Synthesis, characterization, crystal structures, Hirshfeld surface analysis and theoretical calculations of some new bisphosphoramidate derivatives and novel binuclear triorganotin(IV) complexes with diphosphoryl ligand

Article abstract of DOI:10.1016/j.jorganchem.2018.10.012

A series of new bisphosphoramidate and (thio)phosphoramidate derivatives with the general formula R₁R₂P(X)–Y–P(X)R₁R₂ have been synthesized and characterized by IR and NMR spectroscopies (L₁–L₁₂). The crystal structure of compound 1,4-[(C₂H₅O)₂P(S)(CH₂)₃NH]₂C₄H₈N₂ (L₄) is also investigated by X-ray diffraction analysis. Two novel organotin(IV) complexes μ-{1,4-[(C₆H₅)₂P(O)(CH₂)₃NH]₂C₄H₈N₂}[SnR₃Cl]₂, R₃SnCl (R = phenyl/butyl), C₁ and C₂, respectively, are prepared by the reaction of new diphosphoryl ligand L₁ and R₃SnCl under different conditions. C₁ and C₂ are characterized by IR and NMR spectroscopies and X-ray crystallography diffraction analysis. X-ray analysis illustrates that both complexes have similar structures containing binuclear triorganotin(IV) skeletons and ligand coordinates in a bridging mode through two phosphoryl groups. Sn(IV) coordination geometries are distorted trigonal bipyramidal (TBP) for C₁ and C₂ structures contained binuclear arrangement with two SnPh₃Cl/SnBu₃Cl groups linked via the bridging diphosphoryl ligand. The organization of the crystal structures and the intermolecular interactions are discussed. Hirshfeld surfaces and two-dimensional fingerprint plots are used to study short intermolecular contacts in C₁, C₂ and L₄. Finally, the influence of chain length and the effects of various substituents on P=O and P=S bond strength in synthesized ligands (L₁–L₁₂) and optimized ligands (L₁₃–L₁₇) are theoretically investigated by NBO analysis to survey the character of mentioned bonds in these ligands. The AIM analysis is also used to determine the nature of the P=O bond in L₁ and also P=O and O?Sn⁴⁺ bonds in C₁ and C₂. Results show ionic character for O?Sn⁴⁺ interaction in both complexes and mostly electrostatic character for P=O bond in the free ligand, but with a little shift to the covalent character after the complexation.

Full text of DOI:10.1016/j.jorganchem.2018.10.012

Accepted Manuscript
Synthesis, characterization, crystal structures, Hirshfeld surface analysis and
theoretical calculations of some new bisphosphoramidate derivatives and novel
binuclear Triorganotin(IV) complexes with diphosphoryl Ligand
Khodayar Gholivand, Masoumeh Azadbakht, Yazdan Maghsoud, Mahdieh Hosseini,
Canan Kazak
PII:
S0022-328X(18)30715-0
DOI:
https://doi.org/10.1016/j.jorganchem.2018.10.012
JOM 20597
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 14 August 2018
Revised Date: 14 October 2018
Accepted Date: 17 October 2018
Please cite this article as: K. Gholivand, M. Azadbakht, Y. Maghsoud, M. Hosseini, C. Kazak,
Synthesis, characterization, crystal structures, Hirshfeld surface analysis and theoretical calculations
of some new bisphosphoramidate derivatives and novel binuclear Triorganotin(IV) complexes with
diphosphoryl Ligand, Journal of Organometallic Chemistry (2018), doi: https://doi.org/10.1016/
j.jorganchem.2018.10.012.
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Graphical Abstract
Synthesis, Characterization, Crystal Structures, Hirshfeld Surface Analysis
and Theoretical Calculations of some New Bisphosphoramidate Derivatives
and Novel Binuclear Triorganotin(IV) Complexes with Diphosphoryl Ligand
Khodayar Gholivand, Masoumeh Azadbakht, Yazdan Maghsoud, Mahdieh Hosseini and Canan
Kazak
= O or S
= C6H5, OC6H5, NHC6H5, OCH3, OC2H5, NH(CO)CCl3, NH(CO)C6H5

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Synthesis, Characterization, Crystal Structures, Hirshfeld Surface Analysis and
Theoretical Calculations of some New Bisphosphoramidate Derivatives and
Novel Binuclear Triorganotin(IV) Complexes with Diphosphoryl Ligand
Khodayar Gholivand,a,* Masoumeh Azadbakht,a Yazdan Maghsoud,a Mahdieh Hosseini,a Canan
Kazakb
a Department of Chemistry, Faculty of Science, Tarbiat Modares University, Tehran, Iran.
b Department of Physics, Faculty of Art and Science, Ondokuz Mayıs University, Samsun, Turkey
Abstract
A series of new bisphosphoramidate and (thio)phosphoramidate derivatives with the general formula
of R1R2P(X)-Y-P(X)R1R2 have been synthesized and characterized by IR and NMR spectroscopies
(L1–L12). The crystal structure of compound 1,4-[(C2H5O)2P(S)(CH2)3NH]2C4H8N2 (L4) is also
investigated by X-ray diffraction analysis. Two novel organotin(IV) complexes ꢀ-{1,4-
[(C6H5)2P(O)(CH2)3NH]2C4H8N2}[SnR3Cl]2, R3SnCl (R= phenyl/butyl), C1 and C2, respectively, are
prepared by the reaction of new diphosphoryl ligand L1 and R3SnCl under different conditions.
Complexes of C1 and C2 are characterized by IR and NMR spectroscopies and X-ray crystallography
diffraction analysis. X-ray analysis illustrates that both complexes have similar structures containing
binuclear triorganotin(IV) skeletons and ligand coordinates in a bridging mode through two
phosphoryl groups. Sn(IV) coordination geometries are distorted trigonal bipyramidal (TBP) for C1,
and C2 structures contained binuclear arrangement with two SnPh3Cl/SnBu3Cl groups linked via the
bridging diphosphoryl ligand. The organization of the crystal structures and the intermolecular
interactions are discussed. Hirshfeld surfaces and two-dimensional fingerprint plots are used to study
short intermolecular contacts in C1, C2, and L4. Finally, the influence of chain length and the effects
of various substituents on P=O and P=S bond strength in synthesized ligands (L1–L12) and optimized
ligands (L13–L17) are theoretically investigated by NBO analysis to survey the character of
mentioned bonds in these ligands. The AIM analysis is also used to determine the nature of the P=O
bond in L1 and also P=O and O···Sn4+ bonds in C1 and C2. Results show ionic character for O···Sn4+
interaction in both complexes and mostly electrostatic character for P=O bond in the free ligand, but
with a little shift to the covalent character after the complexation.

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Keywords: Triorganotin(IV), Bisphosphoramidate, Crystal structure, Hirshfeld surfaces, Theoretical
studies.
* Corresponding author’s E-mail address: gholi_kh@modares.ac.ir
1. Introduction
Nowadays, considerable interest has been paid to the research on phosphoramide derivatives because
of their valuable and widely applicable properties such as anticancer prodrugs [1–4], antiviral agents
[5–7] and biocidal activity [8, 9]. These compounds are also well-known inhibitors of various
enzymes, including urease and acetylcholinesterase [10, 11]. The ability of phosphoramides to
coordinate to the metal atoms and form coordination compounds is also distinguished, and there are
many studies on their structures [12–16].
Tin compounds play an essential role in inorganic and organometallic chemistry [17, 18].
Investigations on the tin(IV) adducts continue to provide fundamental information about both the
Lewis acid-base model and the reactivity of tin(IV) species [19–21]. Recently organotin(IV)
compounds have received considerable attention because of their biological properties [22],
particularly antitumor [23–28], antibacterial [29–32] and antifungal activity [33–35]. In addition to
their biological activity, the organotin(IV) complexes have also found some applications in catalysis
[36] and nonlinear optics [21, 37].
Phosphoryl-containing compounds have been widely used as active complexation agents in
organotin coordination chemistry [38]. Synthesis of di- and tri-organotin complexes, containing Sn–
O bonds are growing fast [39–41], and the presence of both organotin (IV) and phosphorus moieties
in a single molecular can produce a still more powerful and lasting useful complex [42, 43].
In the present study, some novel bisphosphoramidate and thiophosphoramidate derivatives with the
general formula R1R2P(X)-Y-P(X)R1R2, (X= O and S; Y= NH–(CH2)3–N–(CH2)4–N–(CH2)3–NH
and N–(CH2)4–N–(CH2)2–NH; R1 & R2= (C6H5, OC6H5, NHC6H5, OCH3, OC2H5, NH(CO)C6H5,
1
31
NH(CO)CCl3) (L1–L12), were synthesized and characterized by H, P NMR and IR spectroscopy.
The synthesis and spectroscopic characterization (1H, 31P NMR, and IR) of two new organotin
complexes, obtained from the reaction of R3SnCl (R=phenyl/butyl) with Ph
2
P(O)YP(O)Ph ligand
2
(Y= diamine), were also reported. The structure of the L4 ligand and two complexes, C1 and C2,
were determined using X-ray crystallography, which revealed the similar coordination manner of the
ligand and same structure in C1 and C2 complexes. Furthermore, the crystal structures of L4, C1, and

