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2008, 112, 18727–18729 Published on Web 11/08/2008
How Stable are the Mg-Mg Bonds in Magnesium (I) Compounds toward Hydrogenation? Ayan Datta* School of Chemistry, Indian Institute of Science Education and Research, CET Campus, ThiruVananthapuram, 695016 India ReceiVed: September 16, 2008
DFT calculations show that the Mg(I)-Mg(I) bonded complexes should undergo rapid hydrogenation to form bridged dihydrido complexes. This phenomenon can be understood on the basis of conversion of conventional 2c-2e bonds into 3c-2e bonds that are stronger. Bonding interactions between subvalent metal atoms within the main group elements are rare.1-3 Some of the most prominent examples of such bonding interactions are known for Hg22+ and recently for Ca22+, Zn22+, and Cd22+.4-6 Stabilization of these metal ions in +I oxidation state even though they are most commonly found in the divalent state represents new chemistry and presents novel challenges for synthesis. The common synthetic strategy involves stabilizing these systems within a cavity of sterically hindered ligands (like decamethyl dicyclopentadienyl radicals for Zn22+ and bulky substituted ligands for Cr22+) that impede oxidation and form a cage to discourage stabilization of higher oxidation states by polar solvents.7-9 Possible syntheses of new molecules are also predicted based on computational studies particularly within the framework of the density functional theory wherein calculations can be reliable performed for model systems of interest. A recent report for the synthesis of the first stable Mg(I)Mg(I) bond in (L)MgMg(L), L ) [(Ar)NC(NPr2i)N(Ar)]- or {[(Ar)NC(Me)]2CH}- with Ar ) 2,6-diisopropylphenyl group, is indeed exciting and opens up a new direction in the stabilizationofnewmoleculeswithunusualbondinginteractions.10,11 Comparison of the short Mg · · · Mg contact distances of ∼2.85 Å in both the compounds as obtained from the crystal structure and similar Mg · · · Mg distances as obtained from ab initio calculations for model systems like XMgMgX (X ) η5-Cp, C6H3-2,6-Ph2, Cl, and F) suggested that Mg(I) · · · Mg(I) bonds exist for these newly synthesized molecules.12-14 However, depending only on experimental studies alone, it is difficult to conclusively assert that these molecules exist as XMgMgX and not XMg(µ-H)2MgX. Based on DFT calculations for model systems, {Mg[(Ar’N)2C(NMe2)]}2 and {MgH[(Ar′N)2C(NMe2)]}2 [Ar′ ) C6H3Me2-2,6], the authors predicted that the ligands on either end on the Mg-Mg bond should be almost perpendicular to each other for the former case and in-plane for the dihydrido complex.10 Since, for the experimental molecules, the ligands have almost a similar perpendicular orientation of the rings, it is expected that the experimentally synthesized molecules is of the type as XMgMgX and not XMg(µ-H)2MgX. However, a critical question that remains unanswered is the following: How much are Mg(I)-Mg(I) bonds labile toward hydrogenation as * To whom correspondence should be addressed. E-mail: ayan@ iisertvm.ac.in.
10.1021/jp808973e CCC: $40.75
SCHEME 1: Molecular Structures Considered for the Hydrogenation Reaction in 1 to 2a
a φ[N(R)-Mg(β)-Mg(γ)-N(δ)] represents the dihedral angle between the 4-membered rings on either side of the Mg · · · Mg bonds.
