Dinitrogen Functionalization Affording Structurally Well-Defined Cobalt Diazenido Complexes
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-03-10 |
| Journal | CCS Chemistry |
| Authors | Mingdong Zhong, Xianlu Cui, Botao Wu, Gao‐Xiang Wang, Wen‐Xiong Zhang |
| Institutions | Peking University, The Synergetic Innovation Center for Advanced Materials |
| Citations | 16 |
Abstract
Section titled “Abstract”Open AccessCCS ChemistryCOMMUNICATION1 Feb 2022Dinitrogen Functionalization Affording Structurally Well-Defined Cobalt Diazenido Complexes Mingdong Zhong, Xianlu Cui, Botao Wu, Gao-Xiang Wang, Wen-Xiong Zhang, Junnian Wei, Lili Zhao and Zhenfeng Xi Mingdong Zhong 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 , Xianlu Cui Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816 , Botao Wu 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 , Gao-Xiang Wang 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 , Wen-Xiong Zhang 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 , Junnian Wei Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] 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 , Lili Zhao Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816 and Zhenfeng Xi Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] 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 https://doi.org/10.31635/ccschem.021.202100945 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Cobalt-bis(dinitrogen) complexes [LCo(N2)2]− ( 3,4) with simple and commercially available bidentate phosphine ligands (L: Cy2PCH2CH2PCy2) were synthesized and structurally characterized. Further N2 functionalization by treating the complex 3b with iPr3SiCl afforded the first structurally characterized cobalt diazenido complex 5. These complexes 3-5 were found to be effective catalysts for the transformation of N2 into N(SiMe3)3. The electronic structure of the cobalt diazenido complex 5 is supported by quantum computational calculations based on state-of-the-art energy decomposition analysis (EDA) in conjunction with the natural orbitals for chemical valence (NOCV) method. Download figure Download PowerPoint Introduction Activation and functionalization of N2 using homogeneous metal complexes have attracted considerable interest.1-3 The propensity of coordinated N2 to undergo functionalization correlates with the N2 binding mode and extent of activation. However, the construction of N-C and N-E (E = Si, Ge, B, P, etc.) bonds by the reaction of metal-bound dinitrogen with various functionalizing agents is still very much limited.4-6 Early transition-metal N2 complexes display rich reactivity with respect to electrophilic attack at the terminal N atom.7-24 In comparison, there are fewer examples of N2 functionalization of late transition-metal N2 complexes, besides iron complexes.25-33 The N2 lability and electronegativity are strongly dependent on the metal’s oxidation state, overall charge, and identity of the supporting ligands surrounding the metal center. As the electronegativity of cobalt is higher than iron, the cobalt counterparts are typically less reactive for N2 activation in isostructural Fe- and Co-N2 complexes. Thus, compared with iron, a more electron-rich metal center for cobalt is typically required to activate the N2. In 2003, the first observed dinitrogen functionalization at a cobalt center forming methyl- and silyl-diazenido Co(II) employed an anionic B-triphos ligand.34 However, the cobalt silyldiazenide was only characterized by infrared (IR) and 1H NMR spectra. Subsequently, discoveries of cobalt-based catalysts are the first non-iron or -molybdenum catalyst for NH3 production from N2,35 and the highest turnover number (TON) reported for catalytic conversion of N2 into N(SiMe3)3.36 Although the Lu group37 reported a bimetallic cobalt-mediated catalysis together