ARTICLE pubs.acs.org/JPCA
Infrared Spectra of the Ethynyl Metal Hydrides Produced in Reactions of Laser-Ablated Mn and Re Atoms with Acetylene Han-Gook Cho† and Lester Andrews*,‡ † ‡
Department of Chemistry, University of Incheon, 12-1 Songdo-dong, Yonsu-ku, Incheon, 406-772, South Korea Department of Chemistry, University of Virginia, P.O. Box 400319, Charlottesville, Virginia 22904-4319, United States
bS Supporting Information ABSTRACT: The ethynyl metal hydride molecules (HMCtCH) are identified in the matrix infrared spectra from reactions of laser-ablated Mn and Re atoms with acetylene using D and 13C isotopic substitution and density functional computed frequencies. The assignment of strong MH as well as CtC bond stretching product absorptions suggests oxidative CH insertion during reagent codeposition and subsequent photolysis. The unique linear structure calculated for HMnCtCH is parallel to C3v structures found recently for Mn complexes including CH3MnF.
’ INTRODUCTION Acetylene is a prominent ligand in a wide range of metal complexes.1 Coordination with the π-system is believed to precede and the subsequent steps vary with the metal, leading to its characteristic products. Particularly small acetylene complexes allow high-level theoretical approaches, providing opportunities to investigate the molecular structures and the reaction pathways for better understanding of the related catalytic activities and synthetic routes.27 The best known are the π-complexes (M-η2-(C2H2)),25 ethynyl metal hydrides (HM CtCH),4,5b,5c,6,7 vinylidene derivatives (CH2dCdM),2a,f,3,5b,5c vinyl radicals ( 3 CHdCHM),2ad and ethynyl complexes (HCCM).5c,6a,8 Many transition metal (Group 3, 4, 5,4 Pt,5b Rh,5c) and actinide metal atoms (Th and U)8 generate both the π- and insertion products (M-η2-(C2H2) and HMCtCH) in reactions with acetylene. Recent studies have shown that backdonations of the Group 4 metals to the π*-orbitals in M-η2-(C2H2) lead to the formation of unusually strong πcomplexes, whereas Group 5 metal analogues are relatively weaker.4 Iron is believed to generate the insertion complex via a hydrogen-bonded complex (Fe 3 3 3 HCtCH) without the observation of Fe-η2-(C2H2).7a We are aware of no experimental investigations of Group 7 metal atom reactions with acetylene, but theoretical calculations for the possible Mn π-complex have been reported.7b,c However, π-complexes are the primary products in the Ni and Pd reactions,3,5a although Pt also forms the vinylidene (CH2dCdPt).5b Mn complexes prepared from reactions with methane and halomethanes carry high multiplicities (sextet and quartet), and their structures show that Mn has borderline properties between the early and late transition-metals.9 On the other hand, only the low multiplicity (doublet) methylidyne complexes are observed in Re reactions, and the products show the rarely observed methylidyne CH stretching absorption as well as JahnTeller r 2011 American Chemical Society
distortion.10 In this study, reactions of the Group 7 metals Mn and Re with acetylene have been investigated in excess argon, and the matrix IR spectra reveal strong absorptions from the ethynyl metal hydride molecules (HMCtCH).
’ EXPERIMENTAL AND COMPUTATIONAL METHODS Laser ablated Mn and Re atoms (Johnson-Matthey) were reacted with C2H2 (Matheson, passed through a series of traps to remove acetone stabilizer), C2D2, and 13C2H2 (Cambridge Isotopic Laboratories, 99%) 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.11 Reagent gas mixtures range 0.130.50% in argon. After reaction, infrared spectra were recorded at a resolution of 0.5 cm1 using a Nicolet 550 spectrometer with an 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 Gaussian 09 package,12 B3LYP density functional,13 6-311þþG(3df,3pd) basis sets for C, H, and Mn, and SDD pseudo potential and basis set14 for Re (60 electron core) to provide a consistent set of vibrational frequencies and energies for the reaction products and their analogues. Geometries were fully relaxed during optimization, the optimized geometry was confirmed by vibrational analysis, and BPW9115 calculations were also done to confirm the B3LYP results. The vibrational frequencies were calculated analytically, and the zeropoint energy is included in the calculation of binding energy of a metal complex. Received: March 8, 2011 Revised: April 11, 2011 Published: April 25, 2011 4929
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Figure 1. IR spectra in the product absorptions regions for laser-ablated Mn atoms codeposited with C2H2 in excess argon at 8 K and their variation. (a) Mn þ 0.25% C2H2 in Ar codeposited for 1 h. (b) Same as (a) after photolysis (λ > 420 nm). (c) Same as spectrum b after photolysis (240 < λ < 380 nm). (d) Same as spectrum c after annealing to 28 K. i denotes the product absorption group. p, w, and c stand for precursor, water residue, and common absorptions from this precursor with other metals, respectively.
