 and 1370.3 cm−1 for CH2− YH2 and 1017.8 and 985.5 cm−1 for CD2−YD2.46 The observed/calculated harmonic frequency ratios are 0.972 and 0.966 for the two Y−H stretching modes of (CH2)2−YH2 and 0.977 and 0.973 for the two Y−D stretching modes of (CD2)2−YD2 using the B3LYP functional.40,41 Again the ratios for the more harmonic Y−D modes are slightly higher than their more anharmonic Y−H counterparts. Again the close agreement of these ratios confirms the assignments. Four more m absorptions observed at 1190.8, 798.0, 696.7, and 507.0 cm−1 were designated to the C−C stretching, B2 and A1 CH2 wagging, and YH2 scissoring modes of the Y metallacyclopropane. The observed frequencies are in reasonable agreement with the predicted values (0.92−0.98 of the calculated values), substantiating production of the small transition-metal cyclic product. The h absorptions at 1347.4, 1344.9, and 1338.9 cm−1 and their D counterparts at 967.8 and 959.0 cm−1 arise probably from the Y−H stretching mode of higher-order complex (e.g., CH3CH2−YH···C2H6), and the one at 1168 cm−1 arises from the C−C stretching mode.46

The undetected plausible group 3 metal products in the proposed reaction path19−28 are relatively higher in energy; observed CH3CH2−YH and (CH2)2−YH2 are 64 and 67 kJ/ mol more stable than the reactants [Y(2D) + C2H6], but unobserved CH2CH−YH3 and CH3CH−YH2 are 130 and 14 kJ/mol higher in energy. The ethylidyne complex (CH3C− YH3) is probably not a meaningful energy minimum; all of our attempts to optimize the structure of the Y ethylidyne ended with the CH3CH−YH2 structure. CH3CH−YH2 would show reasonably strong YH2 scissoring band at ∼540 cm−1 in addition to the YH2 stretching bands with intensity variation on photolysis and annealing different from those of the observed products, which were not observed in this study. Undetected (CH3)2−Y is more stable than the observed products (127 kJ/mol more stable than the reactants), parallel to the Sc analogues, but the vibrational bands of the (CH3)2−Y are all relatively weak (Table S10), making it more difficult to detect. Its strongest CH3 deformation bands expected at ∼1090 cm−1 were not observed. The higher-order product absorptions have been observed in reactions of early transition metals,7,8 which can allow further coordination of the precursor to the metal center. R2MX2 (X = H or halogen) are identified in the group 4 and 5 metal reactions with methane and methyl halides. Unlike the group 4 and 5 metal cases, the high-order product absorptions from reactions of group 3 metals with small alkanes are often broad and probably originate from weakly bound complexes (R− MH···RH).46 La + Ethane. Figures 5 and 6 illustrate the product absorption regions of the matrix IR spectra from reactions of

Figure 5. IR spectra in the product absorption region for laser-ablated La atoms codeposited with C2H6 in excess argon at 10 K and their variation: (a) La + 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 > 420 nm and annealing to 20 K in sequence. i, m, and h denote product absorption groups. CH4 and LaH2 absorptions are indicated, and p stands for precursor absorptions.

laser-ablated La atoms with C2H6 and C2D6. The observed new product absorptions are again marked with i, m, and h on the basis of intensity variation on photolysis and annealing and correlation with the predicted values. The i absorptions increased ∼20% on irradiation with λ > 420 nm and ∼40% with 240 < λ < 380 nm. The m absorptions remained almost unchanged on irradiation with λ > 420 nm but more than double on irradiation with 240 < λ < 380 nm. The h absorption increased in the process of irradiation and annealing and at high sample concentration. E


