Phosphenium Hydride Reduction of [(cod)MX2] (M = Pd, Pt; X = Cl, Br): Snapshots on the Way to Phosphenium Metal(0) Halides and Synthesis of Metal Nanoparticles

Article abstract of DOI:10.1021/acs.inorgchem.7b00022

The outcome of the reduction of [(cod)PtX₂] (X = Cl, Br; cod = 1,5-cyclooctadiene) with N-heterocyclic phosphenium hydrides ^RNHP-H depends strongly on the steric demand of the N-aryl group R and the nature of X. Reaction of [(cod)PtCl₂] with ^DippNHP-H featuring bulky N-Dipp groups produced an unprecedented monomeric phosphenium metal(0) halide [(^DippNHP)(^DippNHP-H)PtCl] stabilized by a single phosphine ligand. The phosphenium unit exhibits a pyramidal coordination geometry at the phosphorus atom and may according to DFT calculations be classified as a Z-type ligand. In contrast, reaction of [(cod)PtBr₂] with the sterically less protected ^MesNHP-H afforded a mixture of donor-ligand free oligonuclear complexes [{(^MesNHP)PtBr}_n] (n = 2, 3), which are structural analogues of known palladium complexes with μ₂-bridging phosphenium units. All reductions studied proceed via spectroscopically detectable intermediates, several of which could be unambiguously identified by means of multinuclear (¹H, ³¹P, ¹⁹⁵Pt) NMR spectroscopy and computational studies. The experimental findings reveal that the phosphenium hydrides in these multistep processes adopt a dual function as ligands and hydride transfer reagents. The preference for the observed intricate pathways over seemingly simpler ligand exchange processes is presumably due to kinetic reasons. The attempt to exchange the bulky phosphine ligand in [(^DippNHP)(^DippNHP-H)PtCl] by Me₃P resulted in an unexpected isomerization to a platinum(0) chlorophosphine complex via a formal chloride migration from platinum to phosphorus, which accentuates the electrophilic nature of the phosphenium ligand. Phosphenium metal(0) halides of platinum further show a surprising thermal stability, whereas the palladium complexes easily disintegrate upon gentle heating in dimethyl sulfoxide to yield metal nanoparticles, which were characterized by TEM and XRD studies.

Full text of DOI:10.1021/acs.inorgchem.7b00022

Article
pubs.acs.org/IC
Phosphenium Hydride Reduction of [(cod)MX ] (M = Pd, Pt; X = Cl, Br):
2
Snapshots on the Way to Phosphenium Metal(0) Halides and
Synthesis of Metal Nanoparticles
†
†,§
†
†
‡
Jan Nickolaus, Dominik A. Imbrich, Simon H. Schlindwein, Adrian H. Geyer, Martin Nieger,
,
†
and Dietrich Gudat*
†
Institute of Inorganic Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany
‡
Department of Chemistry, University of Helsinki, P.O Box 55 (A.I. Virtasen Aukio 1), 00014 Helsinki, Finland
*
S Supporting Information
ABSTRACT: The outcome of the reduction of [(cod)PtX ] (X = Cl, Br; cod
2
R
=
1,5-cyclooctadiene) with N-heterocyclic phosphenium hydrides NHP−H
depends strongly on the steric demand of the N-aryl group R and the nature
of X. Reaction of [(cod)PtCl ] with
groups produced an unprecedented monomeric phosphenium metal(0)
halide [( NHP)( NHP−H)PtCl] stabilized by a single phosphine ligand.
The phosphenium unit exhibits a pyramidal coordination geometry at the
phosphorus atom and may according to DFT calculations be classified as a
Dipp
NHP−H featuring bulky N-Dipp
2
Dipp
Dipp
Z-type ligand. In contrast, reaction of [(cod)PtBr ] with the sterically less
2
Mes
Mes
protected NHP−H afforded a mixture of donor-ligand free oligonuclear complexes [{( NHP)PtBr} ] (n = 2, 3), which are
n
structural analogues of known palladium complexes with μ -bridging phosphenium units. All reductions studied proceed via
2
1
31
spectroscopically detectable intermediates, several of which could be unambiguously identified by means of multinuclear ( H, P,
195
Pt) NMR spectroscopy and computational studies. The experimental findings reveal that the phosphenium hydrides in these
multistep processes adopt a dual function as ligands and hydride transfer reagents. The preference for the observed intricate
pathways over seemingly simpler ligand exchange processes is presumably due to kinetic reasons. The attempt to exchange the
Dipp
Dipp
bulky phosphine ligand in [( NHP)( NHP−H)PtCl] by Me P resulted in an unexpected isomerization to a platinum(0)
3
chlorophosphine complex via a formal chloride migration from platinum to phosphorus, which accentuates the electrophilic
nature of the phosphenium ligand. Phosphenium metal(0) halides of platinum further show a surprising thermal stability,
whereas the palladium complexes easily disintegrate upon gentle heating in dimethyl sulfoxide to yield metal nanoparticles, which
were characterized by TEM and XRD studies.
INTRODUCTION
N-Heterocyclic phosphenium (NHP) ions of types Ia and Ib
Chart 1) are isovalent analogues of the widely studied
N-heterocyclic carbenes (NHC), which have found eminent
the carbenes have strong σ-donor and only weak π-acceptor
character, NHPs act predominantly as π-acceptors, and the
metal−ligand bonding in their complexes is in most cases
described as superposition of a dominating dative M → L π-bond
■
1
−3
(
3c,5−8
4
and a weaker dative L → M σ-bond.
As has been demons-
application in coordination chemistry and catalysis. Although
trated recently, these features make NHPs excellent ligands for
the stabilization of complexes with low-valent platinum group
Chart 1. N-Heterocyclic Phosphenium (NHP) Ligands
7
,9−15
(
Ia and Ib) and Phosphenium Metal(0) Halides of Platinum-
metals.
Particular attention has been devoted to phosphe-
a
Group Metals 1−7
nium metal halides in which a formally cationic NHP unit is used
(
0)
−
to support a negatively charged {M X} fragment (M = Pd, Pt)
featuring a zerovalent metal atom with d10 electron count.
Electronic saturation of the resulting neutral {(NHP)MX} units
is achieved by either attachment of additional phosphine donors
1
0−12
to give mononuclear complexes (1−3)
or by oligomeriza-
tion to yield di- (4, 5) or trinuclear arrays (6, 7) of metal centers
held together by μ -bridging phosphenium ligands.
13,15
2
The phosphenium metal(0) halide complexes are accessed via
either insertion of an existing metal(0) center into the P−X bond
Received: January 4, 2017
a
Dipp = 2,6-iPr C H , Xyl = 2,6-Me C H .
2
6
3
2
6
3
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b00022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Electron-beam microprobe measurements were performed on a
CAMECA SX 100 electron probe microanalyzer with a semiconducting
crystal and an excitation voltage of 15 kV. Powder diffraction data were
Scheme 1. Synthetic Pathways to Oligomeric Phosphenium
Metal(0) Halides of Platinum-Group Metals
a13,15
recorded on a STOE and Cie STADI P with a Cu Kα X-ray source.
1
Dipp
Dipp
Synthesis of Complex 9 . A solution of 8
.10 mmol) in THF (5 mL) was cooled to 0 °C, and solid [(cod)PtCl2]
200 mg, 0.55 mmol) was added. The mixture was stirred for 1 h during
(440 mg,
1
(
which the color changed from yellow to green and then brown. The
solvent was removed under reduced pressure, and the solid residue was
washed three times with 10 mL of acetonitrile. Drying of the remaining
yellow solid under reduced pressure gave 370 mg (0.93 mmol, yield
8
6%) of spectroscopically pure product. A crystalline sample suitable
for X-ray diffraction studies was obtained by recrystallization from
THF/dichloromethane (DCM) at −20 °C.
a
M = Pd, Pt; n = 2, 3.
