Metathesis reactivity of bis(phosphinite) pincer ligated nickel chloride, isothiocyanate and azide complexes

Article abstract of DOI:10.1016/j.jorganchem.2015.12.038

A series of nickel pincer complexes of the type [4-Z-2,6-(R₂PO)₂C₆H₂]NiX (R = ^tBu, ⁱPr, Ph; Z = H, CO₂Me; X = NCS, N₃) have been synthesized from the reactions of the corresponding nickel chloride complexes [4-Z-2,6-(R₂PO)₂C₆H₂]NiCl and potassium thiocyanate or sodium azide. X-ray structure determinations of these complexes have shown that the thiocyanate ion binds to the nickel center through the nitrogen. A comparable Ni-N bond length (approx. 1.87 ? for the isothiocyanate complexes and 1.91 ? for the azide complexes) and an almost identical Ni-C_ipso bond length (approx. 1.89 ?) have been observed for these complexes. Metathesis reactivity of [4-Z-2,6-(R₂PO)₂C₆H₂]NiCl and ligand exchange reactions between the nickel isothiocyanate and nickel azide complexes have been investigated. The metathesis reactions with thiocyanate/azide complexes are faster with a less electron rich and more sterically accessible nickel center. The thermodynamic stability of these nickel complexes has been rationalized using hard-soft acid-base theory (HSAB theory); a harder ligand prefers a less electron rich nickel center. These experimental results have been supported by quantum chemical analysis of the coordinating nitrogen atoms in SCN^- and N3.

Full text of DOI:10.1016/j.jorganchem.2015.12.038

Journal of Organometallic Chemistry 804 (2016) 132e141
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Metathesis reactivity of bis(phosphinite) pincer ligated nickel
chloride, isothiocyanate and azide complexes
Huizhen Li a, Wenjuan Meng a, Anubendu Adhikary b, Shujun Li a, Nana Ma a,
Qianyi Zhao a, Qiuyu Yang a, Nathan A. Eberhardt b, Kendra M. Leahy b,
,
,
, ***
Jeanette A. Krause b, Jie Zhang a *, Xuenian Chen a **, Hairong Guan b
a School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, China
b Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA
a r t i c l e i n f o
a b s t r a c t
A series of nickel pincer complexes of the type [4-Z-2,6-(R2PO)2C6H2]NiX (R ¼ tBu, iPr, Ph; Z ¼ H, CO2Me;
X ¼ NCS, N3) have been synthesized from the reactions of the corresponding nickel chloride complexes
[4-Z-2,6-(R2PO)2C6H2]NiCl and potassium thiocyanate or sodium azide. X-ray structure determinations of
these complexes have shown that the thiocyanate ion binds to the nickel center through the nitrogen. A
comparable NieN bond length (approx. 1.87 Å for the isothiocyanate complexes and 1.91 Å for the azide
complexes) and an almost identical NieCipso bond length (approx. 1.89 Å) have been observed for these
complexes. Metathesis reactivity of [4-Z-2,6-(R2PO)2C6H2]NiCl and ligand exchange reactions between
the nickel isothiocyanate and nickel azide complexes have been investigated. The metathesis reactions
with thiocyanate/azide complexes are faster with a less electron rich and more sterically accessible nickel
center. The thermodynamic stability of these nickel complexes has been rationalized using hard-soft
acid-base theory (HSAB theory); a harder ligand prefers a less electron rich nickel center. These exper-
imental results have been supported by quantum chemical analysis of the coordinating nitrogen atoms in
Article history:
Received 22 August 2015
Received in revised form
25 December 2015
Accepted 27 December 2015
Available online 30 December 2015
Keywords:
Metathesis reaction
Synthesis
Crystal structure
Nickel
Pincer complex
SCNe and N3
.
ꢀ
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
a POCOP-pincer ligand, and our efforts have led to the development
of catalysts for the reduction of aldehydes [8a], ketones [8a], CO2
[8bee], cyanomethylation of aldehydes [8f] and CeS cross-coupling
reactions [8g]. To gain a deeper understanding of how structural
modification could change the performance of these complexes in
different catalytic reactions, we have carried out detailed studies
about substituent effects on the reactivity of the metal center. In
one of our recent studies, we have reported how the substituents
on the phosphorus donors influence the NieS bond dissociation
energies and kinetic stability of nickel arylthiolate complexes [9].
The electrostatic component of the bonding appears to contribute
greatly to the strength of the NieS bond. To explore the generality
of this conclusion, herein we extend the chemistry to nickel POCOP-
pincer complexes bearing a nitrogen-based ligand. More specif-
ically, we have focused on the metathesis reactivity of [4-Z-2,6-
(R2PO)2C6H2]NiX (Z ¼ H, CO2Me; R ¼ tBu, iPr, Ph; X ¼ Cl, NCS, N3).
The introduction of an ester group to the pincer backbone has also
allowed us to probe how the remote ligand modification would
impact the bond breaking/forming processes.
Transition metal complexes bearing a bis(phosphinite) pincer or
POCOP-pincer ligand have been extensively studied since the first
reports by Jensen [1] and Bedford [2]. This specific class of com-
plexes has been demonstrated as efficient catalysts for a wide range
of chemical transformations [3]. The popularity of using these
complexes for catalysis has been largely attributed to the high
catalyst thermal stability [4], well balanced ligand properties [5]
and convenient ligand synthesis [1,2,4a,6]. During the past
decade, significant effort has focused on the elucidation of struc-
tureereactivity relationships with the goal of developing more
active catalysts [7].
We are particularly interested in nickel complexes supported by
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: jie.zhang@htu.edu.cn (J. Zhang), xnchen@htu.edu.cn
(X. Chen), hairong.guan@uc.edu (H. Guan).
http://dx.doi.org/10.1016/j.jorganchem.2015.12.038
0022-328X/© 2015 Elsevier B.V. All rights reserved.

H. Li et al. / Journal of Organometallic Chemistry 804 (2016) 132e141
133
2. Results and discussion
trimethylsiloxide complex with Me3SiN3 [7m]. Related nickel azide
complexes bearing a POCsp3OP-pincer ligand [7m] or a PCP-pincer
ligand [11] were prepared in a similar fashion using Me3SiN3.
To accelerate the substitution of the chloride ligand in complex
1 by SCNe and N3ꢀ, each of the reactions shown in Scheme 1 was
conducted under refluxing conditions. However, an extra recrys-
tallization step was needed to ensure the purity of the products,
which resulted in lower isolated yields (60e78%) than those for the
room temperature reactions.
The nickel isothiocyanate and azide complexes reported here
are all air and moisture stable in both solution and solid states. No
significant decomposition was observed when a solid sample or a
solution (in CH2Cl2/n-hexane) was exposed to air for a few weeks.
The new nickel isothiocyanate and azide complexes were fully
characterized by multinuclear NMR, FTIR, X-ray crystallography
and elemental analysis. Notable features of the 13C{1H} NMR
spectra included the ipso carbons appearing as triplets (2JP-
2.1. Synthesis and structure of nickel pincer complexes
Following previously reported procedures for the synthesis of
[2,6-(tBu2PO)2C6H3]NiCl (1aa) [8a], [2,6-(iPr2PO)2C6H3]NiCl (1ab)
[6c] and [2,6-(Ph2PO)2C6H3]NiCl (1ac) [6b], complexes 1ba-c were
synthesized via cyclometalation of the ester-substituted diphos-
phinite ligand with NiCl2 (eq 1). Complex 1bb was reported by
Zargarian and co-workers, who developed a one-pot procedure for
preparing complexes of this type from resorcinol or its derivatives,
R2PCl and nickel powder [10]. The new complexes 1ba and 1bc
were characterized by 1H NMR, 31P{1H} NMR, 13C{1H} NMR and
elemental analysis. The structure of 1ba was also confirmed by X-
ray crystallography (see Appendix B Supplementary data).
