Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 165-168, 2005

Versatility of Bidentate Aniline Derivatives as Ligands for Rhenium(V) and Technetium(V)T. I. A. Gerber*, a and P. Mayerba

Chemistry Department, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa
Chemistry Department, Ludwig-Maximilians University, D-81377 München, Germanyb

Received: July 31, 2005; In Final Form: September 30, 2005
The versatility of aniline derivatives as ligands for Re(V) and Tc(V) has been investigated. The reaction of (nBu4N)[MOCl4] (M=Re, Tc) with 1,2-diaminobenzene (H2dab) and 2,3-diaminopyridine (H2dap) gave the products(Bu4N)[MO(dab)2] (1) and [MO(dapH)2]Cl (2) respectively. Both chelates in 1 and 2 are coordinated as diamides;
however, in 2 the pyridine nitrogens of dapH are protonated. With trans-[ReOCl3 (PPh3)2] as a starting complex, anumber of products were isolated, depending on the reaction conditions. For example, with 3-hydroxy-1,2-diaminobenzene (H2dab-OH) under N2, [Re(dab-OH)Cl3(PPh3)2] (3) was produced, in which dab-OH substituted the oxo groupand is coordinated as a monodentate imide. With 3-nitro-1,2-diaminobenzene (H2dab-NO2), the oxo-free complex[ReCl(PPh3)(dab-NO2 ) 2 ] (4) was obtained, with bidentate diamido coordination of each chelate. Using trans[ReO2(py)4]Cl as a starting complex, complexes of the type [Re(sbqdi-OH)3]Cl (5) and cis-[ReO2(Hdab)(py)2] (6) wereisolated, where sbqdi-OH coordinates as the monoanionic semibenzoquinonediimine and Hdab as a bidentate monoamide. The crystal structures of these complexes are discussed in this account.

1. Introduction
The current widespread interest in the coordination chemistryof rhenium and technetium is mainly due to the potential application of the radioisotopes 186Re and 188Re in radiotherapy andthe utilization of 99mTc in diagnostic nuclear medicine.1 Ourinterest is focused on the synthesis of Re(V) and Tc(V) complexes containing 1,2-diaminobenzene (H2dab) and its derivatives. In fact, increasing attention has lately been devoted tometal complexes derived from H2dab, mainly because of their
unusual structural, magnetic and redox properties.2, 3 In thesecomplexes it was found that the diamine can be present as the1,2-diamide dianion (dab2−), the 1,2-benzosemiquinone diimine(sbqdi) or the neutral 1,2-benzoquinone diimine (bqdi).
NH-NH-e

NH-e

NH -NHdabNH

sbqdi

bqdi

In addition to dab and sbqdi, this account also reports on
unusual Re(V)/Tc(V) complexes containing the following
forms of H2dab derivatives:
N 2-NH -NH2

NH 22. Experimental
Materials. trans- [ReOCl 3 (PPh 3 ) 2 ], (n-Bu 4N)[MOCl 4 ](M=Tc/Re) and trans-[ReO2 (py) 4]Cl were synthesized by literature methods.4−7 H 2 dab and its derivatives were obtainedcommercially (Aldrich), and their purities were checked by
NMR spectroscopy and melting points. Solvents were of reagent
grade and were purified and dried before use. All other chemicals were obtained commercially.
*Corresponding author. E-mail: thomas.gerber@nmmu.ac.za.
FAX: +27-41-5044236.

