Infrared Spectra of M-η2-C2H2, HM–C CH, and HM–C CH

Sep 25, 2012 - Department of Chemistry, University of Incheon, 12-1 Songdo-dong, Yeonsu-gu, Incheon 406-772, South Korea and Department of. Chemistry ...
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Infrared Spectra of M‑η2‑C2H2, HM−CCH, and HM−CCH− Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Acetylene Han-Gook Cho* and Lester Andrews Department of Chemistry, University of Incheon, 12-1 Songdo-dong, Yeonsu-gu, Incheon 406-772, South Korea and Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States S Supporting Information *

ABSTRACT: The major HM−CCH and M-η2-C2H2 products are observed in the matrix infrared spectra from reactions of laser-ablated group 3 metal atoms with acetylene, while the vinylidene product is not detected. These results reveal that coordination of group 3 metal atoms to the acetylene π-bond and H-migration from C to M readily occur during codeposition and photolysis afterward. The product absorption assigned to the La−vinylidene complex in a previous study is reassigned to one of the absorptions of La-η2-C2H2···C2H2. Two strong Sc−H stretching absorptions are assigned to the free HSc−CCH− anion, in accord with a previous study, but a lower frequency counterpart is reassigned to HSc−CCH− coordinated to acetylene based on the increasing relative intensity of the low-frequency component at higher acetylene concentration. The group 3 metals evidently form weaker π-complexes than the group 4 metals. The addition of an electron to HM−CCH elongates the M−H and M−C bonds in the anionic species due to the lower ionic contributions to the bonding.

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find the two products energetically comparable.10a In later work on the reaction of Sc with C2H2, these authors observed two Sc−H stretching absorptions (1387.1 and 1365.3 cm−1) in a relatively low frequency region, which almost disappear on the addition of CCl4, an efficient electron scavenger.10b They have assigned them to HSc−CCH− and HScSc−CCH− on the basis of frequency correlation with the DFT values. The anionic products are in line with the previous report that group 3 metal reactions also produce anionic species owing to the ablation of electrons from these electropositive metals.11 These workers observed two other product absorptions in the Y reactions (1415.3 and 1100.6 cm−1) and assigned them to the strongest bands of the π and insertion complexes (Y-η2-C2H2 and HY− CCH). In part because of inconsistences between these earlier group 3 and our group 4/5 works,5,10 we have reinvestigated the reactions of all group 3 metals (La, Y, and Sc) with acetylene isotopomers in excess argon and performed additional calculations. The higher product yields with additional product absorptions confirm several previously reported products, identify new products, and reassign two other product absorptions. It turns out that La and Sc as well as Y generate the π-complexes and insertion products with no evidence for the vinylidene species and that the strong Sc−H stretching absorptions probably originate from a free anionic species and one coordinated to acetylene.

INTRODUCTION The electron-rich π-system of acetylene, the smallest alkyne, makes an exceptional ligand to generate various types of metal complexes.1 While the bonding in an acetylene complex is similar to that of an ethylene complex, acetylene tends to bind more tightly to a metal center.2 Coordination with the π-system is also often followed by subsequent rearrangements, providing characteristic products. The best known small acetylene complexes are the π-complexes (M-η2-(C2H2)), the C−H insertion products (HM−CCH), vinylidenes (H2CC M), vinyl radicals (•CHCH−M), and ethynyl complexes (HCCM).3−9 Recent studies have revealed that the back-donations from the group 4 metals to the π*-orbitals in M-η2-(C2H2) are extensive, which forms unusually strong π-complexes, and the strengthening of the M−C bonds and the weakening of the C− C bonds decrease with moving to the right in the periodic table.5,6 Although the π and insertion complexes are most common, the primary products vary greatly with the particular transition metal.3−10 Ni and Pd generate the π-complexes with no trace of the insertion and other products,4,6 whereas Fe generates the insertion complex via a hydrogen-bonded complex (Fe···H−CC−H) and without observation of Feη2-(C2H2).8 Only the insertion complexes are identified in the Mn and Re reactions, while the metal vinylidenes are produced in the Ru, Rh, and Pt reactions.6 Teng and Xu have recently reported observation of La− C2H2, La2−C2H2, HLa−CCH, and H2CCLa in the reaction of La, and they have also claimed, we believe incorrectly, that the activation energy for the rare vinylidene complex (193 kJ/mol) is smaller than that of the insertion complex (328 kJ/mol) but © 2012 American Chemical Society

Received: May 29, 2012 Revised: September 21, 2012 Published: September 25, 2012 10917

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Figure 1. IR spectra in the product absorptions regions for laser-ablated La atoms codeposited with C2H2 in excess argon at 8 K and their variation. (a) La + 0.25% C2H2 in Ar codeposited for 1 h. (b) As in panel a, after annealing to 28 K. (c) As in panel b, after photolysis (λ > 420 nm). (d) As in panel c, after photolysis (240 < λ < 380 nm). (e) As in panel d, after annealing to 36 K. π, πA, and i denote the product absorption groups; p, h, and c stand for precursor, high-order product, and common absorptions from this precursor with other metals.

