Synthesis of palladium complexes with anionic N,NR- or neutral NH,NR-theophylline-derived NHC ligands

Article abstract of DOI:10.1016/j.ica.2020.120055

Four C8-halogenated theophyllines (1–4) featuring N7-methyl or N7-allyl and C8-chloro or C8-bromo substituents have been prepared. The halogenotheophyllines react with [Pd(PPh₃)₄] in an oxidative addition to give complexes of type trans-[Pd(theophyllinato)X(PPh₃)₂] (trans-[5]–trans-[8]). The protonation of the unsubstituted theophyllinato ring-nitrogen atom to give the pNHC complexes was achieved either by performing the oxidative addition of 1 in the presence of NH₄BF₄ to give complex trans-[9]BF₄ or by N-protonation of the coordinated theophyllinato ligand in trans-[6]–trans-[8] with HBF₄·Et₂O to give complexes trans-[10]BF₄–trans-[12]BF₄. The molecular structures of trans-[5], trans-[7], trans-[9]BF₄, trans-[11]BF₄ and trans-[12]BF₄ were determined by X-ray diffraction showing significant differences of comparable metric parameters in the theophylline-derived five-membered diaminoheterocycles. No interaction of the N-allyl substituents with the metal center was observed.

Full text of DOI:10.1016/j.ica.2020.120055

InorganicaChimicaActa515(2021)120055
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis of palladium complexes with anionic N,NR- or neutral NH,NR-
theophylline-derived NHC ligands
Mareike C. Jahnke, Alexandre Hervé, Florian Kampert, F. Ekkehardt Hahn⁎
Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, 48149 Münster, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
Four C8-halogenated theophyllines (1–4) featuring N7-methyl or N7-allyl and C8-chloro or C8-bromo sub-
stituents have been prepared. The halogenotheophyllines react with [Pd(PPh3)4] in an oxidative addition to give
complexes of type trans-[Pd(theophyllinato)X(PPh3)2] (trans-[5]–trans-[8]). The protonation of the un-
substituted theophyllinato ring-nitrogen atom to give the pNHC complexes was achieved either by performing
the oxidative addition of 1 in the presence of NH4BF4 to give complex trans-[9]BF4 or by N-protonation of the
coordinated theophyllinato ligand in trans-[6]–trans-[8] with HBF4·Et2O to give complexes trans-[10]BF4–trans-
[12]BF4. The molecular structures of trans-[5], trans-[7], trans-[9]BF4, trans-[11]BF4 and trans-[12]BF4 were
determined by X-ray diffraction showing significant differences of comparable metric parameters in the theo-
phylline-derived five-membered diaminoheterocycles. No interaction of the N-allyl substituents with the metal
center was observed.
8-Halogenotheophyllines
Oxidative addition
Protic N-heterocyclic carbenes
Palladium
1. Introduction
has been developed based on the oxidative addition of the C2-R bond of
azoles to transition metals followed by N-protonation [7]. While the
N-Heterocyclic carbenes (NHCs) have evolved into an important
class of ligands over the last two decades [1], due to the numerous
application of their complexes in catalysis [2], pharmaceutics [3], lu-
minescent materials [4] or as building blocks for metallosupramole-
cular assemblies [5,6]. Most of the known NHC ligands feature an
NR,NR substitution pattern of the five-membered heterocycle, which
prevents a further functionalization of the NHC ligand after complex
formation. Such modifications of an NHC ligand, however, are possible
in complexes bearing protic NHC ligands (pNHCs) [7]. Reactions at
coordinated pNHC ligands have allowed the template-controlled for-
mation of NHC-containing macrcrocycles [8]. In addition, the NH group
of a pNHC ligand can serve as recognition unit for substrate binding via
hydrogen bonds during a catalytic reaction [9]. Today, the number of
complexes possessing pNHC ligands is still low compared to the number
of complexes with the ubiquitous NR,NR-NHC ligands, although a
number of protocols for the synthesis of pNHC complexes exist [7].
Among these are the template-controlled cyclization of β-functionalized
isocyanides [8,10], the reaction of C2-lithiated azoles with metal ions
followed by N-protonation [11], the removal of a protection group (PG)
from a coordinated NR,NPG-NHC ligand [12] or the metal mediated
tautomerization of neutral N-coordinated azoles [13].
oxidative addition of the C2eH bond of an N-alkylated azole has not
been observed so far, functionalization of one of the azole ring-nitrogen
atoms with an amine [14] or phosphine [15] donor group facilitates the
reaction. It is assumed that the N-tethered donor pre-coordinates to the
metal center bringing the C2-H bond of the azole in close proximity to
the metal, which then oxidatively adds yielding an azolato complex.
The initially formed hydrido complex is normally not stable and re-
ductively eliminates a proton which protonates the unsubstituted ring-
nitrogen atom of the azolato ligand, leading to complex A with a pNHC
ligand (Fig. 1). Contrary to the oxidative addition of the C2-H bond, the
oxidative addition of the C2-X (X = Cl, Br, I) bond of N-alkylated azoles
proceeds readily and in the absence of an N-tethered donor group,
leading after protonation of the initially formed azolato ligand, to
complexes with NR,NH-NHC ligands B (Fig. 1) [16]. Even complexes
bearing NH,NH-NHCs [17] or mesoionic pNHC ligands [18] could be
obtained using this method. Recently, we also described the synthesis of
platinum complexes bearing adenine- (C) or caffeine-derived (D) pNHC
ligands (Fig. 1) [19].
In this contribution we describe the oxidative addition of a series of
C8-halogeno-N7-alkyltheophyllines to [Pd(PPh3)4] in order to evaluate
the influence of different C8-halogens present and regarding a potential
involvement of the N-alkyl (alkyl = allyl) function on the reaction
Recently, a new synthetic approach to complexes with pNHC ligands
⁎ Corresponding author.
E-mail address: fehahn@uni-muenster.de (F.E. Hahn).
https://doi.org/10.1016/j.ica.2020.120055
Received 31 August 2020; Received in revised form 25 September 2020; Accepted 28 September 2020
Availableonline02October2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

M.C. Jahnke, et al.
InorganicaChimicaActa515(2021)120055
and 4 as colorless solids.
