Heteroarylphosphorus ligands in platinum(II) complexes. The structure of trans-[Pt(P{2-(N-methylpyrrolyl)}(i)Pr2)2Cl2]

Article abstract of DOI:10.1016/S0020-1693(00)00107-9

Displacement of cod from [Pt(cod)Cl₂] by 2 equiv. of heteroarylphosphorus ligand, PR₂R', in dichloromethane at ambient temperature afforded - but for one exception trans-[Pt(P{2-(N-methylpyrrolyl)}(i)Pr₂)₂Cl₂] (1) - the expected cis complexes, cis-[Pt(P{2-(N-methylpyrrolyl)}Ph₂)₂Cl₂] (2), cis-[Pt(P{2-(5-methylthienyl)}R₂)₂Cl₂] (R = Ph 3, (i)Pr 4) and cis-[Pt(P{2-(5-bromopyridinyl)}(i)Pr₂)₂Cl₂] (5). The complexes were fully characterised and structural assignments were based on ¹J(¹⁹⁵Pt-³¹P) coupling constants from the NMR data. The structure of 1 was determined by X-ray crystallography, which revealed that the two pyrrolyl substituents on the phosphine ligands were oriented on opposite sides of the P-Pt-P axis. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00107-9

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Inorganica Chimica Acta 305 (2000) 32–37
Heteroarylphosphorus ligands in platinum(II) complexes. The
structure of trans-[Pt(P{2-(N-methylpyrrolyl)}iPr2)2Cl2]
a
Janine Chantson , Helmar Go¨rls b, Simon Lotz a,*
a Department of Chemistry, Uni6ersity of Pretoria, Pretoria 0001, South Africa
b Department of Inorganic and Analytical Chemistry, Uni6ersity of Jena, Lessingstraße 8, D-07743 Jena, Germany
Received 19 November 1999; accepted 28 February 2000
Abstract
Displacement of cod from [Pt(cod)Cl2] by 2 equiv. of heteroarylphosphorus ligand, PR2R%, in dichloromethane at ambient
temperature afforded — but for one exception trans-[Pt(P{2-(N-methylpyrrolyl)}iPr2)2Cl2] (1) — the expected cis complexes,
i
cis-[Pt(P{2-(N-methylpyrrolyl)}Ph2)2Cl2] (2), cis-[Pt(P{2-(5-methylthienyl)}R2)2Cl2] (R =Ph 3, Pr 4) and cis-[Pt(P{2-(5-bromopyr-
idinyl)}iPr2)2Cl2] (5). The complexes were fully characterised and structural assignments were based on 1J (195Ptꢀ31P) coupling
constants from the NMR data. The structure of 1 was determined by X-ray crystallography, which revealed that the two pyrrolyl
substituents on the phosphine ligands were oriented on opposite sides of the PꢀPtꢀP axis. © 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Crystal structures; Platinum complexes; Phosphine complexes; Aminoarene complexes
forded cis products, cis-[Pt(PR2R%)2Cl2], which did not
spontaneously convert into the trans isomers. The
mechanisms of cis–trans isomerisations have been the
subject of rigorous investigations and any one of the
precursor, the solvent, phosphine substituents or auxil-
iary ligands in the complex, may determine the struc-
ture of the final product [4]. For complexes of the type
[PtL2X2], the cis isomers are generally enthalpy-driven,
while the trans isomers are entropy-favoured. The posi-
tion of 2-(N-alkylpyrrolyl) in the above sequence as
well as the omission of phosphine ligands displaying
this substituent in the literature in complexes of the
type [Pt(PR2R%)2Cl2], is noteworthy. Furthermore, we
1. Introduction
The development of new ligands is becoming increas-
ingly important to fine tune metal activity in coordina-
tion chemistry [1]. While phosphine ligands with
heteroaryl substituents, such as 2-pyridinylphosphines,
are widely exploited in binuclear complex formation as
bridging ligands, the 2-thienyl, 2-furyl and 2-(N-
methylpyrrolyl) substituents are less inclined to coordi-
nate in a similar fashion through their respective
heteroatoms [2]. In general, 2-thienyl and 2-furyl
groups behave as moderately strong electron-withdraw-
ing units and a comparative study of the magnitude of
31
1J (77Seꢀ P) coupling constants for heteroarylphosphine
observed
in
structural studies that
2,5-(N-
methylpyrrolydene), in sharp contrast to 2,5-thienylene,
displayed a passiveness to stabilise the carbocation
centres in biscarbene complexes [5,6]. Hence, it was of
some interest to us to re-investigate and compare the
structures of [Pt(P{monoheteroaryl}R2)2Cl2] (R =di-
alkyl and diaryl) complexes. In order to minimize vari-
ables all reactions were studied by utilising the cis
selenides established an electron-withdrawing effect
which follows an ordering of 2-furyl\2-thienyl\
phenyl\2-(N-methylpyrrolyl) [3]. Platinum(II) com-
plexes
prepared
in
polar
solvents
from
tetrachloroplatinate(II) and bis(phosphine) ligands con-
taining 2-furyl or 2-thienyl substituents invariably af-
4
directing precursor, dichloro-h -1,5-cyclooctadieneplat-
inum(II), as well as the same reaction conditions and
* Corresponding author. Tel.: +27-12-420 2800; fax: +27-12-362
5297.
