New Lanthanide Alkynylamidinates and Diiminophosphinates

This contribution reports the synthesis and structural characterization of several new lithium and lanthanide alkynylamidinate complexes. Treatment of PhC≡CLi with N,N′-diorganocarbodiimides, R–N=C=N–R (R = iPr, Cy (cyclohexyl)), in THF or diethyl ether solution afforded the lithium-propiolamidinates Li[Ph–C≡C–C(NCy)2] S (1: R = iPr, S = THF; 2: R = Cy, S = THF; 3: R = Cy, S = Et2O). Single-crystal X-ray diffraction studies of 1 and 2 showed the presence of typical ladder-type dimeric structures in the solid state. Reactions of anhydrous LnCl3 (Ln = Ce, Nd, Sm or Ho) with 2 in a 1:3 molar ratio in THF afforded a series of new homoleptic lanthanide tris(propiolamidinate) complexes, [Ph–C≡C–C(NCy)2]3Ln (4: Ln = Ce; 5: Ln = Nd; 6: Ln = Sm; 7: Ln = Ho). The products were isolated in moderate to high yields (61%–89%) as brightly colored, crystalline solids. The chloro-functional neodymium(III) bis(cyclopropylethynylamidinate) complex [{c-C3H5–C≡C–C(NiPr)2}2Ln(μ-Cl)(THF)]2 (8) was prepared from NdCl3 and two equiv. of Li[c-C3H5–C≡C–C(NiPr)2] in THF and structurally characterized. A new monomeric Ce(III)-diiminophosphinate complex, [Ph2P(NSiMe3)2]2Ce(μ-Cl)2Li(THF)2 (9), has also been synthesized in a similar manner from CeCl3 and two equiv. of Li[Ph2P(NSiMe3)2]. Structurally, this complex resembles the well-known “ate” complexes (C5Me5)2Ln(μ-Cl)2Li(THF)2. Attempts to oxidize compound 9 using trityl chloride or phenyliodine(III) dichloride did not lead to an isolable cerium(IV) species. OPEN ACCESS Inorganics 2015, 3 430

What makes the amidinate and guanidinate ligands extremely versatile is the possibility of varying the substituents at all three atoms of the N-C-N unit.Attachment of an alkynyl substituent to the central carbon atom of amidines leads to alkynylamidines, R-C≡C-C(=NR′)(NHR′). Alkynylamidines are of significant interest due to their diverse applications in organic synthesis [20][21][22][23] as well as in biological and pharmacological systems [24][25][26][27].Alkynylamidinate complexes of various transition metals and lanthanides 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 -caprolactone polymerization [28][29][30][31][32][33].Previously used alkynylamidinate ligands include e.g., the propiolamidine derivatives [Ph-C≡C-C(NR)2] − (R = i Pr, t Bu) [16,28,34] and the trimethylsilylacetylene-derived anions [Me3Si-C≡C-C(NR)2] − (R = cyclohexyl (Cy), i Pr) [35].In 2008, the compound Li[Ph-C≡C-C(N i Pr)2] has been reported to be suitable precursor for the preparation of d-transition metal complexes containing bridging alkynylamidinate ligand [36].Moreover, the same ligand has been used in the synthesis of the unsolvated homoleptic Ce(III) complex [Ph-C≡C-C(N i Pr)2]3Ce [16].
In the course of our ongoing investigation of lanthanide amidinates we recently started a project to study lanthanide alkynylamidinates derived from phenylacetylene and cyclopropylacetylene.The cyclopropyl group was chosen because of the well-known electron-donating ability of this substituent to an adjacent electron-deficient center [37][38][39][40][41][42][43].This would provide a rare chance to electronically influence the amidinate ligand system rather than altering only its steric demand.In the course of this study, we first reported the synthesis and full characterization of a series of lithium-cyclopropylethinylamidinates [44].These precursors are readily available on a large scale using commercially available starting materials (cyclopropylacetylene, N,N′-diorganocarbodiimides).In subsequent contributions, we described the first trivalent rare-earth metal complexes comprising the new cyclopropylethynylamidinate ligands and their use as guanylation, hydroamination, and hydroacetylenation catalysts [45][46][47].
We now report the synthesis and structural characterization of two lithium-propiolamidinates and their use as precursors for new homoleptic lanthanide(III)-tris(propiolamidinate) complexes.Moreover, the synthesis and crystal structures of a dimeric neodymium(III) bis(cyclopropylethynylamidinate) complex and a monomeric Ce(III)-diiminophosphinate complex are reported.Scheme 1 shows the anionic N,N′-chelating ligands employed in this study.

