Coordinative versatility in main group complexes of C-2,6-terphenyl substituted amidinates

doi:10.1016/j.poly.2019.06.008

Polyhedron

Volume 170, 15 September 2019, Pages 424-430

https://doi.org/10.1016/j.poly.2019.06.008 Get rights and content

Abstract

The synthesis and characterization of two C-2,6-terphenyl substituted amidines, N,N′-dicyclohexyl-2,6-di(4-tolyl)benzamidine and N,N′-dicyclohexyl-2,6-dimesitylbenzamidine, are described. Both crystallize as the E-syn isomer, though interconversion between the E-syn and Z-syn isomers is observed for both in solution. These amidines are readily deprotonated by either n-butyllithium in THF or AlMe₃ in toluene, affording the THF solvated lithium and dimethylaluminium amidinate complexes respectively. These complexes have been characterized in the solid-state using single-crystal X-ray crystallography. The identity of the 2,6-substituents on the secondary arene rings of the terphenyl group plays a significant role in the coordination motif adopted by the amidinate, as well as the potential to access bis(amidinato)metal complexes.

Graphical abstract

Lithium and aluminium amidinate complexes have been prepared from two new C-terphenyl amidines. The solid-state structures of these complexes show significant congestion about the metal centres, which leads to different ligand binding modes and lability of coordinated solvents.

Introduction

The amidinate, [R¹NC{R²}NR³]⁻, is a ligand set closely related to the β-diketiminate. Amidinate complexes of s-, p-, d- and f-block metals have been extensively studied [1]. Amidinate complexes have been employed as single-source precursors to nitride containing materials [2] and as auxiliary ligands in olefin polymerization precatalysts [3], [4]. Careful tuning of the steric bulk at the N- and C-substituents of the amidinate has led to an array of coordination modes being reported [1]. Notably, the introduction of a sterically demanding C- or N-substituents can prevent the amidinate functioning as a bridging ligand, by inducing a more acute ligand bite angle, thereby enforcing an N,N'-chelating motif [5], [6], [7], [8]. C-2,6-terphenyl substituted amidines were first reported by Arnold [9], and later by Clyburne [10], as a means of introducing steric bulk above and below the plane of the amidinate donor set. C-triptycenyl substituted amidines have also been reported [11]. Lithium complexes of these bulky C-substituted amidinates typically display monodentate binding of the metal (Fig. 1) [9], [11], [12]. Whilst the bidentate N,N'-chelation mode is typically observed for larger metals [9], [10], [12], [13].

Here we report two new C-2,6-terphenyl substituted amidine ligands and explore their coordination chemistries with main group metals.

Section snippets

General considerations

Unless otherwise stated, all manipulations were performed using conventional Schlenk or glovebox techniques under an atmosphere of high purity argon in flame-dried glassware. Diethyl ether, THF, toluene, n-pentane and n-hexane were dried over sodium wire and purged with nitrogen prior to distillation from sodium benzophenone ketyl. Benzene-d₆ (C₆D₆) was dried over sodium and freeze-pump-thaw degassed prior to use. Infrared spectra were recorded as Nujol mulls using sodium chloride plates on a

Results and discussion

Bulky C-2,6-terphenyl amidines, N,N′-dicyclohexyl-2,6-di(4-tolyl)benzamidine (L¹H) and N,N′-dicyclohexyl-2,6-dimesitylbenzamidine (L²H), were prepared similarly to the C-2,6-terphenyl amidines reported by Arnold [9]. Lithium terphenyl was prepared in situ by treating a diethyl ether solution of terphenyl iodide with n-butyllithium. This was then treated with N,N'-dicyclohexylcarbodiimide followed by hydrolysis and recrystallization from hexane (Scheme 1).

The IR spectra of L¹H and L²H are in

Conclusion

In summary, we have successfully prepared two new C-2,6-terphenyl substituted amidines. Solvated lithium complexes derived from these amidines were prepared. The less bulky amidinate coordinates the lithium by a N,N′-chelating mode, whilst the larger amidinate coordinates the lithium in a monodentate fashion. The latter also undergoes partial desolvation in toluene to afford a dimeric lithium amidinate species which features an unprecedented coordination motif. Mono(amidinato)aluminium

Acknowledgement

The authors would like to thank the Australian Research Council (DP110104759) for financial support of this research.

