Coupling of Fluoroborylene Ligands To Give a Viable

Sep 12, 2011 - Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002,. People's Republic of China...
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Coupling of Fluoroborylene Ligands To Give a Viable Cyclopentadienyliron Carbonyl Complex of Difluorodiborene (FBdBF) Liancai Xu,‡ Qian-shu Li,*,†,§ R. Bruce King,*,†,|| and Henry F. Schaefer|| †

Center for Computational Quantum Chemistry, South China Normal University, Guangzhou 510631, People's Republic of China Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, People's Republic of China § Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, People's Republic of China Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States

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bS Supporting Information ABSTRACT: The lowest energy Cp2Fe2(BF)2(CO) structure is predicted by theory to have an iron iron bond bridged by both a difluorodiborene ligand and a carbonyl group. Such a structure is potentially accessible by reaction of B2F4 3 OEt2 with the highly nucleophilic NaFe(CO)2Cp followed by decarbonylation.

he fluoroborylene ligand, BF, is isoelectronic with the ubiquitous carbonyl ligand. Recently Vidovic and Aldridge1 have synthesized the first stable metal fluoroborylene complex, (μ-BF)[Ru(CO)2Cp]2, by reaction of BF3 3 OEt2 with the highly nucleophilic carbonyl anion CpRu(CO)2 (Cp = η5-C5H5). Density functional theory predicts an extensive fluoroborylene chemistry of iron carbonyls that is significantly different from the chemistry of isoelectronic homoleptic iron carbonyls.2 5 In addition, density functional theory has been used to investigate the bonding character of metal and terminal BX groups in the mononuclear complexes [(η5-C5H5)(CO)2M(BX)] (M = Re, Mn; X = F, Cl, Br, I) as well as the geometries of the manganese complexes [CpMn(CO)2]2(μ-BCl), [CpMn(CO)2]2(μ-BCl)2, [CpMn(CO)2]2(μ-CO)(μ-BCl), [CpMn(CO)2]2(μ-B2Cl2), and their relative energies.6 This communication reports a novel system in which two BF ligands couple to form a difluorodiborene ligand stabilized by complexation with a binuclear cyclopentadienyliron carbonyl unit in a thermodynamically viable structure. This Cp2Fe2(μ-BF)2(μ-CO) system is unexpectedly found to be very different from the apparently isoelectronic carbonyl system Cp2Fe2(μ-CO)3. Analogy with the synthesis of (μ-BF)[Ru(CO)2Cp]2 suggests that the structures discussed in this communication are potentially accessible by the reaction of the diethyl ether adduct7 of B2F4 with the highly nucleophilic CpFe(CO)2 . The coordinated B2F2 (FBdBF) ligand is of current interest because of its relationship to the recently discovered stable diborene derivatives LBHdBHL (L = N-heterocyclic carbene ligand).8,9 The related B2Cl2 ligand is found in [(η5-C5H4CH3)Mn(CO)2]2(μ-B2Cl2), which has been synthesized and characterized by X-ray crystallography.10 In addition, the two complexes Rh4{μ-BN(SiMe3)2}2(μ-Cl)4(μ-CO)(CO)4

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and (η5-C5R5)Ir-{BN(SiR3)2}2, having two uncoupled silylaminoborylene ligands without a direct B B bond, have been synthesized and characterized by X-ray diffraction.11,12 Figure 1 shows the two lowest energy Cp2Fe2(BF)2(CO)3 structures predicted using density functional theory.13 The lowest energy structure, 23-1S, has one bridging BF group, one terminal BF group, and an Fe 3 3 3 Fe distance of ∼3.7 Å, too long for a formal iron iron bond. However, in structure 23-2S, which lies only 0.6 kcal/mol above 23-1S by the BP86 method, the two BF ligands have coupled to form an unprecedented B2F2 ligand with a B B bond length of 1.850 Å (B3LYP) or 1.878 Å (BP86). This B B distance is longer than the experimental B B distance of 1.695(7) Å in the B2Cl2 ligand of the binuclear complex [(η5-C5H4CH3)Mn(CO)2]2(μ-B2Cl2).10 Loss of a carbonyl group from Cp2Fe2(BF)2(CO)3 gives Cp2Fe2(BF)2(CO)2, isoelectronic with the well-known14 16 Cp2Fe2(CO)4, which has two bridging CO groups. The two lowest energy Cp2Fe2(BF)2(CO)2 structures are the trans and cis structures 22-1t and 22-1c ,with two bridging BF groups and two terminal CO groups having essentially the same energies within less than 1 kcal/mol (Figure 2). Related Cp2Fe2(BF)2 (μ-CO)2 structures containing bridging CO groups rather than bridging BF groups lie at least 17 kcal/mol above 22-1t and 22-1c. The Fe Fe distances of ∼2.6 Å in 22-1t and 22-1c are consistent with the formal single bonds required to give both iron atoms the favored 18-electron configuration. Photolysis of Cp2Fe2(CO)4 under mild conditions gives the tricarbonyl Cp2Fe2(μ-CO)3, which is a stable triplet state Received: July 20, 2011 Published: September 12, 2011 5084

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Figure 4. Bonding of a difluorodiborene ligand to a pair of iron atoms ([Fe+] refers to a positively charged iron atom with unspecified surrounding ligands).

