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Spiro Metallaaromatics of Pd, Pt, Rh: Synthesis and Characterization Yongliang Zhang, Junnian Wei, Yue Chi, Xuan Zhang, Wen-Xiong Zhang, and Zhenfeng Xi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b02039 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on March 31, 2017
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Spiro Metallaaromatics of Pd, Pt, Rh: Synthesis and Characterization Yongliang Zhang,† Junnian Wei,† Yue Chi,† Xuan Zhang,† Wen-Xiong Zhang,*,† and Zhenfeng Xi*,†,‡ †
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 (China) ‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry (SIOC), Shanghai 200032, China Supporting Information Placeholder ABSTRACT: Since the concept of aromaticity represents one of the most fundamental principles in chemistry, the search for unprecedented and exciting aromatic systems, therefore, continues to drive research in this area. Herein we report the synthesis and characterization of spiro metallaaromatics, in which the transition metal (Pd, Pt, or Rh) is the spiro atom, that cross-conjugates two aromatic five-membered metallacycles. These spiro metallaaromatics tend to take square planar geometries, with the dihedral angle being influenced by the steric repulsion between the α-positioned substituents. Rationalized and classified via both experimental measurements (X-ray structural analysis, NMR spectroscopy, XPS, etc.) and theoretical analysis (DFT calculation, ISE, AICD, NICS, and CMOs), all these fundamental observations extend the concept of aromaticity and organometallic chemistry.
Spiro compounds, typically organic compounds in which 2 or 3 rings are linked together by one common atom (e.g. the spiro atom), are often present in chemistry.1,2 According to the classic “tetrahedral carbon theory” by van’t Hoff,3 spiroaromatic organic compounds I (Scheme 1) with an sp3-C as the spiro atom are impossible. However, when the spiro atom is replaced by a transition metal (II), the whole scenario will change completely, because transition metals are known to make the impossible formation or transformation possible.4 In 2002, Rzepa and co-workers theoretically proposed a novel class of spiroaromatic ring systems, in which the spiro atom (a main group atom such as P, As, Al, etc.) can itself participate in each ring, independently exhibiting πelectron aromaticity.5 In this work, we successfully synthesized and structurally characterized a series of novel spiroaromatic compounds (Scheme 1). The chemistry of metal-containing aromatics is a fascinating topic and has attracted much attention.6,7 Recently, we found that 1,4-dilithio 1,3-butadienes 1 (dilithio reagents for short)8 could react with low-valent transition metal complexes, such as Ni(COD)2, [RhCl(COD)]2 and Cu(I) salts, offering corresponding monocyclic metallaaromatics: dilithionickeloles,9 dilithio 10 rhodacycles and dicupra[10]annulenes,11 in which the butadienyl dianions behaved as non-innocent ligands. Inspired by the unique behavior of dilithio reagents, we envisioned that spiro metallaaromatics might be obtained by fusing two or more dilithio reagents with suitable transition metals. As shown in Scheme 2, the reaction between dilithio reagents 1 with 0.5 equivalent of
Pd(PtBu3)2 in a mixed solvent was carried out at 35 °C (Method A). Tetralithio spiroaromatic palladoles 2a–c were thus obtained in high isolated yields as crystalline compounds. When dilithio reagents 1 were treated with M(COD)Cl2 (M = Pt or Pd) in the presence of an excess amount of lithium (Method B), tetralithio spiroaromatic platinacycles 3a,b could be synthesized also in high isolated yields. Scheme 1. Model of Spiro Metallaaromatics
Scheme 2. Preparation of Tetralithio Spiroaromatic Palladoles 2 and Platinacycles 3
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The molecular structures of 2a–c and 3a,b were confirmed by X-ray crystallographic analysis (see supporting information for details). As they have similar structures, here we discuss the structure of 2a in detail as an example (Figure 1). The structure contains two identical palladoles that share a Pd atom. Each palladole is planar with a 540.0° sum of internal angles. The four Li atoms are located above and below the palladoles and bonded in an η5 fashion. In comparison with the C–C double and single bond distances in the dilithio reagent 1a (1.367(2) Å and 1.533(2) Å, respectively), the C–C bond lengths within the palladole are remarkably averaged (1.413(3), 1.439(3), and 1.419(3) Å), consistent with π-conjugation. The dihedral angle between the two palladoles is 50.2o, which is larger than that of 2c (28.0o), indicating that the dihedral angle is influenced by the steric repulsion between the R1 groups. Thus, the spiropalladole skeleton in 2 would tend toward planar geometry as steric hindrance decreases.
