Re(v) and Re(iii) complexes with sal2phen and triphenylphosphine: Rearrangement, oxidation and reduction

Article abstract of DOI:10.1039/c0dt00993h

Reactions of Re^V, tetradentate Schiff base complexes with tertiary phosphines have previously yielded both rearranged Re^V and reduced Re^III complexes. To further understand this chemistry, the rigid diiminediphenol (N₂O₂) Schiff base ligand sal ₂phen (N,N′-o-phenylenebis(salicylaldimine)) was reacted with (n-Bu₄N)[ReOCl₄] to yield trans-[ReOCl(sal ₂phen)] (1). On reaction with triphenylphosphine (PPh₃), a rearranged Re^V product cis-[ReO(PPh₃)(sal ₂phen*)]PF₆ (2), in which one of the imines was reduced to an amine during the reaction, and the reduced Re^III products trans-[ReCl(PPh₃)(sal₂phen)] (4) and trans-[Re(PPh₃)₂(sal₂phen)]⁺ (5) were isolated. Reaction of sal₂phen with [ReCl₃(PPh ₃)₂(CH₃CN)] resulted in the isolation of [ReCl₂(PPh₃)₂(salphen)] (3). The compounds were characterized using standard spectroscopic methods, elemental analyses and single crystal X-ray crystallography.

Full text of DOI:10.1039/c0dt00993h

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PAPER
Re(V) and Re(III) complexes with sal2phen and triphenylphosphine:
rearrangement, oxidation and reduction†
Stephanie Renee Lane,a Nebiat Sisay,a Brett Carney,a Shorouk Dannoon,a Stephen Williams,a
Hendrik Petrus Engelbrecht,a,b Charles Leslie Barnesa and Silvia Sabine Jurisson*a,b
Received 10th August 2010, Accepted 19th October 2010
DOI: 10.1039/c0dt00993h
Reactions of ReV, tetradentate Schiff base complexes with tertiary phosphines have previously yielded
both rearranged ReV and reduced ReIII complexes. To further understand this chemistry, the rigid
diiminediphenol (N2O2) Schiff base ligand sal2phen (N,N¢-o-phenylenebis(salicylaldimine)) was reacted
with (n-Bu4N)[ReOCl4] to yield trans-[ReOCl(sal2phen)] (1). On reaction with triphenylphosphine
(PPh3), a rearranged ReV product cis-[ReO(PPh3)(sal2phen*)]PF6 (2), in which one of the imines was
reduced to an amine during the reaction, and the reduced ReIII products trans-[ReCl(PPh3)(sal2phen)]
(4) and trans-[Re(PPh3)2(sal2phen)]+ (5) were isolated. Reaction of sal2phen with [ReCl3(PPh3)2-
(CH3CN)] resulted in the isolation of [ReCl2(PPh3)2(salphen)] (3). The compounds were characterized
using standard spectroscopic methods, elemental analyses and single crystal X-ray crystallography.
core and upon chelation with rhenium, four configurations are
Introduction
possible.22 It was suggested that the ligand will lie in the equatorial
plane with respect to the rhenium oxo core when there are less than
five carbons between the two imine nitrogens, resulting in the for-
mation of three chelate rings that assist in stabilizing the rhenium
complex.22 However, there are several examples in which one oxy-
gen of the N2O2 core is coordinated trans to the Re O group.2,20
We recently reported on the reactivity of the monomeric ReV
oxo Schiff base complexes trans-[ReO(OH2)(acac2en)]Cl and trans-
[ReOCl(acac2pn)] with tertiary phosphines20 and with thiocyanate
and cyanide.21 The reaction of the monomeric ReV oxo Schiff
base complexes with phosphines yielded either a rearranged
ReV complex with the tertiary phosphine coordinated cis to
the oxo group or a ReIII complex with two tertiary phosphines
coordinated trans to each other, with the product dependent on
the phosphine and the Schiff base ligand.20 Reaction with non-
reducing nucleophiles such as thiocyanate or cyanide yielded only
the rearranged ReV complex with the nucleophile coordinated cis
to the oxo group.21
Our interest in Re chemistry arises from the potential appli-
cation of two Re radioisotopes, 186Re (90 h t1/2, 1.02 MeV b-,
137 keV g (7%)) and 188Re (17 h t1/2, 2.11 MeV b-, 155 keV g
(15%)), to therapeutic radiopharmaceuticals.23,24 Our earlier work
suggested that ReIII Schiff base phosphine complexes, analogous
to the TcIII Q-series of complexes,25–29 would result if the Schiff
base was sufficiently rigid and the phosphine was not too strong
of a nucleophile (otherwise the rearrangement/substitution path
would dominate and a ReV complex would form).20 As ReIII
complexes (d4) are anticipated to be more substitutionally inert
than ReV complexes (d2), our interest is in a path yielding the ReIII
product, trans-[Re(PR3)2(N2O2)]+.
