Coordination Chemistry of Bidentate Ligands - Research

Published Date: 16 Jan 2018

• Elham Torabi Farkhani
• Mehrdad Pourayoubi
• Pavel V. Avdreev
• Katarina

Introduction

The coordination chemistry of bidentate ligands has been studied for over thirty years [reference]. The bidentate ligands with phosphoryl and thiophosphoryl groups have been used as effective coordinating agents in the different metal chemistry, in most cases the reports were attributed to bonding between the metal cation and specific Lewis sites on the ligand, itself has number sites with potential to bind metal ions, such as nitrogen, sulfur and oxygen. In order to recent report Hg metal ion is known to have strong affinity for nitrogen and sulfur Lewis sites [reference] which our work here is done bonding between Hg metal atom and sulfur in ligands. A search of the Cambridge Structural Database (CSD) [reference] yielded a data set of 76 purely molecular structures to be used for study of coordination of metal atom with a P(S)[N][O]₂ skeleton of ligand. Thus there are a number of reports on molecular structure contain M-S=P fragments with different metal atoms [reference]. An investigation of the reports reveals that there isn’t any publication with Hg metal, also we haven’t found any precise study on the effect of all interactions, including coordinating linkages and intermolecular interactions on the structure of Hg(II) complexes with bisthiophosphoryl ligands. Accordingly we have carried out a study on mercury (II) chloride with two different bidentate ligand with general formula (OEt)₂P(S)-X-P(S)(OEt)₂ where X=1,4-NH-C₆H₅-NH and piperazine (scheme ). Reaction of two ligand with HgCl₂ generated binuclear complex C₁ and C₂. All compounds were charactrized by IR and NMR (¹H, ¹³C and ³¹P NMR) spectroscopy and mass analysis. The structure of ligand 1 and two complexes C₁ and C₂ were determined by X-ray crystallography.

Experimental

Materials and methods

Mercuric chloride (99.5%), O,O′-diethyl chlorothiophosphate (OCH₂CH₃)₂P(S)Cl (97%), diethylenediamine (97%), 1,4-phenylenediamine (99%) (Aldrich), acetonitrile (99%) and methanol (99%) (Merck) were used as supplied. Acetonitrile was dried with P₂O₅ and distilled prior to use. The ¹H, ¹³C and ³¹P NMR spectra were recorded on a Bruker Advance 400 spectrometer at 400, 101 and 162 MHz, respectively. ¹H chemical shifts were determined relative to Si (CH₃)₄. ³¹P chemical shift was measured relative to 85% H₃PO₄ as external standard. Mass spectra were performed using a Varian Star 3400 CX mass spectrometer. Infrared (IR) spectra were recorded on KBr disk using a Buck 500 scientific spectrometer. Elemental analysis was performed using a Thermo Finnigan Flash EA 1112 apparatus. X-ray data collection was performed with a Xcalibur, Sapphire3, Gemini diffractometer with graphite monochromator.

Synthesis

General procedure for the preparation of ligands

The ligands were synthesized from the reaction of 2 mmol (OCH₂CH₃)₂P(S)Cl with 1 mmol of the corresponding diamine (diethylenediamine and 1,4-phenylenediamine) in presence of Et₃N as HCl scavenger in CH₃CN at 0Ëš C. After stirring for 24 h, the solvent was evaporated and the residue was washed with distilled water and dried. Chemical structures are shown in scheme 1. Physical and spectroscopic data of the ligands are presented below:

1, 4 - [(C₂H₅O)₂P(S)N]₂C₄H₈ (L₁): Mp: 105Ëš C. Anal. calc. (%) for C₁₂H₂₈N₂O₄P₂S₂: C: 36.88; H: 7.17; N: 7.17, S: 16.39, found: C: 37.81; H: 7.16, N: 7.26, S: 15.7. IR data (KBr, cm^-1): 2990, 2903, 2864, 1449, 1387, 1339, 1264, 1151, 1098, 1029, 972, 792, 714.¹H NMR (400 MHz, DMSO-d₆) δ: 1.22 (t,³J_H-H= 7.1 Hz, 12H, CH₃), 3.12 (m, 8H, C₄H₈), 7.00 (m, 8H, CH₂), ¹³C NMR (101 MHz, DMSO-d₆) δ: 15.57 (d, ³J_P-C = 8.08 Hz, 4C, CH₃), 44.84 (s, 4C, C₄H₈), 62.45 (s, 4C, CH₂), ³¹P NMR (162 MHz, DMSO-d₆) δ: 73.64. MS (70 eV, EI): m/z (%) = 390 (28), 235 (43), 195 (100), 153 (99), 120 (96), 96 (100), 28 (66).

