Reactivity of [Re{(κ3-H(μ-H)B (timMe)2}(CO)3] (timMe = 2-mercapto-1-methylimidazolyl) toward neutral substrates

Article abstract of DOI:10.1021/ic011171n

The complex [Re{κ³-H(μ-H)B (tim^Me)₂}(CO)₃] (2a) (tim^Me = 2-mercapto-1-methylimidazolyl) reacts with a variety of neutral substrates to afford new complexes featuring the dihydrobis(2-mercapto-1-methylimiclazolyl)borate coordinated in a bidentate or unidentate fashion. By treating 2a with unidentate ligands, the mononuclear complexes [Re{κ²-H₂B (tim^Me)₂}(CO)₃(L)] (L = imidazole (5), 4-(dimethylamino)pyridine (6), tert-butylisonitrile (7), triphenylphosphine (8)) were formed, upon replacement of the agostic B-H???Re bond by the correspondent unidentate ligand. With potentially bidentate substrates, 2a is transformed into mononuclear or dinuclear complexes, depending on the atom donor set of the reacting substrates. Reaction of compound 2a with ethylenediamine (en) gave the complex [Re{κ¹-H₂B (tim^Me)₂}(CO)₃(en)] (9), because of cleavage of the agostic interaction, dechelation of one mercaptoimidazolyl ring, and bidentate coordination of the amine. By contrast, 1,2-bis(diphenyl)phosphinoethane (dppe) is not able to replace the mercaptoimidazolyl ring, and the dimer [Re{κ²-H₂B (tim^Me)₂}(CO)₃]₂(μ-dppe) (10) was formed. The novel Re(I) tricarbonyl complexes (5-10) have been fully characterized, including by X-ray diffraction analysis in the case of 6, 8, 9, and 10. The X-ray diffraction study confirmed the unprecedented unidentate coordination mode of the dihydrobis(2-mercapto-1-methylimidazolyl)borate in complex 9.

Full text of DOI:10.1021/ic011171n

Inorg. Chem. 2002, 41, 2422−2428
Reactivity of [Re{K3-H(µ-H)B(timMe)2}(CO)3] (timMe )
2-Mercapto-1-methylimidazolyl) toward Neutral Substrates
,†
Raquel Garcia,† Aˆngela Domingos,† Anto´nio Paulo,† Isabel Santos,* and Roger Alberto‡
Departamento de Qu´ımica, ITN, Estrada Nacional 10, 2686-953 SacaVe´m, Codex, Portugal, and
Institute of Inorganic Chemistry, UniVersity of Zu¨rich, Winterthurerstr. 190,
CH-8057 Zu¨rich, Switzerland
Received November 15, 2001
The complex [Re{κ3-H(µ-H)B(timMe)2}(CO)3] (2a) (timMe ) 2-mercapto-1-methylimidazolyl) reacts with a variety of
neutral substrates to afford new complexes featuring the dihydrobis(2-mercapto-1-methylimidazolyl)borate coordinated
in a bidentate or unidentate fashion. By treating 2a with unidentate ligands, the mononuclear complexes [Re{κ2-
H B(timMe)2}(CO)3(L)] (L ) imidazole (5), 4-(dimethylamino)pyridine (6), tert-butylisonitrile (7), triphenylphosphine
2
(8)) were formed, upon replacement of the agostic B−H‚‚‚Re bond by the correspondent unidentate ligand. With
potentially bidentate substrates, 2a is transformed into mononuclear or dinuclear complexes, depending on the
atom donor set of the reacting substrates. Reaction of compound 2a with ethylenediamine (en) gave the complex
[Re{κ1-H B(timMe)2}(CO)3(en)] (9), because of cleavage of the agostic interaction, dechelation of one mercap-
2
toimidazolyl ring, and bidentate coordination of the amine. By contrast, 1,2-bis(diphenyl)phosphinoethane (dppe) is
not able to replace the mercaptoimidazolyl ring, and the dimer [Re{κ2-H B(timMe)2}(CO)3]2(µ-dppe) (10) was formed.
2
The novel Re(I) tricarbonyl complexes (5−10) have been fully characterized, including by X-ray diffraction analysis
in the case of 6, 8, 9, and 10. The X-ray diffraction study confirmed the unprecedented unidentate coordination
mode of the dihydrobis(2-mercapto-1-methylimidazolyl)borate in complex 9.
Introduction
Our research group has been involved in the chemistry of
rhenium complexes stabilized with poly(pyrazolyl)borates,15
and recently, we have extended our research work to poly-
(mercaptoimidazolyl)borates.16 Different organometallic com-
plexes of the type [M{κ3-HB(timMe)3}(CO)3] (1) and [M{κ3-
R(µ-H)B(timMe)2}(CO)3] (R ) H (2), Me (3), Ph (4)) (M )
Re, 99/99mTc) have been synthesized and characterized.16
Relative to poly(pyrazolyl)borates, the interesting feature of
the poly(mercaptoimidazolyl)borates, for biomedical ap-
plications, is their high stability in water and the possibility
Poly(mercaptoimidazolyl)borates, which can be seen as
soft analogues of poly(pyrazolyl)borates, have been recently
introduced,1-13 and their use is expected to extend the scope
of the rich chemistry developed with poly(pyrazolyl)-
borates.14
* To whom correspondence should be addressed. E-mail: isantos@
itn1.itn.pt.
† ITN.
‡ University of Zu¨rich.
(1) Garner, M.; Reglinski, J.; Cassidy, I.; Spicer, M. D.; Kennedy, A. R.
Chem. Commun. 1996, 1975.
(2) Kimblin, C.; Hascall, T.; Parkin, G. Inorg. Chem. 1997, 36, 5680.
(3) Santini, C.; Lobbia, G. G.; Pettinari, C.; Pellei, M.; Valle, G.; Calogero,
S. Inorg. Chem. 1998, 37, 890.
(4) Santini, C.; Pettinari, C.; Lobbia, G. G.; Spagna, R.; Pellei, M.;
Vallorani, F. Inorg. Chim. Acta 1999, 285, 81.
