Infrared Spectra of the η2-M (NC)-CH3, CH3-MNC, and CH2 M (H

Article pubs.acs.org/Organometallics

Infrared Spectra of the η2-M(NC)-CH3, CH3-MNC, and CH2M(H)NC Complexes Prepared by Reactions of Thorium and Uranium Atoms with Acetonitrile Han-Gook Cho and Lester Andrews* Department of Chemistry, University of Incheon, 119 Academy-ro, Yonsu-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 η2-M(NC)-CH3, CH3-MNC, and CH2M(H)NC complexes are observed in the matrix IR spectra from reactions of laserablated Th and U atoms with acetonitrile. These actinide products are in line with those from reactions of group 4−6 metals with acetonitrile, and they are again the most stable steps in the previously proposed reaction path. The N-coordination and methylidyne products (M←NCCH3 and HCM(H)2NC) are not observed probably due to their high energies. The actinide methylidenes show agostic distortion, and the nitrile π-complexes (η2-M(NC)-CH3) support strong back-donations to the π*-orbitals. NBO analysis reveals single M−N and M−C bonds with full σ- and π-bonds in the NC subunit. The unidentified N-coordinated-acetonitrile complexes show strong calculated interactions with these early actinide atoms.

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M + CH3CN → M←NCCH3 → η2 ‐M(NC)‐CH3

INTRODUCTION Acetonitrile, the simplest organic nitrile, is often an easily displaceable ligand, which is used for synthesis of more precious compounds.1 It forms various adducts via coordination with its N-end, and its C−N stretching frequency provides valuable information for the coordination strength to an electron-deficient species.2,3 With its simple C3v structure and ability to produce various weakly bound complexes, acetonitrile has long been a subject of vibrational spectroscopy.4 This polar aprotic solvent is also well known with its versatile photoconversions and H-detachment to produce its isomers and fragments, such as HCNCH2, H2CCN, H2CNC, and HCNC.5,6 Recent studies have shown that transition metals including lanthanides and actinides react with small alkanes and halomethanes to generate small insertion and high-oxidationstate complexes, many of which show interesting photochemical properties.7 Recently we have also reported that similar reactions occur with acetonitrile. 8−10 Group 4−6 metals produce the N-coordination, nitrile π-, insertion, methylidene, and methylidyne isocyanide complexes (M← NCCH 3, η 2-M(NC)-CH 3, CH 3-MNC, CH 2M(H)NC, and HCM(H)2NC for group 6) with no trace of the cyanide counterparts. The observed isocyanide products with the N-coordination and nitrile π-complexes and the general assumption that an electron-deficient species is first attracted by the N-end lead to reaction path 1, where the binding between M and N is preserved throughout the course of the reaction.8−10 © 2011 American Chemical Society

→ CH3‐MNC → CH2M(H)NC → HCM(H)2 NC

(1)

DFT computations also show that the observed products for a metal are in fact the most stable steps in reaction path 1.8−10 In contrast, the cyanide complexes, which have not been identified, would be produced in reaction path 2, where the first formed M−N bond dissociates while C−C bond insertion occurs. M + CH3CN → M←NCCH3 → η2 ‐M(NC)‐CH3 → CH3‐MCN → CH2M(H)CN → HCM(H)2 CN

(2)

In this paper we report reactions of laser-ablated Th and U atoms with acetonitrile. The products are identified with isotopic substitution and DFT computations. The observed products are again the most stable steps in reaction path 1, and the products show unusually strong binding of actinide atoms with the N-end and nitrile π-system.

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EXPERIMENTAL AND COMPUTATIONAL METHODS

Laser-ablated Th and U atoms were reacted with acetonitrile isotopomers (CH3CN, CD3CN, and 13CH313CN) in excess argon Received: October 10, 2011 Published: December 21, 2011 535

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Figure 1. Infrared spectra in the product absorption regions from reaction of the laser-ablated thorium atom with CH3CN in excess argon at 10 K. (a) Th and CH3CN (0.50% in argon) co-deposited for 1 h, (b) as (a) after visible (λ > 420 nm) irradiation, (c) as (b) after UV (240−380 nm) irradiation, (d) as (c) after full arc (λ > 220 nm) photolysis, and (e) as (d) after annealing to 28 K. π, i, m, and h designate the product absorption groups, while P and c stand for the precursor and common absorptions in the CH3CN matrix spectra. H2CCNH, H2CCN, H2CNC, and CH4 absorptions are also indicated.