¼ 20e25 Hz) at 123.86e125.37 ppm for 2aa-c and 3aa-c, and
C
131.35e133.25 ppm for 2ba-c and 3ba-c. The chemical shift dif-
ferences for the ipso carbons can be attributed to the electronic
effects of the para-substituents (H vs. CO2Me). The electron-
withdrawing ester group makes the ipso carbons more deshiel-
ded, leading to a downfield shift of 6e9 ppm. All 31P{1H} NMR
spectra displayed a single resonance, consistent with a symmetrical
structure bearing two equivalent phosphorus atoms. For complexes
with the same phosphorus substituents (R groups) and the same
ancillary ligand (NCSe or N3ꢀ), the resonances for the CO2Me-
substituted complexes were located approx. 1 ppm downfield of
resonances associated with the unsubstituted complexes (X ¼ H).
The phosphorus resonances for the isothiocyanate complexes were
found 2e4 ppm downfield from resonances corresponding to the
azide complexes with the same ligand.
Room temperature reactions of nickel chloride complexes 1
with 5 equiv of potassium thiocyanate afforded nickel isothiocya-
nate complexes 2 in high isolated yields (86e93%) (Scheme 1).
Thiocyanate ion is a well-known ambidentate ligand. Depending on
the polarizability and the steric environment of the metal, thiocy-
anate ligand can bind to the metal through either the sulfur or the
nitrogen end, resulting in a thiocyanate or isothiocyanate complex.
In this study, there was no evidence for sulfur binding; the iso-
thiocyanate complex was isolated as the sole product in all cases, as
suggested by X-ray structure analysis of the single crystals as well
as the 31P{1H} NMR spectrum in which only one resonance was
observed.
Selected FTIR data of the isothiocyanate and azide complexes are
listed in Table 1. In general, CN stretching or N3 asymmetric
stretching frequency decreases when changing the phosphorus
substituents from tBu to iPr and from iPr to Ph (e.g., 2aa > 2ab > 2ac
for nCN). The only exception is 3bc which has a slightly higher nN3
value than 3bb. Introducing an electron-withdrawing ester group
at the para position of the ipso carbon results in lower values in nCN
Similarly, mixing nickel chloride complexes 1 with 5 equiv of
sodium azide in THF/MeOH (1: 1) for several days yielded nickel
azide complexes 3 (Scheme 1). Complex 3ab was previously re-
ported by Zargarian et al. through the reaction of the corresponding
t
i
for the Bu and Pr-substituted isothiocyanate complexes (2aa vs.
i
2ba, 2ab vs. 2bb) as well as nN3 for the Pr-substituted azide com-
plexes (3ab vs. 3bb), but has almost no influence on the remaining
pairs (2ac vs. 2bc, 3aa vs. 3ba, or 3ac vs. 3bc).
The nickel isothiocyanate and azide complexes described in this
work crystallized readily and were conveniently studied by X-ray
crystallography. As representative examples, structures of 2aa, 2bb,
3ac and 3ba are shown in Figs. 1e4, respectively. The structure of
3ab has been previously reported by Zargarian and co-workers
[7m]. Structures and crystallographic data of the remaining seven
complexes (2ab, 2ac, 2ba, 2bc, 3aa, 3bb and 3bc) are provided in
the Supplementary data (Appendix B). For comparison purposes,
NieCipso and NieN bond lengths of these complexes are summa-
rized in Tables 2 and 3, respectively, while the NieNeC and
NieNeN bond angles are listed in Table 4.
As shown in Table 2, the NieCipso bond length is essentially
constant for both isothiocyanate and azide complexes
Scheme 1. Synthesis of POCOP pincer ligated nickel isothiocyanate and azide
complexes.
Table 1
CN stretching or N3 asymmetric stretching frequencies of [4-Z-2,6-(R2PO)2C6H2]NiX complexes (cmꢀ1).
Isothiocyanate complexes (X ¼ NCS)
Azide complexes (X ¼ N3)
R ¼ tBu
R ¼ tBu
R ¼ iPr
R ¼ Ph
R ¼ iPr
R ¼ Ph
Z ¼ H
2104(2aa)
2098(2ba)
2091(2ab)
2082(2bb)
2072 (2ac)
2071(2bc)
2072 (3aa)
2073 (3ba)
2053(3ab)
2047(3bb)
2048(3ac)
2049 (3bc)
Z ¼ CO2Me

134
H. Li et al. / Journal of Organometallic Chemistry 804 (2016) 132e141
Fig. 1. ORTEP drawing of [2,6-(tBu2PO)2C6H3]Ni(NCS) (2aa) at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond length (Å) and angles (ꢁ):
Ni1eN1 ¼ 1.8643(18), Ni1eC1 ¼ 1.8787(19), Ni1eP1 ¼ 2.1827(6), Ni1eP2 ¼ 2.1927(6),
Fig. 3. ORTEP drawing of [2,6-(Ph2PO)2C6H3]NiN3 (3ac) at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond length (Å) and angles (ꢁ):
Ni1eN1 ¼ 1.934(2), Ni1eC30 ¼ 1.880(3), Ni1eP1 ¼ 2.1760(9), Ni1eP2 ¼ 2.1587(8),
N1eC23
P1eNi1eN1
C23eN1eNi1 ¼ 178.7(2), N1eC23eS1 ¼ 179.7(2).
¼
1.167(3); C23eS1
¼
1.625(2);
P1eNi1eP2
¼
164.51(2),
179.13(9),
¼
97.51(6), P2eNi1eN1
¼
97.97(6), C1eNi1eN1
¼
N1eN2
P1eNi1eN1
N2eN1eNi1 ¼ 133.7(2), N1eN2eN3 ¼ 175.4(3).
¼
1.153(3);
N2eN3
¼
1.157(4);
P1eNi1eP2
¼
162.95(3),
177.57(12),
¼
96.92(7), P2eNi1eN1
¼
100.04(7), C30eNi1eN1
¼
(1.88e1.89 Å), and unaffected by the substituents on the phos-
phorus donors or at the para position of the aromatic pincer
backbone. Similar results were observed in POCOP-pincer nickel
thiolate complexes [9]. The NieN bond lengths for the isothiocya-
nate complexes (1.86e1.87 Å) are slightly shorter than those for the
azide complexes (1.90e1.94 Å); however the NieN bond lengths for
the unsubstituted azide complexes (1.91e1.94 Å) and those
adorned by an ester group (1.90e1.92 Å) in the para position of the
aromatic pincer ligand are unchanged. In the nickel isothiocyanate
complexes, the NCS ligand maintains its linear geometry (NeCeS
bond angle: 178.7e179.7ꢁ, see Appendix B. Supplementary data)
and deviates slightly from the CipsoeNieN vector in the coordina-
tion plane with a NieNeC bond angle of 160.5e178.7ꢁ (Table 4). In
contrast, the N3 ligand in the nickel azide complexes bends slightly
from its original linear geometry (NeNeN bond angle:
175.1e176.2ꢁ, see Appendix B. Supplementary data) and deviates
significantly from the CipsoeNieN vector with a NieNeN bond
angle of 127.6e139.0ꢁ. Similar bending is observed in other metal
azide complexes [7m,11,12]. Of all the nickel complexes reported
here, 2aa is the most symmetrical molecule in the solid state
(Fig. 5). The linear NCS ligand (NeCeS bond angle: 179.7(2ꢀ)) is
within the coordination plane and essentially extended from the
C
ipsoeNieN vector (NieNeC bond angle: 178.7(2ꢀ)). The NCS ligand
of other isothiocyanate complexes is bent with respect to the
ipsoeNieN vector, likely influenced by the varying conformations
C
of the R groups bound to phosphorus and/or asymmetry induced by
the ester group. Nevertheless, in solution, the rotation of the NieN
bonds are not restricted as all 31P{1H} NMR spectra display a singlet
resonance.
2.2. Metathesis reaction of [4-Z-2,6-(R2PO)2C6H2]NiCl
Although the salt metathesis reactions between POCOP-pincer
nickel chloride complexes and potassium thiocyanate or sodium
azide all ultimately lead to the nickel isothiocyanate or azide
complexes quantitatively, it would be useful to understand the
reactivity differences of the pincer complexes. In particular, we
Fig. 2. ORTEP drawing of [4-CO2Me-2,6-(iPr2PO)2C6H2]Ni(NCS) (2bb) at the 50%
Fig. 4. ORTEP drawing of [4-CO2Me-2,6-(tBu2PO)2C6H2]NiN3 (3ba) at the 50% proba-
probability level. Hydrogen atoms are omitted for clarity. Selected bond length (Å) and
bility level. Hydrogen atoms are omitted for clarity. Selected bond length (Å) and
angles (ꢁ): Ni1eN1
¼
1.867(3), Ni1eC1
Ni1eP2 ¼ 2.1680(10), N1eC21 ¼ 1.169(4); C21eS1 ¼ 1.629(3); P1eNi1eP2 ¼ 163.61(3),
P1eNi1eN1 97.82(10), P2eNi1eN1 98.29(10), C1eNi1eN1 178.09(13),
C21eN1eNi1 ¼ 175.4(3), N1eC21eS1 ¼ 179.0(3).