2.2.1. N7-Allyl-8-chlorotheophylline (3)
Yield: 0.326 g (1.28 mmol, 85%). 1H NMR (400 MHz, DMSO‑d6): δ
6.04–5.92 (m, 1H, H13), 5.22 (d, 3J = 10.4 Hz, 1H, H14a), 4.99 (d,
3J = 17.2 Hz, 1H, H14b), 4.93 (d, 3J = 4.9 Hz, 2H, H12), 3.40 (s, 3H,
H11), 3.22 (s, 3H, H10). 13C{1H} NMR (100 MHz, DMSO‑d6): δ 153.5
(C6), 150.6 (C2), 146.6 (C4), 137.6 (C8), 131.7 (C13), 117.4 (C14),
107.1 (C5), 47.4 (C12), 29.4 (C11), 27.5 (C10). HRMS (ESI, positive
ions): m/z 255.0644 (calcd for [3 + H]+ 255.0649).
2.2.2. N7-Allyl-8-bromotheophylline (4)
Yield: 0.381 g (1.27 mmol, 85%). 1H NMR (400 MHz, DMSO‑d6): δ
6.03–5.89 (m, 1H, H13), 5.21 (d, 3J = 10.5 Hz, 1H, H14a), 4.96 (d,
3J = 17.6 Hz, 1H, H14b), 4.92–4.87 (m, 2H, H12), 3.39 (s, 3H, H11),
3.20 (s, 3H, H10). 13C{1H} NMR (100 MHz, DMSO‑d6): δ 153.4 (C6),
150.5 (C2), 147.6 (C4), 131.8 (C13), 127.8 (C8), 117.4 (C14), 107.9
(C5), 48.4 (C12), 29.4 (C11), 27.5 (C10). HRMS (ESI, positive ions): m/
z 320.9957 (calcd for [4 + Na]+ 320.9963), 299.0138 (calcd for
[4 + H]+ 299.0144).
2.3. Synthesis of trans-[5]
Essentially equimolar amounts of 8-chlorocaffeine 1 (8.9 mg,
0.039 mmol) and [Pd(PPh3)4] (40 mg, 0.035 mmol) were dissolved in
toluene (10 mL). The mixture was heated under reflux for 6 d. After
removal of the solvent in vacuo the residue was washed twice with
hexane (5 mL each) and diethyl ether (5 mL each). Drying in vacuo gave
trans-[5] as colorless solid. Crystals of trans-[5]·CH2Cl2 were obtained
by slow diffusion of diethyl ether into a saturated dichloromethane
solution of trans-[5]. Yield: 17 mg (0.02 mmol, 57%). 1H NMR
(400 MHz, CD2Cl2): δ 7.65–7.56 (m, 12H, Ph-Hortho), 7.44–7.38 (m, 6H,
Ph-Hpara), 7.36–7.27 (m, 12H, Ph-Hmeta), 3.28 (s, 3H, H10), 3.17 (s, 6H,
H11, H12). 13C{1H} NMR (100 MHz, CD2Cl2): δ 162.8 (t, 2JCP = 4.9 Hz,
Fig. 1. Synthesis of complexes possessing pNHC ligands by oxidative addition
of C2-H (A, top) or C2-halogen (B, middle) bonds to selected transition metals
and selected complexes bearing purine-derived pNHC ligands (C and D,
bottom).
C8), 153.7 (C2), 151.5 (C6), 151.4 (C4), 134.8 (v-t, 2/4JCP = 6.4 Hz, Ph-
1/3
C
ortho), 131.0 (Ph-Cpara), 130.6 (v-t,
J
= 24.0 Hz, Ph-Cipso), 128.6
CP
3/5
(v-t,
J
= 5.2 Hz, Ph-Cmeta), 110.1 (C5), 34.4 (C12), 29.4 (C11),
CP
outcome.
27.5 (C10). 31P{1H} NMR (162 MHz, CD2Cl2): δ 21.0 (s). HRMS (ESI,
positive ions): m/z 861.1352 (calcd for [[5]+H]+ 861.1355).
2. Experimental
2.4. Synthesis of trans-[6]
2.1. General procedures and materials
Equimolar amounts of 8-bromocaffeine 2 (27 mg, 0.1 mmol) and
[Pd(PPh3)4] (115 mg, 0.1 mmol) were dissolved in toluene (10 mL) and
stirred at ambient temperature for 2 d. After removal of the solvent in
vacuo the yellow residue was washed with hexane (15 mL) and diethyl
ether (15 mL) and dried in vacuo. Yellowish crystals of trans-[6]·CH2Cl2
were obtained by slow diffusion of diethyl ether into a saturated di-
chloromethane solution of trans-[6]. Yield: 61.8 mg (0.068 mmol,
68%). 1H NMR (400 MHz, CD2Cl2): δ 7.70–7.57 (m, 12H, Ph-Hortho),
7.46–7.38 (m, 6H, Ph-Hpara), 7.37–7.28 (m, 12H, Ph-Hmeta), 3.33 (s, 3H,
H10), 3.19 (s, 6H, H11, H12). 13C{1H} NMR (100 MHz, CD2Cl2): δ
163.9 (t, 2JCP = 4.1 Hz, C8), 153.7 (C2), 151.4 (C6), 151.3 (C4), 134.8
All manipulations were performed under an argon atmosphere using
standard Schlenk techniques or in a glove box. Glassware was oven
dried at 130 °C. The solvents were freshly distilled by standard proce-
dures prior to use. 1H, 13C{1H} and 31P{1H} NMR spectra were re-
corded on Bruker AVANCE I or AVANCE III 400 spectrometers.
Chemical shifts (δ) are expressed in ppm using the residual protonated
solvent as an internal standard. Coupling constants are expressed in
Hertz. Mass spectra were obtained with an Orbitrap LTQ XL (Thermo
Scientific) spectrometer. 8-Chlorotheophylline, 8-bromotheophylline
and [Pd(PPh3)4] were purchased from commercial sources and used as
received. 8-Chloro- and 8-bromocaffeine were synthesized following a
published procedure [20].
