Carbene or zwitterion? Competition in organoplatinum complexes

Article abstract of DOI:10.1039/b517874f

The reaction of a known dimeric dicarbene complex of platinum with a number of ligands results in four new platinum complexes. The structure of the new complexes is described: one complex must exist as a neutral complex with no charge separation, and the other three are assigned a charge-separated (zwitterionic) structure, rather than a carbene form, on the basis of comparative ¹³C NMR shifts. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b517874f

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Carbene or zwitterion? Competition in organoplatinum complexes
Christopher P. Newman, Guy J. Clarkson, Nathaniel W. Alcock and Jonathan P. Rourke*
Received 20th December 2005, Accepted 6th April 2006
First published as an Advance Article on the web 13th April 2006
DOI: 10.1039/b517874f
The reaction of a known dimeric dicarbene complex of platinum with a number of ligands results in
four new platinum complexes. The structure of the new complexes is described: one complex must exist
as a neutral complex with no charge separation, and the other three are assigned a charge-separated
(zwitterionic) structure, rather than a carbene form, on the basis of comparative 13C NMR shifts.
˚
Introduction
the product on the basis of a short Pt–C bond length (1.9519(9) A)
and the 13C chemical shift of the carbon attached to the platinum
(324.3 ppm). Though we did not speculate on the reasons for the
stability of the carbene form of the complex, it is immediately
apparent that the additional hydrogen on the nitrogen, and the
hydrogen bonding to this hydrogen must play a major role. In this
paper we look at the effect of other ligands on the stability of the
carbene complex.
Numerous zwitterionic organometallic species have been reported
in the literature.1 Often, though, there is only a fine distinction
between the formally different resonance structures. One example
of a complex that does not exist in the zwitterionic form is
the platinum carbene complex that we reported earlier.2 In
this paper, we reported the synthesis of this platinum carbene
from the reaction of a 2,6-disubstituted pyridine with potassium
tetrachloroplatinate (Scheme 1). The product (1) formed in
preference to the cyclometallated compound that might have
been expected,3,4 and exhibits remarkable stability, melting in
air without decomposition at 246 ◦C. Two possible extremes
are possible for the compound: a carbene structure (1) and a
zwitterionic structure (2). We confirmed the carbene like nature of
Results and discussion
Reaction of platinum carbene (1) with two equivalents of triph-
enylphosphine per platinum results in the splitting of the dichloride
bridge to give a monoplatinum species (3), whereas reaction with
one equivalent of ligand per platinum results in the formation
of a different monoplatinum species (4), Scheme 2. In practice,
however, we saw mixtures of both compounds regardless of how
much phosphine we used.
Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL.
E-mail: j.rourke@warwick.ac.uk; Fax: 44 (0)24 7652 4112; Tel: 44 (0)24
7652 3263
Scheme 1
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Scheme 2
We consider the reaction with 2 equivalents of phosphine first.
Though we do not have an X-ray crystal structure of compound (3)
all spectroscopic data are consistent with the formulation shown.
The relative sizes of the integrals in the 1H NMR spectrum indicate
two triphenylphosphines present for each diphenylpyridine unit,
and the coupling pattern (in particular the singlet for the protons
on the pyridine ring) indicate that the substitution pattern on the
pyridine ring has not changed. A single peak in the 31P NMR
spectrum indicates a single phosphorous environment (as would
be expected with two triphenylphosphine groups mutually trans)
and the size of the 1J Pt–P coupling (2870 Hz) suggests P trans to
P. Compound (3) does not require the pyridine to be protonated if
it is neutral and there is no low field signal in the 1H NMR spectrum
that might have come from such a proton. Thus we are confident
that the ring attached to the platinum is best represented as a
normal pyridine ring. An analysis of the 13C NMR spectrum of (3)
shows that the carbon directly attached to the platinum resonates
at 176.1 ppm. We are able to clearly assign this signal on the basis
of it having no protons directly attached and it being a triplet
due to coupling to two identical phosphorus atoms (2JPC = 7 Hz);
coupling to platinum was not seen (either because of the weakness
of the sample, or because relaxation due to CSA broadens the
satellites). The chemical shift of this carbon is a bit higher than
might be expected, but it is not in the region that would indicate
a carbene (200+ ppm), and certainly nowhere near the 324 ppm
observed for the original complex (1).
