Synthesis and structural characterization of four dichloridobis(cyclopropylalkynylamidine)metal complexes

Transition-metal amidine complexes of the type MCl2[c-C3H5—C≡C—C(NR′)(NHR′)]2 (M = Mn, Fe, Co) are readily available by a two-step synthesis starting from cycloproplacetylene.


Chemical context
Over the past three decades, chelating anionic 1,3-diazaallyltype ligands such as amidinates, [RC(NR 0 ) 2 ] À , and guanidinates, [R 2 NC(NR 0 ) 2 ] À , have gained tremendous importance in various fields of organometallic and coordination chemistry. These highly versatile N-chelating ligands are generally regarded as steric equivalents of the ubiquitous cyclopentadienyl ligands (Collins, 2011;Edelmann, 2009Edelmann, , 2012Edelmann, , 2013. Unlike the closely related carboxylate anions, [RCO 2 ] À , the steric properties of amidinate anions can be tuned in a wide range by introducing different substituents at all three atoms of the NCN 1,3-diazaallyl unit. A rather interesting and potentially useful variation of the amidinate group is the use of alkinyl groups at the central C atom. Alkinylamidines of the composition RC C-C( NR 0 )(NR 0 ) are of interest because of their applications in organic synthesis (Ong et al., 2006;Xu et al., 2008;Weingä rtner & Maas, 2012) and in biological and pharmacological systems (Rowley et al., 2005;Sienkiewicz et al., 2005). Moreover, alkinylamidinate complexes of transition metals and lanthanides effectively catalyze the addition of C-H, N-H and P-H bonds to carbodiimides as well as the polymerization of polar monomers such as "-caprolactone. ISSN 2056-9890 Previously used alkynylamidinate anions have mainly included the C-phenyl and C-trimethylsilyl derivatives [R-C C-C(NR 0 ) 2 ] À (R = Ph, SiMe 3 ; R 0 = i Pr, Cy; Drö se et al., 2010a,b; Seidel et al., 2012;Xu et al., 2013).
We recently began with an investigation of alkinylamidinate ligands and complexes derived from cyclopropylacetylene. The cyclopropyl group was selected because of its wellestablished electron-donating ability to an adjacent electrondeficient center. This way it is possible to electronically modify the amidinate ligand system rather than just changing its steric demand. In a first study, we described the synthesis and characterization of a series of lithium cyclopropylethinylamidinates, Li[c-C 3 H 5 -C C-C(NR 0 ) 2 ] [R 0 = i Pr, Cy (= cyclohexyl)], which are readily accessible on a large scale using commercially available starting materials (cyclopropylacetylene, N,N 0 -diorganocarbodiimides; Sroor et al., 2013). Subsequently, these ligands have been employed for the preparation of new di-and trivalent lanthanide complexes (Sroor et al., 2015a(Sroor et al., ,b,c,d, 2016Wang et al., 2016). More recently, we became interested in the chemistry of 3d metal complexes containing cyclopropylethinylamidinate ligands. In the course of this work, we occasionally observed and structurally characterized hydrolysis products of the composition MCl 2 [c-C 3 H 5 -C C-C(NR 0 )(NHR 0 )] (M = Mn, Fe, Co; R 0 = i Pr, Cy), which contain the neutral amidines c-C 3 H 5 -C C-C(NR 0 )(NHR 0 ) as new ligands. Neutral amidines are highly versatile ligands in coordination chemistry in their own right (Barker & Kilner, 1994;Coles, 2006). We report here the deliberate synthesis of two new cylopropylalkynylamidines, c-C 3 H 5 -C C-C(NR 0 )(NHR 0 ) (R 0 = i Pr, Cy) as well as the preparation and structural characterization of four first-row transition metal complexes of the type MCl 2 [c-C 3 H 5 -C C-C(NR 0 )(NHR 0 )] (M = Mn, Fe, Co; R 0 = i Pr, Cy).