E-mail address: slotz@nsnper1.up.ac.za (S. Lotz)
solvent. The cis isomers were isolated as expected,
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00107-9

J. Chantson et al. / Inorganica Chimica Acta 305 (2000) 32–37
33
except for the case of trans-[Pt(P{2-(N-methyl-
2.3. Preparation of the phosphine ligands
pyrrolyl)}iPr2)2Cl2], the structure of which was also
confirmed by a crystal structure determination.
2.3.1. Preparation of
diisopropyl-2-(5-methylthienyl)phosphine
A solution of n-BuLi (14.2 cm3 of 1.6 M in hexane,
22.7 mmol) was added dropwise to a solution of 2-
methylthiophene (2.00 cm3, 20.7 mmol) in THF (10
cm3) at room temperature (r.t.). No external cooling
was applied and the resultant mixture was stirred for 1
h. The reaction mixture was then cooled to 0°C over an
ice-bath before PiPr2Cl (3.30 cm3, 20.7 mmol) was
introduced. After allowing the reaction to proceed for 1
additional h at r.t., the solvents were removed under
reduced pressure. Et2O was added, the mixture filtered
through silica gel (layered with anhydrous Na2SO4) and
the filtrate vacuum-evaporated to dryness. The orange
coloured oil obtained was distilled under reduced pres-
2. Experimental
2.1. General
All manipulations were carried out using standard
Schlenk techniques under an atmosphere of dry nitro-
gen. Solvents were distilled under nitrogen from appro-
priate drying agents before use. The [Pt(cod)Cl2]
precursor was prepared from K2[PtCl4] according to a
literature method [7]. TMEDA and N-methylpyrrole
were distilled prior to use. All other reagents and
chemicals were obtained commercially and used as
1
sure to give a colourless liquid. Yield 3.4 g (76%). H
1
received. The H, 31P{1H} and 13C{1H} NMR spectra
3
NMR l (CDCl3): 7.07 (SC4H2, 1H, dd, JHH =3.2,
were recorded on a Bruker ARX300 spectrometer oper-
ating at 300.133, 121.496 and 75.469 MHz for the
respective nuclei. Chemical shifts in 1H and 13C{1H}
NMR spectra were calibrated with reference to residual
proton signals of the deuterated solvent (7.24 and 77.0
ppm, respectively for CDCl3). Chemical shifts of
31P{1H} spectra were relative to an external standard.
Mass spectra (electron impact) were recorded on a
Finnigan Mat 8200 instrument operating at 70 eV.
3JPH =6.6 Hz), 6.71 (SC4H2, 1H, m), 2.47 (SC4H2ꢀCH3,
3
3H, s), 2.00 (P(CH Me2)2, 2H, heptet of d, JHH =7.0,
2JPH =1.1 Hz), 1.09–0.96 (P(CHMe2)2, 12H, m) ppm.
31P{1H} l (CDCl3): 1.2 ppm.
2.3.2. Preparation of
2-(5-methylthienyl)diphenylphosphine
A procedure similar to that described above was
followed, using PPh2Cl (3.80 cm3, 20.6 mmol) instead
1
of PiPr2Cl. Yield 4.9 g (84%). H NMR l (CDCl3): 7.35
3
3
2.2. Crystal structure determination
(Ph, 10H, m), 7.13 (SC4H2, 1H, dd, JHH =3.4, JPH =
6.6 Hz), 6.75 (SC4H2, 1H, m), 2.45 (CH3, 3H, s) ppm.
The intensity data for 1 was collected on a Nonius
Kappa CCD diffractometer, using graphite-monochro-
mated Mo Ka radiation. Data were corrected for
Lorentz, polarisation and absorption effects [8]. The
structure was solved by direct methods and refined by
31P{1H} l (CDCl3): −18.7 ppm.
