Theoretical and experimental studies on formation of diethylzinc-triphenylphosphine complex

Article abstract of DOI:10.1080/10426507.2015.1085037

This work is mainly focused on understanding the complex-forming behavior of diethylzinc with neutral phosphine ligands such as triphenylphosphine (PPh3) to yield [Zn(PPh3)2Et2]. The complex formation in solution is observed in the presence of a large exc

Full text of DOI:10.1080/10426507.2015.1085037

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Theoretical and experimental studies on
formation of diethylzinc-triphenylphosphine
complex
Venkatesh Sadhana, Dhrubjyoti Das, Chinduluri Sravania, Akella
Sivaramakrishna, Kari Vijayakrishna & Hadley S. Clayton
To cite this article: Venkatesh Sadhana, Dhrubjyoti Das, Chinduluri Sravania, Akella
Sivaramakrishna, Kari Vijayakrishna & Hadley S. Clayton (2016) Theoretical and experimental
studies on formation of diethylzinc-triphenylphosphine complex, Phosphorus, Sulfur, and
Silicon and the Related Elements, 191:1, 35-40, DOI: 10.1080/10426507.2015.1085037
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1085037
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Date: 17 January 2016, At: 23:07

PHOSPHORUS, SULFUR, AND SILICON
, VOL. , NO. , –
http://dx.doi.org/./..
Theoretical and experimental studies on formation of diethylzinc-triphenylphosphine
complex
Venkatesh Sadhanaa, Dhrubjyoti Dasa, Chinduluri Sravaniaa, Akella Sivaramakrishnaa, Kari Vijayakrishnaa, and Hadley
S. Claytonb
aChemistry Division, School of Advanced Sciences, VIT University, Vellore  , Tamil Nadu, India; bDepartment of Chemistry, UNISA, Pretoria, South
Africa
ABSTRACT
ARTICLE HISTORY
Received  March 
Accepted  August 
This work is mainly focused on understanding the complex-forming behavior of diethylzinc with neutral
phosphineligandssuchastriphenylphosphine(PPh3)toyield [Zn(PPh3)2Et2]. Thecomplexformation in solu-
tion is observed in the presence of a large excess of diethylzinc, but leaving an insoluble solid on aging. The
product formed in solution was analyzed by spectroscopic data. 31P-NMR was also used as a tool to observe
this behavior, i.e., the disappearance of the chemical shift of PPh3 (δ -5.45) requires 14-fold excess of ZnEt2 in
solution. The alkyl chains reduce the Lewis acidity on Zn and thereby the formation of phosphine adducts
is restricted. Results obtained from orbital analyses calculations reveal that the LUMO appears to be asym-
metrically distributed, and localized on one of the PPh3 ligands. The length of alkyl chains also influence the
stability of [Zn(PPh3)2R2] and the longer chains on Zn impart less stability.
KEYWORDS
Organozinc complexes;
coordination behavior;
triphenylphosphine; DFT
calculation
GRAPHICAL ABSTRACT
ligand, metal, substituents on the alkyl chains.2,3 Metal-dialkyls
[M(CH2R)2] decompose thermally by intermolecular attack on
a C-H bond of an ancillary ligand via several modes such as
α-, β-, or γ -hydrogen elimination, and also reductive elim-
ination.4 For example, the metal dialkyls are ZnR2, PtL2R2,
and MoO2L2R2, which have been studied in detail, have been
Introduction
Metal-dialkyl species are an interesting class of compounds
and known as key intermediates in many catalytic reactions
including alkane functionalization reactions through the acti-
vation of carbon hydrogen bonds.1 In general, the reactiv-
ity of dialkyl-metal complexes depends on the nature of
CONTACT Akella Sivaramakrishna
asrkrishna@vit.ac.in Chemistry Division, School of Advanced Sciences, VIT University, Vellore  , Tamil Nadu, India
Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/gpss.
©  Taylor & Francis Group, LLC