2/4
1/
(v-t,
J
= 6.4 Hz, Ph-Cortho), 130.92 (Ph-Cpara), 130.87 (v-t,
CP
3JCP = 24.3 Hz, Ph-Cipso), 128.6 (v-t, 3/5JCP = 5.2 Hz, Ph-Cmeta), 110.2
(C5), 34.3 (C12), 29.5 (C11), 27.6 (C10). 31P{1H} NMR (162 MHz,
CD2Cl2): δ 20.9 (s). HRMS (ESI, positive ions): m/z 905.0842 (calcd for
[[6]+H]+ 905.0848).
2.2. Synthesis of N7-allyl-8-halogenotheophyllines 3–4
An appropriate 8-halogenotheophylline (X = Cl or Br, 1.50 mmol)
was dissolved in dry DMF (15 mL) and an excess of potassium carbonate
(1.0 g, 7.2 mmol) was added. The resulting suspension was treated with
allyl bromide (0.15 mL, 1.74 mmol) and the reaction mixture was
stirred for 16 h at ambient temperature. Subsequently, water (20 mL)
was added and the suspension was stored at 4 °C for 12 h. A colorless
precipitate formed, which was isolated by filtration and was washed
with cold water (20 mL) and diethyl ether (5 mL) giving compounds 3
2.5. Synthesis of trans-[7]
The synthesis of trans-[7] was performed as described for trans-[5]
by treatment of 3 (8.9 mg, 0.035 mmol) with [Pd(PPh3)4] (40 mg,
0.035 mmol). Yield: 16.8 mg (0.019 mmol, 54%). Colorless crystals of
trans-[7]·CH2Cl2 were obtained by slow diffusion of diethyl ether into a
saturated dichloromethane solution of trans-[7]. 1H NMR (400 MHz,
2

M.C. Jahnke, et al.
InorganicaChimicaActa515(2021)120055
CD2Cl2): δ 7.65–7.55 (m, 12H, Ph-Hortho), 7.45–7.38 (m, 6H, Ph-Hpara),
7.37–7.27 (m, 12H, Ph-Hmeta), 5.74–5.62 (m, 1H, H13), 5.18 (d,
3Jtrans = 17.2 Hz, 1H, H14a), 4.84 (d, 3Jcis = 10.1 Hz, 1H, H14b), 4.36
(d, 3J = 6.4 Hz, 2H, H12), 3.20 (s, 3H, H10), 3.12 (s, 3H, H11). 13C{1H}
(v-t, 2/4JCP = 6.2 Hz, Ph-Cortho), 131.1 (Ph-Cpara), 129.8 (C13), 128.3 (v-
3/5
1/3
t,
J
= 5.3 Hz, Ph-Cmeta), 128.1 (v-t,
J
= 25.4 Hz, Ph-Cipso),
CP
CP
121.1 (C14), 107.1 (C5), 52.8 (C12), 30.3 (C11), 27.6 (C10). 31P{1H}
(162 MHz, CD2Cl2/DMSO‑d6): δ 21.0 (s). HRMS (ESI, positive ions): m/
z 887.1501 (calcd for [10] 887.1512).
2
NMR (101 MHz, CD2Cl2): δ 161.9 (t, JCP = 4.4 Hz, C8), 153.5 (C2),
2/4
151.6 (C4), 151.4 (C6), 134.9 (v-t,
J
J
= 6.4 Hz, Ph-Cortho), 134.5
= 24.1 Hz, Ph-Cipso), 128.6
CP
CP
1/3
(C13), 131.0 (Ph-Cpara), 130.6 (v-t,
2.9. Synthesis of trans-[11]BF4
3/5
(v-t,
J
= 5.2 Hz, Ph-Cmeta), 118.4 (C14), 109.5 (C5), 51.3 (C12),
CP
29.5 (C11), 27.6 (C10). 31P{1H} NMR (162 MHz, CD2Cl2): δ 21.7 (s).
HRMS (ESI, positive ions): m/z 887.1499 (calcd for [[7]+H]+
887.1512).
Samples of 8-bromocaffeine 2 (27.0 mg, 0.1 mmol) and [Pd(PPh3)4]
(115 mg, 0.1 mmol) were dissolved in toluene (10 mL) and stirred at
ambient temperature for 1 d. Then HBF4∙Et2O (0.2 mL of a 1.6 g·mL−1
solution) was added and stirring was continued for 2 h. After removal of
the solvent in vacuo the yellowish residue was washed with hexane
(15 mL) and diethyl ether (15 mL). Then the residue was then dissolved
in MeCN (20 mL) and insoluble materials were separated by filtration.
Removal of the solvent in vacuo gave trans-[11]BF4 as colorless solid.
Crystals of trans-[11]BF4 were obtained by slow diffusion of diethyl
ether into a saturated MeCN solution of [11]BF4. Yield: 87 mg
(0.088 mmol, 88%). 1H NMR (400.0 MHz, CD3CN/DMSO‑d6): δ 13.12
(s, 1H, NH), 7.72–7.62 (m, 12H, Ph-Hortho), 7.52–7.39 (m, 18H, Ph-
2.6. Synthesis of trans-[8]
The synthesis of trans-[8] was performed as described for trans-[6]
by treatment of 4 (29.9 mg, 0.10 mmol) with [Pd(PPh3)4] (115 mg,
0.10 mmol). Compound trans-[8] was obtained as bright yellow solid.
Yield: 65.3 mg (0.070 mmol, 70%). 1H NMR (400 MHz, CD2Cl2): δ
7.65–7.58 (m, 12H, Ph-Hortho), 7.44–7.38 (m, 6H, Ph-Hpara), 7.35–7.29
(m, 12H, Ph-Hmeta), 5.79–5.66 (m, 1H, H13), 5.25 (dd,
3Jtrans = 17.2 Hz, 2J = 1.3 Hz, 1H, H14a), 4.90 (dd, Jcis = 10.1 Hz,
3
H
meta, Ph-Hpara), 3.56 (s, 3H, H12), 3.16 (s, 3H, H11), 3.08 (s, 3H, H10).
