Intermolecular N-H oxidative addition of ammonia, alkylamines, and arylamines to a planar σ3-phosphorus compound via an entropy-controlled electrophilic mechanism

Article abstract of DOI:10.1021/ja412469e

Ammonia, alkyl amines, and aryl amines are found to undergo rapid intermolecular N-H oxidative addition to a planar mononuclear ? ³-phosphorus compound (1). The pentacoordinate phosphorane products (1·[H][NHR]) are structurally robust, permitting full characterization by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. Isothermal titration calorimetry was employed to quantify the enthalpy of the N-H oxidative addition of n-propylamine to 1 (ⁿPrNH₂ + 1 a? 1·[H][NHⁿPr], ?"H _rxn²⁹⁸ = -10.6 kcal/mol). The kinetics of n-propylamine N-H oxidative addition were monitored by in situ UV absorption spectroscopy and determination of the rate law showed an unusually large molecularity (?= k[1][ⁿPrNH₂]³). Kinetic experiments conducted over the temperature range of 10-70 °C revealed that the reaction rate decreased with increasing temperature. Activation parameters extracted from an Eyring analysis (?"H^a§§ = -0.8 ± 0.4 kcal/mol, ?"S^a§§ = -72 ± 2 cal/(mol·K)) indicate that the cleavage of strong N-H bonds by 1 is entropy controlled due to a highly ordered, high molecularity transition state. Density functional calculations indicate that a concerted oxidative addition via a classical three-center transition structure is energetically inaccessible. Rather, a stepwise heterolytic pathway is preferred, proceeding by initial amine-assisted N-H heterolysis upon complexation to the electrophilic phosphorus center followed by rate-controlling N a? P proton transfer.

Full text of DOI:10.1021/ja412469e

Article
pubs.acs.org/JACS
Intermolecular N−H Oxidative Addition of Ammonia, Alkylamines,
and Arylamines to a Planar σ3‑Phosphorus Compound via an
Entropy-Controlled Electrophilic Mechanism
Sean M. McCarthy,† Yi-Chun Lin,† Deepa Devarajan,‡ Ji Woong Chang,§ Hemant P. Yennawar,†
Robert M. Rioux,§ Daniel H. Ess,*,‡ and Alexander T. Radosevich*,†
†Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
§Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
‡Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, United States
S
* Supporting Information
ABSTRACT: Ammonia, alkyl amines, and aryl amines are
found to undergo rapid intermolecular N−H oxidative addition
to a planar mononuclear σ3-phosphorus compound (1). The
pentacoordinate phosphorane products (1·[H][NHR]) are
structurally robust, permitting full characterization by multi-
nuclear NMR spectroscopy and single-crystal X-ray diffraction.
Isothermal titration calorimetry was employed to quantify the
enthalpy of the N−H oxidative addition of n-propylamine to 1
(nPrNH2 + 1 → 1·[H][NHnPr], ΔHrxn = −10.6 kcal/mol).
298
The kinetics of n-propylamine N−H oxidative addition were
monitored by in situ UV absorption spectroscopy and
determination of the rate law showed an unusually large
molecularity (ν = k[1][nPrNH2]3). Kinetic experiments conducted over the temperature range of 10−70 °C revealed that the
reaction rate decreased with increasing temperature. Activation parameters extracted from an Eyring analysis (ΔH⧧ = −0.8 0.4
kcal/mol, ΔS⧧ = −72 2 cal/(mol·K)) indicate that the cleavage of strong N−H bonds by 1 is entropy controlled due to a
highly ordered, high molecularity transition state. Density functional calculations indicate that a concerted oxidative addition via a
classical three-center transition structure is energetically inaccessible. Rather, a stepwise heterolytic pathway is preferred,
proceeding by initial amine-assisted N−H heterolysis upon complexation to the electrophilic phosphorus center followed by rate-
controlling N → P proton transfer.
1. INTRODUCTION
The homogeneous activation of N−H bonds is of significant
interest in connection with the direct catalytic functionalization
of ammonia and amine derivatives for a broad range of
applications.1 The activation of N−H bonds by oxidative
addition to transition metals has been a prime focus of
investigation. For instance, Goldman and Hartwig2 have
described the addition of ammonia to an iridium(I) pincer
complex (Figure 1A), and related examples of N−H cleavage
reactions have been characterized for a range of early,3 late,4
Figure 1. Intermolecular N−H oxidative addition by (A) a transition
metal complex and (B) a nonmetal ccompound.
and polynuclear transition complexes.5
In line with a growing interest in catalysis with abundant
nonprecious elements,6 recent advances in fundamental bond
activation chemistry have shown the cleavage of strong covalent
addition chemistry has been observed with the heavier
bonds is possible by persistent main group compounds under
mild homogeneous conditions.7−9 With respect to N−H bond
persistent divalent group 14 compounds (i.e., silylenes,
germylenes, stannylenes)12 by Power13 and Roesky.14 Alter-
cleavage, Bertrand and Schoeller have demonstrated that
native heterolytic N−H activation strategies including ligand−
nucleophilic singlet carbenes are capable of ammonia N−H
cleavage under ambient conditions (Figure 1B).10 Bielawski
and Siemeling have documented similar reactivity with
Received: December 7, 2013
Published: March 5, 2014
electronically diverse carbenes,11 and related N−H oxidative
© 2014 American Chemical Society
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metalloid cooperativity15 and Lewis acid/base pairing16 have
also been demonstrated.
