Effects of a Nonligating Pendant Hydrogen-Bonding Group in a Metal Complex: Stabilization of an HF Complex

Article abstract of DOI:10.1021/om9809769

An amino group has been appended to a benzoquinolinato ligand (bq-NH₂ or bq-NH(i-Pr)) in such a way that it can hydrogen bond to a ligand that is also bound to the metal. The effects of this two-point binding are studied. The complexes [(bq-NHR)IrH(L)(PPh₃)₂]BF₄ (R = H, i-Pr) were synthesized where L = H₂O, F. The hydrogen-bonding pattern in the water complex is probed by crystallographic, IR, and NMR studies. The fluoro complexes protonate at -90°C to give unstable hydrogen fluoride complexes, characterized by NMR (¹J_HF = 440 Hz, bq-NH₂; 430 Hz, bq-NH(i-Pr)). Comparison with results for the corresponding bq-H and bq-CH₃ species suggests that the hydrogen bonding provided by the pendant amino group is the key factor that allows stabilization of the HF complex, a previously unknown species. Crystal structures of an aqua and a fluoro derivative are reported.

Full text of DOI:10.1021/om9809769

Organometallics 1999, 18, 1615-1621
1615
Effects of a Non liga tin g P en d a n t Hyd r ogen -Bon d in g
Gr ou p in a Meta l Com p lex: Sta biliza tion of a n HF
Com p lex
Dong-Heon Lee, Hye J . Kwon, Ben P. Patel, Louise M. Liable-Sands,
Arnold L. Rheingold,* and Robert H. Crabtree*
Department of Chemistry, Yale University, P.O. Box 208107,
New Haven, Connecticut 06520-8107, and Department of Chemistry, University of Delaware,
Newark, Delaware 19716
Received December 3, 1998
An amino group has been appended to a benzoquinolinato ligand (bq-NH2 or bq-NH(i-Pr))
in such a way that it can hydrogen bond to a ligand that is also bound to the metal. The
effects of this two-point binding are studied. The complexes [(bq-NHR)IrH(L)(PPh3)2]BF4
(R ) H, i-Pr) were synthesized where L ) H2O, F. The hydrogen-bonding pattern in the
water complex is probed by crystallographic, IR, and NMR studies. The fluoro complexes
protonate at -90 °C to give unstable hydrogen fluoride complexes, characterized by NMR
(1J HF ) 440 Hz, bq-NH2; 430 Hz, bq-NH(i-Pr)). Comparison with results for the corresponding
bq-H and bq-CH3 species suggests that the hydrogen bonding provided by the pendant amino
group is the key factor that allows stabilization of the HF complex, a previously unknown
species. Crystal structures of an aqua and a fluoro derivative are reported.
In tr od u ction
We have attached hydrogen-bonding pendant groups
to a number of ligands so that the pendant groups are
unable to bind directly to the metal, but can hydrogen
bond to the adjacent ligand.1 If properly placed, these
groups could have effects similar to those of hydrogen-
bonding residues in the active site cavity of metalloen-
zymes, which are believed to contribute to the accelera-
tion of enzyme reactions by transition-state stabilization.2
In prior work,1 we showed how 2-aminopyridine can
bonding involving a metal fluoride complex and outer
sphere HF. A relevant communication has appeared on
some of our work.6 An article giving X-ray structural
evidence suggesting that discrete HF ligands are coor-
dinated to a lanthanide in a complex inorganic salt has
appeared more recently.7
form an intramolecular H...H hydrogen bond between
the pendant amino group and the adjacent hydride as
in complex 1, but this system is not rigid and the
pendant group can bind to the metal under certain
conditions. For instance, when 2-hydroxypyridine re-
places aminopyridine in 1, H2 is lost and the cyclom-
etalated product shown is formed.3 To avert this possi-
bility, we have now induced rigidity in the system by
using rigid cyclometalated benzoquinoline ligands in
which amino-substitution is easy. We report here the
effect of the pendant group on the stabilization of an
HF complex (M-F-H).
Ch oice of System . 7,8-Benzoquinoline8 and its de-
rivatives6 readily cyclometalate with [Ir(cod)(PPh3)2]BF4
under H2 to give a benzoquinolinato complex, 2 (eq 1).
We have extensive reactivity data8 on both 2 and its
derivatives. The labile coordination site on the metal
in 2, trans to the high trans effect aryl ligand, is the
reactive site for the chemistry of the complex. The
2-position of free benzoquinoline is easy to substitute
selectively with NaNH2 or LiNH(i-Pr) via the Chich-
ibabin reaction.9 These amino groups were chosen
because they are versatile and can in principle act as
hydrogen bond donors via NH, or as acceptors or bases
via the lone electron pair. The amino-substituted ben-
As mentioned in a review by Richmond,4 a number
of groups have described N-H...F species.4 Bifluoride5
complexes, containing the group M-F...H-F, can be
considered as examples of intermolecular hydrogen
(1) (a) Peris, E.; Lee, J . C., J r.; Crabtree, R. H. J . Chem. Soc., Chem.
Commun. 1994, 2573. (b) Peris, E.; Lee, J . C., J r.; Rambo, J . R.;
Eisenstein, O.; Crabtree, R. H. J . Am. Chem. Soc. 1995, 117, 3485.
(2) Walsh, C. Enzymatic Reaction Mechanisms; Freeman: San
Francisco, 1979.
(6) Patel, B. P.; Crabtree, R. H. J . Am. Chem. Soc. 1996, 118, 13105.
(7) Mazej, Z.; Borrmann, K. L.; Zemva, B. Inorg. Chem. 1998, 37,
5912.
