Luminescence of neutral and ionic gold(I) complexes containing pyrazole or pyrazolate-type ligands

Article abstract of DOI:10.1016/j.jorganchem.2006.12.025

A series of pyrazole (Hpz) and pyrazolate (pz^-) Au(I) complexes of types [Au(Hpz^2R(n))(PPh₃)]⁺ (I), [Au(Hpz^2R(n))₂]⁺ (II), [Au(μ-pz^R(n))]₃ (III), [Au(pz^R(n)/2R(n))(PPh₃)] (IV), [AuCl(Hpz^R(n)/2R(n))] (V) and [(PPh₃)Au(μ-pz^R(n))Au(PPh₃)]⁺ (VI), R(n) and 2R(n) represent C₆H₄OC_nH_2n+1 substituents at the 3- or 3- and 5-positions of the heterocyclic ring, respectively, have been shown to be luminescent in the solid state at 77 K, independently of the presence or not of inter-metallic Au-Au interactions. The emission spectra of all complexes consist of structured bands in the region 395-500 nm, attributed to ligand-to-metal charge transfer (LMCT) transitions involving the Hpz or pz^- ligands, the pattern of bands of compounds being related with the molecular structure and/or the nature of the ligands. The thermal behaviour of several complexes of the types III, IV and V containing long-chain substituents (n ≥ 12) was examined by polarising light optical microscopy (POM). The derivative [AuCl(Hpz^R(12))] was proved to have liquid crystal properties exhibiting a mesophase SmA but the remaining complexes were not liquid crystal materials. This complex is one of the scarce examples of Au(I) derivatives exhibiting both liquid crystal and luminescent properties.

Full text of DOI:10.1016/j.jorganchem.2006.12.025

Journal of Organometallic Chemistry 692 (2007) 1690–1697
www.elsevier.com/locate/jorganchem
Luminescence of neutral and ionic gold(I) complexes
containing pyrazole or pyrazolate-type ligands
a
a
a,
b
*
´
´
´
Paloma Ovejero , Marıa Jose Mayoral , Mercedes Cano , Marıa Cristina Lagunas
a
Departamento de Quı´mica Inorga´nica I, Facultad de Ciencias Quı´micas, Universidad Complutense, E-28040 Madrid, Spain
b
School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AG, UK
Received 30 November 2006; received in revised form 19 December 2006; accepted 19 December 2006
Available online 5 January 2007
Abstract
A series of pyrazole (Hpz) and pyrazolate (pzꢀ) Au(I) complexes of types [Au(Hpz2R(n))(PPh3)]+ (I), [Au(Hpz2R(n))2]+ (II), [Au(l-
pzR(n))]3 (III), [Au(pzR(n)/2R(n))(PPh3)] (IV), [AuCl(HpzR(n)/2R(n))] (V) and [(PPh3)Au(l-pzR(n))Au(PPh3)]+ (VI), R(n) and 2R(n) represent
C6H4OCnH2n+1 substituents at the 3- or 3- and 5-positions of the heterocyclic ring, respectively, have been shown to be luminescent in the
solid state at 77 K, independently of the presence or not of inter-metallic Au–Au interactions. The emission spectra of all complexes con-
sist of structured bands in the region 395–500 nm, attributed to ligand-to-metal charge transfer (LMCT) transitions involving the Hpz or
pzꢀ ligands, the pattern of bands of compounds being related with the molecular structure and/or the nature of the ligands. The thermal
behaviour of several complexes of the types III, IV and V containing long-chain substituents (n P 12) was examined by polarising light
optical microscopy (POM). The derivative [AuCl(HpzR(12))] was proved to have liquid crystal properties exhibiting a mesophase SmA
but the remaining complexes were not liquid crystal materials. This complex is one of the scarce examples of Au(I) derivatives exhibiting
both liquid crystal and luminescent properties.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Gold(I) complexes; Pyrazole and pyrazolate ligands; Luminescence; Liquid crystal; Au–Au interactions
1. Introduction
luminescent behaviour can be related to the presence of
aurophilic interactions. In all the studied structures the
Au centres exhibited the characteristic linear coordination,
however only those of complexes of classes III and IV have
shown shorts Au–Au contacts which were intra- or inter-
molecular, respectively. By contrast, the structures of the
cationic derivatives of classes I and II do not exhibit auro-
philic interactions.
