Steric and electronic control over the structural diversity of N-(n-pyridinyl) diphenylphosphinic amides (n = 2 and 4) as difunctional ligands in triphenyltin(iv) adducts

Article abstract of DOI:10.1039/c4ra06212d

Two triphenyltin(iv) adducts of difunctional ligands, N-(n-pyridinyl) diphenylphosphinic amide (n = 2 and 4), have been synthesized and characterized by ¹H, ³¹P, ¹¹⁹Sn NMR and IR spectroscopy. The spectroscopic properties of the complexes were compared with those of corresponding ligands. The crystal structures of the complexes were determined by X-ray crystallography, which reveals a trigonal bipyramidal geometry surrounding the tin(iv). Both of the ligands function in an ambidentate mode, ligating through either the O or N atom. The experimental and theoretical (DFT) studies show that the Sn(iv) interacts more strongly with the N-pyridine atom than the PO functional group. Furthermore, DFT calculations, at the B3LYP level, have been carried out to determine the deeper reasons for the adopted bonding mode in the complexes. The influence of the ligand structure on the coordination behaviour and the contribution of hydrogen bonding to the stability of the resulting complexes were elucidated. The results indicate that the intermolecular hydrogen bonds have an important role in the molecular structures and supramolecular associations of the organotin(iv) compounds. This journal is

Full text of DOI:10.1039/c4ra06212d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Steric and electronic control over the structural
diversity of N-(n-pyridinyl) diphenylphosphinic
amides (n ¼ 2 and 4) as difunctional ligands in
triphenyltin(IV) adducts†
Cite this: RSC Adv., 2014, 4, 44509
K. Gholivand, a A. Gholami,a S. K. Tizhoush,a K. J. Schenk,b F. Fadaeib and A. Bahramia
*
Two triphenyltin(IV) adducts of difunctional ligands, N-(n-pyridinyl) diphenylphosphinic amide (n ¼ 2 and 4),
1
have been synthesized and characterized by H, 31P, 119Sn NMR and IR spectroscopy. The spectroscopic
properties of the complexes were compared with those of corresponding ligands. The crystal structures
of the complexes were determined by X-ray crystallography, which reveals a trigonal bipyramidal
geometry surrounding the tin(IV). Both of the ligands function in an ambidentate mode, ligating through
either the O or N atom. The experimental and theoretical (DFT) studies show that the Sn(IV) interacts
more strongly with the N-pyridine atom than the P]O functional group. Furthermore, DFT calculations,
at the B3LYP level, have been carried out to determine the deeper reasons for the adopted bonding
mode in the complexes. The influence of the ligand structure on the coordination behaviour and the
contribution of hydrogen bonding to the stability of the resulting complexes were elucidated. The results
indicate that the intermolecular hydrogen bonds have an important role in the molecular structures and
supramolecular associations of the organotin(IV) compounds.
Received 25th June 2014
Accepted 9th September 2014
DOI: 10.1039/c4ra06212d
www.rsc.org/advances
catalytic and biological activity.16–21 Studies on organotin(IV)
derivatives containing mixed N,O-ligands have revealed
Introduction
considerable structural diversity which may lead to complexes
with different properties.22–24 However, the basis of their selec-
tivity remains poorly understood.
The development of well-designed ligand systems with which
the properties of their complexes can be easily varied in a
controlled manner is one of the most important goals in
modern inorganic and organometallic chemistry. Among the
many ligand systems in the literature, heterodifunctional
ligands1–8 are intensively studied and applied owing to the
oen unique properties of their metal complexes. The coor-
dination chemistry of such ligands, capable to realize
different binding modes with metal centers,9 is interesting
for synthesis of new selective complexing agents and
analytical reagents.
In this regard, our group has made considerable effort to
study the ligation behavior of phosphoramidate ligands con-
taining additional donor sites such as carbonyl or/and N-pyri-
dine particularly toward organotin(IV) compounds.10–15 Much of
the interest in organotin(IV) complexes arises from their
In the present work, we consider phosphoramidate ligands
based on aminopyridine with different distance between the
donor sites of phosphoryl and N-pyridine. The main reasons for
the selection of these ligands are the different nature of the two
coordinating sites as well as various positions of them relative
to each other that creates variety of potential coordination
modes. From the structural chemistry aspect it is interesting to
give reasons for the type of adopted bonding mode. We
approach the problem by a combined theoretical-practical point
of view.
Accordingly, we have carried out a comparative study on
the triphenyltin(IV) adducts of N-(n-pyridinyl) diphenyl-
phosphinic amides (where n ¼ 2 and 4 in ligand L1 and L2,
respectively). Crystal structures of the complexes C1 and C2
were determined by X-ray crystallography, which revealed
the different coordination behaviour of the two ligands. The
bonding, electronic and energy aspects of all compounds
were considered using quantum mechanical calculations.
Furthermore, the inuence of the ligand structure on the
stereoselectivity of bond formation and the contribution of
hydrogen bonds in the structure and directing the crystal
packing of complexes were elucidated.
aDepartment of Chemistry, Faculty of Basic Sciences, Tarbiat Modares University, P.O.
Box 14115-175, Tehran, Iran. E-mail: gholi_kh@modares.ac.ir; Fax: +98-021-
88009730; Tel: +98-21-82884422
b
´
´ ´
Ecole Polytechnique Federale de Lausanne, CCC-IPSB, Le Cubotron Dorigny, 1015
Lausanne, Switzerland
† Electronic supplementary information (ESI) available: The crystal data and the
details of the X-ray analysis, selected bond lengths and bond angles. CCDC
978396 (C1), 978395 (C2). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ra06212d
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 44509–44516 | 44509

View Article Online
RSC Advances
Paper
Results and discussion
Synthesis and spectroscopic characterization
Various molar ratios of ligand: SnPh3Cl were assessed in the
crystallization, since we were not certain which coordination
modes would prevail. But evaporation from 1 : 1 solutions of
HCCl3: heptane, at room temperature, yielded only two crystals
suitable for X-ray diffraction. For the ligand L1, the ve-
coordinated complex C1 grew from a 1 : 1 ratio, and for the
ligand L2, the ratio 1 : 2 afforded the ve-coordinated mono-
nuclear complex C2. Therefore, we may conjecture that the
formation of C2 is independent of the molar ratio, but is rather Fig. 1 The molecular structure of C1 with its atom (50% probability
level) labelling scheme. Hydrogen atoms were omitted for clarity.
driven by the steric and electronic features of the ligand L2.