(3) Lee, J . L.; Crabtree, R. H. Unpublished data.
(4) Richmond, T. G. Coord. Chem. Rev. 1990, 105, 221.
(5) (a) Murphy, V. J .; Hascall, T.; Chen, J . Y.; Parkin, G. J . Am.
Chem. Soc. 1996, 118, 7248. (b) Whittesley, M. K.; Perutz, R. N.;
Greener, B.; Moore, M. H. J . Chem. Soc., Chem. Commun. 1997, 187.
(c) Hintermann, S.; Pregosin, P. S.; Ru¨egger, H. J . Organomet. Chem.
1992, 435, 225.
(8) (a) Crabtree, R. H.; Lavin, M.; Bonneviot, L. J . Am. Chem. Soc.
1986, 108, 4032. (b) Lavin, M.; Holt, E. M.; Crabtree, R. H. Organo-
metallics 1989, 8, 99.
(9) (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. Compounds 1971, 759.
10.1021/om9809769 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 04/02/1999

1616 Organometallics, Vol. 18, No. 9, 1999
Lee et al.
zoquinoline ligands also readily cyclometalate, and in
the resulting complexes, 3 (eq 2) (R ) NH2, a ; NH(i-
Pr), b), the pendant groups are necessarily adjacent to
the labile site, allowing interaction between the pendant
groups and a ligand bound to Ir. At the same time, the
rigidity of the system prohibits ligation of the amino
group to the metal, which would block reactivity and
complicate the analysis.
trans to N, not to C or P. In addition, compound 3a
shows an Ar-NH2 resonance at 6.09 ppm (broad), and
3b shows an Ar-NH resonance at 6.16 ppm (d, J HP
)
6.6 Hz at -10 °C). The 31P NMR spectra show one peak
(3a , 20.3 ppm; 3b, 20.42 ppm), at a position consistent
with the proposed structures.
Hyd r ogen Bon d in g in th e Aqu a Com p lexes (3).
The hydrogen-bonding pattern in 3 was a point of
particular interest. Of the three most likely structures,
3A, 3B, and 3C, evidence from variable-temperature 1H
NMR (CD2Cl2) and IR spectroscopic studies of 3a and
3b enabled us to choose a plausible model.
Results of the NMR studies of 3a and 3b rule out
structures A and C as possible hydrogen-bonding mod-
els. The two structures require that the two protons of
the bonded water are inequivalent and would thus yield
two peaks when hydrogen-bonded. In the two complexes,
however, the bound water resonance starts appearing
at 233 K as broad single peaks (3a , 3.06 ppm; 3b, 2.78
ppm) and sharpens and shifts (3a , 2.90 ppm; 3b, 2.81
ppm) as the temperature is lowered to 193 K. Structure
B is also apparently inconsistent with the data, which
predict two peaks for the two hydrogens of the pendant
amino group, but from the singlet -NH2 resonance
observed in 3a (6.09 ppm, 293 K; 6.24 ppm, 193 K) we
assume that the two hydrogens (a and b) of the amino
group exchange rapidly by C-N bond rotation in solu-
tion even at low temperatures. The thin-film IR spec-
troscopic study shows two distinctive ν(NH) stretching
bands at 3478 and 3365 cm-1, both shifted from the
single band (3411 cm-1) that we observe in the free
ligand, suggesting that 3B is the likely hydrogen-
bonding mode, at least in the solid state. From the
resonances of the -NH- peak of the 3b pendant group
(6.24 ppm (br s), 293 K; 6.24 ppm (d, J (H-N-C-H) )
8.6 Hz), 263 K; 6.51 ppm (d), 213 K) and one ν(NH) band
observed at 3377 cm-1 (shifted to a lower energy from
3415 cm-1 of the free ligand) we conclude by analogy
that the lone hydrogen of the -NH(i-Pr) group also
hydrogen bonds with the lone electron pair of the water
oxygen in a similar way to structure 3B.
Resu lts
Syn th esis of th e Liga n d a n d Sta r tin g Com -
p lexes. The known 2-amino-7,8-benzoquinoline ligand
(bq-NH2)10 and new 2-isopropylamino-7,8-benzoquino-
line (bq-NH(i-Pr)) ligand were readily synthesized (yield
88%, bq-NH2; 70%, bq-NH(i-Pr)) by a Chichibabin
reaction9 between NaNH2 or LiNH(i-Pr) and 7,8-ben-
zoquinoline in a xylenes/N,N′-dimethylaniline mixture
at 177 °C. Characterization data support the formula-
tion: the microanalysis, the strong parent ion peak at
194 and 236 amu in the GC/MS spectrum, and the
strong NH stretching vibrations at 3411 and 3415 cm-1
in the thin-film IR spectrum are consistent with the
formulation of bq-NH2 and bq-NH(i-Pr), respectively.
The crystal structures of the iridium complexes, dis-
cussed below, indicate that the amino groups are indeed
at the 2-position, consistent with the expected outcome
of the Chichibabin reaction.
