Assessing Phosphine-Chalcogen Bond Energetics from Calculations

Article abstract of DOI:10.1021/acs.organomet.5b00428

Phosphine chalcogenides are useful reagents in chalcogen atom transfer reactions and nanocrystal syntheses. Understanding the strength and electronic structure of these bonds is key to optimizing their use, but a limited number of experimental and computa

Full text of DOI:10.1021/acs.organomet.5b00428

Article
pubs.acs.org/Organometallics
Assessing Phosphine−Chalcogen Bond Energetics from Calculations
Samuel R. Alvarado,†,‡ Ian A. Shortt,§ Hua-Jun Fan,*,§ and Javier Vela*,†,‡
†Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States
‡Ames Laboratory, Ames, Iowa 50011, United States
§Department of Chemistry, Prairie View A&M University, Prairie View, Texas 77446, United States
S
* Supporting Information
ABSTRACT: Phosphine chalcogenides are useful reagents in chalcogen atom
transfer reactions and nanocrystal syntheses. Understanding the strength and
electronic structure of these bonds is key to optimizing their use, but a limited
number of experimental and computational studies probe these issues. Using
density functional theory (DFT), we computationally screen multiple series of
trisubstituted phosphine chalcogenide molecules with a variety of phosphorus
substituents and examine how these affect the strength of the phosphorus−
chalcogen bond. DFT provides valuable data on these compounds including P−
E bond dissociation energies, P−E bond order, Lowdin charge on phosphorus and chalcogen atoms, and molecular geometries.
̈
Experimentally monitoring the 31P and 77Se NMR chemical shifts and published Hammett constants provides good estimates and
confirmation of the relative magnitude of electronic shielding around these nuclei and confirms the predictive value of the
computational results.
phosphine chalcogenide-metal activated complex, which
decomposes into metal chalcogenide nuclei.12 The mechanism
of R3PE decomposition has been studied for the synthesis of
CdSe,13−15 PbSe,16,17 and ZnSe18 nanocrystals. The electron-
donating and -withdrawing effects of different phosphorus
substituents have an effect on the mechanism of InP formation
from triarylsilylphosphines.19
Studies on the electronic structure of R3PE compounds and
the reactive P−E bond are key to guiding their use as both
atom transfer reagents and nanocrystal synthesis precursors. It
is well established that heavier chalcogens form significantly
weaker and longer bonds with phosphorus. Calorimetric
methods and atom transfer reactions have been used to
measure the strength of P−E bonds experimentally.20 Bond
dissociation energies of phosphine sulfides spanned a range of
88−98 kcal/mol, while those of phosphine selenides were in
the range 67−75 kcal/mol.21 Bonding in trialkyl phoosphine
chalcogenides22,23 has also been studied computationally using
density functional theory (DFT)24 and atoms in molecules
(AIM).25 Our group recently used DFT to estimate the P−E
bond strengths of a selection of phosphine sulfide and selenide
derivatives that are particularly useful in the preparation of
colloidal semiconductor nanocrystals (quantum dots and
rods).26
INTRODUCTION
■
Tertiary (trisubstituted) phosphine chalcogenides (R3PE,
where R = alkyl, aryl, amide, alkoxyl, and E = S, Se, Te) are
molecular compounds useful in a variety of chemical trans-
formations including chalcogen atom transfer reactions and
chalcogenide nanocrystal synthesis. In comparison to the
unsupported elemental chalcogens, the substituents (R) on
the phosphine chalcogenide can be used to fine-tune the
solubility and reactivity of the phosphorus−chalcogen (PE)
moiety in these compounds.1
In atom transfer reactions, phosphine chalcogenides donate
sulfur, selenium, or tellurium in a bimolecular fashion.2,3
Current evidence indicates that the rate of atom transfer is
dependent on the relative basicity of the pnictogen center (P,
As, or Sb). This transfer can occur between a phosphine
chalcogenide and another P, As, or Sb atom.4 Computations
suggest that the transfer of S and Se atoms among phosphines
proceeds through chalcogen-philic attack by the pnictide
nucleophile.5 Phosphine sulfide-supported palladium com-
plexes6 as well as Cu(I) and Zn(II) catalysts7 mediate this
transformation. A synthetic application of this strategy is Se
atom transfer from triphenylphosphine selenide to H-
phosphonate diesters.8 Similarly, tricyclohexylphosphine sele-
