Highly Selective Substitution and Insertion Reactions of Silylenes in a Metal-Coordinated Polyphosphide

Article abstract of DOI:10.1021/jacs.9b12151

Selective substitution and insertion reactions of silylenes into the cyclo-P₅ ring of [Cp*Fe(??⁵-P₅)] are reported. The selective substitution of one P atom by an isoelectronic [LSi] fragment (L = PhC(N^tBu)₂) leads to [(??⁴-P₄SiL)FeCp*] and [LSi(Cl)a? P-SiL(Cl)₂]. To elucidate the reaction mechanism, [{LSi(N(SiMe₃)₂)}{(??⁴-P₅)FeCp*}], in which the silicon atom binds to the cyclo-P₅ ring, was synthesized as a model compound for the reaction intermediate. The insertion of [LSi-SiL] into the cyclo-P₅ ring of [Cp*Fe(??⁵-P₅)] resulted in [{??⁴-P₅(SiL)₂}FeCp*] featuring a cyclo-P₅(SiL)₂ ring, which corresponds to the largest silicon-polyphosphorus ring known in a complex.

Full text of DOI:10.1021/jacs.9b12151

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Communication
Highly Selective Substitution and Insertion Reactions
of Silylenes in a Metal Coordinated Polyphosphide
Ravi Yadav, Thomas Simler, Stephan Reichl, Bhupendra Goswami, Dr.
Christoph Schoo, Ralf Köppe, Manfred Scheer, and Peter W. Roesky
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12151 • Publication Date (Web): 20 Dec 2019
Downloaded from pubs.acs.org on December 20, 2019
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Highly Selective Substitution and Insertion Reactions of Silylenes in a
Metal Coordinated Polyphosphide
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Ravi Yadav,† Thomas Simler,† Stephan Reichl,‡ Bhupendra Goswami,† Christoph Schoo,† Ralf
Köppe,† Manfred Scheer,*,‡ and Peter W. Roesky*,†
† Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstraße 15, 76131 Karlsruhe
(Germany).
‡ Institute of Inorganic Chemistry, University of Regensburg, 93040 Regensburg (Germany).
Supporting Information Placeholder
The selective replacement of P atoms in transition metal
ABSTRACT: Selective substitution and insertion reactions of
silylenes into the cyclo-P5 ring of [Cp*Fe(5-P5)] are reported.
The selective substitution of one P atom by an isoelectronic
[LSi] fragment (L=PhC(NtBu)2) leads to [(4-P4SiL)FeCp*] and
[LSi(Cl)=P-SiL(Cl)2] as a byproduct. To elucidate the reaction
mechanism, [{LSi(N(SiMe3)2)}{(4-P5)FeCp*}], in which the
silicon atom binds to the cyclo-P5 ring, was synthesized as a
model compound for the reaction intermediate. The insertion
of [LSi-SiL] into the cyclo-P5 ring of [Cp*Fe(5-P5)] resulted in
polyphosphides such as [Cp*Fe(5-P5)] by other heteroatoms
is still
a a
major challenge.12 In addition, since such
replacement would disturb the five-fold symmetry of
[Cp*Fe(5-P5)], the corresponding products can open new
avenues in the coordination chemistry of polyphosphorus
complexes. The reactivity of [Cp*Fe(5-P5)] with single-
electron reductants usually involves the cyclo-P5 ring in the
reduction process without alteration of the ring itself.13
Interestingly, by using NHCs as reactive donor molecules in the
reactions with different cyclo-Pn ligand complexes, an
extrusion of one P atom is observed, followed by a ring
contraction.14 Therefore, the question arises as to what would
happen if a highly reactive low-valent silicon complex is used
in the reaction with [Cp*Fe(5-P5)], a similar reaction behavior
or a cleavage of a P-P bond in the cyclo-P5 ring and a unique
insertion of heteroatoms in the cyclo-P5 ring or both, P atom
elimination and heteroatom insertion. Herein, we report the
reaction of [Cp*Fe(5-P5)] with low-valent silicon compounds,
resulting in the synthesis of the first sila-phosphaferrocene.
Moreover, experimental investigations of the corresponding
mechanism are reported.
