Formation and structural characterization of a potassium amidinoguanidinate

In the polymeric potassium complex [{iPrN= CHN(iPr)N(NiPr)2K}2(μ-DME)]n, the amidinoguanidinate ligand adopts an unusual mixed σ-/π-coordination mode.


Chemical context
Heteroallylic N,N 0 -chelating donor ligands such as amidinate anions [RC(NR) 2 ] À and guanidinate anions [R 2 NC(NR) 2 ] À are of significant importance in various fields of organometallic and coordination chemistry. It is generally accepted that both types of N,N 0 -chelating ligands can be regarded as 'steric cyclopentadienyl equivalents' (Bailey & Pace, 2001;Collins, 2011;Edelmann, 2013). Over the past three decades, amidinato and guanidinato complexes have been synthesized for nearly every metallic element in the Periodic Table ranging from lithium to the f-block elements (Edelmann, 2009(Edelmann, , 2012(Edelmann, , 2013Trifonov, 2010). Important applications of amidinate and guanidinate ligands include the stabilization of unusually low oxidation states (e.g. Mg I and Fe I ) as well as the design of highly active homogeneous catalysts (Collins, 2011;Edelmann, 2013;Chen et al., 2018). Metal amidinate and guanidinate complexes bearing small aliphatic substituents have also been established as ALD (= atomic layer deposition) and MOCVD (= metal-organic chemical vapor deposition) precursors for the deposition of thin films of metals, metal oxides, metal nitrides etc. (Devi, 2013). Formally, amidinate anions are nitrogen analogues of carboxylate anions, while guanidinates are related in the same way to carbamate anions. However, in contrast to the carboxylates and carbamates, the steric properties of amidinates and guanidinates can be tuned over a wide range by employing different substituents at the outer nitrogen atoms as well as at the central carbon atom of the chelating NCN unit. The most important starting materials in this area are lithium amidinates, which are normally prepared in a straightforward manner by the addition of lithium alkyls to N,N 0 -diorganocarbodiimides in a 1:1 molar ratio. Lithium guanidinates are formed in the same manner by adding lithium-N,N-dialkylamides 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 of these reactions are generally quite straightforward and afford the desired products in high yields. Less investigated are amidinate salts of the heavier alkali metals sodium and potassium Junk & Cole, 2007;Yao et al., 2009;Drö se et al., 2010, Chen et al., 2018.
We recently reported in this journal that, under certain conditions, seemingly straightforward reactions of lithium alkyls with N,N 0 -diorganocarbodiimides can take a different course, leading to lithium salts of dimerized amidinates ligands ('amidinoguanidinates') (Sroor et al., 2016). These could even become the major reaction products when the N,N 0diorganocarbodiimides are used in a twofold molar excess. The first complexes comprising amidinoguanidinate ligands included the lithium precursors Li[ n BuC( NR)(NR)C(NR) 2 ] [R = i Pr, Cy (= cyclohexyl)] and the holmium(III) complex [ n BuC( NCy)(NCy)C(NCy) 2 ]Ho[ n BuC(NCy) 2 ](-Cl) 2 Li-(THF) 2 (Sroor et al., 2016). In this contribution, we report the synthesis and structural characterization of the first potassium amidinoguanidinate derivative, polymeric catena-poly [[bis(-1-amidinato-N,N 0 ,N 00 ,N 000 -tetraisopropylguanidinato-5 N 1 :- As illustrated in Fig. 1, the title compound was formed when N,N 0 -diisopropylcarbodiimide was added to a suspension of potassium hydride in 1,2-dimethoxyethane (DME). With the attempt to prepare the corresponding amidinate K[HC(N i Pr) 2 ], the reactants were used in a molar ratio 1:1. After filtration and concentration of the filtrate to a small volume, the product crystallized directly at room temperature and could be isolated as colorless, plate-like, moisture-sensitive crystals in 76% yield (calculated after determination of the crystal structure). The compound was characterized through elemental analysis as well as IR, NMR ( 1 H and 13 C) and mass spectra. However, the usual set of analytical and spectroscopic methods did not allow for an unequivocal elucidation of the molecular structure. NMR data clearly indicated the presence of coordinated DME. However, both the 1 H and 13 C NMR spectra showed two sets of iso-propyl resonances, thereby ruling out the formation of a simple potassium formamidinate salt of the composition '(DME)K[HC(N i Pr) 2 ]'. Fortunately, the single crystals obtained directly from the filtered and concentrated reaction solution were suitable for X-ray diffraction analysis. This study confirmed the formation of a new amidinoguanidinate complex through dimerization of N,N 0 -diisopropylcarbodiimide in the coordination sphere of potassium.

Structural commentary
The molecular structure of the title compound consists of centrosymmetric dimeric units, being composed of two potassium atoms and two amidinoguanidinate ligands (Fig. 2). The guanidinate unit is attached to potassium in an N,N 0chelating mode, with the K atom in the N 3 C plane of the guanidinate. The same guanidinate moiety is linked to the Molecular structure of the title compound in the crystal. Displacement ellipsoids are drawn at the 50% probability level, hydrogen atoms omitted for clarity. Symmetry codes: ( 0 ) Àx, Ày, 2 À z; ( 00 ) Àx, À1 À y, 2 À z.