Table 1. Frequencies of Product Absorptions Observed from Reactions of La with Acetylene Isotopomers with in Excess Argona C2H2 π

πA

i

2930.8 2913.0 1388.6, 1386.9 1097.5, 1093.2 815.4 601.6 476.6 1673.0 1383.9, 1378.7 1088.4, 1086.7, 1084.7 811.7, 810.7 687.3, 679.8 591.1 559.7 3246.0 1927.7, 1924.7 1302.8, 1263.3,1235.3 687.2 679.9

C2D2

877.6 460.8 468.6 1566.2 873.3 covered

covered 931.3, 926.7, 913.8 covered covered

13

C2H2

HCCD

2922.5 2903.1 covered 1081.4, 1076.0 814.8 585.8 464.2 1615.7 covered 1073.4, 1071.0, 1067.7 811.0, 810.0 667.6, 644.2 585.8 557.1 covered 1302.7, 1263.3, 1235.3 680.8 672.9

covered 1093.2, 1017.0 652.3 529.9 1672.7 covered 1012.6

1928.0, 1924.7, 1794.3, 1792.5 1302.5, 1263.3,1235.3, 931.2, 926.7

description La-η2-C2H2, A1 C−H s.str. La-η2-C2H2, B2 C−H as.str. La-η2-C2H2, A1 C−C str. La-η2-C2H2, B2 CCH ip bend La-η2-C2H2, A1 CCH ip bend La-η2-C2H2, B1 CCH oop bend La-η2-C2H2, A1 LaC2 s. str. La-η2-C2H2···C2H2 C−C str. La-η2-C2H2···C2H2 C−C str. La-η2-C2H2···C2H2, CCH ip bend La-η2-C2H2···C2H2, CCH ip bend La-η2-C2H2···C2H2, CCH ip bend La-η2-C2H2···C2H2, CCH oop bend La-η2-C2H2···C2H2, CCH oop bend HLa−CCH, A′ C−H str. HLa−CCH, A′ C−C str. HLa−CCH, A′ La−H str. HLa−CCH, A″ CCH bend HLa−CCH, A′ CCH bend

All frequencies are in cm−1. Stronger absorptions are bold. Description gives major coordinate. π, πA, and i stand for the π, C2H2-coordinated π, and insertion products, respectively. bOverlapped by precursor absorption.

a

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0.5 cm−1 using a Nicolet 550 spectrometer with a MCT-B detector. Samples were later irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed and a combination of optical filters, and subsequently annealed to allow further reagent diffusion. Complementary density functional theory (DFT) calculations were carried out using the Gaussian09 package,13 B3LYP density functional,14 6-311++G(3df,3pd) basis sets for C, H, and Sc, and SDD pseudopotential and basis set15 for La and Y (28 electron core) to provide a consistent set of vibrational frequencies and energies for the reaction products and their

EXPERIMENTAL AND COMPUTATIONAL METHODS Laser ablated group 3 metal atoms (Johnson−Matthey) were reacted with C2H2 (Matheson, passed through a series of traps to remove acetone stabilizer), C2D2, 13C2H2 (Cambridge Isotopic Laboratories, 99%), and HCCD (prepared from CaC 2 and deuterated water) in excess argon during condensation at 8 K using a closed-cycle refrigerator (Air Products HC-2). These methods have been described in detail elsewhere.12 Reagent gas mixtures range 0.13−0.50% in argon. After reaction, infrared spectra were recorded at a resolution of 10918

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analogues. BPW9116a calculations were also done to confirm the B3LYP results. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. Every minimum identified on the potential energy surface (PES) was found to have all positive harmonic vibrational frequencies and each transition state structure to have only one imaginary frequency, corresponding to the reaction coordinate. Intrinsic reaction coordinate (IRC) calculations16b have been performed to link the transition state structures with the reactants and specific products. The vibrational frequencies were calculated analytically, and the zero-point energy is included in the calculation of binding energy of a metal complex.

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RESULTS AND DISCUSSION

Reactions of group 3 metal atoms with acetylene isotopomers were carried out, and the matrix infrared spectra and their variation upon photolysis and annealing (Figures 1−9 and S1− 2 and Tables 1−3) will be compared with the vibrational characteristics of anticipated products calculated by density functional theory (Supplementary Tables S1−16). In addition, bands from precursor irradiation products common to other metal experiments were also observed.3−10,17 The computed natural atomic charges and bond orders18 of the neutral and anionic insertion complexes are listed in Supplementary Table S17.

Figure 3. IR spectra in the product absorptions regions for laserablated La atoms codeposited with 13C2H2 in excess argon at 8 K and their variation. (a) La + 0.50% 13C2H2 in Ar codeposited for 1 h. (b) As in panel a, after annealing to 28 K. (c) As in panel b, after photolysis (λ > 420 nm). (d) As in panel c, after photolysis (240 < λ < 380 nm). (e) As in panel d, after annealing to 36 K. π, πA, and i denote the product absorption groups; p and h stand for precursor and high-order product absorptions.

