Multimodal study of secondary interactions in Cp*Ir complexes of imidazolylphosphines bearing an NH group

Article abstract of DOI:10.1021/ja906712g

Hydrogen bonding phenomena are explored using a combination of X-ray diffraction, NMR and IR spectroscopy, and DFT calculations. Three imidazolylphosphines R₂PImH (ImH = imidazol-2-yl, R = t-butyl, i-propyl, phenyl, 1a-1c) and control phosphine (i-Pr)₂PhP (1d) lacking an imidazole were used to make a series of complexes of the form Cp Ir(L₁)(L₂)(phosphine). In addition, in order to suppress intermolecular interactions with either imidazole nitrogen, 1e, a di(isopropyl)imidazolyl analogue of 1b was made along with its doubly ¹⁵N-labeled isotopomer to explore bonding interactions at each imidazole nitrogen. A modest enhancement of transfer hydrogenation rate was seen when an imidazolylphosphine ligand 1b was used. Dichloro complexes (L ₁ = L₂ = Cl, 2a-2c,2e) showed intramolecular hydrogen bonding as revealed by four X-ray structures and various NMR and IR data. Significantly, hydride chloride complexes [L₁ = H, L₂ = Cl, 3a-3c and 3e-(¹⁵N)₂] showed stronger hydrogen bonding to chloride than hydride, though the solid-state structure of 3b evinced intramolecular Ir-H...H-N bonding reinforced by intermolecular N...H-N bonding between unhindered imidazoles. These results are compared to literature examples, which show variations in preferred hydrogen bonding to hydride, halide, CO, and NO ligands. Surprising differences were seen between the dichloro complex 2b with isopropyl groups on phosphorus, which appeared to exist as a mixture of two conformers, and related complex 2a with tert-butyl groups on phosphorus, which exists in chlorinated solvents as a mixture of conformer 2a-endo and chelate 5a-Cl, the product of ionization of one chloride ligand. This difference became apparent only through a series of experiments, especially ¹⁵N chemical shift data from 2D ¹H-¹⁵N correlation. The results highlight the difficulty of characterizing hemilabile, bifunctional complexes and the importance of innocent ligand substituents in determining structure and dynamics.

Full text of DOI:10.1021/ja906712g

Published on Web 05/20/2010
Multimodal Study of Secondary Interactions in Cp*Ir
Complexes of Imidazolylphosphines Bearing an NH Group
Douglas B. Grotjahn,*,† John E. Kraus,† Hani Amouri,‡ Marie-Noelle Rager,§
Andrew L. Cooksy,† Amy J. Arita,† Sara A. Cortes-Llamas,†,⊥ Arthur A. Mallari,†
Antonio G. DiPasquale,# Curtis E. Moore,# Louise M. Liable-Sands,|,#
James D. Golen,¶,# Lev N. Zakharov,# and Arnold L. Rheingold#
Department of Chemistry and Biochemistry, 5500 Campanile DriVe, San Diego State UniVersity,
San Diego, California 92182-1030, Institut Parisien de Chimie Mole´culaire, UMR CNRS 7201,
UniVersite´ Pierre et Marie Curie-Paris 6, 4 place Jussieu, case 42, 75252 Paris Cedex 05, France,
NMR Facilities of Ecole Nationale Supe´rieure de Chimie de Paris, 11 Rue Pierre et Marie
Curie, 75231 Paris Cedex 05, France, Department of Chemistry, Widener UniVersity, One
UniVersity Place, Chester, PennsylVania 19013, Department of Chemistry and Biochemistry,
UniVersity of Massachusetts Dartmouth, North Dartmouth, Massachusetts 02747, and
Department of Chemistry and Biochemistry, UniVersity of California, San Diego,
La Jolla, California 92093-0385
Received August 7, 2009; E-mail: grotjahn@chemistry.sdsu.edu
Abstract: Hydrogen bonding phenomena are explored using a combination of X-ray diffraction, NMR and
IR spectroscopy, and DFT calculations. Three imidazolylphosphines R2PImH (ImH ) imidazol-2-yl, R )
t-butyl, i-propyl, phenyl, 1a-1c) and control phosphine (i-Pr)2PhP (1d) lacking an imidazole were used to
make a series of complexes of the form Cp*Ir(L1)(L2)(phosphine). In addition, in order to suppress
intermolecular interactions with either imidazole nitrogen, 1e, a di(isopropyl)imidazolyl analogue of 1b was
made along with its doubly 15N-labeled isotopomer to explore bonding interactions at each imidazole nitrogen.
A modest enhancement of transfer hydrogenation rate was seen when an imidazolylphosphine ligand 1b
was used. Dichloro complexes (L1 ) L2 ) Cl, 2a-2c,2e) showed intramolecular hydrogen bonding as
revealed by four X-ray structures and various NMR and IR data. Significantly, hydride chloride complexes
[L1 ) H, L2 ) Cl, 3a-3c and 3e-(15N)2] showed stronger hydrogen bonding to chloride than hydride, though
the solid-state structure of 3b evinced intramolecular Ir-H· · ·H-N bonding reinforced by intermolecular
N· · ·H-N bonding between unhindered imidazoles. These results are compared to literature examples,
which show variations in preferred hydrogen bonding to hydride, halide, CO, and NO ligands. Surprising
differences were seen between the dichloro complex 2b with isopropyl groups on phosphorus, which
appeared to exist as a mixture of two conformers, and related complex 2a with tert-butyl groups on
phosphorus, which exists in chlorinated solvents as a mixture of conformer 2a-endo and chelate 5a-Cl,
the product of ionization of one chloride ligand. This difference became apparent only through a series of
experiments, especially 15N chemical shift data from 2D 1H-15N correlation. The results highlight the difficulty
of characterizing hemilabile, bifunctional complexes and the importance of innocent ligand substituents in
determining structure and dynamics.
Introduction
or hydrogen bonding interactions to create exquisite catalysts.
Our research group has used phosphine ligands capable of proton
transfer or hydrogen bonding to speed anti-Markovnikov alkyne
hydration by more than 1000 times8 and alkene isomerization
by more than 10 000 times.9 The ligands used in these cases
contained a heterocyclic nitrogen. In the course of structural
Traditional means of fine-tuning the reactivity of organome-
tallic catalysts include changing ligand size or electronic
properties or the oxidation state or charge of the metal. In
contrast, a growing body of research1-7 highlights the benefits
of using secondary interactions provided by a ligand functional
group or groups outside the metal coordination sphere. This
approach to enhancing reactivity resembles features of Nature’s
metalloenzymes, many of which use multiple proton transfer
(1) van den Beuken, E. K.; Feringa, B. L. Tetrahedron 1998, 54, 12985–
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248, 2201–2237.
† San Diego State University.
‡ Universite´ Pierre et Marie Curie-Paris 6.
§ Ecole Nationale Supe´rieure de Chimie de Paris.
⊥ Fulbright Scholar on leave from Universidad de Guadalajara to SDSU.
| Widener University.
(4) Grotjahn, D. B. Chem.sEur. J. 2005, 11, 7146–7153.
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¶ University of Massachusetts Dartmouth.
# University of California, San Diego.
9
10.1021/ja906712g  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 7919–7934 7919

