Synthesis and characterization of rhenium(III) complexes with (Ph2PCH2CH2)2NR diphosphinoamine ligands

Article abstract of DOI:10.1039/c7dt01032j

The synthesis and characterization of a new series of neutral, six-coordinated compounds [Re^IIIX₃(PNPR)], where X is Cl or Br and PNPR is a diphosphinoamine having the general formula (Ph₂PCH₂CH₂)₂NR (R = H, CH₃, CH₂CH₃, CH₂CH₂CH₃, CH₂CH₂CH₂CH₃ and CH₂CH₂OCH₃) are reported. Stable [Re^IIIX₃(PNPR)] complexes were synthesized, in variable yields, starting from precursors where the metal was in different oxidation states (iii and v), by ligand-exchange and/or redox-substitution reactions. The compounds were characterized by elemental analysis, proton NMR spectroscopy, cyclic voltammetry, UV/vis spectroscopy, positive-ion electrospray ionization mass spectrometry (ESI(+)-MS) and X-ray diffraction analysis. Although the formulation of the complexes allows either meridional or facial isomers, the latter arrangement was prevalent both in the solid and solution states. Only [ReCl₃(PNPH)] showed a meridional configuration both in solution and in the crystalline state. [ReBr₃(PNPme)] prefers the meridional configuration in the crystalline state and the facial one in solution. While ESI(+)-MS and voltammetric data seem to indicate some dependency from the nature of the alkyl substituent at the nitrogen, the available structural data of the complexes show only slight differences both for angles and bond lengths upon change of the alkyl chain tethered to the nitrogen.

Full text of DOI:10.1039/c7dt01032j

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Efficient extraction of sulfate from water using
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DOI: 10.1039/C7DT01032J
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ARTICLE
Synthesis and Characterization of Rhenium(III) Complexes with
(Ph2PCH2CH2)2NR Diphosphinoamine Ligands
Nicola Salvarese,†*a,b Fiorenzo Refosco,a Roberta Seraglia,a Marco Roverso,a,c Alessandro
Received 00th January 20xx,
Accepted 00th January 20xx
Dolmella,b Cristina Bolzati†*a,b
The synthesis and characterization of a new series of neutral, six‐coordinated compounds [ReIIIX3(PNPR)], where X is Cl or Br
and PNPR is a diphosphinoamine having the general formula (Ph2PCH2CH2)2NR (R = H, CH3, CH2CH3, CH2CH2CH3,
CH2CH2CH2CH3 and CH2CH2OCH3) are reported. Stable [ReIIIX3(PNPR)] complexes were synthesized, in variable yield, starting
from precursors where the metal was in different oxidation states (III and V), by ligand‐exchange and/or redox‐substitution
reactions. The compounds were characterized by elemental analysis, proton NMR spectroscopy, cyclic voltammetry, UV/vis
spectroscopy, positive‐ion electrospray ionization mass spectrometry (ESI(+)‐MS) and X‐ray diffraction analysis. Although
the formulation of the complexes allows either meridional or facial isomers, the latter arrangement was prevalent both in
solid and in solution state. Only [ReCl3(PNPH)] showed a meridional configuration both in solution and in the crystalline
state. [ReBr3(PNPme)] prefers the meridional configuration in the crystalline state and the facial one in solution. While ESI(+)‐
MS and voltammetric data seems to indicate some dependency from the nature of the alkyl substituent at the nitrogen, the
available structural data of the complexes show only slight differences both for angles and bond lengths upon change of the
alkyl chain tethered to the nitrogen.
DOI: 10.1039/x0xx00000x
www.rsc.org/
to provide very stable complexes, and to full understand the
ligand coordination properties, remain a valid research focus.
Diphosphinoamine (PNP) are an interesting class of ligands
Introduction
Rhenium and technetium represent an attractive pair of metals
in the developing of “matched‐pair” radiopharmaceutical
agents useful in theranostic applications (rhenium‐188 for
therapy and technetium‐99m for Single Photon Emission
Computed Tomography, SPECT).1,2,3
Despite many efforts, however, the current use of 188Re‐based
compounds as therapeutical agents is very rare or, at best, at an
experimental phase, and it is not predictable whether a 188Re‐
radiopharmaceuticals will appear on the market.2 For these
reasons, studies aimed to develop new chelating systems, able
which showed excellent coordination properties toward high
valent technetium(V) and rhenium(V) species containing metal–
nitrogen multiple bonds (terminal nitride4,5 and phenylimido
group6). Over the past years, we have dedicated our efforts to
thoroughly investigate the reactivity of this kind of ligands

toward the nitride‐Tc(V) and ‐rhenium(V) core ([MV N]2+, M =
Tc, Re). By using PNPR ligands, it was possible to prepare
heteroleptic complexes of the type [MV(N)(XY)(PNP)]0/+, where
XY is a bidentate ligand containing soft π‐donor/‐donor
coordinating atoms like the couples: NH2,S− or S,S− or O−,S−.4
Remarkably, this chemistry efficiently works also at the tracer
level both with technetium‐99m5b,e–j and rhenium‐188.5k,l These
a. ICMATE‐CNR, Corso Stati Uniti, 4, 35127 Padova, Italy. E‐mail:
cristina.bolzati@cnr.it; E‐mail: salvarese‐lab@yahoo.it.
complexes are characterized by
a
pseudooctahedral
b. Dipartimento di Scienze del Farmaco, Università di Padova, Via F. Marzolo 5,
35131 Padova, Italy.
geometry5a, in which the two phosphorous atoms of the
diphosphinoamine are reciprocally cis positioned, whereas the
amino nitrogen is positioned in trans with respect to the M≡N
linkages. The nature of the amino group and the length of the
alkyl chains between the nitrogen and the phosphorous atoms
are crucial factors in determining both the stereochemistry and
the reactivity of the intermediate [MV(N)Cl2(PNP)] complexes,
as well as the stability of the final [MV(N)(XY)(PNP)]0/+
compounds. In particular, when a tertiary amine nitrogen (NR;
R = CH3, CH2CH2OCH3) is introduced in the bridging chain, the
PNP always adopts a facial configuration, yielding fac,cis‐
c. Dipartimento di Scienze Chimiche, Università di Padova, Via F. Marzolo 1, 35131
Padova, Italy.
† Working Address: Diparꢀmento di Scienze del Farmaco, Università degli Studi di
Padova, Via F. Marzolo 5, 35131 Padua, Italy
Electronic Supplementary Information (ESI) available: Synthesis of (PH2PCH2CH2)2NR
diphosphinoamine ligands (PNPme, PNPet, PNPpr, PNPbu). List of polydentate
ligands used in substitution reactions with fac‐[ReCl3(PNPet)] (2) and mer‐
[ReCl3(PNPH)] (10). ESI(+)‐MS spectra (range: 600 – 1000 m/z) of [ReBr3(PNPR)]
complexes. MS/MS spectrum of the ion [ReV(O)Cl3(PNPme)+H]+ at m/z 764. MS/MS
spectrum of the [ReCl3(PNP2)+H]+ ion at m/z 791. 1H NMR spectra (300 Mhz, CDCl3,
298 K) of 1 – 11. Selected Bond Lengths (Å) and angles (deg) for the complexes 1 – 4,
10 and 11. A full version of the crystallographic experimental work, including a
detailed discussion of the treatment of disorder in the X‐ray structures. Tightest
intermolecular interactions for
DOI: 10.1039/x0xx00000x. Crystallographic data: CCDC 1532872 (
1532874 ( ), 1532875 ( ), 1532876 (10) and 1532877 (11). For crystallo‐
1
–
4,
10,
11 (discussion and table). See
[MV(N)Cl2(PNP)] compounds as kinetic products.5b,e
A
1
), 1532873 ( ),
2
3
4
meridional arrangement of PNPR is instead adopted when the
graphic data in CIF or other electronic format see DOI: 10.1039/x0xx00000x.