(5) Reglinski, J.; Garner, M.; Cassidy, I. D.; Slavin, P. A.; Spicer, M. D.;
Armstrong, D. R. J. Chem. Soc., Dalton Trans. 1999, 2119.
(6) Reglinski, J.; Spicer, M. D.; Garner, M.; Kennedy, A. R. J. Am. Chem.
Soc. 1999, 121, 2317.
(7) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Chem.
Commun. 1999, 2301.
(8) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew.
Chem., Int. Ed. 1999, 38, 2759.
(10) Kimblin, C.; Bridgewater, B. M.; Hascall, T.; Parkin, G. J. Chem.
Soc., Dalton Trans. 2000, 891.
(11) Kimblin, C.; Bridgewater, B. M.; Hascall, T.; Parkin, G. J. Chem.
Soc., Dalton Trans. 2000, 1267.
(12) Slavin, P. A.; Reglinski, J.; Spicer, M. D.; Kennedy, A. R. J. Chem.
Soc., Dalton Trans. 2000, 239.
(13) Bridgewater, B. M.; Filleben, T.; Friesner, R. A.; Parkin, G. J. Chem.
Soc., Dalton Trans. 2000, 4494.
(14) Trofimenko, S. Scorpionates, the Coordination Chemistry of Poly-
pyrazolylborate Ligands; Imperial College Press: London, 1999.
(15) Paulo, A.; Domingos, A.; Garcia, R.; Santos, I. Inorg. Chem. 2000,
39, 5669 and references therein.
(16) (a) Garcia, R.; Paulo, A.; Domingo, A.; Santos, I.; Ortner, K.; Alberto,
R. J. Am. Chem. Soc. 2000, 122, 11240. (b) Garcia, R.; Paulo, A.;
Domingos, A.; Santos, I. J. Organomet. Chem. 2001, 632, 41.
(9) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Hascall, T.; Parkin,
G. Inorg. Chem. 2000, 39, 4240.
2422 Inorganic Chemistry, Vol. 41, No. 9, 2002
10.1021/ic011171n CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/03/2002

ReactiWity of [Re{K3-H(µ-H)B(timMe)2}(CO)3]
of preparing pure and water stable 99mTc complexes using
very low ligand concentration. All these features highlighted
the potential relevance of model compounds 1-4 as building
blocks for the development of radiopharmaceuticals, namely
for labeling biomolecules with 99mTc or 186/188Re. On the basis
of model complexes 1-4, several approaches can be used
for the labeling of biomolecules.16,17 For complexes [M{κ3-
H(µ-H)B(timMe)2}(CO)3] (M ) Re (2a), 99/99mTc (2b)), which
present an unprecedented agostic B-H‚‚‚Tc interaction, one
of these approaches relies on the cleavage of the agostic
B-H‚‚‚M interaction by neutral substrates (L), potentially
able to be functionalized with biomolecules. An essential
prerequisite to apply this approach is to find substrates strong
enough to cleave the agostic interaction. In the search of
adequate substrates, we have decided to investigate the
possibility of adding different neutral ligands (imidazole
(imzH), 4-(dimethylamino)pyridine (4-NMe2py), tert-butyl-
isonitrile (tBuNC), triphenylphosphine (PPh3), ethylenedi-
amine (en), and 1,2-bis(diphenyl)phosphinoethane (dppe))
to complex [Re{κ3-H(µ-H)B(timMe)2}(CO)3] (2a).
microcrystalline solid was obtained by reacting 112 mg (0.22 mmol)
of 2a with excess (dimethylamino)pyridine (135 mg, 1.11 mmol).
Anal. Calcd for C18H22BN6O3ReS2: C, 34.21; H, 3.48; N, 13.30.
Found: C, 34.78; H, 2.27; N, 13.14. IR(cm-1): 1865, 1880 and
2000 (ν(CO)), 2380 and 2460 (ν(BH)). 1H NMR (300 MHz, CDCl3,
δ (ppm)): 3.02 (s, 6H, N-CH3, py*), 3.75 (s, 6H, CH3), 6.43 (d,
2H, m-py*), 6.73 (d, 2H, CH), 6.98 (d, 2H, CH), 8.52 (d, 2H,
o-py*).
Synthesis of [Re{κ3-H(µ-H)B(timMe)2}(CO)3(tBuNC)] (7). To
a suspension of [Re{κ3-H(µ-H)B(timMe)2}(CO)3] (2a) (100 mg, 0.20
t
mmol) in toluene was added excess BuNC (100 µL, 0.88 mmol),
and the reaction mixture was stirred for 24 h. Compound 7 (100
mg, 0.17 mmol, η ) 85%) was recovered as a white microcrys-
talline solid, after removal of the solvent under vacuum and washing
with n-hexane.
Anal. Calcd for C16H21BN5O3ReS2: C, 32.43; H, 3.55; N, 11.82.
Found: C, 32.58; H, 3.30; N, 11.70. IR(cm-1): 1890, 1950, and
1
2010 (ν(CO)); 2165 (ν(CN)); 2400 and 2460 (ν(BH)). H NMR
(300 MHz, CD3CN, δ (ppm)): 1.31 (s, 9H, CH3, tBu), 3.69 (s, 6H,
CH3), 6.98 (d, 2H, CH), 7.07 (d, 2H, CH).
Synthesis of [Re{κ3-H(µ-H)B(timMe)2}(CO)3(PPh3)] (8). Com-
plex 8 was prepared and recovered as described for 5 and 6. A 178
mg (0.23 mmol, η ) 58%) portion of 8 in the form of a white
microcrystalline solid was obtained by reacting 200 mg (0.40 mmol)
of 2 with excess triphenylphosphine (510 mg, 1.95 mmol).