Scheme 2. Synthesis of the lithium-propiolamidinates 1-3.
Both alkynylamidinates 2 and 3 have been fully characterized by spectroscopic methods and elemental analysis.The IR spectrum of 2 shows a medium band at 2217 cm −1 , which could be assigned to the C≡C triple bond, while in the spectrum of 3 it appears as a weak band at 2211 cm −1 .The C=N stretching vibrations of the NCN unit of amidinate moieties is observed at 1610 cm −1 as a very strong band in the spectrum of 2 and at 1592 cm −1 as strong band in that of 3 [16].The NMR spectra were recorded in toluene-d8 and C6D6 for 2 and 3, respectively.The 1 H and 13 C NMR analyses were in good agreement with the formation of Li[Ph-C≡C-C(NCy)2].Resonances of all protons and carbons were observed except for the carbon atom of the NCN unit and one C atom of the two acetylenic carbon atoms in 3.
Single-crystal X-ray diffraction studies were carried out on the lithium-propiolamidinates 1 and 2. In both cases, X-ray quality, block-like single-crystals were obtained by cooling saturated solutions in THF to 5 °C for a few days.Figures 1 and 2 depict the molecular structures of 1 and 2 along with selected bond lengths and angles, while crystallographic details are listed in Table 1.Full crystallographic data can be found in the Supplementary Information.The crystal structure determinations of Li[Ph-C≡C-C(N i Pr)2] THF (1) and Li[Ph-C≡C-C(NCy)2] THF (2) in both cases revealed the presence of a ladder-type dimeric structure, which is the most characteristic structural motif of most previously characterized lithium amidinates and guanidinates [48][49][50][51][52][53][54][55][56][57][58].The dimers are centrosymmetric with the central Li2N2 ring being strictly planar.In the solid state, both lithium propiolamidinates crystallize in the triclinic space group P-1 with one dimeric molecule in the unit cell.The bond lengths and bond angles are in good agreement with similar structures of amidinate ligands [48][49][50][51][52][53][54][55][56][57][58].The N-C distances in the amidinate N-C-N unit are 1.321(2) and 1.330(2) Å in 1 and 1.3201 (13)
According to the 1 H- 13 C correlation (HSQC) technique, the protons of CH in the cyclohexyl group observed at three different positions, δ = 9.49, 3.76 and 3.56 ppm in 4 (Figure 3) and at δ = 18.33, 3.77 and 3.57 ppm in 5 (Figure S1) are in agreement with the CH-protons in the complex [Ph-C(NCy)2]3Ln (Ln = Pr or Nd), whereas the CH-protons of c-C6H11 in 6 (Figure S2) appear at δ 3.71 and 3.30 ppm [34].The significantly different chemical shifts of the CH proton resonances originate from the different pramagnetism of the Ce 3+ , Nd 3+ , and Sm 3+ ions, respectively.The protons of the phenyl group appear in the range of δ = 7.40-9.41ppm [16,34].The 1 H NMR spectra reflected the stronger paramagnetic properties for the complexes of cerium 4 and neodymium 5 as compared to the less paramagnetic samarium complex 6.On the other hand, the 13   [16] and [Ph-C≡C-C(N i Pr)2]3Y [34].

