Infrared Spectra of CX2 CoX2 and CX3â CoX Complexes from

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J. Phys. Chem. A 2010, 114, 8056–8068

Infrared Spectra of CX2dCoX2 and CX3-CoX Complexes from Reactions of Laser-Ablated Cobalt Atoms with Halomethanes Han-Gook Cho and Lester Andrews* Department of Chemistry, UniVersity of Incheon, 177 Dohwa-dong, Nam-ku, Incheon, 402-749, South Korea, and Department of Chemistry, UniVersity of Virginia, P.O. Box 400319, CharlottesVille, Virginia 22904-4319 ReceiVed: March 24, 2010; ReVised Manuscript ReceiVed: June 15, 2010

Simple cobalt complexes with substantial carbon-cobalt double bond character are produced in Co atom reactions with tetra-, tri-, and dihalomethanes, whereas insertion complexes are identified only in the dihalomethane matrix infrared spectra. These complexes are identified from matrix infrared spectra and comparison with frequencies computed by density functional theory. Exclusive generation of carbenes in the tetrahalomethane systems is consistent with the computational results that the staggered allene-type conformer is the only meaningful energy minimum in the reaction path. Their short C-Co bondlengths of 1.732-1.764 Å and CASSCF computed bond orders near 1.7 are also appropriate for carbon-cobalt double bonds. Hence, reactions of laser-ablated Co atoms are effective means to generate rarely reported high oxidationstate Co complexes with carbon-cobalt double bonds. Unlike the Rh and Ir cases, Co carbynes (with C-Co triple bonds) are not formed. The observation of CH2Cl-CoF with photoreversible intensity variation in the spectra provides unique insight on halogen migration from interconversion with CH2F-CoCl through the CH2dCoFCl carbene. Introduction Numerous high oxidation-state complexes with carbon-metal multiple bonds have been introduced through well developed synthetic routes, and their rich chemistry has become an essential part of coordination chemistry.1 Although many of them are used as C-H insertion agents and important catalysts for metathesis reactions, others are important transient species such as reaction intermediates.2 Cobalt carbenes with a C-Co double bond, however, are rare, and they normally have N, O, or CdO bonded to the C atom, such as [CoCl2(L)2](PF6) (L ) N-methylN-(2-pyrimidinyl)imidazolylidene), which substantially weaken the carbon-metal bond.3,4 Ikeno et al. estimated the C-Co bond orders as 0.793-0.868 for the cobalt carbene intermediates containing the Co-CH-COMe moiety in cyclopropanation reactions on the basis of their time-resolved FT-IR and DFT computation results.4 Parallel to their larger relatives, small transition metal complexes with multiple C-M bonds have also been produced through metal atom reactions, and their structures and reactions have been investigated.5-12 In particular, direct reactions of transition-metal atoms with small alkanes and halomethanes have turned out to be a very efficient means to provide small high oxidation-state complexes, and their matrix infrared spectra allow opportunities to examine photochemical behaviors.6-12 These reactions undergo C-H(X) bond insertion and subsequent H(X) migration to produce the high oxidation-state complexes.6 They are not only model systems for larger complex analogs because they are more amenable to high level theoretical applications,13 but they also show interesting reactions, unique structures, photochemistry, and electronic properties.6-12 Although W, Re, and Os preferentially produce high oxidationstate complexes with C-M triple bonds,6,8,9 the higher oxidationstate complexes become less favored on moving away from * To whom correspondence should be addressed. E-mail: lsa@ virginia.edu.

them in the periodic table.6-11 Very recent studies also show that Rh and Ir form carbynes as well as carbenes in reactions with tetrahalomethanes and produce carbenes preferentially in reactions with tri- and dihalomethanes.11 Larger Ir carbyne complexes containing F also have unusual square planar structures similar to those of Ru carbynes with Ru-X bonds.14 In this study, reactions of cobalt atoms with halomethanes have been carried out, and the products are identified in the matrix spectra through isotopic substitution and helpful predictions from DFT calculations for the plausible products. The identified cobalt carbenes carry short C-Co bonds, suggesting that these small Co complexes are rare examples of C-Co double bonds. Experimental and Computational Methods Laser ablated cobalt atoms were reacted with CCl4 (Fisher), CCl4 (90% enriched, MSD Isotopes), CFCl3, CF2Cl2 (Dupont), CHCl3, CH2Cl2, CH2FCl, CH2F2, CDCl3, CD2Cl2, 13CH2Cl2 (MSD Isotopes), CD2FCl, and CD2F2 (synthesized15) 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.6,16 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 on a rotating metal target (Co, 99.99%, JohnsonMatthey) 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 and annealed to allow further reagent diffusion. To provide support for the assignment of new experimental frequencies and to correlate with related works,6-12 density functional theory (DFT) calculations were performed using the Gaussian 03 program system,17 the B3LYP density functional,18 13

10.1021/jp1026543  2010 American Chemical Society Published on Web 07/21/2010

Infrared Spectra of CX2dCoX2 and CX3-CoX Complexes

J. Phys. Chem. A, Vol. 114, No. 31, 2010 8057

Figure 1. Infrared spectra in the 1000-800 cm-1 and 550-450 cm-1 regions for the reaction products of the laser-ablated cobalt atom with CCl4 isotopomers in excess argon at 10 K. (a) Co and CCl4 (0.5% in argon) codeposited 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) irradiation, and (e) as (d) after annealing to 26 K. (f) Co and 13CCl4 reagent (0.5% in argon) codeposited for 1 h, (g)-(j) as (f) spectra taken following the same sequence of visible, uv, and visible irradiations and annealing to 34 K. m designates the product absorption. Cl2CCl-Cl and CCl3 are produced in CCl4 reactions due to the ablation plume irradiation. Isolated CoCl2 and cyc-(CoO)2 absorptions are also denoted.

the 6-311++G(3df,3pd) basis sets for H, C, F, Cl, and Co19 to provide vibrational frequencies for the reaction products. Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis. The BPW9120 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,6-12,21 and they provide useful predictions for infrared spectra of new molecules. To calculate meaningful bond orders, CASSCF (complete active space, self-consistent field) calculations (CAS(5,6))22 were done for the carbon-metal bonds in CCl2dCoCl2, CFCldCoCl2, CF2dCoCl2, CCl2dRhCl2, and CCl2dIrCl2 using the optimized B3LYP structures. The 6-311++G(3df,3pd) basis sets were used for C, F, Cl, and Co and SDD for Rh and Ir.19 An SCF calculation was done first to generate MO’s, and the CAS routine in Gaussian 03 performed the CASSCF calculation. Finally, CASPT2 calculations were done using Gaussian 03 for comparison with the Rh and Ir methylidene CASSCF calculations. Results and Discussion The matrix infrared spectra (Figures 1-6) from reactions of laser-ablated cobalt atoms with halomethanes are investigated, and DFT frequency calculations (Tables 1-10) of the products and their structures (Figure 7) will be presented in turn. Co + CCl4. Shown in Figure 1 are the reaction product spectra from Co atoms codeposited with carbon tetrachloride in excess argon during condensation at 10 K. The observed product absorptions are all marked “m” (for methylidene), which in concert decrease ∼10%, recover to the original intensity, and decrease ∼15% on visible (λ > 420 nm), uv (240 < λ < 380 nm), and full arc (λ > 220 nm) irradiations, respectively. On annealing they sharpen first and gradually increase until the matrix evaporates. A strong m absorption is observed in the

