J. Phys. Chem. A 2010, 114, 891–897
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Infrared Spectra of CH2dZr(H)NC, CH3-ZrNC, and η2-Zr(NC)-CH3 Produced by Reactions of Laser-Ablated Zr Atoms with Acetonitrile Han-Gook Cho and Lester Andrews* Department of Chemistry, UniVersity of Incheon, 12-1 Songdo-dong, Yonsu-ku, Incheon, 406-840, South Korea, and Department of Chemistry, UniVersity of Virginia, P.O. Box 400319, CharlottesVille, Virginia 22904-4319 ReceiVed: October 16, 2009; ReVised Manuscript ReceiVed: NoVember 20, 2009
The zirconium methylidene isocyanide, methyl isocyanide, and η2-nitrile-π-complexes are observed in the matrix IR spectra from reactions of laser-ablated Zr atoms and acetonitrile isotopomers. The methylidene CH2dZr(H)NC has a C1 agostic structure in line with simple early transition-metal methylidenes recently produced from reactions with small alkanes and methyl halides, and the extent of agostic distortion is also comparable. Formation of the isocyanide complexes from acetonitrile is interesting but not surprising according to previous studies of metal reactions with nitrile-containing compounds, and their stabilities over the cyanide species are reproduced by DFT calculations. Observation of the relatively rare nitrile π-complex and its photodissociation suggests that the reaction proceeds in the order of ZrrNCCH3, η2-Zr(NC)sCH3, CH3sZrNC, and CH2dZr(H)NC. The intermediate transition-state structures are also examined. Introduction Numerous transition-metal high oxidation-state complexes and their chemical properties have added richness to coordination chemistry and help to understand the nature of carbon-metal bonds.1,2 They are now crucial components in numerous synthetic routines for C-H bond insertion and addition reactions, and their industrial importance regarding catalytic properties particularly for metathesis reactions cannot be overemphasized.3 They also often show distinct structures and photochemical properties, and the electronic structures are the subject of various theoretical approaches.4 Recent investigations have shown that small high oxidationstate complexes can be prepared from direct reactions of Groups 3-10 transition-metals and actinides with small alkanes and halomethanes. As a result, C-H(X) bond activation by transition-metal and subsequent H(X)-migration from C to M are now considered as general phenomena.5-11 These simple complexes, cousins of the much larger coordination complexes, are more amenable to high level computations, and the distinct structures particularly due to agostic and halogen-metal interactions are testing grounds for theoretical methods.12 Their dramatic photochemical variations, including photoreversibility, often provide essential information about the related reaction paths.5-11,13 It is an intriguing question whether or not transition metals can undergo similar bond insertion and H-migration with organic species other than alkanes and halomethanes to generate high oxidation-state complexes. In this study, the zirconium reaction with acetonitrile has been carried out. Our earlier studies have shown that Zr is an effective C-H(X) bond insertion agent for alkanes, halomethanes, ethylene, and acetylene.5,13 Particularly, reactions with alkanes and halomethanes generate insertion and high oxidation-state complexes and provide a typical example of the activation reaction by a transition-metal atom. Similar C-H activation and H-migration were observed in reaction with ethylene, and evidence shows exceptionally strong backdonation in the Zr(C2H2) π-complex.13 * Author to whom correspondence should be addressed. E-mail: lsa@ virginia.edu.
