Phosphorus-Chalcogen Ring Expansion and Metal Coordination

Article abstract of DOI:10.1021/acs.inorgchem.7b02217

The reactivity of 4-membered (RPCh)₂ rings (Ch = S, Se) that contain phosphorus in the +3 oxidation state is reported. These compounds undergo ring expansion to (RPCh)₃ with the addition of a Lewis base. The 6-membered rings were found to be more stable than the 4-membered precursors, and the mechanism of their formation was investigated experimentally and by density functional theory calculations. The computational work identified two plausible mechanisms involving a phosphinidene chalcogenide intermediate, either as a free species or stabilized by a suitable base. Both the 4- and 6-membered rings were found to react with coinage metals, giving the same products: (RPCh)₃ rings bound to the metal center from the phosphorus atom in tripodal fashion.

Full text of DOI:10.1021/acs.inorgchem.7b02217

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Phosphorus-Chalcogen Ring Expansion and Metal Coordination
Cameron M. E. Graham,† Juuso Valjus,‡ Taylor E. Pritchard,† Paul D. Boyle,† Heikki M. Tuononen,*,‡
and Paul J. Ragogna*,†
†Department of Chemistry and the Centre for Advanced Materials and Biomaterials Research, Western University, 1151 Richmond
Street, London, Ontario N6A 5B7, Canada
‡Department of Chemistry, Nanoscience Centre, University of Jyvaskyla, FI 40014 Jyvaskyla, Finland
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The reactivity of 4-membered (RPCh)2 rings (Ch = S, Se) that
contain phosphorus in the +3 oxidation state is reported. These compounds
undergo ring expansion to (RPCh)3 with the addition of a Lewis base. The 6-
membered rings were found to be more stable than the 4-membered precursors,
and the mechanism of their formation was investigated experimentally and by
density functional theory calculations. The computational work identified two
plausible mechanisms involving a phosphinidene chalcogenide intermediate,
either as a free species or stabilized by a suitable base. Both the 4- and 6-
membered rings were found to react with coinage metals, giving the same
products: (RPCh)3 rings bound to the metal center from the phosphorus atom
in tripodal fashion.
Although select examples of (RPCh)n have been reported
since the early 1980s, a thorough examination of their
chemistry has not been published, though related P(III)-Ch-
N macrocycles have been reported to form polynuclear Ag(I)
sandwich complexes upon reaction with various amounts of
AgOTf (Figure 1, E).7 In this context, we report herein on the
chemistry of 1Ch and the possibility to use them to access
larger (RPCh)3 rings 2Ch (Ch = S, Se). Although consistent
reactivity between 1S and 1Se is not observed, identical
derivatives can still be prepared using different synthetic
approaches. In particular, the reaction of 1S with Lewis bases
gives 2S, whereas the selenium analogue 2Se can be synthesized
and isolated via cyclocondensation of Ar*PCl2 and Se(TMS)2.
We have also discovered that by introducing coinage-metals Cu
and Ag to 1Ch, ring expansion products (RPCh)3 are observed
for both chalcogens. The rings were found to be further bound
to the employed metal cation in a tripodal fashion through
phosphorus, giving coordination complexes 7ChM (Ch = S, Se;
M = Cu, Ag); the same coordination complexes can also be
prepared directly from 2Ch.
INTRODUCTION
■
Small inorganic ring systems have been of interest to chemists
for decades as they normally offer a window into unique
structure, bonding, and reactivity.1−4 A comprehensive review
regarding the breadth of these compounds was recently
published by Rivard and co-workers.5 Inorganic rings
containing phosphorus-p-block element cores have been
extensively explored and reported in the literature. Of this
large collection of phosphorus-containing main group rings,
phosphorus-chalcogen (P-Ch) heterocycles have been a
particular focus area because of their widespread application
as ligands for transition metals,6,7 and “P” or “Ch” transfer
reagents.8 Major developments in P-Ch heterocyclic chemistry
feature P(V) compounds, with noteworthy examples being
Lawesson’s (Ch = S)9 and Woollins’ (Ch = Se)10 reagents
(Figure 1, A) that contain P2Ch2 rings with each phosphorus
further oxidized by an additional sulfur or selenium atom,
respectively.11−13 These have proven to be indispensable
reagents in a variety of synthetic transformations14−18 and
material applications.19,20 Although there have been extensive
studies on P(V)-Ch derivatives, examples of P(III) within the
ring core are more uncommon, especially in regards to those
with a general formula (RPCh)n. Examples of (RPCh)n have
been reported for n = 2, 3, and 4, where bulky R groups are
required to kinetically stabilize the P(III) center (Figure 1B−
D).21−26 In particular, our group has recently reported the
syntheses of strained (RPCh)2 rings 1Ch (Ch = S, Se) with P in
the +3 oxidation state, the first representatives of these strained
compounds, by installing m-terphenyl substituents at phospho-
rus.24
EXPERIMENTAL METHODS
■
All manipulations were performed under an inert atmosphere either in
a nitrogen-filled MBraun Labmaster 130 glovebox or on a Schlenk line.
HNEt2 was purchased from Sigma-Aldrich and distilled from KOH.