C-Cl stretching region23 at 937.3 cm-1, accompanied with a satellite at 931.7 cm-1 with ∼2/3 of the main band absorption intensity arising from chlorine isotopic splitting (one 35Cl and one 37Cl vs two 35Cl atoms) for a mode involving two equivalent chlorine atoms. Another m absorption is observed at 889.5 cm-1 with satellites at 886.2 and 883.8 cm-1. Unlike the previously studied Rh and Ir cases,11 no absorptions from ClC≡CoCl3 that would be expected on the blue side of the m absorptions are observed, in line with the tendency that higher oxidation-state complexes becomes less favored with moving up in a family group.6-11 Absorptions previously observed through vacuum ultraviolet irradiation at 1036.4 cm-1 (CCl3+),24 1019.3, 926.7, 501.9 cm-1 (Cl2CCl-Cl),25 and 898 cm-1 (CCl3)26 were also observed, which are common to all laser-ablated metal experiments with CCl4 as a result of ablation plume photolysis. A similar experiment with 13CCl4 (90% enriched) shifted the m absorptions to 905.9 cm-1 (with satellite at 900.4 cm-1) and to 860.0 cm-1 (with satellites at 857.9 and 856.1 cm-1) (12/13 isotopic frequency ratios of 1.035 and 1.034). The weak 12C product band at 937.3 cm-1 and its satellite at 931.7 cm-1 with about 1/10 of the 13C product band absorbance in the 13CCl4 spectra indicates single carbon atom participation in the vibrational mode (the other one at 905.9 cm-1 and its satellite at 900.4 cm-1 are weaker but still discernible). On the basis of the previous studies for reactions of late transition-metal atoms with tetrahalomethanes,9-11 the two strong m product absorptions in the C-Cl stretching region are strong evidence for formation of a primary product with a CCl2 moiety, most probably CCl2dCoCl2. They are assigned to the symmetric and antisymmetric CCl2 stretching modes of the small Co methylidene. The observed frequencies and isotopic shifts (31.4 and 29.5 cm-1) correlate well with the DFT values for the doublet ground state as shown in Table 1 (e.g., the B3LYP frequencies of 930.0 and 872.0 cm-1 and their 13C isotopic shifts of 31.9 and 27.8 cm-1). The slightly lower DFT frequencies

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Cho and Andrews

TABLE 1: Observed and Calculated Fundamental Frequencies of CCl2dCoCl2 in the Ground 2A2 Statea 13

CCl2dCoCl2(D) b

approximate description

obs

A1 CCl2 s. str. B2 CCl2 as. str. B1 CoCl2 as. str. A1 C-Co str. B1 CCl2Co deform A1 CoCl2 s. str. A1 CCl2 scis. B2 CCl2 rock A1 CoCl2 scis. B1 CoCl2 rock B2 CoCl2 wag A2 CCl2 tort.

937.3, 931.7 889.5, 886.2, 883.8 483.7, 481.5, 478.9

B3LYP 930.0 872.0 463.3 443.3 436.3 342.7 236.3 185.6 101.2 86.2 71.4 33.0

c

int

c

BPW91

162 213 63 10 34 11 0 0 1 4 1 0

934.5 829.6 457.2 445.9 424.7 341.3 238.2 172.6 104.6 85.4 78.0 55.6

d

int

d

168 200 66 11 15 9 0 0 0 2 0 0

obs

b

905.9, 900.4 860.0, 857.9, 856.1 483.3, 480.0, 477.4

CCl2dCoCl2(D) B3LYPc

intc

BPW91d

intd

898.1 844.2 460.2 443.0 423.2 342.4 236.2 184.9 101.1 86.1 71.4 33.0

151 199 80 10 17 11 0 0 1 4 1 0

902.2 803.2 455.3 445.8 410.9 341.0 238.1 171.9 104.5 85.4 78.0 55.6

156 187 74 11 8 9 0 0 0 2 0 0

a Frequencies and intensities are in cm-1 and km/mol. CCl2dCoCl2 (D) has a staggered C2V structure. The symmetry notations are based on the C2V structure. The two higher B1 modes are mixed. b Observed in an argon matrix. Strongest absorptions in a set are in bold type. c Frequencies computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd).

than the experimental values are common for the late transitionmetal tetrahalomethylidenes, most probably due to underestimation by the DFT methods for the C-Cl bond strengths of the late transition-metal carbenes.9-11 In the low frequency region, another m absorption is also observed at 481.5 cm-1 (with satellites at 483.7 and 478.9 cm-1) on the red side of the isolated CoCl2 antisymmetric Co-Cl stretching at 493.3 cm-1 .27 The lower satellite at 478.9 cm-1 is at the correct position and relative intensity to be due to the 35 37 Cl Cl isotopic splitting for the 481.5 cm-1 main band assuming it is contributed by the most abundant 35Cl2 isotope, based on comparison with the nearby isolated CoCl2 absorption profile of 493.3 and 490.1 cm-1.27b The upper satellite then would be due to a matrix site splitting. Our main band at 481.5 cm-1 is appropriate for the mostly antisymmetric Co-Cl stretching mode in the CCl2dCoCl2 methylidene complex, but the 13C shift of 1.5 cm-1 and smaller chlorine isotopic splitting of 2.6 cm-1 indicate coupling with a close mode of the same B1 symmetry, most likely the CCl2Co deformation mode. Thus, from the infrared spectra alone, this new product contains CCl2 and CoCl2 subunits. Whereas other bands are too weak or too low in frequency to observe, as shown for the frequency calculations for doublet ground state CCl2dCoCl2 in Table 1, the above three strong observed absorptions, which are consistent with DFT predictions, substantiate the formation of the small Co methylidene, CCl2dCoCl2. Only a few Co complexes with C-Co double bonds have been reported, and they normally have N, O, or CdO bonded to the C atom.3,4 The C-Co bond in the Co-CH-COMe moiety is essentially a single bond in the cobalt complex-catalyzed cyclopropanation intermediate on the basis of their time-resolved FT-IR and DFT results.4 The bond orders and lengths of the C-Co bonds are in the range of 0.793-0.868 and 1.886-1.913 Å, respectively. The C-Co bond length of 1.740 Å for CCl2dCoCl2 (Figure 7) is considerably shorter than those of the previously reported Co complexes (1.80-2.10 Å),3,4 indicating that this small Co carbene is a rare example of a Co high oxidation-state complex with effectively a C-Co double bond. Mode analysis shows that the high frequency and large 13C isotopic frequency shift of the symmetric CCl2 stretching band originates from combination of the CCl2 symmetric and C-Co stretching modes, where C vibrates back and forth between heavy Co and two Cl.10 However, the observed CCl2 symmetric and antisymmetric stretching frequencies of 937.3 and 889.5 cm-1 are compared with those of 964.3 and 867.2 cm-1 for

CCl2dRhCl2 and 994.4 and 826.3 cm-1 for CCl2dIrCl2, while the 12/13 ratios of 1.035 and 1.034 with those of 1.036 and 1.032 for both CCl2dRhCl2 and CCl2dIrCl2.11 The increasing and decreasing CCl2 symmetric and antisymmetric stretching frequencies with going down the family column is at least partly due to the decreasing Cl-C-Cl angle (116.2, 114.9, and 113.5° for the Co, Rh, and Ir methylidenes). A smaller ∠ClCCl leads to more effective mode coupling between the CCl2 symmetric and C-M stretching modes and less effective coupling between the two C-Cl vibrators (and thereby the higher and lower 12/ 13 isotopic frequency ratios, respectively). It is also notable that attempts for geometry optimizations of the Co carbyne and insertion complexes (ClC≡CoCl3 and CCl3-CoCl) all converge to the structure of CCl2dCoCl2 in both the doublet and quartet states. This is consistent with

Figure 2. Infrared spectra in the 1400-450 cm-1 region for the reaction products of the laser-ablated cobalt atom with CF2Cl2 and CFCl3 in excess argon at 10 K. (a) Co and CFCl3 (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 irradiation, and (e) as (d) after annealing to 26 K. (f) Co and CF2Cl2 reagent (0.5% in argon) codeposited for 1 h, (g-j) as (f) spectra taken following the same irradiation and annealing sequence. m designates the product absorption, and “P” and “c” stand for the precursor and common absorptions.



