Platinum(II) complexes of cyclic triphosphenium ions: A 31P NMR spectroscopic and computational study

Article abstract of DOI:10.1021/ic3011663

The first transition metal complexes of cyclic triphosphenium ions have been unequivocally identified in solution by ³¹P NMR spectroscopy. The ligands coordinate to platinum(II) via the central phosphorus atom, but only when at least one of the outer phosphorus atoms has non-aromatic substituents. Depending on the system, either trans- (the kinetic reaction product) and/or cis- (the thermodynamic reaction product) complexes are formed. The ¹J coupling constants between ¹⁹⁵Pt and the central phosphorus atom of the CTI (P_A) are small for both cis- and trans-isomers, between 900 and 1300 Hz, whereas other phosphanes in these complexes derived from the platinum(II) starting material show normal ¹J_PtP values. These results suggest a possible long P-Pt bond between the overall positively charged ligand and the platinum(II) cation. Calculations including predicted ³¹P NMR shifts for the CTIs and their Pt(II) complexes largely support our experimental findings.

Full text of DOI:10.1021/ic3011663

Article
pubs.acs.org/IC
31
Platinum(II) Complexes of Cyclic Triphosphenium Ions: a P NMR
Spectroscopic and Computational Study
Philippa K. Coffer (nee Monks), Robert M. K. Deng,† Keith B. Dillon,* Mark A. Fox,
́
and Richard J. Olivey
Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
S
* Supporting Information
ABSTRACT: The first transition metal complexes of cyclic triphosphenium ions have been unequivocally
identified in solution by 31P NMR spectroscopy. The ligands coordinate to platinum(II) via the central
phosphorus atom, but only when at least one of the outer phosphorus atoms has non-aromatic substituents.
Depending on the system, either trans- (the kinetic reaction product) and/or cis- (the thermodynamic
1
reaction product) complexes are formed. The J coupling constants between 195Pt and the central
phosphorus atom of the CTI (PA) are small for both cis- and trans-isomers, between 900 and 1300 Hz,
whereas other phosphanes in these complexes derived from the platinum(II) starting material show normal
1JPtP values. These results suggest a possible long P−Pt bond between the overall positively charged ligand
and the platinum(II) cation. Calculations including predicted 31P NMR shifts for the CTIs and their Pt(II)
complexes largely support our experimental findings.
INTRODUCTION
Cyclic triphosphenium ions (CTIs) are positively charged
heterocycles with a hydrocarbon backbone and three linked
■
phosphorus atoms, the middle one of these being “bare” and
nominally in the +1 oxidation state, Figure 1. The first example
Figure 2. Charge distribution in a CTI.
applications in areas such as catalysis, where displacement of
a weakly bound ligand may be advantageous.
We now report the unequivocal identification by 31P NMR
solution-state spectroscopy of the first platinum(II) complexes
of cyclic triphosphenium ions, clearly coordinated to the metal
via the central phosphorus atom. The platinum(II) precursors
used were dimeric halogen-bridged complexes trans-[Pt(PR3)-
X(μ-X)]2, with R3 = Et3, PhMe2 or Ph2Me and X = Cl or
Br.20−23
Most of the CTIs prepared by other research groups have
aromatic, usually phenyl, substituents on the outer (phospho-
nium) phosphorus atoms.1−17 We have found no evidence for
coordination by these ligands to our platinum(II) substrates,
suggesting that they are not good enough donors (Results and
Discussion). Complexation does occur, however, for alkyl or
alicyclic substituents, and even for an unsymmetrical CTI with
ethyl substituents on one of the outer phosphorus atoms and
phenyl groups on the other. Several examples of both trans-
(the kinetic product) and cis- (the thermodynamic product)
complexes have been identified in solution by 31P NMR
spectroscopy, as described below.
Figure 1. General structure of a CTI.
of a CTI was prepared and structurally characterized by
Schmidpeter and co-workers in 1982.1 Several other examples
have subsequently been synthesized,2−17 particularly by
Macdonald et al.7−10 and by our research group,12−17 with
ring sizes varying from 4 to 8. They feature in a review by Ellis
and Macdonald,18 and may be readily recognized by 31P NMR
solution-state spectroscopy, with a large 1JPP and a marked shift
difference between PA and PB (Figure 1). Where these species
have been characterized by single-crystal X-ray diffraction, the
P−P distances lie between 2.11 and 2.15 Å, intermediate
between normal P−P single and double bonds, providing clear
evidence for a delocalized system.1,7−13,16 The mechanism of
their formation, at least for certain substituents, has been
established.19 The ions may be regarded as consisting of a
phosphide(I) anion coordinated by two phosphonium(V)
cations, Figure 2, which gives rise to the possibility of
coordination to a transition metal fragment via a lone pair on
the central phosphorus atom. No examples of such complexes
have been reported, however. It is reasonable to expect that
coordination may be weak to a metal cation because of
repulsion between the positive charges. This may have
Received: June 1, 2012
Published: September 4, 2012
© 2012 American Chemical Society
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Figure 3. CTIs used in this study.
complexes where SnCl2 is present in the reaction mixture
cannot be specified, although Cl− may clearly be assumed when
no SnCl2 is present. What is certain in all systems is the identity
of the phosphorus ligands attached to platinum(II), and their
relative positions, that is, cis or trans, since the NMR evidence
on these points is clear. The CTI complexes with Pt(II)
described here will also carry an overall charge of +1, for which
various counterions may be present in solution.