Acetonitrile, a C3V symmetric top, has played an important role in the development of vibrational spectroscopy,14 and its photochemical reactions have also drawn much attention.15 Acetonitrile is normally considered as an effective electronpair donor, which readily forms a dative bond with its nitrogenend to various acceptors.16,17 It is also well-known that the lone electron pair provides substantial antibonding character to bonds in the molecular axis, particularly the CN bond.18 As a result, coordination to an acceptor, such as electron-deficient species solvated in acetonitrile, often leads to blue shifts of the stretching frequencies. While the product spectra are complicated with absorptions of photoisomerization products of acetonitrile,15,19,20 the Zr high oxidation-state, insertion, and π-complexes are identified through isotopic substitution and DFT computations. On the basis of the observed spectra, a path for reaction of Zr + CH3CN is suggested and the transition states are also investigated. Experimental and Computational Methods Laser ablated Zr atoms (Johnson-Matthey) were reacted with acetonitrile isotopomers (CH3CN, CD3CN, and 13CH313CN) in excess argon during condensation at 10 K using a closed-cycle refrigerator (Air Products Displex). These methods have been described in detail in previous publications.21 Reagent gas mixtures ranged 0.25-0.50% in argon. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused onto a rotating metal target (Zr, 99.99%, Johnson Matthey) using 5-10 mJ/pulse. After initial reaction, infrared spectra were recorded at 0.5 cm-1 resolution using a Nicolet 550 spectrometer with a Hg-Cd-Te range B detector. Then samples were irradiated for 20 min periods by a mercury arc street lamp (175 W) with the globe removed using a combination of optical filters or annealed to allow further reagent diffusion. To provide support for the assignment of new experimental frequencies and to correlate with related works,5-11 density functional theory (DFT) calculations were performed using the Gaussian 03 program system;22 the B3LYP density functional;23 the 6-311++G(3df,3pd) basis sets for H, C, and N,24 and the
10.1021/jp9099368 2010 American Chemical Society Published on Web 12/16/2009
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Figure 1. Infrared spectra in the 2150-1950, 1700-1500, 1150-850, and 800-550 cm-1 regions for the reaction products of the laser-ablated zirconium atom with CH3CN in excess argon at 10 K. (a) Zr and CH3CN (0.25% in argon) codeposited for 1 h. (b) as (a) after visible (λ > 420 nm) irradiation, (c) as (b) after ultraviolet (240-380 nm) irradiation, (d) as (c) after visible irradiation, (e) as (d) after UV irradiation, and (f) as (e) after annealing to 28 K. m, i, and π designate the product absorption groups while P stands for the precursor absorption and c and c′ indicate the common absorptions in the CH3CN and Zr + acetronitrile matrix spectra. CH2CNH, CH2NCH, and HCCNH2 absorptions are also indicated.
Figure 2. Infrared spectra in the 2150-1950, 1600-1400, and 1200-420 cm-1 regions for the reaction products of the laser-ablated zirconium atom with CD3CN in excess argon at 10 K. (a) Zr and CD3CN (0.5% in argon) codeposited for 1 h, (b) as (a) after visible irradiation, (c) as (b) after UV irradiation, (d) as (c) after full arc (λ < 220 nm) irradiation, and (e) as (d) after annealing to 28 K. m, i, and π stand for the product absorption groups while P designates the precursor absorption and c and c′ indicate the common absorptions in the CH3CN and Zr + acetronitrile matrix spectra. CD2CND and CD2NCD absorptions are also indicated.
SDD pseudopotential and basis set25 for Zr to provide vibrational frequencies for the reaction products. Geometries were fully relaxed during optimization, and the optimized geometry and transition-state structure were confirmed by vibrational analysis. The BPW9126 functional was also employed to complement the B3LYP results. The vibrational frequencies were calculated analytically, and zero-point energy is included in the calculation of binding and reaction energies. Previous investigations have shown that DFT calculated harmonic frequencies are usually slightly higher than observed frequencies,5-11,13 and they provide useful predictions for infrared spectra of new molecules. Results and Discussion Reactions of zirconium atoms with acetonitrile were investigated, and infrared spectra (Figures 1-3) and density functional frequency calculations of the products, transition states, and their structures (Figures 4-6) will be presented in turn.
