Platinum(II) complexes of some unsymmetrical diphosphenes

Article abstract of DOI:10.1016/j.jorganchem.2016.10.005

Reaction of the unsymmetrical diphosphene Ar*P=PAr^F1 (Ar*?=?2,4,6-^tBu₃C₆H₂, Ar^F?=?2,4,6-(CF₃)₃C₆H₂) with the dimeric platinum(II) species trans-[Pt(PEt₃)Cl(μ-Cl]₂led initially to the formation of two different monomeric Pt(II) complexes trans-[Pt(PEt₃)Cl₂(Ar*P=PAr^F)] 2 and trans-[Pt(PEt₃)Cl₂(Ar^FP=PAr*)] 3, where the underlined phosphorus atom coordinates to Pt. These were readily identifiable by³¹P NMR solution-state spectroscopy, but attempts to separate them by column chromatography were unsuccessful. When the reaction was repeated on a larger scale, a third complex cis-[Pt(PEt₃)Cl₂(Ar^FP=PAr*)] 4 was detected in solution, with P-Ar^Fbound to Pt. Calculations of energies and³¹P NMR chemical shifts confirm that this species is expected to be the thermodynamically most stable monomeric reaction product. For comparison, we have also prepared the analogous Pt(II) complex trans-[Pt(PEt₃)Cl₂(Ar*P=PAr*)] 6 of the symmetrical diphosphene Ar*P=PAr* 5, and obtained its³¹P NMR parameters in solution. The mixed diphosphene Ar′P=PAr^F7 (Ar′?=?2,6-(CF₃)₂C₆H₃) reacts with the same platinum(II) dimer to yield a single cis-complex 8. Calculations have enabled us to assign the³¹P chemical shifts of this unsymmetrical diphosphene 7, and to show that the Ar′ group is coordinated to Pt in the unique product cis-[Pt(PEt₃)Cl₂(Ar′P=PAr^F)] 8.

Full text of DOI:10.1016/j.jorganchem.2016.10.005

Accepted Manuscript
Platinum(II) complexes of some unsymmetrical diphosphenes
Keith B. Dillon, Mark A. Fox, Vernon C. Gibson, Helen P. Goodwin, Leila J. Sequeira
PII:
S0022-328X(16)30443-0
10.1016/j.jorganchem.2016.10.005
JOM 19649
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 18 August 2016
Revised Date: 29 September 2016
Accepted Date: 2 October 2016
Please cite this article as: K.B. Dillon, M.A. Fox, V.C. Gibson, H.P. Goodwin, L.J. Sequeira, Platinum(II)
complexes of some unsymmetrical diphosphenes, Journal of Organometallic Chemistry (2016), doi:
10.1016/j.jorganchem.2016.10.005.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Configurations of new platinum(II) complexes with unsymmetrical diphosphenes
were determined by a combination of observed and computed 31P NMR data.

ACCEPTED MANUSCRIPT
Platinum(II) Complexes of some Unsymmetrical Diphosphenes
Keith B. Dillon*a, Mark A. Fox*a, Vernon C. Gibsonb, Helen P. Goodwina and Leila J.
Sequeiraa
a Chemistry Department, University of Durham, South Road, Durham DH1 3LE, UK
b Department of Chemistry, Imperial College London, Exhibition Road, London SW7
2AZ, UK
1