RESULTS AND DISCUSSION
■
Synthesis of CTIs. Using literature methods,13,15,17,19,24
various CTIs (Figure 3) were synthesized and their formation
confirmed using 31P NMR spectroscopy (Table 1).
Table 1. 31P NMR Data for CTIs Used in This Study
CTI
SnCl2
δPB′,
δPB,
ppm
δPA,
ppm
1JPA‑PB
Hz
,
1JPA‑PB′
,
Hz
b
number present ppm
reference
Solution Studies on Pt(II) Complexes of CTIs. Scheme 1
shows the general reaction (for all ring sizes) between one mole
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
×
×
√
×
√
×
√
×
√
×
√
×
√
×
65.0 −229.5
81.5 −269.0
80.5 −264.1
87.3 −289.3
87.2 −289.0
67.1 −251.0
69.4 −251.0
31.1 −253.3
30.5 −252.3
36.5 −297.4
36.6 −292.9
44.7 −299.6
44.8 −299.4
48.3 −262.5
48.1 −260.8
55.9 −264.8
55.5 −262.5
45.5 −237.5
450
437
444
457
455
431
434
417
419
452
456
458
457
472
476
474
480
565
a
13
13
24
17
13
13
15
19
19
a
Scheme 1. Synthesis of Pt(II) Complexes of CTIs
78.0
81.3
460
466
19
17
15
a
√
×
√
√
b
17
17
a
of a dimeric platinum(II) precursor and two moles of a CTI.
The kinetic product is the trans-complex, whereas the
thermodynamic product is the cis-complex.
a
(i). Attempted Formation of Pt(II) Chloro-Complexes of
Five-Membered Ring CTIs. The initial experiment attempted
was a 2:1 reaction of the CTI 1 derived from dppe with trans-
[Pt(PEt3)Cl(μ-Cl)]2; the 31P NMR spectrum was recorded as
soon as possible after addition of the dimer solution. There was
no evidence of complexation of the CTI, even though the NMR
spectrum indicated that some unreacted CTI was still present
This work. B′ = Ph2P in dedppe.
It should be noted that CTIs were sometimes prepared as
their simple chloride salt, but in other reactions tin(II) chloride
was added (Experimental Section), making [SnCl6]2− the most
probable counterion, although the possibility of [SnCl5]−
cannot be discounted. If excess SnCl2 is present, this introduces
−
1
the possibility of SnCl3 rather than Cl− as a ligand on
(δ 31PB 65.0(d), δ 31PB −229.5(t) ppm, JPP 450 Hz). The
platinum(II),25−31 which is sometimes reflected in chemical
shift and coupling constant differences for the phosphorus-
containing ligands, as discussed below. This does not apply to
the five-membered ring CTI systems, however, where no such
pairs of complexes were investigated. Hence the identity of the
two nonphosphorus ligands in monomeric platinum(II)
major products of the reaction were identified as [Pt(dppe)2]2+
and [Pt(dppe)(PEt3)Cl]+, Figure 4. The 31P NMR data for
these complexes, along with literature values, are shown in
Table 2. This result suggests that ring scission is the preferred
reaction with purely aromatic substituents on the outer
phosphorus atoms of the CTI.
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than for the trans-complex. These observations are discussed
further below.
(ii). Formation of Pt(II) Chloro-Complexes of Six-Mem-
bered Ring CTIs. CTIs 5, 6, and 7 (Figure 3) were synthesized
from the diphosphanes depp, dcypp, and dippp, respectively.
Reaction of 5 as its chloride salt with trans-[Pt(PEt3)X(μ-
X)]2 afforded both cis- and trans-complexes (Table 5). The
signals from the trans-isomer were weak, however, and not all
of the coupling constants could be recorded. This system was
also investigated with SnCl2 present, when only the cis-complex
was detected. The differences in the results for the cis-complex
with Cl− only and with SnCl2 also present, particularly δPE,
suggest that a change in one or both coligands from Cl to SnCl3
is quite probable here. The reactions of 5 with trans-
[Pt(PR3)Cl(μ-Cl)]2, where R3 = PhMe2 and Ph2Me, were
both carried out with SnCl2 present; a cis-complex was formed
in each instance.
Figure 4. Reaction products from CTI 1 and trans-[Pt(PEt3)Cl(μ-
Cl)]2.
The reactions with the depe derivative 2 (Figure 3) were
usually carried out with SnCl2 present; for R3 = PEt3 or PhMe2
in trans-[Pt(PR3)X(μ-X)]2, cis-complexes only were observed,
as shown in Table 3. For R3 = Ph2Me, no ring complexation
was apparent, just coordination of depe to platinum(II),
indicating the break-up of the CTI. Similar results were
obtained for this system without SnCl2 present.
Reaction of 6 with trans-[Pt(PEt3)Cl(μ-Cl)]2 and no SnCl2
initially afforded only the trans-complex (Table 5). Weak
signals of the cis-isomer subsequently appeared, although these
signals were recorded more readily in the same reaction with
SnCl2 present. Analogous reactions of 6 with trans-[Pt(PR3)-
Cl(μ-Cl)]2, where R3 = PhMe2 and R3 = Ph2Me, were also
performed. The reaction for R3 = PhMe2 was carried out only
in the presence of SnCl2, yielding a cis-complex. Both trans-
and cis-isomers were observed for R3 = Ph2Me in the absence
of SnCl2, though again the signals from the trans-complex were
weak, and not all coupling constants could be measured.
