Preparation and Protonation Studies of trans-Dioxorhenium(V) Complexes with Imidazoles

Article abstract of DOI:10.1021/ic960451d

Reacting ReO₂I(PPh₃)₂ with imidazole (ImH) and its methylated derivatives 1-MeIm, 1,2-Me₂Im, 2-MeImH, and 4(5)-MeImH yielded salts of the trans-dioxo cation [ReO₂L₄]⁺. Pure stable compounds were isolated for X^- = I^- and L = ImH, 1,2-Me₂Im, and 1-MeIm and X^- = B(C₆H₅)₄^- and L = 1-MeIm, 2-MeImH and 5-MeImH. The ν_as(O=Re=O) IR bands were observed between 765 and 794 cm^-1, whereas the ν_s Raman band appeared in the 900-925 cm^-1 range. The compounds were also characterized by ¹H and ¹³C NMR and UV-vis spectroscopies. Crystal structures were determined for three compounds: [ReO₂(2-MeImH)₄][B(C₆H₅) ₄]·3CH₃-OH, triclinic, P1?, a = 10.696(3) A?, b = 15.128(4) A?, c = 15.497(4) A?, α = 113.57(2)°, β= 97.25(2)°, γ = 95.94(2)°, Z = 2,R = 0.030; [ReO₂(1-MeIm)₄][B(C₆H₅) ₄]·H₂O·0.5CH₃OH, monoclinic, P2₁/n, a = 15.619(2) A?, b = 9.486(2) A?, c = 27.387(4) A?, β= 97.09(1)°, Z = 4, R = 0.057; [ReO₂(5-MeImH)₄][B(C₆H₅) ₄], orthorhombic, P2₁2₁2₁, a = 10.199(2) A?, b = 13.441(5) A?, c = 28.798(10) A?, Z = 4, R = 0.043. The complexes adopt the trans-dioxo geometry, and the octahedra are remarkably regular. Protonation of [ReO₂L₄]I was studied in methanol and in water. Acid dissociation constants were determined in aqueous solution for the [ReO(OH)L₄]²⁺ (K_a1) and [ReO(OH₂)L₄]³⁺ (K_a2) species of 1-MeIm (pK_a1 = 2.0, pK_a2 = ～-4.0) and 1,2-Me₂Im (pK_a1 = ～3.8, pK_a2 = ～-4.1). Iodide salts of [ReO(OH)L₄]²⁺ were not stable enough to be isolated, but the B(C₆H₅)₄^- salt of [ReO(OH)(1,2-Me₂Im)₄]²⁺ was obtained in pure form. The crystal structure of [ReO(OH)(1,2-Me₂Im)₄] (I₃)_0.5-(ReO₄)_1.5 (monoclinic, I2/a, a = 23.290(8) A?, b = 10.863(7) A?, c = 25.709(8) A?, β= 94.36(3)°, Z = 8, R = 0.060) showed the presence of octahedral dications in which the oxo and hydroxo groups are trans to one another and the Re-N bonds are displaced to the Re-OH side by the steric effect of the multiple Re-oxo bond. The σ-donor ability of imidazole, greater than that of pyridine, leads to more stable [ReO₂L₄]⁺ compounds in which the Re=O double bond character is lower and the oxo groups are better nucleophiles.

Full text of DOI:10.1021/ic960451d

7836
Inorg. Chem. 1996, 35, 7836-7844
Preparation and Protonation Studies of trans-Dioxorhenium(V) Complexes with Imidazoles
Suzanne Be´langer and Andre´ L. Beauchamp*
De´partement de Chimie, Universite´ de Montre´al, Montre´al, Que´bec, Canada H3C 3J7
ReceiVed April 25, 1996X
Reacting ReO2I(PPh3)2 with imidazole (ImH) and its methylated derivatives 1-MeIm, 1,2-Me2Im, 2-MeImH, and
4(5)-MeImH yielded salts of the trans-dioxo cation [ReO2L4]+. Pure stable compounds were isolated for X-
)
-
I- and L ) ImH, 1,2-Me2Im, and 1-MeIm and X- ) B(C6H5)4 and L ) 1-MeIm, 2-MeImH and 5-MeImH.
The νas(OdRedO) IR bands were observed between 765 and 794 cm-1, whereas the νs Raman band appeared in
the 900-925 cm-1 range. The compounds were also characterized by 1H and 13C NMR and UV-vis
spectroscopies. Crystal structures were determined for three compounds: [ReO2(2-MeImH)4][B(C6H5)4]‚3CH3-
OH, triclinic, P1h, a ) 10.696(3) Å, b ) 15.128(4) Å, c ) 15.497(4) Å, R ) 113.57(2)°, â ) 97.25(2)°, γ )
95.94(2)°, Z ) 2, R ) 0.030; [ReO2(1-MeIm)4][B(C6H5)4]‚H2O‚0.5CH3OH, monoclinic, P21/n, a ) 15.619(2) Å,
b ) 9.486(2) Å, c ) 27.387(4) Å, â ) 97.09(1)°, Z ) 4, R ) 0.057; [ReO2(5-MeImH)4][B(C6H5)4], orthorhombic,
P212121, a ) 10.199(2) Å, b ) 13.441(5) Å, c ) 28.798(10) Å, Z ) 4, R ) 0.043. The complexes adopt the
trans-dioxo geometry, and the octahedra are remarkably regular. Protonation of [ReO2L4]I was studied in methanol
and in water. Acid dissociation constants were determined in aqueous solution for the [ReO(OH)L4]2+ (Ka ) and
1
[ReO(OH2)L4]3+ (Ka ) species of 1-MeIm (pKa ) 2.0, pKa ) ∼-4.0) and 1,2-Me2Im (pKa ) ∼3.8, pKa
)
2
1
2
1
2
-
∼-4.1). Iodide salts of [ReO(OH)L4]2+ were not stable enough to be isolated, but the B(C6H5)4 salt of
[ReO(OH)(1,2-Me2Im)4]2+ was obtained in pure form. The crystal structure of [ReO(OH)(1,2-Me2Im)4](I3)0.5
-
(ReO4)1.5 (monoclinic, I2/a, a ) 23.290(8) Å, b ) 10.863(7) Å, c ) 25.709(8) Å, â ) 94.36(3)°, Z ) 8, R )
0.060) showed the presence of octahedral dications in which the oxo and hydroxo groups are trans to one another
and the Re-N bonds are displaced to the Re-OH side by the steric effect of the multiple Re-oxo bond. The
σ-donor ability of imidazole, greater than that of pyridine, leads to more stable [ReO2L4]+ compounds in which
the Re)O double bond character is lower and the oxo groups are better nucleophiles.
Introduction
A decade ago, interesting spectroscopic properties were
properties comparable with those of the pyridine analogue,1 but
the emitting state was found to lie at higher energy and coupling
of metal-ligand vibrations was detected in the vibronic
structure. Efforts are being devoted to relate these peculiarities
to structural and other features of these compounds, and for
this reason, it appeared to be desirable to devise routes to pure
and well-characterized salts. This paper describes the prepara-
tion of several trans-[ReO2L4]+ salts with imidazole (ImH) and
its 1-methyl (1-MeIm), 2-methyl (2-MeImH), 4-methyl (4(5)-
MeImH),9 and 1,2-dimethyl (1,2-Me2Im) derivatives. The
protonation of these species is also examined.
1
observed for [ReO2L4]+ complexes (L ) py or
/ en) by
2
Winkler and Gray,1 who discussed promising applications for
such systems in photophysics. In the meantime, several studies
have been devoted to the photochemistry and electrochemistry
of these systems.2-4 Recently, related compounds attracted
attention in connection with the utilization of the 186Re and
188Re nuclei for diagnostic purposes in radiopharmacy.5
The first report on trans-dioxo compounds of rhenium(V)
with imidazoles appeared only recently,6 despite the fact that
Tc analogues had been known for some time.7 Complexes were
described for a few imidazole ligands, but the chloride salts
isolated proved to be unstable: partial decomposition to
perrhenate was frequent, and mixed [ReO2L4]+/[ReO(OH)L4]2+
phases were isolated. In a preliminary spectroscopic study,8
[ReO2(1-MeIm)4]+ exhibited absorption and luminescence
X Abstract published in AdVance ACS Abstracts, November 15, 1996.
(1) Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 1983, 105, 1373.
Winkler, J. R.; Gray, H. B. Inorg. Chem. 1985, 24, 346.
(2) Ram, M. S.; Jones, L. M.; Ward, H. J.; Wong, Y. H.; Johnson, C. S.;
Subramanian, P.; Hupp, J. T. Inorg. Chem. 1991, 30, 2928.
(3) Pipes, D. W.; Meyer, T. J. Inorg. Chem. 1986, 25, 3256.
(4) Ram, M. S.; Johnson, C. S.; Blackbourn, R. L.; Hupp, J. T. Inorg.
Chem. 1990, 29, 238. Liu, W.; Welch, T. W.; Thorp, H. H. Inorg.
Chem. 1992, 31, 4044.
(5) Technetium and Rhenium in Chemistry and Nuclear Medicine;
Nicolini, M., Bandoli, G., Mazzi, U., Eds.; S. G. Editoriali: Padova,
Italy 1995.
