SnIV and ZrIV Compounds of a C 3-Symmetric Ligand with Amine [ ONN ] and [ ONNO ] Coordination Sites

This contribution describes the synthesis and structural characterization of a triaminoguanidinium (TAG)-based ligand [H6(OMe)3Limin]BF4 (1) containing imine bonds (Limin) and its reduction with a dimethylamino borane complex to the corresponding amine compound (Lamin)[H9(OMe)3Lamin]OTs (2). In solution, both ligands are C3-symmetric but crystal structures show the great influence of the reduction on the molecular structure. We show that the planar imine ligand is converted to a highly flexible compound which has nine potential coordination sites, three phenoxy and six amine donors, for binding metal ions. First solid state structures of 1:1 (metal:ligand) coordination compounds with Sn and Zr are presented. Sn exhibits an octahedral coordination sphere and is bound in a facial [ONN] coordination pocket. Zr is pentagonal bipyramidal coordinated and the ligand stabilizes this with its’ [ONNO] binding sites.


Introduction
Triaminoguanidinium salts are easily prepared by a substitution reaction between guanidinium salts with hydrazine [1].Their amine functionalities can be used to perform condensation reactions with aromatic and aliphatic aldehydes and ketones leading to imine compounds [2] [3] [4] [5].The use of salicylalde-International Journal of Organic Chemistry hyde derivatives lead to trigonal building blocks that are known to form trinuclear complexes through coordination of metal ions in their three coplanar [ONN] coordination pockets [6].They have been used for the construction of various supramolecular architectures [7] [8] [9] [10] [11].Nonetheless, condensation reactions are reversible and during our work with these hydrazone derived ligands, we observed their decomposition especially under extreme conditions.To circumvent this problem, the imine double bond can be reduced.Maas et al. [12] reported a method to reduce TAG-based molecules with dimethylamino borane complex according to Ghelfi et al. [13].This resulted in a loss of its threefold symmetry in solid state with an enhancement of structural flexibility.
Applying this method to tris (salicylidene) triaminoguanidinium salts leads to a new ligand type with three phenoxy and six amine donor sites.The new compound type promises to form very stable complexes.Thus, we were interested in the coordination behavior of a ligand with nine potentially strongly coordinating binding sites.The preparation and structural characterization of the ligand before and after the reduction and two coordination compounds with Sn IV and Zr IV are reported herein.

Measurements
NMR spectroscopic data were recorded with UltraShield TH 400 Plus or Avance II Bruker spectrometer at room temperature. 1H and 13 C NMR spectroscopic data are referenced to residual non-deuterated solvent.
Elemental analyses were performed using Heraeus CHN-O-Rapid Vario EL.
Mass spectra were recorded on a ThermoFisher Scientific LTQ-Orbitrap XL.
X-Ray diffraction data of compounds 1, 3 and 4 were obtained with a Bruker Apex-I CCD diffractometer with Mo radiation (λ = 0.71073 Å) und data of 2 were obtained with a Bruker Apex-II CCD diffractometer with Cu radiation (λ = 1.54178Å).Structures were solved by direct methods using SHELXS-2014and refined with SHELXL-2014 [14].

Structural Characterization of Ligands 1 and 2
Following a literature procedure, the imine type ligand is readily prepared via a single step condensation reaction between 2-hydroxy-3-methoxybenzaldehyde with triaminoguanidinium tetrafluoroborate (Scheme 1).[4] The reduction is carried out with dimethylamino borane complex (DMAB) as reducing agent in presence of p-toluenesulfonic acid (pTsOH).The molecules in the asymmetric unit display different conformations of the hydroxyphenyl group to the imine nitrogen (denoted by 1.1 and 1.2 with aromatic rings numbered with Roman numerals).In 1.1, the anti-conformation of the OH groups can be observed in ring I and II and the OH group of ring III exhibits a syn-conformation.This is stabilized by intramolecular hydrogen bonding between OH and N imine (Table 2).Intramolecular hydrogen bonding between OH and methoxy oxygen stabilizes anti.Molecule 1.2 adopts the all-anti-conformation promoted by hydrogen bonds to either counterion (ring I) or methoxy O (ring II and III).
Regarding the molecular structure of 2 (Figure 2), the most striking difference to ligand 1 is its spatial arrangement.Figure 3 illustrates the loss of planarity caused by the reduction.This is in accordance with observations of Maas et al. and following his nomenclature, the ligand exhibits an endo, exo, exo-conformation of the benzyl rings with respect to their position to the central CN 3 unit.The increased flexibility of the reduced ligand results in completely new possibilities to stabilize coordination environments of metal ions.In the following part, two coordination compounds, especially regarding the coordination spheres around the metal centers, are presented.

Structural Characterization of Ligand Complexes
The reaction of [H 9 (OMe      is formed (Scheme 3).The composition could be verified by mass spectrometry and the molecular structure by single crystal X-ray analysis (Figure 5).
The centrosymmetric compound consists of two ligand molecules which are connected by two Zr IV ions binding one acaccoligand each.The Zr IV ions are seven coordinated in a pentagonal bipyramid coordination geometry (Figure 6).
) 3 L amin ]OTs (2) with SnCl 4 •5H 2 O in methanol results in a coordination compound with the molecular formula [SnCl 3 (H 7 (OMe) 3 L amin )](3) (Scheme 2).The X-ray structure is illustrated in Figure4.The Sn IV ion is coordinated in an [ONN] binding pocket.To complete the octahedral coordination sphere, three chloride ions are bound.The ligand occupies facial positions and forms a nearly planar five membered ring and a folded six International Journal of Organic Chemistry

Figure 3 .
Figure 3. ORTEP-drawings at 50% probability level of molecular structures of the ligand before (1) and after (2) the reduction.Counterions are omitted for clarity.