The Coordination Chemistry of Azulene: A Comprehensive DFT

Mar 5, 2010 - The reason for this diversity comes in part from the marked dissymmetry of ... Structure VIII exhibits an anionic and a cationic allyl s...
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Organometallics 2010, 29, 1693–1706 DOI: 10.1021/om901089z

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The Coordination Chemistry of Azulene: A Comprehensive DFT Investigation )

Hanane Korichi,† Fairouz Zouchoune,† Saber-Mustapha Zendaoui,† Bachir Zouchoune,*,†,‡ and Jean-Yves Saillard*,§, †

)

Laboratoire de Chimie Mol eculaire, du Contr^ ole de l’Environnement et des Mesures Physico-Chimiques, Universit e Mentouri-Constantine, 25000 Algeria, ‡Laboratoire de Chimie Mol eculaire et Technologie des Mat eriaux, Universit e Larbi Ben M’Hidi, Oum-el-Bouaghi, 04000 Alg eria, §UMR 6226 Sciences Chimiques de Rennes, CNRS-Universit e de Rennes 1, Campus de Beaulieu, 35042 Rennes-Cedex, France, and Universit e Europ eenne de Bretagne, 5 bd La€ ennec, 35000 Rennes, France Received December 18, 2009

DFT calculations with full geometry optimization have been carried out on a series of real and hypothetical compounds of the type LnMAz, MAz2, LnM2Az, and M2Az2 (Az = azulene). The analysis of their electronic and molecular structures in relation to their electron counts allows a comprehensive rationalization of the bonding within this very large family of compounds. A very rich coordination chemistry of azulene is apparent, even much richer than one could determine from the avalaible experimental data. The reason for this diversity comes in part from the marked dissymmetry of azulene, which is made up of two fused rings of very different sizes. It comes also in large part from the very large electronic and structural flexibility of azulene (in contrast to its isomer naphthalene), which is able to adapt to the electronic demand of the metal(s). Any of the fused C5 and C7 rings of azulene can be coordinated in various hapticities and symmetries, depending on the nature of the MLn moiety (or moieties) they are bonded to. This flexibility favors the possibility of existence of several isomers (sometimes enantiomers) of similar energy and of their interconversion in solution, in particular through haptotropic shifts. The azulene asymmetry causes dinuclear complexes to exhibit very different coordination environments (sometimes different oxidation states). In some of them, M-M bonding is preferred over M-azulene bonding. Most of the investigated complexes are expected to exhibit a rich fluxional behavior.

1. Introduction

*To whom correspondence should be addressed. E-mail: bzouchoune@ gmail.com (B.Z.); [email protected] (J.-Y.S.). (1) Maxwell, G. Chem. Rev. 1952, 50, 127. (2) See the following, for example, and references therein: (a) Sklar, A. L. J. Chem. Phys. 1937, 5, 669. (b) Berthier, G.; Pulman, A. C. R. Acad. Sci. 1949, 229, 761. (c) Julg, A. C. R. Acad. Sci. 1954, 239, 1498. (d) Pariser, R. J. Chem. Phys. 1956, 25, 1112. (e) Murakami, A.; Kobayashi, T.; Goldberg, A.; Nakamura, S. J. Chem. Phys. 2004, 120, 1245. (f) Amatatsu, Y.; Komura, Y. J. Chem. Phys. 2006, 125, 174311. (g) Verdal, N.; Rivera, A.; Hudson, B. S. Chem. Phys. Lett. 2007, 437, 38. (h) Bravaya, K. B.; Grigorenko, B. L.; Nemukhin, A. V.; Zhu, Y.-J.; Zhang, J.-P. J. Mol. Struct. (THEOCHEM) 2008, 855, 40. (3) See, for example, and references therein: Robinson, R. E.; Holovics, T. C.; Deplazes, S. F.; Powell, R. P.; Lushington, G. H.; Thompson, W. T.; Barybin, M. V. Organometallics 2005, 24, 2386.

arises, and a C2v molecular structure is expected. On the other hand, assuming that at least some degree of π localization exists, i.e. one of the Lewis formulas has a larger weight than the other one, a bond alternating structure of Cs symmetry should arise. Up-to-date theoretical calculations indicate that both C2v and Cs structures are close in energy and that their discrimination as being computed to be the ground-state equilibrium structure is very sensitive to the level of calculations and to the basis set choice.2e For example, recent CASSCF calculations found the Cs structure as being the ground-state minimum, whereas CASPT2 calculations found the C2v structure as the unique minimum. Consistently, with the weak energy difference found between both structures, the Cs geometry optimized at the CASSCF level exhibits small differences between long and short alternating C-C bonds and the energy barrier associated with the interconversion of the Cs structures corresponding to isomers of type I and II through a C2v saddle point is found to be less than 0.5 kcal/mol.2e Whatever structures of C2v (full delocalization) or Cs (bond alternation) symmetry are assumed, additional resonant Lewis formulas exhibiting zwitterionic character should be considered (Scheme 1). Structures III-V are the most frequently encountered in textbooks. Assuming a delocalized C2v geometry, they can be merged into the single formula VI. Assuming a partial delocalization of the positive charge in structures III and

r 2010 American Chemical Society

Published on Web 03/05/2010

Azulene (C10H8) and azulene derivatives1 are fascinating molecules which have attracted attention for a very long time, from both the theoretical2 and experimental3 points of view. Despite this longstanding interest, the solid-state and gas-phase structures of azulene have been debated until very recently.2e As is its isomer naphthalene, azulene is a 10-πelectron aromatic species, but the degree of π delocalization in this less symmetrical species is not as straightforward. The resonant Lewis formulas I and II (Scheme 1) can reasonably describe its π-electronic structure. Assuming that they have the same statistical weight, a fully delocalized π bonding

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Scheme 1. Major Lewis Canonical Structures of Azulene

IV generates the formulas X and XI. Structures VII and VIII are also consistent with C2v symmetry. Structure VII exhibits an aromatic cyclotropylium ring coupled with an allyl anion. Structure VIII exhibits an anionic and a cationic allyl system coupled by two double bonds. The azulene molecule can be described from structures VI and VII as resulting from the condensation of a cyclopentadienyl anion with a cyclotropylium cation. As in the case of VI, structures VII and VIII can be split into several unsymmetrical (Cs) forms exhibiting localized positive and/or negative charges (not shown here). The zwitterionic character of azulene is evidenced by a strong charge transfer absorption band, which is responsible for its deep blue color. Azulene is also well-known for its peculiar fluorescent properties associated with a S2 f S0 transition.2f Because of its properties, azulene and its derivatives are commonly used in the design of optical materials and devices. Various applications of azulenic compounds in the field of life sciences are also known.3 Azulene is a 10-π-electron species, as is its C10H8 isomer naphthalene, as well as the pentalene dianion (C8H62-) and the indenyl anion (C9H7-). Similarly to these last three species it can behave as a ligand to transition metals, to which it can donate from 2 to 10 electrons, depending on the metallic demand. It turns out that the coordination chemistry of these three isoelectronic ligands is very rich.4-7 They can bind to one or two metal centers with different hapticities. However, the experimental structural coordination (4) Selected examples of naphthalene complexes: (a) Carter, O. L.; McPhail, A. T.; Sim, G. A. J. Chem. Soc. A 1968, 1866. (b) Albright, J. O.; Datta, S.; Dezube, B.; Kouba, J. K.; Marynick, D. S.; Wreford, S. S.; Foxman, B. M. J. Am. Chem. Soc. 1979, 101, 611. (c) Hull, J. W., Jr.; Gladfelter, W. L. Organometallics 1984, 3, 605. (d) K€undig, E. P.; Perret, C.; Spichiger, S. J. Organomet. Chem. 1985, 286, 183. (e) Sch€aufele, H.; Hu, D.; Pritzkow, H.; Zenneck, U. Organometallics 1989, 8, 396. (f) Thompson, R. L.; Lee, S.; Rheingold, A. L.; Cooper, N. J. Organometallics. 1991, 10, 1657. (g) Pomije, M. K.; Kurth, C. J.; Ellis, J. E.; Barybin, M. V. Organometallics 1997, 16, 3582. (h) Bochkarev, M. N. Chem. Rev. 2002, 102, 2089. (i) K€undig, E. P.; Jeger, P.; Bernardinelli, G. Inorg. Chim. Acta 2004, 357, 1909. (j) Reingold, J. A.; Virkaitis, K. L.; Carpenter, G. B.; Sun, S.; Sweigart, D. A.; Czech, P. T.; Overly, K. R. J. Am. Chem. Soc. 2005, 127, 11146. (5) Bendjaballah, S.; Kahlal, S.; Costuas, K.; Bevillon, E.; Saillard, J.-Y. Chem. Eur. J. 2006, 12, 2048. (6) (a) Calhorda, M. J.; Romao, C. C.; Veiros, V. Chem. Eur. J. 2002, 8, 868. (b) Calhorda, M. J.; Felix, V.; Veiros, L. F. Coord. Chem. Rev. 2002, 230, 49. (c) Zagarian, D. Coord. Chem. Rev. 2002, 233, 157. (7) Ceccon, A.; Santi, S.; Orian, L.; Bisello, A. Coord. Chem. Rev. 2004, 248, 683.

