New adducts of silver(I) halides, AgX (X = Cl, Br), with bidentate phosphine ligands: syntheses and molecular structures of [Ag₃(μ₃-Cl)₂(μ-dppm)₃][PF₆], [Ag₃(μ₃-Br)₂(μ-dppm)₃][AgBr₂], {[Et₄N][Ag₂ (μ-Br)₃(μ-dppe)]}_n, [Et₄N]₂[(AgCl₂)₂(μ-dppe)], and [(AgCl)₂(μ-dppp)₂]

1 Introduction

A family of simple complexes of the form MX:EPh₃ (1:n) [M = univalent copper or silver, X = simple halide or pseudohalide (Cl, Br, I, CN, SCN) or oxyanion (ClO₄, NO₃, or simple carboxylate), E = P, As, Sb, n = 1–4] have been well documented [1], [2], [3], [4]. Continuous advances in the structural characterization of adducts of simple salts of the univalent group 11 metals copper and silver, MX, with simple symmetrical bis(diphenyl-pnicogeno)alkane ligands, Ph₂E(CH₂)_xEPh₂ (E = P, As; x = 1–3, “dpex”), have been stemming from intriguing structural features [5], [6], [7], [8]. In contrast to a number of copper(I) complexes with monodentate ER₃ (E = P, As, Sb) and bidentate Ph₂E(CH₂)_xPPh₂ (E = P, As) dpex ligands [8], [9], [10], analogous silver(I) counterparts are relatively rare due to solubility or other less tangible considerations [7], [11]. Recent interesting reports on the bidentate phosphine ligand arrays have been concerned with systems primarily of the form AgX:dpex of (1:1)_n [12] and (2:3)_n stoichiometeries [10]. It has been noted that the solution and solid-state structures of some 1:2 adducts of Ag(I) salts with 1,3-bis(diphenylphosphino)propane (dppp) were influenced by different halide/pseudohalide anions [5]. Structural features of trinuclear halogen-capped complexes with bidentate bis(diphenylphosphino)methane (dppm) and bis(diphenylphosphino)amide [M₃(μ₃-X)₂(μ-dppx)₃]X (M = Cu, Ag; X = Cl, Br, I; x = m or a) were found to be related to the properties of metal–phosphorus coordination and metal–metal interactions [13], [14]. For continuous understanding of the reactive properties of silver halides and bidentate phosphine ligands and the structural features of the silver-phosphorus complexes with different AgX:dpex stoichiometeries, this work provides more extensive arrays of silver species with bidentate phosphine ligands. The results of complementary studies, including bidentate Ph₂P(CH₂)_xPPh₂ (x = 1–3) ligands and different stoichiometries, are described in this paper.

2 Experimental section

3 Results and discussion

A summary of the synthetic reactions is presented in Scheme 1. The reaction of AgCl and dppm in a 4:3 ratio in the presence of K[PF₆] afforded a typical trinuclear cationic complex [Ag₃(μ₃-Cl)₂(μ-dppm)₃][PF₆] (1). A similar reaction between AgBr and dppm in the presence of [Et₄N]Br gave [Ag₃(μ₃-Br)₂(μ-dppm)₃][AgBr₂] (2). Both complexes 1 and 2 have a [Ag₃(μ₃-X)₂] (X=Cl, Br) core, which was already observed in the previously known complexes [Ag₃(μ₃-X)₂(μ-dppm)₃]X (X=Cl, Br, I) [13], [19], [20], [21], [22]. Treatment of AgBr with dppe in THF-MeCN in the presence of [Et₄N]Br gave a novel polymeric complex {[Et₄N][Ag₂(μ-Br)₃(μ-dppe)]}_n (3) with a {Ag₂(μ-Br)₃} core. The reaction of AgCl with dppe or dppp in the presence of [Et₄N]Cl resulted in the isolation of a dinuclear anionic complex [Et₄N]₂[(AgCl₂)₂(μ-dppe)] (4) with one μ-dppe bridge or a dinuclear neutral complex [(AgCl)₂(μ-dppp)₂] (5) with two μ-dppp bridges and a 12-membered ring, respectively. Usually, silver is four-coordinated; the three-coordinated silver atoms in 4 and 5 are relatively rare in silver halide complexes with phosphine ligands [23].

