Insertions of cyclic and acyclic germanium and tin heterocarbenoids into phosphorus-chlorine bonds: Syntheses, structures, and reactivities

Article abstract of DOI:10.1021/om3000168

Chloro(organo)phosphines are important precursors to diphosphenes and cyclic oligophosphines. Although chloro(organo)phosphines are commonly reduced with bulk metals (e.g., Na, Mg, and Zn), these reactions are much more selective when done homogeneously. To test whether group 14 heterocarbenoid reductions yield isolable insertion products, the mono- and dichloro(organo)phosphines PhPCl₂ (A), Ph₂PCl (B), ^tBuPCl₂ (C), ^tBu₂PCl (D), and bis(dichlorophosphino)methane (PCl ₂)₂CH₂ (E) were treated with the cyclic Me ₂Si(μ-N^tBu)₂El (El = Ge (1), Sn (2)) and the acyclic [(Me₃Si)₂N]₂El (El = Ge (3), Sn (4)) heterocarbenoids. The sterically least-encumbered phosphines were more reactive, while the cyclic stannylene 2 was the most reactive and the acyclic germylene 3 was the least reactive. All but one of the products were either mono- or diinsertion compounds, the lone exception being a distannane derived from 2. Semikinetic and structural data suggest that the tetravalent group 14 compounds are formed via an insertion mechanism whose rate depends on the steric bulk of the reaction partners and the nature of the group 14 elements.

Full text of DOI:10.1021/om3000168

Article
pubs.acs.org/Organometallics
Insertions of Cyclic and Acyclic Germanium and
Tin Heterocarbenoids into Phosphorus−Chlorine Bonds:
Syntheses, Structures, and Reactivities
Joseph K. West and Lothar Stahl*
Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 58202-9024 United States
S
* Supporting Information
ABSTRACT: Chloro(organo)phosphines are important pre-
cursors to diphosphenes and cyclic oligophosphines. Although
chloro(organo)phosphines are commonly reduced with bulk
metals (e.g., Na, Mg, and Zn), these reactions are much more
selective when done homogeneously. To test whether group
14 heterocarbenoid reductions yield isolable insertion pro-
t
t
ducts, the mono- and dichloro(organo)phosphines PhPCl2 (A), Ph2PCl (B), BuPCl2 (C), Bu2PCl (D), and bis(dichloro-
phosphino)methane (PCl2)2CH2 (E) were treated with the cyclic Me2Si(μ-NtBu)2El (El = Ge (1), Sn (2)) and the acyclic
[(Me3Si)2N]2El (El = Ge (3), Sn (4)) heterocarbenoids. The sterically least-encumbered phosphines were more reactive, while
the cyclic stannylene 2 was the most reactive and the acyclic germylene 3 was the least reactive. All but one of the products were
either mono- or diinsertion compounds, the lone exception being a distannane derived from 2. Semikinetic and structural data
suggest that the tetravalent group 14 compounds are formed via an insertion mechanism whose rate depends on the steric bulk of
the reaction partners and the nature of the group 14 elements.
Mont et al., who proved that even GeCl2 can insert into the
P−Cl bonds of chloro(organo)phosphines.11−13
1. INTRODUCTION
Phosphorus is sometimes referred to as the “carbon copy” on
account of the somewhat similar chemical properties that exist
between these two elements.1 This diagonal relationship,
however, is less well developed than that between members
of the earlier groups of the periodic table: cf. lithium/
magnesium or beryllium/aluminum. Unlike carbon,
phosphorus(III) has a lone pair of electrons that renders it
Lewis basic and makes it susceptible to redox reactions. The
group 15 element does, however, have an undeniable tendency
to form phosphorus−phosphorus single and double bonds that
mirror certain aspects of organic chemistry. Phosphorus−
phosphorus bonds,2 just like carbon−carbon bonds, can be
generated by the reduction of the corresponding halides.
A variety of reducing agentsmainly alkali metals, magnesium,
and zinchave been used for this purpose, although the nature
of the metal does seem to matter in the outcomes of the
reductions.3−7 In contrast to alkyl halide reductions, where
species such as R−Mg−X and R−Zn−X are formed,8 no such
intermediates are obtained for bulk metal reductions of the
more reactive halophosphines. The isolation of oxidative
addition products in chlorophosphine reduction requires both
milder reducing agents and milder conditions.
Despite these well-documented cases of group 14 hetero-
carbenoid insertions into P−Cl bonds, there have been no
systematic investigations into these reactions. This dearth of
interest is surprising, because although phosphorus−halogen
bond transformations lack the general utility of carbon−
halogen bond transformations, there are numerous applications
for such reactions. For example, the making of phosphorus−
phosphorus bonds by milder and hence more selective routes is
important in the syntheses of diphosphenes and oligophos-
phines. Detailed studies of heterocarbenoid insertions into P−Cl
bonds may also provide insight into the selective generation of
the long-sought phosphinidenes,14−16 which are, incidentally,
valence isoelectronic with germylenes and stannylenes.
Prior to our investigations no stannylene insertion product
into a phosphorus−halogen bond had ever been isolated; they
had merely been identified by NMR spectroscopy. A few of the
more stable germylene insertion products had been charac-
17
1
terized by in situ H and 31P NMR techniques, but only two
of these (vide supra) had been structurally characterized by
X-ray techniques.10,12 Herein we report on insertions of cyclic
and acyclic germanium and tin heterocarbenoids into the
phosphorus−chlorine bonds of chloro(organo)phosphines.
The emphasis in these studies was on the structures of the
products, as well as the relative reactivities of the hetero-
carbenoids and the phosphine substrates. It will be seen that
P−Cl reductions with homologous heterocarbenoids did not
As long ago as 1983 Bertrand and Veith showed that the
cyclic stannylene Me2Si(μ-tBuN)2Sn reduces dichlorophosphines
to diphosphenes under extremely mild conditions, Me2Si(μ-tBuN)2-
SnCl2 being the only oxidation product.9 The subsequent
isolation of [Me2Si(μ-tBuN)2Ge(Cl)]3P,10 upon treatment of
PCl3 with Me2Si(μ-tBuN)2Ge, provided clear evidence that
such reductions proceed with the intermediacy of insertion
products. This conclusion was supported by the work of du
Received: January 8, 2012
Published: February 22, 2012
© 2012 American Chemical Society
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Me2Si(μ-tBuN)2Sn (1.875 g, 5.87 mmol) was added dropwise by
syringe over 1.5 min. The solution became yellow-orange during the
addition. The flask was removed from the cold bath after 40 min of
stirring at 0 °C. X-ray-quality, yellow crystals were obtained in multiple
fractions from the reaction solution at 3 °C after several days. Yield:
1.351 g (58%). Mp: 112−116 °C. Anal. Calcd for C24H57Cl2N4PSi2Sn2
(797.21): C, 36.16; H, 7.21; N, 7.03. Found: C, 36.10; H, 7.22; N,
always give homologous products and that the reaction rates
differed significantly, with half-lives ranging from seconds to
months. In most cases the metastable insertion products were
isolated and fully characterized, thereby more than quadrupling
the number of such species.
