Stabilization of a diphosphagermylene through pπ-pπ interactions with a trigonal-planar phosphorus center

Article abstract of DOI:10.1002/anie.201308002

N-Heterocyclic carbenes and their heavier homologues are, in part, stabilized by delocalization of the N lone pairs into the vacant p-orbital at carbon (or a heavier Group 14 element center). These interactions are usually absent in the corresponding P-substituted species, owing to the large barrier to planarization of phosphorus. However, judicious selection of the substituents at phosphorus has enabled the synthesis of a diphosphagermylene, [(Dipp) ₂P]₂Ge, in which one of the P centers is planar (Dipp=2,6-diisopropylphenyl). The planar nature of this P center and the correspondingly short Pi￡Ge distance suggest a significant degree of Pi￡Ge multiple bond character that is due to delocalization of the phosphorus lone pair into the vacant p-orbital at germanium. DFT calculations support this proposition and NBO and AIM analyses are consistent with a Gei￡P bond order greater than unity. On a plane: An unusual sterically hindered diphosphagermylene (R ₂P)₂Ge has been synthesized (see picture; R=2,6-iPr ₂C₆H₃). This compound possesses a trigonal planar phosphorus atom; the planarity is due to extensive delocalization of the phosphorus lone pair into the vacant p-orbital at germanium. DFT calculations indicate that the Gei￡P bond has significant multiple bond character.

Full text of DOI:10.1002/anie.201308002

.
Angewandte
Communications
DOI: 10.1002/anie.201308002
Planar Phosphorus
Stabilization of a Diphosphagermylene through pp–pp Interactions
with a Trigonal-Planar Phosphorus Center**
Keith Izod,* Daniel G. Rayner, Salima M. El-Hamruni, Ross W. Harrington, and Ulrich Baisch
Abstract: N-Heterocyclic carbenes and their heavier homo-
logues are, in part, stabilized by delocalization of the N lone
pairs into the vacant p-orbital at carbon (or a heavier Group 14
element center). These interactions are usually absent in the
corresponding P-substituted species, owing to the large barrier
to planarization of phosphorus. However, judicious selection
of the substituents at phosphorus has enabled the synthesis of
a diphosphagermylene, [(Dipp)2P]2Ge, in which one of the P
centers is planar (Dipp = 2,6-diisopropylphenyl). The planar
Scheme 1. Bonding situation in (R2N)2E: and (R2P)2E: species (E=Si,
Ge, Sn, Pb).
À
nature of this P center and the correspondingly short P Ge
À
distance suggest a significant degree of P Ge multiple bond
character that is due to delocalization of the phosphorus lone
pair into the vacant p-orbital at germanium. DFT calculations
support this proposition and NBO and AIM analyses are
tially mitigate the electron deficiency of this center (I,
Scheme 1).
The corresponding heavier Group 14 analogues (diami-
dotetrylenes; (R2N)2E, E = Si, Ge, Sn, Pb) have a much
longer history.[4] It is, perhaps, surprising then that the
corresponding P-substituted species (diphosphatetrylenes;
(R2P)2E)[5–7] and their carbene homologues (PHCs)[8] are
rather less well established. This, at least in part, may be
attributed to the high energetic barrier to achieving the
optimum planar configuration at phosphorus that would
enable efficient pp–pp overlap between the phosphorus lone
pair and the vacant p-orbital on the adjacent Group 14
element center (II). In this regard, it is notable that the P
atoms are distinctly pyramidal in all previously reported
diphosphatetrylenes (III).[5,6]
However, calculations predict that the inherent p-donor
capacity of phosphorus (that is, if the energy required for
planarization is neglected) is almost identical to that of
nitrogen.[9] Thus, if planarity could be induced at phosphorus,
it should be possible to promote efficient pp–pp overlap in
a diphosphatetrylene, stabilizing the electron-deficient Group
14 element center.