Selected spectroscopic data of organotin compounds and their
corresponding ligands are listed in Table 1.
complexes. These data don't provide any concrete information
about the tin-ligand coordination. The single resonance in the
119SnNMR spectrum of C1 at ꢀ50 ppm is in agreement with the
reported value for free SnPh3Cl (ꢀ44.7),27 indicating that the
triphenyltin chloride complex C1 is kinetically labile on the
119SnNMR time scale. This is consistent with other triphenylti-
n(IV) adducts which are not stable in solution.28 Although, the
119Sn resonance for C2 shis to ꢀ74 ppm, such chemical shi is
not still in the range for the ve-coordinated tin compounds
(from ꢀ90 to ꢀ190 ppm).29 This might be a hint to an equilib-
rium between the adduct Ph3SnCl$ligand and Ph3SnCl + ligand
that, at ambient temperature, is fast on the NMR time scale.28
Besides, the 119Sn chemical shi, in comparison with the
shi of pure Ph3SnCl, points to the relative population of free
Ph3SnCl and its complex in the equilibrium. Comparing the
119SnNMR spectrum of two complexes reveals that the Sn–L
interaction is stronger for C2 (through the Npy donor) in solu-
tion, in accordance with the result from solid-state structures
and gas phase computations.
The coordination mode of the ligands can be reliably fol-
lowed by the IR spectroscopy. The considerable negative shi of
the y(P]O) in the spectra of C1 by 24 cmꢀ1 with respect to the free
ligand, demonstrates the coordination of the phosphoryl
group to tin like in many other complexes of phosphor-
amidates.10–15,25,26 The y(py ring) of the complex C1 (1591 cmꢀ1) do
not differ from the frequencies for the free ligand. Interestingly,
an opposite shi was observed for the P]O frequency of C2 with
respect to the free ligand L2. The main change in the spectra of
C2 deal with the increasing of y(py ring) band by 19 cmꢀ1 while the
y(P]O) value is close to that of the free ligand. This observation
can be explained by the coordinating from Npy site to Sn in this
complex. The same results are also, evident from the X-ray
crystallographic structures for C1 and C2 (Fig. 1 and 3).
The absorption band at 3117 cmꢀ1 in L1 is attributed to the
N–H stretching mode which shis toward higher frequency in
C1. This positive shi may be due to the weakening of the
hydrogen bonds from N–H/OP]O in L1, which is a common
hydrogen bonding in the phosphoramide ligands,10–15 to N–H/Npy
in the coordinated species C1. Although the N–H groups of L2
and C2 are involved in the same intermolecular interactions,
N–H/OP]O, the position of this bond in these compounds also
indicates the weaker hydrogen bonding in the structure of C2
relative to the ligand L2.
X-ray crystallography investigation
Colorless single crystals of both complexes were obtained from
solutions of chloroform–heptane by slow evaporation at room
temperature. Crystal data and details of the X-ray analysis are
given in Table S1.† Selected bond lengths and angles are
summarized in Tables S2 and S3† and hydrogen bonds in
Table 2. Molecular structures and packing diagrams are shown
in Fig. 1–4.
1
The HNMR spectral data obtained for the ligands suggest
the expected structure of these molecules. However, the chem-
ical shis and the 1H-coupling constants of ligands are almost
insensitive to complexation. Furthermore, chemical shi of 31
P
for the ligands are quite similar to those of the corresponding
Crystal structure of N-(2-pyridinyl)diphenylephosphinic
amide-k-O chlorotriphenyltin(IV) (C1)
The title complex crystallizes in the space group C2/n with Z0 ¼ 2.
In each conformer (C1 and C10, Fig. 1), the ligand binds in a
unidentate manner through the phosphoryl donor. The coor-
dination polyhedra of tin can be described as distorted trigonal
bipyramid (three phenyls equatorial, chlorine and the phos-
phoryl at the apices). The trans angles around tin are Cl1–Sn1–
O1 ¼ 178.21(5)ꢁ and Cl2–Sn2–O2 ¼ 176.99(4)ꢁ, and the sum of
angles in the trigonal girdle around Snare 358.66ꢁ (C1) and
358.16ꢁ (C10). Bonding parameters agree with values found for
similar geometries.10,30,31 As expected, the shorter Sn–O distance
Table 1 Spectroscopic data of compounds
y(N–H)
(cmꢀ1
y(P]O)
(cmꢀ1
y(py ring)
(cmꢀ1
d(31P)
(ppm)
d(119Sn)
(ppm)
Compound
)
)
)
Ligand
L1
L2
3117
3203
1185
1182
1591
1595
18.8
19.5
—
—
Complex
C1
C2
3442
3440
1161
1187
1591
1614
18.8
19.6
ꢀ50
ꢀ74
44510 | RSC Adv., 2014, 4, 44509–44516
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Table 2 Hydrogen-bonding dataa for the X-ray structures
Compound
D–H–A
d(D–H)
d(H/A)
d(D/A)
<DHA
C1
C2
N(2)–H(200)/N(3)b(i)
N(4)–H(400)/N(1)b(ii)
N(2)–H(22)/O(2)b(iii)
N(4)–H(44)/O(1)b(iv)
0.78 [1.022]
0.74 [1.027]
0.86 [1.025]
0.86 [1.019]
2.57 [2.329]
2.29 [2.006]
2.04 [1.824]
2.19 [1.848]
3.324(3)
3.004(3)
2.835(4)
2.802(4)
163 [163.20]
162 [163.94]
153 [168.21]
128 [154.27]
a
The values in the brackets refer to the calculated parameters at the B3LYP/LANL2DZ/6-311G* level. b Symmetry codes: (i) [x, ꢀ1 + y, ꢀ1 + z]; (ii) [x, 1
+ y, 1 + z]; (iii) [ꢀ1 + x, y, ꢀ1 + z]; (iv) [x, y, 1 + z].