1
2. EXPERIMENTAL SECTION
6.78. H NMR (500 MHz, C6D6, 25 °C): δ 0.41 (s, 6 H, SiMe), 0.48
3
(s, 6 H, SiMe), 1.36 (s, 36 H, NtBu), 1.61 (d, 9 H, JPH = 15 Hz,
2.1. General Procedures. All manipulations and reactions were
carried out under an inert atmosphere of argon, using standard
Schlenk techniques. Solvents were dried over and distilled from Na/
benzophenone (toluene) and K/benzophenone (THF and hexanes).
PhPCl2 and Ph2PCl were purchased from Aldrich Chemical Co. and
4JSnH = 10 Hz, PtBu). 13C{1H} NMR (125.8 MHz, C6D6, 25 °C):
3
3
δ 6.88 (s, JSnC = 50 Hz, SiMe), 7.94 (s, JSnC = 50 Hz, SiMe), 35.9
3
2
3
(d, JSnC = 41.5 Hz, JPC = 12.6 Hz, PCMe3), 36.5 (s, JSnC = 34.0 Hz,
1
2
NCMe3), 38.9 (d, JPC = 37.7 Hz, JSnC = 31.5 Hz, PCMe3), 53.6
(s, 2JSnC = 20.6 Hz, NCMe3). 31P{1H} NMR (202.5 MHz, C6D6, 25 °C):
t
distilled before use. The aliphatic phosphine Bu2PCl was synthesized
1
using a published procedure, and tBuPCl2 was prepared similarly, using
only 1 equiv of tBuLi.18 The heterocarbenoids Me2Si(μ-NtBu)2El, El =
Ge (1), Sn (2),19−21 and [(Me3Si)2N]2El, El = Ge (3), Sn (4),22 were
synthesized according to literature procedures. All 1H, 13C, and 31P NMR
spectra were recorded on a Bruker Avance 500 NMR spectrometer. The
NMR spectra were recorded in C6D6, THF-d8, and CD2Cl2, and the
chemical shifts, δ, are relative to the solvent peak(s) (e.g., C6D5H) for 1H
and 13C spectra and the external standard P(OEt)3 for 31P spectra.
Midwest Micro Labs, LLC, Indianapolis, IN, and Columbia Analytical
Services, Tucson, AZ, performed the elemental analyses.
119,117
δ 7.1 ( J
= 1616, 1543 Hz).
SnP
[Me2Si(μ-tBuN)2GeCl]2PtBu (10). In a 25 mL two-neck flask, a 1.0 M
t
toluene solution of BuPCl2 (2.0 mL, 2.0 mmol) was stirred at 0 °C.
A 1.0 M toluene solution of Me2Si(μ-tBuN)2Ge (4.0 mL, 4.0 mmol) was
added dropwise by syringe over 30 min. The solution was warmed to
room temperature and stirred for 3 days. The product was isolated as
clear colorless crystals in several fractions from toluene at 3 °C. Yield:
0.813 g (58%). Mp: 117−119 °C. Anal. Calcd for C24H57Cl2Ge2N4PSi2
(705.07): C, 40.88; H, 8.15; N, 7.95. Found: C, 40.52; H, 7.98; N,
1
7.81. H NMR (500 MHz, C6D6, 25 °C): δ 0.407 (br s, 6 H), 0.462
2.2. Syntheses. [Me2Si(μ-tBuN)2GeCl]2PPh (5). In a 50 mL
flask, PhPCl2 (0.27 mL, 2.0 mmol) was stirred in pentane
(4 mL) at 0 °C. A 2.0 M toluene solution of Me2Si(μ-tBuN)2Ge
(2.0 mL, 4.0 mmol) was added by syringe over 15 min. After
2 h the reaction mixture was removed from the cold bath and
stirred at room temperature overnight. Colorless crystals were
obtained in an almost quantitative yield from a pentane/toluene
solution at 3 °C. Yield: 1.39 g (96%). Mp: 112−114 °C. Anal.
Calcd for C26H53Cl2Ge2N4PSi2 (725.06): C, 43.07; H, 7.37; N,
(s, 6 H), 1.415 (s, 36 H), 1.684 (d, 9 H, 3JHP = 14.9 Hz). 13C{1H} NMR
(125.8 MHz, C6D6, 25 °C): δ 5.83 (s, SiMe), 7.59 (s, SiMe), 34.99
(d, JPC = 12.5 Hz, PtBu), 35.06 (s, NC(CH3)3), 38.78 (d, JPC
=
2
1
32.6 Hz, PC), 53.41 (s, NC(CH3)3). 31P{1H} NMR (202.5 MHz,
C6D6, 25 °C): δ 3.2.
[(Me3Si)2N]2Ge(Cl)P(tBu)Cl (11). In a 50 mL two-neck flask, tBuPCl2
(0.692 g, 4.35 mmol) was stirred at room temperature in benzene
(5 mL). A sample of [(Me3Si)2N]2Ge (1.712 g, 4.35 mmol) was added
all at once, and the ensuing solution was stirred for 5 days. The solvent
was removed in vacuo, leaving a colorless solid. This was redissolved in
a minimal amount of hexanes, affording colorless, X-ray-quality crystals
overnight at room temperature. Yield: 1.36 g (56%). Mp: 151−153 °C.
Anal. Calcd for C16H45Cl2GeN2PSi4 (552.40): C, 34.79; H, 8.21; N,
1
7.73. Found: C, 43.10; H, 7.30; N, 7.67. H NMR (500 MHz,
CD2Cl2, 25 °C): δ 0.35 (s, 6 H, SiMe), 0.44 (s, 6 H, SiMe),
1.19 (s, 36 H, NtBu), 7.34 (m, 1 H, p-C6H5), 7.36 (m, 2 H,
m-C6H5), 7.94 (m, 2 H, o-C6H5). 13C{1H} NMR (125.8 MHz,
CD2Cl2, 25 °C): δ 5.43 (s, SiMe), 6.87 (s, SiMe), 34.5
1
5.07. Found: C, 35.00; H, 8.08; N, 4.96. H NMR (500 MHz, C6D6,
1
3
25 °C): δ 0.47 (s, 36 H, SiMe3), 1.36 (d, 9 H, JPH = 14.7 Hz, PtBu).
(s, NCMe33), 52.8 (s, NCMe3), 128.6 (d, JPC = 13.8 Hz, PC),
13C{1H} NMR (125.8 MHz, C6D6, 25 °C): δ 7.25 (s, SiMe3), 28.8
128.9 (d, JPC = 8.8 Hz, m-C6H5), 129.9 (s, p-C6H5), 137.2
2
(d, JPC = 17.6 Hz, o-C6H5). 31P{1H} NMR (202.5 MHz,
2
1
(d, JPC = 17.6 Hz, PC(CH3)3), 39.9 (d, JPC = 45.3 Hz, PC(CH3)3).
CD2Cl2, 25 °C): δ −55.2.
31P{1H} NMR (202.5 MHz, C6D6, 25 °C): δ 117.5.
