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Organometallics 2009, 28, 3390–3394
(Acetylene)dicobalt Carbonyl Derivatives: Decarbonylation of the H2C2Co2(CO)6 Tetrahedrane Guoliang Li,† Qian-Shu Li,*,†,‡ Yaoming Xie,§ R. Bruce King,*,†,§ and Henry F. Schaefer III§ Center for Computational Quantum Chemistry, South China Normal UniVersity, Guangzhou 510631, People’s Republic of China, Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, People’s Republic of China, and Department of Chemistry and Center for Computational Chemistry, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed December 22, 2008
The (acetylene)dicobalt carbonyl complexes H2C2Co2(CO)n (n ) 6-4) have been studied using density functional theory. The preferred structure for H2C2Co2(CO)6 is predicted to be an eclipsed structure (C2V symmetry), with the staggered Cs structure as a transition state. An eq-H2C2Co2(CO)5 structure derived by loss of an equatorial carbonyl group from H2C2Co2(CO)6 is predicted to lie ∼10 kcal/mol below an ax-H2C2Co2(CO)5 structure derived by loss of an axial carbonyl group from H2C2Co2(CO)6. This is consistent with the experimental work of Bitterwolf, Scallorn, and Weiss (BSW) on the low-temperature Nujol matrix photolysis of H2C2Co2(CO)6, where ax-H2C2Co2(CO)5 was suggested to be an initial photoproduct at 90 K, which rearranges to the more stable eq-H2C2Co2(CO)5 upon annealing at 140 K. A H2C2Co2(CO)4(µ-CO) structure with a single bridging carbonyl group of intermediate energy is also predicted for H2C2Co2(CO)5 but not found experimentally by BSW. The lowest energy structures for the even more unsaturated H2C2Co2(CO)4 have two terminal carbonyl groups on each cobalt atom rather than three terminal carbonyl groups on one cobalt atom and only one terminal carbonyl group on the other cobalt atom. No structures with bridging carbonyl groups were found for H2C2Co2(CO)4. 1. Introduction The simplest stable closed-shell cobalt carbonyl is Co2(CO)8, which has the well-known C2V doubly bridged crystal structure.1-3 The (alkyne)hexacarbonyldicobalt derivatives R2C2Co2(CO)6 are readily obtained in good yield by reactions of Co2(CO)8 with alkynes under relatively mild conditions. Such reactions lead to replacement of the two bridging carbonyl groups with an alkyne ligand.4-6 They were first synthesized in 1956. Since then they have been extensively used in organic synthesis5,6 for reactions such as the Pauson-Khand synthesis of cyclopentenone derivatives.7 They are also of interest as building blocks for “molecular wires” based on polyyne structures.8 The structures of the R2C2Co2(CO)6 derivatives (Figure 1) are based on a central Co2C2 tetrahedrane with three terminal carbonyl groups on each cobalt and external R groups on each carbon. Since the RC vertices and the Co(CO)3 vertices each contribute three electrons to the skeletal bonding, the Co2C2 tetrahedron in the R2C2Co2(CO)6 derivatives has the 12 skeletal * To whom correspondence should be addressed. E-mail: rbking@ chem.uga.edu (R.B.K.);
[email protected] (Q.-S.L.). † South China Normal University. ‡ Beijing Institute of Technology. § University of Georgia. (1) Sumner, G. G.; Klug, H. P.; Alexander, L. E. Acta Crystallogr. 1964, 17, 732. (2) Leung, P. C.; Coppens, P. Acta Crystallogr. 1983, B39, 535. (3) Braga, D.; Grepioni, F.; Sabatino, P.; Gavezzotti, A. J. Chem. Soc., Dalton Trans. 1992, 1185. (4) Greenfield, H.; Sternberg, H. W.; Friedel, R. A.; Wotiz, J. H.; Markby, R.; Wender, I. J. Am. Chem. Soc. 1956, 78, 120. (5) Dickson, R. S.; Fraser, P. J. AdV. Organomet. Chem. 1974, 12, 323. (6) Went, M. J. AdV. Organometal. Chem. 1997, 41, 69. (7) Casalnuovo, J. A.; Schore, N. E. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Eds.; VCH: Weinheim, Germany, 1995, p 139. (8) Low, P. J.; Rousseau, R.; Lam, P.; Udachin, K. A.; Enright, G. D.; Tse, J. S.; Wayner, D. D. M.; Carty, A. J. Organometallics 1999, 18, 3885.