This journal is © The Royal Society of Chemistry 20xx
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diphosphinoamine incorporates a secondary amine group in the
backbone, thus giving the mer,cis‐[MV(N)Cl2(PNP)] species.5d It
has to be highlighted that only the fac,cis‐[MV(N)Cl2(PNP)]
species promptly undergo ligand exchange reaction, thought
the substitution of the electron withdrawing and geometrically
prone chlorine atoms with a suitable bidentate ligand, to yield
the final heteroleptic complex. On the contrary, the mer,cis‐
[MV(N)Cl2(PNP)] complexes, in the same reaction conditions, do
not exchange the halides5d, thus indicating the fundamental
role played by the nucleophilicity of the amino group.
View Article Online
DOI: 10.1039/C7DT01032J
Beyond these considerations, indeed the PNP ligands are
suitable candidates for the stabilization of rhenium and
technetium in lower oxidation states M(I) – M(IV).7 In particular,
complexes with alkylic PNP ligands (i.e. bis[(2‐
dialkylphosphino)ethyl]amines) were described as useful
compounds in catalytic applications.7g,7l,7n,7o
Thus, the present study combines our past experience on the
[MV(N)(PNP)]‐complexes with our current interest in the M(III)
complexes as potential compounds for radiopharmaceutical
applications,8,9 elucidating the reactivity of a series of
Chart
1
Diphosphinoamines
used
in
our
experiments:
bis[(2‐
diphenylphosphino)ethyl]amine (PNPH); bis[(2‐diphenylphosphino)ethyl]methylamine
(PNPme);
diphenylphosphino)ethyl]propylamine
diphenylphosphino)ethyl]butylamine
bis[(2‐diphenylphosphino)ethyl]ethylamine
(PNPet);
bis[(2‐
bis[(2‐
Bis[(2‐
(PNPpr);
(PNPbu);
diphenylphosphino)ethyl]methoxyethylamine (PNP2).
Elemental analyses (C, H, N) were performed on a Carlo Erba
1106 elemental analyzer. H NMR spectra of the complexes
1
diphosphinoamines,
showing
the
general
formula
were acquired at 298 K in CDCl3 on a Bruker AMX 300
instrument, using SiMe4 as internal reference. Common
abbreviations for signal multiplicity were used (s = singlet, d =
doublets, t = triplets, q = quartets, bs = broad singlet).
Cyclic voltammetry measurements were performed on a BAS
(Bioanalytical System Inc.) CV‐1B cyclic voltammograph at 293
(Ph2PCH2CH2)2NR (here referred as PNPR), toward rhenium(III),
and to assess whether the variation of the R substituent at the
nitrogen atom affects the coordination geometry and the
stability of the complexes as observed for the [MV(N)(PNP)]‐
complexes. The substituents considered are H, CH3, CH2CH3,
CH2CH2CH3, CH2CH2CH2CH3 and CH2CH2OCH3 as sketched in
Chart 1.
K
under an atmosphere of dinitrogen, in anhydrous
deoxygenated dichloromethane solutions (3.5 x 10−3 M) with [n‐
Bu4N][ClO4] (0.1 M) as the supporting electrolyte, by using a
conventional three electrode cell, recording at 0.2 V s−1. A
platinum‐disk electrode (area ca. 1.28 mm2) was used as the
working electrode, a platinum wire as the counter electrode,
and a silver wire as a quasi‐reference electrode. Controlled
potential coulometries were performed using an Amel model
721 integrator, in an H‐shaped cell containing, in arm 1, a
platinum gauze working electrode and an Ag/Ag+ reference
electrode isolated inside a salt bridge by a medium‐glass frit
and, in arm 2, an auxiliary platinum‐foil electrode. All potentials
were internally referred against the ferrocene couple (400 mV
vs NHE).
Starting from the labile oxo‐rhenium(V) and rhenium(III)
precursor, in this paper we report the syntheses and the full
characterization (including the determination of six crystal
structures) of a series of neutral rhenium(III) complexes of the
type [ReX3(PNPR)], where X = Cl and Br.
Experimental
Materials
All chemicals and reagents were purchased from Aldrich
Chemicals (Milano, Italy). The solvents were reagent grade and
were used without further purification.
[NBu4][ReOCl4],
[ReOCl3(PPh3)2],
[ReOBr3(PPh3)2]
and
UV/vis spectra were registered in dichlorometane with a Cary
5E UV/vis spectrophotometer, and normalized at 350 nm
(compounds 1, 2, 4, 6, 8) or 400 nm (compounds 3, 5, 7, 9, 10
[ReIIICl3(MeCN)(PPh3)2] were prepared as previously
described.11
Bis[(2‐diphenylphosphino)ethyl]amine hydrochloride (PNPH ∙
HCl) was purchased from Strem Chemicals (Newbury, Ma, USA).
and 11).
Mass spectra were obtained using a LCQ Fleet Thermo‐Scientific
ion trap mass spectrometer equipped with an heated
electrospray ionization ion source, operating in positive ion
mode. The complexes were dissolved in dichloromethane giving
10–2 M stock solutions, which in turn were diluted with
acetonitrile to obtain 10–5 M final solutions. Experiments were
performed by direct infusion of the sample solution via a syringe
Bis[(2‐diphenylphosphino)ethyl]methoxyethylamine
(PNP2)
was prepared by the method published by Morassi and
Sacconi12 and all the other diphosphinoamines by a slightly
upgraded procedure as described in ESI.
General note: Due to the tendency of this diphosphinoamine
ligands to oxidize, all the operations were carried out under a
dinitrogen atmosphere and the solvents used for their
preparation or in the complexation reactions were previously
degassed.
pump at a flow rate of 13 L/min. The ions were produced using
a spray voltage of 3 kV and entrance capillary temperature of
280 °C in positive ion mode. Other instrumental parameters
were automatically adjusted to optimize the signal‐to‐noise
ratio.
Physical Measurements
2 | Dalton Trans., 2017, 00, 1‐15
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ARTICLE
Synthesis of the [ReX3(PNPR)] Complexes
HBr]+, 36%), 748.41 ([ReBr3(PNPpr)+H–2HBr]+, 55%), 702.49
(100%).
