Spectroscopic detection of a hydrogen fluoride complex of iridium

Full text of DOI:10.1021/ja9628462

J. Am. Chem. Soc. 1996, 118, 13105-13106
13105
Spectroscopic Detection of a Hydrogen Fluoride
Complex of Iridium
Ben P. Patel and Robert H. Crabtree*
Department of Chemistry, Yale UniVersity
225 Prospect Street
New HaVen, Connecticutt 06520-8107
ReceiVed August 14, 1996
The work described here was performed to study the effect
of introducing a non-ligating hydrogen-bonding functionality
into the ligand sphere of a metal complex. We report the
incorporation of an amino group into the benzoquinoline ligand
of an iridium(III) complex and its effect on the stabilization of
an HF ligand. Although there are examples1 of M-F‚‚‚H
hydrogen-bonding interactions, and more recently, of bifluoride
complexes,2 we report here spectroscopic evidence at 183 K
for what appears to be the first M-F-H‚‚‚N interaction in
which a molecule of HF is bound to the metal, stabilized by an
intramolecular hydrogen bond.
The new ligand, 2-amino-7,8-benzoquinoline (1a), was readily
synthesized by a Chichibabin reaction between NaNH2 and 7.8-
benzoquinoline.3 Following the procedure previously reported
for 2b,4 1a reacts smoothly with the complex [Ir(COD)-
(PPh3)2]BF4 and hydrogen (1 atm) in moist CH2Cl2 to give the
new cyclometallated complex, 2a (88% yield). The spectral
data for 2a show that it is a direct analog of the crystallographi-
cally characterized4 species, 2b.
Figure 1. The N-H‚‚‚F-Ir and N‚‚‚H-F-Ir proton resonances in
1
the 300 MHz H NMR spectra of 3a (a) and 5a (b) at 183 K.
shape analysis,6 which allows us to estimate the strength of the
hydrogen bond as 6.6 ( 0.5 kcal/mol by the method previously
described.7
On protonating the fluoro complex 3a with HBF4-Et2O at
183 K in CD2Cl2, we find a new complex is formed (5a) that
1
shows a H NMR spectrum similar to that found for 3a but
having a new doublet at 9.78 ppm with a coupling constant of
440 ( 5 Hz which, for reasons discussed below, we assign to
the Ir-F-H‚‚‚NH2 proton of a molecule of HF which is bound
to the iridium center and intramolecularly hydrogen bonded to
the amino nitrogen (Figure 1). A broad resonance at 6.84 ppm
is assigned to the nonhydrogen bonded NH2 hydrogens. The
identification of the 440 ( 5 Hz splitting as a true coupling
was confirmed by performing the experiment at both 300 and
1
500 MHz and by observing the same JHF coupling in the 19F
NMR spectrum (δ ) -178 ppm);8,9 the origin of the coupling
in the 19F NMR spectrum is further confirmed by its collapse
to a broad singlet when proton decoupled. Selective decoupling
of the NH2 signal led to a narrowing of the HF resonance,
suggesting they are coupled, as required by structure 5a. The
assignment of the resonances at 9.78 and 6.84 ppm to exchange-
able H-F or N-H protons is also supported by their deuteration
1
with CD3OD, which causes these H resonances to disappear
The fluoride complex 3a is easily prepared by stirring 2a
with 1 equiv of [n-Bu4N]F in acetone for 1 h under N2. The
identity of the complex follows from its elemental analysis and
spectral data. As for the closely related complex [IrH2F(2-
aminopyridine)(PPh3)2] (4), previously studied by us,5 we find
that 3a has an intramolecular N-H‚‚‚F-Ir hydrogen bond, as
shown by the 1JHF coupling constant of 52 Hz observed for the
after mixing.
1JHF coupling constants measured for HF in Lewis base
solvents have been shown10 to decrease from experimental gas
phase values (530 Hz)10-12 by as much as 10-20%, as a result
1
of hydrogen bonding. Furthermore, the δ and JHF values for
F-H‚‚‚base systems are related,10 as shown in Figure 2.
