Structures, Electronics, and Reactivity of Strained Phosphazane Cages: A Combined Experimental and Computational Study

Article abstract of DOI:10.1021/acs.inorgchem.5b01292

A series of formamidine-bridged P₂N₂ cages have been prepared. Upon deprotonation, these compounds serve as valuable precursors to hybrid N-heterocyclic carbene ligands, whereas direct metalation gives rearranged dimetallic complexes as a result of cleavage of the formamidine bridge. The latter metal complexes contain an intact cyclophosphazane moiety that coordinates two distinct metal centers in a monodentate and a chelating fashion. A computational study has been carried out to elucidate the bonding within the P₂N₂ framework as well as the reactivity patterns. Natural bond orbital analysis indicates that the cage motif is poorly described by localized Lewis structures and that negative hyperconjugation effects govern the stability of the bicyclic framework. The donor capacity of the cyclophosphazane unit was assessed by inspection of the frontier molecular orbitals, highlighting the fact that π-back-donation from the metal fragments is crucial for effective metal-ligand binding.

Full text of DOI:10.1021/acs.inorgchem.5b01292

Article
pubs.acs.org/IC
Structures, Electronics, and Reactivity of Strained Phosphazane
Cages: A Combined Experimental and Computational Study
Torsten Roth,† Vladislav Vasilenko,† Hubert Wadepohl,† Dominic S. Wright,*,‡ and Lutz H. Gade*,†
†Anorganisch-Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
̈
‡Chemistry Department, Cambridge University, Lensfield Road, Cambridge, CB2 1EW, U.K.
S
* Supporting Information
ABSTRACT: A series of formamidine-bridged P2N2 cages
have been prepared. Upon deprotonation, these compounds
serve as valuable precursors to hybrid N-heterocyclic carbene
ligands, whereas direct metalation gives rearranged dimetallic
complexes as a result of cleavage of the formamidine bridge.
The latter metal complexes contain an intact cyclophosphazane
moiety that coordinates two distinct metal centers in a
monodentate and a chelating fashion. A computational study
has been carried out to elucidate the bonding within the P2N2
framework as well as the reactivity patterns. Natural bond
orbital analysis indicates that the cage motif is poorly described
by localized Lewis structures and that negative hyper-
conjugation effects govern the stability of the bicyclic
framework. The donor capacity of the cyclophosphazane unit was assessed by inspection of the frontier molecular orbitals,
highlighting the fact that π-back-donation from the metal fragments is crucial for effective metal−ligand binding.
INTRODUCTION
■
Dimeric dichlorocyclophosph(III)azanes, [ClP(μ-NR)]2, are
versatile precursor materials for the preparation of a diverse
range of cyclic and acyclic P,N-containing compounds.1 A key
feature of the chemistry of their P2N2 rings is the syn
orientation of the exocyclic Cl substituents and the resulting C2
rotational axis that is frequently retained in the solid and
solution states. A major influence on the relative stability of the
cis and trans isomers is the steric bulk of the acyclic R groups
within the P2N2 motif. A widely used approach to constrain the
cis geometry relies on the introduction of a rigid linker unit,
which bridges one face of the P2N2 plane, by nucleophilic
substitution of the Cl atoms.2,3 Such linkers have been based on
diol or diamino groups or may contain further heteroatoms or
metal centers (A and B in Figure 1).4
Typically, the linking unit comprises at least three atoms,
although recent work by Schulz and co-workers established the
formation of strained P,N-cage compounds (C) via interme-
diary phosphorus biradicaloids.5 Notably, the rearrangement of
Figure 1. Selected P2N2 rings and related frameworks.
strained P2N2 cages may be exploited to access architecturally
novel and diverse heterocyclic frameworks (D and E).6
Recently, we reported the preparation and crystallographic
characterization of a hybrid cyclophosphazane−formamidinium
salt (F) and its subsequent deprotonation and metalation
(Scheme 1).7 In this way, a variety of structurally unique N-
heterocyclic carbene (NHC) metal complexes comprising an
intact bicyclic P,N-framework were accessed via the sequential
introduction of different metal fragments.