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C2 and their packing systems were studied using geometrical parameters, and Hirshfeld surfaces
analysis.
Moreover, NBO analysis was used to investigate the nature of P=O and P=S bonds in studied
bisphosphoramidate and thiophosphoramidate ligands. The AIM analysis was also used to determine
the nature of the P=O bond in L1 and also P=O and O···Sn4+ bonds in C1 and C2.
2. Experimental
2.1. Materials and measurements
All chemicals and solvents are commercially available and were used as received without any further
purification.
1H and 31P NMR spectra were recorded on a Bruker Advance DRX 500 spectrometer. 1H chemical
shifts were determined relative to internal standard TMS, and 31P chemical shifts were determined
relative to 85% H3PO4 as an external standard. Infrared (IR) spectra were recorded on a Shimadzu
model IR-60 spectrometer using KBr pellets. Melting points of the compounds were obtained with
an electrothermal instrument. Elemental analysis was also performed using a Heraeus CHN-O-
RAPID apparatus.
2.2. General Procedure for Synthesis of ligands
The reagents RPOCl2 (R= C6H5(CO)NH) [44], CCl3(CO)NH [45] and R1R2POCl (R1= NHC6H5 & R2
= OC6H5, NH(CO)C6H5, NH(CO)CCl3), were prepared by the reaction of aniline with RPOCl2 (R=
OC6H5, NH(CO)C6H5, NH(CO)CCl3), in 2:1 molar ratio. The aniline was added dropwise to a
CH3CN solution of RPOCl2 at 0 °C. After eight hours stirring, the solvent was removed in vacuum,
and the resulting was washed with distilled water and dried.
All Ligands except L4 were synthesized from the reaction of the corresponding diamine (1mmol) in
the presence of triethylamine (2 mmol) as HCl scavenger in solvent at room temperature was added
dropwise to a stirred solution of RRPXCl (X= S, O) or R1R2POCl (2 mmol) in the same solvent at 0
°C. After 24h stirring, the solvent was evaporated, and the residue was washed with distilled water to
obtain the pure products. Ligand L4 was prepared by using the similar procedure but washed with
THF, then the solution was filtered, and evaporation removed the solvent of the filtrate at room
temperature until yellow oil product was obtained, then it was washed with n-hexane and solid
product obtained. Recrystallizing of Ligand L4 obtained suitable crystal for X-ray diffraction from a

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mixture of CH3CN/CH3CL after the slow evaporation of the solutions at room temperature. The
synthesis procedures of compounds (L1–L12) was represented in Scheme 1. Synthesis and
Characterization of ligands (L13–L17) were reported in [46]. Physical and spectroscopic data of the
new ligands are presented below:
Scheme 1. Preparation pathway of diphosphoryl ligands.
2.2.1. 1,4-[(C6H5)2P(O)(CH2)3NH]2C4H8N2 (L1). powder sample; m.p: 173 °C. Anal. Calc. for
C34H42N4O2P2: IR data (KBr pellet, cm–1): 3190m (υN–H), 1184s (υP=O), 723m, 697m (υP–N), 1115s
1
(υC–N), 2938m, 2806m (υaliph). 31P NMR(DMSO-d6, ppm): δ=21.03m. H NMR (CDCl3, ppm):
δ=7.86 (m, 8H, 4Ph), 7.42 (m, 12H, 4Ph), δ=4.95 (m, 2H, NH), δ=3.03 (m, 4H, 2CH2), δ=2.33 (m,
12H, 6CH2), δ=1.69 (m, 4H, 2CH2).
2.2.2. 1,4-[(C6H5O)2P(O)(CH2)3NH]2C4H8N2 (L2). powder sample; m.p: 100 °C. Anal. Calc. for
C34H42N4O6P2: IR data (KBr pellet, cm–1): 3239m (υN–H), 1195s (υP=O), 760m (υP–N), 935s (υP–O),

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1242s (υC–O), 1101s (υC–N), (2935, 2870, 2811)m (υaliph.). 31P NMR (DMSO-d6, ppm): δ=0.696m. 1H
NMR (CDCl3, ppm): δ=7.33 (t, 8H, 4OPh), δ=7.25 (d, 8H, 4OPh), δ=7.16 (t, 4H, 4OPh), δ=4.74 (m,
2H, NH), δ= 3.19 (m, 4H, 2CH2).
2.2.3. 1,4-[(C6H5O)(C6H5NH)P(O)(CH2)3NH]2C4H8N2 (L3). powder sample; m.p:106 °C. Anal.
Calc. for C34H44N6O4P2: IR data (KBr pellet, cm–1) 3180m (υN–H), 1212s (υP=O), (756, 687, 918)m
(υP–N), 918m (υP–O), 1212s (υC–O), 2940w, 2822w (υaliph.), 1107w (υP–N), 1293w (υP–O). 31P NMR
(DMSO-d6, ppm): δ=5.261m. 1H NMR(DMSO-d6, ppm): δ=7.85 (d, 2H, 2NHAn), δ=7.35 (t, 8H, 2O-
OPh+2OAn), δ=7.143 (2d, 8H, 2m-OPh+2mAn), δ=6.825 (t, 4H, 2p–OPh+2p-An), δ=5.287 (m, 2H,
NH), δ=3.27 (m, 4H, 2CH2), δ= 2.42 (m, 12H, 6CH2), δ=1.443 (m, 4H, 2CH2).
2.2.4. 1,4-[(C2H5O)2P(S)(CH2)3NH]2C4H8N2 (L4). powder sample; m.p: 110 °C. Anal. Calc. for
C18H42N4O4S2P2: IR data (KBr pellet, cm–1) 3083m (υN–H), 806s (υP=S), 608w (υP–N), 948 (υP–O), 1029
1
(υC–O), 1124m (υC–N), (2937, 2824)m (υaliph.).31P NMR(CDCl3, ppm): δ=71.49m. H NMR(CDCl3,
ppm): δ=4.53 (several m, 2H, NH), δ=1.31 (m, 12H, 4Me, OEt), δ=4.04 (m, 8H, 4CH2, OEt),
δ=3.054 (m, 4H, 2CH2), δ=2.686 (m, 12H, 6CH2), δ=7.70 (m,4H, 2CH2).
2.2.5. 1,4-[(CH3O)2P(S)(CH2)3NH]2C4H8N2 (L5). powder sample; m.p: 112 °C. Anal. Calc. for
C14H34N4O4S2P2: IR data (KBr pellet, cm–1) 3079w (υN–H), 799s (υP=S), 627m (υP–N), 1173w (υC–N),
1
1035s (υP–O), 1095m (υC–O), (2944, 2825)m (υaliph). 31P NMR(CDCl3, ppm): δ=75.22m. H NMR
(CDCl3, ppm): δ= 4.74 (several m, 2H, NH), δ=3.62 (s, 12H, 4Me, OMe), δ=3.05 (m, 4H, 2CH2),
δ=2.33 (m, 12H, 6CH2), δ=1.74 (m, 4H, 2CH2).
2.2.6. 1,4-[(CCL3(CO)NH)(C6H5NH)P(O)(CH2)3NH]2C4H8N2 (L6). powder sample; m.p: 176 °C.
Anal. Calc. for C26H36N8O4Cl6P2: IR data (KBr pellet, cm–1): 3303m, 3170w (υN–H), 1621s (υC=O),
31
1169s (υP=O), 1287s,1327s (υC–N), 920s, 748m, 685s (υP–N), 491m (υC–Cl), 2821m (υaliph.). P NMR
1
(DMSO-d6, ppm): δ = –1.244m. H NMR (DMSO-d6, ppm): δ=7.68 (m, 2H, An), δ=6.64–7.60 (m,
HAr), δ=3.15 (m, 4H, 2CH2), δ= 2.423 (m, 12H, 6CH2), δ=1.677 (m, 4H, 2CH2). δ=8.3 (m, 2H,
2NHAmide).
2.2.7. 1,4-[(C6H5(CO)NH)(C6H5NH)P(O)(CH2)3NH]2C4H8N2 (L7). powder sample; m.p: 175-180
°C. Anal. Calc. for C36H46N8O4P2: IR data (KBr pellet, cm–1). 3300s, 3165s (υN–H), 1661s (υC=O),
1202s (υP=O), 1099m (υC–N), (1099, 950, 694)m (υP–N), 2923w, 2807m (υaliph.). 31P NMR (DMSO-d6,
1
ppm): δ= 1.833m, H NMR (DMSO-d6, ppm):δ=9.5 (m, 2H, 2NHAmide), δ=7.855 (m, 2H, An),
δ=4.77 (m, 2H, NH), δ=6.754–7.602 (m, 20H, Ar), δ=2.91 (m, 12H, 6CH2), δ=2.214 (m, 4H, 2CH2),

