Bifunctional imidazolylphosphine ligands as hydrogen bond donors promote N-H and O-H activation on platinum

Article abstract of DOI:10.1021/om060880b

The NH moieties of two imidazolylphosphine ligands on Pt(II) facilitate formation of trans-(hydrido)(R¹O)bis-(phosphine)Pt(II) species from R¹OH (R¹ = CF₃CO, CH₃CO, Ph, CH ₃, H). Evidence points to formation of a reliable binding pocket for coordinated R¹O by hydrogen bond donation from the two NH groups.

Full text of DOI:10.1021/om060880b

Organometallics 2006, 25, 5693-5695
5693
Communications
Bifunctional Imidazolylphosphine Ligands as Hydrogen Bond
Donors Promote N-H and O-H Activation on Platinum
Douglas B. Grotjahn,*,† Yi Gong,† Antonio G. DiPasquale,‡ Lev N. Zakharov,‡ and
Arnold L. Rheingold‡
Department of Chemistry and Biochemistry, San Diego State UniVersity, 5500 Campanile DriVe,
San Diego, California 92182-1030, and Department of Chemistry and Biochemistry, UniVersity of
California, San Diego, La Jolla, California 92093-0385
ReceiVed September 25, 2006
Scheme 1. Different Products as a Function of
N-Substituent
Summary: The NH moieties of two imidazolylphosphine ligands
on Pt(II) facilitate formation of trans-(hydrido)(R1O)bis-
(phosphine)Pt(II) species from R1OH (R1 ) CF3CO, CH3CO,
Ph, CH3, H). EVidence points to formation of a reliable binding
pocket for coordinated R1O by hydrogen bond donation from
the two NH groups.
C-H activation by transition metal complexes has received
intense scrutiny.1 Though less studied, N-H2 and O-H bond
activation3 are of great interest for functionalization of organic
substrates with nitrogen- or oxygen-containing functional
groups. Among reactants that may undergo O-H activation,
water is of special interest because of its importance in green
chemistry and useful processes such as the Wacker oxidation
or hydration of unsaturated species.
Control of substrate binding and activation and catalytic
activity and selectivity by enzymatic4 and synthetic5 systems
can be achieved by using several interactions in concert. The
reactivity of O-H and N-H compounds with organometallic
complexes can be affected profoundly by the use of ligands
containing basic or acidic sites. Heterocyclic phosphine ligands
lead to greatly enhanced rate and selectivity in alkyne addition
reactions such as hydration6 and alkoxycarbonylation.7 Continu-
ing studies of the former5d show that the heterocycle may be
important as a neutral acceptor of a proton or hydrogen bond,
or in protonated form as a donor. Here, we report dramatic
changes in substrate interaction with Pt as a function of whether
a neutral imidazolylphosphine ligand (e.g., 1) bears an N-methyl
(N as hydrogen bond acceptor) or N-H group (e.g., 2, N as
donor).
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* Corresponding author. E-mail: grotjahn@chemistry.sdsu.edu.
† San Diego State University.
‡ University of California. X-ray crystal structures.
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10.1021/om060880b CCC: $33.50 © 2006 American Chemical Society
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5694 Organometallics, Vol. 25, No. 24, 2006
Communications
128°, which are consistent with known values for such
systems.12 Although interesting, this interaction is not respon-
sible for O-H activation of water or methanol using 4,
according to data presented below.
Compared under identical conditions (0.05 M complex in
C6D6 unless otherwise stated), the reactivities of 3 and 4 with
O-H-containing compounds of acidity ranging from that of
trifluoroacetic acid to water were quite different. Complex 3
reacted significantly only with more acidic species, whereas 4
gave O-H activation with all compounds tested. Moreover,
whereas 4 gave symmetrical trans-L2Pt(H)(OR1) complexes 5
with all O-H-containing compounds and under nearly all
conditions, in several cases 3 also gave unsymmetrical species
6 formed by dissociation of the anionic ligand OR.1
Figure 1. Molecular structure of 4.
Reaction of related ligands 1 and 2 with Pt(COD)2 led to
very different products. In the case of 1, the two-coordinate
Pt(0) complex 3 was formed in 66% yield.8 Its 31P{1H} NMR
The first set of experiments involved more acidic reactants,
with pKa less than 10. For example, adding acetic acid (1.0
equiv) to complex 3 in C6D6 led to a mixture containing
unreacted 3 (38%) and one new complex, 5-Me-b (62%). The
spectrum exhibited one sharp singlet with Pt satellites (JPt-P
)
4247.1 Hz). There is no spectroscopic evidence for agostic
interactions of the N-methyl group hydrogens with the metal,
so we conclude that its structure is like that recently reported
for the Pd analogue.9
In contrast, when 2 is reacted with Pt(COD)2, ligand N-H
activation is observed, leading to Pt(II) species 4 in 71% yield.
The unsymmetrical nature of 4 was revealed by two 31P{1H}
doublets (δ 52.6 and 39.4 ppm, 2JP-P-trans ) 327.8 Hz) with Pt
satellites (JPt-P ) 2940.3 and 2341.2 Hz, respectively). Key
proton NMR resonances include a broad one-proton signal at δ
10.57 (N-H) and a sharp doublet (Pt-H, JPH ) 20.5 Hz) at