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chemistry of azulene is quite diverse and we expect more original structures of azulene complexes to be characterized in the future (vide infra). This variety arises from the flexibility of the azulene π system, which lies on the edge between localization and delocalization (vide supra), from the existence of two rings of very different size (more than in the case of indenyl) and from its peculiar polar character. In this paper, we seek to provide a rationalization as complete as possible of the structural coordination chemistry of azulene by the means of density functional theory (DFT) calculations on series of mononuclear and binuclear species of various natures and electron counts. The electronic structure and bonding of the characterized compounds will be described as well as those of hypothetical species, several of them having been computed to be stable enough to be isolated: i.e., satisfying the criteria of “viable” molecules as defined recently by Hoffmann et al.8 Unless specified in the text, all the described computed structures have been characterized as minima on the potential energy hypersurface by vibrational frequency calculations (see Computational Details).

2. Preliminary Considerations 2.1. Electron-Counting Formalism. We use the same electron-counting scheme as in our previous study on pentalene complexes.5 The azulene ligand is a potential donor of 10 electrons, but the actual number of electrons donated to the metal(s) depends on the hapticity of the metal(s) and consequently can be lower than 10. Thus, we will use in the following two different electron counts for the studied compounds. (1) The total number of electrons (TNE) is the sum of all the electrons which can be potentially donated by the azulene ligand (i.e., 10), the metal valence electrons in its actual oxidation state, and the electrons donated by the other ligands. For example, in the case of (CO)3M(Az) (Az = azulene) TNE = (3  2) þ n þ 10, where n is the number of valence electrons of M(0). TNE is indicative of the electron richness of the molecule. (2) The number of metal valence electrons (MVE) corresponds to the number of electrons actually belonging to the metallic sphere. This number depends on the azulene hapticity. For example, in the case of (CO)3M(η5-Az), where all the carbon atoms of the C5 ring are bonded to M, Az behaves as a 6-electron donor, therefore MVE = (3  2) þ n þ 6. MVE indicates the electron richness of the metal(s). It is most often equal or close to 18 and is always less than or equal to TNE. 2.2. The Free Azulene Molecule. The geometry of free azulene was optimized at the same DFT level as the studied complexes in order to compare its metrical data to those of the complexed ligand. At the considered level of theory, the ground-state molecular structure was found to be of C2v symmetry. The optimized bond distances are given in Scheme 2. As in previously reported high-level calculations,2e all the peripheral C-C distances are close to 1.39 A˚ (a value consistent with a formal bond order of 1.5), whereas the central bond C(9)-C(10) exhibits a distance closer to that of a single bond (1.482 A˚). This is indicative of a substantial statistical weight of formulas I and II in Scheme 1. (8) Hoffmann, R.; Schleyer, P. v. R.; Schaefer, H. F.III Angew. Chem., Int. Ed. 2008, 47, 7164.

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Figure 1. The π MO diagram of C2v azulene. Scheme 2. DFT Optimized Distances of Azulene (in A˚) in C2v Symmetry and Atom Numbering Considered throughout the Paper

The π-type MO diagram of azulene is shown in Figure 1. It exhibits a significant gap separating the C5-centered HOMO (π5) from the C7-centered LUMO (π*6). This is the HOMO-LUMO optical transition which is associated with the strong blue color of azulene. This situation makes the C5 ring a better π-donor than the C7 ring, while the latter is a better π-acceptor than the former. Both the occupied and unoccupied sets of MO’s split into 3  b1 and 2  a2.

3. Results and Discussion 3.1. Mononuclear LnM(Az) Complexes. The number of structurally characterized complexes of the general type (9) Koch, O.; Edelmann, F.; Behrens, U. J. Organomet. Chem. 1979, 168, 167. (10) T€ ofke, S.; Behrens, U. Angew. Chem., Int. Ed. Engl. 1987, 26, 147.

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LnMAz is much smaller than that of azulene dinuclear species.9-13 The more electron-rich are 22-TNE/18-MVE species. Two of them, (CO)3Cr(η5-4,6,8-trimethylazulene)9 and (η6-C6H6)Mo(η6-Az),10 are chemically closely related but unexpectedly exhibit somewhat different structures in the solid state. In the former, the pentacoordinated chromium atom symmetrically coordinates the C5 ring.9 Thus, in this compound, the azulenic ligand can be viewed as a coordinated form of type VI (Scheme 1). On the other hand, the benzenemolybdenum species exhibits an unsymmetrical coordination mode in which the metal is bonded not only to all the atoms of the C5 ring but also to one atom of the C7 ring (namely, C(4)), the Mo-C(4) distance being the longest of the six Mo-C distances.10 This η6 coordination mode, which will be denoted η5þ1 in the following, is similar to that observed in some η6-fulvene complexes. Moreover, in this compound the azulene C-C distances exhibit long and short bond alternation. These structural features indicate that the azulene ligand should be viewed as a coordinated form of type I (Scheme 1), which donates to the metal three π(C-C) electron pairs, two from the C5 ring and one from the C7 ring.10 Similarly as in the chromium derivative, the C5 ring of the substituted azulene ligand is regularly η5-cordinated in the 22-TNE/18-MVE cation (η2:η2-COD)Rh(η5-guaiazulene)þ (guaiazulene = 1,4-dimethyl-7-isopropylazulene; COD = cyclooctadiene).11 In the less electron-rich (TNE = 20) complex Cp(DME)Lu(η7-1,4dimethyl-7-isopropylazulene) (DME = dimethoxyethane)12 the Lu(III) atom is (quite unsymmetrically) coordinated to all the atoms of the C7 ring, leading to an MVE count of 16. We have investigated the coordination of azulene to MCp and ML3 moieties for TNE varying from 18 to 26 (even values). The major computed data are given in Tables S1-S3 (Supporting Information), and selected optimized molecular structures are shown in Figure 2. We start with the case of 22-TNE species for which experimental structures are known (see above).9-11 The 18MVE rule requires the azulenic ligand to provide to the d6 metal center 6 among its 10 π electrons. In principle, this could be achieved in three different ways. The first is by coordinating the C5 ring in an η5 fashion (as in the reported Cr(CO)3 species described above9), so that the coordinated azulene would be best described by formula VI. The second is by coordinating all the atoms of the C5 ring and one atom of the C7 ring in a η5þ1 fashion (as in the reported Mo(benzene) species described above10), so that the coordinated azulene would be best described by formula I. A third possibility would be a more or less regular coordination of the C7 ring, with an azulene ligand corresponding to structure VII (heptacoordination), to structure I (unsymmetrical hexacoordination denoted η7-1 in the following), or to some intermediate situation. We have tested these various possibilities of coordination to azulene by the metal moieties Cr(CO)3, Mo(CO)3, Mo(benzene), and MnCp. Interestingly, three minima lying close in energy were found in the case of (CO)3MAz (M = Cr, Mo) (Table S1). The corresponding molecular structures are shown in Figure 2 for the case of M = Cr. Two of them (2.3 and 2.5) correspond to the (11) Rippert, A. J.; Linden, A.; Hansen, H.-J. Helv. Chim. Acta 1993, 76, 2876. (12) Fedyushkin, I. L.; Bochkarev, M. N.; M€ uhle, S.; Schumann, H. Izv. Akad. Nauk SSSR, Ser. Khim. 2003, 1899. (13) (a) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Young, V. G.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 16937. (b) Veiros, L. F. Chem. Eur. J. 2005, 11, 2505.