1
H NMR (THF-d ): δ 8.20 (dd, 1 H, J = 381 Hz, 3JPH = 3.0 Hz,
1
8
PH
of a halophosphine precursor 0−13 or treatment of metal(II) salts
like [(cod)MCl ] (cod = 1,5-cyclooctadiene) with an electron-
rich diphosphine (for M = Pd, Pt) or phosphenium hydride
1
2
3
3
HH
JPtH = 427 Hz, PH), 7.23 (t, 2 H, J = 7.7 Hz, p-CH), 7.16 (t, 2 H,
3
JHH = 7.7 Hz, p-CH), 7.09 (d, 4 H, J = 7.7 Hz, m-CH), 6.91 (d, 2 H,
HH
2
3J = 7.7 Hz, m-CH), 6.76 (d, 2 H, J = 7.7 Hz, m-CH), 6.30 (s, 2 H,
3
HH
HH
3
3
13
NCH), 6.00 (d, 2 H, J = 10.2 Hz, NCH), 3.96 (sept, 2 H, J
=
(for M = Pd) (Scheme 1).
PH
HH
3
9
6.8 Hz, CHCH ), 3.38 (sept, 2 H, J = 6.8 Hz, CHCH ), 3.04 (sept,
3 HH 3
Reaction (1) is viewed as a “nonoxidative” process that leaves
3
3
2
1
H, J = 6.8 Hz, CHCH ), 2.89 (sept, 2 H, J = 6.8 Hz, CHCH ),
HH 3 HH 3
.31 (d, 12 H, J = 6.8 Hz, CHCH ), 1.13 (d, 6 H, J = 6.8 Hz,
CHCH ), 1.04 (d, 12 H, J = 6.8 Hz, CHCH ), 0.94 (d, 6 H, J =
the formal metal oxidation state unchanged, and (2) and (3)
involve reduction of the metal center and constitute new
variants of reductive syntheses that are a standard method for
the preparation of low valent metal complexes or metal nano-
particles. In particular, we consider pathway (2) noteworthy
because on the one hand it is closely related to commonly applied
3
3
HH
3
HH
3
3
3
HH
3
HH
3
6
.8 Hz, CHCH ), 0.33 (d, 6 H, J = 6.8 Hz, CHCH ), 0.26 (d, 6 H,
3 HH 3
3
31
1
2
JHH = 6.8 Hz, CHCH ). P{ H} NMR (THF-d ): δ 234.3 (d, J =
83 Hz, PN ), 81.4 (d, J = 7677 Hz, J = 283 Hz, PH). P NMR
2 PPt PP
(THF-d ): δ 234.3 (d, J = 283 Hz, PN ), 81.4 (dd, J = 7677 Hz,
JPP = 283 Hz, J = 381 Hz, PH). Pt NMR (THF-d ): δ −4128
(d, J = 7677 Hz). Anal. calcd for C H ClN P Pt (1046.64): C
PtP 52 73 4 2
3
8
PP
1
6
1
2
31
2
2
1
8
PP
2
PPt
−
17
2
1
195
reductions with complex hydrides (in particular BH ) but
4
PH
8
1
on the other hand remains peculiar in that the phosphenium
hydride has a dual function by providing both the reduction
equivalents for the generation of the product and the ligand for
its stabilization. The reaction is further mechanistically appealing
because besides the NHP cation the phosphenium hydride
may also act as a supporting ligand that can control the course
and outcome of the reaction. Having established that the phos-
59.67, H 7.03, N 5.35. Found: C 55.23, H 6.95, N 5.02 (the carbon value
that is significantly too low can be explained by incomplete combustion
due to the formation of nonvolatile platinum carbides).
Mes
Preparation of 8 . A stoichiometric amount of LiAlH (120 μL of
4
1
M solution in THF, 0.12 mmol) was added to a precooled (−78 °C),
Mes Mes
19
stirred solution of 2-halogeno-diazaphospholene ( NHP−Br 14
2
(5 mL). The mixture was warmed to ambient temperature and stirred
for 1 h. Reaction control ( P NMR) indicated that formation of 8
:
Mes
Mes 20
00 mg, 0.48 mmol; NHP−Cl 10
: 172 mg, 0.48 mmol) in THF
phenium hydride reduction of [(cod)PdCl ] proceeds rapidly
2
1
3
without observable intermediates, we turned to study the
behavior of analogous platinum salts, expecting that their higher
kinetic stability might render transient intermediates more
easily detectable and thus help in the elucidation of the reaction
pathway. We report here that the phosphenium hydride reduc-
31
Mes
was quantitative, and the solution was used for subsequent reactions
without further purification. If depletion of the formed LiAlX was
4
Mes
intended, the reaction mixture was evaporated to dryness, and 8 was
then extracted with hexane. The filtrate of the extraction was evaporated
once more, and the residue was dissolved in THF.
tion of [(cod)PtX ] (X = Cl, Br) meets this expectation but
2
1
1
H NMR (THF-d ): δ 6.98 (d, 1 H, J = 136 Hz, PH), 6.90 (s, 4 H,
8
PH
also demonstrate that different reaction kinetics have a crucial
effect on the outcome of individual reactions. Furthermore,
we will show that oligomeric phosphenium halides of platinum
and palladium exhibit surprisingly different thermal stabilities,
which has a pronounced effect on their usability for the synthesis
of metal nanoparticles.
3
m-CH), 6.13 (d, 2 H, J = 1.7 Hz, NCH), 2.34 (s, 6 H, p-CH ), 2.28
PH
3
(
s, 12 H, o-CH3). 31P{ H} NMR (THF-d ): δ 63.8 (s).
1
8
Mes
Reaction of [(cod)PtCl ] with 8 . A light yellow solution of
crude 8 prepared as described was treated at room temperature with
(cod)PtCl ] (90 mg, 0.24 mmol) and stirred until a homogeneous
2
Mes
[
2
phase was obtained. The progress of the reaction was monitored by
3
1
P NMR spectroscopy until the spectra indicated complete consump-
Mes
1
31
EXPERIMENTAL SECTION
tion of 8 . Recording of H and P NMR spectra revealed that cod
and 10 had formed as major products along with a minor amount of
■
Mes
All manipulations were carried out under a dry argon atmosphere and
by using standard vacuum line techniques. Solvents were dried by
literature-known procedures. The NMR spectra were recorded on
Mes
complex 11 (≤10% of phosphorus-containing species by integration
of the 31P NMR spectrum). Attempts to identify or isolate a platinum-
containing reaction product failed.
1
31
195
Bruker Avance 400 ( H, 400.1 MHz; P, 161.9 MHz; Pt, 86.7 MHz)
Avance 250 ( H, 250.1 MHz; P, 101.2 MHz; Pt, 54.2 MHz)
NMR spectrometers at ambient temperature if not stated other-
1
31
195
Mes 1
For 11 , H NMR (THF-d ): δ 8.56 (A-part of AA′XX′ pattern,
8
1+3JPH = 404 Hz, 2 H, PH), 6.74 (broad s, 4 H, m-CH), 6.63 (broad s,
1
wise. Chemical shifts are referenced to external TMS ( H, Ξ =
4 H, m-CH), 6.10 (m, 4 H, NCH), 2.30 (broad s, 24H, CH ), 2.13
3
3
1
2
1
1
1
00.000000 MHz) using 85% H PO ( P, Ξ = 40.480742 MHz) and
(broad s, 12H, CH ), −15.24 (t, 1 H, J = 8.5 Hz, J = 1197 Hz,
PtH). P{ H} NMR (THF-d ): δ 80.0 (s, J = 3505 Hz). P NMR
8
3
4
3
PH
PtH
1
95
31 1 1 31
.2 M Na PtCl ( Pt, Ξ = 21.496784 MHz) as secondary references.
2
6
PtP
1+3
1
95
1
1
195
1
Pt chemical shifts were obtained from H-detected H, Pt HMQC
(THF-d ): δ 80.0 (X-part of AA′XX′-pattern, JPH = 405 Hz, J
=
8
PtP
1
95
1
spectra. Coupling constants are given as absolute values. (+)ESI mass
spectra were recorded in MeOH solution on a Bruker Daltonics Micro
TOF Q instrument. Elemental analyses were determined on a Thermo
3505 Hz). Pt NMR (THF-d
Reaction of [(cod)PtBr
8
): δ −4872 (t, JPtP = 3505 Hz).