Gold (I) complexes with neutral N-donor ligands are
much less common than those with P-donor ligands
because of the soft character of the metal centre [1]. How-
ever, our recent studies on the chemistry of substituted
pyrazole-based complexes [2–6] have allowed us to describe
compounds of the types [Au(Hpz2R(n))(PPh3)]+ (I), [Au-
(Hpz2R(n))2]+ (II), [Au(l-pzR(n))]3 (III) and [Au(pzR(n)/2R(n)
)
In this context it is interesting to note that luminescent
properties of the linear Au(I) complexes are usually related
(PPh3)] (IV) in which the pyrazole or pyrazolate groups
carried R(n) substituents (R(n) = C6H4OCnH2n+1) at the
3- (HpzR(n)) or 3- and 5-(Hpz2R(n)) positions of the hetero-
cyclic ring. The X-ray crystal structures of representative
examples of each family have been previously reported
[2–6] and they are now revised in order to clarify if the
˚
to the presence of Au–Au interactions of less than 3.6 A
[5,7–16]. In fact, photoluminescence studies of complexes
of the type [AuY2]ꢀ, where Y represents an anionic ligand,
have been used to elucidate the extension of inter-molecu-
lar Au–Au interactions [16]. Thus, cianurate derivatives
isolated as K+, Cs+, Tl+ salts exhibiting Au–Au contacts
˚
*
of 3.1–3.64 A were proved to have luminescent properties,
Corresponding author. Fax: +34 91 3944352.
E-mail address: mmcano@quim.ucm.es (M. Cano).
however those containing bulkier cations such as NBuþ4 in
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.12.025

P. Ovejero et al. / Journal of Organometallic Chemistry 692 (2007) 1690–1697
1691
which the inter-molecular contacts were prevented behaved
as not luminescent materials [16].
(alkyloxy)phenyl]-1H-pyrazoles (Hpz2R(n)) were prepared
by procedures described in previous works [17–20].
On the other hand, several di and polynuclear pyrazo-
late Au(I) complexes exhibiting luminescent behaviour
have been reported [5,9–12,14]. In particular, for trimetallic
derivatives of the type [Au(l-pz)]3 [11,12], the presence of
Au–Au inter-molecular interactions has been considered
responsible for the emissions whereas no luminescence
was observed in analogous trigold species exhibiting only
intra-molecular contacts [11]. The related digold(I) com-
plexes of the types [Au(pz)(PPh3)]2 [5,9] and [Au2(l-
pz)(L2)] (L2 = (PPh3)2, 1,3-diphenylphosphinopropane)
[9], both exhibiting Au–Au contacts, give rise to structured
emissions, which were attributed to ligand-to-metal charge
transfer (LMCT) transitions. Luminescent tetrametallic
species [Au(l-pz)]4 have also been described, and their
emission attributed mainly to the presence of intra-molec-
ular Au–Au contacts [14].
Elemental analyses for carbon, hydrogen, nitrogen were
carried out by the Microanalytical Service of Complutense
University. IR spectra were recorded on
a FTIR
ThermoNicolet 200 or on FTIR Nicolet Magna-550 spec-
trophotometers with samples as KBr pellets in the
4000–400 cmꢀ1 region: vs (very strong), m (medium).
1H NMR spectra were performed on a Bruker AC200
(200.13 MHz) and 31P{1H} NMR spectrum was recorded
on a Bruker DPX-300 (121.49 MHz) of the NMR Service
of Complutense University from solutions in CDCl3 at
room temperature. Chemical shifts d(H) ( 0.01 ppm) are
listed relative to TMS using the signal of the deuterated sol-
vent as reference (7.26 ppm), and d(P) ( 0.3 ppm) relative
to 85% H3PO4. Coupling constants (J) are in Hz ( 0.3 Hz).
Multiplicities are indicated as s (singlet), d (doublet), t
(triplet), m (multiplet), br (broad signal).
As it can be observed the luminescence properties of
pyrazolate Au(I) complexes have been extensively explored
in contrast to those of related pyrazole derivatives. How-
ever, several types of pyrazole Au(I) compounds have been
described and structurally characterised [2,3].
The electronic absorption spectra were recorded on a
Perkin–Elmer UV/Vis spectrometer Lambda800. The emis-
sion spectra were recorded on a Perkin–Elmer LS55 lumi-
nescence spectrometer equipped with
a
R928
photomultiplier and a low temperature accessory.
In this context we decided to undertake a study of the
structural and optical properties of several derivatives con-
taining pyrazolate or pyrazole ligands. This comprises the
investigation of the luminescence properties of cationic or
neutral complexes of classes I–IV (1–11) [2,6], as well as
those of the new derivatives of class V [AuCl(HpzR(n))]
(12) and [AuCl(Hpz2R(n))] (13,14) and of class VI
[(Ph3P)Au(l-pzR(n))Au(PPh3)]+ (15) (Table 1).