TABLE 2: Observed and Calculated Fundamental Frequencies of CF2dCoCl2 and CFCldCoCl2 in the 2A2 and 2A′′ Ground Statesa CF2dCoCl2

CFCldCoCl2


obsb

B3LYPc

intc

BPW91d

intd

obsb

B3LYPc

intc

BPW91d

intd

CX2 s. str. CX2 as. str. CX2 scis. CX2Co deform C-Co str. CoX2 as. str. CoX2 s. str. CX2 rock CoX2 scis. CoX2 rock CoCl2 wag CX2 tort.

1292.8, 1290.8 1271.9, 1270.0 693.5, 690.7 594.2 461.7

1310.9 1261.4 715.3 559.1 448.0 397.8 324.1 258.9 105.4 98.8 55.0 42.5

583 241 7 13 93 1 2 0 9 3 0 0

1254.5 1176.2 691.7 553.1 452.4 416.2 323.5 263.1 107.4 89.9 77.8 50.9

537 214 3 7 86 4 1 0 1 3 0 0

1226.0, 1223.9 955.2, 953.7

1238.1 938.6 528.7 508.0 454.1 368.6 305.0 207.3 101.5 85.4 68.6 30.9

307 319 3 0 102 7 2 0 2 4 0 0

1186.8 915.0 529.6 492.3 449.5 370.1 316.4 208.4 107.2 82.8 78.7 50.3

285 268 8 0 86 9 0 0 0 2 0 0

474.3

a Frequencies and intensities are in cm-1 and km/mol. CF2dCoCl2(D) and CFCldCoCl2(D) have staggered C2V and Cs structures. b Observed in an argon matrix. Stronger absorptions are in bold type. c Frequencies computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd).

absence of the carbyne and insertion product absorptions in the Co + CCl4 spectra, indicating that the small Co carbene is probably the sole meaningful energy minimum in the reaction coordinate. Therefore, C-X insertion is most likely spontaneously followed by X migration from C to Co to produce the Co carbene. In addition, the doublet and quartet states of CCl2dCoCl2 are energetically comparable, 77 and 73 kcal/mol, respectively, more stable than the reactants (Co(4F) + CCl4). However, the observed frequencies strongly suggest that the primary product is in the doublet ground state on the basis of the good correlation with the predicted frequencies for the isotopomers. It is also consistent with the previously studied Rh and Ir carbenes in the doublet ground states.11 In contrast, the frequencies for the quartet state differ significantly from the observed values (e.g., the B3LYP computed CCl2 symmetric and antisymmetric frequencies are 835 and 809 cm-1). Co + CFCl3 and Co + CF2Cl2. Figure 2 shows infrared spectra from reactions of Co atoms with the chlorofluorocarbones (CFC), where parallel to the CCl4 spectra, only m absorptions are observed. In the CFCl3 spectra, the product absorptions halve, increase to an intensity ∼30% higher than the original intensity, and remain almost the same on visible, uv, and full arc irradiations, respectively. They further increase (∼40%) in the early stage (26 K) of annealing and later gradually increase until the matrix evaporates. Similar to the CCl4 case, the m absorptions at 1226.0 and 955.2 cm-1 (with satellites at 1223.9 and 953.7 cm-1) in the C-Cl and C-F stretching regions23 suggest that a primary product with a CFCl moiety is formed in reaction of Co with CFCl3. They are assigned to the C-Cl and C-F stretching modes of CFCldCoCl2, and the weak m absorptions at 693.5, 594.2, and 461 cm-1 are designated to the CFCl scissoring, CFClCo deformation, and C-Co streatching modes on the basis of correlation with the DFT values (Table 2). The observed five absorptions are consistent with the predicted values for CFCldCoCl2 in the doublet ground state and substantiate formation of the F containing Co carbene. The m absorption intensities in the CF2Cl2 spectra, on the other hand, remain unchanged, triple, and increase further ∼100% (in total 400%) on visible, uv, and full arc photolysis, respectively. They sharpen up in the early stage of annealing and later gradually decrease. Similar to the CCl4 and CFCl3 cases, the product absorptions at 1292.8 and 1271.9 cm-1 (with the satellites at 1290.8 and 1270.0 cm-1, respectively) are assigned to the CF2 symmetric and antisymmetric stretching

modes of CF2dCoCl2. Another product absorption at 474.3 cm-1 is designated to the C-Co stretching mode. Table 2 shows the good correlation between the observed and predicted values. As for the CCl4 case, free radicals and molecular ions derived from the CFC precursor photochemistry provide additional absorptions that are common to different metal experiments.28 No CCl2dCoFCl and CFCldCoFCl absorptions are observed in these experiments, which is similar to the previously studied Rh case, in contrast to the Ir case.11 The carbenes CFCldCoCl2 and CCl2dCoFCl in the doublet ground states are 71 and 58 kcal/mol more stable than the reactants (Co(4F) + CFCl3), and CF2dCoCl2, and CFCldCoFCl in the doublet ground states are 65 and 53 kcal/mol, respectively, more stable than their reactants (Co(4F) + CF2Cl2). The energy differences of 13 and 12 kcal/ mol between the conformers are in fact smaller than those in the Rh (20 and 25 kcal/mol) and Ir systems (16 and 18 kcal/ mol), suggesting that F migration following C-Cl bond insertion is considerably more difficult than Cl migration in the Co system. Again the quartet states are energetically comparable (CFCldCoCl2 (Q) and CF2dCoCl2 (Q) are 73 and 67 kcal/mol, respectively, more stable than the reactants), but the products are most probably in the doublet ground states on the basis of the much better correlation between the observed and predicted values. For example, the CX2 symmetric and antisymmetric stretching frequencies in the quartet states are substantially lower (more than 40 cm-1 on average) than those in the doublet states. Parallel to the CCl4 case, the F containing carbynes and insertion complexes are not identified in the spectra, and it is again consistent with the fact that attempted geometry optimizations of these complexes gave structures of the small carbene complexes in both the doublet and quartet states. Apparently the Co methylidenes with C-Co double bonds are the only energy minima in reactions of Co atoms with tetrahalomethanes, resulting in exclusive generation of the tetrahalo Co methylidenes. Co + CHCl3. Shown in Figure 3 are the product absorptions in the infrared spectra from reactions of Co with CHCl3 isotopomers. The product absorptions (all marked with “m”) are weaker relative to the tetrahalomethane cases, and they remain unchanged, increase more than 20%, and slightly decrease on visible, uv, and full arc irradiations, respectively. They gradually decrease on annealing. The product absorptions are believed to arise from CHCldCoCl2 on the basis of its stability relative to other plausible products and good agreement with the predicted frequencies and isotopic shifts as shown in Table 3.

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Cho and Andrews

Figure 3. Infrared spectra in the 1000-450 cm-1 region for the reaction products of the laser-ablated cobalt atom with CHCl3 isotopomers in excess argon at 10 K. (a) Co and CHCl3 (0.5% in argon) codeposited for 1 h, (b) as (a) after visible irradiation (λ > 420 nm), (c) as (b) after uv irradiation (240 < λ < 380 nm), (d) as (c) after full arc irradiation (λ > 220 nm), and (e) as (d) after annealing to 26 K. (f) Co and CDCl3 reagent (0.5% in argon) codeposited for 1 h, (g-j) as (f) spectra taken following the same irradiation and annealing sequence. m designates a product absorption, and “P” and “c” stand for the precursor and common absorptions.