¼
1.886(3), Ni1eP1
¼
2.1679(10),
angles (ꢁ): Ni1eN1
¼
1.900(6), Ni1eC1
Ni1eP2 ¼ 2.1884(16), N1eN2 ¼ 1.180(7); N2eN3 ¼ 1.159(7); P1eNi1eP2 ¼ 163.49(7),
P1eNi1eN1 98.16(17), P2eNi1eN1 98.28(17), C1eNi1eN1 169.9(2),
N2eN1eNi1 ¼ 138.3(5), N1eN2eN3 ¼ 175.3(6).
¼
1.884(6), Ni1eP1
¼
2.1837(15),
¼
¼
¼
¼
¼
¼

H. Li et al. / Journal of Organometallic Chemistry 804 (2016) 132e141
135
Table 2
NieCipso bond lengths (Å) of nickel isothiocyanate and azide complexes [4-Z-2,6-(R2PO)2C6H2]NiX.
Isothiocyanate complexes (X ¼ NCS)
Azide complexes (X ¼ N3)
R ¼ tBu
R ¼ iPr
R ¼ Ph
R ¼ tBu
R ¼ iPr
R ¼ Ph
Z ¼ H
1.8787(19) (2aa)
1.8765(17) (2ba)
1.8854(11) (2ab)
1.886(3) (2bb)
1.889(2) (2ac)
1.8760(18) (2bc)
1.884(3) (3aa)
1.884(6) (3ba)
1.891(3) (3ab)a
1.880(3) (3ac)
Z ¼ CO2Me
1.8775(14) (3bb)
1.8866(18) (3bc)
a
Data obtained from the cif file of Ref. [7m].
Table 3
NieN bond lengths (Å) of nickel isothiocyanate and azide complexes [4-Z-2,6-(R2PO)2C6H2]NiX.
Isothiocyanate complexes (X ¼ NCS)
Azide complexes (X ¼ N3)
R ¼ tBu
R ¼ iPr
R ¼ Ph
R ¼ tBu
R ¼ iPr
R ¼ Ph
Z ¼ H
1.8643(18) (2aa)
1.8715(10) (2ab)
1.8739(18) (2ac)
1.914(3) (3aa)a
1.936(14)
1.935(2) (3ab)b
1.934(2) (3ac)
Z ¼ CO2Me
1.8667(16) (2ba)
1.867(3) (2bb)
1.8714(17) (2bc)
1.900(6) (3ba)
1.9174(14) (3bb)
1.9036(16) (3bc)
a
The N-atoms of the N3 group in complex 3aa are disordered, both Ni1eN1 and Ni1eN1A bond lengths are listed.
Data obtained from the cif file of Ref. [7m].
b
Table 4
NieNeC bond angles (ꢁ) of [4-Z-2,6-(R2PO)2C6H2]Ni(NCS) and NieNeN bond angles (ꢁ) of [4-Z-2,6-(R2PO)2C6H2]NiN3.
Isothiocyanate complexes (X ¼ NCS)
Azide complexes (X ¼ N3)
R ¼ tBu
R ¼ iPr
R ¼ Ph
R ¼ tBu
R ¼ iPr
R ¼ Ph
Z ¼ H
178.7(2) (2aa)
167.13(12) (2ab)
164.11(18) (2ac)
139.0(7) (3aa)a
132.(5)
130.2(2) (3ab)b
133.7(2) (3ac)
Z ¼ CO2Me
170.39(18) (2ba)
175.4(3) (2bb)
160.51(16) (2bc)
138.3(5) (3ba)
127.60(11) (3bb)
133.21(14) (3bc)
a
The N-atoms of the N3 group in complex 3aa are disordered, both Ni1eN1eN2 and Ni1eN1AeN2A bond angles are listed.
Data obtained from the cif file of Ref. [7m].
b
were interested in knowing how the substituents on the phos-
stirred at 0 ꢁC for 5 min and then quickly quenched by adding 15 mL
of water. This procedure resulted in precipitation of nickel pincer
complexes while keeping KSCN and NaN3 in solution. The ratios
between [4-Z-2,6-(R2PO)2C6H2]NiCl and [4-Z-2,6-(R2PO)2C6H2]NiX
(X ¼ NCS, N3) were determined by 31P{1H} NMR integrations, from
which the conversions were calculated [13]. Table 5 summarizes
the results for eight different reactions with six of them involving
NCSe. Because of the poor solubility of KSCN and NaN3, their con-
centrations are constant throughout the reactions, and low enough
to slow down the reactions so that reactivity difference between
the nickel complexes could be observed. It should be noted that all
the nickel complexes involved in this study tolerate water. No
pincer species other than the starting nickel chloride complexes
and the resulting nickel isothiocyanate or azide complexes was
detected by 31P{1H} NMR. The low concentrations of the starting
nickel chloride complexes (0.017 M each) and KSCN or NaN3 made
the salt metathesis reactions slow enough that the conversions
could be measured with accuracy within the same period of time
(5 min).
phorus donors and the pincer aromatic backbone would influence
the rates of the metathesis reactions. Given the fact that NCSe and
ꢀ
N3 both are linear species and in this case coordinate to nickel via
nitrogen, we were curious as to whether there is a rate difference
upon complexation using the starting chloro complex (refer to
Scheme 1). Unfortunately, the poor solubility of potassium thiocy-
anate and sodium azide in most organic solvents and the relatively
fast reaction rates for this type of reaction make it difficult to obtain
reliable kinetic data. Using Bu4NSCN and Bu4NN3 would solve the
solubility problem; however, with the exception of 1aa and 1ba, the
salt metathesis reactions are too fast (complete in 1 min at 0 ꢁC) to
tell the difference among the nickel pincer complexes. As an
alternative method, the salt metathesis reactions using KSCN and
NaN3 (eq 2) were stopped after a short period of time and the re-
action mixtures were analyzed by 31P{1H} NMR spectroscopy. In a
typical experiment, a dilute THF solution of [4-Z-2,6-(R2PO)2C6H2]
NiCl (0.1 mmol in 3 mL of THF) was mixed with a saturated THF
solution of potassium thiocyanate or sodium azide (0.1 mmol KSCN
or NaN3 mixed with 3 mL of THF) at 0 ꢁC. The resulting mixture was
Fig. 5. Side views of 2aa illustrating the symmetrical geometry.

136
H. Li et al. / Journal of Organometallic Chemistry 804 (2016) 132e141
Table 5
Table 6
Conversions of [4-Z-2,6-(R2PO)2C6H2]NiCl to [4-Z-2,6-(R2PO)2C6H2]NiX after
5 min at 0 ꢁC.
Equilibrium constant for the isothiocyanate/azide exchange reactions (T ¼ 298 K).