Figure 2. Infrared spectra in the product absorption regions from reaction of the laser-ablated thorium atom with CD3CN in excess argon at 10 K. (a) Th and CD3CN (0.50% in argon) co-deposited for 1 h, (b) as (a) after visible (λ > 420 nm) irradiation, (c) as (b) after UV (240−380 nm) irradiation, (d) as (c) after full arc (λ > 220 nm) photolysis, and (e) as (d) after annealing to 28 K. π, i, m, and h stand for the product absorption groups, while P and c designate the precursor and common absorptions in the CD3CN matrix spectra. D2CCND, D2CCN, D2CNC, and water residue absorptions are also indicated. Bands labeled c are common for this precursor and different metals. during condensation at 10 K using a closed-cycle refrigerator (Air Products Displex). These methods have been described in detail in previous publications.11 Reagent gas mixtures are typically 0.5% in argon. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate, 10 ns pulse width) was focused onto a rotating metal target (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. In order to provide support for the assignment of new experimental frequencies and to correlate with related works,7−10,12,13 density functional theory (DFT) calculations were performed using the Gaussian 09 package,14 the B3LYP density functional,15 the 6-311+ +G(3df,3pd) basis sets for H, C, and N, 16 and the SDD pseudopotential and basis sets17 for Th and U 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 BPW9118

functional was also employed to complement the B3LYP results. The vibrational frequencies were calculated analytically, and zeropoint 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,7−10,12,13,19 and they provide useful predictions for infrared spectra of new molecules.

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RESULTS AND DISCUSSION Reactions of Th and U atoms with acetonitrile were investigated, and infrared spectra (Figures 1−6) and observed and predicted frequencies of the products (Tables 1 and 2 and S1−S10) and their relative energies (Figures 7 and 8) and structures (Figures 9 and 10) will be presented in turn. Th + Acetonitrile. Shown in Figures 1, 2, and 3 are the product spectra from reactions of laser-ablated Th atoms with acetonitrile isotopic substitutions and their variation on subsequent photolysis and annealing. The isomerization and dissociation products of acetonitrile (CH2CNH, CH2NCH, 536

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Figure 3. Infrared spectra in the product absorption regions from reaction of the laser-ablated thorium atom with 13CH313CN in excess argon at 10 K. (a) Th and 13CH313CN (0.50% in argon) co-deposited for 1 h, (b) as (a) after visible (λ > 420 nm) irradiation, (c) as (b) after UV (240−380 nm) irradiation, (d) as (c) after annealing to 28 K. π, i, and m stand for the product absorption groups, while P and c designate the precursor and common absorptions in the 13CH313CN matrix spectra. H213C13CNH, H213C13CN, 13CH3N13C, and water residue absorptions are also indicated. Bands labeled c are common for this precursor and different metals.

Figure 4. Infrared spectra in the product absorption regions from reaction of the laser-ablated uranium atom with CH3CN in excess argon at 10 K. (a) U and CH3CN (0.50% in argon) co-deposited for 1 h, (b) as (a) after visible (λ > 420 nm) irradiation, (c) as (b) after UV (240−380 nm) irradiation, (d) as (c) after annealing to 28 K. π, i, and m stand for the product absorption groups, while P and c designate the precursor and common absorptions in the CH3CN matrix spectra. H2CCND, H2CCN, and H2CNC absorptions are also indicated. Bands labeled c are common for this precursor and different metals.

CH3NC, H2CCN, and H2CNC),5,6 due to the laser-plume radiation during ablation, are also observed. Four sets of new product absorptions are marked with π, i, m, and h (for π-, insertion, methylidene, and high-order complexes) on the basis of intensity variation during photolysis and annealing of the deposited samples. The π absorptions remain unchanged on visible (λ > 420 nm) irradiation, but decrease ∼30 and ∼20% on the following UV (240−380 nm) and full arc (λ > 220 nm) irradiation. They increase slightly in the early stage of annealing and later gradually decrease. The i absorptions increase ∼30%, ∼70%, and ∼50% (∼150% in total) on visible, UV, and full arc irradiation and sharpen in the early stage of annealing and gradually decrease later. The m absorptions increase ∼30%, ∼100%, and ∼70% (∼200% in total) on visible, UV, and full arc irradiation, but decrease on annealing. The h absorptions continue to increase in the processes of photolysis and annealing, and they also increase with sample concentration