Scheme 1:

Syntheses of complexes 1–5.

The infrared spectra of all complexes displayed the characteristic ν(P–C) stretching vibration of the dpex ligands at around 500 cm⁻¹. The ³¹P NMR spectra of 1 and 2 displayed dppm ligand signals at δ −4.13 and −4.78 ppm, respectively, which are comparable to those in similar complexes [Ag₃(μ₃-X)₂(μ-dppm)₃]X (X=Cl, Br, I) [13], while the dppe ligands in complexes 3 and 4 exhibited signals at 2.15 and 3.03 ppm, respectively. The dppp ligand in 5 showed a ³¹P signal at 2.48 ppm. The molecular ions corresponding to [Ag(dppm)]⁺ and [Ag₃(dppm)₃]⁺ with the characteristic isotopic distribution patterns can be observed at m/z=492 and 1476 in the mass spectra of 1 and 2, respectively. Accordingly, the molecular ions corresponding to [Ag₂(dppe)]⁺ and [Ag(dppp)]⁺ are found at m/z=614 (for 3 and 4) and 520 (for 5) in their mass spectra. The UV/Vis absorption spectra of complexes 2–5 in CH₂Cl₂ at room temperature are shown in Fig. 1. They are obviously dominated by one intense absorption band at around 265 nm, which may be assigned to a typical metal-to-ligand charge transfer transition [19].

Fig. 1:

Electronic absorption (UV/Vis) spectra of complexes 2 (red), 3 (green), 4 (blue), and 5 (black) in dichloromethane (CH₂Cl₂) with 1 cm optical length.

The solid-state structures of complexes 1–5, as shown in Figs. 2–7 , respectively, were determined by X-ray crystallography. Selected bond lengths and angles of the five complexes are listed in Tables 2–6 , respectively. Complex 1 crystallizes in the monoclinic crystal system with space group Cc with Z=4. It consists of an [Ag₃(μ₃-Cl)₂(μ-dppm)₃]⁺ cation and a PF₆⁻ anion. The Ag atoms are located in distorted tetrahedral P₂Cl₂ environments. The average Ag–μ₃-Cl and Ag–P bond distances are 2.7179(10) and 2.4450(10) Å, respectively, comparable to those in the complexes [Ag₃(μ₃-Cl)₂(μ-dppm)₃]Cl [13] and [Ag₃(μ₃-Cl)₂(μ-dppm)₃][SbF₆] [11]. The Ag···Ag distances range from 3.3489(5) to 3.4490(5) Å, comparable to the sum of the van der Waals radii for silver (3.40 Å) [11]. The Ag–Cl–Ag angles fall in the range of 75.39(3)–78.27(2)°. Complex 2 crystallizes in the triclinic crystal system with space group P1̅ with Z=1. It consists of an [Ag₃(μ₃-Br)₂(μ-dppm)₃]⁺ cation, an [AgBr₂]⁻ anion, and a half diethyl ether solvent molecule. Similar to 1, each Ag atom is coordinated to two bromine atoms and two phosphorus atoms in a distorted tetrahedral geometry in 2. The Ag–μ₃-Br distances range from 2.7642(6) to 2.9082(6) Å, obviously longer than the terminal Ag–Br bond in the counteranion [AgBr₂]⁻ (2.4217(10) and 2.4409(10) Å). The Ag–P bond distances are in the range 2.4519(13)–2.4645(13) Å, similar to those in 1. The Ag···Ag distances range from 3.1845(7) to 3.5069(5) Å. The Ag–Br–Ag angles fall in the range of 68.22(2)–76.64(2)°, comparable to those in [Ag₃(μ₃-Br)₂(μ-dppm)₃]Br (68.5(1)–74.2(2)°) [24], but slightly acuter than those in 1 (75.39(3)–78.27(2)°) probably due to the halogen effect.

Fig. 2:

A view of the [Ag₃(μ₃-Cl)₂(μ-dppm)₃]⁺ cation in 1.

