Pyrazolyl-bridged iridium dimers. 18. Influence of metal-metal bonding on the geometry of diiridium(II) adducts and hydrido-diiridium complexes formed from the diiridium(I) prototype [Ir(μ-pz)(PPh3)(CO)]2 (pzH = pyrazole) by dihydrogen addition or protonation

Article abstract of DOI:10.1021/ic010448g

Slow uptake of molecular dihydrogen by the diiridium(I) prototype [Ir(μ-pz)(PPh₃)(CO)]₂ (1: pzH = pyrazole) is accompanied by formation of a 1,2-dihydrido-diiridium(II) adduct [IrH(μ-pz)(PPh₃)(CO)]₂ (2), for which an X-ray crystal structure determination reveals that (unlike in 1) the PPh₃ ligands are axial, with the hydrides occupying trans coequatorial positions across the Ir-Ir bond (2.672 A). Reaction with CCl₄ effects hydride replacement in 2, affording the monohydride Ir₂H(Cl)(μ-pz)₂(PPh₃)₂(CO) ₂ (3) in which Ir-Ir = 2.683 A. At one metal center, H is equatorial and PPh₃ is axial, while at the other, Cl is axial as is found in the symmetrically substituted product [Ir(μ-pz)(PPh₃)(CO)Cl]₂ (4) (Ir-Ir = 2.754 A) that is formed by action of CCl₄ on 1. Treatment of 1 with I₂ yields the diiodo analogue 5 of 4, which reacts with LiAlH₄ to afford the isomorph Ir₂H(I)(μ-pz)₂(PPh₃)₂(CO) ₂ (6) of 3 (Ir-Ir = 2.684 A). Protonation (using HBF₄) of 1 results in formation of the binuclear cation Ir₂H(μ-pz)₂(PPh₃)₂(CO) ₂⁺ (7: BF₄^- salt), which shows definitive evidence (from NMR) for a terminally bound hydride in solution (CH₂Cl₂ or THF), but 7 crystallizes as an axially symmetric unit in which Ir-Ir = 2.834 A. Reaction of 7 with water or wet methanol leads to isolation of the cationic diiridium(III) products [Ir₂H₂(μ-OX)(μ-pz)₂(PPh₃) ₂(CO)₂]BF₄ (8, X = H; 9, X = Me).

Full text of DOI:10.1021/ic010448g

Inorg. Chem. 2002, 41, 1412−1420
Pyrazolyl-Bridged Iridium Dimers. 18.1 Influence of Metal−Metal Bonding
on the Geometry of Diiridium(II) Adducts and Hydrido-diiridium
Complexes Formed from the Diiridium(I) Prototype [Ir(µ-pz)(PPh3)(CO)]2
(pzH ) Pyrazole) by Dihydrogen Addition or Protonation
Ron D. Brost, Gordon W. Bushnell, Daryll G. Harrison, and Stephen R. Stobart*
Department of Chemistry, UniVersity of Victoria, Victoria, British Columbia, Canada V8W 2Y2
Received April 26, 2001
Slow uptake of molecular dihydrogen by the diiridium(I) prototype [Ir(µ-pz)(PPh3)(CO)]2 (1: pzH ) pyrazole) is
accompanied by formation of a 1,2-dihydrido-diiridium(II) adduct [IrH(µ-pz)(PPh3)(CO)]2 (2), for which an X-ray
crystal structure determination reveals that (unlike in 1) the PPh3 ligands are axial, with the hydrides occupying
trans coequatorial positions across the Ir−Ir bond (2.672 Å). Reaction with CCl4 effects hydride replacement in 2,
affording the monohydride Ir2H(Cl)(µ-pz)2(PPh3)2(CO)2 (3) in which Ir−Ir ) 2.683 Å. At one metal center, H is
equatorial and PPh3 is axial, while at the other, Cl is axial as is found in the symmetrically substituted product
[Ir(µ-pz)(PPh3)(CO)Cl]2 (4) (Ir−Ir ) 2.754 Å) that is formed by action of CCl4 on 1. Treatment of 1 with I2 yields
the diiodo analogue 5 of 4, which reacts with LiAlH to afford the isomorph Ir2H(I)(µ-pz)2(PPh3)2(CO)2 (6) of 3 (Ir−Ir
4
) 2.684 Å). Protonation (using HBF ) of 1 results in formation of the binuclear cation Ir2H(µ-pz)2(PPh3)2(CO)2+ (7:
4
BF - salt), which shows definitive evidence (from NMR) for a terminally bound hydride in solution (CH Cl2 or THF),
4
2
but 7 crystallizes as an axially symmetric unit in which Ir−Ir ) 2.834 Å. Reaction of 7 with water or wet methanol
leads to isolation of the cationic diiridium(III) products [Ir2H (µ-OX)(µ-pz)2(PPh3)2(CO)2]BF (8, X) H; 9, X) Me).
2
4
Introduction
change, but it offers a rationalization in terms of simple
electron counting for the diamagnetism of products that are
formally the result of a unit increase in the oxidation state
at each metal center. It is in this domain that the d7 +2 state
has emerged3 as an important conjunction in the chemistry
Binuclear transition-metal complexes in which a folded
bridging framework holds coordinatively unsaturated d8 sites
in proximity to each other1,2 are characterized by a nominal
bond order of zero between the two metal atoms. This results
from orbital overlap and occupancy that can be represented
in a simplified way as (dσ)2(dσ*)2. It follows that reductive
activation of an external substrate by interaction with the
formally antibonding HOMO (dσ*), which initiates a two-
center oxidative addition reaction,2 will be accompanied by
the development of a metal-metal bond between the two
adjacent centers. Not only is this a physically demonstrable
of iridium. Thus, in particular, it has been shown that d7
2
complexes (IrII-IrII) can function as stable termini to
oxidative steps using binuclear d82 prototypes;4 can exist in
equilibrium with the latter;5 or can be accessed reductively
from oxidized d6 analogues.6 It is striking, therefore, that
2
although concerted addition of dihydrogen to a mononuclear
metal center (including, conspicuously, d8 IrI) is a familiar
event,7 only a handful of reports exist of iridium dimers
* Corresponding author. E-mail: stobartz@uvvm.uvic.ca.
(1) Part 17: Arthurs, M. A.; Bickerton, J.; Stobart, S. R.; Wang, J.
Organometallics 1998, 17, 2743.
(3) Coleman, A. W.; Eadie, D. T.; Stobart, S. R.; Zaworotko, M. J.;
Atwood, J. L. J. Am. Chem. Soc. 1982, 104, 922. Cotton, F. A.;
Laheurta, P.; Sanau, M.; Schwotzer, W. J. Am. Chem. Soc. 1985, 107,
8284.
(4) Bushnell, G. W.; Fjeldsted, D. O. K.; Stobart, S. R.; Wang, J.
Organometallics 1996, 15, 3785.
(5) Brost, R. D.; Fjeldsted, D. O. K.; Stobart, S. R. J. Chem. Soc., Chem.
Commun. 1989, 488.
(6) Bailey, J. A.; Grundy, S. L.; Stobart, S. R. Organometallics 1990, 9,
536.