PCl3 was purchased from Sigma-Aldrich and distilled/degassed prior
to use. Gaseous HCl was prepared in situ by dropping neat H2SO4 to
CaCl2 powder, bubbled through H2SO4 then through the reaction
mixture and ultimately through a NaHCO3 outlet bubbler to neutralize
Received: August 28, 2017
© XXXX American Chemical Society
A
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Figure 1. Left: Previously reported phosphorus-chalcogen rings, (A) Woollins’ and Lawesson’s reagents with P(V), (B−D) (RPCh)n with P(III),
and (E) polynuclear Ag sandwich complex. Right: (i) Synthesis of 2S via base-induced ring expansion, (ii) synthesis of 2Se by cyclocondensation of
Ar*PCl2 and Se(TMS)2, (iii) ring expansion and/or metal coordination to coinage metals. Ar* = 2,6-Mes2C6H3, Mes = 2,4,6-(CH3)3C6H2, Mes* =
2,4,6-(tBu)3C6H2, Im = imidazolium-2-yl.
28−31
excess HCl. Ch(TMS)2 (Ch = S, Se)27 and Ar*PCl2
were made
C−H), 6.83 (br s, 12 H, Mesityl C−H), 7.02 (t, 3 H, JH−H = 7.4 Hz,
3
following known literature procedures. CuCl, AgOTf, (Me2S)AuCl,
and DMAP were purchased from Sigma-Aldrich and used as received.
Solvents were obtained from Caledon and dried using an MBraun
solvent purification system. Dried solvents were collected under
vacuum in a flame-dried Straus flask and stored over activated 4 Å
molecular sieves. Solvents for NMR spectroscopy (CDCl3 and C6D6)
were stored in the drybox over activated 4 Å molecular sieves. Nuclear
Magnetic Resonance (NMR) spectroscopy was recorded on Varian
INOVA 400 MHz (1H 400.09 MHz, 31P{1H} 161.8 MHz) or 600
Aromatic C−H). 31P{1H} NMR (C6D6, 161.8 MHz, δ): 96.7 (s).
13C{1H} NMR (C6D6, 150.8 MHz, δ): 21.5, 21.8, 29.9, 130.1, 130.7,
133.6, 136.9, 137.8, 146.4. EI-MS: 1151.4 m/z, C72H75P3S3Na [M +
Na]+. FT-IR (cm−1 (ranked relative intensities), KBr): 625 (11), 732
(4), 750 (14), 807 (1), 845 (8), 906 (6), 1016 (5), 1216 (7), 1261
(13), 1374 (12), 1445 (2), 1559 (9), 1610 (10), 1643 (15), 2915 (3).
Synthesis of 2Se. A solution of Se(TMS)2 (264 mg, 1.17 mmol,
10 mL THF) was added to a solution of Ar*PCl2 (487 mg, 1.17 mmol,
20 mL THF) portionwise over 3 h. The volatiles were removed in
vacuo, leading to an orange oil. The oil was washed with pentane (3 ×
10 mL) and insoluble 1Se was filtered and collected (yield: 348 mg,
86%). The pentane fractions were combined, concentrated in vacuo,
and 1 mL of DCM was added to the oil, which resulted in the
precipitation of a yellow powder when left at −35 °C for 2 h. X-ray
quality single crystals of 2Se were grown via vapor diffusion using
DCM/Toluene (yield: 49 mg, 10%). mp: decomposes at 215 °C
1
MHz (13C{1H} 150.8 MHz) spectrometers. All samples for H NMR
spectroscopy were referenced to the residual protons in the deuterated
solvent relative to tetramethylsilane (CDCl3; δ = 7.26, C6D6; δ =
7.16). The chemical shifts for 31P{1H} NMR spectroscopy were
referenced using an external standard (85% H3PO4; δ = 0.0). FT-IR
spectroscopy was performed on samples as KBr pellets using a Bruker
Tensor 27 FT-IR spectrometer with 4 cm−1 resolution. Mass
spectrometry was recorded in positive- and negative-ion modes
using an electrospray ionization Micromass LCT spectrometer.
Melting or decomposition points were determined by flame-sealing
samples in capillaries and heating using a Gallenkamp variable heater.
Elemental analyses were performed at the University of Montreal and
are reported as an average of two samples weighed under air and
combusted immediately thereafter. Multiple samples of spectroscopi-
cally pure 2S were sent away for elemental analysis; however, we could
not obtain chemically sensible data. For this case, we have omitted EA
data for this compound.
Note: Metal-coordination reactions were prepared with strict
exclusion from ambient light as compounds 7ChM and 8Ch are
light-sensitive. 7ChM and 8Ch decomposed when left in solution
overnight at room temperature or in the solid-state at room
temperature for more than 2 days. These challenges precluded
obtaining accurate elemental analyses and quaternary carbons in the
corresponding 13C{1H} NMR spectra could not be identified. In the
case of 8Ch, multiple samples were prepared for mass spectroscopic
characterization, including the use of different ionization methods,
however we could not obtain chemically sensible data.
1
(turns dark brown). H NMR (C6D6, 600 MHz, δ): 2.08 (s, 36 H,
3
Mesityl o-CH3), 2.30 (s, 18 H, Mesityl p-CH3), 6.78 (d, 6 H, JH−H
=
7.5 Hz, Aromatic C−H), 6.84 (br s, 12 H, Mesityl C−H), 7.03 (t, 3 H,
3JH−H = 7.5 Hz, Aromatic C−H). 31P{1H} NMR (C6D6, 161.8 MHz,
δ): 87.8 (s, 1JP−Se = 136.6 Hz). 13C{1H} NMR (C6D6, 150.8 MHz, δ):
21.6, 21.8, 128.3, 130.1, 130.3, 137.0, 138.0, 146.0. 77Se NMR (C6D6,
114.4 MHz, δ): 421.0 (ddd, 1JSe−P = 202.6, 133.3 Hz, 3JSe−P = 67.1 Hz).
EI-MS: 1271.3 m/z, C72H75P3Se3 [M+]. FT-IR (cm−1 (ranked relative
intensities), KBr): 85 (4), 159 (15), 236 (12), 321 (9), 391 (7), 414
(13), 576 (3), 738 (11), 1035 (6), 1302 (2), 1381 (8), 1576 (10),
1611 (5), 2915 (1), 3037 (14). Elemental Anal. Calcd 68.08% C,
5.95% H. Exptl 67.60% C, 6.48% H.
