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Water-Soluble Rhenium Phosphine Complexes Incorporating the Ph2C(X) Motif (X = O-, NH-): Structural and Cytotoxicity Studies.
pubs.acs.org/IC Article
Water-Soluble Rhenium Phosphine Complexes Incorporating the
− −
Ph C(X) Motif (X = O , NH ): Structural and Cytotoxicity Studies
2
Abdullah F. Alshamrani, Timothy J. Prior, Benjamin P. Burke, David P. Roberts, Stephen J. Archibald,
* *
Lee J. Higham, Graeme Stasiuk, and Carl Redshaw
CiteThis:https://dx.doi.org/10.1021/acs.inorgchem.9b03239 ReadOnline
ACCESS *
Metrics&More ArticleRecommendations sı SupportingInformation
ABSTRACT: Reaction of [ReOCl (PPh ) ] or [ReO I(PPh ) ]
3 3 2 2 3 2
with 2,2′-diphenylglycine (dpgH ) in refluxing ethanol afforded the 2
air-stable complex [ReO(dpgH)(dpg)(PPh )] (1). Treatment of
3
[ReO(OEt)I (PPh ) ] with 1,2,3-triaza-7-phosphaadamantane
2 3 2
(PTA) afforded the complex [ReO(OEt)I (PTA) ] (2). Reaction 2 2
of [ReOI (PTA) ] with dpgH led to the isolation of the complex
2 3 2
[Re(NCPh )I (PTA) ]·0.5EtOH (3·0.5EtOH). A similar reaction
2 2 3
but using [ReOX (PTA) ] (X = Cl, Br) resulted in the analogous
2 3
halide complexes [Re(NCPh )Cl (PTA) ]·2EtOH (4·2EtOH) and
2 2 3
[Re(NCPh )(PTA) Br ]·1.6EtOH (5·1.6EtOH). Using benzilic
2 3 2 acid(2,2′-diphenylglycolicacid,benzH)with2affordedthecomplex
[ReO(benz) (PTA)][PTAH]·EtOH (6·EtOH). The potential for
2
theformationofcomplexesusingradioisotopeswithrelativelyshort
− half-livessuitable fornuclear medicineapplications bydeveloping conditions for [Re(NCPh )(dpg)I(PTA) ] (7)[ReO ] ina 4 h
2 3 4
timescalewasinvestigated.Aprocedureforthetechnetiumanalogofcomplex[Re(NCPh )I (PTA) ](3)from99mTc[TcO ] − was
2 2 3 4
theninvestigated.Themolecularstructuresof1−7arereported;complexes3−7havebeenstudiedusinginvitrocellassays(HeLa,
HCT116, HT-29, and HEK 293) and were found to have IC values in the range of 29−1858 μM.
50
■
INTRODUCTION potential method for modulating toxicity, localization, etc., in
mammalian cells.11,12 The use of amino acids has allowed for
The diagnosis and early treatment of diseases such as cancer
andheartdiseasehavebenefitedgreatlyfromtheavailabilityof the development of many new chelate families, particularly
technetium-basedimagingagents.199mTcisusedasaγ-emitter where steric bulk is not a prerequisite for complex
formation.13,14 They can also act as bifunctional chelators,
in single-photon emission computed tomography (SPECT).
Moreover, 99mTc has been used for diagnosis as an imaging i.e.,canattachtotargetedbiomoleculesandbearcoordination
agent because of the optimal 140-keV γ-ray emission, the 6 h sites to the radionuclide that can direct the biodistribution of
the
radiotracer.15−17
The use of simple carboxylic acids also
half-life, lower tissue damage, and the availability of a
convenient
generator.2−5
By contrast, rhenium has two
allows for optical properties of metal complexes to be probed
isotopes that are β-emitters, namely, 186Re and 188Re, with with a view to enhancing their bioapplication.18,19 In other
half-lives (t ) of 90 and 17 h, respectively, making them studies, we and others have found that the use of the acids
1/2
suitableastherapeuticsinatheranosticpairwithtechnetium.6 Ph
2
C(X)CO
2
H (X = OH, NH
2
) can impart high crystallinity
Given the similarities between the chemistries of these two in products and be beneficial for characterization.20−23 Thus,
metals, any advances in the synthetic methodology to access herein we report our initial studies on rhenium complexes
one type of metal chelate can usually be applied to the other bearing ligands derived from diphenylglycine (X = NH 2 ) and
metal and vice versa.2,7 Currently, much research effort is the related pro-ligand benzilic acid (X = OH).
devoted to developing targeted agents, thereby ensuring any Convenient entry points into high-oxidation-state rhenium
delivered radiation is localized.3,8,9 chemistry are thephosphine complexes[ReOCl 3 (PPh 3 ) 2 ] and
The use of chelates with added functional groups is a [ReO I(PPh ) ]; however, it must be noted that the low
2 3 2
particularlypromisingavenue,10andwiththisinmind,wehave
initiatedaprogramtoinvestigatetheuseofaminoacidderived Received: November5, 2019
chelates bound to rhenium. We note that data concerning the
cellularuptake,toxicity,andlocalizationofrheniumaminoacid
complexes are scant.11 Moreover, the use of amino acid
derived ligation at the rhenium core has been recognized as a
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Inorganic Chemistry pubs.acs.org/IC Article
biocompatibility,highmolecularweight,andhydrophobicityof (d,J=11.4Hz,2H,NH).31PNMR(CDCl)δ:5.3(s).IR(KBr):
2 3
arylphosphinesrendersthesecomplexesunsuitableformedical 3303(w),3258(w),1584(m),1569(m),1092(s),998(s),980(s),
uses.24Thechallenge,therefore,istoreplacePPh withamore
955(s),936(s),908(s)cm−1.MALDI-MSm/z:917.21[M+H+].
3 Synthesisof[ReO(dpgH)(dpg)(PPh)](1)from[ReO I(PPh)].As
biocompatible and hydrophilic phosphine. 1,2,3-Triaza-7- 3 2 32
above, but using [ReOI(PPh)] (0.50 g, 0.57 mmol) and 2,2′-
phosphaadamantane (PTA) is a phosphine that is widely diphenylglycine (0.26 g, 2 1.14 3 m 2 mol), afforded 1 as green prisms.
used as a water-soluble ligand and possesses an adamantane-
Yield: 0.30 g (57%). Elemental anal. calcd for C H NOPRe: C,
like structure. Daigle et al. first prepared this phosphine in 60.26%;H,4.14%;N,3.05%.Found:C,60.32%;H 4 , 6 3.8 38 9% 2 ;N 5 ,3.07%.
