N-heterocyclic carbene complexes of palladium in oxygen atom transfer reactions involving the making and breaking of N-O bonds

Article abstract of DOI:10.1016/j.ica.2017.05.069

Reaction of three equivalents of MesCNO (Mes = 2,4,6-trimethylphenyl) with one equivalent of [Pd(IPr)(P(p-tolyl)₃)] in toluene yields the solid complex [Pd(IPr)(NCMes)(κ²-O–N[dbnd]C-Mes(–N–C([dbnd]O)Mes))]. Three major steps are proposed to be involved in the reaction based on spectroscopic studies as well as literature precedents for related cycloadditions: i. oxidation of the coordinated phosphine ligand to phosphine oxide ii. oxygen atom transfer forming a C[dbnd]O bond from the N–O bond of MesCNO, and iii. cycloaddition of a final MesCNO ligand to yield product. Addition of two equivalents of ^[rad]NO at low temperature to the in situ generated peroxide complex [Pd(IPr)₂(η²-O₂)] generates the N-bonded complex trans-[Pd(IPr)₂(NO₂)₂] in keeping with a literature precedent reported for similar complexes. Insight into the energetics of this reaction are probed by DFT calculations using the truncated ligand complex [Pd(IMe)₂]. The computed enthalpy of binding of two moles of ^[rad]NO₂ to form [Pd(IMe)₂(NO₂)₂] is ?112 kcal/mol indicating that its preparation from [Pd(IMe)₂], N₂ and 2O₂ is thermodynamically favorable by ?96 kcal/mol. Crystal structures of [Pd(IPr)(NCMes)(κ²-O–N[dbnd]C-Mes(–N–C([dbnd]O)Mes))] and trans-[Pd(IPr)₂(NO₂)₂] are reported.

Full text of DOI:10.1016/j.ica.2017.05.069

Inorganica Chimica Acta xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
N-heterocyclic carbene complexes of palladium in oxygen atom transfer
reactions involving the making and breaking of N-O bonds
Xiaochen Cai a, Subhojit Majumdar a, Leonardo F. Serafim a, Manuel Temprado b, , Steven P. Nolan c,
⇑
⇑
,
a,
a,
Catherine S.J. Cazin c, , Burjor Captain , Carl D. Hoff
⇑
⇑
⇑
a Department of Chemistry, University of Miami, Coral Gables, FL 33146, United States
b Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering, Universidad de Alcalá, Madrid 28801, Spain
c Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281, S-3, Ghent 9000, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of three equivalents of MesCNO (Mes = 2,4,6-trimethylphenyl) with one equivalent of [Pd(IPr)
(P(p-tolyl)3)] in toluene yields the solid complex [Pd(IPr)(NCMes)(
2-O–N@C-Mes(–N–C(@O)Mes))].
Received 14 March 2017
Received in revised form 29 May 2017
Accepted 30 May 2017
Available online xxxx
j
Three major steps are proposed to be involved in the reaction based on spectroscopic studies as well
as literature precedents for related cycloadditions: i. oxidation of the coordinated phosphine ligand
to phosphine oxide ii. oxygen atom transfer forming a C@O bond from the N–O bond of MesCNO, and
iii. cycloaddition of a final MesCNO ligand to yield product.
Addition of two equivalents of ÅNO at low temperature to the in situ generated peroxide complex
[Pd(IPr)2(g
2-O2)] generates the N-bonded complex trans-[Pd(IPr)2(NO2)2] in keeping with a literature
precedent reported for similar complexes. Insight into the energetics of this reaction are probed by
DFT calculations using the truncated ligand complex [Pd(IMe)2]. The computed enthalpy of binding of
two moles of ÅNO2 to form [Pd(IMe)2(NO2)2] is ꢀ112 kcal/mol indicating that its preparation from
[Pd(IMe)2], N2 and 2O2 is thermodynamically favorable by ꢀ96 kcal/mol. Crystal structures of
[Pd(IPr)(NCMes)(j
2-O–N@C-Mes(–N–C(@O)Mes))] and trans-[Pd(IPr)2(NO2)2] are reported.
Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction
Due to the low N-O bond dissociation enthalpy (BDE) in ArCNO
(ꢁ54 kcal/mol) [2], oxygen atom transfer (OAT) can occur readily
to an oxygen atom acceptor. In this context, Beck and coworkers
[3] have reported a highly unusual P-arene bond cleavage reaction
in conjunction with oxidation at P of the PPh3 ligand to form a
coordinated phosphine oxide in reaction of a series of nitrile oxides
with zerovalent Pt phosphine complexes.
Cycloaddition reactions of nitrile oxides are a complex and
important area of organic chemistry. They have been utilized in a
wide range of situations, for example, Echegoyen and co-workers
[1] have reported addition and removal of RCNO to a modified full-
erene in a cycloaddition reaction illustrated in Eq. (1):
Me
R
N
N
O
PPh3
Ph
Ph
Pt NCR
(2)
+
RCN
+
Ph3P=O
Pt(PPh3)3
+ 2 RCNO
R
C N O
P
Ph
(1)
O
N
Me
We have utilized MesCNO in kinetic and mechanistic studies [4]
of cleavage of a range of N-O bonds in reactions such as the one
shown in Eq. (3) for MesCNO:
⇑
Corresponding authors.
E-mail addresses: steven.nolan@ugent.be (S.P. Nolan), c.hoff@miami.edu
t
t
ðArð BuÞNÞ3V þ MesCNO ! ðArð BuÞNÞ3VO þ MesCN
ð3Þ
(C.D. Hoff).
http://dx.doi.org/10.1016/j.ica.2017.05.069
0020-1693/Ó 2017 Elsevier B.V. All rights reserved.
