Ligand Exchange or Reduction at Multiply Bonded Dimetal Units of Molybdenum and Rhenium by 2,6-Bis(diphenylphosphino)pyridine

Article abstract of DOI:10.1021/ic980455d

Reaction of 2,6-bis(diphenylphosphino)pyridine (bdppp or (Ph₂P)₂Py)) with K₄Mo₂Cl₈ in refluxing methanol gives Mo₂Cl₄(bdppp)₂ (1) with retention of the quadruple Mo-Mo bond. However, the quadruply bonded octachlorodirhenate(III) anion, Re₂Cl_82-, reacts with bdppp under similar conditions to afford Re₂Cl₄(bdppp)₂ (2), which has a triply bonded dirhenium(H) core, Re₂₄₊. An intermediate species, [Buⁿ₄N][Re₂Cl₇(bdppp)] (3), containing an Re₂₆₊ core and only one bridging bdppp ligand has also been isolated. The crystal structures of complexes 1-3 have been investigated by X-ray crystallography. In all cases the potentially tridentate bdppp acts as a bidentate ligand using one N and one of the P atoms, leaving the second phosphorus atom noncoordinated. The crystallographic parameters for these structures are as follows: Mo₂Cl₄(bdppp)₂·2CH₂Cl ₂ (1·2CH₂Cl₂), monoclinic space group P2₁/c with a = 15.475(2) A?, b = 11.5958(7) A?, c = 16.7637 (6) A?, β = 98.178(5)°, Z = 2; Re₂-Cl₄(bdppp)₂ (2), monoclinic space group P2₁/n with a = 16.173(2) A?, b = 11.285(1) A?, c = 28.771 (4) A?, β = 96.17(1)°, Z = 4; [Buⁿ₄N][Re₂Cl ₇(bdppp)]·CH₂Cl₂ (3·CH₂Cl₂), triclinic space group P1? with a = 16.054(2) A?, b = 18.562(3) A?, c = 20.205 (6) A?, a = 89.71(2)°, β = 73.21(1)°, γ = 73.47(2)°, Z = 4.

Full text of DOI:10.1021/ic980455d

Inorg. Chem. 1998, 37, 4611-4616
4611
Ligand Exchange or Reduction at Multiply Bonded Dimetal Units of Molybdenum and
Rhenium by 2,6-Bis(diphenylphosphino)pyridine
F. Albert Cotton,*,† Evgeny V. Dikarev,† Glenn T. Jordan IV,† Carlos A. Murillo,*,†,‡ and
Marina A. Petrukhina†
Laboratory for Molecular Structure and Bonding, Department of Chemistry, Texas A&M University,
P.O. Box 300012, College Station, Texas 77842-3012, and Department of Chemistry, Universidad de
Costa Rica, Ciudad Universitaria, Costa Rica
ReceiVed April 22, 1998
Reaction of 2,6-bis(diphenylphosphino)pyridine (bdppp or (Ph2P)2Py)) with K4Mo2Cl8 in refluxing methanol gives
Mo2Cl4(bdppp)2 (1) with retention of the quadruple Mo-Mo bond. However, the quadruply bonded octachlo-
rodirhenate(III) anion, Re2Cl82-, reacts with bdppp under similar conditions to afford Re2Cl4(bdppp)2 (2), which
has a triply bonded dirhenium(II) core, Re24+. An intermediate species, [Bun N][Re2Cl7(bdppp)] (3), containing
4
6+
an Re2 core and only one bridging bdppp ligand has also been isolated. The crystal structures of complexes
1-3 have been investigated by X-ray crystallography. In all cases the potentially tridentate bdppp acts as a
bidentate ligand using one N and one of the P atoms, leaving the second phosphorus atom noncoordinated. The
crystallographic parameters for these structures are as follows: Mo2Cl4(bdppp)2‚2CH2Cl2 (1‚2CH2Cl2), monoclinic
space group P21/c with a ) 15.475(2) Å, b ) 11.5958(7) Å, c ) 16.7637 (6) Å, â ) 98.178(5)°, Z ) 2; Re2-
Cl4(bdppp)2 (2), monoclinic space group P21/n with a ) 16.173(2) Å, b ) 11.285(1) Å, c ) 28.771 (4) Å, â )
96.17(1)°, Z ) 4; [Bun N][Re2Cl7(bdppp)]‚CH2Cl2 (3‚CH2Cl2), triclinic space group P1h with a ) 16.054(2) Å,
4
b ) 18.562(3) Å, c ) 20.205 (6) Å, R ) 89.71(2)°, â ) 73.21(1)°, γ ) 73.47(2)°, Z ) 4.