1974,andbecauseofitssolubilityandstabilityinwater,ithas 1Η ΝΜR (CDCl) δ: 11.24 (s, 1H, NH), 6.91−7.72 (m, 21H,
3
been widely used in the fields of organometallic catalysis and H PPh/CPh),6.66(d,J=11.5,14H,H PPh/CPh),5.92(d,
arom 3 2 arom 3 2
coordinationchemistry;25,26othershaverecentlypublishedon J = 11.5, 2H, NH). 31P NMR (CDCl) δ: 5.3 (s). IR (KBr): 3301
2 3
the coordination chemistry of PTA and outlined its medical (w),3256(w),1584(w),1569(w),1092(s),998(s),979(m),955
and catalytic chemistry.27−30 Thus, herein we have employed (m), 936(m), 908(m)cm−1.MALDI-MSm/z:917.21[M +H+].
Synthesis of [ReO(OEt)I(PTA) ] (2). At room temperature, 1,3,5-
PTA to produce water-soluble rhenium complexes for facile 2 2
triaza-7-phosphaadamantane(0.51g,3.27mmol)wasdissolvedinthe
biological evaluation. We also note that the rhenium oxo minimum volume of dichloromethane (∼30 mL), and the solution
complexes can be isolated from [3 + 2] reactions involving wasaddedtoaconicalflaskcontainingasuspensionof[ReO(OEt)-
diphenyl ketene.31 The dpgH 2 , benzH and complexes 1−7 I 2 (PPh 3 ) 2 ](1.00g,1.09mmol)indichloromethane(20mL),which
prepared herein are shown below in Schemes 1 and 2. causedacolorchangefromlightkhakitoyellow-orange.Thereaction
mixturewasstirredfor1h,andthenthevolatileswereremovedunder
Scheme 1. 2,2′-Diphenylglycine (dpgH ) and Benzilic Acid vacuum. Ethanol (5 mL) was added, and slow evaporation of the
(2,2′-Diphenylglycolic Acid, benzH) 2 solvent at room temperature afforded a yellow-orange solid, which
was collected and washed with EtOH (10 mL) and EtO (10 mL).
2
Orangecrystalsof2suitableforX-raycrystallographywereobtained
by recrystallization from CH Cl /EtOH. Yield: 0.81 g (92%).
2 2
Elemental anal. calcd for C H INOPRe: C, 20.60%; H, 3.55%;
14 292 6 2 2
N, 10.30%. Found: C, 21.01%; H, 3.95%; N, 10.64%. 1Η ΝΜR
(CDCl)δ:4.48−4.74(m,2H,CH),4.03−4.35(m,12H,N−CH−
3 2 2
N),3.87(d,J=9.2Hz,12H,P-CH),1.58(s,3H,CH).31PNMR
2 3
(CDCl) δ: −4.8 (s); in (DO) δ: −2.2 (s). IR (KBr): 1281 (m),
3 2
1040(m),1013(s),969(s),945(s),902(s)cm−1.MALDI-MSm/z:
Finally, given that the aim of this work is to develop new 816.95[M + H+].
Synthesis of [Re(NCPh)I (PTA)]·0.5EtOH (3·0.5EtOH). To [Re-
technetium-based imaging agents, we have conducted the 2 2 3
OI(PTA)](0.50g,0.53mmol)anddpgH (0.24g,1.07mmol)was
preparation of a water-soluble dpgH -derived rhenium 2 3 2
2 addeddryethanol(20mL).Afterthemixturewasrefluxedfor4h,the
complex from (NH 4 )[ReO 4 ] in 4 h and have investigated solution was filtered, and on prolonged standing (2 to 3 days) at
t■his synthetic methodology with [99mTc]TcO − .
4 ambienttemperature,greencrystalsof3formed.Yield:0.40g(68%).
Elementalanal.calcdforC H IN PRe·0.5EtOH:C,34.57%;H,
EXPERIMENTAL SECTION 4.41%;N,12.60%.Found:C 64 ,34 88 .9 4 0% 2 ; 0 H 6 ,4. 2 79%;N,12.72%.1ΗΝΜR
General Information. All manipulations were carried out under (CD)δ:7.05−7.29(m,10H,H CPh),3.85−4.08(m,18H,N−
6 6 arom 2
an atmosphere of dry nitrogen using conventional Schlenk and CH−N), 3.66 (d, J = 41.6 Hz, 18H, P-CH). 31P NMR (CD) δ:
2 2 6 6
cannula techniques or in a conventional nitrogen-filled glovebox. −104.9 (t, J = 11.3 Hz), −120.9 (d, J = 12.5 Hz); in (CDCl) δ:
3
Hexane and ethanol were dried over sodium prior to use.
−104.2(t,J=12.5Hz),−118.7(d,J=12.5Hz);in(DO)δ:−107.3
2
Dichloromethane was refluxed over calcium hydride. Diethyl ether (t,J=11.3Hz),−123.3(d,J=12.5Hz).IR(KBr):1592(w),1416
wasdriedoversodiumbenzophenone.Allsolventsweredistilledand (m),1313(m),1276(s),1241(s),1041(m),1015(s),969(s),946
degassed prior to use. IR spectra (nujol mulls, KBr windows) were (s) cm−1.ES-MS m/z: 1092[M+ H+].
