New mononuclear Pd(II) complexes of sterically hindered bispyrazolylmethanes

Article abstract of DOI:10.1016/j.ica.2009.09.016

Three new palladium(II) complexes incorporating the bispyrazolylmethane core have been synthesised and fully characterised in the solution and solid state. Single crystal X-ray studies revealed almost complete blocking of the upper face of the palladium ion by the substituents at the 3- and 5-positions of the pyrazole rings. Preliminary screening of the complexes for palladium(II) mediated catalysis revealed good catalytic activity for the Heck coupling reaction.

Full text of DOI:10.1016/j.ica.2009.09.016

Inorganica Chimica Acta 363 (2010) 1097–1101
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
New mononuclear Pd(II) complexes of sterically hindered bispyrazolylmethanes
*
Michael W. Jones , Robert M. Adlington, Jack E. Baldwin, Delphine D. Le Pevelen, Nicolas Smiljanic
Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2009
Received in revised form 20 August 2009
Accepted 2 September 2009
Available online 8 September 2009
Three new palladium(II) complexes incorporating the bispyrazolylmethane core have been synthesised
and fully characterised in the solution and solid state. Single crystal X-ray studies revealed almost com-
plete blocking of the upper face of the palladium ion by the substituents at the 3- and 5-positions of the
pyrazole rings. Preliminary screening of the complexes for palladium(II) mediated catalysis revealed good
catalytic activity for the Heck coupling reaction.
Ó 2009 Elsevier B.V. All rights reserved.
Dedicated to Jonathan R. Dilworth
Keywords:
Palladium(II) complexes
Bispyrazolylmethanes
X-ray crystal structures
1. Introduction
plex [26,32]. In addition, some of these complexes have been found
to show biological activity, with cytotoxicity of similar magnitude
Bispyrazolylmethanes (bpm) present a useful template for the
preparation of a range of transition metal complexes [1–13]. As a
result of the stabilised sp2 hybridised nitrogens in the pyrazole
rings, these complexes have given rise to derivatives which
demonstrate potential for applications in catalysis, notably Ir,
and Rh complexes, which have been used to carry out hydroamin-
ation, hydroformylation, hydrothiolation, alcoholysis reactions
[7,14,15]. Bpms are similar to the N-heterocyclic carbenes (NHCs)
which have attracted considerable attention recently as their metal
complexes are extremely active coupling reagents for a wide range
of reactions and substrates [16–18]. Even though palladium
organophosphorane complexes are the first choice for many orga-
nometallic cross-coupling reactions [19–21], palladium complexes
of the NHC ligand set have demonstrated utility and several have
become commercially available [22–24]. The bpm template repre-
sents an easily accessible, robust and stable system which has seen
relatively little study in the area of palladium(II) complexation or
as potential catalysts [1,3,25–29]. The small number of catalytic
studies have found that palladium(II)bpm complexes were able
to catalyse the oligomerization of ethylene [27]. Ullmann-type
homocoupling of aryl halides [30] and asymmetric allylic alkyl-
ation [31]. To date only limited crystallographic examples of
mononuclear palladium(II)bispyrazolylmethane complexes have
been reported [1,25–29]. It is believed that these efforts have been
hampered by the formation of bridged dimers and dimeric species
hence hindering the efforts to synthesise a monomeric LPdX2 com-
to cis-platin in in vitro studies [28].
Previous studies within the group have focussed on the expedi-
ent route towards hindered bispyrazolylmethanes which was orig-
inally disclosed by Burzlaff and co-workers [33,34]. It was
anticipated that by increasing the steric hindrance around the pyr-
azole rings through the installation of aryl and adamantyl substit-
uents at the 3- and 5-positions we would avoid the previously
encountered problems of multinuclear species and drive the for-
mation of a mononuclear complex.