A R T I C L E S
Grotjahn et al.
Scheme 1. Ligands and Complexes Made in This Study
and mechanistic studies,10 we have provided conclusive evidence
in key catalytic intermediates of alkyne hydration for the ability
of a basic heterocyclic nitrogen to accept a hydrogen bond11
and for the ability of a protonated pyridine nitrogen to donate
a hydrogen bond.12,13 In contrast, we have also shown on square-
planar Pt(II) that a neutral heterocyclic NH can donate a
hydrogen bond to an oxygenated ligand.14 In this work, we
investigate secondary interactions on a completely different
metal and ligand set, octahedral Cp*Ir(III) bearing hydride and
chloride ligands. There are several literature studies of hydrogen
bonding to late transition metal complexes (H)(X)MLn, where
X ) halide: in one series of papers, intermolecular approach of
O-H and N-H bonds appears to occur at a chloride ligand
rather than a hydride,15,16 whereas in another study, intramo-
lecular approach of an O-H or N-H bond to hydride is favored
over approach to chloride, bromide, or iodide.17 In still other
hydride complexes, hydrogen bonding to CO or NO ligands
rather than to hydride has been seen.15,18-20 Thus, it is not
entirely clear what will be the preferred bonding site in a given
system, nor why one system would prefer one interaction over
another. Here, through a combination of spectroscopic and X-ray
diffraction data, we demonstrate stronger intramolecular hy-
drogen bonding on Cp*Ir complexes to the chloride than to the
hydride, rather than dihydrogen bonding,21-29 and we further
probe these interactions through quantum chemical modeling.
(8) (a) Grotjahn, D. B.; Lev, D. A. J. Am. Chem. Soc. 2004, 126, 12232–
12233. For subsequent work, see: (b) Labonne, A.; Kribber, T.;
Hintermann, L. Org. Lett. 2006, 8, 5853–5856. (c) Kribber, T.;
Labonne, A.; Hintermann, L. Synthesis 2007, 2809–2818.
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J. Am. Chem. Soc. 2007, 129, 9592–9593.
In the work reported here, three distinctly different hypotheses
were advanced to explain changes in spectroscopic data. Below,
after sections on study design, sample synthesis, and X-ray
diffraction results, we describe the different hypotheses in turn
and the evidence for and against the first two for one complex,
which led us to develop the third. A series of spectral data were
necessary, culminating in 15N chemical shifts obtained by
(10) Grotjahn, D. B. Dalton Trans. 2008, 6497–6508.
(11) Grotjahn, D. B.; Miranda-Soto, V.; Kragulj, E. J.; Lev, D. A.; Erdogan,
G.; Zeng, X.; Cooksy, A. L. J. Am. Chem. Soc. 2008, 130, 20–21.
(12) Grotjahn, D. B.; Kragulj, E. J.; Zeinalipour-Yazdi, C. D.; Miranda-
Soto, V.; Lev, D. A.; Cooksy, A. L. J. Am. Chem. Soc. 2008, 130,
10860–10861.
(13) Pyridylphosphines have also been used to speed alkyne carboxylation.
For mechanistic studies related to this reaction, see: Scrivanti, A.;
Beghetto, V.; Campagna, E.; Zanato, M.; Matteoli, U. Organometallics
1998, 17, 630–635.
1
observing natural abundance samples using H-15N gradient
HMBC and HSQC experiments, as well as data on selected
compounds with 15N isotopic enrichment at both imidazole
nitrogens. Calculations using density functional theory (DFT)
were also needed to discriminate between two very different
explanations of observed spectral changes. This work highlights
the difficulties in studying fluxional or hemilabile organometallic
systems containing bifunctional ligands.
(14) Grotjahn, D. B.; Gong, Y.; DiPasquale, A. G.; Zakharov, L. N.;
Rheingold, A. L. Organometallics 2006, 25, 5693–5695.
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I. L.; Nefedov, S. E. J. Organomet. Chem. 2000, 610, 58–70.
(16) Yandulov, D. V.; Caulton, K. G.; Belkova, N. V.; Shubina, E. S.;
Epstein, L. M.; Khoroshun, D. V.; Musaev, D. G.; Morokuma, K.
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Gruendemann, S.; Golubev, N. S.; Limbach, H.-H. Eur. J. Inorg.
Chem. 2001, 1753–1761.
Results and Discussion
Design of the Study. The rich and unusual chemistry enabled
by the Cp*Ir fragment30-35 led us to explore it as a platform.
In particular, chloro and hydride complexes of Cp*Ir mono-
phosphine fragments have been versatile starting materials or
intermediates. Therefore, using five ligands (1a-1e), we chose
to make a concise series of complexes (Scheme 1) to explore
the inter-relationship of phosphine R group (a, b, c) and hydride
(19) Belkova, N. V.; Besora, M.; Epstein, L. M.; Lledos, A.; Maseras, F.;
Shubina, E. S. J. Am. Chem. Soc. 2003, 125, 7715–7725.
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138.
(21) Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.;
Koetzle, T. F. Acc. Chem. Res. 1996, 29, 348–354.
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181.
(24) Bakhmutov, V. I. Eur. J. Inorg. Chem. 2005, 245–255.
(25) Recent AdVances in Hydride Chemistry; Peruzzini, M., Poli, R., Eds.;
Elsevier: Amsterdam, 2001.
(30) Maitlis, P. M. Coord. Chem. ReV. 1982, 43, 377–384.
(31) Bergman, R. G. J. Organomet. Chem. 1990, 400, 273–282.
(32) Klei, S. R.; Golden, J. T.; Burger, P.; Bergman, R. G. J. Mol. Catal.
A 2002, 189, 79–94.
(26) Bakhmutov, V. I. Dihydrogen Bonds: Principles, Experiments, and
Applications; John Wiley & Sons: Hoboken, NJ, 2008.
(27) Besora, M.; Lledos, A.; Maseras, F. Chem. Soc. ReV. 2009, 38, 957–
966.
(33) Liu, J.; Wua, X.; Iggoa, J. A.; Xiao, J. Coord. Chem. ReV. 2008, 252,
782–809.
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337, 68–76.
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1954.
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9
7920 J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010

