Organometallic rhodium (I) complexes with 1-alkylaminopyrazole ligands

Article abstract of DOI:10.1016/S0022-328X(00)00662-8

New bidentate NN′ and tridentate NN′N 1-alkylaminopyrazoles were synthesized and characterized by elemental analyses and spectroscopic methods. The reaction of [RhCl(cod)₂] (cod=cycloocta-1,5-diene) with one equivalent of L 1-alkylaminopyrazoles afforded Rh₂Cl₂(L)(cod)₂ complexes (L=NN′ and NN′N). These rhodium (I) compounds were studied by IR, ¹H- and ¹³C-NMR and liquid mass (with electrospray and APCI interfaces) spectrometries. The ¹H-NMR spectra and molar conductances of these complexes suggested the presence of 1:1 electrolyte species, [Rh(L)cod]⁺ [RhCl₂(cod)]^-, in solution. A combined electrospray and APCI liquid mass spectroscopy study confirmed the presence of both [Rh(L)cod]⁺ and [RhCl₂(cod)]^- species in solution but the existence of a neutral molecular form of complexes in solution could not be demonstrated.

Full text of DOI:10.1016/S0022-328X(00)00662-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 619 (2001) 14–23
Organometallic rhodium (I) complexes with 1-alkylaminopyrazole
ligands
Glo`ria Esquius, Josefina Pons, Ramo´n Ya´n˜ez, Josep Ros *
Departament de Qu´ımica, Facultat de Ciencies, Uni6ersitat Auto`noma de Barcelona, 08193-Bellaterra-Cerdanyola, Barcelona, Spain
Received 17 April 2000; received in revised form 16 July 2000
Abstract
New bidentate NN% and tridentate NN%N 1-alkylaminopyrazoles were synthesized and characterized by elemental analyses and
spectroscopic methods. The reaction of [RhCl(cod)2] (cod=cycloocta-1,5-diene) with one equivalent of L 1-alkylaminopyrazoles
1
afforded Rh2Cl2(L)(cod)2 complexes (L=NN% and NN%N). These rhodium (I) compounds were studied by IR, H- and 13C-NMR
1
and liquid mass (with electrospray and APCI interfaces) spectrometries. The H-NMR spectra and molar conductances of these
complexes suggested the presence of 1:1 electrolyte species, [Rh(L)cod]+ [RhCl2(cod)]−, in solution. A combined electrospray and
APCI liquid mass spectroscopy study confirmed the presence of both [Rh(L)cod]+ and [RhCl2(cod)]− species in solution but the
existence of a neutral molecular form of complexes in solution could not be demonstrated. © 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Rhodium complexes; Rhodiumꢀamine complexes; 1-Alkylaminopyrazoles; Electrospray mass spectra
1. Introduction
[11]. Reactions of [RhCl(cod)]2 with polydentate N-
donor ligands leading to different types of compounds
have been previously described in the literature. Chelat-
ing bidentate N-donor ligands as phenantrolines [12–
15], a-diimines [16], aminomethylpyridynes [17],
aliphatic diamines [14] and bis(pyrazolyl)methane
[18,19] form three types of rhodium (I) complexes de-
pending on the ligand to metal ratio L:M of the reac-
tion (Fig. 1): mononuclear with a h2-coordinated ligand
(A), dinuclear bridged by the diamino ligands (B) and
The coordination chemistry of pyrazoles is an active
field of interest which has been extensively reviewed by
Trofimenko [1–3] and more recently by La Monica et
al. [4]. Pyrazoles can bear donating groups attached to
any position of the aromatic ring affording a large
family of polydentate ligands. Studies of the coordina-
tion chemistry of these ligands include modeling of
metalloenzymes and organometallic chemistry of
polypyrazolylborate ligands [4].
