Metal complexes of heteroarenes, X[1]. η1 -coordination of phosphinine: Synthesis and structure of cis-dichloro-bis(2,6-dimethyl-4-phenyl-phosphinine)platinum

Article abstract of DOI:10.1515/znb-1999-0209

Cis-Dichloro-bis(2,6-dimethyl-4-phenyl-η ¹-phosphinine)platinum (3) has been prepared by ligand substitution from cis -dichloro(2,5-cyclooctadiene)platinum and characterized by spectroscopy (¹H, ¹³C, ³¹P NMR, IR, UV-Vis), CV and X-ray diffraction (space group P2₁/c, a = 14.998, b = 16.540, c = 11.506 A, α = γ = 90°, β = 92.46°, Z = 4). In line with findings for similar P hybridization states, the Pt-P bond length (221 pm) in 3 equals that in η¹-phosphaalkene Pt complexes and falls short of the respective parameters in Pt phosphane species. The stretching frequencies ν_PtCl and the bond length Pt-Cl indicate that phosphinines and phosphaalkenes, relative to phosphanes, adopt a lower position on the trans-influence scale. This gradation is also suggested by the coupling constants ¹J(¹⁹⁵Pt,³¹P).

Full text of DOI:10.1515/znb-1999-0209

Metal Complexes of Heteroarenes, X [1].
7 7 1 -Coordination of Phosphinine: Synthesis and Structure of
cis-Dichloro-bis(2,6-dimethyl-4-phenyI-phosphinine)platinum
Christoph Elschenbroich*, Steffen Voß, Klaus Harms
Fachbereich Chemie der Philipps-Universität, Hans-Meerwein-Straße, D-35032 Marburg
Z. Naturforsch. 54 b, 209-213 (1999); received September 9, 1998
Phosphinine Complex, Platinum, X-Ray Data, NMR Data, frans-Influence
C/5-Dichloro-bis(2 ,6-dimethyl-4 -phenyl-77'-phosphinine)platinum (3) has been prepared by
ligand substitution from cis -dichloro(2,5-cyclooctadiene)platinum and characterized by spec-
troscopy ('H, 13C, 31P NMR, IR, UV-Vis), CV and X-ray diffraction (space group P2|/c, a =
14.998, b = 16.540, c - 11.506 Ä, a = 7 = 90°, ß = 92.46°, Z = 4). In line with findings for simi-
lar P hybridization states, the Pt-P bond length (221 pm) in 3 equals that in 771-phosphaalkene Pt
complexes and falls short of the respective parameters in Pt phosphane species. The stretching
frequencies u^c\ and the bond length Pt-Cl indicate that phosphinines and phosphaalkenes,
relative to phosphanes, adopt a lower position on the rram'-influence scale. This gradation is
also suggested by the coupling constants 1J(195Pt,31P).
was suggested by the higher stability and superior
Introduction
solubility of the complexes emerging from this syn-
thesis.
Pyridine- and phosphane complexes of Pt(II),
(Py)2PtX2 and (R3P)2PtX2, respectively, are among
the most extensively studied classes of coordination
compounds; reference [2 ] provides access to the vo-
luminous literature in this area. Our concern with
the ligating properties of the heavier heteroarenes of
group 15, C5H5E (E = P, As, Sb) in general [3] and
their role in square planar M(d8) complexes of the
late transition metals in particular [4] initiated our
preparation of the first fully characterized represen-
tative in the class [ds-(phosphinine)2PtX2] which
forms the subject of this Note [5]. Obvious aims in-
clude the description of structural features and, con-
nected with this, a placement of phosphinines on the
trans-influence scale. The current state of phosphi-
nine transition-metal chemistry has been delineated
by Mathey et al. [6 ].
cw-Dichloro-bis-(2 ,6 -dimethyl-4 -phenyl-/71 -
phosphinine)platinum (3) is obtained as rod shaped
yellow crystals which readily dissolve in halocar-
bon solvents. As opposed to the free ligand 2, the
complex 3 is only slightly air-sensitive in solution
and air-stable in the solid state. 3 cannot be sub-
limed and fails to yield a molecular ion peak in the
mass spectrum. As inferred from cyclic voltamme-
try, there are no accessible stable oxidation states
of 3 apart from Pt(II): in the potential range -2.9 V
< E < +1.6 V (DME / 0.1 M rc-Bu4NC104, -40°C),
only one irreversible wave [Epc(0/-) = -1.07 V] is
observed.