View Article Online
DOI: 10.1039/C7DT01032J
General procedure for fac‐[ReCl3(PNPR)] (PNPR = PNPme,
PNPet, PNPpr, PNPbu, PNP2). Chloro‐complexes were
prepared using two different procedures, starting from the
rhenium(V) precursors [RevOCl3(PPh3)2] or [NBu4][RevOCl4], and
the rhenium(III) precursor [ReIIICl3(PPh3)2(MeCN)], as described
below. Final yields were comparable. i) In a two‐neck flask
containing a suspension of [RevOCl3(PPh3)2] (153 mg, 0.18
mmol), or alternatively [NBu4][RevOCl4] (0.18 mmol), in ethanol
(15 mL), a solution of PNPR (0.27 mmol) in ethanol (5 mL) was
added. After the mixture was refluxed for 20 h, an orange
precipitate was formed. After cooling, the solid was filtered off
and washed with ethanol (3x1 mL) and diethyl ether (2x2 mL).
ii) In a two‐neck flask containing the orange suspension of
[ReIIICl3(PPh3)2(MeCN)] (100 mg, 0.116 mmol) in ethanol (10
mL), a solution of PNPR (0.116 mmol) in ethanol (5 mL) was
added. The reaction mixture was refluxed for 6 hours, and
during this time it became clear, while the color turned from
orange to orange‐brown. After cooling, the brown‐orange
precipitate was collected by filtration, washed with ethanol (3
mL) and diethyl ether (3 mL), and dried in vacuum.
fac‐[ReCl3(PNPbu)] (6). Yield 23%. Elem. Anal. Found: C, 48.8%;
H, 4.8%; N, 1.7%. Calc. for C32H37NP2Cl3Re: C, 48.6%; H, 4.7%; N,
1.8%. ESI(+)‐MS: m/z 790.50 ([ReCl3(PNPbu)+H]+, 100%) 754.35
([ReCl3(PNPbu)+H–HCl]+, 10%), 718.46 ([ReCl3(PNPbu)+H–
2HCl]+, 4%).
fac‐[ReBr3(PNPbu)] (7). Yield 9%. Elem. Anal. Found: C, 41.5%;
H, 3.8%; N, 1.4%. Calc. for C32H37NP2Br3Re: C, 41.7%; H, 4.05%;
N, 1.5%.
fac‐[ReCl3(PNP2)] (8). Yield 35%. Elem. Anal. Found: C, 47.4%;
H, 4.6%; N, 1.8%. Calc. for C31H35NOP2Cl3Re: C, 47.2%; H, 4.5%;
N, 1.8%. ESI(+)‐MS: m/z 792.32 ([ReCl3(PNP2)+H]+, 100%),
756.31 ([ReCl3(PNP2)+H–HCl]+, 70%). Another signal in the
spectrum is tentatively assigned to an oxo‐rhenium(V): 772.29
([ReOCl3(PNP2)+H–HCl]+, 8%).
fac‐[ReBr3(PNP2)] (9). Yield 12%. Elem. Anal. Found: C, 40.1%;
H, 3.5%; N, 1.4%. Calc. for C31H35NOP2Br3Re: C, 40.3%; H, 3.8%;
N, 1.5%. ESI(+)‐MS: m/z 947.97 ([ReBr3(PNP2)+Na]+, 19%),
925.16 ([ReBr3(PNP2)+H]+, 100%), 846.21 ([ReBr3(PNP2)+H–
HBr]+, 76%), 764.28 ([ReBr3(PNP2)+H–2HBr]+, 3%), 718.40
(83%).
Procedure for mer‐[ReCl3(PNPH)] (10). In a two‐neck flask
containing the orange dispersion of [ReIIICl3(PPh3)2(MeCN)] (100
mg, 0.116 mmol) in degassed ethanol (10 mL), solid PNPH ∙ HCl
(55.7 mg, 0.116 mmol) was added. To the resulting mixture,
triethylamine (0.017 mL, 0.116 mmol) was added under stirring.
The flask was refluxed 6 hours during which the color of the
mixture turned from yellow to green and a brown precipitate
formed. After filtration and washing with ethanol (3 mL) and
diethyl ether (3 mL) the solid was dried in vacuum. By dissolving
the powder in dichloromethane, brown crystals formed after
few minutes. Yield 89%. Elem. Anal. Found: C, 45.9%; H, 4.1%;
N, 1.8%. Calc. for C28H29NP2Cl3Re: C, 45.8%; H, 4.0%; N, 1.9%.
ESI(+)‐MS: m/z 698.62 ([ReCl3(PNPH)+H–HCl]+, 100%), 662.47
([ReCl3(PNPH)+H–2HCl]+, 58%). Another signal in the spectrum
is tentatively assigned to an oxo‐rhenium(V) species: 678.50
([ReOCl3(PNPH)+H–2HCl]+, 10%).
General procedure for fac‐[ReBr3(PNPR)] (PNPR = PNPet,
PNPpr, PNPbu, PNP2). Bromo‐complexes were prepared
following the procedure i described for fac‐[ReCl3(PNPR)], by
using the precursor [RevOBr3(PPh3)2] (174 mg, 0.18 mmol).
fac‐[ReCl3(PNPme)] (1). Yield 50%. Elem. Anal. Found: C, 46.9%;
H, 4.5%; N, 1.7%. Calc. for C29H31NP2Cl3Re: C, 46.6%; H, 4.2%; N,
1.9%. ESI(+)‐MS: m/z 748.17 ([ReCl3(PNPme)+H]+, 100%),
712.26
([ReCl3(PNPme)+H–HCl]+,
86%),
676.33
([ReCl3(PNPme)+H–2HCl]+, 32%). Other signals in the spectrum
are tentatively assigned to oxo‐rhenium(V) species: 764.26
([ReOCl3(PNPme)+H]+, 93%), 728.23 ([ReOCl3(PNPme)+H–HCl]+,
18%), 692.43 ([ReOCl3(PNPme)+H–2HCl]+, 6%).
fac‐[ReCl3(PNPet)] (2). Yield 45%. Elem. Anal. Found C, 47.4%;
H, 4.5%; N, 1.8%. Calc. for C30H33NP2Cl3Re: C, 47.3%; H, 4.4%; N,
1.9%. ESI(+)‐MS: m/z 762.45 ([ReCl3(PNPet)+H]+, 100%), 726.37
([ReCl3(PNPet)+H–HCl]+, 24%), 690.47 ([ReCl3(PNPet)+H–2HCl]+,
16%). Other signals in the spectrum are tentatively assigned to
oxo‐rhenium(V): 778.35 ([ReOCl3(PNPet)+H]+, 9%), 742.42
([ReOCl3(PNPet)+H–HCl]+, 4%).
fac‐[ReBr3(PNPet)] (3). Yield 14%. Elem. Anal. Found: C, 40.5%;
H, 3.5%; N, 1.4%. Calc. for C30H33NP2Br3Re: C, 40.25%; H, 3.7%;
N, 1.6%. ESI(+)‐MS: m/z 918.00 ([ReBr3(PNPet)+Na]+, 8%),
895.19 ([ReBr3(PNPet)+H]+, 100%), 816.15 ([ReBr3(PNPet)+H–
HBr]+, 11%), 734.32 ([ReBr3(PNPet)+H–2HBr]+, 10%).