Fig. 1 The ORTEP of the structure of ◦(4) (ellipsoids at 30% probability).
˚
Selected bond lengths (A) and angles ( ): Pt(1)–C(119) 1.989(16), Pt(1)–
P(10) 2.219(5), Pt(1)–Cl(1) 2.374(4), Pt(1)–Cl(2) 2.396(4); C(119)–
Pt(1)–P(10) 93.3(4), C(119)–Pt(1)–Cl(1) 85.7(4), P(10)–Pt(1)–Cl(1)
178.0(2), C(119)–Pt(1)–Cl(2) 176.7(4), P(10)–Pt(1)–Cl(2) 89.63(16), Cl(1)–
Pt(1)–Cl(2) 91.36(15).
on the pyridine nitrogen. The Pt–C distance is longer than that
observed in the carbene (1) (though this difference should be
treated with caution as it exists on the limit of experimental error),
but is not in itself diagnostic of either a Pt–C single or double
bond: platinum–carbon single bonds can be found in the literature
When carbene (1) reacts with a single equivalent (per platinum)
of triphenylphosphine, complex (4) results. We do have an X-ray
crystal structure of (4) (Fig. 1), and whilst there was some disorder
in the alkoxy chains leading to a high R value, it does clearly
indicate a cis arrangement of the two chlorides.
6,7
˚
ranging between ∼2.4 and 1.9 A. Spectroscopic data on (4) are
1
revealing: a broad low-field resonance in the H NMR spectrum
Additional features of note include a Pt–C distance of
(12.3 ppm) confirms the presence of a hydrogen on the nitrogen,
which is required if the complex is neutral overall, and a singlet
˚
˚
1.989(16) A and N–O distances of 2.633 and 2.596 A. The N–
1
O distances are less than the sum of the van der Waals radii of
in the 31P NMR spectrum with a J Pt–P coupling (4463 Hz)
5
˚
nitrogen and oxygen (3.07 A) and similar to the N–O distances of
confirming P trans to chloride.
In a similar manner to (1), two resonance structures for (4) are
possible, the zwitterionic (4) and the carbene (5), Scheme 3. Whilst
˚
2.622(9) and 2.720(8) A observed in the starting carbene complex
(1), indicating a hydrogen bonding interaction through a proton
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the p-bond in the Fischer type carbene requires back donation
to the carbon from the platinum. This would suggest that the
carbene structure would only be observed with electron donating
co-ligands on the platinum. This is what is observed with chlorides
(good r- and p-donor ligands). When a chloride is replaced by
the poorer electron donor triphenylphosphine as in compound
(4), or by dmso as in (6), the carbene structure is not observed.
Thus we would expect that the even poorer electron donor, CO,
would be even less likely to exhibit a carbene like structure than
either the triphenylphosphine or the dmso complexes. We thus
assign a zwitterionic structure to (7). It is salient to note that the
zwitterionic structure maintains the aromaticity of the pyridine
ring, which is not present in the carbene form.
There is one example in the literature of a ruthenium complex
that exists in two forms: a metallaquinone (8) or a zwitterionic
compound (9), Scheme 4.10 Both forms of this compound are
seen, with solvent polarity controlling the position of equilibrium.
Thus, in non polar solvents such as benzene the quinoidal form
is adopted, the solution is orange and the carbon bonded to
ruthenium resonates at 303 ppm. In the more polar methanol
the solution becomes yellow and the carbon bonded to ruthenium
resonates at 155 ppm. This situation has analogies to ours and
provides further support for our use of 13C chemical shifts to assign
a zwitterionic structure rather than a carbene. Certainly these
results cast doubt on the assignment of a carbene structure (11)
rather than a zwitterionic structure (10) for an organopalladium
complex in a recent paper.11 The assignment in this paper was
made on the basis of a 13C chemical shift of 176.2 ppm for the
carbon directly bonded to palladium, Scheme 5.