Secondary Interactions in Cp*Ir Complexes
A R T I C L E S
Scheme 3. Syntheses of Imidazolylphosphine Complexes
Scheme 2. Ligand Synthesis
however, reaction progress could be determined not only by
1H and 31P NMR spectroscopy but also visually: the starting
dimer is an intensely blue solid, whereas the products are orange-
yellow, albeit somewhat less intensely colored than the dichlo-
ride complexes. As in the dichloride case, the less hindered
ligands 1b, 1c, and 1e-(15N)2 reacted more quickly (at room
temperature within 1 day), whereas di-tert-butyl ligand 1a
required nearly a week of heating at 70 °C to complete its
reaction. Intriguingly, model ligand 1d, which should have the
same steric profile as 1b or 1c and therefore react just as quickly,
required days at 70 °C to completely break dimer [Cp*Ir(Cl)(µ-
H)]2, which may suggest a role for hydrogen bonding in breaking
the hydride dimer when the imidazolylphosphines are used. In
the case of 3c, cleaner product was obtained by selectively
reducing 2c with sodium formate in methanol.
Dihydride complex 4b was made by reducing 2b with sodium
formate (5 to 6 mol per mol of 4b) in methanol. After 14 h at
ambient temperature, analysis of the mixture by 1H and 31P NMR
spectroscopy showed the complete consumption of 2b, forma-
tion of mostly (>90%) monohydride 3b, along with traces of
4b. In contrast, when a similar mixture was heated at 70 °C for
24 h, reduction was complete, allowing isolation of 4b as a
nearly colorless solid in 69% yield. A solution of 4b in oxygen-
free, dry C6D6 was stable for more than a month at room
temperature, and a solution in CD2Cl2 appeared to contain only
traces (<2%) of 3b after 2 days at room temperature. Model
dihydride complex 4d was made from 2d by a similar procedure,
though interestingly, reduction of 2d seemed to require a greater
excess of formate and longer reaction times.
The monohydride products 3 occasionally contained traces
(<2%) of other hydride species, including the corresponding
dihydride. In addition, some attempts at recrystallizing the
monohydrides 3a and 3b led to isolation of a first crop of
crystals of the corresponding dichlorides 2a or 2b, even when
all chlorinated solvents were avoided. Presumably, this means
that there was a conproportionation of two molecules of 3 to
give one molecule each of 2 and 4, but only 2 was isolated.
Interestingly, solutions of 3a and 3b in oxygen-free, dry C6D6
were stable for weeks at room temperature, suggesting that the
putative conproportionation may be driven by crystallization
of the least-soluble compound 2 under certain conditions.
To explore the effect of ionizing the chloride ligand, 2a and
2b were treated with KB(C6F5)4 to give 5a and 5b, respectively,
in 67-90% isolated yield.
or chloride (2, 3, 4) on hydrogen bonding, using two nonhet-
erocyclic complexes (2d, 3d) incapable of hydrogen bonding
as controls. In addition, later in the study, ligand 1e was made
to examine the role of intermolecular interactions of the
imidazole, which would be reduced or eliminated by the
presence of the isopropyl groups on the heterocycle. Finally,
as discussed in more detail below, 1e-(15N)2 and its complexes
were made to fully characterize intramolecular interactions of
the imidazole nitrogens.
Synthesis. Ligand 1a was made as we previously reported
(Scheme 2),14 by lithiation of N-protected imidazole 6,36
addition of the requisite chlorophosphine, and subsequent
deprotection by a trace of acid generated in situ by adding
chlorotrimethylsilane to methanol. Ligand 1b is new but made
in a similar manner.37 Ligand 1c has been reported but without
details of its synthesis or characterization data;38-40 the
procedure reported here gives 69% yield. Our observations
suggest that better yields result from using as little acid as
possible and using methanol rather than water for the depro-
tection step.41 Ligand 1e was made in a similar fashion from
known precursor 8,36 as was its doubly labeled isotopomer 1e-
(15N)2.37
All five ligands (1a-1e) and isotopomer 1e-(15N)2 were used
to break dimer [Cp*Ir(Cl)(µ-Cl)]2 (Scheme 3). As monitored
by NMR spectroscopy, reactions of the less hindered ligands
1b, 1c, and 1d were complete within minutes at ambient
temperature, whereas that of the di-tert-butyl ligand 1a was
slower. Spectral and other data for the products will be discussed
below.
The monohydride monochloride complexes 3 were made in
a similar fashion by breaking dimer42 [Cp*Ir(Cl)(µ-H)]2. Here,
(36) Curtis, N. J.; Brown, R. S. J. Org. Chem. 1980, 45, 4038–4040.
(37) For details of compound preparation and characterization, see Sup-
porting Information.
(38) Saxon, E.; Armstrong, J. I.; Bertozzi, C. R. Org. Lett. 2000, 2, 2141–
2143.
(39) Spencer, L. P.; Altwer, R.; Wei, P.; Gelmini, L.; Gauld, J.; Stephan,
D. W. Organometallics 2003, 22, 3841–3854.
(40) Chan, K. T. K.; Spencer, L. P.; Masuda, J. D.; McCahill, J. S. J.;
Wei, P.; Stephan, D. W. Organometallics 2004, 23, 381–390.
(41) For synthesis of an analogue of 1 with methyl groups on phosphorus,
see: Chen, Z.; Schmalle, H. W.; Fox, T.; Blacque, O.; Berke, H J.
Organomet. Chem. 2007, 692, 4875–4885.
X-ray Structures. X-ray quality crystals of four dichloride
complexes (2a, 2b, 2c, 2e), two monohydrides [3b and
(42) Gill, D. S.; Maitlis, P. M. J. Organomet. Chem. 1975, 87, 359–364.
9
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A R T I C L E S
Grotjahn et al.
Table 1. Selected Bond Distances (Å) and Angles (deg)a
2a
2b
2c
2e
3b
3e-(15N)2
5b
Ir-Cp* (Cp* ) centroid)
Ir-P
1.844(2)
2.3957(6)
2.4224(5)
1.818(2)
2.3140(7)
2.4336(7
1.827(8)
2.2972(7)
2.4263(7)
1.819(12)
2.3171(13)
2.4419(14)
1.869(2)
2.2645(11)
1.64(2) (Ir-H)
1.866(2)
2.2788(10)
2.4072(11)
1.800(2)
2.3841(8)
2.118(3) (Ir-N)
Ir-A (hydrogen bonded)
(Ir-N for 5b)
Ir-Cl or Ir-X
(non-hydrogen-bonded)
P-C2(Im)
2.4005(5)
2.4032(6)
2.4089(7)
2.4050(14)
2.4097(13)
1.664(19)
2.4001(9)
1.835(2)
3.054(4)
0.88
2.27(1)
0.337
1.807(3)
3.177(8)
0.88
2.44(1)
0.588
1.820(3)
3.202(3)
0.79(3)
2.47(3)
0.115
1.804(5)
3.141(4)
0.87(7)
2.40(7)
0.651
1.821(3)
2.55(2)
0.900(1)
1.913
0.220
2.538
1.820(4)
3.039(4)
0.88
2.29(2)
0.863
1.799(3)
-
0.87
-
-
-
N-A
N-H
H· · ·A
A to imidazole planeb
Cl or X to imidazole
planeb
2.593
0.797
2.741
0.723
1.722
torsional angle C2(Im)
P-Ir-Cp*
-175.65(0.36)
62.58(0.35)
-165.96(0.34)
-66.34(0.52)
163.58(0.53)
-179.59(0.37)
-134.55(0.34)
(θ1) N-H· · ·A
(θ2) Cl-Ir-A,
X-Ir-A, or Cl-Ir-N
(θ3) A-Ir-P
149.1(1)
85.898(19)
141.1(1)
89.39(2)
155(3)
88.74(2)
143(6)
88.05(5)
126.4
84.8(1)
143.3(2)
92.0(17)
-
83.58(8)
91.932(18)
88.090(19)
120.6(1)
86.39(2)
88.84(2)
123.5(1)
91.33(2)
86.84(2)
122.4(3)
87.10(5)
91.78(5)
122.1(4)
79.7(3)
90.5(4)
122.4(1)
95.31(4)
83.1(17)
121.19(4)
66.88(7)
87.64(3)
122.6(1)
or N-Ir-P
(θ4) Cl-Ir-P
or X-Ir-P
(θ5) Cp*-Ir-Cl
or Cp*-Ir-X
(θ6) Cp*-Ir-A
(θ7) Cp*-Ir-P
(θ8) P-C2(Im)-N(H)
(θ9) P-C2(Im)-N
117.5(1)
138.5(1)
124.29(16)
125.20(16)
122.2(1)
133.5(1)
121.9(2)
127.0(2)
123.6(3)
131.4(2)
121.9(2)
125.8(2)
124.0(4)
131.1(4)
121.8(4)
127.5(4)
125(3)
117.55(2)
136.03(3)
123.3(3)
125.8(3)
-
137.3(1)
124.6(6)
124.7(6)
140.6(1)
145.2(3)
105.2(2)
a Cp* ) centroid. b Distance to mean plane defined by the three carbons and two nitrogens of the imidazole ring.
3e-(15N)2], and ionized chelate complex 5b were all obtained.
Collection information is given in Table S1 in Supporting
Information, whereas key bond length and angle data are in
Table 1. Molecular structures are shown in Figures 1-7 and in
Supporting Information Figure S1. In the case of 3b, the
positions of the NH and hydride hydrogens potentially involved
in hydrogen bonding were refined, whereas those of all other
hydrogens were refined as a riding model. For 3e-(15N)2, the
hydrogen atom bound to the Ir was located on the difference
map, its bond length refined to an acceptable range, and its
position refined. All other hydrogen atoms were placed using a
riding model. All structures show the complexes as essentially
octahedral complexes, with distortions detailed below.
Interestingly, all four dichloride structures (Figures 1-3 and
Figure S1) show interaction of the NH with one of the two
chloride ligands and lengthening of the Ir-Cl bond involved
in the hydrogen bonding by 0.0174(14) to 0.0369(28) Å. This
difference varies, as does the orientation of the NH with respect
to the Ir-Cl unit. For the four dichloride complexes, the
Figure 1. ORTEP structure of 2b.
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Secondary Interactions in Cp*Ir Complexes
A R T I C L E S
Figure 2. ORTEP structure of 2c.
Figure 4. ORTEP structure of the hydride complex 3b.
Figure 3. ORTEP structure of 2e.
N-H· · ·Cl angles are in the range of 141 to 155°, which is
reasonably favorable for formation of a hydrogen bond.43
Inspection of the angles formed between Cp* centroid and
the three ligands (two chlorides and one phosphine) shows that
the angles involving the phosphine (θ7, Table 1) are always
significantly larger, by about 18-21° (2a), 10-11° (2b), 8-9°
(2c), and 7-9° (2e), perhaps a reflection of the size of the two
R groups on P. For each complex, the Cp*-Ir-Cl angle
involving the chloride which is the hydrogen bond acceptor (θ6)
was smaller than the angle to the nonbonded chloride (θ5), by
about 1 to 3°.
In monohydride complex 3b (Figure 4), the NH points toward
the hydride ligand. As mentioned above, the position of the
hydride was determined from the data, which show that the
hydride nucleus is only 0.22 Å from the plane defined by
the five atoms of the imidazole ring. This stands in contrast to
the four structures of the dichloride complexes 2a-2c and 2e,
where with the exception of 2c the chloride accepting the NH
hydrogen bond is further (0.34-0.67 Å) away from the
imidazole ring plane. Additional distortions consistent with
favorable hydrogen bonding are shown by comparing the
P-Ir-ClA angle (θ3) in 2b and the analogous P-Ir-HA angle
in 3b, where the latter is 6.7° less. The fact that in 3b the NH
points toward the hydride rather than the chloride and that the
hydride-imidazole plane distance is smaller than analogous
Figure 5. Intermolecular hydrogen bonding by complex 3b.
distances involving chloride in 2a-2c both suggested initially
that the NH · · ·H-Ir dihydrogen bonding interaction is stronger
than the one involving a chloride ligand, or that dihydrogen
bonding is preferred for steric reasons because the hydride is
smaller.44 Below, we give spectroscopic and computational
evidence contradicting this observation, which suggested in turn
that the solution-state structure is different than that in the solid
state. Indeed, inspection of the unit cell for 3b (a part of which
is shown in Figure 5) shows that the NH of one molecule points
not only toward the hydride of the same molecule but also
toward the basic imidazole nitrogen of an adjacent molecule.
This intermolecular interaction appears to be unique among the
complexes of this study.
To help determine whether intermolecular interactions in the
solid state help favor dihydrogen bonding, analogue 3e [made
for other purposes as its (15N2) isotopomer] with added steric
hindrance from two isopropyl groups was analyzed by X-ray
diffraction. Significantly, Figure 6 shows distinct preference for
hydrogen bonding to chloride.
(44) We thank a reviewer for suggesting that we consider the possibility
that hydrogen bonding to hydride in 3b might simply be a function of
smaller size of the hydride ligand.
(43) Jeffery, G. A. An Introduction to Hydrogen Bonding; Oxford: New
York, 1997.
9
J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010 7923