Nonmetal group 15 compounds have the potential to
provide unique geometric structures and electronics for bond
cleavage reactions. The tricoordinate phosphorus compound 1,
initially described by Arduengo (Figure 2),17 displays an
to yield difluorophosphoranes.27 Additional in situ spectral
evidence exists for intermolecular N−H oxidative addition
giving hydrido amido phosphoranes as persistent intermediates
in (1) the substitution reaction of trivalent phosphorus
electrophiles with amines to yield P(III) amides,28 and (2)
the reaction of P(NMe2)3 with diethanolamines en route to
fused bicyclic phosphoramidites.29 With respect to attempted
N−H oxidative addition to 1 itself, Arduengo has noted that
dimethylamine fails to give an adduct with 1, although the same
report does document the O−H oxidative addition product 1·
[H][OMe] as an observable metastable intermediate in the
methanolysis of 1.22a
In this manuscript, we demonstrate intermolecular N−H
bond cleavage of ammonia, alkylamines, and arylamines by
oxidative addition to tricoordinate phosphorus compound 1
proceeding under mild homogeneous conditions and leading to
structurally robust adducts that permit full spectroscopic and
crystallographic characterization. The fidelity and velocity of
these reactions enable quantification of both the reaction
enthalpy and kinetics. Together with these experimental results,
theoretical calculations indicate that the N−H oxidative
addition reaction proceeds in a heterolytic stepwise fashion
initiated by electrophilic reactivity of 1. In view of the typical
Lewis basicity of tricoordinate phosphorus, these results
illustrate the extent to which unusual electronic structure
arising from geometric distortion can lead to high reactivity in
the main group.30
2. RESULTS
Figure 2. Resonance structures, ammonia-borane dehydrogenation,
and amine N−H bond oxidative addition by compound 1.
2.1. Synthesis and Characterization. 2.1.1. Addition of
Ammonia. To initiate the investigation of N−H oxidative
addition, ammonia was condensed onto a solid sample of 1 at
−78 °C (Figure 3). After 30 min, the NH3 was removed under
unusual planar T-shaped geometry at phosphorus with the
tridentate O,N,O-binding motif occupying three adjacent
coplanar sites. The electronic structure of this compound has
been extensively investigated by Arduengo and Dixon.18 As in
related delocalized heterocyclic systems,19 the pincer-like T-
shaped geometry20,21 is stabilized by extensive mesomerism in
the π-plane of the molecule (e.g., resonance structures 1A−C);
several lines of spectroscopic and theoretical evidence suggest
1C is a significant contributor to the overall electronic
structure. The reactivity of this compound with respect to
certain small molecule substrates (including Bronsted acids,
alcohols, halogens, quinones and electron-deficient alkynes)22
and transition metal complexation23 has been investigated by
Arduengo. More recently, Driess has described the nucleophilic
addition of 1 to methyl triflate.24
Figure 3. Ammonia oxidative addition to 1.
Recently, one of our groups has demonstrated that 1 can
serve as a platform for catalytic transfer hydrogenation via the
intermediacy of a pentacoordinate dihydridophosphorane (1·
[H]2) formed from the dehydrogenation of ammonia-borane
(Figure 2).25 Among the possible mechanistic courses, we
considered that the formation of 1·[H]2 might proceed via an
initial N−H oxidative addition to phosphorus. Intramolecular
N−H oxidative addition by P(III) ⇄ P(V) ring−chain
tautomerism is well-known, and structural factors controlling
the position of this reversible cyclization equilibrium have been
discussed.26 Intermolecular N−H oxidative addition to
phosphorus, by contrast, is less well-characterized due to
instability of the P(V) adducts toward subsequent reductive
elimination. The only well-characterized examples of inter-
molecular oxidative addition of N−H bonds to tricoordinate
phosphorus resulting in isolable phosphorane adducts involve
the addition of amines to substituted difluorophosphines RPF2
an N2 sweep yielding a crude solid, which was dissolved in
C6D6. NMR analysis indicated full consumption of 1 (31P δ
187.0 ppm) and clean conversion to a single new phosphorus
containing product with a 31P NMR resonance centered at δ
−43.5 ppm, consistent with formation of a pentacoordinate
phosphorus species.31
The signal multiplicity (Figure 4, bottom) further suggests an
addition reaction with ammonia, and the largest value of J =
1
3
812 Hz is indicative of direct JPH coupling (longer range JPH
coupling to the vinylic ligand C−H protons (31.1 Hz) and 2JPH
coupling to the N−H protons (10.2 Hz) is also evident). The
complementary P−H coupling observed in the 1H NMR
spectrum corroborates this assignment (Figure 4, top);
specifically, a phosphorus-coupled doublet for the P−H is
found at δ 8.79 ppm (1JPH = 812 Hz). Additionally, a feature at
2374 cm−1 in the infrared spectrum may be assigned to the
stretching mode of a P−H unit, and two bands observed in the
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Figure 4. (a) Abridged annotated 1H NMR spectrum for 1·[H][NH2]
1
in C6D6. An additional H signal for −C(CH3)3 is found at δ 1.06
ppm. (b) Annotated 31P NMR spectrum for 1·[H][NH2] in C6D6.
