Synthesis and characterization of Sn(II) complexes supported by amino bis-phenoxide ligands

Abstract

The coordination behavior of a series of [Sn(OR)₂] systems modified by substituted amine-bis(phenolate)ligands was investigated. The complexes were synthesised from the reaction of Sn[N(SiMe₃)₂]₂ and the appropriate pro-ligand: N,N-bis(3,5-dimethyl-2-hydroxybenzyl)ethylamine (L¹H₂), N′,N‘-bis(2-hydroxy-3,5-dimethylbenzyl)-N,N-dimethylethylenediamine (L²H₂), N,N‘-bis(3,5-dimethyl-2-hydroxyphenylmethyl)-N,N‘-dimethylethylenediamine (L³H₂), 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane (L⁴H₂), 2,2′,4,4′-tetramethyl-6,6′-piperazine-1,4-diylbis(methylene)-bisphenol (L⁵H₂) and N′,N′,N′′,N′′-tetrakis(2-hydroxy-3,5-dimethylbenzyl)-1,2-ethanediamine, and the products were identified as follows: [{L¹}Sn]₂ (1), [{L²}Sn] (2), [{L³}Sn] (3), [{L⁴}Sn], (4) and [{L⁶}Sn₂]·(Py)₂ (5). Reaction of complex (1) with O₂ results in the formation of the Sn(IV) species [{L¹}₂Sn] (6), the molecular structure of which was also determined.

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Introduction

Research into low-valent tin compounds has received significant attention since the isolation of the first persistent stannylene system of the type SnX₂ in 1973, i.e., [Sn{CH(SiMe₃)₂}₂].¹ Since then, the chemistry of stannylenes has been investigated extensively with respect to their reactivity, molecular structures, bonding, coordination chemistry, and the possible applications of these heavy carbene analogues in catalysis and the construction of new functional materials.^2–7 More recently, molecular complexes of tin(II) have been investigated for their various applications in the activation of small molecules.^8–11

Even though stannylenes, based on alkyl, aryl, and amide/phosphide, silyl and thiolate ligands have been isolated, tin(II) complexes based on aryloxide ligands are uncommon in contrast. Type A are aryloxides without additional intramolecular coordination Sn(OAr′)₂ (e.g., Ar′ = 4-Me-C₆H₄, 2-ⁱPr-C₆H₄, 2,6-ⁱPr₂C₆H₃ 2,6-^tBu₂C₆H₃, 2,6-^tBu₂-4-Me-C₆H₂, 2,4,6-^tBu₃C₆H₂, 2,6-Mes₂-C₆H₃),^12–16 stabilized by electronic (donation of a lone electron pair of oxygen) and steric (large aryl groups) effects (Fig. 1). While these species can range between monomeric, dimeric or oligomeric, this depends significantly on the steric influence of the aryloxide ligands; for example, while [Sn(OAr′)₂]_x (Ar′ = 4-Me-C₆H₄; x = ∞ and 2-ⁱPr-C₆H₄; x = 4) are oligomeric, sterically more encumbered systems, Sn(OAr′)₂ (Ar′ = 2-^tBu-C₆H₄ or 2,6-Me₂-C₆H₃ and 2,6-ⁱPr₂-C₆H₃)¹⁶ complexes, are dimeric or monomeric (Ar′ = 2,6-^tBu₂-4-Me-C₆H₂, 2,6-dipp-C₆H₃, OC₆H₃-^tBu₂-2,6 or OC₆H₂-^tBu-2,6-Me-4).^12–15 The second type (B–M) are complexes stabilized by the intramolecular interaction of a Sn atom by a Lewis basic group or lariat arm (Fig. 1). One of the possibilities for additional stabilization is achieved by the introduction of phenols containing donor groups, e.g., amino-bisphenols (B) which have been isolated in the monomeric state due to the steric and electronic stabilization of the Sn(II) centre.^17–21 Other donor groups include pyridyl groups (C),²² imine groups (D),^23–25 amino groups (E),^26,27 and dianionic salen-and salen-like tetradentate chelator ligands (F) [salen = ethylene-N,N′- bis(salicylideneimine)] are also known.^28–31 Additionally, Sn(II) systems supported by ligands such as 2-phosphinophenolate (G),³² catecholate (H),^33,34 aminophenolate(I), hydroxypyridine, benzoquinone and hydroxybenzo[h]quinoline derivatives (J–L)³⁵ as well as calix(4)arene derivatives (M) have also been reported.^36–39

Fig. 1 Examples of tin(II) complexes bearing aryloxide ligands.

Obviously, the ligand structure is a significant factor not only in the stabilisation of the structure and the coordination number of the Sn atom but also affects its ability to participate in reactions. Here, we report the synthesis and structural characterisation of a family of amino-bisphenol-supported stannylene systems.

Results and discussion

Synthesis of pro-ligands

The amine-bis(phenol) ligands are versatile and important ligands that can be prepared relatively straightforwardly from common benchtop synthetic methods. The coordination chemistry of these ligands is eminently rich, and the compounds have been exploited in numerous applications in applied coordination chemistry including mimicking the activity of biological compounds, catalysis and molecular magnetism.⁴⁰ Substituents at the phenolate rings as well as the position and nature of the side-chain donor are easily tuneable features; the denticity and bulkiness of the amine-bis(phenol) ligand, as well as different electronic and steric properties, augment the coordination abilities of the ligands and affect the final coordination geometry (Scheme 1).

Scheme 1 Typical metal amine-bis(phenolate) coordination frameworks. (I) tri- or tetradentate amine bis(phenolate) system with a pendant arm (N–D) containing a lariat group (D); (II) Tetradentate amine bis(phenolate) system ligand with a diamino (N–N) bridge.