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Figure 2. Selected optimized molecular structures of LnMAz complexes. The relative energies between isomers are given in kcal/mol. For the experimental structure of (η6-C6H6)Mo(η5þ1-Az) (2.6), see ref 10.

pentacoordination of the C5 ring and differ in the rotational orientations of their M(CO)3 moiety. The structure which has one of the M-CO axis eclipsing C(2) is of Cs symmetry (2.5). The other structure (2.3) corresponds to a rotation of the M(CO)3 tripod of about 180° with respect to the first one but is of C1 symmetry. The asymmetry results from the fact that M is shifted somewhat toward C(8) so that a weak interaction occurs between these atoms (M-C(8) = 2.572, 2.880 A˚ for M = Cr, Mo, respectively). Thus, the C1 structure 2.3 exhibits some loose tendency for the η5þ1 coordination mode experimentally observed in (η6-C6H6)Mo(η6-Az)10 (see above). However, it should be noted that, although it exhibits the same rotational orientation as the computed C1 isomer of (CO)3Cr(Az), the X-ray structure of (CO)3Cr(η5-4,6,8trimethylazulene)9 does not show any tendency for a distortion toward the η5þ1 coordination mode. The geometry optimization of (CO)3Cr(η5-4,6,8-trimethylazulene) in this particular conformation confirms the solid-state structure, giving rise to a regular pentacoordination and long Cr 3 3 3 C(4) and Cr 3 3 3 C(8) separations (2.93 and 3.20 A˚, respectively), secured by steric repulsions between CO ligands and methyl groups. The third computed minimum for the (CO)3MAz (M = Cr, Mo) complexes corresponds to a η7 coordination mode

(2.4) of the C7 ring in which the metal atom is significantly shifted toward C(6), with the M-C(9) and M-C(10) distances being 17% and 15% longer than that of M-C(6) for M = Cr, Mo, respectively. This η7 coordination and the distribution of the optimized C-C distances (Table S1) indicate that the coordinated azulene can be described by structure VII or alternatively by both structures I and II, considering they have equal statistical weight. The potential energy surface was found to be particularly flat around this energy minimum, especially when a distortion away from Cs symmetry favoring the weight of structure I over that of structure II (or inversely) is applied (distortion toward η7-1 coordination). Causing the M-C(9) and M-C(10) distances to differ by an extent of 3% results in virtually no energy change. The 2.3 C1 pentacoordinated and the 2.5 Cs heptacoordinated structures were found to be the most stable in the case of chromium and molybdenum, respectively. This is consistent with the fact that Cr(CO)3 is a better electron acceptor and weaker electron donor than Mo(CO)3. Two energy minima were found in the case of (η6C6H6)MoAz (Table S1 and Figure 2). The most stable (by 6.0 kcal/mol) corresponds to the experimentally observed η5þ1 coordination mode10 of this compound (2.6). The

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optimization under Cs symmetry constraint of a geometry exhibiting a pentacoordinated C5 ring yielded a regular η5 structure which is 2.5 kcal/mol less stable than the η5þ1 geometry and which relaxes to the latter when symmetry constraint is released. This result is stable with respect to an increase of the integration factor. Thus, in contrast to the Mo(CO)3 moiety, the Mo(benzene) fragment cannot coordinate the C5 ring in a regular η5 coordination mode but rather leads to the unsymmetrical η5þ1 bonding mode. Therefore, the η5 coordination mode is the transition state which connects two enantiomeric η5þ1 structures. The metrical, electronic, and energetic differences between the constrained Cs (η5; Mo 3 3 3 C(4) = 2.893 A˚) and relaxed C1 (η5þ1; Mo-C(4) = 2.516 A˚) structures are quite subtle, and it is difficult to clearly identify the various factors explaining why the former is unstable with respect to the latter in the case of (η6-C6H6)MoAz. The other and less stable minimum for (η6C6H6)MoAz is an η7 isomer (2.7), similar to that found for the (CO)3MAz (M = Cr, Mo) systems. Although it is slightly distorted away from Cs symmetry for steric reasons, it does not exhibit any significant tendency for the η7-1 coordination mode. Similar η5þ1 and η7 isomers were also found for the isoelectronic CpMn(Az) complex (Table S1). In contrast to the case for (C6H6)MoAz, the η5þ1 isomer of CpMn(Az) was found to be the less stable one (by 5.5 kcal/mol). Having tested the geometry optimization results with the available experimentally characterized structure of 22-TNE complexes, we move to different TNE counts. We have investigated the ability of azulene for decahapticity, for which there is no reported example. Both 18-TNE tested models CpMAz (M = Sc, Y) converged toward an 18-MVE Cs structure (2.1) exhibiting η10 coordination (Table S2). The results being very similar for both complexes, only the scandium derivative is shown in Figure 2 and discussed thereafter. The shortest Sc-C distances correspond to the C(9) and C(10) hinge atoms (2.274 A˚), whereas the longest (2.863 A˚), associated with C(2) corresponds to a very weak bonding interaction. Thus, the metal atom is closer to the C7 ring than to the C5 ring. The ligand folding angle around the C(9)-C(10) hinge is 39°, indicative of significant ligand flexibility. Interestingly, full optimization of the isoelectronic model CpSc(naphthalene) did not lead to decahapticity, due to the planar rigidity of the naphthalene ligand. In contrast to its naphthalenic isomer, the CpScAz model exhibits closed-shell stability, as exemplified by its large HOMO-LUMO gap (1.90 eV) and the significant computed bonding energy between the azulene and ScCp fragments (8.70 eV). Thus, unlike naphthalene but similarly to pentalene5 and indenyl,13 the azulene ligand is able to donate all its 10 π electrons to a metal center by achieving full hapticity. The CpSc(η10-Az) HOMO can be described as the nonbonding 3dz2 metal orbital of the d2 Sc(I) atom. It lies isolated in the middle of a large energy gap, suggesting the possibility for a closed-shell electron count lower by 2 for the same η10 structure. Such a situation of two closed-shell electron counts for the same full-hapticity structure is known in the case of the 18-TNE and 16-TNE CpM(η8-pentalene) species.5 However, we were not able to characterize a closed-shell energy minimum for any d0 16-MVE LnM(η10-Az) cation. Considering now 20-TNE hypothetical species, the 18electron rule would require azulene donating 8 of its 10 π electrons to the metal. The Lewis structure VIII of Scheme 1 would be appropriate to satisfy the metal electron demand through a η7 coordination mode in which the C5 ring as well