nide and telluride donate a chalcogen atom to N-heterocyclic
carbenes.9
Here we greatly expand our investigation of P−S and P−Se
bond dissociation energies (BDEs) using DFT methods. To
understand how changing the electron density around the PE
moiety influences the electronic structure and strength of the
P−E bond, we closely examine different families of
Because of their desirable reactivity and solubility in low-
volatility (high boiling point) solvents, trialkyl phosphine
chalcogenides have been popular chalcogen sources in
nanocrystal preparations since the early 1990s.10 Cleavage of
the P−E bond is thought to occur by either redox chalcogen
(E0) atom transfer or acid−base chalcogenide (E2−) transfer
mechanisms.11 The latter mechanism proceeds through a
Received: May 20, 2015
© XXXX American Chemical Society
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a
Chart 1. Tertiary (Trisubstituted) Phosphine Chalcogenide Compounds (R3PE, E = S or Se) Studied in This Work
a
The compounds in the first column were calculated previously.26
triarylphosphine chalcogenides containing substituents of
varying resonance and inductive effects. We also investigate
trialkyl, tris-perfluoroalkyl, and caged (Verkade-type) phos-
phine chalcogenides.27 We anticipate that the results of this
large computational screening will be generally applicable to a
variety of problems and applications that make use of
phosphine chalcogenides, including chalcogen atom transfer
and nanomaterial synthesis reactions.
five selenides) were correlated to experimental 31P (and 77Se
NMR) data. Experiments showed that the relative rate of
(homogeneous) CdE nucleation increases more dramatically
than the rate of CdE growth (heterogeneous nucleation) with a
decrease in precursor P−E bond energy (E = S or Se).26
In order to generalize this approach, we have used DFT (see
Computational and Experimental Methods) to expand the
range of computed tertiary phosphine chalcogenide BDEs. For
simplicity, we categorize the specific chalcogenide compounds
in our study into five families based on the type of tertiary
phosphine that they are derived from: (a) triaryl phosphines
monosubstituted with electron-donating or -withdrawing
groups (amino, −NH2; methoxy, −OMe; fluoro, −F; carboxyl,
−CO2H; nitro, −NO2); (b) triaryl phosphines substituted with
one, two, or three methoxy groups; (c) triaryl phosphines
substituted with one, two, or three fluorines; (d) trialkyl and
triperfluoroalkyl phosphines; and (e) caged (Verkade-type)
tertiary phosphines (Chart 1).27 In all cases, we modeled a
RESULTS AND DISCUSSION
■
We have previously shown that bond dissociation energies can
be good indicators of molecular precursor reactivity and
selectivity. Specifically, we have been able to use computed
BDEs of tertiary phosphine chalcogenides26 and disubstituted
dichalcogenides28 to predictably fine-tune the composition,
aspect ratio (of rods), and morphology (from dots to rods to
tetrapods) of CdS−CdSe nanocrystals. In the case of tertiary
phosphine chalcogenides, 10 computed BDEs (five sulfides and
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Figure 1. Calculated P−E bond dissociation enthalpies (ΔH) in tertiary phosphine sulfides (R3PS) and selenides (R3PSe). For consistency with
prior work and discussions, we arbitrarily use values of ΔH as a measure of the P−E BDEs.
homolytic P−E bond dissociation energy for the release of
sulfur or selenium atom from the corresponding phosphine
chalcogenide. In all cases, we first optimized the geometries of
R3PE and R3P, assumed triplet E, and calculated the change in
electronic energy after correcting for zero-point energy
(ΔEZPE), the change in enthalpy (ΔH), and the change in
Gibbs free energy (ΔG) values corrected to 298.15 K (a full list
of all of our results is available in the Supporting Information).
For consistency with prior work and discussions, we arbitrarily
use values of ΔH as a measure of the PE BDEs (for
trisubstituted phosphine chalcogenides calculated here and
elsewhere, we find that the ΔH and ΔG values follow very
similar trends).26
titanocenes also shows a similar correlation between bond
strength and bond length.32
The only exception to the aforementioned trend comes from
discontinuities in bond strengths and lengths observed for the
methoxy- and, to a lesser extent, fluorine-substituted
triarylphosphine chalcogenides (Figure 2b and c, respectively).