The reaction of [LSiCl]15 (L=PhC(NtBu)2) with [Cp*Fe(5-P5)]
in toluene at room temperature resulted in the formation of
[(4-P4SiL)FeCp*] (1) in 40 % yield, consisting of a unique
silatetraphospha-cyclopentadienyl ring, and [LSi(Cl)=P-
SiL(Cl)2] (2) (Route A, Scheme 1). This result reveals that with
silylenes both processes, extraction and insertion, occur. The
isolation of both species was enabled by their slightly different
solubilities in hydrocarbons. Compound 2 can be extracted
with n-hexane while complex 1 is less soluble in this solvent
and can subsequently be extracted using toluene. In a different
approach, complex 1 was synthesized in high yield (80%) by
reacting [K(dme)]2[Cp*Fe(4-P5)]13b with an excess of [LSiCl]
(Route B, Scheme 1). In this case, the recently reported
[L2Si2P2]16 is formed as a by-product.
[{(4-P5(SiL)2}FeCp*] featuring
a cyclo-P5(SiL)2 ring and
corresponds to the largest silicon-polyphosphorus ring known
in a complex.
Selective substitution of ring atoms or ring expansions are
valuable tools in synthetic organic and organometallic
chemistry. Established ring-expansion reactions include, e.g.
the rearrangements of carbocations1 such as the Demjanov and
Tiffeneau-Demjanov
reactions
or
the
Beckmann
rearrangement,2 radical reactions1 and oxidation reactions
such as the Baeyer-Villiger oxidation.3 In contrast, the selective
expansion of phosphorus rings by other elements is not well
explored.4 The most common reaction in this area is the
insertion of heteroatoms into white phosphorus (P4).5
Recently, significant efforts have been devoted to selectively
functionalizing P4 with low-valent main group complexes.5a, 5b,
6
For example, a stepwise P-P bond activation of white
phosphorus using a divalent silicon compound was reported.5g
In contrast to such insertion reactions involving the very
reactive strained P4 cage, we were challenged by selective
substitution and insertion reactions in the non-strained cyclo-
P5 ring of the air-stable [Cp*Fe(5-P5)] (Cp*=C5Me5).7
-
Considering the isolobal analogy between Cp- and cyclo-P5 ,
[Cp*Fe(5-P5)] forms a metallocene structure similar to that of
ferrocene.8 [Cp*Fe(5-P5)] has been employed as a very useful
tool in coordination chemistry due to the presence of five free
lone-pairs on the cyclo-P5 ring.9 Scheer et al. engaged in a
systematic study of the coordination behavior of [Cp*Fe(5-
P5)] with various transition and main-group metals.10 The
synthesis of the first inorganic fullerene-like molecule from
[Cp*Fe(5-P5)] and CuCl is a significant breakthrough.11
The solid-state structure of 1 shows the formation of a five-
membered [cyclo-LSiP4]- ring 4-coordinated to the [Cp*Fe]+
moiety (Figure 1). The Si-P1 (2.1707(13) Å) and Si-P4
(2.1729(12) Å) bond distances are almost equal and lie in-
between single (2.24-2.27 Å) and double bonds (2.06-2.09
Å).16a, 17 Also, the P-P bond lengths (P1-P2 2.1391(13), P2-P3
2.1597(15), and P3-P4 2.1444(13) Å) show a substantial
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double-bond character as observed in the previously reported
K2[Cp*Fe(4-P5)] (2.133(1)-2.153(1) Å).13b
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Scheme 1. Synthesis of 1 and 2.
tBu
tBu
N
N
Si
+ 2 Ph
Fe
Cl
N
Ph
Si
P
P
Cl
N
P
P
tBu
toluene
P
Route A
tBu
Ph
tBu
+
-78 oC to rt
Cl
Si
N
tBu
tBu
N
N
P
N
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tBu
-
Ph
Si
Si
Ph
Fe
Cl
N
N
Fe
Cl
A
P
P
Cl
tBu
P
P
P
P
tBu
tBu
P
2
P
P
N
Si
N
Ph
tBu
dme
tBu
2-
-50 oC to rt
- 2 KCl
1
N
[{K(dme)}+]2
+ ex.
Fe
Ph
Si
Route B
tBu
N
tBu
Cl
P
P
N
P
P
P
N
P
tBu
P
Ph
Ph
Si
Si
-
0.5
N
N
tBu
tBu
L
[
2Si2P2]
Figure 3. Molecular structure of 2. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): P1-Si2
2.1093(8), P1-Si1 2.1925(8).
Figure 1. Molecular structure of 1. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Si-P1
2.1707(13), Si-P4 2.1729(12), P1-P2 2.1391(13), P2-P3
2.1597(15), P3-P4 2.1444(13).
