tert-Butyl is superior to phenyl as an agostic donor to 14-electron Ir(III)

Full text of DOI:10.1021/ja970763v

J. Am. Chem. Soc. 1997, 119, 9069-9070
9069
tert-Butyl Is Superior to Phenyl as an Agostic
Donor to 14-Electron Ir(III)
Alan C. Cooper, William E. Streib, Odile Eisenstein,* and
Kenneth G. Caulton*
Department of Chemistry and Molecular Structure Center
Indiana UniVersity, Bloomington, Indiana 47405-4001
LSDSMS UMR 5636, UniVersite´ de Montpellier 2
34095 Montpellier Cedex 5, France
ReceiVed March 10, 1997
The quest for the synthesis of new coordinatively unsaturated
transition metal complexes has been driven by the role that these
complexes play in many catalytic processes and the useful
reactivity that can be exploited for the functionalization of
simple organic molecules.1 It has been proposed that highly
unsaturated intermediates in homogeneous catalysis may be
stabilized by agostic interactions from ligands on the metal.2
Shaw et al. first observed that the coordination of bulky tertiary
phosphine ligands with the formula PRtBu2 may stabilize
unsaturation in Ir(III) complexes by prohibiting dimerization
and solvent coordination.3 In most studies of unsaturated
complexes which include bulky phosphine ligands, these ligands
have exhibited a “benign” influence in terms of providing steric
protection of empty coordination sites without any direct
interaction beyond metal-phosphorus bonding. However,
notable exceptions exist where intramolecular C-H activation
of alkyl groups on a phosphine occur to give metalated-
phosphine complexes.4 Indeed, the metalation of C-H bonds
has been demonstrated to facilitate reductive elimination from
Ir(III) complexes.5
Figure 1. ORTEP drawings (50% probability ellipsoids) of cations 1
(left) and 3 (right) of [Ir(H)2(PPh(tBu)2)2][BAr′4]. Only the hydrogens
of the agostic methyl groups are shown. Selected bond lengths (Å):
Ir(1)-P(2), 2.323(2); Ir(1)-P(17), 2.319(2); Ir(1)-C(11), 2.811(4);
Ir(1)-C(25), 2.936(4); Ir(63)-P(64), 2.320(2); Ir(63)-P(79),
2.317(2); Ir(63)-C(74), 2.826(4); Ir(63)-C(91), 2.872(4). Selected
bond angles (deg): P(2)-Ir(1)-P(17), 173.66(6); C(11)-Ir(1)-C(25),
99.60(6); P(64)-Ir(63)-P(79), 172.08(8); C(74)-Ir(63)-C(79),
113.28(6).
conclusion from this result is the potent Lewis acidity of sodium
in NaBAr′4, for its ability to abstract the X ligand even when
there is multiple metal-ligand bond character,7 and a 14 e-
species is produced. Crystallization of [Ir(H)2(PPh(tBu)2)2]-
[BAr′4] occurs by slow diffusion of pentane into a concentrated
fluorobenzene solution at -20 °C. This yellow air-sensitive
complex is highly soluble in THF, CH2Cl2, and acetone, and
demonstrates moderate solubility in even nonpolar arene
solvents. The 1H NMR spectrum (25 °C) in toluene-d8 displays
multiple signals for the tBu protons (with virtual triplet splitting),
We report here an example of unusual metal-phosphine
interactions in the form of two separate agostic interactions from
aliphatic phosphine substituents to an unsaturated iridium center.
This extreme case of two agostic interactions is forced by the
exceptionally high Lewis acidity inherent in a 14 e-, non-π-
stabilized Ir(III) complex formed by ligand abstraction from an
already unsaturated metal complex. Despite the potential for
formation of five-membered rings through either tert-butyl or
o-phenyl agostic interactions, the complex is remarkably selec-
tive toward forming agostic interactions with only tert-butyl
groups on the phosphine. The high degree of coordinative
unsaturation present in the molecule ensures that the agostic
t
indicating that the phosphines have inequivalent Bu groups.