Procedure for [ReBr3(PNPme)] (11). The complex was prepared
according to the general procedure for fac‐[ReBr3(PNPR)]
described above. Yield 39%. Elem. Anal. Found: C, 39.7%; H,
3.5%; N, 1.5%. Calc. for C29H31NP2Br3Re: C, 39.6%; H, 3.55%; N,
1.6%. ESI(+)‐MS: m/z 881.16 ([ReBr3(PNPme)+Na]+, 35%),
802.10
([ReBr3(PNPme)+H–2HBr]+, 100%).
([ReBr3(PNPme)+H–HBr]+,
73%),
720.41
fac‐[ReCl3(PNPpr)] (4). Yield 47%. Elem. Anal. Found: C, 48.2%;
H, 4.6%; N, 1.7%. Calc. for C31H35NP2Cl3Re: C, 48.0%; H, 4.55%;
N, 1.8%. ESI(+)‐MS: m/z 776.55 ([ReCl3(PNPpr)+H]+, 89%),
All the complexes are soluble in dichloromethane and
chloroform, slightly soluble in acetone and not soluble in
acetonitrile, ethanol and methanol.
All complexes are paramagnetic. Nevertheless, H NMR data
were collected (see ESI, Tables S1 and S2 and Figures S5 – S15).
740.46
([ReCl3(PNPpr)+H–HCl]+,
100%),
704.56
1
([ReCl3(PNPpr)+H–2HCl]+, 45%). Another signal in the spectrum
is tentatively assigned to an oxo‐rhenium(V) species: 756.50
([ReOCl3(PNPpr)+H–HCl]+, 7%).
X‐ray Crystallography
fac‐[ReBr3(PNPpr)] (5). Yield 11%. Elem. Anal. Found: C, 40.9%;
H, 3.8%; N, 1.5%. Calc. for C31H35NP2Br3Re: C, 41.0%; H, 3.9%; N,
1.55%. ESI(+)‐MS: m/z 932.03 ([ReBr3(PNPpr)+Na]+, 13%),
909.20 ([ReBr3(PNPpr)+H]+, 75%), 830.15 ([ReBr3(PNPpr)+H–
X‐ray quality crystals of
diffusion of methanol in a dichloromethane solution; samples of
and 10 were recrystallized from dimethylformamide and
2 – 4 and 11 were obtained by slow
1
dichloromethane, respectively. Data collection were performed
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precursor/ligand
at room temperature (294 to 302 K) on an Oxford Diffraction [ReIIICl3(MeCN)(PPh3)2], with using
Gemini EOS CCD diffractometer by means of the ω–scans stoichiometric ratio of 1 : 1 ratio.
a
View Article Online
DOI: 10.1039/C7DT01032J
technique, using graphite–monochromated radiation, 1024 × Ligand exchange reaction involving the secondary
1024 pixel mode and 2 × 2 pixel binning. Diffraction intensities diphosphinoamine ligand (PNPH), as hydrochloride salt (PNPH ∙
were corrected for Lorentz and polarization effects; an HCl), and the [ReIIICl3(MeCN)(PPh3)2] precursor afforded the
empirical multi–scan absorption correction, based on complex [ReCl3(PNPH)] (10). The hydrochloride salt was
equivalent reflections, was applied with the scaling algorithm neutralized in situ by addition of triethylamine in stoichiometric
SCALE3 ABSPACK. Two reference frames were collected after amount. Any attempt to prepare 10 from the respective
every 50 frames to check for crystal and instrument stability; no rhenium(V) precursors failed, as well as any attempt to obtain
change in peak positions/intensities was noticed. Data [ReBr3(PNPH)] by reduction substitution reaction starting from
collection, reduction and refinement were carried out by means [RevOBr3(PPh3)2]. This may be due to a lower reducing power of
of the CrysAlis Pro software suite.13
this diphosphinoamine in these reaction conditions. Scheme 1
Accurate unit cell parameters were determined by least– summarizes the reactivity of the PNPR ligands toward
squares refinement of highest intensity reflections chosen from precursors.
the whole experiment. The structures were solved through In all cases, PNPR acts as tridentate ligand generating a six‐
direct methods (complexes
1 – 4) or heavy–atom methods coordinate complexes characterized by an octahedral geometry
(complexes 10
–
11) using SHELXS14 and refined by full–matrix (vide infra), in which the N of the diphosphinoamine occupies
least–squares methods based on Fo with SHELXL–9714 in the an apical position and the two P atoms lie in the equatorial
framework of the OLEX2 software.15 Unless otherwise stated plane; in this situation, the PNPR can assume fac or mer
(see ESI), non‐hydrogen atoms were refined anisotropically. configurations. Characterization data (see below) demonstrate
2
Coordinates of the hydrogen atoms were calculated in idealized that [ReX3(PNPR)] complexes
positions and refined as a riding model. In the structures of isomers both in solid and in solution states. In this configuration,
complexes 10 11 some atoms were disordered over two the phosphorous atoms are in reciprocal cis position, both
1 – 9 were obtained as stable fac
1,
4
,
,
places. Alternate positions of involved atoms were refined with facing a trans halide atom on the octahedral equatorial plane.
site occupation factors constrained to sum to 1.0 and additional [ReBr3(PNPme)] (11) is an unique exception, since it crystallizes
SHELXL14 restraints were applied if necessary; a detailed as mer isomer, but in solution it seems to prefer the fac
description of the modelling strategies is provided in the ESI. In configuration. In particular, once dissolved in chlorinated
all cases, the chemical identity of the compounds was solvents, crystalline mer‐[ReBr3(PNPme)] quantitatively
unambiguously determined, the metal coordination rearranges to the fac isomer and, conversely, the formed fac‐
environments are well characterized and the chosen final [ReBr3(PNPme)] does not rearrange back to the mer form. This
models look acceptable. A summary of the crystallographic and behavior was deduced from the 1H NMR spectrum, the
refinement data is reported in Table 1. The supplementary voltammetric data (in particular, the value of the potentials of
crystallographic data for complexes
at the Cambridge Crystallographic Data Center as .cif files, with are consistent with the spectra and the data of the other fac‐
CCDC numbers 1532872 ( ), 1532873 ( ), 1532874 ( ), 1532875 complexes.
), 1532876 (10) and 1532877 (11). The data can be obtained [ReCl3(PNPH)] (10) is the only one complex which was found to
1 – 4, 10, 11 were deposited the redox processes, vide infra), and the UV/vis spectrum, which
1
2
3
(
4
free of charge via www.ccdc.cam.ac.uk/data_request/cif.
be stable in the mer configuration both in solid and in solution.
In this configuration, the two phosphorous and two chloride
atoms are mutually in trans position.
Results and discussion
Synthesis
Obtained data indicate that also in the present case the
stereochemistry of this series of rhenium(III)‐complexes is
determined by the nature of the amino group framed in the
diphosphine backbone, clearly resembling the behavior
observed in complexes containing the M≡N or the M=NPh
metal‐nitrogen multiple bonds.5,6 Thus, the tertiary amine
group in PNPR privileges the formation of the kinetically and
thermodynamically favored fac conformer, whereas the
secondary amine function promotes the stabilization of the mer
isomer as established by the X‐ray structure of the complex 10
(see below). Deviation from this behavior was observed for
PNPme which yields to both the isomers, when reacted with the
[ReOBr3(PPh3)2]: the mer one is preferred in the crystalline
state, whereas the fac one is favored in solution.