Mes
] with 8 . A light yellow solution of
2
Mes
crude 8 prepared as described was treated at room temperature with
[(cod)PtBr ] (111 mg, 0.24 mmol) and stirred for 2 days. During this
Dipp18
Micro Cube CHN/S analyzer. The secondary diazaphospholene 8
2
was prepared as described. High-resolution transmission electron micros-
copy (HRTEM) images were recorded on a Tecnai G2 Sphera trans-
missionelectron microscopeoperated atanacceleratingvoltageof200 kV.
time, the solution turned deep red, and a reddish precipitate formed.
The mixture was evaporated to dryness, and the residue was washed two
times with water (2 mL), two times with MeOH (2 mL), and five times
B
DOI: 10.1021/acs.inorgchem.7b00022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
with Et O (5 mL). Analytical and spectroscopic studies revealed that
The remaining white solid was dissolved in CDCl . The combined
2
3
the crude product contained a mixture of complexes [(NHP)PtBr]
ether fractions were evaporated, and the off-white residue was like-
n
Mes
Mes
(n = 2, 3; 12 , 13 ) as major components and minor amounts of
wise dissolved in CDCl . NMR spectra disclosed the presence of
3
Dipp
residual solvents and unidentified impurities (no yield determined).
Anal. calcd for {C H N BrPPt} (n = 598.37): C 40.14, H 4.04, N 4.68.
[17 ][AlCl ] (in the ethereal fraction) and complex [ClPt(PMe ) ]-
4
3 3
20
24
2
n
[AlCl ] (in the Et O-insoluble fraction), which were both identified by
4
2
4
8,49
Found: C 40.00, H 4.36, N 4.29. Attempts to recrystallize the crude
their known NMR data.
For [ClPt(PMe ) ][AlCl ], H NMR (CDCl ): δ 1.93 (d, 9 H, J =
PH
Mes
1
2
product yielded a few crystals (no yield determined) of 13 , which
3
3
4
3
were identified by a single-crystal X-ray diffraction study. When reaction
3
2+4
3
1
0.9 Hz, J = 40 Hz), 1.75 (m, 18 H, JPH = 3.9 Hz, J = 24 Hz).
PtH PtH
Mes
monitoring was intended, the initial solution of 8 in THF-d was
31
1
2
1
8
P{ H} NMR (CDCl ): δ −12.5 (d, 2 P, J = 22 Hz, J = 2265 Hz,
3
2
PP
PPt
cooled to −70 °C prior to the addition of [(cod)PtBr ], and multi-
1
195
1
2
PMe ), −27.8 (t, 1 P, J = 22 Hz, J = 3406 Hz, PMe3). Pt{ H}
1
31
195
3
PP
PPt
nuclear ( H, P, Pt) NMR spectra were then recorded while the
temperature was stepwise increased to rt. The spectral data of all
NMR (CDCl ): δ −4627 (q, due to low resolution in the indirect
3
27
domain of the 2D spectrum). Al NMR (CDCl ): δ 103.7 (s).
3
identified intermediates and products detected are listed below.
Dipp
1
For [17 ][AlCl ], H NMR (CDCl ): δ 8.35 (s, 2 H, NCH), 7.61
(t, 2 H, JHH = 7.64 Hz, p-CH), 7.41 (d, 4 H, JHH = 7.86 Hz, m-CH),
.49 (sept, 4 H, J = 6.88 Hz, CH), 1.34 (d, 12 H, J = 6.75 Hz,
For [B]Br, H NMR (THF-d , −50 °C): δ 8.63 (m, 2 H, 1+3J
PtH PH
.18 (m, 2 H, NCH(phosphine)), 6.02 (m, 2 H, NCH(phosphine));
1
4
3
=
8
PH
3
3
2
3
3
6
68 Hz, J = 393 Hz, PH), 7.67 (m, 2 H, J = 5 Hz, NCH(NHP)),
3
3
2
HH
HH
31
3
1
CHCH ), 1.25 (d, 12 H, J = 6.76 Hz, CHCH3). P{ H} NMR
3
HH
signals of NMes substituents could not be unambiguously identified.
2
7
3
1
1
2
1
(CDCl ): δ 204.9 (br s). Al NMR (CDCl ): δ 104.0 (s).
3
3
P{ H} NMR (THF-d , −50 °C): δ 248 (t, J = 202 Hz, J
=
8
2
PP
PtP
1
31
Attempts to Prepare Metal Nanoparticles. The phosphenium
5
(
1
5
(
623 Hz, PN ), 102 (d, J = 202 Hz, J = 5310 Hz, PH). P NMR
2
PP
PtP
Mes
Mes
2
1
metal(0) halide (mixture of 12 /13 or 6) was dissolved in 5 mL
THF-d , −50 °C): δ 248 (broad t, J = 202 Hz, J = 5623 Hz, PN ),
8
PP
PtP
2
2 1 3
of dimethyl sulfoxide (DMSO), and the red solution was heated for 4 h
02 (X-part of AA′XX′ pattern with J = 202 Hz, J = 268 Hz, J
=
PP
PH
PH
Mes
Mes
31
1
195
to 100 °C (12 /13 ) or 50 °C (6), respectively. A P NMR assay
revealed that 12 /13Mes did not undergo any detectable reactions,
whereas in the reaction of 6, a black precipitate formed and the solution
turned orange. The black solid was separated by centrifugation and
6 Hz, J = 5310 Hz, PH). Pt NMR (d -THF, −50 °C): δ −4623
PtP
8
Mes
1
q, J ≈ 5400 Hz).
PtP
Mes 1
For 15 , H NMR (THF-d ): δ 8.75 (A-part of AA′XX′ pattern
8
1
3
2
4
with J = 369 Hz, J = 41 Hz, J = 622 Hz, 2 H, J = 171 Hz,
PH), 7.01 (s, 4 H, m-CH), 6.78 (s, 4 H, m-CH), 6.01 (m, JPH
PH
PH
PP
PtH
3
+5
decantation, washed with Et O (3 × 5 mL) and DCM (3 × 10 mL),
=
2
1
2.1 Hz, 4 H, NCH), 2.28 (s, 12 H, CH ), 2.11 (s, 12 H, CH ), 1.94
dried under vacuum, and characterized as described.
3
3
2
1
(
s, 12 H, CH ), −13.64 (t, 1 H, J = 8 Hz, J = 1272 Hz, PtH).
Crystallographic Studies. Single-crystal X-ray diffraction data were
collected on a Bruker AXS Nanostar C diffractometer equipped with a
Kappa APEX II Duo CCD-detector and a KRYO-FLEX cooling device
3
PH
PtH
3
1
1
1
31
P{ H} NMR (THF-d ): δ 78.5 (s, J = 3444 Hz). P NMR
8
PtP
1
+3
(
J
THF-d ): δ 78.5 (X-part of AA′XX′-pattern with JPH = 410 Hz,
8
1
1
9
5
1
Dipp
Mes
= 3444 Hz). Pt NMR (THF-d ): δ −5007 (t, J = 3444 Hz).