The synthesis of bi (NN%) and tridentate (NN%N)
1-alkylaminopyrazole ligands was developed by
Driessen et al. and, so far, the study of their coordinat-
ing ability has been mainly focused on the design of
chelating systems to mimic metalloenzymes [5–10]. Re-
cently, tridentate ligands bearing three N atoms have
been shown to stabilize unsaturated ruthenium (II)
complexes which made them interesting from the view-
point of potential application in homogeneous catalysis
* Corresponding author. Tel.: +34-93-5811889; fax: +34-93-
5813101.
E-mail address: josep.ros@uab.es (J. Ros).
Fig. 1.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00662-8

G. Esquius et al. / Journal of Organometallic Chemistry 619 (2001) 14–23
15
5989, mass extend to 2000 uma, GC/MS. The chemical
ionization mass spectra were recorded with NH3 as the
reacting gas.
Liquid chromatography mass spectrometry experi-
ments were performed on a Platform II instrument
(Micromass, Manchester, UK) by using a Phoenix 20
syringe pump (C.E. Instruments, Milan, Italy) as an
infusion pump. The carrier was NCMe at a 50 ml
min−1 flow rate. The samples were dissolved in NCMe
at a concentration of 2.0 mg ml−1 and 20 ml of each
solution injected on line by means of a Rheodyne 7125
valve (Rheodyne, Cotati, CA, USA). In the case of
electrospray interface, whole flow was introduced in the
capillary of the source and nebulized with a 20 l h−1
nitrogen flow. The courtain gas was nitrogen at 400 l
h−1 flow rate.
The main electrical conditions were: (a) positive elec-
trospray: capillary at 3500 V; counter-electrode at 500
V, sampling cone was ranged between 50 and 100 V
and the source temperature was 80°C; (b) negative
electrospray: capillary at 3000 V, counter-electrode at 0
V, sampling cone was ranged between 25 and 100 V.
In the case of APCI interface the flow was 1000 ml
min−1 and sample injected volume was 10 ml. The
nebulizer gas was nitrogen set at 200 l h−1 and the
courtain gas was nitrogen at 400 l h−1. The main
electrical conditions were: a) positive APCI: needle at
3000 V; counter-electrode at 500 V, sampling cone was
ranged between 15 and 50 V, the tip temperature was
400°C and the source temperature was 80°C; b) nega-
tive APCI: needle at 4000 V; counter-electrode at 0 V,
sampling cone was ranged between 15 and 50 V, the tip
temperature was 400°C and the source temperature was
80°C.
Fig. 2.
ionic compounds [RhL2(NN)]+[RhCl2L2] (L2=cod)
(C). Non chelating bidentate N-donor ligands have
been also described for aminopyridynes [17], pyrazine
[20], pyrimidyne [21], 4-4%-bipyrazoles [22], benzotria-
zole [23] and imidazole [24]. They form complexes of
the type (B) from a 2:1 L:M ratio, and mononuclear
compounds with a terminal N-bonded ligand from a
1:1 L:M ratio [25]. Some authors have suggested the
existence of an equilibrium between forms B and C in
solution [16,26,27].
In a previous study we reported the synthesis and
coordination chemistry of pyridylꢀpyrazole ligands
[28,29]. Our present research interest is the design and
synthesis of new organometallic compounds containing
polydentate pyrazole-based ligands with two purposes:
potential applications in catalysis and the synthesis of
water-soluble complexes. This paper deals with our first
results about the synthesis of new rhodium (I) com-
plexes with bidentate NN% and tridentate NN%N ligands
(Fig. 2). Some of ligands are new (deai, eae, eai, bdmai
and bmaib) so we describe here their synthesis and
characterization.