absorptions is similar to that of the π absorptions, but they increase more on annealing and are relatively stronger at higher concentration. The i absorptions sharpen in the early stage of annealing (Figure 1b), remain almost unchanged on visible (λ > 420 nm) irradiation (c), and increase 40% on UV (240 < λ < 380 nm) irradiation (d). They sharpen on annealing to 36 K (e) and gradually decrease in later annealing. The observed frequencies are listed in Table 1, and they are compared with the DFT frequencies predicted for the plausible products from reaction of La with C2H2 in Supplementary Tables S1−6 . More π absorptions are observed than found for i, but they are generally weaker as shown in Figures 1−3 and Tables 1 and S1−3. Their dramatic increase in the process of annealing indicate that the La atom readily coordinates to the electronrich π-system. The π absorptions at 1093.2 and 815.3 cm−1 are assigned to the B2 and A1 CCH in-plane (IP) bending modes along with their isotopic counterparts on the basis of correlation with the DFT frequencies (e.g., B3LYP frequencies of 1120.1 and 831.1 cm−1) as shown in Tables 1 and S1. The C−C stretching band, which is originally IR-inactive for acetylene, is observed at 1388.6 cm−1. The π absorptions in the low frequency region at 601.7 and 476.6 cm−1 are assigned to the CCH out-of-plane bending (OOP) and LaC2 symmetric stretching modes. The π absorptions in the high frequency region at 2930.8 and 2913.0 cm−1 are assigned to the A1 and B2 C−H stretching modes. The good correlation of the observed π absorptions with DFT values (Supplementary Table S1) substantiates formation of La-η2-C2H2. Xu et al. have assigned four product absorptions at 2933.8, 1383.9, 1093.4, and 651.5 cm−1 to the C−H stretching, C−C stretching, A2 OOP CCH bending, and B1 CCH bending modes of the π-complex, respectively.10a The B1 OOP bending frequency is predicted to be 988.4 cm−1 (Supplementary Table S1), and the B1 OOP CCH bending frequency in their report instead coincides with the B2 IP CCH bending frequency in this study. In addition, a weak cyc-(LaN)2 absorption is observed in

Figure 2. IR spectra in the product absorptions regions for laserablated La atoms codeposited with C2D2 in excess argon at 8 K and their variation. (a) La + 0.50% C2D2 in Ar codeposited for 1 h. (b) As in panel a, after photolysis (λ > 420 nm). (c) As in panel b, after photolysis (240 < λ < 380 nm). (d) As in panel c, after annealing to 28 K. (e) As in panel d, after annealing to 36 K. π, πA, and i denote the product absorption groups; h and c stand for high-order product and common absorptions from this precursor with other metals.

La + C2H2. Figures 1−3 and S1 show the La + C2H2, C2D2, C2H2, and HCCD spectra in the reaction product absorption regions. The product absorptions are marked π, πA, and i (for π-complex, acetylene coordinated π-complex, and insertion products). The π absorptions are weak in the original deposition spectra but emerge in the early stage of annealing (Figure 1b). The visible irradiation brings a small decrease, but the following UV irradiation depletes them. They are restored on subsequent annealing (e). The behavior of the πA 13

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our spectra at 652.2 cm−1 with a site absorption at 651.2 cm−1,11c and no product absorption is observed at 651.5 cm−1. The C−H stretching and C−C stretching bands are observed at 2930.8 and 1388.6 cm−1 as described above. These authors have attributed the absorptions at 1378.9, 1086.9, and 559.7 cm−1 to the C−C stretching, OOP CCH bending, and C2H2 torsion modes of La2−C2H2.10a The absorption at 1673.2 cm−1, which shows similar intensity variation in the process of photolysis and annealing, is assigned to the C−C stretching mode of the vinylidene complex (H2CCLa) along with its D and 13C counterparts at 1636.0 and 1615.8 cm−1. This vinylidene identification, if correct, would be the first early transition metal vinylidene complex observed in a matrix IR spectrum. These product absorptions increase in concert on annealing and at high concentrations of acetylene, and they deplete on UV irradiation, like the π-complex. However, these previously observed frequencies are higher than the DFT values. The reported frequencies of 1378.9, 1086.9, and 559.7 cm−1 assigned for La2−C2H2 are 1.002, 0.955, and 1.070 of the computed values for the product, and those of 1673.2, 1636.0, and 1615.8 cm−1 assigned to H2CCLa, D2CCLa, and H213C13CLa are 1.032, 1.037, and 1.031 of the predicted values (Supplementary Tables S4 and S5). In addition, the strong CH2 symmetric and C−La stretching absorptions of the vinylidene product are not observed, and a common baseline fluctuation is in fact observed at 1636 cm−1 in matrix IR spectra, as is shown in their spectra. The D counterpart of the absorption at 1673.2 cm−1 is in fact observed at 1566.2 cm−1 in our spectra. We assign these product absorptions to the higher La− C2H2···C2H2 complex, noted πA. Acetylene products are easy to aggregate with the precursor on annealing and at higher concentration.4,6 The πA C−C stretching absorption of the more weakly bound acetylene at 1673.9 cm−1 and its D and 13C counterparts at 1566.2 and 1615.7 cm−1 correlate well with the predicted values of 1743.5, 1682.0, and 1625.3 cm−1 (obs/calcd frequency ratios of 0.960, 0.964, and 0.961). The πA absorptions at 1378.9, 1086.7, and 559.7 cm−1 are assigned to the C−C stretching, A″ CCH bending, and CCH OOP bending modes of the more strongly bound acetylene on the basis of reasonable agreement with the predicted values of 1427.1, 1113.8, and 545.6 cm−1 as shown in Supplementary Table S2 (obs/calcd frequency ratios of 0.966, 0.976, 1.026). Three more πA absorptions are observed at 810.7, 687.3, and 591.1 cm−1 and assigned to the CCH bending modes of La-η2C2H2···C2H2 with good correlation with the predicted values of 835.6, 689.4, and 611.6 cm−1 (obs/calcd frequency ratios of 0.970, 0.997, and 0.966). The frequency of the strongest i absorption (1302.7 cm−1) is essentially the same as the previously reported value, which shows no shift on 13C substitution, but deuteration shifts it to 931.3 cm−1 (H/D ratio of 1.399).10a The observed La−H and La−D stretching frequencies are compared with 1320.9 and 1283.0 cm−1 for LaH2 and 942.6 and 917.6 cm−1 for LaD2.11 Both the La−H and La−D stretching absorptions are observed in the La + HCCD spectra (Figure 3). The La−H stretching absorption is indicative of production of a C−H bond insertion complex (HLa−CCH) in reaction of La with acetylene. The second strong C−C stretching absorption is observed at 1927.7 cm−1. The previously reported absorption at 1938.2 cm−1 does not show the intensity variation of the i absorptions but gradually decreases on successive annealing and photolysis. The weak i absorptions at 687.2 and 679.9 cm−1 are designated to