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Abstract
Reaction of the unsymmetrical diphosphene Ar*P=PArF 1 (Ar* = 2,4,6-tBu3C6H2, ArF
= 2,4,6-(CF3)3C6H2) with the dimeric platinum(II) species trans-[Pt(PEt3)Cl(ꢀ-Cl]2
led initially to the formation of two different monomeric Pt(II) complexes trans-
[Pt(PEt3)Cl2(Ar*P=PArF)] 2 and trans-[Pt(PEt3)Cl2(ArFP=PAr*)] 3, where the
underlined phosphorus atom coordinates to Pt. These were readily identifiable by 31P
NMR solution-state spectroscopy, but attempts to separate them by column
chromatography were unsuccessful. When the reaction was repeated on a larger scale,
a third complex cis-[Pt(PEt3)Cl2(ArFP=PAr*)] 4 was detected in solution, with P-ArF
bound to Pt. Calculations of energies and 31P NMR chemical shifts confirm that this
species is expected to be the thermodynamically most stable monomeric reaction
product. For comparison, we have also prepared the analogous Pt(II) complex trans-
[Pt(PEt3)Cl2(Ar*P=PAr*)] 6 of the symmetrical diphosphene Ar*P=PAr* 5, and
31
obtained its P NMR parameters in solution. The mixed diphosphene Ar’P=PArF 7
(Ar’ = 2,6-(CF3)2C6H3) reacts with the same platinum(II) dimer to yield a single cis-
complex 8. Calculations have enabled us to assign the 31P chemical shifts of this
unsymmetrical diphosphene 7, and to show that the Ar’ group is coordinated to Pt in
the unique product cis-[Pt(PEt3)Cl2(Ar’P=PArF)] 8.
Introduction
Since the first diphosphene Ar*P=PAr* 5 (Ar* = 2,4,6-tBu3C6H2) was prepared by
Yoshifuji and co-workers in 1981, [1] a great deal of interest has focused on their
coordination chemistry. [2,3] They can bind to transition metal fragments either in an
η1 fashion, via a lone pair on phosphorus, or in an η2 mode, utilising the π system of
the P=P double bond; some examples of both coordination types for ArFP=PArF 9
(ArF = 2,4,6-(CF3)3C6H2), [4] with strongly electron-withdrawing substituents, have
been reported by us. [5,6] Occasionally it is possible to coordinate a metal to each of
the phosphorus atoms in an η1 mode, as in the mono- and di-AuCl complexes of
Ar*P=PAr* 5 (Ar* = 2,4,6-tBu3C6H2). [7] For an unsymmetrical diphosphene
R1P=PR2 there is the additional possibility of η1 coordination by either of the
inequivalent phosphorus atoms. Cowley et al. showed that the diphosphene 2,4,6-
tBu3C6H2P(1)=P(2)CH(SiMe3)2 reacted with Fe2(CO)9 to afford a single product in
63% yield, with Fe(CO)4 coordinated to P(2). [8] This observation is consistent with
2

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coordination by the less sterically hindered phosphorus atom, but was not discussed
further. Yoshifuji and co-workers have prepared M(CO)5 complexes (M = Cr, Mo or
W) of the unsymmetrical diphosphene Ar*P=PMes (Mes = 2,4,6-Me3C6H2 or
mesityl), and have shown that these can be photoisomerised from the E- to the Z-
isomer. [9] In these complexes the metal was again coordinated to the less sterically
hindered phosphorus atom. More recent studies on unsymmetrical diphosphenes,
usually with a phosphorus-containing substituent on one of the diphosphene
phosphorus atoms, and some metal complexes thereof, have also appeared in the
literature. [10-14]
We have described the transition metal-catalysed metathesis of phosphorus –
phosphorus double bonds, in which symmetrical or unsymmetrical diphosphenes can
be synthesised from appropriate dichlorophosphanes by interaction with W(PMe3)6, as
shown in equation (1) for an unsymmetrical species. [15]
RPCl2 + R’PCl2 + W(PMe3)6 ꢀ RP=PR’ + 3 PMe3 + WCl4(PMe3)3
(1)
It is also possible to synthesise unsymmetrical diphosphenes by base-catalysed
elimination in one of two ways, equations (2) and (3). [2]
RPH2 + R’PX2 + 2 DBU ꢀ RP=PR’ + 2 DBUH+ X-
RPX2 + R’PH2 + 2 DBU ꢀ RP=PR’ + 2 DBUH+ X-
(DBU = 1,8-diazabicyclo[5,4,0]undec-7-ene)
(2)
(3)
It was therefore of considerable interest to investigate the behaviour of other
unsymmetrical diphosphenes towards a suitable transition metal moiety. We have
synthesised the unsymmetrical diphosphenes Ar*P=PArF 1 (Ar* = 2,4,6-tBu3C6H2,
ArF = 2,4,6-(CF3)3C6H2) via the route shown in equation (1), and the species
Ar’P=PArF 7 (Ar’ = 2,6-(CF3)2C6H3) by both pathways shown in equations (2) and
(3). These compounds have been fully characterised. The unsymmetrical products
have then been reacted in a 2 : 1 molar ratio with the dimeric platinum(II) compound
trans-[Pt(PEt3)Cl(ꢀ-Cl)]2, to investigate whether steric or electronic effects dominate
31
possible complex formation (Scheme 1). While P NMR solution-state spectroscopy
3