The first rhenium Schiff base complexes were reported in 1979
by Middleton and co-workers1 and they have been studied exten-
sively since that time for their potential applications in nuclear
medicine and commercial catalysis.2–6 High oxidation state Re-
oxo complexes (ReVII and ReV) have found utility in catalysis.2,4–10
Rhenium(VII) methylrhenium trioxide complexes (MTO) have
proven particularly useful in oxidation catalysis.8,11 Rhenium(V)
oxo and imido phosphine complexes effect hydrogen release from
organosilane and alcohol mixtures and enantioselective reduction
of imines.9,10 Multiple examples for the use of ReV-oxo Schiff
base complexes as potential catalysts have also been reported. For
example, rhenium oxo N,N¢-ethylenebis(salicylaldimine) (sal2en)
complexes have utility for oxygen atom transfer (OAT), with
their reactivity stemming from rhenium’s high oxidation state
and the cationic charge of the complex.7 Rhenium oxo N,N¢-
cyclohexan-di-yl-bis(salicylaldimine) (sal2dach) complexes are ef-
fective catalysts for hydrosilation of alkyl and aryl ketones and
silane alcoholysis of secondary alcohols.8
Schiff base complexes with ReV and ReIII have been reported
with salicylaldehyde (e.g., sal2en and sal2pn) and acetylacetone
(e.g., acac2en and acac2pn) derived tetradentate ligands.1–3,12–21
This class of Schiff base ligands contains the N2O2 coordination
aDepartment of Chemistry, University of Missouri-Columbia, Columbia,
MO, 65211. E-mail: JurissonS@missouri.edu; Fax: 573-882-2754; Tel: 573-
882-2107
bUniversity of Missouri Research Reactor (MURR), University of Missouri,
Columbia, MO, 65211
† CCDC reference numbers 780134–780137. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00993h
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Fig. 1 Formation of the ReV complex, trans-[ReOCl(sal2phen)].
Here we report on ReV and ReIII Schiff base phosphine
complexes with the tetradentate Schiff base ligand N,N¢-[o-
phenylenebis(salicylaldimine)] (sal2phen) and triphenylphosphine.
Two synthetic paths were pursued, one from ReV and one from
ReIII, to probe the mechanism for forming the ReIII complex,
trans-[Re(PR3)2(N2O2 Schiff base)]+. The sal2phen ligand was
deemed a potentially rigid tetradentate Schiff base ligand with
which to test our hypothesis. The mono-oxo ReV complex
with sal2phen, analogous to trans-[ReOCl(acac2pn)] (acac2pn =
N,N¢-propylenebis(acetylacetoneimine)),3 was synthesized, char-
acterized, and reacted with triphenylphosphine and the Re(III)
starting material [ReCl3(PPh3)2(CH3CN)] was reacted with
sal2phen. Several ReV and ReIII products were isolated and
The first method used to target the desired ReIII complex
utilized the reaction of 1 with triphenylphosphine (PPh3) in
a 3.1 : 1 P : Re ratio. Depending on reaction time and sol-
vent volume (reactant concentrations), this method resulted in
the formation of three products, 2, 4 and 5 (Fig. 2). cis-
[ReO(PPh3)(sal2phen*)]+ (2) is a monosubstituted ReV oxo phos-
phine complex in which one of the imines has been reduced to
an amine. This result suggests that the ligand is reduced and
rearranges faster than the metal center could be reduced.20 trans-
[ReCl(PPh3)(sal2phen)] (4) is a monosubstituted ReIII phosphine
chloride complex. This product resulted from reduction by one
triphenylphosphine via oxygen atom abstraction with only a sin-
gle triphenylphosphine substitution. trans-[Re(PPh3)2(sal2phen)]+
(5) is the target disubstituted Re(III) phosphine complex and
resulted in low yield from triphenylphosphine reduction and
substitution.
1
characterized by their FT-IR, H-, 13C- and 31P-NMR spectra,
ESI-MS spectra, elemental analyses, and single crystal X-ray
structures.