{[(Me3Si)2N]2Sn(Cl)P(Ph)}2 (6). In a 100 mL flask, [(Me3Si)2N]2Sn
(2.79 g, 6.24 mmol) was stirred in 10 mL of hexanes at 0 °C. Exactly
1.56 mL of a 2.0 M toluene solution of PhPCl2 (3.12 mmol) was
added by syringe over 20 min, producing a yellow solution with a
significant amount of a yellow precipitate. Bright yellow crystals were
obtained in several fractions from a concentrated THF solution at
3 °C. Yield: 2.62 g (72%). Mp: 220−221 °C. Anal. Calcd for
C36H82Cl2N4P2Si8Sn2 (1166.02): C, 37.08; H, 7.09; N, 4.80. Found: C,
Me2Si(μ-tBuN)2Sn(Cl)PtBu2 (12). In a 50 mL two-neck flask, a 1.0 M
toluene solution of Me2Si(μ-tBuN)2Sn (6.0 mL, 6.0 mmol) was stirred
at 0 °C. A 0.67 M toluene solution of tBu2PCl (4.0 mL, 9.0 mmol) was
added dropwise by syringe over 25 min. The solution was heated to
60 °C and stirred for 17 days. The product was isolated as yellow
crystals from toluene at 3 °C. Yield: 2.65 g (88%). Mp: 128−129 °C.
Anal. Calcd for C18H42ClN2PSiSn (499.76): C, 43.26; H, 8.47; N, 5.61.
Found: C, 43.05; H, 8.20; N, 5.61. 1H NMR (500 MHz, C6D6, 25 °C):
δ 0.350 (s, 3 H, SiMe), 0.544 (s, 3 H, SiMe), 1.375 (s, 18 H, NtBu),
1
37.05; H, 6.81; N, 4.97. H NMR (500 MHz, C6D6, 25 °C): δ 0.106
(s, 72 H, SiMe), 7.47 (m, 2 H, m-C6H5), 7.52 (m, 2 H, 3JPH = 1.9 Hz,
o-C6H5), 7.98 (m, 1 H, p-C6H5). 13C{1H} NMR (125.8 MHz, C6D6,
25 °C): δ 1.54, (s, SiMe), 130.31 (s, m-C6H5), 130.72 (s, o-C6H5),
131.76 (s, p-C6H5). 31P{1H} NMR (202.5 MHz, C6D6, 25 °C):
3
1.393 (d, 18 H, JPH = 12.4 Hz, PtBu). 13C{1H} NMR (125.8 MHz,
C6D6, 25 °C): δ 7.23 (s, SiMe), 8.52 (s, SiMe), 33.66 (d, PtBu2, 2JPC
=
13.5 Hz), 36.5 (s, NtBu), 36.8 (d, 1JPC = 37.3 Hz, PC), 53.27 (s, NC).
119,117SnP
= 1734,
1
31P{1H} NMR (202 MHz, C6D6, 25 °C): δ 92.8 (1J
119,117
δ −28.5 ( J
= 1470, 1404 Hz).
SnP
1657 Hz).
[(Me3Si)2N]2Ge(Cl)PPh2 (8). In a 25 mL two-neck flask, a 2.0 M
toluene solution of [(Me3Si)2N]2Ge (1.0 mL, 2.0 mmol) was stirred at
0 °C. A 2.67 M toluene solution of Ph2PCl (0.75 mL, 2.0 mmol) was
added dropwise by syringe over 30 min. The product was isolated as
colorless crystals from toluene at 3 °C in a nearly quantitative yield.
Yield: 1.15 g (94%). Mp: 126−128 °C. Anal. Calcd for C24H46ClGeN2PSi4
(614.04): C, 46.94; H, 7.55; N, 4.56. Found: C, 46.54; H, 7.29; N,
Me2Si(μ-tBuN)2Ge(Cl)PtBu2 (13). In a 50 mL two-neck flask, a 1.0 M
toluene solution of Me2Si(μ-tBuN)2Ge (5.0 mL, 5.0 mmol) was treat-
ed dropwise at room temperature with a 0.67 M toluene solution of
tBu2PCl (7.46 mL, 5.0 mmol), which was added by syringe over 20 min.
The solution was kept stirring at 70 °C for 39 days. The product was iso-
lated as colorless crystals from hexanes at −8 °C. Yield: 0.431 g (19%).
Mp: 130−132 °C. Anal. Calcd for C18H42ClGeN2PSi (453.69): C,
1
4.34. H NMR (500 MHz, C6D6, 25 °C): δ 0.30 (s, 36 H, SiMe3),
1
7.01−7.08 (m, 6 H, o, m-C6H5), 7.75−7.78 (m, 4 H, p-C6H5). 13C{1H}
(125.8 MHz, C6D6, 25 °C): δ 7.18 (s, SiMe3), 129.34 (d, 2JPC = 84.3 Hz,
47.65; H, 9.33; N, 6.17. Found: C, 47.24; H, 9.36; N, 6.02. H NMR
(500 MHz, C6D6, 25 °C): δ 0.41 (s, 3 H, SiMe), 0.50 (s, 3 H, SiMe),
1.19 (s, 18 H, NtBu), 1.23 (d, 18 H, 3JPH = 12.5 Hz, PtBu). 13C{1H} NMR
(125.8 MHz, C6D6, 25 °C) δ 7.37 (s, SiMe), 8.43 (s, SiMe), 33.27 (d,
PtBu2, 2JPC = 13.6 Hz), 35.8(s, NtBu), 36.3 (d, 1JPC = 37.3 Hz, PC), 53.02
(s, NC). 31P{1H} NMR (202 MHz, C6D6, 25 °C): δ 63.51.
3
4
o-C6H5), 134.88 (d, JPC = 22.6 Hz, m-C6H5), 136.91 (d, JPC
=
21.4 Hz, p-C6H5). 31P{1H} NMR (202.5 MHz, C6D6, 25 °C): δ −16.0.
[Me2Si(μ-tBuN)2SnCl]2PtBu (9). In a 50 mL two-neck flask, BuPCl2
t
(0.467 g, 2.94 mmol) was stirred in 10 mL of hexanes at 0 °C.
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{[Me2Si(μ-NtBu)2Ge(Cl)]2P}2CH2 (14). In a 50 mL three-neck flask,
Me2Si(μ-tBuN)2Ge (0.557 g, 2.04 mmol) was stirred in hexanes
at −78 °C. (Cl2P)2CH2 (0.069 mL, 0.51 mmol) was added via syringe
dropwise over 9 min. The reaction solution changed from pale yellow
to colorless. After it was warmed to room temperature, a white
precipitate appeared. All solvent was removed, and the solid was
redissolved in toluene. Clear, colorless, X-ray-quality crystals were
obtained from toluene at ca. 3 °C after a few days. Yield: 0.350 g
(52%). Mp: 188 °C dec. Anal. Calcd for C41H98Cl4Ge4N8P2Si4
(1309.93): C, 37.59; H, 7.54; N, 8.55. Found: C, 37.80; H, 7.39; N,
bis(chlorogermyl)phenylphosphine 5 from a pentane/toluene
mixture gave colorless crystals which were subjected to a single-
crystal X-ray analysis. The crystal data and collection and
refinement parameters for 5 are given in Table 1, while its solid-
state structure is depicted in Figure 1. A normal-length P−C
bond (1.837(3) Å) connects the phenyl group to a pyramidal
central phosphorus atom that also bears two bulky chlorogermyl
moieties, which are attached through two almost equidistant P−Ge
bonds of 2.3315(8) and 2.3226(8) Å.