1, 4 - [(C₂H₅O)₂P(S)NH]₂C₆H₄ (L₂): Mp: 105Ëš C. Anal. calc. (%) for C₁₄H₂₆N₂O₄P₂S₂: C: 40.73, H: 6.30, N: 6.78, S: 15.51, found: C: 41.15, H: 6.34, N: 7.01, S: 15.57. IR data (KBr, cm^-1): 3268, 2980, 1515, 1479, 1380, 1278, 1218, 1168, 1023, 959, 816, 726, 646. ¹H NMR (400 MHz, CD₃CN) δ: 1.22 (t,3JH-H= 7.1 Hz, 12H, CH3), 3.12 (m, 8H, C4H8), 7.00 (m, 8H, CH2), ¹³C NMR (101 MHz, CD₃CN) δ: 15.57 (d, 3JP-C = 8.08 Hz, 4C, CH3), 44.84 (s, 4C, C4H8), 62.45 (s, 4C, CH2), ³¹P NMR (162 MHz, CD₃CN) δ: 73.64. MS (70 ev, EI): m/z (%) = 412 (94), 411 (100), 168 (26), 107 (89), 96 (91), 92 (39), 65 (87), 28 (88).

General procedure for the preparation of complexes

The complexes were prepared by a solutions of 2 eq. HgCl₂ in 15 ml of methanol was added drop wise to a solution of 1 eq. the corresponding ligand in 15 ml of methanol. The clear solution was stirred under reflux for 24h. Crystals suitable for X-ray diffraction were obtained from slow evaporation of the solution at room temperature. Physical and spectroscopic data of the complexes are given below:

µ-{1, 4-[(C₂H₅O)₂P(S)N]₂C₄H₈}(HgCl₂)₂ (C₁): Mp: 105Ëš C. Anal. calc. (%) for C₁₂H₂₈Cl₄Hg₂N₂O₄P₂S₂: C: 15.41; H: 2.99; N: 2.99, S: 6.84, found: C: 15.67; H: 2.91, N: 2.99, S: 5.74. IR data (KBr, cm^-1): 2976, 2895, 1444, 1383, 1344, 1266, 1121, 1037, 967, 804, 772, 702.¹H NMR (400 MHz, CD₃CN) δ: 1.22 (t,³J_H-H= 7.1 Hz, 12H, CH₃), 3.12 (m, 8H, C₄H₈), 7.00 (m, 8H, CH₂), ¹³C NMR (101 MHz, CD₃CN) δ: 15.57 (d, ³J_P-C = 8.08 Hz, 4C, CH₃), 44.84 (s, 4C, C₄H₈), 62.45 (s, 4C, CH₂), ³¹P NMR (162 MHz, CD₃CN) δ: 73.64.

µ-{1, 4 -[(C₂H₅O)₂P(S)NH]₂C₆H₄}(HgCl₂)₂ (C₂): Mp: 105Ëš C. Anal. calc. (%) for C₁₄H₂₆Cl₄Hg₂N₂O₄P₂S₂: C: 17.59; H: 2.72; N: 2.93, S: 6.70, found: C: 17.85; H: 2.69, N: 2.93, S: 6.53. IR data (KBr, cm^-1): 3211, 2990, 1615, 1512, 1479, 1380, 1274, 1214, 1161, 988, 824, 633. 1H NMR (400 MHz, CD3CN) δ: 1.22 (t,3JH-H= 7.1 Hz, 12H, CH3), 3.12 (m, 8H, C4H8), 7.00 (m, 8H, CH2), 13C NMR (101 MHz, CD3CN) δ: 15.57 (d, 3JP-C = 8.08 Hz, 4C, CH3), 44.84 (s, 4C, C4H8), 62.45 (s, 4C, CH2), 31P NMR (162 MHz, CD3CN) δ: 73.64.

Result and discussion

IR and NMR spectroscopy

Mass spectroscopy

The nature of the fragments observed in the mass spectrum often provides as clue to the molecular structure. The fragmentation pathways of ligands 1 and 2 were studied by electron ionization at 70 eV experiment and revealed a molecular ion peak [M]⁺ at m/z (%) of 390 (28) and 412 (94) for 1 and 2, respectively. The formation of the [M-1] specie from the parent ion of compound 2 was shown to exclusively involve an aromatic hydrogen atom; our results were in good agreement with previously published results. [reference]. The previous paper has been shown that dialkyl alkanephosphonates ROCH₂CH₂P(O)(OR')₂ undergo a McLafferty rearrangement in which a γ hydrogen from the alkylphosphorous moiety migrates to the phosphoryl group and a molecule of olefin is eliminated from the molecular ion [reference]. The mass spectra of compound 1 and 2 with the same structure have confirmed previously reported mechanism. The peak related to the C₂H₄ radical-cation with m/z = 28 are shown for two structures. Relative peak height = relative abundance as measured from this ion in the compound 1 and 2 are 66 and 88.