(2) Schmidbaur, H.; Franke, R. Inorg. Chim. Acta 1975, 13, 85. Lewis,
N. S.; Mann, K. R.; Gordon, J. G. H.; Gray, H. B. J. Am. Chem. Soc.
1976, 98, 7461. Che, C. M.; Schaefer, W. P.; Gray, H. B.; Dickson,
M. K.; Stein, F.; Roundhill, D. M. J. Am. Chem. Soc. 1982, 104, 4253.
Marshall, J. L.; Stobart, S. R.; Gray, H. B. J. Am. Chem. Soc. 1984,
106, 3027. Fackler, J. P., Jr.; Murray, H. H.; Basil, J. D. Organome-
tallics 1984, 3, 821. Ling, S. S. M.; Jobe, I. R.; Manojlovic-Muir, L.;
Muir, K. W.; Puddephatt, R. J. Organometallics 1985, 4, 1198.
1412 Inorganic Chemistry, Vol. 41, No. 6, 2002
10.1021/ic010448g CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/21/2002

Pyrazolyl-Bridged Iridium Dimers
reacting similarly.8-11 Direct uptake of molecular hydrogen
by any neutral homobinuclear transition-metal complex has
in fact remained distinguished by its rarity,12 although such
addition13 to a mixed-valence (Ir0-IrII, i.e., d9-d7) diiridium
core, together with the demonstration14 that parahydrogen-
induced polarization (PHIP) can be used to characterize its
stereochemical profile, are two very recent and exciting
developments in this context.
reactivity of this protonated complex with dihydrogen, we
have found that it is susceptible to further oxidation. In
particular, it is extremely sensitive to hydrolysis, undergoing
oxidative addition of water, a process that remains19 ex-
tremely rare either at mononuclear or binuclear centers.
Experimental Section
A. General Considerations. Synthetic methodology and instru-
ments used for spectroscopic measurements have been referred to
extensively in earlier papers in this series.1,15,16 The isotopically
labeled reagents 13CO and triphenylphosphine-d15 were used as
received (Aldrich) or obtained by using published procedures.20
Conductivities were measured in acetone solution using a Copen-
hagen conductivity meter with a CDC324 small-volume electrode
(nominal cell constant ) 0.316 cm) and calibration versus NHEt3-
Cl.
B. Synthesis of Neutral Diiridium Complexes. i. [Ir(µ-pz)-
(PPh3)(CO)]2 (1). This complex was synthesized as described15
earlier as an orange-red microcrystalline solid that deteriorates
slowly in air. An isotopomer [Ir(µ-pz)(PPh3)(13CO)]2 (1a) was
prepared as follows: a solution of the precursor [Ir(µ-pz)(COD)]2
(100 mg, 0.14 mmol) in tetrahydrofuran (20 mL) was frozen at
-196° C, evacuated, and then exposed to gaseous 13CO (30 mL at
1.0 atm, 1.34 mmol). On warming to ambient temperature, followed
by rapid stirring, the initially red solution turned intensely yellow
within 25 min. Addition of triphenylphosphine (73 mg, 0.28 mmol)
led to immediate gas evolution and a color change to bright orange;
the product was recovered15 in 91% yield. An isotopomer of 1a,
[Ir(µ-pz)(PPh3-d15)(CO)]2 (1b), may be obtained (>95% atomic
purity) by addition of labeled phosphine to 1a after the carbonylation
step.
ii. [IrH(µ-pz)(PPh3)(CO)]2 (2). A slurry of 1 (1.12 g, 1.02 mmol)
in diethyl ether (35 mL) was formed in a glass liner fitted to a Parr
Instrument Co. model 316 high-pressure reactor, which was then
purged and pressurized to 1000 psig with dihydrogen gas. After
24 h at 35° C,the supernatant was decanted, and the solid residue
composed of a mixture of small colorless crystals and white
microcrystalline material (0.82 g, 0.070 mmol, 73%) was recovered.
A crystal suitable for X-ray diffraction was collected by hand-
sorting. Anal. Calcd for C44H38Ir2N4O2P2: C, 48.0; H, 3.5; N, 5.1.
Found: C, 47.6; H, 3.6; N, 5.3. Stirring solutions of compound 1
in benzene or toluene in an atmosphere of dihydrogen led to very
slow deposition (over 48 h) of 2 as a fine, pale yellow powder.
iii. Ir2H(Cl)(µ-pz)2(PPh3)2(CO)2 (3). Carbon tetrachloride (5
mL) was added to a solution of 2 (100 mg, 0.09 mmol) in
dichloromethane (25 mL). While the mixture was stirred at 20° C,
it turned from colorless to yellow. After 18 h, the solvent was
removed, leaving a yellow solid that was recrystallized from
dichloromethane/hexanes to yield air-stable, X-ray quality material
(80 mg, 0.07 mmol, 78%). Anal. Calcd for C44H37ClIr2N4O2P2: C,
46.5; H, 3.3; N, 4.9. Found: C, 46.2; H, 3.3; N, 4.9.
In our preliminary report9 describing facile two-fragment,
two-center oxidative addition as a characteristic of the
prototypal iridium(I) dimer [Ir(µ-pz)(PPh3)(CO)]2 (1: pzH )
pyrazole), we indicated that as well as reacting in such a
manner with15 dihalogens and alkyl halides, dihydrogen also
adds to 1 to afford a 1,2-dihydrido-diiridium(II) adduct [IrH-
(µ-pz)(PPh3)(CO)]2 (2). In the interim, this has remained one
of the very few8-14 examples of such behavior: it is not
observed for analogues of 1, including16 [Ir(µ-pz)(CO)2]2
and17 [Ir(µ-pz)(COD)]2 (COD ) cycloocta-1,5-diene), nor
is it paralleled in an extensive study by Oro and co-workers18
of the reactivity of other dimers related to 1. We describe
below the verification of the structure of 2 by using X-ray
diffraction as well as by its derivatization to a crystallo-
graphically characterized monohydrido analogue Ir2H(Cl)-
(µ-pz)2(PPh3)2(CO)2 (3), together with the formation of the
iodo analogue of 3 by monosubstitution of the diiodo adduct
[IrI(µ-pz)(PPh3)(CO)]2 of 1. A cationic diiridium(II) product
of protonation of 1, [Ir2H(µ-pz)2(PPh3)2(CO)2]BF4, has also
been isolated. The bimetallic core of the cation adopts an
axially symmetric structure in the crystalline state, with the
single hydride ligand assigned to a bridging position across
an iridium-iridium contact that is contracted to within
bonding range (2.834 Å). This symmetrical structure clearly
does not persist in solution, however, because NMR spec-
troscopy reveals differential coupling (ABX array) of a high-
field proton to nonequivalent phosphorus centers, an obser-
vation that is consistent only with terminal attachment of H
to a unique Ir atom. Although we have yet to investigate the
(7) James, B. R. Homogeneous Hydrogenation; Wiley & Sons: New York,
1973. See also James, B. R. In ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. S., Abel, E. W., Eds.; Pergamon
Press: New York, 1982; Vol. 8, Chapter 51. Collman, J. P.; Hegadus,
L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of
Organotransition Metal Chemistry; University Science Books: Mill
Valley, CA, 1987; Chapter 10.
(8) Bonnet, J. J.; Thorez, A.; Maisonnat, A.; Galy, J.; Poilblanc, R. J.
Am. Chem. Soc. 1979, 101, 5940.
(9) Beveridge, K. A.; Bushnell, G. W.; Dixon, K. R.; Eadie, D. T.; Stobart,
S. R.; Atwood, J. L.; Zaworotko, M. J. J. Am. Chem. Soc. 1982, 104,
920.
(10) Sutherland, B. R.; Cowie, M. Organometallics 1985, 4, 1801. Vaartstra,
B. A.; Cowie, M. Inorg. Chem. 1989, 28, 3138.
(11) Schnabel, R. C.; Roddick, D. M. Organometallics 1996, 15, 3550.