2.2. Ligands
2. Experimental
Among studied bidentate NN% ligands, only the 1-
[(N-ethyl)-2-aminoethyl]-3,5-dimethylpyrazole (deae) [5]
was previously described. The rest of the NN% ligands
were synthesized for the first time following an
analogous method which consists of adding a solution
of 10 mmol of tosylates 1-(2-toluene-parasulfony-
loxyethyl)-3,5-dimethylpyrazole [5,31] or 1-(2-toluene-
parasulfonyloxyethyl)pyrazole [32] in 16 ml of THF
dropwise to a stirred mixture of 2.30 g (58 mmol)
NaOH in 60 ml H2O and 66 mmol of the corresponding
primary amine. The temperature was kept at about
50°C. After the addition, which was completed in 1.5 h,
the temperature was raised to 70°C and the stirring was
continued for 4 h. Then, the mixture was allowed to
cool to room temperature (r.t.). The ligands were ex-
tracted with three portions of 25 ml CHCl3 and dried
overnight with MgSO4. The CHCl3 was removed at low
pressure and products were isolated as yellow oils
(Table 1).
2.1. General procedures
All reactions were carried out with the use of vacuum
line and Schlenk techniques. All solvents were dried
and distilled prior to use, according to standard proce-
dures. Samples of [Rh(cod)Cl]2 [30] (cod=cycloocta-
1,5-diene) were prepared as described in the literature.
The elemental analyses were carried out by the staff
of the Chemical Analysis Service of the Universitat
Auto`noma de Barcelona on a Carlo Erba CHNS EA-
1108 apparatus. IR spectra were obtained on a Perkin–
Elmer 2000 spectrometer with NaCl discs or in KBr
pellets. The conductivity measures were taken with a
Crison, micro CM 2200 conductimeter. 1H- and
13C{1H}-NMR spectra were recorded on a RMN-FT
Bruker AC-250 spectrometer in CDCl3 solutions (1H,
250 MHz; 13C, 62 MHz). Electronic impact and chemi-
cal ionization mass spectra were measured on a HP

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G. Esquius et al. / Journal of Organometallic Chemistry 619 (2001) 14–23
17
The new 1-alkylamino ligands of the type NN%N:
bis[(3,5 - dimethyl - 1 - pyrazolyl)methyl] - 1 - methylethyl-
amine (bdmai) and bis[(1-pyrazolyl)methyl]-1-methyl
ethylamine (bmai) were synthesized following previ-
ously described methods [6,7,33]. The bdmai and bmai
ligands were prepared by mixing 8 mmol of the appro-
priate primary amine with 16 mmol of alcohols 1-(hy-
droxymethyl)pyrazole [33] or 1-(hydroxymethyl)-
3,5-dimethylpyrazole [33,34] in 15 ml of dichloroethane
with stirring at r.t. for 24 h. The mixture was dried
overnight with MgSO4. The solvent was then removed
at low pressure and products were isolated as a solid
(bdmai) or as an oil (bmai) (Table 2).
aminoethylpyrazoles deai, eae and eai were readily pre-
pared from tosylates 1-(2-toluene-p-sulfonyloxoethyl)-
3,5-dimethylpyrazole and 1-(2-toluene-p-sulfonyloxo-
ethyl)pyrazole and the corresponding primary amine.
Yellow oily products were extracted with CHCl3 and
isolated in 80% yield. NN%N tridentate ligands, bdmai
and bmab, were synthesized from the direct reaction of
1-(hydroxymethyl)-3,5-dimethylpyrazole and 1-(hydrox-
ymethyl)pyrazole and the corresponding primary
amine. Colourless oily ligands were obtained by evapo-
rating the solvent in vacuo after drying with anhydrous
MgSO4. Yields were of 89 and 86%, respectively. The
new ligands were obtained as pure products and were
characterized unambigously by C, H, and N elemental
analyses (Tables 1 and 2), IR, 1H- and 13C-NMR
spectroscopies and by Electronic Impact or Chemical
Ionisation Mass Spectra (Tables 4 and 5). Assignment
of RMN signals were assigned by reference to the
literature [31–34] and from DEPT, NOEDIFF and
HETCORR RMN experiments (for bdmai).
