Crystallographic characterization of the palladium(I) dimers, syn-Pd2Cl2(dppmMe)2 and Pd2Cl2(dppm)2

Article abstract of DOI:10.1016/S0020-1693(01)00682-X

An X-ray diffraction study on syn-Pd₂Cl₂(dppmMe)₂, syn-1, shows an unusual boat-like conformation of the eight-membered Pd₂P₄C₂ ring. This conformation, containing equatorial Me groups, facilitates access of the Pd-Pd bond to small molecules such as CO, SO₂, and elemental sulfur or selenium, and makes the syn isomer more reactive than anti-1. A comparison of bond angles around the Pd and P atoms in the syn- and anti-isomers reveals a more strained geometry of the former, which may also contribute to the stronger propensity of syn-1 to form A-frame adducts. Solution NMR/NOE studies on syn- and anti-1 and their (μ-Se) adducts reveal the structural rigidity of these complexes; the Me groups inhibit the interchange of axial and equatorial positions on the bridging methine C-atom, and solution structures correspond to those in the solid state. The X-ray structure of Pd₂Cl₂(dppm)₂ is, as expected, like that of the corresponding bromo complex; both are analogous to that of anti-1 which adopts a chair conformation within both the fused, five-membered rings.

Full text of DOI:10.1016/S0020-1693(01)00682-X

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Inorganica Chimica Acta 327 (2002) 179–187
Crystallographic characterization of the palladium(I) dimers,
syn-Pd2Cl2(dppmMe)2 and Pd2Cl2(dppm)2; solution conformational
behavior of syn- and anti-Pd2Cl2(dppmMe)2 and their (m-Se)
adducts [dppmMe=m-1,1-bis(diphenylphosphino)ethane, and
dppm=m-bis(diphenylphosphino)methane]
Ga´bor Besenyei a,*, La´szlo´ Pa´rka´nyi a, Eszter Ga´cs-Baitz a, Brian R. James b,*
a Chemical Research Center, Institute of Chemistry, P.O. Box 17, 1525 Budapest, Hungary
b Department of Chemistry, The Uni6ersity of British Columbia, Vancou6er, Canada V6T 1Z1
Received 2 June 2001; accepted 4 July 2001
This paper is dedicated to Kees Vrieze, a close friend and colleague for 35 years (at least for BRJ!), for his contributions to organometallic
and coordination chemistry, particularly of Pd
Abstract
An X-ray diffraction study on syn-Pd2Cl2(dppmMe)2, syn-1, shows an unusual boat-like conformation of the eight-membered
Pd2P4C2 ring. This conformation, containing equatorial Me groups, facilitates access of the PdꢀPd bond to small molecules such
as CO, SO2, and elemental sulfur or selenium, and makes the syn isomer more reactive than anti-1. A comparison of bond angles
around the Pd and P atoms in the syn- and anti-isomers reveals a more strained geometry of the former, which may also
contribute to the stronger propensity of syn-1 to form A-frame adducts. Solution NMR/NOE studies on syn- and anti-1 and their
(m-Se) adducts reveal the structural rigidity of these complexes; the Me groups inhibit the interchange of axial and equatorial
positions on the bridging methine C-atom, and solution structures correspond to those in the solid state. The X-ray structure of
Pd2Cl2(dppm)2 is, as expected, like that of the corresponding bromo complex; both are analogous to that of anti-1 which adopts
a chair conformation within both the fused, five-membered rings. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Palladium complexes; A-frame complexes; Selenium compounds; Bis(diphenylphosphino)methane; 1,1-Bis(diphenylphosphino)ethane
1. Introduction
complexes which again have mutually trans-m-dppm
ligands, but have no metal–metal bonding or other
bridging ligands. Introduction of a third bridge, how-
ever, generates the so-called A-frame structures in
which the steric demands of the dppm Ph groups
disfavor the chair conformation, and typically generate
boat-like conformers with the dppm methylenes bent
towards the atom(s) in the apical position. However,
there are a few exceptions to these generalities [2].2 For
example, the M2P4C2 backbones in the side-by-side
The chemistry and structural characterization of
complexes possessing an M2(dppm)2 core1 are well de-
veloped [1]. Complexes with metal–metal bonds and
mutually trans-dppm ligands (side-by-side complexes)
generally prefer the chair-like conformation of the
eight-membered M2P4C2 ring if no other bridging
groups are present; the same is true for face-to-face
* Corresponding authors. Tel.: +1-604-822 6645; fax: +1-604-822
2847.