Figure 2. IR spectra in the product absorptions regions for laser-ablated Mn atoms codeposited with C2D2 in excess argon at 8 K and their variation. (a) Mn þ 0.25% C2D2 in Ar codeposited for 1 h. (b) Same as spectrum a after photolysis (λ > 420 nm). (c) Same as spectrum b after photolysis (240 < λ < 380 nm). (d) Same as spectrum c after annealing to 28 K. i denotes the product absorption group. p, w, and c stand for precursor, water residue, and common absorptions from this precursor with other metals, respectively.
’ RESULTS AND DISCUSSION Reactions of Mn and Re with acetylene isotopomers were carried out, and the matrix infrared spectra and their variation upon photolysis and annealing will be compared with the vibrational characteristics of anticipated products calculated by DFT. In addition, bands from precursor irradiation products common to other metal experiments were also observed.5,6 Mn þ C2H2. Figures 1, 2, and S1 (Supporting Information) show the Mn þ C2H2, C2D2, and 13C2H2 spectra in the reaction product absorption regions. The new product absorptions marked “i” (for insertion product) remain unchanged on visible (λ > 420 nm) irradiation but double upon UV (240 < λ < 380 nm) irradiation (Figures 1, 2, and S1a,b,c). While they decrease about 20% together on annealing, the relative intensity of these presumably matrix environmental site absorptions varies significantly upon annealing, where it appears that some change occurs in the matrix around the MnH bond. We suggest that UV photolysis produces the product in a constrained matrix cage, which is relaxed upon annealing. The product absorption frequencies are compared with DFT computed values for HMnCtCH in Table 1. In addition, the weak MnCO absorption at 1933.6 cm1 attests to the presence and reaction of Mn atoms.11c The strong matrix split i absorptions observed at 1684.6 and 1663.3 cm1 show no 13C shifts, but deuterium substitution shifts them to 1211.7 and 1196.7 cm1 (H/D ratios of both 1.390), which is appropriate for new MnH stretching frequencies. These new MnH stretching frequencies may be compared to the 1592.3 and 1477.9 cm1 values for MnH2 and MnH,16a and the 1582.6 and 1608.6 cm1 observations for CH3MnH and CHMnH.16b,c The MnH2 and MnH absorptions are also observed in this study. The strong new MnH stretching absorptions observed here and their dramatic increase on subsequent UV photolysis suggest oxidative CH bond insertion to form HMnCtCH in the reaction of Mn with acetylene.
Another important i absorption supports the formation of HMnCtCH. The matrix site i absorptions at 1974.2 and 1968.5 cm1 in the high frequency region exhibit D counterparts at 1860.5 and 1856.5 cm1 (H/D ratios of 1.061 and 1.060) and 13 C counterparts at 1903.6 and 1898.6 cm1 (12/13 ratios of both 1.037). These isotopic shifts are appropriate for a CC stretching mode coupled with H/D, and they are in the region for a CtC triple bond mode. The latter frequencies are compared with 1974 cm1 for HCtCH,4d 1956 cm1 for HTiCtCH, 1959 cm1 for HZrCtCH, and 1964 cm1 for HHf-CtCH,4b and they are assigned to the CtC stretching mode on the basis of their high frequencies and relatively large 12/13 ratios. The third strongest mode, CCH bending, is predicted to fall in the region encumbered by CO2 impurity, where it cannot be observed. Thus, we provide evidence for the identification of HMnCtCH, the most stable reaction product, from its two strongest and most characteristic absorptions, the MnH and CtC stretching modes, which correlate with calculated values. Density functional calculations find 3.4 and 0.4% higher frequencies with B3LYP and lower near the observed values using the BPW91 functional, which are in agreement with the calculated and observed frequencies observed previously.4b,c,5a5c On the other hand, unlike the previous studies of Group 3, 4, 5, 10, and 11 metal and acetylene reactions,2a,35 the plausible πcomplex and vinylidene (Mn-η2-(C2H2) and MndCdCH2) are not observed in the product spectra. The strongest absorptions expected at ∼1680 and ∼630 cm1 for the π-complex and at ∼870 cm1 for the vinylidene product (Tables S1 and S2) are not observed in this study. B3LYP results show that HMn CtCH, Mn-η2-(C2H2), and MndCdCH2 all have sextet ground states, and the insertion and π-complexes are 77 and 6 kJ/mol more stable than the reactants (Mn(6S) þ C2H2), whereas the vinylidene derivative is 23 kJ/mol higher energy than the reactants. Our small computed binding energy of the Mn πcomplex is consistent with the previous result of 13 kJ/mol.7b 4930
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a Frequencies and intensities are computed with 6-311þþG(3df, 3pd) for harmonic calculations, and the all-electron basis set is used for Mn. The two listed bands are due to matrix site effects on the same mode. Frequencies and intensities are in cm1 and km/mol.