in the predominating reducing environment, insertion of a H2 molecule to form a bridged hydrido complex is a distinct possibility. Also, it is worthwhile to investigate the stability of the Mg(I)-Mg(I) compounds in reaction surface for the addition of a H2 molecule. Based on our calculations for a series of similar modeled complexes with various ligands, we find that formation of a µ-H2 complex is overwhelmingly exothermic and in the energy surface, the Mg(I)-Mg(I) compound is a metastable intermediate that under thermodynamic equilibrium conditions should transform to the bridged dihydro complex. In scheme I, we show the molecular structures considered in the study. Our calculations were performed using the B3LYP version of DFT, which is comprised of Becke’s hybrid, threeparameter, functional15 and the correlation functional of Lee, Yang, and Parr.16 The 6-31G(d) basis set17 was employed. All of the calculations are done using the Gaussian 03 package.18 Additional frequency calculations were performed to verify the optimized structures. Two major structural changes are noticed in geometries of the molecules on hydrogenation. First, there is a shortening in the Mg · · · Mg bond length on hydrogenation. For example, the Mg · · · Mg bond lengths are 2.835, 2.828, 2.853, and 2.846 Å in 1(a), 1(b), 1(c), and 1(d), respectively, 2008 American Chemical Society
18728 J. Phys. Chem. C, Vol. 112, No. 48, 2008
Letters
Figure 1. Molecular orbitals depicting the 2c-2e Mg-Mg bond and the 3c-2e Mg · · · H · · · Mg bonds for dehydrogenated compound (1a) and the hydrogenated compound (2a), respectively.
whereas they are 2.774, 2.765, 2.780, and 2.776 Å in 2(a), 2(b), 2(c), and 2(d), respectively. Note, that similar calculations by Green et al.10 have come to similar conclusions for 1(a) and 2(a). The shortening of the Mg · · · Mg bond length can be rationalized on the basis of 3c-2e bonds in the two Mg · · · H · · · Mg bonds rather than the conventional 2c-2e Mg · · · Mg bonds in the dehydrogenated compounds.19 This is clearly seen the molecular orbitals shown in Figure 1, wherein the 2c-2e σ(Mg-Mg) bond and the two bent 3c-2e Mg · · · H · · · Mg bonds are shown for 1 and 2. The other interesting structural change associated with hydrogenation is the remarkable change in the dihedral angle defined by, φ[N(R)-Mg(β)-Mg(γ)-N(δ)]. The dihedral angle changes from 77.38° to 0.082°, 75.96° to 0.155°, 71.01° to 0.01°, and 21.70° to 0.703° for 1(a) f 2(a), 1(b) f 2(b), 1(c) f 2(c), and 1(d) f 2(d) respectively. Steric interactions between the R1 groups on either side of Mg-Mg bonds lead to skewed orientation of the 4-membered rings in 1a-1d. The dihedral angle decreases in the order: 1(a) > 1(b) > 1(c) >1(d) in harmony with decreasing steric bulk of R1. The fact that compounds, 2(a)-2(d) have φ[N(R)-Mg(β)-Mg(γ)-N(δ)] almost zero can again be understood on the basis of the 3c-2e interactions that stablize these molecules and essentially require such orbital orientations. The enthalpy of hydrogenation for the compounds are calculated as ∆H298(hydrogenation) ) H298(2a,2b,2c,2d) H298(1a,1b,1c,1d) - H298(H2). ∆H298(hydrogenation) for 1(a) f 2(a), 1(b) f 2(b), 1(c) f 2(c), and 1(d) f 2(d) are -24.2, -23.4, -24.9 and -24.89 kcal/mol respectively. The overwhelming exothermicity for hydrogenation of Mg · · · Mg bonds irrespective of the steric bulk of R1 suggests that the Mg · · · Mg bonds are extremely suspectible toward hydrogenation and based on thermodynamic criteria, Mg · · · Mg bonds should exist rather as Mg(µ-H)2Mg. The stability of the Mg(µ-H)2Mg unit can also estimated by the dissociation energy for XMg(µ-H)2MgX f 2 XMgH which are highly endothermic. It is interesting to note that dissociative hydrogenation was also suggested for XMgMgX by Green et al.10 The dissociation energies for 2(a), 2(b), 2(c), and 2(d) are +29.79, +29.58, +28.41, and +27.90 kcal/ mol, respectively. For a critical understanding of the dramatic change in the dihedral angle defined by, φ[N(R)-Mg(β)-Mg(γ)-N(δ)], we scanned the potential energy surface along it for the model system 2c. The ground-state minimal energy conformations correspond to planar orientation of the angle (φ ) 0 and 180°) with the two hydride ions above and below the plane of the molecule. However, conversion between the two equienergic conformers involve a transition state (shown in Figure 2) with distorted rings and average skewed φ[N(R)-Mg(β)-Mg(γ)N(δ)] ) 55.6°. The transition state involves softening of the vibrational mode that involves superposition of movement along the dihedral angle, φ[N(R)-Mg(β)-Mg(γ)-N(δ)] and movement of the two hydride ions.