Phase studies were carried out by optical microscopy
using an Olympus BX50 microscope equipped with a Lin-
kam THMS 600 heating stage. The temperatures were
assigned on the basis of optic observations with polarised
light.
2.2. Preparation of compounds 1–4 (types I and II)
The photoluminescence behaviour of complexes 1–15
(Table 1) is analysed taking into account the nature of
the ligands, counterions (for the ionic derivatives) as well
as the observed molecular structures, and related to the
presence of inter-metallic interactions. To these effects the
main results of the crystal structure of the new complex
15 are also presented.
In addition, following our previous experience in the
design of liquid crystalline metal complexes based on mes-
ogenic or pro-mesogenic R(n)-mono and disubstituted pyr-
The complexes 1–4 were synthesised as previously
reported [2,3].
2.3. Preparation of compounds 5–7 (type III)
Complex 5 was prepared following our previously
described method for compounds of the type [Au(l-
pzR(n))]3 [6]. Analogous procedures were used to obtain
the new derivatives 6 and 7, as described below.
azole ligands (R(n) = C6H4OCnH2n+1
)
[17–19], the
2.3.1. Preparation of [Au(l-pzR(14))]3 (6)
complexes 6, 7, 11, 12 and 14 were considered as candidates
to present liquid crystal properties and therefore very inter-
esting as potential materials exhibiting both liquid crystal
and luminescent properties. A study on their thermal prop-
erties is now presented.
Complex 6 was obtained as described previously [6]
from [AuCl(tht)] (85.6 mg, 0.240 mmol), HpzR(14)
(77.0 mg, 0.240 mmol) and an excess of 60% NaH.
Yield: 56%. Elemental analyses: found: C 49.7, H 6.0, N
5.1%; calculated for C69H105N6O3Au3: C 50.0, H 6.4, N
5.1. IR (KBr, cmꢀ1): 1612m m(C@N) + m(C@C). 1H
NMR (300 MHz, CDCl3, d in ppm, J in Hz, 298 K):
2. Experimental
3
d = 0.88 (t, JH,H = 6.1, CH3), 1.77–1.82 (m, CH2), 3.88
3
3
2.1. Materials and physical measurements
(t, JH,H = 6.6, OCH2), 4.01 (t, JH,H = 6.6, OCH2), 6.52
3
3
(d, JH,H = 2.2, H-C(4)(pz)), 6.61 (d, JH,H = 8.5,
3
All commercial reagents were used as supplied. The start-
ing Au-complexes [Au(NO3)(PPh3)], [AuCl(tht)] (tht = tet-
rahydrothiophene), [Au(Tf)(PPh3)] (Tf = CF3SO3), and
the 3-substituted and 3,5-disubstituted pyrazoles, 3-[4-
(alkyloxy)phenyl]-1H-pyrazoles (HpzR(n)) and 3,5-bis[4-
H
meta(C6H4)), 6.91 (d, JH,H = 2.2, H-C(4)(pz)), 6.94 (d,
3JH,H = 8.5, 3JH,H = 2.2,
Hmeta(C6H4)), 6.95 (d,
H-C(4)(pz)), 7.40 (d, JH,H = 2.2, H-C(5)(pz)), 7.41
(d, JH,H = 2.2, H-C(5)(pz)), 7.62 (d, JH,H = 8.0,
ortho(C6H4)), 7.90 (d, JH,H = 8.0, Hortho(C6H4)).
3
3
3
3
H

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P. Ovejero et al. / Journal of Organometallic Chemistry 692 (2007) 1690–1697
2.3.2. Preparation of [Au(l-pzR(16))]3 (7)
Complex 7 was synthesised as described [6] from
[AuCl(tht)] (100 mg, 0.202 mmol), HpzR(16) (77.7 mg,
0.202 mmol) and an excess of 60% NaH.
Yield: 41%. Elemental analyses: found: C 51.3, H 6.6, N
4.7%; calculated for C75H117N6O3Au3: C 51.7, H 6.8, N
4.8. IR (KBr, cmꢀ1): 1612m m(C@N) + m(C@C). 1H
NMR (300 MHz, CDCl3, d in ppm, J in Hz, 298 K):
3
d = 0.88 (t, JH,H = 6.7, CH3), 1.76–1.85 (m, CH2), 3.88
3
3
(t, JH,H = 6.6, OCH2), 4.03 (t, JH,H = 6.6, OCH2), 6.57
3
3
(d, JH,H = 2.4, H-C(4)(pz)), 6.60 (d, JH,H = 2.4,
H-C(4)(pz)), 6.64 (d, JH,H = 8.3, Hmeta(C6H4)), 6.65
(d, JH,H = 2.4, H-C(4)(pz)), 6.97 (d, JH,H = 8.3,
meta(C6H4)), 7.39 (d,3JH,H = 2.2, H-C(5)(pz)), 7.51 (d,
H-C(5)(pz)), 7.65 (d,
3JH,H = 2.2, 3JH,H = 8.8,
ortho(C6H4)), 7.92 (d, JH,H = 8.8, Hortho(C6H4)).