2 A search in CSD (October 2000) revealed 353 structures satisfying
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
E-mail addresses: besenyei@cric.chemres.hu (G. Besenyei),
brj@chem.ubc.ca (B.R. James).
the connectivity (C)(C)PCP(C)(C)MP(C)(C)CP(C)(C)M, where M=
Rh, Ir, Ni, Pd, or Au. An additional 18 complexes were found when
one or both H-atoms of the dppm methylene were replaced by
substituents (M=transition metal).
1 Throughout this paper, dppm and dppmMe refer exclusively to
ligands bridging the two metal centres.
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00682-X

180
G. Besenyei et al. / Inorganica Chimica Acta 327 (2002) 179–187
2. Experimental
complexes
like
Pd2(CF3CO2)2(dppm)2
[3],
[Au2(dppm)2]2+ [4] and syn-Ir2Cl2I2(CO)2(dppm)2 [5]
adopt a boat conformation in the solid state, and the
face-to-face cation [Pt2Me2(CO)2(dppm)2]2+ has a boat
conformation with methylene groups bending towards
the Me groups located on the same side of the Pt2P4C2
ring [6]. The complexes [Pt2(Me)2(m-H)(dppm)2]PF6 [7]
and Os2(m-O)Cl6(dppm)2 [8] demonstrate that the pres-
ence of a third bridge does not necessarily result in a
A-frame structure if the steric interactions between the
apical bridge and the Ph groups are weak, and conse-
quently these derivatives form a chair and not the
expected boat conformation. The complex Pd2-
(CF3CO2)2(m-CO)(dppm)2 appears to be the only A-
frame that adopts a chair conformation [9]. Another
type of irregularity is shown by [Rh2Cl2(m-pz)(dppm)2-
(t-BuNC)2]PF6 [10a] (pz=pyrazolate) and Pd2R2(m-
Br)(dppm)2, R=mesityl, [10b] in which both methylene
groups are bent away from the m-apical ligand.
Details for the syntheses of syn- and anti-
Pd2Cl2(dppmMe)2 (1) [12,13], Pd2Cl2(dppm)2 (2)
[12,17a], and syn- and anti-Pd2Cl2(m-Se)(dppmMe)2 (to
be abbreviated syn-1(m-Se) and anti-1(m-Se), respec-
tively) [11,13,14] have been published. Orange crystals
of syn-1 (platelets) and 2 (prisms) were grown from
CH2Cl2 solutions of the complexes by layering with
EtOH. Some details of the crystal data, data collection
and refinement details for these complexes are pre-
sented in Table 1. Intensity data were collected at 293
K on an Enraf–Nonius CAD4 diffractometer. Unit cell
parameters were determined by least-squares of the
setting angles of 25 reflections (syn-1: 12.995q5
13.94°; 2: 27.105q529.50°). The intensities of three
standard reflections were monitored regularly (every 60
min), and were corrected for decay [18]. A psi-scan
absorption correction [19] was applied to the data. The
structure was solved by direct methods [20] and subse-
quent difference syntheses. Anisotropic full-matrix
least-squares refinement [21] on F2 was carried out for
all non-hydrogen atoms. H-atom positions were calcu-
lated from assumed geometries and were included in
structure factor calculations but were not refined. The
isotropic displacement parameters of the H-atoms ap-
proximated from the Ueq value of the atom to which
they were bonded (the riding model was utilized). Neu-
tral atomic scattering factors and anomalous scattering
factors were taken from Ref. [22]. Selected bond lengths
and angles are given in Table 2, and the molecular
structures are shown in Figs. 1 and 2.
We [11–14] and others [15,16] have added an extra
dimension to conformational studies on dppm-type sys-
tems by replacing one of the methylene H-atoms by
Me; the ligand is then 1,1-bis(diphenylphos-
phino)ethane, abbreviated as dppmMe to signify the
methylated dppm. Our initial interest in this ligand
evolved from the possibility of immobilizing a Pd2
moiety akin to Pd2Cl2(dppm)2 (2) on a polystyrene
support for use in separation of gases [12]. The
Pd2Cl2(dppmMe)2 complex now gives rise to both syn-
and anti-isomers, depending on the disposition of the
Me groups with respect to the PdꢀCꢀPd plane.
NOE spectra were recorded at ꢀ300 K on a Varian
Unity Inova (400 MHz) spectrometer on 2.2×10−2
M
CDCl3 solutions of the complexes.
3. Results and discussion
The structure of Pd2Cl2(dppm)2 (2) (Fig. 2) is very
similar to that of the bromo analog [23], and has the
same chair-like conformation of the Pd2P4C2 ring.