New six‐coordinated rhenium(III) complexes of the general
formula [ReX3(PNPR)] (X = Cl, Br and PNPR = PNPme, PNPet,
PNPbu, PNP2, complexes
1 – 9 and 11) were prepared by
reduction/substitution reactions in refluxing ethanol, starting
from rhenium(V) precursors ([RevOX3(PPh3)2] (X = Cl, Br) or
[NBu4][RevOCl4]) and using a precursor/ligand stoichiometric
ratio of 1 : 1.5. Variation of the R substituents on the N nitrogen
of the diphosphinoamine ligand did not substantially affect the
reaction yield; however, the change from chlorine to bromine
dramatically reduced the yield of the reaction, and any attempt
to improve it, by increasing the amount of the
diphosphinoamine ligand and the reaction time or changing the
reaction solvents, were unsuccessful. Chloro‐complexes
1, 2, 4,
6
and where obtained also by ligand exchange reaction from
8
4 | Dalton Trans., 2017, 00, 1‐15
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Table 1 Crystallographic data and structure refinement for the complexes 1 – 4, 10 and 11.
fac‐[ReCl3(PNPme)] 1
C29H31NP2Cl3Re∙DMF
821.13
fac‐[ReCl3(PNPet)] 2
fac‐[ReBr3(PNPet)] 3
C30H33NP2Br3Re
895.44
fac‐[ReCl3(PNPpr)] 4
C31H35NP2Cl3Re
776.09
mer‐[ReCl3(PNPH)] 10
C28H29NP2Cl3Re
734.01
mer‐[ReBr3(PNPme)] 11
C29H31NP2Br3Re
881.42
Empirical formula
Formula weight
Temperature/K
Crystal system
Radiation
C30H33NP2Cl3Re
762.06
302.3(9)
294.4(1)
299.8(4)
297.4(1)
300(3)
298.0(1)
Orthorhombic
MoKα (λ = 0.71073)
P 212121
Monoclinic
MoKα (λ = 0.71073)
P 21/n
Monoclinic
MoKα (λ = 0.71073)
P 21/n
Monoclinic
MoKα (λ = 0.71073)
P 21/n
monoclinic
MoKα (λ = 0.71073)
I 2/a
orthorhombic
MoKα (λ = 0.71073)
Pbca
Space group
a
b
c
/ Å
/ Å
/ Å
/ °
12.61584(9)
14.38916(10)
18.04782(13)
90.00
12.3950(3)
18.9802(4)
13.2976(4)
90.00
12.4502(3)
19.2607(4)
13.4211(3)
90.00
11.44174(12)
14.86579(11)
19.31565(18)
90.00
15.9425(3)
14.9601(3)
25.6721(6)
90.00
17.3668(5)
17.2508(5)
19.9883(6)
90.00
α
β
γ
/ °
90.00
112.107(3)
90.00
111.117(3)
90.00
98.9519(9)
90.00
103.822(2)
90.00
90.00
/ °
90.00
90.00
Volume /Å3
3276.24(4)
4
2898.39(12)
4
3002.25(12)
4
3245.39(5)
4
5945.5(2)
8
5988.3(3)
8
Z
ρcalc / Mg/m3
1.665
1.746
1.981
1.588
1.640
1.955
μ
/ mm−1
(000)
Crystal size/mm
range / °
Reflections collected
Independent reflections/
4.080
4.601
8.168
4.111
4.483
8.188
F
1632
1504
1720
1536
2880
3376
0.76×0.36×0.16
4.3 to 65.6
68698
0.50×0.40×0.20
4.1 to 57.9
38682
0.27×0.22×0.18
4.1 to 53.0
33079
0.60×0.42×0.28
4.5 to 61.0
60358
0.24×0.20×0.11
4.6 to 59.8
46862
0.75×0.20×0.15
4.7 to 61.5
61265
2
θ
R
(int)
11414/0.0283
11414/0/402
1.079
6998/0.0543
6998/0/335
1.053
5597/0.0315
5597/0/335
1.035
9337/0.0263
9337/287/344
1.055
7942/0.0351
7942/334/472
1.077
8716/0.1284
8716/142/355
1.055
Data/restraints/parameters
Goodness‐of‐fita on F2
Final
R
indexes [
Flack parameter 16
Largest diff. peak/hole / e Å−3
I
> 2σ (
I
)]b, c
R1=0.0181, wR2=0.0380
‐0.018(3)
R1=0.0304, wR2=0.0654
‐
R1=0.0208, wR2=0.0456
‐
R1=0.0225, wR2=0.0516
‐
R1=0.0278, wR2=0.0596
‐
R1=0.0446, wR2=0.0993
‐
0.60/−0.61
1.38/−1.47
0.66/−0.68
0.83/−0.56
1.06/−0.47
2.50/−1.16
a Goodness–of–fit = [Σ (w (Fo2 – Fc2)2] / (Nobsevns – Nparams)]1/2, based on all data; b R1 = Σ ( |Fo| – |Fc| ) / Σ |Fo|; c wR2 = [Σ[w (Fo2 – Fc2)2] / Σ[w (Fo2)2]]1/2
.
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DOI: 10.1039/C7DT01032J
CH3
N
Ph
Br
Re
N
Ph
P
Br
Br
Crystallization
CH2Cl2/MeOH
Ph
Ph
Br
P
P
Ph
Ph
Re
Br
P
Br
Ph
Ph
CH3
11
mer-[Re(III)Br3(PNPme)] (
)
11
fac-[Re(III)Br3(PNPme)] (
)
PNPme
H
N
O
O
Ph
Ph
X
PPh3
X
Cl
Cl
Cl
Cl
Cl
P
P
Re
X
Re
or
Re
Cl
Ph3P
Cl
Ph
Ph
X= Cl; Br
mer-[Re(III)Cl3(PNPH)] (10)
PNPR
PNPH
PPh3
Ph
X
Ph
P
X
X
PNPR
Re
N
Cl
Cl
Cl
Ph
Ph
Re
P
MeCN
PPh3
R
fac-[Re(III)X3(PNPR)] (1-9)
X
R
Compound
CH3
CH2CH3
Cl; ---
Cl; Br
Cl; Br
Cl; Br
Cl; Br
1
2; 3
4; 5
6; 7
8; 9
CH2CH2CH3
CH2CH2CH2CH3
CH2CH2OCH3
Scheme 1 Reactivity of the PNPR ligands towards different rhenium(V) and rhenium(III) precursors.
Probably, the less‐encumbering nature of the methyl group trans positioned halide groups). As proof‐of‐concept, ligand
plays a role when the mer isomer is selected in the solid state; exchange reactions were performed on and 10 with the
2
however, since the analogous chloro‐complex was obtained as selected bis‐ or tridentate ligands presented in Fig. S1. All
fac isomer both in solution and in solid state, the bromine attempts to replace the monodentate ligands, in order to mimic
ligands should also influence this behavior, perhaps disfavoring the chemistry exhibited by the corresponding [M(N)(PNP)]‐ and
a facial arrangement of the more bulky halides.
