Synthesis, electrochemical properties and reactivity of [Fe(η5-C5H4PPh2)2]Pt(benzenethiolate)2 complexes: X-ray crystal structure of [Fe(η5-C5H4PPh2</s

Article abstract of DOI:10.1016/j.inoche.2009.11.010

Three heteroleptic Pt(II) complexes with 1,1′-bis(diphenylphosphino)ferrocene (dppf) and benzene-monothiolate (BzT) ligands such as benzenethiolate (BT), 2,3,5,6-tetrafluorobenzenethiolate (TFBT) and 3,5-dimethylbenzenethiolate (DMBT), (dppf)Pt(BzT)₂

Full text of DOI:10.1016/j.inoche.2009.11.010

Inorganic Chemistry Communications 13 (2010) 183–186
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, electrochemical properties and reactivity
of [Fe(
structure of [Fe(
g
5-C5H4PPh2)2]Pt(benzenethiolate)2 complexes: X-ray crystal
g
5-C5H4PPh2)2]Pt(SC6HF4)2
*
Su-Kyung Lee, Dong-Youn Noh
Department of Chemistry, Seoul Women’s University, Seoul 139-774, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Three heteroleptic Pt(II) complexes with 1,10-bis(diphenylphosphino)ferrocene (dppf) and benzene-
monothiolate (BzT) ligands such as benzenethiolate (BT), 2,3,5,6-tetrafluorobenzenethiolate (TFBT) and
3,5-dimethylbenzenethiolate (DMBT), (dppf)Pt(BzT)2, were synthesized from (dppf)PtCl2 and the corre-
sponding benzene-monothiols. X-ray structural analysis of (dppf)Pt(TFBT)2 showed that the two TFBT
ligands are parallel and in an anti-conformation with respect to a slightly distorted P2PtS2 plane. The
redox properties of the three Pt(II) complexes are correlated with the electron-donating capability of
the monothiolate ligands. The more-electronegative TFBT ligand induces a short Pt–P bond distance
Article history:
Received 25 September 2009
Accepted 13 November 2009
Available online 18 November 2009
Keywords:
Pt(II) complex
Benzenethiolate
Electrochemistry
X-ray crystal structures
(2.273(2) Å), a large Pt–P coupling constant (JPt–P = 3171 Hz) and a high oxidation potential (E3
¼
pa
1:355 V). A charge-transfer complex of (dppf)Pt(DMBT)2 was prepared by the reaction with F4TCNQ
and characterized.
Ó 2009 Elsevier B.V. All rights reserved.
The heteroleptic Pt(II) complexes with P2PtS2 core have been
investigated so far, mainly focusing on structural properties [1–5],
and in some cases on redox and luminescent properties [6–8].
These complexes, as other Pd(II) analogues tend to do, show cata-
lytic activity in some organic reactions [9]. Recently, we have
reported serial results of our investigation on the P2Pt(dithiolate)
complexes, where P2 is an electron-rich diphosphine chelate such
as 1,10-bis(diphenylphosphino)ferrocene (dppf), 1,2-bis(diphenyl-
phosphino)acetylene (dppa) and 3,4-dimethyl-30,40-bis(diphenyl-
phosphino)tetrathiafulvalene (P2), and (dithiolate) is either 1,
2-ethylenedithiolate or benzene-1,2-dithiolate (bdt) [10–12]. For
the (dppa)Pt(bdt) complex [11], we elucidated the redox properties
of the P2PtS2 core and luminescent properties varying with substit-
uents such as H, CH3, and Cl on the benzene moiety. As for the
(P2)Pt(1,2-ethylenedithiolate) system, however, it was found that
the redox behavior of the complexes was determined by competi-
tion of redox capability between two heteroleptic ligands [12].
As a next step in ongoing investigation of these compounds, we
expanded our interests to the monothiolate system (P2)Pt(BzT)2
where BzT denotes a benzene-monothiolate such as benzenethiol-
ate (BT), 3,5-dimethylbezenethiolate (DMBT) and 2,3,5,6-tetraflu-
orobenzenethiolate (TFBT), with a different number of functional
groups (H, CH3 and F) on the benzene moiety. Among them, we
present in this paper the synthesis, electrochemical properties,
and reactivity of (dppf)Pt(BzT)2 complexes.