Introduction
Chart 1
2,6-Bis(diphenylphosphino)pyridine (bdppp or (Ph2P)2Py) is
a neutral tridentate ligand (Chart 1) that has been used in the
synthesis of many novel di- and polynuclear complexes.1 The
ability of bdppp to interact with metals in a variety of binding
modes has been demonstrated. Altogether three different
coordination types (Chart 2) have been found so far in all
crystallographically characterized compounds. Most of the
reported complexes1a-g display the first binding mode (Chart
2a). Of particular interest are the doubly bridged compounds
with an [M2{µ-(Ph2P)2Py}2] core (M ) Rh,1a Pt,1b Cu,1c and
Fe1d) and triply bridged molecules with an [M2{µ-(Ph2P)2Py}3]
core (M ) Cu,1c Au1e). Crystal structures are characterized by
long metal-metal separations over a range of 4.74-8.21 Å and
by a relatively open central cavity which is capable of binding
an additional metal atom or ion. An example is provided by
the complexation of tin(II) chloride by a dirhodium macrocycle.1h
The resulting compound has an [RhSnRh{µ-(Ph2P)2Py}2] core
containing two ligands bound in the second coordination mode
(Chart 2b) whereby a nitrogen atom is connected to the tin atom.
Chart 2
The same type of structure has been suggested1d for [FeHgFe{µ-
(Ph2P)2Py}2], but this claim has not yet been supported by a
crystallographic study.
The rhodium complex containing the [Rh4{µ-(Ph2P)2Py}2]
core is the only tetranuclear example of the second type of
coordination mode (Chart 2b) with two ligands trans to one
another and aligned by one unit translation.1f In this complex,
the almost linear chain of four rhodium atoms is also supported
by two bridging Cl and one CO group.
A third coordination type (Chart 2c), which features chelation
by a phosphorus atom and the nitrogen together with the
formation of a bridge to the second phosphorus atom, was
found1i in the dirhodium cation [Rh2(µ-I){(µ-Ph2P)2Py}2(µ-
CO)]+.
The metal-metal separation for the last two coordination
modes is still long (2.57-2.92 Å). Therefore, it was of interest
to study the behavior of the (Ph2P)2Py ligand in reactions with
† Texas A&M University.
‡ Universidad de Costa Rica.
(1) (a) Wood, F. E.; Hvoslef, J.; Balch, A. L. J. Am. Chem. Soc. 1983,
105, 6986. (b) Wood, F. E.; Hvoslef, J.; Hope, H.; Balch, A. L. Inorg.
Chem. 1984, 23, 4309. (c) Field, J. S.; Haines, R. J.; Warwick, B.;
Zulu, M. M. Polyhedron 1996, 15, 3741. (d) Zhang, J.-K.; Zhang,
Z.-M.; Yu, A.; Zhao, S.-L., Zhang, W.-D.; Zhang, Z.-Z. Polyhedron
1996, 15, 2583. (e) Shieh, S.-J.; Li, D.; Peng, S.-M.; Che, C.-M. J.
Chem. Soc., Dalton Trans. 1993, 195. (f) Wood, F. E.; Olmstead, M.
M.; Balch, A. L. J. Am. Chem. Soc. 1983, 105, 6332. (g) Shieh, S.-J.;
Hong, X.; Peng, S.-M.; Che, C.-M. J. Chem. Soc., Dalton Trans. 1994,
3067. (h) Balch, A. L.; Hope, H.; Wood, F. E. J. Am. Chem. Soc.
1985, 107, 6936. (i) Balch, A. L.; Fossett, L. A.; Olmstead, M. M.
Inorg. Chem. 1986, 25, 4526.
S0020-1669(98)00455-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/13/1998

4612 Inorganic Chemistry, Vol. 37, No. 18, 1998
Cotton et al.
1191 (m), 1169 (m), 1088 (s), 1027 (m), 1006 (m), 998 (m), 800 (m),
790 (m), 754 (s), 738 (s), 690 (vs), 664 (s), 575 (m), 552 (m), 530 (s),
518 (s), 503 (s), 493 (s), 476 (w), 446 (m), 417 (m). CV (CH2Cl2, 22
°C, V vs Ag/AgCl): E1/2 (ox) (1) ) -0.07, E1/2 (ox) (2) ) +0.85.
Chart 3
Synthesis of [Bun N][Re2Cl7(bdppp)]‚CH2Cl2 (3‚CH2Cl2).
A
4
suspension of [Bun N]2Re2Cl8 (0.2375 g, 0.20 mmol) and bdppp (0.4503
4
g, 1.00 mmol) in 25 mL of methanol was stirred for 2 h at room
temperature; the color of the solution changed to green-brown. The
solution was filtered, and the solvent was removed under reduced
pressure, leaving a green residue, which was washed with small portions
of toluene (2 × 5 mL), followed by hexanes (2 × 10 mL), and dried.
