Structural, spectroscopic and computational examination of the dative interaction in constrained phosphine-stibines and phosphine-stiboranes

Article abstract of DOI:10.1002/chem.201500281

Abstract A series of phosphine-stibine and phosphine-stiborane peri-substituted acenaphthenes containing all permutations of pentavalent groups -SbCl_nPh_4-n (5-9), as well as trivalent groups -SbCl₂, -Sb(R)Cl, and -SbPh₂ (2-4, R=Ph, Mes), were synthesised and fully characterised by single crystal diffraction and multinuclear NMR spectroscopy. In addition, the bonding in these species was studied by DFT computational methods. The P-Sb dative interactions in both series range from strongly bonding to non-bonding as the Lewis acidity of the Sb acceptor is decreased. In the pentavalent antimony series, a significant change in the P-Sb distance is observed between -SbClPh₃ and -SbCl₂Ph₂ derivatives 6 and 7, respectively, consistent with a change from a bonding to a non-bonding interaction in response to relatively small modification in Lewis acidity of the acceptor. In the Sb^III series, two geometric forms are observed. The P-Sb bond length in the SbCl₂ derivative 2 is as expected for a normal (rather than a dative) bond. Rather unexpectedly, the phosphine-stiborane complexes 5-9 represent the first examples of the σ⁴P→σ⁶Sb structural motif. A complex situation: The strength of a dative phosphine-stiborane (P-Sb) interaction increases with stepwise replacement of phenyl groups on antimony atom with chloride groups. As the Lewis acidity is increased in regular steps (see figure), essentially linear response is observed initially, however then a sudden change in the P-Sb distance takes place during one particular step. This is consistent with a sudden switch from a non-bonding to a bonding interaction, that is, a discrete rather than continuum response.

Full text of DOI:10.1002/chem.201500281

DOI: 10.1002/chem.201500281
Full Paper
&
Donor–Acceptor Systems
Structural, Spectroscopic and Computational Examination of the
Dative Interaction in Constrained Phosphine–Stibines and
Phosphine–Stiboranes
Brian A. Chalmers, Michael Bühl, Kasun S. Athukorala Arachchige, Alexandra M. Z. Slawin,
and Petr Kilian*[a]
Abstract: A series of phosphine–stibine and phosphine–sti-
borane peri-substituted acenaphthenes containing all per-
mutations of pentavalent groups ÀSbClnPh4–n (5–9), as well
as trivalent groups ÀSbCl2, ÀSb(R)Cl, and ÀSbPh2 (2–4, R=
Ph, Mes), were synthesised and fully characterised by single
crystal diffraction and multinuclear NMR spectroscopy. In ad-
dition, the bonding in these species was studied by DFT
computational methods. The P–Sb dative interactions in
both series range from strongly bonding to non-bonding as
the Lewis acidity of the Sb acceptor is decreased. In the pen-
tavalent antimony series, a significant change in the P–Sb
distance is observed between ÀSbClPh3 and ÀSbCl2Ph2 deriv-
atives 6 and 7, respectively, consistent with a change from
a bonding to a non-bonding interaction in response to rela-
tively small modification in Lewis acidity of the acceptor. In
the SbIII series, two geometric forms are observed. The P–Sb
bond length in the SbCl2 derivative 2 is as expected for
a normal (rather than a dative) bond. Rather unexpectedly,
the phosphine–stiborane complexes 5–9 represent the first
examples of the s4P!s6Sb structural motif.
Introduction
a dimeric structure with pseudo-octahedral coordination of the
acceptor P atom (C in Figure 1).[5]
Stabilisation of main-group motifs through Lewis base coordi-
nation is one of the prominent topics of contemporary p-block
chemistry,[1] and while the fundamental bonding aspects in
dative species have been under scrutiny for several decades,
some intense discussions have been ongoing very recently.[2]
Apart from N-heterocyclic carbenes, phosphines remain a popu-
lar choice amongst Lewis bases in such species due to their fa-
vourable electronic properties (strong s-donors and p-accept-
ors, which are widely tunable).[3]
Due to their lesser s-donating capacity, arsines, stibines and
bismuthines act as donors in Main Group complexes less com-
monly. On the other hand, the increased Lewis acidity of heavi-
er Group 15 halides means that they readily become involved
as acceptors, even with weak donors. An example of this
is an early structural report of Me3Sb!SbI2Me.[6] Two mole-
cules of similar geometry are present in the asymmetric unit,
both with Sb–Sb distances corresponding to that of a Sb–Sb
single bond (2.859(1) and 2.868(1) Š). The donor atom adopts
a tetrahedral geometry and the acceptor adopts a tbp geome-
try with one of the equatorial positions occupied by a lone
pair. Weak secondary Sb···I (intermolecular) interactions are
present which extend the coordination of the acceptor Sb
atom to 6.
In connection with our investigations into the chemistry of
peri-substituted naphthalenes,[4] we have established a long-
term interest in backbone supported dative species with
Group 15 atoms acting as both donors and acceptors. Halo-
phosphines are sufficiently electrophilic to act as (lone-pair
possessing) acceptors to other phosphines, although thermally
stable examples of this are rather rare as such species general-
ly disproportionate at or below ambient temperature. Phos-
phine–phosphine donor–acceptor complexes adopt either
a monomeric structure with pseudo-tbp (trigonal bipyramidal)
coordination of the acceptor P atom (A and D in Figure 1); or
The tendency of the heavier Group 15 acceptors to extend
their coordination beyond four (in the solid state) is almost
ubiquitous as secondary interactions result in the formation of
loosely bound oligomeric or polymeric structures consisting of
strongly bound monomers or dimers. An example of this is
[(SbI3)(PMe3)]2, which in the solid state forms a weakly bound
polymer consisting of dimeric units.[7] An exception is the di-
meric adduct [(SbMe3)(SbI3)(THF)]2, in which the THF coordina-
tion seems to block further association of the dimeric units.[8]
An early review by Norman and Pickett gives an overview of
the complexity and structural variety displayed by phosphine–
pnictine adducts, and also provides data on related anionic
species.[9] A comprehensive study on a range of phosphine ad-
ducts with cationic, neutral and anionic antimony halide motifs
[a] B. A. Chalmers, Prof. M. Bühl, Dr. K. S. Athukorala Arachchige,
Prof. A. M. Z. Slawin, Dr. P. Kilian
EaStChem School of Chemistry, University of St. Andrews
St. Andrews, Fife KY16 9ST (UK)
E-mail: pk7@st-andrews.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500281.
