Thiosaccharinate binding to palladium(II) and platinum(II): Synthesis and molecular structures of sulfur-bound complexes [M(κ1-tsac)2(κ2-diphosphane)]
Graphical abstract
Palladium(II) and platinum(II) thiosaccharinate complexes M(κ1-tsac)2{κ2-Ph2P(CH2)nPPh2] (M = Pd, Pt; n = 1–4) have been prepared. In all the thiosaccharinate ligands are S-bound as confirmed in the X-ray crystal structures of Pd(κ1-tsac)2(κ2-dppp) and Pt(κ1-tsac)2(κ2-dppm). With excess dppm the bis(diphosphine) complexes, M(κ1-tsac)2(κ1-dppm)2 (M = Pd, Pt) can be prepared. They are supposed to contain both bound and non-bound phosphorus centers but these interconvert rapidly in solution.
Highlights
► New palladium and platinum thiosaccharinate complexes have been prepared. ► In all the thiosaccharinate ligands bind in a monodentate fashion through sulfur. ► They are fluxional in solution via inversion at sulfur. ► With dppm two bis(diphosphine) complexes have been isolated. ► They are highly fluxional via a process which interconverts bound and free phosphorus atoms.
Introduction
Due to the widespread use of saccharin as an artificial sweetener, the coordination chemistry of this cyclic amide has been widely studied over the past 20 years [1]. Replacement of the carbonyl by a thiocarbonyl gives thiosaccharin and while the two molecules look superficially similar they differ in both their ground state structures and their ligating properties to metal centers. Thus, while saccharin (sacH) exists in a single form (amide), thiosaccharin (tsacH) can adopt two tautomeric forms in solution, namely amide or thiol (Chart 1). Further upon deprotonation the negative charge remains on nitrogen in saccharinate but is primarily located on sulfur in thiosaccharinate (Chart 1) [2], [3]. This becomes important for the coordination chemistry of the two amides since the thiosaccharinate (tsac) ligand is expected to bind strongly to soft metal centers via the exocyclic sulfur atom and consequently the coordination chemistry of thiosaccharinate [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] has been shown to be quite different from that of saccharinate.
In our recent work we have focused on the concurrent binding of diphosphane and amide ligands at platinum(II) and palladium(II) centers [15], [16], [17], [18], [19], [20]. We have recently found that addition of sodium saccharinate to [MCl2(κ2-dppf)] [dppf = 1,1′-bis(diphenylphosphino)ferrocene] affords the mono-substituted complexes [MCl(κ1-sac)(κ2-dppf)] even when a large excess of sodium saccharinate is used, and saccharinate is N-bound [18]. By way of comparison we have now investigated the addition of thiosaccharinate (tsac) to platinum(II) and palladium(II) diphosphane centers and find that complexes of the type [M(κ1-tsac)2(κ2-diphosphane)] readily form, whereby the thiosaccharinate ligands are both S-bound. We report the X-ray crystal structures of two examples of these and variable temperature NMR studies aimed at elucidating fluxionality in solution. While this work was in progress Quinzani and co-workers reported aspects of the palladium chemistry described herein [13].
Section snippets
General
NMR spectra were recorded on a Bruker AMX400 spectrometer at University College London and referenced internally to the residual solvent peak (1H) or externally (31P). IR spectra were recorded on a Shimadzu FT8400 spectrometer as either KBr or CsI discs. Conductivity measurements were made on a Philips PW9526 conductivity meter. Metal salts Na2[PdCl4], K2[PtCl4] and diphosphanes were used as supplied. Thiosaccharin [21], cis-[PtCl2(dmso)2] [22], [Pt(dppm)2]Cl2 [23] and [Pd(tsac)2]·H2O [12] were
Synthesis of [Pt(tsac)2]·H2O (1)
With the aim of preparing complexes of the type [M(tsac)2(diphosphane)] and given the commercial availability of a range of diphosphanes, then a simple approach seemed be the addition of the latter to [M(tsac)2] (M = Pt, Pd). Baran and co-workers have previously reported the synthesis of [Pd(tsac)2]·H2O formed upon treatment of Na2[PdCl4] with thiosaccharin in methanol [12], while very recently Quinzani and co-workers reported formation of [Pd(tsac)2] upon addition of thiosaccharin to [Pd(acac)2]
Summary
In this contribution we have shown that [MCl2(κ2-diphosphane)] complexes react with Na(sac) to afford mono-substituted N-bound complexes [MCl(κ1-sac)(κ2-diphosphane)] as the major products and even in the presence of excess Na(sac) the disubstituted complexes are not generated [19]. In contrast, [M(κ1-tsac)2(κ2-diphosphane)] are isolated when either [Pd(tsac)2].H2O is reacted with diphosphanes or when Na(tsac) is added to [PtCl2(κ2-diphosphane)] and in all the thiosaccharinate ligands are
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2021, Journal of Organometallic ChemistryCitation Excerpt :Over the years, the use of saccharin (sacH) and its water-soluble saccharinate anion (sac−1), which easily forms due to the acidic nature of the heterocyclic NH moiety, as a ligand auxiliary in a variety of metal compounds have attracted continuing interest due to their ability to exhibit versatile and flexible coordination properties by providing up to four potential donor atoms for bonding with transition metal centres [5-12]. Recently, the chemistry of thiosaccharin (tsacH), which is closely related to sacH, with various transition metals has also gained considerable attention [13-20]. The organometallic chemistry of both sacH [7-12] and tsacH [13,14] have been relatively neglected, in particular with low-valent metal clusters, and to our knowledge only four reports appear in the literature on the reactions of sacH [8,12] and tsacH [13,14] with low-valent metal clusters.
Activation of thiosaccharin at a polynuclear osmium cluster
2019, Journal of Organometallic ChemistryCitation Excerpt :While the hydride ligand was not located during data refinement, it may be confidently assigned to a locus at the Os(1)Os(2) vector based on the angular arrangement displayed by the C(1)O(1) and C(4)O(4) groups that reside in the plane of the three osmium atoms. Here the widening of the OC‒Os‒Os angles [C(1)–Os(1)–Os(2) 119.6(4) and C(4)–Os(2)–Os(1) 117.7(4)o] provide space for the hydride to bridge the Os(1)Os(2) edge in a manner similar to the hydride ligand in the corresponding saccharinate analog [HOs3(CO)10(μ-N,O-1,2-sac)] [19]. The location of the hydride at the heterocyclic bridged OsOs bond receives additional support from the comprehensive report recently published by us where the energies of hydride isomers at related heterocyclic-bridged triosmium clusters were examined by DFT [38].
Pd(II) and Pt(II) saccharinate complexes of bis(diphenylphosphino)propane/butane: Synthesis, structure, antiproliferative activity and mechanism of action
2018, European Journal of Medicinal Chemistry
Os edge that is bridged by the heterocyclic ligand. Heating 1 at 80 °C affords 2 and demonstrates that the former cluster is the product of kinetic control. The conversion of 1 → 2 has been investigated by DFT and the isomerization pathway elucidated. The DFT calculations confirm cluster 2 as the thermodynamically preferred isomer in this pair of products. Thermolysis of 2 in refluxing toluene affords the hexanuclear cluster [H2Os6(CO)17(μ-C,N-1,2-C6H4CNSO2)2(μ3-S)(μ4-S)] (3) via carbon-sulfur bond scission and subsequent capture of the extruded sulfur by the cluster core. The molecular structures for the three new clusters have been determined by single-crystal X-ray diffraction analyses.