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2
X. Cai et al. / Inorganica Chimica Acta xxx (2017) xxx–xxx
Due to the strong O=VL3 bond in the product formed through
The Pt(IV) oxo complex is a d6 ion and thus does not violate the
Eq. (3), this reaction is exothermic by nearly 100 kcal/mol [4] and
occurs rapidly even at low temperature. That is not surprising for
early transition metal complexes which are known to have strong
metal-oxo bonds.
This work was begun in attempts to see if OAT to coordinatively
unsaturated low valent Pd complexes could occur as shown in
Eq. (4):
‘‘oxo-wall” axiom [5] related to the unstable nature of metal oxo
complexes lacking an empty d-orbital to participate in
p-accep-
tance of at least one of the oxide ion lone pairs. This complex
undergoes oxidation of the coordinated phosphine of the pincer
ligand as shown in Eq. (9):
+
+
+
+
P(t-Bu)2
P(t-Bu)2
P(t-Bu)2
P(t-Bu)2
O
-Acetone
+Acetone
O
O
Pt
N
Pt
O
Pt
N
(9)
Pt
N
PdðLÞ2 þ MesCNO ! ðLÞ2PdðNCMesÞðOÞ
ð4Þ
OC(CH3
)
2
OC(CH3
)
2
N
Me2
Me2
Me2
Me2
The reaction shown in Eq. (4) would produce a square planar d8
complex of Pd(II) which would not have an empty d orbital on Pd
for
p bonding to lone pairs on O and thus would be predicted to be
The facile OAT to the phosphine arm of the pincer ligand in Eq.
(6) is similar to that observed by Beck and coworkers in Eq. (2). The
Pt@O BDE in the Milstein complex can be estimated from reported
computational data to be on the order of ꢁ30 kcal/mol.
Bergman and coworkers [10] investigated coupling of two
equivalents of benzonitrile at the early transition metal Zr@O com-
plex shown in Eq. (10):
unstable due to the ‘‘oxo-wall” effect [5]. The ‘‘oxo-wall” concept
applies to stable species and does not rule out formation of tran-
sient intermediates. Studies of CO2 exchange with a labelled car-
bonate complex were proposed by Gould and coworkers [6] to
involve generation of a Pt@O oxo complex intermediate as shown
in Eq. (5):
O
P
P
O
L
P
P
O
L
P
P
O
O
-CO2
+L
-L
C
O
(5)
Pt
Pt
Pt
C
O
Ph
O
+CO2
PhCN
O
N
(10)
+
Zr
O
N
C
Ph
Zr
N
Zr
Ph
N
Ph
An analogous mechanism was proposed by Bohle and co-work-
ers [7] in reaction of the hyponitrite complex shown in reaction (6):
In principle, this reaction could be imagined to have begun with
ZrCp⁄2 and PhCNO which would be expected to undergo rapid OAT
to form O-ZrCp⁄2 and PhCN. The first step in (10) involves breaking
the Zr@O bond and its transfer to the C atom of PhCN to form the
chelating benzamide complex. In the second step of the reaction
the highly basic N atom of the benzamido complex performs nucle-
ophilic attack at the C atom of a second equivalent of benzonitrile
to form the metallacyclic complex.
PPh3
Ph3P
Ph3P
O
O
Ph3P
Ph3P
O
O
N
N
CO2
N2O
+
Pt
O
(6)
Pt
C
O
Pt
PPh3
Recently, we have utilized MesCNO (N-O BDE of 54 kcal/mol)
[2] to study the thermodynamics and kinetics of OAT reactions to
PR3, N-heterocyclic carbenes (NHC) and metal containing com-
plexes [4,11]. The mechanism for oxidation of free phosphine
ligands was proposed [11] to occur as shown in Scheme 1.
Central to the cycloadditions is a nucleophilic attack at the C of
the nitrile oxide that removes electron density from the P atom of
the phosphine facilitating formation of the O-P bond essential for
OAT. It would appear, at first sight, that oxidation of basic phos-
phine ligands and coordinatively unsaturated late transition metal
complexes would bear little resemblance. Recent work [12],
however has shown that Lewis acid adduct such as that shown
in Eq. (11) can be formed.
The resonance formulas for N„N–O M N@N@O and their nitrile
oxide counterparts are formally equivalent R–C„N–O M R–
C@N@O. The characteristic 1,3 dipolar addition reactions of the
nitrile oxide, as illustrated in Eq. (1) also have inorganic metal
based counterparts as elucidated in a number of detailed studies
by Kukushkin and coworkers [8]. These types of cycloadditions also
play a prominent role in oxidative reactions such as that shown in
Eq. (7):
R2
R1
Cl
Cl
Cl
Cl
Cl
Cl
N
O
O
N
(7)
R1
C
N
Pt
N
C
R1
O
N
C
R2
+
2
N
Pt
N
Cl
Cl
R1
R2
tBu
Al tBu
tBu
tBu Al
Mes
Mes
Ph
Ph
In this reaction, the metal serves to accelerate the cycloaddition
(11)
process and the metal itself is not oxidized. However as shown in
the work of Beck [3] in Eq. (2) OAT may also play a role culminating
in oxidation of a bound phosphine ligand.
Mes
P
Pt PPh3
P
Pt PPh3
Mes
Related chemistry, and the only isolated late transition metal
oxo complex reported to date has been reported by Milstein [9].
The higher energy OAT reagent peroxyacetone was used as shown
in Eq. (8):
The ability of Vaska’s complex, IrCl(CO)(PPh3)2, to react with
Lewis acids was demonstrated long ago with the formation of its
unusual BF3 adduct [13] and indicates the amphoteric character
of electron rich yet coordinatively unsaturated late transition
metal compounds.
