Reactivity of rhenium(V) oxo Schiff base complexes with phosphine ligands: Rearrangement and reduction reactions

Article abstract of DOI:10.1021/ic048670j

The symmetric rhenium(V) oxo Schiff base complexes frans-[ReO(OH ₂)(acac₂en)]Cl and trans-[ReOCl(acac₂pn)], where acac₂en and acac₂pn are the tetradentate Schiff base ligands N,N'-ethylenebis(acetylacetone) diimine and N,N'- propylenebis(acetylacetone) diimine, respectively, were reacted with monodentate phosphine ligands to yield one of two unique cationic phosphine complexes depending on the ligand backbone length (en vs pn) and the identity of the phosphine ligand. Reduction of the Re(V) oxo core to Re(III) resulted on reaction of frans-[ReO-(OH₂)(acac₂en)]Cl with triphenylphosphine or diethylphenylphosphine to yield a single reduced, disubstituted product of the general type trans-[Re^III(PR ₃)₂(acac₂en)]⁺. Rather unexpectedly, a similar reaction with the stronger reducing agent triethylphosphine yielded the intramolecularly rearranged, asymmetric cis-[Re^VO(PEt ₃)(acac₂en)]⁺ complex. Reactions of frans-[RevO(acac2pn)CI] with the same phosphine ligands yielded only the rearranged asymmetric cis-[Re^VO(PR₃)(acac ₂pn)]⁺ complexes in quantitative yield. The compounds were characterized using standard spectroscopic methods, elemental analyses, cyclic voltammetry, and single-crystal X-ray diffraction. The crystallographic data for the structures reported are as follows: frans-[Re^III(PPh ₃)₂(acac₂en)]PF₆ (H ₄₈C₄₈N₂O₂P₃- Re·PF₆), 1, triclinic (P1), a = 18.8261(12) A, b = 16.2517(10) A, c = 15.4556(10) A, α = 95.522(1)°, β = 97.130(1)°, γ = 91.350(1)°, V = 4667.4(5) A³, Z = 4; trans-[Re^III(PEt₂Ph)₂(acac ₂en)]PF₆ (H₄₈C₃₂N₂O ₂P₂Re·PF₆), 2, orthorhombic (Pccn), a = 10.4753(6) A, b = 18.4315(10) A, c = 18.9245(11) A, V = 3653.9(4) A³, Z = 4; cis-[Re^VO(PEt ₃)(acac₂en)]PF₆ (H₃₃C ₁₈N₂O₃PRe·1.25PF₆), 3, monoclinic (C2/c), a = 39.8194(15) A, b = 13.6187(5) A, c = 20.1777(8) A, β = 107.7730(10)°, V = 10419.9(7) A³, Z = 16; cis-[Re^VO(PPh₃)(acac ₂pn)]PF₆ (H₃₅C₃₁N₂O ₃-PRe·PF₆), 4, triclinic (P1), a = 10.3094(10) A, b = 12.1196(12) A, c = 14.8146(15) A, α = 105.939(2)°, β = 105.383(2)°, γ = 93.525(2)°, V = 1698.0(3) A³, Z = 2; cis-[Re^VO(PEt ₂Ph)(acac₂pn)]PF₆ (H₃₅C ₂₃N₂O₃PRe·PF₆), 5, monoclinic (P2₁/n), a = 18.1183(18) A, b = 11.580(1) A, c = 28.519(3) A, β = 101.861(2)°, V = 5855.9(10) A³, Z = 4.

Full text of DOI:10.1021/ic048670j

Inorg. Chem. 2005, 44, 2381−2390
Reactivity of Rhenium(V) Oxo Schiff Base Complexes with Phosphine
Ligands: Rearrangement and Reduction Reactions
Paul D. Benny, Jenny L. Green, Hendrik P. Engelbrecht, Charles L. Barnes, and Silvia S. Jurisson*
Department of Chemistry, UniVersity of Missouri, 125 Chemistry Building, 601 S. College AVenue,
Columbia, Missouri 65211
Received September 22, 2004
The symmetric rhenium(V) oxo Schiff base complexes trans-[ReO(OH2)(acac2en)]Cl and trans-[ReOCl(acac2pn)],
where acac2en and acac2pn are the tetradentate Schiff base ligands N,N -ethylenebis(acetylacetone) diimine and
N,N -propylenebis(acetylacetone) diimine, respectively, were reacted with monodentate phosphine ligands to yield
′
′
one of two unique cationic phosphine complexes depending on the ligand backbone length (en vs pn) and the
identity of the phosphine ligand. Reduction of the Re(V) oxo core to Re(III) resulted on reaction of trans-[ReO-
(OH2)(acac2en)]Cl with triphenylphosphine or diethylphenylphosphine to yield a single reduced, disubstituted product
of the general type trans-[ReIII(PR3)2(acac2en)]+. Rather unexpectedly, a similar reaction with the stronger reducing
agent triethylphosphine yielded the intramolecularly rearranged, asymmetric cis-[ReVO(PEt3)(acac2en)]+ complex.
Reactions of trans-[ReVO(acac2pn)Cl] with the same phosphine ligands yielded only the rearranged asymmetric
cis-[ReVO(PR3)(acac2pn)]+ complexes in quantitative yield. The compounds were characterized using standard
spectroscopic methods, elemental analyses, cyclic voltammetry, and single-crystal X-ray diffraction. The
crystallographic data for the structures reported are as follows: trans-[ReIII(PPh3)2(acac2en)]PF6 (H48C48N2O2P2-
Re
97.130(1)
2, orthorhombic (Pccn), a
cis-[ReVO(PEt3)(acac2en)]PF6 (H33C18N2O3PRe
‚
PF6), 1, triclinic (P1
h
), a
)
, V
18.8261(12) Å, b
4667.4(5) Å3, Z
18.4315(10) Å, c
1.25PF6), 3, monoclinic (C2/c), a
10419.9(7) Å3, Z
)
)
16.2517(10) Å, c
)
15.4556(10) Å, R ) 95.522(1)
°
,
‚
â )
PF6),
°
,
γ ) 91.350(1)
°
)
)
4; trans-[ReIII(PEt2Ph)2(acac2en)]PF6 (H48C32N2O2P2Re
10.4753(6) Å, b
)
‚
)
18.9245(11) Å, V
)
3653.9(4) Å3, Z
)
4;
)
39.8194(15) Å, b
)
13.6187(5)
Å, c
)
‚
20.1777(8) Å, â ) 107.7730(10)
°
, V
10.3094(10) Å, b
1698.0(3) Å3, Z
)
)
16; cis-[ReVO(PPh3)(acac2pn)]PF6 (H35C31N2O3-
PRe
PF6), 4, triclinic (P1
γ ) 93.525(2)
monoclinic (P21/n), a
4.
h
), a
)
)
12.1196(12) Å, c
2; cis-[ReVO(PEt2Ph)(acac2pn)]PF6 (H35C23N2O3PRe
28.519(3) Å, â ) 101.861(2) , V
5855.9(10) Å3,
)
14.8146(15) Å, R ) 105.939(2)
°, â )
105.383(2)
°
,
°
, V
)
)
‚
PF6), 5,
)
18.1183(18) Å, b
)
11.580(1) Å, c
)
°
)
Z
)
Introduction
(V) oxo Schiff base complexes based on the tetradentate
acetylacetone-derived Schiff base ligands (e.g., acac2en and
acac2pn) have not received as much attention.1,7,10,11 We
Interest in the chemistry of Re(V) oxo complexes stems
from their applications to catalysis and therapeutic nuclear
medicine. Re(V) oxo Schiff base complexes based on the
tetradentate salicylaldehyde-derived Schiff base ligands (e.g.,
sal2en and sal2pn) have been investigated extensively.1-9 Re-
(5) Tisato, F.; Refosco, F.; Mazzi, U.; Bandoli, G.; Nicolini, M. Inorg.
Chim. Acta 1991, 189, 97-103.
(6) Hermann, W. A.; Rauch, M. U.; Artis, G. R. J. Inorg. Chem. 1996,
35, 1988-1991.
* Author to whom correspondence should be addressed. E-mail:
JurissonS@missouri.edu. Phone: 573-882-2107. Fax: 573-882-2754.
(1) Middleton, A. R.; Masters, A. F.; Wilkinson, G. J. Chem. Soc., Dalton
Trans. 1979, 542-546.
(7) Bottomley, L. A.; Wojciechowski, P. E.; Holder, G. N. Inorg. Chim.
Acta 1997, 255, 149-155.
(8) van Bommel, K. J. C.; Verboom, W.; Kooijman, H.; Spek, A. L.;
Reinhoudt, D. N. Inorg. Chem. 1998, 37, 4197-4203.
(9) Bereau, V. M.; Khan, S. I.; Abu-Omar, M. M. Inorg. Chem. 2001,
40, 6767-6773.
(2) Tisato, F.; Refosco, F.; Mazzi, U.; Bandoli, G.; Dolmella, A. Inorg.
Chim. Acta 1989, 164, 127-135.
(3) Banbery, H. J.; McQuillan, F.; Hamor, T. A.; Jones, C. J.; McCleverty,
J. A. J. Chem. Soc., Dalton Trans. 1989, 1405-1407.
(10) Carrondo, M. A. A. F. d. C. T.; Middleton, A. R.; Skapski, A. C.;
West, A. P.; Wilkinson, G. Inorg. Chim. Acta 1980, 44, L7-L8.