Primary Products. Figure 1 shows the product spectra from reaction of laser-ablated Zr atoms with CH3CN and their variation with subsequent photolysis and annealing. The product absorption is marked with m, i, or π (for methylidene, insertion product, or π-complex) depending on the intensity variation upon irradiation (visible (λ > 420 nm), UV (240 < λ < 380 nm), and full arc (λ > 220 nm) and annealing. Due to the laserplume radiation during ablation, isomerization of acetonitrile also occurs, and CH2CNH, CH2NCH, and CH3NC absorptions are observed in the matrix IR spectra.15 These isomers are calculated to be 22, 51, and 23 kcal/mol higher in energy than CH3CN. The CH2CNH absorptions are particularly strong, and they increase ∼15% and decrease ∼5% on UV and full arc irradiations, respectively. Weaker CH2NCH bands remain unchanged on visible photolysis but almost disappear on UV photolysis. CH3NC absorptions19 increase ∼20 and 15% on UV and full arc irradiations.
IR Spectra of Reacted Laser-Ablated Zr and MeCN
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Figure 3. Infrared spectra in the 2100-1900, 1700-1500, 1150-450 cm-1 regions for the reaction products of the laser-ablated zirconium atom with 13CH313CN in excess argon at 10 K. (a) Zr and 13CH313CN (0.5% in argon) codeposited for 1 h, (b) as (a) after visible irradiation, (c) as (b) after UV irradiation, (d) as (c) after visible irradiation, and (e) as (d) after annealing to 28 K. m, i, and π stand for product absorption groups, while P designates the product absorption and c and c′ indicate the common absorptions in the CH3CN and Zr + acetronitrile matrix spectra. 13CH2CNH, 13 CH2N13CH, and H13C13CNH2 absorptions are also indicated.
The m absorptions are relatively weak in the original spectra after deposition and slightly decrease on visible irradiation but show a dramatic increase (more than triple) on UV irradiation. They sharpen in the early stage of annealing and gradually decrease on later annealing. A group of absorptions in the Zr-H stretching region marked “m” are observed at 1585.6, 1574.3, 1569.6, and 1560.5 cm-1. Although the total intensity increases dramatically on UV irradiation like the other m absorptions, the intensity variations of each absorption in the set differ from the others. For example, the strong band at 1585.6 cm-1 decreases ∼25% on visible irradiation, whereas the bands at 1574.3 and 1569.6 cm-1 increase ∼30%. Another strong one at 1560.5 cm-1 increases ∼15 and ∼30% on the first visible and UV irradiations, and it increases dramatically in the process of annealing while the other ones decrease. They show negligible 13 C shifts, and the D counter parts are observed at 1138.2, 1129.8, and 1121.0 cm-1, with a shoulder at 1122.3 cm-1, (average H/D ratio of 1.393). The Zr-H stretching frequencies are compared with 1518.6, 1545.3, and 1623.6 cm-1 for ZrH2, ZrH3, and ZrH427, and also with those for the previously studied Zr methylidenes (1581.0 and 1546.2 cm-1 for CH2dZrH2, 1553.0 and 1551.3 cm-1 for CH2dZrHF, 1570.5 and 1554.0 cm-1 for CH2dZrHCl, and 1573.7 and 1556.6 cm-1 for CH2dZrHBr).5 Matrix site absorptions are common for the Zr-H stretching bands in the previous Zr systems. The other m absorptions also support formation of the nitrogen-containing methylidene. The band at 755.9 cm-1 has D and 13C counterparts at 688.1 and 737.0 cm-1 (H/D and 12/ 13 ratios of 1.099 and 1.026) and is assigned to the C-Zr stretching mode on the basis of the relatively large 13C shift. The high frequency indicates it is most probably a double bond, as the single C-Zr bond stretching band would emerge at ∼500 cm-1. The m absorption at 666.6 cm-1 has its D and 13C counterparts at 529.0 and 662.0 cm-1 (H/D and 12/13 ratios of 1.260 and 1.007) and is designated to the CH2 wagging mode. On the further low frequency side, another m absorption at 587.3 cm-1 is observed along with its D and 13C counterparts at 470.6 and 582.0 cm-1 (H/D and 12/13 ratios of 1.247 and 1.009). The frequency and relatively large D shift (appropriate for a