2J = 1.3 Hz, 1H, H14b), 4.39 (d, 3J = 6.5 Hz, 2H, H12), 3.20 (s, 3H,
H10), 3.13 (s, 3H, H11). 13C{1H} NMR (101 MHz, CD2Cl2): δ 163.1 (t,
13C{1H} NMR (101 MHz, CD3CN/DMSO‑d6): δ 165.1 (t, 2JCP = 9.4 Hz,
C8), 152.2 (C2), 149.1 (C6), 141.4 (C4), 134.2 (v-t, 2/4JCP = 6.4 Hz, Ph-
2JCP = 3.4 Hz, C8), 153.5 (C2), 151.6 (C4), 151.4 (C6), 135.0 (v-t,
2/
1/3
C
ortho), 131.5 (Ph-Cpara), 129.1 (v-t,
J
= 25.6 Hz, Ph-Cipso), 128.7
CP
4JCP = 6.3 Hz, Ph-Cortho), 134.5 (C13), 131.0 (v-t, 1/3JCP = 24.6 Hz, Ph-
3/5
(v-t,
J
= 5.3 Hz, Ph-Cmeta), 108.2 (C5), 36.6 (C12), 30.7 (C11),
CP
3/5
27.6 (C10). 31P{1H} NMR (162 MHz, CD3CN/DMSO‑d6): δ 20.4 (s).
HRMS (ESI, positive ions): m/z 905.0856 (calcd for [11]+ 905.0848),
861.1368 (calcd for {[11] – Br + Cl}+ 861.1355).
Cipso), 130.9 (Ph-Cpara), 128.5 (v-t,
J
= 5.2 Hz, Ph-Cmeta), 118.5
CP
(C14), 109.6 (C5), 51.3 (C12), 29.4 (C11), 27.6 (C10). 31P{1H} NMR
(162 MHz, CD2Cl2): δ 21.3 (s). HRMS (ESI, positive ions): m/z 931.1005
(calcd for [[8] + H]+ 931.1005), 849.1755 (calcd for [[8] – Br]+
849.1756).
2.10. Synthesis of trans-[12]BF4
2.7. Synthesis of trans-[9]BF4
Samples of compound 4 (29.9 mg, 0.1 mmol) and [Pd(PPh3)4]
(115 mg, 0.1 mmol) were dissolved in toluene (10 mL) and stirred in
toluene for 2 d. Then HBF4∙Et2O (0.2 mL of a 1.6 g·mL−1 solution) was
slowly added leading to the formation of a yellow precipitate. After 2 h
of stirring at ambient temperature the solvent was removed in vacuo
and the yellow residue was washed with hexane (20 mL) and diethyl
ether (20 mL) to give trans-[12]BF4 as a yellow solid. Crystals of trans-
[12]BF4 were obtained by slow diffusion of diethyl ether into a satu-
rated MeCN solution of trans-[12]BF4. Yield: 91.5 mg (0.09 mmol,
90%). 1H NMR (400 MHz, CD3CN/DMSO‑d6): δ 12.69 (s, 1H, NH),
7.72–7.63 (m, 12H, Ph-Hortho), 7.54–7.48 (m, 6H, Ph-Hpara), 7.48–7.40
(m, 12H, Ph-Hmeta), 5.74–5.60 (m, 1H, H13), 5.17–5.09 (m, 1H, H14a),
4.83–4.77 (m, 1H, H14b), 4.61 (d, 2H, 3J = 6.4 Hz, H12), 3.19 (s, 3H,
H11), 3.01 (s, 3H, H10). 13C{1H} NMR (101 MHz, CD3CN/DMSO‑d6): δ
165.5 (t, 2JCP = 9.3 Hz, C8), 153.1 (C2), 150.1 (C6), 142.9 (C4), 135.5
(v-t, 2/4JCP = 6.3 Hz, Ph-Cortho), 132.6 (Ph-Cpara), 131.7 (C13), 130.2 (v-
A sample of 8-chlorocaffeine 1 (8.9 mg, 0.039 mmol), [Pd(PPh3)4]
(40 mg, 0.035 mmol) and an excess of NH4BF4 (10.5 mg, 0.10 mmol)
were dissolved in toluene (10 mL) and heated under reflux for 6 d. After
removal of the solvent in vacuo the residue was washed with hexane
(5 mL) and diethyl ether (5 mL). The residue was then dissolved in
dichloromethane (40 mL) and insoluble material was removed by fil-
tration. Removal of the solvent gave trans-[9]BF4 as a colorless solid.
Crystals of trans-[9]BF4·2CH2Cl2 were obtained by slow diffusion of
diethyl ether into a saturated dichloromethane solution of trans-[9]BF4.
Yield: 28.0 mg (0.030 mmol, 86%). 1H NMR (400.0 MHz, CD2Cl2): δ
11.00 (s, 1H, H9), 7.78–7.70 (m, 12H, Ph-Hortho), 7.49–7.38 (m, 18H,
Ph-Hmeta, Ph-Hpara), 3.76 (s, 3H, H12), 3.24 (s, 3H, H11), 3.07 (s, 3H,
H10). 13C{1H} NMR (101 MHz, CD2Cl2): δ 167.3 (t, JCP = 10.1 Hz,
2
C8), 153.0 (C2), 149.5 (C6), 142.0 (C4), 134.7 (v-t, 2/4JCP = 6.4 Hz, Ph-
ortho), 131.8 (Ph-Cpara), 129.3 (v-t, 3/5JCP = 5.4 Hz, Ph-Cmeta), 129.2 (v-
1/3
3/5
C
t,
t,
J
= 25.6 Hz, Ph-Cipso), 129.8 (v-t,
J
= 5.3 Hz, Ph-Cmeta),
CP
CP
1/3
121.9 (C14), 108.7 (C5), 54.4 (C12), 31.8 (C11), 28.7 (C10). 31P{1H}
(162 MHz, CD3CN/DMSO‑d6): δ 20.4 (s). HRMS (ESI, positive ions): m/
z 931.1003 (calcd for [12]+ 931.1005), 887.1512 (calcd for [[12] –
Br + Cl]+ 887.1512).
J
= 25.5 Hz, Ph-Cipso), 108.3 (C5), 32.7 (C12), 29.8 (C11), 27.9
CP
(C10). 31P{1H} NMR (162 MHz, CD2Cl2): δ 20.9 (s). HRMS (ESI, posi-
tive ions): m/z 861.1353 (calcd for [9]+ 861.1355).
