Activation of nitrogen bronsted acids: Synthesis and reactivity of a new class of nitrogen acid complexes

Article abstract of DOI:10.1021/ic5017033

The nitrogen acids RC(O)NHNO₂, N-nitroamide, R = CH₃ (1), C₂H₅ (2) and N-nitrocarbamate, R′OC(O)NHNO₂, R′ = CH₃ (3), C₂H₅ (4) are a class of primary N-nitrocarboxamide compounds that oxidatively add to trans-Ir^(I)(Cl)(N₂)(PPh₃)₂ to give six-coordinate Ir^(III)(η²-(NO₂)-nitrogen acid)(H)(Cl)(PPh₃)₂ complexes 5-8. Unexpected fluxional behavior of the complexes in solution is observed by ¹H NMR spectroscopy. Reaction intermediates of the oxidative addition reactions were also observed and monitored using ³¹P and ¹H NMR and solution IR spectroscopies. Complex 5 reacts with methyl triflate in CH₃CN to generate bis(acetonitrile) complex (9) from a net loss of the nitrogen acid anion. P(CH₃)₂Ph reacts with 5 to give phosphine-substituted and P(CH₃)₂Ph addition isomers (10). Reactivity studies of 5 with CO gave metastable CO adduct isomer 11, which loses CO on prolonged standing in solution. (Chemical Equation Presented).

Full text of DOI:10.1021/ic5017033

Article
pubs.acs.org/IC
Activation of Nitrogen Brønsted Acids: Synthesis and Reactivity of a
New Class of Nitrogen Acid Complexes
D. Scott Bohle* and Zhijie Chua
Department of Chemistry, McGill University, 801 Sherbrooke Street. W., Montreal, Quebec H3A 0B8, Canada
*
S Supporting Information
ABSTRACT: The nitrogen acids RC(O)NHNO , N-nitroamide, R =
2
CH (1), C H (2) and N-nitrocarbamate, R′OC(O)NHNO , R′ = CH
3
2
5
2
3
(
3), C H (4) are a class of primary N-nitrocarboxamide compounds
2 5
(
I)
that oxidatively add to trans-Ir (Cl)(N )(PPh ) to give six-coordinate
2
3 2
(III)
2
Ir (η -(NO )-nitrogen acid)(H)(Cl)(PPh ) complexes 5−8. Unex-
2
3 2
1
pected fluxional behavior of the complexes in solution is observed by H
NMR spectroscopy. Reaction intermediates of the oxidative addition
31
1
reactions were also observed and monitored using P and H NMR and
solution IR spectroscopies. Complex 5 reacts with methyl triflate in
CH CN to generate bis(acetonitrile) complex (9) from a net loss of the
3
nitrogen acid anion. P(CH ) Ph reacts with 5 to give phosphine-
3
2
substituted and P(CH ) Ph addition isomers (10). Reactivity studies of 5 with CO gave metastable CO adduct isomer 11, which
3
2
loses CO on prolonged standing in solution.
INTRODUCTION
introduced into low-valent transition metals by oxidative
addition.
■
The organometallic chemistry of the carbon acids is now well-
developed with CH oxidative addition being a universal
reaction for this species. In contrast, the corresponding class
of nitrogen acids has a much less-developed organometallic
chemistry, with only a handful of N−H oxidative addition
reactions being recognized. Although many nitrogen com-
pounds can coordinate to transition metal complexes as simple
Lewis base ligands, in certain cases they can oxidatively add
Scheme 1). Electron-withdrawing substitutents R favor the
1
EXPERIMENTAL SECTION
General Experimental. Except where noted all reagents and
solvents are used as supplied commercially. Methanol is dried by
distillation from freshly generated magnesium methoxide, and dry
■
2
1
hexane is distilled from sodium benzophenone ketyl. H NMR spectra
are recorded on Varian Mercury 200, 400, or 500 MHz spectrometers,
13
and C NMR spectra are recorded on a Varian Mercury 300 MHz
(
spectrometer. All chemical shifts are recorded in δ (ppm) relative to
latter pathway.
1
13
residual solvent signals for H and C spectra. Melting/decomposition
points are measured by using a TA-Q2000 differential scanning
calorimeter calibrated against an internal standard. The IR spectra are
recorded in KBr disks for solution, KBr matrix for solids, or a gastight
cell for gas-phase spectra by using ABB Bomem MB Series IR
Scheme 1. Carbon and Nitrogen Acids
−1
spectrometer with spectral resolution of 2 cm . Elemental analyses
are performed in the Elemental Analyses Laboratory at University of
Montreal. Sublimation purifications are carried out under vacuum with
cold-finger inserts containing dry ice−acetone mixtures or ice−water
and heated to the required temperatures. Nitramide is synthesized
4
from literature methods with modification. N-nitropropionamide (2)
2
and Ir(η -C H C(O)NNO )(H)(Cl)(PPh ) (6) has been reported
2
5
2
3 2
In prior work, we have established the activation product of
N-nitropropionamide (2) with trans-Ir(Cl)(N )(PPh ) to give
a nitrogen chelate with an unusual O-bound nitro group.
However, the full spectroscopic evaluation and reactivity of this
class of complexes were not examined. In this paper we
describe (1) the synthesis of the ligands, (2) the synthesis of
3
previously. Additional spectroscopic characterization of the transient
intermediates can be found in the Supporting Information.
2
3 2
3
X-ray Crystallography. Crystals are mounted on glass fibers with
epoxy resin or Mitegen mounts using Paratone-N from Hampton
Research, and single-crystal X-ray diffraction experiments are carried
out with a BRUKER SMART CCD or BRUKER APEX-II CCD
diffractometer by using graphite-monochromated Mo Kα radiation (λ
the nitrogen acid ligand complexes using trans-Ir(Cl)(N )-
2
=
0.710 73 Å) and KRYOFLEX for low-temperature experiments.
(
PPh ) , (3) dynamic process of the complexes in solution, (4)
3 2
5
SAINT is used for integration of the intensity reflections and scaling,
observed reaction transient intermediates, and (5) reactivity
studies of Ir(CH C(O)NNO )(H)(Cl)(PPh ) (5). Taken
together these results demonstrate that the N-nitroamides/
3
2
3 2
Received: July 22, 2014
carbamates are good π-acceptor ligands that are readily
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic5017033 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
6
and SADABS is used for absorption correction. Direct methods are
stirring, and extracted with CH Cl (7 × 45 mL). The CH Cl extracts
2
2
2
2
used for structures that do not contain heavy atoms or show
pseudomerohedral twinning, corrected using PLATON, , while
are dried with anhydrous MgSO , and the solvent is removed in vacuo
4
7
until dryness. The white solids are redissolved in ether (20 mL), and
Patterson maps are used for the rest of the structures to generate
the initial solution. Non-hydrogen atoms are located by difference
Fourier maps, and final solution refinements are solved by full-matrix
least-squares method on F of all data, by using SHELXTL software.
The hydrogen atoms are placed in calculated positions.
NH is bubbled into the mixture for 2 min in an ice−water bath to give
3
a white precipitate. The white precipitate is filtered, dried, and
10
collected to give ammonium N-nitroethylcarbamate (4NH4) (1.607
2
5
g, 10.64 mmol, 95% yield).