H-bending mode) lead to an assignment to the C-H in-plane bending mode. On the blue side of the of the strong CCN antisymmetric stretching band at 2037.2 cm-1 for CH2CNH, a strong m absorption is observed at 2042.4 cm-1 along with D and 13C counterparts at 2042.2 and 2001.8 cm-1 (H/D and 12/ 13 ratios of 1.000 and 1.020), and it is assigned to the NC stretching mode on the basis of the negligible D and sizable 13 C shifts. The nitrile (CN) stretching band would have a frequency ∼100 cm-1 higher than that of the m absorption, which is not observed in this study (Table S1). The present results, therefore, show that the major product responsible for the m absorptions carries Zr-H, CdZr, CH2, and NC moieties, and it is therefore most likely to be CH2dZr(H)NC, a simple Zr conjugated methylidene complex. The observed frequencies are well reproduced as shown in Table 1, within the limits of DFT,5-11,13,28,29 which substantiate formation of the small methylidene isocyanide. Formation of the Zr methylidene isocyanide in reaction with CH3CN is interesting but in fact not surprising. The earlier studies of reactions of metals with nitrile (CN) group containing compounds often show preferential generation of the isocyanide species mainly due to their lower energy relative to the cyanide species.30 CH2dZr(H)NC and CH2dZr(H)CN in the singlet ground states are 78 and 73 kcal/mol (B3LYP) more stable than the reactants (Zr(3F) + CH3CN). While formation of the primary product correlates with its stability relative to the cyanide counterpart, the reaction path from methyl cyanide to the methylidene isocyanide remains as an interesting question. The recent studies for reaction of metal atoms with small alkanes and methyl halides show that the C-H(X) bond insertion complex forms first, and subsequent H(X) migration leads to generation of the higher oxidation-state complex.5-11 The insertion complex is normally one of the primary products identified in the matrix spectra as well. The absorption bands for CH3sZrNC are, however, expected to be weaker than those of the methylidene complex as shown in Table 2. The NC stretching band is relatively strong but located in a congested region. The observed i absorptions increase gradually in the process of photolysis (∼5 and ∼10% increase on visible and
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TABLE 1: Observed and Calculated Fundamental Frequencies of CH2dZr(H)NC Isotopomers in the Ground 1A Statea CH2dZr(H)NC
approximate description CH2 as. str. CH2 as. str. NC str. Zr-H str.
obsb
2042.4 1585.6, 1574.3, 1569.6, 1560.5
CH2 scis. C-Zr str. 762.0 CH2 wag 666.6 C-H ip bend 587.3 CH2 twist Zr-NC str. CH2 rock C-H oop bend ZrNC oop bend ZrNC ip bend CZrN bend
13
CD2dZr(D)NC
BPW91c Intc B3LYPd Intd 3132.0 2730.1 2025.6 1623.5
1 5 452 343
3186.9 2830.8 2100.9 1646.2
1302.9 771.4 655.1 616.1 444.0 433.6 392.0 273.9 101.9 87.3 77.2
22 84 112 80 39 60 34 56 17 24 9
1338.0 770.8 696.2 633.1 455.9 432.9 392.8 251.9 116.9 104.7 79.6
obsb
BPW91c Intc B3LYPd Intd
1 7 495 2042.2 408 1138.2, 1129.8, 1122.3, 1121.0 21 91 688.1 142 529.0 77 470.6 42 72 19 69 52 6 15
2316.7 1987.7 2025.5 1155.4
4 3 459 175
2357.2 2061.1 2100.8 1171.6
1000.9 693.8 511.4 479.2 321.6 416.8 289.5 223.3 95.5 81.6 72.8
21 60 84 100 8 38 17 21 6 15 20
1032.7 690.3 544.8 490.3 323.5 414.6 304.5 204.6 104.0 100.2 74.1
CH2dZr(H)N13C
obsb
BPW91c Intc B3LYPd Intd
4 3 502 2001.8 211 1585.6, 1574.3, 1569.5, 1560.5 23 62 737.0 104 662.0 107 582.0 8 43 11 23 2 38 20
3121.4 2723.8 1985.3 1623.5
1 5 448 343
3176.2 2824.2 2059.0 1646.1
1 7 491 409
1295.9 751.2 649.7 613.3 439.9 430.9 390.0 273.3 100.4 86.5 75.2
22 83 108 78 21 74 35 57 18 22 9
1330.1 751.2 690.3 630.6 455.2 426.9 390.0 251.4 116.1 103.5 77.5
20 91 138 73 38 73 20 70 52 5 14
a Frequencies and intensities are in cm-1 and km/mol. b Observed in an argon matrix. The strongest matrix site split absorption is bold. Frequencies computed with BPW91/6-311++G(3df,3pd). d Frequencies and intensities computed with B3LYP/6-311++G(3df,3pd). CH2dZr(H)NC has a near planar C1 structure.