The aminated ligands react smoothly with [Ir(cod)-
(PPh3)2]BF4 (cod ) 1,5-cyclooctadiene) in wet CH2Cl2
under H2 (1 atm) at 0 °C to give the cyclometalated
species, [(bq-NHR)IrH(H2O)(PPh3)2]BF4, 3 (R ) H, a ;
i-Pr, b), with pendant amino groups in 70% (3a ) or 78%
(3b) yield; this is the same route8b that was previously
used for 2. The close analogy between the 1H NMR
spectra of 3a and those of the crystallographically
characterized species 28 and 3b (see below) confirms the
proposed structure. In particular, the Ir-H (2, -16.1
ppm {t, J HP ) 14 Hz}; 3a , -16.4 ppm {t, J HP ) 14.6
Hz}; 3b, -16.1 ppm {t, J HP ) 14.4 Hz}) and Ir-OH2 (2,
2.54 ppm {s}; 3a , 3.06 ppm {s} at -50 °C; 3b, 2.78 {s}
at -40 °C) proton resonances (s ) singlet, t ) triplet)
appear very similar. The hydride chemical shift is
consistent11 with a stereochemistry having the hydride
Cr ysta l Str u ctu r e of 3b. To examine the hydrogen-
bonding pattern in the complexes, we obtained a crystal
structure of 3b as the triflate salt, the only salt that
would crystallize well. The resulting structure (Figure
1, Tables 1 and 2) shows that the benzoquinoline ligand
is indeed 2-substituted as expected and that the triflate
anion is located in the vicinity of the cation.
Hydrogen bonding between the hydrogen of -NH(i-
Pr) and lone pair of bound water oxygen is also
confirmed: the O...H-N distance and angle in the
crystal are calculated as 2.302 Å and 155.6° (on the basis
of a planar sp2 N with d(N-H) ) 1.03 Å), consistent
with a hydrogen-bonding model of type 3B.
(10) Bergstrom, F. W. J . Org. Chem. 1938, 3, 424.
(11) (a) Crabtree, R. H.; Demou, P. C.; Eden, D.; Mihelcic, J . M.;
Parnell, C. A.; Quirk, J . M.; Morris G. E. J . Am. Chem. Soc. 1992,
104, 6994. (b) Crabtree, R. H.; Faller, J . W.; Mellea, M. F. Organome-
tallics 1982, 1, 1361.
Syn th esis of th e F lu or id e. To better test the idea
that the pendant -NH2 group can hydrogen bond to a

Stabilization of an HF Complex
Organometallics, Vol. 18, No. 9, 1999 1617
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t for 3b‚CF 3SO3‚CH2Cl2 a n d 4a
3b‚CF3SO3‚CH2Cl2 4a
54H50Cl2F3IrN2O4P2S
empirical formula
fw
temp (K)
wavelength (Å)
cryst syst
space group
unit cell dimensions
a (Å)
C
C49H40FIrN2P2
929.97
173(2)
0.71073
monoclinic
P21/c
1205.06
173(2)
0.71073
triclinic
P1h
10.6226(2)
11.7354(2)
20.6503(3)
82.3766(7)
87.7899(2)
82.5466(10)
2529.38(5), 2
1.582
2.909
16.0434(4)
13.8092(4)
18.9100(5)
90
114.327(1)
90
3817.46(18), 4
1.618
3.625
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
volume (Å3), Z
density (calcd) (g/cm3)
abs coeff (mm-1
)
F(000)
1208
1856
cryst dimens (mm)
θ range for data collection (deg)
limiting indices
no. of reflns collected
no. of ind reflns
transmission factors
refinement method
0.30 × 0.15 × 0.01
0.30 × 0.20 × 0.20
1.76-26.00
1.89-23.00
-13 e h e 13, -14 e k e 14, 0 e l e 25
16608
9321 (Rint ) 0.1199)
1.000 and 0.590
full-matrix least-squares on F2
9321/0/622
-17 e h e 16, -15 e k e 13, -20 e l e 20
9547
4667 (Rint ) 0.1160)
0.5309 and 0.4093
full-matrix least-squares on F2
4667/0/481
no. of data/restraints/param
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
0.854
1.101
R1 ) 0.0609, wR2 ) 0.1314
R1 ) 0.1145, wR2 ) 0.1615
2.259 and -2.828
R1 ) 0.0382, wR2 ) 0.1029
R1 ) 0.0153, wR2 ) 0.1185
1.108 and -0.791
largest diff peak and hole (e Å-3
)
Ta ble 2. Selected Bon d Len gth s a n d An gles for
Com p lexes 3b‚CF 3SO3‚CH2Cl2 a n d 4a
Bond Distances (Å)
3b
4a
Ir(1)-P(1)
Ir(1)-P(2)
Ir(1)-O(1)
Ir(1)-N(1)
Ir(1)-C(1)
N(1)-C(13)
N(2)-C(13)
N(2)-C(14)
C(14)-C(15)
C(14)-C(16)
2.309(3)
2.349(3)
2.276(6)
2.190(8)
2.005(10)
1.361(12)
1.342(14)
1.469(12)
1.510(16)
1.516(17)
Ir(1)-C(11)
Ir(1)-F(1)
Ir(1)-N(1)
Ir(1)-P(1)
Ir(1)-P(2)
N(1)-C(1)
N(2)-C(1)
2.009(8)
2.143(4)
2.168(6)
2.3047(19)
2.3165(19)
1.331(10)
1.343(10)
Bond Angles (deg)
3b
4a
C(1)-Ir(1)-N(1)
C(1)-Ir(1)-O(1)
C(1)-Ir(1)-P(1)
C(1)-Ir(1)-P(2)
N(1)-Ir(1)-O(1)
N(1)-Ir(1)-P(1)
N(1)-Ir(1)-P(2)
O(1)-Ir(1)-P(1)
O(1)-Ir(1)-P(2)
P(1)-Ir(1)-P(2)
N(1)-C(13)-N(2)
80.3(4)
177.3(4)
87.6(3)
92.3(3)
98.5(3)
92.0(2)
90.8(2)
94.8(2)
85.4(2)
C(11)-Ir(1)-N(1)
80.3(3)
C(11)-Ir(1)-F(1) 174.8(2)
F igu r e 1. ORTEP diagram of [(bq-NH(i-Pr))Ir(H)(H2O)-
(PPh3)2](CF3SO3)‚CH2Cl2 (3b‚CF3SO3‚CH2Cl2). Thermal el-
lipsoids shown at 30% probability. Hydrogen atoms, coun-
terion, and solvent molecule are omitted for clarity.
