Isolable N-heterocyclic carbene adducts of the elusive diiodophosphine

Article abstract of DOI:10.1039/c8cc03972k

Reactions of PH-substituted phosphinidene-imidazolylidenes with I₂ afford isolable NHC-adducts of the transient diiodophosphine PHI₂. The products, which show a surprising structural diversity, are according to DFT studies best formulated as charge-transfer complexes of onio-substituted cationic phosphines with I^? and are capable of reaction under formal transfer of an NHC-P⁺ unit.

Full text of DOI:10.1039/c8cc03972k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Isolable N-heterocyclic carbene adducts of the elusive
diiodophosphine†
M. Cicač-Hudi,a S. H. Schlindwein,a C. M. Feil,a M. Nieger b and D. Gudat*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Reactions of PH-substituted phosphinidene-imidazolylidenes with possible access to the target molecules
2
(Scheme 1).8 The
I2 afford isolable NHC-adducts of the transient diiodophosphine outcome of these investigations reported here reveals that
PHI2. The products, which show a surprising structural diversity, these species, which can formally also be described as
are according to DFT studies best formulated as charge-transfer zwitterionic diiodo-phosphoranides, are not only readily
complexes of onio-substituted cationic phosphines with I– and are isolated, but reveal also a surprising structural variability which
capable of reaction under formal transfer of an NHC-P+ unit.
gives insight into their electronic structure.
Primary and secondary halogenophosphines PXnH3-n (n = 1,2)
are, in contrast to their readily available H/X-substituted silane
analogues, rather elusive compounds. Apart from PF2H, which
is fairly stable as a liquid and was even characterized by XRD,1
primary phosphines PXH2 (X = F, Cl) are short-lived species that
could only be spectroscopically detected in the gas phase at
low temperature,2 and mono- and diiodophosphine were
identified as transient components of dynamically
equilibrating reaction mixtures that cannot be isolated.3 The
stability of mixed H/X-substituted phosphines is greatly
enhanced by introducing a bulky substituent. Species of the
R
N
R
N
R
N
I
I
I2
PH
I
PH
I
PH
N
R
N
R
N
R
1a-d
2a-d
Scheme 1: Iodine oxidation of phosphinidene-imidazolylidenes (R = Me (1a, 2a), iPr (1b,
2b), Mes [2,4,6-Me3C6H2] (1c, 2c), Dipp [2,6-iPr2C6H3] (1d, 2d)).
The reactions of imidazolylidene-phosphinidenes 1a
equimolar amount of iodine proceed readily at -78 °C. The
products 2a were isolated after work-up in 74-94% yield and
-d with an
-
d
fully characterised (see ESI for details). They are yellow to red
crystalline materials that are highly sensitive to moisture but
thermally stable and can, in a dry atmosphere, be stored at
ambient temperature without any decomposition. The N-alkyl-
type RPHCl (R
= Ph3C, Mes*, (Me3Si)3C, (Me2tBuSi)2N,
(Me3Si)2N, tBu)4 or RPHI (R = CF3)5 are stable at ambient
temperature in solution or were even obtained in pure form.
Even if the functionality of the parent compounds is not fully
retained, the concurrent presence of electro- and nucleofugic
substituents awards these derivatives an interesting chemical
reactivity which was e.g. exploited in phosphacumulene
syntheses.6 We presumed that NHC-coordination (NHC =
imidazolylidene), which has been shown to convert kinetically
unstable main-group element halides such as SiX2 (X = Cl, Br)
or PCl2(N3) into isolable products,7 might likewise be suitable
to enhance the kinetic stability of mixed H/X-substituted
phosphines with heavy halogen atoms (X ≠ F) and studied the
oxidation of the PH-substituted derivatives 18 with iodine as a
derivatives 2a,b are readily soluble in THF, toluene, CH2Cl2, or
MeCN. The N-aryl-derivatives 2c,d slowly cleave THF,‡ and
dissolution of isolated, analytically pure samples in CH2Cl2 was
inevitably accompanied by formation of minor amounts (5 –
15%) of 2-phosphino-imidazolium ions [NHC-PH2]+ (NHC = 1,3-
dimesityl-imidazolylidene (3c), 1,3-bis(2,6-di-isopropylphenyl)-
imidazolylidene (3d)10).§
The crystal structures of 2a-d (see Figure 1 for 2b, 2d and ESI
for 2a,c) contain discrete molecular entities with planar N-
heterocyclic rings.§§ The P(1)–H(1) bonds remain, as in the
precursors 1 8
, closely aligned to the ring plane (torsional
angles H(1P)–P(1)–C(1)–N(1) 0.8 – 20.1°), but the P(1)–C(1)
distance is lengthened in relation to 1a-d to values between
1.816(2) and 1.857(6) Å that are indicative of single bonds. In
2a
– 2c, both iodine atoms enclose the phosphorus atom to
form a quasi linear I–P–I unit (angle 166.4(1) - 172.92(3)°) that
is aligned perpendicular to the P–H bond. The P–I distances
vary over a large range (2.611(1) to 3.006(1) Å) and are
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generally longer than single bonds (2.49 – 2.493 Å11). The P–I PII-isomers as the dominant species in solution. However,
distances in each compound are distinctly unequal, with the alternative interpretations are also conceivable and cannot be
difference increasing from 2c (0.110(4) Å) to 2b (0.245(2) Å) ruled out at this stage.