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has been the main experimental means of following these reactions, calculations have
also been carried out, to establish the most stable complexes from a thermodynamic
viewpoint, and to calculate their chemical shifts. In addition to providing convincing
support for the identification of the observed complexes in the Ar*P=PArF 1 reaction,
this procedure has enabled the resonances to be assigned for Ar’P=PArF 7, and
established the identity of the sole product 8 from this reaction. The behaviour of the
platinum(II) dimer with the original Yoshifuji diphosphene Ar*P=PAr* 5 has also
been investigated for comparison purposes. We have previously reported that this
dimer with symmetrical ArFP=PArF 9 forms cis-[PtCl2(PEt3)(ArFP=PArF)] 10 as the
unique product with the diphosphene coordinated η1 by one of the phosphorus atoms.
[5]
Scheme 1. General synthesis of ɳ1-Pt(II) complexes from unsymmetrical
diaryldiphosphenes.
4

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Results and Discussion
(a) Reaction of trans-[Pt(PEt3)Cl(ꢀ-Cl)]2 with Ar*P=PAr* 5
A 1:2 stoichiometric mixture of the reagents in C6D6 was sealed into an NMR tube.
After a few hours at room temperature, some of the starting material had been
consumed, and new signals were apparent in the 31P NMR spectrum. A doublet at 380
ppm (1JP P 548 Hz) was readily assigned to the uncoordinated phosphorus in an η1
A
B
diphosphene complex, while a doublet of doublets with platinum satellites was seen at
1
2
353 ppm (1JPtP 2200 Hz, JP P 548 Hz, JP P 487 Hz). There was also a doublet
A
A
B
A C
signal for the PEt3 group with platinum satellites at 10.4 ppm (1JPt-P 2917 Hz, JP -P
2
C
A
C
1
2
487 Hz). The magnitudes of both JPtP and JP P clearly denote the formation of a
A
C
trans-complex 6, [16] as shown in Figure 1. The NMR data for all new species,
including the parent unsymmetrical diphosphenes 1 and 7, are collected in Table 1.
Figure 1. Compounds prepared in this study.
5

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2
The large JPA-PC values for trans-complexes 2, 3 and 6 in Table 1 are entirely in
keeping with previous work, and predictions from the literature. [17] For example,
2JPP values between 469 and 598 Hz have been reported for nine different trans-
complexes in three recent papers. [18-20]
6

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Table 1. 31P{1H} NMR data (δ 31P in ppm; J values in Hz); PA = bound P in diphosphene for Pt complexes; PB = ‘free’ P in diphosphene for Pt
complexes; PC = PEt3; all RP=PR’ forms are E.
Compound
Group at PA Group at PB δ PA δ PB
1JP P
1JPtP
δ PC
1JP tP
2JP P
cis/trans at Pt
A
B
A
C
A
C
1a
Ar*
Ar*
ArF
ArF
Ar*
Ar*
Ar’
Ar’
ArF
ArF
ArF
ArF
Ar*
Ar*
Ar*
Ar*
ArF
ArF
ArF
ArF
536 417
400 308
317 402
258 360
494
570
529
552
548
2328
2406
3735
18.6
19.2
9.6
3000
3139
3174
528
521
trans
trans
cis
2
3
4b
5
353 380
489 478
386 334
474
548
552
501
2200
2550
2603
10.4
NRc
10
2917
NRc
487
trans
cis
6
7
8
9 Ref. [5]
10 Ref. [5]
347 337
534
4024
-
cis
a 4
J
PBF
= 22 Hz; b 2JP Pt = 343 Hz; c NR = Not Recorded.
B
7