The 31P{1H} NMR spectrum of 1 shows an AA’XX’ spin system
with two sets of multiplets at  -194.4 and 50 ppm (Figures 2,
S25). Such an AA’XX’ spin system is typical for an open-chain P4
unit.18 The multiplet at  -194.4 ppm can be unambiguously
assigned to the P1 and P4 atoms (PAA’) due to the presence of
well-defined Si satellite peaks with a 1JSi-P coupling constant of
145.5 Hz (Table S1). Accordingly, the 29Si{1H} NMR spectrum
showed a triplet at  42.3 ppm (1JSi-P=145.4 Hz). The formation
of complex 1 (Route A) can be rationalized by the insertion of a
formal [LSi] moiety in the cyclo-P5 ring of [Cp*Fe(5-P5)],
accompanied by the elimination of one P atom from the cyclo-
P5 ring and one Cl atom from [LSiCl]. The eliminated P and Cl
atoms are scavenged by two other molecules of [LSiCl], forming
a base-stabilized phosphasilene (2). The solid-state structure
of
2 confirmed the formation of the functionalized
phosphasilene (Figure 3). The Si2-P bond length (2.1093(8) Å)
is in the range of the reported SiP bonds.16a, 17a, 19 Interestingly,
the Si1-P bond distance (2.1925(8) Å) is in-between the
distances for Si-P single and double bonds, suggesting a
partially double-bond character. The 31P{1H} NMR spectrum
showed a singlet at  -182.7 ppm with two Si satellites (1JSi-
Figure 2. 31P{1H} NMR spectrum at 298 K of 1 with nuclei
assigned to an AA’XX’ spin system; insets: extended signals
(upwards) and simulations (downwards).
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P=167 Hz). In solution, complex 2 slowly decomposes into two
in the intermediate after the reduction of the P5 system is of a
polarized strong single-bond character.
known species, [LSiCl3]15 and [L2Si2P2]16 (Figure S11).
1
2
3
4
The electronic properties of 1 are not immediately obvious.
It could be described as either: a) an aromatic 6π-electron
Scheme 2. Synthesis of complex 3.
2-
system, b) a P4 ligand sandwiched between [Cp*Fe]+ and
[LSi]+ or c) a P44- ligand between [Cp*Fe]+ and a [LSi]3+ cation.
The classification of the SiP4 ligand as an aromatic 6π-electron
system may be ruled out because of the strong bending of the
positively charged Si from the P4 plane. To decide whether to
consider silicon formally as silylene or as Si (+IV), the partial
atomic charges Q(Si) from Ahlrichs-Heinzmann population
analyses and the bond distances r(Si-N) can help. The
comparison of the partial charges Q(Si) of the silylenes [LSiCl]
and [LSi(N(SiMe3)2] with those of the molecules with Si in the
formal oxidation state +IV like in 2 shows that, in the former
group, the values are about +0.1, while, in the reference system
2, with Si(+IV) it is about +0.4 to +0.5. Furthermore, in both
silylenes, the Si-N distances to the amidinate ligand are
significantly prolonged. This is also found, e.g., in the
comparison of AlF and AlF3: in the sub-valent compound, the
Al-F distance is significantly longer due to the partial s
character of the lone pair on the central atom.22 Therefore, in 1,
2, A and 3, the silicon atoms are considered to be of the formal
oxidation state +IV (Table S5). On the basis of these findings,
the formal explanation of the bonding situation as a P44- ligand
between [Cp*Fe]+ and a [LSi]3+ cation would be justified. The
bending of Si out of the expected SiP4 plane is explained by the
repelling positive charges.
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tBu
Fe
P
P
N
P
toluene
P
Ph
Si
P
Fe
P
Si
tBu
+
N
-78 C to rt
N
P
P
P
Si
N
Si
N
Si
P
tBu
N
Si
9
Ph
tBu
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3
As a possible reaction pathway, we suggest the formation of
an intermediate, A (Route A, Scheme 1), which further reacts
with [LSiCl], resulting in the formation of 1 and 2. VT-NMR
studies show that the reaction is more complex than indicated
in Scheme 1, explaining the moderate yields of 1 and 2 (Figures
S2-3). To isolate an analog of the possible intermediate A, a
relatively bulkier silylene, [LSi(N(SiMe3)2)],20 was selected as
an analog of [LSiCl]. Reaction of [LSi(N(SiMe3)2)] with
[Cp*Fe(5-P5)] in toluene at room temperature resulted in the
formation of [{LSi(N(SiMe3)2)}{(4-P5)FeCp*}] (3) (Scheme 2).