Scheme 3
the Pt–C distance is of little use in distinguishing these two forms,
the 13C NMR is: the carbon bonded to the platinum resonates at
177.0 ppm. Once again, we are able to clearly identify this signal
on the basis of it having no directly attached protons and it being
a doublet due to coupling to phosphorus (2JPC = 6.5 Hz); again,
coupling to platinum was not seen. The chemical shift value of
177.0 is very similar to that exhibited by complex (3) (171.6 ppm)
which has no possibility of exhibiting a carbenoid form. This leads
us to conclude that complex (4) exists as the zwitterionic form in
solution, not the carbene form.
Two further ligands were used to split the chloride bridged
dimeric core present in (1): carbon monoxide and dmso. Both
were used in excess, with the carbon monoxide being bubbled
through a solution of (1), and (1) being dissolved in dmso, however
spectroscopic and elemental analysis data indicates that in both
cases only one new ligand was coordinated to the platinum.
Scheme 4
The integrals in 1H NMR spectrum of the dmso complex
(6) show the presence of one coordinated DMSO group per
diphenylpyridine unit, and that the pyridine nitrogen is protonated
(a broad resonance at 12.95 ppm). A 3JHPt coupling of 26 Hz gives
strong evidence that, similar to triphenylphosphine complex (4),
the DMSO coordinates trans to chloride.8,9 The signal for carbon
directly bonded to platinum was found at 165.6 ppm in the 13C
NMR spectrum, confirming a zwitterionic structure.
The 1H NMR spectrum of the carbonyl compound (7) is broadly
similar to the mono-triphenylphosphine complex (4) (in terms of
the diphenylpyridine signals), and shows that the pyridine nitrogen
is protonated with a broad resonance at d 10.9 ppm. The carbon
spectrum was too weak to see either the metallated carbon or
the carbonyl carbon, however the infrared spectrum shows mCO
2078 cm−1, relatively high, and consistent with the CO being trans
to Cl and not trans to C.
On the basis of the spectroscopic evidence we were able to
acquire, we are confident of the assignment of cis geometries to
both (6) and (7). The question arises as to the nature of the Pt–
C bond in carbonyl complex (7): is it the carbene like link seen
in (1) or is it a single Pt–C like that in (4)? If we consider the
Scheme 5
Conclusions
The unequivocal assignment of a non-zwitterionic pyridine struc-
ture to triphenylphosphine complex (3) allows us to determine the
13C NMR shift of a carbon directly bonded to a platinum. We have
then been able to use this value to assign a zwitterionic structure
(5), rather than a carbene structure, to another triphenylphosphine
derivative. The assignment of the zwitterionic structure to (5)
allows us to argue that further derivatives also show a zwitterionic
structure, as the ligands present in these new examples (dmso and
CO) are less likely than triphenylphosphine to stabilise a carbene
structure.
=
nature of the Pt C bond in (1), it is apparent that creation of
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4H, Hl); 1.3 (m, 4H, Hm); 1.25 (m, 8H, Hn, o); 1.1 (m, 8H, Hl*, o*);
1.0 (m, 4H, Hm*); 0.9 (m, 4H, Hn*); 0.85 (m, 6H, Hp); 0.75 (m, 6H,
Hp*). dC (CDCl3): 176.1 (t, 2JPC = 7 Hz, Ca); 164.2 (Ci); 152.7 (Cg);
148.2 (Cc); 134.9 (Cx); 131.6 (Cb); 131.1 (Ce); 129.8 (Cv); 128.9
(Cz); 127.6 (Cy); 110.2 (Cd); 104.3 (Cf); 97.8 (Ch); 67.4 (Cj); 66.5
(Cj*); 30.1 (Cn, n*); 28.0 (Ck, k*, l, l*); 24.4 (Cm); 21.0 (Cm*, o, o*); 12.9
(Cp, p*). dP (CDCl3): 23.6 (1JPPt 2870 Hz). MS (LSIMS): m/z 1180
(M+ − PPh3). Microanalysis (%): Found (expected): C 66.9 (67.5);
H 6.6 (6.9); N 0.7 (1.0).
