Synthesis and catalytic activity of homoleptic lanthanide-tris ( cyclopropylethinyl ) amidinates

Reactions of anhydrous lanthanide trichlorides, LnCl3 (Ln = Nd, Sm, Ho), with 3 equiv. of lithiumcyclopropylethinylamidinates, Li[c-C3H5–CRC–C(NR)2] (1a: R = cyclohexyl (Cy), 1b: R = Pr), afforded the new homoleptic lanthanide(III) tris(cyclopropylethinylamidinate) complexes [c-C3H5–CRC–C(NCy)2]3Sm (2a) and [c-C3H5–CRC–C(N Pr)2]3Ln (Ln = Nd (2b), Sm (2c), Ho (2d)) as airand moisture-sensitive crystalline solids in moderate to good isolated yields (45–79%). The formation of unsolvated, homoleptic Ln(III) tris(cyclopropylethinylamidinate) was confirmed by an X-ray diffraction study of the holmium derivative [c-C3H5–CRC–C(N Pr)2]3Ho (2d). EI mass spectra of the new rare-earth metal amidinates indicated a significant volatility. An initial catalysis study revealed that these complexes catalyze the addition of terminal alkynes to carbodiimides to give propiolamidines of the type R–CRC–C(QNR0)(NHR0). The molecular structure of N,N0-dicyclohexyl-phenylpropiolamidine, Ph–CRC–C(NCy)(NHCy) (4), was also determined by X-ray diffraction.


Introduction
In organolanthanide chemistry, steric saturation of the coordination sphere of the large rare-earth metal cations is generally more important than the electron count.Thus the investigation of new spectator ligands which satisfy the coordination requirements of the lanthanides continues to be of significant current interest.Anionic amidinate ligands of the type [RC(NR 0 ) 2 ] À (R = H, alkyl, aryl; R 0 = alkyl, cycloalkyl, aryl, SiMe 3 ) have been demonstrated to be highly useful and versatile in that respect.These readily available N-chelating ligands are generally regarded as steric cyclopentadienyl equivalents. 1 In the case of rare-earth metals, mono-, di-and trisubstituted lanthanide amidinate and guanidinate complexes are all accessible, just like the mono-, di-and tricyclopentadienyl complexes.Over the past ca.25 years, lanthanide amidinates have witnessed an impressive transformation from laboratory curiosities to highly active homogeneous catalysts as well as valuable precursors in materials science.Various rare-earth metal amidinates have been reported to be very efficient homogeneous catalysts e.g. for ring-opening polymerization reactions of lactones, the guanylation of amines or the addition of terminal alkynes to carbodiimides. 2 In materials science, homoleptic alkyl-substituted lanthanide tris(amidinate) complexes are often highly volatile and can be used as promising precursors for ALD (atomic layer deposition) and MOCVD (metal-organic chemical vapor deposition) processes, e.g. for the deposition of lanthanide oxide (Ln 2 O 3 ) or lanthanide nitride (LnN) thin films. 3he introduction of alkinyl groups to the central carbon atom in amidines leads to alkinylamidines (or propiolamidines) of the type R-CRC-C(=NR 0 )(NHR 0 ).In organic synthesis, alkinylamidines have been frequently employed in the preparation of various heterocycles. 4,5More recently, alkinylamidines have attracted considerable attention due to their diverse applications in biological and pharmacological systems. 6Moreover, transition metal and lanthanide alkinylamidinate complexes have been shown to be efficient and versatile catalysts e.g. for C-C and C-N bond formation, the addition of C-H, N-H and P-H bonds to carbodiimides as well as e-caprolactone polymerization. 7Thus far, only very few lanthanide complexes containing alkinylamidinate ligands have been described. 7,8Previously used propiolamidinate ligands include e.g.phenylethinyl derivatives [Ph-CRC-C(NR) 2 ] À (R = i Pr, t Bu) 7a,8 and the trimethylsilylacetylene-derived anions [Me 3 Si-CRC-C(NR) 2 ] À (R = cyclohexyl (Cy), i Pr). 9 In the course of our ongoing investigation of lanthanide amidinates we recently initiated a study of alkinylamidinates derived from cyclopropylacetylene. The resulting anions [c-C 3 H 5 -CR C-C(NR) 2 ] À (R = Cy, i Pr) represent a potentially useful addition to the current library of amidinate ligands.In a first contribution we described the synthesis and full characterization of the lithium-cyclopropylethinylamidinates Li[c-C 3 H 5 -CRC-C(NR) 2 ] (1a: R = cyclohexyl (Cy), 1b: R = i Pr). 10 These precursors are readily available on a large scale and in high yields using commercially available starting materials.In a subsequent study, their use as precursors for new lanthanide amidinates could be demonstrated by the synthesis of a series of new Ln(III) bis(cyclopropylethinylamidinates).In the case of Ce and Nd, the chloro-bridged dimers [{c-C 3 H 5 -CRC-C(NR) 2 } 2 Ln(m-Cl)(THF)] 2 (Ln = Ce, Nd; R = Cy, i Pr) were isolated, whereas the smaller holmium afforded the ''ate'' complex [c-C 3 H 5 -CRC-C(NCy) 2 ] 2 -Ho(m-Cl) 2 Li(THF)(OEt 2 ).An initial study showed that these complexes effectively catalyze the addition of aniline derivatives to carbodiimides to give N-arylguanidines. 11Herein we report the synthesis and structural characterization of the first homoleptic Ln(III) tris(cyclopropylethinylamidinate) complexes as well as an initial study of their possible use as homogeneous catalysts for the addition of terminal alkynes to carbodiimides.