A second method to target the desired ReIII complex in-
volved a substitution reaction between a ReIII starting material,
namely [ReCl3(PPh3)2(CH3CN)], and sal2phen. This resulted in
the isolation of a ReV complex, [ReCl2(PPh3)(salphen)] (3), with
a fragmented salphen ligand in which one salicylaldehyde unit
was cleaved and the imine nitrogen had been reduced to an
amine to yield a coordinated imido group (Fig. 3). This result
was unexpected and may possibly be explained by rhenium’s
catalytic tendencies; ReOI3(PPh3)2 is known to reduce imines
to amines.10 Organic amines have been shown to doubly de-
protonate and coordinate with ReV as imides.31–33 Complex 3 is
the same species reported by Gerber et al. from the reaction
of ReOCl3(PPh3)2 and the tridentate Schiff base ligand N-(2-
aminophenyl)salicylideneamine), H3apa.31
Results and discussion
trans-[ReOCl(sal2phen)] (1) was synthesized in 75% yield from
(n-Bu4N)[ReOCl4] and sal2phen in a manner similar to other ReV
Schiff base complexes (Fig. 1).3 This synthesis is simpler and higher
yielding than that previously reported without the need for column
purification.17 The product was fully characterized using a variety
of spectroscopic methods. The IR band featured at 951 cm-1 is
characteristic of the Re O stretch and consistent with that pre-
viously reported.17 Unique signals were observed for all sal2phen
protons in the 1H NMR spectrum of 1 suggesting that the complex
is slightly asymmetric. The X-ray crystal structure of 1 shows that
the equatorial plane is occupied by the N2O2 donor atoms of
the sal2phen ligand, which is similar to our previously reported
trans-[ReOCl(acac2pn)]3 but differs from Gerber et al. for cis-
[ReOCl(sal2mp)] (sal2mp = 1,2-benzylenebis(salicylideneimine));30
this is likely due to the rigidity of the sal2phen backbone.
The rigidity of the sal2phen ligand coordinated to the ReV center
was deemed sufficient to investigate the formation of the Re(III)
species, trans-[Re(PPh3)2(sal2phen)]+, on reaction with the bulky
triphenylphosphine. We previously reported that the rigidity of the
tetradentate Schiff base ligand and the phosphine nucleophilicity
and reducing power were the likely determining factors in whether
a rearranged ReV complex, cis-[ReO(PR3)(Schiff base)]+, or a
ReIII complex, trans-[Re(PR3)2(Schiff base)]+, analogous to the
Tc–Q compounds was obtained.20,27 Two synthetic approaches
were used to target the desired ReIII complex analogous to
the Tc–Q compounds (trans-[Re(PR3)2(Schiff base)]+), substitu-
tion/reduction on trans-[ReOCl(Schiff base)] and substitution on
[ReCl3(PPh3)2(CH3CN)].
All Re-Schiff base-phosphine products (2–5) were obtained in
low yield, 10–30%, and were difficult to isolate due the similarities
in their chemical properties. Infrared, nuclear magnetic resonance
and mass spectra assisted in determining the product formation
from the various reactions. The FT-IR spectra for 1 and 2 showed
a strong band at ~950 cm-1 corresponding to the Re O stretch,
while this band was not seen in complexes 3–5. However, a strong
absorption at 1092 cm-1 corresponding to the Re N imido group
in 3 is observed.
1
Unique H NMR splitting patterns were observed for each of
the five complexes. Proton identification was difficult in complexes
2–5 due to the multiple aromatic protons. Proton identification
for the ReIII complex 5 was even more difficult due to the fast
relaxation of the unpaired electrons,20,34 with chemical shifts
exhibited over an expanded range (-50 to 50 ppm). A single
peak in the 31P NMR was observed in the ReV complexes 2 (24.89
ppm) and 3 (23.83 ppm). These phosphorous signals are downfield
270 | Dalton Trans., 2011, 40, 269–276
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Fig. 2 Formation of ReV and ReIII phosphine complexes on reaction of triphenylphosphine with trans-[ReOCl(sal2phen)].
Fig. 3 Formation of ReV phosphine complex on reaction of sal2phen with [ReCl3(PPh3)2(CH3CN)].
from unbound triphenylphosphine (-6 ppm) and upfield from
trans-[ReOCl(sal2phen)]·CHCl3 (1)
triphenylphosphine oxide (29.8 ppm). No phosphorous signals
were observed for complexes 4 and 5 due to direct coordination to
the paramagnetic ReIII center.20,34
Complex 1 exhibits distorted octahedral geometry with the
tetradentate N2O2 ligand occupying the equatorial plane and a
Cl coordinated in the axial position trans to the Re O group
˚
(Fig. 4). The Re atom is situated 0.309(2) A above the equatorial
X-ray crystal structures
plane defined by the N2O2 donors towards the oxo group as
is typically found in ReV mono-oxo complexes. This structure
is similar to trans-[ReOCl(acac2pn)]3 but differs from that of
[ReOCl(sal2mp)],30 where the Re O was cis to the chloride. The
trans O Re–Cl angle deviates slightly from linear at 174.2(1)◦
but falls well within the range observed for similar complexes.3,20,27
The ReV complexes, trans-[ReOCl(sal2phen)], 1, cis-[ReO(PPh3)-
(sal2phen*)]PF6, 2, cis-[ReCl2(PPh3)(salphen*)], 3, and the ReIII
complex, trans-[ReCl(PPh3)(sal2phen)], 4, were characterized by
X-ray crystallography (Fig. 4–7 and Tables 1–2). All four exhibited
distorted octahedral geometry and all four complexes exhibited a
small (73–81.21◦) bite angle for the five-membered chelate ring
(NCCN) with the Re center due to the rigidity of the aromatic
backbone.