The pro-ligands L¹H₂–L⁶H₄ involved in this investigation differ primarily in the nature of the phenol linker units, which have been selected on the basis of their ability to provide adaptable coordination environments for the Sn(II) centres, such that L¹ and L⁶ provide an [O^∩N^∩O] coordination environment, and L²−L⁵ provides potential [O^∩N^∩O–N] and [O^∩N^∩N^∩O] coordination environments. In all cases, the phenolic pro-ligands L¹H₂, L²H₂, L³H₄, L⁴H₂, L⁵H₂ and L⁶H₂ are readily prepared by modified Mannich reactions (Scheme 2) between 2,4-dimethylphenol formaldehyde and selected primary or secondary amines, according to literature procedures: L¹H₂,⁴¹L²H₂,⁴²L³H₂,⁴³L⁴H₄,⁴⁴L⁵H₂,⁴⁵ and L⁶H₂.⁴⁶ The ligands were purified accordingly and characterised by comparison with literature data.

Scheme 2 Pre-ligands L¹H₂–L⁵H₂ and L⁶H₄ used in this study.

Synthesis of tin(II) complexes. In order to test the coordination behaviour, pro-ligands L¹H₂, L²H₂, L³H₂, L⁴H₄L⁵H₂ and L⁶H₂ were reacted with [Sn{N(SiMe₃)₂}₂]. The reaction of the pro-ligands L¹H₂, L²H₂, L³H₂, and L⁴H₂ in a 1 : 1 ratio with [Sn{HMDS}₂] in toluene resulted in an immediate colour change of the solutions from orange to pale yellow/colourless (Scheme 3).

Scheme 3 Ligands and complexes prepared in this study.

Concentration in vacuo followed by hot filtration of the reaction mixture resulted in the formation of pale-yellow crystalline materials from the filtrate upon cooling to −15 °C. Complexes [Sn{L¹}]₂ (1), [Sn{L²}] (2), [Sn{L³}] (3) and [Sn{L⁴}] (4) were isolated by filtration and washed with cold hexane. Successive attempts to react the pro-ligand H₂L⁵ with one equivalent of [Sn{(HMDS)₂}₂] in various solvents (toluene, diethyl ether, THF and pyridine) all resulted in the formation of intractable solids, immediately in the case of toluene and diethyl ether, and upon removal of the solvent in the case of THF and pyridine.

The reaction of the tetra-phenolic ligand L⁶H₄ with two equivalents of [Sn{HMDS}₂] resulted in a colour change and precipitation of an intractable colourless solid (Scheme 4). Attempts to dissolve the product in either THF or pyridine were unsuccessful. However, the reaction of two equivalents of [Sn{(HMDS)₂}₂] with L⁴H₄ in anhydrous pyridine resulted in the formation of a pale yellow solution. Removal of the pyridine solvent in vacuo followed by hot filtration from a toluene : THF mixture (5 : 1) resulted in the formation of pale yellow crystals of [Py₂·Sn₂{L⁴}] (5) upon standing of the filtrate at 0 °C. Following recrystallisation, crystals of 1–4 were isolated cleanly in moderate to high yields (58–82%) and were characterised by solution state NMR (¹H, ¹³C and ¹¹⁹Sn) spectroscopy, single crystal X-ray diffraction and elemental analysis.

Scheme 4 Ligands and complexes prepared in this study.

The ¹H and ¹³C NMR spectra of complexes 1–4 were recorded in CD₂Cl₂ as an NMR solvent; both ¹H and ¹³C NMR spectra of complexes 1–4 clearly show the absence of resonances associated with the {HMDS} ligands (∼δ = 0.25 ppm) and are consistent with the formation of the homoleptic complexes. More informative are the ¹¹⁹Sn NMR spectra for 1–4, which show single resonances at δ = −431, −503, −454 and −475 ppm, respectively, indicating the isolation of a single reaction product with comparable Sn-coordination environments, as indicated by the ¹¹⁹Sn chemical shifts. For complex 5, NMR spectra were recorded in d₅-pyridine. Here the ¹H ¹³C and ¹¹⁹Sn NMR spectra again show the presence of a series of resonances associated with the tetra-phenolic ligand as indicated by the presence of resonances associated with two separate {^tBu} groups, two triplet resonances associated with two separate phenolic {C–H} groups alongside two well-separated and defined ‘AB’ doublets for the {CH₂} protons and a single resonance at 3.50 ppm for the eight methyl hydrogen atoms of the {NCH₂CH₂N} group in an 18 : 18 : 2 : 2 : 2 ratio. This is consistent with molecular C₂-symmetry at the centre of the {NCH₂CH₂N} group. Additional resonances associated with coordinated pyridine are also observed in an approximate 5 : 1 ratio of pyridine to ligand. The ¹¹⁹Sn NMR spectra showed a single resonance (in C₅D₅N) at δ = −532 ppm.

Molecular structures of complexes 1–4

Complexes 1–4 were characterized crystallographically and are shown in Fig. 2 (1) and Fig. 3 (2–4), respectively. Selected bond lengths and angles are given in Table 1. In the solid state, 1 crystallises in the triclinic space group P1̄ with two independent dimer molecules in the asymmetric unit cell, as well as one molecule of toluene as a solvent of crystallisation. The ligand [L1] possesses three heteroatoms (2 oxygens and 1 nitrogen) and has been shown to be capable of binding to metals in either a terminal-[O^∩N^∩O] fashion⁴⁷ or as a bridging group in which the [O^∩N^∩O] ligand occupies both terminal and bridging positions.⁴⁸

Fig. 2 (A) The molecular structures of one of the two dimer molecules in the unit cell of complex 1, [{L¹}Sn]₂. (B) Packing of the two molecular dimers in the unit cell. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity.