Complete details of the bromo complex have neither
been published nor deposited in the Cambridge data-
base, but the chloro and bromo complexes have the
same space group and very similar unit cell parameters;
the major difference, besides those expected for PdꢀCl
and PdꢀBr bond lengths, is in the PdꢀPd bond length,
We have established the reactivity order 2\syn-1\
anti-1 for the formation of A-frame species with m-CO,
m-SO2, and m-X (X=S or Se, formed from H2X, or
elemental S or Se), and rationalized the findings on
steric arguments [13,14], based largely on structural
data for anti-1 [12] and anti-Pd2Cl2(m-Se)(dppmMe)2
[14]. The present paper is in part not only a mini review
but also reports new X-ray structural data for syn-1
and 2, which allow a more definitive discussion of the
structure–reactivity relationship, as well as NMR/NOE
data on syn- and anti-1 and their m-Se adducts, which
reflect on the solution structures.
,
which is shorter (2.667(1) A) in 2 than in the bromo
,
complex (2.699(5) A), and is consistent with the weaker
trans effect of the chlorine.
The X-ray analysis on syn-Pd2Cl2(dppmMe)2 (Fig. 1)
confirms our earlier conclusion that this isomer has an
extended boat conformation for the Pd2P4C2 ring, with
the Me groups occupying equatorial positions. One
interesting feature of syn-1 is the significantly shorter

G. Besenyei et al. / Inorganica Chimica Acta 327 (2002) 179–187
181
Table 2
,
PdꢀPd bond (2.569(1) A) than in the anti-isomer
(2.664(1) A) [12]; indeed, this is the shortest PdꢀPd
,
Selected bond lengths (A) and bond angles (°) of syn-1, anti-1, and 2
,
a
bond of all diphosphine-bridged Pd2 and Pt2 deriva-
syn-1
anti-1
2
,
tives, which vary from 2.57 to 2.77 A (see Table 3,
Series A). Generally, the shortest PdꢀPd bond (2.488(1)
Bond lengths
Pd(1)ꢀPd(2)
Pd(1)ꢀCl(1)
Pd(2)ꢀCl(2)
Pd(1)ꢀP(1)
Pd(1)ꢀP(3)
Pd(2)ꢀP(2)
Pd(2)ꢀP(4)
P(1)ꢀC(2)
2.569(1)
2.399(1)
2.374(1)
2.291(1)
2.303(1)
2.320(1)
2.290(1)
1.856(2)
1.875(2)
1.871(3)
1.850(2)
2.664(1)
2.420
2.401
2.297
2.297
2.294
2.294
1.839
1.849
1.861
1.862
2.661(1)
2.410(2)
2.423(2)
2.295(1)
2.278(1)
2.300(1)
2.314(1)
1.827(6)
1.819(5)
1.824(5)
1.836(5)
Table 1
Crystal data and structure refinement parameters for syn-
Pd2Cl2(dppmMe)2(syn-1) and Pd2Cl2(dppm)2 (2)
syn-Pd2Cl2(dppmMe)2 Pd2Cl2(dppm)2
P(2)ꢀC(2)
P(3)ꢀC(1)
P(4)ꢀC(1)
Empirical formula
C52H48Cl2P4Pd2·
2CH2Cl2
C50H44Cl2P4Pd2·
H2O
Formula weight
Radiation and
1250.34
Mo Ka, 0.71073
1070.45
Cu Ka, 1.54184
Bond angles
P(3)ꢀC(1)ꢀP(4)
P(1)ꢀC(2)ꢀP(2)
Cl(1)ꢀPd(1)ꢀPd(2)
Cl(2)ꢀPd(2)ꢀPd(1)
P(1)ꢀPd(1)ꢀP(3)
P(2)ꢀPd(2)ꢀP(4)
104.1(1)
102.1(1)
169.02(3)
171.00(2)
171.5(1)
170.60(2)
105.9
106.5
177.5
177.0
175.5
173.4
105.8(2)
108.7(3)
173.79(4)
178.78(4)
171.9(1)
175.5(1)
,
wavelength (A)
Crystal system
Space group
triclinic
monoclinic
P21/c
(
P1
Unit cell dimensions
,
a (A)
11.974(1)
13.524(1)
18.197(2)
99.66(1)
90.25(1)
110.83(1)
2708.5(4)
2
13.576(4)
16.419(3)
21.603(2)
,
b (A)
,
c (A)
a Data from Ref. [12].