Fig. 3:

A view of the [Ag₃(μ₃-Br)₂(μ-dppm)₃]⁺ cation in 2.

Fig. 4:

A view of the [Ag₂(μ-Br)₃(μ-dppe)]⁻ anion in 3.

Fig. 5:

View of a [Ag₂(μ-Br)₃(μ-dppe)]_nⁿ⁻ chain in 3, four units cells long as drawn with Ortep with 40% displacement ellipsoids.

Fig. 6:

A view of the [(AgCl₂)₂(μ-dppe)]²⁻ anion in 4.

Fig. 7:

A perspective view of the structure of [(AgCl)₂(μ-dppp)₂] (5).

As depicted in Figs. 4 and 5, complex 3 has a polymeric anionic structure with a dinuclear {Ag₂(μ-Br)₃} core, which is quite novel to the best of our knowledge except for the similar structure of [Ag₂(μ-I)₃I₂]³⁻ [25]. Complex 3 crystallizes in the triclinic crystal system with space group P1̅ with Z=2. Each Ag atom is coordinated to three bridging bromine atoms and one phosphorus atom from dppe in a distorted tetrahedral geometry. The bridging Ag–μ-Br lengths in 3 range from 2.5859(14) to 3.0105(14) Å, which are obviously longer than the terminal Ag–Br bond lengths in 2 (2.4217(10) and 2.4409(10) Å). The Ag–P bond lengths in 3 are 2.374(2) and 2.399(2) Å, slightly shorter than those in 2 (2.4519(13)–2.4645(13) Å). The Ag···Ag distance of 2.9039(14) Å is shorter than the sum of the van der Waals radii for silver (3.40 Å) [11], indicating a weak Ag···Ag interaction [26]. The Ag–Br–Ag angles in the {Ag₂(μ-Br)₃} core fall in the range of 60.83(3)–63.74(3)°, obviously acuter than those in 2 bearing a {Ag₃(μ₃-Br)₂} core (68.22(2)–76.640(2)°).

The solid-state structures of complexes 4 and 5 are shown in Figs. 6 and 7, respectively. Complex 4 crystallizes in the monoclinic crystal system with space group P2₁/n with Z=2, while complex 5 crystallizes in the triclinic crystal system with space group P1̅ with Z=1. Complex 4 has a dinuclear anionic structure with one μ-dppe bridge, while 5 is a dinuclear neutral complex with two μ-dppp bridges and a 12-membered ring. Usually, silver is four-coordinated, while in 4 and 5 both the silver atoms adopt three coordination modes: one coordinated to two chlorides and one phosphorus atom from dppe; the other one coordinated to one chloride and two phosphorus atoms from two bridging dppp ligands [27], [28]. The average Ag–Cl bond length of 2.494(4) Å in 4 is shorter than the Ag–Cl bond length in 5 (2.638(2) Å). The Ag–P bond length in 4 is 2.388(5) Å, shorter than the average Ag–P bond length in 5 (av. 2.442(2) Å).

In summary, five silver(I) complexes have been synthesized by the reaction of AgX (X=Cl, Br) with the bidentate phosphine ligands Ph₂P(CH₂)_xPPh₂ (x=1–3) by varying the stoichiometry and the reaction conditions. All presented complexes have been structurally characterized along with spectroscopic properties and microanalyses. In the presence of a chloride or a bromide source, the mononuclear and binuclear Ag-dppm species are converted into typical dichloro- or dibromo-bridged trinuclear complexes. The interaction of silver(I) chloride with dppe or dppp in the presence of a chloride source afforded a dinuclear anionic complex [Et₄N]₂[(AgCl₂)₂(μ-dppe)] with one μ-dppe bridge or a dinuclear neutral complex [(AgCl)₂(μ-dppp)₂] with two μ-dppp bridges and a 12-membered ring, respectively, where the terminal Ag–Cl bonds are kept in both complexes. Further investigations are required to gain insights into the nature of the intermediate species of silver(I) complexes with high nuclearity.

CCDC 1491924–1491928 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.