General Methods for Coordination Chemistry with Coinage
Metals. A solution of 1Ch in DCM was added to a suspension of MX
in DCM and the mixture let stir at room temperature for 1 h. The
volatiles were removed in vacuo and the crude powder was washed
with diethyl ether (3 × 5 mL), giving an insoluble powder that was
collected.
Synthesis of 7SCu. Reagents: 1S (75 mg, 0.097 mmol, 4 mL
DCM) and CuCl (6.0 mg, 0.067 mmol, 4 mL DCM); yield: 55 mg,
61%. Single crystals were grown via vapor diffusion using DCM/
pentane. mp: decomposes at 131 °C (turns brown). 1H NMR
(CDCl3, 600 MHz, δ): 1.80 (s, 36 H, Mesityl o-CH3), 2.24 (s, 18 H,
Synthesis of 2S. Solid 4-dimethylaminopyridine (6.5 mg, 0.053
mmol) was added to a solution of 1S (20 mg, 0.027 mmol, 2 mL
THF) and the mixture was heated for 16 h at 50 °C. The volatiles
were removed in vacuo and the yellow powder was dissolved in 1 mL
DCM and slowly concentrated leading to single crystals of 2S (yield:
13 mg, 70%). Alternatively, the reaction mixture could be heated at 35
°C for 7 days, which resulted in increased yields (>90%). mp: 210−
211 °C. 1H NMR (C6D6, 400 MHz, δ): 2.02 (s, 36 H, Mesityl o-CH3),
2.27 (s, 18 H, Mesityl p-CH3), 6.77 (d, 6 H, 3JH−H = 7.4 Hz, Aromatic
Mesityl p-CH3), 6.81 (br s, 12 H, Mesityl C−H), 6.90 (d, 6 H, 3JH−H
=
3
7.8 Hz, Aromatic C−H), 7.43 (t, 3 H, JH−H = 7.8 Hz, Aromatic C−
H). 31P{1H} NMR (CDCl3, 161.8 MHz, δ): 88.6 (s). 13C{1H} NMR
(CDCl3, 150.8 MHz, δ): 21.4, 21.6, 129.1, 130.8, 131.8, 136.3, 137.5,
146.0. EI-MS: 1191.4 m/z, C72H75P3S3Cu [M - Cl]+. FT-IR (cm−1
(ranked relative intensities), KBr): 733(4), 749 (12), 809 (7), 845 (6),
B
DOI: 10.1021/acs.inorgchem.7b02217
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Figure 2. Cyclocondensation of Ar*PCl2 with Ch(TMS)2 yielding 1S (right), 1Se, and 2Se (left). Although base-induced ring expansion yields 2S
from 1S in high yield, the addition of a base to 1Se results in multiple products, including 2Se.
Figure 3. Top: Using 1Ch as sources of monomeric phosphinidene chalcogenides, trapping 3Ch with dmbd (4Ch, dmbd = 2,3-dimethyl-1,3-
butadiene) or by addition of an NHC (5S, NHC = 1,3-isopropyl-4,5-dimethylimidazol-2-ylidene). Bottom: Proposed reaction pathway for ring
expansion of 1Ch induced by DMAP/heating and proceeding through the suggested intermediate 6Ch.
907 (5), 1030 (10), 1374 (8), 1448 (2), 1480 (14), 1562 (9), 1611
(3), 2853 (13), 2915 (1), 2944 (15), 2968 (11).
intensities), KBr): 634 (3), 731 (5), 807 (8), 854 (6), 910 (13),
1024 (1), 1164 (10), 1213 (15), 1231 (2), 1303 (12), 1378 (11), 1448
(4), 1562 (14), 1608 (9), 2917 (7).
Synthesis of 7SAg. Reagents: 1S (75 mg, 0.097 mmol, 4 mL
DCM) and AgOTf (16 mg, 0.065 mmol, 4 mL DCM); yield: 45 mg,
50%. Single crystals were grown via vapor diffusion using DCM/
pentane. mp: decomposes at 141 °C (turns dark gray). 1H NMR
(CDCl3, 400 MHz, δ): 1.82 (s, 36 H, Mesityl o-CH3), 2.25 (s, 18 H,
Synthesis of 8S. Reagents: 1S (25 mg, 0.033 mmol, 5 mL DCM)
and CuCl (4.3 mg, 0.043 mmol, 5 mL DCM). yield: 12 mg, 61%.
Single crystals were grown via vapor diffusion using DCM/pentane.