The m absorption at 910.1 cm-1 is designated to the C-Cl stretching mode, and its D counterpart expected at ∼790 cm-1 is probably covered by precursor absorption. In the CDCl3 spectra the HCCo bending absorption is observed at 967.0 cm-1 instead, whereas its H counterpart is too weak to observe. The product absorption at 478.2 cm-1 (with a satellite at 475.7 cm-1) below the weak CoCl2 absorption27 at 493.3 cm-1 has its D counterpart at 476.5 cm-1 (with a satellite at 474.1 cm-1). It is assigned to the CoCl2 antisymmetric stretching mode on the basis of its frequency and small D shift. A weak product absorption at 611.4 cm-1 is assigned to the C-Co stretching mode without observation of the D counterpart. The insertion and carbyne complexes (CHCl2-CoCl and HCtCoCl3) are again not identified in the product spectra. Geometry optimization for the carbyne complex also leads to the structure of CHCldCoCl2, indicating that HCtCoCl3 is not a stable conformer. CHCldCoCl2(D) and CHCl2-CoCl(Q) are

Figure 4. Infrared spectra in the 850-450 cm-1 region for the reaction products of the laser-ablated cobalt atom with CH2Cl2 isotopomers in excess argon at 10 K. (a) Co and CH2Cl2 (0.5% in argon) codeposited for 1 h, (b-e) as (a) spectra taken following a sequence of visible, uv, and visible irradiations and annealing to 28 K. (f) Co and CD2Cl2 reagent (0.5% in argon) codeposited for 1 h, (g-i) as (f) spectra taken following a sequence of visible and uv irradiations and annealing to 28 K. (j) Co and 13CH2Cl2 reagent (0.5% in argon) codeposited for 1 h, (k-n) as (j) spectra taken following a sequence of visible, uv, and full arc irradiations and annealing to 28 K. i and m designate the product absorption groups, and “P” and “c” stand for the precursor and common absorptions.

63 and 61 kcal/mol more stable than the reactants, and the higher oxidation-state product is normally expected to be more stabilized in the matrix due to the more polarized bonds. Computations predict the strong C-Cl stretching band of CHCl2-CoCl at ∼750 cm-1, whereas unfortunately the area is covered by precursor absorption. In addition, the weaker C-H bending bands expected at ∼970 and ∼1200 cm-1 and C-Co

TABLE 3: Observed and Calculated Fundamental Frequencies of CHCldCoCl2 Isotopomers in the Ground 2A Statea CHCldCoCl2(D) approximate description C-H str. HCCo bend C-Cl str. C-H oop bend C-Co str. CoCl2 as. str. CoCl2 s. str. CoCCl bend ClCoC bend ClCoCl bend CoCl2 wag CHCl tort

obs

b

910.1 611.4 478.2, 475.7

B3LYP

d

3124.8 1158.0 915.8 693.7 608.6 458.9 341.6 222.2 158.5 106.7 94.3 46.5

int

d

26 6 130 14 26 97 10 6 5 3 5 2

CDCldCoCl2(D) BPW91

int

3053.7 1122.8 889.7 687.8 642.0 464.7 357.2 217.7 178.8 116.4 87.8 26.8

7 55 154 15 18 74 11 0 7 1 2 0

obs

b

967.0

476.5, 474.1

B3LYPd

intd

BPW91

int

2297.3 976.0 796.6 589.0 561.8 452.3 332.6 216.3 141.8 103.3 92.6 44.3

22 86 51 12 14 93 10 5 3 3 5 1

2232.4 965.5 767.4 599.2 574.8 457.4 347.8 213.3 156.1 113.8 86.3 24.6

5 168 42 11 10 70 10 0 5 1 2 0

a Frequencies and intensities are in cm-1 and km/mol. CHCldCoCl2 has a C1 structure with a ClCCoCl dihedral angle of 59.1°. b Observed in an argon matrix. c Frequencies computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/ 6-311++G(3df,3pd).



Figure 6. Infrared spectra in the 1300-500 cm-1 region for the reaction products of the laser-ablated cobalt atom with CH2F2 isotopomers in excess argon at 10 K. (a) Co and CH2F2 (0.5% in argon) codeposited for 1 h, (b-e) as (a) spectra taken following a sequence of visible, uv, and full arc irradiations and annealing to 28 K. (f) Co and CD2F2 reagent (0.5% in argon) codeposited for 1 h, (g-j) as (f) spectra taken following the same irradiation and annealing sequence. m designates a product absorption while P and c stand for the precursor and common precursor product absorptions.

Figure 5. Infrared spectra in the 1300-500 cm-1 region for the reaction products of laser-ablated cobalt atoms with CH2FCl isotopomers in excess argon at 10 K. (a) Co and CH2FCl (0.5% in argon) codeposited for 1 h, (b-f) as (a) spectra taken following a sequence of visible (λ > 420 nm), uv (240 < λ < 380 nm), visible, and uv irradiations and annealing to 28 K. (g) Co and CD2FCl reagent (0.5% in argon) codeposited for 1 h, (h-l) as (g) spectra taken following the same irradiation and annealing sequence. The i, i′ and m labels designate the product absorption groups, and “P” stands for the precursor absorption. “CH4” indicates methane absorption, which is produced due to cracking of silicone diffusion pump oil vapor.

stretching band at ∼550 cm-1 are also not observed, indicating that CHCl2-CoCl is not produced in an amount observable with the weaker bands. The DFT calculations also suggest that the Co carbene in the quartet state is 7 kcal/mol more stable than in the doublet state; however, its vibrational characteristics are not in agreement with the observed values. For example, the strong CoCl2 antisymmetric stretching and CCoCl2 deformation bands expected at ∼550 and 710 cm-1 are not observed, showing that CHCl-CoCl2(D) is the only primary product. Co + CH2X2. Product absorptions from reactions of Co with CH2Cl2, CH2FCl, and CH2F2 isotopomers are shown in Figure 4-6, where both the methylidene (m) and insertion (i) absorptions are observed. It is compared with preferential generation of the carbenes in the Rh and Ir systems.11 Parallel to the tetraand trihalomethanes cases, no carbyne products are identified, and DFT calculations also show that they are not stable conformers. In the CH2Cl2 isotopomer spectra (Figure 4), the m absorptions slightly decrease, increase to an intensity ∼40%

higher than the original intensity, and remain unchanged on visible, uv, and full arc photolysis, respectively. The i absorptions show no intensity change, increase ∼30%, and slightly increase on visible, uv, and full arc irradiations, respectively. They sharpen up in the early stage of annealing and later gradually decrease. The strong m absorption at 478.8 cm-1 has satellites at 475.8 and 472.7 cm-1, and the absorptions, which are separated by 3.0 and 3.1 cm-1, have approximately 9:6:1 relative intensites (the statistical ratio between two 35Cl, one 35Cl and one 37Cl, and two 37Cl in natural abundance). They also show a small D (1.3 cm-1) and no 13C shifts, suggesting that they originate from a primary product with a CoCl2 moiety, as compared to CoCl2 itself at 493.4, 490.2, 487.2 cm-1,27 which is most probably CH2dCoCl2. This band is designated to the CoCl2 antisymmetric stretching mode. Although the B3LYP frequency and D shift (459.3 and 9.8 cm-1) are somewhat lower and higher than the observed values, the BPW91 values of 469.2 and 0.6 cm-1 are in a much better agreement. The weak m absorption at 775.0 cm-1 in the 13CH2Cl2 spectra is assigned to the CH2 wagging mode while its H counterpart is covered by precursor absorption. Unfortunately, all other CH2dCoCl2 bands are too weak to be observed as shown in Table 4. Evidently another primary product is responsible for the i absorptions. The i absorption at 597.2 cm-1 has its D and 13C counterparts at 559.3 and 584.0 cm-1 (H/D and 12/13 ratios of 1.068 and 1.023), and the relatively small D and large 13C shifts lead to an assignment to the C-Cl stretching mode. Its low frequency suggests that the C-Cl bond is relatively weak. The previous studies of late transition-metal reactions with haloalkanes show that many small insertion complexes have bridged structures.9-11 Similarly, computations show that CH2Cl-CoCl also has a bridged structure with bridging Cl as shown in Figure 7. As a result, the C-Cl stretching mode not only has a relatively low frequency but is coupled with the C-Co stretching mode as well.