Fig. 3 The molecular structures of complexes 2, [{L²}Sn] (A), and 3, [{L³}Sn] (B), drawn with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.

Table 1 Single crystal X-ray diffraction experimental details for complexes 1–5

Compound	1	2	3	4	5	6
Chemical formula	C87H108N4O8Sn4	C22H30N2O2Sn	C22H30N2O2Sn	C51H65N5O4Sn2	C63H69N7O4Sn2	C54H66N2O4Sn
Formula Mass	1812.53	473.17	473.17	1049.46	1225.63	925.77
Crystal system	Triclinic	Monoclinic	Orthorhombic	Monoclinic	Monoclinic	Monoclinic
Space group	P	P21/n	Pbca	P21/c	P21/n	P21/c
a/Å	14.5961(4)	11.25310(10)	7.83230(10)	24.51991(12)	12.16953(8)	15.4186(2)
b/Å	14.6600(5)	14.09620(10)	15.26360(10)	12.63699(7)	21.43889(15)	21.9250(2)
c/Å	20.1553(5)	14.02500(10)	35.1984(3)	15.47487(9)	21.98942(15)	14.6746(2)
α/°	72.735(2)	90	90	90	90	90
β/°	83.418(2)	106.1290(10)	90	95.1081(5)	91.0062(6)	110.0141(15)
γ/°	88.995(2)	90	90	90	90	90
Unit cell volume/Å^3	4090.7(2)	2137.16(3)	4207.94(7)	4775.97(5)	5736.18(7)	4661.20(11)
Z (formula units per unit cell)	2	4	8	4	4	4
Crystal Size	0.32 × 0.21 × 0.15	0.34 × 0.23 × 0.18	0.23 × 0.13 × 0.02	0.46 × 0.09 × 0.05	0.76 × 0.07 × 0.06	0.62 × 0.47 × 0.28
Temperature/K	150(2)	150(2)	150(2)	150(2)	150(2)	150(2)
Space group	P	P21/n	Pbca	P21/c	P21/n	P21/c
Radiation type	Mo Kα	Cu Kα	Cu Kα	Cu Kα	Cu Kα	Cu Kα
Absorption coefficient, μ/mm^−1	1.264	9.649	9.801	8.703	7.345	4.725
No. of reflections measured	37 894	18 791	61 969	101 818	46 157	32 217
No. of independent reflections	18 348	4256	4207	9571	11 393	8988
R int	0.0323	0.0295	0.0796	0.0452	0.0338	0.0379
Final R1 values (I > 2σ(I))	0.0368	0.0332	0.0843	0.0248	0.0225	0.0342
Final wR(F^2) values (I > 2σ(I))	0.0683	0.0897	0.1973	0.0647	0.0556	0.0944
Final R1 values (all data)	0.0535	0.0335	0.0848	0.0255	0.0241	0.0353
Final wR(F^2) values (all data)	0.0755	0.0900	0.1974	0.0653	0.0566	0.0955
Goodness of fit on F^2	1.026	1.078	1.268	1.055	1.027	1.070
CCDC number	2465593	2465589	2465592	2465594	2465591	2465590

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As can be seen from Fig. 2A the molecular structure of complex 1 (one of the two dimers in the unit cell is shown) is revealed to possess the latter of these structural motifs with oxygen atoms of a phenolate unit acting as a bridge between the two tin atoms, such that a cis-configured centrosymmetric dimeric structure with a central four-membered {Sn₂O₂} ring is formed via intermolecular O → Sn coordination of two stanna-bicyclodecane units, in a reminiscent of [Sn(OCH₂CH₂)₂NMe]₂ and its derivatives,^49–51 to give 4-coordiante tin atoms with a pseudo-tetragonal pyramidal geometry. This is in contrast to the ortho-^tBu derivative,²⁰ which is monomeric in the solid state, with a 3-coordinate Sn atom and subsequently shorter Sn–O and Sn ← N interactions.

At the core of the {Sn₂O₂} ring, internal Sn–O interactions of 2.227(1) Å [Sn(1)–O(2)], 2.234(1) Å [Sn(1)–O(4)], 2.253(1) Å [Sn(2)–O(2)] and 2.200(1) Å [Sn(2)–O(4)] are augmented by terminal Sn–O interactions of 2.111(1) Å [Sn(1)–O(1)] and 2.098(1) Å [Sn(2)–O(3)], as well as Sn → N interactions of 2.498(1) Å [Sn(1)–N(1)] and 2.522(1) Å [Sn(2)–N(2)], respectively, which complete the coordination of each Sn atom. As a result of the N → Sn coordination, the N(1) and N(2) atoms become chiral with (R) configurations.

Fig. 2B shows the relative arrangement of the two independent dimer molecules in the unit cell, down the approximate c-axis. Analysis of intermolecular contacts would suggest that the arrangement of the two dimer molecules is a result of packing effects.

In contrast to complex 1, complexes 2–4 are all unimolecular and crystallise in the space groups P2₁/n (2), Pbca (3) and P2₁/c (4) respectively.

Single crystals of the stannylenes 2 and 3 suitable for X-ray diffraction analysis (Fig. 3) were grown from concentrated toluene solutions.