42 2
71.4 12
125 2
164.2 199 2 246.9
51.0 92
35 2 159.0 112 2
133.5 71.5
126 2
169.3
12
248.6
158.2
π CMnH bend
π CCMn bend
201 2
43 2
200.1
66 2
154.0
42 2 78 661.9 435.8 46 2 78 529.3 439.7 18 2 77 668.3 447.4 707.8 447.5 π CCH bend σ CMn str.
45 2 82
41 2 82
560.2 439.7
17 2 77
701.0 435.9
75
369 1692.3 336 1692.0 1684.6, 1663.2 199 1206.8 183 1206.8 1211.7, 1196.7 1692.3 1692.0 1684.6, 1663.3 σ MnH str.
336
368
19 3356.2
1892.7 58
25
1970.2
3421.9
1903.6, 1898.6 91
0 2594.2
1845.8 75
2
1914.5
2654.3
1860.5, 1856.5 1963.7
17 3373.2
64 2043.8
24 3439.5
1974.2, 1968.5 σ CH str.
σ CC str.
approximate description
obs
B3LYP
BPW91
83
int B3LYP obs int BPW91 int B3LYP
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int
int
obs
DMnCtCD HMnCtCH
Table 1. Observed and Calculated Fundamental Frequencies of HMnCtCH Isotopomers in the Ground 6Σ Electronic Statea
HMn13Ct13CH
BPW91
int
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Figure 3. IR spectra in the product absorptions regions for laser-ablated Re atoms codeposited with C2H2 in excess argon at 8 K and their variation. (a) Re þ 0.25% C2H2 in Ar codeposited for 1 h. (b) and (c) Same as (a) after annealing to 20 and 28 K. (d) Same as spectrum c after photolysis (λ > 420 nm). (e) Same as spectrum d after photolysis (240 < λ < 380 nm). (f) Same as spectrum e after annealing to 36 K. i denotes the product absorption group. p and c stand for precursor and common absorptions from this precursor with other metals, respectively.
Figure 4. IR spectra in the product absorptions regions for laser-ablated Re atoms codeposited with C2D2 in excess argon at 8 K and their variation. (a) Re þ 0.5% C2D2 in Ar codeposited for 1 h. (b) and (c) Same as spectrum a after annealing to 20 and 28 K, repectively. (d) Same as spectrum c after photolysis (λ > 420 nm). (e) Same as spectrum d after photolysis (240 < λ < 380 nm). (f) Same as spectrum e after annealing to 36 K. i denotes the product absorption group. p and c stand for precursor and common absorptions from this precursor with other metals, respectively.
MnCCH, another possible product, would show the strongest absorptions at ∼1950 and ∼670 cm1 near the absorptions from HMnCtCH, but these are not identified. The π-complexes observed in most metal systems suggest that the metal atom is first attracted to the π-electrons, and the sole primary product (HMnCtCH) in the Mn system is, therefore, a sign of conversion to the insertion complex via oxidative CH bond activation. Re þ C2H2. Figures 3, 4, and S2 show the product absorptions in the Re þ C2H2, C2D2, and 13C2H2 spectra and their variation during photolysis and annealing. The C2H2 and C2D2 spectra are taken in the order of codeposition, annealing, photolysis, and further annealing, whereas the 13C2H2 spectra are in the order of codeposition, photolysis, and annealing. The order of photolysis and annealing was changed to investigate ground-state chemistry versus excited state chemistry in the matrix environment. The product absorptions marked “i” increase ∼20% and another ∼20% upon visible and UV photolysis, and they sharpen in the 4931
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Table 2. Observed and Calculated Fundamental Frequencies of HReCtCH Isotopomers in the Ground 6A0 Electronic Statea HReCtCH approximate description
obs
0
A CH str.