˚
(in C1: 2.4031(14) A) corresponds to the larger P–O–Sn angle (in the structure include: (i) intermolecular hydrogen bonding
C1: 154.60(9)ꢁ). Beside, the Sn–O bond shortening is also including C–H/Cl (with C/Cl distances 3.681(11) and 3.767(4) A)
˚
˚
accompanied by an increase in the Sn–Cl bond length (2.4820(6) and (ii) C–H/p interactions (C5–H5/Cg: dH/Cg ¼ 3.08 A,
ꢁ
˚
˚
A in C1) because of the greater trans inuence.
dC/Cg ¼ 3.927(3) A, q ¼ 152 and C76A–H76A/ Cg: dH/Cg ¼
C1 and C10 are joined to dimers by means of weak N–H/Npy 2.84 A, dC/Cg ¼ 3.72(2) A, q ¼ 159 : Cg is the centroid of py rings
hydrogen bonds between the amidic groups and the pyridine and q is the angle of C–H/Cg). These intermolecular interac-
rings (Table 2). Other weak interactions susceptible to stabilize tions generate a 3D structure in the crystal (Fig. 2).
ꢁ
˚
˚
Crystal structure of N-(4-pyridinyl)diphenylephosphinic
amide-k-N chlorotriphenyltin(IV). N-(4-pyridinyl)
diphenylphosphinic amide (C2)
ꢀ
The compound C2 crystallizes in the space group P1. Its struc-
ture consists of an uncoordinated ligand and a tin adduct held
together by hydrogen bonds. The ligand L2 coordinates in a
monodentate mode, from the nitrogen site of the pyridine ring
toward the tin atom. The central tin atom (Fig. 3) lies in a dis-
torted trigonal bipyramide formed by three phenyl groups (Sn–
˚
C(16), 2.125(3); Sn–C(22), 2.135(3) and Sn–C(28) 2.125(3) A) in
˚
the equatorial plane, one chloride ion (Sn–Cl, 2.5029(9) A) and
˚
the N-pyridine atom (Sn–Npy, 2.553(3) A) in the apical (trans)
positions. The sum of the angles subtended at tin in the trigonal
girdle is 358.8ꢁ, and the trans angle presents little deviation
from linearity (Cl–Sn–Npy 175.82(7)ꢁ). The Sn–C and Sn–Cl bond
lengths are in the range of the covalent band32,33 while the Sn–N
bond is in accordance with the coordinate bond.34 It is worthy to
note that similar bonding parameters of the coordinated and
free ligand present in the crystal C2 do not differ signicantly.
In the structure of C2, moderately strong N-Hamidic/OP¼O
hydrogen bonds (Table 2) build up alternating [100] chains of
tin adducts and free ligands; which are further reinforced by
weaker C–H/Cl hydrogen bonds between a phenyl group, H53,
of the ligand and the chlorine atom on the tin complex. The
same kind of H-bond is also the driving force in generating a 3D
Fig. 2 (a) Dimeric tin aggregate, showing intermolecular hydrogen
bonds between two conformers and (b) packing diagram of C1 formed Fig. 3 The molecular structure of C2 with its atom (50% probability
by supramolecular interactions.
level) labelling scheme. Hydrogen atoms were omitted for clarity.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 44509–44516 | 44511

View Article Online
RSC Advances
Paper
network (Fig. 4); indeed, tin adducts of neighbouring chains are previously.22,30 The most striking feature of the compound C2 is
linked via C–H/Cl interactions between hydrogen atoms of behaviour of the ligand which is unusual in not coordinating to
phenyl rings connected to the chlorine atom of tin adduct (H3 tin, alternately. Adjacent coordinated and uncoordinated
creates a [010] chain, and H9 a [001] one). The donor–acceptor ligands are linked via effective intermolecular N–Hamidic/OP]O
˚
distances of 3.664(5) and 3.717(4) A are in the expected region interactions.
reported for this type of hydrogen bonds.23,35 It is nally worth
As a result of different binding modes in complexes,
noting that the pair of neighbouring free ligands is distin- molecular associations are different in their crystal lattices. 2-
guished by a doublet of parallel CH/p interactionsꢁ (C65– Fold linkages of neighboring conformers C1 and C10 via inter-
˚
˚
H65/Cg: dH/Cg ¼ 3.33 A, dC/Cg ¼ 3.914(5) A, q ¼ 123.07 : Cg is molecular contacts, giving rise to the formation of dimeric tin
the centroid of py ring).
aggregates, while one-dimensional chains in C2 are in conse-
quence of hydrogen bonds between the phosphoryl and amidic
groups which are anti with respect to each other (Fig. 2a and 4a).
Comparison of the structures for complexes C1 and C2
Both of N-(pyridinyl)diphenylephosphinic amides L1 and L2
function as ambidentate ligands, ligating through either the O
or N atom. The ligand L1 coordinates via phosphoryl group in
the structure C1, while the other one acts as an N-donor in the
crystal C2. There is a chlorine atom in the trans position of each
coordinating atom and Sn–Cl distances are in the reverse
correlation with the binding strength of the donors. The
lengthening of Sn–Cl bond in C2 indicates a higher donating
ability of Npy with respect to Op (trans inuence).
