2,2‐Bipyridine Complexes of Antimony: Sequential Fluoride Ion Abstraction from SbF₃ by Exploiting the Fluoride Ion Affinity of Me₃Si⁺

Abstract

The coordination chemistry of the p-block elements as Lewis acceptors is superficially developed relative to that of the transition metals, despite the ability of the heavy p-block elements to access coordination numbers of six or greater. Consequently, many compounds of the heavy elements adopt coordination polymeric arrays in the solid state through intermolecular donor-acceptor interactions. Molecular complexes are known for many p-block element acceptors which are analogous to transition-metal complexes, with a neutral ligand (L) providing an additional bond to a neutral acceptor center (E; p-block element), even with retention of a lone pair at the acceptor site [Eq. (1)]. Alternatively, a neutral ligand can displace an anionic substituent (X; leaving group) to introduce a cationic charge on the complex, depending on the relative bond strengths of EÀX and EÀL, and the lattice energy of the resulting salt [Eq. (2)]. Such coordinative interactions are more generally applicable when the E À X bond is activated by an abstracting agent (A) [Eq. (3)]. The activation of a second E À X bond [Eq. (4)] increases the Lewis acidity of the acceptor, thus enabling a diversification of coordination chemistry, but examples are rare, [1,2] as are examples of heterolytic cleavage of a third EÀX bond [Eq. (5)]. [3][4][5] In an attempt to systematically and rationally augment the Lewis acidity of p-block elements and develop the coordination chemistry of p-block elements, we have examined a variety of reagent combinations of an element halide with a halide abstracting agent and a neutral ligand, and have discovered that mixtures of SbF 3 , Me 3 SiOSO 2 CF 3 (TMSOTf), and 2,2-bipyridine (bipy) in acetonitrile give compounds 1, 2, and 3 in high yield upon isolation depending on the applied stoichiometry (n and m; see Scheme 1). The substantial thermodynamic preference for the formation of Me 3 SiF (b.p. 16.5 8C) is evidenced by gaseous evolution and by the characteristic doublet in the 1 H NMR spectrum of the reaction mixture.Compounds 1, 2, and 3 were isolated and characterized using NMR spectroscopy and elemental analysis. All are extremely moisture sensitive and exposure of the solid results in protonation of the ligands within minutes. The X-ray structures of 1 and 3 are shown in Figures 1 a and b, respectively, and selected structural parameters are listed in Table 1. It has not yet been possible to determine the solidstate structure of compound 2. The solid-state structure of 1 is best described as a triflate salt of a difluorostibenium cation, [SbF 2 ] + , which is asymmetrically chelated at antimony by a bipy ligand. The antimony center also engages in weak interactions with one oxygen atom of the anion and a fluorine Scheme 1. Synthesis of compounds 1 (n = m = 1), 2 (n = m = 2), and 3 (n = 2, m = 3) by the stoichiometric abstraction of fluoride ions from SbF 3 by [Me 3 Si] + .

Coordination Complexes of Ph3Sb2+ and Ph3Bi2+: Beyond Pnictonium Cations

“…Cationic frameworks exhibit an enhanced Lewis acidity compared with neutral analogues and recent reports of cationic complexes of p-block acceptors represent a new direction for coordination chemistry. [2,3] For example, complexes of mono-, [4] di-, [5] and tricationic [6] + , through sequential replacement of an anionic substituent (X) by neutral ligand(s) (L). Indeed, a number of cationic complexes containing tetrahedral P V acceptor centers have been structurally characterized, [7][8][9][10][11] although in all but a single case [7] ligand coordination is facilitated by a low coordination number at a phosphorus center involved in multiple bonding.…”

“…In each case, the second triflate anion is . [6,20,[25][26][27] and N À Bi + bonds [2.50-2.84 ], [20,[27][28][29][30] respectively. We interpret the cations in 3 a/b as ligand-stabilized pseudo-dications, with a short contact to a triflate anion.…”

Coordination Complexes of Ph3Sb2+ and Ph3Bi2+: Beyond Pnictonium Cations

“…Cationic frameworks exhibit an enhanced Lewis acidity compared with neutral analogues and recent reports of cationic complexes of p-block acceptors represent a new direction for coordination chemistry. [2,3] For example, complexes of mono-, [4] di-, [5] and tricationic [6] Sb III centers have been reported with the generic formula [X 3Àn PnL n ] n+ , where X represents a notional anionic substituent and L is a neutral 2e À donor. With this precedent, we envisage the formation of a more extensive series of coordination complexes involving Pn V acceptors with the generic formulae [X 4Àn PnL] n+ , [X 4Àn PnL 2 ] n+ , [X 4Àn PnL 3 ] n+ , etc., representing derivatives of pnictonium cations, [PnX 4 ] + , through sequential replacement of an anionic substituent (X) by neutral ligand(s) (L).…”

“…23-2.81 ]. [6,20,[25][26][27] and N À Bi + bonds [2.50-2.84 ], [20,[27][28][29][30] respectively. We interpret the cations in 3 a/b as ligand-stabilized pseudo-dications, with a short contact to a triflate anion.…”

Anorganische Chemie 2013

“…Unter Ausnutzung der hohen Fluorid‐Ionenaffinität des Silylkations lassen sich ausgehend von Trimethylsilyltriflat und Antimontrifluorid die bipyridinstabilisierten Antimon Mono‐, Di‐ und Trikationen [BipynSbF(3‐m)](OTf)m darstellen 99. Wird Trimethylphosphan anstelle von Bipyridin eingesetzt oder werden die Bipyridinkomplexe mit Me3P zur Reaktion gebracht, so bildet sich ein Tetrastibinotetraphosphonium‐Tetrakation [(Me3P)4Sb4]4+ (50) .…”