h (°)
i (°)
106.37(1)
k (°)
3
,
,
V (A )
Z
4620.2(17)A
4
1.539
8.937
Dcalc (mg m−3
Absorption
)
1.533
1.114
coefficient (mm−1
)
F(000)
1260
2160
Crystal size (mm)
Max/min
0.35×0.30×0.05
0.9871, 0.8445
0.30×0.15×0.15
0.9901, 0.6380
transmission
q Range (°)
Index ranges
2.165q534.98
−195h50;
−205k521;
−295l529
4.335q574.89
−165h517;
−205k50;
−275l50
9702
Reflections collected 23671
Completeness to 2q 0.99
0.965
Number of standard
reflections
3
3
Decay (%)
19.0
9
Reflections observed 13631
I\2|(I)
7062
Independent
reflections
Refinement method
23671 [Rint=0.0095]
9169 [Rint=0.0278]
Fig. 1. Molecular diagram of syn-Pd2Cl2(dppmMe)2 (syn-1) showing
the numbering scheme used. Atomic displacement parameters repre-
sent 50% probabilities. H-atoms and one solvent molecule are omit-
ted for clarity.
full-matrix
full-matrix
least-squares on F2
23671/52/598
least-squares on F2
9169/0/527
Data/restraints/
parameters
Final R indices
[I\2|(I)]
R indices (all data)
R1=0.0477,
wR2=0.1180
R1=0.1035,
wR2=0.1306
0.002, 0.000
R1=0.0639,
wR2=0.1662
R1=0.0813,
wR2=0.1768
0.001, 0.000
,
A) is found in the non-bridged complex
[Pd2(MeCN)4(PPh3)2]2+ (Table 3, Series B), while this
2+
,
bond is stretched to 3.185(1) A in [Pd2(m-C4H6)2]
which has bridging 1,3-butadiene molecules [26].
,
Max. and mean
shift/esd
The other interesting feature of syn-1 is the relatively
large dihedral angle (b) between the two coordination,
least-squares planes formed by the two metal centers
(b=49.1°, see Table 3). The degree of twisting is larger
than that found in anti-1 (37.4°) [12], and in all the
other Series A complexes, except for Pd2Br2(dmpm)2
Goodness-of-fit on
0.993
0.973
F2
Largest difference
2.033 and −1.542
3.307 and −1.825
peak and hole
−3
,
(e A
)

182
G. Besenyei et al. / Inorganica Chimica Acta 327 (2002) 179–187
Fig. 3. Dependence of metal–metal distances on the dihedral angles
of metal ion coordination planes (b) for bis(diphosphine)-bridged
(Series A) and unsupported (Series B) Pd2 and Pt2 complexes.
Fig. 2. Molecular diagram of Pd2Cl2(dppm)2 (2) showing the number-
ing scheme used. Atomic displacement parameters represent 50%
probabilities. H-atoms and solvent molecules are omitted for clarity.
(b=50.5°), where more severe twisting about the
PdꢀPd bond was attributed to the sterically less de-
manding dmpm ligand (vs. dppm) [24c]. Clearly, the
data presented here for syn-1 show that the incorpora-
tion of a Me group at the methylene of dppm can also
lead to increased twisting about the PdꢀPd axis. The
conformation of the eight-membered Pd2P4C2 ring most
likely determines the twisting of the two PdP2Cl coordi-
nation planes about the metal–metal bond. A model of
syn-1 in a boat conformation, with equatorial Me
groups and the two PꢀPdꢀP axes oriented parallel to
each other, shows four axial Ph groups on one side of
the Pd2P4C2 ring. The resulting unfavorable steric inter-
actions are minimized via a twisting about the PdꢀPd
axis; this also minimizes the repulsive overlap of the
out-of-plane, filled metal dp orbitals [24d,25g]. These
two factors are probably synergistic, and result in a
structure with an unusually large dihedral angle and a
short (strong) PdꢀPd bond.
The relationship between dihedral angles and metal–
metal bond lengths in Pd2 complexes has been discussed
earlier [24c,27], nevertheless, we show here more exten-
sive data for both Pd2 and Pt2 complexes (Table 3, Fig.
3) that lead to more definitive conclusions. Two major
factors governing b are: (i) the repulsive interactions of
the metal dp orbitals, which are minimal at 45°; and (ii)
the steric repulsions of ligands cis to the metal–metal
bond, which are minimized at 90° [24c,24d,25g,27]. In
agreement with this, the dihedral angles fall between 45
and 90° in ‘unsupported’, dimeric square-planar com-
plexes (Table 3, Series B). With the exception of
[Pt2(CO)2Cl4]2−, where b=60° [25g], the dihedral an-
gles are larger, in the 69–89° range; ligand–ligand
repulsions clearly decide the molecular geometry, and
there is no obvious relationship between b and the
metal–metal bond length for the very diverse set of
non-bridging ligand systems (Table 3, Fig. 3, line B).