Few compounds with truly planar three-coordinate phos-
phorus centers have been reported.[10] However, it has been
shown that the barrier to inversion (and therefore the barrier
to planarization) at trigonal pyramidal phosphorus may be
significantly reduced by: 1) the incorporation of the phos-
phorus center into a ring; 2) the use of bulky substituents; and
3) the presence of electropositive elements directly adjacent
to phosphorus.[10,11] Strategies (1) and (2) were neatly applied
by Bertrand and co-workers in the synthesis of the first P-
heterocyclic carbene, in which the two phosphorus centers
approach planarity (sum of angles at P = 353 and 3488).[8]
With the foregoing in mind, we posited that a sterically
encumbered diphosphatetrylene, in which one of the phos-
phorus substituents is, necessarily, an electropositive center,
would satisfy the conditions necessary for planarization at
phosphorus and so provide the first example of a tetrylene
À
consistent with a Ge P bond order greater than unity.
S
ince the discovery of the first stable example in 1991, N-
heterocyclic carbenes (NHCs) and their acyclic analogues
have become indispensable, both as ligands for catalytically
active transition-metal complexes and as organocatalysts
themselves;[1,2] recent reports even suggest that NHC com-
plexes of transition metals may exhibit anti-tumor activity.[3]
The versatility of NHCs and related compounds is due to their
unusually strong s-donor (and correspondingly weak p-
acceptor) properties and their remarkable stabilities. The
latter may be attributed to: 1) the presence of electronegative
nitrogen atoms directly adjacent to the carbene center, which
stabilize the singlet over the triplet ground state; and
2) effective pp–pp interactions between the lone pairs on
nitrogen and the vacant p-orbital at carbon, which substan-
[*] Dr. K. Izod, D. G. Rayner, Dr. R. W. Harrington, Dr. U. Baisch
Main Group Chemistry Laboratories, School of Chemistry
Newcastle University, Bedson Building
Newcastle upon Tyne, NE1 7RU (UK)
E-mail: keith.izod@ncl.ac.uk
Homepage:
http://www.ncl.ac.uk/chemistry/staff/profile/keith.izod
Dr. S. M. El-Hamruni
Department of Chemistry, Faculty of Science
Tripoli University, PO Box 13203, Tripoli (Libya)
[**] We thank the University of Newcastle and the Libyan government
for financial support and Prof. S. A. Macgregor and Dr. T. Krꢀmer
(Heriot Watt University (UK)) for their assistance with AIM
calculations. We would like to acknowledge the use of the EPSRC UK
National Service for Computational Chemistry Software (NSCCS) at
Imperial College London in carrying out this work. Solid-state NMR
spectra were obtained at the EPSRC UK National Solid-state NMR
Service at Durham.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308002.
À
stabilized by P E pp–pp interactions.
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À
Treatment of PCl3 with two equivalents of DippLi-
(OEt2),[12] followed by one equivalent of LiAlH4 gave, after
an aqueous work-up, the secondary phosphane (Dipp)2PH (1)
as a colorless, crystalline solid in good yield (Dipp = 2,6-
iPr2C6H3). The reaction between 1 and PhCH2K in THF gave
the potassium complex [(Dipp)2P]K, which was reacted in situ
with half an equivalent of GeCl2(1,4-dioxane) to give a deep
red solid, after removal of KCl and solvent (Scheme 1).
Crystallization of this solid from cold toluene gave single
crystals of the diphosphagermylene [(Dipp)2P]2Ge (2) as air-
sensitive, dark red blocks suitable for X-ray crystallography in
reasonable yield (Scheme 2).
