Synthesis and characterisation of new pyrazole-phosphinite ligands and their ruthenium(II) arene complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.05.047

The new potentially bidentate pyrazole-phosphinite ligands [(3,5-dimethyl-1H-pyrazol-1-yl)methyl diphenylphosphinite] (L¹) and [2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl diphenylphosphinite] (L²) were synthesised and characterised. The r

Full text of DOI:10.1016/j.jorganchem.2005.05.047

Journal of Organometallic Chemistry 690 (2005) 4072–4079
www.elsevier.com/locate/jorganchem
Synthesis and characterisation of new pyrazole–phosphinite
ligands and their ruthenium(II) arene complexes
a
Rosa Tribo , Sergi Mun˜oz , Josefina Pons , Ramon Yan˜ez , Angel Alvarez-Larena ,
a
a
a
b
´
´
´
´
Joan Francesc Piniella , Josep Ros
´
b
a,
*
a
´ `
Departament de Quımica, Universitat Autonoma de Barcelona, 08193-Cerdanyola del Valles, Barcelona, Spain
´
b
`
`
Departament de Geologia, Unitat de CristalÆlografıa, Universitat Autonoma de Barcelona, 08193-Cerdanyola del Valles, Barcelona, Spain
Received 22 March 2005; accepted 24 May 2005
Available online 11 July 2005
Abstract
The new potentially bidentate pyrazole–phosphinite ligands [(3,5-dimethyl-1H-pyrazol-1-yl)methyl diphenylphosphinite] (L1)
and [2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl diphenylphosphinite] (L2) were synthesised and characterised. The reaction of L1 and
L2 with the dimeric complexes [Ru(g6-arene)Cl2]2 (arene = p-cymene, benzene) led to the formation of neutral complexes [Ru(g6-
arene)Cl2(L)] (L = L1, L2) where the pyrazole–phosphinite ligand is j1-P coordinated to the metal. The subsequent reaction of these
complexes with NaBPh4 or NaBF4 produced the [Ru(g6-p-cymene)Cl(L2)][BPh4] and [Ru(g6-benzene)Cl(L2)][BF4] compounds
which contain the pyrazole–phosphinite ligand j2-P,N bonded to ruthenium. All the complexes were fully characterised by analyt-
ical and spectroscopic methods. The structure of the complex [Ru(g6-p-cymene)Cl(L2)][BPh4] was also determined by a X-ray single
crystal diffraction study.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Pyrazole; Phosphinite; Arene ruthenium(II) complexes; X-ray structure
1. Introduction
ꢀsoftꢁ donor atoms, phosphorus is the most common
in homogeneous catalysis and, for this reason, it is
found in many ligands combined with a variety of
ꢀhardꢁ labile donor groups (i.e., N- or O-donor).
Although hemilabile P–O-donor ligands have been
widely studied [1a,1b], increased attention has recently
been given to hemilabile P–N donor ligands [1e,5,7].
The preparation of new nitrogen–phosphinite ligands
and their evaluation in the rhodium-catalysed hydro-
formylation of styrene have recently been undertaken,
by Kostas [9].
In recent years our research group has reported the
synthesis, characterisation and coordination properties
of many N1-substituted pyrazolic ligands containing
supplementary donor groups. The most characteristic li-
gands studied have been: (a) bidentate ligands N(pz)–
N(amine) [10,11], N(pz)–O(alcohol, ether) [12–15],
N(pz)–P(phosphine) [10,16–18], and N(pz)–S(thiolate)
The chemistry of transition-metal complexes con-
taining hemilabile ligands have been the subject of
many studies in recent years [1]. Along with the term
first introduced by Jeffrey and Rauchfuss [2], hemilabile
ligands contain both a strong donor group, fixing the
ligand to the metal, and a weaker donor group, which
can be easily replaced by another ligand. Ligands pos-
sessing both ꢀsoftꢁ and ꢀhardꢁ donor atoms coordinated
to the same metal centre have been found suitable for
catalytic purposes since the stability of intermediate
species is favoured [1]. Recently, many efforts have
been made to improve the catalytic activity of some
complexes by using hemilabile ligands [3–8]. Among
*
Corresponding author. Tel.: +34 93 581 1889; fax: +34 93 581 3101.
E-mail address: josep.ros@uab.es (J. Ros).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.05.047

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R. Tribo et al. / Journal of Organometallic Chemistry 690 (2005) 4072–4079
4073
[19,20]; (b) tridentate ligands (N(pz))2–N(amine) [21,22],
(N(pz))2–O(ether) [23,24], (N(pz))2–S(thioether) [25],
(N(pz))2–N(amine)–O(alcohol)groups [26]; (c) tetraden-
tate ligands (N(pz))2–(S(thioether))2 [27]. These studies
have shown that pyrazole, in general, is the stronger do-
nor group of the ligand, but in the case of ligands con-
taining third period donor atoms (P,S), the pyrazole
group can also behave as a labile donor group [20]. This
fact is apparent in the room temperature 1H NMR spec-
trum of the complex [PdCl2(N2S)] (N2S = 1,5-bis(3,5-di-
methyl-1-pyrazolyl)-3-thiapentane) which shows that
the ligand alternates NN and NS coordination types
[25]. With the aim of extending our studies towards pyr-
azole-derived ligands containing a strong P donor group
we report here the synthesis and characterisation of new
ligands: [(3,5-dimethyl-1H-pyrazol-1-yl)methyl diph-
enylphosphinite] (L1) and [2-(3,5-dimethyl-1H-pyrazol-
1-yl)ethyl diphenylphosphinite] (L2). This study was
completed with the synthesis and full characterisation
of neutral and cationic arene–ruthenium(II) complexes
bearing these pyrazole–phosphinite ligands. A prelimin-
ary test on the catalytic activity in transfer hydrogena-
tion of cyclohexanone by propan-2-ol is also reported.
[Ru(benzene)Cl2]2 [28] and [Ru(p-cymene)Cl2]2 [29] com-
pounds were prepared by using previously published
procedures. The (3,5-dimethyl-1H-pyrazol-1-yl)metha-
nol [30] and 2-(3,5-dimethyl-1H-pyrazol-1-yl)ethanol
[31] were prepared as described in the literature.