at 100(2) K for 9
and 7 (X = Br) and 150(2) K for 13 using Mo
PtP
8
PtP
Mes
1
1
3
For 16 : H NMR (THF-d ): δ 8.65 (dd, 1 H, J = 386 Hz, J =
Kα radiation (λ = 0.71073 Å). Crystals were selected under Fomblin
Y perfluoro oil mounted on nylon loops and immediately placed in a
8
PH
PH
4
45 Hz, J = 153 Hz, PH), 6.89 (s, 2 H, m-CH), 6.83 (s, 2 H, m-CH),
PtH
3
6
.82 (s, 2 H, m-CH), 6.77 (s, 2 H, m-CH), 6.07 (d, 2 H, J = 11 Hz,
cold stream of N . The structures were solved by direct methods
PH
2
4
3
4
21
JPtH = 15 Hz, NCH), 5.87 (d, 2 H, J = 11 Hz, J = 15 Hz, 2 H,
(SHELXS-2014 ) and refined with a full-matrix least-squares scheme
PH
PtH
2
21
NCH), 2.31 (s, 6 H, CH ), 2.29 (s, 6 H, CH ), 2.28 (s, 6 H, CH ), 2.24
on F (SHELXL-2014 ). Semiempirical absorption corrections from
equivalents were applied for all structures. Nonhydrogen atoms were
refined anisotropically. The disordered solvent molecules (DCM)
3
3
3
(
J
(
3
s, 6 H, CH ), 2.03 (s, 6 H, CH ), 1.85 (s, 6 H, CH ), −14.18 (d, 1 H,
3
3
3
2
1 31 1
= 13 Hz, J = 1238 Hz, PtH). P{ H} NMR (THF-d ): δ 116.6
PtH 8
PH
d, J = 700 Hz, J = 4337 Hz, PBr), 79.7 (d, 2J = 700 Hz, J
2
1
1
=
=
Mes
cocrystallizing with complex 13 were refined isotropically. Complex 7
PP
PtP
PP
PtP
31
2
3
487 Hz, PH). P NMR (THF-d ): δ 116.6 (ddm, J = 700 Hz, J
5 Hz, PBr), 79.7 (ddm, J = 700 Hz, J = 386 Hz, PH). Pt{ H}
cocrystallizes with two THF molecules without any disorder. Listings of
crystallographic data and details on the structure solution are given in
8
PP
PH
2
1
195
1
4
PP
PH
1
Dipp
NMR (THF-d ): δ −5051 (dd, J ≈ 4350, 3500 Hz).
the Supporting Information. CCDC-1524319 (9 ), CCDC-1524317
8
PtP
Mes
1
Mes
For 12 , H NMR (THF-d , rt): δ 6.71 (s, 8 H, m-CH), 6.65 (m,
H, NCH), 2.24 (s, 12 H, p-CH ), 2.20 (s, 24 H, o-CH ). P{ H} NMR
8
(7), and CCDC-1524318 (13 ) contain the supplementary crystallo-
31
1
4
3
3
graphic data for this paper. This data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
1
195
(
d -THF): δ 233.0 (s, J = 4157 Hz). Pt NMR (d -THF): δ −4462
8
PtP
8
1
(
t, J = 4157 Hz).
PtP
Mes
1
For 13 , H NMR (THF-d , rt): δ 6.91 (broad s, 6 H, NCH),
8
Computational Studies. All computational studies but the
6
.59 (s, 12 H, m-CH), 2.30 (s, 18 H, p-CH ), 1.96 (s, 36 H, o-CH ).
22
3
3
calculations of NMR properties were performed with the Gaussian09
3
1
1
P{ H} NMR (d -THF): δ 307 (s).
23
8
suite of programs. MOLDEN was used for visualization. Energy-
Dipp
Synthesis of Complex 18 . Trimethylphosphine (170 mg,
optimization of molecular structures was carried out at the bp86/def2-
sv(p) level, which includes relativistic pseudopotentials at the platinum
atom. Calculation of harmonic vibrational frequencies at the same level
of theory ensured that all stationary points located were local minima
on the energy hypersurface. Final energies and population analyses
were then computed at the bp86/def2-tzvp level using the previously
determined geometries. The calculation of NMR properties was carried
0
.23 mL, 2.25 mmol) was added via syringe to a solution of 9Dipp
(150 mg, 0.45 mmol) in THF (5 mL) at room temperature. The mixture
was stirred for 1 h. The orange solid formed was then separated by
filtration, washed with three portions of THF (5 mL), and dried under
1
vacuum to yield 190 mg (0.34 mmol, 76%) of product. H NMR
3
3
(
7
(
1
1
CDCl ): δ 7.35 (t, 2 H, J = 7.7 Hz, p-CH), 7.19 (d, 4 H, J
=
3
HH
HH
3
4
.7 Hz, m-CH), 6.80 (d, 2 H, J = 3.0 Hz, J = 14 Hz, NCH), 2.89
PH HPt
24
3
3
out with the Amsterdam Density Functional package (ADF 2014)
sept, 4 H, J = 6.9 Hz, CH), 1.23 (d, 12 H, J = 6.9 Hz, CHCH ),
HH HH 3
31
1
2
using an all-electron, triple-ζ, double-polarization TZ2P Slater basis.
.13−1.05 (m, 27 H, PCH ). P{ H} NMR (CDCl ): δ 175.0 (q, J =
PP
1 2 1
3
3
25−28
Relativistic two-component zero-order regular approximation (ZORA)
calculations including spin−orbit coupling
13 Hz, J = 5659 Hz, PN ), −47.8 (d, J = 113 Hz, J = 3473 Hz,
PPt
95
2
PP
PPt
29−32
1
1
have been performed
PMe3). Pt NMR (CDCl ): δ −4630 (dq, J = 5659, 3473 Hz).
3
PtP
with the local density approximation (LDA) in the Vosko−Wilk−Nusair
(
−
(
+)-ESI-MS m/z: 786.34 (M − PMe − Cl + MeOH); 754.31 (M − Cl
3
3
3
34
parametrization with nonlocal corrections for exchange (Becke88)
PMe ); 678.27 (M − 2 PMe − Cl). Anal. calcd for C H ClN P Pt
3
3
35 63
2 4
3
5
and correlation (Perdew86) included in a self-consistent manner.
866.31): C 48.52, H 7.33, N 3.23. Found: C 48.01, H 7.13, N 3.17.
Dipp
Reaction of 18
with AlCl . An excess of dry AlCl (120 mg,
.92 mmol) was added to a solution of 18
Chemical shifts were determined as δ
s
= (σref − σ )/(1 − σref) relative to
s
3
3
Dipp
2−
195
0
(100 mg, 0.18 mmol) in
[PtC1l
6
]
P
for Pt and as δ
using the magnetic shielding constants of [PtCl
s
= (σref − σ
s
− 266.1) relative to 85% H
3
PO
4
for 3
36
]
6
2−
(σref
=
DCM (3 mL). The mixture was stirred for 12 h at rt during which time
its yellow color disappeared. The solvent was evaporated, and the
1326 ppm) and PH
putational level.
3
(σref = 590.5 ppm) calculated at the same com-
off-white residue was extracted with five portions (2 mL) of Et O.
2
C
DOI: 10.1021/acs.inorgchem.7b00022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
1
Dipp
RESULTS AND DISCUSSION
Formation of Phosphenium Metal(0) Halides. Reaction
ligand as in 3; Pt−Cl is 2.311(2) Å in 9
vs 2.268(3) Å in
■
Dipp13
Dipp
5
). Altogether, these features suggest viewing 9
containing a pseudolinear {(phosphine)PtCl} unit that is
perturbed by a weakly bound secondary ligand at a right angle
(see also the Discussion below).
The presence of two different P-donor ligands in 9
substantiated by the P{ H} NMR spectrum, which displays an
AX pattern with a large J coupling constant (283 Hz). The
signal at lower frequency (δ 82.8 ppm) is accompanied by Pt
satellites ( J = 7677 Hz) and splits into a doublet of doublets
as
of two equivalents of the secondary N-heterocyclic phosphine
Dipp
8
with [(cod)PtCl ] at room temperature does not proceed
2
13
as with [(cod)PdCl2] to give a mixture of a chlorophosphine
and a dinuclear phosphenium metal(0) halide (Scheme 1) but
Dipp
is
31
1
Dipp
yields mononuclear phosphine-phosphenium complex 9
Scheme 2), which was isolated after workup as a yellow
2
PP
(
1
95
1
PtP
Scheme 2. Reaction of [(cod)PtCl ] with Secondary
2
1
Dipp
a
due to interaction with an adjacent hydrogen atom ( J = 380 Hz)
PH
Diazaphospholene 8
when the decoupling is switched off, backing its assignment to the
phosphorus atom of the secondary phosphine ligand. The second
resonance is attributed to the phosphorus atom in the NHP
unit and displays a substantial deshielding (δ 236.7 ppm) that had
been identified as a characteristic of many phosphenium com-
plexes of platinum group metals.
satellites, and the Pt{ H} NMR signal of 9
9
−11
195
This signal has no Pt
a
R = Dipp.