(12) Jime´nez, M. V.; Sola, E.; Mart´ınez, A. P.; Lahoz, F. J.; Oro, L. A.
Organometallics 1999, 18, 1125.
(13) Heyduk, A. F.; Nocera, D. G. J. Am. Chem. Soc. 2000, 122, 9145.
(14) Oldham, S. M.; Houlis, J. F.; Sleigh, C. J.; Duckett, S. B.; Eisenberg,
R. Organometallics 2000, 19, 2985.
iv. [IrCl(µ-pz)(PPh3)(CO)]2 (4). Carbon tetrachloride (3 mL)
was stirred with 1 (75 mg, 0.07 mmol) in diethyl ether (20 mL)
for 1 h, during which time the solution changed color from bright
orange to yellow. Removal of the solvent left an oil. Redissolution
(15) Atwood, J. L.; Beveridge, K. A.; Bushnell, G. W.; Dixon, K. R.; Eadie,
D. T.; Stobart, S. R.; Zaworotko, M. J. Inorg. Chem. 1984, 23, 4050.
(16) Beveridge, K. A.; Bushnell, G. W.; Stobart, S. R.; Atwood, J. L.;
Zaworotko, M. J. Organometallics 1983, 2, 1447.
(17) Harrison, D. G.; Stobart, S. R. J. Chem. Soc., Chem. Commun. 1986,
285.
(19) Burn, M. J.; Fickes, M. G.; Hartwig, J. F.; Hollander, F. J.; Bergman,
R. G. J. Am. Chem. Soc. 1993, 115, 5875. Yoon, M.; Tyler, D. R. J.
Chem. Soc., Chem. Commun. 1997, 639. Kaplan, A. W.; Bergman,
R. G. Organometallics 1997, 16, 1106. Tani, K.; Iseki, A.; Yamagata,
T. Angew. Chem., Int. Ed. 1998, 37, 3381. Dorta, R.; Togni, A.
Organometallics 1998, 17, 3423. Stobart, S. R.; Zhou, X.; Cea-
Olivares, R.; Toscano, A. Organometallics 2001, 20, 4766.
(18) Tejel, C.; Ciriano, M. A.; Lo´pez, J. A.; Lahoz, F. J.; Oro, L. A.
Organometallics 1997, 16, 4718 and references therein.
(20) Bianco, V. D.; Doronzo, S. Inorg. Synth. 1976, 16, 18.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1413

Brost et al.
in dichloromethane (3 mL) followed by careful addition of a layer
of hexanes led to slow crystallization of the pale yellow, air-stable
product (60 mg, 0.05 mmol, 75%). Anal. Calcd for C44H36Cl2-
Ir2N4O2P2: C, 45.2; H, 3.1; N, 4.8. Found: C, 45.5; H, 3.2; N, 5.0.
v. [IrI(µ-pz)(PPh3)(CO)]2 (5). Crystalline diiodine (94 mg, 0.74
mmol) was dissolved in tetrahydrofuran (3 mL) and then added
drop-by-drop to a slurry of 1 (0.41 g, 0.37 mmol) in diethyl ether
(20 mL). A bright yellow precipitate was immediately deposited;
after 15 min, the precipitate was allowed to settle, and then the
supernatant was removed. The solid residue was washed thoroughly
with hexanes and then dried under vacuum to yield the product as
a light yellow powder (0.49 g, 0.36 mmol, 97%). Anal. Calcd for
C44H36I2Ir2N4O2P2: C, 39.1; H, 2.7; N, 4.1. Found: C, 38.7; H,
2.5; N, 4.0.
vi. Ir2H(I)(µ-pz)2(PPh3)2(CO)2 (6). An excess of solid lithium
aluminum hydride (50 mg, 1.3 mmol) was carefully added to a
slurry of 5 (0.25 g, 0.19 mmol) in diethyl ether (20 mL). After
agitation for 30 min, a pale yellow precipitate was allowed to settle.
Removal of the solvent, washing the precipitate with ether, and
then redissolution of the precipitate in dichloromethane (30 mL)
was followed by filtration through a Celite plug and then concentra-
tion to a 5 mL volume. Thereafter, the product was deposited slowly
as pale yellow, air-stable needles (50 mg, 0.04 mmol, 22%). Anal.
Calcd for C44H37IIr2N4O2P2: C, 43.1; H, 3.0; N, 4.6. Found: C,
43.2; H, 2.9; N, 4.5.
C. Synthesis of Cationic Diiridium Complexes. i. [Ir2H(µ-
pz)2(PPh3)2(CO)2]BF4 (7). To a solution of 1 (180 mg, 0.16 mmol)
in diethyl ether (60 mL) was added tetrafluoroboric acid diethyl
etherate (5 drops, excess). Precipitation of a bright yellow solid
occurred immediately; after 20 min, this solid was recovered by
removal of the supernatant, followed by repeated washing of the
residue with diethyl ether. After the product was dried under
vacuum, it was collected as a bright yellow powder (178 mg, 0.15
mmol, 94%). Anal. Calcd for C44H37BF4Ir2N4O2P2: C, 44.52; H,
3.14; N, 4.72. Found: C, 44.48; H, 3.07; N, 4.66. Conductivity (in
acetone): 99 Ω-1 cm2 mol-1. Crystals suitable for X-ray diffraction
were grown at -30° C from a dichloromethane solution under a
layer of hexanes. In another experiment, addition of tetrafluoroboric
acid diethyl etherate (10 drops, excess) to a rapidly agitated slurry
of 1 (0.75 g, 0.68 mmol) in diethyl ether (40 mL) led to instant
precipitation of a pale yellow solid. This solid was allowed to settle,
and after 10 min, it was collected as a dry, air-stable, pale yellow
powder (0.8 g, 0.68 mmol, 99%). Found: C, 44.3; H, 3.1; N, 4.9.
Addition of diethyl ether to a saturated solution of this material in
tetrahydrofuran resulted in the slow deposition of crystals suitable
for X-ray diffraction.
iv. [Ir2(µ-I)(µ-pz)2(PPh3)2(CO)2]BF4 (10). When silver tetra-
fluoroborate (43 mg, 0.22 mmol) in acetone (5 mL) was added
dropwise to a stirred solution of 5 (150 mg, 0.11 mmol) in acetone
(20 mL), the color of the solution faded in 60 s, and a yellow solid
precipitated. Filtration through Celite followed by removal of
acetone left a yellow oil that was washed with ether (2 × 10 mL)
and then with hexanes (10 mL). This procedure afforded the product
as a deep yellow, air-sensitive powder. Anal. Calcd for C44H36-
BF4IIr2N4O2P2: C, 40.2; H, 2.7; N, 4.3. Found: C, 40.2; H, 2.4;
N, 4.3. Conductivity (in acetone): 108 Ω-1 cm2 mol-1
.
v. [Ir2H(µ-I)(µ-pz)2(I)(PPh3)2(CO)2]BF4 (11). Slow addition of
tetrafluoroboric acid diethyl etherate (5 drops, excess) to a stirred
solution of 5 (200 mg, 0.15 mmol) in dichloromethane (20 mL)
led to a color change from yellow to deep red within 15 s. After
10 min, the volume was reduced to 5 mL, and ether (15 mL) was
added to precipitate a solid that was separated and then washed
with ether. Drying in vacuo left an air-stable, brick-red product
(196 mg, 0.14 mmol, 92%). Anal. Calcd for C44H37BF4I2-
Ir2N4O2P2: C, 36.7; H, 2.6; N, 3.9. Found: C, 36.4; H, 2.4; N,
3.6. Conductivity (in acetone): 101 Ω-1 cm2 mol-1
.