For the compound 1, the base peak is appeared at m/z = 153 (P(S)(OEt)₂) and in the compound 2, the base peak is appeared at m/z = 411 (M-1) fragment. For 1, the main fragmentation is based cleavage of N-P bond then produced A ion and P(S)(OEt)₂ with m/z 153. The ion of A following three pathways: (1): A ion can produce a stabilized ion by loss of ethylene via the McLafferty rearrangement which generate the odd mass ion m/z 181 that it operates for ion m/z 181 capable of electronic shift involving a six-membered cyclic transition state in the molecule skeleton given in scheme 1. This will then stabilize to an even mass ion m/z 180 by elimination of an H radical. (2): in this pathway produce the ion at m/z 147 that formed through a three- membered ring as transition state by loss of two molecule of ethanol. The ion of m/z 147 indicating the relatively low stability of the P-O bond to the molecule of A in comparison with that of the P-N bond. (3). The ion at m/z 84 is formed through two step, the first is cleavage of P-N bond then in second step is formed via a 1,2 hydride shift by loss of a molecule of P(S)(OEt)₂ [reference]. The same kind of rearrangement is observed for 2 and the main fragmentation is based cleavage of N-P bond then produced molecule ion with m/z 107.

Scheme 1. Fragmentation pathway of compound 1

X ray crystallography

Complexes of 1 and 2 were crystallized in the orthorhombic space group Pbca Triclinic with space group P, respectively. Crystal data, data collection and structure refinement details are summarized in Table 1 and selected bond lengths and angles are given in Tables 2 and 3.

The asymmetric unit of complexes of 1 and 2 consist of one Hg²⁺ ion, two Cl and one half crystallographically independent ligand (Fig 1). There are two different types of Hg-Cl bonds that included bridge Hg1-Cl2 bond (2.5904(17) Å in 1 and 2.4852 (7) Å in 2) connect the molecule into one dimensional chain extended along the c axis and terminal Hg1-Cl1 bond (2.369(2) Å in 1 and 2.4295 (9) Å in 2) linked to adjacent ones by intermolecular interaction into a chain parallel to b axis in 1 and a axis in 2. (Fig 2). So, the Hg atom adopts an Hg[Cl]₃[S] coordination environment in this compound with the highly distorted tetrahedral geometry of the Hg(II) center that can be better described as a seesaw structure which two chloride atoms and Hg atom [ Hg1, Hg1, Cl2] is planar, one chloride and sulfur atoms in the pivot position. The different bond distance from the µ-chloride atoms performed and refer to asymmetry of the halogen bonds (2.5904 (17), 2.6820 (17) Å in 1 and 2.4852 (7), 2.8273 (8) Å in 2) and they are compared to the terminal bond of Hg-Cl slightly extended. Some selected bond angles specify the distorted tetrahedral geometry at the Hg(II) center in complex 1 are as follows: Cl1—Hg1—S1 130.91 (7)°, Cl1—Hg1—Cl2 110.98 (7)°, S1—Hg1—Cl2 104.59 (6)°, Cl1—Hg1—Cl2ⁱ 108.29 (8)°, S1—Hg1—Cl2ⁱ 105.96 (6)° , Cl2—Hg1—Cl2ⁱ 87.47 (5)°, Hg1—Cl2—Hg1ⁱ 92.54 (5)° and P1—S1—Hg1 98.40 (8)°.

In ligand L₁, the phosphorus atom has a distorted tetrahedral [N]P(S)[O]₂ configuration with the bond angles in the range of 101.77 (18)° [O2—P1—S1] to 115.80 (19)° [O1—P1—S1]. The P=S bonds of ligand are in a trans orientation is showing respect to each other and that the sulfur atom is coordinated to the mercury center. As a result of coordination to the mercury center, as expected, the P=S bond length (P (1)–S (1) 1.97 (9) Å) is slightly longer than that of the free ligand

The crystal structure of the complex 1 generated by the O1…S=P interaction along c-axis. As a result of these interactions, One-dimensional chain structure is produced. The presence of Hg-Cl and Hg-S moieties in the complex lead to the formation weaker intermolecular C-H…Cl-Hg , C-H…S-Hg interactions between the neighboring 1D chain along b-axis that create a two-dimensional array in the crystal lattice.

Scheme 2. Schematic presentation of bisthiophosphoryl ligands 1 (right), 2 (left)

Fig. 1 Asymmetric unit of complex 1(right) and 2 (left) are shown

Fig.2 Representation of one-dimensional chain of complex 1 along the c-axis. Colour keys for the atoms: Hg …., P orange, O red, N blue, C light grey, H light blue

Fig. 3 The title complex 1, with displacement ellipsoids drawn at the 50% probability level

Fig. 4 The title complex 2, with displacement ellipsoids drawn at the 50% probability level

Table 1. Crystal data, data collection and refinement for complexes 1 and 2

Table 2 Selected bond lengths (AÌŠ) and angles (°) for complex 1

Table 3 Selected bond lengths (AÌŠ) and angles (°) for complex 2