2.2. Synthesis of the ligands
2.2.1. Synthesis of [(3,5-dimethyl-1H-pyrazol-1-yl)
methyl diphenylphosphinite] (L1)
PPh2Cl (0.75 mL, 4.00 mmol) dissolved in 10 mL of
THF were slowly added to a solution of (3,5-dimethyl-
1H-pyrazol-1-yl)methanol (0.5 g, 4.00 mmol) and trieth-
ylamine (0.67 mL, 7.78 mmol) in 20 mL of THF at room
temperature. The mixture was stirred for 12 h and the
triethylammonium chloride was filtered off. Evaporation
of the solvent in vacuo gave [(3,5-dimethyl-1H-pyrazol-
1-yl)methyl diphenylphosphinite] (L1) as a yellowish oil
in 98% yield.
2.2.2. L1: C18H19N2PO (310.0)
Anal. Calc. C, 69.66; H, 6.17; N, 9.03. Found: C,
69.10; H, 6.37; N, 9.36%. IR: (NaCl, cmꢀ1) 3054 (C–
H)ar, 2950 m(C–H)al, 1589, 1558 (m(C@C), m(C@N)),
1480, 1436 (d(C@C), d(C@N)), 1122 d(C–H)ip, 1094
m(P–C), 1053 m(P–O–C), 743, 717, 694 d(C–H)oop. MS
(ESI): m/z (%) 311.0 [MH+] (100%), 233.0
[M+ ꢀ C6H5] (5%), 202.9 [Ph2PO + H+] (7%), 109.0
[pz-CH2 + H+] (43%). 1H NMR (CDCl3 solution,
250 MHz) d: 7.75–7.10 (10H, m, C6H5), 5.65 (2H, s,
pz-CH2), 5.60 (1H, s, pz-CH), 2.15 (3H, s, pz-CH3),
2.06 (3H, s, pz-CH3) ppm. 13C{1H} NMR (CDCl3 solu-
tion, 63 MHz) d: 148.4 (pz-CCH3), 140.6 (pz-CCH3),
135.8, 135.6 (d, JP,C = 7.2 Hz, C6H5), 135.4, 135.3 (d,
JP,C = 7.2 Hz, C6H5), 133.3–128.2 (C6H5), 106.6 (pz-
CH), 60.5, (pz-CH2), 13.8 (pz-CH3) 11.5 (pz-CH3) ppm.
31P{1H} NMR (CDCl3 solution, 81 MHz) 115.9 (s, O–
P–(C6H5)2) ppm.
2. Experimental
2.1. General details
All reactions were performed with the use of vacuum
line and Schlenk techniques. All reagents were commer-
cial grade and were used without further purification.
All solvents were dried and distilled by standard
methods.
The elemental analyses (C,H,N) were carried out by
the staff of the Chemical Analyses Service of the Univer-
`
sitat Autonoma de Barcelona on a Carlo Erba CHNS
EA-1108 instrument. Infrared spectra were run on a Per-
kin Elmer FT-2000 spectrophotometer as KBr pellets or
1
in CH2Cl2 with NaCl mulls. The H NMR, 13C {1H}
2.2.3. Synthesis of [2-(3,5-dimethyl-1H-pyrazol-1-yl)
ethyl diphenylphosphinite] (L2)
NMR and 31P{1H} NMR spectra were run on a
NMR-FT Bruker AC-250 spectrometer in CDCl3 solu-
tions at room temperature. 1H NMR and 13C{1H}
NMR chemical shifts (d) were determined relative to
internal TMS and are given in ppm. 31P {1H} NMR
chemical shifts (d) were determined relative to external
85% H3PO4 and are given in ppm. Electrospray mass
spectra were obtained on an Esquire 3000 ion trap mass
spectrometer from Bruker Daltonics. Quantitative gas
chromatographic measurements were made on a Hew-
lett Packard HP-5890 apparatus equipped with a FID
detector and a HP-5 (30 m, 0.32 mm) capillary column.
Qualitative gas chromatographic measurements were
run o a Hewlett Packard HP-G1800A equipment con-
nected to a MS EID detector and using a HP-5 (30 m,
0.25 mm) capillary column. The precursor complexes
Method 1. PPh2Cl (1.35 mL, 7.13 mmol) dissolved in
10 mL of THF were slowly added to a solution of 2-(3,5-
dimethyl-1H-pyrazol-1-yl)ethanol (1.00 g, 7.14 mmol)
and triethylamine (1.20 mL, 8.57 mmol) in 20 mL of
THF at room temperature. The mixture was stirred
for 12 h and the triethylammonum chloride was filtered
off. Evaporation of the solvent in vacuo gave [2-(3,5-di-
methyl-1H-pyrazol-1-yl)ethyl diphenylphosphinite] (L2)
as a yellowish oil in a 83% yield.
Method 2. A 1.6 M solution of nBuLi in hexane
(2.34 mL, 3.74 mmol) was slowly added to a solution
of 2-(3,5-dimethyl-1H-pyrazol-1-yl)ethanol (0.50 g,
3.57 mmol) in 30 mL of THF at 0 ꢁC. After 1 h of stir-
ring at this temperature,
a solution of PPh2Cl
(0.67 mL, 3.55 mmol) in 10 mL of THF was slowly

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R. Tribo et al. / Journal of Organometallic Chemistry 690 (2005) 4072–4079
4074
added. The mixture was stirred at room temperature for
12 h, and then evaporated to dryness in vacuo. The res-
idue was suspended in toluene (50 mL) and filtered.
Evaporation of the solvent gave [2-(3,5-dimethyl-1H-
pyrazol-1-yl)ethyl diphenylphosphinite] (L2) in 85%
yield.