Entry
Z
Z0
R
R0
Reactions
Keq
Entry
Z
R
X
Reactions
Conversion (%)
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
CO2Me tBu tBu 3aa þ 2ba # 2aa þ 3ba
1.33 0.08
1
2
3
4
5
6
7
8
H
H
H
tBu
iPr
Ph
tBu
iPr
Ph
iPr
iPr
NCS
NCS
NCS
NCS
NCS
NCS
N3
1aa þ NCSe / 2aa þ Cle
1ab þ NCSe / 2ab þ Cle
1ac þ NCSe / 2ac þ Cle
1ba þ NCSe / 2ba þ Cle
1bb þ NCSe / 2bb þ Cle
1bc þ NCSe / 2bc þ Cle
10
76
83
18
82
87
85
91
CO2Me iPr iPr 3ab þ 2bb # 2ab þ 3bb 1.45 0.02
CO2Me Ph Ph 3ac þ 2bc # 2ac þ 3bc
1.35 0.02
3.53 0.08
2.48 0.04
8.4 0.7
H
H
H
tBu iPr 3aa þ 2ab # 2aa þ 3ab
iPr Ph 3ab þ 2ac # 2ab þ 3ac
tBu Ph 3aa þ 2ac # 2aa þ 3ac
CO2Me
CO2Me
CO2Me
H
CO2Me CO2Me tBu iPr 3ba þ 2bb # 2ba þ 3bb 3.75 0.09
ꢀ
1ab þ N3 / 3ab þ Cle
CO2Me CO2Me iPr Ph 3bb þ 2bc # 2bb þ 3bc
CO2Me CO2Me tBu Ph 3ba þ 2bc # 2ba þ 3bc
2.5 0.2
10.1 0.9
ꢀ
CO2Me
N3
1bb þ N3 / 3bb þ Cle
reactions were clean with no other detectable species by 31P{1H}
NMR except the four pincer complexes involved in each equilib-
rium. The equilibria were typically reached at room temperature in
2e3 d from either direction, or by mixing a random ratio of the four
complexes. The equilibrium constants were conveniently measured
by 31P{1H} NMR spectroscopy [13] as the complexes usually showed
well-separated 31P resonances. Table 6 summarizes the equilibrium
constants, which were averaged from three individual experiments.
As shown in Table 5, for nickel chloride complexes bearing the
same pincer backbone, those with phenyl groups as the phosphorus
substituents are the most reactive complexes for the thiocyanate
ion (entries 3 and 6), while those with tert-butyl groups on the
phosphorus atoms are the least reactive ones (entries 1 and 4).
Having an electron-withdrawing CO2Me group at the para position
of the pincer backbone makes nickel complexes consistently more
reactive (entry 1 vs. 4; entry 2 vs. 5; entry 3 vs. 6; entry 7 vs. 8),
although the rate difference is small.
The relatively slow reactions for 1aa and 1ba are possibly due to
sterics. Presumably, the metathesis or substitution reactions are
associative or interchange-associative in nature, and the rates
depend on how easily SCNe and N3ꢀ can approach the metal center.
The bulky tert-butyl substituents are expected to make it more
sluggish. However, electronic effects can also play a role. Among the
three different substituents used in this study (tBu, iPr and Ph), the
tert-butyl group is the most electron-donating substituent while
the phenyl group is the least donating. The nickel center becomes
more electron deficient going from 1aa to 1ab to 1ac or 1ba to 1bb
to 1bc, thus favoring the association of an incoming ligand.
Furthermore, the introduction of an ester group to the para position
of the pincer backbone accelerates the metathesis reactions, likely
due to a more electron deficient metal center. As inferred from the
crystal structures, the steric perturbation should be minimal.
The first three equilibria (entries 1e3) involve the exchange of
isothiocyanate and azide ligands between an ester-substituted
nickel pincer unit and an unsubstituted one. The equilibrium con-
stants are all greater than one, suggesting the azide ligand prefers
the more electron-deficient nickel center. As previously mentioned,
the electron “richness” of the nickel center follows a decreasing
order of {[2,6-(tBu2PO)2C6H3]Ni}þ
>
{[2,6-(iPr2PO)2C6H3]
Ni}þ > {[2,6-(Ph2PO)2C6H3]Ni}þ. Therefore, altering the phosphorus
substituents while keeping the same pincer backbone would
establish an equilibrium in which the azide ligand binds prefer-
entially to {[2,6-(Ph2PO)2C6H3]Ni}þ (entries 5 and 6) followed by
{[2,6-(iPr2PO)2C6H3]Ni}þ (entry 4). Similar results were obtained
when the ester-substituted POCOP-pincer ligands were employed
(entries 7e9).
2.4. Quantum chemical analysis of the isothiocyanate and azide
ions
To gain a better understanding of the aforementioned metath-
esis and ligand exchange reactions, a quantum chemical analysis of
isothiocyanate and azide ions was carried out focusing on the
properties of the coordinating nitrogen atoms. The ground-state
geometries were optimized by the second-order Møller-Plesset
perturbation (MP2) method using the 6e311þþG (d, p) basis set,
and the charges were obtained from the NPA population analysis.
The condensed values of local reactivity descriptors of the coordi-
2.3. Isothiocyanate/azide exchange reactions
nating nitrogen atoms in SCNe and N3 are listed in Table 7.
ꢀ
In our previous study [9], nickel thiolate complexes were shown
to undergo thiolate exchange between two different pincer units.
Interestingly, similar ligand exchange between two different pincer
ligated nickel centers was observed in the present study (eq 3). The
The calculations indicate that with a larger condensed Fukui
function fkꢀ value and a larger local softness sꢀ value, the coordi-
nating nitrogen atom in N3ꢀ is more nucleophilic than that of SCNe
[14]. However, the SCNe nitrogen has a larger global softness value

H. Li et al. / Journal of Organometallic Chemistry 804 (2016) 132e141
137
[2,6-(tBu2PO)2C6H3]NiCl (1aa) [8a], [2,6-(iPr2PO)2C6H3]NiCl (1ab)
[6c] and [2,6-(Ph2PO)2C6H3]NiCl (1ac) [6b] were prepared as
described in the literature.
Table 7
Condensed valuꢀes of local reactivity descriptors of the coordinating nitrogen atoms
in SCNe and N3
.
fkþ
sþ
fkꢀ
s-
fk2
S/eVꢀ1
s2k /meVꢀ1
4.2. Synthesis of [4-CO2Me-2,6-(tBu2PO)2C6H2]NiCl (1ba)
SCNe 0.1691 0.0183 0.2616 0.0283 ꢀ0.0925 0.1081 ꢀ1.0809
ꢀ
N3
0.4514 0.0417 0.5147 0.0475 ꢀ0.0633 0.0923 ꢀ0.5393
fkþ and fkꢀ: condensed Fukui function; sþ and sꢀ: local softness; fk2: dual descriptor;
To
a solution of 3,5-dihydoxybenzoic acid methyl ester
S: global softness; s2k : local hypersoftness.
(1.0 mmol) in 15 mL of THF was slowly added (via a syringe) a
suspension of NaH (2.1 mmol) in 5 mL of THF (caution: hydrogen
gas evolution). The resulting mixture was heated to reflux for 1 h,
followed by the addition (via a syringe) of a solution tBu2PCl
(2.1 mmol) in 5 mL of THF, and then refluxed for an additional 1 h.
The solvent was evaporated under vacuum, and the residue was
extracted with 40 mL of pentane. The pentane solution was
concentrated under vacuum, first at room temperature and then at
55 ꢁC for 2 h to remove the remaining tBu2PCl. The crude 3,5-
(tBu2PO)2C6H3CO2Me was obtained as a colorless viscous oil which
occasionally solidified upon standing. This crude product was used
in the next step without further purification and characterization.
S and a more negative local hypersoftness s2k value. This implies
that the nitrogen end of SCNe is a softer base than the terminal
ꢀ
nitrogen of N3
.
Substituent effect on the electrophilicity of the nickel center of
this type of complexes has been studied by Zargarian and co-
workers using cyclic voltammetry [7f,h]. It was found that the
oxidation potentials of this type of complexes are affected by both
P- and ring-substituents. For complexes with the same aromatic
pincer backbone, oxidation of the nickel center becomes more
difficult when the P-substituent changes from tBu to iPr, or from iPr
to Ph. Oxidation is also more difficult for complexes containing an
electron-withdrawing substituent at the para-position.
According to hard-soft acid-base theory (HSAB theory), the acid/
base interaction between two hard or two soft species are stronger
and generally form a more stable complex than the interaction
between one hard and one soft species. The calculations are
consistent with the equilibrium constant measurements described
in Table 6; the azide ligand, which is a harder base, prefers a less
electron rich nickel center (i.e., a harder acid) whereas the iso-
thiocyanate ligand, which is a softer base, prefers a more electron
rich nickel center (i.e., a softer acid).
Under
a
nitrogen atmosphere, the crude 3,5-(tBu2
-
PO)2C6H3CO2Me (1.38 g, 3.0 mmol) was mixed with anhydrous
NiCl2 (389 mg, 3.0 mmol) in toluene (50 mL). The resulting mixture
was refluxed under nitrogen for 18 h, cooled to room temperature
and filtered to remove the small amount of solid formed. The or-
ange filtrate was concentrated under vacuum to 5 mL, after which
30 mL of Et2O was added. The desired product formed as a pre-
cipitate, which was collected by filtration and washed with Et2O.