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δ=1.52 (m, 4H, 2CH2).
2.2.8. 1,4-[(C6H5)2P(O)N]2(CH2)2C4H9N (L8). powder sample; m.p: 57 °C. Anal. Calc. for
C30H33N3O2P2: IR data (KBr pellet, cm–1): 3208w (υN–H), 1188s (υP=O), 1119s (υC–N), 962m, 723s,
31
1
697s (υP–N), (3059, 2934, 2860)w (υaliph.). P NMR (CDCl3, ppm): δ =29.14m, 24.06m. H NMR
(CDCl3, ppm): δ=7.83 (m, 8H, 4Ph), δ=7.42 (m, 12H, 4Ph), δ=3.72 (m, 1H, NH), δ =3.192 (m, 6H,
3CH2), δ=2.55 (m, 6H, 3CH2).
2.2.9. 1,4-[(C6H5O)2P(O)N]2(CH2)2C4H9N (L9). oil sample; Anal. Calc. for C30H33N3O6P2: IR data
(KBr pellet, cm–1): 3232w (υN–H), 1193s (υP=O), 930s, 769m (υP–N), 930s (υP–O), 1266m (υC–O), (2931,
1
2815, 2675)w (υaliph.). 31P NMR (DMSO-d6, ppm): δ=0.749m, 0.625m. H NMR (DMSO-d6, ppm):
δ=7.37 (m, 8H, 4OPh), δ=7.24 (m, 12H, 4OPh), δ=5.74 (m, 1H, NH), δ=3.07 (m, 6H, 3CH2), δ=2.23
(m, 6H, 3CH2).
2.2.10. 1,4-[(C6H5O)(C6H5NH)P(O)N]2(CH2)2C4H9N (L10). Powder sample; m.p: 54 °C. Anal.
Calc. for C30H35N5O4P2: IR data (KBr pellet, cm–1
) 3186m (υN–H), 1211s (υP=O), 1148w (υC–N), 756m,
691w (υP–N), 924s (υP–O), 1292w (υC–O), 2967w, 2893w, 2817w (υaliph.). 31P NMR (DMSO-d6, ppm):
1
δ=4.524m, 5.298m. H NMR (DMSO-d6, ppm): δ=8.05 (m, 1H, 1NHAn), δ= 7.87 (m, 1H, 1NHAn),
δ=7.28–7.395 (2t, 8H, 2O-OPh+2O-An), δ=7.04–7.16 (2d, 8H, 2m-OPh+2m-An), δ=6.81–6.89 (2t,
4H, 2p-OPh+2p-An), δ= 5.1(m, 1H, NH), δ= 3.035 (m, 6H, 3CH2), δ= 2.11 (m, 6H, 3CH2).
2.2.11. 1,4-[(C2H5O)2P(S)N]2(CH2)2C4H9N (L11). powder sample; m.p: 85 °C. Anal. Calc. for
C16H33N3O4S2P2: IR data (KBr pellet, cm–1): 3211m (υN–H), 798s (υP=S), 1164m (υC–O), 1028s (υC–N),
633m (υP–N), 957s (υP–O), (2982, 2905, 2502)w (υaliph.). 31P NMR (DMSO-d6, ppm): δ=73.2031m,
1
72.3935m. H NMR (CDCl3, ppm): δ= 5.755(several m, 1H, NH), δ=3.93 (m, 8H, 4CH2, OEt),
δ=1.21 (m, 12H, 4Me, OEt), δ= 3.192 (m, 6H, 3 CH2), δ=2.482 (m, 6H, 3CH2).
2.2.12. 1,4-[(C6H5(CO)NH)(C6H5NH)P(O)N]2(CH2)2C4H9N (L12). powder sample; m.p: 145-150
°C. Anal. Calc. for C32H37N7O4P2: IR data (KBr pellet, cm–1
): 3400w, 3197m (υN–H), 1667s, 1601m
(υC=O), 1209s, 1276m (υP=O), 1072 (υC–N), (1072, 975, 704) (υP–N), 2923s, 2900w (υaliph.). 31P
1
NMR(DMSO-d6, ppm): δ= 2.398m, 1.27, H NMR (DMSO-d6, ppm): δ=9.63 (m, 2H, 2NHAmide),
δ=8.8 (m, 2H, NH), δ=4.64 (m,1H, 1NH), δ=6.7–7.9 (m, 20H, Ar), δ=3.03 (m, 6H, 3CH2), δ= 2.355
(m, 6H, 3CH2).
2.3. Synthesis of metal complexes

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Complex C1: A solution of Sn(Ph)3Cl (2 mmol) in methanol was added dropwise to a solution of
ligand L1 (1 mmol) in the same solvent at room temperature.
Complex C2: The synthesis of C2 was carried out identically to C1, using Sn(Bu)3Cl instead of
Sn(Ph)3Cl and reaction solvent was toluene.
Suitable crystals of C1 and C2 for X-ray diffractions were obtained from slow evaporation of the
clear solutions at room temperature. Physical and spectroscopic data of the synthesized complexes
are presented below:
2.3.1. ꢀ-{1,4-[(C6H5)2P(O)(CH2)3NH]2C4H8N2}[SnPh3Cl]2 (C1). Mp: 202 °C. Anal. Calc. for
C70H72Cl2N4O2P2Sn2: IR data (KBr pellet cm–1): 3053w (υN–H), 1156s (υP=O), 730s, 693m (υP–N),
1
1122s (υC–N), 2945w, 2820w (υaliph.).31P NMR(DMSO-d6, ppm): δ=21.33m, H NMR (DMSO-d6,
ppm), δ=5.32m (several m, 2H, NH), δ=7.4–7.5 (m, 31H, 4Ph, Ph3Sn), δ=7.75–7.79 (m, 10H, 4Ph,
Ph3Sn), δ=7.84–7.9 (m, 9H, 4Ph, Ph3Sn), δ=3.28 (m, 4H, 2CH2), δ=2.77 (m, 12H, 6CH2), δ=1.58 (m,
4H, 2CH2).
2.3.2. ꢀ-{1,4-[(C6H5)2P(O)(CH2)3NH]2C4H8N2}[SnBu3Cl]2 (C2). Mp: 194 °C. Anal. Calc. for
C58H60Cl2N4O2P2Sn2: IR data (KBr pellet cm–1): 3063w (υN–H), 1162s (υP=O), 731w, 694m (υP–N),
1127s (υC–N), 2950s, 2921s, 2854m (υaliph.). 1P NMR (DMSO-d6, ppm): δ=21.14m, 1H NMR(DMSO-
d6, ppm), δ=5.4m (several m, 2H, NH), δ=7.76 (m, 8H, 4Ph), δ=7.469 (m, 12H, 4Ph), δ=3.3 (m, 4H,
2CH2), δ=2.75 (m, 12H, 6CH2), δ= 2.22 (m, 4H, 2CH2).
2.4. Crystal structure determination
X-ray intensity data were collected at 100 K on a Bruker SMART APEXII CCD diffractometer (L4),
and at 0 K on an Xcalibur, Eos diffractometer (C1 and C2) with graphite-monochromatized Mo Kα
radiation (λ = 0.71073 Å). The structures were solved by direct methods using SHELXTL-2009 [47],
ShelXT-2015 [48] and ShelXS-2008 [47] for L4, C1, and C2, respectively. Then the structures were
refined with the full-matrix least-squares procedure on F2 by SHELXTL-2009 for L4 and ShelXT-
2015 for C1 and C2. All non-hydrogen atoms have been refined anisotropically. Hydrogen atoms
have been added at calculated positions and refined using a riding model based on the parent atom.
In the structure of C2, the butyl Ligands are disordered over two positions with 0.65/0.35
occupancies. In the crystal packing figures of this complex, the minor component of the disordered
atoms is omitted for clarity. Structural artworks have been drawn with MERCURY [49]. The CIF
files have been deposited with the CCDC and have been given the deposition numbers 1847678,