A R T I C L E S
Grotjahn et al.
Figure 7. ORTEP structure of the cation of 5b.
Figure 6. ORTEP structure of the hydride complex 3e-(15N)2.
B appears to apply to all complexes in this work and to 2a
under conditions (C6D6, toluene-d8, THF-d8) where ionization
is disfavored. In even slightly more polar or hydrogen bonding
solvents, 2a undergoes ionization (hypothesis C) to varying
degrees.
In the absence of samples suitable for X-ray diffraction, NMR
relaxation methods are often used to examine dihydrogen
bonding to hydride ligands; fortunately, because the hydride
position was located in the structure of 3b, we have a more
direct determination of the H · · ·H distance (1.91 Å). This value
is in line with distances seen in a variety of inter- and
intramolecular dihydrogen bonds.15,21,22,45-47
The solid-state structures of the dichloride complexes and
3b and 3e-(15N)2 show two basic conformational preferences.
If one considers the torsional angle defined by the Cp* centroid,
Ir, P, and imidazolyl C2 atoms, then the conformations seen in
the solid state can be called anti if the angle is near (180° or
gauche if near (60°. Following this scheme, 2a, 2c, 3b and
3e-(15N)2 would be anti and 2b and 2e gauche. Below, we show
calculated structures of the two conformers that match the solid-
state structures very well and low-temperature NMR data that
also are completely consistent with the presence of gauche and
anti conformers in solution.
Finally, the structure of 5b (Figure 7) verifies that the
imidazolylphosphine ligand is chelating and also verifies the
distortions one would expect from a four-membered ring, such
as a small (ca. 70°) P-Ir-N angle and therefore other larger
intraligand angles.14,48,49
NMR Spectroscopy and Three Hypotheses To Explain the
Data. A combination of IR and NMR spectroscopic data, the
latter at ambient and low temperatures, shows the effects of
hydrogen bonding. In the course of this study, three distinctly
different hypotheses (A, B, and C below) were advanced to
account for the data obtained. Ultimately, complex 2a was
shown to differ significantly even from its homologue 2b,
revealing intriguing and unexpected differences caused by the
size of R groups on phosphorus. Experiments and calculations
allowed us to reject hypothesis A and conclude that hypothesis
Table 2 shows selected 1H and 31P NMR data for the
complexes in this study. Similar data for the ligands and full
tables of 13C NMR data appear in Supporting Information, along
1
with details of their assignments using a combination of H,
13C{1H}, COSY, HSQC, and HMBC data. In addition, in
selected cases, ROESY (4b) and 1H{31P} NMR data were
obtained in order to differentiate the two imidazolyl CH protons
and the isopropyl methyl protons and to resolve the small
couplings shown by the imidazole CH proton peaks. The
assignments for the imidazolyl protons and carbons match those
found for simple imidazole derivatives.50,51
From inspection of the NMR data, several trends emerge.
All of the hydride complexes of imidazolylphosphines (3a, 3b,
3c, 4b) appeared as single species in all solVents examined
(C6D6, toluene-d8, CDCl3, and CD2Cl2), and at all temperatures
examined. For dichloride complex 2c, the same observation was
made, whereas for 2b or 2e, signals for two species could be
seen at low temperatures in either toluene-d8 or CD2Cl2. In
contrast, solutions of 2a in toluene-d8 or C6D6 revealed the
presence of a single species at all temperatures examined, but
solutions in CDCl3 and CD2Cl2 showed two sets of NMR
resonances, even at ambient temperatures.
Our initial hypothesis (A in Scheme 4) was that, for both 2a
and 2b, the two species differ in orientation of the imidazole
ring, where in conformer 2-endo the imidazole NH is pointed
toward one chloride ligand, as shown in the solid-state structures,
and in 2-exo the imidazole ring would be turned 180°, with the
NH pointing away from the chlorides. The ratios of these two
proposed species could have been taken as indications of relative
hydrogen bonding strength. However, we note that the conformers
shown in hypothesis B are precisely what are seen in the solid-
state structures of 2a and 2c (endo-anti) and 2b and 2e (endo-
gauche). Furthermore, calculations (see below) also ultimately lead
to rejection of hypothesis A because the exo conformers were
predicted to be 5.8 to 12.5 kcal mol-1 less stable, depending on
(45) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155–284.
(46) Shubina, E. S.; Belkova, N. V.; Krylov, A. N.; Vorontsov, E. V.;
Epstein, L. M.; Gusev, D. G.; Niedermann, M.; Berke, H. J. Am. Chem.
Soc. 1996, 118, 1105–1112.
(47) Bakhmutov, V. I.; Bakhmutova, E. V.; Belkova, N. V.; Bianchini,
C.; Epstein, L. M.; Masi, D.; Peruzzini, M.; Shubina, E. S.; Vorontsov,
E. V.; Zanobini, F. Can. J. Chem. 2001, 79, 479–489.
(48) Grotjahn, D. B.; Gong, Y.; Zakharov, L. N.; Golen, J. A.; Rheingold,
A. L. J. Am. Chem. Soc. 2006, 128, 438–453.
(50) Padilla-Martinez, I. I.; Ariza-Castolo, A.; Contreras, R. Magn. Reson.
Chem. 1993, 2, 189–193.
(49) Miranda-Soto, V.; Perez-Torrente, J. J.; Oro, L. A.; Lahoz, F. J.;
Martin, M. L.; Parra-Hake, M.; Grotjahn, D. B. Organometallics 2006,
25, 4374–4390.
(51) Takeuchi, Y.; Yeh, H. J. C.; Kirk, K. L.; Cohen, L. A. J. Org. Chem.
1978, 43, 3565–3570.
9
7924 J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010

Secondary Interactions in Cp*Ir Complexes
A R T I C L E S
9
J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010 7925

A R T I C L E S
Grotjahn et al.
9
7926 J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010

Secondary Interactions in Cp*Ir Complexes
A R T I C L E S
9
J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010 7927

A R T I C L E S
Grotjahn et al.
Scheme 4. Three Hypotheses To Explain Observation of Two
Species in Solution
a shift of the NH from one chloride ligand to the other one
nearby and is predicted to have such a low barrier (1.4 kcal
mol-1) that the isopropyl groups of 2b-endo-anti would be
effectively equivalent. Very similar spectral changes were seen
for more substituted complex 2e, though here the endo-gauche
conformer was slightly favored over the endo-anti one. The
reversal of preference is not dramatic, however, involving a
change of no more than about 1 kcal mol-1. In principle, similar
changes could be expected for spectra of 2c because the phenyl
rings in 2c-endo-gauche would be diastereotopic, but either
there is a stronger preference for one of the two conformers in
this case or the activation energy for gauche-anti conversion is
low enough that even at -90 °C sharp peaks could still be seen.
Calculations (see below) predict that the barrier between gauche
and anti conformers is 2.3-3.5 kcal mol-1 lower for 2c than
for 2b and 2e, which could preclude detection of discrete
conformers for 2c.
Using line-shape analyses on 31P{1H} NMR spectra, the
activation parameters for interconversion of two conformers of
2b in CD2Cl2 were measured as ∆Hq ) 15.3 kcal mol-1 and
∆Sq ) +16 cal mol-1 K-1. It should be noted that the value of
∆Hq cannot be assigned only to breaking of whatever hydrogen
bond might be present but also to changes in steric interactions
between the imidazole ring and the phosphine R substituents
during rotation around the bond between phosphorus and iridium
C2. The strength of hydrogen bonding might be expected to be
different in benzene or toluene than in either CDCl3 or CD2Cl2
[ET(30) values: CHCl3, 39.1; CH2Cl2, 40.7; benzene, 34.3;
toluene, 33.9].52 To test this hypothesis, VT NMR data for 2b
in toluene-d8 were obtained, again showing two species
equilibrating, but with ∆Hq ) 9.4 kcal mol-1 and ∆Sq ) +3
cal mol-1 K-1, values lower than those seen in CD2Cl2, rather
than higher as might be expected for breaking a stronger
hydrogen bond.53 Calculations (below) predict ∆Gq ) 14.1 kcal
mol-1 for interconversion from gauche to anti in the gas phase,
in reasonable agreement with our experimental results.
NMR Evidence for Ionization of 2a (Hypothesis C). Ionization
of 2a in CDCl3 or CD2Cl2 to create a mixture of 5a-Cl and 2a
seemed unlikely at first, given the fact that these solvents are
not particularly polar. Nonetheless, numerous studies have
shown the ability of chloroform or dichloromethane to donate
hydrogen bonds,54-60 which (along with the NH of the cation
of 5a-Cl) could solvate chloride ion.
whether a hydride or chloride was available for hydrogen bonding.
All available NMR evidence is in accord with hypothesis C for
2a in chlorinated or more polar solvents and hypothesis B for 2b
and all other cases in this study, as described next.
NMR Evidence Allowing Identification of Endo-anti and
Endo-gauche Conformers. The 31P{1H} NMR spectrum of 2b
in CD2Cl2 at -60 °C shows two sharp singlets at 7.3 (major)
and -4.3 ppm (minor), with a ratio of 1.4:1, corresponding to
a free energy difference of only 0.14 kcal mol-1. Warming the
latter solution to near 0 °C led to coalescence of the two peaks,
which at 30 °C appeared as one slightly broad singlet at 1.7
ppm. Similar features were seen in proton NMR spectra of 2b,
with NH chemical shifts of 11.00 and 12.16, as well as 31P and
1H spectra of analogous complex 2e.
(52) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 2nd
ed.; VCH: New York, 1988.
(53) As for 2a, it exhibited a single set of NMR peaks in C6D6, just like
2b. However, unlike 2b, for a sample of 2a in toluene-d8 (which is
similar in solvent polarity to C6D6) between 30 and-60 °C, at all
temperatures only a single set of resonances was seen, although the
broadness of some resonances changed as a function of temperature.
Some minor resonances were seen, but if these were indeed attributable
to a second conformer, its amount would be at most 5%. Because for
2a two species were easily detectable in either CDCl3 or CD2Cl2 at
30 °C, it seemed much more probable that, in the aromatic solvents,
a single species was seen rather than two very rapidly equilibrating
ones. Observing a single species for 2a in aromatic solvents is
completely consistent with hypothesis C (see below) because it would
be expected that ionization of chloride and formation of 5a-Cl would
be more favorable in chlorinated solvents because of their higher
polarity and the possibility of hydrogen bonding of solvent to chloride
ion.
Though still slightly broadened at -90 °C, the 1H resonances
for the isopropyl groups on phosphorus in 2b give a striking
indication of the symmetry of each species: for the minor
conformer, two broadened one-proton peaks ascribed to in-
equivalent methine protons were seen, whereas for the major
conformer, only a single two-proton peak was observed.
Moreover, for the minor species, four three-proton doublets of
doublets between 0.76 and 1.73 ppm were visible, whereas for
the major component, only two closely spaced six-proton
doublets of doublets at 0.98 and 1.07 ppm were found. This
would be consistent with the minor conformer being 2b-endo-
gauche, in which each methine is diastereotopic and also all
four isopropyl methyls would be inequivalent. In contrast,
although 2b-endo-anti is chiral, it puts the isopropyl groups in
a much more symmetrical environment; moreover, intercon-
version of the two enantiomers of 2b-endo-anti involves only
(54) Gordy, W. J. Chem. Phys. 1939, 7, 167–171.
(55) Kamlet, M. J.; Taft, R. W. J. Chem. Soc., Perkin Trans. 2 1979, 349–
356.
(56) Kamlet, M. J.; Dickinson, C.; Taft, R. W. J. Chem. Soc., Perkin Trans.
2 1981, 353–355.
(57) Oszczapowicz, J.; Thanh, B. P.; Oszczapowicz, I.; Wielogorska, E.
Mol. Phys. Rep. 2001, 33, 87–89.
9
7928 J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010