˚
˚
The Re–O (1.990(3) A), Re–N (2.054–2.061(3) A), and Re
O
V
˚
(1.655(3) A) bond distances were consistent with similar Re
oxo Schiff base complexes.2,3,12–16,18–21,30 For comparison, the trans
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Table 1 X-ray crystal data, data collection parameters, and refinement parameters for 1, 2, 3 and 4
1
2
3
4
CCDC #
Formula
fw
780136
780134
780135
780137
-
C20H14ClN2O3Re·CHCl3
C38H31N2O3PRe+PF6 ·0.5CH2Cl2
C31H24N2OPCl2Re·2.5CHCl3
C38H29ClN2O2PRe·2CH2Cl2
671.35
Orthorhombic
Pbca
968.25
Orthorhombic
Pnn2
21.0703(15)
11.3780(8)
15.0859(11)
90
90
90
3616.7(4)
4
1.778
173(2)
3.595
0.71073
7838
0.0589
25028
0.0374
0.0608
1.013
1027.01
Triclinic
968.10
Monoclinic
P21/c
23.0805(7)
13.1946(4)
25.9868(8)
90
Crystal system
¯
Space group
P1
˚
a/A
19.4392(14)
10.9257(8)
21.4868(15)
90
90
90
4563.5(6)
8
1.954
173(2)
5.820
0.71073
5045
11.1410(8)
11.5874(8)
17.7337(12)
91.854(1)
108.169(1)
115.297(1)
1929.7(2)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
104.447(1)
90
7663.7(4)
8
1.678
173(2)
3.600
0.71073
16877
0.0494
53711
0.0398
0.0666
1.032
3
˚
V/A
Z
rc/g cm-3
T/K
1.768
173(2)
3.879
0.71073
8290
0.0251
13762
0.0515
0.1139
m/mm-1
˚
l source (A)
Indep. Refln.
Rint
0.0499
30828
0.0350
0.0886
1.046
Obs. Refln.
R(F)
Rw(F)2
GoF
1.101
R = (RꢀFO| - |FCꢀ/R|FOꢀ). RW = [Rv(|FO2| - |FC2|)2/Rv(|FO2|2]1/2
.
◦
˚
Table 2 Selected bond angles ( ) and distances (A) for 1, 2, 3, 4a and 4b
1
2
3
4a
4b
Re–N1
Re–N2
Re–O1
Re–O2
Re–O3
Re–P1
Re–Cl1
Re–Cl2
2.061(3)
2.054(3)
1.990(3)
1.990(3)
1.665(3)
2.087(5)
2.199(5)
1.973(4)
1.943(3)
1.697(4)
2.4923(16)
—
2.208(7)
1.775(6)
1.891(5)
—
2.026(4)
2.037(4)
2.002(3)
2.001(3)
—
2.413(1)
2.455(1)
—
1.318(6)
1.312(6)
97.14(12)
90.28(13)
170.48(13)
—
86.85(9)
—
93.94(9)
170.81(13)
91.29(13)
—
87.88(9)
87.90(9)
80.92(14)
—
87.13(10)
97.01(10)
—
2.024(4)
2.032(3)
1.996(3)
2.011(3)
—
2.422(1)
2.456(1)
—
1.317(5)
1.308(5)
96.08(11)
91.20(13)
171.35(12)
—
86.29(9)
—
88.85(9)
171.64(13)
91.22(12)
—
89.84(9)
87.00(9)
81.21(14)
—
86.51(10)
97.29(10)
—
—
2.4244(16)
2.4422(15)
2.3621(15)
1.233(11)
—
2.568(1)
—
—
N1–C3/C7
N2–C6/C14
O1–Re–O2
O1–Re–N1
O1–Re–N2
O1–Re–O3
O1–Re–Cl1
O1–Re–Cl2
O1–Re–P1
O2–Re–N1
O2–Re–N2
O2–Re–O3
O2–Re–Cl1
O2–Re–P1
N1–Re–N2
N1–Re–O3
N1–Re–Cl1
N1–Re–P1
N1–Re–Cl2
N2–Re–O3
N2–Re–Cl1
N2–Re–P1
N2–Re–Cl2
O3–Re–P1
P1–Re–Cl1
P1–Re–Cl2
Cl1–Re–Cl2
1.294(5)
1.299(5)
85.28(12)
94.27(13)
163.34(14)
100.29(15)
83.04(10)
—
1.283(8)
1.511(7)
91.61(19)
92.08(17)
168.84(16)
102.69(17)
—
—
84.8(2)
157.8(2)
—
87.85(14)
100.01(15)
87.03(14)
—
—
—
—
—
73.5(3)
—
87.07(17)
92.61(16)
173.39(17)
—
95.20(17)
89.59(17)
101.99(18)
—
174.89(6)
92.14(5)
88.61(5)
—
—
82.92(12)
84.50(17)
80.15(19)
165.6(2)
—
88.14(10)
79.72(18)
96.48(18)
—
170.99(13)
—
85.88(18)
—
104.15(13)
—
91.95(13)
—
—
—
161.21(13)
94.10(12)
102.90(14)
82.02(10)
—
80.99(13)
95.68(15)
79.29(10)
—
—
96.08(15)
80.38(10)
—
—
—
—
—
—
—
—
89.06(9)
90.77(9)
—
—
175.77(4)
—
89.10(10)
96.19(10)
—
—
173.89(4)
—
—
—
272 | Dalton Trans., 2011, 40, 269–276
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Fig. 4 ORTEP representation of trans-[ReOCl(sal2phen)] (1) with 50%
probability ellipsoids (CCDC# 780136). The CHCl3 of solvation is omitted
for clarity.