recorded on a Nicolet Avatar 360 FT IR spectrometer; 1H NMR Synthesis of [Re(NCPh 2 )Cl 2 (PTA) 3 ]·2EtOH (4·2EtOH). To [Re-
spectra were recorded at room temperature on a Varian VXR 400 S OCl(PTA)] (0.50 g, 0.68 mmol) and dpgH (0.27 g, 1.10 mmol)
2 3 2
spectrometerat400 MHzor aGemini 300 NMRspectrometer or a
wasaddeddryethanol(20mL).Afterthemixturewasrefluxedfor4
Bruker Advance DPX-300 spectrometer at 300 MHz. The 1H NMR h,thesolutionwasfiltered,andonprolongedstanding(2to3days)
spectra were calibrated against the residual protio impurity of the at ambient temperature, green crystals of 4 formed. Yield: 0.40 g
deuterated solvent. Elemental analyses were performed by the (67%).Elementalanal.calcdforC H ClN PRe(sampledriedin-
31 46 2 10 3
elemental analysis service at the London Metropolitan University vacuum −2EtOH): C, 40.93%; H, 5.06%; N, 15.40%. Found: C,
andtheDepartmentofChemistryandBiochemistryattheUniversity 40.52%; H, 4.95%; N, 15.01%. 1Η ΝΜR (CD) δ: 7.06−7.29 (m,
6 6
ofHull. 10H,H CPh),3.55−3.63(m,18H,N−CH−N),3.19(d,J=8.7
arom 2 2
The precursors [ReOCl (PPh)], [ReOBr(PPh )], [ReOI- Hz,18H,P-CH).31PNMR(CD)δ:−82.0(t,J=10.0Hz),−94.9
3 3 2 3 3 2 2 2 6 6
(PPh)], [ReO(OEt)I(PPh)], and [ReOCl(PTA)] were pre- (d,J=10.0Hz);in(CDCl)δ:−81.2(t,J=10.0Hz),−94.2(d,J=
3 2 2 3 2 2 3 3
pared by the literature methods; [ReOX(PTA)] (X = Br, I) was 10.0Hz);in(DO)δ:−77.4(t,J=10.0Hz),−91.6(d,J=7.5Hz).
preparedfollowingthemethodusedfor[R
2
eOCl(
3 PTA)].24,32−34All
IR(KBr):1598
2
(w),1462(s),1315(w),1281(w),1242(w),1040
otherchemicals werepurchasedfrom Sigma-Ald
2
rich.
3 (w),1015(m),972(m),947(m)cm−1.MALDI-MSm/z:909.20[M
Synthesis of [ReO(dpgH)(dpg)(PPh)] (1). To [ReOCl(PPh)] + H+].
3 3 3 2
(0.50 g, 0.60 mmol) and 2,2′-diphenylglycine (0.27 g, 1.20 mmol) Synthesis of [Re(NCPh )Br (PTA) ]·1.6EtOH (5·1.6EtOH). To
2 2 3
wasaddeddryethanol(20mL).Afterthemixturewasrefluxedfor4 [ReOBr(PTA)] (0.50 g, 0.60 mmol) and dpgH (0.27 g, 1.20
2 3 2
h, volatiles were removed in vacuum and the residue was extracted mmol) was added dry ethanol (20 mL). After the mixture was
intoCHCl (20mL).AdditionofalayerofEtO(20mL)afforded, refluxedfor4h,thesolutionwasfiltered,andonprolongedstanding
2 2 2
on prolonged standing, green prisms of 1. Yield: 0.40 g (74%). (2 to 3 days) at ambient temperature, green crystals of 5 formed.
Elementalanal.calcdforC H NOPRe:C,60.26%;H,4.14%;N, Yield: 0.35 g (58%). Elemental anal. calcd for C H BrN PRe·
46 38 2 5 31 46 2 10 3
3.05%.Found:C,59.93%;H,4.00%;N3.02%.1ΗΝΜR(CDCl)δ: 0.5EtOH (sample dried in-vacuum, −1.1EtOH): C, 37.64%; H,
3
11.24(s,1H,NH),7.13−7.51(m,21H,H PPh/CPh),6.98(d,J 4.80%;N,13.72%.Found:C,37.83%;H,5.18%;N,13.26%.1ΗΝΜR
arom 3 2
=8.2Hz,6H,H PPh),6.67(d,J=11.4Hz,8H,H CPh),5.92 (DMSO−D)δ:7.53(t,J=7.8Hz,6H,H CPh),6.94−7.11(m,
arom 3 arom 2 6 arom 2
B https://dx.doi.org/10.1021/acs.inorgchem.9b03239
Inorg.Chem.XXXX,XXX,XXX−XXX
Inorganic Chemistry pubs.acs.org/IC Article
Scheme 2. Complexes 1−7 Prepared Herein
4H, H CPh), 3.91−4.39 (m, 18H,N−CH−N),3.73 (d, J = 9.6 underrefluxfor15min.Theobtainedolive-greensolidwasfilteredoff
arom 2 2
Hz,18H,P-CH).31PNMR(DMSO−D)δ:−87.5(t,J=10.0Hz), andwashedwithethanol(25mL)anddiethylether(25mL),withno
2 6
−103.6 (d, J = 10.0 Hz); in (CDCl) δ: −89.4 (t, J = 10.0 Hz), furtherdrying.Thiscompoundwasthenaddedtothesolventmixture
3
−103.3(d,J=10.0Hz);in(DO)δ:−70.8(m),−82.2(d,J=5Hz). (1:25)(water/acetone),whichwasstirredinanicebathfor30min.A
2
IR(KBr):1459(s),1376(s),1281(m),1277(m),1239(m),1094 violetcompound[ReOI(PPh)]precipitatedwhichwasthenfiltered
(s), 1040 (m), 1011 (s), 969 (s), 945 (s) cm−1. MALDI-MS m/z: off and driedinvacuum 2 . 1,3,5 3 -T 2 riaza-7-phosphaadamantane(2.17 g,
840.68[M− PTA]. 13.8mmol)wasdissolvedinaminimumvolumeofdichloromethane,
Synthesis of [ReO(benz)(PTA)][PTAH]·EtOH (6·EtOH). To [ReO- and the solution was then mixed (at room temperature) with the
2
(OEt)I(PTA)](0.61g,0.74mmol)andbenzilicacid(0.34g,1.48 preprepared [ReOI(PPh)] (2.00 g, 2.30 mmol) dissolved in
2 2 2 3 2
mmol) was added dry ethanol (20 mL). After the mixture was dichloromethane (30 mL); the color changed from dark brown to
refluxed for 4 h, the solution was allowed to cool to room pale brown. The reaction mixture was stirred for 40−50 min,
temperature, whereupon purple crystals of 6·EtOH formed. Yield: following which the volume of the solution was reduced in under
0.50 g (68%). Elemental anal. calcd for C H N O PRe: C, vacuumtoremoveanyvolatiles.Ethanol(5mL)wasthenadded,and
82 96 12 15 4 2
49.47%;H,4.87%;N,8.47%.Found:C,49.35%;H,4.60%;N,8.65%. after 5 min, the remaining solvent was removed using a rotary
