Using CS2 to Probe the Mechanistic Details of Decarboxylation of Bis(phosphinite)-Ligated Nickel Pincer Formate Complexes

Article abstract of DOI:10.1021/acs.organomet.6b00759

The reaction of the formate complex {2,6-(R₂PO)₂C₆H₃}Ni(OCHO) (R = ^tBu, 5; R = ⁱPr, 6) with CS₂ shows first-order kinetics in nickel concentration and zero-order in [CS₂

Full text of DOI:10.1021/acs.organomet.6b00759

Article
pubs.acs.org/Organometallics
Using CS2 to Probe the Mechanistic Details of Decarboxylation of
Bis(phosphinite)-Ligated Nickel Pincer Formate Complexes
Qiang-Qiang Ma,† Ting Liu,† Anubendu Adhikary,‡ Jie Zhang,*,† Jeanette A. Krause,‡
and Hairong Guan*,‡
†School of Chemistry and Chemical Engineering, Henan Key Laboratory of Boron Chemistry and Advanced Energy Materials, Henan
Normal University, Xinxiang, Henan 453007, China
‡Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States
S
* Supporting Information
ABSTRACT: The reaction of the formate complex {2,6-(R2PO)2C6H3}-
Ni(OCHO) (R = tBu, 5; R = iPr, 6) with CS2 shows first-order kinetics in
nickel concentration and zero-order in [CS2] when CS2 is used in large
excess. Rate measurement at different temperatures gives activation
parameters ΔH⧧ = 22.6
0.9 kcal/mol and ΔS⧧ = −5.2
3.0 eu for
the decarboxylation of 5 and ΔH⧧ = 22.6 1.0 kcal/mol and ΔS⧧ = −4.3
3.2 eu for the decarboxylation of 6. Comparing the decarboxylation rate
constants for 6 and {2,6-(iPr2PO)2C6H3}Ni(OCDO) (6-d) yields KIE
values of 1.67−1.90 within the temperature range 30−45 °C. On the basis of these experimental results and DFT calculations, an
ion pair mechanism has been proposed for the decarboxylation process. The CS2 insertion products {2,6-(R2PO)2C6H3}Ni-
t
i
(SCHS) (R = Bu, 3; R = Pr, 4) have been characterized by X-ray crystallography.
straightforward, as demonstrated with CpRe(NO)(PPh3)-
INTRODUCTION
Decarboxylation of transition metal formate complexes (eq 1)
is an important step in catalytic processes such as
■
(OCHO).7a It is more challenging when decarboxylation is
thermodynamically unfavorable. One solution is to study the
exchange reaction between the formate complex and 13CO2 as
an indirect probe to the decarboxylation process. For instance,
the isotopic exchange of trans-(Cy3P)2M(H)(OCHO) (M =
7c
Ni, Pd)7b or [Cr(CO)5(OCHO)]− with excess 13CO2 is first-
order in formate concentration and independent of 13CO2
pressure. The observed exchange rate is therefore equivalent to
the rate of formate decarboxylation.
decomposition of formic acid to H2 and CO2,1 transfer
hydrogenation reactions using HCO2H, HCO2H−NEt3, or
HCO2 as the hydrogen source,2 and isomerization of methyl
−
In this paper we investigate the reactions of formate
complexes with CS2 as an alternative but less expensive method
to elucidate the mechanistic details of formate decarboxylation.
We focus specifically on bis(phosphinite)-ligated nickel pincer
formate complexes due to their involvement in nickel-catalyzed
reduction of CO2 with boranes.9 Our previous research has
shown that decarboxylation of these formate complexes is
kinetically accessible but thermodynamically unfavorable.9a,b
Given the notion that CS2 is more reactive than CO2,10 we
hypothesize that CS2 will outcompete CO2 for the insertion
into the nickel hydride intermediates, leading to a kinetic
scenario in which the decarboxylation step becomes rate
determining.
formate to acetic acid.3 It also constitutes a strategy to
synthesize transition metal hydrides,4 provided that thermody-
namics favor decarboxylation or the removal of CO2 can drive
the equilibrium to favor hydride formation. Double decarbox-
ylation of bis(formato) complexes is a convenient way to
generate low-valent metal species by taking advantage of the
subsequent H2 elimination.5 The reverse step of decarbox-
ylation (i.e., insertion of CO2 into metal hydrides) is often the
key to the success of developing catalytic reduction of CO2.6
Understanding the mechanistic details of decarboxylation
reactions can thus shed light on how CO2 insertion occurs
because, according to the principle of microscopic reversibility,
these two steps proceed via the same transition state.