2. Experimental
2.1. Materials and instrumentation
All reagents and solvents were obtained from commercial
sources (Sigma–Aldrich and Alfa Aesar) and, unless otherwise sta-
ted, were used as received. 1H NMR spectra were recorded on Brü-
ker DPX400 (400 MHz) and Brüker AVB500 (500 MHz)
spectrometers at 298 K and chemical shifts (dH) are reported in
parts per million (ppm) and are referenced to the residual solvent
peak. 13C NMR spectra were recorded on Brüker DPX400
(100 MHz) and Brüker AVB500 (125 MHz) spectrometers at 298 K
and chemical shifts (dC) are reported in parts per million (ppm)
and are referenced to the residual solvent peak. Assignments are
supported by DEPT analysis and HSQC experiments where neces-
sary. Elemental analyses were performed by the University of Ox-
ford microanalysis service. Mass spectra were recorded using
Fisons Platform spectrometer (ES mode). High resolution mass
spectra were recorded on a micromass LCT time-of-flight mass
spectrometer using the positive ion electrospray (ES+) technique.
* Corresponding author. Tel.: +44 1865 285153; fax: +44 1865 272690.
E-mail addresses: michael.jones@chem.ox.ac.uk, jonesmicheal@gmail.com
(M.W. Jones).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.09.016

1098
M.W. Jones et al. / Inorganica Chimica Acta 363 (2010) 1097–1101
Accurate masses are reported to four decimal places using tetra-
octylammonium bromide (466.5352 g molÀ1) as an internal refer-
ence. Infrared spectra were recorded on a Perkin Elmer 1000
Paragon Fourier Transform spectrometer as KBr discs. Absorption
concentrated in vacuo to afford a colourless solid. This was recrys-
tallised from abs EtOH to afford the title compound 3c as small col-
ourless prisms (18.764 g, 95%). m.p. >300 °C. m
max (KBr, cmÀ1) 2903,
2848, 1541, 1451, 1344, 1302, 1251, 1102, 793. 1H NMR (CDCl3,
400 MHz) d: 1.74 (24H, m, 12ÂCH2), 1.89 (12H, d, J 2.6 Hz,
6ÂCH2), 1.96 (12H s, 6ÂCH2), 2.02 (12H, m, 12ÂCH), 5.90 (2H, s,
CH2), 6.51 (2H, s, 2ÂpzCH). 13C NMR (CDCl3, 100 MHz) d: 28.6
(CH), 28.7 (CH), 33.9 (Cq), 34.3 (Cq), 36.5 (CH2), 37.0 (CH2), 41.1
(CH2), 42.4 (CH2), 66.4 (CH2), 100.9 (CH), 153.5 (Cq), 159.8 (Cq).
MS-ESI m/z 685 (M+, 100%), 592 (6), 325 (7), 192 (20) (HRMS Found
685.5197 C47H64N4 + H+ requires 685.5204).
maxima (m
max) are reported as wavenumbers (cmÀ1). Melting
points were recorded using a Reicher–Koffler block apparatus
and are uncorrected. The bpms 3a and b were available through
previously developed routes [35–37]. Adamantane carboxylic acid
methyl ester was prepared by the method of Chakraborti et al. [38].
2.2. Synthetic work
2.2.1. Preparation of 1,3-bis(adamantyl)-1,3-propanedione 1c
2.2.4. General procedure for formation of the palladium complexes
To a stirred solution of bpm derivative in CH2Cl2/MeOH (1:1)
was added PdCl2(MeCN)2 at room temperature. The resultant mix-
ture was stirred at room temperature for 3 h, and the solid col-
lected by filtration to afford the desired complex.
Sodium hydride (60% in mineral oil, 4.71 g, 117.80 mmol) was
washed with petroleum ether (2 Â 25 mL) at room temperature
under an atmosphere of nitrogen. Anhyd. THF (140 mL) was then
added, followed by adamantane carboxylic acid methyl ester [38]
(8.004 g, 41.23 mmol) and adamantane methyl ketone (7.00 g,
39.27 mmol). The resultant mixture was heated to reflux for
16 h. The mixture was cooled to room temperature and quenched
by the slow addition of H2O (20 mL). It was then added to a rapidly
stirred mixture of conc. HCl (50 mL) and ice (200 g). The mixture
was then extracted with Et2O (4 Â 60 mL), and the combined or-
ganic extracts were dried over anhyd. Na2SO4, filtered and concen-
trated in vacuo to afford an off-white coloured solid. This was
recrystallised from MeOH to afford the title compound 1c as col-
ourless needles (8.987 g, 67%). m.p. 268–270 °C (lit. [39] m.p.