(acetonitrile in Ar). The product absorption frequencies are listed in Table 1 and compared with the DFT computed values in Tables S1−S5 for plausible reaction products. The π absorption observed at 1536.3 cm−1 (with a matrix site absorption at 1514.1) is accompanied by D and 13C counterparts at 1523.8 and 1495.3 cm−1 (with matrix site absorptions at 1505.7 and 1481.6 cm−1, respectively) (H/D and 12/13 frequency ratios of 1.008 and 1.027). Its small D and large 13C shifts (12.5 and 41.0 cm−1) suggest a C−N stretching mode, and the low frequency is a sign of a weakened nitrile bond. In line with the previous reports,9,10 we assign this π absorption to the C−N stretching mode of the Th π-complex, η2-Th(NC)-CH3. The C−N stretching mode is considerably mixed with the C−C stretching mode, resulting in essentially a C−C−N antisymmetric stretching mode. The C−C stretching mode is expected at about 920 cm−1, but covered by precursor absorption, while its D counterpart is observed at 709.6 cm−1. 537

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Figure 5. Infrared spectra in the product absorption regions from reaction of the laser-ablated uranium atom with CD3CN in excess argon at 10 K. (a) U and CD3CN (0.50% in argon) co-deposited for 1 h, (b) as (a) after visible (λ > 420 nm) irradiation, (c) as (b) after UV (240−380 nm) irradiation, (d) as (c) after annealing to 28 K. π, i, and m stand for the product absorption groups, while P and c designate the precursor and common absorptions in the CH3CN matrix spectra. D2CCNH, D2CCN, D2CNC, and water residue absorptions are also indicated. Bands labeled c are common for this precursor and different metals.

Figure 6. Infrared spectra in the product absorption regions from reaction of the laser-ablated uranium atom with 13CH313CN in excess argon at 10 K. (a) U and 13CH313CN (0.50% in argon) co-deposited for 1 h, (b) as (a) after visible (λ > 420 nm) irradiation, (c) as (b) after UV (240−380 nm) irradiation, (d) as (c) after annealing to 28 K. π, i, m, and h stand for the product absorption groups, while P and c designate the precursor and common absorptions in the 13CH313CN matrix spectra. H213C13CNH, H213C13CN, and water residue absorptions are also indicated. Bands labeled c are common for this precursor and different metals.

Other π absorptions all support generation of the η2-Th nitrile π-complex. The π absorptions observed at 1425.2, 1345.1, and 1118.3 cm−1 are assigned to the CH3 bending and deformation modes with good correlation with the B3LYP values of 1459.1, 1382.9, and 1143.5 cm−1. The strong π absorption observed at 604.8 cm−1 (with a site absorption at 587.3 cm−1) carries its D and 13C counterparts at 572.5 and 601.2 cm−1 (with matrix site absorptions at 559.3 and 581.9 cm−1). It is assigned to the N−Th stretching mode on the basis of its frequency, small D and 13C shifts, and good correlation with the DFT frequencies and isotopic shifts (Table S2). Table S2 also shows that the observed π absorptions are the strongest bands for η2-Th(NC)-CH3, which substantiate formation of the thorium nitrile π-complex.20 The strong i absorption at 2034.6 cm−1 shows a negligible D shift, but its 13C counterpart is observed at 1993.8 cm−1 (12/13 frequency ratio of 1.020). While its high frequency suggests

a triple-bond stretching band, the negligible D shift indicates the triple-bond stretching motion is isolated from the C−H stretching motion. We assign it to the N−C stretching band of methyl thorium isocyanide, CH3-ThNC, consistent with the previous results.8−10 The C−N stretching band of its cyanide counterpart (CH3-ThCN) would appear ∼120 cm−1 on its blue side, which is not observed in this study. A weak i absorption is observed at 1094.0 cm−1 with its D counterpart at 876.5 cm−1 and is assigned to the CH3 deformation mode. The two observed i bands are in good agreement with the DFT results (Table S3), showing that the N−C stretching band is predominantly strong and the CH3 deformation band next strong in the observation range. On the red side of the i N−C stretching absorption at 2034.6 m−1, an m absorption is observed at 2030.4 cm−1, showing dramatic increases on photolysis and decrease in annealing. Deuteration causes virtually no frequency shift, but 538

