Communication pubs.acs.org/Organometallics
Aluminabenzene−Zirconium Complexes: Intramolecular Coordination of Chloride to Aluminum Taichi Nakamura, Katsunori Suzuki,* and Makoto Yamashita* Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Bunkyo-ku, Tokyo 112-8551, Japan S Supporting Information *
ABSTRACT: The reaction of the previously reported anionic aluminabenzene with zirconium chlorides (CpZrCl3, Cp*ZrCl3, and ZrCl4) gave novel zirconium complexes bearing aluminabenzene ligand(s). The X-ray crystallographic analysis of the resulting complexes revealed that the aluminabenzene ligand(s) coordinated to the zirconium center in an η5-fashion as a six-electron donor. Chloride ligand(s) on the zirconium center intramolecularly coordinated to the Lewis acidic aluminum atom in the aluminabenzene ring. The variable-temperature 1H NMR experiment showed a dynamic process for dissociation and recoordination of chloride to the aluminum center.
T
cyclic six-π-electron system to push the negative charge to the five carbon atoms. Ashe also reported intramolecular coordination of a tethered amino group in a boratabenzene− manganese complex possessing an η5-coordination mode (E; Chart 1).4 In addition, it was reported that formation of a Lewis acid−base pair between pyridine and a boratabenzene−cobalt complex induces the η5-coordination mode (F; Chart 1).5 Thus, the boron atom in boratabenzene ligands is sufficiently Lewis acidic to accept a nucleophile.6 In contrast to the widely studied boratabenzene complexes, there is only one example of a heterobenzene complex having a group 13 element heavier than boron. The reported sole example is a gallatabenzene− manganese complex with an η5-coordination mode and the potentially Lewis acidic three-coordinate gallium atom.7 Recently, we have reported the synthesis of the anionic aluminabenzene as the first example of an aluminum-containing benzene ring (Chart 2).8 Its structural analysis indicated the
he incorporation of heteroatom(s) into the benzene molecule can alter its electronic and structural features. Replacement of one of the six carbon atoms in benzene to an anionic group 13 element gives anionic heterobenzenes having six π electrons (A; Chart 1).1 These anionic heterobenzenes Chart 1. Anionic Heterobenzene Containing a Group 13 Element and Its Coordination Complexes
Chart 2. Mesomeric Resonance Structures of Aluminabenzene 1A,B
could be utilized as an anionic six-electron ligand, which is isoelectronic with an anionic cyclopentadienyl (Cp) ligand. In this context, the coordination chemistry of boratabenzenes has been widely explored.1,2 The anionic boratabenzene usually coordinates to transition metals in an η6-coordination mode (B; Chart 1), being similar to η6-benzene complexes. The coordination mode of the anionic borabenzene ligand could be changed to η5-coordination mode, depending on the substituents on the boron atom (C; Chart 1). In fact, Ashe and Bazan et al. reported an anionic boratabenzene ligand having an amino group on the boron atom coordinated to the zirconium atom in an η5-mode (D; Chart 1).3 The π-type interaction between the empty p orbital of the boron atom and a lone pair on the nitrogen atom of the amino group broke the © XXXX American Chemical Society
presence of mesomeric resonance structures 1A with aromatic six π electrons and a negative charge on Al and 1B bearing an ambiphilic character described by the combination of a neutral aluminum center and a pentadienyl anion moiety. According to the structure 1B, the aluminabenzene would be an anionic η5pentadienyl ligand bearing a very Lewis acidic aluminum atom. Here, we report the first synthesis and characterization of zirconium complexes having aluminabenzene ligand(s). In the Received: January 26, 2015
A
DOI: 10.1021/acs.organomet.5b00073 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics solid state, the zirconium complex had an interaction between the chlorine atoms and the Lewis acidic aluminum atoms. The anionic aluminabenzene 1 could be introduced to zirconium complexes (Scheme 1). The salt elimination reaction Scheme 1. Synthesis of the Aluminabenzene−Zirconium Complexes 2a,b and 3
Figure 2. Molecular structure of 2b (50% probability thermal ellipsoids). Hydrogen atoms are omitted for clarity.