and finally 2a (0.395(2) Å). The whole PHI2-unit adopts thus an The proposed description of 2a
-d
is further backed by the
unsymmetrical T-shape, and the phosphorus atom attains a results of DFT studies on 2a,c,d (see ESI for details). Energy
pseudo-trigonal-bipyramidal coordination geometry with optimization of the molecular geometry allowed us to identify
iodine atoms in the axial and the hydrogen and the NHC unit in three different isomers of each species that can be denoted as
the equatorial positions. Similar structural features as for 2a
12
–
c
charge-transfer complexes with I…P–I (
2
-IPI) or P–I…I (
2-PII)
had been reported for NHC-adducts of PCl3 and PBr3,13 topologies and a contact ion pair (
2-cip) that lacks any
respectively.
significant charge transfer, respectively (Figure 2). Computed
distances (Table 1) and Wiberg bond indexes14 (WBI, see ESI)
suggest addressing the cation fragment of
substituted phosphine with single bonds between phosphorus
and all its neighbour atoms. In -PII and -IPI, the P–H and P–C
2-cip as onio-
2
2
single bond character is retained while the P–I bonds are
elongated and the covalent bonding contribution (as judged by
the WBIs) falls off with increasing transfer of electron density
from the iodide. Moreover, P–I and I…I distances (in
2-PII) and
bond indexes respond perceptibly to inductive influences of
the N-substituents, and the ion pair character of these isomers
increases significantly upon their embedding in dipolar
solvents (see ESI). These features are typical for dative bonds
Figure 1: Molecular structure of 2b (left) and 2d (right) in the crystal. Thermal
ellipsoids are drawn at the 50% probability level. All hydrogen atoms except H1
were omitted for clarity. Selected distances [Å] and angles [°]: 2b: P1–C1
1.819(2), P1–I1 2.9053(7), P1–I2 2.6599(7), P1–H1 1.22(2), C1–P1–H1 102(2), I1–
P1–I2 172.92(3); 2d: P1–C1 1.816(2), P1–I1 2.4966(6), I1–I2 3.3418(3), P1–H1
1.27(2), C1–P1–H1 99.8(13), P1–I1–I2 178.13(2).
arising from charge transfer between
a donor and an
acceptor,15 and occur also in P-halogeno-diazaphospholenes.16
Relaxed potential energy scans indicate also that distortion of
the nearly symmetrical I–P–I moieties in the energy optimized
The molecular structure of 2d displays a totally different
layout. The three heavy atoms form in this case a quasilinear
P–I–I array with a central iodine atom (P–I–I 178.13(2)°). The
P1–I1 distance (2.4966(6) Å) falls definitely into the single
bond range, while the I1–I2 distance (3.3418(3) Å) is halfway
between a single bond and a van-der-Waals distance. The
phosphorus atom adopts pyramidal coordination geometry.
geometries of 2a-c to dissymmetric structures with unequal P-I
distances is facile (inducing a difference of 0.5 Å in the P–I
distances of 2a-IPI requires 2.4 kJ mol-1 in the gas phase and <1
kJ mol-1 in CH3CN), so that the experimentally observed
distortions are as in other cases17 attributable to crystal
packing effects.