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The dramatic increase of the i and m absorptions in photolysis with a decrease of the π absorptions suggests that the nitrile π-complex produced during co-deposition of Th atoms and acetonitrile undergoes insertion reaction to CH3-ThNC and subsequent H-migration to produce to CH2Th(H)NC similar to Zr.8 Analogous N-coordinated and methylidyne complexes observed in the previous group 5 and 6 metal studies9,10 are not observed in this Th study. The observed Th products are in fact the most stable steps in reaction path 1. Th-NCCH3, η2-Th(NC)-CH3, CH3-ThNC, CH2Th(H)NC, and HC÷ Th(H)2NC in their singlet, singlet, singlet, singlet, and triplet states are 95, 252, 305, 305, and 129 kJ/mol more stable than the reactants (Th(3F) + CH3CN), respectively. U + CH3CN. The product absorption regions from reactions of laser-ablated U atoms with acetonitrile isotopomers are shown in Figures 4, 5, and 6. Parallel to the Th case, four product absorption groups (π, i, m, and h) are observed based on intensity variation in the processes of photolysis and annealing. In the original spectra after co-deposition, the π absorptions are predominantly strong. They remain almost the same on visible irradiation, but decrease ∼30% on UV irradiation and diminish on annealing. The i absorptions increase ∼20% and ∼100% on visible and UV irradiation and decrease fast in the early stage of annealing. The m absorptions are weak in the original co-deposition spectrum, remain almost unchanged on visible irradiation, but double on UV irradiation and gradually decrease on annealing. The h absorptions, which most likely arise from a higher-order product, continuously increase on photolysis and annealing. The π absorptions in the U spectra are the strongest among those observed from reactions of group 4−6 metals8−10 and Th with acetonitrile. The strongest π absorption is observed at 584.6 cm−1 (with a matrix site absorption at 599.2 cm−1), and its D and 13C counterparts are at 552.7 and 581.1 cm−1 (with matrix site absorptions at 570.1 and 596.4 cm−1) (H/D and 12/13 frequency ratios of 1.058 and 1.006). Its low frequency and small isotopic shifts lead to assignment to the U−N stretching mode of η2-U(NC)-CH3. The second strong π absorption at 1091.2 cm−1 is assigned to the CH3 rocking mode on the basis of it frequency and sizable D and small 13C shifts (Tables 2 and S7). The π absorptions at 1421.5 and 1342.3 cm−1 are believed to be the A′ CH3 deformation and bending modes on the basis of their frequencies, large D and small 13C shifts, and good correlation with DFT values (Table S7). The A′′ CH3 deformation band expected at ∼1442 cm−1 is covered by precursor absorption, whereas its D and 13C counterparts are observed at 1025.5 and 1439.7 cm−1. Lastly the weakened C−N bond, due to coordination of U, leads to the low C−N stretching frequency of 1503.6 cm−1 and its D counterpart at 1496.8 cm−1. The six π absorptions substantiate formation of the U nitrile π-complex, η2-U(NC)-CH3. The strong i absorption at 2029.7 cm−1 is compared with that at 2034.6 cm−1 in the Th spectrum and shows a negligible D shift but a 13C shift of 41.1 cm−1 (12/13 frequency ratio of 1.021). Its frequency, high intensity, isotopic shifts, and correlation with DFT results lead to an assignment to the N−C stretching mode of CH3-UNC. The cyanide counterpart (CH3-UCN), whose C−N stretching band is expected at ∼2150 cm−1, is again not observed, consistent with the previous results. Another i absorption is observed at 1108.9 cm−1 along with its D and 13C counterparts at 874.5 and 1099.2 cm−1

Table 1. Matrix Infrared Frequencies Observed for Product Absorptions of Reactions of Th with Acetonitrile Isotopomers in Excess Argona CH3CN π

1536.3, 1514.1 1425.2 1345.1 1118.3

604.8, 587.3 h 1684.8 i 2034.6 1094.0 m 2030.4 covered (1408) 682.7 647.9

CD3CN

13

CH313CN

description η -Th(NC)-CH3, A′ CCN as. str. 2

1523.8, 1505.7 1128.4 covered 982.9, 966.5 709.6 572.5, 559.3 1671.3 2035.0 876.5 2030.6 1013.7

1495.3, 1481.6 covered covered 1093.0

1989.3 1408.1

high order product, C−N str. CH3−ThNC, N−C str. CH3−ThNC, CH3 deform. CH2Th(H)NC, NC str. CH2Th(H)NC, Th−H str.