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The observed/calculated ratio for the La−H mode of CH3CH2−LaH using the B3LYP functional is 0.945, and it is 0.959 for the La−D mode of the deuterium counterpart. These ratios are appropriate for this density functional,40,41 and the slightly higher ratio for the La−D mode is due to its lower anharmonicity. The strong m absorptions at 1303.8 and 1252.5 cm−1 are accompanied with their D counterparts at 931.2 and 898.3 cm−1 (H/D ratios 1.400 and 1.394) and the former bands are designated for the symmetric and antisymmetric LaH 2 stretching modes of (CH2)2−LaH2. They are close to the previously observed LaH2 antisymmetric stretching frequency of CH2−LaH2 (1246.8 cm−1) and the LaD2 stretching frequencies of CD2−LaD2 (924.5 and 892.9 cm−1).46 The m absorptions observed at 1201.0 and 497.0 cm−1 are assigned to the C−C stretching and LaH2 scissoring modes. The m absorption at 1282.2 cm−1 in the C2D6 spectra is assigned to the A1 CH2 scissoring mode, while its H counterpart is believed to be covered by precursor absorption. The observed/calculated frequency ratios (data from Table 2) for the La−H stretching modes of (CH2)2−LaH2 are 0.968 and 0.972, while the analogous ratios for the deuterium counterparts are both 0.979. Here we again see that the latter are slightly higher than the former, because the La−D mode is more harmonic, and its observed frequencies provide a better match for the calculated harmonic frequencies. Parallel to the Sc and Y cases, the observed insertion and metallacyclopropane complexes are considerably more stable than the undetected vinyl trihydrido and ethylidene complexes, and the ethylidyne is again probably not a meaningful minimum. CH3CH2−LaH, (CH2)2−LaH2, and CH3CH− LaH2 are 106, 96, and 17 kJ/mol more stable than the reactants [La(2D) + C2H6], while CH2CH−LaH3 is 109 kJ/mol higher. The C−C bond insertion complex [(CH3)2−La] was again not detected, although it is more stable than the observed products (164 kJ/mol lower in energy than the reactants). CH3CH−LaH2 is expected to show its relatively strong LaH2

Figure 6. IR spectra in the product absorption region for laser-ablated La atoms codeposited with C2D6 in excess argon at 10 K and their variation: (a) La + 1.0% C2D6 in Ar codeposited for 1 h. (b−d) As (a) after photolysis with λ > 420 nm and 240 < λ < 380 nm and annealing to 20 K in sequence. i, m, and h denote product absorption groups, and p stands for precursor absorption.

The i absorption at 1263.1 cm−1 is accompanied by its D counterpart at 908.8 cm−1 (H/D ratio 1.390), and they are assigned to the La−H stretching mode of CH3CH2−LaH and the La−D stretching mode of CD3CD2−LaD on the basis of good correlation with the predicted values (1337.2 and 947.7 cm−1) as shown Tables 1 and S11. They are also compared with the previously reported La−H stretching frequency of CH3−LaH and its D counterpart (1294.8 and 925.9 cm−1)46 and also with 1320.9 and 1283.0 cm−1 for LaH2 and 942.6 and 917.6 cm−1 for LaD2.44,45 Ethyl substitution for methyl lowers the M−H stretching frequency by 31.7 cm−1, consistent with the previous results (e.g., the Ta−H stretching frequency of CH3CH2−TaH is 34.5 cm−1 lower than that of CH3− TaH).7,8,19−28 Other absorptions of the C−H bond insertion complex were unfortunately too weak to observe (Table S7).

Figure 7. B3LYP structures of plausible products from reaction of Sc with ethane. The bond lengths and angles are in angstroms and degrees, respectively. The insertion and metallacyclopropane products [CH3CH2−ScH and (CH2)2−ScH2] were observed in the matrix IR spectra, but the energetically higher vinyl trihydrido and ethylidene [CH2CH−ScH3 and CH3CScH2] products were not. The energetically lower C−C bond insertion product [(CH3)2Sc] is not observed probably due to the high energy barrier. F


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Figure 8. B3LYP structures of plausible products from reaction of Y with ethane. The bond lengths and angles are in angstroms and degrees, respectively. The insertion and metallacyclopropane products [CH3CH2−YH and (CH2)2−YH2] were observed in the matrix IR spectra, but the energetically higher vinyl trihydrido and ethylidene [CH2CH−YH3 and CH3CYH2] products were not. The energetically lower C−C bond insertion product [(CH3)2Y] is not observed probably due to the high energy barrier.