BF4
BF4
In the first part of this work, we report the synthesis, structural
characterization and preliminary mechanistic studies for the
formation of the Pd(II) metallacyclic complex [Pd(IPr)(NCMes)
P(t-Bu)2
Pt solv
P(t-Bu)2
O
O
(8)
Pt
O
Acetone
N
(j
2-O-N=C-Mes(-N-C(@O)Mes))] by reaction of 3 equivalents of
N
Me2
Me2
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X. Cai et al. / Inorganica Chimica Acta xxx (2017) xxx–xxx
3
Scheme 1. Selected species in proposed oxidation of PR3 by MesCNO [11].
MesCNO with [Pd(IPr)(P(p-tolyl)3)] where two OAT reactions from
the nitrile oxide to the ligands bound to the metal center occur.
In the second part of this work, we describe studies of addition
of NO to a metal peroxide complex. The reaction of the peroxide
complex to form a N-bonded nitro complex has been known since
reaction (12) was reported fifty years ago by Collman [14].
in a vial in 2 mL of toluene and left standing for approximately
12 h. During that time, a copious amount of red-orange crystals
of high quality precipitated on the walls of the vial. The super-
natant liquid was decanted into a Schlenk tube, evacuated to dry-
ness and the solid residue dissolved in C6D6. The NMR spectrum
showed near quantitative production of O@P(p-tolyl)3, no remain-
ing unreacted MesCNO and only traces of free MesCN. The solid
material was isolated in approximately 80% yield. Examination
under a microscope showed the crystals to be of high quality as
formed. The crystals were suitable for structure determination as
described in a later section.
Å
L2PdðO2Þ þ 2ÅNO ! L2PdðNO2Þ2
ð12Þ
The reaction in Eq. (12), involving two equivalents of nitric
oxide with bound molecular oxygen, can be compared to the step-
wise reaction (13) for the free gaseous molecules.
The red-orange complex is sparingly soluble in C6D6 and
appears to change color in CD2Cl2. The ESI-TOF mass spectrum
showed a peak at 962.5 assigned to P + 1, at 817.4 assigned to
P ꢀ NCMes + 1, and at 387.3 assigned to IPr + 1. NMR data of the
complex are reported in the Supporting Information.
2 " NOþ # O2 # ¢ ONOO # þ " NO ¢ ONOONO ¢ " NO2þ # NO2
ð13Þ
Reaction (13) is important in atmospheric chemistry and has
been extensively studied. Computational work [15] provides
insight into this reaction. The authors are aware of no experimental
or computational reports of the mechanism or energetics of reac-
tion (12) in which the metal-based counterpart of reaction (13)
occurs. Preliminary synthetic, structural, and computational
results are reported in the second part of this contribution in which
coordinated oxygen atoms are transferred to rather than from N.
2.3. Spectroscopic studies of reactions of [Pd(IPr)(P(p-tolyl)3)] and
MesCNO
In the glove box a solution was made of 20.8 mg (0.026 mmol)
[Pd(IPr)(P(p-tolyl)3)] and 8.1 mg (0.0503 mmol) MesCNO in 3 mL
C6D6 to form a solution with initial [MesCNO] = 0.0168 M and
[Pd(IPr)(P(p-tolyl)3)] = 0.0087 M at a 1.94 to 1 MesCNO/Pd molar
ratio. An IR cell and an NMR tube were filled with the solution
and its reactivity monitored at 22 °C by IR and 25 °C by NMR.
The first FTIR spectrum was run six minutes after mixing the
reagents in the glove box and difference spectra were recorded fol-
lowing at regular time intervals. Related reactions were performed
on solutions of [Pd(IPr)(P(p-tolyl)3)] and MesCNO in C6D6, prepared
at 1/1, 1/3 and 1/10 M ratios. Reaction of MesCNO and P(p-tolyl)3
was also investigated using techniques strictly analogous to those
2. Experimental
2.1. General considerations
Unless stated otherwise, all operations were performed in a
Vacuum Atmospheres or MBraun glovebox under an atmosphere
of purified nitrogen or argon or utilizing standard Schlenk tech-
niques under an Argon atmosphere. Special Schlenk lines were
developed that allowed rapid switching from Argon to Oxygen to
Argon or other gas atmosphere during the reaction. Solvents were
degassed and purified using standard techniques. Anhydrous
deuterobenzene or toluene-d8 were purchased from Aldrich chem-
ical, degassed, and stored over 4 Å molecular sieves in the glove
box. Mesityl nitrile oxide was prepared and recrystallized accord-
ing to the literature [16]. [Pd(IPr)2] and [Pd(IPr)(P(p-tolyl)3)] were
prepared as described previously [17]. 1H NMR spectra were
recorded on a Bruker Avance-400 spectrometer at room tempera-
ture. 1H NMR chemical shifts are reported in parts per million
(ppm) with respect to the protio impurities referenced at
7.16 ppm for C6D6 and 2.09 ppm for toluene-d8. FTIR spectra were
obtained using a Perkin Elmer Spectrum 400 FTIR Spectrometer.
reported earlier [11]. In most cases 1H NMR were obtained but 31
P
NMR spectroscopy was also performed to confirm unequivocally
the production of free O@P(p-tolyl)3 as product.