36
V. SADHANA ET AL.
shown to undergo these types of reactions.5 Among them, the
chemistry of organozinc is very well established6 and it is well
known that zinc can form a variety of compounds with 6–
8 coordination numbers, relatively little has been reported on
the coordination behavior of diethylzinc. The Zn-alkyls, espe-
cially diethylzinc are important reagents for chemists to perform
asymmetric organic transformations including cyclopropana-
tion of alkenes with CH2I2,7–9 asymmetric additions to alde-
hydes and ketones10,11 and other addition reactions.12 It has
been found that organozinc compounds are capable of forming
stable coordination complexes having well-defined stoichiom-
etry with a variety of oxygen-, nitrogen-, phosphorus-, and
arsenic-containing ligands.13 The present work reports an unex-
pected and unusual reactivity of ZnR2 complex with phosphine
ligands.
at δ -5.45. There is an additional signal which appears at δ 29.6
due to the formation of [Zn(PPh3)2Et2]. The changes in the sig-
nal intensities at different ratios of PPh3 and ZnEt2 are given in
Figure 1 (A and B). Initially the molar concentration of PPh3 was
kept constant and a molar concentration of ZnEt2 was increased
and then vice versa. The peak intensity of the product is found
to increase with the increase of ZnEt2 concentration and then
decrease as the PPh3 concentration is increased.
From 31P{1H}-NMR data, it was observed that the signal at δ
29.6 increases gradually with an increase in the concentration of
ZnEt2. It is significant to note that one mole of PPh3 requires 14
moles of ZnEt2 to cause the free PPh3 signal at complex δ -5.45
to disappear as shown in Figure 1.
1
As expected, the H-NMR spectrum of 2a or 2b was not
much use in the structural analysis due to the presence of a large
excess of diethylzinc as shown in Figure 2. In this context, the
31P-NMR was a key tool for the investigations. On addition of 7
and 14 moles of PPh3 to the 1:14 molar ratio of the mixture, a
new peak was observed at δ -5.43 in 31P spectrum, which indi-
cates the presence of free ligand. When the molar concentration
was increased up to 14 moles, the peak intensity of the free lig-
and also increases.
Results and discussion
Initially, the complex 1 was prepared by reacting anhydrous zinc
chloride with PPh3 at room temperature in CH2Cl2.14 Upon
addition of 1 to freshly prepared n-BuMgBr or n-octylMgBr
at 0°C in dry ether, 31P{1H}-NMR chemical shift of the crude
product mixture showed the free PPh3 (a signal at δ -5.45) due
to the dissociation of PPh3 and subsequent formation of dialkyl-
zinc (3) (Scheme 1). Also, the presence of 3 was confirmed
It is found that when the mixture containing the product,
[Zn(PPh3)2Et2] and an excess of ZnEt2 was subjected to a vac-
uum for long period, the product completely decomposed and
1
this was evident from the H-NMR spectrum of the decom-
1
by H-NMR spectrum. This dissociation phenomenon can be
posed crude (i.e., phenyl protons). On aging, this mixture turns
to a turbid solution slowly and thus leading to a colorless prod-
uct, which is insoluble in common solvents. It is predicted
that this unknown product is a combination of zinc phosphide
species.
accounted that the Zn center becomes electrophilic when it is
bound to the electronegative Cl group. As a consequence Zn
then prefers to accept the electron pair from PPh3 and subse-
quently forms [Zn(PPh3)2Cl2]. It is predicted that transmetala-
tion of 1 with RMgX may lead to formation of the intermedi-
ates either 2a or 2b or both, where two electron donating alkyl
groups donate electrons sufficiently to zinc to make electron rich
leading to the formation of 3 by expelling PPh3.
UV/Vis absorption studies
The solutions of diethylzinc (0.01 g, 0.0823 mmol) in 100 mL
of chloroform and triphenylphosphine (0.02 g, 0.0763 mmol)
in 100 mL of chloroform were prepared. These solutions were
used recording the absorption spectra. Solutions of diethyl-
Addition of PPh3 to ZnEt2 in solution: An exploratory
analysis
This result prompted us to study further the coordination behav- zinc (0.130 g, 1.07 mmol) and triphenylphosphine (0.02 g,
ior of ZnR2, which is forced in the presence of a large excess of 0.0763 mmol) were mixed in 100 mL of chloroform. This solu-
ligands. Different ratios of diethylzinc and triphenylphosphine tion was used for recording the absorption spectrum.
were mixed in CDCl3 and the 31P NMR shifts were measured,
The absorption spectrum of complex [Zn(PPh3)2Et2] shows
which showed two distinct signals in 31P{1H}-NMR spectra of two broad bands with maximum at 263 and 223 nm. The λmax of
the mixture and one signal is assigned to uncoordinated PPh3 the metal complex is significantly different from the free ligand
Scheme . Proposed mechanism for the formation of dialkylzinc.