1
mp: decomposes at 119 °C (turns brown). H NMR (CDCl3, 400
Mesityl p-CH3), 6.87 (br s, 12 H, Mesityl C−H), 6.97 (d, 6 H, 3JH−H
=
MHz, δ): 1.92 (s, 36 H, Mesityl o-CH3), 2.12 (s, 18 H, Mesityl p-
3
3
CH3), 6.78 (br s, 12 H, Mesityl C−H), 6.88 (d, 6 H, JH−H = 6.4 Hz,
7.6 Hz, Aromatic C−H), 7.53 (t, 3 H, JH−H = 7.6 Hz, Aromatic C−
H). 31P{1H} NMR (CDCl3, 161.8 MHz, δ): 109.0 (br s). 13C{1H}
NMR (CDCl3, 150.8 MHz, δ): 21.2, 21.5, 128.8, 131.1, 132.8, 136.6,
138.6, 146.2. 19F{1H} NMR (CDCl3, 376.3 MHz, δ): −77.4 (s). EI-
MS: 1237.3 m/z, C72H75P3S3Ag [M - OTf]+. FT-IR (cm−1 (ranked
relative intensities), KBr): 635 (9), 749 (13), 807 (11), 852 (7), 1027
(4), 1113 (14), 1162 (10), 1182 (15), 1232 (2), 1305 (8), 1377 (6),
1449 (3), 1563 (12), 1610 (5), 2918 (1).
3
Aromatic C−H), 7.46 (t, 3 H, JH−H = 6.4 Hz, Aromatic C−H).
31P{1H} NMR (CDCl3, 161.8 MHz, δ): 61.3 (s). 13C{1H} NMR
(CDCl3, 150.8 MHz, δ): 21.4, 22.2, 130.1, 130.8, 131.7, 132.1, 137.0,
146.7. FT-IR (cm−1 (ranked relative intensities), KBr): 728 (3), 748
(12), 807 (5), 850 (2), 908 (6), 1031 (7), 1113 (13), 1182 (11), 1377
(9), 1443 (1), 1561 (10), 1609 (8), 2854 (14), 2914 (4), 2946 (15).
Synthesis of 8Se. Reagents: 1Se (50 mg, 0.0590 mmol, 7 mL
DCM) and CuCl (7.8 mg, 0.0788 mmol, 7 mL DCM); yield: 32.6 mg,
80%. Single crystals were grown via vapor diffusion using DCM/
pentane. mp: decomposes at 160 °C (turns black). 1H NMR (CDCl3,
400 MHz, δ): 1.94 (s, 36 H, Mesityl o-CH3), 2.24 (s, 18 H, Mesityl p-
Synthesis of 7SeCu. Reagents: 1Se (40 mg, 0.047 mmol, 5 mL
DCM) and CuCl (3.0 mg, 0.032 mmol, 5 mL DCM). 31P NMR
spectra of crude reaction mixture showed formation of 7SeCu (δP =
72) and free 2Se (δP = 88). Attempts to separate 2Se and 7SeCu by
fractional crystallization were unsuccessful. Attempts to convert 2Se to
7SeCu resulted in multiple products in the 31P NMR spectrum,
including 8Se (Figure S3).
3
CH3), 6.82 (br s, 12 H, Mesityl C−H), 6.86 (d, 6 H, JH−H = 7.6 Hz,
3
Aromatic C−H), 7.45 (t, 3 H, JH−H = 7.6 Hz, Aromatic C−H).
31P{1H} NMR (CDCl3, 161.8 MHz, δ): 45.9 (br s). 13C{1H} NMR
(CDCl3, 150.8 MHz, δ): 21.5, 22.0, 130.0, 130.8, 131.7, 132.0, 135.8,
136.9, 138.9, 146.5. FT-IR (cm−1 (ranked relative intensities), KBr):
719 (15), 748 (5), 807 (1), 846 (6), 1030 (2), 1090 (14), 1110 (10),
1183 (12), 1263 (7), 1376 (9), 1445 (3), 1562 (11), 1610 (8), 1720
(13), 2964 (4).
Synthesis of 7SeAg. Reagents: 1Se (50 mg, 0.039 mmol, 4 mL
DCM) and AgOTf (6.7 mg, 0.026 mmol, 4 mL DCM); yield: 27 mg,
69%. Single crystals were grown via vapor diffusion using DCM/
pentane. mp: decomposes at 171 °C (turns dark gray). 1H NMR
(CDCl3, 400 MHz, δ): 1.92 (s, 36 H, Mesityl o-CH3), 2.27 (s, 18 H,
3
Mesityl p-CH3), 7.03 (d, 6 H, JH−H = 7.6 Hz, Aromatic C−H), 7.15
(s, 12 H, Mesityl C−H), 7.63 (t, 3 H, 3JH−H = 7.6 Hz, Aromatic C−H).
31P{1H} NMR (CDCl3, 161.8 MHz, δ): 113.3 (d, 1J109Ag−P = 213.2 Hz,
1J107Ag−P = 185.8 Hz, 1JSe−P = 151.8 Hz). 13C{1H} NMR (CDCl3, 150.8
MHz, δ): 20.9, 21.4, 129.7, 131.3, 133.7, 135.1, 136.8, 140.5, 146.4.
19F{1H} NMR (CDCl3, 376.3 MHz, δ): −76.4 (s). EI-MS: 1379.2 m/
z, C72H75P3Se3Ag [M - OTf]+. FT-IR (cm−1 (ranked relative
RESULTS AND DISCUSSION
■
Formation of P/Ch Heterocycles. The synthesis of 1Ch
resulted from the cyclocondensation of Ar*PCl2 with Ch-
(TMS)2 (Figure 2). Although the sulfur chemistry proceeds
cleanly, multiple products are present in the crude reaction
C
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Figure 4. Reaction coordinate diagram for DMAP-induced (RPS)n ring expansion (phosphorus atoms are colored orange, sulfur being yellow, and
nitrogen blue). For simplicity, interconversion of different conformers of 2S is omitted (dashed arrow). Gibbs energies are reported in kJ mol−1 and
relative to 1S + 6S.