Units are ppm relative to Me4Si (1H) and H3PO4 (31P). See
Supporting Information for full spectra.
Figure 5. Examples of intermolecular N−H oxidative addition of alkyl
and aryl amines to 1. Reactions with 2,6-disubstituted arylamines were
conducted at 55 °C.
infrared spectrum at 3516 and 3411 cm−1 are consistent with
the symmetric and antisymmetric stretching bands of a terminal
−NH2 group.
2.1.4. Structure and Bonding of N−H Addition Product. A
diffraction quality single crystal of 1·[H][NHMes] was
prepared from a concentrated pentane solution following
filtration over neutral alumina. As depicted in the thermal
ellipsoid plot in Figure 6, this N−H cleavage adduct adopts an
Taken together, these spectroscopic details are consistent
with the formation of an authentic N−H bond cleavage product
1·[H][NH2] resulting from the reaction of ammonia and 1.32
1
According to Martin’s empirical correlation between JPH and
ligand electronegativity,33 the observed coupling constant is
best accommodated by a trigonal bipyramidal structure for 1·
[H][NH2] having an equatorial hydride and amide as depicted
in Figure 3.
2.1.2. Addition of Alkyl Amines. Exposure of 1 to primary
alkyl amines similarly leads to N−H bond cleavage (Figure 5).
For instance, the addition of 1 equiv of n-propylamine to a
benzene-d6 solution of 1 in a sealable NMR tube at ambient
temperature results in quantitative conversion of 1 to hydrido
propylamido phosphorane 1·[H][NHnPr] (δ −46.4 ppm, J =
815 Hz). This reaction is rapid, proceeding to completion
within minutes of mixing. No intermediates are evident in
1
either the H or 31P NMR spectra. Analogous reactivity was
observed with other primary aliphatic amines, giving 1·
[H][NHiPr] (δ −47.1 ppm, J = 819 Hz) and 1·[H][NHBn]
(δ −46.0 ppm, J = 824 Hz) by the addition of isopropylamine
and benzylamine, respectively.
2.1.3. Addition of Aryl Amines. Reaction of 1 with p-
anisidine was also found to yield hydrido anilido phosphorane
1·[H][NHPMP] (δ −48.7 ppm, J = 831 Hz) by oxidative
addition (Figure 5); however, the qualitative rate of reaction in
this case is somewhat decreased relative to the primary alkyl
amines. This effect is especially pronounced with more
sterically encumbered anilines. Specifically, the reaction of
2,6-disubstituted anilines with 1 in benzene solution proceeds
only sluggishly at ambient temperature, but heating the mixture
in a sealed NMR tube at 55 °C results in complete conversion
over 2 h. Under these conditions, both 2,4,6-trimethylaniline
and 2,6-diisopropylaniline were observed to give the corre-
sponding hydrido anilido phosphoranes 1·[H][NHMes] (δ
−48.9 ppm, J = 840 Hz) and 1·[H][NHDipp] (δ −51.4 ppm, J
= 841 Hz).
Figure 6. (Left) Molecular structure of 1·[H][NHMes]. All hydrogen
atoms on carbon are omitted for clarity. Thermal ellipsoids are shown
at the 50% probability level. (Right) Top view of equatorial plane with
schematic orbital representation of nN → σ*P−N interaction .
essentially trigonal bipyramidal monomeric structure about
phosphorus in which the O,N,O-chelating motif retains an
approximately planar pincer-like arrangement about phospho-
rus, with oxygen atoms spanning apical positions at an O−P−O
angle of 174.0(1)°. When compared to the parent compound 1,
the supporting ligand in 1·[H][NHMes] exhibits a significant
contraction in both P−O and P−N1 bond distances (the
relevant structural metrics are tabulated in Table 1), in line with
the increase in valency. The addend nitrogen (N2) and
hydrogen (H1) atoms occupy equatorial positions of the
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Table 1. Comparison of Selected Crystallographic Bond
Lengths (Å) and Angles (deg) for 1 and 1·[H][NHMes]
a
b
metric
P1−O1
P1−O2
P1−N1
P1−N2
1
1·[H][NHMes]
1.792(2)
1.703(1)
1.726(1)
1.674(1)
1.645(1)
1.314(15)
174.0(1)
123.3(1)
123.7(7)
1.835(2)
1.703(2)
-
P1−H1
-
O1−P1−O2
N1−P1−N2
N1−P1−H1
167.7(1)
-
-
a
b
Data from ref 17. See Supporting Information for full crystallo-
graphic details.
trigonal bipyramid (P1−N2 = 1.645(1) Å, P1−H1 = 1.315(15)
Å) along with N1 of the bicyclic supporting structure. The
hydrogen atoms on P and N2 could be detected in the
experimental electron density map and were refined isotropi-
cally. In accord with Bent’s rule,34 the substituents around the
phosphorane-bound anilide nitrogen atom N2 are found to be
planarized. The orientation of the coordination plane around
N2 is disposed orthogonally with respect to the equatorial
plane at phosphorus, indicative of a stabilizing interaction with
35
phosphorus (likely via nN2 → σ*(P−N1) as in Figure 5) and is
consistent with the well-known preference for π-donor
substituents to occupy the equatorial plane of pentacoordinate
phosphorus.36 An NBO analysis37 of 1·[H][NHMes] sub-
stantiates this orbital interaction (see Supporting Information
for details).