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1847679 and 1847680 for L4, C1, and C2, respectively.
2.5. Computational details
All of the quantum chemical computations were implemented by the use of Gaussian 03 program
package [50]. Density functional theory (DFT) was utilized, which can characterize the structure of
ligands and complexes quantitatively. Coulomb-attenuating method (CAM-B3LYP) was used as a
hybrid exchange-correlation functional, which combines the hybrid qualities of B3LYP and the long-
range correction presented by Tawada et al. [51]. All the atoms were treated by the def2-TZVP [52],
basis set in H2O and the temperature of 293 K (room temperature) using the SMD solvation model.
Frequency calculations were accomplished at the same level of theory as those for the structural
optimization. Vibrational frequency analysis, calculated at the same levels of theory, indicates that
optimized structures are at the stationary points corresponding to local minima without any
imaginary frequency. The atoms in molecules theory (AIM) analysis was carried out using AIM2000
program [53], in which the wave function was obtained from the aforementioned basis sets at the
CAM-B3LYP level of theory. Natural bond orbital (NBO) analysis was performed with the NBO
(ꢂ)
ꢀꢁ
program, embedded in the Gaussian package. Delocalization stability energy, ∆E
in NBO
approach is defined by the following equation:
ꢋ
ꢈ
ꢃꢄꢅ ꢃꢇꢃꢅ ꢊꢃ
(ꢂ)
ꢀꢁ
ꢆ
ꢉ
∆E
=
ꢌ ꢍꢌ
ꢆ
ꢉ
ꢎ
Where, F is the Fock operator, φꢀ and φꢁ are the electron donor and electron acceptor orbitals
respectively, and εꢀ and εꢁ are their corresponding energies [54].
3. Results and discussion
3.1. Spectral Study
All compounds were characterized by IR and NMR spectroscopy. Relevant spectroscopic data of
new bisphosphoramidate derivatives and complexes C1 and C2 are presented in supplementary data.
The analysis of the IR spectra indicated that the fundamental υ(P=O) stretching modes and υ(P=S)
for compounds appeared at 1169–1276 and 798–806 cm–1, respectively. Other fundamental
characteristics of these compounds are N–H and P–N stretching modes, appear at the values around
3079–3400 and 608–1099 cm–1, respectively. Besides, a significant decreasing of the N–H stretching

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frequencies is observed in complexes C1 and C2. As well the N–H bond participates in the hydrogen
bonding, the positive shift of υ(N–H) may be attributed to the weakening of the hydrogen bonds
from NH···Op=o in the free ligands to NH···Cl in its complexes. Moreover, The C=O stretching
modes for L6, L7, and L12 compounds appear at 1601–1667cm–1.
The stretching frequencies of P=O groups in complexes C1 and C2 appear at 1156 and 1162 cm–1,
respectively, which are at a lower frequency concerning the related free ligand (1184 cm–1). That
support coordination of ligand to the metal via the oxygen atom of phosphoryl group in both
complexes which are confirmed by X-ray diffraction studies. Besides, the low-frequency shifts are
,
higher in the complex C1 (including with substituents SnPh3Cl) confirming that the Sn–O interaction
is stronger than C2.
Phosphorus chemical shift values δ(31P) for compounds were observed in the range of –1.24 to 29.14
31
ppm for P=O and 71.49 to 75.22 ppm for P=S derivatives. As PNMR spectra reveal, compounds
with P=S group indicated a higher upfield shift in comparison to compounds with the P=O group.
31
The PNMR spectra for compounds indicated similar and multiplet splitting pattern; This splitting
pattern arising from spin couplings between the phosphorus nucleus with NH and protons of the
CH2. 31P NMR spectroscopic results indicated that chemical shift δ(31P) for the complexes were also
very close to the value of the related free ligand. The 1H NMR spectral data obtained for ligands are
indicative of the expected structure for these molecules.
1
The integrated H NMR spectra in complexes C1 and C2 are in accordance with the binuclear
structures, which exhibit the expected proton signals for a bridging ligand and two triphenyl and
tributyltin(IV) respectively.
3.2. Single crystal X-ray diffraction studies
3.2.1. Descriptions of crystal structures
Single crystals of L4, C1, and C2 were obtained by slow evaporation of solvent at room temperature
with excellent quality. Crystallographic data of mentioned compounds as well as their geometric
parameters are summarized in Tables 1 and 2, respectively. Molecular structures of L4, C1, and C2
are presented in Fig. 1.

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Fig. 1. Ball and stick diagram of the molecular structure of (a) L4; (b) C1 and (c) C2. Symmetry
codes: i: –x,2–y,1–z; ii: 1–x,1–y, –z; iii: 2–x,1–y,1–z.
ꢏ
L4 crystallizes in the triclinic crystal system with space group P1. Both complexes C1 and C2 form in
the monoclinic crystal system with space groups P21/n and P21/c, respectively.
Table 1. Crystal data and structural refinement for compounds L4, C1 and C2.
L4
C1
C2
Empirical formula
Formula weight
Temperature (K)
C18H42N4O4P2S2
504.62
100(2)
C70H72Cl2N4O2P2Sn2 C58H42Cl2N4O2P2Sn2
1371.53
295(2)
1197.17
100(2)
Wavelength (A)˚
Crystal system, space group
a/Å
b/Å
c/Å
α (°)
β (°)
γ (°)
0.71073
Triclinic, P1
0.71073
Monoclinic, P21/n
9.8673(4)
20.4833(7)
16.4063(6)
90
90.269(4)
0
3315.9(2)
2
0.71073
Monoclinic, P21/c
10.9616(9)
31.314(2)
9.8585(9)
90
104.461(9)
90
3276.7(5)
2
ꢏ
7.7306(6)
8.2289(6)
10.7584(8)
80.5560(10)
81.3250(10)
81.3250(10)
635.09(8)
1
V (A˚3)
Z
µ (mm−1)
F(000)
0.21
272
0.928
1400.0
0.930
1196.0

