Formation and structure of the first metal complexes comprising amidinoguanidinate ligands

The first metal complexes comprising amidinoguanidinate ligands have been prepared and structurally characterized, namely bis[μ-N,N′,N′′,N′′′-tetraisopropyl-1-(1-butylamidinato)guanidinato-κ3 N 1,N 2:N 2) bis[(tetrahydrofuran)lithium] and [bis(tetrahydrofuran)lithium]-di-μ-chlorido-[N,N′,N′′,N′′′-tetracyclohexyl-1-(1-butylamidinato)guanidinato-κ2 N 1,N 2](N,N′-dicyclohexyl-1-butylamidinato-κ2 N 1,N 2)holmate(III).


Chemical context
Anionic N-chelating donor ligands such as the amidinates [RC(NR) 2 ] À and the guanidinates [R 2 NC(NR) 2 ] À have gained tremendous importance in various fields of organometallic and coordination chemistry during the past two decades. Both types of N-chelating ligands are often regarded as 'steric cyclopentadienyl equivalents' (Bailey & Pace, 2001;Collins, 2011;Edelmann, 2013). Meanwhile, amidinato and guanidinato complexes are known for virtually every metallic element in the Periodic Table ranging from lithium to uranium (Edelmann, 2009(Edelmann, , 2012(Edelmann, , 2013Trifonov, 2010). Amidinate and guanidinate ligands have been successfully employed in the stabilization of unusual oxidation states such as magnesium(I) and iron(I) as well as the design of various homogeneous catalysts (Collins, 2011;Edelmann, 2013). Alkyl-substituted amidinate and guanidinate complexes of various metals have also been established as ALD and MOCVD precursors for the deposition of thin layers of metals, metal oxides, metal nitrides etc. (Devi, 2013). Formally, the amidinate anion is the nitrogen analogue of the carboxylate anion, while guanidinates are similarly related to the carbamates. However, in contrast to the carboxylates and carbamates, the steric properties of amidinates and guanidinates can be widely tuned through the use of different substituents, both at the outer nitrogen atoms as well as at the central carbon atom of the NCN unit. Lithium amidinates are normally prepared in a straightforward manner by addition of lithium alkyls to N,N 0 -diorganocarbodiimides in a 1:1 molar ratio, while lithium guanidinates are formed when lithium-N,N-dialkylamides are added to N,N 0 -diorganocarbodiimides (Stalke et al., 1992;Aharonovich et al., 2008;Chlupatý et al., 2011;Nevoralová et al., 2013;Hong et al., 2013). All these reactions are generally quite straightforward and afford the desired products in high yields. We have now discovered that, under certain conditions, reactions of lithium alkyls with N,N 0diorganocarbodiimides can afford different products which can be named 'amidinoguanidinates' (cf. reaction scheme, Fig. 1). These can even become the major reaction products when the stoichiometry of the reactands is changed from 1:1 to 1:2, i.e. when the N,N 0 -diorganocarbodiimide is used in a twofold molar excess. We report here the synthesis and characterization of the first metal complexes comprising 'amidinoguanidinate' ligands which can be viewed as dimers of the amidinate anions. The first amidinoguanidinate complexes described here include the lithium precursors Li[ n BuC( NR)(NR)C(NR) 2 ] (1: R = Cy (cyclohexyl), 2: R = i Pr) and the holmium(III) 'ate' complex [ n Bu-C( NCy)-(NCy)C(NCy) 2 ]Ho[ n BuC(NCy) 2 ](-Cl) 2 Li(THF) 2 (3).