Synthesis and structure
The starting materials used in this study, the lithium-cyclopropylethinylamidinates Li[c-C 3 H 5 -CRC-C(NR) 2 ] (1a: R = Cy, 1b: R = i Pr), were prepared in a straightforward manner according to Scheme 1 by in situ-deprotonation of commercially available cyclopropylacetylene followed by treatment with either N,N 0 -diisopropylcarbodiimide or N,N 0 -dicyclohexylcarbodiimide according to the published procedure.These lithium amidinates can be isolated in the form of stable, crystalline solids as adducts with donor solvent like diethyl ether, THF or DME (1,2-dimethoxyethane). 10However, for the reactions with lanthanide trichlorides, the reagents 1a and 1b were conveniently prepared in THF solution and used in situ.
Subsequent reactions of the lithium-cyclopropylethinylamidinates 1a and 1b with anhydrous lanthanide trichlorides, LnCl 3 (Ln = Nd, Sm, Ho) were carried out in a 1 : 3 molar ratio in THF solutions according to Scheme 2. Evaporation of the volatiles and recrystallization of the crude products from n-pentane afforded the new lanthanide(III) tris(cyclopropylethinylamidinate) complexes 2a-d in moderate (2b: 54%, 2c: 45%, 2d: 55%) to good (2a: 79%) yields.The samarium and holmium derivatives 2a, 2c, and 2d were isolated as yellow, air-and moisture-sensitive crystals, while the neodymium complex 2b is a green, crystalline solid.All four compounds are highly soluble in THF, diethyl ether, toluene and n-pentane.The very high solubility even in non-polar solvents like n-pentane certainly accounts for the relatively low yields in the case of complexes 2b-d.A single-crystal X-ray diffraction study of the holmium derivative 2d (vide infra) confirmed the presence of the expected unsolvated, homoleptic lanthanide(III) tris(cyclopropylethinylamidinate) complex.
All four compounds were characterized by their NMR ( 1 H, 13 C) and IR spectra as well as elemental analyses.Despite the paramagnetic nature of the Ln 3+ ions employed here, meaningful NMR spectra could be obtained for all four compounds with the exception of the 1 H NMR spectrum of the Ho 3+ complex 2c.The data were in good agreement with the formation of unsolvated lanthanide(III) tris(cyclopropylethinylamidinates).No signals attributable to coordinated THF could be observed.The IR spectra of 2a-c were found to be almost superimposable.IR bands resulting from the CQN stretching vibrations of the N-C-N units appear at around 1606-1612 cm À1 , whereas very strong bands at 2220-2227 cm À1 can be assigned to the CRC vibrations.In all cases the EI mass spectra indicated good volatility of the new homoleptic lanthanide amidinates as they all showed the molecular ions in an intensity range of 20-45% relative intensity.
As a typical representative of the new homoleptic lanthanide tris(amidinates), the holmium derivative 2d was structurally authenticated through single-crystal X-ray diffraction (Fig. 1).
Pale yellow, block-like single-crystals of 2d were obtained by cooling of a very concentrated solution in n-pentane to À30 1C over a prolonged period of time.Crystallographic data of 2d are listed in Table 1, while selected bond lengths and angles are summarized in the caption of Fig. 1.Compound 2d crystallizes in the triclinic space group P% 1.The crystal structure determination clearly confirmed the presence of the first unsolvated homoleptic lanthanide(III) tris(cyclopropylethinylamidinate) complex.The central Ho 3+ ion is coordinated by three chelating [c-C 3 H 5 -CR C-C(N i Pr) 2 ] À ligands in a highly distorted octahedral fashion.To our knowledge, only three closely related homoleptic Ln(III) tris(phenylethinylamidinate) complexes of the type [Ph-CR C-C(N i Pr) 2 ] 3 Ln (Ln = Y, 8b Ce, 8a Lu 8b ) have been reported in the previous literature.All three complexes have also been structurally characterized by X-ray diffraction.The overall structural features of 2d are very similar to those reported for [Ph-CR C-C(N i Pr) 2 ] 3 Ln (Ln = Y, Ce, Lu).The Ho-N distances in 2d are in the very narrow range of 2.342(2)-2.383(3)Å.As a result of the lanthanide contraction, 12 these values are virtually identical with those reported for the yttrium(III)-tris(phenylethinylamidinate) complex [Ph-CRC-C(N i Pr) 2 ] 3 Y (Y-N 2.363(4) and 2.356(4) Å).The average N-Ho-N bite angle to the chelating amidinate ligands in 2d is 57.33( 9)1.This is also favorably comparable to the corresponding N-Ln-N angles found in the three phenylethinylamidinates [Ph-CRC-C(N i Pr) 2 ] 3 Ln (Ln = Y, Ce, Lu) and in other homoleptic lanthanide tris(N,N 0 -dialkylamidinates). 1,8he bond lengths of the triple bonds in the cyclopropylethinyl units in 2d are 1.182( 6) Å (C2-C3), 1.185(4) Å (C14-C15) and 1.184( 5) Å (C22-C23).