relatively short Ge P distance (2.291(4) ꢀ) is also observed
in the two-coordinate phosphagermylene [2,6-
(Tripp)2C6H3]Ge[P(SiMe3)2] (4), although this compound
contains a pyramidal phosphorus center and so does not
À
possess significant Ge P multiple bond character (Tripp =
2,4,6-iPr3C6H2).[7d] The Ge P1 distance in 2 is somewhat
À
longer than the corresponding distances in previously
reported phosphagermene compounds, although we note
that these involve a GeIV P double bond; for example, the
=
Ge P distances in (Mes)2Ge P(Mes*) (5)[15] and
À
=
(6)[16]
are
2.138(3)
and
=
(tBu2MeSi)2Ge P(Mes*)
2.1748(14) ꢀ, respectively (Mes = 2,4,6-Me3C6H2, Mes* =
2,4,6-tBu3C6H2). The P1-Ge-P2 angle of 107.40(4)8 is similar
to the P-Ge-C angle (106.89(5)8) in 4.[7d]
Compound 2 crystallizes as discrete monomers with a V-
shaped P2Ge core and is, to the best of our knowledge, the
first crystallographically characterized diphosphagermylene
with a two-coordinate germanium center (Figure 1).[5,13] The
In solution, 2 is highly fluxional; the 1H NMR spectrum of
2 at room temperature exhibits a single set of broad signals,
consistent with rapid exchange between the two (Dipp)2P
À
ligands and with rapid rotation about the P C bonds. At low
temperatures these signals broaden and decoalesce, such that,
at À958C, the spectrum consists of numerous broad over-
lapping signals that are not possible to assign unambiguously.
The 31P{1H} NMR spectra of 2 are more informative: at room
temperature, a single, broad singlet is observed at 3.2 ppm
(Figure 2), which is consistent with rapid exchange between
Scheme 2. Synthesis of 2. Dipp=2,6-iPr2C6H3.
Figure 1. Molecular structure of 2 with H atoms omitted for clarity.
Selected bond lengths [ꢁ] and angles [8]: Ge–P1 2.2337(11), Ge–P2
2.3823(12), P1–C1 1.839(4), P1–C13 1.846(4), P2–C25 1.866(4), P2–
C37 1.851(4); P1-Ge-P2 107.40(4), C1-P1-Ge 108.33(14), C13-P1-Ge
144.33(15), C1-P1-C13 105.69(19), C25-P2-Ge 89.38(13), C37-P2-Ge
119.39(14), C25-P2-C37 102.76(18).
Figure 2. Variable temperature 31P{1H} NMR spectra of 2 in [D8]toluene
[the low intensity peaks between À20 and À40 ppm are due to minor
impurities].
two phosphorus centers in 2 are distinctly different: while P2
adopts a typical trigonal pyramidal geometry (sum of angles
at P2 = 311.538), with its lone pair directed towards the rear of
the molecule, P1 is essentially planar (sum of angles at P1 =
358.358) and the C1-P1-C13 and P1-Ge-P2 planes are nearly
the pyramidal and planar phosphorus centers. As the temper-
ature is reduced this signal broadens and shifts to higher field,
until, at À808C, the signal decoalesces into two broad, equal
intensity singlets at 8.0 and À42.0 ppm, along with two very
broad, low intensity signals at approximately 95 and À80 ppm.
These signals sharpen as the temperature is reduced further,
such that at À958C (the lowest temperature that we were able
to attain) the spectrum consists of broad singlets at 98.5 (A),
7.8 (B), À43.1 (C), and À82.0 ppm (D) in the ratio 1:2:3:1;
coupling between the 31P nuclei is not resolved. A 31P EXSY
NMR spectrum at this temperature shows that there is rapid
exchange between A and D and between B and C, but not
À
coincident. Furthermore, while the Ge P2 distance
II
À
(2.3823(12) ꢀ) is similar to previously reported Ge
P
distances,[5–7] the Ge P1 distance is approximately 6%
shorter (2.2337(11) ꢀ). This is consistent with a significant
À
À
degree of multiple bond character in the Ge P1 bond (IV,
À
Scheme 1); for comparison, the Ge P distance in [CH-
{MeCN(Dipp)}2]Ge[P(SiMe3)2] (3) is 2.3912(8) ꢀ.[7a,14]
A
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À
between the pairs A/D and B/C (see Supporting Informa-
tion).
this orbital lies just 0.3 eV higher in energy than the P Ge
s bond and that it is 77% P-based. The corresponding p*
orbital is found as the LUMO + 1 and lies 0.3 eVabove the p-
orbital. In contrast, the lone pair at P2 has essentially sp-
character and is oriented towards the rear of the molecule,
while the germanium lone pair has predominantly s-charac-
ter.