Compound 4NH (1.607 g, 10.64 mmol) is gradually dissolved in
4
Crystals of 1−4 are grown from slow evaporation of CH Cl at
H SO (1 mL of concentrated H SO , 3.0 g of ice, and 10 mL of
2
2
2
4
2
4
room temperature, while those of 5−8 are grown from CH Cl /
deionized H O) and stirred until homogeneous. The solution is
2
2
2
CH OH layered solutions at −21 °C, and those of 10 are from
extracted with ethyl acetate (7 × 10 mL), and the extracts are dried
3
MeCN/ether layered solutions. Crystallographic data and data-
collection parameters for the nitrogen acids and Ir(III) complexes
are shown in Supporting Information, Table S1.
over anhydrous MgSO . The solvent is removed in vacuo, and the
4
white residue is purified by vacuum sublimation (dry ice−acetone) at
65−70 °C to give N-nitroethylcarbamate (4) (1.244 g, 9.21 mmol,
87% yield). Complete spectroscopic data for this preparation follows
Theoretical Calculations. Density functional theory (DFT)
calculations are performed with the Gaussian03 program with the
B3LYP functionals and 6-311++g** basis sets. The Z and E ground-
state structures correspond to local minima with all-positive vibrational
modes; the transition states, located by the quadratic synchronous
transit algorithm, are then checked with an intrinsic reaction
coordinate(IRC) analysis following calculations to ensure that the
transition state corresponds to the E and Z isomerization.
−1
and matches the limited literature data. mp: 67.7 °C (14.5 kJ mol ).
−1
IR (KBr, cm ): 3235s, 3007m, 2988m, 2947w, 2880w, 2810w,
1741vs, 1605vs, 1548m, 1518m, 1453s, 1391s, 1368m, 1329s, 1227s,
1
1116m, 1017m, 997m, 878m, 798m, 768m, 731w, 603m, 458w. H
NMR (200 MHz, CDCl ) ppm: δ = 1.36 (t, CH , J = 7.1 Hz), 4.36 (q,
3
3
CH , J, = 7.1 Hz), 9.90 (s, broad, NH).
2
2
Synthesis of Ir(η -CH C(O)NNO )(H)(Cl)(PPh ) (5). trans-Ir-
3
2
3 2
8
Synthesis of N-Nitroacetamide CH C(O)NHNO (1). A
(Cl)(N )(PPh ) (0.110 g, 0.141 mmol), 1 (0.020 g, 0.192 mmol),
3
2
2 3 2
4b
modified literature preparation of N-nitroacetamide was devised.
Briefly, nitramide (0.500 g, 8.06 mmol) is dissolved in ether (5 mL)
and cooled to 0 °C in an ice-bath. Acetic anhydride (10 mL, 105.8
mmol) is then added followed by LiCl (0.4 mg, 9.41 μmol), and the
reaction mixture is allowed to warm to room temperature. The
reaction mixture is stirred for 24 h and subjected to vacuum
evaporation at T < 40 °C to give white solids, which are purified by
vacuum sublimation (dry ice−acetone) at 70 °C to give colorless
crystals of N-nitroacetamide (1) (0.350 g, 3.37 mmol, 42% yield). IR
and degassed CHCl3 (5 mL) are mixed and stirred under a N2
atmosphere. Gas evolution is observed, and the reaction mixture is
stirred for 3 h. The reaction mixture is dried in vacuo to give a yellow
residue, which is recrystallized from CH Cl /CH OH to give yellow
2
2
3
2
crystals of Ir(η -CH C(O)NNO )(H)(Cl)(PPh ) (5) (0.104 g, 0.121
3
2
3 2
−1
mmol, 86% yield). IR (cm ): 3056w, 2963vw, 2322vw, 2068vw,
1696m, 1572vw, 1514s, 1483m, 1435s, 1363w, 1261w, 1241w, 1198s,
1096s, 1035m, 806m, 745m, 707m, 695vs, 621w, 520vs, 504m. Raman
(cm ): 3067vw, 2326w, 1590m, 1190w, 1098m, 1030m, 1002vs,
891w, 704m, 617m, 542w, 255m. H NMR (200 MHz, CDCl ) ppm:
−1
−1
1
(
KBr, cm ): 3433w, 3252m, 3156s, 3018s, 2856m, 2811m, 2735w,
3
2
570w, 1962w, 1728vs, 1623vs, 1443s, 1309vs, 1196vs, 1016vs,
δ = −31.78 (s, Ir-H, broad), 1.23 (s, CH ), 7.38 (m, 18H-PPh ), 7.62
3
3
1
1
007vs, 952m, 764m, 723m, 605s, 569m, 454w. Gas-phase IR
(m, 12H-PPh ). H NMR (400 MHz, CDCl ) ppm: δ = −31.74 (s, Ir-
3
3
−1
1
(
cm ): 1747s, 1738s, 1732s, 1623vs, 1430m, 1371s, 1316s, 1230s,
H, broad), 1.25 (s, CH ). H NMR (200 MHz, C D ) ppm: δ =
3
6
6
−1
1
185s, 1011m. Raman (cm ): 2952vw, 1718m, 1638w, 1307m,
−31.22 (t, Ir-H, J
= 13.4 Hz), 1.47 (s, CH ), 6.92 (m, 18H-PPh ),
P−H 3 3
1
1
1
194m, 1005m, 954m, 763m, 608vs, 454m, 389m, 246w. H NMR
7.88 (m, 12H-PPh ). H NMR (400 MHz, C D ) ppm: δ = −31.22 (s,
3
6
6
13
31
(
200 MHz, CDCl ) ppm: δ = 2.52 (s, CH ), 10.52 (NH, broad).
C
Ir-H, broad), 1.48 (s, CH ). P NMR (81 MHz, CDCl ) ppm: δ =
3 3
3
3
NMR (75 MHz, CDCl ) ppm: δ = 24.13 (s), 168.08 (s).
11.42 (s). 31P NMR (81 MHz, C D ) ppm: δ = 10.92 (s). Electrospray
3
6 6
9
+
Synthesis of N-Nitromethylcarbamate CH OC(O)NHNO (3).
ionization mass spectrometry (ESI-MS): 878.92 [M + Na] . Anal.
3
2
−1
Methyl carbamate (1.000 g, 13.3 mmol) is gradually added at 25 °C to
a mixture of concentrated H SO (15 mL) and KNO (2.03 g, 20.01
Calcd for C H N O P ClIr·CH Cl (941.24 g mol ) % C, 49.72; H,
38 34 2 3 2 2 2
3.82; N, 2.97. Found % C, 49.22; H: 3.74; N, 2.85.
Synthesis of Ir(η -CH OC(O)NNO )(H)(Cl)(PPh ) (7). trans-
2
4
3
2
mmol). The mixture is stirred for 15 min at 25 °C, poured onto
3
2
3 2
crushed ice (14.2 g) with stirring, and extracted with ether (7 × 6 mL).
Ir(Cl)(N )(PPh ) (0.015 g, 0.0192 mmol), 3 (0.003 g, 0.002
2
3 2
The ether extracts are dried with anhydrous MgSO , and the solvent is
mmol), and degassed CHCl (3 mL) are mixed and stirred under a
4
3
removed in vacuo to dryness. The white solids are redissolved in ether
N atmosphere. Gas evolution is observed, and the reaction mixture is
stirred for 3 h. The reaction mixture is evacuated in vacuo to give
2
(
20 mL), and NH is bubbled into the mixture in an ice−water bath to
3
give a white precipitate. The white precipitate is filtered, dried, and
yellow solids, which are recrystallized from CH Cl /CH OH to give
2
2
3
1
0
2
collected to give ammonium N-nitromethylcarbamate (3NH4)
1.682 g, 12.3 mmol, 92% yield).