Comparison with our values supports our belief that we are
dealing with an F-H‚‚‚base hydrogen-bonded system. In
contrast, the 1JHF value for the [F‚‚‚H‚‚‚F]- ion (120 Hz)10,13 is
much smaller than for F-H‚‚‚base, suggesting the presence of
an Ir-F-H‚‚‚N rather than an Ir-F‚‚‚H-N arrangement.
1
N-Ha proton resonance in the H NMR spectrum at 183 K,
and the shift of the ν(N-H) band in the IR spectrum to low
energy (free 1a 3411 cm-1; 3a 3175 cm-1). These values are
similar to those previously observed (1JHF ) 65 Hz; ∆ν ) 230
cm-1) for 4. The ∆Gq barrier at 193 K for Ha/Hb exchange in
1
the H NMR spectrum of 3a is 12.4 ( 0.5 kcal/mol by line
(6) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press: New
York, 1982.
(1) (a) Yap, G. P. A.; Rheingold, A.; Das, P.; Crabtree, R. H. Inorg.
Chem. 1995, 34, 3474-6 (b) Albinati, A.; Bakhmutov, V.; Caulton, K.
G.; Clot, E.; Eckert, J.; Eisentein, O.; Gusev, D. G.; Grushin, V. V.; Hauger,
B. E.; Klooster, W. T.; Koetzle, T. F.; McMullan, R. K.; O’Loughlin, T.
J.; Pelissier, M.; Ricci, J. S.; Sigalas, M. P.; Vymenits, A. B. J. Am. Chem.
Soc. 1993, 115, 7300-12. (c) Veltheer, J. E.; Burger, P.; Bergman, R. G.
J. Am. Chem. Soc. 1995, 117, 12478-82.
(2) (a) Murphy, V. J.; Hascall, T.; Chen, J. Y.; Parkin, G. J. Am. Chem.
Soc. 1996, 118, 7248-429. (b) Whittlesey, M. K.; Perutz, R. N.; Greener,
B.; Moore, M. H. J. Chem. Soc., Chem. Commun. Submitted. (c)
Hintermann, S.; Pregosin, P. S.; Ru¨egger, H. J. Organomet. Chem. 1992,
435, 225-34. (d) Caulton et al.2e have found a hydrogen chloride complex.
(e) Caulton, K. Personal communication (1996).
(7) (a) Lee, J. C., Jr.; Peris, E.; Rheingold, A. L.; Crabtree, R. H. J. Am.
Chem. Soc. 1994, 116, 11014-9. (b) The method assigns a value of 5.8
kcal/mol to the intrinsic barrier for Ar-NH2 rotation in the absence of
H-bonding, based on theoretical studies and NMR studies of the free ligands.
(8) 19F NMR chemical shifts are reported relative to external CFCl3 in
CD2Cl2.
(9) No other 19F NMR signals were found over a 400 ppm range. For
19F NMR chemical shifts of neutral Ir(III) fluoro complexes, see: Cockman,
R. W.; Ebsworth, E. A. V.; Holloway, J. H.; Murdoch, H.; Robertson, N.;
Watson, P. G. ACS Symp. Ser. 555, Thrasher, J. S., Strauss, S. H., Eds.;
Chapter 20.
(10) Martin, J. S.; Fujiwara, F. Y. J. Am. Chem. Soc. 1974, 96, 7632-7.
(11) Emsley, J. W.; Phillips, L.; Wray, V. Prog. Nucl. Magn. Reson.
Spectrosc. 1976, 10, 83-756 (see: Barlow, M. G.; Dean, R. R.; Lee, J.
Trans. Faraday Soc. 1969, 65, 321 (ref 54).
(3) (a) Vorbru¨ggen, H. AdV. Heterocycl. Chem. 1990, 49, 117. (b)
Khristich, B. I.; Kruchinin, V. A.; Pozharskii, A. F.; Siminov, A. M. Chem.
Hetrocycl. Compds. 1971, 759-62.
(12) A 1JHF value of 615 ( 50 Hz has been reported for the HF coupling
constant measured in neat HF. Solomon, I.; Bloembergen, N. J. Chem.
Phys. 1956, 25, 261-6.