Altogether, their structural features suggest to describe 2a
charge-transfer complexes formed by donation of electron
density from an I– anion into the *(P–I) orbital of a [NHC-PHI]+
cation unit (n(I–)
*(P–I) charge-transfer). The structure of 2d
-c as
σ
ꢀσ
may be rationalised by assuming that the bulky substituents
block the access to the phosphorus terminus of the P–I bond in
the cation fragment, and the donor approaches the
acceptor orbital from the side of the iodine atom.
σ*(P–I)
Figure 2: Wireframe representations of the energy-optimized molecular gas-phase
structures of 2a-IPI, 2-PII, and 2-cip (from left to right) computed at the RI-B3LYP-
3DBJ/def2-tzvp level of theory.
The analytical and spectral data of 2a
with the structures derived from the crystallographic studies.
The shielding of the 31P NMR nuclei is in 2a
higher and in
2c lower than in the appropriate precursors 1a-d, revealing
-d (see ESI) are in accord
,
b
The relative free energies (ꢀꢀG) of the three types of isomers
depend like their structures on solvent influences (see Table 1
and ESI). In the gas phase, the IPI-isomer is always the most
stable one, but the relative free energy of the PII-isomer
decreases upon formal replacement of N-alkyl by N-aryl groups
from 2a (69 kJ mol-1) to 2c,d (28 – 19 kJ mol-1). The
consideration of solvent effects results in relative stabilization
of the PII- and cip-isomers which renders in case of 2d the PII-
isomer generally the energetically most favourable solution
,
d
opposite trends for derivatives with N-alkylated and N-arylated
1
NHC-units, respectively. The magnitudes of the JPH coupling
constants in 2a-d (214 Hz – 238 Hz) are larger than in 1a-d (164
– 165 Hz) but remain in the typical range for phosphines.
Interestingly, the 1H and 31P NMR spectra show an appreciable
solvent dependence which affects not only the chemical shifts
1
of the nuclei in the PH-unit but also the JPH coupling constant
and is most prominent for 2c (
δ31P[1JPH] -138.2[244 Hz] in THF
structure. In case of 2c
, the PII- and IPI-isomers are
vs. -127.3[228 Hz] in CD2Cl2). We interpret the observed
variations as a hint to solvent-induced changes in the solute
structure which may, with reference to the different solid state
structures of 2a-c and 2d, arise from a switch between IPI- and
energetically close, and their energetic ranking may vary with
the solvent polarity. The increasing stabilization of the PII-
relative to the IPI-isomers in the series 2a
– 2c – 2d correlates
with a shrinking difference ꢀq = q(P) – q(I) in NBO charges on
2 | J. Name., 2012, 00, 1-3
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the covalently bonded phosphorus and iodine atoms in the cip-
isomers (in CH3CN: q = 0.55 (2a), 0.51 (2c), 0.44 (2d); see ESI
R
N
R
N
I
I3
PH
I
ꢀ
(i)
PH
I
for further details) which suggests that the P–I bond polarity in
the NHC-PHI+ fragment drops off when the electron donating
character of the NHC unit declines upon formal replacement of
the N-Me substituents in 2a by Mes (2c) or Dipp (2d),
respectively. Since this trend implies a concomitant increase in
N
R
N
R
2d
2b
4d
R'
N
R'
N
R'
N
I
F
(ii)
-
PH
I
P +
F
+ [PF6
]
the contribution from the iodine atom to the
which is expected to encourage P–I…I contacts, these findings
σ*(P-I) orbital,
N
R'
N
R'
5
N
R'
7
indicate that the structural change between 2a-c and 2d is not
R'
N
exclusively attributable to steric factors.
F
P
F
F
Considering that the ordering of dipolar molecules in a crystal
may induce similar geometric distortions as dipolar
solvent,17b these findings reproduce the structural dichotomy
between 2a and 2d, and back as well the assumption of
+
X + PF3
(iii)
N
R'
F
a
R'
N
R'
N
6 (X = F)
8 (X = H)
-c
P
I
9
several co-existing isomers in solutions of 2c. Regarding
1
further that the calculated JPH coupling constant is for 2c-IPI
N
R'
N
R'
Scheme 2: Selected reactions of 2b (R'
= iPr) and 2d (R = Dipp). Reagents and
significantly larger than for the two other isomers (see ESI for
details), a solvent induced a solvent induced switch in the
solute structure can also account for the observed solvent
conditions: (i) 1 equiv. I2, THF, -78 °C to r.t.; (ii) 1 – 3 equivs. XeF2, CD3CN; (iii) 3 equivs.