The (dppf)Pt(BzT)2 complexes [13] were synthesized by the
reaction of (dppf)PtCl2 with monothiols such as 2,3,5,6-tetrafluoro-
benzenethiol ((TFBT)H) and 3,5-dimethylbezenethiol ((DMBT)H),
and with a lead derivative (Pb(BT)2) [14], as shown in Scheme 1.
The (dppf)Pt(BT)2 complex prepared using Pb(BT)2 is chemically
and spectroscopically identical with that prepared by using ben-
zenethiol [6]. The yields (56–72%) were comparatively lower than
those of the dithiolate analogues such as (dppf)Pt(C3S5) (91%) and
(dppf)Pt(S4C4H4) (86%) [10], possibly due to the lack of a chelate ef-
fect in the monothiolate system. The low stability of the monothi-
olate system is demonstrated in its FAB-MS data, in which the most
intense peak corresponds to that with a loss of one thiolate ligand
([MꢀBzT]+), while the mother peak [M]+ is less than 11%.
The X-ray crystal structure of (dppf)Pt(TFBT)2 (Fig. 1) [15] shows
that the two TFBT ligands are in an anti-conformation with respect
to the P2PtS2 plane, as in many metal complexes with a fluorinated
benzenethiolate ligand [2–5]. Two TFBT ligands are parallel with
the dihedral angle of 15.2(1)°, which is somewhat larger than that
found in (dppf)Pt(SC6F5)2 (2.7(2)° and 9.8(3)°) [2] and (dppe)-
Ni(SC6F5)2 (4.9°) [5]. Therefore, the centroid-centroid distance be-
tween the two TFBT rings (4.76(1) Å) is more than 3.6 Å, the
maximum inter-planar separation between two neighboring
p-sys-
tems for an intramolecular interaction [17]. The hP1P2S2S1
p–p
plane in the P2PtS2 core is almost planar, with a torsion angle of
9.52(3)°. The averaged Pt–S bond distance in the (dppf)Pt(TFBT)2
* Corresponding author. Tel.: +82 2 970 5656; fax: +82 2 970 5972.
E-mail address: dynoh@swu.ac.kr (D.-Y. Noh).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.11.010

184
S.-K. Lee, D.-Y. Noh / Inorganic Chemistry Communications 13 (2010) 183–186
The results of cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) of (dppf)Pt(BzT)2 are shown in Figs. 2 and 3.
Their parameters are compared with those of (dppf)PtCl2 in Table
1. The DMBT and BT complexes exhibit two cycles, within the mea-
sured potential range (0.0–2.0 V). The first one, observed at around
E31=2 ¼ 1:228 V, is reversible, no doubt associated with the redox
process of the coordinated dppf ligand ([dppf]+/0), as compared to
that of (dppf)PtCl2 (E31=2 ¼ 1:139 V). The second one observed at a
lower potential region is irreversible, as shown in Fig. 2; since it
is not observed in the CV of (dppf)PtCl2, it must be responsible
for the PtS2 centered oxidation. Moreover, it is noteworthy that this
peak is not observed in CV of TFBT complex (Fig. 3) even though it
also has a P2PtS2 core as do the DMBT and BT complexes. This can
be explained by the electronegativity difference of the BzT ligands:
Pauling’s electronegativity (PEN) [20] of the F atom (3.98) is much
larger than that of H atom (2.20) and the group electronegativity
(GEN) of –CH3 (2.27–2.55) [21], but is relatively close to that of
Cl atom (3.16), and the GEN of TFBT ligand (2.92) [9] is close to
the PEN of Br atom (2.96). These facts together suggest that the
reason that no redox peak in the Pt(TFBT)2 moiety was observed
in the measured potential region is because of the high electron-
withdrawing capability of the TFBT ligand, as in the case of
(dppf)PtCl2. On the contrary, the DMBT and BT complexes, with a
weaker electron-withdrawing capability of the ligands, show such
redox cycles at 0.613 V and 0.696 V, respectively. This rationale is
also supported by the 31P NMR results. The P–Pt coupling constant
of the TFBT complex (JP–Pt = 3171 Hz) is much larger than that of
DMBT and BT complexes (3000 and 3001 Hz, respectively), which
means that the more electron-withdrawing TFBT ligand induces
the stronger P–Pt coupling and the larger JP–Pt value than those
of the DMBT and BT complexes (see Table S1 in Supplementary
data).
