Synthesis and crystal structures of two new tin bis(carboranylamidinate) complexes

Tin forms mononuclear chelate complexes with κC,κN-coordinated carboranylamidinate ligands, in which the Sn atom exhibits a trigonal–bipyramidal (SnIV) or pseudo-trigonal–bipyramidal (SnII) coordination.


Chemical context
Amidinates of the general formula [RC(NR 0 ) 2 ] À are the nitrogen analogs of carboxylate anions. These versatile N,N 0chelating ligands form stable coordination compounds with nearly every metallic element in the Periodic Table. In view of this rich coordination chemistry, amidinate ligands are frequently regarded as 'steric cyclopentadienyl equivalents'. Metal complexes comprising amidinato ligands are readily available by insertion of the 1,3-diorganocarbodiimide, R--N C N-R, into an M-C bond of an organometallic precursor compound. Another common synthetic route involves salt metathetical reactions between lithium amidinates and metal halides (Collins, 2011;Edelmann, 2008Edelmann, , 2013a. Metal amidinates comprising small alkyl substituents are often surprisingly volatile and may serve as useful precursors for metal oxides and nitrides by means of ALD (atomic layer deposition) or MOVCD (metal-organic chemical vapor deposition) processes (Devi 2013;Lim et al., 2003;Li et al., 2005).
A key advantage of the amidinate anions [RC(NR 0 ) 2 ] À is the fact that the substituents R and R 0 attached to the N-C-N unit can be varied in many ways. With R = ortho-C 2 H 11 B 10 ('ortho-carboranyl') we introduced a sterically demanding and chemically versatile moiety in the backbone of the amidinate ligand. Carboranes are of tremendous scientific and technological interest due to their various applications ever since their discovery in the 1960 0 s. These applications include the ISSN 2056-9890 synthesis of polymers and ceramics, catalysts, radiopharmaceuticals and non-linear optics, as well as the BNCT (= boron neutron capture therapy) technique (Belmont et al., 1989;Brown et al., 1992;Felekidis et al., 1997;Murphy et al., 1993;Teixidor et al., 1996;Vaillant et al., 2002). The first orthocarboranylamidinate ligand [o-(C 2 H 10 B 10 )C(N i Pr)(NH i Pr)] À (= [HL iPr ] À ) was synthesized in our lab in 2010 by in situ lithiation of the parent o-carborane, ortho-C 2 H 12 B 10 (= orthodicarba-closo-dodecaborane), followed by treatment with 1 equiv. of N,N 0 -diisopropylcarbodiimide (Drö se et al., 2010) as shown in Fig. 1. Subsequent reactions of the so-obtained lithium ortho-carboranylamidinate Li[HL iPr ] or the related Li[HL Cy ] with various metal and non-metal chlorides have been reported by us and others to yield carboranylamidinates of e.g. Sn II and Cr II , Rh I and Ir I , Fe II and Fe III , Ti IV , Zr IV , Si and P (Harmgarth et al., 2014;Hillebrand et al., 2014;Yao et al., 2011Yao et al., , 2012Yao et al., , 2013Xu et al., 2014). In all of these compounds, the ligand adopts a specific C,N-chelating mode instead of the N,N 0 -chelating mode usually observed for simple amidinate ligands (Collins, 2011;Edelmann, 2008Edelmann, , 2013a. In the case of the carboranylamidinates [HL R ] À (R = i Pr, Cy), a proton is formally shifted from the carborane C atom to the amidinate unit, resulting in an amidine moiety that usually acts as a monodentate donor functionality as shown in Fig. 2a.
In some cases, subsequent deprotonation of the NH functionality results in formation of a formally dianionic ligand [L R ] 2-, whose favored coordination mode is still C,N (Fig. 2b). Derivatives of Si, P, Ge, Sn II , Sn IV , Fe II and Fe III , Rh I and Ir I containing this ligand system have been prepared by double lithiation of the parent ortho-carboranylamidine followed by treatment with appropriate element chloride precursors (Yao et al., 2011;Harmgarth et al., 2014Harmgarth et al., , 2017, or through spontaneous disproportionation of in situ formed [HL R ] À to [L R ] 2and free carboranylamidine H 2 L R . The latter reaction has been found to be favored in the case of strongly Lewis-acidic metal precursors, namely Cp 2 TiCl 2 , Cp 2 ZrCl 2 and various chlorosilanes (Harmgarth et al., 2014(Harmgarth et al., , 2017. While dichlorosilanes R 2 SiCl 2 react with Li[HL iPr ] readily to form R 2 Si[L iPr ]-type products, we recently found that for the heavy group 14 analogues Sn and Pb the formation of R 2 ECl[HL iPr ]type products is much more preferred (Harmgarth et al., 2017).
Among the known carboranylamidinate complexes are only very few compounds with more than one carboranylamidinate ligand per metal atom, and these are exclusively of the type M[HL R ] 2 (M = Sn II , Cr II  The mono-lithio-ortho-carboranylamidinate precursors Li[HL Cy ] and Li[HL iPr ] were readily available following a published procedure by reaction of the mono-lithiated o-carborane Li-o-C 2 B 10 H 11 with a stoichiometric amount of the carbodiimides i PrN C N i Pr or CyN C NCy, respectively, in THF (cf. Fig. 1) (Harmgarth et al., 2014). In a first experiment, reaction of Li[HL Cy ] with 0.5 equiv. of SnCl 2 in THF afforded the stannylene compound Sn[HL Cy ] 2 (1) as colorless, block-like single crystals after recrystallization from toluene. The low isolated yield of ca 20% can be traced back to the very high solubility of 1 even in non-polar organic solvents. In addition to the X-ray diffraction study, compound 1 was also characterized through elemental analysis and the usual set of spectroscopic methods. In the IR spectrum, a characteristic (NH) band at 3423 cm À1 confirmed the presence of monoanionic [HL Cy ] À ligands. The NH functionalities were also observed in the 1 H NMR spectrum through a broad singlet at 4.50 ppm. A single 119 Sn NMR resonance at À46 ppm was in agreement with the formation of a single Sncontaining species. The mass spectrum of 1 showed the molecular ion at m/z 818 with 27% relative intensity.
A similar reaction of SnCl 4 with 2 equiv. of Li [HL iPr ] was carried out with the aim of synthesizing the hitherto unknown tin(IV) bis(carboranylamidinate) Sn[L iPr ] 2 . Cooling of the reaction mixture afforded a fairly large amount of wellformed, colorless crystals, which turned out to be the known solvated pentachloridostannate(IV) salt [Li(THF) 4 ][SnCl 5 -(THF)]. This compound was first prepared and structurally characterized by Junk & Leary (2000). From the concentrated mother liquid of the pentachloridostannate salt, only a small amount (ca 5% isolated yield) of the unexpected tin(IV) carboranylamidinate SnCl[L iPr ][HL iPr ] (2) could be obtained. The X-ray crystal structure determination of 2 revealed the presence of the first complex containing both a mono-and a dianionic carboranylamidinate ligand in one molecule. As in 1, the IR spectrum of 2 showed a characteristic (NH) band at 3410 cm À1 . Elemental analysis and a single resonance in the 119 Sn NMR spectrum ( 290 ppm) confirmed the purity of 2. In the mass spectrum, the molecular ion was observed at m/z 692 with 47% relative intensity.