Herein, we report on the synthesis, characterization, and
stability of the resulting mixed compounds [Re{κ2-H2B-
t
(timMe)2}(CO)3(L)] (L ) imzH (5), 4-NMe2py (6), BuNC
(7), PPh3 (8)), [Re{κ1-H2B(timMe)2}(CO)3(en)] (9), and
[Re{κ2-H2B(timMe)2}(CO)3]2(µ-dppe) (10).
Anal. Calcd for C29H27BN4O3PReS2: C, 45.10; H, 3.50; N, 7.26.
Found: C, 45.10; H, 2.92; N, 6.83. IR(cm-1): 1860, 1905 and 2000
1
(ν(CO)), 2380 and 2440 (ν(BH)). H NMR (300 MHz, CDCl3, δ
Experimental Section
(ppm)): 3.70 (s, 6H, CH3), 6.69 (d, 2H, CH), 6.98 (d, 2H, CH),
7.32-7.42 (m, 9H, CH, m+p-Ph), 7.74 (m, 6H, CH; o-Ph). 31P
NMR (121.3 MHz, CDCl3, δ (ppm)): 13.38.
General Procedures. The reactions were carried out under air,
except when indicated to the contrary. Chemicals and solvents were
of reagent grade and were used without further purification.
[Re{κ3-H(µ-H)B(timMe)2}(CO)3] (2a) was prepared as described
previously.16 1H and 31P NMR spectra were recorded on a Varian
Unity 300 MHz spectrometer; 1H chemical shifts were referenced
with the residual solvent resonances relative to tetramethylsilane,
and the 31P NMR chemical shifts with external 85% H3PO4 solution.
NMR spectra were run in CD3CN or in CDCl3. IR spectra were
recorded as KBr pellets on a Perkin-Elmer 577 spectrometer.
Carbon, hydrogen, and nitrogen analyses were performed on a
EA110 CE Instruments automatic analyzer. Although all complexes
were isolated with high chemical purity, as indicated by 1H NMR,
low values were systematically obtained for the hydrogen analysis.
Synthesis of [Re{κ3-H(µ-H)B(timMe)2}(CO)3(imzH)] (5). To
a suspension of [Re{κ3-H(µ-H)B(timMe)2}(CO)3] (2a) (200 mg, 0.40
mmol) in toluene was added imzH (130 mg, 1.9 mmol). The
reaction mixture was stirred for 24 h, and after this period of time,
the solvent was removed under vacuum. After washing the residue
with diethyl ether, to remove free imidazole, a white microcrys-
talline solid was recovered which has been formulated as 5 (140
mg, 0.24 mmol, η ) 60%).
Synthesis of [Re{κ1-H2B(timMe)2}(CO)3(en)] (9). To a solution
of compound 2a (100 mg, 0.196 mmol) in methanol was added
excess ethylenediamine (65 µL, 0.973 mmol). The resulting solution
was heated under reflux for 2 h, precipitating a white solid. After
this time, the reaction mixture was dried under vacuum, to remove
1
the solvent and excess ethylenediamine. H NMR analysis of the
crude material revealed that it contained essentially complex 9.
Further purification of 9 was achieved by recrystallization from
methanol. Yield: 70 mg (0.123 mmol, 63%).
Anal. Calcd for C16H21BN7O3ReS2: C, 27.42; H, 3.51; N, 14.76.
Found: C, 27.59; H, 2.16; N, 14.21. IR(cm-1): 1880 and 2000
(ν(CO)), 2390 and 2430 (ν(BH)), 2990-3650 ((ν(NH)). 1H NMR
(300 MHz, CD3CN, δ (ppm)): 2.72 (m, 2H, CH2, en), 3.00 (m,
2H, CH2, en), 3.43 (s, 3H, CH3), 3.72 (s, 3H, CH3), 4.10 (br, 4H,
N-H, en), 6.63 (d, 1H, CH), 6.71 (d, 1H, CH), 6.93 (d, 1H, CH),
6.96 (d, 1H, CH).
Synthesis of [Re{κ2-H2B(timMe)2}(CO)3]2(µ-dppe) (10). While
under nitrogen, dppe (20 mg, 0.05 mmol) was added to a methanolic
solution of compound 2a (50 mg, 0.098 mmol). The reaction
mixture was heated under reflux for 2 h, and while refluxing,
complex 10 began to precipitate as a white microcrystalline solid.
After cooling to room temperature, 10 was recovered by filtration,
followed by washing with methanol (2 × 5 mL) and drying under
vacuum. Yield: 46 mg (0.032 mmol, 58%).
Anal. Calcd for C14H16BN6O3ReS2: C, 29.12; H, 2.77; N, 14.67.
Found: C, 29.57; H, 2.54; N, 14.67. IR(cm-1): 1860, 1880, 1905
1
and 2000 (ν(CO)), 2400 and 2460 (ν(BH)). H NMR (300 MHz,
CD3CN, δ (ppm)): 3.65 (s, 6H, CH3), 6.92 (d, 2H, CH), 7.02 (d,
2H, CH), 7.10 (br, 1H, CH, imzH), 7.27 (br, 1H, CH, imzH), 8.09
(br, 1H, CH, imzH), 10.77 (s, 1H, NH, imzH).
Synthesis of [Re{κ3-H(µ-H)B(timMe)2}(CO)3(4-NMe2py)] (6).
Complex 6 was prepared and recovered as described for 5. A 95
mg (0.15 mmol, η ) 85%) portion of 6 in the form of a white
Anal. Calcd for C48H48B2N8O6P2Re2S4: C, 40.68; H, 3.39; N,
7.91. Found: C, 40.35; H, 2.87; N, 7.67. IR(cm-1): 1865, 1910
1
and 2000 (ν(CO)), 2400 and 2460 (ν(BH)). H NMR (300 MHz,
CDCl3, δ (ppm)): 2.70 (s, 4H, CH2, dppe), 3.66 (s, 12H, CH3),
6.67 (d, 4H, CH), 6.94 (d, 4H, CH), 7.37 (br, 12H, m+p-Ph, dppe),
7.59 (br, 8H, o-Ph, dppe). 31P NMR (121.3 MHz, CDCl3, δ
(ppm)): 7.79.