618.7 512.0

661.9 640.9

CH2Th(H)NC, C−Th str. CH2Th(H)NC, Th−H ip bend

601.2, 581.9 1651.2 1993.8

η2-Th(NC)-CH3, A′ CH3 bend η2-Th(NC)-CH3, A′ CH3 deform. η2-Th(NC)-CH3, A′ CH3 rock η2-Th(NC)-CH3, A′ CCN s. str. η2-Th(NC)-CH3, A′ N−Th str.

All frequencies are in cm−1. Stronger absorptions in a set are bold. Description gives major coordinate.

a

Table 2. Matrix Infrared Frequencies Observed for Product Absorptions of Reactions of U with Acetonitrile Isotopomers in Excess Argona CH3CN π 1503.6, 1491.9 covered 1421.5 1342.3 1091.2 599.2, 584.6 h 1674 1535 i 2029.7 1108.9 m 2014.7 1456.2

CD3CN 1496.8, 1481.1 1025.5 covered 1032.8 955.4 570.1, 552.7 1660 1535 2029.7 874.5 2014.7 1050.8

13

CH313CN

description

covered

η2-U(NC)-CH3, A′ C−N str.

1439.7 1415.7 1331.4 1069 596.4, 581.1 1638

η2-U(NC)-CH3, A” CH3 bend η2-U(NC)-CH3, A′ CH3 bend η2-U(NC)-CH3, A′ CH3 deform. η2-U(NC)-CH3, A′ CH3 rock η2-U(NC)-CH3, A′ N−U str. high order, C−N str. high order, C−N str. CH3−UNC, A′ N−C str. CH3−UNC, A′ CH3 deform. CH2−U(H)NC, N−C str. CH2−U(H)NC, U−H str.b

1988.6 1099.2 1973.5 1458.0

All frequencies are in cm−1. Stronger absorptions in a set are bold. Description gives major coordinate. bTentative assignment.

a

C substitution causes a substantial shift to 1983.8 cm−1. Another m absorption at 1408.1 cm−1 in the 13CH313CN spectrum (Figure 3) carries its D counterpart at 1013.7 cm−1 (H/D ratio of 1.389). The Th−H stretching absorption in the CH3CN spectra in Figure 1 is believed to be covered by precursor absorption. The previously studied ThH (ThD) and ThH2 (ThD2) absorptions at 1485.2 (1060.2) and 1480.1 and 1455.6 (1055.6 and 1040.3) cm−1 are not observed.21a The N−C and Th−H stretching absorptions indicate formation of the thorium methylidene isocyanide. The m absorptions are assigned to CH2Th(H)NC. The other m absorptions in the low-frequency region at 682.7 and 647.9 cm−1 are assigned to the C−Th stretching and Th−H in-plane bending modes on the basis of a good correlation with the B3LYP values of 684.7 and 655.2 cm−1. The observed and predicted D and 13C shifts are also in excellent agreement, as shown in Table S4. The four observed m absorptions substantiate formation of CH2Th(H)NC. 13

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Figure 7. Energies of the plausible Th products in the proposed reaction path (1) relative to the reactants (Th(3F) + CH3CN). η2-Th(NC)-CH3, CH3-ThNC, and CH2Th(H)NC are observed in the matrix IR spectra. On UV photolysis, the η2-Th(NC)-CH3 absorptions decrease, while those of CH3-ThNC and CH2Th(H)NC increase. Th←NCCH3 and CH2Th(H)2NC are not observed probably due to their high energies (see text). S, T, and Qi indicate singlet, triplet, and quintet states, respectively.