A reaction between N,N 0 -dicyclohexylcarbodiimide and n BuLi in a 2:1 molar ratio in THF afforded the first lithium amidinoguanidinate, Li[ n BuC(=NCy)(NCy)C(NCy) 2 ]ÁTHF (1), in 60% yield. This reaction represents the first case of dimerization of a carbodiimide under formation of a novel amidinoguanidinate anion. The lithium-amidinoguanidinate salt 1 is partially soluble in THF, Et 2 O, and DME and slightly soluble even in toluene and n-pentane. The new sterically bulky amidinoguanidinate 1 has been fully characterized by spectroscopic methods and elemental analysis to confirm the product as shown in Fig. 1. DMSO-d 6 (DMSO = dimethyl sulfoxide) was found to be the best solvent for measuring the NMR spectra of Li[ n BuC(=NCy)(NCy)C(NCy) 2 ]ÁTHF. A mass spectrum of 1 showed only fragments for the monomeric compound. Interestingly, the reaction using N,N 0 -dicyclohexylcarbodiimide and n-butyllithium in THF according to Fig. 1 represents the only case thus far where a pure amidinoguanidinate salt (1) could be isolated. A similar reaction carried out with N,N 0 -diisopropylcarbodiimide produced the isopropyl-substituted amidinoguanidinate salt 2 in >70% yield, although NMR data indicated the presence of significant amounts of an impurity, presumably the 'normal' lithium amidinate Li[ n BuC(N i Pr) 2 ], which could not be separated by fractional crystallization from solvents like THF, DME or diethyl ether. However, occasionally a small amount of well-formed single-crystals of 2 were obtained directly from the reaction mixture which allowed a structural characterization of the new amidinoguanidinates through X-ray diffraction. Apparently the formation of the new amidinoguanidinate anions is critically influenced not only by the stoichiometric ratio of the starting materials, but also by the substituents at the N-atoms and the solvents employed. The solvent effect became apparent when reactions of N,N 0 -dicyclohexylcarbodiimide with 0.5 or 0.3 equiv. of n-butyllithium were carried out in Et 2 O solution. Using this solvent, the reactions produced a variable mixture of amidinoguanidinate and amidinate salts, Li[ n BuC(=NCy)(NCy)C(NCy) 2 ] and Li[ n BuC(NCy) 2 ], respectively, as illustrated in the reaction scheme ( Fig. 1). This was clearly indicated by the rather 'messy' NMR spectra of the reaction products. Attempts to separate the product mixture by fractional crystallization from THF, DME, or diethyl ether were unsuccessful.
The presence of both types of anions in the reaction mixture obtained was also confirmed by the subsequent reaction of the in situ-prepared mixture of Li[ n BuC(=NCy)(NCy)C(NCy) 2 ] and Li[ n BuC(NCy) 2 ] with anhydrous HoCl 3 . In detail, treatment of N,N 0 -dicyclohexylcarbodiimide with 0.5 equiv. of n BuLi in Et 2 O followed by addition of anhydrous HoCl 3 (Freeman & Smith, 1958) in THF produced a yellow solution. Separation of the LiCl by-product and recrystallization from n-pentane afforded the unexpected holmium complex [ n BuC(=NCy)(NCy)C(NCy) 2 ]Ho[ n BuC(NCy) 2 ](-Cl) 2 Li-(THF) 2 (3) in 71% yield. This compound is a mixed-ligand complex containing both the new amidinoguanidinato ligand and the normal amidinato ligand [ n BuC(NCy) 2 ] À in the coordination sphere of holmium. Compound 3 was fully characterized by its IR spectrum, elemental analysis and single-crystal X-ray diffraction. As a result of the highly paramagnetic nature of the Ho 3+ ion, it was impossible to obtain interpretable NMR data for 3. Yellow, air-and moisture-sensitive, needle-like single-crystals of 3 were obtained by slowly cooling a saturated solution in n-pentane to 268 K.
In summarizing the results reported here, we prepared the first metal complexes containing novel amidinoguanidinate ligands obtained by dimerization of N,N 0 -diorganocarbodiimides in the presences of sub-stoichiometric amounts of n-butyllithium. The cyclohexyl-substituted lithium-amidinoguanidinate salt Li[ n BuC(=NCy)(NCy)C(NCy) 2 ]ÁTHF (1) is readily available as a pure solid in fairly good yield (60%). This compound could play an interesting role as a precursor for the synthesis of new transition metal and lanthanide amidinoguanidinate complexes. The first lanthanide complex comprising the new ligand system is the holmium 'ate' complex [ n BuC(=NCy)(NCy)C(NCy) 2 ]Ho[ n BuC(NCy) 2 ](-Cl) 2 Li(THF) 2 (3).