This is attributed to agostic bonding, which causes the inequiva-
t
t
lence of agostic Bu vs pendant Bu groups. In addition, there
1
are two resolved signals in the upfield region of the H NMR
spectrum for the hydride resonances of two diastereomers. The
variable-temperature 1H NMR spectra reveal two dynamic
t
processes involving Bu protons. Coalescence of pendant and
agostic tBu groups is evident at 75 °C. There is also an observed
t
broadening of the agostic Bu resonances below -20 °C,
attributed to decoalescence of the methyl groups within an
agostic tBu. Due to the complexity of the NMR spectra resulting
from these several dynamic processes, using a technique with
a faster time scale for spectroscopic characterization of agostic
interactions in solution was necessary. The IR spectrum of [Ir-
(H)2(PPh(tBu)2)2][BAr′4] in C6D6 displays three bands of
medium intensity at 2625, 2593, and 2552 cm-1. These are
assigned to the agostic C-H stretches of two diastereomers in
solution.8 The X-ray structure9 shows that there are three
independent [Ir(H)2(PPh(tBu)2)2]+, three noncoordinating10
[BAr′4]-, and one (noninteracting) fluorobenzene per asym-
metric unit cell. Two of the cations (Figure 1) are very similar,
with only minor conformational differences within the phosphine
ligands, while the third cation shows opposite chirality at one
phosphine relative to the first two cations. In all three cations,
there are no close Ir/C(phenyl) contacts (<3.48 Å), but there
are two separate agostic interactions from tert-butyl C-H bonds
t
interactions yield resolved agostic and pendant Bu groups in
1
the H NMR spectrum at room temperature.
Abstraction of the X ligand from coordinatively unsaturated
Ir(H)2(X)(PPh(tBu)2)2 (X ) Cl, F, OSO2CF3) by NaBAr′4 (Ar′
) 3,5-bis(trifluoromethyl)phenyl)6 in fluorobenzene yields the
solvent-ligand-free cationic iridium(III) complex [Ir(H)2(PPh-
(tBu)2)2][BAr′4] in nearly quantitative yield. An important
(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. In
Principles and Applications of Organotransition Metal Chemistry; Univer-
sity Science: Mill Valley, CA. (b) Bengali, A. A.; Arndtsen, B. A.; Burger,
P. M.; Schultz, R. H.; Weiller, B. H.; Kyle, K. R.; Moore, C. B.; Bergman,
R. G. Pure Appl. Chem. 1995, 67 (2), 281.
(2) (a) Margl, P.; Lohrenz, J. C. W.; Ziegler, T.; Blo¨chl, P. E. J. Am.
Chem. Soc. 1996, 118, 4434. (b) Grubbs, R. H.; Coates, G. W. Acc. Chem.
Res. 1996, 29, 85.
(3) Empsall, H. D.; Hyde, E. M.; Shaw, B. L.; Uttley, M. F. J. Chem.
Soc., Dalton Trans. 1976, 2069.
(4) (a) Cooper, A. C.; Caulton, K. G. Inorg. Chim. Acta 1996, 251, 41.
(b) Cooper, A. C.; Huffman, J. C.; Folting, K.; Caulton, K. G. Organome-
tallics 1997, 16, 505. For an example of phosphite metallation see: Bedford,
R. B.; Castillo`n, S.; Chaloner, P. A.; Claver, C.; Fernandez, E.; Hitchcock,
P. B.; Ruiz, A. Organometallics 1996, 15, 3990.
(7) Caulton, K. G. New J. Chem. 1994, 18, 25 and references therein.
(8) For comparison see: Wasserman, H. J.; Kubas, G. J.; Ryan, R. R. J.
Am. Chem. Soc. 1986, 108, 2294.
(5) (a) Albe´niz, A. C.; Schulte, G.; Crabtree, R. H. Organometallics 1992,
11, 242. (b) Aizenberg, M.; Milstein, D. Organometallics 1996, 15, 3317.
(c) Cooper, A. C.; Huffman, J. C.; Caulton, K. G. Organometallics 1997,
16, 1974.
(6) Brookhart, M.; Grant, R. G.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920.
(9) Crystal data for C180H180B3F72P6Ir3; C6H5F (-167 °C): a ) 20.023-
(4) Å, b ) 28.492(6) Å, c ) 19.579(4) Å, R ) 109.47(1)°, â ) 89.89(1)°,
γ ) 110.40(1)° with Z ) 2 in space group P1h. R(F) ) 0.0660 for 18776
reflections with F > 3σ(F).