PHOSPHORUS, SULFUR, AND SILICON
37
Figure . P{H}-NMR spectral evidence for the formation of [Zn(PPh)Et] in CDCl solution at room temperature; 1A: at constant PPh concentration, 1B: at constant
ZnR concentration.
which indicates that the formation of complex. The free ligand Effect of temperature on the formation of [Zn(PPh3)2Et2]
PPh3 show two broad bands with maximum at 261 and 215 nm,
The mixtures of ZnEt2 and PPh3 with molar ratios of 1:1, 5:1,
and the ZnEt2 does not show any significant band under the con-
10:1, and 14:1 in CDCl3 were heated at 50°C for different time
centrations taken as shown in Figure 3.
intervals (3h, 24 h and 72 h). No significant changes were
observed in the peak intensities even after prolonged heating at
50°C and this suggests that the product(s) formation does not
depend on the temperature.
Influence of other ligands
Addition of other ligands like 1,10-phenanthroline (1,10-Phen)
to [Zn(PPh3)2Et2] formed by a 1:2 molar ratio of ZnEt2 and
PPh3 increased the complexation from 6.6% to 13.4%. However,
there is no significant change on addition of dppe to the com-

Figure . H-NMR spectrum of the reaction mixture containing unreacted diethyl-
zinc.
plex. This experiment was performed to check whether the addi-
tion of other ligands may retard the complex formation com-
pletely due to competitive reactions.
Effect of dilution
The dilutions were performed with chloroform solutions of 5.0,
10.0, 15.0, and 20.0 mL containing mixtures of PPh3 and ZnEt2
where the ratio was maintained throughout as 1:14. No peak
other than the one at 29 ppm was observed under these dif-
ferent dilution conditions. The dilution factor did not influ-
ence the dissociation of complex as evidenced by 31P-NMR data.
Attempts to isolate the product [Zn(PPh3)2Et2] from the mix-
ture completely failed.
Figure . Comparison of absorption spectrum of the product, [Zn(PPh)Et]
formed with the precursors.