2.3.3. Preparation of
diisopropyl-2-(N-methylpyrrolyl)phosphine
N-Methylpyrrole (2.50 cm3, 28.2 mmol) was reacted
with n-BuLi (16.0 cm3 of a 1.6 M hexane solution, 25.6
mmol) in the presence of TMEDA (3.84 cm3, 25.6
mmol). The reaction mixture was heated for 20 min at
50°C, and then allowed to cool to r.t. before the
temperature was decreased to −60°C (acetone–dry
ice), before PiPr2Cl (4.07 cm3, 25.6 mmol) was added.
The product was filtered through silica gel. Yield 2.5 g
2
full-matrix least-squares techniques against Fo [9,10].
The hydrogen atoms of the compound were included at
calculated positions with fixed thermal parameters. All
non-hydrogen atoms were refined anisotropically [10].
XP (Siemens Analytical X-ray Instruments) was used
for structure representations. Crystal data for 1:
C22H40Cl2N2P2Pt, Mr =660.49 g mol−1, colourless
prism, size 0.14×0.12×0.10 mm3, monoclinic, space
1
(50%). H NMR l (CDCl3): 6.72 (NC4H3, 1H, m), 6.26
3
group P 21/n, a =8.1993(3), b=8.2423(3), c=
(NC4H3, 1H, m), 6.14 (NC4H3, 1H, dd, JHH =3.7,
3
19.4461(6) A, i=94.303(2)°, V=1310.48(8) A , T =
3JPH =2.5 Hz), 3.70 (NCH3, 3H, s), 1.96 (P(CH Me2)2,
,
,
3
2
−90°C, Z =2, Dcalc=1.674
g
cm−3
,
v(Mo
2H, heptet of d, JHH =6.90, JPH =0.9 Hz), 0.99
Ka)=56.92 cm−1, ꢀ-scan, transmin: 0.553, transmax:
0.968, F(000)=656, 6702 reflections in h(−10/10),
k(−10/8), l(−24/23), measured in the range 3.51°5
q526.32°, completeness qmax=99%, 2638 independent
reflections, Rint =0.041, 2173 reflections with Fo \
4|(Fo), 133 parameters, 0 restraints, R1(obs)=0.032,
wR2(obs)=0.072, R1(all)=0.042, wR2(all)=0.076, Good-
(PCHMe2, dd, 6H, JHH =7.1, JPH =16.3 Hz), 0.90
3
3
3
3
(PCHMe2, dd, 6H, JHH =7.0, JPH =11.7) ppm.
31P{1H}?l (CDCl3): −18.8 ppm.
2.3.4. Preparation of
2-(N-methylpyrrolyl)diphenylphosphine
The phosphine ligand was prepared as described
above, adding PPh2Cl (4.60 cm3, 25.6 mmol) instead of
PiPr2Cl. The product was purified as described above.
ness-of-fit on F2=1.120, largest difference peak and
−3
,
hole: 1.246/−1.779 e A
.

34
J. Chantson et al. / Inorganica Chimica Acta 305 (2000) 32–37
1
Yield 5.6 g (82%). H NMR l (CDCl3): 6.84 (NC4H3,
1H, m), 6.15 (NC4H3, 1H, m), 5.88 (NC4H3, 1H, dd,
5 cm3 CH2Cl2. Yield 0.66 g (83%). Anal. Calc. for
C34H32Cl2N2P2Pt: C, 51.27; H, 4.05; N, 3.52. Found: C,
3JHH =3.62, JPH =1.81 Hz), 3.62 (NCH3, 3H, s).