-19.17 ppm with satellites (JPt-H ) 1169.3 Hz).
Clarification of which NMR resonances were associated with
the monodentate and bidentate phosphines in 4 came from
decoupling experiments. The hydride proton was coupled only
to the 31P nucleus with downfield chemical shift, which has a
value of JPt-P (2940.3 Hz) normal for a Pt(II) complex with
two phosphines mutually trans, and thus is assigned to the
monodentate phosphine. In the chelate, the effects of the
constrained four-membered ring on the 31P nucleus within it
include a small value of JPt-P (2341.1 Hz)10 and the lack of
observable coupling to the hydride proton.10c
N-H activation of heterocycles on low-valent metals includ-
ing Pt(0) is known to form metal hydride complexes.2d,l
Interestingly, as seen in the X-ray diffraction structure of 4 in
Figure 1,11 although the hydride could not be located with
certainty, the imidazole N-H could be, and the fact that it is
directed toward the apparently vacant site on Pt is consistent
with interaction of the form M-H- - -H-N, studied in detail
by a number of other groups.12 Reasonably assuming a Pt-H
distance of 1.6 Å and two equal P-Pt-H angles leads to a
calculated H- - -H distance of 1.80 Å and H- - -H-N angle of
1
symmetry of the latter was identified in H NMR spectra by a
triplet at δ -22.60 ppm (JP-H ) 14.6 Hz, JPt-H ) 1077.4 Hz)
and in 31P{1H} by a singlet with reduced JPt-P (2949.7 vs 4247.1
Hz for 3). When another equivalent of acid was added, the
reaction mixture still contained 62% of 5-Me-b, but less of 3
(17%) and in addition 6-Me-b (21%). In another experiment,
dissolving 3 in acetic acid-d4 led to complete conversion to
6-Me-b-d4. The unsymmetrical nature of 6-Me-b was revealed
by two 31P{1H} doublets, 2JP-P-trans ) 321.8 Hz, at δ 52.5 (JPt-P
) 2348.2 Hz) and 43.9 ppm (JPt-P ) 2938.8 Hz). Formation
of 6-Me-b from 5-Me-b with increasing amount of acid may
be viewed as an effect of stabilizing ionic species 6-Me-b
through hydrogen bonding of excess acid to the carboxylate
anion. Similarly, with CF3CO2H (1.0 equiv), 3 was completely
converted to 5-Me-a (80%) and 6-Me-a (20%).
In sharp contrast, reaction of 4 in C6D6 with either CF3CO2H
or CH3CO2H (1.0 equiv) gave only 5-H-a or 5-H-b.13a Remark-
ably, even in pure CD3CO2D, 4 was converted to 5-H-b-d4
(90%), with only 10% of an unsymmetrical species, either 4 or
6-H-b-d4.
With phenol (5.0 equiv), a mixture of unreacted 3 (45%) and
6-Me-c (55%) was produced without detectable amounts of
5-Me-c.13a In contrast, reaction of 4 with phenol (2 equiv) gave
a mixture of 4 (39%) and 5-H-c (61%), without 6-H-c.13a
Thus, whereas both 3 and 4 react with phenol and carboxylic
acids, as do several known phosphine-Pt(0) complexes,14
3
without NH moieties leads to mixtures of 5-Me and 6-Me in
varying amounts, whereas 4 shows a pronounced ability to form
symmetrical species 5-H in a predictable manner.
Perhaps the most remarkable differences between 3 and 4
are reactivities toward methanol and water, which are both O-H
compounds of low acidity. Whereas 3 is completely unreactiVe
toward either of these reagents,13b even after days at 85 °C, 4
reacts reVersibly at 25 °C within minutes. Starting with 4 in
C6D6, methanol (5 equiv) gave 4 (50%) and 5-H-d (50%). In
the product, the Pt-O-CH3 moiety produced a singlet at 3.43
ppm with Pt satellites (3JPt-H ) 25.6 Hz). Starting with 4 in
C6D6, water (5 equiv) gave 4 (33%) and 5-H-e (67%). Although
formation of 5-H-e is reversible, a successful X-ray diffraction
study of this product (Figure 2)11 verified its identity as a square-
(8) See Supporting Information for details.
(9) Grotjahn, D. B.; Gong, Y.; Zakharov, L. N.; Golen, J. A.; Rheingold,
A. L. J. Am. Chem. Soc. 2006, 128, 438.
(10) (a) Garrou, P. E. Chem. ReV. 1981, 81, 229. (b) Hietkamp, S.;
Stufkens, D. J.; Vrieze, K. J. Organomet. Chem. 1979, 169, 107. (c) Goel,
A. B.; Goel, S. Inorg. Chim. Acta 1983, 69, 233.
(11) For 4: monoclinic, P21, colorless, 0.32 × 0.20 × 0.04 mm, a )
8.0597(4) Å, b ) 13.7541(6) Å, c ) 12.0198(5) Å, â ) 105.7600(10)°, V
) 1282.35(10) Å3, Z ) 2, T ) 218(2) K, Dcalc ) 1.602 g/cm3, R1 ) 1.96%
for 5228 independent reflections, GOF ) 0.819. For 5-H-e: monoclinic,
P21, colorless, 0.10 × 0.10 × 0.05 mm, a ) 15.938(2) Å, b ) 11.0650(15)
Å, c ) 16.233(2) Å, â ) 110.883(2)°, V ) 2674.7(6) Å3, Z ) 4, T )
100(2) K, Dcalc ) 1.583 g/cm3, R1 ) 2.38% for 11 778 independent
reflections, GOF ) 0.821. Hydrogen atoms involved in hydrogen bonding
were located from the difference map and refined isotropically.
(12) (a) Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold,
A. L.; Koetzle, T. F. Acc. Chem. Res. 1996, 29, 348. (b) Brammer, L. Dalton
Trans. 2003, 3145.
(13) (a) In these experiments, it is estimated that as little as 5% of 6
could have been detected. (b) As little as 1% of 5 could have been seen.
(14) Examples: (a) Goel, R. G.; Ogini, W. O.; Srivastava, R. C.
Organometallics 1982, 1, 819. (b) Thomas, K.; Dumler, J. T.; Renoe, B.
W.; Nyman, C. J.; Roundhill, D. M. Inorg. Chem. 1972, 11, 1795. (c)
Seligson, A. L.; Cowan, R. L.; Trogler, W. C. Inorg. Chem. 1991, 30, 3371;
also ref 2a.

