Synthesis, structure and catalytic activity of rare-earth metal amino complexes incorporating imino-functionalized indolyl ligand

doi:10.1016/j.jorganchem.2020.121661

Journal of Organometallic Chemistry

Volume 934, 15 February 2021, 121661

https://doi.org/10.1016/j.jorganchem.2020.121661 Get rights and content

Highlights

•
A new imino-functionalized indolyl ligand were synthesized.
•
Three kinds of rare-earth metal amido complexes have been obtained.
•
Complexes could catalyze the guanylation reaction under solvent-free condition.

Abstract

The reactions of the imino-functionalized indolyl ligand (HL, L = 3-(4-Me₂N-C₆H₄CH=N-CH₂CH₂)C₈H₅N) with the rare-earth metal amides [(Me₃Si)₂N]₃RE(µ-Cl)Li(THF)₃ producing different types of rare-earth metal amido complexes were investigated. The reactions of HL with 1 equiv. of [(Me₃Si)₂N]₃RE(µ-Cl)Li(THF)₃ generated a series of hetero-nuclear bimetallic rare-earth metal amino complexes {[η¹:µ-η²-3-(4-Me₂N-C₆H₄CH=N-CH₂CH₂)C₈H₅]RE[N(SiMe₃)₂]₂(µ-Cl)Li(THF)} (RE = Y(1), Sm(2), Gd(3), Er(4), Yb(5)). By extending the reaction time, only the reaction of HL with [(Me₃Si)₂N]₃Gd(µ-Cl)Li(THF)₃ gave an unexpected binuclear rare-earth metal complex {[(µ-η⁵:η¹):η¹:η¹-3-[(Me₂N)₂-C₁₄H₉]-(NCH₂CH₂-C₈H₅N)₂]Gd₂[N(SiMe₃)₂]₃} (6) incorporating a novel polycyclic ligand through C-C and C-N coupling. Treatment of HL with [(Me₃Si)₂N]₃Sm(µ-Cl)Li(THF)₃ in a 2:1 ratio generated the bis(indolyl) heteronuclear bimetallic rare-earth metal amino complex {(η¹:η¹-[µ-η²:η¹-3-(4-Me₂N-C₆H₄CH=N-CH₂CH₂)C₈H₅]Li[µ-η²:η¹-3-(4-Me₂N-C₆H₄CH=N-CH₂CH₂)C₈H₅])Sm[N(SiMe₃)₂]₂} (7) in low yield probably due to accompanying with the formation of the complex 2. The above results indicated that reaction conditions play important roles in the formation of different coordination modes of the imino-functionalized indolyl rare-earth metal amido complexes. All new complexes 1−7 are fully characterized including X-ray structural determination. The catalytic activity of complexes 1-7 for the addition of amines to carbodiimides was explored. The results showed that all complexes displayed an excellent activity towards the addition of amines to carbodiimides producing guanidine under solvent-free condition.

Graphical abstract

Introduction

Functionalized indolyl ligands are used widely in organometallic and coordination chemistry of rare-earth metals for their strong electron-donating indolyl ring and the precise tailoring of the metal coordination sphere, and enable the complexes to be used as homogeneous catalysis [1]. Various rare-earth metal complexes (alkyls, amides) stabilized by functionalized indolyl ligands have been synthesized and structurally characterized [2]. Moreover, these complexes have been found to be efficient catalysts or pre-catalysts for C single bond C [2c-d, 2e, 2g] C-N [2h] and C-P [2a-b, 2i] bond construction. Among these work, the reactions of different pyrrolyl-functionalized indoles with rare-earth metal amides [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ generated different kinds of rare-earth metal amido complexes [2f-g]. The findings encouraged us to explore the reactivity of imino-functionalized indolyl ligand with rare-earth metal amides.

Many substituted guanidines have biologically and pharmaceutically activities [3], such as antimicrobial [4], anti-cancer agents [5]. They are also widely used as ligands in organometallic and coordination chemistry [6]. Meanwhile guanidine derivatives are used in many kinds of base-catalyzed reactions [7]. As a result, various methods have been explored for the synthesis of guanidines [8], among them, metal catalytic addition of amines to carbodiimides is a straightforward and atom-economic route to substituted guanidine [9]. Since Hou's group employed the half-sandwich lanthanide alkyl complexes as catalysts for this reaction [10a], the development of rare-earth metal catalysts for the transformation has attracted increasing attention [10]. Shen's group found ytterbium triflate to be efficient catalyst for the reaction under solvent-free condition [10c]. This finding aroused chemists’ enthusiasm and many kinds of rare-earth metal complexes with different ancillary ligands had been found for this reaction under solvent-free condition subsequently, such as β-diketiminate [10d, 10f], N-heterocyclic carbene [10e], and bridged bis(amidate) [10g]. However, the drawbacks such as steric of ligands and the ionic radii of rare-earth metals in most cases remain to be solved. Thus, searching for new kinds of high efficiency catalysts to synthesis of substituted guanidine under solvent-free condition is still required.