Ligands [L²] and [L³] possess four heteroatoms (2 oxygens and 2 nitrogens) with similar ligands having been shown to support the formation of unimolecular complexes through binding to metals in a tetradentate terminal-[O^∩N^∩O–N]²⁻ fashion,^52,53 as well as a bridging group in which the [O^∩N^∩O–N]²⁻ ligand spans terminal and bridging positions through phenolate oxygen atoms.^42,54,55

The central tin atom in 2 possesses a [3 + 1] coordination mode due to the formation of two transannular Sn ← N interactions (Fig. 3A): a strong bond with the bridgehead-amine N atom [Sn(1)–N(1) = 2.364(2) Å] and a weaker interaction with the N atom of the {NMe₂} lariat arm [Sn(1)–N(1) = 2.773(2) Å]. Similarly, the distances between the Sn(II) atom and the oxygen atoms are notably different; the Sn–O bonds are asymmetric, with short and long bonds [Sn(1)–O(1) = 2.069(3) Å, Sn(1)–O(2) = 2.103(3) Å]. As a result, the coordination environment of the Sn atom in 2 is best described as a 4-coordinate distorted pseudo-trigonal bipyramid [τ_Sn = 1.14: O(2)–Sn(1)–N(2) = 153.60(8)°, O(1)–Sn(1)–N(1) 84.98(8)°]⁵⁶ with O(2) and N(2) occupying axial positions, and O(1) and N(1) the equatorial positions. A direct interpretation of the geometry about the tin atom would suggest that the tin center possesses a stereoactive lone pair of electrons. However, inspection of the bond angles about the Sn centre [O(1)–Sn(1)–O(2) = 92.84(8)°; O(2)–Sn(1)–N(1) = 85.07(7)°; N(1)–Sn(1)–N(2) = 70.307(7)°; O(1)–Sn(1)–N(1) = 92.84(8)°; O(1)–Sn(1)–N(2) = 94.29(7)°] would suggest a lack of hybridisation, i.e., a 5s lone pair of electrons.

A very similar structural geometry is observed for the stannylene 3 (Fig. 3B) grown from i.e. a 4-coordinate distorted pseudo-trigonal bipyramid geometry about the Sn centre [τ_Sn = 0.94: O(1)–Sn(1)–N(2) = 149.8(3)°, O(2)–Sn(1)–N(2) = 93.7(3)°], with asymmetric Sn–O bonds [Sn(1)–O(1) = 2.069(3) Å, Sn(1)–O(2) = 2.103(3) Å] and Sn ← N interactions [Sn(1)–N(1) = 2.387(9) Å, Sn(1)–N(2) = 2.599(8) Å]. Similarly, angles at the Sn centre in 3 do not suggest hybridization at the metal centre [O(1)–Sn(1)–O(1) = 85.8(3)°; O(1)–Sn(1)N(1) = 92.84(8)°; O(2)–Sn(1)–O(2) = 85.07(7)°; N(1)–Sn(1)–N(2) = 73.1(3)°]. Both 2 and 3 have structural similarities to the bis(phenoxy)-amine tin(II) complexes reported by Zaitsev et al.¹⁹ and Praban et al.²¹

As part of this study, the molecular structure of the {^tBu} analogue of 3 was also isolated and collected. The molecular structure and full characterisation data are included in the SI.

Similarly, [L⁴] acts as an [O^∩N^∩N^∩O]²⁻ coordinating ligand and has been shown to support the formation of unimolecular complexes.^45,57 In the case of complex 4, the compound crystallises with two independent molecules in the monoclinic unit cell, one of which is shown in Fig. 4. As no substantial differences in bond lengths and angles are found between the two crystallographically non-equivalent molecules, only one ORTEP diagram for the structure of compound 4 is displayed in Fig. 4.

Fig. 4 The molecular structure of one of the two complexes in the unit cell of complex 4, [{L⁴}Sn], (50% probability ellipsoids). Hydrogen atoms and solvent of crystallisation (pyridine) are omitted for clarity.

While the Sn–O bonds [Sn(1)–O(1) = 2.065(3) Å; Sn(1)–O(2) = 2.095(3) Å; Sn(2)–O(3) = 2.062(2) Å; Sn(2)–O(4) = 2.086(2) Å] and Sn ← N interactions [Sn(1)–N(2) = 2.402(2) Å; Sn(1)–N(1) = 2.478(2) Å; Sn(2)–N(4) = 2.405(3) Å; Sn(2)–N(3) = 2.523(3) Å] are comparable to those found in complexes 1–3, there is an obvious reduction in the asymmetry of the bond lengths. This is concomitant with a change in the geometry of the stannylene: from a distinctly distorted pseudo-trigonal bipyramidal geometry in 2–3, to a geometry between trigonal bipyramidal and square-based pyramidal in 4 [τ_Sn = 0.72/0.64: O(1)–Sn(1)–N(2) = 140.85(5)°, O(2)–Sn(1)–N(1) = 97.70(6)°, O(4)–Sn(2)–N(3) = 139.14(6)°, O(3)–Sn(2)–N(4) = 101.11(6)°], presumably a result of restricted flexibility in the homopiperazine backbone. The remaining angles in 4 show a greater deviation away from 90° (more acute) compared to comparable angles in 2 and 3 [O(1)–Sn(1)–O(2) = 85.74(6), O(2)–Sn(1)–N(2) = 82.45(5), O(1)–Sn(1)–N(1) = 77.71(5), N(2)–Sn(1)–N(1) = 65.28(5), O(3)–Sn(2)–O(4) = 84.62(6), O(3)–Sn(2)–N(4) = 101.11(6), O(4)–Sn(2)–N(4) = 83.75(6), N(4)–Sn(2)–N(3) = 64.68(6)].