195
1
Dipp
thus exhibits
a doublet structure arising from spin-coupling to the single
phosphorus atom in the phosphine ligand. The unusually small
JPtP coupling constant is presumably caused by the combined
solid and identified by spectral data and a single-crystal X-ray
diffraction study. The product is thermally stable in solution and
does not react under loss of the phosphine to the previously
1
described dimer 5Dipp.
1
3
influences of a large interatomic distance (which decreases the
1
37,38
Dipp
magnitude of JPt,P
) and, perhaps more importantly, the lone
Crystalline 9
contains isolated molecular complexes featur-
pair at the NHP phosphorus atom (which shifts the coupling
ing a three-coordinate platinum atom with nearly T-shaped
geometry (Figure 1). The Pt−P2 (2.156(2) Å) bond to the
39
constant from a large positive number to more negative values ).
1
Numerically small J couplings were also observed for other
PtP
platinum complexes featuring P-donor ligands with lone pairs
such as pyramidal PR units (2659−445 Hz for [(PPP)PtX] or
2
+
[
(PPP)Pt(PR )] , where (PPP) denotes pincer ligands with
3
11,40
a central PR donor unit framed by two phosphine moieties
as well as phosphavinyl or side-on-bound phosphaalkene and
phosphaalkyne
)
2
41
41
4
2,43
ligands (19−446 Hz).
Dipp
The partitioning of 9
into a {(phosphine)PtCl} unit and
an additional secondary ligand suggests describing the bonding
situation in terms of the interaction of an anionic 14 VE
{
(phosphine)PtCl} fragment with a phosphenium cation acting
as a Z-type ligand (Chart 2(a)). The anion fragment is then
Figure 1. View of the molecular structure of 9Dipp (left) and a reduced
plot in which the atoms of the N-Dipp substituents are omitted.
Hydrogen atoms of CH bonds are omitted for clarity. Thermal ellipsoids
are drawn at the 50% probability level. Selected distances [Å] and angles
Chart 2. Limiting Descriptions of the Metal−Ligand
Dipp
a
Interaction in 9
[
deg]: Pt1−P1, 2.154(3); Pt1−Cl1, 2.313(3); Pt1−P2, 2.319(3); P1−
N2, 1.692(8); P1−N1, 1.716(9); P2−N3, 1.699(9); P2−N4, 1.710(8);
P1−Pt1−Cl1, 162.77(11); P1−Pt1−P2, 90.96(11); Cl1−Pt1−P2,
1
06.21(11).
a
R = 2,6-iPr C H .
2
6
3
phosphine ligand and the Pt−Cl bond (2.311(2) Å) include an
angle of 162.86(8)° and form the horizontal bar of the T-shape,
whereas its vertical axis is associated with the Pt−P1 bond to the
NHP moiety, which is nearly perpendicular to the Pt-phosphine
bond (P2−Pt−P1, 90.97(7)°) and significantly elongated
−
isovalent to [AuCl ] , and the platinum atom has a formal
2
oxidation state of zero. However, because pyramidal geometry
and a long M−P bond of a metal-bound NHP unit have also been
viewed as indicators of a phosphide nature of the ligand,
can also be depicted as a coordinatively unsaturated Pt(II)
phosphido complex (Chart 2(b)). Hoping to cast further light
onto this dichotomy, we carried out DFT calculations on model
8
,11 Dipp
(
Pt−P1, 2.319(2) Å). The coordination geometry at the NHP-
phosphorus atom P1 is pyramidal (sum of bond angles,
97.3(7)°) and the five-membered rings in both P-ligands
9
2
display a similar degree of puckering. The perception of a
lengthened Pt−P1(NHP) bond is underlined by a comparison
with the distances in related NHP complexes with a zerovalent
Me
complex 9 featuring N-Me instead of the computationally
expensive N-Dipp substituents. The molecular structure
obtained after energy optimization reproduces the T-shaped
metal coordination and the relative differences in Pt−P and
11
platinum atom (Pt−P, 2.2446(11)−2.2606(9) Å ). In contrast,
the Pt−P(phosphine) and Pt−Cl distances are significantly
Dipp
shorter than in structurally related complexes (Pt−P is
Pt−Cl distances observed for 9 , even if all bond distances are
Dipp
2
.156(2) Å in 9
vs 2.290(2)−2.3787(6) Å in 3 (X = Br)
overestimated by 4−7 pm. Assuming that the deviations arise,
at least in part, from the structural simplification of the model
+
and [(PPP)Pt(PR )] , where (PPP) denotes the same pincer
3
D
DOI: 10.1021/acs.inorgchem.7b00022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
compound, we refrained from using more sophisticated models
and conclude that the present results should allow deductions
that are qualitatively reasonable but not numerically exact.
results of computational modeling studies on a series of
Me
(1−n)+
Me
complexes [{( NHP) PtH}Cl ]
Me-substituted N-heterocyclic phosphenium; see SI for details).
(n = 0−2, NHP =
2
n
4
4
195
NBO analysis reveals that the partial charges on the phos-
phine (+0.40), NHP (+0.55), and chloro ligands (−0.53) deviate
considerably from the ideal values of 0 and ±1, indicating
the presence of substantial charge transfer and strong covalent
bonding contributions. This is confirmed by Wiberg bond
indexes (WBI) of 0.88 (Pt−phosphine), 0.71 (Pt−NHP), and
Using the match between predicted and observed (δ Pt =
195
45
−4410 ppm) Pt NMR chemical shifts as selection criterion,
Me
+
195
a cationic complex [( NHP) PtH] (δ Ptcalcd = −4511 ppm)
2
emerged as the best candidate. The construction of a logical
scenario explaining both the formation of an intermediate
Dipp
[(NHP) PtH][Cl] (A) from 8
further isomerization to 9
and [(cod)PtCl ] and its
2
2
Dipp
0.61 (Pt−Cl). The enhanced positive charge of the NHP
is in principle feasible, but a
as compared to the phosphine ligand gives a first hint to a
phosphenium rather than phosphide nature of the former,
which is further emphasized by the different composition of the
appropriate natural bond orbitals. Thus, the NBO representing
the Pt−phosphine bond (76.0% P, 24.0% Pt) is like the Pt−Cl
NBO (85.4% Cl, 14.6% Pt) strongly polarized toward the ligand
atom, which is commonly interpreted as indicating a dative bond
dominated by L → M donation from ligand-based lone pairs
to the metal center. In contrast, the Pt−NHP NBO (33.7% P,
detailed mechanistic discussion seems premature as long as no
definitive identification of A has been achieved and shall thus be
postponed. We note, however, that the metal hydride nature of A
proves, regardless of the incomplete constitutional assignment,
Dipp
that activation of the P−H bond of 8
and transfer of a
hydrogen atom to platinum occur in a very early reaction stage
and justifies viewing the studied reaction as another manifestation
of the phosphenium hydride nature of the starting material.