Å
2.4. Reaction of in situ prepared [Pd(IPr)2(
g
2-O2)] and NO
In the glove box, a Schlenk tube was loaded with 20 mg
[Pd(IPr)2] and approximately 10 mL of toluene. The mixture was
shaken to completely dissolve the solid yielding an orange colored
solution. The Schlenk tube was taken from the glovebox and placed
in a ꢀ78 °C bath all while under a positive pressure of Argon. The
reaction atmosphere was then changed to O2 producing a dark yel-
low solution together with a milky white precipitate in keeping
with generation of in situ [Pd(IPr)2(g
2-O2)] [17]. The oxygen was
2.2. Reaction of [Pd(IPr)(P(p-tolyl)3)] and MesCNO
removed by vacuum and replaced with argon. To this was added
10 mL ÅNO by gas tight syringe. The color slowly changed to orange
and then to light yellow and then to light yellow-green. The
solution was warmed to room temperature under continuous
In the glove box, 48.5 mg (0.061 mmol) [Pd(IPr)(P(p-tolyl)3)]
and 29.5 mg (0.183 mmol) MesCNO (1/3 M ratio) were dissolved
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X. Cai et al. / Inorganica Chimica Acta xxx (2017) xxx–xxx
evacuation to give a yellow-green solid. Extraction with heptane
under an argon atmosphere allowed clean separation of the yellow
product and filtration of the heptane solution yielded a clear yel-
low solution which upon cooling to ꢀ20 °C in a freezer yielded
pure crystals of trans-[Pd(IPr)2(NO2)2] (2) in approximately 25%
yield. These proved suitable for structural study as described
below. NMR data are in agreement with the structure of 2 and
are provided in the Supporting Information.
empirical dispersion correction [24] and the Def2-TZVP [25] basis
set for Pd along with the corresponding ECP [26] and the 6-311G
(d,p) basis set for the rest of the atoms. We refer to this basis set
combination in this work as BSI. All stationary points were opti-
mized by computing analytical energy gradients. The obtained
minima were characterized by performing energy second deriva-
tives, confirming them as minima by the absence of negative
eigenvalues of the Hessian matrix of the energy. To further refine
the energies, single-point calculations were performed using the
larger Def2-QZVP [26] and 6-311+G(2d,p) basis set for Pd and the
rest of the atoms respectively. We refer to this basis set combina-
tion in this work as BSII. In addition, analogous single-point calcu-
lations were also carried out with the B3LYP [27] and BP86 [28]
density functionals. Computed electronic energies were corrected
for zero-point energy, thermal energy and entropic effects to deter-
2.5. Crystallographic analyses
Red-orange crystals of [Pd(IPr)(NCMes)(j
2-O-N@C-Mes(-N-C
(@O)Mes))] (1) were obtained by slow crystallization from the
forming solution in toluene as described earlier. Pale yellow crys-
tals of trans-[Pd(IPr)2(NO2)2] (2) were obtained by slow crystalliza-
tion from a toluene/heptane mixture in a freezer at ꢀ20 °C also for
a period of weeks. The data crystal of 2 was mounted onto the end
of a thin glass fiber using Paratone-N for data collection at 100 K
under N2. The data crystals of 1 and 2 were glued onto the end
of a thin glass fiber. X-ray intensity data were measured by using
mine
D
H0(298 K).
3. Results
3.1. Synthesis of Pd(IPr)(NCMes)(j
2-O-N(@C)-Mes(-N-C(@O)Mes)) (1)
a Bruker SMART APEX2 CCD-based diffractometer using Mo K
a
radiation (k = 0.71073 Å) [18]. The raw data frames were inte-
grated with the SAINT+ program by using a narrow-frame integra-
Reaction of MesCNO and [Pd(IPr)(P(p-tolyl)3)] occur in a 3/1 M
ratio as shown in Eq. (14) as confirmed by IR and NMR studies.
IPr
Pd
N
IPr
O
O
N
P
Pd
(14)
+
3 MesCNO
+
N
O
P
tion algorithm. Corrections for Lorentz and polarization effects
were also applied with SAINT+. An empirical absorption correction
based on the multiple measurement of equivalent reflections was
applied using the program SADABS [19]. All non-hydrogen atoms
were refined with anisotropic displacement parameters. Hydrogen
atoms were placed in geometrically idealized positions and
included as standard riding atoms during the least-squares refine-
ments. All structures were solved by a combination of direct meth-
ods and difference Fourier syntheses, and refined by full-matrix
least-squares on F2, by using the SHELXTL software package [20].
Crystal data, data collection parameters, and results of the analyses
are listed in Table 1.
Compounds 1 and 2 crystallized in the monoclinic crystal sys-
tem. For 2, the space group P21/c was confirmed on the basis of
the systematic absences in the data. For 1, the space groups P21
and P21/m were indicated on the basis of the systematic absences
in the data. The structure could only be solved in the former space
group.
Due to low solubility, the pure crystalline product can be
obtained in ꢁ80% yield. This is best achieved when a slight excess
of [Pd(IPr)(P(p-tolyl)3)] is present since excess MesCNO appears to
be capable of further reaction with 1 slowly to yield unknown
products.
3.2. Mechanistic investigation of reaction of MesCNO and [Pd(IPr)
(P(p-tolyl)3)]
A representative plot of IR spectra run to determine the approx-
imate rate law for the reaction is shown in Fig. 1 below. Under
these conditions [Pd(IPr)(P(p-tolyl)3)] is present in slight excess.
A plot of the difference spectra is shown in Fig. 1 below:
Fig. 1 shows the smooth decrease in concentration of free
MesCNO at 1348 cmꢀ1 and concurrent increase in free O@P(p-tolyl)3
at 1217 cmꢀ1
.
The band at 1217 cmꢀ1 is assigned to the
O@P(p-tolyl)3 stretch in keeping with literature assignments for
O@PPh3 [29] and also based on independent study of the reaction
of MesCNOand P(p-tolyl)3 under similar conditions [11]. No attempt
was made to interpret the other bands that rise and fall during the
reaction and are most likely due to differences in free MesCNO
versus the metallacyclic complex. The two bands followed are
unequivocally assigned to MesCNO and O@P(p-tolyl)3.