mixture for Ch = Se. The major product 1Se (δP = 23) could be
isolated by washing the crude reaction mixture with pentane,
yielding an orange powder. A minor product from this reaction
was isolated by precipitating out a yellow powder from a DCM
solution at −35 °C (δP = 88). Slowly concentrating a DCM
solution resulted in single crystals suitable for X-ray diffraction
that confirmed the preparation of 2Se. Although numerous
examples of (RPS)3 are known, 2Se represents the first
selenium derivative, and is one of the few known examples of
heavier chalcogens in such a ring system.23
After successfully isolating 1Ch, we endeavored to utilize
these strained compounds as precursors to the monomeric R-
PCh, which have been elusive intermediates in low valent
phosphorus-chalcogen chemistry. In our initial work (Figure 3,
top), 1Ch could be monomerized by gentle heating, giving
solution accessible R-PCh that could be trapped using 2,3-
dimethyl-1,3-butadiene (dmbd) to give 4Ch. Alternatively,
monomerization of 1S could be induced through the addition
of 1,3-isopropyl-4,5-dimethylimidazol-2-ylidene (NHC), which
resulted in the formation of the adduct 5S even without
heating.24 Taking our lead from these results, the reaction of
DMAP with 1S in 2:1 stoichiometry yielded one major product
in the 31P{1H} NMR spectrum at δP = 97, an upfield shift from
the parent 1S (δP = 127), and significantly downfield shifted
from 5S (δP = 28). Slowly concentrating a DCM solution at
−35 °C resulted in pale-yellow single crystals. Single-crystal X-
ray diffraction experiments revealed that the product is not a
simple DMAP adduct of 1S but instead the 6-membered ring
expansion species 2S, analogous to 2Se. 31P{1H} NMR spectra
obtained from solutions containing redissolved crystals of 2S
showed the same distinct singlet (δP = 97) observed in the
crude reaction mixture, whereas the 1H NMR spectra showed a
single set of terphenyl protons, indicating equivalence of
aromatic substituents on the NMR time scale.
analogous reaction using 1Se gave numerous products, as
determined by 31P{1H} NMR spectroscopy. Even though none
of these products could be isolated and characterized, the NMR
spectroscopic data clearly indicated that ring expansion to 2Se
did occur (δP = 88). Other structurally related Lewis bases such
as pyridine and 2,6-lutidene also generated 2Se, but again with
multiple other side products (Figure S1). Alternatively to
nitrogen bases, phosphine-based donors (PEt3, PCy3, and
PPh3) produced multiple products as determined by 31P{1H}
NMR spectroscopy. Interestingly, there was no evidence for a
ring expansion product 2Se, however evidence for the
corresponding phosphine-selenides were present in the
31P{1H} NMR spectra (R3PSe; Figure S2), indicative of a
competing process upon addition of alkyl/aryl phosphines.
Taking all of the above into account, a possible mechanism
for the formation of 2Ch can be envisioned (Figure 3, bottom).
The reaction most likely begins by breaking up of 1Ch that is
induced either by the employed base (cf. formation of 5S) or
simply by heating (cf. formation of 4Ch). In the presence of
excess DMAP, the initial product should in both cases be the
adduct 6Ch that could subsequently undergo insertion into
1Ch, leading to dissociation of DMAP and formation of 2Ch
(Figure 3, bottom). This mechanism is experimentally
supported by the appearance of free DMAP in the crude
reaction mixture once 1S is fully consumed. Furthermore,
heating samples of 1S without DMAP does not lead to the
formation of 2S. However, as 2Se is formed even during the
synthesis of 1Se, the potential energy surface for the formation
of 2Ch is obviously not completely independent of the
chalcogen atom.
The reaction pathway for forming 2S was examined using
density functional theory at the PBE1PBE/TZVP level
augmented with an empirical dispersion correction. The
calculations show that DMAP does not monomerize 1S as
efficiently as was observed earlier in the case of an N-
Although the ring expansion product was the only species
obtained from the reaction between 1S and DMAP, the
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Figure 5. Ring expansion of 1Ch (Ch = S, Se) using coinage metals (MX = CuCl, AgOTf) resulting in 7ChM. Similar products were obtained from
the addition of 2Ch to MX. Addition of excess CuCl to either 1Ch or 2Ch resulted in the formation of 8Ch.
Figure 6. Stack plot of 31P{1H} NMR spectra of different sulfur derivatives reported herein: parent 1S and ring expansion products 2S, 7SCu, 7SAg,
and 8S.
heterocyclic carbene. However, the calculated activation barrier
for the formation of two stoichiometric equivalents of 6S (95 kJ
mol−1) is in any case considerably lower than that found for the
direct, heat induced, monomerization of 1S to two equivalents
of 3S (124 kJ mol−1).32 Furthermore, while the formation of 3S
is highly endergonic (118 kJ mol−1), the base-assisted
monomerization of 1S to two equivalents of 6S is only slightly
so (44 kJ mol−1). This shows that DMAP efficiently stabilizes
the R-PS unit, as expected, but does it so that further
reactivity of the monomer is not hindered. This is in contrast to
what was observed for the NHC adduct 5S, the formation of
which was calculated to be exergonic by as much as 106 kJ
mol−1.
for this process, but the most likely one is a direct reaction
between 1S and 6S. Computational work showed that the
lowest energy pathway on this particular potential energy
surface involves the breakup of one P−S bond in 1S to give 1S-
b that then binds 6S and forms a 6-membered acyclic
intermediate Int1 (Figure 4). This reaction is almost energy
neutral and proceeds through a transition state that is
comparable in energy (TS1, 103 kJ mol−1) to that found for
the formation of 6S. The intermediate Int1 can subsequently
close in on itself (TS2, 75 kJ mol−1), dissociating the
coordinated DMAP and forming 2S. The calculations showed
that 2S can adopt several conformers that are interconnected
via low-energy transition states, with the experimentally
observed chair conformer of 2S as the global energy minimum.