[M(N‐Ph)(PNP)]‐compounds, failed, indicating that in these
Considering the molecular environments of fac versus mer [ReCl3(PNPR)] complexes the halides are chemically inaccessible
compounds, it is reasonable to suppose that the fac isomers for this purpose.
might be good candidates for further substitution of
monodentate halide groups with polydentate ligands, both for Characterization
steric (a complete face of the octahedron is prone for Elemental analyses are in good agreement with the proposed
substitution) and electronic reasons (phosphine P might labilize formulations.
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NMR. All the obtained complexes are paramagnetic, as it is protons and PCH2CH2N sets of signals consistent with those
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typical for octahedral complexes with a low‐spin d4 rhenium(III) observed in the spectra of the other fac cDoOmI:p1l0e.1x0e3s9,/cCo7DnfTi0r1m03in2gJ
center. No signals are displayed in the 31P NMR spectra, the quantitative conversion of the mer isomer to the fac one as
consistently with previously reported paramagnetic discussed above.
1
rhenium(III) complexes.17 Instead, the H NMR spectra show Cyclic voltammetry. Cyclic voltammetry was performed on all
mostly sharp peaks in a relatively narrow window (about 50 the complexes to explore their redox properties. Data are
ppm), in which some 1H‐1H couplings are visible and reported in Table 2 along with cyclic voltammograms of the
integrations are consistent (see ESI, Figures S5 – S15). Thus, it selected compounds
was possible to tentatively assign the resonances on the basis 1). All fac complexes (
7
1
and
8 as representative examples (Fig.
– 9 and 11 which in solution is in the fac
of their multiplicities and integrations, as well as through a configuration), show a reversible oxidation Re(III)/Re(IV) with
careful comparison between the spectra. In general, the spectra potentials in the range 0.185 – 0.005 V. The trend within the
of the fac complexes (1 – 9 and 11) show three sets of signals. chloride series is directly proportional to the inductive effect +I
The first one is assignable to the PCH2CH2N bridges and of the N‐R substituent: the bigger is the +I effect, the lower is
comprises four broad singlets or multiplets, each one the oxidation potential. The increased electron density at
integrating for two protons, at about: −35.48, −27.33, −3.84 and nitrogen makes the complex easier to oxidize. This trend is not
7.88 ppm for the chloro‐complexes; −33.50, −25.34, −4.21 and observed in the bromide series: likely, the bromine atoms favor
8.16 ppm for the bromo‐complexes (mean values; exact the delocalization of the electron density, and contribute to
chemical shifts are given in Table S1). The second set is even the +I effect of the different R substituent. Complex 10
represented by the resonances of aromatic hydrogens and does not follow the same course.
includes six signals: two doublets (four protons each) around In the cathodic region, all the complexes show an intense peak
14.90 and 11.15 ppm respectively, which are assignable to the due to the Re(III)/Re(II) reduction, as established by coulometry;
ortho hydrogens; two triplets (four protons each) around 10.23 this process is irreversible for
and 9.08 ppm respectively, given by the meta hydrogens; two whereas it is reversible for the mer complex 10. Moreover, for
triplets (two protons each) around 10.47 and 9.55 ppm and all the bromo‐complexes, it is clearly visible a
respectively, corresponding to the para hydrogens. From the reversible couple Re*(III) Re*(II) due to the product of a
observed multiplicities, it looks clear that 1H‐31P and long range chemical transformation following the Re(III)/Re(II) reduction.
1H‐1H couplings are absent. Exact chemical shifts are listed in Conversely, for
and 11 this reversible couple cannot be seen
Table S2. The third set of signals is assignable to the R chain, as even at 500 mVs−1 scan speed. Within the chloride series,
is
specified in Table S1. The spectrum of the mer complex 10 the easiest to reduce (−1.215 V), whereas is the most stable
shows significant differences in the chemical shifts of both the (−1.48 V), in perfect agreement with the inducꢀve effect. On the
aromatic protons and PCH2CH2N sets of signals with respect to contrary, within the bromide series, is the easiest to reduce
the fac complexes. Notably, the mer configuration of 11 and 11 the most stable, probably owing to the ability of the
1 – 9 and 11 (fac complexes),
1, 6, 8

2, 4
1
8
9
,
established by the X‐ray analysis (vide infra), was not confirmed bromine atoms to better delocalize the electron density with
1
in solution: indeed, the H NMR spectrum shows aromatic respect to the chloride analogues.
Table 2 Voltammetric Data for complexes 1 – 11 in dichloromethanea
E°ox Re(III)Re(IV) Ered Re(III)Re(II) ∆(E°ox‐Ered) E° Re*(III)Re*(II)
Complex
(Volts)
(Volts)
(Volts)
(Volts)
mer‐[ReCl3(PNPH)]
fac‐[ReCl3(PNPme)]
fac‐[ReCl3(PNPet)]
fac‐[ReCl3(PNPpr)]
fac‐[ReCl3(PNPbu)]
fac‐[ReCl3(PNP2)]
10
1
0.040
−1.600b
1.640
‐
0.185
0.150
0.140
0.110
0.005
0.185
0.130
0.140
0.140
0.160
−1.215
−1.460
−1.430
−1.410
−1.480
−1.380
−1.230
−1.250
−1.270
−1.120
1.400
1.610
1.570
1.520
1.485
1.565
1.360
1.390
1.410
1.280
−0.800
‐
2
4
‐
6
−0.690
−0.830
‐
8
fac‐[ReBr3(PNPme)] 11
fac‐[ReBr3(PNPet)]
fac‐[ReBr3(PNPpr)]
fac‐[ReBr3(PNPbu)]
fac‐[ReBr3(PNP2)]
3
5
7
9
−0.710
−0.660
−0.680
−0.760
aData internally referred to the Fc/Fc+ couple. Data recorded at 200 mVs−1, in dry and degassed dichloromethane with [n‐Bu4N][ClO4] (0.1 M) as supporting electrolyte,
a platinum‐disk working electrode, and a silver‐wire quasi‐reference electrode. bReversible process.
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Fig. 1 Cyclic voltammograms of complexes 7 (a) and 8 (b). Data recorded at 200 mVs−1
,
in dry and degassed dichloromethane with [n‐Bu4N][ClO4] (0.1 M) as supporting
electrolyte, a platinum‐disk working electrode, and a silver‐wire quasi‐reference
electrode. Here, peaks are referred to the Ag/Ag+ couple. Voltammetric data internally
referred to the Fc/Fc+ couple are given in Table 2.
The latter feature is underlined also by the difference between
the anodic and cathodic potentials ∆(E°ox‐Ered) of the main redox
processes.
Complex
8 is worth of mention, because it is oxidized (E° =
+0.005V) and reduced (E = −1.48 V) at the lowest potenꢀals of
the fac‐series. Despite the absence of the corresponding anodic
peak and according to the coulometry, this reduction is a one‐
electron transfer. However, once reduced to Re(II), 8 undergoes
a chemical transformation whose product showed a reversible
couple at E° = −0.83 V (Fig. 1).
Moreover, it appears that the ether substituent at the nitrogen
significantly affects the reduction potential of the complex,
probably because of the withdrawing effect of the oxygen,
which consistently contributes to shift the reduction potential
at the lowest value of the chloride series (E = −1.480 V).