the A″ and A′ CCH bending modes and the one at 3246.0 cm−1 to the C−H stretching mode. The observed frequencies are in reasonable correlation with the DFT frequencies for HLa−CCH as shown in Supplementary Table S3, supporting that lanthanum readily undergoes C−H insertion to produce HLa−CCH. Observation of the π and insertion complexes in matrix IR spectra from La reaction with C2H2 is in line with the previously investigated transition metal analogues (groups 4− 11, actinides, and Y).3−10 The possible anionic insertion complex (HLa−CCH−) is not detected, as its strongest La−H stretching absorption would appear at ∼1200 cm−1. The La vinylidene is also not observed. Its strong CH2 stretching, C−C stretching, and CH2 wagging bands, which would appear at ∼2895, 1555, and 900 cm−1, are not observed. The observed products are derived from the two most stable among the plausible products: HLa−CCH, La-η2-C2H2, and H2CCLa are only 242, 204, and 109 kJ/mol more stable than the reactants (La(2D) + C2H2). Y + C2H2. The product absorptions from reactions of Y with acetylene isotopomers (C2H2, C2D2, HCCD, and 13C2H2) are shown in Figures 4−6 and S2. The observed frequencies are

Figure 4. IR spectra in the product absorptions regions for laserablated Y atoms codeposited with C2H2 in excess argon at 8 K and their variation. (a) Y + 0.50% C2D2 in Ar codeposited for 1 h. (b) As in panel a, after annealing to 28 K. (c) As in panel b, after photolysis (λ > 420 nm). (d) As in panel c, after photolysis (240 < λ < 380 nm). (e) As in panel d, after annealing to 36 K. π and i denote the product absorption groups; p and c stand for precursor and common absorptions from this precursor with other metals.

listed in Table 2 and compared with the DFT frequencies in Supplementary Tables S7−10. Parallel to the La case, the Y-η2C2H2 and HY−CCH absorptions marked π and i are observed in the product spectra. The π absorptions increase dramatically, but almost disappear on UV irradiation, and they reappear in the process of further annealing. The i absorptions, however, are observed as a group of site absorptions (particularly the Y− H stretching band), and the total intensity increases ∼50% on UV irradiation. Teng and Xu have observed two product absorptions at 1100.6 and 1415.3 cm−1 and assigned them to the in-plane CCH bending mode of the π-complex (Y-η2-C2H2) and the Y− H stretching mode of the insertion product (HY−CCH), respectively.10b These are the strongest absorptions for the aforementioned products. The in-plane CCH bending absorption of Y-η2-C2H2 marked π is observed here essentially at the same frequency (1100.5 cm−1) with a site absorption at 1104.0 cm −1 . The weaker CCH out-of-plane bending absorption is also observed here at 618.0 cm−1, and the YC2 symmetric and antisymmetric stretching absorptions are 10920