Figure 9. B3LYP structures of plausible products from reaction of La with ethane. The bond lengths and angles are in angstroms and degrees, respectively. The insertion and metallacyclopropane products [CH3CH2−LaH and (CH2)2−LaH2] were observed in the matrix IR spectra, but the energetically higher vinyl trihydrido and ethylidene [CH2CH−LaH3 and CH3CLaH2] products were not. The energetically lower C−C bond insertion product [(CH3)2La] is not observed probably due to the high energy barrier.

scissoring band at ∼460 cm−1 as well as the LaH2 stretching bands (Table S14) with intensity variation on photolysis and annealing different from those of the observed products. These spectral features were not observed in this study. The C−C bond insertion complex [(CH3)2La] was again not detected, although it is more stable than the observed products,

being 164 kJ/mol more stable than the reactants [La(2D) + C2H6]. The strongest CH3 deformation bands expected at ∼1080 cm−1 were not observed. Structures. Figures 7−9 illustrate the B3LYP structures of the plausible products from reactions of group 3 metals and ethane, and the key geometric parameters are summarized in G


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The Journal of Physical Chemistry A Table 2. The insertion (CH3CH2−MH) and ethylidene (CH3CH−MH2) complexes have C1 structures, whereas the metallacyclopropane and C−C insertion complexes [(CH2)2− MH2 and (CH3)2−M] have C2v structures, and the vinyl trihydrido complex (CH2CH−MH3) has a Cs structure. A group 3 metal contributes high d characters to the C−M and M−H bonds of CH3CH2−MH (73.5 and 54.7%, 76.5 and 62.9%, and 76.2 and 63.6% for the Sc, Y, and La complexes), which leads to the bent C−M−H structure,47 like most other previously studied transition-metal analogues.7,8,19−28 In contrast, CH3CH2−MnH, where the metal atom gives much higher s contribution to the C−Mn (32.9% s, 40.3% p) and Mn−H (37.4% s, 51.0% p) bonds to preserve its half-filled dorbitals, has a nearly linear C−Mn−H moiety.22 The Re and Fe analogues also have less bent (larger angle) structures.23,24 The metallacyclopropane products have relatively short C−C bonds among the plausible products (1.394, 1.407, and 1.398 Å for the Sc, Y, and La complexes, respectively). They are also considerably shorter than those of the group 4−6 metal analogues (e.g., 1.516, 1.490, and 1.458 Å for Zr, Nb, and Mo), probably due to weaker back-donation from the group 3 metal center to the C−C π* orbital.19−21 It is also consistent that the observed C−C stretching frequencies of 1201.9, 1190.8, and 1201.0 cm−1 for the Sc, Y, and La metallacyclopropanes are noticeably higher than those of 958.8, 1024.7, 1123.3, and 1035.0 cm−1 for the Zr, Nb, V, and W analogues.19−21 Apparently the three valence electrons of group 3 metal cannot afford strong electron-donation to the empty C−C π* orbital of the metallacyclopropane. The metallacyclopropane, a common product in reactions of ethane with transition metals, has a unique symmetric structure with a small C−M−C angle (34.3, 32.2, and 30.0° for the Sc, Y, and La complexes). Moreover, the ethylene moiety has a nearly planar structure [e.g., dihedral angle Φ(HCCH) = 162.4° for (CH2)2−YH2], and the C−C bond length (1.407 Å) is slightly longer than that for ethylene calculated at the same level of theory (1.325 Å). The ethylene moiety is more planar than those of the previously studied analogues, for example, Φ(HCCH) = 134.7, 140.1, and 145.9° for the Zr, Nb, and Mo metallacyclopropanes.19−28 The C−C−H bond angles, 120.8, 120.4, and 120.6° are also comparable to 121.7° calculated for ethylene. The natural bond orders29 [(occ− occ*)/2] of the C−C bonds of the group 3 metal metallacyclic complexes are all 1.46, indicating sizable π character in the bonds. The C−C bond of the undetected vinyl trihydrido product (CH2CH−MH3) is evidently a true double bond (natural bond orders 1.97, 1.97, and 1.96 for the Sc, Y, and La complexes), which is the shortest among those of the plausible products [1.284, 1.297, and 1.329 Å for the Sc, Y, and La complexes being compared with that of ethylene (1.325 Å)]. The MH3 group located above the vinyl group forms a nearly bridged structure (C−C−M angles 86.3, 91.2, and 94.3°). The energetically high group 3 metal ethylidenes (CH3CH− MH2) do not show agostic distortion of the C−H bond toward the metal center, unlike the group 4,5,6 metal analogues.15−17,48 Roos et al. have concluded from their theoretical study that CH2−YH2 is not agostic in contrast to the other early transition-metal methylidenes because the long C−Y bond (2.31 Å) prevents effective interaction between the C−H bond and the metal center.16 Similarly, the C−M bonds of these ethylidenes (2.143, 2.317, and 2.427 Å for Sc, Y, and La) are evidently too long for effective electron donation from the C−