Here, we report the synthesis of a series of sterically and
electronically diverse cationic cages of type F, a systematic
investigation of their properties, and their coordination to late-
transition-metal fragments. Such coordination is accompanied
by cage opening at the formamidine bridge and contrasts with
Received: June 10, 2015
© XXXX American Chemical Society
A
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Scheme 1. Deprotonation/Metalation Sequence of Cationic
a
P2N2 Cages Leading to NHC Metal Complexes
a
Reaction conditions: (i) KHMDS, toluene, room temperature; (ii)
[Ir(cod)Cl]2, toluene, room temperature.
retention of the cage structure in the carbene complex G. The
observed reactivity motivated a computational study, and the
bonding situation encountered was analyzed by means of
natural bond orbital (NBO) theory.
Figure 2. Molecular structure of 4a. Thermal ellipsoids are set at the
50% probability level. H atoms have been omitted for clarity. Selected
bond lengths [Å] and angles [deg]: P(1)−N(1) 1.7221(12), P(1)−
N(3) 1.7296(13), N(1)−C(1) 1.3911(17), N(2)−C(1) 1.2737(17);
N(1)−P(1)−P(2) 116.04(4), N(3)−P(1)−N(4) 79.96(5), N(2)−
C(1)−N(1) 122.75(12).
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of Cationic
Cyclophosphazane Cages. We initially focused on the
stepwise formation of the desired cationic cage compounds,
using the dichlorocyclophosph(III)azane 1a, the formamidine
2a, and an excess of triethylamine to produce the unsymmetri-
cally substituted P2N2 species 4a (Scheme 2, top). The latter
compound displays two doublet signals in the 31P{1H} NMR
spectrum (δP1 = 188.42 ppm; δP2 = 130.64 ppm; JPP = 38.6 Hz)
and was structurally characterized by X-ray diffraction (Figure
2).
generalized one-pot process starting from dichlorocyclophosph-
(III)azanes 1a−1d and using the corresponding silyl-protected
formamidines 3a−3d proved to be the most efficient route to
the target cages 5a−5h (Scheme 2), giving the products in
moderate to high yields (40−90%). Single crystals suitable for
X-ray diffraction were obtained for 5b−5g, and their solid state
structures were determined. Selected structural parameters of
all cationic cages are summarized in Table 1.
A comparison of the molecular structural parameters
established for 5b−5g supports the view of a rigid central
cage framework that is only marginally influenced by the
peripheral substituents. In contrast, the organic substituents on
the P2N2 ring themselves are oriented in such a way as to avoid
Although condensation of 1a and 2a selectively yielded the
monosubstituted cyclophosphazane, the cationic product (5a)
of the reaction of 4a with Me3Si-OTf as the chloride scavenger
degraded in solution.8 For this reason, more sterically shielding
and electron-rich formamidines were chosen to stabilize the
cationic frameworks. From a practical point of view, a
Scheme 2. Preparation of Cationic Cyclophosph(III)azane Cages 5a−5h via a Two-Step Procedure (Top) or a One-Pot
Protocol (Bottom)
B
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Table 1. Schematic Representation of the Cage Motif and Selected Structural Parameters of Compounds 5b−5g
d [Å]
angle [deg]
a
b
c
P−Nc
P−Nf
N−C−N
ϕ
5b
5c
5d
1.7001(13)−1.7399(14)
1.711(2)−1.732(2)
1.6987(12)−1.7412(12)
1.704(3)−1.716(3)
1.706(3)−1.723(3)
1.699(2)−1.724(2)
1.7960(13), 1.8076(13)
1.787(2), 1.797(2)
122.6(1)
122.1(2)
122.6(1)
122.7(3)
124.3(4)
123.3(2)
25.50(8)
23.5(1)
30.53(7)
31.4(1)
30.7(2)
31.2(1)
1.8049(12), 1.8082(12)
1.823(3), 1.837(3)
d
5e
5f
1.821(3), 1.834(3)
5g
1.810(2), 1.815(2)
a
b
P−Nc denotes the range of P−N bond distances within the P2N2 ring. P−Nf denotes the range of P−N bond distances between the P2N2 P atom
and the formamidine N atom. ϕ denotes the angle between the planes P−N(1)−P and P−N(2)−P. Taken from ref 7.