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absorptions at 1946.9, 1944.8, and 1941.0 cm−1) is accompanied with its D and 13C counterparts at 1832.2 and 1873.3 cm−1 (with their site absorptions). It is assigned to the C−C stretching mode of HY−CCH, which is the second strongest band for the insertion complex as shown in Supplementary Table S8. The observed i absorptions along with the previous result substantiate formation of the Y insertion complex. Parallel to the La case, H2CCY and HY−CCH− are not observed, which would show their strong CH2 wagging and Y− H stretching absorptions at ∼920 and ∼1240 cm −1 (Supplementary Tables S9−10). DFT results also indicate that the identified products are the most stable: HY−CCH, Yη2-C2H2, and H2CCY are only 183, 170, and 90 kJ/mol more stable than the reactants (Y(2D) + C2H2). A series of recent studies reveal that the electron-deficient metal atom (M) is first attracted to the π-system, forming M-η2-C2H2, and the excess reaction energy or the photon energy leads to H-migration from C to M, yielding the insertion complex (HM−CCH). Further conversion to the vinylidene product (H2CCY), however, requires relatively high activation energy.6a Sc + C2H2. Figures 7−9 show the product absorptions from reactions of Sc with acetylene isotopomers. Three sets of product absorptions marked i− as well as π and i are observed unlike the La and Y cases. The product absorptions are relatively weak in the original deposition spectrum but increase in the early stage of annealing or emerge on photolysis. The i− absorption increases dramatically in annealing, but decreases ∼50% on UV irradiation. The π absorptions also increase in annealing but decrease substantially (more than 50%) on UV irradiation. The i absorption emerges on UV irradiation and gradually decreases in annealing. Most noticeable in the product spectra are the two strong i− Sc−H stretching absorptions at 1386.9 and 1364.9 cm−1, which shift to 996.7 and 981.7 cm−1 on deuteration (D/H ratios of 1.391 and 1.390) but show essentially no shifts on 13C substitution. The Sc−H stretching frequencies are comparable to 1305.1 cm−1 for the IR-active frequency of ScH4−.11 Teng and Xu have shown that these i− absorptions almost disappear with the addition of CCl4, an electron scavenger, in the reagent mixture, and they have assigned the two product absorptions to HSc−CCH− and HScSc−CCH−, respectively.10 Figure 7 shows the Sc product spectra with (a−c) 0.13% C2H2 + 0.13% C2D2, (d,e) 0.13% C2H2, and (f−j) 0.50% C2H2, where the absorption on the red side at 1364.9 cm−1 becomes relatively stronger than the other one at 1386.9 cm−1 at higher concentration. If the lower frequency component originates from HScSc− CCH−, its intensity relative to that of the other one will decrease at higher acetylene concentration because the concentration ratio between Sc and C2H2 decreases. The observed frequency (1364.9 cm−1) is higher than the B3LYP and BPW91 frequencies of 1283.8 and 1280.9 cm−1 for HScSc−CCH− as shown in Supplementary Table S15. In addition, HScSc−CCH− is 74 kJ/mol higher in energy than Sc2−C2H2−. Therefore, observation of the insertion complex with no trace of the energetically considerably more favorable discandium π-complexes (Sc2−C2H2 and Sc2−C2H2−) is unlikely. HScSc−CCH− is also 141 kJ/mol higher in energy than HSc−CC-ScH−, whose strong Sc−H stretching bands expected at ∼1270 and 1240 cm−1 with a 1:3 intensity ratio are not observed in this study. We assign these two nearby Sc−H stretching absorptions to the free anionic insertion complex (HSc−CCH−), in accord with the previous report, and (HSc− CCH − ) coordinated with more acetylene (HSc−

Figure 5. IR spectra in the product absorptions regions for laserablated Y atoms codeposited with C2D2 in excess argon at 8 K and their variation. (a) Y + 0.50% C2D2 in Ar codeposited for 1 h. (b) As in panel a, after annealing to 28 K. (c) As in panel b, after photolysis (λ > 420 nm). (d) As in panel c, after photolysis (240 < λ < 380 nm). (e) As in panel d, after annealing to 36 K. π and i denote the product absorption groups; p and c stand for precursor and common absorptions from this precursor with other metals.

Figure 6. IR spectra in the product absorptions regions for laserablated Y atoms codeposited with HCCD in excess argon at 8 K and their variation. (a) Y + 0.25% HCCD in Ar codeposited for 1 h. (b) As in panel a, after photolysis (λ > 420 nm). (c) As in panel b, after photolysis (240 < λ < 380 nm). (d) As in panel c, after annealing to 28 K. (e) As in panel d, after annealing to 36 K. π and i denote the product absorption groups; p and c stand for precursor and common absorptions from this precursor with other metals.

observed here at 537.7 and 530.0 cm−1. In the high frequency region, the A1 and B2 C−H symmetric and antisymmetric stretching bands are observed at 2945.3 and 2919.7 cm−1 along with their isotopic counterparts as shown in Table 2. The observed π absorptions, whose frequencies are in good correlation with the DFT values as shown in Supplementary Table S7, confirm the previous report for production of Y-η2C2H2 with additional observed absorptions. The Y−H stretching absorption marked i is observed at 1415.6 cm−1 (with site absorptions at 1448.0, 1434.6, and 1396.3 cm−1) with the D and 13C counterpart as listed in Table 2. The observed Y−H stretching frequency is comparable to 1459.8 and 1397.8 cm−1 for YH2.11 Our HCCD reaction product spectra show both the Y−H and Y−D stretching absorptions. The weaker i absorption at 1942.7 cm−1 (with site 10921

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Table 2. Frequencies of Product Absorptions Observed from Reactions of Y with Acetylene Isotopomers in Excess Argona C2H2 π

i

13

C2D2

C2H2

HCCD

2945.3 2919.7 1104.0, 1100.5

2217.1 2165.3 917.8, 915.3

2935.0 2912.2 1085.9, 1082.5

2932.8, 2199.0 2919.4, 2183.7

618.0

528.5, 525.9

613.2

587.1

540.5, 537.7, 529.9 530.0 1946.9, 1944.8, 1942.7, 1941.0 1448.0, 1434.6, 1415.6, 1396.3

468

525.3, 522.3 516.0 1877.0, 1875.3, 1873.3, 1871.7 1448.3, 1434.9, 1415.3, 1395.9

485.7

1843.6, 1837.3, 1832.2 1028.1, 1014.3

1946.6, 1944.7, 1942.7, 1941.0 1844.2, 1838.6, 1830.2 1448.0, 1434.6, 1415.2, 1396.0 1026.9, 1014.3

description Y-η2-C2H2, A1 C−H s.str. Y-η2-C2H2, B2 C−H as.str. Y-η2-C2H2, B2 CCH ip bend Y-η2-C2H2, B1 CCH oop bend Y-η2-C2H2, A1 YC2 s. str. Y-η2-C2H2, B2 YC2 as. str. HY−CCH, A′ C−C str. HY−CCH, A′ Y−H str.

