Communication pubs.acs.org/Organometallics
Isolation of a Heavier Cyclobutadiene Analogue: 2,4-Digerma-1,3diphosphacyclobutadiene Yile Wu,† Liu Liu,*,† Jue Su,† Jun Zhu,† Zhe Ji,† and Yufen Zhao*,†,‡ †
Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, People’s Republic of China ‡ Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: The heavier cyclobutadiene analogue 2,4-digerma1,3-diphosphacyclobutadiene ([L12Ge2P2], 4; L1 = CH{(CMe)(2,6-iPr2C6H3N)}2), featuring a planar Ge2P2 four-membered ring, has been synthesized via the elimination of carbon monoxide from the corresponding phosphaketenyl germylene [L1GePCO] (2) under UV irradiation.
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Scheme 1. Preparation of Compound 2 from 1
yclobutadiene (Figure 1), the smallest antiaromatic fourπ-electron species that has a planar structure, is usually hard to prepare and isolate due to its unfavorable energy and high reactivity.1 For the last 60 years, many efforts have been undertaken to isolate stable cyclobutadiene and its derivates.2 Isolation of cyclobutadiene has become viable by coordination to metal atoms, thereby stabilizing the molecule.3 In addition, having a bulky substituent makes cyclobutadiene too sterically encumbered to react further and only produces monomer.4 In
1996, Frank et al. reported the isolation of a P4 cycle that contains a dispirocyclic system with a tetraphosphete fourmembered-ring center.5 Similar P4 cycles were obtained in some transition-metal complexes.6 Following this, tetrasilacyclobutadiene7 and tetragermacyclobutadiene,8 demonstrated by Tamao et al. and Sekiguchi et al., were isolated as transitionmetal complexes featuring a Si4 or Ge4 cyclobutadiene fragment. Compounds involving multiple bonds between phosphorus atoms and group 14 elements, in particular phosphacyclobutadiene, are fascinating and have long interested chemists because of their unique electronic properties.9 A cobalt complex of 1,3-phosphacyclobutadiene was synthesized by Schneider and co-workers via dimerization of two CP compounds.10 In 2011, Roesky et al. and Driess et al. independently reported 2,4-disila-1,3-diphosphacyclobutadiene with two-coordinated phosphorus atoms and zwitterionic Si−P bonds.11 However, phosphacyclobutadiene and compounds with Ge−P multiple bonds are still rare.12 Very recently, Driess et al. reported the first example of the 1,3-digerma-2,4-diphosphacyclobutadiene LH2Ge2P2 (LH = CH(CHNDipp)2, Dipp = 2,6-iPr2C6H3), which is a back-to-back study with ours.12a It is important to Received: March 7, 2016
Figure 1. Cyclobutadiene and heavier cyclobutadiene analogues. © XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.6b00187 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
Figure 2. Molecular structure of compound 2 with anisotropic displacement parameters depicted at the 50% probability level. The hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ge(1)−P(1) 2.5138(8), P(1)−C(1) 1.609(4), Ge(1)− N(1) 1.990(2), Ge(1)−N(2) 1.990(2); N(2)−Ge(1)−N(1) 90.92(9), Ge(1)−P(1)−C(1) 87.32(13), P(1)−C(1)−O(1) 178.7(3).
Figure 3. Molecular structure of compound 4 with anisotropic displacement parameters depicted at the 50% probability level. The hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ge(1)−P(1) 2.2540(8), Ge(1)−P(1A) 2.2611(8), Ge(1)−N(1) 1.995(2), Ge(1)−N(2) 1.991(2); N(2)−Ge(1)−N(1) 89.77(9), P(1)−Ge(1)−P(1A) 103.25(3), Ge(1)−P(1)−Ge(1A) 76.75(3). Symmetry generated (A): −x + 1/2, −y + 3/2, −z.
Scheme 2. Proposed Mechanism for the Formation of 4
Since a previous report showed that Si2P2 can be prepared via the dimerization of the SiP moiety,11 it can be assumed that a similar reaction may take place employing a germanium precursor. In addition, Bertrand presented the isolation of a free phosphinophosphinidene by irradiation-induced elimination of CO from the corresponding phosphanyl phosphaketene employing very sterically demanding substituents.17 Indeed, the photolysis of 2 (254 nm) leads to the formation of red block crystals of 4 in high yield within 10 min (Scheme 2; 86%), which is stable in air and in water even at a temperature of 100 °C. Compound 4 is the dimer of the proposed GeP intermediate 3. On the basis of density functional theory (DFT) calculations (M06-2X/Def2-SVP), intermediate 3 has a singlet ground state with 5.1 kcal/mol below the triplet state, indicating the difficulty of characterization of 3. Due to the poor solubility of 4 in all usual solvents, a solidstate NMR rather than a solution spectrum of 4 was performed. The 31P NMR spectrum exhibits a singlet at 131.9 ppm, which is notably deshielded (ca. 432 ppm) in comparison to that in 2. In the solid-state 13C NMR spectrum of 4, only one set of signals belonging to the ligand backbone was observed with the γ-CH group displaying a singlet at 97.89 ppm. The structure of 4 (Figure 3) has been characterized by single-crystal XRD; 4 crystallized in the monoclinic space group C2/c (see Table S1 in the Supporting Information for crystallography details). In the structure of 4, the Ge2P2 core is planar and diamond-shaped. The four Ge−P bond lengths of 4 (2.2540(8), 2.2540(8), 2.2611(8), and 2.2611(8) Å) are nearly equal and represent values intermediate between the GeP bond lengths of [Mes2GePMes*] (Mes = 2,4,6Me3C6H2; Mes* = 2,4,6-tBu3C6H2) (2.138(3) Å)12c and [(tBu2MeSi)2GePMes*] (2.1748(14) Å),12d and the fourmembered-ring Ge−P bond length of [{Ge(μ-PAr)}2] (Ar = 2,6-Ph2C6H3) (2.313 Å average).18 The powder X-ray diffraction pattern of 4 is consistent with that calculated from