The structure of 3 as determined by X-ray diffrac-
tion (Table I, [8 ]) is depicted in Fig. 1 where a stere-
oplot of the unit cell is also given; bond lengths and
angles are collected in Table II with the parameters
for the free ligand 2 [9] provided for comparison.
The square-planar arrangement within the PtP2Cl2
frame is somewhat distorted in that angles P-Pt-Cl
(trans) fall short of 180° . The angle Cl-Pt-Cl (cis)
is smaller than 90°. The dihedral angle between the
phosphinine- and the P2PtCl2 coordination planes
amounts to 59.7 and 51.0°, respectively, placing
the ortho-methyl groups of the ligands in positions
where they can protect the complex against nucleo-
Results and Discussion
Following
a well established general proce-
dure [7], the complex 3 was prepared via substitu-
tion of 1,5-cyclooctadiene in 1 for the phosphinine 2
(eq. (1)). The choice of this phosphinine derivative
* Reprint requests to Prof. Dr. Ch. Elschenbroich;
Fax: +49 6421 28 8917;
e-mail: Eb@psl515.chemie.uni-marburg.de.
0932-0776/99/0200-0209 $ 06.00 © 1999 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com
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210
Ch. Elschenbroich et al. ■Metal Complexes of Heteroarenes, X
Table I. Crystal and refinement data for the structure de-
termination of 3.
Habitus, colour
Crystal size
Crystal system
Block, yellow
0.30 x 0.25 x 0.20 mm
Monoclinic
Space group
P2,/c, Z= 4
Unit cell dimensions
a = 14.9985(10) Ä
b= 16.5396(7) Ä
c = 11.5064(7) Ä
ß= 92.466(6)°
2851.7(3) A
Volume
Cell refinement
Chemical formula
Formula weight
F(000)
5000 selected reflections
C27H28CI4P2Pt
751.32
1464
Density (calculated)
Absorption coefficient
Diffraktometer type
Wavelength
1.750 Mg/m3
5.423 mm“ 1
Stoe IPDS
MoKa(71.073 pm)
Temperature
193(2)K
0-Range for data coll.
Index ranges
2.16- 25.92°
A: -18/18 A:: -20/20
/: -14/14
Data collection software
Cell refinement software
Data reduction software
Reflections collected
Stoe Expose
Stoe Cell
Stoe Integrate
24543
Independent reflections
Observed reflections
Refl. used for refinement
Absorption correction
5531 [/?(int) = 0.0717]
4353 [I > 2cr(I)]
5531
Fig. 1. Structure of 3 in the crystal; top: SHELXTL plot,
the ellipsoids are drawn at the 50 % level; bottom: stereo-
view of the unit cell.
Numerical (SHELXTL)
Max. and min. Transmiss. 0.4102, 0.2930
Solution
Refinement
Direct methods
Full-matrix refinement at F2
philic attack from an axial direction. A discussion
of the Pt-P and Pt-Cl bond lengths benefits from
a comparison with the respective parameters in 4
[10], 5 [11] and 6 [12] (Fig. 2).
Treatm. of hydrogen atoms Calculated positions,
fixed isotr. U's
Largest diff. peak and hole 1.083 and -0.854 *1030 e/m3
Programs used
SHELXS-97
(Sheldrick, 1997),
SHELXL-97
(Sheldrick, 1997),
SHELXTL
5531/ 311
l/[cr2(Fo2)+(0.0293P)2]
where P=(F02+2Fc2)/3
0.917
wR2 = 0.0584
R = 0.0276
The Pt-P distances in 3 lie between those for 4 and
5 whereas the Pt-Cl distances in 3 are shorter than
in 4 and 5. The inference of a 7r-acceptor strength of
phosphinine approaching that of the trisorganophos-
phite ligand (MeO^P from the Pt-P bond lengths
data alone is flawed, however, since differing hy-
bridization states of phosphorus in phosphanes («
sp3) and phosphinines (« sp2) must be taken into
account, the latter leading to Pt-P bond shortening,
Data / parameters
Weighting scheme
Goodness-of-fit on F2
R index (all data, F2)
R index conventional
[I > 2 ( 7 ( 1 ) ]
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211
Ch. Elschenbroich et al. • Metal Complexes of Heteroarenes, X
Table II. Selected bond lengths (pm) and
bond angles (°) of the ligand 2 and the
complex 3.