c
d
Figure 3. Solid-state structures of 5c (left), 5d (center), and 5g (right). In the graphical representation of 5d, the biphenyl units are drawn as
wireframes. H atoms, except for C(1)−H, are omitted for clarity. The disorder in 5g is not shown. For the molecular structures of 5b, 5e, and 5f, see
the SI.
Scheme 3. Ring-Opening Reaction of Cationic Cages Induced by Late-Transition-Metal Fragments
unfavorable steric interactions with the o-iPr and Me
substituents of the formamidine aryl groups (see Figure 3
and the Supporting Information, SI). A consistent feature of all
of the cationic cages is the elongated bond between the P2N2 P
atom and the formamidine unit (P−Nf = 1.79−1.83 Å)
compared to the P−N distances within the P2N2 rings (P−Nc =
1.71−1.72 Å).9 In contrast, the exo-P−N bonds in unstrained
bis(amino)cyclophosph(III)azanes are typically significantly
shorter (1.68−1.71 Å).9 Moreover, the N−C−N angles at the
formamidine bridges of the cationic cages 5b−5g closely
resemble those found in the protonated formamidinium salts of
[2a + H][OTf] and [2b + H][OTf], indicating that any strain
within the cage structures is mainly located at the P2N2 motif.
This is supported by the significantly larger puckering angles
corresponding dichlorocyclophosphazanes (e.g., 1a, ϕ =
9.13°; 1b, ϕ = 0.18°).10
Metalation of the Cationic P2N2 Cages. The deproto-
nation of compound 5e yielding an NHC and its coordination
chemistry with selected late-transition-metal fragments has
been reported by us previously.7 In the current work, the focus
is on the strained cationic cages themselves and their potential
use as P donors for late-transition-metal complexes.11 The
coordination properties were assessed in reactions with
iridium(I), rhodium(I), and palladium(II) precursors (giving
the products 6a−6d; Scheme 3). In all of the reactions
investigated, clean formation of single products was observed
after the reagents were mixed for 30 min at room temperature.
Significantly, each of the products ([6a−6c]OTf) exhibited two
doublet signals in the 31P NMR spectrum, indicating
desymmetrization of the P2N2 unit.12 The molecular structures
of the three isostructural derivatives [6a−6c]OTf were
(ϕ) of the P2N2 units found in compounds 5b−5g [ϕrange
=
23.5(1)−31.4(1)°; Table 1] compared to those in the
C
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Figure 4. Molecular structure of [6b]OTf (left) and [6c]OTf (right). Thermal ellipsoids are set at the 50% probability level. H atoms, except for
H(1), have been omitted for clarity. Phenyl rings of the benzhydryl substituents are drawn as wireframes. For selected bond lengths and angles, see
Table 2.
a
Table 2. Selected Structural Parameters of Compounds [6a−6c]OTf
d [Å]
angle [deg]
b
M(1)−P(1)
M(2)−P(2)
P−Nc
P(1)−N(3)
C(1)−N(3)
C(1)−N(4)
M(1)−N(4)
N−C−N
6a
6b
6c
2.217(2)
2.232(1)
2.2131(4)
2.215(2)
2.218(1)
1.696
1.710
1.695
1.779(5)
1.764(3)
1.790(3)
1.370(8)
1.366(5)
1.372(2)
1.285(8)
1.301(5)
1.294(2)
2.071(5)
2.079(3)
2.063(1)
123.5(5)
123.9(4)
123.6(1)
2.2096(4)
a
b
For the atom labeling, see Figure 4. P−Nc denotes the average P−N bond distances within the P2N2 ring.