Catalytic activity
For a first study of the possible catalytic activity of the new Ln(III) tris(cyclopropylethinylamidinate) we chose the catalytic addition of alkynes to carbodiimides to give substituted propiolamidines.The lanthanide-catalyzed synthesis of propiolamidines R-CR C-C(=NR 0 )(NHR 0 ) was first reported in 2005 by Hou et al. using rare-earth metal half-sandwich complexes as catalysts.In a second set of experiments, the Ln-catalyzed addition of three different terminal alkynes to both N,N 0 -diisopropylcarbodiimide and N,N 0 -dicyclohexylcarbodiimide was studied.For these tests, the most active complex [c-C 3 H 5 -CRC-C(NCy) 2 ] 3 Sm (2a) was used as the precatalyst.The reactions were again carried out in THF at 60 1C according to Scheme 4.
As can be seen from the results listed in Table 3, this short screening produced mixed results.Reactions of phenylacetylene with both N,N 0 -diisopropylcarbodiimide and N,N 0 -dicyclohexylcarbodiimide gave good yields of the hydroacetylenation products 3 and 4, while cyclopropylacetylene could be added only to N,N 0 -dicyclohexylcarbodiimide affording a moderate yield of propiolamidine 5.In sharp contrast, virtually no reactions were observed when trimethylsilylacetylene was employed.Thus the use of the new homoleptic lanthanide(III)-tris(cyclopropylethinylamidinates) as catalysts for the addition of terminal acetylenes to carbodiimides appears to be quite limited.Obviously these amidinate complexes cannot seriously compete with previously reported rare-earth metal catalysts comprising cyclopentadienyl 7a or pyrazolylborate 7g ligands.These compounds all contain additional s-alkyl groups such as -CH 2 Ph or -CH 2 SiMe 3 which certainly account for the significantly higher activity of such catalysts systems.7a,g In the course of the present study, the molecular structure of the propiolamidine 4 has been verified by single-crystal X-ray diffraction (cf.Table 1).X-Ray-quality single-crystals of 4 were grown by slowly cooling a solution in hot acetonitrile to room temperature.The molecular structure of 4 is shown in Fig. 2