note that several theoretical studies of germaphosphacyclobutadiene have been reported.13 Additionally, the groups of Grützmacher, Goicoechea, and Bertrand showed that phosphaethynolate anion has been widely used as a building block to prepare phosphacycles in the past few years.14 Herein, we report the facile synthesis of a 2,4-digerma-1,3-diphosphacyclobutadiene prepared by ultraviolet-light-induced elimination of CO from the N-heterocyclic phosphaketene germylene. The starting material [L1GePCO] (2, L1 = CH{(CMe)(2,6-iPr2C6H3N)}2) was readily prepared in high yield by the salt elimination reaction of the corresponding [L1GeCl] compound with NaPCO14k (Scheme 1). The 31P NMR spectrum of 2 shows a high-field singlet at −301.7 ppm which is deshielded in comparison to that in Ph3GePCO (−344.0 ppm).15 The IR spectrum of 2 shows a characteristic band at 1879.2 cm−1 (νasym), demonstrating the presence of a PCO moiety, which is close to that found for Ph3GePCO (1954 cm−1).14 The molecular structure of 2 (Figure 2) has been solved, refined, and analyzed by single-crystal X-ray diffraction (XRD) in the triclinic space group P1̅ (see Table S1 in the Supporting Information for crystallography details). This structure shows that it is a thermodynamic product featuring the phosphaketene moiety (attack by the P atom during the reaction process).15 The Ge−P distance of 2 (2.5138 Å) is longer than that in L1Ge−PPh2 (2.4746(11) Å).16 B
DOI: 10.1021/acs.organomet.6b00187 Organometallics XXXX, XXX, XXX−XXX
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electronic ground state with an energy difference of 47.1 kcal/ mol. Natural bond orbital (NBO) calculations (M06-2X/ TZVP//M06-2X/Def2-SVP) show that the Wiberg bond indices (WBIs) of the Ge−P bonds are 1.12 and 1.10. The NBO charges of Ge and P are +1.27 and −0.81 a.u., respectively. Furthermore, the NBOs corresponding to the Ge2P2 fragment show four Ge−P single bonds and two lone pairs of electrons at each P center (Figure 4b−g). Two lonepair orbitals on each P are further confirmed in the highest occupied molecular orbital (HOMO) and HOMO-1 of 4 (Figure 5). Therefore, the electronic structure of 4 is best described by the ylide-like resonance structure form 4A (Scheme 2), similar to that observed for 2,4-disila-1,3diphosphacyclobutadiene.10 The second-order perturbation theory of the NBO method shows high stabilization energy (approximately 50.0 kcal/mol) by two-electron donor−acceptor interactions from the lone pairs of P atoms into the vacant orbitals of Ge atoms (see Figure S9 in the Supporting Information for details). The computed nucleus independent chemical shift19 (M06-2X/6-311G(d,p)//M06-2X/Def2-SVP) NICS(0) and NICS(1) values are −5.97 and −2.95 ppm, respectively. These data are an indication that the Ge2P2 fourmembered ring of 4 has little to no aromaticity. In summary, we have reported the synthesis and characterization of a heavier analogue of cyclobutadienes, namely the 2,4-digerma-1,3-diphosphacyclobutadiene 4. This compound was prepared by irradiation-induced elimination of carbon monoxide from the corresponding phosphaketenyl germylene 2. On the basis of DFT calculations, the most plausible electronic structure of 4 is the zwitterionic form 4A with a planar Ge2P2 four-membered ring. The coordination ability of 4 toward transition-metal complexes is currently under active investigation.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00187. NMR spectra, crystal data and structure refinement details of 2 and 4, powder X-ray diffraction pattern of 4, and computational details (PDF) Crystallographic data for 2 (CIF) Crystallographic data for 4 (CIF)
Figure 4. Natural bond orbital analysis of 4: (a) NBO charges given in atomic units and WBIs given in parentheses; (b) Ge1−P1 σ bond; (c) Ge2−P1 σ bond; (d) Ge2−P2 σ bond; (e) Ge1−P2 σ bond; (f) lone pair of electrons at P1; (g) lone pair of electrons at P1.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for L.L.:
[email protected]. *E-mail for Y.Z.:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 5. (left) HOMO of 4. (right) HOMO-1 of 4.
ACKNOWLEDGMENTS This work was supported by the Chinese National Natural Science Foundation (No. 21375113) and the National Basic Research Program of China (2012CB821600). We thank the reviewers for their valuable remarks. Thanks are also given to Dr. D. A. Ruiz from UCSD for English improvement.
the single-crystal X-ray diffraction data (see Figure S7 in the Supporting Information), indicating the phase purity of 4. To gain more insight into the electronic structure of 4, DFT calculations were carried out at the M06-2X/Def2-SVP level of theory. The optimized structural parameters are in good agreement with the crystal structure of 4 (see Figure S8 in the Supporting Information for details). Calculations at both the singlet and triplet states demonstrate the singlet state to be the
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DOI: 10.1021/acs.organomet.6b00187 Organometallics XXXX, XXX, XXX−XXX
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NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper that was published on May 5, 2016, a reference was omitted. In 1986, Nixon et al. reported the first synthesis of a 1,3-diphosphacyclobutadiene ring as a transitionmetal complex. In the version of this paper that appears as of May 27, 2016, a new reference is given (ref 20) and a revised Figure 1 is provided.
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DOI: 10.1021/acs.organomet.6b00187 Organometallics XXXX, XXX, XXX−XXX