281–283 °C). mmax (KBr, cmÀ1) 2903, 2844, 1603 (C@O), 1452,
1239, 974, 776, 620. 1H NMR (CDCl3, 400 MHz) d: 1.73 (12H, q, J
12.4 Hz, 6ÂCH2), 1.84, (12H, brs, 6ÂCH2), 2.05 (6H, brs, 6ÂCH),
5.64, (2H, s, CH2). 13C NMR (CDCl3, 100 MHz) d: 28.1 (CH2), 36.6
(CH), 39.1 (CH), 41.4 (Cq), 90.3 (CH2), 201.1 (Cq).
2.2.5. [bppmPdCl2ÁCH2Cl2] 4a
Bis[3,5-diphenyl-1H-pyrazolyl]methane (bppm) 3a [37]
(0.200 g, 0.442 mmol), PdCl2(MeCN)2 (0.115 g, 0.442 mmol),
CH2Cl2 (2.5 mL) and MeOH (2.5 mL) were treated according to
the general procedure to afford a tan coloured solid. This was crys-
tallised from CH2Cl2 and Et2O infused to afford the complex 4a as
orange prisms of X-ray quality (0.263 g, 95%). m.p. >250 °C (Found:
C, 57.98; H, 4.31; N, 8.31. C31H24Cl2N4PdÁ0.2CH2Cl2 requires: C,
57.95; H, 3.95; N, 8.66%). mmax (KBr, cmÀ1) 3443, 3048, 1556,
1484, 1463, 1451, 1380, 1265, 757, 694. 1H NMR (CDCl3,
500 MHz) d: 6.53 (1H, d, J 14.3 Hz, CH2a), 6.61, (2H, s, 2ÂpzCH),
7.16 (4H, d, J 7.7 Hz, Ph), 7.31 (4H, t, J 7.6 Hz, Ph), 7.46 (2H, t, J
7.5 Hz, Ph), 7.57 (2H, t, J 7.4 Hz, Ph), 7.64 (4H, t, J 7.5 Hz, Ph),
7.96 (1H, d, J 14.3 Hz, CH2b), 8.41 (4H, d, J 7.3 Hz, Ph). 13C NMR
(CDCl3, 125 MHz) d: 60.4 (CH2), 108.0 (CH), 126.8 (Cq), 128.57
(CH), 128.65 (CH), 128.68 (CH), 129.5 (CH), 129.6 (Cq), 130.1
(CH), 130.5 (CH), 147.2 (Cq), 156.1 (Cq). MS-ES+ m/z 597
[(MÀ35Cl)+, 88%], 588 (100), 556 (26).
2.2.2. Preparation of 3,5-bis(adamantyl)-1H-pyrazole 2c
Hydrazine monohydrate (2.51 mL, 51.64 mmol) was added
drop-wise to a stirred suspension of 1,3-bis(adamantyl)-1,3-prop-
anedione 1c (8.79 g, 25.82 mmol) in MeOH (125 mL) at room tem-
perature under atmosphere of nitrogen. The resultant mixture was
heated at reflux for 3 h. The mixture was cooled to room tempera-
ture and concentrated in vacuo. The residue was dissolved in
CH2Cl2 (150 mL) and added to H2O (150 mL). The layers were sep-
arated and the aqueous layer extracted with CH2Cl2 (3 Â 40 mL).
The combined organic extracts were dried over anhyd. Na2SO4, fil-
tered and concentrated to dryness in vacuo. The residue was then
dissolved in CH2Cl2 (30 mL) and added to pentanes (300 mL). The
resulting precipitate was collected by suction filtration to afford
the title compound 2c as colourless prisms (7.046 g, 81%). m.p.