Compound 3NH (1.682 g, 12.3 mmol) is gradually dissolved in
yellow crystals of Ir(η -CH OC(O)NNO )(H)(Cl)(PPh ) (7)
3 2 3 2
−
1
(
(0.012 g, 0.0133 mmol, 69% yield). IR (KBr, cm ): 3056w,
2953vw, 2920vw, 2850vw, 2298vw, 1747s, 1518m, 1483m, 1434s,
4
H SO (1 mL of concentrated H SO , 3.0 g of ice, and 10 mL of
1266m, 1212m, 1185m, 1161w, 1097s, 1028w, 998w, 843w, 795w,
2
4
2
4
1
deionized H O) and stirred until the mixture is homogeneous. The
747m, 695s, 918vw, 520vs, 503m, 462w. H NMR (200 MHz, CDCl )
2
3
solution is extracted with ethyl acetate (7 × 10 mL), and the extracts
ppm: δ = −31.69 (t, Ir−H, J
= 13.2 Hz), 2.97 (s, CH ), 7.34 (m,
18H-PPh ), 7.62 (m, 12H-PPh ). P NMR (81 MHz, CDCl ) ppm: δ
P−H 3
31
are dried over anhydrous MgSO . The solvent is removed in vacuo,
4
3
3
3
1
and the white residue is purified by vacuum sublimation (dry ice−
= 9.97 (s) but the H coupled spectrum is a exchange broadened
acetone) at 65−70 °C to give N-nitromethylcarbamate (3) (1.230 g,
doublet (JP−H = 4.5 Hz Hz). Anal. Calcd for C H N O P ClIr·
38
34
2
4 2
−
1
−1
−1
1
3
1
6
1
0.3 mmol, 77% yield). mp: 91.0 °C (8.65 kJ mol ). IR (KBr, cm ):
245s, 3206s, 3060w, 3024w, 2972w, 2850vw, 1748vs, 1615vs, 1549w,
458s, 1414m, 1325m, 1244s, 1092w, 1003m, 947m, 771m, 736,w,
CH Cl (957.23 g mol ) % C, 48.89; H, 3.76; N, 2.93. Found % C,
2 2
49.11; H, 3.62; N, 2.85.
2
Synthesis of Ir(η -C H OC(O)NNO )(H)(Cl)(PPh ) (8). trans-
2
5
2
3 2
1
25m, 456w. H NMR (200 MHz, CDCl ) ppm: δ = 3.91 (s, CH ),
Ir(Cl)(N )(PPh ) (0.010 g, 0.100 mmol), 4 (0.014 g, 0.104 mmol),
3
3
2 3 2
0.12 (s, broad, NH).
Synthesis of N-Nitroethylcarbamate C H OC(O)NHNO (4).
and degassed CHCl3 (5 mL) are mixed and stirred under a N2
atmosphere. Gas evolution is observed, and the reaction mixture is
stirred for 3 h. The reaction mixture is evacuated to give yellow
residues, which are recrystallized from CH Cl /hexanes to give yellow
1
1
2
5
2
This preparation is a combination of separate methods for the
synthesis of the ammonium salt and the free base of the nitrogen acid
with modifications. Ethyl carbamate (1.000 g, 11.2 mmol) is gradually
added at temperature less than 25 °C to a mixture of concentrated
H SO (15 mL) and KNO (2.03 g, 20.01 mmol). The mixture is
2
2
2
crystals of Ir(η -C H OC(O)NNO )(H)(Cl)(PPh ) (8) (0.053 g,
2
5
2
3 2
−1
0.060 mmol, 60% yield). IR (KBr, cm ): 3056w, 2988vw, 2923vw,
1747m, 1517m, 1483m, 1435s, 1207s, 1095vs, 1028m, 999w, 745m,
2
4
3
1
stirred for 15 min at 25 °C, poured onto crushed ice (12 g) with
695vs, 542m, 522vs H NMR (200 MHz, CDCl ) ppm: δ = −31.80 (t,
3
B
dx.doi.org/10.1021/ic5017033 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Ir-H, JP−H = 13.6 Hz), 0.88 (t, CH , J = 7.2 Hz), 3.32 (q, CH , J = 7.2
3
2
RESULTS AND DISCUSSION
Synthesis of N-Nitro Compounds. Primary N-nitro-
amides synthesis have been described previously and are
1
■
Hz), 7.40 (m, 18H-PPh ), 7.64 (m, 12H-PPh ). H NMR (400 MHz,
3
3
CDCl ) ppm: δ = −31.81 (t, Ir-H, J
= 14 Hz), 0.87 (t, CH , J = 8
3
P−H
3
Hz), 3.32 (q, CH , J = 8 Hz), 7.39 (m, 18H-PPh ), 7.65 (m, 12H-
2
3
1
prepared by acylation of nitramide with acid anhydrides
PPh ). H NMR (200 MHz, C D ) ppm: δ = −30.88 (t, Ir-H, J
=
3
6
6
P−H
3
(
Scheme 2a). This contrasts with the synthesis of N-
1
1
=
3.5 Hz), 0.61 (t, CH , J = 7.2 Hz), 3.23 (q, CH , J = 7.2 Hz), 6.96 (m,
3
2
1
8H-PPh ), 7.87 (m, 12H-PPh ). H NMR (400 MHz, C D ) ppm: δ
3
3
6
6
−30.91 (t, Ir-H, J
= 12 Hz), 0.61 (t, CH , J = 8 Hz), 3.23 (q,
Scheme 2. Synthesis of (a) N-Nitroamides and (b) N-
Nitrocarbamates
P−H
3
31
CH , J = 8 Hz), 6.96 (m, 18H-PPh ), 7.87 (m, 12H-PPh ). P NMR
2
3
3
31
(
81 MHz, CDCl ) ppm: δ = 10.35 (s). P NMR (81 MHz, C D )
3
6
6
+
ppm: δ = 10.71 (s). ESI-MS: 884.96 [M − H] . Anal. Calcd for
−1
C H N O P ClIr·0.3C H (912.19 g mol ) % C, 53.72; H, 4.44; N,
3
9
36
2
4
2
6
14
3
.07. Found % C, 53.30; H, 4.82; N, 2.58.
General Procedure for Solution IR Reaction Monitoring. In a
typical experiment, trans-Ir(Cl)(N )(PPh ) (0.010 g, 0.013 mmol)
2
3 2
and nitrogen acid (slight excess of 0.013 mmol) are charged into a
reaction flask under a N atmosphere. Cooled degassed CHCl (5 mL)
2
3
is added, and the reaction is monitored at −10 °C, during which time,
aliquots are removed for IR analysis and returned.
Reaction of 5 with Methyl Trifluoromethanesulfonate.
Complex 5 (0.050 g, 0.058 mmol) is dissolved in dry CH CN (4
3
mL) under a N atmosphere. Methyl trifluoromethanesulfonate (7 μL,
2
0
.064 mmol) is added and stirred for 2 h. The reaction mixture is
observed to change from pale yellow to colorless. The solvent is
removed in vacuo to give tan solids, which are recrystallized from
CH CN/ether to give [Ir(H)(Cl)(CH CN) (PPh ) ][SO CF ] (9)
nitrocarbamates, where nitration of the corresponding primary
alkylcarbamates with KNO in concentrated H SO is sufficient
3
2
4
3
3
2
3
2
3
3
to give very good yields of the nitro compounds (Scheme 2b).