The phosphorus−germanium bonds are only slightly longer
than in the tri-insertion product [Me2Si(μ-NtBu)2Ge(Cl)]3P10
and the di-insertion product (adamantyl)P(GeCl3)2,12 where
their average lengths were determined to be 2.310(4) and
2.3100(14) Å, respectively. Despite the steric bulk of the chloro-
germyl substituents, the bond angles about phosphorus are close to
those expected for an sp3-hybridized atom, ranging from 103.85(10)
to 108.78(3)°, with an angle sum of 317.87°. This is in contrast to
the larger Ge−P−Ge angles in [Me2Si(μ-NtBu)2Ge(Cl)]3P,
whose values sum to 345.0(2)°, consistent with a much flatter
pyramid that approaches a trigonal-planar arrangement.10
The reaction of acyclic germylene 3 with A was not as clean
as that with 1, giving instead a complex product mixture.
Although a monoinsertion intermediate was identified spec-
troscopically, the reaction ultimately yielded the cyclic oligo-
phosphines (PhP)n, n = 4, 5, as the only identifiable products.
1
8.44. H NMR (500 MHz, C6D6, 25 °C): δ 0.455 (s, 12H, SiMe),
0.539 (s, 12H, SiMe), 1.443 (s, 72H, NtBu), 3.623 (t, 2H, PCH2P,
2JPH = 3.15 Hz). 13C{1H} NMR (125.8 MHz, C6D6, 25 °C): δ 6.19
1
(s, SiMe), 7.16 (s, SiMe), 12.13 (t, PCH2P, JPC = 157 Hz), 35.16
(s, NCCH3), 52.97 (s, NCCH3). 31P{1H} NMR (202.5 MHz, C6D6,
25 °C): δ −51.9.
2.3. X-ray Crystallography. Suitable single crystals of 5−14 were
coated with Paratone N oil, affixed to Mitegen or Litholoop crystal
holders, and centered on the diffractometer under a stream of cold
nitrogen. Reflection intensities were collected with a Bruker Apex
diffractometer, equipped with an Oxford Cryosystems 700 Series
Cryostream cooler, operating at 173 K. Data were measured with ω
scans of 0.3° per frame for 20 s until complete hemispheres of data had
been collected. Data were retrieved using SMART software and
reduced with SAINT-plus,23 which corrects for Lorentz and
polarization effects and crystal decay. Empirical absorption corrections
were applied with SADABS.24 The structures were solved by direct
methods with SHELXS-97 and refined by full-matrix least-squares
methods on F2 with SHELXL-97.25
3. RESULTS AND DISCUSSION
3.1.1. Syntheses and Structures. Reactions of Chloro-
(phenyl)phosphines with Group 14 Heterocarbenoids. Pre-
vious NMR spectroscopic studies on reductions of halo(organo)-
phosphines with the cyclic diaminogermylenes and -stannylenes
1 and 2, respectively, had detected the intermediacy of both
short-lived and persistent oxidative addition products.17 Although
none of these intermediates had been fully characterized, the
studies clearly demonstrated that steric bulk on phosphorus
slows the reaction rates and that germylene insertions are
slower than those of the corresponding stannylenes. These
findings served as benchmarks for our synthetic studies, and we
began our investigation by repeating the reduction of
dichloro(phenyl)phosphine with the cyclic stannylene 2.
The reaction of dichloro(phenyl)phosphine (A) with 2 equiv
of 2 did indeed yield only the cyclic oligophosphines (PhP)4
26
and (PhP)5 but no characterizable tin-containing product,
even when done at −78 °C. The germylene analogue 1 reacted
In contrast, and as eq 2 shows, the more reactive acyclic
stannylene 4 did furnish an isolable insertion product, albeit not
the expected one. It was not possible to assign a structure to
this insertion product by NMR spectroscopy alone, but
fortunately copious amounts of bright yellow crystals were
available, and one of these was subjected to a single-crystal
X-ray analysis. Pertinent crystal and refinement data for 6 are
collected in Table 1, while selected bond parameters appear in
the caption of Figure 2. The thermal ellipsoid plot shows that 6
is not a conventional monoinsertion product, but rather a bulky
diphosphine that was likely created in a reaction step that
followed an initial insertion.
The compound is a symmetrical diphosphine whose most
intriguing features are the two bulky (diamino)(chloro)stannyl
substituents. Despite this steric bulk, the phosphorus atoms
feature surprisingly small bond angles, which range from
99.3(6) to 103.631(17)°, thereby creating highly pyramidalized
phosphorus atoms that are connected by a normal-length P−P
significantly more slowly than 2, as shown in eq 1, the reaction
requiring 1 h at 0 °C and affording the diinsertion product 5 in
almost quantitative yields. Because the intermediate 5 was
sufficiently kinetically stable, no cyclic oligophosphines or other
phosphorus−phosphorus-bonded species were isolated, nor
were they observed by NMR techniques. As is typical for
compounds with direct phosphorus−germanium bonds, the
thermally stable, colorless 5 exhibited a high-field-shifted signal
at −55.2 ppm in the 31P NMR spectrum.Recrystallization of the
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Table 1. Crystallographic Data for 5−14
5
6
7
8
9
10
chem formula
C26H47Cl2Ge2N4PSi2 C36H82Cl2N4P2
Si8Sn2
C20H48Cl2N4Si2Sn2
C24H46ClGeN2PSi4
C24H57Cl2N4PSi2Sn2
C24H57Cl2Ge2N4PSi2
formula wt
space group
T, °C
718.91
1166.00
709.08
614.00
797.17
704.97
P21/n (No. 14)
−100
P1 (No. 2)
̅
−100
P21/c (No. 14)
−100
P1 (No. 2)
̅
−100
P1 (No. 2)
̅
−100
P21/c (No. 14)
−100
a, Å
17.330(3)
10.0397(16)
22.406(4)
90
9.5087(10)
10.5988(16)
15.3895(15)
75.362(2)
78.652(2)
70.7470(10)
1405.7(2)
1
16.175(10)
12.434(7)
17.246(10)
90
9.0100(10)
11.5526(13)
17.337(2)
89.039(2)
78.051(2)
67.828(2)
1631.2(3)
2
12.4363(14)
16.8886(19)
19.353(2)
93.611(2)
106.664(2)
100.286(2)
3803.3(7)
4
16.782(3)
18.699(3)
11.7227(19)
90
b, Å
c, Å
α, deg
β, deg
γ, deg
107.715(2)
90
108.827(9)
90
100.186(2)
90
V, Å3
3713.5(10)
4
3283(3)
4
3620.7(10)
4
Z
ρcalcd, g cm−3
1.286
1.377
1.435
1.250
1.392
1.293
λ, Å
μ, mm−1
0.710 73
1.891
0.710 73
1.239
0.710 73
1.771
0.710 73
1.232
0.710 73
1.577
0.710 73
1.937
no. of data collected 30 238
11 738
22 017
12 778
29 902
29 782
no. of unique data
8071 (0.0242)
5976 (0.0166)
6175 (0.0346)
6335 (0.0186)
14 842 (0.0167)
7863 (0.0242)
(Rint)
a
R(F), I > 2σ(I)
0.0273
0.0834
0.0248
0.0685
11
0.0336
0.1462
12
0.0342
0.0922
0.0316
0.0853
0.0238
0.0659
14
b
Rw(F2), all data
13
chem formula
formula wt
space group
T, °C
C16H45Cl2GeN2PSi4
552.36
C18H42ClN2PSiSn2
499.74
C18H42ClGeN2PSi
453.64
C41H98Cl4Ge4N8P2Si4
1309.73
P1 (No. 2)
̅
−100
P21/c (No. 14)
−100
P21/c (No. 14)
−100
P4 (No. 81)
̅
−100
a, Å
10.6676(9)
11.8453(10)
12.1007(11)
77.3000(10)
89.9670(10)
76.1580(10)
1446.1(2)
2
16.0010(14)
9.0520(8)
18.8509(16)
90
11.2153(12)
11.8227(13)
18.439(2)
90
17.6548(7)
17.6548(7)
10.5683(8)
90
b, Å
c, Å
α, deg
β, deg
110.2880(10)
90
93.948(2)
90
90
γ, deg
V, Å3
90
2561.0(4)
4
2439.2(5)
4
3294.1(3)
2
Z
ρcalcd, g cm−3
1.269
1.296
1.235
1.320
λ, Å
μ, mm−1
0.710 73
1.471
0.710 73
1.215
0.710 73
1.484
0.710 73
2.124
no. of data collected
12 177
20 788
19 840
45 380
8131 (0.0212)
0.0218
0.0600
no. of unique data (Rint)
6172 (0.0130)
0.0226
5543 (0.0164)
0.0249
5315 (0.0188)
0.0256
a
R(F), I > 2σ(I)
b
Rw(F2), all data
0.0655
0.0676
0.0742
a
b
2
2
2
R = ∑|Fo − Fc|/∑|Fo|. Rw = {[∑w(Fo − Fc2)]/[∑w(Fo )2]}1/2; w = 1/[σ2(Fo)2 + (xP)2 + yP], where P = (Fo + 2Fc2)/3.