D. Reaction of 1 with Trifluoroacetic Acid. To 1 (0.37 g, 0.34
mmol) in diethyl ether (20 mL) was added trifluoroacetic acid (10
drops, excess) with stirring (1 h). Evaporation of the clear yellow
solution to dryness followed by washing with hexanes (3 × 10
mL) left behind the majority of a solid white product, 12 (0.32 g,
0.23 mmol, 71%). Anal. Calcd for C48H38F6Ir2N4O6P2: C, 43.4;
H, 2.9; N, 4.2. Found: C, 43.2; H, 2.7; N, 4.3. Conductivity (in
acetone): 0.7 Ω-1 cm2 mol-1
.
E. X-ray Crystallography. All procedures conformed to those
adopted earlier.4,15,16 Additional details, including metric parameters,
are included in the Supporting Information. Crystal data for
compounds 2-4, 6, and 7 are collected in Table 1. For structures
2-4 and 6, phenyl carbon atoms as well as hydrogen atoms were
refined isotropically, as is discussed further in the Supporting
Information.
Results
Uptake of gaseous hydrogen by orange-yellow solutions
of the diriidium(I) complex [Ir(µ-pz)(PPh3)(CO)]2 (1) is very
slow at ambient pressure. At 70 bar, however, this process
accounts for complete conversion within 24 h to an almost
colorless solid, [IrH(µ-pz)(PPh3)(CO)]2 (2), that was char-
acterized analytically as a product of dihydrogen addition.
Further insight into the nature of this compound is provided
by the spectroscopic data collected in Table 2. IR spectra
(using KBr pellets) show a very strong band attributable to
ii. [Ir2H2(µ-OH)(µ-pz)2(PPh3)2(CO)2]BF4 (8). Compound 7 (100
mg, 0.09 mmol) was dissolved in dichloromethane (20 mL), water
(3 drops) was added, and then the solution was stirred for 6 h.
Volatile material was pumped away, and then the yellow oily
residue was washed with hexanes (3 × 10 mL). Drying overnight
in vacuo afforded a light yellow, air-stable solid (93 mg, 0.08 mmol,
93%). Anal. Calcd for C44H39BF4Ir2N4O3P2: C, 43.2; H, 3.1; N,
4.6. Found: C, 43.7; H, 3.2; N, 4.3. Conductivity (in acetone): 100
Ω-1 cm2 mol-1. Use of dideuterium oxide instead of water yielded
an isotopomer of 8, 8-2H2.
iii. [Ir2H2(µ-OMe)(µ-pz)2(PPh3)2(CO)2]BF4 (9). Compound 7
(80 mg, 0.07 mmol) was stirred in methanol (10 mL) for 12 h. The
slightly turbid solution was evaporated under vacuum, leaving a
light yellow oil. This oil was washed with hexanes and then dried
in vacuo to give a yellow powder (65 mg). The product was shown
by NMR spectroscopy to contain ca. 30% of 8, which could not be
removed by fractional crystallization.
ν
CO, which is shifted to higher wavenumbers compared to
the corresponding band of 1. A weak, broad feature above
2150 cm-1 is also tentatively assigned to νIrH, although here
as well as in related hydrides (see Table 2) identification of
the band center was somewhat arbitrary, and its position is
likely to be influenced by vibrational mixing with νCO.
21 In
1
the H NMR (see Table 2) spectrum, three resonances of
equal intensity for hydrogens on the bridging-pz ligands
correspond to15 symmetrical disubstitution along the Ir2 axis,
and a single high-field proton signal centered at -16.15 ppm
shows triplet structure, J ) 7.6 Hz. The latter is indicative
of “virtual coupling” to two chemically equivalent P nuclei
that are strongly coupled to each other, suggesting a diaxial
(21) Poilblanc, R. Inorg. Chim. Acta 1982, 62, 75. Guilmet, E.; Maisonnat,
A.; Poilblanc, R. Organometallics 1983, 2, 1123.
1414 Inorganic Chemistry, Vol. 41, No. 6, 2002

Pyrazolyl-Bridged Iridium Dimers
Table 1. Crystal Dataa
compound
2
3
4
6
7
Ir2P2O2N4C44H38
1101.2
4241.0
I2/a (No. 15)
33.065(6)
13.657(8)
19.357(8)
90
109.20(4)
90
8255(7)
8
Ir2ClP2O2N4C44H37
1135.6
4249.0
Pbca
16.748(4)
20.027(5)
24.462(4)
90
90
90
8205(3)
8
1.84
Ir2Cl2P2O2N4C44H38
1170.1
967.0
Ir2IP2O2N4C44H37
1227.1
4284.0
Pbca
16.772(4)
20.513(6)
24.287(6)
90
90
90
8356(3)
8
1.95
Ir2P2F4O2N4C44BH37
1187.0
2284.0
fw (g/mol)
F(000) (e)
space group
a (Å)
b (Å)
c (Å)
P1h (No. 2)
11.002(1)
13.035(2)
15.281(2)
110.42(1)
93.47(2)
93.83(1)
2041(3)
2
C2/c
10.303(5)
23.372(9)
17.931(8)
90
96.85(7)
90
4287(3)
4
1.84
R (deg)
â (deg)
γ (deg)
cell volume
Z
D(calc) (g cm-3
cryst size (mm)
)
1.77
1.91
0.15 × 0.21 × 0.06
0.1 × 0.1 × 0.05
0.1 × 0.1 × 0.05
0.3 × 0.1 × 0.03
0.2 × 0.2 × 0.2
abs coeff (cm-1
)
65.36
66.43
67.43
71.90
63.12
data quadrant (2θ)
no. of unique reflections
no. of reflections in L.S.
no. of parameters.
GOF
(h,+k,+l < 451
3843
1461
162
1.6557
0.0956
0.1001
0.0005
+h,+k,+l < 401
3837
2732
+h, (k, (l < 401
3429
2381
255
1.8718
0.0688
0.0850
0.0005
+h,+k,+l < 451
5472
3866
(h,+k,+l < 50
3773
2871
286
1.3460
0.0409
0.0543
0.0005
246
246
2.1104
0.0741
0.0921
0.007
1.6914
0.0652
0.0786
0.0005
R
Rw
max shift/esd
a λradiation ) 0.71069 Å, T ) 295 K, weak reflection cutoff I/σ(I) < 2.5.
Scheme 1
relationship between the phosphine ligands in a structure
derived from that of 1 by entry of H at an equatorial site at
each Ir center. Accordingly, the 31P{1H} NMR spectrum
consists of a singlet (Table 2); there is no NMR evidence
for the existence of more than one geometrical isomer in
solution. The availability of small, thin crystals allowed a
crystallographic structure determination to be performed on
2, although only to R(final) > 9%. Connectivity is clearly
established, however, and the contraction of the Ir2 separation
to within bonding range as well as the presence of an axial
phosphine ligand at each Ir center are each unambiguously
recognizable. The carbonyl ligands occupy distal coordina-
tion sites that are related by C2 rotation (i.e., the two CO
ligands are trans to one another across the Ir-Ir bond) rather
than by a proximal relationship (i.e., with the two CO ligands
cis to one another, as is found in the thiolato-bridged
analogue described8,21 by Poilblanc et al.). Such a structure
is fully consistent with all the spectroscopic data and shows
that addition of H2 to 1 produces a 1,2-dihydrido geometry
in which the H atoms are trans to one another across the Ir2
vector (see Scheme 1).