established by X-ray diffraction and revealed monodentate
coordination of one metal fragment to one of the P atoms of
the opened cage and chelating κ2-P,N coordination of the
second metal center (Figure 4 and SI). The five-membered
PN2CM metallacyclic fragments of [6a−6c]OTf adopt
essentially planar conformations and are arranged orthogonally
to the intact P2N2-ring units. In addition, cis coordination of the
terminal and chelated metal centers with respect to the P2N2
cores is also observed. The M−C (cod/nbd) bonds that are
located trans to the P atoms are slightly longer than the M−C
bonds that are trans to chloride ligands, suggesting a significant
trans influence of the P2N2−P unit compared to the chloride
ligand. The M−P bond lengths are, however, all within the
expected range of 2.21−2.23 Å, as are the metal−imine bonds
(ca. 2.07 Å).13 The differing bond lengths within the N−C−N
unit (mean 1.29 vs 1.37 Å within [6a−6c]OTf) may indicate
little or no delocalization in contrast to the cationic cages.
A variable-temperature (VT) 31P NMR spectroscopic study
of the palladium complex [6d]OTf revealed dynamic behavior
in solution (Figure 5). At low temperatures (−40 °C), the
resonances of a 1:1 mixture of prone and supine isomers of
[6d]OTf (depending on the relative orientation of the two η3-
methallyl ligands) were observed, while at elevated temper-
atures, a new set of signals emerged.14 This second species is
interpreted as resulting from the reversible decoordination of a
[Pd(2-Me-allyl)Cl] fragment from the palladium(II) complex
[6d]OTf, giving the mononuclear complex [6d-mono]OTf
(Scheme 4).15 The reaction enthalpy for decoordination (ΔH =
42 kJ mol−1) was found to be similar to that previously
reported for the analogous process in the related NHC
cyclophosphazane iridium complex shown in Scheme 6.7
The cage-opening reaction seen in the formation of the metal
complexes 6a−6d (Scheme 3) can be regarded as occurring by
chloride-induced ring opening and subsequent metal coordi-
nation.16 This view is supported by the finding that the cationic
cage 5a readily reacts with a stoichiometric amount of [nBu4N]
Figure 5. VT 31P NMR study of [6d]OTf. Signals marked by asterisks
correspond to the dimetallic complex 6d; the hashtag denotes the
resonances corresponding to the monometallic complex [6d-mono]-
OTf.
Cl as a chloride source to form the corresponding ring-opened
product 4a (Scheme 5). In this context, the (reverse) ring-
closing reaction of species like 4a using Me3Si-OTf can be
regarded as part of an anion-dependent equilibrium (Scheme
5). Notably, a similar ring-opening reaction to 5a is not
observed when the cationic cage was deprotonated prior to
metalation to give the corresponding carbene ligand.7 This
contrasting reactivity (Scheme 6) indicated that the stability of
the molecular cages reported in this work is governed by subtle
electronic effects within the hybrid P2N2 framework, a feature
that is explored using density functional theory (DFT)
calculations later in the current work.
Computational Studies. In order to probe the reasons for
the differences in reactivity between the phosphazane cations
and the carbenes, DFT calculations were performed on a model
cation (type I), free carbene (type II), and dimetallic iridium(I)
complex (type III) (Figure 6). Notably, representatives of all of
D
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a
Scheme 4. Reversible Coordination/Decoordination of [Pd(2-Me-ally)Cl]2 to the Palladium Complex [6d-mono]OTf
a
In solution, diastereomers of [6d]OTf are accessible by rotation around the highlighted bonds.