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as C(4) and C(8) would be bonded to the metal (denoted η5þ2 hereafter). Structure I suggests another possibility through an η7þ1 coordination mode with the C7 ring and C(1) bonded to the metal. None of these hypothetical closed-shell situations were found to be stable for the 20-TNE (CO)3TiAz and CpVAz complexes, for which the ground state was found to be a 16-MVE η7-coordinated structure (see Table S2 and 2.2 in Figure 2). The electron deficiency of these species is exemplified by the small HOMO-LUMO gap of (CO)3Ti(η7-Az) (0.45 eV) and the triplet ground state of CpV(η7-Az) (Table S2). Structures with 16 MVE are also favored for the related 20-TNE (CO)2MoAz complex, for which the η7 (global minimum) and η5þ1 (less stable by 2.5 kcal/mol) coordination modes were found. Going to TNE = 24, the electronic demand of the metal to the azulene ligand for satisfying the 18-electron rule is 4 electrons. Several possible η4 coordination modes can be anticipated from structure I of Scheme 1. Structure VIII suggests the possibility of η3 or η4 coordination, whereas structure VI is designed for η5 coordination of the C7 ring. The global mimimum computed for the 24-TNE model (CO)3FeAz adopts an η4 coordination mode of the C5 ring (see Table S3 and 2.8 in Figure 2). Another low-energy minimum (2.9) was found. It exhibits an η4 coordination mode of the C7 ring and is also consistent with structure I. Interestingly, the global minimum of the isoelectronic CpCoAz model (not shown here) is not a η4 species. It adopts a structure of perfect Cs symmetry in which the metal atom is bonded to the C5 ring in a η5 fashion (Table S3). Inspection of its HOMO indicates that it has a large π*7(Az) character (61% localization on C(4), C(6), and C(8)) with 25% of metal contribution mixed in a nonbonding way). Thus, this CpCo(η5-Az) species is better described as an 18-MVE Co(II) complex of Az2- rather than a 20-MVE Co(0) complex of Az. Consistently, the corresponding triplet state was found lying significantly higher (by 13.2 kcal/mol) than its singlet counterpart. The second minimum of CpCoAz (not shown in Figure 2) is identical with 2.8. With 26-TNE species, the azulene ligand is expected to donate 2 π electrons to the metal fragment, presumably through η2 coordination, although η1 coordination should not be excluded. The search for low-energy minima of the 26TNE model (CO)3NiAz leads to five isomers (2.11-2.15) lying in an energy range of less than 2 kcal/mol (Table S3 and Figure 2). Each of them corresponds to the coordination of a C-C bond. Only the C(9)-C(10) hinge bond cannot be complexed. The η2 coordination found for each of these isomers is dissymmetrical, showing a displacement toward η1 coordination favoring the more electron-rich carbon atom. The computed energetic and metric data suggest that these types of 24-TNE/18-MVE complexes should be stable enough to be isolated. They should exhibit a rich dynamic behavior through interconversion between all these almost isoenergetic isomers. 3.2. Mononuclear Bis(azulene) Complexes. To our knowledge, there is only one reported complex related to the MAz2 family, namely Fe[4-endo,60 -endo-bis(azulene)], in which the two ligands are linked through a single bond of length 1.585 A˚ connecting C(4) of one ligand to C(6) of the other one.14 The Fe atom is η5-bonded to both C5 rings. To be able to dimerize, the azulenyl fragments needs to be in their -I oxidation state, leaving a 18-MVE Fe(II) center in (14) Churchill, M. R.; Wormald, J. Inorg. Chem. 1969, 8, 716.

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Figure 3. Selected optimized molecular structures of MAz2 complexes. The relative energies between isomers are given in kcal/mol.

a ferrocene-type coordination sphere. Interestingly, a related compound has been reported in the pentalene family.15,5 In principle, the lowest TNE count for a transition-metal bis(azulene) complex would correspond to a d0 species: i.e., 20 TNE. Our investigation on the hypothetical [M(Az)2]3þ (M = Sc, Y) systems did not result in the finding of any stable closed-shell situation. Thus, no analogy can be traced with the stable 20-TNE M(η8-pentalene)2 (M = Ti, Zr, Hf) species,5,16,17 presumably because of the overly large cationic charge required for this electron count. On the other hand, a stable closed-shell structure was found for the 22-TNE [M(η7þ2-Az)(η5-Az)]þ (M = Sc, Y) models with HOMO/ LUMO gaps of 0.71 and 0.68 eV for M=Sc, Y, respectively (see Table S4 in the Supporting Information and 3.1 in Figure 3 for M = Sc). These species, which favor structure IX of Scheme 1, are 16-electron systems. The search for alternative 18-TNE minima was unsuccessful. TNE = 24 is the lowest even value possible for neutral MAz2 species, the Ti and Zr cases of which were investigated. Since (15) (a) Katz, T. J.; Acton, N.; McGinnis, J. J. Am. Chem. Soc. 1972, 94, 6205. (b) Churchill, M. R.; Kuo-Kuang, G. P. Inorg. Chem. 1973, 12, 2274. (16) Jonas, K.; Kolb, P.; Kollbach, G.; Gabor, B.; Mynott, R.; Angermund, K.; Heinemann, O.; Kruger, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 1714. (17) Costuas, K.; Saillard, J.-Y. Chem. Comm. 1998, 2047.

they provided rather similar results, only the Ti system is commented. Several low-energy minima were characterized (Figure 3 and Table S4). Three singlet-state low-energy minima were found, namely Ti(η7-Az)2 (3.2), Ti(η5þ1-Az)(η7-Az)2 (3.3), and Ti(η5þ1-Az)2 (3.5), which can thus be described as 16-electron species. Consistently, they exhibit small or moderate HOMO/LUMO gaps. Such a situation favors the occurrence of triplet ground states. Indeed, two low-energy triplet state minima were found, one for a C2v Ti(η5-Az)2 structure (3.4) and one for a Cs Ti(η5-Az)(η7-Az) structure (3.6), with relative energies of 0.4 and 5.2 kcal/mol, respectively. Clearly, the 24-TNE count does not favor strong chemical stability. Going to M = Cr, Mo (TNE = 26) leads to two conformations close in energy in which both C5 rings are pentacoordinated (3.8) or both C7 rings are heptacoordinated (3.9) (with long M-C(9) and M-C(10) bonds), respectively (Figure 3 and Table S4 for M = Cr). In both conformations, each coordinated ring donates 6 electrons to the metal (structures VI and VII or Scheme 1, respectively) allowing them to reach the 18MVE count. In these corresponding C2v structures, the uncoordinated rings are eclipsed, a situation often encountered in bis(indenyl) complexes.18 In these eclipsed conformations, some π-π bonding interaction between the ligands is induced (18) Calhorda, M. J.; Veiros, L. F. J. Organomet. Chem. 2001, 635, 197.