H bond to the metal center. The relatively short C−Sc bond, which is still considerably longer than those of the Ti and V analogues (1.814 and 1.797 Å),20,21 reflects the smallest covalent radius of Sc (1.44, 1.62, and 1.69 Å for Sc, Y, and La).49 The C−C bond insertion products have C2v structures with computed C−M−C bond angles of 115.9, 111.6, and 108.8°, respectively. Apparently the C−M bonds are single bonds (natural bond orders29 all 0.99), and the unpaired electron mostly stays at the metal atom (Mulliken atomic spin densities50 on the metal center 0.922, 0.872, and 1.241). To further investigate the electronic structure of small group 3 metal metallacyclopropanes, CASSCF calculations51 were also performed for an active space of (9,10) involving five bonding and five antibonding orbitals (those for the C−C σ and π bonds, MC2 bond, and two M−H bonds). Figure 10 shows the

Figure 10. CASSCF structure and orbitals using an (9,10) active space involved in the C−C and C−Y bonds of (CH2)2−YH2 and plotted with isodensity 0.04 e/Å3. The occupation numbers are given in parentheses: the C−C bond is a double bond (C−C bond order 1.978), and the yttrium dihydride group is weakly bound to the ethylene moiety (the YC2 orbital is half-filled). H


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Figure 11. Energy diagram for reaction of Sc(2D) with ethane on the doublet potential surface. The energy barrier to (CH3)2Sc is 70 kJ/mol higher than that to CH3CH2−ScH. The energy barrier for the metallacyclopropane (69 kJ/mol) is also considerably lower than that to the ethylidene complex (128 kJ/mol). The insertion and metallacyclopropane products [CH3CH2−ScH and (CH2)2−ScH2] were observed in the IR matrix spectra. The numbers are the energies of the products relative to the reactants Sc(2D) + C2H6.

CASSCF structure and orbitals for (CH2)2−YH2 (those for the Sc analogue are shown in Figure S2). The C−C, C−Y, and Y− H bonds are longer in the CASSCF than B3LYP structure, but the C−H bonds are shorter. Clearly, on the one hand, the YH2 group is weakly bound to the nearly planar ethylene moiety; the CASSCF distance between C and Y is 2.590 Å, and YC2 bonding orbital is only half-filled (occupation number 1.001, lowest unoccupied molecular orbital (LUMO)). On the other hand, the σ and π bonding orbitals of the C−C bond are both doubly occupied. Figure 10 also shows that the C−C π and π* orbitals are considerably overlapped with the d orbitals of Y, opening a channel for strong electron interaction between the metal center and the π system. A group 3 metal atom allows only a small occupation number for the π* orbital (0.031), while the doubly occupied π-orbital is expected to allocate a substantial amount of electron density to the metal center. Natural bond order (NBO) analysis29 also shows that electron donation from the C−C π bond to the metal center is sizable (124, 94, and 85 kJ/mol for the Sc, Y, and La metallacyclopropanes). Both donation of the π electrons to the metal center and backdonation from the metal atom to the π* orbital weaken the C− C bond. The weak back-donation (essentially empty π* orbital) in a group 3 metal metallacyclopropane apparently leads to the relatively short C−C bond among the early transition-metal analogues. Reactions. The earlier studies have revealed that the transition metals in a group at the right and left sides of the periodic table produce similar products. In contrast, high oxidation-state products become clearly more favored in the heavier metal systems in the middle of the d block. For example, only the C−H bond insertion products (R−MH) were provided in reactions of Mn and Fe with small alkanes, but Re and Os generated only the methylidynes (e.g., HC

