Dinitrogen Functionalization Affording Chromium Hydrazido Complex

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Dinitrogen Functionalization Affording Chromium Hydrazido Complex Jianhao Yin, Jiapeng Li, Gao-Xiang Wang, Zhu-Bao Yin, Wen-Xiong Zhang, and Zhenfeng Xi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00822 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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Journal of the American Chemical Society

Dinitrogen Functionalization Affording Chromium Hydrazido Complex Jianhao Yin,†,# Jiapeng Li,†,# Gao-Xiang Wang,†,# Zhu-Bao Yin,† 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 Supporting Information Placeholde ABSTRACT: A series of trinuclear and dinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were synthesized and characterized. Further reduction of the Cr(I)-N2 complexes generated anionic Cr(0)-N2 complexes, which could react with Me3SiCl to afford the first chromium hydrazido complex from N2 functionalization. These complexes were found to be effective catalysts for the transformation of N2 into N(SiMe3)3.

Dinitrogen activation and functionalization with transition metals is one of the most challenging topics in chemistry.[1] Complexes based on Fe,[2] Mo[3] and others have been investigated for homogeneous catalytic N2 reduction into ammonia.[4] On the other hand, catalytic reduction of N2 into N(SiMe3)3 is a complementary route for ammonia production, as N(SiMe3)3 can be converted into ammonia upon hydrolysis quantitatively.[5-10] The first successful example of this type reduction was reported by Shiina using CrCl3 as catalyst, generating 5.4 equivalents of N(SiMe3)3.[5] Since then, several homogeneous catalyst systems based on different transition metals, such as Fe,[6] Co,[7] V,[8] Mo[9] and W[9f] have been reported to be effective for catalysing N2 silylation. However, in contrast to the fact that many metals, as mentioned above, have been found to form dinitrogen complexes and some of them can catalyse the transformation of N2 into ammonia or N(SiMe3)3, examples of Cr(N2) complexes are very rare.[11]. In fact, as far as we are aware, there is only one example of well-defined chromium(0) dinitrogen complex (I, Scheme 1) capable of catalysing N2 silylation,[10] in which the intermediates of the catalytic process is not yet clear.[12,13] Rational design of ligands is crucial for transition metalmediated or catalysed N2 activation and functionalization. We have developed a convenient synthetic method for multi-substituted cyclopentadienyl-phosphine compounds (L, Scheme 1),[14,15] in which two commonly used strong coordination moieties of different steric and electronic properties are linked together. With these unique ligands, constrained geometry complexes (CGCs) of transition metals such as Cr could be anticipated to form [LCr-N2] complexes capable of N2 functionalization. Herein we report the synthesis and structural characterization of novel examples of trinuclear and dinuclear Cr(I)-N2 complexes bearing the cyclopentadienyl-phosphine ligands. The first chromium hydrazido complex and anionic [Cr(0)-N2]- complexes were realized and structurally characterized. A plausible intermediate involved in the Cr-catalyzed N2 silylation cycle was experimentally observed. In addition, the electronic structures and bondings of these Cr-N2 complexes were theoretically investigated.

Scheme 1. Top: Selected chromium dinitrogen complexes. Bottom: The first Chromium hydrazido complex and anionic LCr(0)-N2 complexes in this work.

The multi-substituted cyclopentadienyl-phosphine ligands and their corresponding potassium salts 1 could be readily prepared by our reported methods (Scheme 2).[15] Reaction of the potassium salts 1 with CrCl2 in THF at room temperature for 12 h generated chromium chloride complexes 2 in good isolated yields. Complexes 2a-c were characterized by single-crystal X-ray structural analysis (For details see Figures S30-S32 in SI). Complexes 2a-c have solution magnetic moments of 2.7 ± 0.1μB, 2.9 ± 0.1μB and 2.6 ± 0.1μB (measured by Evans’ method in C6D6) at 296 K, respectively, which are consistent with the spin-only value for an S = 1 spin state (2.83 μB). Reduction of the chromium chlorides 2 with 1 equivalent of KHBEt3 in THF under N2 gave a dark green solution from which the Cr(I)-N2 complexes 3 (3a: R = Ph, 3b: R = Cy) were obtained in 71% and 64% isolated yields as green powder (Scheme 2). Generation of H2 was detected in this reaction process at 4.51 ppm by 1H NMR, although chromium hydride species could not be isolated neither in N2 or Ar atmosphere. Complexes 3a and 3b have solution magnetic moments of 4.1 ± 0.2μB and 2.9 ± 0.1μB at 296 K, respectively.

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Selected bond lengths [Å] and angles [°]: Cr1–N1 1.825(2), Cr1– N3 1.894(3), Cr2–N2 1.826(3), Cr2–N5 1.863(3), N1–N2 1.169(4), N3–N4 1.081(5), N5–N6 1.108(5), N3–Cr1–N1 92.61(11), N2– Cr2–N5 91.95(12).