Complex 1ba was isolated as an orange-green powder (725 mg, 44%
yield). 1H NMR (400 MHz, CDCl3,
d): 7.06 (s, ArH，2H), 3.85 (s,
OCH3, 3H), 1.48 (t, JP-H ¼ 6.8 Hz, C(CH3)3, 36H). 13C{1H} NMR
(101 MHz, CDCl3,
d
): 169.33 (t, JP-C ¼ 9.1 Hz, ArC), 167.15 (s, CO2CH3),
133.86 (t, JP-C ¼ 20.2 Hz, ArC), 130.47 (s, ArC), 105.98 (t, JP-C ¼ 6.1 Hz,
ArC), 52.12 (s, CO2CH3), 39.58 (t, JP-C ¼ 7.1 Hz, C(CH3)3), 28.06 (t, JP-
3. Conclusions
¼ 2.8 Hz, C(CH3)3). 31P{1H} NMR (162 MHz, CDCl3,
d): 188.54 (s).
C
A dozen new nickel isothiocyanate and azide complexes have
been synthesized from bis(phosphinite) pincer ligated nickel
chloride complexes and KSCN/NaN3. In order to better understand
how substituents influence the kinetic and thermodynamic
behavior of these complexes, complexes were synthesized with
varying substituents on the phosphorus donor atoms and pincer
aromatic backbone. X-ray crystallographic studies show little
change in NieN and NieCipso bond lengths of the resulting nickel
pincer isothiocyanate and azide complexes by the P-substituents or
aromatic pincer ring modification. However, substituent effects
have been observed on the rates of the NieN bond formation
during the metathesis reactions of the nickel chloride complexes
with KSCN/NaN3, and on the strength of the NieN bonds that in-
fluence the exchange of isothiocyanate and azide ligands on two
different nickel pincer units. The metathesis reaction is faster with
a less electron rich nickel center. Calculations have shown that the
Anal. Calcd for C24H41O4P2ClNi: C, 52.44; H, 7.52. Found: C, 52.61; H,
7.58.
4.3. Synthesis of [4-CO2Me-2,6-(iPr2PO)2C6H2]NiCl (1bb)
To a Schlenk flask containing 3,5-dihydroxybenzoic acid methyl
ester (840 mg, 5.0 mmol), anhydrous NiCl2 (972 mg, 7.5 mmol), and
4-dimethylaminopyridine (1.28 g, 10.5 mmol) in THF (30 mL) was
added iPr2PCl (1.6 mL, 10.1 mmol) at room temperature. The
mixture was stirred under a nitrogen atmosphere and heated to
80 ꢁC for 18 h. After cooling to room temperature, the volatiles were
evaporated, and the resulting solid residue was extracted with n-
hexane (3 ꢂ 20 mL) and filtered. Slow evaporation of the combined
solutions at ambient temperature produced 1bb as yellow-orange
crystals (2.05 g, 83% yield). The 1H NMR data are consistent with
the literature values [10].
terminal nitrogen of N3 is harder than the nitrogen end of SCNe,
ꢀ
thus explaining why the azide ligand prefers a less electron rich
nickel center while the isothiocyanate ligand prefers a more elec-
tron rich nickel center.
4.4. Synthesis of [4-CO2Me-2,6-(Ph2PO)2C6H2]NiCl (1bc)
To the mixture of 3,5-dihydroxybenzoic acid methyl ester (3.8 g,
23.0 mmol) and Ph2PCl (8.4 mL, 46.0 mmol) in toluene (40 mL) was
added triethylamine (7.0 mL, 50.0 mmol) dropwise. The resulting
mixture was heated to reflux for 18 h. After cooling to room tem-
perature, the volatiles were removed under vacuum and the res-
idue was extracted with THF (3 ꢂ 20 mL) and then filtered through
a pad of Celite. After the removal of THF under vacuum, 3,5-
(Ph2PO)2C6H3CO2Me was isolated as a yellow-orange solid. This
crude product was used in the next step without further purifica-
tion and characterization.
4. Experimental section
4.1. General procedures
Unless otherwise indicated, all the syntheses were carried out
under a nitrogen atmosphere using standard Schlenk and glove box
techniques. Solvents used for the reactions were degassed and
dried using standard procedures. The pincer-ligated nickel chloride,
isothiocyanate and azide complexes reported in this work, once
isolated, can be handled in air without noticeable decomposition.
Under a nitrogen atmosphere, the crude 3,5-(Ph2PO)2C6H3
CO2Me (1.01 g, 1.89 mmol) and NiCl2 (245 mg, 1.89 mmol) were
-

138
H. Li et al. / Journal of Organometallic Chemistry 804 (2016) 132e141
mixed with toluene (50 mL). The resulting mixture was refluxed for
24 h. After cooling to room temperature, solvent was evaporated
under vacuum and the residue was extracted with CH2Cl2. Removal
of CH2Cl2 followed by recrystallization from diethyl ether produced
1bc as a yellow-green crystalline solid (865 mg, 73% yield). 1H NMR
(KBr): nCN ¼ 2072 cmꢀ1. Anal. Calcd for C31H23NO2P2SNi: C, 62.66;
H, 3.90; N, 2.36. Found: C, 62.38; H, 4.12; N, 2.40.
4.8. Synthesis of [4-CO2Me-2,6-(tBu2PO)2C6H2]Ni(NCS) (2ba)
(400 MHz, CDCl3,
12H), 7.28 (s, ArH, 2H), 3.87 (s, OCH3, 3H). 13C{1H} NMR (101 MHz,
CDCl3,
d
): 7.94e8.02 (m, ArH, 8H), 7.44e7.56 (m, ArH,
Complex 2ba was prepared from 1ba and KSCN in 86% (room
temperature) or 65% (under reflux) yield by procedures similar to
d
): 166.94 (t, JP-C ¼ 12.1 Hz, ArC), 166.69 (s, CO2CH3), 134.85
those used for 2aa. 1H NMR (400 MHz, CDCl3,
d): 7.07 (s, ArH, 2H),
(t, JP-C ¼ 23.2 Hz, ArC)，132.14 (t, JP-C ¼ 25.2 Hz, ArC)， 132.08 (t, JP-
3.85 (s, OCH3, 3H), 1.45 (t, JP-H ¼ 7.2 Hz, C(CH3)3, 36H). 13C{1H} NMR
¼ 6.1 Hz, ArC)， 131.91 (t, JP-C ¼ 7.1 Hz, ArC)，131.84 (s, ArC),
(101 MHz, CDCl3,
d
): 169.53 (t, JP-C ¼ 9.1 Hz, ArC),166.78 (s, CO2CH3),
C
128.99 (t, JP-C ¼ 5.1 Hz, ArC), 107.65 (t, JP-C ¼ 7.1 Hz ArC), 52.32 (s,
147.65 (s, NCS), 133.16 (t, JP-C ¼ 20.2 Hz, ArC), 131.52 (s, ArC), 106.39
(t, JP-C ¼ 6.1 Hz, ArC), 52.22 (s, CO2CH3), 39.42 (t, JP-C ¼ 7.1 Hz,
C(CH3)3), 27.79 (t, JP-C ¼ 3.0 Hz, C(CH3)3). 31P{1H} NMR (162 MHz,
CO2CH3). 31P{1H} NMR (162 MHz, CDCl3,
d): 141.55 (s). Anal. Calcd
for C32H25O4P2ClNi: C, 61.04; H, 4.00. Found: C, 61.27; H, 4.18.
CDCl3,
d
): 195.18 (s). FTIR (KBr): nCN ¼ 2098 cmꢀ1
,
nCO ¼ 1723 cmꢀ1
.
4.5. Synthesis of [2,6-(tBu2PO)2C6H3]Ni(NCS) (2aa)
Anal. Calcd for C25H41NO4P2SNi: C, 52.47; H, 7.22; N, 2.45. Found: C,
52.37; H, 7.23; N, 2.45.