Scheme 5. Chloride-Induced Ring-Opening and Ring-
Closing Reactions of the Cationic Cage 5a (Ar = 4-F-Ph)
Figure 6. Structural types explored using model DFT calculations
([Ir] is an organometallic iridium(I) fragment, e.g., G; Scheme 1).
these species have been characterized previously by us or
reported in this study, and there is thus a range of
crystallographic data available for comparison with computed
structures.
All computations were performed on simplified model
systems (M1−M3 in Figure 7), where substituents on the
cage frameworks were replaced by methyl groups and the
ligand set on IrI consisted of two π-bonded ethylene ligands
and a chloride. During the course of the structural studies
described above, we had found that the bicyclic core motif is
only negligibly perturbed by the N substituents, rendering this
simplification a legitimate approximation (see Table 1).
Selected computed geometrical parameters as well as highest
occupied molecular orbital (HOMO)/lowest unoccupied
molecular orbital (LUMO) and promotion energies are
presented in Table 3. Initial computational studies employing
the standard B3LYP functional and 6-311++G** basis set gave
poor agreement with experimentally determined molecular
geometries, even when the full structures were modeled. In
Figure 7. Molecular structures of models M1−M3 at the PBE0-D/
def2-TZVP level of theory. From left to right: M1 is the cationic cage,
M2 the carbene, and M3 the iridium(I) dimetallic complex. Color
code: turquoise, Ir; dark green, Cl; purple, P; blue, N; gray, C; white,
H.
particular, the most significant deviations were observed for the
P−Nf bond to the formamidine bridge, which was computed to
be generally more than 0.05 Å longer than that found in crystal
structure analyses. In order to ensure that the computed
geometries were accurate, a screening of basis sets and
functionals was performed, and the combination of the
Scheme 6. Complementary Reactivity of Cationic P2N2 Cages in the Presence of Chloride-Containing Metal Precursors: (Left)
a
Ring-Opening Reaction; (Right) Deprotonation and Metalation
a
Reaction conditions: (i) [Ir(cod)Cl]2, CH2Cl2, room temperature; (ii) KHMDS, toluene, room temperature; (iii) [Ir(cod)Cl]2, toluene, room
temperature.
E
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see Figure 8 and the SI. While the FMOs share several common
features, a few significant differences can be found. The Kohn−
Sham HOMO of the carbene model M2 is mainly located along
the P−Nf bonds to the formamidine bridge, with additional
contributions from the lone pairs of Nc and a large coefficient
for the carbene lone pair. The HOMO of M2 is therefore
perfectly suitable for σ coordination to a metal fragment at the
P and carbene centers. In contrast, the HOMO of the cationic
cage M1 is located around the P−Nc bonds of the P2N2 ring,
with the P lobes being much closer to the P2N2 plane.18 All
LUMOs of M1−M3 were found to have π symmetry.
Table 3. Core Structure of the Model Systems and Selected
Calculated Metric Parameters of Models M1−M3 at the
PBE0-D/def2-TZVP Level of Theory (Top) and Kohn−
Sham HOMO/LUMO Energies and Gaps (Bottom)
The composition, symmetry, and relative energies of the
FMOs as well as their ordering have profound implications for
the bonding of metals to the cation M1 and the carbene M2.
Given the FMOs of the free carbene M2, σ donation via the
carbene center should lead to weaker, longer P−Nf bonds by
depletion of the electron density from the HOMO. This is in
line with the particularly long P−Nf bonds of the cationic cage
M1. In contrast, the structures of the free carbene M2 and the
iridium complex M3 are almost identical with respect to the P−
Nf bond length. In part, this difference between M1 and M2
and M3 can be attributed to the lower Lewis acidity of the
neutral iridium fragment compared to a proton. Additionally, π-
back-donation into the LUMO of M2 can occur for electron-
rich d metals like iridium, thus reenforcing the P−Nf bonds.