Assuming steric protection by the bulky NHP ligand as the key
Dipp
6
6.3% Pt) displays a reverse polarization, which we then interpret
as evidence for a “retro-dative” M → L donation and thus
phosphenium” character of the NHP ligand. These findings
reason for the surprising thermal stability of 9 , we decided
to also study the behavior of [(cod)PtCl ] toward the sterically
2
Mes
“
less demanding phosphenium hydride 8 . NMR spectroscopic
indicate that, even if the assignment of formal oxidation states
may seem questionable in light of the large covalent contribu-
tions to the metal−ligand bonding, the phosphenium complex
reference structure (Chart 2(a)) is closer to the actual electronic
situation than the representation of a phosphido-complex (b),
studies disclosed that the reaction yielded chlorophosphine
Mes
10
and a small amount (<10 mol %) of a phosphine
Mes
platinum hydride, which we assign as 11 (Scheme 3). Further
Scheme 3. Reactions of 8Mes with [(cod)PtX2]a
Me
Dipp
and we consider 9 , and therefore 9 , as another example of a
phosphenium-stabilized metal(0) halide. It is further noteworthy
Me
that calculations of NMR data of 9 reveal quite good agree-
1
95
195
ment of the predicted Pt NMR chemical shift (δ Pt
=
calcd
195
Dipp
−
4118 ppm) with the observed value for 9
4149 ppm) and also confirm the postulated trend in
JPtP couplings: the coupling constant to the phosphine ligand
is large and positive as expected ( J
862 Hz for 9 ) whereas that to the NHP-phosphorus atom is
smaller in magnitude and of opposite sign ( J
compared to |J | < 30 Hz for 9 ).
Having noticed that reacting mixtures of 8
(δ Pt =
−
1
1
= 6556 Hz compared to
PtP,calcd
Dipp
7
1
= −946 Hz
PtP,calcd
Dipp
PtP
Dipp
a
and
X = Cl, Br; R = Mes.
[
(cod)PtCl ] show a transient green color before adopting the
2
Dipp
orange color of 9 , we reckoned that the reaction involves
reaction intermediates or platinum-containing products were
intermediates that might possibly be spectroscopically detect-
neither spectroscopically detected nor isolated. Gratifyingly,
1
31 195
Mes
able. Reaction monitoring by multinuclear ( H, P, Pt) NMR
we observed that the reaction of 8 with [(cod)PtBr ] took a
2
Dipp
spectroscopy at −70 °C revealed that the formation of 9
was
similar course as that of phosphenium hydrides with [(cod)-
Dipp
13,15
Mes
indeed preceded by that of another complex. Formation of 9
PdCl2]
(Scheme 1(2)) to yield a mixture of oligomers 12
Mes
Mes
occurred with appreciable rates above −30 °C and thwarted
and 13 , bromophosphine 14 , and dihydrogen as the final
3
1
1
Mes
attempts to isolate this intermediate. The P{ H} NMR signal of
the transient species appears as a singlet that is accompanied by
products (Scheme 3). An analytically impure mixture of 12
Mes
and 13
was isolated after workup, and recrystallization
1
95
Mes
satellites arising from coupling to a single Pt nucleus. The large
produced a very small amount of crystalline 13 , which was
identified by a single-crystal X-ray diffraction study (Figure 2).
The structural features of the Pt Br unit and the bridging
31
1
chemical shift (δ P = 206.8 ppm) and small magnitude of J
PtP
(
360 Hz) suggest the presence of an NHP ligand in a similar
3
3
Dipp
1
coordination mode as in 9 . The H NMR spectrum
NHP ligands are closely comparable to those of the previously
15
contains, besides the signals of the NHP unit, solely a resonance
described trimeric phosphenium platinum(0) chloride 7
1
attributable to a metal-bound hydride (δ H = −14.7 ppm,
(Chart 1), the crystal structure of which had been unknown
and was now newly determined for comparison (see SI for a
pictorial representation and a list of metrical parameters).
The Pt−Pt distances in both compounds (7, 2.6855(5)−
1
195
J
PtH
= 1563 Hz). The signal multiplicities in Pt and selectively
31
decoupled P NMR spectra as well as heteronuclear correlation
spectra allow for assembly of the individual building blocks to a
Mes
{
(NHP) PtH} partial structure with two equivalent NHP units.
2.7016(5) Å; 13 , 2.6901(6)−2.6983(5) Å) do not signifi-
2
However, the data give no clue about the presence or absence
of additional halide ligands, and the precise structure of the
intermediate thus remains unknown.
Lacking further experimental evidence, we attempted to derive
a tentative structural assignment of the intermediate from the
cantly differ, whereas the Pt−P distances in 7 (2.2721(17)−
Mes
2.2909(16) Å) are slightly longer than in 13
(2.268(2)−
Mes
2.269(2) Å). The deviation in Pt−X distances (13 : Pt−Br,
2.4778(9)−2.4867(13) Å; 7: Pt−Cl, 2.357(2)−2.383(2) Å)
mainly reflects the different covalent radii of the halogens, and
E
DOI: 10.1021/acs.inorgchem.7b00022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. View of the molecular structure of 13Mes (left) and reduced plot (right) displaying only the Pt Br core and the diazaphospholene rings.
3
3
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected distances [Å]: Pt1−P1, 2.268(2); Pt1−Br1,
2.4867(13); Pt1−Pt2, 2.6983(5); Pt2−P2, 2.268(2); Pt2−P1, 2.269(2); Pt2−Br2, 2.4778(9); Pt2−Pt2#1, 2.6901(6).
the intraligand bond lengths in the NHP units are similar as in
Chart 3. Possible Structures of Intermediates in the Reaction
a
13,15
Mes
other complexes of type 5 and 7.
The formation of trinuclear
of 8 with [(cod)PtBr2]
Mes
complex 13
is remarkable as it contradicts earlier assump-
1
5
tions that the presence of N-aryl substituents in the NHP units
favors the formation of dinuclear phosphenium metal(0) halides
for steric reasons.
Even if the final products of the reaction of 8Mes with
[
(cod)PtBr ] are formed at ambient temperature, the mecha-
2
nism is quite intricate and involves several detectable inter-
a
1
31 195
R = Mes.
mediates. Multinuclear ( H, P, Pt) VT-NMR studies disclosed
that the reaction sets off at −50 °C to yield a major product along
with several minor byproducts that gave strongly exchange-
broadened NMR signals and could not be identified further.
The main product gives rise to AM /AM X-type signal patterns
those of 11Mes, and the P and Pt chemical shifts match those
of known complexes trans-[Pt(R P) (H)(Cl)], which can also
be readily prepared by hydride reduction of Pt(II) precursors.
31
195
4
6
3
2
4
7
2
2
A, M = 3 P; X = Pt) in the P{ H} and Pt{ H} NMR
1
195
31
1
195
1
The structural assignment is also backed by the good match of
(
the observed 1 Pt NMR chemical shift with the calculated value
95
spectra and is therefore assigned as a mononuclear platinum
Me
31
of −5114 ppm for the N-Me substituted model compound 11 .
complex with three P-based ligands. The P NMR spectrum
Mes
3
1
Further slow conversion of 15 at room temperature pro-
reveals for the M -signal (δ P = 102 ppm) an additional AA′XX′-
2
ceeded under formal H/Br exchange at one phosphine ligand to
yield 16 (Chart 3), which is recognized by its AB(X)-type
signal pattern in the P{ H} and Pt NMR spectra, a similar
metal chemical shift (δ Pt −5051) as 15 , and the persistence
of H NMR signals for P- and Pt-bound hydrogen atoms. This
species eventually evolved into a mixture of 12 and 13
Complex 12 gives rise to a broadened line in the room temp-
erature P{ H} NMR spectrum, which narrows upon cooling
to yield a sharp singlet (δ P = 233) with two sets of satellites
that form the A parts of A X and A X spin systems with J
type splitting, which arises from coupling with P-bound hydrogen
atoms and allows for identifying two of the ligands as 8
Mes
Mes
.
31
1
195
The A-signal (triplet at 248 ppm) is only slightly broadened by
long-range couplings and exhibits a similar chemical shift as that
195
Mes
1
Dipp
9−15
of the cationic units in 9
and other Pt-NHP complexes
and
Mes
Mes
.
is attributed to a metal-bound phosphenium ion. Structural
assignments compatible with these data are cationic complex B
and the isomeric neutral species C and D (Chart 3). Even if the
spectral data allow no definite decision, we favor assignment
as (solvent separated or contact) ion pair [B]···[Br] based on
Mes
31
1
31
1
=
2
2
2
2
PtP
195
4
157 Hz and arise from isotopomers with one and two Pt
1
95
the finding that the computed Pt NMR chemical shift for the
nuclei, respectively. The reason for the signal broadening, which
is completely reversible, is currently not known (VT-NMR
studies gave no indication of a relation to dynamic exchange
Me
model cation B of −4701 ppm gives the best match with the
1
95
31
experimental value (δ Pt = −4623) and the similarity of the P
Mes
Mes
NMR data with those of previously reported analogous complexes
(NHP)Pt(PPh ) ][OTf].