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is driven by differences in local environment polarity in the in-
dividual crystal lattices; indeed calculations show that polarity
effects can result in switch of the energy minima for the two
forms shown in Figure 1.[13]
The b-diketiminate scaffold has also been used to support
the phosphine–stibine and phosphine–arsine donor–acceptor
complexes L by Burford (Figure 1).[14]
In this study, a comprehensive series of phosphine–stibine
and phosphine–stiborane species are reported. The peri-substi-
tution supported motifs have been designed with view of ob-
serving a series of P!Sb dative interactions using a fixed
donor motif and Sb acceptor with progressively increasing
Lewis acidity. We wished to establish whether sudden structur-
al changes consistent with a clear distinction between bonding
and non-bonding (repulsive, frustrated) interaction will be ob-
served, or whether smooth trend of P–Sb distances will be pro-
duced in the series consistent with a “spectrum-like” character
of the dative interaction. Indeed, differentiation of “primary”
and “secondary” coordination in main group chemistry was
mentioned by Levason in a recent review as one of the issues
in contemporary Main Group chemistry.[3b] In this study we
provide insight into these interactions using combination of
experimental (structural, spectroscopic) and computational ap-
proaches.
Figure 1. Structural formulae of species mentioned in the text.
Results and Discussion
Synthesis
([SbCl]2+, [SbCl2]+, SbCl3 and [SbCl4]À) has been reported more
recently.[10]
A comprehensive series of phosphorus–antimony peri-substi-
tuted species was synthesised to study the onset of the
donor–acceptor P!Sb bonding. While the donor (ÀPiPr2
group) is maintained throughout this series, the acceptor
groups include all permutations of pentavalent groups À
SbClnPh4–n (n=0–4, compounds 5–9), as well as trivalent
groups ÀSbCl2, ÀSb(Ph)Cl, ÀSb(Mes)Cl and ÀSbPh2 (compounds
2–4, Scheme 1).
Employing a rigid organic backbone provides a degree of
molecular geometry control with respect to the donor–accept-
or interaction. Norman utilised peri-substitution and other re-
lated systems to study amine–stibine interactions.[11] The single
donor complex E displayed short N!Sb interaction. In the
double donor species F only one amine group is strongly inter-
acting with the Sb acceptor, whilst the other shows a weak
non-bonding interaction as reflected by the respective Sb–N
distances (2.219(4) and 2.903(4) Š).
A small series of SbIII and SbV donor–acceptor complexes
with naphthalene and o-phenylene scaffolds was synthesised
by Yamaguchi.[12] Only a weakly bonding N···Sb interaction was
found in G, whilst the shorter N–Sb distance in H indicated
a dative bond was present. Interestingly the derivative with an
extra methylene bridge I showed a slightly shorter N–Sb dis-
tance than H (2.590(6) vs. 2.658(4) Š), indicating the rigidity of
the peri backbone is hindering efficient N–Sb interaction slight-
ly.[12]
5-Bromo-6-(diisopropylphosphino)acenaphthene (1) was
used as a principal precursor in all syntheses described in this
study.[5b] The majority of the new compounds were prepared
by low temperature halogen–lithium exchange reaction yield-
ing 1’, which was (without isolation) subjected to CÀSb cou-
pling reaction with a variety of (aryl)antimony chloride species
(Scheme 1). In case of 9, Ph4SbBr was used in the coupling re-
action. The compounds were isolated as white to yellow pow-
ders with yields ranging from moderate to excellent (70–91%).
Compounds 5 and 6 were prepared by chlorination of the
corresponding SbIII species 2 and 3 with sulfuryl chloride
(Scheme 1); both these reactions afforded near quantitative
yields. Our attempts to prepare these highly chlorinated SbV
species (5 and 6) by treating the lithiated acenaphthene pre-
cursor 1’ with SbCl5 and PhSbCl4, respectively, resulted in com-
plex mixtures as indicated by 31P NMR spectra. In the case of
the reaction with PhSbCl4, somehow unexpectedly, a product
of a scrambling reaction (compound 7) was observed as
a minor component in the mixture after the reaction. The ob-
served lack of chemoselectivity of these couplings is likely
Very recently, Beckmann reported a comprehensive series of
phosphine–pnictine species shown in Figure 1.[13] All species
show a strong attractive P–E interaction. Interestingly, the
structure of the antimony species K differs fundamentally from
all others by possessing a near-linear P-Sb-Cl motif. Extensive
analysis by a variety of computational methods is given in the
study, however it seems there is no simple explanation as to
why the structure of K is anomalous. Most likely the structure
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the acenaphthene backbones
remain essentially planar, while
the out-of-plane displacements
are small to moderate at 0.111–
0.240 Š for the
P atom and
0.011–0.207 Š for the Sb atom
(Figure 6). Splay angles increase
uniformly in this series (from
À1(1) in 2 to +15.7(4) in 4; see
Figure S5 in Supporting informa-
tion for splay angle definition).
The phosphorus atoms in the
phosphine groups attain distort-
ed tetrahedral geometry; in all
cases the conformation with the
lone pair pointing (at least ap-
proximately) towards the anti-
mony atom is observed in the
solid state structures as shown
in Figure 6.
The structure of 2 in the crys-
tal is shown in Figure 2 and
Table 1. A loosely bound centro-
symmetric dimeric assembly is
formed in the solid state
through two Sb···Cl interactions
(3.4233(17) Š); both contacts are
Scheme 1. Preparation of phosphorus–antimony peri-substituted acenaphthenes 2–9.
stemming from the extremely electrophilic nature of the
highly chlorinated SbV precursor species.
significantly shorter than the sum of the van der Waals radii
(4.15 Š).[16] The Sb atom in 2 adopts a rather distorted pseudo-
trigonal bipyramidal geometry with chlorine atoms occupying
the axial positions (Cl1-Sb1-Cl2 angle of 171.35(5)8) and atoms
P9, C1 and the lone pair occupying the equatorial positions.
The P–Sb distance in 2 is 2.5923(17) Š, which is within the
normal range for l3P–l3Sb bonds (2.49–2.59 Š).[17]
Compound 5 was isolated as a thermally unstable yellow
powder, which can however be stored at À308C. In the solid
state visible signs of decomposition (colour change to brown)
are observed within 2–3 days at room temperature when
stored under argon. The decomposition is instant in chlorinat-
ed solvents, while in benzene or toluene solutions the decom-
position was slow enough that it was possible to acquire
a complete set of solution NMR spectra.