Figure 8. Energies of the plausible U products in the proposed reaction path (1) relative to the reactants (U(5L) + CH3CN). η2-U(NC)-CH3, CH3UNC, and CH2-U(H)NC are observed in the original matrix IR spectra, and on UV photolysis, the absorptions of CH3-UNC and CH2-U(H)NC increase, whereas those of η2-U(NC)-CH3 decrease. U←NCCH3 and CH2-U(H)2NC are not observed probably due to their high energies (see text). S, T, and Qi indicate singlet, triplet, and quintet states, respectively.

The m absorption at 2014.7 cm−1 in the N−C stretching region has its D and 13C counterparts at 2029.7 and 1988.6 cm−1. In line with the Th case and previous results, we assign it to the

(H/D and 12/13 frequency ratios of 1.268 and 1.009), and its frequency and large D and small 13C shifts lead to an assignment to the CH3 deformation mode. 540

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Figure 9. B3LYP structures of the plausible Th products with the 6-311++G(3df,3pd) basis sets for H, C, and N and SDD pseudopotential and basis for Th. Bond distances and angles are in Å and deg. While CH-Th(H)2NC has a triple ground state, all other thorium products have singlet ground states. Notice the agostic distortion of the Th methylidene complex.

Figure 10. B3LYP structures of the plausible U products with the 6-311++G(3df,3pd) basis sets for H, C, and N and SDD pseudopotential and basis for U. Bond distances and angles are in Å and deg. The U←NCCH3, η2-U(NC)-CH3, and CH3-UNC have quintet ground states, while the U methylidene and methylidyne have triplet and singlet ground states. CH2-U(H)NC has an agostic C1 structure.

and UH2 bands at 1423.6 and 1406.1 and 1370.7 cm−1 21b were not observed in this study. The observed U products are also the most stable in reaction path 1. The uranium nitrile π-, insertion, and methylidene products in their ground quintet, quintet, and triplet states are

N−C stretching mode of CH2-U(H)NC. Another m absorption observed at 1456.2 cm−1 on the edge of a precursor absorption shows a large D shift to 1050.8 cm−1 (H/D frequency ratio of 1.386) and a negligible 13C shift. It is assigned to the U−H stretching mode (Tables 2 and S9). On the other hand, the UH 541

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π-complexes are substantially longer than calculated for CH3CN itself (1.149 Å) and most of those in the previously studied group 4−6 metal analogues (1.239−1.280 Å),8−10 while the electronegativities of Th and U, 1.3 and 1.7, are comparable to those of group 4−6 metals.22 This indicates that backdonations from the actinides to the nitrile π*-orbitals are considerable, but we cannot draw conclusions about the relative metal−π back-bonding in these two actinide metal cases from the present information. Interestingly, the NBO analysis23 provides nearly the same natural charges computed (B3LYP) for Th(0.98), N(−0.83), C(−0.17) and for U(1.02), N(−0.85), C(−0.16). Although the NC stretching frequency for the Th complex is ∼30 cm−1 higher than for the U complex, this frequency is not a pure N−C stretching mode, and we cannot jump to conclusions from this difference. However, the NBO analysis23 reveals some interesting differences in these two actinide π-complexes. First, the groundstate Th atom is a 3F2 state,24 but the η2-Th(NC)-CH3 complex is computed to have a 1A′ ground state, which suggests that the Th(6d) orbitals are involved in the metallacyclic bonding to the NC subunit. The NBO descriptions of the Th−N, Th−C, and NC bonds in these actinide π-complexes are summarized in Table 3. Note that the thorium contributions to the bonding to N and to C are dominated by the d orbitals, and their natural bond orders are each 0.95. Second, the ground-state U atom is a 5 L6 state,24 and the η2-U(NC)-CH3 complex is computed to have a 5.A′ ground state, which means that more orbital mixing is going on in the U-complex. The Milliken spin densities are U(4.26), N(0.07), C(0.15), C′(0.04), and H(0.002, 0.0007, 0.0006), which shows clearly that the unpaired spins remain on the U center. Note that the uranium contributions to the bonding to N and to C are also dominated by the 6d orbitals, and their natural bond orders are each 0.93, but that the U(5f) contribution is greater in the U−N than in the U−C bond. Finally, the oxidation states appear to be +2 for each metal, and the NC double bonds are made up of 0.99 σ- and 0.91 π-bond order in each case. Figures 9 and 10 also illustrate the agostically distorted structures of actinide methylidenes.7,13 The agostic angles (∠HCM) are 94.2° and 92.2° for the Th and U methylidenes, which are compared with 91.3°, 85−90°, and 86−87° for Zr, group 5 metals, and Mo and W analogues.8−10 The extent of distortion is similar to that of Zr methylidene.7a These agostic angles are also compared with those of 95.2° and 92.9° for CH2ThHF and CH2ThHCl and 87.5° and 89.6° for their U analogues.25 The agostic distortion, the interaction between the metal center and adjacent C−H bond, distorts not only the CH2 group but often the planar structure around the C and M centers as well, leading to the C1 structures of many transition metal methylidenes.7 Our NBO23 analyses also show that the C−Th and C−U bonds in these methylidene complexes are true double bonds (natural bond orders of both calculated as 1.95). The calculated Cs structures of the actinide methylidynes (Figures 9 and 10) are similar to those reported in our previous studies.7,10 The Th−C and U−C bond lengths, 2.367 and 1.896 Å, respectively, are considerably longer than those for the group 6 metal analogues (1.591−1.744 Å), and the NBO analyses further show that they are double and triple bonds (natural bond orders of 1.96 and 2.89) in their triplet and singlet ground states.