Experiments with 7 were only performed with trans-
[Pt(PEt3)Cl(μ-Cl)]2; in the absence of SnCl2, the cis-complex
was detected, but in the presence of SnCl2 both cis- and trans-
species were apparent, Figure 5. The signals from the trans-
complex were again weak, and it was not possible to assign
δPE(trans) with certainty (Table 5). Comparison of the results for
the cis-complex with and without SnCl2 again suggests the
strong possibility of a change of coligand, with differences in
both chemical shifts and coupling constants being apparent.
As for the five-membered ring CTIs, the six-membered ring
analogues show a very significant shift to higher frequency for
the central coordinated phosphorus atom, which is larger for
cis-complexes than for their trans-isomers. All of the 1JPtP values
for this atom lie between 1100 and 1300 Hz, where they could
be recorded.
For CTI 3 synthesized from dcype (Figure 3), the reactions
with R3 = Et3 were performed for the chloride salt, since the
hexachlorostannate appeared to be sparingly soluble and no
reliable results could be obtained. Both the trans- and cis-
complexes were identified in solution (Table 3); the results
clearly demonstrate coordination by the central phosphorus
atom of the CTI to Pt(II). For reactions of 3 with the Pt(II)
precursors where R3 = PhMe2 or Ph2Me, SnCl2 was present.
The trans- and cis-complexes were both detected for PPh2Me
as coligand, but only the cis-isomer for PPhMe2 (Table 3).
In view of the results above, a particularly interesting system
to study was CTI 4, derived from the unsymmetrical
diphosphane dedppe, Et2P(CH2)2PPh2 (Figure 3).13 The
NMR spectra from the CTI itself and its potential platinum(II)
complexes are more complicated than usual, because all three
phosphorus atoms in the CTI are nonequivalent. The results in
Table 4 clearly show that this CTI, too, will coordinate via the
central phosphorus atom to form a cis-complex upon reaction
with trans-[Pt(PEt3)Cl(μ-Cl)]2. (We did not have enough
material to investigate its reactions with the other Pt dimers.)
In each of the new Pt(II) complexes containing a five-
membered ring CTI the most notable feature is the
1
comparatively low value for JPtP to the CTI, irrespective of
whether the geometry is cis or trans, as shown in Tables 3 and
1
4. The J values range from 1023 to 1183 Hz, and where both
isomers have been identified in solution these are higher for the
cis-complex than for the trans-complex, as is usually the case for
other phosphorus donors to platinum(II).34 The phosphanes
PR3 introduced with the platinum(II) precursor have
completely normal 1J values for cis- (3385−3625 Hz) or
trans- (2783−2972 Hz) Pt(II) complexes. The chemical shift of
the central phosphorus atom of the CTI also moves to much
higher frequency on complexation, as shown by a comparison
of the data in Table 1 with those in Tables 3 and 4. Where
isomeric pairs have been identified, this shift is larger for the cis-
(iii). Formation of Pt(II) Chloro-Complexes of Seven-
Membered Ring CTIs. CTIs 8, 9, and 10 (Figure 3) were
synthesized from the diphosphanes dcypb, dippb, and dtbpox,
respectively. Reaction of 8 as its chloride salt with trans-
[Pt(PEt3)Cl(μ-Cl)]2, yielded a cis-complex, whereas in the
presence of SnCl2 a trans-complex was detected, Table 6. It is
probable that there is also a change in coligand from Cl to
SnCl3, since 1JPtP to the central phosphorus atom of the CTI is
slightly higher for the trans-complex than for the cis-complex,
Table 2. 31P NMR Data for Products from the Reaction of CTI 1 and trans-[Pt(PEt3)Cl(μ-Cl)]2
δPA, ppm
1JPt‑PC, Hz
[Pt(dppe)2]2+
literature data for [Pt(dppe)2]2+
48.3
47.0
2356
2360
1JPt‑PE, Hz
32
δPC, ppm
δPD, ppm
δPE, ppm
1JPt‑PC, Hz
1JPt‑PD, Hz
2JPC‑PE, Hz
[Pt(dppe)(PEt3)Cl ]+
literature data for [Pt(dppe)(PEt3)Cl ]+
53.5
53.1
43.7
43.3
17.2
16.6
2260
2263
3542
3527
2282
2282
369
368
33
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Table 3. 31P NMR Data for 5-Membered Ring Chloro-Complexes of CTIs
complex number CTI SnCl2 present
PR3
PEt3
cis/trans δPD ppm δPC, ppm δPE, ppm 1JPC‑PD, Hz 1JPt‑PC, Hz 1JPt‑PE, Hz 2JPC‑PE, Hz
11
12
13
14
15
16
17
2
2
3
3
3
3
3
√
√
×
cis
70.0
69.3
69.9
72.6
69.9
72.1
72.9
−104.3
−113.2
−107.2
−148.2
−111.6
−124.7
−104.1
11.4
−14.0
8.9
363
359
384
408
381
380
392
1059
1120
1099
1023
1179
1032
1183
3407
3534
3422
2972
3492
2783
3625
PPhMe2
PEt3
cis
cis
×
PEt3
trans
cis
16.3
−19.6
14.4
0.2
321
215
√
√
√
PPhMe2
PPh2Me
PPh2Me
trans
cis
Table 4. 31P NMR Data for 5-Membered Ring Chloro-Complexes of CTI 4
cis/
a
1
1
1
compound number CTI SnCl2 present PR3
18 PEt3
trans
δPD′ , ppm δPD, ppm δPC, ppm δPE, ppm 1JPC‑PD′, Hz JPC‑PD, Hz JPt‑PC, Hz JPt‑PE, Hz
4
√
cis
72.9
50.8
−89.5
10.7
359
365
1096
3385
a
D′ = Ph2P in dedppe.