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through the interaction with the metal. Indeed, the out-ofphase combination of the π5 azulene HOMO’s and the in-phase combination of the π6 LUMO’s interact preferencially with the metal AO’s, due to their closer proximity to them. The result is a significant depopulation of the former and population of the latter, both effects contributing to the existence of interligand π-π bonding. The 28-TNE count is formally reached for M=Fe, Ru. In the case of Fe, an 18-MVE structure has been experimentally characterized (see above).14 In this compound the four C5 rings are pentacoordinated and the two azulenic units are dimerized, with C(4) of one ligand being linked to C0 (6) of the other ligand.14 Other interligand couplings are possible, namely C(4)-C0 (4), C(6)-C0 (6), and C(4)-C0 (8). Interestingly, two different couplings are found to be close in energy in the cases of both metals (see Table S5 in the Supporting Information and Figure 3 for M = Fe): the unsymmetrical C(4)-C0 (6) coupling (3.9), as in the reported X-ray structure,14 and the symmetrical C(6)-C0 (6), coupling which yields a C2v symmetry (3.10). In the case of M = Fe, the experimentally observed unsymmetrical structure 3.9 is found to be the most stable, the symmetrical structure (3.10) being only 2.1 kcal/mol less stable. In the case of M = Ru, both structures are computed to have almost the same energy. The optimized structures corresponding to the C(4)-C0 (4) and C(4)-C0 (8) couplings (3.11 and 3.12) are found to lie slightly higher in energy in the case of both metals (see Tables S5 and S13). Finally, a minimum without any interligand coupling was found (3.13), lying at higher energy. In this structure, the metal coordinates in an η5 mode the C5 ring of one ligand as well as the C7 ring of the other ligand, leading to a d8 18-MVE system. Finally, in the 30-TNE Ni case, two low-energy minima were found (Table S5 and Figure 3). In the lowest one (3.14) the metal atom coordinates the C(1), C(2), and C(3) atoms of both ligands in a pseudotetrahedral conformation, leading to a Ni(η3-Az)2 18-MVE closed-shell species. In the second minimum (3.15), one of the C5 rings is better described as η4 bonded, with a rather long Ni-C0 (9) distance (2.198 A˚). This Ni(η3-Az)(η4-Az) isomer is also an 18-MVE species. The search for a Ni(η4-Az)2 minimum did not succeed. 3.3. Dinuclear Azulene Complexes. 3.3.1. 34-TNE Complexes. The structurally characterized dinuclear complexes (19) Edelman, F.; Tofke, S.; Behrens, U. J. Organomet. Chem. 1986, 308, 27. (20) Churchill, M. R.; Bird, P. H. Inorg. Chem. 1968, 7, 1545. (21) Cotton, F. A.; Lahuerta, P.; Stults, B. R. Inorg. Chem. 1976, 15, 1866. (22) Cotton, F. A.; Hanson, B. E. Inorg. Chem. 1976, 15, 2806. (23) Tofke, S.; Behrens, U. J. Organomet. Chem. 1988, 338, 29. (24) (a) Churchill, M. R.; Bird, P. H. Chem. Comm. 1967, 746. (b) Churchill, M. R. Inorg. Chem. 1967, 6, 190. (25) McKechnie, J. S.; Paul, I. C. Chem. Commun. 1967, 747. (26) Schlueter, A. W.; Jacobson, R. A. Inorg. Chim. Acta 1968, 2, 241. (27) Cotton, F. A.; Hanson, B. E.; Kolb, J. R.; Lahuerta, P.; Stanley, G. G.; Stults, B. R.; White, A. J. J. Am. Chem. Soc. 1977, 99, 3673. (28) Matsubara, K.; Oda, T.; Nagashima, H. Organometallics 2001, 20, 881. (29) Arce, A. J.; De Sanctis, Y.; Galarza, E.; Garland, M. T.; Gobetto, R.; Machado, R.; Manzur, J.; Russo, A.; Spodine, E.; Stchedroff, M. J. Organometallics 2001, 20, 359. (30) Cabeza, J. A.; del Rio, I.; Garcia-Granda, S.; Martinez-Mendez, L.; Moreno, M.; Riera, V. Organometallics 2003, 22, 1164. (31) Matsubara, K.; Mima, S.; Oda, T.; Nagashima, H. J. Organomet. Chem. 2002, 650, 96. (32) Churchill, M. R.; Bird, P. H. Inorg. Chem. 1968, 7, 1793. (33) Schneider, J. J.; Wolf, D.; Janiak, C.; Heinemann, O.; Rust, J.; Kruger, C. Chem. Eur. J. 1998, 4, 1982. (34) Klein, H.-F.; Hammerschmitt, B.; Lull, G.; Florke, U.; Haupt, H.-J. Inorg. Chim. Acta 1994, 218, 143.

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Figure 4. Simplified interaction MO diagram for an (ML3)2 (10-π-electron ligand) complex with 34 TNE (d6 M).

have TNE counts varying from 34 to 38.19-35 All the known 34-TNE compounds adopt the syn conformation and are, apart from two exceptions (heteronuclear species; see below),23 of the general type (ML3)2(η5:η5-Az) (M = Cr, Mo, W).19-22 In these complexes the M 3 3 3 M distance suggests the existence of weak metal-metal bonding. One of the metals is pentacoordinated to the C5 ring, and the other one is pentacoordinated to the C7 ring (C(4)-C(8)). In such a coordination mode, the azulene ligand can be described by the Lewis structure VI of Scheme 1. It donates 6 π electrons to the metal atom coordinated to the C5 ring and 4 to the other one. Thus, the overall MVE count is 34. We have shown that this particular electron count is the best one to provide closed-shell stability to the related dinuclear complexes of pentalene.5 Typical examples are anti-(Cp*Fe)2(η5:η5-C8H6)36 and synand anti-[(CO)3M]2(η5:η5-C8H6) (M = Mn, Re).37 Indeed, these species exhibit a large HOMO/LUMO gap, although they are formally electron deficient with respect to the 18electron rule. The “electron deficiency” is associated with a vacant combination of metal AO’s which is nonbonding with respect to the metal-ligand interactions but which is high-lying due to its large s and p character. The existence of this highlying nonbonding hybrid combination is due to the fact that there are six accepting metallic hybrid combinations and only five π-type donor orbitals on the conjugated ligand. This situation, which can be extended to azulene, is shown in Figure 4. Thus, the 34-TNE/34-MVE count does not require the building of a formal metal-metal bond for satisfying the closed-shell principle. Nevetheless, the known syn configurations of the pentalene analogs exhibit short M 3 3 3 M nonbonding contacts.36 This tendency is even more pronounced in all the characterized 34-TNE (ML3)2(η5:η5-Az) complexes (M 3 3 3 M ≈ 3.2-3.3 A˚ for M = Cr, Mo, W).19-22 (35) Matsubara, K.; Mima, S.; Oda, T.; Nagashima, H. J. Organomet. Chem. 2002, 650, 96. (36) Bunel, E. E.; Valle, L.; Jones, N. L.; Carrol, P. J.; Barra, C.; Gonzalez, M.; Mu noz, N.; Visconti, G.; Aizman, A.; Manrı´ quez, J. M. J. Am. Chem. Soc. 1988, 410, 6596. (37) Jones, S. C.; Hascall, T.; Barlow, S.; O’Hare, D. J. Am. Chem. Soc. 2002, 124, 11610.

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Figure 5. Selected optimized molecular structures of 34-TNE LnM2Az complexes. The relative energies between isomers are given in kcal/mol.

The 34-TNE models [M(CO3)]2(Az) (M = Cr, Mo), [Mo(C6H6)]2(Az), [Mo(C6H6)][Cr(CO)3](Az), (MnCp)2(Az), and {[Mo(C6H6)][Rh(CO)2](Az)}þ were investigated. Both syn and anti configurations were considered. The major computed data are given in Table S6 in the Supporting Information, and some relevant structures are shown in Figure 5. All the homonuclear syn and anti optimized structures exhibit the η5:η5-Az coordination mode. Some deviation from the ideal Cs symmetry is obtained in the cases of anti-(MnCp)2(η5:η5-Az) and syn-[Mo(C6H6)]2(η5:η5-Az) and is associated with rotations of the Cp and C6H6 ligands, respectively. All the computed homonuclear complexes are found to be more stable in the syn configuration (5.1, 5.3, 5.5, 5.7, and 5.9), a result fully consistent with the known experimental structures.19-22 The preference for the syn configuration is due to presence of a weak M 3 3 3 M attractive interaction resulting from the mixing of the vacant s/p hybrid combination (Figure 4) into some of the occupied weakly M-M bonding/ antibonding “t2g” combinations. The energy difference between the syn and anti configurations gives an order of magnitude of the M 3 3 3 M bond energy (∼10 kcal/mol), a value far from being negligible. In the case of the isoelectronic 34-TNE (ML3)2(η5:η5-pentalene) species, this interaction is weaker and generally dominated by steric repulsions so that the anti