Bonding of metal fragments via the P atoms can be explained in
a similar fashion. Following the analysis outlined above, σ
coordination of the P atom to a metal should, in principle,
produce weaker P−Nf bonds. In practice, however, complexes
containing P-coordinated metals feature much shorter P−Nf
bonds, indicating that σ coordination merely plays a secondary
role compared to π-back-bonding. In addition to its donation
properties, the cage M2 is also an excellent π acceptor featuring
a low-lying LUMO. Accordingly, donation of electron density
from filled metal d orbitals into the LUMO accounts for a
major part of the P−M bond. This is in agreement with the
results reported by Frenking et al., who found that bonding of
weak donor ligands such as PCl3 to transition metals,
electrostatic effects aside, mainly consists of π-back-bonding.19
Most importantly, interactions of this type produce stronger
P−Nf bonds by donation of electron density into the LUMO,
that is, bonding with respect to the P−Nf bond.
d (Å)/α (deg)
bond/angle
M1
M2
M3
a
b
Nf−P
Nc−P
1.81
1.71
1.76
1.73
1.75, 1.72
a
1.71, 1.73
b
1.68, 1.71
Nf−C−Nf
123.7
114.9
117.2
E/ΔE (eV)
orbital
M1
M2
M3
HOMO
LUMO
gap
−11.25
−5.96
5.29
−5.39
−1.42
3.97
−5.62
−2.26
3.37
a
b
Bond lengths for the noncoordinated P. Bond lengths for the
coordinated P.
dispersion-corrected PBE0 functional and def2-TZVP basis set
was found to give the best results (for details, see the
Experimental Section and SI).
A comparison of theoretical models M1−M3 (Figure 7) with
the experimentally determined structures (see Table 1) reveals
close agreement with respect to the P2N2 units (Table 3). A
noticeable contraction of the P−Nf bond length is seen upon
going from the cation (M1, 1.81 Å) to the carbene (M2, 1.76
Å) and to the dimetallic complex (M3, 1.72 and 1.75 Å for the
Ir-coordinated and uncoordinated P atoms, respectively). This
trend is in excellent agreement with the parameters found in
the experimental structures of the cations (range 1.79−1.83 Å;
Table 1) and in the previously reported iridium(I) dimetallic
complex G [1.756(3) and 1.760(3) Å; Scheme 1].7,17
In order to obtain a qualitative understanding of the
underlying bonding in these species and the changes that
accompany metal deprotonation and metal coordination, we
analyzed the Kohn−Sham frontier molecular orbitals (FMOs).
For a graphical representation of selected FMOs for M1−M3,
To explore the bonding situation in the cages further, all
model compounds were investigated by means of NBO
analysis. Analysis of the models M1−M3 shows that these
cages are only poorly described by localized bonding alone.
This delocalization extends beyond that expected over the
Figure 8. Kohn−Sham FMOs of models M1 and M2 computed at the PBE0-D/def2-TZVP level of theory. An isovalue of 0.05 was used.
F
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Figure 9. Stabilizing (left) and destabilizing (right) donor−acceptor contributions to the P−Nf bonds in the model cation M1.
formamidine bridges of M1−M3. Owing to strongly
delocalized bonding, small changes in the cage structures, e.g.,
protonation of the carbene center or metal coordination to the
P centers, can have a dominant effect on the properties of the
cage as a whole. An overview of the most prominent NBO
interactions in M1 is shown in Figure 9.
Two major types of interactions can be distinguished in the
cages: (i) P−Nf destabilizing contributions and (ii) P−Nf
stabilizing contributions (see Table 4). These are shown for
Table 5. (Lone-Pair) Hybridization for Model Compounds
M1−M3
hybridization (spn)
#
Nf
Nc
P
C
M1
M2
M3
p1
p1
p1
sp9.28
sp5.72
sp8.00
sp0.63
sp0.71
sp1.44
sp2.17
sp1.21
sp1.60
a
a
Coordinated to iridium.