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(b) Reaction of trans-[Pt(PEt3)Cl(ꢀ-Cl)]2 with Ar*PA=PBArF 1
A similar reaction was carried out with the unsymmetrical diphosphene 1; in this
instance the mixture was warmed to 60°C for 1 h in the NMR tube. The 31P{1H}
NMR spectrum confirmed the presence of a small amount of unreacted starting
material, together with two new complexes 2 and 3 (Table 1). Analysis of the spectra
1
2
was not trivial because of the close similarity between the values of JP P and JP P
,
A
B
A C
particularly for one of the compounds. The results clearly demonstrate, however, that
both species have trans configurations, with either PA(Ar*) 2 or PA(ArF) 3 η1-
coordinated to Pt (Figure 1). The expected doublet for the uncoordinated P atom of
the diphosphene ligand in each complex could be readily identified (Table 1). Signal
intensities suggested a slight prevalence of complex 3 with PA(ArF) bonded to Pt.
Attempts to separate these isomers by column chromatography were unsuccessful, as
31
they decomposed during the process. When a scaled-up reaction was performed, P
NMR signals for a third complex 4 were detected; the latter gave a clean spectrum for
cis-[PtCl2(PEt3)(ArFPA=PBAr*)], with PA(ArF) coordinated to Pt (Figure 2), as
1
confirmed by the higher JPtP than for the corresponding trans-isomer 3, and the
A
absence of any large 2J coupling between PA and PC in the PEt3 group (Table 1).
Since cis complexes are usually more stable than trans, 4 may well be the
thermodynamic reaction product, suggesting that P(ArF) is a rather better donor than
P(Ar*). A reasonable assumption is that 2 and 3 are initially formed in an equilibrium
mixture from 1; reaction was not complete, as shown by the presence of a small amount of
starting material. As trans-isomer 3 converts to the thermodynamically more stable cis-
isomer 4, removal of 3 from the equilibrium will result in conversion of 2 to 3.
Eventually 4 will remain as the only product. This conclusion is fully supported by the
calculations discussed below, which indicate that complex 4 is the most stable of all
four possible single η1-bonded species. Although the electronegative CF3 groups in
ArF would tend to reduce the effectiveness of donation from the phosphorus lone pair,
electronic effects seem to be outweighed by the steric hindrance to complexation
caused by the very bulky ortho- tbutyl groups in PAr*.
8

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Figure 2. 31P NMR spectrum for the cis-complex, [PtCl2(PEt3)(ArFP=PAr*)], 4.
31
There are two other points of particular interest in the P{1H} NMR spectrum
of 4 (Figure 2 and Table 1). Firstly, the 1JPtP value (for the PEt3 group) is smaller than
C
usually observed in cis-complexes, although slightly larger than that in 3. Secondly,
2JPtP coupling (to the PAr* group) is clearly apparent, with a coupling constant of 343
B
Hz; this behaviour was not observed in any of the other systems studied. These results
may indicate a slightly unusual configuration for the coordinated diphosphene in 4,
bringing PB somewhat closer to platinum than normal and forcing PC further away.
(c) Reaction of [Pt(PEt3)Cl2]2 with Ar’P=PArF 7
A parallel reaction to those described above was carried out between the platinum
dimer and Ar’P=PArF 7 at room temperature. The mixture was stirred for 1 h. 31P{1H}
NMR spectroscopy revealed the presence of a single η1-bonded cis-isomer 8 (Figure 1
and Table 1). This result was not unexpected, in view of the formation of a similar
cis-complex 10 by the symmetrical diphosphene ArFP=PArF.[5] In itself the NMR
spectrum of 8 did not allow the coordinating atom to be distinguished in the
9

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unsymmetrical diphosphene, since similar 31P spectra would result, irrespective of
whether P(Ar’) or P(ArF) was the donor atom. Calculations described below indicate
that P(Ar’) coordinates to Pt, and also enable the resonances to be assigned for the
starting material 7. In this system, comparable steric hindrance would be expected
from both potential donor sites, because each phosphorus has two ortho-CF3 groups.
The Ar’ moiety is expected to be rather less electronegative than ArF, however, hence
a better donor, and this is entirely in keeping with the deductions above.
(d) Calculations
Geometries of known and new diaryldiphosphenes were optimised at
B3LYP/6-31G*, to assess the accuracy of the computations by comparing with
31
reported X-ray data [21-23] where available, and by comparing computed P GIAO-
NMR data on the optimised geometries with observed 31P NMR shifts, which are
summarised in Table 2. The calculated P=P bond lengths are consistently longer by
0.02 Å, whereas the agreement between computed and observed 31P NMR data [24] is
good. The more computationally intensive model chemistries B3LYP/6-311G** and
B3LYP/cc-pVDZ gave geometric data no better than B3LYP/6-31G* (Table S1) thus
31
B3LYP/6-31G* is used here. For the new diphosphene 7, the P peaks may thus be
assigned with confidence. Similar computations were also carried out for known
diphosphenes coordinated to cationic methyl group [25] or metals in an η1 fashion
[7,9,26] as listed in Table 2, and Figure 3 demonstrates the good agreement between
observed and computed (GIAO) 31P NMR data.
10