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3
0.31 × 0.18 × 0.16
0.625 × 0.345 × 0.21
0.558 × 0.354 × 0.251
2θ = 6.732 -51.45°
h = -13→13
k = -17 →38
l = -7→12
10831
0.992
6210/0/250
1.019
Crystal size (mm )
θ Range for data collection (°) θ = 1.93 -30.00°
h = -10→10
2θ= 6.24 - 51.362°
h = -12→ 10
k = -19→ 24
l = 20→ 19
16287
0.998
6263/0/374
1.030
Limiting indices
k = -11→11
l = -15→15
13421
99.3
3678 / 0 /138
1.001
Reflections collected
Completeness to theta
Data/restraints/parameters
Goodness-of-fit on F2
R1 (I > 2σ(I))
0.0294
0.0348
0.0851
wR2 (I > 2σ(I))
0.0677
0.0673
0.2125
R1 = 0.0342, wR2 =
0.0709
R1=0.0553, wR2 =
0.0754
R1 = 0.1719, wR2 =
0.2682
R indices (all data)
Largest diff. peak and hole
0.648 and -0.368
0.35-0.32
0.78-0.72
-3
(e.Å )
Table 2. Selected bond lengths (Å) and angles (˚) for the crystal structures of L4, C1, and C2.
Compound
L4
P-(C/ O) P-N
P=O(S)
Sn-O
Sn-Cl
Sn-C
< O-Sn-Cl
< C-Sn-C
1.588(1)
1.619(1) 1.9376(6)
-
-
-
-
-
1.5907(9)
1.795(3)
1.795(3)
1.606(3) 1.481(2)
2.299(2) 2.4882(9) 2.126(3) 176.38(6)
117.9(1)
116.8(1)
123.7(1)
358.4(1)
358(1)
2.127(3)
2.115(3)
C1
C2
1.79(1)
1.818(9)
1.626(8) 1.460(7)
2.421(6) 2.504(3)
2.21(3)
2.16(3)
2.30(3)
178.7(2)
As shown in Fig. 1, the structure of C1 and C2 complexes are quite similar. X-ray analysis revealed
that the asymmetric unit of these two complexes consists of one Sn(Ph/Bu)3 metal center, half of the
bidentate bisphosphoramidate ligand, and one coordinated chloride anion. So C1 and C2 contain
binuclear arrangement with two SnPh3Cl/SnBu3Cl groups linked via the bridging diphosphoryl
ligand. The bidentate ligand has P=O groups in an anti-conformation related to each other.
The coordination polyhedra around tins can be described as slightly distorted trigonal bipyramidal
with three phenyl/butyl carbon atoms occupying the equatorial positions and the chlorine atom and
the phosphoryl group at the apices positions. The trans angles found around the metal are 176.38(6)
(C1), and 178.7(2) (C2), as well as the sum of the angles subtended at tin in the trigonal girdle, is
358.4(1) and 358(1) for complexes C1 and C2, respectively. The value of the Sn–C distances for
complexes are in good agreement with published values [55] and corresponds well to the sum of the
covalent radii (2.15 Å) of Sn and C atoms [56]. The Sn-Cl bonds (2.4882 (9) in C1 and 2.504 (3) Å in

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C2) are in accordance with the sum of the covalent radii of Sn and Cl atoms (2.37–2.60 Å) [57]. The
Sn–O bond distances (2.299(2) (C1), 2.421(6) Å (C2)) are longer than the sum of the covalent bond
radii of Sn and O atoms, (2.038–2.115 Å) [58] but considerably shorter than the amount of their van
der Waals radii of (3.71 Å) [59]. The P=O bond length in C1 (1.481(2) Å) and C2 (1.460 (7) Å) are
longer than the standard P=O bond length (1.45 Å) [60]. The lengthening of the P=O bond is
described merely by the polarization of the phosphoryl group in the electrostatic field of the tin(IV)
atom. This fact can be observed in lower stretching frequency of the P=O bond in the IR spectra for
complexes concerning the related free ligand. The Sn–O bond shortening is also accompanied by an
increase in the Sn–Cl bond length and the reverse, because of the trans influence. The P=O bond
elongation is in almost linear correlation with shortening of Sn–O bonds. C1 has the longer P=O
distance and the shorter Sn–O distance comparing to C2. Moreover, The Sn-Cl bond in C1 is shorter
than it is expected. This shortening of this bond distance can be considered as a result of the packing
effects and intermolecular interactions. The phosphorus atom shows a tetrahedral configuration, the
average of surrounding angles around the P atoms are (109.23, 109.466 and 109.3 Å), for ligand L4,
complexes C1 and C2, respectively. All of the P–N bonds are shorter than the typical P–N single
bond length (1.77 Å) [60].
3.2.2. Crystal packing
L4: The molecular structure of this compound is shown in Fig. 1(a). Two thio-phosphoryl groups
adopt the anti-conformation. In the crystal structure of L4, classical hydrogen bonds which are those
between N–H amide group donor and piperazine nitrogen acceptors (N–Hamidic···N(C–N)) plus non-
classical hydrogen bonds between sulfur atoms and methyl groups (C–H···S(P=S)) create one-
dimensional hydrogen bonding chains along b-axis. As it is presented in Fig. 2 (a), the head to tail
dimeric N–H···N hydrogen bonds accumulate centrosymmetric Rꢂꢂ (12) amide-piperazine synthons.
ꢂ
(
)
Besides the N–Hamidic···N(C–N) and C–H···S(P=S) hydrogen bonds together build up two Rꢂ 17
ꢂ
ꢂ
(
)
rings between two adjacent dithiophosphoryl units. So, there are two Rꢂ 17 and one Rꢂ (12)
synthons between adjacent molecules in a chain along b-axis. The one-dimensional molecular chains
are linked together in ab-plane by head-to-tail hydrogen bond interactions between sulfur atoms and
methyl hydrogens (C4–H4A···S1) which result in Rꢂꢂ (12) graph set motif. Therefore, P=S group is
involved in bifurcated hydrogen bonds which participate in a Rꢂꢐ (8) ring motif formed with
assistance between four molecules of L4 as it is shown in Fig. 2 (b). Besides, a head to tail hydrogen
bonding between the oxygen atom of diethyl group (O1) and methylic hydrogen atom (H2C) link the
chain in the c-direction producing Rꢂꢂ (8) ring motif, Fig. 2 (c). All of these intermolecular hydrogen

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bonds complete the whole 3D architecture of this compound in the solid state. A summary of
parameters for the interactions mentioned above is presented in Tables 3 and 4.
(a)
(b)
(c)
Fig. 1. Crystal packing of L4: a) intermolecular hydrogen bonding linking the neighboring
molecules and chains in ab-plane; b) head-to-tail hydrogen bonds connecting adjacent strings in bc-

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plane; c) side view of the crystal packing of L4 in the ac-plane (In (a) and (b) different colors
illustrate different molecular chains).
Table 3. Hydrogen bond geometries for L4, C1, and C2.
Structure
L4
...
X-ray Geometry (Å/ deg)*
Symmetry
D–H A
C4–H4C···S1
N1–H1N···N2
C4–H4A···S1
C2–H2C···O1
C1–H1A···Cl1
C17–H17···Cl1
C14–H14A···Cl1
C2–H2···Cl1
2.993/3.731(2)/133
2.216/2.988(2)/157
2.999/3.716(1)/131
2.758/3.737(2)/179
2.936/3.890(6)/160
2.900/3.684(3)/143
3.009/3.94(1)/162
2.895/3.731(7)/150
3.018/3.864(7)/152
x,–1+y,z
–x,1–y,1–z
1–x,2–y,–z
–x,–y, 2–z
1/2–x,1/2+y,1/2–z
1/2–x,1/2+y,1/2–z
1+x,y,z
C1
C2
1+x,y,z
C10–H10···Cl1
1+x,y,1+z
*Geometrical parameters of HB-bond: H...A/D...A/<D–H...A.
Table 4. C–H···π interaction Geometries for C1 and C2.
Structure
C1
Interaction/CH···CgI
X-ray Geometry (Å/deg)*
Symmetry
C21–H21···Cg(5)
C15–H15···Cg(3)
C32–H32···Cg(3)
C26–H26···Cg(4)
C16–H16···Cg(5)
C4–H4A···Cg(6)
C10–H10···Cg(5)
C21A–H21C···Cg(2)
C4–H4···Cg(2)
3.936/127
3.980/130
3.624/135
3.329/140
3.327/148
3.450/149
3.982/170
3.304/162
3.750/136
3.309/127
–1+x,y,z
–x,1–y,1–z
1–x,1–y,1–z
1/2+x,1/2–y,–1/2+z
1/2–x,1/2+y,1/2–z
–1/2+x,1/2–y,–1/2+z
3/2–x,1/2+y,1/2–z
–1+x,y,z
C2
x,1/2–y,–1/2+z
x,1/2–y,–1/2+z
C4–H4···Cg(3)
Cg stands for the center of gravity of the mentioned ring: For C1: Cg(3): C12–C17; Cg (4): C18–
C23; Cg(5): C24–C29; Cg(6): C30–C35; For C2: Cg(2): C1–C6; Cg(3): C7–C12.
C1: The molecular structure of this compound is shown in Fig. 1(b). In the crystal packing of this
complex, the molecular units of C1 are joined through a C–H···π contact (C21–H21···Cg(5)) in the
a-direction (Fig. 3(a)). The result chains are further connected to each other by two C–H···π
interactions (C15–H15···Cg(3) and C32–H32···Cg(3)) in the c- direction (Fig. 3(b)) to generate 2D
sheets in the ac-plane.