−1
(
18 mg, 0.022 mmol, 37% yield). IR (KBr, cm ): 3059w, 3024vw,
There is only limited spectroscopic information for primary
3
1
6
005vw, 2990vs, 2931vw, 2296vw, 2272vw, 2251vw, 1483m, 1435s,
11
N-nitroamides and N-nitrocarbamates except for 4 with most
270vs, 1224m, 1187m, 1154m, 1096m, 1031s, 999m, 753m, 709m,
1
of the prior studies concerning secondary N-nitrocarboxami-
95s, 638s, 524vs, 504m. H NMR (200 MHz, CDCl ) ppm: −20.67
3
9b
des. There are some rare instances of limited IR character-
(
t, Ir-H, J
= 12.3 Hz), 1.91 (s, 3H), 2.00 (s, 3H), 7.46 (m. PPh ),
P
−
H
3
8
3
1
7
7
.69 (m, PPh3). P NMR (81 MHz, CDCl ) ppm: 4.43 (s). ESI-MS:
ization of the N-nitroamides. There are, however, numerous
3
+
93.81 [M − CH CN] .
data for the related nitramines R NNO , which have similar
3
2
2
Reaction of 5 with P(CH ) Ph. Complex 5 (0.060 g, 0.070 mmol)
vibrational data to the N−NO compounds reported here.
3
2
2
is dissolved in dry CHCl (5 mL). P(CH ) Ph (21 μL, 0.147 mmol) is
3
3
2
The primary N-nitroamides and N-nitrocarbamates have
added and stirred for 3 h. The solution turned slightly pale yellow over
time, and the solvent was removed in vacuo. The oil that remained
1
amidic protons that are observed between 9−11 ppm in H
NMR. This chemical shift is similar to that of nitramide where
2
contains Ir(η -CH C(O)NNO )(H)(Cl)(P(CH ) Ph) (10A) and
3
2
3
2
2
the amide proton is observed between 8−11 ppm depending
1
isomers of Ir(η -CH C(O)NNO )(H)(Cl)(P(CH ) Ph) (10B−
0D) in ratio of 0.18:0.42:0.10:0.30, respectively (approximately
3,12
13
3
2
3
2
3
on solvent. For simple nitramines, the chemical shift effect
1
0
1
on the α-H of the −N(NO ) group is similar to that of alkoxy
.043 g). H NMR (200 MHz, CDCl ) ppm: (10C) −21.48 (dt, Ir-H,
2
3
14
substituents. The alkyl substituents of the N-nitroamides and
J
= 19.2 Hz, J
= 12.2 Hz, fraction of minor isomer = 0.10),
cis P−H
cis P−H
N-nitrocarbamates are all observed slightly downfield (approx-
imately 0.2 to 0.4 ppm difference) from the generic region of
the regular amides and esters probably due to the inductive
effect of the nitro group.
(
10B) −19.96 (dt, Ir-H, J
= 19.8 Hz, Jcis P−H = 12.2 Hz, fraction
cis P−H
of major isomer = 0.42), (10A) −19.68 (t, Ir-H, J
= 13.2 Hz
P−H
fraction of minor isomer = 0.18), (10D) −11.63 (dt, Ir-H, J
=
P
trans P−H
= 16.0 Hz, fraction of minor isomer = 0.30). 3
1
1
57.8 Hz, J
cis P−H
NMR (81 MHz, CDCl ) ppm: −44.37 (s, free P(CH ) Ph), fraction
3
3
2
The N-nitroamides and N-nitrocarbamates also exhibit
interesting IR bands (Figure 1). In particular, the presence of
the acyl- and nitro-conjugated system gives rise to unique bands
that do not reside in the usual ranges for the respective
functional groups. Some suitable references that would help in
the assignment of these vibrational bands include the regular
of combined three isomers (10B−10D) = 0.82; −34.56 (t,
P(CH ) Ph, J
1
= 19.9 Hz), −20.12 (d, PPh , J
=
trans P‑cis P
3
2
cis P‑trans P
3
9.9 Hz), −4.38 (s, free PPh ), (10A) 35.13 (s, minor isomer = 0.18).
3
Reaction of 5 with CO. Complex 5 (0.030 g, 0.035 mmol) is
dissolved in CHCl3 (5 mL). CO is vigorously bubbled into the
solution. The yellow solution turned pale yellow after 10 min. The
solvent is removed in vacuo, and the pale yellow solids recrystallized
from CH Cl /hexanes to give pale yellow solids that contain isomers
15
15,16
14
amides, carbamates,
and nitramines (Figure 1).
For the N-nitroamides, a new strong band is observed
2
2
1
−1
(
(
0.05:0.56:0.39) of Ir(η -CH C(O)NNO )(H)(Cl)(CO)(PPh )
between 1000−1100 cm , which is assigned to a mode that is
3
2
3 2
−1
11) (0.030 mg, 0.034 mmol, 97% yield). IR (cm ): 3056w,
predominantly a N−N bond-stretching band. For the N-
nitrocarbamates, several new bands are also observed around
the 900 to 1150 cm− region. These bands are assigned to the
carbamate stretching modes. The lowest energy, strong
2
1
7
962vw, 2185vw (broad), 2047m (CO), 1695vw, 1648vw (broad),
1
511m, 1483m, 1435s, 1362w, 1262w, 1188m, 1095s, 1029m, 1000m,
1
45m, 694vs, 524vs H NMR (400 MHz, CDCl ) ppm: δ = −8.31 (t,
3
Ir-H, J
= 16 Hz, minor isomer = 0.05), −7.68 (t, Ir-H, J
= 12
−1
P−H
P−H
absorption band in this region, 900−1000 cm , has significant
MHz, major isomer = 0.56), −7.54 (t, Ir-H, J
= 12 MHz, minor
P−H
ν(N−N) character. The IR bands of 4 match well with the
isomer = 0.39), 1.15 (s, 3H, minor isomer = 0.41), 1.57 (s, 3H, major
11
isomer = 0.59). 31P NMR (81 MHz, CDCl ) ppm: δ = 3.61 (s, minor
literature assignment.
3
Synthesis of Ir(III) N-Nitrocarboxamides Complexes.
The amide proton of the N-nitro compounds 1−4 is acidic due
to the electron-withdrawing nature of the nitro group, and
isomer = 0.43), 3.92 (s, major isomer = 0.57). Upon leaving the
reaction mixture in solution overnight, 5 is regenerated, as confirmed
by the loss of the 2047 cm band. P NMR shows only the
appearance of a single resonance at 11.45 ppm (5). Repeated CO
bubbling regenerated the three isomers of 11 again.
−1
31
these nitrogen acids oxidatively add to trans-Ir(Cl)(N
)(PPh )
2
3 2
(Scheme 3).
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Figure 1. Infrared bands for primary N-nitroamides, N-nitrocarbamates, and related compounds.
Scheme 3. Reaction of trans-Ir(Cl)(N )(PPh ) with
a deep yellow to pale yellow. Subsequent workup of the
2
3 2
Nitrogen Acids 1−4
reaction mixtures gave good yields of the new Ir complexes 5−
8
.
31
The P NMR spectroscopy of the Ir(III) complexes all show
a single resonance signal, which corresponds to a single isomer
in solution. The Ir hydride resonance is located between −30 to
−
40 ppm and appears as triplets for 6, 7, and 8 but is a broad
signal for 5 in CDCl , which will be discussed below. The alkyl
3
and alkoxy groups are also located downfield compared to the
free acid.