Figure 1. Structure of the (R2C2)2Co2(CO)6 derivatives, showing the axial (ax) and equatorial (eq) carbonyl groups.
electrons required for localized bonds along its six edges. These six two-center-two-electron bonds include four Co-C bonds, one C-C bond, and one Co-Co bond. In such a localized bonding model the Co-Co bond can be considered to be a formal single bond. However, a recent theoretical study9 suggests some singlet diradical character of the Co-Co interaction. The six carbonyl groups of the R2C2Co2(CO)6 derivatives are partitioned into two sets (Figure 1): namely, the two carbonyl groups nearly collinear with the Co-Co bond (the axial carbonyl groups) and the remaining four carbonyl groups (the equatorial carbonyl groups). Unsaturated R2C2Co2(CO)n derivatives (n < 6) are of interest in providing insight into the mechanisms of R2C2Co2(CO)6 reactions.5-7 In this connection Bitterwolf, Scallorn, and Weiss (BSW) have studied the photochemical decarbonylation of R2C2Co2(CO)6 derivatives in frozen Nujol at 90 K.10 They present evidence, based on infrared ν(CO) spectra, for two isomers of R2C2Co2(CO)5 in these low-temperature matrices. The R2C2Co2(CO)5 isomer initially formed at 90 K, conveniently designated as ax-R2C2Co2(CO)5, was suggested to have a structure in which an axial CO group is lost from the original (9) Platts, J. A.; Evans, G. J. S.; Coogan, M. P.; Overgaard, J. Inorg. Chem. 2007, 46, 6291. (10) Bitterwolf, T. E.; Scallorn, W. B.; Weiss, C. A. J. Organomet. Chem. 2000, 605, 7.
10.1021/om801211g CCC: $40.75 2009 American Chemical Society Publication on Web 05/07/2009
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Figure 2. Optimized structures of H2C2Co2(CO)n (n ) 6, 5). In Figures 2 and 3 the upper bond distances were determined by the B3LYP method and the lower bond distances were determined by the BP86 method. All structures are confirmed to be genuine (local) minima by the absence of significant imaginary vibrational frequencies.
R2C2Co2(CO)6 structure. This isomer was reported to rearrange to a second isomer upon heating to 140 K. This second isomer, conveniently designated as eq-R2C2Co2(CO)5, was suggested to have a structure derived from the original R2C2Co2(CO)6 structure by loss of an equatorial CO group. No experimental evidence was found by BSW for any R2C2Co2(CO)4(µ-CO) isomers with a bridging carbonyl group. This paper describes our theoretical studies on H2C2Co2(CO)n (n ) 6-4) using density functional theory (DFT). Formal CodCo double and CotCo triple bonds, respectively, are required to give both cobalt atoms the favored 18-electron configurations in H2C2Co2(CO)5 and H2C2Co2(CO)4. Such cobalt-cobalt multiple bonds within the Co2C2 tetrahedron should be recognizable by shorter cobalt-cobalt distances relative to that in H2C2Co2(CO)6. The unsubstituted derivatives, which were included in the experimental work by BSW,10 were used to simplify the computations.
2. Theoretical Methods Density functional theory (DFT) methods have been acknowledged to be a practical and effective computational tool, especially for organometallic compounds.11-19 Two density functional theory (DFT) methods were used in this study. The first functional, designated B3LYP, is an HF/DFT hybrid method using Becke’s three-parameter functional (B3)20 and the Lee-Yang-Parr generalized gradient correlation functional (LYP).21 The BP86 functional is a pure DFT method combining Becke’s 1988 exchange functional (B)22 with Perdew’s 1986 gradient correlation functional (P86).23 All-electron double-ζ plus polarization (DZP) basis sets were used herein. The DZP basis sets for C and O, which are designated (11) (12) (13) 486. (14) (15) 1926. (16) (17) (18) 93. (19) (20) (21) (22) (23)
Ehlers, A. W.; Frenking, G. J. Am. Chem. Soc. 1994, 116, 1514. Delley, B.; Wrinn, M.; Lu¨thi, H. P. J. Chem. Phys. 1994, 100, 5785. Li, J.; Schreckenbach, G.; Ziegler, T. J. Am. Chem. Soc. 1995, 117, Jonas, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474. Barckholtz, T. A.; Bursten, B. E. J. Am. Chem. Soc. 1998, 120, Niu, S.; Hall, M. B. Chem. ReV. 2000, 100, 353. Macchi, P.; Sironi, A. Coord. Chem. ReV. 2003, 238, 383. Carreon, J.-L.; Harvey, J. N. Phys. Chem. Chem. Phys. 2006, 8, Bu¨hl, M.; Kabrede, H. J. Chem. Theory Comput. 2006, 2, 1282. Becke, A. D. J. Chem. Phys. 1993, 98, 5648. Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. Becke, A. D. Phys. ReV. A 1988, 38, 3098. Perdew, J. P. Phys. ReV. B 1986, 33, 8822.
