One step conversion of perrhenate into an Re(IV) complex: Synthesis and molecular structure of trans-[ReCl4(PMePh2)2]

Article abstract of DOI:10.1016/S0020-1693(96)05574-0

KReO₄ reacts in a one-pot reaction with 4 mole of diphenylmethylphosphine and aqueous hydrochloric acid in boiling ethanol to yield trans-tetrachloro-bis(diphenylmethylphosphine) rhenium (1) in high yield. The almost quantitative, direct reduction of perrhenate to Re(IV) by PPh₂Me/HCl has been observed for the first time. Bright red 1 can be crystallized from chloroform/hexane to give bright red plates. Crystals of 1 are triclinic with a=8.898(6), b=9.543(4), c=9.713(3) A, α=66.68(2), β=88.54(4), γ=63.10(3)°, Z=1 and D_obs=1.83 g cm^-3. The structure was solved from 2327 diffraction data (F_o²≥3σ(F_o²)) collected at 20(2)° and refined to R=0.029 (R_w=0.039).

Full text of DOI:10.1016/S0020-1693(96)05574-0

Inorganica Chimica Acta 261 (1997) 109–112
Note
One step conversion of perrhenate into an Re(IV) complex:
synthesis and molecular structure of trans-[ReCl4(PMePh2)2]
F. Ekkehardt Hahn U, Lutz Imhof, Thomas Lu¨gger
Institut fu¨r Anorganische und Analytische Chemie, Freie Universita¨t Berlin, Fabeckstrasse 34-36, D-14195 Berlin, Germany
Received 16 April 1996
Abstract
KReO4 reacts in a one-pot reaction with 4 mole of diphenylmethylphosphine and aqueous hydrochloric acid in boiling ethanol to yield
trans-tetrachloro-bis(diphenylmethylphosphine) rhenium (1) in high yield. The almost quantitative, direct reduction of perrhenate to Re(IV)
by PPh2Me/HCl has been observed for the first time. Bright red 1 can be crystallized from chloroform/hexane to give bright red plates.
˚
Crystals of 1 are triclinic with as8.898(6), bs9.543(4), cs9.713(3) A, as66.68(2), bs88.54(4), gs63.10(3)8, Zs1 and Dobss1.83
g cmy3. The structure was solved from 2327 diffraction data (Fo G3s(Fo )) collected at 20(2)8 and refined to Rs0.029 (Rws0.039).
2
2
Keywords: Perrhenate reduction; Rhenium(IV) complexes; Phosphine complexes; Chloride complexes; Crystal structures
1. Introduction
dation of Re(III) complexes of the type mer-[ReCl3L3] or
[Re2Cl6(L2)2] with chlorinated solvents [6–8] or chlorine
[9]. While bidentate phosphines like dppe yield cis-
[ReCl4(dppe)] [6], trans-[ReCl4(PR3)2] is the product if
Re(III) complexes with monodentate phosphines PR3 are
oxidized [7,9,10].
In contrast to the preparation of Re(V)oxotrichloro and
Re(III)trichloro complexes by reduction of perrhenate with
PR3/HCl, the direct preparation of Re(IV)tetrachloro com-
plexes from such reactions has not been reported. We have
now found that KReO4 reacts in ethanol with 4 equiv. of
PMePh2 and aqueous HCl to give directly the Re(IV) com-
plex trans-[ReCl4(PMePh2)2] (1). In this contribution we
report on the preparation and molecular structure of 1.
Rhenium tetrachloride fragments are useful precursors for
the preparation of Re(IV) complexes. However, the exis-
tence of pure ReCl4 remains in dispute and materials
described to have the compositionReCl4 areinsolubleinmost
organic solvents [1]. To avoid these problems we investi-
gated the synthesis of Re(IV) coordination compounds con-
taining the ReCl4 core.
Compared to Re(V) complexes containing the Re(O)Cl3
core [2] and Re(III) complexes with the ReCl3 moiety [3],
less is known about Re(IV) complexes containing the ReCl4
core and no simple, one-step synthetic procedure to such
complexes starting with the most versatile rhenium precursor
perrhenate has been published. The Re(IV) complexes cis-
[ReCl4(THF)2] [4], cis-[ReCl4(dme)] (dmes1,2-di-
methoxyethane) [5] and cis-[ReCl4(dppe)] (dppes
bis(diphenylphosphino)ethane) [5] have been prepared
from ReCl5 or from Re(IV) acetylene complexes of the type
[ReCl4(RC^CR)(OPCl3)]. The cis-geometry is not sur-
prising for complexes with bidentate ligands, which form
five-membered chelate rings upon complexation, while
electronic reasons are assumed to be responsible for the cis-
geometry in cis-[ReCl4(THF)2] [4]. The oldest method for
the preparation of complexes [ReCl4L2] (L2stwo mono-
dentate phosphines or one bidentate diphosphine) is the oxi-
2. Experimental
All solvents were freshly distilled prior to use. IR spectra
were recorded as KBr pellets on a Perkin-Elmer 983 spec-
trometer. Elemental analyses were performed on a Vario EL
elemental analyzer at the Institut fu¨r Anorganische und
Analytische Chemie, Freie Universita¨t Berlin. Mass spectra
(EI) were recorded on a Finnigan MAT 112 instrument.
KReO4 and PMePh2 were obtained from Aldrich and were
used as obtained.
U Corresponding author.
0020-1693/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved
PII S0020-1693(96)05574-0