C(11)-Ir(1)-P(1)
C(11)-Ir(1)-P(2)
N(1)-Ir(1)-F(1)
N(1)-Ir(1)-P(1)
N(1)-Ir(1)-P(2)
F(1)-Ir(1)-P(1)
F(1)-Ir(1)-P(2)
86.7(2)
86.2 (2)
95.54(19)
92.85(16)
92.39(16)
90.29(12)
97.18(12)
170.42(7)
117.7(7)
ligand bound to Ir, we synthesized analogous fluoro-
complexes, with the hope that J (H,F) coupling between
the amino and fluoro groups would unequivocally iden-
tify any hydrogen bonding. The aqua complexes proved
to react rapidly with [n-Bu4N]F (TBAF) in acetone at
room temperature to give analogous fluoride complexes,
(bq-NHR)IrH(F)(PPh3)2, 4 (eq 3) (R ) H, a ; i-Pr, b). In
addition, (bq-CH3)IrH(F)(PPh3)2 (4c) was synthesized as
177.17(11) P(1)-Ir(1)-P(2)
116.5(10) N(1)-C(1)-N(2)
C(13)-N(2)-C(14) 125.1(10)
N(2)-C(14)-C(15) 110.7(9)
N(2)-C(14)-C(16) 108.3(10)
obtained in 70% yield after recrystallization. 4c was
obtained in 73% yield.
12
Apart from the microanalysis, crystallographic (see
below) and spectroscopic evidence support the structure
of 4a , 4b, and 4c. In particular, the Ir-H resonances
at -16.13 ppm (4a ), -16.09 ppm (4b), and -17.5 ppm
(4c) show coupling not only to the cis PPh3 groups
(2J (H,P) {t} 17 Hz, a ; {t} 16.7 Hz, b, {t} 15 Hz, c) but
also to the cis fluoride (2J (H,F) {d} 4.8 Hz, a ; {d} 5.6
a control: bq-CH3 has the same steric effects as the
aminated ligands but cannot hydrogen bond and is thus
an ideal control ligand. 4a was obtained in 65% yield
after recrystallization from CH2Cl2/hexanes, and 4b was
(12) Arai, S.; Ishikura, M.; Sato, K.; Yamagishi, T. J . Heterocycl.
Chem. 1995, 32, 1081.

1618 Organometallics, Vol. 18, No. 9, 1999
Lee et al.
Hz, b, {d} 4.7 Hz, c). The 19F NMR of 4a shows a
resonance at -328 ppm (vs CFCl3) with a triplet
coupling to the cis PPh3 groups (2J (F,P) {t} 21 Hz).
The key observation supporting a hydrogen-bonding
interaction is the presence at 193 K of a J (Ha,F) coupling
of 52 Hz in one of the amino resonances at 10.1 ppm in
the fluoride 4a , assigned to Ha. Similar J (H,F) coupling
of 49.6 Hz for the lone proton of -NH(i-Pr) is also shown
at 193 K. These numbers are appropriate for a hydrogen-
bonding interaction of the type N-Ha‚‚‚F by comparison
with the value of 63.5 Hz previously observed1a for the
appropriate rotamer of the related 2-aminopyridine
fluoride 5. This is confirmed in 4a by the presence of
F igu r e 2. ORTEP diagram of [(bq-NH2)Ir(H)(F)(PPh3)2]
(4a ). Thermal ellipsoids shown at 30% probability. Hydro-
gen atoms, except for hydride, are omitted for clarity.
Carbon atoms that were refined isotropically are shown as
spheres.
ν(NH) bands in the IR spectrum at 3470 and 3354 cm-1
,
shifted relative to the 3411 cm-1 band in the free ligand,
and comparable to that previously found in fluoride 5.
to one of the hydrogens of the NH2 pendant group. The
H‚‚‚F distance and N-H‚‚‚F angle found in the crystal
are 1.813 Å and 165.6°, respectively, consistent with the
classical hydrogen-bonding model.
In 4b, the ν(NH) band is too broad (3300-3450 cm-1
)
for a precise wavenumber to be reported.
Sp ectr oscop ic Detection of a Hyd r ogen F lu or id e
Com p lex. In their classic 1958 text, Basolo and Pear-
son14 indicate that the acid catalysis of fluoride substi-
tution in Co(III) complexes implies that the fluoride
ligand can be protonated to give a transient hydrogen
fluoride complex of the type M-F-H, which rapidly
loses HF. Only one proposed HF complex7 has so far
been isolated, however, and the alternative hydrogen-
bonded adduct, M-F‚‚‚H-A (HA ) acid), cannot be
excluded on kinetic evidence alone.
A variable-temperature proton NMR study of the
fluoride 4a gave further information. On cooling, the
-NH2 resonance decoalesces, and by 193 K, Ha and Hb
appear as broad singlets at 4.8 and 10.1 ppm. Averaging
these shifts, we infer that the coalesced signal must
occur near 7.45 ppm, hidden under the large aromatic
signal at room temperature. Line shape analysis be-
tween 183 and 193 K reveals a significant initial
broadening that was interpreted in terms of an ex-
Protonation of the pendant amine fluoro-complex, 4a ,
with HBF4‚Et2O at 195 K in CD2Cl2 leads to the
formation of the protonated form, 6a , in solution (eq 4).