38
V. SADHANA ET AL.
energies (I) and electron affinities (A) according to the Koop-
mans’ theorem as given in Eqs. 1 and 2.16,17 Diagrams and ener-
gies of the calculated HOMO and LUMO orbitals are given in
Table 1. The calculated reactivity indices are given in Table 2.
A = −ε
(1)
LUMO
I = −ε
(2)
HOMO
Chemical hardness (η) is associated with the stability and
reactivity of a chemical system. In a molecule, it measures
the resistance to change in the electron distribution or charge
transfer. On the basis of frontier molecular orbitals, chemical
hardness corresponds to the HOMO-LUMO energy gap and is
approximated using Eq. 3. The larger the HOMO-LUMO energy
gap, the more stable/less reactive the molecule.
Scheme . Complexes resulting from insertion reactions of C(Ni-Pr) with ZnMe
and ZnEt reported by Schulz et al.
η = (εLUMO − ε
)/2.
(3)
HOMO
Theoretical calculations
Schulz and coworkers15 have recently reported on a cluster for-
mation from the insertion reactions of carbodiimide, C(NPri)2,
with ZnMe2 and ZnEt2. Surprisingly, different cluster mor-
phologies were obtained depending on the alkyl chain length
(see Scheme 2). It is assumed that the chelate structure of tetra-
coordinated zinc for the 1:1 complexes with bidentate ligands is
possible along with the planar complexes by a monodentate lig-
and. By increasing the electronegative character of the organic
group R on zinc, the polar character of the zinc-carbon bond
will be enhanced. The negative charge is being pulled away from
the zinc atom causing the electron affinity of the vacant orbitals
of zinc and, consequently, the tendency to form donor-acceptor
complexes to be increased.12 The influence of the organic sub-
stituent (R) on the acceptor properties of the zinc atom has
been investigated by suitable variation of R. It is the fact that
phenyl groups are more electron attracting than butyl groups
and subsequently diphenylzinc forms coordination complexes
more readily than does dibutylzinc. This phenomenon is further
supported by DFT calculations.
The electronic chemical potential (μ) is defined as the neg-
ative of electronegativity of a molecule and determined using
Eq. 4. Physically, μ describes the escaping tendency of electrons
from an equilibrium system. The greater the electronic chemical
potential the less stable or more reactive the molecule.18
μ = (εHOMO + ε
)/2.
(4)
LUMO
The electrophilicity index (ω), introduced by Parr, is calcu-
lated using the electronic chemical potential and chemical hard-
ness as shown in Eq. 5. ω measures the capacity of a species to
accept electrons. Accordingly, it is a measure of the stabiliza-
tion in energy after a system accepts additional electronic charge
from the environment.
ω = μ2/2η.
(5)
All calculated structures indicate the HOMO orbital contain
The intriguing behavior exhibited by the diethylzinc com- significant contribution from the Zn center. This strongly sug-
plexes in solution with PPh3 led us to investigate the nature of gests that the metal center is directly involved in the reactivity of
these species computationally in order to obtain evidence, which cluster formation. In addition, in the ZnEt2 complex, the LUMO
supports, or refutes, cluster formation. While calculations on the is nearly completely positioned on the Zn metal, indicating the
proposed clusters complexes were not computationally feasible metal center is susceptible to nucleophilic attack. There is a note-
due to their size of fluxional behavior, we examined possible key worthy difference in the HOMO energies of ZnMe2(PPh3)2 and
intermediates involved in the clustering mechanism.
ZnEt2(PPh3)2 which influences the reactivity indices of the com-
The electronic structures and properties of five Zn species pounds and suggests that these might not react to form clusters
were calculated using density functional theory (DFT). The DFT in the same way. The reactivity indices indicate that relative to
global chemical reactivity descriptors (chemical hardness (η), ZnEt2, its phosphine adduct ZnEt2(PPh3) is significantly more
electronic chemical potential (μ), and electrophilicity (ω)) were reactive.
calculated for the compounds and used to predict their rela-
tive stability and reactivity. In addition, the activation energy
for PPh3 addition to diethylzinc, a key step in the formation of
Conclusion
the cluster, has been determined. The possible formation mecha- The addition of large excess of diethylzinc to PPh3 yields the
nism of zinc clusters is discussed on the basis of these theoretical corresponding zinc complex in solution. It was not possible to
results.
isolate the product from solution and subsequently the prod-
The energies of HOMO and LUMO are popular quantum uct in solution was identified by UV-Vis, IR and 31P-NMR
mechanical descriptors as these orbitals have been shown to play spectral data. The transformation requires 14 times excess
a major role in governing many chemical reactions. The HOMO of ZnEt2 in solution for the disappearance of chemical shift
and LUMO orbital energies are related to gas phase ionization of PPh3 to form [(PPh3)2ZnEt2] complex. The longer alkyl

PHOSPHORUS, SULFUR, AND SILICON
39
Table . Optimized structures and calculated frontier orbitals.
Optimized structure
LUMO orbital
HOMO orbital
ZnEt
E = -.eV
E = -.eV
ZnEt(PPh)
ZnMe(PPh)
ZnEt(PPh)
ZnCl(PPh)
E = -.eV
E = -. eV
E = -.eV
E = -.eV
E = -.eV
E = -. eV
E = -.eV
E = -.eV
chains reduces the Lewis acidity on Zn and thereby the forma- phosphoric acid external peak at 0 ppm. Electronic spectra were
tion of phosphine adducts is restricted. Results obtained from recorded by JASCO–V-550 UV/Vis Spectrophotometer. Varian
orbital analyses calculations reveal that the LUMO appears to Inc, USA (410 Prostar Binary LC with 500 MS IT PDA Detec-
be asymmetrically distributed, and localized on one of the PPh3 tors) Direct Infusion Mass with ESI & APCI Negative & Positive
ligands.
mode ionization Mass spectrometer was used to record the spec-
trum.
The precursor complex [Zn(PPh3)2Cl2] (1) was prepared
by reacting ZnCl2 (2.014 g, 14.78 mmol) with PPh3 (8.14 g,
31.04 mmol) in 20 mL of CH2Cl2 for 2 h as per the literature.14
The obtained solid product (9.2 g, 94%) was filtered and washed
with 2 × 5 mL of petroleum ether and dried under high vacuum
pump for 2 h.
Procedure for the preparation of [Zn(PPh3)2Et2] (2): Diethylz-
inc [3.28 g (26.7 mmol)] was added to a clean 25 mL RB flask,
and evaporated the hexane at RT by rotavap. To this, PPh3 [0.5 g
(1.91 mmol)] was added followed by CHCl3 (10.0 mL). The reac-
tion was monitored by 31P-NMR to make sure that there is no
free triphenylphosphine left. As the product was not possible to
isolate from the mixture in the presence of excess diethylzinc
even under high vacuum.
Experimental
All the reactions were carried out under dry nitrogen using
Schlenk techniques. Solvents were dried and purified by
standard methods. Reagents like diethylzinc (in hexane)
and triphenylphosphine were purchased from Sigma-Aldrich.
Grignard reagents were prepared as described from the lit-
erature reports.19 NMR spectra were recorded on a Bruker
Avance DMX-400 spectrometer and operating frequencies were
400.13 MHz (1H), 100.61 MHz (13C), and 161.92 MHz (31P). All
the spectra were recorded in CDCl3 solvent. Spectra are reported
in δ (ppm) relative to TMS, as determined from standard resid-
ual solvent-proton (or carbon) signals for H and 13C. All 31P
chemical shifts were H decoupled and referenced to an 85%
1
1
Table . Global chemical reactivity indices for ZnR compounds.
ZnEt
ZnEt(PPh)
ZnMe(PPh)
ZnEt(PPh)
ZnCl(PPh)
LUMO (eV)
HOMO (eV)
η (eV)
μ (eV)
ω (eV)
− .
− .
.
− .
− .
.
− .
− .
.
− .
− .
.
− .
− .
.
− .
− .
− .
− .
− .
.
.
.
.
.