(11) Benny, P. B.; Barnes, C. L.; Piekarski, P. M.; Lydon, J. D.; Jurisson,
S. S. Inorg. Chem. 2003, 42, 6519-6527.
(4) Banbery, H. J.; McQuillan, F. S.; Hamor, T. A.; Jones, C. J.;
McCleverty, J. A. Inorg. Chim. Acta 1990, 170, 23-26.
10.1021/ic048670j CCC: $30.25
Published on Web 02/17/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 7, 2005 2381

Benny et al.
recently reported on the syntheses of the monomeric Re(V)
oxo Schiff base complexes with acac2en (N,N′-ethylenebis-
(acetylacetone) diimine) and acac2pn (N,N′-propylenebis-
(acetylacetone) diimine).11 In this paper, we report on the
reactivity of the monomeric Re(V) oxo Schiff base com-
plexes trans-[ReO(OH2)(acac2en)]Cl and trans-[ReOCl(acac2-
pn)] with tertiary phosphines.
Our interest in the Re(III) complexes stems from the
expectation that d4 Re(III) complexes will be kinetically more
inert than d2 Re(V) complexes and, thus, may be useful for
radiotherapeutic applications. Two Re radioisotopes, 186Re
and 188Re, have potential utility in therapeutic radiopharma-
ceuticals.12,13 The impetus for this work arose from our
interest in targeted radiotherapy using the radionuclides 186Re
(90 h t1/2, 1.02 MeV â-, 137 keV γ (7%)) and 188Re (17 h
t1/2, 2.11 MeV â-, 155 keV γ (15%)) and is based on earlier
promising work on 99mTc-based diagnostic agents. Rhenium
is the third row congener of Tc, and thus their chemistry is
closely related, making them a matched pair for formulation
of radiodiagnostic (99mTc) and radiotherapeutic (186/188Re)
agents. In the earlier work, Tc(V) oxo Schiff base complexes
analogous to those reported in this paper (trans-[TcOX(Schiff
base)]0/+ (X ) Cl-, OH2)) were shown to react with
phosphines to yield Tc(III) complexes of the type trans-[Tc-
(PR3)2(Schiff base)]+ that showed reversible Tc(III)/Tc(IV)
and Tc(III)/Tc(II) redox couples.14-16 This chemistry led to
the development of a series of Tc(III) Schiff base phosphine
complexes, referred to as the Q-series, as potential myocar-
dial imaging agents.17,18 The Q-series of complexes were
more recently evaluated for utility in assessing multidrug
resistance to chemotherpeutic agents.19 The reactivity of the
Re(V) oxo Schiff base complexes with phosphines resulted
in the formation of the Re(III) analogues to the Tc(III)
complexes in some cases and to rearranged Re(V) oxo
complexes in other cases, with the product dependent on both
the Schiff base ligand and the phosphine. The Re(V) and
Re(III) Schiff base phosphine complexes synthesized were
fully characterized, including single-crystal X-ray diffraction
and cyclic voltammetric analysis.
Figure 1. Formation of rhenium(III) complexes or asymmetric rhenium-
(V) phosphine complexes.
where X ) Cl- or OH2 and L ) acac2en or acac2pn, yielded
some rather unexpected results. The reactions of simple alkyl-
and arylphosphines with the Re(V) complexes resulted in
the formation of two different types of complexes containing
coordinated phosphine ligands. Depending on the Schiff base
ligand (acac2en or acac2pn) and the phosphine ligand (PPh3,
PEt2Ph or PEt3), either a reduced and disubstituted trans bis-
(phosphine) Re(III) or a cis monosubstituted phosphine Re-
(V) oxo complex was isolated (Figure 1). The Re(III)
complexes resulted from the reduction of the metal center,
possibly through yl-oxygen atom abstraction by the phos-
phine ligand following initial phosphine coordination trans
to the oxo group, to yield symmetric complexes of the type
trans-[ReIII(PR3)2(L)]+ and the phosphine oxide. The Re(V)
oxo complexes resulted from phosphine substitution of the
trans ligand (Cl- or H2O) and intramolecular rearrangement
to generate asymmetric complexes of the type cis-[ReVO-
(PR3)(L)]+. The specific type of substituted complex isolated,
either through reduction or rearrangement, appears to be
dependent on the nature of the Schiff base ligand and the
phosphine ligand and may be the result of the kinetics of
rearrangement versus reduction.
The general synthesis of compounds 1-6 employed two
different methods that each yielded single products. One
method involved a two step in situ approach, where 2 equiv
of the ligand (acac2en or acac2pn) was reacted with a
common Re(V) starting material, [n-Bu4N][ReOCl4], fol-
lowed by the addition of 4 equiv of phosphine ligand. In
this approach, the yields of 1-6 depended on the percent of
rhenium complexation (40-50%) with the Schiff base ligand,
which was essentially the same as that observed if the Re-
(V) Schiff base complex were isolated.11 The second method
involved prior isolation of the Re(V) oxo Schiff base
complex, followed by direct reaction with the phosphine
ligands. The Re(V) Schiff base complexes utilized, trans-
[ReO(OH2)(acac2en)]Cl and trans-[ReOCl(acac2pn)], were
prepared as previously reported.11 Reaction of the Re(V) oxo
Schiff base complexes directly with the phosphine ligands
(4 equiv for 1 and 2; 1 equiv for 3-6) resulted in compounds
1-6 in excellent yields and higher purity than the two-step
in situ method. The two-step in situ method resulted in
Results and Discussion
The reactions of tertiary phosphine ligands (PR3) with
symmetric rhenium(V) oxo N2O2 acetylacetone-derived
Schiff base complexes of the type trans-[ReVOX(L)]0/+
,
(12) Heeg, M. J.; Jurisson, S. S. Acc. Chem. Res. 1999, 32, 1053-1060.
(13) Volkert, W. A.; Hoffman, T. J. Chem. ReV. 1999, 93, 1137-1156.
(14) Jurisson, S.; Lindoy, L. F.; Dancey, K. P.; McPartlin, M.; Tasker, P.
A.; Uppal, D. K.; Deutsch, E. Inorg. Chem. 1984, 23, 227-231.
(15) Jurisson, S. S.; Dancey, K.; McPartlin, M.; Tasker, P. A.; Deutsch, E.
Inorg. Chem. 1984, 23, 4743-4749.
(16) Ichimura, A.; Heineman, W. R.; Deutsch, E. Inorg. Chem. 1985, 24,
2134-2139.
(17) Marmion, M. E.; Woulfe, S. R.; Neumann, W. L.; Nosco, D. L.;
Deutsch, E. Nucl. Med. Biol. 1999, 26, 755-770.
(18) Rosetti, C.; Vanoli, G.; Paganelli, G.; Kwiakowski, M.; Zito, F.;
Colombo, F.; Bonino, C.; Carpinelli, A.; Casati, R.; Deutsch, K.;
Marmion, M.; Woulfe, S. R.; Lunghi, G.; Deutsch, E.; Fazio, F. J.
Nucl. Med. 1994, 35, 1571-1580.
(19) Crankshaw, C. L.; Marmion, M.; Luker, G. D.; Rao, V.; Dahlheimer,
J.; Burleigh, B. D.; Webb, E.; Deutsch, K. F.; Piwnica-Worms, D. J.
Nucl. Med. 1998, 39, 77-86.
2382 Inorganic Chemistry, Vol. 44, No. 7, 2005

Re(V) Oxo Schiff Base Complexes with Phosphines
Table 1. X-ray Crystal Data, Data Collection Parameters, and Refinement Parameters for 1-5
1
2
3
4
5
-
formula
fw
cryst system Triclinic
C48H46N2O2P2Re+PF6-‚0.5H2O C32H48N2O2P2Re+PF6 H33C18N2O3PRe‚1.25PF6 C31H35N2O3PRe+PF6-‚CH2Cl2 C47H74N4O7P2Re2‚2PF6
1085.99
885.83
orthorhombic
Pccn
723.85
monoclinic
C2/c
893.21
1531.38
monoclinic
P21/n
triclinic
P1h
space group P1h
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
18.8261(12)
10.4753(6)
18.4315(10)
18.9245(11)
90
90
90
39.8194(15)
13.6187(5)
20.1777(8)
90
107.733(1)
90
10.3094(10)
12.1196(12)
14.8146(15)
105.939(2)
105.383(2)
93.525(2)
1698.0(3)
2
18.1183(18)
11.580(1)
28.519(3)
90
101.861(2)
90
16.2517(10)
15.4556(10)
95.522(1)
97.130(1)
91.350(1)
4667.4(5)
4
3653.9(4)
4
10419.9(7)
16
5855.9(10)
4
F
calc (g/cm3) 1.545
T (K)
1.610
1.846
1.747
1.737
173
2.772
173(2)
3.519
173
3.820
173
3.820
173
4.327
µ (mm-1
)
λ source (Å) 0.710 73
0.710 73
0.0197
0.0430
1.036
0.710 73
0.0355
0.0945
1.035
0.710 73
0.0277
0.0701
0.710 73
0.0305
0.0675
1.040
R(F)a
Rw(F)2 a
GoF
0.0377
0.1007
1.049
1.073
a R ) (Σ||Fo| - |Fc||/Σ|Fo||). Rw ) [Σw(|Fo | - |Fc |)2/Σw(|Fo | ]1/2
.