51.48; H, 4.31; N, 3.38%. H NMR l (CDCl3): 7.44 (Ph
3
1
31P{1H} l (CDCl3): −29.3 ppm.
o-H, 8H, m), 7.32 (Ph p-H, 4H, m), 7.13 m-H, 8H, m),
6.74 (NC4H3, 2H, m), 5.99 (NC4H3, 2H, m), 5.96
(NC4H3, 2H, m), 3.48 (NCH3, 6H, s) ppm. 13C{1H} l
(CDCl3): 134.4 (o-C), 130.9 (p-C), 129.7 (ipso-C6H5),
128.7 (NCCH), 128.1 (m-C), 122.7 (NCCH) 108.2
2.3.5. Preparation of
2-(6-bromopyridinyl)diisopropylphosphine
2,6-Dibromopyridine (1.137 g, 4.80 mmol) was re-
acted with n-BuLi (3.30 cm3 of a 1.6 M hexane solu-
tion, 5.28 mmol) in CH2Cl2 at −78°C for 20 min.
Thereafter, PiPr2Cl (0.76 cm3, 4.8 mmol) was added and
after 30 min the reaction mixture was allowed to equili-
brate to r.t. Water was added and the aqueous layer
extracted with Et2O. The combined organic fractions
were dried (Na2SO4), filtered, and the solvents were
removed by rotary evaporation at reduced pressure.
The ligand was purified by column chromatography
(silica gel with Et2O–hexane mixtures as eluant). Yield
(NCCH), 37.5 (NMe) ppm, (ipso-NC4H3) not ob-
1
served. 31P{1H} l (CDCl3): −3.5 (Pt satellites JPtP
=
3644.3 Hz) ppm.
2.4.3. Dichlorobis[2-(5-methylthienyl)-
diphenylphosphine]platinum(II) (3)
Complex 3 was isolated as a white solid from 2-(5-
methylthienyl)diphenylphosphine (0.700 g, 2.48 mmol)
and [Pt(cod)Cl2] (0.464 g, 1.24 mmol) under the same
reaction conditions as for 1. Yield 0.94 g (91%). Anal.
Calc. for C34H30Cl2P2S2Pt: C, 49.16; H, 3.64. Found: C,
1
0.79 g (60.0%). H NMR l (CDCl3): 7.42 (NC5H3, 1H,
d, JHH =3.0 Hz), 7.40 (NC5H3, d, 1H, JHH =2.4 Hz),
7.35 (NC5H3, 1H, m), 2.25 (P(CH Me2)2, 2H, heptet of
3
3
1
49.6; H, 3.91%. H NMR l (CDCl3): 7.46 (Ph o-H, 8H,
3
m), 7.30 (Ph p-H, 4H, t, JHH =7.6 Hz), 7.15 (Ph m-H,
3
2
d, JHH =7.1, JPH =2.6 Hz), 1.09 (PCMe2, dd, 6H,
3
3
8H, td, JHH =7.6, JHH =2.0 Hz), 6.77 (SC4H2, 4H,
m), 2.42 (CH3, 6H, s) ppm. 13C{1H} l (CDCl3): 149.6
(ipso-CCH3), 141.0 (SC4H2), 125.7 (SC4H2), 134.2 (o-
C), 130.6 (p-C), 130.0 (ipso-C6H5), 128.4 (ipso-SC4H2),
127.6 (m-C), 30.9 (PCPh2), 15.0 (CH3) ppm. 31P{1H} l
3JHH =7.1, JPH =14.7 Hz) and 0.92 (PCMe2, dd, 6H,
3
3JHH =7.0, JPH =12.1 Hz) ppm. 31P{1H} l (CDCl3):
3
16.4 ppm.
2.4. Preparation of Pt(II) complexes
1
(CDCl3): 5.1 (Pt satellites JPtP =3695.8 Hz) ppm. EI
MS (70 eV): m/z 835 [M+, 5%], 797 [M+−Cl, 3%], 759
[M+−2Cl, 1%], 476 [Pt(MeSC4H2PPh2)+, 0.6%], 282
[MeSC4H2PPh+2 , 100%], 205 [MeSC4H2PPh+, 42%], 97
[MeSC4H+2 , 12%].
2.4.1. Dichlorobis[diisopropyl-2-
(N-methylpyrrolyl)phosphine]platinum(II) (1)
A
solution
of
diisopropyl(N-methylpyrrolyl)-
phosphine (0.274 g, 1.39 mmol) in 5 cm3 of CH2Cl2 was
slowly added to a suspension of [Pt(cod)Cl2] (0.260 g,
0.695 mmol) in 5 cm3 CH2Cl2. The resulting mixture
was stirred at r.t. for 14 h. After removal of the solvent,
a solid was obtained, which was washed with hexane
and with Et2O. Yield 0.42 g (89.0%). X-ray quality
crystals were grown from a solution of 1 in CH2Cl2,
layered with hexane at −20°C over several days. Anal.