On boiling in CCl4, substitution of only one H on Ir in 2
by Cl takes place, yielding the first of two isomorphous
monohydrido dimers related to 2 that have also been
identified crystallographically. The product 3 exhibits (Table
2) two 31P NMR signals and six bridging-1H resonances,
which is consistent only with a binuclear structure that is
unsymmetrically substituted along the Ir2 axis. A high-field
1H resonance (δ -16.72) appears as a doublet of doublets,
with coupling constants of 15 and 1.5 Hz that are typical of
2Jcis to PA and 3J to PB across an Ir-Ir bond. The geometry
suggested by these data (Scheme 1) has been verified by a
single-crystal X-ray structure determination (see below). The
hydride ligand at one metal center has been replaced by
chloride. Entry of the chloride at an axial site relocates one
PPh3 to a position that is equivalent to that found in 1.
Refluxing unoxidized 1 in CCl4 or allowing it to react under
mild conditions with I2 leads to an archetypal adduct
geometry of [IrX(µ-pz)(PPh3)(CO)]2 (4, X ) Cl; 5, X ) I)
through occupation of both axial sites at diiridium(II) by
halide. Additional reaction of 5 with LiAlH4 generates 6,
which is an analogue of 3. Thus, 6 has spectroscopic
properties that closely resemble those of 3 (Table 2), although
the 3J coupling to PB is not resolved. We used X-ray
diffraction to confirm the unsymmetrical binuclear config-
uration of 6, whose crystals are isomorphous with those of
3, resulting from replacement of a single axial iodide by an
equatorial hydride (Scheme 1).
Inorganic Chemistry, Vol. 41, No. 6, 2002 1415

Brost et al.
of solid 7 (in KBr) showed a weak band at 2220 cm-1 that
was attributable to terminal Ir-H stretching, as well as νCO
at 2050 and 2010 cm-1 and νBF at 1050 cm-1. Dissolution
of 7 in dichloromethane led to a 1H NMR spectrum in which
five of the six pz hydrogens are distinguishable (Table 2)
from one another, while a conspicuous high-field resonance
at -25.5 (dd) ppm was split by couplings of J ) 15.4 and
Table 2. Spectroscopic Data
1H NMRb
compound
IRa (cm-1
1962 (s)
)
µ-pz
otherc
31P NMR
1d
7.58 (2H),
6.52 (2H),
5.84 (2H)
6.60 (2H),
6.39 (2H),
5.65 (2H)
-123.4
2
3
2169 (w)e,
-16.15 (2H)
-16.72 (1H)
-143.2
2156 (m)e,
2002 (s),
1976 (s)
2
3
4.2 Hz, which are attributable to cis JHIrP and JHIrIrP,
respectively. The 31P NMR spectrum showed singlets at
-133 and -136 ppm. To our surprise, quite different
parameters were recorded for 7 in THF-d8 (Table 2). Six
bridging-hydrogen environments were again resolved, but
at high-field a signal centered at -15.7 ppm appeared to be
a doublet with J ) 17.3 Hz, and the two 31P shifts were
more widely separated (-135, -142 ppm). Using dichloro-
methane and tetrahydrofuran, crystals that differed in their
cell dimensions were grown independently; however, on
investigation of both materials crystallographically, the data
from the product obtained from dichloromethane (which led
to the more accurate of the two independent structure
determinations) led to the conclusion that these materials
have rigorous two-fold symmetry, with hydrogens that are
tentatively located at bridging rather than at terminal sites.
Traces of water in NMR solvents led to progressive
conversion of the cation in 7 to a hydrido species that is
characterized by a new high-field resonance at -17.9 ppm
2230 (m)e,
2080 (s),
2060 (s)
7.73 (1H),
6.81 (1H),
6.58 (1H),
6.07 (1H),
5.93 (1H),
5.49 (1H)
7.67 (2H),
6.31 (2H),
5.58 (2H)
7.88 (2H),
6.54 (2H),
5.54 (2H)
7.80 (1H),
6.71 (1H),
6.53 (1H),
5.98 (1H),
5.83 (1H),
5.40 (1H)
8.05 (1H),
7.89 (1H),
6.90 (1H),
6.75 (1H),
6.01 (1H),
5.73 (1H)
7.69 (2H),
6.81 (2H),
5.91 (2H)
7.83 (2H),
6.37 (2H),
5.83 (2H)
6.69 (2H),
5.84 (2H)k
8.43 (1H),
7.85 (1H),
6.68 (2H),
5.96 (1H),
5.77 (1H)
7.60 (2H),
6.64 (2H),
5.87 (2H)
-155.7,
-137.0f
4
5
6
2034 (s),
2013 (sh)
-147.2
-148.9
2028 (s),
2012 (s)
2200 (m)e,
2020 (s),
1990 (s)
-16.67 (1H)
-25.50 (1H)
-17.95 (2H)
-151.1,
-138.5
7
2220 (w)e,
2055 (s),
20l6 (s)
-135.5,
-133.2
2
with Jcis (to P) ) 16.5 Hz. Addition of water accelerated
8g
9
2200 (w)e,h
2185e,
,
-140.2
-144.1
-140.5
the formation of this product, 8, which showed only three
NMR signals attributable to bridging-pz protons and a single
31P resonance. A solution of 7 in methanol generated a very
2057 (s, br)
n.m.i
3.57 (3H)j,
-19.00 (2H)
1
similar H NMR spectrum for a new complex, 9 (Table 2),
with a doublet at -19.0 ppm (2Jcis ) 15.8) and an additional
singlet at 3.57 ppm (OCH3). Product 9 was found to be
contaminated with 8. The spectroscopic, analytical, and
conductivity data are consistent with the formulation of 8
as a diiridium(III) cation (Scheme 2) that has a terminal H
at each metal center and a bridging-OH group that is replaced
by OMe in the methoxy-bridged analogue 9. Compound 7
also changed color under an atmosphere of H2S gas, and
new high-field hydrogen resonances were observed, but no
single major product could be separated.
10
11
2040 (s),
2020 (s)
2190 (m)e,
2065 (s),
2010 (sh)
-13.93 (1H)
-17.24 (2H)
-162.3,
-144.4
12l
2256 (w)e,
2238 (w)e,
2059 (s)
-143.0
a νco (KBr) unless otherwise indicated. b δ (CD2Cl2 soln). c δ (IrH) unless
otherwise indicated. PPh3 resonances observed in δ 7.60-7.10 range (30H).
e
d Reference 25. νIrH
.
f CDCl3 soln. g 1H signal attributable to δ (OH) not
Access to binuclear cationic fluoroborates from the di-
iododiiridium(II) adduct 5 was also explored. Thus, treatment
of neutral 5 with AgBF4 afforded a deep yellow powder that,
on the basis of NMR spectra (Table 2) as well as micro-
analytical and conductivity data, was formulated as the
symmetrical diiridium(II) cation 10 (Scheme 2). After
abstraction of one iodide ligand (as AgI), this product is
presumably formed by a rearrangement in which the second
ligand undergoes terminal-bridge relocation. In accordance
with this conclusion, further reaction of 10 with sodium
hydride led metathetically to the formation of the protonated
analogue 7, which was formed as a mixture with its H2O
adduct 8. Neutral 5 also reacted immediately with HBF4,
yielding a brick-red solid that, when analyzed, indicated 1:1
addition; in solution, this product showed two rather widely
separated 31P signals (Table 2) and a doublet 1H NMR signal
at -13.9 ppm (2Jcis ) 14.7) that was accompanied by six
observed. νIrD at 1580 (w). i n.m. ) not measured. j δ (OCH3). k Third
µ-pz resonance obscured by PPh3 hydrogens. l δ 19F ) -75.3.
h
Because 3 and 6 are putatively hydrogen halide adducts
of the parent 1, the effect of bubbling gaseous, anhydrous
HX (X ) Cl, Br, or I) into solutions of 1 was also examined,
although stoichiometric addition of these reagents was not.