All frequencies are in cm−1. Stronger absorptions are bold. Description gives major coordinate. π and i stand for the π and insertion products, respectively.

a

Figure 7. IR spectra in the product absorptions regions for laserablated Sc atoms codeposited with C2H2 in excess argon at 8 K and their variation. (a) Sc + 0.13% C2H2 + 0.13% C2D2 in Ar codeposited for 1 h. (b) As in panel a, after photolysis (λ > 420 nm and 240 < λ < 380 nm). (c) As in panel b, after annealing to 36 K. (d) Sc + 0.13% C2H2 in Ar codeposited for 1 h. (e) As panel d, after photolysis (λ > 420 nm and 240 < λ < 380 nm) and annealing to 36 K. (f) Sc + 0.50% C2H2 in Ar codeposited for 1 h. (g) As in panel f, after photolysis (λ > 420 nm). (h) As in panel g, after photolysis (240 < λ < 380 nm) and annealing to 20 K. (i) As in panel h, after annealing to 28 K. (j) As in panel i, after annealing to 36 K. i−, i−A, π, and i denote the product absorption groups; p and c stand for precursor and common absorptions from this precursor with other metals; w indicates water residue absorption.

Figure 8. IR spectra in the product absorptions regions for laserablated Sc atoms codeposited with C2D2 in excess argon at 8 K and their variation. (a) Sc + 0.50% C2D2 in Ar codeposited for 1 h. (b) As in panel a, after photolysis (λ > 420 nm). (c) As in panel b, after photolysis (240 < λ < 380 nm). (d) As in panel c, after annealing to 28 K. (e) As in panel d, after photolysis (240 < λ < 380 nm). (f) As in panel e, after annealing to 36 K. i−, i−A, π, and i denote the product absorption groups; p and c stand for precursor and common absorptions from this precursor with other metals; h stands for highorder product.

On the blue side of the strong Sc−H stretching absorptions of HSc−CCH−, a weak absorption marked i is observed at 1501.0 cm−1, which is accompanied with its D counterpart at 1082.8 cm−1 (H/D ratio of 1.386) and shows a negligible 13C shift. The ScH and ScD diatomic molecule absorptions are also observed at 1530.3 and 1102.9 cm−1.11 We assign this new Sc− H stretching absorption, which is 114.1 and 136.1 cm−1 higher than those of the anionic species, to the neutral insertion complex (HSc−CCH) on the basis of the good correlation with the DFT frequencies as shown in Supplementary Table S12 (e.g., B3LYP frequency and D shift of 1529.9 and 435.5 cm−1). Other bands of HSc−CCH are too weak to observe here (Supplementary Table S12). Observation of this neutral insertion complex shows that the Sc atom also undergoes C− H bond insertion like the previously studied transition metals.5−10 The vinylidene H2CCSc and HScSc−CCH−, which would show their strong CH2 wagging and Sc−H stretching bands at ∼920 and ∼1230 cm−1, respectively, are not identified in the matrix IR spectra. The vinylidene complex is again less stable

CCH−···C2H2) based on the present results. [This band is labeled i−A in our figures.] Weaker i− absorptions are also observed at 660.0 and 619.1 cm−1 and assigned to the CCH out-of-plane and in-plane bending modes of the anionic insertion complex. New π absorptions at 1063.6 and 810.5 cm−1 are assigned to the B2 and A1 CCH in-plane bending modes of Sc-η2-C2H2 along with their D and 13C counterparts, and the one at 621.4 cm−1 to the B1 CCH out-of-plane bending mode as shown in Tables 3 and S11. The C−C stretching absorption is observed at 1416.1 cm−1 and its D counterpart at 1364.5 cm−1. The π absorptions at 593.1 and 547.6 cm−1 in the congested low frequency region are assigned to the ScC2 symmetric and antisymmetric stretching modes. Those in the high frequency region at 2972.3 and 2933.1 cm−1 are assigned to the A1 and B2 C−H stretching modes. The observed π absorptions are in reasonable agreement with the DFT results (Supplementary Table S11), supporting formation for the Sc π-complex. 10922

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transition states in the La, Y, and Sc systems are estimated to be 260, 281, and 277 kJ/mol higher in energy than the insertion complexes and 56, 112, and 135 kJ/mol higher in energy than the reactants (M(2D) + C2H2). Direct conversion of M-η2-C2H2 to H2CCM is also examined. Shown in Figure 10 are the plausible reaction

Figure 9. IR spectra in the product absorptions regions for laserablated Sc atoms codeposited with 13C2H2 in excess argon at 8 K and their variation. (a) Sc + 0.25% 13C2H2 in Ar codeposited for 1 h. (b) As in panel a, after photolysis (λ > 420 nm). (c) As in panel b, after photolysis (240 < λ < 380 nm). (d) As in panel c, after annealing to 28 K. (e) As in panel d, after annealing to 36 K. i−, i−A, π, and i denote the product absorption groups; p and c stand for precursor and common absorptions from this precursor with other metals.