(17) Garcia, R.; Xing, Y. H.; Paulo, A.; Domingos, A.; Santos, I.
Unpublished results.
Inorganic Chemistry, Vol. 41, No. 9, 2002 2423

Garcia et al.
Table 1. Crystallographic Data for 6 and 8-10
6
8
9
10
formula
mol wt
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C18H22BN6O3S2Re
631.55
orthorhombic
Cmc21
29.462(3)
14.386(2)
C29H27BN4O3PS2Re
771.65
orthorhombic
Pbca
10.3595(4)
18.0988(12)
33.845(2)
C13H18BN6O3S2Re
567.46
triclinic
P1h
8.4512(7)
11.591(2)
11.980(2)
117.409(14)
98.312(14)
94.345(14)
1017.2(3)
2
C48H48B2N8O6PS4Re2‚2C4H8O
1561.36
monoclinic
P21/n
10.0319(12)
13.517(3)
17.546(2)
23.508(3)
98.706(12)
7437(2)
12
1.692
5.100
0.0652
0.0971
6345.8(6)
8
1.615
4.048
0.0591
0.1020
3151.0(9)
2
1.646
4.079
0.0543
0.0808
Z
F
calcd, g cm-3
1.853
6.202
0.0252
0.0530
µ(Mo KR), mm-1
R1a
wR2a
a R1 ) ∑||Fo| - |Fc||/∑|Fo| and wR2 ) [∑(w(Fo - Fc )2)/∑(w(Fo )2)]1/2; w ) 1/[σ2(Fo ) + (aP)2 + bP] where P ) (Fo + 2Fc )/3. The values were
2
2
2
2
2
2
calculated for data with I > 2σ(I) only.
Scheme 1
X-ray Crystallographic Analysis. X-ray data were collected
from white crystals of 6 (0.31 × 0.22 × 0.16 mm3), 8 (0.29 ×
0.16 × 0.15 mm3), 9 (0.63 × 0.14 × 0.09 mm3), and 10 (0.45 ×
0.22 × 0.18 mm3). The crystals were obtained by recrystallization
from THF/n-hexane (6, 8, and 10) or acetonitrile (9) and mounted
in thin-walled glass capillaries. The data were collected at room
temperature on an Enraf-Nonius CAD-4 diffractometer with
graphite-monochromated Mo KR radiation, using an ω-2θ scan
mode. Unit cell dimensions were obtained by least-squares refine-
ment of the setting angles of 25 reflections with 14.8° < 2θ <
23.9° for 6, 20.8° < 2θ < 31.8° for 8, 15.2° < 2θ < 26.9° for 9,
and 15.3° < 2θ < 25.7° for 10. The crystal data are summarized
in Table 1.
2a and 2b remains intact in most common coordinating
solvents, such as alcohols, water, dimethyl sulfoxide, tet-
rahydrofuran, or acetonitrile.16a Despite this robustness, we
anticipated that the finding of neutral substrates that break
the B-H‚‚‚M interaction, in a selective and efficient way,
should allow the use of complexes 2a and 2b as preformed
synthons for the labeling of biomolecules. Obviously, the
choice and screen of adequate substrates should be focused
on those with a well recognized affinity for the fac-[Re-
(CO)3]+ moiety and which offer the possibility of being
functionalized with biomolecules. To get chemical informa-
tion helpful in the choice of co-ligands for further function-
alization with biomolecules, we have evaluated the reactivity
of [Re{κ3-H(µ-H)B(timMe)2}(CO)3] (2a) toward monodentate
and potentially bidentate substrates such as imidazoles,
pyridines, isonitriles, phosphines, and amines.
The data were corrected18 for Lorentz polarization effects and
for absorption (Ψ scans). The heavy atom positions were located
by Patterson methods using SHELXS-86.19 The remaining atoms
were located by sucessive least-squares refinements on F2 using
SHELXL-93.20 Complex 6 crystallizes with two independent
molecules in the asymmetric unit. In one of these molecules, the
Re and B atoms, one of the CO ligands, and the pyridine ligand lie
on a plane of symmetry. The atoms from this pyridine ligand display
large displacement parameters, and their refinement was performed
with some geometrical constraints on distances and imposing a flat
geometry for the ring. All the non-hydrogen atoms were refined
with anisotropic thermal motion parameters. For complex 10, there
are two molecules of THF of crystallization per formula unit. The
contributions of the hydrogen atoms were included in calculated
positions, constrained to ride on their carbon or boron atoms.
Atomic scattering factors and anomalous dispersion terms were as
in SHELXL-93.20 The drawings were made with ORTEP-II21a and
ORTEP-3,21b and all the calculations were performed on a Dec-
alpha 3000 computer.
As indicated in Scheme 1, complex 2a reacts with excess
(1:5) of the monodentate neutral substrates leading to the
novel mixed complexes [Re{κ2-H2B(timMe)2}(CO)3(L)] (L
t
) imzH (5), 4-NMe2py (6), BuNC (7), PPh3 (8)).
In the reactions of 2a with potentially bidentate ligands,
such as ethylenediamine and 1,2-bis(diphenyl)phosphino-
ethane, a different reactivity was observed, and the com-
pounds isolated were monomer [Re{κ1-H2B(timMe)2}(CO)3-
(en)] (9) and dimer [Re{κ2-H2B(timMe)2}(CO)3]2(µ-dppe)
(10), respectively (Scheme 2).
Complexes 5-10 are white microcrystalline solids, which
were obtained in moderate to high yield (60%-85%).