(9s5p1d/4s2p1d), begin with Dunning’s standard double-ζ contraction24 of Huzinaga’s primitive sets (DZ)25 and add one set of pure spherical harmonic d functions with orbital exponents (Rd(C) ) 0.75 and Rd(O) ) 0.85). For H, a set of p polarization functions (Rp(H) ) 0.75) is added to the Huzinaga-Dunning DZ set. For Co, the DZP basis set, designated as (14s11p6d/10s8p3d), uses the Wachters primitive set26 augmented by two sets of p functions and one set of d functions and contracted following the method of Hood et al.27 For H2C2Co2(CO)6, there are 318 contracted Gaussian functions with the present DZP basis set. Conceivable structures of H2C2Co2(CO)n (n ) 6-4) were fully optimized using B3LYP and BP86 methods. All of the computations were carried out with the Gaussian 03 program,28 in which the fine grid (75, 302) is the default for evaluating integrals numerically and the tight (10-8 hartree) designation is the default for the SCF convergence. The optimized geometries from the B3LYP/DZP and BP86/DZP computations are depicted in Figures 2 and 3 with all bond distances given in angstroms, while Table 1 gives their electronic states and energies and Table 2 shows their ν(CO) frequencies. All of the structures reported in Table 1 and Figures 2 and 3 have a central Co2C2 tetrahedrane unit consisting of six bonds: namely, one Co-Co bond, one C-C bond, and four Co-C bonds. Diamond structures were also investigated for H2C2Co2(CO)6 and H2C2Co2(CO)4, in which the Co · · · Co and C · · · C distances are too long for bonding (see Figure S1 in the Supporting Information). However, such structures were found to be very high energy structures at >80 kcal/mol above the corresponding global minima with significant imaginary vibrational frequencies. Therefore, such diamond structures are almost certain to be chemically irrelevant and thus are not considered in detail in this paper.
3. Results and Discussion 3.1. H2C2Co2(CO)6. Structures of H2C2Co2(CO)6 were investigated in which the three carbonyls on one cobalt atom are in either eclipsed positions (C2V symmetry) or staggered positions (Cs symmetry) relative to the three carbonyls on the other cobalt atom. All of our DFT calculations predict that the eclipsed C2V (24) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823. (25) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. (26) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (27) Hood, D. M.; Pitzer, R. M.; Schaefer, H. F. J. Chem. Phys. 1979, 71, 705. (28) Frisch, M. J., Gaussian 03, Revision C 02; Gaussian, Inc., Wallingford, CT, 2004 (see the Supporting Information for details).