3
3
3
H
3
H
2.4. Preparation of compounds 8–11 (type IV)
The compounds 8–10 were synthesised as reported by us
in a previous work [5]. A similar method was used to obtain
11, as indicated below.
2.4.1. Preparation of [Au(pzR(14)) (PPh3)]2 (11)
Complex 11 was prepared as described [5] from
[Au(NO3)(PPh3)] (44 mg, 0.084 mmol) and NapzR(14). The
latter was obtained in situ from HpzR(14) (30.7 mg,
0.084 mmol) and an excess of 60% NaH.
Yield: 60%. Elemental analyses: found: C 60.1, H 6.0, N
3.4%; calculated for C82H100N4O2P2Au2: C 60.4, H 6.2, N
3.4%. IR (KBr, cmꢀ1): 1613m m(C@N) + m(C@C). 1H
NMR (300 MHz, CDCl3, d in ppm, J in Hz, 298 K):
3
d = 0.71 (t, JH,H = 6.6, CH3), 1.62 (m, CH2), 3.80 (t,
3JH,H = 6.6, OCH2), 6.5 (br, H-C(4)(pz)), 6.72 (d,
3JH,H = 8.6, Hmeta(C6H4)), 7.34 (br, H-C(5)(pz)), 7.63 (d,
3JH,H = 8.6, Hortho(C6H4)). 31P{1H} NMR (121.49 MHz,
CDCl3, d in ppm, 298 K.): d = 32.8.
2.5. Preparation of complexes 12–14 (type V)
2.5.1. Preparation of [AuCl(HpzR(12))] (12)
A
THF solution (20 mL) of HpzR(12) (90 mg,
0.274 mmol) was added to a solution of [AuCl(tht)]
(87.9 mg, 0.274 mmol) in dry THF (20 mL). After 24 h of
stirring, the resulting mixture was filtered through a plug
of Celite and the colourless filtrate was evaporated to dry-
ness. The residue was dissolved in CH2Cl2 and precipitated
with hexane.
Yield: 40%. Elemental analyses: found: C 44.6, H 5.6, N
4.9%; calculated for C21H32ClN2OAu: C 45.0, H 5.7, N 5.0.
IR (KBr, cmꢀ1): 3198m m(NH), 1615vs m(C@N) + m(C@C).
1H NMR (300 MHz, CDCl3, d in ppm, J in Hz, 298 K):
3
d = 0.90 (t, JH,H = 6.7, CH3), 1.20–1.70 (m, CH2), 1.83
3
(m, CH2), 4.03 (t, JH,H = 6.4, OCH2), 6.61 (s, H-
3
C(4)(pz)), 6.99 (d, JH,H = 8.6, Hmeta(C6H4)), 7.39 (s, H-
3
C(5)(pz)), 7.45 (d, JH,H = 8.6, Hortho(C6H4)), 12.54 (s,
NH).

P. Ovejero et al. / Journal of Organometallic Chemistry 692 (2007) 1690–1697
1693
2.5.2. Preparation of [AuCl(Hpz2R(4))] (13)
The analytical and spectroscopic data of the new com-
plexes (Section 2) are in agreement with the proposed for-
mulation. Selected spectroscopic and structural features are
discussed below.
As described for 12, from Hpz2R(4) (85.1 mg,
0.223 mmol) and [AuCl(tht)] (74.9 mg, 0.233 mmol).
Yield: 52%. Elemental analyses: found: C 46.4, H 5.1, N
4.5%; calculated for C23H28ClN2O2Au: C 46.3, H 4.7, N
4.7. IR (KBr, cmꢀ1): 3234m m(NH), 1615vs
m(C@N) + m(C@C). 1H NMR (300 MHz, CDCl3, d in
The new complexes of the families III (6,7) and IV (11)
containing long-chain R(n)-substituents (n P 14) on the
pyrazole have been synthesised by a procedure similar to
that described for related complexes [5,6].