In contrast to ligands [L¹]–[L⁵], ligand [L⁶] possesses six heteroatoms (4-oxygens and 2-nitrogens) and has been shown to be capable of binding to metals in various modes dependent upon the degree of de/protonation in the ligand and the coordination demands of the metal.¹⁸ Single crystal X-ray diffraction studies revealed that the product 5 comprises two independent halves of the tetraphenolate ligand coordinated to two Sn(II) in a tridentate [O^∩N^∩O]²⁻ fashion, wherein the “N(CH₂)₂N” moiety also acts as a bridge to connect each Sn metal center. No substantial differences in bond lengths and angles are found between the two crystallographically non-equivalent half-molecules, as such only one ORTEP diagram for the structure of compound 5 is displayed in Fig. 5. Each Sn(II) is additionally coordinated by a pyridine molecule rendering each Sn(II) centre 4 coordinate, with distorted pseudo-trigonal bipyramidal geometries about the Sn centre [τ_Sn = 0.86: N(1)–Sn(1)–N(3) = 150.55(5)°, O(1)–Sn(1)–O(2) = 99.15(5)°], [N(2)–Sn(2)–N(4) = 147.67(5)°, O(5)–Sn(2)–O(6) = 100.99(5)°], with Sn–O bonds [Sn(1)–O(1) = 2.076(2) Å, Sn(1)–O(2) = 2.057(3) Å, Sn(2)–O(5) = 2.078(2) Å, Sn(2)–O(6) = 2.047(3) Å,] and Sn ← N interactions [Sn(1)–N(1) = 2.551(3) Å, Sn(2)–N(2) = 2.528(3) Å] which are similar to those found in 1–4, with Sn ← pyridine interactions [Sn(1)–N(3) = 2.434(2) Å; Sn(2)–N(4) = 2.471(3) Å] which are comparable to those found in similar Sn(II) systems.¹⁸ Again, angles about the Sn centres in 5 do not suggest hybridization at the metal centre [O(1)–Sn(1)–N(1) = 81.23(5)°; O(1)–Sn(1)–N(3) = 80.97(5)°; O(2)–Sn(1)–N(1) = 81.23(5)°; O(2)–Sn(1)–N(3) = 78.79(5)°; O(5)–Sn(2)–N(2) = 81.32(5)°; O(5)–Sn(1)–N(4) = 78.55(5)°; O(6)–Sn(1)–N(2) = 81.48(5)°; O(6)–Sn(1)–N(4) = 77.89(5)°]. Three additional pyridine solvent molecules were also located in the unit-cell lattice.

Fig. 5 Thermal displacement ellipsoid drawing (50% probability) showing one of the two [{L⁶}Sn₂]·Py₂ half-molecules in the unit cell of 5. Symmetry-generated atoms (A) generated by the symmetry operation (1 − X, 1 − Y, 1 − Z). All hydrogen atoms and solvent of crystallisation (pyridine) are omitted for clarity.

Reactivity

To gain insight into the reactivity of these complexes, a simple redox reaction was attempted. Oxygen gas was bubbled through a toluene solution of complex 1 (Scheme 5). Addition of oxygen gas to the pale-yellow solution resulted in darkening of the solution and precipitation of an insoluble white solid. The solution was filtered, and the filtrate was evaporated to dryness. Recrystallisation from hot toluene yielded crystals of 6, which were isolated cleanly in moderate to high yields (44% based on Sn) and were characterised by solution-state NMR (¹H, ¹³C and ¹¹⁹Sn) spectroscopy, single crystal X-ray diffraction and elemental analysis. The ¹H and ¹³C NMR spectra of 6 (CD₂Cl₂ was used because of solubility issues) reveal a single set of resonances consistent with molecular C₁-symmetry, which are shifted with respect to those of complex 1, indicating asymmetry of the ligand. The ¹¹⁹Sn NMR spectra similarly showed a shift to δ = −670 ppm (cf. −431 ppm for 1). We believe that the changes in the NMR spectra are indicative of a change in the oxidation state of the Sn centre from Sn(II) to Sn(IV). Single crystal X-ray diffraction studies reveal that the crystals, isolated from the reaction mixture, are the octahedral tin(IV) complex trans/mer-[{L¹}₂Sn], 6. This is presumably the result of oxidation of 1 to form a putative “[{L¹}Sn=O]” species, which then undergoes ligand exchange to form 6 with expulsion of SnO₂, as a white solid (Scheme 5).

Scheme 5 Oxidative conversion of 1 into 6.

The molecular structure of 6 is shown in Fig. 6 and clearly shows that the overall geometry around the Sn(IV) atom is best described as a distorted meridional octahedron with the nitrogen atoms N(1) and N(2) of the amine bis(phenolate)-ligand {L¹} positioned trans to each other. The doubly deprotonated ligand, [O^∩N^∩O]²⁻, is bound in a meridional fashion, with the facial mode being sterically unavailable. The distortion from an octahedral geometry is small in the [ONO]²⁻ ligand: N(1)–Sn(1)–O(1) = 90.51(7), N(1)–Sn(1)–O(2) = 90.30(7)°, N(2)–Sn(1)–O(3) = 89.21(6)°, and N(2)–Sn(1)–O(4) = 90.79(6)°. The O(1)–Sn(1)–O(2) and O(3)–Sn(1)–O(4) angles are 176.49(7)° and 177.04(7)°, respectively, while the corresponding N(1)–Sn(1)–N(2) angle at 177.53(6)° deviates from linearity. The planarity of the {SnO₄} fragment is exceedingly good; the maximum deviation from the mean plane is 0.008 Å. The average Sn–O and Sn–N distances of 2.021(3) and 2.238(3) Å fall in the range reported for the structurally characterized mononuclear Sn(IV) complexes.^58–60

Fig. 6 (A) Thermal displacement ellipsoid drawing (50% probability) showing [{L¹}₂Sn] in the unit cell of 6. (B) View of the molecule down the N(1)–Sn(1)–N(2) axis showing the relative orientation of the two tridentate amino-bis(phenolate) ligands, which coordinate to Sn(IV) in a meridional fashion. All hydrogen atoms and solvent of crystallisation (toluene) are omitted for clarity.