2
3
2
3
Ptl -C ll
Ptl -C12
234.62(11)
- Ptl -C12
91.88(4)
91.04(4)
86.37(4)
90.80(4)
176.51(4)
176.87(4)
107.71(19)
118.7(3)
126.9(4)
Cll
233.84(10) Cll - Ptl - PI
222.00(10) C12 -Ptl -P15
220.91(11) PI - Ptl - P15
Ptl
-PI
Ptl -P15
PI - C2
171.9(4)
PI -
174.8(5)
173.6(5) 171.2(4)
C2 --C3 138.7(7)
-C4 139.4(7)
-C5 138.7(7) 139.0(5)
C5 -C6 138.2(7) 138.7(5)
Ptl -■C12
P15 -Ptl -Cll
PI - C2
C6 -
PI -
C6
102.4(2)
122.9(4)
124.7(5)
138.2(6)
140.0(6)
PI - C2 -
C2 - C3 -
C3
C4
C3
C4
C5
C4 -
121.7(4) 120.7(4)
C3 -
C2
C4 -
C5 - C6 - PI
PI - C2 - C13 116.7(4) 120.1(3)
126.7(4)
119.1(3)
-C13 150.7(8) 150.7(5)
C6 -C14 151.1(8) 150.1(5)
C4 -
C5 - C6
125.4(5)
122.9(4)
-C7 149.3(7) 148.7(5)
PI -
C14 117.5(4)
C6 -
119.8(3)
Tab. III. 'H, l3C, and 31P NMR data for the ligand 2 and
the complex 3, solvent: CD2CI2, 6 (ppm), J (Hz).
236.4
- i
.......2.2..5..6....
3
2
^
Prlt^'238.8
Me3P2r23.9
’ Cl
7,75
6.9
2.74
15.0
7.60
7.5
7.94(0.19)
27.5b
2.74(0.0)
22.6b
7.53(-0.7)
7.2
<5H-3,5(z^)a
37(31P,'H)
<SH-7,8(AS)
3y(31p,'H)
<5H-10,14(zA<5)
3y('H,'H)
4
-1
vp,ci = 303 cm'
= 280 cm'1
vPtC1= 312 cm'1
= 298 cm'1
‘J(195Pt,3lP) = 3907 Hz
'J(195Pt,3lP) = 3520 Hz
7.47(0.0)
7.7
7.41(0.02)
7.47
7.4
7.39
<$H-11,13(Z\<5)
3y(1H,1H)
bH-\2(Ab)
?h
Ph^=
1
233.5
240.8
219.2
p ,
>
P
„
„
. „ c i
.
...............
194.1
128.4(-65.7)
3907.4
bP(Ab)
'y('95pt,31p)
^238.4
*C1
(Me°)3P2- Pt'
J
53
Ph^=P^2T9.5^(
’Cl
P h
M e s
168.5
49.9
150.4(-18.1)
27.8
20.1
137.5(5.4)
n.r.
6.3
bC-2,6(Ab)
'7(31P,13C)
37(3iP,13C)
<5C-3,5(^^)
6
5
.-1
vPtci= 320 cm'1
= 298 cm'1
vPtci= 326 cm’
= 298 cm'1
132.1
154.3
13.3
'J(195Pt,31P) = 3950 Hz
‘J(195Pt,31P) = 5671 Hz
7( P, C)
Fig. 2. Structural and spectroscopic parameters for 3 and
the related molecules 4 -6 relevant to the rrans-influence
gradation.
141.3(-2.0)
15.1
21.6(-3.0)
7.0
139.9(-2.3)
128.5(-0.3)
126.8(-0.8)
127.8(0.2)
143.3
15.6
24.6
^c - 4 ( ^ )
7( P, C)
bC-lM Ab)
37(3iP ,C )
bC-9(Ab)
<5C-10,14(Z\<5)
<5C-11,13(Z^6)
bC-\2(Ab)
6.8
142.2
128.8
127.6
127.6
reflecting the inherently smaller covalent radius
for P(sp2). Not surprisingly, the distance Pt-P in 3
matches that for Pt-P (phosphaalkene) in 6 where the
ligand, as in 3, possesses two-coordinate phospho-
rus. Yet, the significantly shortened Pt-Cl distances
in 3 attest to a pronounced 7r-acceptor character of
ry’-phosphinine [13] which favors multiple bonding
a Ab = ^(complex) - ^(ligand);b filled-in doublet.
the direction toward the ideal benzene structure.
They include partial rehybridization which provides
the P lone pair with higher 3p content, resulting in
between Pt and Cl, effected by Pt
Cl x-donation.