Supramolecular features
In both 1 and 2, the molecules are well separated in the crystal and no unusually short intermolecular contacts have been observed. The shortest intermolecular contacts are found between cyclohexyl groups and carborane backbones in 1 [B5Á Á ÁC14 3.727 (3) Å ] and between isopropyl groups in 2 [C5Á Á ÁC15 3.670 (7) Å ], respectively. In both compounds, the free N-H groups are not involved in hydrogen bonding.
For reviews on the chemistry of carboranylamidinates, see: Edelmann (2013b)

Synthesis and crystallization
All operations were performed under an argon atmosphere using standard Schlenk techniques. THF and toluene were distilled from sodium/benzophenone under argon. NMR spectra were recorded on a Bruker DPX400 ( 1 H: 400 MHz) spectrometer in THF-d 8 at 295 (2) K. 1 H and 13 C NMR shifts are referenced to Si(CH 3 ) 4 , 119 Sn shifts to Sn(CH 3 ) 4 (each = 0 ppm). IR spectra were measured on a Bruker Vertex V70 spectrometer equipped with a diamond ATR unit, electron impact mass spectra on a MAT95 spectrometer with an ionization energy of 70 eV. Elemental analyses (C, H and N) were performed using a VARIO EL cube apparatus.

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. The CH 3 groups in 2 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 B and N atoms were located in the difference-Fourier map, B-H distances were restrained to 1.12 (2) Å and N-H distances to 0.88 (2) Å . The U iso (H) values were set at 1.5U eq (C) for the methyl groups in 2, and at 1.2U eq (X) (X = B, C, N) in all other cases. For 1, the reflections (100) and (010) disagreed strongly with the structural model and were therefore omitted from the refinement.

Bis(N,N′-dicyclohexylamidinatocarboranate)tin(II) (compound_1)
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.