Monitoring by NMR indicated the prompt formation of
mononuclear products,22 with no evidence that any binuclear
adduct intermediates formed. By contrast, unoxidized 1 was
found to react rapidly with HBF4. Analytical data supported
the incorporation of 1 mol equiv of HBF4 into 1 to yield a
bright yellow product, 7, that precipitated from diethyl ether
and behaved in acetone as a 1:1 electrolyte. The IR spectrum
(22) Bailey, J. A.; Grundy, S. L.; Stobart, S. R. Inorg. Chim. Acta 1996,
243, 47.
1416 Inorganic Chemistry, Vol. 41, No. 6, 2002

Pyrazolyl-Bridged Iridium Dimers
Scheme 2
distinguishable resonances attributable to pz hydrogens.
These data are consistent with an unsymmetrically15 substi-
tuted binuclear cation 11, in which, as in 10, an iodide rather
than a hydride occupies the endo bridging site (Scheme 2).
Finally, addition to the prototype 1 of anhydrous trifluoro-
acetic acid was investigated, with the expectation that a
cationic complex related to 7-9 or 11 would be formed.
Instead, a colorless, neutral product was isolated and was
shown by microanalytical data to contain one carboxylato
group per Ir atom. The IR spectrum contained weak, very
high-energy features attributable to νIrH (2256, 2238 cm-1),
bands near 1670 cm-1 assigned to CF3CO2, and typically
strong bands assigned to νCO. A polyisotopic parent peak in
the FAB mass spectrum corresponded to the binuclear ion
[Ir2H2(pz)2(PPh3)2(CO)2(CF3CO2)2]+. Singlet resonances were
observed in both the 31P and 19F NMR spectra, which are
indicative of a highly symmetrical configuration. Accord-
one another across the Ir2 axis so that the molecule is
atropisomeric.4 Dihydrogen addition to this dimer affords9
2, the X-ray structure of which establishes the molecular
geometry that is shown in Figure 1. The phosphines have
been twisted into the axial sites, leaving a conspicuous
equatorial vacancy at each metal atom that corresponds to
the position of a nonlocated hydride ligand. Both the latter
and the two carbonyl ligands are trans to one another across
the metal-metal vector, thus preserving in 2 the C2 sym-
metry4,15 of unoxidized 1; however, the Ir-Ir distance is
contracted to within bonding range (2.672 Å, see Table 3),
as it should be for a diamagnetic diiridium(II) adduct.4 These
features correspond in all respects to those reported by
Poilblanc et al.8,21,23 for the only direct analogue [IrH(µ-
SBut)(CO){P(OMe)3}]2, except that in the latter, the non-
located hydrogen atoms are located cis coequatorial (Cs
symmetry) to one another. Either type of coequatorial site-
1
ingly, in the H spectrum, three (rather than six, q.v.) pz
hydrogen resonances were evident and were accompanied
2
by a high-field feature (-17.2 ppm, Jcis ) 18.1) that
accounted for two hydrogens upon signal integration. All
these observations are consistent with the oxidation of
precursor 1 to a formal diiridium(III) state in a complex
possessing a structure unrelated to any of those of 2-11,
but rather one based on an edge-sharing bioctahedral
arrangement, 12.
Discussion
In the crystalline state, 1 adopts15 classic folded binuclear
geometry in which the pyrazolyl-bridged metallacyclic unit
acts as a hinge between the square coordination environments
that surround the adjacent d8 metal centers. The phosphine
ligands occupy in-plane (equatorial) sites that are trans to
Figure 1. Molecular geometry of compound 2.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1417

Brost et al.
Table 3. Selected Interatomic Distances and Angles for Compounds 2-4, 6, and 7
compound
2
3
4
6
7
Interatomic Distances (Å)
Ir(2)-Ir(1)
P(1)-Ir(1)
P(2)-Ir(2)
N(1)-Ir(1)
N(2)-Ir(2)
N(3)-Ir(1)
N(4)-Ir(2)
C(1)-Ir(1)
C(2)-Ir(2)
2.672(4)
2.34(2)
2.33(2)
2.11(5)
2.01(5)
2.19(5)
2.20(4)
1.79(2)a
1.76(2)a
Ir(2)-Ir(1)
Cl-Ir(1)
2.683(2)
2.485(7)
2.328(8)
2.328(8)
2.07(3)
2.20(3)
2.05(2)
2.05(2)
1.78(3)
1.71(3)
Ir(2)-Ir(1)
Cl(2)-Ir(1)
Cl(1)-Ir(2)
P(2)-Ir(1)
P(1)-Ir(2)
N(1)-Ir(1)
N(2)-Ir(2)
N(3)-Ir(1)
N(4)-Ir(2)
C(7)-Ir(2)
C(8)-Ir(1)
2.754(2)
2.448(8)
2.410(9)
2.325(8)
2.320(9)
2.06(3)
2.05(2)
2.07(3)
2.05(2)
1.83(3)
1.75(4)
Ir(2)-Ir(1)
I-Ir(1)
2.684(1)
2.782(2)
2.318(5)
2.326(5)
2.10(2)
2.16(2)
2.05(2)
2.10(2)
1.74(2)
1.78(2)
Ir(1′)-Ir(1)
P(1)-Ir(1)
N(1)-Ir(1)
N(2′)-Ir(1)
C(1)-Ir(1)
2.834(1)
2.302(2)
2.069(6)
2.053(6)
1.844(11)
P(1)-Ir(1)
P(2)-Ir(2)
N(1)-Ir(1)
N(2)-Ir(2)
N(3)-Ir(2)
N(4)-Ir(1)
C(7)-Ir(2)
C(8)-Ir(1)
P(1)-Ir(1)
P(2)-Ir(2)
N(1)-Ir(1)
N(2)-Ir(2)
N(3)-Ir(2)
N(4)-Ir(1)
C(7)-Ir(2)
C(8)-Ir(1)
Interatomic Angles (deg)
P(1)-Ir(1)-Ir(2) 169.1(5)
P(2)-Ir(2)-Ir(1) 166.2(5)
C(1)-Ir(1)-N(3) 164.6(14)
C(2)-Ir(2)-N(2) 165.(2)
Cl-Ir(1)-Ir(2)
156.8(2)
Cl(2)-Ir(1)-Ir(2) 151.3(2)
Cl(1)-Ir(2)-Ir(1) 150.8(2)
I-Ir(1)-Ir(2)
158.2(1)
N(1)-Ir(1)-P(1) 174.9(2)
N(2′)-Ir(1)-C(1) 175.3(3)
P(2)-Ir(2)-Ir(1) 167.0(2)
C(8)-Ir(1)-N(4) 169.8(11)