Figure 1
Formation of the title compound by reaction of potassium hydride with N,N 0 -diisopropylcarbodiimide in DME. symmetry-equivalent K atom in an 3 -diazaallyl mode, i.e. the metal atom is situated above the N1/C1/N2 plane. The exposed nitrogen donor of the amidinate backbone (N4) in the title compound is attached to the metal center in a simple monodentate coordination, with the N atom having a perfectly planar environment (sum of bond angles = 360.0 ). This is in agreement with the expected sp 2 hybridization of atom N4 (cf. Scheme). As a result of the -bridging coordination of the amidinoguanidinate ligand, the potassium atom is surrounded by a -chelating guanidinate group, a -diazaallyl-coordinated guanidinate group, and a single amidinate nitrogen atom in a T-shaped fashion. A pseudo-square-planar coordination is completed by one oxygen atom of a -O:O 0 -coordinated DME ligand. Through this bridging DME coordination, the dimeric units are interconnected into a one-dimensional coordination polymer (Fig. 3).
An increased tendency towards -coordination modes is characteristic for the heavier alkali metals and has frequently been observed in other complexes with nitrogen ligands (e.g. von Bü low et al., 2004;Liebing & Merzweiler, 2015). However, in potassium amidinates and guanidinates, a symmetric double-chelating coordination is usually preferred over coordination modes with a contribution of the -electron system ( Fig. 4) (Giesbrecht et al., 1999;Benndorf et al., 2011). A similar mixed -/-coordination to that in the title compound has been recently observed by us in a potassium dithiocarbamate (Liebing, 2017).

Supramolecular features
The crystal structure of the title compound does not display any specific interactions between the polymeric chains. The closest interchain contact is 3.632 (3) Å (C5Á Á ÁC14) between the methyl carbon atoms of isopropyl groups.

Synthesis and crystallization
General Procedures: The reaction was carried out under an inert atmosphere of dry argon employing standard Schlenk and glove-box techniques. The solvent dimethoxyethane (DME) was distilled from sodium/benzophenone under nitrogen atmosphere prior to use. All glassware was ovendried at 393 K for at least 24 h, assembled while hot, and cooled under high vacuum prior to use. The starting material N,N 0 -diisopropylcarbodiimide was obtained from Sigma-Aldrich and used as received. Commercially available potassium hydride was freed from protecting paraffin oil by thoroughly washing with n-pentane and stored in a glove-box. The Illustration of the polymeric chain structure of the title compound, extending along the crystallographic b axis.

Figure 4
Coordination modes of 1,3-diazaallyl-type ligands (= amidinate or guanidinate) observed in potassium complexes: symmetric doublechelating (A), single-chelating and 3 -coordination of the 1,3-diazaallyl -system (B). 1 H and 13 C NMR spectra were recorded in solutions on a Bruker Biospin AVIII 400 MHz spectrometer at 298 K. Chemical shifts are referenced to tetramethylsilane. The IR spectrum was measured with a Bruker Optics VERTEX 70v spectrometer, and the electron impact mass spectrum was recorded using a MAT95 spectrometer with an ionization energy of 70 eV. Microanalysis of the title compound was performed using a 'vario EL cube' apparatus from Elementar Analysensysteme GmbH. The melting/decomposition point was measured on a Bü chi Melting Point B-540 apparatus.
Synthesis of [{ i PrN CHN( i Pr)N(N i Pr) 2 K} 2 (l-DME)] n : 1.6 mL (1.26 g, 10.0 mmol) of N,N 0 -diisopropylcarbodiimide were added to a stirred suspension of 0.41 g (10 mmol) of KH in 50 ml of DME. The reaction mixture was stirred for two days and refluxed for an additional 2 h. After cooling to room temperature, all insoluble solid parts were filtered off and the volume of the resulting clear solution was reduced to ca 25 ml. After three days at room temperature, the title compound crystallized as colorless, plate-like crystals suitable for singlecrystal X-ray diffraction. Yield: 1.3 g (76%). M.p. 378 K (dec.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms attached to C atoms were fixed geometrically and refined using a riding model. CH 3 groups were allowed to rotate freely around the C-C vector, and the corresponding C-H distances were constrained to 0.98 Å . C-H distances within CH 2 groups were constrained to 0.99 Å , C-H distances within the i Pr CH groups to 1.00 Å , and the C-H distance within the amidinate group (i.e. at C2) to 0.95 Å . The U iso (H) values were set at 1.5U eq (C) for methyl groups and at 1.2U eq (C) in all other cases. The reflections (001) and (010) disagreed strongly with the structural model and were therefore omitted from the refinement.

Funding information
Financial support of this work by the Otto-von-Guericke-Universitä t Magdeburg is gratefully acknowledged.  ; program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010). 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.