1ΗΝΜR(CD)δ:7.24−7.96(m,20H,H CPh),3.22−3.29(m, evaporator. The obtained product was then washed with ethanol to
6 6 arom 4
12H, N−CH−N), 2.91 (d, J = 7.8 Hz, 12H, P-CH). 31P NMR give a yellow-orange solid [ReOI(PTA)]. Finally, [ReOI(PTA)]
2 2 2 3 2 3
(CD)δ:−90.4(s),−94.5(s);in(DO)δ:−88.5(s),−91.6(s).IR (2.00 g, 2.44 mmol)and dpgH (1.11 g, 4.88 mmol)were mixedin
6 6 2 2
(KBr):1656(s),1631(s),1025(s),1011(m),981(w),969(s),945 ethanol(20mL).Thereactionwasthenstirredat100°Cunderreflux
(s),923 (w) cm−1.MALDI-MS m/z: 971.24[M+ H+]. for 90 min to produce a green solution. This green solution was
Synthesis of [Re(NCPh)(dpg)I(PTA)]·2CD (7·2CD). Ammo- filtered, and the solvent was then removed via rotary evaporation to
2 3 6 6 6 6
niumperrhenate(1.09g,4.10mmol)andtriphenylphosphine(5.00g, produceafinegreensolidofcomplex7.Yield:1.80g(62%).Green
19.0mmol)werestirredinethanol(30mL).Hydroiodicacid(56%,5 crystalssuitableforX-rayanalysiswereobtainedbyrecrystallizationof
mL) was then added carefully, and the brown mixture was heated the product [Re(NCPh)(dpg)I(PTA)] (7) in benzene. Elemental
2 3
C https://dx.doi.org/10.1021/acs.inorgchem.9b03239
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)atadlla(
Rw 2
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anal. calcd for C H IN PRe: C, 45.34%; H, 4.86%; N, 12.93%. removed from the wells and 100 μL of the compound in the media
45 581 11 3
Found:C,45.12%;H,5.12%;N12.72%.1ΗΝΜR(CD)δ:7.64(d, wasadded.Variousconcentrationsofcompoundsintherangeof6.25
6 6
J = 8.3 Hz, 4H, H CPh), 6.82−7.34 (m, 6H, H CPh), 3.55− mMto6.25nMweretested.After24hofincubation,thecontentsof
arom 2 arom 2
4.34 (m, 32H, CH(PTA)), 3.17−3.40 (m, 2 H, NH). 31P NMR the wells were removed using a multipipette, and then 180 μL of
2 2
(CD)δ:−4.7(s),−102.3(s);in(CDCl)δ:−4.8(s),−98.2(s);in sterilizedPBSwasaddedfollowedbytheadditionof20μLofMTS
6 6 3
(DO)δ:−2.2(s),−95.3(s).IR(KBr):3409(w),1633(m),1035 reagent(Promega,U.K.).Plateswerethenreturnedtotheincubator
2
(w), 1012 (s), 968 (m), 945 (m) cm−1. MALDI-MS m/z: 1191.80 for 4 h. Color intensity (absorbance) of the treated wells was
[M]+,1175.85 [M+− NH]. measured at 490 nm using a Synergy HT microplate reader. The
Synthesisof[99mTc][Tc(N 2 CPh)I(PTA) ].[99mTc]TcO−waseluted percentages of the cell viability of the treated cells were calculated
2 2 3 4
in saline from an Ultra Technekow FM 99Mo/99mTc generator based on positive and negative control where they represent 100%
(Curium,U.K.),and100μL(∼200MBq,insaline)wasaddedtoa2 and 0% viable cells, respectively. IC values were calculated using
50
mL glass vial containing a mixture of 900 μL of dimethylformamide G■raphPad Prismsoftware.
(DMF),triphenylphosphine(66.5mg,0.255mmol),andhydroiodic
acid (66.5 μL, 0.883 mmol). The vial was heated at 137 °C for 30 RESULTS AND DISCUSSION
m ph i o n s . p A h f a t a e d r m th a e nt v a i n a e l w (3 a . s 7 a m llo g w , e 0 d .02 to 35 co m ol m , o a l) so i l n ut D io M n F of ( 1 1 , 0 3 0 ,5- μ tr L ia ) za w -7 as - Use of 2,2′-Diphenylglycine. A convenient entry point
into much rhenium(V) oxo chemistry is the readily available
addedtothereactionmixture,andthemixturewasstirredfor40min
at99°C.Finally,asolutionof2,2′-diphenylglycine(0.17mg,0.00074 [ReOCl
3
(PPh
3
)
2
].38 Reaction of this complex with two
mmol)inDMF(10μL)wasaddedtothereactionmixture,andthe equivalents of dpgH
2
in refluxing ethanol afforded, following
mixture was stirred at 137 °C for 2 h. Quality control analysis was workup, the green complex [ReO(dpgH)(dpg)(PPh )] (1) in
3
carried out by HPLC or radio-TLC. Radio-TLC experiments were moderate-to-goodyield(≤74%).TheIRspectrumcontaineda
carriedoutonaluminum-backedsilica TLCplateswithsalineasthe strong band at 955 cm −1 assigned to ν along with strong
Re=O
m TL o C bile de p te h c a t s o e r . . D [9 e 9m te T c c ti ] o T n cO w 4 a − s R p F erf = orm 0.9 e 6 d 7; us [ i 9 n 9 g mT a c] Sc [ a T n c - O RA 2 I M (PP R h a 3 d ) i 2 o ] R N e M − R P s s p tr e e c t t c r h u e m sa d t is 1 p 0 l 9 a 2 ye c d m a −1 si a n n g d let ν N a − t H δ a 5 t .3 33 p 0 p 3 m cm fo − r 1. th T e h P e P 31 h P
RF= 0.04; and[99mTc][TcOI(PTA)] RF= 0.087. 3
2 3 ligand. Single crystals suitable for an X-ray crystal structure
HPLC Conditions. HPLC analysis was performed on an Agilent
determination were grown by diffusing diethyl ether into a
1100SeriessystemequippedwithadiodearrayUV(detectionat254
nm)andLabLogicNaIcrystalgammadetectorscontrolledbyLaura saturated dichloromethane solution at ambient temperature.