Calc. for C22H40Cl2N2P2Pt: C, 40.01; H, 6.10; N, 4.24.
Found: C, 39.9; H, 6.00; N, 4.36%. 1H NMR l
(CDCl3): 6.84 (NC4H3, 2H, m), 6.34 (NC4H3, 2H, dd,
2.4.4. Dichlorobis[diisopropyl-2-
(5-methylthienyl)phosphine]platinum(II) (4)
Complex
4 was formed from diisopropyl-2-(5-
methylthienyl)phosphine (0.520 g, 2.4 mmol) and [Pt(-
cod)Cl2] (0.425 g, 1.14 mmol) using the same procedure
as for 3. A white powder was obtained. Yield 0.75 g
(95%). Anal. Calc. for C22H38Cl2P2S2Pt: C, 38.04; H,
1
5.51. Found: C, 38.23; H, 5.31%. H NMR l (CDCl3):
3
3
6.72 (SC4H2, 2H, dd, JHH =3.5, JPH =6.1 Hz), 6.42
3JHH =1.3, JPH =8.8 Hz), 6.22 (NC4H3, 2H, m), 4.16
(SC4H2, 2H, d, JHH =3.5 Hz), 3.08 (P(CH Me2)2, 4H,
3
3
(NCH3, 6H, s), 2.79 (P(CH Me2)2, 4H, m), 1.22
(P(CMe2)2, 24H, m) ppm. 13C{1H} l (CDCl3): 128.2
(NCHCH), 119.1 (NCCH), 114.7 (ipso-CP), 108.1
(NCCH), 37.4 (NMe), 22.3 (P(CMe2), 18.6 (P(CMe2),
17.7 (P(CMe2) ppm. 31P{1H} l (CDCl3): 14.4 (Pt satel-
m), 2.41 (SC4H2ꢀCH3, 6H, s), 1.49 (P(CHMe2), 12H,
3
3
dd, JHH =7.1, JPH =17.1 Hz), 1.11 (P(CHMe2), 12H,
3
3
dd, JHH =6.9, JPH =16.3 Hz) ppm. 13C{1H} l
(CDCl3): 145.4 (ipso-CCH3), 135.9 (SCCH), 126.3
1
(SCCH), 121.3 (ipso-CP, d, JPC =46.4 Hz), 27.7
1
1
3
lites JPtP =2430.0 Hz) ppm. EI MS (70 eV): m/z no
(P(CMe2), d, JPC =39.7 Hz, JPtC unresolved), 19.6
M+ was observed.
(P(CMe2), d, JPC =12.2 Hz, JPtC unresolved), 15.0
2
3
(SC4H2ꢀCH3) ppm. 31P{1H} l (CDCl3): 19.2 (Pt satel-
1
2.4.2. Dichlorobis[2-(N-methylpyrrolyl)-
diphenylphosphine]platinum(II) (2)
A solution of (N-methylpyrrolyl)diphenyl phosphine
(0.56 g, 2.11 mmol) in 10 cm3 CH2Cl2 was slowly added
to a suspension of [Pt(cod)Cl2] (0.376 g, 1.00 mmol) in
lites JPtP =3763.4 Hz) ppm. EI MS (70 eV): m/z 694
[M+, 8%], 659 [M+−Cl, 2%], 624 [M+−2Cl, 2%], 581
[M+−2Cl−iPr, 1%], 538 [M+−2Cl−2iPr, 1%], 410
[Pt(MeSC4H2PiPr2)+, 0.2%], 367 [Pt(MeSC4H2PiPr)+,
1%], 323 [Pt(MeSC4H2)+, 4%], 214 [MeSC4H2PiPr+2 ,

J. Chantson et al. / Inorganica Chimica Acta 305 (2000) 32–37
35
100%], 129 [MeSC4H2P+, 72%] and 97 [MeSC4H+2 ,
4%].
washing with various solvents. cis Products were ex-
pected to form from the direct replacement of a 1,5-cy-
clooctadiene ligand in the metal precursor by two
phosphine ligands. This was indeed the case in all the
reactions with the exception of dichlorobis[diiso-
propyl-2-(N-methylpyrrolyl)phosphine]platinum(II) (1)
(Scheme 1).