Computational study
The optimized structures of ligands and resulting complexes
are represented in Fig. S1† and some selected bond lengths for
fully optimized geometries given in Table 3. It should be
mentioned that the corresponding distances in hydrogen-
bonded clusters are equal to the experimental values, due to
the freezing of non-hydrogen atoms in the calculations. In order
to evaluate the electronic and energy aspects of compounds, the
NBO calculations were performed and the results are shown in
Table 4. Moreover, electron delocalization energies, E2, from
the NBO analysis and binding energies, DE, of some notable
interactions including coordination and hydrogen bonds
(Table 5) are calculated to characterize the strength of these
donor–acceptor interactions. The former refers to stabilization
energy of electron delocalization between the donor–acceptor
orbitals, while the binding energy is related to the sum of total
attractive and repulsive forces between two bonded fragments.
Since the Sn–N bond is stronger than the Sn–O one, it was
expected that the nitrogen atom coordinated to tin rather than
the phosphryl oxygen. However, it seems that coordination of
N
py is inhibited in C1, owing to a steric factor. The ambidentate
manner of L1 instead of the chelating arrangement in question
may be also caused by steric hindrances due to the presence of
three bulky phenyl groups on tin. The steric control over the
molecular structure of organotin compounds have been noticed
Electronic parameters of compounds
As seen in Tables 3 and 4, from structural and electronic points
of view, the fully optimized ligands L1 and L2 are very close to
each other and in a good agreement with free ligand in the
0
cluster C2 (L2 ). However, in the ortho-pyridinyl phosphor-
amidate L1, the negative charge localized on NPy atom, q(NPy), is
larger in magnitude and also the hybridization of the lone pair
of this atom, LP(NPy), takes a more p character. The charge
transfer from non-bonding orbital of the Namidic atom to anti-
bonding orbital C–Npy in the ligand L1 is responsible for the
different electronic features of L1 and L2. The NBO analysis
shows the stabilizing energy E2 of 37.04 kcal molꢀ1 for the
electronic delocalization LP(Namidic) / s*(C–Npy). Beside, the
Table 3 Striking calculated bond lengths (d, A˚) at B3LYP/6-311G*-
LANL2DZ level for the optimized geometries
Compound
L1
L2 (L20 )
Binuclear model
d(P]O)
1.495
1.701
1.011
—
—
—
1.494 (1.484)
1.710 (1.657)
1.010 (1.019)
—
—
—
—
1.503
1.706
1.010
2.496
2.674
2.459
2.466
d(P–Namide
d(H–Namide
d(Sn–OP]O
d(Sn–Npy
d(Sn–Cltrans to O
d(Sn–Cltrans to N
)
)
)
)
Fig. 4 (a) 1D polymeric chain along a-axis, showing intermolecular
hydrogen bonds and (b) packing diagram of C2 formed by supramo-
lecular interactions.
)
)
—
44512 | RSC Adv., 2014, 4, 44509–44516
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Table 4 Atomic charges, natural electron configurations, hybridizations at B3LYP/6-311+G*-LANL2DZ level (from the NBO analysis)
Atomic Charge
NEC
Hybridization
Compound
q(OP]O
)
q(Npy
)
q(Sn)
OP¼O
Npy
LP(OP]O
)
LP(Npy)
Free Ligands
L1
L2
ꢀ1.067
ꢀ1.062
ꢀ1.096
ꢀ0.502
ꢀ0.478
ꢀ0.460
—
—
—
[Core] 2s1.80 2p5.25
[Core] 2s1.80 2p5.25
[Core] 2s1.80 2p5.26
[Core] 2s1.36 2p4.12 3d0.01 4p0.01
[Core] 2s1.38 2p4.08 3p0.01 3d0.01 4p0.01
[Core] 2s1.36 2p4.33 3p0.01 d0.01
sp0.51
sp0.52
sp0.56
sp2.54
sp2.39
sp2.34
0
L2
Real complexes
C1
C2
a
ꢀ1.134
ꢀ1.122
ꢀ0.526
ꢀ0.547
2.091
2.119
[Core] 2s1.76 2p5.36 3p0.01
[Core] 2s1.79 2p5.33
[Core] 2s1.36 2p4.15 3p0.01 3d0.01 4p0.01
[Core] 2s1.35 2p4.16 3s0.01 3p0.01 4p0.01
sp0.74
sp0.60
sp2.53
sp2.55
Model complex
Binuclear
ꢀ1.114
ꢀ0.542
2.160
[Core] 2s1.77 2p5.33 3p0.01
[Core] 2s1.36 2p4.16 3p0.01 4p0.01
sp0.58
sp2.58
a
The mean values are represented for two conformers.
Table 5 D–A distances, E2 (Electron delocalization energies (kcal molꢀ1)) and DE (binding energies (kcal molꢀ1))
Sn–OP]O
Sn–Npy
P]O/HNamide
N
py/HNamide
Compound
d
E2a
DE
d
E2a
DE
d
E2b
DE
d
E2b
DE
C1
C2
2.403
2.463
—
26.28
22.20
—
12.14
11.78
—
—
—
—
—
—
—
3.323
3.004
—
5.02
13.66
—
4.88
—
2.553
2.674
34.64
27.76
12.43
8.82
2.801
2.836
—
10.80
10.24
—
8.23
5.05
—
Binuclear model
2.496
25.89
8.78
—
—
—
a
b
LP(1) donor to LP* Sn. LP(1) donor to s* HNamid
.
intermolecular hydrogen bonding in the H-bonded cluster C2
explains the more q(OP]O) for the uncoordinated ligand L20,
compared to the free ligand L2. Similarly, along with increase in
negative charge on the oxygen atom, hybridization of LP(OP]O),
sp0.56, takes the more p character in L20 versus sp0.52 in L2. The
electronic charge is transferred from the neighbors to the O
atom involved in hydrogen bonding due to the polarization
effect.