The specific outliers are compounds that feature substitution at
the 2 or ortho position, i.e., adjacent to the P−E bond, such as
(2,6-MeO2-C6H3)3PE (Chart 1). A possible explanation is that
p orbitals from the adjacent heteroatom substituents (O, F)
may be interacting with the P−E bond itself. This interaction
could potentially increase the P−E distance and weaken the
bond accordingly. To further investigate the presence and effect
of this interaction, we visualized the total electron density of
three molecules: Ph3PS, (2,6-(MeO)2-C6H3)3PS, and
(C6F5)3PS (Figure 3a−c). In the optimized geometries of the
latter two compounds, electron density from the neighboring
ortho substituents may interact with the P−E bond. Therefore,
while the methoxy groups should increase electron density at
the P−E bond via resonance effectsthe reported pKa of
(2,4,6-(MeO)3-C6H2)3PS is 11.233their steric bulk may
actually cause significant weakening of the P−E bond.
Calculated average distances for these interactions in (2,6-
(MeO)2-C6H3)3PS, (2,4,6-(MeO)3-C6H2)3PS, (2,6-(MeO)2-
C6H3)3PSe, and (2,4,6-(MeO)3-C6H2)3PSe are 3.15, 3.16,
3.23, and 3.24 Å, respectively. Interestingly, in (C6F5)3PE,
one of the three aryl rings in these compounds is rotated nearly
coplanar with the P−E bond (Figure 3c). The nearest
calculated F−E interaction is 3.09 Å in (C6F5)3PS and 3.17 Å
in (C6F5)3PSe. The van der Waals radii of F, S, Se, and P are
1.47, 1.80, 1.90, and 1.80 Å, respectively.34 Interactions
between S and F atoms were previously revealed by single-
crystal X-ray crystallography.35,36
General Observations: Bond Strength vs Bond
Length. Figure 1 shows the calculated BDEs (ΔHs) for all
the compounds we have studied. Across the five compound
families (a through e above), the heavier phosphine selenides
exhibit significantly longer (by ca. 0.15 Å) and weaker (by ca.
13−15 kcal/mol) bonds compared to the corresponding
phosphine sulfides (Figure 2). This is expected given that the
larger selenium atom has larger, more diffuse orbitals, resulting
in poorer orbital overlap with the phosphorus orbitals as
compared to the smaller sulfur atom. Interestingly, this is not
the case within each of the sulfide or selenide series, where the
BDE actually increases with increasing P−E distance (with
some exceptions, see below). For example, the monosub-
stituted triarylphosphine chalcogenides show a near-linear
increase in BDE across the P−S and P−Se series individually
(Figure 2a). This trend is also seen in fluorinated triaryl
phosphines substituted with one, two, or three fluorines (Figure
2c), trialkyl and triperfluoroalkyl phosphines (Figure 2d), and
caged Verkade phosphines (Figure 2e), although these are not
as linear as the aromatic phosphine chalcogenides. While this
trend was unexpected and may seem counterintuitive, there are
several well-documented examples of longer bonds being
stronger in covalent compounds of tin29 and its lighter
̈
Bond Strength vs Lowdin Charges. To understand why
longer bonds are slightly stronger among homologous
(chalcogen constant) families of phosphine chalcogenides, we
examined the partial charge on the phosphorus and chalcogen
analogues,30 as well as S−F bonds in SF2 dimers.31
crystallographic study of phosphine adducts of open
A
atoms in these compounds using a Mulliken population
̈
analysis37−40 of symmetrically orthogonalized orbitals.41
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Figure 2. Calculated P−E bond dissociation enthalpies (ΔH) (E = S or Se) vs P−E distances for monosubstituted triarylphosphine chalcogenides
(a), fluorinated triarylphosphine chalcogenides (b), methoxy-substitued triarylphosphine chalcogenides (c), trialkyl and triperfluoroalkyl phosphine
chalcogenides (d), and caged (Verkade-type) tertiary phosphine chalcogenides (e).
Mulliken population analysis assigns a partial charge to each
̈
atom in the molecule. Subsequent Lowdin analysis prevents
̈
excessive charge buildup on any given atom, which in some
cases can lead to more than two electrons sharing a single
orbital.