NaH, THF, r.t.
dependence of 1JPH in 2c
.
Crystalline 4d (Figure 3) is composed of pairs of cations and
nearly symmetrical tri-iodide anions (I1A–I2A 2.8973(5) Å, I2A–
I3A 2.9215(5) Å) linked by a short P–H…I contact (H1A–I1A
3.12(8) Å) that can possibly be interpreted as weak hydrogen
bonding interaction. The structural features of the cation
match, apart from a slight shortening of the P–I distance (P1–
I1 2.442(2) Å), those of 2d. This similarity supports, in
connection with DFT studies on 4d (see ESI), the formulation of
both species as onio-substituted phosphines.
Table 1: Relative free energies (in kJ mol-1) and selected distances (in Å) for 2a,c,d
calculated at the RI-B3LYP/def2-tzvp level. Values given refer to isolated molecules in
the gas phase and on molecules embedded in a dielectric medium (in brackets;
simulation of solvent using COSMO-RS with the parameters for MeCN), respectively.
Compound
2-IPI
0.0[0.0]
2-PII
2-cip
88.0[21.4]
2.462[2.497]
--
a)
a)
a)
2a (R = Me)
ꢀGrel
P–I
68.9[18.4]
2.791[2.534]
--
2.792[2.812]
2.793[2.819]
--
I–I
P–C
ꢀGrel
P–I
2.956[3.487]
1.795[1.817]
28.0[-1.7]
2.710[2.531]
--
--
1.824[1.821]
0.0[0.0]
1.843[1.815]
48.5[7.6]
2.539[2.485]
--
2c (R = Mes)
2d (R = Dipp)
2.800[2.838]
2.794[2.800]
--
I–I
P–C
ꢀGrel
P–I
3.002[3.431]
1.801[1.820]
19.3[-12.5]
2.662[2.497]
--
--
1.855[1.846]
0.0[0.0]
1.790[1.821]
46.8[1.9]
2.457[2.442]
--
2.733[2.748]
2.735[2.752]
--
I–I
2.955[3.310]
1.795[1.814]
--
P–C
1.849[1.844]
1.801[1.818]
a) relative to the 2-IPI isomer
Figure 3: Molecular structure of 4d in the crystal. Thermal ellipsoids are drawn at the
50% probability level and N-DIpp substituents using a wire model. Hydrogen atoms
except H1A were omitted for clarity. Selected distances [Å] and angles [°]: P1–C1
1.828(5), P1–I1 2.4424(16), P1–H1A 1.22(3), P1…I1A 3.5626(16), I1A–I2A 2.8973(5), I2A–
The results of an initial exploration of representative chemical
reactions of 2b and 2d are summed up in Scheme 2. Treatment
of 2d with an excess of iodine affords the tri-iodide 4d which I3A 2.9215(5), C1–P1–H1A 96(4), C1–P1–I1A 150.92(17), I1A–I2A–I3A 176.85(2).
was isolated in crystalline form and fully characterised. It is
interesting to note that any 2-phosphino-imidazolium cation
3d contained as an impurity in the original reaction mixture
(vide supra) reacts under these conditions to the same
product, presumably via similar H/I-exchange processes as had
been observed3 during the synthesis of PHI2.
Oxidation of the phosphorus atom of 2b is accomplished by
treatment with XeF2. Reaction monitoring by multinuclear
NMR spectroscopy revealed that the rather unselective
reaction affords, besides trace amounts of several unidentified
species, zwitterionic phosphoranide 618 and imidazolium salt
-
[PF6 ] as final phosphorus-containing products. In addition,
7
transient species assigned as PF3, onio-substituted cationic
difluoro-phosphine
, and phosphoranide zwitterion 812 were
5
detected. These findings suggest that 2b and XeF2 react
initially via oxidative I/F- and H/F-exchange to afford I2 and
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transient cationic phosphines [NHC–PXF]+ (X = H, F) which are
5
6
R. C. Dobbie, P. D. Gosling and B. P. Straughan, Dalton Trans.,
1975, 2368.
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then oxidised by XeF2 to the phosphoranides
respectively. The formation of is assumed to arise from
fission of the P–C bond in by initially formed HF.