2.6. Computational methods
All calculations were carried out at the DFT level with an ultra-
fine grid [21] as implemented in Gaussian 09 suite of programs
[22]. Geometry optimizations were performed without any sym-
metry restrictions using the PBE0 functional [23], the D3(BJ)
Compared to oxidation of the free phosphine by MesCNO, oxi-
dation of the bound phosphine was observed to occur more rapidly
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X. Cai et al. / Inorganica Chimica Acta xxx (2017) xxx–xxx
5
Table 1
Detailed kinetic studies including temperature variation to deter-
mine activation parameters were not performed. The primary con-
clusions regarding the mechanism are qualitative in nature at this
time but suggest that any proposed mechanism should be consis-
tent with the following observations:
(i) The rate determining step is approximately first order in
both MesCNO and [Pd(IPr)(P(p-tolyl)3)] (ii). The course of the reac-
tion is progressive with the decay of 3 mol of MesCNO paralleling
the production of 1 mol of O@P(p-tolyl)3. (iii) There are no indica-
tions in the FTIR of bands that rise and fall and no stable interme-
diate complex is formed at detectable levels during the reaction.
(iv) From the beginning, with no induction period there is a smooth
buildup of dissociated O@P(p-tolyl)3. There is no spectroscopic evi-
dence of free MesCN or free P(p-tolyl)3 during the reaction.
Crystallographic data for 1 and 2.
Compound
1
2
Empirical formula
Formula weight
Crystal system
Lattice parameters
a (Å)
PdO2N5C57H69
962.57
Monoclinic
PdO4N6C54H72
975.57
Monoclinic
11.3388(7)
19.7405(11)
12.0599(7)
90
91.505(1)
90
2698.5(3)
P21(# 4)
2
1.185
0.387
296
50.0
7583
604
1.012
0.000
0.0394; 0.0685
Multi-scan
0.7457/0.6692
0.386
21.1672(9)
12.5519(6)
20.2658(10)
90
97.801(1)
90
5334.6(4)
P21/c(# 14)
4
1.215
0.395
296
50.0
7102
602
1.070
0.008
0.0567; 0.1435
Multi-scan
0.5776/0.7457
1.094
b (Å)
c (Å)
a
(deg)
b (deg)
c
(deg)
V (Å3)
Space group
Z value
qcalc (g/cm3)
(Mo K ) (mmꢀ1
)
3.3. Structure of [Pd(IPr)(NCMes)(j
2-O-N@C-Mes(-N-C(@O)Mes))] (1)
l
a
Temperature (K)
Hmax (°)
No. Obs. (I > 2
No. Parameters
2
The crystal structure of complex 1 is shown in Fig. 3. Compound
1 is a mononuclear Pd complex that contains a terminal IPr ligand,
r
(I))
a terminal NCMes ligand and a
j
2-O-N@C-Mes(-N-C(@O)Mes)
Goodness of fit GOF*
Max. shift in cycle
Residuals*: R1; wR2
group. As seen from the angles around Pd in Fig. 3 caption, the
geometry around the Pd atom may be viewed as approximately
square planar, and atoms C1-N3-N4-O1 are slightly offset from
all lying within a plane. Compound 1 can be considered formally
as a Pd(II) complex with a 16-electron configuration where the
Absorption Correction, Max/min
Largest peak in Final Diff. Map (eꢀ/Å3)
wR2 = [Rhklw(|Fobs| ꢀ |Fcalc|)2/RhklwFo2bs
]
,
1/2
*R1 = Rhkl(kFobs| ꢀ |Fcalck)/Rhkl|Fobs|;
2-O-N@C-Mes(-N-C(@O)Mes) group serves as a 4 electron donor
w = 1/r
2(Fobs); GOF = [Rhklw(|Fobs| ꢀ |Fcalc|)2/(ndata ꢀ nvari)]1/2
.
j
(2ꢀ charge) ligand. The Pd1ꢀN3 distance of 2.020(6) Å to the
nitrile group is similar in value to the PdꢀN distances in the com-
pounds [Pd(NCPh)2X2] (X = Cl or Br) which range from 1.94(2)ꢀ
1.99(2) Å [30]. Also the Pd1ꢀN4 distance of 2.050(4) Å to the amide
group is close in values to that found in a related cyclopalladium
complex, 1.956(2) Å [31]. The cyclic arrangement of atoms with
palladium is quite rare and there are only two other reported crys-
tal structures that have that same arrangement of 5 atoms [32].
3.4. Synthesis and structural determination of trans-[Pd(IPr)2(g1
-
NO2)2] (2)
As described in the experimental section, reaction of in situ pre-
pared [Pd(IPr)2(
2-O2)] when exposed to ÅNO under an argon atmo-
g
Fig. 1. Difference spectra (observed spectrum – first spectrum) in the region 1400–
1150 cmꢀ1
.
but to produce only about 1/3 of an equivalent of O@P(p-tolyl)3 in
keeping with the stoichiometry of the reaction. Also of note is fail-
ure to observe bands in the IR that rise and fall which could be
attributed to an intermediate species that accumulates during
the reaction. The presence of several isosbestic points in the spec-
tra also point to a clean reaction without accumulation of interme-
diate complexes.
A plot of the absorbance of MesCNO versus time is shown in
Fig. 2 as well as a simulation of absorbance change data based on
a second order rate law: d[P]/dt = k[Pd(IPr)(P(p-tolyl)3)][MesCNO].
The graph in Fig. 2 is in keeping with this being a second order
reaction overall displaying first order dependence on each reactant.
That is also in keeping with other qualitative and quantitative
studies of the reaction made by IR and NMR of the reaction.