a
Table 5. 31P NMR Data for Chloro-Complexes of 6-Membered Ring CTIs
compound number CTI SnCl2 present
PR3
PEt3
cis/trans δPD, ppm δPC, ppm δPE, ppm 1JPC‑PD, Hz 1JPt‑PC, Hz 1JPt‑PE, Hz 2JPC‑PE, Hz
19
20
21
22
23
24
25
26
27
28
29
30
31
32
5
5
5
5
5
6
6
6
6
6
6
7
7
7
×
×
√
√
√
×
cis
19.1
26.1
18.0
18.8
18.8
20.3
27.5
20.5
20.4
20.0
27.6
27.6
27.7
34.8
−102.3
−136.0
−98.6
−108.3
−111.6
−102.7
−151.9
−102.3
−104.7
−97.0
3.2
9.3
340
348
344
338
344
371
369
375
372
380
366
374
369
380
1104
nr
3369
nr
PEt3
trans
cis
170
PEt3
9.7
1117
1166
∼1200
nr
3392
3550
3642
nr
PPhMe2
PPh2Me
PEt3
cis
−14.4
−11.0
3.4
cis
cis
×
PEt3
trans
cis
15.6
3.6
nr
2815
3456
3513
3628
nr
172
√
√
×
×
×
√
√
PEt3
1154
1227
1267
nr
PPhMe2
PPh2Me
PPh2Me
PEt3
cis
−13.6
0.1
cis
trans
cis
−148.0
−94.6
−99.1
3.9
266
172
0.2
1223
1133
1115
3628
3447
nr
PEt3
cis
4.7
PEt3
trans
−122.4
nr
a
nr = not recorded.
Figure 5. 31P NMR spectrum of the reaction between 7 and trans-[Pt(PEt3)Cl(μ-Cl)]2 to give complexes 31 and 32.
opposite to the normal trend if all other ligands are kept
constant.
repeated, to ascertain whether the trans-complex would convert
to its cis-isomer on standing. Very weak signals for a cis-
complex were obtained soon after mixing the solutions (Table
6), but because of weak satellite peaks one of the coupling
constants could not be measured. After 1 day, these signals had
disappeared, and no CTI complexes were observed in solution.
Reactions of 9 with trans-[Pt(PEt3)Cl(μ-Cl)]2 were
performed both with and without SnCl2. A trans-isomer was
Reactions of 8 with trans-[Pt(PR3)Cl(μ-Cl)]2 for R3
=
PhMe2 and Ph2Me were only carried out with SnCl2 present.
The initial product was a trans-isomer in each case (Table 6).
The 31P NMR spectrum of the trans-complex formed in the
reaction of 7 with trans-[Pt(PPhMe2)Cl(μ-Cl)]2 is depicted in
Figure 6. The experiment with PPhMe2 as coligand was
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a
Table 6. 31P NMR Data for 7-Membered Ring Chloro-Complexes of CTIs
Compound number CTI SnCl2 present
PR3
PEt3
cis/trans δPD, ppm δPC, ppm δPE, ppm 1JPC‑PD, Hz 1JPt‑PC, Hz 1JPt‑PE, Hz 2JPC‑PE, Hz
33
34
35
36
37
38
39
40
41
8
×
cis
38.5
43.3
40.0
44.0
43.7
50.9
50.9
51.1
50.8
−78.6
−127.8
−78.5
−124.1
−124.1
−133.1
−132.5
−127.7
−136.5
3.9
14.6
−15.1
3.4
418
423
421
420
421
418
420
417
550
1222
1245
nr
3431
2842
3630
2512
2586
2840
2842
2873
nr
8
√
√
√
√
×
√
×
√
PEt3
trans
cis
281
8
PPhMe2
PPhMe2
PPh2Me
PEt3
8
trans
trans
trans
trans
trans
trans
1017
1030
943
949
1013
nr
291
287
275
275
287
375
8
2.3
9
15.7
15.6
5.1
9
PEt3
9
PPh2Me
PEt3
10
11.4
a
nr = not recorded.
Figure 6. 31P NMR spectrum of the reaction between 8 and trans-[Pt(PPhMe2)Cl(μ-Cl)]2 to give complex 36.
Table 7. 31P NMR Data for 5-Membered Ring Bromo-Complexes of CTI 2
compound number
CTI
PR3
cis/trans
cis
δPD, ppm
δPC, ppm
−103.9
δPE, ppm
1JPC‑PD, Hz
1JPt‑PC, Hz
1032
1JPt‑PE, Hz
42
2
PEt3
71.0
3.6
369
3584
formed in each case. The NMR data (Table 6) are so similar
here that the coligands may be reasonably assumed to be
chlorides in both reactions. A trans-complex was also produced
from reaction of 9 with trans-[Pt(Ph2Me)Cl(μ-Cl)]2 with no
SnCl2 added (Table 6).
complexation, as is reasonable for coordination via the central P
atom.