Along with the splay angle of À1(1)8 this is consistent with
an attractive interaction (i.e., a standard bond) between the
phosphorus and antimony atom. Taking into account the
Sb···Cl contacts within the dimer, the antimony atom geometry
can be seen as distorted pseudo-octahedral with the chlorine
atoms occupying the meridional positions and a close to linear
arrangement of P9–Sb1···Cl1 motif (170.34(5)8). Natural bond
orbital (NBO) analysis indicates large s character of the lone
pair on the antimony atom (87% s- and 13% p-character,
Figure 3), which makes it difficult to assign the antimony ge-
ometry as tbp or octahedral based on the lone-pair location.
Interestingly, the two SbÀCl bond lengths in 2 are statistical-
ly indifferent, although Cl1 atom is involved in sub-van der
Waals intramolecular contacts, whilst Cl2 atom is not. The C1-
Sb1-P9 angle (80.6(2)8) is rather acute due to the constraints of
the acenaphthene backbone.
All new species with the exception of 5 are hydrolytically
stable, including those containing SbÀCl linkages. This allowed
efficient removal of ionic impurities after the SbÀC coupling re-
actions through washing with degassed water. In the series of
EIII congeners A[5b] (E=P), B[15] (E=As) and 2 (E=Sb, reported
herein) an expected trend is observed of increasing hydrolytic
stability with the increase of metallic character of pnictogen
atom (on descending Group 15), with phosphorus species A
hydrolysing instantly, arsenic species B being moderately sensi-
tive and antimony species 2 being fully resistant to hydrolysis.
Novel compounds 2–9 were fully characterised by multinu-
clear NMR spectroscopy (1H, 13C and 31P), single crystal X-ray
diffraction, MS, IR and Raman spectroscopy. The homogeneity,
where possible, was confirmed using microanalysis.
The structure of 3 in the crystal is shown in Figure 2 and
Table 1. The PÀSb distance of 2.7104(8) Š in 3 indicates an at-
tractive interaction across the peri gap, although noticeably
weaker than that found in 2 and slightly outside the normal
range for l3P–l3Sb bonds (2.49–2.59 Š).[17a] The splay angle in 3
is accordingly slightly larger (2.8(6)8) versus that in 2. The ge-
ometry around antimony in 3 is distorted pseudo-trigonal bi-
Structural investigations
Antimony(III) species
The series of antimony(III) species includes 2, 3, 3-Mes and 4.
Despite the P–Sb distances varying widely in these species,
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thene plane rather than perpen-
dicular to it as in 2. The proximal
location of one of the isopropyl
groups to the phenyl rings is
such that a weak intramolecular
CH···p interaction can be ob-
served, with H20B···Cg(13–18)
distance of 2.720 Š (Figure 6).
The structure of 3-Mes in the
crystal is shown in Figure 2 and
Table 1. The overall structural
characteristics of 3-Mes are very
similar to those of 3. The PÀSb
bond length of 2.7244(9) Š is
only marginally longer than that
in 3, and the splay angle of
3.1(5)8 is correspondingly slightly
more obtuse. The Cl1-Sb1-P9
angle is 168.07(2)8, the Sb1ÀC13
bond lengths differ slightly in 3
(2.152(3) Š) and in 3-Mes
(2.176(2) Š). The largest structur-
al difference between 3 and 3-
Mes is in the C1-Sb1-C13 angle
(95.1(1)8 vs. 108.58(8)8). Hence
the inclusion of the methyl
groups results not only in
a slight increase of electron-re-
leasing character of the mesityl
Figure 2. X-ray structures of 2 (showing the loosely bound dimeric assembly), 3, 3-Mes and 4. Hydrogen atoms
and a molecule of solvating dichloromethane (from 2) are omitted for clarity.
group (mirrored in minor P9ÀSb1 and Sb1ÀC13 elongation),
but also in an increased steric clash between the aryl group (in
particular the ortho-methyl groups) and both the acenaph-
thene skeleton and the isopropyl groups.
The structure of 4 in the crystal is shown in Figure 2 and
Table 1. The P···Sb distance of 3.191(1) Š combined with the
splay angle of 15.7(4)8 indicates there is no conventional bond-
ing interaction between antimony and phosphorus. The anti-
mony atom adopts a distorted trigonal pyramidal geometry
with rather acute angles (S=2858). Close to linear arrange-
ment of P9···Sb1-C13 motif (168.14(6)8) and the phosphorus
lone pair pointing towards the antimony however indicates an
onset of 3c–4e interaction (n(P9)!s*(Sb1-C13); this is further
corroborated by
a
slightly elongated Sb1ÀC13 bond
(2.183(2) Š) with respect to the Sb1ÀC19 bond (2.152(2) Š).
Two distinct structural types (X and Y, Figure 4) have been
identified in the series of the SbIII species 2–4. Compound 2 at-
tains structure X in the crystal, featuring linear Cl-Sb-Cl ar-
rangement and a standard PÀSb covalent bond. The other
structural type (Y) has approximately linear P-Sb-R arrange-
ment (R=Cl, Ph, Mes) and features either a weakened P–Sb
bond (as in 3 and 3-Mes), or an essentially non-bonding P···Sb
interaction as in 4. In a recent study,[13] a related peri-substitut-
ed species K (Figure 1) was reported, which features a less
Lewis basic phosphine group. In the crystal, K adopts structure
Y with a P···Sb distance of 2.808(1) Š, which is only slightly
elongated with respect to that in 3 (2.7104(8) and 3-Mes
(2.7244(9) Š). Extensive computations indicated that “electric
Figure 3. Plot of the lone pairs at the Sb atoms in the dimeric assembly of 2
(NBOs at the B3LYP/CPCM level, 87 and 13% s and p character, respectively).
Hydrogen atoms are omitted for clarity. See the Supporting Information for
the colour version.
pyramidal with the chlorine and phosphorus atoms occupying
the axial positions (with Cl1-Sb1-P9 angle of 168.20(3)8) and
atoms C1, C13 and the lone pair occupying the equatorial po-
sitions. Most remarkably, the geometric orientation of the sub-
stituents with respect to the acenaphthene ring differs in 2
and 3, with the tbp axis in 3 being parallel with the acenaph-
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Table 1. Selected interatomic distances, displacements [Š], angles and torsion angles [8] for 2–9.
