Charge-Assisted phosph(v)azane anion receptors

Article abstract of DOI:10.1039/d0dt00489h

Coordination of Cu(i) or Pd(ii) to seleno-cyclodiphosph(v)azanes of the type [RNH(Se)P(μ-N^tBu)]₂ results in positively charged anion receptor units which have increased anion affinity over the neutral seleno-phosph(v)azanes, due to the increase in electrostatic interactions between the receptor and the guest anions. The same effect is produced by replacement of one of the PSe units by a P-Me⁺ unit.

Full text of DOI:10.1039/d0dt00489h

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Charge-assisted phosph(V)azane anion receptors†
Alex J. Plajer,* Sanha Lee, Andrew D. Bond,
Dominic S. Wright
Jonathan M. Goodman
and
Cite this: DOI: 10.1039/d0dt00489h
*
Received 10th February 2020,
Accepted 25th February 2020
DOI: 10.1039/d0dt00489h
rsc.li/dalton
Coordination of Cu(I) or Pd(II) to seleno-cyclodiphosph(V)azanes of result of electron transfer to the transition metal and the
the type [RNH(Se)P(μ-NtBu)]2 results in positively charged anion resulting polarisation of the N–H bonds, as well as pre-organis-
receptor units which have increased anion affinity over the neutral ation of the N–H groups for optimum anion coordination.16
seleno-phosph(V)azanes, due to the increase in electrostatic inter- For example, the binding constant and anion transport activity
actions between the receptor and the guest anions. The same of the neutral metal-phosphazane receptor [(5-CF3-2-Pyridyl)
effect is produced by replacement of one of the PvSe units by a NHP(µ-NtBu)]2(Rh(CO)Cl)2 is one to two orders of magnitude
P–Me+ unit.
greater than the phosphazane [(3,5-(CF3)2C6H3NH)(Se)P
(μ-NtBu)]2, despite the presence of only one CF3 group within
the pyridyl substituent.
Small organic molecules such as ureas (A) and squaramides
(B), which are capable of acting as H-bonding anion
receptors (Fig. 1), have been employed in a number of impor-
tant areas such as anion-sensing, counter-anion catalysis and
membrane transport.1–4 Recently, inorganic systems of this
type based on non-carbon frameworks have emerged as viable
alternatives.5–11 Goldfuss and coworkers were the first to
explore the anion binding of cyclodiphosph(V)azanes of the
type [(3,5-(CF3)2C6H3NH)(E)P(μ-NtBu)]2 (C) (E = O, S) (Fig. 1)
and found that the high anion affinity of the oxygen derivative
makes it useful in counterion-catalysis of the N-acyl-Mannich
reaction.12–14 Contrary to our initial expectations, we found
recently that selenium derivatives of this class of anion recep-
tor have similar if not higher anion binding constants than
those containing more electronegative oxygen.15 One particu-
lar advantage of the selenium receptors is that they are air-
and water-stable and can therefore be used as synthetic anion
transporters under aqueous conditions through phospholipid
bilayers.
In this communication we investigate the effects on anion
binding of using cationic receptors based on seleno-cyclodi-
phosph(V)azanes. We show that receptors of the type [RNH(Se)
P(μ-NtBu]2 have the ability to coordinate the anion and cation
components of a salt cooperatively (Fig. 2) and that the chela-
tion of a cationic metal component to the Se atoms has two
major effects, (i) increasing electrostatic interaction between
the receptor unit and coordinated anions by giving the recep-
tor net positive charge, and (ii) increasing the polarisation of
the N–H bonds by net electron transfer to the transition metal
fragment. We also show that the geometry of the N–H binding
site can be altered drastically by transition metal coordination
Metal coordination to phosph(III)azanes has an even greater
effect on anion binding than oxidation of the PIII centres with
Group 16 elements. This is particularly the case for P,N-chela-
tion of transition metals by the pyridyl-functionalised phosph
(III)azanes of the type [(2-py′)NHP(μ-NtBu)]2 (D) (Fig. 1), as a
Chemistry Department, Cambridge University, Lensfield Road, Cambridge CB2 1EW,
UK. E-mail: dsw1000@cam.ac.uk
†Electronic supplementary information (ESI) available: Syntheses of com-
pounds, spectroscopic data, single-crystal X-ray analysis, DFT calculations of
anion binding. CCDC 1890515, 1890518 and 1890520. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/d0dt00489h
Fig. 1 Anions receptors, (A) urea, (B) squaramide, (C) cyclodiphosph(V)
azane, (D) metal-cyclodiphosph(III)azane.