(10) The closest iridium-anion contact is 4.246 Å. For a recent review
see: Strauss, S. H. Chem. ReV. 1993, 93, 927.
S0002-7863(97)00763-4 CCC: $14.00 © 1997 American Chemical Society

9070 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Communications to the Editor
to the unsaturated iridium center.11 The six independently
measured agostic interactions are characterized by Ir-C dis-
tances of 2.81-2.94 Å. These Ir-C distances are longer than
reported agostic interactions among crystallographically char-
acterized Ir(III) complexes,5a,12 but within the range of reported
agostic interactions involving other 5d metals.13,14 Due to the
presence of two tBu groups on each phosphine, it is possible to
have an “internal standard” to accurately gauge the magnitude
of bond deformation inherent in the agostic interactions. For
example, in cation 3 shown in Figure 1, the Ir(63)-P(64)-
[Ir(H)2(PPh(tBu)2)2][BAr′4] in CD2Cl2 shows only one signal
(virtual triplet) for the tBu protons and one hydride resonance,
which indicates CD2Cl2 coordination and the loss of agostic
bonding, and demonstrates that the agostic interactions are
readily displaced by even weak Lewis bases.18 The preparation
of [Ir(H)2(PPh(tBu)2)2][BAr′4] as a solvent-ligand-free material
thus demands the use of fluorobenzene as a solvent; dichlo-
romethane is present in the solid product obtained from the
reaction of Ir(H)2(X)(PPh(tBu)2)2 (X ) Cl, F, OSO2CF3) with
NaBAr′4 in CH2Cl2.
t
[C(tBu)] angle of the agostic Bu group is 20.5° less than the
Addition of 1 atm of H2 to a solution of [Ir(H)2(PPh(tBu)2)2]-
[BAr′4] in CD2Cl2 results in broadening of the hydride resonance
in the room temperature 1H NMR spectrum. As the temperature
t
non-interacting Bu group of the same phosphine. The six
t
agostic (96.7-98.4°) / non-agostic (114.3-117.3°) Bu in the
three cations give an average ∆Ir-P-C of 18.6°.
1
is lowered, new signals appear in the H and 31P{1H} NMR
Since the two agostic interactions are mutually cis, the
hydrides must be cis also. Because of their low scattering power
of X-rays, they were not found experimentally. In an effort to
understand whether the cis orientation of the hydride ligands is
a result of the agostic interactions from phosphine C-H bonds,
the geometry of Ir(H)2(PH3)2+ was optimized at the Becke3LYP
level with no symmetry constraint (Gaussian 94).16 Despite the
lack of steric hindrance or Ir-agostic interactions from PH3
ligands, the optimized geometry shows a preference17 for trans
phosphines and a cis arrangement for the hydrides (to minimize
mutual influence of trans hydrides). The calculated H-Ir-H
angle of 88.2° in the absence of agostic interactions shows that
the cis hydride orientation in the experimental structure is
consistent with a minimum-energy conformation in the absence
of, and not caused by, the cis agostic interactions. The bent
ML4 structure (essentially an octahedron with two cis ligands
missing) of d6 Ir(III) is more stable than square-planar or
tetrahedral since it is the only geometry associated with three
nonbonding d-orbitals and since it maximizes the HOMO-
LUMO gap between the three filled nonbonding d orbitals and
the two empty antibonding d orbitals. Additionally, the presence
of hydride ligands of low electronegativity and strong σ-donor
ability trans to the empty coordination sites minimizes the metal
character of the unoccupied orbitals and consequently minimizes
the Lewis acidity of these vacant sites for maximum (overall)
molecular stability.
spectra which reveal reversible dihydrogen binding, yielding
spectroscopically observable mono- and bis-dihydrogen adducts
(eq 1; H chemical shifts shown beside associated H or H2).
1
The equilibrium constant for formation of the mono-H2 complex
(K1) increases with decreasing temperature. By 213 K, K1[H2]
) 1 and signals for the bis-H2 complex are observed. At 183
K, the relative concentration of bis-H2/mono-H2/H2-free com-
plexes in solution is ca. 1:4:2. From these data and the observed
displacement of agostic interactions by CH2Cl2, the relative
strength of ligand binding to the empty coordination sites of
[Ir(H)2(PPh(tBu)2)2][BAr′4] is H2 > CH2Cl2 > agostic C-H.