478.8, 475.8, 472.7

775.0

0 0 4 10 0 2 76 6 6 0 1 0 2322.8 2151.1 1011.7 686.2 669.3 482.6 468.6 362.0 173.8 124.9 106.4 83.5 1 5 1 10 12 13 96 5 0 7 4 5 477.5, 474.6, 471.8

2406.1 2225.0 979.2 646.9 610.7 519.7 449.5 344.8 166.5 153.6 102.1 55.3 0 0 6 20 0 5 76 6 7 0 0 0 3117.5 2982.1 1281.1 861.8 751.0 638.3 469.2 362.8 191.0 125.6 118.2 109.1 1 7 0 21 6 13 106 5 0 9 3 7 covered

3228.2 3086.7 1297.1 817.7 676.9 656.1 459.3 345.6 207.8 167.8 103.9 62.6 B as. CH2 str. A s. CH2 str. A CH2 scis. B CH2 wag B CH2 rock A C-Co str. B as. CoCl2 str. A s. CoCl2 str. A CH2 twist B CoCl2 rock B CoCl2 wag A CoCl2 bend

632.9? 478.8, 475.8, 472.7

int BPW91 intd B3LYPd approximate description

obsb

B3LYPd

intd

BPW91

int

obsb

CD2dCoCl2(D) CH2dCoCl2(D)

a Frequencies and intensities are in cm-1 and km/mol. CH2dCoCl2 has a C2 structure with two equal Co-Cl bonds. b Observed in an argon matrix. Strongest absorptions in bold type. c Frequencies computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd).

0 0 6 21 0 5 76 6 7 0 0 0 3104.3 2977.9 1272.1 853.3 733.0 634.7 469.2 362.4 186.9 125.3 118.1 108.7 1 7 0 22 6 13 106 5 0 8 3 7 3214.5 3082.4 1291.3 810.0 673.4 638.8 459.3 345.3 207.5 164.1 103.6 62.5

intd B3LYPd obsb

13CH2dCoCl2(D)

BPW91

int


TABLE 4: Observed and Calculated Fundamental Frequencies of CH2dCoCl2 Isotopomers in the Ground 2A Statea

8062

Cho and Andrews The i absorption at 578.8 cm-1 has 13C counterpart at 575.9 cm-1 and is assigned to the CH2 rocking mode on the basis of the small 13C shift, whereas the D counterpart is too low in frequency to observe. Another i absorption at 530.0 cm-1 shows a small D and sizable 13C shifts of 16.0 and 11.3 cm-1, and it is designated to the C-Co stretching mode, which is coupled with the C-Cl stretching mode. The observed frequencies are compared with the DFT frequencies computed for CH2Cl-CoCl in the quartet ground state. The DFT frequencies show a reasonable agreement with the observed values, and the BPW91 CH2 rocking frequency (470.9 cm-1) is somewhat lower than the observed and B3LYP values. CH2dCoCl2(D) and CH2Cl-CoCl(Q) are 49 and 61 kcal/mol more stable than the reactants. The m absorptions in the CH2FCl spectra (Figure 5), which are relatively weak, halve and recover repeatedly on visible and uv irradiations, and they sharpen up in the early stage of annealing and later gradually decrease. In addition to the m absorptions, two more groups of absorptions (i and i′) are observed. The i absorptions also halve and recover on visible and uv irradiations, and gradually decrease on annealing. The i′ absorptions, on the other hand, double on the first visible irradiation and decrease ∼30% on the following uv irradiation. They increase another 100% (to three times the original intensity) on the second visible irradiation and decrease back to their intensity after the first uv irradiation on the second uv irradiation. The photoreversible intensity variations strongly suggest that interconversion occurs between the primary product responsible for the i′ absorptions and those for the i and m absorptions. The m absorption at 645.0 cm-1 has its D counterpart at 533.3 cm-1 (H/D ratio of 1.210), and its frequency and large D shift with a good correlation with computed values (e.g., B3LYP values of 651.7 and 102.9 cm-1) lead to an assignment to the CH2 rocking mode of CH2dCoFCl. Another m absorption at 854.5 cm-1 is designated to the CH2 wagging mode, and its D counterpart expected at ∼660 cm-1 is believed to be covered by precursor absorption. The strong one at 685.5 cm-1 (with a satellite at 687.6 cm-1) in the CD2FCl spectra is assigned to Co-F stretching mode (Table 6) whereas its H counterpart, expected much weaker at ∼750 cm-1, is most probably covered by precursor absorption. The Co-F stretching mode is coupled with the CH2 rocking mode in its planar structure (Figure 7), and deuteration substantially increases the Co displacement, making it vibrate back and forth between F and Cl and dramatically increasing the absorption intensity as well. The strongest i absorption is observed at 952.8 cm-1, and deuteration moves it to 912.6 cm-1 (H/D ratio of 1.044). It is assigned to the C-F stretching mode on the basis of the frequency, relatively small D shift, and strong intensity. The i absorption at 1208.8 cm-1 has it D counterpart at 967.6 cm-1 (H/D ratio of 1.249) and is designated to the CH2 wagging mode on the basis of the frequency and large D shift. Another i absorption is observed at 551.6 cm-1 (with satellites at 558.7 and 546.4 cm-1) along with its D counterpart at 517.7 cm-1 (H/D ratio of 1.065), and it is assigned to the C-Co stretching mode on the basis of the frequency and relatively small D shift. The most probable primary product that carries C-F, CH2, C-Co moieties is CH2F-CoCl and the predicted frequency and D shift are in a reasonable agreement with the observed values as shown in Table 7; for example, the B3LYP frequencies of 547.8, 959.8, and 1224.1 cm-1 show less than 2% discrepancies from the observed values. Another i absorption at 1048.1 cm-1



Figure 7. The B3LYP computed structures of the identified Co methylidene and insertion products in the doublet ground states with the 6-311++G(3df,3pd) basis sets for H, C, F, Cl, and Co. Bond distances and angles are in Å and degrees. The tetra- and trihalomethylidenes have allene-type staggered structures, and the F-containing dihalo methylidenes have planar structures. CHCldCoCl2 and CH2dCoCl2 have C1 and C2 symmetry (the ClCCoCl and HCCoCl dihedral angles are shown). The molecular symmetry is denoted under the structure.