Our Lowdin analysis clearly shows that the P atom is much
̈
more positively charged (by ca. 0.11+) in the R3PSe
compounds than in the analogous R3PS compounds (Figure
4). Similarly, the chalcogen (E) atom is much more negatively
charged (by ca. 0.10+) on the R3PSe compounds than in the
R3PS compounds (Figure 4). Interestingly, in most compounds
studied, a larger positive charge on P and a larger negative
charge on E leads to an increase in the P−E BDE. This is true
across chalcogens as well as within each separate (sulfide or
selenide) family. Small deviations from this trend in the
methoxy- and fluorine-ortho-substituted cases can be attributed
to a redistribution of electron density around the P−E moiety
due to close steric contacts as shown above (Figure 3).
Figure 3. Total electron density, visualized at a contour value of 0.05,
for Ph3PS (a), (2,6-MeO2-C6H3)3PS (b), and (C6F5)3PS (c). We note
the close contact of p orbitals from methoxy groups to the P−E
moiety.
The observation that a more positively charged P atom and a
more negatively charged E atom result in a stronger P−E bonds
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Figure 4. Calculated P−E bond dissociation enthalpies (ΔH) (E = S or Se) vs Lowdin charges on P and E atoms in monosubstituted
̈
triarylphosphine chalcogenides (a), fluorinated triarylphosphine chalcogenides (b), methoxy-substitued triarylphosphine chalcogenides (c), trialkyl
and triperfluoroalkyl phosphine chalcogenides (d), and caged (Verkade-type) tertiary phosphine chalcogenides (e).
implies that, in R3PE compounds, the P−E bond strength increases
with ionic character. Three possible structures may be
envisioned to rationalize the distribution of electron density
around the P−E moiety in these compounds (Scheme 1).
Structure I is a resonance form that features a double bond
between formally neutral P and E atoms. Structure II is a
zwitterionic resonance form that has a single bond between a
positively charged P atom and a negatively charged E atom.
Structure III is an intermediate between the first two resonance
structures, containing some partial double-bond character and
some partial charge on each P and E (Scheme 1). In line with
previous investigations,25 our results above strongly suggest
that the P−E bond has an order between 1 and 2, is composed
of a mixture of covalent and ionic character, and becomes
stronger as the ionic character increases.
Substituent Effects. To see how resonance or inductive
effects influence the strength of the P−E bond, we compared
Scheme 1. Three Resonance Structures to Rationalize the
Distribution of Electron Density around the P−E Bond in
Tertiary Phosphine Chalcogenides: A Formal Double Bond
(I), a Formal Single Bond with a Phosphorous Cation and a
Chalcogenide Anion (Zwitterionic Structure) (II), and a
Hybrid (Intermediate) Resonance Form between I and II
with a P−E Bond Order between 1 and 2 (III)
the bond dissociation ΔH(BDE) and Lowdin charges of para-
̈
and meta-monosubstituted (with NH2−, MeO−, H−, F−,
HO2C−, and F− groups) triarylphosphine chalcogenides
against known Hammett constants (σp or σm).42 As shown in
Figure 5, an increase in P−E bond strength, as measured by the
ΔH(BDE), is accomplished by increasing the electron-donating
ability of the para substituent, while the meta substituent has a
smaller effect. However, both para and meta substituents have
minor effects on the buildup of positive and negative Lowdin
̈
charges on P and chalcogen (E), respectively (Figure 6).
E
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GAMESS from the sum of the density matrices of the atoms in
question, as described previously.43−45 As could be expected
based on simple atom size and orbital overlap considerations
(see above), we find that the P−Se bond order (1.4−1.5) is
generally lower than P−S bond order (1.7−1.8). For each
chalcogenide family (sulfides or selenides), there is a stronger
linear correlation between the P−E bond order and the identity
of para substituents than that of the meta substituents (Figure
7). Increasing the electron-donating ability of the para
Figure 5. Calculated P−E bond dissociation ΔH(BDEs) vs Hammett
constants in 4- (para) (a) and 3-substituted (meta) triarylphosphine
chalcogenides (b).