Examples of decarboxylation of formate complexes are
known for virtually every transition metal; however, very few
systems have been subjected to in-depth analysis of this
transformation.7 In particular, kinetic and mechanistic
information pertaining to decarboxylation reactions is limited
in the literature and relies mostly on density functional theory
(DFT) calculations.8 For a thermodynamically favorable
decarboxylation, direct measurement of the kinetics should be
RESULTS AND DISCUSSION
■
Reactivity of Nickel Pincer Hydride Complexes
toward CS2. Although the insertion of CS2 into metal hydrides
is a well-known process,11 examples for nickel complexes are
Received: September 27, 2016
© XXXX American Chemical Society
A
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surprisingly rare. Liaw and co-workers have used CS2 to trap a
transient Ni(III) hydride species bearing a tripodal [P(C6H3-3-
SiMe3-2-S)3]3− ligand,12 which, to our knowledge, is the only
known case for inserting CS2 into a Ni−H bond. To establish
the insertion chemistry, nickel hydrides 1 and 2 were treated
with excess CS2 at room temperature (eq 2), which resulted in
an immediate color change from light yellow to golden yellow.
The insertion products 3 and 4 were isolated in good yield and
characterized by elemental analysis, NMR spectroscopy, and X-
ray crystallography. As expected for a dithioformate complex, 3
or 4 dissolved in CDCl3 displays characteristic low-field 1H and
13C resonances for the dithioformato group (δH 11.50 and δC
234.0 for 3, δH 11.55 and δC 233.7 for 4).
Figure 2. ORTEP drawing of 4 at the 50% probability level. Selected
bond lengths (Å) and angles (deg): Ni−C1 1.8975(19), Ni−P1
2.1686(6), Ni−P2 2.1568(6), Ni−S1 2.2269(6), C23−S1 1.681(2),
C23−S2 1.646(3), P1−Ni−P2 163.60(2), C1−Ni−S1 170.24(6), Ni−
S1−C23 107.40(8), S1−C23−S2 128.65(14). Interatomic Ni···S2
distance: 3.4813(7) Å.
Our previous work on the analogous formate complexes
t
i
{2,6-(R2PO)2C6H3}Ni(OCHO) (R = Bu (5), Pr (6), or
cyclopentyl) shows that in the solid state the formato group
prefers an in-plane conformation with respect to the
coordination plane, unless the pincer ligand is sufficiently
when a solid sample or a toluene/hexane solution was exposed
to air for several weeks. Unlike nickel formate complexes 5 and
6, which lose CO2 readily under a dynamic vacuum while being
heated at 60 °C, dithioformate complexes 3 and 4 show no sign
of CS2 deinsertion under the same conditions. Bubbling CO2
through a solution of 3 or 4 in C6D6 for 2 h led to no change in
NMR spectra, further illustrating that CS2 insertion is
irreversible or the dithioformate complex is more stable than
the formate complex. Complex 3 underwent alkylation with
allylic or benzyl bromide at 90 °C, producing the known {2,6-
(tBu2PO)2C6H3}NiBr13 and alkyl dithioformate11j cleanly (eq
3). In the latter case, the NMR yield for PhCH2SCHS was
9a,c
t
bulky (e.g., R = Bu).
The dithioformato group in both 3
(Figure 1) and 4 (Figure 2) adopts an almost perpendicular
determined to be 74%. Had CS2 deinsertion occurred, one
would anticipate dehalogenation of the alkyl bromides by the
nickel hydride intermediate 1.
Decarboxylation of Nickel Formate Complexes in the
Presence of CS2. The resistance of 3 and 4 against CS2
deinsertion and the reversible nature of CO2 insertion into 1
and 2 suggest that for the bis(phosphinite)-based nickel pincer
system the formate complexes should react with CS2 via
decarboxylation. Indeed, such reactions were found to take
place at room temperature, albeit slowly. At 80 °C with a large
excess of CS2 (∼30 equiv), complexes 5 and 6 were fully
converted to 3 and 4, respectively, within 12 h and the
dithioformate complexes were isolated in high yield (eq 4).