Yield: 0.094 g (0.07 mmol, 34.6%). Green plate-shaped crystals were
obtained after a few days from CH2Cl2 solutions layered with hexanes;
they were always contaminated by a small amount of crystals of
[Bun4N]2Re2Cl8, which were separated manually. IR (KBr, cm-1): 1617
(w), 1585 (w), 1570 (w), 1543 (w), 1482 (s), 1434 (vs), 1379 (m),
1368 (m), 1263 (w), 1233 (w), 1192 (m), 1152 (s), 1091 (s), 1070 (w),
1024 (s), 998 (m), 925 (w), 879 (m), 809 (m), 785 (w), 740 (vs), 689
(vs), 531 (s), 512 (s), 488 (s), 466 (s), 442 (m). CV (CH2Cl2, 22 °C,
V vs Ag/AgCl): E1/2 (red) (1) ) + 0.03, E1/2 (red) (2) ) -0.75.
X-ray Crystallographic Procedures. Single crystals of 1-3 were
obtained as described above. Unit cell determinations and data
collections followed routine procedures and practices of this laboratory.3
Oscillation photographs of principal axes were taken to confirm Laue
class and axial lengths. All data were corrected for Lorentz and
polarization effects. The intensities for 1 and 3 were corrected for
absorption using a local adaptation of the program SORTAV.4
All calculations were done on a DEC Alpha running VMS. The
coordinates of the heavy atoms for all of the structures were found in
direct methods E-maps using the structure solution program SHELX-
TL.5 The positions of the remaining atoms were located by a
combination of least-squares refinements and difference Fourier maps
using the SHELXL-93 program.6 Details on data collection and
structure refinement for 1-3 are summarized in Table 1.
Mo2Cl4(bdppp)2‚2CH2Cl2. A green block-shaped crystal of size
0.20 × 0.10 × 0.05 mm was mounted and placed in a cold nitrogen
stream (-31 °C) on a Nonius FAST diffractometer equipped with an
area detector and monochromated Mo KR radiation (λ ) 0.710 73 Å).
Indexing revealed a primitive monoclinic cell. On the basis of
systematic absences, the space group was uniquely determined to be
P21/c. Complex 1 crystallizes with two interstitial dichloromethane
molecules per dimolybdenum unit. All of the non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen atoms,
except those from the solvent molecule, were found in a difference
map and also refined. A final difference map was essentially
featureless.
dinuclear units containing short metal-metal bonds, for example
with the quadruply bonded Mo2Cl84- and Re2Cl82- anions. This
study reveals still another coordination mode for the bdppp
ligand (Chart 3) in which the metal atoms are bridged by the
ligand through P and N atoms, leaving the remaining phosphorus
atom noncoordinated.
The synthesis, structures, and spectroscopic and electrochemi-
cal properties of a molybdenum compound and two rhenium
complexes containing the (Ph2P)2Py ligand are described in this
paper.
Experimental Section
General Procedures. All syntheses and purifications were carried
out under a N2 atmosphere using standard Schlenk techniques. All
solvents were freshly distilled under N2 from suitable drying agents.
Aldrich, Inc., supplied [Bun N]2Re2Cl8; 2,6-bis(diphenylphos-
4
phino)pyridine1d and K4Mo2Cl8 were synthesized using literature
2
methods. Dichloromethane-d2 was obtained from Cambridge Isotope
Laboratories.
Physical Measurements. Electrochemical measurements were
carried out on dichloromethane solutions that contained 0.1 M tetra-
n-butylammonium hexafluorophosphate (TBAH) as the supporting
electrolyte. A stream of nitrogen was bubbled through the solution
during the measurements. E1/2 values, determined as (Ep,a + Ep,c)/2,
were referenced to the Ag/AgCl electrode at room temperature (E1/2
)
+0.47 V for the ferrocenium/ferrocene couple). Voltammetric experi-
ments were carried out on a Bioanalytical Systems Inc. electrochemical
analyzer, Model 100; the scan rate was 100 mV/s at the Pt disk
electrode. The IR spectra were performed in the range 4000-400 cm-1
on a Perkin-Elmer 16PC FT-IR spectrometer using KBr pellets. The
1H and 31P {1H} NMR data were recorded at room temperature on a
UNITY-plus 300 multinuclear spectrometer operated at 300 and 121.4
MHz, respectively. Reference standards were 1H, CH2Cl2 (5.32 ppm),
and 31P, 85% H3PO4 (0.00 ppm). Elemental analyses were done by
Canadian Microanalytical Services, Ltd.; they were satisfactory.
Synthesis of Mo2Cl4(bdppp)2‚2CH2Cl2 (1‚2CH2Cl2). In a 100 mL
round-bottom flask, 0.164 g (0.260 mmol) of K4Mo2Cl8 and 1.60 g
(3.51 mmol) of bis(diphenylphosphino)pyridine were suspended in 25
mL of methanol. The mixture was refluxed for 5-6 h. Upon heating,
the color of the mixture changed from red to lime green. After
refluxing, the green solid was collected by filtration and washed
repeatedly with warm H2O, followed by methanol. Excess bdppp was
removed by washing the product with small portions of toluene.