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Figure 3. Optimized structures of the H2C2Co2(CO)4 derivatives. Table 1. Total Energies (Etot, in hartrees), Relative Energies (∆E, in kcal/mol), and Bond Distances (in Å)a for the Co-Co and C-C Bonds in the Optimized Structures of the (H2C2)Co2(CO)n Complexes B3LYP/DZP ∆E
Co-Co
HC-CH
Co-Co
HC-CH
1
3 523.219 57
0.0
2.485
1.347
1
3 523.747 39
0.0
2.502
1.360
5-1S 5-2S 5-3S
1
A (C1) A1 (C2v) 1 A′ (Cs)
3 409.843 01 3 409.835 22 3 409.826 80
0.0 4.9 10.2
2.355 2.562 2.467
1.326 1.352 1.349
1
A (C1) A1 (C2V) 1 A′ (Cs)
3 410.360 87 3 410.357 43 3 410.344 15
0.0 2.1 10.5
2.373 2.524 2.498
1.349 1.370 1.368
4-1S 4-2S 4-3S 4-4S 4-5S 4-6S
1
3 296.459 56 3 296.455 58 3 296.454 59 3 296.449 20 3 296.441 64 3 296.431 86
0.0 2.5 3.1 6.5 11.2 17.4
2.317 2.334 2.363 2.343 2.343 2.426
1.324 1.341 1.338 1.352 1.334 1.350
1
3 296.962 89 3 296.966 93 3 296.963 31 3 296.957 22 3 296.947 22 3 296.936 96
0.0 -2.5 -0.3 3.5 9.8 16.3
2.344 2.338 2.396 2.363 2.373 2.478
1.353 1.356 1.365 1.369 1.355 1.373
structure
6
6-1S
5
4
a
BP86/DZP
-Etot
n
state (sym) A1 (C2V)
1
A′ (Cs) A (C2) 1 A′ (Cs) 1 A′ (Cs) 1 A (C1) 1 A1 (C2V) 1
state (sym) A1 (C2V)
1
A (C1) A (C2) A′ (Cs) 1 A′ (Cs) 1 A (C1) 1 A1 (C2V) 1 1
-Etot
∆E
The C-C bond distance in the free HCtCH molecule is 1.214 Å (B3LYP/DZP) or 1.225 Å (BP86/DZP).
structure 6-1S (Figure 2) of H2C2Co2(CO)6 is a genuine minimum. The staggered Cs structure of H2C2Co2(CO)6 is a transition state with an imaginary vibrational frequency. Following the normal mode of this imaginary frequency leads from the staggered Cs structure to the eclipsed C2V structure. The optimized bond distances for the eclipsed C2V structure 6-1S of H2C2Co2(CO)6 are shown in Figure 2. The Co-Co bond distance of 2.485 Å (B3LYP) or 2.502 Å (BP86) in 6-1S can be considered as indicative of the formal single bond necessary to give both cobalt atoms the favored 18-electron configuration. The two axial carbonyls (COax) in 6-1S are approximately trans to this Co-Co bond if the cobalt coordination polyhedra in 6-1S are considered as distorted octahedra with two bonds to the acetylene carbons, three bonds to terminal carbonyl groups, and the cobalt-cobalt bond as the sixth “ligand”. The remaining four equatorial carbonyls (COeq) are then in approximate trans positions relative to the Co-C bonds to the acetylene carbon atoms. The B3LYP and BP86 methods predict the four equivalent Co-COeq bonds to be longer than the two Co-COax bonds by ∼0.04 Å suggesting that the acetylene ligand has a stronger trans effect than the Co-Co bond. The predicted bond distances in Figure 2 can be compared with the corresponding experimental bond distances29 by X-ray crystallography for (tBu2C2)Co2(CO)6, which are 2.462 Å for the Co-Co bond, 1.983-1.988 Å for the Co-C(H) bond to the acetylene carbons, 1.791-1.796 Å for the Co-COax bonds, 1.819-1.839 Å for the Co-COeq bonds, and 1.349 Å for the C-C bond. Our
B3LYP results agree much more closely with experiment than the BP86 results. 3.2. H2C2Co2(CO)5. The three structures found for H2C2Co2(CO)5 (Figure 2 and Table 1) have all real vibrational frequencies, indicating that they are genuine minima. The lowest energy H2C2Co2(CO)5 structure 5-1S is derived from the H2C2Co2(CO)6 structure 6-1S by removal of an equatorial carbonyl group: i.e., a carbonyl group trans to the acetylene ligand. This structure can be designated as eq-H2C2Co2(CO)5, as noted in the Introduction. The predicted CodCo distance of 2.355 Å (B3LYP) or 2.373 Å (BP86) in 5-1S is ∼0.13 Å shorter than the Co-Co single bond in 6-1S and thus can be considered to correspond to the formal double bond needed to give both cobalt atoms the favored 18-electron configuration in H2C2Co2(CO)5. Furthermore, this predicted CodCo double bond distance in 5-1S is very similar to the experimental FedFe double-bond distance of 2.316 Å determined by X-ray crystallography30 on (t-Bu2C2)Fe2(CO)6. This is of some significance, since iron and cobalt have very similar covalent radii of 1.25 and 1.26 Å, respectively. The C2V singly bridged H2C2Co2(CO)4(µ-CO) structure 5-2S lies 4.9 kcal/mol (B3LYP) or 2.1 kcal/mol (BP86) above the global minimum 5-1S. The Co-Co distance of 2.562 Å (B3LYP) or 2.524 Å (BP86) in 5-2S is ∼0.05 Å longer than that in H2C2Co2(CO)6 and thus corresponds to a formal single bond rather than the formal double bond in 5-1S. A dipolar CofCo single bond gives one cobalt atom in 5-2S the favored
(29) Baert, F.; Guelzim, A.; Poblet, J. M.; Wiest, R.; Demuynck, J.; Benard, M. Inorg. Chem. 1986, 25, 1830.