2.2. Thermodynamic and Mechanistic Investigations.
2.2.1. Enthalpy of N−H Oxidative Addition. In an effort to
ascertain the driving force for N−H oxidative addition to 1, we
have undertaken thermodynamic investigations using isother-
mal titration calorimetry (ITC). ITC experiments were carried
out by titration of a solution of 1 (1 mM in PhH, 1 mL total
volume) with 6 μL aliquots of an n-propylamine solution (12.5
mM in PhH) at 25 °C under an inert atmosphere. Figure 7
shows an isotherm for the addition of n-propylamine to 1 (top)
and the integrated heats measured by ITC (bottom). Utilizing
an independent binding site model (red line in Figure 7), we
confirm a 1:1 stoichiometry for the binding of n-propylamine to
Figure 7. Thermochemistry of N−H oxidative addition of n-
propylamine to 1. (Top) ITC thermogram at 25 °C in benzene.
(Bottom) Plot of integrated heat evolved with fitted model.
1. An enthalpy of reaction of −10.6
0.1 kcal/mol and an
equilibrium constant of (3.03
0.17) × 104 M−1 were
extracted from the independent binding sites model.
Experimental data regarding bond energies in hydridophos-
phoranes are limited; therefore, the P−H and P−N strengths in
1·[H][NHnPr] were estimated with calculations. In accordance
with the ITC values, M06-2X/6-311++G(2d,2p) calculations
estimate ΔH298 = −7.4 kcal/mol. The homolytic P−H and P−
N bond dissociation energies (ΔH298) were computed using a
model of 1·[H][NHnPr] (Figure 8). The computed phosphor-
anyl radicals arising from P−H and P−N bond scission exhibit
seesaw (monovacant trigonal bipyramidal) structures with
minimal structural reorganization of the remaining substituents.
The calculated P−H bond dissociation energy is DP−H = 70.6
kcal/mol, which for comparison is somewhat less than the
experimental value for trivalent PH3 (DP−H = 83.9 0.5 kcal/
mol)38 but is consistent with Bentrude’s observation of radical
P−H abstraction in a hydridophosphorane by thiyl radicals.39
The calculated P−N bond dissociation energy (DP−N = 68.3
kcal/mol) is in line with the experimental values obtained for
both trivalent (DP−N = 66.8 0.8 kcal/mol in P(NEt2)3)40 and
Figure 8. M06-2X/6-311++G(2d,2p) P−H and P−N bond enthalpies.
pentavalent (DP−N = 74.3 kcal/mol in (Cl3PNMe)2)41
phosphorus compounds.
2.2.2. Attempted Reductive Elimination and Amine
Exchange Experiments. Attempts to promote the reductive
elimination of amine from several adducts 1·[H][NHR] by
heating solid samples in vacuo at elevated temperature have
been unsuccessful to date. For instance, ammonia adduct 1·
[H][NH2] sublimes without decomposition at 40 °C and 1
mmHg. Similarly, heating of heavier amine adducts such as 1·
[H][NHMes] to higher temperatures under vacuum does not
drive reductive elimination, but results instead only in
nonspecific decomposition.
By contrast, heating adducts 1·[H][NHR] with an excess of
an exogenous amine H2NR′ in benzene solution leads to amine
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ligand exchange. Specifically, exposure of a benzene solution of
1·[H][NHnPr] to 5 equiv. of benzylamine (H2NBn) for 24 h at
80 °C in a sealed NMR tube results in formation of 1·
[H][NHBn] (Figure 9). Likewise, when a benzene solution of
1·[H][NHBn] is heated with 5 equiv n-propylamine (H2NnPr)
at 80 °C in a sealed NMR tube, 1·[H][NHnPr] is observed.
Figure 9. Amine exchange in benzene solution.
2.2.3. Isotopic Tracer Study. We have employed isotopic
labeling experiments to track the course of the N−H bond.
Specifically, the presence of the electron-rich O,N,O-supporting
structure in 1 suggested the potential for a phosphorus-ligand
cooperative mechanism for this oxidative addition. However,
exposure of 1 to deuterated benzylamine (D2NBn) affords
1
deuterated 1·[D][NDBn] (δ −47.0 ppm, JPD = 126 Hz) and
related isotopologues with incorporation of isotopic label
exclusively at the phosphorus and nitrogen centers (Figure 10).
Figure 10. Oxidative addition of N-deuterated benzylamine proceeds
without incorporation of the isotopic label in the O,N,O-chelating
framework.