Experimental
All chemicals were used as supplied, unless noted otherwise. All
NMR spectra were obtained on a Bruker Avance 400 or 500 in
CDCl3 and are referenced to external TMS, assignments being
made with the use of decoupling, NOE and the DEPT and
COSY pulse sequences. The labelling scheme used in the NMR
assignments of all complexes is shown in Fig. 2. All elemental
analyses were performed by Warwick Analytical Services. Crystal
structures were collected on a Siemens Smart 1 K. Platinum
carbene complex (1) was synthesised as we previously reported.2
Cis-Dichloro(2,6-bis(2,4-
diheptyloxyphenyl)pyridine)(dmso)platinum (6)
A sample of platinum carbene (1) (40 mg, 2.08 × 10−5 mol)
was dissolved in DMSO (3 ml) and heated at 50 ◦C for 15 h.
Upon cooling, the mixture was flooded with water and extracted
with dichloromethane. The organic washings were washed with
water and dried over MgSO4. Concentration of the solution
followed by addition of diethyl ether resulted in precipitation of
the cis-dichloro(2,6-bis(2,4-diheptyloxyphenyl)pyridine)(dmso)-
platinum product which was collected by filtration. Yield: 34 mg,
78% (based on Pt).
Fig. 2 The labelling scheme used in the NMR assignments of all
complexes.
dH (CDCl3): 12.9 (br, 1H, Hq); 8.1 (s, 2H, 3JHPt 43 Hz, Hb); 7.7
3
3
4
Triphenyl phosphine complexes (3) and (4)
(d, 2H, JHH 7 Hz, He); 6.6 (dd, 2H, JHH 7 Hz, JHH 2 Hz, Hf);
4
3
6.5 (d, 2H, JHH 2 Hz, Hh); 4.0 (m, 8H, Hj, j*); 3.55 (s, 6H, JHPt
26 Hz, dmso); 1.7 (m, 4H, Hk); 1.6 (m, 4H, Hk*); 1.4 (m, 4H, Hl);
1.3 (m, 12H, Hm,n,o); 1.1 (m, 4H, Hl*); 1.05 (m, 4H, Ho*); 1.0 (m,
4H, Hn*); 0.9 (m, 4H, Hm*); 0.85 (m, 6H, Hp); 0.7 (m, 6H, Hp*). dC
(CDCl3): 165.6 (Ca); 161.2 (Ci); 155.6 (Cg); 143.8 (Cd); 132.4 (Ce);
113.1 (Cc); 111.6 (Cb); 105.9 (Cf); 99.2 (Ch); 69.4 (Cj); 67.8 (Cj*);
46.8 (Cr); 31.3 (Cm/m*–o/o*); 30.1 (Ck, k*); 26.5 (Cl); 23.0 (Cl*); 14.6
(Cp, p*). MS (LSIMS): m/z 1032 (M+). Microanalysis (%): found
(expected) C 53.9 (54.7); H 6.9 (7.3); N 1.1 (1.4).
Triphenylphosphine (18 mg, 6.71 × 10−5 mol) was added to a
solution of platinum carbene (1) (64 mg, 3.33 × 10−5 mol) in
◦
CHCl3. The mixture was heated at 80 C for approximately 4 h.