Synthesis of complexes(n-Bu 4N)[MO(dab) 2] (M = Tc(1a), Re(1b)). A mixture of150 µmol of (n-Bu4N)[MOCl4] and 450 µmol of H 2dab in 30
mL of ethanol was stirred at room temperature for 18 h (for Tc)
or heated under reflux for 90 min (for Re) in air. After thereaction periods orange precipitates were collected, washedwith ethanol and diethyl ether, and dried. Recrystallisationfrom acetone/ethanol gave crystals suitable for X-ray analysis.
IR: 1a: ν(Tc = O) 891, ν(Tc-N)432; 1b: ν(Re = O) 936, ν(ReN) 452 cm−1.
[MO(dapH) 2]Cl·2H2O (M=Tc(2a), Re(2b)). (n-Bu4N)[MOCl4](150 µmol) and 300 µmol of H2dap were dissolved in 20 mL ofa 10:1 benzene/ethanol solvent mixture, and heated underreflux for 60 min. After cooling to room temperature, the solutions were filtered and left to evaporate slowly at room temperature. After two days crystals, suitable for X-ray studies, werecollected. IR: 2a: ν(Tc = O) 931, ν(Tc-N) 454; 2b: ν(Re = O)956, ν(Re-N) 469 cm−1.
[Re(dab-OH)Cl3 (PPh3) 2] (3). To a quantity of 180 µmol oftrans-[ReOCl3 (PPh 3) 2] in 20 mL of ethanol were added 200µmol of H2dap-OH, and the mixture was heated under reflux
under a N2 atmosphere for 90 min. After cooling to room temperature, a precipitate was filtered off, washed with ethanoland diethyl ether, and dried. Further slow evaporation of thefiltrate gave crystals suitable for X-ray crystallography. IR:
ν(Re = N-) 1092; ν(Re-Cl) 313,295 cm−1.
[ReCl(PPh3)(dab-NO2) 2] (4). A mixture of 130 µmol of trans[ReOCl3(PPh3)2] and 260 µmol of H2dab-NO2 in 30 mL of ethanol was heated under reflux for 3 h. After cooling a red-brownsolid was removed by filtration, washed with EtOH and Et2O
and dried under vacuum. Recrystallization was from acetone.
IR: ν(Re-Cl) 301 cm−1.
[ReIV (sbqdi-OH) 3]Cl (5). A mixture of 170 µmol of trans[ReO2 (py) 4]Cl and 520 µmol of H2dab-OH in 30 mL of ethanol
was stirred at room temperature for 18 h in air. After the reaction period the solution was filtered and left to evaporate slowly
at room temperature. After 3 days green-black crystals, suitable for X-ray analysis, were collected. IR: ν(Re-N) 507 cm−1.
cis-[ReO2 (Hdab)(py) 2] (6). A mixture of 150 µmol of trans[ReO2 (py4]Cl and 305 µmol of H2dab in 15 mL of ethanol wasstirred at room temperature for 30 h. After the reaction period© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

166 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

GerberTABLE 1: Crystal and Structure Refinement Data
Crystal system
Space group1
Orthorhombic
P212121

2bTriclinic–
P1

3
Monoclinic
P21/n4
Monoclinic
P21/c

5
Hexagonal
P66Triclinic–
P1a(Å)
b(Å)
c(Å)
α (˚)
β (˚)
γ (˚)
V(Å3)
Z
Reflections obs.
RGOF

11.644(3)15.303(4)16.950(5)9090903020.3(2)
4
15681
0.058
1.29

7.1477(2)9.2464(4)12.1316(5)73.838(1)78.658(2)73.786(1)733.29(5)230042
0.02991.08

12.127(1)14.385(1)21.711(1)9099.09(1)903740.0(6)
4
47022
0.061
1.07

14.937(11)15.017(8)15.394(12)90112.12(6)90
3199(4)417392
0.08071.07

13.535(2)13.535(2)8.394(2)9090
1201331.7(3)221942
0.05661.10

9.570(2)12.493(3)12.851(3)61.60(3)70.56(3)78.68(4)1273.1(4)235192
0.048
1.06

1

2

for |Fo| > 3σ(Fo); for |Fo| > 2σ(Fo)a purple precipitate was collected. The slow evaporation of themother liquor over a period of 3 days at room temperature gave
crystals suitable for X-ray crystallography. IR: νs (ReO2) 905,
νa (ReO2) 876 cm−1.
Physical measurements. The instrumentation used is thesame as reported earlier.8 A Nicolet Siemens R3m/V (for 1a,3, 4) and a Nonius Kappa CCD (for 2b, 5, 6) diffractometer
were used. Crystal and structure refinement data are given inTable 1. Structural refinements were made by the full-matrix
least-squares method on F2 using the program SHELXL97.9
3. Results and Discussion
Structure of 1a. This complex (Figure 1) shows a squarepyramidal geometry with the oxo group occupying the apicalposition, with the Tc atom lying above the mean plane definedby the four amido nitrogens by 0.67 Å. On coordination to theTc(V) center, the two bidentate ligands both lose two amine