Cpd 3-Mes
peri-region bond lengths
2
3
4
5
6
7
8
9
P9···Sb1 2.5923(17)
62.5
1.802(6)
2.7104(8)
65.3
1.807(3)
2.182(2)
2.6798(8)
2.7244(9)
65.6
1.805(2)
2.181(2)
2.6682(9)
3.191(1)
76.9
1.847(2)
2.183(2)
–
2.6199(8)
63.1
2.659(2)
64.1
2.9925(8)
72.1
3.088(3)
74.4
1.810(7)
2.180(7)
2.640(2)
3.1247(16)
75.3
1.843(3)
2.218(4)
–
[a]
% SrvdW
P9ÀC9
1.796(3)
2.132(3)
2.373(1)–
2.4159(9)
–
1.815(9)
2.149(8)
2.449(2)–
2.469(2)
2.158(9)
1.829(6)
2.132(8)
2.466(2)
2.486(2)
2.131(7)
2.171(7)
Sb1ÀC1
Sb1ÀCl
2.153(6)
2.6160(17)
2.3124(17)
–
Sb1ÀCipso
2.152(3)
2.176(2)
2.152(2)
2.183(2)
2.170(6)–
2.191(9)
2.156(5)–
2.242(5)
peri-region bond angles
C9-P9-Sb1
C1-Sb1-P9
P9-Sb1-Etrans
98.3(2)
80.6(2)
–
À1.0(1)
90.08(9)
78.12(7)
168.20(3)
2.8(6)
98.55(7)
78.03(6)
168.07(2)
3.1(5)
90.36(7)
73.88(5)
97.48(8)
15.7(4)
95.3(1)
83.51(8)
179.08(3)
1.1(7)
95.7(3)
81.5(3)
173.30(8)
3.0(1)
92.4(2)
75.4(2)
169.1(2)
10.0(1)
91.6(2)
74.5(2)
171.6(2)
13.9(7)
91.7(2)
73.9(1)
172.1(1)
14.0(1)
[b]
splay angle[c]
out-of-plane displacement
P9
Sb1
0.271
À0.201
0.240
À0.011
0.190
0.207
0.111
À0.039
0.185
0.084
0.038
À0.044
0.289
À0.137
0.041
À0.012
0.102
À0.097
peri-region torsion angle
P9-C9···C1-Sb1 11.9(3)
7.5(1)
0.52(9)
2.7(1)
1.8(1)
3.3(4)
11.4(3)
0.8(4)
4.9(2)
[a] van der Waals radii used, P: 1.95 Š, Sb: 2.20 Š.[16] [b] E=Cl for 2, 3, 3-Mes, 5 and 6, E=C for 4, 7–9. [c] Splay angle: S of the bay region angles À3608
(see Figure S5 in the Supporting Information).
field” (such as from polar crystal lattice or polar solvent) pro-
motes adoption of structure type X by K, but in the gas phase
structure Y is the ground state.[13] Our calculations (including
polarizable continuum model for CHCl3) indicate that enthalpy
difference for 3/3’’ is 51.3 kJmolÀ1 (Figure 8), that is, the struc-
tural type Y is strongly preferred for 3 even in the polar envi-
ronment. On the other hand, when modelling in the same en-
vironment the structural preference is reversed for 2, the
isomer 2’ (type Y) being 15.3 kJmolÀ1 higher in enthalpy than
isomer 2 (type X). This leads us to conclude that structure X is
stable when a very strong donor–acceptor interaction is in
place, such as in 2, which combines very basic donor and
rather acidic acceptor. Interestingly, the structure X appears to
be limited to phosphine-donor systems as the dimethylamino
derivative E adopts structure Y in the crystal despite strong
N!Sb interaction (2.460(4) Š).[11]
Figure 4. The two structural types identified in the reported SbIII series (X
and Y) and structural formulas of compounds mentioned in the discussion.
The P!Sb derivatives with either a less Lewis basic donor
(K) or less Lewis acidic acceptor (3, 3-Mes and 4) adopt struc-
ture Y, which is driven by a formation of the n(P)!s*(Sb–R)
3c–4e interaction. Indeed, 3c–4e bonding has been shown to
drive conformational preferences in a wide range of peri-sub-
stituted species, stretching from 1-fluoro-8-anisylselenyl naph-
thalene, M,[4a,18] to heteroleptic bis(phosphino)acenaphthene, N
(Figure 4).[19]
hand, lessening of the accepting ability of Sb moiety by replac-
ing one or two strongly electronegative chlorine ligands with
more electron releasing aryl groups leads to small (in 3 and 3-
Mes) or significant (in 4) elongation of the P–Sb distance, con-
comitant with a major change in bonding geometry. Hence,
the latter species comply with Haaland’s definition of DA
bonding more than the former.
The observations above prompt a query as to how the
bonding can be classified in the compounds of the SbIII series.
Using definitions of donor–acceptor (DA) bonding delineated
in Haaland’s seminal review,[20] 2 would fail to be classified as
phosphine–stibine DA complex, as its PÀSb bond length lies
within the range of normal covalent bonds.[21] This corresponds
well with our computed Wiberg bond index (WBI) of 0.73 (gas
phase, see the computational part for details). On the other
According to the VSEPR model, normal bond pairs (NBP) are
more spatially demanding than the accepted bond pairs
(ABP).[20] When bonded via a dative bond, it is expected that
the P atom (bearing an ABP) should occupy an axial position
in the tbp environment of Sb atoms in 2–4, similar to, for ex-
ample, NMe3 donor occupying an axial position in the tbp
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adduct of Me3N!SiF4.[20] Indeed, the P donor atom is occupy-
ing the axial position in 3, 3-Mes and 4 (structure Y). In con-
trast, in 2 (structure X), the two Cl atoms occupy axial positions
in the antimony atom tbp environment, with the P donor
being in equatorial position. By this, the P–Sb bond in 2 again
resembles more the “normal” bond rather than the dative one.