Compounds 6-8 and 10 are highly soluble in most common
polar organic solvents; 5 and 9 are only sparingly soluble in
alcohols and halogenated hydrocarbons but highly soluble
Results and Discussion
Syntheses. As we reported in a previous communication,
the agostic B-H‚‚‚M (M ) Re, Tc) interaction in complexes
(18) Fair, C. K. MOLEN; Enraf-Nonius: Delft, The Netherlands, 1990.
(19) Sheldrick, G. M., SHELXS-86: Program for the Solution of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(20) Sheldrick G. M. SHELXL-93: Program for the Refinement of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
(21) (a) Johnson, C. K. ORTEP II; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976. (b) Farrugia, L. J. Appl. Crystal-
logr. 1997, 32, 565.
2424 Inorganic Chemistry, Vol. 41, No. 9, 2002

ReactiWity of [Re{K3-H(µ-H)B(timMe)2}(CO)3]
Scheme 2
in acetonitrile or dimethyl sulfoxide. All compounds are
stable toward aerial oxidation and hydrolysis, either in solid
state or in solution. However, with the exception of the
isonitrile derivative (7), the adducts with monodentate ligands
(5, 6, 8) tend to regenerate slowly starting compound [Re-
{κ3-H(µ-H)B(timMe)2}(CO)3] (2a), if kept in solution without
excess of the corresponding unidentate co-ligand (imidazole,
pyridine, or phosphine). The follow-up of this process by
1H NMR has shown that, after 24 h at room temperature,
there is approximately a 50% conversion of complexes 3, 4,
and 6 into 2a, which is the unique species formed. In the
case of n-butylamine, we could observe the formation of the
compound [Re{κ2-H2B(timMe)2}(CO)3(nBuNH2)] by reacting
2a with this aliphatic amine and following the reaction by
1H NMR spectroscopy. However, we were unable to isolate
the mixed complex in the solid state because, after drying
the reaction mixtures under vacuum, pure [Re{κ3-H(µ-H)B-
(timMe)2}(CO)3] (2) was the only compound recovered. This
can be explained by the rebuilding of the agostic B-H‚‚‚Re
interaction, concomitantly with the removal of the volatile
amine, and it indicates a certain dynamic character for the
process. In summary, these data indicate that the electronic
properties of the unidentate co-ligand have a marked influ-
ence on the rebuilding of the agostic B-H‚‚‚Re interaction.
In complex 7, the strong π-acceptor character of the tert-
butylisonitrile ligand probably prevents such rebuilding.
is the formation of such an intermediate that, when formed,
is fast and irreversibly converted into complex 9.
In contrast with ethylenediamine, the softer 1,2-bis-
(diphenyl)phosphinoethane is unable to replace the sulfur
atoms of the dihydrobis(mercaptoimidazolyl)borate. There
was no evidence for the formation of a monomer analogous
to 8, even when a large excess of diphosphine (5:1 molar
ratio) was used. In the case of the reaction with dppe, the
hypothetical intermediate “[Re{κ2-H2B(timMe)2}(CO)3(κ1-Ph2-
PCH2CH2PPh2)]” is converted into [Re{κ2-H2B(timMe)2}-
(CO)3]2(µ-dppe) (10), upon replacement of the B-H‚‚‚Re
bond in a second molecule of complex 2a. The differences
found in the coordination behavior of the potentially bidentate
chelating ligands (en or dppe) must reflect electronic factors
instead of stereochemical ones. The ability of the ethylene-
diamine ligand to coordinate in a bidentate fashion can be
certainly accounted for by the enhanced hard character of
the d6 fac-[Re(CO)3]+ moiety, as a consequence of the strong
π-acceptor properties of the carbonyl ligands. This enhanced
hard character is well illustrated by the remarkable stability
of different Re(I) tricarbonyl complexes containing relatively
hard ligands with nitrogen (i.e., imidazole, amines) and
oxygen donor atoms (i.e., carboxylate).22
Spectroscopic Data. The collected spectroscopic data (IR,
1H and 31P NMR) are consistent with the molecular structures
of complexes 5-10, which for some of them were confirmed
by X-ray diffraction analysis.
The IR spectra of 5-10 display very strong ν(CO) bands,
ranging from 1860 to 2010 cm-1, with the typical pattern of
complexes with the “fac-Re(CO)3” moiety. In the spectrum
of complex 7, these frequencies are slightly blue-shifted in
comparison with the ν(CO) bands in the IR spectra of the
other complexes. A possible explanation for this trend is
Interestingly, the reaction of 2a with ethylenediamine
involves cleavage of the B-H‚‚‚Re bond and replacement
of one of the sulfur atoms by the second nitrogen of the
diamine. The most probable intermediate is a species such
as “[Re{κ2-H2B(timMe)2}(CO)3(κ1-H2NCH2CH2NH2)]”, which
is analogous to the mixed complexes (5-8) discussed
previously. However, we did not detect the formation of such
an intermediate, even when the reaction was followed by
1H NMR. Most probably, the rate limiting step of the process
(22) Alberto, A.; Schibli, R.; Waibel, R.; Abram, U.; Schubiger, A. P.
Coord. Chem. ReV. 1999, 190-192, 901.
Inorganic Chemistry, Vol. 41, No. 9, 2002 2425

Garcia et al.
Table 2. Selected Bond Lengths and Angles for the Two Independent
Molecules of 6
competition of the isonitrile in π-back-bonding with the
metal, as previously invoked by other authors.23 However,
this is not corroborated by the ν(CN) stretching frequency
(2165 cm-1) observed for the coordinated tert-butylisonitrile
in 7, which is even higher than in free tert-butylisonitrile.
Then, apparently, the isonitrile is acting mainly as a σ donor.
The presence of two medium intensity bands in the range
2380-2460 cm-1 is another significant feature of the IR
spectra of complexes 5-10. These bands were attributed to
the ν(B-H) stretching vibrations of the bidentate (5-8 and
10) or unidentate (9) dihydrobis(mercaptoimidazolyl)borate.