Scheme 2. Synthesis of complexes 2 and 3.

The molecular structures of 3a and 3b were characterized by single-crystal X-ray structural analysis. The ORTEP drawings of 3a and 3b are shown in Figure 1 and 2. In 3a, it shows a trinuclear structure with three chromium centers, coordinating four molecular N2 ligands: two in terminal end-on mode and the other two in bridging end-on mode (Figure 1). The N–N bond lengths of the two bridging N2 are 1.175 and 1.178 Å, which are longer than the two terminal N2 ligands (1.087, 1.051 Å). Compared with dinitrogenbridged dinuclear complexes, trinuclear complexes with multiple dinitrogen ligands are very rare.[16] When changing the phenyl groups on the phosphorus center in 2a to cyclohexyl groups (2b), dinitrogen-bridged dinuclear complex 3b was generated, probably due to the increase of steric hindrance on the phosphorus center. The structure of 3b is shown in Figure 2. Each Cr center in 3b coordinates one bridging N2 and one terminal N2, showing identical coordination environment. The N-N bond lengths of bridging and terminal N2 ligands in 3b are comparable to those found in 3a. The IR spectra of 3a and 3b showed strong vibration peaks (1739 cm-1, 1981 cm-1 for 3a; 1751 cm-1, 1962 cm-1 for 3b) assignable to the bridging and terminal dinitrogen ligands (For details see Figures S10 and S11 in SI), respectively. The 15N2-labeled sample of 3b, which was prepared under 15N2 atmosphere at room temperature, showed 15N–15N stretching vibrations at 1696 cm-1 and 1896 cm-1, being consistent with the mass difference between 15N2 and 14N2 (Figure S3).

Scheme 3. Further reduction of 2b (or 3b) affording anionic chromium dinitrogen complexes 4.

Figure 1. Molecular structure of complex 3a with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Cr1–N1 1.875(6), Cr1– N5 1.873(6), Cr2–N3 1.879(8), Cr2–N2 1.816(6), Cr3–N6 1.832(6), Cr3–N7 1.902(8), N1–N2 1.171(8), N3–N4 1.169(8), N5–N6 1.092(10), N7–N8 1.054(9), N3–Cr2–N2 94.40(3), N1–Cr1–N5 99.40(3), N6–Cr3–N7 95.50(3).

Figure 2. Molecular structure of complex 3b with thermal ellipsoids at 30% probability. H atoms are omitted for clarity.

Chemical reduction of 2b with excess K, Rb, or Cs in THF under N2 atmosphere resulted in a dark red solution (Scheme 3). Reduction of 3b with excess K gave the same result. With the aid of 2.2.2-cryptand, after further workup and recrystallization, the corresponding products 4 (4a: M = K; 4b: M = Rb; 4c: M = Cs) were isolated in 57%, 51% and 46% yields as red crystalline solids, respectively. Complexes 4a−c are diamagnetic. The molecular structure of 4a is depicted in Figure 3 (for X-ray structural information of 4b and 4c, please see Figures S36 and S37 in SI). In the solid states, complexes 4a-c showed similar structures with a little difference in the interaction between the alkali metal cations M+ and the N atoms in the anionic complexes.[2h,17] The anionic parts in 4a-c are almost identical, in which each chromium center coordinates with two terminal N2 ligands. In complex 4a, there is


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no coordination interaction between the K+ and the N2 ligands. Whereas in complexes 4b and 4c, the Rb+ and Cs+ coordinate with one and four N atoms, respectively, which is probably due to the increasing size of ionic radius. The Cr–N and N–N distances in these complexes are similar (1.820 to 1.834 Å) and (1.132 to 1.151 Å), respectively. Accordingly, the N–N stretching frequencies of 4a-c in solid states (1822 cm-1, 1906 cm-1 for 4a; 1823 cm-1, 1906 cm-1 for 4b; 1822 cm-1, 1907 cm-1 for 4c; 1765 cm-1, 1844 cm-1 for 15N -4a) (Figure S13-S15) showed stronger activation of N 2 2 ligands compared with those in 3. The key bond lengths and stretching frequencies of N2 ligands in 3 and 4 are summarized in Table 1. Figure 3. Molecular structure of 4a with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Cr1–N1 1.834(10), Cr1–N3 1.823(11), N1–N2 1.144(15), N3–N4 1.141(15), N1–Cr1–N3 93.79(5).