Yield: 0.289 g (0.235 mmol, 90.5%). Crystals were grown by diffusion
of hexanes into a concentrated CH2Cl2 solution. 1H NMR (CD2Cl2):
δ 7.72 (mult), 7.45 (mult), and 7.25 (mult) ppm. 31P NMR (CD2Cl2):
δ +36.19 (s) and -9.60 (s) ppm. IR (KBr, cm-1): 3052 (w), 1585
(w), 1566 (m), 1544 (m), 1483 (s), 1433 (vs), 1365 (m), 1189 (m),
1138 (w), 1087 (s), 1027 (m), 1014 (m), 998 (m), 795 (m), 744 (vs),
691 (vs), 518 (vs).
Re2Cl4(bdppp)2. A green plate-shaped crystal with dimensions 0.20
× 0.18 × 0.10 mm was selected and fixed on the tip of a quartz fiber
with grease. It was then placed in a nitrogen stream at -100 °C in a
CAD-4S diffractometer equipped with monochromated Mo KR radia-
tion. Indexing based on 25 reflections in the range 20° < 2θ < 28°
resulted in a primitive monoclinic cell. No significant decay was found
during the data collection. An empirical absorption correction based
on azimuthal scans of 6 reflections with their ψ angles near 90° was
applied (0.5471-0.9986). The analysis of the systematic absences
unambiguously assigned the space group as P21/n. Anisotropic
displacement parameters were used for all non-hydrogen atoms.
Hydrogen atoms were included in the structure factor calculations at
idealized positions and were allowed to ride on the corresponding
carbon atoms. The final difference Fourier map showed no peaks higher
than 1 e/Å3.
Synthesis of Re2Cl4(bdppp)2 (2). A suspension containing [Bun N]2-
4
Re2Cl8 (0.1845 g, 0.15 mmol) and bdppp (0.3440 g, 0.75 mmol) in 20
mL of methanol was refluxed for 15 h. A green solid was collected
by filtration, washed with methanol (2 × 10 mL) and with small
portions of toluene, and then dried. Yield: 0.175 g (0.12 mmol, 77.1%).
Dark-green plate-shaped crystals were obtained by diffusion from a
CH2Cl2 solution after layering with hexanes. 1H NMR (CD2Cl2): δ
7.94 (mult), 7.47 (mult), and 7.24 (mult) ppm. IR (KBr, cm-1): 1584
(w), 1572 (w), 1560 (w), 1479 (s), 1433 (vs), 1350 (vs), 1263 (w),
(3) (a) Bino, A.; Cotton, F. A.; Fanwick, P. E. Inorg. Chem. 1979, 18,
3558. (b) Cotton, F. A.; Frenz, B. A.; Deganello, G.; Shaver, A. J.
Organomet. Chem. 1973, 50, 227. (c) Cotton, F. A.; Dikarev, E. V.;
Feng, X. Inorg. Chim. Acta 1995, 237, 19.
(4) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(5) SHELXTL V.5; Siemens Industrial Automation Inc.: Madison, WI,
1994.
(6) Sheldrick, G. M. In Crystallographic Computing 6; Flack, H. D.,
Parkanyi, L., Simon, K., Eds.; Oxford University Press: Oxford, U.K.,
1993; pp 111-122.
(2) Brencic, J. V.; Cotton, F. A. Inorg. Chem. 1970, 9, 351.

Multiply Bonded Dimetal Units of Mo and Re
Inorganic Chemistry, Vol. 37, No. 18, 1998 4613
Table 1. Crystallographic Data for Mo2Cl4(bdppp)2‚2CH2Cl2 (1‚2CH2Cl2), Re2Cl4(bdppp)2 (2), and [Bun N][Re2Cl7(bdppp)]‚CH2Cl2 (3‚CH2Cl2)
4
1‚2CH2Cl2
2
3‚CH2Cl2
formula
fw
space group
a, Å
b, Å
c, Å
Mo2P4N2Cl8C60H50
1898.38
P21/c (No. 14)
15.475(2)
11.5958(7)
16.7637(6)
Re2P4N2Cl4C58H46
1409.05
P21/n (No. 14)
16.173(2)
11.285(1)
28.771(4)
Re2P2N2Cl9C46H61
1395.36
P1h (No. 2)
16.054(2)
18.562(3)
20.205(6)
89.71(2)
73.21(1)
73.47(2)
5507(2)
4
R, deg
â, deg
γ, deg
V, Å3
Z
98.178(5)
96.17(1)
2977.6(4)
2
1.560
5221(1)
4
1.793
5.002
F
calc., g cm-3
1.683
4.920
µ, mm-1
0.929
radiation (λ, Å)
temp, °C
Mo KR (0.710 73)
Mo KR (0.710 73)
Mo KR (0.710 73)
-31
-100
-60
R1,a wR2b [I > 2σ(I)]
R1,a wR2b (all data)
0.0518, 0.1341
0.0807, 0.1479
0.0372, 0.0766
0.1072, 0.0961
0.0816, 0.1859
0.1189, 0.2217
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
Scheme 1
Scheme 2
[Bun N][Re2Cl7(bdppp)]‚CH2Cl2. All of the crystals were very
4+
favors the formation of complexes containing the Re2 core.