Grim and co-workers showed that aryl substituents
promote, to an extent, the stabilisation of the cis iso-
mers [12]. The synthesis of bis(phosphine)platinum(II)
complexes under analogous reaction conditions, with
2.4.5. Preparation of dichlorobis[5-(2-bromopyridinyl)-
diisopropylphosphine]platinum(II) (5)
A similar procedure was followed as described above.
[Pt(cod)Cl2] (0.386 g, 1.03 mmol) was reacted with
2-(6-bromopyridinyl)diisopropylphosphine (0.565 g,
2.06 mmol). The complex was isolated as white, crys-
talline flat needles. Yield 0.797 g (95%). Anal. Calc. for
C22H34Br2Cl2N2P2Pt: C, 32.45; H, 4.21; N, 3.44. Found:
2-(5-methylthienyl)diphenylphosphine
and
2-(N-
1
C, 32.81; H, 4.11%. H NMR l (CDCl3): 7.13 (NC5H3,
methylpyrrolyl)diphenylphosphine, respectively, led to
the isolation of dichlorobis[2-(N-methylpyrrolyl)-
diphenylphosphine]platinum(II) (2), and dichlorobis[2-
6H, m), 3.06 (P(CH Me2)2, 4H, m), 1.55 and 1.07
3
3
(P(CMe2)2, dd, JHH =7.2, JPH 16.3 Hz and m, 24H)
ppm. 13C{1H} l (CDCl3): 141.7 (ipso-CP), 139.75 (ipso-
CBr), 137.3 (p-C), 128.4 (m-C), 125.6 (m-C), 25.6
(P(CHCH3)2), 19.5 and 19.0 (P(CHCH3)2) ppm.
31P{1H} l (CDCl3): 28.2 (Pt satellites 1JPtP =3711.7 Hz)
ppm. EI MS (70 eV): m/z 817 [M+, 63%], 310 [PtBrCl+
, 100%], 232 [BrpyPiPr+, 84%], 230 [PtCl+, 100%].
(5-methylthienyl)diphenylphosphine]platinum(II)
(3).
1
31
The magnitude of the J (195Ptꢀ P) coupling constants
of 3644.3 and 3695.8 Hz for 2 and 3, respectively, fit
into the range of 1J (195Ptꢀ P) values reported for other
31
cis-dichlorobis(phosphine)platinum(II) complexes [13].
A
heteroaryl substituent with strong coordinating
power via the N-heteroatom, such as a pyridinyl deriva-
tive [14] also yields a cis product, bis(2-{6-bromopy-
ridinyl}diphenylphosphine)dichloroplatinum(II), 5 (1J
(
3. Results and discussion
195Ptꢀ P)=3711.7 Hz). It is interesting to note that in
31
5 there is evidence (the resonance is broadened and
The phosphine ligands were prepared by utilising
standard synthetic organic methods which involved the
monolithiation of N-methylpyrrole, 2-methylthiophene,
and 2,6-dibromopyridine [11], followed by quenching
with the chlorodiphenylphosphine or chlorodiisopropy-
lphosphine electrophile. The platinum(II) complexes
were obtained by the displacement of 1,5-cyclooctadi-
1
shifted upfield) from the H NMR spectrum that hy-
drogen bonding occurs between methyl hydrogen atoms
of the isopropyl group and the nitrogen atom of the
pyridine ring. On comparison with related complexes
31
reported in the literature, the 1J (195Ptꢀ P) coupling
constants for 3 and for 5 correspond favourably with
the 3681 Hz for cis-bis(diphenyl-2-thienylphos-
phine)dichloroplatinum(II) [15] and 3675.6 Hz for cis-
bis(diphenyl-2-pyridinylphosphine)dichloroplatinum(II),
respectively (dichloromethane-d2) [16]. Incomplete spec-
troscopic data were reported for the former complex.
On replacing the two phenyl groups on the het-
eroarylphosphine ligands with isopropyl groups, cis-
dichlorobis(2-{5-methylthienyl}diisopropylphosphine)-
4
ene from dichloro(h -1,5-cyclooctadiene)platinum(II)
by 2 equiv. of the appropriate phosphine ligand at
ambient temperature in dichloromethane. All the com-
plexes were isolated as white solids and purified by
31
platinum(II) (4), was obtained. Its 1J (195Ptꢀ P) cou-
pling constant of 3763.4 Hz is once again indicative of
a cis configuration. However, replacement of phenyl
groups of the 2-(N-methylpyrrolyl)phosphine ligand by
isopropyl groups results in a platinum(II) complex, 1,
31
displaying a significantly smaller1J (195Ptꢀ P) coupling
constant of 2430.0 Hz. This value is characteristic of
trans-dichlorobis(phosphine)platinum(II)
complexes
[13].