software. The crystal structure of 1 is rather complex because there are
TheanalyticalHPLCwasperformedonaGemini5μmC18110Å four chemically equivalent, but symmetry-independent mole-
LC column (150 × 4.6 mm, Phenomenex) at a flow rate of 1 mL cules in the asymmetric unit. Each of these four molecules
min−1, with a mobile phase consisting of methanol with 0.1% TFA features the same binding of the ligands, but these molecules
(solventA)andwaterwith0.1%TFA(solventB).Gradient1[time/ havephenylringsindifferentorientationswhichrendersthem
min](solvent A/solvent B): [0−2](50:50), [2−17](95:5), [17−22]
symmetry independent. Rather than describe the four related
(95:5),[22−26](50:50). The HPLC sample was prepared by taking
molecules in words, the geometry about the metal for one of
10 μL of sample dissolved in 90 μL of methanol. Semipreparative
HPLC was carried out on a Luna 5 μm C18(2) 100 Å LC column theseisreportedanddisplayedinFigure1(thefullasymmetric
(250 × 10 mm; Phenomenex) and a Pursuit 200 Å C18 10 × 250
mm, 10 μm, HPLC column (Agilent) at a flow rate of 5 mL min−1,
withamobilephaseconsistingofmethanolwith0.1%TFA(solvent
A) and water with 0.1% TFA (solvent B). Semipreparative gradient
[time/min](solvent A/solvent B): [0−2](95:5), [2−22](95:5−
50:50), [0−2](95:5), [2−22](95:5−50:50). Prior to the HPLC
purification, the reaction mixtures were passed through C-18
cartridges totrapunreacted [99mTc]TcO−.
4
X-rayCrystallography.X-raydiffractiondatafromsinglecrystals
of 3·0.5EtOH and 6·EtOH were collected at the EPSRC
crystallography service inSouthampton, U.K. For the other samples,
fullsets of X-ray diffraction intensitydata werecollected inseriesof
ω-scansusingaStoeIPDS2imageplatediffractometeroperatingwith
MoKαradiationat150(2)K.Amultiscanmethodwasappliedforthe
absorptioncorrections ofthe collected data.35
The structures were solved using dual-space methods within
SHELXT and full-matrix least-squares refinement was carried out
withinSHELXL-2018viatheWinGXprograminterface.36,37Allnon-
hydrogen positionswerelocated inthe direct and differenceFourier
mapsandrefined usinganisotropicdisplacementparameters.Crystal
structure data for the compounds reported here are summarized in
Table1.
Cytotoxicity. MTS assay was usedto calculate the percentageof
viable cells in the culture media. This assay depends on the
transformation of a tetrazolium salt into formazan in viable cells by
mitochondrial dehydrogenase enzyme activity. There is a positive
correlationbetweentheamountofformazanandthenumberofviable Figure 1. Molecular structure of one metal ion in [ReO(dpgH)-
cellsintheculturemedia.HeLa,HCT116,HT-29,andHEK293cells (dpg)(PPh)] (1). Selected bond lengths (Å) and angles (deg):
3
wereseeded in 96flat-bottomed microliter tissue culture plates with Re(1)−O(1) 1.705(4), Re(1)−O(5) 2.064(4), Re(1)−O(7)
20000 cells per well in 200 μL media of McCoy’s and Dulbecco’s 2.025(3), Re(1)−N(1) 2.250(5), Re(1)−N(2) 1.889(5), Re(1)−
Modified Eagle’s Medium (DMEM). In order to attach the cells to P(1) 2.4487(13); O(5)−Re(1)−N(1) 73.85(15), O(1)−Re(1)−
the well base in the microliter plates, the plates were incubated O(7) 104.88(15), N(2)−Re(1)−O(7) 80.47(16), O(5)−Re(1)−
overnightina5%CO incubatorat37°C.After24h,themediawas P(1) 88.69(10).
2
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Figure 2. Molecular structure of [ReO(OEt)I(PTA)] (2). Selected bond lengths (Å) and angles (deg): Re(1)−O(1) 1.706(5), Re(1)−O(3)
2 2
1.906(5), Re(1)−I(1) 2.8117(5), Re(1)−I(2) 2.7450(5), Re(1)−P(1) 2.4673(17), Re(1)−P(2) 2.4523(17); O(1)−Re(1)−O(3) 172.6(2),
P(2)−Re(1)−P(1) 176.27(6), O(1)−Re(1)−I(2) 95.59(16), C(31)−O(3)−Re(1) 146.9(4), O(3)−C(31)−C(32) 111.9(6). Re(2)−O(2)
1.705(5), Re(2)−O(4) 1.906(5), Re(2)−I(3) 2.7519(5), Re(2)−I(4) 2.8080(5), Re(2)−P(3) 2.4555(17), Re(2)−P(4) 2.4631(17); O(2)−
Re(2)−O(4) 174.3(2), P(3)−Re(2)−P(4) 176.24(6), O(2)−Re(2)−I(3) 94.42(15), C(33)−O(4)−Re(2) 152.0(4), O(4)−C(33)−C(34)
111.2(5).
unit and full details of bond lengths about each metal center adopts a distorted octahedral geometry with one bulky PTA
are in the SI.) The metal ion adopts a distorted octahedral ligand trans to the one other PTA. An ethoxide ligand is
geometry where the main distortion occurs as a result of positioned trans to the oxo group.
− −
binding through COO and NH of the two diphenylglycine- The twoindependentcomplexes intheasymmetric unit are
derived ligands. One of these (dpgH) is only deprotonated at related by pseudo-translational symmetry. Upon warming of
the acid, but is bound through the NH (Re(1)−N(1) the crystal, the structure simplifies and the pseudo-translation
2
2.250(5)Å).Thesecondistwice-deprotonated,i.e.,dpg,once becomes a strict symmetry element. The structure at 298 K
at the carboxylic acid and once at the amine, and it binds crystallizesinthespacegroupC2/cwithasingleRecomplexin
through COO − and NH − . Accordingly, the two Re−N bond the asymmetric unit (Z = 4) (Figure S3). There is small-scale
lengths are very different (Re(1)−N(2) 1.889(5) Å). disorder of the position of the ethoxide alkyl chain in this
Itprovedpossibletoobtain1byuseofasimilarreactionbut structure,whichsuggeststhatthepresenceoftwoindependent
employing [ReO I(PPh ) ] as the rhenium source. complexes at low temperature is a result of ordering of the
2 3 2
With a view to conducting biological assays, we decided to alkyl chains.