After cooling the solution was concentrated in vacuo and excess
hexane added, precipitating a dark yellow solid. 31P NMR showed
the precipitate to contain three different phosphorus containing
compounds. The mixture was eluted on a silica column (5%
methanol–95% chloroform). The first fraction contained an un-
identified impurity. Fraction 2 contained cis-dichloro(2,6-bis(2,4-
diheptyloxyphenyl)pyridine)(triphenylphosphine)platinum(II),
complex (4); yield: 26 mg, 32% (based on Pt). Fraction 3 contained
trans-chloro(2,6-bis(2,4-diheptyloxyphenyl)pyridine)bis(triphen-
ylphosphine)platinum(II), complex (3); yield: 25 mg, 27% (based
on Pt).
cis-Dichloro(2,6-bis(2,4-
diheptyloxyphenyl)pyridine)(carbonyl)platinum (7)
A sample of platinum carbene (1) (40 mg, 2.08 × 10−5 mol) was
dissolved in chloroform and gaseous carbon monoxide bubbled
through for 90 min. The solvent was removed and the remaining
residue was run down a column of silica with CHCl3 as the
eluent. A brownish band contained the cis-dichloro(2,6-bis(2,4-
diheptyloxyphenyl)pyridine)(carbonyl)platinum product. Yield:
24 mg, 59% (based on Pt).
dH (CDCl3): 10.85 (br, 1H, Hq); 7.5 (d, 2H, 3JHH 9 Hz, He); 6.55
(s, 2H, Hb); 6.5 (d, 2H, 3JHH 9 Hz 4JHH 2 Hz, Hf); 6.4 (d, 2H, 4JHH
2 Hz, Hh); 3.9 (t, 8H, 3JHH 6 Hz, Hj, j*); 1.7 (m, 4H, Hk); 1.55 (m,
4H, Hk*); 1.4 (m, 4H, Hl); 1.25 (m, 12H, Hm, n, o); 1.15 (m, 4H,
Hl*); 1.05 (m, 4H, Ho*); 1.0 (m, 4H, Hn*); 0.9 (m, 4H, Hm*); 0.85
(m, 6H, Hp); 0.75 (m, 6H, Hp*). dC (CDCl3): 162.0 (Ci); 157.3 (Cg);
146.4 (Cd); 131.1 (Ce); 114.3 (Cc); 114.2 (Cb); 106.6 (Cf); 100.5 (Ch);
69.7 (Cj); 68.210 (Cj*); 32.8 (Cm/m*-o/o*); 29.3 (Ck, k*); 26.1 (Cl); 22.7
(Cl*); 14.2 (Cp, p*). MS (LSIMS): m/z 983 (M+). Microanalysis (%):
found (expected): C 55.7 (56.3); H 6.9 (7.1); N 1.3 (1.4). IR (CHCl3
solution): 2078 cm−1 (CO stretch).
cis-Dichloro(2,6-bis(2,4-diheptyloxyphenyl)pyridine)(triphenyl-
phosphine)platinum(II) (4). dH (CDCl3): 12.3 (br t, 1H, Hq); 7.7
(d, 2H, 4JHH 2 Hz, Hb); 7.65 (m, 6H, PPh3); 7.3 (d, 2H, 3JHH 9 Hz,
3
4
He); 7.15 (m, 9H, PPh3); 6.5 (dd, 2H, JHH 9 Hz, JHH 2 Hz, Hf);
4
3
6.45 (d, 2H, JHH 2 Hz, Hh); 3.9 (t, 8H, JHH 7 Hz, Hj, j*); 1.72
(m, 4H, Hk); 1.52 (m, 4H, Hk*); 1.4 (m, 4H, Hl); 1.3 (m, 4H, Hm);
1.25 (m, 8H, Hn,o); 1.1 (m, 4H, Hl*); 1.05 (m, 4H, Ho*); 0.95 (m,
4H, Hm*); 0.85 (m, 6H, Hp); 0.8 (m, 4H, Hn*); 0.7 (m, 6H, Hp*).
2
dC (CDCl3): 177.0 (d, JPC = 6.5 Hz, Ca); 161.8 (Ci); 156.3 (Cg);
142.3 (Cc); 133.8 (Cx); 132.0 (Cb); 130.5 (Ce); 129.7 (Cv); 129.2 (Cz);
126.8 (Cy); 110.5 (Cd); 105.6 (Cf); 99.3 (Ch); 68.3 (Cj); 67.4 (Cj*);
30.5 (Cn, n*); 28.1 (Ck,k*,l,l*); 24.8 (Cm); 21.6 (Cm*,o,o*); 13.1 (Cp,p*).
dP (CDCl3): 14.4 (1J(PPt) 4463 Hz). MS (LSIMS): m/z 1216 (M+).