Fig. 6 ORTEP representation of [ReCl2(PPh3)(salphen*)] (3) with 50%
probability ellipsoids (CCDC# 780135). The CHCl3 molecules of solvation
are omitted for clarity.
Fig. 5 ORTEP representation of cis-[ReO(PPh3)(sal2phen*)]+ (2) with
50% probability ellipsoids (CCDC# 780134). The anion (PF6-) and solvent
(0.5 CH2Cl2) are omitted for clarity.
˚
Re–Cl bond (2.568(1) A) was slightly longer than that of
Fig. 7 ORTEP representation of trans-[ReCl(PPh3)(sal2phen)] (4) with
50% probability ellipsoids (CCDC# 780137). The CH2Cl2 molecules of
solvation are omitted for clarity.
˚
cis-[ReOCl(sal2mp)] (2.420(1) A) and of trans-[ReOCl(acac2pn)]
(2.468(2) and 2.464(2) A),3,30 and is attributed to the trans influence
˚
of the oxo moiety; this effect may be further enhanced by the
more efficient donation of electron density to the Re atom
by the sal2phen ligand. The planar two carbon backbone led
to a N1–Re–N2 angle of 80.99(13)◦ compared to 82.2(2)◦ in
trans-[ReO(OH2)(acac2en)]Cl (also a 5-membered chelate ring).3
Hydrogen bonding interactions (2.715–2.818 A) are observed in
the solid state between the O and/or Cl coordinated to the Re
and the phenyl protons in the equatorial plane, and between
spans the equatorial and axial positions). This is very different
from previous reports with the acac2en and acac2pn complexes, in
that an imine was reduced.20 One oxygen and two nitrogen donor
atoms from sal2phen* occupy three equatorial coordination sites,
while the remaining phenolic oxygen donor (from the reduced sal
imine fragment) coordinates in the axial position trans to the ReV
oxo group. This results in a distorted octahedral conformation
˚
˚
with the Re atom situated 0.1517(20) A above the equatorial
˚
the chloroform hydrogen atom (H21) (2.417–2.421 A) and the
plane (N2OP) toward the oxo group. The Re O bond distance
coordinated sal2phen oxygen atoms.
3,12–16,18–21,30
˚
of 1.697(4) A falls within the range typically observed.
The “bite-angle” of 79.72(18)◦ observed for N1–Re–N2 is in line
with other five-membered ring systems.3,9,20 The Re–O and Re–N
cis-[ReO(PPh3)(sal2phen*)]PF6·0.5CH2Cl2 (2)
˚
˚
The X-ray crystal structure of 2 illustrates the asymmetric coor-
dination of the sal2phen ligand resulting from the coordination
of triphenylphosphine cis to the ReV oxo group (Fig. 5), and the
reduction of one imine to an amine (from the sal fragment that
bond distances of 1.943(3)–1.973(4) A and 2.087(5)–2.199(5) A,
respectively, are typical for Re–O and Re–N bond distances in
similar complexes.2,3,12–16,18–21,30 The longer Re–N2 bond distance
of 2.199(5) A is expected for Re-amine bonds. The Re–P bond
35
˚
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V
˚
distance of 2.4923(16) A is within the range of Re –P bond
lengths.9,20 The ReV core (O2–Re–O3) deviates from linear at
165.6(2)◦, most likely due to the bulky triphenylphosphine cis to
the oxo group and the constraints of the rearranged sal2phen*
ligand spanning an axial and three equatorial positions. An
extensive hydrogen interaction network was observed in the solid
state between the PF6 anion and the complex, with distances
for both molecules. The two molecules (4a and 4b) have very
similar bond distances and bond angles. The cis-N1–Re–N2
“bite-angle” was 80.92(14)◦ for 4a and 81.21(14)◦ for 4b. The
Re–O (1.996(3)–2.011(3) A) and Re–N (2.024(4)–2.037(4) A)
bond distances are similar to other ReIII–N and ReIII–O bond
distances.37–40 The Re–P and bond lengths in the axial positions
˚
˚
˚
ranged from 2.4128(11)–2.4218(12) A and were shorter than
˚
ranging from 2.171 to 2.870 A. The dichloromethane solvent
the Re–P bonds of trans-[Re(PPh3)2(acac2en)]PF6 (2.4820(11)–
20
˚
molecule also exhibited hydrogen interactions ranging from 2.628
2.4992(11) A; acac2en = N,N¢-ethylenebis(acetylacetoneimine)).