6 and 8,
7
8
Finally, 2b was found to react with NaH to afford the
phosphamethine cyanine19 salt 920 as only soluble phosphorus-
containing species. The generation of H2 as co-product implies
that formation of the [(NHC)2P]+ cation involves deprotonation
7
of the P–H function of 2b as one step. However, even if
9 can
Ferguson, R. McDonald and E. Rivard, Can. J. Chem. 2018
,
formally be considered to arise from quenching the resulting
hypothetical "NHC-P+" unit by the NHC fragment of another
DOI:10.1139/cjc-2017-0607.
a) A. M. Tondreau, Z. Benkő, J. R. Harmer and H.
Grützmacher, Chem. Sci., 2014, 5, 1545; b) M. Cicač-Hudi, J.
Bender, S. H. Schlindwein, M. Bispinghoff, M. Nieger, H.
Grützmacher and D. Gudat, Eur. J. Inorg. Chem., 2016, 649.
The reaction parallels the easy oxidation of P-phenylated
8
9
molecule of 2b 21
,
the reaction mechanism is presumably more
complex and involves yet unknown redox steps. The spectral
and structural data of the cation of
previously reported BPh4 salt.20
9 match those of the
analogues of 1 with chalcogens, see: D. Bockfeld, T.
Bannenberg, P. G. Jones and M. Tamm, Eur. J. Inorg. Chem.,
In summary, we have established iodine oxidation of PH-
substituted imidazolylidene-phosphinidenes as a viable route
2017, 3452.
10 L. Liu, D. A. Ruiz, F. Dahcheh and G. Bertrand, Chem.
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1
to stable PHI2-NHC-adducts that are best described as charge-
transfer complexes of cationic, onio-substituted phosphines
[NHC–PHI]+. Apart from showing a remarkable structural
diversity, these species may undergo controlled reactions
under transfer of the cationic "NHC-P+" fragment. The further
elucidation of this reactivity as well as the exploration of
oxidations of related PI species is currently under way.
The authors acknowledge support by the state of Baden-
Württemberg through bwHPC and the German Research
Foundation (DFG) through grants no INST 40/467-1 FUGG and
Gu 415/17-1.
Röschenthaler, Chem. Sci., 2013,
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18 NHC-PF5 adducts: a) A. J. Arduengo, R. Krafczyk, W. J.
Conflicts of interest
Marshall and R. Schmutzler, J. Am. Chem. Soc., 1997, 119
,
3381; b) T. Böttcher, O. Shyshkov, M. Bremer, B. S. Bassil and
G.-V. Röschenthaler, Organometallics, 2012, 31, 1278; c) C.
Tian, W. Nie, M. V. Borzov and P. Su, Organometallics, 2012,
31, 1751.
There are no conflicts to declare.
19 Review: B. D. Ellis and C. L.B. Macdonald, Coord. Chem. Rev.,
2007, 251, 936.
20 For a salt with the same cation, see: F. O. Elnajjar, J. F.
Binder, S. C. Kosnik and C. L. B. Macdonald, Z. Anorg. Allg.
Chem., 2016, 642, 1251.
21 For the synthesis of phosphamethine cyanines from NHCs,
see: B. D. Ellis, C. A. Dyker, A. Decken and C. L. B. Macdonald,
Chem. Commun., 2005, 1965.
Notes and references
‡ The reaction affords as main products isolable imidazolium
salts and
HP(O)(OCH2CH2X)2 (X currently unknown). That THF is the source
a
species identified as
a
dialkoxyphosphite
3
of the CH2CH2X group is confirmed by observation of a JPH
coupling to two OCH2 groups, which is absent when the sample
was dissolved in THF-d8 (see ESI for details). The observed ring-
opening points to an enhanced Lewis acidity of 2c
to 2a which correlates well with the donating ability of the
proximal NHC-fragments.
§ Even if the exact mechanism of this reaction is yet unknown,
3c are likely to arise from reaction with trace amounts water.
,d compared
,b
,d
§§ CCDC 1842618-1842623 contain the supplementary
crystallographic data for this work. These data can be obtained
free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
1
a) R. W. Rudolph and R. W. Parry, Inorg. Chem., 1965, 4,
1339; b) M. F. Klapdor, H. Beckers, W. Poll and D. Mootz, Z.
Naturforsch., 1997, 52b, 1051.
2
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DOI: 10.1039/C8CC03972K
TOC entry:
Isolable NHC‐adducts of PHI2 display surprising structural diversity and are capable of reaction under
formal transfer of an NHC‐P+ unit.