Figure 1. ORTEP drawing of 3 at the 50% probability level. Selected
bond length (Å) and angles (deg): Ni−C1 1.9051(19), Ni−P1
2.1955(6), Ni−P2 2.2119(6), Ni−S1 2.2203(6), C23−S1 1.694(2),
C23−S2 1.638(2), P1−Ni−P2 163.44(2), C1−Ni−S1 166.41(6), Ni−
S1−C23 120.47(8), S1−C23−S2 132.22(13). Interatomic Ni···S2
distance: 3.9841(6) Å.
conformation with a dihedral angle of 84.0(1)° between the
P1−P2−C1−Ni and SC(S)H planes. The S1 atom is displaced
out of the least-squares plane defined by the P1, P2, C1, and Ni
atoms; however, the long interatomic Ni···S2 distance suggests
a κ1-coordination mode for the dithioformato ligand. Complex
3 displays a more bent C1−Ni−S1 angle but a longer
interatomic Ni···S2 distance than 4, further supporting that
the displacement of S1 from the coordination plane is not due
to Ni···S2 interaction.
Complexes 3 and 4 are moisture and air stable both as solids
and in solution. No significant decomposition was observed
Kinetics for the aforementioned reactions in toluene-d8 were
1
studied by H NMR spectroscopy using 1,4-dioxane as an
B
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internal standard. The nickel formate complexes were first
treated with 20 equiv of CS2, and the initial formate
concentration was kept at 0.013 M. Monitoring the
disappearance of the nickel formate complexes at 30 °C
showed a first-order decay with a rate constant of (1.8 0.1) ×
10−5 s−1 for 5 and (3.7 0.1) × 10−5 s−1 for 6. Increasing the
initial concentration or the equivalence of CS2 accelerated the
reaction of 5 until a plateau was reached at [CS2] ≈ 0.8 M,
giving a saturation rate constant of (2.5 0.1) × 10−5 s−1. In
contrast, the rate for the decarboxylation of 6 was not enhanced
with increasing [CS2], suggesting that with 20 equiv of CS2 the
rate was already saturated.
Transition State for the Decarboxylation Reaction. We
have previously proposed a four-membered-ring transition state
featuring axial hydride abstraction by the nickel, first based on
DFT calculations9b and more recently from a reactivity study of
P-stereogenic nickel formate complexes as shown in Figure 3.16
The kinetics data can be rationalized by a reaction scheme
involving reversible CO2 deinsertion followed by CS2 insertion
(Scheme 1). The steady-state approximation applied to the
Figure 3. Reactivity difference between meso and racemic isomers of
nickel formate complexes.
The current work provides additional experimental evidence
supporting the transition-state structure. The experimentally
determined ΔH⧧ value (22.6 kcal/mol) for 5 is in good
agreement with the computed value (24.0 kcal/mol),9b
especially considering the absolute error in ΔH⧧ (2.2 kcal/
mol). The measured ΔS⧧ values for 5, 6, and 6-d are slightly
Scheme 1. Decarboxylation of Nickel Formate Complexes
Assisted by CS2
negative, comparable to the ΔS⧧ value (−6.3
1.3 eu)
determined for the decarboxylation of CpRe(NO)(PPh3)-
(OCHO).7a However, the absolute errors in ΔS⧧ are too large
to discern the gain or loss of degrees of freedom. Nevertheless,
the sterically less demanding nickel formate complex 6 loses
CO2 at a rate consistently faster than 5 (by 30−60%) and
appears to be controlled by the entropy term.
nickel hydride intermediate (which was not observed during
the reaction) gives a rate law shown in eq 5. Saturation occurs
when CO2 insertion becomes negligible (k−1[CO2] ≪
k2[CS2]), at which point the observed rate constant is for the
decarboxylation step (eq 6).