The complex decomposed on attempted isolation, but the
1H NMR data at 183 K of 6a indicated it is a hydrogen
fluoride complex. In particular, the resonances at 10.1
and 4.8 ppm, assigned to the -NH2 group, are replaced
by new ones at 9.8 and 6.8 ppm in a 1:2 integral ratio
and assigned to Ha and (Hb + Hc) (see eq 4), respectively.
This assignment is supported by the observation of rapid
(,1 min) isotope exchange of Ha, Hb, and Hc with CD3-
OD (1H NMR). The key observation is the presence of a
1J (H,F) coupling of 440 Hz for the Ha resonance,
consistent with 6a being a true N‚‚‚H-F complex,
rather than having a hydrogen-bonded N-H‚‚‚F system,
change rate at 193 K. The free energy barrier, ∆Gq
,
193K
for Ha/Hb exchange was then estimated by standard
methods as 12.4 ((0.5) kcal/mol.1a As we have discussed
previously,13 this barrier is expected to include the
intrinsic Ar-NH2 rotation barrier, previously estimated
as 5.7 kcal/mol, and the contribution from the hydrogen
bond strength, which is therefore estimated as 6.7 ((1)
kcal/mol. This N-H‚‚‚F hydrogen bond strength is
comparable to the value of 5.2 ((1) kcal/mol previously
estimated for the aminopyridine analogue, 5.
Cr yst a llogr a p h ic St u d y of 4a . Complex 4a gave
crystals suitable for a complete X-ray diffraction study,
shown in Figure 2 and Tables 1 and 2.
This shows that the structure of the complex is
consistent with the postulated structure: the fluoride
is located trans to the aryl ligand and hydrogen bonds
1
as in 4a , where J (H,F) is much lower (e.g., 52 Hz for
4a ). Figure 3 illustrates the Ha resonances of 4a and
6a .
A similar effect is observed upon protonation of the
1
related complex 4b at 183 K, where J (H,F) is 430 Hz.
As expected, the 1J (H,F) coupling was the same at
300 and at 500 MHz. The 440 Hz coupling involves Ha
and F because the same 440 Hz coupling appears in the
19F NMR spectrum but disappears on decoupling the
(13) Sandstrom, J . Dynamic NMR Spectroscopy; Academic Press:
New York, 1982.
(14) Basolo, F.; Pearson R. G. Mechanisms of Inorganic Reactions;
Wiley: New York, 1958; p 153.

Stabilization of an HF Complex
Organometallics, Vol. 18, No. 9, 1999 1619
F igu r e 3. N-H...F-Ir and N...H-F-Ir proton resonances
in the 300 MHz 1H NMR spectra of 4a (a) and 6a (b) at
183 K.
Ha resonance of 6. The protonation of the bq-CH3 species
4c did not lead to a stable H-F complex, indicating that
the N‚‚‚H-F stabilization is not the result of a steric
effect, but of the hydrogen-bonding ability of the NH2
pendant ligand.
1
F igu r e 4. J HF vs δ correlation plot of data from ref 17
(bifluoride complex from ref 5a), showing close agreement
with the data points for 6a and 6b.
1
The large value of J (Ha,F), 440 Hz, observed for 6a
is reasonable for an H-F complex, as indicated by
comparison with literature data.15 Figure 4 illustrates
the relation between J and δ values for HF in the gas
phase, in various hydrogen-bonded adducts, such as
[C5H5N‚‚‚H-F], in solution, and in [FHF]- ion, also in
solution.
The hydrogen-bonded adducts of nitrogen bases with
HF and the HF complex 6 are clustered in the same
region of the graph, while HF (gas phase) and [FHF]-
ion have quite different spectral properties. This is
powerful supporting evidence that the Ha resonance
indeed arises from a hydrogen-bonded HF group. The
recently discovered bifluoride complexes also have
distinctly different NMR spectral behavior.5a
The kinetic site of protonation16 of 4a to give 6a could
not be determined experimentally, but protonation of
the basic amino nitrogen seems most likely. This is
expected to be followed by proton transfer from the
+
resulting NH3 group to the hydrogen-bonded fluoride
to give 6a . The pKa of HF (ca. 4 in water) is at the high
+
end of the reported pKa range for ArNH3 (-10 to 5),
consistent with the postulated direction of the proton
transfer to give 6a .17
The stability of the HF complex is indeed a result of
the presence of the pendant hydrogen-bonding amino
group: as already stated, by substituting bq-CH3 for bq-
NH2, the fluoro complex no longer gives an HF complex
on protonation under identical conditions and merely
loses HF. Free HF is not observed in solution, but
control experiments with deliberate addition of HF show
that this is because free HF does not survive under the
conditions used; recent data from Caulton18 indicate
that free HF can rapidly react with the glass of an NMR
tube to give [SiF5]-.
We needed to establish if the fluoride remains bound
to the metal on protonation. The Ir-F bond in 4a is
indeed retained in 6a , as shown by the persistence of a
cis-coupling between the phosphorus nuclei of L and the
1
cis fluoride in the H NMR spectrum on going from 4a
(J (P,F) ) 21 Hz.) to 6a (J (P,F) ) 12 Hz). In further
support of this structure, a doublet resonance at 16.8
ppm with a 2J (P, F) coupling of 21 Hz in the 31P {1H}
NMR spectrum of 4a moves in 6a to 21.5 ppm but
retains some coupling, 12 Hz. Protonation of the fluoride
is expected to weaken the Ir-F bond, and so the
observed decrease in the coupling is reasonable.