(6) Lebuis, A. M.; Young, J. M. C.; Beauchamp, A. L. Can. J. Chem.
1993, 71, 2070.
Experimental Section
Materials. Reactants were used as received from Aldrich (KReO4,
PPh3, imidazoles, NH4PF6), Fisher (NaBPh4), Johnson Matthey (HBF4),
or Anachemia (HI). Spectrograde acetone and methanol (American
Chemicals or ACP Chemicals) and all other solvents (reagent grade)
were used without further purification. Deuterated solvents were from
Cambridge Isotope Laboratory or Aldrich.
Physical Measurements. Solution H and 13C NMR spectra were
1
recorded on a Varian VXR 300 MHz spectrometer. 1H spectra were
referenced to the residual solvent signal (D2O, δ 4.80; CD3OD, δ 3.30;
(7) Fackler, P. H.; Lindsay, M. J.; Clarke, M. J.; Kastner, M. E. Inorg.
Chim. Acta 1985, 109, 39.
(8) Savoie, C.; Reber, C.; Be´langer, S.; Beauchamp, A. L. Inorg. Chem.
1995, 34, 3851.
(9) Conventional ring numbering starting with the N-H(R) position is
used. The two tautomeric forms are designated as 4-MeImH (4-methyl-
1H-imidazole) and 5-MeImH (5-methyl-1H-imidazole), respectively.
The equilibrium mixture is represented by 4(5)-MeImH.
S0020-1669(96)00451-X CCC: $12.00 © 1996 American Chemical Society

Reaction of ReO2I(PPh3)2 with Imidazoles
Inorganic Chemistry, Vol. 35, No. 26, 1996 7837
dimethyl-d6 sulfoxide (DMSO-d6), δ 2.49 ppm) and converted to the
conventional Me4Si scale. 13C spectra were recorded on the same
instrument at 75.431 MHz. For solution spectra, dioxane was added
to the D2O samples as internal standard (δ 67.4 ppm). The CP-MAS
spectra were referenced to the aromatic carbons of hexamethylbenzene
(δ 132.1 ppm). Infrared spectra were recorded as CsI or KBr pellets
on a Perkin-Elmer 783 spectrophotometer. The Raman spectra were
recorded on a Jobin Yvon/ISA Mole S-3000 spectrophotometer,
equipped with Spectra Physics Stabilite 2016 argon ion (514.5 nm)
and Lexel 3500 krypton (647.1 nm) lasers. Electronic absorption
spectra were obtained using a Varian/CARY-5E UV-vis-near-IR
spectrophotometer. Elemental analyses were performed by Guelph
Chemical Labs.
Preparation of [ReO2L4]I. The purple starting material ReO2I-
(PPh3)2 was prepared by the literature method10 using KReO4 instead
of HReO4, as described by Brewer and Gray.11 We found ReO2I(PPh3)2
to be moderately sensitive to O2, even in the solid state, decomposing
to the dimer [ReOI2(PPh3)2(OReO3)], as evidenced from the unit cell
and 31P NMR spectrum being identical with those obtained from an
authentic sample.12,13 The Re(V) starting material was therefore stored
under argon, and all further manipulations involving this compound
were carried out under controlled atmosphere. The [ReO2L4]I salts
were obtained by reacting ReO2I(PPh3)2 with excess of the imidazole
(L) in methanol.
When excess ligand was used, the yield was improved, but the raw
product contained an appreciable amount of free imidazole identified
by NMR. Most of the free ligand could be removed by stirring in
CHCl3, filtering, and washing with CH2Cl2.
Preparation of [ReO2L4][B(C6H5)4]. The salts were obtained by
adding Na[B(C6H5)4] to the iodide generated in situ or isolated as
described above. They are virtually insoluble in water, but somewhat
soluble in methanol and acetone.
[ReO2(2-MeImH)4][B(C6H5)4]‚2H2O. 2-MeImH (0.367 g, 4.470
mmol) was added to a suspension of ReO2I(PPh3)2 (0.966 g, 1.11 mmol)
in methanol (30 mL). The reaction mixture was heated for 35 min,
after which time the greenish brown solution was allowed to cool.
Excess toluene was added, and the solvent was evaporated until a dark
brown oil (1 mL) was obtained. The iodide was not isolated as a solid,
but an NMR spectrum run on the oily sample confirmed the presence
of [ReO2(2-MeImH)4]+. 1H NMR (D2O; ppm): δ 7.07 (d, H5), 6.78
(d, H4), 2.20 (s, NsCH3), 2J(H4-H5) ) 1.8 Hz. The oil was diluted
in methanol (5 mL) and Na[B(C6H5)4] (0.406 g, 1.186 mmol) was added
to the solution. The solvent was allowed to evaporate slowly in open
air, and the complex was isolated as a mustard yellow powder; yield,
0.954 g (95%). IR (CsI; cm-1): 765 (sh) or 775 (vs) νas(OdRedO)
(interference from the [B(C6H5)4]- band). Raman (cm-1): 924
νs(OdRedO). UV-vis (CH3OH; λ, nm (ꢀ, M-1 cm-1)): 207 (8.7 ×
105, this absorption band includes a strong [B(C6H5)4]- component),
467 (2.1 × 102). 1H NMR (CD3OD; ppm): δ 2-MeImH, 7.10 (d, H5),
6.79 (d, H4), 2.27 (s, NsCH3); [B(C6H5)4]-, 7.28 (m, 2H), 6.95 (t,
2H), 6.81 (t, 1H). Anal. Calcd for C40H48BN8O4Re: C, 53.27; H, 5.36;
B, 1.19; N, 12.42. Found: C, 53.04; H, 5.24; B, 1.19; N, 12.42.
Crystals suitable for X-ray diffraction were grown in methanol by slow
evaporation. The recrystallized sample has the formula [ReO2(2-
MeImH)4][B(C6H5)4]‚3CH3OH.
[ReO2(1,2-Me2Im)4]I. ReO2I(PPh3)2 (1.17 g, 1.35 mmol) and 1,2-
Me2Im (2.64 g, 27.5 mmol) were stirred in methanol (10 mL). A light
brown solution was obtained within 2 min and the mixture was refluxed
for 20 min. After the dark brown solution was cooled to room
temperature, toluene (10 mL) was added and the solvent was evaporated
until the mother liquor lost its brown color. The mustard yellow powder
was filtered and washed with toluene and diethyl ether; yield, 0.96 g
(98%). IR (CsI, cm-1): 790 (vs) νas(OdRedO). Raman (cm-1): 907
νs(OdRedO). UV-vis (CH3OH; λ, nm (ꢀ, M-1 cm-1)): 211 (3.2 ×
104), 255 (6.2 × 103), 467 (3.5 × 102). UV-vis (H2O; λ, nm (ꢀ, M-1
cm-1)): 194 (5.4 × 104), ∼223 (sh) (∼2.5 × 104), ∼255 (sh) (∼5 ×
103), 480 (3.2 × 102). 1H NMR (D2O, ppm): δ 7.10 (d, H5), 6.78 (d,
[ReO2(1-MeIm)4][B(C6H5)4]. [ReO2(1-MeIm)4]I (0.232 g, 0.344
mmol) was dissolved in methanol (20 mL). Na[B(C6H5)4] was added
in excess (0.297 g, 0.868 mmol) as a concentrated methanol solution.
The solvent was allowed to partly evaporate, and a homogeneous brick-
red crystalline solid precipitated; yield, 0.240 g (80%). Composition
was determined from the X-ray diffraction work described below. IR
(KBr; cm-1): 794 (vs) νas(OdRedO). 1H NMR (CD3OD; ppm): δ
1-MeIm, 7.73 (s, H2), 7.11 (d) and 7.10 (d) (H5/H4), 3.71 (s, NsCH3)
3
H4), 3.74 (s, NsCH3), 2.28 (s, CsCH3), J(H4sH5) ) 1.7 Hz. 13C
CP-MAS (ppm): δ 146.5 (C2), 130.6 (C4), 122.4 (C5), 33.3, and 32.0
(NsCH3), 12.1 (CsCH3). Anal. Calcd for C20H32IN8O2Re: C, 32.92;
H, 4.42; I, 17.39; N, 15.36. Found: C, 32.86; H, 4.36; I, 17.15; N,
15.00.
4
4
(3J(H4-H5) ∼ J(H2-H4) ∼ J(H2-H5) ∼ 1.3 Hz); [B(C6H5)4]-, 7.26
(m, 2H), 6.93 (t, 2H), 6.80 (t, 1H).
[ReO2(1-MeIm)4]I‚2H2O. As above. IR (KBr, cm-1): 793 (vs)
νas(OdRedO). Raman (cm-1): 905 νs(OdRedO). UV-vis (CH3-
OH, nm): 211 (2.9 × 105), 265 (9.2 × 103), 473 (2.0 × 102). UV-
vis (H2O; λ, nm (ꢀ, M-1 cm-1)): 193 (6.3 × 104), 221 (2.5 × 104),
255 (9.2 × 103). 1H NMR (D2O; ppm): δ 7.65 (s, br, H2), 7.21 (t,
H5), 7.00 (t, H4), 3.80 (s, NsCH3), 3J(H4sH5) ∼ 4J(H2sH4) ∼
4J(H2sH5) ∼ 1.5 Hz. 13C NMR (D2O; ppm): δ 140.0 (C2), 129.5
(C4), 123.7 (C5), 35.0 (NsCH3). 13C CP-MAS (ppm; two non-
equivalent ligands detected in the solid): δ 137.5, 135.9 (C2); 130.6,
128.9 (C4); 126.3, 122.3 (C5); 35.4, 34.2 (NsCH3). Anal. Calcd for
C16H28IN8O4Re: C, 27.08; H, 3.98; I, 17.89; N, 15.79. Found: C,
27.35; H, 3.70; I, 17.73; N, 16.00.