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Table 2. Comparison of computed (B3LYP/6-31G*//GIAO-NMR) and observed P=P bond lengths (in Å) and 31P chemical shifts (δ 31P in ppm)
for known diaryl diphosphenes Ar1PA=PBAr2, [Ar1(Me)PA=PBAr2]+ cation and complexes of type Ar1{R}PA=PBAr2.
Ar1
Ar2
R
P=P
E/Z
P=P
P=P
Reference
[21]
δ PA
δ PA
δ PB
δ PB
Reference
(calc)
(obs)
(calc)
(obs)
(calc)
(obs)
ArF
Ar*
Ar’
ArF
Ar’
Ar’
Ar*
Ar’
ArF
Mes
Mes
Ar*
Ar*
Ar*
ArF
ArF
Ar’
Ar*
Ar*
Ar’
ArF
Ar*
Ar*
Ar*
-
E
E
E
E
E
E
E
E
E
E
Z
E
2.052
400.5
487.9
479.1
467.9
477.5
408.7
349.1
439.5
437.8
415.8
374.5
320.1
417.0
492.4
489.0
473.9
477.0
422.1
332.2
431.0
430.2
412.3
384.9
339.0
536.7
536.0
This work
This work
This work
This work
[24]
1
5
7
9
-
2.062 2.034(2)
2.040
-
466.8
478.0
-
2.040 2.022(2)
2.040 2.019(2)
2.051
[22]
[23]
-
-
535.1
216.8
440.5
437.4
527.0
408.3
395.5
533.0
237.0
432.6
427.6
500.9
393.9
386.0
[24]
-Me+
2.046 2.024(2)
2.043
[25]
[25]
-Cr(CO)5
-Cr(CO)5
-Cr(CO)5
-Cr(CO)5
-AuCl
[26]
2.046
[26]
2.052
[9]
2.055 2.039(3)
2.051 1.975(5)
[9]
[7]
[9]
[7]
11

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550
500
450
400
350
300
250
200
Best line-fitting error, R = 0.987
200
250
300
350
400
450
500
550
Observed 31P (ppm)
Figure 3. Comparison between computed and observed 31P chemical shifts for diaryl
diphosphenes and related derivatives listed in Table 2.
The platinum complexes observed experimentally are more complicated
computationally, as there are many possible conformers and orientations. For
simplification and reduced computational times, PMe3 was used instead of PEt3 in the
model geometries 11-22 where all possible isomers were investigated at
B3LYP/LANL2DZ:6-31G*. Table 3 compares the computed 31P GIAO-NMR data for
11-22 with observed complexes 2-4, 6, 8 and 10. The agreement with observed
complexes is less precise, but all observed data fit best with the data of the expected
isomer out of the possible structures. The relative energies between the model isomers
11-22 are listed in Table 4. The most stable isomers computationally are all found
experimentally i.e. 13, 16, 17 and 21 for 4, 6, 8 and 10 respectively. The less
thermodynamically stable isomers 2 and 3 observed are the next two most stable
forms computationally, whereas the cis-isomer with the Ar* group at PA is
unfavourable due to sterics, as shown pictorially in Figure 4.
12