The actual reducing mechanism of this process is still not
defined. Nevertheless, it has been found that the use of other
solvents9 or “mild” reaction conditions may prevent or slow
down the reduction processes.10 In this work, when we reacted
4
weak diffractors; a green plate with dimensions 0.18 × 0.08 × 0.05
mm was selected. Indexing and refinement of 238 reflections gave a
primitive triclinic cell. Space group P1h was chosen and confirmed by
the successful refinement of the structure. Both tetrabutylammonium
cations and the CH2Cl2 solvent molecules were highly disordered. Thus,
only the non-hydrogen atoms of two crystallographically independent
2-
Re2Cl8 with bdppp in methanol at room temperature and
stopped the reaction after 1.5-2 h, we isolated the rhenium-
dirhenium complexes and those of one of the [Bun N]+ cations were
4
(III,III) complex 3, [Bun N][Re2Cl7(bdppp)]; it was soluble in
4
refined anisotropically. All hydrogen atoms were included in the
structure calculations but were not refined. The final difference map
contained some peaks associated with solvent molecules.
methanol in contrast to 2. Complex 3 has one bridging bdppp
ligand and can be regarded as an intermediate in the formation
of the doubly bridged rhenium(II,II) complex 2.
Results and Discussion
The identities of compounds 1, 2, and 3 have been established
by X-ray crystallographic studies and further confirmed by their
properties. Thus, complexes 2 and 3 exhibit typical electro-
chemical behavior for Re24+ and Re26+ cores, respectively. For
2 two couples are observed at -0.07 and +0.85 V, each
corresponding to a reversible one-electron oxidation, while the
CV of 3 shows a reversible one-electron reduction at +0.03 V
and an irreversible reduction at -0.75 V.
Description of Structures. Mo2Cl4(bdppp)2‚2CH2Cl2 (1‚2CH2-
Cl2) crystallizes in the monoclinic space group P21/c; there are
two dimolybdenum molecules and four dichloromethane mol-
ecules per unit cell. Re2Cl4(bdppp)2 (2) forms crystals in the
monoclinic space group P21/n with four dirhenium molecules
Reaction Chemistry. The dimolybdenum complex 1, iso-
lated in crystalline form with two molecules of CH2Cl2, was
synthesized by refluxing a mixture of K4Mo2Cl8 and bdppp in
methanol (Scheme 1). The conversion to Mo2Cl4(bdppp)2
proceeds in 90% yield. The analogous rhenium complex Re2-
Cl4(bdppp)2 (2) was obtained using [Bun N]2Re2Cl8 as a starting
4
material and by following a similar route (Scheme 2). This
reaction not only results in a ligand substitution but also is
accompanied by reduction of rhenium(III) to rhenium(II). It is
worth mentioning that reduction of Re2Cl82- by phosphines has
often been observed, especially when refluxing alcohols were
used as reaction media.7,8 Prolonging the reaction time also
(7) Ebner, J. R.; Walton, R. A. Inorg. Chem. 1975, 14, 1987.
(8) Root, D. R.; Blevins, C. H.; Lichtenberger, D. L.; Sattelberger, A. P.;
Walton, R. A. J. Am. Chem. Soc. 1986, 108, 953.
(9) Cotton, F. A.; Dikarev, E. V.; Petrukhina, M. A. J. Am. Chem. Soc.
1997, 119, 12541.
(10) Cotton, F. A.; Dikarev, E. V. Inorg. Chem. 1996, 35, 4738.

4614 Inorganic Chemistry, Vol. 37, No. 18, 1998
Cotton et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Mo2Cl4(bdppp)2 in 1‚2CH2Cl2 and Re2Cl4(bdppp)2 (2)
2
1
M-M
2.149(1)
2.228(5)
2.411(2)
2.402(2)
2.544(2)
3.259(2)
106.1(1)
103.56(5)
106.97(5)
87.18(5)
161.94(5)
166.6(1)
149.29(6)
2.2342(6)
2.097(9)
M-N
2.109(9)
2.374(3)
2.362(3)
2.397(3)
3.113(3)
102.2(3)
101.70(7)
105.91(7)
88.91(8)
160.41(6)
168.9(3)
151.6(1)
M-Cl
2.384(3)
2.377(3)
2.388(2)
2.980(3)
102.8(3)
100.42(7)
105.47(8)
90.14(8)
162.63(6)
166.8(3)
153.1(1)
M-Cl
M-P
M‚‚‚Pa
M-M-N
M-M-Cl
M-M-Cl
M-M-P
M-M-Pa
N-M-P
Cl-M-Cl
a Noncoordinated P atom.