A
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frequency resonance is evidently caused by the strong shielding effect of the diatropic ring current, clearly showing the aromaticity of 2. To gain more insight into the aromaticity and electronic states of 2, density functional theory (DFT) calculations were carried out using Gaussian 09 (for more calculation details, see supporting information).13 The nucleus-independent chemical shift (NICS) is a simple and efficient tool to judge aromaticity.14 In general, negative values indicate aromaticity and positive values mean anti-aromaticity. To decrease the effect of the sterically encumbered substituents, theoretical calculations on 2c (without coordinated THF) were performed (STable 8). The large negative values of the calculated NICS (at the center of the PdC4 ring, NICS(1)zz: –15.7 ppm) indicate considerable aromaticity, consistent with the above NMR experimental results. Another convenient tool to measure the magnitude of aromaticity is the “isomerization stabilization energy” (ISE) method.15 We calculated the ISE values of 2c analogue and 2a analogue in which the coordinated THF were omitted. The ISE reactions retained the same degree of unsaturation in the reactants and products (Scheme 3). The negative energies (–21.6 kcal/mol) of 2c analogue and corresponding spirenes demonstrated and quantified its aromaticity. The ISE value of 2a analogue was also calculated to be –13.8 kcal/mol, which is smaller than that of 2c analogue and consistent with that the steric effect reduces the aromaticity. Scheme 3. ISE Evaluations of Aromaticity of Tetralithio Spiroaromatic Palladoles and Corresponding Spirenes (kcal/mol) Li
Et Et
Li
B
Li
Et Et
Li
TMS
TMS
Et
C
1.419(3) 1.439(3)
TMS
Li
Et Et
Li Me
Et
Li Et
TMS Li Me
TMS
ISE = -13.8
TMS
Li
Pd Et Li
Li Me
Pd Me Li
Et Et
ISE = -21.6
Pd Et Li
Li
Et
Pd Me Li
TMS
TMS
Li
Me
2.111(2)
Pd
1.413(3) 2.119(2)
Figure 1. (A) ORTEP drawing of 2a. The ellipsoids represent a probability of 30%. H atoms are omitted for clarity. (B) The spiroring skeleton of 2a without the Li atoms and THF for clarity. (C) Selected bond lengths (Å) of 2a. X-ray photoelectron spectroscopy (XPS) measurements were carried out to get information about the valence of the Pd atom in 2a. The XPS data detected the Pd 3d5/2 binding energy at 337.02 eV (SFigure 9), thus falling within the range for Pd(II),12 which corresponds well with a dsp2 hybridization. To further confirm and measure the aromaticity of 2, 7Li NMR was carried out in d8-THF with 0.1 M LiCl in d8-THF as the external standard. The signal attributable to 2a was found at –5.20 ppm (–5.13 ppm for 2b and –4.10 ppm for 2c). This low-
The theoretical calculations of the electronic structure were also performed. For clarity, the calculated analysis of model complex 2’ (R1 = R2 = Me, without coordinated THF) was conducted. The optimized structure of 2’ is in good agreement with the experimental structure. The selected canonical molecular orbitals indicate the conjugation across the spiroring (Figure 2, top). The HOMO orbital clearly shows the interactions between the π* orbitals of two butadienyl dianion moieties and the pz orbital of the Pd atom. The orbital structure could also explain the +2 valence of Pd. The aromaticity of 2’ was further confirmed by the anisotropy of the induced current density (AICD) analysis, which is a general method to investigate and quantify the delocalization in molecules.16 The clockwise current density vectors plotted on the AICD isosurface indicate that a diatropic ring current along the periphery of the spiropalladole ring (Figure 2, bottom), confirming the aromaticity in 2’. The AICD analysis also implies that the spiroring skeleton of 2’ is more likely to be planar geometry excluding the steric hindrance.