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31
31
Table 3. Computed P chemical shifts (δ P in ppm) for η1-coordinating diphosphene-platinum complexes [PtCl2(PCR3)(R1PA=PBR2)]. PA =
bound P in diphosphene; PB = ‘free’ P in diphosphene; PC = PMe3 or PEt3; All R1P=PR2 forms are E. Observed data of reported compounds are
included in italics for comparison.
Compound
δ PA
δ PB
δ PC
Compound
δ PA
δ PB
δ PC
cis-[PtCl2(PMe3)(Ar*P=PArF)]
344.8
285.2
5.1
trans-[PtCl2(PMe3)(Ar*P=PArF)]
trans-[PtCl2(PEt3)(Ar*P=PArF)]
407.2 314.0
400 308
7.0
11
12
2
21
cis-[PtCl2(PMe3)(ArFP=PAr*)]
cis-[PtCl2(PEt3)(ArFP=PAr*)]
285.2
384.1
10.4
trans-[PtCl2(PMe3)(ArFP=PAr*)]
trans-[PtCl2(PEt3)(ArFP=PAr*)]
318.1 417.1
317 402
8.2
13
4
14
3
258
360
10
19
cis-[PtCl2(PMe3)(Ar*P=PAr*)]
317.5
372.0
4.4
trans-[PtCl2(PMe3)(Ar*P=PAr*)]
trans-[PtCl2(PEt3)(Ar*P=PAr*)]
374.9 391.2
353 380
3.4
15
16
6
10
cis-[PtCl2(PMe3)(Ar’P=PArF)]
cis-[PtCl2(PEt3)(Ar’P=PArF)]
342.0
315.4
10.3
trans-[PtCl2(PMe3)(Ar’P=PArF)]
382.1 345.4
6.6
17
8
18
386
334
NR
cis-[PtCl2(PMe3)(ArFP=PAr’)]
329.3
326.4
11.8
trans-[PtCl2(PMe3)(ArFP=PAr’)]
trans-[PtCl2(PMe3)(ArFP=PArF)]
371.6 364.2
374.9 352.8
6.7
8.0
19
20
22
cis-[PtCl2(PMe3)(ArFP=PArF)]
cis-[PtCl2(PEt3)(ArFP=PArF)]
336.1
327.2
9.9
21
10
347
337
10
13

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Table 4. Relative energies in kcal mol-1 for cis- and trans- isomers of model platinum
complexes, 11-22. PA = bound P in diphosphene. All ArP=PAr forms are E. The
observed complexes 2-4, 6, 8 and 10 are assigned to the corresponding model isomer.
Compound
Group at PA
Ar*
cis
trans
Group at PA
ArF
cis
trans
[PtCl2(PMe3)(Ar*P=PArF)]
11
12
3.8
2
13
0.0
4
14
3.4
3
4.4
[PtCl2(PEt3)(Ar*P=PArF)]
Ar*
ArF
[PtCl2(PMe3)(Ar*P=PAr*)]
[PtCl2(PEt3)(Ar*P=PAr*)]
Ar*
Ar*
15
16
0.0
6
0.4
[PtCl2(PMe3)(Ar’P=PArF)]
[PtCl2(PEt3)(Ar’P=PArF)]
Ar’
Ar’
ArF
ArF
17
0.0
8
18
19
20
1.8
0.1
2.2
[PtCl2(PMe3)(ArFP=PArF)]
[PtCl2(PEt3)(ArFP=PArF)]
ArF
ArF
21
0.0
10
22
1.3
14

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Figure 4. Optimised geometries of the model platinum complex,
[PtCl2(PMe3)(Ar*P=PArF)], 11-14.
15