employ the water soluble phosphine 1,2,3-triaza-7-phosphaa- The reaction of [ReOI (PTA) ] with two equivalents of
2 3
damantane (PTA). Treatment of [ReO(OEt)I (PPh ) ] with dpgH in ethanol afforded, after workup, the complex
2 3 2 2
three equivalents of PTA in dichloromethane/ethanol [Re(NCPh )(PTA) I ]0.5EtOH (3·0.5EtOH). The infrared
2 3 2
afforded, after workup, the complex [ReO(OEt)I (PTA) ] spectrumof3exhibitedthesignalsofPTAligandsat946,969,
2 2
(2) in high yield. The IR spectrum was dominated by the and 1015 cm −1, together with a weak band for ν at 1592
N=C
signalsofν Re=O at945and902cm −1forν (O−CH2) .Also,theIR cm −1.Also,intheIRspectrum,symmetricν P−CH2 wasfoundat
spectrumcontainedstrongbandsofcoordinatedPTAligandat 1313 cm −1, while ν N−CH2 and ν N−C−N were observed at 1276
969 and 1013 cm −1. The 1H NMR spectrum exhibited and1241cm −1,respectively.The1HNMRspectrumdisplayed
methyleneprotonsforPTAbetween4.03and4.74ppm,while signals for methylene protons of PTA between 3.66 and 4.08
therewerenosignalsdisplayedforthearomaticgroupsrelated ppmaswellasaromaticsbetween7.05and7.29ppm.The31P
to a PPh ligand. The 31P NMR spectrum showed a singlet at NMRspectrum(C D )exhibited(similarshiftswerenotedin
3 6 6
−4.8 ppm, suggesting the presence of two equivalent PTA CDCl ,seetheExperimentalSection)atripletanddoubletatδ
3
ligands. −104.9 and −120.9 ppm; these shifts (and those of 4 and 5)
Single crystals suitable for a structure determination were are somewhat downfield of those reported for the complex
grown via diffusion of dichloromethane/ethanol at ambient [ReNCl (PTA) ] (−78.2 and −74.5 ppm in CDCl ).24 This
2 3 3
temperature. Structure determination for the crystal at 150 K dataisconsistentwiththepresenceoftwoequivalentandone
wasroutine,andthisdemonstratedthat2crystallizedwithtwo otherPTAligands.Inordertoassessthestabilityofcomplex3,
chemicallyidentical molecules in the asymmetric unit (Z = 2) a sample of 3 was dissolved in D O and monitored by 31P
2
in space group P2 /n. NMR spectroscopy. The spectrum showed the peak of one
1
The molecular structure is shown in Figure 2, with bond PTA at −107.3 ppm and two PTA ligandstrans to eachother
lengths and angles given in the caption. Each rhenium center at −123.3 ppm. The two signals look similar to the triplet-
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doublet pattern of 3 in C D (−104.9 and −120.9 ppm). view see Figure S4), with selected bond lengths and angles
6 6
Complex 3 was isolated as single crystals from EtOH, and the given in the caption. The rhenium center adopts a distorted
crystal structure was determined by X-ray diffraction. The octahedral geometry in which the phosphine Re(1)−P(1)
molecular structure is shown in Figure 3 (for an alternative binds trans to the Re(1)−P(3). The distortion occurs as a
result of the bulky nature of all the ligands present.
In accordance with the structure of 3, the Re(1)−P(1) and
Re(1)−P(3) bonds found in the trans positions each display
longerdistancesof2.3929(12)and2.4004(12)Å.TheRe(1)−
I(2)bondlengtheningof2.8652(4)Åcanbeascribedtotrans
influence exerted by the Re(1)−N(10) that itself has a bond
length of 1.817(4) Å. We have noted the loss of CO from
2
dpgH during complexation in previous studies,39,40 while a
2
search of the CSD for MNCPh or MNCHPh type motifs
2 2
revealed 19 hits.39,41,42
A similar reaction but using [ReOCl (PTA) ] and
2 3
[ReOBr (PTA) ] also resulted after workup in the isolation
2 3
of a green complex of formula [Re(NCPh )(PTA) Cl ]·
2 3 2
2EtOH (4·2EtOH) and [Re(NCPh )(PTA) Br ]·1.6EtOH
2 3 2
(5·1.6EtOH), respectively. However, a single-crystal X-ray
diffraction study revealed that although the geometry at the
metal is essentially the same as in 3, there are different
amounts of solvent present. The molecular structures for
complexes 4·2EtOH and 5·1.6EtOH are shown in Figure 4
(for alternative views see Figures S5 and S6), with selected
bond lengths and angles given in the caption.
Use of Benzilic Acid. The reaction of [ReO(OEt)-
I (PTA) ](2)withtwoequivalentsofbenzilicacidinethanol
2 2
afforded, after workup, the complex [ReO(benz) (PTA)]-
2
[PTAH]·EtOH (6·EtOH). The infrared spectrum of 6
exhibited bands for the PTA ligand at 1025 cm
−1,
together
Figure 3. Molecular structure of 3·0.5EtOH (unbound ethanol not with ν stretching at 945 cm −1. The 1H NMR spectrum
shown). Selected bond lengths (Å) and angles (deg): Re(1)−I(1) Re=O
2.7939(4),Re(1)−I(2)2.8652(4),Re(1)−P(1)2.3929(12),Re(1)− displayed signals for the methylene protons of PTA between
P(2) 2.3911(11), Re(1)−P(3) 2.4004(12), N(10)−Re(1) 1.817(4), 2.91and3.29ppmaswellasaromaticsbetween7.24and7.96
N(10)−C(19) 1.279(6); I(1)−Re(1)−I(2) 82.391(11), P(2)− ppm. The 31P NMR spectrum showed two singlet peaks for
Re(1)−P(1) 94.14(4),Re(1)−N(10)−C(19)179.3(3). two PTA ligands at −90.4 and −94.5 ppm. The stability of 6
was examined in D O via 31P NMR spectroscopy, which
2
Figure4.Molecularstructuresof4·2EtOHand5·1.6EtOH.Selectedbondlengths(Å)andangles(deg):For4·2EtOH:Re(1)−Cl(1)2.4517(7),
Re(1)−Cl(2) 2.4608(8), Re(1)−P(1) 2.4095(8), Re(1)−P(2) 2.4009(7), Re(1)−P(3) 2.4310(8), Re(1)−N(10) 1.804(2), N(10)−C(19)
1.300(3);Cl(1)−Re(1)−Cl(2)85.79(3),P(2)−Re(1)−P(1)92.46(3),Re(1)−N(10)−C(19)177.5(2).For5:Re(1)−Br(1)2.5960(9),Re(1)−
Br(2) 2.6499(9), Re(1)−P(1) 2.3875(19), Re(1)−P(2) 2.387(2), Re(1)−P(3) 2.3923(19), Re(1)−N(10) 1.801(6), N(10)−C(19) 1.307(9);
Br(1)−Re(1)−Br(2)82.88(3),P(2)−Re(1)−P(1)95.69(7),Re(1)−N(10)−C(19) 177.4(5).