It is interesting to note that while 3 and 3-Mes are molecu-
lar entities with only slightly elongated SbÀCl bonds
(2.6798(8) Š and 2.6682(9) Š) compared to the axial SbÀCl
bonds in Ph3SbCl2 (2.4820(5) and 2.4925(5) Š),[22] the related
phosphorus species O (Figure 4) display PÀP bonded ionic sep-
arated structure with ClÀ as counterion.[23]
Crystals suitable for X-ray diffraction were grown from toluene
at À358C; these contained one molecule of solvated toluene
per molecule of 5. The structure of 5 in the crystal is shown in
Figure 5 and Table 1. The P–Sb distance of 2.6199(8) Š is the
shortest amongst the SbV compounds 5–9, and is only slightly
longer than the shortest known l3P!l5Sb bond, which was
observed
in
a
tetrameric
bis(phosphine)
complex
[(SbBr3)4(dmpe)4] (PÀSb bond lengths in this complex range
from 2.498(6) to 2.656(4) Š, bridging halide atoms provide the
extra coordination to Sb centres).[7] Comparison with normal
(i.e., non-dative) l3P–l5Sb bond length is not possible as no
structural reports on species containing a non-dative bond
have appeared in the literature to date. The splay angle of
1.1(7)8 in 5 is the smallest observed amongst the SbV com-
pounds 5–9. The antimony atom adopts a distorted octahedral
geometry with cis angles ranging from 83.51(8)8 (P9-Sb1-C1) to
95.63(8)8 (Cl4-Sb1-C1). The SbÀCl distances in 5 are the short-
est observed in this series ranging from 2.3732(10) Š [Sb1ÀCl1]
to 2.4159(9) [Sb1ÀCl2] (cf. 2.4820(5) and 2.4925(5) Š in
Ph3SbCl2).[22]
Antimony(V) species
Since an interesting structural variety and differing strengths of
DA bonding were observed in the SbIII series (see above), we
expanded our study towards the related SbV compounds 5–9.
We expected that the lesser structural flexibility in the SbV
series (only octahedral coordination of Sb was anticipated) will
result in a smoother onset of the DA bonding. In addition, in-
creased coordinative saturation of Sb centres was expected to
eliminate formation of secondary (intermolecular) contacts re-
sulting in less complex bonding.
The structure of 6 in the crystal is shown in Figure 5 and
Table 1. The antimony atom adopts a distorted octahedral ge-
ometry with phenyl group placed trans to the backbone’s C1
atom. The P–Sb distance of 2.659(2) Š in 6 is only very moder-
ately elongated versus that in 5 (2.6199(8) Š), indicating
a bonding interaction. This is further supported by only a mar-
ginal increase in splay angle to 3(1)8. The cis angles around the
Sb atom range from 86.7(3)8 (C13-Sb1-Cl3) to 99.9(3)8 (C13-
Sb1-P9). The SbÀCl distances in 6 are slightly, but noticeably
elongated versus those in 5 to 2.449(2)–69(2) Š, in agreement
with increased electron density at the antimony atom in 6
versus 5.
The structure of 7 in the crystal is shown in Figure 5 and
Table 1. The octahedral geometry around antimony is noticea-
bly distorted in 7, with cis angles of 88.10(18)8 [C1-Sb1-Cl1] to
102.4(3)8 [C1-Sb1-C13] and the trans angle C1-Sb1-C19
157.7(3)8. Nevertheless, the fact that Sb adopts a near-octahe-
dral geometry indicating the phosphorus atom is part of its co-
ordination sphere. The most striking feature is a significant
elongation of the P···Sb distance to 2.9925(8) Š, which is 11 %
longer than that in 6 and 12% longer than that in 5. Despite
the elongation, the distance remains significantly sub-van der
Waals at 73% of the SrvdW(P, Sb)=4.15 Š.[16] In addition, peri
atoms are significantly displaced from the acenaphthene mean
plane (P9 0.289, Sb1 À0.137 Š), and the splay angle is in-
creased to 10(1)8, indicating noteworthy out-of-plane as well
as in-plane distortions in the peri region. These structural fea-
tures indicate a rather ambiguous bonding situation in 7, with
both repulsive and attractive terms, perhaps best described as
weakly bonding P···Sb interaction. Our computations give WBI
of 0.29 in 7 (in the crystal structure), which is significantly
lower than those in 5 (0.57) and 6 (0.53), yet indicate the pres-
ence of a significant covalent bonding component. The two
chlorine atoms in 7 are placed trans to each other, with the Cl-
Sb-Cl motif perpendicular to the acenaphthene plane, hence
ruling out potential 3c–4e bonding involving donation of lone
pair density on P atom into low lying s*(SbÀCl) orbital as seen
Rather strikingly, whilst the chemistry of phosphine com-
plexes of SbIII halides has been well explored and a number of
structural reports appeared in the literature,[7,10,24] no structural
reports on phosphine or arsine complexes of SbV halides (or,
more generally, stiboranes R5Sb) have been found in the litera-
ture to date. Compound 5 and other species in this series thus
represent the first examples of such electroneutral complexes
with the general formula R’3P!SbR5. This is rather surprising,
especially considering that stiboranes and in particular SbV hal-
ides are expected to be more Lewis acidic than the respective
stibines and SbIII halides and hence DA complexes in which sti-
boranes act as acceptors should be stable. Indeed, calculated
binding enthalpies (B3LYP/CPCM level) for prototypical PMe3
adducts of SbCl3 and SbCl5 are À54.9 kJmolÀ1 (À72.2 kJmolÀ1
)
and À147.3 kJmolÀ1 (À175.5 kJmolÀ1), respectively (values in
parentheses include BSSE and dispersion corrections at the
B3LYP-D3 level). The early literature describes reaction of PMe3
with SbCl5 as rapid,[25] however no follow up of this reactivity is
available. We believe this unexpected absence may be due to
the redox reactions plaguing this and similar systems, with SbV
oxidising the phosphine.[26]
Despite differing P–Sb distances in the series of 5–9 (range
of 2.6199(8)–3.1247(16) Š) the acenaphthene backbones
remain essentially planar, while the out-of-plane displacements
are small to moderate at 0.038–0.289 Š for the P atom and
0.012–0.137 for the Sb atom (Figure 6). The phosphorus atoms
in the phosphine groups attain distorted tetrahedral geometry;
in all cases the conformation with the lone pair pointing to-
wards the antimony atom is observed (Figure 6) and the angle
C9-P9-Sb1 is rather acute (91.6(2)–95.7(3)8) due to constraints
of the backbone.