The title compounds were first discovered serendipitously when studying reactions of anhydrous metal(II) chlorides MCl 2 (M = Mn, Fe, Co) with 2 equiv. of the lithium cyclopropylethinylamidinates, Li[c-C 3 H 5 -C C-C(NR 0 ) 2 ] (R 0 = i Pr, Cy) in THF solution. Occasionally, small amounts of wellformed crystals were obtained, which turned out (by X-ray diffraction studies) to be the aforementioned hydrolysis products MCl 2 [c-C 3 H 5 -C C-C(NR 0 )(NHR 0 )] (M = Mn, Fe, Co; R 0 = i Pr, Cy). We then decided to prepare these complexes in a deliberate manner. As illustrated in Fig. 1, the bottom-up synthesis starts with the readily available lithium cyclopropylethinylamidinates, Li[c-C 3 H 5 -C C-C(NR 0 ) 2 ] (R 0 = i Pr, Cy; Sroor et al., 2013), which were made by addition of c-C 3 H 5 -C C-Li (prepared in situ from cyclopropylacetylene and n BuLi) to the carbodiimides R 0 -N C N-R 0 (R = i Pr, Cy). The lithium amidinate intermediates were then carefully hydrolyzed under controlled conditions to afford the neutral amidines c-C 3 H 5 -C C-C(NR 0 )(NHR 0 ) [R 0 = i Pr (1), Cy (2)] in >70% isolated yields. Both compounds form yellow oils, which were characterized by the usual set of spectroscopic data (MS, 1 H NMR, 13 C NMR, IR) and elemental analysis. With the free amidine ligands in hand, the metal complexes with first-row transition metals could easily be prepared by treatment of metal(II) chlorides MCl 2 (M = Mn, Fe, Co) with 2 equiv. of either 1 or 2 in THF solution. The manganese(II) complex 3 as well as the two iron(II) complexes 4 and 5 form colourless crystals, while the cobalt(II) complex 6 is blue. The compositions of all four products as 1:2 complexes were confirmed by elemental analyses. The title compounds 3-6 were also characterized by their IR and mass spectra. The mass spectra showed a number of readily interpretable peaks resulting e.g. from loss of one amidine ligand or one or both chlorine atoms. IR bands in the region above ca 3100 cm À1 could be assigned to the (N-H) vibrations, while strong bands around 1570 cm À1 were characteristic for the C N double bond in the amidine ligands. In the far-infrared region, the M-Cl bands could be clearly assigned by comparison with literature values (Clark & Williams, 1965;Takemoto et al., 1974) and IR spectra of the respective anhydrous metal(II) chlorides, MCl 2 (M = Mn, Fe, Co; for details see the Synthesis and crystallization section).
Compound 3 represents a rare example of a complex of tetra-coordinated manganese with nitrogen ligands, while a larger number of the corresponding iron and cobalt complexes are known. The Mn-N bond length in 3 is 2.160 (2) Å and therefore comparable with literature data (Handley et al., 2001;Wang, 2009). In the iron complexes, the Fe-N distances are very similar at 2.088 (3) Å (4), and 2.073 (2)-2.079 (2) Å (5). These values are in the range of Fe-N distances usually observed in MCl 2 L 2 -type complexes, where L is a ligand with an sp 2 -hybridized nitrogen donor (Benson et al., 2010;Xiao et al., 2011;Batcup et al., 2014). The same is true for the cobalt complex 6, having Co-N bond lengths of 2.041 (2) and 2.043 (2) Å (Riggio et al.;Jian et al., 2003;Xiao et al., 2011). The set of C-N bond lengths within the NCN group of the amidine ligands is virtually equal in 3-6, including one formal C=N double bond at 1.309 (2)-1.315 (4) Å , and one formal C-N single bond at 1.337 (4)-1.340 (2) Å . The small difference between single-and double-bond length may indicate some degree of delocalization of the -electron density.