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configuration is computed to be more stable than the syn configuration.5 The reason for a stronger M 3 3 3 M bonding in the case of the 34-TNE azulenic derivatives lies in its dissymmetry. The metal bonded to the C7 ring is formally a 16-MVE pentacoordinated center which has a low-lying σtype vacant hybrid orbital suited for accepting electrons from the occupied dx2-y2 (“t2g”) orbital of the other metal, which is a hexacoordinated 18-MVE center. If this interaction were strong, a real M-M single bond would be present and the two metals would be real 18-MVE centers. However, the poor overlap and the large energy difference between the two involved metal orbitals allow only weak bonding. The computed metrical data (Table S6), in particular the computed M 3 3 3 M values, are in agreement with the reported X-ray data (3.154 and 3.177 A˚ in [M(CO3)]2(Az) for M=Cr, Mo, respectively).19-22 Two 34-TNE heteronuclear species, namely syn-[Mo(C6H6)][Cr(CO)3](Az) and {syn-[Mo(C6H6)][Rh(CO)2](Az)}þ, have been structurally characterized.24 In both complexes, the Mo atom adopts the η5þ1 hapticity, whereas the other metal is unsymmetrically tetracoordinated to the C7 ring, suggesting the azulene ligand best described by the Lewis structure I of Scheme 1. We have optimized the syn and anti isomers of [Mo(C6H6)][Cr(CO)3](Az) and [Mo(C6H6)][Rh(CO)2](Az)þ, assuming that the Mo atom coordinates the C5 ring, as experimentally observed.23 Interestingly, although being of C1 symmetry, the optimized structure of the syn isomer of [Mo(C6H6)][Cr(CO)3](Az) (5.5) exhibits the “regular” η5:η5 hapticity (in contrast to the experimental η5þ1:η4 structure), whereas the less stable anti isomer (5.6) adopts the η5þ1:η4 bonding mode. These results illustrate the large flexibility of the coordination of the Mo(C6H6) moiety to the azulene ligand discussed above. The calculated structure of the syn isomer of [Mo(C6H6)][Rh(CO)2](Az)þ (5.9) exhibits the η5þ1:η4 hapticity and is very close to the experimental structure. On the other hand, the optimization of the anti conformation yielded an original structure of near Cs symmetry (5.10) in which the Mo atom is coordinated to the C5 ring and (more weakly) to C(4) and C(8) (η5þ2 hapticity) and the Rh atom is η3 coordinated to C(5), C(6), and C(7) (Table S6 and Figure 5). The analysis of the electronic structure of this {anti-[Mo(C6H6)][Rh(CO)2](η5þ2:η3-Az)}þ isomer indicates that the Mo and Rh atoms are best described as being in the þII and þI oxidation states, respectively. The azulene ligand is formally a dianion which can be described by adding two electrons to structure VIII of Scheme 1. It donates 4 (from C5) þ 4 (from C7) = 8 electrons to the Mo(II) metal and 4 (from C7) to the Rh(I) metal. Thus, the former is an 18-MVE center and the latter is a 16-MVE square-planar center. This anti isomer being only less stable by 10.5 kcal/mol than the syn one, it should be possible to experimentally characterize this new coordination mode. 3.3.2. 36-TNE Complexes. Apart from two exceptions, all the structurally characterized 36-TNE dinuclear complexes of azulenic ligands are of the type (ML2)(ML3)(η5:η3-Az) (M = Fe, Ru).24-31 These compounds exhibit a regular metal-metal single bond (M-M = 2.8-2.9 A˚), with the ML2 moiety pentacoordinated to the C5 ring and the ML3 moiety coordinated to the C(4), C(5), and C(6) atoms of the C7 ring. Thus, the coordinated azulene is best described by formula XI of Scheme 1 and acts as an 8-electron donor to the metallic system, keeping the C(7)-C(8) double bond uncoordinated. Therefore, these 36-TNE complexes are 34MVE species. The existence of an M-M single bond allows

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Figure 6. Selected optimized molecular structures of 36-TNE LnM2Az complexes. The relative energies between isomers are given in kcal/mol.

both metal centers to satisfy the closed-shell 18-electron rule. We should note that if the azulene ligand were donating all its 10 π-electrons to the metal centers (η5:η5 hapticity), there would be no need for an M-M bond to reach the 18-electron count at each metal center. The optimization of the models syn-[M(CO)2][(M(CO)3](Az) (M = Fe, Ru) leads to geometries similar to those of the known experimental structures (6.1): i.e. exhibiting η5:η3 hapticity and an M-M bond (2.702 and 2.934 A˚ for M = Fe, Ru, respectively). Their computed HOMO-LUMO gaps are large (2.18 and 2.29 eV for M = Fe, Ru, respectively). indicating significant thermal stability (see Table S7 in the Supporting Information and Figure 6). When the Cs symmetry constraint is applied, the optimization leads to the η5:η5 hapticity and no M-M bond is present. This structure, which is unstable with respect to a distortion toward the unsymmetrical η5:η3 6.1 minimum, lies 35 and 36 kcal/mol above it for M = Fe, Ru, respectively. Surprisingly, the optimized structure of the hypothetical anti configuration (6.2 in Table S7 and Figure 6) exhibits the same η5:η3 coordination mode as in its cis counterpart, although no M-M bond is possible. Thus the metal atom coordinated to the C7 ring prefers to keep a 16-MVE count rather than achieving the 18-electron rule in coordinating the C7 ring in a η5 fashion. Consistently, the anti configuration is significantly less stable than the syn structure. The M-M bond energy in the syn configuration can be estimated by the energy difference between the syn and anti isomers: i.e., ∼28 kcal/mol for M = Fe, Ru. The existence of a strong metal-metal bond in the syn-[M(CO)2][(M(CO)3](η5:η3-Az) (M = Fe, Ru) complexes is due to the fact that both metal atoms are in a pseudo-square-pyramidal environment of ligands, thus having a good d/s/p hybrid orbital pointing to the other metal to make a 2-electron/

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2-center M-M bond. Such a situation disappears in the case of isoelectronic (ML3)2(Az) species, in which the metal coordinating the C5 ring uses its hybrid to bind to the extra L ligand. In such a case, only a weak metal-metal bond (if any) is possible, involving donation of a contracted “t2g” lone pair on the metal coordinating the C5 ring into the accepting hybrid on the η3coordinated metal. This is what happens in the 36-TNE heteronuclear complex syn-[(Mo(C6H6)][Fe(CO)3]2(η5:η3-Az), the X-ray structure of which is known.23 The experimental Mo-Fe distance (3.09 A˚) is indicative of a weak bonding interaction. The calculations reproduce the syn-[(Mo(C6H6)][Fe(CO)3]2(η5:η3-Az) X-ray structure (see 6.3) and confirm the existence of a weak metal-metal bond (3.022 A˚). Thus, the 36-TNE/34MVE complexes syn-[(Mo(C6H6)][Mn(CO)3]2(η5:η3-Az) and syn-[Fe(CO)2][(Fe(CO)3](η5:η3-Az) differ only in the strength of their metal-metal bond. However, the less stable anti isomers of [(Mo(C6H6)][Fe(CO)3]2(Az) and [Fe(CO)2][(Fe(CO)3](Az) differ in their coordination mode. While in the latter (6.2) the η3-coordinated Fe atom is an electron-deficient 16-electron center, it is an η5-coordinated 18-electron center in the former (6.4). Another 36-TNE (ML3)2(Az) complex has been structurally characterized, namely anti-[Mn(CO)3]2(η5:η5-Az).32 In this compound, all the azulene π electrons are donated to the metals so that each of them reaches the 18-MVE count and there is no need for a metal-metal bond. The optimized geometry is in full agreement with the experimental one (see 6.8 in Table S7 and Figure 6). It is slightly distorted away from Cs symmetry to minimize steric repulsions between the carbonyl ligands. Since the C5 ring donates 6 electrons to Mn(1) and the C7 ring donates 4 electrons to Mn(2), the formal oxidation states of the metals are þI for Mn(1) and -I for Mn(2). This is reflected in the computed Mulliken charges, which are þ0.27 and -0.57 for the former and the latter, respectively. The anti isomer (6.8) is 8 kcal/mol more stable than the syn isomer (6.7), which exhibits the same η5:η5 coordination mode. The major difference between the structures of syn-[Mn(CO)3]2(η5:η5-Az) (6.7; 36 TNE) and syn-[Cr(CO)3]2(η5:η5-Az) (5.1; 34 TNE) is that a weak M 3 3 3 M attraction exists in the latter (Cr 3 3 3 Cr = 3.15 A˚), whereas the M 3 3 3 M interaction is repulsive in the former (Mn 3 3 3 Mn = 4.30 A˚). Calculations on the isoelectronic model (FeCp)2(Az) lead to similar results. Thus, the existence of a more or less strong metal-metal bond in the 36TNE species depends largely on the nature of the MLn units. 3.3.3. 38-, 40-, and 42-TNE Complexes. Two different types of 38-TNE cobalt species have been structurally characterized. The first one is exemplified by anti-[Co(PMe3)2][Co(PMe3)3](η5:η3-Az).34 It is a saturated 36-MVE species (no M-M bond) directly related to that of the 36-TNE/34MVE complex syn-[Fe(CO)2][(Fe(CO)3](η5:η3-Az) (strong M-M bond, see above). Calculations on the model [Co(CO)2][(Co(CO)3](Az) reproduced the experimentally observed η5:η3 coordination mode for both syn (7.1) and anti (7.2) isomers, which were found to lie very close in energy (Table S8 in the Supporting Information, Figure 7). The second type corresponds to the series anti-(CoCp0 )2(η4:η4-Az*) (Cp0 = C5H2tBu3, Az* = azulene or substituted azulene).33 Interestingly, a close inspection of the Cambridge Structural Database38 files corresponding to the three published X-ray structures33 revealed two different η4:η4 (38) CSD version 2009: Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380.