The perchlorinated compound 5 was isolated as a yellow
powder which is thermally unstable at room temperature.
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Figure 5. X-ray structures of 5–9. Hydrogen atoms and solvated molecules of toluene in 5 and acetonitrile in 8 are omitted for clarity.
in 3 and 3-Mes. Instead, the n(P9)!s*(Sb1ÀC13) interaction is
present with the P9···Sb1-C13 angle of 169.07(17)8.
NMR spectroscopy
Crystals of 8 suitable for X-ray diffraction were grown from
acetonitrile at 08C. The structure of 8 in the crystal is shown in
Figure 5 and Table 1, the molecule crystallises with a solvated
molecule of MeCN. The geometry around Sb atom in 8 is
rather similar to that in 7, with the P···Sb distance of 3.088(3) Š
(3% elongation on 7) and splay angle of 13.9(7)8 (48 more
obtuse than 7). Interestingly, a much smaller out-of-plane dis-
tortion in the peri-region is observed with the P and Sb atoms
being displaced from the mean acenaphthene plane only by
0.041 and À0.012 Š. As in 7, the near-linear arrangement of
P9···Sb1-C19 motif (171.6(2)8) indicates a n(P9)!s*(Sb1ÀC19)
3c–4e bonding interaction, resulting in a P···Sb WBI of 0.25.
The structure of 9 in the crystal is shown in Figure 5 and
Table 1. The replacement of the last Cl atom on Sb atom with
phenyl group leads to small elongation of the P···Sb distance
to 3.1247(16) Š, whilst other metric parameters (including
splay angle, P and Sb displacements) closely resemble those of
8. An overview of the crystal structures 2–9 aligned along the
acenaphthene plane to show the geometry of the substituents
around Sb and P atoms and relevant distortions of the back-
bone is provided in Figure 6.
Selected NMR and structural parameters of compounds 2–9
are shown in Table 2. All compounds exhibit sharp singlets in
the 31P{1H} NMR spectra. It is expected that upon coordination
(i.e., sequestration of its lone pair to a bonding interaction),
the phosphorus atom becomes deshielded. In agreement with
this, dP in compounds 2–4 (the SbIII series) are increasingly
shifted to low frequency as the Lewis acidity of the antimony
centre decreases (most Lewis acidic 2>3>3-Mes>4 least
Lewis acidic).
However, only compounds 5–7 in the SbV series follow the
same trend. Whilst the Lewis acidity of antimony centres in 8
and 9 is clearly lower than those in 5–7 (as indicated by longer
P···Sb distances and decreasing WBIs, Table 3), their dP increase
from À45.6 in 7 to À42.3 in 8 and À33.8 ppm in 9. Hence, an-
other effect appears to override the reduced removal of elec-
tron density from phosphorus due to decreasing dative bond
strength. Aromatic ring currents from phenyl rings placed cis
to Sb atom may (at least partially) contribute to the observed
deshielding of the 31P nucleus in 8 and 9 as the phosphorus
atom is forced relatively close to the outside of these rings
due to the congested nature of peri-substituted molecules.
Compounds 4 and 6–9 feature large 13C–31P through-space
coupling (tsJCP) values (31–87 Hz) associated with the ipso
5
carbon atoms of the phenyl substituents. These formally JCP
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Figure 6. Molecular structures of 2–9 in the crystal. Molecules are aligned along the acenaphthene plane to show the geometry of the substituents around
Sb and P atoms. In each of the compounds 5–9, which have octahedral Sb environment, one of the substituents bound to Sb atom (pointing towards the
viewer) has been omitted for clarity.
Table 2. Selected NMR parameters of compounds 2–9.
Table 3. PhosphorusÀantimony bond lengths [Š] observed in the crystal
and calculated at the B3LYP level. Wiberg bond indices obtained at the
same level for the respective structures are displayed in square parenthe-
ses. For clarity, data for compounds for which X-ray diffraction data are
given are in bold.
5ts
Cpd
Sb moiety
dP [ppm]
J
[Hz][a]
CP
SbIII series
2
3
SbCl2
51.0
1.5
À3.6
À21.9
–
–
–
Sb(Ph)Cl
Sb(Mes)Cl
SbPh2
Cpd
d(P-Sb)X-ray
[WBI][a]
d(P-Sb)calcd
gas [WBI][a]
d(P-Sb)calcd
DHrel CPCM
3-Mes
4
[c]
CPCM [WBI][b] [kJmolÀ1
]
40.3
2
2’
3
2.5923(17) [0.74] 2.628 [0.73] 2.629 [0.76]
0
15.3
0
SbV series
n/a
2.974 [0.33]
2.820 [0.49]
5
6
7
8
9
SbCl4
À20.1
À28.4
À45.6
À42.3
À33.8
–
52.3
86.4, 77.8
32.6
35.7, 31.1
2.7104(8) [0.48]
2.978 [0.32] 2.713 [0.60]
Sb(Ph)Cl3
Sb(Ph)2Cl2
Sb(Ph)3Cl
SbPh4
3’
3’’
n/a
n/a
3.208 [0.17]
2.621 [0.73]
2.966 [0.34] 2.794 [0.51]
3.238 [0.16] 3.242 [0.16]
3.240 [0.14] 3.246 [0.14]
2.679 [0.62] 2.638 [0.68]
3.215 [0.17]
2.606 [0.77]
53.7
51.3
0
55.8
0
153.7
0
0
7.0
0
15.5
0
58.2
0
3-Mes
3’-Mes n/a
4
4’
5
6
6’
7
7’
8
8’
9
2.7244(9) [0.48]
3.191(1) [0.15]
n/a
[a] Through-space coupling from 31P nucleus, observed on ipso-carbon
atom(s) in the phenyl group(s).
2.6199(8) [0.57]
2.659(2) [0.53]
n/a
2.9925(8) [0.29]
n/a
3.088(3) [0.25]
n/a
3.1247(16) [0.22] 3.223 [0.19] 3.214 [0.20]
2.709 [0.54] 2.682 [0.59]
2.789 [0.47] 2.742 [0.54]
2.750 [0.52]
3.069 [0.27] 3.057 [0.29]
2.799 [0.46] 2.737 [0.54]
3.167 [0.22] 3.185 [0.22]
2.994 [0.35] 2.892 [0.44]
2.722 [0.58]
couplings are listed in Table 2. Since the P···Sb distances in 2–9
are shorter than the sum of the van der Waals radii (SrvdW
=
4.15 Š) there is the opportunity for an efficient overlap of the
phosphorus lone pair and the s*(SbÀC) orbital (facilitated by
the quasi linear P···SbÀC) arrangement, or the phosphorus lone
pair and the s(SbÀC) orbital as shown in Figure 7.[27]
[a] SDD/6-31G* basis. [b] SDD/6-31(+)G* basis, CPCM model (parameters
of CHCl3). [c] Computed enthalpy of isomers (marked with primes) relative
to the observed species at the B3LYP/SDD/6-31(+)G*/CPCM level in
kJmolÀ1; n/a=not applicable.