Confirmation of the trans configuration of phosphine
ligands in 1 was obtained from a single crystal X-ray
structure determination. X-ray quality crystals of 1
were grown from a dichloromethane solution layered
with hexane. The molecular structure of 1 is given in
Fig. 1 and selected bond lengths and angles are listed in
Table 1. A square planar structure is adopted with two
molecules of phosphine ligands occupying trans posi-
Scheme 1.

36
J. Chantson et al. / Inorganica Chimica Acta 305 (2000) 32–37
116.2(2)° while the other is 109.7(2)°. Furthermore,
while the C7ꢀC6ꢀP and C10ꢀC9ꢀP angles are identical
at 112.1(4)°, the other methyl group of each isopropyl
substituent gives different angles viz. 109.3(4)° for
C11ꢀC9ꢀP and 111.3(4)° for C8ꢀC6ꢀP. The N-methyl
group is slightly tilted away from the phosphorus and
inclined to the far side of the pyrrole ring (C1ꢀNꢀC5=
128.8(5), and C4ꢀNꢀC5=122.2(5)°).
The cis complexes did not afford crystals suitable for
single-crystal X-ray diffraction studies. We ascribe this
to better packing of the molecules resulting from the
centrosymmetric trans complex compared to the cis
complexes.
The reason for 1 assuming a trans conformation
while 2–5 are cis isomers, we believe, lies in electronic
rather than steric effects. Even large bulky phosphines
can stabilise the cis isomer. For example, the intermesh-
ing ability of the phosphine ligand in cis-dichlorobis(di-
tert-butylphenylphosphine)platinum(II) alleviates steric
hindrance and allows for the cis configuration to be
adopted [19]. An example of a case where electronic
interactions lead to quite different properties in pyrrole-
or thiophene-like molecules is given by the comparison
of the 2,2%-diaryl molecules. A twisted conformation is
observed for 2,2%-bi(N-methylpyrrole) in the biscarbene
Fig. 1. Molecular structure of trans-[Pt(P{2-(N-methylpyrrolyl)}-
iPr2)2Cl2] (1).
Table 1
a
,
Selected bond lengths (A) and angles (°) for 1
PtꢀCl(1)
PtꢀP(1)
2.321(1)
2.334(1)
1.808(6)
1.855(6)
1.858(6)
1.385(8)
1.380(7)
1.467(7)
P(1)ꢀC(1)
P(1)ꢀC(9)
P(1)ꢀC(6)
N(1)ꢀC(1)
N(1)ꢀC(4)
N(1)ꢀC(5)
complex
[(CO)5WC(OEt)C5H5NꢀC5H5NC(OEt)W-
(CO)5], where the two pyrrole rings are twisted away
from each other [5]. By contrast, the analogous biscar-
bene complex with a 2,2%-biphenyl spacer [20] and 5,5%-
substituted 2,2-bithiophene compounds [21] have
planar structures with rotation about the 2,2%-linkage
bond severely restricted. This implies that the p-orbitals
of the sulfur atom are involved in delocalization of
electron density over the two thiophene rings, whereas
this is not the case for dipyrroles. The difference in the
electronic properties between the 5-membered heterocy-
cles of 2-methylthiophene and N-methylpyrrole deter-
mines the bonding properties of these phosphine
ligands and plays a role in the reasons for 1 adopting a
trans configuration.
Cl(1A)ꢀPtꢀCl(1)
Cl(1A)ꢀPtꢀP(1)
Cl(1)ꢀPtꢀP(1)
C(1)ꢀP(1)ꢀPt
C(9)ꢀP(1)ꢀPt
C(6)ꢀP(1)ꢀPt
C(8)ꢀC(6)ꢀP(1)
C(7)ꢀC(6)ꢀP(1)
C(10)ꢀC(9)ꢀP(1)
C(11)ꢀC(9)ꢀP(1)
180.0
89.49(5)
90.52(5)
116.2(2)
109.7(2)
116.5(2)
111.3(4)
112.0(4)
112.1(4)
109.3(4)
a Symmetry transformations used to generate equivalent atoms: A:
−x, −y, −z.