bond (2.2360(10) Å). The elongated phosphorus−tin bonds
(2.5706(6) Å) are likely due to the steric bulk of the acyclic
chlorostannyl unit. Because the tin−phosphorus bonds are
formed by phosphorus hybrid orbitals with high p character, the
Sn−P coupling constants are expectedly small, being 1470 and
1404 Hz for the tin-119 and tin-117 isotopes, respectively.
Although the tin atoms are approximately tetrahedral, they are
surrounded by widely varying bond angles that range from
103.633(17) to 119.35(5)°. The central portion of the
molecules is structurally related to the dication of [Ph3P−
PhP−PPh−PPh3][OTf]2, reported by Burford et al.,27 and the
dianionic tetraphosphorus chains [PhP−PhP−PPh−PPh]2−
bond. From eq 2 it is apparent that this dimer cannot be the
sole product of the reaction, and this begs the question as to
the fate of the second chlorostannyl moiety. A plausible
scenario is the loss of a Me2Si(μ-tBuN)2SnCl radical from a
diinsertion product, two of which then dimerized to give
the ditin compound [Me2Si(μ-tBuN)2(Cl)Sn−Sn(Cl)
(μ-NtBu)2SiMe2]2.30
Support for this hypothesis is provided by the isolation of
just such a tin dimer, namely 7, which was obtained in small
amounts from the reaction of B with 1 equiv of the cyclic
stannylene 2, as shown in eq 3. This reaction is seemingly
instantly complete at −78 °C, and just like the reaction of 2
with PhPCl2, it produces Ph2PPPh2, but here the distannane
[Me2Si(μ-NtBu)2(Cl)Sn−Sn(Cl)(μ-NtBu)2SiMe2]2 (7) was
also formed. In previous chlorophosphine reductions by the
stannylene 2 only the final oxidation product, namely Me2Si
(μ-NtBu)2SnCl2, had been isolated. Although it is not clear
and [CyP−CyP−PCy−PCy]2−, reported by Grutzmacher and
̈
co-workers.28,29 It therefore comes as no surprise that the bond
parameters for the central PhP−PPh moiety in all four
compounds are similar, the P−P bonds ranging from
2.163(2) to 2.2360(10) Å, compound 6 having the longest
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Figure 1. Solid-state structure and partial labeling scheme of 5.
Hydrogen atoms have been omitted, and all non-hydrogen atoms are
drawn as 50% probability thermal ellipsoids. Selected bond lengths (Å)
and angles (deg): Ge1−P1 = 2.3334(6), Ge2−P1 = 2.3234(6), Ge1−
Cl1 = 2.1985(6), Ge2−Cl2 = 2.1924(6), Ge1−N(av) = 1.8276(15),
Ge2−N(av) = 1.8314(15); Ge1−P1−Ge2 = 108.792(19), Cl1−Ge1−
P1 = 99.40(2), P1−Ge2−Cl2 = 100.29(2), C50−P1−Ge1 = 103.82(6),
C50−P1−Ge2 = 105.25(6).
Figure 3. Solid-state structure and partial labeling scheme of 7.
Hydrogen atoms have been omitted, and all non-hydrogen atoms are
drawn as 50% probability thermal ellipsoids. Selected bond lengths (Å)
and angles (deg): Sn1−Sn2 = 2.7797(12), Sn1−Cl1 = 2.3701(17),
Sn2−Cl2 = 2.3561(17), Sn1−N(av) = 2.005(4), Sn2−N(av) =
2.015(4); Cl1−Sn1−Sn2 = 103.94(6), Cl2−Sn2−Sn1 = 106.58(5),
N1−Sn1−N2 = 77.1(2), N3−Sn−N4 = 76.7(2).
Figure 2. Solid-state structure and partial labeling scheme of
centrosymmetric 6. Hydrogen atoms have been omitted, and all
non-hydrogen atoms are drawn as 50% probability thermal ellipsoids.
Selected bond lengths (Å) and angles (deg): Sn1−P1 = 2.5706(6),
P1−P1 = 2.2360(10), Sn1−Cl1 = 2.3652(5), Sn1−N(av) =
2.0464(16), P1−C13 = 1.839(2); Sn1−P1−P1′ = 100.57(3), Cl1−
Sn1−P1 = 103.631(17), Sn1−P1−C13 = 99.34(6), P1′−P1−C13 =
100.26(8), N1−Sn1−N2 = 113.87(7), N1−Sn1−Cl1 = 101.87(5),
N2−Sn1−Cl1 = 105.79(5), N1−Sn1−P1 = 110.11(5), N2−Sn1−P1 =
119.35(5).
whether all stannylene reductions go through the one-electron
process shown in reaction 3, the isolation of 7 lends support to
the hypothesis that a similar dimer is produced in reaction 2
above.