Table 1. Cr-N, N-N bond lengths, and N-N bond stretching frequencies for Cr-N2 complexes

Complex

Formal oxidation state of Cr

Cr-N (Å)

N-N (Å)

uNN (cm-1)

ref

Cr(N2) 2(PPh4NBn4)

0

1.930, 1.884

1.112, 1.120

2072, 1918

10

(dmpe) 4Cr2(C 2SiiPr3) 2( -N2)

1

1.881

1.187

11g

[(Me 3SiCC)(dmpe)2Cr]2(-N 2)

1

1.870

1.177

1680 ___

3a

1

1.816-1.902

1.054-1.171

1981(terminal), 1739(bridging) 1962(terminal), 1751(bridging)

11h This work

3b

1

1.825-1.863

1.081-1.169

4a

0

1.823, 1.834

1.141, 1.144

4b

0

1.834, 1.836

1.142, 1.146

1906, 1823

This work

4c

0

1.820-1.830

1.132-1.151

1907, 1822

This work

In order to get deeper understanding of the electronic structures of complexes 3 and 4, density function theory (DFT) calculations on 3b and 4a were carried out using Gaussian 09 (For detailed calculations, please see SI).[18] Calculations showed strong Cr(3d)to-N2(π*) back-donation in the HOMO orbitals of 3b and 4a (Figure 4). In addition, the Mulliken atomic charge distributions on the two distal N atoms (-0.270 and -0.268) in 4a are higher than those (-0.172) in 3b, indicating a more polarized nature of the terminal N2 ligand (Figure S43 and S44). The Mayer bond orders of the N–N bonds (2.35 and 2.36) in 4a are smaller than the bond orders of terminal N2 ligands in 3b (2.48), showing a higher activation of N2 in 4a. Furthermore, from the space-filling models based on the crystal structures, the terminal N2 unit had a more open site in 4a than that in bridging complexes, which might be attacked more easily by electrophiles (Figure S39).

1906, 1822

This work This work

Scheme 4. Formation of chromium hydrazido complex 5.

4a

1.0 eq. Me 3SiCl -78 °C to RT, 1 h

Cy P Cy

Et

Et

Et

Et Cr

N N SiMe3 SiMe3 5: 45%

Et

Et

Et Et Cy P Cr Cl Cy 2b: 24%

The N2 functionalization reactions were explored preliminarily. Reactions of complexes 3 and 4 with acids (H(OEt2)2BArF4, [LutH]Cl, [LutH]OTf, etc.) gave detectable amounts (~5% based on N atoms) of ammonia or hydrazine. When 4a was reacted with 1 equivalent of Me3SiCl, hydrazido complex 5 was isolated in 45% yield, along with regeneration of 2b (Scheme 4). Formation of mono-silylation product was not observed. The structure of 5 is given in Figure 5. The Cr–N distance is 1.680 Å, indicating a double bond character. The N–N distance is 1.372 Å, similar with the N–N distances in known transition metal hydrazido complexes.[6a,9b,9c,12a] Geometry optimization on 5 is in good consistence with the crystal structure. The Mayer bond order of the Cr–N and N–N bonds are 1.649 and 1.130, respectively. Calculations showed that the unpaired spin was located mainly on the Cr center as an S = 3/2 spin state in the Mulliken atomic spin density distribution (Figure S45).

Figure 4. HOMO orbitals of 3b and 4a.



Figure 5. Molecular structure of 5 with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Cr1–N1 1.680(6), N1–N2 1.372(8), N2–Si1 1.756(6), N2–Si2 1.769(6), Cr1–N1–N2 175.4(5), Si1–N2–Si2 129.8(3). Hydrazido complexes are proposed as key intermediates in N2 catalytic reduction process.[6a,9b,9c,12,13] Inspired by the isolation of 5, we explored the catalytic silylation process with complexes 2-5 to form N(SiMe3)3. The best catalytic efficiency was found when using 2c as catalyst and the product/catalyst ratio was 26.0 based on Cr atom. Although the efficiency in our system is not as high as that in Mock’s system,[10] these seminal experimental results and the isolation of complex 5 provided complementary insights for the Cr-catalyzed silylation process (For the detailed screening table, please see Table S1). In summary, we have synthesized a series of novel Cr(I)-N2 complexes as trinuclear or binuclear structures, coordinated with multiple N2 molecules. Further reduction of the dinitrogen complexes gave the anionic dinitrogen complexes as the mononuclear structure, which could react with Me3SiCl to form chromium hydrazido complex. The isolation of the chromium hydrazido complex represented one of the key intermediates in the Cr-catalyzed N2 reduction cycle. This study should have a major scientific impact on the chemistry of transition-metal (Cr in particular) mediated or catalyzed dinitrogen fixation and transformation.

ASSOCIATED CONTENT Supporting Information Experimental details, calculation details, copies of NMR and IR spectra, X-ray data for 1-5. 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.

Author Contributions #

These authors contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (Nos. 21690061, 21725201). We thank Mr. Chao Yu for crystal structure refinement and Mr. Botao Wu for his help in computational calculations.

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Dinitrogen Functionalization Affording Chromium Hydrazido Complex

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