In total, a reaction energy of −127 kJ mol−1 were obtained for
the overall transformation of three equivalents of 1S to two
Having established the possible role of DMAP in stabilizing
the transient R-PS monomer, we turned our attention to the
formation of 2S. Many different mechanisms can be envisioned
E
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Figure 7. Stack plot of 31P{1H} NMR spectra of different selenium derivatives reported herein: parent 1Se and ring expansion products 2Se, 7SeCu,
7SeAg, and 8Se.
equivalents of 2S, which confirms that the 6-membered rings
are more stable than their 4-membered precursors.
prevented coordination. We then turned to the coinage metals
(M = Cu, Ag, Au) because of their linear coordination modes
and affinity for soft, phosphine donors (Figure 5).
As 2Se was found to form even during the synthesis of 1Se,
there must also be a base-free mechanism for the formation of
2Ch. To allow comparison to extant data, we examined this
possibility for 2S with computational methods. The results
show that 3S, being a highly reactive species, binds very easily
to 1S via formation of two P−S interactions. This gives a stable
intermediate Int2 that is 33 kJ mol−1 lower in energy compared
to free 3S and 1S. The intermediate Int2 can subsequently
form 2S via simple rearrangement of its P−S bonds (TS3, 57 kJ
mol−1). Even though all of these transformations involve only
low-energy transition states, such a pathway is not feasible in
practice because of the instability of the monomer 3S, in
particular with respect to the dimer 1S; the conversion of two
equivalents of 3S to 1S has a calculated barrier of only 6 kJ
mol−1. Thus, simply heating 1Ch is unlikely to lead to the
formation of significant amounts of 2Ch. However, as the
synthesis of 1Ch proceeds via 3Ch, it is possible that some 2Ch
forms during the process, as observed experimentally for Ch =
Se.
Reactivity of P/Ch Heterocycles with Coinage Metals.
Both 1Ch and 2Ch have a lone pair of electrons on each
phosphorus, leaving a functional group for onward reactivity,
especially metal coordination. Our initial approach was to
explore their reactivity as ligands to transition metals, first with
Group 16 metal carbonyls (M = Cr, Mo, W). However, no
reaction occurred upon reaction of the (M(CO)5THF)
precursor to either 1Ch/2Ch, even after prolonged reaction
times and varying reaction conditions. We hypothesized that
the steric demand imposed by the m-terphenyl ligands
The addition of 1Ch to a metal salt (CuCl or AgOTf) in 3:2
stoichiometry at room temperature showed single resonances
in the 31P{1H} NMR spectra (Ch = S: Cu δP = 89, Ag δP = 109;
Ch = Se: Ag δP = 113; Figures 6 and 7). Slow layering of
pentane onto a DCM solution resulted in single crystals
suitable for X-ray diffraction, which confirmed the structures to
be ring-expansion products from the parent 1Ch, with the
metal being bound to the phosphorus atoms in tripodal fashion
(7ChM). The mechanism of this transformation was not
examined computationally, but it can be easily envisaged that
the coordination of a metal fragment to 1Ch leads to
weakening of P−S bonds, which provides a pathway for the
formation of 3Ch stabilized by the coordinated metal center.
Although the addition of 1Ch with MX proceeds cleanly for
7SM and 7SeAg, the addition of 1Se to CuCl resulted in
multiple products. However, a major product at δP = 72, which
has been hypothesized to be 7SeCu based on the similarity of
the shift to 7SCu, and a minor product at δP = 88 (2Se), with a
relative integration of 60:40, were dominant (Figure 7). The
addition of more CuCl to the crude reaction mixture in an
attempt to convert 2Se to 7SeCu did not proceed cleanly but
resulted in multiple phosphorus containing products (Figure
S3). Attempts to use a different Cu+ source CuOTf[MeCN]4
yielded similar results as reactions performed with CuCl. As an
alternative to the coinage metal induced ring expansion of 1Ch,
the parent 6-membered rings 2Ch could be reacted directly
with the metal species (CuCl, AgOTf) in 1:1 stoichiometry to
yield 7ChM that were isolated in yields comparable to those
F
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Figure 8. Solid-state structures of the reported compounds. From left to right, top to bottom, 2S, 2Se, 7SCu, 7SAg, 8S, and 8Se. Thermal ellipsoids
are drawn at 50% probability level. Mesityl (2,4,6-trimethylphenyl) rings are omitted from 2S, 2Se, 7SCu, and 8Ch for clarity. Terphenyl ligands are
omitted from 7SAg for clarity.
obtained for reactions performed with 1Ch. Similar to the
reactions with Cu and Ag, the low temperature addition of 1S
to Me2SAuCl resulted in one major product (δP = 82). Leaving
a solution state sample at −35 °C for overnight in an attempt to
grow single crystals resulted in numerous phosphorus-
containing products (Figure S4) and repeated attempts to
isolate the observed major product were met only with
evidence for decomposition.
8Ch can be interpreted as containing two bridging sulfur sites,
four 3-coordinate sulfides and four Cu(I) centers, where the
cage is charge-neutral.