A mixture of [2,6-(tBu2PO)2C6H3]NiCl (1aa) (492 mg, 1.0 mmol)
and KSCN (486 mg, 5.0 mmol) in THF (20 mL) was stirred in a sealed
flask at room temperature for 3e4 d. The volatiles were removed
under vacuum and the residue was extracted with toluene and then
filtered through a pad of Celite. Removal of toluene under vacuum
produced 2aa as an orange solid (452 mg, 88% yield). 1H NMR
4.9. Synthesis of [4-CO2Me-2,6-(iPr2PO)2C6H2]Ni(NCS) (2bb)
Complex 2bb was prepared from 1bb and KSCN in 93% (room
temperature) or 78% (under reflux) yield by procedures similar to
those used for 2aa. 1H NMR (400 MHz, CDCl3,
d
): 7.08 (s, ArH, 2H),
3.85 (s, OCH3, 3H), 2.39e2.47 (m, CH(CH3)2, 4H), 1.30e1.44 (m,
CH(CH3)2, 24H). 13C{1H} NMR (101 MHz, CDCl3,
): 168.80 (t, JP-
(400 MHz, CDCl3,
d
): 6.94 (t, JH-H ¼ 8.0 Hz, ArH, 1H), 6.41 (d, JH-
¼ 8.0 Hz, ArH, 2H), 1.46 (t, JP-H ¼ 7.6 Hz, CH3, 36H). 13C{1H} NMR
d
H
(101 MHz, CDCl3,
d
): 169.89 (t, JP-C ¼ 9.1 Hz, ArC), 146.87 (s, NCS),
¼ 10.1 Hz, ArC), 166.62 (s, CO2CH3), 147.11 (s, NCS), 132.58 (t, JP-
C
129.31 (s, ArC), 124.70 (t, JP-C ¼ 20.0 Hz, ArC), 105.42 (t, JP-C ¼ 6.1 Hz,
¼ 20.2 Hz, ArC), 131.93 (s, ArC), 106.58 (t, JP-C ¼ 7.1 Hz, ArC), 52.23
C
ArC), 39.21 (t, JP-C ¼ 7.5 Hz, C(CH3)3), 27.88 (t, JP-C ¼ 3.0 Hz, CH3). 31
P
(s, CO2CH3), 28.31 (t, JP-C ¼ 12.1 Hz, CH(CH3)2), 17.41 (t, JC-P ¼ 3.0 Hz,
{1H} NMR (162 MHz, CDCl3,
d): 193.94 (s). FTIR (KBr):
CH(CH3)2), 16.80 (s, CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3,
d):
nCN ¼ 2104 cmꢀ1. Anal. Calcd for C23H39NO2P2SNi: C, 53.72; H, 7.64;
189.85 (s). FTIR (KBr): nCN ¼ 2082 cmꢀ1
,
nCO ¼ 1721 cmꢀ1. Anal.
N, 2.72. Found: C, 53.53; H, 7.54; N, 2.86.
Calcd for C21H33NO4P2SNi: C, 48.86; H, 6.44; N, 2.71. Found: C,
48.88; H, 6.44; N, 2.72.
Alternatively,
a
mixture of [2,6-(tBu2PO)2C6H3]NiCl (1aa)
(492 mg, 1 mmol) and KSCN (486 mg, 5 mmol) in THF (20 mL) was
refluxed under an nitrogen atmosphere for 24 h. After cooling to
room temperature, the volatiles were removed under vacuum and
the residue was extracted with toluene and then filtered through a
pad of Celite. Removal of toluene under vacuum followed by
recrystallization in CH2Cl2/n-hexane produced 2aa as orange crys-
tals (318 mg, 62% yield).
4.10. Synthesis of [4-CO2Me-2,6-(Ph2PO)2C6H2]Ni(NCS) (2bc)
Complex 2bc was prepared from 1bc and KSCN in 90% (room
temperature) or 65% (under reflux) yield by procedures similar to
those used for 2aa. 1H NMR (400 MHz, CDCl3,
d
): 7.83e7.88 (m, ArH,
8H), 7.51e7.60 (m, ArH, 12H), 7.28 (s, ArH, 2H), 3.87 (s, OCH3, 3H).
13C{1H} NMR (101 MHz, CDCl3,
d
): 167.17 (t, JP-C ¼ 11.1 Hz, ArC),
4.6. Synthesis of [2,6-(iPr2PO)2C6H3]Ni(NCS) (2ab)
166.34 (s, CO2CH3), 146.33 (s, NCS), 133.27 (t, JP-C ¼ 23.2 Hz, ArC),
132.69 (s, ArC), 132.58 (s, ArC), 131.51 (t, JP-C ¼ 7.1 Hz, ArC), 131.35 (t,
JP-C ¼ 25.3 Hz, ArC), 129.38 (t, JP-C ¼ 5.1 Hz, ArC), 107.89 (t, JP-
Complex 2ab was prepared from 1ab and KSCN in 93% (room
temperature) or 78% (under reflux) yield by procedures similar to
¼ 8.1 Hz, ArC), 52.37 (s, CO2CH3). 31P{1H} NMR (162 MHz, CDCl3,
C
d
those used for 2aa. 1H NMR (400 MHz, CDCl3,
d): 6.94 (t, JH-
): 145.02 (s). FTIR (KBr): nCN ¼ 2071 cmꢀ1
,
nCO ¼ 1715 cmꢀ1. Anal.
¼ 8.0 Hz, ArH, 1H), 6.39 (d, JH-H ¼ 8.0 Hz, ArH, 2H), 2.35e2.41 (m,
Calcd for C33H25NO4P2SNi: C, 60.77; H, 3.86; N, 2.15. Found: C,
60.43; H, 3.94; N, 2.12.
H
CH(CH3)2, 4H), 1.28e1.42 (m, CH(CH3)2, 24H). 13C{1H} NMR
(101 MHz, CDCl3,
d
): 169.06 (t, JP-C ¼ 9.8 Hz, ArC), 146.27 (s, NCS),
129.74 (s, ArC), 124.22 (t, JP-C ¼ 21.3 Hz, ArC), 105.57 (t, JP-C ¼ 6.0 Hz,
ArC), 28.17 (t, JC-P ¼ 11.3 Hz, CH(CH3)2), 17.45 (t, JC-P ¼ 3.0 Hz,
4.11. Synthesis of [2,6-(tBu2PO)2C6H3]NiN3 (3aa)
CH(CH3)2), 16.82 (s, CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3,
d
):
Anal. Calcd for
19H31NO2P2SNi: C, 49.81; H, 6.82; N, 3.06. Found: C, 49.85; H, 6.68,
A mixture of [2,6-(tBu2PO)2C6H3]NiCl (1aa) (492 mg, 1 mmol),
NaN3 (325 mg, 5 mmol), THF (10 mL) and MeOH (10 mL) was stirred
in a sealed flask at room temperature for 3e4 d. The volatiles were
removed under vacuum and the residue was extracted with toluene
and then filtered through a pad of Celite. Removal of toluene under
vacuum produced 3aa as an orange solid (451 mg, 91% yield). 1H
188.57 (s). FTIR (KBr): nCN
¼
2091 cmꢀ1
.
C
N, 3.06.
4.7. Synthesis of [2,6-(Ph2PO)2C6H3]Ni(NCS) (2ac)
NMR (400 MHz, CDCl3,
d
): 6.92 (t, JH-H ¼ 8.0 Hz, ArH, 1H), 6.38 (d, JH-
Complex 2ac was prepared from 1ac and KSCN in 91% (room
temperature) or 75% (under reflux) yield by procedures similar to
¼ 8.0 Hz, ArH, 2H), 1.46 (t, JP-H ¼ 8.0 Hz, CH3, 36H). 13C{1H} NMR
H
(101 MHz, CDCl3,
d
): 169.79 (t, JP-C ¼ 9.6 Hz, ArC), 128.77 (s,
those used for 2aa. 1H NMR (400 MHz, CDCl3,
d
): 7.86e7.89 (m, ArH,
ArC),123.86 (t, JP-C ¼ 20.2 Hz, ArC), 105.24 (t, JP-C ¼ 5.9 Hz, ArC),
8H), 7.51e7.55 (m, ArH, 12H), 7.11 (t, JH-H ¼ 8.0 Hz, ArH, 1H), 6.65 (d,
38.94 (t, JP-C ¼ 7.2 Hz, C(CH3)3), 27.83 (t, JP-C ¼ 3.0 Hz, CH3). 31P{1H}
JH-H ¼ 8.0 Hz, ArH, 2H). 13C{1H} NMR (101 MHz, CDCl3,
d
): 167.51 (t,
NMR (162 MHz, CDCl3,
d
): 189.85. FTIR (KBr): nN3 ¼ 2072 cmꢀ1
.