The solid-state structure of 3 (Figure 4) resembles that of the
suggested intermediate A. At room temperature, the 31P{1H}
NMR spectrum of 3 shows a broad signal at  35.3 ppm. A well-
resolved spectrum with an AA’MXX’ spin system could be
observed at 193 K, with signals at  -29.9 (PAA’), 28.4 (PM), and
30.7 (PXX’) ppm (Figures S17, S26-S27). The 29Si{1H} NMR
spectrum exhibits three resonances at =-34.9 (SiMe3), 6.0
Scheme 3. Synthesis of complex 4.
1
(SiMe3) and 10.8 (Si1) ppm (Figures S12). No JSi-P coupling
could be identified at room temperature, indicating a possible
lability of the Si1-P bond.
tBu
N
tBu
N
Fe
P
+
2
2
Ph
Si
Si
Ph
P
P
N
N
P
P
tBu
tBu
toluene
-78 C to rt
Fe
P
P
P
P
tBu
N
tBu
N
P
tBu
Ph
tBu
Ph
P
P
N
N
Si
Si
1
Ph
+
+
0.5Ph
Si
Si
N
N
N
N
tBu tBu
tBu
tBu
4
L
(
2Si2P2)
Figure 4. Molecular structure of 3. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Si1-P1
2.2722(5), P1-P2 2.1725(6), P1-P5 2.1654(6), P2-P3
2.1483(8), P3-P4 2.1252(10), P4-P5 2.1533(9).
To examine the possibility of ring expansion reactions, we
studied the reaction between [LSi-SiL]23
formal Si(I)
,
a
compound, and [Cp*Fe(5-P5)] (Scheme 3). The 31P{1H} NMR
spectrum of the crude reaction mixture showed the formation
of 1, [L2Si2P2] and [{(4-P5(SiL)2}FeCp*] (4) as a new product
(Figure S22). Compound 4 was isolated after work-up as
yellow-orange colored crystals. The analysis of the solid-state
structure of 4 (Figure 5) revealed the insertion of two [LSi]
silicon moieties into two P-P bonds of the cyclo-P5 ring. In a
similar approach as for the synthesis of 1, we also tried to
access 4 from a reduced anionic form of [Cp*Fe(5-P5)]
([K(dme)2K(dme)][{Cp*Fe}2(4:4-P10)]13b) and [LSiCl]. In situ
NMR studies showed that 4 is formed in low yields. As by-
To gain insight into the formation process and the electronic
properties of 1, density functional theory (DFT) calculations
(RI-BP86 level, basis sets of each atom of def-SV(P)-quality)
were carried out. Especially, the formation of the suggested
elusive intermediate A, which may be structurally similar to 3,
was investigated theoretically to confirm the process proposed
in Scheme 1. The formation of A is exothermic by -5.2 kJ mol-1,
the further reaction with two more silylenes forming 1 and 2 is
exothermic by -136.8 kJ mol-1. In comparison, complex 3
formed by using [LSi(N(SiMe3)2] is much more stable than the
educts (by -26.2 kJ mol-1). According to an Ahlrichs-Heinzmann
population analysis21 of intermediate A, the Si-P bond formed
products, [(LSi)2P2], [Cp*Fe(η5-P5)],
1 and two unstable
intermediates were observed by VT-NMR studies (Figure S23).
No further attempts were made to explore this reaction on a
preparative scale.
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The 31P{1H} NMR spectrum of 4 reveals an AMM’XX’ spin
system. Simulations using an iterative fitting procedure
revealed two sets of multiplets for the four atoms
coordinated to the [Cp*Fe]+ moiety at =-47.6 (PMM’) and 80.8
(PXX’) ppm (Figure S28). The P atom between the two Si atoms
(PA) in the ring is upfield shifted to  -163.5 ppm (1JSi-P=118.3
Hz). The multiplet at  -47.6 ppm is unambiguously assigned to
P2 and P5 on the basis of the well-defined Si satellite signals
(1JSi-P=169.3 Hz). Accordingly, the 29Si{1H} NMR spectrum
showed a doublet of doublets at  31.9 ppm with the same 1JSi-P
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Experimental procedures and characterization details, NMR,
IR, computational and structural information (PDF).
X-ray crystallographic data CCDC 1919229-1919231 and
1919233 (CIF).