(p-cym-Cq), 90.6 (p-cym-CH), 88.2 (p-cym-CH), 73.9
(pz-CH2–O), 30.3 (p-cym-CH(CH3)2), 22.1 (p-cym-
CH(CH3)2), 17.4 (p-cym-CCH3), 13.7 (pz-CH3), 10.7
(pz-CH3) ppm. 31P{1H} NMR (CDCl3 solution,
81 MHz) 112.0 (s, O–P–(C6H5)2) ppm.
2.3.1.2. C29H35Cl2N2OPRu Æ 0.5CH3OH (646.57). Anal.
Calc. C, 54.80; H, 5.77; N, 4.33. Found: C, 54.68; H, 5.70; N,
4.18%. IR: (KBr, cmꢀ1) 3040 m(C–H)ar, 2920 m(C–H)al, 1553
(m(C@C), m(C@N)), 1482, 1435 (d(C@C), d(C@N)), 1093
2.2.4. L2: C19H21N2PO (324.4)
Anal. Calc. C, 70.36; H, 6.52; N, 8.64. Found: C,
70.22; H, 6.78; N, 8.97%. IR: (NaCl, cmꢀ1) 3053 (C–
H)ar, 2926 m(C–H)al, 1554 (m(C@C), m(C@N)), 1481,
1435 (d(C@C), d(C@N)), 1131 d(C–H)ip, 1092 m(P–C),
1047 m(P–O–C), 739, 697 d(C–H)oop. MS (ESI): m/z
(%) 347.1 [MNa+] (100%), 325.1 [MH+] (87%), 247.0
[M+ ꢀ C6H5] (11%), 203.0 [Ph2PO+H+] (4%), 123.1
1
m(P–C), 1043 m(P–O–C), 746, 696 d(C–H)oop. H NMR
(CDCl3 solution, 250 MHz) d: 7.79-7-36 (10H, m, C6H5),
5.83 (1H, s, pz-CH), 5.20 (2H, d, JH,H = 6.6 Hz, p-cym-
3
CH), 5.10 (2H, d, 3JH,H = 6.6 Hz, p-cym-CH), 4.16 (2H, t,
3
3JH,H = 5.1 Hz, CH2), 4.04 (2H, dt, JH,H = 5.1 Hz,
1
3
[pz-CH2–CH2 + H+] (41%). H NMR (CDCl3 solution,
3JP,H ꢁ 4 Hz, CH2), 2.57 (1H, sp, JH,H = 6.6 Hz, p-cym-
250 MHz) d: 7.31–7.17 (10H, m, C6H5), 5.66 (1H, s,
pz-CH), 4.05 (4H, m, pz-CH2–CH2–O), 2.12 (3H, s,
pz-CH3), 2.04 (3H, s, pz-CH3) ppm. 13C{1H} NMR
(CDCl3 solution, 63 MHz) d: 148.1 (pz-CCH3), 141.8,
141.5 (d, JP,C = 18.2 Hz, C6H5), 140.3 (pz-CCH3),
134.5, 134.2 (d, JP,C = 16.8 Hz, C6H5), 131.5–128.5
CH (CH3)2), 2.22 (3H, s, pz-CH3), 2.20 (3H, s, pz-CH3),
3
1.77 (3H, s, p-cym-CH3), 1.04 (6H, d, JH,H = 7.3 Hz, p-
cym-CH(CH3)2) ppm. 13C{1H} NMR (CDCl3 solution,
63 MHz) d: 147.7 (pz-CCH3), 140.0 (pz-CCH3), 136.6–
127.9 (C6H5), 112.0 (p-cym-Cq), 105.3 (pz-CH), 98.4
(p-cym-Cq), 90.2 (d, JP,C = 3.7 Hz, p-cym-CH), 87.9
(d, JP,C = 7.4 Hz, p-cym-CH), 65.8 (O–CH2), 48.9 (pz-
CH2), 30.3 (p-cym-CH(CH3)2), 22.0 (p-cym-CH(CH3)2),
17.4 (p-cym-CCH3), 13.7 (pz-CCH3), 11.4 (pz-CCH3) ppm.
31P{1H} NMR(CDCl3 solution, 81 MHz) 114.1 (s, O–P–
(C6H5)2) ppm.
2
(C6H5), 105.3 (pz-CH), 68.9, 68.6 (d, JP,C = 17.8, pz-
3
CH2–CH2–O), 49.8, 49.7 (d, JP,C = 8.2, pz-CH2–
CH2O), 14.0 (pz-CH3), 11.5 (pz-CH3) ppm. 31P{1H}
NMR(CDCl3 solution, 81 MHz) 116.4 (s, O–P–
(C6H5)2) ppm.
2.3. Synthesis of the complexes
2.3.1.3. C25H27Cl2N2OPRu Æ 0.5CH2Cl2 (616.91). Anal.
Calc. C, 49.65; H, 4.57; N, 4.54. Found: C, 50.01; H, 4.51;
N. 4.50%. IR: (KBr, cmꢀ1) 3070 m(C–H)ar, 2957 m(C–H)al,
1550 (m(C@C), m(C@N)), 1483, 1436 (d(C@C), d(C@N)),
2.3.1. Complexes [Ru(p-cymene)Cl2L] (L = L1 (1), L2
(2)) and [Ru(benzene)Cl2L](L = L2 (3))
The appropriate ligand (0.32 mmol: L1, 0.101 g; L2,
0.106 g), dissolved in dichloromethane (5 mL) was
added to a solution of the complex (0.16 mmol: [Ru(p-
cymene)Cl2]2, 0.100 g; [Ru(benzene)Cl2]2, 0.080 g) in
dichloromethane (20 mL). The solution was stirred at
room temperature for 16 h. The resulting solution was
concentred and the corresponding product was precipi-
tated with cold diethylether and filtered off. Yields:
85% (1), 85% (2), 50% (3).
1095 m(P–C), 1038 m(P–O–C), 744, 698 d(C–H)oop. H
1
NMR (CDCl3 solution, 250 MHz) d: 7.80–7.40 (10H, m,
C6H5), 5.84 (1H, s, pz-CH), 5.34 (6H, s, C6H6), 4.21 (2H,
br, CH2), 4.15 (2H, br, CH2), 2.25 (3H, s, pz-CH3), 2.21
(3H, s, pz-CH3) ppm. 13C{1H} NMR (CDCl3 solution,
63 MHz) d: 147.7 (pz-CCH3), 140.0 (pz-CCH3), 132.3–
128.1 (C6H5), 105.3 (pz-CH), 90.2 (C6H6), 66.3 (O-CH2),
48.9 (pz-CH2), 13.5 (pz-CCH3), 11.3 (pz-CCH3) ppm.
31P{1H} NMR(CDCl3 solution, 81 MHz) 114.1 (s, O–P–
(C6H5)2) ppm.