MH3 and CH3CMH3) with small alkanes and halomethanes.7,8,23,25 The previous studies of ethane reactions with transitionmetal atoms have shown that C−H bond insertion to form CH3CH2−MH precedes β-H-migration to generate the stable metallacyclopropane [(CH2)2−MH2], which is initiated by excess reaction energy or photon energy supplied separately.19−28 Another H-migration from C to M and ring opening result in formation of the vinyl trihydrido complex (CH2CH−MH3), which has been previously observed in group 4 and 5 metal reactions.19,20 Following H-migration from α to β carbon makes ethylidyne (CH3CMH3), reaction 1.20,21,23,25,28 The H-migration from α-C to M of CH3CH2− MH would produce the ethylidene, reaction 2, and C−C bond insertion of ethane would lead to (CH3)2−M, reaction 3. Both ethyl radical (530 cm−1)52 and ethylene (947 cm−1) are observed in these experiments owing to CH3CH3 photodissociation from far UV photons in the laser ablation plume7 reaction 4. M + C2H6 → CH3CH 2−MH (observed) → (CH 2)2 −MH 2 (observed) → CH 2CH−MH3 (not observed) → CH3C−MH3 (not observed)

(1)

CH3CH 2−MH (observed) → CH3CH−MH 2 (not observed)

I

(2)

M + C2H6 → (CH3)2 −M (not observed)

(3)

C2H6 + hν → CH3CH 2 + H → CH 2CH 2 + H 2

(4)


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Figure 12. Energy diagram for reaction of Y(2D) with ethane on the doublet potential surface. The energy barrier to (CH3)2Y is 56 kJ/mol higher than that to CH3CH2−ScH. The energy barrier for the metallacyclopropane (68 kJ/mol) is also considerably lower than that to the ethylidene complex (140 kJ/mol). The insertion and metallacyclopropane products [CH3CH2−YH and (CH2)2−YH2] were observed in the IR matrix spectra. The numbers are the energies of the products relative to the reactants Y(2D) + C2H6.

Figure 13. Energy diagram for reaction of La(2D) with ethane on the doublet potential surface. Notice the energy barrier to (CH3)2La is 71 kJ/mol higher than that to CH3CH2−LaH. The energy barrier for the metallacyclopropane (76 kJ/mol) is considerably lower than that to the ethylidene complex (134 kJ/mol). The insertion and metallacyclopropane products [CH3CH2−LaH and (CH2)2−LaH2] were observed in the IR matrix spectra. The numbers are the energies of the products relative to the reactants La(2D) + C2H6.

The observed products in the previous studies are basically the ones that are most stable along the reaction path after initial C−H bond insertion, supporting the proposed reaction mechanisms.19−28 The insertion and metallacyclopropane complexes are the energetically most favorable ones in these group 3 metal systems as described above. Because of the small number of valence electrons in group 3 metals, the ethylidynes, which require at least four chemical bonds around the metal

center, are not a meaningful energy minimum. Similarly, the methylidynes were not observed in reactions of group 3 metals with methane and halomethanes.7,8,46 The stability of metal− halogen bonds was also not enough to provide the high oxidation-state complex; reactions with tri- and tetrahalomethanes generated the bridged methylidenes (ZC(X)−MX2, Z = H, halogen) instead of ZC−MX3.53,54 J