Notably, magnitudes of the observed through-space cou-
1
plings are larger than JCP through-bond couplings involving PIII
(e.g., Me3P 1JCP =13.6 Hz; tBu3P 1JCP =33.9 Hz) and of similar
1
magnitude to JCP values in PV species (e.g., Ph4P+ 88.4 Hz;
Compound 8 shows only one 5tsJ(Cipso–P) value (32.6 Hz)
since all the phenyl rings are isochronous, presumably due to
Ph3PO 104.4 Hz).[28]
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compared to those with no chlorine atoms trans to the phos-
phorus (Table 3). While PÀSb distances, when optimized in the
gas phase, are significantly overestimated with respect to the
values observed in the crystal (by up to 0.27 Š, see entry for 3
in Table 3), they shorten significantly when optimized in a con-
tinuum modelling a moderately polar solvent (chloroform in
this case). Compounds without Cl atoms in trans positions are
much less affected by the environment (see for instance entry
for 2 in Table 3). This observation is readily explained by meso-
meric resonance according to [P:Sb–Cl$P+–Sb:ClÀ], where
a polar environment favours the latter, ionic resonance struc-
ture.
Figure 7. Diagram showing the origin of the through-space coupling in 7
through overlap of the phosphorus lone pair with the s*(SbÀC) orbital (left)
5ts
and the s(SbÀC) orbital (right), observed magnitudes of the
in 7 are 86.4 and 77.8 Hz.
J constants
CP
The PÀSb distances, related WBIs and relative enthalpies of
selected isomers of compounds 2–4 and 6–8 (Figure 8) were
their rapid interchange in solution akin to Berry pseudo-rota- computed and are shown in Table 3. The strongest PÀSb inter-
tion. This is different to 9 (displaying 5tsJ(Cipso–P)=35.7, 31.1 action is found for 2, where it amounts to almost a full single
and 0.0 Hz) where three anisochronous phenyl environments bond (WBI exceeding 0.70). The isomer 2’ with a bent Cl-Sb-Cl
are observed. Similar to 9, no scrambling of the phenyl sub- moiety (angle 89.38) has a longer P–Sb distance and concomi-
stituents is observed in 7, presumably owing to the stronger tantly, a smaller, but still substantial WBI of about 0.49. An
P···Sb interaction.
even larger WBI of 0.60 is obtained for 3 (and 3-Mes, WBI
The 1H NMR spectrum of 9 shows a large difference (ca. 0.51). Also, the WBIs for the SbÀCl bonds trans to the phos-
0.93 ppm) in the dH of the CH3 environments, indicating re- phorus are much reduced (0.44 in 2’, 0.24 in 3), indicating true
stricted rotation of the iPr groups and spatial proximity to the multicentre bonding in these (type Y) species.
ring current of the phenyl substituents orientated perpendicu-
In most cases unconstrained search for localised molecular
lar to the P···Sb vector. Similar differences can be seen in the orbitals (MOs) in NBO[33] analyses did not find any SbÀCl
1H NMR spectra of 3, 3-Mes and 8 as these compounds also bonds. Instead chlorine atoms are almost invariably localised
possess phenyl groups perpendicular to the P···Sb vector.
as ClÀ ions (with four lone pairs), underscoring the highly polar
nature of these bonds. The only exception to this was for the
perchloroantimony compound 5, where the NBO analysis
found one PÀSb, one SbÀC and four SbÀCl bonds, each with
Computational analysis
To complement the experimental findings, we performed den- relatively low occupancies of 1.8 electrons (instead of the ex-
sity functional theory (DFT) calculations at the B3LYP/SDD/6- pected 2.0 electrons for a 2c–2e bond).
31(+)G* level[29] (compatible with our previous work). Opti-
The weakest phosphorus–antimony interaction is found in 4.
mised P–Sb distances and WBIs[30,31] are collected in Table 3. Despite the long P···Sb distance around 3.2 Š (the longest of
Consistent with results for related P–Sn species,[32] the PÀSb all the compounds in this study), the WBI is still noticeable at
distances in compounds with chlorine atoms trans to the phos- 0.15. This value is comparable to those of related systems such
phorus were considerably more affected by the environment as Acenap(PiPr2)(SnPh3) or Acenap(TePh)2 (Acenap=acenaph-
Figure 8. Compounds synthesised in this study and their higher lying computed isomers. Computed enthalpy of isomers (marked with primes) are relative to
the observed species at the B3LYP/SDD/6-31(+)G*/CPCM level in kJmolÀ1
.
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thene-5,6-diyl), where the onset of the 3c–4e bonding be-
tween the formally non-bonded atoms has been shown
through the sizable J(31P–119Sn) or J(125Te–125Te) coupling con-
stants.[32,34] Unfortunately, the unfavourable properties of the
NMR-active antimony isotopes (121Sb and 123Sb), in particular
with varying polarity and was compared to the experimental
X-ray structure. The results of these computations are shown
in Table 4.
Regardless of the polarity of the modelling solvent (CHCl3,
MeCN, H2O), the PÀSb bond length in 3 does not vary too dra-
their high quadrupolar moments, prevent the use of direct PÀ matically from the distance observed in the X-ray structure
Sb NMR probe in this series. On the other hand, compound 4
(largest change is ca. 0.1 Š). However, the PÀSb distance does
show a large increase (ca. 0.26 Š) when modelled without sol-
vent (i.e., in gas phase). The SbÀCl bond length depends heav-
ily on the medium in which the optimisation is modelled. In
highly polar solvents, such as water and acetonitrile, the SbÀCl
distance increases by about 0.28 Š as compared to the X-ray
structure with the computed WBIs also showing a complemen-
tary change (Scheme 2 and Table 4).