3
ppm, J in Hz, 298 K): d = 0.85 (t, JH,H = 6.1, CH3),
3
1
1.11–1.72 (m, CH2), 3.86 (t, JH,H = 6.4, OCH2), 6.54 (s,
The H NMR data for new trinuclear pyrazolate com-
3
H-C(4)(pz)), 6.81 (d, JH,H = 8.6, Hmeta(C6H4)), 7.49 (d,
plexes (type III) [Au(l-pzR(14))]3 (6) and [Au(l-pzR(16))]3
(7) indicate the presence of an asymmetric trimetallic unit.
Thus, the spectra exhibit three H-C(4) and two H-C(5)
signals in a 1:1:1 and 2:1 ratio, respectively, in agreement
with the presence of non-equivalent pyrazolate groups.
Accordingly, a similar pattern is observed for the OCH2
and aromatic protons of the R substituents. The latter
appear as two doublets (3JH,H ꢁ 8 Hz) for each the ortho
and meta protons at ca. 7.6 and 7.9 ppm, and 6.6 and
6.9 ppm, respectively, whereas the OCH2 groups give
two triplets (3JH,H ꢁ 6.6 Hz) at ca. 3.9 and 4.0 ppm, in
a 4:2 ratio.
3JH,H = 8.6, Hortho(C6H4)).
2.5.3. Preparation of [AuCl(Hpz2R(12))] (14)
As described for 12, from Hpz2R(12) (105.9 mg,
0.181 mmol) and [AuCl(tht)] (58.0 mg, 0.181 mmol).
Yield: 45%. Elemental analyses: found: C 57.0, H 7.4, N
3.3%; calculated for C39H60ClN2O2Au: 57.0, H 7.4, N
3.4%. IR (KBr, cmꢀ1): 3430m m(NH), 1615vs
m(C@N) + m(C@C). 1H NMR (300 MHz, CDCl3, d in
3
ppm, J in Hz, 298 K): d = 0.88 (t, JH,H = 5.6, CH3),
3
1.74–1.87 (m, CH2), 3.99 (t, JH,H = 6.0, OCH2), 6.69 (s,
3
H-C(4)(pz)), 6.95 (d, JH,H = 8.8, Hmeta(C6H4)), 7.64 (d,
In the absence of adequate crystals of 6 and 7 for X-
ray structural determination, the structure of these com-
pounds has been considered similar to that found for the
related complex [Au(l-pz2pp)]3(pz2pp = 3,5-bis (4-phen-
oxyphenyl)pyrazolate) (16) [6] (Fig. 1b), which shows a
trigonal geometry in which the three metal centres are
connected through intra-molecular Au–Au contacts of
3JH,H = 8.8, Hortho(C6H4)).
2.6. Preparation of [(Ph3P)Au(l-pzR(8))Au(PPh3)]Tf
(15) (type VI)
To a solution of [Au(Tf)(PPh3)] (60 mg, 0.098 mmol)
in 25 mL of dry THF was slowly added NapzR(8), pre-
pared by treating HpzR(8) (13,32 mg, 0,049 mmol) with
an excess of 60% NaH in dry THF. The mixture was
stirred for 24 h, then filtered through Celite. The filtrate
was evaporated to dryness and the residue was dissolved
in CH2Cl2. Upon addition of hexane, a white precipitate
formed, which was filtered and dried in vacuo. Crystals
were obtained by slow diffusion of hexane into a CH2Cl2
solution of 15.
˚
3.309(1) A [6].
The 1H NMR spectrum of complex 11 exhibits the char-
acteristic signals of monosubstituted pyrazole ligand. All
attempts to get adequate crystals for X-ray structural
determination of this complex were unsuccessful and then
the X-ray crystal structure of the related compound (8)
[4] was used as reference (Fig. 1b). The molecular structure
of the later shows a dimeric nature in which the individual
molecules are connected through inter-molecular Au–Au
˚
Yield: 60%. Elemental analyses: found: C 48.2, H 4.0, N
2.1%; calculated for C54H53F3N2O4P2SAu2: C 48.4, H 4.0,
N 2.1. IR (KBr, cmꢀ1): 1611vs m(C@N) + m(C@C). 1H
NMR (300 MHz, CDCl3, d in ppm, J in Hz, 298 K):
contacts of 3.029(1) A.
The compounds 12, 13, 14 (type V) and 15 (type VI)
involved new structural types to respect to those previously
described.
3
d = 0.88 (t, JH,H = 6.1, CH3), 1.11–1.72 (m, CH2),
Type V
complexes [AuCl(HpzR(12))] (12), [AuCl-
3
3
3.86 (t, JH,H = 6.4, OCH2), 6.85 (d, JH,H = 8.6,
(Hpz2R(4))] (13) and [AuCl(Hpz2R(12))] (14) were prepared
from equimolecular reactions between [AuCl(tht)] and the
corresponding pyrazole ligand. The pyrazole ligands,
HpzR(12), Hpz2R(4) and Hpz2R(12), were selected to study
the influence of the alkyl chain length as well as the effect
of mono- versus di-substitution of the pyrazole ring on
the luminescence properties of the complexes.