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Figure 7. Selected optimized molecular structures of 38-TNE LnM2Az complexes (Az* = 3,5,8-trimethylazulene). The relative energies between isomers are given in kcal/mol.

coordination modes. Whereas the coordinated atoms of the C5 ring are C(9), C(1), C(2), and C(3) in the three compounds, those of the C7 ring are C(5), C(6), C(7), and C(8) (structure of type a) for two of them and C(10), C(4), C(5), and C(6) (structure of type b) in the third one. In both cases the coordinated azulenic ligand is best described by the Lewis structure I (Scheme 1) and donates 4 electrons to each metal center. The calculations on anti-(CpCo)2Az reproduced the structure of type a as the lowest energy minimum (7.4). On the other hand, all attempts to optimize the structure of type b led to a different coordination mode (7.5; type c), best described as η5:η3, in which the electron-deficient Co(2) metal is bonded to C(4), C(5), and C(6). The small energy difference between types a and c suggests their easy interconversion in solution at room temperature. In the case of the syn configuration, a unique energy minimum was found (7.3). It is of Cs symmetry, and its coordination mode is best described as η5:η3, with both CpCo moieties trying to keep as far apart as possible to minimize steric repulsions. Clearly, this electron-deficient isomer of rather high energy has little chance to be isolated. We have tried to look for a structure of type b in the isoelectronic and less sterically crowed

hypothetical models [M(CO)3]2(Az) (M = Fe, Ru) but found a unique energy minimum of type a, in both the syn and anti configurations (Table S8), with the anti isomer (7.6) being the most stable by 3.1 and 4.2 kcal/mol for M = Fe, Ru, respectively. Thus, the coordination mode of these 38-TNE complexes is very sensitive to the nature of the MLn moieties and to the nature and position of the occasional substituents on the azulenic ligand. Mention should also be made of the 38-TNE syn-[Ru(CO)2][Ru(CO)3L](η5:η1-guaiazulene) (L = PR3, CNR) series,28,35 in which the Ru(CO)3L unit is bonded to the only C(8) atom. With C(4)-C(5) and C(6)-C(7) double bonds, the formally neutral guaiazulene can be described by structure IV in Scheme 1, assuming it receives a metal lone pair through the building of the Ru(2)-C(8) retrodative bond. This latter metal receives in turn a lone pair from Ru(1) through the building of a Ru-Ru bond, allowing it to achieve the 18-electron configuration. Surprisingly, calculations on the isoelectronic [Ru(CO)2][Ru(CO)4](Az) model yielded a different structural arrangement, namely [Ru(CO)2][Ru(CO)4](η5:η2 -Az), in both the syn and anti configurations (Table S8). In these isomers, with the Ru(CO)4

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Figure 9. Selected optimized molecular structures of 30- and 32-TNE LnM2Az complexes (singlet states).

Figure 8. Selected optimized molecular structures of 40- and 42-TNE LnM2Az complexes. The relative energies between isomers are given in kcal/mol.

moiety coordinating to the C(7)-C(8) double bond, the metal reaches the 18-MVE count. Therefore, there is no need for a metal-metal bond and the anti configuration (7.7) is preferred by 11.2 kcal/mol for steric reasons. Incidentally, anti-[Ru(CO)2][Ru(CO)4](η5:η2-Az) is more stable than its isomer anti-[Ru(CO)3]2(Az) by 3.2 kcal/mol. Interestingly, the disagreement between the optimized structure of the model syn-[Ru(CO)2][Ru(CO)4](η5:η2-Az) and the experimental structure of syn-[Ru(CO)2][Ru(CO)3L](η5:η1-guaiazulene) disappears when the Az ligand of the model is substituted by methyl groups at C(3), C(5), and C(8), to mimic guaiazulene. The optimized structure of syn-[Ru(CO)2][Ru(CO)4](Az*) (Az*=3,5,8-trimethylazulene) reproduces the η1 coordination of the C7 ring and the existence of a Ru-Ru bond (2.836 A˚; see 7.8 in Table S8 and Figure 7). The corresponding anti configuration exhibits the η5:η2 cordination mode and lies 26.7 kcal/ mol above its syn isomer (Table S8). Although no TNE count larger than 38 has been structurally characterized, we have investigated the possibility for stable 40- and 42-TNE complexes. The [Co(CO)3]2Az 40-TNE model was found to adopt the η3:η3 coordination mode shown in Figure 8 in both its syn (8.1) and anti (8.2) configurations. In such a bonding mode, assuming the azulene ligand to be formally neutral (formula III of Scheme 1), both Co(I) and Co(-I) centers achieve the 18-electron rule. A similar situation occurs in the isoelectronic (NiCp)2Az model (8.3 and 8.4). However, the moderate computed HOMO/ LUMO gap (1.00 eV) is consistent with the existence of a lowlying triplet state which was computed to lie ∼5 kcal/mol above the singlet ground state (Table S9 in the Supporting Information). Interestingly, alkyl-substituted derivatives of (NiCp)2Az have been synthesized and shown to be paramagnetic in solution.39 Finally, with 42 TNE’s, the η2:η2 coordination mode is adopted by the [Ni(CO)3]2Az model (see 8.5 (39) Schneider, J. J.; Denninger, U. Z. Anorg. Allg. Chem. 2004, 630, 1908.

Figure 10. Simplified interaction MO diagram for a M2 (10-πelectron ligand)2 complex with 34 TNE (M = d7).

and 8.6), the most stable conformations of which are given in Table S9 and Figure 8. 3.3.4. Complexes with Less Than 34 TNE. With TNE counts lowest than 34, electron deficiency occurs, even in the case of full ligand hapticity (i.e., η5:η5), unless it is compensated by metal-metal bonding, thus favoring the syn configuration. We have shown in the 28- to 32-TNE pentalene series that the M-M bonding created by this electron deficiency is always weak, associated with small HOMO/ LUMO gaps and low-lying triplet states.5 Our exploration of the 28- to 32-TNE (CpM)2Az and [M(CO)3]2Az series indicate that the M-M bonding is even weaker in the azulene series, suggesting that such species are likely not to be viable,8 with the possible exceptions of syn-[V(CO)3]2(η5:η5-Az) (9.2; TNE = 32) and syn-(VCp)2(η5:η5-Az) (9.1; TNE = 30) (see Table S10 in the Supporting Information and Figure 9). In these compounds, the V-V distance is rather short (2.817 and 2.777 A˚ for the former and the latter, respectively). However, the computed HOMO/LUMO gaps are small or relatively small (0.74 and 0.38 eV, respectively) and the lowest unoccupied MO’s are best described as nonbonding metallic orbitals rather than antibonding ones. The corresponding anti configurations are less stable than their syn relatives by only 9.9 and 8.6 kcal/mol, respectively. Clearly, the short V-V contacts in these compounds are not the consequence of strong metal-metal bonding. 3.4. Dinuclear Bis(azulene) Complexes. 3.4.1. General Electron-Counting Scheme. No complexes of the type M2Az2 are known so far. However, M2(pentalene)2 species

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have been isolated40 and their electronic structures has been investigated.5,40a In these compounds, the most favored closed-shell TNE count (assuming no M-M) bond is 34, Scheme 3. M2Az2 Projected structures of C2v (a) and C2h (b) Symmetriesa

a Atom labels refer to Tables S11 and S12 in the Supporting Information.

