A five coordinate PdII complex stable in solution and in the solid state

Article abstract of DOI:10.1039/b305547g

The treatment of the strained complex TrpyPdOAc^F 1 with NaBAr^F, followed by the addition of trimethylphosphine, yields the stable cationic 16VE- or 18VE-complexes 3 and 4, depending on the amount of phosphane added.

Full text of DOI:10.1039/b305547g

A five coordinate PdII complex stable in solution and in the solid state
Martin Bröring* and Carsten D. Brandt
Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße, D-35043 Marburg, Germany.
E-mail: Martin.Broering@chemie.uni-marburg.de
Received (in Cambridge, UK) 19th May 2003, Accepted 7th July 2003
First published as an Advance Article on the web 18th July 2003
The treatment of the strained complex TrpyPdOAcF 1 with
NaBArF, followed by the addition of trimethylphosphine,
yields the stable cationic 16VE- or 18VE-complexes 3 and 4,
depending on the amount of phosphane added.
The cationic monophosphane complex 38 displays the
expected pseudoplanar coordination geometry at the PdII centre.
As indicated by the N(2)–Pd–P angle of 141.48°, the PdN3P
core of 3 deviates markedly from planarity. The phosphane-
…tripyrrin distances C(1)…C(30) and C(16)…C(30) are as
short as 3.591 and 3.450 Å, and the methyl termini of 3 are bent
out of the planes of the adjacent pyrrole moieties by 7.6° [C(1)]
and 9.0° [C(16)], respectively. This data accounts for a large
amount of intramolecular strain, equal to a significant destabili-
sation of the 16VE species.
The molecular structure of the 18VE diphosphane complex
49 shows a non-strained square-pyramidal coordination of the
PdII ion with two equatorial P donors trans to each other and
with the central N(2) in the apical position. Within the PdN3P2
polyhedron the angles deviate from the ideal 90° by a maximum
of 5.1°. The deviation from the mean square plane through N(1),
N(3), P(1) and P(2) is small (0.132 Å), and the PdII ion is located
undistorted in the centre of this plane. Due to the antibonding
interaction of the lone pair of N(2) with the filled dz2 orbital at
PdII ions usually form very stable square planar 16VE
2
complexes due to a high energy non-occupied 4dx 2y2 orbital.
Ligand exchange reactions on these tetracoordinate complexes
always proceed via an associative mechanism, with a square-
pyramidal or trigonal-bipyramidal 18VE species being the
reactive intermediates.1 Amatore and Jutand have recently
shown by electrochemical methods, that anionic 18VE com-
plexes play a major role in all important palladium catalysed C–
C coupling reactions.2 In the past, many such five-coordinate
PdII species have been characterised in the solid state by X-ray
diffraction. In solution, however, these compounds are mainly
dissociated, and only four-coordinate 16VE species could be
observed directly.
We have attempted to prepare a five-coordinate PdII
complex, which is stable towards dissociation, by destabilising
the respective 16VE species. PdII complexes of the new
tripyrrin ligands3 are of particular use in this regard, since the
terminal functionalities at the C14N3 backbone are positioned
directly in the donor plane of square-planar metal complexes,
and effectively shield the space segment required for the
binding of the fourth donor. As we have shown earlier on
methyl terminated palladium tripyrrinates, the desired destabili-
sation of a four-coordinate PdII ion originates as a consequence
of the steric interaction between the fourth ligand and the
methyl termini, and becomes visible in strained, non-planar
coordination modes and a fluxional behaviour (Scheme 1).4
Starting from the neutral trifluoroacetate complex 1 we
sought to optimize the effect of destabilisation by exchanging
the trifluoroacetate ligand against the more bulky PMe3 donor.
This was performed in two steps. Salt metathesis of 1 with
NaBArF (sodium tetrakis[3,5-bis(trifluormethyl)phenyl]bor-
ate5) quantitatively gave the cationic complex 2 (most likely
with L = CH2Cl2) as a green solution. This solution of 2 was
filtered from precipitated sodium trifluoroacetate and then
treated with trimethylphosphine to produce the mono- and
diphosphane complexes 36 and 4,7 respectively, depending on
the amount of phosphane used (Scheme 2). No excess
phosphane was necessary to obtain the 18VE species 4. Both
new complexes 3 and 4 were analysed by X-ray diffraction. The
molecular structures and selected data of the respective cations
are shown in Figs. 1 and 2.
Scheme 2 PMe3 addition to [TrpyPd(L)]+BArF2 2.
Fig. 1 ORTEP plot of the molecular structure of 3. Selected bond lengths
[Å] and angles [°]: Pd–N1 2.020(3), Pd–N2 2.052(3), Pd–N3 2.021(3), Pd–
P 2.3136(12), N1–Pd–N3 167.90(12), N2–Pd–P 141.48(8).
Scheme 1 Fluxional structure of the strained tripyrrin complex TrpyPd-
OAcF 1.
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CHEM. COMMUN., 2003, 2156–2157
This journal is © The Royal Society of Chemistry 2003