The metal-bound H-F in 6a is certainly hydrogen-
bonded, as indicated by the data15 of Figure 4, and the
pendant amino group of the ligand seems the most
natural H-bond acceptor group to consider. Consistent
with this proposal, irradiation of the Ha proton causes
a slight narrowing of the broad Hc,d amine resonance,
probably as a result of unresolved coupling between Ha
Discu ssion
The data indicate that the presence of a pendant
-NH2 or -NH3+ group can have significant “neighbor-
ing group” effects on the reactivity of ligands bound to
a metal at an adjacent site. In the case of the fluoro
complex, 4a , the pendant -NH2 group allows stabiliza-
tion of a very labile HF complex to a sufficient degree
that it can be spectroscopically detected.
(16) Kramarz, K. W.; Norton, J . R. Prog. Inorg. Chem. 1990, 42, 1.
(17) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
(18) Cooper, A. C.; Huffman, J . C.; Caulton, K. G. Inorg. Chim. Acta
1998, 270, 261.
and Hc,d
.
(15) Martin, J . S.; Fujiwara, F. Y. J . Am. Chem. Soc. 1974, 96, 7632.

1620 Organometallics, Vol. 18, No. 9, 1999
Lee et al.
2
2
m), 4.72 (NH, d, J HH ) 7.6 Hz), 6.69 (1H, d, J HH ) 8.5 Hz),
7.50-7.62 (4H, m), 7.83-7.89 (2H, m), 9.10-9.13 (1H, m).
Anal. Calcd for C16H16N2: C, 81.32; H, 6.82; N, 11.90. Found:
C, 81.84; H, 6.61; N, 11.61.
The observation of an HF complex helps confirm the
proposal that acid-catalysis of fluoride substitution
occurs by prior protonation at the fluoride and loss of
the labile HF ligand. We and others have previously
provided evidence for halocarbon complexes where the
order of binding is RF < RCl < RBr < RI.19 Here we
see that, at least in the special case of HF, a weak acid,
a similar type of complex can also exist.
2-Meth yl-7,8-ben zoqu in olin e. Synthesis of this ligand
followed a known literature preparation method.12 1H NMR
2
(CD2Cl2) δ in ppm: 2.83 (CH3, s), 7.42 (1H, d, J HH ) 5.1 Hz),
7.67-7.89 (4H, m), 7.89-7.94 (1H, m), 8.08 (1H, d, 2J HH ) 5.2
Hz), 9.30-9.33 (1H, m). NMR data closely match literature
values.12
Con clu sion
(2-Am in o-7,8-b e n zoq u in olin a t o)h yd r id o(a q u a )b is-
(tr iph en ylph osph in e)ir idiu m (III) Tetr aflu or obor ate (3a).
[Ir(cod)(PPh3)2]BF4 (200 mg, 0.242 mmol) and 2-amino-7,8-
21
The presence of a potentially hydrogen-bonding but
nonligating group at the 2-position of a cyclometalated
benzoquinolinate iridium complex has significant effects
on the chemistry of ligands adjacent to the group, as
compared to a control compound with a pendant group
showing similar steric effects without the hydrogen-
bonding ability. Here we show that the group hydrogen
bonds to an adjacent Ir-F fluoride and stabilizes an Ir-
(FH) complex. In other work,20 we find it also acts as a
base in facilitating the activation of molecular hydrogen
and modifies the structure of a trihydride derived from
protonating the neutral dihydride starting complex (1).
For example, in the absence of the amino group, a stable
H2 complex is formed by displacement of H2O; with the
amino group present, proton transfer from this H2
complex to the bq-NH2 group occurs.20 This general
strategy may in the future offer other unusual applica-
tions for modifying ligand structure and reactivity in
metal complexes.
benzoquinoline (40 mg, 0.21 mmol) in CH2Cl2 (10 mL) were
treated with H2 (1 atm, 20 mL/min) at 0° (ice bath) until the
color of the solution changed from red to pale yellow (5 min).
The volume of the solution was reduced by 50% in vacuo and
hexanes-Et2O added (1:1 v/v, 10 mL) with stirring to precipi-
tate a pale yellow solid (30 min), which was filtered, washed
(hexanes, 5 mL), and dried in vacuo to give 3 (158 mg, 70%
yield). 1H NMR (298 K, CD2Cl2) δ in ppm: -16.43 (Ir-H, t,
2J HP ) 14.6 Hz); 1.92 (H2O, br s); 6.09 (NH2, br s); 6.5-8.8 (37
H, br m). 31P{H} NMR (298 K, CD2Cl2) δ in ppm: 20.3 (s).
Anal. Calcd for C39H42N2OP2BF4Ir: C, 55.93; H, 4.14; N, 2.76.
Found: C, 55.82; H, 4.17; N, 3.03.
(2-Isopr opylam in o-7,8-ben zoqu in olin ato)h ydr ido(aqu a)-
bis(tr ip h en ylp h osp h in e)ir id iu m (III) Tetr a flu r obor a te or
Hexa flu or p h osp a te (3b). Preparation of 3b was analogous
to the synthesis of 3a , substituting 2-isopropylamino-7,8-
benzoquinoline for 2-amino-7,8-benzoquinoline. 3b was ob-
tained in 78% yield. 1H NMR (CD2Cl2) δ in ppm: -16.10
2
2
(Ir-H, t, J HP ) 14.4 Hz); 1.30 (CH(CH3)2, d, J HH ) 5.9 Hz);
3.93 (-CH(CH3)2, m); 6.16 (NH, br s), 6.6-7.8 (37H, m). 31P{H}
NMR (CD2Cl2) δ in ppm: 20.42 (s). Anal. Calcd for C52H48N2-
OP2PF6Ir: C, 56.0; H, 4.33; N, 2.51. Found: C, 55.91; H, 4.76;
N, 2.70.