than the π and insertion products: Sc-η2-C2H2, HSc−CCH, and H2CCSc are only 168, 142, and 73 kJ/mol lower in energy than the reactants (Sc(2D) + C2H2). It is interesting that the binding energy of HSc−CCH − and acetylene to form HSc− CCH···C2H2 is 225 kJ/mol, which is much higher in energy than those of Sc(2D) + C2H2 to form Sc-η2-C2H2 and HSc− CCH−, indicating that the anionic Sc tetravalent complex is energetically highly favorable. With an extra electron, the group 3 metal becomes like a group 4 metal, which generates stable tetravalent complexes.12e Reactions in the Matrix. The group 3 metals all produce the π and insertion complexes in reactions with acetylene with no trace of the higher-energy vinylidenes and other rearrangement products. The excess energy from metal coordination to the acetylene π-system or the photon energy (λ > 420 nm and 240 < λ < 380 nm) leads to H-migration from C to M to form HM−CCH, but it is evidently not enough for further conversion to H2CCM (1). Recent studies have shown that conversion from the π to vinylidene complex probably occurs via the insertion complex and that the conversion from HM− CCH to H2CCM requires a sizable activation energy.6a The

Figure 10. Energies of the plausible La products and transition states relative to the reactants (La (2D) + C2H2). The π and insertion complexes (La-η2-C2H2 and HLa−CCH) are identified in this study while the energetically less favorable vinylidene product is not.

paths in the La + C2H2 system. TS1 and TS3, the transition states to the insertion and vinylidene complex in direct conversion from La-η2-C2H2, are 175 and 282 kJ/mol higher in energy than the π-complex. The high activation energy to the vinylidene product is consistent with our previous studies.5,6 The energy barrier of 282 kJ/mol is clearly too high to overcome in the process of annealing under 40 K. The energy barriers for direct conversion of the Y and Sc π-complexes to the vinylidenes, 277 and 274 kJ/mol, are comparable. UV irradiation is in fact required to generate the vinylidenes in the previously studied late transition metal systems.6 Figures 1−3 and S1 also show that even the i absorptions for the insertion

Table 3. Frequencies of Product Absorptions Observed from Reactions of Sc with Acetylene Isotopomers in Excess Argona π

i i−

i−A

C2H2

C2D2

2972.3 2933.1 1416.1 1063.6 814.6, 810.5 621.4, 620.5, 618.9 598.4, 596.5, 593.1 547.6, 545.3 1512.1, 1501.0 1386.9 660.0, 649.1 628.7 1364.9

2247.8 2184.2 1364.5 881.7 597.0 479.2, 476.7, 475.6, 474.3 591.0, 590.1, 587.0 473.3, 471.8 1082.8 996.7 covered 495.6 981.7

13

C2H2 + C2D2

description

2960.2

C2H2

2272.3, 2247.9

1051.7, 1047.0, 1041.9 811.9

1063.6, 881.5 810.5 621.0 593.3, 586.6 473.7 1530.3, 1102.9 1386.9, 996.7 660.0 628.8, 495.9 1364.9, 981.7

Sc-η2-C2H2, A1 C−H s.str. Sc-η2-C2H2, B2 C−H as.str. Sc-η2-C2H2, A1 C−C str. Sc-η2-C2H2, B2 CCH ip bend Sc-η2-C2H2, A1 CCH ip bend Sc-η2-C2H2, B1 CCH oop bend Sc-η2-C2H2, A1 ScC2 s. str. Sc-η2-C2H2, B2 ScC2 as. str. HSc−CCH, A′ Sc−H str. HSc−CCH−, A′ Sc−H str. HSc-CCH−, A″ CCH oop bend HSc−CCH−, A′ CCH ip bend [HSc−CCH···C2H2]−, Sc−H str.

579.4 532 1501.0 1386.8 654.0 (?) 622.4 (?) 1364.8

All frequencies are in cm−1. Stronger absorptions are bold. Description gives major coordinate. π, i, i−, and i−A stand for the π, insertion, anionic insertion, and C2H2-coordinated anionic insertion products, respectively. a