Conclusions
In summarizing the work reported here, we succeeded in the straightforward preparation of a series of new homoleptic lanthanide tris(cyclopropylethinylamidinate) complexes comprising neodymium, samarium, and holmium as central metals.The lithium-cyclopropylethinylamidinate precursors employed in these preparations are readily available in one step from commercially available starting materials.The new complexes 2a-d are highly soluble even in non-polar solvents such as n-pentane.The presence of unsolvated, homoleptic tris(cyclopropylethinylamidinate) complexes could be verified by an X-ray crystal structure determination of the holmium complex 2d.An initial catalysis study revealed that the new amidinates effectively catalyze the addition of phenylacetylene to N,N 0 -diisopropylcarbodiimide and N,N 0 -dicyclohexylcarbodiimide but have insufficient activity with other terminal acetylenes.
A 100 ml Schlenk flask was charged with phenylacetylene (1.40 ml,  12.8 mmol) and N,N 0 -diisopropylcarbodiimide (2.0 ml, 12.8 mmol) in 20 ml of THF.To the mixture was added the catalyst (2a, 2b, 2c, or 2d) (0.5 or 1.0% mmol), dissolved in 5 ml of THF.The resulting mixture was stirred at 60 1C or at room temperature for a fixed time.The solvent was completely removed under vacuum and the product was purified by crystallization from a minimum amount of dry acetonitrile in air to give 3 in yields as shown in Table 2. 4.4 General procedure for the addition of terminal alkynes to N,N 0 -diisopropylcarbodiimide catalyzed by 2a A 100 ml Schlenk flask was charged with the terminal alkyne (1.0 mmol) and N,N 0 -diisopropylcarbodiimide (1.0 mmol) in 15 ml of THF.To the mixture was added the catalyst 2a (0.01 mmol), dissolved in 5 ml of THF.The resulting mixture was stirred at 60 1C for a fixed time, as shown in Table 2.The solvent was removed under vacuum and the product was purified by crystallization from a minimum amount of dry acetonitrile in air.The resulting propiolamidines 3-5 were identified through their 1 H and 13 C NMR data (cf.ESI ‡). 7,14

X-Ray crystallographic studies
The intensity data of 2d and 4 were collected on a Stoe IPDS 2T diffractometer with MoKa radiation.The data were collected with the Stoe XAREA 16 program using o-scans.The space groups were determined with the XRED32 24 program.Absorption corrections were applied using the multi-scan method.The structures were solved by direct methods (SHELXS-97) 17a and refined by full matrix least-squares methods on F 2 using SHELXL-97.17b Data collection parameters are given in Table 1.
The pre-catalysts used in this study were constrained-geometry-type complexes such as [Me 2 Si(C 5 Me 4 )(NPh)]Y(CH 2 SiMe 3 )(THF) 2 .It was found that half-sandwich complexes comprising a propiolamidinate ligand play an important role in the catalytic cycle.Upon treatment with excess acetylene, they release the propiolamidine product.7aMost recently, Zhang and Zhou et al. employed rare-earth metal alkyl complexes stabilized by the bulky pyrazolylborate ligand Tp Me2 (=hydro-tris(3,5-dimethylpyrazolyl)-borate) as catalysts for the synthesis of N-aryl-substituted propiolamidines.7gIn an initial screening test, we examined the Ln-catalyzed addition of phenylacetylene to N,N 0 -diisopropylcarbodiimide in the presence of all four compounds 2a-d as illustrated in Scheme 3.All four new lanthanide(III)-tris(cyclopropylethinylamidinates) 2a-d were used as precatalysts, and the reactions were carried out in concentrated THF solutions at 60 1C.The results are summarized in Table2.The isolated yields of the known compound Ph-CR C-C(N i Pr)(NH i Pr) (3)13 varied from 27 to 85% depending of the lanthanide metal employed.Clearly the highest activity was observed for the samarium complex [c-C 3 H 5 -CRC-C(NCy) 2 ] 3 Sm (2a), while the lowest yields were obtained when using the holmium catalyst [c-C 3 H 5 -CRC-C(N i Pr) 2 ] 3 Ho (2d).In a control experiment(Table 2, entry 11), an equimolar mixture of phenylacetylene and N,N 0 -diisopropylcarbodiimide were heated in concentrated THF solution at 60 1C for 1 h in the absence of a rare-earth metal compound.Under these conditions, no trace of Ph-CR C-C(N i Pr)(NH i Pr) (3) could be detected in the reaction mixture.

Scheme 3 a
Scheme 3 Synthesis of Ph-CRC-C(N i Pr)(NH i Pr) (3) using 2a-d as catalyst.

Table 1
Crystallographic data and structure refinement parameters for compounds 2d and 4 À13 r h r 13 À12 r h r 11 À18 r k r 18 À12 r k r 12 À19 r l r 23 À13 r l r 12 Data/restraints/parameters 10 209/38/461 3625/157/267 Goodness-of-fit on F

Table 3
Catalytic addition of terminal alkynes to N,N 0 -diisopropylcarbodiimide catalyzed by 2a General condition: THF as solvent at 60 1C.b All reactions carried out using 1.0% mol of 2a.
a c Isolated yield.