The vibrational spectra of complexes 5−8 have unusual
bands associated with the coordinated ligand. Of great interest
is the weak ν(Ir−H) band for 5, 6, and 7 at 2322, 2311, and
−
1
2
298 cm , respectively, while no such band is observed in 8. A
In all of the above reactions, vigorous evolution of gas was
observed at the start of the reaction, which is most likely from
possible reason for the low intensity could be the Ir−H bond
has little polarity, resulting in a small change in dipole moment.
There is, however, a medium-intensity band observed around
the loss of N . The reaction mixture was stirred at room
2
temperature over 3 h, during which time, the color turned from
Figure 2. ORTEP plots of the nitrogen acids at 50% thermal ellipsoid. (a) N-nitroacetamide (1). (b) N-nitromethylcarbamate (3). (c) N-
nitroethylcarbamate (4).
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1
8
05 cm− in 5−7, which can be assigned to the δ(Ir−H)
A list of selected experimentally determined bond lengths
and bond angles of the free ligands of the nitrogen acids are
presented in Table 1. In general, all the molecules are planar
with most of the dihedral angles around the amide/carbamate
groups at or close to 180°. The planarity and the short C−N,
1
7
bending mode. Other unique vibrational bands depicted in
Figure 1 are roughly assigned to the functional groups in the
coordinated conjugate base.
The successful reaction of trans-Ir(Cl)(N )(PPh ) with the
nitrogen acids differs from that of Vaska’s complex, trans-
Ir(Cl)(CO)(PPh ) . With Vaska’s complex, stoichiometric
2
3 2
1
.38 to 1.40 Å, and N−N, 1.38 to 1.40 Å, bond lengths of these
acids is consistent with considerable delocalization of the N
lone pair across the framework. An interesting observation in
this series of compounds is that the N-nitrocarbamates 3 and 4
are slightly nonplanar due to deviations from 0 or 180° in the
dihedral angles of C−C−N−O and C−N−N−O. This could be
the result of competing conjugation requirements of the
carbamate functional group. Increasing the alkyl chain length of
the alkoxy group seems to reduce the planarity too.
Although the bond angles of the N-nitro compounds are
similar, for the nitro groups the N−N−O angles for the nitro
group are different. The O(1) atom, which is trans to the acyl
group, tends to have smaller N−N−O angles compared to the
O(2) atom cis to the acyl group. This asymmetric effect could
be due to packing constraints. The presence of extensive
hydrogen bonds between the amide protons and the acyl
oxygen atoms in the crystal array, which stabilize the Z
conformation in 1, may also lead to these variations in nitro
group geometry.
3
2
addition of 1 results in some formation of a new compound,
as indicated in 3 P and H NMR monitoring, but the reaction
1
1
does not go to completion even under reflux conditions.
Furthermore, increasing the stoichiometry of the nitrogen acid
results in formation of complex mixtures. The implication for
the above observations is that the reaction could be reversible
or in equilibrium initially, but with excess 1 this leads to
product inhibition due to the formation of small amounts of
base derived from some decomposition of 1. It has been
established that the extent of oxidative addition of an acid to
Vaska’s complex is related to the strength of the acid (pK ) or
the coordinating ability of the conjugate base. It is therefore
very likely that 1 has a pK value higher than the mineral acids
such as HCl. In comparison with the strong organic acids such
as trifluoroacetic acid and pentadifluoropropionic acid, 1 has
similar acid strength as the fluoroalkyl acids that react reversibly
a
18
a
19
with Vaska’s complex to give multiple isomers as observed
with 1. The labile nature of the N2 ligand in trans-
Ir(Cl)(N )(PPh ) also facilitates the eventual bidentate
The molecular structures of 5, 6, and 7 are shown in Figure
4
. In all of the three examples, the new Ir(III) complexes have a
distorted octahedral geometry with the triphenylphosphine
ligands trans to each other. The Ir(III) complexes are shown to
have an unexpected bidentate coordination of the N-nitro-
amide/carbamate conjugate base through the nitrogen on the
amide functionality and the oxygen of the nitro group. The
nitrogen acid conjugate bases form a four-membered Ir−N−
N−O chelate ring. The chloride ligand is positioned trans to
the N atom of the amide group with Cl(1)−Ir(1)−N(1)
between 164° to 173°. The most interesting aspect of the solid-
state structures for the complexes is that one of the oxygen
atoms of the nitro group out-competes the acyl oxygen that is
present in 5 and 6 as well as an additional methoxy subsituent
in 7 for coordination to the Ir center (Figure 4). Our previous
report for 6 had included a comparison of the different isomers
2
3 2
coordination of the nitrogen acids, which is not possible for
the more inert carbonyl complex. During the course of the
reaction, transient intermediates are observed in the P and H
NMR spectra, which will be discussed later.
Structural Results. The solid-state molecular structures for
the nitrogen acids 1, 3, and 4 are shown in Figure 2.
Compound 1 is shown to adopt the Z conformation in the
solid state similar to 2. However, DFT calculations indicate that
the most stable gas-phase isomer is the E isomer (Figure 3a).
For 3, the Z isomer is calculated to be only slightly more stable,
which is the same as that observed in the solid state (Figure
3
1
1
3
b).
3
with DFT calculations. For 7, the oxygen of the alkoxy group
can also coordinate to the Ir center giving four additional
possible isomers; however, the preferred mode of coordination
is still the oxygen of the nitro group.
A different type of isomerism encountered within these
Ir(III) complexes involves the nitrogen acid ligands. As
discussed above, there are two possible conformations for the
nitrogen acids, E or Z. On the basis of conventions, the E/Z
isomerism for ligands coordinated to transition metal
complexes may be different from the free acid or the anionic
salts (Figure 5).
For 5 and 6, the N-nitroamide conjugate base has a flipped
geometry around the C−N bond, while for 7 the ligand C−N
conformation corresponds to that also found in the free acid 3.
The selected structural data for 5, 6, and 7 are shown in
Table 1. The Ir(1)−N(1) bond length for all three complexes
are approximately 2.04 Å, which corresponds to an Ir−N single
bond. The Ir(1)−O(1) bond lengths of the three complexes are
more than 2.33 Å, which is longer than most Ir−O single bonds
and indicates a weak Ir−O bond. The reason for the observed
structural feature may be due to the trans influence of the
Figure 3. DFT calculated bond lengths and relative energy levels of
the isomers of (a) 1 and (b) 3. Measured bond distances are shown in
italics.