186, 247, and 158 kJ/mol more stable the reactants (U(5L) + CH3CN), whereas the unidentified N-coordinated and methylidyne complexes (U-NCCH3 and HCU(H)2NC) in their quintet and singlet ground states are 129 kJ/mol lower and 55 kJ/mol higher in energy than the reactants. Reactions. As described above, Th and U both produce η2-M nitrile π-complex, insertion, and methyidene products in reactions with acetonitrile. The actinide η2-M(NC)-CH3 complexes are the primary product observed in the original infrared spectrum of the co-deposited samples, and evidently subsequent photolysis converts it to CH3-MNC and further to CH2M(H)NC via H migration. The energies of the plausible products along reaction path 1 relative to the reactants are illustrated in Figures 7 and 8. The metal atom is most probably attracted by the lone electron pair of the N-end, but the N-coordination complex (M←NCCH3) is not trapped in the matrix, probably due to its relatively high energy (94 and 57 kJ/mol higher than the π-complexes in the Th and U systems). The excess reaction energy and following photolysis lead to conversion of η2-M(NC)-CH3 to CH3-MNC and CH2M(H)NC. Further H-migration to CHM(H)2NC evidently did not occur, probably due to its high energy. The Th and U methylidynes are 176 and 213 kJ/mol higher than the methylidenes in their ground states, and in the U case, the methylidyne is energetically even higher than the reactants. Parallel to the previous studies,8−10 no cyanide products are identified. The C−N stretching band is much weaker than the N−C stretching band of the isocyanide complex, due to the sequential increase of electronegativities of the constituting members in the M−C−N moiety. In the effectively M−C−N antisymmetric stretching mode, the polarization change in the C−N bond is reduced by that of the M−C bond. Even so, the absence of the absorptions from the cyanide complexes (even the strong M−H stretching bands) in reactions of group 4−6 metals and actinides indicates that production of the cyanide products (reaction path 2) is not favored in the metal reactions with acetonitrile, although the cyanide and isocyanide complexes are energetically comparable. The products along reaction path 1 all contain an M−N bond. Once the metal atom is attracted to the N-end of acetonitrile to form an M−N bond, it is retained throughout the conversions in the reaction path.8−10 Molecular Structures. The computed molecular structures of the plausible products along reaction path 1 are shown in Figures 9 and 10. The N-coordination, nitrile π-, insertion, and methylidyne complexes (M←NCCH3, η2-M(NC)-CH3, CH3-MNC, and HCM(H)2NC) have Cs structures, whereas the methylidene products (CH2M(H)NC) C1 structures. The actinide N-coordination complexes, though not observed in this study, have bent computed structures. The previously studied N-coordination complexes mostly have linear structures, and only the Cr and Mo products have bent MNC moieties (∠MNC = 160° and 148°).10 It is interesting that unlike the previously introduced complexes, these actinide N-coordination analogues have bent NCC moieties (116.5° and 137.7° for Th and U). The short M−N bonds (2.005 and 2.202 Å for Th and U) reflect strong bonding between M and N and lead to lower bond orders for the C−N bonds (natural bond orders of 1.89 and 2.43 for Th and U), and as a result, the NCC moieties are bent. Some comparison of the η2-Th(NC)-CH3 and η2-U(N C)-CH3 nitrile π-complexes is in order. The almost identical C−N bond lengths of 1.281 and 1.278 Å in the Th and U 542