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of the metal atom determines the possible number of chemical bonds, and more chemical bonds lead to the relative stability of the product. As a result, the primary products of a group 4 metal are (CH3)2−MH2 and CH2CH−MH3, those of heavy group 6−8 metals are CH3CMH3, and a group 5 metal forms the anionic ethylidyne (CH3CMH3−).19−21,23,25 The metallacyclopropanes (three chemical bonds around the metal center) are the most stable products in group 3 metal systems along the reaction path (1). While the reaction products from reactions of ethane with transition metals are comparable with those from reactions of methane, the high oxidation-state products are in general slightly more favored (higher yields) in the ethane reactions.7,8,19−28 However, unlike in methane reactions, products with C−C double bonds are also produced (CH2CH−MH3), and elimination of H2 can occur from the product (CH2CH−MnH).19−28 The absorptions of the metallacyclopropane, the smallest metal cyclic product, which corresponds to stable (CH3)2MH2 in the methane reactions, are often dominating spectral features if produced in the matrix spectra reflecting its high yield.19−21,28 The ethylidene (CH3CHMH2), which corresponds to the methylidene in methane reactions, is in most cases less stable than the metallacyclopropane and as a result, rarely observed.28 Like in methane reactions, the higher preference for higher oxidation-state products in reaction of a heavier metal in a group is not observed in reactions of ethane with group 3 metals. The high preference for the high oxidation-state products in the transition-metals systems in the middle of the d block weakens gradually with moving to the left in the periodic table,19−25 but it is still observable in group 4 metal systems.19 While Ti, Zr, and Hf all produced CH3CH2−MH and (CH2)2−MH2, the absorptions arising from (CH2)2−MH2 became relatively stronger than those from CH3CH2−MH from Ti to Zr to Hf. The absorptions of the trihydrido product CH2CH−MH3 were prominent in the Hf spectra, but they were invisible in the Ti spectra.19 However, group 3 metals all gave the same products, CH3CH2−MH and (CH2)2−MH2, and systematic variation in the relative yields between products was also not noticeable. Rh and Pt, the second- and third-row elements all produced only insertion complexes in reactions with ethane,26,27 indicating that the preference for higher oxidation-state products in heavier metal systems is not as obvious as in reactions of early transition metals. The previous results from methane and halomethanes also revealed that the high oxidation-state products were less favored in the late transition-metal systems.7,8 The group 3 metal reactions provide us the best possibility to observe the C−C insertion product [(CH3)2−M], which is unique in the ethane reactions, because it is clearly the most stable product in the metal systems unlike in other transitionmetal systems.19−28 However, it was not detected in this study as described above. The high energy barrier (230, 153, and 133 kJ/mol), statistical factor (only one C−C bond vs six C−H bonds), deeper location of the bond, and low absorption intensities all played together to prevent its observation.

The transition states of the reactions (1−3) are calculated, and IRC calculations55 followed to confirm the transition states. Figures 11−13 show energy diagrams for the Sc, Y, and La reactions with ethane in the doublet potential surface, showing that CH3CH2−MH and (CH2)2−MH2 are not only relatively stable but also easier to produce over the rather low energy barriers toward them. The activation barriers for the C−H insertion reactions (160, 97, and 62 kJ/mol for Sc, Y, and La) are considerably lower than those of the competing C−C insertion reactions (230, 153, and 133 kJ/mol). Similarly the activation energy toward the metallacyclopropanes (69, 68, and 76 kJ/mol) are significantly smaller than those of the competing isomerization toward the ethylidenes (128, 140, and 134 kJ/mol). As a result, (CH3)2−M is more difficult to produce, although it is more stable than the observed two products [CH3CH2−MH and (CH2)2−MH2]. Likewise the metallacyclopropane is expected to be more favored over the ethylidene. The high energy barrier toward less stable CH2 CH−MH3 (219, 200, and 211 kJ/mol) also prevents generation of the vinyl trihydrido product. The earlier studies have shown that the undetected C−C insertion complexes are energetically comparable to the observed products in other metal systems.19−28 For example, (CH3)2−Zr is 69 kJ/mol more stable than CH3CH2−ZrH, but it is 25 and 7 kJ/mol higher in energy than (CH2)2−ZrH2 and CH2CH−ZrH3, whereas (CH3)2−Nb is 34 and 11 kJ/mol more stable than CH3CH2−NbH and CH2CH−NbH3, but it is 31 kJ/mol higher in energy than (CH2)2−NbH2. (CH3)2−Mo is 44, 41, and 41 kJ/mol more stable than CH3CH2−MoH, (CH2)2−MoH2, and CH3CMoH3. However, in group 3 metal systems the C−C insertion complexes are considerably more stable than the observed products as described above, but they are still not produced in an observable amount. The C−C bond, which is located at the center of ethane is evidently more difficult to approach than the six C−H bonds. Comparisons within Transition-Metal and Ethane Reactions. The identified products in the matrix spectra from reactions of ethane varies substantially with the reacting transition metals.7,8,19−28 While Cr, Fe, Rh, and Pt generated only the insertion complexes (CH3CH2−MH),21,24,26,27 Re and Os produce exclusively the ethylidynes (CH3CMH3).23,25 In addition experiments with Ru also gave only the insertion product (2024.4 cm−1 for the Ru−H mode and 1456.9 cm−1 for the Ru−D mode with H/D frequency ratio 1.390; unpublished results). In contrast, V yielded the insertion and metallacyclopropane [(CH2)2−MH2] products,21 and Mn gave the insertion and vinyl (CH2CH−MH) products.22 Mo and W produced CH3CH2−MH, (CH2)2−MH2, and CH3CMH3,21 whereas Nb and Ta gave CH3CH2−MH, (CH2)2−MH2, CH2CH−MH3, and CH3CMH3−.20 The group 4 metals in comparison yield CH3CH2−MH, (CH2)2− MH2, and CH2CH−MH3 with no trace of ethylidyne.19 The group 3 metals as described in this paper generate the insertion and metallacyclopropane complexes, and the further reactions toward vinyl trihydrido and ethylidyne are energetically uphill processes, and the energy barriers are also high. Actinides also produced similar products.28 CH3CH2−MH, (CH2)2−MH2, CH3CHMH2, and CH2CH−MH3 are observed in reactions of Th and U with ethane. The observed products in the earlier and present studies reflect the variation in relative stability of the products among the plausible ones originated from the electronic structures of the transition metals. The number of available valence electrons