The most notable change which the ligand 2 expe-
riences upon coordination in 3 is the increase of the enhanced M-P bonding. The small but significant
C6-P-C2 bond angle by 5.3° with attendant decrease shortening of the P-C2 resp. P-C6 bond lengths is
of the P-C2-C3 angle by 4°. These changes are in probably an outcome of two countervailing effects:
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212
Ch. Elschenbroich et al. • Metal Complexes of Heteroarenes, X
P-C bond shortening caused by the build up of pos-
itive partial charge on P as a result of the P —>M o-
donor contribution and the population of the intrali-
gand antibonding MO bj (7r*) effected by P <—M
7r-backbonding.
nitrogen by standard procedures. Instrumental analysis
was performed with equipment listed previously [1], 2,6-
Dimethyl-4-phenyl-phosphinine (2) was prepared via the
Märkl route [16], ds-dichloro-(2,5-cyclooctadiene)pla-
tinum (1) is a commercial product.
Spectral features which have served to place a
specific ligand L on the rrans-influence scale [14]
include the stretching frequency vptx, observed in
complexes of the type ds-L2PtX2, and the scalar
coupling 'y(193Pt,31P). In Fig. 2, the relevant data
for complex 3 are compared with those of related
compounds. Based on the IR and NMR criteria,
the frans-influence of the phosphinine in 3 mimicks
that of phosphaalkene in 6 and is smaller than in 4
and 5; the agreement of the Pt-Cl bond lengths in 3
and 6 further illustrates this kinship. Close resem-
blance is also exhibited by the values of ’7(195Pt,31 P)
for 3 and 6 (Table III). According to ref. [15], this
would imply similar s-orbital contributions to the
Pt-P bonds in 3 and 6 . Somewhat disturbingly, how-
ever, 'y(195Pt,31 P) for complex 5, in which the s-
character of the P lone pair at the ligand P(OMe)3
should be lower than in a phosphaalkene or phos-
phinine, is particularly large. Apparently, the elec-
tronegativity of the substituents at a phosphorus lig-
and and the positive partial charge they generate on
the ligating atom also exert an influence on 1y(195Pt,
P) [12]. In view of these uncertainties, the dubious
exercise of apportioning the overall rrans-influence
of phosphinine into a- and 7r-components will not
be attempted here.
cis-Dichlom-bis(2,6-dimethyl-4-phenyl-r^ -phosphini-
ne)platinum (3)
To a solution of 202 mg (0.54 mmol) of m-dichloro-
(2,5-cyclooctadiene)platinum (1) in 30 ml of methylene
chloride are added during 15 min at r.t. 16 mg (1.08 mmol)
of 2,6-dimethyl-4-phenyl-phosphinine (2), dissolved in
20 ml of methylene chloride. After stirring for 2 h and
reduction to a volume of 3 ml the pale yellow solution is
filtered and layered with 15 ml of benzene. Within one
week at room temperature3 • CH2CI2 is obtained as light
yellow cubes. Yield: 284 mg (71%).
C26H26Cl2P2Pt • CH2CI2 (751.36)
Calcd C 46.86 H 4.93 %,
Found C 45.93 H 4.70 %.
IR (4000-500 cm-1 KBr pellet, 500-100 cm- 1Nujol):
3044 s, 2974 s, 2914 s, 1960 m, 1811 w, 1600 w, 1576 w,
1490 m, 1437 m, 1373 m, 1283 w, 1266 m, 1171 w, 1131
vw, 1078 w, 1027 vw, 971 vw, 887 w, 847 vw, 765 s, 739
vs, 731 vs, 700 vs, 646 w, 529 vw, 448 w, 251 vw, 312
vw, 298 vw.
UV-vis (CH2CI2): Amax = 304 nm, E = 36800
[lmol"'em]. MS (El, 70 eV): m/z = 200 (100, 2+), 185
(33, 2+-CH3). NMR data: see Table III.
Acknowledgements
Experimental Section
We are indebted to the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie for the
support of this work.
All manipulations were carried out under nitro-
gen protection, solvents were dried and saturated with
[1] Part IX: Ch. Elschenbroich, S. Voß, O. Schiemann,
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[4] S. Voß, Ph. D. Thesis, Univ. Marburg (1998).
[5] A complex P2PtCb, where P is the unsymmetrically
substituted ligand 2-phenyl-4,5-dimethyl-phosphi-
nine has been reported previously. Structural char-
acterization by X-ray diffraction has not been forth-
coming, however, and the m-configuration has been
proposed on the basis of 31P NMR: M. Shiotsuka,
T. Tanamachi, Y. Matsuda, Chem. Lett. 1995, 531.
[3] see [1] and preceding papers in the series.
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Ch. Elschenbroich et al. • Metal Complexes of Heteroarenes, X
213
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