Figure 7. Calculated P−E bond order vs Hammett constants in 4-
(para) (a) and 3-substituted (meta) (b) triarylphosphine chalcoge-
nides.
substituent decreases the P−E bond order by up to ca. 0.1
(Figure 7a). This is consistent with the stronger electronic
influence of substituents in the para and ortho positions
compared to the meta position in organic chemistry. Thus,
while bonding in the phosphine chalcogenides appears to have
significant ionic character as noted above (Scheme 1),
computational24 and experimental studies show that a double-
bond resonance structure (or at least partial multiple-bond
character) is still important in rationalizing trends within closely
related families of compounds such as para-monosubstituted
triarylphosphine chalcogenides.
Correlating Experimental and Computational Results.
Finally in this study, we sought to gain new insight into the
predictive value of our calculations by investigating selected
triarylphosphine compounds experimentally with 31P and 77Se
spectroscopy (see Supporting Information for a complete table
of NMR chemical shifts). The compounds we monitored by
NMR include (2,4,6-MeO3-C6H2)3PE, (4-MeO-C6H4)3PE, (4-
F-C6H4)3PE, and Ph3PE; additionally, we added data from
compounds we studied previously,26 namely, (PhO)3PE,
(Et2N)3PE, (n-Pr)Ph2PE, (n-Bu)3PE, and (n-octyl)3PE. Pre-
viously, the effect of substituents around P−E bonds on NMR
spectra has been investigated in related arylphosphorothio-
Figure 6. Calculated Lowdin charges on phosphorus and chalcogen
atoms vs Hammett constants in 4- (para) (a) and 3-substituted (meta)
(b) triarylphosphine chalcogenides.
̈
Bond Order and Substituent Constants. To further
examine the P−E bond in para- and meta-monosubstituted
triarylphosphine chalcogenides, we calculated bond orders in
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nates46 and triarylselenophosphates.47 The 31P−77Se coupling
constant is also influenced by adjacent substituents.48
With multiple factors influencing the electron density on the
calculations in Gaussian 03 using the optimized molecular
geometries. There is a correlation between calculated NMR
tensor and increasing ΔH(BDE). These results imply that
increasing the electronic shielding around the Se nucleus leads
to a stronger P−E bond with a stronger ionic character.
P atom, it is not surprising to find out the scatter plot of all 31
P
NMR chemical shifts (sulfides and selenides) and 1J(31P−77Se)
coupling constants (for selenides only) available to us does not
show an immediate correlation with our calculated P−E bond
dissociations, ΔH(BDEs) (Figure 8a). The lack of correlation
CONCLUSIONS
■
We have investigated the bond strength and nature of bonding
in multiple families of phosphine chalcogenide compounds.
Generally, within a specific chalcogen family (sulfides or
selenides), DFT computations show that the ΔH(BDE)
increases as the P−E bond distance increases. This may be
due to increasing partial positive and negative charges on the
phosphorus and chalcogen atoms, respectively, which results in
a stronger, slightly longer bond with greater ionic character.
Monosubstituted triarylphosphine chalcogenides with electron-
donating groups and negative Hammett constants exhibit
stronger bonds, while the same class of compound with
electron-withdrawing groups and positive Hammett constants
exhibit weaker bonds. Electron-donating groups add electron
density to the P−E unit, allowing for a bond of more ionic
character by stabilization of the positive charge buildup on P.
The net result of this effect is a P−E bond order between 1 and
2. We show that the computational results can be verified by
77Se NMR spectroscopy, where more shielded nuclei having
more negative 77Se chemical shifts have stronger P−Se bonds
as measured by higher ΔH(BDE) values. We anticipate that our
results will allow for the design of new atom transfer and
colloidal nanocrystal synthesis reagents and reactions with full
reproducibility and enhanced utility.