Table 4. Summary of Selected Donor−Acceptor Interactions
a
in Models M1−M3
As has been shown by Frenking and co-workers, weak
donors like PF3 and PCl3 have a very high s character in the free
ligand (76% and 80%), which is substantially reduced upon
coordination to transition metals, e.g., to 19% and 30% in
chromium pentacarbonyl complexes [Cr(CO)5PX3].19 Sim-
ilarly, deprotonation of the cationic cage M1 decreases the s
character only to a small extent (61% in M1 vs 58% in M2),
whereas coordination of iridium gives more directional bonds
with 41% s character in M3. Remarkably, the p character of the
metal-coordinated cage M3 is even lower than that in
complexes of the extremely weak donors PCl3 and PF3, thus
underlining the extraordinarily low P−σ-donor strength of this
class of compounds. These findings are in agreement with the
results obtained from analysis of the FMOs, where the P2N2
backbone was identified as excellent π-acceptor and poor σ-
donor units.
E(D→A) (kcal mol−1
)
n(Nc) →
σ(C−X) →
σ*(Nf−P)
n(Nf) →
σ*(P−Nc)
n(P) →
#
σ*(P−Nf)
σ*(Nf−C)
M1
M2
M3
41.0
30.4
36.4
10.1
28.8
19.2
7.6
11.6
10.8
10.5
14.2
14.3
a
In the case of several analogous interactions, the total sum is
provided. Stabilizations ΔE were determined as estimates calculated
from second-order perturbation theory.
the cation M1 in Figure 9. The relative strength of these
interactions is critically dependent on the structure of the
complex. Donation of electron density from the n(Nc) lone pair
into the σ*(P−Nf) orbital is particularly pronounced in the
cationic cage compound M1, resulting in weakened P−Nf
bonds (Figure 9, right). This is caused by a smaller energetic
splitting between the donor and acceptor orbitals in the
cationic model. Additional destabilization of this bond arises
from donation of electron density from the (pre)carbenic
center into the σ*(P−Nf) orbital. The extent of this interaction
depends on the bonding partner of the carbene C atom and
follows the order n(C) > σ(C−Ir) > σ(C−H). Stabilization of
the P−Nf bonds mainly occurs through donation of electron
density from the Nf lone pairs into the σ*(P−Nc) orbital and
from the P lone pairs into the σ*(Nf−C) orbitals (Figure 9,
left). In line with the lability of the P−Nf bond in the cationic
cage, these stabilizing interactions were found to play only a
minor role in M1, overall leading to a significantly weaker
attachment of the bridging formamidinium unit to the
cyclophosphazane ring, compared to all other structures. For
a summary of the most prominent donor−acceptor inter-
actions, see Table 4.
CONCLUSION
■
We have developed a general method for the preparation of
bicyclic formamidinium-bridged cyclophosphazanes via either a
two-step protocol or a one-pot procedure. The molecular
structures of these compounds were established by X-ray
structure analysis, corroborating a highly strained cage
framework shielded by the peripheral substituents. Subsequent
metalation of the cationic cages without prior conversion to a
carbene ligand results in rearrangement of the phosphazane
frameworks, as seen in the resulting dinuclear rhodium, iridium,
and palladium complexes, in which cleavage of the formamidine
bridges had occurred. This reactivity pattern contrasts with that
of the carbenes (derived from deprotonation of the cations),
which coordinate transition-metal fragments without degrada-
tion of the phosphazane units.
Analysis of the Kohn−Sham FMOs and NBOs in model
cation, carbene, and carbene−transition metal complexes
highlights the important influence of delocalized bonding on
their reactivities, stabilities, and coordination properties. In
particular, calculations reveal that, like the P atoms of the
derived carbene ligands, the P atoms of the cations have
predominantly π-acceptor character and very low σ-donor
character. These studies inform future work in this area by
providing a potential basis for the ways in which the acceptor
The poor P−σ-bonding properties of cages are reflected in
the hybridization of P lone pairs (Table 5) because
coordination of a metal to the P atoms usually leads to
significant polarization of the lone pair and thus to higher p
character. Therefore, the donor ability of a phosphine is best
assessed by comparing the p character observed upon
coordination of the metal fragment (M2 → M3).