The caption of Figure 3 contains selected bond parameters,
while crystal and refinement data of 7 are given in Table 1. As
the thermal ellipsoid plot of 7 shows, the two chlorostannyl
moieties are staggered with respect to each other and
connected by a tin−tin bond (2.7799(12) Å) that is slightly
shorter than the sum of two covalent tin radii.31 Equivalent
bond parameters in both units are isometric; this includes the
Sn−Cl and Sn−N bonds, which are 2.3631(16) and 2.010(4) Å
long on average and thus have lengths similar to those in
related compounds.32
tin analogue 4 yielded only the diphosphine Ph2PPPh2 and
unidentified metal-containing products. 31P NMR spectra
showed that the stannylene reaction proceeded with the
intermediacy of a tin analogue of 8 that did not persist long
enough for isolation. The absence of satellite peaks in
germylene derivatives does not allow structural conclusions
based on NMR data alone to be made, but the diagnostic shift
As shown in eq 4, acyclic 3 inserted cleanly into the P−Cl
bond of B to furnish 8 in almost quantitative yields, while the
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of 96.5 ppm to high field was a strong indication that an
insertion product had formed.
The solid-state structure of 8, which is depicted in Figure 4,
shows a distorted tetrahedrally coordinated germanium(IV)
Figure 5. Solid-state structure and partial labeling scheme of one of
the two independent molecules of 9. Hydrogen atoms have been
omitted for clarity, and all other atoms are drawn as 50% probability
thermal ellipsoids. Selected bond lengths (Å) and angles (deg): Sn1−
P1 = 2.5083(8), Sn2−P1 = 2.4937(8), Sn1−Cl1 = 2.3831(10), Sn2−
Cl2 = 2.3561(9), Sn1−N(av) = 2.026(3), Sn2−N(av) = 2.026(3) P−
C = 1.901(3); Sn1−P1−Sn2 = 104.04(3), Cl1−Sn1−P1 = 99.04(3),
P1−Sn2−Cl2 = 103.38(3), Sn1−P1−C1 = 109.71(11), Sn2−P1−C1 =
106.49(10).
Figure 4. Solid-state structure and partial labeling scheme of 8.
Hydrogen atoms have been omitted, and all non-hydrogen atoms are
drawn as 50% probability thermal ellipsoids. Selected bond lengths (Å)
and angles (deg): Ge1−P1 = 2.3432(7), Ge1−Cl1 = 2.1918(6), Ge1−
N1 = 1.8448(18), Ge1−N2 = 1.8497(18); N1−Ge1−N2 = 112.27(8),
N1−Ge1−Cl1 = 110.02(6), N2−Ge1−Cl1 = 103.09(6), N1−Ge1−P1 =
110.84(6), N2−Ge1−P1 = 116.45(6), P1−Ge1−Cl1 = 103.35(2).
center, whose bond angles vary from 103.09(6) to 116.45(6)°.
Pertinent crystal and refinement data are collected in Table 1;
selected bond parameters appear in the caption of Figure 4.
The Ge−Cl and Ge−P bonds are almost identical in length to
those in 5, but the Ge−N bonds in 8 are longer (mean
1.8473(18) Å) due to the much bulkier nitrogen substituents in
this acyclic germylene.
3.1.2. Reactions of Chloro(tert-butyl)phosphines with
Group 14 Heterocarbenoids. Although previously published
reports9,17 suggested that chloro(alkyl)phosphines undergo car-
benoid insertions substantially more slowly than those with
aromatic groups, BuPCl2 reacted rapidly with 2 equiv of 2,
even at 0 °C, as shown in eq 5, to quantitatively afford the
Figure 6. Solid-state structure and partial labeling scheme for 10.
Hydrogen atoms have been omitted, and all other atoms are drawn as
50% probability thermal ellipsoids. Selected bond lengths (Å) and
angles (deg): Ge1−P1 = 2.3365(5), Ge2−P1 = 2.3286(5), Ge1−Cl1 =
2.2059(5), Ge2−Cl2 = 2.1836(5), Ge1−N(av) = 1.8373(13), Ge2−
N(av) = 1.8357(13); Ge1−P1−Ge2 = 107.664(17), Cl1−Ge1−P1 =
100.494(17), P1−Ge2−Cl2 = 104.535(16), Ge1−P1−C1 =
113.33(5), Ge2−P1−C1 = 104.50(5).
t
given in Table 1; selected bond parameters for these com-
pounds appear in their respective figure captions. Compound 9
is the first fully characterized diinsertion product of a
stannylene into a dichlorophosphine. The central phosphorus
atom is trigonal pyramidally surrounded by its three bulky sub-
stituents, as indicated by an angle sum of 320.18°. There are slight
asymmetries in both the tin−phosphorus (2.5082(8) and
2.4937(8) Å) and the Sn−Cl bonds (2.3837(9) and 2.3570(9) Å),
but the lengths are typical for such bonds.
While the germylene analogue 10 is not isomorphous with 9,
the molecular structures of both products are similar, as Figure
6 shows. The central phosphorus atom in 10 is more crowded
and therefore slightly less pyramidal (325.51°) than that in 9,
and this greater steric strain is also reflected in a slightly
elongated P−tBu bond (1.9067(16) Å). Consistent with the
difference in the covalent radii of Ge (1.22 Å) and Sn (1.41 Å),
both the Ge−P (2.3365(5) and 2.3285(5) Å) and the Ge−Cl
bonds (2.2058(5) and 2.1837(5) Å) are 0.18−0.19 Å shorter
than those in 10.
bis(chlorostannyl)-tert-butylphosphine 9 after less than 1 h.
The integrations of the lone phosphorus signal and those of its
tin satellites confirmed the presence of two tin-phosphorus
bonds. The cyclic germylene 1 reacted with C in a manner
analogous to that of 2, albeit much more slowly, the reaction
requiring 3−4 days at room temperature rather than 1 h at
0 °C.
The diinsertion products 9 and 10 are structural analogues,
and one of the independent molecules of the tin derivative, 9, is
shown in Figure 5, while Figure 6 depicts the germanium
counterpart 10. Crystal and refinement data for 9 and 10 are
In contrast to cyclic 1 and 2, the bulkier acyclic hetero-
carbenoids 3 and 4 did not yield diinsertion products with tBuPCl2.
Stannylene 4, in fact, gave no isolable insertion product at all,
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but a monoinsertion intermediate was observed by NMR methods.
As is typical for reductions having unstable insertion products, the
reaction furnished cyclic tetra- and tri-tert-butylcyclophosphines,33
with a particularly large amount of the latter.
40 days at 60 °C. Because the cyclic germylene 1 showed no
more than 20% conversion, we chose not to investigate the re-
activity of this bulky monochlorophosphine with 3. Cyclic 2
inserted into the lone P−Cl bond, as shown in eq 7, to form
The acyclic germylene 3, however, did give the stable mono-
insertion product 11 in ca. 56% yield, after being allowed to
react with C for 5 days, as shown in eq 6. Notably, compound
[(Me3Si)2N]2SnP(Cl)tBu2 (12), which was isolated as yellow
crystals in a yield of 88%.
Figure 8 shows the solid-state structure of 12. Selected bond
parameters are given in the figure caption, while data collection
11 is the only product described herein in which an available
P−Cl bond did not oxidatively add to a second heterocarbenoid,
even though the latter was present in excess.
Compound 11 was isolated in the form of colorless crystals
which melt at 151−153 °C; one of these was subjected to an
X-ray analysis, the results of which may be seen in Figure 7.