Secondary Interactions in Cp*Ir Complexes
A R T I C L E S
a
1
Table 3. 15N Chemical Shifts and Coupling Constants JNH
compound(s) solvent (temp) species
CD2Cl2
N1 (NH or NCH3)
-201.9
-223.5
-220.5
-216.0
-219.2
N3 (sp2 N)
δN3-δN1
1JNH (Hz)
1a
not vis
-115.7
-122.8
-133.1
-135.4
nd
not vis
1-methylimidazole
C6D6
CDCl3
CD3OD
C6D6 + CF3CH2OH
107.8
99.7
82.9
83.8
1-methylimidazole +
2 CF3CH2OH
1e-(15N)2
e
C6D6
C6D6
CDCl3
-210.4 (d, J ) 21.3)
-193.3
-102.8
-99.4
107.6
93.9
92.3
-17.2
93.9
-17.8
99.0
95.4
nd
101.0
93.3
103.5
103.4
96.0
103.8
96.5
98.2
93.53
2a
not vis
not vis
b
2a
5a-Cl
-102.1 (2a)
-203.9 (5a-Cl)
-100.7 (2a)
-203.3 (5a-Cl)
-103.1
-194.4 (2a)
b c
,
-186.7
(5a-Cl)
b
CD2Cl2
-194.6 (2a)
-185.5 (5a-Cl)
-202.1
2b
C6D6
CDCl3
CDCl3
toluene-d8
toluene-d8
(-80 °C)
CD2Cl2
not vis
not vis
-202.8
nd
-107.4
nd
d
2e
95.07
2
2
e
2e-(15N)2
-205.9 (d, JNP ) 8.4)
-196.7 (d, JNP ) 9.7)
-212.2 (d, JNP ) 9.3)
-208.3 (d, JNP ) 9.7)
-199.7 (d, JNP ) 9.3)
-213.6 (d, JNP ) 9.3)
-104.9 (d, JNP ) 10.2)
95.57
2
2
d
endo-anti (minor)
endo-gauche (major)
-103.4 (d, JNP ) 10.2)
95.91
2
2
d
-108.7 (d, JNP ) 10.2)
94.97
2
2
e
-104.9 (sl br d, JNP ≈ 11)
95.40
2
2
d
CD2Cl2 (-80 °C)
endo-anti (minor)
endo-gauche (major)
-103.7 (d, JNP ) 10.2)
95.21
2
2
d
-109.8 (d, JNP ) 10.2)
95.45
d
3a
C6D6
C6D6
-197.6
-201.0
-101.1
-102.8
96.84
d
3b
96.92
3e-(15N)2
toluene-d8
toluene-d8 (-60 °C)
C6D6
d
-203.0 (d, J ) 10.7)
-201.6
-105.1 (d, J ) 9.7)
-106.4
-107.1 (d, J ) 9.3)
-200.7
97.9
95.2
99.1
-3.4
-4.0
95.16
d
4b
97.12
e
4e-(15N)2
5a
C6D6
-206.2 (dd, J ) 1.0, 11.1)
-197.4
95.32
acetone-d6
not vis
not vis
b
acetone-d6 (-40 °C)72
-197.0
5b
-201.0
a Samples at 30 °C unless otherwise specified. Unlabeled samples were examined using 1H-15N gHMBC and in some cases gHSQC experiments.
Parameters were chosen so as to give digital resolutions in the 15N dimension of 0.5 to 1.5 ppm. Labeled samples were examined using 15N{1H} NMR
experiments, typically with digital resolution of 0.8 Hz (0.015 ppm at 50.7 MHz). The 15N chemical shift of a standard reference sample of formamide
solution in DMSO-d6 (90%) was determined and set to be -267.8 ppm. Then, using the same sweep width and offsets, samples of nitromethane (1.0 M
in CDCl3) and quinine (0.5 M in CDCl3) gave 15N chemical shifts of -4.2 for CH3NO2 and -72.4 and -349.2 ppm for the sp2- and sp3-hybridized
nitrogens of quinine, respectively. b Identity verified by gHSQC experiment. c Identity verified by separate gHSQC experiment at -30 °C, where the low
temperature was needed to allow the cross-peak to be seen. d Digital resolution 0.05 Hz. e Digital resolution 0.025 Hz.
15N NMR Data as Probe of Hydrogen or Metal Bonding to
To gain insight into the bonding environment of the second
species formed from 2a, an authentic sample of ionic complex
5a was made from 2a using KB(C6F5)4, and data for it and 5b
the Imidazolyl Group on the Phosphine. 15N chemical shifts
can reveal a great deal about bonding at the nitrogen.62-67
Previous studies67 of histidine and the model compound
1-methylimidazole showed the power of using 15N NMR data
as a probe for hydrogen bonding or protonation of the imidazole
nucleus. Thus, a series of 15N chemical shift data were obtained
(Table 3) using natural abundance materials and 1H-15N
gHMBC and in some cases gHSQC experiments.68-70 These
techniques worked well for imidazole derivatives without ring
isopropyl groups because of strong cross-peaks between the ring
protons and nitrogens; for 1e and its complexes, the labeled
derivative 1e-(15N)2 and 15N direct detection were used instead.
were obtained, but 1H and 31P NMR data did not allow
conclusive identification of 5a-Cl. However, one unusual 13C
NMR resonance which strongly suggests formation of 5a-Cl
from 2a in chlorinated solvents is the signal for C2 of the
imidazole ring, whose chemical shift and small one-bond
coupling constant [δ 153.4 ppm (d, 1JCP ) 52.0 Hz)] match the
1
data for chelate complexes 5a and 5b [5a: 155.1 (d, JCP
)
1
44.3)] much better than data for 2a [141.5 (d, JCP ) 69.2)]
and related 2b, 2c, and 2e. We have previously observed
downfield shift of C2 and diminution of 1JCP upon chelation by
an imidazolylphosphine,14,48,61 along with other unusual NMR
couplings which can be explained by altered bonding in the
four-membered ring chelate. In striking contrast, the two species
observed in low-temperature NMR spectra of 2b show com-
(62) Schilf, W.; Stefaniak, L. In New AdVances in Analytical Chemistry;
Atta-ur-Rahman, Ed.; Harwood Academic: Amsterdam, 2000; p P1/
659-P1/696.
(63) Pazderski, L.; Szlyk, E.; Sitkowski, J.; Kamienski, B.; Kozerski, L.;
Tousek, J.; Marek, R. Magn. Reson. Chem. 2006, 44, 163–170.
(64) Marek, R.; Lycka, A.; Kolehmainen, E.; Sievanen, E.; Tousek, J. Curr.
Org. Chem. 2007, 11, 1154–1205.
1
pletely normal JCP for imidazole C2 (75 and 72 Hz), which
would be consistent with relatively minor differences between
endo-anti and endo-gauche conformers of 2b.
(65) Marek, R.; Lycka, A. Curr. Org. Chem. 2002, 6, 35–66.
(66) Andreeva, D. V.; Ip, B.; Gurinov, A. A.; Tolstoy, P. M.; Denisov,
G. S.; Shenderovich, I. G.; Limbach, H.-H. J. Phys. Chem. A 2006,
110, 10872–10879.
(58) Grushin, V. V.; Marshall, W. J. Angew. Chem., Int. Ed. 2002, 41,
4476–4479.
(67) Farr-Jones, S.; Wong, W. Y. L.; Gutheil, W. G.; Bachovchin, W. W.
J. Am. Chem. Soc. 1993, 115, 6813–6819.
(59) Chisholm, M. H.; Gallucci, J.; Hadad, C. M.; Huffman, J. C.; Wilson,
P. J. J. Am. Chem. Soc. 2003, 125, 16040–16049.
(68) Foltz, C.; Enders, M.; Bellemin-Laponnaz, S.; Wadepohl, H.; Gade,
L. H. Chem.sEur. J. 2007, 13, 5994–6008.
(60) Cerda, J. F.; Koder, R. L.; Lichtenstein, B. R.; Moser, C. M.; Miller,
A.-F.; Dutton, P. L. Org. Biomol. Chem. 2008, 6, 2204–2212.
(61) Grotjahn, D. B.; Zeng, X.; Cooksy, A. L.; Kassel, W. S.; DiPasquale,
A. G.; Zakharov, L. N.; Rheingold, A. L. Organometallics 2007, 26,
3385–3402.
(69) Baird, B.; Pawlikowski, A. V.; Su, J.; Wiench, J. W.; Pruski, M.;
Sadow, A. D. Inorg. Chem. 2008, 47, 10208–10210.
(70) Delp, S. A.; Munro-Leighton, C.; Khosla, C.; Templeton, J. L.; Alsop,
N. M.; Gunnoe, T. B.; Cundari, T. R. J. Organomet. Chem. 2009,
694, 1549–1556.
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J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010 7929