2.3.1.1. C28H33Cl2N2OPRu (616.52). Anal. Calc. C,
54.55; H, 5.40; N, 4.54. Found: C, 54.16; H, 5.55; N,
4.23%. IR: (KBr, cmꢀ1) 3041 m(C–H)ar, 2963 m(C–H)al,
1552 (m(C@C), m(C@N)), 1482, 1435 (d(C@C),
d(C@N)), 1093 m(P–C), 1043 m(P–O–C), 746, 696 d(C–
2.3.2. Complexes [Ru(p-cymene)ClL2][BPh4] (4) and
[Ru(benzene)ClL2][BF4] (5)
0.18 mmol of 2 (0.113 g) or 3 (0.105 g) were dissolved
in 10 mL of dichloromethane and 0.18 mmol of NaBPh4
(0.062 g) or NaBF4 (0.020 g) dissolved in 2 mL of meth-
anol to this solution. The mixture was stirred at room
temperature for 20 h and the solvent was evaporated
to dryness in vacuo. The resulting solid was suspended
in 10 mL of dichloromethane and filtered of to remove
the NaCl. The addition of hexane resulted in the precip-
itation of the product. The solid was filtered off and
dried in vacuo. Yields: 60% 4, 60% 5. Complexes can
be crystallised in dichloromethane/methanol mixtures.
1
H)oop. H NMR (CDCl3 solution, 250 MHz) d: 7.88–
7.37 (10H, m, C6H5), 5.78 (1H, s, pz-CH), 5.63 (2H, d,
3JP,H = 4.4 Hz, pz-CH2–O), 5.42 (4H, s(br), p-cym-
3
CH), 2.60 (1H, sp, JH,H = 7.3 Hz, p-cym-CH (CH3)2),
2.21 (3H, s, pz-CH3), 2.02 (3H, s, pz-CH3), 1.84 (3H,
3
s, p-cym-CH3), 1.07 (6H, d, JH,H = 7.3 Hz, p-cym-
CH(CH3)2) ppm. 13C {1H} NMR (CDCl3 solution,
63 MHz) d: 176.9 (pz-CCH3), 149.6 (pz-CCH3), 141.1–
128.1 (C6H5), 111.9 (p-cym-Cq), 106.6 (pz-CH), 98.6

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˚
2.3.2.1. C53H55BClN2OPRu Æ 0.25 CH2Cl2 (935.56). Anal
Calc. C, 68.36; H, 5.98; N, 2.99. Found: C, 68.0; H, 5.64;
N, 2.50%. IR: (KBr, cmꢀ1) 3055 m(C–H)ar, 2930 m(C–H)al,
1558 (m(C@C), m(C@N)), 1477, 1434 (d(C@C), d(C@N)),
mated Mo Ka radiation (k = 0.71069 A) and a x-2h
scan with an x scan width = 0.80 + 0.35 tanh, and an
x scan speed = 1.3–5.5ꢁ. Reflection ranges for the
data collection: 1 < h < 25; ꢀ15 < h < 15, ꢀ16 < k < 15,
ꢀ2 < l < 16. Lp and empirical absorption corrections
[32] were applied, Tmin = 0.960, Tmax = 1.000. 8498 un-
ique reflections, 6988 with I > 2r(I), were used. The
structure was solved by direct methods (SHELXS-86) [33]
and refined by full-matrix least-squares procedures on
F2 for all reflections (SHELXL-97) [34]. One molecule of
CH2Cl2 was found in the asymmetric unit. Restraints
were applied in order to have sensible distances in phenyl
groups and solvent. All non hydrogen atoms were refined
anisotropically. Hydrogen atoms were placed in calcu-
lated positions with isotropic displacement factors 1.5
times (methyl H) or 1.2 times (the rest) times the Ueq val-
ues of corresponding carbons. The final weighting
scheme was w ¼ 1=½r2ðF o2Þ þ 0.0806P2þ 1.0526Pꢂ, where
P ¼ ½maxðF 2o; 0Þ þ 2F 2cꢂ=3. Crystal and other structure
refinement data are displayed in Table 1.
1
1085 m(P–C), 1034 m(P–O–C), 746, 696 d(C–H)oop. H
NMR (CDCl3 solution, 250 MHz) d : 7.98–6.44 (10H,
m, C6 H5), 6.16 (1H, s, pz-CH), 5.42 (1H, d,
3JH,H = 5.8 Hz, p-cym-CH), 4.93 (1H, d, 3JH,H = 5.8 Hz,
p-cym-CH), 4.72 (2H, m, p-cym-CH), 3.88/3.55/3.21/
2.99 (4H, 4ddd, CH2H2), 2.74 (3H, s, p-cym-CH3), 2.49
(1H, sp, 3JH,H = 6.6 Hz, p-cym-CH (CH3)2), 1.73 (3H, s,
pz-CH3), 1.77 (3H, s, p-cym-CH3), 1.20 (3H, d,
3JH,H = 7.3 Hz, p-cym-CH(CH3)2), 1.10 (3H, d,
3JH,H = 7.3 Hz, p-cym-CH(CH3)2) ppm. 13C{1H} NMR
(CDCl3 solution, 63 MHz) d: 165.3–121.9 (C6H5), 158.3
(pz-CCH3), 147.9 (pz-C CH3), 119.1 (p-cym-Cq), 110.1
(pz-C H), 101.5 (p-cym-Cq), 93.7, 91.5, 89.2, 85.4
(p-cym-C H), 65.7 (O–CH2), 49.6 (pz-CH2), 30.3 (p-
cym-C H(CH3)2), 22.6 (p-cym-CH(C3)2) 21.4 (p-cym-
CH(C3)2), 17.9 (p-cym-CCH3), 17.3 (pz-CCH3), 12.1
(pz-CCH3) ppm. 31P{1H} NMR(CDCl3 solution, 81
MHz) 114.1 (s, O–P–(C6H5)2) ppm.