2
2
2 2
unidentified byproducts, resulting from the reaction of
unreacted rhenium starting material with the phosphine ligand
to form Re(IV) or Re(V) chloro-phosphine complexes,
which were separated using chromatographic methods.
General Characterization. Elemental analyses and elec-
trospray ionization mass spectrometry (ESI-MS) of com-
pounds 1-6 confirmed the identities of the products. The
molecular ions with the expected rhenium isotope pattern
were observed in the positive mode of ESI-MS. The FT-IR
spectra of the Re(V) complexes showed the presence of the
RedO stretches between 960 and 970 cm-1, typical of Re-
(V) monooxo complexes.1-7,9-11 This band was absent in
the spectra of the Re(III) complexes.
Reaction of trans-[ReO(OH2)(acac2en)]Cl with phosphine
ligands resulted in either the reduced Re(III) core or the
rearranged Re(V) oxo complex. Aryl-containing phosphine
ligands (i.e., PPh3 or PEt2Ph) reacted with trans-[ReVO(OH2)-
(acac2en)]Cl to generate ReIII complexes of the type trans-
[ReIII(PR3)2(acac2en)]+, 1 and 2. The trialkylphosphine (PEt3),
however, resulted only in substitution to form the asymmetric
cis-[ReVO(PEt3)(acac2en)]+ complex, 3. This outcome was
unexpected considering the increased basicity and reducing
power of the trialkylphosphine. The triethylphosphine was
expected to favor the formation of the ReIII complex relative
to the aryl phosphines because of its strong reducing power.
It is hypothesized that kinetics control this reaction; the
rearrangement reaction is favored over oxygen atom abstrac-
tion for the more nucleophilic PEt3. The reduction reaction
is apparently not sufficiently facile compared to rearrange-
ment situating the very nucleophilic PEt3 cis to the oxo group.
The resulting product, cis-[ReO(PEt3)(acac2en)]+, appears
resistant to subsequent reduction; use of a large excess of
phosphine, higher temperatures, and different solvents re-
sulted in the same product.
NMR Characterization. The asymmetric Re(V) products
were easily characterized by their distinctive 1H NMR
spectra, with virtually all protons unique in these complexes.
The acac2en/pn ligands in cis-[ReVO(PR3)(L)]+, compounds
3-6, exhibit characteristic asymmetric splitting patterns
1
(Table 3). Analysis of the H NMR spectra can be divided
on the basis of the orientation of the N, O donors relative to
the Re(V) oxo group: one acac of acac2en/pn is cis (or
perpendicular) to the Re(V) oxo group while the second acac
is parallel to the Re(V) oxo group. The acac moiety cis to
the Re(V) oxo group exhibits chemical shifts for the methyl
and vinyl protons similar to the Re(V) oxo starting materials
trans-[ReOX(acac2en/pn)]0/+. The signals observed for the
acac moiety parallel to the Re(V) oxo group were shifted
dramatically (Table 3). The alkyl backbone, en or pn, in the
asymmetric cis complexes showed increased splitting due
to the inequivalency of these protons. Phosphorus coupling
1
was observed in both the H and 13C NMR spectra. The
The reactions of trans-[ReOCl(acac2pn)] with the three
phosphine ligands yielded only the asymmetric cis-[ReV-
O(PR3)(acac2pn)]+ complexes, 4-6. No reduced Re(III)
complexes were detected in the reaction mixtures with the
acac2pn complexes. Attempts to convert the cis-[ReVO(PR3)-
(acac2pn)]+ complexes to the corresponding reduced com-
pounds trans-[ReIII(PR3)2(acac2pn)]+ were unsuccessful, even
with greater than a 10-fold excess of phosphine, higher
reaction temperatures, and/or longer reaction times. Although
the trans-[ReIII(PR3)2(acac2en)]+ complexes were not reacted
with oxidants, exposure of solutions of these complexes to
air did not appear to yield the cis-[ReVO(PR3)(acac2pn)]+
complexes as determined by NMR and color (red vs green).
aromatic carbons are assigned on the basis of the relative
coupling constants expected for a quaternary P (assuming
the Re as the fourth site) and the relative intensities of the
signals.20 The 31P NMR chemical shifts for the coordinated
phosphines were comparable in complexes 3-6 and observed
as singlets between -15 and -17 ppm, a very narrow range
considering that the free phosphines ranged from -5 to -20
ppm depending on the R groups (ca. -5 for triphenylphos-
phine and ca. -20 for triethylphosphine). The chemical shifts
(20) Levy, G. C.; Lichter, R. L.; Nelson, G. L. Carbon-13 Nuclear Magnetic
Resonance Spectroscopy, 2nd ed.; John Wiley & Sons: New York,
1980.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2383

Benny et al.
Table 2. Selected Bond Angles (deg) and Distances (Å) for 1-5
1
3
5
molecule a
molecule b
2
molecule a
molecule b
4
molecule a
molecule b
Re-N1
Re-N2
Re-O1
Re-O2
Re-O3
Re-P1
Re-P2
2.041(4)
2.039(4)
2.027(3)
2.028(3)
2.045(4)
2.044(4)
2.037(3)
2.025(3)
2.0381(18)
2.0381(18)
2.0226(15)
2.0226(15)
2.070(5)
2.034(6)
2.021(4)
2.078(4)
1.692(5)
2.4860(18)
2.059(6)
2.025(6)
2.029(5)
2.079(5)
1.687(5)
2.484(2)
2.095(3)
2.081(3)
2.026(3)
1.998(2)
1.688(3)
2.5252(9)
2.091(3)
2.084(3)
2.024(3)
2.010(3)
1.691(3)
2.478(1)
2.084(3)
2.088(3)
2.032(3)
2.019(3)
1.690(3)
2.492(1)
2.4827(11)
2.4868(11)
2.4992(11)
2.4820(11)
2.4579(8)
2.4579(8)
O1-Re-O2
O1-Re-N1
O1-Re-N2
O1-Re-O3
O1-Re-P1
O1-Re-P2
O2-Re-N1
O2-Re-N2
O2-Re-O3
O2-Re-P1
O2-Re-P2
N1-Re-N2
N1-Re-O3
N1-Re-P1
N1-Re-P2
N2-Re-O3
N2-Re-P1
N2-Re-P2
O3-Re-P1
P1-Re-P2
98.41(13)
89.66(15)
171.82(14)
98.58(12)
89.60(14)
172.65(13)
94.79(9)
91.12(7)
174.10(7)
80.92(18)
90.84(19)
158.2(2)
102.2(2)
85.98(13)
81.33(19)
90.2(2)
158.8(2)
101.9(2)
87.58(14)
83.89(10
89.91(11)
166.76(11)
99.74(11)
89.69(8)
86.52(11)
88.82(12)
168.48(12)
99.18(13)
84.92(8)
84.24(11)
89.33(12)
167.08(12)
98.04(13)
86.60(8)
86.63(9)
88.60(9)
171.34(15)
89.33(15)
84.13(9)
87.56(9)
171.44(13)
88.64(14)
88.64(4)
87.98(4)
174.10(7)
91.12(7)
91.03(19)
80.3(2)
166.2(2)
83.01(13)
91.4(2)
80.4(2)
166.4(2)
83.45(14)
88.98(11)
83.04(11)
168.96(11)
86.71(8)
86.06(11)
82.93(11)
167.63(12)
85.24(7)
86.89(11)
83.06(12)
168.65(12)
84.09(8)
91.43(9)
86.59(9)
82.77(16)
91.64(9)
87.02(9)
83.25(15)
87.98(4)
88.64(4)
82.98(11)
78.5(2)
102.3(2)
173.62(16)
79.6(2)
101.7(2)
174.70(16)
87.73(12)
101.40(12)
175.69(8)
85.86(12)
104.92(13)
169.57(9)
87.56(13)
104.21(13)
170.44(9)
92.18(11)
90.50(11)
91.70(10)
90.84(10)
92.03(5)
91.72(5)
90.56(11)
94.53(11)
94.33(10)
94.25(10)
91.72(5)
92.03(5)
102.65(17)
83.81(16)
100.93(17)
83.48(17)
91.69(9)
82.90(9)
98.75(9)
84.35(9)
94.49(10)
84.94(10)
174.51(4)
171.29(4)
175.00(3)
Table 3. 1H NMR Spectral Assignments for 3-6 (CDCl3, TMS, 500 MHz, Room Temperature)
A/A (ppm)
B (ppm)
C/C (ppm)
X
3 (n ) 2)
4 (n ) 3)
2.21 (s, 3H)
2.25 (s, 3H)
2.34 (s, 3H)
3.02 (s, 3H)
1.73 (s, 3H)
2.21 (s, 3H)
2.29 (s, 3H)
2.83(s, 3H)
3.59-3.73 (m, 2H, CH2CH2)
4.00-4.07 (m, 1H, CH2CH2)
5.29-5.41 (m, 1H, CH2CH2)
5.45 (s, 1H)
6.06 (s, 1H)
1.02-1.15 (d of t, 9H, PCH2CH3)
1.67-1.82 (m, 6H, PCH2CH3)
7.40-7.63 (m, 15H, PPh3)
2.01-2.20 (m, 2H, CH2CH2CH2)
3.12-3.19 (m, 1H, CH2CH2CH2)
3.48-3.61 (m, 1H, CH2CH2CH2)
4.21-4.27 (m, 1H, CH2CH2CH2)
4.93-5.04 (m, 1H, CH2CH2CH2)
2.35-2.42 (m, 2H, CH2CH2CH2)
3.08-3.19 (m, 1H, CH2CH2CH2)
3.66-3.75 (m, 1H, CH2CH2CH2)
4.15-4.24 (m, 1H, CH2CH2CH2)
4.99-5.10 (m, 1H, CH2CH2CH2)
1.97-2.10 (m, 2H, CH2CH2CH2)
3.07-3.18 (m, 1H, CH2CH2CH2)
3.72-3.81 (m, 1H, CH2CH2CH2)
4.12-4.19 (m, 1H, CH2CH2CH2)
5.11-5.20 (m, 1H, CH2CH2CH2)
5.35 (s, 1H)
5.58 (s,1H)
5 (n ) 3)
6 (n ) 3)
1.52 (s, 3H)
2.18 (s, 3H)
2.39 (s, 3H)
2.94 (s, 3H)
5.30 (s, 1H)
5.76 (s, 1H)
0.96-1.10 (d of t, 6H, PCH2CH3)
1.84-2.23 (m, 4H, PCH2CH3)
7.40-7.61 (m, 5H, PPh)
1.01-1.15 (d of t, 9H, PCH2CH3)
2.16 (s, 3H)
2.23 (s, 3H)
2.30 (s, 3H)
3.22 (s, 3H)
5.23 (s, 1H)
6.00 (s, 1H)
1.61-1.81 (m, 6H, PCH2CH3)
of monodentate phosphines coordinated to metal centers can
be affected by several factors including the oxidation state
of the metal, the geometry of the metal complex, the steric
bulk of the phosphine, and the chemical shift of the
uncoordinated phosphine. These various factors may have
additive, subtractive, or canceling effects on the chemical
shift of the coordinated phosphine. The general tendency
observed is that shielded ligands become less shielded on
coordination (PEt3 is the most shielded phosphine in our
study), while deshielded ligands become more shielded (PPh3
is the least shielded phosphine in our study).21 Additionally,
the higher the oxidation state of the metal center (+5 here)
is, the greater sensitivity of the chemical shift of the
coordinated phosphine, and shielding tends to increase as
the group is descended.21 Some combination of these various
effects results in the coincidental narrow range of 31P NMR
(21) Mason, J., Ed. Multinuclear NMR; Plenum Press: New York, 1987;
pp 385-389.