Structural commentary
The crystal structure determination of 2 revealed the presence of ladder-type centrosymmetric dimers (space group P2 1 /c, Z = 2), which is the most characteristic structural motif of most previously characterized lithium amidinates and guanidinates (Stalke et al., 1992;Snaith & Wright, 1995;Downard & Chivers, 2001). Fig. 2 shows the molecular structure of compound 2, while crystallographic data are summarized in Table 1. The central building unit of the dimer is a typical planar Li 2 N 2 ring, formed by -bridging coordination of one of the guanidinate N atoms (N2). The Li-N distances within this ring are 2.0528 (17) and 2.1559 (17) Å and therefore in the expected range. The second N atom of the guanidinate unit (N1) is attached to only one Li atom with a shorter Li-N bond of 2.0177 (18) Å . Through this -3 N,N 0 :N-coordination mode of the guanidinato moiety, a 'ladder' consisting of three four-membered rings is formed. By coordination of a solvent THF molecule, a typical distorted tetrahedral coordination of the Li atom is completed. The free N donor of the amidinate unit (N4) does not contribute to coordinative saturation of the Li atom. The bonds C1-N1 [1.3197 (12) Å ] and C1-N2 [1.3396 (11) Å ] are similar in length, indicating a common delocalization of the negative charge within the Li-coordinating N-C-N fragment. By contrast, the third C-N bond of the guanidinate unit C1-N3 is considerably longer at 1.4528 (11) Å and can therefore be interpreted as a pure single bond. The 1-butylamidinate fragment does not show any delocalization of the -electron density, with one distinct double bond [C8-N4, 1.2808 (12) Å ] and one single bond [C8-N3, 1.3940 (11) Å ]. The amidinate C3-C8-N3 fragment is twisted out of the guanidinate C1/N1/N2/N3 plane by approx. 75 , similar to that found earlier for this type of ligands (Zhou et al., 1998;Wood et al., 1999;Lu et al., 2001).
The holmium complex 3 crystallizes in the triclinic space group P1 with one molecule in the asymmetric unit. The molecular structure is shown in Fig. 3. The X-ray diffraction study revealed the presence of an 'ate' complex formed through retention of a [LiCl(THF) 2 ] fragment by the five-coordinate unit [ n BuC(=NCy)(NCy)C(NCy) 2 ]Ho[ n BuC-(NCy) 2 ]Cl. The phenomenon of 'ate' complex formation via retention of alkali metal halides in the products is quite common in organolanthanide chemistry (Edelmann, 2006). It can be traced back to the strong tendency of the large Ln 3+ ions to adopt high coordination numbers. In the resulting sixcoordinate bimetallic complex 3, the central holmium(III) ion is coordinated by two -bridging chloride ions, one chelating amidinoguanidinate ligand and one chelating amidinate ligand. The Ho atom is located in the C1N1N2N3 plane of the amidinoguanidinate ligand and, just like in the case of the lithium derivative 2, the amidinate N atom N4 does not contribute to metal coordination. The Ho-N distances are in The molecular structure of compound 2 in the crystal. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity. Symmetry operator to generate equivalent atoms: 2 À x, 1 À y, Àz.

Figure 3
The molecular structure of compound 3 in the crystal, illustrating the disorder of one cyclohexyl group and both THF ligands. Displacement ellipsoids drawn at the 50% probability level and H atoms have been omitted for clarity. a narrow range of 2.327 (3)-2.354 (3) Å that is in good agreement with the values observed in related lanthanide amidinate and guanidinate complexes (Edelmann, 2009(Edelmann, , 2012. The same applies to the corresponding coordination angles N1-Ho-N2 [57.0 (1) ] and N5-Ho-N6 [57.3 (1) ]. The guanidinate and the amidinate moiety in compound 3 are arranged nearly perpendicular to each other, gaining a minimal contact between the bulky cyclohexyl substituents. The [LiCl 2 (THF) 2 ] fragment is attached to the Ho atom in a formally chelating mode, leading to the formation of a regular kite-shaped Ho/Cl1/Li/Cl2 ring [Ho-Cl 2.6326 (13) and 2.6453 (15) Å , Ho-Cl-Li 87.0 (2) and 88.0 (3) ]. The Li atom exhibits a typical tetrahedral coordination by the two -bridging Cl atoms and two THF ligands. Within the chelating NCN units of the amidinato and the amidinoguanidinato ligands, the C-N distances are nearly equal [1.324 (5)-1.336 (5) Å ], indicating a typical -electron delocalization within these units. The conformation of the amidinatoguanidinate ligand is very similar to that in compound 2 (angle between guanidinate and amidinate plane approx. 75 ), and the localization of single and double bonds within the 1-butylamidinate backbone is identical with that in the lithium derivative [C-N 1.272 (5)-1.429 (5) Å ].