This electronic redistribution occurs even more when the
ligand is coordinated to SnPh3Cl, as the polarization effect
arises from the electrostatic eld of the Sn(IV) atom. Natural
electronic conguration (NEC) of the OP]O atom is [core] 2s1.80
2p5.25 in the free ligands that almost changes to [core] 2s1.76
2p5.36 3p0.01 by coordinating to the tin in C1. In the same way,
electronic population in 2p orbital of the nitrogen increases
from the free ligand L2 to the complex C2. The hybridization of
the lone pair of the mentioned atoms, as expected, is affected
upon complexation (Table 4). Furthermore, the atomic charges
of donor sites become more negative when the ligand is coor-
dinated. It is worth noting that the negative charge difference
(Ddꢀ ¼ dꢀ(donor atom)complex ꢀ dꢀ(donor atom)free ligand) is
approximately equal for both the donors (0.067e for OP]O and
0.069e for Npy). The more value of Ddꢀ (0.087e) is obtained for
Npy using dꢀ(Npy)free ligand of L20 instead of L2. Other electronic
differences between the ligands and the corresponding adducts
can be viewed as a result of the intermolecular hydrogen bonds.
Structural preference of the complexes with the difunctional
ligands
As mentioned in the previous part, the electronic properties of
the ligands have not been signicantly affected by the difference
in the structures. This key result indicates that any differences
between the C1 and C2 structures are not related to the elec-
tronic features of the ligands. In return, from the structural
point of view, position of nitrogen in the pyridine ring on the
one hand and the tendency of SnPh3Cl to produce a trigonal-
bipyramidal stereochemistry on the other is of prime impor-
tance for the association. In the case of C1, coordination of the
phosphoryl donor instead of N-pyridine which can be explained
by steric hindrance, regardless of electronic priority (see section
X-ray crystallography investigation).
Interestingly, the low preference of L2 for bidentate binding,
despite the lack of steric demands, may be attributed to
competition between two types of possible interactions for
the P(O)NH functional group. It seems that the intermolecular
P]O/HNamidic interactions would be preferable to tin–oxygen
coordination for the association in L2. To verify the assumption
made above, we have carried out a comparative study on the real
complex C2 and a model binuclear complex in which the biden-
tate ligand L2 adopts the bridging mode of coordination. As given
in Table 5, the sum of two P]O/HNamidic hydrogen-binding
energies (13.28 kcal molꢀ1) in C2 is larger than the energy gain
from Sn–O interaction in the model (8.78 kcal molꢀ1). This shows
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 44509–44516 | 44513

View Article Online
RSC Advances
Paper
that the monodentate binding mode of L2 is preferred, although lengthening of the N–H distances in the H-bonded clusters with
the difference of DE with the bridging mode is small (4.50 kcal respect to the fully optimized ligands. Table 5 shows that the
molꢀ1). It should be stressed here that the highest occupied stabilizing energy increases from the LP(Npy) / s*(N–H)
molecular orbital, HOMO, of the ligand L1 is localized on the delocalization (5.02 and 13.66 kcal molꢀ1) to LP(OP]O) /
N-pyridine atom, while the HOMO of L2 is located on the oxygen. s*(N–H) (10.80 and 10.24 kcal molꢀ1), in line with a decrease in
Both of the mentioned atoms are involved in hydrogen bonding in the donor–acceptor distance. Coincident with this, the values of
the related complexes. It suggests that the coordinating modes 8.23 and 5.05 kcal molꢀ1 for binding energies were calculated
adopted in both complexes are not only the result of stereo- for the OP]O/H–N hydrogen bonds, while the DE value of 4.88
electronic features of the donor sites but also of the inuence of kcal molꢀ1 is related to the sum of two Npy/H–N interactions in
H-bonding interactions in the ligands.
the cluster C1.
The substantial contribution of the hydrogen bonds around
5–8 kcal molꢀ1 towards the stability of the resulting complexes
makes these an essential factor for the interesting coordination
behavior of the ligands. The inuence of H-bonding on ligation
manner of multifunctional compounds has also been realized
previously.3,30
Comparison between the coordination ability of the P]O and
Npy functional groups
The NBO analysis of both ligands L1 and L2 shows that the
hybridization of LP(NPy) possesses a remarkable more p char-
acter in comparison with LP(OP]O). Furthermore, the natural
electronic conguration of OP]O includes more electronic
population in the electronegative 2s orbital (2s1.8) compared to
the case of Npy (2s1.36). The obtained results reveal a higher
electron availability and thus the higher basic potential of the
nitrogen atom than the phosphoryl oxygen atom. In contrast,
the negative charge on OP]O atom is approximately two times
higher than that of NPy, and makes the phosphoryl oxygen more
suitable for the electrostatic interaction with the positive
charged tin(IV) atom (about +2.1e). Finally, the inuential elec-
tronic factor which determines the winner site depends on the
type of substituents on the P atom.10 In other words, the donor
character of the P]O group depends on the electron donation
ability of substituents, bounded to the phosphorus, and the
magnitude of q(OP]O). In the following, we will show that the
former is more effective in the title ligands including two phenyl
groups on the P atom.
Experimental
Synthesis of ligands
The ligands were synthesized by the reaction of n-amino-
pyridine (n ¼ 2 and 4 respectively) with (C6H5)2P(O)Cl in 2 : 1
molar ratio. The amine was added dropwise to a CH3CN solu-
tion (20 ml) of (C6H5)2P(O)Cl at 0 ꢁC. Aer 24 h, the solvent was
evaporated and the residue was washed with distilled water and
dried. Physical and spectroscopic data of them are presented
below:
N-(2-pyridinyl)diphenylphosphinic amide (L1)..8 Yield: 85%.
m.p. 176 ꢁC. Anal. calc. for C17H15N2OP : C 69.38, H 5.13, N
9.52; found: C, 69.16; H, 5.21; N, 9.26.31P NMR (121.49 MHz,
CDCl3): d (ppm) ¼ 18.8. 1HNMR (300.13 MHz, CDCl3): d (ppm) ¼
3
6
6.77 (ddd, 3JHH ¼ 7.3 Hz, JHH ¼ 5.0 Hz, JPH ¼ 0.8 Hz, 1H-py),
3
Here the size of stabilizing energies E2 for the electronic
delocalization LP(donor) to LP*(Sn) can be used to characterize
the strength of the donor–acceptor coordination interactions.