University Science Books, New York, 1987 ; C. Elschenbroich,
Organometallchemie, 4th Edn., Teubner, Stuttgart, 2003.
2 C. Amatore and A. Jutand, Acc. Chem. Res., 2000, 33, 314.
3 C. Mazet and L. H. Gade, Chem. Eur. J., 2003, 9, 1759; A. Krivokapic,
A. R. Cowley and H. L. Anderson, J. Org. Chem., 2003, 68, 1089; H.
Furuta, H. Maeda and A. Osuka, Inorg. Chem. Commun., 2003, 6, 162;
M. Bröring and C. D. Brandt, Chem. Commun., 2001, 499; J. L. Sessler,
A. Gebauer, V. Král and V. Lynch, Inorg. Chem., 1996, 35, 6636; A.
Jaumà, J.-A. Farrera and J. M. Ribó, Monatsh. Chem., 1996, 127, 936.
4 M. Bröring and C. D. Brandt, J. Chem. Soc., Dalton Trans., 2002, 1391;
M. Bröring, C. D. Brandt and S. Prikhodovski, J. Porphyrins Phthalocya-
nins, 2003, 7, 17.
5 M. Brookhart, B. Grant and A. F. Volpe, Organometallics, 1992, 11,
3920.
6 Spectroscopic data for 3: mp. 160 °C (decomp.); 1H NMR (CD2Cl2): d =
1.09 (t, 6H, 3J = 7.6 Hz, 2 3 CH2CH3), 1.18 (t, 6H, 3J = 7.6 Hz, 2 3
CH2CH3), 1.21 (t, 6H, 3J = 7.6 Hz, 2 3 CH2CH3), 1.33 [d, 9H, 2JPH
=
11.4 Hz, P(CH3)3], 2.42 (q, 4H, 3J = 7.6 Hz, 2 3 CH2CH3), 2.65 (q, 8H,
3J = 7.6 Hz, 4 3 CH2CH3), 2.69 (s, 6H, 2 3 terminal CH3), 6.92 (s, 2H,
2 3 meso-H), 7.58 (s, 4H, 4 3 para-H), 7.75 (s, 8H, 8 3 ortho-H); 13
C
Fig. 2 ORTEP plot of the molecular structure of 4. Selected bond lengths
[Å] and angles [°]: Pd–N1 2.0410(18), Pd–N2 2.298(2), Pd–N3 2.0354(18),
Pd–P1 2.3337(7), Pd–P2 2.3207(6), N1–Pd–N3 175.97(7), P1–Pd–P2
170.15(2), N2–Pd–P1 94.44(6), N2–Pd–P2 95.10(6).
NMR (CD2Cl2): d = 14.7, 16.9, 17.2, 17.6, 17.9, 18.0, 18.5, 20.8 [q, 1JCP
1
= 10.5 Hz, P(CH3)3], 117.9 (s, 4 3 para-C), 120.9, 125.0 (q, JCF
=
272.7 Hz, 8 3 CF3), 129.3 (qq, 2JCF = 31.5 Hz, 2JCF = 2.9 Hz, 8 3 meta-
C), 135.2 (s, 8 3 ortho-C), 138.0, 138.8, 139.0, 140.1, 150.9, 162.1 (q,
1JBC = 49.9 Hz, 4 3 ipso-C), 170.4; 19F NMR (CD2Cl2): d = 262.6; 31
P
NMR (CD2Cl2): d = 223.1; MS (FAB): m/z 598 [M 2 BArF]+; calc. for
palladium the Pd–N(2) bond is elongated by ~ 0.3 Å to 2.298
C
63H59BF24N3PPd 3 2 CH2Cl2: C 47.83, H 3.89, N 2.57; found: C 47.47,
Å.
H 3.63, N 2.47%.
The stepwise association of trimethylphosphine to the cation
7 Spectroscopic data for 4: mp. 129 °C (decomp.); 1H NMR (CD2Cl2): d =
0.71 (pseudotriplet, 18H, 6 3 PCH3), 1.08 (t, 6H, 3J = 7.6 Hz, 2 3
CH2CH3), 1.17, 1.20 (2 3 t, 12H, 3J = 7.6 Hz, 4 3 CH2CH3), 2.43 (q,
4H, 3J = 7.6 Hz, 2 3 CH2CH3), 2.66, 2.69 (2 3 q, 8H, 4 3 CH2CH3),
2.80 (s, 6H, 2 3 terminal CH3), 7.06 (s, 2H, 2 3 meso-H), 7.57 (s, 4H,
4 3 para-H), 7.73 (s, 8H, 8 3 ortho-H); 13C NMR (CD2Cl2): d = 13.6
[vt, N = 25.4 Hz, 2 3 P(CH3)3], 15.1, 17.6, 17.7, 18.1, 18.6, 18.8, 20.2,
117.9 (s, 4 3 para-C), 123.1, 125.0 (q, 1JCF = 272.6 Hz, 8 3 CF3), 129.4
(q, 2JCF = 31.5 Hz, 8 3 meta-C), 135.2 (s, 8 3 ortho-C), 135.7, 139.0,
2 in solution can easily be monitored by H- and 31P-NMR,
1
respectively. The 31P-NMR resonance shows a low-field shift
from 223.1 ppm to 218.8 ppm upon going from the tetra- to the
pentacoordinate species. The same process is indicated in the