The 31P{1H} CP-MAS solid-state NMR spectrum of 2
exhibits peaks with isotropic chemical shifts of 81.9 and
À61.6 ppm in a 1:1 ratio, the former exhibiting substantial
chemical shift anisotropy consistent with a planar phosphorus
environment; no other significant signals are observed (see
the Supporting Information for further details). Compounds
À
Consistent with the presence of Ge P1 multiple bond
with a planar phosphorus center involved in a GeIV P bond
character, the Wiberg bond indices for the Ge P1 and Ge P2
bonds are 1.33 and 0.89, respectively. Analysis of the bonding
in 2calc using the atoms-in-molecules approach similarly
=
À
À
typically have downfield 31P chemical shifts (for example,
175.4 (5), 416.3 (6) ppm),[15,16] while diphosphagermylenes
with pyramidal phosphorus centers typically have 31P chem-
ical shifts close to those of the free phosphine.[5,6] We
therefore assign the broad signals A and D in the low-
temperature solution-state spectrum (and the corresponding
signals at 81.9 and À61.6 ppm in the solid-state NMR
spectrum) to the planar and pyramidal phosphorus centers
in 2, respectively. This assignment is supported by DFT
calculations (see below), which predict chemical shifts for the
planar and pyramidal phosphorus centers in 2 of 100 and
À61 ppm, respectively (see the Supporting Information for
details). The identity of the species that account for signals B
and C is less clear. These signals appear to arise from two
separate species owing to their unequal intensities. As all four
peaks A–D coalesce to a single peak at room temperature and
as both elemental analysis and solid-state 31P NMR data
indicate the presence of a single species in the solid state, we
attribute peaks B and C to alternative conformers of 2, each
possibly containing two pyramidal phosphorus centers; how-
ever, on the current evidence it is not possible to confirm this.
DFT calculations provide further insight into the nature of
the bonding in 2. The calculated structure (2calc) at the B97D/
6-311G(2d,p) level of theory is very similar to that obtained
by X-ray crystallography. In particular, the planar nature of
one phosphorus center is reproduced extremely well; the sum
of angles about P1 in 2calc is 359.718.
À
indicates the presence of double bond character in the Ge
P1 bond.[17] A bond critical point (BCP) was located between
Ge and P1 with 1 = 0.091 and ellipticity 0.297, compared with
the BCP between Ge and P2 with 1 = 0.083 and ellipticity
0.064. The ellipticity is a quantitative measure of the
anisotropy of the electron density (that is, its deviation from
cylindrical symmetry) at the BCP and provides information
on the p-character of the bond; for example, the ellipticities of
À
=
ꢀ
the C C, C C, and C C bonds in butane, ethylene, and
ethyne are 0.01, 0.30, and 0.00, respectively.[18] Thus, the Ge
À
À
P2 bond has essentially single bond character, while the Ge
P1 bond has substantial double bond character. An alter-
native estimate of bond order may be obtained from the
delocalization index (DI), which is equivalent to a bond order
when two atoms are connected by a bond path;[19] for the Ge
À
À
P1 and Ge P2 bonds the DIs are calculated to be 1.275 and
0.843, respectively, which is consistent with the Wiberg bond
orders obtained for these bonds.[20]
Attempts to locate a minimum energy geometry in which
both phosphorus centers were pyramidal, and which might
shed light on the species observed in the low-temperature
31P{1H} NMR spectrum of 2, were unsuccessful. These led
only to the identification of a second conformer, 2’calc, in which
the two aromatic rings adjacent to the planar P center are
twisted in the opposite direction; this second conformer lies
Inspection of the MOs reveals that the HOMO and
LUMO are centered on the aromatic rings of the Dipp
substituents. However, the HOMOÀ1 consists of a p-orbital
arising from overlap of a phosphorus lone pair of essentially
pure p-character with the vacant p-orbital on the germanium
atom (Figure 3). Natural bond orbital analysis indicates that
just 1.9 kJmolÀ1 higher in energy than 2calc
.
In summary, we report the synthesis of a uniquely steri-
cally hindered phosphanide ligand and its use in the synthesis
of the first diphosphatetrylene with a trigonal planar P atom.