The lack of deuteration at the vinylic C−H positions evident in
2
the H NMR spectrum makes an active functional role for the
O,N,O-supporting structure unlikely; instead, an oxidative
addition with reactivity localized at the phosphorus center
appears to be operative.
Figure 11. (A) Experimental electronic absorption spectra for 1 (red
line) and 1·[H][NHnPr] (blue line) superimposed with corresponding
TDDFT (CAMB3LYP/6-311++G**) excited state transitions and
oscillator strengths. (B) Molecular orbital isosurfaces and energies for
models of 1 and 1·[H][NHnPr]. See Supporting Information for
details.
2.2.4. Kinetic Study of N−H Oxidative Addition. An
interrogation of the kinetics of the N−H oxidative addition
process by in situ UV absorption spectroscopy has permitted an
experimental determination of the rate law and related
activation parameters. Tricoordinate 1 exhibits electronic
absorption features (Figure 11A) at 313 nm (ε = 7.9 × 103
M−1·cm−1), 220 nm (ε = 10.5 × 103 M−1·cm−1), and 207 nm (ε
= 10.5 × 103 M−1·cm−1). On the computational model of 1
shown in Figure 11, TDDFT calculations indicate that the low
energy band is dominated by π → π* and nP → π* electronic
transitions (the CAMB3LYP/6-311++G** method was used
for TDDFT calculations due to significant charge-transfer
regions for excitation; see Supporting Information for full
details). Oxidative addition of n-propylamine to 1 gives the
pentacoordinate hydrido amide 1·[H][NHnPr], in which the
lowest energy band is shifted hypsochromically to 239 nm (ε =
6.7 × 103 M−1·cm−1). The progress of the oxidative addition
process can therefore be conveniently monitored at λ = 313
nm.
Kinetic experiments were conducted on solutions of 1 in dry
benzene (0.27 mM) in sealable quartz cuvettes. In the presence
of at least a 10-fold excess of n-propylamine, the N−H oxidative
addition reaction proceeds cleanly to 1·[H][NHnPr] within
minutes and without intervention of observable intermediates
or side products (Figure 12A). Under these conditions, kinetic
plots follow a pseudo first-order decay with respect to
consumption of 1 (Figure 12B). Variation of the concentration
of n-propylamine (4.3−689 mM, 14−2222 equiv) shows that
the pseudo-first order rate constants depend linearly on
[H2NnPr]3 (Figure 12C). Thus, the experimentally determined
rate law is given by eq 1:
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Figure 12. Mechanistic data for the oxidative addition of nPrNH2 to 1 in PhH. (A) Time stacked absorption spectra indicating consumption of 1 and
formation of 1·[H][NHnPr] with time. (B) Plot of [1] vs time monitored at λmax(1) = 314 nm. (Inset) Plot of ln [1] vs time with linear least-squares
fit. (C) Plot of kobs vs [nPrNH2]3 with linear least-squares fit. (D) Eyring plot for kinetic data collected over the temperature range 10−70 °C.
−d[1]
dt
= k′[1][H2NnPr]3 = kobs[1]
(1)
An Eyring plot was constructed for kinetic data collected in
at least triplicate at 10 °C temperature intervals between 10 and
70 °C (Figure 12D). Unexpectedly, the observed reaction rate
was found to decrease with increasing temperature. The
activation parameters extracted from least-squares treatment
of the relationship of ln k′/T vs 1/T (Figure 12) yield values of
ΔH⧧ = −0.8 0.4 kcal/mol and ΔS⧧ = −72 2 cal/(mol·K).
Figure 13. Concerted N−H oxidative addition to 1 with computed
2.3. DFT Energy Landscape. It is evident from the kinetics
metrics and enthalpies in kcal/mol. Hydrogens and t-butyl methyl
investigations (i.e., large molecularity of the experimental rate
law, large negative entropy of activation) that the oxidative
groups were omitted for clarity in TS-conc.
addition reaction under study here does not proceed by a
62.6 kcal/mol) imposes a large electronic barrier to direct
concerted mechanism via a three-centered transition state.42
oxidative addition.
Indeed, a calculated transition structure (M062X, 6-311+
+G(2d,2p)) corresponding to the concerted process (TS-conc,
Figure 13) resides at high energy (ΔH⧧ = 50.1 kcal/mol). The
relatively high barrier to concerted N−H cleavage can be
rationalized within the valence bond configurational mixing
We therefore examined two polar stepwise mechanisms
differing in the sequence of bond forming events (Figure 14):
(A) a nucleophilic mechanism involving initial P−H formation
by proton transfer to 1 to yield a phosphonium intermediate,
followed by subsequent P−N bond formation, and (B) an
electrophilic mechanism involving initial P−N formation by
framework,43 where the large singlet−triplet gap for 1 (ΔEST
=
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bond lengths (1.83 and 1.36 Å, respectively) in TS1 are very
similar to those found in TS-conc. This observation suggests
that the main stabilizing effect in TS1 is the basicity of the two
explicit methylamine molecules involved in hydrogen bonding.