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Conclusions
Symmetrical diphosphene Ar*P=PAr* 5 forms a trans-complex 6 with trans-
[Pt(PEt3)Cl(ꢀ-Cl)]2, whereas the unsymmetrical diphosphene ArFP=PAr’ 7 yields a
cis-complex 8, parallel to the behaviour of ArFP=PArF, [5] with the PAr’ group
bonded to platinum, as shown by theoretical calculations. The most interesting results
were obtained with the unsymmetrical diphosphene Ar*P=PArF 1. Initially a mixture
of two trans-complexes was formed, corresponding to separate η1 coordination of
ArFP 3 and Ar*P 2, possibly with a slight preference for 3. Subsequently a single cis-
complex 4 was detected, with ArFP coordinated to Pt. Calculations confirm that this
complex is expected to be the most thermodynamically stable of the four possible
products with η1 coordination. The results illustrate well the interplay of steric and
electronic effects in complexes of unsymmetrical diphosphenes. The most sterically
demanding group studied is Ar*, so that for Ar*P=PArF the least stable complex is
calculated to be the one where the Ar*P group would be cis to the PEt3 ligand on Pt.
This is in full agreement with the experimental observations, even though Ar*P
should be a better donor than ArFP. For Ar’P=PArF, where the steric effects should be
similar for both potential donor sites, electronic effects take over, and coordination is
observed only via the Ar’P group, which should be the better donor. Again theory and
experiment are in agreement. The present work illustrates the value of combining
experimental and theoretical studies of such systems.
Experimental
All manipulations, including the preparation of NMR samples, were carried out under
an inert atmosphere of dry nitrogen, either using standard Schlenk and cannula
techniques, or in a nitrogen-filled glovebox. Solvents were refluxed over an
appropriate drying agent, and distilled and degassed prior to use. 31P{1H} NMR
spectra were recorded in either CDCl3 or C6D6 at ambient temperature on a Bruker
AC250 spectrometer operating at 101.256 MHz; chemical shifts were measured
relative to external 85% H3PO4. 19F NMR spectra were recorded on the same
instrument at 235.36 MHz; chemical shifts were measured relative to external CFCl3.
Mass spectra were recorded on a VG7070E instrument by Dr. M. Jones or Miss L.
Turner. Elemental analyses were obtained by the microanalytical services of the
Chemistry Department, University of Durham.
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Synthesis of Ar*P=PAr* 5
This known diphosphene 5 [1] was prepared via dechlorination of Ar*PCl2 by
W(PMe3)6.[15] W(PMe3)6 (0.500 g, 0.78 mmol) and Ar*PCl2 (0.520 g, 1.5 mmol)
were placed in an ampoule, and benzene (30 ml) was condensed into the vessel at -
196 °C; the reaction mixture was then allowed to warm to room temperature (RT)
under vacuum. The mixture was stirred at RT for 36 h. During this time the solution
changed colour from yellow/gold to orange. 31P{1H} solution-state NMR
spectroscopy indicated that the reaction had reached completion, and all of the
Ar*PCl2 had been consumed. The solution was transferred to a Schlenk vessel and the
benzene was removed in vacuo. The residue was extracted with n-pentane (3 x 15 ml).
The solution was reduced in volume to ca. 20 ml, and cooled to -78 °C to give a
bright orange crystalline product. It was identified by its characteristic 31P NMR shift
(δ 494 ppm).
Synthesis of Ar*P=PArF 1
This new unsymmetrical diphosphene was similarly prepared. W(PMe3)6 (0.300 g,
0.46 mmol), Ar*PCl2 (0.156 g, 0.45 mmol) and ArFPCl2 (0.175 g, 0.46 mmol) were
placed in an ampoule and benzene (30 ml) was condensed into the vessel at -196 °C.
The reaction mixture was allowed to warm to RT under vacuum. This mixture was
stirred at RT for 36 h, during which time the solution changed colour from
31
yellow/gold to bright red, with some precipitation of a pale solid. After this time P
NMR indicated that the reaction had reached completion.
The solution was filtered and benzaldehyde (5 ml, 5.22 g, 49 mmol) [10]was
added to the filtrate. The mixture was stirred at RT for 48 h, during which time much
31
pale solid precipitated; P NMR indicated that this solid contained no diphosphene.
After further filtration, Et2O (15 ml) was added and the solution cooled to 4 °C for 48
h. Further solid precipitated and was filtered off. The benzene and Et2O were
removed in vacuo. 31P NMR of the resultant orange oil indicated that the diphosphene
was ca. 90 % pure. The residue was passed down a silica column using CH2Cl2 as
eluant. Removal of the solvent in vacuo afforded the pure diphosphene 1 as an orange
oil. After standing at room temperature for a few days, small florets were observed to
crystallise, but these were unsuitable for X-ray diffraction. (Yield 185 mg, 70 %)
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Found: C, 55.12, H, 5.30 %; C27H31F9P2 requires C, 55.11, H, 5.31 %. M/Z 588 [M+],
31
1
307 [M+ - ArF], P{1H} NMR (CDCl3) 536 (d, Ar*P, JPP 570 Hz), 417 (doublet of
1
4
19
4
septets, ArFP, JPP 570 Hz, JPF 22 Hz). F NMR (C6D6) -56.8 (t, 6F, o-CF3, JPF 20
Hz), -63.7 (s, 3F, p-CF3).
Synthesis of Ar’P=PArF 7
As indicated above, this unsymmetrical diphosphene was prepared in two different
ways
(a) From ArFPCl2 and Ar’PH2
DBU (1.44 ml, 9.65 mmol) in THF (15 ml) was added dropwise over 5 min. to a
stirred solution of Ar’PH2 (1.18 g, 4.80 mmol) and ArFPCl2 (1.84 g, 4.80 mmol) in
THF (45 ml) at 0 °C. The solution turned brown-yellow, with formation of a
precipitate. This mixture was stirred for 30 min after being allowed to warm to RT.
The solid was removed by filtration and the filtrate was concentrated to yield a pale
yellow powder (1.9 g, 3.4 mmol, crude yield 71%). This solid was washed with
several 10 ml aliquots of Et2O to yield a pure white solid (1.2 g, 2.2 mmol, 46%).
Recrystallisation from CH2Cl2 (10 ml) gave small crystals unsuitable for X-ray
diffraction.
(b) From ArFPH2 and Ar’PCl2
A similar reaction between DBU (1.90 ml, 12.7 mmol), ArFPH2 (1.97 g, 6.3 mmol)
and Ar’PCl2 (1.98 g, 6.3 mmol) in THF (100 ml) yielded 1.64 g (2.9 mmol, 46%) of
the same product 7 after work-up. The ripple tank oscillation technique was employed
between 10 and -30 °C for several days in an attempt to obtain better crystals of the
diphosphene (0.9 g) in toluene (7 ml), but again only very small crystals resulted. M.
Pt. 131 °C. Found, C, 36.33, H, 0.60 %; C17H5F15P2 requires C, 36.71, H, 0.91 %.
M/Z 343 [M+ - Ar’], 324 [M+ - Ar’ –F], 31P{1H} NMR (CDCl3) 489 (d, 1JPP 552 Hz),
478 (d, 1JPP 552 Hz). 19F NMR (CDCl3) ca. -53 (12F, o-CF3, complex region with two
overlapping doublets of doublets), -64.1 (s, 3F, p-CF3).
Representative synthesis of
a
platinum complex
–
Synthesis of cis-
[PtCl2(PEt3)(ArFP=PAr’)] 8
The platinum dimer trans-[Pt(PEt3)Cl2]2 (0.58 g, 0.75 mmol) was added to a stirred
solution of 7 (0.85 g, 1.5 mmol) in CH2Cl2 at room temperature. The mixture was
18