Similar structural systems have also been reported for homo- and heteroleptic [O^∩N^∩O] complexes of titanium,^47,61,62 zirconium and hafnium.⁶³ Jones and co-workers also reported independently a series of structurally characterized homoleptic group 4 complexes of sterically less demanding [O^∩N^∩O] ligands.⁶⁴

The reactivity of complexes 2 and 3 toward molecular oxygen was also investigated during our study. In the case of complex 2, reaction with O₂ in toluene resulted in the formation of a clear solution from which crystals of the pro-ligand L²H₂ were isolated. However, this may have been the result of accidental exposure to moisture during prolonged standing. As such, the precise nature of the reaction, if any, is not clear; what is clear is that the ¹¹⁹Sn NNR spectra of the as-reacted reaction mixture revealed the absence of Sn species in solution.

In the case of complex 3, the attempted reaction with molecular O₂ in a toluene solution resulted in a slight clouding of the solution. Nonetheless, extraction and filtration, followed by recrystallisation, yielded crystals of 3, indicating a lack of reactivity.

Experimental

Complexes 1–6 were synthesised under air- and moisture-sensitive conditions. All manipulations were carried out under an atmosphere of nitrogen or argon using standard Schlenk-line or glovebox techniques. Solvents were dried according to standard methods and collected by distillation. [Sn(HMDS)₂] was produced according to a literature procedure.⁶⁵ The pro-ligands L¹–L⁵H₂ and L⁶H₄ were produced according to literature procedures.^41–46

Synthesis of [Sn{L¹}]₂ (1). A toluene solution (5 ml) of H₂L¹ (0.94 g, 3 mmol) was added to Sn(HMDS)₂ (1.318 g, 3 mmol) in toluene (5 ml), at 0 °C. The reaction mixture was left stirring for one hour, and the colour changed from orange to pale yellow. The solution was filtered to remove impurities. The filtrate was concentrated in vacuo, and the product was recrystallised from toluene. Yield = 0.95 g, 74%. ¹H NMR (400 MHz, CD₂Cl₂, 25 °C); δ_ppm 1.38 (t, ³J_HH = 7.27 Hz, 6H, NCH₂Me), 2.18 (s, 12H, ArMe), 2.19 (s, 12H, ArMe), 2.97 (quartet, ³J_HH = 7.27 Hz, 4H, NCH₂Me), 3.61 (s, 4H, ArCH₂N), 6.64 (d, ⁴J_HH = 1.62 Hz, 4H, ArH), 6.93 (d, ⁴J_HH = 1.62 Hz, 4H, ArH); ¹³C{¹H} NMR (100 MHz); δ_ppm 9.8 (NCH₂Me), 17.0 (Ar–Me), 20.5 (Ar–Me), 50.8 (NCH₂Me), 55.0 (Ar–CH₂N), 122.7 (Ar–C), 127.2 (ArC), 129.2 (Ar–CH), 129.4 (Ar–CH), 132.5 (ArC), 155.6 (Ar–CO); ¹¹⁹Sn NMR (186 MHz); δ_ppm −431; elemental analysis for C₄₀H₅₀N₂O₄Sn₂: calc. C, 55.85; H, 5.86; N, 3.26; Found C, 55.82; H, 5.88; N, 3.22.

Synthesis of [Sn{L²}] (2). A toluene solution (5 ml) of H₂L² (0.36 g, 1 mmol) was added to Sn(HMDS)₂ (0.44 g, 1 mmol) in toluene (5 ml), at room temperature. The reaction mixture was left stirring for one hour, and the colour changed from orange to pale yellow. The solution was filtered to remove impurities. The filtrate was concentrated in vacuo, and the product was recrystallised from toluene (1 ml). Yield = 0.31 g, 66%. ¹H NMR (500 MHz, CD₂Cl₂, 298 K) δ_H: 1.90 (s, 6H, N(CH₃)₂), 2.14 (s, 6H, Ar–CH₃), 2.16 (s, 6H, Ar–CH₃), 2.46 (m, 2H, NCH₂CH₂N(CH₃)₂), 2.95 (m, 2H, NCH₂CH₂N(CH₃)₂), 3.64 (s, 4H, ArCH₂N), 6.57 (s, 2H, Ar–H), 6.88 (s, 2H, Ar–H); ¹³C{¹H} NMR (125 MHz, CD₂Cl₂, 298 K) δ_C: 16.9 (Ar–CH₃), 20.5 (Ar–CH₃), 44.1 (N(CH₃)₂), 55.1 (NCH₂CH₂N(CH₃)₂), 57.6 (NCH₂CH₂N(CH₃)₂), 59.4 (Ar–CH₂N), 122.6 (Ar–C), 125.6 (Ar–C), 128.5 (Ar–C), 128.9 (Ar–CH), 132.1 (ArCH), 157.2 (Ar–CO) ¹¹⁹Sn{¹H} NMR (186 MHz, CD₂Cl₂, 298 K) δ_Sn: −503; ¹¹⁹Sn NMR (186, C₆D₆, MHz); δ_ppm −495; elemental analysis for C₂₂H₃₀N₂O₂Sn·C₆H₈: calc. C, 60.78; H, 6.92; N, 5.06; Found C, 60.05; H, 5.95; N, 5.03.