Exp er im en ta l Section
All reactions were carried out with standard Schlenck
techniques and degassed analytical grade solvents under N2
or Ar. The following spectrometers were used: a 300 MHz GE-
Omega (1H, 31P NMR); a 500 MHz Bruker (1H NMR); a 490
MHz instrument (19F NMR); a MIDAC M1200 (FTIR); a HP-
5971A MSD interfaced to a HP-5890 Series II GC (GC/MS).
Elemental analysis was performed in Robertson Microlit
Laboratories, Inc. (NJ ).
(2-Me t h yl-7,8-b e n zoq u in olin a t o)h yd r id o(a q u a )b is-
(tr ip h en ylp h osp h in e)ir id iu m (III) Tetr a flu or obor a te (3c).
Preparation of 3c was analogous to the synthesis of 3a ,
substituting 2-methyl-7,8-benzoquinoline for 2-amino-7,8-ben-
1
zoquinoline. 3c was obtained in 80% yield. H NMR (CD2Cl2)
δ in ppm: -14.06 (Ir-H, br s); 2.35 (CH3, s); 6.7-8.0 (37H,
m). 31P{H} NMR (CD2Cl2) δ in ppm: 22.97 (s). Anal. Calcd for
C
50H43IrNOP2BF4: C, 59.20; H, 4.27; N, 1.38. Found: C, 58.88;
2-Am in o-7,8-ben zoqu in olin e. Preparation was adapted
from a known literature synthesis.9 A suspension of 7,8-
benzoquinoline and 4 equiv of NaNH2 in N,N-dimethylaniline
(DMA) was heated to 177 °C for 3 h under a continuous purge
of dinitrogen. During this time the suspension underwent a
color change from pale yellow, to dark green, and finally to
dark brown. After cooling to room temperature, the mixture
was hydrolyzed. The resulting organic layer was separated,
dried over anhydrous MgSO4, filtered, and concentrated under
vacuum to less than 1 mL. The amine was isolated from the
mixture by flash chromatography over alumina, using hexanes
to elute the DMA and unreacted benzoquinoline, followed by
acetone to elute the ligand. After combining the fractions,
solvent removal by rotoevaporation produced the compound
as a pale orange solid (88% yield). 1H NMR (CD2Cl2) δ in
ppm: 9.07 (1H, m), 7.94 (1H, d, J HH ) 7.6), 7.83-7.87 (1H,
m), 7.58-7.63 (4H, m), 6.83 (1H, d, 2J HH ) 8.6 Hz), 4.87 (NH2,
s). Mp: 98-102 °C (lit.10 97-100 °C).
H, 4.44; N, 1.20.
(2-Am in o-7,8-b en zoq u in olin a t o)h yd r id o(flu or o)b is-
(tr ip h en ylp h osp h in e)ir id iu m (III) (4a ). To complex 3a (250
mg, 0.25 mmol) in stirred acetone (10 mL) was added [n-Bu4N]F
(0.25 mL of 1 M thf solution, 0.25 mmol). After 10 min, hexanes
(10 mL) were added to give a pale yellow solid, which was
filtered, washed (hexanes, 10 mL), and dried in vacuo (230
mg). Recrystallization from CH2Cl2-hexanes gave 4a (163 mg,
65% yield). 1H NMR (298 K, CD2Cl2) δ in ppm: -16.13 (Ir-H,
2
2
2
td, J HP ) 17 Hz, J HF ) 4.8 Hz); 6.03 (1H, J HH ) 9 Hz); 6.42
2
2
(1H, t, J HH ) 7.2 Hz); 6.67 (1H, d, J HH ) 6.6 Hz); 6.9-8.5
1
(36H, m). H NMR (193 K, CD2Cl2) δ in ppm: 10.05 (N-H‚‚‚
1
F, d, J HF ) 52 Hz); 4.85 (N-H, br s). 31P{H} NMR (183 K,
CD2Cl2) in ppm: 16.4 (d, 2J PF ) 21 Hz). 19F NMR (298 K, CD2-
2
Cl2, with CFCl3 ref) δ in ppm: -328 (t, J PF ) 21 Hz); (193 K
CD2Cl2) -323 (br s). Anal. Calcd for C49FH40IrN2P2: C, 63.29,
H, 4.31; N, 3.01. Found: C, 63.11; H, 4.17; N, 3.05.
(2-Isop r op yla m in o-7,8-ben zoqu in olin a to)h yd r id o(flu -
or o)bis(tr ip h en ylp h osp h in e)ir id iu m (III) (4b). With 3b as
the starting material, the synthesis of 4b was analogous to
the preparation of 4a . 4b was obtained in 70% yield after
2-Isop r op yla m in o-7,8-ben zoqu in olin e. Synthesis of this
ligand was analogous to the preparation of 2-amino-7,8-
benzoquinoline: LiNH(i-Pr) was used instead of NaNH2,
yielding the ligand as a bright yellow oil in 70% yield. 1H NMR
1
recrystallization. H NMR (CD2Cl2) δ in ppm: -16.09 (Ir-H,
2
(CD2Cl2) δ in ppm: 1.36 (2 CH3, d, J HH ) 6.1 Hz), 4.39 (CH,
2
2
2
td, J HP ) 16.7 Hz, J HF ) 5.6 Hz); 0.96 (CH(CH3)2, d, J HH
)
2
6.6 Hz); 3.33 (CH(CH3)2, m); 6.13 (1H, d, J HH ) 8.7 Hz); 6.36
(19) Kulawiec, R. J .; Crabtree, H. C. Coord. Chem. Rev. 1990, 99,
89.
(20) Lee, D.-H.; Patel, B. P.; Clot, E.; Eisenstein, O.; Crabtree, R.