The P−P coupling constant within the CTI decreases
somewhat on complexation to platinum(II) in all cases,
irrespective of ring size, although the change is less marked
than that which occurs on either protonation3,15,24,35 or
alkylation3,14,36−38 of the central phosphorus atom. The latter
reactions result in a change of formal oxidation state of PA from
+1 to +3, and the P−P bonds become normal single bonds,
with distances between 2.191(1) and 2.233(1) Å in all the
structures that have been determined by single-crystal X-ray
diffraction.37,38 Complexation to a transition metal such as
platinum(II) changes the geometry at the central phosphorus
atom of the CTI, but not its formal oxidation state. A reduction
A new CTI, 10, was prepared from dtbpox by reaction with
PCl3 in the presence of SnCl2. Formation of the CTI was
confirmed by a doublet, δ 31P 45.5 ppm, and a triplet, δ 31P
−237.5 ppm, for PB and PA respectively, 1JPP 565 Hz, in the 31
P
NMR solution spectrum. Only the reaction of 10 with trans-
[Pt(PEt3)Cl(μ-Cl)]2 was carried out, because of only a small
amount of the diphosphane being available. Weak signals from
a trans-complex were apparent in the 31P NMR spectra (Table
1
6), but reliable values could not be obtained for either JPtP
,
1
because of the very low intensities of the satellite signals.
in JPP is expected on complexation because of the transfer of
The same overall trends are again apparent in Table 6 that
were noted for the smaller ring CTIs. Where they could be
measured, the JPtP values to the central phosphorus atom of
the CTI are again low, between 940 and 1250 Hz, whereas the
phosphane coligands show normal 1JPtP values for cis- or trans-
complexes. There is a large shift to higher frequency for the
central phosphorus atom of the CTI on complexation, which is
greater for cis-complexes than for their trans-isomers. The outer
phosphorus atoms of the CTI are very little affected by
electron density away from phosphorus to the metal, but it
seems entirely reasonable that this reduction should be smaller
than that caused by protonation or alkylation. The consequence
may well be a slight lengthening of the P−P bonds in the CTI
complexes relative to those in the free ligand, but they will
probably still be shorter than usually found for P−P single
bonds.
1
(iv). Attempted Formation of Pt(II) Bromo-Complexes of
Five-Membered Ring CTIs. In an attempt to obtain a
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crystallizable complex, reactions were carried out between 2 as
its bromide salt and the platinum(II) bromo-dimers trans-
[Pt(PR3)Br(μ-Br)]2 (where R3 = Et3, PhMe2 or Ph2Me).21,23
Formation of a platinum(II) complex was only observed for
values is acceptable, with a line fitting error value of R = 0.982.
This is in spite of the probable free rotations of the PPh3 groups
in solution. This computed NMR procedure was applied to the
CTIs 1−10 here. Figure 7 shows the excellent agreement
the reaction between 2 and trans-[Pt(PEt3)Br(μ-Br)]2 ; the 31
P
NMR parameters for this cis-species are given in Table 7. The
data are entirely in keeping with those for the analogous chloro-
complex in Table 3. In the chloro-systems a cis-complex was
also formed for R3 = PhMe2, though not for R3 = Ph2Me,
possibly suggesting that a better donor phosphane as coligand
is necessary in the bromo-complexes. The same feature of a
normal 1JPP for a cis-PEt3 group but a comparatively low 1JPP for
the coordinated CTI is apparent as in the chloro-complexes.
While these species are the first transition metal derivatives
of CTIs, complexes of phosphenium ions with heavier
transition metals, including rhodium(I),39 platinum(0),40,41
and palladium(0)41,42 have been described. In these, the
phosphenium ion is usually stabilized by two adjacent nitrogen
atoms in a heterocyclic system. The platinum derivatives were
formed by reaction with Pt(PPh3)4. They have very short Pt−P
bond distances to the phosphenium ion in structurally
characterized compounds, regarded as double bonds, between
2.1073(9) and 2.116(3) Å, with correspondingly very high 1JPtP
values, ranging from 6446 to 7354 Hz (not in the same
sequence).40,41 The platinum(II) complexes in the present
work are much more similar in their NMR behavior to terminal
phosphido complexes with PR1R2 ligands, which have long Pt−
P distances in structurally characterized compounds, from
Figure 7. Comparison between computed and observed values for
CTIs 1−10.
between observed and computed 31P NMR shifts. The line
fitting error values of R = 0.999 were found for the 5- and 6-
membered ring CTIs whereas a slightly less accurate fit of R =
0.998 was estimated for the 7-membered ring CTIs. The
optimized geometries of 1−10 are therefore appropriate and
likely to be found experimentally.
1
2.351(2) to 2.4117(10) Å, and low JPtP values, from 643 to
1239 Hz.43 The small coupling constants and long Pt−P
distances in these complexes have been attributed to the
bonding phosphorus atom remaining pyramidal,43a,e and to a
low degree of s character in the Pt−P bond.43e The conclusion
that phosphido complexes form a better model for CTI
complexes than the phosphenium complexes mentioned above
is supported by theoretical calculations described below.
DFT Calculations. (i). CTIs and Their Protonated
Derivatives. As we have not yet succeeded in obtaining
suitable crystals of the platinum(II) complexes of CTIs 2−10
for X-ray structural determinations, density functional theory
(DFT) calculations were carried out on model systems to give
insight into their geometries. As far as we are aware, there are
no reported X-ray data on monocationic CTIs with alkyl
substituents on the PB atom. There are also no X-ray structures
of any protonated forms of CTIs with PA-H bonds.