2.8. Synthesis of trans-[10]BF4
2.11. X-ray crystallography
A sample of trans-[7] (8.9 mg, 0.01 mmol) was dissolved in THF
(20 mL) and HBF4∙Et2O (0.2 mL of a 1.6 g·mL−1 solution) was added
slowly to this solution. A yellow precipitate formed, which was sepa-
rated by filtration and washed twice with ice cold diethyl ether (5 mL
each). The residue was dried in vacuo to give trans-[10]BF4 as a yellow
solid. Yield: 8.8 mg (0.009 mmol, 90%). 1H NMR (400 MHz, CD2Cl2/
Diffraction data for trans-[5]·CH2Cl2 and trans-[9]BF4·2CH2Cl2 were
collected at T = 153(2) K with a Bruker AXS APEX I CCD diffractometer
equipped with a rotation anode using graphite-monochromated MoKα
radiation (λ = 0.71073 Å). Diffraction data for trans-[7]·CH2Cl2, trans-
[11]BF4 and trans-[12]BF4 were collected with a Bruker APEX II CCD
Diffractometer equipped with a microsource using MoKα radiation
(λ = 0.71073 Å) at T = 153(2) K (trans-[7]·CH2Cl2) or T = 100(2) K
(trans-[11]BF4 and trans-[12]BF4). Diffraction data were collected over
the full sphere and were corrected for absorption. Structure solutions
were found with the SHELXT [21a] package using direct methods and
were refined with SHELXL [21b] against all |F2| using first isotropic and
DMSO‑d6):
δ 12.91 (s, 1H, NH), 7.68–7.61 (m, 12H, Ph-Hortho),
7.53–7.47 (m, 6H, Ph-Hpara), 7.46–7.38 (m, 12H, Ph-Hmeta), 5.74–5.61
3
(m, 1H, H13), 5.07 (d, Jtrans = 17.0 Hz, 1H, H14a), 4.80 (d,
3Jcis = 10.2 Hz, 1H, H14b), 4.55 (d, 2H, 3J = 6.5 Hz, H12), 3.23 (s, 3H,
H11), 3.03 (s, 3H, H10). 13C{1H} NMR (101 MHz, CD2Cl2/DMSO‑d6): δ
163.6 (t, 2JCP = 9.7 Hz, C8), 151.5 (C2), 148.4 (C6), 141.4 (C4), 133.7
3

M.C. Jahnke, et al.
InorganicaChimicaActa515(2021)120055
later anisotropic thermal parameters (for exceptions see description of
the individual molecular structures). Hydrogen atoms were added to
the structure models on calculated positions if not noted otherwise.
2.11.1. Crystal data for trans-[5]·CH2Cl2
Formula C45H41N4Cl3O2P2Pd, M = 944.51 g·mol−1, colorless prism,
0.12
×
0.09 × 0.05 mm3, a = 14.2664(7), b = 10.7009(5),
c
ρ
=
28.3413(13) Å,
β
=
91.3750(10)°,
V
=
4325.4(4) Å3,
calc = 1.450 g·cm−3, μ = 0.731 mm−1, monoclinic, space group P21/c,
Z = 4, semiempirical absorption correction (0.827 ≤ T ≤ 0.954), ω-
and φ-scans, 41742 measured intensities (2.9° ≤ 2Θ ≤ 55.0°), 9932
independent (Rint = 0.0261) and 9126 observed intensities (I ≥ 2σ(I)),
refinement of 516 parameters against |F2| of all measured intensities
with hydrogen atoms on calculated positions.
R = 0.0798,
wR = 0.1749, Rall = 0.0849, wRall = 0.1764. The asymmetric unit
contains one molecule of trans-[5] and one molecule of CH2Cl2. One of
the phenyl substituents and the CH2Cl2 molecule are disordered. No
hydrogen atoms were added to the structure model for the disordered
CH2Cl2 molecule.
Scheme 1. Synthesis 8-halogenated theophylline derivatives 1–4 (the num-
bering refers to the assignment of the NMR resonances).
2.11.2. Crystal data for trans-[7]·CH2Cl2
ρcalc = 1.571 g·cm−3, μ = 1.495 mm−1, monoclinic, space group P21/c,
Z = 4, semiempirical absorption correction (0.648 ≤ T ≤ 0.746), ω-
and φ-scans, 67400 measured intensities (6.2° ≤ 2Θ ≤ 57.6°), 11191
independent (Rint = 0.0373) and 9732 observed intensities (I ≥ 2σ(I)),
refinement of 556 parameters against |F2| of all measured intensities
Formula C47H43N4Cl3O2P2Pd, M = 970.54 g·mol−1, colorless prism,
0.19
×
0.16 × 0.12 mm3, a = 11.2988(4), b = 12.1975(4),
c = 16.3021(6) Å, α = 80.668(2)°, β = 83.396(2)°, γ = 75.886(2)°,
V = 2143.35(13) Å3, ρcalc = 1.504 g·cm−3, μ = 0.739 mm−1, triclinic,
space group P-1,
Z = 2, semiempirical absorption correction
with hydrogen atoms on calculated positions.
R = 0.0269,
(0.871 ≤ T ≤ 0.916), ω- and φ-scans, 38151 measured intensities
(6.1° ≤ 2Θ ≤ 62.8°), 13159 independent (Rint = 0.0300) and 11750
observed intensities (I ≥ 2σ(I)), refinement of 513 parameters against
|F2| of all measured intensities with hydrogen atoms on calculated
positions. R = 0.0456, wR = 0.1189, Rall = 0.0514, wRall = 0.1243.
The asymmetric unit contains one molecule of trans-[7] and one mo-
lecule of CH2Cl2. One of the phenyl substituents and the CH2Cl2 mo-
lecule are disordered. No hydrogen atoms were added to the structure
model for the disordered phenyl group and the CH2Cl2 molecule.
wR = 0.0664, Rall = 0.0346, wRall = 0.0697. The asymmetric unit
contains one formula unit of trans-[12]BF4.
3. Results and discussion
The 8-halogenocaffeines 1 and 2 were prepared from the appro-
priate 8-halogenotheophylline and methyl iodide following a published
procedure [20]. The N7-allyl derivatives 3 and 4 were prepared in a
similar manner from allyl bromide and the appropriate 8-halogen-
otheophylline (Scheme 1). Compounds 3 and 4 were characterized by
NMR spectroscopy and mass spectrometry. The formation of the theo-
phylline derivatives 3 and 4 was concluded from their NMR spectra
exhibiting the typical chemical shifts for the allylic protons and carbon
atoms in the expected ranges and multiplicity compared to other re-
ported N-allyl-substituted azole derivatives [22].