3. Results and discussion
2.3.2.2. C25H27BClF4N2OPRu Æ 0.25CH2Cl2 (647.03).
Anal. Calc. C, 46.87; H, 4.28; N, 4.33. Found: C, 46.70;
H, 4.45; N, 4.19%. IR: (KBr, cmꢀ1) 3070 m(C–H)ar,
2950 m(C–H)al, 1558 (m(C@C), m(C@N)), 1483, 1436
(d(C@C), d(C@N)), 1090 m(P–C), 1040 m(P–O–C),
3.1. Synthesis of the ligands
The synthetic procedure for the preparation of the li-
gands is shown in Scheme1. Pyrazole–phosphinite ligands
[(3,5-dimethyl-1H-pyrazol-1-yl)methyl diphenylphosphi-
nite] (L1) and [2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl
diphenylphosphinite] (L2) were synthesised by hydrogen
1
1080 m(B–F), 744, 700 d(C–H)oop. H NMR (CDCl3
solution, 250 MHz) d: 8.05–6.53 (10H, m, C6H5),
6.21 (1H, s, pz-CH), 5.78 (6H, s, C6 H6), 4.64 (2H, br,
CH2), 4.21 (2H, br, CH2), 2.87 (3H, s, pz-CH3), 2.09
(3H, s, pz-CH3) ppm.13C{1H} NMR (CDCl3 solution,
63 MHz) d: 158.1 (pz-C H3), 148.2 (pz-CCH3), 133.6–
128.0 (C6H5), 110.0 (pz-CH), 92.6 (C6H6), 66.7
(O-CH2), 51.4 (pz-CH2), 17.4 (pz-CCH3), 12.6
(pz-CCH3) ppm. 31P{1H} NMR(CDCl3 solution,
81 MHz) 111.9 (s, O–P–(C6H5)2) ppm.
Table 1
Crystal data and structure refinement for 4
Empirical formula
Formula weight
Temperature (K)
Crystal system
C53H55BClN2OPRu Æ CH2Cl2
999.22
293(2)
Triclinic
ꢀ
Space group
Unit cell dimensions
P1
˚
A (A)
12.965(2)
13.691(5)
14.269(5)
100.80(3)
99.68(2)
2.4. Catalytic experiments
˚
B (A)
˚
c (A)
Under N2 atmosphere, cyclohexanone (10 mmol,
0.981 g), the catalyst precursor (0.05 mmol), and
15 mL of a 0.094 M solution of NaOH (0.14 mmol,
5.6 · 10ꢀ3 g) in 2-propanol, were introduced in to a
Schlenk flask fitted with a condenser and heated at
82 ꢁC. The reaction was monitored by gas chromatogra-
phy. Cyclohexanol and acetone were the only detected
products.
a (ꢁ)
b (ꢁ)
c (ꢁ)
97.32(2)
3
˚
V (A )
2419.3(13)
2
1.372
Z
qcalc (g cmꢀ3
)
Absorption coefficient (mmꢀ1
F(000)
)
0.564
1036
h Range for data collection (ꢁ)
Data/restraints/parameters
Goodness-of-fit on F2
1.48–24.97
8498/38/568
1.092
2.5. X-ray crystal structure analysis of complex 4
R indices [I > 2r(I)]
R(F) = 0.044,
Rw(F2) = 0.132
R(F) = 0.057,
Rw(F2) = 0.126
0.69 and ꢀ0.81
Suitable crystals for X-ray diffraction of complex 4
were obtained by crystallisation in a dichloromethane/
methanol mixture. Data were collected on an Enraf-
Nonius CAD4 diffractometer using graphite-monochro-
R indices (all data)
Largest difference peak
ꢀ3
˚
and hole (e A
)

´
R. Tribo et al. / Journal of Organometallic Chemistry 690 (2005) 4072–4079
4076
(i), (ii)
Cl
P
+
N
N
OH
N
N
O
P
(CH2)x
(CH2)x
x = 1 ( L1), 2 (L2)
(i): NEt3, THF, 12 h, room temperature; (ii) Alternatively for x = 2: nBuLi, THF, 12 h, 0 o
C
Scheme 1.
abstraction of the previously described N1-hydroxy-
alkylpyrazoles Me2Pz(CH2)xOH: (3,5-dimethyl-1H-pyr-
azol-1-yl)methanol (x = 1) [30] and 2-(3,5-dimethyl-1H-
pyrazol-1-yl)ethanol (x = 2) [31], respectively, by a base
(NEt3) and the subsequent reaction with one equivalent
of PPh2Cl, in anhydrous THF and inert atmosphere
(N2). The ammonium salt was separated by filtration
and the ligands were obtained by extracting the solvent
in vacuoin goodyields(98%and83%, respectively). Alter-
natively, nBuLi was also used as a base in the synthesis of
L2. These synthetic methodologies led to L1 and L2 li-
gands, as a yellowish oil, with enough purity to give good
analytical and spectroscopic data (see Section 2). C, H, N
elemental analyses and IR, 1H, 13C{1H}, 31P{1H} NMR
and MS spectroscopies are in agreement with the pro-
posed structures for ligands. Singlets at d = 115.9 and
116.4 ppm, respectively, in the 31P{1H} NMR spectrum
of ligands L1 and L2 correspond to diphenylphosphinite
groups [9]. The rest of NMR signals are consistent with
those reported for related ligands [10,11].
3.2. Synthesis and characterisation of complexes
The whole of reactions with of [Ru(g6-arene)Cl2]2
with ligands L1 and L2 are depicted in Scheme 2. The
reactions of [Ru(g6-arene)Cl2]2 (arene = p-cymene, ben-
zene) with an equimolar amount of L1 or L2 in dichloro-
methane at room temperature gave the red compounds
1, 2 and 3 in moderate yields (85% (1), 85% (2), and
50% (3)). Elemental analyses of products 1–3 are consis-
tent with the suggested molecular formulas containing
solvent molecules. The absorption bands corresponding
R1
R1
Cl
Me
Cl
Me
Me
Me
(i)
Ru
Ru
+ 1/2
N
N
P
O
O
P
N
N
(CH2)x
(CH2)x
Cl
Cl
Ru
R2
R2
R1
Cl
Cl
x = 1 ( L1), 2 (L2)
Me
R2
(ii)
N
(CH2)x
O
x = 1, R1 = Me, R2 = iPr: (1)
x = 2, R1 = Me, R2 = iPr: (2)
x = 2, R1= R2 = H: (3)
Me
N
Cl
[BR3
]
Ru
4
P
R1
R2
x = 2, R1 = Me, R2 = iPr, R3 = Ph: (4)
x = 2, R1= R2 = H, R3 = F: (5)
(i): CH2Cl2, 16 h, room temperature; (ii): NaBR34, CH2Cl2/MeOH, 20 h, room temperature
Scheme 2.