Fig. 2 Normalized electronic spectra of 1, 2, 4, 6, 8 and 10 (a) and 3, 5, 7, 9, and 11 (b) in
dichlorometane.
another indication that in solution it seems to prefer the facial
configuration.
Conversely, the reduction potential for the analogous 9 (−1.120
Mass Spectrometry. All complexes were analyzed by
electrospray ionization mass spectrometry (ESI‐MS). Full
spectra of fac‐[ReCl3(PNPR)] complexes (Fig. 3b‐e) display the
protonated molecular ion ([ReCl3(PNPR)+H]+) and ionic species
generated by the consecutive loss of one and two HCl
V) is the highest of the bromide series. This indicates that the
electron density delocalizing ability of the bromine atoms
prevails on the ether‐oxygen inductive effect.
As an exception, the mer complex 10 show a reversible
reduction process, having the highest reduction potential
within the series (E° = −1.600 V). Whether this is due to the
effect of the different configuration or of the absence of alkyl
substituents at the amino nitrogen, is difficult to ascertain.
UV/vis spectroscopy. Electronic spectra of all fac chloro‐
([ReCl3(PNPR)+H–HCl]+
and
[ReCl3(PNPR)+H–2HCl]+
respectively). All these fragmentations are confirmed by
tandem mass spectra of the single peaks (data not shown). The
protonated molecular ion is not present in the spectrum of 10
(Fig. 3a).
complexes (
maximum at ca. 350 nm, whereas for fac bromo‐complexes
and an absorbtion band centered at ca. 400 nm with a tail
extending up to 550 nm is observed (Fig. 2b). Accordingly, the
dichlorometane solutions of choloro‐ and bromo‐complexes are
nearly colorless and strong yellow colored, respectively. For the
unique mer chloro‐complex (10) a red shift to ca. 400 nm is
observed (indeed, the solution is yellow), along with another
absorption band centered at ca. 500 nm. 11 shows no significant
differences with the other fac bromo‐complexes, which is
1, 2, 4, 6 and 8, Fig. 2a) show an adsorption
A further group of signals might be generated from an oxidation
through the insertion of an oxygen atom. The latter species are
tentatively assigned to the complexes [ReV(O)Cl3(PNPR)+H]+,
[ReV(O)Cl3(PNPR)+H–HCl]+ and [ReV(O)Cl3(PNPR)+H–2HCl]+.
Notably, the abundance of such species is inversely
proportional to the length of the R group: it reaches its
3
,
5,
7
9
maximum in the spectrum of
the spectrum of
1, while the species are absent in
6
. As an example, collisional experiments on
the ion [ReV(O)Cl3(PNPme)+H]+ were performed; the spectrum
is reported in Fig. S2. Two consecutive HCl losses originates to
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Fig. 4 ORTEP view of complex 1. Thermal ellipsoids were drawn at the 40% probability
level. Hydrogen atoms and the crystallization dimethylformamide molecule omitted for
clarity. Bonds with alternate positions of disordered atoms in the diphosphinoamines
moiety were drawn in white.
Fig. 3 Partial full scan Full ESI(+)‐MS spectra (range: 640 – 850 m/z) of compounds 10 (a),
1 (b), 2 (c), 4 (d), 6 (e) and 8 (f). Red arrows indicates the putative oxo‐rhenium(V) ions
as discussed in the main text. [
= [ReCl3(PNPR)+H–2HCl]+.
= [ReCl3(PNPR)+H]+; = [ReCl3(PNPR)+H–HCl]+;
the ions [ReV(O)Cl3(PNPme)+H–HCl]+ (m/z 728) and
[ReV(O)Cl(PNPme)+H–2HCl]+ (m/z 692). A further CO loss is
responsible for the ion at m/z 664, while the most abundant
ions, at m/z 499 and 471, can be explained by rearrangement of
the PNPR moiety after oxygen loss, with a consequent reduction
of the rhenium(V) metal center and might be assigned to the
species
[ReIIICl2(Ph2PCH2CH3)]+, respectively.
[ReIVCl2(Ph2PCH2CH2N‐CH3)]+
and
fac‐[ReBr3(PNPR)] complexes show similar spectra, with similar
fragmentation patterns (Fig. S3). No oxo‐species were detected
in the ESI‐MS spectra of the corresponding fac‐[ReBr3(PNPR)]
compounds.
The collision of [ReX3(PNP2)+H]+ ions induces the formation of
fragments generated not only by the loss of the two HX, but also
by the further cleavage of –CH3 and –CH2CH2OCH3 radicals from
the alkoxy‐alkyl chain of PNP2, as shown in Fig. S4 in the case of
[ReCl3(PNP2)+H]+ ions. This kind of fragmentations at the
expense of the alkoxy‐alkyl chain were previously observed for
other complexes, characterized by the presence of the
[Tc(N)(PNP2)]‐ building block.18 The fragmentation of the alkyl
chain on the N atom was not observed for the other complexes.
X–Ray structure characterization of the complexes 1 – 4, 10
and 11
Details of the crystal structure determinations are listed in Table
1; a selection of bond distances and angles is reported in Table
S3. Fig. 4 – 7 report the ORTEP19 diagrams of the compounds.
All the complexes share a distorted octahedral geometry. In fac
complexes (Table S3), the N(1)–Re–X(3) angle shows a weak
tendency to widen, depending on the nature of the alkyl R
substituent. The angle grows by 1.2° when R changes from
methyl (
a propyl (
trend (+0.3° compared with
1
4
) to ethyl (
); however, the bromide complex
, but –0.9° compared with
2
) and by an additional 3.1° when R becomes
fit less well to the
).
Fig. 5 ORTEP views of complex 2 (a) and 3 (b). Thermal ellipsoids were drawn at the 40%
probability level, with hydrogen atoms omitted for clarity.
3
1
2
Likewise, the X(1)–Re–X(2) angle opens up by 0.5° when R
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Fig. 6 ORTEP view of complex 4. Thermal ellipsoids were drawn at the 40% probability
level, with hydrogen atoms omitted for clarity. Bonds with alternate positions of atoms
in the disordered phenyl ring were drawn in white, with disordered atoms refined only
isotropically.
changes from methyl (
when R becomes a propyl (
distances also show a similar behaviour; the bond grows by
about 0.05 Å when R changes from methyl ( ) to propyl ( ).
Similarly, in mer complexes, the Re–N distance grows of ca. 0.10
1
) to ethyl (
2
) and then by a further 1.3°
4), with
3
along the trend. Re–N
1
4
Fig. 7 (a) ORTEP view of complex 10 (a) and 11 (b). Thermal ellipsoids were drawn at the
40% probability level, with hydrogen atoms omitted for clarity. Bonds with alternate
Å passing from 2.156(2) (R = H, 10) to 2.252(4) Å (R = CH3, 11). positions of disordered atoms in the phenyl rings (a) and in the diphosphinoamine
ethylene bridges (b) were drawn in white.