2.11.3. Crystal data for trans-[9]BF4·2CH2Cl2
Formula C46H44N4BCl5F4O2P2Pd, M = 1117.25 g·mol−1, colorless
prism, 0.29 × 0.14 × 0.11 mm3, a = 15.9986(8), b = 18.1562(10),
c
ρ
=
17.2072(9) Å,
β
=
97.3880(10)°,
V
=
4956.7(5) Å3,
calc = 1.497 g·cm−3, μ = 0.765 mm−1, monoclinic, space group P21/
n, Z = 4, semiempirical absorption correction (0.809 ≤ T ≤ 0.921), ω-
and φ-scans, 49266 measured intensities (3.3° ≤ 2Θ ≤ 60.0°), 14438
The 8-chlorotheophylline derivatives 1 and 3 were treated with [Pd
(PPh3)4] in toluene at 110 °C for 6 d to give the azolato complexes trans-
[5] and trans-[7] in moderate yields of 57% and 54%, respectively. In
contrast to the harsh reactions conditions employed for the synthesis of
trans-[5] and trans-[7], the brominated theophylline derivatives 2 and 4
react with [Pd(PPh3)4] already at ambient temperature over 2 d to yield
azolato complexes trans-[6] and trans-[8] in good yields of 68% and
70%, respectively (Scheme 2). Complexes with the two phosphine li-
gands in cis-disposition were not observed [16a]. Complexes trans-[5]
-trans-[8] were completely characterized by NMR spectroscopy and
independent (Rint
= 0.0307) and 11847 observed intensities
(I ≥ 2σ(I)), refinement of 589 parameters against |F2| of all measured
intensities with hydrogen atoms on calculated positions. R = 0.0362,
wR = 0.0884, Rall = 0.0473, wRall = 0.0947. The asymmetric unit
contains one formula unit of trans-[9]BF4 and two molecules of CH2Cl2.
2.11.4. Crystal data for trans-[11]BF4
Formula C44H40N4BBrF4O2P2Pd, M = 991.86 g·mol−1, colorless
prism, 0.31 × 0.16 × 0.14 mm3, a = 14.2526(2), b = 10.3371(2),
c
ρ
=
28.7506(5) Å,
β
=
92.8790(10)°,
V
=
4230.50(13) Å3,
calc = 1.557 g·cm−3, μ = 1.519 mm−1, monoclinic, space group P21/c,
Z = 4, semiempirical absorption correction (0.664 ≤ T ≤ 0.764), ω-
and φ-scans, 78192 measured intensities (3.9° ≤ 2Θ ≤ 64.6°), 13999
independent (Rint
= 0.0310) and 12366 observed intensities
(I ≥ 2σ(I)), refinement of 539 parameters against |F2| of all measured
intensities with hydrogen atoms on calculated positions. R = 0.0313,
wR = 0.0805, Rall = 0.0379, wRall = 0.0868. The asymmetric unit
contains one formula unit of trans-[11]BF4.
2.11.5. Crystal data for trans-[12]BF4
Formula C46H42N4BBrF4O2P2Pd, M = 1017.89 g·mol−1, colorless
prism, 0.26 × 0.12 × 0.06 mm3, a = 14.2481(2), b = 10.57620(10),
4303.96(9) Å3,
Scheme 2. Synthesis of palladium complexes trans-[5]-trans-[8].
c
=
28.6188(4) Å,
β
=
93.6260(10)°,
V
=
4

M.C. Jahnke, et al.
InorganicaChimicaActa515(2021)120055
Table 1
Selected 13C{1H} and 31P{1H} NMR parameters for complexes trans-[5]-trans-
[8].
Complexa
δ(C8) (ppm)
δ(P) (ppm)
2
2
2
2
trans-[5]
trans-[6]
trans-[7]
trans-[8]
162.8 (t, JCP = 4.9 Hz)
21.0
20.9
21.7
21.3
163.9 (t, JCP = 4.1 Hz)
161.9 (t, JCP = 4.4 Hz)
163.1 (t, JCP = 3.4 Hz)
a
NMR spectra were measured in CD2Cl2.
mass spectrometry.
Selected 13C{1H} and 31P{1H} NMR data of complexes trans-[5]
-trans-[8] are summarized in Table 1. The 13C{1H} NMR spectra of the
four complexes feature the resonances for the carbene carbon atom as
triplet in a narrow range between δ 161.9 ppm and δ 163.9 ppm. The
resonances for the C8 atom in the bromido complexes trans-[6] and
trans-[8] are located slightly more downfield shifted (Δδ ≈ 1 ppm) than
2
their chlorido analogues. The JCP coupling constants for carbon atom
C8 also fall in a narrow range for all four complexes. The observation of
triplet resonances for C8 indicates the formation of mononuclear
complexes possessing two chemically identical phosphor nuclei occu-
pying trans-positions. No dinuclar complexes obtained by attack of the
negatively charged azolato ring-nitrogen atom of one complex at a
second metal center with substitution of a phosphine ligand, as often
observed for the analogous imidazolato and benzimidazolato complexes
[16a,b,18b], were observed. The difference in the coordination chem-
istry of theophyllinato and azolato complexes rests with the difference
in nucleophilicity of the unsubstituted ring-nitrogen atom in the theo-
phyllinato ligand compared to the azolato ligands. The anionic purine-
base derived ligands feature an electron-withdrawing six-membered
ring fused to the diaminoheterocycle thereby reducing the electron
density at the unsubstituted ring-nitrogen atom [19]. The 31P{1H} NMR
spectra of trans-[5]-trans-[8] each show only one singlet (Table 1). The
resonances for the allylic protons of trans-[7] and trans-[8] did not shift
significantly when compared to the chemical shifts observed for the
parent 8-halogenotheophylline derivatives 3 and 4. Thus, coordination
of the N-allyl function in both complexes can be ruled out, since such a
coordination would have caused a significant upfield shift of the re-
sonances for the allylic carbon atoms and protons [22].