Communications
Organometallics, Vol. 25, No. 24, 2006 5695
Scheme 2. Reactivity Depends on Imidazole Substituents
Figure 2. Conformers of 5-H-e. The three interactions of form
N-H- - -O have N-H-O angles 160(4)-166(5)°, N- - -O distances
2.545(4)-2.596(5) Å, and for the O-H- - -N interaction in B,
O-H-N ) 154.3° and O- - -N ) 2.567(5) Å.8 Hydrogen atoms
involved in hydrogen bonding were located from the difference
map and refined isotropically.
accepting ability of the metal center, but rather to create a
reliable binding pocket for the formally anionic oxygenated
ligands on Pt through hydrogen bonding. A relevant control
experiment started with 3, which is incapable of hydrogen bond
donation, and water (10 equiv) under the same conditions as in
Scheme 2. The strong acid CF3SO3H (1 equiv) was added;
remarkably, 90% of 3 remained unreacted, whereas the remain-
ing 10% formed the unsymmetrical cation of 6-Me. There was
no evidence13b of a product containing a coordinated water
molecule.
In summary, the results presented here show that ligands 1
and 2 produce very different behavior of Pt centers toward O-H
bonds. The effects of NH moieties provided by ligand 2 on Pt
include N-H activation in the formation of 4 and stabilization
of H-Pt-OR1 units in 5-H-a to -e by hydrogen-bonding
interactions with the oxygen atom of the attached ligand. Further
exploration and utilization of phosphine substituents as hydrogen
bond donors and acceptors is under active investigation in these
laboratories and is expected to be useful in fields such as
catalysis, small molecule activation, and molecular recognition.
planar species with a hydroxo ligand and trans phosphines.
Significantly, evidence for the effects of the ligand NH moieties
could be gathered. Two distinct conformers (A and B) appear
in the unit cell. In A, both imidazole NH units donate a hydrogen
bond to the hydroxo oxygen, whereas in B, there is only one
such interaction, and the other imidazole acts as an acceptor of
a hydrogen bond donated by the hydroxo ligand. Both cases
show interactions between the imidazoles and the hydroxo ligand
rather than with the hydride as in 4 (Figure 1).
We conclude by noting that because water, all O-H species,
and most heterocyclic N-H compounds are at least somewhat
acidic (having pKa values below 20), their bond activations by
necessity must be considered from the viewpoint of acid-base
chemistry. Both 3 and 4 react with the stronger acids in this
study (carboxylic acids or phenol) to give hydridic products
(5-Me and 6-Me from 3, only 5-H from 4).15 In contrast, only
4 reacts with water or methanol. From both these reactivity data
and the structure of 5-H-e in Figure 2, we conclude that the
function of the ligand NH moieties is not to increase the proton-
Acknowledgment. The NSF is thanked for support.
Supporting Information Available: Compound preparation,
characterization, selected spectra, hydrogen bonding in 5-H-e, and
CIF files. This material is available free of charge via the Internet
at http://pubs.acs.org.
(15) The reversibility of the O-H bond activations in this work precluded
a facile comparison of the pKa values of 5 and 6.
OM060880B