C(7)-Ir(2)-N(3) 170.7(11)
N(1)-Ir(1)-P(1) 171.5(6)
N(4)-Ir(1)-Cl
N(3)-Ir(2)-P(2)
C(8)-Ir(1)-Cl
C(7)-Ir(2)-P(2)
C(8)-Ir(1)-Ir(2)
C(7)-Ir(2)-Ir(1)
N(4)-Ir(1)-Ir(2)
N(3)-Ir(2)-Ir(1)
N(1)-Ir(1)-Ir(2)
N(2)-Ir(2)-Ir(1)
N(4)-Ir(1)-N(1)
N(3)-Ir(2)-N(2)
N(1)-Ir(1)-Cl
P(2)-Ir(2)-Ir(1) 168.1(1)
C(8)-Ir(1)-N(4) 169.1(7)
C(7)-Ir(2)-N(3) 168.1(8)
N(1)-Ir(1)-P(1) 170.1(4)
N(4)-Ir(1)-I
N(3)-Ir(2)-P(2)
C(8)-Ir(1)-I
C(7)-Ir(2)-P(2)
C(8)-Ir(1)-Ir(2)
C(7)-Ir(2)-Ir(1)
N(4)-Ir(1)-Ir(2)
N(3)-Ir(2)-Ir(1)
N(1)-Ir(1)-Ir(2)
N(2)-Ir(2)-Ir(1)
N(4)-Ir(1)-N(1)
N(3)-Ir(2)-N(2)
N(1)-Ir(1)-I
N(3)-Ir(1)-P(2)
N(2)-Ir(2)-P(1)
172.3((8)
176.4(6)
C(1)-Ir(1)-P(1)
C(1)-Ir(1)-N(1)
N(2′)-Ir(1)-P(1)
N(2′)-Ir(1)-N(1)
92.3(3)
92.7(3)
92.1(3)
82.8(3)
N(3)-Ir(1)-P(1)
N(2)-Ir(2)-P(2)
N(3)-Ir(1)-Ir(2)
N(2)-Ir(2)-Ir(1)
C(1)-Ir(1)-Ir(2)
C(2)-Ir(2)-Ir(1)
95.2(15)
93.1(14)
74.5(14)
76.3(14)
91.(2)
N(1)-Ir(1)-C(8) 169.2(13)
N(4)-Ir(2)-C(7) 173.5(12)
88.7(6)
93.1(7)
98.5(9)
95.7(8)
98.4(9)
97.0(8)
72.8(6)
74.4(7)
72.6(7)
71.9(6)
83.0(8)
85.9(8)
91.8(7)
92.7(4)
96.5(5)
95.0(6)
95.3(7)
99.0(6)
95.9(7)
71.5(4)
72.5(4)
72.3(4)
72.0(4)
82.7(6)
86.8(6)
91.3(4)
N(1)-Ir(1)-P(2)
N(4)-Ir(2)-P(1)
C(8)-Ir(1)-P(2)
C(7)-Ir(2)-P(1)
N(3)-Ir(1)-C(8)
N(2)-Ir(2)-C(7)
N(3)-Ir(1)-N(1)
N(4)-Ir(2)-N(2)
P(2)-Ir(1)-Cl(2)
P(1)-Ir(2)-Cl(1)
C(8)-Ir(1)-Cl(2)
C(7)-Ir(2)-Cl(1)
N(1)-Ir(1)-Cl(2)
N(4)-Ir(2)-Cl(1)
N(3)-Ir(1)-Cl(2)
N(2)-Ir(2)-Cl(1)
N(1)-Ir(1)-Ir(2)
N(4)-Ir(2)-Ir(1)
N(3)-Ir(1)-Ir(2)
N(2)-Ir(2)-Ir(1)
P(2)-Ir(1)-Ir(2)
P(1)-Ir(2)-Ir(1)
C(8)-Ir(1)-Ir(2)
C(7)-Ir(2)-Ir(1)
92.0(8)
93.3(7)
Ir(1′)-Ir(1)-N(2′) 69.6(3)
Ir(1′)-Ir(1)-N(1) 68.3(3)
Ir(1′)-Ir(1)-P(1) 110.6(3)
Ir(1′)-Ir(1)-C(1) 107.6(3)
97.5(11)
86.4(10)
89.7(14)
97.1(12)
81.1(11)
83.3(9)
86.5(3)
89.6(3)
97.8(11)
98.3(10)
87.9(8)
88.1(7)
90.0(8)
89.0(7)
69.5(7)
70.3(6)
69.7(8)
69.7(7)
93.(2)
C(1)-Ir(1)-P(1) 100.(2)
C(2)-Ir(2)-P(2) 99.(2)
N(1)-Ir(1)-P(1) 104.2(13)
N(4)-Ir(2)-P(2) 101.5(12)
N(1)-Ir(1)-N(3) 86.(2)
N(4)-Ir(2)-N(2) 88.(2)
N(1)-Ir(1)-Ir(2)
N(4)-Ir(2)-Ir(1)
N(1)-Ir(1)-C(1)
N(4)-Ir(2)-C(2)
72.0(12)
69.8(10)
94.7(12)
98.(3)
N(2)-Ir(2)-P(2) 104.2(6)
C(8)-Ir(1)-N(1)
C(7)-Ir(2)-N(2)
P(1)-Ir(1)-Ir(2) 100.3(2)
P(1)-Ir(1)-N(4)
P(1)-Ir(1)-Cl
N(2)-Ir(2)-P(2) 103.7(4)
89.6(11)
94.7(10)
C(8)-Ir(1)-N(1)
C(7)-Ir(2)-N(2)
P(1)-Ir(1)-Ir(2)
P(1)-Ir(1)-N(4)
P(1)-Ir(1)-I
89.4(7)
91.8(8)
99.5(1)
89.5(4)
95.2(1)
97.5(6)
90.4(6)
93.4(3)
96.3(9)
P(1)-Ir(1)-C(8)
P(1)-Ir(1)-C(8)
111.0(2)
110.4(2)
102.1(11)
103.7(10)
a These two distances were influenced by a constraint in the refinement.
occupation by H (i.e., trans, 2, or cis8) is at conspicuous
variance with the axial entry of addenda that is followed in
the more typical oxidative pathway, where nucleophilic (SN2-
like) behavior of the bimetallic center is recognizable.3-5,9
That the structure of the thermodynamic product of this type
of dihydrogen addition says nothing about the mechanism
by which it is formed has been amply demonstrated in a
very interesting reinvestigation of corresponding diiridium
“A-frame” chemistry by Eisenberg and co-workers.14 The
related but reversible reaction of a novel mixed-valence bis-
(bis(trifluoroethoxy)phosphino)methylamine-bridged Ir0-IrII
(d9-d7) dimer with H2 affords a IrI-IrIII dihydride (d8-d6)
in which Ir-Ir ) 2.7775(11) Å. In this product, the two
hydrogen atoms (not located) that are attached to different
iridium centers are found13 at sites that are distal (i.e., like
those in 2) rather than proximal (i.e., cis8).
Cl, affording compound 3 (Scheme 1). Under the same
conditions, 1 undergoes two-center oxidative addition to give
the 1,2-dichloro adduct 4 (the 4-chloro-pyrazolyl-bridged
analogue of which is formed15 by dichlorine addition to 1).
Oxidation of 1 using diiodine yields an analogue [IrI(µ-pz)-
(PPh3)(CO)]2 (5) of 4, which reacts with tetrahydroaluminate
ion to replace one (but not both) I by H, affording the
homologue 6 of 3. While the X-ray structure of 4 is
unremarkable (molecular C2 symmetry; see Figure 2, which
shows the axial disposition of each Cl and where the Ir-Ir
bond distance is 2.754 Å), those of 3 and 6 (which are
isomorphous) are consistent with the idea that the accom-
modation of the trans influences exerted by Ir-Ir and Ir-H
bonds is a primary factor in determining the equilibrium
geometry. Thus, in each molecule, as is shown in Figures 3
and 4, the (single) hydride is equatorial (detected as an
obvious vacancy), and the phosphine ligand that is attached
to the same Ir center is oriented axially. By contrast, at the
second Ir site, the phosphine and halide occupy positions
(equatorial and axial, respectively) corresponding to those
The binuclear dihydride 2 reacts in boiling carbon
tetrachloride with replacement of one (but not both) H by
(23) Thorez, A.; Maisonnat, A.; Poilblanc, R. Inorg. Chim. Acta 1977, 25,
L19.