Fig. 2. Molecular structures of trans-[5] in trans-[5]·CH2Cl2 (top) and of trans-
[7] in trans-[7]·CH2Cl2 (bottom, hydrogen atoms have been omitted for clarity,
ellipsoids drawn at 50% probability). Selected bond lengths (Å) and angles
(deg) in trans-[5] [trans-[7]]: Pd–Cl 2.367(2) [2.3632(6)], Pd–P1 2.325(2)
[2.3465(6)], Pd–P2 2.330(2) [2.3270(6)], Pd–C8 2.004(6) [1.980(2)], N7–C8
1.356(7) [1.362(3)], N9–C8 1.324(7) [1.338(3)]; Cl–Pd–P1 90.48(5)
[91.93(2)], Cl–Pd–P2 92.29(6) [91.53(2)], Cl–Pd–C8 179.2(2) [178.89(7)],
P1–Pd–P2 176.11(6) [173.69(2)], P1–Pd–C8 88.80(2) [88.56(6)], P2–Pd–C8
88.43(2) [88.07(6)], C8–N7–C5 105.5(6) [106.5(2)], C8–N9–C4 104.4(5)
[104.4(2)], N7–C8–N9 113.4(5) [112.1(2)].
observed in the molecular structure of trans-[7] in accord with the NMR
data.
Next, the N9-protonated derivatives of chloride complexes trans-[5]
and trans-[7] were prepared. Two different synthetic strategies were
employed. Complex trans-[9]BF4 was obtained as described for the
trans-[5] except that 8-chlorocaffeine 1 was reacted with an equimolar
amount of [Pd(PPh3)4] in the presence of an excess of NH4BF4 as proton
source. This protocol yielded the pNHC complex trans-[9]BF4 bearing
an NH,NMe-theophyllin-8-ylidene ligand in good yield of 86% (Scheme
3, top). However, the oxidative addition of N-allyl-8-chlorotheophylline
3 to [Pd(PPh3)4] in the presence of NH4BF4 did not proceed well and
yielded only a small amount of trans-[10]BF4. The reasons for the
The formation of mononuclear complexes trans-[5] and trans-[7]
was confirmed by X-ray diffraction studies with suitable crystals of
composition trans-[5]·CH2Cl2 and trans-[7]·CH2Cl2. The molecular
structures are depicted in Fig. 2. Both structure analyses confirm the
formation of square planar palladium complexes with a trans-arrange-
ment of the two PPh3 groups and a C8-bound theophyllinato ligand
oriented essentially perpendicular to the coordination plane of the
metal center.
The Pd-C8 bond lengths (trans-[5]: 2.004(6) Å, trans-[7]:
1.980(2) Å) as well as the Pd-P bond lengths (trans-[5]: 2.325(2) and
2.330(2) Å, trans-[7]: 2.3465(6) and 2.3270(6) Å) in both complexes
fall in the range previously observed for related complexes bearing
benzimidazole-derived pNHC ligands [16a]. The C8-N9 bond lengths
involving the unsubstituted ring-nitrogen atom are significantly shorter
than the C8-N7 bond lengths in both complexes. This difference is best
explained with the formally negative charge residing at the N9 nitrogen
atom. A similar effect has been observed for platinum [19] or cobalt
[23] complexes bearing anionic C8-bound caffeine-derived azolato li-
gands. The different number of substituents of the two ring-nitrogen
atoms N7 and N9 is also reflected in the C8–N7–C5 (105.5(6) and
106.5(2)° for trans-[5] and trans-[7], repectively) and C8–N9–C4
(104.4(5) and 104.4(2)° for trans-[5] and trans-[7], respectively) bond
angles, where in accord with VSEPR expectations the larger angles are
observed for the alkylated ring-nitrogen atom N7. Various complexes
bearing N-allyl substituted NHCs feature an interaction of the allylic
double bond with the metal center [22]. No such interaction was
Scheme 3. Synthesis of trans-[9]BF4 and trans-[10]BF4.
5

M.C. Jahnke, et al.
InorganicaChimicaActa515(2021)120055
different behavior of the two theophyllines are not clear as the N-sub-
stituent should not have a significant influence on the oxidative addi-
tion or protonation reaction. In order to obtain pNHC complex trans-
[10]BF4, the previously isolated complex trans-[7] was treated with the
strong acid HBF4·Et2O to give trans-[10]BF4 in good yield of 90%
(Scheme 3, bottom). This procedure has previously been employed for
the preparation of a related platinum complex bearing a pNHC ligand
obtained from a coordinated caffeinato ligand [19].
Scheme 4. Synthesis of pNHC complexes trans-[11]BF4 and trans-[12]BF4.
The formation of the compounds trans-[9]BF4 and trans-[10]BF4
was confirmed by NMR spectroscopy and mass spectrometry. The 1H
NMR spectra of the complexes feature the characteristic N-H resonances
at δ 11.00 ppm and δ 12.91 ppm for trans-[9]BF4 and trans-[10]BF4,
respectively. The stronger downfield shift of the N-H resonance in trans-
[10]BF4 can be attributed to the solvent mixture used (CD2Cl2/
DMSO‑d6), which allows the formation of N-H⋯O hydrogen bonds to
DMSO which is not possible for trans-[9]BF4 measured in CD2Cl2.
Similar observations have been made for related complexes bearing
pNHC ligand [16a,c]. The observation of only one resonance in the 31P
{1H} spectra and of a triplet resonance for the carbon atom C8 in the
13C{1H} spectra (δ 167.3 ppm, 2JCP = 10.1 Hz for trans-[9]BF4 and at δ
163.6 ppm, 2JCP = 9.7 Hz for trans-[10]BF4) indicated the formation of
complexes with two trans-arranged phosphine ligands. A comparison of
the 13C{1H} NMR spectra of the two azolato complexes trans-[5] and
trans-[7] with the pNHC complexes trans-[9]BF4 and trans-[10]BF4 re-
veals a significant downfield shift of the C8 resonance upon N-proto-
nation. This is in accord with previous observation made for complexes
bearing benzimidazolato ligands and NH,NR-NHC complexes obtained
from N-alkylated benzimidazoles [16b]. No coordination of the N-allyl
substituent was observed in trans-[10]BF4 as judged from the chemical
shifts of the protons and carbon atoms of the N-allyl group, which are
rather similar to the values observed for N7-allyl-8-chlorotheophylline
3.
caffeinato complex trans-[5]. The N9-protonation causes significant
changes of the endocyclic bond angles. In accord with previous ob-
servations [16b,19], the N7-C8-N9 bond angle shrinks upon N9-proto-
nation by about 6 deg and the endocyclic angles at the ring-nitrogen
atoms N7 and N9 become essentially identical in trans-[9]+ (109.8(2)
and 109.6(2)°).