2384 Inorganic Chemistry, Vol. 44, No. 7, 2005

Re(V) Oxo Schiff Base Complexes with Phosphines
Table 4. 1H NMR Spectral Assignments for 1 and 2 (CDCl3, TMS,
500 MHz, Room Temperature)
Table 5. E°′ Values for the Re(III) Complexes and Their Tc(III)
Analoguesa,b
Assgnt
δ (ppm)
mult; J (Hz)
integratn
Complex
redox couple E°′(Re) E°′(Tc)14
diff
1
2
ortho H (P-Ph)
meta H (P-Ph)
para H (P-Ph)
NCH2CH2N
acac CH3
+6.36
br s
12 H
12 H
6 H
4 H
6 H + 6 H
2 H
4 H
4 H
2 H
4 H
4 H + 4 H
12 H
6 H + 6 H
2 H
[M(PEt2Ph)2(acac2en)]PF6
[M(PEt2Ph)2(acac2en)]PF6
[M(PPh3)2(acac2en)]PF6
[M(PPh3)2(acac2en)]PF6
III/II
III/IV
III/II
-1.431
0.118
-1.575
-0.014
-0.889
0.716
-1.037
0.674
0.542
0.598
0.538
0.688
+7.28
t; 7.3
t; 7.6
s
2 s
br s
d; 6.85
t; 7.5
t; 7.3
s
2 v br s
br m
2 s
+8.88
+40.71
III/IV
-13.36, -29.50
-13.50
a Conditions: cyclic voltammetry in propylene carbonate, 0.1 M TEAP;
Pt working electrode; Ag/AgCl reference electrode; Pt auxiliary electrode;
scan rate of 100 mV/s. b E°′ values in V.
acac CH
ortho H (P-Ph)
meta H (P-Ph)
para H (P-Ph)
NCH2CH2N
PCH2CH3
PCH2CH3
acac CH3
acac CH
+12.51
+7.09
+8.92
ortho, meta, and para protons, with cross correlation peaks
observed for the meta protons with both the ortho and para
protons in each complex. The meta and para protons are
observed as triplets in both compounds with coupling
constants of ca. 7.3-7.5 Hz. The methylene and methyl pro-
tons of the coordinated PEt2Ph do not exhibit the usual proton
coupling (i.e., quartet and triplet) but rather are observed as
two very broad singlets (4.55 and 1.27 ppm) for each
methylene proton and a broad multiplet (2.86 ppm) for the
methyl protons. Each methylene proton is chemically in-
equivalent, as previously reported for Re(III) coordination.31
No 31P coupling is observed. The paramagnetic Re(III) center
results in very fast relaxation of the phosphorus center and,
thus, loss of coupling.27-30,32,36,37 The two acac2en methyl
groups (-11 to -30 ppm) and the methine proton (ca. -13
ppm) are observed upfield from TMS while the backbone
methylene protons are observed significantly downfield (ca.
40-50 ppm). The chemical shifts in the 13C NMR spectra
were observed between -3 and +280 ppm relative to TMS,
and short acquisition times were required because of the very
fast relaxation times due to the paramagnetic center. The
aromatic C directly bound to the phosphorus was not
observed, as also reported by Randall et al. for [ReX3(PR2-
Ph)3].37 The aromatic carbon signals were very weak. The
phosphines coordinated to the paramagnetic Re(III) center
were not observed by 31P NMR, consistent with previously
reported paramagnetic Re(III) complexes.30,32-34
Electrochemistry. The cyclic voltammograms of the Re-
(III) and Re(V) Schiff base phosphine complexes were
determined for comparison with the Tc(III) analogues, which
showed both reversible Tc(III)/Tc(IV) and Tc(III)/Tc(II)
couples.15,16 The Re(III) analogues exhibited the same two
reversible couples as did the related Tc(III) complexes, with
the E°′ values about 600 mV more negative for the Re
compounds (Table 5), indicating that they are more difficult
to reduce than their Tc analogues. The CV for 2 is shown in
Figure 2. The very negative values of the Re(III)/Re(II)
couples indicate that in vivo reduction will not be accessible,
as these values are significantly more negative (more difficult
to reduce) than those for the TcIIIQ12 compounds that have
been evaluated for human use.15-19 Additionally, trans-[Re-
(PEt2Ph)2(acac2en)]+ is harder to reduce than the PPh3
analogue, as would be expected (the more electron-
withdrawing triphenylphosphine groups allows for a more
facile reduction) while it will be easier to oxidize than the
PPh3 analogue (increased σ-electron density on the phos-
+45.09
+4.60, +1.71
+2.88
-11.26, -27.60
-12.30
br s
chemical shifts observed in these Re(V) complexes. The PPh3
became significantly more shielded on coordination to Re-
(V) (-17.01 ppm vs -4.45 ppm) and also has the longest
Re(V)-P bond distance (2.5252(9) Å) (ca. 2.48-2.49 Å for
the PEt3 and PEt2Ph analogues, which became less shielded
on coordination; Table 2). Other Re(V) complexes in which
a phosphine group is cis to the oxo group show the expected
variation in chemical shifts on coordination (i.e., are as
variable as are their free phosphines).22-26
The paramagnetic d4 ReIII metal center of 1 and 2 increased
the complexity of the H and 13C NMR spectra and made
1
them more difficult to interpret. The 1H NMR spectra of the
Re(III) complexes exhibited sharp signals; however, the
chemical shifts were observed in an expanded window of
about -30 to +50 ppm (Table 4), consistent with previously
reported paramagnetic Re(III) complexes.27-36 The aromatic
protons of the coordinated phosphine are observed between
6.2 and 13 ppm. The ortho protons for trans-[Re(PEt2Ph)2-
(acac2en)]PF6 are split into a doublet and observed at 12.51
ppm, while the ortho protons for trans-[Re(PPh3)2(acac2en)]-
PF6 are observed as a broad singlet at 6.36 ppm (on the basis
of the integration). The former is consistent with other Re-
(III)-PPh3 reports while the latter is not.27-37 1H-1H COSY
NMR spectra were used to confirm the assignments of the
(22) Bolzati, C.; Porchia, M.; Bandoli, G.; Boschi, A.; Malago, E.; Uccelli,
L. Inorg. Chim. Acta 2001, 315, 205-212.
(23) Mazzi, U.; Refosco, F.; Bandoli, G.; Nicolini, M. Transition Met.
Chem. 1985, 10, 121-127.
(24) Connac, F.; Lucchese, Y.; Dartiguenave, M.; Beauchamp, A. L. Inorg.
Chem. 1997, 36, 256-259.
(25) Cavell, R. G.; Hilts, R. W.; Luo, H.; McDonald, R. Inorg. Chem. 1999,
38, 897-905.