Darensbourg and co-workers have reported a small KIE (kH/
kD = 1.13 at 15 °C) for the decarboxylation of [Cr-
(CO)5(OCHO)]− and have accordingly proposed an early
transition state.7c Creutz et al. have shown a similarly small KIE
(kH/kD = 1.15 0.10 at 25 °C) for the limiting rate at which
formate ion binds to [(η6-C6Me6)Ru(bipy)(H2O)]2+ and have
used it as the rate for the decarboxylation of [(η6-C6Me6)Ru-
(bipy)(OCHO)]+.17 The KIE values (1.67−1.90 at 30−45 °C)
obtained in this work are larger but comparable to the KIE (kH/
kD = 1.55 0.19 at 112 °C) reported for the decarboxylation of
CpRe(NO)(PPh3)(OCHO).7a Our results suggest that at the
transition state 5 or 6 has a more elongated formate C−H bond
than [Cr(CO)5(OCHO)]−. The larger KIE values are likely
due to [{2,6-(R2PO)2C6H3}Ni]+ and [CpRe(NO)(PPh3)]+
being better hydride acceptors than the neutral Cr(CO)5
fragment. Compounds 5 and 6 are 16-electron complexes
and therefore can undergo β-hydrogen elimination directly as
the decarboxylation mechanism. Coordinatively saturated
complexes, on the other hand, would require ligand dissociation
prior to β-hydrogen elimination.18 Alternatively, decarboxyla-
tion of these complexes may proceed via an ion pair mechanism
k1k2[CS2][NiOCHO]
k−1[CO2] + k2[CS2]
−d[NiOCHO]
dt
=
(5)
When k2[CS2] ≫ k−1[CO2],
k1k2[CS2][NiOCHO]
−d[NiOCHO]
dt
≈
k2[CS2]
= k1[NiOCHO]
(6)
The rate constants for the decarboxylation of 5 and 6 at
higher temperatures were determined in a similar fashion (see
Supporting Information). These data yielded activation
parameters ΔH⧧ = 22.6
0.9 kcal/mol and ΔS⧧ = −5.2
3.0 eu for the decarboxylation of 5 and ΔH⧧ = 22.6 1.0 kcal/
mol and ΔS⧧ = −4.3 3.2 eu for 6. It should be noted that the
errors provided here were derived from the standard least-
squares procedures. The uncertainties in the activation
parameters should be higher due to the very narrow
temperature range applied (30 to 45 °C, limited by the NMR
technique). Using the error propagation formulas described by
Girolami et al.,14 the absolute errors in ΔH⧧ and ΔS⧧ were
estimated to be 2.2 kcal/mol and 7.2 eu, respectively.15
involving the dissociation of HCO2 accompanied by H−
−
transfer from carbon to metal, as proposed for CpRe(NO)-
(PPh3)(OCHO)7a and (RPNHPR)Fe(CO)H(OCHO)
(RPNHPR = HN(CH2CH2PR2)2).8b On the basis of DFT
calculations, Catlow, Xiao, and co-workers have proposed a
similar mechanism for the decarboxylation of Cp*Ir(imino-
aryl)(OCHO).8e Their experimental data on Cp*Ir(imino-
aryl)-catalyzed transfer hydrogenation of imines with HCO2H−
NEt3 have suggested that the decarboxylation step is rate
limiting with a kHCO2H/kDCO2D value of about 1.9 (at 60 °C).
For our system, the calculated transition state for the
decarboxylation of 5 (Figure 4) shows a relatively short Ni···
H distance (1.757 Å) between nickel and the formate
To further understand the decarboxylation process, deute-
rium-labeled nickel formate complex 6-d was prepared from
protonation of hydride 2 with formic acid-d2. Its decarbox-
ylation in the presence of 20 equiv of CS2 was markedly slower
than the protio species 6. Comparing their rate constants (kH/
kD) revealed a kinetic isotope effect (KIE) of 1.90 0.12 at 30
°C and 1.85 0.19 at 45 °C. Rate measurements for 6-d at
different temperatures gave activation parameters ΔH⧧ = 23.4
0.4 kcal/mol and ΔS⧧ = −2.9 1.2 eu. The absolute errors
σΔH⧧ (2.3 kcal/mol) and σΔS⧧ (7.4 eu) again are higher than
the errors obtained from fitting the Eyring equation.14,15
C
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yellow residue was subjected to recrystallization from toluene/hexane
1
(1:1) to yield pure 3 (155 mg, 97% yield). H NMR (CDCl3, 400
MHz, δ): 11.50 (t, JP−H = 2.0 Hz, SCHS, 1H), 7.00 (t, JH−H = 8.0 Hz,
ArH, 1H), 6.48 (d, JH−H = 8.0 Hz, ArH, 2H), 1.45 (vt, JP−H = 7.2 Hz,
1
CH3, 36H). H NMR (C6D6, 400 MHz, δ): 11.70 (t, JP−H = 2.4 Hz,
SCHS, 1H), 6.87 (t, JH−H = 8.0 Hz, ArH, 1H), 6.55 (d, JH−H = 8.0 Hz,
ArH, 2H), 1.34 (vt, JP−H = 7.2 Hz, CH3, 36H). 13C{1H} NMR
(CDCl3, 101 MHz, δ): 234.0 (s, SCHS), 168.8 (t, JP−C = 8.7 Hz, ArC),
129.7 (s, ArC), 128.2 (t, JP−C = 18.9 Hz, ArC), 104.9 (t, JP−C = 5.8 Hz,
ArC), 40.1 (t, JP−C = 7.8 Hz, C(CH3)3), 28.4 (t, JP−C = 2.5 Hz, CH3).