with a computationally illustrated catalytic reaction mechanism, studies on direct N2 functionalization at a cobalt center are still relatively unexplored. Long et al.38 attempted to isolate CoNNSiMe3 species using neutral triphos ligands (EP3Ph, E = N or CMe) (Scheme 1a). However, due to the highly reactive silyldiazenido species interacting with other active species, further X-ray diffraction (XRD) characterization of (EP3Ph)Co(NNSiMe3) failed. In 2018, Deng and co-workers reported a cobalt(−1)bis(dinitrogen) complex with N-heterocyclic carbene (NHC) ligation and the conversion to side-on bound diazene complexes (Scheme 1a). It provided the first well-characterized intermediates in N2-reduction catalyzed by cobalt complexes.39 To the best of our knowledge, the isolation and theoretical analysis of cobalt end-on bound diazenido complexes in N2 silylation have not yet been reported. Scheme 1 | (a and b) Different modes of cobalt-mediated N2 functionalization by silylation. Download figure Download PowerPoint Results and Discussion Herein, we report the first structurally certified cobalt diazenido complex with nonclassical valence π-bonding by using a simple and commercially available diphosphine ligand (Scheme 1b). The Co(II)-chloride complex 1 in Scheme 2a could be easily prepared in a one-pot reaction from CoCl2, Cy2PCH2CH2PCy2 (dcpe), and Li[(3,5-Me2-C6H3)NPiPr2] in excellent yields (see Supporting Information). Compound 1 was characterized by single-crystal X-ray structural analysis and has solution magnetic moments of 3.1 ± 0.1 (as measured by Evans’ method40,41) in d8-tetrahydrofuran (THF) at 295 K. When 1 was treated with 1.0 equiv of potassium graphite under an atmosphere of N2, a diamagnetic species 2 was formed as judged by a sharpening resonance in its 31P NMR spectrum ( Supporting Information Figure S4). Stirring the THF solution of 2 with Na, K, or Cs (3.0 equiv) at room temperature under N2 atmosphere gave brown solutions, with the appearance of a new singlet resonance in their respective 31P NMR spectra ( Supporting Information Figures S7, S11, and S15). After work-up and recrystallization, the cobalt-bis(dinitrogen) complexes (dcpe)Co(N2)2M (sol = THF, Et2O) 3a-3c were isolated in 37%, 37%, and 34% yields, respectively. However, when the (dcpe)CoCl2 complex was directly treated with reductants such as K and KC8 under N2 atmosphere, only [(dcpe)2Co] complex was isolated (see Supporting Information Page S7). Compared with cobalt mono(dinitrogen) complexes, the binding of two or more N2 molecules to cobalt has only been observed in NHC-supported cobalt(−1)bis(dinitrogen) complexes.39,42-46 Alternatively, 3 can be prepared directly from the reaction of 1 with excess alkaline metals (Scheme 2a). Scheme 2 | (a and b) Synthetic routes to cobalt-N2 complexes. Download figure Download PowerPoint Single-crystals ( 3a, 3b, and 3c) suitable for XRD were obtained from concentrated THF/Et2O solutions at −30 °C. The structure of 3b (Figure 1) reveals that N−N bond lengths [1.128(3)-1.150(3) Å] are longer than free N2 (NN = 1.0975 Å), indicating a certain degree of N2 activation.47 This is also supported by the IR spectrum of 3b showing stronger vibration peaks (1837 and 1931 cm−1). The two bands in the IR resonance spectra are assignable to the symmetric and asymmetric stretching of the tetrahedral anion [(dcpe)Co(N2)2]-. The 15N2-labeled sample of 3b, which was prepared under 15N2 atmosphere at room temperature, showed strong IR bands at 1803 and 1881 cm−1, which could be assigned to the 15N2 stretch and then shifted relative to the 14N2 ( Supporting Information Figure S38). In solid state, irrespective of the different alkali metal ions, the Co-N [1.750(2)-1.7784(19) Å] and N−N [1.128(3)-1.150(3) Å] bond lengths in 3b are comparable with those found in 3a and 3c ( Supporting