Scheme 1. Synthesis of (dppf)Pt(BzT)2 complexes.
Also noteworthy from the CV results of TFBT complex is that
two more irreversible anodic peaks (E2pa ¼ 1:125 V and 1:224 V)
were detected in the lower potential region of the coordinated dppf
cycle (E3pa ¼ 1:355 V). The shape and potential of these two irre-
versible peaks are very sensitive to the scan rate and solvent (see
Figs. S1 and S2 in Supplementary data). These two peaks are tenta-
tively thought to represent a rearrangement of the electron-with-
drawing TFBT ligand [3].
Fig. 1. The molecular structure of (dppf)Pt(TFBT)2 with a selected numbering
scheme. Selected bond lengths (Å) and angles (°): Pt1–P1 2.273(2), Pt1–P2 2.273(2),
Pt1–S1 2.347(2), Pt1–S2 2.358(2), P1–Pt1–P2 98.65(7), P1–Pt1–S1 83.58(7), P2–
Pt1–S2 87.90(7), S1–Pt1–S2 90.66(7).
As a preliminary study on the reactivity of the complexes, the
charge-transfer (CT) complex of (dppf)Pt(TFBT)2 was prepared by
reacting it with F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-
quinodimethane) in CH2Cl2/CH3CN, giving a species with composi-
tion [(dppf)Pt(TFBT)2]3(F4TCNQ)2 as indicated by elemental analysis
data [22]. The UV–Vis spectra of the CT complex in CH2Cl2 show a
red-shift of two intense absorption peaks of a neutral F4TCNQ
appearing at 394 and 372 nm to 458 nm [23] (see Fig. S3 in Supple-
mentary data). The IR peaks for the C„N vibration observed at 2215
compound (2.353(2) Å) is very close to that found in other monothi-
olate systems such as (dppf)Pt(SC6F5)2 (2.369(1) Å) [2] and
(dppf)Pt(SC6H5)2 (2.359(1) Å) [6], but longer than that in the corre-
sponding dithiolate system such as (dppf)Pt(C3S5) (2.327(2) Å) [10]
and (dppf)Pt(S2C6H4) (2.309(1) Å) [18]. It coincides with the well-
known fact that the dithiolate system contains a shorter Pt–S bond
distance than that of the monothiolate one because of the delocal-
ized five-membered metallacyclic system [19].
Fig. 2. The cyclic voltammogram (A) and its differential pulse voltammogram (B) of (dppf)Pt(DMBT)2 measured in CH2Cl2. The (dppf)Pt(BT)2 shows the same CV and DPV
cycles. Their potentials are compared in Table 1.

S.-K. Lee, D.-Y. Noh / Inorganic Chemistry Communications 13 (2010) 183–186
185
Fig. 3. The cyclic voltammogram (A) and its differential pulse voltammogram (B) of (dppf)Pt(TFBT)2 measured in CH2Cl2. It was measured between 0.0 V and 2.0 V but only
the observed redox cycle is represented (0.8–1.7 V).
[2] A. Garcés-Rodríguez, D. Morales-Morales, S. Hernández-Ortega, Acta Cryst. E63
Table 1
(2007) m479.
The cyclic voltammetry parameters of (dppf)Pt(BzT) and (dppf)PtCl2 (in volt).a
[3] F. Estudiante-Negrete, R. Redón, S. Hernández-Ortega, R.A. Toscano, D.
Morales-Morales, Inorg. Chim. Acta 360 (2007) 1651.
[4] G. Rivera, S. Bernès, H. Torrens, Polyhedron 26 (2007) 4276.
[5] M.A. Usón, J.M. Llanos, J. Organomet. Chem. 663 (2002) 98.
[6] W.S. Han, Y.-J. Kim, S.W. Lee, Bull. Korean Chem. Soc. 24 (2003) 60.