>300 °C. mmax (KBr, cmÀ1) 3228 (NH), 2903, 2848, 1568, 1451,
1343, 1315, 1155, 1053, 995, 795. 1H NMR (CDCl3, 400 MHz) d:
1.78 (12H, q, J 13.0 Hz, 6ÂCH2), 1.95 (12H, m, 6ÂCH2), 2.07 (6H,
brs, 6ÂCH), 5.88 (1H, s, pzCH). 13C NMR (CDCl3, 125.8 MHz) d:
28.5 (CH2) 33.4 (Cq), 36.8 (CH), 42.6 (CH), 96.1 (CH), 157.8 (Cq).
MS-ESI m/z 336 (M+, 18%), 335 [(MÀH)+, 95%], 255 (45), 243 (75),
236 (46), 212 (100), 180 (46) (HRMS Found 337.2638
C23H32N2 + H+ requires 337.2638).
2.2.6. [bppm*PdCl2ÁCH2Cl2] 4b
Bis[3,5-bis(2-methoxyphenyl)-1H-pyrazolyl]methane (bppm*)
3b [37] (0.200 g, 0.350 mmol), PdCl2(MeCN)2 (0.091 g,
0.350 mmol), CH2Cl2 (2.5 mL) and MeOH (2.5 mL) were treated
according to the general procedure to afford a tan coloured solid.
This was crystallised from Et2O infused into a CH2Cl2 solution to af-
ford the complex 4b as orange prisms of X-ray quality (0.236 g,
81%). m.p. 232–234 °C (dec.) (Elemental microanalysis: Found: C,
55.41; H, 4.39; N, 7.09. C35H32Cl2N4O4PdÁ0.2CH2Cl2 requires: C,
55.12; H, 4.26; N, 7.30%). mmax (KBr, cmÀ1) 3454, 2938, 2836,
1606, 1583, 1512, 1485, 1279, 1250, 1119, 1020, 754. 1H NMR
(CDCl3, 400 MHz) d: 3.81 (6H, s, 2ÂOCH3), 3.90 (6H, s, 2ÂOCH3),
6.02 (1H, d, J 14.7 Hz, CH2a), 6.56 (2H, s, 2ÂpzCH), 6.77 (2H, t, J
7.5 Hz, Ph), 6.82 (2H, d, J 8.5 Hz, Ph), 6.94 (2H, dd, J 7.5, 1.6 Hz,
Ph), 7.03 (2H, d, J 8.3 Hz, Ph), 7.26 (2H, t, J 7.4 Hz, Ph), 7.31 (2H,
ddd, J 8.3, 7.5, 1.4 Hz, Ph), 7.48 (2H, td, J 8.1, 1.4 Hz, Ph), 7.56
(1H, d, J 14.7 Hz, CH2b), 8.53 (2H, dd, J 7.5, 1.4 Hz, Ph). 13C NMR
(CDCl3, 100 MHz) d: 53.5 (CH2), 55.6 (OCH3), 55.9 (OCH3), 60.6
(CH2), 110.9 (CH), 111.2 (CH), 111.4 (CH), 115.9 (Cq), 119.8 (Cq),
120.5 (CH), 120.9 (CH), 131.0 (CH), 131.1 (CH), 131.9 (CH), 132.0
(CH), 142.4 (Cq), 152.8 (Cq), 156.0 (Cq), 157.3 (Cq). MS-ES+ m/z
712 [(MÀ35Cl)+, 25%], 676 (100).
2.2.3. Preparation of bis[3,5-bis(adamantyl)-1H-pyrazol-1-yl]methane
(bapm) 3c
A vigorously stirred mixture of 3,5-bis(adamantyl)-1H-pyrazole
2c (19.40 g, 57.69 mmol), KOH (12.95 g, 230.77 mmol), anhyd.
K2CO3 (31.90 g, 230.77 mmol), CH2Br2 (2.02 mL, 28.85 mmol) and
benzyltriethylammonium chloride (2.143 g, 11.54 mmol) in anhyd.