2.3. Complexes
Rh2Cl2(deae)(cod)2 (1), Rh2Cl2(deai)(cod)2 (2),
Rh2Cl2(eae)(cod)2
(3),
Rh2Cl2(eai)(cod)2
(4),
Rh2Cl2(bdmae)(cod)2 (5), Rh2Cl2((bdmai)(cod)2 (6),
Rh2Cl2(bmae)(cod)2 (7), Rh2Cl2(bmai)(cod)2 (8).
A solution of 0.079 g (0.160 mmol) of [RhCl(cod)]2 in
CH2Cl2 was treated with a solution of 0.320 mmol of
the corresponding ligand in 5 ml of CH2Cl2. After 6 h
of stirring, the CH2Cl2 was evaporated and the remain-
ing solid was washed with cold Et2O. The addition of
hexane to a solution of the solid in a minimum amount
of CH2Cl2 gave yellow–orange complexes, which were
filtered off, and vacuum dried (Table 3).
3.2. Reaction of 1-alkylaminopyrazoles with
[RhCl(cod)]2
The reaction of the rhodium (I) [RhCl(cod)]2 com-
plex with 1-alkylaminopyrazoles (NN% and NN%N) in a
1:2 molar ratio in CH2Cl2 solution led to Rh2Cl2(NN%)-
(cod)2 and Rh2Cl2(NN%N)(cod)2 complexes, respectively
(NN%=deae (1), deai (2), eae (3), eai (4) and NN%N=
bdmae (5), bdmai (6), bmae (7) and bmai (8)) (Scheme
1). Compounds were characterized by elemental analy-
ses, IR and 1H- and 13C-NMR spectroscopies and
conductivity measurements. Table 3 displays yields, C,
H and N elemental analyses and molar conductances of
complexes in MeOH and NCMe. Tables 6 and 7 dis-
play spectroscopic data of complexes. IR spectra of
3. Results and discussion
3.1. Synthesis of ligands
All ligands were synthesized following previously de-
scribed Driessen procedures [5,31,33]. New NN%
Table 3
Ligand weigths, yield, elemental analyses and molar conductances of complexes (1–8)
Complexes Ligand Weight (g) Yield (%) Anal. Found (%)
Anal. Calc. (%)
\
(V−1 cm2
M
mol−1) 10−3
M
MeOH NCMe
Rh2Cl2(deae)(cod)2 (1) Deae
0.054
0.058
0.044
0.048
0.084
85
74
76
81
71
C, 45.14; H, 6.44; N, 6.01
C, 46.37; H, 6.50; N, 6.28
C, 43.48; H, 5.89; N, 6.42
C, 44.78;H, 5.89; N, 6.23
C, 47.47; H, 6.23; N, 9.23
C, 45.47; H, 6.26; N, 6.36
C, 46.31; H, 6.43; N, 6.23
C, 43.69; H, 5.91; N, 6.65
C, 44.60; H, 6.09; N, 6.50
C, 47.75; H, 6.29; N, 9.28
78.1
77.9
81.6
91.7
80.1
59.5
56.1
58.2
59.0
54.2
Rh2Cl2(deai)(cod)2 (2)
Rh2Cl2(eae)(cod)2 (3)
Rh2Cl2(eai)(cod)2 (4)
Rh2Cl2(bdmae)(cod)2
(5)
Deai
Eae
eai
bdmae
Rh2Cl2(bdmai)(cod)2
(6)
bdmai
0.088
78
C, 48.25; H, 6.27; N, 9.26
C, 48.45; H, 6.43; N, 9.11
81.2
55.0
Rh2Cl2(bmae)(cod)2 (7) bmae