Recently, the synthesis of polynuclear silver complexes
stabilized by macrocyclic (PNPCh)2 ligands and accommodat-
ing up to four metal centers have been reported by varying the
metal:ligand ratio.7 In this respect, attempts to use a large
excess of either AgOTf or 1Ch did not influence the outcome
of the reaction, and 7ChAg were the only appreciable products
formed alongside 8Ch. The increased steric bulk at phosphorus
(terphenyl vs phenyl) could rationalize why sandwich metal
complexes could not be formed.
Although 2Ch showed long-term stability when stored under
ambient conditions (no decomposition after 2 months at room
temperature for either chalcogen derivative) and high thermal
stability (2S mp = 210−211 °C; 2Se decomposes at 215 °C),
the metal-containing compounds 7ChM and 8Ch were found to
be unstable at room temperature, with decomposition
occurring either in the solid state or in solution (Figures S-5
and S-6). This inherent instability of 7ChM and 8Ch precluded
our ability to fully characterize these compounds.
Compounds 7SM contain a single resonance in the 31P{1H}
NMR spectrum (Figure 6) as well as one set of terphenyl
1
signals in the H NMR spectrum, indicating 3-fold symmetry
among the 6-membered ring in solution. The 19F{1H} NMR of
7ChAg contained singlets with the chemical shifts reminiscent of
a covalent cation−anion interaction (Ch = S: δF = −77.4; Ch =
Se: δF = −76.4; c.f. [Bu4N][OTf]: δF = −79.0 (ionic) and
MeOTf: δF = −74.1 (covalent)).33,34 The 31P{1H} NMR
spectrum of 7SAg displayed a broad singlet, likely indicating a
dynamic process in solution (Figure 6). In contrast, the
selenium congener 7SeAg showed two sharp doublets with
1
1
selenium satellites (107Ag: JAg−P = 185.8 Hz; 109Ag: JAg−P
=
1
213.2 Hz; JSe−P = 151.8 Hz; Figure 7).
The addition of excess CuCl to either 1Ch or 2Ch in DCM
resulted in the consumption of the starting material and the
appearance of a broad resonance in the 31P{1H} NMR
spectrum (Ch = S: δP = 61; Ch = Se: δP = 46; Figures 6 and
7). After removing the volatiles in vacuo and washing the crude
powder with Et2O, the slow layering of a DCM solution with
pentane at −35 °C resulted in single crystals that confirmed the
product to be 8Ch. Redissolving the single crystals in CDCl3
gave the same broad 31P{1H} NMR resonance, consistent with
X-ray Crystallography. Images of the solid-state structures
are shown in Figure 8, metrical parameters are listed in Table 1,
and important X-ray parameters are found in Table S-1. X-ray
diffraction experiments were performed on 2Ch and 7SM
where structure solutions were refined anisotropically and
confirmed the ring expansion products. A structure solution of
7SeAg was obtained that verified atom connectivity, however
the data was of poor quality and could only be refined
isotropically (Figure S31) and therefore an in-depth analysis of
this particular structure is not appropriate.
1
the crude reaction mixture, and a H NMR spectrum that
shows one distinct set of m-terphenyl signals indicating a
symmetrical core on the NMR time scale. The structures are
unexpected, but a close examination shows insertion of Cu(I)
into 1Ch with Cu2Ch2 bridging another P2Ch2Cu. Compound
The solid-state structures of 2Ch confirmed the production
of 6-membered ring expansion products from the parent P2Ch2
rings, and were observed to have alternating P/Ch atoms that
adopt a chair conformation with P−S−Pavg = 90.19(7)° and S−
G
DOI: 10.1021/acs.inorgchem.7b02217
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Table 1. Selected Metric Parameters of the Reported Compounds
2S
2Se
7SCu
7SAg
8S
8Se
Bond Length (Å)
P(1)−Ch(1)
P(1)−Ch(3)
P(2)−Ch(1)
P(2)−Ch(2)
P(3)−Ch(2)
P(3)−Ch(3)
P(1)−Ch(2)
Ch(1)−Cu(1) (avg)
P(1)−C(1)
P(2)−C(25)
P(3)−C(49)
P(1)−M(1)
P(2)−M(1)
P(3)−M(1)
Ag(1)−O(1)
2.1459(18)
2.1367(19)
2.1352(17)
2.1556(19)
2.1395(17)
2.1455(18)
2.3021(19)
2.273(2)
2.278(2)
2.283(2)
2.286(2)
2.290(2)
2.1320(17)
2.131(3)
2.125(3)
2.128(4)
2.132(3)
2.141(3)
2.112(4)
2.1258(10)
2.2569(16)
2.1318(17)
2.1364(17)
2.1305(18)
2.1322(17)
2.1343(17)
2.1458(10)
2.3098(9)
1.859(3)
2.2757(15)
2.3948(10)
1.867(6)
1.854(4)
1.855(4)
1.860(4)
2.273(2)
2.278(2)
2.283(2)
1.850(4)
1.846(10)
1.828(9)
1.825(10)
2.689(3)
2.705(3)
2.746(3)
2.259(8)
1.853(4)
1.862(5)
2.4535(16)
2.4179(15)
2.5095(16)
2.2864(9)
2.2881(17)
Bond Angle (deg)
P(1)−Ch(1)−P(2)
P(2)−Ch(2)−P(3)
P(3)−Ch(3)−P(1)
Ch(1)−P(2)−Ch(2)
Ch(2)−P(3)−Ch(3)
Ch(3)−P(1)−Ch(1)
P(1)−Ch(2)−P(1C)
C(1)−P(1)−Ch(1)
C(1)−P(1)−Ch(3)
C(25)−P(2)−Ch(1)
C(25)−P(2)−Ch(2)
C(49)−P(3)−Ch(2)
C(49)−P(3)−Ch(3)
P(1)−M−X
90.31(7)
90.39(7)
89.88(7)