(4) Crabtree, R. H.; Lavin, M.; Bonneviot, L. J. Am. Chem. Soc. 1986,
108, 4032-7.
(5) Peris, E.; Lee, J. C., Jr.; Rambo, J.; Eisenstein, O.; Crabtree, R. H.
J. Am. Chem. Soc. 1995, 117, 3485-91.
(13) Gouin, L.; Cousseau, J.; Smith, J. A. J. Chem. Soc., Faraday Trans.
2 1977, 73, 1878-83.
S0002-7863(96)02846-6 CCC: $12.00 © 1996 American Chemical Society

13106 J. Am. Chem. Soc., Vol. 118, No. 51, 1996
Communications to the Editor
the HF resonance at -178 ppm is lost, and in the 31P{1H} NMR
spectrum the 2JPF coupling is lost. Free HF was never observed
in the spectrum, possibly due to rapid proton exchange with
trace amounts of basic materials present or reaction with the
glass of the NMR tube.16
In a control experiment on the fluoro complex lacking the
NH2 group, [Ir(7,8-benzoquinoline)(PPh3)2(H)F] (3b), protona-
tion under the same conditions described above for 3a leads to
immediate and irreversible loss of the fluoride ligand as
determined by 1H, 31P{1H}, and 19F NMR spectroscopies; free
HF is not seen.
The NMR evidence of Figure 2 supports the idea that the
coordinated HF is hydrogen bonded as F-H‚‚‚base and the NH2
lone pair of the benzoquinoline group in 5a is the most likely
candidate for the base. We therefore believe the HF ligand is
stabilized by an intramolecular hydrogen bond, showing the
utility of this strategy.
1
Figure 2. JHF vs δ correlation plot of data taken from ref 10, showing
close agreement with the data point for 5a.
In order to show that the F of this HF is still bound to Ir, as
shown in 5a, and not dissociated from the metal center and
merely hydrogen-bonded to the amino functionality, shown as
6a, we turned to the 31P{1H} NMR, where the unprotonated
2
species 3a has a doublet at 16.8 ppm with a cis JPF of 21 Hz.
Protonation at 183 K to give the HF complex 5a causes the 31
P
2
resonance to move to 21.5 ppm, but a cis JPF coupling of 12
Hz is retained, showing that F is still bound to iridium.14
This new HF complex is analogous to our previously reported
halocarbon complexes.15 The bent R-X-M arrangement
always found for halocarbon complexes ( RXM ) ca. 100-
120°)15 implies that H-F-M in 5a might also be bent, allowing
for a hydrogen bond with the adjacent amine as shown. This
prior work15 also shows that the NMR properties of RX are
little affected by coordination for R ) alkyl; in this work, we
see the same is true for HF.
We conclude that the presence of reactive nonligating
hydrogen-bonding groups in the ligand sphere of a metal
complex can play an important role in modifying the chemistry
observed, and this strategy may prove generally useful.
Acknowledgment. We thank Professor J. W. Faller for several
helpful suggestions and the NSF for support.
Warming the sample above 210 K leads to the irreversible
Supporting Information Available: Spectroscopic and analytical
data, as well as experimental details for the preparation of 1a, 2a, 3a,
and 5a (2 pages). See any current masthead for ordering and Internet
access instructions.
1
loss of HF. The H NMR resonance at 9.78 ppm assigned to
coordinated HF and the 2JHF coupling of the hydride resonance
at -16.1 ppm are both lost, while in the 19F NMR spectrum
2
(14) No JFP coupling was observed in the 19F NMR of 5a because the
JA9628462
peaks for the HF ligand were very broad (W1/2 ) 190 Hz).
(15) (a) Kulawiec, R. J.; Holt, E. M.; Lavin, M.; Crabtree, R. H. Inorg.
Chem. 1987, 26, 2559-61. (b) Burk, M. J.; Segmuller, B.; Crabtree, R.
H. Organometallics 1987, 6, 2241-6. (c) Kulawiec, R. J.; Crabtree, R. H.
Coord. Chem. ReV. 1990, 99, 89-115.
(16) Nor could HF be observed in attempts to introduce small quantities
of authentic HF into the same solvent under the conditions of the NMR
experiments.