HRe13Ct13CH
DReCtCD
B3LYP
int.
BPW91
int
obs
B3LYP
int.
BPW91
int
obs
B3LYP
int
BPW91
int
3439.4
34
3375.3
33
2636.8
5
2579.5
9
3422.5
35
3359.1
34
A0 CC str.
1920.6
1970.0
34
1892.9
8
1814.8
1855.6
65
1786.2
39
1846.0
1900.0
18
1838.6
74
A0 ReH str.
218
1824.2
1811.3
349
1817.7
287
1305.8
1285.4
167
1291.8
134
1818.0
1810.0
361
1803.4
A00 CCH oop bend
648.3
696.7
37
650.7
36
523 sh
556.2
15
519.9
14
642.8
689.6
38
644.1
36
A0 CCH ip bend
645.7
685.8
39
628.8
39
547.4
15
501.6
14
639.8
678.9
40
622.7
40
A0 CRe str.
424.3
94
452.1
73
405.8
70
417.1
52
413.1
93
444.3
70
A0 CReH bend
349.6
40
389.8
19
285.4
33
323.4
20
434.8
36
380.3
18
A00 CCRe oop bend A0 CCRe ip bend
239.5 177.0
43 3
241.1 173.6
37 4
217.8 147.7
30 3
219.5 148.0
26 3
232.5 172.8
41 3
234.0 169.1
36 4
a Frequencies and intensities are computed with 6-311þþG(3df, 3pd) for harmonic calculations, and the SDD core potential and basis set are used for Re. Frequencies and intensities are in cm1 and km/mol.
early stage of annealing and decrease upon later annealing. A weak 1940.1 cm1 band was also observed for the very strong ReNN absorption.11d The distinctively strong product absorption at 1824.2 cm1 with C2H2 shifts to 1305.8 cm1 with C2D2 and to 1818.0 cm1 with 13C2H2 (H/D and 12/13 ratios of 1.397 and 1.003) upon deuteration and 13C substitution. [The 1305.8 cm1 band in C2D2 experiments is not due to methane impurity: this band is not present in the initial sample spectrum, and its observation requires laser ablation of Re. It further increases on photolysis together with the 1920.6 cm1 band as rhenium atom reaction products (Figure 4). However, a 1304.6 cm1 shoulder absorption, which does not increase upon photolysis, is probably due to methane since a similar band is produced in the C2H2 experiment.] The small 13C shift of this predominantly ReH stretching absorption originates from slight mixing with the associated CtC stretching mode, whose absorption is observed at 1920.6 cm1 along with the D and 13C counterparts at 1814.8 and 1846.0 cm1. This ReH stretching frequency is compared with 1946.4, 1819.9, and 1804.0 cm1 values for HCtReH3,10 which was exclusively produced in the reaction with CH4. The above ReH stretching and CtC stretching absorptions provide strong evidence for formation of the ethynyl hydride product (HReCtCH). In the low frequency region, two CCH bending (out-of-plane and in-plane bending modes) absorptions are observed, suggesting a bent (Cs) structure for HReCtCH, which is consistent with the DFT results (Table 2 and Figure 6). The one at 648.3 cm1, accompanied by D and 13C counterparts as a weak shoulder at 523 cm1 and a band at 642.8 cm1, is assigned to the out-of-plane bending mode, whereas the other one at 645.7, with its 13C counterpart at 639.8 cm1, is designated to the in-plane bending mode without observation of the D counterpart. The observed four product absorptions substantiate formation of the Re and CH insertion product, HReCtCH. Frequencies calculated using the DFT approximation in Table 2 compare favorably with the matrix observations. Again, the B3LYP value is higher, 2.5%, for the CC mode, but 0.7% lower for the ReH mode. Since BPW91 predicts these two modes closer together, the mode mixing and carbon-13 shift for the ReH stretching mode are greater than those found with the B3LYP functional, and the observed values fall intermediate between the two functional results.