20
hydride ligand. The N(1)−N(2) bond lengths of 5 and 7
decreased on coordination to the Ir center in comparison to the
E
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Table 1. Selected Bond Lengths (Å) Bond and Torsion Angles (deg) of N-Nitroamides, N-Nitrocarbamates, and Ir(η2-
X)(H)(Cl)(PPh ) Complexes from X-ray Diffraction
3
2
3
a
b
compound
N(1)−N(2)
N(1)−C(1)
N(2)−O(1)
N(2)−O(2)
Ir(1)−N(1)
Ir(1)−O(1)
N(1)−N(2)−O(1)
N(1)−N(2)−O(2)
N(2)−N(1)−C(1)
O(1)−N(2)−O(2)
Ir(1)−N(1)−N(2)
O(1)−Ir(1)−N(1)
N(2)−O(1)−Ir(1)
1
2
3
4
5
6
7
1.3835(19)
1.389(2)
1.3858(18)
1.3949(19)
1.2183(16)
1.2141(16)
1.382(3)/1.380(3)
1.394(3)/1.396(3)
1.222(3)/1.226(3)
1.212(3)/1.214(3)
1.385(2)
1.387(2)
1.216(2)
1.2154(19)
1.372(5)
1.398(6)
1.287(5)
1.199(5)
2.030(3)
2.329(3)
109.4(3)
128.1(4)
121.9(4)
122.5(4)
101.2(2)
59.26(12)
90.1(2)
1.389(6)
1.372(7)
1.298(6)
1.193(6)
2.042(4)
2.357(4)
109.6(4)
126.6(5)
120.7(5)
123.8(5)
101.3(3)
59.38(16)
89.7(3)
1.349(4)
1.405(5)
1.275(4)
1.223(4)
2.050(3)
2.323(3)
110.8(3)
126.5(3)
118.9(3)
122.7(4)
100.4(2)
58.80(11)
89.97(19)
178.3(4)
−1.2(7)
1.2230(17)
1.2144(17)
114.14(12)
119.62(13)
125.47(13)
126.24(13)
114.40(11)
119.18(11)
125.27(12)
126.42(12)
114.0(2)/113.9(2)
119.7(2)/119.3(2)
124.5(2)/124.6(2)
126.3(2)/126.8(2)
115.17(14)
118.41(15)
123.87(14)
126.41(16)
O(1)−N(2)−N(1)−C(1)
O(3)−C(1)−N(1)−N(2)
180.0(1)
0.0(2)
180.0(1)
0.0(2)
−178.2(2)/-178.8(2)
−1.3(4)/0.5(4)
168.1(2)
173.7(4)
−174.3(4)
−179.9(5)
173.3(5)
−1.8(3)
a
b
2
independent molecules. O(5), O(6), O(7), O(8), N(3), N(4), C(3), C(4) in corresponding order for second molecule. Atom sequence of
complex 6 has been change from reference 3
Figure 4. Molecular structures of (a) 5 at 45% thermal ellipsoid, (b) 6 and (c) 7 at 50% thermal ellipsoid. Hydrogen atoms and solvent molecules
were omitted for clarity.
The N−O bond lengths in 5, 6, and 7 differ significantly
between the metal-bound N(2)−O(1) and the free N(2)−
O(2) by at least 0.5 Å. This represents a more localized
bonding arrangement for the shorter noncoordinating N−O
fragment, resonance form C, which is formally the N-oxide of
the acetyldiazotate anion. The bite angle of the Ir−N(1)−
N(2)−O(1) chelate is typical of four-membered rings, which is
Figure 5. E/Z isomers of nitrogen conjugate acids coordinated to
very acute compared to five- or six-membered rings. This
transition metal.
bonding mode is, however, preferred in these systems even
though alternative less-strained chelate rings are possible with
the acyl group. The length of the alkyl chain on the acyl
fragment does not have a significant effect on the planarity
between the amide and nitro section of the nitrogen acid
ligands on coordination.
free acid 1 and 3. For 6, the N(1)−N(2) bond length is similar
to that of 2. The N(1)−C(1) bond lengths of 5 and 7 both
increased compared to 1 and 3. For 6, a reverse in the trend is
observed as the N(1)−C(1) bond length actually decreases
compared to 2.
Dynamic Process in Solution. As mentioned above, the
1
observation of the H NMR Ir−H resonance signal confirms
the presence of a metal hydride, which is not located in the X-
ray diffraction experiments. However, the appearance of a
broad signal for the Ir−H resonance in 5 is unexpected. A
broad NMR signal could be due to the presence of a
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1
Figure 6. Variable-field H NMR 400 MHz (upper) and 200 MHz (lower) in C D (left) and CDCl (right) for Ir−H region of 5 shown with
6
6
3
presence of the transient species.
paramagnetic metal center or a dynamic process occurring in
solution. The Ir(III) metal centers have a low-spin d6
configuration, and coupled with the general sharp signals
observed in the rest of the NMR spectra, the latter is more
likely. For 5, the appearance of the broad signal is also solvent-
dependent (Figure 6).
The additional metal hydride resonance that is located
further upfield at approximately −35 ppm in Figure 6 is a
separate species that does not interact with the product
resonance at −30 ppm as both of the resonance signals broaden
and resolve independently. This upfield resonance is assigned
to an intermediate (see below).
Since the broad resonance is only observed for the Ir hydride
31
and neither in the other proton resonances nor in the
P
NMR, it is very likely that the dynamic process significantly
affects the Ir−H environment. We propose that this dynamic
process is due to a labile reversible O atom coordination trans
to the hydride (Scheme 4). The reason for such a phenomenon
20
could be due to the strong trans effect of the hydride ligand.
Scheme 4. Proposed Labile Coordination of O Atom to Ir
Center
In CDCl , at the effective field strength of 200 MHz, the Ir
3
hydride signal is very broad, but this sharpens slightly when the
effective field strength is increased to 400 MHz. When C D is
6
6
used instead, the Ir hydride signal is observed to be a triplet at
the effective field strength of 200 MHz. When the effective field
strength is increased to 400 MHz, the same signal is now
observed as a broad signal. The conclusion for the observations
is that in C D the dynamic process is at the fast-exchange limit.
III
2
The related Ir(III) acetate complex, Ir (η -CH CO )(H)-
3
2
(Cl)(PPh ) (12), synthesized by Kubota and co-workers, was
3
2
6
6
prepared and does not exhibit this fluxional behavior in either
the 31P or H spectra (Supporting Information). The Ir
hydride signal observed in 12 stands in stark contrast with that
of 5 and provides more evidence for the dynamic behavior of 5
in solution.
1
21
In CDCl , the dynamic process is also at the fast-exchange limit
3
but is closer to the lower coalescence temperature; at effective
field strength of 200 MHz, the lower coalescence temperature
has already been breached giving a broad signal that sharpens as
the effective field strength is increased. It is likely that the
dynamic process is faster in C D than it is in CDCl .
Reaction Intermediates and Transient Species. In the
course of treating trans-Ir(Cl)(N )(PPh ) with 1 to give 5, a
6
6
3
2
3 2
From the collective Ir hydride resonances observed for 5, 6,
, and 8, it is very likely that an increase in the alkyl chain
transient species is observed to form initially, which slowly
7
converts into the final product, which is isolated in excellent
3
1
length or change to an alkoxy chain length causes the rate of the
dynamic process to increase such that all the triplet resonances
observed are actually in the fast-exchange region. This is
supported to some extent since the Ir hydride resonance signal
in 6 is a broad signal that shows some coupling pattern that can
yield. This intermediate, 5A, can be observed using P NMR
1
(Figure 7), H NMR (Figure 8), and solution IR (Figure 9)
spectroscopy techniques. In this and subsequent discussion the
term intermediate is used for the transient species that are
observed to form and then convert to the final product and are
not isolated.
3
be resolved as a triplet (Supporting Information, Figure S3).
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Figure 7. 31P NMR reaction monitoring of trans-Ir(Cl)(N )(PPh ) with 1 in CDCl to give 5.
2
3
2
3
1
Figure 8. H NMR reaction monitoring of trans-Ir(Cl)(N )(PPh ) with 1 in CDCl to give 5.
2
3
2
3
In 31P NMR (Figure 7), a resonance signal at 14.5 ppm is
observed to form initially together with the signal at 11.5 ppm.
The 14.5 ppm signal initially increases and subsequently
decreases over time to give only the signal at 11.5 ppm. Thus,
H
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−
1
Figure 9. Solution IR spectra for the reaction of trans-Ir(Cl)(N )(PPh ) with 1 to give 5 between 2200 and 1300 cm . Reaction carried out at −10
2
3 2
°
C in CHCl over 24 h. Aliquots taken at the following time intervals: (red) Initial trans-Ir(Cl)(N )(PPh ) complex immediately after addition of 1;
3
2
3 2
(
blue) after 15 min; (green) after 1.5 h; (black) after 17 h.
the signal at 14.5 ppm corresponds to an intermediate that
slowly converts to 5, which is at 11.5 ppm. This similar trend is
1
also observed in H NMR (Figure 8), where the resonance
signals at 1.36 and −35.1 ppm assigned to 5A increased and
subsequently decrease to resonance signals at 1.23 and −31.8
ppm observed in 5.