(P-P ) dmpm, bis(dimethylphosphino)methane; dppm, bis-
(diphenylphosphino)methane; tdpm, tris(diphenylphosphino)-
methane; and dmdppm, dimethyl(diphenylphosphino)methane)
that have the same trans arrangement of bridging diphosphine
ligands with one bridgehead atom. The corresponding distances
in 1 fall in the ranges found for the â-isomers, 2.54-2.66 Å
for the Mo-P and 2.39-2.41 Å for the Mo-Cl. Molecule 1
has a totally eclipsed conformation, similarly to Mo2Cl4(dmpm)2
and Mo2Cl4(dppm)2, but unlike Mo2(NCS)4((Ph2P)Py)2 where
the twist angle, P-Mo-Mo-N, is 10.8°.
Figure 1. Perspective drawing of Mo2Cl4(bdppp)2 in 1‚2CH2Cl2. Atoms
are represented by their thermal ellipsoids at the 40% probability level.
Carbon atoms are shown as arbitrarily sized spheres and for clarity are
not labeled. Hydrogen atoms are omitted.
The Re-Re distance in 2 (2.2342(6) Å) matches well those
of other similar doubly bridged Re(II)-Re(II) species containing
a Re-Re triple bond.11b The average Re-P (2.393(3) Å),
Re-N (2.103(9) Å), and Re-Cl (2.374(3) Å) distances are
shorter than those of the Mo complex 1 (Table 2) as typically
found in analogous rhenium and molybdenum (or tungsten)
compounds. Unfortunately, the crystal structure of the di-
rhenium complex with two trans (diphenylphosphino)pyridine
(Ph2PPy) ligands is still unknown, so we cannot make direct
comparison to the metal-to-ligand distances in 2. Both Re-P
and Re-Cl bond distances seem normal if compared to those
of the â-isomers of Re2Cl4(P-P)214 (P-P ) dppm and dppa, bis-
(diphenylphosphino)amine). However, it is well-known that the
latter possess staggered conformations; this contrasts to the
ligand arrangement in 2, which is essentially eclipsed. The
torsion angles for this triply bonded Re complex 2 (1.1-9.3°)
are a little bit larger than for the quadruply bonded Mo analogue
1.
We can now turn to the central point of the discussion,
namely, the coordination mode of the (Ph2P)2Py ligand. In 1
and 2 the ligand, 2,6-bis(diphenylphosphino)pyridine, exhibits
a structural feature never seen before: it bridges over the M-M
multiply bonded units through the phosphorus and nitrogen
atoms, while the remaining phosphorus atom does not coordinate
to any metal atom. This appears to be due to the short metal-
metal separations found in the multiply bonded dimetal units
and the limited flexibility of the bdppp ligand. Another
contributing factor is the reluctance of the Mo24+ units to attach
axial ligands.15
Figure 2. Perspective drawing of Re2Cl4(bdppp)2 (2). Atoms are
represented by their thermal ellipsoids at the 40% probability level.
Carbon atoms are shown as arbitrarily sized spheres and for clarity are
not labeled. Hydrogen atoms are omitted.
per unit cell. The molecular structures 1 and 2 are very similar
(Figures 1 and 2). Both are composed of two trans bdppp
ligands bridging the dinuclear metal units in a head-to-tail
arrangement through one phosphorus and one nitrogen atom of
each ligand, forming an M2P2N2 framework; each metal center
also has two equatorial chloride ligands. The molecule of Mo2-
Cl4(bdppp)2 (1) (Figure 1) has a crystallographically imposed
inversion center located at the midpoint of the Mo-Mo bond,
whereas in the Re2Cl4(bdppp)2 molecule (Figure 2) the two
halves of the structure are crystallographically independent.
The metal-metal distance of 2.149(1) Å in 1 is in the normal
range for Mo-Mo quadruple bonds.11a The bond distances for
the bridging ligand Mo-P (2.544(2) Å) and Mo-N (2.228(5)
Å) are similar to those found in an analogous dimolybdenum
complex with monosubstituted pyridine, Mo2(NCS)4((Ph2P)-
Py)2,12 which are 2.545(2) and 2.228(5) Å, respectively. The
Mo-P and Mo-Cl distances in 1 (Table 2) may also be
(13) (a) Cotton, F. A.; Falvello, L. R.; Harwood, W. S.; Powell, G. L.;
Walton, R. A. Inorg. Chem. 1986, 25, 3949. (b) Abbott, E. H.; Bose,
K. S.; Cotton, F. A.; Hall, W. T.; Sekutowski, J. C. Inorg. Chem.
1978, 17, 3240. (c) Campbell, F. L., III; Cotton, F. A.; Powell, G. L.