DFT calculations indicate that the electron-poor germaniu-
m(II) center in this compound is stabilized by delocalization
of the lone pair of the planar phosphorus atom into the vacant
=
p-orbital at germanium, effectively generating a Ge P p-
interaction.
Experimental Section
Synthesis of (Dipp)2PH (1): PCl3 (0.85 mL, 1.34 g, 9.7 mmol) was
added dropwise by syringe to a solution of (Dipp)Li(OEt2)[12] (4.74 g,
19.6 mmol) in cold (08C) diethyl ether (40 mL). This mixture was
allowed to attain room temperature and was stirred for 16 h. The
solids were removed by filtration and volatiles were removed in vacuo
from the filtrate to give a sticky yellow solid. This was dissolved in
diethyl ether (30 mL) and this solution was added dropwise to
a suspension of LiAlH4 (0.37 g, 9.7 mmol) in diethyl ether (10 mL),
and this mixture was stirred for 3 h. Degassed water (30 mL) was
carefully added and the organic phase was extracted into diethyl ether
(3 ꢁ 20 mL) and dried over activated 4 ꢀ molecular sieves. The dried
solution was filtered and solvent was removed in vacuo from the
Figure 3. HOMOÀ1 of 2calc showing the P-Ge p-orbital.
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filtrate to give 1 as a colorless solid. Yield 2.63 g, 76%. 1H NMR
D. M. Giolando, R. A. Jones, C. M. Nunn, J. M. Power, Poly-
(CDCl3): d = 1.01 (d, JHH = 6.4 Hz, 12H, CHMeMe), 1.04 (d, JHH
6.9 Hz, 12H, CHMeMe), 3.51 (m, 2H, CHMeMe), 5.47 (d, JPH
=
=
hedron 1988, 7, 1909; c) S. C. Goel, M. Y. Chiang, D. J. Rauscher,
W. E. Buhro, J. Am. Chem. Soc. 1993, 115, 160; d) C. Druck-
enbrodt, W.-W. du Mont, F. Ruthe, P. G. Jones, Z. Anorg. Allg.
Chem. 1998, 624, 590; e) E. Rivard, A. D. Sutton, J. C. Fettinger,
P. P. Power, Inorg. Chim. Acta 2007, 360, 1278; f) K. Izod, W.
McFarlane, B. Allen, W. Clegg, R. W. Harrington, Organo-
metallics 2005, 24, 2157; g) K. Izod, J. Stewart, E. R. Clark, W.
McFarlane, B. Allen, W. Clegg, R. W. Harrington, Organo-
metallics 2009, 28, 3327; h) K. Izod, J. Stewart, W. Clegg, R. W.
Harrington, Organometallics 2010, 29, 108; i) K. Izod, J. Stewart,
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Organometallics 2012, 31, 246.
232.6 Hz, 1H, PH), 7.08 (m, 2H, ArH), 7.24 ppm (m, 1H, ArH).
13C{1H} NMR (CDCl3): d = 23.85 (CHMeMe), 24.26 (CHMeMe),
32.84 (d, JPH = 12.5 Hz, CHMeMe), 123.54, 128.83, 132.75 (Ar),
152.61 ppm (d, JPC = 11.0 Hz, Ar). 31P NMR (CDCl3): d = À101.1 ppm
(d, JPH = 232.6 Hz).
Synthesis of (Dipp2P)2Ge (2): A solution of PhCH2K[21] (0.42 g,
3.3 mmol) in THF (10 mL) was added to a solution of 1 (1.13 g,
3.2 mmol) in THF (30 mL) and this mixture was stirred for 2 h. The
resulting red solution was added dropwise to a solution of GeCl2(1,4-
dioxane) (0.37 g, 1.6 mmol) in THF (10 mL). This mixture was stirred
for 16 h in the absence of light. Solvent was removed in vacuo and the
sticky red solid was extracted into toluene (40 mL) and filtered. The
filtrate was concentrated to about 5 mL and cooled to À208C; dark
red blocks of 2 suitable for X-ray crystallography were isolated after 2
weeks. Yield 0.47 g, 38%. Anal. calcd. for C48H68GeP2 (779.58):
C 73.95, H 8.79%; found C 73.81, H 8.72%. 1H NMR ([D8]toluene,
295 K): d = 1.08 (br. s, 48H, CHMe2), 3.94 (br. s, 8H, CHMe2), 6.99
(m, 8H, ArH), 7.11 ppm (m, 4H, ArH). 13C{1H} NMR ([D8]toluene,
295 K): d = 24.73 (CHMe2), 34.14 (CHMe2), 124.45, 129.13, 137.61,
153.38 ppm (Ar). 31P{1H} NMR ([D8]toluene, 295 K): d = 3.2 ppm (br.