40
V. SADHANA ET AL.
Computational analysis
1995, 785; (c) Crabtree, R. H.; Mingos, D. M. P. (Eds.), Comprehen-
sive Organometallic Chemistry 3, Elsevier, 2007, 10-11; (d) Cotton, F.
A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic
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All calculations were carried out using the DMol3 density
functional theory (DFT) code as implemented in the Accelrys
Material Studio 6.0 software package.20 The nonlocal general-
ized gradient approximation (GGA) using the PW91 exchange-
correlation functional was used for geometry optimizations in
all cases.21 A double numeric, polarized split valence (DNP)
basis set was used in this study with a DFT semicore pseudopo-
tential to account for the core electrons of P. The size of the DNP
basis set is comparable to Gaussian 6–31 G∗∗, but the DNP is
more accurate than the Gaussian basis set of the same size.22
Geometry optimizations were performed without any symme-
try constraints. The convergence criteria for these optimizations
consisted of the following threshold values: 1 × 10−5 Ha for
4. (a) Rothwell, I. P. Polyhedron 1985, 4, 177-200; (b) Bulls, A. R.; Schae-
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Gaggelli, E.; Valensin, D.; Valensin, G. Coord. Chem. Rev. 2009, 253,
2665-2685; (d) Zhai, T.; Fang, X.; Zeng, H.; Xu, X.; Bando, Y.; Gol-
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1539.
−1
˚
˚
energy; 0.002 HaA for gradient and 0.005A for displacement
convergence, while a self-consistent field density convergence
threshold of 1 × 10−6 Ha was specified. All optimized geome-
tries were subjected to a full frequency analysis at the same level
of theory (GGA/PW91/DNP) to verify the nature of the sta-
tionary points. Optimized geometries were characterized by the
absence of imaginary frequencies.
Funding
7. Smith, M. B.; March, J. March’s Advanced Organic Chemistry, 5th ed.,
Wiley: New York, 2001, Ch. 15 and references therein.
8. Nishimura, J.; Kawabata, N.; Furukawa, J. Tetrahedron, 1969, 25, 2647-
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1996, 61, 8000-8001.
Dr. ASRK is highly grateful to Council of Scientific and Industrial Research
(CSIR, New Delhi, India) (Ref. No. 01(2541)/11/EMR-II) for the financial
support. Our sincere thanks are due to DST-VIT-FIST for NMR and SIF-
VIT for GC-MS facilities. Mr. Venkatesh Sadhana is thankful to CSIR for
the Junior Research Fellowship.
10. Jeon, S.-J.; Li, H.; Garcia, C.; LaRochelle, L. K.; Walsh, P. J. J. Org. Chem.
2005, 70, 448-455.
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Organomet. Chem. 2012, 696, 4327-4331.
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