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Fig. 2. Crystal packing of C1: (a) CH⋯π interaction within a chain; (b) CH⋯π interactions link the
strings in the ac-plane. Non-interacting hydrogen atoms in (b) have been omitted for visual clarity. In
(b) different colors illustrate different molecular chains.
On the other hand, various hydrogen bonds and C–H···π contacts, including C1–H1A···Cl1, C17–
H17···Cl1, C26–H26···Cg(4), C16–H16···Cg(5), C4–H4A···Cg(6) and C10–H10···Cg(5) are
involved in the joining of 2D networks in the ab-plane which complete a 3D architecture (Fig. 4(a)
and (b)). A summary of the parameters for the interactions mentioned above is presented in Tables 3
and 4.

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Fig. 3. Crystal packing of C1: (a) Three-dimensional supramolecular structure of compound C1; (b)
The intermolecular interactions linking the adjacent 2D layers in the ab-plane; Non-interacting
hydrogen atoms in (b) have been omitted for visual clarity; different colors present different 2D
sheets.
C2: The molecular structure of this compound is shown in Fig. 1(c). In the solid state of this
binuclear complex, molecules are connected to each other by bifurcated non-classical hydrogen
bonds (C14–H14A···Cl1 and C2–H2···Cl1) accompanied with a C–H···π contact (C21A–
H21C···Cg(2)) which build up a chain directed along the a-axis (Fig. 5a). These chains are held
together by C4–H4···Cg(2) and C4–H4···Cg(3) linkages along the b-axis and C10–H10···Cl1
hydrogen bonding along the c-direction, which complete a 3D network (Figs. 5b-d).

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Fig. 4. Self-assembly of C2: (a) H-bonds and C–H···π linkages generating [100] chains; (b) C–H···π
interactions which link the chains in the ab-plane; (c) Hydrogen bond connecting the strings in the
ac-plane; (d) overall supramolecular array containing layers connected to each other. Non-interacting
hydrogen atoms in (c) have been omitted for visual clarity. Different colors display different
molecular chains.
3.2.3. Hirshfeld surface analysis
Hirshfeld surface analysis is a robust technique for the quantitative study of intermolecular
connections in crystal packing [61]. Consuming graphical tools based on Hirshfeld surfaces and two-
dimensional (2D) fingerprint graphs to assess and relate the proportion of intermolecular associates
in crystalline structures would be advantageous [62, 63]. The Hirshfeld surfaces mapped with the
normalized contact distance, dnorm, range 0.5 to 1.5 Å, and full fingerprint plots were made using
the program CrystalExplorer3.0 [64], which accepts a structure input file in CIF format. The
Hirshfeld surfaces of L4, C1, and C2 mapped with dnorm are illustrated in Fig. 6. The relative

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contributions of different intermolecular contacts to the Hirshfeld surface area of L4, C1, and C2 are
shown as a chart in Fig. 7.
Fig. 5. Hirshfeld surfaces of L4, C1 and C2 and effective interaction in 2D finger plots of compounds
derived from Hirshfeld surfaces. For L4, Red, blue, and green plots indicate O···H and N···H and
S···H interactions, respectively. In C1 and C2, Red and blue plots indicate Cl⋯H and C⋯H
interactions, respectively.

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In the Hirshfeld surface of L4, the relatively large red circles specify the classic NH⋯N hydrogen
bonds which are established between the piperazine and amine moieties. As it can be seen in the
chart in Fig. 7, after H⋯H contact which has the most contribution in the Hirshfeld surface area, the
S⋯H, and O⋯H and N⋯H interactions play a crucial role in the solid state of this compound.
The Hirshfeld surfaces of C1 and C2 are presented in Fig. 6. The red circles belong to Cl⋯H and
C⋯H contact. In both structures after H⋯H contact which is dominant, with 82.6 and 66.3% for C1
and C2, respectively, the C⋯H interaction which is attributed to C–H⋯π contact, and Cl⋯H contact
are the most significant contacts. All of these contacts between molecular components play the
leading role in directing the crystal packing, while strong interaction such as H⋯H is anticipated not
to impact on the crystallization. It is worthy to mention that the C⋯H contact percentage is higher in
C1 than C2 because of the presence of phenyl groups in the skeleton of this complex.
Fig. 6. The relative contributions of different intermolecular interactions to the Hirshfeld surface area
in L4, C1, and C2.
3.3. Theoretical studies
Ligands studied here have the general formula R1R2P(X)–Y–P(X)R1R2, in which Y can be NH–
(CH2)3–N–(CH2)4–N–(CH2)3–NH, NH–(CH2)2–N–(CH2)4–N, N–(CH2)4–N and connected R– groups
to P=O vary as; Ph/Ph (L1, L8, L13), PhO/PhO (L2, L9, L14), PhO/PhNH (L3, L10, L15),
PhNH/Ph(CO)NH (L7, L12), PhNH/Cl3C(CO)NH (L6) and connected R– groups to P=S are EtO/EtO
(L4, L11, L16) and MeO/MeO (L5, L17). In order to study the effectiveness of chain length and also
the effects of these various substituents on phosphoramidate and thiophosphoramidate groups, which
are the coordinating parts of the ligands, we have divided studied ligands (L1–L17) into seven
different groups (Scheme 1 and Table S1). The optimized structures of all synthesized ligands and

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complexes at the CAM-B3LYP/def2-TZVP level of theory are represented in Fig. S1.
3.3.1. Structural parameters. P=O, P=S, and P–N calculated bond lengths in optimized free ligands
are available in Table 5 (Cartesian atomic coordinates of studied structures are listed in
supplementary data). Two different kinds of nitrogen may exist in the main body of ligands, which
are named Nchain and Ncycle. Nchain is the nitrogen atom connected to the chain of carbon site, while
Ncycle is the piperazine nitrogen atom (Scheme 2).
Scheme 2. Two different kinds of nitrogen in studied ligands.
According to Table 5, P=O, P=S, and P–N calculated bond lengths are in the range of 1.460–1.477
Å, 1.918–1.929 Å and 1.626–1.674 Å, respectively. Among bisphosphoramidate ligands L1, L8 and
L13, in which R1 and R2 are phenyl groups, have the most extended P=O and P–N bond lengths.
These parameters reach their shortest values in the case of phenoxy substituted ligands (L2, L9, and
L14). In all bisphosphoramidate ligands, calculated P=O bond lengths are relatively equal in both
Ncycle and Nchain, while P–N bonds are longer in Ncycle than Nchain. Obtained values also demonstrate
that the length of the chain (e.g., between L1, L8, and L13) has a negligible effect on P=O bond
lengths.
Table 5. P–N and P=O/P=S bond lengths (Å) in optimized free ligands Ln (n=1–17) at CAM-
B3LYP/def2-TZVP level of theory.
Ligand
No.
d(P–N)/Å
Nchain
d(P=O/P=S)/Å
Nchain Ncycle
1
2
Ncycle
L1
L8
L13
1.666
1.664
–
–
1.476
1.477 1.477
–
–
Bisphosphoramidate
1.672
1.674
1.477
L2
L9
L14
1.626
1.627
–
–
1.460
1.460 1.463
–
–
1.642
1.630
1.460
L3
1.635
–
1.462
–