References (38)

J. Barker et al.
Coord. Chem. Rev.
(1994)
S.T. Barry
Coord. Chem. Rev.
(2013)
R.J. Baker et al.
J. Organomet. Chem.
(2006)
H. Hart et al.
Tetrahedron
(1995)
M.L. Cole et al.
J. Organomet. Chem.
(2004)
T. Chlupatý et al.
J. Organomet. Chem.
(2011)
L. Falivene et al.
Organometallics
(2016)
M.P. Coles et al.
J. Am. Chem. Soc.
(1997)
S. Dagorne et al.
J. Am. Chem. Soc.
(2000)
S.L. Aeilts et al.
Organometallics
(1998)

M.P. Coles et al.

Organometallics

(1998)

R.T. Boeré et al.

J. Chem. Soc., Dalton Trans.

(1998)

P.C. Junk et al.

Chem. Commun.

(2007)

J.A.R. Schmidt et al.

Chem. Commun.

(1999)

D. Abeysekera et al.

Organometallics

(2001)

J.A.R. Schmidt et al.

J. Chem. Soc., Dalton Trans.

(2002)

J.A.R. Schmidt et al.

Organometallics

(2002)

G.R. Fulmer et al.

Organometallics

(2010)

K. Ruhlandt-Senge et al.

J. Am. Chem. Soc.

(1993)

Cited by (7)

Heteroleptic lead and aluminium complexes ligated by a bulky non-symmetrical triazenide
2020, Journal of Organometallic Chemistry
Citation Excerpt :
We immediately focused on dimethyl aluminium complexes, as they are readily prepared by treating the parent triazene with trimethylaluminium [33,58,59] and the tetrahedral geometry about aluminium potentially minimizes the steric influence of the spectator methyl ligands. To this end, we have previously used this metal-based model to compare the steric bulk of a number of benzamidinate ligands [60]. Addition of a slight excess of AlMe3 to a THF solution of 1 at room temperature (Scheme 3), resulted in an immediate color change of the solution, colorless to bright yellow, with accompanying effervescent.
The sterically bulky non-symmetrical triazene, DmpN₃(H)Dipp∗, was prepared. Amido lead triazenide and dimethyl aluminium triazenide complexes were readily prepared by metalation of the parent triazene. The former is a rare example of a stable heteroleptic lead triazenide complex. Analogous compounds are inaccessible when smaller triazenes are employed under these conditions, as they readily form homoleptic bis(aryl)triazenide complexes. Quantification of the bulk of this and other bidentate monoanionic N,N′-donor ligands, using the solid-state structures of model dimethyl aluminium complexes, help to rationalize the superior stability of the amido lead triazenide complex reported herein with regards to subsequent metalation.
Synthesis, structure, and catalytic activity of the neodymium tris(amidinate) complex [Bu<sup>t</sup>C(N-2,6-Me<inf>2</inf>C<inf>6</inf>H<inf>3</inf>)<inf>2</inf>]<inf>3</inf>Nd in ring-opening polymerization of ε-caprolactone
2023, Russian Chemical Bulletin
Molecular Main Group Metal Hydrides
2021, Chemical Reviews
Aluminum Amidinates: Insights into Alkyne Hydroboration
2021, Inorganic Chemistry
Stereoelectronic Characterization and Catalytic Potential of a 1,3-Bis(2,6-terphenyl)-Substituted N-Heterocyclic Carbene
2021, European Journal of Inorganic Chemistry
Bulky bis(aryl)triazenides: just aspiring amidinates? A structural and spectroscopic study
2020, Dalton Transactions

View all citing articles on Scopus

View full text

Coordinative versatility in main group complexes of C-2,6-terphenyl substituted amidinates

Abstract

Graphical abstract

Introduction

Section snippets

General considerations

Results and discussion

Conclusion

Acknowledgement

Coord. Chem. Rev.

Coord. Chem. Rev.

J. Organomet. Chem.

Tetrahedron

J. Organomet. Chem.

J. Organomet. Chem.

Organometallics

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Organometallics

Organometallics

J. Chem. Soc., Dalton Trans.

Chem. Commun.

Chem. Commun.

Organometallics

J. Chem. Soc., Dalton Trans.

Organometallics

Organometallics

J. Am. Chem. Soc.