of 1-Li(Et2O) with CpZrCl3 in hexane gave (aluminabenzene)CpZrCl2 2a in 54% yield. The corresponding Cp* derivative 2b was also synthesized by a similar reaction with Cp*ZrCl3 in 63% yield. In the case of the reaction of ZrCl4 in toluene, the bis(aluminabenzene)−zirconium complex 3 could be obtained in 88% yield. The resulting 2a,b and 3 were completely characterized by 1H and 13C NMR spectroscopy and elemental analysis. The molecular structures of complexes 2a,b and 3 obtained by X-ray crystallography are shown in Figures 1−3, and selected structural parameters are summarized in Table 1. These complexes adopted sandwich-type structures having the
Figure 3. Molecular structure of 3 (50% probability thermal ellipsoids). Hydrogen atoms, the disordered isopropyl group, and solvent molecules are omitted for clarity.
aluminabenzene ligand(s) with/without a cyclopentadienyl ligand. One of the two chloride ligands of 2a,b interacted with both zirconium and aluminum atoms in a bridging μ2coordination mode. In the case of complex 3 having two aluminabenzene ligands and C1 symmetry in the solid state, each aluminum atom interacted with each chloride ligand coordinating to the zirconium atom. Due to these intramolecular interactions between Al and Cl, the aluminabenzene ligand in 2a,b and 3 coordinated to the zirconium atom in a η5coordination mode. The coordination distances between
Figure 1. Molecular structure of 2a (50% probability thermal ellipsoids). Hydrogen atoms are omitted for clarity. B
DOI: 10.1021/acs.organomet.5b00073 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
(WBI) of the Zr−Cl bonds with bridging chloride were estimated to be 0.81, 0.78, and 0.71 for 2a,b and 3 (Table 2).
Table 1. Selected Distances (Å) and Angles (deg) of 2a,b and 3 Zr−Cl Zr−Cortho Zr−Cmeta Zr−Cpara Al−Zr Al−Cl Al−Cortho ∑Alb
2a
2b
3a
2.5575(11) 2.4300(11) 2.778(4) 2.605(4) 2.589(4) 2.538(4) 2.340(4) 3.0062(13) 2.3763(13) 1.994(4) 1.993(4) 345.8
2.5417(16) 2.4278(13) 2.898(7) 2.678(6) 2.649(6) 2.550(6) 2.337(6) 3.0858(15) 2.4510(19) 1.993(5) 1.992(5) 348.0
2.6328(9) 2.6305(9) 2.599(3) 2.698(4) 2.568(3) 2.539(3) 2.545(3) 2.572(3) 2.543(3) 2.534(3) 2.393(3) 2.399(9) 2.9581(11) 2.9836(11) 2.3148(13) 2.3163(13) 2.029(3) 2.026(3) 1.993(4) 1.987(3) 344.1 344.9
Table 2. Selected WBI Values of 2a,b and 3 2a
2b
3
Zr−Cl (−Al)
0.81
0.78
0.71 0.71
Zr−Cl (no Al−Cl) Al−Cl
1.18 0.30
0.97 0.35
Zr−Al
0.13
0.11
Cp2ZrCl2
1.17 0.43 0.41 0.15 0.14
These values were smaller than those of the Zr−Cl single bonds without the Al−Cl interaction (0.97−1.18). On the other hand, the WBI bond orders between the aluminum and chlorine atoms were in the range 0.30−0.43. These values were greater than those of Al−Zr (0.11−0.15), suggesting the existence of an aluminum−chlorine interaction. The UV−vis spectra of complexes 2a,b and 3 were measured in hexane solution. The absorption maxima of 2a,b were observed at 498 nm (ε = 1500 cm−1 M−1) for 2a and 483 nm (ε = 1600 cm−1 M−1) for 2b. Since the calculated HOMOs and LUMOs of 2a,b were mainly located around the aluminabenzene ligand and zirconium atom (Figure S13 in the Supporting Information), the observed absorptions would be assignable to the LMCT-type transitions from the aluminabenzene ligand to the zirconium center. Accordingly, the HOMO−LUMO transitions of 2a,b were predicted to be 515 nm for 2a and 508 nm for 2b by the TD-DFT calculations. The absorption wavelengths of 2a,b were comparable to those of the previously reported zirconium complexes bearing pentadienyl ligands,14 supporting the pentadienyl-type η5-coordination of the aluminabenzene ligand in solution. On the other hand, the absorption bands of 3 were red-shifted relative to those of 2a,b. In the