protons to form an anionic complex. The Tc= O distance of1.668(7) Å is typical, and the Tc-N distance (mean 1.98(1) Å)is intermediate between Tc=N (1.91 Å) and Tc-NH2 (2.08 Å).10
The angles around the nitrogens are in the order of 120˚, consistent with sp2-hybridization. The average bite angle of thetwo dab chelates is 78.9˚.
Structure of 2b. The complex (Figure 2) has a square-pyramidal geometry with the Re atom lying 0.68 Å above the plane
defined by the four amido nitrogen atoms. The Re = O distanceof 1.688(4) Å) is typical for monooxorhenium(V) complexes,
and the Re-N distances (mean = 1.993(5) Å) are similar to thosefound for Re-amide bonds.11 The average bite angle of the twochelates is 77.4˚. Protonation of the pyridyl nitrogen atoms has aminor effect on the aromaticity of the rings, with a slight deviation from planarity. The packing of the molecules in the crystal
is essentially achieved by hydrogen bonds between Cl−, NH andH2O. The Cl− counterion hydrogen-bonds to the protons on thepyridyl N(1) [N(1)–H···Cl = 3.380(5) Å] and N(2) [N(2)–H···Cl= 3.378(5) Å]. The proton on the pyridyl N(4) is hydrogenbonded to O(2) of a water molecule (2.982(7) Å).
Structure of 3. The complex (Figure 3) exhibits an approximate octahedral geometry with the Re(V) atom displaced fromthe mean Cl(1)P(1)Cl(3)P(2) plane by 0.08 Å towards N(1).
The 2,3-diaminophenol ligand acts as a dianionic moiety, andthe Re-N(1)-C(1) bond angle (179.5(7)˚) emphasizes the linear
coordination mode of the triply bonded phenylimido unit, andFigure 1. Structure of the [TcO(dab)2]− anion. (Tc-O = 1.668(7), TcN(1) = 1.99(1), Tc-N(2) = 1.98(1), C(1)-N(1) = 1.38(2) Å; O-Tc-N(1) =109.3(4), O-Tc-N(2) = 110.0(4), N(1)-Tc-N(2) = 78.8(5)˚).
O3_2
ClO2O1N2N5C1C6N4C10C5Re

C7C9N1C8N6N3C2C4C3Figure 2. The molecular structure of [ReO(dapH)2]Cl. (Re-O(1) =1.688(4), Re-N(2) = 2.002(4), Re-N(3) = 1.995(4), C(1)-N(1) = 1.346(7)Å; O(1)-Re-N(2) = 108.7(2), O(1)-Re-N(3) = 110.1(2), Re-N(3)-C(2) =117.6(3), N(2)-Re-N(6) = 141.4(2)˚).

Figure 3. An ORTEP view of [Re(dab-OH)Cl3(PPh3)2]. (Re-N(1) =1.730(9), Re-P(1) = 2.475(3), Re-P(2) = 2.502(3), Re-Cl(1) = 2.421(3),Re-Cl(2) = 2.442(3), Re-Cl(3) = 2.420(2) Å; N(1)-Re-Cl(2) = 176.7(3),P(1)-Re-P(2) = 177.8(1), N(1)-Re-P(1) = 89.7(3), N(1)-Re-Cl(1) =93.5(3)˚).

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 167

Versatility of Bidentate Aniline Derivatives as Ligands

Figure 4. The molecular structure of [ReCl(PPh3)(dab-NO2)2]. (ReCl = 2.393(9), Re-P(1) = 2.431(9), Re-N(1) = 1.98(3), Re-N(2) =1.96(2) Å; N(1)-Re-N(2) = 77(1), Re-N(1)-C(1) = 120(2), Cl-Re-N(1) =139.1(8)˚).