Finally, the N9-protonated analogs of palladium complexes trans-
[6] and trans-[8] were prepared in a two step protocol similar to the
preparation of trans-[10]BF4 from trans-[7]. Thus, the 8-bromotheo-
phylline derivatives 2 and 4 were initially reacted with [Pd(PPh3)4] in
toluene at room temperature for 2 d. Subsequently, the reaction mix-
ture was treated with HBF4·Et2O giving complexes trans-[11]BF4 and
trans-[12]BF4 in good yields (Scheme 4). While not studied here, we
assume that the pNHC complexes trans-[11]BF4 and trans-[12]BF4 can
be N-deprotonated to give the previously isolated stable azolato com-
plexes trans-[6] and trans-[8] which were prepared by oxidative addi-
tion of 8-bromotheophyllines in the absence of a proton acid (Scheme
2). The reversible N-protonation/N-deprotonation has been demon-
strated multiple times for various azolato/pNHC complexes [7,16,17].
Both compounds were characterized by NMR spectroscopy and mass
spectrometry. The 1H NMR spectra of complexes trans-[11]BF4 and
trans-[12]BF4 are rather similar to those of the chlorido complexes
trans-[9]BF4 and trans-[10]BF4. The characteristic N9-H resonances
Crystals of composition trans-[9]BF4·2CH2Cl2 were obtained be re-
crystallization of trans-[9] from CH2Cl2/diethyl ether. The X-ray dif-
fraction analysis with these crystals revealed the expected square-
planar molecular structure of cation trans-[9]+ with the two phos-
phines in trans-configuration (Fig. 3).
were observed at δ 13.12 ppm and δ 12.69 ppm, respectively. Both, 31
P
{1H} (only one phosphine resonance) and 13C{1H} NMR spectroscopy
(triplet resonance for C8), confirm the trans-configuration of the
phosphine ligands in trans-[11]BF4 and trans-[12]BF4. The resonances
of the phosphorous atoms in the azolato complexes trans-[6] (δ
20.9 ppm) and trans-[8] (δ 21.3 ppm) do not differ significantly from
those observed for the pNHC complexes trans-[11]BF4 (δ 20.4 ppm) and
trans-[12]BF4 (δ 20.4 ppm). While the chemical shifts for the C8 carbon
atoms in the azolato complexes trans-[6] and trans-[8] and the pNHC
complexes trans-[11]BF4 and trans-[12]BF4 do not differ significantly,
A comparison of the metric parameters of trans-[5] and trans-[9]+
reveals some changes caused by protonation of ring-nitrogen atom N9.
First, the Pd-C8 bond slightly shortens upon N9 protonation in trans-
[9]+ in accord with previous observations [19]. The endocyclic C8-N
bond lengths in trans-[9]+ become more similar compared to the
2
the latter ones feature an significantly larger JCP coupling constant. In
summary, the substitution of the chlorido ligands in trans-[9]BF4 and
trans-[10]BF4 for a bromido ligand in trans-[11]BF4 and trans-[12]BF4
does not lead to a significant change of the NMR spectra in general.
Crystals of trans-[11]BF4 and trans-[12]BF4 were obtained by slow
diffusion of diethyl ether into saturated acetonitrile solutions of the
compounds. The molecular structures of the cations trans-[11]+ and
trans-[12]+ are depicted in Fig. 4.
Both complex cations trans-[11]+ and trans-[12]+ exhibit a nearly
square-planar coordination geometry with the pNHC ligand plane or-
iented almost perpendicular to the metal coordination plane.
Comparable metric parameters in trans-[11]+ and trans-[12]+ are ra-
ther similar to those of trans-[9]+. Substitution of the chloro ligand in
trans-[9]+ for a bromo ligand in trans-[11]+ and trans-[12]+ does not
significantly influence the three other Pd-L bond lengths which fall in
the range previous described for complexes bearing NH,NR- [19,24] or
NR,NR-NHC ligands [25]. As indicated by the NMR data, no interaction
of the N-allyl function with the palladium atom was observed.
Fig. 3. Molecular structure of trans-[9]+ in trans-[9]BF4·2CH2Cl2 (hydrogen
atoms have been omitted for clarity, except for N9-H, ellipsoids drawn at 50%
probability). Selected bond lengths (Å) and angles (deg) in trans-[9]+: Pd–Cl
2.3309(5), Pd–P1 2.3453(5), Pd–P2 2.3315(5), Pd–C8 1.982(2), N7–C8
1.340(3), N9–C8 1.365(3); Cl–Pd–P1 90.62(9), Cl–Pd–P2 89.82(2), Cl–Pd–C8
178.13(6), P1–Pd–P2 178.81(2), P1–Pd–C8 89.62(6), P2–Pd–C8 89.91(6),
C8–N7–C5 109.8(2), C8–N9–C4 109.6(2), N7–C8–N9 106.6(2).
4. Conclusions
A series of C8-halogeno-N7-alkyltheophyllines have been prepared
6

M.C. Jahnke, et al.
InorganicaChimicaActa515(2021)120055
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.120055.
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ellipsoids drawn at 50% probability). Selected bond lengths (Å) and angles
(deg) for trans-[11]+ [trans-[12]]: Pd–Br 2.4522(2) [2.4574(2)], Pd–P1
2.3333(5) [2.3528(5)], Pd–P2 2.3345(5) [2.3389(5)], Pd–C8 1.982(2)
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90.228(12) [89.707(13)], Br–Pd–P2 90.139(13) [91.428(13)], Br–Pd–C8
177.39(5) [179.70(6)], P1–Pd–P2 174.03(2) [174.32(2)], P1–Pd–C8 90.45(5)
[90.36(5)], P2–Pd–C8 88.93(5) [88.45(5)], C8–N7–C5 109.6(2) [109.3(2)],
C8–N9–C4 109.7(2) [109.6(2)], N7–C8–N8 106.5(2) [106.7(2)].
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Declaration of Competing Interest
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