(26) Nock, B.; Maina, T.; Tisato, F.; Papadopoulos, M.; Raptopoulou, C.
P.; Terzis, A.; Chiotellis, E. Inorg. Chem. 1999, 38, 4197-4202.
(27) Shaw, D.; Randall, E. W. J. Chem. Soc., Chem. Commun. 1965, 82-
83.
(28) Randall, E. W.; Shaw, D. Mol. Phys. 1965, 10, 41-48.
(29) Chatt, J.; Leigh, G. J.; Mingos, D. M. P.; Randall, E. W.; Shaw, D. J.
Chem. Soc., Chem. Commun. 1968, 419-420.
(30) Randall, E. W.; Shaw, D. J. Chem. Soc. A 1969, 2867-2872.
(31) Rossi, R.; Duatti, A.; Magon, L.; Casellato, U.; Graziani, R.; Toniolo,
L. J. Chem. Soc., Dalton Trans. 1982, 1949-1952.
(32) Cotton, F. A.; Luck, R. L. Inorg. Chem. 1989, 28, 2181-2186.
(33) Over, D. E.; Critchlow, S. C.; Mayer, J. M. Inorg. Chem. 1992, 31,
4643-4648.
(34) Pearson, C.; Beauchamp, A. L. Inorg. Chim. Acta 1995, 237, 13-18.
(35) Tisato, F.; Refosco, F.; Bolzati, C.; Cagnolini, A.; Gatto, S.; Bandoli,
G. J. Chem. Soc., Dalton Trans. 1997, 1421-1427.
(36) Bhattacharyya, S.; Banerjee, S.; Drghangi, B. K.; Menon, M.;
Chakravorty, A. J. Chem. Soc., Dalton Trans. 1999, 155-159.
(37) Mitsopoulou, C. A.; Mahieu, N.; Motevalli, M.; Randall, E. W. J.
Chem. Soc., Dalton Trans. 1996, 4563-4566.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2385

Benny et al.
Figure 3. ORTEP representation of trans-[ReIII(PPh3)2(acac2en)]PF6, 1,
with 40% probability ellipsoids.
Figure 2. Cyclic voltammogram (CV) for trans-[Re(PEt2Ph)2(acac2en)]-
PF6.
phorus atom of PEt2Ph makes the Re(III) complex easier to
oxidize to the Re(IV) state). Another series of homologous
Tc(III)/Re(III) phosphine complexes of the type trans-[MCl2-
(diphosphine)2]+ were reported to show reversible III/II and
II/I redox couples, with the Re(III) couples about 200 mV
more negative than the Tc(III) couples, while here about a
600 mV difference between analogous Tc and Re complexes
was observed.38
The Re(V) complexes reported in our study were also
evaluated electrochemically, but not surprisingly no revers-
ible couples were observed. The Re(V) complexes contain
an axial oxo group, which would most likely be lost on
reduction to Re(IV) and thus result in a nonreversible Re-
(V)/Re(IV) couple. Oxidation of the Re(V) complexes might
result in a Re(VI) complex or more likely the thermody-
namically stable Re(VII) perrhenate. Very few Re(VI)
complexes have been reported (mostly halide and oxide
halide complexes)39,40 and oxidation to Re(VI) may continue
to Re(VII), in which case the couple would definitely be
nonreversible. Both reduction to Re(IV) (loss of the oxo
group) and oxidation to Re(VII) (octahedral to tetrahedral)
would result in changes to the Re coordination sphere,
making the process nonreversible.
X-ray Crystal Structures. The Re(III) complexes trans-
[Re(PPh3)2(acac2en)]PF6, 1, and trans-[Re(PEt2Ph)2(acac2-
en)]PF6, 2, and the Re(V) complexes cis-[ReO(PEt3)(acac2-
en)]PF6, 3, cis-[ReO(PPh3)(acac2pn)]PF6, 4, and cis-[ReO-
(PEt2Ph)(acac2pn)]PF6, 5, were characterized by X-ray
crystallography (Figures 3-7). The two Re(III) complexes
are close to octahedral in geometry with the acac2en ligand
occupying the equatorial plane and two phosphine ligands
trans to each other. The three Re(V) complexes are distorted
octahedra with the RedO group somewhat above the
Figure 4. ORTEP representation of trans-[ReIII(PEt2Ph)2(acac2en)]PF6,
2, with 40% probability ellipsoids.
equatorial plane, as typically observed with Re(V) monooxo
complexes, and the single phosphine ligand oriented cis to
the RedO group and with the trans oxo site occupied by a
ligand (acac2en/pn) oxygen.
trans-[Re(PPh3)2(acac2en)]PF6 (1) and trans-[Re(PEt2Ph)2-
(acac2en)]PF6 (2). The bond angles within the Re(III)
complexes are near their expected values, with the angles
for the cis and trans ligands approximately 82-98 and 171-
174°, respectively, with the Re atoms in the ligand planes
(Table 2).41 The Re-O (2.0226-2.037 Å) and Re-N
(2.038-2.045 Å) bond distances are consistent with the Re-
(V) oxo Schiff base complexes3-6,8-11 and similar to other
Re(III)-N and Re(III)-O bond distances.42-45 The Re-P
(38) Kirchhoff, J. R.; Heineman, W. R.; Deutsch, E. Inorg. Chem. 1987,
26, 3108-3113.
(41) Available as Supporting Information.
(39) Housecroft, C. E.; Sharpe, A. G. Inorganic Chemistry; Prentice Hall:
New York, 2001; pp 554-558.
(42) Lock, C. J. L.; Murphy, C. N. Acta Crystallogr. 1979, B35, 951-
954.
(40) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Butterworth Heinemann: Oxford, U.K., 1997; pp 1040-1069.
(43) Rossi, R.; Duatti, A.; Magon, L.; Casellato, U.; Graziani, R.; Toniolo,
L. J. Chem. Soc., Dalton Trans. 1982, 1949-1952.
2386 Inorganic Chemistry, Vol. 44, No. 7, 2005

Re(V) Oxo Schiff Base Complexes with Phosphines
Figure 5. ORTEP representation of trans-[ReVO(PEt3)(acac2en)]PF6, 3,
with 40% probability ellipsoids.
Figure 7. ORTEP representation of trans-[ReVO(PEt2Ph)(acac2pn)]PF6,
5, with 40% ellipsoids.
bond angles, and bond distances of 1 are isostructural with
those observed in the technetium analogue, trans-[99Tc(acac2-
en)(PPh3)2]PF6.15 The bond distances of the Schiff base ligand
(oxygen and nitrogen) to technetium ranged from 2.007 to
2.084 Å, and the trans phosphine ligands ranged from 2.499
to 2.511 Å,15 very similar to those reported here for the Re-
(III) analogues.
cis-[ReO(PEt3)(acac2en)]PF6(3),cis-[ReO(PPh3)(acac2pn)]-
PF6 (4), and cis-[ReO(PEt2Ph)(acac2pn)]PF6 (5). The X-ray
crystal structures of 3-5 illustrate the asymmetric coordina-
tion of the acac2en or acac2pn ligand resulting from the co-
ordination of the phosphine ligand cis to the Re(V) oxo group
(Figures 5-7). Complexes 3-5 exhibit distorted octa-
hedral coordination geometry with the rhenium lying above
the equatorial ligand plane (0.2 Å) toward the oxo group
with the bond angles about the Re center ranging from 82
to 104° (cis) and 167 to 175° (trans). One oxygen and two
nitrogen donor atoms from the tetradentate Schiff base ligand
occupy three equatorial coordination sites, while the remain-
ing oxygen donor coordinates in the axial position trans to
the Re(V) oxo group. The Re-O bond distances for the
equatorial oxygen atoms (2.010-2.026 Å) are comparable
to those for the axial oxygen (1.998-2.078 Å) atoms. The
Re-O, Re-N, and RedO (1.688-1.692 Å) bond distances
are comparable to those observed in other Re(V) complex-
es.3-6,8-11,22,23 The Re(V) oxo core (OdRe-O) in 3-5
deviates slightly from linear (166.2-168.96°) due to ligand
constraints from the asymmetric coordination. The Re-P
bond distances in 3-5 (2.478-2.525 Å) are typical for Re-
(V) oxo cis phosphine complexes (2.39-2.55 Å).11,13-17,22,23
Reactivity of Phosphines with trans-[ReOX(Schiff base)]
Complexes. The reactivity of monodentate phosphine ligands
with the Re(V) complexes, trans-[ReOX(Schiff base)]0/+, was
investigated with the intent of generating the Re(III) com-
plexes, trans-[Re(PR3)2(Schiff base)]+, analogous to the Tc-
(III) complexes previously reported.15,19 Rhenium(III) com-
plexes should be more kinetically inert to substitution than
Re(V) complexes and, thus, may yield more stable 186/188Re
complexes under in vivo conditions, and Re(III) is reasonably
Figure 6. ORTEP representation of trans-[ReVO(PPh3)(acac2pn)]PF6, 4,
with 40% probability ellipsoids.