Figure 1. The two lowest energy Cp2Fe2(BF)2(CO)3 structures. In all of the figures in this paper, the energies (in kcal/mol) are given in parentheses, first for the B3LYP method and then for the BP86 method. The distances indicated in the figures were obtained by the BP86 method.

Figure 2. The two lowest energy Cp2Fe2(BF)2(CO)2 structures.

Figure 3. The four lowest energy singlet Cp2Fe2(BF)2(CO) structures.

molecule in which all three carbonyl groups bridge the pair of iron atoms.17 20 The lowest energy structure of the isoelectronic Cp2Fe2(BF)2(CO) is a singlet structure (21-1S in Figure 3), with both BF groups and the CO group bridging the pair of iron atoms. However, the two bridging BF groups have coupled to form a B2F2 (difluorodiborene) ligand with a predicted B B distance of 1.877 Å (B3LYP) or 1.906 Å (BP86). Free B2F2 is not

known but is predicted21 to have a linear structure with a BdB double bond distance of 1.487 Å. The lengthening of the BdB distance in B2F2 by ∼0.4 Å upon complexation with the diiron system can be related to the donation of electrons from the iron atoms into the antibonding π* orbitals of the BdB double bond so that it effectively is a single bond. In fact, the B B bond in 21-1S is even ∼0.15 Å longer than the B B single bond of length 1.720(4) Å in B2F4, determined by electron diffraction,22 and is longer than the experimental B B distance of 1.695(7) Å in [(η5C5H4CH3)Mn(CO)2]2(μ-B2Cl2).10 The neutral bridging B2F2 ligand in 21-1S can be considered to be a two-electron donor to the diiron system, as indicated by the zwitterionic structure in Figure 4 with formal negative charges on the tetracoordinate boron atoms and formal positive charges on the iron atoms. This is in contrast with two separate bridging BF ligands which together pairwise donate four electrons to a diiron system such as in the Cp2Fe2(μ-BF)2(CO)2 structures 22-1t and 22-1c (Figure 2). The FetFe distance in 21-1S with two bridging groups of ∼2.35 Å is ∼0.25 Å shorter than the Fe Fe single bonds in the likewise doubly bridged Cp2Fe2(μ-BF)2(CO)2 structures 22-1t and 22-1c. The FetFe bond in 21-1S can be interpreted as a formal triple bond, thereby giving each iron atom the favored 18-electron configuration. The second lowest energy singlet Cp2Fe2(BF)2(CO) structure, 21-2S, is related to 21-1S, since in both structures the BF groups and the CO group are located between the two iron atoms. However, in 21-2S one of the BF groups has coupled with the CO group to form a bridging FBdCdO ligand with a predicted B C distance of ∼1.94 Å. Free FBdCdO is unknown but is predicted to have a linear structure with a B C distance of 1.382 Å.21 The lengthening of the B C distance in 21-2S by ∼0.5 Å over than in free FBCO, like the lengthening of the B B distance in 21-1S over that in free B2F2, can be a consequence of donation of iron electrons into the π* antibonding orbitals of the BdC double bond in FBCO. The remaining two higher energy singlet Cp2Fe2(BF)2(CO) structures 21-3S and 21-4S (Figure 3) are unexceptional structures with two bridging BF groups and a terminal CO ligand. Structures 21-3S and 21-4S differ in location of the central Fe2(μ-BF)2 unit relative to the terminal CO group. The FedFe distances of ∼2.5 Å in 21-3S and 21-4S are ∼0.1 Å shorter than the Fe Fe single bond distances in 22-1t and 22-2c and thus can be interpreted as the formal double bonds required to give both iron atoms in 21-3S and 21-4S the favored 18-electron configuration. Since the lowest energy Cp2Fe2(CO)3 structure is the experimentally known triplet triply bridged structure,17 20 triplet Cp2Fe2(BF)2(CO) structures were also investigated. The lowest energy triplet Cp2Fe2(BF)2(CO) structure has the two BF groups and the CO group bridging the iron iron bond (Figure 5). The predicted FedFe distance of ∼2.30 Å in 21-1T is actually ∼0.05 Å shorter than the FetFe distance of ∼2.35 Å in 21-1S. However, because of the presence of three bridging groups in 21-1T 5085

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Figure 5. The lowest energy triplet Cp2Fe2(BF)2(CO) structure.