1
2
3
4
5
6
7
8
P
9
coupling constants. The ring-expanded complex
4 also
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represents the largest structurally characterized silicon-
polyphosphorus ring in a coordination sphere.24 The Si1-P2
(2.2151(8) Å) and Si2-P5 (2.2126(9) Å) bond lengths are
slightly shorter than Si-P single bonds (2.24-2.27 Å).16a, 17a, 19 In
contrast, the Si1-P1 (2.1419(8) Å) and Si2-P1 (2.1407(9) Å)
distances are in-between single (2.24-2.27 Å) and double
bonds (2.06-2.09 Å).16a, 17a, 19 The P-P bond lengths (2.1416(9)-
2.1612(9) Å) in 4 are also in the same range as in 1
(2.1391(13)-2.1597(15) Å), indicating a partially double bond
character.17a Interestingly, the Si-P seven-membered ring is
isolobal to the tropylium anion. Therefore, the expansion of the
cyclo-P5 ring to a cyclo-Si2P5 ring is a significant advancement
in the silicon and phosphorus chemistry.
AUTHOR INFORMATION
Corresponding Authors
*roesky@kit.edu, manfred.scheer@chemie.uni-regensburg.de.
ORCID
Ravi Yadav: 0000-0003-2375-7607
Thomas Simler: 0000-0002-0184-303X
Ralf Köppe: 0000-0002-0492-0803
Peter W. Roesky: 0000-0002-0915-3893
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We are grateful to the Deutsche Forschungsgemeinschaft
(DFG) (No. 266153560, Ro 2008/17-2 and Sche 384/33-2) for
financial support. TS thanks the Alexander von Humboldt
Foundation for
a postdoctoral fellowship. The authors
acknowledge computational support by the state of Baden-
Württemberg through bwHPC and the Deutsche
Forschungsgemeinschaft (DFG) through grant No INST
40/467-1 FUGG. We dedicate this contribution to Prof. Herbert
W. Roesky on the occasion of his 85th birthday.
REFERENCES
Figure 5. Molecular structure of complex 4. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å): Si1-P1
2.1419(8), Si2-P1 2.1407(9), Si1-P2 2.2151(8), Si2-P5
2.2126(9), P2-P3 2.1416(9), P3-P4 2.1612(9), P4-P5
2.1454(9).
(1) Carey, F. A.; Sundberg, R. J., Advanced Organic Chemistry : Part B:
Reactions and Synthesis. Fifth Edition ed.; Springer US: Boston, MA,
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(2) (a) Smith, M. B.; March, J., Marchs advanced organic chemistry :
reactions, mechanisms, and structure. 7. ed. ed.; Wiley: Hoboken, NJ,
2013. (b) Smith, P. A. S.; Baer, D. R., The Demjanov and Tiffeneau-
Demjanov Ring Expansions. In Organic Reactions, 1960; Vol. 11, pp
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(3) ten Brink, G. J.; Arends, I. W. C. E.; Sheldon, R. A., The Baeyer−Villiger
Reaction:ꢀ New Developments toward Greener Procedures. Chem. Rev.
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Phosphaethynolate Anion. Angew. Chem. Int. Ed. 2018, 57 (52), 16968-
16994. (b) Coburger, P.; Grützmacher, H.; Hey-Hawkins, E., Molecular
In summary, we have shown that (i) the P atoms in P-cycles
can be removed and selectively substituted by isoelectronic
[LSi] fragments and (ii) that the [LSi] fragment can be inserted
via a ring expansion into phosphorus cycles. As a result, we
have synthesized the first example of a sila-phosphaferrocene
(1) by substituting one P atom in the cyclo-P5 ring of [Cp*Fe(5-
P5)]. In addition, the by-product of the reaction, an
amidinatedichlorosilyl group-functionalized phosphasilene
(2), was isolated, allowing a better understanding of the
reaction pathway between silylene and [Cp*Fe(5-P5)]. The
isolation of the possible analog of the reaction intermediate (3)
was achieved by using a silylene bearing a bulkier group. To
examine further ring expansions, the insertion of a base-
stabilized di-silylene into the cyclo-P5 ring of [Cp*Fe(5-P5)]
was studied. The resulting complex 4 possesses the largest
silicon-polyphosphorus ring as a ligand known to date. This
work bridges a gap between the low-valent silicon chemistry
and transition metal polyphosphides, both of which are
important parts of the current research in the field of
organometallic and heterocyclic chemistry.
doping:
accessing
the
first
carborane-substituted
1,2,3-
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