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Journal of the American Chemical Society when we reduced our previously reported compound 410 with an excess amount of lithium in THF at room temperature, the pentalithio spiroaromatic rhodacycle 5 was successfully obtained in 65% isolated yield and undoubtedly confirmed by single-crystal X-ray structural analysis (Figure 4).
HOMO (-2.95 eV)
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HOMO-1 (-4.54 eV)
HOMO-3 (-4.94 eV)
Scheme 4. Synthesis of Pentalithio Spiroaromatic Rhodacycle 5 THF Li HOMO-9 (-7.70 eV)
Me
HOMO-13 (-8.21 eV)
Me
THF THF TMS Li
TMS
Me Me
Rh TMS TMS
Li
Me
TMS Li
excess Li THF r.t., 24 h
THF 4
THF TMS Li Me Rh
Me Li Li Me Li TMS TMS THF THF 5: 65%
A
Figure 2. Selected canonical molecular orbitals (CMOs) (top) and AICD isosurfaces (bottom) of 2’ (R1 = R2 = Me). Computed AICD plots of 2’ by the total contribution with an isosurface value of 0.03. The magnetic field vector is orthogonal with respect to the ring plane and points upward (clockwise currents are diatropic). The adaptive natural density partitioning (AdNDP) method is useful and reliable for gaining theoretical insight into the nature of the delocalized bonding.17 Thus, the AdNDP analysis of 2’ was then carried out using Multiwfn (for details see supporting information).18 As Figure 3 shows, there are two delocalized 7c-2e πbonds on each PdC4 ring, together with one 13c-2e delocalized bond.19 Thus, the tetralithio spiroaromatic palladoles 2 could be regarded as a 10π-system.
B
1.442(8)
C
2.046(6)
13c-2e ON = 1.96 |e| 1.419(9)
Rh
1.455(8) Li 2.192(6)
7c-2e ON = 1.98 |e|
7c-2e ON = 1.96 |e|
Figure 3. Results of AdNDP delocalization. Isovalue = 0.04. The other two corresponding 7c-2e bonds are not shown. With 2 and 3 in hand, we continued to search for the possibility of other metal-containing analogues. Thus, as given in Scheme 4,
2.298(14) 2.075(9)
Figure 4. (A) ORTEP drawing of 5. The ellipsoids represent a probability of 30%. H atoms are omitted for clarity. (B) The spiroring skeleton of 5 without the four η5-coordinated Li atoms and THF for clarity. (C) Selected bond lengths (Å) of 5. The molecular structure of 5 showed similar pseudo-square planar geometry as compounds 2 and 3. It is noteworthy that five Li atoms were found in the structure of 5. The extra Li (Li3 in Figure 4) atom is located between the two rhodacycle planes. The
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Li3 has no THF coordination probably due to the steric effect of TMS. The signal attributable to Li3 in 7Li NMR was found at 6.37 ppm, a very remarkable low-field shift probably caused by the deshielding effect of the rhodacycles in 5. The charge balance in 5 is kept due to the existence of Li3. Thus, this pentalithio spiroaromatic rhodacycle 5 has the same electron numbers and similar orbital structures as 2a. In summary, this work described the synthesis, experimental measurements and theoretical analysis of a series of spiro metallaaromatics. Unique structural characteristics and bonding models were revealed. This work fills the void of spiro skeletons and aromaticity theory, and shall lead to a novel field of organometallic chemistry and aromatic system.
ASSOCIATED CONTENT Supporting Information Experimental details, calculation details, X-ray data for 2a–c, 3a,b, 5 and NMR spectra of new products. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
[email protected] *
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (Nos. 21372012, 21690061)
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