1418 Inorganic Chemistry, Vol. 41, No. 6, 2002

Pyrazolyl-Bridged Iridium Dimers
Scheme 3
Figure 2. Molecular geometry of compound 4.
unambiguously recognizable on the basis of temperature-
1
invariant H and 31P NMR data as an unsymmetrically
substituted, binuclear species with a single hydrogen atom
bound to only one metal center (i.e., it shows two distin-
guishable environments for µ-pz 3-, 4-, and 5-hydrogens and
for P nuclei). Protonation (by HBF4 or HO3SCF3) of a
diamidonaphthalene-bridged diiridium(I) complex that is a
direct analogue of 1 has been described very recently by
Oro and co-workers.24 The cation so obtained, which displays
NMR parameters that are almost identical to those of 7 and
was likewise concluded to contain a hydride ligand bound
terminally at one Ir atom, undergoes a sequence of chemical
changes that lend convincing support to such a formulation.
Further consideration of data from the same source indicates24
that if terminal H at Ir occupies an axial site, a 1H resonance
is observed as a doublet of doublets (i.e., through coupling
to a distant PB, across the Ir-Ir bond, as well as to an
adjacent (cis, PA) phosphorus nucleus). By contrast, equato-
rial disposition allows only cis coupling (to PA) to be
resolved, and the pattern simplifies to a doublet. We believe
that this accounts for the difference in the NMR spectra of
7/CH2Cl2 and 7/THF, with the donor capacity of THF leading
to the solvation of vacant axial sites in 7, which destabilizes
axial hydride coordination (see Scheme 3, 7-ii versus 7-i).
The earlier data also suggest24,25 that there is a delicate
energetic balance between terminal- and bridging-hydride
geometries. It is perhaps not surprising, therefore, that upon
crystallization the binuclear cations of 7 are ordered into a
Figure 3. Molecular geometry of compound 3.
Figure 4. Molecular geometry of compound 6.
-
in compound 4 (see Table 3). The resistance of these
diiridium(II) adducts to reductive elimination (i.e., of HX
for 3 and 6, X ) Cl or I, respectively) is underscored by the
independent observation that dimer 1 is quantitatively cleaved
to mononuclear species22 using HX (X ) Cl, Br, or I) as
reagents.
rigorous two-fold relationship, with each BF4 counterion
in a position that, while distant, is centered on the same C2
(24) Jime´nez, M. V.; Sola, E.; Lo´pez, J. A.; Lahoz, F. J.; Oro, L. A. Chem.s
Eur. J. 1998, 4, 1398. See also Oro, L. A.; Sola, E.; Lo´pez, J. A.;
Torres, F.; Elduque, A.; Lahoz, F. J. Inorg. Chem. Commun. 1998, 1,
64. Torres, F.; Sola, E.; Elduque, A.; Martinez, A. P.; Lahoz, F. J.;
Oro, L. A. Chem.sEur. J. 2000, 6, 2120.
(25) Sola, E.; Bakhmutov, V. I.; Torres, F.; Elduque, A.; Lo´pez, J. A.;
Lahoz, F. J.; Werner, H.; Oro, L. A. Organometallics 1998, 17, 683.
The reaction of 1 with fluoroboric acid affords a chrome-
yellow product, 7, having a structure in solution that is
Inorganic Chemistry, Vol. 41, No. 6, 2002 1419

Brost et al.
of its NMR properties. Observation of a doublet signal at
-17.95 ppm in the 1H NMR spectrum that is attributable to
a terminal hydride, the relative intensity of which is reduced
2
by half when H2O is used as a reactant instead of water,
together with two 31P resonances that are resolved (-140.28,
-140.37 ppm) from one another, are consistent with forma-
tion of the isotopomer [Ir2(H)(2H)(µ-pz)2(µ-O2H)(PPh3)2-
(CO)2]BF4. This reaction was observed to be complete, with
no side products formed, after 10 d in a solution containing
only 5 ppm H2O. It is not possible to distinguish axial (exo)
hydride attachment from equatorial hydride attachment (see
Scheme 2) on the basis of these data, although the thermo-
dynamic preference in related chemistry appears to be axial.24
A parallel reaction in wet methanol yielded an approximate
30:70 mixture of 8/9, in which 8 is the minor product and 9
(Scheme 2) is a symmetrical analogue of 8. These results,
when taken with the fact that 1 is unaffected by water or
methanol, imply that the products are formed by nucleophilic
attack on the cation of 7. There are very few other examples
of discrete transition-metal (hydroxo)hydrido complexes, and
none of them have been synthesized in a similar manner.19
Reaction of 1 with trifluoroacetic acid afforded a neutral
complex, rather than a cationic product like 7, that was shown
by mass spectrometry to be dimeric, although it was not
obtained as crystalline material. While the cations of 8-11
are expected, by comparison with related species,6,17,22 to
adopt a bioctahedral arrangement that is triply bridged,
consideration of the stoichiometry and spectroscopic proper-
ties of the 12 are indicative of a more open structure in which
the octahedra containing the adjacent d6 Ir(III) centers are
likely to be connected only through the pyrazolyl-bridged
framework (see Scheme 2).
Figure 5. Molecular geometry of compound 7.
axis. The resulting arrangement is shown in Figure 5 (50%
thermal ellipsoids are larger for the isolated tetrahedral anion,
as is typical for such units). The bridging hydrogen was not
found, possibly because of thermal motion that may represent
positional averaging of an atom that is unsymmetrically24
bonded between the metal nuclei, but the remaining hydro-
gens were all successfully included in the refinement. This
showed that an ortho H of a Ph substituent at P (i.e., H
attached to C-16, Figure 5) is placed close enough to Ir (i.e.,
3.06 Å), by rotation of the phosphine ligand, to rule out the
occupancy of an exo (axial) site by a terminal hydride.
Occupation of a terminal site by hydride would therefore
necessarily lead to a H‚‚‚H approach e1.75 Å. The cation
configuration is superficially identical to that of the neutral
prototype 1, but the Ir2 distance is contracted from 3.163 to
2.834 Å (i.e., into bonding range), although it is evidently
somewhat longer than those in 2-4 and 6 (Table 3). This
observation is also consistent with the presence of a bridging-
hydride interaction. The solid-state, hydride-bridged, metal-
metal-bonded representation for 7 (Scheme 3, 7-iii) clearly
Acknowledgment. We thank the N.S.E.R.C., Canada, for
financial support, Dr. R. Vefghi for conducting certain
exploratory experiments, and Mrs. K. Beveridge for technical
assistance with crystallography.
corresponds to a d7 diiridium(II) formalism, in contrast to
2
Supporting Information Available: Full crystallographic
details including tables of fractional atomic coordinates, anisotropic
temperature factors, interatomic distances, and bond angles for
compounds 2-4, 6, and 7. This material is available free of charge
via the Internet at http://pubs.acs.org.
the solution structure (7-i or 7-ii), which might equally well
be viewed24 as d6d8 (i.e., IrIIIIrI).
The reactivity of 7 in ostensibly dry solvents provided the
first clue to the sensitivity of the complex to hydrolysis.
Subsequent experiments led to the isolation of another cation,
8, identified as a symmetrical binuclear species on the basis
IC010448G
1420 Inorganic Chemistry, Vol. 41, No. 6, 2002