´
R. Tribo et al. / Journal of Organometallic Chemistry 690 (2005) 4072–4079
4077
to pyrazole–phosphinite ligands in the infrared spectra
of compounds 1–3 do not show significant differences
expected absorptions due to the presence of the anions
[BPh4]ꢀ and [BF4]ꢀ, respectively. The room temperature
1H NMR spectra of compounds 4 and 5 display com-
plex multiplets, corresponding to the OCH2H2Pz frag-
ment, characteristic of pairs of diastereotopic
hydrogen atoms in a rigid ethylene alkyl chain [25]. In
complex 4, four multiplets (ddd) at 3.88, 3.55, 3.21
and 2.99 ppm (1H each) are observed, but, in complex
5, two broad unresolved multiplets centred at 4.64
(2H) and 4.21 (2H) are observable. Although the proton
NMR signals for the OCH2H2Pz agreed with a rigid
conformation of the chain, the two broad resonances
observed for complex 5 would suggest some degree of
flexibility, probably attributable to the smaller steric
requirement of the benzene ligand compared with that
of the p-cymene. The proton spectrum of complex 4
exhibited the CH p-cymene signals as three signals:
two doublets at 5.42 (1H) and 4.93 (1H) ppm
(JH,H = 5.8 Hz), and a multiplet at 4.72 (2H) ppm. The
appearance of separated signals for CH p-cymene
hydrogens was consistent with a C1 symmetry of the
complex, which was expected for a [(g6-arene)R-
uL1L2L3] complex core [7,35,36]. In addition, the bulk
of the pyrazole–phosphinite L2 chelated ligand seems
to prevent a free rotation of the p-cymene ligand around
the arene–Ru axis. This phenomenon is common in [(g6-
arene)RuCl(L–L)]+(L–L = neutral bidentate ligand)
complexes [35,36]. With reference to complex 5, the
C6H6 proton signal is observed at 5.78 ppm as a singlet
(6H). The 13C{1H} NMR spectra of complexes 4 and 5
display signals of the OC2C2Pz chain at 65.7, 66.7 ppm
(OCH2) and 49.6, 51.4 ppm (CH2Pz), respectively. In
concordance with the proton spectrum of 4, the CH p-
cymene signals are observed as four signals at 93.7,
91.5, 89.2 and 85.4 ppm. The C6H6 carbon resonance
occurred at 92.6 (s) ppm. Concerning the 31P{1H}
NMR spectra of complexes 4 and 5, expected singlets
at 114.1 and 111.9 ppm are observed. It is significant
to remark that 31P NMR signals of ligands and com-
plexes do not differ significantly [38,39].
1
with respect to those of the free ligands. H and 13C
NMR spectra of compounds 1–3 display all the signals
of coordinated ligands. In the 1H NMR of 1 the methy-
lene CH2 protons of L1 appear at 5.63 ppm as a doublet
(JP,H = 4.4 Hz) and the p-cymene CH protons are ob-
served as a broad singlet, that integrates 4H, at
1
5.42 ppm. The H NMR of 2 shows the CH2H2 signals
of L2 ligand as two separated resonances: a triplet at
4.16(JH,H = 5,1 Hz) and a double triplet at 4.04 ppm
(JH,H = 5,1 and JH,P ꢁ 4 Hz). This spectrum also dis-
plays the p-cymene CH protons as two doublets at
5.20 and 5.10 ppm (3JH,H = 6.6 Hz). The presence of
one (broad) or two signals corresponding to CH p-cym-
ene hydrogens in the 1H NMR spectra of 1 and 2 is con-
sistent with a Cs symmetry of complexes and with a free
rotation of the arene ligand [35,36]. The proton NMR
spectrum of complex 3 displays the CH2H2 resonances
of L2 ligand as two broad signals at 4.21 and 4.10 ppm
and the C6H6 protons as a singlet at 5.34 ppm (6H).
The proton signals corresponding to pyrazolyl group
of coordinated L1 and L2 in complexes 1–3 do not differ
significantly from those of free ligand spectra, which is
in agreement with a free orientation of this group. The
most relevant signals of 13C{1H} NMR spectra of com-
plexes 1–3 are those corresponding to arene ligands
(p-cymene 1, 2), C6H6 (3) and to (C2)x (x = 1 (1), 2 (2, 3))
chain. Carbon atoms of the arene ring in p-cymene
ligand are observed as two singlets at 90.6 and
88.2 ppm in compound 1 and as a two doublets at
90.2 and 87.9 ppm (JC,P = 3.7 and 7.4 Hz, respectively).
A similar pattern of signals was observed in the 13C{1H}
NMR spectra of [RuCl2 (p-cymene)(PR3)] (R = Ph,
OMe, OPh) complexes [37]. The signal corresponding
to the CH2 link of complex 1 is observed at 73.9 ppm,
whereas the signals of the CH2CH2 fragment are found
at: (a) 65.8 ppm (OCH2) and 48.9 ppm (CH2Pz) for
complex 2, and (b) 66.3 ppm (OCH2) and 48.9 ppm
(CH2Pz) for complex 3. Regarding 31P{1H} NMR spec-
tra, singlets at 112.0, 114.1 and 114.1 ppm are found for
complexes 1, 2 and 3, respectively, which are in the ex-
pected range for coordinated phenylphosphinite ligands
[7,30,38,39]. All these data are in agreement with [RuCl2-
(g6-arene)(j1-P-pyrazole–phosphinite)] type structures
for compounds 1–3.
3.3. Catalysis
In a preliminary study, some of the synthesised com-
plexes were evaluated as a precursor for the catalytic
transfer hydrogenation of cyclohexanone by 2-propanol
(Table 2). The activity of ruthenium(II) arene complexes
is well known in this catalytic process [7, (and references
therein)]. In a typical experiment, 0.05 mmol of the com-
plex and 10 mmol of cyclohexanone were added to a
0.094 M solution of NaOH in 2-propanol (0.14 mmol
of NaOH in 15 mL of 2-propanol) and refluxed at
82 ꢁC, the reaction being monitored by GC. With a com-
plex/NaOH ratio of 1/24, complexes 1–4 are rather ac-
tive in the transfer hydrogenation. Complex 3 is the
most active one which leads to a quantitative transfor-
The reaction in CH2Cl2 of complexes 2 and 3 with
equimolar amounts of Na[PPh4] or Na[BF4], respec-
tively, yielded complexes 4 and 5, which showed a j2-
PN mode of coordination of the pyrazole–phosphinite
ligand L2. Yields were of 60% for both complexes. Ele-
mental analyses of orange products 4 and 5 agree with
the molecular formula [RuCl2(g6-arene)(L2)][BR4] Æ 0.25
CH2Cl2 (R = Ph (4), F (5)). The infrared spectra of these
ionic compounds 4 and 5 show a similar pattern of
bands to that in neutral complexes 1–3 but with the

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R. Tribo et al. / Journal of Organometallic Chemistry 690 (2005) 4072–4079
4078
Table 2
Catalytic transfer hydrogenation of cyclohexanonea
mation of the ketone in 2 h, with a moderate TOF50 of
152 hꢀ1. From these preliminary results, it can be seen
that neutral complexes are more active than cationic
compound 4 and that the g6-benzene ruthenium(II)
complex 4 is more effective than g6-p-cymene deriva-
tives. These results are consistent with those reported
in the literature [7,40]. The decrease of the quantity of
base leads to the deactivation of the catalyst. It should
be pointed out that complexes 1–4 are more active
catalysts than the corresponding precursors: [Ru(p-
cymene)Cl2]2 (41% maximum yield in 24 h) and
[Ru(benzene)Cl2]2 (37% maximum yield in 24 h) with a
1/14 complex/NaOH ratio.