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CONCLUSIONS Reactions of laser-ablated group 3 metal atoms with ethane have been performed, and the products were observed in the matrix IR spectra on the basis of the isotopic shifts, comparison with calculated results, and related earlier observations. They all K


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(4) Billups, W. E.; Konarski, M. M.; Hauge, R. H.; Margrave, J. L. Activation of Methane with Photoexcited Metal Atoms. J. Am. Chem. Soc. 1980, 102, 7393−7394. (5) Greene, T. M.; Andrews, L.; Downs, A. J. The Reaction of Zinc, Cadmium, and Mercury Atoms with Methane: Infrared Spectra of the Matrix-Isolated Methylmetal Hydrides. J. Am. Chem. Soc. 1995, 117, 8180−8187. (6) Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. Interactions of Atomic and Molecular Iron with Methane in an Argon Matrix. J. Am. Chem. Soc. 1985, 107, 6134−6135. (7) Andrews, L.; Cho, H.-G. Matrix Preparation and Spectroscopic and Theoretical Investigations of Simple Methylidene and Methylidyne Complexes of Group 4−6 Transition Metals. Organometallics 2006, 25, 4040−4053. (8) Cho, H.-G.; Andrews, L. Matrix Preparation and Spectroscopic and Theoretical Investigation of Small High Oxidation-State Complexes of Group 3−12 and 14, Lanthanide and Actinide Metal Atoms. Coord. Chem. Rev. 2017, 335, 76−102. (9) Labinger, J. A.; Bercaw, J. E. Understanding and Exploiting C-H Bond Activation. Nature 2002, 417, 507−514. (10) Shilov, A. E.; Shul’pin, G. B. Activation of C-H bonds by Metal Complexes. Chem. Rev. 1997, 97, 2879−2932. (11) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Selective Intermolecular Carbon-Hydrogen Bond Activation by Synthetic Metal Complexes in Homogeneous Solution. Acc. Chem. Res. 1995, 28, 154−162. (12) Schrock, R. R. High Oxidation State Multiple Metal-Carbon Bonds. Chem. Rev. 2002, 102, 145−180. (13) Mindiola, D. J. Foreword for the Special Issue Hydrocarbon Chemistry: Activation and Beyond. Responsible or Irresponsible Use of Hydrocarbons? You Be the Judge! Organometallics 2017, 36, 5−7. (14) Crabtree, R. H. Aspects of Methane Chemistry. Chem. Rev. 1995, 95, 987−1007 and references therein.. (15) Berkaine, N.; Reinhardt, P.; Alikhani, M. E. Metal (Ti, Zr, Hf) Insertion in the C-H Bond of Methane: Manifestation of an Agostic Interaction. Chem. Phys. 2008, 343, 241−249. (16) Roos, B. O.; Lindh, R.; Cho, H.-G.; Andrews, L. Agostic Interaction in the Methylidene Metal Dihydride Complexes H2MCH2 (M = Y, Zr, Nb, Mo, Ru, Th, or U). J. Phys. Chem. A 2007, 111, 6420− 6424. (17) von Frantzius, G.; Streubel, R.; Brandhorst, K.; Grunenberg, J. How Strong is an Agostic Bond? Direct Assessment of Agostic Interactions Using the Generalized Compliance Matrix. Organometallics 2006, 25, 118−121. (18) Hinrichs, R. Z.; Schroden, J. J.; Davis, H. F. Competition between C-C and C-H Insertion in Prototype Transition MetalHydrocarbon Reactions. J. Am. Chem. Soc. 2003, 125, 860−861. (19) Cho, H.-G.; Andrews, L. C-H Activation of Ethane by Group 4 Metal Atoms: Observation and Characterization of the MH-CH2CH3, MH2-(CH2)2, and MH3-CHCH2 Complexes. J. Phys. Chem. A 2008, 112, 1519−1525. (20) Cho, H.-G.; Andrews, L. Observation and Characterization of CH3CH2-MH, (CH2)2-MH2, CH2CH-MH3, and CH3-CMH3− Produced by Reactions of Group 5 Metal Atoms with Ethane. J. Phys. Chem. A 2017, 121, 6766−6777 W. Lester S. Andrews Festschrift. (21) Cho, H.-G.; Andrews, L. Observation and Characterization of CH3CH2-MH, (CH2)2-MH2, and CH3-CMH3 Prepared in Reactions of Ethane with Laser-Ablated Group 6 Metal Atoms. Organometallics 2017, 36, 1479−1487. (22) Cho, H.-G.; Andrews, L. Infrared Spectra of Manganese Insertion, Vinyl, and Cyclic Complexes Prepared in Reactions of Laser-Ablated Mn Atoms with Methane, Ethane, Ethyl Chloride, and 1,2-Dichloroethane. Organometallics 2013, 32, 3458−3468. (23) Cho, H.-G.; Andrews, L. Infrared Spectra of Methylidynes Formed in Reactions of Re Atoms with Methane, Methyl Halides, Methylene Halides, and Ethane: Methylidyne C-H Stretching Absorptions, Bond Lengths, and s Character. Inorg. Chem. 2008, 47, 1653−1662.