Figure 6. The cis-[ReO2 (Hdab)(py) 2] neutral complex. (Re-O(1) =1.723(7), Re-O(2) = 1.716(6), Re-N(1) = 2.053(8), Re-N(2) = 2.042(9),Re-N(3) = 2.163(9), Re-N(4) = 2.155(9) Å; O(1)-Re-O(2) = 118.2(3),O(1)-Re-N(1) = 157.8(3), O(1)-Re-N(2) = 85.2(3), N(3)-Re-N(4) =174.3(3), N(1)-Re-N(2) = 72.7(3)˚).

Re1N2O1C2C3N1C1C4C6C5

Figure 5. Molecular view for the monocation [Re(sbqdi-OH)3]+. (ReN(1) = 2.049(10), Re-N(2) = 1.887(19), N(1)-C(1) = 1.29(1), N(2)-C(2)= 1.53(2) Å; N(1)-Re-N(2) = 77.5(3), C(1)-N(1)-Re = 118(1)˚).

the Re-N(1) distance of 1.730(9) Å agrees with the values foundin other Re(V)-phenylimido octahedral complexes.12 The phenylimido moiety is virtually perpendicular to the mean equatorial plane (dihedral angle of 89.2(3)˚), and to minimize stericcongestion around the metal atom, the ligand fits between two
phenyl rings.
Structure of 4. The crystal structure (Figure 4) shows that
4 exhibits a skew-trapezoidal bipyramidal geometry. The complex contains two dinegative dab-NO2 ligands symmetricallycoordinated to the Re(V) centre. The four Re-NH bond distances have an average of 1.98(2) Å, with the Re-P bond (2.43(1)Å) shorter than in other Re(V) complexes.13 The Re-Cl bondlength (2.393(9) Å) is within the range observed for a large variety of Re(V) oxo complexes.14 The Re, Cl and P(1) atoms areabove the N(1), N(2), N(1′), N(2′) mean plane by 0.72, 2.51 and2.57 Å, respectively. The two dab-NO2 ligands make a dihedral
angle of 60.3˚.
Structure of 5. The rhenium atom is at the center of a trigonal-prismatic environment (Figure 5). The Re-N(2) bond is short
at 1.89(2) Å, and is indicative of bonding of an amide (–NH−)
to Re. The longer Re-N(1) bond (2.05(1) Å) indicates a single
bond with the quinone imine (=NH) nitrogen. This value compares well with other Re-N(imine) bonds, which vary in therange 2.03(1)-2.15(1) Å.15 Bonding in the ligand is of particularinterest. The C(1)-N(1) bond (1.29(2) Å) is short, consistent
with a localized double bond of the sbqdi form. The C(2)-N(2)bond is long (1.53(2) Å) and indicates a localized single bond.
The six-membered ring of the benzosemiquinone diimine displays typical quinoid distortions. The C(5)-C(6) length of
1.26(4) Å is short and indicative of a localized double bond, aswould be expected for the sbqdi form with a C(1)-N(1) doublebond. The assignment of a +IV oxidation state to Re is thus justified on the basis of the crystallographic data.
Structure of 6. The rhenium atom is at the center of a distorted octahedral environment (Figure 6). The small bite angle(72.7˚) of Hdab may be responsible for the cis orientation of thetwo oxo groups, which have an O(1)-Re-O(2) angle of 118.2˚.
The mean planes of the pyridines define angles of 97.1˚ and87.5˚ with the Hdab plane, and the mean planes passing throughthe pyridines define an angle to each other of only 12.5˚. TheN(3)-Re-N(4) angle deviates from linearity (174.3(3) Å), andthe Re = O, Re-Npy and Rer-Namine bond lengths fall within thereported limits.16
4. Conclusions
This report illustrates the versatility of 1,2-diaminobenzene
and its derivatives as ligands for Re(V) and Tc(V). A varietyof unusual complexes of these metals, containing different
coordination modes of the chelate H 2dab, were prepared byusing different starting materials and reaction conditions.
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