Entry
Catalyst
Yield (%)b,c
TOF50 (hꢀ1 d
)
1
2
3
4
5
a
1
55 (>99)e
25 (75)c
98 (>99)f
23 (60)c
10 (12)c
55
10
2
3
152
16
4
3g
h
Reaction conditions: 82 ꢁC, 2-propanol (15 mL), NaOH
(0.14 mmol), catalyst (0.05 mmol) and cyclohexanone (10 mmol).
Ketone/catalyst/NaOH ratio: 200/1/24 Yields were determined by GC.
b
Yield of cyclohexanol after 2 h.
Yield after 24 h in parenthesis.
c
d
Turnover frequencies [(mol cyclohexanol/mol catalyst)/time] at
50% conversion.
e
Yield after 12 h in parenthesis.
Yield after 3 h in parenthesis.
f
3.4. Crystal and molecular structure of complex 4
g
Catalyst/NaOH ratio: 1/14.
Maximum yield of 12%.
h
The structure of complex 4 was confirmed by a single-
crystal X-ray diffraction study. Complex 4 consisted of a
cationic complex [Ru(p-cymene)Cl(L2)]+ and an anion
[BPh4]ꢀ with a solvent molecule (CH2Cl2). A view of
the molecular structure of the cation complex is shown
in Fig. 1. Selected bond lengths and angles of the struc-
ture are displayed in Table 3. The molecule displayed a
pseudooctahedral three-legged piano-stool geometry
around the ruthenium atom with the arene, the L2 li-
gand and the chloro ligand completing the coordination
around the metal. The distortion of the octahedral
geometry was evident from the values of the P–Ru–
N(82) (89.69(9)ꢁ), P–Ru–Cl (84.31(4)ꢁ) and N(82)–Ru–
Cl (91.18(9)ꢁ) angles. It is interesting to note that the
flexibility of the bidentate L2 ligand makes possible a
nearly equal distribution of the N, P, Cl donor atoms
around the metal. The angles between the centroid of
the arene ring (C*), the metal and the N, P and Cl atoms
Table 3
˚
Selected bond distances (A) and angles (ꢁ) for 4
Ru–Cl
Ru–P
2.407(1)
2.305(1)
1.614(3)
1.434(5)
2.141(3)
2.195(4)
2.329(4)
2.202(4)
2.250(4)
2.181(4)
2.293(5)
1.745(5)
91.18(9)
89.69(9)
127.0(1)
128.7(1)
84.31(4)
114.0(1)
123.2(1)
121.6(2)
P–O
O–C(1)
Ru–N(82)
Ru–C(92)
Ru–C(94)
Ru–C(96)
Ru–C(91)
Ru–C(93)
Ru–C(95)
Ru–C*
Cl–Ru–N(82)
P–Ru–N(82)
C*–Ru–N(82)
C*–Ru–P
Cl–Ru–P
Ru–P–O
C*–Ru–Cl
P–O–C(1)
C* = centroid ring
are in the 123–129ꢁ range. The Ru(II) atom is g6-
bonded to a p-cymene ring, to L2 in a j2-P,N coordina-
tion mode and to a Cl atom. The Ru–Cl bond length of
˚
2.407(1) A was consistent with those reported in the lit-
erature [35]. The Ru–C(p-cymene) bond lengths average
*
˚
˚
2.242(4) A and the Ru–C distance is of 1.745(5) A
(C* = centroid ring). The coordination of p-cymene to
metal was slightly unsymmetrical, with Ru–C(94)
˚
˚
(2.329(4) A) and Ru–C(95) (2.293(4) A) bond lengths
significantly long. This distortion possibly comes from
the steric constraint imposed by the pyrazole ring, whose
methyl C(86) points directly to C(94) and C(95) atoms.
The L2 ligand is j2-bonded to metal forming a seven-
membered metallacycle with Ru–P and Ru–N(82) dis-
˚
tances of 2.305(1) and 2.141(3) A, respectively. These
values are of the same order as those found in the related
Fig. 1. Molecular structure of the cation complex of 4.

´
R. Tribo et al. / Journal of Organometallic Chemistry 690 (2005) 4072–4079
4079
[10] G. Esquius, J. Pons, R. Yan˜ez, J. Ros, J. Organomet. Chem. 619
(2001) 14.
[Ru(p-cymene)Cl(Me2HPz)(PPh2OH)] complex [16].
˚
The P–O bond length of 1.614(3) A is coherent with oth-
ers reported in the literature [41].
[11] G. Esquius, J. Pons, R. Yan˜ez, J. Ros, R. Mathieu, B. Donnadieu,
N. Lugan, Eur. J. Inorg. Chem. (2002) 2999.
[12] A. Boixassa, J. Pons, A. Virgili, X. Solans, M. Font-Bardia, J.
Ros, Inorg. Chim. Acta 340 (2002) 49.
[13] A. Boixassa, J. Pons, X. Solans, M. Font-Bardia, J. Ros, Inorg.
Chim. Acta 346 (2003) 151.
Acknowledgement
[14] A. Boixassa, J. Pons, X. Solans, M. Font-Bardia, J. Ros, Inorg.
Chim. Acta 355 (2003) 254.
´
Support by the Spanish Ministerio de Educacion y
Cultura (Project BQU2003-03582) is gratefully acknow-
ledged.
[15] A. Boixassa, J. Pons, X. Solans, M. Font-Bardia, J. Ros, Inorg.
Chim. Acta 357 (2003) 733.
[16] R. Tribo, J. Pons, R. Yan˜ez, J.F. Piniella, A. Alvarez-Larena, J.