COMPUTATIONAL AND EXPERIMENTAL
METHODS
■
Computations. All calculations were carried out using
GAMESS49,50 (May 2013 version, revision 1) with DFT and the
Tao−Perdew−Staroverov−Scuseria (TPSS)51,52 functional. The accu-
racy of the new generation functional TPSS is known to match or
exceed almost all prior functionals, including the popular hybrid
functional B3LYP.53 TPSS reproduces geometric properties at least as
precisely as B3LYP and can recognize relatively weak interactions
(such as agostic interactions), while B3LYP significantly under-
estimates them.54 Since hydrogen atoms in the systems we modeled
did not play significant roles, we used the 6-311G* basis set55 for all
elements. By not applying polarization functions on H atoms far from
the phosphorus center, the calculations are accelerated considerably
without significantly degrading computational precision or accuracy.56
All structures were fully optimized and Hessian calculations (frequency
analyses) were performed to ensure a minimum was achieved with
zero imaginary vibrational frequencies. Thermodynamic functions,
including enthalpies, entropies, and free energies, were calculated at
298.15 K and 1 atm. Results were visualized with MacMolPlt.57
Calculations of NMR tensors were carried out using Gaussian 0358 at
the same level of theory as used above with the gauge-independent
atomic orbital (GIAO) method.59−63
Figure 8. 31P NMR chemical shifts and 1J(31P−77Se) coupling
constants vs calculated P−E ΔH(BDEs) of tertiary phosphine
chalcogenides (a). 77Se NMR chemical shifts and calculated 77Se
tensors vs calculated P−E ΔH(BDEs) of tertiary phosphine selenides.
between 31P and P−E bond energy data is more generally a sign
that bond angles and geometry, which vary widely among all
the calculated structures, heavily impact 31P chemical shifts. On
the other hand, we find there is a very strong correlation
between our experimentally measured 77Se NMR chemical
shifts and our calculated P−E bond dissociations, ΔH(BDEs),
for the trisubstituted phosphine selenides (Figure 8b). More
specifically, the 77Se NMR chemical shifts move upfield and
become more negative, as the P−Se ΔH(BDE) values increase.
As mentioned above, a larger ΔH(BDE) value corresponds to a
Materials. Unless otherwise noted, all chemicals were used as
received without further purification. Triphenylphosphine (99%) was
purchased from Acros; sulfur (99.999%), selenium (99.999%), and
tris(4-fluorophenyl)phosphine (98%) were purchased from Alfa Aesar;
tris(pentafluorophenyl)phosphine (98%), tris(4-methoxyphenyl)-
phosphine (98%), and tris(2,4,6-trimethoxyphenyl)phosphine (98%)
were from Strem; toluene (99.9%), xylenes (99.9%), and chloroform
(99.9%) were from Fisher.
more polarized P−Se bond and a higher partial (Lowdin)
̈
negative charge on Se; this increase in electron density at Se
helps explain the consequent and progressive upfield shift of
the 77Se resonances as the P−Se becomes stronger. We
corroborated these experimental results with 77Se NMR tensor
Characterization. 31P NMR chemical shifts were referenced to
85% phosphoric acid, H3PO4 (δ 0 ppm). 77Se NMR spectra were
referenced to Ph3PSe/CDCl3 (δ 266.20 ppm vs Me2Se δ 0 ppm).
G
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Synthesis. Triphenylphosphine selenide,15 tris(4-fluorophenyl)-
phosphine sulfide, tris(4-fluorophenyl)phosphine selenide, tris(2,4,6-
trimethoxyphenyl)phosphine sulfide, and tris(2,4,6-trimethoxyphen-
yl)phosphine selenide64 were prepared as described previously.
Tris(pentafluorophenyl)phosphine sulfide was prepared with a
modified procedure;65−67 briefly, tris(pentafluorophenyl)phosphine
(50.3 mg, 0.0945 mmol) and sulfur (2.8 mg, 0.0873 mmol) were
heated to reflux in xylenes for 4 days. Solvent was removed under
vacuum. The crude was recrystallized from ethanol to give white
needles (29.5 mg, 60.1%). 31P NMR: −8.28 ppm; 19F NMR: 131.54
ppm (d, 22.20 Hz), 143.53 ppm (t, 21.07 Hz), 157.84 ppm (t, 20.89
Hz).
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ASSOCIATED CONTENT
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̈
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Corresponding Authors
*E-mail: hjfan@pvamu.edu.
*E-mail: vela@iastate.edu.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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J.V. gratefully acknowledges the National Science Foundation
for funding of this work through the Division of Materials
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DMR-1309510). H.-J.F. thanks the Department of Chemistry
at Prairie View A&M University for release time and a 2014
Summer Research mini-grant (115103-00011), and the U.S.
Department of Energy, National Nuclear Security Admin-
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