110
F.E. Hahn et al. / Inorganica Chimica Acta 261 (1997) 109–112
Table 2
2.1. Synthesis of trans-[ReCl4(PPh2Me)2] (1)
Positional parameters and isotropic equivalent thermal parameters for 1 a
2
KReO4 (1.0 g, 3.46 mmol) and diphenylmethylphosphine
(2.77 g, 13.84 mmol) were dissolved in a mixture of ethanol
(55 ml) and aqueous HCl (10 ml, 24%). The reaction mix-
ture was heated under reflux for 15 h. Within 30 min the
colorless solution turned green. After another 4 h the solution
had turned yellow and a red precipitate had formed. The
reaction was complete (TLC control) after 15 h. The red
precipitate was separated by filtration and washed with cold
ethanol. It was analyzed as trans-[ReCl4(PPh2Me)2]. The
product can be recrystallized unchanged from chloroform/
hexane. Yield 2.14 g (85%) of bright red crystals. Anal. Calc.
for C26H26Cl4P2Re (Ms728.46): C, 42.87; H, 3.60. Found:
C, 42.55; H, 3.55%. IR (KBr): n 3050 (Ar), 1434 (P–C),
324 (Re–Cl). MS (EI, 80 eV, m/z): 727 (Mq, 0.11), 692
(MqyCl, 0.15), 657 (Mqy2Cl, 1.15), 492 (MqyCly
PPh2Me, 0.55), 200 (PPh2Me, 100); correct isotope pattern
for rhenium containing fragments.
˚
Atom
x
y
z
Beq (A )
Re
0.000
0.500
0.000
0.0507(1)
0.1530(1)
2.337(4)
3.82(2)
3.56(2)
2.31(2)
3.4(1)
2.68(8)
3.7(1)
4.3(1)
4.3(1)
4.4(1)
3.5(1)
2.68(8)
3.4(1)
4.0(1)
4.2(1)
4.1(1)
3.22(9)
Cl1
Cl2
P
0.2195(1)
0.1703(1)
0.1006(1)
0.2286(1)
0.5311(1)
0.6432(1)
0.7828(5)
0.7893(4)
0.9227(5)
1.0396(5)
1.0240(5)
0.8943(6)
0.7757(5)
0.4898(4)
0.3767(5)
0.2602(6)
0.2511(5)
0.3621(5)
0.4812(5)
y0.2321(1)
y0.3960(5)
y0.2221(4)
y0.1834(5)
y0.1789(6)
y0.2106(6)
y0.2497(6)
y0.2549(5)
y0.2844(4)
y0.1828(5)
y0.2198(6)
y0.3551(6)
y0.4568(5)
y0.4213(5)
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
y0.0727(6)
0.1727(5)
0.0654(6)
0.1139(7)
0.2706(7)
0.3778(6)
0.3301(5)
0.2739(5)
0.4289(6)
0.5640(6)
0.5454(6)
0.3907(7)
0.2555(5)
a Re resides on
a
special position (inversion center); Beqs8/3p2-
[8i8jUijaiUajUaiaj].
2.2. Crystal structure analysis
collected at 20(2)8C using v–2u scans. Raw data were
reduced to structure factors [11] (and their e.s.d.s) by cor-
recting for scan speed, Lorentz and polarization effects. An
empirical absorption correction was applied to the data. The
Crystals of 1 were grown at room temperature from a
chloroform/hexane solution and are air stable. A suitable
specimen was selected in air and mounted at 20(2)8C on an
Enraf-Nonius CAD-4 diffractometer. Search and autoindex-
ing procedures gave a triclinic cell. Important crystal anddata
collection details are listed in Table 1. Diffraction data were
#
space group was assumed to be P1. Solution by Patterson
methods and refinement of the structure confirmed this
choice. The positional parameters for all non-hydrogenatoms
were refined by using first isotropic thermal parameters. A
difference Fourier map calculated after refinement with ani-
sotropic thermal parameters showed almost all hydrogen
positions. However, hydrogen atoms were added to the struc-
Table 1
Selected crystal and data collection details
Crystal size (mm)
Formula
0.30=0.14=0.12
C26H26Cl4P2Re
728.46
8.898(6)
9.543(4)
˚
ture model at calculated positions (d(C–H)s0.95 A) and
Formula weight (a.m.u.)
are unrefined [12]. The isotropic temperature factors for
hydrogens were fixed to be 1.3 times the Beq of the parent
atom. All calculations were carried out with the MolENpack-
age [13] 1. Atomic coordinates and equivalent isotropicther-
mal parameters for 1 are listed in Table 2.
˚
a (A)
˚
b (A)
˚
c (A)
9.713(3)
a (8)
b (8)
66.68(2)
88.54(4)
g (8)
V (A )
Space group
63.10(3)
3
˚
663.4(7)
#
P1 (No. 2)
3. Results and discussion
Z
D
1
calc (g cmy3
)
1.823
1.83
51.81
Mo Ka, 0.71073
Dobs (g cmy3
)
The reaction of phosphines and hydrochloric acid with
perrhenate salts has been known for quite some time to yield
complexes of types mer-[ReCl3(PR3)3] [2] or trans-
[ReOCl3(PR3)2] [2,7,9] depending on the acidity of the
solution and the phosphine used. In these reactions normally
concentrated hydrochloric acid is used and the phosphines
are added in excess and function as reducing agent and as
ligands for the formed complexes. Under these conditions
m (cmy1
)
˚
Radiation, l (A)
Data collection temperature (8C)
2u Range (8)
20(2)
2F2uF50
hkl Range
0FhF10, y9FkF10,
y10FlF10
2336
No. unique data
2
2
No. observed data, Fo G3s(Fo )
2327
2.89
3.90
1.077
152
R a (%)
Rw a (%)
GOF a
1 Definition of residuals: Rs8NNFoNyNFcNN/8NFoN, Rws[8wNNFoN
2
2
2
No. variables
yNFcNN /8wNFoN ]1/2
,
GOFs[8wNNFoNyNFcNN /(noynp)]1/2 with
nosnumber of structure factors and npsnumber of parameters, ws1/
a For definition of residuals see footnote 1.
[s(F)]2.