A R T I C L E S
Grotjahn et al.
In order to calibrate our data, we observed 1-methylimidazole
(Table 3) in C6D6, CDCl3, and CD3OD as did Farr-Jones et al.67
and observed similar trends: the 15N chemical shift of the basic
nitrogen (N3) moved progressively upfield (by nearly 20 ppm)
as the solvent’s hydrogen bond donor ability increased, and the
difference in nitrogen shifts δN3-δN1 decreased. Hydrogen bond
donor CF3CH2OH added to a solution of 1-methylimidazole in
C6D6 gave 15N data resembling those in pure CD3OD. Thus, if
the basic imidazole nitrogen is involved in accepting a hydrogen
bond from solvent (e.g., CDCl3 or CD2Cl2), we should see
upfield shifts for the N3 resonance or decrease in δN3-δN1. For
2b, on going from C6D6 to CDCl3, these values change only
about half as much as for 1-methylimidazole N3 in the same
two solvents.
ined. The least polar solvents [C6D6, toluene-d8, and THF-d8;
ET(30) values52 34.3, 33.9, 37.4] afforded 2a exclusively. In
contrast, CDCl3 or CD2Cl2 gave mixtures with roughly similar
amounts [ET(30) values: CHCl3, 39.1; CH2Cl2, 40.7). The fact
that CHCl3 and THF are so similar by this measure but CDCl3
gives so much more ionization suggests that chloride in 5a-Cl
may be solvated by hydrogen bonding to the C-D of CDCl3.
Acetone was a poor solvent, but CD3NO2 gave 2a and 5a-Cl in
a 1:2 ratio, and what part of the sample did dissolve in i-PrOH-
d8 was more than 90% ionized (5a-Cl). Nitromethane and
i-PrOH have ET(30) values of 46.3 and 48.4,52 respectively; the
latter solvent could be expected to promote somewhat greater
ionization by virtue of its polarity but in addition would be
expected to solvate the chloride ion by hydrogen bonding to it.
Why would 2a form ionized chelate 5a-Cl, whereas 2b would
not form a similar species? Why would the un-ionized complex
2a (whether observed alone or as a mixture with 5a-Cl under
conditions favoring ionization) appear to exist as a single
conformer at all temperatures, whereas 2b exists as a mixture
of two conformers, 2b-endo-gauche and 2b-endo-anti? Both
differences can be explained by the propensity of large sub-
stituents on acyclic systems to promote ring formation or cyclic
intramolecular interactions. In organic chemistry, the Thorpe-
Ingold effect and gem-dimethyl effect are classic expressions
of this concept.73,74 In organometallic chemistry, it has been
observed75 that large substituents R on R2PR1 favor cyclom-
etalation reactions at the third phosphine substituent R1, or that
large ligand substituents can favor chelation.76-78 In this work,
we find that hemilabile complexes show similar trends.
IR Spectroscopy. IR data (Table 4) were obtained in three
ways, on freshly prepared C6H6 or CH2Cl2 solutions in CaF2
cells or using reflectance on solid samples. The data for the
three ligands in CH2Cl2 show the NH stretching frequency value
in the absence of hydrogen bonding, which is approximately
3438 cm-1. In chelate complexes 5a and 5b, which also would
not have an intramolecular hydrogen bond, the NH absorption
was slightly shifted but within 1% of the ligand value. Isotopic
substitution by 15N changes the position of absorption by about
0.2%.
Much more dramatic and diagnostic changes are seen in
CDCl3 when 2a (δN1 ) -194.4 and δN3 ) -102.1 ppm), with
shifts similar to those for 2b, is partially converted to 5a-Cl.
The ionized species shows two 15N peaks very close in chemical
shift (δN1 ) -186.7, but δN3 ) -203.9 ppm), with the N1-H
peak clearly assigned using gHSQC. The 15N chemical shift
data for preionized complexes 5a and 5b in acetone-d6 match
those for putative 5a-Cl in CDCl3 very well.
1
Careful observation of H NMR spectra for the NH peak of
1
complex 3b dissolved in C6D6 allowed determination of JNH
1
(Table 3). A series of H decoupling experiments determined
that narrow sharp triplets (J ) 1.8 Hz) on either side of the
broad major peak were the result of splitting by the imidazole
ring protons, not the phosphorus. Integration of the minor peaks
relative to the central broad peak was difficult due to overlap,
but the integrals were consistent with the minor peaks being
satellites from natural abundance 15N at N1.71 The 15N satellite
peaks were only seen in spectra of hydride complexes 3a, 3b,
or 4b, whether in C6D6 or CD2Cl2, but not for dichloride
complexes 2a, 2b, or 2c, even when samples of these species
were cooled to -40 °C or lower to slow any possible NH
1
exchange which might be masking JNH
.
Labeled compounds derived from 1e-(15N)2 allowed deter-
mination of 1JNH under a wider variety of conditions, presumably
because the ring isopropyl groups slowed NH exchange and
also allowed observation at low temperatures of two distinct
sets of 15N peaks and δN3-δN1, which also appears to be
sensitive to the nature of hydrogen bond donation by the
imidazole NH. Absolute values of 1JNH increase from 93.53 Hz
in free ligand 1e-(15N)2 to 94.97-95.91 in the conformers of
2e, to 95.16-95.32 in 3e and 4e. Shifting the NH interaction
from chloride to hydride in 4e weakens the interaction (because
the chloride has a greater negative partial charge) but shortens
the interaction distance (because hydride has a smaller ionic
radius). Rather surprising is the sensitivity of δN3-δN1, where
this value changes by nearly 10% between endo-anti and endo-
gauche conformers of 2e. This sensitivity to environmental
change may be useful in other studies of bifunctional imidazole-
based systems. Gratifyingly, the agreement between observed
15N chemical shift changes in Table 3 and calculated values
(Supporting Information, Table S33) is very good.
In contrast, samples of the complexes 2b, 2c, 2e and 3b, 3c,
3e showed one or two bands for NH absorption, both in the
range of 3189-3300 cm-1, and much broader than that seen
for ligands 1a-1c. In most cases when two bands were seen,
one was a shoulder on the other, perhaps evidence of the two
endo-anti and endo-gauche conformers. Calculated vibrational
frequencies (Table S32) suggest that bands between 29 and 103
cm-1 apart might be seen for the two conformers, with the higher
wavenumber value for the anti conformer. However, it appears
that because anti and gauche conformers are so close in energy
(e.g., from ratios seen for 2b and 2e-(15N)2 in CD2Cl2 at low
temperature) variations in solvent and temperature could reverse
which conformer is preferred.
The NH absorption for 4b and 4e-(15N)2 appeared outside
the range seen for the complexes just discussed and closer to
Influence of Solvent on Equilibrium between 2a and 5a-Cl
and the Role of R on P in Promoting This Reaction. Having
conclusively identified 5a-Cl, the ratio of 2a to 5a-Cl as a
function of solvent polarity and hydrogen bonding was exam-
(73) Jung, M. E.; Piizzi, G. Chem. ReV. 2005, 105, 1735–1766.
(74) Bachrach, S. M. J. Org. Chem. 2008, 73, 2466–2468.
(75) Shaw, B. L. J. Organomet. Chem. 1980, 200, 307–318.
(76) Sekabunga, E. J.; Smith, M. L.; Webb, T. R.; Hill, W. E. Inorg. Chem.
2002, 41, 1205–1214.
(77) Gallo, V.; Mastrorilli, P.; Nobile, C. F.; Braunstein, P.; Englert, U.
Dalton Trans. 2006, 2342–2349.
(71) Katritzky, A. R.; Akhmedov, N. G.; Doskocz, J.; Mohapatra, P. P.;
Hall, C. D.; Guven, A. Magn. Reson. Chem. 2007, 45, 532–543.
(72) Low temperature (-40 °C) was needed to resolve various 13C
resonances, as well.
(78) Arthur, K. L.; Wang, Q. L.; Bregel, D. M.; Smythe, N. A.; O’Neill,
B. A.; Goldberg, K. I.; Moloy, K. G. Organometallics 2005, 24, 4624–
4628.
9
7930 J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010