We herein report the synthesis and structural characterization of new rare-earth metal amides supported by imino-functionalized indolyl ligand. The reaction of imino-functionalized indolyl ligand with rare-earth metal amides [(Me₃Si)₂N]₃RE(µ-Cl)Li(THF)₃ obtained three kinds of rare-earth metal complexes under different conditions. The structures of new complexes 1-7 are elucidated by X-ray crystallography and spectrometry analysis. Their catalytic addition of arylamines to carbodiimides under solvent-free condition was examined.

Section snippets

Ligand synthesis

The reaction of tryptamine with p-(N, N-dimethylamino) benzaldehyde in ethanol at room temperature gave 4-(((2-(1H-indol-3-yl)ethyl)imino)methyl)-N, N-dimethylaniline (HL) in high yield (Scheme 1).

Synthesis and characterization of the hetero-nuclear bimetallic rare-earth metal amino complexes

Treatments of [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with 1 equiv. of 4-(((2-(1H-indol-3-yl)ethyl)imino)methyl)-N,N-dimethylaniline (HL) at 80 °C in toluene for 24 h generated a series of hetero-nuclear bimetallic rare-earth metal amido complexes{[η¹:µ-η²-3-(4-Me₂N-C₆H₄CH=N-CH₂CH₂)C₈H₅]RE[N(SiMe₃)₂]₂(µ

Conclusions

In summary, the reaction of rare-earth metal amides [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with the imino-functionalized indolyl ligand (HL) under different reaction conditions generated three kinds of novel rare-earth metal amido complexes, including heteronuclear bimetallic monoindolyl complexes 1-5, homonuclear bimetallic bis(indolyl) complex 6 and heteronuclear bimetallic bis(indolyl) complex 7. The results indicated that the imino-functionalized indolyl ligand had multiple reactivity with rare-earth

Materials and procedures

All syntheses and manipulations of air- and moisture-sensitive materials were performed under dry argon and an oxygen-free atmosphere using standard Schlenk techniques or in a glovebox. All solvents were refluxed and distilled over sodium benzophenone ketyl under argon prior to use unless otherwise noted. Other reagents were used in their commercial form without further purification. [(Me₃Si)₂N]₃RE(µ-Cl)Li(THF)₃(RE = Y, Sm, Gd, Er, Yb) were prepared according to literature methods [15].

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant No. 21602157), Anhui Provincial Natural Science Foundation (grant No. 1908085QB50), Anhui Laboratory of Molecule-Based Material (grant No. fzj19015), The Excellent Young Talents Fund Program of Higher Education Institutions of Anhui Province (grant No. gxyq2019041).

References (15)

Y. Shimazaki et al.
Coord. Chem. Rev.
(2009)
T. Matsugi et al.
Macromolecules
(2002)
Y. Yang et al.
J. Polym. Sci., Part A
(2008)
P.L. Diaconescu
Acc. Chem. Res.
(2010)
S.-J. Lou et al.
J. Am. Chem. Soc.
(2020)
X. Cong et al.
J. Am. Chem. Soc.
(2020)
X. Zhu et al.
Chem. Commun.
(2012)
X. Zhu et al.
Inorg. Chem.
(2012)
G. Zhang et al.
Chem. Eur. J.
(2015)
G. Zhang et al.
Dalton Trans.
(2016)
Z. Feng et al.
Dalton Trans.
(2015)
S. Yang et al.
Dalton Trans.
(2014)
S. Yang et al.
Sci China Chem
(2014)
L. Guo et al.
Inorg. Chem.
(2015)
G. Zhang et al.
Organometallics
(2015)
X. Zhu et al.
Organometallics
(2016)
X. Zhu et al.
Chin. J. Chem.
(2020)
R.G.S. Berlinck
Nat. Prod. Rep.
(1996)
R.G.S. Berlinck et al.
Nat. Prod. Rep.
(2010)
S. Tahir et al.
Bioorg. Chem.
(2015)
R. Kuppusamy et al.
Org. Biomol. Chem.
(2018)
Y.-C. Lin et al.
J. Am. Chem. Soc.
(2020)
V. Previtali et al.
Med. Chem. Commun.
(2018)
M. Staszewski et al.
Med. Chem. Commun.
(2019)
R. Blaszczyk et al.
ACS Med. Chem. Lett.
(2020)

There are more references available in the full text version of this article.