Fig. 2. Plot of [DA1348cm-1] versus time (seconds) for reaction of [MesCNO] =
0.0168 M and [Pd(IPr)(P(p-tolyl)3)] = 0.0087 M at a 1.94 to 1 MesCNO/Pd molar
ratio in C6D6 at 22 °C. The orange curve is actual data and the green curve is the
computed data based on an overall second order rate law with first order
dependence in each reactant at this unequal concentration of reactants.
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6
X. Cai et al. / Inorganica Chimica Acta xxx (2017) xxx–xxx
Fig. 3. An ORTEP of the molecular structure of [Pd(IPr)(NCMes)(j
2-O-N@C-Mes(-N-
C(@O)Mes))] (1) showing 30% thermal probability ellipsoids. Selected bond
distances (in Å) and angles (in °) are as follows: Pd1–O1 = 1.944(3), Pd1–
C1 = 2.007(5), Pd1–N3 = 2.020(6), Pd1–N4 = 2.050(4), O1–N5 = 1.386(4), O2–C48
1.236(6), N3–C28 = 1.143(7), N4–C48 = 1.362(6), N4–C38 = 1.402(6), N5–
C38 = 1.275(5), O1–Pd1–C1 = 85.07(17), O1–Pd1–N3 = 169.6(3), C1–Pd1–N3 = 96.4
(2), O1–Pd1–N4 = 80.76(14), C1–Pd1–N4 = 165.70(18), N3–Pd1–N4 = 97.93(17).
Fig. 4. An ORTEP of the molecular structure of trans-[Pd(IPr)2(NO2)2] (2) showing
30% thermal probability ellipsoids. Selected bond distances (in Å) and angles (in °)
are as follows: Pd1–N2 = 2.024(4), Pd1–N1 = 2.028(4), Pd1–C31 = 2.070(4), Pd1–
C1 = 2.075(4), O1–N1 = 1.145(6), O2–N1 = 1.229(6), O3–N2 = 1.193(5), O4–
N2 = 1.213(5), N2–Pd1–N1 = 179.49(18), C31–Pd1–C1 = 179.18(15), N2–Pd1–
C31 = 90.94(15), N1–Pd1–C31 = 89.05(16), N2–Pd1–C1 = 88.79(16), N1–Pd1–
C1 = 91.22(16).
sphere at ꢀ78 °C was observed to change from yellow to orange to
dark yellow. The light-yellow complex could be extracted using
heptane or toluene and recrystallized by cooling in a freezer at
ꢀ20 °C. The crystal structure of 2 is shown in Fig. 4.
Compound 2 is also a square planar Pd(II) 16 electron complex
with the IPr ligands trans to each other. The NO2 ligands are
arranged such that all four O atoms do not lie in the same plane.
There is indication of some interaction between C–H bonds on
the pendant isopropyl groups and N–O bonds of the coordinated
nitro groups as shown in Supporting Information. The NO2 groups
are equivalent to each other by C2 symmetry. The Pd–N and N–O
distances are similar to those found in the complex trans-[Pd
(PPh3)2(NO2)2] [33]. The fact that in trans-[Pd(PPh3)2(NO2)2] all
four O atoms lie in the same plane, while in (2) they do may be
due to the C–H——O bonding interactions. A figure showing these
interactions is provided in Supporting Information. Insufficient
data are available at this time to assess the significance of these
interactions.
Scheme 2. Enthalpies of reaction (kcal/mol) relevant to formation of trans-[Pd
(IMe)2(NO2)2] computed at the BP86-D3(BJ)/BSII//PBE0-D3(BJ)/BSI level (see com-
putational details section).
3.5. Computational results
Density functional theory (DFT) was employed to provide
insights into the reaction energetics of formation of in situ pre-
pared [Pd(IPr)2(
trans-[Pd(IPr)2(
g
2-O2)] and 2 ÅNO. The optimized structure of
g
1-NO2)2] 2 was found to be in excellent agreement
4. Discussion
with the X-ray structural determination. In order to gain insight
about reaction energetics, computations were done using a trun-
cated model of the Pd complex, [Pd(IMe)2], where the 2,6-diiso-
propylphenyl substituents on the nitrogen atoms of the IPr
carbenes coordinated to the Pd center are replaced by methyl
The goal of this work was to study reactions involving the mak-
ing and breaking of N-O bonds in reactions with low valent Pd
complexes. That is part of a long-range goal aimed at understand-
ing catalytic processes involving transition metal catalyzed OAT
reactions of NxOy compounds. The first part of this work involved
our initial studies of the reaction between MesCNO and [Pd(IPr)
(P(p-tolyl3)]. This work builds upon earlier mechanistic and ther-
mochemical studies of OAT reactions utilizing MesCNO as a potent
groups. The truncated models [Pd(IMe)2], [Pd(IMe)2(g
2-O2)] and
trans-[Pd(IMe)2(NO2)2] were computed (see SI for more details)
and enthalpies of reaction computed as shown in the thermochem-
ical cycle in Scheme 2.
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X. Cai et al. / Inorganica Chimica Acta xxx (2017) xxx–xxx
7
agent for transfer of a single oxygen atom to metals and also non-
metals [4,11]. It was hoped that direct evidence for formation of an
intermediate ‘‘LnPdO” complex would emerge, either by detection
of an intermediate or in isolation of a stable dimeric complex of
The potential complexity of these steps is shown in Scheme 4
which illustrates several possible mechanisms for the initial reac-
tion of MesCNO and [Pd(IPr)(P(p-tolyl)3)].
The first step shown is formation of cis-[ML1L2(j
2-RCNO)] in a
the form [Pd(IPr)(L)(
l
-O)]2 in which L = MesCN or P(p-tolyl)3
rapid pre-equilibrium step. This would involve [ML1L2] attacking
the C atom of RCNO and also coordinating through the O atom in
a manner resembling that proposed for PR3 oxidation as shown
in Scheme 1. This is in keeping with the recent work [12]
highlighting the Lewis base reactivity shown in Eq. (11). It is also
in accord with failure to observe reaction of [Pd(IPr)2] and MesCNO
under similar conditions since adoption of a cis geometry for the
bis IPr system results in significant steric as is the case [17] for
formed by dissociation of one ligand to form a stable oxo bridged
square planar structure. No such direct evidence was found.