Secondary Interactions in Cp*Ir Complexes
A R T I C L E S
Table 4. Infrared Data for Ligands and Complexes in This Studya (Absorptions Listed Are for N-H Stretches unless Otherwise Specified)
sample medium
compound
C6H6
CH2Cl2
ATR
1a
1b
1c
1e
nd
nd
nd
nd
nd
3438.3 (m)b
nd
nd
nd
nd
nd
3439.0 (m)c
3438.1 (m)c
3438.2 (m)
3430.8 (m)
1e-(15N)2
2a
3305.3 (m)
3210.7 (m)
3193.5 (m, br)
∼3240 (sh)
3225.4 (s, br)
∼3300 (sh)
∼2400-2700 (m, very br)
3240.7 (m, br)
291.3, 259.4 (s, Ir-Cl)
3258.9 (s)
2b
3241.2 (m, br)
∼3290 (sh)
3102.1
287.1, 253.1 (s, Ir-Cl)
3279.3 (br)
290.8, 264.5 (s, Ir-Cl)
nd
2c
3299.5 (m, br)
∼3240 (sh)
3281.6 (m, br)
3267.4 (m, br)
3260.1 (m, br)
2e
3248.8 (m, br)
∼3290 (sh)
2e-(15N)2
3a
3238.1 (m, br)
∼3290 (sh)
nd
∼3230 (m, very br)c d
2131.0 (s, Ir-H)
3388.1 (m, br)c
∼3223 (w, very br)
2130.2 (s, Ir-H)
3203.2 (m, br)
3313.4 (br)
,
2159.2 and 2140.0 (Ir-H)
282.0 (s, Ir-Cl)
3257.0 (m, br)
2137.1 (s, Ir-H)
285.2 (s, Ir-Cl)
3221.9 (br)
3b
3c
3201.9 (m, br)
2102.9 (s, Ir-H)
2097.2 (s, Ir-H)
3240.9 (m, br)
3251.1 (m, br)
2099.7 (s, Ir-H)
2091.6 (s, Ir-H)
2090.8 (br, Ir-H)
283.4 (s, Ir-Cl)
2142.5 and 2104.6
281.5 (s, Ir-Cl)
nd
3d
nd
2137.5 (s, Ir-H)c
3e-(15N)2
3188.8 (m, br)
∼3240 (shoulder)
2104.5 (s, Ir-H)
3384.4 (m)
3191.4 (m, br)
∼3240 (shoulder)
2103.6 (s, Ir-H)
3381.3 (m)
4b
3227.0 (br)
2107.4 (s, Ir-H)
2104.8 (s, Ir-H)
2183.1 and 2111.3 (Ir-H)
nd
nd
4d
2120.4 (s, Ir-H)c d
2116.6 (s, Ir-H)c d
,
,
4e-(15N)2
3382.4 (m)
3379.6 (m)
2102.4 (s, Ir-H)
nd
nd
2107.3 (s, Ir-H)
3405.9 (m, br)
3407.0 (m, br)
5a
5b
nd
nd
b
a Solution samples measured in CaF2 cells unless otherwise specified. Measured as 3484.4 cm-1 in NaCl cells. c Value measured on solution in
NaCl cells. d In C6D6.
where the free ligands absorbed. This would indicate weaker
hydrogen bonding, consistent with solid-state data on 3e-(15N)2
and calculations below.
many others have reported a single absorption for other
Cp*Ir(H)2(PR3) complexes,80,81 and others have discussed why
only one absorption might be seen for other metal dihydride
species.15 Changing the R groups on P can easily change the
The wider spectral window afforded by the ATR instrument
allowed one to observe bands for Ir-Cl stretches. As expected,
the dichloro complexes showed two bands (in the ranges of
253.1-264.5 and 287.1-291.3 cm-1), whereas the hydride chloride
complexes showed only one (in the narrower range of 281.5-285.2
cm-1). Significantly, the data for 3d were in the same narrow range
as the data for the other complexes with imidazolyl substituents,
which would suggest that in complexes 3a-3c the Ir-Cl bond is
relatively unperturbed by hydrogen bonding in the solid state,
consistent with the X-ray structure of 3b.79
Observation of Ir-H stretches was possible both in solution
and in ATR samples. In the latter case, for some reason, two
bands were seen even for the monohydride complexes 3.
Therefore, greater attention is paid to solution-phase spectra,
where single strong bands for Ir-H absorption were seen. Only
a single band was seen for dihydride 4b, as well; we note that
Ir-H stretching frequency by more than 10 cm-1 80,81 whereas
,
previous work from our lab10-12,61,82 has shown that imidazol-
2-yl and phenyl groups are electronically very similar and therefore
should have little effect. A change in the value of νIr-H from 2137.5
(3d) to 2091.6 cm-1 (3c) suggests dihydrogen bonding in the latter
species,29 contradicting other evidence in this study. More compel-
ling are the very similar νIr-H values for 4b, 4d, and 4e-(15N)2,
which are all within 12 cm-1 of each other.
Catalysis. Three complexes were compared as catalysts for
base-promoted transfer hydrogenation of acetophenone using
i-PrOH; results are shown in Table 5. Conditions used were
modeled after those reported elsewhere.83,84 Phosphine-free
complex [Cp*IrCl(µ-Cl)]2 was reasonably efficient, completing
(80) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105, 3929–
3939.
(81) Paisner, S. N.; Burger, P.; Bergman, R. G. Organometallics 2000,
(79) For example, hydrogen bonding to Cl of (H)(Cl)Os(CO)(PR3)2 caused
19, 2073–2083.
the Os-Cl stretching frequency to decrease from 398 to 378 cm-1
,
(82) Grotjahn, D. B.; Zeng, X.; Cooksy, A. L.; Kassel, W. S.; DiPasquale,
whereas the Os-H and C-O stretching frequencies increased by 11-
16 cm-1. Yandulov, D. V.; Caulton, K. G.; Belkova, N. V.; Shubina,
E. S.; Epstein, L. M.; Khoroshun, D. V.; Musaev, D. G.; Morokuma,
K. J. Am. Chem. Soc. 1998, 120, 12553.
A. G.; Zakharov, L. N.; Rheingold, A. L. Organometallics 2008, 27,
3626.
(83) Miecznikowski, J. R.; Crabtree, R. H. Organometallics 2004, 23, 629–
631.
9
J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010 7931

A R T I C L E S
Grotjahn et al.
Table 5. Yields (%) of Transfer Hydrogenation of Acetophenone
Using Three Catalystsa
Catalyst
time (h)
2b
[Cp*IrCl(µ-Cl)]2
2d
1
3.9
7.8
0
2
4
32.0
41.3
11.9
12.8
6
93.3
7
8
>97.0
83.9
>97.0
24.7
49.6
24
a Reaction conditions: refluxing i-PrOH, substrate/Ir/KOH
)
100:1:50, 0.1 M substrate in i-PrOH. Yields were determined by 1H
NMR spectroscopy (CDCl3).
reduction within 24 h. The complex made using a non-
heterocyclic phosphine (2d) was approximately 2-4 times
slower, whereas the analogue 2b was about 3 times faster.
Additional experiments would be needed to elucidate the
mechanistic reasons for acceleration provided by the NH
phosphine, which is somewhat modest. However, under the basic
conditions employed, one can imagine that the NH may be
deprotonated and the resulting pendant base may assist in one
or more steps of the transfer hydrogenation, for example, by
deprotonating the alcohol as it binds to the metal and forms an
alkoxide, several examples of which are known on Cp*Ir
fragments.85-87
Calculations. Computational modeling of several complexes
was carried out using Gaussian 0388 in order to provide a
theoretical basis for some of the observations. Unless indicated
otherwise, all calculations were carried out using the B3LYP
density functional method89,90 and Dunning’s cc-pVDZ basis
set91 for the main group atoms and the CEP-121G basis set of
Stevens et al.92 for the Ir. The conformations of the i-Pr groups
were not rigorously surveyed but are consistent among the
structures tabulated here. Tests of several 3b structures suggest
that the i-Pr conformations impact the calculated energies by 2
kcal/mol or less.
A survey of the conformations in 2, 3, and 4 consistently
predicts that endo-NH-Cl hydrogen-bonded structures are
stabilized by free energies of 10 to 12 kcal/mol, while endo-
NH-H structures are stabilized by 6-7 kcal/mol, in comparison
to exo structures with the NH oriented away from the other
ligands (Table 6). Furthermore, each NH-Cl structure has two
distinct conformations, anti and gauche (with respect to rotation
about the Ir-P bond, Figure 8). In 2 and 3, the anti is always
Figure 8. Optimized gauche, eclipsed (transition state), and anti conforma-
tions of 2b. All hydrogens but the NH atom are omitted for clarity.
higher in energy, but by less than 2.5 kcal/mol. In contrast, for
the endo geometries of 4, only the anti was found to be stable.
Table 6. Relative B3LYP/cc-pVDZ,CEP-121G Free Energies
(kcal/mol) of Selected Conformers
(84) Corbera´n, R.; Sanau´, M.; Peris, E. Organometallics 2007, 26, 3492–
3498.
(85) Newman, L. J.; Bergman, R. G. J. Am. Chem. Soc. 1985, 107, 5314–
5315.
(86) Ritter, J. C. M.; Bergman, R. G. J. Am. Chem. Soc. 1998, 120, 6826–
6827.
(87) Glueck, D. S.; Winslow, L. J. N.; Bergman, R. G. Organometallics
1991, 10, 1462–1479.
(88) Frisch, M. J.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT,
2004.
(89) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(90) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(91) Dunning, J. T. H. J. Chem. Phys. 1989, 90, 1007–1023.
(92) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem.
1992, 70, 612–630.
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7932 J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010