Figure 6. The lowest energy singlet Cp2Fe2(BF)2 structure.

Table 1. Carbonyl Dissociation Energies (kcal/mol) of the Lowest Energy Cp2Fe2(BF)2(CO)n (n = 3 1) Structures B3LYP

BP86

Cp2Fe2(BF)2(CO)3 f Cp2Fe2(BF)2(CO)2 + CO

0.1

2.2

Cp2Fe2(BF)2(CO)2 f Cp2Fe2(BF)2(CO) + CO

45.7

46.6

Cp2Fe2(BF)2(CO) f Cp2Fe2(BF)2 + CO

45.5

58.6

(two BF groups and one CO group) rather than only two bridging groups in 21-1S (one B2F2 group and one CO group), an iron iron bond of a given order is expected to be significantly shorter in the triply bridged structure 21-1T than in the doubly bridged structure 21-1S. The FedFe bond in 21-1T is therefore interpreted as a formal double bond, giving each iron atom the favored 18-electron configuration. The triplet spin multiplicity arises from the FedFe double bond being a σ + 2/2 π bond with two single-electron “half bonds” in orthogonal π orbitals. Structure 21-1T may be considered to be a direct analogue of the known19 Cp2Fe2(μ-CO)3, in which two of the bridging CO groups are replaced by bridging BF groups. The experimental FedFe distance in Cp2Fe2(μ-CO)3 of 2.265 Å, determined by X-ray crystallography,20 is close to the predicted FedFe distance of ∼2.30 Å in the Cp2Fe2(μ-BF)2(μ-CO) structure 21-1T. Both 21-1S and 21-1T are genuine minima without any imaginary vibrational frequencies. The B3LYP method predicts the triplet Cp2Fe2(μ-BF)2(μCO) structure 21-1T to lie 6.4 kcal/mol in energy below 21-1S, whereas the BP86 method predicts the singlet Cp2Fe2(μ-B2F2)2(μ-CO) structure 21-1S to lie 2.6 kcal/mol below 21-1T. This is consistent with the tendency for B3LYP to favor higher spin states relative to BP86 discussed by Reiher, Salomon, and Hess.23 In addition, the energy differences between the Cp2Fe2(μ-BF)2(μ-CO) structures are much smaller than those reported by Braunschweig and co-workers6 for related dichlorodiborene dimanganese complexes. For the latter complexes, the Cp2Mn2(μ-B2Cl2)2(CO)2 structure containing the coupled μ-B2Cl2 dichlorodiborene ligand lies 26.3 kcal/mol below the Cp2Mn2(μ-BCl)2(CO)2 structure with two separate μ-BCl ligands.

The CO dissociation energies of the Cp2Fe2(BF)2(CO) derivatives provide information regarding their viabilities. In order to provide information on the CO dissociation energy of Cp2Fe2(BF)2(CO), the structure of the carbonyl-free Cp2Fe2(BF)2 was optimized. In this connection, the lowest energy Cp2Fe2(BF)2 structure 20-1S by ∼15 kcal/mol relative to other singlet structures was found to have two bridging BF groups and a very short FetFe distance of ∼2.1 Å (Figure 6). The latter suggests the formal triple bond required to give both iron atoms the favored 18-electron configuration. The tricarbonyl Cp2Fe2(BF)2(CO)3 is predicted to lose a carbonyl group very easily to give the dicarbonyl Cp2Fe2(BF)2(CO)2, since its CO dissociation energy is essentially thermoneutral (Table 1). This suggests that structure 23-2S (Figure 1), with a bridging difluorodiborene ligand, is not a viable structure. However, the dissociation energies of Cp2Fe2(BF)2(CO)2 to give Cp2Fe2(BF)2(CO) and of Cp2Fe2(BF)2(CO) to give Cp2Fe2(BF)2 are both appreciable, more than 40 kcal/mol (Table 1). This suggests that Cp2Fe2(μ-B2F2)(μ-CO) (21-1S in Figure 3) is a viable difluorodiborene complex. In summary, this theoretical study indicates that the cyclopentadienyliron carbonyl system is a promising system for stabilizing the unprecedented difluorodiborene ligand. A possible synthesis of such species might use the reaction of B2F4 3 OEt2 with the highly nucleophilic CpFe(CO)2 anion followed by decarbonylation.