H
H
meta(C6H4 + H-C(4)(pz)),
7.49
(d,
3JH,H = 8.6,
ortho(C6H4) + H-C(5)(pz)). 31P{1H} NMR (121.49 MHz,
CDCl3, d in ppm, 298 K): d = 32.1.
3. Results and discussion
3.1. Spectroscopic and structural characterisation
In addition, the thermal behaviour of the new complexes
12 and 14 with potential for liquid crystal properties was
also examined and the results will be discussed below.
The type VI complex, 15, was prepared by reaction of
[Au(Tf)(PPh3)] [17] and NapzR(8) in a 2:1 molar ratio.
The crystal structure of 15 was determined by X-ray dif-
fraction and we describe here only the results conducting
to show the presence of metal–metal interactions. The
Typical structures for each class of compounds are
established on the basis of a single crystal X-ray data of
representative examples described in previous works [2,6]
and in this work (Fig. 1b). Fig. 1a depicts a schematic rep-
resentation of molecular structures for the complexes of
classes I–VI.

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P. Ovejero et al. / Journal of Organometallic Chemistry 692 (2007) 1690–1697
a
R= C6H4OCnH2n+1
R
R
N
R
N
N
R
Au
Au
N
N
Ph3P
N
N
Au
Au
R
R
N
N
N
R
N
N
R
H
H
H
Au
type III
[Au( pzR(n))]3
R
type I
type II
[Au(Hpz2R(n))(PPh3)]+
[Au(Hpz2R(n))2]+
R1
R1
R1
R
N
N
R2
Cl
Au
N
N
N
R2
R2
N
H
N
N
Au
Au
Au
PPh3
Au
Ph3P
Ph3P
PPh3
type V
type IV
[AuCl(HpzR(n))]
[AuCl(Hpz2R(n))]
type VI
[Au(pzR(n))(PPh3)]
[Au(pz2R(n))(PPh3)]
1
2
1
2
R =H; R =R
R1=R2=R
R =H; R =R
R1=R2=R
[(Ph3P)Au( -pzR(n))Au(PPh3)]+
type III
type I
type II
b
[Au( pz2pp)]3 (16)
2pp = C6H4OC6H5
[Au(Hpz2R(4))(PPh3)]NO3 (1)
[Au(Hpz2R(4))2]PTS (4)
d(Au---Au)inter= 7.910(1)2
Å
d(Au---Au)inter= 8.650(1)3
Å
d(Au---Au)intra= 3.309(1)6Å
type IV
type VI
[Au(pz2R(4))(PPh3)] (8)
[(Ph3P)Au( -pzR(8))Au(PPh3)]Tf (15)
d(Au---Au)intra= 3.029(1)4
Å
d(Au---Au)intra= 3.387(1)*Å
* To be published
Fig. 1. (a) Schematic representation of molecular structures for the complexes of types I–VI and (b) crystal structures of classes I–IV and VI.
molecular structure (Fig. 1b) consists of one bimetallic unit
in which each Au(I) centre is coordinated to a PPh3 ligand
and N-bonded to a bridging pyrazolate ligand. The coordi-
nation around each metal is almost linear (177.87°) and the
two Au(I) atoms in the molecule are separated by
The crystal structures of complexes [Au(Hpz2R(4))-
(PPh3)]NO3 (1) and [Au(Hpz2R(4))2]PTS (4) (PTS = CH3-p-
C6H4SO3) and [Au(pzR(4))(PPh3)] (8), from classes I, II and
IV, respectively, are included in Fig. 1b, to aid the
discussion of the optical results. Compounds 1 and 4
(Fig. 1b) show monomeric structures in which neither inter-
nor intra-molecular Au–Au contacts were found [2,3], how-
ever, the complex 8 (Fig. 1b) forms dimers in the solid state
˚
3.387(1) A, in agreement with a weak intra-molecular
Au–Au interaction. Full structural data will be published
as part of a comparative study of Au-pyrazolate com-
plexes, which is currently in progress.
˚
through strong Au–Au interactions of 3.029(1) A [4].

P. Ovejero et al. / Journal of Organometallic Chemistry 692 (2007) 1690–1697
1695
3.2. Thermal studies
The complexes 6, 7, 11, 12 and 14 having long chain sub-
stituent on the pyrazole ring were studied by optical
microscopy under polarized light (POM). Differential scan-
ning calorimetry (DSC) was used for those complexes
exhibiting liquid crystalline behaviour.