Figure 8 shows three optimized model geometries of 43, 44,
and 45 to represent the 5- (2−4), 6- (5−7), and 7- (8−10)
Figure 8. Calculated geometries of model CTIs 43 (n = 2), 44 (n = 3),
and 45 (n = 4) derived from Me2P(CH2)nPMe2.
To estimate the accuracy of the optimized geometries, the
experimental and computed geometries of [(Ph3P)2P]+ and
[(Ph3P)2PH]2+ were compared, as crystal structures of both
compounds have been published.35,44,45 The calculated P−P
bond lengths of [(Ph3P)2P]+ and [(Ph3P)2PH]2+ are 2.186 and
2.273 Å, respectively, whereas the corresponding observed
values are 2.131 and 2.215 Å. While the calculated P−P bonds
are 0.055−0.058 Å longer than observed, the differences in the
bond lengths between the two cations in both calculated and
observed values are virtually the same.
One way to confirm good agreement between computed and
experimental geometries is by comparison of the observed and
calculated 31P NMR shifts. Experimentally, these are 30 and
−174 ppm for [(Ph3P)2P]+ and 23 and −120 ppm for
[(Ph3P)2PH]2+, whereas computationally they are 37.5 and
−103 ppm for [(Ph3P)2P]+ and 32.3 and −44.1 ppm for
[(Ph3P)2PH]2+, respectively.35,44 While the actual shift values
are different, the correlation between observed and computed
membered CTIs, respectively. It is shown here that the P−P−P
angles and P−P−P−C torsion angles widen as the ring size
increases, yet the averaged P−P bond lengths are similar. The
seven-membered ring CTI has two nonequivalent P−P bonds
and P−P−P−C torsion angles; thus, it is likely that the two
mirror image conformers are fluxional in solution, which may
explain the slightly lower R = 0.998 value for the 7-membered
ring CTIs. The methyl groups point toward the area of the PA
phosphorus atom as the ring size increases. Hence steric effects
increase, especially with more bulky groups than methyl, when
coordination to the PA atom is considered.
(ii). Pt(II) Complexes of CTIs. The geometries of the
platinum complexes were optimized with the model CTIs 43,
44 and 45 as the CTI ligand and trimethylphosphine, PMe3,
was used to model the phosphine ligand, PEt3, PMe2Ph or
PMePh2. Figures 9 and 10 depict the most stable geometries
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Figure 9. Optimized geometries of trans-Pt(PMe3)(CTI)Cl2 com-
plexes.
Figure 11. Comparison between computed and observed values for
platinum complexes of CTIs 2−10 with the PMe3 ligand in the
optimized geometries and the PEt3 ligand in experimental complexes.
6- membered ring CTIs (R = 0.998 and 0.992 respectively) are
considered as appropriate models, whereas the optimized
geometries for the trans- and cis-complexes with 7-membered
CTIs are likely to have the environment of the PC atom
incorrectly modeled (with R values between 0.572 and 0.965).
Despite the poor agreement for the PE and PC atoms in some
complexes, computed NMR shifts for all complexes show that
the predicted 31P shift for PC is always at a higher frequency
than for PE within each complex. This is in accord with
experimental 31P data where both isomers of the complex could
be observed.
The cis-isomer is considered as the thermodynamic product
whereas the trans-isomer is the kinetic product according to
NMR observations. Here, relative energies computed using the
solvation model (with a dielectric constant, ε, of 35.7) support
this deduction (Table 8).46 For complexes from the CTIs 1−9,
the energies required for cis to trans isomerization are between
4.1 and 10.0 kcalmol−1. The calculated isomerization energy at
0.0 kcal mol−1 for the complexes from the 7-membered CTI 10,
suggests that the steric effects of the 7-membered CTI 10 are
considerable compared to other CTIs in the cis-complexes
here.
The question of why the CTI 1 derived from dppe does not
form a Pt complex has been explored here by looking at the
relative energies of the CTI Pt complexes and [(Me3P)PtCl2]2
with different CTIs (Table 8). The reaction of [(Me3P)PtCl2]2
with CTI 1 to form the cis-complex is endothermic (5.6
kcalmol−1) whereas the reaction of [(Me3P)PtCl2]2 with CTI 2
to form the cis-complex is exothermic (−5.5 kcalmol−1). The Pt
complex with CTI 10 is endothermic at 13.8 kcalmol−1 because
of the considerable steric effects of the tert-butyl groups in the
cis-complex. The cis-complex from CTI 10 was not observed
experimentally. Examination of the optimized geometry of the
Pt complex with CTI 1 did not show any obvious steric effects.
It is likely that formation of the Pt complex would not be
favorable because of electronic effects, where the phenyl groups
reduce the electron density on the PA atom, making it a poor
donor, so that it does not coordinate to the platinum
experimentally.
Figure 10. Computed geometries of cis-Pt(PMe3)(CTI)Cl2 com-
plexes.
located for the trans- and cis-platinum complexes. The three
trans-complexes show different CTI orientations, reflecting the
steric influences of the different CTI ring sizes. These
orientations affect the P−P bond lengths from 2.206 for the
5-membered, 2.218 for the 6-membered, and 2.224 Å for the 7-
membered CTI ligand. The Pt−P bonds are, however,
consistent across the series with ranges of 2.338−2.342 Å
determined for Pt−P(phosphine, PMe3) bonds and 2.391−
2.431 Å for the Pt−P(CTI) bonds. The short Pt-phosphine and
1
long Pt−P(CTI) bonds are in accord with the large J(Pt−P)
values observed for the P(phosphine) peaks and small 1J(Pt−P)
coupling constants for the P(CTI) peaks.