CH2Cl2 (300 mL) was heated at reflux under an atmosphere of
nitrogen for 20 h. The mixture was cooled to room temperature, di-
luted with CH2Cl2 and filtered through a glass sinter. The solids
were washed with CH2Cl2 (2 Â 100 mL) and the combined filtrate
2.2.7. [bapmPdCl2Á3CH2Cl2] 4c
Bis[3,5-bisadamantyl-1H-pyrazolyl]methane
(bapm)
3c
(0.300 g, 0.438 mmol), PdCl2Á(MeCN)2 (0.114 g, 0.438 mmol),
CH2Cl2 (5 mL) and MeOH (5 mL) were treated according to the gen-
eral procedure to afford an orange coloured solid (0.274 g, 76%).
This was crystallised from toluene to afford the complex 4c as or-

M.W. Jones et al. / Inorganica Chimica Acta 363 (2010) 1097–1101
1099
ange prisms of X-ray quality. m.p. 247–250 °C (Elemental micro-
analysis: Found: C, 53.82; H, 6.16; N, 5.05. C47H64Cl2N4PdÁ3CH2Cl2
formed crystals that were analysed by X-ray crystallography
(Fig. 1). Complex formation was enacted readily at room tempera-
ture with equimolar PdCl2Á(MeCN)2 in dichloromethane and meth-
anol to afford the complexes 4a–c as orange solids in high yields.
The installation of the six-membered chelate was evident by the
appearance of two mutually coupled doublets (ca. J 14.5 Hz) for
the methylene protons, with separations of 1.6–2.5 ppm, as a result
of the rigid six-membered chelate ring and deshielding of the pro-
ton projected towards the palladium centre.
requires: C, 53.76; H, 6.32; N, 5.02%). m
max (KBr, cmÀ1) 3484, 2904,
2849, 1625, 1539, 1450, 1361, 1314, 1273, 1230, 1104, 796, 730. 1H
NMR (CDCl3, 400 MHz) d: 1.77 (24H, m, 12ÂCH2), 1.95, (24H, m,
12ÂCH2), 2.32 (6H, d, J 11.6 Hz, 6ÂCH), 2.62, (6H, d, J 11.6 Hz,
6ÂCH), 6.09 (2H, s, 2ÂpzCH), 6.97 (1H, d, J 14.3 Hz, CH2a), 8.61
(1H, d, J 14.3 Hz, CH2b). 13C NMR (CDCl3, 100 MHz) d: 28.2 (CH),
28.6 (CH), 35.1 (Cq), 35.5 (Cq), 36.0 (CH2), 36.4 (CH2), 41.9 (CH2),
42.3 (CH2), 64.2 (CH2), 77.2 (CH), 105.2 (CH), 156.2 (Cq), 166.5
(Cq). MS-ES+ m/z 864 (MNa+, 33%), 821 (MH+, 13%), 788 (MÀ35Cl+,
100%).
X-ray quality crystals were available for all three complexes
giving rise to further information about the solid state behaviour
of the materials. Both of the aryl derivatives formed by the infusion
of diethyl ether into a saturated dichloromethane solution of com-
plex (Fig. 2). Significant solvent was trapped in the voids for each of
the crystals, which was also evident in the elemental microanaly-
ses. The crystals of the bapmPdCl2ÁÂCH2Cl2 complex obtained by
this method experienced problems during the collection of X-ray
data due to crystal cracking and decomposition at low tempera-
tures. The likely source of this was the forced reduction in the
mobility of the adamantyl groups and also loss of the volatile sol-
vents from the crystals, damaging the structural integrity of the
crystals. Changing the solvent to toluene to avoid cracking and
decomposition of the crystals through solvent evaporation yielded
X-ray quality crystals of complex 4c. All crystal structures indi-
cated the formation of mononuclear complexes as a result of the
steric bulk on the ligands preventing chloro-bridging or dimeric
systems (Fig. 2). This complete shielding of one face above the me-
tal centre (Fig. 3) is encouraging for catalytic purposes as this is a
feature in several other very active catalytic systems [47,48].