Korichi et al.

which corresponds to the crude MO diagram sketched in Figure 10. Assuming the highest symmetry possible with full ligand hapticity and no formal metal-metal bond, the 10 occupied π-type ligand combinations (left side) interact with 18 metal-metal nonbonding combinations (right side), of which 10 are of d -type and 8 of s and p type. In a picture of a one-to-one interaction diagram, each of the 10 ligand MO’s seeks for a symmetry counterpart on the M2 fragment, favoring interaction with the metallic s and p types, of which only 7 can interact by symmetry, 1 of them remaining nonbonding. Thus, 3 ligand orbitals need d type counterparts, leaving 7 nonbonding d-type combinations. The filling of this nonbonding d block corresponds to TNE = 34 and provides the molecule a significant HOMO-LUMO gap. With TNE > 34, partial decoordination is anticipated due to the

Figure 11. Selected optimized geometries of M2Az2 complexes. The relative energies between isomers are given in kcal/mol.

Article

filling of antibonding metal-ligand levels. With TNE < 34, depopulation of out-of-phase d-type combinations induces the building of metal-metal bonding. The situation is expected to remain similar in the case of hypothetical M2Az2, despite the lower symmetry. Indeed, azulene is an aromatic 10-π-electron donor, as are naphthalene and pentalenediide. Moreover, the reason one s-/p-type combination remains nonbonding in the diagram of Figure 10 should remain in the case of bis(azulene) complexes. This is one of two out-of-phase combinations of s and pσ character, only one of which is needed for interacting with the ligands. A series of M2Az2 models with 28 < TNE < 40 has been investigated in the two conformations depicted as projections in Scheme 3. The highest symmetries possible are C2v and C2h for the two possible types of conformations (a and b) depicted in Scheme 3, respectively. The major results are provided in Figure 11 and in Tables S11 and S12 in the Supporting Information. 3.4.2. 34-TNE Complexes. We start with this reference closed-shell count for which the M2(η5:η5-Az)2 (M=Mn, Re) 34-MVE structure shown in Figure 11 is found, in agreement with the general bonding picture of Figure 10. A conformation of type b, which keeps the metals equivalent, is preferred (11.1). Although no formal 2-electron/2-center M-M bond is required in the closed-shell situation sketched in Figure 10, the intermetallic distance lies within bonding values (2.78 and 2.92 A˚ for Mn and Re, respectively). This is the consequence both of a geometric constraint effect and of the presence of a significant stabilizing through-bond interaction similar to that which exists in the 34-TNE syn-(ML3)2(η5:η5-Az) complexes (vide supra). 3.4.3. Complexes with TNE > 34. With more than 34 TNE’s, the closed-shell situation illustrated in Figure 10 disappears and some distortion is expected to occur in order to restore a significant HOMO/LUMO gap. Indeed, rather unsymmetrical M2(η5:η4-Az)2 (M = Fe, Re) structures of types a and b were found for TNE = 36, in which each metal center lies in a pseudo-ML5 environment (see 11.9 and 11.10 in Figure 11 and Table S11). Not considering M-M bonding, each metal is an 18-MVE center. Structure b, in which both metals have the same oxidation state, is the most stable. The M-M distance is longer than in the 34-TNE relatives (2.860 and 2.845 A˚ in 11.9 and 11.10, respectively), whereas the metal radii are smaller. This indicates weaker through-bond interaction (if any). In the case of M = Co (TNE = 38), structure a leads to different coordination modes for the two ligands: namely, Co2(η3:η3-Az)(η5:η5-Az) (11.11) and Co2(η5:η3-Az)2 (11.12). With 18 electrons donated by the ligands in 11.11 and on the assumption that the C5-bonded metal is Co(I) and the C7-bonded metal is Co(-I), both metals are 18-MVE centers. Nevertheless, the Co-Co distance is rather short (2.693 A˚). In the most stable isomer, 11.12, both metal centers are equivalent and each Az ligand donates 8 electrons. The metal centers reach the 18-MVE count through the building of a rather long Co-Co bond (2.807 A˚). Going to M = Ni (TNE = 40) leads to a structure of type a (i.e., Ni2(η3:η3-Az)2, 11.13), lying at high energy because it requires large redox internal reorganization for the metal centers to reach the 16-MVE count associated with their pseudo-square-planar coordination. In the most stable Ni2(η4:η2-Az)2 type b isomer (11.14), the Az ligands are best (40) (a) Cloke, F. G. N.; Green, J. C.; Jardine, C. N.; Kuchta, M. C. Organometallics 1999, 18, 1087. (b) Balazs, G.; Cloke, F. G. N.; Harrison, A.; Hitchcock, P. B.; Green, J.; Summerscales, O. T. Chem. Commun. 2007, 873.

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described by structure I (or II), and the Ni atoms can be seen as 16-MVE pseudo-square-planar centers, although again the Ni-Ni distance is rather short (2.663 A˚). 3.4.4. Complexes with TNE < 34. Lowering the TNE count below 34 creates electron deficiency, which in principle should induce metal-metal bonding. This is what happens in the case of M = Mo, which is more stable in a structure of type a (11.5), with a rather small HOMO/LUMO gap of 0.50 eV, whereas the triplet state of type b (11.6) lies at almost the same energy (Table S12 and Figure 11). These results suggest weak metal-metal bonding. A similar situation occurs for the 30TNE models (M=V, see 11.3 and 11.4), which are found to have triplet ground states. Only in the case of the 28-TNE Ti2Az2 model with a structure of type b (11.2) is a significant HOMO/LUMO gap found (0.86 eV). This situation is reminiscent of the M2(pentalene)2 series,5 in which 28-TNE species have been isolated and structurally characterized.40

4. Concluding Remarks In this paper, we have investigated the electronic and molecular structures of LnMAz, MAz2, LnM2Az, and M2Az2 complexes for a large range of electron counts and provided a comprehensive rationalization of the bonding within this very large family of compounds. A particularly rich coordination chemistry of azulene is apparent, even much richer than one could determine from the published structural data. The reason for this diversity comes in part from the marked dissymmetry of azulene, which is made up of two fused rings of very different sizes. It comes also in large part from the very large electronic and structural flexibility of azulene (in contrast to its isomer naphthalene) which is able to adapt to the electronic demand of the metal(s) in a much easier way than for its isomer naphthalene, for example, which is much more rigid due to its very large aromaticity and to its ring size. It turns out that any of the Lewis formulas shown in Scheme 1 can be “frozen” (and all the other ones neglected) by complexing azulene with the proper MLn moiety or moieties. This property favors the possibility of existence of several isomers of similar energy and of their interconversion in solution, in particular through haptotropic shifts. For instance, in the case of the hypothetical (CO)3Ni(η2-Az) complex, the Ni(CO)3 unit is able to coordinate any of the peripheral C-C bonds of azulene (Figure 2), suggesting that it could move all around azulene, dynamics reminiscent of what occurs in naphthalene and anthracene mononuclear nickel complexes.41 The interconversion between enantiomers in, for example, (C6H6)Mo(η5þ1-Az) (Figure 2), syn-(CO)5Fe2(η5:η3-Az) (Figure 6), and anti-(CO)5Co2(η5:η3-Az) (Figure 7) or on substituted related complexes (to slow down some processes) should also be worth exploring. We are currently investigating by means of DFT calculations the dynamics of several of the systems described in this paper, including the dinuclear species in which M-M bonding is competing with M-azulene bonding, a situation which is not common in organometallic chemistry, where M-L bonding is often largely preferred over M-M bonding. The electronic communication between two metal centers in the dinuclear species where no M-M bond is present is also worth investigating. The fact that the two metal atoms lie in rather different environments (sometimes different oxidation states) should confer to these systems properties which are different from those of the (41) (a) Benn, R.; Mynott, R.; Topalovic, I.; Scott, F. Organometallics 1989, 8, 2299. (b) Stanger, A. Organometallics 1991, 10, 2979.

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naphthalene, pentalene, or indacene analogues, for instance. The odd-electron species and the complexes with less than 34 TNE’s should be particularly interesting. It is also worthmentioning that, although as general as possible, such a theoretical investigation cannot be fully exhaustive, especially in the field of dinuclear electron-poor species (TNE