The mean Re–N distance in fac complexes is 2.265 Å (estimated
standard deviations for reported mean values not given
becomes bulkier, besides to the differences due to the fac or
mer arrangement of the halide ligands. This effect is
transferred, in an attenuated manner, to the mean Re–X
distance, that correspondingly shows a very subtle tendency to
shorten when the R residue becomes longer. The donating
ability of the tertiary amine nitrogen is then slightly weakened
by a growing alkyl residue, allowing a tighter halide–metal
bond. Accordingly, a weak variation in the Re–X bonds stability
along the series can be expected.
In the CCDC repository21 we found only a few mononuclear
rhenium(III) compounds showing three chloride (or bromide)
anions in an X3P2N donor set22a‐h. All the members of this
restricted group are mer complexes and only one show a
bis(diphenylphosphino)ethylamine tridentate ligand similar to
those described in this work. Interestingly, there is also another
complex showing a similar tridentate ligand, that is bis[(2‐di‐
tbutylphosphino)ethyl]amine, but in this case the metal is in the
+4 oxydation state22i. Accordingly, to the best of our knowledge,
because of the very limited number of observations involved),
0.06 Å longer than that observed in mer complexes, 2.204 Å;
complex
1 fits less well to the trend (Re–N distance
intermediate between those of 10 and 11).
The comparison of the average Re–Cl, Re–Br bond lengths
indicates that these bonds are longer (+0.02 Å) in fac than in
mer complexes, regardless of the halogen type (2.397 vs. 2.373
Å in 1, 2, 4 and 10, respectively; 2.538 vs. 2.516 Å in 3 and 11,
respectively). These variations do not appear affected by the
variation of the R residue.
Mean equatorial Re–X distances in fac complexes (for
, 2.425 Å; for , 2.566 Å) are longer than mean trans Re–P
bonds (for and , 2.392 Å; for , 2.409 Å). Hence, Re–X
1, 2 and
4
3
1,
2
4
3
distances look trans–labilized by more efficient P donors; a
reverse trend looks true for mer complexes.
The average Re–Cl, Re–Br bond lengths also reveal a very weak
influence of the R residue. In complexes 1, 2 and 4, the mean
Re–X distances decreases by 0.01 Å when R changes from
methyl to propyl, becoming shorter when R becomes heavier.
Along with this perspective, our data show that fac and mer
complexes cannot be compared, whereas an attempted
comparison between the mer complexes 10 and 11, taking into
account the difference in vdW radia of Br (1.85 Å) and Cl (1.75
Å)20, does not reveal the same trend. All the above comparisons
were made in the attempt of spotting any influence due to the
R residue. Our data suggest that such influence exist, but it is
very faible and involves primarily the N(1) atom, which becomes
a little less tightly bound to the metal when the R residue
compounds
1 – 4 are the first reported rhenium(III) fac
complexes showing the the X3P2N donor set provided by an
alkylphosphinoethylamine ligand.
Intermolecular contacts are not much represented in
complexes
molecule present in the unit cell of
efficient 3D interaction network. A full description of
2
–
11. Instead, the dimethylformamide (DMF)
1
substantiates a rather
nonbonding interactions in complexes
(Table S4).
1–11 is given in the ESI
10 | Dalton Trans., 2017, 00, 1‐15
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ed. J. A. McCleverty, T. J. Meyer, Elsevier, Amsterdam, 2003,
View Article Online
271−402; (c) W. C Eckelman, JACC CardDioOvIa: 1s0c..1I0m39a/gCi7nDgT,021003029J,
2
, 364.
Conclusion
2
3
C. Bolzati, D. Carta, N. Salvarese and F. Refosco, Anti‐Cancer
Agents Med. Chem., 2012, 12, 428.
This work is part of our ongoing efforts to explore the
coordination chemistry of rhenium along with the prospect of
developing metal‐based radiopharmaceuticals for nuclear
medicine applications. Here we have presented, following
previous extensive studies about PNPR complexes of rhenium
and technetium in high oxidation states, a further investigation
describing rhenium complexes in the +3 oxidation state.
A series of neutral rhenium(III) mixed‐ligand complexes,
encompassing different PNPR ligands and the Cl− and Br− halides
have been prepared and fully characterized. As previously
observed for the corresponding compounds incorporating the
technetium/rhenium nitride or phenylimido cores,5,6 the
stereochemistry of these compounds is determined by the
nature of the amino group inserted on the diphosphine
backbone as well as by the length of the group chain. Thus,
PNPR ligands incorporating a highly nucleophilic tertiary amino
group, always stabilize fac isomer, except for the less
encumbered PNPme donor. In this case, only for fac‐
[ReBr3(PNPme)] complex, a facial to meridional isomerization
takes place, when the compound was recrystallized, giving rise
to the mer‐[ReBr3(PNPme)] species. This behavior reproduces
that previously found with the PNPme ligand in nitride‐Tc(V)
complexes5 (vide supra), which gave pacing for embarking this
R. Alberto, H. Braband, D. Can, Y. Tooyama, S. Imstepf, in
Technetium and Other Metals in Chemistry and Medicine, ed.
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Am. Chem. Soc., 2002, 124, 11468; (b) A. Boschi, C. Bolzati, E.
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work. Since the analogous chloro‐complex
1 assumes instead
the fac conformation both in solid state and in solution, the
bromine ligands likely count in this behavior. Indeed, ligand
characterized by the less nucleophilic secondary amino group
(PNPH) yield only the stable mer form (complex 10).
Although the fac compounds are characterized by molecular
properties that in principle may allow the replacement of the
three halides with other chelating systems, the complex are
inert toward ligand exchange reaction.
Considering the important deferent chemical properties
between rhenium and technetium, a possible future extension
of this work may be addressed to the translation of this
chemistry to the technetium congener.
6
7
M. Porchia, F. Tisato, F. Refosco, C. Bolzati, M. Cavazza‐
Ceccato, G. Bandoli and A. Dolmella, Inorg. Chem., 2005, 44
,
4766.
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Bandoli, Coord. Chem. Rev., 1994, 135‐136, 325; (c) G. Bandoli,
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Acknowledgements
The authors thank the Ministero dell’Istruzione, dell’Università
e della Ricerca (MIUR) and the Associazione Italiana per la
Ricerca sul Cancro (AIRC) for financial support (MIUR:
PON01_2388 and FIRB‐MIUR RBAP114AMK_006 RINAME; AIRC:
IG 13121). The authors also thank Ms. Anna Moresco for
elemental analysis, Dr. Francesco Tisato for his help with NMR,
Dr. Gregorio Bottaro for the UV/vis spectroscopy and Dr.
Valentina Peruzzo for the mass spectrometry data collection.
P. E. Fanwick and R. A. Walton, Inorg. Chim. Acta, 2002, 329
,
1; f) Y.‐S. Kim, Z. Hea, R. Schibli, S. Liu, Inorg. Chim. Acta, 2006,
359, 2479. g) A. Choualeb, A. J. Lough and D. G. Gusev,
Organometallics, 2007, 26, 3509; h) A. T. Radosevich, J. G.
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DOI: 10.1039/C7DT01032J
The reactivity of diphosphinoamine ligands [(Ph2PCH2CH2)2NR] toward rhenium(III, V) precursors have been
studied. The obtained complexes have been fully characterized.