Recently, it has been claimed that the ν(B-H) frequency
can be an useful indicator of the hapticity of poly(pyrazolyl)-
borates in coordination complexes of d-transition metals.24
For the family of complexes herein described, the ν(B-H)
frequencies observed for 9 ({κ1-H2B(timMe)2}) and for 5-8
and 10 ({κ2-H2B(timMe)2}) are not significantly different.
However, so far, the number of complexes with this type of
ligand is limited, with it being premature to correlate hapticity
with the ν(B-H) frequencies.
Molecule 1
Distances (Å)
av Re(1)-C
Re(1)-N(1)
av C-S
1.87(3)
2.20(2)
1.73(3)
av Re(1)-S
av C-O
2.537(7)
1.18(3)
Angles (deg)
C(1)-Re(1)-C(2)
C(2)-Re(1)-C(3)
C(1)-Re(1)-S(1)
C(2)-Re(1)-N(1)
C(2)-Re(1)-S(2)
C(3)-Re(1)-S(1)
N(1)-Re(1)-S(1)
S(1)-Re(1)-S(2)
89.1(13)
89.5(4)
91.8(10)
93.9(9)
92.3(9)
92.5(8)
84.1(5)
86.7(2)
C(1)-Re(1)-C(3)
C(1)-Re(1)-N(1)
C(1)-Re(1)-S(2)
C(2)-Re(1)-S(1)
C(3)-Re(1)-N(1)
C(3)-Re(1)-S(2)
N(1)-Re(1)-S(2)
87.1(11)
94.0(9)
176.7(8)
177.9(8)
176.5(10)
95.9(8)
82.9(5)
Molecule 2
Distances (Å)
av Re(2)-C
Re(2)-N(7)
C(19)-S(3)
1.83(4)
2.18(4)
1.64(3)
Re(2)-S(3)
av C-O
2.535(8)
1.19(4)
Angles (deg)
88.7(13) C(24)-Re(2)-C(24′)a 86(2)
C(23)-Re(2)-C(24)
C(23)-Re(2)-N(7)
C(24)-Re(2)-N(7)
The 1H NMR spectra of complexes 5-8 and 10 display a
set of two doublets due to the C-H protons of the
2-mercapto-1-methylimidazolyl rings and a unique singlet
for the CH3 groups of these rings. These data indicate that
the two coordinated rings are magnetically equivalent, in
agreement with the Cs symmetry of the complexes, which
was confirmed for some of them (6, 8, and 10) by X-ray
175(2)
C(23)-Re(2)-S(3)
95.9(8)
91.6(11)
80.6(7)
95.0(13) C(24)-Re(2)-S(3)
C(24)-Re(2)-S(3′)a 174.6(11) N(7)-Re(2)-S(3)
S(3)-Re(2)-S(3′)a
a Atoms related by the symmetry operation: -x, y, z.
90.8(4)
1
DMSO-d6 is not the solvent of choice to run the H NMR
spectra of 9, as one of the resonances due to the methyl
groups of the dihydrobis(mercaptoimidazolyl)borate is hidden
by the residual water present in the solvent. For [Re{κ2-H2B-
(timMe)2}(CO)3]2(µ-dppe) (10), only one singlet is observed
for the methylenic protons of the diphosphine, in agreement
with the centrosymmetric molecular structure of 10, which
was confirmed by X-ray diffraction analysis. Consistently,
only one signal was observed in the 31P NMR spectrum of
10.
1
diffraction analysis. By contrast, in the H NMR spectrum
of [Re{κ1-H2B(timMe)2}(CO)3(en)] (9), the protons from the
mercaptoimidazolyl rings appear as a set of four doublets
(C-H protons) and as a set of two singlets (CH3 protons).
This pattern is consistent with the presence of one coordi-
nated mercaptoimidazolyl ring and a noncoordinated one,
as confirmed in the solid-state molecular structure of 9 (see
1
later). In the H NMR spectra of complexes 5-10, the
resonances due to the protons of the correspondent unidentate
co-ligands appear within the usual range of chemical shifts,
displaying the expected splitting patterns. However, the
spectrocopic data obtained for 9 and 10 justify some further
Molecular Structures of Complexes 6, 8, 9, and 10.
Single-crystal X-ray diffraction analysis confirmed the
identity of [Re{κ2-H2B(timMe)2}(CO)3(L)] (L ) 4-NMe2py
(6), PPh3 (8)), [Re{κ1-H2B(timMe)2}(CO)3(en)] (9), and [Re-
{κ2-H2B(timMe)2}(CO)3]2(µ-dppe) (10). The structures of the
complexes consist of discrete mononuclear (6, 8, and 9) or
dinuclear (10) units with the rhenium atoms in a distorted
octahedral environment. The facial arrangement of the three
carbonyl ligands is a common feature of all structures.
Selected bond lengths and angles are presented in Tables
2-5, and ORTEP diagrams are shown in Figures 1-4.
The structures of complexes [Re{κ2-H2B(timMe)2}(CO)3-
(L)] (L ) 4-NMe2py (6), PPh3 (8)) are analogous to the
structure of [Re{κ2-H2B(timMe)2}(CO)3(imzH)] (5), which
was briefly reported in a previous communication.16a As can
be seen in Figures 1 and 2, [H2B(timMe)2]- is coordinated in
a bidentate way through the two sulfur atoms, defining an
eight-membered chelating ring and occupying two of the
equatorial positions. The remaining coordination position is
occupied by the 4-(dimethylamino)pyridine (6) or phosphine
(8) co-ligands.
1
comments. In CD3CN, the H NMR spectrum of [Re{κ1-
H2B(timMe)2}(CO)3(en)] (9) shows two multiplets of equal
intensity for the methylenic protons of neutral and bidentate
ethylenediamine. This is consistent with the diastereotopic
character of these protons, which point either toward one of
the CO ligands or toward the coordinated sulfur from the
unidentate dihydrobis(mercaptoimidazolyl)borate, as con-
firmed by the X-ray diffraction analysis of 9 (see later). One
would expect the same type of splitting for the N-H protons
of ethylenediamine, but in CD3CN, only one broad signal is
observed at 4.10 ppm for these protons. Most probably, this
is due to occasional overlapping of the two N-H resonances.