c
TABLE 2: Observed and Calculated Fundamental Frequencies of CH3-ZrNC Isotopomers in the Ground 3A′′ Statea approximate description
CH3sZrNC obs
b
A′ CH3 as. str. A′′ CH3 as. str. A′ CH3 s. str. A′ NC str. 2016.7 A′′ CH3 bend A′ CH3 bend A′ CH3 deform A′ C-Zr str. 496.1 A′ Zr-NC str. 460.8 A′ CH3 rock A′′ CH3 rock A′ ZrNC ip bend A′′ ZrNC oop bend A′′ CH3 tort A′ CZrN bend
BPW91 3053.1 2963.5 2899.3 2000.2 1365.9 1355.8 1102.2 516.8 448.2 357.5 328.6 161.0 151.9 111.5 72.3
c
B3LYP
1 2 5 299 7 1 10 38 90 4 4 0 0 0 3
3093.0 3015.6 2955.5 2091.6 1417.2 1407.4 1153.3 519.0 451.1 372.8 359.3 162.2 143.7 107.7 75.8
Int
13
CD3sZrNC
c
d
Int
d
obs
b
2 4 5 385 2016.5 6 1 10 884.1 39 119 448.6 5 5 0 0 0 4
BPW91 2254.0 2191.5 2080.9 2000.2 991.3 984.3 872.3 453.8 445.3 286.0 245.7 154.6 143.8 85.0 67.3
c
c
B3LYP
1 1 2 299 1 1 22 29 84 3 2 1 0 0 4
2283.4 2227.1 2121.6 2091.6 1028.5 1021.6 908.2 455.4 447.2 298.2 268.3 156.5 137.8 81.0 70.4
Int
d
Int
d
obs
b
1 1 2 385 1971.1 4 1 23 36 488.7 106 4 4 1 0 0 4
CH3sZrN13C
BPW91c Intc B3LYPd Intd 3043.1 2957.3 2895.9 1960.0 1362.8 1352.7 1092.5 505.7 442.1 353.0 326.9 159.9 151.0 110.6 70.3
1 2 5 297 6 1 8 35 88 4 4 0 0 0 3
3082.9 3005.3 2952.0 2049.5 1414.0 1404.2 1143.6 508.2 445.0 367.8 357.5 161.0 142.6 106.9 73.7
2 4 5 383 5 1 9 36 116 5 5 0 0 0 4
a Frequencies and intensities are in cm-1 and km/mol. b Observed in an argon matrix. c Frequencies computed with BPW91/6-311++G(3df,3pd). d Frequencies and intensities computed with B3LYP/6-311++G(3df,3pd). CH3–ZrNc has a Cs structure, and the symmetry notations are based on the Cs structure.
UV irradiations). The NC stretching absorption is observed at 2016.7 cm-1 and its D and 13C counterparts at 2016.5 and 1971.1 cm-1 (H/D and 12/13 ratios of 1.000 and 1.023). The i absorption at 496.1 cm-1 has its 13C counterpart at 488.7 cm-1 (12/13 ratio of 1.015) and is assigned to the C-Zr stretching mode without observation of the D counterpart. Another i absorption on the red side at 460.8 cm-1 shows a weak D counterpart at 448.6 cm-1 (H/D ratio of 1.027) while the 13C counterpart, which is expected too close to our observation limit, is not observed. The i absorptions, which correlate nicely with the DFT frequencies, support formation of the Zr insertion complex containing the isocyanide group. CH3-ZrNC is in fact the most stable among the plausible products; CH3sZrNC and CH3sZrCN are 82 and 78 kcal/mol lower in energy than the reactants. The CN stretching band of the methyl cyanide complex, one of the plausible products, would appear at ∼2150 cm-1, which is not observed in this study. The DFT frequencies for the cyanide complex are listed in Table S2. The present and previous results,5-11 therefore, suggest that the methyl isocyanide insertion complex is formed prior to generation of the methylidene complex, and subsequent H migration from C to Zr produces the higher oxidation-state complex.