Closer inspection of the crystal data of the three structures
(Table 1) has highlighted the orientation about the Pd bonds to
be deviated from the standard square planar ideal of 720° for the
sum of six angles by an order of 10.21–12.93°, most likely from
the distortions to the molecule caused by the steric bulk immedi-
ately above the palladium centre. The manner in which the bond-
ing arrangement about the metal centre departs from square
planar is interesting; the N1–Pd–N4 bite angle is lower than 90°,
which adds strain and the steric hindrance about the ligand system
appears to increase the N–Pd–Cl angles to above 90° forcing the
chlorides to lie closer in space than in the optimised square planar
structure. There appears to be little difference in the bond dis-
tances between either the sp2-nitrogens or the chlorides when
coordinated to palladium. The bond angles about the palladium
do, however, show some differences when the bulkier adamantyl
substituents are introduced. Compared to the aryl substituents,
they appear to increase the N–Pd–Cl angle, ‘squeezing’ the chlo-
2.3. Crystallographic methods
Single crystal X-ray diffraction data for compounds 3c, 4a, 4b
and 4c was collected at 150 K with an Oxford Cryosystems open
flow N2 cooling device [40] mounted on a Nonius KappaCCD dif-
fractometer. Data collection and cell refinement were carried out
using DENZO-SMN [41,42]. Intensity data were processed and cor-
rected for absorption effects by the multiscan method, based on
multiple scans of identical and Laue equivalent reflections within
SCALEPACK. Structure solution was carried out with direct meth-
ods using the programme SIR-92 [43], within the CRYSTALS [44]
software suite. In general, coordinates and anisotropic displace-
ment parameters of all non-hydrogen atoms were refined except
where this was not possible due to the presence of disorder. The
discrete Fourier transform of the void regions were treated as con-
tributions to the A and B parts of the calculated structure factors.
The complete details of each structure have been deposited with
the CCDC (including CIF files) and have been included in the Sup-
plementary data.
3. Results and discussion
Bispyrazolylmethanes 3a–c were synthesised in three steps
from commercially available materials (Scheme 1). To begin, a Cla-
isen condensation from the respective ketone/ester pair was per-
formed [45,39], followed by treatment with hydrazine
monohydrate to form the pyrazole [46]. The final installation of
the methylene bridge achieved by reaction with dibromometh-
ane/dichloromethane under phase transfer conditions. The tetra-
adamantyl derivative, which has not been previously described,
Scheme 1. (i) N2H4ÁH2O, MeOH, reflux, 3 h; (ii) CH2Br2, K2CO3, KOH, benzyltrieth-
ylammonium chloride, CH2Cl2, reflux, 12 h; and (iii) PdCl2Á2MeCN, MeOH, CH2Cl2, rt
(R = Ph (a), PhOMe (b) and Ad (c)).
Fig. 1. ORTEP depiction of bispyrazolylmethane 3c with thermal ellipsoids at 50%
probability.

1100
M.W. Jones et al. / Inorganica Chimica Acta 363 (2010) 1097–1101
Fig. 2. ORTEP depiction of complexes 4a–c with thermal ellipsoids at 50% probability. Solvent molecules and the minor component of disorder (for complex 4b) have been
removed for clarity.
rides closer together (87.83° for 4c), while this is seen to a lesser
extent when the ortho-methoxyl substituent is placed on the phe-
nyl ring (90.94° 4b cf. 91.62° 4a). While appearing strained in the
solid state, the complexes all proved to be air and temperature sta-
ble, and could be stored for considerable lengths of time without
fear of decomposition.
A brief assessment of the catalytic capabilities of the complexes
was conducted. The well-studied and industrially important Heck
reaction of a simple model system of iodobenzene and methyl
methacrylate was selected and catalyst loadings of 5 mol% used
for the coupling reaction in acetonitrile. This afforded quantitative
conversion to cinnamyl methyl ester within 3 h of heating at re-
flux. Further decreasing the catalyst loading to 0.24 mol% did not
affect the time taken for the reaction to go to completion, giving
quantitative conversion to the desired adduct. The coupling reac-
tions demonstrate that the complexes synthesised have catalytic
activity for the Heck coupling reaction, however, further work is
required to establish the scope of their catalytic potential with
Fig. 3. Space-filling model (MERCURY) of bapmPdCl2 4c indicating the complete
‘blocking’ of one face of the metal centre. Solvent removed for clarity.