After the P−N bond is formed in INT2, the methylammo-
nium reorients and then protonates the phosphoranide center
via TS2 (Figure 16) with a small calculated activation enthalpy
Figure 14. Outline of polar stepwise pathways.
amine addition to 1, followed by subsequent P−H bond
formation.
Calculations indicate that deprotonation of the weakly acidic
methylamine substrate by compound 1, as in pathway A, is
energetically prohibitive. Electronically, this fact is suggested in
the relatively low energies of the filled frontier orbitals (−7.5,
−7.9, and −8.7 eV; for HOMO, HOMO−1 and HOMO−2,
respectively) and a large computed ionization potential (IP =
6.4 eV). The reaction enthalpy for the proton transfer to 1 from
a single methylamine molecule is ΔH = 87.7 kcal/mol. A
marginal decrease in enthalpy is found by the inclusion of
additional explicit methylamine molecules. The unfavorable
thermodynamics of all these proton transfer events effectively
rule out the nucleophilic pathway A shown in Figure 14.44
The presence of a low-lying LUMO (−1.0 eV) and the
relatively favorable electron affinity (EA = −2.2 eV) indicate
that 1 may, by contrast, exhibit electrophilic character. Indeed,
examination of a stepwise electrophilic pathway (Figure 15)
reveals a significantly lower energy pathway compared to either
the concerted pathway or nucleophilic pathway. While attempts
to identify a stable P−N adduct between 1 and a single
methylamine molecule failed, inclusion of two additional
explicit methylamine molecules allowed location of an adduct
(INT1) with a P−N distance of 1.97 Å (Figure 15). INT1 is
exothermic by −3.8 kcal/mol relative to compound 1 and a
hydrogen-bonding cluster of three methylamine molecules on
the enthalpy surface but is endergonic by 10.1 kcal/mol on the
free energy surface, suggesting that this complex would not be
long-lived. Subsequent deprotonation of the phosphorus-bound
amine then gives INT2, an amidophosphoranide anion45 and
amine-stabilized ammonium cation pair, via TS1. The potential
energy surface along reaction coordinate INT1 → TS1 →
INT2 is found to be quite flat, with ΔH⧧ = −2.2 kcal/mol and
ΔG⧧ = 13.0 kcal/mol for TS1. The partial P−N and N−H
Figure 16. Transition structures along the stepwise electrophilic
pathway for N−H oxidative addition to 1 with computed metrics.
Hydrogens and t-butyl methyl groups were omitted for clarity.
(ΔH⧧ = 2.4 kcal/mol). TS2 is the global maximum on the free
energy reaction surface, with a ΔG⧧ value (18.4 kcal/mol) that
is controlled by entropic effects due to organization of the
methylamine hydrogen bonding network.
The computed stepwise electrophilic mechanism for N−H
oxidative addition is consistent with a number of the key
experimental observations, including the high molecularity of
the rate law and the corresponding large negative ΔS‡. The
apparent negative ΔH‡ is also well-accommodated by the
proposed intervention of an initial exothermic P−N adduct.46
All the intermediates identified by calculations (INT1, INT2)
are endergonic on the free energy surface, which is consistent
with the absence of any observable intermediates in the in situ
spectroscopy. The endergonic pre-equilibria are energetically
subsumed in the apparent activation enthalpy of the subsequent
rate controlling proton-transfer via TS2. The experimental
298
activation free energy (ΔG‡
= 20.7 0.5 kcal/mol) and
expt
calculated activation free energy (ΔG‡
= 18.4 kcal/mol)
298
calc
Figure 15. Calculated stepwise electrophilic reaction pathway for the N−H oxidative addition of methylamine to 1 (M06-2X/6-311++G(2d,2p)),
with enthalpies (free energies in parentheses) in kcal/mol. Hydrogens and t-butyl methyl groups were omitted for clarity on INT1 and INT2.
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are nearly identical and their magnitudes are clearly dominated
by entropic contributions. In short, calculation of the
electrophilic stepwise mechanism is in good qualitative
agreement with experiments.
heterolytic mechanism to other bond making and breaking
reactions at tricoordinate phosphorus are ongoing.
5. EXPERIMENTAL SECTION
A full of description of the general experimental methods, including
purification and preparation of starting materials, can be found in the
Supporting Information.
3. DISCUSSION
The notion that N−H oxidative addition to 1 proceeds via an
initial electrophilic step stands in contrast to the anticipated
Lewis basicity of trivalent phosphorus compounds but is in line
with previous observations from Pudovik28b and Quin.28c
Indeed, the electrophilic or Lewis acid behavior of certain
tricoordinate phosphorus compounds is well-established.
Diverse phosphorus(III) compounds of the formulas PX3 and
RPX2 are known to associate Lewis bases including amines,47
phosphines,48 and N-heterocyclic carbenes.49 In these struc-
tures, the tetracoordinate phosphorus adopts a seesaw shape
analogous to the computed structure of the amidophosphor-
anide intermediate proposed above. Consequently, the starting
planar T-shaped geometry of 1 may be considered to
predispose the compound to its Lewis acidic function.