On the contrary, the metal–metal bond lengths in
complexes with bridging diphosphines appear to de-
crease with an increase in the dihedral angle (Table 3,
Fig. 3, line A). That the values are in the 33–50.5°
range suggests that the orbital repulsive interactions are
dominant, although rotation around the metal–metal
Table 3
A compilation of data for some dimeric Pd(I) and Pt(I) complexes a
,
Compound
M–M (A)
b (°) Reference
Series A, bis(diphosphine)-bridged complexes
Pd2Br2(dppm)2
Pd2Cl2(dppm)2
2.699(5)
2.661(1)
2.644(2)
2.670(2)
2.594(2)
2.665(1)
2.664(1)
39
[23]
t.w.
[24a]
[24b]
[3]
38.7
41.3
45.0
44.5
40.0
37.4
Pd2Cl(SnCl)3(dppm)2
Pd2(C6Cl5)2(dppm)2
Pd2(CF3CO2)2(dppm)2
[Pd2(HpyS)2(dppm)2]2+
anti-Pd2Cl2(dppmMe)2
(anti-1)
[24c]
[12]
syn-Pd2Cl2(dppmMe)2 (syn-1) 2.569(1)
49.1
50.5
38.5
38.6
40.1
42
40.0
33.5
37.1
39.6
47.2
t.w.
Pd2Br2(dmpm)2
2.603(1)
2.646(1)
2.651(1)
2.620(1)
2.665(2)
2.642(1)
2.769(1)
2.615(1)
2.627(2)
2.580(1)
[24d]
[24e]
[24f]
[24g]
[24h]
[24i]
[24j]
[24k]
[24k]
[24k]
Pd2Cl2(dcpm)2
Pt2Cl2(dppm)2
[Pt2Cl(CO)(dppm)2]+
[Pt2Cl(Ph3P)(dppm)2]+
[Pt2(CO)2(dppm)2]2+
[Pt2H(p1-dppm)(dppm)2]+
[Pt2(dppm)2(quin)2]2+
[Pt2Cl(dppm)2(tmpy)]+
[Pt2(dppm)2(mim)2]2+
Series B, non-bridged complexes
[Pd2(CH3NC)6]2+
Pd2I2(CH3NC)4
2.531(1)
2.533(1)
2.518(3)
2.599(2)
2.617(2)
2.602(1)
2.598(1)
2.532(2)
2.488(1)
2.584(2)
86.2
85.3
81
69
86
78.0
89.0
82.7
76.3
60
[25a]
[25b]
[25c]
[25c]
[25d]
[25d]
[25e]
[25f]
[26]
[Pd2(bipy)2(ArNC)2]2+
[Pd2(phenMe2)2(ArNC)2]2+
[Pd2(dppp)2(ArNC)2]2+
[Pd2(dppen)2(ArNC)2]2+
[Pd2(PMe3)6]2+
Pd2Cl2(t-BuNC)4
[Pd2(MeCN)4(PPh3)2]2+
[Pt2(CO)2Cl4]2−
[25g]
a t.w.=this work; HpyS=4-mercaptopyridine, bound as a thione;
dmpm=bis(dimethyl-phosphino)methane; dcpm=bis(dicyclohexyl-
phosphino)methane;
quin=quinoline;
tmpy=2,4,6-trimethyl-
pyridine; mim=1-methyl-imidazole; bipy=2,2%-bipyridine; Ar=
aryl; phenMe2=2,9-dimethyl-1,10-phenanthroline; dppen=cis-1,2-
bis(diphenylphosphino)ethene.

G. Besenyei et al. / Inorganica Chimica Acta 327 (2002) 179–187
183
bond is restricted by the bridging nature of the PꢀP
ligand. For a fixed ring size, steric factors depending on
the nature of the substituent(s) at the P-atoms, the size
of the ligand trans to the M–M bond, and sometimes
the intramolecular H-bonded interactions [27], must
also play a role and could, for example, contribute to
the scatter around the least-squares line A.
Besides the PdꢀPd distance, other bond lengths of
syn-1 are close to those reported for anti-1 (Table 2).
The bond angles, however, reveal the more strained
geometry of syn-1. For example, the XPdY bond angles
in syn-1 deviate more from 90°; the differences in the
largest and smallest angles around Pd(1) and Pd(2) are
12.3 and 11.1°, respectively, while the respective num-
bers are 2.1° and 6.7° in anti-1. Severe distortions from
the tetrahedral geometries around P(1) and P(4) with
125.3 and 121.9° angles are seen in syn-1, accompanied
by PdꢀPdꢀCl and PꢀPdꢀP angles of 169.5 and 171.0°,
respectively, with the deviation from linearity for the
anti isomer (177.2 and 174.5°) being much smaller.