(2.4579-2.4992 Å) bond distances are comparable to those
observed in other Re(III) complexes.46-53 The near linear
trans phosphine ligands (P-Re-P) are equidistant (2.45-
2.49 Å) from the rhenium center. Two crystallographically
unique complexes are observed in the unit cell of 1 with
comparable bond angles and bond distances. The unit cell,
(44) Rall, J.; Weingart, F.; Ho, D. M.; Heeg, M. J.; Tisato, F.; Deutsch, E.
Inorg. Chem. 1994, 33, 3442-3451.
(45) Fortin, S.; Beauchamp, A. L. Inorg. Chem. 2001, 40, 105-112.
(46) Duckworth, C. F.; Douglas, P. G.; Mason, R.; Shaw, B. L. J. Chem.
Soc., Chem. Commun. 1970, 1083-1084.
(47) Vanderheyden, J.-L.; Heeg, M. J.; Deutsch, E. Inorg. Chem. 1985,
24, 1666-1673.
(48) Pombeiro, A. J. L.; Hughes, D. L.; Pickett, C. J.; Richards, R. L. J.
Chem. Soc., Chem. Commun. 1986, 246-247.
(49) Bakir, M.; Fanwick, P. E.; Walton, R. A. Polyhedron 1987, 6, 907-
913.
(50) Cotton, F. A.; Luck, R. L. Inorg. Chem. 1989, 28, 2181-2186.
(51) Chang, L.; Aizawa, S.-I.; Heeg, M. J.; Deutsch, E. Inorg. Chem. 1991,
30, 4920-4927.
(52) Coutinho, B.; Dilworth, J. R.; Jobanputra, P.; Thompson, R. M.;
Schmid, S.; Strahle, J.; Archer, C. M. J. Chem. Soc., Dalton Trans.
1995, 1663-1669.
(53) Luo, H.; Liu, S.; Rettig, S. J.; Orvig, C. Can. J. Chem. 1995, 73,
2272-2281.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2387

Benny et al.
accessible from perrhenate, the starting compound on the
radiotracer level. Although it was anticipated that the Re-
(V) complexes would be more difficult to reduce to Re(III)
than their Tc(V) analogues, the formation of cis-[ReO-
(PEt3)(acac2en)]+ was unexpected since the less nucleophilic
and less reducing phosphines, PPh3 and PEt2Ph, resulted in
the formation of the Re(III) complexes while the PEt3
resulted in the formation of the asymmetric cis Re(V) oxo
complex. Re(V) complexes with phosphine ligands bound
cis to the Re(V) oxo group are known. Mazzi et al. reported
Re(V) oxo complexes with a tridentate ONO Schiff base in
which a tertiary phosphine was coordinated cis to the oxo
group, as we observed for some of the Re(V) oxo tetradentate
Schiff base complexes.23 Softer σ donors, such as phosphines,
prefer coordinating cis to the oxo group as observed with
other Re(V) complexes.22-26,54-58
The reactions of tertiary phosphines with trans-[ReVOX-
(Schiff base)]0/+ complexes may involve initial loss of the
labile water or chloride group trans to the oxo group,
followed by formation of a 5-coordinate intermediate. This
intermediate then rearranges on reaction with phosphine to
give the cis-Re(V) product or it undergoes phosphine
coordination, either trans to the oxo group or on the oxo
group to yield the trans-Re(III) product on reaction with an
additional 2 equiv of phosphine. One possible mechanism
to the Re(III) product involves formation of a coordinated
phosphine oxide followed by phosphine coordination trans
to this group, loss of the phosphine oxide, and then finally
coordination of the third 1 equiv of phosphine to yield the
final product. A second possible mechanism involves initial
phosphine coordination trans to the oxo group followed by
attack of the second phosphine on the coordinated oxo group
to form the coordinated phosphine oxide, loss of the
phosphine oxide, and finally coordination of the third 1 equiv
of phosphine to yield the Re(III) product. Whether oxygen
atom abstraction and reduction to Re(III) occurs or whether
rearrangement of the coordinated phosphine to the more
favorable position cis to the oxo group occurs is most likely
a kinetic phenomenon. When acac2pn is the Schiff base,
sufficient flexibility in the backbone allows the rearrangement
reaction to be facile, and the resultant product is exclusively
cis-[ReO(PR3)(acac2pn)]+. This is consistent with the reports
of the reaction of other Re(V) oxo complexes with
phosphines.22-26,54-58 When acac2en is coordinated to the Re-
(V) oxo complex, the coordinated Schiff base ligand is quite
rigid and the reduction reaction is sufficiently facile to
compete favorably with the rearrangement reaction. How-
ever, when PEt3 coordinates trans to the oxo group, the
reduction reaction is not favored. The major product (85-
90% on the basis of NMR) is the rearranged Re(V) oxo
complex, cis-[ReO(PEt3)(acac2en)]+, even though PEt3 is the
strongest reducing agent and the best oxygen atom abstractor
of the phosphines investigated. Triethylphosphine is also the
strongest nucleophile, and because of its strong σ-donating
and π-back-bonding characteristics (or tendencies), the
reduction reaction is not able to effectively compete with
the rearrangement reaction thus yielding the Re(V) product.
The pathway to the Re(III) complexes is probably similar
to the phosphine oxygen abstraction mechanism proposed
for the analogous technetium complexes.15 The mechanism
suggested involves a three-step process: coordination of a
phosphine in the labile position trans to the oxo moiety;
abstraction of the oxo group by a second phosphine;
coordination of a third phosphine in the position originally
occupied by the yl oxygen. The surprising observations for
trans-[ReO(OH2)(acac2en)]+ are that triphenylphosphine and
diethylphenylphosphine reduce the Re(V) core to Re(III)
while triethylphosphine does not, perhaps due to the weaker
σ donation of the former phosphines compared to trieth-
ylphosphine, possibly allowing coordination trans to the oxo
group followed by oxygen atom abstraction and reduction,
or to steric interaction(s) of the aryl group(s) inhibiting
rearrangement. In the Tc system all of the phosphines
investigated resulted in the formation of Tc(III) bis(phos-
phine) complexes,15,17 consistent with the more facile reduc-
tion of Tc(V) compared to Re(V).
Conclusions
The reactions of tertiary phosphine ligands with the Re-
(V) oxo Schiff base complexes, trans-[ReOX(acac2en/pn)]0/+
,
result in either the reduced and disubstituted ReIII product,
trans-[ReIII(PR3)2(acac2en)]+, or the ReV monosubstituted
product, cis-[ReVO(PR3)(Schiff base)]+. The nature of the
phosphine ligand (i.e., alkyl or aryl) and the ligand backbone
(i.e., en vs pn) determine the product formed, and the reaction
path appears to be kinetically driven. This work demonstrates
that it is possible to prepare Re(III) complexes of the type
trans-[Re(PR3)2(Schiff base)]+ with due consideration of the
Schiff base ligand, possible steric effects, and the properties
of the phosphine ligand. Extension of this chemistry to the
radiotracer level is underway, with preliminary 186Re ra-
diotracer studies indicating that phosphine coordination prior
to Schiff base complexation will probably be necessary at
the radiotracer level (µM or less) rather than formation of
the Re(V) Schiff base complex first, as is done in forming
99mTcQ12. The Re(III) complexes will not be accessible to
in vivo reduction, on the basis of their III/II redox potentials,
and the strong σ-donating and π-back-bonding properties of
the phosphines should stabilize Re(III) to oxidiation. Water-
soluble, triarylphosphines may allow facile access to Re-
(III) and sufficiently decrease the lipophilicity of the resultant
complexes to make them biologically compatible.
(54) Sacerdoti, M.; Bertolasi, V.; Gilli, G.; Duatti, A. Acta Crystallogr.
1984, C40, 968-970.
(55) Battistuzzi, R.; Manfredini, T.; Battaglia, L. P.; Corradi, A. B.;
Marzotto, A. J. Crystallogr. Spectrosc. Res. 1989, 19, 513-534.
(56) Gerber, T. I. A.; Perils, J.; Du Preez, J. G. H.; Bandoli, G. Acta
Crystallogr. 1997, C53, 217-219.
(57) Perez-Lourido, P.; Romero, J.; Garcia-Vazquez, J.; Sousa, A.; Maresca,
K. P.; Rose, D. J.; Zubieta, J. Inorg. Chem. 1998, 37, 3331-3336.
(58) Chen, X.; Femia, F. J.; Babich, J. W.; Zubieta, J. Inorg. Chim. Acta
2000, 307, 149-153.
Experimental Section
General Considerations. Unless noted, all common laboratory
chemicals were of reagent grade or better. Solvents were degassed
with nitrogen gas prior to use, and all experiments were carried
1
out under a nitrogen atmosphere unless otherwise noted. H and
2388 Inorganic Chemistry, Vol. 44, No. 7, 2005

Re(V) Oxo Schiff Base Complexes with Phosphines
13C NMR spectra were recorded on a Bruker 250 or 500 MHz
instrument at 25 °C in deuterated chloroform with TMS as an
internal reference. 31P NMR spectra were recorded on a Bruker
250 MHz instrument at 25 °C in deuterated chloroform with H3-
PO4 as an external reference. Two-dimensional NMR correlation
spectra were only obtained for the Re(III) complexes to allow
chemical shift assignments, and only 1H-1H chemical shift
correlation (COSY) experiments were run. All other NMR spectra
were run in one-dimensional mode only. FT-IR spectra were
obtained as KBr pellets on a Nicolet Magna-IR spectrometer 550.