Comparing the stabilizing energy of coordination bond in C1
(22.2. and 26.28 kcal molꢀ1) and C2 (34.64 kcal molꢀ1), it is
found that the Sn–Npy is slightly stronger than the Sn–OP]O
bond. This is also evident from Sn–Cl bond lengths in the model
complex (Table 3). The longer distance belonging to the chlo-
rine atom in trans position to Npy owing to the greater trans
inuence. Furthermore, the expected trend obtained for the
stabilizing energies in the binuclear compound, although the
difference is not very signicant (Table 5). The higher electron
donating capacity of Npy compared to OP]O is conrmed by the
experimental results.
6.99 (d, JHH ¼ 8.3 Hz, 1H-py), 7.39–7.55 (m, 6H-Ph, 1H-py),
3
3
7.90 (dd, JHH ¼ 6.8 Hz, JPH ¼ 12.6 Hz, 4H; o-Ph), 7.98
(d, 3JHH ¼ 5.0 Hz, 1H-py). IR (KBr, cmꢀ1): n ¼ 3117 (m, NH), 1591
(s, py ring), 1185 (vs., P]O).
N-(4-pyridinyl)diphenylphosphinic amide (L2). Yield: 72%.
m.p. 193 ꢁC. Anal. calc. for C17H15N2OP : C 69.38, H 5.13, N
9.52; found: C, 69.14; H, 5.19; N, 9.36%. 31P NMR (121.49 MHz,
CDCl3): d (ppm) ¼ 19.5. 1H NMR (300.13 MHz, CDCl3): d (ppm)
¼ 6.85 (d, 3JHH ¼ 6.2 Hz, 2H-py), 7.44 (td, 3JHH ¼ 6.1 Hz, 4JPH
¼
3
3.4 Hz, 4H; m-Ph), 7.53 (t, JHH ¼ 7.2 Hz, 2H; p-Ph), 7.81 (dd,
3JHH ¼ 7.0 Hz, 3JPH ¼ 12. 7 Hz, 4H; o-Ph), 8.14 (d, 3JHH ¼ 6.1 Hz,
2H-py). IR (KBr, cmꢀ1): n ¼ 3203 (s, NH), 1595 (s, py ring), 1182
(vs., P]O).
Synthesis of complexes
Hydrogen bonding
N-(2-pyridinyl)diphenylephosphinic
amide-k-O
chloro-
While the donor–acceptor distances of H-bonds in the triphenyltin(IV) (C1). 0.1 mmol triphenyltinchloride (0.04 g) was
hydrogen-optimized clusters are the same experimental values, added to a solution of 0.1 mmol ligand 1 (0.03 g) in 10 ml
the other optimized parameters differ from the X-ray values chloroform and the mixture stirred for some hours. The single
(Table 2). The DFT calculations yield the shorter DH/A crystals suitable for X-ray analysis were isolated by slow evapo-
distances compared to the solid state structures.
ration of a 1 : 1 solution chloroform–heptan at room tempera-
ꢁ
The NBO analysis reveals an electronic delocalization LP(A) ture. Yield: 85%. m.p: 150 C. Anal. calc. for C35H30ClN2OPSn
/ s*(N–H) (A is Npy in C1 and OP]O in C2) among the subunits (679.77): C, 61.84; H, 4.44; N, 4.12. Found: C, 61.79; H, 4.65; N,
of the clusters. Such an electronic density transfer explains the 4.14%. 31P NMR (121.49 MHz, CDCl3): d (ppm) ¼ 18.8. 1H NMR
44514 | RSC Adv., 2014, 4, 44509–44516
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
(300.13 MHz, CDCl3): d (ppm) ¼ 6.77 (dd, 3JHH ¼ 7.3 Hz, 3JHH
¼
points for the geometrical calculation. For the structure C1 two
5.1 Hz, 1H-py), 6.94 (d, 3JHH ¼ 8.3 Hz, 1H-py), 7.36–7.70 (m, 22H; hydrogen-bonded conformers were used in the calculations,
H-Ph, py), 7.87 (dd, 3JHH ¼ 8.1 Hz, 3JPH ¼ 12.6 Hz, 4H; o-Ph), 7.98 while the structure C2 was modelled as a cluster in which the
3
(d, JHH ¼ 4.5 Hz, 1H-py). 119Sn NMR (111.86 MHz, CDCl3): d target complex is surrounded by two neighbouring uncoordi-
(ppm) ¼ ꢀ50. IR (KBr, cmꢀ1): n ¼ 3442 (s, NH), 1591 (s, py ring), nated ligands. Although normalized hydrogen positions from
1161 (vs, P]O).