˚
˚
to 2.833 A.
The Re–Cl bonds were 2.4550(11)–2.4563(11) A and were within
the range of Re–Cl bond lengths. The dichloromethane is involved
in hydrogen bonding interactions in the solid state with the Cl atom
[ReCl2(PPh3)(salphen)]· 2.5 CHCl3 (3)
˚
at 2.847 A and with the phenyl groups coordinated to phosphorus
˚
Compound 3 shows a slightly distorted octahedral geometry
(Fig. 6). One oxygen and two nitrogen donor atoms from the frag-
mented tridentate N2O Schiff base ligand (salphen) occupy three
equatorial coordination sites along with one chloride. This ligand
consists of one salicylaldehyde moiety linked to one diaminoben-
zene and is thought to be the remnant of a reduced sal2phen ligand
in which the imine was completely reduced to an amine resulting in
the loss of the second salicylic moiety. The reduction of the imine
may have occurred as a result of the oxidation of ReIII to ReV or
from metal catalyzed oxidation of PPh3; this type of reaction is not
unprecedented in the literature as ReOI3(PPh3)2 has been used in
the enantioselective reduction of imines to amines.10 The axial sites
contain a PPh3 molecule with a trans-coordinated chloride. The
ReV core (PPh3–Re–Cl) deviates slightly from linear at 174.89(6)◦
and is similar to that of the ReV cores (PPh3–Re–Cl) of the triden-
tate phosphine complexes, cis-[Re(apa)Cl2(PPh3)], at 173.36(5)◦
and cis-[Re(mps)Cl2(PPh3)], at 173.15(3)◦.5,36 The cis-N1–Re–
N2 “bite-angle” for complex 3 is 73.4(3)◦ and falls within the
range of the similar tridentate complexes, cis-[Re(apa)Cl2(PPh3)],
at 70.1(3)◦ and cis-[Re(mps)Cl2(PPh3)] (mps = N-(2-amino-3-
methylphenyl)salicylideneimine), at 76.7(1)◦.5,30,34 The other angles
(82.6(1)–87.7(3)◦) around the Re center fall within the range
of angles observed for similar complexes.5,31,36 The Re–O bond
at 2.844 A. The bound Cl atom also showed a hydrogen bonding
˚
interaction of 2.870 A with a hydrogen atom on the phenyl group
of an adjacent PPh3.
Experimental
General considerations
All chemicals were of regent grade or better and solvents were
degassed before using. All experiments were carried out under a
nitrogen atmosphere unless otherwise noted. H and 13C NMR
spe◦ctra were recorded on a Bruker 250 or 500 MHz instrument at
25 C in deuterated solvent (d6-DMSO or CDCl3) with TMS as
an internal reference. 31P NMR spectra were recorded on a Bruker
250 MHz instrument at 25 C in deuterated solvent (d6-DMSO
or CDCl3) with H3PO4 as an external reference. FT-IR spectra
were recorded in the range of 4000–400 cm-1 as KBr pellets on
a Nicolet Magna-IR spectrometer 550. Electrospray Ionization
Mass Spectra (ESI-MS) were obtained on a Thermo Finnigan
TSQ7000 triple-quadrupole instrument with an API2 source.
Elemental analyses were performed by Quantitative Technologies
Inc. (QTI; Whitehouse, NJ).