It is noteworthy that while three of the four ‘diago-
nal’ Pd···P distances in the two Pd2P2C rings in syn-1
time (Table 4). Two sets of four Ph groups are distin-
guished in syn- and anti-1, and in syn-1(m-Se), while
four pairs of equivalent Ph rings are seen for anti-1-
(m-Se). The enhancements of resonance induced by
intramolecular dipole–dipole relaxation between inter-
acting nuclei are given in Table 5.
For anti-1, irradiation of the Me hydrogens results in
the enhancement of both sets of the o-protons, showing
that the Me groups are roughly equidistant from both
sets of Ph rings, i.e. they maintain their equatorial
position in solution. The axial orientation of the CH
protons allows steric interactions only with the o-hy-
drogens of the equatorial Ph rings, their distances from
the other set of o-protons being much larger. These
observations confirm that the conformations of anti-1
in solution and in the solid state are the same, viz. an
extended chair arrangement with equatorial Me groups.
Unlike the dppm-bridged complexes [17], interchange
of the axial and equatorial positions at the bridging
C-atoms is hampered by the Me substituents.
The NOE difference spectra of syn-1 reveal corre-
sponding, intramolecular steric effects, with the conclu-
sion that the solution conformation has equatorial Me
groups, but now with an extended boat arrangement.
Fig. 4, showing a superimposition of the central core of
syn- and anti-1 looking down the PdꢀPd axis, clearly
illustrates the positioning of the equatorial Me groups
in both isomers.
The NMR spectra of syn-1(m-Se) (see above) reveal
the C26 symmetry of the solution geometry, but do not
give information on whether the equivalent Me groups
are equatorial or axial. The NOE spectra, however,
reveal dipole-dipole interactions of the CH-hydrogens
only with one set of the o-hydrogens, while both sets of
o-protons give NOE effects when the Me resonance
was saturated; the Me groups thus maintain their equa-
torial orientation in the (m-Se) derivative within an
extended boat conformation. This conserved orienta-
tion, evident in solution reactivity, is readily pictured as
it involves no steric interaction of the approaching
reagent with axially oriented Me groups, and is the
major factor making syn-1 more reactive than anti-1
(see below).
,
are somewhat shorter (3.245–3.377 A) than the sum of
,
the van der Waals radii (3.45 A), there are no such
contacts in anti-1 [12]. This feature resembles the inter-
actions observed sometimes in non-bridged dimeric
complexes where equatorial phosphine and isonitrile
ligands are bent towards their non-bonded metal cen-
ters [25a,e]. The contacts may just result from the larger
dihedral angle discussed above, as other close contacts
,
are also apparent; e.g. in syn-1, [P(1)···P(2) 2.902 A and
,
P(3)···P(4) 2.933 A] compared with the corresponding
,
values of 2.955 and 2.970 A in anti-1, and 2.962 and
,
2.919 A in 2.
Before solution reactivity is considered, it is impor-
tant to establish the solution structures in relation to
the solid-state structures discussed above. Complexes of
type 2 are known to be fluxional in solution with the
methylene H-atoms being rendered equivalent by a
rapid interchange of the axial and equatorial positions
via a ring flipping motion; the same motion also makes
the Ph groups equivalent, and only one set of o-, m-,
and p-protons is seen in 1H NMR [17]. In order to shed
light on the conformational features of Pd2Cl2-
(dppmMe)2 complexes in solution, 1H NMR/NOE
spectra were recorded to map the intramolecular steric
The reaction of anti-1 to form anti-1(m-Se) results in
a change from a chair into boat conformation, and one
of the Me groups flips from an equatorial to axial
position [14]. Accordingly, the equatorial Me in anti-
1(m-Se) interacts with two sets of o-hydrogens, while
the other axial Me detects the proximity of only one set
of o-protons (Table 5); consistent with this, the CH-hy-
drogen adjacent to the axial Me shows dipole–dipole
relaxation with two sets of o-hydrogens (Table 5). Of
the complexes studied here, this is the only case where
a CH-hydrogen interacts with two sets of o-hydrogen
atoms.
1
interactions. The H and 31P{1H} NMR spectra of syn-
and anti-1, and their derived m-Se adducts (Table 4),
have been reported earlier [11–13]. The assignments are
straightforward: in syn- and anti-1, and syn-1(m-Se), the
two CH (and Me) protons are equivalent, as are the
P-atoms, and virtual coupling to the remote P-atoms
gives the expected splitting patterns. For anti-1(m-Se),
the two CH (and Me) protons become inequivalent,
and coupling occurs only with the adjacent P-atoms.