Microanalysis (%): Found (expected): C 61.6 (62.2); H 6.2 (7.0);
N 0.8 (1.2).
trans-Chloro(2,6-bis(2,4-diheptyloxyphenyl)pyridine)bis(triphen-
ylphosphine)platinum(II) (3). dH (CDCl3): 7.6 (m, 12H, PPh3);
7.3 (s, 2H, Hb); 7.25 (m, 18H, PPh3); 6.9 (d, 2H 3JHH 9 Hz, He); 6.5
Crystal structure data for compound (5)
3
4
(dd, 2H, JHH 9 Hz, 4JHH 2 Hz, Hf); 6.45 (d, 2H, JHH 2 Hz, Hh);
Colourless plates 0.16 × 0.12 × 0.04 mm, C63H84Cl2NO4PPt, M =
¯
3.95 (m, 8H, Hj, j*); 1.7 (m, 4H, Hk); 1.5 (m, 4H, Hk*); 1.35 (m,
1216.27, triclinic, space group P1; a = 11.9888(3), b = 13.3067(4),
3324 | Dalton Trans., 2006, 3321–3325
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◦
◦
˚
I > 2r(I)] = 0.0898, wR2 = 0.2044. Data/restraints/parameters
c = 21.2775(6) A, a = 75.4120(10) , b = 85.0940(10) , c =
◦
−3
3
˚
10273/22/626. Largest difference Fourier peak and hole 1.178
66.880(2) ; Dc = 1.337 Mg m , U = 3020.82(15) A (by least-
squares refinement on 2891 reflection positions), T = 180(2) K,
−3
˚
and −1.137 e A . The largest peaks are all close to the Pt atom.
−1
˚
The relatively high R-value is not unexpected in view of the high
thermal motion of the chains and the partial disorder.
CCDC reference number 294673.
k = 0.71073 A, Z = 2, F(000) = 1256; l(Mo-Ka) = 2.482 mm .
Maximum h was 29.46◦. The hkl ranges were −7/14, −13/15,
−25/25. 15439 reflections measured, 10273 unique (Rint = 0.1747).
Absorption correction semi-empirical from equivalents; minimum
and maximum transmission factors: 0.5998, 0.90723.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b517874f
¯
No systematic absences; space group P1 was chosen on the
basis of intensity statistics and shown to be correct by successful
refinement. The structure was solved in P1 by direct methods using
Acknowledgements
¯
SHELXS12 and then the inversion centre located; additional light
atoms were found by Fourier methods. One chain was found to
be disordered, some of the partly-occupied positions coinciding
with those in the alternative position related by symmetry. Rather
than model these as overlapped atoms, both chains were included
completely at partial occupancy. The occupancy was refined
but converged to about 0.5, as is required for such alternating
positions; it was then held at 0.5. The possibility of a super-
lattice with ordered molecules was considered but no evidence
for this was seen. Refinement was by full-matrix least squares
on F2 for 2851 reflection positions using SHELXTL. Hydrogen
atoms were added at calculated positions, including all disordered
positions, and refined using a riding model with freely rotating
methyl groups. Anisotropic displacement parameters were used
for all non-H atoms apart from the disordered chain and some
terminal chain atoms with high displacement parameters; H-
atoms were given isotropic displacement parameters equal to
1.2 (or 1.5 for methyl hydrogen atoms) times the equivalent
isotropic displacement parameter of the atom to which the H-
atom is attached. The weighting scheme was calculated using
The authors would like to thank Johnson-Matthey for loan of
chemicals.
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2
Goodness of fit on F2 = 0.878; R1 [for 3225 reflections with
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The Royal Society of Chemistry 2006
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