Information Figures S51 and S53). In comparison with the NHC ligand-supported Co(−1)−N2 complexes [N−N bond lengths: 1.145(6)-1.162(5) Å] reported by Deng et al.,39 the N−N distances in 3 are shorter, which might be due to the different electron-donating ability of the ligands. Figure 1 | Molecular structure of the dimer of 3b with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1-P1 2.1766(6), Co1-P2 2.1771(6), Co1-N1 1.750(2), Co1-N3 1.7703(19), Co2-P3 2.1714(6), Co2-P4 2.1734(6), Co2-N5 1.7784(19), Co2-N7 1.751(2), N1-N2 1.150(3), N3-N4 1.135(3), N5-N6 1.128(3), N7-N8 1.139(3), N1-Co1-N3 112.15(8), N5-Co2-N7 111.97(9). Download figure Download PowerPoint Further reaction of 3 with 2.2.2-cryptand resulted in the formation of [(dcpe)Co(N2)2M(crypt-222)] ( 4) (M = Na, K, Cs) via encapsulation of M+ (Scheme 2b). The crystal structures of 4 reveal that N-N bond lengths [1.129(2)-1.1418(19) Å] are similar to those in 3. These alkali metal exchanges were also monitored by IR spectroscopy, showing similar frequencies of N2 stretches compared with 3 (1897 and 1974 cm−1 for 4a; 1892 and 1970 cm−1 for 4b; 1876 and 1956 cm−1 for 4c; 1831 and 1905 cm−1 for 15N2 -4b). Overall, encapsulation of the M+ adducts by 2.2.2-crypt resulted in decreasing N2 activation. In addition, there is a less dramatic change going from 3c to 4c because Cs+ is still partially coordinated to the N2 fragments. In the crystal structure of 4c, the cobalt center is coordinated by a diphosphine ligand and two end-on N2 ligands, forming a distorted tetrahedral geometry (Figure 2). In addition to the cobalt center, the two N2 ligands are also coordinated with Cs. Figure 2 | Molecular structure of complex 4c with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1-P1 2.1472(4), Co1-P2 2.1520(4), Co1-N1 1.7686(11), Co1-N3 1.7623(11), N1-N2 1.1305(17), N3-N4 1.1357(17), N1-Co1-N3 115.18(5). Download figure Download PowerPoint Reactions of complexes 3 with Brönsted acids such as [H(OEt2)2BArF4, (ArF = 3,5-(CF3)2C6H3)) [Ph2NH2]BArF4, [Ph2NH2]OTf, etc.] failed to form their corresponding cobalt hydrazido or hydrazine species, probably due to the high sensitivity of the cobalt phosphine ligands toward acid. In being isolobal and isocharged with H+, silylium ions are known to impart enhanced kinetic stability to the resultant silyldiazenides, thus making their isolation and characterization easier.48,49 Therefore, silylation reactions of 3b were investigated. Reaction of iPr3SiCl with 3b in THF resulted in the formation of a purple solution of complex 5 (Scheme 3). The crystals of 5 suitable for XRD analysis were obtained from concentrated hexane solutions at −35 °C. Scheme 3 | Formation of cobalt diazenido complex 5. Download figure Download PowerPoint The structure of 5 in Figure 3 reveals pesudotetraheral geometry around the Co center with two phosphorus atoms and two nitrogen atoms. The Co-N bond lengths [Co1-N3 1.6741(11) Å, Co2-N1 1.6746(12) Å] are significantly shorter than typical single bonds, indicating a multiple bond characteristic.50-53 Slight bending is observed in the Co-N-N angle [Co1-N3-N4 168.94(12)°, Co2-N1-N2 163.58(12)°], which is consistent with the π-interaction between 3d(Co) and π(N=N-TIPS) (TIPS = triisopropylsilyl) orbitals. The N-N bond distances [N1-N2 1.2080(18) Å, N3-N4 1.2051(17) Å] are in the range of typical N=N double bonds, and a more acute N-N-SiiPr3 angle [N1-N2-Si1 147.54(13)°, N3-N4-Si2 143.98(12)°] suggests an sp to sp2 hybridized geometry for the N atom bound to the TIPS group.31 Its IR spectrum shows a characteristic N=N band at 1696 cm−1 further supporting the assignment of the N2 unit as being reduced to a diazenide fragment.34,38 A 15N-15N stretching vibration at 1655 cm−1 of the 15N2-labeled sample of 5 is consistent with the mass difference of 15N2 to 14N2. The N-N bond length