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L10
L15
1.636
–
1.650
1.637
1.462 1.464
–
1.462
L4
L11
L16
1.643
1.645
–
–
1.920
1.920 1.929
–
–
Bisthiophosphoramidate
1.653
1.654
1.928
L5
L17
1.641
–
–
1.918
–
–
1.653
1.926
L7
L12
1.633
1.634
–
1.462
1.462 1.465
–
Bisphosphoramidate
1.640
L6
1.635
–
1.464
–
1, 2
Nchain and Ncycle refer to two different kinds of nitrogen atoms which exist in studied ligands and
investigated parameters are related to that kind of nitrogen.
In bisthiophosphoramidate, calculated P=S and P–N bond lengths in L4, L11, and L16, in which R1
and R2 are ethoxy groups, are a bit longer than that in methoxy substituted ligands (L5 and L17). Both
ethoxy and methoxy electron donating groups elongate the P=S and P–N bond lengths, but the effect
in Ncycle site is more than that in Nchain. Overall, the length of the chain does not make any tangible
change in P=O/P=S and P–N bond lengths.
3.3.2. NBO analysis. Investigation on phosphorus interaction with oxygen and sulfur have been
done by NBO analysis to survey the character of mentioned bonds, which is an essential factor in
coordination behavior of these ligands to a metal cation.
3.3.2.1. Bond hybridization. The hybridization of P, O, and S in related P=O and P=S bonds are
illustrated in Table 6. In the case of bisphosphoramidates, the p character of O decreases slightly
from Ph/Ph substituted ligands (≈ sp 1.60d 0.02) to PhO/PhNH (≈ sp 1.55d 0.02) and PhO/PhO ones (≈ sp
1.51d 0.02). The p character of the phosphorus atom in L2, L9, L14 and L3, L10, L15 ligands is also
considerably smaller in comparison with other ligands. These two facts attest that the bond between
P and O in these ligands is stronger than the others. It is also apparent that there is not any practical
difference between P=O bonds which are connected to Nchain or Ncycle and the length of the chain also
doesn’t make any remarkable effect on the P=O bond strength. The hybridization of P in P–N bonds
demonstrate maximum and minimum p character in Ph/Ph and PhO/PhO substituted ligands (Table
S2) which means that both kinds of P–N bonds (Nchain and Ncycle) in L1, L8, and L13 are considerably
weaker in comparison to other ligands.

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Table 6. Hybridization of P and O/S atoms in related P=O/P=S bonds in studied ligands at CAM-
B3LYP/def2-TZVP level of theory.
No.
Hybridization
P in P=O/P=S
Nchain
O/S in P=O/P=S
Nchain
Ncycle
Ncycle
sp 2.43d 0.04
sp 2.45d 0.05
–
–
sp 1.60d 0.02
sp 1.60d 0.02
–
–
L1
L8
L13
sp 2.47d 0.05
sp 2.47d 0.05
sp 1.60d 0.02
sp 1.58d 0.02
sp 1.93d 0.04
sp 1.93d 0.04
–
–
sp 1.51d 0.02
sp 1.50d 0.02
–
–
L2
L9
L14
sp 1.89d 0.04
sp 1.86d 0.04
sp 1.48d 0.02
sp 1.48d 0.02
sp 1.96d 0.04
sp 1.97d 0.04
–
–
sp 1.55d 0.02
sp 1.54d 0.02
–
–
L3
L10
L15
sp 1.96d 0.04
sp 2.00d 0.04
sp 1.55d 0.02
sp 1.53d 0.02
sp 2.47d 0.55
sp 7.92d 5.30
sp 2.48d 0.57
sp 7.87d 5.12
–
–
sp 4.31d 0.06
sp 1.00d 0.01
sp 4.29d 0.06
sp 1.00d 0.01
–
–
L4
sp 2.36d 0.06
sp 5.32d 0.09
sp 35.87d 0.24
sp 5.37d 0.09
sp 34.15d 0.24
L11
L16
sp 99.99d 99.99 1.33
f
sp 2.35d 0.06
sp 99.99d 99.99 1.39
f
sp 2.41d 0.49
sp 7.62d 5.35
–
–
sp 4.11d 0.06
sp 1.00d 0.01
–
–
L5
sp 2.31d 0.05
sp 5.17d 0.09
sp 32.41d 0.24
L17
sp 95.99d 97.82 1.31
f
sp 2.29d 0.04
sp 2.29d 0.04
–
sp 1.53d 0.02
sp 1.53d 0.02
–
L7
L12
sp 2.32d 0.06
sp 1.52d 0.02
sp 2.26d 0.04
–
sp 1.55d 0.02
–
L6
In bisthiophosphoramidates, the P=S bonds are composed of one sigma (σ) and one pi (π) bond. This
kind of bond has an energy less than twice that of a single bond, indicating that the stability added by
the π bond is less than the stability of a σ bond. This weakness is referring to significantly less
overlap between the component p-orbitals due to their parallel orientation. This is in contrast to σ
bonds, which form bonding orbitals directly between the nuclei of involved atoms, resulting in more
great overlap and a strong σ bond. The σ-bonding hybridization of sulfur and phosphorus atoms in
(L4, L11 and L16) show a bit more p character than L5 and L17 (see Table 6). When S is located in
Nchain site (L4, L11 and L5) σ-bonding hybridization is about sp 4.24d 0.06 (average hybridization), while
the hybridization is around sp 5.29d 0.09 in the case of Ncycle (L11, L16, and L17), but the hybridization of
P is nearly close in both Nchain and Ncycle. The π-bonding hybridization of S and P represent the same

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behavior as the σ-bonding hybridization when its compared between methoxy and ethoxy ligands,
but nearly different in Nchain and Ncycle sites. When S is connected to P–Ncycle, the hybridization is
around sp 34.14d 0.24 (average value), while this value is precisely sp 1.00d 0.01 in Nchain. In the case of P,
when it is connected to Nchain, hybridization is approximately sp 8d 5, but in Ncycle site hybridization is
almost sp99.99d99.99 1.3. By mentioned facts, the σ and π bonds would be stronger in P=S bonds
f
connected to Nchain rather than Ncycle, because the participation percentage of p, d, and f (in
phosphorus and sulfur) orbitals are higher in hybridization of P=S, connected to Ncycle rather than
Nchain
.
3.3.2.2. Wiberg bond index. Another factor in probing the bond strength is Wiberg bond index
(WBI). Wiberg was initially defined for closed-shell and semi-empirical wave functions. For
common chemical bonds, it’s value is usually very close to formal bond order [65].
Wiberg bond indices of P−N, P=O, and P=S bonds are represented in Table 7. In
bisphosphoramidate ligands, maximum WBI values of P=O and P−N bonds are obtained for L2, L9
and L14, while minimum values are related to L1, L8 and L13. The effect of the chain length is
insignificant on bond orders. To some extent, WBIs of P=O bonds are the same when they are
connected to Nchain and Ncycle sites, while in the case of P−N bonds, WBIs vary as Nchain ˃ Ncycle
.
In bisthiophosphoramidates, P=S and P−N calculated Wiberg bond indices are so close to each other,
but when Nchain and Ncycle sites are comparing, both P=S and P−N bonds demonstrate greater bond
orders in Nchain sites. As can be seen, the results of hybridization and WBI are in accordance with
each other and unanimously attest that unlike P=O, P=S and P−N bonds are stronger when connected
to Nchain
.
Table 7. Wiberg bond indices (WBI) of P−N, P=O/P=S bonds and the values of ƩE(ꢂ) from different
atoms LP (lone pair) to BD* (antibonding) P=O and P=S in studied ligands at CAM-B3LYP/def2-
TZVP level of theory.
No.
WBI
P−N
Donor-Acceptor
ꢓ
Ʃꢑ(ꢒ)
P=O/P=S
Nchain
0.812
0.816
–
Ncycle
–
0.785
0.779
Nchain
1.210
1.200
–
Ncycle
–
1.204
1.206
Nchain
14.85
14.95
–
Ncycle
–
15.08
14.86
L1
L8
L13
0.835
0.834
–
–
1.269
1.269
–
–
22.97
22.96
–
–
L2
L9
L14
0.828
0.804
1.267
1.266
23.02
24.46

