All chemicals were used without further purification. Borane dimethylamine complex and Zr(acac)₄(acac: acetylacetonate) were purchased from abcr, p-to- luenesulfonic acid monohydrate, AgBF₄ and 2-hydroxy-3-methoxybenzaldehyde were purchased from Sigma-Aldrich, Inc., and SnCl₄∙5H₂O from ACROS. TAGCl was synthesized according to published procedures [1] .

NMR spectroscopic data were recorded with UltraShield TH 400 Plus or Avance II Bruker spectrometer at room temperature. ¹H and ¹³C 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 = 1.54178 Å). Structures were solved by direct methods using SHELXS-2014and refined with SHELXL-2014 [14] .

In 2, one toluene molecule was found to be disordered over an inversion center. Due to the low data to parameter ratio, it was eliminated by the SQUEEZE procedure [15] . In 3, the disorder of one phenyl group could not be modelled properly. Thus, EADP and SIMU restraints are applied to the corresponding hydroxyl and methoxy oxygen atoms and the carbon atoms.

Crystal data can be taken from Table 1.

TAGBF₄ To a solution of TAGCl (1.00 g, 7.11 mmol, 1 eq.) in 10 mL water was added a solution of AgBF₄ (1.39 g, 7.11 mmol, 1 eq.) in 5 mL water under exclusion from light. The mixture was stirred overnight. The precipitate was removed by filtration and the solvent was removed under reduced pressure. A colorless powder could be isolated Yield: 1.32 g, (6.89 mmol, 97%). CH₉N₆BF₄ (191.93): calcd. C 6.26, H 4.73, N 39.59; found C 6.18, H 5.19, N 39.65. ¹H NMR (400 MHz, (CD₃)₂SO): 8.59 (s, 3H, NH), 4.48 (s, 6H, NH₂) ppm. ¹³C NMR (100 MHz, (CD₃)₂SO): δ = 159.0 ppm.

Tris(2-hydroxy-3-methoxybenzylidene) triaminoguanidiniumtetrafluoroborate ([H₆(OMe)₃L]BF₄ (1)) TAGBF₄ (161.5 mg, 0.84 mmol, 1 eq.) and 2-hydroxy-3-methoxybenzaldehyde (383.6 mg, 2.52 mmol, 3 eq.) were dissolved in 5 mL EtOH/H₂O (1:3) at 50˚C. Both solutions were combined and stirred over night at room temperature. The solvent was removed under reduced pressure to obtain a yellowish solid. Yield: 482.5 mg (0.81 mmol, 97%). C₂₅H₂₇N₆O₆BF₄ (594.32): calcd. C 50.52, H 4.58, N 14.14; found C 50.49, H 4.83, N 14.10. ¹H NMR (400 MHz, (CD₃)₂SO): δ = 11.88 (s, 3H, NH), 9.58 (s, 3H, OH), 9.08 (s, 3H, CH = N), 7.76 (dd, ³J_HH = 8.0 Hz, ⁴J_HH = 1.5 Hz, 3H, CH_arom), 7.08 (dd, ³J_HH = 8.2 Hz, ⁴J_HH = 1.5 Hz, 3H, CH_arom), 6.89 (t, ³J_HH = 7.9 Hz, 3H, CH_arom), 3.86 (s, 9H, OCH₃) ppm. ¹³C NMR (100 MHz, (CD₃)₂SO): δ = 148.7, 148.0, 147.4, 146.8, 119.9, 119.1, 118.4, 113.9, 55.9 ppm. MS-ESI: m/z 507.20 [M⁺]. Crystals suitable for single crystal analysis were obtained by slow evaporation of an acetonitrile/toluene mixture (1:1) at room temperature.

Tris(2-hydroxy-3-methoxybenzyl) triaminoguanidiniumtosylate ([H₉(OMe)₃L_amin]OTs (2)) To a stirred solution of tris(2-hydroxy-3-methox- ybenzylidene) triaminoguanidinium tetrafluoroborate (1) (1.36 g, 2.29 mmol, 1 eq.) and dimethylamino borane complex (862.2 mg, 14.6 mmol, 5 eq.) in 30 mL acetonitrile was added a solution of p-toluenesulfonic acid monohydrate (13.42 g, 78.58 mmol, 24 eq.) in 9 mL of methanol/acetonitrile (1:2) dropwise. The solution was stirred for 12 hours, diluted with 30 mL dichloromethane and 15 mL of a saturated solution of Na₂CO₃ was added. The phases were separated and the aqueous phase was extracted two times with dichloromethane. The organic phases were combined and the solvent was removed under reduced pressure to obtain a brown solid. Yield: 1.81 g (91%). C₃₂H₄₀N₆O₉S (684.76): calcd. (2・3H₂O) C 52.02 H 6.28 N 11.38; found C 52.61 H 5.90 N 11.46. ¹H NMR (400 MHz, (CD₃)₂SO): 8.73 (s, 3H, NH), 8.57 (s, 3H, OH), 7.47 (m, 2H, CH_arom(OTs)), 7.11 (m, 2H, CH_arom(OTs)), 6.89 (dd, 3H, ³J_HH = 8,0 Hz, ⁴J_HH = 1.6 Hz, CH_arom), 6.80 (dd, 3H, ³J_HH = 7.8 Hz, ⁴J_HH = 1.6 Hz, CH_arom), 6.73 (t, 3H, ³J_HH = 7.8 Hz, CH_arom), 5.18 (t, 3H, ³J_HH = 5.9 Hz, NH), 3.78 (s, 9H, OCH₃), 3.76 (d, 6H, ³J_HH = 5.7 Hz, CH₂), 2.29 (s, 3H, CH₃(OTs)) ppm. ¹³C NMR (100 MHz, (CD₃)₂SO) δ = 157.2, 147.5, 145.9, 144.6, 137.7, 128.1, 125.6, 123.6, 122.2, 118.7, 111.2, 55.9, 49.6, 20.9 ppm. MS-ESI: m/z 513.24 [M⁺]. Crystals suitable for single crystal analysis were obtained by slow evaporation of a chloroform/toluene mixture (1:1) at room temperature.