[ReO2(5-MeImH)4][B(C6H5)4]. Conversion to the tetraphenylborate
salt was effected in methanol, as described for [ReO2(1-MeIm)4]-
[B(C6H5)4], using 3 equiv of Na[B(C6H5)4]. The crystals that grew
out of the solution within 2 days were used for the crystal structure
study; yield, 64%. IR (CsI; cm-1): 775 (vs, br) νas(OdRedO). 1H
NMR (CD3OD; ppm): δ 5-MeImH, 7.59 (s, br, H2), ∼6.84 (H4,
interference with anion signal), 2.20 (s, br, C-CH3); [B(C6H5)4]-, 7.26
(m, 2H), 6.94 (t, 2H), 6.83 (t, 1H). Anal. Calcd for C40H44BN8O2Re:
C, 55.49; H, 5.12; N, 12.94. Found: C, 55.42; H, 5.09; N, 12.76.
[ReO(OH)L4]2+ Iodides. In a typical run aimed at preparing [ReO-
(OH)(ImH)4](I)2, a methanol solution (0.22 M) of HI (4.5 mL, 0.99
mmol) was added to a suspension of [ReO2(ImH)4]I (0.140 g, 0.227
mmol) in methanol (20 mL). The reaction mixture immediately took
a purple color, and all of the solid eventually dissolved. After the
mixture was stirred for 2 min, the solvent was evaporated to a residual
volume of 1 mL, without formation of a precipitate. The purple
complex was precipitated by adding excess diethyl ether, filtered and
washed with diethyl ether. The presence of [ReO(OH)(ImH)4]2+ in
the sample is deduced from the strong IR band at 957 cm-1 for
ν(RedO), whereas the strong νas(OdRedO) band of [ReO2(ImH)4]+
at ∼780 cm-1 is absent. The species remains [ReO(OH)(ImH)4]2+ in
[ReO2(ImH)4]I‚2.5CH3OH. ReO2I(PPh3)2 (2.39 g, 2.74 mmol) and
imidazole (0.073 g, 11.7 mmol) were heated in methanol (25 mL) for
10 min. The brown solution was cooled to room temperature, and
toluene (40 mL) was then added. The solution was evaporated, and
the brick red oily residue was dried by pumping in vacuum. The sample
was recrystallized in methanol; yield, 0.75 g (38%). IR (KBr; cm-1):
780 (vs) νas(OdRedO). UV-vis (CH3OH; λ, nm (ꢀ, M-1 cm-1)): 221
(2.2 × 105), 258 (7.5 × 103), 474 (2.2 × 102). 1H NMR (D2O; ppm):
4
δ 7.77 (t, H2), 7.29 (t, H5), 7.07 (t, H4), 3J(H4-H5) ∼ J(H2-H4) ∼
4J(H2-H5) ∼ 1.3 Hz. 13C NMR (D2O; ppm): δ 139.7 (C2), 128.8
(C4), 119.4 (C5). Anal. Calcd for C14.5H26IN8O6.5Re: C, 23.87; H,
3.59; I, 17.40; N, 15.36. Found: C, 24.00; H, 3.86; I, 17.53; N, 15.50.
1
DMSO, as evidenced from the violet color of the solution. The H
NMR resonances (DMSO-d6) at relatively high field (7.95 (H2), 7.46
(H5), and 6.96 ppm (H4)) are consistent with the higher positive charge
on the complex. From the obtained yield (0.33 g), it is clear that the
product is not [ReO(OH)(ImH)4](I)2, but it could correspond to
essentially quantitative conversion to [ReO(OH)(ImH)4](I3)2 or a mixed
I3-/I- salt.
[ReO(OH)(1-MeIm)4](I)2 behaved similarly, giving a ν(RedO) IR
band at 955 cm-1 and 1H NMR signals (DMSO-d6) at 8.01 (H2), 7.50
(H5), 6.94 (H4), 3.86 (NsCH3) ppm. [ReO(OH)(1,2-Me2Im)4]2+ is
(10) Ciani, G. F.; D’Alfonso, G.; Romiti, P. F.; Sironi, A.; Freni, M. Inorg.
Chim. Acta 1983, 72, 29.
(11) Brewer, J. C.; Gray, H. B. Inorg. Chem. 1989, 28, 3334.
(12) Ciani, G.; Sironi, A.; Beringhelli, T.; D’Alfonso, G.; Freni, M. Inorg.
Chim. Acta 1986, 113, 61.
(13) Freni, M.; Giusto, D.; Romiti, P.; Minghetti, G. Gazz. Chim. Ital. 1969,
99, 286.

7838 Inorganic Chemistry, Vol. 35, No. 26, 1996
Table 1. Crystallographic Data for Structures I-IV
Be´langer and Beauchamp
[ReO2(2-MeImH)4][B(C6H5)4]‚
3CH3OH (I)
[ReO2(1-MeIm)4][B(C6H5)4]‚
H2O‚0.5CH3OH (II)
[ReO2(5-MeImH)4]-
[B(C6H5)4] (III)
[ReO(OH)(1,2-Me2Im)4]-
(I3)0.5(ReO4)1.5 (IV)
mol formula
fw
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C43H56BN8O5Re
961.99
C40.5H48BN8O3.5Re
899.90
15.619(2)
9.486(2)
27.387(4)
90
97.09(1)
90
4026.7(12)
4
C40H44BN8O2Re
865.86
10.199(2)
13.441(5)
28.798(10)
90
90
90
3948(2)
4
C20H33I1.5N8O8Re2.5
1169.42
23.290(8)
10.863(7)
25.709(8)
90
94.36(3)
90
6486(5)
8
10.696(3)
15.128(4)
15.497(4)
113.57(2)
97.25(2)
95.94(2)
2247.1(1)
2
Z
space group
temp, °C
λ, Å
P1h (No. 2)
P21/n (No. 4)
-58
P212121 (No. 19)
-98
I2/a (No. 15)
-58
20
0.709 30 (Mo KR)
1.422
27.9
0.50-0.72
0.030
1.541 78 (Cu KR)
1.484
1.541 78 (Cu KR)
1.457
1.541 78 (Cu KR)
2.395
F
calcd, g cm-3
µ, cm-1
transm coeff
Ra
60.6
0.14-0.34
0.057
61.3
0.46-0.68
0.043
290.4
0.08-0.36
0.060
a
Rw
0.029
0.062
0.046
0.060
2
a R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
very sensitive to ligand loss, and an iodide salt could not be isolated,
although a stable salt was obtained when Na[B(C6H5)4] was added early
to the mixture, before the imidazolium cation began to appear (see
below).
stable [ReO2(1,2-Me2Im)4]I complex could be studied at room tem-
perature over the entire pH range. In both cases, decomposition to an
insoluble black material occurred at acidities above pH -4.
Crystallographic Measurements and Structure Determination.
X-ray work for all compounds was carried out on an Enraf-Nonius
CAD-4 diffractometer using graphite-monochromatized copper radia-
tion. The reduced triclinic cell was obtained from 15-25 reflections
found on a rotation photograph. Conversion to the conventional cell,
when needed, was then made in accordance with the Niggli matrix.
The cell parameters given in Table 1 were calculated from 25 higher-
angle reflections. Laue symmetry was eventually checked by compar-
ing intensities between the various octants. Systematic absences were
determined by inspection of the full data set.
[ReO(OH)L4]2+ Tetraphenylborates. [ReO(OH)(1,2-Me2Im)4]-
[B(C6H5)4]2 was isolated when [ReO2(1,2-Me2Im)4]I (1.24 g, 1.71
mmol) was dissolved in methanol (20 mL) and protonated by addition
of a methanol solution of HBF4 (3.4 mL, 2.51 mmol). The protonated
complex was immediately precipitated by adding Na[B(C6H5)4] (1.29
g, 3.77 mmol) as a concentrated methanol solution. The purple solid
was filtered and washed with ethanol until the filtrate lost its yellow
color; yield, 1.515 g (70%). IR (CsI; cm-1): 962 (m, br) ν(RedO).
13C CP/MAS (ppm): δ 1,2-Me2Im, 148.1 (C2), 128.8 (C4), 122.7 (C5),
33.8 (NsCH3), 11.0 (CsCH3); B(C6H5)4-, 164.2, 136.0, 125.7. Anal.
Calcd for C68H78B2N8O2Re: C, 65.75; H, 5.92; B, 1.74; N, 9.02.
Found: C, 65.22; H, 5.81; B, 1.66: N, 9.03.
In all cases, intensity data (2θmax ) 140°) were collected over more
than the minimum number of octants required by Laue symmetry. The
orientation was monitored every 200 measurements, and the intensity
was checked every 60 min with four to six standard reflections. Data
were corrected for the effects of Lorentz, polarization, and absorption15
(Gaussian integration, 10 × 10 × 10 grid).