M.W. Jones et al. / Inorganica Chimica Acta 363 (2010) 1097–1101
1101
Table 1
[4] X.-Y. Chen, W. Shi, J. Xia, P. Cheng, B. Zhao, H.-B. Song, H.-G. Wang, S.-P. Yan, D.-
Z. Liao, Z.-H. Jiang, Inorganic Chemistry 44 (2005) 4263.
[5] D.L. Reger, R.P. Watson, M.D. Smith, P.J. Pellechia, Organometallics 24 (2005)
1544.
[6] C.-C. Chou, C.-C. Su, A. Yeh, Inorganic Chemistry 44 (2005) 6122.
[7] E. Teuma, M. Loy, C. Le Berre, M. Etienne, J.-C. Daran, P. Kalck, Organometallics
22 (2003) 5261.
[8] M. Porchia, G. Papini, C. Santini, G.G. Lobbia, M. Pellei, F. Tisato, G. Bandoli, A.
Dolmella, Inorganic Chemistry 44 (2005) 4045.
[9] B. Machura, R. Penczek, J. Kusz, Structural Chemistry 19 (2008) 165.
[10] G.F. Zhang, M.H. Yin, Y.L. Dou, Q.P. Zhou, J.B. She, Journal of Coordination
Chemistry 61 (2008) 1272.
Selected bond lengths and angles from the X-ray data.
Cl2 Cl1
R
R
Pd
N4
N1
N3 N2
C1
R
R
[11] F. Mohr, E. Cerrada, M. Laguna, Dalton Transactions (2006) 5567.
[12] H.C. Clark, G. Ferguson, V.K. Jain, M. Parvez, Organometallics 2 (1983) 806.
[13] E.V. Lider, O.L. Krivenko, E.V. Peresypikina, A.I. Smolentsev, Y.G. Shvedenkov,
S.F. Vasilevskii, L.G. Lavrenova, Russian Journal of Coordination Chemistry 33
(2007) 896.
bppmPdCl2 4a bppm*PdCl2 4b bapmPdCl2 4c
Pd–Cl(1) (Å)
Pd–Cl(2) (Å)
2.287
2.282
2.279
2.279
2.277
2.276
[14] S. Burling, L.D. Field, B.A. Messerle, K.Q. Vuong, P. Turner, Dalton Transactions
(2003) 4181.
[15] L.D. Field, B.A. Messerle, M. Rehr, L.P. Soler, T.W. Hambley, Organometallics 22
(2003) 2387.
[16] W.A. Herrmann, Angewandte Chemie – International Edition 41 (2002) 1290.
[17] M. Weck, C.W. Jones, Inorganic Chemistry 46 (2007) 1865.
Pd–N(1) (Å)
Pd–N(4) (Å)
2.036
2.019
85.57
91.55
173.52
176.09
90.98
2.054
2.044
86.75
91.01
174.17
177.50
91.17
2.057
2.062
84.09
92.23
174.31
173.32
95.29
\N(1)–Pd–N(4)°
\N(1)–Pd–Cl(1)°
\N(1)–Pd–Cl(2)°
\N(4)–Pd–Cl(1)°
\N(4)–Pd–Cl(2)°
\Cl(1)–Pd–Cl(2)°
Sum of six angles
\N(2)–C1–N(3)°
h\N(1)–N(2)–C(1)–N(3)°
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more difficult coupling reactions on more challenging substrates.
The bpm template provides potential for chirality being induced
(through chiral groups being attached at the methylene carbon),
and also for regiochemical site discrimination (by way of the bulky
substituents giving rise to the less hindered product) and allowing
for further reaction by judicious choice of coupling agent.
4. Conclusions
Three mononuclear palladium(II) complexes of sterically hin-
dered bpms have been prepared and characterised by X-ray single
crystal analysis, NMR, IR, elemental analysis and mass spectrome-
try. In addition a new sterically hindered ligand with four pendant
adamantyl groups has been prepared and the structure solved by
X-ray crystallography. Preliminary screening found that the new
complexes had good catalytic activity for the Heck coupling
reaction.
Acknowledgements
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The authors wish to thank the EPSRC for funding and Dr. Amber
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2009.09.016.
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