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give the complexes [(Ph3P)Cu(3)BF4] (4) and [(Ph3P)2Pd(3)
BF4][BF4] (5), respectively, which were isolated in high crystal-
line yield after workup (74% for 4, 95% for 5) (see ESI†). While
4 forms neutral molecules, [{tBuNH(Sev)P(µ-NtBu)}2(CuBF4)],
in the solid state,
5 has an ion-separated structure,
−
[{tBuNH(Sev)P(µ-NtBu)}2(PdBF4)]+BF4
.
Neutral 4 and the
cation of 5 (Fig. 4) exhibit the expected chelation of the copper
−
(I) and palladium(II) centres by the Se atoms of 3, with a BF4
anion being chelated on the other side of the phosph(V)azane
framework by two N–H⋯F H-bonds. It should be noted in this
respect that there is some uncertainty concerning the precise
coordination mode in both cases due to apparent rotational
Fig. 2 The cooperative effect concept explored in the current study.
−
disorder of the BF4 anions in both crystal structures.
and that this effect can be used to switch on or off the binding
of anions by the N–H groups. Finally, replacement of one of
the PvSe units by a P–Me+ unit, giving non-metal based recep-
tors with net positive charge, has the same effect as metal
coordination in enhancing anion binding to the N–H groups.
An initial indication of the ability of seleno-cyclodiphosph
(V)azanes to coordinate the cation and anion components of a
salt was provided by the formation and solid-state structure of
the complex [(NH2)(Se)P(µ-NtBu)]·CuCl·THF (2), obtained from
the reaction of the new seleno-cyclodiphosph(V)azane [(NH2)
(Se)P(µ-NtBu)] (1) with CuCl in THF. The single-crystal X-ray
structure shows a polymeric arrangement in which intact CuCl
monomers are coordinated side-on by the Lewis base donor Se
and H-bond donor NH2 groups of 1, with the retention of Cu–
Cl bonding (Fig. 3). In addition, 1 also employs its N–H func-
tionality in a bifurcated H-bonded interaction with a THF
molecule.
However, the N–H⋯F–B interactions involved are well within
the sum of the van der Waals radii of H and F (ca. 2.0 Å for 4
and 5 vs. ∑vdW = 2.5 Å).
A measure of the effect of metal coordination on the polar-
ization of the N–H protons is provided by the chemical shifts
of the N–H protons in the room-temperature, solution 1H
NMR spectra of 3, 4 and 5 in CD2Cl2, with a large increase in
The observation of cooperative cation and anion coordi-
nation in 2 led us to explore the impact of chelation of the Se
atoms on the H-bonding binding site of the related receptor
[tBuNH(Sev)P(µ-NtBu)]2 (3).17 The reactions of 3 with [(Ph3P)
Cu(CH3CN)3](BF4) and [(Ph3P)2Pd(CH3CN)2](BF4)2 in CH2Cl2
Fig. 4 Molecular structures of (a) the CuI complex 4 and (b) the cation
−
of 5. Disorder of the BF4 ions in both and H-atoms, except those
involved in the H-bonding interactions, and a CH2Cl2 solvent molecule
in the lattice of 4 have been omitted for clarity. Selected bond length [Å]
and angles [°]: 4, Nendo–P av. 1.68, Nexo–P av. 1.61, P–Se av. 2.14, Se–Cu
av. 2.40, P–Cu 2.24, Nexo–P–Nendo av. 112.5, P–N–P av 94.5, Se–Cu–Se
122.2, P–Cu–Se av. 118.8; 5, Nendo–P av. 1.68, Nexo–P av. 1.61, P–Se av.
2.16, Se–Pd av. 2.52, P–Pd av. 2.34, Nexo–P–Nendo av. 114.4, P–N–P av.
Fig. 3 Molecular structure of 2. H-Atoms, except those involved in the
H-bonding interactions, have been omitted for clarity. Selected bond
length [Å] and angles [°]: Nendo–P av. 1.68, Nexo–P av. 1.61, P–Se av. 2.13,
Se–Cu av. 2.35, N⋯Cl 3.18, Nexo–P–Nendo av. 112.0, P–N–P av. 96.2,
Nendo–P–Se av. 118.1; P (orange), Se (yellow), N (blue), Cl (green), Cu
92.9, Se–Pd–Se 103.0, P–Pd–P 92.3. Colour code, P (orange), Se
(orange/brown).