[SnCl₃(H₇(OMe)₃L_amin)] (3) [H₉(OMe)₃L_amin]OTs (2) (11.7 mg, 0.02 mmol, 1 eq.) and SnCl₄・5H₂O (7.0 mg, 0.02 mmol, 1 eq.) were dissolved in 1 mL methanol. Slow evaporation of the solvent at room temperature led to colorless crystals of compound 3. Yield: 8.4 mg (57%) C₂₅H₃₁Cl₃N₆O₆Sn (736.60).

[Zr(acac)(H₅(OMe)₃L_amin)]₂ (4) [H₉(OMe)₃L_amin]OTs (2) (10.2 mg, 0.02 mmol, 1 eq.) and Zr(acac)₄ (9.8 mg, 0.02 mmol, 1 eq.) were dissolved in 1 mL acetonitrile and triethylamine (109.5 µL, 0.79 mmol, 90 eq.) was added. Slow evaporation of the solvent at room temperature led to colorless crystals of compound 4. Yield: 12.7 mg (43%) C₆₀H₇₂N₁₂O₁₆Zr₂・2CH₃CN (1481.84). MS-ESI: m/z 1405.35 [M-Li]⁺.

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). ¹H and ¹³C NMR spectra clearly reveal signals of the successfully reduced species and the corresponding TsO⁻ counterion. In solution, both ligands exhibit a C₃-symmetric conformation on the NMR time scale. Due to their high exchange rates in solution, acidic protons (OH and NH) usually do not show a coupling to protons of neighboring carbon atoms. In the ¹H nmr spectrum of compound 2 it is striking that the signal of the outer NH protons splits to a triplet indicating a low exchange rate meaning a low acidity in comparison to the neighbored amine hydrogen atoms which show singlet signals. Both compounds have been subjected to single crystal X-ray analysis. The molecular structure of 1 is shown in Figure 1.

Scheme 1. Formation of [H₆(OMe)₃L_imin]BF₄ (1) inEtOH/H₂O (1:3) at 50˚C and reduction with dimethylamino borane complex (DMAB) in presence of p-toluenesulfonic acidin acetonitrile at room temperature to [H₉(OMe)₃L_amin]OTs (2).

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 CN₃ 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.

The reaction of [H₉(OMe)₃L_amin]OTs (2) with SnCl₄・5H₂O in methanol results in a coordination compound with the molecular formula [SnCl₃(H₇(OMe)₃L_amin)] (3) (Scheme 2).

The X-ray structure is illustrated in Figure 4.

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

Scheme 2. Formation of [SnCl₃(H₇(OMe)₃L_amin)] (3).

membered ring with Sn^IV. Therefore, the coordination sphere is slightly distorted (N(11)-Sn(1)-N(20) 73.6(2)˚, Cl(1)-Sn(1)-Cl(2) 100.14(7)˚, O(12)-Sn(1)-Cl(3) 171.8(1)˚). Nitrogen N(11) stays protonated verifying the low acidity indicated by ¹H NMR spectroscopy. Thus, the Sn(1)-N(11) bond (2.294(5) Å) is significantly longer than the Sn(1)-N(20) bond (2.126(5) Å). Additionally, a hydrogen bond between N(11) and a methoxy oxygen of another molecule can be observed (N(11)H∙∙∙O 2.15 Å, N(11)∙∙∙O 3.064(6) Å, N-H∙∙∙O 152˚). The aromatic rings are arranged in an endo,exo,exo conformation. It is conceivable to bind further metal ions in remaining free coordination sites. The ligand’s general ability of binding adjacent metal ions is confirmed by compound 4.

By reacting [H₉(OMe)₃L_amin]OTs (2) with Zr(acac)₄ in acetonitrile in the presence of NEt₃ compound 4 with the molecular formula [Zr(acac)(H₅(OMe)₃L_amin)]₂ 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^IVions binding one acaccoligand each. The Zr^IV ions are seven coordinated in a pentagonal bipyramid coordination geometry (Figure 6). The ligand coordinates one Zr^IV ion in an [ONNO] binding pocket formed by O(22), N(21), N(10), and O(12).The outer nitrogen N(21) is not deprotonated despite the presence of a base in the reaction mixture. The presence and the position of the hydrogen atom is confirmed by an intramolecular hydrogen bond between N(21) and a methoxy oxygen of a symmetry generated molecule (N(21)H∙∙∙O 2.08(3)˚, N(21)∙∙∙O 2.936(4)˚, N-H∙∙∙O 157(4)˚). The N(10)-Zr(1)

Scheme 3. Formation of [Zr(acac)(H₅(OMe)₃L_amin)]₂ (4).

bond (2.277(4) Å) is much shorter than the N(21)-Zr(1) bond (2.466(4) Å). The phenolate oxygen O32 coordinates the second Zr^IV ion (Zr(1)’). The ligand forms three chelate rings. N(10) to O(12) is a facial, folded seven membered ring. N(10) to N(21) is a nearly planar, five membered ring and N(21) to O(22) is a meridional, folded six membered ring. The acaccoligand coordinates equatorial. The O(12)-Zr(1)-O(32)’ angle is about 168.6(1)˚ and thus distorted away from linearity. The ligand is exhibits an exo,exo,exo conformation.