Figure 8. Solid-state structure and partial labeling scheme for 12, with
the major conformation of the P(tBu)2 moiety shown. Hydrogen
atoms have been omitted, and all other atoms are drawn as 50%
probability thermal ellipsoids. Selected bond lengths (Å) and angles
(deg): Sn1−P1 = 2.4718(10), Sn1−Cl1 = 2.3877(5), Sn1−N1 =
2.0312(17), Sn1−N2 = 2.0371(16); P1−Sn1−Cl1 = 109.84(3), N1−
Sn1−N2 = 76.32(7), C30−P1−Sn1 = 108.94(9), C40−P1−Sn1 =
102.8(3), C30−P1−C40 = 110.6(4), N1−Sn1−Cl1 = 107.74(5), N2−
Sn1−Cl1 = 107.45(5).
Figure 7. Solid-state structure of and partial labeling scheme for 11.
Hydrogen atoms have been omitted. All atoms are drawn as 50%
probability thermal ellipsoids. Selected bond lengths (Å) and angles
(deg): Ge1−P1 = 2.3992(4), Ge1−Cl1 = 2.1869(4), Ge1−N1 =
1.8414(11), Ge1−N2 = 1.8468(11), P1−Cl2 = 2.0795(6), P1−C13 =
1.8825(15); Ge1−P1−Cl2 = 97.14(2), Cl1−Ge1−P1 = 104.429(15),
P1−Ge1−N1 = 114.69(4), P1−Ge1−N2 = 111.49(4), Ge1−P1−C13 =
109.82(5), N1−Ge1−N2 = 112.09(5).
and refinement parameters are given in Table 1. There are two
distinct conformations for the Sn−PtBu2 fragments in the unit
cell, both differing substantially in their Sn−P bond lengths.
The Sn−P bond in the major conformer is comparatively short
(2.4718(10) Å), while that in the minor conformer
(2.5945(11) Å) is the longest of any of the compounds
described herein. The germylene derivative 13 is depicted in
Figure 9 in a perspective that emphasizes the relative sizes of
the di-tert-butylphosphinyl and chlorogermyl moieties and their
orientations with respect to each other. Homologues 12 and 13
join the diinsertion products 9 and 10 as only the second pair
of homologous insertion compounds, and here too it was the
cyclic heterocarbenoids that gave isolable products. Although
the Ge−P and Ge−Cl bonds are slightly elongated, they are not
significantly longer than those in 5, 8, or 10.
Crystal and data collection parameters for 11 are given in Table 1,
while selected bond parameters appear in the figure caption.
This monoinsertion product is a close structural relative of 8,
both compounds differing only in the phosphorus substituents,
namely phenyl/phenyl in 8 versus tert-butyl/chlorine in 11.
The lone Ge−P bond (2.3991(4) Å) in 11 is noticeably longer
than that in 8 (2.3432(7) Å), but it is not clear whether this is
due to steric or electronic effects. In contrast, the Ge−Cl bonds
in 11 and 8 have virtually identical lengths, being 2.1869(4) and
2.1918(6) Å long, respectively.
The di-tert-butylchlorophosphine D was the by far least reactive
chlorophosphine in its oxidative additions to the electron-
deficient germanium and tin compounds, requiring elevated
temperatures and prolonged reaction periods. Even for the
most reactive of the group 14 heterocarbenoidsthe cyclic
stannylene 2the reaction was only 90% complete after 17
days at 60 °C. The acyclic stannylene 4 reacted significantly
more slowly, the reaction being only 60% complete after
3.1.3. Reactions of Bis(dichlorophosphino)methane with
Group 14 Heterocarbenoids. All the reactions described above
involved monophosphines with either one or two chlorine
atoms. There are, to our knowledge, no reports on reactions of
bis(chlorophosphines) or bis(dichlorophosphines) with group
14 hetercarbenoids. In light of the isolation of the P−P-bonded
dimer 6, the generation of oligomers having [−P−C−P−]n
segments appeared a conceivable outcome for such inter-
actions. Bis(dichlorophosphino)methanethe simplest bis-
(dichlorophosphine)has four P−Cl bonds available for
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Figure 9. Solid-state structure and partial labeling scheme for 13.
Hydrogen atoms have been omitted, and all other atoms are drawn as
50% probability thermal ellipsoids. Selected bond lengths (Å) and
angles (deg): Ge1−P1 = 2.3425(4), Ge1−Cl1 = 2.2170(4), Ge1−N1 =
1.8366(12), Ge1−N2 = 1.8461(12); P1−Ge1−Cl1 = 109.121(17),
N1−Ge1−N2 = 83.20(6), C30−P1−Ge1 = 108.41(5), C40−P1−Sn1 =
104.79(5), C30−P1−C40 = 111.65(7), N1−Ge1−Cl1 = 109.09(4),
N2−Ge1−Cl1 = 107.46(4).
Figure 10. Solid-state structure and partial labeling scheme of C2-
symmetric 14. Hydrogen atoms have been omitted. All other atoms
are drawn as 50% probability thermal ellipsoids, except the tert-butyl
groups, which are shown as sticks for better visibility. Selected bond
lengths (Å) and angles (deg): P1−Ge1 = 2.3306(5), P1−Ge2 =
2.3357(5), Ge1−Cl1 = 2.2075(5), Ge2−Cl2 = 2.1979(5), Ge1−N(av) =
1.8249(16), Ge2−N(av) = 1.8338(16), P1−C5 = 1.8699(17); P1−C5−
P1′ = 108.99(14), Ge1−P1−Ge2 = 108.422(19), P1−Ge1−Cl1 =
95.654(19), P1−Ge2−Cl2 = 105.166(19), N1−Ge1−N2 = 83.56(8),
N3−Ge2−N4 = 83.57(8).
reaction with heterocarbenes, and a number of scenarios,
beginning with monoinsertion and ending with tetrainsertions,
seemed plausible.
When we added 2 equiv of 1 to Cl2PCH2PCl2, 31P NMR
spectroscopy revealed the formation of an unsymmetrical di-
insertion productpossibly [Me2Si(μ-NtBu)2Ge(Cl)]2-
PCH2PCl2that eluded isolation. When 4 equiv of 1 or 2
was added to hexanes solutions of Cl2PCH2PCl2 at −78 °C, as
groups the phosphorus−carbon bonds have normal lengths
(1.8699(17) Å). The two unique phosphorus−germanium
bonds are virtually isometric, being 2.3306(5) and 2.3357(5) Å
long and almost identical in length with those in 5. There is a
minor discrepancy in the Ge−Cl bonds, the sterically more
encumbered Ge1−Cl1 being slightly longer (2.2075(5) Å) than
the Ge2−Cl2 bond (2.1979(5) Å), but both bond lengths are
comparable to those in 5 and 10.
1
shown in eq 8, rapid color changes were observed. H and 31P
3.2. Product Distribution and Constitution. The
thermodynamic products of germylene and stannylene reduc-
tions of chloro(organo)phosphines are phosphorus−phosphorus-
bonded species (mainly diphosphines and cyclic oligophosphines)
and the corresponding dihalides, namely [(Me3Si)2N]2ElCl2 and
Me2Si(μ-NtBu)2ElCl2 (El = Ge, Sn). As we showed in the pre-
vious section, these reductions proceed with the intermediacy
of oxidative addition products of the type R3−xP-
{[(Me3Si)2N]2ElCl}x and R3−xP{[Me2Si(μ-NtBu)2El]Cl}x (x = 1, 2;
El = Ge, Sn) of varying stabilities.