Instead, relatively clean formation of 1 occurred. The stoichiometry
of the product forming reaction involves assembly of three equiv-
alents of MesCNO with one equivalent of [Pd(IPr)(P(p-tolyl)3)].
The following overall processes must occur during this reaction:
(a) One mole of MesCNO serves to oxidize the PR3 ligand. The
MesCN produced in this reaction coordinates to the metal; (b) A
second mole of MesCNO undergoes OAT to form the C@O bond
found in the product; (c) The remaining MesCNO ligand does a
cycloaddition reaction yielding product. A general mechanism for
these steps is shown in Scheme 3:
The order of these steps does not have to occur as shown, but it
seems most reasonable to the authors that the rate determining
step which is first order in both MesCNO and Pd(IPr)(P(p-tolyl)3)]
would be OAT to produce O@P(p-tolyl)3. The phosphine oxide pro-
duced should be a weakly bound token ‘‘ligand” and readily
undergo rapid ligand substitution. Dissociation of O@P(p-tolyl3)
and its replacement by MesCN or MesCNO would be expected to
allow for facile rearrangements in subsequent steps due to reduc-
tion in steric hindrance. This is in keeping with our initial kinetic
studies showing a rate limiting second order reaction step between
reactants with no buildup of intermediates.
The driving force for the proposed first reaction would be
strong. Replacement of the weak MesCN-O bond (BDE =
54 kcal/mol) [2] by the much stronger O@P(p-tolyl)3 bond
(BDE = 138 kcal/mol) [11] which should be exothermic by more
than 80 kcal/mol. The second major step proposed in Scheme 3 is
OAT from MesCNO to a C atom to form ultimately a benzamido
complex similar to that shown in the mechanism by Bergman
[10] in Eq. (10). The driving force for this reaction is again replace-
ment of the weak MesCN-O bond by the relatively strong C@O
bond formed as well as oxidation of Pd from formally Pd(0) to
Pd(II). Preliminary computational results attest to the thermo-
chemical feasibility of this reaction [34].
The final step of the sequence would then involve a cycloaddi-
tion of MesCNO at the highly basic imido N atom of the benzamido
complex at the C atom of the MesCNO-this would lead to the final
product by establishment of a Pd-O bond to the O of the incoming
MesCNO while cleaving the bond to the coordinated ketone yield-
ing product. This proposal was also motivated by the mechanism
proposed for O@ZrCp⁄2 (Eq. 10). Steric strain, as well as a negative
formal charge on the highly basic N of the benzamido complex
should provide the driving force for the nucleophillic attack at
the C atom of the third mole of MesCNO yielding 1. While the
authors view this as a reasonable overall mechanism that is consis-
tent with observed experimental results and also literature reports
by other groups, it is not established. Details of the precise formu-
lation of the complexes are not shown, however, these three steps
appear key.
[M(IPr)2(
Formation of low steady state concentrations of cis-
[ML1L2(
2-RCNO)] is believed to occur prior to rate determining
g
2-O2)].
j
OAT to either the Pd center or to the P(p-tolyl)3 ligand in our sys-
tem (L1 in Scheme 4) by one of two routes in the probable rate
determining step for the overall reaction. This can occur by two
potential pathways that would be kinetically indistinguishable.
The first would be formation of a transition state oxo complex
[ML1L2(RCN)(O)] which then undergoes OAT as shown in reactions
1 followed by reaction 2 in Scheme 4. This complex resembles
intermediates proposed by Gould [6] and Bohle [7], and discussed
earlier (see reactions 5 and 6). However, a pathway in which the
j
2-RCNO ligand attacks first the Pd-PR3
r
⁄ bond and then forms
a P-O bond cannot be ruled out and is shown in the single con-
certed step reaction (1,2) Concerted.
In addition to OAT to L1, two additional key reactions of the pos-
tulated ‘‘[ML1L2(RCN)(O)]” complex are also shown in Scheme 4.
The Keq dimer step shows the well-established formation of the
dimeric bridging oxo structure which is well documented [35].
One reason for the successful isolation of the Milstein Pt(IV)-O
complex in Eq. (3) is that the bulky ancillary pincer ligand serves
to inhibit dimerization of this type. Recently, we have reported
the role of reaction sterics on the monomer/dimer bridging hydride
reaction for bulky triorganotin platinates with ancillary NHC
ligands [36]. Varying the NHC ligand was shown to play a key role
in influencing this reaction.
The final key reaction of ‘‘L1L2(RCN)M-O” shown is oxygen
migration to the C atom of the coordinated nitrile. This is shown
in reaction 3 followed by reaction 4 which results in formation of
a benzamido complex. This resembles the mechanism proposed
by Bergman and coworkers [10] and shown in Eq. (10). Not shown
in the scheme (for clarity) is that the step 3 concerted followed by
4 can also lead to the benzamido complex. The authors could only
find one crystallographically determined true benzamido complex
in the Cambridge database [10c]. The proposed benzamido com-
plex in Scheme 4 fits the requirement that OAT has occurred to
the C atom, and the basic N should be capable of attack at the C
of the third MesCNO to yield the final product as shown. Precedent
for this type of reactivity is shown in the work of Kukushkin [8].
The second part of this work involves OAT to rather than from
N. In spite of the report fifty years ago by Collman [14], to the
authors’ knowledge, the mechanism for conversion of a bound
Å
Å
peroxide by two moles of NO to two moles of NO2 has not been
Scheme 3. Major steps proposed for the formation of 1.