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revealed two singlets at −88.5 and −91.6 ppm, similar to the Scheme 3. One-Pot 4 h Preparation of 3 and 7 from
signals observed for complex 6 in C D albeit with slightly Perrhenate
6 6
downfieldshifts.Thecomplexiscompletelysolubleinbenzene
andlesssolubleinwater.For example,in5mLofD O,when
2
100 mg of 6 was dissolved, after filtration, the remaining
undissolved solid weighed 40 mg, i.e., 60 wt % uptake.
Complex 6 was isolated as single crystals from EtOH and
examined by X-ray diffraction. The molecular structure is
showninFigure5(foranalternativeviewseeFigureS7),with
TheIRspectrumof7exhibitedsignalsassociatedwithPTA
ligands in the range of 1012 and 1035 cm
−1,
together with
ν Re−O stretching at 945 cm −1 and ν NH2 at 3409 cm −1. The 1H
NMRspectrumdisplayedsignalsforthemethyleneprotonsof
PTAbetween3.55and4.34ppmaswellasaromaticsbetween
6.82 and 7.64 ppm. Complex 7 was isolated as single crystals
from benzene-d and was examined by X-ray diffraction. The
6
molecular structure is shown in Figure 6 (for an alternative
view see Figure S8), with selected bond lengths and angles
Figure5.Molecularstructureof6·EtOH.Thedashedlineindicatesa given in the caption. The rhenium center adopts a distorted
hydrogenbond.Selectedbondlengths(Å)andangles(deg):Re(1)− octahedral geometry in which the phosphine Re(1)−P(1)
O(1) 2.0697(14), Re(1)−O(3) 1.9624(14), Re(1)−O(4) bindstranstotheRe(1)−P(3).Oneofthedpgligandshaslost
2.1238(14), Re(1)−O(6) 1.9677(14), Re(1)−O(7) 1.7009(15), CO and binds to the rhenium via the nitrogen atom, in a
Re(1)−P(1) 2.3904(5); O(7)-Re(1)-O(1) 97.28(6), O(3)-Re(1)- sim 2 ilar fashion to that observed in 3. The second
O(1) 81.02(6), O(6)-Re(1)-O(4) 75.49(6), O(7)-Re(1)-P(1) diphenylglycine-derived ligand is deprotonated at the carbox-
89.32(5).
ylic acid and binds in a monodentate fashion through a single
oxygenatomofthecarboxylate.Similartothestructure3,the
selected bond lengths and angles given in the caption. The Re(1)−P(1) and Re(1)−P(3) bond in a trans fashion and
rhenium center adopts a distorted octahedral geometry in display long distances at 2.3932(18) and 2.4017(18) Å,
which the phosphine (Re(1)−P(1) = 2.3904 (5) Å) binds respectively. The Re(1)−I(1) bond lengthening of 2.8101(5)
trans to the oxygen of an acid group. The main distortion Å can be ascribed to the trans influence exerted by Re(1)−
occurs as a result of the chelate binding of the two P(2) that itself has a bond length 2.3980(16) Å. The Re(1)−
deprotonated benzilic acid derived ligands. A protonated O(1) bond lengthening of 2.158(5) Å can also be ascribed to
PTA forms a hydrogen bond with one of the CO benzilic thetransinfluenceexertedbyRe(1)−N(10)whichhasabond
groups.TheReObonddistanceat1.7009(15)Åistypical.43 length of 1.801(5) Å.
The Re(1)−O(1) bond lengthening of 2.1238(14) Å can be The geometrical parameters associated with the MNCPh
2
ascribed to the trans influence exerted by the Re(1)−P(1). motiffor3,4,5,and7arecompared inFigureS9. Thismotif
Potential to Form a Technetium-99m Analogue hasbeencalledazavinylideneoralkyleneamide,41anditisclear
Compound. In order that the systems herein could be thereisdoublebondcharacterassociatedwiththeR−Nbond.
investigated and developed as radiopharmaceuticals, it is Given the linearity observed here for the Re−N−C angles
necessary (given the 6 h half-life of 99mTc, used for SPECT (>177°),wefavorthedescriptionputforwardbyPombeiroet
imaging) that the product preparation can be conducted from al.,41 whereby thus linkage acts as a three electron donor, i.e.,
perrhenate in about 4 h. We have chosen perrhenate as the as
starting material, given the usual source of 99mTc generated
Re3N = CPh
from clinical grade generators in hospital radiopharmacies is ̅ 2
pertechnetate.5 With this in mind, the product [Re(NCPh)- This then, in terms of electron counting, gives an 18 electron
(dpg)I(PTA) ] (7) was obtained in 4 h by preparing count for the complexes 3, 4, 5, and 7.
3
[ReO I(PPh ) ] (from (NH )[ReO ]) and then exchanging Cell Viability Studies. The complexes 3, 4, 5, 6, and 7
2 3 2 4 4
the PPh ligands with PTA to afford ReO I(PTA) ; then, the weretestedforcytotoxicityagainstcancerouscelllines(HeLa,
3 2 3
ReO I(PTA) was reacted with two equivalents of dpgH in HCT116, and HT-29) to show that they are nontoxic at
2 3 2
ethanol to afford, after workup, a mixture comprising 85% of concentrations used for imaging. The IC values were
50
complex 7 and 15% of complex 3 (as evidenced by NMR determinedusingthecellviabilityassay,MTS.Allcompounds
spectroscopy), see Scheme 3. in this study are nontoxic in the concentration range used in
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trendofI>Br>Cl,whichcorrelatestothesizeofthehalogen
andamorelabileatomunderaqueousenvironment.Thebond
distance betweenthe Re metalandthe halogen increaseswith
toxicity from 2.86 Å (Re−I), 2.64 Å (Re−Br), and 2.46 Å
(Re−Cl), suggesting that the mechanism of action is direct
binding of a protein or DNA to the metal center, in a similar
manner to thatof cisplatin. The IC valuesobservedhere are
50
slightly higher than that of cisplatin for the cancer cell lines
(Table 2).44,45 The addition of the two benzilic acid or
diphenylglycine ligands in 6 and 7 with regards to HeLa cells
must allow for access to mechanism of resistance in this cell
line.Althoughthemechanismofactionhasnotbeenexplored,
it is clear that the change in structure of either amino acid
ligandsorhalogenhasaneffectontheIC value.Tocheckthe
50
biostability of compound 3 and 7, a 10 μL sample of
compound (6.25 mM) was diluted in 100 μL of biological
media (DEME) and monitored by 31P NMR spectroscopy
over a period of 5 days; the spectral form does not change
across the study (Figures S32−S35). This suggests that the
coordination of the water-soluble phosphine ligand (PTA)
remainsonthemetalcenterandthatthesearenotremovedas
part of the mechanism of action within the cells. This also
suggest that the complexes have a good stability under
biological conditions.