case of 3, the absorption edge reached to 850 nm with an absorption maximum of 720 nm. The TD-DFT calculation of 3 provided the HOMO−LUMO transition of 795 nm to support the experimentally obtained value. The calculated HOMO energy level of 3 was comparable to those of 2a,b, whereas the LUMO level of 3 was stabilized relative to 2a,b to lead the narrower HOMO−LUMO gap of 3 (Figure S14 in the Supporting Information). The lowering of LUMO level would be explained by the decreasing the pπ-dπ interaction between Cl and Zr atoms due to the two Al−Cl interaction in 3 to induce the decreasing electron density of Zr. The variable-temperature 1H NMR spectra of the aluminabenzene−zirconium complex 2a (Figures S1−S3 in the Supporting Information) showed the dynamic behavior due to the exchange of the Al−Cl interactions (Scheme 2). The signals assignable to the meta protons (Ha and Hb) were broad at
a
Structural parameters around the two aluminabenzene rings are described separately. bThe sum of the three Al−C bond angles.
bridged chloride and the zirconium center in 2a,b were 2.5575(11) and 2.5417(16) Å. These values were longer than the distances of conventional Zr−Cl bonds (2.446(5) and 2.436(5) Å for Cp2ZrCl2)9 with no such Al−Cl interaction. In the case of complex 3, the two Zr−Cl distances (2.6328(9), 2.6305(9) Å) were slightly elongated relative to the bridged Cl−Zr distances of 2a,b. In these complexes, the chloride ligand(s) was close to the aluminum atom rather than to the zirconium atom, reflecting the difference in atomic sizes of aluminum and zirconium atoms.10 In fact, the upper limit of the range for Al−Cl distances (2.3148(13)−2.4510(19) Å) in 2a,b and 3 was slightly shorter than those observed in the previously reported intermolecular Al−Cl interaction in zirconocene dichloride 4 (Chart 3),11 indicating the strong interaction Chart 3. Example of Intermolecular Al−Cl Interaction of Zirconocene Dichloride
between the aluminum atom and chloride ligand. In the aluminabenzene ligands in 2a,b, the two Zr−Cortho distances (2.778(4), 2.605(4) Å for 2a, 2.898(7), 2.678(6) Å for 2b) were significantly different. This unsymmetrical coordination could be attributed to the steric repulsion between Mes or TIPS substituents of aluminabenzene and the Cp/Cp* ligand. Due to the Al−Cl interactions, the geometry around the aluminum atoms in 2a,b and 3 was distorted from planar, as observed in free aluminabenzene 1, to tetrahedral (sum of the surrounding C−Al−C bond angles 344.1−348.0°). In the aluminabenzene ligand, the Al−C distances of the sixmembered ring (1.987(3)−2.029(3) Å) were elongated relative to those of the free aluminabenzene 1 (1.924(2), 1.922(2) Å), indicating the disappearance of the unsaturated character of the Al−C bonds similar to that for the reported aluminabenzene− Lewis base adduct.8 To get further insight into the structures of 2a,b and 3, we performed DFT calculations by using the Gaussian09 suite of programs.12,13 The structural optimizations of 2a,b and 3 with C1 symmetry reproduced the structures experimentally observed by X-ray crystallography. The Wiberg bond indexes
Scheme 2. Dynamic Exchange of the Coordinating Chloride to the Aluminum Atom in 2a,b
C
DOI: 10.1021/acs.organomet.5b00073 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