G
DOI: 10.1021/acs.inorgchem.5b01292
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
dispersion-corrected PBE0 functional,27,28 employing the RIJCOSX
approximation, as implemented in the ORCA 3.0 software suite.29 The
absence of imaginary frequencies was confirmed by numerical
frequency calculations.
and donor character of related main-group ligand systems
might be tuned by structural changes.
EXPERIMENTAL SECTION
■
All manipulations were carried out under exclusion of air and moisture
using standard Schlenk and glovebox techniques. As inert gas, Argon
5.0, purchased from Messer Group GmbH, was used after drying over
Granusic phosphorpentoxide granulate. Solvents were dried over
activated alumina columns using a solvent purification system (M.
Braun SPS 800) or according to standard literature methods20 and
stored in glass ampules under an argon atmosphere. Degassed solvents
were obtained by three successive freeze−pump−thaw cycles.
Phosphorus trichloride was distilled prior to use, and triethylamine
was degassed. NMR spectra were recorded on Bruker Avance (400,
500, and 600 MHz) instruments. Chemical shifts (δ) are reported in
parts per million (ppm) and are referenced to residual proton solvent
signals or carbon resonances.21 H3PO4 (31P) and CCl3F (19F) were
used as external standards. High-resolution mass spectrometry (MS)
spectra were acquired on Bruker ApexQe hybrid 9.4 T FT-ICR (ESI)
and JEOL JMS-700 magnetic sector (FAB, EI, and LIFDI)
spectrometers at the mass spectrometry facility of the Institute of
Organic Chemistry, University of Heidelberg. Elemental analyses were
carried out in the Microanalysis Laboratory, Chemistry Department,
University of Heidelberg, on a vario MICRO cube (Elementar). All
chemicals were obtained from commercial suppliers and were used
without further purification. The dichlorocyclophosphazanes 1a−1d,3
formamidines,22 and silylated formamidines23 were prepared following
established procedures.
ASSOCIATED CONTENT
■
S
* Supporting Information
Methods and characterization data, details of the computational
studies, optimized structures, and CIF files giving crystallo-
graphic data. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.inorgchem.5b01292.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: dsw1000@cam.ac.uk.
*E-mail: lutz.gade@uni-heidelberg.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the award of a Ph.D. grant to T.R.
from Landesgraduiertenforderung (LGF Funding Program of
̈
the State Baden-Wurttemberg), the award of a national
̈
scholarship (Deutschlandstipendium) to V.V., the University
of Heidelberg for generous funding, and the ERC-Advanced
Investigator Grant to D.S.W. We are grateful to Sebastian
Intorp and Clemens Blasius for experimental support.
General Procedure for the One-Pot Preparation of Cationic
Cyclophosphazane Cages 5a−5h. To a solution of the
dichlorocyclophosphazane 1 (1 equiv) in toluene was added a
solution of the silylated formamidine 3 (1 equiv) in toluene dropwise
at room temperature. The mixture was stirrred for 1 h and then cooled
to −40 °C, and a cooled solution of TMS-OTf (1 equiv) in toluene
(10 mL) was added dropwise. After the indicated time, the reaction
mixture was filtered, and the residue was washed with toluene and
dried in vacuo, affording product 5 as a colorless solid. For full
analytical and spectroscopic characterization of 5a−5h, see the SI.
REFERENCES
■
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Exemplary Data for 5c: colorless solid, 44%. H NMR (THF-d8,
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COMPUTATIONAL DETAILS
■
The computational software ORCA 3.0 was used for geometry
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H
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Inorganic Chemistry
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1
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I
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Inorg. Chem. XXXX, XXX, XXX−XXX