Mo-data are also quite reliable, we preferred to optimize the
N-(4-pyridinyl)diphenylephosphinic
amide-k-N
chloro- hydrogen atoms, and kept all other atoms invariant in the
triphenyltin(IV). N-(4-pyridinyl)diphenylphosphinic amide (C2). optimization process. We denote these structures as hydrogen-
0.2 mmol triphenyltinchloride (0.04 g) was added to a solution optimized systems. Moreover, full geometrical optimizations of
of 0.1 mmol ligand 3 (0.03 g) in 10 ml chloroform and the the ligands and a binuclear model (wherein the ligand L2 is
mixture stirred for some hours. The suitable crystals for X-ray bridged between two SnPh3Cl moieties) were computed. The
analysis were obtained by slow evaporation of a 1 : 1 solution calculations were performed at the B3LYP/6-311G* level of
chloroform–heptane at room temperature. Yield: 55%. m.p: theory41 for all atoms except the tin(IV) which were described by
110 C. Anal. calc. for C52H45ClN4O2P2Sn (974.02): C, 64.12; H, the LanL2DZ basis set42 without any symmetry restrictions. The
ꢁ
4.65; N, 5.75. Found: C, 64.16; H, 4.26; N, 5.83%. 31P NMR electronic features of the optimized structures were studied by
1
(121.49 MHz, CDCl3): d (ppm) ¼ 19.6. H NMR (300.13 MHz, natural bound orbital43(NBO) analysis at the B3LYP method and
CDCl3): d (ppm) ¼ 6.86 (d, 3JHH ¼ 6.2 Hz, 4H-py), 7.15 (br, NH), the LanL2DZ and 6-311+G* basis sets. Besides, some surprising
3
3
7.32–7.70 (m, 27H; H-Ph), 7.82 (dd, JHH ¼ 6.9 Hz, JPH ¼ 12.7 binding energies were calculated for the binuclear model and the
Hz, 8H; o-Ph), 8.08 (d, 3JHH ¼ 5.7 Hz, 4H-py). 119Sn NMR (111.86 real hydrogen-bonded clusters based on the energy difference
MHz, CDCl3): d (ppm) ¼ ꢀ74. IR (KBr, cmꢀ1): n ¼ 3440 (s, NH), between the total energy of the systems and their fragments, as
1614 (m, py ring), 1187 (m, P]O).
represented in the equation DE ¼ Etotal – (Efrag1 + Efrag2).
Fragments 1 and 2 are two subunits which are connected by the
corresponding bond. The interaction energies were corrected
for basis set superposition error (BSSE) based on the counter-
poise correction method of Boys and Bernardi.44
Instrumentation
1H, 31P and 119Sn NMR spectra were recorded on a Bruker-
Avance DRS 500 spectrometer at 300.13, 121.50 and 111.86
MHz, respectively. 1H, 31P and 119Sn chemical shis were
measured relative to Si(CH3)4, 85% H3PO4 and Sn(CH3)2 as Conclusion
external standard respectively. Infrared (IR) spectra were
We have prepared and characterized two organotin(IV) adducts
recorded on KBr disk using a Shimadzu model IR-60 spec-
trometer. Elemental analysis was performed using a Heraeus
CHN-O-RAPID apparatus. Melting points were obtained with an
electrothermal instrument.
with phosphoramidate derivatives based on aminopyridine as
heterodifunctional ligands. The crystal structures of the
complexes reveal Sn(IV) to be ve-coordinated and both ligands
to function in an ambidentate mode. The ligand L1 coordinates
via a phosphoryl group in the structure C1, while the other one
acts as an N-donor in the crystal C2. The calculations of the
complexes in the gas phase, with respect to the solid-state
structures and the chemical behavior in solution, are compat-
ible with each other; they show that Sn(IV) interacts more
strongly with the N-atom in pyridine than the with the P]O
functional group. In the case of C1, coordination of the phos-
phoryl donor instead of N-pyridine can be explained by steric
hindrance, regardless of the electronic priorities. In contrast,
the low preference of L2 for bidentate binding, despite the lack
of steric demands, can be attributed to a competition between
two types of possible interactions for the P(O)NH functional
group. Indeed, theoretical calculations indicate the intermo-
lecular P]O/HNamidic interactions would be preferable to tin–
oxygen coordination for the association in C2. The substantial
contribution of the hydrogen bonds around 5–8 kcal molꢀ1
towards the stability of the resulting complexes makes these an
essential factor for the interesting coordination behavior of
both ligands.
Crystal structure determination
Bragg-intensities of the C1 and C2 were collected (T ¼ 292 K) on
a Stoe IPDS II with graphite-monochromatized MoKa (l ¼
˚
0.71073 A) radiation. Cell renement and integration by X-AREA
(1.62),36 data reduction and a numerical absorption correction
by XRED32 (1.31).37 C1 was solved with direct methods (SHELXS-
97)38 and C2 with DIRDIF-2008.39 Renement on |F|2 with the
program SHELXL-97.38 All non-H atoms in C1 and C2 were
rened anisotropically and the aromatic H were made to ride on
their carrier atoms, except the amine-H which were optimized
isotropically aer having been located in a difference map. In C1
most of the carbons appeared as prolate ellipsoids; the worst
rings C17 / C21 and C72 / C77 were split into two, their s.o.f.
adding up to one. Nevertheless, SAME, ISOR and FLAT restraints had
to be imposed in order to hinder them from diverging. SAME
restraints also had to be used for obtaining reasonable Sn–C
distances. The crystallographic and renement data are
summarized in Table S1† for both complexes.
Computational details
Acknowledgements
Density functional theory (DFT) calculations were carried out
using the Gaussian 03 program-package.40 In the case of C1 and Financial support of this work by Tarbiat Modares University is
C2, the X-ray structures of complexes were used as starting gratefully acknowledged.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 44509–44516 | 44515

View Article Online
RSC Advances
Paper
22 D. R. Smyth, C. P. D. Stapleton and E. R. T. Tiekink,
Organometallics, 2003, 22, 4599.
Notes and references
23 R. Zhang, G. Tian and C. Ma, J. Organomet. Chem., 2005, 690,
4049.
24 C. L. Ma, Y. F. Han, R. F. Zhang and D. Q. Wang, Dalton
Trans., 2004, 1832.
25 R. Murugavel, R. Pothiraja, S. Shanmugan, N. Singh and
R. J. Butcher, J. Organomet. Chem., 2007, 692, 1920.
26 K. E. Gubina, V. A. Ovchynnikov, J. Swiatek-Kozlowska,
V. M. Amirkhanov, T. Yu. Sliva and K. V. Domasevitch,
Polyhedron, 2002, 21, 963.
1 V. V. Grushin, Chem. Rev., 2004, 104, 1629.
2 S. M. Aucott, A. M. Z. Slawin and J. D. Woollins, Dalton
Trans., 2000, 2559.