31P{1H} NMR (CDCl3, 162 MHz, δ): 190.5 (s). Anal. Calcd for
C23H40NiO2P2S2: C, 51.80; H, 7.56. Found: C, 51.96; H, 7.45.
Synthesis of {2,6-(iPr2PO)2C6H3}Ni(SCHS) (4). Method A from
2. Under an argon atmosphere, 2 (200 mg, 0.50 mmol) was dissolved
in 20 mL of toluene, followed by the addition of CS2 (0.60 mL, 9.98
mmol). The resulting mixture was stirred at room temperature for 2 h.
The volatiles were then removed under vacuum, and the yellow
residue was subjected to recrystallization from toluene/hexane (1:4) to
yield pure 4 (157 mg, 66% yield).
Method B from 6. Under an argon atmosphere, 6 (100 mg, 0.22
mmol) was dissolved in 20 mL of toluene, followed by the addition of
CS2 (0.41 mL, 6.82 mmol). The resulting mixture was stirred at 80 °C
for 12 h. The volatiles were then removed under vacuum, and the
yellow residue was subjected to recrystallization from toluene/hexane
(1:4) to yield pure 4 (96 mg, 90% yield). 1H NMR (CDCl3, 400 MHz,
δ): 11.55 (s, SCHS, 1H), 7.00 (t, JH−H = 7.6 Hz, ArH, 1H), 6.71 (d,
JH−H = 7.6 Hz, ArH, 2H), 2.29−2.26 (m, CH(CH3)2, 4H), 1.38−1.32
(m, CH3, 12H), 1.26−1.20 (m, CH3, 12H). 13C{1H} NMR (CDCl3,
101 MHz, δ): 233.7 (t, JP−C = 3.3 Hz, SCHS), 168.2 (t, JP−C = 9.3 Hz,
Figure 4. Calculated transition state for the decarboxylation of 5 (for
clarity, methyl groups on the pincer ligand are not shown).
hydrogen.9b By comparison, the calculated Ni−H distance of
nickel hydride 1 (1.535 Å) is about 0.22 Å shorter.9b In
contrast, going from the ground state of 5 to the
decarboxylation transition state, the Ni−O bond is significantly
stretched from 1.934 to 2.492 Å. Thus, the decarboxylation
pathway is best described as an ion pair mechanism. The
boundary to the β-hydrogen elimination mechanism could be
blurred, as with the mechanistic scenarios for the insertion of
CO2 into pincer-ligated group 10 metal hydride complexes.
Hazari, Kemp, and co-workers have shown in a palladium
system that varying the central donor of the pincer ligand could
shift the mechanism from typical 1,2-insertion to nucleophilic
attack on CO2 by the metal hydride.19 The ion pair mechanism
implicates that increasing the polarity of the solvent or adding
salts may enhance the rate of decarboxylation or CO2 insertion,
as observed in a number of cases.20
ArC), 129.8 (s, ArC), 129.7 (t, JP−C = 19.8 Hz, ArC), 105.1 (t, JP−C
=
6.2 Hz, ArC), 29.6 (t, JP−C = 11.9 Hz, CH(CH3)2), 17.5 (s, CH3), 16.9
(s, CH3). 31P{1H} NMR (CDCl3, 162 MHz, δ): 190.7 (s). Anal. Calcd
for C19H32NiO2P2S2: C, 47.84; H, 6.76. Found: C, 48.03; H, 6.88.