Rh2Cl2(bmai)(cod)2 (8) bmai
0.066
0.070
80
73
C, 44.09; H, 5.13; N, 10.49 C, 44.71; H, 5.64; N, 10.03 79.6
C, 45.19; H, 5.66; N, 10.05 C, 45.51; H, 5.81; N, 9.83 77.8
54.5
46.9

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G. Esquius et al. / Journal of Organometallic Chemistry 619 (2001) 14–23
19
Table 5
IR a, 1H- and 13C-NMR data of ligands of the type NN%N
Ligand
bdmai
IR (NaCl–KBr) w (cm−1
)
1H-NMR 250 MHz CDCl3 l (ppm)
13C{1H}-NMR 62 MHz CDCl3 l (ppm)
2967–2919 (wCꢀHal), 1558 (wCꢁCar,
wCꢁNar), 1460 (l CH3as), 1422–1404
(lCꢁCar, lCꢁNar), 1379–1150 (wCꢀN),
5.68 [s, 2H,CH pyrazole], 4.78 [s, 4H,
CH2], 3.06 [septd, 3JHꢀH=6.7 Hz, 1H,
CH(CH3)2], 2.09 [s, 6H, CH3 pyrazole],
1.97 [s, 6H, CH3 pyrazole], 0.94 [d,
3JHꢀH=6.7 Hz, 6H, CH(CH3)2]
146.8 [CCH3], 139.2 [CCH3], 105.6 [CH
pyrazole], 61.8 [CH2], 47.9 [CH(CH3)2],
18.2 [CH(CH3)2], 13.2–10.4 [CCH3]
776 (l CꢀHoop
)
bmai
3105 (wCꢀHar), 2967 (wCꢀHal), 1512
7.35 [d, 3JHꢀH=2.2 Hz, 2H, CH
138.7 [CH pyrazole], 128.6 [CH pyrazole],
105.3 [CH middle pyrazole], 64.9 [CH2],
50.2 [CH(CH3)2], 19.5 [CH(CH3)2]
(wCꢁCar, wCꢁNar), 1465–1443 (l CꢁCar, l pyrazole], 7.34 [d, 3JHꢀH=2.2 Hz, 2H,
CꢁNar), 1395–1119 (wCꢀN), 751 (l
CꢀHoop
CH pyrazole], 6.08 [t, 3JHꢀH=2.2 Hz,
2H, CH middle pyrazole], 4.90 [s, 4H,
CH2], 3.09 [septd, 3JHꢀH=6.7 Hz, 1H,
CH(CH3)2], 0.79 [d, 3JHꢀH=6.7 Hz, 6H,
CH(CH3)2]
)
a al=aliphatic, ar=aromatic, as=asymmetric, oop=out of plane, s=singlet, d=doublet, t=triplet, septd=septuplet.
of complexes in KBr pellets display absorptions of both
1-alkylaminopyrazole and cod ligands. IR spectra of
complexes 1–4 show moderated shifts of the w(NH)
band (3200–3150 cm−1) to lower energies than in the
free ligands (3300 cm−1) whereas the l (NH) band is
observed at 1677–1654 cm−1 [35]. The characteristic
w(CN)+w(CꢁC) absorption for the pyrazolyl group
appears at 1595–1512 cm−1 [28,29]. The 1H-NMR
spectra of complexes are in accordance with the pres-
ence of 1-alkylaminopyrazoles [33] and cod [36] ligands.
Most of the signals of 1-alkylaminopyrazole ligands
shifted downfield by coordination. On the other hand,
the corresponding signal of the NH hydrogen for com-
plexes 1–4 could not be assigned. These data are in
agreement with a bidentate coordination of NN% lig-
ionic forms [Rh(L)(cod)]+ [RhCl2(cod)]− in solution.