(yellow), N (blue), B (pink), F (bright yellow).
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the chemical shift being observed moving from 3 (δ =
3.2 ppm), to 4 (δ = 5.3 ppm), to 5 (δ = 5.6 ppm), indicating that
the increase in the polarization of the phosphazane framework
results in strengthening of the NH⋯F interactions. Whereas
the room-temperature 19F NMR spectrum of a solution of two
equivalents of TBABF4 (TBA = tert-butylammonium) and 3 in
CD2Cl2 shows only a negligible shift from that of TBABF4 in
CD2Cl2 (Δδ < 0.1 ppm), suggesting at best weak H-bonding of
BF4 to 3, the 19F NMR spectra of 4 and 5 in CD2Cl2 show sig-
−
nificantly larger shifts of Δδ = 0.6 and 1.66 ppm. The observed
Fig. 5 Distortion of the P2N2 ring units of 3 (left) caused by metal
coordination in 4 (middle) and 5 (right), resulting in an increase in the
H-bond donor bite.
−
19F NMR shifts in 4 and 5 not only indicate that BF4 is
H-bonded to the receptors in both, but also that metal coordi-
nation actually switches on anion coordination ability.
DFT calculations at the B3LYP-D3/def2-sv(p) level of theory
were used to investigate further the activating effect of metal
coordination. Gas-phase optimizations reproduce the solid-
state structures of 4 and the cation of 5 closely. The receptor-
coordination and hence activation for larger anions. This
ability to adapt upon metal coordination is in contrast to
established anion receptor systems such as geometrically-rigid
squaramide and ureas.
BF4 association energy increases from 4 (49.4 kJ mol−1) to 5
−
(58.3 kJ mol−1), supporting the increase in BF4 binding
−
In order to quantify the extent to which anion binding is
enhanced in solution, we carried out 1H NMR spectroscopic
titrations.18 We chose nitrate as the target anion since we
reasoned that its relative chemical hardness would not result
in displacement of the metal fragments and would leave the
Se–Pd bonds intact. Addition of increasing amounts of
TBANO3 in CDCl3 to 5 or 3 (in the presence of 2 eq. TBABF4)
results in gradual downfield shifting of the N–H resonances
for both, indicative of fast exchange binding of nitrate to the
N–H binding sites. In both cases the binding equilibrium
could be described with a 1 : 1 binding model with KA = (6.6
0.2) M−1 for 3 and KA = (661.3 51.6) M−1 for 5. It is clear in
the case of 5 that attachment of the Pd(II) fragment to 3 results
in a large (two orders of magnitude) increase in binding con-
stant compared to 3 alone. This is despite the fact that the
measured binding constant for 5 is actually a competitive
binding constant between BF4− and NO3−, rather than with the
free receptor (as in the case of 3). Unfortunately, complex 4
proved to be too unstable in the presence of excess TBANO3 so
that binding data could not be determined.
affinity indicated experimentally in the NMR spectroscopic
experiments in CD2Cl2 solution. NBO analysis reveals that
−
BF4 binding primarily stems from F(n) → N–H(σ*) donation,
but also to a lesser extent (10–15%) from F(n) → C–H(σ*)
t
donation to the Bu groups of 3 (see ESI†). The increase in the
F(n) → N–H(σ*) component of ca. 11 kJ mol−1 going from 4 to
5 supports the view of increased polarisation of the N–H
bonds in 5 (with the F(n) → C–H(σ*) component being similar
in both). Topological analysis is consistent with this, showing
an increase in electron density at the NH⋯F bond critical
points (BCPs) by 0.013e going from 4 to 5. Together with this,
NBO partial charge analysis also shows an increase in the posi-
tive charge of the H atom of the N–H groups moving from 3
(+0.39e) to 4 (+0.46e) and 5 (+0.47e). Transition metal com-
plexation to 3 also results in a decrease in the HOMO–LUMO
gap by 3.6 kJ mol−1 for 4 and 10.0 kJ mol−1 for 5, indicating
−
greater electron delocalisation from the BF4 anion. Although
only marginally affecting the charge on the H atoms of the N–
H groups of 4 and 5, the increase in the oxidation state of the
transition metal has a large effect on the partial charges of the
phosphorus and selenium atoms, which are significantly more
positively charged in 5. Overall, the increase in the binding of
Addition of harder anions results in displacement of the
palladium complex fragment in 5. For example, addition of
TBACl to 5 in CDCl3 cleaves the Pd–Se bonds and gives
(Ph3P)2PdCl2, as shown by in situ 31P NMR spectroscopy
(Scheme 1). This is accompanied by release of the H-bonded
−
the H-bonding of the BF4 anions of 4 and 5 can be seen as
the consequence of the increase in the net positive charge of
the receptor units, and the resulting increase in electrostatic
interaction, together with the polarisation of the N–H bonds.