Scheme 4. Preparation of 8.
Dark green, needle-like single crystals of 8 suitable for X-ray diffraction were obtained by slow cooling of saturated solutions in n-pentane to −30 °C.The single-crystal X-ray diffraction study clearly established the presence of a chloro-bridged dimer of the composition [{c-C3H5-C≡C-C(NR)2}2Nd(µ-Cl)(THF)]2.The molecular structure of 8 is depicted in Figure 4; the crystal and structural refinement data are summarized in Table 1.Complete structural data of 8 can be found in the Supplementary Information.The molecule is a centrosymmetric dimer of the type [L2Ln(µ-Cl)(THF)]2 comprising a planar four-membered Nd2Cl2 ring.With angles of 106.85(5)° (Nd-Cl-Nd) and 73.15(5)° (Cl-Nd-Cl), the central Nd2Cl2 moiety is rhomb-shaped.The cyclopropyl-ethinylamidinate ligands are coordinated in the "classical" N,N′-chelating coordination mode, and the chloro ligands act as bridging ligands.The coordination sphere around the large Nd 3+ ion still leaves room for an additional THF ligand, resulting in a formal coordination number of seven around neodymium.It has been pointed out earlier [45], that in this respect the cyclopropylethynylamidinates differ from the related dimeric yttrium, ytterbium, and lutetium bis(guanidinato) complexes such as [{(Me3Si)2NC(N i Pr)2}2Y(µ-Cl)]2 [60] and [{(Me3Si)2NC(NCy)2}2Ln(µ-H)]2 [61].In these compounds, the higher steric demand of the supporting guanidinate ligands prevents the formation of THF adducts, resulting in six-coordinate lanthanide ions.This shows that the steric demand of the cyclopropylethynylamidinate anions is significantly lower than that of the N,N-bis(trimethylsilyl)-substituted guanidinate anion [(Me3Si)2NC(NCy

A Cerium(III)-bis(Diiminophosphinate) Complex
Rare-earth metal complexes comprising related N-P-N chelating diiminophosphinate ligands [R2P(NR′)2] − were first reported in the literature in 1989 [62], although the number of well-characterized examples remains small [63,64].However, there is renewed interest in this class of compounds.A recent report by Cui et al. demonstrated that new diiminophosphinato-supported lanthanide dialkyl complexes are valuable catalysts for the 3,4-polymerization of isoprene.The NPN-bidentate diiminophosphinate-ligated rare-earth metal complexes provided both high syndio-and iso-3,4-selectivities (3,4 > 99%, rr = 66%, mmmm = 96%), depending on the frameworks, steric environment and geometry of the ligands.The regio-and stereoselective mechanisms proceeding in these systems were explicated by DFT simulation [65].A new monomeric Ce(III)-diiminophosphinate complex, [Ph2P(NSiMe3)2]2Ce(µ-Cl)2Li(THF)2 ( 9), has been synthesized in the course of this work.The starting material Li[Ph2P(NSiMe3)2] was prepared according to the literature method [66,67].According to Scheme 5, a suspension of anhydrous CeCl3 in THF was added to a solution of Li[Ph2P(NSiMe3)2] in THF.The reaction mixture was stirred over night at room temperature.The product 9 was extracted as a golden-yellow solution in n-pentane.Complex 9 was isolated as an exceedingly air-and moisture-sensitive bright yellow, block-like crystals at 5 °C in 78% yield.