For the cis-complexes with model CTIs 43, 44, and 45, the
orientation of the CTI ligand is inevitably identical across the
series, because of unfavorable steric interactions with the
neighboring phosphine, PMe3. The Pt-phosphine bond lengths
are 2.306−2.309 Å, which are some 0.03 Å shorter that those
found for the trans-complexes. This is reflected in the larger
Pt−P coupling constants seen for the phosphine peaks in the
cis-isomers compared to those couplings observed for the
phosphine peaks in the trans-complexes. However, the
geometries for the cis-complexes show comparatively short
Pt−P(CTI) bonds of 2.328−2.332 Å which, while still longer
than the Pt−P phosphine bonds, do not support the small
1J(Pt−P) coupling constants observed for the P(CTI) peaks,
assuming that these coupling constants are directly related to
Pt−P distances. The experimental results are entirely self-
consistent, however, suggesting that there may be other factors
involved which we are unable to model satisfactorily. The long
calculated P−P distances of 2.244 to 2.258 Å suggest that they
are single bonds.
Figure 11 shows the comparison between observed and
computed 31P NMR shifts for the trans- and cis-complexes of
the CTIs 2−10 with the phosphine, PMe3, used to model the
PR3 ligand in all cases. It is clear that while the shifts of the
phosphines (PE) and the PD atoms of the CTI ligands are in
broad agreement, the shifts for the PC atoms vary considerably.
The optimized geometries for the trans-complexes with 5- and
The relationship between the CTI, the heterocyclic N−P−N
phosphenium cation, and the Ph2P phosphide anion was
explored using models 43, 46, and 47 for these respective
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a
Table 8. Energies for the cis to trans Isomerizations of the Model Pt Complexes and for Formations of the Model Pt
Complexes
CTI
cis to trans isomerization of the Pt complex
formation of the cis-complex 0.5 [(Me3P)PtCl2]2 and CTI 1 to (Me3P)Pt(CTI)Cl2
1
2
4.4
5.5
7.2
6.9
7.3
9.2
5.2
10.0
4.1
0.0
5.6
−5.5
0.9
3
4
−2.7
−12.5
−0.3
−4.5
1.3
5
6
7
8
9
−2.8
13.8
10
a
In kcal mol−1.
ligands (Figure 12). Calculated natural charges for these models
show that the CTI ligand has the charge distribution shown in
dimer starting material or the decomposition products.
Nevertheless, the geometries of the complexes are supported
by DFT calculations. Complete removal of solvent was not
attempted, since it was clear from the NMR spectra (see
Figures 5 and 6 for typical examples) that although the CTI
complexes were the predominant species in solution, there
were minor amounts of impurities present. Hence the
likelihood of obtaining pure material for further character-
ization was remote. Since these complexes are necessarily ionic,
with very large cations, it seems probable that they will have
very low lattice energies, and will be soluble in nonpolar as well
as polar solvents, similar to salts of large tetra-alkyl ammonium
cations. This may explain why techniques such as layering the
reaction mixture with a nonpolar solvent such as hexane did not
lead to the formation of crystals of the CTI complexes.
Irrespective of CTI ring size or geometry of the complex, the
most remarkable feature of the solution-state NMR results is a
1
comparatively small JPtP to the central phosphorus atom (PC)
of the CTI, between 943 and 1267 Hz, whereas the phosphane
coligand which is an artifact of the preparative procedure has a
normal 1JPtP value (2200−3000 Hz for trans, 3200−4000 Hz for
cis), consistent with the geometry. In this respect the CTI
complexes behave similarly to terminal phosphido-complexes.43
The data strongly suggest that the CTIs coordinate
comparatively weakly and that the bond from platinum to PC
is a long one, possibly because of repulsion between two species
with overall positive charges, although the geometry at
phosphorus may also play an important role.
Figure 12. Comparison of (above) natural charges at P and N atoms
for model ligands 43, 46, and 47 and (below) of P−Pt bond lengths
and pyramidal angles at P for model complexes 48, 49, and 50.
Figure 2, and the charge on the PA atom is more like that on the
P atom in the phosphide 47 than in the phosphenium ion 46.
Optimized geometries for model platinum complexes
[Pt(dpme)Cl(L)]n+ 48, 49, and 50 with L as 43, 46, and 47
respectively (Figure 12) reveal long Pt−P bonds of 2.45−2.46
Å and pyramidal coordination at P for the ligands L in 48 and
50. By contrast, the Pt−P bond is shorter at 2.36 Å and a more
planar conformation is found at the P atom in the
phosphenium ligand. Thus the CTI as a ligand behaves
similarly to the Ph2P monoanion (terminal phosphide) in
complex 50 rather than to the phosphenium cation in complex
49.
EXPERIMENTAL SECTION
■
All manipulations, including NMR sample preparation, were carried
out either under an inert atmosphere of dry nitrogen or in vacuo, using
standard Schlenk line or glovebox techniques. Chemicals of the best
available commercial grade were used, in general without further
purification. The 31P NMR spectra of diposphanes were recorded, to
verify that no major impurities were present. 31P NMR spectra were
recorded on a Varian Mercury 300 or Varian VXR 400 Fourier-
transform spectrometer at 121.4 and 161.9 MHz, respectively.
Chemical shifts were measured relative to external H3PO4 (31P),
with the higher frequency direction taken as positive.