F.E. Hahn et al. / Inorganica Chimica Acta 261 (1997) 109–112
111
triphenylphosphine will reduce perrhenate only to trans-
[ReOCl3(PR3)2] while dialkylarylphosphines reduce
perrhenate [9] or Re(V)oxotrichloro complexes [7] to mer-
[ReCl3(PR3)3]. Rhenium(IV) complexeshavenotbeeniso-
lated in these reactions. They can be obtained by reoxidation
of mer-[ReCl3(PPh2R)3]withchlorinatedhydrocarbons[7]
or chlorine [9]. This oxidation is strongly dependent on the
phosphine coordinated to rhenium with the shortest reaction
time (10 min) observed for P(nBu)2Ph [7].
phenylphosphines. In accordance with our observation, di-
phenylmethylphosphine is not well suited for the reduction
of Re(V)oxotrichloro complexes to Re(III)trichloro
complexes.
The reduction of perrhenate or Re(V)oxotrichloro com-
plexes with phosphines to Re(III) normally requires an
excess of phosphine (1:6 for Re(VII)™Re(III) [9], 1:4.8
for ReO(V)™Re(III) [7]). In contrast to this we used only
4 equiv. of diphenylmethylphosphine for the Re(VII)™
Re(IV) reduction. Since 2 equiv. of phosphine are used for
coordination in 1, only 2 equiv. are available for the three-
electron reduction. This constitutes a much smaller excess
than in the previously described reductions.
We believe that the choice of diphenylmethylphosphine as
the reducing agent and its almost stoichiometric use are
responsible for the unusual, direct conversion of perrhenate
to 1. This appears to be another example of the significant
effect of small changes in a phosphine ligand upon the
reactivity.
The molecular structure of 1 was established by X-ray
crystallography (Fig. 1). The molecule resides on an inver-
sion center. Only small deviations from a perfectlyoctahedral
geometry were observed in 1. The Re–Cl and Re–P distances
(Table 3) fall in the range observed previously for
[Re(IV)Cl4L2] complexes (L2stwo monodentate phos-
phines [5,6] or one bidentate phosphine [10]).
Since the reduction of Re(VII) to Re(V) or Re(III) as
well as the oxidation of Re(III) to Re(IV) depends strongly
on the phosphine present, we became interested in the pos-
sibility of stopping the reduction Re(VII)™Re(V)™
Re(III) at the Re(IV) stage by an appropriate phosphine.
Since triphenylphosphine can reduce Re(VII) only toRe(V)
[7] and dialkyphenylphosphines reduce Re(VII) directly to
Re(III) [9] we decided to investigate the reaction of per-
rhenate salts with the rarely used alkyldiphenylphosphines.
For our investigations we chose diphenylmethylphosphine
PMePh2. It was hoped that the reducing power of this phos-
phine would lie betweenPPh3 andP(alkyl)2Ph, thusallowing
the direct reduction of perrhenate to Re(IV)tetrachlorocom-
plexes without formation of Re(III)trichloro complexes.
The reaction of KReO4 with exactly 4 equiv. of PMePh2
and aqueous HCl (24%) in ethanol yields the bright red