The multiple solution IR spectra of the reaction mixture over
time gave the most information regarding the functional group
changes taking place over time (Figure 9). It is shown that the
Figure 10. Possible structures for the intermediates observed in trans-
Ir(Cl)(N )(PPh ) reaction with 1 and 3 to give 5 and 7, respectively.
2
3 2
band at 2104 cm− corresponding to the N band in trans-
1
2
Ir(Cl)(N )(PPh ) rapidly disappears, which indicates the loss
The 31P NMR is observed to show two transient species,
namely, 7A at 14.4 ppm and 7B at 7.5 ppm, with intermediate
7B in greater proportion. Both these intermediates are
observed to disappear within 3 h to give 7, which has the
2
3 2
−1
of N ligand. Similar bands at 1736 and 1610 cm , which
belong to 1 and a band near 1300 cm , are also observed to
decrease overtime. The band at 1570 cm is observed to
2
−
1
−1
1
resonance signal at 10.0 ppm. Similarly, in H NMR,
increase initially but decrease over the course of the reaction.
Finally product bands at 1695, 1510, and 1360 cm−1 are
resonances corresponding to the transient species are observed;
7
A at 3.4 and −36.7 ppm and 7B at 3.6 and −26.0 ppm with
observed to increase over time.
We conclude that the band at 1570 cm most likely
corresponds to 5A that is observed in the P and H NMR
−1
7B in greater proportion. These resonances disappear over time
3
1
1
to give resonance signals at 3.0 and 35.7, which are assigned to
7
.
spectra. Comparing the IR bands in some carboxamide
2
2
The solution IR monitoring formation of 7 reveals some
complexes and the related Ir(III) carboxylate complexes
similar changes observed in the synthesis of 5. The decrease of
2
1
−1
such as 12, the 1570 cm band may be assigned to a
coordinated acyl ligand. Togther with the NMR data obtained,
a few possible conformations of the intermediate are proposed
−1
the 2104 cm band of the starting N ligand with concurrent
2
−1
decrease in the 1790 and 1620 cm bands are assigned to 3.
The increase in the bands at 1750 and 1260 cm and the
general increase in the bands between 1570 and 1500 cm are
−1
(Figure 10).
−1
The reaction of trans-Ir(Cl)(N )(PPh ) with 3 to give 7 was
2
3 2
due to the formation of 7. In this case, no initial increase and
subsequent decrease and loss of any bands could be observed.
A possible reason is that the rate of reaction of trans-
Ir(Cl)(N )(PPh ) with 3 is much more rapid than it is with 1.
3
1
also monitored by P NMR (Supporting Information, Figure
S4), H NMR (Supporting Information, Figure S5), and IR
1
spectroscopy (Supporting Information, Figure S6) and reveals
the possible presence of two species that eventually convert to
the product 7.
2
3 2
−1
This is shown by the rapid loss of the 2104 cm band and the
overall time scale of the transformations. In the reaction of
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Scheme 5. Reaction of Methyl Triflate with 5 To Form 9
trans-Ir(Cl)(N )(PPh ) with 1, the reaction time monitored in
unsuccessful using chemical ionization/ESI-MS or GC-MS of
the reaction mixture.
Tertiary phosphines react rapidly but incompletely with 5.
2
3 2
NMR is over 24 h long, while that with 3 is observed to be
complete within 2 h! Thus, despite starting the solution IR
experiment at −10 °C, the rate of conversion between the
intermediates to 7 was too rapid for observation of significant
quantities of any transient species.
For example, 1 equiv of P(CH ) Ph did not result in
3
2
conversion of all starting complex. Complex 5 was still
3
1
dominant after extended reaction times. Several new
P
The possible structures of the intermediates 7A and 7B are
shown in Figure 10, which corresponds with the spectroscopic
data. With the limited data, it is not possible to determine if
intermediates 7A and 7B interconvert between them to give 7.
The key difference between the formation of 5 versus that of
NMR resonance signals were observed, one of which with
significant intensity at −4.38 ppm is assigned to free PPh . A set
3
of doublet and triplet signals with similar coupling constants of
19.9 Hz in the 31P NMR spectrum represents a phosphine
ligand that is positioned cis to two identical trans phosphine
ligands. It is very likely that the initial substitution to one of the
7
is that the rate of conversion of the intermediates in 7 is much
faster than that of 5. This difference may be driven by the
alkoxy versus the alkyl functional group or may reflect the
PPh ligands of 5 results in the phosphine-substituted product
3
to be more reactive toward further PPh substitution. (Scheme
3
different pK of the free acids.
6)
a
We were unsuccessful in obtaining the crystal structure for 8;
3
1
1
however, from the P and H NMR reaction monitoring
results (Supporting Information, Figures S7 and S8, respec-
tively) together with the IR results of the isolated solids, it is
determined that 8 has similar reaction intermediates and
product geometry as 7. Thus, the structure of 8 is very likely an
analogue of 7 with an ethoxy group.
Scheme 6. Proposed Reaction Isomers of 10 from the
Addition of P(CH ) Ph to 5 with Fractional Proportions
3
2
2
Reactivity Studies of Ir(η -CH C(O)NNO )(H)(Cl)(PPh )
3
2
3 2
(
5). Upon methylation with methyl triflate 5 there is a loss of
the N-nitroacetamide ligand, to give the bis(acetonitrile) Ir(III)
complex 9 (Scheme 5).
The IR spectrum of 9 shows multiple weak bands in the 2270
−
1
cm region, which can be assigned to the coordinated
acetonitrile ligands and the Ir hydride stretch. The appearance
−1
of the bands at 1270, 1154, 1031, and 638 cm confirms the
presence of the triflate anion, while the disppearance of bands
−1
at 1695, 1514, 1262, 1198, and 621 cm indicates the loss of
3
1
the coordinated N-nitroacetamide ligand. The P spectrum
1
shows only a single singlet at 4.43 ppm, and the H NMR
spectrum has only a single triplet at −20.67 ppm for the Ir
hydride, indicating a single species. New singlets at 1.90 and
2.00 ppm can be assigned to the methyl in the coordinated
CH CN ligands. A crystal structure determination for 9
3
confirms the above assignments (Supporting Information,
Table S1 and Figure S2).