(30) Cotton, F. A.; Jamerson, J. D.; Stults, B. R. J. Organomet. Chem. 1975, 94, C53.
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Table 2. ν(CO) Frequencies (in cm-1) and Infrared Intensities (in km/mol, Given in Parentheses) of the (H2C2)Co2(CO)n Complexes at the B3LYP/DZP and BP86/DZP Levelsa n
structure
B3LYP/DZP
BP86/DZP
exptlb
6
6-1S
2155 (181), 2109 (2034), 2098 (1636), 2092 (1005), 2076 (0.3), 2075 (0)
2067 (147), 2032 (1414), 2013 (1424), 2005 (879), 1992 (53), 1992 (0)
2099 (w), 2058 (s), 2032 (s), 2027 (sh), 2016 (w)
5
5-1S
2141 (273), 2096 (2185), 2083 (926), 2071 (883), 2063 (371) 2141 (197), 2095 (2139), 2084 (1940), 2079 (0), 1936 (384) 2147 (321), 2096 (1666), 2092 (1461), 2074 (622), 2059 (393)
2052 (235), 2013 (1630), 2000 (733), 1987 (681), 1976 (438) 2047 (152), 2009 (1784), 1994 (1661), 1988 (0), 1878 (290) 2059 (268), 2011 (1509), 2007 (1155), 1994 (174), 1972 (437)
2084 (w), 2043 (s), 2019 (m), 2014 (sh), 2000 (w)
2129 (579), 2081 (1694), (570), 2063 (1046) 2121 (148), 2076 (2737), (1150), 2050 (575) 2129 (153), 2087 (2600), (1737), 2053 (93) 2121 (270), 2074 (1500), (1400), 2059 (993) 2131 (497), 2085 (1443), (939), 2050 (810) 2136 (476), 2078 (2043), (1333), 2055 (0)
2035 (393), 2000 (1282), 1978 (1000), 1973 (439) 2030 (131), 1994 (1867), 1977 (929), 1967 (614) 2034 (170), 1998 (2001), 1980 (1293), 1964 (45) 2037 (364), 1991 (800), 1972 (1275), 1983 (839) 2045 (404), 1998 (1036), 1987 (840), 1964 (632) 2043 (380), 1988 (1744), 1993 (929), 1970 (0)
5-2S 5-3S 4
4-1S 4-2S 4-3S 4-4S 4-5S 4-6S
a
2066 2063 2072 2064 2070 2067
2084 (w), 2080 (w), 2034 (s), 2019 (m), 2011 (sh), 1980 (m)
Experimental data are also given for comparison. b Taken from ref 10.