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stirred for 1 h, then cooled to -40° C. The ɳ1 cis-complex 8 was isolated as small
transparent crystals, which proved to be unsuitable for X-ray crystallography. Yield
0.62 g (43 %).
Computational Details
All computations were carried out with the Gaussian 09 package.[27] The geometries
of compounds listed in Table 2 and the model systems 11-22 were fully optimised
with the B3LYP functional[28] with no symmetry constraints using the 6-31G* basis
set [29] for all atoms apart from Au and Pt, where LANL2DZ pseudopotentials[30]
were employed. The much larger 6-311G** and cc-pVDZ basis sets did not show
more accurate data (Table S1) compared with results from the 6-31G* basis set and
experimental data, thus the 6-31G* basis set is used here for all computations. The
polarised continuum solvation model (PCM) with ε = 35.7 was applied in all cases to
reflect solvent effects.[31] Calculated 31P NMR chemical shifts at the GIAO-
B3LYP/6-31G* level were obtained for the optimised geometries using the δ(31P) =
300.0 – 0.9σ(31P) scale.[32]
Acknowledgements
We thank the EPSRC for financial support, Johnson-Matthey plc for the generous
loan of platinum compounds, and Professor M. Yoshifuji for a gift of Ar*PCl2.
Supporting Information
Table of bond lengths of optimised geometries for selected diphosphenes obtained at
6-31G*, 6-311G** and cc-pVDZ basis sets.
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Highlights
•
•
•
some new unsymmetrical diphosphenes and their platinum(II) complexes
synthesised
configurations assigned by a combination of 31P solution-state NMR and high-
level theoretical calculations
steric and electronic effects both important influences on the configuration
adopted