proton NMR by a high-field shift and change in habitus of the
PMe3-proton resonance from 1.23 ppm (doublet, 3) to 0.71 ppm
(pseudotriplet, 4). No free phosphane was detected in either
spectra. These NMR results prove the five-coordinate structure
of 4 to be stable in solution.
Of particular interest is the seemingly paradoxical finding,
that the coordination number of a metal ion can be increased by
decreasing the number of accessible coordination sites. This
aspect is the major difference to the usual approach of blocking
coordination sites by large, bulky substituents, which always
renders higher coordination numbers sterically impossible. The
use of small, well-positioned substituents to introduce steric
repulsion only at very small segments of the coordination
sphere of a metal ion may be a generally applicable method for
the stabilisation of “reactive” intermediates. We are currently
investigating this opportunity in our laboratory.
140.1, 144.1, 152.4, 162.1 (q, 1JBC = 49.8 Hz, 4 3 ipso-C), 168.3; 19
F
NMR (CD2Cl2): d = 262.7; 31P NMR (CD2Cl2): d = 218.8; MS
(FAB): m/z 674 [M 2 BArF]+; calc. for C66H68BF24N3P2Pd 3 CH2Cl2:
C 48.83, H 4.22, N 2.59; found: C 49.10, H 4.16, N 2.47%.
8 Crystal data for C63H59BF24N3PPd 3: violet blocks, M = 1462.31,
monoclinic, space group P 21/c, a = 12.9944(11), b = 20.5527(17), c =
25.0962(21) Å, b = 104.331(1)°, U = 6493.9(9) Å3, Z = 4, Dc = 1.496
g cm23, m = 0.421 mm21, F(000) = 2960, 66958 reflections collected
(1.30 < q < 26.37°) at 173(2) K, 13271 independent (Rint = 0.0619),
10167 used in the structure refinement; R1 = 0.0630 [I > 2s(I)], wR2
=
0.1456 (all data), GOF = 1.099 for 1048 parameters and 6 restraints,
largest difference peak, hole = 0.847, 20.517 e Å23. CCDC 195926. See
http://www.rsc.org/suppdata/cc/b3/b305547g/ for crystallographic data
in .cif or other electronic format.
This work was funded by the Deutsche Forschungsge-
meinschaft (Emmy-Noether-Programm) and the Fonds der
Chemischen Industrie. The authors thank Professor Helmut
Werner for his generous support.
9 Crystal data for C66H68BF24N3P2Pd 4: violet blocks, M = 1538.38,
¯
triclinic, space group P 1, a = 14.5341(11), b = 15.1714(11), c =
17.1442(12) Å, a = 95.9710(10), b = 97.2310(10), g = 105.8260(10)°,
U = 3570.0(4) Å3, Z = 2, Dc = 1.431 g cm23, m = 0.408 mm21, F(000)
= 1564, 67837 reflections collected (1.21 < q < 26.37°) at 173(2) K,
14572 independent (Rint
= 0.0257), 13891 used in the structure
refinement; R1 = 0.0423 [I > 2s(I)], wR2 = 0.1012 (all data), GOF =
1.143 for 1056 parameters and 0 restraints, largest difference peak, hole
=
suppdata/cc/b3/b305547g/ for crystallographic data in .cif or other
electronic format.
Notes and references
1 J. P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke, Principles
and Applications of Organotransition Metal Chemistry, 2nd Edn.,
0.652, 20.413 e Å23. CCDC 195925. See http://www.rsc.org/
CHEM. COMMUN., 2003, 2156–2157
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