Parallel to the Mn case, no other Re acetylene reaction products are identified in the matrix IR spectra. Both Reη2-(C2H2) and RedCCH2 would show their strongest absorptions near ∼1560 and ∼660 cm1, which were not observed in this study. ReCCH, which would show its strongest absorptions at ∼3310 and ∼620 cm1, was also not observed. The insertion, π-complex, and vinylidene products (HReCtCH, Re-η2-C2H2, and RedCdCH2) in the sextet, quartet, and quartet ground states are 74, 62, and 73 kJ/mol more stable than the reactants (Re(6S) þ C2H2), respectively. However, in the sextet states, Re-η2-C2H2 and RedCdCH2 are 36 and 63 kJ/mol higher than HReCtCH. Both the Mn and Re primary products (HMnCtCH and HReCtCH) are most stable in the sextet ground states. Weak benzene absorptions are observed in this and previous laser ablated metal reactions with acetylene,4,5 but they are stronger here due to the use of higher laser energy required to ablate the refractory metal Re. Benzene is produced from photocyclotrimerization of acetylene.17 Reactions in the Matrix. Figures S3 and S4 show the relative energies for the possible products. In the Mn system, these products all have sextet ground states, and the sole primary product is undoubtedly the lowest in energy. On the contrary, in the Re system, the π and vinylidene complexes in their quartet ground states are in fact energetically comparable to HReCtCH, but in its sextet state HReCtCH is again clearly the most stable. However, the fact that new products are formed on reactions of laser ablated Mn atoms during sample deposition and on subsequent photolysis suggests that these reactions require excited Mn atoms, which is consistent with the failure to observe product growth upon annealing of cold reagents. This also follows the earlier observation that thermal Mn atoms failed to react with pure solid methane, but CH3MnH was formed on UV (300340 nm) photolysis,16b and similar behavior was observed in recent Mn and CX4 reactions performed in this laboratory.9 Previous laser ablation investigations reveal the presence of Mn* (8P, 4s3d54p) metastable states 222 kJ/mol above the ground state (6S, 4s23d5), which likely are responsible for our reactions during sample deposition.18 Clearly, UV irradiation of the cold sample leads to reaction most likely through photoexcitation of Mn. The excited metal atom is attracted by the π-electrons and rapidly undergoes oxidative CH insertion forming 4932
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Figure 5. The B3LYP structures of the acetylene π, insertion, and vinylidene complexes of Mn. The π, insertion, and vinylidene complexes have C2v, C¥, and planar Cs structures, respectively. The bond lengths and angles are in Å and degrees.
Figure 6. The B3LYP structures of the acetylene π, insertion, and vinylidene complexes of Re. The π, insertion, and vinylidene complexes have C2v, planar Cs, and C2v structures, respectively. The bond lengths and angles are in Å and degrees.
HMCtCH, but cannot proceed to MdCdCH2 due to its higher energy.
The absence of the π-complex product absorptions is surprising due to the fact that acetylene is one of the best known ligands for generating various π-complexes.1 However, acetylene π-complexes of Mn and Re are rare.1b It is notable that the π-complex is more stable than the insertion product for Group 3, 4, 5, and 10 metals as shown in Table S5. Only in the Fe system is the insertion product slightly more stable than the π-complex. Therefore, the stability of HMCtCH over M-η2-(C2H2) in the Group 7 metal systems is distinctive and also consistent with the present results. In addition, the computational results also show that the π-complexes would have relatively low absorption intensities (Tables S1 and S3). Evidently, the Mn and Re reactions with acetylene did not produce the π-complexes in enough quantity for observation in the matrix infrared spectra. Structures of HMnCtCH and HReCtCH. The optimized structures of the Mn and Re π, insertion, and vinylidene complexes are illustrated in Figures 5 and 6. It is notable that the Mn insertion product (ethynyl metal hydride) has a linear (C¥) structure unlike the previously studied analogues, while the structures of the π and vinylidene complexes resemble those of the formerly introduced complexes.28 The binding energies of the unidentified Mn and Re π-complexes in their ground electronic states (6 and 62 kJ/mol) are much smaller than those for the previously studied Group 4, 5, and actinide analogues (151314 kJ/mol) and Group 10 counterparts (109205 kJ/ mol). They are also compared to those for the Li, Ga, In, and Fe π-complexes (4663 kJ/mol). The CC bond length in the Mn π-complex (1.265 Å) is much shorter than those for Group 4 and