Figure 3
Molecular structure of 4 in the crystal. Displacement ellipsoids are drawn at the 50% level, C-bound H atoms omitted for clarity. Symmetry code: ( 0 ) 1 À x, 1 À y, z.

Figure 4
Molecular structure of 5 in the crystal. Displacement ellipsoids are drawn at the 50% level, C-bound H atoms omitted for clarity.

Figure 5
Molecular structure of 6 in the crystal. Displacement ellipsoids are drawn at the 50% level, C-bound H atoms omitted for clarity.

Chemistry of related structures
For reviews on the coordination chemistry of neutral amidines, see Barker & Kilner (1994) and Coles (2006).

Synthesis and crystallization
General Procedures: All reactions were carried out in ovendried or flame-dried glassware in an inert atmosphere of dry argon employing standard Schlenk and glovebox techniques. The solvent THF was distilled from sodium/benzophenone in a nitrogen atmosphere prior to use. n-Butyllithium (1.6 M in hexanes) was purchased from Sigma-Aldrich. 1 H NMR (400 MHz) and 13 C NMR (100.6 MHz) spectra were recorded in THF-d 8 solutions using a Bruker DPX 400 spectrometer at 298 K. Chemical shifts are referenced to tetramethylsilane. IR spectra were measured with a Bruker Vertex 70V spectrometer equipped with a diamond ATR unit between 4000 and 50 cm À1 . The relative intensities of the absorption bands are given as very strong (vs), strong (s), medium (m), weak (w) and shoulder (sh). Electron impact mass spectra were measured on a MAT95 spectrometer with an ionization energy of 70 eV. Microanalyses of the compounds were performed using a vario EL cube apparatus from Elementar Analysensysteme GmbH.
Synthesis of 3-cyclopropyl-N,N 0 -diisopropylpropynamidine, c-C 3 H 5 -C C-C(N i Pr)(NH i Pr) (1): A THF (80 ml) solution of cyclopropylacetylene (4.2 ml, 50 mmol) in a Schlenk flask (250 ml) was cooled to 253 K and treated slowly with n-butyllithium (50 mmol, 1.6 M solution in hexanes). After 30 min, neat N,N 0 -diisopropylcarbodiimide (7.8 ml, 50 mmol) was added and the mixture was stirred for 15 min at 253 K. The solution was warmed to room temperature and stirred for 1 h. During this time, the solution colour turned yellow. 20 ml of distilled water were added and stirring was continued for 30 min. The solution was separated using a separatory funnel and allowed to stand overnight after adding 3.0 g of anhydrous magnesium sulfate to remove the remaining water. The solvents were removed under vacuum to obtain 1 as a yellow oil. Yield: 6.9 g, 72%. Elemental analysis for C 12 H 20 N 2 (192.30 g mol À1 ): C, 74.95; H, 10.48; N, 14.57; found C, 74.74; H, 10.46; N, 14.58 Synthesis of 3-cyclopropyl-N,N 0 -dicyclohexylpropynamidine, c-C 3 H 5 -C C-C(NCy)(NHCy) (2): A THF (100 ml) solution of cyclopropylacetylene (4.2 ml, 50 mmol) in a Schlenk flask (250 ml) was cooled to 253 K and treated slowly with n-butyllithium (50 mmol, 1.6 M solution in hexanes). After 30 min, N,N 0 -dicyclohexylcarbodiimide (10.3 g, 50 mmol) was added and the rest of the reaction mixture was worked up as described for 1. The solvent was removed under vacuum to obtain 2 as a yellow oil. Yield: 10.1 g, 74%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. H atoms attached to C atoms were fixed geometrically and refined using a riding model. The CH 3 groups in 3 and 4 were allowed to rotate freely around the C-C vector, 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 CH groups to 1.00 Å . H atoms attached to N atoms were located in the difference-Fourier map and refined, the N-H distances were restrained to 0.88 (2) Å . The U iso (H) values were set at 1.5U eq (C) for the methyl groups in 3 and 4, and at 1.2U eq (X) (X = C, N) in all other cases. For 6, the reflections (011) and (002) disagreed strongly with the structural model and were therefore omitted from the refinement.

Dichloridobis(3-cyclopropyl-N,N′-diisopropylprop-2-ynamidine)manganese(II) (3)
Crystal data 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.

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.

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.18374 (11) 0.73239 (12) 0.51035 (7 (7) −0.0013 (7) 0.0031 (7)  C19 0.0177 (7) 0.0127 (7) 0.0125 (7) 0.0012 (6)  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.