dx.doi.org/10.1021/om200957j | Organometallics 2012, 31, 535−544

Article

CONCLUSIONS Actinide nitrile π-, insertion, and methylidene complexes are prepared from reactions with acetonitrile. The π-complexes (η2-M(NC)-CH3) are the prominent products in the original co-deposition spectra, but they apparently convert to CH3MNC and CH2M(H)NC in the subsequent photolysis. These products are the most stable steps in the reaction path previously proposed for reactions of group 4−6 metals with acetonitrile. The actinide N-coordination and methylidyne complexes with higher energies are not observed. The absence of cyanide products in this and previous studies indicates the preferential binding between N and M in the course of the reaction. The structures of the Th and U products reveal unusually strong bonding between these actinides with the N-end and nitrile π-system. Strong binding to the N-end leads to a decrease of the C−N bond order and a bent NCC moiety in the N-coordination complexes. The considerably long C−N bond in the nitrile π-complex indicates efficient back-donation from the actinide center to the π*-orbitals of the C−N bond. NBO analysis reveals very similar bonding in the two actinide η2-M(NC)-CH3 complexes. The actinide methylidenes also show agostically distorted structures, and the extents of distortion are smaller than those of group 5 and 6 metal analogues but comparable to that of the methylidene complex CH2ZrH2.7a,26

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ASSOCIATED CONTENT S Supporting Information * Tables of computed frequencies. This material is available free of charge via the Internet at http://pubs.acs.org.

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Based on NBO analysis (ref 23) using the B3LYP density functional. bNatural bond order.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS We gratefully acknowledge financial support from DOE Grant DE-SC0001034 to L.A. and support from the Korea Research Foundation (KRF) funded by the Korean government (MEST) (No 2009-0075428) and the KISTI supercomputing center.

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REFERENCES

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a

β-spin N: 99.5% C: 99.8% 0.99 + 0.91 β-spin 77.2% C: 27.8% s, 72.2% p 0.93 β-spin 78.7% N: 3.5% s, 95.9% p 0.93 0.99 + 0.91 0.95 0.95 BOb

α-spin 81.7% N: 5.7% s, 93.7% p

NC bonds σ-orbital α-spin 59.8% N: N: 41.1% s, 58.4% p 40.2% C: C: 31.1% s, 68.8% p β-spin 60.1% N: N: 40.3% s, 59.2% p 39.9% C: C: 32.4% s, 76.5% p π-orbital α spin N: 99.5% p C: 99.9% p U−C bond α-spin 17.7% U: 7.3% s, 1.0% p, 78.9% d, 12.8% f 82.3% C: 26.6% s, 73.4% p β-spin 22.8% U: 62.7% s, 0.06% p, 28.9% d, 8.4% f 77.2% C: 27.8% s, 72.2% p α-spin 82.3% C: 5.7% s, 93.7% p Th−C bond 21.1% Th: 7.4% s, 2.7% p, 75.6% d, 14.4% f 78.9% C: 25.9% s, 74.0% p Th−N bond 20.3% Th: 3.0% s, 2.3% p, 68.7% d, 26.1% f 79.7% N: 5.6% s, 93.4% p

NC bonds σ-orbital 59.3% N N: 39.4% s, 60.0% p 40.7% C C: 30.1% s, 69.8% p π-orbital N: 99.3% p C: 99.8% p

U−N bond α-spin 21.3% U: 3.7% s, 2.7% p, 65.6% d, 28.1% f 81.7% N: 5.7% s, 93.7% p β-spin 18.3% U: 3.3% p, 67.8% d, 28.3% f

π-complex

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nitrile η2-U(NC)-CH3 π-complex nitrile η2-Th(NC)-CH3

Table 3. NBO Descriptions for the M−N, M−C, and NC Bonds in the η2-M(NC)-CH3 Nitrile π-Complexesa

Organometallics

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