The initial product reacted further to yield 14 and a new
between 12
and 13 ). The observed splitting pattern is
7,10
13
[
typical for dimeric phosphenium platinum(0) halides, and the
3
2
Mes
Mes
Dipp
structural analogy of 12 and 5
is further corroborated by
Mes
matching of the values of J and δ195P of both species. The
1
complex identified as platinum hydride 15 (Chart 3). This
irreversible transformation was very slow at −50 °C, occurred at
an increased rate above −30 °C, and went rapidly to completion
PtP
31
Mes
P NMR signal of 13 , which is much less intense than that
Mes
1
31
of 12 but readily detectable in H and P HMQC spectra,
consists of a parent singlet accompanied by a complex set of
31
195
1
Mes
at 0 °C. The P, Pt, and H NMR data of 15 are similar to
F
DOI: 10.1021/acs.inorgchem.7b00022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
195Pt satellites. Concise analysis of the satellite spectrum was
precluded by the limited resolution and signal intensity in the
indirect dimension of the 2D spectrum, but the similarity to the
directed to the NHP-phosphorus atom, and this initial attack
may then be assumed to facilitate breakdown into a bromo-
phosphine and a neutral fragment {(8) Pt}, which is in turn
2
15
Mes
Mes
signal pattern of 7 is taken as a strong argument for support-
trapped by HBr to yield 15. The formation of 16 from 15
Mes
ing that the trinuclear structure of crystalline 13 persists in
may likewise be considered a phosphorus- rather than a platinum-
centered reaction, which proceeds as in free phosphenium
solution.
1
8,20
The outcome of the reaction monitoring allows for outlining a
hydrides
via P−H/P−Br bond metathesis; as this process
Mes
mechanistic description of the reduction of [(cod)PtBr ] by 8
leaves the Pt−P bond intact, it may be kinetically preferable to an
exchange of the complete phosphine ligand, which is often
notoriously slow.
2
(Scheme 4). We propose that the reactants undergo an initial
Scheme 4. Proposed Mechanism of the Reaction of 8Mes with
Reactions of Phosphenium Metal(0) Complexes. The
a
electronically unsaturated nature of the metal atom suggests that
[(cod)PtBr2]
Dipp
9
can in principle add further ligands. Because the increase in
metal electron density associated with such extra ligation should
strengthen the bonding to a Z-type (phosphenium) and weaken
the bonding to an X-type (phosphide) ligand, characterization of
the reaction products might help to cast further light on the
electronic properties of the NHP unit. Anticipating that sterically
nondemanding and strongly σ-donating ligands are particularly
effective, we considered PMe an ideal substrate for these studies.
3
Dipp
Treatment of 9
with an excess of PMe gave an orange
3
precipitate, which was separated by filtration from the liberated
Dipp
phosphenium hydride 8
and purified by several washings with
31
1
195
THF. The P{ H} and Pt NMR spectra of the product display
31
195
an AM (X) (A, M = P, X = Pt) pattern. The downfield
3
31
P NMR signal at 175 ppm is attributable to an NHP unit that
a
R = Mes.
carries no hydrogen atom and the upfield signal at −47.8 ppm to
a set of three equivalent PMe ligands. These data led us to
anticipate an ionic product with cation E (Scheme 5), which is
metathesis to give HBr (formation of HCl as a coproduct is
3
proven for reactions of phosphenium hydrides with [(cod)PdCl ]
2
13
under similar conditions ) and a phosphenium complex, which
is stabilized by incorporation of two further molecules of 8Mes as
the observable intermediate B[Br]. Action of the HBr formed on
this species is then considered to give bromo-diazaphospholene
Scheme 5. Reaction of 9Dipp with an Excess of PMe and
3
Constitution of Complex Cation E
Mes
Mes
14
and complex 15 , which reacts further by hydride/halide
Mes
exchange at one phosphine ligand to give 16 . This trans-
formation may once more be initiated by HBr as shown in
Mes
Scheme 4 or, alternatively, by the bromo-diazaphospholene 14
(
intermolecular hydride/halide-exchange processes between
18
free diazaphospholenes have precedence ). Finally, formation
of 12 and 13 can be rationalized by oligomerization of a
{
from 16 . Dihydrogen, which was also positively identified as
reaction product by H NMR spectroscopy, can arise from the
action of HBr on 15 (see Scheme 4) as well as from a side-
reaction of HBr with 8 . We note that the initial consumption
Mes
Mes
R
Mes
( NHP)PtBr} unit arising from formal cleavage of 8 and HBr
Mes
1
Mes
2
0
Mes
Mes
of an overstoichiometric amount of 8 (three instead of two
structurally related to the cations in salts [(NHP)M(PMe ) ]-
3
3
equivalents as suggested by reaction (2) in Scheme 1) is also
in accordance with the finding that the H NMR signal of
OTf (M = Pd, Pt) that had been accessed via analogous
1
7
phosphine exchange reactions. Interestingly, further reaction
[
(cod)PtBr ] remained visible during a major part of the reaction,
with AlCl did not only result in a simple conversion of free
2
3
−
which indicates that total consumption of this starting material
occurred only during a later reaction stage.
It is noteworthy that a reaction of key intermediate B via
capture of the halide anion and cleavage of the two phosphine
chloride into [AlCl ] but induced a breakdown of the original
4
Dipp
48
complex into phosphenium salt [17 ][AlCl4] and complex
4
9
[ClPt(PMe ) ][AlCl ], which were identified by their
3 3 4
known NMR data (Scheme 5; the additional chlorides originate
presumably from disintegration of the solvent DCM). These
findings imply that the chloride anion is an essential constituent
Mes
Mes
ligands would provide a more direct route to 12 and 13
than the detour via 15Mes and 16 . The failure to observe this
pathway is obviously due to kinetic reasons. A hint to a possible
explanation can be derived from population analysis of the
Mes
of the original PMe complex and led us to reassign its
3
Dipp
50
constitution as 18
with a neutral Cl−NHP ligand. Such a
structure is not only well in accordance with the close match of
the computed Pt NMR chemical shift of model compound
18 (−4439 ppm) with the value for 18
can also explain the fact that the P NMR chemical shift of the
seems too low for a cationic phosphenium
complex. Furthermore, even if the reaction of 9
Me
computed electron density for the model intermediate B .
195
The LUMO of this species is centered on the platinum and
the phosphorus atom of the NHP unit with a slightly larger
coefficient on the latter, and the natural charges are positive on
the phosphorus (+1.13) but negative on the metal atom (−0.62).
In the light of these findings, attack of an anion on B should be
Me
Dipp
(−4630 ppm) but
31
Dipp
NHP unit in 18
Dipp
with PMe3
G
DOI: 10.1021/acs.inorgchem.7b00022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
did not result in the expected cationic complex, the proposed
constitution of 18Me may still serve as corroboration of the
electrophilic rather than nucleophilic character of the metal-
products depending on the nature of the N-aryl substituent
in the phosphorus and the halogen atom in the platinum-
containing reactant. Oligomeric phosphenium platinum(0)
Mes
Mes
Mes
bound NHP fragment.