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complex, which need 107 kJ/mol less activation energy than H2CCLa, do not increase on annealing. These present and previous results do not agree with the report by Xu et al. claiming that TS1 is 134 kJ/mol higher in energy than TS3, a rationale for the strongly increasing absorption at 1673.2 cm−1 on annealing, which they assigned to the La vinylidene.10a We assign the absorption along with other πA absorptions to the C−C stretching mode of La-η2-C2H2···C2H2 as described above. Production of the anionic insertion complex in the Sc system is not in line with the electron affinities of the group 3 metals (45, 30, and 18 kJ/mol for La, Y, and Sc, respectively).19 Anionic insertion complexes have also been observed in reactions of CH4 and CH3F with coinage metals as well as the group 5 metals,20 and it is shown that the electron affinity of the insertion complex is substantially higher than that of the atom. Therefore, it is expected that the insertion complex is generated first from coordination of the electron-deficient metal atom to the acetylene π-system and following H-migration, and electron capture occurs afterward to form the anionic insertion product. The electron affinities of the La, Y, and Sc insertion complexes (HM−CCH → HM−CCH−) are 85, 85, and 58 kJ/ mol, respectively, which are higher than the atomic electron affinities, but still do not explain exclusive production of the Sc anionic species. The smallest atomic radius and highest electronegativity of scandium among the group 3 metals perhaps lead to more effective electron capture. The preference of the anionic Sc insertion complex over the neutral counterpart in the reaction of laser-ablated Sc atoms and electrons with C2H2 would be a good subject for future theoretical studies. Molecular Structures. Figures 11−13 show the B3LYP structures of the plausible products from group 3 metal reactions with acetylene. The insertion complexes have planar Cs structures, whereas the π and the vinylidene complexes have planar C2v structures. The C−M and M−H bond lengths largely reflect the metal atom radius, and as a result, the CMC of the π-complex increases in the order of La, Y, and Sc. The observed C−C stretching frequencies from the group 3 metal π-complexes are substantially higher than those of the group 4 metal analogues; 1405.4 and 1416.1 cm−1 for the La and Sc complexes are compared to 1393.9, 1316.9, and 1364.6 cm−1 for the Hf, Zr, and Ti analogues. The C−M bond lengths of the La, Y, and Sc π-complexes (2.307, 2.208, and 2.049 Å) are also significantly longer than those of the Hf, Zr, and Ti πcomplexes (2.117, 2.089, and 1.970 Å). The binding energies of 242, 184, and 168 kJ/mol for the La, Y, and Sc π-complexes are also compared with those of 243, 247, and 188 kJ/mol for the Hf, Zr, and Ti analogues, indicating that the group 3 metals form weaker π-complexes than the group 4 metals. Because of its largely empty d-orbitals, the back-donation of a group 3 metal to the acetylene π*-orbital is believed to be weaker than that of a group 4 metal, consistent with the observed higher C−C stretching frequency, longer C−M bonds, and higher binding energy. However, the C−C bond lengths of the La, Y, and Sc π-products (1.342, 1.345, and 1.337 Å) are longer than those of the Hf, Y, and Ti (1.323, 1.343, and 1.332 Å) and Ta, Nb, and V (1.328, 1.330, and 1.342 Å) analogues. More acetylene π-electron is probably delocalized to the empty d-orbitals of the group 3 metal atom, elongating in the C−C bond.

Figure 11. B3LYP structures of the acetylene π, insertion, vinylidene, and anionic insertion complexes of La, La−C2H2···C2H2, and transition states shown in Figure 10. The bond lengths and angles are in Å and degrees, and the ground electronic state and symmetry are also shown.

Addition of an electron to the insertion complex brings only slight changes in the C−H and C−C bond lengths but substantial increases in the C−M and M−H bond lengths. NBO analyses18 reveal that the bond orders show little changes on the addition of an electron as listed in Supplementary Table S17, whereas the atomic charge of the metal decreases significantly, 1.25 → 0.40, 1.29 → 0.43, and 1.17 → 0.31 for La, Y, and Sc, respectively. As a result, the large ionic contributions in the M−H and M−C bonds of the neutral insertion complex are expected to decrease accordingly, leading to the weaker C−M and M−H bonds in the anionic species. In contrast to previous work, we find for HScSc−CCH− a near linear planar Cs structure, which is different from the doublebent C1 structure reported by Teng and Xu.10 The predicted Sc−H stretching frequency (1283.8 cm−1) for this planar Cs structure is also substantially lower than the reported value for the double-bent structure (1328.0 cm−1).

■

CONCLUSIONS The π and insertion complexes (M-η2-C2H2 and HM−CCH) are produced in reactions of all group 3 metals (La, Y, and Sc) and acetylene. The higher-order π-complex (M-η 2 C2H2···C2H2) and anionic electron-capture species (HM− CCH−) are observed in the La and Sc reactions, respectively. No higher-energy vinylidene products (H2CCM) are identified in this study. The absorption at 1673.0 cm−1 assigned to the vinylidene product in the La + C2H2 system by Xu et al.,10a which increases on annealing and depletes on UV irradiation, is 10924

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absorptions at 1386.9 and 1364.9 cm−1 are assigned here to HSc−CCH− free and coordinated to acetylene, in contrast to the previous report, due to the increasing relative intensity of the red side absorption and considerably higher calculated energy of HScSc−CCH− in comparison with the corresponding π-complex. La-η2-C2H2···C2H2, Sc-η2-C2H2, HSc−CCH, and HSc−CCH−···C2H2 are newly identified in this study. The observed π and insertion products reveal that the group 3 metals readily coordinate to the acetylene π-bond and that oxidative C−H bond insertion also occurs. The high activation barriers to the vinylidene products (in both direct conversion from the π-complex and conversion via the insertion complex) are consistent with the absence of the product. The group 3 metals form weaker π-complexes than the group 4 metals on the basis of the C−C stretching frequencies and C−M bond lengths. The addition of an electron to the insertion complex barely changes the bond orders of M−H and M−C bonds but significantly decreases the ionic contribution, resulting in the longer bonds in HM−CCH−. The electron affinity of HM− CCH is higher than the atomic value, similar to the previously studied insertion complexes; however, it does not explain the exclusive generation of anionic Sc species.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 12. B3LYP structures of the acetylene π, insertion, vinylidene, HCCYH−, and Y−C2H2···C2H2 complexes of Y. The bond lengths and angles are in Å and degrees, and the ground electronic state and symmetry are also shown.

Tables of computed frequencies and natural atomic charges and bond orders. Figures of La + HCCD and Y + 13C2H2 spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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 the Korea Research Foundation (KRF) grant funded by the Korean government (MEST) (No 20090075428) and KISTI supercomputing center.

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REFERENCES

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Figure 13. B3LYP structures of the acetylene π, insertion, vinylidene, HCCScH−, HCCScScH−, and Sc−C2H2···C2H2 complexes of Sc. The bond lengths and angles are in Å and degrees, and the ground electronic state and symmetry are also shown.

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