Closer inspection of the solid-state structures of 4 and 5
indicates that the coordination of the Se-atoms to the Cu(I)
and Pd(II) centres has an additional geometric effect, resulting
in buckling of the essentially planar P2N2 ring unit in 3, with
an associated increase in the bite of the (N–H)2 H-bond recep-
tor site. The increase in the bite (5 > 4 > 3) can be traced
directly to the metal geometry, with the trigonal planar geome-
try of Cu(I) in 4 (with Se–Cu–Se ca. 120°) causing less distortion
than the square-planar geometry of Pd(II) in 5 (with Se–Pd–Se
ca. 103°) (Fig. 5). The flexibility of the phosph(V)azane back-
−
BF4 anion since the activating effect of metal complexation is
bone therefore allows adaptation of the receptors upon metal Scheme 1 Chloride-triggered release of BF4− from 5.
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isation of the N–H bonds in 6 is similar to that in 5 (and 4).
Although we also observe strong interaction between 6 and
nitrate, 1H NMR titration of 6 with TBANO3 in CDCl3 indicated
that more than one binding process occurs (e.g., N–H chela-
tion and potentially side-on N/Me). This prevented quantitative
analysis of the process.
In conclusion, we have shown that metal coordination to
seleno-cyclodiphosph(V)azanes (producing cationic receptor
units) increases their affinity for anions, largely by the receptor
units possessing net positive charge which leads to greater
electrostatic interaction. Metal coordination can also switch on
anion binding, as in the case of 5 with BF4−; a process that can
be reversed by removal of the metal. Non-metal based, posi-
tively charged phosphazane receptors which also have high
binding constants can also be used, in which one of the PvSe
groups is replaced with a P–Me+ functionality.
Scheme 2 Synthesis and binding behaviour of 6.
reversed, with the 19F NMR spectrum after chloride addition
−
showing free BF4 alone (having the same chemical shift as
that in the TBABF4 reference solution). This provides the
potential for a capture-release strategy using phosph(V)azane
receptors of this type.19,20
Given that a major contribution to the increase in the
anion affinity is the net positive charge of the metal-bonded
receptor units in 4 and 5, we wondered whether a similar
increase in anion binding could be promoted by alkylation of
the P-centres, producing
phosphonium receptor. In order to investigate this we syn-
thesized [tBuNH(Sev)P(µ-NtBu)2P(Me)HNtBu]BArF (6) (BArF
Conflicts of interest
a (metal-free) positively-charged
There are no conflicts to declare.
=
[(3,5-CF3)2C6H3]4B−) from a modified synthetic protocol to that
reported previously by Balakrishna for the synthesis of the
iodide salt of 6 (Scheme 2).21,22 We chose BArF in our case in
order to minimize coordination of the counter-anion to the
[tBuNH(Sev)P(µ-NtBu)2P(Me)HNtBu]+ cation and to improve
solubility in organic solvents. 6 is stable under benchtop
conditions for at least 6 months and does not require the use
of pre-dried solvents. Importantly, the room-temperature
1H NMR spectrum of 6 in CDCl3 shows that both of the N–H
protons have similar chemical shifts [δ(SevPNH) = 3.43 ppm,
δ(Me-PNH) = 3.30 ppm], suggesting that methylation of only
one of the phosphorus(III) centres results in similar polaris-
ation of the two N–H protons. In addition, the chemical shifts
of the N–H protons in 6 are similar to those in 3 (δ = 3.20)
implying similar polarisation. Addition of increasing amounts
of TBABF4 in CDCl3 solution results in gradual downfield shift-
Acknowledgements
We thank the Cambridge Trust (Vice Chancellor Scholarship
for AJP).
Notes and references
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−
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