In previous works, we have shown that both 3-mono-
substituted (HpzR(n)
) )
and 3,5-disubstituted (Hpz2R(n)
pyrazoles containing alkyloxyphenyl substituents C6H4O-
CnH2n+1 can induce mesomorphism upon coordination to
different metal fragments. However, only the disubstituted
pyrazole (Hpz2R(n)) (n = 4–18 carbon atoms) presents
liquid crystal properties as free ligands [17–19], in contrast
with the non-mesogenic behaviour of the related monosub-
stituted HpzR(n) ligands. In view of these results, we under-
took the study of the thermal behaviour of selected
compounds of types III, IV and V with n P 12.
The trimeric complexes of class III (6,7) were proposed
as adequate candidates for liquid crystal properties. In fact,
the related complex [Au(l-pzR(n))]3 (n = 12) had been
proved to have columnar mesophases consistent with the
columnar distribution of disc-like non-planar trimers
observed in the X-ray structure of 16 [6]. However, those
with shorter (n = 4) or longer (n = 18) chains behaved as
non-liquid crystal materials [6].
Fig. 2. Texture of the SmA mesophase on second cooling of
[AuCl(HpzR(12))] (12) at 60 °C.
first heating
first cooling
second cooling
POM observations of complexes 6 and 7 having 14 and
16 carbon atoms in the alkylic chains showed that the com-
plexes were non-mesomorphic and simply melted to the
isotropic state. It appears that the increase to 14, 16 or
18 carbon atoms in the alkylic chains disrupts the ordering
on the columnar arrangement of mesophases established
for the complex with 12 carbon atoms.
50
55
60
65
70
75
80
85
Temperature (ºC)
Quite interestingly the coordination of the non-meso-
genic HpzR(12) as pyrazolate ligand to the AuCl fragment
(complex 12) results in the appearance of a mesomorphic
behaviour which was monotropic. So after the phase tran-
sition from solid to the isotropic liquid at 82.4 °C, the mes-
ophase was observed on cooling (Fig. 2) at 69.2 °C
exhibiting an oily streak texture characteristic of SmA mes-
ophases. Crystallization begins to take place close to the
temperature of formation of mesophase. As a consequence
only a broad peak, attributed to an overlapping of both
phase transitions I-SmA and SmA-Cr, could be observed
on the thermogram of DSC. In agreement with that consid-
eration the enthalpy value (DH = ꢀ39.49 kJ molꢀ1) was
high enough to involve both sequential and overlapped
processes (Fig. 3).
Fig. 3. DSC thermogram of [AuCl(HpzR(12))] (12).
melts at 155 °C to the isotropic liquid. Upon cooling a sin-
gle phase transition corresponding to the crystallization
from the isotropic liquid was observed.
The different thermal behaviour of 12 and 14 can be
explained by considering the more appropriate molecular
shape of complex 12, compared to that of 14 for liquid
crystal properties. The former, containing a sole R substi-
tuent at the 3-position of the pyrazole ring, could form a
rod-like elongated dimeric structure, such as that depicted
in Fig. 4, which would be responsible for the smectic mes-
ophase observed. That structure should not be possible for
the related compound 14 with 3,5-disubstituted pyrazole.
Subsequent DSC scans show on heating a behaviour
analogous to that observed on the first cycle. However,
on the second cooling the thermogram exhibits an addi-
tional peak at 52.2 °C that should correspond to the crys-
tallization process from the mesophase as it had also
been observed under MOP. The mesophase was then stable
between 65 and 52 °C.
Complex [AuCl(Hpz2R(12))] (14) did not exhibit meso-
morphic properties. It is solid at room temperature and
Fig. 4. Schematic representation of [AuCl(HpzR(12))] (12) in which the
rod-like elongated dimeric structure is visualised.

1696
P. Ovejero et al. / Journal of Organometallic Chemistry 692 (2007) 1690–1697
Unfortunately, all attempts to get crystals of type V
chain length of the R(n)/2R(n)-substituents (compare 13–
14, 8–10, or 5–7).
complexes suitable for X-ray diffraction studies were
unsuccessful, that feature being commonly found for com-
plexes based on long chained pyrazole ligands.
None of type IV complexes exhibited mesomorphism,
which should be expected on the basis of their molecular
shape containing the bulky PPh3 ligand.
The room temperature solid-state emission of some of
the free pyrazoles was also measured for comparison.