of the bridging N2 is 1.1296(17) Å, which is longer than free N2 (NN = 1.0975 Å).47 In contrast to the iron phenyldiazenido complex {(SiPPh3)Fe(N2C6H5)} [SiPR3 represents (Si(o-C6H4PR2)3)−, R = Ph or i-Pr], diazenido 5 is diamagnetic.25 No EPR signals were observed. One 29Si-NMR resonance is present in the 29Si-NMR spectrum at −1.94 ppm. A 15N-NMR spectrum of the labeled complex 15N- 5 shows three resonances at −51.13(d, Nα), −192.26, −200.39(d, Nβ) ppm, respectively. Large separation between Nα and Nβ (Δδ 149 ppm) is fully consistent with functionalization at the dinitrogen ligand by the trimethylsilyl (TMS) group, as it is for the related Fe silyldiazenido complexes [Fe(PP)2(NN-SiMe3)][BArF4] (PP = dmpe/depe).31 Figure 3 | Molecular structure of complex 5 with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): N1-N2 1.2080(18), N3-N4 1.2051(17), N5-N6 1.1298(16), Co2-N1 1.6746(12), Co1-N3 1.6741(11), Co1-N5 1.8584(11), Co2-N6 1.8583(11), N1-N2-Si1 147.54(13), N3-N4-Si2 143.98(12), Co1-N3-N4 168.94(12), Co2-N1-N2 163.58(12). Download figure Download PowerPoint To gain insight into the bonding nature of the interactions between the RCo-center and activated N2 speices (i.e., N=N-TIPS) in molecule 5, we performed bonding analysis using energy decomposition analysis (EDA)-natural orbitals for chemical valence (NOCV) method (see Supporting Information for details).54-58 As shown in Supporting Information Table S13, more appropriate fragments for the Co-N bond in 5 can be better described as the neutral RCo(0) and N=N-TIPS species in their electronic doublet states, because of the lesser ΔEorb energy.57,58 The contribution of the orbital interaction term ΔEorb (45.5%) in molecule 5 is slightly stronger than the electrostatic attraction ΔEelstat (44.5%). The dispersion forces provide the remaining 10.0% to the total attraction, which is nonnegligible. The most important information about the orbital interactions comes from the breakdown of the ΔEorb term into the pairwise orbital contributions. Four major orbital interactions can be identified by inspecting the deformation densities Δρ(1)∼Δρ(4) associated with ΔEorb(1)∼ΔEorb(4) (Figure 4 and Supporting Information Table S13). The strongest orbital interaction ΔEorb(1) (i.e., −65.3 kcal/mol) comes mainly from the π-back donation from the 3d orbital of the Co-center to the in-plane π (antibonding orbital) of N2 species of the N=N-TIPS ligand ( Supporting Information Figures S60 and S61). The second larger (ΔEorb(2) = −29.5 kcal/mol) contribution primarily corresponds to the σ-donation from the N=N-TIPS fragment to the 4s orbital of the Co-center, as well as some polarization inside the two fragments. The third and fourth contribution (Δρ(3) and Δρ(4)) arise from the delocalized out-of-plane π bonding among the N2 species and Co-center, but obviously polarized as shown by the larger ΔEorb(3) than ΔEorb(4). The individual components and most important interacting molecular orbitals (MOs) of the neutral fragments are shown in Supporting Information Figure S61, while the two MO correlation diagrams of both neutral and ionic Co and N=N-TIPS species from resonant of complex 5 are given in Supporting Information Figure S62. Above all, the recommended electronic resonance formulas for complex 5 are shown in Scheme 4, where the interactions between Co and N=N-TIPS can be described as the combination of three effects: (1) strong π-back donation from Co-center to the in-plane π of N2 species; (2) σ-donation from N to Co center; and (3) delocalized out-of-plane π bonding among the N2 species and Co-center. Figure 4 | Shape of the deformation densities Δρ(1)-(4) associated with the orbital interactions ΔEorb(1)-(4) for complex 5 at the UB3LYP+D3(BJ)/TZ2P//UPBE0+D3(BJ)/def2-SVP level. The eigenvalues υ of the fragment orbitals give the size of