(4) Ashe, A. J., III; Kampf, J. W.; Waas, J. R. Organometallics 1997, 16, 163. (5) Herberich, G. E.; Engelke, C.; Pahlmann, W. Chem. Ber. 1979, 112, 607. (6) Putzer, M. A.; Rogers, J. S.; Bazan, G. C. J. Am. Chem. Soc. 1999, 121, 8112. (7) Ashe, A. J. III; Al-Ahmad, S.; Kampf, J. W. Angew. Chem., Int. Ed. Engl. 1995, 34, 1357. (8) Nakamura, T.; Suzuki, K.; Yamashita, M. J. Am. Chem. Soc. 2014, 136, 9276. (9) Prout, K.; Cameron, T. S.; Forder, R. A.; Critchley, S. R.; Denton, B.; Rees, G. V. Acta Crystallogr., Sect. B 1974, B30, 2290. (10) Emsley, J. The Elements, 3rd ed.; Oxford University Press: New York, 1998. (11) Harlan, C. J.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1995, 117, 6465. (12) Frisch, M. J., et al. Gaussian09, revision C.01; Gaussian Inc., Wallingford, CT, 2009. (13) The structure optimizations were carried out at the B3LYP/ [LanL2DZ for Zr, 6-31G(d) for Cl, Si, Al, C, H] level of theory. For details of the calculations see the Supporting Information. (14) Rajapakshe, A.; Gruhn, N. E.; Lichtenberger, L. D.; Basta, R.; Arif, A. T.; Ernst, R. D. J. Am. Chem. Soc. 2004, 126, 14105.
room temperature, whereas the signals were separated at 233 K (7.35 and 8.36 ppm). With an increase in temperature, the two signals coalesced to be one doublet signal at 7.69 ppm at 373 K. The results indicate that the dynamic exchange of the Al−Cl interactions is comparable to the NMR time scale. On the other hand, the meta protons in 2b were observed as one doublet at 7.62 ppm, indicating that the exchange of the Al−Cl interactions is faster than the NMR time scale. The meta protons of the C2-symmetrical bis(aluminabenzene)−zirconium complex 3 were observed as two doublets at 7.70 and 8.36 ppm at room temperature. The broadening of the signals could not be observed even under 373 K in toluene-d8. Due to two Al−Cl interactions, the exchange process in 3 may be more restricted in comparison with that in complexes 2a,b. In summary, aluminabenzene−zirconium complexes have been synthesized and structurally characterized. As a result of structural analysis in the solid state and in solution, interactions between the chlorine ligands on the zirconium center and aluminum atom in the aluminabenzene ligand were observed. This result indicates that the aluminum atom in the aluminabenzene has Lewis acidic character even in the ligand form on transition-metal complexes. The study of the reactivity of the aluminabenzene−zirconium complexes is currently underway.
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, tables, and CIF and xyz files giving details of experiments, crystallographic analysis, DFT calculations, and full citation of ref 12. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for K.S.:
[email protected]. *E-mail for M.Y.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Young Scientist (B) (No. 25810029) from the Japan Society for the Promotion of Science (JSPS), a Grant-in-Aid for Scientific Research on Innovative Areas “Stimulus-Responsible Chemical Species for Creation of Functional Molecules” (No. 24109012), and “New Polymeric Materials Based on Element-Blocks” (No. 25102538) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), and a Sasagawa Scientific Research Grant from the Japan Science Society. The computations were performed using the Research Center for Computational Science, Okazaki, Japan. We thank Prof. T. Hiyama, of Research and Development Initiative, Chuo University, for providing an X-ray diffractometer.
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REFERENCES
(1) (a) Fu, G. C. Adv. Organomet. Chem. 2001, 47, 101. (b) Bosdet, M. J. D.; Piers, W. E. Can. J. Chem. 2008, 86, 8. (2) (a) Sadimenko, A. P. Adv. Organomet. Chem. 2005, 89, 125. (b) Ashe, A. J., III; Al-Ahmad, S.; Fang, X. J. Organomet. Chem. 1999, 581, 92. (c) Herberich, G. E.; Ohst, H. Adv. Organomet. Chem. 1986, 25, 199. (d) Ashe, A. J., III Organometallics 2009, 28, 4236. (3) Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III.; Al-Ahmad, S.; Müller, C. J. Am. Chem. Soc. 1996, 118, 2291. D
DOI: 10.1021/acs.organomet.5b00073 Organometallics XXXX, XXX, XXX−XXX