102.01(7)
101.83(7)
102.62(7)
86.36(8)
85.77(7)
89.09(7)
102.94(8)
102.55(8)
101.67(8)
89.07(6)
89.56(6)
89.34(7)
104.16(7)
103.93(7)
104.37(7)
91.10(14)
91.20(13)
91.61(13)
103.44(13)
103.74(15)
102.88(13)
92.92(9)
90.55(8)
98.41(14)
105.84(14)
105.81(14)
99.02(14)
106.98(14)
98.03(14)
96.83(15)
107.54(16)
106.73(16)
100.65(17)
107.23(17)
96.88(16)
106.07(15)
99.74(16)
100.14(14)
106.12(15)
99.80(14)
106.15(16)
135.49(6)
136.68(6)
133.59(6)
107.0(13)
101.3(3)
100.9(3)
107.7(13)
100.2(3)
106.3(3)
148.9(2)
130.3(2)
138.1(2)
P(2)−M−X
P(3)−M−X
P−Savg = 102.15(7)° for 2S and P−Se−Pavg = 87.07(7)° and
Se−P−Seavg = 102.37(7)° for 2Se. The average P−Ch bond
length for 2S is 2.1393(2) Å, whereas significant lengthening of
P−Ch occurred in 2Se with an average P−Se bond length of
2.285(2) Å. The 6-membered P3Ch3 cores in both 2Ch were
found to be occupationally disordered, with the major
component occupying 79% for 2S and 78% for 2Se (see
Figure S31). The solid-state structures of 2Ch resemble those
previously reported.23,25,26
The structures of 7SM confirm the formation of a ring
expansion product from the parent 4-membered ring, and
possess the same P3Ch3 cores as 2Ch. However, in 7SM, the
metal is bound by each phosphorus atom from within the 6-
membered ring. 7SCu has P−S−Pavg = 89.32(7)° and S−P−Savg
= 104.15(7)°, whereas 7SAg has P−S−Pavg = 91.30(13)° and
S−P−Savg = 103.35(13)°. This data is similar to that found for
2S. The same is true also for P−Ch bond distances in 7SCu,
7SAg, and 2S: 2.1334(16), 2.128(2) and 2.1393(2) Å,
respectively. Compound 7SCu has an average P−Cu distance
of 2.4603(16) Å that is considerably longer than that found for
other tripodal phosphine complexes bound to CuCl (2.246−
2.359 Å).35−39 Compound 7SAg has an average P−Ag bond
distance of 2.713(3) Å that is considerably longer than that
found for one other reported tripodal phosphine-AgOTf
complex (2.537 Å)40 and longer than P−Ag bonds in
polynuclear silver(I)-phosphine compounds (average P−Ag
bond distance of 2.676 Å).7
The structures of 8Ch are isomorphous and contain four 4-
coordinate phosphorus atoms, four μ3-chalcogen centers, four
3-coordinate copper centers, and two μ2-chalcogen centers.
Compounds 8Ch crystallize in the space group I-4 and sit on a
crystallographic center of symmetry with Ar*PCh2Cu as the
asymmetric unit. 8Ch has P−Ch(μ3) bond distances (S:
2.1258(10) Å, Se: 2.2559(16) Å) that are slightly different from
P−Ch(μ2) distances (S: 2.1458(10) Å, Se: 2.2757(15) Å), but
which all fall in the range of typical P−Ch single bond lengths
(the sum of Pyykko & Atsumi single bond covalent radii for P
̈
and S/Se is 2.14/2.27 Å)41,42 and vary only slightly from the
parent 1Ch (1S, P−Savg = 2.1461(2) Å; 1Se, P−Seavg
=
2.303(2) Å).24 The P−Cu bond lengths in 8Ch are not
significantly different (S: 2.2864(9) Å, Se: 2.2881(17) Å),
whereas the Ch−Cu distances (S−Cu = 2.3098(9) Å and Se−
Cu = 2.3948(10) Å) naturally depend on the identity of the
employed chalcogen atom.
CONCLUSION
■
We have shown the successful synthesis of (RPCh)3 rings 2Ch:
the sulfur derivative through ring expansion of the parent 4-
membered rings 1Ch by the addition of DMAP, and the
selenium derivative by cyclocondensation of Ar*PCl2 and
Se(TMS)2. The mechanism for the formation of 2S was
H
DOI: 10.1021/acs.inorgchem.7b02217
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Article
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ASSOCIATED CONTENT
* Supporting Information
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Figures S1−S31, Table S1, computational details,
complete experimental section and associated spectro-
scopic data not included in the manuscript (PDF)
Optimized structures of investigated compounds (XYZ)
Accession Codes
CCDC 1569577−1569582 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: pragogna@uwo.ca.
*E-mail: heikki.m.tuononen@jyu.fi.
■
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ORCID
Heikki M. Tuononen: 0000-0002-4820-979X
Paul J. Ragogna: 0000-0003-2727-7635
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Science and Engineering Research
Council of Canada (NSERC), Canada Foundation for
Innovation, Ontario Ministry of Research and Innovation,
Western University, and University of Jyvaskyla for funding. We
̈
̈
also acknowledge grants of computer capacity from the Finnish
Grid and Cloud Infrastructure (persistent identifier urn:nbn:-
fi:research-infras-2016072533).
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DOI: 10.1021/acs.inorgchem.7b02217
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