[7] (a) K.A. Van Houten, D.C. Heath, C.A. Barringer, A.L. Rheingold, R.S. Pilato,
Inorg. Chem. 37 (1998) 4647;
E1pa
E2pa
E3pa
E3pc
E31=2
Compounds
(dppf)Pt(TFBT)2
(dppf)Pt(DMBT)2
(dppf)Pt(BT)2
(dppf)PtCl2
–
1.125/1.224
1.355
1.343
1.341
1.191
1.221
1.114
1.114
1.087
1.288
1.229
1.228
1.139
0.613
0.696
–
–
–
–
(b) S.P. Kaiwar, A. Vodacek, N.V. Blough, R.S. Pilato, J. Am. Chem. Soc. 119
(1997) 3311;
a
Scan rate: 50 mV sꢀ1
;
supporting electrolyte: 0.10 M n-Bu4NꢁPF6; working
(c) J.M. Bevilacqua, R. Eisenberg, Inorg. Chem. 33 (1994) 2913;
(d) J.M. Bevilacqua, J.A. Zuleta, R. Eisenberg, Inorg. Chem. 33 (1994) 258.
[8] (a) K.S. Shin, D.Y. Noh, Bull. Korean Chem. Soc. 25 (2004) 130;
(b) K.S. Shin, Y.K. Han, D.Y. Noh, Bull. Korean Chem. Soc. 24 (2003) 235.
[9] C. Herrera-Álvarez, V. Gómez-Benítez, R. Redón, J.J. García, S. Hernández-
Ortega, R.A. Toscano, D. Morales-Morales, J. Organomet. Chem. 689 (2004)
2464.
[10] D.Y. Noh, E.M. Seo, H.J. Lee, H.Y. Jang, M.G. Choi, Y.H. Kim, J. Hong, Polyhedron
20 (2001) 1939.
[11] (a) K.S. Shin, K.I. Son, J.I. Kim, C.S. Hong, M. Suh, D.Y. Noh, Dalton Trans. (2009)
1767;
electrode: Pt-button; counter electrode: Pt-wire; ref. electrode: Ag/AgCl; 1.0 mM
samples in CH2Cl2; E1/2 = 0.565 V for Fc/Fc+ couple.
and 2227 cmꢀ1 also shifted to 2137 and 2197 cmꢀ1 upon formation
of the CT complex. Furthermore, the IR peaks of the ferrocenyl group
observed in the region of 456–553 cmꢀ1 were not greatly altered by
the F4TCNQ complexation, implying that it may be the P2PtS2 core,
rather than the dppf moiety, that is oxidized.
(b) D.Y. Noh, K.S. Shin, K.I. Son, Bull. Korean Chem. Soc. 28 (2007) 343.
[12] (a) K.S. Shin, Y. Jung, S.K. Lee, M. Fourmigué, F. Barrière, J.F. Bergamini, D.Y.
Noh, Dalton Trans. (2008) 5869;
In summary, we successfully synthesized a series of (dppf)
Pt(BzT) complexes with BzT = benzenethiolate (BT), 3,5-dimethyl-
benzenethiolate (DMBT) and 2,3,5,6-tetrafluorobenzenethiolate
(TFBT) for the purpose of investigating the dependency of their re-
dox properties on the electronegativity of the functionalized ben-
zenethiolate ligand. The more-electronegative TFBT ligand can
induce a shorter Pt–P bond distance, a larger Pt–P coupling con-
(b) S.K. Lee, K.S. Shin, D.Y. Noh, O. Jeannin, F. Barrière, J.F. Bergamini, M.
Fourmigué, Chem. Asian J. (2009) in print.
[13] Synthesis of (dppf)Pt(BzT)2:
A CH2Cl2 solution (25 mL) of (dppf)PtCl2
(0.2 mmol, 164 mg), triethylamine (0.8 mmol, 81 mg) and benzenethiols
(0.4 mmol, 73 mg of 2,3,5,6-tetrafluorobenzenethiol, 55 mg of 3,5-
dimethylbenzenethiol) was stirred for 24 h at room temperature under
argon atmosphere. The yellow solution was evaporated to obtain the solid
residue. (dppf)Pt(TFBT)2 was purified by column chromatography (SiO2,
CHCl3) and recrystallized from CHCl3/MeOH. Yield 56% (124 mg). Mp. > 211 °C
(decomp.). HR-FABMS Calc. for C46H30F8FeP2PtS2 1111.0133 Found 1111.0126
stant (JPt–P), and a higher oxidation potential of dppf ligand (E3
than those of the less electronegative DMBT and BT ligands. Fur-
ther investigation on the CT complexes of (dppf)Pt(BzT)2, as well
as (P2)Pt(BzT)2, are currently under investigation.