5.1. Synthesis. 1·[H][NH2]. A nitrogen-flushed 25 mL round-
bottom flask equipped with a magnetic stirrer was charged with 1 (200
mg, 0.83 mmol). Ammonia (ca. 5 mL) was condensed into the flask
cooled to −78 °C. The suspension was stirred at −78 °C for 30 min,
and then warmed to room temperature to remove the excess ammonia,
yielding 1·[H][NH2] as a yellow solid (214 mg, 0.83 mmol,
1
quantitative yield). H NMR (360 MHz, C6D6) δ 8.79 (1H, d, J =
812 Hz), 5.35 (2H, d, J = 31 Hz), 1.98 (2H, d, J = 10 Hz), 1.12 (18H,
s) ppm. 13C NMR (90 MHz, C6D6): δ 149.6 (d, J = 3.6 Hz), 100.4 (d,
J = 16.3 Hz), 31.9 (d, J = 3.6 Hz), 27.7 (s) ppm. 31P NMR (145 MHz,
C6D6): δ −43.5 (dtt, J = 813, 31, 10 Hz) ppm. HRMS (EI) calcd for
[C12H23N2O2P]+, 258.14972; found, 258.15011. FT-IR (ATR): 3516,
3411, 3128, 2959, 2374, 1655, 1213 cm−1. Anal. Calcd (found) for
C12H23N2O2P: C, 55.83 (55.68); H, 8.98 (8.71); N, 10.85 (10.51); P,
12.00 (12.35).
5.1.2. 1·[H][NHnPr]. To a solution of n-propylamine (17.0 μL, 0.21
mmol) in benzene-d6 (2 mL) was added 1 (50 mg, 0.21 mmol). The
solution was transferred to a sealed J. Young NMR tube and allowed to
stand at room temperature for 20 min, yielding 1·[H][NHnPr]: (62
mg, 0.21 mmol, quantitative by NMR). Removal of volatiles in vacuo
gave 1·[H][NHnPr] as a light yellow solid. 1H NMR (360 MHz,
C6D6) δ 8.89 (1H, d, J = 822.0 Hz), 5.43 (2H, d, J = 30.5 Hz), 2.82
(2H, sextet, J = 8.2 Hz), 2.22 (1H, br), 1.23 (2H, t, 8.6 Hz), 1.16
(18H, s), 0.72 (3H, t, J = 8.8 Hz) ppm. 13C NMR (90 MHz, C6D6): δ
149.2 (d, J = 3.6 Hz), 100.5 (d, J = 16.3 Hz), 43.4 (s), 32 (d, J = 3.6
Hz), 27.6 (s), 26.2 (d, J = 3.6 Hz), 11.0 (s) ppm. 31P NMR (145 MHz,
C6D6): δ −46.4 (dtq, J = 815, 30.5, 12.7 Hz) ppm. HRMS (ESI) calcd
for [C15H29N2O2P]+, 301.19667; found, 301.19580. FT-IR (ATR):
3436, 2959, 2871, 2372, 1655, 1215 cm−1. Anal. Calcd (found) for
C15H29N2O2P: C, 60.01 (60.03); H, 9.74 (9.31); N, 9.33 (9.19); P,
10.32 (10.05).
In light of the importance of amine-assisted proton transfer
to the proposed mechanism for N−H oxidative addition,50 we
speculate that the failure of the reductive elimination
experiments described in Section 2.2.2 may have a mechanistic
basis. Only in the presence of exogenous amine capable of
assisting in proton-shuttling is the amine exchange observed.
Absent the exogenous amine, as would be the case in the solid
state, reductive elimination would require transit via high
energy concerted transition structure like TS-conc, and
therefore, the N−H addition adducts are thermally robust.
There are parallels between our findings and other N−H
oxidative addition systems. A similar two-step complexation/
proton-transfer mechanism for E−H oxidative addition has
been established in reactions of the tetrelenes (e.g., carbenes,11
silylenes,51 and germylenes52). This electrophilic activation
mode for p-block elements bears significant resemblance to the
mechanisms that have been advanced for analogous N−H
oxidative addition to d-block transition metals, where metal
ammine complexes are believed to be common intermediates
that precede the oxidative addition product along the reaction
coordinate.53 The qualitative functional equivalency between
transition metal and main group compounds has been
noted.8−10,54
5.1.3. 1·[H][NHMes]. To a solution of 2,4,6-trimethylaniline (29.1
μL, 0.21 mmol) in benzene-d6 (2 mL) was added 1 (50 mg, 0.21
mmol). The solution was transferred to a sealed J. Young NMR tube
and heated to 55 °C for 2 h, yielding 1·[H][NHMes] (78 mg, 0.21
mmol, quantitative yield by NMR). The adduct is stable to column
chromatography under nitrogen atmosphere (neutral alumina, 12:1
pentane/DCM). A single crystalline sample was obtained by slow
evaporation of a concentrated pentane solution of 1·[H][NHMes]
1
under an inert atmosphere at −30 °C. H NMR (360 MHz, C6D6) δ
9.03 (1H, d, J = 839.0 Hz), 6.72 (2H, s), 5.41 (2H, d, 30.7 Hz), 3.43
(1H, d, J = 10.1 Hz), 2.27 (6H, s), 2.13 (3H, s), 0.97 (18H, s) ppm.