JP-C ¼ 11.3 Hz, ArC), 145.69 (s, NCS), 132.37 (s, ArC), 131.81 (t, JP-
Anal. Calcd for C22H39N3O2P2Ni: C, 53.04; H, 7.89; N, 8.43. Found: C,
53.17; H, 7.93. N, 8.39.
¼ 24.8 Hz, ArC), 131.55 (t, JP-C ¼ 7.5 Hz, ArC), 130.65 (s, ArC), 129.29
C
(t, JP-C ¼ 5.2 Hz, ArC), 125.13 (t, JP-C ¼ 23.9 Hz, ArC), 107.01 (t, JP-
Alternatively,
(492 mg, 1 mmol), NaN3 (325 mg, 5 mmol), THF (10 mL) and
a
mixture of [2,6-(tBu2PO)2C6H3]NiCl (1aa)
¼ 7.0 Hz, ArC). 31P{1H} NMR (162 MHz, CDCl3,
d): 144.59 (s). FTIR
C

H. Li et al. / Journal of Organometallic Chemistry 804 (2016) 132e141
139
methanol (10 mL) was refluxed under a nitrogen atmosphere for
4.16. Synthesis of [4-CO2Me-2,6-(Ph2PO)2C6H2]NiN3 (3bc)
24 h. After cooling to room temperature, the volatiles were
removed under vacuum and the residue was extracted with toluene
and then filtered through a pad of Celite. Removal of toluene under
vacuum followed by recrystallization in CH2Cl2/n-hexane produced
3aa as orange crystals (348 mg, 70% yield).
Complex 3bc was prepared from 1bc and NaN3 in 91% (room
temperature) or 60% (under reflux) yield by procedures similar to
those used for 3aa. 1H NMR (400 MHz, CDCl3,
d
): 7.96e8.01 (m, ArH,
8H), 7.46e7.55 (m, ArH, 12H), 7.28 (s, ArH, 2H), 3.87 (s, OCH3, 3H).
13C{1H} NMR (101 MHz, CDCl3,
d): 166.93 (t, JP-C ¼ 11.1 Hz, ArC),
166.57 (s, CO2CH3), 133.96 (t, JP-C ¼ 22.2 Hz, ArC), 132.42 (s, ArC),
131.92 (s, ArC), 131.73 (t, JP-C ¼ 8.1 Hz, ArC), 131.73 (t, JP-C ¼ 24.2 Hz,
ArC), 129.24 (t, JP-C ¼ 5.1 Hz, ArC), 107.71 (t, JP-C ¼ 7.1 Hz, ArC), 52.31
4.12. Synthesis of [2,6-(iPr2PO)2C6H3]NiN3 (3ab)
Complex 3ab was prepared from 1ab and NaN3 in 89% (room
temperature) or 78% (under refluxing) yield by procedures similar
(s, CO2CH3). 31P{1H} NMR (162 MHz, CDCl3,
d): 143.09 (s). FTIR
(KBr): nN3 ¼ 2049 cmꢀ1
,
nCO ¼ 1716 cmꢀ1. Anal. Calcd for
to those used for 3aa. 1H NMR (400 MHz, CDCl3,
d): 6.94 (t, JH-
C
32H25N3O4P2Ni: C, 60.41; H, 3.96; N, 6.60. Found: C, 60.57; H, 3.93;
¼ 8.0 Hz, ArH, 1H), 6.39 (d, JH-H ¼ 8.0 Hz, ArH, 2H), 2.36e2.43 (m,
H
N, 6.65.
CH(CH3)2, 4H), 1.31e1.45 (m, CH(CH3)2, 24H). 13C{1H} NMR
(101 MHz, CDCl3,
d
): 168.85 (t, JP-C ¼ 10.1 Hz, ArC), 129.11 (s, ArC),
4.17. Comparative study of the reactions of [4-Z-2,6-(R2PO)2C6H2]
NiCl with KSCN or NaN3
124.44 (t, JP-C ¼ 20.2 Hz, ArC), 105.42 (t, JP-C ¼ 6.1 Hz, ArC), 27.96 (t,
JC-P ¼ 10.1 Hz, CH(CH3)2), 17.33 (t, JC-P ¼ 4.0 Hz, CH(CH3)2), 16.82 (s,
CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3,
d): 185.17 (s). FTIR (KBr):
In a 25 mL flask, 0.10 mmol of potassium thiocyanate or sodium
azide was mixed with 3 mL of THF. The mixture was stirred at 0 ꢁC
for 10 min, followed by the addition of a [4-Z-2,6-(R2PO)2C6H2]NiCl
(R ¼ tBu, iPr, Ph; Z ¼ H, CO2Me) solution (0.10 mmol in 3 mL of THF).
The resulting reaction mixture was further stirred at 0 ꢁC for 5 min,
after which 15 mL of water was quickly added with stirring. The
precipitate was collected by filtration, dried and analyzed by 31P
{1H} NMR. The ratio between [4-Z-2,6-(R2PO)2C6H2]NiCl and [4-Z-
2,6-(R2PO)2C6H2]NiNCS or [4-Z-2,6-(R2PO)2C6H2]NiN3 was deter-
mined based on 31P{1H} NMR integrations, from which the con-
version was calculated.
nN3 ¼ 2053 cmꢀ1. Anal. Calcd for C18H31N3O2P2Ni: C, 48.90; H, 7.07;
N, 9.50. Found: C, 48.73; H, 7.05; N, 9.40.
4.13. Synthesis of [2,6-(Ph2PO)2C6H3]NiN3 (3ac)
Complex 3ac was prepared from 1ac and NaN3 in 88% (room
temperature) or 60% (under reflux) yield by procedures similar to
those used for 3aa. 1H NMR (400 MHz, CDCl3,
d): 7.88e7.92 (m, ArH,
8H), 7.49e7.57 (m, ArH, 12H), 7.07 (t, JH-H ¼ 8.0 Hz, ArH, 1H), 6.60 (d,
JH-H ¼ 8.0 Hz, ArH, 2H). 13C{1H} NMR (101 MHz, CDCl3,
d): 167.26 (t,
JP-C ¼ 11.4 Hz, ArC), 132.23 (s, ArC), 132.13 (t, JP-C ¼ 23.8 Hz, ArC),
131.76 (t, JP-C ¼ 8.0 Hz, ArC), 130.00 (s, ArC), 129.16 (t, JP-C ¼ 5.6 Hz,
4.18. Equilibrium constant measurement for the isothiocyanate/
azide exchange reactions
ArC), 125.37 (t, JP-C ¼ 24.4 Hz, ArC), 106.81 (t, JP-C ¼ 7.2 Hz, ArC). 31
P
{1H} NMR (162 MHz, CDCl3,
d): 142.47 (s). FTIR (KBr):
nN3 ¼ 2048 cmꢀ1. Anal. Calcd for C30H23N3O2P2Ni: C, 62.32; H, 4.01;
The exchange reactions were carried out at room temperature
(T ¼ 298 K) in sealed NMR tubes using CDCl3 as the solvent. The
reactions were monitored by 31P{1H} NMR spectroscopy. Three
individual experiments were carried out for any given exchange
reaction. As a representative example, for the exchange reaction of
3aa þ 2ba # 2aa þ 3ba , the three experiments were: (1) 5 mg
of 3aa and 5 mg of 2ba were mixed with 0.5 mL of CDCl3 in a NMR
tube; (2) 5 mg of 2aa and 5 mg of 3ba were mixed with 0.5 mL of
CDCl3 in a NMR tube; (3) 2e3 mg of 3aa, 2e3 mg of 2ba, 2e3 mg of
2aa and 2e3 mg of 3ba were mixed with 0.5 mL of CDCl3 in a NMR
tube. The NMR tubes were then sealed and the 31P{1H} NMR spectra
were recorded every 8 h. The equilibrium was considered to be
established when the ratios of the NMR integration values were
unchanged. Each reported equilibrium constant was the average of
the values from the three experiments.