The cis complexes do not spontaneously undergo
cis–trans isomerisation to the thermodynamically more
stable trans isomer. It is difficult to establish a trend
amongst the various interrelating factors that affect the
isomerisation equilibrium, although the effect of the
substituent at phosphorus is marked. As discussed be-
low, the diisopropyl-2-(N-methylpyrrolyl)phosphine lig-
and is the most shielded of the series of phosphines in
this investigation, suggesting that the phosphine ligand
is a relatively stronger electron-donor. This may affect
the reaction kinetics leading to the trans rather than the
cis isomer. The presence of a trace amount of free
phosphine is known to catalyse the isomerisation [4]. It
can thus be argued that platinum(II), being a relatively
electron-rich metal centre, could have a greater ten-
dency to labilise the stronger donor phosphine ligand.
tions. Deviations of the bond angles at platinum from
90° are minimal and the PꢀPtꢀCl atoms are almost
co-planar. The PtꢀCl and PtꢀP bond lengths are
,
2.321(2) and 2.334(1) A, respectively. Both bond
lengths are in the normal range for trans-PtꢀCl and
trans-PtꢀP bonds [17]. In the analogous compound
trans-dichlorobis(triethylphosphine)platinum(II)
the
PtꢀCl bond length is also reported to be slightly shorter
than the PtꢀP bond length [18]. The average
PꢀC(isopropyl) bond distance is 1.857(6), which is 0.049
,
A longer than the PꢀC(pyrrolyl) bond length of
,
1.808(6) A. This feature has been observed in other
alkyl–aryl phosphines [19]. The two isopropyl groups
attached to a phosphorus atom display slightly different
orientations. One methyne group has a CꢀPꢀPt angle of

J. Chantson et al. / Inorganica Chimica Acta 305 (2000) 32–37
37
Chemical shifts of phosphines in the 31P NMR spec-
Acknowledgements
tra are affected by the electronegativity of the group
attached to phosphorus. The increased shielding of the
phosphorus centre in the order triphenylphosphine (−
4.7 ppm [3]), 2-{5-methylthienyl}diphenylphosphine
(−18.7 ppm) and 2-{N-methylpyrrolyl}diphenyl-
phosphine (−29.3 ppm) suggests that the electron
donor ability of the aryl substituents increases from
phenyl, to 5-methylthienyl, to N-methylpyrrolyl. From
the view of group electronegativities of the heteroatom
of thienyl and pyrrolyl compared to phenyl, the ob-
served trend is in apparent contradiction. Studies of
quarterization kinetics by Allen et al. has shown that
diphenyl-2-thienylphosphine reacts half as fast as does
triphenylphosphine, indicating that the thienyl sub-
stituent behaves as a stronger electron withdrawing
substituent than phenyl [22]. However, as thienyl, and
pyrrolyl, are classified as ‘p-excessive’ systems, these
studies suggest that electron density is returned to
phosphorus via the p-system by pp–dp interactions.
Electron-withdrawing or -donating abilities of aryl and
heteroaryl sustituents involves both s (inductive) and p
(resonance) properties [22]. Cone angles also affect 31P
NMR chemical shifts [13b]. For example, the phenyl
groups in triphenylphosphine are more electron-with-
drawing than the isopropyl groups in triisopropylphos-
phine, yet the phosphorus centre in the latter is more
deshielded than in the former. This is due to the
increase in cone angle from 145° in triphenylphosphine
to 160° in triisopropylphosphine [23]. An analogous
type of effect is presumed to account for the differences
in 31P chemicals shifts for 2-{5-methylthienyl}diphenyl-
phosphine (−18.7 ppm) and 2-{5-methylthienyl}-
diisopropylphosphine (1.2 ppm); and 2-{N-methyl-
pyrrolyl}diisopropylphosphine) (−18.3 ppm) and 2-
{N-methylpyrrolyl}diphenylphosphine (−29.3 ppm).
Upon coordination with platinum, the phosphorus is
deshielded in each of the complexes.
We thank the Foundation for Research and Develop-
ment, Pretoria, for financial assistance.
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4. Supplementary material
Further details of the crystal structure investigations
are available on request from The Director of the
Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, on quoting the depository
number CCDC-136338 (1), the names of the authors,
and the journal citation.
.