UV-vis spectra were recorded on a Hewlet Packard 8452A diode
array spectrophotometer. 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; White-
house, NJ).
ethanol. The solution was refluxed overnight and then cooled to
room temperature. The reaction mixture was purified by silica gel
chromatography as indicated in method 1. Yield: 82% (∼45 mg).
1H NMR [CDCl3, 500 MHz; δ (ppm)]: -29.50 (s; 6 H; acac CH3);
-13.50 (br s; 2 H; acac CH); -13.36 (s; 6 H; acac CH3); 6.36 (br
s; 12 H; ortho H); 7.28 (t; J ) 7.3 Hz; 12 H; meta H); 8.88 (t; J
) 7.6 Hz; 6 H; para H); 40.71 (s; 4H; NCH2CH2N). 13C NMR
[CDCl3, 500 MHz; δ (ppm)]: 118.99, 126.07 (acac CH3), 126.82
(acac CH3), 132.07, 132.77, 133.63, 133.78, 141.34 (NCH2CH2N),
174.66, 275.69. UV-vis [CH2Cl2, λ in nm (ꢀ in cm-1 M-1)]: 238
(27 970), 280 sh (13 400), 378 (5990), 422 (8320), 538 (1520).
MS (m/z): [M+] 933, 935; calcd 931.81, 933.81. Anal. Calcd
(found) for ReC48H48N2O2P3F6: C, 53.48 (53.48); H, 4.46 (4.25);
N, 2.60 (2.47); P, 8.64 (8.48); F, 10.58 (10.39).
trans-[ReIII(PEt2Ph)2(acac2en)][PF6] (2). The synthesis of 2
followed the procedures described for method 2 of compound 1,
substituting diethyphenylphosphine in THF for triphenylphosphine
in dichloromethane. Yield: 80% (37 mg). Recrystallization from
CH2Cl2/hexane (1:1) by slow evaporation yielded X-ray-quality
Materials. 2,4-Pentanedione, 1,2-ethylenediamine, and 1,3-
propylenediamine were purchased from Aldrich and used without
further purification. The ligands, L1 ) N,N′-ethylenebis(acetylac-
etone imine) (acac2en) and L2 ) N,N′-propylenebis(acetylacetone
imine) (acac2pn) were prepared according to the previously reported
methods.59,60 Ligands (L1 and L2) were recrystallized prior to use
from either absolute ethanol, dry isopropyl alcohol, hexane, or ethyl
ether. Tetrabutylammonium tetrachlorooxorhenium(V) ([n-Bu4N]
[ReVOCl4]) was prepared by bubbling anhydrous HCl(g) into an
ethanol solution of n-NBu4[ReO4] for 30 min at 25 °C; gold-orange
crystals formed upon concentrating and then cooling the solution.61
The purified complexes trans-[ReVO(OH2)(acac2en)]Cl and trans-
[ReOCl(acac2pn)] were prepared as reported previously.11
trans-[ReIII(PPh3)2(acac2en)][PF6]‚H2O (1). Method 1. (n-
Bu4)[ReVOCl4] (50 mg, 0.085 mmol) was added to 20 mL of a
degassed solution of acac2en, L1 (42.5 mg, 0.189 mmol), in absolute
ethanol, and the mixture was refluxed for 1.5 h under a nitrogen
atmosphere. After 1.5 h, triphenylphosphine (89.1 mg, 0.34 mmol)
dissolved in 2 mL of dichloromethane was added to the reaction
vessel, and the resultant reaction mixture was refluxed for an
additional 3 h to yield a red solution and a brown precipitate. The
mixture was then cooled to room temperature and filtered, and the
supernatant was evaporated to dryness in vacuo, to yield a product-
containing residue. Purification of the complex was achieved by
silica gel column chromatography. The reaction mixture was
reconstituted in the minimum volume of dichloromethane and
adsorbed on a column (1 cm × 20 cm) equilibrated in dichlo-
romethane. The column was eluted with CH2Cl2 until a yellow band
(containing triphenylphosphine) was removed. The eluent was then
changed to acetone, which eluted several unidentified bands. The
desired red product was finally displaced with methanol. After
addition of tetrabutylammonium hexafluorophosphate (n-Bu4NPF6)
(36.3 mg, 0.094 mmol), the solution was filtered and the filtrate
concentrated to ∼5 mL and placed in the freezer for several days.
Dark red X-ray-quality crystals of 1 were collected by filtration
and washed with three 5 mL aliquots of toluene, followed by three
5 mL aliquots of ether. Yield: 48% (45 mg).
1
crystals. H NMR [CDCl3, 500 MHz; δ (ppm)]: -27.60 (s; 6 H;
acac CH3); -12.30 (br s; 2 H; acac CH); -11.26 (s; 6 H; acac
CH3); 1.71, 4.60 (2 v br s; 4 H; PCH2); 2.80 (br s; 12 H; PCH2CH3);
7.09 (t; J ) 7.5 Hz; 4 H; meta H); 8.92 (t; J ) 7.3 Hz; 2 H; para
H); 12.51 (d; J ) 6.85 Hz; 4 H; ortho H); 45.09 (s; 4 H;
NCH2CH2N). 13C NMR [CDCl3, 500 MHz; δ (ppm)]: -2.52
(PCH2CH3), 117.72, 119.06, 120.09, 120.33 (acac CH3), 123.16
(acac CH3), 142.91 (NCH2CH2N), 161.78, 166.26 (PCH2), 189.79.
UV-vis [CH2Cl2, λ in nm (ꢀ in cm-1 M-1)]: 234 (24 100), 264
(18 000), 382 (9010), 406 (8690), 428 (8650), 490 (1390), 530
(1160). MS (m/z): [M+] 739, 741; calcd 739.64, 741.64. Anal.
Calcd (found) for ReC32H48N2O2P3F6: C, 43.39 (43.76); H, 5.42
(5.29); N, 3.16 (2.99); P, 10.51 (8.11); F, 12.88 (12.45).
cis-[ReVO(PEt3)(acac2en)][PF6] (3). trans-[ReO(OH2)(acac2en)]-
Cl (25 mg, 0.0523 mmol) was dissolved in 10 mL of absolute
ethanol. Triethylphosphine (6.8 mg, 0.057 mmol) in THF was added
to the reaction mixture. The solution was refluxed for 1 h followed
by the addition of tetrabutylammonium hexaflurophosphate (NBu4-
PF6) (22.3 mg, 0.058 mmol). The solution was filtered, concentrated
to 2 mL by rotary evaporation, and cooled to -30 °C. Analytically
pure dark brown X-ray-quality crystals of 3 were obtained after
several days. Yield: 75% (27 mg). 1H NMR [CDCl3, 250 MHz; δ
(ppm)]: 1.02-1.15 (PCH2CH3, d of t, 9H, JP-H ) 15.6 Hz); 1.67-
1.82 (PCH2CH3, m, 6H); 3.59-3.73 (2H), 4.00-4.07 (1H), 5.29-
5.41 (1H) (NCH2CH2N, m, 4H); 2.21, 2.25, 2.34, 3.02 ((CH3)-
CCHC(CH3)O, four s, 12 H); 5.45, 6.06 ((CH3)CCHC(CH3)O, s,
2H). 13C NMR [CDCl3, 250 MHz; δ (ppm)]: 7.09, 7.14 (PCH2CH3;
JP-C ) 3.1 Hz); 17.74, 18.17 (PCH2CH3 ; JP-C ) 27.5 Hz); 21.78,
23.96, 24.70, 25.73 ((CH3)CCHC(CH3)O); 59.06, 63.90 (NCH2CH2-
N); 102.94, 109.80 ((CH3)CCHC(CH3)O); 175.40, 178.29 (-(CH3)-
C)N-); 184.43, 186.62 (-(CH3)CO). 31P NMR: -15.21 ppm.
UV-vis [CH2Cl2, λ in nm (ꢀ in cm-1 M-1)]: 232 (2200), 272
(2390), 346 (1420), 414 (936). IR (KBr, ν in cm-1): 964 (RedO).
MS (m/z): [M+] 542, 544; calcd 541.39, 543.40. Anal. Calcd
(found) for ReC18H33N2O3P2F6: C, 31.44 (31.48); H, 4.80 (4.82);
N, 4.08 (3.88); P, 9.02 (9.08); F, 16.59 (16.59).
Method 2. Triphenylphosphine (45.0 mg, 0.172 mmol) in 2 mL
of degassed dichloromethane was added to trans-[ReVO(OH2)(acac2-
en)]Cl (25 mg, 0.0523 mmol) in 10 mL of degassed absolute
cis-[ReVO(PPh3)(acac2pn)][PF6] (4). trans-[ReO(acac2pn)Cl]
(25 mg, 0.053 mmol) was dissolved in 10 mL of degassed absolute
ethanol. Triphenylphosphine (15.2 mg, 0.058 mmol) dissolved in
2 mL of degassed dichloromethane was added to the reaction vessel.
The solution was heated for ca. 1 h under a nitrogen atmosphere.
Tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) (22.4 mg,
0.058 mmol) was then added to the reaction mixture while the
solution was still warm. Upon cooling of the sample to room
(59) McCarthy, P. J.; Hovey, R. J.; Ueno, K.; Martell, A. E. J. Am. Chem.
Soc. 1955, 77, 5820-5824.
