Synthesis and structure of the first tin(II) amidinato-guanidinate [DippNC(ⁿBu)NDipp]Sn{pTol-NC[N(SiMe₃)₂]N-pTol}

Metal complexes containing various types of N,N-chelating ligands are, for reasons of new catalytic or material application exploration, frequently studied chemical entities nowadays. One of the most prominent classes of the metal complexes are species with possible delocalization of π electrons over the ligand skeleton and thus the possible stabilization of the low-valent metal centers: amidinates (Jones et al., 2011; Benndorf et al., 2012; Gallego et al., 2013; Schwamm et al., 2013), guanidinates (Bonyhady et al., 2010; Fohlmeister et al., 2012; Kelley et al., 2013) and β-diketiminates (Green et al., 2007; Rodriguez et al., 2011; Lehenmeier et al., 2013; Zhao et al., 2013). Such monoanionic or dianionic ligand systems form the four- or six-membered diazametallacycles (Chart 1) with a high degree of the electron density donation ability.

Chart 1

Monoanionic amidinato (A), guanidinato (B) and β-diketiminato (C) ligands.

In the chemistry of main group metals, there is an interest to investigate the unusual, generally low oxidation states of the central metal atoms and the reactivity of its complexes in order to replace the expensive and low-abundance metals (Pd, Pt, Rh, Ru) in some catalytically driven organic transformations (Power, 2010). The main group metal amidinates and guanidinates fulfill such demands (Green et. al., 2007; Junold et al., 2012) and became one of the most studied types of compounds in the area, although the tin(II) or tin(IV) amidinates and guanidinates, mostly studied as perfect models for structure and reactivity predictions of some applications in homogeneously catalyzed ring-opening polymerization of lactones and carbonates, appeared as well (Aubrecht et al., 2002).

The title compound 1, bearing both types of NCN unsaturated chelated units, amidinato and guanidinato ligand, in the molecule, was prepared by the transmetallation reaction of lithium N,N′-bis[2,6-di(propan-2-yl)phenyl]n-butylamidinate with an equimolar amount of heteroleptic stannyl chloride in Et₂ O at room temperature via elimination of lithium chloride in moderate yield (Scheme 1). To the best of our knowledge, 1 is the first example of a main group metal compound containing two different ligands belonging to a large family of structurally similar NCN systems. Structural behaviors as well as the purity of the studied complex 1 were confirmed with ¹H and ¹¹⁹Sn nuclear magnetic resonance (NMR) spectroscopy in tetrahydrofuran (THF)-d₈ solution. The ¹H NMR spectrum reveals only one set of signals corresponding to each geometrically independent group of the aliphatic chain or the aromatic ring, and the supposed existence of C₂ symmetry through both central ipso carbon atoms of NCN units and the presence of the tin atom in the structure are found with respect to ¹H NMR spectra. On the contrary, the absence of symmetry across the Dipp substituents in the amidinato ligand is reflected in the presence of two multiplets belonging to CH groups and four doublets for CH₃ groups, respectively, of isopropyl fragments in the proton NMR spectrum. The values of the proton chemical shifts of 1 are comparable to the corresponding parameters of starting components. In the ¹¹⁹Sn NMR spectrum of 1 [δ(¹¹⁹Sn)=-443.9], the upfield shift about 200 ppm in comparison to starting stannyl chloride {pTol-NC[N(SiMe₃)₂]N-pTol}SnCl (-220 ppm in THF-d₈) (Chlupatý et al., 2012a) is observed probably due to an increase in the coordination number of tin atom from three to four. However, the chemical shift value of 1 is comparable to that of the reported homoleptic four-coordinated tin(II) amidinate [Dipp-NC(Me)N-Dipp]₂ Sn (-394 ppm in C₆ D₆) (Nimitsiriwat et al., 2007) and the solvent-independent guanidinate {pTol-NC[N(SiMe₃)₂]N-pTol}₂ Sn (-427.8 in THF-d₈) (Chlupatý et al., 2012a).

Scheme 1

Reaction route to [DippNC(nBu)NDipp]Sn{pTol-NC[N(SiMe₃)₂]N-pTol} (1); Dipp=2,6-di(propan-2-yl)phenyl.

A comparison of the structural motifs of both homo- and heteroleptic tin(II) amidinates and guanidinates, containing additional amido or halogen ligands, is reported (Chlupatý et al., 2012a,b), but to the best of our knowledge there are only two reports on metal complexes where both ligand types are present in the same molecule. The first one is the masked amidinato-guanidinate tetranuclear ytterbium complex (Pi et al., 2007), and the second one is the mononuclear zirconium amidinato-guanidinato-amide (Sun et al., 2009). The structure of the prepared compound 1 (Figure 1) with the desired combination of both types of ligands is comparable to that of both homoleptic tin(II) amidinates and guanidinates. The tin atom in 1 is four-coordinated with distorted (screwed) pseudo-square pyramidal configuration similarly to the rest of the reported tin(II) bis-amidinate complexes (Zhou and Richeson, 1996; Hitchcock et al., 2007; Nimitsiriwat et al., 2007) and some bis-guanidinates (Chlupatý et al., 2012a).

Figure 1

ORTEP view of the molecular structure of compound 1 at 40% probability level.

Hydrogen atoms are omitted for clarity.

However, the tin(II) bis-guanidinates, which reveal the C₂ symmetry with the geometry of distorted pseudo-trigonal bipyramid around the tin atom, were also reported (Foley et al., 2000). Unexpectedly, only small differences were found between appropriate interatomic distances and angles describing both diazametallacycles (Figure 2) and the values found in the literature for both homoleptic tin(II) amidinates and guanidinates. High degree of π-electron delocalization in nearly planar N₂ C systems is proven by the sum of angles around nitrogen atoms approaching 360°. The tin atom as well as the peripheral nitrogen and carbon atoms (N3 and C47) is located above the N₂ C planes of the amidinate and guanidinate units [0.561(2) for Sn1-amidinate, 0.792(3) for Sn1-guanidinate, 0.103(3) for N3 and 0.115(3)Å for C47]. The interplanar (bite) angle between planes defined by N1, Sn1 and N2 and N4, Sn1 and N5 atoms is 71.56(8)°, which is value about 15° lower than is typical for metal complexes of guanidinates as well as of amidinates (Zhou and Richeson, 1996; Tin et al., 1999; Thirupathi et al., 2000).

Figure 2

Schematic drawing of the bonding situation in 1.

Peripheral functional groups are omitted for clarity, bond lengths are given in angstroms, and interatomic distances are highlighted by black arrows and red arcs.