In fact, in DMSO-d6, two well separated signals at 5.32 and
4.37 ppm are observed for the N-H protons of ethylene-
diamine, confirming their diastereotopic character. However,
(23) Spies, H.; Glaser, M.; Hahn, F. E.; Lu¨gger, T.; Scheller, D. Inorg.
Chim. Acta 1995, 232, 235.
(24) Akita, M.; Ohta, K.; Takahashi, Y.; Hikichi, S.; Moro-oka, Y.
Organometallics 1997, 16, 4121.
The X-ray diffraction analysis of [Re{κ1-H2B(timMe)2}-
(CO)3(en)] (9) confirmed the unidentate coordination of
2426 Inorganic Chemistry, Vol. 41, No. 9, 2002

ReactiWity of [Re{K3-H(µ-H)B(timMe)2}(CO)3]
Table 3. Selected Bond Lengths and Angles for 8
Distances (Å)
Re-C(1)
Re-C(3)
Re-S(2)
C(1)-O(1)
C(3)-O(3)
C(8)-S(2)
1.89(2)
1.94(2)
2.523(4)
1.17(2)
1.14(2)
1.727(14)
Re-C(2)
Re-S(1)
Re-P
C(2)-O(2)
C(4)-S(1)
1.88(2)
2.544(3)
2.498(4)
1.17(2)
1.73(2)
Angles (deg)
C(1)-Re-C(2)
C(2)-Re-C(3)
C(1)-Re-S(1)
C(2)-Re-P
89.8(9)
92.0(6)
176.3(6)
94.9(5)
174.6(6)
91.3(5)
89.93(13)
87.65(13)
C(1)-Re-C(3)
86.6(8)
92.5(6)
95.4(6)
87.2(6)
173.0(4)
89.9(4)
83.29(13)
C(1)-Re-P
C(1)-Re-S(2)
C(2)-Re-S(1)
C(3)-Re-P
C(3)-Re-S(2)
P-Re-S(2)
C(2)-Re-S(2)
C(3)-Re-S(1)
P-Re-S(1)
S(1)-Re-S(2)
Table 4. Selected Bond Lengths and Angles for 9
Figure 1. ORTEP view of [Re{κ2-H2B(timMe)2}(CO)3(4-NMe2py)] (6).
Vibrational ellipsoids are drawn at the 30% probability level.
Distances (Å)
Re-C(1)
Re-C(3)
Re-N(1)
C(1)-O(1)
C(3)-O(3)
C(8)-S(2)
1.910(5)
1.891(5)
2.206(4)
1.145(6)
1.158(6)
1.702(4)
Re-C(2)
1.900(4)
2.550(1)
2.223(3)
1.153(6)
1.731(4)
Re-S(1)
Re-N(2)
C(2)-O(2)
C(4)-S(1)
Angles (deg)
C(1)-Re-C(2)
C(2)-Re-C(3)
C(1)-Re-S(1)
C(2)-Re-N(1)
C(2)-Re-N(2)
C(3)-Re-S(1)
N(1)-Re-S(1)
N(2)-Re-S(1)
87.7(2)
88.2(2)
94.8(1)
93.9(2)
94.1(2)
92.8(1)
84.98(9)
83.27(9)
C(1)-Re-C(3)
C(1)-Re-N(1)
C(1)-Re-N(2)
C(2)-Re-S(1)
C(3)-Re-N(1)
C(3)-Re-N(2)
N(1)-Re-N(2)
N(3)-B-N(5)
89.7(2)
94.9(2)
172.4(2)
177.3(1)
175.1(2)
97.7(2)
77.7(1)
108.1(4)
Table 5. Selected Bond Lengths and Angles for 10
Distances (Å)
Re-C(1)
Re-C(3)
Re-S(2)
C(1)-O(1)
C(3)-O(3)
C(8)-S(2)
1.890(11)
1.898(11)
2.539(2)
1.159(11)
1.156(11)
1.728(10)
Re-C(2)
Re-S(1)
Re-P
C(2)-O(2)
C(4)-S(1)
1.922(11)
2.551(3)
2.481(2)
1.152(11)
1.724(10)
Figure 2. ORTEP view of [Re{κ2-H2B(timMe)2}(CO)3(PPh3)] (8). Vibra-
tional ellipsoids are drawn at the 30% probability level.
Angles (deg)
C(1)-Re-C(2)
C(2)-Re-C(3)
C(1)-Re-S(1)
C(2)-Re-P
88.1(4)
88.3(4)
176.3(3)
179.6(3)
93.7(3)
92.9(3)
89.91(8)
88.04(9)
C(1)-Re-C(3)
90.5(4)
91.4(3)
88.6(3)
90.5(3)
91.8(3)
177.8(3)
86.20(8)
C(1)-Re-P
C(1)-Re-S(2)
C(2)-Re-S(1)
C(3)-Re-P
C(3)-Re-S(2)
P-Re-S(2)
C(2)-Re-S(2)
C(3)-Re-S(1)
P-Re-S(1)
S(1)-Re-S(2)
[H2B(timMe)2]- and the bidentate coordination of ethylene-
diamine (Figure 3).