’ ASSOCIATED CONTENT

bS

Supporting Information. Tables S1 S4, giving theoretical harmonic vibrational frequencies for Cp2Fe2(BF)2(CO)n (n = 3 0) (10 structures) using the B3LYP/DZP and BP86/ DZP methods, Tables S5 S14, giving theoretical Cartesian coordinates for Cp2Fe2(BF)2(CO)n (n = 3 0) (10 structures) using the B3LYP/DZP method and BP86/DZP methods, and text giving the complete Gaussian 03 reference. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Authors

[email protected]; [email protected].

’ ACKNOWLEDGMENT We are indebted to the scientific research fund of the State Key Laboratory of Explosion Science and Technology (Grant No. 2DkT10-01a), the Research Fund for the Doctoral Program of Higher Education (Grant No. 20104407110007), the National Natural Science Foundation of China (Grant No. 20973066), and the U.S. National Science Foundation (Grant Nos. CHE1054286 and CHE-0716718) for support of this research. ’ REFERENCES (1) Vidovic, D.; Aldridge, S. Angew. Chem., Int. Ed. 2009, 48, 3669. (2) Xu, L.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. Inorg. Chem. 2010, 49, 1046. (3) Xu, L.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. Inorg. Chem. 2010, 49, 2996. (4) Xu, L.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. Inorg. Chim. Acta 2010, 363, 3538. 5086

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(5) Xu, L.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. New J. Chem. 2010, 34, 2813. (6) Pandey, K. K.; Braunschweig, H.; Dewhurst, R. D. Eur. J. Inorg. Chem. 2011, 2045. (7) Finch, A.; Schlesinger, H. J. J. Am. Chem. Soc. 1958, 80, 3573. (8) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King, R. B.; Schaefer, H. F.; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2007, 129, 12412. (9) Wang, Y.; Quillian, B.; Wei, P.; Xie, Y.; Wannere, C. S.; King, R. B.; Schaefer, H. F.; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2008, 130, 3298. (10) Braunschweig, H.; Coling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2002, 41, 1359. (11) Braunschweig, H.; Forster, M.; Radacki, K. Angew. Chem., Int. Ed. 2006, 45, 2132. (12) Bertsch, S.; Braunschweig, H.; Christ, B.; Forster, M.; Schwab, K.; Radacki, K. Angew. Chem., Int. Ed. 2010, 49, 9517. (13) Density functional theory studies were performed using the B3LYP and BP86 functionals as cited in previous work.2 5 For the basis sets one set of pure spherical harmonic d functions with orbital exponents αd(B) = 0.7, αd(C) = 0.75, αd(O) = 0.85, and αd(F) = 1.0 for boron, carbon, oxygen, and fluorine, respectively, was added to the standard Huzinaga Dunning contracted DZ sets, designated as (9s5p1d/4s2p1d). The loosely contracted DZP basis set for iron is the Wachters primitive set augmented by two sets of p functions and one set of d functions, contracted following the method of Hood, Pitzer, and Schaefer, designated as (14s11p6d/10s8p3d). All calculations were performed with the Gaussian 03 program package. A given Cp2Fe2(BF)2(CO)a structure is designated as 2a-bA, where 2 is the number of BF groups and iron atoms, a is the number of CO groups, and b orders the structures according to their relative energies using the BP86 method. A indicates whether the structure is a singlet (S) or triplet (T). Thus, the lowest energy structure of singlet Cp2Fe2(BF)2(CO)3 is designated 23-1S. (14) Mills, O. S. Acta Crystallogr. 1958, 11, 620. (15) Bryan, R. F.; Greene, P. T. J. Chem. Soc. A 1970, 3068. (16) Mitschler, A.; Rees, B.; Lehmann, M. S. J. Am. Chem. Soc. 1978, 100, 3390. (17) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1980, 102, 7794. (18) Hooker, R. H.; Mahmoud, K. A.; Rest, A. J. Chem. Commun. 1983, 1022. (19) Hepp, A. F.; Blaha, J. P.; Lewis, C.; Wrighton, M. S. Organometallics 1984, 3, 174. (20) Blaha, J. P.; Bursten, B. E.; Dewan, J. C.; Frankel, R. B.; Randolph, C. L.; Wilsonand, B. A.; Wrighton, M. S. J. Am. Chem. Soc. 1985, 107, 4561. (21) Kurkin, A. A.; Byllkova, A.; Bartlett, R. J.; Boyd, R. J.; Schleyer, P. v. R. J. Phys. Chem. 1996, 100, 5702. (22) Donaldson, D. D.; Patton, J. V.; Hedberg, K. J. Am. Chem. Soc. 1977, 99, 6484. (23) Reiher, M.; Salomon, O.; Hess, B. A. Theor. Chem. Acc. 2001, 107, 48.

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