Cited by (4)

Synthesis and characterization of Bis(β-diketiminato) rare-earth amido complexes, their activity for catalytic addition of amines to carbodiimides
2022, Journal of Organometallic Chemistry
Citation Excerpt :
The catalytic formation of guanidines by addition of amines to carbodiimides is of great interest as it provides a straightforward and atom-economical approach to access multi-substitution guanidines, which plays an importance role in organic transformations and many biological and pharmaceutical compounds [14–17]. A great many rare-earth metal complexes have been found to serve as pre-catalysts in this catalytic addition reaction [18–28]. Among them, rare-earth amides are particularly attractive.
Reactions of anhydrous RECl₃ (RE = Sm, Nd) with 2 equiv. of NaL^2-Me (L^2-Me = [N(2-MeC₆H₄)C(Me)]₂CH⁻) afforded the dimeric chlorides [Sm(L^2-Me)₂(μ-Cl)]₂ (1) and [Nd(L^2-Me)₂(μ-Cl)]₂ (2). Salt metathesis reactions between chloride complexes and NaN(SiMe₃)₂ in THF yielded a series of bis(β-diketiminato) rare-earth amido complexes RE(L^2-Me)₂[N(SiMe₃)₂] (RE = Sm (3), Nd (4), Yb (5), Y (6), Pr (7)). Complexes 1−7 were fully characterized including X-ray single crystal structure analyses. Complexes 3−7 can serve as efficient pre-catalysts for catalytic addition of amines to carbodiimides to afford multi-substituted guanidines. Catalytic activity was influenced by the rare-earth metals with an active sequence of Y (6) ∼ Yb (5) < Pr (7) < Nd (4) < Sm (3). The present catalytic addition reaction using complex 3 showed a wide scope of substrates including primary and secondary aromatic amines and aliphatic cyclic amines. The mechanism investigation by the isolation and characterization of the intermediates of the amido complexes RE(L^2-Me)₂(NHPh)(THF) (RE = Sm (8), Nd (9), Yb (10)) and guanidinate complexes Sm(L^2-Me)₂[(C₆H₅NH)C(NCy)₂] (11) and Nd(L^2-Me)₂[(C₆H₅N)C(NHCy)(NCy)] (12). Their reactivity revealed that the present catalytic addition proceeded through amine-exchange reaction between the pre-catalyst amido complex and an amine to a novel amido complex, nucleophilic addition of the formed amido species to a carbodiimide to yield a guanidinate species, and protonation reaction of the guanidinate species by an amine to regenerate amido species and release the final product.
Hydroamination of Carbodiimides Catalyzed by Lithium Triethylborohydride
2023, Synthesis (Germany)
Unusual selective reactivity of the rare-earth metal complexes bearing a ligand with multiple functionalities
2023, Science China Chemistry
Rare-earth metal dialkyl complexes supported by 1,3-disubstituted indolyl ligand: Synthesis, characterization and catalytic activity for isoprene polymerization
2021, Jiegou Huaxue

View full text

Synthesis, structure and catalytic activity of rare-earth metal amino complexes incorporating imino-functionalized indolyl ligand

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Ligand synthesis

Synthesis and characterization of the hetero-nuclear bimetallic rare-earth metal amino complexes

Conclusions

Materials and procedures

Declaration of Competing Interest

Acknowledgements

Coord. Chem. Rev.

Macromolecules

J. Polym. Sci., Part A

Acc. Chem. Res.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Chem. Commun.

Inorg. Chem.

Chem. Eur. J.

Dalton Trans.

Dalton Trans.

Dalton Trans.

Sci China Chem

Inorg. Chem.

Organometallics

Organometallics

Chin. J. Chem.

Nat. Prod. Rep.

Nat. Prod. Rep.

Bioorg. Chem.

Org. Biomol. Chem.

J. Am. Chem. Soc.

Med. Chem. Commun.

Med. Chem. Commun.

ACS Med. Chem. Lett.