SynthesisofNovelTechnetium-99mPhosphadaman-
tane Precursor. An attempt was made to translate these
protocols to produce technetium-99m analogues of the
Figure6.Molecularstructureof7·2CD.Benzenemoleculeswithin
6 6 rhenium complexes. Multistep protocols, such as this, are
the crystal structure are not shown. Selected bond lengths (Å) and
challenging to translate to radiochemistry where the amounts
angles (deg): Re(1)−O(1) 2.158(5), Re(1)−N(10) 1.801(5),
Re(1)−P(1) 2.3932(18), Re(1)−P(2) 2.3980(16), Re(1)−P(3) of the metal ion isotope precursor are limited and the
2.4017(18), Re(1)−I(1) 2.8101(5), N(10)−C(19) 1.305(8); concentrations are low (due to accessible amounts of the
Re(1)−N(10)−C(19) 177.8(5), N(10)−Re(1)−O(1) 173.3(2), radioisotope and radiation protection issues). After several
N(10)−Re(1)−P(1) 89.17(16), O(1)−Re(1)−P(1) 84.32(15), attempts and development of an optimized protocol to
N(10)−Re(1)−P(2) 96.65(16). produce [99mTc][TcO I(PPh ) ] and, subsequently, [99mTc]-
2 3 2
[TcOI (PTA) ] from the commercial radioisotope generator
2 3
SPECT medical imaging (nM−pM) with IC values in the supplying [99mTc]pertechnetate, the reaction with diphenyl-
50
rangeof29.87−1858μM.MTSgraphsfortreatmentofHeLa, glycine was attempted. Despite the use of various different
HCT116,HT-29,andHEK293cellswithcompounds3,4,5, conditions (including varying temperature, dpgH concen-
2
6, and 7 are shown in the Supporting Information, Figures tration, and time of reaction), the target compound [99mTc]-
S10−S31. Table 2 summarizes the IC values of the [Tc(NCPh )I (PTA) ]couldnotbeaccessed.Furthermethod
50 2 2 3
complexes synthesized. development is required to produce the technetium analogues
The analogues compounds 3, 4, and 5 show enhanced from these precursors in an applicable time scale for
toxicity in HeLa cells in comparison to that of compounds 6 radiopharmaceutical synthesis.
■
and 7, although in colon cancer cell lines (HCT116 and HT-
29) compounds 6 and 7 show the higher toxicity (Table 2).
CONCLUSION
Whencompounds3,6,and7werestudiedinanoncancerous
cell line, there was a dramatic decrease in toxicity to the high In conclusion, we have used the precursors [ReOCl (PPh ) ]
3 3 2
micro−millimolar (μM−mM) range, suggesting a preference and [ReO I(PPh ) ] to access a number of new water-soluble
2 3 2
for antitumor toxicity. The most potent compounds in colon phosphinecomplexesofrheniumviareactionwith1,2,3-triaza-
cancer cell lines (3, 6, and 7) appear to be derived from the 7-phosphaadamantane(PTA)andtheacidsdiphenylglycineor
iodo starting materials. When looking close at the change of benzilic acid. Seven complexes have been successfully
halogenwithrespecttoIC values,weobservedthefollowing synthesized and fully characterized, and in a number of cases
50
Table 2. IC (μM) Values of Re Complexes Synthesized Herein and of Cisplatin44−46
50
complex HeLaIC (μM) HCT116IC (μM) HT-29IC (μM) HEK293IC (μM)
50 50 50 50
3 47.17±1.207 40.69±2.584 49.57±5.833 574.7±0.836
4 86.62±1.032 84.47±1.876 88.22±2.491 −
5 71.72±1.415 37.12±2.358 72.04±1.745 −
6 123.60±0.6227 29.87±2.312 37.29±1.699 662.1±1.062
7 140.10±1.045 42.92±2.117 34.38±3.073 1858±1.377
cisplatin 5.45±0.4444 8.2±0.1445 2.7±0.8645 109±1246
I https://dx.doi.org/10.1021/acs.inorgchem.9b03239
Inorg.Chem.XXXX,XXX,XXX−XXX
Inorganic Chemistry pubs.acs.org/IC Article
(e.g., 3 and 7), the acid-derived ligand has lost CO during Complete contact information is available at:
2
complexation. https://pubs.acs.org/10.1021/acs.inorgchem.9b03239
Complexes 3, 4, 5, 6, and 7 have been screened for their
antiproliferative activity against various cancer cell lines in Notes
ordertoshowanontoxicbehavioratinjecteddosesforSPECT T■he authors declare no competing financial interest.
medical imaging. Preliminary studies to form a technetium-
99m analogue show that the [99mTc][TcOI 2 (PTA) 3 ] complex ACKNOWLEDGMENTS
wassuccessfullysynthesized;however,thefinaldpgH complex
with 99mTc remains elusive, under the reaction c 2 onditions We thank the Saudi Cultural Bureau for its sponsorship (of
A.F.A.). C.R. thanks the Whitelaw Frater Cancer Trust for
attempted in this study. Further development of routes to the
[99mTc][TcdpgH ] complexes is required to allow future funding. The EPSRC Mass Spectrometry Service Center at
2 Swansea University and the EPSRC National Crystallographic
potentialapplicationsinSPECTimagingtobeexplored.Itisa
significantadvancetohavesynthesizedthesenovelcompounds S■ervice at Southampton are thanked for data collection.
withtherheniumanaloguesinaprocedurethatcanbecarried
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