Scheme 5. Preparation of 9.
The new compound [Ph2P(NSiMe3)2]2Ce(µ-Cl)2Li(THF)2 ( 9) has been fully characterized by spectroscopic methods, elemental analysis and single-crystal X-ray diffraction.The molecular structure of 9 is in good agreement with related structures such as [Ph2P(NSiMe3)2]2Sm(µ-I)2Li(THF)2 [63].The IR spectrum showed medium bands at 1180 and 1116 cm −1 , which can be assigned to the PNSi unit.The strong bands that appear at 1246, 933 and 840 cm −1 can be attributed to the SiMe3 groups.The 1 H NMR spectrum of 9 shows sets of signals assignable to the phenyl groups and the coordinated THF molecules.Due to the paramagnetic nature of the Ce 3+ ion, the protons of the Si(CH3)3 groups appear as singlets at high magnetic field at δ = −6.50ppm.The X-ray study revealed the presence of the "ate" complex [Ph2P(NSiMe3)2]2Ce(µ-Cl)2Li(THF)2 (9).The cerium(III) ion is coordinated with four nitrogen atoms of the diiminophosphinate ligands and two bridging chloride ions, giving a formal coordination number of six, as shown in Figure 5.The phenomenon of "ate" complex formation via retention of alkali metal halides in the products is quite common in organolanthanide chemistry [68].It can be traced back to the strong tendency of the large Ln 3+ ions to adopt high coordination numbers.

General Procedures
All experiments were carried out under an inert atmosphere of dry argon employing standard Schlenk and glovebox techniques (<1 ppm O2, <1 ppm H2O).n-Pentane and THF were distilled from sodium/benzophenone under nitrogen atmosphere prior to use.All glassware was oven-dried at 120 °C for at least 24 h, assembled while hot, and cooled under high vacuum prior to use.The starting materials, anhydrous LnCl3 (Ln = Ce, Nd) [76] and Li[Ph2P(NSiMe3)2] [66,67], and compound 8 [45] were prepared according to the literature methods.n BuLi, phenylacetylene, and N,N′-dicyclohexylcarbodiimide were purchased from Aldrich and used as received. 1H-NMR (400 MHz) and 13 C-NMR (100.6 MHz) were recorded in C6D6 or CDCl3 solutions on a Bruker DPX 400 spectrometer at 25 °C.Chemical shifts were referenced to TMS.Assignment of signals was made from 1 H- 13 C HSQC 2D NMR experiments.IR spectra were recorded using KBr pellets on a Perkin Elmer FT-IR spectrometer system 2000 (Perkin Elmer, Rodgau, Germany) between 4000 and 400 cm −1 .Microanalyses of the compounds were performed using a LECO CHNS 932 apparatus (LECO Corp., St. Joesph, MI, USA).

X-ray Crystallography
The intensity data of 1, 2, 8, and 9 were collected on a Stoe IPDS 2T diffractometer with MoKα radiation.The data were collected with the Stoe XAREA [77] program using ω-scans.The space groups were determined with the XRED32 [78] program.Absorption corrections were applied using the multi-scan method.The structures were solved by direct methods (SHELXS-97) [79] and refined by full matrix least-squares methods on F2 using SHELXL-97 [80].Data collection parameters are given in Table 1.CCDC 1419935 (1), 1419937 (2), 1400418 (8), and 1419934 (9) contain the supplementary crystal data for this article.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Conclusions
In summarizing the results reported here, we extended the series of homoleptic lanthanide tris(propiolamidinate) complexes by preparing four new representatives, [Ph-C≡C-C(NCy)2]3Ln
C{ 1 H} NMR spectra showed that the signals of the phenyl carbon atoms are observed in the range of δ = 125-140 ppm in all three cases.The carbon atoms of the CH of the cyclohexyl groups are observed at δ = 67.1,61.1 and 56.3 ppm in 4, and at δ = 78.5, 61.1 and 50.0 ppm in 5, whereas in 6 they appear at δ = 59.5 and 57.4 ppm.The carbon atoms of the CH2 in the cyclohexyl group appear in the same range at δ = 25.7-36.5 ppm for all of three complexes.All data clearly showed that the new complexes 4-7 are unsolvated, homoleptic lanthanide(III)-tris(propiolamidinates) like the previously reported )2] − .Within the N-C-N groups the C-N distances are equal (average C-N = 1.335Å), indicating full π-electron delocalization within the chelating amidinate units.The bond lengths of the triple bonds in the cyclopropylethynyl units are C(2)-C(3) 1.186(9) Å and C(22)-C(23) 1.188(9) Å.