Synthesis of [Sn{L³}] (3). A toluene solution (5 ml) of H₂L³ (0.36 g, 1 mmol) was added to Sn(HMDS)₂ (0.44 g, 1 mmol) in toluene (5 ml) at room temperature. The reaction mixture was left stirring for one hour, and the colour changed from orange to pale yellow. The solution was filtered to remove impurities. The filtrate was concentrated in vacuo, and the product was recrystallised from fresh toluene (5 mL) and THF (1 mL) at −25 °C. Yield = 0.19 g, 40%. ¹H NMR (500 MHz, CD₂Cl₂, 298 K) δ_H: 2.17 (s, 6H, NCH₃), 2.21 (s, 6H, ArCH₃), 2.25 (s, 6H, Ar–CH₃), 2.72 (q, J = 6.4 Hz, 2H, NCH₂CH₂N), 3.00–3.09 (m, 4H, ArCH₂N and NCH₂CH₂N), 4.16 (s, 1H, Ar–CH₂N), 4.19 (s, 1H, Ar–CH₂N), 6.66 (s, 2H, Ar–H), 6.95 (s, 2H, Ar–H); ¹³C{¹H} NMR (125 MHz, CD₂Cl₂, 298 K) δ_C: 17.1 (Ar–CH₃), 20.5 (Ar–CH₃), 42.0 (NCH₃), 55.8 (NCH₂CH₂N), 60.2 (Ar–CH₂N), 123.5 (Ar–C), 125.5(Ar–C), 128.8 (Ar–H), 129.3 (Ar–C), 131.7 (Ar–CH), 157.4 (Ar–CO); ¹¹⁹Sn{¹H} NMR (186 MHz, CD₂Cl₂, 298 K) δ_Sn: −454; elemental analysis for C₂₂H₃₀N₂O₂Sn: calc. C, 55.84; H, 6.39; N, 5.92; Found C, 55.86; H, 6.23; N, 5.67.

Synthesis of [Sn{L⁴}] (4). H₂L⁴ (0.37 g, 1 mmol) in toluene (5 ml) was added to Sn(HMDS)₂ (0.44 g, 1 mmol) in toluene at 0 °C. The reaction mixture was left stirring for one hour, initially turning colourless, and later turning pale yellow. The solvent was removed in vacuo and replaced with fresh toluene (5 ml). The reaction mixture was then filtered hot (110 °C) through Celite to remove impurities. The reaction was concentrated in vacuo, and the product was recrystallised at −25 °C. Yield = 0.28 g, 58%. ¹H NMR (500 MHz, CD₂Cl₂, 298 K) δ_H: 2.10 (s, 6H, ArCH₃), 2.14 (s, 6H, ArCH₃), 2.20–2.35 (m, 2H, NCH₂CH₂CH₂N), 2.40–2.47 (m, 2H, NCH₂CH₂N), 2.55–2.62 (m, 2H, NCH₂CH₂CH₂N), 3.04 (s, 1H, Ar–CH₂N), 3.06 (s, 1H, Ar–CH₂N), 3.07–3.11 (m, 2H, NCH₂CH₂N), 3.27–3.36 (m, 2H, NCH₂CH₂CH₂N), 4.12 (s, 1H, ArCH₂N), 4.14 (s, 1H, ArCH₂N), 6.51 (s, 2H, Ar–H), 6.84 (s, 2H, Ar–H); ¹³C{¹H} NMR (125 MHz, CD₂Cl₂, 298 K) δ_C: 17.3 (Ar–CH₃), 20.4 (ArCH₃), 24.4 (NCH₂CH₂CH₂N), 46.8 (NCH₂CH₂N), 56.1 (NCH₂CH₂CH₂N), 60.3 (ArCH₂N), 120.6 (Ar–C), 124.7 (Ar–C), 128.4 (Ar–CH), 129.6 (Ar–C), 131.3 (Ar–CH), 158.6 (Ar–CO); ¹¹⁹Sn{¹H} NMR (186 MHz, CD₂Cl₂, 298 K) δ_Sn: −475; elemental analysis calculated for C₂₃H₃₀N₂O₂Sn: C, 56.93; H, 6.23; N, 5.77; Found C, 56.23; H, 6.23; N, 5.22.

Synthesis of [{L⁶}Sn₂]·Py₂ (5). H₄L⁴ (0.597 g, 1 mmol) was added to Sn(HMDS)₂ (0.88 g, 2 mmol) in pyridine, at 0 °C. The reaction mixture was left stirring for one hour. It was concentrated in vacuo, and the product was recrystallised from pyridine. Yield = 82%. ¹H NMR (500 MHz, Pyr-d⁵, 298 K) δ_H: 2.12 (s, 12H, Ar4CH₃), 2.22 (s, 12H, Ar–CH₃), 3.50 (s, 4H, NCH₂CH₂N), 3.79 (d w/shouldering, 4H, Ar–CH₂N), 3.97 (m, 4H, Ar–CH₂N), 6.79 (d, J = Hz, 4H, Ar–H), 6.99 (d, J = Hz, 4H, Ar–H), 7.19 (m 8H, m-C₅H₅N), 7.55 (m 4H, p-C₅H₅N), 8.70 (m 8H, o-C₅H₅N); ¹³C{¹H} NMR (126 MHz, 25 °C); δ_ppm 18.2 (Ar–Me), 20.0 (Ar–Me), 48.7 (NCH₂CH₂N), 55.9 (Ar–CH₂N), 124.1 (m-C₅H₅N), 124.5 (Ar–C) 125.9, (Ar–CH), 129.2 (Ar–C), 130.1 (Ar–C), 132.6 (Ar–CH), 136.2 (Ar–C), 136.1 (p-C₅H₅N), 150.4 (o-C₅H₅N), 150.0, 158.7 (Ar–CO); ¹¹⁹Sn NMR (186 MHz, 25 °C); δ_ppm −532; ¹¹⁹Sn NMR (125 MHz, C₆D₆, 25 °C); δ_ppm −477; elemental analysis for C₄₈H₅₄N₄O₄Sn₂: calc. C, 58.33; H, 5.51; N, 5.67; Found C, 58.31; H, 5.18; N, 5.61.