The 13C NMR spectra are reported here for the first

184
G. Besenyei et al. / Inorganica Chimica Acta 327 (2002) 179–187
Table 4
NMR spectroscopic data for syn- and anti-1 and their m-Se adducts
Compound
Carbon atoms
Hydrogen atoms
a
a
lC
JCP (Hz)
Position
lH
J
HH/JHP (Hz)
Position
syn-1
15.16 (qn)
41.64 (qn)
aromatic carbons
135.90
132.85
130.92
1.6
9.8
methyl (e)
methine
1.09 (d qn)
4.84 (q qn)
set 1
7.54
7.33
7.0/6.6
7.0/5.5
methyl (e)
methine (a)
n.a.
n.a.
quaternary
n.a.
ortho
para
meta
7.16
set 2
130.59
129.68
129.63
128.18
quaternary
n.a.
n.a.
7.63
7.31
7.23
ortho
para
meta
127.69
n.a.
anti-1
15.02 (m)
42.22 (qn)
Aromatic carbons
137.01
132.81
132.00
methyl (e)
methine
1.05 (d qn)
4.94 (q qn)
set 1
7.76
7.46
6.8/6.1
6.8/6.2
methyl (e)
methine (a)
9.0
n.a.
n.a.
quaternary
n.a.
ortho
para
meta
7.45
set 2
130.54
129.70
129.47
128.45
n.a.
quaternary
n.a.
7.22
7.13
6.80
para
ortho
meta
127.36
n.a.
syn-1(m-Se)
14.87 (b, m)
30.65 (qn)
Aromatic carbons
Set 1
methyl (e)
methine
0.98 (d qn)
5.61 (q qn)
7.2/6.1
7.2/7.3
methyl (e)
methine (a)
14.7
132.48 (4C)
132.10 (8C)
129.69 (4C, bs)
128.43 (8C)
set 2
quaternary
ortho
para
7.71
7.34
7.33
ortho
para
meta
meta
136.74 (8C)
130.64 (4C)
127.39 (8C)
127.3 (4C)
ortho
para
meta
quaternary
7.54
7.32
7.21
ortho
para
meta
anti-1(m-Se)
14.74 (t)
30.37 (t)
3.8
26.7
methyl (e)
methine
0.98 (d t)
5.88 (t q)
7.2/10.1
7.2/13.8
methyl (e)
methine (a)
Aromatic carbons
Set 1
132.17 (4C)
131.9 (2C)
129.83 (2C, bs)
128.67 (4C)
Set 2
ortho
quaternary
para
7.89
ortho
7.47
7.48
para
meta
meta
137.01 (4C)
130.62 (2C, bs)
127.32 (4C)
126.6 (2C)
21.99 (t)
ortho
para
meta
quaternary
methyl (a)
methine
7.38
7.23
7.02
ortho
para
meta
3.0
22.2
1.38 (d t)
4.08 (m)
7.1/13.0
methyl (a)
methine (e)
37.38 (t)
Aromatic carbons
Set 3
137.32 (4C)
132.3 (2C)
130.62 (2C, bs)
127.90 (4C)
Set 4
ortho
quaternary
para
7.92
ortho
7.37
7.32
para
meta
meta
134.34 (4C)
133.2 (2C)
129.68 (2C, bs)
127.52 (4C)
ortho
quaternary
para
7.66
ortho
7.26
7.18
para
meta
meta
a In ppm, relative to TMS; n.a.=not assigned; bs=broad singlet; d=doublet; q=quartet; qn=quintet; t=triplet; a=axial; e=equatorial.

G. Besenyei et al. / Inorganica Chimica Acta 327 (2002) 179–187
185
Table 5
Enhancements of selected resonances in NOE difference spectra a
Irradiated resonances
Resonances observed
CH3
CH
o-Protons
l
p
l
p
l
p
anti-1
CH (l 4.94)
CH3 (l 1.05)
1.05
2.1
7.76
7.13
7.76
9.9
3.8
2.4
4.94
4.84
5.62
7.2
syn-1
CH (l 4.84)
CH3 (l 1.09)
1.09
0.98
2.2
1.7
7.63
7.54
7.63
9.7
4.0
3.0
7.7
7.9
syn-1(v-Se)
CH (l 5.62)
CH3 (l 0.98)
7.71
7.55
7.71
9.0
5.2
3.5
anti-1(v-Se)
CH (l 5.88)
CH3 (l 0.98)
0.98
1.38
2.3
7.89
7.38
7.89
7.66
7.92
7.92
12
5.88
4.08
7.0
6.7
5.1
2.9
6
3.5
4.1
b
CH (l 4.08)
CH3 (l 1.38)
a p=100 (I−Io)/Io, where I is the integrated intensity for one proton.
b Not determined.