The 3-substituted pyrazoles (HpzR(n)) showed an intense
band at 320 nm when excited at 275 nm, whereas the 3,5-
disubstituted (Hpz2R(n)) emitted at 340 nm (kexc = 290 nm).
Taken the above observations into account, and given
the similarity of all the solid-state emission spectra of the
Au(I) compounds as well as their vibronic structures, the
most plausible origin for the luminescence of the complexes
is a ligand (Hpz/pzꢀ) to metal charge transfer (LMCT)
transition [9,10]. The energy separation between the two
first (and well-defined) emission maxima in the spectra
ranges between 1100 and 1470 cmꢀ1, and corresponds well
with m(C@C) and/or m(C@N) vibrations of the pyrazole/
pyrazolate rings. However, given the presence of Au–Au
contacts in the pyrazolate derivatives of the families III,
IV and VI, some ligand-to-metal–metal charge transfer
(LMMCT) character of the emission cannot be ruled out
in these complexes.
3.3. Luminescence studies
All compounds 1–15 are luminescent in the solid state at
77 K, and give rise to structured bands in the region 395–
500 nm. The emission and excitation maxima are recovered
in Table 1. Some of the complexes (1, 6, 11 and 14) also
emit at room temperature, showing emission bands at sim-
ilar energies that those at 77 K, but generally less resolved
and of lower intensity.
The emission spectra at 77 K of the complexes exhibit
two types of patterns of bands, namely A or B. Represen-
tative examples of these two types of emission pattern
observed for 7 (type A) and 4 (type B) are shown in
Fig. 5. They both consist on a three component emission
band, with the component of higher intensity being the first
one in B and the middle one in A.
Complexes containing pyrazolate ligands (classes III, IV
and VI) give type A spectra, whereas those with pyrazole
ligands (classes I, II and V) exhibit a type B pattern, inde-
pendently of whether they are cationic or neutral. For com-
plexes with type B spectra, however, the substitution of a
neutral Hpz ligand by an anionic Cl ligand (compare com-
pounds of classes II and V) results in a shift of the emission
maxima to lower wavelengths, but no shift is observed
when Hpz is exchanged by another neutral ligand, PPh3
(compare compounds of classes II and I).
As it can be deduced from our structural data (Fig. 1b)
the complexes 8 and 15 whose emission pattern was analo-
gous to that of all complexes of classes III, IV and VI
(Table 1) exhibited short inter and intra Au–Au contacts
˚
of 3.029(1) and 3.387(1) A, respectively. On the other hand
for trimeric complexes of class III (5, 6 and 7) intra-molec-
ular Au–Au contacts as those found on 16 can also be
suggested.
In agreement with those results we should also consider
that several trimetallic pyrazolate complexes exhibiting
both intra- and inter-molecular Au–Au contacts have
shown to give metal-centered emissions consisting of broad
unstructured bands at 625–630 nm [11,21]. By contrast the
complex [Au(l-3-Me-5-Ph-pz)]3, where inter-molecular
contacts are prevented by bulky substituents on the pyraz-
olate rings, was not luminescent at room temperature [11].
On the other hand, pyrazolate phosphine complex of the
type [(PPh3)2Au(l-3,5-Ph2pz)]NO3 analogous to those of
classes IV and VI [9] have shown similar spectra to those
described here. The presence of Au–Au interactions not
being solely responsible for the observed emission.
In summary, despite their structural variety, the lumi-
nescence of complexes 1–15 appears to be mainly domi-
nated by the Hpz/pzꢀ ligands, with the emissive states
perturbed by their coordination to the gold atoms but
not determined by the presence of Au–Au interactions.
The different spectra observed for complexes of classes
III, IV and VI, containing Au–Au contacts, compared to
those of compounds of types I and II, without Au–Au
interactions, could be related not only to the different nat-
ure of the ligands, but also to the presence of the interac-
tions. Similarly, and given that the presence of Au–Au
contacts in type V complexes cannot be ruled out, the shift
observed in the emission of these compounds, compared to
that of I or II, could be partly attributed to those
interactions.
The presence of one or two chains on the pyrazole
ligand does not significantly affect the emission (i.e., com-
pare 12 and 14). The spectra are also unaffected by the
600
pattern of bands A: [Au(μ−pzR(16))]3(7)
pattern of bands B: [Au(Hpz2R(4))2]PTS (4)
450
300
150
0
450
600
λ
(nm)
Fig. 5. Emission spectra of the two types of pattern of bands A or B.

P. Ovejero et al. / Journal of Organometallic Chemistry 692 (2007) 1690–1697
1697
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4. Conclusions
This paper has shown the use of pyrazolate or pyrazole
ligands bonded to Au(I) metal centres or AuL metal frag-
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