Re(IV) complex 1 (Scheme 1). The reaction can be fol-
lowed optically. The initially colorless reactionsolutionturns
yellow–green within 30 min, indicating the formation of
trans-[ReOCl3(PMePh2)2] [2]. After heating for another
4 h a red precipitate begins to form. The reaction is completed
after 15 h. The red precipitate was isolated and characterized
as 1.
The direct formation of an Re(IV) complex like 1 by
reduction of perrhenate has not been observed previously.
We attribute the formation of 1 to (i) the use of diphenyl-
methylphosphine as reducing agent and (ii) the use of only
4 equiv. phosphine in the reduction reaction.
Alkyldiphenylphosphines have rarely been used for the
reduction of perrhenate or Re(V)oxo complexes. The only
example we found in the literature was the reduction of trans-
[ReOCl3(PPh3)2] to mer-[ReCl3(PPh2Me)3] with a large
excess of phosphine, which proceeded in exceptionally low
yield compared to the same reaction carried out with dialkyl-
phenylphosphines [7]. We take this as an indication for the
weaker reducing power of PMePh2 compared to dialkyl-
Fig. 1. Molecular structure of 1 with the crystallographical numbering
scheme. Atom Re resides on an inversion center; starred atoms represent
transformed coordinates of the type (yx, yy, yz).
Table 3
˚
Selected bond distances (A) and angles (8) in 1
Re
Re
Re
P
P
P
Cl1
Cl2
P
C1
C2
C8
2.2863(11)
2.3382(11)
2.5133(10)
1.810(5)
1.812(4)
1.813(4)
Cl1
Cl1
Cl2
Re
Re
Re
C1
C1
C2
Re
Re
Re
P
P
P
P
P
P
Cl2
P
P
C1
C2
C8
C2
C8
C8
89.61(4)
92.66(4)
90.42(4)
110.8(2)
116.00(14)
113.48(14)
102.9(2)
106.9(2)
105.9(2)
Scheme 1. Preparation of complex 1 from KReO4.

112
F.E. Hahn et al. / Inorganica Chimica Acta 261 (1997) 109–112
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4. Supplementary material
A full list of data collection and refinement details, tables
of positional parameters for all atoms, all bond distances and
angles, anisotropic temperature factors, as well as calculated
and observed structure factor amplitudes have beendeposited
and can be obtained from the Fachinformationszentrum
Karlsruhe, D-76433 Eggenstein-Leopoldshafen (Germany)
on quoting the depository number CSD-59293, the names of
the authors and the journal citation.
[5]
[6]
[7]
[8]
[9]
[10]
Acknowledgements
[11]
Neutral scattering factors were used: International Tables for X-Ray
Crystallography, Vol. IV, Kynoch, Birmingham, UK, 1974, Table
2.2B. Terms of anomalous dispersion from International Tables for
X-Ray Crystallography, Vol. IV, Kynoch, Birmingham, UK, 1974,
Table 2.3.1.
We thank the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for financial support.
[12]
[13]
M.R. Churchill, Inorg. Chem., 12 (1972) 1213.
MolEN: Molecular Structure Solution Procedures, program
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