It is noteworthy that the NOE experiments reveal an
erroneous assignment regarding the Me groups and
CH-hydrogens in anti-1(m-Se) [14], where the lMe 1.38
and lH 5.88 resonances were assigned to geminal posi-
tions on the methine C-atom of dppmMe; the NOE
data show that the lMe 0.98 is ‘associated with’ the lH
5.88 resonance.
A-frame adducts formed from anti-1 (see above). The
reactivity trend: 2\syn-1\anti-1 (e.g. for the reaction
with CO in CH2Cl2, the normalized equilibrium con-
stants are in the following order 1:ꢀ10−3:ꢀ10−5
[13,14]) is obviously governed by the effects of the Me
groups.
Essentially, formation of a boat conformer within an
A-frame adduct takes place when a reagent approaches
the PdꢀPd bond of any of the Pd2Cl2(dppm/dppmMe)2
complexes. Irrespective of the side attacked in anti-1,
the entering reagent will experience steric hindrance by
an axial Me. In 2, formation of the boat conformer
generates only axial H-atoms. In syn-1, the boat con-
formation with equatorial Me groups is not changed
during a reaction at the PdꢀPd bond, and interactions
with the Me-groups are irrelevant; the steric effects of
the axial Ph-groups on the other side of the Pd2P4C2
plane probably make syn-1 less reactive than 2.
The spontaneous but slow isomerization of syn-1 to
anti-1 (t1/2ꢀ15 days in CHCl3 at ꢀ20°) thus involves
the conversion of an extended boat conformation to a
chair form, with Me groups in both isomers occupying
equatorial positions. The strains reflected in steric con-
gestion in syn-1 versus anti-1 must contribute to this
conversion, as well as to a higher reactivity of the
former (see Section 1). The other key factor is the
configuration adopted by the respective A-frame ad-
ducts: e.g. in anti-1, the Me groups occupy the less
sterically crowded pseudoequatorial positions of the
fused five-membered, chair conformation chelate rings
[12], while in the A-frame product anti-1(m-Se) both
the twisted rings adopt boat conformations with the
methylene C-atoms being bent towards the Se bridge
and one of the Me groups is twisted into a less steri-
cally favored axial position (inside the boat) [12].
The structure of 2 is remarkably similar to that of
anti-1 (except for the presence of the pseudoequatorial
Me group) and the much higher reactivity of 2 [12]
must again result from the unfavorable sterics in the
Fig. 4. A superimposition of the central core of syn-1 (solid line) and
anti-1 (dashed line) looking down the PdꢀPd axis.

186
G. Besenyei et al. / Inorganica Chimica Acta 327 (2002) 179–187
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4. Summary
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The structural data for Pd2Cl2(dppm)2 (2) and syn-
Pd2Cl2(dppmMe)2 (syn-1) support the earlier conclu-
sions regarding their geometries. The boat
conformation of the Pd2P4C2 ring and the equatorial
orientation of the Me groups in syn-1 (both in solid
state and solution) have been confirmed. NOE spectra
show that the Me groups retain their equatorial posi-
tions in the A-frame adduct syn-Pd2Cl2(m-
Se)(dppmMe)2. The higher reactivity of syn-1 can be
ascribed to the presence of a relatively open space
about the PdꢀPd bond on one side of the Pd2P4C2 ring,
as well as possible contributions from its distorted
geometry manifested by the strained bond angles at the
Pd- and P-atoms. Because of steric congestion exercised
by four Ph-rings on the ‘methyl side’ of the Pd2P4C2
ring, large dihedral angles of the two metal-centered
planes can be expected for syn-[Pd2Cl2(m-X)(dppmMe)2]
A-frame derivatives.
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5. Supplementary material
[18] K. Harms, XCAD4 Data Reduction Program for CAD4 Diffrac-
tometers, Philipps-Universita¨t, Marburg, Germany, 1996.
[19] (a) A.C. North, D.C. Philips, F. Mathews, Acta Crystallogr. A
24 (1968) 350;
Crystallographic data for structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 172852 for complex syn-1,
Pd2Cl2(dppmMe)2 and 172851 for structure 2,
Pd2Cl2(dppm)2. Copies of this information may be ob-
tained free of charge from The Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ, UK (fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
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[21] G.M. Sheldrick, SHELXL-97 Program for Crystal Structure
Refinement, University of Go¨ttingen, Go¨ttingen, Germany,
1997.
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Acknowledgements
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