Because dichlorophosphines can undergo both mono- and
di-insertions, the combination of 4 heterocarbenoids and 5
different chloro(organo)phosphines offered the opportunity to
potentially isolate more than 20 such insertion products. For a
variety of reasons, the major one being kinetic instability, we
were unable to obtain all conceivable products for the reactions
studied. Nonetheless, 10 intermediates, which are depicted in
Table 2, were isolated and fully characterized.
Depending on one’s vantage point, the products may be
described as either phosphinyl- and chloro-substituted
germanium(IV) and tin(IV) compounds or as chlorogermyl-
and chlorostannyl-substituted phosphines.34 Because of the
ubiquity of phosphines in chemistry, the latter nomenclature is
perhaps more appropriate for the compounds shown in Table 2.
In the solid state most of the insertion products show remarkable
stability to air and elevated temperatures, and even in solution
many do not decompose upon prolonged heating.
NMR spectra on aliquot samples of both mixtures sug-
gested very clean transformations and the presence of only
one symmetrical product in each case. Solely in the case of 1,
however, were we able to isolate a stable product: namely 14.
A
31P{1H} NMR spectrum on the isolated colorless crystals of
14 showed one singlet, shifted to high field (−51.9 ppm) and
thereby signaling a product with direct Ge−P bonds.
This was confirmed by a single-crystal X-ray analysis of 14,
the results of which may be seen in Figure 10. The data collec-
tion and refinement parameters are given in Table 1, while the
figure caption contains selected bond lengths and angles. The
molecule crystallized on a 2-fold rotation axis of the tetragonal
space group P4, with the methylene carbon located on that axis.
̅
The central methylene group is connected to two bis-
(chlorogermyl)phosphines, which enclose a perfectly tetrahe-
dral angle of 108.99(14)° at the carbon atom. Despite the
substantial steric bulk of the bis(chlorogermyl)phosphino
Phosphorus−germanium bonds are stronger than the
corresponding phosphorus−tin bonds, however, and this likely
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Table 2. Complete Listing of All Products Isolated from the Reactions of Chloro(organo)phosphines with Cyclic and Acyclic
a
Group 14 Heterocarbenoids
a
All numbered compounds were fully characterized; unnumbered compounds were not structurally characterized by X-ray studies. Empty spaces
denote unsuccessful or unattempted reactions.
accounts for the greater stability of the germylene deriva-
tives.35,36 As Table 2 shows, there is a preponderance of such
speciesspecifically insertion products involving the cyclic
germylene 1that is in stark contrast to the two compounds
featuring the cyclic stannylene 2. Compounds 6 and 7 stand out
in that they are not true insertion products but rather derived
from these intermediates in subsequent reactions. It is
significant that both were produced in stannylene, rather than
germylene, reductions.
The compounds shown in Table 2 are structural relatives of
the insertion products obtained from the interactions of
silylenes,37,38 germylenes,39 and stannylenes40,41 with haloal-
kanes. Oxidative additions of haloalkanes with group 14 car-
benoids, of course, yield products with the much stronger El−C
bonds that are less likely to undergo decomposition. Reduction
products featuring C−C bonds are therefore rarely isolated.42
An even closer structural relationship exists between the
group 14 heterocarbenoids presented herein and N-heterocyclic
carbenes (NHCs), although this structural similarity does not
translate to reactivity trends. NHCs do not insert into the
phosphorus−chlorine bonds, but depending on their steric bulk
they either form simple Lewis acid−base adducts with PCl3 or
displace all chlorine substituents as chlorides to furnish cationic
phosphorus species with chloride counterions.43−46
3.3. Reactivity Trends. Perhaps more surprising than the
varying stabilities of the insertion products is the very wide
range of reaction rates for the oxidative addition reactions, with
some being complete in seconds, while at least one interaction
showed no product even after being kept at 60 °C for days.
Dichlorophosphines can and do undergo diinsertions, and only
in the case of 11 were we able to stop the reaction after the first
step. In those reactions where diinsertions were observed, the
second insertion was almost always slower than the first one,
but not significantly so. The half-lives for the various reactions
show that, among the chlorophosphines, dichlorophenylphosphine
reacts somewhat more quickly than chlorodiphenylphosphine
and bis(dichlorophosphino)methane, while chlorodi-
tert-butylphosphine reacts exceedingly slowly, or not at all.
Dichloro-tert-butylphosphine is intermediate in its reactivity
with group 14 heterocarbenoids, the reactions requiring from
hours to days in their oxidative additions on these divalent
germanium and tin compounds. For any given chlorophos-
phine, the cyclic stannylene 2 reacted the fastest, while the
cyclic germylene 1 and the acyclic stannylene 4 reacted approxi-
mately equally quickly, with acyclic germylene 3 being the least
reactive. Of the two homologous pairs of heterocarbenoids, 1, 2
and 3, 4, the tin derivatives always reacted noticeably faster
than their germanium analogues.
The semikinetic data in Table 3 suggest that steric bulk on
both the phosphines and the heterocarbenoids slows the
reactions, and this is consistent with an associative mechanism.
Group 14 elements have two readily accessible oxidation states
(II and IV), the lower oxidation state becoming more stable as
one descends the group.47−49 Accordingly, the tendency of the
group 14 carbenoids to undergo oxidative additions decreases
with increasing atomic number of the central element, silylenes
being most reactive and plumbylenes least reactive. If the
insertion step were the rate-determining step, the germylenes
should react more quickly than the stannylenes. The opposite,
however, was consistently observed, suggesting that the rate-
determining step precedes the oxidative addition. This step
likely involves a Lewis acid−base adduct between phosphine
and heterocarbenoid. Although we did not detect such
intermediates spectroscopically, adducts between electron-
deficient group 14 heterocarbenoids and tertiary phosphines
are well-known.50,51
The proposed mechanism is analogous to that suggested by
du Mont et al. for the insertion of germanium(II) halides into
chloro(organo)phosphines,11,12 but it differs from those
proposed for the reactions of electron-deficient group 14
species with haloalkanes. These latter reactions are generally
believed to go by radical pathways.38,52,53 We saw no evidence
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Organometallics
Article
a
Table 3. Semi-Kinetic Data (Half-Lives) for the Reactions Investigated in This Study
a
Reaction rates ranged from immeasurably fast to imperceptibly slow.
in this study that would lead us to propose a radical mechanism
for the formation of the insertion products described above.
Unlike carbon, phosphorus has a sufficiently high-lying electron
pair to form a Lewis acid/base adduct with the electron-
deficient carbenoids. To shed more light on the mode of
formation and decomposition of the heterocarbenoid insertion
products, more detailed kinetic and mechanistic studies are
necessary. In particular, we intend to probe the relative
importance of steric and electronic substituent effects on
phosphorus, because the electron-rich chlorophosphines,
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (Grant No. EPS-
0814442) for financial support of this work.
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