5 metal and actinide analogues (1.3231.351 Å) but comparable to those for the Group 10 metal, In, Ga, Li, and Fe counterparts (1.2481.286 Å), indicating weak back-donation of Mn to the acetylene π*-orbital. The unique linear structure of HMnCtCH, which is in contrast to the planar Cs structures with a bent CMH moiety for the previously studied acetylene insertion complexes,28 is in fact parallel to the unusual C3v structures for the recently studied Mn complexes prepared in reaction with halomethanes,9 such as CH3MnF. Our natural bond orbital (NBO)19 analysis shows that Mn in CH3MnF carries high s (66%) and low d (34%) characters in the CMn bond, while the MnF bond is ionic.9 In the case of linear HMnCtCH, the Mn contributes 34% s, 49% p, and 17% d character in the CMn bond and 37% s, 50% p, and 26% d character in the MnH bond, whereas for nonlinear HReCtCH, Re holds 25% s, 31% p, and 44% d character in the CRe bond and 28% s, 34% p, and 38% d character in the ReH bond. For 6Σ MnH2, the MnH bond has 36% s, 50% p, and 14% d character, a 1.692 Å bond length, and a natural charge of 1.02 on Mn, which is less than the 1.14 charge on Mn and longer than the 1.675 Å MnH bond in HMnCtCH. The lower d characters of Mn in the CMn and MnH bonds become more obvious when compared with those of the previously studied transition-metal analogues as shown in Table S6. Landis et al. have shown that the d-contribution to hybridization of valence electrons for metal atoms plays a key role in determination of the bond angles in a metal hydride.20 On the basis of the linear structure and low d-characters in the MnH and CMn bonds, Mn evidently supports a hybridization for HMnCtCH somewhat similar to the sp hybridization. The low d-participations in the Mn bonds at least partly originate from the stability of the half-filled 3d-orbitals, leading to the 4933
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The Journal of Physical Chemistry A unique linear structure of the ethynyl hydride similar to those of the previously studied Group 2 metal6a and methyl Mn halide9 complexes. The nonlinear HReCtCH structure, which is, however, still less bent than the previously studied analogues, reflects the higher d-character in the Re bonds, indicating the lower preference of the half-filled 5d-orbitals.
’ CONCLUSIONS The ethynyl metal hydrides (HMCtCH) are identified in the product spectra of Mn and Re reactions with acetylene from isotopic substitution and density functional computed frequencies, while other plausible products are not observed. The strong MH and CtC stretching mode product absorptions reveal oxidative CH insertion to form the ethynyl metal hydride molecules. The ethynyl Mn hydride (HMnCtCH) has a unique linear structure in contrast to the planar Cs structures of the previously studied transition-metal and Re analogues. The exceptionally low d-character in the CMn and MnH bonds, parallel to those of the recently reported CX3MnX (X = H or halogen),9 reflect the stability of the half-filled 3d-orbitals, whereas the higher d-characters in the Re bonds and the bent structure indicate the relatively lower preference of the half-filled 5d-orbitals. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figures of carbon-13 isotopic spectra. Tables of computed frequencies and product energies. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT 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 2010-0016527). ’ REFERENCES (1) (a) Bowden, F. L.; Lever, A. B. P. Organomet. Chem. Rev. 1968, 3, 227. (b) Haiduc, I; Zuckerman, J. J. Basic Organometallic Chemistry; Walter de Gruyter: New York, 1985. (c) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; Wiley: New York, 2001. (2) (a) Chenier, J. H. B.; Howard, J. A.; Mile, B.; Sutcliffe, R. J. Am. Chem. Soc. 1983, 105, 788 (Cu, Ag, Au). (b) Kasai, P. H. J. Am. Chem. Soc. 1983, 105, 6704. (c) Kasai, P. H. J. Am. Chem. Soc. 1982, 104, 1165. (d) Chertihin, G. V.; Andrews, L.; Taylor, P. R. J. Am. Chem. Soc. 1994, 116, 3515 (Al). (e) Manceron, L.; Andrews, L. J. Am. Chem. Soc. 1985, 107, 563 (Li). (f) Kasai, P. H. J. Am. Chem. Soc. 1992, 114, 3299 (Li, Na, K). (3) Ozin, G. A.; McIntosh, D. F.; Power, W. J.; Messmer, R. P. Inorg. Chem. 1981, 20, 1782 (Ni, Cu). (b) Kline, E. S.; Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1987, 109, 2402 (Ni). (4) (a) Teng, Y.-L.; Xu, Q. J. Phys. Chem. A 2010, 114, 9069 (Sc, Y). (b) Cho, H.-G.; Kushto, P.; Andrews, L.; Bauschlicher, C. W., Jr. J. Phys. Chem. A 2008, 112, 6295 (Ti, Zr, Hf). (c) Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2010, 114, 10028 (V, Nb, Ta). (d) Herzberg, G., Infrared and Raman Spectra; Van Nostrand: Princeton, NJ, 1945.
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