Knowing that 4
donating phosphines to yield defined metal(0) complexes,
halides [{( NHP)PtBr} ] (12 : n = 2; 13 : n = 3)
n
Dipp
Mes
disassembles in the presence of strongly
accessible from 8 and [(cod)PtBr ] are structural analogues to
2
13
known palladium complexes, but phosphine-stabilized complex
Dipp
we wondered if oligonuclear phosphenium halides [(NHP)MX]n
M = Pd, Pt; n = 2, 3) might in the absence of a stabilizing
Lewis base form metal nanoparticles. The Pt-based oligomers
9
is unprecedented in representing the first coordinatively
(
unsaturated monomeric phosphenium metal(0) halide featuring
a formal 14 VE count at the metal center. The reactions leading
to these products are slower than those of analogous palladium
compounds and proceed via detectable intermediates. Even
if not all transient species were unambiguously identified, their
observation provides some insight into a rather complicated
mechanistic pathway and confirms that the phosphenium
hydrides adopt a dual function in the reactions studied in acting
both as ligands and (reducing) hydride donors. Accordingly,
all reactions involve the transfer of a hydride from phosphorus
to platinum in an early reaction stage, which is subsequently
Mes
Mes
Dipp
12
/13 proved, like 9 , thermally quite stable and suffered
no visible disintegration even upon prolonged heating to 100 °C
1
5
in DMSO. In contrast, the previously described trinuclear
palladium(0) complex 6 (X = Cl) reacted in the same solvent
at 50 °C to yield elemental palladium and a solution containing
tBu
several unidentified decomposition products of chloro-NHP 10
.
The solid was identified by micro probe analysis and a powder
diffraction study (Figure 3). Further characterization by trans-
eliminated, presumably as HX or H , when the final products
2
(
mostly phosphenium metal(0) halides) are formed. The pre-
ference for the observed multistep mechanisms can be explained
by assuming that transformations initiated by nucleophilic attack
at the ligating phosphorus atoms may be kinetically favored over
the displacement of intact phosphine ligands from the metal. The
Dipp
outcome of the reaction of 9
with trimethyl phosphine, which
proceeds under formal migration of a chloride from platinum to
phosphorus to yield a platinum(0) chlorophosphine complex, is
in line with evidence from quantum chemical studies suggesting
Dipp
that the phosphenium unit in monomeric complex 9
still
exhibits electrophilic character and might thus be classified as a
Z-type ligand.
Figure 3. X-ray powder diffraction pattern of Pd nanoparticles resulting
from thermal disintegration of 6 at 50 °C in DMSO. The black and red
curves represent experimental results and simulated data for elemental
Pd, respectively.
Phosphenium platinum(0) halides derived from platinum and
palladium show further remarkable differences in their thermal
stability. The kinetically inert platinum complexes are surpris-
ingly thermally stable, whereas the kinetically more labile
palladium complexes are easily disintegrated by gentle heating
in DMSO to yield metal nanoparticles. Further investigations
to explore the use of these reactions as a new approach to the
deposition of nanoparticles on suitable carrier materials and to
test the catalytic activity of these materials are in progress.
Figure 4. TEM image of Pd nanoparticles produced by thermolysis of 6
at 50 °C in DMSO.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b00022.
■
*
S
mission electron microscopy (TEM) revealed the presence
of small nanoparticles that assemble to yield irregularly shaped
larger aggregates (Figure 4). Analysis of 60 randomly selected
particles resulted in an average diameter of 3.3 ± 1 nm, which
is in reasonable agreement with a value of 2.9 ± 0.4 nm
computed from the width of the (111) reflection in the powder
Dipp
Details of the crystal structure determinations of 7, 9
,
,
Mes
Dipp
Dipp
Dipp
Dipp
and 13 ; NMR spectra of 9 , 12 /13 , 15
Dipp
16
Dipp, and 18 ; details of the characterization of Pd
51
metal powders, and results of DFT calculations (PDF)
Crystal structure determination of 7 (CIF)
Crystal structure determination of 9
Crystal structure determination of 13 (CIF)
diffractogram using a modified version of the Scherrer equation.
The particle size and distribution compare to those of reported
monodisperse nanoparticles generated by size-controlled syn-
Dipp
(CIF)
52,53
Mes
thetic approaches.
In summary, these findings indicate that
the synthesis of metal nanoparticles from oligomeric phos-
phenium metal(0) halides is in principle feasible, but platinum
AUTHOR INFORMATION
Corresponding Author
E-mail: gudat@iac.uni-stuttgart.de.
Mes
Mes
■
*
complexes 12 /13
thermal stability, which is, like in the case of 9 , presumably a
failed to react due to their superior
Dipp
consequence of their increased kinetic inertness.
ORCID
CONCLUSIONS
The reactions of phosphenium hydrides 8
Dietrich Gudat: 0000-0002-9196-0466
Present Address
D.A.I.: Institute of Polymer Chemistry, University of Stuttgart,
■
[
Dipp
and 8Mes with
§
(cod)PtX ] (X = Cl, Br) proceed less uniformly than those
2
with analogous palladium salts and yield different types of
Pfaffenwaldring 55, 70550 Stuttgart, Germany
H
DOI: 10.1021/acs.inorgchem.7b00022
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Notes
Structure of Platinum Bis(N-arylamino)phosphenium Complexes
including [Pt(phosphane) (phosphenium)-(N-heterocyclic carbene)].
Angew. Chem., Int. Ed. 2004, 43, 1955−1958.
The authors declare no competing financial interest.
(
10) Caputo, A.; Brazeau, A. L.; Hynes, Z.; Price, J. T.; Tuononen, H.;
ACKNOWLEDGMENTS
■
Jones, N. D. A Cation-Captured Palladium(0) Anion: Synthesis,
−
We thank the Institut fu
support. We further thank B. Fo
̈
r Anorganische Chemie for financial
rtsch for elemental analyses,
Structure, and Bonding of [PdBr(PPh ) ] Ligated by an N-
3
2
̈
Heterocyclic Phosphenium Cation. Organometallics 2009, 28, 5261−
5265.
J. Trinkner and Dr. W. Frey (both from Institute of Organic
Chemistry, University of Stuttgart) for the measurement of mass
spectra and the collection of X-ray data sets, and Dr. N. Preisig
and C. Schlick (both from Institute of Physical Chemistry,
University of Stuttgart) for the transmission electron microscopy
(11) Pan, B.; Xu, Z.; Bezpalko, M. W.; Foxman, B.; Thomas, C. M. N-
Heterocyclic Phosphenium Ligands as Sterically and Electronically-
Tunable Isolobal Analogues of Nitrosyls. Inorg. Chem. 2012, 51, 4170−
4
179.
(12) Pan, B.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M.
(
TEM) measurements. The computational studies were carried
out at the JUSTUS HPC facility at the University of Ulm, funded
by the state of Baden-Wurttemberg through the bwHPC
Heterolytic addition of E−H bonds across Pt−P bonds in Pt N-
heterocyclic phosphenium/phosphido complexes. Dalton Trans. 2012,
̈
4
1, 9083−9090.
initiative and the bwHPC-C5 project and the German Research
Foundation (DFG) through Grant INST 40/467-1 FUGG.
(13) Forster, D.; Nickolaus, J.; Nieger, M.; Benko, Z.; Ehlers, A.;
Gudat, D. Donor-Free Phosphenium−Metal(0)−Halides with Unsym-
metrically Bridging Phosphenium Ligands. Inorg. Chem. 2013, 52,
̈
̈
7
(
699−7708.
DEDICATION
■
14) Pan, B.; Evers-McGregor, D. A.; Bezpalko, M. W.; Foxman, B. M.;
This paper is dedicated to Prof. Dr. Evamarie Hey-Hawkins on
the occasion of her 60th birthday.
Thomas, C. M. Multimetallic Complexes Featuring a Bridging N-
heterocyclic Phosphido/Phosphenium Ligand: Synthesis, Structure,
and Theoretical Investigation. Inorg. Chem. 2013, 52, 9583−9589.
(15) Nickolaus, J.; Bender, J.; Nieger, M.; Gudat, D. Sterically
Controlled Synthesis and Nucleophilic Substitution Reactions of Di-
and Trimeric N-Heterocyclic-Phosphenium-Metal(0)-Halides. Eur. J.
Inorg. Chem. 2014, 2014, 3030−3036.
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