The existence of an equilibrium between binuclear neu-
tral and ionic forms which was suggested for related
NN% bidentate ligands (Scheme 2) could not be proved
from NMR data of products. Since efforts to grow
crystals from solutions of complexes were unsuccessful,
we recorded electrospray mass spectra of complexes 2
(deai; NN% type ligand) and 6 (bdmai; NN%N type
ligand) in NCMe in order to confirm the presence of
those ions in solution. This technique is effective for the
study of inorganic complexes in solution, allowing ions
present in solution to be observed in the mass spectra
[38,39]. The positive ionization spectrum of 2 measured
at +50 V cone voltage gave peaks with m/z values of
339 [Rh(L)(cod)+H]+ (molecular peak of the cation),
304 [Rh(cod)(C5H13N)]+, 211 [Rh(cod)]+ and 182 [L+
H]+ (100%). The negative ionization spectrum of 2 at
1
ands. The H-NMR signals of complexes 5–8 indicate
that pyrazolyl groups in NN%N coordinated ligands are
equivalents. This fact suggests also a bidentate coordi-
nation of these ligands by means of N (pyrazolyl) donor
atoms. The cod resonances appear as broad signals,
which could not be resolved at low temperatures. This
can be attributed to the existence of different ‘Rh(cod)’
forms in solution or a possible reorientation of the
coordinated cod ligand, as it has been established in
pyrazolato rhodium (I) complexes [36]. The 13C-NMR
spectra of complexes show resonances for the carbon
atoms of the 1-alkylaminopyrazole and cod ligands. No
significant differences between 13C-NMR spectra of free
and coordinated 1-alkylaminopyrazole ligands were ob-
served. The corresponding signals of the diolefinic lig-
and show the expected 13C chemical shifts [36]. Molar
conductances of complexes measured in MeOH are
between neutral molecules and 1:1 electrolytes. Mea-
surements in NCMe give values, which would be con-
cordant with a neutral formulation of compounds
[27,37].
+50
V cone voltage gave peaks at m/z 281
[RhCl2(cod)ꢀH]− (molecular peak of the anion), 171
[RhC5H8]− (100%) and 113 [C8H17]−. The ESMS spec-
tra of 6 are similar to those of complex 2. The positive
spectrum (+50 V) gave peaks at m/z 487
[Rh(L)(cod)+H]+ (molecular peak of the cation), 307
[Rh(cod)(C5H8N2)]+ and 211 [Rh(cod)ꢀH]+ (100%).
1
The broad signals observed in the H-NMR spectra
of the synthesized Rh2Cl2(L)(cod)2 complexes (L=NN%
and NN%N) are consistent with the presence of both
Scheme 1.

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G. Esquius et al. / Journal of Organometallic Chemistry 619 (2001) 14–23
21

22
G. Esquius et al. / Journal of Organometallic Chemistry 619 (2001) 14–23
Scheme 2.
species we registered positive and negative electrospray
mass spectra of compounds. The results confirmed our
hypothesis and cationic [Rh(L)(cod)]+ and anionic
[RhCl2(cod)]− species have been detected in the ES
mass spectra. In addition, the atmospheric pressure
chemical ionization mass spectra of some complexes
were also registered to detect the molecular form of
complexes but only ionic species were found.
Acknowledgements
Support by the Ministerio de Educacio´n y Cultura of
Spain (Project PB96-1146 and grant to G.E.) is grate-
fully ackowledged.
Fig. 3.
The negative electrospray spectrum (−50 V) shows
peaks at m/z 283 [RhCl2(cod)+2H]− (molecular peak
of the anion), 281 [RhCl2(cod)ꢀH]−, 171 [Rh(C5H9)]−
and 113 [C8H17]− (100%). In order to identify the
molecular complexes of the type Rh2(L)Cl2(cod)2 (L=
deai NN% and bdmai NN%N) we also recorded the APCI
(atmospheric pressure chemical ionization) mass spectra
of complexes 2 and 6. This technique shows a different
fragmentation pattern but peaks of some cationic and
anionic species can be observed. APCI(+) spectra
displays m/z peaks:(2) 25 V, 392 [Rh(L)(cod)]+ and 182
[L+H]+ (100%); (6) 50 V, 211 [Rh(cod)]+ (100%).
APCI(−) spectra of complexes 2 and 6 show identical
patterns with a m/z peak at 381 [RhCl2(cod)]−. Neither
mass spectrum spectroscopy shows peaks correspond-
ing to the molecular complex Rh2Cl2(L)(cod)2.
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