s).
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DFT calculations: Geometry optimizations were performed with
the Gaussian09 suite of programs (revision D.01) using the B97D
functional, which includes a correction for dispersion effects, and with
automatic density fitting; the 6-311G(2d,p) all-electron basis set was
used on all atoms (default parameters were used throughout). The
location of minima was confirmed by the absence of imaginary
vibrational frequencies. NMR shielding tensors were calculated for
the optimized structure using the GIAO method at the B97D/6-311 +
+ G(2d,p) level of theory and chemical shifts are quoted relative to
85% H3PO4. Natural bond orbital analyses were performed using the
NBO 3.1 module of Gaussian09. Further details of these calculations
can be found in the Supporting Information.
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Received: September 11, 2013
Revised: January 6, 2014
Published online: March 3, 2014
[12] DippLi(OEt2) was prepared by the same method as 2,4,6-
iPr3C6H2Li(OEt2); R. A. Bartlett, H. V. R. Dias, P. P. Power, J.
Organomet. Chem. 1988, 341, 1. See the Supporting Information
for more details.
Keywords: density functional calculations · germanium ·
NMR spectroscopy · P ligands · solid-state structure
.
[13] Crystal data for 2: C48H68GeP2, Mr = 779.55, red blocks, 0.32 ꢁ
0.20 ꢁ 0.20 mm3, monoclinic, space group P21/c, a = 13.8221(9),
[1] For recent treatises on NHC-metal based catalysis, see: a) N-
Heterocyclic Carbenes in Transition Metal Catalysis and Organo-
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Heterocyclic Carbenes: From Laboratory Curiosities to Efficient
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Chemistry, Cambridge, 2011.
[2] For recent reviews of NHC-mediated organocatalysis see: a) S. J.
Ryan, L. Candish, D. W. Lupton, Chem. Soc. Rev. 2013, 42, 4906;
b) A. Grossmann, D. Enders, Angew. Chem. 2012, 124, 320;
Angew. Chem. Int. Ed. 2012, 51, 314; c) X. Bugaut, F. Glorius,
Chem. Soc. Rev. 2012, 41, 3511.
b = 17.1511(13),
c = 18.7976(14) ꢀ,
b = 91.137(6)8,
V=
4455.4(6) ꢀ3, Z = 4, m = 0.790 mmÀ1, T= 150(2) K, R1 = 0.060
(F2 > 2s), wR2 = 0.1449 (all data), 7838 independent reflections
(4869 with F2 > 2s) and 476 parameters. CCDC 960358 contains
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[14] Although [CH{MeCN(Dipp)}2]Ge[P(SiMe3)2] (and its tin and
lead homologues) contains a near-planar P atom, the germanium
center in this compound is electron-precise and so the planarity
of the P center may be attributed to steric rather than electronic
factors.
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www.angewandte.org
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Angewandte
Communications
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ꢀ
[20] An alternative interpretation of the bonding in 2 involving a Ge
À ꢀ
P triple bond, analogous to the proposed bonding in F3C C SF3,
À ꢀ
À ꢀ À
F5S C SF3 and H C S OH, is not supported by the present
data (see Supporting Information for a more detailed discus-
sion); a) D. Christen, H. G. Mack, C. J. Marsden, H. Ober-
hammer, G. Schatte, K. Seppelt, H. Willner, J. Am. Chem. Soc.
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[21] Details of the synthesis of this compound are given in the
Supporting Information.
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