H. J . Chem. Soc., Chem. Commun. 1999, 297.
(21) Haines, L. M.; Singleton, E. J . Chem. Soc., Dalton Trans. 1972,
1891.

Stabilization of an HF Complex
Organometallics, Vol. 18, No. 9, 1999 1621
2
2
(1H, t, J HH ) 6.0 Hz); 6.57 (1H, d, J HH ) 6.9 Hz); 6.8-7.5
capillary with epoxy cement. The data were collected on a
Siemens P4 diffractometer equipped with a SMART/CCD
detector.
(34H, m); 10.26 (N-H‚‚‚F, d, J HF ) 49.6 Hz). 31P{H} NMR
(CD2Cl2) δ in ppm: 14.89 (d, J PF ) 21.5 Hz). Anal. Calcd for
52FH46IrN2P2: C, 64.20, H, 4.77; N, 2.88. Found: C, 64.55;
1
2
C
The structures were solved by direct methods, completed
by subsequent difference Fourier syntheses, and refined by
full-matrix, least-squares procedures. An empirical absorption
correction (DIFABS) was applied to the data of 3b‚CF3-
SO3‚CH2Cl2, based on a Fourier series in the polar angles of
the incident and diffracted beam paths, and was used to model
an absorption surface for the difference between the observed
and calculated structure factors. No corrections were required
for 4a because there was less than 10% variation in the
integrated ψ-scan intensities. The carbons C(5), C(24), C(44),
and C(56) of 3b‚CF3SO3‚CH2Cl2 were persistently nonpositive
definite and were refined isotropically. All other non-hydrogen
atoms were refined with anisotropic displacement parameters.
The hydride hydrogen atoms and the hydrogen atoms on the
aqua ligand of 3b‚CF3SO3‚CH2Cl2 could not be located and
were ignored in the refinement, but not in the calculation of
the intensive properties of the crystal. The hydride hydrogen
atom of 4a was located from the difference map and allowed
to refine. All other hydrogen atoms were treated as idealized
contributions. Several peaks from the final difference map of
3b‚CF3SO3‚CH2Cl2 (1.3-2.3 e/Å3) remained, but were in
chemically unreasonable positions (0.9-1.5 Å from iridium)
and were considered noise.
H, 4.32; N, 2.49.
(2-Met h yl-7,8-b en zoq u in olin a t o)h yd r id o(flu or o)b is-
(tr ip h en ylp h osp h in e)ir id iu m (III) (4c). With 3c as the
starting material, the synthesis of 4c was analogous to the
preparation of 4a . 4c was obtained in 73% yield. 1H NMR (CD2-
2
2
Cl2) δ in ppm: -17.5 (Ir-H, td, J HP ) 15 Hz, J HF ) 4.7 Hz);
2
2.12 (CH3, s); 6.49 (1H, t, J HH ) 7.0 Hz); 6.8-7.4 (35H, m);
2
7.59 (1H, d, J HH ) 8.5 Hz). 31P{H} NMR (CD2Cl2) δ in ppm:
2
12.46 (d, J PF ) 22 Hz). Anal. Calcd for C50FH41IrNP2: C,
64.60, H, 4.45; N, 1.51. Found: C, 64.73; H, 4.53; N, 1.43.
Obser va tion of HF Com p lex 6a . To complex 4a (5 mg,
5.4 µmol) in CD2Cl2 (0.5 mL) in an NMR tube at 195 K
(acetone-dry ice) was added HBF4‚Et2O (1µL, 5.8 µmol). After
rapid transfer to a precooled NMR probe, NMR data were
1
obtained. H NMR (183 K, CD2Cl2) δ in ppm: -16.31 (Ir-H,
br s); 6.64 (1H, t); 6.84 (NH2, br s); 6.99-8.25 (36H, m); 9.77
1
(Ir-F-H, d, J HF ) 440 ( 5 Hz). 19F NMR (183 K, CD2Cl2) δ
in ppm: -178 (Ir-F-H, d, 440 ( 5 Hz). 31P{1H} NMR (183 K,
2
CD2Cl2) δ in ppm: 21.54 (d, J PF ) 12 Hz). The compound
decomposes above 210 K; therefore no isolation or elemental
analysis was possible.
Obser va tion of HF Com p lex 6b. To complex 4b (5.5 mg,
5.7 µmol) in CD2Cl2 (0.5 mL) in an NMR tube at 195 K
(acetone-dry ice) was added HBF4‚Et2O (1 µL, 5.8 µmol). After
rapid transfer to a precooled NMR probe, NMR data were
Ack n ow led gm en t. We thank the NSF for support.
1
obtained. H NMR (183 K, CD2Cl2) δ in ppm: -16.25 (Ir-H,
br s); 1.16 (-CH(CH3)2, d, 2J HH ) 63.8 Hz), 3.91 (CH(CH3)2,
s), 6.5-7.5 (38H, m), 9.89 (Ir-F-H, d, 1J HF ) 430 ( 5 Hz). No
isolation or elemental analysis was possible due to decomposi-
tion of the product.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, bond distances, bond angles, anisotropic displace-
ment coefficients, and hydrogen atom coordinates for the
structural analyses of compounds 3b‚CF3SO3CH2Cl2 and 4a .
This material is available free of charge via the Internet at
http://pubs.acs.org.
.
X-r ay Str u ctu r al An alysis of Com plex 3b.CF3SO3 CH2Cl2
a n d 4a . Suitable crystals for single-crystal X-ray diffraction
(0.30 × 0.15 × 0.01 mm, 3b‚CF3SO3‚CH2Cl2; 0.30 × 0.20 ×
0.20 mm, 4a ) were selected and mounted on the tip of a
OM9809769