Reaction of 3 with Allylic or Benzyl Bromide. In a glovebox, 3
(8.0 mg, 15 μmol) and alkyl bromide (30 μmol) were mixed in ∼0.6
mL of toluene-d8. The resulting mixture was heated by a 90 °C oil
bath, and the progress of the reaction was monitored by 1H and
31P{1H} NMR spectroscopy. The reaction was complete in 8 h (for
benzyl bromide) or 10 h (for allylic bromide). The organic product
was also confirmed by GC-MS. 1H NMR (toluene-d8, 400 MHz, δ) of
{2,6-(tBu2PO)2C6H3}NiBr: 6.83 (t, JH−H = 8.0 Hz, ArH, 1H), 6.48 (d,
JH−H = 8.0 Hz, ArH, 2H), 1.43 (vt, JP−H = 6.8 Hz, CH3, 36H). 31P{1H}
NMR (toluene-d8, 162 MHz, δ) of {2,6-(tBu2PO)2C6H3}NiBr: 189.4
CONCLUSIONS
■
Through this study, we have demonstrated that CS2 is an
effective mechanistic probe for the decarboxylation of transition
metal formate complexes, particularly when the decarboxylation
process is thermodynamically unfavorable. Because CS2 is more
reactive than CO2 toward insertion, the limiting rates for the
reactions of formate complexes and CS2 are in fact the rates for
the decarboxylation reactions. We believe that the method
developed here can be applied to a wide variety of formate
complexes, thus providing the opportunity to study the
mechanistic details of decarboxylation reactions. Our ongoing
efforts focus on identifying factors that promote this trans-
formation.
1
(s). H NMR (toluene-d8, 400 MHz, δ) of CH2CHCH2SC(S)H:
10.66 (s, SCHS, 1H), 5.51−5.40 (m, CH2CH, 1H), 4.94−4.90 (m,
CH2CH, 1H), 4.85−4.82 (m, CH2CH, 1H), 3.50 (d, JH−H = 7.2
Hz, CH2, 2H). 1H NMR (toluene-d8, 400 MHz, δ) of PhCH2SC(S)H:
10.67 (s, SCHS, 1H), 6.97−6.91 (m, ArH, 5H), 4.13 (s, CH2, 2H).
Synthesis of {2,6-(iPr2PO)2C6H3}Ni(OCDO) (6-d). Under an
argon atmosphere, 2 (80 mg, 0.20 mmol) was dissolved in 20 mL of
toluene, followed by the addition of formic acid-d2 (0.02 mL, 0.53
mmol). The resulting mixture was stirred at room temperature for 2 h.
The volatiles were then removed under vacuum, and the resulting
residue was subjected to recrystallization from toluene/hexane (1:4) to
EXPERIMENTAL SECTION
■
General Comments. All air-sensitive compounds were prepared
and handled under an argon atmosphere using standard Schlenk line
and glovebox techniques. Dry and oxygen-free solvents were collected
from an Innovative Technology solvent purification system and used
throughout the experiments. Toluene-d8 was distilled from Na and
benzophenone under an argon atmosphere. NMR spectra were
recorded on a Bruker Avance-400 MHz NMR spectrometer. {2,6-
(tBu2PO)2C6H3}NiH (1),21 {2,6-(iPr2PO)2C6H3}NiH (2),21 {2,6-
(tBu2PO)2C6H3}Ni(OCHO) (5),9a and {2,6-(iPr2PO)2C6H3}Ni-
(OCHO) (6)9c were prepared as described in the literature.
1
yield pure 6-d (79 mg, 88% yield). H NMR (toluene-d8, 400 MHz,
δ): 6.84 (t, JH−H = 7.9 Hz, ArH, 1H), 6.48 (d, JH−H = 7.9 Hz, ArH,
2
2H), 2.34−2.27 (m, CH(CH3)2, 4H), 1.38−1.17 (m, CH3, 24H). H
NMR (toluene, 61 MHz, δ): 8.25 (s, NiOCDO). 31P{1H} NMR
(toluene-d8, 162 MHz, δ): 182.6 (s).