Complex 9 is the unique example of a coordination
complex having a dihydrobis(2-mercapto-1-methyl-imida-
zolyl)borate coordinated in a κ1-S fashion. To the best of
our knowledge, even for the much more studied poly-
(pyrazolyl)borates, the unidentate coordination mode is quite
rare.25,26 Ethylenediamine forms a five-membered [ReN(1)C-
(12)N(2)C(13)] chelating ring, which adopts a gauche
configuration with twisting of the C(12)-C(13) bond in order
Figure 3. ORTEP view of [Re{κ1-H2B(timMe)2}(CO)3(en)] (9). Vibrational
ellipsoids are drawn at the 40% probability level.
to achieve an unstrained conformation. Within the chelating
ring, the dihedral angle between planes [ReN(1)C(12)] and
[ReN(2)C(13)] is 18.4°, which is smaller than the corre-
sponding values (23.5°-29°) found in related complexes
[ReBr(CO)3(N,N,N′,N′-tetramethylene-1,2-diamine)] and
[ReBr(CO)3(N,N′-dimethylethane-1,2-diamine)]. However,
the torsional angle of 54.1° across the C-C bond of
(25) Gutie´rrez, E.; Hudson, S. A.; Monge, A.; Nicasio, M. C.; Paneque,
M.; Carmona, E. J. Chem. Soc., Dalton Trans. 1992, 2651.
(26) Gutie´rrez, E.; Hudson, S. A.; Monge, A.; Nicasio, M. C.; Paneque,
M.; Ruiz, C. J. Organomet. Chem. 1998, 551, 215.
Inorganic Chemistry, Vol. 41, No. 9, 2002 2427

Garcia et al.
(µ-dppe) (10). In complex [Re{κ1-H2B(timMe)2}(CO)3(en)]
(9), the Re-S bond distance for unidentate [H2B(timMe)2]-
(2.550(1) Å) is slightly larger than the average Re-S bond
distances in monomeric 6 and 8, reflecting the presence of
eight-membered chelating rings in the later complexes. For
6 and 8-10, the Re-S bond distances are larger than the
values obtained for the precursor [Re{κ3-H(µ-H)B(timMe)2}-
(CO)3] (2) (av Re-S of 2.508(1) Å).16a Most probably, this
is justified by presence of the agostic B-H‚‚‚Re interaction
that obliges the ligand to assume a boat conformation with
a closer approach to the Re(I) atom.
Concluding Remarks. We have shown that ligand
[H2B(timMe)2]- is very versatile toward the fac-[Re(CO)3]+
moiety. It can switch from tridentate (κ3-HS2) to bidentate
(κ2-S2) or monodentate (κ1-S), depending on the nature of
the neutral co-ligands. For σ + π donor co-ligands such as
isonitriles, pyridines, imidazoles, or phosphines, disruption
of the B-H‚‚‚Re bond in [Re{κ3-H(µ-H)B(timMe)2}(CO)3]
(2a) led to monomeric or dimeric complexes containing the
fragment “Re{κ2-H2B(timMe)2}(CO)3”. By contrast, ethyl-
enediamine, a harder and exclusive σ donor, tranforms 2a
into a mixed complex, [Re{κ1-H2B(timMe)2}(CO)3(en)] (9),
where the ligand displays the unprecedented κ1-S coordina-
tion mode. The monomeric mixed complexes, 5-9, are
valuable models for the labeling of biomolecules. Attachment
of the biomolecules to the incoming co-ligands enables an
easy control of the physicochemical properties of the
complexes, which are very important in targetting. Because
of their enhanced stability in solution, the isonitrile and
ethylenediamine derivatives, 7 and 9, appear as the most
promising for the development of specific 99mTc and 186/188Re
radiopharmaceuticals.
Figure 4. ORTEP view of [Re{κ2-H2B(timMe)2}(CO)3]2(µ-dppe) (10).
Vibrational ellipsoids are drawn at the 30% probability level.
ethylenediamine is within the range (51.4°-57.3°) previously
reported for the latter complexes.27,28
The coordination environmental of each rhenium in
dimeric [Re{κ2-H2B(timMe)2}(CO)3]2(µ-dppe) (10) is identical
to the one found for 8, with the diphosphine bridging the
two rhenium units. The dimeric structure of 10 contains an
inversion center which makes each unit chemically and
crystallographic equivalent (Figure 4).
In complexes 6, 8, 9, and 10, the Re-CO, Re-N, and
Re-P bond lengths and the molecular parameters of the CO,
pyridine, diamine, and phosphine ligands are normal, com-
paring with those found in previously described octahedral
Re(I) tricarbonyl complexes with the fac-[Re(CO)3]+ moiety
and containing this type of ligand.27-32 Concerning bidentate
[H2B(timMe)2]-, the mean values found for the Re-S
distances are 2.536(8) Å in [Re{κ2-H2B(timMe)2}(CO)3(4-
NMe2py)] (6), 2.534(4) Å in [Re{κ2-H2B(timMe)2}(CO)2-
(PPh3)] (8), and 2.545(3) Å in [Re{κ2-H2B(timMe)2}(CO)3]2-
Acknowledgment. This work has been partially sup-
ported by the FCT (POCTI/2001/QUI/42939) and by the
Commission of the European Communities (COST action
B12). R.G. thanks the FCT for a BIC research grant.
(27) Couldwell, M. C.; Simpson, J. J. Chem. Soc., Dalton Trans. 1979,
1101.
(28) Abel, E. W.; Bhatti, M. M.; Hursthouse, M. B.; Malik, K. M. A.;
Mazid, M. A. J. Organomet. Chem. 1980, 197, 345.
(29) Winslow, L. N.; Rillema, D. P.; Welch, J. H.; Singh, P. Inorg. Chem
1989, 28, 1596.
(30) Yam, V. W. W.; Wong, K. M. C.; Lee, V. W. M.; Lo, K. W.; Cheung,
K. K. Organometallics 1995, 14, 4034.
(31) Wallace, L.; Woods, C.; Rillema, D. P. Inorg. Chem. 1995, 34, 2875.
(32) Abram, U.; Alberto, R.; Dilworth, J. R.; Zheng, Y.; Ortner, K.
Polyhedron 1999, 18, 2995.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determination of 6, 8, 9, and 10.
This material is available free via the Internet at http://pubs.acs.org.
IC011171N
2428 Inorganic Chemistry, Vol. 41, No. 9, 2002