TABLE 5: Observed and Calculated Fundamental Frequencies of CH2Cl-CoCl Isotopomers in the Ground 4A′′ Statea CH2Cl-CoCl(D) approximate description A′′ CH2 as. str. A′ CH2 s. str. A′ CH2 scis. A′ CH2 wag A′′ CH2 twist A′ C-Cl str. A′′ CH2 rock A′ C-Co str. A′ Co-Cl str. A′ CoCCl bend A′′ CCoCl oop bend A′ CCoCl ip bend

obs

b

597.2 578.8 530.0

B3LYP

d

3174.3 3093.4 1423.2 1081.4 1038.4 588.1 569.8 547.1 379.9 140.6 87.4 61.1

int

d

0 4 2 4 0 63 23 21 51 5 6 4

13

CD2Cl-CoCl(D)

BPW91

int

3109.1 3023.1 1368.7 1021.0 1001.1 606.0 470.9 534.7 379.8 152.7 55.7 59.1

0 5 2 7 0 47 29 14 43 10 7 2

obs

b

559.3 514.0

B3LYP

d

2358.0 2240.7 1050.5 840.5 755.6 563.0 428.4 515.5 372.4 140.1 82.6 60.8

int

d

0 3 3 0 0 49 14 42 41 15 6 4

BPW91

int

2309.2 2188.0 1010.9 798.7 731.1 581.1 371.5 500.9 346.1 152.2 58.8 52.4

0 3 2 1 0 42 34 27 16 10 2 7

obs

b

584.0 575.9 518.7

CH2Cl-CoCl(D)

B3LYPd

intd

BPW91

int

3161.8 3087.9 1419.0 1074.3 1036.5 574.1 566.7 534.9 377.5 140.0 86.6 61.1

0 4 2 4 0 64 23 21 48 15 6 4

3096.9 3017.9 1364.7 1013.9 999.0 590.8 469.2 523.5 377.3 152.2 59.1 54.7

0 5 2 8 0 46 29 14 41 10 2 7

Frequencies and intensities are in cm-1 and km/mol. CH2Cl-CoCl has a Cs structure. b Observed in an argon matrix. computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd). a

c

Frequencies

8064


Cho and Andrews

TABLE 6: Observed and Calculated Fundamental Frequencies of CH2dCoFCl Isotopomers in the ground 2A′′ Statea CH2dCoFCl(D) approximate description A′ as. CH2 str. A′ s. CH2 str. A′ CH2 scis. A′′ CH2 wag A′ Co-F str. A′ C-Co str. A′ CH2 rock A′ Co-Cl str. A′ CCoF bend A′ CCoCl bend A′′ CoFCl wag A′′ CH2 twist

obs

b

B3LYP

854.5 covered 645.0

d

3251.9 3101.6 1298.6 841.2 763.9 663.3 651.7 400.2 230.6 182.2 133.8 121.5

int

d

0 3 2 41 25 10 121 19 11 9 1 10

CD2dCoFCl(D) BPW91

int

3148.9 3009.0 1300.5 911.8 756.5 729.2 639.1 401.9 224.9 186.4 151.8 118.6

2 0 4 35 15 2 105 14 8 6 0 6

obs

b

covered 685.5 533.3

B3LYPd

intd

BPW91

int

2425.7 2235.8 980.2 658.5 686.7 619.4 548.8 397.4 220.4 174.3 122.6 94.1

1 2 2 23 130 8 16 21 10 7 11 0

2347.4 2171.7 1005.5 715.4 669.9 665.5 546.0 399.0 215.5 177.6 119.7 106.8

2 1 4 19 104 2 15 15 8 5 6 1

a Frequencies and intensities are in cm-1 and km/mol. CH2dCoFCl(T) has a planar Cs structure. b Observed in an argon matrix. c Frequencies computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd).

TABLE 7: Observed and Calculated Fundamental Frequencies of CH2F-CoCl Isotopomers in the 4A′′ Statea CH2F-CoCl(Q) A′′ CH2 as. str. A′ CH2 s. str. A′ CH2 scis. A′ CH2 wag A′′ CH2 twist A′ C-F str. A′′ C-Co str. A′ CH2 rock A′ Co-Cl str. A′ CoCF bend A′ CCoCl ip bend A′′ CCoCl oop bend

CD2F-CoCl(Q)

obsb

B3LYPd

intd

BPW91

int

952.8, 930.5 558.7, 551.6, 546.4

3064.5 3008.7 1431.3 1224.1 1212.3 959.7 547.8 492.7 386.7 176.8 62.0 45.2

12 14 4 49 4 177 56 20 65 9 8 9

3024.4 2935.8 1383.2 1148.3 1165.6 947.4 528.0 488.5 384.3 155.2 51.1 43.4

8 10 3 41 5 217 8 33 36 3 4 6


1208.8

obsb

1048.1 967.6 912.6 517.7

B3LYPd

intd

BPW91

int

2271.5 2178.5 1057.2 967.7 900.5 931.5 504.1 380.8 377.9 175.9 61.4 44.4

5 9 17 70 2 107 74 36 26 9 8 10

2242.3 2124.3 1028.8 904.2 869.7 920.4 466.7 409.8 358.4 153.5 52.7 43.3

4 7 32 55 1 143 34 29 12 3 4 6

a Frequencies and intensities are in cm-1 and km/mol. CH2F-CoCl has a Cs structure. b Observed in an argon matrix. computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd).

c

Frequencies

TABLE 8: Observed and Calculated Fundamental Frequencies of CH2Cl-CoF Isotopomers in the 4A′′ Statea CH2Cl-CoF approximate description A′′ CH2 as. str. A′ CH2 str. A′ CH2 scis. A′ CH2 wag A′′ CH2 twist A′ C-F str. A′ C-Cl str. A′′ CH2 rock A′ C-Co str. A′ ClCCo bend A′ CCoFip bend A′′ CCoF oop bend

obsb

669.5 594.6 572.4 .

CD2Cl-CoF

B3LYPd

intd

BPW91

int

3181.6 3102.6 1413.8 1092.0 1032.2 668.5 568.1 567.5 502.8 136.7 84.9 28.5

1 2 3 0 0 141 14 19 26 27 6 13

3103.9 3020.5 1370.4 1036.4 1000.6 645.9 591.9 564.3 508.6 156.7 79.5 113.9

1 5 2 1 0 141 15 18 18 15 4 6

obsb

B3LYPd

intd

BPW91

int

658.0 556.5

2362.8 2248.9 1042.4 847.1 748.8 654.9 546.9 426.3 473.7 136.2 84.5 28.5

0 1 5 1 0 141 20 12 13 27 6 14

2306.0 2187.0 1012.9 812.1 729.8 631.4 574.7 427.8 470.5 156.1 79.2 107.8

0 3 3 2 0 142 23 11 5 15 4 6

Frequencies and intensities are in cm-1 and km/mol. CH2Cl-CoF has a Cs structure. b Observed in an argon matrix. computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd). a

in the CD2FCl spectra is assigned to the CD2 scissoring mode without observation of the H counterpart. Another group of product absorptions marked i′ is also shown in Figure 5. The strongest i′ absorption at 669.5 cm-1 is accompanied with its equally strong D counterpart at 658.0 cm-1 (H/D ratio of 1.016). The distinctively strong product absorption with a relatively small D shift is most probably a Co-F stretching band, and among the plausible products, only CH2Cl-CoF carries a Co-F stretching band consistent with the observed frequency, intensity, and isotopic shifts. The

c

Frequencies

B3LYP frequency and D shifts are 668.5 and 13.6 cm-1 (Table 8). Mode analysis shows that the Co-F stretching mode is strongly coupled with the C-Co stretching mode, making it essentially a C-Co-F antisymmetric stretching mode. On the red side of the strong Co-F stretching absorption, two weak i′ absorptions are observed at 594.6 cm-1 (with D counterpart at 556.5 cm-1) and 572.4 cm-1, and they are designated to the C-Cl stretching and CH2 rocking modes. The two modes are substantially mixed, and the D counterpart of the CH2 rocking mode is too low in frequency to observe.