)
pa
(M+). FAB-MS (m/z, %) 1111(M+, 7), 930([M-C6HF4S]+, 100). FT-IR (KBr, cmꢀ1
)
3054 (Ar C–H), 1624, 1588, 1481, 1436, 1425 (Ar C@C), 1216 (Ar C–F str), 1097
(P–Ph), 1041, 1025, 999 (Ar C–H ip def), 910 (C-F def), 884, 824, 742, 712, 692,
638 (Ar C–H oop def), 556, 515, 494, 471, 461 (Fe-ring vib). 1H NMR (500 MHz,
CDCl3, ppm) d 7.85 (8H, m, Ph), 7.42 (4H, t, Ph), 7.35 (8H, t, Ph), 6.55 (2H, m,
SC6F4H), 4.39 (4H, s, Fc), 4.27 (4H, d, J = 1.60 Hz, Fc). 31P NMR (202 MHz, CDCl3,
ppm) d 17.47 (JPt–P = 3171 Hz). 19F NMR (282 MHz, CDCl3, ppm) ꢀ134.6 (qn,
o-F), ꢀ142.9 (qr, m-F). UV–Vis (CH2Cl2, nm) 238s, 268sh, 356w. (dppf)Pt
(DMBT)2 was purified by column chromatography (SiO2, CHCl3:MeOH = 30:1)
and recrystallized from acetone/n-hexane. Yield 62% (127 mg). Mp. 224–
225 °C. HR-FABMS Calc. for C50H46FeP2PtS2 1023.1514 Found 1023.1518 (M+).
FAB-MS (m/z, %) 1023(M+, 11), 866([MꢀC8H9S]+, 100). FT-IR (KBr, cmꢀ1) 3021,
3053 (Ar C–H), 2913, 2855 (–CH3), 1592, 1573, 1480, 1436 (Ar C@C), 1095 (P–
Ph), 1029, 999 (Ar C–H ip def), 833, 744, 690, 639 (Ar C–H oop def), 553, 515,
494, 471, 456 (Fe-ring vib). 1H NMR (500 MHz, CDCl3, ppm) d 7.80 (8H, m, Ph),
7.36 (4H, t, Ph), 7.28 (8H, t, Ph), 6.78 (4H, s, CH in DMBT), 6.37 (2H, s, CH in
DMBT), 4.28 (4H, s, Fc), 4.13 (4H, d, J = 1.05 Hz, Fc), 2.02 (12H, s, CH3). 31P NMR
(202 MHz, CDCl3, ppm) d 17.68 (JPt–P = 3000 Hz). UV–Vis (CH2Cl2, nm) 232s,
262sh. Synthesis of (dppf)Pt(BT)2 An acetone solution (20 mL) of Pb(BT)2
(0.05 mmol, 21 mg) was added to a CH2Cl2 solution (10 mL) of (dppf)PtCl2
(0.05 mmol, 41 mg) with stirring for 24 h at room temperature. The PbCl2
precipitate was separated by filtration with Celite, and the purified product
was recrystallized from CH2Cl2/n-hexane. Yield 72% (36 mg) Mp.
245 °C > (decomp.). HR-FABMS Calc. for C46H38FeP2PtS2 967.0857 Found
967.0893 (M+). FAB-MS (m/z, %) 967.1 (M+, 10), 858.1 ([MꢀC6H5S]+, 100). FT-
IR (KBr, cmꢀ1) 3057 (Ar C–H), 1577, 1477, 1435 (Ar C@C), 1095 (P–Ph), 1024,
999 (Ar C–H ip def), 824, 735, 696, 639 (Ar C–H oop def), 552, 515, 494, 470
(Fe-ring vib). 1H NMR (500 MHz, CDCl3, ppm) d 7.79 (8H, m, Ph), 7.37 (4H, m,
Ph), 7.29 (8H, t, Ph), 7.17 (4H, m, CH in BT), 6.77 (6H, m, CH in BT), 4.29 (4H, s,
Acknowledgments
This work was supported by a special research grant from Seoul
Women’s University (2009). We gratefully appreciate Dr. M. Fou-
rmigue (CNRS-Univ. of Rennes 1, France) for the supply of F4TCNQ
compound.