Figure 2. Potential energy surface for twisting along φ[N(R)-Mg(β)Mg(γ)-N(δ)] in the hydrogenated complex, 2c.
In conclusion, our calculations show that hydrogenation of the XMg · · · MX bonds to XMg(µ-H)2MgX is highly exothermic and under thermodynamic equilibrium, should be the stable product. The role of steric interactions among R1 is found to be negligible in the exothermicity of the reaction. However, based on the fact that φ[N(R)-Mg(β)-Mg(γ)-N(δ)] for the dehydrogenated complexes agrees well with the experimentally synthesized molecules, we believe that under the synthesis conditions, the thermodynamically less stable XMg · · · MgX is formed. It would be an interesting experiment to try to isolate the actual thermodynamically stable, XMg(µ-H)2MgX product under slow and large doses of H2 in the reaction environment particularly when Green et al.10 could not synthesize the dihydrogen bridged product at 80 °C or under UV radiation. A potential application of such molecular systems would to be explore if the Mg · · · Mg bonds are capable for intake to more than one H2 molecule to utilize these molecules for storage of hydrogen. Acknowledgment. Computations were carried out at the computer center of IISER-Thiruvananthapuram. Supporting Information Available: Optimized B3LYP/631G(d) geometries, energies, harmonic frequencies, and the complete lists of authors for ref 18. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) von Schnering, H. G. Angew. Chem., Int. Ed. 1981, 20, 33–51. (b) Breher, F. Coord. Chem. ReV. 2007, 251, 1007–1043. (2) (a) Hino, S.; Olmstead, M. M.; Philips, A. D.; Wright, R. J.; Power, P. P. Inorg. Chem. 2004, 43, 7346–7352. (b) Wright, R. J.; Phillips, A. D.; Hino, S.; Power, P. P. J. Am. Chem. Soc. 2005, 127, 4794. (3) (a) Poli, R.; Rheingold, A. L. J. Chem. Soc. Chem. Commun. 1990, 552. (b) H-Molina, R.; Edwards, A. J.; Clegg, W.; Sykes, A. G. Inorg. Chem. 1998, 37, 2989. (4) (a) Leong, W. K.; Chen, G. Organometallics 2001, 20, 5771. (b) Llusar, R.; Beltran, A.; Andres, J.; Fuster, F.; Silvi, B. J. Phys. Chem. A 2001, 105, 9460. (c) Robinson, G. H. Acc. Chem. Res. 1999, 32, 773. (d) Li, Q. S.; Xu, Y. J. Phys. Chem. A 2006, 110, 11898. (e) Xiao, Z. L.; Hauge, R. H. High Temp Sci. 1991, 31, 59. (f) Blaisten-Barojas, E.; Chien, C.-H.; Pederson, M. R.; Mirick, J. W. Chem. Phys. Lett. 2004, 395, 109. (5) Grirrane, A.; Resa, I.; Rodriguez, A.; Carmona, E.; Alvarez, E.; G-Puebla, E.; Monge, A.; Galindo, A.; del Rio, D.; Andersen, R. A. J. Am. Chem. Soc. 2007, 129, 693. (6) Zhu, Z.; Fischer, R. C.; Fettinger, J. C.; Rivard, E.; Brynda, M.; Power, P. P. J. Am. Chem. Soc. 2006, 128, 15068. (7) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science 2004, 305, 1136.
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