Synthesis of [{L¹}₂Sn] (6). Complex 1 (1 mmol, 0.4321 g) was dissolved in toluene (20 mL) and cooled to 0 °C before a stream of oxygen was bubbled through the solution. The solution was stirred for 30 minutes. The solvent was removed in vacuo, and 10 ml of fresh toluene was used to extract the residue. The reaction mixture was filtered through Celite and concentrated in vacuo (5 ml). The product was recrystallised from toluene at −5 °C. Yield, 0.32 g (44%). Elemental analysis calculated for C₄₀H₅₀N₂O₄Sn: C, 64.78; H, 6.80; N, 3.78; Found C, 60.26; H, 5.91; N, 3.29.0. ¹H NMR (500 MHz, CD₂Cl₂, 298 K) δH 1.03 (m, 6H, NCH₂CH₃), 1.46 (s, 6H, Ar–Me), 2.18 (s, 6H, Ar–Me), 2.20 (s, 6H, Ar–Me), 2.22 (s, 6H, Ar–Me), 2.98 (m, 2H, NCH₂CH₃), 3.16 (m, 2H, NCH₂CH₃), 3.63 (m, 4H, ArCH₂N), 5.10 (q, J = 13.0 Hz, 2H, ArCH₂N), 5.33 (q, J = 13.0 Hz, 2H, ArCH₂N), 6.70 (s, 2H, ArH), 6.77 (s, 2H, ArH), 6.83 (s, 2H, ArH), 6.90 (s, 2H, ArH); ¹³C{¹H} NMR (125 MHz, CD₂Cl₂, 298 K) δC: 4.70 (NCH₂CH₃), 16.4 (Ar–Me), 17.4 (Ar–Me), 20.4 (Ar–Me), 20.5 (Ar–Me), 44.2 (NCH₂CH₃), 56.4 (ArCH₂N), 57.2 (ArCH₂N), 119.1 (Ar–C), 120.6 (Ar–C), 126.3 (Ar–C), 126.9 (Ar–C), 127.7 (Ar–C), 128.4 (Ar–CH), 128.5 (Ar–CH), 129.0 (Ar–C), 131.9 (Ar–CH), 132.2 (Ar–CH), 157.6 (Ar–CO), 157.6 (Ar–CO); ¹¹⁹Sn NMR (186 MHz, CD₂Cl₂, 298 K) δSn: −670.

Crystallographic details

Single crystal X-ray diffraction data were collected on a SuperNova, EosS2 diffractometer using CuKα (λ = 1.54184 Å) radiation, except for 1 which was collected using MoKα (λ = 0.71073 Å) radiation. In each case, the crystals were maintained at 150 K during data collection. Using Olex2, the structures were solved with the olex2. solve structure solution program or ShelXT and refine with the ShelXL refinement package using least−squares minimisation.

Table 1 contains crystal and structural refinement data for 1, 2, 3, 5 and 6. The SI contains data for the {^tBu} analogue of 3 (^tBu₄Sn).

Conclusions

A series of Sn(II) complexes (1–5) bearing amine bis(phenolate) were obtained cleanly in quantitative yields, having different steric crowding around the metal, and their structures were confirmed by X-ray crystallography. The [O^∩N^∩O]-type ligands {L¹} and {L⁶} bind to the metal in a tridentate fashion. In the case of 1, the {L¹} bridges to Sn(II) centres. On the other hand, in the [O^∩N^∩O–N]-ligands, {L²}, and [O^∩N^∩N^∩O]-ligands, {L³}–{L⁴} bind to the metal centres in a tetradentate fashion. In the case of the pre-ligand L⁵H₂, reaction with Sn(HMDS)₂ did not yield tractable products.

Single crystal X-ray diffraction studies of the direct reaction of 1 with O₂ reveal the formation of a Sn(IV) complex, presumably by a ligand exchange process, with concomitant formation of SnO₂.

Taken together, these efforts demonstrate the complex coordination behaviour and utility of the ligands in terms of producing controlled structured M(OR)₂ systems. Further development of these types of ligands will facilitate controlled construction of even more complex M(OR)_x systems for materials applications.

Author contributions

Conceptualization, A. L. J.; methodology, A. R. and H. dL.; crystallography, A. L. J. and G. K. K.; writing—draft preparation, A. L. J.; writing—review/editing, A. L. J.; supervision, A. L. J.; project administration, A. L. J.; and funding acquisition, A. L. J. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available in the SI of this article. General experimental details, the synthesis of [^tBu₄Sn] and crystallographic data for [^tBu₄Sn]; experimental details of the data collection/X-ray crystallographic studies of 1–6 and [^tBu₄Sn]; and characterisation data for the {^tBu} analogue of 3, ^tBu₄Sn. See DOI: https://doi.org/10.1039/d5dt01442e.

CCDC 2465589–2465595 contain the supplementary crystallographic data for this paper.^66a–g

Where possible, additional data that support the findings of this study are available from the corresponding authors upon reasonable request.

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