TABLE 9: Observed and Calculated Fundamental Frequencies of CH2dCoF2 Isotopomers in the 2A2 Statea CH2dCoF2(D) approximate description B2 CH2 as. str. A1 CH2 s. str. A1 CH2 scis. B1 CH2 wag B2 CoF2 as. str. B2 CH2 rock A1 C-Co str. A1 CoF2 s. str. A1 CoF2 scis. B2 CoF2 rock A2 CH2 twist B1 CoF2 wag

obs

b

B3LYP

d

3251.1 3099.5 1293.4 845.7 787.0 691.1 674.8 605.5 235.8 219.8 179.9 146.5

781.0 775.4 676.4 covered

int

d

CD2dCoF2(D) BPW91

int

3150.2 3007.9 1293.1 911.7 767.0 678.8 729.6 592.6 231.7 213.2 176.4 139.8

1 0 2 42 66 88 0 5 15 5 0 11

0 2 1 49 93 89 13 11 19 6 0 18

obs

b

600.5 744.9 539.7

B3LYP

d

2425.8 2234.2 981.9 663.0 750.0 554.4 627.6 605.1 234.1 200.5 145.7 127.9

intd

BPW91

int

0 2 2 28 170 13 14 9 19 4 18 0

2349.3 2170.2 1004.2 715.8 726.0 549.1 662.3 592.4 230.1 194.2 125.6 139.1

1 1 1 23 143 11 0 8 14 3 0 11

a Frequencies and intensities are in cm-1 and km/mol. CH2dCoF2 has a C2v structure with two equal Co-F bonds. b Observed in an argon matrix. c Frequencies computed with B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd).

The identified primary products are energetically comparable: CH2dCoFCl(D), CH2F-CoCl(Q), and CH2Cl-CoF(Q) are 40.1, 48.5, and 46.4 kcal/mol more stable than the reactants (4F + CH2FCl). Previous studies on reactions of metal atoms with small haloalkanes suggest that C-Cl bond insertion occurs first with electron-rich Cl attracting the metal atom and generating the insertion complex (CH2F-CoCl).6-11 Subsequent F migration occurs from C to Co during codeposition and following photolysis to form the Co methylidene (CH2dCoFCl). In addition, the other identified insertion complex (CH2Cl-CoF) and observed photo interconversions reveal that forward and reverse Cl migrations occur interchanging the carbene and CH2Cl-CoF. Visible irradiation favors CH2Cl-CoF whereas uv photolysis promotes CH2F-CoCl and CH2dCoFCl (reaction 1). Thus, a dynamic photochemical equilibrium can be established among these three product species of comparable stability. uv

Co(4F) + CH2FCl f CH2F-CoCl {\} vis

uv

CH2dCoFCl {\} CH2Cl-CoF (1) vis

In Figure 6 (Co + CH2F2), two groups of product absorptions (marked “m” and “i”) are shown from Co reactions with CH2F2 isotopomers. The m absorptions are in general weaker than the i absorptions. The m absorptions show a ∼20% decrease, no change, and an increase to an intensity ∼10% higher than the original intensity on visible, uv, and full arc irradiations, respectively. They sharpen on the early stage of annealing and later gradually decrease. The i absorptions increase about 5, 30, and 5% (in total ∼40%) on visible, uv, and full arc photolysis, respectively, and they gradually decrease in annealing. The strongest m absorption at 775.4 cm-1 and its D counterpart at 744.9 cm-1, on the basis of the frequency position and small D shift, are assigned to the CoF2 antisymmetric stretching mode of CH2dCoF2(D), which is 29 kcal/mol more stable than the reactants (Co(4F) + CH2F2). On the red side, an almost as strong m absorption and its D counterpart are observed at 676.4 and 539.7 cm-1 (H/D ratio of 1.253), and the large D shift leads to an assignment to the CH2 rocking mode. The CoF2 antisymmetric stretching and CH2 rocking modes are substantially coupled, leading to intensity borrowing from the stronger former mode to the latter. The CH2 wagging absorption, which is expected to be third-strongest, is observed at 781.0 cm-1, and its D counterpart is observed at 600.5 cm-1 (H/D ratio of

1.301). Table 9 shows a reasonable correlation between the observed and DFT values. For example, the observed frequencies of 781.0, 775.4, and 676.4 and their D shifts of 180.5, 30.5, and 136.7 cm-1 are compared with the B3LYP frequencies of 845.7, 787.0, and 691.1 cm-1 and the D shifts of 182.7, 37.0, and 136.7 cm-1, respectively. The distinctively strong i absorption at 655.4 cm-1 with D counterpart at 650.0 cm-1 (H/D ratio of 1.008), and its strong intensity, frequency, and very small D shift lead to a designation to the Co-F stretching mode. The second strong one is observed at 947.7 cm-1 in the C-F stretching region, and deuteration moves it to 908.5 cm-1 (H/D ratio of 1.043), leading to an assignment to the C-F stretching mode. The primary product with C-F and Co-F moieties is most probably CH2F-CoF(Q), which is 34 kcal/mol more stable than the reactants. Another i absorption at 1214.1 cm-1 is accompanied with its D counterpart at 945.4 cm-1 (H/D ratio of 1.284), and it is designated to the CH2 wagging mode. The observed values have a good correlation with the predicted values as shown in Table 10 (for instance, the B3LYP frequencies of 1214.1, 947.7, and 655.4 cm-1 and their dyz shifts of 258.6, 39.2, and 5.4 cm-1). The absorption at 591.9 cm-1 shows the same intensity variation on photolysis and annealing, but its frequency differs significantly from the predicted values. We tentatively assign it to the CH2 rocking mode (predicted at 499.5 cm-1) without observation of its D counterpart. Structures and Bonding in Small Model Co Complexes. The structures of the Co carbene and insertion complexes identified and calculated in this study are illustrated in Figure 7. These Co carbene and insertion complexes are believed to have the doublet and quartet ground states, respectively, in line with the previous Rh and Ir cases.11 As described above, the C-Co bond lengths computed for the carbene complexes (1.732-1.764 Å) are considerably shorter than the 1.80-2.10 Å values previously reported for Co complexes and attributed to C-Co double bonds, which shows that these small Co carbenes are good model systems for the C-Co double bond. Our computed C-Co double bond lengths are nearly the same as those calculated for similar Ni complexes.10a The tetrahalo Co carbenes all have allene-type staggered conformations with the X-C-Co-X dihedral angle of 90°, and CHCldCoCl2 and CH2dCoCl2 also have staggered structures but with dihedral angles of ∼60°. On the other hand, CH2dCoFCl and CH2dCoF2 have planar structures, unlike the Rh and Ir dihalocarbenes, which all have allene-type staggered conformations. Planar late transition-metal

8066


Cho and Andrews

TABLE 10: Observed and Calculated Fundamental Frequencies of CH2F-CoF Isotopomers in the 4A” Statea CH2F-CoF(Q) approximate description A′′ CH2 as. str. A′ CH2 s. str. A′ CH2 scis. A′ CH2 wag A′′ CH2 twist A′ C-F str. A′ Co-F A′ C-Co str. A′′ CH2 rock A′ CoCF bend A′ FCoC ip bend A′′ FCoC oop bend

obs

b

1214.1 947.7 655.4 591.9

B3LYP

d

3193.7 3088.6 1448.5 1178.8 1113.1 783.9 668.5 646.9 559.8 287.9 155.4 116.0

int

d

1 7 1 33 3 110 66 10 35 4 13 13

CD2F-CoF(Q) BPW91

int

2990.3 2927.7 1383.3 1175.5 1165.1 944.8 640.0 494.7 479.4 170.0 80.0 74.5

14 16 2 21 3 125 179 1 19 13 9 9

obs

b

945.4 908.5 650.0

B3LYPd

intd

BPW91

int

2378.4 2232.3 1075.0 931.8 833.5 753.7 666.8 519.0 478.8 287.4 151.5 114.6

1 5 7 15 2 109 76 21 6 5 12 13

2217.0 2117.8 1025.1 938.9 868.8 912.5 633.6 449.8 369.3 168.7 79.7 75.5

7 10 17 46 1 66 182 1 12 13 9 10

a Frequencies and intensities are in cm-1 and km/mol. b Observed in an argon matrix. B3LYP/6-311++G(3df,3pd). d Frequencies and intensities computed with BPW91/6-311++G(3df,3pd).