3 A. M. Z. Slawin, J. Wheatley, M. V. Wheatley and
J. D. Woollins, Polyhedron, 2003, 22, 1397.
4 D. A. Padron and K. K. Klausmeyer, Inorg. Chim. Acta, 2013,
405, 511.
5 D. A. Padron and K. K. Klausmeyer, Inorg. Chim. Acta, 2013,
405, 511.
27 Wrackmeyer, in: Tin Chemistry: Fundamentals, Frontiers, and
Applications, ed. A. G. Davies, M. Gielen, K. H. Pannell, E. R.
T. Tiekink, Wiley, Chichester, UK, 2008, ch. 2, pp. 17–53.
28 J. Lorberth, S. Wocadlo, W. Massa, E. V. Grigoriev,
N. S. Yashina, V. S. Petrosyan and P. Finocchiaro, J.
Organomet. Chem., 1996, 510, 287; M. Wagner, M. Henn,
6 S. Pailloux, C. E. Shirima, A. D. Ray, E. N. Duesler,
K. A. Smith, R. T. Paine, J. R. Klaehn, M. E. McIlwainb and
B. P. Hay, Dalton Trans., 2009, 7486.
7 F. D. Sokolov, N. G. Zabirov, L. N. Yamalieva, V. G. Shtyrlin,
R. R. Garipov, V. V. Brusko, A. Y. Verat, S. V. Baranov,
P. Mlynarz, T. Glowiak and H. Kozlowski, Inorg. Chim. Acta,
2006, 359, 2087.
¨
C. Dietz, M. Schurmann, M. H. Prosenc and K. Jurkschat,
Organometallics, 2013, 32, 2406; B. Z. Momeni, S. Shahbazi
and H. R. Khavasi, Polyhedron, 2010, 29, 1393.
29 J. Holecek, M. Nadvornik, K. Handlir and A. Lycka, J.
Organomet. Chem., 1986, 315, 299.
30 A. R. Salimi, M. Mirzaei, M. Chahkandi, A. Azadmeher,
H. Eshtiagh-Hosseini, H. R. Khavasi and M. M. Amini, J.
Mol. Struct., 2009, 937, 44.
31 R. Zhang, M. Yang and C. Ma, J. Organomet. Chem., 2008,
693, 2551.
32 A. Bondi, J. Phys. Chem., 1964, 68, 441.
8 W. Lackner-Warton, S. Tanaka, C. M. Standfest-Hauser,
¨
¨
O. Oztopcu, J. Hsieh, K. Mereiter and K. Kirchner,
Polyhedron, 2010, 29, 3097.
9 A. D. Garnovskii, D. A. Garnovskii, I. S. Vasil'chenko,
A. S. Burlov, A. P. Sadimenko and I. D. Sadekov, Russ.
Chem. Rev., 1997, 66, 389.
10 K. Gholivand, N. Oroujzadeh and F. Afshar, J. Organomet.
Chem., 2010, 695, 1383.
11 K. Gholivand, S. Farshadian and Z. Hosseini, J. Organomet.
Chem., 2012, 696, 4298.
33 A. R. Narga, M. Schuermann and C. Silvestru, J. Organomet.
Chem., 2001, 623, 161.
12 K. Gholivand and S. Farshadian, Inorg. Chim. Acta, 2011, 368,
111.
34 S. Chandrasekar, B. S. Krishnamoorthy, V. S. Sridevi and
K. Panchanatheswaran, J. Coord. Chem., 2005, 58, 295.
35 M. Nishio, CrystEngComm, 2004, 6, 130.
36 X-AREA, Version1.62, StoeandCie, Darmstadt, Germany, 2011.
37 X-RED32, Version 1.31, Stoe and Cie GmbH, Darmstadt,
Germany, 2005.
13 K. Gholivand, N. Oroujzadeh and M. Rajabi, J. Iran. Chem.
Soc., 2012, 9, 865.
14 K. Gholivand, H. R. Mahzouni, M. Pourayoubi and S. Amiri,
Inorg. Chim. Acta, 2010, 363, 2318.
15 K. Gholivand, M. Rajabi, F. Molaei, H. R. Mahzouni and
S. Farshadian, J. Coord. Chem., 2013, 66, 4199.
16 S. E. Denmark and J. J. Fu, J. Am. Chem. Soc., 2003, 125, 2208.
17 S. E. Denmark, T. Wynn and G. L. Beutner, J. Am. Chem. Soc.,
2002, 124, 13405.
38 G. M. Sheldrick, Acta Crystallogr., 2008, A64, 112.
39 P. T. Beurskens, G. Beurskens, R. De Gelder, J. M. M. Smits,
´
S. Garcıa-Granda and R. O. Gould, DIRDIF, Crystallography
Laboratory, Radboud University Nijmegen, The
Netherlands, 2007.
18 S. K. Hadjikakou and N. Hadjiliadis, Coord. Chem. Rev., 2009,
253, 235.
40 M. J. Frisch, et al., Gaussian 03, Revision D.01, Gaussian, Inc.,
Wallingford, CT, 2004.
41 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter
Mater. Phys., 1988, 37, 785.
19 X. Shang, X. Meng, E. C. B. A. Alegria, Q. Li, M. F. C. Guedes
da Silva, M. L. Kuznetsov and A. J. L. Pombeiro, Inorg. Chem.,
2011, 50, 8158.
´
20 E. Lopez-Torres, F. Zani and M. A. Mendiola, J. Inorg.
42 P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 299.
43 P. Foster and F. Weinhold, J. Am. Chem. Soc., 1980, 102, 7211.
44 S. F. Boys and F. Bernardi, Mol. Phys., 1970, 19, 553.
Biochem., 2011, 105, 600.
21 T. Gianferrara, I. Bratsos and E. Alessio, Dalton Trans., 2009,
37, 7588.
44516 | RSC Adv., 2014, 4, 44509–44516
This journal is © The Royal Society of Chemistry 2014