Inorg. Chem. 1984, 23, 4222. (d) Cotton, F. A.; Eglin, J. L.; James,
C. A. Inorg. Chim. Acta 1993, 204, 175.
13
compared with those in the class of â-isomers Mo2Cl4(P-P)2
(14) (a) Barder, T. J.; Cotton, F. A.; Dunbar, K. R.; Powell, G. L.;
Schwotzer, W.; Walton, R. A. Inorg. Chem. 1985, 24, 2550. (b)
Costello, M. T.; Derringer, D. R.; Fanwick, P. E.; Price, A. C.; Rivera,
M. I.; Scheiber, E.; Siurek, E. W., III; Walton, R. A. Polyhedron 1990,
9, 573.
(11) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
2nd ed.; Oxford University Press: Oxford, U.K., 1993; (a) Table 3.1.1,
p 143; (b) Table 2.1.4, p 72; (c) Table 2.1.1, p 33.
(12) Cotton, F. A.; Matusz, M. Inorg. Chim. Acta 1989, 157, 223.

Multiply Bonded Dimetal Units of Mo and Re
Inorganic Chemistry, Vol. 37, No. 18, 1998 4615
Table 3. Selected Bond Distances (Å) and Angles (deg) for Two
Crystallographically Independent Molecules [Bun N][Re2Cl7(bdppp)]
4
in 3‚CH2Cl2
Re-Re
2.275(1)
2.487(5)
2.15(2)
2.646(5)
2.348(6)
2.299(6)
2.326(5), 2.345(5),
2.307(5), 2.330(5)
3.140(5)
88.2(1)
102.4(4)
166.0(1)
160.6(1)
2.280(1)
2.460(5)
2.15(2)
2.636(5)
2.353(5)
2.290(6)
2.328(6), 2.336(5),
2.314(5), 2.333(6)
3.132(5)
88.0(1)
100.6(4)
165.5(1)
159.7(1)
Re-P
Re-N
Re-Clax
Re-CltransP
Re-CltransN
Re-CltransCl
Re‚‚‚Pa
Re-Re-P
Re-Re-N
Re-Re-Clax
Re-Re-Pa
a Noncoordinated P atom.
Figure 3. Perspective drawing of one crystallographically independent
anion [Re2Cl7(bdppp)]-. Atoms are represented by their thermal
ellipsoids at the 40% probability level. Carbon atoms are shown as
arbitrarily sized spheres and for clarity are not labeled. Hydrogen atoms
are omitted.
(for two molecules) is 3.136(5) Å with a Re-Re-P angle of
160.2(1)°. The other axial position of the Re26+ unit is occupied
by a chlorine atom. Thus the two rhenium atoms have
coordination numbers 5 and 6, for Re(1) and Re(2), respectively
(Figure 3). The axial chloride has a relatively weak interaction
with the Re center as reflected by a bond length of 2.641(5) Å,
which is ca. 0.3 Å larger than the other Re-Cl distances in 3
(average 2.325(5) Å).
The Re-Re distances (2.275(1) and 2.280(1) Å) in the two
molecules are among the longest known for quadruply bonded
dirhenium species.11c They are even longer than that in the triply
bonded complex 2. The reason for the elongation must be the
presence of an axial chloride ligand in 3. This phenomenon
was also observed in Re2(DMF)4L2,18 which have Re-Re bond
lengths of 2.276 and 2.305 Å for L ) Cl and OMe, respectively.
Other examples are the complexes (Ph4As)2Re2(NCS)8‚2L19 with
rhenium-rhenium distances of 2.270 Å for L ) (CH3)2O and
2.296 Å for L ) C5H5N.
The Mo‚‚‚P distances on both ends of molecule 1 are 3.259(2)
Å, and the angles Mo-Mo-P are 161.94(5)°. Recently, similar
molybdenum complexes bridged by dpmp ligands (dpmp ) bis-
((diphenylphosphino)methyl)phenylphosphine), trans-[Mo2(O2-
CR)2(dpmp)2](BF4)2 (R ) Me, But, Ph), were reported16 to have
the same type of coordination mode. The Mo‚‚‚P separations
vary from 3.234 to 3.446 Å, indicative of the absence of any
bonding interaction. Additional evidence of the nonbonding
character of this separation in 1 is the fact that the distance
from the Mo(1) atom to the P(2A) atom connected to the other
side of the dimolybdenum unit (Figure 1) is even shorter (3.248
Å).