generate the same products, the insertion and metallacyclopropane complexes (CH3CH2−MH, and (CH2)2−MH2). In contrast, on the one hand, the plausible ethylidene and vinyl trihydrido products (CH3CHMH2 and CH2CH−MH3) are energetically too high, and the high barriers are also difficult to overcome, and the ethylidyne is probably not a meaningful energy minimum for group 3 metals. The C−C insertion complex [(CH3)2−M], on the other hand, is energetically lower than the observed products, but the higher barrier to its formation and its lower absorption intensities have prevented its observation. The ethylene moiety of a group 3 symmetrical metallacyclopropane is structurally similar to ethylene itself. The C− C bond is shortest among the early transition metal analogues, and its stretching frequency is also the highest. Both the NBO and CASSCF analyses indicate that it has a strong π character and that the metal dihydride group is weakly bound to the ethylene moiety. The CASSCF results also show that π and π* orbitals are largely incorporated with the d orbitals of the metal center. However, the antibonding orbital is essentially empty due to the smaller number of valence electrons of the group 3 metal, resulting in weak back-donation from the metal center. Parallel to the previously studied methylidenes, the undetected ethylidenes are not agostic due to the long C−M bonds unlike other early transition-metal analogues. The C−C bond of the vinyl trihydrido complex (CH2CH−MH3) is a true double bond.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b06785. Calculated vibrational frequencies, intensities, NBO results, CASSCF results of (CH2)2−ScH2, Cartesian coordinates of the observed products, and full 4000−400 cm−1 matrix IR spectra. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-32-835-8236. ORCID

Han-Gook Cho: 0000-0003-0579-376X Lester Andrews: 0000-0001-6306-0340 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by an Incheon National University Research Grant in 2017.

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REFERENCES

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Matrix Infrared Spectra of Insertion and Metallacyclopropane Complexes

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