Synthesis, characterization and structural systematics in diorganotin complexes with O,N,O'-tris-chelating semirigid diaza-scaffolds: Mono- vs. di-nuclear compounds

Highlights

• Nine novel diorganotin(IV) complexes with O,N,O'-tris-chelating semirigid diaza-scaffolds have been synthesized.

• Structural characterizations of the complexes were accomplished from IR, ¹H, ¹³C, ¹⁵N and ¹¹⁹Sn NMR spectroscopy.

• All the complexes have been characterized by single crystal X-ray crystallography.

• X-ray analysis reveals dinuclear and mononuclear structures.

• Steric and electronic trends decide about the nuclearity in nine diorganotin complexes.

Abstract

A series of nine novel diorganotin(IV) complexes of O,N,O' chelating ligands were synthesized viz., [n-Bu₂Sn(L²)]₂·0.25 (C₆H₁₄) (1), [Me₂Sn(L¹)]₂·(C₇H₈) (2), [n-Bu₂Sn(L¹)]₂·(C₇H₈) (3), [Me₂Sn(L³)]₂·(C₇H₈) (4), 2 [n-Bu₂Sn(L³)]₂·4 (n-Bu₂Sn(L³)) (5), [Ph₂Sn(L³)] (6), [Ph₂Sn(L¹)] (7), [Ph₂Sn(L⁴)] (8) and [Ph₂Sn(L²)] (9) and structurally characterized. The ligand scaffolds differ with respect to the chemical link between the coordinating N and O atoms, which is either an alkyl or an aryl moiety. Diffraction results indicate that the smallest methyl groups favor dimerization via Sn–O–Sn bridging and six-coordination at the cation. Among these dinuclear derivatives, more asymmetric oxygen bridges and longer Sn⋯Sn separations are found for the less nucleophilic phenolate O. In contrast, the bulky phenyl substituents prevent aggregation for both classes of ligands and always lead to five-coordinated mononuclear species. The n-Bu groups are sterically more demanding than Me but flexible, resulting in an intermediate behavior. When the O,N,O' ligand with phenolate O coordinates a Sn(n-Bu)₂ fragment, a borderline situation occurs and both mono- and dinuclear complexes coexist in the same crystalline solid. The overall structural variety is reflected in a range of different Addison-τ₅ [Addison et al., J. Chem. Soc., Dalton Trans. 1984, 1349-1356] descriptors for five-fold coordination. For better comparability we introduce a slightly modified geometry index τ'₅, in which the basal angle α is subtended by the organic substituents, regardless of its absolute value. τ'₅ represents a sensitive indicator for the coordination geometry about Sn^IV. Tin NMR results revealed that all compounds exist as mononuclear pentacoordinated species in solution.

Introduction

Organotin(IV) compounds have attracted interest due to their biological and catalytic activity [1], [2]. Other applications include wood preservation [2], carbon dioxide capture [3], homogenous catalysts in PVC stabilization, polyurethane formation and trans esterification [4], strong cytotoxic agents and diverse medicinal applications [1], [5], [6], [7], [8], [9].

Schiff bases are versatile ligands and have contributed enormously to the progress of coordination chemistry [10], [11], [12]. Therefore the large number of recently published reports regarding organotin complexes with tridentate O,N,O’-ligands is not surprising. Recent developments in the synthesis and applications of organotin(IV) complexes have focused mainly on the use of Schiff bases functionalized with (i) amino acids [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] and (ii) amino alcohol/aminophenol [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], giving potentially tridentate O,N,O’-ligands. This is mainly because these ligands can be tuned easily which allows for convenient variation of their steric and electronic properties. As a result, organotin(IV) compounds of Schiff bases derived from amino acids have found great utility in cytotoxicity and antibacterial activity [14], [15], [16], [17], [18], [19], photophysical properties useful for nonlinear optics (NLO) applications [20], fluorescent staining of silk fibroin [18], and sensing [21], [22] as well as the detection of warfare agents (organophosphate based chemical weapons) [23]. On the other hand, Schiff bases of amino alcohol/aminophenol have also witnessed diverse applications such as cytotoxicity and antibacterial activity [31], [33], [34], [37], [38], [40], [41], [42], [44], [45], photophysical properties useful for NLO applications [29], [32], [36], [46], [47] and catalysts for solvent-free cycloaddition of CO₂ to epoxides at ambient pressure [43]. Hydroxyl groups in such Schiff base pro-ligands may be deprotonated and act as terminal coordinating atoms. The overall tridentate dianionic ligands match the geometric requirements of group 14 elements i. e., Ge, Pb and in particular Sn. The resulting complexes are either mononuclear with five-coordinate distorted trigonal bipyramidal coordination or dinuclear. In the second case two additional O⋯Sn bonds join two mononuclear entities and form a central planar Sn₂O₂ four-membered ring, with both tin atoms in distorted octahedral coordination. This second category comprises most diorganotin(IV) complexes with tridentate O,N,O’-ligands [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44].

Fusion of two such four-membered Sn₂O₂ rings to a trinuclear aggregate has recently been reported [45]; in this structure, tin cations in six- and in five-coordinated geometry coexist. In the O,N,O’-tridentate ligands described above, the Schiff base backbone typically utilized 2-hydroxybenzaldehydes and an amino alcohol or aminophenol. The versatile Sn^IV center allows for variable coordination but also implies an obvious challenge for the rational design of complexes.

We have recently focused on the use of azo substituted 2-hydroxybenzaldehydes as one of the components in the synthesis of Schiff bases to regulate the resultant structures [13], [22], [48]. As part of our studies on the design of organotin(IV) complexes of O,N,O’-tridentate Schiff bases, we have prepared four new pro-ligands (H₂L¹-H₂L⁴) by condensing (E)-2-hydroxy-5-(phenyldiazenyl)benzaldehyde or methyl 2-[(E)-(3-formyl-4-hydroxyphenyl)diazenyl]benzoate with amino alcohol/aminophenol in order to obtain new materials with interesting properties and structures. The incorporation of azo substituted 2-hydroxybenzaldehydes in these pro-ligands was motivated by recent applications such as potential photoprobes [49] and low molecular dopants for creating polymer composites [50], [51]; such applications probably led to the larger scale preparation of 5-arylazosalicylaldehyde using the Zn/SBA-15 catalyst [52].

Diorganotin(IV) compounds derived from Schiff bases with O,N,O’ donor ligands are either mono- or dinuclear. We here address the decisive arguments for the observed nuclearity via systematic structural variation: different alkyl and aryl substituents (R = Me, n-Bu and Ph) were attached to the tin cations, and the imine part of the pro-ligands (E)-2-hydroxy-5-(phenyldiazenyl)benzaldehyde and methyl 2-[(E)-(3-formyl-4-hydroxyphenyl)diazenyl]benzoate was functionalized with either amino alcohol or aminophenol. Table 1 compiles the substitution pattern for the resulting series of nine complexes. The nuclearity of the target compounds in the solid state was analyzed by single-crystal X-ray diffraction (SCXRD), and we have rationalized why a mononuclear structure is preferred over a dinuclear or vice versa. The structures of the complexes in solution were investigated based on results of NMR spectroscopy.