In the case of the rhenium complex 2 these distances are
significantly different at both ends of molecule: the Re(2)-
P(3) distance is 2.980(3) Å, while the Re(1)-P(1) distance is
longer (3.113(3) Å). This shortening of the Re‚‚‚P separations
is in accord with the longer M-M distances and shorter M-L
distances in the rhenium compound 2 when compared to the
molybdenum complex 1. Still the Re(2)‚‚‚P(2) contact cannot
be considered a bonding interaction. There is no dpmp analogue
of complex 2. All known rhenium complexes with dpmp
ligands of the formula [Re2Cl3(dpmp)2]X (X ) Cl, PF6)17
possess a structure in which the triphosphine is doubly bridging
the dimetal unit with the central phosphorus atom coordinated
to one rhenium atom, and both terminal phosphorus atoms
coordinated to the other rhenium atom. This can be attributed
to the greater flexibility of the methylene-bridged triphosphine
ligands compared to the (Ph2P)2Py.1i
Complex 3 crystallizes in the space group P1h with two pairs
of crystallographically independent dirhenium anionic molecules
per unit cell. There are also four highly disordered tetrabutyl-
ammonium cations and dichloromethane molecules. The dirhe-
nium anion [Re2Cl7(bdppp)]- (Figure 3) is the first example of
a compound with only one bdppp ligand bridging a dimetal unit.
The ligand coordination mode is of the same type as for the
molecules 1 and 2 (Chart 3) with one noncoordinated phos-
phorus atom “blocking”, but not binding to, one of the axial
positions of the dirhenium unit. The average Re‚‚‚P separation
The Re-Clax distances in 3 (Table 3) are comparable to those
in other dirhenium complexes having axial chlorides, e.g., Re2-
Cl5(dppm)220 (2.575 Å), Re2Cl5(NO)(dmpm)221 (2.621 Å), and
Re2(DMF)4Cl2 (2.527 Å).18 The rest of the metal-ligand bond
lengths in [Re2Cl7(bdppp)]- are very similar to the correspond-
ing distances in 2. However, there is a clear trans influence in
3 but not in 2. The Re-P and Re-N bonds in 3 are trans to
the Re-Cl bonds, while in 2 the former are trans to the Re-N
and Re-P bonds and all Re-Cl bonds are trans to other Re-
Cl bonds. That led to the slight elongation of the Re-P and
Re-N distances for 3 but not to the doubly bridged complex
2. Also, there are three sets of Re-Cl bond lengths among
them: those trans to N atoms are the shortest, while those trans
to P atoms are the longest. Like Re2Cl4(bdppp)2 (2), the
molecule of 3 is slightly twisted from the eclipsed configuration
although the L-Re-Re-L torsion angles are all less than 12°.
Conclusions
Dimolybdenum(II) and dirhenium(II) complexes M2Cl4-
(bdppp)2 (1, 2) were obtained by refluxing quadruply bonded
Mo2Cl84- and Re2Cl82- anions with bdppp in methanol. A rare
example of a quadruply bonded Re26+ unit bridged by only one
neutral ligand [Bun N][Re2Cl7(bdppp)] (3) was found to be an
4
intermediate in the latter process. In all cases coordination of
the bdppp ligand to multiply bonded units with short metal-
metal separations dictates the novel type of bonding for this
(15) Collins, D. M.; Cotton, F. A.; Murillo, C. A. Inorg. Chem. 1976, 15,
1861.
(18) Cotton, F. A.; Ren, T. J. Am. Chem. Soc. 1992, 114, 2495.
(19) Cotton, F. A.; Matusz, M. Inorg. Chem. 1987, 26, 3468.
(16) Tanase, T.; Igoshi, T.; Yamamoto, Y. Inorg. Chim. Acta 1997, 256,
61.
(20) Cotton, F. A.; Shive, L. W.; Stults, B. R. Inorg. Chem. 1976, 15, 2239.
(21) Ara, I.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 1991, 30, 1973.
(17) Cotton, F. A.; Matusz, M. Inorg. Chem. 1987, 26, 984.

4616 Inorganic Chemistry, Vol. 37, No. 18, 1998
Cotton et al.
ligand: a bidentate coordination through the nitrogen and only
one phosphorus atom with the second phosphine unit remaining
noncoordinated.
Acknowledgment. We are grateful to the National Science
Foundation for support of this work at Texas A&M University
and the Department of Chemistry at UCR.
We believe that the new compounds obtained in this work
could serve as precursors for the synthesis of polynuclear
bimetallic complexes containing metal-metal bonds. One of
the possible ways to achieve this goal is to use mononuclear
chlorides of Co and Sn, for example, capable of coordinating
through the dangling phosphine end. These studies are currently
under way.
Supporting Information Available: Three X-ray files, in CIF
format, are available on the Internet only. Access information is given
on any current masthead page.
IC980455D