Appendix A. Supplementary material
CCDC 748161 contains the supplementary crystallographic data
for (dppf)Pt(TFBT)2. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.inoche.2009.11.010.
References
[1] (a) H. Torrens, Coord. Chem. Rev. 196 (2000) 331;
(b) V.K. Jain, L. Jain, Coord. Chem. Rev. 249 (2005) 3075.

186
S.-K. Lee, D.-Y. Noh / Inorganic Chemistry Communications 13 (2010) 183–186
Fc), 4.13 (4H, s, Fc). 31P NMR (202 MHz, CDCl3, ppm) d 18.85 (JPt–P = 3001 Hz).
UV–Vis (CH2Cl2, nm) 232s, 262sh.
[14] S.E. Appleton, G.G. Briand, A. Decken, A.S. Smith, Dalton Trans. (2004)
3515.
[19] U.T. Mueller-Westerhoff, B. Vance, Comp. Coord. Chem. 2 (1987) 595.
[20] P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong, Inorganic Chemistry,
fourth ed., Oxford, 2006. p. 31.
[21] L.D. Garner-O’Neale, A.F. Bonamy, T.L. Meek, B.G. Patrick, J. Mol. Struc.
(Theochem) 639 (2003) 151.
[22] Preparation of [(dppf)Pt(TFBT)2]3(F4TCNQ)2: To a 10 mL CH2Cl2 solution of
(dppf)Pt(TFBT)2 (0.01 mmol, 11 mg) was added a 10 mL CH3CN solution of
F4TCNQ (0.01 mmol, 3 mg), and the orange solution was heated for 40 min.
Some black precipitates began to form as the reaction solution cooled down to
room temperature, and then was stored in a freezer. The product was filtered
off, washed with cold CH3CN and dried under vacuum. M.p > 208 °C (dec.). EA:
calc. for C162H90F32N8P6Pt3S6 C 50.73, H 2.74, N 2.82; obsd. C 51.62, H 2.83, N
2.88. FT-IR (KBr, cmꢀ1) 3057 (Ar @C–H), 2197, 2137 (–C„N), 1626, 1484, 1436
(Ar C@C), 1220, 1168 (Ar C–F str), 1096 (P–Ph), 1041, 1026, 999 (Ar C–H ip
def), 911(C–F def), 884, 825, 745, 711, 692, 638 (Ar C–H oop def), 561, 516,
493, 471 (Fe-ring vib). UV–Vis (CH3CN, nm) 234s, 268sh, 458m.
[15] Crystal data for (dppf)Pt(TFBT)2: C46H30F8FeP2PtS2, Mw = 1111.70, T = 173(2) K,
Monoclinic, space group P2(1)/n, a = 10.7588(5) Å, b = 20.6302(9) Å, c =
18.4938(8) Å, b = 94.493(1)°, V = 4092.2(3) Å3, Z = 4, F(0 0 0) = 2176, Crystal
size = 0.43 ꢂ 0.10 ꢂ 0.08 mm3, GOF = 1.153, R1 = 0.0444, wR2 = 0.1087 [I > 2
r(I)].
The data were collected at 173(2) K on a SMART APEX CCD SYSTEM (Bruker)
equipped with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). The
structures were solved by the direct method and refined by full-matrix least-
squares analysis using anisotropic thermal parameters for non-hydrogen
atoms with the SHELXL-97 program [16].
[16] G.M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal Structures,
University of Gottingen, Gottingen, Germany, 1997.
[17] C.A. Hunter, J.K.M. Sanders, J. Am. Chem. Soc. 112 (1990) 5525.
[18] L.L. Maisela, A.M. Crouch, J. Darkwa, I.A. Guzei, Polyhedron 20 (2001) 3189.
[23] D.A. Dixon, J.C. Calabrese, J.S. Miller, J. Phys. Chem. 93 (1989) 2284.