(60) Lindoy, L. F.; Lip, H. C.; Moody, W. E. J. Chem. Soc., Dalton Trans.
1974, 44-47.
(61) Alberto, R.; Schibli, R.; Egli, A.; Schubiger, A.; Herrmann, W.; Artus,
G.; Abram, W.; Kaden, T. J. Organomet. Chem. 1995, 492, 217-
224.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2389

Benny et al.
temperature, a green precipitate 4 resulted. The product was
collected by filtration and washed with three 5 mL aliquots of
toluene, followed by three 5 mL aliquots of cold ethanol and finally
three 5 mL aliquots of ether. Yield: >99% (46 mg) based on the
starting material trans-[ReOCl(acac2pn)]. X-ray-quality crystals of
4 were obtained by slow crystallization of a concentrated solution
with three 5 mL aliquots of toluene and three 5 mL aliquots of
ether. The crystals were analytically pure and required no additional
purification. Yield: >99%. 1H NMR [CDCl3, 250 MHz; δ (ppm)]:
1.01-1.18 (PCH2CH3, d of t, 9H, JP-H 15.7 Hz); 1.65-1.81
(PCH2CH3, m, 6H); 1.97-2.10 (NCH2CH2CH2, m, 2H); 2.16, 2.23,
2.30, 3.22 ((CH3)CCHC(CH3)O, 4 s, 12H); 3.07-3.18, 3.72-3.82,
4.12-4.19, 5.11-5.20 (NCH2CH2CH2, 4 m, 4H); 5.23, 5.99 ((CH3)-
CCHC(CH3)O, 2 s, 2H). 13C NMR [CDCl3, 250 MHz; δ (ppm)]:
6.88, 6.936 (PCH2CH3, d, JP-C 3.7 Hz); 17.05, 17.49 (PCH2CH3,
d, JP-C 27.6 Hz); 19.47, 21.93, 23.65, 24.96 ((CH3)CCHC(CH3)O);
27.11 (NCH2CH2CH2N); 49.56, 56.71 (NCH2CH2CH2N); 104.47,
106.67 ((CH3)CCHC(CH3)O); 172.5, 177.5 (-(CH3)C)N-); 178.5,
184.5 (-(CH3)CO). 31P NMR: -16.22 ppm. UV-vis [CH2Cl2, λ
in nm (ꢀ in cm-1 M-1)]: 212 (11 070), 274 (9960), 344 (4860). IR
(KBr, ν in cm-1): 967 (RedO). MS (m/z): [M+] 555, 557; calcd
555.42, 557.42. Anal. Calcd (found) for ReC19H35N2O3P2F6: C,
32.52 (32.59); H, 4.99 (4.87); N, 3.99 (3.90); P, 8.84 (8.91); F,
16.26 (16.50).
1
of ethanol/chloroform (1:1) in the freezer. H NMR [CDCl3, 250
MHz; δ (ppm)]: 2.01-2.20 (NCH2CH2CH2, m, 2H); 1.73, 2.21,
2.29, 2.83 ((CH3)CCHC(CH3)O, four s, 12 H); 3.12-3.19, 3.48-
3.61, 4.21-4.27, 4.93-5.04 (NCH2CH2CH2, 4 m, 4H); 5.35, 5.58
((CH3)CCHC(CH3)O, two s, 2H); 7.40-7.46, 7.55-7.62 (PPh, 2
m, 15H). 13C NMR [CDCl3, 250 MHz; δ (ppm)]: 21.98, 22.29,
24.37, 25.26 ((CH3)CCHC(CH3)O); 27.12 (NCH2CH2CH2N); 50.98,
56.82 (NCH2CH2CH2N); 104.99, 107.69 ((CH3)CCHC(CH3)O);
128.75 and 128.92 (PPh; JC-P 10.2 Hz, meta), 130.79 and 131.538
(JP-C 47.2 Hz, PC), 131.28 and 131.31 (JP-C 2.2 Hz, para), 133.52
and 133,68 (JP-C 10.0 Hz, ortho); 172.99, 177.28 ((CH3)C)N-);
177.69, 184.60 (-(CH3)CO). 31P NMR: -17.10 ppm. UV-vis
[CH2Cl2, λ in nm (ꢀ in cm-1 M-1)]: 238 (21 540), 274 (12 774),
344 (4790). MS (m/z): [M+] 699, 701; calcd 699.55, 701.55. IR
(KBr, ν in cm-1): 960 (RedO). Anal. Calcd (found) for
ReC31H35N2O3P2F6: C, 44.02 (43.88); H, 4.14 (4.01); N, 3.31
(3.28); P, 7.34 (7.82); F, 13.49 (13.34).
Electrochemical Studies. Electrochemical data were obtained
with a Bioanalytical Systems Inc. (BAS) CV-50 instrument.
Tetraethylammonium perchlorate (TEAP; 0.1 M) in propylene
carbonate (Burdick and Jackson, high-purity solvent for GC and
spectrophotometry) was used as the electrolytic solution. A non-
aqueous Ag/AgCl solution with 0.1 M TEAP in propylene carbonate
was used as the reference electrode, in conjunction with Pt auxiliary
and Pt working electrodes, for analyzing the Re complexes (1 mM).
Ferrocene (3 mM in propylene carbonate) was used as a reference
standard with a Ag/AgCl aqueous reference electrode.
cis-[ReVO(PEt2Ph)(acac2pn)][PF6] (5). The synthesis and pu-
rification of 5 followed the procedure described for compound 4,
substituting diethylphenylphosphine for triphenylphosphine. Yield:
1
>99% (40 mg). H NMR [CDCl3, 250 MHz; δ (ppm)]: 0.90-
1.10 (PCH2CH3, d of t, 6H); 1.84-2.25 (PCH2CH3, 2 complex m,
4H); 1.86-1.99 (NCH2CH2CH2, m, 2H); 1.52, 2.18, 2.39, 2.94
((CH3)CCHC(CH3)O, 4 s, 12 H); 3.08-3.19, 3.66-3.75, 4.15-
4.24, 4.99-5.10 (NCH2CH2CH2, 4 m, 4H); 5.30, 5.76 ((CH3)-
CCHC(CH3)O, 2 s, 2H); 7.44-7.61 (PPhEt2, m, 5H). 13C NMR
[CDCl3, 250 MHz; δ (ppm)]: 6.78, 6.86, 7.00, 7.04 (PCH2CH3, 2
d); 17.81, 18.16, 18.26, 18.61 (PCH2CH3, 2 d); 21.52, 22.20, 24.27,
25.35 ((CH3)CCHC(CH3)O); 27.44 (NCH2CH2CH2N); 50.49, 56.60
(NCH2CH2CH2N); 104.60, 108.16 ((CH3)CCHC(CH3)O); 128.82
and 129.55 (JP-C 45.4 Hz, PC), 128.92 and 129.07 (JP-C 9.4 Hz,
meta), 130.79 and 130.83 (JP-C 2.4 Hz, para), 131.54 and 131.67
(JP-C 7.9 Hz, ortho) (PPhEt2, 4 d); 172.68, 177.99 ((CH3)C)N-);
179.30, 185.05 (CH3)CO). 31P NMR: -15.82 ppm. UV-vis [CH2-
Cl2, λ in nm (ꢀ in cm-1 M-1)]: 234 (16 150), 276 (11 590), 346
(5130). MS (m/z): [M+] 603, 605; calcd 603.46, 605.47. IR (KBr,
ν in cm-1): 967 (RedO). Anal. Calcd (found) for ReC23H35-
N2O3P2F6: C, 36.85 (36.73); H, 4.67 (4.50); N, 3.74 (3.64); P, 8.28
(8.32); F, 15.22 (15.02).
X-ray Structure Determination and Refinement for 1-5.
Intensity data were obtained at -100 °C on a Bruker SMART CCD
area detector system using the ω scan technique with Mo KR
radiation from a graphite monochromator. Intensities were corrected
for Lorentz and polarization effects. Equivalent reflections were
merged, and absorption corrections were made using the multiscan
method. Space group, lattice parameters, and other relevant
information are given in Table 1. The structure was solved by direct
methods with full-matrix least-squares refinement, using the SHELX
package.62,63 All non-hydrogen atoms were refined with anisotropic
thermal parameters. The hydrogen atoms, except those of the waters
of crystallization, were placed at calculated positions and included
in the refinement using a riding model, with fixed isotropic U. The
final difference map contained no features of chemical significance.
Supporting Information Available: X-ray crystallographic data
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
cis-[ReVO(PEt3)(acac2pn)][PF6] (6). The synthesis and purifica-
tion of 6 followed the procedure outlined in method 2 for compound
4, substituting triethylphosphine for triphenylphosphine with slight
modifications. After the reaction mixture was refluxed and the
n-Bu4NPF6 added to the reaction mixture, the solution was
concentrated to approximately 5 mL and then placed in the freezer
overnight to yield green crystals that were collected and washed
IC048670J
(62) Sheldrick, G. M. SHELXS-97, Crystal Structure Solution; University
of Gottingen: Gottingen, Germany, 1997.
(63) Sheldrick, G. M. SHELXL-97, Crystal Structure Refinement; University
of Gottingen: Gottingen, Germany, 1997.
2390 Inorganic Chemistry, Vol. 44, No. 7, 2005