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8
X. Cai et al. / Inorganica Chimica Acta xxx (2017) xxx–xxx
Scheme 4. Possible first steps in reaction of L1ML2 and RCNO (L1 = PR3 or other oxidizable ligand, L2 = NHC or other less readily oxidized ligand, M = Pd, Pt.
elucidated. As with the formation of 1, the formation of 2 presents
a number of challenges in mechanistic inorganic chemistry.
Four potential mechanisms appear reasonable for this reaction.
The first, and one that can be ruled out would involve loss of coor-
dinated O2 which then reacts with 2 ÅNO as shown in reaction (13)
to yield 2ÅNO2 as is well known. Two equivalents of ÅNO2 so formed
would be expected to undergo rapid coordination as shown in
Eq. (15)
[Pd(IPr)(NCMes)(j
2-O-N@C-Mes(-N-C(@O)Mes))]. During the reac-
tion two of the three N-O bonds of the reacting nitrile oxides
undergo OAT. A possible mechanism is proposed based on qualita-
tive and semi-quantitative studies. The possibility of formation of
unstable intermediate complexes containing LnPd-O bonds is men-
tioned as a possibility. The premise that such bonds should be
weak remains unchallenged but the active participation of such
species and even possibly their eventual isolation remains possible.
MesCNO itself is unstable with respect to MesCN and ½ O2 from a
thermodynamic perspective. The mechanism outlined is one of
many possible and incorporates plausible driving forces for the
individual steps. Due to the weakness of the N-O bond broken in
MesCNO as well as the weakness of any temporary Pd-O bond
formed as an intermediate a number of possible routes exist until
the weak N-O bond is converted to a strong P@O or C@O bond in
the product.
½PdðIPrÞ2ꢂ þ 2ÅNO2 ! trans-½PdðIPrÞ2ðNO2Þ2ꢂ
ð15Þ
The complex [Pd(IPr)2(g
2-O2)] has a weak Pd-O2 bond [17] and
readily dissociate O2. However, preliminary studies [34] appear to
indicate that reaction with 2 ÅNO occurs at an overall rate faster at
low temperatures than O2 loss. In addition, [Pd(IMes)2(g
2-O2)]
which requires temperatures [37] on the order of ꢁ60 °C to lose
The reaction of [Pd)(IPr)2(g
2-O2)] with two equivalents of ÅNO to
form trans-[Pd(IPr)2(NO2)2] resembles reaction (12) reported by
O2, readily adds ÅNO at ambient temperatures. For that reason, dis-
sociation of bound O2 from [Pd(L)2(g
2-O2)] followed by reaction of
free O2 with 2 ÅNO is not viewed as a likely mechanism. The
remaining three mechanisms all involve attack of ÅNO on the
Collman of the PPh3 analog reported nearly fifty years ago [14].
Å
Å
Reaction (13) for the O2(g) and 2 NO(g) to make 2 NO2(g) occurs
with an enthalpy of activation near 0 kcal/mol and a nearly con-
stant rate over a 600 °C temperature range [15]. Detailed study
Pd of [Pd(L)2(
g g
2-O2)], or on the O of either [Pd(L)2( 2-O2)] or
[Pd(L)2(
g
1-O2)]. Kinetic studies of this reaction for a range of low
Å
of reaction of a range of Pd and Pt peroxo complexes and NO is
valent peroxide complexes are planned.
underway in our laboratory to allow comparison of free versus
bound dioxygen reactivity with nitric oxide. The computational
result that formation of 2 from N2 and 2O2 is thermochemically
favorable provides incentive for investigation of chemical/electro-
chemical catalytic oxidation of N2 itself as first suggested almost
a century ago by Lewis and Randall [38].
The most surprising aspect of this second project was the com-
puted reaction energetics shown in Scheme 2. In particular reac-
tion D that forms the bound bis-nitro complex from N2 and O2 is
computed to be highly favorable from a thermodynamic point of
view. Utilizing these data and tabulated literature values [2], the
reactions shown Eqs. (16) and (17) can be compared:
½PdðIMeÞ2ꢂþ2ÅNO2 ! trans-½PdðIMeÞ2ðNO2Þ2ꢂ
D
H0 ¼ ꢀ112kcal=mol
ð16Þ
Acknowledgments
This article is dedicated to Professor Luis Echegoyen. Support of
this work by the National Science Foundation (Grant No. CHE-
1300206 to BC), the Brazilian National Council for Scientific and
Technological Development (CNPq process 213876/2014-0), the
ERC (FUNCAT to SPN), CTQ2016-80600-P (MINECO, Spain to MT)
is gratefully acknowledged.
H2ðgÞ þ 2ÅNO2 ! 2HNO2ðgÞ
The computed energy of reaction (16) is 60 kcal/mol more
exothermic than (17). Attempts to directly measure by solution
calorimetric methods the enthalpy of formation of 2 by reaction
(16) are planned to test the accuracy of our computed data.
D
H0 ¼ ꢀ52 kcal=mol
ð17Þ
5. Conclusion
Appendix A. Supplementary data
This work reports the surprisingly efficient reaction of three
moles of MesCNO with one mole of [Pd(IPr)(Pd(p-tolyl)3)] to form
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2017.05.069.
Please cite this article in press as: X. Cai et al., Inorg. Chim. Acta (2017), http://dx.doi.org/10.1016/j.ica.2017.05.069

X. Cai et al. / Inorganica Chimica Acta xxx (2017) xxx–xxx
9
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Please cite this article in press as: X. Cai et al., Inorg. Chim. Acta (2017), http://dx.doi.org/10.1016/j.ica.2017.05.069