CONCLUSIONS
■
We have shown conclusively that CTIs of various ring sizes are
capable of coordinating to a platinum(II) moiety via the central
phosphorus atom, PA, provided that the substituents on the
outer phosphorus atoms (PB and/or PB′) are not all aromatic
groups. Even the CTI derived from dedppe formed complexes
in this way. Both trans- (the kinetic reaction product) and/or
cis- (the thermodynamic reaction product) complexes have
been identified in solution by 31P NMR spectroscopy; over 30
complexes have been thus produced. Unfortunately, despite our
best efforts to obtain one of the complexes in crystalline form,
any crystals isolated have been from either the platinum(II)
Example Reaction. Reaction of the CTI (2) with trans-
[Pt(PPhMe2)Cl(μ-Cl)]2. depe (0.0300 g, 0.14 mmol) and SnCl2
(0.0265 g, 0.14 mmol) were dissolved in 1.0 mL of CDCl3. PCl3
(0.014 mL, 0.14 mmol) was then added, and a 31P{1H} NMR
spectrum recorded soon after mixing to confirm the formation of the
cyclic triphosphenium ion. When necessary, residual PCl3 was
removed in vacuo. trans-[Pt(PPhMe2)Cl(μ-Cl)]2 dimer (0.0560 g,
0.07 mmol) was then added to the solution.
Quantities of reagents used in all reactions are shown in Table 9.
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Table 9. Quantities of Reagents Used
quantity of diphosphane
quantity of PCl3
quantity of SnCl2
quantity of Pt dimer
compound number
g
mmol
mL
mmol
g
mmol
g
mmol
11
0.0530
0.0300
0.2061
0.0193
0.0310
0.4228
0.0642
0.0230
0.0140
0.0210
0.0808
0.0239
0.0260
0.0446
0.0397
0.0136
0.1411
0.0900
0.0733
0.0569
0.0305
0.0280
0.0235
0.1550
0.0386
0.26
0.14
0.49
0.05
0.07
1.40
0.19
0.10
0.06
0.10
0.18
0.05
0.06
0.10
0.14
0.05
0.31
0.20
0.16
0.13
0.11
0.10
0.08
0.40
0.18
0.020
0.014
0.030
0.004
0.010
0.14
0.23
0.16
0.34
0.05
0.11
1.60
0.17
0.11
0.06
0.11
0.11
0.06
0.06
0.11
0.11
0.05
0.23
0.20
0.16
0.11
0.11
0.11
0.08
0.44
0.18
0.0474
0.0265
0.25
0.14
0.0960
0.0560
0.1409
0.0202
0.0342
a
0.13
0.07
0.18
0.03
0.04
a
12
13/14
15
0.0095
0.0139
0.2654
0.05
0.07
1.40
16/17
18
19/20
21
0.015
0.010
0.005
0.010
0.010
0.005
0.005
0.010
0.010
0.004
0.020
0.018
0.014
0.010
0.010
0.010
0.007
0.040
0.016
0.0456
0.0398
0.0242
0.0242
0.0547
0.0192
0.0242
0.0466
0.0513
0.0191
0.1071
0.1532
0.0647
0.0498
0.0466
0.0583
0.0323
a
0.05
0.05
0.03
0.03
0.07
0.03
0.03
0.05
0.06
0.03
0.14
0.20
0.08
0.05
0.05
0.08
0.04
a
0.0197
0.0114
0.0190
0.10
0.06
0.10
22
23
24/25
26
0.0157
0.0114
0.08
0.06
27
28/29
30
31/32
33
0.0100
0.05
34
0.0400
0.0303
0.20
0.16
35/36
37
38
39
0.0291
0.0151
0.0810
0.15
0.08
0.43
40
41
42
0.0850
0.09
a
Dimer was not added quantitatively.
Chemistry, University of Durham, for a gift of depdppe; and
Johnson Matthey for the loan of Pt(II) salts.
COMPUTATIONAL SECTION
■
All computations were carried out with the Gaussian 09 package.47
Geometry optimizations for all compounds discussed here were
carried out with the DFT-hybrid functional B3LYP48 using the 6-31G*
basis set49 for all atoms except for Pt where the pseudopotential
LANL2DZ50 was used. No symmetry constraints were used in all
cases. Calculated 31P NMR chemical shifts at the GIAO51-B3LYP/6-
31G* level were obtained from the optimized geometries using the
δ(31P) = 350.0 − σ(31P) scale.
ABBREVIATIONS
■
CTI, cyclic triphosphenium ion; dcypb, 1,4-bis-
(dicyclohexylphosphino)butane; dcype, 1,2-bis-
(dicyclohexylphosphino)ethane; dcypp, 1,3-bis-
(dicyclohexylphosphino)propane; dedppe, (1-diethylphosphi-
n o - 2 - d i p h e n y l p h o s p h i n o ) e t h a n e ; d e p e , 1 , 2 -
bis(diethylphosphino)ethane; depp, 1,3-bis(diethylphosphino)-
propane; dippb, 1,4-bis(di-iisopropylphosphino)butane; dippp,
1,3-bis(di-isopropylphosphino)propane; dppe, 1,2-bis-
(diphenylphosphino)ethane; dtbpox, bis(α,α-di-t-butylphosphi-
no)-o-xylene
ASSOCIATED CONTENT
* Supporting Information
Cartestian coordinates of optimized geometries of CTIs 1−10
and their (Me3P)Pt(CTI)Cl2 complexes. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
REFERENCES
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: k.b.dillon@durham.ac.uk.
■
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