1
◦
Materials. (n-Bu4N)(ReOCl4)41
and
[ReCl3(PPh3)2-
˚
distance for the equatorial oxygen atom is 1.890(5) A, comparable
(CH3CN)]42 were prepared as reported in the literature. The
sal2phen ligand was synthesized according to the literature.1
˚
to the values observed in literature of 1.866(4) and 1.928(2) A
for similar tridentate ligands.5,31,35 The Re–N bond distances in
˚
the equatorial plane were 1.775(6) and 2.209(7) A, indicating one
trans-[ReOCl(sal2phen)], (1). (n-Bu4)[ReOCl4] (186 mg,
0.318 mmol) in CHCl3 (10 mL) was added to a solution of
sal2phen (207 mg, 0.653 mmol) in ethanol (15 mL). The solution
was refluxed for 3 h under a nitrogen atmosphere and placed in a
freezer overnight. The maroon product was collected by vacuum
filtration. The product washed with hot toluene and ether to
remove any remaining starting material and byproducts. The solid
product was dissolved in chloroform and placed in a screw-top
vial with the cap slightly opened. After several days, crystals had
imido31,36 and one imine bond, respectively. The Re–P bond of
2.4244(16) A is in close agreement with the Re–P bond of 2.429(2)
observed in cis-[Re(apa)Cl2(PPh3)] by Gerber et al.31 The Re–Cl
bond length trans to the PPh3 was longer than the cis-Re–Cl bond
length, and is attributed to the trans influence of the PPh3 moiety.
Two of the three solvent chloroform molecules exhibit hydrogen
bonding interactions in the solid state of 2.744–2.863 A with the
two chlorides coordinated to Re. The coordinated Cl atoms show
additional hydrogen bonding interactions with hydrogen atoms on
˚
˚
1
formed. Yield: 75%. H NMR [d6-DMSO, 500 MHz; d (ppm)]:
˚
adjacent phenyl groups of 2.741–2.834 A. The third chloroform is
7.16 (t, 1H), 7.26 (t, 1H), 7.37 (d, 1H), 7.50 (d, 1H), 7.78 (sextet,
1H), 7.82 (sextet, 1H), 7.84 (ddd, 1H), 7.95 (ddd, 1H), 8.03 (dd,
1H), 8.16 (dd, 1H), 8.53 (sextet, 1H), 8.60 (sextet, 1H), 10.41 (s,
1H), 10.42 (s, 1H). 13C NMR [d6-DMSO, 250 MHz; d (ppm)]:
179.43, 176.28, 173.29, 172.98, 151.47, 150.02, 141.00, 140.06,
139.82, 139.59, 131.44, 131.27, 122.31, 121.54, 121.45, 120.61,
119.56. IR (KBr, n in cm-1): 951 (m, Re O). MS (m/z): [M–Cl]+
517.04 (517.07 calcd). Anal. Found: C, 43.36; H, 2.91; N, 5.06;
Cl, 6.40. Calcd for ReC20H14N2O3Cl: C, 43.13; H, 2.50; N, 5.27;
Cl, 6.71.
disordered about an inversion center.
trans-[ReCl(PPh3)(sal2phen)]·2 CH2Cl2 (4)
Two independent molecules of 4 (4a and 4b) were found within
the unit cell. The sal2phen ligand occupies the equatorial plane
with the Re atom situated within the ligand plane, and PPh3
and chloride occupy the axial sites. The ReIII core (PPh3–Re–
Cl) deviates slightly from linear at 175.77(4)◦ and 173.89(4)◦
274 | Dalton Trans., 2011, 40, 269–276
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cis-[ReO(PPh3)(sal2phen*)]PF6·CH2Cl2, (2). Triphenylphos-
phine (143 mg, 0.546 mmol) in CH2Cl2 (10 mL) was added to
a solution of 1 (100 mg, 0.181 mmol) in ethanol (10 mL). The
solution was refluxed for 2.5 h under a nitrogen atmosphere,
after which the solvent was removed under vacuum. The maroon-
colored product was washed with hot hexane and ether to remove
ligand by-products (i.e., phosphine oxide, sal2phen fragments).
Ammonium hexafluorophosphate (NH4PF6) (11 mg, 0.067 mmol)
was dissolved in CH2Cl2 (5 mL) and added to the product
in CH2Cl2. X-ray quality crystals were obtained by the slow
evaporation of CDCl3 in an NMR tube at ambient temperature.
Yield: 30%. 1H NMR [CDCl3, 500 MHz; d (ppm)]: 3.96 (d, 2H);
6.65 (m, 2H); 6.87 (m, 2H); 6.95 (m, 2H); 7.31 (m, 3H); 7.44 (m,
7H); 7.51 (m, 6H); 7.64 (m, 6H); 7.90 (m, 2H); 8.55 (s, 1H); 9.55 (s,
1H). 31P NMR [d6-DMSO, 250 MHz; d (ppm)]: 21.87. IR (KBr, v
in cm-1): 951 (m, Re O). MS (m/z): [M–Cl–M, Chloride bridged
dimer]+ 1597.78 (1597.32).
X-ray structure determinations and refinements for 1, 2, 3 and 4
Intensity data were obtained at -100 ◦C on a Bruker SMART
CCD Area Detector system using the w scan technique with
Mo-Ka radiation from a graphite monochromator. Intensities
were corrected for Lorentz and polarization effects. Equivalent
reflections were merged, and absorption corrections were made
using the multi-scan method. Space group, lattice parameters and
other relevant information are given in Table 1. The structures were
solved by direct methods with full-matrix least-squares refinement,
using the SHELX package.43,44 All non-hydrogen atoms were
refined with anisotropic thermal parameters. The hydrogen atoms
were placed at calculated positions and included in the refinement
using a riding model, with fixed isotropic U. The final difference
maps contained no features of chemical significance.