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0.817
0.815
–
–
1.246
1.245
–
–
21.14
21.45
–
–
L3
L10
L15
0.792
0.799
1.242
1.244
23.07
22.78
0.823
0.822
–
–
1.592
1.593
–
–
37.79
37.63
–
–
L4
L11
L16
0.780
0.776
1.457
1.450
52.59
52.38
0.826
–
–
1.597
–
–
37.88
–
–
L5
L17
0.779
1.452
51.60
0.833
0.831
–
1.248
1.248
–
7.91
7.83
–
8.46
L7
L12
0.808
1.245
0.827
–
1.254
–
7.97
–
L6
a The values are reported in kcal.mol–1.
3.3.2.3. Perturbation theory energy analysis. The next factor which can interpret the strength of
the bond is charge transfer. This segment summarizes the second-order perturbative estimates of
donor-acceptor (bond-anti bond) interactions. This analysis is executed by examining all possible
interactions between filled (donor) Lewis-type NBOs and empty (acceptor) non-Lewis NBOs, and
estimating their energetic importance by 2nd-order perturbation theory.
As discussed before, investigations on P=O/P=S bond lengths, bond hybridizations, and bond orders
exemplified that in bisphosphoramidates, the strength of P=O bonds are relatively equal to each other
when Nchain and Ncycle sites are comparing, while in bisthiophosphoramidates, P=S bonds
demonstrated greater bond strength in Nchain sites. To authenticate mentioned results, the interactions
of different atoms lone pairs with P=O/P=S antibonds (σP=O*, σP=S* and πP=S*) are represented in
detail in Tables S3 and S4. These data accurately explain the numerical values of the charge transfer
from donor orbitals to P=O/P=S antibonds. Furthermore, total E(2) values from different lone pairs to
σP=O* in Table 7 show negligible differences between Nchain and Ncycle, but charge transfers from
available lone pairs to σP=S* and πP=S* illustrate a considerable discrepancy among Nchain and Ncycle
.
Greater values of ƩE(ꢂ)to P=S antibonds in Ncycle site attenuate the bond, while smaller values in
Nchain site reinforce the bond.
Overall, P=O bonds in bisphosphoramidate ligands, which are involved bonds in complexation with
metal cations, can be affected by vicinal groups. This investigation illustrated that electron donating
groups such as phenyl would make the P=O bond weaker, so the oxygen atom would perform a
better interaction with cations by its lone pairs. P=S bonds in bisthiophosphoramidate ligands are

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sensitive to Nchain and Ncycle sites, and when P=S is connected to Ncycle, the bond becomes weaker.
3.3.3. AIM analysis. The AIM analysis is an efficient tool to determine the presence of bond critical
points (BCPs). The most often used criteria for the existence of covalent or ionic bonding
interactions are the electron density ρ(r) and the Laplacian of the electron density ν2ρ(r) at the BCPs.
2
The Laplacian of charge density at the BCP ( ρ(BCP)) is the sum of the curvatures in the electron
density along any orthogonal coordinate axes at the BCP. The sign of 2ρ(BCP) indicates that
2
whether the charge density is locally depleted ( 2ρ(BCP) > 0) or locally concentrated ( ρ(BCP) <
0). The negative curvatures for the λ1 and λ2, i.e., dominate at the BCP, the electronic charge is
locally concentrated within the region inter atoms and leading to the formation of covalent or
polarized bonds and is characterized by large ρ(BCP) values, 2ρ(BCP) < 0, and | λ1| / λ3 > 1. On the
other hand, if the positive curvature λ3, i.e., is dominant at the BCP, the electronic density is locally
concentrated in each of the atomic basins. In this case, the interaction leads to the formation of
highly ionic bonds, hydrogen bonds (HBs) and van der Waals interactions. It is characterized by
relatively low ρ(BCP) values, 2ρ(BCP) > 0 and | λ1| / λ3 < 1. The AIM analysis was used to
determine the presence of bond critical points (BCPs) of the P=O bond in L1 and also P=O and
O···Sn4+ bonds in two synthesized complexes (Table 8). This comparison not only performs an
authentic insight into the changes of P=O bond character before and after the complexation but also
determine the nature of the O···Sn4+ bond.
Table 8. Calculated d(P=O)/Å bond lengths and main BCP parameters of L1 and related complexes
at CAM-B3LYP/def2-TZVP level.
2ρ (in au)
Complex
d(P=O)/Å
ρ (in au)
P=O
0.293
0.205
0.217
|λ1|/ λ3
P=O
0.199
0.214
0.203
Sn4+···O
–
0.045
0.031
P=O Sn4+···O
Sn4+···O
–
0.162
0.154
1.476
1.491
1.487
1.35
1.18
1.27
L1
C1
C2
–
0.201
0.191
2
Calculated AIM charge density (ρ), Laplacian ( ρ) at the P=O BCP and also at the BCP of O···Sn4+
interactions are given in Table 8. Smallish values of ρ and |λ1| / λ3 at P=O BCP in the free ligand L1
and large values of 2ρ show mostly electrostatic character for this bond. The charge density at the
P=O BCP decreases when the ligand is coordinated to the metal cation. The ρ values at the P=O BCP
in ligand is 0.293 au but decreases to 0.225 and 0.217 au in C1 and C2 complexes, which is in
accordance with the lengthening of the P=O bond after complexation. Analysis of the obtained bond
critical points for O···Sn4+ interactions, suggests a closed−shell interaction and ionic character of

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O···Sn4+. Hessian eigenvalues (λ1, λ2, and λ3) of the charge density at the main BCP of the
complexes are presented in Table 8. Very small values of ρ, 2ρ > 0 and |λ1| / λ3 = (0.162 and 0.154,
in C1 and C2) which is << 1 at the BCP of O···Sn4+ confirm the presence of ionic interactions. The
values of 2ρ at P=O BCP in both complexes are ˃˃ 0 and |λ1| / λ3 <<1, which attest that the P=O
bond still demonstrates electrostatic nature, but with a little shift to the covalent character after the
complexation.
4. Conclusions
In this study, some of the novel bisphosphoramidate and (thio)phosphoramidate derivatives (L1–L12),
as well as two organotin(IV) complexes with diphosphoryl ligand L4 have been prepared and
characterized by spectroscopic methods. Furthermore, the crystal structure of compound L4 and
complexes C1 and C2 were investigated. The crystal structures of the complexes revealed that the
Sn(IV) atoms are five-coordinated. The coordination geometry around tin(IV) center could be
described as slightly distorted trigonal bipyramidal and confirmed the binuclear structures for the
tin(IV) adducts with two trans SnPh3Cl/SnBu3Cl linked via the bridging diphosphoryl ligand. The
bidentate ligand in complexes has P=O groups in an anti conformation to each other, and the
phosphorus atom showed a tetrahedral configuration as well as for ligand L4 in which the P=S
groups are on opposite sides of the molecule. The crystal packing and the intermolecular interactions
were studied. The relative contributions of different intermolecular contacts in crystal packing L4, C1
and C2 studied by Hirshfeld surface analysis.
Theoretical investigations on the effect of chain lengths on P=O bond strength by bond lengths,
hybridization, WBI and donor-acceptor in bisphosphoramidate derivatives unanimously attested
that the chain length did not make any considerable effect on P=O bond strength in both Nchain and
Ncycle positions, but in the case of bisthiophosphoramidates, chain length influence on P=S bond
strength is remarkable, and the bond length is larger in Ncycle than Nchain. The effect of different
substituent groups in bisphosphoramidate ligands demonstrated that L1, L8, and L13 have the largest
P=O and P–N bond lengths, while L2, L9, and L14 have the shortest ones. The AIM analysis was also
showed ionic character for O···Sn4+ interaction in C1 and C2 synthesized complexes and mostly
electrostatic character for P=O bond in free ligand L1, but with a little shift to the covalent character
after the complexation.
Ethical Statement/Conflict of Interest

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The authors declare no conflict of interest.
Supplementary Material
Supplementary data associated with this article can be found in the online version.
Acknowledgment
Financial support of this work by Tarbiat Modares University is gratefully acknowledged.
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