Secondary Interactions in Cp*Ir Complexes
A R T I C L E S
Table 7. Experimental and Calculated Properties of the N-H
Bonds in 1e, 3e, and 4e
species
1JNH (exp) (Hz)
1JNH (calcd) (Hz)
RNH (Å)
1e
3e
4e
93.5
95.1
95.3
92.0
92.6
94.0
1.013
1.026
1.017
As a result, the isopropyls are aligned roughly perpendicular
to the Cp* plane at the eclipsed transition state, and distances
as small as 2.1 Å occur between atoms of the Cp* and atoms
of the i-Pr groups. In contrast, the corresponding interaction in
the transition state for 2c places a flat phenyl group roughly
parallel to the Cp* plane, and minimum distances between the
two groups are at least 2.7 Å. As a result, the free energy of
activation from anti to gauche is predicted to be 11.8 kcal/mol
for 2c but 14.1 and 15.3 kcal/mol for 2b and 2e, respectively.
The 15N-1H J couplings in 1e, 3e, and 4e were predicted
using the GIAO approximation. Although the calculated values
uniformly underestimate the coupling constant, they correctly
reproduce the observed ordering of the values (Table 7), with
both 3e and 4e having larger couplings than 1e, despite the
lengthening of their N-H bonds by hydrogen bonding to
the neighboring Cl. Interestingly, the major contributor to the
increase in 1JNH from 1e to 3e and 4e is the Fermi contact term,
yet the s character of the orbitals centered at the N and H atoms
1
decreases as this bond stretches. The increase in JNH appears
Figure 9. Eclipsed conformations of 2b and 2c, optimized geometries as
transition states between gauche and anti conformations, showing the greater
steric interaction in the 2b transition state.
to be caused by the small, added electron density at the N and
H atoms donated by the Cl at the other end of the hydrogen
bond. A comparable increase in 1JNH (0.4-0.6 Hz) is predicted
for imidazole when a HCl molecule is placed at Cl-N
separations of 3.0-3.4 Å.
Four initial geometries of 3b, including the X-ray structure,
were constructed specifically to test the viability of the
NH-hydride interaction indicated by the X-ray structure. Upon
optimizing these geometries, the only one of these structures
to maintain the NH-hydride bond has the imidazole placed in
a gauche conformation, such that the Cl atom is unavailable
for hydrogen bonding. All others converged to either the endo-
gauche or endo-anti with hydrogen bonding to the chloride.
Extending the basis set on the interacting atoms to include
diffuse functions failed to make the NH-hydride conformation
more stable than the NH-Cl, as did replacing the DFT
calculations by a much more demanding CISD method.93
In all cases, the energy ordering of the gauche and anti
conformers may be predicted on the basis of steric interactions
with the Cp* group. The more stable conformer has the largest
minimum distance between atoms on the Cp* and on the
phosphine R groups.
By this reasoning, the gauche conformers are favored by 2
and 3 because the P-Ir-Cp* angle decreases by about 7° on
replacement of the H atoms in 4 by Cl atoms. This increases
the importance of Cp* steric interactions with the R groups of
the phosphine in 2 and 3. It is then more favorable to place a
flat imidazole near the Cp* ring, rotated so its plane is roughly
parallel to the Cp*, than an i-Pr with hydrogens sticking out in
several directions. We emphasize, however, that other steric
interactions, as between the phosphine R groups themselves,
are expected to play a role in these energetics, and these
dependencies have not been rigorously tested. Furthermore, the
predicted energy differences between the gauche and anti
conformers are comparable to the expected error of the
calculations.
Conclusions
All imidazolylphosphine complexes in this study show
evidence of hydrogen bonding involving the NH, as evidenced
by a combination of data. In all solvents examined and at all
temperatures, all monohydride [3a-3c, 3e-(15N)2] and dihydride
[4b, 4e-(15N)2] complexes with an imidazolyl group appeared
Only gauche appears to be stable for the exo geometries,
perhaps because in this conformation the rotation of the
imidazole to the endo form is hindered by the Cp*. In 2, with
two chlorides, the endo-anti conformer effectively has Cs
symmetry, assuming rapid motion across the calculated barrier
of 1.4 kcal/mol for rotating the NH from one Cl to the other.
1
as a single species in H, 31P, 13C, and 15N NMR spectra.
Meanwhile, dichloride complex 2c gave evidence of only one
species, whereas 2a, 2b, and 2e showed evidence of more than
one species, all involving hydrogen bonding, but with distinct
differences. Examination of crystal structures and DFT calcula-
tions both point to the existence of two low-lying conformers,
each defined as endo because hydrogen bonding of the NH to
chloride (2, 3) or hydride (4) is stabilizing. The two conformers
are defined as anti and gauche, using the relationship of the
Cp* and imidazolyl substituents on Ir and P as reference.
To better characterize the conformational dynamics of 2, the
transition states between the anti and gauche forms were
optimized. As expected, the transition states correspond to
eclipsed conformations (Figure 9), and the activation barriers
are dominated by the steric interaction between the Cp* and
the R group on the phosphine. For 2b and 2e, the two R groups
are isopropyls, which align roughly parallel to each other to
minimize their steric interaction with each other.
(93) Raghavachari, K.; Pople, J. A. Int. J. Quantum Chem. 1981, 20, 1067–
1071.
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J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010 7933

A R T I C L E S
Grotjahn et al.
Significantly, for complexes with isopropyl groups on P, low-
temperature NMR spectra could be used to identify the anti
and gauche conformers on the basis of symmetry.
2d. Imidazolylphosphines with a free NH promise to be the
subject of additional structural and catalytic studies because of
their unusual ability to either accept or donate a hydrogen bond,
thereby affecting catalytic turnover and performance.
Several lines of evidence (supported by DFT calculations)
point to stronger hydrogen bonding to chloride than hydride in
this system, which stands in distinct contrast to another Ir(III)
system studied previously.17 Significantly, the X-ray diffraction
structure of 3e-(15N)2, showing hydrogen bonding to chloride
rather than hydride, differs from that of less hindered analogue
3b because competing intermolecular hydrogen bonding is
suppressed. Infrared data for NH stretching show greater changes
from free ligand values when at least one chloride ligand is
present in the coordination sphere.
The structural studies highlighting the difference between 2a
and 2b show how relying on a single kind of spectroscopic
1
evidence (IR, H, 13C, or 15N NMR) to characterize hydrogen
bonding can be very risky, even when seemingly similar species
are compared.
Acknowledgment. The NSF is thanked for support of the
research done for this project at San Diego State University under
CHE 0415783 and CHE 0719575, and for travel support under INT
0128861. H.A. would like to thank UPMC-Paris 6 for travel support
under ongoing collaboration (DRI) between SDSU and UPMC-
Paris 6. S.C.-L. thanks Fulbright for an award funding her stay at
San Diego State University.
For 2a, NMR data, particularly 15N chemical shift informa-
tion, point to ionization of chloride in chlorinated solvents,
nitromethane, or isopropyl alcohol, forming chelate 5a-Cl
(hypothesis C), a result also seen on addition of protic solute
HOCH2CF3 to a toluene solution of 2a. In comparison, in either
benzene or toluene, 2a exists as one conformer. In contrast, for
2b, strong evidence was found for the presence of anti and
gauche conformers (hypothesis B) in solution. Low-temperature
Supporting Information Available: Preparative details for all
compounds, graphical representations of 2D NMR data used in
compiling Tables 3 and 4, the molecular structure of 2a (Figure
S1), 1H NMR spectrum of 2e-(15N)2 (Figure S2) showing peaks
for anti and gauche conformers, crystallographic data and details
for X-ray diffraction studies, CIF files for all structures, tables
for DFT calculations of structures, energies, IR and NMR data,
and complete ref 88. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
NMR data involving H, 13C, 31P, and, where available, 15N in
2b and its analogues 2e and 2e-(15N)2 are completely consistent
with calculations. The difference between 2a and 2b, which
are homologues, is striking and points to the importance of
innocent ligand substituents in determining the behavior of
hemilabile and bifunctional systems. Finally, imidazolylphos-
phine complex 2b was somewhat more effective as a transfer
hydrogenation catalyst than non-heterocyclic control compound
JA906712G
9
7934 J. AM. CHEM. SOC. VOL. 132, NO. 23, 2010