Recently, Arndtsen and Bergman have reported C-H bond
activation under mild conditions with a cationic Ir(III) com-
plex.19 However, addition of 1 atm of D2 to a solution of
[Ir(H)2(PPh(tBu)2)2][BAr′4] in THF-d8 leads to hydrogen/
deuterium scrambling at the hydride positions within 1 h at room
temperature, but no exchange of deuterium into the phosphine
ligands (determined by 2H NMR), even after extended heating
(70 °C, 12 h).
While the molecular structure of [Ir(H)2(PPh(tBu)2)2][BAr′4]
minimizes Lewis acidity, and allows for the isolation of this
complex, the agostic interactions do not “poison” the reactivity
1
of the two available orbitals. Thus, the H NMR spectrum of
(11) For an example of a bis-agostic interaction, involving two phenyls,
to a single coordination site see: King, W. A.; Luo, X-L.; Scott, B. L.;
Kubas, G. J.; Zilm, K. W. J. Am. Chem. Soc. 1996, 118, 6782.
(12) (a) Crabtree, R. H.; Holt, E. M.; Lavin, M.; Morehouse, S. M. Inorg.
Chem. 1985, 24, 1986. (b) Robertson, G. B.; Wickramasinghe, W. A. Acta
Crystallogr. Sect. C (Cryst. Struct. Commun.) 1988, 44, 1383.
(13) (a) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983,
250, 395. (b) Crabtree, R. H. Chem. ReV. 1985, 85, 245.
Preliminary studies in our laboratory have shown that the
selective abstraction of one chloride ligand from IrHCl2L2 (L
) bulky phosphine) can generate [IrHClL2][BAr′4] in high yield.
Acknowledgment. This work was supported by the National
Science Foundation, by an international NSF/CNRS(PICS) grant
(K.G.C. and O.E.) and by material support by Johnson Matthey/Aesar.
(14) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J. Am. Chem.
Soc. 1976, 98, 5850. The lack of compression of the M-P-C angles
(observed at 108-113°) and the long (2.7-2.83 Å) M/H distances indicate
that any agostic interactions in Pd(PPh(tBu)2)2 and Pt(PPh(tBu)2)2 are much
weaker than those in [Ir(H)2(PPh(tBu)2)2][BAr′4]. For reference, the Pd/H
distances in PdI2(PMe2Ph)2 are 2.84-2.85 Å.15
Supporting Information Available: Full crystallographic details
and positional and thermal parameters for [Ir(H)2(PPh(tBu)2)2][BAr′4]
and synthesis, ambient and variable temperature 1H, 19F, 31P{1H}, and
13C{1H} NMR data for [Ir(H)2(PPh(tBu)2)2][BAr′4] (11 pages). See any
current masthead page for ordering and Internet access instructions.
(15) Bailey, N. A.; Mason, R. J. Chem. Soc. A 1968, 2594.
(16) The calculations were performed with the Gaussian 94 package.
The pseudopotential and basis sets for Ir, P, and H are those of LANL2DZ.
Polarization functions were added to P and the hydrides. The hydrogens of
PH3 are calculated with a minimal basis set. Optimization was carried out
at the DFT(B3LYP) level. Gaussian 94: Revision, D. I.; Frisch, M. J.;
Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M.
A.; Cheeseman, J. R.; Keith, T.; Peterson, G. A.; Montgomery, J. A.;
Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.;
Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.;
Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.;
Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.;
Gonzalez, C.; Pople, J. A., Gaussian, Inc.: Pittsburgh, PA, 1995.
(17) For relevant theoretical precedent, see: Burdett, J. K. J. Chem. Soc.,
Faraday Trans. 2 1974, 70, 1599. Elian, M.; Hoffmann, R. Inorg. Chem.
1975, 14, 1058.
JA970763V
(18) For examples of halocarbon coordination to iridium see: Crabtree,
R. H.; Faller, J. W.; Mellea, M. F.; Quirk, J. M. Organometallics 1982, 1,
1361. Burk, M. J.; Segmuller, B.; Crabtree, R. H. Organometallics 1987,
6, 2241. Kulawiec, R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89.
(19) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970.