18-electron configuration and the other cobalt atom a 16-electron configuration. Structure 5-2S can then be considered to be a resonance hybrid involving the two canonical structures of this type. The Co-CO bond distance to the bridging carbonyl group in 5-2S is 1.897 Å (B3LYP) or 1.899 Å (BP86), which, as expected, is significantly longer than the four Co-CO bonds of 1.820 Å (B3LYP) or 1.805 Å (BP86) to the terminal carbonyl groups. This bridging carbonyl group in 5-2S exhibits a relatively low ν(CO) frequency of 1878 cm-1 (BP86 in Table 2), as is typical for bridging carbonyl groups compared with terminal carbonyl groups. The final H2C2Co2(CO)5 structure, namely 5-3S at 10.2 kcal/ mol (B3LYP) or 10.5 kcal/mol (BP86) above 5-1S, can be derived from the H2C2Co2(CO)6 structure 6-1S by removal of an axial carbonyl group and can thus be designated as axH2C2Co2(CO)5. The Co-Co distance in 5-3S of 2.467 Å (B3LYP) or 2.498 Å (BP86) is within 0.02 Å of that in H2C2Co2(CO)6 (6-1S) and may thus be regarded as a formal single bond. This gives the cobalt atom in 5-3S bearing three terminal carbonyl groups (the “right” cobalt atom in Figure 2) the favored 18-electron configuration. However, the cobalt atom in 5-3S bearing only two terminal carbonyl groups (the “left” cobalt atom in Figure 2) has only a 16-electron configuration. 3.3. H2C2Co2(CO)4. Two carbonyl groups can be removed from H2C2Co2(CO)6 in six different ways to give six different structures of H2C2Co2(CO)4 (Figure 3 and Table 1). All six structures are genuine minima, as indicated by the absence of imaginary vibrational frequencies. The three lowest energy H2C2Co2(CO)4 structures, namely 4-1S, 4-2S, and 4-3S, as well as structure 4-6S, are derived from the H2C2Co2(CO)6 structure 6-1S by removal of the two carbonyl groups from different cobalt atoms so that they each have two Co(CO)2 units in their structures. The two higher energy structures 4-4S and 4-5S are derived from 6-1S by removal of two carbonyl groups from the same cobalt atom. The B3LYP method predicts Cs symmetry for the H2C2Co2(CO)4 global minimum 4-1S. However, with the BP86 method, this Cs structure is a transition state. Following the normal mode of the imaginary frequency leads to a C1symmetric structure (Figure 2). The Co-Co distances of the five lowest energy H2C2Co2(CO)4 structures (Figure 2) are predicted by the B3LYP
Table 3. Dissociation Energies (kcal/mol) for the Successive Removal of Carbonyl Groups from the Lowest Energy Structures of the H2C2Co2(CO)n Derivatives process
B3LYP
BP86
H2C2Co2(CO)6 f H2C2Co2(CO)5 + CO H2C2Co2(CO)5 f H2C2Co2(CO)4 + CO
30.1 34.4
37.2 44.4
method to fall in the range 2.31-2.37 Å, respectively, which is significantly shorter than the 2.485 Å Co-Co distance in the canonical H2C2Co2(CO)6 structure 6-1S, reflecting the unsaturated nature of the H2C2Co2(CO)4 structures. The highest energy H2C2Co2(CO)4 structure 4-6S, i.e., the ax,ax′-H2C2Co2(CO)4 structure with no carbonyl groups nearly axial to the Co-Co bond, has a significantly longer Co-Co distance than the other H2C2Co2(CO)4 structures: namely, 2.426 Å (B3LYP). 3.4. Carbonyl Dissociation Energies. The bond dissociation energies for the loss of one carbonyl group from the acetylenic dicobalt carbonyls H2C2Co2(CO)n (n ) 6, 5) using the global minima are seen to be 30.1 kcal/mol (B3LYP) or 37.2 kcal/ mol (BP86) for H2C2Co2(CO)6 and 34.4 kcal/mol (B3LYP) or 44.4 kcal/mol (BP86) for H2C2Co2(CO)5 (Table 3). Thus, the loss of the first carbonyl group from H2C2Co2(CO)6 requires less energy than the loss of the second carbonyl group, as is typical for metal carbonyl chemistry. These carbonyl dissociation energies are similar to those found for the similar binary metal carbonyls, as indicated by the reported31 experimental carbonyl dissociation energies of 37 kcal/mol for Cr(CO)6 and 41 kcal/ mol for Fe(CO)5. 3.5. Comparison with Experiment. Bitterwolf, Scallorn, and Weiss (BSW)10 have studied the photolysis of H2C2Co2(CO)6 in frozen Nujol matrices at 90 K using changes in the ν(CO) infrared spectrum to follow the reaction. They suggest that the initial photoproduct is the H2C2Co2(CO)5 isomer 5S-3, namely ax-H2C2Co2(CO)5, in which an axial carbonyl group is lost from H2C2Co2(CO)6. Upon annealing the matrix at 140 K, they observe conversion of the initial photoproduct to a second H2C2Co2(CO)5 isomer, which they suggest to be 5S-1, namely eq-H2C2Co2(CO)5, in which an equatorial carbonyl group is lost from H2C2Co2(CO)5. No evidence for any H2C2Co2(CO)5 (31) Sunderlin, L. S.; Wang, D.; Squires, R. R. J. Am. Chem. Soc. 1993, 115, 12060.