Supramolecular features
Due to an effective 'packaging' of the molecules by the sterically demanding alkyl substituents, both title compounds do not feature any specific intermolecular interactions. In the lithium derivative 2, the closest intermolecular contacts are between two isopropyl-CH 3 groups [C3Á Á ÁC10, 3.740 (3) Å ] and between an isopropyl-CH 3 and a butyl-CH 3 group [C13Á Á ÁC18, 3.744 (4) Å ]. The crystal structure of the holmium complex 3 comprises a close package of cyclohexyl groups, butyl groups and THF ligands with a minimal H 2 CÁ Á ÁCH 2 distance of 3.64 (4) Å , and one H 2 CÁ Á ÁCH 3 contact of at least 3.73 (6) Å (C6 and C21B of disordered cyclohexyl group).

Synthesis and crystallization
General Procedures: All reactions were carried out under an inert atmosphere of dry argon employing standard Schlenk and glovebox techniques. THF and n-pentane were distilled from sodium/benzophenone under nitrogen atmosphere prior to use. All glassware was oven-dried at 393 K for at least 24 h, assembled while hot, and cooled under high vacuum prior to use. Anhydrous holmium(III) chloride was prepared according to the literature method (Freeman & Smith, 1958). n-Butyllithium solution, N,N 0 -diisopropylcarbodiimide and N,N 0 -dicyclohexylcarbodiimide were purchased from Aldrich and used as received. 1 H NMR (400 MHz) and 13 C NMR (100.6 MHz) spectra were recorded in DMSO-d 6 solution on a Bruker DPX 400 spectrometer at 298 K. Chemical shifts are referenced to TMS. IR spectra were recorded using KBr pellets on a Perkin Elmer FT-IR spectrometer system 2000 between 4000 cm À1 and 400 cm À1 . Microanalyses (C, H and N) of compounds 1 and 3 were performed using a Leco CHNS 932 apparatus.
Synthesis of Li[ n BuC( NCy)(NCy)C(NCy) 2 ]ÁTHF (1): A solution of N,N 0 -dicyclohexylcarbodiimide (10.30 g, 50 mmol) in 100 ml of THF at 253 K was treated slowly with n-butyllithium (16 ml, 1.6 M solution in hexanes). The reaction mixture was stirred for 10 min at 253 K, then warmed to room temperature and stirred overnight to give a white suspension in THF. Synthesis (2): In a similar manner as for compound 1, N,N 0 -diisopropylcarbodiimide (4.2 g, 50 mmol) was treated with n-butyllithium (10 ml, 2.5 M solution in hexanes) in THF solution (80 ml). From this reaction 14.3 g of colorless 2 were isolated. X-ray quality single crystals (colorless rods) were occasionally obtained directly upon cooling of the reaction mixture to 278 K. However, NMR data showed that the bulk product was heavily contaminated with the lithium amidinate salt Li[ n BuC(N i Pr) 2 ] (10-20%) which could not be separated by fractional crystallization.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were fixed geometrically and refined using a riding model with U iso (H) = 1.2 U eq (C). C-H distances in CH 3 groups were constrained to 0.98 Å , those in CH 2 groups to 0.99 Å and those in CH groups to 1.00 Å . Methyl H atoms were allowed to rotate around the C-C vector but not to tip to best fit the experimental electron density (AFIX 137 in SHELXL). In the crystallographic dataset of compound 3, the intensities of reflections (111) and (111) strongly disagreed with the structural model and were therefore omitted from the refinement. One of the cyclohexyl groups (C19-C24) and both THF ligands (O1, C48-C51 and O2, C52-C55) in compound 3 are disordered. The aforementioned atoms were each split over two sites (site occupancy factors refined freely). Equivalent disordered THF and cyclohexyl moieties were restrained to have similar geometries (SAME restraint in SHELXLL), and U ij components of ADPs were restrained to be similar for atoms closer than 1.7 Å (SIMU restraint in SHELXL; the esd applied was 0.01 Å 2 ). Occupancy ratios refined to 0.760 (6) and 0.240 (6) for the cyclohexyl group (C19-C24), and to 0.663 (11) and 0.337 (11) (O1, C48-C51) and to 0.823 (11) and 0.177 (11) (O2, C52-C55) for the THF moieties.  (Sheldrick, 2008); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015). Molecular graphics: Diamond (Brandenburg, 1999) for compound_2; DIAMOND (Brandenburg, 1999) for compound_3. For both compounds, software used to prepare material for publication: publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.31 e Å −3 Δρ min = −0.21 e Å −3 Extinction correction: SHELXL2016 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00067 (16) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.