N, 7.27. Found: C, 62.56; H, 3.98; N, 7.46.
4.14. Synthesis of [4-CO2Me-2,6-(tBu2PO)2C6H2]NiN3 (3ba)
Complex 3ba was prepared from 1ba and NaN3 in 90% (room
temperature) or 60% (under reflux) yield by procedures similar to
those used for 3aa. 1H NMR (400 MHz, CDCl3,
d): 7.05 (s, ArH, 2H),
3.85 (s, OCH3, 3H), 1.46 (t, JP-H ¼ 7.2 Hz, C(CH3)3, 36H). 13C{1H} NMR
(101 MHz, CDCl3,
d
): 169.46 (t, JP-C ¼ 10.1 Hz, ArC), 166.97 (s,
CO2CH3), 132.85 (t, JP-C ¼ 20.2 Hz, ArC), 131.03 (s, ArC), 106.23 (t, JP-
¼ 5.9 Hz, ArC), 52.18 (s, CO2CH3), 39.20 (t, JP-C ¼ 7.1 Hz, C(CH3)3),
C
27.76 (t, JP-C ¼ 3.0 Hz, C(CH3)3). 31P{1H} NMR (162 MHz, CDCl3,
d):
191.13 (s). FTIR (KBr): nN3 ¼ 2073 cmꢀ1
,
nCO ¼ 1719 cmꢀ1. Anal. Calcd
for C24H41N3O4P2Ni: C, 51.82; H, 7.43; N, 7.55. Found: C, 51.98; H,
7.53; N, 7.46.
4.19. Quantum chemical analysis
4.15. Synthesis of [4-CO2Me-2,6-(iPr2PO)2C6H2]NiN3 (3bb)
A quantum chemical analysis for the coordinating nitrogen
atoms of SCNe and N3 was performed. The ground-state geome-
ꢀ
Complex 3bb was prepared from 1bb and NaN3 in 92% (room
temperature) or 60% (under reflux) yield by procedures similar to
tries were optimized by the second-order Møller-Plesset pertur-
bation (MP2) method using the 6e311þþG (d, p) basis set.
Condensed values of local reactivity descriptors, described by
conceptual density functional theory (CDFT) [15] were calculated
according to the literature procedure [16]. The charges were ob-
tained from the NPA population analysis. All calculations were
carried out using the Gaussian 09 software package [17].
those used for 3aa. 1H NMR (400 MHz, CDCl3,
d
): 7.06 (s, ArH, 2H),
3.85 (s, OCH3, 3H), 2.38e2.45 (m, CH(CH3)2, 4H), 1.30e1.45 (m,
CH(CH3)2, 24H). 13C{1H} NMR (101 MHz, CDCl3,
): 168.52 (t, JP-
d
¼ 10.1 Hz, ArC), 166.84 (s, CO2CH3), 133.25 (t, JP-C ¼ 21.2 Hz, ArC),
C
131.28 (s, ArC), 106.40 (t, JP-C ¼ 6.1 Hz, ArC), 52.18 (s, CO2CH3), 28.04
(t, JP-C ¼ 11.1 Hz, CH(CH3)2), 17.25 (t, JC-P ¼ 3.0 Hz, CH(CH3)2), 16.75
(s, CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3,
d
): 186.56 (s). FTIR
4.20. X-ray structure determinations
(KBr): nN3 ¼ 2047 cmꢀ1
,
nCO ¼ 1715 cmꢀ1. Anal. Calcd for
C
20H33N3O4P2Ni: C, 48.03; H, 6.65; N, 8.40. Found: C, 48.04; H, 6.67;
Single crystals of the nickel isothiocyanate and azide complexes
N, 8.35.
were obtained from recrystallization in CH2Cl2/n-hexane. Single

140
H. Li et al. / Journal of Organometallic Chemistry 804 (2016) 132e141
Table 8
Summary of crystallographic data and structure refinement for representative nickel complexes.
2aa
2bb
3ac
3ba
Empirical formula
Formula weight
Temp, K
C23H39NNiO2P2S
514.26
103(2)
C21H33NNiO4P2S
516.19
103(2)
C30H23N3NiO2P2
578.14
296(2)
C24H41N3NiO4P2
556.25
103(2)
Crystal system
Space group
a, Å
Triclinic
P ꢀ1
Orthorhombic
Pca21
22.3789(10)
7.9931(4)
14.1551(6)
90.00
90.00
90.00
2532.0(2)
4
1.354
Orthorhombic
P212121
9.290(2)
14.067(3)
19.763(4)
90.00
90.00
90.00
2582.8(10)
4
1.487
Orthorhombic
Pca21
25.048(2)
8.6616(8)
12.8253(7)
90.00
90.00
90.00
2782.5(4)
4
1.328
8.3477(10)
12.0771(14)
13.5837(16)
102.639(2)
93.182(3)
103.943(2)
1288.4(3)
2
b, Å
c, Å
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
Volume, Å3
Z
dcalc, g cmꢀ3
1.326
l
, Å
0.71073
0.977
0.71073
1.001
0.71073
0.910
0.71073
0.845
m
, mmꢀ1
No. of data collected
No. of unique data
Rint
49168
13206
0.1101
0.977
23360
7884
0.0686
0.833
23478
5960
0.0489
1.059
23633
7688
0.1216
0.986
Goodness-of-fit on F2
R1, wR2 (I > 2
R1, wR2 (all data)
s
(I))
0.0597, 0.1219
0.1208, 0.1483
0.0385, 0.0616
0.0479, 0.0643
0.0390, 0.0988
0.0432, 0.1051
0.0627, 0.1100
0.1017, 0.1270
crystals of 1ba were grown from CH2Cl2. Crystal data collection and
refinement parameters of 2aa, 2bb, 3ac and 3ba are summarized in
Table 8. The data for the remaining nickel complexes can be found
in Supplementary data (Appendix B). Intensity data were collected
at 103 K for 2aa, 2ba, 2bb, 2bc, 3aa, 3ba, 3bb and 3bc, and 296 K for
2ab and 3ac on a Bruker SMART6000 CCD diffractometer using
Light Source) program at Beamline 11.3.1 at the Advanced Light
Source (ALS), Lawrence Berkeley National Laboratory. The ALS is
supported by the U.S. Department of Energy, Office of Energy Sci-
ences Materials Sciences Division, under contract DE-AC02-
05CH11231. Data for 2ac were collected at the University of Cin-
cinnati on a Bruker SMART6000 diffractometer which was funded
by an NSF-MRI grant (CHE-0215950).
graphite-monochromated MoK
data of 2ac were collected at 150 K on a Bruker SMART6000 CCD
diffractometer using graphite-monochromated CuK radiation,
a
radiation,
l
¼ 0.71073 Å. Intensity
a
Appendix A. Supplementary material
l
¼ 1.54178 Å. Intensity data of 1ba were collected at 150 K on a
Bruker PHOTON100 CMOS detector at Beamline 11.3.1 at the
Advanced Light Source (Lawrence Berkeley National Laboratory)
CCDC 1404255 (1ba), 1404256 (2ac), 1034585 (2ab), 1034586
(2bb), 1034587 (3ac), 1404901 (2ba), 1404902 (2bc), 1404903
(3aa), 1404904 (3ba), 1404905 (3bc), 1404906 (2aa) and 1404907
(3bb) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
using synchrotron radiation tuned to
l
¼ 0.7749 Å. The frames were
integrated with the Bruker APEX2 software package using a
narrow-frame algorithm. The data were corrected for decay, Lor-
entz, and polarization effects as well as absorption and beam cor-
rections based on the multi-scan technique. The structures were
solved by a combination of direct methods in SHELXTL and the
difference Fourier technique and refined by full-matrix least-
squares procedures. Nonhydrogen atoms were refined with aniso-
tropic displacement parameters. The H-atoms were either located
or calculated and subsequently treated with a riding model. Two
independent molecules of 1ba crystallize in the lattice. Disorder is
observed for some of the carbon atoms of the tBu groups of 1ba and
the nitrogen atoms of the N3 group of 3aa; for 1ba a two-
component disorder models is presented for C10 B/C and C14 B/C,
and for 3aa a two-component model with 85:15 occupancy is
given. No solvent of crystallization is present in the lattice for any of
the structures.
Appendix B. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.12.038.
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