Synthesis of {2,6-(tBu2PO)2C6H3}Ni(SCHS) (3). Method A from
1. Under an argon atmosphere, 1 (200 mg, 0.44 mmol) was dissolved
in 20 mL of toluene, followed by the addition of CS2 (0.53 mL, 8.80
mmol). The resulting mixture was stirred at room temperature for 2 h.
The volatiles were then removed under vacuum, and the yellow
residue was subjected to recrystallization from toluene/hexane (1:1) to
produce orange crystals of 3 (207 mg, 88% yield).
Method B from 5. Under an argon atmosphere, 5 (150 mg, 0.30
mmol) was dissolved in 20 mL of toluene, followed by the addition of
CS2 (0.54 mL, 8.98 mmol). The resulting mixture was stirred at 80 °C
for 12 h. The volatiles were then removed under vacuum, and the
Rate Constant Measurement for the Reactions of Nickel
Formate Complexes with CS2. In a typical experiment, a 0.6 mL
solution of complex 5, 6, or 6-d in toluene-d8 (13 mM) was transferred
into a resealable NMR tube. For the kinetic studies of 5 or 6, 2 μL of
1,4-dioxane was added as an internal standard. For 6-d, a sealed
capillary tube containing P(OEt)3 was inserted into the solution as an
external standard. To each solution was added 20 to 100 equiv of CS2.
The sealed NMR tube was immediately placed into an NMR probe
that was precalibrated to the desired temperature (using 100%
ethylene glycol). The first 1H NMR (for 5 or 6) or 31P{1H} NMR (for
6-d) spectrum was recorded within 5 min, and spectra were taken
D
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every 5−30 min for 3−5 half-lives. The integration for the NiOCHO
resonance of 5 or 6 was compared to that of the internal standard,
whereas the integration for the phosphorus resonance of 6-d was
compared to that of the external standard.
4, 323−325. (i) Mellmann, D.; Sponholz, P.; Junge, H.; Beller, M.
Chem. Soc. Rev. 2016, 45, 3954−3988.
(2) For some recent examples, see: (a) Wienhofer, G.; Sorribes, I.;
̈
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X-ray Structure Determination. Single crystals of 3 and 4 were
obtained from recrystallization in toluene/n-hexane. Crystal data
collection and refinement parameters are provided in the Supporting
Information. Intensity data were collected at 150 K on a standard
Bruker SMART6000 CCD diffractometer using graphite-monochro-
mated Cu Kα radiation, λ = 1.54178 Å. The data frames were
processed using the program SAINT. The data were corrected for
decay, Lorentz, and polarization effects as well as absorption and beam
corrections based on the multiscan technique. The structure was
solved by a combination of direct methods in SHELXTL and the
difference Fourier technique and refined by full-matrix least-squares on
F2 for reflections. Non-hydrogen atoms were refined with anisotropic
displacement parameters. The H atoms were either located or
calculated and subsequently treated with a riding model. No solvent
of crystallization is present in the lattice for both structures.
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* Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00759.
Kinetic data, plots of these data, NMR spectra of the
nickel pincer complexes, and crystallographic details
(PDF)
Crystallographic data (CIF)
Optimized Cartesian coordinates for compounds 3
(XYZ) and 4 (XYZ)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: jie.zhang@htu.edu.cn.
*E-mail: hairong.guan@uc.edu.
■
Mercado, B. Q.; Wurtele, C.; Bernskoetter, W. H.; Hazari, N.;
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Schneider, S. J. Am. Chem. Soc. 2014, 136, 10234−10237. (c) Nova, A.;
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ORCID
Hairong Guan: 0000-0002-4858-3159
Notes
The authors declare no competing financial interest.
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Chem. Soc. 2010, 132, 8872−8873. (b) Huang, F.; Zhang, C.; Jiang, J.;
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ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(No. 21571052), the Key Science and Technology Project of
Henan Province (No. 152102210085), and the U.S. National
Science Foundation (CHE-1464734) for support of this
research. We also thank Prof. Gregory Girolami (University
of Illinois−Urbana−Champaign) for sharing the method of
calculating absolute errors in activation parameters. Crystallo-
graphic data were collected on a Bruker SMART6000
diffractometer, which was funded by an NSF-MRI grant
(CHE-0215950).
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