Ros, Inorg. Chem. Commun. 3 (2000) 545.
[17] R. Tribo, J. Ros, J. Pons, R. Yan˜ez, A. Alvarez-Larena, J.F.
Piniella, J. Organomet. Chem. 676 (2003) 38.
Appendix A. Supplementary data
[18] G. Esquius, J. Pons, R. Yan˜ez, J. Ros, R. Mathieu, N. Lugan, B.
Donnadieu, J. Organomet. Chem. 667 (2003) 126.
[19] J. Garcia-Anton, J. Pons, X. Solans, M. Font-Bardia, J. Ros,
Inorg. Chim. Acta 355 (2003) 87.
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC reference number 266007 for com-
pound 4. Copies of this information may be obtained
free of charge from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK, fax: +44 1223 336
033, email: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.uk. Supplementary data associated with
this article can be found, in the online version at
doi:10.1016/j.jorganchem.2005.05.047.
[20] J. Garcia-Anton, J. Pons, X. Solans, M. Font-Bardia, J. Ros,
Inorg. Chim. Acta 357 (2004) 571.
[21] R. Mathieu, G. Esquius, N. Lugan, J. Pons, J. Ros, Eur. J. Inorg.
Chem. (2001) 2683.
[22] G. Aullon, G. Esquius, A. Lledos, F. Maseras, J. Pons, J. Ros,
Organometallics 23 (2004) 5530.
[23] A. Boixassa, J. Pons, J. Ros, R. Mathieu, N. Lugan, J.
Organomet. Chem. 682 (2003) 233.
[24] A. Boixassa, J. Pons, X. Solans, M. Font-Bardia, J. Ros, Inorg.
Chim. Acta 357 (2004) 827.
[25] J. Garcia-Anton, J. Pons, X. Solans, M. Font-Bardia, J. Ros,
Eur. J. Inorg. Chem. (2003) 3952.
References
[26] J. Garcia-Anton, J. Pons, X. Solans, M. Font-Bardia, J. Ros,
Eur. J. Inorg. Chem. (2003) 2992.
[1] For reviews on hemilabile ligands see: (a) A. Bader, E. Lindner,
Coord. Chem. Rev. 108 (1991) 27;
[27] J. Garcia-Anton, J. Pons, X. Solans, M. Font-Bardia, J. Ros,
Eur. J. Inorg. Chem. (2002) 3319.
(b) E. Lindner, S. Pautz, M. Haustein, Coord. Chem. Rev. 155
(1996) 145;
[28] R.A. Zelonka, M.C. Baird, J. Organomet. Chem. 35 (1972) C43.
[29] M.A. Bennett, G.B. Robertson, A.K. Smith, J. Organomet.
Chem. 43 (1972) C41.
(c) C.S. Slone, D.A. Weinberger, C.A. Mirkin, Prog. Inorg.
Chem. 48 (1999) 233;
[30] I. Dvoretzky, G.H. Richter, J. Org. Chem. 15 (1950) 1285.
[31] W.G. Haanstra, W.L. Driessen, J. Reedijk, U. Turpeinen, R.
Hamalainen, J. Chem. Soc., Dalton Trans. (1989) 2309.
[32] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta. Crystallogr.,
Sect. A 24 (1968) 351.
(d) P. Espinet, K. Soulantica, Coord. Chem. Rev. 147 (1999) 1;
(e) P. Braunstein, F. Naud, Angew. Chem., Int. Ed. 40
(2001) 680;
(f) L. Pascale, N. Le Bris, H. des Abbayes, Trends
Organomet. Chem. 4 (2002) 131;
[33] G.M. Sheldrick, SHELXS86, in: G.M. Sheldrick, C. Kruger, R.
Goddard (Eds.), Crystallographic Computing 3, Oxford Univer-
sity Press, 1985, pp. 175–189.
(g) U. Shubert, J. Pfeiffer, F. Stohr, D. Sturmayr, S.
Thompson, J. Organomet. Chem. 646 (2002) 53–58;
(h) P. Braunstein, J. Organomet. Chem. 689 (2004) 3953.
[2] J.C. Jeffrey, T.B. Rauchfuss, Inorg. Chem. 18 (1979) 2658.
[3] H. Yang, M. Alvarez-Gressier, N. Lugan, R. Mathieu, Organo-
metallics 16 (1997) 1401.
[34] G.M. Sheldrick, SHELXL 97, Program for Crystal Structure
Refinement, University of Go¨ttingen, Germany, 1997.
[35] E. de la Encarnacion, J. Pons, R. Yan˜ez, J. Ros, Inorg. Chim.
Acta 358 (2005) 3272.
[4] K.K. Hii, M. Thornton-Pett, A. Jutand, R.P. Tooze, Organo-
metallics 18 (1999) 1887.
[36] I. Moldes, E. de la Encarnacion, J. Ros, A. Alvarez-Larena, J.F.
Piniella, J. Organomet. Chem. 566 (1998) 165.
[5] I.D. Kostas, J. Organomet. Chem. 626 (2001) 221.
[6] V. Cadierno, P. Crochet, J. Diez, J. Garcia-Alvarez, S.E. Garcia-
Cadierno, J. Gimeno, Inorg. Chem. 42 (2003) 3293.
[7] A. Caballero, F.A. Jalon, B.R. Manzano, G. Espino, M. Perez-
Manrique, A. Mucientes, F.J. Poblete, M. Maestro, Organomet-
allics 23 (2004) 5694.
[37] E. Hodson, S.J. Simpson, Polyhedron 23 (2004) 2695.
[38] P. Le Gendre, M. Offenbecher, C. Bruneau, P.H. Dixneuf,
Tetrahedron: Asymm. 9 (1998) 2279.
[39] I.D. Kostas, B.R. Steele, A. Trezis, S.V. Amosova, Tetrahedron
59 (2003) 3467.
[40] P. Crochet, M.A. Fernandez-Zumel, C. Beauquis, J. Gimeno,
Inorg. Chim. Acta 356 (2003) 114.
[8] M. Dieguez, O. Pamies, A. Ruiz, Y. Diaz, S. Castillon, C. Claver,
Coord. Chem. Rev. 248 (2004) 2165.
[9] I.D. Kostas, Inorg. Chim. Acta 355 (2003) 424.
[41] E. Cesarotti, L. Prati, A. Sironi, G. Ciani, J. Chem. Soc., Dalton
Trans. (1987) 1149.