13C NMR (90 MHz, C6D6): δ 150.0 (d, J = 3.9 Hz), 137.0 (d, J = 3.1
Hz), 135.5 (d, J = 3.6 Hz), 134.6 (d, J = 1.7 Hz), 100.3 (s), 100.1 (s),
31.8 (d, J = 2.6 Hz), 27.3 (s), 20.7 (s), 19.0 (s) ppm. 31P NMR (145
MHz, C6D6): δ −48.9 (dtd, J = 839.7, 30.5, 9.6 Hz) ppm. HRMS
(ESI) calcd for [C21H33N2O2P]+, 376.2358; found, 377.2351. Anal.
Calcd (found) for C21H33N2O2P: C, 67.04 (67.32); H, 8.84 (8.66); N,
7.45 (7.49); P, 8.23 (8.18).
4. SUMMARY
The cleavage of the N−H bonds of ammonia, alkylamines, and
arylamines by oxidative addition to a geometrically distorted
tricoordinate phosphorus compound presented here represents
a rare well-characterized example of intermolecular N−H
addition to a σ3-phosphorus compound. These reactions
proceed with facility under mild conditions in homogeneous
solution to give structurally robust phosphorane adducts. On
the basis of mechanistic investigations, we conclude that the
observed cleavage of strong N−H bonds by 1 is an entropically
controlled, stepwise process initiated by electrophilic activation
of the amine substrate to give a phosphoranide intermediate.
The rate-determining second step involves a slow amine-
assisted proton transfer to furnish the product hydrido amido
phosphorane. Investigations delineating the extent to which the
planar geometry and corresponding electronic structure of 1
facilitate the electrophilic intermolecular E−H oxidative
addition and the potential transferability of this stepwise
5.2. Calorimetric Methods. Solutions were prepared immediately
before each titration by dissolving 1 (1.0 mM) and n-propylamine
(12.5 mM) in dried, degassed benzene.
ITC experiments were performed using a NanoITC III calorimeter
(TA Instruments, New Castle, DE) equipped with hastelloy cells (V =
1.056 mL). Titrations were carried out at 25 °C using a 250 μL syringe
at a stirring rate of 250 rpm. The sample cell contained 1 and the
reference cell contained benzene. The ‘‘heat flow’’ baseline was allowed
to equilibrate once the reference and sample solutions were loaded
into the cells. Titrations were run as an incremental series of 6 μL
injections of the n-propylamine solution into the solution of 1. Blank
experiments were conducted under identical conditions with only
solvent in the sample cell to experimentally determine the heat of
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mixing of the n-propylamine with the pure solvent. These blanks were
subtracted from the experiments with 1 in the cell and integrated to
isolate the heat evolved from oxidative addition. Data analysis was
performed using NanoAnalyze v2.1 from TA Instruments using the
independent binding sites algorithm. The first integrated heat point in
the data set is anomalous because the syringe allows for a small
amount of n-propylamine to mix during equilibration.
5.3. Computational Methods. Geometries were optimized in
Gaussian 0955 using the M06-2X56 density functional with the 6-311+
+G(2d,2p) basis set. Optimizations were performed with the
continuum CPCM model for acetonitrile.57 Methylamine was used
for a model of n-propylamine; the complete compound 1 was
modeled. TDDFT calculations were performed with CAMB3LYP58
since this method provides accurate estimation of charge-transfer
excited states. Singlet−triplet gaps, electron affinities, and ionization
potentials were calculated using ground-state geometries. See
Supporting Information for further details.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Additional synthetic procedures including the preparation of 1;
1H, 13C, and 31P spectra; crystallographic details for 1·
[H][NHMes]; procedures and data for kinetic studies,
computational details including Cartesian coordinates for
stationary points. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
dhe@chem.byu.edu
radosevich@psu.edu
Notes
■
(14) (a) Jana, A.; Schulzke, C.; Roesky, H. W. J. Am. Chem. Soc. 2009,
131, 4600. (b) Jana, A.; Roesky, H. W.; Schulzke, C.; Samuel, P. P.
Organometallics 2009, 28, 6574.
The authors declare no competing financial interest.
(15) (a) Jana, A.; Objartel, I.; Roesky, H. W.; Stalke, D. Inorg. Chem.
2009, 48, 798. (b) Meltzer, A.; Inoue, S.; Prasang, C.; Driess, M. J. Am.
̈
ACKNOWLEDGMENTS
Chem. Soc. 2010, 132, 3038. (c) Wang, W.; Inoue, S.; Yao, S.; Driess,
M. Organometallics 2011, 30, 6490. (d) Myers, T. W.; Berben, L. A. J.
Am. Chem. Soc. 2013, 135, 9988.
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(19) Arduengo, A. J.; Dixon, D. A. In Heteroatom Chemistry; Block,
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■
Financial support for this work was provided by the
Pennsylvania State University. We thank Profs. Mark
Maroncelli and Christine Keating (Penn State) for use of
spectrophotometric equipment and Ting-Yi Lai (Penn State)
for experimental assistance. D.H.E. thanks BYU and the Fulton
Supercomputing Lab.
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