Figures 1-3 show another group of product absorptions marked “π”, which remain unchanged on visible photolysis but almost disappear on UV irradiation, whereas the m absorptions increase dramatically. The π absorption at 634.9 cm-1 has its D and 13C counterparts at 612.4 and 631.5 cm-1 (H/D and 12/ 13 ratios of 1.037 and 1.005). They are 0.982, 0.985, and 0.981 of the B3LYP values of 646.3, 622.0, and 643.5 cm-1 for the N-Zr stretching mode of the η2-π-complex, which is the expected agreement for this density functional.5-11,13,28,29 The other observed π absorptions also correlate well with the DFT frequencies for the nitrile π-complex. The one at 927.6 cm-1 shows its D and 13C counterparts at 780.3 and 913.2 cm-1 (H/D and 12/13 ratios of 1.189 and 1.016) and is designated to the CCN symmetric stretching mode. The broad π absorption at 1101.1 cm-1 (with a site absorption at 1092.1 cm-1) is accompanied with its D and 13C counterparts at 969.2 and 1083.0 cm-1 (with site absorptions at 965.4 and 1068.9 cm-1) (H/D and 12/13 ratios of 1.136 and 1.017). It is assigned to the CH3 rocking mode, which is substantially mixed with the CCN symmetric stretching mode. The weak π absorption at 1545.8 cm-1 has its D counterpart at 1464.4 cm-1 and is assigned to the CCN asymmetric stretching mode without observation of the 13C counterpart. The H counterpart of the absorptions at
IR Spectra of Reacted Laser-Ablated Zr and MeCN
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TABLE 3: Observed and Calculated Fundamental Frequencies of the η2-π-Complex Zr(NC)-CH3 Isotopomers in the Ground 3 A′′ Statea approximate description A′ CH3 as. str. A′′ CH3 as. str. A′ CH3 s. str. A′ CCN as. str. A′′ CH3 bend A′ CH3 bend A′ CH3 deform A′ CH3 rock A′′ CH3 rock A′ CCN s. str. A′ N-Zr str. A′ C-Zr str. A′′ CCNZr deform A′ CCZr bend A′′ CH3 tort
Zr(NC)sCH3 b
obs
1545.8 covered 1101.1, 1092.1 927.6 634.9
c
c
Zr(N13C)s13CH3
Zr(NC)sCD3
B3LYP
d
d
BPW91
Int
Int
3036.7 3029.1 2957.8 1484.4 1422.8 1416.5 1334.0 1086.7
20 4 41 11 10 27 4 64
3088.0 3082.0 3014.8 1565.1 1470.4 1465.4 1384.5 1113.5
21 6 42 23 10 21 4 81
966.2 912.8 633.6 465.0 296.7 252.7 27.3
0 21 52 0 4 15 0
1005.7 938.3 646.3 463.5 308.3 252.0 47.9
0 27 82 1 3 14 0
b
obs
1464.4 1109.8 969.2, 965.4 780.3 612.4
BPW91
c
c
d
d
Int
B3LYP
Int
2250.5 2239.4 2123.0 1475.5 1025.9 1092.4 1021.6 963.0
11 2 16 23 5 38 13 50
2288.9 2279.5 2164.2 1559.4 1060.6 1125.8 1054.9 984.4
12 3 17 31 5 41 11 78
780.5 756.3 611.7 437.5 267.9 231.9 20.9
1 4 46 0 3 13 1
811.9 792.6 622.0 436.7 278.4 231.8 36.8
1 4 69 1 2 13 0
obs
b
1413.6 1083.0, 1068.9 913.2 631.5
BPW91c
Intc
B3LYPd
Intd
3025.7 3018.6 2954.7 1452.4 1420.6 1408.4 1323.8 1062.6
19 4 41 5 10 33 4 57
3076.8 3071.3 3011.6 1528.1 1468.2 1461.4 1373.9 1090.0
21 6 43 20 10 25 4 71
954.2 898.2 630.6 454.2 288.2 248.1 27.3
0 24 52 0 4 14 0
993.3 921.3 643.5 452.8 299.4 247.4 47.9
0 31 81 1 3 14 0
a Frequencies and intensities are in cm-1 and km/mol. b Observed in an argon matrix. The strongest matrix site split absorption is bold. Frequencies computed with BPW91/6-311++G(3df,3pd). d Frequencies and intensities computed with B3LYP/6-311++G(3df,3pd). The π-complex has a Cs structure, and the symmetry notations are based on the Cs structure. c