The same method applied to [ReO2(1-MeIm)4]+ yielded a solid
whose microanalysis corresponds to [ReO(OH)(1-MeIm)4][B(C6H5)4]4/3
(BF4)2/3. IR (CsI; cm-1): 973 (m) ν(RedO). 13C CP/MAS (ppm): δ
1-MeIm, 137.0 (C2), 126.7 (C4), 123.6 (C5), 33.1 (NsCH3); B(C6H5)4-
-
,
Structure determination for [ReO(OH)(1,2-Me2Im)4](I3)0.5(ReO4)1.5
raised particular problems, which are discussed below. The three other
structures were solved by the heavy-atom method. The Re atoms were
located from a Patterson map. All other non-hydrogen atoms were
located by successive ∆F maps. Refinement was anisotropic for all
non-hydrogen atoms. Non-methyl hydrogen atoms were calculated at
idealized positions (0.95 and 0.85 Å for CsH and NsH, respectively).
The thermal parameters B(H) were fixed at [B(non-H) + 1] Å2. One
hydrogen of each methyl group was located from the ∆F map, the
others were placed at idealized positions. Hydrogen parameters were
not refined, but their positions were recalculated after each cycle.
Details specific to each structure are given below. Refinement was
done using NRCVAX15 and SHELX-76.16 In the latter case, the atomic
scattering factors and anomalous dispersion terms (Re, I) were taken
from standard sources.17-19 Full sets of atom parameters are provided
in the supporting information.
166.9, 141.2, ∼ 127 (interference from ligand). Anal. Calcd for
C48H51.7B2N8O2ReF2.7: C, 55.92; H, 5.05; B, 2.10; N, 10.87. Found:
C, 56.08; H, 5.14; B, 1.85; N, 10.89. It is unclear whether this sample
-
-
is a mixture of the BF4 and B(C6H5)4 salts or a single crystalline
-
-
phase with partial BF4 substitution like the I3-/ReO4 salt described
in this paper.
Protonation of [ReO2L4]+ Solutions. Protonation of [ReO2(1-
MeIm)4]I was followed spectrophotometrically in methanol by recording
spectra (200-350 nm) of ∼10-5 M solutions at room temperature as
a function of pH. Under these conditions, equilibrium H+ concentra-
tions are essentially equal to added HBF4.
Protonation of ∼10-3 M solutions of [ReO2(1-MeIm)4]I and [ReO2(1,2-
Me2Im)4]I in aqueous solutions was monitored from the weaker d-d
spectral region (300-900 nm). Data were obtained in the +7 to -4
pH range for the 1-MeIm complex and in the +8 to -8 pH range for
the 1,2-Me2Im complex. For HCl concentrations < 0.1 M, pH was
measured using a combined glass/calomel electrode calibrated just
before use with standard buffers (pH 4 and 7). Acidities for higher
concentrations of HCl and H2SO4 were expressed in terms of Hammett’s
acidity function |Ho|.14 Each data point was collected on a fresh
solution, and absorbance was measured 2-3 min after dissolution.
Measurements at intermediate acidities for the determination of Ka1 of
the 1-MeIm complex were obtained at 3 °C using an ice-water-cooled
cell. Since temperature had no effect on the more acidic solutions,
the data to estimate Ka2 were obtained at room temperature. The more
[ReO2(2-MeImH)4][B(C6H5)4]‚3CH3OH (I). A thick yellow plate
(0.13 × 0.38 × 0.49 mm3) was used for the X-ray diffraction study.
The whole sphere (15774 intensity measurements) was averaged to 7887
unique hkl, hkhl, hhkl, and hhkhl reflections (Rint ) 0.026), of which 6127
with I/σ(I) g 3 were used for structure determination. The structure
(15) Gabe, E. J.; Le Page, Y.; Charland, J. P.; Lee, F. L. J. Appl. Crystallogr.
1989, 22, 1021.
(16) Sheldrick, G. M. SHELX-76, Crystallographic Program System;
University of Cambridge: Cambridge, England, 1976.
(17) Cromer, D. T.; Libermann, D. J. Chem. Phys. 1970, 53, 1891.
(18) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J. Chem. Phys. 1965,
42, 3175.
(14) Rochester, C. H. Organic Chemistry: A Series of Monograms;
Academic Press: London, U.K., 1970; Vol. 17, p 38.
(19) Cromer, D. T. Acta Crystallogr. 1965, 18, 17.

Reaction of ReO2I(PPh3)2 with Imidazoles
Inorganic Chemistry, Vol. 35, No. 26, 1996 7839
solved and refined well in the centric P1h space group. No disorder
was detected. The hydroxyl hydrogens of the lattice methanol
molecules were located from the ∆F map, and their positions and
temperature factors were refined. A few electron-density peaks in the
range -0.63 to +0.42 e/Å3 remained near the Re atom in the final ∆F
map.
[ReO2(1-MeIm)4][B(C6H5)4]‚H2O‚0.5CH3OH (II). The crystal
used for the X-ray work was a brick-red prism (0.24 × 0.34 × 0.60
mm3). A total of 15 267 reflections (hkl, hkhl, hhkl, hhkhl) were averaged
to 7639 unique hkl and hkhl reflections (Rint ) 0.055), of which 6446
with I/σ(I) g 3 were used for structure determination. Space group
P21/n was uniquely determined from Laue symmetry and systematic
absences. The non-hydrogen atoms in [ReO2(1-MeIm)4]+ and
[B(C6H5)4]- were well-ordered, but disorder was found to be present
in two different regions containing lattice solvent molecules.20 Residual
peaks within (0.60 e/Å3 remained in the final ∆F map near the metal
center.
[ReO2(5-MeImH)4][B(C6H5)4] (III). A brick-red prism of dimen-
sions 0.14 × 0.19 × 0.26 mm3 was used for the data collection. A set
of 14 440 hkhl, hkhhl, hhkhl, and hhkhhl reflections was measured. Out of the
7483 hkl and hhkhhl obtained after averaging (Rint ) 0.056), 6336 with
I/σ(I) g 3 were used in the structural study. The space group was
defined uniquely from Laue symmetry and systematic absences. No
evidence of disorder, such as that resulting from imidazole binding as
the 4-methyl, instead of the 5-methyl, tautomer, was detected. The
absolute configuration was determined using the BIVOET routine of
the NRCVAX program.21 Residual peaks within (1.3 e/Å3 remained
near the metal center.
[ReO(OH)(1,2-Me2Im)4](I3)0.5(ReO4)1.5 (IV). These crystals were
obtained as a side product during attempts to prepare ReO(OEt)I2(1,2-
Me2Im)2 as described for pyridine analogues.22 Concentrated HBF4
(2.5 mL, 13.8 mmol) was added to a suspension of [ReO2(1,2-Me2-
Im)4]I‚2CH3OH (1.88 g, 2.60 mmol) in ethanol (100 mL), and the purple
solution was refluxed for 90 min. The very dark blue solution was
cooled to room temperature, and a black solid was filtered and washed
with diethyl ether. By recrystallization from ethanol, a small amount
of royal-blue diamond-shaped crystals suitable for X-ray diffraction
were obtained. This complex is clearly a minor component of the black
mixture.
Nevertheless, the mixture was heated for a short time to ensure
complete reaction. To obtain samples with better crystallinity,
attempts were made to replace the counterion by adding NH4-
PF6 or NH4BF4 to [ReO2L4]I in methanol. However, the
+
samples were contaminated by the NH4 salt when a large
excess was added, or only partial replacement occurred when a
stoichiometric amount or a small excess was used. A typical
example is [ReO2(5-MeImH)4](PF6)0.94(I)0.06, whose composition
was deduced from an X-ray crystallographic study.23 However,
-
clean conversion to less soluble B(C6H5)4 salts could be
achieved.
Spectroscopic Data of trans-Dioxo Complexes. The
OdRedO unit gives rise to strong characteristic features in both
infrared and Raman spectra. For all of our [ReO2L4]X com-
pounds with imidazoles and the chloride salts described earlier,6
the νas(OdRedO) band is found between 765 and 794 cm-1
.
This is comparable with the frequencies reported for
[ReO2(CN)4]3- 24 and trans-ReO2N4 cores with cyclam25-28 and
azaindole.29 This vibration tends to appear at 810-825 cm-1
for complexes with pyridines and aliphatic amines.30-33 The
Raman active νs(OdRedO) vibration was found in the 900-
925 cm-1 range, in agreement with the value (905 cm-1 8
)
calculated from the luminescence spectrum of [ReO2(1-
MeIm)4]+. For [ReO2py4]+, a frequency of 916 cm-1 was
observed in acetonitrile solution34 and calculated from lumi-
nescence spectra.1
The electronic spectra of the trans-dioxo imidazole complexes
exhibit three major components. The high-energy region below
225 nm includes imidazole π f π* transitions.35 The absorp-
tion near 260 nm (ꢀ ) 6000-10 000 M-1 cm-1) probably has
the same origin as the 331 nm band (ꢀ ∼ 25 000 M-1 cm-1) of
the corresponding pyridine complex, which has been attributed
to a [dxy f πL*] MLCT transition.11 Finally, the third absorption
found in the visible region (467 nm) is due to a ligand field
1
1A1g[(b2g)2] f Eg[(b2g)1(eg)1] transition. It was found at 445
A small crystal (0.058 × 0.058 × 0.076 mm3) was used for X-ray
work. An entire sphere (22 974 reflections) was collected, which was
averaged to 6155 unique hkl and hkhl reflections. The standards decayed
by 16% during data collection, and scaling did not perfectly correct
for decomposition, as indicated by the relatively high Rint value (0.147).
The data with I/σ(I) g 3 totaled 2509 reflections. Space groups I2/a
and Ia were consistent with Laue symmetry and systematic absences
(hkl, h + k + l * 2n; h0l, h, l * 2n; 0k0, k * 2n). The structure was
solved and refined in the centric I2/a space group. The unexpected
nm in the pyridine complex.
The NMR data given in the experimental section follow the
trend discussed in a previous study.6 The 13C chemical shifts
as assigned in the C2 > C4 > C5 order as noted by other
(23) [ReO2(5-MeImH)4](PF6)0.94(I)0.06 was obtained from metathesis of the
corresponding iodide with 2 equiv of NH4PF6 in methanol. Dark red
crystals formed after several days. The orthorhombic unit cell (P21212,
a ) 12.225(5) Å, b ) 20.617(6) Å, c ) 11.031(6) Å, Z ) 4) contains
[ReO2L4]+ in which all ligands exist as the 5-MeImH tautomer. The
counterions occupy two sets of 2-fold special positions, in one of which
-
presence of ReO4 and disorder made the Patterson map difficult to
unravel, but some of the Re and I atoms could be located. The
remaining non-hydrogen atoms were found from ∆F maps. The anions
PF6 is replaced 12% of the time by I-. The two types of PF6 are
disordered, most of the F atoms being distributed over two or three
positions. The structure could not be refined below R ) 0.07, and the
more reliable crystallographic study on the BPh4- salt is reported here.
(24) Howard-Lock, H. E.; Lock, C. J. L.; Turner, G. Spectrochim. Acta
1982, 38A, 1283.
-
-
providing the -2 charge should actually be represented as (I3)0.5
-
(ReO4)0.5(ReO4). The triiodide anions are not disordered, but both types
-
of ReO4 are.20 Refinement was anisotropic for Re and I atoms and
isotropic for C, N, and O atoms. Hydrogen atoms were treated as they
were in the previous structures. Residual electron density within (0.78
e/Å3 was located near Re and I atoms.
(25) Winckler Tsang, B.; Reibenspies, J.; Martell, A. E. Inorg. Chem. 1993,
32, 988.
(26) Luna, S. A.; Bolzati, C.; Duatti, A.; Zucchini, G. L.; Bandoli, G.;
Refosco, F. Inorg. Chem. 1992, 31, 2595.
Results
(27) Blake, A. J.; Greig, J. A.; Schro¨der, M. J. Chem. Soc., Dalton Trans.
1988, 2645.
(28) Wang, Y. P.; Che, C. M.; Wong, K. Y.; Peng, S. M. Inorg. Chem.
1993, 32, 5827.
(29) Lebuis, A. M.; Beauchamp, A. L. Can. J. Chem. 1993, 71, 2060.
(30) Jezowska-Trzebiatowska, B.; Hanuza, J.; Baluka, M. Spectrochim. Acta
1971, 27A, 1753.
(31) Johnson, N. P.; Lock, C. J. L.; Wilkinson, G. J. Chem. Soc. 1964,
1054.
trans-Dioxo compounds [ReO2L4]I of imidazoles were pre-
pared by adapting the method developed by Brewer and Gray11
for [ReO2py4]I. Yields generally >70% were obtained by
reacting ReO2I(PPh3)2 with 4-20 equiv of the ligand in
methanol. The more basic imidazoles reacted much faster than
pyridines. Almost instantaneous change in color indicated that
the reaction had taken place quickly at room temperature.
(32) Johnson, J. W.; Brody, J. F.; Ansell, G. B.; Zentz, S. Inorg. Chem.
1984, 23, 2415.
(20) A full description of the disorder is available in the Supporting
Information.
(21) Hynes, R. C.; Le Page, Y. J. Appl. Crystallogr. 1991, 24, 352.
(22) Ram, M. S.; Hupp, J. T. Inorg. Chem. 1991, 30, 130.
(33) Beard, J. H.; Casey, J.; Murmann, R. K. Inorg. Chem. 1965, 4, 797.
(34) Thorp, H. H.; Van Houten, J.; Gray, H. B. Inorg. Chem. 1989, 28,
889.
(35) Grebow, P. E.; Hooker, T. H., Jr. Biopolymers 1975, 14, 871.

7840 Inorganic Chemistry, Vol. 35, No. 26, 1996
Be´langer and Beauchamp
Figure 1. ORTEP drawings showing numbering schemes for (a) [ReO2(2-MeImH)4]+ (I), (b) [ReO2(1-MeIm)4]+ (II), (c) [ReO2(5-MeImH)4]+
(III), and (d) [ReO(OH)(1,2-Me2Im)4]2+ (IV). Thermal ellipsoids drawn at the 50% probability level. H atoms omitted for simplicity.
workers.36,37 C4/C5 averaging due to fast tautomeric exchange
in non-N-methylated ligands is removed by complexation.
Most of our 1H NMR spectra were run in D2O. The lowest-
field signal is found for H2, when present, at 7.6-7.8 ppm,
whereas H4 and H5 occur in the 6.8-7.3 ppm range, H5
appearing ∼0.2 ppm downfield from H4. However, the spectra
are solvent sensitive. For related [TcO2L4]+ in CD3OD,7 H2
was found at lower field and the H4-H5 separation was much
smaller, an unresolved H4/H5 signal being observed for the
1-MeIm complex. This is consistent with our observations for
[ReO2(1-MeIm)4]+: the 7.65 (H2), 6.99 (H4), and 7.20 (H5)
ppm signals for the iodide salt in D2O are displaced to 7.73
shows, in addition to the highly predominant set of signals, a
series of very weak peaks that could be due to such isomers.
However, since the homogeneous crystalline material used for
X-ray diffraction was shown to contain only [ReO2(5-Me-
ImH)4]+ and this isomer was the only one detected in the solid
state for [Co(NH3)5(5-MeImH)]Cl3‚2H2O36 and [Cu(IDA)(5-
MeImH)] (IDA ) iminodiacetate dianion),38 the strong signals
can confidently be assigned to the isomer containing only the
1
remote 5-MeImH tautomer. The H spectrum of the CD3OD
solution (7.59 (H2), ∼6.84 (H4), and 2.20 (CH3) ppm) being
similar to the one reported for the Tc analogue (7.76, 6.88 and
2.17 ppm), the latter species should also be formulated as the
[TcO2(5-MeImH)4]+ isomer, rather than [TcO2(4-MeImH)4]+
as proposed.7
Crystal Structures of the trans-Dioxo Complexes. The
structures of [ReO2(2-MeImH)4][B(C6H5)4]‚3CH3OH (I), [ReO2-
(1-MeIm)4][B(C6H5)4]‚H2O‚0.5CH3OH (II), and [ReO2(5-
MeImH)4][B(C6H5)4] (III) were determined. ORTEP drawings
of the three [ReO2L4]+ ions are shown in Figure 1. Selected
distances and angles are listed in Table 2. The RedO distances
in the 2-MeImH and 5-MeImH compounds lie in the 1.761-
1.769 Å range typical of such complexes.6,27-29,32,39-41 The
-
(H2) and 7.10/7.11 (H4/H5) ppm, respectively, for the B(C6H5)4
salt in methanol. For [TcO2(1-MeIm)4]+, H4 was assigned
downfield from H5: the order of these resonances is not likely
to be changed by solvent effect, and these assignments should
be interchanged.
In all cases, various rotamers could exist since a 2-fold
rotation of the ligand about the Re-N bond generates two non-
equivalent configurations (see crystallographic work below).
However, NMR provides no evidence for multiple rotamers in
solution, either because one rotamer is favored or, more likely,
because all ligands become equivalent by fast interconversion.
The situation is more complex with 4(5)-MeIm, which can
(38) Campos, A. C.; Busnot, A.; Garc´ıa, M. E. A.; Zafra, A. G. S.; Pe´rez,
J. M. G.; Gutie´rrez, J. N. Inorg. Chim. Acta. 1994, 215, 73.
coordinate as the 4-MeIm or 5-MeIm tautomers and produce
+
(39) Lock, C. J. L.; Turner, G. Acta Crystallogr. 1978, B34, 923.
(40) Murmann, R. K.; Schlemper, E. O. Inorg. Chem. 1971, 10, 2352.
(41) Glowiak, T.; Kubiak, M.; Jezowska-Trzebiatowska, B. Bull. Acad. Pol.
Sci., Ser. Sci. Chim. 1977, 25, 271. Lis, T.; Glowiak, T.; Jezowska-
Trzebiatowska, B. Bull. Acad. Pol. Sci., Ser. Sci. Chim. 1975, 23, 417.
Glowiak, T.; Lis, T.; Jezowska-Trzebiatowska, B. Bull. Acad. Pol.
Sci., Ser. Sci. Chim. 1972, 20, 957. Sergienko, V. S.; Porai-Koshits,
M. A.; Khodashova, T. S. Zh. Strukt. Khim. 1974, 15, 275.
various [ReO2(5-MeIm)n(4-MeIm)4-n
]
isomers. In fact, the
-
1H NMR spectrum of a fresh CD3OD of the B(C6H5)4 salt
(36) Henderson, W. W.; Shepherd, R. E.; Abola, J. Inorg. Chem. 1986,
25, 3157.
(37) Costes, J. P.; Commenges, G.; Laurent, J. P. Inorg. Chim. Acta 1987,
134, 237.

Reaction of ReO2I(PPh3)2 with Imidazoles
Inorganic Chemistry, Vol. 35, No. 26, 1996 7841
Table 2. Comparison of Selected Bond Lengths (Å) and Angles (deg) for the Complex Cations
[ReO2(2-MeImH)4]+ (I)
[ReO2(1-MeIm)4]+ (II)
[ReO2(5-MeImH)4]+ (III)
[ReO(OH)(1,2-Me2Im)4]2+ (IV)
Re-O1
1.768(3)
1.761(3)
2.138(3)
2.137(4)
2.146(3)
2.148(3)
178.13(13)
89.88(13)
87.76(14)
90.57(14)
89.56(14)
91.99(13)
92.09(14)
87.56(14)
90.54(14)
92.01(13)
178.20(14)
89.34(13)
86.26(13)
176.99(13)
92.41(13)
130.1(3)
123.2(3)
130.6(3)
123.2(3)
130.0(3)
122.9(3)
129.3(3)
123.7(3)
1.749(5)
1.782(6)
2.112(7)
2.120(6)
2.138(7)
2.113(5)
179.5(2)
89.6(2)
89.9(3)
89.2(3)
89.4(2)
90.1(2)
89.8(3)
91.1(3)
91.0(2)
90.4(3)
178.8(3)
90.4(2)
89.8(3)
178.9(2)
89.4(2)
1.769(7)
1.763(7)
2.111(8)
2.132(8)
2.140(9)
2.108(8)
175.5(4)
87.9(3)
90.9(3)
92.5(4)
88.1(3)
89.0(3)
92.3(3)
90.7(3)
88.8(3)
88.1(3)
179.2(3)
92.2(3)
91.1(3)
1.896(13)a
1.732(15)
2.137(20)
2.106(21)
2.117(22)
2.139(22)
179.1(7)
84.3(7)
86.7(7)
86.1(7)
85.1(7)
95.7(8)
94.2(8)
93.8(8)
94.0(8)
94.4(8)
168.9(8)
86.0(8)
90.7(8)
171.7(8)
87.6(8)
Re-O2
Re-N31
Re-N32
Re-N33
Re-N34
O1-Re-O2
O1-Re-N31
O1-Re-N32
O1-Re-N33
O1-Re-N34
O2-Re-N31
O2-Re-N32
O2-Re-N33
O2-Re-N34
N31-Re-N32
N31-Re-N33
N31-Re-N34
N32-Re-N33
N32-Re-N34
N33-Re-N34
Re-N31-C21
Re-N31-C41
Re-N32-C22
Re-N32-C42
Re-N33-C23
Re-N33-C43
Re-N34-C24
Re-N34-C44
88.5(3)
178.8(3)
127.8(7)
129.5(7)
123.5(7)
129.0(7)
128.5(8)
126.8(8)
130.0(7)
124.1(6)
126.2(5)
127.0(5)
127.2(6)
127.0(6)
126.1(6)
130.2(7)
126.4(4)
127.6(5)
127.8(18)
120.5(16)
133.6(17)
123.1(15)
131.6(18)
122.4(17)
130.5(18)
122.2(17)
a The Re1 atom is involved.
Table 3. O1-Re-N3-C(R) Torsion Angles (deg) Describing Ring Orientation in Crystal Structuresa
ring 1
ring 2
ring 3
ring 4
ref
[ReO2(1-MeIm)4]+
20 [C4]
-16 [C2]
-22 [C2]
14 [C4]
29 [C4]
-44 [C4]
-36 [C2]
-46 [C4]
-26
-4 [C2]
-41 [C2]
19 [C2]
6 [C2]
46 [C4]
-42 [C4]
-36 [C4]
-46 [C2]
20
1 [C4]
6 [C4]
22 [C4]
-14 [C2]
35 [C2]
-50 [C2]
-36 [C2]
-46 [C4]
-18
38 [C2]
20 [C4]
-19 [C4]
-6 [C4]
34 [C2]
-35 [C4]
-36 [C4]
-46 [C2]
18
this work
this work
7
7
this work
this work
6
[ReO2(5-MeImH)4]+
[TcO2(1-MeIm)4]+
[TcO2(ImH)4]+
[ReO2(2-MeImH)4]+
[ReO(OH)(1,2-Me2Im)4]2+
[ReO(O/OH)(1,2-Me2Im)4]+/2+
[ReO2py4]+
39
32
[ReO2(4-Mepy)4]+
-6
-21
6
21
6
17
15
17
-6
-17
[TcO2(4-(t-Bu)py)4]+
12
-15
-12
43
-6
-17
6
a The four torsion angles are calculated with respect to the same oxo atom, and the R-carbon is chosen so as to keep the angle in the -90° e
τ e 90° range. For imidazoles, the R-carbon involved is identified in brackets.
slight differences (1.749(5) and 1.782(6) Å) observed for the
1-MeIm complex could be an artifact due to disorder in the
solvent molecules hydrogen-bonded to the oxo groups. The
12 independent Re-N distances, ranging from 2.111 to 2.148
Å, are also similar to those found in complexes with other
N-heterocycles. The angles correspond within 3° to a regular
octahedron, the only exception being the OdRedO angles of
175.5(4)° in the 5-MeImH complex.
In the 1-MeIm and 5-MeImH compounds, coordination takes
place along the lone pair direction, as evidenced from the
approximately equal ResN3sC2 and ResN3sC4 angles
(Table 2). On the other hand, for the C2-methylated complex,
the ResN3sC2 angles (129.3-130.6°) are, on average, 7°
greater than the ResN3sC4 angles (122.9-123.7°). Steric
strain is also revealed by the NsC2sCH3 angles: N3sC2sC6
is 3.2-7.0° greater than N1sC2sC6 (Supporting Information).
Both of these distortions, which have been noted for other
systems,42 tend to displace the methyl group away from the
OdRedO axis. Steric hindrance also affects the overall
complex conformation by rotating the imidazole ligands about
the ResN bond. As shown by the torsion angles listed in Table
3, in the absence of a 2-methyl group, at least three and often
all four imidazole rings are roughly aligned with the OdRedO
direction (-22° e τ e +22°). The same trend is exhibited by
compounds with pyridines, which are not R-substituted.32,39,43
On the other hand, with 2-MeImH and 1,2-Me2Im, the R-methyl
group rotates the rings ∼45° away from the OdRedO direction
and generates a propeller-like conformation. For imidazole
ligands, there are two unequivalent ring orientations about the
ResN bond, corresponding to C2 being on the same side (up)
as or opposite (down) to the reference RedO bond, respectively.
The structures listed in Table 3 do not follow a unique pattern:
(42) Geiser, U.; Ramakrishna, B. L.; Willett, R. D.; Hulsbergen, F. B.
Reedijk, J. Inorg. Chem. 1987, 26, 3750. Horrocks, W. D., Jr.; Ishley,
J. N.; Whittle, R. R. Inorg. Chem. 1982, 21, 3270. Bencini, A.;
Gatteschi, D.; Zanchini, C. Inorg. Chem. 1985, 24, 700. Phillips, F.
L.; Shreeve, F. M.; Skapski, A. C. Acta Crystallogr. 1976, B32, 687.
Reedijk, J.; Verschoor, G. C. Acta Crystallogr. 1973, B29, 721.
(43) Kastner, M. E.; Fackler, P. H.; Clarke, M. J.; Deutsch, E. Inorg. Chem.
1984, 23, 4683.

7842 Inorganic Chemistry, Vol. 35, No. 26, 1996
Be´langer and Beauchamp
the up-up-down-down and up-down-up-down sequences around
the OdRedO axis are the most common, but the up-up-up-
down arrangement has been found in an oxo-hydroxo com-
pound.
Views of molecular packing are given and commented on
in the Supporting Information. Both oxo groups form hydro-
gen bonds in the three structures. In [ReO2(2-MeImH)4]-
[B(C6H5)4]‚3CH3OH, lattice MeOH molecules form strong
hydrogen bonds with O‚‚‚O separations of 2.64 and 2.69 Å.
An extensive network of hydrogen bonds involving these MeOH
molecules and imidazole NsH groups provides an important
contribution to lattice cohesion. Hydrogen bonding is less
extensive in [ReO2(1-MeIm)4][B(C6H5)4]‚H2O‚0.5CH3OH, since
the imidazole is N-methylated, but the oxo groups retain weaker
interactions with disordered water or MeOH molecules (2.79
Å). In solvent-free [ReO2(5-MeImH)4][B(C6H5)4], imidazole
NsH groups act as donors in strong hydrogen bonds (N‚‚‚O,
2.71 and 2.76 Å).
Figure 2. Protonation of [ReO2(1-MeIm)4]I (3.30 × 10-5 M) with
HBF4 in methanol. H+/Re ratio (from top to bottom at 270 nm): 0,
0.12, 0.23, 0.70, 1.10.
Unit cell packing has been compared for the above structures
and the [TcO2(1-MeIm)4]+ and [TcO2(ImH)4]+ complexes.7
[ReO2(2-MeImH)4][B(C6H5)4]‚3CH3OH differs from the other
crystals by having the OdRedO axes of all cations oriented
approximately along the same direction. More commonly, in
[ReO2(5-MeImH)4][B(C6H5)4] for instance, adjacent ions adopt
mutually perpendicular orientations. In the case of [ReO2(1-
MeIm)4][B(C6H5)4]‚H2O‚0.5CH3OH, an intermediate situation
is observed: the cations are linked into a chain with which the
OdRedO units are roughly aligned, but adjacent chains are
running in mutually perpendicular directions in the crystal.
Protonation of [ReO2L4]+. ReO(OEt)I2py2 has been re-
ported to form when [ReO2py4]+ is refluxed in the presence of
HI in ethanol.22 With imidazoles, oxo-alkoxo compounds were
not obtained under these conditions from either ethanol or
methanol. As soon as HI is added, the solution takes the bright
purple color of [ReO(OH)L4]2+ and salts can be isolated. A
broad range of conditions was tested with methanol. If the
solution is allowed to stand or if acid concentration is high, the
color changes from purple to brownish red. Under refluxing
conditions, decomposition is complete after 2-4 h. The
decomposition products include free protonated ligand (identi-
(3200-3500 cm-1) is usually masked by bands due to lattice
solvent. As to the ν(ResOH) and δ(ResO-H) modes, expected
in the 550-575 and 800-1200 cm-1 ranges,30 respectively, they
are usually weak and not detected in the present case.
Upon protonation, the color of the complex changes from
orange ([OdRedO]+) to purple ([OdRe-OH]2+) to deep blue
([OdResOH2]3+). Protonation of [ReO2(1-MeIm)4]I was
studied in methanol at room temperature in the 200-350 nm
spectral region. Figure 2 shows the changes in the UV region
during the first protonation in methanol. As HBF4 is added to
the solution of the trans-dioxo complex, a net drop in the
absorption near 265 nm occurs, while a slight increase near 240
nm is observed. Protonation is quantitative in methanol, since
addition of excess HBF4 results in loss of the isosbestic point
at 250 nm (see H+/Re ) 1.10, Figure 2). In water, protonation
is not quantitative in a 1:1 complex:HBF4 mixture, but proto-
nation can be achieved in concentrated acids (see below).
The effect of protonation on the UV-visible spectra parallels
that observed for the pyridine analogue. In an aqueous solution,
[ReO2(1-MeIm)4]+ and [ReO2(1,2-Me2Im)4]+ show ligand-field
bands at 431 nm (ꢀ ) 183 M-1 cm-1) and 480 nm (ꢀ ) 321
M-1 cm-1), respectively, characteristic of the [OdRedO]+
chromophore. This absorption moves to lower energies upon
protonation ([OdResOH]2+: 1-MeIm, 536 nm, ꢀ ) 148 M-1
fied from H NMR), I3 and ReO4-, and an insoluble black
material which has not been identified. Decomposition could
not be totally avoided even under the mildest conditions used,
making it difficult to isolate pure stoichiometric phases, probably
because I-, I3-, and ReO4- can be interchanged as counterions
and lead to nonstoichiometric materials (see below) or salt
mixtures. Decomposition similarly occurred when HBF4 was
1
-
cm-1; 1,2-Me2Im, 556 nm, ꢀ ) 287 M-1 cm-1; [O-Re-OH2]3+
:
1-MeIm, 624 nm; 1,2-Me2Im, 683 nm). The corresponding
values for the pyridine complex are as follows: [OdRedO]+,
445 nm, ꢀ ) 1200 M-1 cm-1; [OdResOH]2+, 504 nm, ꢀ )
540 M-1 cm-1; [OdResOH2]3+, 605 nm.3
-
used instead of HI. The B(C6H5)4 salts are more stable but
less soluble.
The ligand-based transitions near 195 and 220 nm are not
greatly influenced by the first protonation of an oxo ligand. The
LMCT band of [ReO2(py)4]+ moves from 331 nm to 297 nm
upon protonation.3 The 1-MeIm and 1,2-Me2Im complexes
exhibit the same behavior, the LMCT band moving to higher
energies, from 255 to below 240 nm, where it gives rise to an
intensity gain at ∼220 nm.
Even though the reactions were run in methanol, [ReO(OMe)-
L4]2+ is not formed in these samples isolated quickly after
addition of acid. An oxo-methoxo species would be detected
by NMR, since rhenium-oxygen bonds are not labile.44 Re-
OCH3 exchange with CD3OD to give Re-OCD3 should not
take place over the few minutes needed to record the spectrum
1
and a high-field H NMR signal should be found for the Re-
The second protonation is more problematic, since 100%
diprotonation is not reached at the highest acidity accessible
(pH -4). The absorption near 500 nm (monoprotonated form)
OCH3 group in fresh solutions. Similarly, any solvolysis of
[ReO(OH)L4]2+ to [ReO(OCD3)L4]2+ should produce an extra
set of imidazole signals, which are not observed.45
In the infrared, the [OdResOH]2+ core shows a well-defined
ν(RedO) band in the 957-973 cm-1 range, in agreement with
the published results for [ReO(OH)L4]2+ species with ethyl-
(45) The relevant point concerning oxo-methoxo species was raised by
one of the reviewers. Although there is no evidence that [ReO(OMe)-
L4]2+ appears during the short residence times in solution used here,
their formation over periods of days cannot be ruled out. This aspect
could not be examined because an unidentified insoluble black solid
precipitated. [ReO(OH2)L4]3+/[ReO(HOMe)L4]3+ exchange in the
trication could not take place under the conditions used here, all
experiments being run in concentrated acids containing no methanol.
enediamine (981 cm-1),30,31 pyridines (945-975 cm-1 33
and
)
macrocyclic ligands (952-969 cm-1).25 The ν(OsH) band
(44) Kashani, F. F.; Murmann, R. K. Int. J. Chem. Kinet. 1985, 17, 1007.

Reaction of ReO2I(PPh3)2 with Imidazoles
Inorganic Chemistry, Vol. 35, No. 26, 1996 7843
Since spectra of very dilute solutions ∼10-5 M were
particularly sensitive to decomposition, ∼10-3 M samples were
used and protonation was studied in the visible ligand-field range
(300-900 nm), where extinction coefficients are appropriate.
Spectra were obtained for aqueous HCl solutions of pH from
-4 to +7.
The acidity constant Ka (eq 1) of the monoprotonated species
1
with 1-MeIm was determined at 3 °C, because decomposition
took place much more slowly than at room temperature in the
pH 2-6 range. The less sensitive 1,2-Me2Im complex was
[OdResOH]2+ h [OdRedO]+ + H+
Ka ) ([OdRedO]+)(H+)/([OdResOH]2+) (1)
1
studied at room temperature. The changes in the visible region
when the pH is varied from 6 to 1 are shown in Figure 3a. The
pKa values of 2.0 and ∼3.8 were determined from the pH at
1
half-neutralization in a plot of ꢀ(550 nm) vs pH for the 1-MeIm
and 1,2-Me2Im complexes, respectively.
trans-Dioxo systems with chelating amines have pKa’s of ∼3
(ethylenediamine, 3.3;47,48 1,3-diaminopropane, 3.2;48 1,4,8,11-
tetraazacyclodecane, 2.95 25), whereas [ReO2(CN)4]3- protonates
at pH 3.7-4.249 and [ReO2(CH3NH2)4]+ is strongly acidic (pKa
1
) -0.43).33 For a series of substituted pyridines, the proto-
nation constant of the rhenium complex roughly follows that
of the free ligand. The values for pyridine complexes typically
range from -0.6 to +0.8 2,33 with the 4-(dimethylamino)pyridine
complex having a pKa value of 2.3.2
1
[OdResOH2]3+ h [OdResOH]2+ + H+
Figure 3. (a) Spectral changes accompanying the protonation of [ReO2-
(1-MeIm)4]+. The pH varies from -1 to +6 from top to bottom at 550
nm. Inset: Absorbance measured at 550 nm. (b) Spectral changes
accompanying the protonation of [ReO(OH)(1-MeIm)4]2+. The pH
varies from -4 to +1 top to bottom at 624 nm. Inset: Absorbance at
624 nm.
Ka ) ([OdResOH]2+)(H+)/([OdResOH2]3+) (2)
2
The pKa values were only estimated. At acidities above pH
2
-4, dissolution in viscous H2SO4 is slow and decomposition
to a black insoluble product occurs in the meantime. The
spectral changes in the +1 to -4 pH range are shown in Figure
3b, as well as the ꢀ(624 nm) vs pH curve for the 1-MeIm
complex. Curve fitting over this limited pH range could not
be applied in the absence of a clear plateau a high acidities to
provide ꢀ for [OdResOH2]3+. However, an approximate ꢀ
value of 450 M-1 cm-1 was obtained by quickly scanning the
visible region of a solution in concentrated HBF4, whose blue
color indicated a very high degree of protonation. From the
never totally disappears as the absorption near 640 nm grows.
At the same time, an intense band appears at ∼325 nm and it
trails in the visible, accounting for the greenish color of the
solution (oxo-aqua complexes are expected to be blue).46
[ReO(OH)L4]2+ dissociates in water and NMR spectra were
essentially those of [ReO2L4]+. Spectra were recorded in
DMSO, where the violet color indicates that the complex is
still largely present in the [ReO(OH)L4]2+ form. The 1H signals
are shifted to a greater extent relative to the free ligand in DMSO
than for the trans-dioxo species. Comparisons can be made
between the [ReO(OH)L4]2+ and cobalt(III)-imidazole com-
plexes.36 For the latter complexes, the three ring protons were
displaced downfield by complexation, the shifts being greater
for H2 and H5 (ca. 0.5 ppm) than for H4 (ca. 0.15 ppm).
Similar trends are followed by our protonated Re compounds:
downfield shifts of 0.3/0.4 ppm were observed for H2 and 0.4/
0.5 ppm for H5. The shift for H4 is large downfield (0.65 ppm)
for 1,2-Me2Im, smaller (0.08 ppm) for 1-MeIm and in the
opposite direction (-0.08 ppm) for ImH. It should be noted,
however, that, in the last case, these displacements are expressed
with respect to the averaged H4/H5 signal of the ligand in fast
tautomeric exchange.
ꢀ(624 nm) vs pH curve, pKa can be estimated to be close to
2
-4. Using the same strategy, a pKa value of ∼-4.1 was obtained
2
for the second protonation of the 1,2-Me2Im complex.
Literature records on the second protonation for related
systems are scarce. To our knowledge, the only published data
on rhenium systems are those for [ReO2(CN)4]3- (pKa ) 1.3-
2
1.4)49 and [ReO2(L-L)2]3+ (L-L ) ethylenediamine, -0.9;47,48
1,3-diaminopropane, -0.7 48). The imidazole complexes studied
in this paper are definitely more acidic than these systems.
Crystal Structure of [ReO(OH)(1,2-Me2Im)4](I3)0.5(ReO4)1.5.
The environment of the rhenium atom defines a distorted
octahedron (Figure 1d). The two rhenium-oxygen bonds are
not equivalent: RedO2 (1.732(15) Å) is definitely shorter than
ResO1 (1.896(13) Å), and both significantly deviate from those
found in the OdRedO units. The difference between our
ResO1 and ResO2 bond lengths is significant, although
smaller than what has been observed (1.685(8) and 1.970(8)
Determination of Protonation Constants in Aqueous
Solutions. The complexes are unstable in acidic solutions,
decomposition producing visible spectral changes in both HCl
and HBF4 over periods of ∼10 min. Results for [ReO2(1-
MeIm)4]+ and [ReO2(1,2-Me2Im)4]+, the least unstable com-
plexes in the series, are described here.
(47) Murmann, R. K.; Foerster, D. R. J. Phys. Chem., 1963, 67, 1383.
(48) Lawrance, G. A.; Sangster, D. F. Polyhedron 1986, 5, 1553.
(49) Roodt, A.; Leipoldt, J. G.; Helm, L.; Merbach, A. E. Inorg. Chem.
1992, 31, 2864, and references cited therein.
(46) Parker, D.; Roy, P. S.; Inorg. Chem. 1988, 27, 4127.

7844 Inorganic Chemistry, Vol. 35, No. 26, 1996
Be´langer and Beauchamp
Å) for [ReO(OH)(O1cyclam)]+ (where O1cyclam ) 1,4,8,11-
tetraazacyclotetradecan-2-one).25 However, it is in good agree-
ment with those found for [ReO(OH)(CN)4]2- (ResO ) 1.70(1)
and 1.90(1) Å).50 Clearly, the complex studied is the oxo-
hydroxo complex and not the oxo-aqua ion: in the case of
Re(V) oxo-aqua species, the long ResO bond is much longer
(2.142(7) Å).51 The imidazole rings are tilted to minimize steric
hindrance, and coordination does not take place along the lone
pair direction, the ResN3sC2 angles (mean 130.9°) being
complexes with pyridines and other imidazoles to keep the rings
roughly aligned (within 22°) with this direction. There are no
particular constants in molecular packing that could explain this
feature, since the available series of structures includes com-
plexes with pyridines and imidazoles, various counteranions,
and different lattice solvents present. This could reflect a certain
degree of π-bonding in the ResN bond. Johnson and co-
workers54 have proposed that imidazole can act as a π-donor
in the iron(III)-cyano core, where metal d orbitals of suitable
symmetry are available and a number of CN- ligands can act
as a good π-acceptor sink. The possibility of π-donation is
less likely here, because the upright ligand orientation would
require a participation of the filled dxy orbital. Metal-to-ligand
back-bonding remains possible, however, involving donation
of the two electrons of the metal dxy orbital into an empty
imidazole π*-level. Nevertheless, the effect would likely be
small, since some non-2-substituted ligands deviate from the
upright orientation in the crystal structures, the loss of this
interaction in the 2-methylimidazole derivatives does not
appreciably affect stability, and spectroscopic properties are
similar for all systems.
Even though trans-dioxo imidazole complexes show greater
stability towards ligand loss than those of pyridines, protonated
ligand was detected in aged solutions of [ReO2(1,2-Me2Im)4]-
Cl,6 and some of our compounds sometimes showed small
amounts of byproducts yet to be identified. Ligand dissociation
was also noted for water solutions of [TcO2L4]Cl (L ) ImH,
1-MeIm, 4(5)-MeImH) even at pH 7.7 Under the same
conditions, free ligand was not usually detected in our samples,
but, at intermediate pH (2-4), the complexes are not stable over
long periods of time. Work is underway to determine the nature
of these new species, which are probably those responsible for
the spurious NMR signals.
-
larger than ResN3sC4 (mean 122.1°). I3 is linear by
symmetry, and the IsI distance (2.925(3) Å) is normal.52
Discussion
trans-[ReO2L4]I complexes were obtained in very high yields,
and conversion to the tetraphenylborate salts provided clean
samples suitable for X-ray diffraction. The iodides are also less
prone to decomposition than the chlorides prepared earlier.
Generally speaking, the imidazole compounds form more
readily than the pyridine analogues. The reaction with ReO2I-
(PPh3)2 takes place within minutes, even under stoichiometric
conditions, whereas longer reaction time and large excess have
to be used with pyridines. The imidazole complexes are also
less sensitive to decomposition, which can be ascribed to greater
ResN bond strength due to imidazoles being better σ-donors
(pKa of LH+ ∼ 7) than pyridines (pKa ) 5-6).53 Greater
σ-donation also makes the Re center electron richer and reduces
its ability to act as π-acceptor for the oxo group. As a
consequence, the RedO double bond character is decreased,
as indicated by the νas(OdRedO) IR stretching frequency
(810 cm-1 for pyridine, 780 cm-1 for imidazole), whereas the
oxo groups become more nucleophilic (greater pKa of the
OdResOH2+ unit). In addition, imidazoles are less sterically
demanding than pyridines: the ring being five-membred, the
adjacent NsC bonds are displaced away from the ResN bond
and the substituents on the R-carbons more remote from the
OdRedO axis. These factors probably explain that stable
complexes were obtained with 2-methylated imidazoles, whereas
compounds with 2-substituted pyridines have not been reported.
The conformation of the complex is sensitive to the presence
of a 2-methyl group: the ligands are rotated ∼45° with respect
to the OdRedO axis, although there is a clear trend among
Acknowledgment. We wish to thank M. Simard, F. Be´-
langer-Garie´py, and A. M. Lebuis for help in collecting the
X-ray data and G. Timmins (McMaster University) for running
the Raman spectra. Special thanks to M. Simard for help in
solving the structure of the oxo-hydroxo complex and to C.
Reber and C. Savoie for assistance with the UV-vis experi-
ments. The financial support of the Natural Sciences and
Engineering Research Council of Canada is also acknowledged.
Supporting Information Available: Figures showing views of
molecular packing, tables listing final coordinates, temperature factors,
bond lengths and angles, distances to the weighted least-squares planes
for the four structures, and text describing solvent disorder for II (43
pages). Ordering information is given on any masthead page.
(50) Purcell, W.; Roodt, A.; Basson, S. S.; Leipoldt, J. G. Transition Met.
Chem. 1989, 14, 5.
(51) Purcell, W.; Roodt, A.; Basson, S. S.; Leipoldt, J. G. Transition Met.
Chem. 1990, 15, 239.
(52) Be´langer, S.; Beauchamp, A. L. Acta Crystallogr. 1993, C49, 388.
(53) Scriven, E. F. V. In ComprehensiVe Heterocyclic Chemistry; Katritzki,
A. R., Rees, C. W., Eds.; Pergamon Press: Oxford, U.K., 1984; Vol.
2, Chapter 2.05. Grimmett, M. R. V. In ComprehensiVe Heterocyclic
Chemistry; Katritzki, A. R., Rees, C. W., Eds.; Pergamon Press:
Oxford, U.K., 1984; Vol. 5, Chapter 4.07.
IC960451D
(54) Johnson, C. R.; Jones, C. M.; Asher S. A.; Abola, J. E. Inorg. Chem.
1991, 30, 2120.