c

Frequencies

computed

with

CCl2dIrCl2 σ (1.94980), π (1.90836), Ir 5dyz (0.99666) σ* (0.04594), π* (0.09256); Ir, C, Cl (0.00668) Fluorine substitution increases the effective bond orders to 1.74 and 1.80 stepwise with the number of fluorine substituents on the carbon center in CFCldCoCl2 and CF2dCoCl2, respectively, with the following occupancies, where the effect appears to increase both bonding and decrease both antibonding orbital occupancies:

Figure 8. CAS orbitals computed for CCl2dRhCl2 at the B3LYP minimum and plotted with isodensity 0.03 e/Å3.

carbenes are reported in a recent matrix infrared study for Pt reactions with halomethanes, where the F containing tri-, di-, and monohalocarbenes have planar structures.10d Introduction of F elongates the C-Co bond as well probably because the higher electronegativity F contracts the carbon 2p orbitals and weakens the carbon-metal bond. For instance, the C-Co bond lengths of CCl2dCoCl2 and CF2dCoCl2 are 1.740 and 1.757 Å, and those of CH2dCoCl2 and CH2dCoFCl are 1.745 and 1.764 Å. CASSCF calculations determine a 1.67 effective bond order for the CCl2dCoCl2 complex, which may be compared with 1.81 computed by CASPT2 for the CCl2dNiCl2 complex10a or 1.83 recomputed here by CASPT2. Similar CASSCF calculations for CCl2dRhCl2 and CCl2dIrCl2 find slightly larger bond orders 1.83 and 1.86, respectively, which are in line with the CASPT2 values of 1.83 and 1.86 computed here or 1.89 and 1.89, respectively computed earlier for CCl2dPdCl2 and CCl2dPtCl2.10 The orbital occupancies for the five active electrons in the six orbitals are as follows (note that the sigma and pi bonding and antibonding occupancies result in the effective bond orders given above), and the CAS orbitals for CCl2dRhCl2 are illustrated in Figure 8. The CAS orbitals for CCl2dCoCl2 and CCl2dIrCl2 are similar. CCl2dCoCl2 σ (1.85757), π (1.80967), Co 3dyz (0.99434) σ* (0.14346), π* (0.18831); Co, C, Cl (0.00755) CCl2dRhCl2 σ (1.93221), π (1.90006), Rh 4dyz (0.99634) σ* (0.06301), π* (0.10155); Rh, C, Cl (0.00683)

CFCldCoCl2 σ (1.88089), π (1.85386), Co 4dyz (0.99269) σ* (0.11913), π* (0.14462); Rh, C, Cl (0.00880) CF2dCoCl2 σ (1.90684), π (1.89592), Co 3dyz (0.99258) σ* (0.09229), π* (0.10299); Co, C, Cl (0.00937) The Co insertion complexes with C-Cl bonds have bridged structures as illustrated in Figure 7, with Cl being located above the C-Co bond, due to interaction with the electron-deficient metal center. Similar structures with bridging Cl have often been observed from late transition-metal insertion complexes. On the other hand, a bridged structure with bridging F, similar to that of previously studied CH2F-NiCl,10a is not observed, indicating that the interaction between F and the metal center is not strong enough for such distortion. The C-Co bonds (1.951-1.968 Å) are markedly longer than those of the carbenes while they are compared with typical C-Co single bond lengths of ∼2.0 Å.29 Reactions. Transition-metal reactions with halomethanes are believed to proceed through C-X bond insertion to form the insertion complexes, probably on the quartet potential energy surface, and following X migration from C to M, to generate the high oxidation-state complexes during codeposition and subsequent photolysis.6 Thus, the carbene complex is probably produced first in the excited quartet state, and then relaxed to the doublet ground state by the cold matrix. In the cobalt system, reactions with tetrahalomethanes lead to exclusive formation of the carbenes, C-X bond insertion and X migration being a continuous process to form the small Co carbene.

Co* + CX4 98 CX2dCoX2 CX3-CoX*

(2)

Only the Co carbenes with Co-Cl bonds are identified in the tetra- and trihalomethane spectra. Evidently, C-Cl bond insertion and subsequent Cl migration are considerably more efficient

Infrared Spectra of CX2dCoX2 and CX3-CoX Complexes than corresponding processes with F or H both during codeposition and in the process of subsequent photolysis. In contrast, in reactions of dihalomethanes the insertion complexes are identified as well as the carbenes (3), in line with the fact that the two primary products become the most stable conformations. As described above, the identification of CH2Cl-CoF and the observed photoreversibility in the Co + CH2FCl system demonstrate conversion of CH2F-CoCl f CH2dCoFCl f CH2Cl-CoF and the reverse on visible and uv irradiations, respectively, probably on the quartet potential energy surface.

Co* + CH2X2 f CH2X-CoX* f CH2dCoX2

(3)

Conclusions Reactions of laser-ablated Co atoms with halomethanes have been carried out, and the products are identified from the matrix IR spectra on the basis of frequencies, isotopic shifts, and correlation with the DFT results. Unlike the Rh and Ir cases,11 no Co carbynes are identified. Instead, only carbene complexes are identified in the tetra- and trihalomethane spectra, whereas both the carbene and insertion products are observed in the dihalomethane spectra. Computations also reveal that only carbenes are meaningful energy minima in the tetrahalomethane systems, whereas both carbene and insertion complexes are comparably stable configurations in the tri- and dihalomethane cases. The calculated C-Co bond lengths of 1.732-1.764 Å for the identified carbene complexes are appropriate for the carbon-metal double bonds. These are much shorter than those for the previously synthesized Co complexes described with C-Co double bonds but with bond orders estimated to be considerably smaller than 2. CASSCF calculations provide a 1.67 bond order for the CCl2dCoCl2 complex, which may be compared with 1.83 and 1.86 for CCl2dRhCl2 and CCl2dIrCl2, respectively, or 1.81 as computed by CASPT2 for the CCl2dNiCl2 complex.10a The new Co carbenes with Co-C double bonds demonstrate that direct reactions of Co atoms with halomethanes are efficient means to produce Co complexes with Co-C double bonds, which have been little studied. Although the small Co carbenes in doublet ground states mostly have allene-type staggered structures, F-containing dihalocarbenes show planar structures. The CH2Cl-CoF insertion product and its photoreversible intensity variations in the Co + CH2FCl system indicate that reversible F and Cl-migrations occur for conversions between CH2Cl-CoF, CH2dCoFCl, and CH2FCoCl. 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 2009-0075428). References and Notes (1) (a) Crabtree, R. H. Chem. ReV. 1995, 95, 987, and references therein. (b) Wada, K.; Craig, B.; Pamplin, C. B.; Legzdins, P; Patrick, B. O.; Tsyba, I.; Bau, R. J. Am. Chem. Soc. 2003, 125, 7035. (c) Ujaque, G.; Cooper, A. C.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1998, 120, 361. (2) (a) Herndon, J. W. Coord. Chem. ReV. 2009, 253, 1517. (b) Herndon, J. W. Coord. Chem. ReV. 2009, 253, 86. (c) Herndon, J. W. Coord. Chem. ReV. 2007, 251, 1158. (d) Herndon, J. W. Coord. Chem. ReV. 2006,

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