Conclusions
In conclusion, the steric constraints imposed by the o-
phenylenediimine backbone did not afford a ReIII complex without
complications and ligand fragmentation. Obviously more than
just size and rigidity of the tetradentate Schiff base ligand and
nucleophilicityandreducingpower ofthe phosphine determine the
final product(s). The sal2phen ligand’s delocalized electron density
did not assist in the reduction of ReV to ReIII or afford substitution
onto the ReIII starting material. Rhenium’s catalytic tendencies,
which are said to approach those of Ni and Pt, must be considered
when working with rhenium compounds. One of the most versatile
transition metal catalysts is methyltrioxorhenium(VII) (MTO).8,12
Rhenium(V) oxo complexes have been reported to be involved
in reduction, oxygen atom transfer, hydrosilation and silane
alcoholysis reactions.4–10 This reactivity is very different from that
reported for analogous Tc complexes, and perhaps more than
redox and substitution rate differences must be considered in
designing radiopharmaceuticals based on Re.
[ReCl2(PPh3)(salphen)]·2 CHCl3, (3). [ReCl3(PPh3)2(CH3CN)]
(50 mg, 0.063 mmol) in CHCl3 (5 mL) was added to a solution
of sal2phen (20 mg, 0.063 mmol) in ethanol (5 mL). The orange-
brown solution was refluxed for 2 h, and filtered through a medium
glass frit to remove the yellow unreacted sal2phen starting material.
The filtrate was then dried under vacuum and sonicated in toluene
to remove the green-brown unreacted [ReCl3(PPh3)2(CH3CN)]
starting material. The purified product was then dissolved in
CDCl3 and placed in an NMR tube. Slow evaporation of the
1
solvent resulted in crystal formation. Yield: 15%. H NMR [d6-
DMSO, 500 MHz; d (ppm)]: 5.84 (t, 1H); 6.13 (d, 1H); 6.21 (d,
1H); 7.15 (t, 1H); 7.29 (m, 9H); 7.54 (m, 1H); 7.61 (m, 2H); 7.68 (m,
6H); 8.78 (m, 1H); 13.81 (d, 1H). 31P NMR [d6-DMSO, 250 MHz;
d (ppm)]: 24.67. IR (KBr, n in cm-1): 1092 (Re N). MS (m/z):
[M+Na]+ 751.16 (751.05).
trans-[ReCl(PPh3)(sal2phen)]·2 CH2Cl2 (4). Triphenylphos-
phine (74 mg, 0.282 mmol) in CH2Cl2 (5 mL) was added to 1
(50 mg, 0.091 mmol) in ethanol (20 mL). The burgundy solution
was refluxed under N2 for 12 h. The brown reaction mixture
volume was reduced to dryness under vacuum. The dark brown
solid was washed with toluene and filtered. The dark brown
solid was washed with ether and then dried in air overnight.
Small, orange crystals were obtained by slow vapor deposition
of dichloromethane into hexane in a scintillation vial. Yield: 10%.
1H NMR [CDCl3, 500 MHz; d (ppm)]: -22.44 (d; 2H); -9.18 (t;
2H); -6.68 (d; 2H); 3.46 (m; 2H); 7.30 (m; 2H); 7.61 (b; 6H); 7.90
(t; 6H); 8.26 (t; 3H); 21.65 (t; 2H); 30.89 (d; 2H). MS (m/z): [M∑]+
798.05 (798.12).
Acknowledgements
The authors acknowledge the University of Missouri Mass
Spectroscopy Facility; National Science Foundation grants NSF
CHE-89-08304 for the use of the NMR Facility and NSF CHE-
90-110804 for the use of the X-ray Facility; National Institutes of
Biomedical Imaging and Bioengineering (NIBIB) Training Grant
NIBIB 5 T32 EB004822 (SRL); National Science Foundation
Research Experience for Undergraduates Grant NSF CHE-
035891 (BC and SW); and University of Missouri Department
of Chemistry Stevens’ Undergraduate Research Program (NS).
trans-[Re(PPh3)2(sal2phen)]Cl
(5). Triphenylphosphine
References
(1.47 g, 5.61 mmol) was added to 1 (1 g, 1.81 mmol) in ethanol
(10 mL). The burgundy solution was refluxed for 12 h. The
reaction mixture volume was reduced to dryness in vacuum. The
grey solid was washed with toluene that resulted in a red-brown
solid once filtered through a glass frit. The solid filtrate was
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