13
displays large magnitude (40.1 Hz) of a 31PÀ C coupling involv-
ing the ipso-carbon from a phenyl group on the antimony
5ts
atom (atom C13 in Figure 2, hence formally JCP). This is likely
to involve a strong through-space component, indicating sig-
nificant overlap of orbitals on P and Sb atoms.
The SbV compounds (5–9) all show significant PÀSb interac-
tions. These are particularly strong when a chlorine atom is lo-
cated trans to the phosphorus atom, as in 5 and 6 (WBIs ex-
ceeding 0.5), but are still substantial with phenyl groups trans
to phosphorus, as in 7–9 (WBIs ca. 0.2–0.3). The WBI data for
the corresponding hypervalent tin compounds Acenap-
(PiPr2)(SnPhnCl3–n) in a previous study were found to be around
0.3–0.4 for the mixed Ph/Cl substituents (where chlorine is
always trans to phosphorus), and 0.12 for the SnPh3 deriva-
tive.[32] The covalent multicentre character is thus more pro-
nounced for the P–Sb than for the PÀSn motif.
Table 4. Medium effects on PÀSb and SbÀCl bond distances [Š] calculat-
ed for compound 3 at the B3LYP/SDD level.
Medium
Basis set[a]
dP-Sb
(WBI)[b]
dSb-Cl
(WBI)[b]
crystal (X-ray)
n/a
6-31G*
6-31+G*
6-31+G*
6-31+G*
6-31+G*
2.710 [0.48]
2.978 [0.32]
2.966 [0.33]
2.800 [0.50]
2.721 [0.59]
2.713 [0.60]
2.680 [0.46]
2.533 [0.57]
2.550 [0.56]
2.753 [0.37]
2.935 [0.26]
2.959 [0.24]
gas
gas
CHCl3
[c]
For the mixed Ph/Cl compounds (6, 7 and 8), the computed
stereoisomers (6’, 7’ and 8’; see Figure 8) are all energetically
disfavoured with DHrel lying between about 7 and 60 kJmolÀ1
higher in energy (Table 3). The B3LYP/CPCM WBIs are plotted
against the optimised PÀSb distances in Figure 9, illustrating
the expected inverse relationship.
MeCN[c]
H2O[c]
[a] SDD on Sb and 6-31G* elsewhere. [b] WBIs obtained at the same level
are given in square parentheses. [c] CPCM model; n/a=not applicable.
As mentioned earlier, compounds with chlorine atoms trans
to the phosphorus atom were more affected by the environ-
ment used to model them owing to the resonance P-Sb-
Cl$P+ÀSb···ClÀ. In order to gain further insight into this, the
structure of 3 was optimised using a set of modelling solvents
Scheme 2. Structures of 3 modelled in different media with P–Sb and Sb–Cl
distances given along with their corresponding WBIs (in parentheses).
In the gas phase (using both 6-31G* and 6-31+G* basis) the
P–Sb distance is long enough to be considered a non-bonding
interaction, and the SbÀCl bond length is short (of typical co-
valent length). The other extreme of this is using the CPCM cal-
culations for water and acetonitrile in which the P–Sb distance
is shorter (WBI reaching 0.60), yet the Sb···Cl distance is long
enough for the compound to be consider a phosphonium salt
with a chloride counter ion. The observed X-ray structure is
middle ground, with WBIs in between those of the two ex-
tremes (Scheme 2). This short study serves to show the effect
the solvating medium has on the compound with tendency to
ionise in highly polar medium.
Figure 9. Plot of computed P–Sb WBIs against optimised P–Sb distances;
data taken from Table 3 (B3LYP/CPCM level). Unfilled squares represent com-
pounds 2–9 and filled squares represent calculated stereoisomers (2’, 3’, 3’’,
3’-Mes, 4’, 6’, 7’ and 8’). Included is also a hypothetical compound Me2P–
SbMe2 with a full formal single bond (filled square, top left). An exponential
fit to all points is included to guide the eye.
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Conclusion
Acknowledgements
Two distinct comprehensive series of intramolecular phosphine
complexes with antimony-centred groups of varied Lewis acidi-
ty were synthesised and fully characterised. The presence of
the supporting backbone in these species allows observation
of the full scale of the PÀSb interaction, from strongly bonding
to non-bonding (i.e., repulsive). The structural and computa-
tional data demonstrate the ambiguity of the assignment of
a bond as a standard covalent or strong dative, and also high-
lights the existence of situations in which description of the in-
teraction as bonding or non-bonding is somehow arbitrary.
No fundamental geometry change (rehybridisation) is ob-
served within the series of SbV compounds, with the coordina-
tion environment of the Sb acceptor remaining octahedral
throughout. The two extremes, compounds 5 and 9, can be
described as PÀSb covalently bonded (5) and non-bonded (9).
Within this series the PÀSb bond length increases by 19%,
while WBI drops from 0.57 to 0.22. Despite the Lewis acidity of
the SbV-centred groups being modified gradually (in regular
steps) by varying the ratio of Cl and Ph substituents, the P–Sb
bond strength trend within the series is not smooth. A sudden
structural change is observed between 6 and 7, with the P–Sb
distance being elongated by 12%, and WBI dropping from
0.53 to 0.29 on replacing just one of the chlorine groups with
phenyl group. This can be considered as the dividing line be-
tween the bonding and non-bonding (i.e., repulsive) interac-
tion, although this division is somewhat arbitrary as the WBI in
7 is still 54% of that in 6, and certainly remains significant. The
P–Sb contacts in 6 and 7 are both strongly sub-van der Waals
(64 and 72% of respective SrvdW of 4.15 Š). However, the rela-
tively sudden drop in WBI between 6 and 7 serves as an illus-
tration of non-linear relationship of Lewis acidity/Lewis basicity
difference and strength of dative interaction.
This work was financially supported by the EPSRC and COST
actions CM0802 PhoSciNet and CM1302 SIPs. The authors
would also like to thank the University of St. Andrews NMR
Service and to Mrs. Caroline Horsburgh for running the MS
spectra. M.B. thanks the School of Chemistry and EaStCHEM for
support and for access to a computer cluster maintained by
Dr. H. Früchtl.
Keywords: antimony · dative bond · donor–acceptor systems ·
phosphorus · synthesis
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Experimental Section
For full experimental details see the Supporting Information.
Chem. Eur. J. 2015, 21, 7520 – 7531
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Published online on March 27, 2015
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim