Dirhenium Polyhydrides Containing Bridging 2-Mercaptoquinoline Ligands

Article abstract of DOI:10.1021/ic960187o

The dirhenium polyhydride complex [Re₂H₆(μ-mq)₂(PPh₃) ₄](BF₄)2 (1), where mq represents the 2-mercaptoquinoline ligand, can be deprotonated in two steps to afford the diamagnetic complexes [Re₂H₅(μ-mq)₂(PPh₃) ₄]BF₄ (2) and Re₂H₄μ-mq)₂(PPh₃)₄ (3) through the use of the bases PMe₃ and DBU, respectively. The treatment of 2 and 3 with HBF₄·Et₂O regenerates 1. The basic structure of the [Re₂(μ-mq)₂(PPh₃)₄] unit is retained throughout these deprotonations/protonations as demonstrated by ¹H and ³¹P{'H} NMR spectroscopy. The neutral tetrahydrido complex 3 undergoes a two-electron oxidation when treated with the oxidants [(η⁵-C₅H₅)₂Fe]BF₄ and [(η⁵-C₅H₅)₂-Fe]PF₆ to afford the salts [Re₂H₄(μ-mq)₂(PPh₃) ₄](BF₄)₂ (4a) and [Re₂H₄(μ-mq)₂(PPh₃) ₄](PF₆)₂ (4b), respectively. The [Re₂H₄(μ-mq)₂(PPh₃) ₄]²⁺/Re₂H₄(μ-mq) ₂(PPh₃)₄ couple has an E_1/2 value of -0.47 V vs Ag/AgCl as measured by the cyclic voltammetric technique on solutions of 3 and 4 in 0. l M Bu₄NPF₆/CH₂Cl₂. The chemical reversibility of this process has been confirmed by the use of (η⁵-C₅H₅)₂Co to reduce 4a back to 3. An X-ray crystal structure on a salt of the [Re₂H₄(μ-mq)₂(PPh₃) ₄]²⁺ cation, established that this complex is very similar structurally to 1 (see J. Am. Chem. Soc., 1995, 117, 9715). The most important structural differences are (1) the different numbers of hydrido ligands present and (2) the presence of a Re-Re single bond in 4a (the Re-Re distance is 3.000(1) A? compared to 3.9034(8) A? in [Re₂H₆(μ-mq)₂(PPh₃) ₄)²⁺. Crystal data for [Re₂H₄(μ-mq)₂(PPh₃) ₄]-(ReO₄)_1.18(BF₄) _0.82·3CH₂Cl₂ at 203 K: monoclinic space group P2_1/n (No. 14) with a = 14.716(4) A?, b = 43.908(13) A?, c = 14.860(3) A?, β= 110.164(19)°, V = 9013(4) A?³, Z = 4. The structure was refined to R = 0.071 (R_w = 0.159) for 14128 data (F_o²> 2σ(F_o²)).

Full text of DOI:10.1021/ic960187o

Inorg. Chem. 1996, 35, 5249-5253
5249
Dirhenium Polyhydrides Containing Bridging 2-Mercaptoquinoline Ligands
Andrea L. Ondracek, Wengan Wu, Phillip E. Fanwick, and Richard A. Walton*
Department of Chemistry, Purdue University, 1393 Brown Building,
West Lafayette, Indiana 47907-1393
ReceiVed February 22, 1996X
The dirhenium polyhydride complex [Re2H6(µ-mq)2(PPh3)4](BF4)2 (1), where mq represents the 2-mercaptoquinoline
ligand, can be deprotonated in two steps to afford the diamagnetic complexes [Re2H5(µ-mq)2(PPh3)4]BF4 (2) and
Re2H4(µ-mq)2(PPh3)4 (3) through the use of the bases PMe3 and DBU, respectively. The treatment of 2 and 3
with HBF4‚Et2O regenerates 1. The basic structure of the [Re2(µ-mq)2(PPh3)4] unit is retained throughout these
1
deprotonations/protonations as demonstrated by H and 31P{1H} NMR spectroscopy. The neutral tetrahydrido
complex 3 undergoes a two-electron oxidation when treated with the oxidants [(η5-C5H5)2Fe]BF4 and [(η5-C5H5)2-
Fe]PF6 to afford the salts [Re2H4(µ-mq)2(PPh3)4](BF4)2 (4a) and [Re2H4(µ-mq)2(PPh3)4](PF6)2 (4b), respectively.
The [Re2H4(µ-mq)2(PPh3)4]2+/Re2H4(µ-mq)2(PPh3)4 couple has an E1/2 value of -0.47 V vs Ag/AgCl as measured
by the cyclic voltammetric technique on solutions of 3 and 4 in 0.1 M Bu4NPF6/CH2Cl2. The chemical reversibility
of this process has been confirmed by the use of (η5-C5H5)2Co to reduce 4a back to 3. An X-ray crystal structure
on a salt of the [Re2H4(µ-mq)2(PPh3)4]2+ cation, established that this complex is very similar structurally to 1
(see J. Am. Chem. Soc., 1995, 117, 9715). The most important structural differences are (1) the different numbers
of hydrido ligands present and (2) the presence of a Re-Re single bond in 4a (the Re-Re distance is 3.000(1)
Å compared to 3.9034(8) Å in [Re2H6(µ-mq)2(PPh3)4]2+). Crystal data for [Re2H4(µ-mq)2(PPh3)4]-
(ReO4)1.18(BF4)0.82‚3CH2Cl2 at 203 K: monoclinic space group P21/n (No. 14) with a ) 14.716(4) Å, b )
43.908(13) Å, c ) 14.860(3) Å, â ) 110.164(19)°, V ) 9013(4) Å3, Z ) 4. The structure was refined to R )
2
2
0.071 (Rw ) 0.159) for 14128 data (Fo > 2σ(Fo )).
Introduction
bridging unit. A common reaction pathway which accesses
many of these dinuclear species involves the thermal or
photochemical loss of H2 from a mononuclear rhenium poly-
hydride complex to afford an electronic and coordinatively
unsaturated species which subsequently dimerizes. A recent
example is the facile thermal loss of H2 from ReH5(triphos) to
produce the dinuclear complex Re2(µ-H)3H(triphos)2, where
triphos ) CH3C(CH2PPh2)3.25 A new entry into dirhenium
polyhydride chemistry has been provided by our finding26 that
when the mononuclear Re(V) polyhydride ReH4(mq)(PPh3)2
(mq is the monoanion of 2-mercaptoquinoline) is treated with
the electrophiles H+ and Ph3C+, abstraction of H- to afford
the 16-electron [ReH3(mq)(PPh3)2]+ cation is followed by its
dimerization to produce [Re2H6(µ-mq)2(PPh3)4]2+. This reaction
course ensues in the absence of a complexing reagent, such as
an alkyne, which can stabilize the mononuclear cation.26 We
have now examined the acid/base chemistry of [Re2H6(µ-mq)2-
(PPh3)4]2+, a study which has provided access to other poly-
hydride species that contain the {Re2(µ-mq)2} unit. These
compounds differ from other dirhenium polyhydrides1-25 in that
they possess no Re-H-Re bridging bonds. A report on this
chemistry and the structure of one of these new compounds is
provided herein.
A large body of data now exists on dirhenium polyhydride
complexes, a class of compounds which display a fascinating
array of structures and reactivities.1-25 In these systems, the
bonding in the dinuclear units involves at least one Re-H-Re
X Abstract published in AdVance ACS Abstracts, August 1, 1996.
(1) Chatt, J.; Coffey, R. S. J. Chem. Soc. A 1969, 1963.
(2) (a) Green, M. A.; Huffman, J. C.; Caulton, K. G. J. Am. Chem. Soc.
1981, 103, 695. (b) Roberts, D. A.; Geoffroy, G. L. J. Organomet.
Chem. 1981, 214, 221.
(3) Brant, P.; Walton, R. A. Inorg. Chem. 1978, 17, 2674.
(4) Bruno, J. W.; Caulton, K. G. J. Organomet. Chem. 1986, 315, C13.
(5) Lyons, D.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1985, 587.
(6) Fanwick, P. E.; Root, D. R.; Walton, R. A. Inorg. Chem. 1989, 28,
395.
(7) Fanwick, P. E.; Root, D. R.; Walton, R. A. Inorg. Chem. 1989, 28,
3203.
(8) Costello, M. T.; Moehring, G. A.; Walton, R. A. Inorg. Chem. 1990,
29, 1578.
(9) Bau, R.; Carroll, W. E.; Teller, R. G.; Koetzle, T. F. J. Am. Chem.
Soc. 1977, 99, 3872.
(10) Cotton, F. A.; Luck, R. L. Inorg. Chem. 1989, 28, 4522.
(11) Cotton, F. A.; Luck, R. L.; Root, D. R.; Walton, R. A. Inorg. Chem.
1990, 29, 43.
(12) (a) Allison, J. D.; Walton, R. A. J. Chem. Soc., Chem. Commun. 1983,
401. (b) Allison, J. D.; Walton, R. A. J. Am. Chem. Soc. 1984, 106,
163.
(13) Moehring, G. A.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 1987,
26, 1861.
(14) Root, D. R.; Meyer, K. E.; Walton, R. A. Inorg. Chem. 1989, 28,
2503.
(15) Costello, M. T.; Fanwick, P. E.; Meyer, K. E.; Walton, R. A. Inorg.
Chem. 1990, 29, 4437.
(16) Allison, J. D.; Cotton, F. A.; Powell, G. L.; Walton, R. A. Inorg. Chem.
1984, 23, 159.
(17) Rhodes, L. F.; Huffman, J. C.; Caulton, K. G. J. Am. Chem. Soc. 1983,
105, 5137.
(18) Mueting, A. M.; Bos, W.; Alexander, B. D.; Boyle, P. D.; Casalnuovo,
J. A.; Balaban, S.; Ito, L. N.; Johnson, S. M.; Pignolet, L. H. New J.
Chem. 1988, 12, 505.
(20) Green, M. A.; Huffman, J. C.; Caulton, K. G. J. Am. Chem. Soc. 1982,
104, 2319.
(21) (a) Fanwick, P. E.; Root, D. R.; Walton, R. A. Inorg. Chem. 1989,
28, 2239. (b) Meyer, K. E.; Fanwick, P. E.; Walton, R. A. J. Am.
Chem. Soc. 1990, 112, 8586.
(22) Meyer, K. E.; Root, D. R.; Fanwick, P. E.; Walton, R. A. Inorg. Chem.
1992, 31, 3067.
(23) Meyer, K. E.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 1992, 31,
4486.
(24) Abrahams, A. C.; Ginsberg, A. P.; Koetzle, T. F.; Marsh, P.; Sprinkle,
C. R. Inorg. Chem. 1986, 25, 2500.
(25) Costello, M. T.; Fanwick, P. E.; Green, M. A.; Walton, R. A. Inorg.
Chem. 1992, 31, 2359.
(19) Westerberg, D. E.; Sutherland, B. R.; Huffman, J. C.; Caulton, K. G.
J. Am. Chem. Soc. 1988, 110, 1642.
(26) Leeaphon, M.; Ondracek, A. L.; Thomas, R. J.; Fanwick, P. E.; Walton,
R. A. J. Am. Chem. Soc. 1995, 117, 9715.
S0020-1669(96)00187-5 CCC: $12.00 © 1996 American Chemical Society

5250 Inorganic Chemistry, Vol. 35, No. 18, 1996
Ondracek et al.
OC(O)CH3 (2/1) solvent mixture by the slow evaporation of the solvents
at 25 °C under a nitrogen atmosphere. An orange platelike crystal
having approximate dimensions 0.35 × 0.23 × 0.13 mm was mounted
on a glass fiber in a random orientation. The data collection was
performed on an Enraf-Nonius CAD4 computer-controlled diffracto-
meter with graphite-monochromatized Mo KR radiation at 203 ( 1 K.
The cell contents were based on 25 reflections obtained in the range
17° < θ < 21° measured by the computer-controlled diagonal-slit
method of centering. Three standard reflections were measured after
every 5000 s of beam time during data collection to monitor the crystal
stability. Calculations were performed on VAX computers. The
structure was solved by using the Enraf-Nonius structure determination
package (MolEN) and was refined using the SHELXL-93 program.28
Lorentz and polarization corrections were applied to the data set. An
empirical absorption correction was also applied,29 but no correction
for extinction was made.
Experimental Section
Starting Materials and Reaction Procedures. The compound
[Re2H6(µ-mq)2(PPh3)4](BF4)2 (1) was prepared as described in the
literature except that HBF4‚Et2O was used in place of HPF6(aq).26 The
compounds [(η5-C5H5)2Fe]BF4 and [(η5-C5H5)2Fe]PF6 were prepared
by the literature method.27 The following compounds were obtained
from Aldrich Chemical Co.: 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU);
HBF4‚Et2O; (η5-C5H5)2Co. Trimethylphosphine was purchased from
Strem Chemicals Inc. All liquid reagents and solvents were deoxy-
genated by purging with dinitrogen prior to use, and all reactions, unless
otherwise noted, were performed under an atmosphere of dry dinitrogen.
Synthesis of [Re2H5(µ-mq)2(PPh3)4]BF4 (2). A mixture of trim-
ethylphosphine (0.027 mL, 0.310 mmol), [Re2H6(µ-mq)2(PPh3)4](BF4)2
(0.150 g, 0.078 mmol), and THF (10 mL) was stirred for 24 h. During
this time, a red-brown solution formed and a similarly colored solid
precipitated from the solution. The precipitate was collected via
filtration, washed with diethyl ether, and dried under vacuum; yield
0.118 g (82%). Anal. Calcd for C90H85BF4N2O4P4Re2S2: C, 56.72;
H, 4.49 (i.e. [Re2H5(µ-mq)2(PPh3)4]BF4‚4H2O). Found: C, 55.88; H,
4.41. This complex displayed a broad, fairly intense ν(O-H) band at
3425 cm-1 in its IR spectrum (KBr pellet).
Synthesis of Re2H4(µ-mq)2(PPh3)4 (3). A mixture of [Re2H6(µ-
mq)2(PPh3)4](BF4)2 (0.107 g, 0.056 mmol) and THF (7 mL) was treated
with DBU (0.033 mL, 0.221 mmol) and allowed to stir for 3 h,
whereupon a purple solution formed. The contents of the reaction
vessel were filtered, and the resulting filtrate was layered with diethyl
ether, which led to the slow formation of a purple crystalline solid.
The solid was collected by filtration, washed with diethyl ether, and
dried under vacuum; yield 0.089 g (91%). Anal. Calcd for C90H76-
N2P4Re2S2: C, 61.91; H, 4.39. Found: C, 61.03; H, 4.82.
An alternative route to Re2H4(µ-mq)2(PPh3)4 involves the use of
[Re2H5(µ-mq)2(PPh3)4]BF4 in place of [Re2H6(µ-mq)2(PPh3)4](BF4)2.
The reaction conditions and procedure were otherwise very similar;
yield 85%.
Synthesis of [Re2H4(µ-mq)2(PPh3)4](BF4)2 (4a). A mixture of
Re2H4(µ-mq)2(PPh3)4 (0.075 g, 0.043 mmol), [(η5-C5H5)2Fe]BF4 (0.024
g, 0.088 mmol), and THF (10 mL) was stirred for 45 min, during which
a yellow solid precipitated. A quantity of diethyl ether (15 mL) was
added to the reaction mixture, which was stirred for a further 5 min.
The yellow-green solid was collected by filtration, washed with diethyl
ether, and dried under vacuum; yield 0.073 g (88%). Anal. Calcd for
C90H82B2F8N2O3P4Re2S2 (i.e. [Re2H4(µ-mq)2(PPh3)4](BF4)2‚3H2O): C,
54.77; H, 4.19. Found: C, 54.67; H, 4.14. This complex displayed a
broad, fairly intense ν(O-H) band at 3420 cm-1 in its IR spectrum
(KBr pellet).
When a quantity of complex 4a (0.094 g, 0.049 mmol) was treated
with cobaltocene (0.019 g, 0.100 mmol) in 5 mL of THF and the
mixture stirred, conversion to the neutral purple complex 3 occurred;
yield 0.067 g (78%).
Synthesis of [Re2H4(µ-mq)2(PPh3)4](PF6)2 (4b). A procedure
similar to that reported for 4a, but with the use of [(η5-C5H5)2Fe]PF6
in place of [(η5-C5H5)2Fe]BF4, afforded the yellow-green title complex
4b; yield 96%. Anal. Calcd for C90H76F12N2P6Re2S2: C, 53.09; H,
3.76. Found: C, 52.74; H, 3.99.
The compound crystallized in the monoclinic crystal system. The
space group of P21/n was determined on the basis of systematic
absences observed on the data set. The structure was solved by the
use of the Patterson heavy-atom method to reveal the positions of the
Re atoms. The remaining non-hydrogen atoms were located in
succeeding difference Fourier syntheses. During the course of the
structural analysis, the two [BF4]- anions were found to be disordered
with two [ReO4]- anions which had formed during the slow crystal-
growing procedure. This disordered model was included in the analysis,
and the two anionic sites were finally refined to occupancies of 0.404-
(5) and 0.596(5) for [B(3)F4]- and [Re(3)O4]- and of 0.413(6) and
0.587(6) for [B(4)F4]- and [Re(4)O4]-, respectively. Three molecules
of CH2Cl2 from the crystallization solvents were found to be present
in the asymmetric unit. They were also included in the analysis and
refined satisfactorily. All non-hydrogen atoms in this structure were
refined with anisotropic thermal parameters, and corrections for
anomalous scattering were applied to these atoms.30 The positions of
the hydride ligands were calculated by using the energy-minimizing
program HYDEX,31 and the positions of the hydrogen atoms on all
organic groups were calculated by the use of the idealized geometries
with C-H ) 0.95 Å and U ) 1.3Ueq(C). They were added to the
structure factor calculations, but their positions were not refined. The
structure was refined by full-matrix least-squares calculations where
2
the function minimized was ∑w(Fo - Fc2)2 and the weighting factor
2
w is defined as w ) 1/[σ2(Fo ) + (0.0001P)2 + 718.55P], where P )
(Fo2 + 2Fc2)/3. The final residuals for the structure determination were
2
R ) 0.071 and Rw ()[∑w(Fo2 - Fc2)2/∑w(Fo )2]1/2) ) 0.159 with GOF
) 1.19. The largest remaining peak in the final difference Fourier
map was 3.67 e/Å3, the magnitude of which probably reflects the
relatively poor quality of the crystal. The minimum negative peak was
at -2.71 e/Å3.
Physical Measurements. A Perkin-Elmer 1800 FTIR spectrometer
was used to record the IR spectra of the compounds as mineral oil
(Nujol) mulls or KBr pellets. Electrochemical measurements were
carried out on dichloromethane solutions that contained 0.1 M tetra-
n-butylammonium hexafluorophosphate (TBAH) as supporting elec-
trolyte. E1/2 values, determined as (Ep,a + Ep,c)/2, were referenced to
the silver/silver chloride (Ag/AgCl) electrode at room temperature and
are uncorrected for junction potentials. Under our experimental
conditions, E1/2 ) +0.47 vs Ag/AgCl for the ferrocenium/ferrocene
couple. Voltammetric experiments were performed with a BAS Inc.
Model CV-27 instrument in conjunction with a BAS Model RXY
recorder. 1H, 31P{1H}, and 31P NMR spectra were obtained with a
Varian Gemini XL-200A spectrometer. Proton resonances were
referenced internally to the residual protons in the incompletely
deuteriated solvent, while phosphorus resonances were referenced
externally to a sample of 85% H3PO4. Elemental microanalyses were
performed by Dr. H. D. Lee of the Purdue University Microanalytical
Laboratory.
Protonation Reactions of 2 and 3. Both complexes were recon-
verted to [Re2H6(µ-mq)2(PPh3)4](BF4)2 when treated with HBF4‚Et2O.
(a) An excess of HBF4‚Et2O (0.15 mL) was added to a solution of
2 (0.055 g, 0.030 mmol) in dichloromethane (5 mL). The mixture was
stirred for 10 min and then treated with an excess of diethyl ether (30
mL). A yellow precipitate of 1 formed and was filtered off, washed
with diethyl ether, and dried under vacuum; yield 0.042 g (73%).
(b) A procedure similar to (a) was used to convert 3 to 1, but 31P-
{1H} NMR spectroscopy showed the product to be contaminated with
appreciable amounts of 4a (i.e. [Re2H4(µ-mq)2(PPh3)4](BF4)2); total yield
(1 + 4a) 65%.
(28) Sheldrick, G. M. SHELXL-93: A Program for Crystal Structure
Refinement. University of Go¨ttingen, Germany, 1993.
(29) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, A39, 158.
(30) Cromer, D. T. International Tables for X-ray Crystallography;
Kynoch: Birmingham, England, 1974, Vol. IV: (a) Table 2.3.1; (b)
Table 2.2B.
X-ray Crystallography. Single crystals of composition [Re2H4(µ-
mq)2(PPh3)4](ReO4)1.18(BF4)0.82‚3CH2Cl2 suitable for X-ray diffraction
analysis were grown from a solution of complex 4a in a CH2Cl2/C2H5-
(27) Hendrickson, D. N.; Sohn, Y. S.; Gray, H. B. Inorg. Chem., 1971,
10, 1560.
(31) Orpen, A. G. J. Chem. Soc., Dalton Trans. 1980, 2509.

Dirhenium Polyhydrides
Inorganic Chemistry, Vol. 35, No. 18, 1996 5251
Scheme 1. Reaction Chemistry of
[Re2H6(µ-mq)2(PPh3)4](BF4)2 (1)
Chart 1. Representation of the Structural Relationships
among 1-3
The pentahydridodirhenium complex 2 displays an IR spec-
trum with an intense and characteristic ν(B-F) mode for the
[BF4]- anions at 1058 cm-1 and weak bands at 1966 and 1944
cm-1, which are assigned to ν(Re-H). A cyclic voltammogram
of a solution of 2 in 0.1 M TBAH-CH2Cl2 shows an irreversible
oxidation at Ep,a ≈ +1.20 V, a couple at E1/2 ) +0.15 V, which
corresponds to an oxidation of the bulk complex (∆Ep ()Ep,a
- Ep,c) ) 100 mV), and a further coupled process which is
a
DBU ) 1,8-diazabicyclo[5.4.0]undec-7-ene. b The analogous
[PF6]- salt (4b) is formed when 3 is reacted with [(η5-C5H5)2Fe]PF6.
Results and Discussion
The reactions of the eight-coordinate rhenium(V) polyhydride
characterized by Ep,c ) -0.55 V and Ep,a ≈ -0.40 V (ip,c
>
32
complex ReH4(mq)(PPh3)2 with the electrophiles H+ and
ip,a) vs Ag/AgCl.
Ph3C+ result in the loss of H- and the dimerization of the 16-
electron fragment [ReH3(mq)(PPh3)2]+ to produce stable salts
of the [Re2H6(µ-mq)2(PPh3)4]2+ cation.26 The complex [Re2H6-
(µ-mq)2(PPh3)4](BF4)2 (1) does not react with alkynes; this
demonstrates the stability of the Re2(µ-mq)2 unit toward
cleavage and the formation of the same alkylidyne complexes
which are generated when the putative species {[ReH3(mq)-
(PPh3)2]+} is produced in the presence of alkynes. Likewise,
we find that the reactions of 1 with the bases PMe3 and DBU
do not give mononuclear [ReH3(mq)(L)(PPh3)2]BF4 (L ) PMe3,
DBU). Instead, we observe a combination of deprotonation and
redox reactions as shown in Scheme 1.
The NMR spectral properties of 2 (recorded in CDCl3) are
quite complicated. The relatively low symmetry of this complex
is reflected by the 31P{1H} NMR spectrum, which displays four
distinct resonances of approximately equal intensity, two as
broad singlets at δ +51.3 and +30.4 and two as doublets at δ
1
+36.0 and +29.5 (JP-P′ ) 11.0 Hz). The H NMR spectrum
consists of three distinct Re-H resonances. There are doublets
of doublets at δ -6.35 (JP-H ) 64 Hz, JP′-H ) 21 Hz) and δ
-1.93 (JP-H ) 60 Hz, JP′-H ) 15 Hz), which are assigned to
the two magnetically inequivalent hydrido ligands at the rhenium
center that has been deprotonated, and a broad triplet (relative
intensity 3) centered at δ -0.67, which is associated with the
three hydrido ligands that are bound to the other rhenium atom.
The magnetic equivalence of these three hydrides in the NMR
spectrum implies the occurrence of a fluxional process. This
was confirmed by recording the 1H NMR spectrum of 2 in CD2-
Cl2 over the temperature range +20 to -80 °C. Under these
conditions, the two doublets of doublets remained unchanged
but the triplet at δ -0.67 broadened, then collapsed as the
temperature was lowered (coalescence temperature ca. -30 °C),
and by the low temperature limit of -80 °C had been replaced
by a very broad doublet at δ ca. -4.0 and another broad
resonance at δ ca. +2.7 possessing relative intensities of ca.
1:2. Over this same temperature range, the 31P{1H} NMR
spectrum (in CD2Cl2) changed very little except for a small
downfield shift of the resonances at δ +51.3 and +36.0, so the
fluxional process must involve only motion of the three hydrido
ligands at the saturated rhenium center. The low-temperature
1H NMR spectrum associated with the rhenium center that
contains these three hydrido ligands resembles that reported
previously26 for the fluxional species [Re2H6(µ-mq)2(PPh3)4]2+
(1), in which the halVes of the dication are equiValent. The
31P{1H} spectrum of 1 shows two separate resonances (dou-
blets), while the 1H NMR spectrum at room temperature has a
broad feature at δ ca. +0.9 for the six hydrido ligands, which
collapses when the temperature is lowered and splits out into
two broad multiplets at δ ca. -2.35 and ca. +2.1 (intensity
ratio 1:2) by -80 °C. Accordingly, we can confidently conclude
that 2 is similar structurally to 1 but with a 3:2 distribution of
terminal hydrido ligands (rather than 3:3 in the case of 1). This
structure change is represented in Chart 1.
Our initial attempts to cleave 1 into mononuclear species were
carried out in the presence of PMe3. Under these conditions,
the red-brown, singly deprotonated product [Re2H5(µ-mq)2-
(PPh3)4]BF4 (2) was obtained in high yield. This complex,
which behaves as a 1:1 electrolyte in acetonitrile (Λm ) 171
Ω-1 cm2 mol-1 for cm ) 1.0 × 10-3 M), can be deprotonated
further by DBU to produce the neutral tetrahydrido complex
Re2H4(µ-mq)2(PPh3)4 (3) in essentially quantitative yield.
Complex 3 can also be accessed by treating 1 with DBU.
Complex 3 is readily oxidized to the stable dicationic species
[Re2H4(µ-mq)2(PPh3)4]2+, isolable as both its [BF4]- salt 4a and
[PF6]- salt 4b, in the presence of [(η5-C5H5)2Fe]BF4 and [(η5-
C5H5)2Fe]PF6, respectively. Solutions of these complexes in
acetonitrile (cm ) 1.0 × 10-3 M) had conductivities in accord
with 1:2 electrolyte behavior (Λm ) 240 and 262 Ω-1 cm2
mol-1 for 4a and 4b, respectively). The reversibility of the
reaction 3 a 4 was demonstrated by the reduction of 4a to 3 in
the presence of 2 equiv of cobaltocene.
The protonation of 2 and 3 occurs upon their treatment with
HBF4‚Et2O to regenerate 1 (Scheme 1). This reaction proceeds
cleanly in the case of 2, but the conversion of 3 to 1 is
complicated by the competing oxidation of 3 to 4a due to the
presence of small amounts of oxidants in the reaction medium.
The oxidant appears to be adventitious O2 and not H+, since
the yield of 1 increases relative to that of 4a as the reaction
mixture becomes more thoroughly deoxygenated. Under our
reaction conditions, the lowest yield of 4a was obtained in the
case of a 1:4a product ratio of 5:1, but we were unable to obtain
1 free from contamination by 4a when using this protonation
reaction.
As was mentioned previously, the purple neutral tetrahydrido
complex 3, which is the product of the double deprotonation of
1, is readily oxidized to the yellow dicationic species 4, which
(32) Leeaphon, M.; Rohl, K.; Thomas, R. J.; Fanwick, P. E.; Walton, R.
A. Inorg. Chem. 1993, 32, 5562.

5252 Inorganic Chemistry, Vol. 35, No. 18, 1996
Ondracek et al.
Table 1. Crystallographic Data for
[Re2H4(µ-mq)2(PPh3)4](ReO4)1.18(BF4)0.82‚3CH2Cl2 (4a)
empirical formula
fw
space group
a, Å
b, Å
Re3.18Cl6S2P4F3.28O4.72N2C93B0.82H82
2367.24
P21/n (No. 14)
14.716(4)
43.908(13)
14.860(3)
110.164(19)
9013(4)
4
c, Å
â, deg
V, Å3
Z
T, K
λ(Mo KR), Å
203
0.710 73
1.745
57.12
0.46, 0.31
0.071
F
calc, g cm-3
µ(Mo KR), cm-1
transm coeff: max, min
Ra
b
Rw
0.159
GOF
1.194
Figure 1. ORTEP33 representation of the structure of the dirhenium
cation [Re2H4(µ-mq)2(PPh3)4]2+ present in complex 4a. The thermal
ellipsoids are drawn at the 50% probability level except for those of
the hydrido ligands and the phenyl group atoms of the PPh3 ligands,
which are circles of arbitrary radius.
R ) ∑|Fo| - |Fc|/∑|Fo|. Rw ) [∑w(Fo - Fc2)2/∑w(Fo )2]1/2; w
a
b
2
2
) 1/[σ2(Fo ) + (0.0001P)2 + 718.55P] where P ) (Fo + 2Fc2)/3.
2
2
Table 2. Selected Intramolecular Bond Distances (Å) and Angles
(deg) for the Dirhenium Cation of
[Re2H4(mq)2(PPh3)4](ReO4)1.18(BF4)0.82‚3CH2Cl2 (4a)a
is isolable as its [BF4]- and [PF6]- salts 4a and 4b. Since the
properties of 4a and 4b are essentially identical, except for the
spectral changes associated with the different anions, only the
properties of 4a will be discussed. Both 3 and 4a are
diamagnetic and show identical cyclic voltammograms. Solu-
tions of 3 and 4a in 0.1 M TBAH-CH2Cl2 show a reversible
one-electron oxidation at E1/2(ox) ) +1.27 V vs Ag/AgCl, with
Distances
Re(1)-S(1)
Re(1)-S(2)
Re(1)-P(1)
Re(1)-P(2)
Re(1)-N(11)
Re(1)-H(1)
Re(1)-H(2)
S(2)-C(12)
N(11)-C(12)
N(11)-C(110)
Re(1)-Re(2)
2.394(5)
2.454(6)
2.422(6)
2.475(6)
2.19(2)
1.68(2)
1.68(2)
1.76(3)
1.39(3)
1.38(3)
3.000(1)
Re(2)-S(2)
Re(2)-S(1)
Re(2)-P(3)
Re(2)-P(4)
Re(2)-N(21)
Re(2)-H(3)
Re(2)-H(4)
S(1)-C(22)
N(21)-C(22)
N(21)-C(210)
2.374(6)
2.449(6)
2.471(6)
2.410(6)
2.14(2)
1.63(2)
1.69(2)
1.79(2)
1.30(3)
1.38(3)
∆Ep ) 90 mV, and a reversible two-electron process at E1/2
)
-0.47 V vs Ag/AgCl, with ∆Ep ) 120 mV. The ease of
oxidizing 3 to 4a is reflected by the very low value for Ep,a
-0.41 V.
)
The IR spectrum of 3 shows the absence of [BF4]- and reveals
a weak, broad ν(Re-H) mode at ca. 1960 cm-1. In the case of
4a, the ν(B-F) mode of [BF4]- appears at 1060 cm-1 while a
Angles
S(1)-Re(1)-S(2)
S(1)-Re(1)-P(1)
S(1)-Re(1)-P(2)
S(1)-Re(1)-N(11)
S(2)-Re(1)-P(1)
S(2)-Re(1)-P(2)
S(2)-Re(1)-N(11)
P(1)-Re(1)-P(2)
P(1)-Re(1)-N(11)
P(2)-Re(1)-N(11)
H(1)-Re(1)-H(2)
Re(2)-S(1)-Re(1)
99.2(2)
82.2(2)
169.1(2)
87.8(5)
146.3(2)
87.3(2)
66.5(5)
97.4(2)
146.8(5)
86.8(5)
139(2)
S(2)-Re(2)-S(1)
S(2)-Re(2)-P(3)
S(2)-Re(2)-P(4)
S(2)-Re(2)-N(21)
S(1)-Re(2)-P(3)
S(1)-Re(2)-P(4)
S(1)-Re(2)-N(21)
P(3)-Re(2)-P(4)
P(3)-Re(2)-N(21)
P(4)-Re(2)-N(21)
H(3)-Re(2)-H(4)
Re(2)-S(2)-Re(1)
99.9(2)
168.3(2)
84.1(2)
89.9(5)
86.2(2)
143.6(2)
65.7(5)
97.0(2)
83.5(5)
150.6(5)
137(2)
1
weak ν(Re-H) mode is located at 2012 cm-1. The H NMR
spectra of 3 (recorded in CDCl3) shows broad Re-H resonances
at δ -6.25 and -2.88 which have the appearance of broad
doublets (JP-H ca. 63 Hz), although in reality they are probably
unresolved doublets of doublets. The 31P{1H} NMR spectrum
of 3 (recorded in CDCl3) consists of broad singlets at δ +52.3
and +30.3. These spectral features are in accord with the
structure for 3 as shown in Chart 1, in which there is a
symmetrical 2:2 distribution of terminal hydrido ligands. This
is further supported by the 1H and 31P{1H} NMR spectra of 4a
(recorded in CDCl3), which show doublets of doublets for the
Re-H resonances at δ -5.39 (JP-H ) 55 Hz, JP′-H ) 23 Hz)
and δ +2.26 (JP-H ) 54 Hz, JP′-H ) 25 Hz) and doublets in
76.5(2)
76.8(2)
a
Numbers in parentheses are estimated standard deviations in the
least significant digits.
multiplicities for the formula unit totaled 2.0. An ORTEP33
representation of the structurally significant tetrahydridodirhe-
nium cation is shown in Figure 1, while key crystallographic
data and important structural parameters are given in Tables 1
and 2.
the 31P{1H} NMR spectrum at δ +21.9 and +15.7 (JP-P′
)
10.5 Hz). The similarities between the NMR spectra and cyclic
voltammograms of 3 and 4a (and 4b) imply a very close
structural relationship; consequently, the two-electron redox
process does not involve a major structural rearrangement. The
structure for 4a, and by implication that of 3, has been confirmed
by a single-crystal X-ray structure determination on a crystal
of composition [Re2H4(µ-mq)2(PPh3)4](ReO4)1.18(BF4)0.82‚3CH2-
Cl2, which was obtained from a solution of 4a in a CH2Cl2/
ethyl acetate (2/1) solvent mixture.
The basic structure of the [Re2H4(µ-mq)2(PPh3)4]2+ cation
resembles closely the previously determined structure of the
[Re2H6(µ-mq)2(PPh3)4]2+ cation (1), as present in its [H2PO4]-
salt,26 with the difference that there are two less terminal Re-H
bonds present in the former species. The Re-P distances in
4a are a little longer on average than they are in the structure
of the [Re2H6(µ-mq)2(PPh3)4]2+ cation; the range is 2.410(6)-
2.475(6) Å for 4a (Table 2) compared to 2.400(2)-2.419(2) Å
for 1.26 Also, the disparity between the Re-P distances of the
two PPh3 ligands that are bound to each Re center in the
structures of 4a and 1 is greatest in 4a (ca. 0.06 Å versus 0.02
The nature of the anions in the crystal of 4a was confirmed
by the IR spectrum of the batch of crystals from which the single
crystal was selected; bands at 1054 (s) and 906 (m-s) cm-1 are
assigned to the ν(B-F) and ν(Re-O) modes of the [BF4]- and
[ReO4]- anions, respectively. The proportion of [BF4]- to
[ReO4]- (0.82:1.18) was established by the successful refine-
ment of the structure, with the constraint that the anion
(33) Johnson, C. K. ORTEP II. Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.

Dirhenium Polyhydrides
Inorganic Chemistry, Vol. 35, No. 18, 1996 5253
1
Å). These structural features are in accord with the H and
31P{1H} NMR spectral properties of 4a, as discussed previously.
While the Re-N distances in the structure of 4a (2.19(2) and
2.14(2) Å) are not significantly different from the Re-N
distance in the asymmetric unit of the analogous [Re2H6(µ-mq)2-
(PPh3)4]2+ cation (2.191(6) Å),26 the Re-S distances are much
shorter in 4a; the range is 2.374(6)-2.454(6) Å for 4a (Table
2) compared to distances of 2.490(2) and 2.505(2) Å in the
structure of [Re2H6(µ-mq)2(PPh3)4](H2PO4)2.26 This result is
probably a consequence of a stronger Re(µ-S)2Re bridging unit
because of the presence of a Re-Re single bond in 4a. The
Re-Re distance of 3.000(1) Å in the structure of 4a can be
contrasted with a nonbonding distance of 3.9034(8) Å in the
structure of the hexahydridodirhenium cation.26 The formation
of a Re-Re bond in 4a, which accords with the observed
diamagnetism of the [Re2H4(µ-mq)2(PPh3)4]2+ species, results
in an opening of the S-Re-S bond angles; these angles are
99.2(2) and 99.9(2)° in the case of 4a (Table 2) and 76.56(7)°
in the structure of [Re2H6(µ-mq)2(PPh3)4](H2PO4)2.26 This is
also mirrored in the changes in the Re-S-Re angles which
are much smaller in 4a (76.5(2) and 76.8(2)°) than in 1
(102.8(1)°).
Acknowledgment. Support from the National Science
Foundation through Grant No. CHE94-09932 to R.A.W. is
gratefully acknowledged.
Supporting Information Available: For compound 4a, Tables S1-
S5, giving full details of crystal data and data collection parameters,
positional parameters, thermal parameters, and complete bond distances
and bond angles (19 pages). Ordering information is given on any
current masthead page.
IC960187O