The methylation of 5 only occurs in acetonitrile and is
unsuccessful in other solvents such as CHCl and benzene. The
3
use of alternative methylating agents such as CH I and
3
dimethylsulfate did not alter the reaction outcome in other
solvents. It is very likely that coordination of the acetonitrile
ligand to 5 is necessary for the methylation reaction to proceed
and is related to the proposed labile behavior of the
coordinated oxygen atom of the nitro group. It is interesting
that successful methylation of the N-nitroacetamide ligand
results in the loss of the methlylated ligand through substitution
by acetonitrile. We were not able to isolate the methylated N-
nitroacetamide ligand but propose the methylation site to be on
the oxygen atom of the nitro group. Attempts to identify the
methylation product of the conjugate base of 1 were
This is supported by the absence of two sets of doublets that
have large coupling being observed in the 31P NMR for
inequivalent trans phosphines. The substituted phosphine
complex 10A also undergoes further nucleophilic addition of
an additional P(CH ) Ph molecule preferentially to give the
3
2
isomers 10B, 10C, and 10D. This is evidenced by the
J
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Inorganic Chemistry
Article
appearance of three sets of doublets of triplets resonance signals
and a single triplet in the upfield region of the H NMR
The carboxylate complex 12 also undergos CO addition to
1
1
21
give Ir(η -CH CO )(H)(Cl)(CO)(PPh ) (14). Complex 14
3 2 3 2
spectrum. With excess P(CH ) Ph added, total conversion of 5
occurs to give the four complexes. The single Ir hydride triplet
is reported to be only one isomer based on IR analysis of the
3
2
Ir−H, Ir−Cl, and Ir-CO stretches. The trans labilization effect
23
of carbon monoxide on the Ir hydride affects ν(Ir−H) stretch
is assigned to the PPh substituted 10A as it is not coupled to
3
−1
that is found at 2112 cm , which is much lower than that of 12
any additional phosphine.
−1
The three complexes, 10B−D, will have identical 31P NMR
coupling systems and are assigned to the doublet and triplet
at 2307 cm . The observation of hydridocarbonyl vibrational
24
coupling also supports a trans H−Ir−CO structure from
1
2
31
1
isotopic analysis using H/ D Ir hydride samples. The
P
resonance signals (Scheme 6). One of the doublet of triplet H
1
single singlet resonance at 5.41 ppm, H NMR signals of singlet
at 1.06 ppm for the methyl group, and triplet at −7.83 ppm (14
Hz) for the Ir hydride indicate the presence of only a single
isomer for 14.
signals for the Ir hydride complexes has a significantly large
doublet coupling constant of 158 Hz. This isomer is most likely
1
0D, which has the hydride located trans to the dimethylphe-
nylphosphine ligand. The remaining two hydric doublet of
triplets have the hydride located cis to the equatorial
phosphine, and the assignment of the signals assumes that
P(CH ) Ph preferentially adds from the less-hindered site.
The formation of multiple isomers in 11 compared to 14
could be due to the labile coordination of the oxygen of the
nitro group. Isomeric carbonyl complexes of 11 are metastable
and reversibly lose CO to regenerate 5. Ir(III) complexes with a
labile carbonyl are rare, but there are precedents of labile CO
3
2
Therefore, 10B is formed in larger proportions than 10C.
For the corresponding carboxylate complex 12, addition of
6
1
on transition metal complexes such as the d Re(η -CH CO )-
1
3
2
P(CH ) Ph gives only a single isomer of Ir(η -CH CO )(H)-
25
3
2
3
2
(
CO) (PPh ) complex. The presence of three π-accepting
3 3 2
(
Cl)(P(CH ) Ph)(PPh ) (13), with the Ir hydride trans to the
3 2 3 2
CO ligands causes the Re center to be electron-deficient
resulting in a labile CO. Thus, the electron-withdrawing or π-
backbonding effect of the N-nitroacetamide ligand may be the
cause for the hemilabile nature of CO and substitution of PPh3
by P(CH ) Ph. Furthermore, the bidentate nature of the N-
2
1
P(CH ) Ph ligand. The original article characterized this
3
2
21
isomer by its ν(Ir−Cl). A separate synthesis of 13 confirms
−1
31
the presence of ν(Ir−H) at 2187 cm . The P NMR contains
two sets of resonances, namely, a doublet at −2.77 ppm (13.5
3
2
Hz) assigned to P(CH ) Ph and a triplet of doublet at −42.50
3
2
nitroacetamide ligand allows efficient chelation to the Ir center.
These two factors together may play a role in the labile nature
of the carbonyl ligand.
ppm assigned to PPh from coupling to both the P (13.5 Hz)
3
and CH groups (7.3 Hz) in P(CH ) Ph. Significantly, these
3
3 2
NMR results confirm only one isomer is present.
The reactivity studies of 12 are in great contrast with 5 with
regard to CO and P(CH ) Ph addition. In both the CO and
The carbonylation of 5 gives a single new broad IR band at
3
2
1
2
047 cm− This band is assigned to the carbonyl ligand of the
P(CH ) Ph additions to 12 only one ligand is added to
3 2
new Ir(III) carbonyl complex 11. The Ir hydride stretch is also
generate single isomers, which are stable. For 5, the addition of
P(CH Ph results in PPh substitution by P(CH Ph, which
is further observed to preferentially add an additional
P(CH Ph to give isomers of 10. The introduction of CO
−1
observed as a low-intensity broad band centered at 2185 cm .
3
)
2
3
)
3 2
The 31P and H NMR spectra, however, indicate the presence
1
3
1
of multiple isomers of 11. In the P NMR spectrum, two
)
3 2
to 5 generates isomers of 11, which on prolonged duration in
solution loses CO and regenerates 5. The above reactivity for 5
can be attributed to both the electron-withdrawing nature of
the N-nitroacetamide ligand and the labile coordination of the
resonance signals are observed with unequal intensity. In the
H NMR, three Ir hydride triplet resonances are observed, with
two signals that are significantly stronger and the third weaker
in a ratio of 0.56:0.39:0.05. Thus, there are three isomers of 11
that can form depending, on the site of CO addition (Scheme
1
1
oxygen atom of the nitro group observed earlier from the H
NMR of the Ir hydride resonance. It seems that the trans
7).
20
labilizing effect of the Ir hydride in 5 is not sufficient to
enforce the formation of a single isomer for nucleophiles.
The carbon monoxide addition reaction to 5 to give isomers
of 11 is in contrast to the reaction of trans-Ir(Cl)(CO)(PPh3)2
with 1 (Scheme 8). The former results in formation of isomers
that reversibly lose the CO ligand, while the latter gives
incomplete conversion of the starting complex trans-Ir(Cl)-
Scheme 7. Possible CO Addition Coordination Isomers with
5
(
CO)(PPh ) , which, with excess amounts of 1, generates
3 2
multiple products. Significantly, reductive elimination of the
conjugate base of the nitrogen acid ligand to give Vaska’s
complex is not observed.
CONCLUSION
■
Compounds 1−4 undergo oxidative addition with trans-
Ir(Cl)(N )(PPh ) to give new Ir(III) hydrido nitrogen acid
2
3 2
complexes 5−8, respectively, which exhibit diagnostic vibra-
tional bands. The Ir(III) complexes all feature an unusual four-
membered N,O chelate with the nitrogen of the amide and the
1
oxygen of the nitro group to the Ir center. The H NMR
spectrum for 5 is observed to have solvent-dependent line
shape behavior for the metal hydride resonance, which can also
be observed on varying the effective field strength. This effect is
K
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Inorganic Chemistry
Article
Scheme 8. Reaction of 5 with CO Versus Reaction of trans-Ir(Cl)(CO)(PPh ) with 1
3
2
proposed to be due to the labile Ir−O bond and is believed to
undergo rapid coordination and release in solution. The
fluxional behavior of the nitrogen acid in 5 is believed to
contribute to the unusual reactivity with nucleophiles such as
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ASSOCIATED CONTENT
■
*
S
Supporting Information
Information on the crystallographic parameters of the
determined structures, ORTEP plot of 9, NMR and IR
reaction profile of 7 and 8 are included. NMR characterization
of the transient species in the syntheses of 5−8 are also
included. This material is available free of charge via the
Internet at http://pubs.acs.org. The CIF files for the X-ray
structure determinations of 1, 3, 4, 5, 7, and 9 are available at
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ACKNOWLEDGMENTS
■
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