3394 Organometallics, Vol. 28, No. 12, 2009
isomers with bridging carbonyl groups, corresponding to structure 5S-2, was found in the BSW study. Our theoretical studies support BSW’s interpretation of their results.10 The experimental ν(CO) frequencies found under their conditions for the H2C2Co2(CO)6 starting material are 2099 w, 2058 s, 2032 s, 2027 sh, and 2016 w cm-1, which correspond to our predicted ν(CO) frequencies of 2067, 2032, 2013, 2005, and 1992 cm-1. Our predicted ν(CO) frequencies average 25 cm-1 below the BSW experimental ν(CO) frequencies with a similar pattern of relative infrared intensities. Similarly, the BSW experimental ν(CO) frequencies for the initial photoproduct of 2084 w, 2080 w, 2034 s, 2019 w, 2011 sh, and 1980 m cm-1 correspond to our predicted ν(CO) frequencies of 2059, 2011, 2007, 1994, and 1972 cm-1 for ax-H2C2Co2(CO)5 (5S-3) with an average error of 16 cm-1 in the same direction as for H2C2Co2(CO)6. Furthermore, the BSW experimental ν(CO) frequencies of 2084 w, 2043 s, 2019 w, 2014 sh, and 2000 w for the H2C2Co2(CO)5 isomer produced upon annealing at 140 K correspond to our predicted ν(CO) frequencies of 2052, 2013, 2000, 1987, and 1976 cm-1 for eq-H2C2Co2(CO)5 (5S-1) with an average error of 21 cm-1 in the same direction as for H2C2Co2(CO)6. Thus, our theoretical studies discussed in this paper are in excellent agreement with BSW’s assignment of the ax-H2C2Co2(CO)5 structure 5S-3 as the initial photoproduct and the eq-H2C2Co2(CO)5 structure 5S-1 to the product obtained after annealing at 140 K. Furthermore, the initially produced ax-H2C2Co2(CO)5 (5S-3) is predicted from our theoretical studies to have an energy of ∼10 kcal/mol (Table 1) above the subsequently produced eq-H2C2Co2(CO)5 (5S-1). Thus, the conversion of ax-H2C2Co2(CO)5 to eq-H2C2Co2(CO)5, as observed by BSW,10 is seen to be thermodynamically favorable.
4. Summary The preferred structure for H2C2Co2(CO)6 is predicted to be an eclipsed structure (C2V symmetry) with the staggered Cs
Li et al.
structure as a transition state. For H2C2Co2(CO)5 an eqH2C2Co2(CO)5 structure derived by loss of an equatorial carbonyl group from H2C2Co2(CO)6 is predicted to lie ∼10 kcal/ mol below an ax-H2C2Co2(CO)5 structure derived by loss of an axial carbonyl group from H2C2Co2(CO)6. This is consistent with the experimental work of Bitterwolf, Scallorn, and Weiss (BSW)10 on the low-temperature Nujol matrix photolysis of H2C2Co2(CO)6, where ax-H2C2Co2(CO)5 is suggested to be an initial photoproduct at 90 K, which rearranges to the more stable eq-H2C2Co2(CO)5 upon annealing at 140 K. A H2C2Co2(CO)4(µCO) structure with a single bridging carbonyl group of intermediate energy is also predicted for H2C2Co2(CO)5. The lowest energy structures for the even more unsaturated H2C2Co2(CO)4 have two terminal carbonyl groups on each cobalt atom rather than three terminal carbonyl groups on one cobalt atom and only one terminal carbonyl group on the other cobalt atom. No structures with bridging carbonyl groups were found for H2C2Co2(CO)4.
Acknowledgment. We are indebted to the 111 Project (B07012) and the National Natural Science Foundation (20873045) of China as well as the U.S. National Science Foundation (Grants CHE-0749868 and CHE-0716718) for support of this research. Supporting Information Available: Table S1, giving the vibrational frequencies and infrared intensities of the H2C2Co2(CO)n (n ) 6-4) complexes at the B3LYP/DZP and BP86/DZP levels, Table S2, giving the Cartesian coordinates of the optimized H2C2Co2(CO)n (n ) 6-4) structures at the B3LYP/DZP and BP86/ DZP levels, Figure S1, giving high-energy Diamond structures of H2C2Co2(CO)6 and H2C2Co2(CO)4, and the complete Gaussian 03 reference.28 This material is available free of charge via the Internet at http://pubs.acs.org. OM801211G