the charge migration. The color code for the charge transfer is red → blue. Download figure Download PowerPoint With these cobalt-bis(dinitrogen) bidentate phosphine compounds in hand, we investigated their catalytic ability for N2 silylation ( Supporting Information Table S1). The compound 3c afforded up to 51.8 N(SiMe3)3 per complex using K and Me3SiCl as the electron and silyl cation sources, respectively. This yield is comparable with that previously reported using simple cobalt complexes bearing carbonyl, cyclopentadienyl, and bipyridyl (bpy) ligands.59 Dzik60 used PPh3 as monodentate phosphine and small amounts of N(SiMe3)3 were obtained. In comparison, when 3b was applied as the catalyst, up to 89.4 N(SiMe3)3 was obtained per complex. The TMS-TMS dimer was the major side-product, based on gas chromatography (GC) analysis, presumably from radical recombination of in situ-generated Me3Si• from the reaction of KC8 and Me3SiCl, which was studied by the Nishibayashi group.61 These experimental results and isolation of cobalt silyldiazenido complex 5 provide complementary insights for understanding the Co-catalyzed silylation process. Scheme 4 | Electronic resonance valence bond (VB) formulas for complex 5. Download figure Download PowerPoint Conclusion Novel cobalt-bis(dinitrogen) complexes with simple and commercially available diphosphine ligand have been synthesized and structurally characterized. The first structurally certified cobalt-silyldiazenido complex 5 has been thoroughly characterized. Quantum chemical calculations indicate that molecule 5 has an open-shell singlet ground state. The in-plane [TIPS-N=N] ← RCo(3d) π-back-donation is remarkably stronger than [TIPS-N=N] → RCo(4S) σ-donation/polarization, which indicates a conjugation effect and an electron transfer feature between the Co-center and N=N-TIPS fragment. Supporting Information Supporting Information is available and includes experimental details, NMR spectra (Figures S1-S33) of new products, IR spectra (Figures S34-S45), UV-vis spectra (Figures S46-S48), X-ray data for 1-5 (Figures S49-S58 and Tables S2-S11), the catalytic ability for N2 silylation (Table S1), and calculation details (Figures S59-S62 and Tables S12-S14). Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the Basic Science Center of Transformation Chemistry of Key Components of Air, National Natural Science Foundation of China (no. 21988101). M.Z. acknowledges China Postdoctoral Science Foundation (no. 2019M650294). L.Z. acknowledges the financial support from NSFC (nos. 21973044 and 21703099), Natural Science Foundation of Jiangsu Province for Youth (no. BK20170964). Acknowledgments The DFT calculations were supported by the High-Performance Computing Platform of Peking University, and the High-Performance Center of Nanjing Tech University supported the computational resources. References 1. Fryzuk M. D.Side-On End-On Bound Dinitrogen: An Activated Bonding Mode that Facilitates Functionalizing Molecular Nitrogen.Acc. Chem. Res.2009, 42, 127-133. Google Scholar 2. Burford R. J.; Fryzuk M. D.Examining the Relationship between Coordination Mode and Reactivity of Dinitrogen.Nat. Rev. Chem.2017, 1, 0026. Google Scholar 3. Chalkley M. J.; Drover M. W.; Peters J. C.Catalytic N2-to-NH3 (or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes.Chem. Rev.2020, 120, 5582-5636. Google Scholar 4. 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Google Scholar Nishibayashi Transformation of Molecular into under Reaction Google Scholar of by 4, Google Scholar Nishibayashi Transformation of Molecular into under Google Scholar Information phosphine diazenido DFT calculations were supported by the High-Performance Computing Platform of Peking University, and the High-Performance Center of Nanjing Tech University supported the computational resources.