1109.8 and 1413.6 cm-1 in the CD3CN and 13CH313CN spectra is believed to be covered by precursor absorption, which is designated to the A′ CH3 bending mode of the π-complex. The observed frequencies in good agreement with the DFT values in Table 3 substantiate production of the η2-π-complex, Zr(NC)sCH3. The nitrile π-complexes are rare relative to the π-complexes to C-C multiple bonds or conjugated systems.31 Acetonitrile normally forms a dative bond to an electrondeficient species with its electron-rich nitrogen-end.16 It is, therefore, reasonable to expect first coordination of acetonitrile with its N-end to the Zr atom, forming a dative bond (ZrrNCCH3). However, the coordination complex is far less stable than the η2-π-complex: η2-Zr(NC)sCH3 and Zrr NCCH3 are 68 and 44 kcal/mol more stable than the reactants. Moreover, the absorption bands for ZrrNCCH3 are expected to be weak as shown in Table S3. Although anticipated to form first, no product absorptions from ZrrNCCH3 are observed in this study. Evidently the coordination complex using the lone electron pair on the N-end is not present in the sample in an observable amount. A broad absorption is also observed at 554.3 cm-1 along with its D and 13C counterparts at 487.8 and 552.3 cm-1 (H/D and 12/13 ratios of 1.136 and 1.004) and it gradually decreases during photolysis. Although it does not belong to any known product absorption groups, we tentatively assign it to the NH2 wagging mode of HCC-NH2. Although ethynyl amine, which is 39 kcal/mol higher than acetonitrile, is expected to form on UV photolysis, it has not been identified in the matrix spectra.15 More recently its production on a Pt(111) surface is proposed in reaction of NH3 and C2 molecules by Deng and Trenary.20 The observed frequencies correlate well with the B3LYP frequencies of 568.2, 499.9, and 565.3 cm-1 for the isotopomers. Another absorption feature showing similar behavior on photolysis is observed at 457.7 cm-1 with a 13C counterpart at 451.7 cm-1 (12/13 ratio of 1.013), and it is assigned to the C-H bending mode on the basis of the B3LYP frequencies of 482.1 and 480.0 cm-1. The observed frequencies are compared with the DFT values in Table S4. Molecular Structures. The calculated structures of the primary products are illustrated with their electronic states and molecular symmetries in Figure 4. The conjugated methylidene complex CH2dZr(H)NC has a C1 structure with the markedly distorted CH2 group in its ground singlet state, parallel to the
Figure 4. Structures calculated for the identified reaction products of zirconium with acetonitrile at the B3LYP level of theory using the 6-311++G(3df,3pd) basis sets for H, C, and N, and SDD pseudopotential and basis set for Zr. Bond distances and angles are in Å and deg. The molecular symmetry and electronic state are given under each structure.
previously studied CH2dZrH2 and its halide derivatives.5 Distortion of the CH2 group results in one of the methylene hydrogen atoms very close to the metal center (
